Abstract
The aim of this report is to explain the processes of high temperature incineration of hazardous waste, as well as its environmental risks and benefits and its advantages and disadvantages. Modern industrial processes generate a certain amount of hazardous waste that must be dealt with somehow. High temperature incineration aims to dispose of this waste as completely and safely as possible. However, even state of the art incinerators produce emissions and residues which are themselves hazardous. Modern incineration operations aim to minimize the environmental impacts of these emissions and residues and must abide by environmental legislation. Research completed in 2011 showed that continuous monitoring of dioxins in incinerator stack emissions is viable. Due to a lack of high temperature incineration facilities in Australia, considerable effort has been put into developing processes to recycle hazardous waste. The availability of incineration is a disincentive to such research and development, however it may reduce the risks of accidental pollution from stockpiles and the incidence of illegal dumping.
Introduction
Incineration refers to burning waste in a controlled manner in an enclosed environment. It is a common method of general waste disposal in some parts of the world (WasteNet, 2005). The aim of high temperature incineration is to ensure that the combustion process is as effective as possible and produces a minimum of emissions and residues. This is achieved partly by the high temperature and partly by long residence time in the combustion chamber along with good mixing with excess oxygen (WasteNet, 2005).
Hazardous waste that cannot be disposed of in landfill because of its toxic nature can be partly disposed of by high temperature incineration. Incineration doesn’t get rid of it completely, but it reduces its volume considerably and may render the remaining part of the waste safer for landfill disposal. (WasteNet, 2005)
Even the most efficient forms of combustion result in the production of undesirable gases and particulates, as well as solid, non-combustible residues. A high proportion (but not all) of the gases and particulates can be removed from the exhaust gas by air pollution control devices (Wu, H. et al., 2011).
Exhaust gases and particulates from incineration are a serious health issue. The two main concerns are persistent organic pollutants – of which the most concerning are the class of chemicals known as dioxins – and fine and ultrafine particulates (Cernuschi et al., 2012; Unilabs, n.d.).
The term “dioxin” is commonly used to refer to a class of several hundred chemical compounds, including polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and co-planar polychlorinated biphenyls (PCBs) (DEWHA, 2011), the widespread environmental effects of which have been recognised for many years. They are persistent and accumulate in animal tissues and can have teratogenic, immunosuppressant, and carcinogenic effects (USEPA, 2001).
The adverse health effects of fine particulates include lung cancer and cardiopulmonary mortality (Cohen et al., 2005) and are well documented, but recent research has also found evidence of harmful effects of ultrafine particulates. (Cernuschi et al., 2012).
A wide array of technological solutions has been developed to remove as much of these pollutants from the exhaust gases as possible. These devices include scrubbers and baghouses (fabric filters) (Cernuschi et al., 2012).
The other main product of incineration is ash. This is still a hazardous waste, potentially containing leachable heavy metals, dioxins and other pollutants, and must be disposed of safely (WasteNet, 2005). One method of disposing of incinerator ash is stabilization and disposal in landfill (Kalliopi et al., 2010).
Examples
Veolia Environmental Services
Veolia Environmental Services are a waste management company in the UK. They run a high temperature incineration plant for hazardous wastes. Their plant incinerates waste at up to 1200°C and they claim to comply with environmental protection regulations and international standards.
The types of waste the plant accepts include gases, solids, liquids, highly toxic liquids, organic wastes including PCBs, controlled drugs, oily sludges, contaminated soils, bulk powders, and halogenated wastes. It can handle 100,000 tonnes of waste per year.
Veolia’s plant uses a rotary kiln with a waste residence time of 30 to 90 minutes. The slag from this kiln flows into a water quench, where it cools to an inert solid which can then be disposed of in landfill. The exhaust gas from the kiln flows into a secondary combustion chamber where it is burnt along with liquid wastes. A residence time of over two seconds, along with turbulence, excess air, and a combustion temperature of over 1100°C ensure maximum destruction of waste is achieved.
