Transcript
Catalytic Steam Gasification of MSW
Chapter No. 1
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Catalytic Steam Gasification of MSW
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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Catalytic Steam Gasification of MSW 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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5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 Lahore Faisalabad Hyderabad Gujranwala Peshawar Waste Generation Tons/day Waste Collectin Tons/day
Figure 1.1: Rate of Generation and Collection of SW in a Few Major Cities of Pakistan
Hyderabad, 51
Faisalabad, 54 Faisalabad Lahore Peshawar
Gujranwala, 52
Lahore, 45
Gujranwala Hyderabad
Peshawar, 61
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 pathways²thermochemical, 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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Catalytic Steam Gasification of MSW 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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Catalytic Steam Gasification of MSW 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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Catalytic Steam Gasification of MSW
Chapter No. 2
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Catalytic Steam Gasification of MSW
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 ³reforming´ and results in a hydrogen and carbon dioxide rich ³synthetic´ 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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Catalytic Steam Gasification of MSW 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:
Process 1. 2. 3. Pyrolysis Catalytic Pyrolysis Steam Gasification Catalytic 4. Steam Gasification 5. Plasma Gasification 100 0 18.18 1.06 9.09 83.48 0 7.36 1.65 18.86 Carbon 22.82 34.14 44.07 Tar Yield 38.54 18.75 0.23 Char Yield (weight%) 25.86 11.45 7.95 Dry gas Yield (weight%) 0.21 0.34 0.51 Heating Value of gas (MJ/kg) 4.13 6.75 7.66
Conversion(%) (weight%)
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 costcompetitive 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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Catalytic Steam Gasification of MSW 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 2. C + H2O 3. C + CO2 4. C + 2H2 5. CO + H20 6. CO + 3H2 CO2 CO + H2 2CO CH4 CO2 + H2 CH4 + H20 -393 kJ/mol (exothermic) +131 kJ /mol (endothermic) +172 kJ/mol (endothermic) -74 kJ/mol -41 kJ/mol (exothermic) (exothermic)
-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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Catalytic Steam Gasification of MSW 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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Catalytic Steam Gasification of MSW 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 noncondensable, 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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Catalytic Steam Gasification of MSW 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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Catalytic Steam Gasification of MSW 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 68.96 Paper 9.95 Textile 2.17 Wood 7.40 Plastic 11.52
Table3. 2 ± Ultimate and proximate analysis of MSW samples (Dry Basis) [ref. 1] Ultimate analysis Proximate analysis C H O (by difference) N S 51.81 (wt.%) 5.76 (wt.%) 30.22 (wt.%) 0.26 (wt.%) 0.36 (wt.%) Volatile matter Fixed carbon Ash Low heating value Apparent density 82.28 (wt.%) 11.79 (wt.%) 5.93 (wt.%) 21 306 kJ/kg 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 +
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Catalytic Steam Gasification of MSW C2H4 x 59.036 + C2H6 x 63.772)/1000 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 xC2H4% + 2 x C2H6) 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)/(W 1+ W 2) x 100% where, SD is steam decomposition, W 1 is steam flow rate and W 2 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 H2O + CO = = 1.28H2 + CO H2 + CO2 -41.2 MJ/kmol (1) (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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Catalytic Steam Gasification of MSW 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 310mm 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 BET surface area (m2/g) 9.96 Micropore area (m2/g) 1.73 External surface area (m2/g) 8.23 Total pore volume (cm3/g) 2.27
Calcined Dolomite
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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 rC, and last until a temperature of 700 rC or even higher.
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Catalytic Steam Gasification of MSW 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 C + H2O
CH4 + H2O
2CO CO + H2 CO + 3 H2 n2CO2 + n3H2
+162 kJ/mol (endothermic) +131 kJ /mol (endothermic) +206.3kJ/mol (endothermic) (¨H298K>0)
(4) (5) (6) (7)
Tar + n1H2O
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
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Catalytic Steam Gasification of MSW participation of 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.
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Catalytic Steam Gasification of MSW
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] LHV (MJ/Nm3) 19.68 19.84 15.02 11.43 Carbon conversion (%) 22.82 34.14 44.07 83.48 Tar (wt.%) 38.54 18.75 0.23 0 yield Char (wt.%) 25.86 11.45 7.92 7.36 yield Dry yield (N.m3/kg) 0.21 0.34 0.51 1.65 gas
Run
1 2 3 4
Conditions: gasifier temperature, 900 ÛC. 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
27
I . C. E. T, U N I V ER S I TY OF TH E P U N J A B
Catalytic Steam Gasification of MSW 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
I . C. E. T, U N I V ER S I TY OF TH E P U N J A B
28
Catalytic Steam Gasification of MSW 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 3Qm 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:
I . C. E. T, U N I V ER S I TY OF TH E P U N J A B
29
Catalytic Steam Gasification of MSW 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.
