Torrefaction and Gasification of Hydrolysis Residue - …€¦ · · 2008-01-29Torrefaction and Gasification of Hydrolysis Residue UMEÅ UNIVERSITY 2007 5 ABBREVATIONS ASTM American

Torrefaction and Gasification of Hydrolysis Residue

from the Wood‐to‐Ethanol Pilot Plant in

Örnsköldsvik

Katarina Håkansson

Master of Science Thesis in Energy Engineering Umeå Institute of Technology

i

ABSTRACT Transportation converts high amounts of energy to economic activity and high living standard and is an important and indispensible part of the modern welfare state. As more countries develop toward stable economies, the transportation demands increase concurrently. The limited availability of oil will in time lead to extreme price developments and ultimately, oil deficiency. Combined with the domination of petroleum fuels in the transportation sector, it becomes an economical and energy security issue to find suitable replacements. Furthermore, as the impact of fossil fuels on the greenhouse gas concentrations in the atmosphere is now considered an undisputable fact, finding an environmentally friendly replacement of oil for all energy purposes and especially, the transportation sector, becomes more and more urgent. Ethanol produced from biomass is one such replacement.

When producing ethanol from lignocellulosic material biochemically using hydrolysis combined with fermentation, a large amount of residue consisting of mainly lignin is generated. Significant amount of energy is retained in this residue and must be utilized for the process to become economically viable. This report presents an investigation of the possibility to gasify the residue, thereby increasing the fuel yield as well as the overall efficiency. Gasification is a thermal conversion method, minimally described as combustion with insufficient oxygen supply. The final product is a gas mixture of carbon monoxide and hydrogen called synthesis gas (syngas). Using a variety of catalysts, both inorganic and biological, different transportation fuels can be produced from the syngas, such as methanol, DME, FT‐diesel and ethanol.

The pre‐treatment method torrefaction was also studied, as it has been shown to improve biomass fuel qualities. The biomass is heated to 200‐300°C for 10‐30 minutes resulting in an easily feed able and hydrophobic material. By the use of torrefaction, a simpler and less expensive gasifier can be used as the torrefied material is more easily milled into powder due to the more porous structure.

The hydrolysis residue was torrefied at different settings in an electrical furnace, according to an experimental design. Fuel composition, milling cost and circularity was analyzed and investigated for all batches and compared to non‐torrefied material. The gasification experiments were performed using a bubbling fluidized bed gasifier in bench‐scale (5kW). Pelletized torrefied and non‐torrefied hydrolysis residue was gasified with a 30% peat addition. For reference, wood pellets were gasified. Investigations of impurities and gas content were performed on all materials to evaluate the fuel suitability for gasification applications.

The results from the multivariate data analysis performed show that torrefaction of hydrolysis residue result in; lower milling cost, hydrogen content and oxygen content and higher ash, sulphur, carbon and nitrogen content and heating value compared to un‐treated hydrolysis residue. The torrefaction temperature and reaction time were the most influential factors, resulting in a high level of torrefaction (low solid yield). The visual analysis demonstrates no effect of torrefaction on the particle circularity of the material. This may be caused by an unsuitable analysis method and must be investigated further.

The syngas content examinations show that gasification of hydrolysis residue and torrefied hydrolysis residue result in equivalent gas composition compared to that of the wood reference material.

The complete dust load was lowered for both investigated materials compared to wood but more so for the non‐torrefied hydrolysis residue (1987 mg/Nm3 and 2771 mg/Nm3 for HR resp. TFHR compared to 3920 mg/Nm3 for wood). The particle size examination show the largest distribution in the 1.2‐1.4 µm region for both investigated materials compared to wood (largest distribution in the 0.4‐0.5 µm region).

The ASTM ash fusion tests performed in both oxidizing and reducing environment show a low initial deformation temperature of the ash (IT) for both torrefied and non‐torrefied hydrolysis residue (980°C and 970°C for oxidizing and 890°C and 830°C for reducing environment respectively). This was also observed during

ii

experiments, as the gasification temperature had to be lowered for the hydrolysis residue compared to wood due to bed agglomeration. The ash component analysis showed almost a hundred times higher sodium content in the residue (0.36 %‐D.S. and 0.45 %‐D.S. for HR and TFHR respectively compared to 0.004 %‐D.S. for wood), which explains the observed agglomeration and low ash fusion temperatures. The high sodium levels is retained from sodium hydroxide added to pH‐adjust the ethanol‐residue mixture (hydrolysate) before the filter press in the pilot plant.

The thermogravimetric analysis used for a relative fuel reactivity analysis provides information on material weight changes during a thermal process. From this, no conclusion on whether or not torrefaction lowers the material reactivity could be drawn. However, from the weight temperature derivative it was concluded that TFHR show a different chemical structure compared to HR and wood, further demonstration the molecular decomposition resulting from the torrefaction process.

From this work it is concluded that hydrolysis residue has the potential to be a suitable gasification fuel, especially if the pre‐treatment torrefaction is used. However, as long as the ash fusion problem remain (i.e. as long as sodium hydroxide is added before the filtration press), the hydrolysis residue is of no use in combustion or gasification applications.

iii

CONFIDENTIAL INFORMATION Certain results from this report is pending patent and are therefore concealed in grey.

ACKNOWLEDGEMENTS I would like to thank my experimental supervisors Kristoffer Persson and Ingemar Olofsson at ETPC for all their help, support and assistance. Thank you for always taking time to answer my questions, no matter the topic. I would also like to thank my supervisor Anders Nordin, ETPC for his help, ideas and continued confidence in me and this project.

My supervisor Torbjörn van der Meulen, SEKAB E‐Technology, is greatly acknowledged for his constant support and guidance all through this work and for providing insight and knowledge on the pilot plant. I would also like to thank Hans Grundberg, Processum, who has been very helpful to me with information on the hydrolysis residue.

I am also very grateful to Håkan Örberg and Mikael Thyrel, BTC for all hard work when producing hydrolysis residue pellets.

Last but certainly not least, I would like to thank Ulf Nordström, ETPC for his invaluable help with practical problem solving and construction. I could not have done this work without your help.

29 may 2007,

Katarina Håkansson

iv

TABLE OF CONTENTS ABSTRACT ..................................................................................................................................................... I CONFIDENTIAL INFORMATION ..................................................................................................................... III ACKNOWLEDGEMENTS ................................................................................................................................ III TABLE OF CONTENTS ................................................................................................................................... IV ABBREVATIONS ............................................................................................................................................. 5 1. INTRODUCTION .................................................................................................................................... 6

1.1. BACKGROUND ....................................................................................................................................... 6 1.1.1. Increasing Energy Usage and Oil Dependency ................................................................................................. 6 1.1.2. Global Warming from Fossil Fuel Use .............................................................................................................. 7 1.1.3. Renewable Fuels for Transportation ............................................................................................................... 8

1.2. OBJECTIVE ............................................................................................................................................ 8 2. THEORY ................................................................................................................................................ 9

2.1. ETHANOL FROM LIGNOCELLULOSIC MATERIAL .............................................................................................. 9 2.1.1. Ethanol Production ........................................................................................................................................... 9 2.1.2. The Pilot Plant in Örnsköldsvik ....................................................................................................................... 10

2.2. HYDROLYSIS RESIDUE .............................................................................................................................10 2.3. TORREFACTION.....................................................................................................................................11

2.3.1. Torrefaction Mechanisms for Lignocellulosic Biomass ................................................................................... 12 2.4. GASIFICATION ......................................................................................................................................13

2.4.1. Fluidized Bed Gasifiers.................................................................................................................................... 14 2.4.2. Entrained Flow Gasifiers ................................................................................................................................. 15

3. MATERIALS AND METHODS ................................................................................................................ 16 3.1. TORREFACTION OF HYDROLYSIS RESIDUE ....................................................................................................16

3.1.1. Equipment and Materials ............................................................................................................................... 16 3.1.2. Moisture Measurements ................................................................................................................................ 16 3.1.3. Torrefaction Experiments ............................................................................................................................... 16 3.1.4. Milling Analysis .............................................................................................................................................. 17 3.1.5. Visual Analysis ................................................................................................................................................ 17

3.2. GASIFICATION EXPERIMENTS ...................................................................................................................18 3.2.1. Gasification materials .................................................................................................................................... 18 3.2.2. Fluidized Bed Gasification at Bench Scale ...................................................................................................... 19 3.2.3. Syngas Analysis Methods ............................................................................................................................... 19

3.3. REACTIVITY TESTING USING TGA ..............................................................................................................21 4. RESULTS AND DISCUSSION .................................................................................................................. 22

4.1. TORREFACTION OF HYDROLYSIS RESIDUE ....................................................................................................22 4.1.1. Milling calculations ........................................................................................................................................ 22 4.1.2. Multivariate Data Analysis ............................................................................................................................. 23 4.1.3. Visual Analysis ................................................................................................................................................ 25

4.2. GASIFICATION OF HYDROLYSIS RESIDUE .....................................................................................................26 4.2.1. Ash Analysis of Hydrolysis Residue ................................................................................................................. 26 4.2.2. Particle Emission Analysis .............................................................................................................................. 27 4.2.3. ........................................................................................................................................................................... 4.2.4. Results from Gas Analysis .............................................................................................................................. 30

4.3. REACTIVITY ANALYSIS ............................................................................................................................31 4.3.1. Reactivity theories and discussion .................................................................................................................. 33

5. CONCLUSIONS .................................................................................................................................... 35 6. FUTURE WORK ................................................................................................................................... 36 7. REFERENCES ....................................................................................................................................... 37 8. APPENDIX ........................................................................................................................................... 39

APPENDIX 1: TORREFACTION SETTINGS AND RESULTS .................................................................................................39 APPENDIX 2: TORREFACTION PHOTOS .....................................................................................................................39 APPENDIX 3: MULTIVARIATE ANALYSIS PLOTS ...........................................................................................................41 APPENDIX 4: GASIFICATION RESULTS ......................................................................................................................45

Torrefaction and Gasification of Hydrolysis Residue UMEÅ UNIVERSITY 2007

5

ABBREVATIONS ASTM American Society for Testing and MaterialsBFBG Bubbling Fluidized Bed GasifierBTC Biobränsletekniskt Centrum (Biofuel Technical Centre)CFBG Circulating Fluidized Bed GasifierDME Dimethyl Ether DoE Design of Experiments D.S. Dry Substance E Energy ECN Energy research Centre of the NetherlandsETC Energitekniskt Centrum (Energy Technology Centre)ETPC Energy Technology and Thermal Process Chemistry (Umeå University)FBG Fluidized Bed Gasifiers FT Fisher‐Tropsch GC Gas Chromatography HHV Higher Heating Value HR Hydrolysis Residue IEA International Energy AgencyLHV Lower Heating Value m Mass MC Milling Cost MFV Minimum Fluidizing VelocityOECD Organization for Economic Co‐operation and Development

Members: Australia Austria Belgium Canada Czech Republic Denmark

Finland France Germany Greece Hungary Iceland

IrelandItaly Japan Korea Luxemburg Mexico

NetherlandsNew Zealand Norway Poland Portugal Slovak Republic

Spain Sweden Switzerland Turkey United Kingdom United States

P Power PAH Poly Aromatic HydrocarbonsPLS Projections to Latent StructuresSLU Sveriges Lantbruksuniversitet (Swedish University of Agricultural Sciences) SSF Simultaneous Saccharification and FermentationSY Solid Yield TFHR Torrefied Hydrolysis Residue TGA Thermogravimetric AnalysisU Voltage


6

1. INTRODUCTION For the last fifty years, oil has been one of the most indispensable physical substances in the modern welfare state. It is used as raw material in production of many everyday products such as plastics and paint, for domestic heating (in decreasing extent) and last but not least, as the dominating source of fuel for the transportation sector. The transportation system translates high amounts of energy to economic activity and high living standard and is an important part of industrialized society. Unfortunately, it also contributes to a large energy dependency and environmental degradation.

1.1. Background 1.1.1. Increasing Energy Usage and Oil Dependency The International Energy Agency’s (IEA) data for the total oil consumption (Figure 1.1) show that transport energy has been the single most important factor behind the increasing oil demand. According to estimations from the IEA, this domination will further continue over the following twenty years [1].

