Effect Of Pretreatment On The Structure Properties And Pyrolysis Behaviours Of Lignite

Low temperature pyrolysis is one of effective means for lignite utility. However, due to the high contents of oxygen and ash of lignite, the crosslinking reaction between the oxygen-containing functional groups and the exchangeable ions of mineral material acted as the cross-linking site during pyrolysis process, which could produce carbon dioxide and water, and suppress the pyrolysis tar generation. Moisture of lignite needs to absorb heat for evaporation, while it could participle pyrolysis.In this paper, the thermogravimetric analysis and a self-built fixed reactor were used to study the effect of hot pressing drying and fixed reactor drying under different atmosphere on lignite structure and pyrolysis properties. At the same time, effect of acid-washed, thermal pretreatment and solvent swelling pretreatment on lignite pyrolysis was investigated. The results were obtained as follows:1. Effect of acid-washed pretreatment. Acid-washed can eliminate ash and transfer carboxylate into carboxyl, which could suppress crosslinking reaction in pyrolysis, at the same time; new channels were made in lignite surface after acid-washed, which could increase volatile evolution, and reduce secondary pyrolysis. The yield of tar after acid-washed pretreatment was increased by 1.69 percentage points, from 6.88% to 8.57%. However, the quality of tars was decreased. In the same time, acid-washed pretreatment can reduce the yield of water and carbon dioxide.2. Effect of thermal and solvent swelling pretreatment. Thermal and solvent swelling pretreatment can destroy hydrogen bonds of lignite, which can suppress crosslinking reaction in pyrolysis, and reduce the yield of carbon dioxide and water. The average pore diameter of lignite was enlarged and specific surface area was reduced after swelling pretreatment, but no change was found for the main structure. After thermal pretreatment, lignite tar yield was increased by 0.35 percentage points, from 6.88% to 7.23%, and the n-hexane soluble increased 2.18 percentage points. Tar yield of thermal pretreatment acid-washed lignite higher than acid-washed lignite by 0.7 percentage points, from 8.57% to 9.27%, and the n-hexane soluble increased 2.49 percentage points. The yield of tar of the lignite swelling pretreated by methanol, tetralin, and tetrahydrofuran were higher than raw lignite by 0.85, 0.8 and 0.65 percentage points, respectively. Meanwhile, the yield of n-hexane soluble greater than raw lignite by 4.52, 1.36, and 2.2 percentage points, respectively.3. Effect of moisture and drying on lignite pyrolysis.(1) Moisture could participle chemical reaction in pyrolysis, but moisture evaporates are endothermic, which could extend the residence time of volatiles, and intensify secondary pyrolysis. At the beginning, the yield of tar and n-hexane soluble increased, and then decreased with moisture content growing and its reach maximum when moisture content was 17.32%. The yields of carbon dioxide and water were reduced with the increase of moisture content.(2) Drying temperature is one significant of hot pressing drying. Hot pressing drying could destroy hydrogen, and suppress crosslinking reaction in pyrolysis. When temperature of hot pressing drying is 200 oC, lignite tar yield was increased by 0.47 percentage points, from 6.88% to 7.35%.(3) The pore block and shrinkage in lignite when drying under nitrogen and air atmosphere. Non-covalent bond of lignite could break after drying, which make lignite structure steady, and reduce decomposition in pyrolysis. The yield and quality of tar were reduced after drying.