Real-Time Emission, Chemical Properties, and Dynamic Evolution Mechanism of Volatile Organic Compounds during Co-Pyrolysis of Rice Straw and Semi-Bituminous Coal

The co-pyrolysis of biomass–coal blends improves energy utilization efficiency; however, the synergistic mechanisms behind thermal degradation and volatile formation remain unclear. We combined online thermogravimetry–Fourier transform infrared spectrometry–gas chromatography/mass spectrometry (TG–FTIR–GC/MS), Gaussian deconvolution, and two-dimensional correlation spectrometry (2D-COS) to reveal the component degradation, sequential response, and evolution mechanism of volatiles during co-pyrolysis of rice straw (RS) and semi-bituminous coal (SBC), which were mixed in three proportions of 1:3, 1:1, and 3:1. The activation energies (24.70–53.43 kJ mol–1) and preexponential factors (44.67–7663.43 min–1) for decomposition and average emission intensity coefficient (EIC) (0.06–0.12) of volatiles exhibited significant heterogeneity and were highly dependent on pyrolysis temperature and blend proportion. The EIC values of phenols/esters, alcohols/ethers, ketones, aldehydes, and acids increased with increasing RS proportion. The volatile distribution of blends with high SBC proportions was mainly located in the decarbonylation/dehydration reaction region. Moreover, the volatile organic compound (VOC) and intermediate VOC percentages were 59–83 and 17–39%, respectively, with N-containing species contributing the most to the intermediate VOC fraction. Most of the volatiles mainly exhibited reducing character, with average carbon oxidation state below zero. An increase in the proportion of RS and SBC contributed to high unsaturation and small carbon skeletons of volatiles, respectively. Notably, the primary sequential temperature response of volatiles was hydrocarbons, alcohols/phenols/ethers/esters, and (aldehydes/ketones/acids, aromatics), in that order. Furthermore, we proposed a novel synergistic mechanism to demonstrate that the heterogeneous degradation of RS/SBC components contributed significantly to the dynamic formation of volatiles during the co-pyrolysis process. These novel insights into the mechanisms of biomass–coal co-pyrolysis are useful for energy optimization and pollution control.