Figure 1.1: Evolution from 1971 to 2004 of world total oil consumption by sector. *Other sectors include agriculture, commercial and public service, residential and non‐specific [2].

Economic activity and population growth are the key factors that determine the transportation energy demand for a specific country. Projections from the Population Reference Bureau [3] show that the 6.1 billion human inhabitants present in the year 2000 will increase to more than 9 billion by the year 2050. Nearly all of this annual growth is projected to occur in the developing countries in Africa, Asia, and Latin America over the coming 30 years.

During the same period of time, estimations show that over 70% of the predicted increase in total energy demand will come from these developing countries, with China alone accounting for 30% [4]. Increased economic activities raise the per capita income and as the living standard improve, so do the demand for personal transportation. The OECD countries will continue to hold the largest share of transport energy use over the next 20 years, but the primary growth will come from non‐OECD countries. The oil demand for developing countries is estimated to more than double by the year 2020 compared to today’s level [1].

Consequently, as the populations and economic activity simultaneously expand, a near exponential increase in world transportation fuel demand will emerge. Figure 1.2 shows the total energy consumption by regional separation 1971‐2004, where the tendencies towards a more rapid increase begin to show after 2002.

Non‐energy use Other* Transport Industry

1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004

Year

Oil consum

ption [M

illion Tons] 3500

3000

2500

2000

1500

1000

500

0


7

Figure 1.2: Evolution from 1971 to 2004 of World Total Energy Consumption by region. Prior to 1994 combustible renewable and waste final consumption have been estimated. *Asia excludes China, Japan and Korea (Japan and Korea are OECD countries) [2].

It is an accepted fact that the available oil reserves are limited and that the world oil supply will run out at some point, probably during this century. In specific terms, the fossil energy reserves on this planet will never actually run out [5]. Firstly because of fossil energy stored further down toward the earth’s core and other places we cannot, or cannot afford (economically or environmentally), to reach. Secondly, because the free market forces of supply and demand ensure that the available oil reserves will not be completely emptied but rather too expensive to use.

When the global so called oil peak (point of highest oil production) will occur is greatly debated among scientist. Some predict it to occur around 2020, others up to ten years later. The Association for the Study of Peak Oil and Gas (ASPO) has a more pessimistic view and expect the oil peak to occur before 2010 or may have taken place already [5]. Sooner or later, the oil production will decrease or simply not increase at the same rate as the oil consumption. At some point, the available oil supply will be insufficient to meet the market demands, creating a gap between production and consumption. The forces of the free market will trigger the oil prices to extreme heights as this gap is widened further, resulting in oil shortage and economic disorder, long before there is an actual deficiency of oil. This prospect alone should be sufficient incitement for world governments to reduce the oil dependency and thereby increase the energy security for their country.

1.1.2. Global Warming from Fossil Fuel Use Even if the oil had been an endless resource, it would be important for our continued habitation of this planet to significantly reduce the use of fossil fuels. The impact of combustion of fossil fuels on the greenhouse gas concentrations in the atmosphere is now considered an undisputable fact. As the composition of the atmosphere changes, so does the earth environment, resulting in a climate change of which we can only speculate the full effects. The amount of carbon dioxide in the atmosphere has increased steadily since industrialization commenced in the end of the 18th century (Figure 1.3) and will continue to increase during this century [6].

Figure 1.3. Carbon dioxide concentration in the atmosphere over the last millennium [6].

1.5

1.0

0.5

0.0

CO2 con

centratio

n [ppm

] 380

360

340

320

300

280

260

240 1000 1200 1400 1600 1200 2000

Year

8000

7000

6000

5000

4000

3000

2000

1000

0 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004

Year

Energy con

sumption

[Million To

ns]

Africa Latin America Asia* China Non‐OECD Europe Former USSR Middle East OECD

Radiative forcing [W

m‐2]


8

The enhanced concentrations of greenhouse gases in the atmosphere result in a global temperature raise between 1 to 6‐7 °C by the year 2100 [6]. This in turn, will influence the polar ices and glaciers, causing the sea level to rise due to melt ice water. The increased temperature also results in elevated evaporation from land and sea, causing draught as well as floods at different locations around the world. Other effects of global warming include more frequent and more energetic weather disasters such as tornados, hurricanes, cyclones and heavy storms.

1.1.3. Renewable Fuels for Transportation There is no single solution to the global warming and oil shortage known at present time, therefore several options must be researched and studied simultaneously. A variety of second generation transportation fuels can be produced from biomass such as ethanol, methanol, FT‐diesel, DME, hydrogen gas and more, so called biofuels. The use of biofuels to replace petroleum transportation fuels such as gasoline and diesel has considerable possibilities. The CO2 emitted from the use of biofuels are equal to the amount absorbed by the plant during the photosynthesis, and biofuels are therefore considered to be CO2‐neutral.

Bioethanol is believed to have the highest potential to replace fossil fuel over the next decade as the current transportation system can be utilized with only few moderations. Several vehicle brands now offer flexi‐fuel cars where both gasoline and ethanol can be utilized as fuel in various combinations. Ethanol as fuel has higher octane number but lower heating value and ignition difficulties at low temperatures compared to gasoline [7]. This can be overcome by the use of engine pre‐heaters and suitable additives. Bioethanol can be produced from a variety of biomass feedstock using hydrolysis and fermentation (Section 2.1).

From a mixture of carbon monoxide and hydrogen gas produced during gasification of biomass (Section 2.3), several fuels suitable for transportation use can be produced, using different catalysts and after‐treatment methods. These include methanol, DME and FT‐diesel. Methanol has similar fuel characteristics to ethanol but is more hazardous and corrosive. DME is a suitable diesel replacement but has the drawback of being gaseous at atmospheric conditions. On the other hand, the exhaust gases are much cleaner than those from diesel and it is not corrosive to metals. FT‐diesel consists of –CH2– chains of 9 to 25 carbon atoms. It can be used instead of regular diesel in a diesel engine, but additives are needed to improve lubricity, anti‐statics and cold flow properties [8]. Additionally, ethanol can be produced from the gas produced in the gasification process, but the method is currently emerging demonstration stage.

1.2. Objective When producing ethanol from lignocellulosic material by hydrolysis combined with fermentation, a large amount of residue consisting of mainly lignin is generated. In order to make the wood‐to‐ethanol hydrolysis process economically viable, the large amount of energy remaining in the residue must be utilized. Previous research [9] has indicated low alkali and ash content as well as high heating value for the residue making studies of its properties during combustion and gasification very interesting. Ek, Monica (2005) [10] states that the most beneficial and realistic use of the large amounts of hydrolysis residues is to use them as a feedstock for the production of liquid fuels through gasification.

The objective of this thesis was to evaluate the qualities and properties for the hydrolysis residue as gasification fuel. If suitable for gasification, the fuel yield as well as the process overall efficiency will increase. The content and quality of the gasification product gas, as well as the process parameters, was investigated through bench scale experiments using a bubbling fluidized bed gasifier.

The pre‐treatment method torrefaction has been shown to improve fuel parameters such as energy density and water content in addition to having a substantial effect on the operation of particle size reduction of biomass [11]. It was therefore included in this thesis to investigate the effect of torrefaction on hydrolysis residue.


9

2. THEORY 2.1. Ethanol from Lignocellulosic Material Bioethanol can be produced biochemically from any biological feedstock containing either sugar or materials that can be converted into sugar such as starch or cellulose. At present time, commercial bioethanol is produced mainly from material containing either pure sugar or starch, as cellulosic biomass (containing mainly cellulose, hemicellulose and lignin) are more difficult to convert to sugar [7]. However, as most plant matter is neither sugar nor starch but cellulosic material, substantial research in this area is presently conducted in several countries around the world.

A description of the major steps in ethanol production from lignocellulosic material is provided in this section together with brief description of the ethanol pilot plant in Örnsköldsvik.

2.1.1. Ethanol Production The wood‐to‐ethanol fermentation process consists of two main steps; hydrolysis and fermentation (More detailed description in Figure 2.1). Hydrolysis or saccharification is primarily the breakdown of cellulose and hemicellulose to pentose and hexose sugars. Fermentation is the conversion of sugars to ethanol using suitable microorganisms. After that, several steps follow with purpose to concentrate and purify the ethanol, such as distillation and evaporation (not described in this report).

Figure 2.1: Schematic description of Wood‐to‐Ethanol production using hydrolysis and fermentation [12].

Raw Material Wood are composed mainly (around 90‐98 % D.S.) of the polymers cellulose, hemicellulose and lignin, which provide plant strength. The few remaining percent are ash and wood extractive. The ratio of the main components is plant specific but generally 35‐50% is cellulose, 20‐30% hemicellulose and 15‐30% lignin [13].

Cellulose is a long linear polysaccharide built of up to 10 000 linked D‐glucose units, written (C6H10O5)n [14]. When hydrolyzed, cellulose split into hexose sugars which are easily fermented to ethanol. Hemicellulose is a branched polysaccharide with a low degree of polymerization compared to cellulose and is built of both hexose and pentose sugars [13]. In comparison to cellulose, hemicellulose is more easily hydrolyzed due to its lesser molecular strength. The last main component, lignin, is an aromatic polymer which acts as glue between fibers of cellulose and hemicellulose. Lignin molecules cannot be hydrolyzed to sugar and are consequently not fermented to ethanol (more on lignin, Section 2.2) [13].

Hydrolysis Saccharification or hydrolysis is the decomposition of the cellulose and hemicellulose polymers into sugar units with the addition of the hydrogen cation and the hydroxide anion of water [15]. Essentially there are three hydrolysis methods available; concentrated‐acid hydrolysis, dilute‐acid hydrolysis and enzymatic hydrolysis. The latter two are processes researched and evaluated at the ethanol pilot plant in Örnsköldsvik and will therefore be discussed more thoroughly.

Distillation

Fermentation

Fermentation

Enzymatic hydrolysis

Pre‐hydrolysis

Acid hydrolysis

Pre‐treatment


10

Concentrated acid hydrolysis is the use of concentrated acid to dissolve cellulose and hemicellulose into sugars. The process takes place around room temperature in a non‐pressurized system. The problems associated with this method are the need for corrosion resistant materials and expensive acid recovery systems [13]. At present time this method is not economically viable.

Dilute acid hydrolysis is equivalently the use of dilute acid to dissolve cellulose and hemicellulose into sugars. The process usually operates with an acid concentration of 0.5‐1.5 vol% in a pressurized system with temperatures around 160‐220°C [13]. Sulphuric acid is the most commonly used acid in hydrolysis processes. A frequent problem with both acid hydrolysis methods is that the acid may break down molecular bonds in the freed glucose molecules, resulting in a lower yield of sugars to enter the next process stage. The formed products of the continued breakdown inhibit the microorganisms of the fermentation stage, which lower the ethanol yield further. To overcome this problem it is common that dilute acid hydrolysis is performed in two steps at slightly different conditions [7]. The first step is generally called pre‐treatment or pre‐hydrolysis, where more easily degraded hemicellulose molecules are hydrolyzed. The hydrolysate from the pre‐hydrolysis is by‐passed and re‐joined with the process stream after the second hydrolysis step. The second step, where cellulose molecules are broken down has more severe conditions with higher temperature and acid concentration.

Enzymatic hydrolysis is a method where a specific set of enzymes is used to hydrolyze wood materials. This method requires a pre‐treatment step, such as dilute‐acid hydrolysis, in order to make the cellulose molecules more accessible to the enzymes. The process is slower than dilute acid‐hydrolysis but a high yield (~90%) can be achieved after a few days [13]. Unfortunately, the enzymes are inhibited by their own products and therefore the reaction rate decreases at high sugar concentrations. To overcome this problem it is possible to combine the hydrolysis and fermentation steps into one, called Simultaneous Saccharification and Fermentation (SSF). Sugars produced are simultaneously fermented to ethanol and the inhibiting factor is diminished.

