Field of Disclosure
The present disclosure relates to pollution control processes. Particularly, the
present disclosure deals with the pollution control processes in coal combustion
power plants.
Background
Coal accounts for a major fraction of the combustion material used worldwide
to produce electricity, and coal based generation plants account for a major
fraction of the stationary sources of air pollution worldwide.
There are a variety of combustion processes that have variable impacts on the
environment. Coal reserves are vital for providing global primary energy
needs. Studies suggest that energy production from coal combustion process is
also recognized for more than 50% of the increase in global C02 levels in the
atmosphere. Energy production from coal combustion power plants, like any
other industrial process, results in various by-products, including flue gas (e.g.,
CO2, S02 NOx,N2) and solid wastes (e.g., fly ash and bottom ash).
The fly ash produced from coal combustion is entrained in flue gas and
dispersed into the atmosphere. This creates environmental and health concerns
that prompted laws which have reduced fly ash emissions to less than I% of
ash produced. Worldwide, around 65% of fly ash produced from coal power
stations is disposed of in landfills and ash ponds.
Bottom ash and Fly ash art? primary products of coal combustion and is
obtained as a fine particulate mineral residue, left behind after all combustibles
in coal are burnt out. The current output rate of fly ash from plants utilizing
pulverized coal is about 10 million tons per annum. The accumulation without
adequate avenues for consumption or disposal otherwise creates a problem of
storage space. Air polluted by boiler chimneys or carried away by prevailing
winds from open heaps also causes lung disorders such as breathing problems.
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A need is therefore felt to discover new areas of usage of fly ash such that the
twin problems of storage and pollution can be mitigated.
Ash is also produced by the burning of various materials, such as Wood ash
generated from boilers where wood (or bark) is used as a heating source, Coal
Ash generated from coal fired electrical generating power plants and
Incinerator ash. Some ash contains significant amount of alkali and alkaline
earth metal oxides, hydroxides and carbonates.
The recycling of ash has become an increasing concern in recent years due to
increasing landfill costs and current interest in sustainable development. The
accumulation without adequate avenues for consumption or disposal otherwise
of ash creates a problem of storage space.
The flue gas, another by product of coal combustion refers to the combustion
exhaust gas produced at power plants. Its composition depends on what is
being burned, but it will usually consist of mostly nitrogen (typically more than
two-thirds) derived from the combustion air, carbon dioxide (C02), and water
vapor as well as excess oxygen (also derived from the combustion air). It
further contains a small percentage of a number of pollutants, such as
particulate matter, carbon monoxide, nitrogen oxides, and sulfur oxides.
At power plants, flue gas is sometimes treated with a senes of chemical
processes and scrubbers, which remove pollutants. Electrostatic precipitators or
fabric filters remove particulate matter and flue-gas desulfurization captures the
sulfur dioxide produced by burning fossil fuels, particularly coal. Nitrogen
oxides are treated either by modifications to the combustion process to prevent
their formation, or by high temperature or catalytic reaction with ammonia or
urea. In either case, the aim is to produce nitrogen gas, rather than nitrogen
oxides. In USA, there is a rapid deployment of technologies to remove mercury
from flue gas-typically by adsorption on sorbents or by capture in inert solids
as part of the flue-gas desulfurization product. Such scrubbing can lead to
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•
meaningful recovery of sulfur for further industrial use. (C.Michael Hogan.
2011).
Technologies based on regenerative capture by amines for the removal of CO2
from flue gas have been deployed to provide high purity CO2 gas to the food
industry and for Enhanced Oil Recovery. They are now under active research
as a method for CO2 capture for long-term storage as a means of greenhouse
gas remediation, and have begun to be implemented in a limited way
commercially (e.g. the Sleipner West field in the North Sea in Norway,
operating since 1996).
There are a range of emerging technologies for removing pollutants emitted
from power plants. As yet, there is very little performance data available from
large-scale industrial applications of such technologies, and none has achieved
significant penetration ofthe enormous worldwide market.
Currently several techniques exist to capture CO2 from coal combustion
processes such as Pre-combustion methods (fuel decarbonization), Combustion
in 02/C02 atmospheres (oxy-fuel firing), and Post-combustion capture
methods.
However, all of the above techniques have their own drawbacks. For example,
these techniques are energy intensive and produce additional by-products
which require special handling and disposal methods.
US 2008/0267838 discloses a fluidized bed reactor device for sequestering flue
gas C02 from a flue gas source. The fluidized bed reactor device comprises an
operating portion having a first end and a second end. A flue gas inlet is formed
at the first end of the operating portion with the flue gas inlet receiving flue gas
from the flue gas source. A flue gas outlet formed at the second end of the
operating portion. A distributor plate is mounted within the operating portion
adjacent the first end of the operating portion. A volume of fly ash is encased
within the operating portion between the second end and the distributor plate
with the flue gas traveling through the distributor plate and the fly ash creating
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reacted flue gas wherein the reacted flue gas exits the operating portion through
the flue gas outlet.
