Dry/Hot Gas Cleaning

In dry/hot gas cleaning, residual contaminations are removed by chemical absor­bents at elevated temperature. In the methanol process, hot gas cleaning has few energy advantages as the methanol reactor operates at 200-300°C, especially when preceding additional compression is required (efficient compression requires a cold inlet gas). However, dry/hot gas cleaning may have lower oper­ational costs than wet gas cleaning (Mitchell 1998). Within ten years hot gas cleaning may become commercially available for BIG/CC applications (Mitchell 1998). However, requirements for methanol production, especially for catalyst operation, are expected to be more severe (Tijmensen 2000). It is not entirely clear to what extent hot gas cleaning will be suitable in the production of meth­anol.

Tars and oils are not expected to be removed during the hot gas cleaning since they do not condense at high temperatures. Therefore, they must be removed prior to the rest of the gas cleaning, as discussed above.

For particle removal at temperatures above 400°C, sliding granular bed filters are used instead of cyclones. Final dust cleaning is done using ceramic candle filters (Klein Teeselink et al. 1990; Williams 1998) or sintered-metal barriers operating at temperatures up to 720°C; collection efficiencies greater that 99.8% for 2-7 pm particles have been reported (Katofsky 1993). Still better ceramic filters for simultaneous SOx, NOx, and particulate removal are under development (White et al. 1992).

Processes for alkali removal in the 750-900°C range are under development and expected to be commercialized within a few years. Lead and zinc are not removed at this temperature (Alderliesten 1990). High-temperature alkali removal by passing the gas stream through a fixed bed of sorbent or other material that preferentially adsorbs alkali via physical adsorption or chemisorption was dis­cussed by Turn et al. (1998). Below 600°C alkali metals condense onto particu­lates and can more easily be removed with filters (Katofsky 1993).

Nickel-based catalysts have proved to be very efficient in decomposing tar, ammonia, and methane in biomass gasification gas mixtures at about 900°C. However, sulfur can poison these catalysts (Hepola et al. 1997; Tijmensen 2000). It is unclear if the nitrogenous component HCN is removed. It will probably form NOx in a gas turbine (Verschoor et al. 1991).

Halogens are removed by sodium and calcium-based powdered absorbents. These are injected in the gas stream and removed in the dedusting stage (Ver — schoor et al. 1991).

Hot gas desulfurization is done by chemical absorption to zinc titanate or iron oxide-on-silica. The process works optimally at about 600°C or 350°C, respectively. During regeneration of the sorbents, SO2 is liberated and has to be processed to H2SO4 or elemental sulfur (Jansen 1990; Jothimurugesan et al. 1996). ZnO beds operate best close to 400°C (van Dijk et al. 1995).

Early compression would reduce the size of gas cleaning equipment. How­ever, sulfur and chloride compounds condense when compressed and they may corrode the compressor. Therefore, intermediate compression to 6 bar takes place only after bulk removal and 60 bar compression just before the guardbed.

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