Fermentation Fermentation is the last major step in producing ethanol from lignocellulosic material. The microorganism used to convert hexose sugars into ethanol is regular bakers’ yeast (Saccharomyces cerevisiae) [7]. It can unfortunately not ferment pentose sugars and research is underway to find a microorganism that can, in order to maximize the ethanol yield. The hydrolysate may contain substances with inhibiting effect on the microorganisms, as mentioned earlier, which lowers the ethanol yield. After the fermentation step, a solution of ~4.5 % ethanol is attained using SSF [16].

2.1.2. The Pilot Plant in Örnsköldsvik The ethanol pilot research plant in Örnsköldsvik has a capacity of 400‐500 liters of ethanol per day, which corresponds to around 2 tons D.S. of wood chips per day. SEKAB E‐Technology (former Etek Etanolteknik AB) was in charge of the construction of the plant and is now responsible for the process research and development, though the plant is formally owned by Umeå University and Luleå University of Technology [17]. The pilot plant was constructed for both dilute‐acid hydrolysis and enzymatic hydrolysis but until recently the research was focused solely on the dilute‐acid process. As of august 2006, research on the enzymatic hydrolysis process is performed as well [18]. As the plant is a research and development facility, the produced ethanol is not commercially utilized.

2.2. Hydrolysis Residue Lignin, the main component of hydrolysis residue, is an aromatic polymer, consisting mostly of phenyl propane units forming a very complex three‐dimensional structure (Figure 2.2). In biomass, lignin acts as glue between fibers of cellulose and hemicellulose thereby contributing to the stability of plant tissue and the protection against degrading microorganisms [13].


11

Figure 2.2: The complex structure of the lignin molecule [10].

Lignocellulosic material usually contains 15‐30% lignin as explained in Section 2.1.1. This large amount of material cannot be hydrolyzed in an ethanol process and will therefore end up as residue. As result, a significant amount of energy is not utilized which lower the overall yield of the process. In order for ethanol production from lignocellulosic material to become economically viable, it is important that the lignin component in the material is used to full extent. In the former Soviet Union, extensive studies were carried out on hydrolysis residue as they had several hydrolysis plants in operation. Research about different chemical reactions that could modify the residue to change its chemical properties was performed and the end use of the product ranged from soil additives to dispersants. However, most of the hydrolysis residue was never utilized as the modifications were either to expensive, to advanced or did not function as desired. Today, the most realistic use of the large amount of hydrolysis residue that will be produced if ethanol from biomass production is expanded is through gasification or combustion [10].

In the ethanol pilot plant, the hydrolysis residue (consisting of 40‐45% of the original feedstock [19]) is filtered out from the hydrolysate through a membrane filter press before the fermentation step [17]. It contains mainly lignin, un‐hydrolyzed cellulose and water but also small amounts of sugars and extractives (~1%). The precise composition of the residue from the ethanol pilot plant in Örnsköldsvik depends on the origin of the residue; i.e. if enzymatic or dilute‐acid hydrolysis has been operated. If dilute‐acid hydrolysis is used, the dried residue contains ~50% lignin and ~50% cellulose. When enzymatic hydrolysis is the origin, the cellulose content is lowered to ~30% with a lignin content of ~70%. In both cases, the residue has a moisture content of 50‐55%.

The material has a rather small size distribution compared to wood or other biomass, making it very well suited for powder gasification or combustion utilization. Dried hydrolysis residue unfortunately result in a significant dust formation, which may create hazardous working environments or even dust explosions.

2.3. Torrefaction Biomass as fuel source has lower energy density, higher moisture content and is more difficult to mill/grind into small particles than more traditional energy sources such as fossil fuels. The material is highly hydrophilic and thereby absorbs moisture when transported and stored, which may lead to storage and transportation problems, uncontrolled biological degradation processes and in worst case scenario, self ignition [20][21].

Torrefaction is a pre‐treatment technology that improves all of the above mentioned drawbacks of biomass application. It is a thermochemical pre‐treatment method utilizing an operating temperature of 200°C to 300°C. In literature, several synonyms can be found for the torrefaction process such as roasting, slow‐ and mild pyrolysis, wood cooking and high‐temperature drying. The connection to pyrolysis is especially easy to make as torrefaction covers the very first decomposition reactions of pyrolysis at different process conditions [22]. The process is characterized by atmospheric pressure, oxygen free (reducing) environment, low particle heating rates (<50°C/min) and relatively long residence time (up to 60 minutes, drying included) [22].

Approximately 70% of mass and 90% of the energy content is retained after the torrefaction process is completed, resulting in higher energy density and heating value [22]. After the torrefaction process, the material has a brittle structure, similar to that of roasted coffee, which makes it easier to mill compared to


12

untreated biomass. Torrefaction, followed by milling, has been shown to result in a more homogeneous, spherical powder with better feeding properties to a gasifier [22] [23]. Furthermore, the material attains hydrophobic qualities due to the removal of OH‐groups and the creation of unsaturated non‐polar structures, thus preventing many logistic problems [24]. A sufficiently high reactivity of the biomass powder is also essential for cost efficient gasification systems, and torrefaction may aid the attainment of sufficiently small size distributions for this reactivity.

The total torrefaction process typically consists of three stages; drying, torrefaction and cooling (Figure 2.3, stages marked with arrows). In the drying stage, the biomass is heated and dried, as water molecules and some light organic compounds are evaporated. When the temperature reaches around 200°C, torrefaction commences and the material is partly decomposed, releasing various types of volatiles. The biomass is now completely dried and has obtained hydrophobic qualities. The final step consists of cooling the torrefied biomass to the desired final temperature [22].

Figure 2.3: Heating stages of moist biomass during a typical torrefaction batch process. th = heating time to drying, tdry = drying time, th,int =heating time to torrefaction, ttor = reaction time torrefaction temperature, ttor,h = heating time from 200°C to torrefaction temperature (Ttor), ttor,c = cooling time from Ttor to 200 °C, tc = cooling time to ambient temperature [11].

2.3.1. Torrefaction Mechanisms for Lignocellulosic Biomass During the actual torrefaction step, the thermal decomposition mechanisms vary depending on chemical structure of the biomass. As described by Bergman, P.C.A. et al (2005), different reaction pathways can be defined and grouped in to main regions (Figure 2.4).

Figure 2.4: Main chemical phenomena from heating of lignocellulosic materials during

torrefaction. The lettering is described in the section below. Original image [22] has been modified.

Time

Tempe

rature [°C]

300

200

100

0

Temperature [°C]100 150 200 250 300

Hemicellulose

Lignin

Cellulose

Torrefaction

Ttor

ttor,h ttor,ctc

ttor

th,int

tdry

th

heating drying torrefaction cooling Intermediate

heating


13

Similar decomposition regions can be defined for all three polymers in lignocellulosic biomass, although the location on the temperature axis varies. (A) represents the initial drying of the material and as the temperature is increased to region (C), depolymerization commences and new, shorter polymers re‐condense within the remaining physical structure. (D) represents limited devolatilization and carbonization of the structures formed in the previous stage and the remaining intact polymers. The end region (E) consists of extensive devolatilization and carbonization of the products of step C and D. For lignin, the main component of hydrolysis residue, an additional region (B) has been defined where softening of the molecules occur which densifies the biomass [22].

2.4. Gasification Fuels such as coal, petroleum or biomass contain carbonaceous material that when mixed with heat and oxygen will be thermally converted by combustion, gasification or pyrolysis. Complete combustion is when all carbon (C) and hydrogen (H2) present in the fuel are fully oxidized to carbon dioxide (CO2) and water (H2O). This is achievable only when the supplied oxygen together with the oxygen present in the fuel is sufficient (i.e. stoichiometric amount). If the amount of C and H present in the fuel are known, the stoichiometric amount can be calculated using the reactions, and ½ . The oxygen ratio or the lambda‐value (λ) is defined as the ratio between the actual oxygen supply and the stoichiometric amount. By definition, combustion thus has λ ≥ 1 with typical values around λ=1‐1.5.

Pyrolysis is the thermal conversion process at the opposite end of the oxygen‐ratio spectra when λ = 0‐0.2 and essentially no oxygen is present. Products formed are mainly carbon monoxide (CO), methane (CH4) and charcoal. Extreme pyrolysis, that leaves only carbon as residue, is called carbonization.

Gasification is the processes between combustion and pyrolysis with λ‐values from 0.2 to <1.0, though values above 0.6 are rarely utilized with CO and H2 as the desired products. Significantly low oxygen content is necessary as higher λ (more oxygen added) shift the equilibrium state to more of the unwanted products CO2 and H2O. The product of the gasification process is called product gas or producer gas and beside CO and H2 it contains more or less of CO2, CH4, nitrogen (N2), H2O, hydrocarbons (CxHy) and other impurities such as tar, soot and PAH. Cleaned product gas, where impurities and inhibiting products are removed, is called synthesis gas (abbreviated syngas) or biosynthesis gas (biosyngas) if non‐fossil feedstock is used.

There are two main steps in traditional gasifiers; oxidization and reduction. In the first oxidizing step, the fuel is partially combusted by introducing air or oxygen. The heat can also be supplied from an external source such as steam (gasifying medium). This releases the energy needed to initiate the reactions of the second step. When selecting the gasifying medium, economics are most often the deciding factor. Air is cheaper than oxygen (oxygen plant needed) but introduces a large amount of inert N2(g) to the product gas, which have to be removed in the after‐treatment steps. Using superheated steam is economically viable only if excess process heat is easily obtainable. A drawback is that extra heat might be required since reactions with steam are highly endothermic. Finally, in the reducing step, the gases released from the fuel feedstock in the oxidizing process are reduced. As result, the released energy from the initial step can be captured and stored within the product gas.

A great advantage of gasification is that the end product (syngas) is almost independent of the fuel input and the process can therefore be applied to most biomass feedstock or residues. The process also provides great end product flexibility where several different transportation fuels can be produced from the exact same product, the syngas, such as methanol, FT‐diesel (synthetic diesel) and DME as well as different green chemicals. The gas can also be burned in a gas turbine, producing power and heat or be used directly as fuel in combustion engines and fuel cells.


14

The content of the product gas depends on the parameter settings in the gasification process, which in turn is influenced by the type of gasifier used. The following is a brief description of the most commonly used types that provide a syngas of a good quality; fluidized bed and entrained flow gasifiers.

2.4.1. Fluidized Bed Gasifiers Gasification in fluidized bed gasifiers (FBG) normally takes place at temperatures around 800‐900°C. The bed consists of a mixture of inert bed material, usually quartz sand with a size distribution around 250 µm, and fuel material [25]. The mixture is fluidized (i.e. has fluid‐like properties) by the gasifying medium, typically air. The main advantages of FBG is the homogeneous conditions and improved mixing, kinetics and heat exchange, which leads to increased overall gasifier efficiency.

There are two types of FBG, bubbling fluidized bed gasifier (BFBG), see Figure 2.5, and circulating fluidized bed gasifier (CFBG), see Figure 2.6. The key difference between the two is the velocity of the fluidizing air. In a BFBG, the velocity is slightly above the so called minimum fluidizing velocity (MFV), which evidently is the minimum velocity to fluidize the bed. At this velocity, bubbles are formed and the bed consists of a fluid‐like mixture of bed material and fuel. When the velocity is increased, the bubble size builds up and the bed particles tend to follow the air up through the gasifier [25]. In a CFBG the bed particles follow the fluidizing medium to the top of the reactor where they are recycled into the bed using a cyclone separator.

Fluidized bed gasifiers are easy to upscale but when using biomass as feedstock the risk for bed agglomeration is introduced, induced by alkali‐rich ashes present in the fuel. The bed particles obtain a sticky coating that cause them to attach to each other, producing so called agglomerates. To overcome this problem the temperature is kept sufficiently beneath the ash melting temperature of the fuel (normally <900°C) and the bed material is replaced continuously. The low gasification temperature unfortunately results in higher amount of tars in the product gas that must be removed at a later stage. Another drawback from FBG is the increased amount of particulates and soot in the product gas [25].