Because of the expense of operating these pollution control devices, there has
been a tendency to avoid scrubbing of flue gas and reduction of release of
atmospheric pollutants where possible, and to reduce operating costs where
scrubbing is required.
Accordingly, there exists a need for a system to capture and mineralize flue gas
carbon dioxide generated for coal combustion for reducing carbondioxide
emissions and at the same time utilize ash effluent.
OBJECT
The object of the present invention is to provide an integrated pollution control
process in coal combustion power plants, whereby alkaline ash generated from
power plants boilers is converted into useful and non hazardous products and at
the same time the carbon, sulfur and nitrogen oxides content from the exhaust
flue gas generated from power plants boilers is reduced to a large extent.
SUMMARY
The present disclosure provides an integrated process for pollution control in
coal combustion process, said process comprising:
passing exhaust flue gases containing oxides of carbon, sulfur and
nitrogen, through an alkaline ash slurry, at a temperature range of 40 to
60°C to obtain a neutralized slurry and scrubbed flue gases; and
filtering the neutralized slurry to obtain carbonated ash.
In a preferred embodiment of the present disclosure, the exhaust flue gas is
mixed at a temperature in the range of 40 to 60°C, with said ash slurry.
Typically, the exhaust flue gases contain oxides of carbon, sulfur and nitrogen.
Typically, the ash slurry comprises calcium oxide.
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Typically, the carbon dioxide content of the exhaust flue gases is reduced by
10-30%.
Typically, the sulfur oxide content of the exhaust flue gases is minimized to
more than 80-90%.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIG. 1 shows the operation of the preferred embodiment of the system in
schematic fonn.
DETAILED DESCRIPTION
The present disclosure provides an integrated process for pollution control in
coal combustion power plant. The process involves reacting a by product of
coal combustion process such as exhaust flue gas with another waste product
such as ash slurry. The process reduces the oxides of carbon, sulfur, and
nitrogen present in the exhaust flue gases and simultaneously neutralizes the
ash slurry to produce carbonated ash.
Exhaust flue gases are byproducts of coal combustion process. Exhaust flue
gases contains Carbon dioxide up to 10-15%, Nitrogen oxides up to 50 - 110
ppm and sulfur oxide up to 180 to 2000ppm.
Carbon dioxide (C02): Coal-fired electric power plants comprise the single
biggest source of CO2 emissions in the world. Burning fossil fuels such as coal
releases carbon dioxide (C02) pollution, electricity generation is the single
largest source of greenhouse gases in the world. Currently there is 30% more
CO2 in the atmosphere than there was at the start of the Industrial Revolution,
and we are well on the way to doubling CO2 levels in the atmosphere during
this century. Coal-fired power plants have the highest output rate of CO2 per
unit of electricity among all fossil fuels.
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Sulfur Dioxide (S02): Gas emitted through burning coal and oil, that converts
into acid gases (sulfuric acid) and sulfur particulate matter (pm). Health effects
include: airway irritation, heart rhythm destabilization, and asthma attacks.
Nitrogen Oxide (NOx): General term for NO/N02 hazes formed from burning
coal, oil, natural gas, and gasoline. It is a main ingredient in acid rain and
ozone smog.
Acid Rain: Acid rain, or acid deposition, occurs when sulfur dioxide (S02)
and nitrogen oxide (NOx) react with water and oxygen in the atmosphere to
form acidic compounds, most commonly sulfuric and nitric acid. These acidic
compounds then either mix with natural precipitation and fall to the earth as
acid rain, or remain dry and then settle to the ground. Acid rain destroys
ecosystems, including streams and lakes, by changing their delicate pH balance
making them unable to support life. Acid rain can destroy forests, devastate
plant and animal life, and eat away at man-made monuments and buildings to
effectively destroy our natural and historical treasures.
The power plants pulverize coal to powder fineness before it is burned. Fly
ash - the mineral residue produced by burning coal - is captured from the
power plant's exhaust gases and collected for use. Fly ash is a fine, silicate
powder recovered from the flue gas after burning coal during the production of
electricity. These micron-sized earth elements consist primarily of silica,
alumina and iron. Some ash contains significant amount of alkali and alkaline
earth metal oxides, hydroxides and carbonates.
Other sources of ash are burning of biomass, municipal and industrial waste
and incinerator ash. Ash derived from coal combustion contain calcium oxide,
ash derived from biomass generally contains potassium metal salt. Industrial or
municipal waste, incinerator ash contain generally potassium and sodium salts.
