Methods Of Recovering Energy From Waste
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Biomass (waste) energy is increasingly attracting attention worldwide because it is a renewable source of energy and potentially CO2 neutral. At present, most waste materials are converted into electricity often by combustion. Waste combustion is widely applied for district heating and combined heat and power for electricity generation. This report describes waste, waste reduction and treatment regulations in Europe as well as different types of waste generated, an overview of waste to energy technologies applied throughout the world today.
Energy from waste has been evaluated based on their ability to reduce the emission of pollutant into the atmosphere. In order to mitigate climate change which is gaining increasing awareness, recent developments of different technologies that have been able to process waste to generate heat and power with high efficiencies can be considered to be the most viable option to replace fossil fuels.
Due to the rising cost and the ecological disadvantages of fossil fuels, there as been concerns about the future of energy supply in the world. According to the World Energy Council, fossil fuel covers about 82% of the world's energy. And this has caused severe damages for the environment in terms of greenhouse emissions, sea level rising, air pollution, etc. Moreover, as the World continues to experience globalization, rapid industrialization and technological advancement, it will certainly get to a point, where supply of these fossil fuels will not meet demand. Therefore, it is imperative to find an alternative source of energy (Soetaert and Vandamme, 2006).
Research and development of renewable sources of energy and energy from biomass is expected to be of immense benefit to the society as it reduces the concentration of carbon dioxide in the atmosphere and it is not exhaustible. Biomass is a major source of biologically derived bio fuels (bio-ethanol) and biogas. This is considerably becoming a reality in energy/electricity generation. Biomass can be burnt directly to produce heat or electricity, or it can be converted into solid, liquid or gaseous fuel through fermentation process into alcohol and anaerobic digestion into biogas. There are many environmental and social benefits associated with biomass energy. These include reduction in CO2 levels, energy carriers to rural communities, waste control, etc. (Calbe, Bajay, Rothman and Harry, 2000).
Biomass raw materials for energy generation includes the first generation feedstocks i.e. energy crops (corn, sugarcane, wheat, etc) and the second generation feedstock mainly lignocellulosic materials (wood and agricultural residues). Hence, using biomass as a substitute for fossil fuels is sustainable and beneficial. However, there have been contentions on the use of human food (energy crop) for energy generation especially in developing countries where there is shortage of food. Hoffert et al. (2002), "Dismiss the use of biomass for energy, others take the opposite view" (Dewulf and Langenhove 2006). To this effect, energy recovery from waste can be an economical viable option.
Waste arises as a result of human technological development and social activities. The Uk's Environmental Protection Act 1990 indicated waste includes any substance which constitutes a scrap material, an effluent or other unwanted surplus arising from the application of any process or any substance or article which requires to be disposed of which as been broken, worn out, contaminated or otherwise spoiled, this is supplemented with anything which is discarded otherwise dealt with as if it were waste shall be presumed to be waste unless the contrary is proved. Murphy et al. (2002) defined "waste" as a material with no further beneficial use. The figure below shows the Schematic illustration of the EU Legal definition of waste
3.2 EU WASTE FRAMEWORK DIRECTIVE
"The Directive requires all Member States to take the necessary measures to ensure that waste is recovered or disposed of without endangering human health or causing harm to the environment and includes permitting, registration and inspection requirements. The Directive also requires Member States to take appropriate measures to encourage firstly, the prevention or reduction of waste production and its harmfulness and secondly the recovery of waste by means of recycling, re-use or reclamation or any other process with a view to extracting secondary raw materials, or the use of waste as a source of energy" (Department for Environment, Food and Rural Affairs, 2009).
3.3 UK WASTE MANAGEMENT POLICY
The UK Waste Policy is developed from the idea of sustainable development. This policy encourage diversion from landfills by imposing high tax levies on landfill site, reduce the amount of waste produced by the commercial sector, and ultimately encourage re-use and recycling of materials.
3.4 WASTE HIERARCHY
Figure 3.1 Waste hierarchy
3.5 TYPES OF WASTE
Municipal Solid Waste
Industrial and Commercial Waste
Other Waste e.g. construction and demolition industry waste, mines and quarry waste and power station ash, iron and steel slags (Williams, 1999).
There are also two different kinds of component fraction in a waste stream; The Organic fraction i.e. Biodegradable and the Inorganic fraction.
