Main Routes For the Thermo-Conversion of Biomass into Fuels and Chemicals. Part 2: Gasification Systems
Exploring the Potential of Biomass Gasification for Sustainable Energy Production
by Ku. Ranjana Seth*,
- Published in Journal of Advances in Science and Technology, E-ISSN: 2230-9659
Volume 1, Issue No. 1, Feb 2011, Pages 0 - 0 (0)
Published by: Ignited Minds Journals
ABSTRACT
Gasification as a thermo-chemicalprocess is defined and limited to combustion and pyrolysis. The gasification ofbiomass is a thermal treatment, which results in a high production of gaseousproducts and small quantities of char and ash. The solid phase usually presentsa carbon content higher than 76%, which makes it possible to use it directlyfor industrial purposes. The gaseous products can be burned to generate heat orelectricity, or they can potentially be used in the synthesis of liquidtransportation fuels, H2, or chemicals. On the other hand, the liquid phase canbe used as fuel in boilers, gas turbines or diesel engines, both for heat orelectric power generation. However, the main purpose of biomass gasification isthe production of low- or medium heating value gas which can be used as fuelgas in an internal combustion engine for power production. In addition tolimiting applications and often compounding environmental problems, thesetechnologies are an inefficient source of usable energy.
KEYWORD
gasification systems, thermo-chemical process, biomass, fuels, chemicals, gaseous products, solid phase, carbon content, industrial purposes, liquid transportation fuels
Introduction
asification is a thermo-chemical conversion technology that carbonaceous materials (coal, etroleum coke, biomass, etc.) into a combustible gas called producer gas. The production of ombustible gas from carbonaceous materials is already an old technology. The first record of its ommercial application origins from so called dry distillation (or pyrolysis – heating of feedstock n absence of O2, resulting in thermal decomposition of fuel into volatile gases and solid carbon) rigins from year 1812 . The first commercial gasifier was installed in 1839, when Bischaf atented a simple process for gasifying coke. The first attempt to use producer gas to fire internal ombustion (IC) engine was carried out in 1881 . Gasifiers were subsequently further developed or different fuels and were in widespread use in specific industrial power and heat applications up o the 1920s, when oil fuelled systems gradually took over the producer gas fuelled systems . efore the construction of natural gas pipelines, there were many ‘‘gasworks” serving larger town nd cities in Europe and the United States. During the 2nd World War (1939–1945), almost a illion gasifiers were used to run cars, trucks, and buses using primarily wood as a fuel . ince the energy crises of the 1970s, many countries have become interest in biomass as a fuel ource. Biomass is the most abundant renewable energy source on earth and is considered by far he highest quality form of indirect solar energy. Biomass energy is more economic to produce nd it provides more energy than other energy forms. The technology was perceived as a relatively heap indigenous alternative for small-scale industrial and utility power generation in those eveloping countries that suffered from high world market petroleum prices and had sufficient ustainable biomass resources . The manufacturing took off with increased interest shown in asification technology. A review of gasifier manufacturers in Europe, the United States, and anada identified 50 manufacturers offering commercial gasification lants from which: (1) 75% of the designs were fixed-bed downdraft type, (2) 20% of the designs ere fluidized-bed systems, (3) 2.5% of the designs were updraft type, and (4) 2.5% were of arious other designs. he remainder of this paper is organized as follows. Section 2 presents a detailed review of iomass gasification and gasification technologies. Section 3 discusses the major applications of roducer gas produced from biomass gasification. Finally, Section 4 draws the main conclusions f this paper.