Fly-ash is mostly disposed off using either Dry or Wet disposal procedure. (a)
In the case of dry disposal method, the fly-ash is transported by truck, chute or
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conveyor from the power plant to the site of disposal where these are disposed
off by constructing a dry embankment (ash dyke). (b) In wet disposal, the flyash
is transported as slurry through pipe and disposed off in impoundment
called ash pond. Most ofthe power plants use wet disposal system.
In a preferred embodiment of the present disclosure, the process comprises,
passing exhaust flue gas into the alkaline ash slurry, whereby the CO2, SOx,
NOx from the flue gases are captured by alkali/alkaline earth metal oxides
present in the ash to yield a mixture consisting of carbonates, bicarbonates,
nitrates and sulfate salts and mineral acids. The mineral acids so formed during
the reaction further neutralize the alkaline ash slurry. The neutralization of ash
slurry reduces scale deposition in ash slurry pipelines caused by high alkalinity
and pH.
The carbonated salts and mineral salts so obtained during the reaction and after
neutralization can be used in agriculture, landfills, construction etc.
Thus the process of the present disclosure utilizes the waste generated for the
production of value added products for construction/agriculture and also
reduces the carbon dioxide emission and improves plant performance.
Figure 1 shows the operation of the preferred embodiment of the process in
schematic form. The exhaust gas (2) is passed through alkaline ash slurry
source (1) in a reactant vessel (3) under controlled flow and temperature. The
flow of the gas is continued till the slurry is neutralized. The treated exhaust
gas from the vent (4) is analyzed before discharging in to the atmosphere.
The neutralized slurry is pumped into a filtration tank, the precipitate is
collected and the neutralized supernatant liquid is passed as an effluent. The
exhaust gas stream is obtained during combustion of coal. The exhaust gas
stream contains oxides of sulfur, nitrogen, carbon, and the halogens. The gas
stream generated during the combustion is passed through the reactor tank (3).
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To prevent settling of the solids to the bottom of reactor (3), the slurry may be
stirred or recirculated by suitable means.
The ash slurry is produced by mixing ash with water at system temperature f,
additional calcium oxide may be added if required.
The exhaust gas (2) reacts with slurry (1) in the reactor (3) at a temperature in
the range of 40-60 DC.
Following Table 1 illustrates the physicochemical properties of the alkaline ash
slurry before and after neutralization.
pH 12.3 7.8
Coduetivity IlS/cm 6050 1890
P-alk as CaC03 ppm 1348 0
M-alk as CaC03 ppm 1388 364
TH asCaC03 ppm 1560 965
CaH asCaC03 ppm 1552 876
CI- ppm 160 160
sol- ppm 160 265
N03- ppm 3.0 3.2
Table I
The analysis of vent exhaust gas indicates that around 10-30% of CO2, 80-90%
of S02 and 90% SPM from the flue gases are absorbed by the ash slurry.
The reactor (3), with suitable volume is provided with gas distribution and
stirring means, both being constructed from stainless steel, or other suitable
materials, such as rubber, which can tolerate highly alkaline or acidic solutions.
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In another preferred embodiment of the present disclosure, the apparatus may
consist of more than one reactor in series.
The process of the present disclosure provides for removal of the caustic
components as neutral solids suitable for disposal as non-hazardous waste. At
the same time, the exhaust gas passing through the slurry is cleansed of a
significant portion of the compounds of the halogens and oxides of sulfur,
nitrogen and cabon.
Technical advantages
I. Reduction in green house gases (C02) by 10-30%.
2. Reduction of pollutants such as S02, SPM from the flue gases.
3. Minimizing the mineral acid consumption for neutralizing the alkalinity of
ash slurry.
4. Reduction in scaling of slurry pipe lines, due to neutralized effluent.
5. Carbonated ash for agriculture, construction purpose.

We Claim:
1. An integrated process for pollution control in coal combustion power
plant, said process comprising:
a) passing exhaust flue gas containing oxides of carbon, sulfur and
nitrogen, through an alkaline ash slurry, at a temperature in the range
of 40 to 60°C to obtain a neutralized slurry and scrubbed flue gases; and
b) Filtering the neutralized slurry to obtain carbonated ash.
2. The process of claim 1, wherein the exhaust flue gas is mixed at a
temperature in the range of 40 to 60°C, before reacting with said ash slurry.
3. The process of claim 1, wherein the exhaust flue gas is generated from coal
combustion power plants, contains oxides of carbon, sulfur and nitrogen.
4. The process of claim 1, wherein the ash slurry comprise of mainly calcium
oxide.
S. The process of claim 1, wherein the carbon dioxide content of the exhaust
flue gases after passing through the ash slurry is reduced by 10-30%.
6. The process of claim 1, wherein the sulfur oxide content of the exhaust flue
gases after passing through the ash slurry is minimized to more than 80-90%.