Waste to Energy

The consumption habits of modern consumer lifesyles are causing a huge worldwide waste problem.

 

Waste Technologies 

 

Organic waste-to-energy technologies can be broadly classified as either biochemical, chemical or thermal processes.

 

Bio-chemical Conversion 

Digestion is a bio-chemical process by which organic waste is broken down by the action of bacteria into simple molecules, either aerobically (with oxygen) or anaerobically (without oxygen). Aerobic digestion takes place where the waste is aerated, such as in the early stages of decomposition of municipal solid waste (MSW) and during composting. Anaerobic digestion takes place where the waste has restricted aeration, such as in the later stages of the decomposition of MSW or in the digestion of sludges or wastewater in enclosed digestion vessels. Aerobic digestion produces carbon dioxide and water, whereas anaerobic digestion produces methane and water, and also some carbon dioxide and hydrogen sulphide. The gas produced by anaerobic digestion can therefore be combusted and used, either to produce electricity or heat, thereby converting the methane gas to carbon dioxide (with a lower global warming potential).

 

Landfill Gas

Landfill gas is an adventitious fuel that is a by-product of current landfilling practices and hence occurs only after MSW has been disposed of in a totally non-sustainable way. The anaerobic digestion of the buried solid organic waste produces the landfill gas naturally, as the bacterial decomposition of the organic matter continues over time. It is an extremely low efficiency way of recovering energy from MSW. In the long run, as the use of landfills necessarily dwindle, landfill gas will disappear as a resource.

The methane produced in landfill sites normally escapes into the atmosphere, unless the landfill gas is captured and extracted by inserting perforated pipes into the landfill.

In this process, the gas will travel through the pipes under natural pressure or a slight vacuum to be collected and used as an energy source, rather than simply escaping into the atmosphere.

As well as electricity production, landfill gas can be used in thermal applications, where the gas is burnt to provide heating for buildings and industrial processes. Whilst this application can be less economic than electricity production due to transmission costs associated with taking the gas to the desired location, the use of gas on site (such as the construction of facilities on reclaimed landfill sites) increases the viability.

 

Thermo-chemical Conversion

Thermal processing of organic waste materials can produce heat or a number of liquid or gaseous fuels. There are three main options for recovering energy from solid refuse: 

  • by mass burn (combustion or direct incineration) of MSW without pre-treatment,
  • by production of more or less refined fuels out of the main waste stream either partially processed or more highly processed refuse derived fuels (RDF) as pellets for later combustion in incinerators (such as rotary kilns) or via new pyrolysis or gasification techniques, and
  • by the development of new approaches involving the recovery of chemicals such as plastic monomers combined with gasification, pyrolysis, hydrogenation and/or reforming of the gases and oils produced.

 

Direct Combustion and Incineration

Also described as mass burn or direct incineration, direct combustion is the burning of waste to produce heat for cooking, space heating, industrial processes or for electricity generation. Ash from the incineration process can also be sold to the construction and road building industry to further reduce the amount of material to be ultimately disposed. Dry wastes are required for direct combustion, and dried sludge from wastewater can also be used as a feedstock.

On a larger scale, solid waste (including agricultural and forestry residues), can be combusted in furnaces to produce process heat to feed steam turbine generators. Power plant size is often constrained by the availability of local feedstock and is generally less than 25 - 40 MWe. However, by using dedicated feedstock supplies, such as the co-location of incinerators at waste disposal sites, the size can be increased to 50 -75 MWe, gaining significant economies of scale.

 

Refuse Derived Fuels (RDF)

Using raw unprocessed MSW as a fuel is problematic due to the heterogeneous nature of the material, which varies from suburb to suburb and season to season. It also has a low heat value and high ash and moisture content. This makes it difficult for plant designers and operators to always provide acceptable pollution free levels of combustion. Processing of the waste to RDF partially overcomes these problems and the fuel can then be used more successfully in different ways. 
Waste with a high organic (carbon) content is suitable for briquetting and pelletising after non-combustible and recyclable materials have been separated. These processes involve the compaction of the waste at high temperatures and very high pressures. The organic matter is compressed in a die to produce briquettes or pellets. It is important to note that using processed waste (where recyclable and non combustible components have been removed), for power generation will dramatically increase the efficiency of the waste to energy process, but at an increased cost due to the increased handling of the product.

