Biofuels || Thermochemical Conversion of Biomass to Biofuels

Biofuels: Alternative Feedstocks and Conversion P

C H A P T E R

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Thermochemical Conversionof Biomass to Biofuels

Thallada Bhaskar*, Balagurumurthy Bhavya,Rawel Singh, Desavath Viswanath Naik, Ajay Kumar,

Hari Bhagwan GoyalBio-Fuels division (BFD), Indian Institute of Petroleum (IIP),

Council of Scientific and Industrial Research (CSIR), Dehradun 248005, India

*Corresponding author: Thallada Bhaskar; E-mail: [email protected]; [email protected]

1 INTRODUCTION

The demand for energy sources to satiate human energy consumption continues toincrease. Currently, the main energy source in the world is fossil fuels. Although it is notknown how much fossil fuel is still available, it is generally accepted that it is being depletedand is nonrenewable. Prior to the use of fossil fuels, biomasswas the primary source of energyfor heat via combustion. With the introduction of fossil fuels in the forms of coal, petroleum,and natural gas, the world increasingly became dependent on these fossil fuel sources.Renewable energy is of growing importance in responding to concerns over the environmentand the security of energy supplies. Given these circumstances, searching for other renewableforms of energy sources is reasonable. Other important consequences associated with fossilfuel uses include global warming. Also, fossil fuel resources are not distributed evenlyaround the globe, which makes many countries heavily dependent on imports.

Governments across the world are stimulating the utilization of renewable energies andresources such as solar, wind, hydroelectricity, and biomass. The threemajor forces that drivethem are (i) secured access to energy; (ii) threat of climate change; (iii) develop/maintainagricultural activities (Lange, 2007). Agricultural economies could be supported by promot-ing the exploitation of local (bio) resources for food, energy, and material. Interestingly, eachof these major drivers also represents one of the three dimensions of sustainability, namely,profitability (affordable energy), planet (climate change), and people (social stability).

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52 3. THERMOCHEMICAL CONVERSION OF BIOMASS TO BIOFUELS

Current use of fossil fuels is split, with about three-quarters for heat and power generation,about one-quarter for transportation fuel, and just a few percent for chemicals and materials(US Department of energy, 2006). The heat and power sector can be supplied with a varietyof renewable sources, namely wind, solar, hydropower, and biomass. The transportationsector has a much more limited choice, however. At this time, biomass is the only resourcethat can provide renewable liquid fuels. Apart from the transportation sector, biomass is alsoa promising feedstock for the chemical industry due to the presence of a wide range offunctionalities available with biomass, the natural polymer.

Biomass is unique in providing the only renewable source of fixed carbon,which is an essen-tial ingredient inmeetingmanyofour fuelandconsumergoodsrequirements.Woodandannualcrops and agricultural and forestry residues are some of the main renewable energy resourcesavailable (Bridgewater, 2006). Biofuel production has been growing rapidly in recent years.

Biomass, a renewable energy source, via photosynthesis, has provided energy for life forthe longest period of existence. Industrial processes that take in biomass can be integratedwith the natural photosynthesis/respiration cycle of vegetation. If used in this manner,biomass is a renewable energy source and by its utilization, much less CO2 is added overallto the atmosphere compared with the fossil fuel counterpart processes. When combined withCO2 sequestration, biomass-based processes can actually lower the CO2 concentrated in theatmosphere (Van swaaij et al., 2004). Lignocellulosic biomass, which is not competing withthe food chain, should be used for the production of fuels, chemicals, power, and heat. Thiscompetition can be avoided by first using the abundant residues from forests, agriculture,and subsequently energy crops. The potential of special energy crops is estimated to be inthe range of 50-250 EJ/annum (Berndes et al., 2003).

Biomass combines solar energy and carbon dioxide into chemical energy in the form ofcarbohydrates via photosynthesis. The use of biomass as a fuel is a carbon neutral process sincethe carbon dioxide captured during photosynthesis is released during its combustion. Biomassincludes agricultural and forestry residues, wood, byproducts from processing of biologicalmaterials, and organic parts ofmunicipal and sludgewastes. Photosynthesis by plants capturesaround 4000 EJ/year in the form of energy in biomass and food (Kumar et al., 2009a).

The most important factor is that all fossil fuels are taken out from under the earth’ssurface, and its continuous excavation creates many geothermal disturbances. Biomass isgrown and consumed only over the earth’s surface and hence does not create such problems.

The events of the last few years have brought into sharp focus the need to developsustainable green technologies for many of our most basic manufacturing and energy needs.Since the beginning of the new millennium, we have witnessed an ever-increasing merger oftechnical, economic, and societal demands for sustainable technologies. As such, this seeksto develop a new “carbohydrate-lignin economy” that will initially supplement today’spetroleum economy and, as these nonrenewable resources are consumed, will become theprimary resource for fuels, chemicals, and materials (Yunqiao et al., 2008).

2 FEEDSTOCKS FOR BIOFUELS

Biomass is harvested as part of a constantly replenished crop. This maintains a closedcarbon cycle with no net increase in atmospheric CO2 levels. There are five basic categoriesof material, that is, virgin wood, forestry materials, materials from arboricultural activities

533 COMPOSITION OF LIGNOCELLULOSIC BIOMASS

or from wood processing; energy crops: high-yield crops grown specifically for energyapplications; agricultural residues: residues from agriculture harvesting or processing; foodwaste, from food and drink manufacture, preparation and processing, and postconsumerwaste; industrial waste and coproducts from manufacturing and industrial processes.

Feedstocks that are used directly in a manner that is given to us by nature fall underthe category of natural feedstocks. The first-generation biofuels use the edible biomass forproducing biofuels. Some of them are sunflower seeds, jojoba oil, soya bean oil, safflowerseeds for biodiesel production, and corn and sugar cane for producing ethanol. In contrast,the second-generation biofuels are produced from non edible feedstocks like lignocellulosicfeedstocks which include agro residue (stalk, husk), forest residue (branch, twigs, bark,leaves), and several others.

In addition to growing currently available feedstocks on available land to produce biofuels,the realization of dedicated energy crops with enhanced characteristics would representa significant step forward. The genetic sequences of a few key biomass feedstocks arealready known, such as Poplar (Tuskan et al., 2006), and there are more in the sequencingpipeline. This genetic information gives scientists the knowledge required to developstrategies for engineering plants with far superior characteristics, such as diminishedrecalcitrance to conversion (Himmel et al., 2007).

Another area where genetic engineering could produce dramatically positive results isthe development of perennial feedstocks that can reach high-energy densities over a shorttime with minimal fertilization and water consumption. By combining the known targetedclimates and soil types present in the available conservation reserve program (CRP) andmar-ginal lands with tailored feedstocks, it may be possible to develop grasses and short-rotationwoody crops thatmaximize carbon and nitrogen fixationwithin these ecosystems. In additionto modifying the intrinsic polysaccharide/lignin composition and central metabolism of thefeedstock itself, several research groups are attempting to express enzymes that are capableof breaking down cellulose into glucose directly within plants.

3 COMPOSITION OF LIGNOCELLULOSIC BIOMASS

Biomass is an organic material which stores sunlight in the form of chemical energy. It isavailable on a renewable basis. Here, we specifically mention the lignocellulosic biomassfrom plants and residues from various agricultural activities. Biomass is an organic mate-rial that is composed of polymers that have extensive chains of carbon atoms linked tomacromolecules. The polymer back bone consists of chemical bonds linking carbon withcarbon, or carbon with oxygen, or sometimes other elements such as nitrogen or sulfur.Instead of describing polymers in terms of the atomic structure of the chain, most can beviewed as assemblies of some larger molecular unit. In the case of cellulose, that unit isthe glucan moiety, essentially a molecule of glucose with one molecule of water missing(C6H10O5)n. For hemicellulose, the unit is often a 5-carbon sugar, called xylose. However,hemicellulose polymers are not linear chains as in the cellulose polymer. Some are branchedand othermonomer units have side chains, with acetyl groups being very common. The ligninpolymers are composed of phenyl propane subunits linked at various points on themonomerthrough C22C and C22O bonds. In addition, there are often side chain moieties such as


methoxy groups. Wood-based biomass is available in large quantities and is cheap. It consistsof three major components, that is, lignin, cellulose, and hemicellulose.

(i) Cellulose: It contains linear polysaccharides in the cell walls of wood fibers, consisting ofD-glucose molecules bound together by b-1,4-glycoside linkages. Biomass comprises40-50% cellulose.

