Effects of inorganic sodium on the soot generation of coal particle: Insights with PLIF and DFT calculation

Inorganic sodium in low rank coal exerts significant influence on the soot generation in the early stage of combustion. The effect of inorganic sodium on the PAHs, soot formation and OH generation of coal particle was investigated on a concentrating light heating platform using Planar Laser Induced Fluorescence (PLIF) of OH radicals and PAHs. The generation characteristics of PAHs during coal particles pyrolysis with the temperature of 500 °C and 800 °C were also investigated. Raw coal, acid-washed (AW) coal and AW impregnated with different mass ratio of NaCl (1 %, 3 % and 5 % NaCl-loaded) were studied. Theoretical calculations using density functional theory was performed to explore the effect of sodium on the conversion of small molecule to macromolecular substances during early coal combustion. The results showed that the NaCl addition inhibited the OH radical and soot generation in the early stage of coal combustion, as demonstrated by the reduced OH intensity, and decreased soot intensity by observing the flame luminosity for the NaCl-loaded coals, compared with AW coal. Simultaneously, the PAHs generation was significantly reduced for NaCl-loaded coals, both at the early stage of coal combustion and under pyrolysis conditions, compared with AW coal. The quantum chemistry calculation revealed that the formed C-Na structure enhanced coal structural stability, making the chemical bonds less prone to cleavage, thereby inhibiting the volatile and PAHs release during the early stage of coal combustion. On the other hand, the released Na+ can catalyze the addition reactions of small ring PAHs (naphthalene, naphthalene derivatives) to larger ring PAHs (phenanthrene, pyrene), which would promote the PAHs and soot generation in homogeneous reactions, explaining the highest PAHs with 3-to-5 rings for 5 % NaCl-loaded coal among NaCl-loaded coals. The above two kings of reaction pathways were related to soot formation, but considering the results of reduced soot yield after NaCl addition, the former dominated. Furthermore, the results from the two pathways also illustrated that the Na inhibit the OH formation, which was corresponding to the decreased OH intensity for NaCl-loaded coals, as detected by OH-PLIF, and thus was detrimental to soot consumption. Therefore, the presence of Na in the early stage of combustion inhibited the release of volatiles and PAHs, which was the main reason for the decreased soot generation amount of NaCl-loaded coal.