Complete MSW

1

Catalytic Steam Gasification of MSW

I .C .E .T , UNIVERSITY OF THE PUNJAB

Chapter No. 1

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INTRODUCTION

In recent years, the quantity of municipal solid waste (MSW) has increased

significantly in the industrialized and developing countries raising the question of its

sustainable disposal management yields of MSW reach approximately 900 million tones

in the world each year. Recently, MSW increased at an annual rate of 8-10%, and it

reached 150 x 106 tones in 2004. Lots of energy and money was used for

transportation, treatment, and final disposal of MSW, and thus the disposal of MSW is

one of the most important and urgent problems in environmental management in the

world because of the decrease in the available space for land-filling and the growing

concern about the living environment.

Solid Waste Management (SWM) can be defined as the discipline associated

with the control of generation, storage, collection, transfer, processing and disposal of

Municipal Solid Waste aesthetics and other environmental considerations. The

municipalities in developing countries typically lack the financial resources and skills

needed to cope with this crisis. Several countries have realized that the way they

manage their solid wastes does not satisfy the objectives of sustainable development .

This raises the important issue of how to deliver quality service in the face of the

financial and skill constraints of the public sector.

1.0 CURRENT STATUS OF SWM PRACTICES

Currently solid waste in Pakistan has not been carried out in a sufficient and proper

manner in collection, transportation and disposal or dumping regardless of the size of

the city; therefore the environmental and sanitary conditions have become more serious

year by year, and people are suffering from living such conditions. The scope of

problems regarding solid waste management is very wide and involves the

consideration of all the aspects relating to solid waste and its management, either

directly or indirectly. These aspect may include rate of urbanization, pattern and density

of urban areas, physical planning and control of development, physical composition of

waste, density of waste, temperature and precipitation, scavenger’s activity for

recyclable separation, the capacity, adequacy and limitations of respective


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municipalities to manage the solid waste i.e. storage, collection, transportation and

disposal. According to the 1998 census, of the 130.579 million persons living in

Pakistan, 67% live in rural areas, while 33 % live in urban areas. Furthermore, out of 33

% of persons living urban areas, 54 % of them live in ten major cities of Pakistan (GOP,

1996). During the last several decades, migration has occurred from rural to urban

areas.

1.1 Population and Household Estimates

The number and growth of population and households is the foremost factor

affecting the solid waste and its management at various stages. The selected cities are

growing at a rate ranging between 3.67% to 7.42%, which is much higher than the

overall growth rate of Pakistan, i.e. 2.8%. Major cities of them are estimated to double

their population in next ten years. These cities are generating high amounts of solid

waste which is increasing annually with the respective population growth. The numbers

of households also play an important role in generation and collection of the solid waste.

The average household size in the selected cities varies from 6.7 to 7.3 persons.

1.2 Waste Generation and Collection Estimates

The average rate of waste generation from all type of municipal controlled areas

varies from 1.896 kg/house/day to 4.29 kg/house/day in a few major cities (Pak-EPA,

2005). It shows a trend of waste generation wherein increase has been recorded in

accordance with city's population besides its social and economic development. Figure

1 presents city wise waste generation rate with respective daily and annual estimate of

solid waste. In Pakistan, solid waste is mainly collected by municipalities and waste

collection efficiencies range from 0 percent in low-income rural areas to 90 percent in

high income areas of large cities (Pak-EPA, 2005). Collection rate of solid waste by

respective municipalities ranges from 51% to 69% of the total waste generated (Figure

2) within their jurisdiction. The uncollected waste, i.e., 31% to 49% remains on street or

road corners, open spaces and vacant plots, polluting the environment on continuous

basis


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Lahore Faisalabad Hyderabad Gujranwala Peshawar0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Waste Generation Tons/dayWaste Collectin Tons/day

Figure 1.1: Rate of Generation and Collection of SW in a Few Major Cities of

Pakistan

Faisalabad, 54

Lahore, 45

Peshawar, 61

Gujranwala, 52

Hyderabad, 51

FaisalabadLahorePeshawarGujranwalaHyderabad

Figure 1.2: Solid Waste Collection Rate in a Few Major Cities of Pakistan


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1.3 Physical Composition of Waste

In the gasification of MSW, it requires a greater knowledge of the composition of

municipal solid waste. Solid waste in Pakistan is generally composed of three

categories i.e. biodegradable such as food waste, animal waste, leaves, grass, straws,

and wood. Non-biodegradable are plastic, rubber, textile waste, metals, fines, stones

and recyclable material includes paper, card board, rags and bones(Figure 1.3).

Pakistan's urban (municipal) solid waste differs considerably from that of cities in

developed countries (which is to be expected).One reason for this is that there is a wide

range from poverty to affluence in Pakistan’s urban population; another is that much of

the waste is reclaimed for recycling at various stages from arising to final disposal.

Figure 1.3: Physical Composition of Solid Waste in Pakistan (% Weight)

Source: EPMC Estimates, 1996


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1.4 Current Waste Treatment and Disposal in Pakistan:

The waste is disposed off within or outside municipal limits into low lying areas like

ponds etc, without any treatment except recyclable separation by scavengers. The land

is also hired/leased on long term basis for disposal. Moreover, the least mitigating

measures have also never been reported from any municipality. Treatment and disposal

technologies such as sanitary land filling, composting and incineration are

comparatively new in Pakistan. Crude open dumping is the most common practice

throughout Pakistan and dump sites are commonly set to fire to reduce the volume of

accumulating waste, hence adding to the air pollution caused by the uncovered dumped

waste itself. At present, there are no landfill regulations or standards that provide a

basis for compliance and monitoring, but national guidelines for these standards are

being prepared by the Consultant under National Environmental Action Plan Support

Program (NEAP SP).

1.5 Conversion Pathways

Energy conversion of organic materials can proceed along three main pathways

—thermochemical, biochemical, and physicochemical. Currently, all three pathways are

utilized to varying degrees with fossil fuel feedstocks.

Thermochemical conversion processes include combustion, gasification, and

pyrolysis. Thermochemical conversion is characterized by higher temperatures and

faster conversion rates. It is best suited for lower moisture feedstocks. For biomass

feedstocks, the lignin fraction currently can not be converted biochemically, although

research is investigating lignin fermentation processes. On the other hand,

thermochemical routes can convert all of the organic portion of suitable feedstocks. The

inorganic fraction (ash) of a feedstock does not contribute significantly to the energy

products but does participate in important ways including fouling of high temperature

equipment, increased nutrient (e.g. K and P) loading in facility waste water treatment

and disposal, and in some cases by providing marketable co-products or adding

disposal cost. Inorganic constituents may also be catalytic for some of the conversion

reactions.


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Biochemical conversion processes include aerobic conversion (i.e.,

composting), anaerobic decomposition or digestion (which occurs in landfills and

controlled reactors or digesters) and anaerobic fermentation (for example, the

conversion of sugars from hydrolyzed cellulose and hemicellulose by ethanol producing

yeasts and recombinant bacteria. Biochemical conversion proceeds at lower

temperatures and lower reaction rates. Higher moisture feedstocks are generally good

candidates for biochemical processes.

Physicochemical conversion involves the physical and chemical synthesis of

products from feedstocks (for example, biodiesel).

Some literatures shown that thermal disposal especially incineration is a desired

and viable option with energy recovery in forms of heat and electricity, and has the

advantage of reducing the amount of MSW by weight and volume when compared with

landfilling and composition. However, incineration has drawbacks as well particularly

harmful emissions of acidic gases (SOx, HCl, HF, NOx, etc.) and volatile organic

compounds (VOCs) especially polyaromatic hydrocarbons (PAH), polychlorinated

biphenyls (PCBs) and polychlorinated dibenzo-p-dioxine/ furans (PCDD/Fs) and

leachable toxic heavy metals. Furthermore, more and more stringent environmental

regulations are being imposed to control the environmental impact of MSW and

pollutant emissions of MSW cineration. Nevertheless, different waste management,

treatment and disposal methods have been adopted besides the traditional methods of

landfilling and incineration. Now attentions are being paid to energy efficient,

environment friendly and economically sound technologies of gasification processing of

waste. Gasification is defined as thermo-chemical conversion of a carbon-containing

material through the addition of heat in an oxygen-starved environment using a gaseous

compound such as water, air, oxygen and their mixtures, producing a gaseous product.

MSW gasification obviously reduces and avoids corrosion and emissions by retaining

alkali and heavy metals (except mercury and cadmium), sulphur and chlorine within the

process residues, prevents largely PCDD/F formation and reduces thermal NOx

formation due to lower temperatures and reducing conditions. The gasification


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technology of MSW can, however, avoid these problems, and have promising

application in waste-to-energy (WTE) technology.