. Gasification of biomass .1. Principles of biomass gasification
he gasification of biomass is a thermal treatment, which results in a high production of gaseous roducts and small quantities of char and ash . It is a well-known technology that can be classified epending on the gasifying agent: air, steam, steam–oxygen, air–steam, oxygen-enriched air, etc. . asification is carried out at high temperatures in order to optimize the gas production. The esulting gas, known as producer gas, is a mixture of carbon monoxide, hydrogen and methane, ogether with carbon dioxide and nitrogen . Yield a product gas from thermal decomposition omposed of other gaseous hydrocarbons (CHs), tars, char, inorganic onstituents, and ash. Gas composition of product from the biomass gasification depends heavily n the gasification process, the gasifying agent, and the feedstock composition . Gasification of iomass is generally observed to follow the reaction: ssuming a gasification process using biomass as a feedstock, the first step of the process is a hermochemical decomposition of the cellulose, hemicelluloses and lignin compounds with roduction of char and volatiles . Further the gasification of char and some other equilibrium eactions occur. har gasification is the rate-limiting step in the production of gaseous fuels from biomass. rrhenius kinetic parameters have been determined for the reaction of chars prepared by pyrolysis f cottonwood and Douglas fir at 1275 K with steam and CO2 The results indicate that both reactions are approximately zero order with respect to char; the verall reaction rate is fairly constant throughout and declines only when the char is nearly epleted. This suggests that the reaction rate depends on such factors as total available active urface area or interfacial area between the char and catalyst particles. These parameters would emain relatively constant during the gasification process. Softwood and hardwood chars exhibited imilar gasification behavior. Results indicate that the mineral (ash) content and composition of he original biomass material, and pyrolysis conditions under which char is formed significantly nfluence the char gasification reactivity. ne of the major problems in biomass gasification is how to deal with the tar formed during the rocess . Tar is a complex mixture of condensable hydrogen which includes single ring to 5-ring romatic compounds along with other oxygen containing hydrocarbons and complex the olycyclic aromatic hydrocarbons (PAHs) . Control technologies of tar production can broadly be ivided into two approaches : (1) treatments inside the gasifier (primary methods) and (2) hot gas leaning after the gasifier (secondary methods). Although secondary methods are proven to be ffective, treatments inside the gasifier are gaining much attention due to economic benefits. In rimary methods, the operating parameters such as temperature, gasifying agent, equivalence atio, residence time and catalytic additives play important roles in the formation and ecomposition of tar. Primary methods are not yet fully understood and have not to be mplemented commercially . Pilot-scale tests have shown that catalytic cracking of tars can be ery effective. Tar conversion in excess of 99% has been achieved using dolomite, nickel-based nd other catalysts at elevated temperatures of typically 1075–1175 K . Coconut shell gasification y steam reforming in the presence of nickel-dolomite. They reported that the tar yield was ecreased from 19.55–1.4% at temperature 1075 K, feed rate 0.5 g min_1 and steam to carbon atio 0.95. 2.2. Types of gasifiers Several types of gasifiers have been developed; an overview is hown in Fig. 1. These gasifiers have different hydrodynamics (especially the way in which the olid fuel and the gasification agent are contacted), gasification agents (air, oxygen and/or steam) nd operating conditions such as temperature and pressure . The most important types are fixed ed (updraft or downdraft fixed beds) gasifiers, fluidized-bed gasifiers, and entrained flow asifiers. ixed-bed gasifiers are the most suitable for biomass gasification. Fixed-bed gasifiers involve eactor vessels in which the biomass material is either packed in or moves slowly as a plug, with ases flowing in between the particles . Fixed-bed gasifiers re usually fed from the top of the reactor and can be designed in either updraft or downdraft onfigurations. With fixed-bed updraft gasifiers, the air or oxygen passes upward through a hot eactive zone near the bottom of the gasifier in a direction countercurrent to the flow of solid aterial . They can be scaled up; however, they produce a product gas with very high tar oncentrations. This tar should be removed for the major part from the gas, creating a gas-cleaning roblem. Fixed-bed downdraft gasifiers are limited in scale and require a well-defined fuel, aking them not fuel-flexible . Small scale fixed-bed downdraft gasifier installations (150 kWe–1 We) can be employed for on-site conversion of biomass to electricity and heat . luidized-bed gasifiers are a more recent development that takes advantage of the excellent ixing characteristics and high reaction rates of this method of gas–solid contacting . Fluidized- ed gasifiers are typically operated at 1075–1275 K (limited by the melting properties of the bed aterial) and are therefore not generally suitable for coal gasification, as due to the lower eactivity of coal compared to biomass, a higher temperature is required (>1575 K) . Examples of he fluidized-bed gasifier systems are bubbling fluidized-bed gasifiers, entrained bed gasifiers, and irculating fluidized-bed gasifiers. he bubbling fluidized-bed gasifier tends to produce a gas with tar content between that of the pdraft and downdraft gasifiers. Some pyrolysis products are swept out of the fluid bed by asification products, but are then further converted by thermal cracking in the freeboard region . he circulating fluidized-bed gasifiers employ a system where the bed material circulates between he gasifier and a secondary vessel. The circulating fluidized-bed gasifiers are suitable for fuel apacity higher than 10 MWth .
