Very high temperature bio-waste treatment using plasma-based modules has been tested in Sweden and at IEERAS in Russia, producing impressive results in terms of CO2 and toxin reduction, and in production of syngas.
The demand for more effective and ecologically clean processes for the treatment and neutralisation of waste is rising. Of the methods available the plasma approach promises to be the most effective and ecologically clean, emitting only minute amounts of CO2 and methane and none of the dioxins and other harmful emissions that are inherent to conventional incineration. It offers the potential to solve three problems simultaneously: the production of ecologically clean energy, the disposal of waste, and the release of landfill sites for other uses.
Experience to date at the handful of actual plants built has been mixed. At least one of the three medium scale plants built in Japan is rumoured to have have failed to produce more power than it consumes, while two plants, in Australia and Germany, closed down because their emissions performance failed to meet environmental standards. The IEERAS and ScanArc plants described here have been designed to overcome these drawbacks.
The owners of a large scale plant in St Lucie, Florida, due to come on line in 2009, are similarly optimistic. It is designed to generate 120 MWe from syngas, and produce 600 t/day of solid material, from 40 MW of power consumed and 3000 t/day of waste.
Generally, waste can be divided into two groups: highly toxic matter that must be decontaminated and neutralised, and low-toxicity materials that may be treated, to produce secondary raw materials such as synthetic gas (syngas) that can be used directly to generate heat and electricity, or as a raw material for the production of liquid fuels and hydrogen.
Of special interest in this regard is waste containing organic components (paper, plastic, wood, tyres, etc), such as solid municipal waste, and products of the wood, food, and agricultural industries. Such waste is very profitable and productive of raw materials since it is available in large quantities, and has high calorific value and low toxicity. The application of plasma technology to this kind of waste has several advantages, mainly for the production of syngas. Conventional incineration is significantly less productive of raw material and syngas.
The decontamination and neutralisation of ‘super toxic’ waste (mixtures of freons, PCV, and so on) can be achieved with absolute reliability by plasma methods, an assurance that cannot be provided by conventional incineration.
Plasma technologies are all based on generators in which plasma is created by passing a suitable gas, typically air or nitrogen, through stabilised electric arcs (illustration below). The composition of the gas is critical in determining plasma arc efficiency and electrode life.
Plasma treatment exposes the waste to extremely high temperatures leading to the molecular dissociation and complete transformation of organic components into a syngas composed mainly of carbon monoxide (CO) and hydrogen (H2) in approximately equal volumes. The non-organic components (metals, glass, and so on) melt down and are removed from the reactor after which they can be re-used as secondary raw materials.
It is the plasma process temperature of more than 12000°C that leads to the main advantages of the technology:
• decreased emissions of CO2 and methane into the atmosphere, and the absence of harmful elements, first, because there are no dioxins created in the plasma gasification process and second, because the process is closed;
• the quantity of solid particulates (in particular, of ash) is decreased to a minimum;
•the quantity of non-organic exhaust components is minimised;
• the useful yield of syngas is significantly higher than with other methods;
• the effectiveness of the gasification process provides opportunities for the application of combined cycle options.
Conventional incineration has been in use for a long time and has been widely applied as the basic method for waste treatment, but has the crucial drawback that dioxins are created and released into the atmosphere, since it is impossible to remove them by the usual methods of flue gas clean-up. Moreover, the equipment necessary to remove the large amount of particulates created increases the cost of plant.by approximately 30%. This waste must be taken to landfill, which leads to the risk of polluting ground water with unprocessed particulate and toxic content. The toxic content of the ash is significantly decreased with plasma treatment, and it may become economic to recover metals such as lead and zinc from the ash, owing to their increased concentration.
Comparing the technologies
The conventional incineration process consists of burning the delivered waste and reburning the exhaust gases. With plasma technology the waste is processed at a constant temperature of 12000-20 000°C. The product syngas can be used to produce electric energy via a gas turbine or gas engine. In combined cycle, a generation efficiency of 55–60% may be achieved, and with full use of the heat energy, the thermal efficiency can be 90%. The latter value is higher than usual because the exhaust exit temperature is so high – about 12000°C.
A principal advantage of the plasma method lies in its ability to regenerate energy from organic components during the treatment process. In conventional incineration the waste heat is only used to raise steam for heating, so the main goals are to decrease the volume of product waste and produce energy in the form of heat.
In the countries of the European Union, the law defines norms on waste incineration and processing of exhaust gases and now requires that the combustion temperature is maintained at more that 850°C for at least 2 seconds to minimise the creation of toxic components. This requirement has led to the enforced reconstruction of many incineration plants in Europe. Plasma technologies run at temperatures greater than 12000°C, so no unprocessed components leave the reactor.
