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The '''acid dew point''' (also '''acid dewpoint''') of a [[flue gas]] (i.e., a [[combustion]] product [[gas]]) is the [[temperature]], at a given [[pressure]], at which any gaseous [[acid]] in the flue gas will start to [[Condensation (phase transition)|condense]] into [[liquid]] acid.<ref>{{cite book|author=David A. Lewandowski|title=Design of Thermal Oxidation Systems for Volatile Organic Compounds|edition=1st Edition|publisher=CRC Press|year=2000|id=ISBN 1-56670-410-3}}</ref><ref>{{cite book|author=Walter R. Niessen|title=Combustion and Incineration Processes|edition=3rd Edition|publisher=CRC Press|year=2002|id=ISBN 0-8247-0629-3}}</ref><ref>{{cite journal|author=W.M.M. Huijbregts and  R. Leferink|title=Latest Advances in the Understanding of Acid Dewpoint Corrosion: Corrosion and Stress Corrosion Cracking in Combustion Gas Condensates|journal=Anti-Corrosion Methods and Materials|volume=51|issue=3|pages=173 - 188|date=2004|id=|url=http://www.hbscc.nl/pdf/56%20ACMM%20Condensate%20%20SCC.pdf}}</ref>


The acid dew point of a flue gas, at a given pressure, is often referred to as the point at which the flue gas is "saturated" with gaseous acid, meaning that the flue gas cannot hold any more gaseous acid.
In many industrial combustion processes, the flue gas is cooled by the recovery  of [[heat]] from the hot flue gases before they are emitted to the [[Earth's atmosphere|atmosphere]] from the final [[flue gas stack]] (commonly referred to as a chimney). It is very important not to cool the flue gas below its acid dew point because the resulting liquid acid condensed from the flue gas can cause serious [[corrosion]] problems for the equipment used in transporting, cooling and emitting the flue gas.
==Chemistry and mechanism==
===Sulfuric acid dew point===
As a broad generality, flue gases from the combustion of [[coal]], [[fuel oil]], [[natural gas]], or [[biomass]] are primarily composed of gaseous [[carbon dioxide]] (CO<sub>2</sub>) and [[water]] vapor (H<sub>2</sub>O) as well as gaseous [[nitrogen]] (N<sub>2</sub>) and excess [[oxygen]] (O<sub>2</sub>) remaining from the intake combustion [[air]]. Typically, more than two-thirds of the flue gas is nitrogen. The combustion flue gases may also contain small amounts of [[particulate matter]], [[carbon monoxide]], [[nitrogen oxide]]s, and [[sulfur oxide]]s in the form of gaseous [[sulfur dioxide]] (SO<sub>2</sub>) and gaseous [[sulfur trioxide]]  (SO<sub>3</sub>). The SO<sub>3</sub> is present because a portion of the SO<sub>2</sub> formed in the combustion of the [[sulfur]] (S) [[Chemical compound|compounds]] in the combustion fuel is further [[Oxidation|oxidized]] to SO<sub>3</sub>. The gas phase SO<sub>3</sub> then combines the vapor phase H<sub>2</sub>O to form gas phase [[sulfuric acid]] H<sub>2</sub>SO<sub>4</sub>:
{{Image|Sulfuric acid dew point.png|right|397px|Calculated sulfuric acid dew points of typical combustion flue gases, as a function of SO<sub>3</sub> content, and water [[vapor]] content <ref>[http://www.condexenergy.com/Condensing_Economizer_Article.pdf Condensing Economizer Article]</ref>}}
:: &nbsp; &nbsp; H<sub>2</sub>O + SO<sub>3</sub> → H<sub>2</sub>SO<sub>4</sub>
:water + sulfur trioxide → sulfuric acid
Because of the presence of gaseous sulfuric acid, the sulfuric acid dew point of most flue gases is much higher than the [[water dew point]] of the flue gases. For example, a flue gas with 12 volume % water vapor and containing no acid gases has a water dew point of about 49.4 °[[Celsius (unit)|C]] (121 °[[Fahrenheit (unit)|F]]). The same flue gas with the addition of only 4 [[Parts-per notation|ppmv]] (0.0004 volume %) of SO<sub>3</sub> will have a sulfuric acid dew point of about 130.5 °C (267 °F).
The acid dew point of a combustion flue gas depends upon the composition of the specific fuel being burned and the resultant composition of the flue gas. The adjacent graph depicts how the amounts of water vapor and gaseous SO<sub>3</sub> present in a flue gas affect the sulfuric acid dew point of the flue gas.