1.6 Potential of MSW to Produce Energy

The heat content of raw MSW depends on the concentration of combustible

organic materials in the waste and its moisture content. On the average, raw MSW has

a heating value of roughly 13,000 kJ/kg or about half that of bituminous coal. The

moisture content of raw MSW is 20% on average. Figure-2.6 shows how the heating

values of MSW and its components change with moisture content. Points shown are

experimental values, and solid lines show the thermochemical calculations for various

organic compounds. Mixed plastics and rubber contribute the highest heating values to

municipal solid waste. Moist food and yard wastes have the lowest heating value and

are better suited for composting, rather than for combustion or gasification.


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Figure 1.4: Effect of moisture on heating value of MSW materials


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Figure 1.5: Heating Values of Various Fuels

Source: ECN Website (2002)

With its recovery of the chemical energy of MSW, and the generated residue is

disposed on landfilling sites or applied in cement and construction, thus, MSW can be

seen as a kind of valuable fuel able to substitute or supplement fossil fuels in power

generation and other industrial processes. Waste management system consists of

reuse/recycling, biological treatment of organic waste (i.e. land filling, compost) and

thermal treatment (i.e. incineration, pyrolysis, gasification).


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Chapter No. 2

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INTRODUCTION:

Gasification is defined as thermo-chemical conversion of a carbon-containing

material through the addition of heat in an oxygen-starved environment using a gaseous

compound such as water, air, oxygen and their mixtures, producing a gaseous product.

Gasification converts low quality carbon containing feed stocks, such as coal, oil sand

or even municipal waste into valuable output.

2.0 Principle of Gasification:

A basic law of physics i.e. Law of Conservation of Matter says that "Matter can

neither be created nor it can be destroyed, but it can be transformed from one form to

another" and this is the basic of Gasification.

Gasification converts low quality carbon containing feed stocks, such as coal, oil

sand or even municipal waste into valuable output.

2.1 Gasification Process:

Gasification is a thermochemical process that generates a gaseous, fuel rich

product. Regardless of how the gasifier is designed, two processes must take place in

order to produce a useable fuel gas.

In the first stage, pyrolysis releases the volatile components of the fuel at

temperatures below 600°C (1112°F). The by-product of pyrolysis that is not vaporized is

called char and consists mainly of fixed carbon and ash.

In the second gasification stage, the carbon remaining after pyrolysis is either

reacted with steam or hydrogen or combusted with air or pure oxygen. Gasification with

air results in a nitrogen-rich, low BTU fuel gas. Gasification with pure oxygen results in a

higher quality mixture of carbon monoxide and hydrogen and virtually no nitrogen.

Gasification with steam is more commonly called “reforming” and results in a hydrogen

and carbon dioxide rich “synthetic” gas (syngas). Typically, the exothermic reaction

between carbon and oxygen provides the heat energy required to drive the pyrolysis

and char gasification reactions.


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The product yield during the gasification of MSW depends on temperature,

pressure, time, reaction conditions, and added reactants or catalysts. Several studies

on the gasification of MSW have already been investigated. MSW gasification

processes have been studied previously by using several different types of equipments

such as fixed bed, fluidized beds, rotary kilns, plasma furnace.

Table 2.1: Comparison of different Gasification Techniques:

ProcessCarbon

Conversion(%)

Tar Yield

(weight%)

Char Yield

(weight%)

Dry gas Yield

(weight%)

Heating Value

of gas (MJ/kg)

1. Pyrolysis 22.82 38.54 25.86 0.21 4.13

2.Catalytic

Pyrolysis34.14 18.75 11.45 0.34 6.75

3.Steam

Gasification44.07 0.23 7.95 0.51 7.66

4.

Catalytic

Steam

Gasification

83.48 0 7.36 1.65 18.86

5.Plasma

Gasification100 0 18.18 1.06 9.09

2.2 Applications of Syn-gas Produced by Gassificatin:

In general, the products of gasification of MSW are ash, oils and combustible

gases (carbon monoxide, hydrogen, carbon dioxide and hydrocarbon). The catalytic

gasification of MSW has been considered to be a promising method for future energy

systems to meet environmental requirements, and provides one of the most cost-

competitive means of obtaining hydrogen-rich gas or syngas from renewable resources,

which are used as feedstock for producing hydrogen for methanol and ammonia

synthesis or for fuel cell applications and hydrogen combustion engines to release its


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stored energy. Hydrogen-rich gas can also be converted to liquid transportation fuels

using Fischer–Tropsch synthesis. Furthermore, the hydrogen-rich gas could be directly

used in the production of electrical power in fuel cells or by combustion in gas turbines.

2.3 Reactions involved in Gasificaton

The basic gasification reactions that must be considered are:

1. C + O2 → CO2 -393 kJ/mol (exothermic)

2. C + H2O → CO + H2 +131 kJ /mol (endothermic)

3. C + CO2 → 2CO +172 kJ/mol (endothermic)

4. C + 2H2 → CH4 -74 kJ/mol (exothermic)

5. CO + H20 → CO2 + H2 -41 kJ/mol (exothermic)

6. CO + 3H2 → CH4 + H20 -205 kJ/mol (exothermic)

All of these reactions are reversible and their rates depend on the temperature,

pressure and concentration of oxygen in the reactor.

2.4 Gasifier Designs

The reactors used for the gasification process are very similar to those used in

combustion processes. The main reactor types are fixed beds and fluidized beds.

2.4.1 Fixed Beds

Fixed bed gasifiers typically have a grate to support the feed material and

maintain a stationary reaction zone. They are relatively easy to design and

operate, and are therefore useful for small and medium scale power and thermal

energy uses. It is difficult, however, to maintain uniform operating temperatures

and ensure adequate gas mixing in the reaction zone. As a result, gas yields can

be unpredictable and are not optimal for large-scale power purposes (i.e. over 1

MW). The two primary types of fixed bed gasifiers are updraft and downdraft.

2.4.1.1 Downdraft


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Downdraft gasifiers (Figure 2.1) have a long history of use in cars and

buses to produce a wood-derived gas for internal combustion engines. In a

downdraft gasifier, air is introduced into a downward flowing packed bed or

solid fuel stream and gas is drawn off at the bottom. The air/oxygen and fuel

enter the reaction zone from above decomposing the combustion gases and

burning most of the tars. As a result, a simple cooling and filtration process is

all that is necessary to produce a gas suitable for an internal combustion

engine. Downdraft gasifiers are not ideal for waste treatment because they

typically require a low ash fuel such as wood, to avoid clogging. In addition,

downdrafts have been difficult to scale up beyond 1MW because of the

geometry of their throat section.

Figure 2.1: Down Draft Gasifier

Source: Scottish Agricultural Web Site 2002

2.4.1.2 Updraft

In updraft gasifiers, the fuel is also fed at the top of the gasifier but the

airflow is in the upward direction (Figure 2.2). As the fuel flows downward

through the vessel it dries, pyrolyses, gasifies and combusts. The main use of

updraft gasifiers has been with direct use of the gas in a closely coupled

boiler or furnace. Because the gas leaves this gasifier at relatively low

temperatures, the process has a high thermal efficiency and, as a result, wet

MSW containing 50% moisture can be gasified without any predrying of the

waste. Moreover, size specifications of the fuel are not critical for this gasifier.


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Ash is removed from the bottom, where the gasification air and steam are

introduced. However the product gas exits at low temperatures, (typically less

than 500°C), yielding a tar rich gas. For heating applications, this is not a

problem as long as blocking of pipes can be overcome. To minimize the tar in

the product gas high temperature and a suitable catalyst may be used (e.g.

Dolomite as catalyst).

Figure 2.2: Updraft Gasifier

Source: Source: Scottish Agricultural Web Site 2002

2.4.1.3 Slagging Fixed Beds

One particular updraft gasifier is the high-pressure, oxygen- injected

slagging fixed bed (Figure 2.3). Originally developed for the gasification of

coal briquettes, these units operate at a maximum temperature of around

3000° F, above the grate and at pressures of approximately 450 psi. In

theory, the high temperatures crack all tars and other volatiles into non-

condensable, light gases. Also under these conditions, the ash becomes

molten and is tapped out, as is done in iron blast furnaces. The potential

problems for such a system are maintaining the furnace for extended periods

of time at such high temperatures and pressures, overcoming blockages in

the outlet by accretions, and tapping a slag from the bottom of the furnace.


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Figure 2.3: Slagging Fixed Bed Gasifier for Mixed MSW & Coal

2.4.2 Fluidized Beds

Fluidized beds offer the best vessel design for the gasification of MSW. In a

fluidized bed boiler, inert material and solid fuel are fluidized by means of air

distributed below the bed. A stream of gas (typically air or steam) is passed upward

through a bed of solid fuel and material (such as coarse sand or limestone). The gas

acts as the fluidizing medium and also provides the oxidant for combustion and tar

cracking. The fluidized bed behaves like a boiling liquid and has some of the

physical characteristics of a fluid. Waste is introduced either on top of the bed

through a feed chute or into the bed through an auger. The two main types of

fluidized beds for power generation are bubbling and circulating fluidized beds.