.2. Composition of producer gas
he composition of the gas obtained from a gasifier depends on a number of parameters such as : 1) fuel composition, (2) gasifying medium, (3) operating pressure, (4) temperature, (5) moisture ontent of the fuels, (6) mode of bringing the reactants into contact inside the gasifier, etc. It is ery difficult to predict the exact composition of the gas from a gasifier . Introduction of the ater–gas equilibrium concept provides the opportunity to calculate the gas composition heoretically from a gasifier, which has reached equilibrium at a given temperature . Table 1 hows typical gas composition data as obtained from commercial wood and charcoal downdraft asifiers operated on low to medium moisture content fuels.
. Applications of biomass gasification
asification may be defined as a process by which a remnant – biomass, carbon, etc. – is onverted into gases by means of a partial oxidization carried out at high temperature . At emperatures of approximately 875–1275 K, solid biomass undergoes thermal decomposition to orm gas-phase products that typically include and other gaseous CHs. In ost cases, solid char plus tars that would be liquids under ambient conditions are also formed . he solid phase usually presents a carbon content higher than 76%, which makes it possible to use t directly for industrial purposes . The gaseous products can be burned to generate heat or lectricity , or they can potentially be used in the synthesis of liquid transportation fuels , H2 , or hemicals . On the other hand, the liquid phase can be used.
able 1 : Typical gas composition data as obtained from commercial wood and charcoal owndraft gasifiers operated on low to medium moisture content fuels (wood 20%, charcoal 7%). s fuel in boilers, gas turbines or diesel engines, both for heat or electric power generation. owever, the main purpose of biomass gasification is the production of low- or medium heating alue (LHV, MHV) gas which can be used as fuel gas in an IC engine for power production. ossible products obtained from gasification process are given in Fig. 2.
.1. Heat and power generation
enerating electricity and useful heat from the same power plant is called ‘‘cogeneration” in orth America and ‘‘combined heat and power (CHP)” in Europe. CHP plants the product gas is ired on a gas engine. Modified gas engines can run without problems on most product gases even hose from air-blown gasification that have calorific values of approximately 5–6 MJ/m3. ypically, the energetic output is one-third electricity and two-third heat The use of biomass for istrict heating and CHP has been expanding rapidly in countries such as Austria and Germany. In inland, biomass-based fuels are used nearly completely in heat and CHP production. The number f large scale CHP plants in Finland is nearly 100MWand the total capacity is over 1500 MW. he Alholmens Kraft CHP plant in Pietarsaari, Finland, is the largest biofuelled power plant in the orld. The plant produces steam for the adjacent paper mill and for a utility generating electricity nd heat. iomass integrated gasification combined cycle (BIGCC) technology holds the promise of fficient, clean and cost-effective power generation from biomass. But, this technology is not yet ommercially available. There are experiments with gasification for use in high efficiency ombined-cycle power plants, which are in the demonstration phase. Several projects have been nitiated for IGCC applications over the last decade, however, only two have been implemented, he SYDKRAFT plant at Värnamo based on FOSTER WHEELER technology and the ARBRE lant based on TPS technology. iomass can be used as a primary energy source or as a secondary energy source to power gas urbines. As a secondary energy source, biomass is used to make a fuel, which can be used to fire a as turbine. Biomass gasification is the latest generation of biomass energy conversion processes, nd is being used to improve the efficiency, and to reduce the investment costs of bio-electricity eneration through the use gas turbine technology. Biomassgasifier/gas turbines are projected to ave bio-electricity efficiencies of 40–45%, or more than double those of Rankine-cycle systems. he costs of steam-Rankine systems vary widely depending on the level of sophistication. A ypical installed capital cost for a 25MW unit is $1600–2100/kW. The heat produced from the lectricity generating process is captured and utilized for domestic purposes and can be used in team