View source for Flue gas

{{Short description|Gas exiting to the atmosphere via a flue}}
[[File:Bankside power station and St George the Martyr church - geograph.org.uk - 1607688.jpg|thumb|Flue gas from London's [[Bankside Power Station]], 1975]]

'''Flue gas''' is the [[gas]] exiting to the [[atmosphere]] via a [[flue]], which is a pipe or channel for conveying [[Exhaust gas|exhaust gases]], as from a fireplace, oven, [[Furnace (house heating)|furnace]], [[boiler]] or [[Boiler (power generation)|steam generator]]. It often refers to the exhaust gas of [[combustion]] at [[Power plant|power plants]]. Technology is available to remove pollutants from flue gas at power plants.

Combustion of [[Fossil fuel|fossil fuels]] is a common source of flue gas. They are usually combusted with ambient air, with the largest part of the flue gas from most [[Fossil fuel|fossil-fuel]] combustion being [[nitrogen]], [[carbon dioxide]], and [[water vapor]].

== Description ==
Flue gas is the [[gas]] exiting to the atmosphere via a [[flue]], which is a pipe or channel for conveying [[Exhaust gas|exhaust gases]] from [[combustion]], as from a [[fireplace]], [[oven]], [[Furnace (house heating)|furnace]], [[boiler]] or [[Boiler (power generation)|steam generator]].<ref>{{Cite web |year=2022 |title=Flue gas definition and meaning |url=https://www.collinsdictionary.com/dictionary/english/flue-gas |website=Collins English Dictionary}}</ref> The term can also be used to refer to the gas produced from chemical or physical processes that do not involve combustion, such as [[Natural-gas processing|natural gas processing]].<ref>{{cite journal |last1=Abotaleb |first1=Ahmed |last2=Gladich |first2=Ivan |last3=Alkhateeb |first3=Alaa |last4=Mardini |first4=Nour |last5=Bicer |first5=Yusuf |last6=Sinopoli |first6=Alessandro |title=Chemical and physical systems for sour gas removal: An overview from reaction mechanisms to industrial implications |journal=Journal of Natural Gas Science and Engineering |date=October 2022 |volume=106 |pages=104755 |doi=10.1016/j.jngse.2022.104755}}</ref>

== Power plants ==
Quite often, the flue gas 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 of air, [[carbon dioxide]] ({{CO2}}), 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 [[Atmospheric particulate matter|particulate matter]] (like [[soot]]), [[carbon monoxide]], [[nitrogen oxides]], and [[sulfur oxides]].<ref>[http://www.eoearth.org/article/Fossil_fuel_combustion_glue_gases?topic=74180 Fossil fuel combustion flue gases] Milton R. Beychok, Encyclopedia of Earth, 2012.</ref>

=== Scrubbing ===
[[File:Air_pollution_by_industrial_chimneys.jpg|right|thumb|Before [[flue-gas desulfurization]] was installed, emissions from [[Four Corners Generating Station]], [[New Mexico]] contained excessive amounts of [[sulfur dioxide]].]]
At power plants, flue gas is often treated with a series of chemical processes and [[scrubber]]s, which remove pollutants. [[Electrostatic Precipitator|Electrostatic precipitators]] or [[Dust collector#Fabric filters|fabric filters]] remove particulate matter and [[flue-gas desulfurization]] captures the [[sulfur dioxide]] produced by burning fossil fuels, particularly coal.<ref>{{Cite web |title=Flue gas treatment {{!}} technology |url=https://www.britannica.com/technology/flue-gas-treatment |access-date=2021-10-13 |website=Encyclopedia Britannica |language=en}}</ref> 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 the United States, there is a rapid deployment of technologies to remove [[mercury (element)|mercury]] from flue gas—typically by [[absorption (chemistry)|absorption]] on sorbents or by capture in inert solids as part of the flue-gas desulfurization product. Such scrubbing can lead to meaningful recovery of sulfur for further industrial use.<ref>[http://www.eoearth.org/article/Sulfur?topic=49557 Sulfur] {{webarchive|url=https://web.archive.org/web/20121028080550/http://www.eoearth.org/article/Sulfur?topic=49557|date=October 28, 2012}} C. Michael Hogan, Encyclopedia of Earth, 2011.</ref>

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 gas field#Carbon capture and storage project|Sleipner West field]] in the [[North Sea]], operating since 1996).<ref>{{Cite web |title=Technology - Technology and innovation - statoil.com |url=http://www.statoil.com/en/TechnologyInnovation/ProtectingTheEnvironment/CarboncaptureAndStorage/Pages/CarbonDioxideInjectionSleipnerVest.aspx |url-status=dead |archive-url=https://web.archive.org/web/20091216072031/http://www.statoil.com/en/TechnologyInnovation/ProtectingTheEnvironment/CarboncaptureAndStorage/Pages/CarbonDioxideInjectionSleipnerVest.aspx |archive-date=2009-12-16 |access-date=2017-12-09 |website=www.statoil.com |language=en}}</ref>