Figure 2.5: Bubbling fluidized bed gasifier [25] Figure 2.6: Circulating fluidized bed gasifier [25]

Raw biosyngas

Cyclone

Gas phase reactions

Circulating fluidized bed

Fuel

Additional sand

biosyngasRaw

Cyclone

Additional sand Fuel

Bubbling fluidized bed

Fluidizationmedium

Fluidization medium

GrateGrate Bottom ash andBottom ash and bed materialbed material

Fly ash andparticles

Inert + Char


15

2.4.2. Entrained Flow Gasifiers Entrained flow gasifiers (Figure 2.7), normally operates at temperatures around 1200‐1400°C and are often pressurized. Due to this higher temperature, the dust load of products of incomplete gasification is highly reduced and an almost tar free product gas is produced [25] [26].

The fuel is normally introduced as gas, solid powder or slurry and mixed with a steam/oxygen stream. After entering the reaction chamber, the mixture is gasified in a high temperature powder flame [25].

However, the high temperature of the flame results in production of a lot of excess heat which must be utilized somehow in order to achieve adequate overall process efficiencies. Process integration of a gasification based conversion system is therefore an important aspect.

When biomass is used as fuel feedstock it must be milled into a fine powder (or pyrolyzed to gas, pyrolysis oil and coke) [25]. This is a big and expensive process step that hopefully can be significantly reduced by the pre‐treatment method torrefaction (Section 2.3). A significant benefit with the entrained flow principle is the simple technology, which in turn lead to cheaper and less complicated full‐scale gasifiers.

Figure 2.7: Entrained Flow gasifier [25]

Slag

Syngas

Steam

Coal Slurry Oxygen

Feedwater


16

3. MATERIALS AND METHODS 3.1. Torrefaction of Hydrolysis Residue 3.1.1. Equipment and Materials The torrefaction experiments were performed at facilities used by ETPC at Umeå University. The experimental equipment is illustrated in Figure 3.1. A 3x330 volt, 9 kW electric furnace from Chemantex was used as reactor for the torrefaction batch process. The power supply to the furnace was regulated by a PID‐regulator using the process temperature as controlled variable. A thermo‐element (type N) placed in the back measured the actual furnace temperature, during experiment. Four additional thermo‐elements were placed at different locations in the furnace space and connected to a 48 channel tri‐multi‐thermo field point system, serving as the basis of the temperature measurements. The field point system was further connected to a laptop where a control program (LabView) was used to log and view the temperature signals 20 times/second.

As torrefaction requires reducing environment, inert nitrogen gas was constantly entering the furnace during experiment. Minimizing the oxygen contact until the sample has reached sufficiently low temperature is crucial in order to avoid undesired combustion. Therefore, a nitrogen filled cooling space was constructed and used to shield the sample after completed torrefaction and through the entire cooling process.

3.1.2. Moisture Measurements The hydrolysis residue used for the torrefaction experiments could not be retrieved directly from the pilot plant as plant operation was currently down. Instead, it was arranged so that hydrolysis residue (enzymatic origin) shipped earlier from the pilot plant to BTC in Umeå could be used. The material had been pre‐dried to a moisture content of around 8% and in order to attain more initial moisture values, water was added to a moisture content of ~35‐40%.

Measurements were performed on the material according to the Swedish standard SS 18 71 70 to establish the actual moisture content. After calculations, 500 g of water was added per 1000 g of material in order to achieve the desired moisture content. The material was left to soak for three days and then new measurements were performed to validate the new moisture content.

3.1.3. Torrefaction Experiments The experiments were planned according to a full factorial design (2x) to avoid correlation between factors (variables) and to span a larger experimental space [27]. This makes it easier to find the optimal settings or a direction to the optimal settings when evaluating the responses (result variables, description in Section 4.1).

To establish the proper settings for the designed factors, a few trial runs were performed. Three factors were used in the full factorial experimental design together with three centre points (replicates). The centre points are required in order to evaluate the quality of the model and its reproducibility. Table 3.1 show a list of the varied factors and the corresponding design settings.

(4)

Figure 3.1: Experimental equipment for torrefaction and moisture testing. (1) Electric furnace, (2) nitrogen gas, (3) cooling space.

(3)

(2)

(1)


17

Table 3.1: Description of designed factors

Factors Description ‐ 0 + Input temperature* [°C] Furnace temperature at sample input 100 200 300 Output temperature [°C] Desired torrefaction temperature 270 285 300 Torrefaction time [min] Total reaction time over 200°C 15 20 25

The reaction time and reaction temperature are two obviously significant variables and was therefore chosen as factors. The final factor, the input temperature, influences the heat rate and was used to represent that process parameter as the two variables are positively correlated, i.e. the higher input temperature, the higher the heat rate. The final experimental design is shown in Table 3.2 below, together with a randomized run order. Figure 3.2 illustrates how the experiments span the experimental space.

Each experimental run described in Table 3.2 was initiated when the furnace reached the input temperature. A nitrogen flow of 17 l/min was constantly entering the furnace during experiment, ensuring inert atmosphere as described above. The sample, 200 g of hydrolysis residue, was weighed on a metal plate and then placed inside the furnace. The desired value on the regulator was then raised to the output temperature. When the sample temperature reached 200°C a timer was set to monitor the desired torrefaction time. After completed time, the plate was removed from the furnace and placed in the cooling space, previously filled with nitrogen gas. When sufficient low temperature (~50‐70°C) had been reached, the plate was weighed and the solid yield calculated.

3.1.4. Milling Analysis As mentioned in the theory section, torrefaction of woody material result in a more brittle material, easier to grind/mill. However, as hydrolysis residue is rather porous and consists of small particles to begin with, due to the extensive acid treatment during the hydrolysis, significant reduction in milling power may not occur. To validate this assertion, milling experiments on the torrefied hydrolysis residue were performed.

The analysis equipment consisted of a small rotor mill from Retsch, able to produce a fineness of 0.25, 0.50 or 0.75 mm. A clamp meter was attached to the electrical cord and connected to the PC and the LabView control program, thereby measuring the electrical current needed to grind the added amount of material. As the heating value for each run was determined through chemical analysis, the milling cost in fuel energy could be calculated. Three or four milling replicates were performed for each torrefaction run.

3.1.5. Visual Analysis Another interesting assertion of the torrefaction process (combined with milling) is that it may result in more spherical particles.

Table 3.2: Experimental design for torrefaction experiments

No Time (X1) Input Temp(X2)

Output Temp(X3)

Run

1 15 100 270 12 25 100 270 83 15 300 270 24 25 300 270 95 15 100 300 116 25 100 300 37 15 300 300 78 25 300 300 49 20 200 285 1010 20 200 285 611 20 200 285 5

Figure 3.2: Experimental space in a full factorial design.


18

This in turn could greatly simplify the feeding process to a gasifier as pneumatic transportation can more easily be utilized. To investigate this, visual analysis was performed using an online digital camera model Micro Eye (uEye, Camera Model UI‐2220‐M C‐ganga) connected to a PC and mounted onto a microscope. The samples were prepared using the setup displayed in Figure 3.3. Five glass plates were placed in a 40‐litre glass container, covered by a glass lid with one small corner‐piece removed. A small amount of sample placed on the tip of a spoon was positioned 2‐3 centimeters under the lid in a 45° angle to the container wall. Compressed air was used to give the sample a rapid blow of air to spread the particles over the entire container. After a few minutes, each glass plate was removed from the container and placed under the microscope for examination. Four pictures were shot on each plate to attain adequate number of photographed particles per plate. The photos were analyzed investigating the circularity of each particle.

3.2. Gasification Experiments 3.2.1. Gasification materials The gasification tests were performed on three different materials; wood pellets, hydrolysis residue (HR) and torrefied hydrolysis residue (TFHR). The latter two was pelletized by BTC and ETC. The TFHR was produced in the reactor furnace using the method described in section 3.1; however, four plates were used per batch instead of one. The variable settings for the torrefaction are shown in Table 3.3.

The trial runs indicated a problematically low agglomeration temperature for HR and TFHR. Therefore, peat from Saltmyran was added to a content of 30%, as research show peat to have a heightening effect on the agglomeration temperature [28].

Initially, it was decided to perform all three runs at 900°C but due to previously mentioned agglomeration issues, only wood was gasified at 900°C and HR and TFHR were gasified at 800°C and 780°C respectively. As the gasification conditions were different for each run the experimental analysis will unfortunately not be directly comparable and the variation in process conditions must be taken into account during analysis. The fuel compositions for all fuels and fuel mixtures are given in table 3.3 below (Colored columns indicate fuels/fuel mixes used in the gasification tests). Included in the last column are HR data from 2005, analyzed by SLU, for comparison.

Table 3.3: Fuel composition for all gasification fuels used and torrefaction settings for TFHR. Wood HR

(unmixed) TFHR(unmixed)

Peat(Saltmyran)

HR/Peat (70/30)

TFHR/Peat (70/30)

HR1

Input temp [°C] 200 Output temp [°C] 290 Torrefaction time [min] 20 Average Solid Yield [%] 77.8 Dry content [%] 92.5 88.4 80.2 70.6 83.1 77.3 48.3Moisture cont. [%] 7.5 11.6 19.8 29.4 16.9 22.7 51.7 Ash [%] 0.4 0.6 0.7 4.8 1.86 1.93 0.7 HHV [MJ/kg DS] 20.55 22.71 26.34 21.88 22.46 25.00 23.91LHV [MJ/kg DS] 19.22 21.41 25.29 20.65 21.18 23.90 22.70S [% DS] 0.00734 0.050 0.064 0.181 0.089 0.099 0.06 C [% DS] 50.3 56.4 67.3 53.3 55.5 63.1 58.6 H [% DS] 6.1 6.0 4.8 5.7 5.9 5.0 5.6 N [% DS] 0.2 0.3 0.5 2.7 1.0 1.2 0.2O [% DS] 43.1 36.4 26.0 33.6 35.6 28.3 34.81: Data for hydrolysis residue from the pilot plant analyzed by SLU 2005

Figure 3.3: Experimental setup for particle distribution


19

3.2.2. Fluidized Bed Gasification at Bench Scale The gasification experiments were performed in a bubbling fluidized bed gasifier in bench‐scale (5kW) see (Figure 3.4). The gasifier is constructed by stainless steel and has a bed inner diameter of 100 mm and freeboard inner diameter of 200 mm. The 3 m high reactor is mounted to the ceiling, allowing thermal expansion to occur. Electrical wall heaters surround the reactor to ensure isothermal conditions and thereby a more controllable freeboard environment (reducing cold wall effect). The gasification medium, air, is preheated before fluidizing the quartz sand bed through a perforated stainless steal distribution plate located at the bottom of the reactor. The syngas pass through a cyclone separator before analysis and is subsequently combusted in a syngas burner.

Figure 3.4: BFBG at bench scale. The box to the left is a pellets burner, not part of the gasification setup [29].

Each experimental run was initiated with the reactor being slowly heated (~3°C/min) by the electrical wall heaters to ~50°C lower than the desired gasification temperature. The bed, 1100g of quartz sand from Askania AB, was added and fluidization initiated. The gasifier was run by a PC, utilizing the LabView control program. The bed temperature and lambda were controlled by varying the fuel feed as the air flow was set at a fixed value. Therefore, to achieve gasification and not combustion, a rather high fuel feed was necessary (~2‐4kg/h depending on fuel type). To control the gasification conditions, the oxygen content in the flue gas after the syngas burner was kept at a sufficient low and steady value.

3.2.3. Syngas Analysis Methods During gasification of biomass, a variety of gas impurities are also formed such as soot, tars, alkali salts, sulphur and halogen compounds. Some of these impurities can cause problem further down the process stream or be environmentally hazardous. The amount of remaining impurities must fall within the limit of the specifications for the after‐treatment processes, which may vary accordingly. Therefore, examining the syngas for impurities as well as gas content is highly relevant when evaluating the fuel quality.

In this section, methods are described for all analysis performed during the gasification experiments.

Pellets burner


20

3.2.3.1. Particle Emission Analysis

To examine the particle emission of the fuels i.e. the dust content of the syngas, complete dust analysis and size distribution determinations were performed. The complete dust load analysis was performed according to the Swedish standard, SS‐EN 13284‐1. The hot syngas was sampled for 120 seconds and passed through two quartz filters, heated to a temperature of ~260°C. The dust load was then established by weighing the quartz filters before and after the sampling. Three replicates were performed for each gasification fuel.