(ii) Hemicellulose: It is an amorphous and heterogeneous group of branched polysaccharides(copolymer of any of the monomers of glucose, galactose, mannose, xylose, arabinose,and glucuronic acid). Hemicellulose surrounds the cellulose fibers and is a linkagebetween cellulose and lignin (15-30%). Hemicelluloses are heterogeneous polymersof pentoses (e.g., xylose, and arabinose), hexoses (e.g., mannose, glucose and galactose),and sugar acids. Unlike cellulose, hemicelluloses are not chemically homogeneous.Hemicelluloses are relatively easily hydrolyzed by acids to their monomer componentsconsisting of glucose, mannose, galactose, xylose, arabinose, and small amounts ofrhamnose, glucuronic acid, methylglucuronic acid, and galacturonic acid. Hardwoodhemicelluloses contain mostly xylans, whereas softwood hemicelluloses contain mostlyglucomannans. Xylans are the most abundant hemicelluloses. Xylans of many plantmaterials are heteropolysaccharides with homopolymeric backbone chains of 1, 4-linkedb-D-xylopyranose units. Xylans from different sources, such as grasses, cereals,softwood, and hardwood, differ in composition. Besides xylose, xylans may containarabinose, glucuronic acid, and acetic, ferulic and p-coumaric acids. The degree ofpolymerization of hardwood xylans (150-200) is higher than that of softwoods.

(iii) Lignin: It is a highly complex three-dimensional polymer of different phenylpropaneunits bound together by ether (C22O) and carbon-carbon (C22C) bonds. Lignin isconcentrated between the outer layers of the fibers, leading to structural rigidity andholding the fibers of polysaccharides together (15-30%). Generally, softwoods containmore lignin than hardwoods. Lignins are divided into two classes, namely, guaiacyllignins and guaiacyl-syringyl lignins. Although the principal structural elements inlignin have been largely clarified, many aspects of their chemistry remain unclear.

In addition, small amounts of extraneous organic compounds, that is, extractives, proteins,and inorganic constituents are found in lignocellulosic materials (about 4%; Stocker, 2008).Biomass residues like wheat straw, corn stover, or sugar cane bagasse contain much ashand N, S, Cl, and these quantities also depend on the geographical source.

4 LIGNOCELLULOSIC BIOMASS PRETREATMENT TECHNIQUES

Lignocellulosic biomass mainly consists of three components, namely, cellulose, hemi-cellulose, and lignin. Cellulose (major component) susceptibility to hydrolysis is restricteddue to the rigid lignin and hemicellulose protection surrounding the cellulose micro fibrils.Therefore, an effective pretreatment is necessary to liberate the cellulose from the lignin-hemicellulose seal and also reduce cellulosic crystallinity. Some of the available pretreatmenttechniques include acid hydrolysis, steam explosion, ammonia fiber expansion (AFEX),alkaline wet oxidation, and hot water pretreatment. Besides reducing lignocellulosic recalci-trance, an ideal pretreatment must also minimize formation of degradation products that

554 LIGNOCELLULOSIC BIOMASS PRETREATMENT TECHNIQUES

inhibit subsequent hydrolysis and fermentation. Pretreatmentmethods are subject to ongoingand intense research worldwide. Possible pretreatment methods can be classified as follows,although not all of them have been developed yet enough to be feasible for applicationsin large-scale processes (Taherzadeh and Karimi, 2008):

i. Physical pretreatments: milling (ball milling, two-roll milling, hammer milling, colloidmilling, vibroenergy milling), irradiation (gamma ray, electron beam, microwave),others (hydrothermal, high-pressure steaming, expansion, extrusion, pyrolysis)

ii. Chemical and physicochemical pretreatment methods: explosion (steam explosion,ammonia fiber explosion, CO2 explosion, SO2 explosion),alkali treatment (treatmentwith sodium hydroxide, ammonia or ammonium sulfite), acid treatment (sulfuric acid,hydrochloric acid, phosphoric acid), gas treatment (chlorine dioxide, nitrogen dioxide,sulfur dioxide), addition of oxidizing agents (hydrogen peroxide, wet oxidation, ozone),solvent extraction of lignin (ethanol-water extraction, benzene-water extraction,ethylene glycol extraction, butanol-water extraction, swelling agents)

iii. Biological pretreatments (fungi and actinomycetes)

Mechanical comminuting reduces cellulose crystallinity, but power consumption is usu-ally higher than inherent biomass energy. Steam explosion causes hemicellulose degradationand lignin transformation and is cost effective but destroys a portion of the xylan fraction,causes incomplete disruption of the lignin-carbohydrate matrix, and generates compoundsinhibitory to microorganism. AFEX is an important pretreatment technology that utilizesboth physical (high temperature and pressure) and chemical (ammonia) processes to achieveeffective pretreatment. Besides increasing the surface accessibility for hydrolysis, AFEXpromotes cellulose decrystallization and partial hemicellulose depolymerization and reducesthe lignin recalcitrance in the treated biomass. This process is not efficient for biomass withhigh lignin content. CO2 explosion increases accessible surface area; are cost effective and donot cause formation of inhibitory compounds but does not modify lignin or hemicelluloses.Ozonolysis reduces lignin content and do not produce toxic residues, but a large requirementof ozone makes it very expensive. Acid hydrolysis hydrolyzes hemicellulose to xyloseand other sugars and alters lignin structure. Its disadvantages are high cost, equipmentcorrosion, and formation of toxic substances. Alkaline hydrolysis removes hemicellulosesand lignin and increases accessible surface area but long residence times are required, irre-coverable salts are formed and incorporated into biomass. Organosolv hydrolyzes ligninand hemicelluloses but solvents need to be drained from the reactor, evaporated, condensed,and recycled; hence, the process cost becomes high. Pulsed electrical field process is carriedout in ambient conditions which disrupts plant cells and is simple equipment, but this pro-cess needs more research. Biological process involves degradation of lignin and hemi-celluloses and has low-energy requirements, but the rate of hydrolysis is very low (Kumaret al., 2009b).

Lignocellulosic biomass has lignin, cellulose, and hemicelluloses with the complexstructures with high molecular weight. The selective and effective lignocellulosic biomassconversion methods are highly desirable to produce the wide range of usable hydrocarbonsas fuels, chemicals, and other products. The decomposition of complex structure canbe performed by using biochemical and thermochemical methods using conventional andnonconventional energy sources.


5 BIOTECHNOLOGICAL CONVERSION

Following pretreatment, woody biomass can be converted into simple sugars by enzymaticdeconstruction via a cellulase treatment. This remains the second most expensive componentin the bioconversion of wood to bioethanol, despite the fact that research studies over the pastdecade have decreased cellulase costs by greater than a 10-fold basis. Numerous publicationsand reviews have highlighted the use of (i) separate hydrolysis and fermentation (SHF) and(ii) simultaneous saccharification and fermentation (SSF) to convert pretreated wood toethanol (Wingren et al., 2003; Wyman, 1994). A process challenge in the conversion ofwood to biofuels is the efficient conversion of all wood sugars (i.e., C5 and C6) to ethanol,especially for hardwoods which have greater amounts of pentoses.

One promising strategy has been to take a natural hexose ethanologen and add thepathways to convert other sugars (Helle et al., 2004; Lawford and Rousseau, 2002). An alter-native approach to minimize the cost of cellulose deconstruction and conversion to ethanolis consolidated bioprocessing (CBP). CBP involves (i) bioproduction of cellulolytic enzymesfrom thermophilic anaerobic microbes, (ii) hydrolysis of plant polysaccharides to simplesugars and (iii) their subsequent fermentation to ethanol all in one stage (Lynd et al.,2005). This bioprocess is projected to reduce the cost of bioethanol by a factor of fourover SSF, and these reduced costs and simplicity of operation have heightened research inthis field.

6 THERMOCHEMICAL CONVERSION

The base of thermochemical conversion is the pyrolysis process in most cases. Theproducts of conversion include water, charcoal (carbonaceous solid), biocrude, tars, andpermanent gases including methane, hydrogen, carbon monoxide, and carbon dioxidedepending upon the reaction parameters such as environment, reactors used, finaltemperature, rate of heating, and source of heat.

6.1 Combustion

Combustion is the sequence of exothermic chemical reactions between a fuel and an oxi-dant accompanied by the production of heat and conversion of chemical species. During thecombustion of lignocellulosic biomass, the heat is generated due to oxidation reaction, wherecarbon, hydrogen, oxygen, combustible sulfur, and nitrogen contained in biomass react withair or oxygen. By far the most common means of converting biomass to usable heat energy isthrough straightforward combustion, and this account for around 90% of all energy attainedfrom biomass (http://www.esru.strath.ac.uk/EandE/Web_sites/06-07/Biomass/HTML/combustion_technology.htm). It contributes over 97% of bioenergy production in the world.Combustion is a proven low-cost process, highly reliable technology, relatively well under-stood and commercially available. There are three main stages that occur during biomasscombustion: drying, pyrolysis and reduction, and combustion of volatile gases and solid char.

Typically, the biomass contains high moisture and high oxygen content, which causes tohave low heating values for biomass. The high moisture content is one of the most significant

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disadvantage features. Although the combustion reactions are exothermic, the evaporation ofwater is endothermic. As themoisture content increases, both the higher heating value (HHV)and lower heating value (LHV) decrease. HHV and LHV are used to describe the heat pro-duction of a unit quantity of fuel during its complete combustion. In determining the HHVand LHV values of a fuel, the liquid and vapor phases of water are selected as the referencestates, respectively. The negative linear relationship exists between the moisture content andthe heating value. Fouling (alkali and other elements) and corrosion (alkali, sulfur, chlorine,etc.) of the combustor are typical issues associated with biomass combustion. These are con-sidered to be detrimental because of the resulting reduction in heat transfer in the combustor.

