Identification and control of metal pollutant spikes in municipal solid waste incinerators
The emission of metals during municipal solid waste incineration has become a question of considerable public and scientific concern in the light of evidence of their extreme toxicity. Sophisticated and expensive gas cleaning systems are required to meet the increasingly stringent EC atmospheric emission limits. While the technology for the clean-up of particulate matter and acid gases in flue gas is comparatively straightforward, the emissions of micro-pollutants such as heavy metals and dioxins remain a concern. Previous research on metal emissions has concentrated on overall mass balances, working either with laboratory or plant based measurements, or with computational models (frequently with little correlation between the techniques), leading to incomplete information on the system concerned. The waste incineration process is highly inhomogeneous, due to the changing nature of the waste feed, but the effects of changing waste feed and combustion conditions on the concentration and distribution of metals in incinerator residues has not been investigated fully until now. Therefore the main objective of this PhD research project was to develop a comprehensive understanding of metal behaviour during municipal solid waste incineration, including an assessment of the importance and effect of temporal variation in waste composition and incineration conditions, through a co-ordinated programme of experimental measurements and mathematical modelling. A new on-line continuous monitoring technique was developed in order to obtain temporally resolved data on metal concentrations in incinerator flue gases. This system was used to obtain unique data on the distribution of nineteen different metals in a UK municipal solid waste incinerator. It was found that not only was metal distribution highly dependent on the volatility of the metal concerned, but also that temporal fluctuations could be extremely significant. These were caused by changing waste feed and changing incinerator conditions. In parallel with the experimental programme, state-of-the-art thermodynamic modelling techniques were used to predict the distribution of metallic pollutants during waste incineration, for a range of waste compositions and combustion conditions. The thermodynamic equilibrium around various heterogeneous items in municipal waste was studied in order to a) assess the importance of non-uniformity in the waste stream, and b) investigate whether this could lead to the observed periodic changes in metal concentrations. These calculations, in conjunction with the experimental data obtained, have helped to show how unusual partitioning behaviour can be the result of local or transient behaviour on the burning bed, as well as changing bulk incinerator combustion conditions or bulk waste feed properties. These two approaches to the problem of understanding metal behaviour during municipal solid waste incineration have complemented each other extremely well. The presence of spikes in metal emissions has clearly been demonstrated, and probable causes have been identified. Heterogeneous items containing high proportions of volatile metal are the most likely source of the large spikes observed for some heavy metals, whilst other changes are linked to more general incinerator conditions. These results indicate that more careful sorting of municipal waste prior to incineration could significantly reduce the toxic metal spikes in the flue gas, and therefore the overall environmental impact of incineration, although the atmospheric emissions are already near-zero and are therefore of minimal impact.