Production Of Sludge Coke With Microwave Drying Techniques

With the increasing of municipal sewage sludge production, more attention has been paid onhow to effectively dispose of the sludge in recent years. Resources reuse of sludge is considered as afundamental way to reduce the sludge volume. In this paper, possibility of using sludge to producesludge coke by microwave drying techniques was studied. Excess sludge was taken from YanTaiXin’an river Municipal Wastewater Treatment Plant. Impacts of radiation time, microwave inputpower and radiation intensity on the dehydration process were analysed, and also cell composition ofsewage sludge. Tests were carried out to study effect of microwave energy consumption on sludgecoke heat value increasing(ΔNCV). To obtain economical combustible sludge coke, pulverized coalor sawdust were added to ensure the dehydrated sludge coke heat value exceed12500J/g. The mainconclusions were drawn as follows.①The basic characteristics of dewatering sludge were shown as that volatile matter was54.10%, the ash was34.92%, water content was72.70%and moisture content (after centrifugation)was65.37%, which indicated that dewatering sludge possessed the typical characteristics of highwater content, volatile matter and ash and low dewatering performance.②Microwave drying techniques could effectively realize the disposal purpose of reduction,dehydration and freetreatment of sewage sludge. Compared with traditional hot drying techniques,sewage sludge was treatmented more quickly, economically, efficiently and lower energyconsumption by microwave with the condition of longer radiation time, higher input power, lessweight and lower moisture content. The water content of sludge could be reduced to10%.③Along with the time for microwave radiation, cell composition of sewage sludgeconstantly changed in the way of cell cytoderm shrinked and broken and then broken cytodermpolymerizing. After90s, because of serious composition deformation, the most of cells cytodermwere broken so that the water in cell was released and dewatering performance was improved.④The process of microwave drying shew3different stages as preheating speed up dryingstage, uniform drying and speed down drying stage. In spite of different time for every stage, actually,with comprehensive analysis, it shew that the drying speed was related to energy consumption. Thepoints of boundary for3stages were1.0±0.2KJ/g and3.0±0.2KJ/g. ⑤The NCV of sludge coke was increasing with wanter content, and the liner relationshipbetween them was observed as: NCV=(115.1X+11280)J/g. And the highest NCV of sludgecoke was about10000J/g.⑥In this paper, the best condition of microwave drying was that100.0g sludge sample wasdried by800W input power. To maximum utilize the microwave energy and avoid loss of sludgecoke NCV, energy consumption should less than2.95KJ/g (water content higher than18%). Theoptimum energy utilization for sludge coke preparation should be at range of2.90~2.95KJ/g. Afterdrying6min10s, we produced sludge coke whose moisture content was18.66%, NCV was9100J/gand energy consumption was2.96KJ/g.⑦Microwave sludge coke (18.66%water content) could not meet combustible requirement(12500J/g), and at least14.66%of pulverized coal (￥137.4/t) or51.9%of sawdust (￥215.8/t) shouldbe added. By comparing the heating efficiency and cost between these2kinds additives, it could beconcluded that pulverized coal was much better than sawdust.⑧Production of sludge coke by microwave dehydration has industrial prospect. But duringdehydration, odor is produced, also there is a fire threaten in microwave oven, further study should becarried out to solve these problems.