The Oxygen Migration Regularity During Lignite Pyrolysis

Grading lignite pyrolysis technology is a more efficient and cleaner way for lignite conversion. Pyrolysis is an important initial unit for this technology. Lignite, with high oxygen content, the oxygen-containing functional groups in it influence its pyrolysis conversion processes and utilization of the downstream products directly. It has important academic significance and potential application value to study the oxygen migration characteristics and influencing factors in pyrolysis process for the oriented conversion of oxygen under different technical conditions.The effect of operating conditions(temperature, heating rates) and particle size on the oxygen migration characteristics were investigated on a fixed-bed reactor. The evolutions of main oxygen-containing functional groups in lignite during pyrolysis were analyzed by FT-IR and chemical titration. And the effect of oxygen migration during hydro-pyrolysis was also discussed. Moreover, solvent pretreatment and acid pretreatment affect oxygen migration under the conditions of isothermal pyrolysis and solid heat carrier pyrolysis were investigated. The main results of this paper are obtained as follows:1. Pyrolysis temperature has a significant effect on oxygen migration: the proportion of oxygen transformed into pyrolysis water and gas increased with increasing pyrolysis temperature. After 650 °C, the trend of oxygen migrated into pyrolysis water slowed down. With the increasing pyrolysis temperature, the proportion of oxygen migrated into tar first increased until 650 °C then decreased. 70.71% of oxygen in lignite migrated into volatile products when lignite pyrolyzed at 800 °C. Carboxyl group had poor thermal stability during pyrolysis: its fastest decomposition rate was in the range of 250 °C-400 °C, and there had no carboxyl group exist in solid products obtained at 600 °C. The decomposition of phenolic hydroxyl mainly occurred at 450 °C-600 °C. For carbonyl and ether linkage, their contents firstly decreased, then increased to maximum, and then decreased with the increasing pyrolysis temperature.2. For temperature-programmed pyrolysis and isothermal pyrolysis, pyrolysing small particle coal obtained more oxygen in solid products and the content of oxygen that migrated to tar also increased slightly. The impact of coal particle size was not obvious under solid heat carrier pyrolysis. At the same pyrolysis temperature, heating rate was the main factor that affected the oxygen migration.3. Providing external hydrogen during pyrolysis could increase the tar yields both for raw coal and the extracted coal. This effect was more obvious on the extracted coal. Meanwhile, more oxygen migrated into tar if external hydrogen was supplied during pyrolysis.4. Both of solvent-swelling and acid-washing pretreatment could increase the tar yield and deceased the yield of pyrolysis water. For swelled coal, more oxygen migrated into volatile products(the proportion of oxygen in water is the majority during isothermal pyrolysis, while more oxygen remained in solid product during solid heat carrier pyrolysis). Acid-washing could both improve the oxygen content in tars that obtained from isothermal pyrolysis and solid heat carrier pyrolysis. Acid-washing inhibited oxygen migrate to water and CO2.