Alkali and Gas Emissions from PFB Combustion of Lignite

Abstract

Abstract—The results of pressurized fluidized hed combustion (PFBC) tests of lignite in a 780 mm diameter bed operated at 7 atm, using three types of bed material (dolomite, alumina and sand) are presented. The alkali and gas emissions are correlated with bed operating parameters, i.e., excess air, bed temperature and bed materials Sulfur dioxide and nitric oxide emissions, well below EPA limits, were attained, even when inert bed materials (sand or alumina) were used. The inherent alkali oxides in lignite ash have been identified as responsible for removal of SO₂ from flue gas. Negligible SO₂ emissions were measured with dolomite as a bkd material.

However, the measured vapor phase alkali concentrations (−0.2 to 4 ppmw) are one to two orders of magnitude larger than suggested by gas turbine manufacturers (0.024 ppmw). Alumina, as a bed material, results in lowered alkali vapor emissions, due to fixation of alkali in the bed as stable, high melting temperature alumino-silicates.

While some results on alkali emissions for lignite combustion at atmospheric conditions have heen published previously, to the authors knowledge. these are the first results that are made available for the combustion of lignite at pressurized conditions.