The effect of preparation process on combustibility of Robinia pseudoacacia charcoal and its structure-performance relationship

Using Robinia pseudoacacia (RP) branches as raw materials, charcoal was prepared in a nitrogen atmosphere. The optimal conditions for preparing charcoal were determined based on energy yield, mass yield, and combustion characteristics (R0.5), ignition characteristic index (Di), comprehensive combustion index (S), flame stability combustion index (Fith), and combustion time (tc) in co-combustion with anthracite, which turned out to be a pyrolysis temperature of 500 °C, a heating rate of 4 °C/min, and an isothermal residence time of 60 min. Elemental analysis, X-ray Diffraction (XRD), and Raman fitting were used to analyze the microstructure of the RP Charcoal (RPC). Under optimal conditions, the charcoal exhibited numerous active sites, high disorderliness, large microcrystalline size, maximum activity, and high energy efficiency. It exhibited the lowest ignition temperature when co-combusted with anthracite. Its single combustion followed the F1 mechanism model, with an activation energy of 87.27 kJ/mol. The structure-activity relationship between the microstructure parameters of RPCs and their combustion performance indicators was established. R0.5 increased linearly with the crystalline carbon content (A(D1+G)/Aall), the heteroatom nitrogen and sulphur content (n(N+S)/nC), and the carbon microcrystal interlayer spacing (d002). It decreased linearly with the amorphous carbon content (A(D3+D4)/Aall) and the stacking thickness of the crystal plates (Lc). Di showed a parabolic change with the increase in the Lc, reaching a minimum value at 0.94, indicating that thicker carbon crystal plates result in poorer combustion performance, making it harder to ignite, with the best ignition performance at Lc = 0.94. Di also decreased linearly with the increase in the heteroatom nitrogen and sulphur content (n(N+S)/nC). The S and Fith decreased linearly with the increase in the heteroatom oxygen, nitrogen, and sulphur content (n(O+N+S)/nC) and proximate analysis (PA) but increased linearly with the carbon content, indicating that nitrogen and sulphur heteroatoms are detrimental to combustion. Both indices showed a parabolic change with the increase in the Lc and the d002, reaching minimum values at Lc = 0.91 and d002 = 0.398, indicating the poorest comprehensive combustion performance and combustion stability at these points. The tc exhibited a parabolic change with the oxygen content (nO/nC) and the d002, reaching a minimum value at nO/nC = 0.022 and tc = 0.365.