| Sludge is being generated from municipal sewage treatment plant in an ever-increasing amount due to the rapid urbanization and higher effluent criteria implemented in recent decades. However, how to dispose of tremendous sewage sludge to eliminate the potential hazardous effect on environment has become a serious challenge with which people are currently confronted. Pyrolysis is receiving more and more attention as an economic efficient and environmental friendly route to waste treatment. Pyrolysis of sewage sludge is a promising thermal conversion technology aiming at sludge reduction, stabilization, detoxification and recycle. With the support of the cooperation between State Key Laboratory of Coal Combustion in Huazhong University of Science and Technology (China) and Institute of Environmental Science and Engineering in Nanyang Technological University (Singapore) on the project of high efficient conversion of municipal sewage sludge to energy, a detailed research on pyrolysis of sewage sludge samples from Hong Kong was carried out.The pyrolysis of three types of sewage sludge samples from primary and/or secondary treatment process was investigated using thermogravimetry analyzer (TGA), and their kinetic parameters were calculated through Coats-Redfern integral method. The influence of sample mass, particle size and heating rate was analyzed. The product gas was detected on-line using Fourier Transform Infrared Spectrometer (FTIR) coupled with TGA. Moreover, the catalytic effects of inorganic mineral matters, including Al2O3, CaO, Fe2O3, TiO2, and ZnO were studied due to the high ash content of sludge samples. It shows that TiO2 promotes the pyrolysis of sewage sludge; CaO, Fe2O3, and ZnO mainly inhabited the pyrolysis, while the effect of Al2O3 on pyrolysis was neglectable. The pretreatment of sludge demineralization was helpful for the pyrolysis of sewage sludge.To get further information of the forming characteristics of gas products, liquid oil and solid char from pyrolysis of sewage sludge, the experiments on the pyrolysis of three types of sludge samples were performed in a fixed bed reactor under N2 atmosphere from 500℃to 900℃. The pyrolysis products were analyzed with Micro-GC, GC-MS, FTIR, SEM, ASAP 2010, ICP-OES and so on. The optimal operating condition for pyrolysis products with high efficiency of energy recovery was suggested. With the increase of final pyrolysis temperature, the yield of char decreased, while that of product gas increased. The maximum yields of bio-oil were obtained at 600℃for all the three types of sewage sludge samples. The content of volatile matter in char decreased as the temperature increasing, and the heating value of char also decreased, however the content of ash increased. The porosity of char was developed during pyrolysis and the fractal dimension increased with the upgoing temperature, which was beneficial for sludge pyrolysis. The composition of bio-oil was very complex. With the temperature increasing, oxygen-containing compounds in bio-oil decreased, while aromatic compounds increased. The major compounds in bio-oil obtained at 600℃were acetic acid, 3-penten-2-one 4-methyl-, and phenol. The pyrolysis gas was mainly H2, CO, CO2, and CH4 with few of C2H4, C2H6, and other hydrocarbons. The concentrations of H2 and CO increased with the temperature increasing, while the concentration of CO2 decreased, which indicated that higher temperature enhanced the formation of H2 and CO. The optimal pyrolysis temperature for three studied sludge samples was 900℃, 800℃, and 600℃, respectively.The potential secondary pollutants are critical to the energy recycle of sewage sludge, therefore the emission of H2S and fate of heavy metals in the pyrolysis of sewage sludge were investigated, meanwhile, the leaching behavior of heavy metals from chars was also studied. The maximum yield of H2S was found at about 350℃, after that the yield of H2S decreased. The release of H2S was related to the amount of sulfur and its existing status in raw sludge. Most of heavy metals (Cu, Mn, Ni, V, and Zn) were fixed in the char during pyrolysis of sewage sludge, except Cr and Co, which were possibly emitted to gas phase. It was found that the amount and rate of heavy metals leached from chars were very low with pH value ranging from 5 to 9.The kinetic property of sewage sludge pyrolysis was simulated with three-component kinetic model, moreover the gas distribution was simulated with thermodynamic equilibrium model. The simulation results agreed very well with the experimental ones, which would be beneficial for optimization of reactor design.
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