Gasification followed by FTS is currently the most promising method for upgrad­ing low-value coal and biomass to high-value liquid fuels and chemicals. The total biomass produced each year as waste material from agriculture and forest operations could be converted into roughly 40 billion gal/year of liquid fuels (roughly 25% of the current US gasoline usage).

Tijmensen et al. (2002) review the technical feasibility and economics of BIG — FT process and also point out the key R&D issues involved in the commercializa­tion of this process. Boerrigter and den Uil (2002) give a similar review identifying a potential BIG-FT process configuration. The FTS for the production of liquid hy­drocarbons from coal-based synthesis gas has been the subject of renewed interest for conversion of coal and natural gas to liquid fuels (Jin and Datye 2000).

Gasification is a complex thermochemical process that consists of a number of elementary chemical reactions, beginning with the partial oxidation of a biomass fuel with a gasifying agent, usually air, oxygen, or steam. The chemical reactions involved in gasification include many reactants and many possible reaction paths. The yield from the process is a product gas from thermal decomposition com­posed of CO, CO2, H2O, H2, CH4, other gaseous hydrocarbons, tars, char, inor­ganic constituents, and ash. The gas composition of product from biomass gasi­fication depends heavily on the gasification process, the gasifying agent, and the feedstock composition. A generalized reaction describing biomass gasification is as follows:

Biomass + O2 ! CO, CO2, H2O, H2, CH4 + other (CHs) + tar + char + ash (3.1)

The relative amount of CO, CO2, H2O, H2, and (CHs) depends on the stoichiometry of the gasification process. If air is used as the gasifying agent, then roughly half of the product gas is N2.

Most biomass gasification systems utilize air or oxygen in partial oxidation or combustion processes. These processes suffer from low thermal efficiencies and low Btu gas because of the energy required to evaporate the moisture typically inherent in the biomass and the oxidation of a portion of the feedstock to produce this energy.

Syngas (a mixture of carbon monoxide and hydrogen) produced by gasification of fossil fuels or biomass can be converted into a large number of organic com­pounds that are useful as chemical feedstocks, fuels, and solvents. Many conversion technologies were developed for coal gasification but process economics have re­sulted in a shift to natural-gas-derived syngas. These conversion technologies suc­cessively apply similarly to biomass-derived biosyngas. Franz Fischer and Hans Tropsch first studied the conversion of syngas into larger, useful organic compounds in 1923 (Spath and Mann 2000).

The reasons for using biofuels are manifold and include energy security, envi­ronmental concerns, foreign exchange savings, and socioeconomic issues related to the rural sector. Catalytic conversion will be a primary tool for industry to produce valuable fuels, chemicals, and materials from biomass platform chemicals. Catalytic conversion of biomass is best developed for synthesis gas, or syngas. Economic con­siderations dictate that the current production of liquid fuels from syngas translates into the use of natural gas as the hydrocarbon source. Biomass is the only renewable that can meet our demand for carbon-based liquid fuels and chemicals. Biofuels as well as green motor fuels produced from biomass by FTS are the most modern biomass-based transportation fuels. Green motor fuels are the renewable replace­ment for petroleum-based diesel. Biomass energy conversion facilities are important for obtaining bio-oil by pyrolysis. The main aim of FTS is the synthesis of long — chain hydrocarbons from a CO-H2 gas mixture. The products from FTS are mainly aliphatic straight-chain hydrocarbons (CxHy). Besides the CxHy, branched hydro­carbons, unsaturated hydrocarbons, and primary alcohols are also formed in minor quantities. The FTS process is a process capable of producing liquid hydrocarbon fuels from biosyngas. The large hydrocarbons can be hydrocracked to form mainly diesel of excellent quality. The process for producing liquid fuels from biomass, which integrates biomass gasification with FTS, converts a renewable feedstock into a clean fuel.

FTS is a process for producing mainly straight-chain hydrocarbons from a syn­gas rich in CO and H2. Catalysts are usually employed. Typical operating condi­tions for FTS are temperatures of 475 to 625 K and very high pressure depending on the desired products. The product range includes light hydrocarbons such as meth­ane (CH4) and ethane (C2H6), propane (C3H8), butane (C4H10), gasoline (C5-Ci2), diesel (C13-C22), and light waxes (C23-C33). The distribution of the products de­pends on the catalyst and the process conditions (temperature, pressure, and resi­dence time). The syngas must have very low tar and particulate matter content.

The literature dealing with the actual conversion of biosyngas to fuels using FTS is smaller. Jun et al. (2004) report experimental results of FTS carried out using a model biosyngas. In his review on biofuels, Demirbas (2007) considers FTS using biosyngas as an emerging alternative.