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2.4.2.1 Bubbling Fluidized Bed (BFB)

In a BFB, the gas velocity must be high enough so that the solid particles,

comprising the bed material, are lifted, thus expanding the bed and causing it

to bubble like a liquid. A bubbling fluidized bed reactor typically has a

cylindrical or rectangular chamber designed so that contact between the gas

and solids facilitates drying and size reduction (attrition). The large mass of

sand (thermal inertia) in comparison with the gas stabilizes the bed

temperature (Figure 2.4). The bed temperature is controlled to attain

complete combustion while maintaining temperatures below the fusion

temperature of the ash produced by combustion. As waste is introduced into

the bed, most of the organics vaporize pyrolytically and are partially

combusted in the bed. The exothermic combustion provides the heat to

maintain the bed at temperature and to volatilize additional waste. The bed

can be designed and operated by setting the feed rate high relative to the air

supply, so that the air rate is lower than the theoretical oxygen quantity

needed for full feed material oxidation. Under these conditions, the product

gas and solids leave the bed containing unreacted fuel. The heating value of

the gases and the char increases as the air input to the bed decreases

relative to the theoretical oxygen demand. This is the gasification mode of

operation. Typical desired operating temperatures range from 900° to 1000

°C. Bubbling fluidized-bed boilers are normally designed for complete ash

carryover, necessitating the use of cyclones and electrostatic precipitators or

bag houses for particulate control.

Figure 2.4: Bubbling Fluidized Bed


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Source: Scottish Agricultural Web Site 2002

2.4.2.2 Circulating Fluidized Bed (CFB)

As the gas velocity increases in a turbulent fluidized chamber, the bed of

solids continues to expand, and an increasing fraction of the particles is blown

out of the bed. A low efficiency particle collector can be used to capture the

larger particles that are then returned to the bed. This suspended-combustion

concept is a called a circulating fluid bed. A circulating fluid bed is differentiated

from a bubbling fluid bed in that there is no distinct separation between the dense

solids zone and the dilute solids zone (Figure 2.5). Circulating fluid bed densities

are on the order of 560 kg/m, as compared to the bubbling bed density of about

720 kg/m. To achieve the lower bed density, air rates are increased from 1.5-3.7

m/s (5 - 12 ft/s) of bubbling beds to about 9.1 m/s (30 ft/s). The particle size

distribution, attrition rate of the solids and the gas velocity determine the optimal

residence time of the solids in a circulating fluid bed.

Figure-2.5 Circulating fluidized bed gasifier


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Chapter No. 3

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3.1 Method of Sampling and Analysis of MSW:

The low heating value of the MSW samples can be estimated using a bomb

calorimeter with accuracy of <0.15%. Ultimate analysis of the MSW samples can be

obtained with a CHNS/O analyzer. This analysis gives the weight percent of carbon,

hydrogen, oxygen, nitrogen, and sulphur in the samples simultaneously (Table 3.2), and

the weight percent of oxygen is determined by difference. A TA Instruments system was

used to obtain proximate analysis of the MSW samples (that is, moisture, volatile

matter, fixed carbon, and ash content of the material (Table 3.1). X-ray diffraction

(XRD) measurements of catalysts were carried out to determine main components and

investigate the catalytic performance before and after the experiment. Gas compositions

analysis was conducted with a dual channel micro-gas chromatography that is able to

provide precise analysis of the principal gas components (H2, CO, CO2, CH4, C2H4, and

C2H6). [ref. 1]

Table 3.1 – Components in MSW samples (wt.%) [ref. 1]

Kitchen Garbage Paper Textile Wood Plastic

68.96 9.95 2.17 7.40 11.52

Table3. 2 – Ultimate and proximate analysis of MSW samples (Dry Basis) [ref. 1]Ultimate analysis Proximate analysis

C 51.81 (wt.%) Volatile matter 82.28 (wt.%)

H 5.76 (wt.%) Fixed carbon 11.79 (wt.%)

O (by difference) 30.22 (wt.%) Ash 5.93 (wt.%)

N 0.26 (wt.%) Low heating value 21 306 kJ/kg

S 0.36 (wt.%) Apparent density 280.5 kg/m3

3.1.1 Methods of data processing [ref. 1]:

The lower heating value (LHV) of hydrogen-rich gas is calculated by,

LHV (MJ/Nm3) = CO x 126.36 + H2 x 107.98 + CH4 x 358.18 + C2H2 x 56.002 + C2H4 x 59.036 + C2H6 x 63.772)/1000


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where,

CO, H2, CH4, C2H4 and C2H6 are the molar percentages of components of

hydrogen-rich gas.

The carbon conversion efficiency (%) is calculated by,

XC(%) = 12Y(CO% + CO2% + CH4% + 2 x C 2H4% + 2 x C 2H6) x100%

22.4 x C%

where,

Y is the dry gas yield (N.m3/kg), C% is the mass percentage of carbon in

ultimate analysis of MSW feedstock, and the other symbols are the molar

percentage of components of hydrogen-rich gas.

Steam decomposition (%) is calculated by,

SD(%) = 1000Y(H2% + 2x CH4% + 2 x C2H6%) x 18/22.4)/(W1+ W2) x 100%

where,

SD is steam decomposition, W1 is steam flow rate and W2 represent the

total moisture content in the MSW feedstock.

The molecular formula of MSW (daf.) can be expressed as CH1.53O0.49

based on the ultimate analysis (Table 3.2). The stoichiometric yield of H2 from

MSW is 106.58 mol H2/kg MSW (daf.) calculated by the follow equations:

CH1.53O0.49 + 0.51H2O = 1.28H2 + CO (1)

H2O + CO = H2 + CO2 -41.2 MJ/kmol (2)

H2 potential yield is defined as the sum of measured hydrogen in product gas and

the theoretical hydrogen that could be formed by completely shifting carbon

monoxide as in reaction (2) and completely reforming hydrocarbon mspecies in

product gas according to reaction (3), given below


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CnHm + 2n H2O = (2n + (m/2))H2(∆H298>0) (3)

3.2 Catalyst

The tar formed during gasification is one of the major issues, catalytic pyrolysis

or gasification for tar reduction has been extensively reported in the literatures. The use

of dolomite as a catalyst in biomass gasification had attracted much attention, because

it is inexpensive and abundant and can significantly reduce the tar content of the

product gas from a gasifier, but they are significantly active only above 800 °C.

Likewise, during MSW gasification process tar was formed, calcined dolomite was used

to eliminate tar. Natural dolomite was ground and sieved, the particle with a size of 3-

10mm was calcined in muffle oven at 900 °C for 4 hr. The surface characteristics and

XRD patterns of the calcined dolomite were listed in Table 3.3 and figure 3.1

respectively.

Table 3.3 – Surface characteristics of catalyst [ref. 1]

Catalyst BETsurface

area (m2/g)

Microporearea

(m2/g)

Externalsurface

area (m2/g)

Total porevolume(cm3/g)

CalcinedDolomite

9.96 1.73 8.23 2.27


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Fig. 3.1 – XRD patterns of catalysts. (1) Natural dolomite, (2) Calcined dolomite. [ref. 1]

3.2.1 Mechanism of Catalytic steam Gasification of MSW

The purpose of using catalyst includes:

i. Cracking of tar;

ii. To decrease the gasification temperature;

iii. To enhance steam reforming and water gas shift reactions in order to produce

hydrogen-rich gas and more product gas.

In general, steam gasification reactions include two steps.

The first step is a thermo-chemical decomposition of MSW with production of tar,

char and volatiles, this step termed primary pyrolysis, could perform at a lower

temperature approx. 300 C, and last until a temperature of 700 C or even higher.


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The second step includes reactions of CO, CO2, H2 and H2O with the

hydrocarbon gases and carbon in MSW, thereby producing gaseous products.

The catalytic steam gasification mechanism of MSW might be described by the

following reactions as shown in Eqs. (4)–(7):

C + CO2 → 2CO +162 kJ/mol (endothermic) (4)

C + H2O → CO + H2 +131 kJ /mol (endothermic) (5)

CH4 + H2O → CO + 3 H2 +206.3kJ/mol (endothermic) (6)

Tar + n1H2O → n2CO2 + n3H2 (∆H298K>0) (7)

Calcined dolomite can accelerate the reaction rate of the steam with tar and char,

also participate in the secondary reactions. calcined dolomite consists of CaO, and

MgO, which convert to Ca(OH)2 and Mg(OH)2 quickly at the presence of moisture,

some Ca(OH)2 and Mg(OH)2 can convert to CaCO3 and MgCO3 using CO2 as a

sorbent by reacting with CO2 produced during gasification reaction, CO2 absorbing

contributes to water gas shift reaction Eq. (2) and carbon gasification reaction

(Eq.5), which lead to production of hydrogen-rich gas and high content of

combustible gas.