There are a number of combustion methods/technologies/reactors available for biomasscombustion and themain ones can be categorized under two headings: Fixed-bed combustionsystems and fluidized-bed combustion systems.

6.1.1 Fixed-Bed Combustion

There are two prominent types of fixed-bed combustion: underfeed stokers and gratefirings. With these methods of combustion, air is primarily supplied through the grate frombelow, and initial combustion of solid fuel takes place on the grate and some gasificationoccurs. This allows for secondary combustion in another chamber above the first wheresecondary air is added. Generally, fixed-bed combustion is used in small-scale batch furnacefor biomass containing little ash. Typical examples of fixed-bed systems are manual-fedsystems, spreader-stoker systems, underscrew systems, throughscrew systems, static grates,and inclined grates.

6.1.1.1 UNDERFEED STOKERS

Generally suitable only for small-scale systems, underfeed stokers are a relatively cheapand safe option for biomass combustion. They have the advantage of being easier to controlthan other technologies, since load changes can be achieved quickly andwith relative simplic-ity due to the fuel feed method. Fuel is fed into the furnace from below by a screw conveyorand then forced upward onto the grate where the combustion process begins. Underfeedstokers are limited in terms of fuel type to low ash content fuels such as wood chips. Dueto ash removal problems, it is not feasible to burn ash-rich biomass as this can affect theair flow into the chamber and cause combustion conditions to become unstable.

6.1.1.2 GRATE FIRINGS

There are several different types of grate firing, with both fixed and moving gratescommonplace. They have the distinct advantage over underfeed stokers in that they canaccommodate fuels with high moisture and ash content as well as with varying fuel sizes.It is very important that fuel is spread evenly over the grate surface in order to ensure thatair is distributed uniformly throughout the fuel and thus combustion is kept homogeneousand stable. There are a number of different types of grate firing including fixed grates,movinggrates, rotating grates, horizontal/inclined grate, water cooling grate, dumping grate, andtravelling grates.

The simplest fixed-bed system is composed of one combustion room with a grate.Generally, as soon as the new biomass feed is added into the furnace, it is pyrolyzed intovolatile gases and chars. Primary and secondary air supplies are provided under and above


the grate for the combustion of chars and volatile gases, respectively. The heat generatedthrough the combustion of chars is responsible for providing enough heat for the pyrolysisof newly added biomass. Because of the high content of volatile matter in biomass fuels,a greater secondary air supply is required than the primary air supply. This is one of themajordifferences from the process of coal combustion. Recent developments have been made toenhance the combustion efficiency. One example is the cyclonic combustion system, whichmay be viewed as a modified fixed-bed system, suitable for the combustion of agriculturalresidues and particulate wood wastes at a high efficiency (Quaak et al., 1999).

6.1.2 Fluidized-Bed Combustion

Fluidized-bed furnaces operate in quite a different manner from fixed-bed furnaces andhave a number of advantages associated with them. Fluidized-bed combustion uses silicasand (lime stone, dolomite, or other noncombustible materials) for bed material, keeps fueland sand in furnace in boiling state with high-pressure combustion air, and burns throughthermal storage and heat transmission effect of sand. It is suitable for high-moisture fuelor low-grade fuel. The typical operating temperatures are lower than fixed-bed systems.Depending on the blowing air velocity, fluidizing-bed systems can be further divided intoBubbling Fluidized-Bed (BFB) and Circulating Fluidized-Bed (CFB).

6.1.2.1 BUBBLING FLUIDIZED BED (BFB) COMBUSTION

The fundamental principle of a BFB furnace is that the fuel is dropped down a chute fromabove into the combustion chamber where a bed, usually of silica sand, sits on top of a nozzledistributor plate, through which air is fed into the chamber with a velocity of between1 and 2.5 m/s (http://www.esru.strath.ac.uk/EandE/Web_sites/06-07/Biomass/HTML/combustion_technology.htm). The bed normally has a temperature of between 800 and900 �C and the sand accounts for about 98% of the mixture, with the fuel then making upa small fraction of the fuel and bed material. BFBs have two main advantages in terms offuel size and type over more traditional fixed-bed systems. First, they can cope with fuelof varying particle size and moisture content with little problem, and second, they can burnmixtures of different fuel types such as wood and straw. BFBs are only a practical option withlarger plants with a nominal boiler capacity greater than 10 MWth.

6.1.2.2 CIRCULATING FLUIDIZED BED (CFB) COMBUSTION

If the air velocity is increased to 5-10 m/s then a CFB system can be achieved, where thesand is carried upward by the flue gases and amore thorough mixing of the bedmaterial andfuel takes place. The sand is then separated from the gas in a hot cyclone or U beam separatorat the top of the furnace and fed back into the combustion chamber where the whole processbegins again. CFBs deliver very stable combustion conditions, but it involves higher cost.CFB systems exhibit several advantages, such as the adaptation to various fuels withdifferent properties, sizes, shapes, and high moisture (up to 60%), and ash contents up to50% (http://www.esru.strath.ac.uk/EandE/Web_sites/06-07/Biomass/HTML/combustion_technology.htm).








6.1.3 Entrained Flow Combustion

The fuel particles are transported into an externally heated silicon carbide (SiC) tubepneumatically through an insulated andwater-cooled injector. Prior to the injection, the feed-ing stream, composed of air and fuel particles, has to pass through an agitation chamber for“disaggregation and filtering of pulses in the feeding.” The feeding fuel is ignited by a naturalgas/air burner at the reactor entrance (Jimenez and Ballester, 2006). There are three mainstages that occur during biomass combustion: drying, pyrolysis and reduction, and com-bustion of volatile gases and solid char (IEA, International Energy Agency, Task 32: biomasscombustion and co-firing: an overview. http://www.ieabioenergy.com/MediaItem.aspx?id¼16).The combustion of volatile gases contributes to more than 70% of the overall heatgeneration. It takes place above the fuel bed and is generally evident by the presence ofyellow flames.

Combined Heat and Power (CHP): Production of electricity and heat from one energy sourceat the same time is called CHP. In almost all cases, the production of electricity from biomassresources is most economical when the resulting waste heat is also captured and used asvaluable thermal energy—known as CHP or cogeneration (http://www.epa.gov/chp/documents/biomass_fs.pdf). Biomass is most economical as a fuel source when the CHPsystem is located at or close to the biomass feed stock. In some cases, the availability of bio-mass in a location may prompt the search for an appropriate thermal host for a CHP applica-tion. In other circumstances, a site may be driven by a need for energy savings to search forbiomass fuel within a reasonable radius of the facility (http://www.epa.gov/chp/basic/renewable.html).

Using biomass instead of fossil fuels tomeet energy needswith CHPprovidesmany poten-tial environmental and economic benefits, which can include (i) reduced greenhouse gasand other emissions, (ii) reduced energy costs, (iii) improved local economic development,(iv) reduced waste, (v) expanded domestic fuel supply, (vi) reduced transmission anddistribution losses. CHP offers distributed generation of electrical and/or mechanical power;waste heat recovery for heating, cooling, or process applications; and seamless systemintegration for a variety of technologies, thermal applications, and fuel types into existingbuilding infrastructure. CHP systems typically achieve total system efficiencies of 60-80%for producing electricity and thermal energy (http://www.epa.gov/chp/documents/biomass_fs.pdf).

6.2 Carbonization

Biomass such as woody waste and food waste can be converted to a renewable energysource by means of carbonization processes. Carbonization processes for biomass is one ofseveral technologies concerned with producing renewable energy sources and effectivelyreducing greenhouse gas production. Carbonization is done to obtain charcoal by heatingsolid biomass in the absence of air or oxygen. Carbonization is the term for the conversionof an organic substance into carbon or a carbon-containing residue through pyrolysis ordestructive distillation. When biomaterial is exposed to sudden searing heat, it can becarbonized extremely quickly, turning it into solid carbon. From the point of view of waste,

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woody waste, food waste, and sewage sludge can be considered to contribute to biomass.The basic characteristics of woody waste and food waste, such as proximate analysis andheating value, are evaluated before carrying out carbonization tests. Medium-sized and smallenterprises have been using carbonization technology for biomass, but themethod is not usedin large-scale operations because the production of carbonization residue by conventionaltechnology is inefficient and uneconomical.

