| Global biomass ranks fourth as an energy resource, supplying approximatively 14% of the world’s energy demand. It is well-known that the high volatile matter content of biomass dominates the burning process, while char burning step plays a less important role. Biomass combustion through primitive low efficiency and highly polluting combustion way accounts for a major share of the world’s population.However, compared to traditional biomass, pellet fuels (PF) can be used in fluidized bed and grate furnaces combustion with many advantages, such as lower transportation and storage costs, higher energy density, lower dust levels, less pollution and combined with standardized sizes and composition. Moreover, the pellets offer the parallel superiorities for automation and optimization as the petroleum-derived fuels, with less products of incomplete combustion and high combustion efficiency.The increasing production of municipal sewage sludge (MSS) is a huge challenge for the sustainable development of society. In China, more than 8 million tons (in dry matter) of MSS are produced every year. With rapid development of urbanization and industrialization in China, the production of MSS had increased at a rate between 8% and 10% annually. The environmental issues associated with disposal of MSS are complicated, since MSS usually contains heavy metals, endocrine disrupter, pesticide, pathogens, polychlorinated biphenyl, polycyclic aromatic hydrocarbon, pathogens, other microbial contaminants and nutrients such as nitrogen and phosphorus causing eutrophication. Combustion had the benefits of both energy recovery and treatments of MSS. Therefore, sludge combustion has been quickly increased recently. The boiler efficiency and the secondary pollutants emissions during incineration have been attracted more concern with the development of municipal sewage sludge (MSS) combustion.On the basis of the present research on characteristics of PF, boiler efficiency, secondary pollutant emission from the combustion, Chemical speciation, mobility, phyto-accessibility and risk assessment of heavy metals in fly ash and slag, we carried out four areas of research work.The first section describes the characteristics of PF in varying proportions and combustion characteristics of the PF. The lower heating value decreased, the color deepened, density increased, diameter decreased with an increase in the sludge ratio. The Meyer hardness of 30% MSS PF was lowest among PF, and it’s percentage of particle breakage was highest. On the contrary, The Meyer hardness of 50%MSS PF was highest among PF, and it’s percentage of particle breakage was lowest. The density, moisture content, and calorific value of 10%and 30%MSS PF met the requirement of standards in Beijing (DB11/T 541-2008). But that of 50%MSS PF can’t met the requirement of that standards.It was found that 10%and 30%MSS PF could be successfully used in the traveling grate boiler rations as an energy source instead of about 5000 kcal coal. These two kinds of PF can completely combust. However, steam of 50%MSS PF can’t meet the requirements demand, and it incompletely combusted due to its low calorific value. The 50%MSS PF can’t been applied to practice due to its low calorific value, and the combustion of 10%MSS PF can only dispose a small amount of sludge. As a result, the 30%MSS PF should practiced in some boiler due to its high calorific value and high capability of sludge disposal.The second section describes the boiler efficiencies, normal pollutants, the emission, distribution characteristics and mass balance of PCDD/Fs and heavy metals by combustion of 10%and 30%MSS PF. The results showed that the boiler efficiencies of 10%and 30%(MSS content%) MSS pellet fuels were 76.63%and 75.85%, respectively. The concentrations of CO in stack flue gas were very low for PF (56-64 Nm’3). The losses on ignition values for slag of pellet fuels were only 0.66% and 0.76%, respectively.The emission, mass balance and distribution characteristics of PCDD/Fs and heavy metals during the combustion of 10%and 30%MSS pellet fuels (PF) were also carried out in a traveling grate boiler. The international toxic equivalency quantities (I-TEQ) of the PCDD/Fs in the stack flue gas from the combustion of 10% and 30%PF were 0.191 and 0.157 ng I-TEQ Nm-3, respectively. Although the combustion processes of the traveling grate boiler are operated between 653 and 667 ℃, this temperature range covered the upper window for the formation PCDD/Fs. However, the completeness of combustion and combustion stability for PF and relatively low chlorine levels in PF resulted in lower PCDD/Fs emissions. MSS exhibited a remarkable difference in ratios of PCDFs to PCDDs and congener profiles compared with stack flue gas, fly ash and slag. Ratio of PCDFs to PCDDs and congener profiles for slag also differed significantly from those of stack flue gas and fly ash.The distribution characteristics of the heavy metals researched in this study are primarily dependent on their evaporation characteristics.The emitted pollutants in the traveling grate boiler met the requirement of standards in China (GB 18485-2014).The third section qualitatively describes the chemical speciation, mobility and phyto-accessibility of heavy metals in MSS, PF, fly ash and slag from combustion of PF. Fly ash and slag are important byproducts of combustion of PF. The comparisons of sequential extraction consequences among MSS, fly ash and slag showed that after combustion of pelletized fuel, the bioavailable heavy metal fractions (acid soluble/exchangeable, reducible and oxidizable fractions) were mostly transformed into the very stable heavy metal fractions (residual fractions). The proportion of bioavailable fractions (F1+F2+F3) of heavy metals in slag during incineration of hospital waste and municipal solid waste were higher those in this study.In comparison to slag, fly ashes contain more exchangeable and carbonate heavy metals except As. It displayed that heavy metals except As in fly ash is more bioavailable and mobile than those in slag.On the other hand, combustion after pelletization markedly reduced heavy metals mobility evaluated with toxicity characteristic leaching procedure (TCLP), phyto-accessibility (diethylenetriamine pentaacetic acid and HC1 extraction) and thus reduced the toxicity of metals. The results of sequential extraction, TCLP and phyto-accessibility show that heavy metals in slag are more stable than those in fly ash. The direct and long-term bioavailability and eco-toxicity of heavy metals in fly ash and slag were relieved, which implies that combustion after pelletization could be a promising and completely safe disposal technology for MSS treatment in practice.The fourth section quantitatively evaluates the pollution risk of heavy metals in MSS, fly ash and slag. The quantitative environmental impact assessment of heavy metals in fly ash and slag, compared to MSS, were performed in accordance with bioavailability and eco-toxicity, geo-accumulation index (GAI), risk assessment code (RAC) and potential ecological risk index (PERI). The results demonstrated not only direct but also long-term bioavailability and eco-toxicity of heavy metals in fly ash and slag significantly decreased (P< 0.05) except direct bioavailability and eco-toxicity of Pb in fly ash. The GAI demonstrated that combustion significantly weakened (P< 0.05) the pollution levels of heavy metals. PERI indicated that all risks attributed to heavy metals were significantly lowered (P< 0.05) from 777.07 (very high risk) in MSS to 288.72 (moderate risk) and 64.55 (low risk) in fly ash and slag, respectively. In MSS, fly ash and slag, Cu and Cd contributed majority of the PERI’s amount. In terms of the RAC, seven heavy metals had low even no risk to the environments after combustion besides As in slag. The environmental risk of heavy metals in fly ash and slag was decreased compared with MSS. However, the results of PERI showed that fly ash had a moderate risk. Thus, fly ash from combustion of PF should be pretreated or upgraded before reutilization or landfill. In conclusion, the pollution hazards of heavy metals in fly ash and slag were obviously mitigated. The results revealed that combustion of pelletized MSS could be a suitable disposal route for MSS. |
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