FTS was established in 1923 by German scientists Franz Fischer and Hans Tropsch. It is described by the following set of equations (Schulz 1999):

nCO C (n C m/2)H! CnHm C «H2O (3.2)

where n is the average length of the hydrocarbon chain and m is the number of hy­drogen atoms per carbon. All reactions are exothermic and the product is a mixture of different hydrocarbons where paraffin and olefins are the main constituents.

In FTS one mole of CO reacts with two moles of H2 in the presence of a cobalt (Co)-based catalyst to yield a hydrocarbon chain extension (-CH2-). The reaction of synthesis is exothermic (AH = —165kJ/mol):

CO C 2H2 ! — CH2- C H2O AH = — 165kJ/mol (3.3)

The — CH2- is a building block for longer hydrocarbons. A main characteristic re­garding the performance of FTS is the liquid selectivity of the process (Stelma- chowski and Nowicki 2003). When iron (Fe)-based catalysts are used in WGS re­action activity, the water produced in Reaction 3.2 can react with CO to form ad­ditional H2. The reaction of synthesis is exothermic (AH = —204kJ/mol). In this case a minimal H2/CO ratio of 0.7 is required:

2CO C H2 ! — CH2- C CO2 AH = -204 kJ/mol (3.4)

The kind and quantity of liquid product obtained in FTS is determined by the re­action temperature, pressure and residence time, type of reactor, and catalyst used. Fe catalysts have a higher tolerance for sulfur, are cheaper, and produce more olefin products and alcohols. However, the lifetime of Fe catalysts is short and in commer­cial installations generally limited to 8 weeks (Davis 2002). Bulk Fe catalysts are the catalysts of choice for converting low H2/CO ratio syngas produced by gasifi­cation of biomass or coal to fuels via FTS. These relatively low-cost catalysts have low methane selectivity and high WGS activity. However, development of a bulk Fe FTS catalyst that combines high FT activity, low methane selectivity, high attrition resistance, and long-term stability is still elusive and presents a widely recognized barrier to the commercial deployment of FTS for biomass conversion. The critical property determining the activity and deactivation of Fe catalysts for FTS appears not to be Fe in the metallic state but the carburized Fe surface.

The design of a biomass gasifier integrated with an FTS reactor must be aimed at achieving a high yield of liquid hydrocarbons. For the gasifier, it is important to avoid methane formation as much as possible and convert all carbon in the biomass to mainly carbon monoxide and carbon dioxide (Prins et al. 2004). Gas cleaning is an important process before FTS and is even more important for the integration of a biomass gasifier and a catalytic reactor. To avoid the poisoning of the FTS catalyst, tar, hydrogen sulfide, carbonyl sulfide, ammonia, hydrogen cyanide, alkali, and dust particles must be removed thoroughly (Stelmachowski and Nowicki 2003).

FTS has been widely investigated for more than 70 years, and Fe and Co are typ­ical catalysts. Co-based catalysts are preferred because their productivity is better than that of Fe catalysts thanks to their high activity, selectivity for linear hydrocar­bons, and low activity for the competing WGS reaction.

There has been increasing interest in the effect of water on Co FTS catalysts in recent years. Water is produced in large amounts with Co catalysts since one wa­ter molecule is produced for each C atom added to a growing hydrocarbon chain and due to the low WGS activity of Co. The presence of water during FTS may affect the synthesis rate reversibly as reported for titania-supported catalysts and the deactivation rate as reported for alumina-supported catalysts; water also has a sig­nificant effect on the selectivity for Co catalysts on different supports. The effect on the rate and on deactivation appears to depend on the catalyst system studied, while the main trends in the effect on selectivity appear to be more consistent for different supported Co systems. There are, however, also some differences in the selectivity effects observed. The present study deals mainly with the effect of water on the se­lectivity of alumina-supported Co catalysts, but some data on the activity change will also be reported. The results will be compared with those for other supported Co systems reported in the literature.

The activity and selectivity of supported Co FTS catalysts depends on both the number of Co surface atoms and on their density within support particles, as well as on transport limitations that restrict access to these sites. Catalyst preparation vari­ables available to modify these properties include Co precursor type and loading level, support composition and structure, pretreatment procedures, and the presence of promoters or additives. Secondary reactions can strongly influence product se­lectivity. For example, the presence of acid sites can lead to the useful formation of branched paraffins directly during the FTS step. However, product water not only oxidizes Co sites, making them inactive for additional turnovers, but it can inhibit secondary isomerization reactions on any acid sites intentionally placed in FTS re­actors.

