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

Analyses of solid streams from a pilot-scale coal gasifier equipped with ho t-gas filtration at 580 degreesC have indicated that oil coal-derived mercu ry was carried away by the fuel gas. Calculations using a thermodynamic equ ilibrium model have confirmed that there was Little potential for removal o f mercury in the hot gas filter at these temperatures. In this work, severa l experimental capture/recovery methods have been used in attempting to qua ntify the amount of mercury carried in the fuel gas. Standard 'wet' samplin g methods have not worked well. A 'dry' mercury-capture method has been eva luated for determinations of mercury release from a pilot-scale gasifier eq uipped with a hot gas filter. At temperatures below 200 degreesC, the fine dust on the hot gas filter was able to capture significant amounts of mercu ry. The findings from pilot-scale experiments have bean examined by work in a specially designed bench-scale apparatus. Mercury removal was shown to b e due to a reaction with the carbonaceous dust from the gasifier; the dust had substantial capacity for mercury capture at temperatures below 200 degr eesC. 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 p ilot plant. This observation is believed to be due to the main mechanism of dust collection by the filter being gravity separation directly to the hop per, rather than deposition as a filter cake. (C) 2001 Elsevier Science Ltd . All rights reserved.