Brief Introduction: There have been lots of reports from all over the world about utilization of municipal solid waste(MSW) for conversion to energy. In India too, we have been talking and discussing about segregation of household waste to enable subsequent processing activities. But it has not taken off in any significant way due to a number of reasons. In the next couple of posts I intend to summarize the approaches followed worldwide along with Technology options available. Hopefully, it will throw light on what we need to do in India to put our act together for exploiting municipal solid waste (MSW) and its conversion to energy. Globally too, although there are quite a few success stories to relate, it has been rather difficult to sustain interest. This post is based on a recent extensive report from EPRI (Electrical Power Research Institute) on this subject. I do believe there are several…
Thailand’s annual energy consumption has risen sharply during the past decade and will continue its upward trend in the years to come. While energy demand has risen sharply, domestic sources of supply are limited, thus forcing a significant reliance on imports. To face this increasing demand, Thailand needs to produce more energy from its own renewable resources, particularly biomass wastes derived from agro-industry, such as bagasse, rice husk, wood chips, livestock and municipal wastes.
In 2005, total installed power capacity in Thailand was 26,430 MW. Renewable energy accounted for about 2 percent of the total installed capacity. In 2007, Thailand had about 777 MW of electricity from renewable energy that was sold to the grid. Several studies have projected that biomass wastes can cover up to 15 % of the energy demand in Thailand (Thailand-Danish Country Programme for Environmental Assistance 1998-2001, Ministry of Environment and Energy, 2000). These estimations are primarily made from biomass waste from the extraction part of agricultural activities, and for large scale agricultural processing of crops etc. – as for instance saw and palm oil mills – and do not include biomass wastes from SMEs in Thailand. Thus, the energy potential of biomass waste can be much larger if these resources are included. The major biomass resources in Thailand include the following:
Wood residues from wood and furniture industries (bark, sawdust, etc.)
Biomass for ethanol production (cassava, sugar cane, etc.)
Biomass for biodiesel production (palm oil, jatropha oil, etc.)
Industrial wastewater from agro-industry
Livestock manure
Municipal solid wastes and sewage
Thailand’s vast biomass potential has been partially exploited through the use of traditional as well as more advanced conversion technologies for biogas, power generation, and biofuels. Rice, sugar, palm oil, and wood-related industries are the major potential biomass energy sources. The country has a fairly large biomass resource base of about 60 million tons generated each year that could be utilized for energy purposes, such as rice, sugarcane, rubber sheets, palm oil and cassava. Biomass has been a primary source of energy for many years, used for domestic heating and industrial cogeneration. For example, paddy husks are burned to produce steam for turbine operation in rice mills; bagasse and palm residues are used to produce steam and electricity for on-site manufacturing process; and rubber wood chips are burned to produce hot air for rubber wood seasoning.
In addition to biomass residues, wastewater containing organic matters from livestock farms and industries has increasingly been used as a potential source of biomass energy. Thailand’s primary biogas sources are pig farms and residues from food processing. The production potential of biogas from industrial wastewater from palm oil industries, tapioca starch industries, food processing industries, and slaughter industries is also significant. The energy-recovery and environmental benefits that the KWTE waste to energy project has already delivered is attracting keen interest from a wide range of food processing industries around the world.
Food waste is an untapped energy source that mostly ends up rotting in landfills, thereby releasing greenhouse gases into the atmosphere. Food waste is difficult to treat or recycle since it contains high levels of sodium salt and moisture, and is mixed with other waste during collection. Major generators of food wastes include hotels, restaurants, supermarkets, residential blocks, cafeterias, airline caterers, food processing industries, etc.
In United States, food waste is the third largest waste stream after paper and yard waste. Around 12.7 percent of the total municipal solid waste (MSW) generated in the year 2008 was food scraps that amounted to about 32 million tons. According to EPA, about 31 million tons of food waste was thrown away into landfills or incinerators in 2008. As far as United Kingdom is concerned, households throw away 8.3 million tons of food each year. These statistics are an indication of tremendous amount of food waste generated all over the world.
The proportion of food waste in municipal waste stream is gradually increasing and hence a proper food waste management strategy needs to be devised to ensure its eco-friendly and sustainable disposal. Food waste can be recycled via:
In-vessel composting (IVC): A treatment that breaks down biodegradable waste by naturally occurring micro-organisms with oxygen, in an enclosed vessel or tunnel;
Anaerobic digestion (AD): A treatment that breaks down biodegradable waste in the absence of oxygen, producing a renewable energy (biogas) that can be used to generate electricity and heat.
Currently, only about 3 percent of food waste is recycled throughout U.S., mainly through composting. Composting provides an alternative to landfill disposal of food waste, however it requires large areas of land, produces volatile organic compounds and consumes energy. Consequently, there is an urgent need to explore better recycling alternatives. Anaerobic digestion has been successfully used in several European and Asian countries to stabilize food wastes, and to provide beneficial end-products. Sweden, Austria, Denmark, Germany and England have led the way in developing new advanced biogas technologies and setting up new projects for conversion of food waste into energy.
