Around 130 million tonnes of municipal solid waste (MSW) are combusted annually in over 600 waste-to-energy (WTE) facilities globally that produce electricity and steam for district heating and recovered metals for recycling. Since 1995, the global WTE industry increased by more than 16 million tonnes of MSW. Incineration, with energy recovery, is the most common waste-to-energy method employed worldwide. Over the last five years, waste incineration in Europe has generated between an average of 4% to 8% of their countries’ electricity and between an average of 10% to 15% of the continent’s domestic heat.
Currently, the European nations are recognized as global leaders of the SWM and WTE movement. They are followed behind by the Asia Pacific region and North America respectively. In 2007 there are more than 600 WTE plants in 35 different countries, including large countries such as China and small ones such as Bermuda. Some of the newest plants are located in Asia.
The United States processes 14 percent of its trash in WTE plants. Denmark, on the other hand, processes more than any other country – 54 percent of its waste materials. As at the end of 2008, Europe had more than 475 WTE plants across its regions – more than any other continent in the world – that processes an average of 59 million tonnes of waste per annum. In the same year, the European WTE industry as a whole had generated revenues of approximately US$4.5bn. Legislative shifts by European governments have seen considerable progress made in the region’s WTE industry as well as in the implementation of advanced technology and innovative recycling solutions. The most important piece of WTE legislation pertaining to the region has been the European Union’s Landfill Directive, which was officially implemented in 2001 which has resulted in the planning and commissioning of an increasing number of WTE plants over the past five years.
Solid waste is a growing problem in all countries, and a critical problem in almost all the cities of the developing world. Developed countries have in recent years reduced the environmental impact of solid waste through sanitary landfills and high-temperature incineration, as well as conserving natural resources and energy through increased recycling, but the volume of waste generated in developing countries is rising astronomically. Very few cities have adequate solid waste collection and disposal systems, and the accumulating waste threatens health, damages the environment, and detracts from the quality of life. Therefore, it is necessary to make use of all possible waste management technologies to arrest the degradation of environment and foster waste-to-energy technologies.
Urban areas of Asia produce about 760,000 tonnes of municipal solid waste (MSW) per day, or approximately 2.7 million m3 per day. In 2025, this figure will increase to 1.8 million tonnes of waste per day, or 5.2 million m3 per day. These estimates are conservative; the real values are probably more than double this amount. Most common method of disposing of wastes is to dump them in low-lying areas on the outskirts of towns which is very haphazard and unscientific. This has serious environmental impacts like water pollution, methane emissions, and soil degradation.
The advantages of the refuse-derived fuel plant type are focused mainly on the relatively higher energy content of the RDF fuel, which originates from the pre-combustion separation processing.
RDF plant employs mechanical processes to shred incoming MSW separating the non-combustibles in order to produce a high-energy fuel fraction and thus improved efficiency.
One of the most appealing aspects of RDF is that it can be employed as a supplementary fuel in conventional boilers. Furthermore, RDF’s energy content is around half that of UK’s industrial coals and nearly two thirds that of low grade US coal.
Pelletization scores over mass-burning, anaerobic digestion and composting because the pellets’ energy content is close to that of coal and can be substituted in local industry.
A number of widely employed industrial and utility-scale coal utilisation technologies have the potential to co-utilise RDF and coal, such as large-scale pulverised coal-fired power plant boilers, cement kilns, fluidised bed or stoker-fired boilers, coal gasification plant
Due to reduction in fuel particle size and reduction in non-combustible material, RDF fuels are more homogeneous and easier to burn than the MSW feedstock.
RDF has been successfully burned in a variety of stoker boilers and in suspension as a stand-alone fuel in bubbling and circulating fluidized bed combustion technology boilers. It needs lower excess air and hence works at better efficiency. Also, handling is easier since non-combustibles have been already removed.
In utilizing MSW through a pelletization process, additional tonnage of recyclables will be positively selected to take to that market, assisting the developing regions to work towards sustainable development goals, while positively selecting appropriate materials to mix with purchased high BTU materials in the production of the high BTU pellets, that can be used either to replace coal or coke in industrial processes.
RDF is a much more uniform fuel than MSW with regard to fuel particle sizing and heating value resulting in a more efficient combustion process. In addition, a majority of the non-combustible material is removed from the RDF before the fuel is fed into the boiler which reduces the size of both the fuel and ash handling systems. These fuel characteristics result in a RDF boiler system which is generally less expensive than a mass-burn system, thereby offsetting the cost of the RDF processing equipment.
It is much easier to transport and store fuel pellets to power plants or industries than raw MSW.
Advanced thermal technologies like gasification, pyrolysis, and depolymerization are unattractive to developing countries due to their prohibitive costs.
Technologies to control the release of air contaminants have improved substantially in the past decade and that, therefore, the releases from modern RDF plants are not high enough to have negative impacts on human health.
Air emissions from incinerators should be compared with air emissions from other methods of generating energy. When such comparisons are made, it is found that natural gas power plants are the cleanest way to generate energy but that emissions from waste incinerators are equivalent to or less than the emissions from a coal or oil power plant.
Incinerator proponents also assert that the health risks from exposure to the releases from a RDF power plant are substantially less than the risks from many other common activities.
The conversion of organic waste material to energy can proceed along three main pathways – thermochemical, biochemical and physicochemical. Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products.
Thermochemical conversion includes incineration, pyrolysis and gasification. The incineration technology is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine.
Bio-chemical conversion processes, which include anaerobic digestion and fermentation, are preferred for wastes having high percentage of organic biodegradable (putrescible) matter and high moisture content. Anaerobic digestion is a biological treatment method that can be used to recover both nutrients and energy contained in organic wastes such as animal manure. The process generates gases with a high content of methane (55–70 %) as well as biofertilizer. Alcohol fermentation is the transformation of organic fraction of waste to ethanol by a series of biochemical reactions using specialized microorganisms.
The physico-chemical technology involves various processes to improve physical and chemical properties of solid waste. The combustible fraction of the waste is converted into high-energy fuel pellets which may be used in steam generation. The waste is first dried to bring down the high moisture levels. Sand, grit, and other incombustible matter were then mechanically separated before the waste is compacted and converted into pellets. Fuel pellets have several distinct advantages over coal and wood because it is cleaner, free from incombustibles, has lower ash and moisture contents, is of uniform size, cost-effective, and eco-friendly.
Waste-to-energy plants offer two important benefits of environmentally safe waste management and disposal, as well as the generation of clean electric power. Waste-to-energy facilities produce clean, renewable energy through thermal, biochemical and physicochemical methods. The growing use of waste-to-energy as a method to dispose off solid and liquid wastes and generate power has greatly reduced environmental impacts of municipal solid waste management, including emissions of greenhouse gases.
Waste-to-energy conversion reduces greenhouse gas emissions in two ways. Electricity is generated which reduces the dependence on electrical production from power plants based on fossil fuels. The greenhouse gas emissions are significantly reduced by preventing methane emissions from landfills. Moreover, waste-to-energy plants are highly efficient in harnessing the untapped sources of energy from a variety of wastes.
An environmentally sound and techno-economically viable methodology to treat biodegradable waste is highly crucial for the sustainability of\ modern societies. A transition from conventional energy systems to one based on renewable resources is necessary to meet the ever-increasing demand for energy and to address environmental concerns.