Kinetic analysis of the fast reactions on the basis of the oxy-combustion of coal chars

摘要

Recently, a great amount of attention has been focused on the carbon dioxide emissions from the power sector due to the significant impact on the greenhouse effect. To reduce the emissions of greenhouse gases from fossil fuel-fired power generation, oxy-combustion of coal seems to be a promising future technology while the retrofitting of existing boilers to enable O_2-enriched atmosphere combustion [1]. Thermogravimetric analysis is a commonly used technique to investigate and compare thermal behavior and kinetics during the combustion of solid raw materials, such as coal, biomass, wastes and its chars. Therefore the TG analysis is simple and rapid technique for solid fuels oxy-combustion kinetic study [2]. Combustion and oxy-combustion of coal consists of several consecutive and parallel processes and reactions i.e.: drying, pyrolysis, volatile matter combustion and in-situ char combustion, which is the slowest step of whole process, but it is still the very fast reaction. Therefore kinetic analysis of coal char is crucial for modelling and design of industrial boilers. The issue of present work was to evaluate the oxy-combustion kinetics of two different chars derived from Polish coals (subbituminous and lignite). The chars for the studies of the oxy-combustion processes were obtained via pyrolysis of coal in a bench scale laboratory reactor at a temperature of 1273 K and at atmospheric pressure. The kinetics of the char oxy-combustion was examined by using two types of thermobalances: Netzsch STA 409PG and TA Instruments TG-HP150s pressurized thermogravimetric analyzer with Rubotherm magnetic suspension balance. The experiments were performed at isothermal conditions and at a wide range of temperatures (723 - 1273 K) at atmospheric pressure. Gas mixtures that contained 20% and 30% of O_2 in CO_2 were used as an oxidant. A comparative study of the data received from both thermobalances, which differ in construction, design of crucibles, sample size and gas flow quantity and direction, were made. The influence of individual factors such as temperature, O_2 concentration, dimensions of crucibles and the dimensions of char bed on the reaction rate were analyzed. Impact of these parameters were analyzed which affecting the interpretation of the TG results. On the basis of these results, kinetic model showing the influence of all the factors on reaction rate were presented. This model can be applied for other fast reactions to ensure the chemical reaction kinetics control regime during TG tests.