The syngas produced by plasma technology may be sold as fuel or as a raw material for the chemical and fuel industries for the production of liquid fuels and hydrogen. The latter process has significantly higher effectiveness than electrolysis. Under the proper conditions the volume fraction of hydrogen in the syngas can be as high as 45%, when, for instance, waste with high wood content is processed.
The comparison tests for which the results are shown in Tables 1 and 2 were implemented in the following way. The common features, for instance building and maintaining engineering structures, and the cost of energy, were ignored, as was the water content limit placed on the raw waste. This figure is usually given as not more than 35%.
Waste treatment plants were compared on the baseline of a similar waste handling rate (about 5 tonne/h); and it was assumed that the cost of waste disposal and management were the same in each case. Performance was assessed on the basis of the value of the energy produced, the value of the energy consumed, and investment and payback figures. The usual size of a plasma module corresponds to a unit handling 5 tons/hour. Larger plants would be created by linking modules together, with a common system for syngas collection.
Data on conventional waste incineration plant came from the Swedish company KMW Weckman which specialises in the incineration of dried waste from the wood processing industry. The reference plant handles 5 tons/h, and supplies a 4 MW turbine. Of the two plasma plants the first was made by the Swedish company ScanArc, the second by IEERAS at the Institute for Electro Physics and Electric Power of the Russian Academy of Sciences (IEE RAS), Russia. The two plants are engineered differently, but each uses a plasma generator as the basic working component and they present similar indices of effectiveness and performance.
The waste materials used consist of wood chips, waste material from the wood processing industry, solid municipal waste, and wood gathered at building demolition sites. The waste can contain dangerous substances, such as arsenic, copper, and chrome. Its humidity level of not more than 30% rendered it commercially profitable for treatment by a plasma based system, but it is not the only fuel available: RDF (refuse derived fuel), a mixture of plastics, wood, and paper obtained from the process of sorting household waste into various categories, is another possibility.
SWOT’ analysis of waste incineration
Waste incineration technology has been developed over a long period, during which there have been many new developments in flue gas cleaning. There are now more then 40 000 plants around the world, representing a very large investment. A great deal of experience has been gained in constructing and maintaining them.
It requires considerable investment in flue gas cleaning, the largest single proportion (about 30%) of the whole plant cost. A more serious issue is the creation of CO2, which is highly favoured at the process temperature of about 850°C. Significant amounts of dangerous toxins can be found in the fly ash, leading to difficulties in managing safe landfill. Ash from waste incineration contains a high proportion of unprocessed carbon which could otherwise be a source of energy. The quantity of ash is a high proportion of the input. The coefficient of energy transformation is rather low.
As conventional incineration has wide applicability throughout the world and is already in place it will continue for some time as the mainstay technology even if gradually replaced.
There is growing concern among local communities that waste incineration is a short term solution. It is argued by some that instead of putting off the problem we should be making efforts to find a lasting and robust solution.
SWOT analysis of plasma treatment
The system does not contribute to emissions of polluting solvents and chemicals. It does not increase the amount of CO2 in the atmosphere. All inorganic waste is solidified, can be utilised, and does not leach. All organic waste material is converted into syngas, which can be used for fuel or as a raw material for synthetics such as FT-diesel fuel, which can be used unmodified in conventional diesel engines.
The technology is quite new in waste applications and has not yet been carried out on a large scale. The investment level required is high, as in other high-tech areas.
Given the environmentally friendly properties of the plasma based system, there is great potential in the future especially if legislation on the polluting processes inherent in waste management becomes more severe.
The system has a very wide range in terms of its ability to neutralise different types of waste including hazardous materials.
For plasma treatment to become the primary choice in waste management, its full potential has to be more familiar to the world at large.
When switching to a new technology there are many political and economic obstacles that have to be overcome, including the achievement of the stability required to minimise risk during the transition from old to new. New legislation would help to alleviate this threat.
Not all types of waste are profitable avenues, financially or in terms of the energy equation, for plasma treatment. For example, where there is a large volume of material that has to crushed and remelted. A key factor is the water content of the waste, and whether it is efficient to pre-dry it.
All existing incineration facilities have a long depreciation period and it is not likely that they will be replaced by plasma based systems at a fast rate: nor is the benefit of changing over great enough to accelerate this transition. Nonetheless the new technologies are expected to undergo extremely rapid development, especially in USA, Japan and Taiwan.
A critical factor may be that plasma based systems are much better at dealing with certain waste materials than are incineration plants. The most important benefit is in the ability of such systems to transform the energy contained in the waste into the energy content of the syngas. A good example is the gasification of biomass for fuel production. The biomass-to-liquid fuel industry is a growing area in biofuel energy technologies, and plasma systems have great benefits over existing technologies owing to the low tar content in the syngas product. This is particularly applicable to the conversion of municipal waste to biofuel.