From the secondary combustion chamber, gases flow through a heat exchanger and are then quenched to below 80°C. This rapid cooling helps avoid the formation of dioxins, which occurs around 250°C to 400°C. The gases then flow through two scrubbing towers to remove hydrochloric acid, sulphur oxides, bromine, and some particulates, before passing through a fabric and lime filter to remove any residual particulates and neutralize any remaining acids.
Effluent from the scrubbers is neutralized, mixed with a flocculant, and discharged into settlement tanks. The settlement tank sludge is thickened and dewatered before disposal offsite.
Emissions testing is carried out as well as regular monitoring by the Environment Agency, to ensure compliance with standards. Veolia claim to consistently beat the limits for gaseous emissions. (Veolia, n.d.).
ICI Australia
Historically there have been no hazardous waste incineration facilities in Australia. That means that companies producing hazardous waste have been faced with the challenge of what to do with their waste products. It created a situation where they have been forced to either stockpile it in the hope that an incinerator would one day be built, or to develop innovative schemes for dealing with it.
In the 1970s and 1980s, most of the hazardous wastes in Australia were generated by ICI, who had no alternative but to spend a lot of money on research into minimizing and recycling their waste products. Until the mid 1970s, their research efforts were directed into development of a method to recycle liquid wastes from dichloroethane (DCE) manufacturing. Later on, they developed processes to recycle liquid waste from their solvents plant and more of the waste from the DCE plant.
Where their overseas plants were relying on high temperature incineration to dispose of these wastes, ICI Australia were recycling them and saving money. In 1986, a spokesman for ICI said that until the early 1980s, high temperature incineration had been considered the most economically and technically appropriate disposal method for these waste products, but by the late 1980s the economic benefit of incineration was not so obvious when it was compared with recycling.
If there had been an incinerator available to ICI during those years, these recycling processes would probably never have been developed. (Beder, S., 1990).
Research
A paper titled “Assessment of polychlorinated dibenzo-p-dioxin and dibenzofuran emissions from a hazardous waste incineration plant using long-term sampling equipment” by J. Rivera-Austrui, M.A. Borrajo, K. Martinez, M.A. Adrados, M. Abalos, B. Van Bavel, J. Rivera, and E. Abad, published in 2011, details an evaluation of the performance of a system for continuously monitoring the concentration of dioxins in the stack emissions of a hazardous waste incineration plant.
Emissions of toxic substances in the flue gas of incinerators is a serious concern. The exact nature of these emissions varies with the configuration of the plant and the substances being incinerated, but commonly include nitrogen oxides, sulfur dioxide, hydrogen chloride, metals, and volatile organic compounds such as trihalomethanes and fluorobenzines. They also commonly include persistent organic pollutants (POPs), in considerably smaller quantities. Despite their relatively low levels, POPs are of major concern because of their tendency to bioaccumulate. Of particular concern among POPs is the class of chemicals known as dioxins, which are considered to be a serious danger to animals (including humans).
In Spain in the last twenty years many studies have been carried out on emission levels from incinerators and similar plants. However, all these studies have been based on short term sampling, carried out over periods of no more than 8 hours. Technical advances have now enabled continuous sampling to occur and it is expected that continuous sampling will be mandatory in the European Union in the foreseeable future. The aim of this study was to evaluate the performance of such long term monitoring.
The study was carried out at a state of the art hazardous waste incinerator – the only hazardous waste incinerator in Spain. Information on organic pollutant emissions from incinerators is scarce and the study aimed to address that, as well as to provide reliable information on that particular plant. Sampling covered 93% of the operating time over two years. Long term sampling was compared to short term samples collected twice a year throughout the sampling period.
This study showed long term sampling to be more reliable for monitoring dioxin emissions than occasional short term sampling. Multivariate statistics were found to be useful for evaluating the data from the monitoring, showing subtle differences in the dioxin pattern. The dioxin emission factor for state of the art hazardous waste incineration was found to be overestimated compared with what the continuous monitoring data showed. The study also showed the plant was operating within the statutory limits of dioxin emissions for the whole period of sampling.
