Characteristics of CO, CO2 generation and reactive group conversion in isothermal smoldering combustion of coal

Abstract

The process of coal smoldering combustion will produce a significant number of CO and CO2 gases, causing environmental pollution and casualties. It is important to elucidate the law and mechanism of CO and CO2 generation during coal smoldering combustion. In this paper, the CO and CO2 production patterns and the relationship of reactive group conversion during isothermal smoldering combustion were investigated by TG-MS and FT-IR. Results reveal that the CO and CO2 generation curves with time during isothermal smoldering combustion of coal in the first stage conform to the polynomial function Y1=A1+B1t+C1t2+D1t3+E1t4, in the second stage accord with the complex exponential function Y2=A2+B2ekt (k < 0), as well as the CO + CO2 production curve. The consumption rate function of functional groups during isothermal smoldering combustion is calculated from the CO + CO2 production curve as v=L·YCO + CO21000000Vm. As the isothermal temperature rises, the rate of coal combustion in smoldering combustion accelerates, causing an exponential increase in CO + CO2 emissions and reactive group consumption. The CO and CO2 emissions vary considerably at different isothermal temperatures, which is due to the increase in the number and type of reactive groups involved in the reaction with rising temperature. During isothermal smoldering combustion of coal, in exception to the direct generation of CO and CO2 from carboxyl and carbonyl groups, aliphatic hydrocarbons are key groups for the continuous generation of CO and CO2, and are also important mediators for the formation of carboxyl and carbonyl groups. The hydroxyl group acts as a reactant at relatively low temperatures and plays an essential role in driving the reaction. The aromatic hydrocarbon is the principal source of CO and CO2 at higher temperatures. This study will help to forecast the degree of coal smoldering combustion and to control coal smoldering combustion.