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[[Image:Four Corners Power Plant.jpg|right|thumb|250px|{{#ifexist:Template:Four Corners Power Plant.jpg/credit|{{Four Corners Power Plant.jpg/credit}}<br/>|}}Before flue gas desulfurization was installed, the emissions from this power plant in [[New Mexico]] contained excessive amounts of [[sulfur dioxide]]]]
A '''flue gas stack''' is a a vertical pipe, channel or similar structure through which [[combustion]] product gases called ''flue gases'' are exhausted to the outside air. It is sometimes referred to as a ''smokestack''.


'''Flue gas desulfurization''' (FGD) is the technology used for removing [[sulfur dioxide]] (SO<sub>2</sub>) from the exhaust [[flue gases]] of [[power plant]]s that burn coal or oil to produce steam for the [[steam turbines|turbines]] that drive their electricity [[generators]]. The most common types of FGD contact the flue gases with an [[alkaline]] [[sorbent]] such as [[lime]] or [[limestone]]. <ref name=Schnelle>{{cite book|author=Karl B. Schnelle and Charles A. Brown|title=Air Pollution Control Technology>|edition= |publisher=CRC Press|year=2001|id=ISBN 0-8493-9599-7}}</ref><ref name=ICAC>[http://www.icac.com/i4a/pages/index.cfm?pageid=3401 SO<sub>2</sub> Control Technologies] (from website of the Institute of Clean Air Companies)</ref><ref name=EPA>[http://www.epa.gov/ttn/catc/dir1/ffdg.pdf Air Pollution Control Technology Fact Sheet] [[U.S. EPA]] publications EPA-452/F-03-034</ref>
Flue gases are produced when [[coal]], [[fuel oil]], [[natural gas]], [[wood]] or any other fuel is [[combustion|combusted]] in an industrial [[furnace]] or [[boiler]], a [[power plant]]'s [[steam-generator]] or [[boiler]], or other large combustion device. Flue gas is usually composed of [[carbon dioxide]] (CO<sub>2</sub>) and water vapor as well as [[nitrogen]] and excess [[oxygen]] remaining from the intake combustion air. It also contains a small percentage of pollutants such as [[particulate matter]], [[carbon monoxide]], [[nitrogen oxides]] and [[sulfur oxides]]. The flue gas stacks are often quite tall, up to 400 meters (1300 feet) or more, so as to disperse the exhaust [[pollutant]]s over a greater area and thereby reduce the [[ground-level concentration]] of the pollutants to comply with governmental [[air pollution]] control regulations.  
As sulfur dioxide is responsible for [[acid rain]] formation, stringent environmental protection regulations have been enacted in many countries to limit the amount of sulfur dioxide emissions from power plants and other industrial facilities.  


Prior to the advent of strict environmental protection regulations, tall flue gas stacks (i.e., chimneys) were built to disperse rather than remove the sulfur dioxide emissions. However, that only led to the transport of the emissions to other regions. For that reason, a number of countries also have regulations limiting the height of flue gas stacks.
When the flue gases are exhausted from stoves, ovens, fireplaces, or other small sources within residential abodes, restaurants, hotels, or other public buildings and small commercial enterprises, their flue gas stacks are referred to as ''chimneys''.
 
For a typical coal-fired power station, FGD technology will remove 95 percent or more of the SO<sub>2</sub> in the flue gases.


==History==
==History==


Methods for removing sulfur dioxide from flues gases have been studied for over 150 years.  Early concepts useful for flue gas desulfurization appear to have germinated in 1850 in [[England]].
The first industrial chimneys were built in the mid-17th century when it was first understood how they could improve the combustion of a [[furnace]] by increasing the draft (draught) of air into the combustion zone. <ref>Douet, James (1988). ''Going up in Smoke:The History of the Industrial Chimney'', Victorian Society, London, England.  [http://www.victoriansociety.org.uk/caserpts.html Victorian Society Casework Reports]</ref> As such, they played an important part in the development of [[reverberatory furnace]]s and a coal-based metallurgical industry, one of the key sectors of the early [[Industrial Revolution]]. Most 18th century industrial chimneys (now commonly referred to as '''flue gas stacks''') were built into the walls of the furnace much like a domestic chimney. The first free-standing industrial chimneys were probably those erected at the end of the long condensing chimneys associated with smelting [[lead]].
 