turbines to generate additional electricity. Fig. 3 shows the comparison between energy nputs to separate heat and power system and cogeneration system. Cogeneration is the imultaneous production of electricity and useful thermal energy from a single source. he gasification of biomass in fixed-bed reactors provides the possibility of combined heat and ower production in the power range of 100 kWe up to 5 MWe. A system for power production by eans of fixed-bed gasification of biomass consists of the main unit gasifier, gas cleaning system nd engine (Fig. 4) . asifiers are used to convert biomass into a combustible gas (biogas). The biogas is than used to rive a high-efficiency, combined- cycle gas turbine. Gaseous fuels consist of low and medium- alorific-value gases; the liquid is a primary-pyrolysis oil alled biocrude. A number of gasifiers have been developed to produce biogases from biomass nd peat Biogas is a mixture of mainly CH4 and CO2 with very small amounts of sulfuric omponents. The gas generally composes of CH4 (55–65%), CO2 (35–45%), N2 (0–3%), H2 (0– %), and H2S (0–1%) . Methane gas that is produced from manure is around 4800–6700 kcal/m3. s compare with pure methane gas contain energy of 8900 kcal/m3 . Typically between 20% and 0% of the heating value of the feedstock is contained in the biogas. For electricity generation, iogas is commonly burnt in IC engines, which may include heat recovery for able 2 : Typical data and figures for power generation from biomass. ombined heat and power production. Electrical capacities range from tens of kW to several MW. iogas may also be burnt in gas turbines; at larger scales, combined-cycle systems may be conomically justified . he capital cost of power plants with biomass gasification in the United States is about 2000– 000/kW and generation cost is in the order of 90/MWh. Such plants may be cost-effective in HP mode if connected to district heating schemes. The cost of biomass combustion steam cycle nd CHP plants can be lower, with 1000/ kW as the cost target. In Europe, the investment cost of iomass plants varies considerably from 1000 to 5000/kW, depending on plant technology, level f maturity and plant size . Data regarding investments required and the cost of bio-electricity with tilization of different technologies are presented in Table 2.
.2. Transportation fuels via biomass gasification .2.1. Hydrogen
any experts think that hydrogen has a major role to play as an energy carrier in future energy upply. Hydrogen can be used as a transportation fuel, whereas neither nuclear nor solar energy an be used directly. It has good properties as a fuel for IC engines in automobiles. Hydrogen can e used as a fuel directly in an IC engine not much different from the engines used with gasoline. ydrogen has very special properties as a transportation fuel, including a rapid burning speed, a igh effective octane number, and no toxicity or ozone-forming potential. It has much wider limits f flammability in air (4–75% by volume) than methane (5.3– 15% by volume) and gasoline (1– .6% by volume). A stoichiometric hydrogen–air mixture has very low minimum ignition energy f 0.02 mJ. A hydrogen engine is easy to start in cold winter because hydrogen remains in a aseous state until it reaches a low temperature uch as 20 K. With proper measurements it is believed that this amount of NOx can be reduced, ven attaining 1/200 as low as diesel engines . The advantage is that hydrogen stores pproximately 2.6 times more energy per unit mass than gasoline, meaning that hydrogen is more nergy efficient than gasoline. The disadvantage is that it needs an estimated four times more olume than gasoline to store that energy . he methods available for the hydrogen production from biomass can be divided into two main ategories: thermo-chemical able 3 : Main advantages and limitations of biomass to hydrogen. able 4 : Comparison of hydrogen yields were obtained by use of three different processes. nd biological routes. The production of hydrogen from renewable biomass has several advantages nd limitations compared to that of fossil fuels, as shown in Table 3. The yield of hydrogen that an be produced from biomass is relatively low, 12–14% based on the biomass weight . ydrogen can be produced from biomass by pyrolysis , gasification , steam gasification , steam- eforming of bio-oils , and enzymatic decomposition of sugars . Table 4 shows comparison of ydrogen yields were obtained by use of three different processes. A certain