Formation And Emission Of PAHs From Coal Partial Gasification And Residual Char Combustion In Fluidized Bed

The second generation pressurized fluidized bed combustion combined cycle (2G PFBC-CC) based on coal partial gasification (CPG) and residual char combustion (RCC) is one of the promising coal clean technologies in the future. The hazardous organic pollutants emission from 2G PFBC-CC system, such as polycyclic aromatic hydrocarbons (PAHs), have been paid considerable attention. Formation and emission of PAHs is one of the very active fields in modern research of energy and environment.To investigate the formation and emission of PAHs in CPG and RCC for controlling the pollution of PAHs in 2G PFBC-CC system, the CPG and RCC were carried out in two bench-scale test rigs and two pilot-scale test rigs at atmospheric and elevated pressure, respectively. After Soxhlet extraction and Kuderna-Danish (K-D) concentration, the 16 PAHs specified by US EPA were analyzed by a high performance liquid chromatography (HPLC) coupled with fluorescence and diode-array detection.The effects of coal, air/coal ratio, steam/coal ratio, bed temperature and additives on the formation and emission of PAHs from CPG were studied in an atmospheric bench-scale fluidized bed. The results show that the total-PAHs produced in CPG are higher than in coal. The low and middle molecular weight PAHs dominate in the gas phase while the PAHs in solid phase are dominated by the middle and high molecular weight PAHs. The formation and distribution of PAHs is affected deeply by the inherent characters of coal. For air/coal ratio, steam/coal ratio and bed temperature, the formation of PAHs and CPG have a common optimal reaction zone. The use of additives not only increases carbon conversion efficiency, but also improves the formation of PAHs. It is also found that the PAHs concentration is in direct proportion to monoxide carbon content, and the fabric filter has better removal efficiency for PAHs than the cyclone.Based on the tests in the bench-scale and pilot-scale facilities at elevated pressure, the influence of pressure on the formation and emission of PAHs was investigated and proved. The proportion of low molecular weight PAHs in gasifying products is higher than that at atmospheric pressure. The total-PAHs produced in pressurized CPG are more than in coal. The yield of PAHs increases with the rise of pressure.The RCC at elevated pressure can effectively destroy PAHs in char and prevent PAHs from forming again. PAHs in fuel gas are difficult to be oxidized and decomposed during combustion, but the toxicity equivalent quantity (TEQ) of PAHs drops. Limitation of PAHs in fuel gas or transfer of PAHs from fuel gas to char during CPG can realize the minimum emission of PAHs in 2G PFBC-CC system.The total-PAHs TEQs in CPG and RCC are dominated by the five-ringed PAHs, and benzo(a)pyrene and dibenzo(a,h)anthracene are two main contributors of PAHs TEQ.Three PAHs formation theories are considered possible in CPG and RCC:â‘ from the undecomposed PAHs originally present in the feedstock;â‘¡from the pyrosynthesis PAHs at high temperature;â‘¢from the polymerization of free radical at high temperature. PAHs from CPG mainly come from Methodâ‘¡andâ‘¢while PAHs from RCC mostly are derived from Methodâ‘¢.Based on the multivariate statistic analysis and SAS software, the predicted model of PAHs in coal and gasifying products has been constructed. The values calculated from the predicted model are basically consistent with the measured values.