coal emission control

8/7/2019 Coal Emission Control

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COAL AND THE ENVIRONMENT and WAYS OF EMMISION CONTROL

The deployment of all energy generating technologies invariably leads to some degree ofenvironmental impact. The nature of the impact is dependent on the specific generation

technology used and are categorized into four concerns over land and water resource use,

pollutant emissions, waste generation and public health and safety concerns. The use of coal forpower generation is not exempt from these impacts. (Coal Use and the Environment)

From mining to coal cleaning, from transportation to electricity generation to disposal, coal

releases numerous toxic pollutants into our air, our waters and onto our lands. Coal fired power

plants largely emit nitrogen oxides, sulfur dioxides, mercury, particulate matter and trace

elements which are all harmful to human health and the environment. (Engineered Coal Fuel

Technologies, 2010)

The trace elements contained in coal (and others formed during combustion) are a large group of

diverse pollutants. They are a public health concern because at sufficient exposure levels they

adversely affect human health. Some are known to cause cancer, others impair reproduction and

the normal development of children, and still others damage the nervous and immune systems.

Many are also respiratory irritants that can worsen respiratory conditions such as asthma. They

are an environmental concern because they damage ecosystems.

Power plants also emit large quantities of carbon dioxide (CO2), the greenhouse gas largely

responsible for climate change. The health and environmental effects caused by power plant

emissions may vary over time and space, from short-term episodes of coal dust blown from a

passing train to the long-term global dispersion of mercury, to climate change. (Keating, 2001)

Promoting more coal use without also providing additional environmental safeguards will onlyincrease this toxic abuse of our health and ecosystems. (Keating, 2001)

Scientists have made significant progress in the last two decades to ensure that coal can be used

without harming the environment. Technologies are being developed that change coal into clean-

burning gases and liquid fuels. Many of these modern processes belong to an energy generation

family called clean coal technologies (CCT). (Illinois Coal and Clean Coal Technologies)

Clean Coal Technology is an umbrella term used to describe technologies being developed that

aim to reduce the environmental impact of coal energy generation. (Vlaicu, 2010) Clean coal

technologies are being classified into four, depending on where in the whole coal combustion

process a certain technology is deployed and these are namely precombustion, combustion,

postcombustion and conversion.

Precombustion technologies are technologies that are used to clean the coal before it is used and

include actual physical, chemical and biological coal washing or cleaning. These methods

remove sulfur and ash from coal. (Illinois Coal and Clean Coal Technologies) Lower ash levels

result to in decreased SO2 and Hg emissions. (Engineered Coal Fuel Technologies, 2010)


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Coal cleaning processes are called coal beneficiation in general. (Illinois Coal and Clean Coal

Technologies)

Coal cleaning offers a way of improving its quality both economically and environmentally.

(Breeze, Coal Cleaning and Processing, 2005) Environmentally as it of course reduces the

amount of contaminants formed during combustion and economically as it allows coal-fueled boilers to operate more efficiently as there are fewer impurities and other chemicals that can

decrease heat and combustion rates. More efficient combustion in turn reduces NOX and

CO2 emissions. (Engineered Coal Fuel Technologies, 2010)

Physical coal cleaning techniques take advantage of the differences in specific gravity of coal

and its impurities and are often done with the use of water. (Coal Cleaning Methods, 2000)

These however only remove inorganic sulfur from coal. Some of the devices usually used are

shown in figures below.

Figure 1: Hydrocyclone. Figure 2: Dense-Medium Vessels. Figure 3: Froth Floation.

Hydrocyclones are water-based cyclones where the heavier particles accumulate near the walls

and are removed via the base cone. Lighter (cleaner) particles stay nearer the center and are

removed at the top via the vortex finder. The cyclone diameter has a significant influence on the

sharpness of separation. Figure 1 is an example of hydrocyclones.

Figure 2 on the other hand are examples of dense-medium vessels. The picture at the right of thefigure is an example of a dense-medium bath while the one at the left is an example of a dense-

medium cyclone. They as well separate by specific gravity difference. However they differ fromhydrocyclones as instead of water, they use a suspension of magnetic and water. This suspension

has a specific gravity between that of coal and the refuse and a better separation can be obtained.Dense-medium cyclones clean coal by accelerating the dense-medium, coal and refuse by

centrifugal force. The coal exits the cyclone from the top and the refuse from the bottom. Betterseparation of smaller-sized coals can be achieved by this method. (Coal Cleaning Methods,

2000)