3.2.2 Catalyst Activity

Dolomites were employed in biomass steam gasification processes to enhance

the yield and quality of product gas and decrease tar yield by cracking and

reforming the high molecular weight organic components with steam. The catalytic

activity of calcined dolomite was extensively investigated in different reactors such

as fixed bed and fluidized bed reactors, but few literatures have been found on

catalytic behaviors of calcined dolomite in the steam gasification of MSW. In Fig.

3.3, H2, CO, CO2, CH4, C2H4 and C2H6 contents are represented for the catalytic

and non-catalytic pyrolysis and gasification. In contrast, there was a great

difference between pyrolysis (run 1) and steam gasification (run 3), it was

concluded that the introduction of steam increased H2, CO and CO2 contents, while

CH4, C2H4 and C2H6 contents decreased, which was caused by the participation of


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the steam in gas-phase reactions and gasification of tar and char, thus tar yield

and char yield decreased, and dry gas yield increased. The decrease of CH4, C2H4

and C2H6 contents led to the decrease of LHV of syngas is because their heating

value is higher. Especially, there was a little tar during steam gasification reaction

(run 3), the presence of the steam can significantly decreased the tar, and caused

a drastic decrease of 38.31% in the tar yield. Calcined dolomite improves the

quality of the product gas and diminishes significantly the tar yield. At the presence

of catalyst, the results of catalytic steam gasification (run 4) were compared with

those of catalytic pyrolysis (run 2), a crucial increase of 32.41% in H2 content and

13.00% in CO2 content as well as a remarkable decrease of CO, CH4, C2H4 and

C2H6 contents was achieved, which attributed to water gas shift reaction and steam

reforming of hydrocarbon reactions, resulting in an increase of 42.39% in the lower

heating value of the hydrogen-rich gas as shown in Table 3.4, The dry gas yield

and carbon conversion efficiency drastically increased by 385.29% and 144.52%,

respectively, and char yield decreased by 35.72%. It was concluded that the

presence of steam increased the H2 and CO2 contents, and decreased CO, CH4,

C2H4 and C2H6 contents. More tar and char participated in steam gasification, which

led to a rapid increase of dry gas yield and carbon conversion efficiency.

Interestingly, there was no tar detected during catalytic steam gasification reaction

(run 4) owing to steam and calcined dolomite significantly eliminating the tar, which

agreed with the results of several authors.


27


Fig. 3.3: Gas composition in steam gasification andpyrolysis for non-catalytic and

catalytic processes. [ref. 1]

Table 3.4 – Results of pyrolysis and steam gasification [ref. 1]

RunLHV

(MJ/Nm3)

Carbon

conversion

(%)

Tar yield

(wt.%)

Char yield

(wt.%)

Dry gas

yield

(N.m3/kg)

1 19.68 22.82 38.54 25.86 0.21

2 19.84 34.14 18.75 11.45 0.34

3 15.02 44.07 0.23 7.92 0.51

4 11.43 83.48 0 7.36 1.65

Conditions: gasifier temperature, 900 ˚C. 1, pyrolysis; 2, catalytic

pyrolysis; 3, steam gasification; 4, catalytic steam gasification.

In the present study, the tar yield was lower than those data because of the

presence of calcined dolomite. Table 3.3 shows calcined dolomite is porous with


28


high external surface area and micropore area, the large external surface area of

calcined dolomite particles accounts for the high chance of gas contacting solid

particles and long gas residence time of >4 s, which can adsorb tar and promote

the catalytic cracking of hydrocarbon and the elimination of tar.

At the presence of steam, if the results of catalytic steam gasification (run 4)

were compared with steam gasification (run 3), The presence of the calcined

dolomite can increase H2 content, CO2 content, carbon conversion efficiency and

the dry gas yield, while CO, CH4, C2H4 and C2H6 contents diminished. In the catalytic

process (runs 4 and 2) the effects originated by the steam were greater than those

of the non-catalytic process (runs 1 and 3).

3.4 Process Description:

The sun dried and pre-treated MSW is subjected to manual segregation and is

then shredded into 1 inch size. The shredded waste will be conveyed and heaped in a

hopper. The size of hopper will depend upon the volume of the MSW to be contained.

The hopper will be fitted with an auger or screw conveyer at its bottom. The rpm of the

auger will be set to meet the MSW demand in the gasifier.

3.4.1 Gasifier:

The gasifier is an internally heated vessel. The heating source comprises of

electrical coils. The primary purpose of gasifier is to convert MSW into synthesis

gas. The gasifier operates at an internal pressure of 170 kPa and at an internal

temperature of 950oC.

The MSW enters the gasifier almost at room temperature. As it moves down

the gasifier, through different temperature zones, it becomes almost moisture free.

During the coarse of its downward fall, it interacts counter currently with steam and

gasifies giving synthesis gas, tar and leaving behind char. The char leaves from the

gasifier at the bottom through similar auger conveyer setting as described above,

whereas, due to high temperature inside the gasifier, the tar gets vaporized and


29


moves upward with the synthesis gas towards the outlet of the gasifier where the

suction is created. On the upper portion of the gasifier, a bed of calcined dolomite

catalyst is placed. At a temperature of 950oC, when the tar mixed gases pass

through the bed of catalyst, the tar gets decomposed giving the valuable products.

3.4.2 Waste Heat Boiler:

The synthesis gas, coming out of the gasifier will be at a temperature of about

900-950 oC. This excess energy will be recovered by passing it through a series of

heat exchangers and will be utilized to generate steam. The gas first passes

through a super heater. Almost 150-170 MJ of energy will be recovered here, using

shell and tube exchanger. The gas will pass through the shell side whereas the

steam (coming from thermosyphone /steam drum) will be passed through tube side.

The synthesis gas will leave the super heater at about 500 oC. The gas will then

pass through the evaporator and finally through the economizer.

Boiler feed water will enter the W.H.B at about room temperature and a steam of

600 psig and 400 oC will be generated from super heater .Almost 90% energy will

be recovered from the synthesis gas by the W.H.B. The synthesis gas leaves the

boiler at about 125 oC.

3.4.3 Cyclone Separator:

The gas is then passed through cyclone separator to remove any dirt particles

larger the 3m size. The dirt is collected at the bottom whereas the gas leaves from

the top.

3.4.4 Condenser:

The gas stream is then passed through a condenser unit, where the moisture is

condensed and removed from the gas stream. The stream leaves the condenser at

about 35 oC, which is moisture free gas.

3.4.5 Absorption Tower:


30


The moisture free gas coming out of the condenser is then send to the CO2

absorption tower. 14.5% MEA solution is used as solvent in CO2 absorption tower. MEA

is preferred as CO2 absorbent because of its ease of regeneration. The CO2 content of

the synthesis gas is reduced up to 95% in this unit. SO2 which is present in very low

proportion in the synthesis gas also gets absorbed into the solvent because the

conditions are very much favorable for its absorption in the tower. The very low CO2 and

SO2 content gas is obtained from the tower.


31



PRO

CESS

(M

SW

ST

EAM

GA

SIFC

ATIO

N)

Sun

Dry

ing

for

7 da

ys

*

Shr

eddi

ng1

inch

Fil

tera

tion

Con

dens

er

Was

te

Hea

t Boi

ler

Cyc

lone

Sep

erat

er

CO

2 Abs

orbe

rS

crub

ber

GA

SIF

IER

Ste

am

Ele

ctri

cal C

oils

Hop

per

MSW

32


3.5 INFLUENCE OF TEMPERATURE

3.5.1 Influence of temperature on product distribution

Table 3.5 shows the product distribution (char, tar and gas) by catalytic steam

gasification of MSW at different reactor temperatures with calcined dolomite. The

data indicated that dry gas yield and mass balance exceed 100% due to the

introduction of steam. With the temperature increasing from 700 to 950˚C, the char

decreased gradually from 21.68% to 8.12%, while dry gas yield increased from

81.84% to 104.16%. In regard to the gas fraction, the increase of gas fraction was

mainly attributed to the decomposition of char and the secondary reaction of the tar

vapor as temperature increases, more carbon and steam can be converted into gas

through Eqs. (4) and (5), therefore, carbon conversion efficiency and steam

decomposition increases, accordingly char decreased markedly. Especially, tar

catalytic gasification was improved significantly, tar decreased drastically from

0.42% at 700 °C to 0.14% at 800 °C, in particular, no condensed matter was

observed in the cleaning system as temperature increases from 850 to 950 °C. This

variation was probably dependent on the more favorable thermal cracking and

steam reforming reactions at higher temperatures, which resulted in the secondary

cracking reactions into the gas fraction. Subsequently, at the presence of catalyst,

higher temperature favored the carbon conversion efficiency, tar decomposition and

char further gasification with steam.