A Dekati Low Pressure Impactor (DLPI) was used to determine the particle size distribution. The DLPI is a cascade impactor having two co‐linear plates of which one has a small nozzle in it, see Figure 3.5 below. When the sample stream has passed through the nozzle it is forced to make a sharp turn. Particles not able to follow the stream, impact on the second plate and particles with small enough inertia remain in the gas flow. The impactor consists of 13 successive stages with decreasing cut diameters and thereby decreasing particle sizes [30]. To obtain the particle size distribution, each plate is weighed before and after sampling.

Figure 3.5: Schematic description of the Dekati Low Pressure Impactor [30].

3.2.3.2.

3.2.3.3. Gas Analysis To analyze the syngas content, samples were taken from each of the gasification runs. The hot gas was sampled from the gas canal for 60 seconds and at constant gas flow. The gas was filtered from dust and particles and cooled to remove tars before collected into the sample bag. Three replicates for each fuel was the initial plan

Stage 1: > 5 µm

Stage 2: > 1 µm

Stage 3: > 0.5 µm

Collectionsubstrates

Collection plate

Jet to Plate distance

Jet Velocity, U

Nozzle Diameter, D

Flow


21

but due to bed agglomeration, only 2 replicates were prepared for HR/Peat. The gas bags were sent to ETC where a Micro GC from Chrompack was used to analyze the gas content.

3.3. Reactivity Testing using TGA By thermogravimetric analysis (TGA), weight changes in a material is determined as a function of temperature (or time) in a controlled atmosphere and the method is principally used to measure a material's thermal stability and composition [32]. TGA is a commonly used method to obtain first estimates reactivities for fuels, relating weight changes to devolatilization and oxidizing reactions and describing the overall reaction rates of the event. However, no information about the composition of the volatiles is provided [9]. In this work, TGA was used to obtain some initial data on material reactivity for the two hydrolysis residue fuels and the reference fuel, wood. As it is difficult to exclude the influence of the experimental conditions and equipment on the results, a comparative analysis of data are desired. The reactivity data produced are therefore considered to be relative results rather than actual kinetic data.

Experiments were performed using a TA Instruments Q5000IR TGA available at ETPC for all three unmixed fuels; HR, TFHR and Wood, milled to similar particle size. A small amount of material (6‐8 mg) was placed in the sample holder and heated in the TGA furnace from ambient to 800°C with a 40°C/min heat rate. Isothermal conditions at 800°C were then kept for 10 minutes before the run was completed. The surrounding atmosphere was kept constant by a flow of 25 ml/min CO2 and one run for each fuel material was performed.


22

4. RESULTS AND DISCUSSION 4.1. Torrefaction of Hydrolysis Residue The torrefaction experiments were performed as batch processes in an electrical furnace as described in section 3.1. From the thermo‐elements placed in the furnace, uneven heat distribution was observed. This temperature gradient results in varied heat rate and torrefaction over the plate surface and during the course of the experimental run. The influence on the results is hard to estimate but as the plate was located at the same depth and height for all runs, the comparability should not be affected.

As described in the theory section, dried hydrolysis residue result in a significant dust formation. One initial observation for the torrefied hydrolysis residue was the significantly lower dust formation tendency. This effect would greatly improve the handling and storage of the material.

The chemical composition and energy content for each torrefied batch and a reference sample (Ref) of dried hydrolysis residue was determined (Table 4.1).

Table 4.1: Chemical analysis for all torrefaction runs (N1‐N11) and dried hydrolysis residue (Ref).

N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 Ref DS [%] 98.1 98.4 98.2 98.9 99.0 97.9 98.9 98.0 98.8 98.7 98.7 99.1

Ash [% DS] 1.0 1.6 1.8 1.6 1.2 1.8 1.3 1.8 1.4 1.3 1.5 0.9

HHV [MJ/kg DS] 23.93 27.71 23.42 27.87 25.34 27.76 25.95 28.10 26.67 26.43 27.29 22.71

LHV [MJ/kg DS] 22.82 26.95 22.18 27.09 24.24 27.11 24.87 27.43 25.65 25.39 26.38 21.41

S [% DS] 0.06 0.07 0.05 0.07 0.06 0.08 0.06 0.07 0.07 0.06 0.07 0.05

C [% DS] 60.6 73.4 58.2 73.0 64.0 74.6 65.8 75.2 68.1 67.4 71.4 56.4

H [% DS] 5.1 3.5 5.7 3.6 5.1 3.0 5.0 3.1 4.7 4.8 4.2 6.0

N [% DS] 0.4 0.6 0.3 0.6 0.4 0.7 0.4 0.6 0.5 0.5 0.6 0.3

O [% DS] 32.8 20.8 34.8 21.1 29.2 19.8 27.4 19.2 25.2 25.9 22.2 36.4

The most relevant variables from this analysis were further analyzed through multivariate analysis described in Section 4.1.2 and the variation in chemical composition between batches and reference will therefore be more thoroughly discussed later. Though, clearly noticeable from Table 4.1 is the grouping of N1, N3, N5 and N7 (batches with time = 15 min) as well as N2, N4, N6 and N8 (batches with time = 25 min).

4.1.1. Milling calculations The milling energy for each run was calculated from the collected data from the clamp meter and the lower heating value. An example of the milling output is displayed in Figure 4.1 below.

Figure 4.1: The clamp meter measurements for the first replica for N2. The circled area indicates where the actual milling occurred.

0 5 10 15 20 25 30 35 40 45 50 Time [s]

8

7

6

5

4

3

2

1

0

Electrical Current [A

]


23

The control program collects data on the electrical current 20 times per second. From those data, the milling power for each point was calculated by integrating the current using, P= I · t (t=0.05 s for each point). In order to calculate the total power used, a baseline was constructed using the average of the current before and after the milling (Figure 4.2). This average baseline was subtracted from the point effects of the

target area and added to obtain the total milling power.

Emilling = P · U and Efuel = msample · LHVsample were used to calculate the milling cost in terms of fuel energy;

MC = kJe/MJth . The milling results and the calculated solid yield, SY 100 , was further analyzed using multivariate methods discussed in the following section.

4.1.2. Multivariate Data Analysis The experimental design for the torrefaction experiments were orthogonally constructed (Figure 3.2); hence the data generated are very well suited for multivariate analysis. By using multivariate data analysis, all measurements are simultaneously analyzed instead of one variable at a time. For the torrefaction data, a projection method called PLS (Projections to Latent Structures) was used by the software program SIMCA‐ P, developed by Umetrics AB [33]. PLS is a method relating two data matrices, X and Y, to each other by a linear multivariate model. Selected chemical analysis results, milling result and the calculated solid yield were used as responses (Y‐variables) and the varied factors described in table 3.1 and 3.2 were used as factors (X‐variables).

The multivariate analysis was used to answer the following questions:

• Will the pre‐treatment process, torrefaction reduce the milling cost of the material?

• What are the optimal factor settings to obtain a high quality torrefied product?

A so called Coefficient Plot can be constructed (Figure 4.3) using a suitable software (SIMCA‐P). The PLS model has been rewritten as a regression model, i.e. Y Yavg X B F , where B is the coefficient matrix. As design of experiments was used for the experimental design, the coefficients are independent and express a casual relationship [33].

Figure 4.3: Coefficient Plot for two of the most interesting responses; Solid Yield (left), Milling cost (right). The error bars indicate one standard deviation. The Y‐scale goes from ‐1 to 1 where high (close to 1) and low values (close to ‐1) indicate a large variable importance and values close to 0 indicate low variable importance. Positive values correspond to a positive correlation between the factor and response and negative value to a negative correlation.

The solid yield response can be viewed as a measurement of the torrefaction level. A low solid yield represents a more torrefied sample, and high solid yield indicates a less torrefied sample. Therefore, all other responses can be viewed against the solid yield for information on the effect of torrefaction upon that particular

Figure 4.2: Baseline construction.

Solid Yield Milling Cost

Time over Input Temperature Time over Input Temperature 200 °C Temperature 200 °C Temperature

CoeffCS(1) Solid Yield

CoeffCS(1) M

illing Co

st

0.1 0.0 ‐0.1 ‐0.2 ‐0.3 ‐0.4 ‐0.5 ‐0.6 ‐0.7 ‐0.8 ‐0.9 ‐1.0

0.10.0‐0.1‐0.2‐0.3‐0.4‐0.5‐0.6‐0.7‐0.8‐0.9‐1.0

mtorrefied

DS · msample

Emilling Efuel


24

response. The milling energy coefficient plot shows three relevant variables. This clearly demonstrates that torrefaction has an impact on the milling cost of the material. The analysis show that low milling cost is attained with long residence time, high torrefaction temperature and low input temperature. With reference to the solid yield it can be deduced that a high level of torrefaction result in low milling cost, as was expected from studies of previous research.

A plot of the calculated milling cost values for all runs is shown in Figure 4.4 for further clarification of the results. Three torrefaction runs have higher milling cost than the reference sample; N1, N3 and N5, corresponding to the three runs with the highest solid yield and thereby the lowest level of torrefaction. One explanation could be that the improvement in milling energy is not enough to compensate for the loss of energy through volatile compounds during the torrefaction process. In addition, N2, N4, N6 and N8 all have significantly lower milling cost compared to the reference sample and as these four have the lowest solid yield, the relation between the torrefaction level and the milling cost is further supported.

Figure 4.4: Results from milling cost calculation, not analyzed by multivariate methods. Each value is an average of tree or four replicas represented together with the standard deviation.

For all responses investigated (Figure 4.5), the time over 200°C, X1, is the most influential factor, followed by the temperature, X3. Both of these factors are positively correlated for all investigated responses, meaning that high temperature and long reaction time both have the same influence in the material. The input temperature, X2, (heat rate), seems to have little or no effect on the investigated responses. This conclusion was further supported by the Variable Importance Plot (VIP) (Figure 9.7, displayed in Appendix 3).

Figure 4.5: Coefficients Overview Plot. All coefficients for all responses are displayed. They have been normalized, i.e. the coefficients have been divided by the standard deviation of the respective response. The Y‐axis is scaled as described in Figure 4.3.

The Coefficient Overview Plot allows us to see how the variables affect all the responses. The results show that a high torrefaction level result in low milling cost, hydrogen content and oxygen content but high ash content, heating value, sulphur content, carbon content and nitrogen content.

Time over 200°C Input Temperature Temperature

Solid Milling Moisture Ash HHV LHV Sulphur Carbon Hydrogen Nitrogen Oxygen Yield Cost content content content content content content

1.0

0.8

0.6

0.4

0.2

0.0

1.0

0.5

0.0

‐0.5

‐1.0


25

A high heating value is considered as a desired fuel parameter that heightens the quality of the fuel. The torrefaction process results in an energy densification that increases the heating value of the material. However, high heating value combined with a significant loss of mass may also indicate that a large amount of energy is lost through the released volatile gases. The energy yield and solid yield are highly correlated (Figure 4.6) and hence a high torrefaction level leads to a significant energy loss. The key element is to find the ideal settings were the benefits from torrefaction can be attained while at the same time sufficient amount of energy is retained. As mentioned in the theory section, a 70% solid yield and a 90% energy yield is desired for torrefaction. Figure 4.6 show indications that such ideal settings could be found for hydrolysis residue.

Figure 4.6: Solid Yield, Energy Yield and Lower Heating Value (LHV) for all torrefaction runs.

4.1.3. Visual Analysis The freeware program ImageJ1 was applied to each picture and used to calculate the circularity for each identifiable particle (Figure 4.7). The circularity was defined by the program as;

Circularity = 4π ∙ ,

ranging from 0.0‐1.0 where values approaching 0.0 indicates an increasingly elongated polygon and 1.0 a perfect circle [34]. As the values may not be valid for very small particles (pixel size sets the boundary) the decision was made to exclude particles with area less than 0.00012 mm2 when performing the analysis. The output from each picture varied but more than 700 particles were examined for each batch and the circularity calculated (Figure 4.8).

Figure 4.7: An example of a sample picture. This is one of the 20 pictures shot for N2.

Area Perimeter2

1 Available: http://rsb.info.nih.gov/. Research Service Branch, National Institute of Mental Health, USA


26

Figure 4.8: Circularity averages, for each torrefaction run and the reference sample together with the standard deviation.

