Environmental characteristics of thermal utilization of waste with external and internal supply of thermal energy

Abstract

THE PURPOSE. Identification of optimal regimes for autothermal and allothermic methods of gasification of plant biomass in terms of energy parameters of generator gases, as well as determination of environmental indicators during subsequent combustion of generator gases to obtain thermal energy.METHODS. When modeling gasification processes, a nonstoichiometric model was used, based on the assumption that a chemically reacting multicomponent mixture is in a state of thermodynamic and chemical equilibrium, which corresponds to the minimum value of the isobaric-isothermal potential. When modeling the combustion of generator gas in a mixture with air, a kinetic model of a perfectly mixed flow reactor was used and the detailed mechanism of chemical interaction for the C-H-O-N-S reacting system was taken into account. The calorific value of generator gas obtained by steam gasification and external supply of thermal energy is significantly higher than the calorific value of gas obtained by internal supply of thermal energy. However, the values of the energy potential and thermochemical efficiency are very close for both types of gasification.RESULTS. For plant biomass with a given averaged elemental composition, gasification conditions are determined that increase the degree of conversion of initial materials into generator gas. In particular, for the autothermal gasification method, the maximum calculated values of the energy potential of dry ash-free generator gas and thermochemical efficiency were obtained at an excess air coefficient α ≈ 0.32. For the allothermic gasification method, the maximum calculated values of the energy potential of the generator gas and the thermochemical efficiency correspond to the gasification temperature range T ≈ 1050 -1100 K and the mass fraction of the supplied steam gH2O ≈ 0.217. To ensure these conditions, it will be necessary to supply thermal energy through combustion of ≈ 37 wt. % generator gas. Generator gas produced by the allothermic method has higher energy performance, and the negative impact on the environment during its subsequent combustion is characterized by lower specific CO and CO2 emissions in terms of a ton of reference fuel.