There are a number of proven technologies for removing pollutants emitted from power plants that are now available. There is also much ongoing research into technologies that will remove even more air pollutants.{{citation needed|date=February 2017}}

== Fossil fuels ==
Most [[fossil fuels]] are combusted with ambient air (as differentiated from combustion with pure [[oxygen]]). Since ambient air contains about 79 volume percent gaseous [[nitrogen]] (N<sub>2</sub>),<ref name=":0">{{Cite web|last=Change|first=NASA Global Climate|title=10 interesting things about air|url=https://climate.nasa.gov/news/2491/10-interesting-things-about-air|access-date=2021-11-23|website=Climate Change: Vital Signs of the Planet}}</ref><ref>[http://www.eoearth.org/article/Sulfur?topic=49557 Sulfur] {{webarchive |url=https://web.archive.org/web/20121028080550/http://www.eoearth.org/article/Sulfur?topic=49557 |date=October 28, 2012 }} C. Michael Hogan, Encyclopedia of Earth, 2011. {{verify source |date=September 2019 |reason=This ref was deleted Special:Diff/893261757 by a bug in VisualEditor and later restored by a bot from the original cite located at Special:Permalink/867902840 cite #2 - verify the cite is accurate and delete this template. [[User:GreenC bot/Job 18]]}}</ref> which is essentially non-combustible, the largest part of the flue gas from most fossil-fuel combustion is uncombusted nitrogen. [[Carbon dioxide]] (CO<sub>2</sub>), the next largest part of flue gas, can be as much as 10−25 volume percent or more of the flue gas. This is closely followed in volume by water vapor (H<sub>2</sub>O) created by the combustion of the hydrogen in the fuel with atmospheric oxygen. Much of the 'smoke' seen pouring from [[flue gas stack]]s is this water vapor, forming a cloud as it contacts cool air.

A typical flue gas from the combustion of fossil fuels contains very small amounts of [[nitrogen oxide]]s ({{NOx}}), [[sulfur dioxide]] (SO<sub>2</sub>) and [[Atmospheric particulate matter|particulate matter]].<ref>[http://www.eoearth.org/article/Fossil_fuel_combustion_glue_gases?topic=74180 Fossil fuel combustion flue gases] Milton R. Beychok, Encyclopedia of Earth, 2012. {{verify source |date=September 2019 |reason=This ref was deleted Special:Diff/893261757 by a bug in VisualEditor and later restored by a bot from the original cite located at Special:Permalink/867902840 cite #1 - verify the cite is accurate and delete this template. [[User:GreenC bot/Job 18]]}}</ref> The nitrogen oxides are derived from the nitrogen in the ambient air, as well as from any nitrogen-containing compounds in the fossil fuel. The sulfur dioxide is derived from any [[sulfur]]-containing compounds in the fuels. The particulate matter is composed of very small particles of solid materials and very small liquid droplets which give flue gases their smoky appearance.

The steam generators in large [[power plant]]s and the process [[Industrial furnace|furnace]]s in large [[Oil refinery|refineries]], [[petrochemical]] and [[chemical plant]]s, and [[incinerator]]s burn considerable amounts of fossil fuels and therefore emit large amounts of flue gas to the ambient atmosphere. The table below presents the total amounts of flue gas typically generated by the burning of fossil fuels such as [[natural gas]], [[fuel oil]] and [[coal]]. The data were obtained by [[Stoichiometry|stoichiometric]]<ref>{{Cite web|url=http://www.statoil.com/en/TechnologyInnovation/ProtectingTheEnvironment/CarboncaptureAndStorage/Pages/CarbonDioxideInjectionSleipnerVest.aspx|title=Technology - Technology and innovation - statoil.com|website=www.statoil.com|language=en|access-date=2017-12-09|url-status=dead|archive-url=https://web.archive.org/web/20091216072031/http://www.statoil.com/en/TechnologyInnovation/ProtectingTheEnvironment/CarboncaptureAndStorage/Pages/CarbonDioxideInjectionSleipnerVest.aspx|archive-date=2009-12-16}} {{verify source |date=September 2019 |reason=This ref was deleted Special:Diff/893261757 by a bug in VisualEditor and later restored by a bot from the original cite located at Special:Permalink/867902840 cite #3 - verify the cite is accurate and delete this template. [[User:GreenC bot/Job 18]]}}</ref> calculations.<ref>Citation error. See inline comment how to fix. {{verify source |date=September 2019 |reason=This ref was deleted Special:Diff/893261757 by a bug in VisualEditor and later identified by a bot. The original cite can be found at Special:Permalink/867902840 (or in a rev close to it) as cite #4 - find and verify the cite and replace this template with it (2). [[User:GreenC bot/Job 18]]}}</ref>

The total amount of wet flue gas generated by coal combustion is only 10 percent higher than the flue gas generated by natural-gas combustion (the ratio for dry flue gas is higher).

