Effect of alkali pretreatment on pyrolysis kinetics, thermodynamics, mechanism and product distribution of rice‑wheat straw blends via TG and Py‑GC/MS

抄録

Taking rice-wheat straw blends (1:1, wt%) as the research object, thermogravimetric analysis (TGA) was used to investigate the co-pyrolysis behavior of the untreated sample (RA) and the alkali-treated blend (RB) at four heating rates. The Kissinger-Akahira-Sunose (KAS) method, Friedman (FR) method and Coats method were combined to analyze the kinetic parameters, thermodynamic parameters and pyrolysis mechanism, and the product distribution was characterized. The results showed that the average activation energy (E_α) of RA was 173.82 kJ/mol (KAS method) and 176.57 kJ/mol (FR method), with the average Gibbs free energy change (ΔG) of 171.68 kJ/mol, enthalpy change (ΔH) of 171.63 kJ/mol, and entropy change (ΔS) of −0.07 J/(mol·K). After alkali treatment, the average E_α of RB increased to 180.18 kJ/mol (KAS method) and 186.67 kJ/mol (FR method), along with the average ΔG of 180.50 kJ/mol, ΔH of 181.49 kJ/mol, and ΔS of 1.58 J/(mol·K). The pyrolysis mechanism of RA evolved from the three-dimensional diffusion model (D3) to the random nucleation model (F2), while RB followed the D3 model throughout the entire pyrolysis process. Through comparison with individual feedstocks, RA exhibited a complex coexistence of positive (lowered E_a and ΔH) and negative (elevated temperature) synergies, whereas RB showed predominantly negative synergy with elevated E_a, ΔG, ΔH and a shifted temperature range to 260–407°C, suggesting that alkali treatment shifts the predominant synergistic effect from coexistence to a predominantly inhibitory mode. In terms of product distribution, the biochar proportion of RB (36.70%) was markedly higher than that of RA (24.32%), while the bio-oil proportion of RB (31.12%) was lower than that of RA (44.44%). The average pre-exponential factors (A) of RA and RB were 2.0 × 10¹⁶ s^-1 and 1.8 × 10¹⁵ s^-1, respectively. This study shows that alkali treatment slightly increases the pyrolysis difficulty of the rice-wheat straw blends, but can effectively regulate the product distribution and promote biochar formation, providing theoretical support for the thermochemical conversion and efficient utilization of rice straw and wheat straw.