With the construction of large-scale power plants in England in the 1920s, the problems associated with large volumes of SO<sub>2</sub> emissions began to concern the public. The problem did not receive much attention until 1929, when the British government upheld the claim of a landowner against the Barton Electricity Works for damages to his land resulting from SO<sub>2</sub> emissions.  Shortly thereafter a press campaign was launched against the erection of power plants within the confines of [[London]]. This led to the imposition of SO<sub>2</sub> controls on all such power plants.<ref name=Biondo>Biondo, S.J. and Marten,J.C., ''A History of Flue Gas Desulfurization Systems Since 1850'', Journal of the Air Pollution Control Association, Vol. 27, No. 10, pp 948-961, October 1977.</ref>
 
During this period, major FGD installations went into operation in England at three power plants. The first one began operation at the Battersea Station in London in 1931.  In 1935, the second one went into service at the Swansea Power Station. The third one was installed in 1938 at the Fulham Power Station. All three installations were abandoned during [[World War II]].
 
Large-scale FGD units did not reappear in commercial operation until the 1970s, and most of the activity occurred in the [[United States]] and [[Japan]].<ref name=Biondo/> As of June 1973, there were 42 FGD units, ranging in size from 5 to 250 [[megawatts]], in operation: 36 in Japan and 6 in the United States.<ref name=Beychok>Beychok, Milton R., ''Coping With SO<sub>2</sub>'', Chemical Engineering/Deskbook Issue, October 21, 1974</ref>  
 
As of about 1999-2000, there were 678 FGD units operating worldwide (in 27 countries) producing a total of about 229 [[gigawatts]]. About 45% of that FGD capacity was in the United States, 24% in [[Germany]], 11% in Japan and 20% in various other countries. Approximately 79% of the units, representing about 199 gigawatts of capacity, were using lime or limestone wet scrubbing. About 18% (or 25 gigawatts) utilized spray-dry scrubbers or dry sorbent injection systems.<ref name=BW>Nolan, Paul  S., ''Flue Gas Desulfurization Technologies for Coal-Fired Power Plants'', The Babcock & Wilcox Company, U.S., presented by Michael X. Jiang at the Coal-Tech 2000 International Conference, November, 2000, Jakarta, Indonesia</ref><ref name=CMU>Rubin, E.S., Yeh, S., Hounsell, D.A., and Taylor, M.R., ''Experience curves for power plant emission control technologies'', Int. J. Energy Technology and Policy, Vol. 2, Nos. 1/2, 2004</ref><ref>Beychok, Milton R., ''Comparative economics of advanced regenerable flue gas desulfurization processes'', EPRI CS-1381, Electric Power Research Institute, March 1980</ref>
 
== FGD chemistry ==
 
SO<sub>2</sub> is an acid gas. Therefore, the most common large-scale FGD systems use an alkaline sorbent such as [[lime]] or [[limestone]] to neutralize and remove the SO<sub>2</sub> from the flue gas. Since lime and limestone are not soluble in water, they are used either in the form of an aqueous slurry or in a dry, powdered form.
 
When using an aqueous slurry of sorbent, the FGD system is referred to as a ''wet scrubber''. When using a dry, powdered sorbent, the system is referred to as a ''dry'' system. An intermediate or semi-dry system is referred to as a ''spray-dry'' system.
 
