Modeling of alkali metal release during biomass pyrolysis

The release of alkali metals during biomass pyrolysis was numerically and experimentally studied. The concentration of sodium and potassium in the gas phase above a biomass particle was measured; quantitative and time resolved data were acquired by means of the Laser-Induced Breakdown Spectroscopy (LIBS) technique. LIBS made it possible to extend the measurements of alkali metal concentration to the sooty pyrolysis stage. Data from the measurements revealed a staged release of alkali metals from biomass. Two distinct peaks of concentrations were observed, one associated with the pyrolysis stage and the other with the gasification stage. Since during the pyrolysis stage a large temperature gradient exists inside the particle, numerical simulations were carried out to explain the experimental measurements and extract the kinetic data. Using a detailed particle model, the rates of potassium and sodium release from the particle during the pyrolysis stage were attained. For sodium release the activation energy was found to be in the range of 218 to 248 kJ/mole and for potassium release it was found to be between 168 and 198 kJ/mole. Furthermore, equilibrium calculations were performed to identify the stable sodium and potassium compounds and their phases during the pyrolysis stage of the particle.