Fabric filter bag investigation following field testing of sorbent injection for mercury control at TXU's Big Brown Station

Field testing of mercury control sorbent injection options with a TOXECON™ configuration has been completed at TXU's Big Brown Station. Mercury control results at Big Brown were promising and have been previously reported. However, the high air-to-cloth ratio of operations at this unit results in significant differential pressure, and thus there was little operating margin before differential pressure limits were encountered, especially at high loads. This limited the use of sorbent injection as the added material contributes to the overall differential pressure. After field testing, the residual differential pressure across the test fabric filter module had increased relative to baseline conditions to the point that the plant performed a filter change of the test module several months ahead of schedule. An investigation was conducted on pre- and posttest filter samples from the test module and a parallel nontest module to examine the effect of activated carbon injection. Analysis of the samples indicates an increase in residual dust embedded in the filters which appears to explain the low fabric permeabilities. The long-term increase in differential pressure did not appear to be associated with activated carbon injection, but instead was due to a gradual buildup of embedded material on the filters that was not cleaned away by the pulse cleaning system. The injected activated carbon appeared to behave like additional fly ash in terms of baghouse differential pressure but did not appear to accelerate the buildup of residual material.