The reaction taking place in wet scrubbing using
a CaCO<sub>3</sub> (limestone) slurry produces CaSO<sub>3</sub> ([[calcium sulfite]]) and can be expressed as:
 
:'''CaCO<sub>3</sub> (solid) + SO<sub>2</sub> (gas) → CaSO<sub>3</sub> (solid) + CO<sub>2</sub> (gas)'''
 
When wet scrubbing with a Ca(OH)<sub>2</sub> (lime) slurry, the reaction also produces CaSO<sub>3</sub> (calcium sulfite) and can be expressed as:
 
:'''Ca(OH)<sub>2</sub> (solid) + SO<sub>2</sub> (gas) → CaSO<sub>3</sub> (solid) + H<sub>2</sub>O (liquid)'''
 
When wet scrubbing with a Mg(OH)<sub>2</sub> ([[magnesium hydroxide]]) slurry, the reaction produces MgSO<sub>3</sub> ([[magnesium sulfite]]) and can be expressed as:
 
:'''Mg(OH)<sub>2</sub> (solid) + SO<sub>2</sub> (gas) → MgSO<sub>3</sub> (solid) + H<sub>2</sub>O (liquid)'''
 
Some FGD systems go a step further and oxidize the CaSO<sub>3</sub> (calcium sulfite) to produce marketable CaSO<sub>4</sub> · 2H<sub>2</sub>O ([[gypsum]]):
 
:'''CaSO<sub>3</sub> (solid) + ½O<sub>2</sub> (gas) + 2H<sub>2</sub>O (liquid) → CaSO<sub>4</sub> · 2H<sub>2</sub>O (solid)'''


Aqueous solutions of [[sodium hydroxide]] (known as ''caustic soda'' or simply ''caustic'') may also be used to neutralize and remove SO<sub>2</sub> from flue gases. However, caustic soda is limited to small-scale FGD systems, mostly in industrial facilities  other than power plants because it is more expensive than lime. It has the advantage that it forms a solution rather than a slurry and that makes it easier to operate. It produces a solution of [[sodium sulfite]] or [[sodium bisulfite]] (depending on the [[pH]]), or [[sodium sulfate]] that must be disposed of. This is not a problem in a paper mill for example, where the solution can be recycled and reused within the paper mill.
The powerful association between industrial chimneys and the characteristic smoke-filled landscapes of the industrial revolution was due the universal application of the [[steam engine]] for most manufacturing processes. The chimney is part of a steam-generating boiler, and its evolution is closely linked to increases in the power of the steam engine. The chimneys of Thomas Newcomen’s steam engine were incorporated into the walls of the engine house. The taller, free-standing industrial chimneys that appeared in the early 19th century were related to the changes in boiler design associated with James Watt’s "double-powered" engines, and they continued to grow in stature throughout the [[Victorian]] period. Decorative embellishments are a feature of many industrial chimneys  from the 1860s.


== Types of FGD systems ==
The invention of fan-assisted draft (draught) in the early 20th century removed the industrial chimney's original function, that of drawing air into the steam-generating boilers or other furnaces. With the replacement of the steam engine as a prime mover, first by [[diesel engine]s and then by [[electric motor]]s, the  early industrial chimneys began to disappear from the industrial landscape. Building materials changed from stone and brick to steel and later reinforced concrete, and the height of the industrial chimney was determined by the need to disperse combustion flue gases to comply with governmental [[air pollution]] control regulations.


The major types of large-scale, power plant FGD systems include spray towers, spray dryers and dry sorbent injection systems.
==Flue gas stack draft (or draught)==


===Spray tower===
[[Image:Chimney effect.svg|thumb|180px|right|The stack effect in chimneys: the gauges represent absolute air pressure and the airflow is indicated with light grey arrows. The gauge dials move clockwise with increasing pressure.]]


[[Image:FGD Locations.png|right|thumb|274px|{{#ifexist:Template:FGD Locations.png/credit|{{FGD Locations.png/credit}}<br/>|}}Location of the various FGD options]]  
The combustion flue gases inside the flue gas stacks are much hotter than the ambient outside air and therefore less [[density|dense]] than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower [[pressure]] than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney.  That movement or flow of combustion air and flue gas is called "natural draft (or draught)", "natural ventilation"]], "chimney effect", or "[[stack effect]]". The taller the stack, the more draft (or draught) is created.


