| Sewage sludge is the solid residue produced in the process of wastewater treatment. The production of sewage sludge has been gradually increased along with the rapid development of urbanization and perfect construction of wastewater treatment facilities. Sewage sludge has the characteristics of high moisture content, poor mechanical properties, containing a lot of poisonous and harmful substances as well as covering a big area. If not properly disposed, it would bring serious problems to environment.With the change of people’s conscious and the progress of technologies, kinds of so-called "pollutant" have been considered as "resources located in wrong places", and being recycled. Researches have indicated that sludge contains a lot of organic matters accounting for about50%of the dry weight, and it could be adopted as energy due to that its calorific value is the same as that of lignite. Up to now, the major methods of energy-oriented utilization of sludge have included anaerobic digestion, microbial fuel cell, gasification and pyrolysis, combustion and co-combustion. Wherein, the combustion and co-combustion with the advantage of the realization of direct energy conversion and relatively mature equipments and technology, is considered as the most economically viable way for utilization of sludge as energy nowadays.With the purpose of energy-oriented utilization of sewage sludge without increase of energy consumption, the paper adopted the process of sludge-coal mixed and molding, drying, heat utilization, and ash utilization, and consequently realized the recycle of sewage sludge and fuel ash.The main obstacle of sewage sludge combustion is the80%moisture content. By means of compound preparation into composite molding fuel, the mixture of coal and sludge can achieve the balance of energy supply in the combustion process, and can simultaneously accelerate the drying of sludge in the mixture. Experiments were conducted in this paper including the effects of initial water content, amount of addition of sewage sludge and moulding pressure on characteristics of combined fuel, as well as the drying characteristics of the combined fuel at different temperatures. Results showed that the optimized conditions for moulding combined fuel were60-70%of initial moisture content of sludge and70-80%of solid content in the mixture of sludge and coal. The drop shatter had been less affected by moulding pressure at range of10-30MPa. Combined fuel prepared from sludge had good strength of drop shatter, which was satisfied with the commercial leve of briquette. Compared with the sewage sludge alone, the moulded sludge-coal could be dried more easily and quickly at room temperature or lower than100℃. Thus, the purposes of rapid dehydration of sewage sludge as well as its utilization as a potential energy were achieved.Thermal characteristics of sludge, coal and mixture fuel were analyzed under air and N2condition, respectively. The parameters of total loss of weight, weight loss rate, heating effect, combustion and pyrolysis parameters, coke formation were studied. Results showed that compared with coal, sewage sludge had higher content of ash and volatile, however, combustion and pyrolysis temperature were lower than those of the coal. The addition of sludge improved the ignition temperature and activation energy of the combined fuel. Sludge or coal was expressed its own thermal characteristics during combustion and pyrolysis process of the combined fuel, and no obvious interaction was observed. The content of coke in the sludge was lower than that in the coal, and was completely decomposed at1000℃.The emission of pollutants during the combustion process of sewage sludge, coal and the combined fuel was studied, including the characteristics of SO2and NOx emissions under different ratio of sludge/coal and temperatures and also the leaching characteristics of heavy metals in the ash. Results showed that addition of sludge could maintain sulfur and nitrogen retention during combustion process of the combined fuel. The absolute quantities of SO2and NOx emission were increased because of the higher content of S and N in the sludge, but the capture efficiency of sulfur and nitrogen was enhanced with the increasing addition of sludge in the combined fuel. Leaching of heavy metal from the ash satisfied the national related standards.The combustion ash of sludge and combined fuel were prepared for ceramsite products under different conditions with two step sintering process, respectively, and different performances of haydite were obtained. Water absorption and bulk density were characterized, and the mineral phases of raw materials and the formed haydite were determined by X-ray Diffraction (XRD). Results showed that for the sludge ash, the products of haydite with the range of45.32-4.11%of water absorbent and1.67-0.84g/cm3of bulk density, respectively, were obtained by the control of material formula and process conditions such as temperature and sintering time. As for the ash of combined fuel, products with56.35-1.12%of water absorbent range and1.609-0.758g/cm3of bulk density, were obtained at1150℃and sintering time range of5-30min. Higher sintering temperature and longer sintering time were needed during the process of sintering the ash of the combined fuel as compared with the ash of sludge. XRD analysis showed that material phase changed with the increase of temperature, the peak of quartz phase obviously decreased, and the phases such as mullite, rhombic sodium zeolite and amorphous appeared. Sintering temperature of combined fuel ash increased might be related to the presence of mullite content.By means of the processes discussed in the paper, the purpose of energy-oriented utilizationof sludge could be realized, which can make full use of the existing combustion equipments, and avoid the defects of high moisture content of the sludge. The methods in this paper perform good practicability and popularization.Meanwhile, the final ash of sludge and combined fuel can be adopted for preparation of ceramic materials, and consequently achieve the sludge "zero emissions" and complete utilization as resources. |
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