Nonchemical Conditioning of Simulated Coal Gasification Streams Prior to Real-Time Hg Determinations Using Atomic Absorption

摘要

An experimental Hg CEM intended for use at coal gasification plants was developed and tested in the laboratory. The CEM uses atomic absorption (AA) with a deuterium-based background correction system. Prior to the Hg analyzer, aromatic hydrocarbons and ammonia (NH_3) are removed by catalytic oxidation with platinum (Pt). The Pt catalyst effectively removed heavy tars from a slipstream in field tests at a gasification facility. The catalyst also effectively destroyed NH_3 and light aromatic hydrocarbons from gas streams in the laboratory, and was able to pass low levels of elemental Hg in air. Full Hg recoveries were still obtained when using a simulated gasifier stream, but 25% of the Hg was in the oxidized form when HCl was present, even when using a pyrolytic converter downstream from the catalyst. HgCl_2 in air was effectively converted to elemental Hg with a pyrolyzer, and little or no reoxidation of Hg occurred as the gases cooled. However, when HCl was also present, results indicated that Hg oxidation downstream from the pyrolyzer is of concern. When simulated gasifier streams were passed over the catalyst, various aerosols and solid condensates formed as the gases cooled. The amount and composition of the aerosols and condensates depended on various experimental parameters. Even under optimal conditions, white aerosols were observed exiting the catalyst tube. Those aerosols formed easily in the presence of a catalyst and appeared to be H_2SO_4 mist from the oxidation of H_2S. After the catalyst, a Nafion-based dryer is used for moisture removal. Low levels of elemental Hg (in air) could be successfully transported through a laboratory-scale Nafion dryer, while 15-20% of the Hg was lost when passing the gases through a field-scale Nafion-based dryer. Sample gas flow rate was an important variable.