Gasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures (>700°C), without combustion, with a controlled amount of oxygen and/or steam. The gasification processes may be distinguished according to the gasification agent used. When biomass is heated with no oxygen or only about one-third of the oxygen needed for efficient combustion, it gasifies to a mixture of primarily carbon monoxide and hydrogen — called synthesis gas or syngas (typically 40% CO, 40% H2, 3% CH4 and 17% CO2, dry basis), which can be used to make methanol, ammonia and diesel fuel with known commercial catalytic processes. When air is the oxidant, nitrogen accounts for about half of the product gas. This dilutes the concentration of hydrogen and carbon monoxide gases, resulting in a low-energy fuel gas or producer gas (typically 22% CO, 18% H2, 3% CH4, 6% CO2 and 51% N2) [16].

After leaving the gasifler, the product gas has to be cleaned and, depending on further pro­cessing steps, upgraded. The reasons for gas cleaning are to prevent corrosion, erosion and deposits in the process lines as well as to prevent poisoning of catalysts. Contaminants such as tars and inorganic components (halides, alkalis, ash) present in the syngas can deactivate the catalysts and must be removed prior to catalytic conversion [52]. Upgrading encom­passes modification of the CO/H2 ratio or removal of inert gas fractions, mainly CO2 [16].

Starting from the cleaned and upgraded synthesis gas, several fuel processing pathways are possible: the application of (thermo)chemical processes such as Fischer-Tropsch (pro­viding diesel/gas like biofuel) or methanol synthesis as well as biotechnological processing towards alcohols is possible [16].

The syngas produced by gasification of biomass can be converted into a large number of organic compounds that are useful as chemical feedstocks, fuels and solvents (Figure 2.4). Collectively, the process of converting CO and H2 mixtures to liquid hydrocarbons over a transition metal catalyst has become known as the Fischer-Tropsch (FT) synthesis, invented in the 1920s by the German engineers Franz Fischer and Hans Tropsch. At the center of this transformation is a selective catalyst that works under heat and pressure to convert the carbon monoxide and hydrogen into larger, more useful compounds. Currently the FT reaction is successfully used for fuel production from coal (CtL = Coal-to-Liquid) or natural gas (GtL = Gas-to-Liquid). Variations on this synthesis pathway soon followed for the selective production of methanol, mixed alcohols, and isosynthesis products. Another outcome of Fischer-Tropsch Synthesis was the hydroformylation of olefins, discovered in 1938 [53]. Catalysts play a pivotal role in syngas conversion reactions. In fact, fuels and chemicals synthesis from syngas does not occur in the absence of appropriate catalysts [53].

The formation of tars, and measures to deal with their removal, are significant challenges in biomass gasification. Advances in catalyst preparations are also needed in order to make large-scale biomass to liquid facilities practical [52].

Alternatively, syngas can be converted into alcohols, such as ethanol and butanol, or other chemicals, such as organic acids and methane, through syngas fermentation. The main advantages of this microbiological process are the mild process conditions (ambient temperature and pressure), lower sensitivity of the used microorganisms towards sulfur (resulting in reduced gas cleaning costs), independence of the H2:CO ratio for bioconver­sion, aseptic operation of syngas fermentation due to generation of syngas at higher tem­peratures, no issue of noble metal poisoning and a higher reaction specificity. Biological catalysts (such as Clostridium ljungdahlii, Clostridium autoethanogenum, Acetobacterium woodii, Clostridium carboxidivorans and Peptostreptococcus productus) are able to fer­ment syngas into liquid fuel more effectively than the chemical catalysts (e. g. iron, copper or cobalt) [54, 55]. Low volumetric productivity, poor solubility of gaseous substrates in the liquid phase, inhibition of microorganisms, syngas quality and product recovery are the major issues to be addressed in order to make syngas fermentation more economically feasible [16, 56, 57].