Hydrogen-rich Gas Production Characteristics During Microwave-induced High Temperature Pyrolysis Of Sewage Sludge

Nowadays, energy shortage and enviorment pollution are the key problems restricting the development of the society. It is urgent to develop a clean and efficient technology. Biomass is a kind of clean energy with high abundance, and it is widely researched in the worldwide. Sewage sludge can be treated as a type of biomass and has the potential for energy-oriented use due to its high organic content and heating value. As an economic and environmentally acceptable route for waste disposal, microwave pyrolysis is becoming one of the most promising methods for sludge utilization and disposal.In this paper, microwave pyrolysis of sewage slugde from Shanghai has been conducted. Firstly, the temperature evolution during microwave treatment of wet and dry sludge,as well as the microwave adsorbing performance of active carbon, SiC and graphite were detaild studied. Secondly, pyrolysis of sewage sludge were conducted on a single-mode microwave reactor and an electric-heated tube furnace, respectively. The effects of particle size, moisture content, pyrolysis temperature, and structure of microwave receptor on the yield and composition of hydrogen-rich gas were studied. Finally, the experiments of sludge high temperature steam gasification and catalytic pyrolysis of CaO, ZnCl2were carried out in a single-mode microwave reactor, and the production composition and distribution were investigated. The main conclusions of this paper are lised as follows:(1) It was found that the SiC powder, graphite powder and activated carbon powder are able to absorb microwave energy effectively, thus they can be treated as good microwave absorbing materials. Activated carbon (microwave absorber) of fixed shape is also effective in absorbing microwave energy. The temperature of the activated carbon can reach about862℃when the microwave input power was1056W. It was found that when the raw sludge without microwave absorber was put into the microwave oven, the sludge can only be dried, and no pyrolysis of sludge will take place. Therefore, the addition of microwave absorber into sludge sample is necessary for achieving high temperature pyrolysis of sludge.(2) The sludge particle size in the range of0-5.0mm had no obvious effect on the mass distribution of pyrolysis products. However, the H2and CO concentrations will increase with decreasing of sludge particle size. When sludge particle size decreases from2.5-5.0mm to<0.45mm, the H2concentration will increase from31vol.%to34vol.%, and the CO concentration will increase from17vol.%to22vol.%. The results also indicated that both the sludge moisture content and the pyrolysis temperature had great influences on the distribution of the pyrolysis products. Higher moisture content or higher pyrolysis temperature will lead to markedly higher concentrations of Hi and CO. When the sludge moisture content increases from0to83wt.%, the H2concentration will increase from32vol.%to42vol.%, and the CO concentration will increase from20vol.%to31vol.%. Compared with microwave adsorber in powder phase, the microwave adsorber in fixed structure can transform more volatile compounds to uncondensable gas, and the concentrations of H2and CO also increased slightly.(3) It was found that the water vapor favored the steam reforming reaction of volatile compounds, and promoted the production of hydrogen-rich gas.In the microwave pyrolysis process, the addition of water vapor increased the volatile matter content of the pyrolysis product, and the content of solid residue was reduced. The results also indicated that the temperature and steam flow rate had great effects on the component of the gas product. The yield of hydrogen was obviously promoted by increasing the pyrolysis temperature or by injecting steam. However, the heating value of the gas product was decreased with the increased injecting content of steam at high temperature.(4) It was found that CaO and ZnCl2were able to markedly reduce the activation energy of sludge pyrolysis, which can effectively decrease the sludge pyrolysis temperature, the results also indicated that the CaO and ZnCl2can promote the production of H2and CO. The CaO can react with CO2emitted from sludge pyrolysis, thus greatly reduced the CO2concentration in gas product gas and promoted the production of H2.