Hydrogen-rich Gas Production From Catalytic Gasification Of Municipal Solid Waste (MSW) With In-situ Steam Agent

With the shortage of fossil fuels and the environmental pollution during its utilization, people are more focusing on the development and utilization of a clean energy-hydrogen. Moreover, the amount of municipal solid waste (MSW) increases dramatically, which leads to serious environmental pollution. Therefore, the treatment of MSW is becoming an urgent problem to be solved. The organic components in MSW with high content of hydrocarbon can be considered as a renewable energy. The application of MSW pyrolysis-gasification technology not only reduces MSW but also recovers hydrogen energy.In this thesis, MSW with a certain content of moisture is used as raw material. The water in MSW turns into steam at high temperature, forming an auto-generated steam atmosphere. The steam produced can be used as gasifying agent during the gasification of MSW. CaO is added into the raw material for hydrogen production with in-situ CO2removal and catalytic cracking of tar at high reaction temperature. Since the tar yield is still high under this condition, the NiO supported on modified dolomite catalyst is developed for further catalytic cracking of tar and hydrogen-rich gas production. According to this process, the following work was carried out in this thesis:(1) The organic components of MSW including fabric, sawdust, paper and plastic were selected as raw material. The proximate analysis shows that the MSW is rich in volatile matter and fixed carbon, but the ash content is lower than2wt%. From the ultimate analysis of MSW, the main elements are C, H and O. The lower heating value of MSW is17.09MJ/kg.(2) Through the TG/DTG/DTA method, the pyrolysis characteristics of each organic component and mutual influence of the various components in the pyrolysis process were discussed respectively, and the thermogravimetric behavior of MSW sample was analyzed and pyrolysis kinetic parameters were calculated too. The results show that there is only one main weight loss stage during the individual pyrolysis of fabric, plastic and wood, while there are two mass loss processes appeared in waste paper pyrolysis. Studying the co-pyrolysis characteristics of the mixed components, it is found that when the composition of each component is similar, the mixing pyrolysis does not subject to the mutual influence between the single-component. If the composition of each component varies greatly, the overlapping of the conversion stage temperature distribution in the individual component pyrolysis process becomes the main influence factor of mixed pyrolysis. Thermogravimetric analysis of MSW sample indicates that the cellulose, hemicelluloses and some plastics are decomposed in low temperature zone (240～385℃). The second weightlessness peak between385～500℃is mainly attributed to the degradation of plastic and lignin components. The pyrolysis kinetic parameters of MSW were calculated by iso-conversional methods. The apparent activation energy of MSW pyrolysis is in the range of170～225kJ/mol.(3) The characteristics of MSW gasification with in-situ steam agent has been systematic studied, which was carried out in a lab-scale fixed bed. The effects of heating rate, temperature, moisture content, and residence time on product distribution and gas composition were investigated. The material balance and energy analysis of this gasification process were also studied to discuss the feasibility of MSW in-situ steam gasification process. The results show that the fast heating method improves the quality of gas and reduces the yield of tar. The gas yield increases with temperature rising, but the yield of tar and char show an opposite trend. Meanwhile, the contents of H2and CO increase, while those of the other gases such as CO2, CH4and C2hydrocarbon gas decrease. When the moisture content of MSW is39.45wt%, the highest value of H2content in gas production is25.8vol%. The flow rate of N2can indirectly reflect the gas residence time, and the H2content increases from22.84vol%to28.49vol%as N2flow rate reduced. The error of material balance is6.8%under this experimental condition. The energy evaluation on the gasification process showed that cold gas efficiency, energy recovery and steady-state theoretical energy consumption ratio are54.24%,85.56%and2.78, respectively.(4) The TG-MS was used to analyze the pyrolysis characteristics of MSW with CaO addition. Meanwhile, hydrogen-rich gas production from in-situ steam gasification of MSW with CaO addition was experimental investigated. The results show that the addition of CaO is favorable to the pyrolysis of MSW in lower temperature. Adding CaO in MSW can reduce the escape of tar components and weaken the escape intensity of CO2. With the [Ca]/[C] ratio increasing from0to1.5, the hydrogen content and hydrogen yield increase from25.89%to45.90%and10.86g H2/kg MSW to31.56g H2/kg MSW, respectively. The introduction of steam can improve the activity of CaO carbonation reaction, which promotes the hydrogen production. However, the higher content of moisture would reduce the quality of gas production, and the optimal moisture content of MSW is found to be39.45wt%. Higher temperature could strengthen the thermal decomposition of MSW, which is greatly benefit for hydrogen production but unfavorable to CaO carbonation reaction. The best operating temperature is in the range of700-750℃.(5) The NiO supported on modified dolomite (NiO/MD) was prepared by deposition-precipitation (DP) method. The effect of catalyst on hydrogen production from catalytic gasification of MSW with in-situ steam agent was investigated in a two-stage fixed bed reactor. The GC-MS was used to analyze the main chemical components of tar product with different catalysts. The results show that the NiO/MD catalyst is better at catalytic activity and stability than that calcined MD and NiO/γ-Al2O3catalyst. With the NiO/MD catalyst, the removal ratio of tar is over90%, and the content of hydrogen in gas production reaches52.79vol%. As the catalyst changed from calcined MD to NiO/γ-Al2O3and NiO/MD, the content of PAHs in tar components continues to decrease, while that of single-ring aromatic increases. The higher catalytic temperature could favor the steam cracking and tar reforming, and promote H2and CO production.