There are various types of wet scrubbers. For example, spray towers, [[venturi]] scrubbers, packed towers and trayed towers. Slurries would cause serious erosion problems in a venturi scrubber because of the high speeds at the throat of the venturi section. Packed towers or trayed towers would plug up if handling slurries. For handling slurries, the spray tower is a good choice and it is in fact a commonly used choice in large-scale FGD systems.<ref name=EPA/><ref>
The equation below provides an approximation of the pressure difference, Δ''P'', (between the bottom and the top of the flue gas stack) that is created by the draft:<ref>[http://www.arch.hku.hk/teaching/lectures/airvent/sect02.htm Natural Ventilation Lecture 2]</ref><ref>{{cite book|author=Perry, R.H. and Green, Don W.|title=Perry's Chemical Engineers' Handbook|edition=6th Edition (page 9-72)|publisher=McGraw-Hill Book Company|year=1984|id=ISBN 0-07-049479-7}}</ref>
[http://www.babcock.com/library/pdf/br-1643.pdf Wet FGD System Materials Cost Update, by M.G. Milobowski, Babcock & Wilcox] (Presented at EPRI-DOE-EPA Combined Utility Air Pollutant Control
Symposium, August 1997)</ref><ref name=EPA2>[http://www.epa.gov/ttncatc1/dir1/fsprytwr.pdf Air Pollution Control Technology Fact Sheet] [[U.S. EPA]] publications EPA-452/F-03-016</ref>


Spray towers are used downstream of the particulate equipment ([[electrostatic precipitator]] or [[baghouse]]) where the flue gas contains very little, if any, combustion [[fly ash]]. In a spray tower system, the sorbent slurry is simply injected via spray nozzles into a vertical tower where the slurry droplets are  contacted with the upflowing flue gas.
:<math>\Delta P =\; C\, a\; h\; \bigg(\frac {1}{T_o} - \frac {1}{T_i}\bigg)</math>


Part of the water in the slurry is evaporated by the hot flue gas and the flue gas becomes saturated with water vapor.
:{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
!align=right| Δ''P''
|align=left|= available pressure difference, in [[Pascal (unit)|Pa]]
|-
!align=right|''C''
|align=left|= 0.0342
|-
!align=right| ''a''
|align=left|= absolute [[atmospheric pressure]], in Pa
|-
!align=right| ''h''
|align=left|= height of the flue gas stack, in [[metres|m]]
|-
!align=right| ''T<sub>o</sub>''
|align=left|= absolute outside air [[temperature]], in [[kelvin|K]]
|-
!align=right| ''T<sub>i</sub>''
|align=left|= absolute average temperature of the flue gas inside the stack, in K
|}


The SO<sub>2</sub> dissolves into the slurry droplets and reacts with the alkaline sorbent particles. The slurry falls to the bottom of the spray tower and is sent to a reaction tank where the reaction is completed and a neutral salt is formed. In a regenerable system, the residual slurry is recycled back for reuse in the spray tower. In a once-through system, the residual slurry is dewatered and either disposed of or oxidized to CaSO<sub>4</sub> · 2H<sub>2</sub>O and sold as a by-product gypsum.
The above equation is an approximation because it assumes that the [[molar mass]] of the flue gas and the outside air are equal and that the pressure drop through the flue gas stack is quite small.  Both assumptions are fairly good but not exactly accurate.


===Spray-dryer===
== The flue gas flow rate induced by the draft==


Spray-dryers are used upstream of the particulate removal equipment (electrical precipitator or baghouse) where the flue gas contains the combustion fly ash. In a spray-dryer system, the alkaline sorbent is usually lime slurry. The slurry is atomized and sprayed into a vessel as a cloud of fine bubbles where it contacts the hot flue gas. The water is completely evaporated by the hot gas and the residence time in the vessel (about 10 seconds) allows the SO<sub>2</sub> and any other acid gases, such as SO<sub>3</sub> and HCl, to react with the lime to form a dry powder of calcium sulfite, calcium sulfate and unreacted lime.<ref name=EPA/><ref name=IEA>[http://www.iea-coal.org.uk/content/default.asp?PageID=1002 IEA Clean Coal Center: Spray dry scrubbers for SO<sub>2</sub> removal]</ref><ref name=Babcock2>[http://www.babcock.com/library/pdf/E1013178.pdf Dry Flue Gas Desulfurization (FGD)Systems] (From Babcock and Wilcox website)</ref>
As a "first guess" approximation, the following equation can be used to estimate the flue gas flow rate induced by the draft of a flue gas stack. The equation assumes that the molar mass of the flue gas and the outside air are equal and that the [[friction|frictional resistance]] and heat losses are negligible:<ref>[http://www.arch.hku.hk/teaching/lectures/airvent/sect03.htm Natural Ventilation Lecture 3]</ref> <br><br>