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Figure 3.4: FLOW SHEET OF THE PROCESS (MSW STEAMGASIFCATION)
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Catalytic Steam Gasification of MSW
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 950ÛC, 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 (ÛC) 700 750 800 850 900 950
Product distribution (wt.%) Gas Tar Char 81.84 0.42 21.68 88.54 0.37 18.03 90.50 0.14 15.82 95.99 0 11.23 97.19 0 9.87 104.16 0 8.12
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Catalytic Steam Gasification of MSW Gas composition (mol%, dry basis) H2 CO CO2 CH4 C2H4 C2H6 Gas characterization H2/CO (mol/mol) LHV (MJ/kg of fed MSW) LHV (MJ/kg of fed MSW) H2 yield (mol/kg) H2 yield potential (mol/kg) Steam decomposition (%) Carbon conversion efficiency (wt.%) Dry gas yield (Nm3/kg) 2.89 14.44 3.11 13.47 3.21 12.62 3.39 11.74 3.27 10.77 3.15 9.36 27.01 9.34 35.25 20.23 6.31 1.79 34.70 11.16 30.86 17.89 4.02 1.37 40.46 12.65 26.61 15.57 3.85 0.68 45.60 13.46 24.23 12.24 3.60 0.87 48.63 14.85 23.59 9.62 2.38 0.93 53.29 16.92 22.05 5.76 1.01 0.97
10.69 9.78 55.48
11.45 14.43 57.35
12.37 19.49 62.75
13.15 34.99 67.86
13.79 30.46 70.14
13.85 38.60 70.00
42.96
62.13
64.74
68.18
72.37
74.51
62.05
62.13
64.74
68.18
72.37
74.51
0.74
0.85
0.98
1.12
1.28
1.48
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Catalytic Steam Gasification of MSW 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
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Catalytic Steam Gasification of MSW 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(at700ÛC ) > 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 ÛC. 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
35
I . C. E. T, U N I V ER S I TY OF TH E P U N J A B
Catalytic Steam Gasification of MSW 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 ÛC, 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]
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Catalytic Steam Gasification of MSW
Fig. 3.6 ± Variations of instantaneous dry gas yield rates at different temperatures with the time.
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Catalytic Steam Gasification of MSW
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.
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Table. 3.6: Char Element Analysis [ref. 1] Temperature (ÛC) 700b 700 750 800 850 900 950 a: by difference, C 45.87 31.20 23.25 16.40 14.71 9.02 4.09 H 1.19 1.07 0.90 0.88 0.72 0.70 0.40 Oa 35.86 36.59 33.83 25.04 26.01 17.49 11.50 ash 17.08 31.14 42.02 57.68 58.56 72.79 84.01
b: Catalytic Pyrolysis
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Catalytic Steam Gasification of MSW
Table. 3.7: Comparison of different Gasification Techniques: Carbon Tar Yield Char Yield (weight%) 25.86 11.45 Dry gas Yield (weight%) 0.21 0.34 Heating Value of gas (MJ/kg) 4.13 6.75
Process 1. 2. Pyrolysis Catalytic Pyrolysis Steam Gasification Catalytic 4. Steam Gasification 5. Plasma Gasification
Conversion(%) (weight%) 22.82 34.14 38.54 18.75
3.
44.07
0.23
7.95
0.51
7.66
83.48
0
7.36
1.65
18.86
100
0
18.18
1.06
9.09
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Catalytic Steam Gasification of MSW
Chapter No. 4
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Catalytic Steam Gasification of MSW
4.1
Material Balance around Gasifier:
MSW
Gasifier T = 950 deg C Steam P = 170 kPa
Product (Syn. Gas)
Char
Feed flow rate = F = 100 kg/hr MSW Ultimate Analysis of Feed Component C H O Weight % 51.81 5.76 30.22 Moles 4.3175 5.76 1.8888 Mole% * 35.99 48.02 15.74
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Catalytic Steam Gasification of MSW
N S Gross total Ash Total 0.26 0.36 88.41 11.59 100.00 0.0186 0.0113 11.9962 0.16 0.09 100.00
* Ash Free Basis
Steam flow rate = M = 77kg/hr Composition 100% H2O Moles 4.2778 Temperature (deg C) 109.6 Pressure (kPa) 141.3
Applying mass balance for Carbon: mass of carbon in = mass of Carbon out mass of carbon in 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 Syn. Gas H2 CO Weight 9.207 40.925 Moles 3.521 1.118 Mole% 53.29 16.92
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Catalytic Steam Gasification of MSW
CO2 CH4 C2H4 C2H6 1. Gross Total NOx
Impurities
83.803 7.959 2.4376 2.511 112.3243 0.558 0.7232 33.2368 34.518 146.8423
1.457 0.381 0.0666 0.0640 6.607 0.0186 0.0113 1.8465 1.8764 8.4834
22.05 5.76 1.01 0.97 100 0.9912 0.6022 98.406 100
SO2 Moisture 2. Gross Total Total
Char flow rate = Ch = 30.1577 kg/hr Composition of Char Component C H O Ash Total Weight 13.206 2.1213 3.24 11.59 30.1577 Weight % 43.7911 7.034 10.7435 38.4313 100.00
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Chapter No. 5
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Catalytic Steam Gasification of MSW
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 + m = 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)
