Study On Combustion Characteristic Of Municipal Solid Waste （msw） In O₂/N₂and O₂/CO₂Atmospheres And Corrosion Of Water-wall Tubes In AMSW Incinerator

With the development of economy and the expansion of urban, the wastetreatment becomes a big problem of city development in China. The wasteincineration has become a realistic choice of municipal solid waste （MSW） treatmentowing to its characteristics of small occupation, big capacity, short period and energyrecovery. The waste incineration technology starts relatively late in China, and thecomponents and calorific value of foreign MSW differ from domestic MSW, therefore,the direct adoption of foreign incineration technology don’t fit the domestic conditionwell. O₂/CO₂combustion technology is one of the several promising new technologiesassociated with carbon capture and contributes to carbon reduction and sociallow–carbon development. The study on the MSW O₂/CO₂combustion technology canenrich and develop the waste incineration technology.The present study aimed at investigating the pyrolysis and combustioncharacteristics theoretically and experimentally, and discussing the effects ofReplacement of N₂by CO₂on pyrolysis and combustion characteristics and gasemissions. The corrosion of water-wall tubes in a MSW mechanical grate incineratoris also studied. The severe corrosion zone is sampled and tested to analyze the depositcorrosion mechanism of water-wall tubes and influencing factors, combined withthermal calculation, theoretical analysis and numerical simulation.（1） The thermal decomposition behavior and kinetic characteristics of the MSWwere studied at different N₂/CO₂atmospheres in a thermogravimetric analyzer. TheTGA and kinetic study revealed the following conclusions: The DTG curves below650℃changed with atmosphere indistinctively, but the location and mechanism ofthe peak in the high temperature range were affected by atmosphere. Above650℃,the DTG curve showed two peaks in80%N₂/20%CO₂atmosphere obviously, whereasonly an obvious peak in other atmospheres. Replacement of N₂by CO₂promoted thechar gasification in high temperature range and influenced the residual mass. Theresidual mass decreased from39.2%（in100%N₂） to36.9%（in80%N₂/20%CO₂） and33.2%（in60%N₂/40%CO₂）. But when the CO₂concentration was over60%, theresidual mass almost remained the same （32.2%）. Several independent fractions of nthorder reaction model fitted the weight loss well.（2） The oxygen-enriched combustion characteristics and kinetic behavior of theMSW are studied at different O₂/N₂and O₂/CO₂atmospheres. The following conclusions can be drawn as a result of TGA and kinetic study: All the samples losemost their weight between200℃-540℃. Higher oxygen concentration promotescombustion. The conversion rate curves and DTG curves shift to lower temperaturewithout significant change in its shape as the oxygen concentration increases. At thesame oxygen concentration, the peak values in O₂/CO₂atmosphere are lower thanthose in O₂/N₂atmosphere, indicating that CO₂has a higher inhibitory effect than N₂on MSW combustion. Above600℃, the weight loss peak appears much later inO₂/CO₂atmosphere than it dose in O₂/N₂atmosphere. The three-step reaction of nthorder reaction model could fit the weight loss very well.（3） The thermal decomposition behavior and kinetic characteristics of the typicalcomponents of the MSW were studied at different O₂/N₂and O₂/CO₂atmospheres in athermogravimetric analyzer. The TGA and kinetic study revealed the followingconclusions: Regardless of individual material or mixture, replacement of80%N₂by80%CO₂influenced combustion negatively and decreased the burnout rate. Thereplacement of N₂by CO₂had more obvious effect on combustion of fixed carbonthan volatiles. An oxygen content of30%in the O₂/CO₂atmosphere achieved asimilar combustion performance as air. Therefore, the development of MSW oxy–fuelcombustion technology required support from oxygen-enriched combustiontechnology. The combustion characteristics of different materials of a kind hadgenerality and individuality. For example, among textiles, the ignition temperature ofchemical fiber was higher than cotton textile by100℃. Therefore, when designingand operating MSW oxy–fuel combustion equipments, the applicability of specifickinds of material that would be used must be taken into consideration.（4） The gases emissions from the combustion of paper mixture, plant residuemixture, fruit waste and textiles were studied using a lab-scale electrically heated tubefurnace. The variations of CO, H2, SO₂and NO_xwere analyzed. The followingconclusions can be drawn: For the variations of gases emissions concentration withatmosphere, the similarity was observed for paper mixture, pericarp mixture andfoliage mixture. In O₂/CO₂and O₂/N₂atmospheres, CO, H2, SO₂and NO_xconcentration first increased and then decreased with the increase of heating time.Below800℃, the emissions of CO, H2, and NO_xall decreased with temperature underboth atmospheres, but above800℃, the increment of temperature increased NO_xemissions from800℃to1000℃. NO_xemission first decreased and then increased, because the dominant role changed from reduction reaction to oxidation reaction withthe increment of temperature. Replacement of N₂by CO₂increased reducing gases（CO and H2）, promoted reduction reaction, and reduced NO_xemissions at hightemperatures. There was no clear correlation between the nitrogen content and theNO_xemission. Instead, fuels with higher volatile content lead to higher NO_xemission.（5） Various deposits and corrosion product were taken from the main corrosionareas on the water-wall tubes in a MSW grate incinerator and their morphologicalcharacteristics and component analysis were conducted using SEM-EDS, XRF andXDR. The following conclusions can be drawn: SEM showed that all of the depositscontain more or less molten （spherical） or semi-molten （hemispherical, globe-like）materials. The molten materials are due to the chemical reaction heat during thesulphation, the existence of low melting point compounds and molecular cramming.The content of K, Na, Cl or Fe in the deposits is bigger than in fly ash, but fly ashcontain more Ca, S, Si and Al, due to their different source and deposition mechanism.The alkali-acid ratio of three deposits are greater than2.5, indicating that MSWdeposit is obviously alkaline and has a high deposition trend. XRF and XRD provedeposits contain chlorides and alkali metal compounds, and are therefore consideredrisky materials prone to causing corrosion.（6） The corrosion mechanism of water-wall tubes is discussed using thermalcalculation, theoretical analysis and numerical simulation. As shown in the CFDresults, two rotational flow zones appeared in the junction of top of first pass and theforward part of ceiling （Ⅰ#zone） and in the top of front second pass （Ⅱ#zone）. Therotational flow zones extended the residence time of flue gas and resulted in theaccumulation and growth of deposit. Ⅰ#zone coincided with the wet areas in thefurnace, and it accelerated deposit growth and corrosion rate. The urea solution wasseparated into NH3and HNCO rapidly. HNCO is lowly reactive in the actualoperation temperature range. The unreacted HNCO came into the rotational flow zone,formed corrosive acid fog in wet condition and resulted in direct corrosion ofwater-wall tubes. Under different conditions, the corrosion rate was60～200timesfaster than the depreciation rate. In actual operation, we should separated chlorineeffectively from the sources, chose appropriate spray location of SNCR reducingagent, apply highly alloyed materials, chose appropriate soot blower and addappropriate additives （such as kaolinite）, in order to decrease the corrosion rate of water-wall tubes. In order to mitigate high–temperature corrosion and prevent theformation of dioxins, the furnace temperature should be controlled and thetemperature of the exit of the first pass must be held within850℃～972℃. Theproportion of primary air should be not less than85%to assure primary air pressure.