Of the different types of organic wastes available, food waste holds the highest potential in terms of economic exploitation as it contains high amount of carbon and can be efficiently converted into biogas and organic fertilizer. Food waste can either be used as a single substrate in a biogas plant, or can be co-digested with organic wastes like cow manure, poultry litter, sewage, crop residues, abattoir wastes, etc.
Renewable energy systems have been used in Jordan since early 1970s. Infact, Jordan has been a pioneer in renewable energy promotion in the Middle East with its first wind power pilot project in Al-Ibrahemiya as early as 1988. Systematic monitoring of the technological developments and implementation/execution of demonstration and pilot projects has been the hallmark of Jordan’s foray into clean energy sector.
Municipal solid wastes represent the best source of biomass in Jordan. In terms of quantity per capita and constituents, the waste generated in Jordan is comparable to most semi-industrialized nations. The per capita of waste generated in Jordan is about 0.9 kg/day. The total generation of municipal waste in Jordan is estimated at 1.84 million tons per year.The main resources of organic waste in Jordan that can be potentially used to produce biogas are summarized as follows:
Municipal waste from big cities
Organic wastes from slaughterhouse, vegetable market, hotels and restaurants.
Organic waste from agro-industries
Animal manure, mainly from cows and chickens.
Sewage sludge and septic.
Olive mills.
Organic industrial waste
According to a study conducted by the Greater Amman Municipality, around 1.5 million tonnes of organic waste was generated in Jordan in 2009. In addition, an annual amount of 1.83 million cubic meter of septic and sewage sludge from treatment of 44 million cubic meter of sewage water is generated in greater Amman area. The potential annual sewage sludge and septic generated in Amman can be estimated at 85,000 tons of dry matter.
The Government of Jordan, in collaboration with UNDP, GEF and the Danish Government, established 1MW Biomethanation plant at Rusaifeh landfill near Amman in 1999. The Plant has been successfully operating since its commissioning and efforts are underway to increase its capacity to 5MW. Infact, the project has achieved net yearly profit from electricity sale of about US $ 100, 000. The project consists of a system of twelve landfill gas wells and an anaerobic digestion plant based on 60 tons per day of organic wastes from hotels, restaurants and slaughterhouses in Amman. The successful installation of the biogas project has made it a role model in the entire region and several big cities are striving to replicate the model.
Front end MSW pre-processing is used to prepare MSW for treatment and separate any recyclables
Conversion unit (reactor)
Gas and residue treatment plant (optional)
Energy recovery plant (optional): Energy / chemicals production system includes gas turbine, boiler, internal combustion engines for power production. Alternatively, ethanol or other organic chemicals can be produced
Emissions clean up
Incineration
Combustion of raw MSW, moisture less than 50%
Sufficient amount of oxygen is required to fully oxidize the fuel
Combustion temperatures are in excess of 850oC
Waste is converted into CO2 and water concern about toxics (dioxin, furans)
Any non-combustible materials (inorganic such as metals, glass) remain as a solid, known as bottom ash (used as feedstock in cement and brick manufacturing)
Fly ash APC (air pollution control residue) particulates, etc
Needs high calorific value waste to keep combustion process going, otherwise requires high energy for maintaining high temperatures
Anaerobic Digestion
Well-known technology for domestic sewage and organic wastes treatment, but not for unsorted MSW
Biological conversion of biodegradable organic materials in the absence of oxygen at temperatures 55 to 75oC (thermophilic digestion – most effective temperature range)
Residue is stabilized organic matter that can be used as soil amendment after proper dewatering
Digestion is used primarily to reduce quantity of sludge for disposal / reuse
Methane gas generated used for electricity / energy generation or flared
Gasification
Can be seen as between pyrolysis and combustion (incineration) as it involves partial oxidation.
Exothermic process (some heat is required to initialize and sustain the gasification process).
Oxygen is added but at low amounts not sufficient for full oxidation and full combustion.
Temperatures are above 650oC
Main product is syngas, typically has net calorific value of 4 to 10 MJ/Nm3
Other product is solid residue of non-combustible materials (ash) which contains low level of carbon
Pyrolysis
Thermal degradation of organic materials through use of indirect, external source of heat
Temperatures between 300 to 850oC are maintained for several seconds in the absence of oxygen.
Product is char, oil and syngas composed primarily of O2, CO, CO2, CH4 and complex hydrocarbons.
Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes
Syngas typically has net calorific value (NCV) of 10 to 20 MJ/Nm
Plasma Gasification
Use of electricity passed through graphite or carbon electrodes, with steam and/or oxygen / air injection to produce electrically conducting gas (plasma)
Temperatures are above 3000oC
Organic materials are converted to syngas composed of H2, CO
Inorganic materials are converted to solid slag
Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes
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.
Biomass energy projects provide major business opportunities, environmental benefits, and rural development. Feedstocks can be obtained from a wide array of sources without jeopardizing the food and feed supply, forests, and biodiversity in the world.
