Use of Cement Kilns in Managing Solid and Hazardous Wastes: Implementation in Australia
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The use of solid and hazardous wastes as supplementary fuel or raw material substitutes in cement kilns is rapidly being accepted worldwide as one of the best technologies for complete and safe destruction of these wastes, while simultaneously recovering their resource values for the use in cement manufacture. High temperatures (up to 1500°C), long gas residence times (up to 10 seconds) and high turbulence in the cement kiln ensure complete destruction of organic constituents in the waste materials. The main benefits in using solid and hazardous wastes in cement kilns include energy recovery, conservation of non-renewable fuels, reduction in cement production costs and the use of existing facilities. Although the technology of using cement kilns in waste management was first developed in 1975, this technically innovative and environmentally safe technology is not presently implemented to the extent one would expect in many countries including Australia. There is also opposition from environmental groups to implement this technology in Australia. This thesis investigates the key technical, environmental and social issues preventing or delaying the introduction of this technology in Australia and suggests solutions to some of these concerns. Unburnt wastes in stack gases, emissions of Products of Incomplete Combustion (PICs) such as chlorinated dioxins and furans and the fate of metal constituents in waste materials were identified as three major concerns in using cement kilns to manage solid and hazardous wastes. The principal source of metals entering a cement kiln was identified as the raw material feed and not the waste derived fuels. The metals such as arsenic, barium, vanadium and zinc are added by coal fuel and the major source of metals cadmium, chromium, lead and nickel was identified as the waste derived fuels. The metals entering a cement kiln via waste derived fuels or raw material feed were categorised as volatile, semi-volatile and refractory, based on their volatilities. The analysis of data on metal emissions indicated that the emissions of refractory metals are not increased as a result of an increase in the input of these metals. A moderate correlation was found between the semi-volatile metal emissions and their input. The study found no correlation between the chlorine input and the metal emissions in stack gases. The comparison of metal emissions with total metal input demonstrated that less than 0.1 % of the total input of refractory metals appear in the stack gases indicating that over 99.9% of these metals are retained by the cement kiln. A similar comparison for semi-volatile metals demonstrated that only less than 0.75% of their input appear in the stack gas indicating over 99% retained by the cement kiln. Although, the semi-volatile metals such as lead and cadmium were found to concentrate more on the cement kiln dust than cement clinker, it was found that the use of waste derived fuels is not the determining factor for this distribution. All the refractory metals were found to concentrate in the cement clinker. The analysis of organic compound emissions indicated that extreme high combustion conditions in a cement kiln combined with high turbulence and long residence times readily overcome any oxygen deficiencies inside the kiln to achieve destruction efficiencies exceeding the 99.99% regulatory requirement and even reaching 99.9999% in many cases. The emissions of chlorinated dioxins/furans from a cement kiln burning hazardous wastes, under worst case scenarios, were found to be very much lower than the emissions of such compounds from a properly operated commercial waste incinerators. The data on the emissions of dioxins/furans also suggested that if the cement kiln operators can control the kiln exit temperatures below 250°C, then it will be possible to reduce the emissions of these compounds to even lower levels. No correlations were found between dioxin/furan emissions and chlorine input or waste fuel input. However, a strong correlation was found between the dioxinlfuran emissions and carbon monoxide emissions suggesting that completeness of combustion has a major influence on the emissions of these compounds. Investigations into waste generation in Australia indicate that Australia generates about 12.8 million tonnes of solid wastes and about 400,000 tonnes of industrial hazardous wastes annually. Furthermore, over 100,000 tonnes of scheduled wastes are stockpiled around the country. There are 22 cement kilns around Australia burning about a 1 million tonne of coal to produce 5.6 million tonnes of cement each year. The present study found that if the Australian cement kilns could replace 31% of its thermal energy requirement by hazardous wastes, then it would totally dispose the industrial hazardous wastes generated in Australia annually. Public perception, incomplete waste profiles and lack of regulatory guidelines were identified as the main constraints to implement this technology in Australia. Public involvement, which comprises public education and public consultation, was identified as the most important task in the implementation of this technology in any country. The present study discusses some of the techniques that can be used for effective public involvement and the role of international conferences to enhance the public education process. It was recommended that the cement plant management should allocate the necessary resources to the public involvement process rather than allocating all the available resources for conducting test burns, as has been done in the past.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environmental Engineering
Solid and hazardous wastes