The dry powder is removed from the flue gas along with the combustion fly ash in the [[particulate matter|particulate]] removal equipment. Some of the solids from the particulate removal equipment (i.e., fly ash, calcium sulfite, calcium sulfate and unreacted lime) may be recycled and reused as part of the sorbent slurry.<ref name=Babcock2/>
:<math>Q = C\; A\; \sqrt {2\;g\;H\;\frac{T_i - T_o}{T_i}}</math>
{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
!align=right| ''Q''
|align=left|= flue gas flow rate, [[cubic metre|m³]]/s 
|-
!align=right| ''A''
|align=left|= cross-sectional area of chimney, [[square metre|m²]] (assuming it has a constant cross-section) 
|-
!align=right| ''C''
|align=left|= discharge coefficient (usually taken to be from 0.65 to 0.70)
|-
!align=right| ''g''
|align=left|= [[standard gravity|gravitational acceleration at sea level]], 9.807 m/s²
|-
!align=right| ''H''
|align=left|= height of chimney, [[metre|m]]
|-
!align=right| ''T<sub>i</sub>''
|align=left|= absolute average temperature of the flue gas in the stack, [[kelvin|K]]
|-
!align=right| ''T<sub>o</sub>''
|align=left|= absolute outside air temperature, K 
|}


===Dry sorbent injection===
Designing chimneys and stacks to provide the correct amount of natural draft involves a great many factors such as:


The dry FGD system simply injects powdered lime or limestone sorbent directly into the flue gas. As shown in the adjacent location diagram, the dry sorbent may be injected into any one of three locations:  (1) the upper section of the [[steam generator]], (2) the [[economizer]] section of the steam generator  or the ducting between the [[air preheater]] and the electrostatic precipitator.<ref name=ICAC/><ref name=EPA/><ref name=IEA/><ref>{{cite book|author=Barbara Toole-O'Neil and Ohio Coal Development Office (Editors)|title=Dry Scrubbing Technologies for Flue Gas Desulfurization|edition=|publisher=Springer|year=1998|id=ISBN 0-7923-8346-X}}</ref>
* The height and diameter of the stack.
* The desired amount of excess combustion air needed to assure complete combustion.
* The temperature of the flue gases leaving the combustion zone.
* The composition of the combustion flue gas, which determines the flue gas [[density]].
* The frictional resistance to the flow of the flue gases through the chimney or stack, which will vary with the materials used to construct the chimney or stack.
* The heat loss from the flue gases as they flow through the chimney or stack.
* The local atmospheric pressure of the ambient air, which is determined by the local elevation above sea level.


The powdered sorbent is pneumatically injected through lances designed to distribute the sorbent evenly across the flow path of the flue gas.  
The calculation of many of the above design factors requires trial-and-error reiterative methods.


When injected into the upper section of the steam generator, it should enter at a point where the temperature range is about 900 to 1200 °C. Injection into the economizer should be at a point where the temperature range is about 400 to 600 °C. Injection into the ducting between the preheater and the precipitator should be at point where the flue gas temperature is about 150 to 180 °C.<ref name=EPA/><ref name=IEA/>
Governmental agencies in most countries have specific codes which govern how such design calculations must be performed. Many non-governmental organizations also have codes governing the design of chimneys and stacks (notably, the [[American Society of Mechanical Engineers|ASME]] codes).