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The most widely used method in cleaning fine coal but the most complicated and expensive isshown in Figure 3 and is called Froth Flotation. (Coal Cleaning Methods, 2000) Froth flotation is

a surface-chemistry process of separation of fine solids that takes advantage of the differences ofwettability at solids particle-surfaces. Solid surfaces are usually water-loving and are termed

hydrophilic. A surface that is non-wettable is water repelling and termed hydrophobic. If a

surface is hydrophobic, it is also typically air attracting termed aerophilic, and is stronglyattracted to an air interface, which readily displaces water at the solid's surface. In froth flotation,separation of a coal-water mixture may be accomplished by the selective attachment of coal to

air bubbles. The refuse particles which are hydrophilic remain in the water. The difference in thedensity between the air bubbles and water provides buoyancy that preferentially lift the coal to

the surface where they remain entrained in a froth which can be drain off or mechanicallyskimmed away, thus, effecting the separation. (Froth Flotation, 2005)

As mentioned earlier, only inorganic sulfur can be removed by physical coal cleaning so to lower

the amount of sulfur chemically combined with carbon in coal or organic sulfur, chemical coal

cleaning techniques are used. Chemical coal cleaning techniques remove pollutants from coal by

reacting them with chemicals. (Andrew, 1990) One of the most popular example of which isMolten-Caustic Leaching a process quite effective in removing not only organic and pyritic

sulfur but mineral matter and trace materials from coal as well. (Illinois Coal and Clean Coal

Technologies)

Molten Caustic Leaching is where coal is treated with molten caustic (usually a eutectic mixture

of sodium and potassium hydroxides) at 350 to 400 for up to four hours. After treatment of

coal, it is washed with water and dilute acid to produce a low-sulfur, low-ash product. (Riegel,

2003)

Another area on which coal beneficiation is also centered is biological coal cleaning. Biologicalcoal cleaning involves the use of bacteria which literally eats the sulfur out of the coal. (IllinoisCoal and Clean Coal Technologies) Biological coal cleaning or biodesulfurization process

basically aims to remove the organically bound sulfur from coal while retaining the fuel value ofthe coal. (Noyes, 1991) Today, scientists are trying to improve the characteristics of bacteria

through experimentation. Others use fungi while still others are trying to duplicate the enzyme orchemical in bacteria that removes sulfur. These enzyme or chemical can then be injected directly

to coal for a faster process. (Illinois Coal and Clean Coal Technologies)

Another set of techniques used to clean coal thus reducing the pollutants it produces are applied

inside the furnace where the coal is actually burned and are called Combustion Clean Coal

Technologies. These methods remove nitrogen oxides and sulfur dioxides. (Coal CleaningMethods, 2000)

Nitrogen oxides refer to nitric oxide and nitrogen dioxide and are produced by the reaction of

nitrogen and oxygen found in air during combustion. This production is strongly affected by

temperature of combustion and amount of oxygen during combustion. (Breeze, Low Nitrogen

Oxide Burners, 2005) Controlling these factors can offers a way of controlling the quantity of


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nitrogen oxides produce and these can be achieved simply by the use of low nitrogen oxide

burners but mixing it with primary measures like over-fire air and reburning give higher removal

efficiency.

Low nitrogen oxide burners are burners designed to create an initial combustion region for the

pulverized coal particles where the proportion of oxygen is kept low. (Breeze, Low Nitrogen

Oxide Burners, 2005) Low nitrogen oxide burners reduces the amount of nitrogen oxides by

staging the combustion process by creating fuel lean and fuel rich zones within the flame. These

large and branched flames are created by controlling the mixing of air and fuel which results to

low nitrogen oxide production as peak flame temperatures are reduced and higher burner

efficiency as improve flame structure reduces the amount of oxygen available in the hottest part

of the flame.

There are three stages within a Low Nitrogen Oxide Burner incorporated with over fire air and

reburning. In the initial stage, combustion occurs in the fuel rich zone of the flame, or otherwisecalled as the primary combustion zone. In this zone, oxygen is low in concentration as some of

the air needed to burn the coal completely is prevented from entering so formation of thermal

NOx (NOx caused by high flame temperatures) is stifled. The stage that follows is the reburning

zone or the secondary combustion zone wherein more coal is simply introduced into the

combustion gases after they have left the primary combustion zone. The secondary combustion

zone is operated substoichiometrically to generate hydrocarbon radicals that reduce nitrogen

oxides that were formed to nitrogen. (Nitrogen Oxides: Pollution Prevention, 1998) After then,

the combustion gases enter the last stage which is fuel lean. In this stage, further air is admitted

to completely combust coal. However, this occurs in low temperature and the air admitted at this

stage is called the over-fire air. Fuel lean zone therefore prevents thermal and fuel NOx(formation of NOx resulting from the oxidation of fuel bound nitrogen). (Breeze, Low Nitrogen

Oxide Burners, 2005) Low nitrogen oxide burners can reduce NOx emissions by as much as 70

percent.