No significant difference in the circularity between the torrefied and dried hydrolysis residue was observed. As mentioned in section 3.1.5 hydrolysis residue already has rather spherical particles and there may be little to improve in that parameter by torrefaction. From these results it is hard to conclude whether torrefaction has no effect or if the analysis methods are insufficient. The mill used is a high energy hammer mill with significant effects on particle morphology whereas the typically used mill in industrial systems would be a ball mill that retains more of the initial biomass structure. Therefore the mill process used may not represent the actual industrial method of milling biomass. Further investigations must be performed before a definitive conclusion can be drawn.

4.2. Gasification of Hydrolysis Residue 4.2.1. Ash Analysis of Hydrolysis Residue The initial gasification tests indicated agglomeration tendencies for both HR and TFHR at a temperature far beneath the known agglomeration temperature for wood. Bed agglomeration occurs when ash melts deposit on the bed material creating a sticky coating, which in turn cause the bed particles to attach to each other creating so called agglomerates. The ash melting point may be reduced by a various amount of elements present in the ash such as potassium (K), sodium (Na) and chlorine (Cl). To examine the cause of the premature agglomeration, the material was sent for an ash composition analysis (Figure 4.9).

Figure 4.9: Ash content for the unmixed fuels. Data for wood is displayed as reference. The data for wood contained no measurements for MnO and TiO2; therefore these are displayed as 0.

1.0

0.8

0.6

0.4

0.2

0.0

0.25

0.20

0.15

0.10

0.05

0.00

Amou

nt [%

of D

.S.]

Wood HR TFHR

Circularity


27

The lower content of K2O seems to support previous assessment of the hydrolysis process working as an “alkali‐wash”. Research by Eriksson et al (2004) [9] on hydrolysis residue (spruce feedstock) from the Rundvik process (Nordmaling, Sweden), indicated low ash content (<0.1 %) and lower alkali content than softwood (0.006% compared to 0.021%). The 100 times higher sodium (Na) content present in the residue seemed therefore completely out of order. When investigating the matter further, new information provided an explanation. Sodium hydroxide (NaOH) had been added to the material before the filter press in the pilot plant to pH‐adjust from pH 2 to pH 4. When the residue was filtered out, a large amount of sodium ions was retained in the solid residue. This unnaturally high concentration of sodium in the hydrolysis residue is therefore the cause of the premature agglomeration tendencies observed and such high levels of alkali metals spoil the material for any energy applications.

The ASTM D1857‐68 ash fusion test was also performed in both reducing and oxidizing environment (Table 4.2). The method generally represents an overstatement of the agglomeration temperature, as the real value usually lies about 100°C below the initial deformation temperature of the ash (IT). For the gasification runs, the reducing results are most representative of the environment in the bed. The last column in Table 4.2 contains results from an ash fusion analysis of hydrolysis residue from the pilot plant performed in 2005.

Table 4.2: Results from the ASTM ash fusion test.

Oxidizing HR [°C] TFHR [°C] Reducing HR [°C] TFHR [°C] Oxidizing HR1 [°C] IT 980 970 IT 890 830 IT 1140 ST 100 1000 ST 900 840 ST 1220 HT 1020 1010 HT 910 860 HT 1220 FT 1040 1020 FT 920 880 FT 1220

1)Data for hydrolysis residue from the pilot plant analyzed by SLU 2005

The results from the ash fusion test confirm the observed low agglomeration temperature for the hydrolysis residue. The lower ash fusion results for torrefied hydrolysis is a consequence of the higher ash content, induced by the loss of volatile compounds during the torrefaction process. The 2005 values represent significantly higher ash fusion temperatures, indicating that a significantly higher agglomeration temperature could be achieved for hydrolysis residue under different conditions, i.e. if no sodium hydroxide was added.

4.2.2. Particle Emission Analysis The gasification operation conditions and the type of gasifier used are two important factors when it comes to particle emission during gasification. Fluidized bed gasifiers often emit quite large particles, usually separated from the product gas by a cyclone. The feedstock properties are also a very important factor determining the amount and composition of particulates emitted. The particulates are comprised by different types of inorganic compounds such as alkali metals, ash, unconverted feedstock, bed material and soot [35]. The results from the total particle load analysis are shown in Figure 4.10.

Figure 4.10: The complete particle load for all three gasification runs. Each value is an average of three replicate and the error bars show the calculated mean error.

Wood HR/Peat TFHR/Peat


28

Ek (2005) [10] has shown the possibility of low particle emissions for hydrolysis residue making it suitable for combustion and gasification as well as firing fuel in gas turbines. The results from the complete particle load analysis seem to support that thesis.

The particle size distribution has been calculated from the mass of the material deposit on each of the impactor levels (Figure 4.11). The large variation between the three replicas for wood particle diameter between 0.1 and 1.0 µm could result from several different factors such as sampling procedure and gasification conditions. It is still clear that wood has a lower amount of large particles in the region above 1 µm than HR/Peat and TFHR/Peat. The fact that TFHR/Peat has the highest amount of large particles in the product gas and higher total particle load than HR/Peat could be explained by the molecular depolymerization resulting from the torrefaction process and the lower gasification temperature.

Figure 4.11: Particle size distribution. The averages of the three replicas are plotted for all three gasification fuels together with one standard deviation.

4.2.3.

WoodHR/PeatTFHR/Peat

12000

10000

8000

6000

4000

2000

0

Particle Loa

d [m

g/Nm

3 ]

0.01 0.10 1.00 10.00 Particle Diameter, Dp [µm]


29


30

4.2.4. Results from Gas Analysis As the syngas is the final product of gasification, its content and thereby quality is important to investigate. Dilution of the syngas reduces the heating value and depending on the compound (hydrocarbons), the


31

gasification efficiency. For all gasification runs performed, air was the gasifying medium. Therefore, the main gas component in the syngas is N2 (Figure 4.14) which is considered to be inert in the subsequent process steps, thereby diluting the syngas and lowering the heating value. To avoid high N2 levels, oxygen or indirect gasification could be used instead of air as gasifying medium.

Depending on the usage of the product gas, the methane level is desired to be kept high or low. If the syngas is intended to be used in fuel catalysts, the methane level must be kept at a low level to avoid further dilution of the syngas and lowered product efficiency [25]. However, if the syngas is intended to be used as fuel directly, the methane will increase the heating value of the gas.

Figure 4.15: Gas content in the product gas. The values for wood and TFHR/Peat are based on three replicas and the values for HR/Peat on two. Adjustments have been made to compensate for possible air inflow. Error bars show one standard deviation.

The higher N2 and CO2 and lower CH4 and CO content for TFHR/Peat can be explained by the difference in gasification conditions. The TFHR/Peat fuel mixture was gasified at a lower temperature and a slightly higher lambda value than the other two. Higher lambda equals more air, resulting in an equilibrium shift toward more CO2. With that factor taken into account, the gas content is fairly the same for all fuels.

4.3. Reactivity Analysis As, explained in the Materials and Methods section, the experimental condition will not allow for any exact kinetic data determinations. Instead, a relative estimation of the reactivity is obtained. Reactivity is a property with no clear and commonly accepted definition; hence no directives on how to measure and determine reactivity exist. The overall description of reactivity can be summarized as an estimation of how quickly a specific reaction or process occurs, i.e. the rate of the reaction or reactions. For thermal processing of biomass, several different reactions occur simultaneously and the material reactivity is thus the combined rate of these reactions, i.e. the overall process reactivity.

In this work, the objective was to evaluate whether or not torrefaction result in a relatively lower reactivity than the original fuel. The reactivity for the three unmixed gasification fuels was therefore estimated using thermogravimetric analysis (TGA). The total amount of volatiles and char, as well as the weight derivative were used as basis for the fuel comparison. Figure 4.16 summarize the results from the TGA runs for all fuels.

Wood HR/Peat TFHR/Peat

H2 N2 CH4 CO CO2 Ethene Ethane Acetylene


32

Figure 4.16: Results from the TGA log. From this plot the total weight loss for each fuel can be estimated.

The first initial weight loss (~70‐200°C) is attributed to water vaporization during the drying process. At about 300°C, volatile compounds are released from the investigated materials at different rates (total weight loss and volatiles for all fuels was established from the runs and summarized in Table 4.4). Important to keep in mind is the influence of other parameters that most probably have more impact on the reactivity than the specific fuel type, such as particle size. In this thesis, the particle size was milled the same way to an approximately comparable particle size but no complete size determination was made. The influence of this parameter on the results are difficult to determine but was considered to have little effect on the results as char combustion/gasification was not investigated.

Table 4.4: Weight loss, amount volatiles, char and the temperature of highest volatilization rate for all investigated fuels. The retained char amount is estimated at the end of the 10 minute isothermal.

Fuels Total weight loss [%]

Moisture release [%]

Volatile release[%‐D.S.]

Residual char [%‐D.S.]

Temperature of highest volatilization rate [°C]

Wood 90.52 9.27 89.55 10.45 398 HR 83.14 6.88 81.89 18.11 394 TFHR 70.59 15.22 65.31 34.69 353

As expected due to the partial volatilization that has already occurred during the torrefaction process, TFHR released the least amount of volatiles of all three investigated fuels and therefore also contain the highest amount of char residue. This higher char content indicate that the time for complete carbon conversion may be longer than for HR and wood. The same conclusion can be drawn when comparing HR and wood separately where HR has higher char content than wood. The char combustion/gasification usually takes longer time than the volatilization and the reaction rate is highly determined by process variables, especially particle surface and structure.

However, the highest rate of volatilization occurred at 40‐45°C lower temperature for TFHR than the other two fuels indicating that the torrefied material had been made more accessible to thermal conversion than the non‐torrefied material. The volatilization process is initiated when sufficient temperature is reached for the volatile compounds to be released and have no direct influence on the material reactivity but provide information on

0 200 400 600 800 Temperature [°C]

Wood HR TFHR

100

80

60

40

20

0

Weight [%

]


33

the material behavior. The temperature of the highest volatilization rate presented in Table 4.4 was determined by plotting the weight temperature derivates for all fuels (Figure 4.17) which also provide some indications to the thermal process mechanisms that has occurred during each run.

Figure 4.17: Weight temperature derivate for all fuels vs. temperature up to 800°C. More thorough description and analysis provided in the text.

The first peak (1) show the drying process occurring for all measured fuels and is higher for TFHR as more water evaporated due to the higher moisture content. The thermal destruction process for wood is well known and peak (2) is thereby identified as the destruction of hemicellulose molecules (no such peak present for the hydrolysis residue fuels) [38]. Further, the peak for cellulose and lignin occur generally at similar temperature, resulting in one large peak representing the destruction of both molecule types which is shown for both wood and HR (3). TFHR has no such high, distinct peak for lignin/cellulose further supporting the assumption that a completely new molecular structure has been formed during the torrefaction process. The largest peak for TFHR (4) occurs at a lower temperature compared to the lignin/cellulose peak for HR and Wood, demonstrating that more easily degraded molecules have been formed during the torrefaction process. Another distinct peak occurs for TFHR at a higher temperature (5), and a small elevation at this temperature can also be identified for HR. The exact thermal process mechanism this peak represents has not been concluded.

When considering the volatilization process, HR and Wood show similar characteristics whereas TFHR initiates the volatilization at a higher temperature but peaks in volatilization rate at a lower temperature compared to the other fuels. Furthermore, the volatilization rate at this peak is only ~0.4 %/°C compared to over 0.8 %/°C for Wood and HR. Hence, TFHR has a lower volatilization rate compared to HR and Wood but not necessarily a lower reactivity.

4.3.1. Reactivity theories and discussion In order to estimate the relative reactivity of the three investigated fuels some deliberation is necessary due to the insufficient definition of reactivity, hence the following section is to be considered as theory and discussion for future work rather than fact.