numbers of efforts ave been made by researchers to test gasification of various types of biomass for the production f hydrogen. Biomass gasification has been identified as a possible system for producing enewable hydrogen, which is beneficial to exploit biomass resources, to develop a highly efficient lean way for large-scale hydrogen production, and has less dependence on insecure fossil energy ources. n the pyrolysis and gasification processes, water–gas shift is used to convert the reformed gas into ydrogen, and pressure swing adsorption is used to purify the product. The cost of hydrogen roduction from supercritical water gasification of wet biomass was several times higher than the urrent price of hydrogen from steam methane reforming. The price of hydrogen obtained by irect gasification of lignocellulosic biomass, however, is about three times higher than that for ydrogen produced by steam reforming of natural gas . Estimated cost comparison of hydrogen roduction by biomass gasification and natural gas steam reforming is shown in Fig. 5 . ydrogen is produced from the steam gasification of beech wood, olive waste, and wheat straw, azelnut shell, wood sawdust, and waste wood. Modeling of biomass steam gasification to ynthesis gas is a challenge because of the variability (composition, structure, reactivity, physical roperties, etc.) of the raw material and because of the severe conditions (temperature, residence ime, heating rate, etc.) required .This is well-illustrated in a fluidized gasification system as hown in Table 5. he production of hydrogen gas on a pilot scale by steam gasification of charred lignocellulosic aste material. In the study, the hydrogen gas was freed from moisture and CO2. They nvestigated the beneficial effect of some inorganic salts such as chlorides, carbonates and hromates on ig. 5. Estimated cost comparison of hydrogen production by biomass gasification and natural gas team reforming.. Table 5 : Biomass and process characteristics.. he reaction rate and production cost of the hydrogen gas. Steam reforming C1–C5 hydrocarbons, afta, gas oils, and simple aromatics are commercially practiced, well-known processes. Steam eforming of hydrocarbons; partial oxidation of heavy oil residues, elected steam reforming of aromatic compounds, and gasification of coals and solid wastes to ield a mixture of H2 and CO (syngas), followed by water–gas shift conversion to produce H2 and O2, are well-established processes. When the objective is to maximize the production of H2, the toichiometry describing the overall process is he simplicity of Eq. (2) hides the fact that, in a hydrocarbon reformer, the following reactions ake place concurrently: t normal reforming conditions, steam reforming of higher hydrocarbons (CnHm) is irreversible Eq. (2)), whereas the methane reforming (Eq. (2)) and the shift conversion (Eq. (3)) reactions pproach equilibrium. A large molar ratio of steam to hydrocarbon will ensure that the equilibrium or Eqs. (2) and (3) is shifted toward H2 production. s a part of the European research project AER-GAS, the Absorption Enhanced Reforming (AER) echnique is being utilized for the unpressurized steam gasification of biomass (Eq. (4)). Through imultaneous CO2 absorption (with CaO as the sorbent in the example, Eq. (6)), the equilibrium of he homogenous water gas shift reaction (Eq. (5)) is shifted towards H2 and CO2 and all of the arallel reforming/gasification reactions are also influenced in favour of the desired products. ccordingly, a hydrogen-rich product gas results with reduced CO and CO2 concentration. Eq. (7) epresents the idealized sum reaction for AER gasification – the formations of secondary products like methane, coke, and tars) are neglected here he yield of H2 from steam gasification increases with increasing water-to-sample (W/S) ratio . he yields of hydrogen from the pyrolysis and the steam gasification increase with increasing of emperature. the yields of H2 from pyrolysis and steam gasification of hazelnut shell at different emperatures. Steam gasification runs were carried out over a temperature range from 925 to 1225 . W/S ratios were 0.7 and 1.9 in steam gasification runs. The highest H2 yield (59.5%) was btained from the gasification run (W/S = 1.9) at 1225 K. In general, the gasification temperature s higher than that of pyrolysis and the yield of hydrogen from the gasification is higher than that f the pyrolysis. the yield of hydrogen from supercritical fluid (water) extraction (SFE), pyrolysis nd steam gasification of wheat straw and olive waste at different temperatures. The highest yields % dry and ash free basis) were obtained from the pyrolysis (46%) and steam gasification (55%) f wheat straw while the lowest yields from olive waste. the yield of hydrogen from SFE, yrolysis and steam gasification of beech wood at different temperatures. Distilled water was used n the SFE (the critical temperature of pure water is 647.7 K). From Fig. 6, the yield of hydrogen rom SFE was considerably high (49%) at lower temperatures. The pyrolysis was carried out at the oderate temperatures and steam gasification at the highest temperatures. he effect of catalyst on gasification products is very important. Catalysts not only reduce the tar ontent, but also improve the gas product quality and conversion efficiency. Dolomite, Ni-based atalysts and alkaline metal oxides are widely used as gasification catalysts Alumina, luminosilicate material, and nickelsupported catalysts were tested by Corte et al. [72]. K2CO3 atalyst shows a destructive effect on the organic compounds, and H2 and CO2 form at the end of he steam gasification (catalytic steam reforming) process. Lv and co-workers investigated the ig. 6. Plots for yield of hydrogen from supercritical fluid (water) extraction, pyrolysis and steam asification. [(Water/Solid) = 2] of beech wood at different temperatures.. Table 6 Composition of bio-syngas from biomass gasification. ield of hydrogen from biomass with the use of dolomite in the fluidized- bed gasifier and the use f nickel-based catalysts in the fixed-bed reactor downstream from the gasifier. They obtained a aximum hydrogen yield (130.28 g H2/kg biomass) over the temperature range of 925–1125 K. apagna et al. studied catalytic steam gasification of biomass in a bench-scale plant containing a luidized-bed gasifier and a secondary fixed-bed. The influence of the operating conditions in the atalytic converter on the production of gases, especially H2, was investigated over the emperature range of 935–1105 K. They obtained a maximum hydrogen yield (60 vol.%) by tilizing the fresh catalyst at the highest temperature level.
.2.2. Fisher–Tropsch diesel via bio-syngas
asification technologies provide the opportunity to convert renewable biomass feedstocks into lean fuel gases or synthesis gases. The synthesis gas includes mainly hydrogen and carbon onoxide (H2 + CO) which is also called as bio-syngas [75–78]. Biomass can be converted to io-syngas by non-catalytic, catalytic, and steam gasification processes. n the steam-reforming reaction, steam reacts with hydrocarbons in the feed to predominantly roduce bio-syngas. Steam reforming can be applied various solid waste materials including, unicipal organic waste, waste oil, sewage sludge, paper mill sludge, black liquor, refuse-derived uel, and agricultural waste . Bio-syngas is a gas rich in CO and H2 obtained by gasification of iomass. Table 6 shows the composition of bio-syngas from biomass gasification. Bio-syngas can e used in turbines and boilers or as feed gas for the production of liquid alkanes by Fischer– ropsch Synthesis (FTS). he production of liquid fuels from syngas has a long history, which goes back to the pioneering ork of Fisher and Tropsch to synthesize hydrocarbon fuels in Germany in the 1920s. The first TS plants began operation in Germany in 1938 but closed down after the Second World War. hen in 1955, Sasol, a worldleader in the commercial production of liquid fuels and chemicals rom coal and crude oil, started Sasol I in Sasolburg, South Africa. Following the success of Sasol , Sasol II and III, located in Secunda, South Africa, came on line in 1980 and 1982, respectively. he FTS is a process by which gasoline, diesel oil, wax, and alcohols are produced from CO and 2 gas mixture. Basic FTS reactions are: here n is the average length of the hydrocarbon chain and m is the number of hydrogen atoms er carbon. All reactions are exothermic and the product is a mixture of different hydrocarbons in hat paraffin and olefins are the main parts. he distribution of products is described by so called Schulz–Flory equation: here Xn is the mole fraction of the product n. The composition of the synthesis gas, temperature, ressure, and the composition of the catalyst affect on the value of the parameter a. The effect of he parameter a on the composition of the FTS products is given in Table 7. The catalyst activation ffects the reaction rate and synthesis gas conversion. TL