By: Boghos Ghougassian

The world is now discovering new means to meet the growing demands for energy, to fight climate change. And “Waste-to-energy” (WtE) is the slogan, which is a sustainable waste management paradigm and at the same time a renewable energy generator with proven success stories.

Waste sources such as mixed municipal solid waste (MSW) and agro-industrial residues (farm waste and food industry waste), apart from wind and solar energy, constitute the most abundant renewable energy reservoir on earth. Waste-to-energy technologies are the processes of creating energy in the form of electricity or heat or bio-fuels or synthetic fuels (syn-fuel) from the conversion of waste source.

Modern WtE technologies can be broadly classified as thermal and non-thermal processes. Most of the modern or advanced WtE processes produce electricity directly through combustion, or produce a combustible fuel commodity, such as methane gas, methanol, ethanol, hydrogen or synthetic fuels. Worldwide confidence on advanced WtE processes are increasing as safe and proven renewable energy technologies that meet stringent environmental standards. In 2007 there were more than 600 large-scale WtE plants in 35 different countries. Denmark processes more than any other country – 54 % of its waste goes into WtE plants. Sweden, Belgium, Germany, the Netherlands, Japan, Spain and France process more than one third of their wastes through WtE plants. The United States processes 14 % of its trash in WtE plants. Overviews on WtE technologies are presented here after.

Thermal processes include incineration, pyrolysis and gasification technologies. Incineration is the combustion of solid waste with energy recovery and it is the most common WtE implementation in the world that produces electricity and heat. All new incineration plants in OECD countries meet strict emission standards and they reduce the volume of the original waste by 95-96 %, or 75% by weight. Norms for conventional incinerators are 90% reduction by volume and 70% reduction by weight. The energetic use of waste continues to grow throughout the world – global incineration capacities increased by 3% per year from 2005 to 2010. Japan is the largest user of thermal treatment of MSW in the world with 40 million tons per year. However, incineration based processes for MSW treatment are a subject of intense debate around the world. In general, concerns regarding their operation include the production of air pollutants such as fine particulates, heavy metals, dioxins, acid gas emissions and adding to climate change, even though these emissions are very low from modern incinerators. In addition, most of the current WtE incinerators generate electricity through the steam cycle which has low levels of electric efficiencies, on the order of 14-28%. The rest of the produced energy is utilized as heat, but is otherwise lost as waste heat. In the Arab region, the use of incinerators for MSW treatment is limited to hospital waste management. Currently the only operational incinerator is in Qatar, operated by Keppel Seghers Co.

Some countries like Lebanon and Morocco have installed incinerators during the 1970s and 1980s. But their operations have stopped after some years due to the citizen’s opposition, high operational costs, and the high humidity (70%) content in the collected MSW that rendered it difficult and expensive to burn. Pyrolysis is another thermal technology which is often referred to as destructive distillation. This thermal process involves the superheated degradation of carbon-rich organic matter, in the absence of oxygen, in order to produce three forms of energy: solid char, 35 wt %, liquid oil (bio-fuel), 40 wt%, and a synthetic gas (syn-gas), a gas mixture of carbon monoxide, hydrogen and carbon dioxide, 10 wt%. The first two are storable, while the gas is burned in the pyrolytic process, which is far less regressive than incineration. Pyrolysis provides an option which puts process products to direct use. Recent trend is to convert non-recycled plastics into oil or gas. This saves 1.8 to 3.6 million Btu per ton of plastic. Data from industrialized countries indicates that the cost to process the MSW is approximately $50 per ton for pyrolysis technologies. Germans, British, Swiss and Italians are the countries most active in pyrolysis technologies that combine with recycling and composting.

The major advantages of the pyrolysis processes include: few air emissions and more efficient in energy production than incineration (70% vs. 40%). The main disadvantage is that it might generate possible toxic residues similar to incinerators. Another disadvantage is the need for grinding the waste, to make the particle size suitable for feeding into the pyrolysis system, making waste treatment expensive. On the other hand, Gasification technology is an efficient means of converting low-value fuels and residuals into a synthesis gas (or syn-gas or producer gas). A wide variety of feedstock can be gasified, such as MSW, refuse-derived-fuel (RDF), non-recycled plastics, agro-industrial residues, dried sewage sludge, and coal. These fuels are converted into synthesis gas containing mainly carbon monoxide and hydrogen gases. This is achieved by reacting the material at high temperatures (>700 °C), without combustion, with a controlled amount of oxygen and/or steam. The heating value of the resulting syn-gas mixture is10-15% that of natural gas.

The process of producing energy using the gasification method has been in use for more than 180 years. Wood gas generators, called Gasogene, were used in vehicles to power their engineers. By 1945 there were more than 9 million trucks, buses and agricultural machines that were powered by gasification in Europe and elsewhere. In large-scale plants, the syn-gases are utilized directly in engines for electric power generation and heat production, which is economically and environmentally attractive. The syn-gas in turn can be converted to methanol, ammonia, synthetic gasoline, or used directly as a natural gas substitute and even blended with it in a gas supply line. When compared to landfill disposal, gasification of MSW saves 7 to 14 million Btu per ton and 0.33 to 0.66 tons of carbon equivalent emissions per ton of MSW. The cost to process the MSW is approximately $50 per ton for gasification technologies. The pyrolysis and gasification reactions are presented in the following diagram. Pyrolysis of carbonaceous fuels Gasification of char Several types of gasifiers are currently available for commercial uses including: counter-current fixed bed, “down draft”, fluidized bed, plasma and other processes. Main advantages of waste gasification over incineration is that electric power may be generated in engines and gas turbines, which are much cheaper and more efficient than the steam cycle used in incineration. Also lower emissions are achieved and the ash (2%) can be released in a glassy and chemically stable form, while up to 98% of the waste stream can be converted into energy. During the last decade the gasification technology has become more popular and it is being adopted for large-scale plants that are being installed all over the world.

