Investigation On Drying Characteristic And Gas Emissions From Municipal Solid Waste Combustion

With the development of economy and urbanization, production of municipal solidwaste(MSW) was gradually increasing. Incineration power generation is an effective meansof making MSW harmness,reduction and resource-oriented. Compared with developedcountry and district, MSW in China has of high moisture content, low heat value and changeswith season and district. Employing imported equipment in local MSW combustion reducedcombustion efficiency. Large amounts of gas products would be released from incinerationprocess, including nitrogen oxides and other gases harm to environment and human healthseriously. Experiments were conducted on drying characteristics and gas emissioncharacteristics of Guangzhou MSW. It is hoped to provide reference for controlling pollutantsemission from source, designing and improving incineration equipment suitable to local MSW.The experimental research and theoretical analysis on drying and combustion process ofMSW were carried on. The drying characteristics of MSW and typical kichen disposal werediscussed and the dynamic analysis was studied. The factors on gas emissions from MSWcombustion were investigated, such as furnace temperature, combustion atmosphere, blendingratio of straw and coal, catalyst type and blending rate, moisture content and so on. The heatand mass transfer process on MSW combustion in grate furnace, was simulated by CFD onthe condition of changing moisture content and blending rate of straw.（1)A series of drying experiments on municipal solid waste(MSW) in Guangzhou wereconducted in air dry oven，simulating the drying process in incineration. Temperatureinfluence on drying characteristics of MSW was analyzed. The optimal drying models wereobtained to predict experimental drying process. The results show that drying time is shorter,maximum drying rate is higher and the corresponding moisture content is lower with thehigher drying temperature. The experimental drying process could be described accurately byModified page and Weibull distribution models. The effective diffusivity of MSW wascalculated with fick’s diffusivity model. The MSW activation energy was determined throughArrhenius equation.（2)Experiments on gas emissions from municipal solid waste (MSW) and typicalelements combustion were conducted in a lab-scale electrically heated tubular furnace. Thegas emission characteristics were discussed on the factors of furnace temperature andatmosphere. MSW samples with different ratio of waste cotton and branch were taken asexperimental material in order to find the gas emission characteristics. The results indicatedthat the locations on the trough of O2, peaks of CO and CO2emerged simultaneously. The CO emission curves expressed single peak and NO curves had two peaks. Compared with fourwaste elements, waste cotton combustion produced highest peak concentration of CO andCO2, kitchen disposal combustion consumed maximum oxygen and brought highest NO peakvalue. It means that kitchen disposal is the main factor on NO emission. With the temperatureincreasing, the residual rates and CO peak values decreased, time of gas emission shiftedearlier, the peak value of CmHnincreased, NO peak values decreased first and then increased.The higher the oxygen content in atmosphere, the higher the two peak values of NO and CO2peak values, the lower the CO peak values and residual rates. CO peak values and the secondNO peak values were improved with the increasing mass ratio of waste cotton. CO peakvalues elevated with growing mass ratio of branch, but not for NO curves.（3)Experiments on MSW co-combustion with straw and coal were conducted in alab-scale electrically heated tubular furnace at850°C. The gas concentration was measuredwith gas analyzer. The mathematic models were researched on NO production and Nconversion rate. The following conclusions could be drawn: the shapes of NO and CO curveswere not affected by co-combustion with coal. The increasing blending ratio of coal resultedin shortening burning time, reducing CO peak value, improving NO second peak value andproduction. MSW co-firing with straw could change CO curves, but not for the NO curves.With the increasing blending ratio of straw, the second peak value of NO curve was increased,burning time was shortened and the N conversion rate was decreased. The sequence of NOproduction and N conversion rate was described as: co-combustion with coal>MSWcombustion> co-combustion with straw.Model of NO production could predict the value withdifferent straw and coal blending rate. The N conversion rate of experimental combustionprocess could be described accurately by Logistic model.（4)Five kinds of alkaline compounds with different blending rate were chosen ascatalysts applied in MSW combustion in terms of NO emission and burnout rate.Mathematical model were constructed on N conversion rate. The results showed that catalystaddiition in MSW combustion could increase O2trough concentration, change the CO2peakvalue little, make the CO curves broad and delay the location of NO second peak. The peakvalues and peak locations of CO and NO emission curves changed with catalyst type andblending ratio. In comparison with15samples,9%CaO、5%and7%K2CO3、7%and9%Na2CO3addition in MSW combustion could reduce NO average concentration and Nconversion rate, enhance burnout rate, so they could be the better choices as catalyst for MSWcombustion. There are higher determination coefficients utilizing Logistic model to simulateN conversion rate of experimental process, especially for Ba2CO3. （5)Combustion experiments on MSW with different moisture content were carried on at850°C. Gas emissions and the models describing N, C and H conversion rate were the mainpoints. The residual rates were calculated. It could be concluded that with higher moisturecontent, the drying process was longer, the process of gas emission was slower, the O2troughconcentration and CO2peak concentration were higher. There were two preaks in the COcurves and the first peak value decreased with the improved moisture content. The single peakappeared in the curves of CmHnand NO curves. With the higher moisture content, the NOpeak values enhanced, the peak concentration of CmHnascent firstly and then dropped. Thehigher moisture content, the higher C, N and H conversion rate. The Logistic model simulatedthe experimental process of C, N and H conversion rate accurately with R2more than0.99303.（6) The operating grate furnace in Guangzhou was simulated with the method of CFD.The effect of moisture content and blending rate of straw on combustion efficiency wasdisscussed. The following results could be concluded: the decreasing moisture content andincreasing blending rate of straw could improve heat value and temperature distribution in thefurnace. On the condition of satisfying the resistance time of flue gas and temperature level,the excess air ratio should be increased in order to avoid imcomplete combustion owing tolack of oxygen.