 

Gasification 

This process of partial incineration with restricted air supply to create an air-deficient environment, can be used to convert biomass and plastic wastes into synthesis gas with a heating value 10-15% that of natural gas. When integrated with electricity production it can prove economically and environmentally attractive, though it appears better suited for clean biomass, such as wood wastes. The synthesis gas (CO + H) in turn can be converted to methanol, synthetic gasoline, or used directly as a natural gas substitute and even blended with it in a gas supply line. Even at a larger scale (say >50MW), such processes are not usually cost effective compared with using natural gas.

 

Fermentation

Organic wastes can be converted to ethanol, the alcohol found in beverages, through bacterial fermentation, which converts carbohydrates in the feedstock to ethanol. Feedstocks to date have included agricultural wastes, such as molasses or waste starch, with more recent developments focusing on municipal organics, including food and sewage sludge. The production of ethanol from cellulose components, such as corn cobs and rice straw is under development.

The overall chemical reaction conducted by the yeast is represented by the chemical equation: C6H12O6 → 2 CH3CH2OH + 2 CO2. The vast majority of ethanol is produced by fermentation and is used as fuel.

 

Esterification

Biodiesel can be produced from straight vegetable oil, animal oil/fats, tallow and waste oils.

Almost all biodiesel is produced using base catalysed transesterification as this is the most economical process. It requires only low temperatures and pressures and produces a 98% conversion yield. The Transesterification process is the reaction of a triglyceride (fat/oil) with an alcohol to form esters and glycerol. The heavier co-product, glycerol, settles out and may be sold as it is or it may be purified for use in other industries, e.g. the pharmaceutical, cosmetics, etc.

Biodiesel is a less toxic and more biodegradable fuel than is petroleum diesel and is often blended with petroleum diesel to provide a renewable energy component in the fuel.

 

Pyrolysis

Pyrolysis is defined as incineration under anaerobic conditions and is another option for waste-to-energy that is being investigated. Pilot projects using pyrolysis for plastic wastes, and for mixed municipal solid waste potentially have very high-energy efficiencies. Combined pyrolysis and gasification systems and combined pyrolysis and combustion have also been developed and implemented. 

 

Enersludge

An alternative to incineration or anaerobic digestion of sewage sludge (or dumping it out at sea, which is still often used as the disposal method) is the innovative Enersludge process, which coverts the sludge into useful bio-oil. The concept was first promulgated by Prof Bayer in Germany in the early 1980s but it is only recently that environmental pressures and the economics of other treatment options have made it competitive. 

 

Combustors

Technology for coal combustion has been adapted for combustion of biofuels and waste products. Combustion of biomass is more complex than coal combustion, due to the non-homogeneity, variation in moisture content and composition of the feedstock.

 

 

Best and Biggest

California becomes home to the world's largest waste-to-energy plant converting landfill gas into usable natural gas. Major landfill operator Waste Management, in partnership with the Linde Group, just opened its $13.5 million facility outside of Livermore, with plans to produce 4 million gallons of fuel a year.

 

South African thermal processing specialists Prestige Thermal are leading the way in the global waste-to-energy conversion industry, having opened the world's largest conversion plant in March 2008.

Employing these new technologies, Prestige Thermal's plant in Wadeville, has the capacity to produce 3MW of electrical energy from three tons of municipal solid waste (MSW).

In the meantime, the company has received considerable attention from Europe. The company is recognised one of the global industry leader particularly in the area of waste management best practice.

OTTAWA, ONTARIO--(Marketwire - Oct. 22, 2009) - A technology that converts municipal solid waste into electricity moved toward commercialization today. Sustainable Development Technology Canada (SDTC) announced that the Plasma Gasification for Municipal Solid Waste project led by Plasco Energy Group Inc. (Plasco) has reached completion. It is result of more than 25 years of focused research and development.

 

The project demonstrated a Plasma Gasification process that converts 75 tonnes a day of municipal solid waste into synthetic gas, inert solid material and heat. The gas is utilized in a power plant to produce electricity for sale into the electricity grid.

To date, the City of Ottawa has signed a letter of intent to bring a 400 tonne per day Plasco facility to the community and the Central Waste Management Commission in Red Deer, Alberta has signed a contract for a 200 tonne per day Plasco facility. Several other municipalities in Canada and around the world have shown interest in Plasco's technology.

Europe is the largest waste to energy plants market in the world with a very well developed infrastructure and over 429 installed plants in 2008. New analysis from Frost & Sullivan (European Waste to Energy Plants Market), finds that the market earned revenues of €3.10 billion in 2008.

Countries such as France and Germany have the largest number of waste to energy plants. Such plants have facilitated the effective treatment of waste diverted from landfills, enabling these countries to reach successfully their landfill diversion targets.