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Garbage disposal has been a long-standing problem and will continue to be a
problem in the future. As the population of the world continues to increase, so
will the garbage produced. It is therefore important to seek out the ways that can
best be employed to minimize the amount of garbage. Incineration is a method
that has become more widely used as the garbage problem has worsened.
1.3 Waste Treatment methods:
a) Anaerobic digestion:
It is used as part of the process to treat biodegradable waste and sewage sludge.
As part of an integrated waste management system, anaerobic digestion reduces
the emission of landfill gas into the atmosphere. Anaerobic digesters can also be
fed with purpose-grown energy crops, such as maize.
It is a collection of processes by which micro-organisms breakdown biodegradable
material in the absence of oxygen. The process is used for industrial
or domestic purposes to manage waste and/or to produce fuels. Much of the
fermentation used industrially to produce food and drink products, as well as
home fermentation, uses anaerobic digestion. Silage is produced by anaerobic
digestion.
The digestion process begins with bacterial hydrolysis of the input materials.
Insoluble organic polymers, such as carbohydrates, are broken down to soluble
derivatives that become available for other bacteria. Acidogenic bacteria then
convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia,
and organic acids. These bacteria convert these resulting organic acids into
acetic acid, along with additional ammonia, hydrogen, and carbon dioxide.
Finally, methanogens convert these products to methane and carbon dioxide.
The methanogenic archaea populations play an indispensable role in anaerobic
wastewater treatments.
Anaerobic Digestion
Anaerobic digestion is widely used as a source of renewable energy. The
process produces a biogas, consisting of methane, carbon dioxide and traces of
other ‘contaminant’ gases. This biogas can be used directly as fuel, in combined
heat and power gas engines or upgraded to natural gas-quality biomethane. The
nutrient-rich digestate also produced can be used as fertilizer.
With the re-use of waste as a resource and new technological approaches which
have lowered capital costs, anaerobic digestion has in recent years received
increased attention among governments in a number of countries, among these
the United Kingdom (2011), Germany and Denmark (2011).
If the put rescible waste processed in anaerobic digesters were disposed of in a
landfill, it would break down naturally and often anaerobically. In this case, the
gas will eventually escape into the atmosphere. As methane is about 20 times
more potent as a greenhouse gas than carbon dioxide, this has significant
negative environmental effects.
In developing countries, simple home and farm-based anaerobic digestion
systems offer the potential for low-cost energy for cooking and lighting. From
1975, China and India have both had large, government-backed schemes for
adaptation of small biogas plants for use in the household for cooking and
lighting. At present, projects for anaerobic digestion in the developing world can
gain financial support through the United Nations Clean Development
Mechanism if they are able to show they provide reduced carbon emissions.
Some countries offer incentives in the form of, for example, feed-in tariffs for
feeding electricity onto the power grid to subsidize green energy production.
b) Pyrolysis:
Pyrolysis is a thermochemical decomposition of organic material at elevated
temperatures in the absence of oxygen (or any halogen). It involves the
simultaneous change of chemical composition and physical phase, and is
irreversible. The word is coined from the Greek-derived elements pyro "fire"
and lysis "separating".
Pyrolysis is a type of thermolysis, and is most commonly observed in organic
materials exposed to high temperatures. It is one of the processes involved in
charring wood, starting at 200–300 °C (390–570 °F). It also occurs in fires
where solid fuels are burning or when vegetation comes into contact with lava in
volcanic eruptions. In general, pyrolysis of organic substances produces gas and
liquid products and leaves a solid residue richer in carbon content, char.
Extreme pyrolysis, which leaves mostly carbon as the residue, is called
carbonization.
The process is used heavily in the chemical industry, for example, to produce
charcoal, activated carbon, methanol, and other chemicals from wood,
to convert ethylene
dichloride into vinyl chloride to make PVC,
to
produce coke from coal,
to convert biomass into syngas and biochar, to
turn
waste into safely disposable substances, and for transforming medium-weight
hydrocarbons from oil into lighter ones like gasoline. These specialized uses
of pyrolysis may be called various names, such as dry distillation, destructive
distillation, or cracking 2 Pyrolysis
Pyrolysis also plays an important role in several cooking procedures, such as
baking, frying, grilling, and caramelizing. In addition, it is a tool of chemical
analysis, for example, in mass spectrometry and in carbon-14 dating. Indeed,
many important chemical substances, such as phosphorus and sulfuric acid,
were first obtained by this process. Pyrolysis has been assumed to take place
during catagenesis, the conversion of buried organic matter to fossil fuels. It is
also the basis of pyrography. In their embalming process, the ancient Egyptians
used a mixture of substances, including methanol, which they obtained from the
pyrolysis of wood.