3.5.1Municipal Solid Waste
This is composed of/includes mainly household waste, with commercial and trade waste which is collected or disposed of by a municipality within an area. The composition of MSW streams varies, depending upon socio-economic factors, geographical locations, climate, population density and level of industrialization etc. for example in US and other industrialized countries where value is placed on time, consumers have adapted to the mentality of using disposable at all times, thereby driving packaging and wrapping technologies to a new level, in addition with the putrescible waste from food stuffs. The energy fraction contained in materials of this type of waste stream can be between 75 to 90%. Whereas in less developed countries where the waste stream is composed dominantly of putrescible fraction i.e. foodstuffs, vegetable and organic materials with minute packaging materials, the energy content will be much lower (Murphy, 2002).
3.5.2 Hazardous Waste
Hazardous Waste is waste which contains substances that are considered to be dangerous to health and society. This includes substances which are reactive, infectious, harmful, toxic and corrosive etc. examples are chemicals such as, hydrogen cyanide, sulphuric acid, hydrofluoric acid etc. explosives such as dynamite, ammunition etc. water reactive chemicals such as potassium, phosphorous, sodium hydride etc. In USA, estimate arisings of hazard waste is 275 million tonnes, UK 4.5 million tonnes, Germany 6 million tonnes, Spain 1.7 million tonnes (Williams, 1999).
3.5.3 SEWAGE SLUDGE
Sewage Sludge is usually generated at the waste water treatment facility. It is a by-product of the treatment of raw sewage from domestic households, which may also include commercial and industrial waste. The sewage is composed mainly of water, but after treatment, the moisture material is concentrated to form sewage sludge (Williams, 1999). They are solid, semi-solid or bio solid in nature. Harper-Collins Dictionary of Environmental Science defines Sewage Sludge as "a semi-solid mixture of bacteria, virus-laden organic matter, toxic metals, synthetic organic chemicals, and settled solids removed from domestic and industrial waste at sewage treatment plants." (Renewable Energy Institute) The sewage sludge treatment undergoes both aerobic and anaerobic digestion and the final treated sewage sludge is either land filled, spread on land as fertilizers or incinerated. Recent developments have shown that biogas can be produced during anaerobic digestion.
3.5.4 CLINICAL WASTE
Clinical Waste is mainly produced/waste arising from health centres, hospitals and nursing homes etc. Examples include drugs, syringes, needles, blood, human or animal tissue etc. Approximately 0.3 millions tonnes of clinical waste arises in the UK annually. Majority of clinical waste are incinerated (Williams, 1999).
3.5.5 AGRICULTURAL WASTE
Agricultural waste is waste streams generally produced from agricultural activities. They are produced within agricultural premises and they include organic materials such as slurry, manure from livestock, silage effluent and crop residues. It is estimated that approximately 700 million tonnes of agricultural waste is produced in OECD countries. UK also produces large tonnage, 80 million tonnes estimated from housed livestock alone (Williams 1999).
3.5.6 INDUSTRIAL AND COMMERCIAL WASTE
These are types of waste stream that arises from both industrial and commercial sectors such as hotels and catering, food, drink and tobacco manufacturing industries, metal manufacturing industries, timber and wooden furniture industries, mechanical and electrical industries, transport and communication industries etc. The typical composition of this waste streams differs and very wide. A survey carried out by Environmental Agency of some 4,500 commercial and industrial businesses in England in 2002/2003 showed that commercial waste amounted to 30 million tonnes and industrial waste 38 million tonnes (mt). "In that survey, the main sectors producing C&I wastes were retail (12.7mt), food, drink and tobacco (7.2mt), "professional services and other" (7.1mt), utilities (6.2mt), the chemicals industries (including fibre, rubber and plastics) (5.3mt), basic metal manufacture (4.8mt) and hotels/catering (3.4mt)" (Defra, commercial and industrial waste in England, 2009).
4 ENERGY RECOVERIES FROM WASTE TECHNOLOGIES
Energy is recovered from waste either through thermal combustion or biological/chemical reactions. The energy recovery process produces electricity directly through combustion, or produces synthetic and combustible fuel i.e. methane.
Incineration with energy recovery is the controlled combustion of waste and it is the most wide spread waste to energy implementation. It involves the combustion of waste streams at high temperatures and the heat produced can be used to drive a turbine in order to produce electricity and district heating. Waste materials or fuel are fed into incinerators in two ways, the mass fired/burning systems and refuse derived fuel (RDF) fired systems. The mass burning involves minimal processing; the entire mixed municipal solid waste is fed into a furnace without any removal/separation of recyclable and non combustible materials. For RDF fired technologies, MSW undergoes pre-treatment, separation of non-combustible and recyclable material which is known as RDF. RDF fired systems has a higher energy content compared to unprocessed MSW because of its homogeneity (Tchobanoglous, Thiesen and Vigil, 1993).
