The product gas cooler has to cool the gas from 850°C at the outlet of the gasifier to 400°C. i.e. the operational temperature of the cyclone. Cooling of product gas is not a standard operation and in most biomass plants cooler fouling is a major source of reduced availability. There are very few examples of functioning gas coolers. Conventional water-tube coolers will foul very rapidly (within several hours) resulting in reduction of the cool capacity of up to 80%. Reference is made to the operational (and still unsolved) problems in the AMER 80 MWth CFB gasifier in Geertruidenberg, The Netherlands, which co-fires the cooled gas in a coal boiler after dust removal in a cyclone [1].

The only approach with positive references to prevent significant cooler fouling is to use a dedicated fire tube cooler upstream of the dust removal cyclone and to keep the cooler surfaces at high temperature. The coarse solids in the gas will continuously clean the inner pipe wall, i.e. erode the surface to prevent the build-up of deposit layers. This cooler approach has successfully been applied in the Värnamo 18 MWth pressurised air-blown CFB gasifier and at ECN in the 0.5 MWth atmospheric air-blown CFB gasifier. ECN uses a single-tube fire tube cooler with air as cooling medium. In Värnamo a fire tube cooler (multiple tubes; inner diameter of 5 cm) is used in which water is evaporated at 40 bar (250°C) to produce steam.

References:

1. A. van der Drift and J.R. Pels: Product gas cooling and ash removal in biomass gasification. ECN, ECN-C--04-077, 2004, The Netherlands.