processes use catalysts based mainly on iron (Fe), cobalt (Co), ruthenium (Ru), and otassium (K), and have been extensively characterized. They operate at high pressures between .5 and 4.5 MPa, and temperatures between 495 K and 725 K . In the FTS one mole of CO reacts ith two moles of H2 in the presence Co-based catalyst to afford a hydrocarbon chain extension (– H2–). The reaction of synthesis is exothermic (DH = –165 kJ/mol) [75– 77]: he –CH2– is a building stone for longer hydrocarbons. A main characteristic regarding the erformance of the FTS is the liquid selectivity of the process. For this reaction given with Eq. 12) is necessary a H2/CO ratio of at least two for the synthesis of the hydrocarbons. The reaction f synthesis is exothermic (DH = –42 kJ/ mol). When the ratio is lower it can be adjusted in the eactor with the catalytic water–gas shift (WGS) reaction. hen Fe-based catalysts are used with WGS reaction activity the water produced in the reaction 13) can react with CO to form additional H2. The reaction of synthesis is exothermic (DH = –204 J/ mol). In this case a minimal H2/CO ratio of 0.7 is required: ompared to other metal catalysts for FTS, a Fe-based catalyst is distinguished by higher onversion, selectivity to the lower olefins, and flexibility to the process parameters . The lifetime f the Febased catalysts is short and in commercial installations generally limited to 8 weeks . Co- ased catalysts have the advantage of a higher conversion rate a longer life (over 5 years). These atalysts are in general more reactive for hydrogenation and produce. Table 7 : Effect of the parameter a on the composition of the FTS products (% by mole). herefore less unsaturated hydrocarbons and alcohols compared to Fe-based catalysts. Ni-based atalysts are also very active but their high hydrogenation activity leads to a higher level of CH4 electivity. Ru-based catalysts are equally good but their high cost tends to exclude them; thus, the se of Ru is considered only as a promoter in support of the more economic Fe and Co catalysts 88]. Although a number of catalysts for FT process are developed, the new effective catalysts of arious chemical compositions and geometric shapes are foreseen. The efficiency of a number of atalytic reactions and hence the catalytic performance, among other important factors, depend on he capability of the catalyst for heat transfer and diffusion. he products from FTS are mainly aliphatic straight-chain hydrocarbons (CxHy). Besides the xHy also branched hydrocarbons, unsaturated hydrocarbons, and primary alcohols are formed in inor quantities. The product distribution obtained from FTS includes the light hydrocarbons ethane (CH4), ethene (C2H4) and ethane (C2H5), LPG (C3–C4, propane and butane), gasoline C5–C12), diesel fuel (C13–C22), and light and waxes (C23–C33). The products from the FTS ary depending on the catalyst formulation and process conditions. Typical product distributions btained with iron based for low temperature Fischer–Tropsch (LTFT) and high temperature ischer–Tropsch (HTFT) are shown in Table 8. he Fischer–Tropsch catalytic conversion process can be used to synthesize diesel fuels from bio- yngas. Fig. 7 shows the production of diesel fuel from bio-syngas by FTS. The design of a iomass gasifier integrated with a FTS reactor must be aimed at achieving a high yield of liquid ydrocarbons. For the gasifier, it is important to avoid methane formation as much as possible, nd convert all carbon in the biomass to mainly CO and CO2. Bio-syngas can be cleaned to meet T specifications with proven and commercial available technologies. There are no biomass- pecific impurities that require a totally different gas cleaning approach. Synthetic FT diesel fuels an have excellent autoignition characteristics. he synthetic FT diesel fuel can provide benefits in terms of both PM and NOx emissions. The FT rocess is particularly suitable for the production of high-quality diesel, since the products are ainly straight-chain paraffins that possess a high cetane number. A high cetane number results in cleaner burning of the diesel with reduced harmful emissions. Physical properties of synthetic T diesel fuel are very similar to No. 2 diesel fuel, and its chemical properties are superior in that he FT process yields middle distillates that, if correctly processed (as through a Co-based atalyst), contain no aromatics or sulfur compounds. Properties of FT diesel fuel and No. 2 diesel uels are given in Table 9.
