The exit gases and solids coming out of any type of gasification unit (com­bustion, gasification, high severity pyrolysis, plasma technology, etc.) should not only meet EPA standards but also facilitate subsequent processing of gases (such as for Fischer-Tropsch) and solids (for the construction and fer­tilizer industries).

7.4.3.1 Syngas Treatment

In a combustion process, gases do not have a significant fuel value (they mostly contain CO2, O2, H2O, and N2) and as shown earlier, the main objec­tive of gas purification is to remove any impurities and particulate materi­als. Earlier sections outline the steps taken by commercial processes for this purpose. The extent to which impurities are present in syngas (by gasifica­tion, high severity pyrolysis, and plasma technology) will be a function of the nature of the process and the nature of the feedstock. The presence of minor impurities such as sulfur, chlorine, ammonia, and soot will, however, be inevitable. Because concentrations of these compounds normally exceed specification of a gas turbine or a catalytic synthesis reactor such as Fischer — Tropsch, methanol, and the like, which process the syngas (see Table 7.9), the gas cleaning is necessary.

The impurities in syngas can be poisonous to the catalysts used in FT and methanol syntheses. The required minimum for these impurities in syn­gas is outlined in Table 7.9. The definition of gas cleaning is therefore based on the economic consideration of investment in cleaning versus accepting a lower catalyst life. Generally, an investment in cleaning is less expensive than replacing expensive catalyst materials. Co-gasification adds another complexity because different products coming out of coal and biomass need to be handled. Additional compounds will include chlorides, sulfur compounds, and very toxic carcinogenic tars (unless tar is recycled back in the reactor such as in the slagging entrained flow reactor). For mixed feed­stock of coal and biomass, higher hydrogen chloride, and more toxic organic compounds need to be treated compared to those found in coal gasification alone. The gas cleaning associated with the mixed feedstock is not the same as that for a single feedstock. As long as there is a single syngas purification system, it needs to handle impurities coming from all components of the mixed feed stock whether the components of mixed feedstock are gasified together or separately.

TABLE 7.9

Allowable Concentrations of Contaminants in Syngas for Catalytic Systems

Syngas Contaminants Maximum Allowable Concentrations

Total in each category <1 ppmv

Total in each category <10 ppmv

Almost completely removed Lower the better; however, "loose maximum" CH4 of total up to 15 vol % is acceptable Should be at the level that no condensation occurs when syngas is pressurized to the required pressure of approx. 25-60 atm for Fischer-Tropsch synthesis

Source: Ratafia-Brown et al. 2007. Assessment of Technologies for Co-converting Coal and Biomass to Clean Syngas-Task 2 Report (RDS), NETL report (May 10); Boerrigter, H. et al. 2005. OLGA Tar Removal Technology-Proof of Concept for Application in Integrated Biomass Gasification Combined Heat and Power (CHP) Systems, ECN-C-05-009 (January); and Boerrigter et al. 2004. Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer — Tropsch (FT) Systems, ECN, Petten, The Netherlans, ECN — report number ECN-C-04-056, 59 pp.

Sulfur compounds generally result in the production of sulfur dioxide, and this can be removed by several existing processes. More development is, however, being pursued to remove sulfur at high syngas temperature to improve the energy efficiency of an integrated FT plant. Co-gasification of coal with low sulfur substances such as biomass or waste reduces hydrogen sulfide partial pressure in an FT reactor and hence makes the gas cleaning system larger. The tar from wood gasification and hydrochloric acid from waste gasification may further complicate the issue, as will the presence of liquid products (e. g., tar) in syngas.

One problem for syngas treatment for gasification of mixed feedstock is the production of a relatively large amount of tar and the possibility of this condensing as the gas is cooled. For biomass, tar condensation occurs in the temperature range of 200-500°C and this can rapidly blind the filters. The tar generation is not a problem for an entrained bed gasifier because the tem­perature in such a gasifier is greater than about 2,300°F and tar cracks at such a high temperature. In processing a mixed feedstock, the composition and rates of feedstock should be designed such that the tar formation is not an issue [111]. For an entrained bed gasifier, the biosyngas is cleaned with standard techniques used for fossil syngas: dust filters, wet scrubbing tech­niques for the removal of NH3 and HCl, and zinc oxide (ZnO) filters for the removal of H2S. After adjusting the required H2/CO ratio and CO2 removal (which may require a steam or a tri-reforming step), the gas is compressed
to the required FT synthesis pressure (approximately 40 atm) and fed to the FT reactor.

In other types of gasifiers where generally the temperature in the gasifier is low, tar in the exit gas stream needs to be handled. For example, in a CFB gasifier, tar is removed by either installing a tar cracking unit operated at 1,300°C or by using OLGA tar removal technology. In a tar cracker all organic compounds in the product gas (i. e., tar, BTX, CH4, and C2-hydrocarbons) are destroyed to produce additional syngas. In the OLGA technique, tar and BTX are separated and recycled back into the gasifier until they are extinct. Lower hydrocarbons are not removed by OLGA technology. Hence, in this case wet cleaning and the filter process are followed by a reforming step to convert lower hydrocarbons into additional syngas. There are reports [109, 112] that the tar problem can also be reduced by an addition of dolomite in the gasification process. In both cases, the gas purification step is followed by a gas conditioning step before gas is introduced in the FT reactor.

In co-gasification, gas is used for further conversion to products via FT and methanol synthesis, and solids are directly used for industrial purposes. The exact species formed from the ash constituents when biomass is gas­ified depends on the reactor temperature and oxygen partial pressure. For example, in the IGCC system, the sulfur species will be present as sulfides rather than as sulfates (alkalis and alkaline earths as sulfides; Fe as FeSx), whereas Al— and Si—containing species probably would present as oxides. This means the gas coming from the gasifier may encounter contaminants in their reduced form. To some extent this also depends upon the interac­tion between solids and gas as they travel through the reactor. It is therefore important to monitor gas and solids at several locations within the various process streams.