6.2.1 Hydrothermal Carbonization (HTC)

HTC is a thermochemical conversion process for biomass to yield a solid, coal-like product.It has been used for almost a century in different sciences, mainly to simulate natural coalifi-cation in the laboratory. Due to the need for efficient biomass conversion technologies,HTC has attracted some interest as a possible application for biomass in recent years, andR&D projects have been launched to assess its feasibility and discover additional possibilitiesfor applications. HTC has been in use as a method for simulating natural coalification in coalpetrology for nearly a century, and many experimental results have been published. It wasintroduced to this research field by Bergius as early as 1913 andwas discussed controversiallyfrom then on. HTC is an exothermic process that lowers both the oxygen and hydrogencontent of the feed (described by the molecular O/C and H/C ratio) by mainly dehydrationand decarboxylation to raise its carbon content with the aim of achieving a higher calorificvalue. This is achieved by applying temperatures of 180-200 �C in a suspension of biomassand water at saturated pressure for several hours. With this conversion process, a lignite-like,easy-to-handle fuel with well-defined properties can be created from biomass residues, evenwith high moisture content. Thus, it may contribute to a wider application of biomass forenergetic purposes (Behar and Hatcher, 1995; Funke and Ziegle, 2009; Mukherjee et al.,1996; Payne andOrtoleva, 2001; Ross et al., 1991; Siskin andKatritzky, 1991;Wolfs et al., 1960).

Many chemical reactions that might appear during HTC have been mentioned throughoutthe literature, but just few have been the focus of detailed investigations, for example,the hydrolysis of cellulose. It has been realized that the process is governed in sum by dehy-dration and decarboxylation, which means that it is exothermal. Simultaneously, functionalgroups are being eliminated to some extent. But the complex reaction network is not knownin detail. So, for the time being, only a separate discussion of general reaction mechanismsthat have been identified can provide useful information about possibilities of manipulatingthe reaction. These mechanisms include hydrolysis, dehydration, decarboxylation, conden-sation polymerization, and aromatization. They do not represent consecutive reaction stepsbut rather form a parallel network of different reaction paths. It is understood that thedetailed nature of these mechanisms, as well as their relative significance during the courseof reaction, primarily depends on the type of feed.

Although HTC has been known for nearly a century, it has received little attention incurrent biomass conversion research. Although it received great attention for biomass lique-faction and gasification, a technical implementation of HTC has only been developed withcomparably low effort. This may be due to the fact that coal as an energy carrier is inferiorto liquid or gaseous fuels. On the other hand, process requirements of HTC are comparablylow while producing a fuel that is easier to handle and store because it is stable and nontoxic.Due to these facts, HTC may provide some advantages when considering small-scale,


decentralized applications. Moreover, it might become a viable option for the productionof functional carbonaceous materials.

The mildest reaction conditions in terms of temperature and pressure are employed inHTC. Lignocellulosic substrates have been extensively examined (Titirici et al., 2007) asreactants at temperatures from 170 to 250 �C over a period of a few hours to a day(Heilmann et al., 2010). Latest research on HTC focused on the preparation of functionalcarbonaceous materials and achieved interesting results for a future application to produceevenmore value-addedmaterials. Low-value andwidely available biomass can be convertedinto interesting carbon nanostructures using environment-friendly steps. These low-costnanostructured carbon materials can then be designed for applications in crucial fields suchas separation, energy conversion, and catalysis. Besides controlling the chemistry of carboni-zation (i.e., C22C linkage), two other important prerequisites for the achievement of usefulproperties are the control overmorphology both at nano- andmacroscale and the control overfunctionality by chemical means in HTC (Titirici and Antonietti, 2010).

6.2.2 Microwave-Assisted Hydrothermal Carbonization (MAHC)

The process uses microwave heating at 200 �C in acidic aqueous media to carbonize pinesawdust (Pinus sp.) and a-cellulose (SolucellW) at three different reaction times. Elementalanalysis showed that the lignocellulosic samples subjected to MAHC yielded carbon-enriched material 50% higher than raw materials. In order to qualitatively evaluate thecarbonization process, H/C and O/C were plotted using the van Krevelen (1950) diagram,which provides information about the changes in chemical structure after carbonization.These results showed that microwave-assisted HTC is an innovative approach to obtaincarbonized lignocellulosic materials (Guiotoku et al., 2009).

6.3 Gasification

Gasification is the conversion of solid raw material into fuel gas or chemical feedstock gasotherwise called as synthesis gas, which can be upgraded to liquid fuels (diesel and gasoline)by Fischer-Tropsch synthesis. Biomass gasification is a process that converts carbonaceousbiomass into combustible gases (e.g., H2, CO, CO2, and CH4) with specific heating valuesin the presence of partial oxygen (O2) supply (typically 35% of the O2 demand for completecombustion) or suitable oxidants such as steam and CO2.

When air or oxygen is employed, gasification is similar to combustion, but it is considereda partial combustion process. In general, combustion focuses on heat generation, whereasthe purpose of gasification is to create valuable gaseous products that can be used directlyfor combustion, or be stored for other applications. In addition, gasification is consideredto be more environmentally friendly because of the lower emissions of toxic gases intothe atmosphere and the more versatile usage of the solid byproducts (Rezaiyan andCheremisinoff, 2005).

Gasification can be viewed as a special form of pyrolysis, taking place at highertemperatures to achieve higher gas yields. Biomass gasification offers several advantages,such as reduced CO2 emissions, compact equipment requirements with a relatively smallfootprint, accurate combustion control, and high thermal efficiency (Marsh et al., 2007;


Rezaiyan and Cheremisinoff, 2005). Gasification is normally carried out at temperatures over(727 �C)1000 K, but recently it has been demonstrated that H2 and CO can be producedthrough the aqueous phase reforming of glycerol at lower temperatures <347 �C (<620 K)(Simonetti et al., 2007; Soares et al., 2006) at which integration of syngas production withFT upgrading is feasible. The ratio of CO/H2 can be modified by the water gas shift reaction(CO þ H2O ! CO2 þ H2).

The classification of gasification is based on several parameters such as types of gasifiers,gasification temperature, heating (direct or indirect), and gasification agent.

6.3.1 Types of Gasifiers

6.3.1.1 FIXED-BED GASIFIERS

Fixed-bed gasifiers generally produce low-heating-valued syngas. They are suitable forsmall or medium-scale thermal applications.

6.3.1.1.1 UPDRAFT (COUNTER-CURRENT) GASIFIERS The updraft gasifier is the simplesttype of gasifier. The biomass is fed at the top while the air is injected at the bottom. Biomassand air move in a countercurrent direction. During its downward movement, biomassis firstly dried passing through a “drying zone.” In the “distillation zone,” biomass under-goes decomposition and is converted into volatile gases and solid char. The gases and charwill be further converted into CO and H2 as they pass through “reduction zone”. Sincesome of the char settles down in the bottom of the reactor, heat is generated through itscombustion in the “hearth zone” and is transported upward by the upflowing gas to main-tain the pyrolysis and drying processes. In addition, CO2 and H2O vapor is also producedfrom char combustion.

Updraft gasifiers can accept biomass with relatively high moisture content (up to 60%).However, the resulting product gas has high tar content because the tar, newly formed duringpyrolysis, does not have the opportunity to pass through the combustion zone.

6.3.1.1.2 DOWNDRAFT (CO-CURRENT) GASIFIERS The downdraft gasifier is currently oneof the most widely used fixed-bed gasification systems. Different from the updraft gasifier,air in the downdraft gasifier is introduced into the reactor from the middle part. This designleads to the reversed order of the hearth zone and the reduction zone. In this gasifier, theinjected air and biomass move cocurrently.

6.3.1.1.3 CROSS-FLOW GASIFIERS In a crossflow gasifier, biomass is added at the top ofthe reactor and moves downward. Air is introduced from one side of the reactor and thegas products are released from the other side of the reactor on the same horizontal level.

6.3.1.1.4 OPEN-CORE GASIFIERS Open-core gasifiers are generally employed to gasifybiomass with low bulk density and high ash content. An example of this kind of biomassis rice husk. Instead of the narrow throat characteristic of other gasifiers, the open-coregasifier has a wide mouth for biomass injection to prevent fuel flow inhibition caused bybridging.


6.3.1.2 FLUIDIZED-BED GASIFIERS

Fluidized-bed reactors are widely employed as gasifiers. Fluidized-bed gasifiers can alsobe further classified into bubbling fluidized gasifiers and circulating fluidized gasifiers.In a bubbling fluidized gasifier, air is injected from the bottom of a grate, above whichthe moving bed is mixed with the biomass feed. The bed temperature is maintained at700-900 �C. Biomass is pyrolyzed and cracked through contact with the hot bed material.In a circulating fluidized gasifier, the hot bed material is circulated between the reactorand a cyclone separator. During this circulation, bedmaterials and char go back to the reactor,while the ash is separated and removed from the system.

6.3.1.3 ENTRAINED FLOW GASIFIERS

In an entrained flow gasifier, the feed and air move cocurrently and the reactions occur ina dense cloud of very fine particles at high pressures, varying between 19.7 and 69.1 atmand very high temperatures >1000 �C. This type of gasifier has an elevated throughputof syngas (Zhang et al., 2010).