Table 3.5 – Influence of temperature on product distribution and gas characterization

[ref. 1]

Temperature (˚C) 700 750 800 850 900 950

Product distribution (wt.%)

Gas 81.84 88.54 90.50 95.99 97.19 104.16

Tar 0.42 0.37 0.14 0 0 0

Char 21.68 18.03 15.82 11.23 9.87 8.12


33


Gas composition (mol%, dry basis)

H2 27.01 34.70 40.46 45.60 48.63 53.29

CO 9.34 11.16 12.65 13.46 14.85 16.92

CO2 35.25 30.86 26.61 24.23 23.59 22.05

CH4 20.23 17.89 15.57 12.24 9.62 5.76

C2H4 6.31 4.02 3.85 3.60 2.38 1.01

C2H6 1.79 1.37 0.68 0.87 0.93 0.97

Gas characterization

H2/CO (mol/mol) 2.89 3.11 3.21 3.39 3.27 3.15

LHV (MJ/kg of fed

MSW)14.44 13.47 12.62 11.74 10.77 9.36

LHV (MJ/kg of fed

MSW)10.69 11.45 12.37 13.15 13.79 13.85

H2 yield (mol/kg) 9.78 14.43 19.49 34.99 30.46 38.60

H2 yield potential

(mol/kg)55.48 57.35 62.75 67.86 70.14 70.00

Steam

decomposition (%)42.96 62.13 64.74 68.18 72.37 74.51

Carbon conversion

efficiency (wt.%)62.05 62.13 64.74 68.18 72.37 74.51

Dry gas yield

(Nm3/kg)0.74 0.85 0.98 1.12 1.28 1.48


34


3.5.2 Influence of temperature on the gas fraction

The gas component distribution profile from catalytic steam gasification of MSW at

different reactor temperatures was plotted in Table 3.5. It indicated that the main

components are H2, CO, CO2, CH4 and small quantities of low molecular

hydrocarbons, such as C2H4 and C2H6. Water gas shift reaction (Eq. (2)) is

exothermic and thus less important at higher temperature. The main reactions (Eqs.

(2), (4)–(7)) are endothermic strengthened by increasing temperature. Therefore, the

reactor temperature had a significant influence on the syngas compositions. As

shown in Table 3.5, higher temperatures significantly resulted in higher H2 contents.

It can be concluded that Boudouard reactions (Eq. (4), carbon gasification reaction

(Eq. (5)), together with the secondary cracking reactions of tar (Eq. (7), were the

main factors responsible for the increase in H2 and CO contents. H2 content almost

doubled from 27.01% to 53.29%, CO content increased by 22.29%, while CO2

content decreased by 37.45%.

Because of some CO2 reacting with calcined dolomite. Methane decomposition

(Eq. (4)) was favored at higher temperature, which accounted for a significant

decrease of 71.63% in CH4 content as temperature increases. C2H4 and C2H6 content

were relatively small, and slightly decreased. This shown that temperature had

strong influence on the decomposition of CH4, that agreed with Turn et al higher

temperature provided more favorable conditions for thermal cracking and steam

reforming, so steam decomposition and dry gas yield increased. Furthermore,

temperature had remarkable influence on H2 yield, H2 yield significantly increased

from 9.78 to 38.60 mol/kg. However, H2 potential yield increased first and

subsequently remained almost unchanged, it was inferred that middle-low

temperature (700–900 °C) favored H2 potential yield with an increase of 26.42%

obtained.

With respect to the different gas compositions, the increase of H2 content was

greater than that of CO content, thus H2 to CO ratio (H2/CO) in the syngas slowly

increased from 2.89 to 3.15 over the range of temperature from 700 to 950 °C, this


35


kind of syngas was advisable for producing hydrogen for ammonia synthesis or for

fuel cell applications.

The influences of reactor temperature on the O/C and H/C atomic ratios of

hydrogen-rich gas were plotted in Fig.3.4, H/C atomic ratio at lower temperature

(700–850 °C) increased more markedly than that at higher temperature (850–900

°C), which was explained by more quickly increasing in H2 content at lower

temperature, it was concluded that higher temperature was not favorable for H/C

atomic ratio. Meanwhile, O/C atomic ratio almost remianed constant at lower

temperature, and increased very slightly from 1.05 to 1.25 at higher temperature, it

was concluded that reactor temperature almost had no influence on the O/C atomic

ratio of hydrogenrich gas.

Furthermore, the O/C and H/C atomic ratios of the syngas product, pyrolytic gas

and MSW feedstock were in the same following order due to decomposition of

hydrocarbon and formation of H2-rich gas:

Syngas > Pyrolyticgas(at700˚C ) > MSW feedstock

Furthermore, the lower heating value (LHV) of syngas decreased from 11.85

MJ/Nm3 to 10.08 MJ/Nm3, when the Reactor temperature increased from 700 to

950 ˚C. Methane had the highest heating value in syngas, the sharp decrease of

methane content led to decrease LHV of syngas. However, on the other hand, the

energy content of the total syngas shown a slight increase from 8.77 MJ/kg of MSW

to 14.91 MJ/kg of MSW. Fig. 3.6 shows time profiles of instantaneous dry gas yield

rate at different reactor temperatures, higher temperature exerted a pronounced

influence on the reaction time, this is because that higher temperature can

accelerate gasification of MSW with steam, and increase significantly the mean

reaction rate. Variation trend of instantaneous dry gas yield rate with the gasification

time remained the same. With respect to a specific temperature, instantaneous dry

gas yield rate changed drastically during the gasification process, at the beginning,

instantaneous dry gas yield rate increased drastically, and then decreased, higher

temperature remarkably enhanced the instantaneous dry gas yield rate after only the


36


first 10 min, and decreased subsequently. Furthermore, Fig. 3.7 shows the

maximum instantaneous dry gas yield rate increased from 0.053 Nm3/kg min to

0.203 Nm3/kg min with temperature increasing from 700 to 950 ˚C, which can be

explained by MSW feedstock absorbing more heat energy and being converted into

product gas at higher temperature at a very short interval.

Fig. 3.5. – H/C and O/C atomic ratios of the syngas at different temperatures.[ref. 1]


37


Fig.

3.6 – Variations of instantaneous dry gas yield rates at different temperatures with the

time.


38


Fig. 3.7 – Influence of temperature on the maximum instantaneous gas yield rate.

Influence of temperature on the solids fraction:

Table 6 reports elemental analysis and ash content of char from catalytic steam

gasification of MSW at different temperatures, the data show that the increase of

temperature can significantly enhance the ash content in the char, the char almost was

solid ash with a maximum value of 86.01% in content, which may be accounted for by

effective gasification of MSW with steam at the presence of catalyst at higher

temperature. The char with high ash content can recycle in cement and construction

industry [38], or be disposed of for landfilling application. Elemental analysis data show

a significant decrease in carbon content and oxygen content from 31.20% to 4.09% and

from 35.4% to 8.5%, respectively, and a gradual decrease in hydrogen content over the

temperature range of 700–950 C, which was caused by the dehydrogenation and

carbon gasification of MSW, therefore, there existed little hydrogen and carbon in the

residual char.


39


Table. 3.6: Char Element Analysis [ref. 1]

Temperature (˚C) C H Oa ash

700b 45.87 1.19 35.86 17.08

700 31.20 1.07 36.59 31.14

750 23.25 0.90 33.83 42.02

800 16.40 0.88 25.04 57.68

850 14.71 0.72 26.01 58.56

900 9.02 0.70 17.49 72.79

950 4.09 0.40 11.50 84.01

a: by difference, b: Catalytic Pyrolysis


40


Table. 3.7: Comparison of different Gasification Techniques:

ProcessCarbon

Conversion(%)

Tar Yield

(weight%)

Char Yield

(weight%)

Dry gas Yield

(weight%)

Heating Value

of gas (MJ/kg)

1. Pyrolysis 22.82 38.54 25.86 0.21 4.13

2.Catalytic

Pyrolysis34.14 18.75 11.45 0.34 6.75

3.Steam

Gasification44.07 0.23 7.95 0.51 7.66

4.