Figure 4: Fuel Reburning Diagram.


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As for sulfur dioxide removal, there is no strategy like low nitrogen oxide burners that can be

employed. Once sulfur is contained in coal, it is converted to sulfur dioxide during combustion.

Sulfur dioxide quantity can only be reduced therefore by either capturing sulfur with processes

that belonged to the precombustion clean coal technologies discussed earlier or capturing sulfur

dioxide after combustion with the use of a chemical reagent. (Breeze, Coal Cleaning and

Processing, 2005)

The use of a chemical reagent in capturing sulfur dioxide after combustion is one of the many

examples of Post Combustion Technologies. Post Combustion Technologies, as the name

already suggests, are technologies employed after the coal is burned.

There are many chemicals available today that are potentially capable of capturing sulfur dioxide

before combustion gases or emissions reach the smokestack and released into the air but the

cheapest would be lime [Ca(OH)2] or lime stone (CaCO3). Both react with sulfur dioxide and

produce calcium sulphate. (Breeze, Sulfur Dioxide Removal, 2005)

Reactions are CaCO3 (s) + SO2 (g) CaSO3 (s) + CO2 (g) and Ca(OH)2 (s) + SO2 (g)

CaSO3 (s) + H2O (l).

This method of removing sulfur dioxide is often called flue gas desulfurization or scrubbing. The

efficiency of this method depends on which part of the process the sorbent is injected. Sorbent

can either be injected directly on top of the furnace or into the hot flue gas stream with products

filtered downstream the injection point. Injecting above the furnace is the cheapest however the

least effective method as it only removes 30% of sulfur dioxide compared to injecting later in the

flue gas which can remove up to 90%. (Breeze, Sulfur Dioxide Removal, 2005)

Removing sulfur from the flue gas stream is often achieved with a flue gas desulfurization unit,

also called a wet scrubber. The unit has a chamber through which the flue gas stream passes and

reacted with a slurry of 10% lime or limestone and water. This captures the sulfur dioxide and

also produces gypsum. The excess lime or unreacted lime and gypsum contained in the slurry

after reaction is collected at the bottom of this chamber and recycled. This method can achieve

up to 97% sulfur removal but with special additives can be raised to 99%.

Like sulfur dioxide, nitrogen oxides can also be captured after combustion, in the flue gas

stream. This involves the injection of ammonia gas or urea in the flue gas stream that reactionwith the nitrogen oxides present converting it to nitrogen and water. (Breeze, Nitrogen Oxides

Capture Strategies, 2005)

Urea separates into ammonia (NH3) and isocyanic acid (HNCO) during the process. HNCO

contributes partly to nitrogen oxide reduction and partly form NCO which in turn forms to

laughing gas, N2O. By using ammonia, the process is more direct. (Runnger, 2008)


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Reactions are NO + NH3 +

O2 = N2 +

H2O (using ammonia) and 2NO + (NH2)2CO + O2 =

2N2 + 2H2O + CO2 (using urea). Depending on the spontaneity of this process, it is either called

Selective Non-Catalytic Reduction (SNCR) or Selective Catalytic Reduction (SCR).

If ammonia or urea is injected in a flue gas stream where the temperature is between 870 and

1200, the reaction takes place spontaneously and is called Selective Non-Catalytic Reduction.

As the name implies, this method doesnt need a catalyst for the reaction to proceed. (Runnger,

2008)

On the other hand, if ammonia or urea is injected in a flue gas stream where the temperature is

between 340 and 380, typical flue gas temperature, a special metal catalyst is needed to

stimulate the reaction process. This process is called Selective Catalytic Reduction. The heart of

this process is the catalysts as they create a surface for reacting the nitrogen oxide and ammonia

and allow the reaction to occur at typical flue gas temperature ranges. Catalysts are normally

made from vanadium-titanium or zeolite. (Selective Catalytic Reduction, 2010)

Figure 5: Diagram showing reaction of Ammonia and NOx at the catalyst pores.