In a thermal conversion process, a particle undergoes volatilization followed by char conversion. In a gasifier, the released gaseous compounds are ignited after leaving the particle by the oxygen in the surrounding

0 200 400 600 800 Temperature [°C]

Weight ‐Tem

perature Derivate [%

/°C] 8.0

6.0

4.0

2.0

0.0

Wood HR TFHR


34

atmosphere potentially creating local conditions around the particle with elevated temperature (Figure 4.18). This oxidization may provide sufficient energy and heat for the subsequent reduction of the carbon retained in the particle. As the volatile compounds leave the particle, oxygen diffuse in through the emitted gases creating a decreasing oxygen gradient towards the particle surface and centre. The ignition level being located some distance from the particle surface is a direct result of the oxygen gradient as the volatile gases will not be ignited until sufficient oxygen is provided.

The lower rate of volatilization and the amount of volatile compounds left for the torrefied material could result in increased oxygen diffusion rate. As result, the ignition level is brought to a closer proximity to the particle surface

improving the char gasification kinetics. If so, torrefaction may in fact increase the particle reactivity. On the other hand, the increased char content resulting from the torrefaction process may reduce the material reactivity. However, as the content of the volatiles and char have been altered by the torrefaction process it is impossible to draw any correct conclusion on the reactivity of the fuels investigated from the work performed in this thesis.

Figure 4.18: Schematic view of particle oxidization and reduction. High temperature region within ignition level.


35

5. CONCLUSIONS Torrefaction of hydrolysis residue lowers the milling cost for the material, if the torrefaction level is high

enough. If low level of torrefaction, the improvement in milling energy will not compensate for the energy loss from the torrefaction process.

From the multivariate analysis, it can be concluded that the torrefaction reaction time (the total time above 200 °C) is the most influential factor for the chemical composition, solid yield and milling cost for the material. The torrefaction temperature has correlated influence on the responses but with lesser impact.

The reaction time and temperature have positive correlation to the solid yield and hence to the torrefaction level. The input temperature does not seem to influence any of the result variables.

This investigation shows that torrefaction of hydrolysis residue result in; ‐ lower milling cost, hydrogen content and oxygen content ‐ higher ash, sulphur, carbon and nitrogen content and heating value.

Torrefaction does not seem to have any effect on the particle circularity for hydrolysis residue. However, this may be an effect of the type of energetic an efficient mill used and probably not a general conclusion. This work confirms that both hydrolysis residue and torrefied hydrolysis residue have lower particle emission levels than wood. The particle size distribution, however, lean toward the larger end of the spectra for the hydrolysis residue materials.

Z z z

Further, the product gas from hydrolysis residue and torrefied hydrolysis residue show no significant difference to that from wood.

A very low agglomeration temperature were shown for hydrolysis residue, caused by the pH‐adjustments using sodium hydroxide. To continue with this additive before the press would be a disaster for any subsequent energy use of the residue and should be avoided.

From the TGA analysis performed it is impossible to draw any conclusion as to whether or not torrefaction increase or decrease the particle reactivity. Torrefaction will most probably influence the reactivity as it lowers the volatilization rate and amount of volatiles of the material. The possibility exists that the loss of volatiles and the molecule decomposition of the material when torrefied, result in increased material reactivity. On the other hand, the increased char content may instead lower the reactivity. In addition, a lower reactivity for TFHR compared to HR does not necessarily provide a problem in thermal conversion processes as the material is more easily milled and hence a smaller particle size can be obtained by the use of torrefaction.

Finally, hydrolysis residue is a suitable gasification fuel based on the lower particle emission and the product gas content. Xxx xx The low ash melting temperature caused by the high sodium content of the material unfortunately diminishes the fuel qualities of the material to a large extent. This problem has to be solved if the hydrolysis residue is to be used as fuel in gasification or combustion applications.


36

6. FUTURE WORK Further investigations should focus on evaluating the ash melting temperature of the residue without the addition of sodium hydroxide in the process stream. If significantly improved, new gasification test could be performed at higher gasification temperature without agglomeration predicaments. If the material were to be gasified without peat additive, more comparable results to wood could be provided.

Milling analysis from a different type of mill could clarify the visual analysis. There are indications that the mill used in this work was too efficient compared to actual mills used on biomass industrially, pulverizing the material rather than milling it. The circularity issue is not fully investigated without further testing.

The theories deliberated in the discussion concerning the material reactivity (Section 4.3.1) need to be investigated further with additional testing and methods.


37

7. REFERENCES [1] International Energy Agency (IEA) (2002), Transportation & Energy. Brochure. Available for download at:

http://www.iea.org/Textbase/work/2002/WSSD/transportation.pdf

[2] International Energy Agency (IEA) (2006), Key World Energy Statistics – 2006 Edition. Statistics Book. Available for download at: http://www.iea.org/textbase/nppdf/free/2006/key2006.pdf

[3] Population Reference Bureau. Teachers Guide. Available at: http://www.prb.org/Educators/TeachersGuides/HumanPopulation/PopulationGrowth.aspx

[4] International Energy Agency (IEA) (2006). World Energy Outlook 2006 – Summary and Conclusions. Available at http://www.worldenergyoutlook.org

[5] Swedish Commission on Oil Independence (2006, June). Making Sweden an OIL‐FREE Society. Information material from the Prime Minister's Office, Commission on Oil Independence, Government of Sweden. Available at Government Offices of Sweden, http://www.sweden.gov.se

[6] Intergovernmental Panel on Climate Change (IPCC) (2001). Summary for policy makers. Working group 1. Available at: http://www.ipcc.ch

[7] International Energy Agency (IEA) (2004), Biofuels for Transport – An International Perspective. Study Book. Available for download at: http://www.iea.org

[8] Course material from Renewable Propellants (2006), Umeå University.

[9] Eriksson, Gunnar L. (2005) Combustion of Solid Waste from Wood‐Based Ethanol Production. Licentiate Thesis. Luleå, Sweden. Luleå University of Technology, Department of Applied Physics and Mechanical Engineering.

[10] Ek, Monica (2005, August). The Status of Applied Lignin Research. Processum Technology Park AB.

[11] Bergman, P.C.A. Boersma, A.R. et al (2004). Torrefaction for Entrained‐Flow Gasification of Biomass. Energy research Centre of the Netherlands.

[12] Crowtz, Timothy (2005). Industrial scale cultivation of Saccharomyces cerevisiae on dilute‐acid spruce hydrolysates. Master Thesis. Stockholm, Sweden. Royal Institute of Technology (KTH), Department of Chemical Engineering and Technology.

[13] Pettersson, Pär (2003, October). Countercurrent operation in the dilute‐acid hydrolysis of cellulose for ethanol production. Ph.D. Thesis. Lund, Sweden. Lund Institute of Technology, Department of Chemical Engineering.

[14] Wingren A., Ethanol from Softwood Techno‐Economic evaluation for Development of the Enzymatic Process, Department of Chemical Engineering, Lund University, Sweden, 2005

[15] Merriam‐Webster Online Dictionary and Thesaurus. Retrieved: September 10, 2006 Keyword Dictionary: hydrolysis. Available: http://www.m‐w.com

[16] Sweden Energy Agency (2006). Programmet etanol från skogsråvara. Report. Responsible Publisher: Lars Vallander. ID‐NR: ER2006:23. ISSN: 1403‐1892. Can be purchased at: http://www.energimyndigheten.se

[17] Etek Etanolteknik AB (2005). A unique facility for ethanol production – for future independence. Örnsköldsvik, Sweden. Ågrenshuset.

[18] Etek Etanolteknik AB webpage. Retrieved: September 12, 2006. Now unavailable. For information: http://www.sekab.se/

[19] Öhman, M., Boman, C. (2006). Residential Combustion Performance of Pelletized Hydrolysis Residue from Lignocellulosic Ethanol Production. Energy & Fuels, 20, 1298‐1304.

[20] Nimlos, M., et al. (2003). Biomas Torrefaction Studies with a Molecular Beam Mass Spectrometer. National Bioenergy Center, National Renewable Energy Laboratory, Golden, USA.

[21] Lipinsky, Edward S., Arcate, James R. and Reed, Thomas B. (2002). Enhanced Wood Fuels via Torrefaction. ACS 2002 National Meeting, Chemistry of Renewable Fuels.


38

[22] Bergman, P.C.A. Boersma, A.R. et al (2005, July). Torrefaction for biomass co‐firing in existing coal‐fired power stations “BIOCOAL”. Energy research Centre of the Netherlands (ECN).

[23] Bergman, P.C.A. Boersma, A.R. et al (2005). Torrefaction for entrained‐flow gasification of biomass. ECN report, ECN‐C—05‐067

[24] Ouwens, C. Daey, Küpers, G. (2004). Lowering the Costs of Large‐Scale, Biomass Based, Production of Fischer Tropsch. Eindhoven University of Technology, Eindhoven, The Netherlands.

[25] Olofsson, I., Nordin, A. and Söderlind, U. (2005). Initial Review and Evaluation of Process Technologies and Systems Suitable for Cost‐Efficient Medium‐Scale Gasification for Biomass to Liquid Fuels. Energy Technology & Thermal Process Chemistry, Umeå University, Umeå and Department of Engineering, Physics and Mathematics, Mid Sweden University, Sundsvall, Sweden.

[26] Energy Information Administration (EIA) (2007, May). International Energy Outlook 2007. Report number: DOE/EIA‐0484(2007). Available at: http://www.eia.doe.gov/oiaf/ieo

[27] Umetrics Academy (2004). Design of Experiments. Book. Pharma Applications.

[28] Lundholm, K.N., A. Öhman, M. Näslund, B.O. (2002). Experimental Studies on the Role of Peat Fuel in Preventing Bed Agglomeration during Fluidized Bed Combustion of Biomass Fuels. In 12th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection. (2002). The Netherlands.

[29] Olofsson, Ingemar (2005). Low‐Tar‐Formation using High‐Temperature Flash‐Gasification of Intelligent Biomass Fuel Mixtures. Master Thesis. Umeå, Sweden. Umeå University, Energy Technology and Thermal Process Chemistry.

[30] Dekati Low Pressure Impactor constructer webpage: http://www.dekati.com/dlpi.shtml

[31]

[32] Constructor webpage: http://www.tainstruments.com

[33] Eriksson, Lennart (2001). Multi‐ and Megavariate Data Analysis: Principles and Applications. Book. Umetrics Academy, ISBN: 91‐973730‐1‐X

[34] The webpage manual for ImageJ: http://rsb.info.nih.gov/ij/docs/menus/analyze.html.

[35] Ståhlberg, P. et al. (1998). Sampling of contaminants from product gases of biomass gasifiers. VTT Energy, Technical Research Centre of Finland. ISBN: 951‐38‐5297, ISSN: 1235‐0605.

[36]

[37]

[38] Rainer Backman, Personal Communication, June 2007


39

8. APPENDIX Appendix 1: Torrefaction Settings and Results Settings and results from all torrefaction batches are summarized in table 8.1.

Table 8.1: Settings and results from all torrefaction runs and the reference sample of dried hydrolysis residue. N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 RefTime >200°C [min] 15 25 15 25 15 25 15 25 20 20 20 ‐ ‐ ‐

Input temperature [°C]

100 100 300 300 100 100 300 300 200 200 200 ‐ ‐ ‐

Output temperature [°C]

270 270 270 270 300 300 300 300 285 285 285 ‐ ‐ ‐

Solid Yield [%] 83.8 51.0 91.0 53.2 73.6 44.9 69.0 46.7 63.0 65.0 55.9 ‐ ‐ ‐

Milling Cost [kJe/MJth]

0.9666 0.6906 0.9499 0.5440 0.8385 0.5716 0.8137 0.8249 0.7316 0.6528 0.8983 0.8591

Moisture [%] 1.9 1.6 1.8 1.1 1.0 2.1 1.1 2.0 1.2 1.3 1.3 0.9

Ash [% DS] 1 1.6 1 1.6 1.2 1.8 1.3 1.8 1.4 1.3 1.5 0.9

HHV [MJ/kg DS] 23.93 27.71 23.42 27.87 25.34 27.76 25.95 28.10 26.67 26.43 27.29 22.71

LHV [MJ/kg DS] 22.82 26.95 22.18 27.09 24.24 27.11 24.87 27.43 25.56 25.39 26.38 21.41

S [% DS] 0.06 0.07 0.05 0.07 0.06 0.08 0.06 0.07 0.07 0.06 0.07 0.05

C [% DS] 60.6 73.4 58.2 73.0 64.0 74.6 65.8 75.2 68.1 67.4 71.4 56.4

H [% DS] 5.1 3.5 5.7 3.6 5.1 3.0 5.0 3.1 4.7 4.8 4.2 6.0

N [% DS] 0.4 0.6 0.3 0.6 0.4 0.7 0.4 0.6 0.5 0.5 0.6 0.3

O [% DS] 32.8 20.8 34.8 21.1 29.2 19.8 27.4 19.2 25.2 25.9 22.2 36.4

Circularity 0.7021 0.6994 0.6964 0.7093 0.7052 0.6573 0.7355 0.6848 0.7024 0.7001 0.6766 0.6795

Appendix 2: Torrefaction Photos Below are photographs of all torrefied batches. Except for the centre runs (N9‐N11), two images on the same row have the same settings on input and output temperature but differ in reaction temperature (15 minutes to the left and 25 minutes to the right). The influence of the reaction time is clearly demonstrated.