Catalytic

Steam

Gasification

83.48 0 7.36 1.65 18.86

5.Plasma

Gasification100 0 18.18 1.06 9.09


41



42

Steam

GasifierT = 950 deg CP = 170 kPa

MSW

Char

Product (Syn. Gas)


4.1 Material Balance around Gasifier:

Feed flow rate = F = 100 kg/hr MSW

Ultimate Analysis of Feed

Component Weight % Moles Mole% *

C 51.81 4.3175 35.99

H 5.76 5.76 48.02

O 30.22 1.8888 15.74


Chapter No. 4

43


N 0.26 0.0186 0.16

S 0.36 0.0113 0.09

Gross total 88.41 11.9962 100.00

Ash 11.59

Total 100.00

* Ash Free Basis

Steam flow rate = M = 77kg/hr

Composition Moles Temperature (deg C) Pressure (kPa)

100% H2O 4.2778 109.6 141.3

Applying mass balance for Carbon:

mass of carbon in = mass of Carbon out

mass of carbon in ¿51.81100

x 100

mass of carbon out = P x( Xco + Xco2 + XcH4 + Xc2H4 + Xc2H6 )p + Ch x Xcch

Similarly applying mass balance for other components and following table is generated:

Product (Synthesis Gas) flow rate = P = 146.8423 kg/hr

Composition of Product Gas

Components Weight Moles Mole%

Syn

. Ga

s

H2 9.207 3.521 53.29

CO 40.925 1.118 16.92

CO2 83.803 1.457 22.05


44


CH4 7.959 0.381 5.76

C2H4 2.4376 0.0666 1.01

C2H6 2.511 0.0640 0.97

1. Gross Total 112.3243 6.607 100

Impu

ritie

s

NOx 0.558 0.0186 0.9912

SO2 0.7232 0.0113 0.6022

Moisture 33.2368 1.8465 98.406

2. Gross Total 34.518 1.8764 100

Total 146.8423 8.4834

Char flow rate = Ch = 30.1577 kg/hr

Composition of Char

Component Weight Weight %

C 13.206 43.7911

H 2.1213 7.034

O 3.24 10.7435

Ash 11.59 38.4313

Total 30.1577 100.00


45



46


5.1 Gasifier:

5.1.1 Energy Balance around Gasifier:

1. Energy input by feed = mCp∆T

As ∆T = 0

Energy input by feed = 0

2. Energy input by steam:

hcv = 2221.23 kJ/kg

Cp = 4.187 kJ/ kg K

Heat input = mCp∆T + mλ

= 77 x 4.187 x (109.6 - 25) + 77 x 2231.23

= 199.1 MJ/kg

3. Energy output by product gas:

Average Cp of Product = 420607 kJ/kmol

Total moles of Product gas = 8.4834 kmol

Energy Output = 8.4834 x 42.607 x 925

= 33433.3 kJ

= 334.34 MJ

4. Heat output = 913.9 MJ + 334.34 – 199.09

= 1049.16 MJ

5.1.2 Heat loss at walls of Gasifier:

Resistance to heat flow = R

R = L/KA

let area of heat flow = 1m3

R1 = resistance of refractory brick:

So R = 0.2032 / (.8 x 1)


Chapter No. 5

47


= 0.607 s.°C / kJ

R1 = Thickness of sheet plate / H.T.C.C for plate carbon steel x 1m2

= (7mm + 2mm) / (K2 x 1)

= 9 x 10-3 / (K2 x 1)

so q = ∆T / R

= (950 - 75) / (R1 – R2)

= 314957.23 kJ

so Heat losses through walls = 87.49 kW

5.1.3 Reaction wise Energy Production:

Reaction 1: C + O2 → CO2

Exothermic (-393 kJ/mol)

=100 x (-393) x 0.2729

= 107249.7 kJ

Reaction 2: C + H2O → CO + H2

Endothermic (+131 kJ/mol)

= 2.941 x 131000

= 385677.1 kJ

Reaction 3: C + CO2 → 2CO

Endothermic (+172 kJ/mol)

=0.2729 x 172000

= 46938.8 kJ

Reaction 4: CO + H20 → CO2 + H2


48



=1.7185 x 41000

= 70458.5 kJ

Reaction 5: CO + 3H2 → CH4 + H20


= 0.3806 x 205000

= 78023 kJ

Reaction 6: H2 + ½O2 → H2O

Exothermic (-243.276 kJ/mol)

= 1.8144 x 243.276 x 1000

= 441399.9744 kJ

Reaction 7: CO2 + 6H2O → 7O2 + C2H6

Exothermic (-767.448 kJ/mol)

= 0.0113 x 767.448 x 1000

= 8672.1624 kJ

Reaction 8: N + ½ O2 → NO

Endothermic

= 0.1282 x 745.8 x 1000 x 4.18

= 399656 kJ

Reaction 9: CO2 + 2H2O → C2H4 + 2O2

Endothermic

= 0.1335 x 1411.1 x 1000 x 4.18 = 7847436.133 kJ


49


5.2 Energy Balance around Boiler:

For our process conditions, we have:

Gas Side flow rate = 146.85 kg/hr

Gas stream composition:

Compositions Mole%

H2 0.41507

CO 0.131784

CO2 0.171727

CH4 0.04485

C2H4 0.007849

C2H6 0.007547

SO2 0.001332

NOx 0.002192

Moisture 0.217649


50


Specific heat of inlet syn. gas can be evaluated as follows:

ComponentMoles in

productMole% T

Constants for Equation of Sp. Heat Capacity Specific

Heat

Mole% x

CpA B C D

H2 3.521367 0.41507 398 27.143 0.009278 -1.4E-05 7.65E-09 29.13019 12.09106

CO 1.118028 0.131784 398 30.809 -0.01285 2.79E-05 -1.3E-08 29.31097 3.862715

CO2 1.456897 0.171727 398 19.795 0.073436 -5.6E-05 1.72E-08 41.23331 7.080874

CH4 0.380499 0.04485 398 19.251 0.05213 1.2E-05 -1.1E-08 41.1818 1.847005

C2H4 0.066592 0.007849 398 3.806 0.1566 -8.3E-05 1.76E-08 54.01567 0.423988

C2H6 0.064025 0.007547 398 5.409 0.1781 -6.9E-05 8.71E-09 65.85202 0.496967

SO2 0.0113 0.001332 398 16.37 0.1459 -0.00011 3.24E-08 58.74105 0.07824

NOx 0.0186 0.002192 398 29.345 -0.00094 9.75E-06 -4.2E-09 30.25175 0.066324

Moisture 1.846489 0.217649 398 32.243 0.001924 1.06E-05 -3.6E-09 34.45392 7.498857

8.483797 1 33.44603

For our case, we will make the following assumptions:

Tube Side = Steam at outlet, Maximum Flow at 600 psig and 750 °F

Feed water at 227 °F and pressure required at inlet

Pressure Drop in Super-heater, 15.0 psi

Pressure Drop in Economizer, 10.0 psi

Now, we have set all of our conditions, putting these known values in our diagram, so we can

proceed with a heat balance,


51


Now we can calculate the missing data,

Heat available to Super-heater = Q = n x Cp x dT

=8.3545 x 42.165 x ((950+273)-(25+273))

=329.33MJ/hr

Heat available to Evaporator = Q = n x Cp x dT

=8.3545 x 37.95 x ((500+273)-(25+273))

=150.64MJ/hr

Heat available to Economizer = Q = n x Cp x dT

=8.3545 x 35.08 x ((125+273)-(25+273))

=65.96MJ/hr

Heat available in outlet syn. gas= Q = n x Cp x dT

=8.3545 x 42.165 x ((950+273)-(25+273))

=27.9MJ/hr

Total heat recovered = 329.33 – 27.9

= 301.43 MJ/hr


52


Heat recovered by cold water = m x Cp x dT + m

301426 = m x 4.187 x (100-25) + m x 2210

m = 119.42kg/hr

Therefore,

Flowrate of boiler feed water = 119.42kg/hr

Now, we can now complete our schematic with all known values.


53



Chapter No. 6

54


6.1 Mechanical Design of Gasifier:

From Experimental data:

Feed rate = 0.257 kg/hr

Complete decomposition time of MSW at 950 ˚C = 30 min

Internal Diameter of vessel = 81 cm

So Internal Radius of vessel = 40.5 cm

Internal cross sectional area of gasifier = π r2

= π (40.51000

)2

= 5.15 x 10-3 cm2

So Decomposition rate of MSW at 950 ˚C per unit area = 0.257/(30 x 5.15 x 10-3)

= 1662 g/min- cm2

Now

Our feed rate = 100 kg/hr

= 1666 g/min

Hence Area Required = 1m2

Internal Diameter = 1 meter + (8 inch) x 2

(Using Vessel Thickness Table from Reference 5)

Minimum thickness required = 7 mm = e

Also

Design Temperature at vessel = 50˚C

as we are using the vessel with FIRE Clay bricks

So, Design Stress can be evaluated from Table 13.2 of M.O.C Carbon Steel []ref. 5]


55


Tensile Strength = 360 N/mm2

Design Stress = 135 N/mm2 (i.e. between 0-50 ˚C)

Design Pressure:

Take as 10% above operating Pressure i.e. 170 kPa

= (170 x 0.1) + 170 kPa

= 0.18695 N/mm2

Design Temperature = 50 ˚C

Design Stress = 135 N/mm2

Cylindrical Section:

Plate Thickness = e = Pi x Di2 f x Pi

[ref. 5]

= Pi x D 0.18695 x1.4064 x 103

2x 135−0.18695

= 0.975 mm

Corrosion Allowance = 2mm [ref. 5]

So,

Plate Thickness = 2.975 mm

≈ 3mm

Conical Section:

Plate Thickness = e = Pi x Dc

2 f J x Pi x 1cosα [ref. 5]

Here


56


J = joint co-efficient

For welding joint using typical value i.e. J=1 [ref. 5]

α = 30˚

f = 135

Dc = Cone diameter

Let’s suppose that our feed inlet enter through the opening of 1 ft

Then Dc = 1ft + 4 inch x 2

= 0.3048 m +0.2032 m

= 0.508 m

Hence

Plate Thickness = e = 0.18695 x 508

2x 135x 1−0.18695 x 1cos30 ˚ [ref. 5]

= 0.406 mm

Volume of MSW = decomposition rate

Average density of MSW

Average Density = 366.25 kg/m3

So, Volume rate of MSW = 16.66

366.25

= 0.0455 m3/hr

Accumulation time of MSW in gasifier = 10 min.