The SO<sub>2</sub> reacts directly with the powdered sorbent and the spent sorbent is removed from the flue gas along with the combustion fly ash in the [[particulate matter|particulate]] removal equipment
==Other items of interest==


==Facts and statistics==
It should be noted that not all fuel-burning industrial equipment rely upon natural draft.  Many such equipment items use large fans or blowers to accomplish the same objectives, namely: the flow of combustion air into the combustion chamber and the flow of the hot flue gas out of the chimney or stack.


:''The information in this section was obtained from a [[U.S. EPA]] published fact sheet.''<ref name=EPA/>
A great many power plants are equipped with facilities for the removal of [[sulfur dioxide]] (i.e., [[flue gas desulfurization]]) and [[nitrogen oxides]] (i.e, [[selective catalytic reduction]], [[exhaust gas recirculation]], thermal de[[NOx]], or low NOx burners).  
Flue gas desulfurization scrubbers have been applied to combustion units firing coal and oil that range in size from 5 MW to 1500 MW. Scottish Power are spending £400 million installing FGD at Longannet power station which has a capacity of over 2 GW. Dry scrubbers and spray scrubbers have generally been applied to units smaller than 300 MW.


Approximately 85% of the flue gas desulfurization units installed in the US are wet scrubbers, 12% are spray dry systems and 3% are dry injection systems.
In the United States and a number of other countries, [[atmospheric dispersion modeling]]<ref>{{cite book|author=Beychok, Milton R.|title=[[Fundamentals Of Stack Gas Dispersion]]|edition=4th Edition|publisher=author-published|year=2005|id=ISBN 0-9644588-0-2}} [http://www.air-dispersion.com www.air-dispersion.com]</ref> studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United States also limits the maximum height of a flue gas stack to what is known as the "Good Engineering Practice (GEP)" stack height.<ref>''Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations), Revised'' (1985), EPA Publication No. EPA–450/4–80–023R, U.S. Environmental Protection Agency (NTIS No. PB 85–225241)</ref><ref>Lawson, Jr., R.E. and W.H. Snyder (1983). ''Determination of Good Engineering Practice Stack Height: A Demonstration Study for a Power Plant'', EPA Publication No. EPA–600/3–83–024. U.S. Environmental Protection Agency (NTIS No. PB 83–207407)</ref> In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modeling studies for such stacks must use the GEP stack height rather than the actual stack height.
 
The highest SO2 removal efficiencies (greater than 90%) are achieved by wet scrubbers and the lowest (less than 80%) by dry scrubbers. However, the newer designs for dry scrubbers are capable of achieving efficiencies in the order of 90%.
 
The capital, operating and maintenance costs per [[short ton]] of SO<sub>2</sub> removed (in 2001 US dollars) are:
 
*For wet scrubbers larger than 400 MW, the cost is $200 to $500 per ton
*For wet scrubbers smaller than 400 MW, the cost is $500 to $5,000 per ton
*For spray dry scrubbers larger than 200 MW, the cost is $150 to $300 per ton
*For spray dry scrubbers smaller than 200 MW, the cost is $500 to $4,000 per ton
 
== Alternative methods of reducing sulfur dioxide emissions ==
 
An alternative to removing [[sulfur]] from the flue gases after burning is to remove the sulfur from the fuel before or during combustion.  [[Hydrodesulfurization]] of fuel has been used for treating [[fuel oil]]s. 
 
[[Fluidized bed combustion]] adds lime to the fuel during combustion. The lime reacts with the SO<sub>2</sub> to form [[sulfate]]s which become part of the combustion ash.
 
== Sulfuric acid mist formation ==
 
[[Fossil fuel]]s such as coal and oil contain significant amounts of sulfur. When burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide (SO<sub>2</sub>). This happens under normal conditions of temperature and of oxygen present in the flue gas. However, there are circumstances under which this may not be the case.
 