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Catalytic Steam Gasification of MSW
= 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
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Catalytic Steam Gasification of MSW
Exothermic (-41 kJ/mol) =1.7185 x 41000 = 70458.5 kJ Reaction 5: CO + 3H2 CH4 + H20
Exothermic (-205 kJ/mol) = 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: Endothermic = 0.1282 x 745.8 x 1000 x 4.18 = 399656 kJ Reaction 9: Endothermic = 0.1335 x 1411.1 x 1000 x 4.18 = 7847436.133 kJ CO2 + 2H2O C2H4 + 2O2 N + ½ O2 NO
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Catalytic Steam Gasification of MSW
5.2
Energy Balance around Boiler:
For our process conditions, we have: Gas Side flow rate = 146.85 kg/hr Gas stream composition: Compositions H2 CO CO2 CH4 C2H4 C2H6 SO2 NOx Moisture Mole% 0.41507 0.131784 0.171727 0.04485 0.007849 0.007547 0.001332 0.002192 0.217649
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Specific heat of inlet syn. gas can be evaluated as follows:
Moles in product 3.521367 1.118028 1.456897 0.380499 0.066592 0.064025 0.0113 0.0186 1.846489 8.483797 Constants for Equation of Sp. Heat Capacity Mole% T A 0.41507 0.131784 0.171727 0.04485 0.007849 0.007547 0.001332 0.002192 0.217649 1 398 398 398 398 398 398 398 398 398 27.143 30.809 19.795 19.251 3.806 5.409 16.37 29.345 32.243 B 0.009278 -0.01285 0.073436 0.05213 0.1566 0.1781 0.1459 -0.00094 0.001924 C -1.4E-05 2.79E-05 -5.6E-05 1.2E-05 -8.3E-05 -6.9E-05 -0.00011 9.75E-06 1.06E-05 D 7.65E-09 -1.3E-08 1.72E-08 -1.1E-08 1.76E-08 8.71E-09 3.24E-08 -4.2E-09 -3.6E-09 Specific Heat 29.13019 29.31097 41.23331 41.1818 54.01567 65.85202 58.74105 30.25175 34.45392 Mole% x Cp 12.09106 3.862715 7.080874 1.847005 0.423988 0.496967 0.07824 0.066324 7.498857 33.44603
Component
H2 CO CO2 CH4 C2H4 C2H6 SO2 NOx Moisture
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,
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Catalytic Steam Gasification of MSW
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
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Catalytic Steam Gasification of MSW
Heat recovered by cold water = m x Cp x dT + mP 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.
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Chapter No. 6
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6.1 Mechanical Design of Gasifier:
From Experimental data: Feed rate = 0.257 kg/hr Complete decomposition time of MSW at 950 ÛC = 30 min Internal Diameter of vessel = 81 cm So Internal Radius of vessel = 40.5 cm Internal cross sectional area of gasifier = r2 = ( )2
= 5.15 x 10-3 cm2 So Decomposition rate of MSW at 950 ÛC 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 = 50ÛC 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]
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Tensile Strength = 360 N/mm2 Design Stress = 135 N/mm2 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 ÛC Design Stress = 135 N/mm2 Cylindrical Section: (i.e. between 0-50 ÛC)
Plate Thickness = e =
=
[ref. 5]
= 0.975 mm Corrosion Allowance = 2mm So, Plate Thickness = 2.975 mm § 3mm Conical Section:
[ref. 5]
Plate Thickness = e =
x
[ref. 5]
Here
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Catalytic Steam Gasification of MSW
J = joint co-efficient For welding joint using typical value i.e. J=1 = 30Û 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 =
[ref. 5]
x
[ref. 5]
= 0.406 mm
Volume of MSW
=
Average Density = 366.25 kg/m3 So, Volume rate of MSW
=
= 0.0455 m3/hr Accumulation time of MSW in gasifier = 10 min. Volume of Gasses = V = nRT / P T = 950 °C of outlet gases
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Catalytic Steam Gasification of MSW
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 Similarly at 1000 deg C Specific volume = 3.81877 m3/kg Now, T1 X Y=sp. Vol. 900 3.51855 T 950 ? T2 1000 3081877 (at 700 deg C, 170 kPa)
Y = (((950-900)(3.81877-3.51855))/(1000-900)) + 3.51855 So
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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 = = r2 l (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
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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 ³r´ 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]
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Catalytic Steam Gasification of MSW Here, Pi Di f 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) Here, J = Welding joint factor a = Angle of cone Minimum thickness = e = = 1 450 [ref. 5] = Internal Pressure = 1 atm = 0.103 N/mm2 = Internal Diameter = 646 mm = Design Stress =135 (From table [ref. 5])
= 0.343 mm + 2 mm Corrosion allowance = 2.343 mm
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Catalytic Steam Gasification of MSW 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
