Time to Address the Air Pollution Control Residue Problem
The incineration of municipal solid waste is gaining increasing importance in the UK, especially as the burning of waste can be used to generate energy, providing a more sustainable solution to our waste problem by diverting it from landfill sites.
An issue with incineration is that some of our waste contains hazardous chemicals such as chlorine and lead which are volatilised during the process. They need to be removed from the gaseous emission streams to stop them being released into the environment. These chemicals are removed via a gas abatement facility and end up in a solid residue called air pollution control residue (APCr).
APCr is classified as an absolute hazardous waste in the European Waste Catalogue because of its high alkalinity due to the addition of scrubbing agents such as lime to the abatement process.
Typically 2-5% of the mass of waste incinerated becomes APCr and so as incineration activities are increasing in the UK so is the generation of APCr. Currently around 300,000 tonnes of this residue is generated annually but the figure is expected to rise to 500,000 tonnes per year in the next 4 to 5 years making APCr one of the UKs fastest growing hazardous waste problems.
There are already established technologies which have the capability to treat and transform APCr into a safe, non-hazardous material. One example of this is the vitrification process, which is one of the most established methods; during this process the APCr is thermally treated to transform it into a uniform glass-like material. The glassy matrix of this material traps the hazardous elements, thereby greatly reducing their mobility and migration into the surrounding environment. As a result the glass-like product displays extremely low leaching characteristics, so much so that this glass can be classified as inert and safe for use in commercial applications.
Despite the availability of such technologies, the majority of APCr is sent to hazardous landfill or underground salt mine storage. Although this is perhaps a simpler approach, it is also an unsustainable approach since the current associated costs range from £100 – £140 per tonne and are expected to continue to rise.
To be accepted for landfilling, the material must meet the Waste Acceptance Criteria (WAC) for the particular class of landfill site (inert, non-hazardous and hazardous). The WAC is primarily concerned with the leaching of chemicals from the material into the environment. APCr exhibits high leaching of lead and chlorine but it is chlorine, whose leaching value is typically 5 times higher than the WAC for hazardous landfill, which is the most troublesome. Consequently APCr must undergo some form of pre-treatment to meet the WAC before it can be accepted for landfill.
There are a small number of landfill sites which have three times WAC derogation permits, which are primarily designed for the management of hard to deal with hazardous wastes. The 3xWAC permits are intended to offer a temporary waste management solution whilst there are no better solutions available; however this can only ever be a short term fix that offers neither a sustainable nor environmentally friendly remedy for this hazardous waste management challenge. DEFRA have indicated a desire to phase out these sites from the UK’s waste management policy saying the UK should “cease to rely on these sites for hazardous waste management”. There have been consultations on this issue between the Environmental Agency and the relevant industries and a decision is expected sometime in the autumn as to whether, or how, these permits will be phased out.
The phasing out of the permits would rely on the state of the technology available to treat the material but the removal of the 3xWAC would further increase the cost of the landfilling of APCr as it would require more intensive pre-treatment, making alternative technologies more economically attractive. Importantly however, it would move the treatment of wastes such as APCr in a more sustainable direction and would aid the development of an environmentally favourable long term vision of waste management.
Tetronics International’s plasma vitrification process which has been successful in the Far East, particularly in Japan, uses thermal plasma technology to transform APCr into what is known as Plasmarok®, an environmentally benign and an Environment Agency approved product, which can be used in industrial applications, for example in construction. Hydrochloric acid is also produced during this process, a saleable by-product extracting more value out of the hazardous APCr which would otherwise be sent to landfill.
Thermal plasma is an intensive heat source which is produced by ionising an inert gas, such as argon or nitrogen, using electrical energy. A plasma arc is chemically neutral and so allows for close control of the furnace conditions. This creates high temperatures without requiring the use of fuels for combustion avoiding any unwanted chemical interactions with the hazardous material. Dangerous organic chemicals, which are often present in wastes like APCr, are destroyed when using plasma due to the effect of the intense heat and ultraviolet light emitted from the arc, which photo-catalytically cracks complex hydrocarbons into simpler, less hazardous, molecules such as CO2.
The small physical footprint of a Tetronics vitrification plant means that one could be retrofitted onto an existing incineration plant, enabling the on-site treatment of the material and therefore reducing transport costs of the material. Despite the small footprint of the plant, one would have the capability of treating 30,000 to 50,000 tonnes per year of APCr and so just a small number of these plants would have a considerable impact on the UK’s ability to sustainably manage the generation of APCr as incineration itself increases.
Recent research and development at Tetronics International has been focused around the reduction of the specific energy requirement in the treatment of APCr. The significant improvements made during the research trials of the energy requirement mean that the operational cost and the demand for electricity is reduced. Consequently the operational costs for the treatment of APCr using thermal plasma are lower than the current costs for integrated hazardous landfill. The reduced energy requirement for the treatment also reduces the environmental impact of the vitrification process and therefore the overall environmental impact of the incineration process. Adding to the environmental benefit of treating APCr, the significant investment which has to be made on gas abatement facilities in incineration plants makes it logical to treat the product formed as a resource rather than a waste improving the return on investment of the plant. Both hydrochloric acid and Plasmarok® are resource-efficient products generated from a waste material which can substitute for other raw materials, something which is emphasised in the long term vision of resource management in today’s society.
Diverting APCr away from landfill is becoming a necessity as the negative environmental impacts of the landfilling of hazardous wastes cannot be ignored and are a blight on the green credentials of incineration processes. The adoption of technologies such as the vitrification process would advance the sustainability and resource efficiency of the overall waste management philosophy of the UK, which in the long term would be a more cost effective solution rather than a myopic “stick it in the ground” approach. It may however take a change in industrial focus, coupled with regulatory change by removing the 3xWAC permits, to ensure that one of the UK’s fastest growing hazardous wastes is no longer one of the fastest growing hazardous waste problems.