For example, when the flue gas has too much [[oxygen]] and the SO<sub>2</sub> is further oxidized to [[sulfur trioxide]] (SO<sub>3</sub>). Actually, too much oxygen is only one of the ways that SO<sub>3</sub> is formed. Gas temperature is also an important factor. At about 800 °C, formation of SO<sub>3</sub> is favored. Another way that SO<sub>3</sub> can be formed is through catalysis by trace metals in the fuel. This is particularly true for heavy fuel oil, where small amounts of [[vanadium]] are present. In whatever way that SO<sub>3</sub> is formed, it does not behave like SO<sub>2</sub> in that it forms a liquid [[aerosol]] known as [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>) mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to SO<sub>3</sub>. Since SO<sub>3</sub> an acid gas, just as is SO<sub>2</sub>, it is also removed by the alkaline sorbents used in FGD systems.


==References==
==References==
{{reflist}}
{{reflist}}


== External links ==
==External links==
 
*[http://www.ashrae.org/publications/page/158 ASHRAE's Fundamentals Handbook] is available here from [[ASHRAE]]  
* Schematic of [http://www.mhi.co.jp/mcec/product/fgd.htm desulfurization plant]
*[http://www.asme.org/Codes/ ASME Codes and Standards] available from [[ASME]]
*[http://www.babcock.com/pgg/tt/pdf/BR-1666.pdf 5000 MW FGD Plant] (includes a detailed process flow diagram)
*[http://www.skyscraperpage.com/diagrams/?20374745 Diagram of 25 tallest flue gas stacks worldwide]
*New innovative design using [http://www.owr.ehnr.state.nc.us/ref/16/15870.pdf Gas Suspension Absorption]

Revision as of 16:18, 20 February 2008

A flue gas stack is a a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. It is sometimes referred to as a smokestack.

Flue gases are produced when coal, fuel oil, natural gas, wood or any other fuel is combusted in an industrial furnace or boiler, a power plant's steam-generator or boiler, or other large combustion device. Flue gas is usually composed of carbon dioxide (CO2) and water vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants over a greater area and thereby reduce the ground-level concentration of the pollutants to comply with governmental air pollution control regulations.

When the flue gases are exhausted from stoves, ovens, fireplaces, or other small sources within residential abodes, restaurants, hotels, or other public buildings and small commercial enterprises, their flue gas stacks are referred to as chimneys.

History

The first industrial chimneys were built in the mid-17th century when it was first understood how they could improve the combustion of a furnace by increasing the draft (draught) of air into the combustion zone. [1] As such, they played an important part in the development of reverberatory furnaces and a coal-based metallurgical industry, one of the key sectors of the early Industrial Revolution. Most 18th century industrial chimneys (now commonly referred to as flue gas stacks) were built into the walls of the furnace much like a domestic chimney. The first free-standing industrial chimneys were probably those erected at the end of the long condensing chimneys associated with smelting lead.

The powerful association between industrial chimneys and the characteristic smoke-filled landscapes of the industrial revolution was due the universal application of the steam engine for most manufacturing processes. The chimney is part of a steam-generating boiler, and its evolution is closely linked to increases in the power of the steam engine. The chimneys of Thomas Newcomen’s steam engine were incorporated into the walls of the engine house. The taller, free-standing industrial chimneys that appeared in the early 19th century were related to the changes in boiler design associated with James Watt’s "double-powered" engines, and they continued to grow in stature throughout the Victorian period. Decorative embellishments are a feature of many industrial chimneys from the 1860s.

The invention of fan-assisted draft (draught) in the early 20th century removed the industrial chimney's original function, that of drawing air into the steam-generating boilers or other furnaces. With the replacement of the steam engine as a prime mover, first by [[diesel engine]s and then by electric motors, the early industrial chimneys began to disappear from the industrial landscape. Building materials changed from stone and brick to steel and later reinforced concrete, and the height of the industrial chimney was determined by the need to disperse combustion flue gases to comply with governmental air pollution control regulations.

Flue gas stack draft (or draught)

The stack effect in chimneys: the gauges represent absolute air pressure and the airflow is indicated with light grey arrows. The gauge dials move clockwise with increasing pressure.

