The chemical and physical effects of CO2 on the homogeneous and heterogeneous ignition of the coal particle in O2/CO2 atmospheres

Oxy-fuel combustion technology is deemed a promising technology to the reduction of CO2 emissions during fossil fuel combustion process and has been comprehensively investigated. Many researchers found that the ignition of pulverized coal delayed in O2/CO2 atmospheres compared to in O2/N2 atmospheres with an identical mole fraction of O2 and attributed the ignition delay to the higher volume specific heat of CO2 compared with N2. This paper studied in detail the chemical effects and physical effects (specific heat, radiation characteristics and mass diffusivity) of CO2 on both coal homogeneous and heterogeneous ignitions in O2/CO2 atmospheres. Jovanovic's ignition experiments (Jovanovic et al., 2011, 2012) were adopted to verify the simulated results. The artificial materials X, Y, Z and W were introduced to isolate the effects of chemical properties, specific heat, radiation characteristics and mass diffusivity of CO2 during the homogeneous and heterogeneous ignition process of the coal particle, respectively (as can be seen in Table 3). The results showed that in the homogeneous ignition mechanism of coal in O2/CO2 atmospheres, the reasons for the ignition delay compared to the corresponding O2/N2 atmosphere were higher specific heat and chemical properties of CO2. The chemical properties of CO2 resulted in the ignition delay during the homogeneous ignition of pulverized coal in O2/CO2 atmospheres. High concentration of CO2 resulted in the increase in the amount of heat absorption in the chemistry reaction system, especially R99 (CO2 + H ⇔ OH + CO), which was an exothermic reaction in the X case, whereas an endothermic reaction in the CO2 case. Higher specific heat, higher thermal radiation and lower mass diffusion coefficient of CO2 played important roles in the ignition delay in the heterogeneous ignition of pulverized coal in O2/CO2 atmospheres compared to O2/N2 atmospheres.