Pyrolysis differs from other high-temperature processes like combustion and
hydrolysis in that it usually does not involve reactions with oxygen, water, or
any other reagents. In practice, it is not possible to achieve a completely
oxygen-free atmosphere. Because some oxygen is present in any pyrolysis
system, a small amount of oxidation occurs.
c) GASIFICATION:
Gasification is a process that converts organic or fossil based carbonaceous
materials into carbon monoxide, hydrogen and carbon dioxide. This is achieved
by reacting the material at high temperatures (>700 °C), without combustion,
with a controlled amount of oxygen and/or steam. The resulting gas mixture is
called syngas (from synthesis gas or synthetic gas) or producer gas and is itself
a fuel. The power derived from gasification and combustion of the resultant gas
is considered to be a source of renewable energy if the gasified compounds were
obtained from biomass.
The advantage of gasification is that using the syngas is potentially more
efficient than direct combustion of the original fuel because it can be combusted
at higher temperatures or even in fuel cells, so that the thermodynamic upper
limit to the efficiency defined by Carnot's rule is higher or not applicable.
Syngas may be burned directly in gas engines, used to produce methanol and
hydrogen, or converted via the Fischer–Tropsch process into synthetic fuel.
Gasification can also begin with material which would otherwise have been
disposed of such as biodegradable waste. In addition, the high-temperature
process refines out corrosive ash elements such as chloride and potassium,
allowing clean gas production from otherwise problematic fuels. Gasification of
fossil fuels is currently widely used on industrial scales to generate electricity
Scottish engineer William Murdoch gets credit for developing the basic process.
In the late 1790s, using coal as a feedstock, he produced syngas in sufficient
quantity to light his home. Eventually, cities in Europe and America began using syngas or “town gas” as it was known then to light city streets and homes.
Eventually, natural gas and electricity generated from coal-burning power plants
replaced town gas as the preferred source of heat and light.
Today, with a global climate crisis looming on the horizon and power-hungry
nations on the hunt for alternative energy sources, gasification is making a
comeback. The Gasification Technologies Council expects world gasification
capacity to grow by more than 70 percent by 2015. Much of that growth will
occur in Asia, driven by rapid development in China and India. But the United
States is embracing gasification, as well.
Coal Gasification
The heart of a coal-fired power plant is a boiler, in which coal is burned by
combustion to turn water into steam. The following equation shows what
burning coal looks like chemically: C + O2 --> CO2. Coal isn't made of pure
carbon, but of carbon bound to many other elements. Still, coal's carbon content
is high, and it's the carbon that combines with oxygen in combustion to produce
carbon dioxide, the major culprit in global warming. Other byproducts of coal
combustion include sulfur oxides, nitrogen oxides, mercury and naturally
occurring radioactive materials.
The heart of a power plant that incorporates gasification isn't a boiler, but a
gasifier, a cylindrical pressure vessel about 40 feet (12 meters) high by 13 feet
(4 meters) across. Feedstocks enter the gasifier at the top, while steam and
oxygen enter from below. Any kind of carbon-containing material can be a
feedstock, but coal gasification, of course, requires coal. A typical gasification
plant could use 16,000 tons (14,515 metric tons) of lignite, a brownish type of
coal, daily.
A gasifier operates at higher temperatures and pressures than a coal boiler --
about 2,600 degrees Fahrenheit (1,427 degrees Celsius) and 1,000 pounds per
square inch (6,895 kilopascals), respectively. This causes the coal to undergo
different chemical reactions. First, partial oxidation of the coal's carbon releases
heat that helps feed the gasification reactions. The first of these is pyrolysis,
which occurs as coal's volatile matter degrades into several gases, leaving
behind char, a charcoal-like substance. Then, reduction reactions transform the
remaining carbon in the char to a gaseous mixture known as syngas.
Carbon monoxide and hydrogen are the two primary components of syngas.
During a process known as gas cleanup, can remove harmful impurities,
including sulfur, mercury and unconverted carbon. Even carbon dioxide can be
pulled out of the gas and either stored underground or used in ammonia or
methanol production.
That leaves pure hydrogen and carbon monoxide, which can be combusted
cleanly in gas turbines to produce electricity. Or, some power plants convert the
syngas to natural gas by passing the cleaned gas over a nickel catalyst, causing
carbon monoxide and carbon dioxide to react with free hydrogen to form
methane. This "substitute natural gas" behaves like regular natural gas and can
be used to generate electricity or heat homes and businesses.
Biomass gasification works just like coal gasification: A feedstock enters a
gasifier, which cooks the carbon-containing material in a low-oxygen
environment to produce syngas.
