Derivation of accurate rate constants for char CO_2, H_2O gasification using a pressurized drop tube furnace

摘要

Drop tube furnace (DTF) test is a routinely applied technique to analyze the reactivity of solid fuels such as coal or biomass under controlled atmospheres. For char conversion, the rate constants for a reaction model are derived from the reaction conditions varying the temperature and the partial pressure of reactants. The rate constants can be used in computational fluid dynamic (CFD) simulations for combustion or gasification systems utilizing the fuel. However, the reaction conditions of particles inside a DTF are often assumed to be identical to the controlled reactor conditions. This study assesses the validity of such approach and proposes a new method to derive accurate rate constants. For this purpose, coal char was gasified by CO_2 and H_2O in a pressurized DTF, and the rate constants of a char conversion model was derived for the controlled reactor conditions. CFD simulations using the rate constants significantly underestimated the char conversion. Three reasons were identified for the discrepancies: i) shorter residence time of char particles concentrated more on the high velocity region, ii) lower particle temperatures by the endothermic reactions, and iii) lower partial pressure of reactants by its consumption. A new method was proposed to estimate the actual reaction conditions. The CFD results adopting the corrected rate constants based on the method well matched with the measured data.