6.3.2 Low/High-Temperature Gasification

High-temperature gasification (typically above 1200 �C) results in a gas, which merelycontains H2 and CO as combustible components. At low-temperature however (typicallybelow 1000 �C), also hydrocarbons are present in the gas. A CFB gasifier operated on biomassoperated at 900 �C typically produces a gas containing 50% hydrocarbons (mainly methane,ethylene, and benzene) on energy basis (http://www.biosng.com/experimental-line-up/gasification-technology/).

6.3.3 Heating Source for Gasification

6.3.3.1 INDIRECT (OR ALLOTHERMAL) GASIFICATION

It is characterized by the separation of the processes of heat production and heat consump-tion. It therefore generally consists of two reactors connected by an energy flow. The biomassis gasified in the first reactor and the remaining solid residue (char) is combusted in thesecond reactor to produce the heat for the first process. Hot sand is circulated to transportthe heat from the combustor to the gasifier. These indirect gasifiers theoretically are operatedat an equilibrium based on the temperature dependence of the char yield in the gasifier.This means that at a low temperature, much char is remaining from the gasifier. Since thischar is combusted to produce the heat, the temperature will rise until char yield matchesthe energy demand of the gasification (http://www.biosng.com/experimental-line-up/gasification-technology/).

6.3.3.2 PLASMA GASIFICATION

Plasma gasification is a gasification process that decomposes biomass into basiccomponents, such as H2, CO, and CO2 in an oxygen-starved environment at an extremelyhigh temperature. Plasma is regarded as the 4th state of matter; it is an ionized gas producedby electric discharges. A plasma torch is a tubular device that has two electrodes to producean arc. It is an independent heat source that is neither affected by the feed characteristics northe air/oxygen/steam supply. When electricity is fed, an arc is created, and the electricity is

http://www.biosng.com/experimental-line-up/gasification-technology/







converted into heat through the resistance of the plasma. A plasma torch can heat the biomassfeedstock to a temperature of 3000 �C or higher (up to 15,000 �C). Under such extremelyelevated temperature, the injected biomass stream can be gasified within a few millisecondswithout any intermediate reactions. The plasma technique has high destruction and reduc-tion efficiencies. Any form of wastes, for example, liquid or solid, fine particles or bulkitems, dry or wet, can be processed efficiently. In addition, it is a clean technique with littleenvironmental impact. Plasma technique has great application potential for treating a widerange of hazardous wastes (Zhang et al., 2010).

6.3.3.3 CONCENTRATING-SOLAR BIOMASS GASIFICATION (CSBG)

The concept’s key feature is the use of high-temperature heat from a solar-concentratingtower to drive the chemical process of converting biomass to a biofuel, obtaining a near-complete utilization of carbon atoms in the biomass. The aim of the concept is to obtain aneasy to handle fuel with near-zero CO2 emission and reduced land-use requirements com-pared to first- and second-generation biofuels. H2 from water electrolysis with solar poweris used for reverse water gas shift to avoid producing CO2 during the process. The solar-driven third-generation biofuel requires only 33% of the biomass input and 38% of totalland as the second-generation biofuel, while still exhibiting a CO2-neutral fuel cycle. WithCO2 capture, second-generation biofuel would lead to the removal of 50% of the carbon inthe biomass from the atmosphere. There is a trade-off between reduced biomass feedcosts and the increased capital requirements for the solar-driven process; it is attractive atintermediate biomass and CO2 prices (Hertwich and Zhang, 2009).

6.3.4 Gasification Agent

6.3.4.1 OXYGEN-BLOWN GASIFICATION

It is an alternative route for the production of a nitrogen-free product gas. To preventlocal hotspots in the reactor, the oxygen is normally diluted with steam or CO2. The methanecontent drops with increasing steam/O2 and CO2/O2 ratio. The decrease on dry gas basisis mainly caused by the dilution by CO2 or H2 that is produced from steam by the CO shiftreaction.

A low steam or CO2/O2 ratio produces a product gas with the highest CH4 content, whichis desired for synthetic natural gas (SNG) production. A low amount of CO2 or steam alsoincreases the gasifier efficiency, because less “inert” gas needs to be heated to the processtemperature. A certain amount of oxygen dilution is required to prevent possible agglomera-tion of biomass (http://www.biosng.com/experimental-line-up/o2-blown-gasification/).

6.3.4.2 SUPER CRITICAL GASIFICATION OF BIOMASS

Super critical water gasification (SCWG) technology is suitable for wet biomasses andorganic wastes. This technology takes advantage of the large amount of water in biomassesby using the water as a reaction medium, eliminating the costly feedstock-drying step.Supercritical water has a low dielectric constant close to that of organic compounds. Theorganic reactions under supercritical water, therefore, become more homogeneous, resultingin a higher reaction rate. The free radical condition of supercritical water also enhances thegas formation, leading to the high gas yield. As compared to conventional dry gasification,

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SCWG produces a lower amount of tarry material and char as byproduct, due to the highersolubility and reactivity of the organic compounds in supercritical water. Nevertheless,because tar and char are difficult to gasify, they act as a drier to achieve complete gasification.The formation of tar and char also causes a reduction in the energy efficiency of the processby means of reactor plugging, heat exchanger fouling, and catalyst deactivation(Chuntanapum and Matsumura, 2010).

6.3.4.3 HYDROTHERMAL GASIFICATION OF BIOMASS

Hydrothermal gasification is the conversion of solid biomass into gaseous and/or liquidproducts in the presence of steam. Different hydrothermal biomass gasification processesare under development. In contrast to biomass gasification processes without water, biomasswith the natural water content (“green biomass”) can be converted completely and energeti-cally efficiently to gases. Depending on the reaction conditions, methane or hydrogen is theburnable gas produced. Some processes use catalysts. In recent years, significant progresswas achieved in the development of various hydrothermal biomass gasification processes.However, some challenges still exist and technical solutions are needed before large-scaleproduction facilities can be built (Kruse, 2009).

6.4 Pyrolysis

Pyrolysis is the fundamental chemical reaction process that is the precursor of both thegasification and combustion of solid fuels, and is simply defined as the chemical changesoccurring when heat is applied to a material in the absence of oxygen. The products ofbiomass pyrolysis include water, charcoal (carbonaceous solid), pyrolysis oils or tars, andpermanent gases including methane, hydrogen, carbon monoxide, and carbon dioxide.The nature of the changes in pyrolysis depends on the material being pyrolyzed, the finaltemperature of the pyrolysis process, and the rate at which it is heated up. The pyrolysis pro-cess is a mildly endothermic reaction. The heat of vaporization of pure water is 2.26 KJ g�1

at 100 �C, while the chemical energy content of wood is only about 18.6 KJ g�1. Most ofthe energy obtained from biomass goes in moisture removal. This reinforces the facts thatlower the moisture content, greater is the energy obtained.

As typical lignocellulosic biomass materials such as wood, straws, and stalks are poor heatconductors, management of the rate of heating requires that the size of the particles beingheated be quite small. Otherwise, in massive materials such as logs, the heating rate isvery slow, and this determines the yield of pyrolysis products. Depending on the thermalenvironment and the final temperature, pyrolysis will yield mainly char at low temperatures,<450 �C, when the heating rate is quite slow, andmainly gases at high temperatures,>800 �C,with rapid heating rates. An intermediate temperature and under relatively high heatingrates, the main product is a liquid bio-oil, a relatively recent discovery, which is just beingturned to commercial applications. There are 3 stages in the pyrolysis process: The first stage,prepyrolysis, occurs between 120 and 200 �C with a slight observed weight loss, when someinternal rearrangements, such as bond breakage, the appearance of free radicals, and theformation of carbonyl groups take place, with a corresponding release of small amounts ofwater (H2O), carbon monoxide (CO), and CO2. The second stage is the main pyrolysis pro-cess, during which solid decomposition occurs, accompanied by a significant weight loss


from the initially fed biomass. The last stage is the continuous char devolatilization causedby the further cleavage of C22H and C22O bonds.

In reacting chemical systems, the term severity is used to capture the idea that both theduration of heating and the final temperature influence the chemical products of pyrolysis.Very-low-severity treatments of short duration to a maximum temperature of about 250 �Care sometimes called torrefaction and result in a product that has lost some H2O and CO2

from pyrolysis while retaining almost all of the heat value. Traditional charcoaling is amedium-severity process, while the production of bio-oils is a short-duration high-severityprocess, which, if the duration at high temperature is maintained, will go all the way togas and soot.

Depending on the reaction temperature and residence time, pyrolysis can be dividedinto fast pyrolysis, intermediate pyrolysis, and slow pyrolysis. Typically, fast pyrolysishas an extremely short residence time (�1 s); the reaction temperature is approximately100 �C higher than that of slow pyrolysis (e.g.�500 �C vs.�400 �C). Short reaction times com-bined with an elevated temperature generally result in a higher yield of liquid product.A conventional moderate or slow pyrolysis process, with a relatively long vapor residencetime and low heating rate, has been used to produce charcoal for thousands of years(Zhang et al., 2010).