As can be seen in figure 5, the reaction occurs in the catalyst bank which could consist of one or

more layer of catalysts for treatment. The nitrogen oxides will be selectively reduced by reacting

with ammonia in the presence of oxygen to form water and nitrogen as by products. (Selective

Catalytic Reduction, 2010)

Besides sulfur dioxides and nitrogen oxides, particulate matter can also be collected from flue

gas streams after combustion. This is done either through the use of fabric (baghouse) filters or

electrostatic precipitators. (Breeze, Particulate Matter Removal, 2005)

Electrostatic precipitators are particle control devices that use electrical forces to move the

particles out of the flowing gas stream. Figure shows how exactly an electrostatic precipitator

works.


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Figure 6: Diagram showing how ESP operates. Figure 7: Conventional Electrostatic Precipitator.

Electrostatic Precipitators utilize a system of plates and wires to apply a large voltage across the

flue gas as it passes through the ESPs chamber. This causes an electrostatic charge to build up

on the solid particles in the flue gas which are then attracted to the oppositely-charged plates of

the ESP where they are collected. The particles then fall into a large storage container called a

hopper and the clean air is brought out.

Like Electrostatic Precipitators, baghouses or fabric filters are particulate air pollution control

devices. Baghouses have hoppers through which flue gas enters. Larger particles drop out while

smaller dust particles collect on filter bags. When the dust layer thickness reaches a level where

flow through the system is restricted, the bag cleaning process is initiated. Cleaning can be done

Figure 8: Examples of Baghouses Pulse Jet Baghouse (L) and Reverse Air Baghouse (R).


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while the baghouse is still online (filtering) or in isolation (offline). Once cleaned, the

compartment is placed back in service and the filtering process starts over. Fabric filters

normally operate in the temperature range of 120-180. (Fabric Filter Bags, 2010)

As carbon is coals primary combustible material, and complete combustion of this material with

air produces carbon dioxide, coal-fired power plants emit large quantities of carbon dioxide in

their operation. Though not a pollutant, carbon dioxide is mainly responsible for climate change

enough reason why its production must be minimized or controlled. Methods of carbon dioxide

are now being developed these days and these methods can be broadly classified into physical

absorption, chemical absorption and membrane separation. (Breeze, Carbon Dioxide Removal,

2005)

Among these methods, chemical absorption is the most preferred option at the present day. In

chemical absorption, CO2 is captured in the flue gas by a reactive liquid solvent. Before

absorption, the flue gas is first treated in a cooling system to reach the right temperature and in apurifying section to remove particulates and other possible impurities especially nitrogen oxides

and sulfur dioxides that could cause solvent losses for degradation. After then, the flue gas is

delivered to the absorption tower where in CO2 reacts with aqueous solutions of amines while

the cleaned flue gas, essentially hot air, is released in the atmosphere. The widely used amines

are alkanolammines, such as monoethanolammina (MEA), diethanolamine(DEA),

methyldiethanolamine (MDEA). (Carbon Dioxide Capture, 2008)

The reaction actually creates a compound in which CO2 is loosely bound to. This compound is

then delivered to a stripper section where it is heated obtaining CO2 and regenerating the

solvent. After that, CO2 captured is dried, compressed and transported in a dense phase to thestorage site while the regenerated solvent is recycled back to the absorption tower.

Figure 9: Chemical Absorption of Carbon Dioxide.


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In physical absorption on the other hand, solid sorbents are used to adsorb CO2 from flue gas.These are typically made of materials with high surface areas, as zeolites and activated carbon,

which make them able to capture carbon dioxide, obtaining carbon free flue gas. The separationof CO2 and the regeneration of the adsorbent are based on the increase of temperature or

pressure. (Carbon Dioxide Capture, 2008)

However, this technology is not fit for large scale applications at the moments because of its lowCO2 selectivity and high energy requirement for regeneration.

The last method of carbon dioxide capture is membrane separation and like its application in

other fields, membrane separation uses the properties of a special membrane which will allow

carbon dioxide pass through it but hinder oxygen or nitrogen. Membrane separation operates on

the basis of carbon dioxides partial pressure across the membrane. (Breeze, Carbon Dioxide

Removal, 2005)

These methods however, solve only half of the problem the capture or separation of carbon

dioxide from the plant. The second half of the problem would be where to put it.

Most coals contain a small amount of mercury and can be easily discharged to the atmosphere in

which it could accumulate and increase in quantity. High levels of mercury have toxic effect on

our nervous system reason enough to control or lessen its release in the environment. (Pre-

Combustion and Combustion Technology for Control of Mercury Emissions from Coal Fired

Boilers, 2007)

Mercury can be removed through precombustion, combustion and postcombustion technologies

along with the removal of other pollutants. It can be removed through an ESP or fabric filter,scrubbing or injection of powdered activated carbon.

