| Due to the ever increasing amount of sewage sludge and increasingly stringent environmental standards, the problem of sewage sludge disposal is becoming more serious than before. Meanwhile, as a type of biomass, sewage sludge has the potential for energy-oriented use because of its high organic content and heating value. As an economic and environmental acceptable route for waste disposal, pyrolysis is one of the most promising methods of sludge resource utilization in the future and has attracted increasingly worldwide attention. The moisture content of sewage sludge has an important effect on the pyrolysis process. On the one hand, the increasing moisture content improves the energy consumption of pre-drying process, leading to a cost increase in the pyrolysis; on the other hand, the presence of water in the raw sewage sludge favors the production of pyrolysis gas and the generation of hydrogen-rich fuel gas. This thesis studied the pyrolysis process of wet sewage sludge at high temperatures which completing the drying, pyrolysis and gasification of wet sewage sludge in a single thermal process. The main findings of this study are summarized as follows:The effects of heating mode, temperature, moisture content and heating rate on the characteristics of products yield and gas composition were analyzed in a tubular furnace. The results indicate that the main components of pyrolysis gas include H2,CO,CO2,CH4,C2H2,C2H4,C2H6,C3H6 and C3H8. High quality hydrogen-rich fuel gas can be obtained when wet sewage sludge is sent into the reactor quickly at the setting temperature. High temperature is an advantageous circumstance for reducing the yields of solid and liquid and increasing the production of hydrogen-rich gases. The increase of moisture content favors the steam reforming of the volatile compounds and a partial gasification of the solid char residue, which leads to the generation of H2 and other fuel gases and the heating value of generated gas could reach over 12 MJ-m-3. Additionally, high heating rate prolongs the residence time of the volatiles in the hot zone, resulting in an increase in non-condensable gases and a decrease in the yields of tar and char.The kinetic experiments of dried sewage sludge and wet sewage sludge with a moisture content of 84 wt% were carried out by using thermogravimetry-differential thermal analysis (TG-DTA) at different heating rates. The TG curves of dried sludge have an obvious weight loss peak, indicating that the pyrolysis process of dried sewage sludge can be divided into 3 stages:dehydration, main stage of pyrolysis and carbonization stage; while the TG curves of wet sewage sludge have two significant weight loss peaks, indicating that its pyrolysis process can be divided into 4 stages:dehydration, early stage of pyrolysis, main stage of pyrolysis and carbonization stage. The pyrolysis process should be promoted by high heating rate, although the effect of high heating rate on the pyrolysis of wet sewage sludge is much more significant than that on the pyrolysis of dried sewage sludge. The Coats-Redfern method was employed to seek the most probable kinetic mechanism function of the main stage of pyrolysis from 11 kinetic functions. The Malek method was used to screen the best mechanism equation, and the kinetic parameters were calculated according to the mechanism function determined. With a sufficient pyrolysis, the effect of heating rate on the activation energy of the main stage of pyrolyzing dried sewage sludge is insignificant; however, the activation energy of the main pyrolysis stage of wet sewage sludge increases with heating rate increasing. Moreover, the activation energy of the main pyrolysis stage of wet sewage sludge is proved to be much higher than that of dried sewage sludge at the same heating rate.Sewage sludge with different moisture content was pyrolyzed at different temperatures in a tubular furnace. The distribution and composition of pyrolysis products were detected by GC-MS and FTIR, respectively, in order to investigate the influence of temperature and moisture content on the characteristic of pyrolysis products. Further, a preliminary mechanistic understanding of wet sewage sludge decomposition from starting constituents to final products was developed. The results show that the high moisture content of wet sewage sludge favors the steam reforming reaction of volatile compounds, which promotes the production of hydrogen-rich fuel gas and reduces the generation of toxic substances and coking phenomena. Meanwhile, the presence of moisture favors the ejection of the volatile matter of sewage sludge, and reduces the organic content of char. Normally, a steam-rich atmosphere would be generated as soon as the wet sewage sludge is sent into the furnace at high temperature, which gave rise to in situ steam reforming reaction with volatile compounds and a partial gasification of the solid char reidue, generating H2,CH4,CO2 and CO etc. With temperature increasing, a secondary cracking of large organic compounds would occur, generating a large amount of H2, CO, C2 and C3 hydrocarbons; simultaneously, some PAHs are formed through condensation,cyclization and aromatization of small organic substance, which is accompanied by the release of H2. Therefore, H2 is considered to be an indicator for the tar cracking. The Diels-Alder reaction mechanism was used to explain the PAHs formation.According to the mass and energy balance evaluation of the pyrolysis process of dried sewage and wet sewage sludge with a moisture content of 84% at 1000C, the error of mass balance evaluation of dried and wet sewage sludge pyrolysis are 1.0% and 6.4%, respectively. The energy consumption (Qp) of wet sewage sludge is much higher than Qp of dried sewage sludge. However, more than 70% of Qp of wet sewage sludge is spent on the vaporization of water and the heating up of steam, and the effective pyrolysis inputs (Qpyrolysis) of wet sewage sludge is significantly lower than Qpyrolysis of dried sewage sludge. The pyrolysis process recovers hydrogen-fuel gas,tar and solid char which can be used as fuels or as sources of chemical feedstock. In addition, the recovery of flue gas waste heat is very important to the technique of wet sewage sludge pyrolysis.In order to expand the pyrolysis form of wet sewage sludge, enhance product additionally value and reduce the cost of pyrolysis process, the effect of blend ratio of wet sewage sludge/straw and dried sewage sludge/straw on the distribution of pyrolysis products, the gas composition and heating value, and gas energy conversion were analyzed in this thesis. The results show that the increase of sludge percent leads to the reduction of gas and tar yield. The gas produced by wet sewage sludge/straw copyrolysis is higher than that of dried sewage sludge/straw copyrolysis at the same blend ratio, because the addition of straw can supplement the carbon source and improve the volatile content, meanwhile, water can be used as gasification agent and hydrogen source, which is conducive to the generation of hydrogen-rich fuel gas. However, futher increase of the wet sewage sludge decreases the volatile content, which leads to the significant reduction of gas yield as well as H2, CO, CnHm production, and consequently decreases the heating value and the energy conversion of gas. |
PDF Full Text Request