A kinetic study of gaseous potassium capture by coal minerals in a high temperature fixed bed reactor
Multi-scale and/or multi-disciplinary approach to process-product innovation
Analysis of Energy-Environmental Issues (T3-3P)
Keywords: kinetics, potassium vapor, coal minerals, high temperature, gas-solid reaction
Co-firing of biomass and coal in suspension fired power plant boilers is an attractive option for utilizing a CO2 neutral fuel and obtaining a high electrical efficiency. The high content of Cl and K in the straw fuel ash may lead to problems regarding fly ash utilization, deposit formation, corrosion and SCR catalyst deactivation. To obtain the benefits of straw and coal co-firing both the coal type and the straw fuel share have to be appropriately chosen. The interactions of the inherent coal ash and potassium from straw have a strong influence on ash deposits formation and corrosion, and fly ash quality. However, due to the complexity of coal and straw composition, complicated conditions in full-scale boilers and pilot combustor, the kinetics and mechanisms of potassium capture by coal minerals are difficult to obtain. To alleviate this problem, the reactions between gaseous potassium compounds and coal minerals were investigated in a lab-scale high temperature fixed bed reactor under well-defined conditions.
The applied coal minerals included kaolin, silica, alumina, bituminous, lignite coal ash and were formed into cylindrical pellets. Alumina pellets saturated with potassium salts were used as alkali source and the applied reaction temperatures were 900-1500C. Capture of potassium by kaolin is independent of oxygen content in the gas. Estimation by initial rate method shows that capture of potassium by kaolin is a first-order reaction concerning bulk KCl concentration when water vapor is in excess. The metakaolin conversion was calculated by comparing the measured weight gain with the maximum achievable weight gain. With 1000 ppmv KCl in the gas the metakaolin conversion increases quickly up to an exposure time of 2 h and then increases slowly to a maximum value of 40% with further increasing exposure time. Interestingly, metakaolin conversion decreases with increasing temperature in the range of 900-1300C and increases again with further increasing temperature up to 1500C. Metakaolin can be converted to mullite by high temperature treatment. The results show that no reaction of mullite with KCl was observed at temperatures below 1300C indicating that formation mullite may be an important deactivation mechanism of the kaolin pellet.
A simple model was developed for the gas solid reaction between potassium vapor and metakaolin pellet below 900C assuming no deactivation of the pellet and that external mass transfer can be neglected. The model can reasonably predict the pellet conversion as function of potassium vapor pressure, exposure time, and also the concentration profile of captured potassium inside the pellet.
Presented Tuesday 18, 13:30 to 15:00, in session Analysis of Energy-Environmental Issues (T3-3P).
