Effects Of Sludge Cofiring On Environmental Security And Processing Stability Of Coal-Fired Power Plants

Cofiring of Municipal sewage sludge with coal in coal-fired power plants cannot only significantly reduce sludge disposal costs, improve efficiency ofsludge treatment and disposal, but also sludge can be used as alternativeenergy or raw materials, thus reducing production costs. With thedevelopment of thermo-electric and sludge projects carried out rapidly acrossour country, the security applications of sludge in thermal power plants willbe the focus of industry and government. This paper introduces the presenttechnology of sludge disposal in coal-fired power plants at home and abroad.Air pollutant emissions including CO, NOx, SO₂, heavy metals and dioxin etc.,as well as utilization of ash were investigated. In addition, a case when coalwas blended with wet sludge in coal-fired power plant were analyzed andevaluated on electricity production stability as well as secondary pollutions.Thermogravimetric analysis using different sludge ratios to investigeateweight loss during cofiring process showed effects of combustion propertiesof different blending ratios of sludge and coal. Through the Arrheniusequation and Ozawa, Vyazovkin model, analysis of correlation coefficient,activation energy required by different blending ratios was obtained.According to results of both experiments and models, the appropriate ratio ofsewage sludge is about10²0%.The emission patterns of NOx, SO₂, CO and changes of heavy metals in ashfor different blending ratios of wet sewage sludge had been studied. Theresults showed that, when the blended proportion of wet sludge was less than30%, with the increase of blending ratio, the conversion rate of NOx and totalemission of SO₂reduced. A better effect of fixing sulfur could be obtainedwhen blending ratio was30%. The distribution of heavy metals in ash varied from heavy metal characteristics and combustion conditions, causing thedifference proportion of heavy metals in ash and smoke.Based on the results of laboratory analysis, a case study was carried out.Results showed that when less than25%of sewage sludg was co-combustedwith coal in coal-fired power plants with technology of circulated fluidizedbed （CFB）, quality of flue gas after being processed in the existing flue gascleaning equipment could meet the existing requirements in "Solid WasteIncineration Pollution Control Standard"（GB18485-2001） and "Fire PowerPlant Air Pollutant Emission Standards "（GB13223-2003） in the relevantprovisions. Contents of heavy metals in the ash could be used for buildingmaterials in line with industry and national standards. Once the highestblending ratio of sludge was about25to30%, it could be directly transportedto the circulating fluidized bed to combust, and the sludge with water contentof about85%wouldn’t affect the normal operation of the boiler.