US
=
V V
[ref. 5]
= = We know that
5.56 x 10-4 m/s
US
=
S V T
=
0.2 x 0.5Dc x 9.8 x (0.5Dc x 0.25Dc)2 V T x 4Dc x G x Dc
=
0.00039g Dc3V TG
Dc3
=
T V
[ref. 5]
=
T x 0.041 x 5.56 x 10-4 0.00039 x 9.8 x 0.626
Dia. of Cyclone Separator = 0.031m
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Chapter No. 7
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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
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Catalytic Steam Gasification of MSW 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 ³from scratch´. Elements of Automatic Process Control The following are some important elements of Automatic Process Control. 1. 2. 3. 4. 5. 6. Sensors Valves Signal Transmitter and trasducer Transducer Controller 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
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Catalytic Steam Gasification of MSW 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
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Catalytic Steam Gasification of MSW 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.
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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.
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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 ³leg´ which acts as a manometer. When in the vessel, float and displacement sensors are usually placed in a ³stilling chamber´ which reduces the effects of flows in the vessel.
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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
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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
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Figure 7.1: INSTRUMENTATION OF THE GASIFIER
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Figure 7.2: INSTRUMENTATION OF THE W.H.B
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Chapter No. 8
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Cost Estimation and Evaluation 8.1 Individual Cost of Each Equipment:
1)
Hopper: Capacity of hopper Type Constant Cost Index Purchase Cost =0.3m3 =Vertical storage vessel =$2400(2004) =0.6 =2400 x (0.3)0.6 =$1165.42 [from ref. 5] [from ref. 5]
2)
Gasifier: Diameter of Gasifier =1m Material factor (C.S) =1 Pressure factor (1 bar) =1 [from ref. 5] [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 Constant Cost Index =Cylindrical =540 =0.77 [from ref. 5] [from ref. 5]
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Purchase Cost =540 x (253.8)0.77 =$38,364.65
4)
Boiler: Pressure of steam Steam Produced Constant Cost Index =600 psig=42.38 bar =118 kg/hr =100 =0.8 [from ref. 5] [from ref. 5]
Purchase Cost
=100 x (118) 0.8 =$4544.7
5)
Absorber(CO2, SO2): Diameter =1m
Vessel overall height =5m Packed height Packing Size of Packing Material of Packing Volume of packing =3m = Intalox saddle =38mm =Ceramic = (pi/4) x 3 =2.35m3 Cost of packing =1020 x 2.35 =$2397 [from ref. 5]
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M.O.C of Vessel Operating Pressure Material factor (C.S) =Carbon steel =3 bar =1 [from ref. 5] [from ref. 5]
Pressure factor (1 bar) =1
Cost of Column
= Cost from graph x M.F x P.F = (10 x 1000) x 1 x 1 = $10,000
[from ref. 5]
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 Operating Pressure Type factor (T.F) = Fixed tube sheet = 1 bar = 0.8 [from ref. 5] [from ref. 5] [from ref. 5]
Pressure factor (1 bar) =1 Purchase cost = Cost from graph x T.F x P.F = (10 x 1000) x 0.8 x 1 = $8,000
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8.2 Total Purchase Cost of Major Equipments
Hopper: Gasifier: Furnace: Boiler: Absorber(CO2 & SO2): Condenser : Total Cost = $1165.42 = $10,000 = $38,364.65 = $4544.7 = $12,397 = $8,000 = $74,471.77
8.3 Fixed Capital Cost
Estimation of fixed capital cost for fluid processing plant, following items are to be considered: Items Equipment Erection Piping Instrumentation Electrical Contigencies So, Total Fixed Cost = 74,471.77 x (0.4+0.7+0.2+0.1+0.1) = $111,707.65 Factors [from ref. 5] 0.40 0.70 0.20 0.10 0.1
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Chapter No. 9
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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 ²²p 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 n²p 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: y y y Methanol Bio-diesel Petroleum like fuels
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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. 2. 3. 4. 5. 6. 7. 8. www.pc-education.mcmaster.ca www.sciencedirect.com www.hrsgdesign.com www.engineeringtoolbox.com www.safewasteandpower.com www.egr-group.com/process.html www.westinghouse-plasma.com www.norres.ca
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