=== Composition of flue-gas emissions from fossil-fuel combustion ===
{| class="wikitable"
|+Exhaust flue gas generated by combustion of fossil fuels
(In SI metric units and in US customary units)
! align="left" |Combustion data
! align="right" |[[Fuel gas]]
! align="right" |[[Fuel oil]]
! align="right" |[[Coal]]
|-
!Fuel properties:
!
!
!
|-
| align="left" |[[Heating value|Gross caloric value]], MJ/m<sup>3</sup>
| align="right" |43.01
|
|
|-
| align="left" |[[Heating value|Gross heating value]], Btu/scf
| align="right" |1,093
|
|
|-
| align="left" |Gross caloric value, MJ/kg
|
| align="right" |43.50
|
|-
| align="left" |Gross heating value, Btu/gal{{vague|date=June 2025}}
|
| align="right" |150,000
|
|-
| align="left" |Gross caloric value, MJ/kg
|
|
| align="right" |25.92
|-
| align="left" |Gross heating value, Btu/lb
|
|
| align="right" |11,150
|-
| align="left" |[[Molecular mass|Molecular weight]]
| align="right" |18
|
|
|-
| align="left" |[[Relative density|Specific gravity]]
|
| align="right" |0.9626
|
|-
| align="left" |[[API gravity|Gravity]], °API
|
| align="right" |15.5
|
|-
| align="left" |[[Carbon]]/[[hydrogen]] ratio by weight
|
| align="right" |8.1
|
|-
| align="left" |weight % carbon
|
|
| align="right" |61.2
|-
| align="left" |weight % hydrogen
|
|
| align="right" |4.3
|-
| align="left" |weight % [[oxygen]]
|
|
| align="right" |7.4
|-
| align="left" |weight % sulfur
|
|
| align="right" |3.9
|-
| align="left" |weight % nitrogen
|
|
| align="right" |1.2
|-
| align="left" |weight % [[Coal assay#Ash|ash]]
|
|
| align="right" |12.0
|-
| align="left" |weight % [[Coal assay#Moisture|moisture]]
|
|
| align="right" |10.0
|-
! align="left" |Combustion air:
!
!
!
|-
| align="left" |Excess combustion air, %
| align="right" |12
| align="right" |15
| align="right" |20
|-
! align="left" |Wet exhaust flue gas:
!
!
!
|-
| align="left" |Amount of wet [[exhaust gas]], m<sup>3</sup>/GJ of fuel
| align="right" |294.8
| align="right" |303.1
| align="right" |323.1
|-
| align="left" |Amount of wet exhaust gas, scf/10<sup>6</sup> Btu of fuel
| align="right" |11,600
| align="right" |11,930
| align="right" |12,714
|-
| align="left" |{{CO2|link=yes}} in wet exhaust gas, volume %
| align="right" |8.8
| align="right" |12.4
| align="right" |13.7
|-
| align="left" |O<sub>2</sub> in wet exhaust gas, volume %
| align="right" |2.0
| align="right" |2.6
| align="right" |3.4
|-
| align="left" |Molecular weight of wet exhaust gas
| align="right" |27.7
| align="right" |29.0
| align="right" |29.5
|-
! align="left" |Dry exhaust flue gas:
!
!
!
|-
| align="left" |Amount of dry exhaust gas, m<sup>3</sup>/GJ of fuel
| align="right" |241.6
| align="right" |269.3
| align="right" |293.6
|-
| align="left" |Amount of dry exhaust gas, scf/10<sup>6</sup> Btu of fuel
| align="right" |9,510
| align="right" |10,600
| align="right" |11,554
|-
| align="left" |CO<sub>2</sub> in dry exhaust gas, volume %
| align="right" |10.8
| align="right" |14.0
| align="right" |15.0
|-
| align="left" |O<sub>2</sub> in dry exhaust gas, volume %
| align="right" |2.5
| align="right" |2.9
| align="right" |3.7
|-
| align="left" |Molecular weight of dry exhaust gas
| align="right" |29.9
| align="right" |30.4
| align="right" |30.7
|}

: <small>m<sup>3</sup> are standard cubic meters at 0 °C and 101.325 kPa, and scf is standard cubic feet at 60 °F and 14.696 psia.</small>

==See also==
*[[AP 42 Compilation of Air Pollutant Emission Factors]]
*[[Carbon capture and storage]]
*[[Emission standard]]
*[[Exhaust gas]]
*[[Flue gas stacks]]
*[[Aviation biofuel#ATJ-SPK|Flue gas to fuel]]
*[[Flue-gas desulfurization]]
*[[Integrated gasification combined cycle]] (often referred to as IGCC)
*[[Landfill gas]]

==References==
{{Reflist}}

{{Coal}}
{{Authority control}}

[[Category:Air pollution]]
[[Category:Combustion]]
[[Category:Environmental engineering]]
[[Category:Industrial gases]]