Since 2009 gasification of waste is expanding across the USA for MSW conversion to energy and industrial-scale gasification of low quality fossil fuels such as coal. In the case of Non-Thermal processes there are a number of new and emerging technologies that are able to produce energy from waste and other fuels without direct combustion. Many of these technologies have the potential to produce more electric power from the same amount of waste than would be possible by direct combustion. Some of the non-thermal technologies are able to efficiently convert the waste into liquid or gaseous fuels. Bio-digestion of wastes have high potential in the non-thermal WtE processes. Digestion is a biologic process by which organic waste is broken down by the action of bacteria (or enzymes) into simple molecules, either an-aerobically (without oxygen) or aerobically (with oxygen) and renewable fuels are produced. Anaerobic digestion (AD) of biomass is a true renewable energy technology. It relies on anaerobic bacteria to break down biodegradable waste material in the absence of oxygen. The anaerobic digestion produces three by-products including: biogas, bio liquid (or liquid digestate), and fiber digestate (compost).

Biogas is a gaseous mixture composed of 60% methane (CH4) and about 40% carbon dioxide (CO2), with trace amounts of hydrogen sulfide and ammonia. Biogas can be used in combustion engines to generate electricity and heat. In addition, liquid and fiber digestates can be used as fertilizers or compost to improve soil fertility. An-aerobic digestion offers potential energy savings and is a more stable process for medium and high strength organic effluents. Other advantages of the AD include: Generation of renewable energy in the form of electricity and heat; Avoidance of emissions in the atmosphere and smell; Avoidance of liquid effluents, hence no water pollution occurs in case of waste digestion that reuses the process water; Reduction of waste volume by 85%; Waste treatment cost is as low as 10 Euro per ton; Capital and operational costs of AD plants are lower than that of aerobic waste treatment plants. The main disadvantage of AD of waste is when poor feedstock is used in the digestion process, which may result in the production of unusable by-products. WtE technology includes also aerobic digestion or fermentation processes, which can take biomass and create ethanol (ethyl alcohol) and other products, using cellulosic waste or organic material.

In the bacterial fermentation process (enzymes may be used to speed up the process), the sugar in the waste is converted to carbon dioxide and alcohol (ethanol), in the same general process that is used to make wine. Production of ethanol from food crops, such as sugar cane, is another example of aerobic digestion. Brazil since decades produces ethanol from crops which is blended with gasoline and used as car fuel. Ethanol can be also produced from MSW. E.g., Recently (2010), Fulcrum BioEnergy Inc. located in Pleasanton, CA, has established a large scale WtE fermentation plant near Reno, NV. that was planned to produce approximately 40 million liters per year of ethanol from 90,000 tons per year of MSW. On the other hand, Bio-chemical Technologies are other non-combustion type WtE processes for the treatment of MSW and production of energy. The Dendro Liquid Energy (DLE) technology is a recent innovation and has high potential in the WtE field. DLE is close to “zero-waste” technology. In this case all sorts of mixed waste, including plastics and large size wooden logs are treated in a reactor, to produce carbon monoxide and hydrogen that are clean fuels for generating electric power. The German developers of the technology claim that DLE is four times more efficient in power generation, compared with anaerobic digestion. No emissions, no effluents and no nuisance problems results at the DLE plant sites.

At the end of the process 4 to 8 % inert residues (sand, gravel, etc.) remain that are used as aggregates or for land-filling. This technology is a proven tool that solves the waste management issue of cities and farms and at the same time contributes to the renewable energy basket of countries. Specific advantages of DLE technology include: Small decentralised low-cost units; No-burn, no-emission technology involved; CO2 – neutral, eligible for CDM certificates; High energy efficiency and 80% energy conversion; Moderate process conditions, at 150 – 250 °C, depending on type of input material; Accepts large variety of input materials, both in dry and wet states; Output is a clean synthesis gas (CO + H2), free from tar and particulates A typical 30.000 t/year installation (based on dry input material), needs an investment of Euro 14.5 million and an annual operating costs of about Euro 1,750,000. Annual total revenues of the plant would be about Euro 16,960,000.

This implies that the DLE plant will cover its cost within one year and secure high profits during its lifetime of more than 15 years. Finally, Mechanical Treatment of waste is another common approach for MSW management. In this regards, refuse derived fuel (RDF) production is the common practice at MSW recycling centers. RDF in pellet form is then used as a uniform fuel in either incineration or gasification plants, if transport is not over long distances or in cement factory kilns as a substitute for fuel and its ashes enter into cement composition. In conclusion, present trends for waste management indicate a move away from conventional single solutions, such as mass burn or landfill, towards the integration of more advanced WtE technologies, based on setting priorities for waste treatment methods.

These include waste minimization, recycling, materials recovery, composting, anaerobic digestion and biogas production, energy recovery through RDFs, and finally residual landfilling. This approach favors the integration of combustion technologies (incineration, pyrolysis, gasification) for energy production, within a range of complementary approaches.