N1: Uneaven and incomplete torrefaction. The larger

particles show complete surface torrefaction.

N2. Complete torrefaction, all material is black.

Some combustion have occurred (ash present).


40

N3: Uneaven and incomplete torrefaction. Some part

of the sample has only been dried (light brown).

N4: Same as N2, but less undesired combustion.

N5: Still uneaven torrefaction but the largest portion of the sample is fully torrefied.

N6: Similar to N2 but more undesired combustion

have occurred.

N7: Almost complete torrefaction. N8: Completely torrefied and no undisired

combustion can be observed.


41

N9: Slightly lighter brown in the center, but overall completely torrefied

N10: Slightly lighter brown in the center, but overall completely torrefied

N11: Very slightly lighter brown in the center, but overall completely torrefied

Appendix 3: Multivariate Analysis Plots More plots from the multivariate data analysis, for interested readers. Knowledge about multivariate data analysis is assumed and therefore only brief plot description and analysis are presented.

Figure 8.1: Model Overview Plot. Two significant components describe: Observations (N) = 38, Variables (K) = 14. R2 X = 0.667; R2Y = 0.831, Q2(cum) = 0.809.

High R‐values indicate that the model describe the variation between the variables and observations well and high Q‐value indicate that the model have good prediction capacity. From this plot it can be concluded that the model describe the data very well and that the main part of the variation are described by the first component.


42

Figure 8.2: X/Y Overview Plot. Shows the cumulated R2 and Q2 values for each “focus” variable of the training set (Y for PLS).

Well modeled variables have high green bars, R2, and high blue bars, Q2, at levels of 0.5 or above. Here R2 indicates how well the variation of the variable is explained, while Q2 indicates how well a variable can be predicted. The latter is estimated by cross validation. A variable may be poorly modeled because its variation is mainly noise or in PLS, if the Y‐variable is not related to the X‐variables [33]. From this it can be concluded that the moisture content is not influenced by the torrefaction settings, rather by the storage and handling of the samples. Based on this analysis, the moisture content was excluded from further modeling.

Figure 8.3: Score scatter plot, comp 1 vs. comp 2.

As design of experiments was used as experimental design a very structural displacement of the scores are shown. The scatter plot of t1 vs. t2 is a window in the X space, displaying how the X observations are situated with respect to each other. The score plot is a map of the observations. This plot shows no extreme outliers in the dataset.


43

Figure 8.4: Loadings scatter plot, component 1 vs. component 2.

To interpret this plot, identify the important responses. From one response, imagine a line through the origin and project other responses and factors on this line. Responses opposite to each other are negatively correlated and positively correlated to responses situated near them. Variables situated near responses are positively correlated to them and those situated opposite are negatively correlated to the responses [33]. From this plot, the same conclusions can be drawn as from Figure 4.5, Section 4.1.1 i.e. that the time above 200°C and torrefaction temperature are positively correlated to each other (have the same effect) and that the input temperature have little to no effect on all responses.


44

Figure 8.5: Coefficient Plot for all responses not displayed previously. The error bars indicate one standard deviation. The Y‐scale goes from ‐1 to 1 where high (close to 1) and low values (close to ‐1) indicate a large variable importance and values close to 0 indicate low variable importance.

Figure 8.5 display all responses except solid yield and milling cost (displayed in Figure 4.3). Positive values correspond to a positive correlation between the factor and response and negative value to a negative correlation. Principally the same information as displayed in Figure 8.4 but more accessibly presented. The torrefaction time and temperature is positively correlated to; ash content, HHV, S‐content, C‐content and N‐content and negatively correlated to; H‐content and O‐content (and milling cost and solid yield). The input temp has a very insignificant effect on the responses and has opposite correlation compared to the other two variables.

Figure 8.6: DModX plot for both components. DModX is the distance of the observation in the training set to the X model plane or hyper plane [33]

The DModX plots for both components show that the model has no moderate outliers (no values above D‐crit).


45

Figure 8.7: Variable Importance Plot. VIP‐values larger than 1 indicates “important” X‐variables, and values lower than 0.5 indicate “un‐important” X‐variables.

As previously discussed, the reaction temperature (T>200°C) has a large influence on the responses and output temperature (Temp) also show significance. The input temperature shows no importance.

Appendix 4: Gasification Results Analysis results from the syngas sampling and ash analysis are shown below.

Ash analysis

Table 8.2: Complete results from ash analysis of hydrolysis residue and torrefied hydrolysis residue.

HR TFHR Unit: [mg/kg D.S.]: HR TFHR

TS [%] 88.4 80.2 Cr 12.2 15.8 Ash content [% D.S.] 0.6 0.7 Cu 1.92 2.22 SiO2 [% D.S.] 0.0613 0.0585 Hg <0.01 <0.01 Al2O3 [% D.S.] 0.0110 0.0120 La 0.0655 0.0878 CaO [% D.S.] 0.0275 0.0290 Mo 5.95 7.31 Fe2O3 [% D.S.] 0.0250 0.0315 Nb <0.04 <0.04 K2O [% D.S.] 0.0123 0.0140 Ni 4.94 6.2 MgO [% D.S.] 0.0045 0.0043 Pb 0.426 0.524 MnO [% D.S.] 0.0028 0.0033 S 896 962 Na2O [% D.S.] 0.1800 0.2230 Sc <0.007 <0.009 P2O6 [% D.S.] 0.0129 0.0105 Sn <0.1 <0.2 TiO2 [% D.S.] 0.0007 0.0008 Sr 1.78 1.97

Unit: [mg/kg D.S.]: V 0.116 0.105

As <0.1 <0.1 W 0.610 0.779 Ba 27.9 34.5 Y 0.0297 0.0194 Be <0.004 <0.004 Zn 2.93 3.62 Cd <0.006 <0.006 Zr 0.326 0.25 Co 0.128 0.153


46

Complete Load Dust

Gas Analysis Results Table 8.4: Results from GC analysis for syngas composition.

H2 N2 CH4 CO CO2 Ethene Ethane Acetylene

Wood 1 14.25 47.53 4.34 19.65 11.95 0.44 0.013 0.13 Wood 2 14.82 45.57 4.79 20.39 12.08 0.49 0.017 0.14 Wood 3 15.02 45.44 4.59 20.38 12.2 0.48 0.016 0.15

HR 1 14.92 45.5 4.7 20.39 12.14 0.49 0.016 0.15 HR 2 14.97 45.47 4.64 20.38 12.17 0.48 0.016 0.15

TFHR 1 15.24 50.84 3.27 12.24 15.89 0.77 0.05 0.033 TFHR 2 16.04 49.08 3.4 12.34 16.42 0.89 0.061 0.034 TFHR 3 14.68 51.91 2.98 12.18 15.69 0.72 0.047 0.025

Table 8.3: Calculations of dust load for all samplings.

nr m1 [g] m2 [g] m(sample) [mg]

Volume[m3] @ ambient T Volume [Nm3]

Dust Load [mg/nm3] Error [%]

Wood 1 59.6701 59.7158 45.7 0.009 0.008122749 5077.78 5.56 Wood 2 59.2342 59.2666 32.4 0.008 0.007220221 4050.00 6.25 Wood 3 58.9545 58.9782 23.7 0.009 0.008122749 2633.33 5.56

HR 1 58.9943 59.0165 22.2 0.010 0.009025277 2220.00 5.01 HR 2 58.9625 58.9798 17.3 0.010 0.009025277 1730.00 5.01 HR 3 59.6839 59.702 18.1 0.009 0.008122749 2011.11 5.56

TFHR 1 59.2101 59.2331 23 0.010 0.009025277 2300.00 5.01 TFHR 4 59.2009 59.2356 34.7 0.015 0.013537915 2313.33 3.34 TFHR 5 59.1886 59.2182 29.6 0.008 0.007220221 3700.00 6.25


47


48

Particle Size Distributions Data Table 8.5: Results from the DLPI worksheet. Di = Particle size, Dp = Particle Load. Run 1 for TFHR + Peat was ruined and data therefore excluded.

Wood Pellets Run 1 Run 2 Run 3 Stage Di dm/d log(Dp) Di dm/d log(Dp) Di dm/d log(Dp) # [µm] [mg/Nm³] [µm] [mg/Nm³] [µm] [mg/Nm³]

1 0.024 13.725 0.025 8.237 0.024 2.827 2 0.043 54.499 0.044 10.171 0.043 0.000 3 0.081 35.58 0.082 33.315 0.08 2.270 4 0.158 304.437 0.16 366.019 0.156 124.861 5 0.279 9866.305 0.281 4526.095 0.276 1422.196 6 0.462 12289.092 0.465 4213.614 0.457 1465.510 7 0.768 2765.166 0.771 998.04 0.761 194.045 8 1.28 595.516 1.284 416.764 1.269 285.534 9 2.071 336.56 2.076 371.246 2.054 332.373

10 3.31 122.668 3.317 214.235 3.284 115.875 11 5.579 75.109 5.589 68.364 5.536 76.216 12 8.829 115.172 8.843 17.157 8.761 105.568 13 23.204 33.643 23.222 276.922 23.116 85.916

Hydrolysis Residue + Peat Run 1 Run 2 Run 3 Stage Di dm/d log(Dp) Di dm/d log(Dp) Di dm/d log(Dp) # [µm] [mg/Nm³] [µm] [mg/Nm³] [µm] [mg/Nm³]

1 0.025 0.000 0.025 0.000 0.025 0.000 2 0.044 3.749 0.044 0.000 0.044 0.000 3 0.082 9.059 0.082 1.139 0.083 1.124 4 0.160 15.479 0.159 15.563 0.161 17.599 5 0.282 182.565 0.281 66.593 0.284 349.217 6 0.465 266.958 0.465 0.000 0.468 744.223 7 0.772 103.984 0.773 355.405 0.775 117.575 8 0.124 242.361 1.288 491.303 1.289 203.027 9 2.076 855.985 2.083 874.034 2.083 671.817

10 3.317 1948.556 3.328 1518.866 3.327 1478.884 11 5.589 1899.337 5.609 1509.147 5.605 1392.010 12 8.843 618.576 8.875 555.985 8.867 779.936 13 23.223 23.297 23.265 51.545 23.253 68.060

Torrefied Hydrolysis Residue + Peat Run 1 Run 2 Run 3 Stage Di dm/d log(Dp) Di dm/d log(Dp) #. [µm] [mg/Nm³] [µm] [mg/Nm³]

1 0.024 0 0.025 0 2 0.043 0 0.045 0 3 0.081 12.509 0.083 4.525 4 0.159 43.267 0.162 34.318 5 0.281 6.483 0.284 92.289 6 0.464 78.755 0.467 159.835 7 0.772 210.676 0.774 179.536 8 1.286 722.268 1.287 661.813 9 2.08 2588.253 2.079 2143.088

10 3.324 4406.829 3.319 4401.318 11 5.602 3819.458 5.591 3980.863 12 8.865 1893.017 8.845 3152.679 13 23.251 143.592 23.224 204.996

Documents

Torrefaction and Gasification of Hydrolysis Residue - …€¦ · · 2008-01-29Torrefaction and Gasification of Hydrolysis Residue UMEÅ UNIVERSITY 2007 5 ABBREVATIONS ASTM American