Volume of Gasses = V = nRT / P

T = 950 °C of outlet gases


57


P = 170 kPa = 1677 bar

= ((606070) x 82.06 x (950 + 273)) / 1.677

= 0.395 m3

Volume of Moisture = 33.2368 kg

P = 100kPa

P = 170 kPa

P = 200kPa

(y2 – y1)/(x2 – x1) = (x – x1)/(y – y1)

y = ((y2 – y1)/ (y2 – y1)) x (x – x1) + y1

y = (((170 – 100) (2.70643 – 5.4135))/ (200-100)) + 504135

i.e.

Specific volume = 3.518 m3/kg (at 700 deg C, 170 kPa)

Similarly at 1000 deg C

Specific volume = 3.81877 m3/kg

Now,

T1 T T2

X 900 950 1000

Y=sp. Vol. 3.51855 ? 3081877

Y = (((950-900)(3.81877-3.51855))/(1000-900)) + 3.51855

So


58


sp. Vol. = 3.5200 m3/kg

density (at 950°C and 170kPa) = 1/ 305200

= 0.2840 kg/m3

Volume of steam = 33.2368 / 60

= 1.95 m3

so total Volume = 1.95 + 0.395 + 0.0455

= 2.39 m3

so volume of vessel should be greater than this value,

as diameter is already specified

so volume = π r2 l

= π (0.5)2 x 3.5 (proposed height = l = 305 m)

= 2.75 m3

hence our gasifier has diameter to height ratio = 1/3.5


59


6.2 Design of Hopper:

6.2.1 Volume:

Basis = 1 hour operation

Amount of MSW to be stored in the hopper = 100 kg

Average density of MSW = 366.25 kg/m3

So, Volume of Hopper required = 100/366.25

=0.273m3

As,

Volume of Hopper = Volume of cylinder + Volume of cone – Volume of cone (lower

end shown by dashed line)

= pi x r2 x h + (1/3) x pi x r2 x h - (1/3) x pi x r2 x h [ref. 5]

As, we have already calculated the volume of hopper, So, inserting a value

greater then we have already calculated, say 0.3 m3 , and we have fixed the outlet dia

of hopper to be 1 ft. and supposing the height of cylindrical and conical sections of

hopper to be 0.8 m & 0.3 m respectively,

Then

0.3 = pi x r2 x h + (1/3) x pi x r2 x h - (1/3) x pi x r2 x h

Value of “r” obtained =0.323 m

6.2.2 Material of Construction: Plain Carbon Steel

6.2.3 Plate thickness required:

Cylindrical section:

Minimum thickness = e = (Pi x Di)/(2f – Pi) [ref. 5]

Here,


60


Pi = Internal Pressure = 1 atm = 0.103 N/mm2

Di = Internal Diameter = 646 mm

f = Design Stress =135 (From table [ref. 5])

So,

Minimum thickness = e = 0.242 mm + 2 mm Corrosion allowance

= 2.242 mm

But from table, practical wall thickness for a 2 ft. dia. vessel is 5mm +2 mm Corrosion

allowance

Therefore,

Wall thickness of cylindrical section = 7 mm

Conical Section:

Minimum thickness = e = (Pi x Di)/(2fJ – Pi) cos(a) [ref. 5]

Here,

J = Welding joint factor = 1

a = Angle of cone = 450

Minimum thickness = e = 0.343 mm + 2 mm Corrosion allowance

= 2.343 mm


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6.3 Design of Cyclone Seperator:

To remove particle having diameter greater then 3µm

G = 146.85 kg/hr

= 146.85/3600

= 0.041 kg/s

So, by Stokes law

U = ¿¿ [ref. 5]

=(1500−0.626)x (3 x 10−6)x 9.8

18 x1.32 x10−5

= 5.56 x 10-4 m/s

We know that

U =0.2d g A i2

zgD c

= 0.2 x 0.5Dc x 9.8 x (0.5Dc x 0.25Dc) 2

x 4Dc x G x Dc

= 0.00039g Dc3

G

Dc3 =

G0.00039g

[ref. 5]

= x 0.041 x 5.56 x 10 -4

0.00039 x 9.8 x 0.626

Dia. of Cyclone Separator = 0.031m


62



Chapter No. 7

63


INTRODUCTION

The important feature common to all process is that a process is never in state of

static equilibrium except for a very short period of time. Process is a dynamic entity

subject to continual upset or disturbance which tend to drive it away from the desired

state of equilibrium; the process must then be manipulated upon or corrected to derive

some disturbances bring about only transient effect of process behavior. These passes

away and they never occur again. Others may apply periodic or cycle forces which may

make the process respond in a cyclic or periodic fashion. Most disturbances are

completely random w.r.t time and show no repetitive pattern. Thus, their occurrence

may be accepted but cannot be predicted at any particular time. If a process is to

operate efficiently the disturbances process must be controlled.

A process is design for a particular objective or output and is then found,

sometimes by trial and error and some time by previous experience that control of a

particular variable associated with some stages of the process is necessary to achieve

the desired efficiency.

Each process will have associated with it a number of variables which are likely

to change at random. Each such change will lead to changes in the dependent variable

of the process. One of which is selected as being indicative of successful operation.

One of the input variables will be manipulated to cause further changes in the output

variables to restore the original conditions.

Process may be controlled more precisely to give more uniform and high quality

products by the application of automatic control, which often leads to highest profits.

Additionally, process which response too rapidly, and is to be controlled by human

operators, can be controlled automatically. Automatic control is also beneficial in certain

remote, hazardous or routine operations. Automatically control processing systems

which may too large and too complex for effective direct human control.

Sensors to measure process conditions and valves to influence process


64


operations are essential for all aspects of engineering practice. While sensors and

valves are important in all aspects of engineering, they assume greatest importance in

the study of automatic control, which is termed process control when applied in the

process industries. Process control deals with the regulation of processes by applying

the feedback principle using various computing devices, principally digital computation.

Process control requires sensors for measuring variables and valves for implementing

decisions. Therefore, the presentation of this material is designed to complement other

learning topics in process control.

Since successful process control requires appropriate instrumentation, engineers

should understand the principles of common instruments introduced in this section. The

descriptions in this section cover the basic principles and information on the

performance for standard, commercially available instruments. Thus, selection and

sizing of standard equipment is emphasized, not designing equipment “from scratch”.

Elements of Automatic Process Control

The following are some important elements of Automatic Process Control.

1. Sensors

2. Valves

3. Signal Transmitter and trasducer

4. Transducer

5. Controller

6. Final Control Element

7.1 Sensors

Sensors are used for process monitoring and for process control. These are

essential elements of safe and profitable plant operation that can be achieved only if the

proper sensors are selected and installed in the correct locations. While sensors differ

greatly in their physical principles, their selection can be guided by the analysis of a


65


small set of issues, which are presented in this section.

7.1.1 Temperature Measuring Sensors

Temperature control is important for separation and reaction processes, and

temperature must be maintained within limits to ensure safe and reliable operation of process

equipment. Temperature can be measured by many methods; several of the more common

are described below:

Table 7.1: Summery of Temperature Sensors


66


7.1.2 Flow Measuring Sensors

Flow measurement is critical to determine the amount of material purchased and sold,

and in these applications, very accurate flow measurement is required. In addition, flows

throughout the process should the regulated near their desired values with small variability; in

these applications, good reproducibility is usually sufficient. Flowing systems require energy,

typically provided by pumps and compressors, to produce a pressure difference as the driving

force, and flow sensors should introduce a small flow resistance, increasing the process energy

consumption as little as possible. Most flow sensors require straight sections of piping before

and after the sensor; this requirement places restrictions on acceptable process designs, which

can be partially compensated by straightening vanes placed in the piping. The sensors

discussed in this subsection are for clean fluids flowing in a pipe; special considerations are

required for concentrated slurries, flow in an open conduit, and other process situations.Several

sensors rely on the pressure drop or head occurring as a fluid flows by a resistance; an

example is given in Figure 1. The relationship between flow rate and pressure difference is

determined by the Bernoulli equation, assuming that changes in elevation, work and heat

transfer are negligible.


