Direct Sampling of Inorganic Vapors Released During Biomass Combustion

Alkali metal containing vapors released during biomass combustion cause significant problems in power generating facilities that convert biomass to electricity. Gas-phase transport of alkali vapors increases the potential for fouling and slagging heat transfer surfaces in boilers, and will cause accelerated erosion and corrosion of turbine blades in future direct biomass-fired facilities. Biomasspower is an attractive renewable energy resource; however, alkali deposits that form on surfaces in power generating facilities reduce the efficiency of electricity production. As a result, there is a need to develop methods or define combustion conditions for reducing the transport of alkali metal containing vapors during biomass combustion. One solution to fouling and slagging problems is todevelop methods of hot gas cleanup that reduce the amount of alkali vapor to acceptable levels. This requires a detailed understanding of the mechanisms of alkali metal release during biomass combustion as well as identifying these alkali metal containing vapor species, how these vapors lead to fouling and slagging, and how they may be sequestered. Our approach is to directly sample the hotgases liberated from the combustion of small biomass samples in a variable temperature quartz tube reactor employing a molecular beam mass spectrometer system, constructed and operated at NREL, to monitor the combustion event. This system comprises a three-stage, differentially pumped vacuum chamber. High-temperature, ambient-pressure biomass combustion gasses are admitted into the first stagethrough a stainless steel molecular beam sampling cone. Chemical reactions are quenched and condensation is inhibited as these combustion gases undergo free-jet expansion into stage one. As a result, reactive and condensable alkali species remain in the gas phase at temperatures far below their condensation point for long periods compared to reaction rates. A molecular beam is formed bycollimating the free-jet expansion with a conical skimmer, located at the entrance to stage two. The molecular beam is directed into the ionization region of an Extrell C-50 triple quadruple mass spectrometer housed in stge three. Ions are produced by electron impact ionization and detected with an off-axis electron multiplier. We have successfully used this experimental technique to identifyalkali species released during the combustion of selected biomass feedstocks. A detection limit of 9 ppm of potassium chloride has been achieved in diluted, post-combustion gases. The mass spectral results have been used to correlate alkali release with feedstock composition in addition to changes in alkali speciation under various combustion conditions. Contact (e-mail): [email protected]