.2.3. Biomethanol
ethanol is mainly produced from natural gas, but biomass can also be gasified to methanol biomethanol). Table 10 shows main production facilities of methanol and biomethanol. iomethanol. able 8 : FT product distribution for Fe-based catalyst (per 100 carbon atoms). ig. 7. Production of diesel fuel from bio-syngas by Fisher–Tropsh synthesis. Table 9 : Properties of FT diesel fuel and No. 2 diesel fuel.. Table 10 : Main production facilities of methanol and biomethanol. an be produced from H2/CO2 mixtures by means of the catalytic reaction of CO and some CO2 ith hydrogen. The requirements for bio-syngas production from biomass for the subsequent ethanol synthesis are not fulfilled by conventional gasification processes. In contrast to asification processes for electricity production, the bio-syngas for the methanol generation rocess is limited by inert gas components (CH4, N2), which are not converted during methanol ynthesis. A second requirement for the bio-syngas composition is a high hydrogen content, ecause a main part of the biomass carbon is converted to CO2 in the gasification step (CO2 needs mol of H2 for hydrogenation to methanol). The preferable H2/CO ratio in the gasifier raw gas as to be >2 In this case, a shift reactor and therefore the additional appliances are not required. he gasification of biomass always results in a gas containing a too low hydrogen portion, espective of a too high carbon portion (CO2) for the methanol synthesis, even if the requirement entioned above is fulfilled. he gases produced can be steam reformed to produce H2 and followed by WGS reaction to urther enhance H2 production. When the moisture content of biomass is higher than 35%, it can e gasified in a supercritical water condition. The gas is converted to methanol in a conventional team-reforming/WGS reaction followed by high-pressure catalytic methanol synthesis: ig. 8. Biomethanol from carbohydrates by gasification and partial oxidation with O2 and H2O. eaction [103]. Fig. 8 shows production of biomethanol from carbohydrates by gasification and artial oxidation with O2 and H2O. variety of catalysts are capable of causing the conversion, including reduced NiO-based reparations, reduced Cu/ZnO shift preparations, Cu/SiO2 and Pd/SiO2, and Pd/ZnO. Typical ynthesis conditions are a pressure of 50–100 bar and a temperature of 495–575 K using a u/Zn/Al catalyst. An ideal synthesis gas should have a ratio H2/(2CO + 3CO2) at about 1.05, and low CO2 content of approximately 3%. n comparison with gasoline, methanol is a superior engine fuel. Thermal efficiency values for the ngine are higher, and there are no emission problems. Because of a high octane number (1 0 6), ethanol is an excellent fuel for high-compression engines. Methanol benefits the environment, conomy and consumers. Its physical and chemical characteristics result in several inherent dvantages as an automotive fuel. Some methanol benefits include low emissions, high- erformance, and less flammable than gasoline. On the basis of the mass unit, methanol has a ower energy value than gasoline. The lower heating value of the liquid fuel is 19.9 MJ/kg for ethanol and 44.4 MJ/kg for C8H18. As a fuel, methanol is most often used as a blend with asoline called M85 (85% methanol and 15% gasoline), although the fuel can also be used in an lmost pure form (M100). M85 vehicles tend to emit 30–50% less ozone-forming compounds. nd while formaldehyde emissions tend to be higher with methanol than gasoline, M85 vehicles ould likely be able to meet new emissions standards
onclusions
he term gasification stands for a sequence of the sub-processes drying, pyrolysis, oxidation, and har gasification. Gasification is accompanied by chemical reactions that proceed at high emperature with gasifying agent and (occasionally) with steam as moderating agent. In general, he gasifying agent can be air, oxygen or oxygen-enriched air. The main product of gasification is mixture of gases (‘‘producer gas”) with the main components H2, CO, CO2, H2O, CH4 and air itrogen. enewable biomass has been considered as potential feedstock for gasification to produce syngas, he economics of current processes favor the use of light hydrocarbons (in natural gas) and coal. asification converts biomass to a low to medium calorific value gaseous fuel, which can be used o generate heat and electricity by direct firing in engines, turbines and boilers. Alternatively, the roduct gas can be reformed to produce fuels such as methanol or hydrogen, which could then be sed in fuel cells. An alternative approach to the production of H2 from biomass begins with asification of biomass. The production of hydrogen from renewable biomass has several dvantages and limitations compared to that of fossil fuels. io-fuels as well as green diesel produced from biomass by Fischer–Tropsch Synthesis (FTS) are he most modern biomassbased transportation fuels. The FTS produces hydrocarbons of different ength from a gas mixture of H2 and CO (syngas) from biomass gasification called as bio-syngas. he FTS process is a process capable of producing liquid hydrocarbon fuels from biosyngas. The arge hydrocarbons can be hydrocracked to form mainly diesel of excellent quality. The process or producing liquid fuels from biomass, which integrates biomass gasification with FTS, converts renewable feedstock into a clean fuel. Physical properties of synthetic FT diesel fuel are very imilar to No. 2 diesel fuel, and its chemical properties are superior in that the FT process yields iddle distillates that, if correctly processed, contain no aromatics or sulfur compounds.
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