Feedstock
There are a large number of different feedstock types for use in a gasifier, each
with different characteristics, including size, shape, bulk density, moisture
content, energy content, chemical composition, ash fusion characteristics, and
homogeneity of all these properties. Coal and petroleum coke are used as
primary feedstocks for many large gasification plants worldwide. Additionally, a
variety of biomass and waste-derived feedstocks can be gasified, with wood
pellets and chips, waste wood, plastics and aluminium, Municipal Solid Waste
(MSW), Refuse-derived fuel (RDF), agricultural and industrial wastes, sewage
sludge, switch grass, discarded seed corn, corn stover and other crop residues all
being used.
Reception Area/Tipping Hall
On entering the incineration facility, waste trucks make their way to the tipping
hall. Here the waste is offloaded into large bunkers for storage. The air in the
reception area and in the bunkers is maintained at a lower pressure than outside
(negative pressure) and this prevents odours escaping.
Grab System
Waste will be received from both household and commercial sources. Some of
the waste will be bulky in nature, so it may need to be broken down into smaller
pieces. Overhead traverse cranes fitted with grapples mix the waste before
feeding it into the furnace hoppers. The mixing of the waste is useful in
producing a more uniform fuel that will help maintaining a steady combustion
process within the desired operating conditions
Combustion Chamber
The cranes and grabs transfer the mixed waste from the bunker to the furnace
'hopper'. This process can be fully automatic in modern incineration plants. At
the bottom of the hopper a metering ram pushes the waste onto the combustion
grate, which agitates and transports the waste through the combustion chamber.
Combustion takes place at temperatures of 850 – 1100 o
C, the temperature at
which odour of gases and all dioxins will be destroyed.
Fly Ash
Fly ash is the particulate removed during the gas cleaning phase. It is generally
about 1-3% by weight of the original waste. Fly ash is considered hazardous and
so it must be disposed of in a specially designed facility. At present there are no
hazardous waste facilities in Ireland, so hazardous material will be exported for
safe disposal.
Flue Gases
The combustion process produces flue gas containing water vapour, nitrogen,
carbon dioxide, nitrogen oxides, oxygen and particulate matter. Some of these
compounds are harmful to health and therefore the flue gas is thoroughly
cleaned before it is discharged to the air. The flue gas cleaning equipment of a
modern incineration plant is complex and can take up about half of the space
within the plant. There are various flue gas cleaning designs, but modern plants
generally include the following stages:
Electrostatic Precipitators (ESP): The ESP will initially remove 99% or more of
particulates. These are primarily dust and ash particles, but may also include
minute quantities of heavy metals, dioxins and furans.
Acid Gas Scrubbing: This consists of a lime mixture being injected into the gas
stream. This reacts to neutralise acid gases such as sulphur dioxide, hydrogen
fluoride and hydrogen chloride. Activated Carbon Activated carbon injection is
used to remove organic compounds such as dioxins and also volatile metals
such as mercury or cadmium. The activated carbon provides a surface onto
which the heavy metals can adhere and these will then be filtered out at a later
stage.
Filtration : The final filtration of particulate matter typically uses a bag house
filter (fabric filter). At this stage of the cleaning process, particulate matter is
primarily made up of spent activated carbon and spent hydrated lime (from the
earlier part of the cleaning process). This material is usually recycled back into
the combustion chamber to ensure that dioxins are properly destroyed in the
high temperatures.
Electricity Generation
A boiler converts the energy from the combustion into high pressure steam. The
combustion chamber is surrounded by water tube walls, which are heated by
radiation from the combustion. The hot flue gases release additional heat in
additional tube panels in the boiler. The steam goes into a turbine, which drives
an electric generator. Generally, about 10% of the electricity is used on site and
the remainder is fed into the national grid. The incineration of 400,000 tonnes of
waste can supply the annual electricity consumption of more than 30,000
homes. The heat remaining after the electricity production can be used to heat
water, which can be directly piped to people's homes in a district heating
system. This can supply the annual requirements for heating and domestic hot
water for approximately 25,000 homes.
2.5 Bottom Ash
At the end of the grate the solid waste has been completely burned out. The
remaining residue is called bottom ash, which is ejected at the bottom of the
combustion chamber. The bottom ash corresponds to about 15 - 20% by weight
or 4-6% by volume of the original waste. After storage the bottom ash may be
screened into fine and coarse fractions and the ferrous metals (iron or steel) in
the ash will be extracted using large magnets. The metals represent 5 - 10% of
the bottom ash and are sent to the steel works for recycling. The remaining
bottom ash is non-hazardous and is typically used in other applications such as
an aggregate in concrete or for road building.
2.6 Selection & Installation:
As we have discussed the basics of incineration process, it is equally important
to be aware of the dimensions of Incineration plants, converting Waste-toEnergy,
serving as a means of disposing vast amount of waste for benefit of
masses. An Incinerator manufacturing company “S.A. Incinerator Co. (Pty) Ltd”
provides the sizes of plant, room, chimney as per the Kg/hr of general waste,
figure and table are to be included as below