There are various types of incinerator plant design: moving grate, fixed grate, rotary-kiln, and fluidized bed incinerator.
4.1.1 Moving Grate
This can also be called Municipal Solid Waste incinerators. The moving grate enables the movement of waste by a waste crane at one end of the grate through the combustion chamber to the ash pit at the lower end. The combustion air is supplied through the grate lying below. Cooling of the grate itself is essential for the mechanical strength of the grate. One single moving grate boiler can handle 35 metric tonnes of waste per hour, and 8,000 hours per year.
4.1.2 Fixed Grate
This is a simpler type of incinerator. It is made of a brick lined compartment with a fixed metal grate above the lower ash pit, with one opening for loading and another opening in the side for removing incombustible solids known as clinkers.
4.1.3 Rotary kiln
It is mostly used by municipalities and large industrial plants. Rotary kiln incinerators have 2 chambers, primary and secondary chamber. In the primary chamber, movement and conversion of solid fraction of the waste to gases and partial combustion occurs while the secondary chamber completes the gas phase combustion reactions.
4.1.4 Fluidized Bed
Fluidized Bed Combustion (FBC) is a combustion technology used in power plants. FBC developed from efforts to find a combustion process that is able to control pollutant emissions. Advanced fluidized bed combustion offers a viable power generation technique. In fluidized bed combustion, a strong airflow is forced through a sand bed, which keeps the waste suspended on pumped air currents and takes on fluid like character. Due to the turbulent mixing, the waste and sand are fully circulated through the furnace.
4.2 BALDOVIE WASTE TO ENERGY PLANT
The Baldovie waste to energy plant is a state of the art facility commissioned in Dundee in 1999, to replace a waste disposal incinerator due to its inability to meet EU requirements on emissions. The plant is run by a joint venture between Dundee city council and private sector partners. The joint venture is collectively called Dundee energy recycling limited (DERL). The plant processes 120,000 tonnes of waste annually and generates electricity to meet its own demand (about 2.2MW) and also supply to the public (about 8.8MW). Separation techniques carried out before combustion removes and recovers ferrous metals which can be resold (Gazetteer for Scotland, 2010).
4.3 LANDFILL GAS CAPTURE
Landfill is a waste disposal site where waste is deposited onto or into the land. When waste is deposited, it undergoes various degradation process which produces gas mainly methane. Modern landfills have gas recovery systems, where the landfill gas is extracted and can be used for production of steam, heat and electricity (Dewulf and Langenhove 2006, p.248)
4.4 ANAEROBIC DIGESTION
Anaerobic treatment technologies are used throughout the world for effective treatment of organic waste. This technology is particularly attractive because the energy required for operating the process is minimal compared to energy required for aerobic process. Anaerobic digestion is a complex biogenic process which involves the microbial degradation or conversion of organic waste in a closed reactor vessel (absence of air) to produce gas chiefly methane (55-65%), CO2 (35-45%), and trace amounts of N2, H2 and H2S, depending on factors such as the composition of waste, organic loadings applied to the digester, time and temperature. The methane-rich biogas which can be combusted to generate heat and electricity
In general anaerobic digestion is considered to occur in three stages:
A] Liquefaction or polymer breakdown;
B] Acid formation; and
C} Methane formation
Substrates for anaerobic digestion includes waste water from food processing, breweries, distilleries etc. sewage sludge, animal waste, farm residues.
4.5 GASIFICATION AND PYROLYSIS
Gasification is defined as a partial oxidation/combustion of biomass and various combustible waste/carbonaceous fuels to produce gas. This gas can be used in internal combustion engines and gas turbines to generate electric power.
Pyrolysis is a thermal process which involves the breakdown of organic materials in waste under pressure in the absence of oxygen. The pyrolysis process produces a liquid residue and gas which can be combusted to generate electricity.
The principal difference between the two systems is that, pyrolysis system use an external source of heat to drive the endothermic pyrolysis reaction in an oxygen free environment, whereas gasification systems are self-sustainable and use air or oxygen for partial combustion of solid state (Tchobanoglous, Thiesen and vigil 1993).
Energy from waste has been evaluated based on their ability to reduce the emission of pollutant into the atmosphere, minimise waste, and generate heat and power.
With the recent development of different technologies that have been able to process waste to generate energy with high efficiencies, waste to energy technologies can be considered to be viable substitute/option for fossil fuels for electricity generation and also in minimizing of waste accumulation.
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