Compared to fossil fuels, biomass is rich in alkali salts that typically vaporize at high gasifier temperatures but condense downstream below 600 °C. Because condensation of alkali salts causes serious corrosion problems, efforts are made to strip the gas of alkali. If the gas can be cooled to below 600 °C, the alkali will condense onto fine solid particles (<5 microns) that can be captured in a cyclone, ESPs, or filters. Some applications do not permit cooling of the gas. In such cases, the hot gas may be passed through a bed of active bauxite main­tained at 650 to 725 °C.

Disposal of Collected Tar

Tar removal processes produce liquid wastes with higher organic compound concentrations, which increase the complexity of water treatment. Wastewater contaminants include dissolved organics, inorganic acids, NH3, and metals. Collected tars are classified as hazardous waste, especially if they are formed at high temperatures (Stevens, 2001). Several technologies are available for treatment of these contaminants before their final disposal. Hasler et al. (1997) presented a description of the available technologies that comprise extraction with organic solvent, distillation, adsorption on activated carbon, wet oxidation, oxidation with hydrogen peroxide (H2O2), oxidation with ozone (O3), incinera­tion, and biological treatment.

Cracking

Cracking involves breaking large molecules into smaller ones. It converts tar into permanent gases such as H2 or CO. The energy content of the tar is thus mostly recovered through the smaller molecules formed. Unlike in physical cleaning, the tar need not be condensed for cracking. This process involves heating the tar to a high temperature (~1200 °C) or exposing it to catalysts at lower temperatures (~800 °C). There are two major types of cracking: thermal and catalytic.

Thermal Cracking

Thermal cracking without a catalyst is possible at a high temperature (~1200 °C). The temperature requirement depends on the constituents of the tar. For example, oxygenated tars may crack at around 900 °C (Stevens, 2001). Oxygen or air may be added to allow partial combustion of the tar to raise its tempera­ture, which is favorable for thermal cracking. Thermal decomposition of biomass tars in electric arc plasma is another option. This is a relatively simple process but it produces gas with a lower energy content.

Catalytic Cracking

Catalytic cracking is commercially used in many plants for the removal of tar and other undesired elements from product gas. It generally involves passing the dirty gas over catalysts. The main chemical reactions taking place in a cata­lytic reactor are represented by Eq. (4.5) in the presence of steam (steam reforming) and Eq. (4.6) in the presence of CO2 (dry reforming). The main reactions for tar conversion are endothermic, so a certain amount of combustion reactions are allowed in the reactor by adding air.

Nonmetallic catalysts include less-expensive disposable catalysts: dolomite, zeolite, calcite, and so forth. They can be used as bed materials in a fluidized bed through which tar-laden gas is passed at a temperature of 750 to 900 °C. Attrition and deactivation of the catalyst are a problem (Lammars et al., 1997). A proprietary nonmetallic catalyst, D34, has been used with success in a fluid­ized bed at 800 °C followed by a wet scrubber (Knoef, 2005, p. 153).

Metallic catalysts include Ni, Ni/Mo, Ni/Co/Mo, NiO, Pt, and Ru on sup­ports like silica-alumina and zeolite (Aznar et al., 1997). Some of them are used in the petrochemical industry and are readily available. A Ni/Co/Mo blend converts NH3 along with tars. Catalysts deactivate during tar cracking and so need reactivation. Typically the catalysts are placed in a fixed or fluidized bed. Tar-laden gas is passed through at a temperature of 800 to 900 °C.

Dolomite (calcined) and olivine sand are very effective in in-situ tar reduc­tion. This type of catalytic cracking takes place at the typical temperature of a fluidized bed. Good improvement in gas yield and tar reduction is noted when catalytic bed materials are used.