Agricultural Residues
Crop residues encompasses all agricultural wastes such as bagasse, straw, stem, stalk, leaves, husk, shell, peel, pulp, stubble, etc. Large quantities of crop residues are produced annually worldwide, and are vastly underutilised. Rice produces both straw and rice husks at the processing plant which can be conveniently and easily converted into energy. Significant quantities of biomass remain in the fields in the form of cob when maize is harvested which can be converted into energy. Sugar cane harvesting leads to harvest residues in the fields while processing produces fibrous bagasse, both of which are good sources of energy. Harvesting and processing of coconuts produces quantities of shell and fibre that can be utilized.
Current farming practice is usually to plough these residues back into the soil, or they are burnt, left to decompose, or grazed by cattle. These residues could be processed into liquid fuels or thermochemical processed to produce electricity and heat. Agricultural residues are characterized by seasonal availability and have characteristics that differ from other solid fuels such as wood, charcoal, char briquette. The main differences are the high content of volatile matter and lower density and burning time.
Animal Waste
There are a wide range of animal wastes that can be used as sources of biomass energy. The most common sources are animal and poultry manures. In the past this waste was recovered and sold as a fertilizer or simply spread onto agricultural land, but the introduction of tighter environmental controls on odour and water pollution means that some form of waste management is now required, which provides further incentives for waste-to-energy conversion.
The most attractive method of converting these waste materials to useful form is anaerobic digestion which gives biogas that can be used as a fuel for internal combustion engines, to generate electricity from small gas turbines, burnt directly for cooking, or for space and water heating.
Forestry Residues
Forestry residues are generated by operations such as thinning of plantations, clearing for logging roads, extracting stem-wood for pulp and timber, and natural attrition. Harvesting may occur as thinning in young stands, or cutting in older stands for timber or pulp that also yields tops and branches usable for biomass energy. Harvesting operations usually remove only 25 to 50 percent of the volume, leaving the residues available as biomass for energy.
Stands damaged by insects, disease or fire are additional sources of biomass. Forest residues normally have low density and fuel values that keep transport costs high, and so it is economical to reduce the biomass density in the forest itself.
Wood Wastes
Wood processing industries primarily include sawmilling, plywood, wood panel, furniture, building component, flooring, particle board, moulding, jointing and craft industries. Wood wastes generally are concentrated at the processing factories, e.g. plywood mills and sawmills. The amount of waste generated from wood processing industries varies from one type industry to another depending on the form of raw material and finished product.
Generally, the waste from wood industries such as saw millings and plywood, veneer and others are sawdust, off-cuts, trims and shavings. Sawdust arise from cutting, sizing, re-sawing, edging, while trims and shaving are the consequence of trimming and smoothing of wood. In general, processing of 1,000 kg of wood in the furniture industries will lead to waste generation of almost half (45 %), i.e. 450 kg of wood. Similarly, when processing 1,000 kg of wood in sawmill, the waste will amount to more than half (52 %), i.e. 520 kg wood.
Industrial Wastes
The food industry produces a large number of residues and by-products that can be used as biomass energy sources. These waste materials are generated from all sectors of the food industry with everything from meat production to confectionery producing waste that can be utilised as an energy source.
Solid wastes include peelings and scraps from fruit and vegetables, food that does not meet quality control standards, pulp and fibre from sugar and starch extraction, filter sludges and coffee grounds. These wastes are usually disposed of in landfill dumps.
Liquid wastes are generated by washing meat, fruit and vegetables, blanching fruit and vegetables, pre-cooking meats, poultry and fish, cleaning and processing operations as well as wine making.
These waste waters contain sugars, starches and other dissolved and solid organic matter. The potential exists for these industrial wastes to be anaerobically digested to produce biogas, or fermented to produce ethanol, and several commercial examples of waste-to-energy conversion already exist.
Pulp and paper industry is considered to be one of the highly polluting industries and consumes large amount of energy and water in various unit operations. The wastewater discharged by this industry is highly heterogeneous as it contains compounds from wood or other raw materials, processed chemicals as well as compound formed during processing. Black liquor can be judiciously utilized for production of biogas using anaerobic UASB technology.
Municipal Solid Wastes and Sewage
Millions of tonnes of household waste are collected each year with the vast majority disposed of in open fields. The biomass resource in MSW comprises the putrescibles, paper and plastic and averages 80% of the total MSW collected. Municipal solid waste can be converted into energy by direct combustion, or by natural anaerobic digestion in the engineered landfill. At the landfill sites the gas produced by the natural decomposition of MSW (approximately 50% methane and 50% carbon dioxide) is collected from the stored material and scrubbed and cleaned before feeding into internal combustion engines or gas turbines to generate heat and power. The organic fraction of MSW can be anaerobically stabilized in a high-rate digester to obtain biogas for electricity or steam generation.
Sewage is a source of biomass energy that is very similar to the other animal wastes. Energy can be extracted from sewage using anaerobic digestion to produce biogas. The sewage sludge that remains can be incinerated or undergo pyrolysis to produce more biogas.
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