Effects of inorganic sodium on the combustion characteristics of Zhundong coal with fast-heating rate.

• Inorganic sodium inhibits coal combustion in early stage under fast-heating rate. • Ignition delay time of NaCl-loaded coal increases visibly under fast-heating rate. • Evolution pathways of condensation reactions catalyzed by Na are studied by DFT. • Na in studied coal reduces energy barrier of condensation reactions in early stage. The combustion characteristics of raw (RAW), acid-washed (AW) and inorganic sodium impregnated (NaCl-loaded) Zhundong (ZD) coals were researched in a fast-heating rate combustion system and thermal gravimetric (TG) analyzer, aiming to exhibit the effect of inorganic sodium on coal combustion processes under different heating rates. The results showed that the ignition delay time decreased in the order: NaCl-loaded coal > AW coal > RAW coal in the early stage with fast-heating rate. The surface temperature of NaCl-loaded coal increased the slowest, while with the highest T is , reflecting that the ignition and combustion behavior became more difficult after adding NaCl in the early stage. However, in the later stage, the fastest increase in temperature and the shortest t b , together with the shorter extinction time of volatile flame, were observed on NaCl-loaded coal. The size of NaCl-loaded coal decreased faster than that of AW coal. Thus, it can be concluded that inorganic sodium will inhibit coal combustion in the early stage, while promotes coal combustion in the later stage under fast-heating rate. In the TG experiment, the T i-TG and T b-TG of NaCl-loaded coal were lower than that of AW coal, and the T m-TG of NaCl-loaded coal moved to the low temperature area, revealing that inorganic sodium can promote coal combustion with low-heating rate during the whole combustion process. The quantum chemistry calculation at M06-2X/6-31G(d,p)//CBS-QB3 level reveals that inorganic sodium can catalyze the addition and cyclization reactions of small chain molecules in the early stage with fast-heating rate, thereby inhibiting coal combustion in the early stage, which was different from that in the later stage. Inorganic sodium will promote the formation of OH radical, and the organic sodium obtained from sodium-containing surface compounds of the char can be beneficial for the oxidation reaction of the carbon residue, thereby promoting coal combustion in the later stage with fast-heating rate, which was consistent with the references. [ABSTRACT FROM AUTHOR]