The combustion flue gases inside the flue gas stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draft (or draught)", "natural ventilation"]], "chimney effect", or "stack effect". The taller the stack, the more draft (or draught) is created.

The equation below provides an approximation of the pressure difference, ΔP, (between the bottom and the top of the flue gas stack) that is created by the draft:[2][3]

where:  
ΔP = available pressure difference, in Pa
C = 0.0342
a = absolute atmospheric pressure, in Pa
h = height of the flue gas stack, in m
To = absolute outside air temperature, in K
Ti = absolute average temperature of the flue gas inside the stack, in K

The above equation is an approximation because it assumes that the molar mass of the flue gas and the outside air are equal and that the pressure drop through the flue gas stack is quite small. Both assumptions are fairly good but not exactly accurate.

The flue gas flow rate induced by the draft

As a "first guess" approximation, the following equation can be used to estimate the flue gas flow rate induced by the draft of a flue gas stack. The equation assumes that the molar mass of the flue gas and the outside air are equal and that the frictional resistance and heat losses are negligible:[4]

where:  
Q = flue gas flow rate, m³/s
A = cross-sectional area of chimney, m² (assuming it has a constant cross-section)
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration at sea level, 9.807 m/s²
H = height of chimney, m
Ti = absolute average temperature of the flue gas in the stack, K
To = absolute outside air temperature, K

Designing chimneys and stacks to provide the correct amount of natural draft involves a great many factors such as:

  • The height and diameter of the stack.
  • The desired amount of excess combustion air needed to assure complete combustion.
  • The temperature of the flue gases leaving the combustion zone.
  • The composition of the combustion flue gas, which determines the flue gas density.
  • The frictional resistance to the flow of the flue gases through the chimney or stack, which will vary with the materials used to construct the chimney or stack.
  • The heat loss from the flue gases as they flow through the chimney or stack.
  • The local atmospheric pressure of the ambient air, which is determined by the local elevation above sea level.

The calculation of many of the above design factors requires trial-and-error reiterative methods.

Governmental agencies in most countries have specific codes which govern how such design calculations must be performed. Many non-governmental organizations also have codes governing the design of chimneys and stacks (notably, the ASME codes).

Other items of interest

It should be noted that not all fuel-burning industrial equipment rely upon natural draft. Many such equipment items use large fans or blowers to accomplish the same objectives, namely: the flow of combustion air into the combustion chamber and the flow of the hot flue gas out of the chimney or stack.

A great many power plants are equipped with facilities for the removal of sulfur dioxide (i.e., flue gas desulfurization) and nitrogen oxides (i.e, selective catalytic reduction, exhaust gas recirculation, thermal deNOx, or low NOx burners).

In the United States and a number of other countries, atmospheric dispersion modeling[5] studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United States also limits the maximum height of a flue gas stack to what is known as the "Good Engineering Practice (GEP)" stack height.[6][7] In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modeling studies for such stacks must use the GEP stack height rather than the actual stack height.

References

  1. ↑ Douet, James (1988). Going up in Smoke:The History of the Industrial Chimney, Victorian Society, London, England. Victorian Society Casework Reports
  2. ↑ Natural Ventilation Lecture 2
  3. ↑ Perry, R.H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition (page 9-72). McGraw-Hill Book Company. ISBN 0-07-049479-7. 
  4. ↑ Natural Ventilation Lecture 3
  5. ↑ Beychok, Milton R. (2005). Fundamentals Of Stack Gas Dispersion, 4th Edition. author-published. ISBN 0-9644588-0-2.  www.air-dispersion.com
  6. ↑ Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations), Revised (1985), EPA Publication No. EPA–450/4–80–023R, U.S. Environmental Protection Agency (NTIS No. PB 85–225241)
  7. ↑ Lawson, Jr., R.E. and W.H. Snyder (1983). Determination of Good Engineering Practice Stack Height: A Demonstration Study for a Power Plant, EPA Publication No. EPA–600/3–83–024. U.S. Environmental Protection Agency (NTIS No. PB 83–207407)

External links

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