On the thermodynamics and dynamics of coal and biomass combustion

• Main Author: Wang, Linwei
• Published: University of Glasgow 2019
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.768791

The combustion of solid fuels is a complex multi-phase, multi-scale and multi-component process. While oxy-fuel combustion is under development and given the promising future of biomass in energy generation, further research on oxy-fuel coal and biomass combustion is demanded. In particular, the combustion characteristics of coal and biomass in oxygenated environments need to be better understood. To respond to these needs, transient combustion of a single coal and biomass particle in a drop-tube furnace is investigated numerically. The burning process takes place in preheated O2/N2 and O2/CO2 atmospheres with varying concentration of oxygen and under either quiescent or active flows. The oxygen concentration varies from 20% to 100% in both nitrogen and carbon dioxide background environments. The simulations are rigorously validated against the existing experimental data by matching the particle ignition delay time, particle life time and particle temperature. The unsteady temperature and species concentration fields are calculated in the course of transient burning process and the subsequent diffusion of the combustion products into the surrounding gases. The spatio-temporal evolutions of the temperature, major chemical species including CO, CO2, O2, H2 and H2O are obtained. It is shown that the homogenous combustion of the products of devolatilisation process dominates the temperature and chemical species fields at low concentrations of oxygen, and the rates of production and transport of chemical species reach their maximum level during the homogenous combustion of volatiles and decay subsequently. Yet, by oxygen enriching of the atmosphere the post-ignition heterogeneous reactions become increasingly more influential. However, the transient transfer of heat of combustion continues for a relatively long time after the termination of particle life time. It should be noted that, the results show that the combustion behaviours are different under CO2 background gas compared with N2 environments. Whether the volatiles and char burn simultaneously is distinguished in these two different gas atmospheres. Through using the validated numerical combustion model, analysis of NOx and SOx emissions is conducted by investigating the overall NOx and SOx PPM emissions rate and formation process of nitrogen and sulphur pollutant species. It is shown that CO2 has a significant inhibitory effect on NOx formation, while it promotes SO2 emissions generally. The overall NO and SO2 PPM decrease in oxygenated conditions in both O2/N2 and O2/CO2 atmospheres. The emission rate of NO and SO2 decreases when increasing oxygen concentration under both types of gas atmospheres. Further, oxygen concentration has a greater influence on the formation of NOx in O2/CO2 gas conditions. Also, SOx pollutants are sensitive to oxygen concentration in both O2/N2 and O2/CO2 environments. Further, the numerical results are post processed to reveal the temporal rates of unsteady entropy generation by transport of heat and chemical species and through chemical reactions in the transient oxy-combustion of single coal and biomass particles combustion system. Analysis of the total entropy generation shows that the chemical entropy is the most significant source of irreversibility and is generated chiefly by the ignition of volatiles. However, thermal entropy continues to be produced well after termination of the particle life time through diffusion of the hot gases into the surrounding environment. Mass transfer irreversibility is found to have a negligibly small contribution. Most importantly, it is demonstrated that a slight oxygenation of the atmosphere results in major increases in the total chemical entropy generation and thus in the global irreversibility of the process. Nevertheless, upon exceeding a certain mole fraction of oxygen in the atmosphere, further addition of oxygen only causes minor increases in entropy generation. This trend is observed consistently in both quiescent and active flow cases. In addition, an experimental work about the smouldering combustion of biomass packed bed under different ignition temperatures and air mass flow conditions is conducted. The instabilities and the speed of the smouldering combustion front, and the combustion temperature and heating rate during the smouldering combustion process are investigated in detail. The results show that, the smouldering reaction front keeps flat at lower ignition temperatures and slower air mass flow rate conditions, and the instabilities appear when the ignition temperature and air mass flow rate exceed a certain value. Increases in ignition temperature and air mass flow rate both lead to the increases in the reaction front speed, smouldering combustion temperature and heating rate. Moreover, the results indicate that the ignition temperature has a greater influence on the smouldering combustion of biomass packed bed than air mass flow rate.