Effects of nickel additives with different anions on the structure and pyrolysis behavior of Hefeng coal

The effects of nickel salts with different anions as the additive on structure and pyrolysis behavior of Hefeng coal were studied by using a thermogravimetric analyzer (TGA) and a fixed-bed reactor. Four Ni2+ salts (Ni(NO3)2, NiCl2, NiSO4 and Ni(C2H3O2)(2)) with the same mass ratio of Ni2+ to coal being 0.01 were impregnated in de mineralized coal (DC), separately. Elemental analysis, FTIR, SEM, simulated distillation and GC MS were adopted to characterize the structural characteristics of char and the quality of tar. The ion exchange reaction between Ni2+ and hydrogen-containing functional groups in coal occurs, and the surface of Ni-loaded coal samples becomes rough. The anion of nickel salts obviously influences the coal pyrolysis. The total weight loss decreases while the peak intensity of CH4 obviously increases for NiSO4-loaded DC sample. The char yield of all Ni-loaded coal samples increases but the hydrogen content in all char decreases compared with that in DC-Char. The addition of NiCl2 inhibits the evolution of volatiles, resulting in a 19.73% decrease compared with DC. The addition of NiSO4 results in a 4.33% increase of tar yield although gas yield decreases 22.35% owning to the inhibition of CO production. Part of nickel species could be reduced to metal Ni by the produced char and/or reductive gases during coal pyrolysis, but reduction temperature is different for various Ni species. In comparison with other nickel salts, NiSO4 plays a major role to affect the volatile release. Under the influence of these metal species, the percentage of light components in all of the tar increases, especially a 27.32% increase for NiSO4. GC MS analysis exhibited that long chain alkanes account for about 70% of the tar, while Ni (C2H3O2)(2) and NiSO4 could promote the cracking of long chain alkanes. The highest fraction and yield of light tar reveal that NiSO4 is in favor of tar upgrading. This work provides a potential route to control products distribution and tar quality of coal pyrolysis by using appropriated catalyst precursor.