Among the short residence-time processes (0.5-5 s) under development are vacuumpyrolysis at about 300-400 �C and 0.3 atm (U. of Sherbrooke, Canada), flash pyrolysis at about500-650 �C and 1 atm (U. ofWaterloo, Canada), hydropyrolysis in an atmosphere of hydrogenat about 500-600 �C and 5-6 atm (HYFLEX TM, IGT), and flash pyrolysis in atmospheresof hydrogen or methane at 600-1000 �C and 1-70 atm (Brookhaven National Laboratory).An interesting report of a relatively long residence time (10-15 min heat up, several hoursat temperature) pyrolysis study at reduced pressures of 0.0004-0.004 atm and temperaturesof 250-320 �C of wild cherry wood seems to contrast with the results of several reports onflash pyrolysis (http://journeytoforever.org/biofuel_library/liquefaction.html).

6.4.1 Slow Pyrolysis

Heating of the lignocellulosic biomass in inert atmosphere for hours to a maximumtemperature of 400-500 �C is called slow pyrolysis. The charcoal yield is 35-40% by weight.In general, the yield of liquid products would be less than the fast pyrolysis of biomass.Several types of catalysts can be employed for the pyrolysis of biomass and/or upgradationof the vapors produced from the thermal pyrolysis.

6.4.2 Fast Pyrolysis

The goal of fast pyrolysis is to produce liquid fuel from lignocellulosic biomass thatcan substitute for fuel oil in any application. The liquid can also be used to produce a rangeof specialty and commodity chemicals. The essential features of a fast pyrolysis process arevery high heating and heat transfer rates, which often require a finely ground biomass feed.Carefully controlled reaction temperature of ca. 500 �C in the vapor phase and residence timeof pyrolysis vapors in the reactor less than 1 s; and then quenching (rapid cooling) ofthe pyrolysis vapors to give the bio-oil product. The main product of fast pyrolysis isbio-oil, which is obtained in yields of up to 80 wt% of dry feed.

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Fast pyrolysis is a promising process to produce transportable oil with a high volumetricenergy density from bulky and inhomogeneous biomass. There are several applicationsforeseen for pyrolysis oil. It has been tested as a substitute for fuel oil or diesel in boilers,furnaces, engines, and turbines for heat and power generation and has been considered asa precursor for transportation fuels and chemicals. Water is the most abundant componentin pyrolysis oil; typically, it is present in the range of 15-35 wt%. Probably all applicationsrequire different specifications with respect to the water content of pyrolysis oil. For fuelinginto a diesel engine, the water content should be below 30 wt% to decrease emissions ofparticles and to prevent ignition delay and phase separation. But there should also be a mini-mum amount of water present to limit NOx emissions and to ensure a uniform temperaturedistribution in the cylinders. For cofeeding pyrolysis oil in a mineral oil refinery, nearlyall water and most organically bound oxygen must be removed. Generally, less water isbeneficial for the energy density, transportation costs, stability, and acidity of pyrolysis oil.Fast pyrolysis oil possesses many undesirable properties including a high total acid number(TAN �200), low heating value (�6560 BTU/lb), high oxygen content (�40%), chemicalinstability, high water content (20%), and incompatibility with petroleum fractions. Inherentlow-energy density makes pyrolysis oil expensive to transport, and the high TAN makes itmetallurgically incompatible with conventional transport vessels and refinery hydro-conversion equipment, both designed for feeds with TANs less than 2. In addition to theseundesirable properties, pyrolysis oil is not miscible with petroleum fractions and if addedinto existing refinery equipment (hydrotreaters or hydrocrackers) will require a separatepyrolysis-oil feed system. Thus, pyrolysis oil needs effective pretreatment and upgradationbefore it is used as crude oil replacement. Depending on the reactors used, we have manykinds of fast pyrolysis processes.

6.4.2.1 ABLATIVE FAST PYROLYSIS

Ablative pyrolysis, in which much larger particle sizes can be employed than in othersystems, as the heat is transferred from a hot surface to the biomass particle and the process,is limited by the rate of heat supply to the reactor rather than the rate of heat absorption by thepyrolyzing biomass. Ablative pyrolysis is fundamentally different from fluid bed processesfrom the mode of heat transfer through a molten layer at the hot reactor surface, use of largeparticles, and absence of a fluidizing gas.

6.4.2.2 VORTEX REACTOR

A vortex tube has certain advantages as a chemical reactor, especially if the reactionsare endothermic, the reaction pathways are temperature dependent, and the products aretemperature sensitive. With low-temperature differences, the vortex reactor can transmitenormous heat fluxes to a process stream containing entrained solids. This reactor has nearlyplug flow and is ideally suited for the production of pyrolysis oils from biomass at lowpressures and residence times to produce about 10 wt% char, 13% water, 7% gas, and 70%oxygenated primary oil vapors based on mass balances. This product distribution wasverified by carbon, hydrogen, and oxygen elemental balances. The oil production appearsto form by fragmenting all of the major constituents of the biomass. Cyclonic fast pyrolysis,also called vortex fast pyrolysis, separates the solids from the noncondensable gases andreturns them to the mixer.


6.4.2.3 ROTATING CONE FAST PYROLYSIS: ROTATING CONE REACTOR

The rotating cone reactor is a novel reactor type for fast pyrolysis of biomass with negli-gible char formation, in which rapid heating and short residence time of the solids can berealized. Particles fed into the reactor first enter an impeller which is mounted in the baseof the heated cone. After leaving the impeller, the particles flow outward over the conicalsurface and experience a high heat transfer rate due to their small distance from the heatedsurface. Biomass materials like wood, rice husks, or even olive stones can be pulverized andfed to the rotating cone reactor. Flash heating of the biomass will suppress coke-formingcracking reactions. Since no carrier gas is needed (cost reducing), the pyrolysis productswill be formed at high concentrations. If additional thermal quenching of the gas outlet flowis applied, the amount of secondary tar decomposition reactions can be suppressed. In therotating cone reactor, wood particles fed to the bottom of the rotating cone, together with anexcess of inert heat carrier particles, are converted while being transported spirally upwardalong the cone wall. The cone geometry is specified by a top angle of p/2 radians and amax-imum diameter of 650 mm. Products obtained from the flash pyrolysis of wood dust in arotating cone reactor are noncondensable gases, bio-oil, and char. The biomass decomposesinto 70% condensable gases with 15% noncondensable gases and 15% char.

6.4.2.4 BUBBLING FLUIDIZED BED (BFB) PYROLYSIS

A simple method for the rapid heating of biomass particles is to mix themwith the movingsand particles of a high-temperature fluid bed. High heat transfer rates can be achieved, as thebed usually contains small sand particles, generally about 250 mm. The heat required isgenerated by combustion of the pyrolysis gases, and/or char, and eventually transferredto the fluid bed by heating coils. While the sand to biomass heat transfer is excellent (over500 W/m2 K), the heat transfer from the heating coils to the fluid bed will be low, due tothe resistance inside the coils (gas to coil wall heat transfer estimated 100-200 W/m2K),and the limiting driving force of around 300 �C as a maximum. In an optimistic case, at least10-20 m2 surface area is required per ton/h of biomass fed.

6.4.2.5 CIRCULATING FLUIDIZED BED (CFB) PYROLYSIS

CFB reactor has been widely used for the pyrolysis of lignocellulosic biomass into highyield of liquid products (Rapid Thermal Process, RTP; UOP).The CFB reactor has manyadvantages, for example, the simple structures, high production capacity, favorableconditions of heat andmass transfer, and the convenience of operation, etc., the CFBwas usedas the main reactor in this study. To reduce the operation cost, part of the pyrolysis gas wasused as the carrier gas, while the rest and the pyrolysis char were recycled as heat.

The CFB could be divided into two zones corresponding to the main chemical processes.(i) pyrolysis zone: In this zone, feedstockwas loaded into the bed and pyrolyzed very quickly.Since the feedstock particles were small and the heat exchanged rapidly, the heating rate wasvery high. For example, a small particle at 0.1-0.2 mm diameter could be heated at the rateof about 103 �C/s in an atmosphere at 1000 �C. In this zone, the main chemical process couldbe described as

Biomass ! char þ tarþH2Oþ gas ðCO2;CO;CH4;CnHm;H2Þ:


Temperature was another essential factor affecting the pyrolysis besides heating rate.Because the relatively high temperature was favorable to form more noncondensable gasand decrease the tar yield, moderate and carefully controlled temperature was needed.

