DIRECT OBSERVATION OF ALKALI VAPOR RELEASE DURING BIOMASS COMBUSTION AND GASIFICATION .1. APPLICATION OF MOLECULAR-BEAM MASS-SPECTROMETRY TO SWITCHGRASS COMBUSTION

Electricity from biomass and biomass-derived fuels has become an attra ctive and viable alternative energy source. Alkali metal release durin g biomass combustion can cause significant problems in terms of severe fouling and slagging of heat transfer surfaces in boilers thus reduci ng efficiency, and in the worst case, leading to unscheduled plant shu tdown. Future biomass to electricity facilities will benefit from incr eased efficiencies by incorporating integrated gasification combined c ycle systems that use biomass combustion gases to directly drive an ae roderivative turbine. These systems will have even lower tolerances fo r alkali vapor release because accelerated erosion and corrosion of tu rbine blades results in shorter turbine lifetimes. One solution to the fouling and slagging problem is to develop methods of hot gas cleanup to reduce the amount of alkali vapor to acceptable levels. A detailed understanding of the mechanisms of alkali metal release during biomas s combustion as well as identifying alkali metal containing vapors and how the vapors lead to fouling and slagging could greatly benefit the development of hot gas cleanup technology. This paper demonstrates th e application of molecular beam/mass spectrometry to the study of alka li metal speciation and release during switchgrass combustion. We have successfully used this experimental technique to identify alkali meta l containing species released during the combustion of switchgrass at four different conditions: 1100 degrees C in He/O-2(20%), 800 degrees C in He/O-2(20%), 1100 degrees C in He/O-2(5%), and 1100 degrees C in He/O-2(10%)/steam(20%). These conditions were chosen to study the effe ct of temperature, oxygen concentration, and excess steam on alkali me tal release and speciation. Initial feedstock composition is the most significant factor which affects the amount and species of alkali meta l released during biomass combustion. The switchgrass sample screened in the present study is high in both alkali metal (potassium) and chlo rine. As a result, the predominant alkali metal containing species rel eased during switchgrass combustion is potassium chloride. Varying the combustion condition affects the amount of alkali metal released by a factor of 2 or less. Adding excess steam to the combustion environmen t tends to shift the form of alkali metal release from the alkali chlo ride to the hydroxide.