The raw synthesis gas produced by gasification contains impurities. The most typical impurities are organic impurities like condensable tars, BTX (benzene, toluene, and xylenes), inorganic impurities (NH3, HCN, H2S, COS, and HCl), volatile metals, dust, and soot (Tijmensen 2000; van Ree et al. 1995). These contaminants can lower catalyst activity in reformer, shift, and methanol reactor, and cause corrosion in compressors, heat exchangers and the (optional) gas turbine.

The estimated maximal acceptable contaminant concentrations are summa­rized in Table 2.2 together with the effectiveness of wet and dry gas cleaning, as described below.

The gas can be cleaned using available conventional technology, by applying gas cooling, low-temperature filtration, and water scrubbing at 100-250°C. Alter­natively, hot gas cleaning can be considered, using ceramic filters and reagents at 350-800°C. These technologies have been described thoroughly by several authors (Consonni et al. 1994; Kurkela 1996; Tijmensen 2000; van Dijk et al. 1995; van Ree et al. 1995). The considered pressure range is no problem for

TABLE 2.1 (CONTINUED) Characteristics of Gasifiers

1 640 ktonne dry wood annual, load is 8000 h.

2 Calculated from LHVwet = HHV^ X (1 — W) — Ew X (W + Hwet x mH2O); with Ew the energy needed for water evaporation (2.26 MJ/kg), Hwet the hydrogen content on wet basis (for wood Hdry = 0.062) and mH2O the amount of water created from hydrogen (8.94 kg/kg).

3 Wet biomass: 80/0.7 = 114 tonne/hr to dry biomass 80/0.85 = 94.1 tonne/hr for IGT П evaporate water 20.2 tonne/hr at 1.3 ts/twe in Niro (indirect) steam dryer. Calculation for BCL is alike. The steam has a pressure of 12 bar and a temperature of minimally 200°C (Pierik et al. 1995).

4 Pressure is 34.5, 25, or 1.2 bar, temperature is minimally 250, 240, or 120°C.

5 Calculated from the total mass stream, 188.5 tonne/hr.

6 Quoted from OPPA (1990) by Williams et al. (1995).

7 Knight (1998).

8 Compiled by Williams et al. (1995).

either of the technologies. Hot gas cleaning is advantageous for the overall energy balance when a reformer or a ceramic membrane is applied directly after the cleaning section, because these processes require a high inlet temperature. However, not all elements of hot gas cleaning are yet proven technology, while there is little uncertainty about the cleaning effectiveness of low temperature gas cleaning. Both cleaning concepts are depicted in Figure 2.4.