(ii) Reduction and cracking zone: Before the pyrolysis vapors were quenched by thecondenser, further reactions had taken place; for example, the tar cracked and the charwas reduced. These processes produced more noncondensable gas such as CO and H2.Some CnHm also cracked at the same time. The main reactions could be expressed as

CþH2O ! COþH2;

Cþ CO2 ! 2CO;

CH4 þH2O ! COþ 3H2;

COþH2O ! CO2 þH2;

Tar ! CH4 þH2Oþ CnHm þH2:

Pyrolysis char contributed to secondary cracking by catalyzing secondary cracking in the
vapor phase; rapid object of gasification is to get high-quality gas product. Thus, the hightemperature of up to 900 �C is wanted to increase the gas product and decrease the tar,while the relatively long residence time contributes to the secondary reactions includingchar reduction, tar cracking, shift reaction, etc. So the amount of CO2, CO, CH4, and H2 isfar more, and the amount of CnHm is less in the gas product of gasification. By contrast,the objective of fast pyrolysis is to obtain more liquid product; it determines the operationconditions of moderate temperature and short residence time to increase the liquid pro-duction rate. Such operation conditions lead to the higher amount of CnHm and less amountof CO, CH4, and H2, which indicate that the degree of pyrolysis is not excessive.
6.4.2.6 AUGER (SCREW) REACTOR

The auger type of pyrolyzer has been identified as especially appealing for its potentialto reduce operating costs associated with bio-oil production. This design may also be wellsuited for small, portable pyrolysis systems in a highly distributed or decentralized biomassprocessing scheme. The operating principle of this design is that biomass is continuouslypyrolyzed by being brought into direct contact with a bulk solid heat transfer mediumreferred to as a “heat carrier.” The heat carrier material, such as sand or steel shot, is heatedindependently before being metered into the reactor. On a gravimetric basis, thermodynamiccalculations suggest a heat carrier feed rate 20 times the biomass feed rate. Two intermeshing,co-rotating 1inch augers quickly combine biomass and heat carrier in a shallow bed to effec-tively carry out the pyrolysis reactions. This mechanical mixing process, though not wellunderstood, appears to be the essence of this alternative pyrolyzer design. Volatile vaporsand aerosols exit at various ports, while char is transported axially through the 20 inch longreactor section and stored in a canister with the heat carrier (Brown, 2009).

6.4.3 Ultra Fast Pyrolysis

The ultra fast high-temperature pyrolysis will be carried out in a high-temperature fluid-wall reactor which can withstand working temperatures of up to 2200� C. The biomass is fedto the top of the reactor (rate of 1.0-1.8 kg/min). The feed falls and, at the same time is very


quickly heated by radiation to the reaction temperature. The estimated heating rate is on theorder of 106 �C/s for reactant surfaces. The fluid wall, produced by a nitrogen flow throughthe 30-cm diameter porous reactor core, prevents both reactants and products from reachingthe reactor wall. The product distribution at the reactor exit has been determined for differentoperating conditions. The influence of reactor temperature, biomass feed rate, and biomassparticle size on the product distribution and on the heating value of the exit gas has beeninvestigated (Corella et al., 1988).

6.4.4 Hydropyrolysis

A better approach for biomass conversion is the integrated hydropyrolysis and hydro-conversion of biomass to directly produce fungible gasoline and diesel fuel or blendingcomponents is carried out in two integrated stage. The first stage is a medium pressure,catalytically assisted, fast hydropyrolysis step completed in a fluid bed under moderatehydrogen pressure. Vapors from the first stage pass directly to a second-stage hydro-conversion step where a hydrodeoxygenation catalyst removes all remaining oxygen andproduces gasoline and diesel boiling range material. All the process steps are completed atessentially the same pressure, so that compression costs are minimized. A unique featureof this process is that all the hydrogen required for this process is produced by reformingthe C122C3 hydrocarbons, so no additional hydrogen is required. Pyrolysis is carried outin the presence of hydrogen at high pressure. The advantage of hydropyrolysis is thehigh quality of the products at the maximum liquid yield. The disadvantage is the highhydrogen consumption, which leads to high processing costs, but this is only a short-termeconomic consideration. If the H2 from a carbon-free source becomes cost competitive, thehydropyrolysis can become commercially exploitable technique for the conversion of ligno-cellulosic biomass with complete utilization of carbon content (Agrawal and Singh, 2009;Marker et al., 2009).

6.4.5 Vacuum Pyrolysis

Vacuum or vacuum moving bed pyrolysis includes a combination of slow and fast pyrol-ysis conditions. Course solids are heated relatively slowly to temperatures higher than thoseof slow pyrolysis, while the gas is removed from the hot temperature zone relatively quicklyby applying a reduced pressure of less than 0.20 atm in the process. Vacuum pyrolysis isnot a rapid heating technique and is in the same thermal regime of time and temperatureas slow pyrolysis (charcoal production); however, yields of liquid that are over 50% of theoriginal biomass are achieved by removing the vapors as soon as they are formed byoperating under a partial vacuum—essentially the converse of work at high pressures,in which the liquids are held in the charring mass to increase the yield of char.

6.5 Liquefaction

Hydrothermal liquefaction is the conversion of solid biomass into gaseous and/or liquidproducts in the presence of water. Liquefaction consists of the catalytic thermal decomposi-tion of large molecules to unstable shorter species that polymerize again into a bio-oil.


Biomass is mixed with water and basic catalysts like sodium carbonate, and the processis carried out at lower temperatures than pyrolysis (252-472 �C) but higher pressures(50-150 atm) and longer residence times (5-30 min.). These factors combine to make lique-faction a more expensive process; however, the liquid product obtained contains less oxygen(12-14%) than the bio-oil produced by pyrolysis and typically requires less extensiveprocessing (Elliott, 2007; Inoue et al., 1999; Karagoz et al., 2006; Kruse et al., 2003; Minowaet al., 1997).

6.5.1 Direct and Indirect Liquefaction

Currently, more research is being done on direct and indirect thermal liquefactionmethodsfor biomass andwastes than on the othermethods. Direct liquefaction is either reaction of bio-mass components with smaller molecules such as H2 and CO (e.g., Pittsburg Energy ResearchCentre (PERC) and Lawrence Berkeley Laboratory, Berkeley, USA (LBL) processes) or short-term pyrolytic treatment, sometimes in the presence of gases such as H2. Indirect liquefactioninvolves successive production of an intermediate, such as synthesis gas or ethylene, and itschemical conversion to liquid fuels, In 1983, after several years of laboratory and pilot-plantwork on the PERC and LBL processes, which involve reaction of product oil or water slurriesof wood particles with H2 and CO at temperatures up to about 370 �C and pressures up to 272atm in the presence of sodium carbonate catalyst, researchers concluded that neither processcan be commercialized for liquid fuel productionwithout substantial improvement. Themostattractive approach to such improvement is believed to be a combination of solvolysis with apyrolysis or reduction step. However, the oxygen content of the resulting complex liquidmix-ture is still high (6-10 wt%), and considerable processing is necessary to upgrade this material(http://journeytoforever.org/biofuel_library/liquefaction.html).

Direct liquefaction has some similarity with pyrolysis in terms of the target products(liquid products). However, they are different in terms of operational conditions. Specifi-cally, direct liquefaction requires lower reaction temperatures but higher pressures thanpyrolysis (0.5-2 atm for liquefaction vs. 0.01-0.05 atm for pyrolysis). In addition, dryingof the feedstock is not a necessary step for direct liquefaction, but it is crucial for pyrolysis.Moreover, catalysts are always essential for liquefaction, whereas they are not as critical forpyrolysis. At the beginning of the liquefaction process, biomass undergoes depolymeriza-tion and is decomposed into monomer units. These monomer units, however, may berepolymerized or condensed into solid chars, which are undesirable (Zhang et al., 2010).

6.6 Co-processing

Investigation and large-scale application of co-gasification and co-pyrolysis of biomassand coal are becoming more common recently. In addition to the reduction of CO2 emission,cogasification of biomass provides several advantages over biomass or coal gasification(Kumabe et al., 2007). One of the advantages is the reduction of sulfur and ash that causeequipment corrosion and environmental problems in coal gasification (Chmielniak andSciazko, 2003; McLendon et al., 2004). It can also reduce the high cost of the feedstockand high tar generation in biomass gasification.

Likewise, co-pyrolysis has advantages over sole biomass or coal pyrolysis. Althoughpyrolysis of coal is a good method for producing liquid fuels, the yields of these products




are limited because of the low H/C ratio in coal. The high H/C ratio in biomass renders bio-mass to act as a hydrogen donor in co-pyrolysis of biomass/coal blends. Moreover, the highthermochemical reactivity and high content of volatiles of biomass facilitate the conversionand the upgrading of the fuel. Therefore, it’s considered promising to co-fire the two fuelsas a step toward valid, sustainable utilization of coal and biomass and to minimize the impacton the environment.

Pyrolysis gas has a high heating value, 17 MJ/kg (http://www.nh.gov/oep/programs/energy/documents/biooil-nrel.pdf), and both pyrolysis oil and char can be gasified to pro-duce syngas; it is a promising technique to further process the pyrolysis products throughgasification to produce syngas more efficiently. Here, the process consists of the pyrolysisand subsequent gasification sections. In the first reactor, biomass is pyrolyzed with coalat 500-700 �C. The pyrolysis gas is quenched to produce liquid oil, and the char is flowedto the gasifier where steam and limited air are supplied to produce syngas.