67


Table 7.2: Summery of Flow Sensors:

Pressure Measuring Sensors

Most liquid and all gaseous materials in the process industries are contained within

closed vessels. For the safety of plant personnel and protection of the vessel, pressure in the

vessel is controlled. In addition, pressured is controlled because it influences key process

operations like vapor-liquid equilibrium, chemical reaction rate, and fluid flow.

The following pressure sensors are based on mechanical principles, i.e., deformation

based on force.


68


Table 7.3: Summery of Pressure Sensors:

Level Measuring Sensors

Level of liquid in a vessel should be maintained above the exit pipe because if

the vessel empties the exit flow will become zero, a situation that would upset

downstream processes and could damage pumping equipment that requires liquid.

Also, the level should not overflow an open vessel nor should it exit through a vapor line

of a closed vessel, which could disturb a process designed for vapor. In addition, level

can influence the performance of a process; the most common example is a liquid

phase chemical reactor. Level is usually reported as percent of span, rather than in

length (e.g., m). Level sensors can be located in the vessel holding the liquid or in an

external “leg” which acts as a manometer. When in the vessel, float and displacement

sensors are usually placed in a “stilling chamber” which reduces the effects of flows in

the vessel.


69


Table 7.4: Summery of Level Sensors:

7.2 On Stream Analyzers

The term analyzer refers to any sensor that measures a physical property of the process

material. This property could relate to purity (e.g., mole % of various components), a basic

physical property (e.g., density or viscosity), or an indication of product quality demanded by the

customers in the final use of the material (e.g., gasoline octane or fuel heating value).

Analyzers rely on a wide range of physical principles; their unifying characteristic is a

greatly increased sensor complexity when compared with the standard temperature, flow,

pressure and level (T, F, P, and L) sensors. In many situations, the analyzer is located in a

centralized laboratory and processes samples collected at the plant and transported to the

laboratory. This procedure reduces the cost of the analyzer, but it introduces long delays before

a measurement is available for use in plant operations.

7.3 Control Valves

The most common method for influencing the behavior of chemical processes is through

the flow rate of process streams. Usually, a variable resistance in the closed conduit or pipe is

manipulated to influence the flow rate and achieve the desired process behavior. A valve with a

variable opening for flow is the standard equipment used to introduce this variable resistance;

the valve is selected because it is simple, reliable, relatively low cost and available for a wide

range of process applications. In some cases the valve resistance is set by a person adjusting


70


the opening, like a home faucet. In many cases the valve resistance is determined by an

automatic controller, with the valve designed to accept and implement the signal sent from the

controller. These are control valves. A multitude of commercial control valves are available


71



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73



Chapter No. 8

74


Cost Estimation and Evaluation

8.1 Individual Cost of Each Equipment:

1) Hopper:

Capacity of hopper =0.3m3

Type =Vertical storage vessel

Constant =$2400(2004) [from ref. 5]

Cost Index =0.6 [from ref. 5]

Purchase Cost =2400 x (0.3)0.6

=$1165.42

2) Gasifier:

Diameter of Gasifier =1m

Material factor (C.S) =1 [from ref. 5]

Pressure factor (1 bar) =1 [from ref. 5]

Purchase cost =Cost from graph x M.F x P.F

= (10 x 1000) x 1 x 1

=$10,000

3) Furnace:

Energy requirement of furnace= 253.8 kW

Type of furnace =Cylindrical

Constant =540 [from ref. 5]



75


Purchase Cost =540 x (253.8)0.77

=$38,364.65

4) Boiler:

Pressure of steam =600 psig=42.38 bar

Steam Produced =118 kg/hr

Constant =100 [from ref. 5]


Purchase Cost =100 x (118) 0.8

=$4544.7

5) Absorber(CO2, SO2):

Diameter =1m

Vessel overall height =5m

Packed height =3m

Packing = Intalox saddle

Size of Packing =38mm

Material of Packing =Ceramic

Volume of packing = (pi/4) x 3

=2.35m3

Cost of packing =1020 x 2.35 [from ref. 5]

=$2397


76


M.O.C of Vessel =Carbon steel

Operating Pressure =3 bar

Material factor (C.S) =1 [from ref. 5]


Cost of Column = Cost from graph x M.F x P.F [from ref. 5]

= (10 x 1000) x 1 x 1

= $10,000

Total cost = Cost of column + Cost of packing

= 10,000 + 2397

= $12,397

6) Condenser :

Heat transfer area =10m2

M.O.C of Condenser = Carbon steel (shell)

Stainless steel (tube)

Type of condenser = Fixed tube sheet

Operating Pressure = 1 bar

Type factor (T.F) = 0.8 [from ref. 5]


Purchase cost = Cost from graph x T.F x P.F [from ref. 5]

= (10 x 1000) x 0.8 x 1

= $8,000


77


8.2 Total Purchase Cost of Major Equipments

Hopper: = $1165.42

Gasifier: = $10,000

Furnace: = $38,364.65

Boiler: = $4544.7

Absorber(CO2 & SO2): = $12,397

Condenser : = $8,000

Total Cost = $74,471.77

8.3 Fixed Capital Cost

Estimation of fixed capital cost for fluid processing plant, following items are to be considered:

Items Factors [from ref. 5]

Equipment Erection 0.40

Piping 0.70

Instrumentation 0.20

Electrical 0.10

Contigencies 0.1

So,

Total Fixed Cost = 74,471.77 x (0.4+0.7+0.2+0.1+0.1)

= $111,707.65


78



Chapter No. 9

79


Future Considerations:

We have made the design of gasification plant up to the best of our knowledge. Although the

energy balance around the plant yielded a net positive energy of 0.13MW/hr, but after detailed

analysis, thorough study and research, we have come to a conclusion, that there may be some

aspects, which can be modified in order to improve the overall efficiency of the plant. These can

be summarized as under:

1. The increased amounts of CO2 in the synthesis gas are a big factor in its relatively low

heating value. This problem can be overcome by feeding the gasifier with Coal or Coke

along with MSW. This will favour the following reaction

CO2 + C —— 2CO

This will decrease the CO2 contents of the synthesis gas and will ultimately increase the

calorific value of synthesis gas.

2. Methane is a high calorific value gas. In the gasification process, it is yielded by the

following reaction

CO + 3H2 — C2H4 + H2O

The above-depicted reaction is a shift reaction in the forward direction, depending on the

pressure. Increasing the pressure inside the gasifier can eventually increase the yield of

methane contents of synthesis gas.

3. In the gasifier plant we have designed, the furnace is the most energy-consuming unit.

This can be lower down by heating the gasifier externally, using a part of synthesis gas

produced.

4. The synthesis gas produced, can be used to generate following fuels y Fischer-Tropsch

process:

Methanol

Bio-diesel

Petroleum like fuels


80


BIBLIOGRAPHY

1. INTERNATIONAL JOURNAL OF HYDROGEN ENERGY: Hydrogen-rich gas from

catalytic steam gasification of municipal solid waste (MSW), Volume 34, Issue 5 , March

2009,Pages 2174-2183.

2. CARL R.BRANAN: Rules of Thumb for Chemical Engineers, 2nd edition, GULF, 2001.

3. J.F.RICHARDSON, J.H.HARKER, J.R.BACKHURST: Coulson & Richardson Chemical

Engineering, volume 2, 5th edition, ELSEVIER 2008.

4. ALBERT.H.PERRY, DON W.GREEN: Perry’s Chemical Engineer’s Handbook, 7th

edition, MC-GRAW HILL, 1999.

5. R.K.SINNOTT: Chemical Engineering Design, volume 6, 4th edition, ELSEVIER 2006.

6. CARL.L.YAWS: Chemical Properties Handbook, MC-GRAW HILL HANDBOOKS 1999.

7. OM PRAKASH GUPTA: Elements of Fuels, Furnaces, and Refractories, 4th edition,

KHANNA PUBLISHERS 2000.

8. STANLEY M. WALES: Chemical Process Equipment Selection and Design, 1990.

9. DAVID M.HIMELBLAU: Basic Principle and Calculations in Chemical Engineering,6th

edition, PRENTICE HALL,1996.

Web Links:

1. www.pc-education.mcmaster.ca

2. www.sciencedirect.com

3. www.hrsgdesign.com

4. www.engineeringtoolbox.com

5. www.safewasteandpower.com

6. www.egr-group.com/process.html

7. www.westinghouse-plasma.com

8. www.norres.ca


http://www.norres.ca/

http://www.westinghouse-plasma.com/

http://www.egr-group.com/process.html

http://www.safewasteandpower.com/

http://www.engineeringtoolbox.com/

http://www.hrsgdesign.com/

http://www.sciencedirect.com/

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