Formation and distribution of polycyclic aromatic hydrocarbons during large size coal pyrolysis

Abstract：Being an important process of coal fires and underground coal gasification, coal seam pyrolysis is a crucial source of PAHs discharging into the air. Four kinds of low rank coals were pyrolyzed in the horizontal tube furnace to simulate coal seam pyrolysis ,and the formation and distribution of the 16 kinds of US EPA prior controlled PAHs at different temperatures were studied. The results show that with temperature increasing, four kinds of large size coals produce PAHs continually and PAHs yield is remarkably affected by temperature. 3-ring PAHs are the main species which takes up more than 80%. The yield of 2-ring PAHs attains the maximum at 400 ~C , and decreases rapidly with increasing temperature. Above 700 ~C, the yield of 4-ring PAHs rises significantly, and the yields of 5-ring and 6-ring are less than 1.0 mg/kg. The formation of the most toxic specie, benzo（a） pyrene, peaks between 500-700℃, and the maximum discharge of Dibenz（ a, h）anthracene occurs at 400℃. Resistance of heat conduction and mass transfer dur- ing the low speed pyrolysis process for the large size coal is the main affecting factor for the formation and discharge of PAHs. With the coal rank increasing, the discharge of toxic PAHs is enhanced. The maximum of TEQ occurs at 400 and 900 ℃.