Effects of potassium salt on the combustion characteristics of corn stalk with fast-heating rate: Insights with OH-PLIF measurement and DFT calculation

The effect of potassium salt on biomass ignition and combustion behaviors was investigated using a fast-heating rate platform with Planar Laser-Induced Fluorescence of OH radicals. Acid-washed corn stalks (CS-aw) and CS-aw impregnated with varying mass ratios of K2CO3 (CS-1% K, CS-3% K, and CS-5% K) were studied. Theoretical calculations using density functional theory at the M06-2X/6-31G(d,p)//CBS-QB3 level were performed to reveal the effects of potassium salt on the CS combustion process. The results showed that the order of increasing ignition delay time was CS-aw < CS-1% K < CS-3% K, indicating prolonged ignition of CS with potassium salt, with no ignition occurring for CS-5% K. Similarly, K2CO3-loaded CS samples exhibited lower peak temperatures compared to CS-aw. However, in the later stages of combustion, CS-1% K and CS-3% K had shorter char burnout times and higher char conversion rates than CS-aw. The increase in K2CO3 decreased OH intensity and the duration of the OH signal, but the presence of K promoted the generation of PAHs. This corresponded to the lower combustion intensity of K2CO3-loaded CS samples in the early combustion stage. The TGA experiment indicated that the maximum weight loss temperature for K2CO3-loaded CS samples shifted to a lower temperature range, and increased addition of K2CO3 led to a reduction in burnout temperature. Thus, unlike under low heating rates, the addition of K2CO3 had a dual effect on CS combustion under fast-heating rate: it inhibited combustion in the early stage while promoting it in the later stage. Quantum chemical calculations support this, suggesting that potassium salts promote the formation of larger and more stable hydrocarbons in the early stage of CS combustion under fast-heating rate rather than the cracking of these molecules into active radicals such as OH radicals through H + O2→OH + O reaction, and thus the ignition and early-stage combustion were inhibited for K2CO3-loaded CS samples.