Successful conversion of waste to energy is based on the production of high-grade recovered fuels from dry and combustible wastes from enterprises, shops and households. In special cases, other waste streams, e.g., sludges from sewage treatment plants, electronic scrap, tyres, can also be used, but these wastes require special treatment techniques.
In addition to conventional power plant boilers, certain waste fractions can also be burned in cement kilns or blast furnaces. Utilisation methods, economy and environmental impacts of waste treatment are dependent on the composition and sources of wastes. Hence, such material choices should be preferred that reduce wastes and contribute to recovery and energy use of wastes.
What is waste?
Waste is any kind of material that has been already used and left discarded. This waste can be either reduced, exchanged, recycled or disposed off.
Waste can be classified in many ways :
- Solid waste
- Waste water
- Industrial/commercial waste
- Residential/community waste
- Radioactive waste
Thermal power stations, in addition to several captive power plants, use bituminous or sub-bituminous coal and produce large volumes of flyash. High ash content (30-50%) of Indian coals is contributing to these large volumes of flyash.
Fly ash is the residue of the coal combustion process. Its indiscriminate disposal requires large volumes of land, water and energy. The fine particles of flyash by virtue of their lightness can become air borne, if not managed well. Indian flyashes are safer than those produced in other countries (especially on account of lower content of sulphur, heavy / toxic elements and radio nuclides) however management of large volumes produced poses a big challenge to the country.
At present, nearly 90 million tonnes of flyash is being generated annually in India and nearly 65,000 acres of land is presently occupied by ash ponds.
Various applications of Flyash:
- Cement manufacturing
- Part replacement of cement in mortar and concrete
- Roads & Embankment construction
- Dyke raising
- Structural fill for reclaiming low lying areas
- Hydraulic structures
- Stowing material for mines
- Agriculture & Forestry
- Other medium & high value added products (tiles, wood, paints, light weight aggregate, extraction of alumina, cenospheres, tc.)
As is often the case with natural resources, a major problem arises from their uneven distribution. Sustainable design for waste water management should respect the water resources with diligence whatever the natural distribution may be. The challenge of sustainable design applies more to areas where freshwater is not limited than to dry areas (economics of high-cost water tends to promote wise stewardship). The principles of sustainable design apply, without reservation, to all types of climates. However in a city, water issues center on providing safe drinking, washing, cooking, and toilet flushing water.
These waters are produced by some industrial processes and include the undesired liquid product of any unit operation. The major concern with these wastes is the reactions that may occur with the environment being either direct or indirect. Some may rob oxygen from the environment, while others may be toxic.
There isn’t a standard design to treat all wastewaters due to the characteristics of the process specific waste. Each site requires a design specific to the process at hand, but some combination of the following pretreatment, primary treatment, secondary treatment, and processing are normally used. As a rule of thumb, we would like to separate several pollutants in one step and use several steps to increase the degree of treatment.
The Pretreatment Process
This is the process of adjusting the concentration of wastes, flow of wastes, mixing, oxygen characteristics, temperature, and so on.
This is the process of adding materials, mixing and adjusting pH measurements for the bulk of the discharge to adequate values.
Due to solubility characteristics, we can separate oils by gravity, or something as simple as skimming. We can also add chemicals to break down or separate oils.
These must be separated from the discharge to insure that federal guidelines are met. Chemical addition and reaction can often aid in the separation
This process is used for materials with slower settling times. They are designed on the basis of the retention time necessary for adequate waste removal. Operation is essentially the same as for the grit chamber, except in flow rates are much slower, and a scum layer can be removed from the surface due to poor mixing.
A lagoon containing compost liquor
If sludge has a low enough water composition we can reduce the volume by combustion. Air and a startup fuel are normally added, making cost large. Using the released heat to further drying of the incoming sludge, or using the heat to produce energy can offset some of the costs. The ash is normally buried in a landfill.
These are non-leaching storage areas which are buried. They must be monitored regularly to prevent environmental damage.
Because sludges are rich in nutrients, using them as a fertilizer is a viable method of disposal. Agricultural areas normally benefit, as does the industry.
Biomass is organic material made from plants and animals. biomass contains stored energy from the sun. plants absorb the sun’s energy in a process called photosynthesis. the chemical energy in plants gets passed on to animals and people that eat them. biomass is a renewable energy source because we can always grow more trees and crops, and waste will always exist. some examples of biomass fuels are wood, crops, manure, and some garbage.
When burned, the chemical energy in biomass is released as heat. if you have a fireplace, the wood you burn in it is a biomass fuel. wood waste or garbage can be burned to produce steam for making electricity, or to provide heat to industries and homes.
Another source of biomass is our garbage. trash that comes from plant or animal products is biomass. materials that are made out of glass, plastic, and metals are not biomass because they are made out of non-renewable materials. waste can be a source of energy by either burning in waste to energy plants, or by capturing biogas. in waste-to-energy plants, trash is burned to produce steam that can be used either to heat buildings or to generate electricity.
Combustion to heat homes
The combustion of biomass, particularly firewood, is a long-standing tradition in many of the rural areas of Canada. Chunkwood that is split and dried is burned in high-efficiency enclosed fireplaces, box-stoves, furnaces, and cook-stoves. Many cook-stoves and furnaces are fitted with a heating coil to provide domestic hot water and space heating.
Stoves that use pellets and corn for fuel are now being used to heat homes. These stoves start automatically and supply fuel in response to a thermostat signal. They produce low exhaust temperatures that allow the exhaust to exit through walls without the need for traditional masonry or an insulated steel chimney.
Gasification Methods Convert Wastes Into Useable Fuel
Gasification converts carbon-containing materials, under high temperature and pressure, into synthesis gas. Synthesis gas or “syngas” is generally composed of hydrogen and carbon monoxide, but can be manufactured to contain methane and other higher molecular weight compounds. Syngas can be used as a fuel to generate electricity, or as a basic chemical building block for use in the petrochemical and refining industries. Generally, syngas has a heating value that is approximately two-thirds that of natural gas, and when burned as fuel produces emissions that are similar to natural gas.
Four R’s (Refuse, Reuse, Recycle, Reduce) to be followed for waste management :
1. Refuse. Instead of buying new containers from the market, use the ones that are in the house. Refuse to buy new items though you may think they are prettier than the ones you already have.
2. Reuse. Do not throw away the soft drink cans or the bottles; cover them with homemade paper or paint on them and use them as pencil stands or small vases.
3. Recycle. Use shopping bags made of cloth or jute, which can be used over and over again. Segregate your waste to make sure that it is collected and taken for recycling.
4. Reduce. Reduce the generation of unnecessary waste, e.g. carry your own shopping bag when you go to the market and put all your purchases directly into it.
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