The last set of technologies in cleaning coal is called Conversion technologies. Conversion

technologies are technologies that turn coal into gas or liquid that can be cleaned and used as

fuel. These are advanced coal-fired power plant technologies.

There are three most widely known conversion technologies today. The first one is the Fluidized

Bed Combustion (FBC).

Fluidized Bed Combustion is a method of burning coal in a bed of heated particles suspended in

a gas flow. Envision a layer of sand, of finely ground coal, or of another fine solid material is

placed in a container and high-pressure air is blown through it from below, the particles,

provided they are small enough, become entrained in the air and form a floating, or fluidised, bed

of solid particles above the bottom of the container. At sufficient flow rates, this bed behaves like

a fluid in which the constituent particles constantly move to and fro and collide with one another,

resulting in rapid mixing of the particles. Coal is added to the bed and the continuous mixing


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encourages complete combustion and a lower temperature than that of the conventional coal-

fired powerplant.

During start-up, the fluidized bed of inert solids are first heated up by start-up burners and when

a high enough temperature is reached at least 600 - coal is added to the bed and after it

ignites, the startup firing can be shut down. The combustion of coal can then be maintained at a

temperature of around 850. This combustion temperature is maintained by in-furnace cooling

surfaces that are usually located at the furnace walls. Figure below is an diagram showing the

FBC technology. (Johnsson, 2007)

Figure 10: Fluidized Bed Combustion System Diagram.

Because FBC have lower combustion temperatures, they produce less nitrogen oxides in theoutlet gas. Also, as limestone is continuously added with coal, they produce less sulfur oxides.

(Johnsson, 2007)

Besides these advantages, the boiler pipes in FBC are immersed in the bed itself, allowing

extremely efficient heat capture. (Breeze, Fluidized Bed Combustion, 2005)

Another advance coal-fired power plant technology is the Integrated Gasification Combined

Cycle or IGCC.

The Integrated Gasification Combined Cycle produces electricity from coal by converting it first

to synthetic gas or syngas which is a mixture of hydrogen and carbon monoxide. Then syngas is


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converted to electricity in a combined power cycle block consisting of a gas turbine process and

a steam turbine process which includes a heat recovery steam generator. (Breeze, Integrated

Gasification Combined Cycle, 2005)

Figure below shows how an IGCC power plants works.

Figure 11: Integrated Gasification Combined Cycle Diagram.

As can be seen in Figure, coal is fed to the gasifier where it is partially oxidized under pressure

(30-80 bar). If a plant uses oxygen as oxidant, an air separation unit (ASU) will be needed. In the

gasifier, the temperature may exceed 1500 which ensures that the ash is converted to a liquid

slag with low viscosity for easy flow out of the gasifier. (Maurstad, 2005)

Besides its chemical energy (heating value), the hot raw syngas contains sensible heat which

may be recovered in heat exchangers to produce steam for the steam turbine. The use of syngascoolers for this purpose increases efficiency, but adds capital costs. In theory, it would be

desirable to clean the raw syngas without cooling (as the sensible heat would be utilized mostefficiently when delivered to the gas turbine), but the proven technologies for gas clean up

operate at near ambient temperatures. In the gas clean up process, particles, sulfur and otherimpurities are removed. At this point, CO

2may also be captured. Because of the high partial

pressures of the species and the low volume flow of syngas, the gas clean up process is very


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efficient and low cost compared to traditional flue gas cleaning. The clean syngas is then fed tothe gas turbine for production of electricity. (Maurstad, 2005)

Most of the sensible heat in the hot gas turbine exhaust gas is recovered in the heat recovery

steam generator (HRSG) which supplies the steam to a turbine for additional electricity

production. (Maurstad, 2005)

An IGCC can reach up to 45% efficiency, can remove up to 99% of sulfur from coal and can

reduce the emission of nitrogen oxides to 50ppm. (Breeze, Integrated Gasification CombinedCycle, 2005)

The last type of advanced coal-fired power plant technology is Oxygen Firing or

OxyCombustion. Oxy-combustion is simply the combustion of coal with mixture of nearly pureoxygen and recycled flue gas. Since nearly pure oxygen is fed instead of air, atmospheric

nitrogen is not introduced into the products of combustion and a concentrated CO2 flue gasstream is produced. This CO2 stream can be dried and compressed for sequestration, or

processed further into a high-purity CO2 product for varied uses. (Stamatelopoulos, 2007)

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