Fe catalysts used commercially in FTS for the past five decades (Dry 2004) have several advantages: (1) lower cost relative to Co and ruthenium catalysts, (2) high WGS activity allowing utilization of syngas feeds of relatively low hydrogen con­tent such as those produced by gasification of coal and biomass, (3) relatively high activity for the production of liquid and waxy hydrocarbons readily refined to gaso­line and diesel fuels, and (4) high selectivity for olefinic C2-C6 hydrocarbons used as chemical feedstocks. The typical catalyst used in fixed-bed reactors is an unsup­ported Fe/Cu/K catalyst prepared by precipitation. While having the aforementioned advantages, this catalyst (1) deactivates irreversibly over a period of months to a few years by sintering, oxidation, formation of inactive surface carbons, and transforma­tion of active carbide phases to inactive carbide phases and (2) undergoes attrition at unacceptably high rates in the otherwise highly efficient, economical slurry bubble — column reactor.

It is well known that the addition of alkali to iron causes an increase in both the 1-alkene selectivity and the average carbon number of produced hydrocarbons. While the promoter effects on iron have been thoroughly studied, few and, at first glance, contradictory results are available for Co catalysts. In order to complete the experimental data, the carbon number distributions are analyzed for products obtained in a fixed-bed reactor under steady-state conditions. Precipitated Fe and Co catalysts with and without K2CO3 were used.

Activated carbon (AC) is a high-surface-area support with the very unique prop­erty that its textural and surface chemical properties can be changed by an easy treatment like oxidation, and these changes affect the properties of the resultant cat­alysts prepared with AC.

Fe catalysts have a higher tolerance for sulfur, are cheaper, and produce more olefin products and alcohols. However, the lifetime of Fe catalysts is short and in commercial installations generally limited to 8 weeks. Co catalysts have the advan­tage of a higher conversion rate and a longer life (over 5 years). Co catalysts are in general more reactive for hydrogenation and therefore produce fewer unsaturated hydrocarbons and alcohols compared to Fe catalysts.

Low-temperature Fischer-Tropsch (LTFT) reactors, either multitubular fixed-bed (MTFBR) or slurry reactor (SR) operating at approx. 25 bar and 495 to 525 K, use a precipitated FE catalyst. High-temperature Fischer-Tropsch (HTFT) fluidized bed reactors, either fixed (SAS) or circulating (Synthol) operating at approx. 25 bar and 575 to 595 K, use a fused Fe catalyst. In their experiments with Fe for CO2 hydro­genation, Riedel et al. (1999) found that alumina was the best support and potassium acted as a powerful promoter. Copper was added to the catalyst to enable its easy re­duction. They report that the hydrocarbon distribution from the H2/CO2 and H2/CO syngas is the same, but the reaction rate for CO2 syngas was about 43% lower than that of the CO-rich syngas.

The Al2O3/SiO2 ratio has a significant influence on Fe-based catalyst activity and selectivity in the process of FTS. Product selectivities also change significantly with different Al2O3/SiO2 ratios. The selectivity of low-molecular-weight hydro­carbons increases and the olefin-to-paraffin ratio in the products shows a monotonic decrease with an increasing Al2O3/SiO2 ratio. Table 3.6 shows the effects of the Al2O3/SiO2 ratio on hydrocarbon selectivity (Jothimurugesan et al. 2000). Recently, Jun et al. (2004) studied FTS over Al2O3- and SiO2-supported Fe-based catalysts from biomass-derived syngas. They found that Al2O3 as a structural promoter fa­cilitated the better dispersion of copper and potassium and gave much higher FTS activity. Table 3.7 shows properties of FT diesel and No. 2 diesel fuels.

Biosyngas consists mainly of H2, CO, CO2, and CH4. FTS has been carried out using a CO/CO2/H2/Ar (11/32/52/5 vol.%) mixture as a model for biosyngas on co­precipitated Fe/Cu/K, Fe/Cu/Si/K, and Fe/Cu/Al/K catalysts in a fixed-bed reactor. Some performances of catalysts that depended on the syngas composition have also been presented (Jun et al. 2004). The kinetic model predicting product distribution is taken from Wang et al. (2003) for an industrial Fe-Cu-K catalyst.

FTS for the production of transportation fuels and other chemicals from synthesis has attracted much attention due to the pressure from the oil supply. Interest in the use of Fe-based catalysts stems from its relatively low cost and excellent WGS reaction activity, which helps to make up the deficit of H2 in the syngas from coal gasification (Wu et al. 2004; Jothimurugesan et al. 2000; Jun et al. 2004). Riedel et al. (1999) have studied the hydrogenation of CO2 over both these catalysts. In the absence of any WGS reaction promoter like Mn, CO2merely behaves as a diluting gas as it is neither strongly adsorbed nor hydrogenated on Co catalysts. When Mn is added to Co catalysts, reverse WGS is possible. The FT chain growth on Co occurs due to strongly adsorbed CO on the surface. With a low partial pressure of CO, these inhibitions are removed and the regime moves from an FT to a methanation regime, yielding more CH4. It was observed that even when the r-WGS reaction was fast, the attainable CO concentration was not sufficient to attain an FT regime. It was hence concluded that CO2 hydrogenation is not possible even with a hybrid Co catalyst containing a shift catalyst like Mn.