A fuel gas can be produced from biomass or other feed stocks by partial oxidation at high temper­ature using oxidizing agents such as air, oxygen, steam, carbon dioxide or combination of these. In case of gasification, the temperatures used are typically between 600 and 1000°C. The differ­ent steps occurring in gasification of biomass or other feedstocks are graphically represented in Figure 6.2.

The first step in the thermo-chemical conversion of a feedstock, such as biomass, is the drying of the biomass followed by the pyrolysis, of the cellulose, hemicellulose and lignin to produce char and volatiles, such as permanent gases, light hydrocarbons and tars. Introduction of reducing agents and sometimes also a catalyst further decompose the char and the tars to permanent gases. Constituents such as tars may not be completely converted and also ash from the char will be present in the formed product gas. In view of this description gasification is only one-step of many although they together generally is referred to as only gasification.

The processes graphically represented in Figure 6.3 can generally be specified in the main chemical reactions as described in reactions (6.1)-(6.6) (Engvall, 2011):

Feedstock ^ char + tars + CO2 + H2O + CH4 + CO

+ H2 + (C2 — C5) + impurities (pyrolysis) (6.1)

C + %O2 ^ CO AH°r = -109kJ/mol (partial oxidation) (6.2)

Reaction (6.1) describes the pyrolysis, an endothermic process, which is a very important step for biomasses due to the large fraction of volatiles (70-80% dry basis) in these feedstocks. The reactions (6.2)-(6.6) are the common reactions included in gasification of biomass. Heat for the endothermic reactions can be supplied either by direct partial oxidation directly, as e. g. described in reaction (6.2), or from an external source transferring heat to the gasifier. Other reactions that influence the product gas yield and composition are the cracking of the tars due to thermal conversion at high temperatures (reaction 6.7), or catalytic tar reforming with steam (reaction 6.8) or dry (reaction 6.9) in the presence of, for instance, a catalytically active bed material used in fluidized bed gasification:

The thermal conversion in reaction (6.7) is a simplification. The decomposition is generally much more complex and many different paths as proposed by Devi et al. (2005).

The decomposition of the feedstock is a complex process and depends on parameters, such as biomass feedstock composition, gasifying agent and the gasification process (Dayton, 2002). The gasification process produces a raw gas, generally called producer gas that consists of the permanent gases CO, CO2, H2O, H2, CH4, other gaseous hydrocarbons (C2-C5), char, tars, inorganic constituents and ash. The impurities in form of ash and char particulates, tar, and inorganic impurities, such as H2S, CS2, COS, AsH3, PH3, HCl, NH3, HCN, and alkali salts, have to be removed before utilizing the gas, depending on the application of interest. Of significant importance is the tar produced, present in different amounts, depending on the gasifier technique, feedstock used and process conditions. Tar is often a confusing term because different definitions of tar are used depending on the gasifier type and the gasification operating conditions. Tar usually consists of condensable highly aromatic hydrocarbon organic compounds, ranging from molecular weight above 78 (benzene) and can generally be divided into so-called water-soluble (phenolic) and non-water-soluble (aromatic) compounds (Moersch, 2000).

The formation of tar is one of the major obstacles in the commercialization of biomass gasifi­cation technologies (Yung, 2009), causing problems in downstream process equipment, such as blocking of pipes and filters, as well as coking on catalysts in gas upgrading processes (Dayton, 2002), even at very low concentrations in the gas. Beside the tars, other impurities important to remove before utilizing the gas are generally particulates, alkali salts and sulfur-containing compounds.

In the case where black liquor is used as a fuel in the gasification, also reactions (6.10) and (6.11) take place due to the presence of high contents of alkali salts in the feedstock (Grace, 1994):

C + 1/2Na2SO4 ^ CO2 + 1/2Na2S (6.10)

C + 1/4Na2SO4 ^ CO + 1/4Na2S (6.11)

As the black liquor droplet enters the recovery unit it is exposed to hot gases and will undergo drying, pyrolysis and char conversion. The amount of salts is very high in the black liquor. This
causes some problems with respect to corrosion, but it also is acting as a very strong catalyst, and thus black liquor can be gasified at 100-200°C lower temperature than the same amount of “normal” biomass.