6.7 Hydrolysis

Hydrolysis pathways are appropriate for lignocellulose processing if higher selectivity isdesired in biomass utilization, for example, in the production of chemical intermediatesor targeted hydrocarbons for transportation fuel. Selective transformations require isolationof sugar monomers, a step which is complex and expensive for lignocellulosic feedstocks.Once sugar monomers are isolated, however, they can be processed efficiently at relativelymild conditions by a variety of catalytic technologies (Alonso et al., 2010).

The ability to recover and use the major components of lignocellulosic biomass (cellu-lose, hemicellulose, lignin) is critical in developing economically viable bioproducts andbiorefineries. This project focuses on the biomass pretreatment step of hemicellulose acidhydrolysis to recover the hemicellulose sugars and prepare the biomass for subsequentenzymatic or acid cellulose conversion. The ultimate goal is to identify promising routesto reduce the sugar production cost by 30% compared with establishedmethods. Researchersare investigating three hydrolysis systems: water-rich hydrolysis, water-restricted, andnear neutral pH. Using different reactor configurations (e.g., batch tube, Parr, flow through)with varying solids and pH levels, researchers have developed comprehensive data on thedestructuring, disaggregation, and depolymerization of hemicellulose to sugars. Flow ratehas been found to enhance hemicellulose removal, which is inconsistent with models typi-cally applied to describe hemicelluloses hydrolysis. New models have been defined thatreveal mass transfer could be important in explaining this anomaly. The flow throughreactor experiments showed that lignin is modified as hemicellulose reacts, and the resultingdisruption of lignin may play a significant role in enhancing cellulose digestion. In addition,researchers have shown that nonproductive adsorption on lignin can be reduced byprior treatment with low-cost proteins, thereby substantially cutting enzyme costs(Iranmahboob et al., 2002; Mosier et al., 2005; Patrick Lee et al., 1997; Wang et al., 2007;Yat et al., 2008).

The ideal process for cellulosic biomass conversion would be the production of liquidfuels from biomass in a single step at a short residence time. The liquid product producedin pyrolysis is called bio-oil, which is an acidic combustible liquid containing more than300 compounds (Wang et al., 2008). Bio-oils are not compatible with existing liquid

http://www.nh.gov/oep/programs/energy/documents/biooil-nrel.pdf



738 TYPICAL ISSUES FOR LIFE-CYCLE ANALYSIS

transportation fuels including gasoline and diesel. To use bio-oil as a conventional liquidtransportation fuel, it must be catalytically upgraded (Carlson et al., 2008). Zeolite catalystsadded into the pyrolysis process can convert oxygenated compounds generated by pyrolysisof the biomass into gasoline-range aromatics.

7 BIO-REFINERIES AND BIOFUELS

In addition to being a resource for energy generation, lignocellulosic biomass has potentialto serve for multiple purposes. There is not necessarily a concurrence of various options.Highest value can be achieved by diverting individual components to optimum routes, thusaiming to achieve complete valorization of the material. Among possible target utilizationoptions are to be mentioned in particular: (i) electricity and fuel generation; (ii) productionof chemicals; (iii) precursors for industrial products such as biodegradable plastics;(iv) utilization as soil amendment.

The idea of so-called biorefineries is to process bioresources such as agricultural or forestbiomass to produce energy and awide variety of precursor chemicals and bio-basedmaterials(Sigrid andMorar, 2009). Petroleum refineries are already built, and use of this existing infra-structure for the production of biofuels requires little capital investment (Marinangeli et al.,2006). Furthermore, the infrastructure for blending fuels as well as their testing and dis-tribution is already in place at oil refineries. Three options are available for using petroleumrefineries to convert biomass-derived feedstocks into fuels and chemicals: (i) fluid catalyticcracking (FCC), (ii) hydrotreating-hydrocracking, and (iii) utilization of biomass-derivedsynthesis gas (syngas) or hydrogen.

Cofeeding biomass-derived molecules into a petroleum refinery could rapidly decreaseour dependence on petroleum feedstocks. Petroleum-derived feedstocks are chemically dif-ferent than biomass-derived feedstocks; therefore a new paradigm in how to operate andmanage a petroleum refinery is required. Another improvement toward the productionof biofuels in a petroleum refinery would be if governments were to offer tax exemptionsand subsidies to all types of biofuels, and not only for selected biofuels such as ethanol andbiodiesel. As the price of petroleum continues to increase, we project that refining tech-nology will be developed to allow the coproduction of bio- and petroleum-based fuels inthe same (petroleum) refinery and even using the same reactors. A realistic practical sce-nario will be one in which both industries cooperate, with one producing the biofuelprecursors and the other processing and converting them into valuable fuels (Huber andCorma, 2007).

8 TYPICAL ISSUES FOR LIFE-CYCLE ANALYSIS

(i) Use of fossil fuel and raw materials to produce biofuels: The whole life cycle of theproduction of biofuels involves the use of fossil fuel and raw materials to some extent.Whether the net gain balance out of the fossil input obtained in terms of low emissionsis positive or not remains under discussion.


(ii) Availability of land for fuel (food vs. fuel issue): Current biofuel producers do not alwayshave a secure access to raw materials due to limited grain reserves and the fact that thecurrent costs of crude vegetable oil from “food crops” are variable. Bio-based energyindustries are also currently in competition with food producers, and we perceive themas being a primary cause of the increase in food prices. In order to make biofuelproduction profitable and more sustainable, avoiding as much as possible competitionwith the foodmarket, companies have to focus on second-generation biofuelsmade fromalternative cheap feedstocks (e.g., (waste)-biomass, waste oils and fats, residues, etc.).(Ligno) cellulosic ethanol and biodiesel from waste oils, nonfood crops, or algae emergeas real alternatives to tackle this problem.

(iii) Environmental impact: Despite the fact that some studies carried out to date showfirst-generation biofuels may offer a low carbon balance, fossil fuel usage and GHGbalance, further outputs and environmental indicators must be addressed. Waterusage (in the growth of the crops), eutrophication (run off of lawn fertilizers into naturalwaters), and soil erosion are some of them. Second- and potential third-generationbiofuels are more attractive in terms of crop economy.

(iv) Socioeconomic impact: Some sectors of the industry estimated that a robust globalbiofuel market will be fully established around 2012. The implementation of biofuelswill also be highly dependent on the feasibility of the technologies employed fortheir production and the economics of the processes play a fundamental role in thisregard.

At the moment, there is not yet a widely accepted definition of “sustainable biofuels,”or a scheme for certification and labeling (Mol, 2007). Nevertheless, some agreement canbe observed on four ecological issues that should be included in sustainability schemessuch as GHG emissions, energy balance, biodiversity loss, specific environmental effects(i.e., soil condition and water use).The problem is that each feedstock is different andmany crops produce their best yields in specific regions of the world or require certain soilor water conditions. These local differences demand specific attention and are not easilygeneralized. Furthermore, there is wide disagreement on the implementation of internationalconventions, while inclusion of social criteria is even more difficult (Oosterveer andMol, 2010).

A holistic approach to valorization of lignocellulosic biomass needs to take into accountsustainability of chosen options. If concepts are too heavily orientated toward energy produc-tion or industrial use, this can even be at the expense of environmental protection. If cropresidues such as straw are no longer left on field, this will result in depletion of soil organicmatter. While anaerobic digestion results in a digestate, which can be brought back to fieldto supply not only nutrients but also organic matter, thermal valorization and production ofsecond-generation biofuels result in complete consumption of the biomass and consequentlya lack of nutrients and organic matter. Soil requirements vary within a wide range and needto be assessed locally. Only lignocellulosic biomass which is in surplus of soil demand fororganic matter should be considered for treatment options with complete consumption ofthe substrate. In regions with concern about declining organic content of soils, anaerobic diges-tion should be given special attention even if the net energy recovery is lower compared to thatof alternative technologies with total consumption of the biomass (Sigrid and Morar, 2009).

75REFERENCES

9 PERSPECTIVES AND CHALLENGES

Several potential scenarios for biofuels can be foreseen in the future. The big hopes forthe transport sector are second- and future third-generation biofuels, including biodieselfrom microbial oil, the production of biobutanol (from nonedible feedstocks) as a morepetrol-like fuel, and the preparation of biofuels from cellulosic and biomass nonediblefeedstocks.

One of the critical factors that will influence the future prospects of biofuels is diversifica-tion. The future of biofuels as a sustainable (economic, social, and environmental definitions)technology is directly linked to themaximumuse of byproducts that will make its productionmore cost effective.

Biology and synthetic biology would have the opportunity to design plants with specialproperties through genetic engineering to produce biomass feedstocks with requisite ratioand functionalities of lignin/cellulose (Luque et al., 2008). Such discoveries can significantlychange the future of biofuels, as a major contribution to the GHG reduction through theregulation of the CO2 fixed by plants in crops.

Acknowledgments

The authors thank The Director, Indian Institute of Petroleum, Dehradun, for his constant encouragementand support. RS thanks Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing JuniorResearch Fellowship (JRF).

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