Thermal Behavior Of Chlorine During Municipal Solid Waste (MSW) Combustion

High chlorine content in municipal solid waste (MSW) and refuse derived fuel(RDF) results in the challenge for operating waste-to-energy (WTE) plants, such ashigh temperature corrosion on the superheater surface and correspondingly lowefficiency.Currentknowledgeisstillinsufficientinthefieldofchlorinequantificationin MSW and chlorine thermal conversion in WTE boiler. This thesis is thereforefocusing on chlorine characterization in MSW and RDF and its thermal behaviorduringcombustionprocess.For chlorine characterization, five analytical methods (bomb combustion,Schoeniger flask combustion, elemental analyzer, muffle furnace and elution test)were applied and compared to determine chlorine content in eight combustiblefractions of household waste. Moreover, total chlorine content (TCC) is furtherdistinguished, in terms of chlorine species, to organic chlorine and inorganic chlorine,combustiblechlorineandincombustiblechlorine.To study the thermal behavior of chlorine, chlorine volatilization wasinvestigated in micro-combustion unit to determine the influencingfactors byvaryingfuel matrix and operating conditions of thermal conversion. Information on chlorinedeposits formation was obtained in a practical WTE plant, by inserting probes atdifferent flue gas convective passes in the boiler, including pre-protector, 3rdsuperheater, 1st superheater, evaporator and economizer. After 9-month exposure, thedeposits were collected for physical characterization and chemical analysis usingscanning electron microscope (SEM) and energy dispersive spectrometer (EDS).Moreover, a lab-scale corrosion test facility has been established to predict thecorrosionrateanddistinguishtheinfluenceofdifferentdeposits(NaClandflyash)oncorrosion.The results show: chlorine content in MSW varies from 0.1 wt.% in wood to >6wt.% in non-packaging plastics (drybasis). The Schoeniger flask combustion methoddetects the highest chlorine content in plastic samples, while the elution test recoversmore chlorine in organic waste fractions which probably distinguishes organic andinorganic chlorine. Elemental analyzer and muffle furnace provide the opportunity toanalyze the combustible and incombustible chlorine. Sulfur addition leads to 20-40%higher HCl formation rate in most fractions. Silica supports the chlorine release at relatively low temperatures between 700℃and 850℃. The experimental resultsobserved werein goodagreement withthermodynamicequilibrium calculations usingFactsagesoftware.After volatilization, high concentration of chlorine lead to the formation ofeutectics in fly ashes with a relatively low melting point and these fine fly ashparticles condense on the superheaters resulting in ash deposit formation and possiblecorrosion phenomenon. Observation in a WTE plant shows that with the depositsprogress, the sodium and sulfur contents are rich in interface, silicon and magnesiumare rich in inner layer; and calcium and chlorine are very high in the outer layer. ThepresenceofNaClinthedepositsisapremiseforseverecorrosionofsuperheatertubes.Thus lab-scale corrosion test indicates that the corrosion attack is quite uniform andthe corrosion products consist mainly of oxides of iron and chlorides of iron. Thecorrosion rate for 20G and 15CrMoG covered with NaCl is 3.4*10-5mm²/h and1.6*10-5mm²/h, respectively. Comparatively, corrosion rate for 20G and 15CrMoGcovered with fly ash is 3.1*10^-6 mm²/h and 1.7*10^-6mm²/h, respectively. Moistureplaysasignificantroleinchlorineinducedcorrosionmechanism.These findings enhance to understand the thermal behavior of chlorine in MSWand RDF (refuse derived fuel) in WTE plants and lead to specific fuel qualitycontrolforwastederivedfuelsinordertominimizechlorineinducedcorrosion.