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Waste incineration on a grate is a well-established thermal treatment technology in Denmark and several other countries. The method is flexible with respect to operation, it allows for recovery of energy, it reduces the volume of solid waste significantly (by a factor of 8-10), and advanced flue gas cleaning technologies ensures very low emissions from modern incineration plants. In 2005, 24% of the total reported Danish waste production was incinerated. However, the presence of inorganic constituents such as alkali metals, Cl, S, and heavy metals in the waste constitute an essential challenge in waste incineration, both with respect to operational issues and due to environmental concerns. Formation of acidic pollutants, high mass loading of aerosols, and deposition of potentially corrosive components on heat transfer surfaces are among the encountered problems caused by volatile alkali compounds and heavy metals during combustion, and the potential leaching of heavy metals from the solid residues upon disposal is another major environmental concern. In this work the partitioning of trace elements in a waste-to-energy (WtE) boiler was studied experimentally, based on results from a fullscale measuring campaign at a Danish WtE plant while applying changes in waste composition and grate operation, respectively. The changes in waste composition were applied by adding (one-by-one): dedicated waste fractions, comprising road salt (NaCl), household batteries, automotive shredder waste, CCA (copper-chromate-arsenate)-impregnated wood, PVC, and, shoes, to a base-load waste. Special focus in the present work was on the partitioning of the elements Pb, Zn, Cl, S, Na, and K, which are all considered problematic elements with respect to deposition and corrosion problems. The partitioning of the elements was found to be influenced by various effects, and there was not necessarily a correlation between the input concentration of an element in the feedstock and the amounts recovered in the fly ash and flue gas fractions. The study indicated that Cl and S played an important role for the partitioning of Pb, and maybe also Zn. When firing Cl-rich waste fractions (PVC, salt, shoes), the partitioning of Pb seemed to shift toward increased vaporization (and subsequent recovery in fly ash and aerosol fractions). The full-scale study also implied that organically bound Cl (as in PVC, shoes) was preferably released (as HCl(g)) to the gas phase, whereas the inorganically bound Cl (salt) was recovered in the bottom ash and fly ash fractions (indicating alkali-chloride bonding). Overall, Cl was concluded to be a critical element with respect to deposition and corrosion problems as it (1) enhanced the vaporization of heavy metals, (2) caused increased mass-load of aerosols if present as alkali-chloride in the waste, and (3) caused increased deposition fluxes and increased concentrations of Cl in the deposits.

Incineration of municipal solid waste is a debated waste management technology. In some countries it is the main waste management option whereas in other countries it has been disregarded. The main discussion point on waste incineration is the release of air emissions from the combustion of the waste, but also the energy recovery efficiency has a large importance. The historical development of air pollution control in waste incineration was studied through life-cycle-assessment modelling of eight different air pollution control technologies. The results showed a drastic reduction in the release of air emissions and consequently a significant reduction in the potential environmental impacts of waste incineration. Improvements of a factor 0.85-174 were obtained in the different impact potentials as technology developed from no emission control at all, to the best available emission control technologies of today (2010). The importance of efficient energy recovery was studied through seven different combinations of heat and electricity recovery, which were modelled to substitute energy produced from either coal or natural gas. The best air pollution control technology was used at the incinerator. It was found that when substituting coal based energy production total net savings were obtained in both the standard and toxic impact categories. However, if the substituted energy production was based on natural gas, only the most efficient recovery options yielded net savings with respect to the standard impacts. With regards to the toxic impact categories, emissions from the waste incineration process were always larger than those from the avoided energy production based on natural gas. The results shows that the potential environmental impacts from air emissions have decreased drastically during the last 35 years and that these impacts can be partly or fully offset by recovering energy which otherwise should have been produced from fossil fuels like coal or natural gas.

A model for life-cycle assessment of waste incinerators is described and applied to a case study for illustrative purposes. As life-cycle thinking becomes more integrated into waste management, quantitative tools for assessing waste management technologies are needed. The presented model is a module in the life-cycle assessment model EASEWASTE. The module accounts for all uses of materials and energy and credits the incinerator for electricity and heat recovered. The energy recovered is defined by the user as a percentage of the energy produced, calculated on the lower heating value of the wet waste incinerated. Emissions are either process-specific (related to the amount of waste incinerated) or input-specific (related to the composition of the waste incinerated), while mass transfer to solid outputs are governed by transfer coefficients specified by the user. The waste input is defined by 48 material fractions and their chemical composition. The model was used to quantify the environmental performance of the incineration plant in Aarhus, Denmark before and after its upgrading in terms of improved flue gas cleaning and energy recovery. It demonstrated its usefulness in identifying the various processes and substances that contributed to environmental loadings as well as to environmental savings. The model was instrumental in demonstrating the importance of the energy recovery system not only for electricity but also heat from the incinerator.

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Life-cycle assessment (LCA) models are becoming the principal decision support tools of waste management systems. This paper describes our experience with the use of EASEWASTE (Environmental Assessment of Solid Waste Systems and Technologies), a new computerized LCA-based model for integrated waste management. Our findings provide a quantitative understanding of waste management systems and may reveal consistent approaches to improve their environmental performances. EASEWASTE provides a versatile system modelling facility combined with a complete life-cycle impact assessment and in addition to the traditional impact categories addresses toxicity-related categories. New categories dealing with stored ecotoxicity and spoiled groundwater resources have been introduced. EASEWASTE has been applied in several studies, including full-scale assessments of waste management in Danish municipalities. These studies led to numerous modelling issues: the need of combining process-specific and input-specific emissions, the choice of a meaningful time horizon, the way of accounting for biological carbon emissions, the problem of stored ecotoxicity and aspects of crediting the waste management system with the savings inherent in avoided production of energy and materials. Interpretation of results showed that waste management systems can be designed in an environmentally sustainable manner where energy recovery processes lead to substantial avoidance of emissions and savings of resources.

Waste incineration can be considered a robust technology for energy recovery from mixed waste. Modern incinerators are generally able to maintain relatively stable performance, but changes in waste input and furnace operation may affect emissions. This study investigated how inorganic air emissions and residue composition at a full-scale incinerator were affected by known additions of specific waste materials to the normal municipal solid waste (MSW) input. Six individual experiments were carried out (% ww of total waste input): NaCl (0.5%), shoes (1.6%), automobile shredder waste (14%), batteries (0.5%), poly(vinyl chloride) (5.5%) and chromate-cupper-arsenate impregnated wood (11%). Materials were selected based on chemical composition and potential for being included or excluded from the waste mix. Critical elements in the waste materials were identified based on comparison with six experiments including 'as-large-as-possible' changes in furnace operation (oxygen levels, air supply and burnout level) only using normal MSW as input. The experiments showed that effects from the added waste materials were significant in relation to: air emissions (in particular As, Cd, Cr, Hg, Sb), element transfer coefficients, and residue composition (As, Cd, Cl, Cr, Cu, Hg, Mo, Ni, Pb, S, Sb, Zn). Changes in furnace operation could not be directly linked to changes in emissions and residues. The results outlined important elements in waste which should be addressed in relation to waste incinerator performance. Likely ranges of element transfer coefficients were provided as the basis for sensitivity analysis of life-cycle assessment (LCA) results involving waste incinerator technologies.

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