Control of gasifier mercury emissions in a hot gas filter: the effect of temperature

Abstract

Analyses of solid streams from a pilot-scale coal gasifier equipped with hot-gas filtration at 580°C have indicated that all coal-derived mercury was carried away by the fuel gas. Calculations using a thermodynamic equilibrium model have confirmed that there was little potential for removal of mercury in the hot gas filter at these temperatures. In this work, several experimental capture/recovery methods have been used in attempting to quantify the amount of mercury carried in the fuel gas. Standard ‘wet’ sampling methods have not worked well. A ‘dry’ mercury-capture method has been evaluated for determinations of mercury release from a pilot-scale gasifier equipped with a hot gas filter. At temperatures below 200°C, the fine dust on the hot gas filter was able to capture significant amounts of mercury. The findings from pilot-scale experiments have been examined by work in a specially designed bench-scale apparatus. Mercury removal was shown to be due to a reaction with the carbonaceous dust from the gasifier; the dust had substantial capacity for mercury capture at temperatures below 200°C. The effect of the dust in modifying the bulk thermodynamic properties of the hot gas filter system composition is attributed to chemisorption of HgS. The capacity of the dust for mercury capture was observed to be much greater in the laboratory-scale reactor than in the hot gas filter of the pilot plant. This observation is believed to be due to the main mechanism of dust collection by the filter being gravity separation directly to the hopper, rather than deposition as a filter cake.