Discussion
There are several advantages of high temperature incineration of hazardous waste. The most important one is that it disposes of intractable wastes that are potential sources of pollution and could cause serious environmental damage if they were disposed of in other ways. The lack of high temperature incinerators in Australia has led to a lot of highly toxic waste being stockpiled as there were no disposal options available (Beder, 1990). Incineration is also usually the lowest cost option for dealing with such waste.
The disadvantages of this form of waste disposal are that even the best designed, state of the art incinerators cannot completely dispose of all wastes, resulting in solid residues, and they also produce some toxic emissions in their exhaust gases. Although these emissions are at relatively low levels, some have the ability to bioaccumulate, which means their effects increase over time. The residues of incineration may themselves be hazardous and, in turn, require some form of safe disposal. The convenience of being able to incinerate waste acts as a disincentive to researching and developing ways to avoid, reduce, reuse, or recycle such waste – which would be much better options in the long term, both economically and environmentally.
Ideally all hazardous waste would either be avoided, reused, or recycled – and the availability of high temperature incineration seems to be a disincentive to that. Because there were no hazardous waste incinerators in Australia, a vast amount of such wastes was stockpiled. Eventually, the economics of stockpiling led to development of processes to recycle a significant portion of that stockpiled waste – something that can be seen as a benefit of the absence of incineration as an option.
However, when such wastes are stockpiled, there is always a risk of accidental release into the environment. Disposing of such wastes promptly reduces that risk. Illegal dumping of hazardous waste is also a serious problem (ACC, 2011). It seems likely that without incineration as a legal and relatively safe means of disposal, levels of illegal dumping would be higher.
Conclusion
The aim of high temperature incineration is to dispose of hazardous waste as completely as possible. However, even the most modern, state of the art incinerators release some toxic emissions to the atmosphere. Combustion is rarely total so a residue remains, which may itself be a hazardous waste.
The two main atmospheric pollutants of concern are dioxins and fine particulates. These are both potentially serious hazards to human and animal health. Modern technology has minimized these emissions and they are routinely monitored to ensure they fall within legislated limits.
Solid residues may be stabilized and disposed of in suitable landfill.
The availability of high temperature incineration is a disincentive to the development of technologies and processes that avoid creation of hazardous waste in the first place, or make it possible to reuse or recycle it, rather than disposing of it as a waste product.
However, without the option of incineration, waste is likely to be stockpiled for long periods of time, creating the possibility of accidental discharge into the environment. It is also likely that there would be more illegal dumping of waste if there was no way to dispose of it legally.
References
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Beder, S. (1990) The Case Against an Australian High Temperature Incinerator for Hazardous Wastes. Chemical Engineering in Australia, vol. 76, no. 1, March 1990, pp12-14. Retrieved from http://www.uow.edu.au/~sharonb/incinerator2.html on 17/10/12.
Cernuschi, S., Giugliano, M., Ozgen, S., Consonni, S. (2012) Number concentration and chemical composition of ultrafine and nanoparticles from WTE (waste to energy) plants. Science of the Total Environment, 420 (2012) 319–326.
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Rivera-Austrui, J., Borrajo, M.A., Martinez, K., Adrados, M.A., Abalos, M., Van Bavel, B., Rivera, J., Abad, E., (2011). Assessment of polychlorinated dibenzo-p-dioxin and dibenzofuran emissions from a hazardous waste incineration plant using long-term sampling equipment. Chemosphere, Volume 82, Issue 9, February 2011, Pages 1343–1349.
Unilabs Environmental (no date) Characterisation and Estimation of Dioxin and Furan Emissions from Waste Incineration Facilities. Report for Environment Australia. Retrieved from http://www.environment.gov.au/settlements/publications/chemicals/dioxins/pubs/dioxinemissions.pdf on 17/10/12.
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Veolia Environmental Services (no date) High temperature incineration – the science of managing waste. High temperature incineration brochure. Veolia Environmental Services, UK. Retrieved from http://www.veoliaenvironmentalservices.co.uk on 18/10/12.
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Wu, H., Lu, S., Yan, J., Li, X., Chen, T. (2011). Thermal removal of PCDD/Fs from medical waste incineration fly ash – Effect of temperature and nitrogen flow rate. Chemosphere, Volume 84, Issue 3, June 2011, Pages 361–367.