Given a flue gas composition, its acid dew point can be predicted fairly closely. As an approximation, the sulfuric acid dew points of flue gases from the combustion of fuels in [[thermal power plant]]s range from about 120 °C to about 150 °C (250 to 300 °F).
===Other acid dew points===
====Sulfurous acid====
Some of the sulfur dioxide in flue gases will also combine with water vapor in the flue gases and form gas phase [[sulfurous acid]] (H<sub>2</sub>SO<sub>3</sub>):
:: &nbsp; &nbsp; H<sub>2</sub>O + SO<sub>2</sub> → H<sub>2</sub>SO<sub>3</sub>
:water + sulfur dioxide → sulfurous acid
====Nitric acid====
The nitrogen in flues gases is derived from the combustion air as well as from nitrogen compounds contained in the combustion fuel. Some small amount of the nitrogen is oxidized into gaseous nitrogen dioxide (NO<sub>2</sub>) and some of that gas phase nitrogen oxide then combines with water vapor to form gas phase [[nitric acid]] (HNO<sub>3</sub>):
:: &nbsp; &nbsp; H<sub>2</sub>O + NO<sub>2</sub> → H<sub>2</sub>NO<sub>3</sub>
:water + nitrogen dioxide → nitric acid
====Hydrochloric acid====
Some flue gases may also contain gaseous [[hydrochloric acid]] (HCl) derived from [[chloride]] compounds in the combustion fuel. For example, [[municipal solid wastes]] contain chloride compounds and therefore the flue gases from municipal solid waste [[incinerator]]s may contain gaseous hydrochloric acid which will condense into liquid hydrochloric acid if those flue gases are cooled to a temperature below the acid dew point of hydrochloric acid.
==Prediction of acid dew points==
These equations can be used to predict the acid dew points of the four acids that most commonly occur in typical combustion product flue gases:
'''''Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) dew point:'''''&thinsp;<ref>{{cite journal|author=F.H. Verhoff and J.T. Banchero|title=Predicting Dew Points of Gases |journal=Chemical Engineering progress|volume=78|issue=8|pages=71 - 72|date=1974|id=|url=}}</ref><ref>{{cite journal|author=R.R. Pierce|title=Estimating Acid Dewpoints in Stack Gases|journal=Chemical Engineering|volume=84|issue=8|pages=125 - 128|date=1977|id=|url=}}</ref>
:(1) &nbsp; <math>1000/T = 1.7842\, +\, 0.0269\, \log_{10}\,(P_\mathrm{H_2O})\, -\, 0.1029\, \log_{10}\,(P_\mathrm{SO_3})\, +\, 0.0329\, \log_{10}\,(P_\mathrm{H_2O})\, \log_{10}\,(P_\mathrm{SO_3})</math>
:or this equivalent form:
:(2) &nbsp; <math>1000/T = 2.276\, +\, 0.02943\, \log_e\,(P_\mathrm{H_2O})\, -\, 0.0858\, \log_e\,(P_\mathrm{SO_3})\, +\, 0.0062\, \log_e\,(P_\mathrm{H_2O})\, \log_e\,(P_\mathrm{SO_3})</math>
'''''Sulfurous acid (H<sub>2</sub>SO<sub>3</sub>) dew point:'''''&thinsp;<ref name=Kiang>{{cite journal|author=Yen Hsiung Kiang|title=Predicting Dewpoints of Gases|journal=Chemical Engineering|volume=88|issue=3|pages=127|date=1981|id=|url=}}</ref>
:(3) &nbsp; <math>1000/T = 3.9526\, -\, 0.1863\, \log_e\,(P_\mathrm{H_2O})\, -\, 0.000867\, \log_e\,(P_\mathrm{SO_2})\, +\, 0.000913\, \log_e\,(P_\mathrm{H_2O})\, \log_e\,(P_\mathrm{SO_2})</math>
'''''Hydrochloric acid (HCl) dew point:'''''&thinsp;<ref name=Kiang/>
:(4) &nbsp; <math>1000/T = 3.7368\, -\, 0.1591\, \log_e\,(P_\mathrm{H_2O})\, -\, 0.0326\, \log_e\,(P_\mathrm{HCl})\, +\, 0.00269\, \log_e\,(P_\mathrm{H_2O})\, \log_e\,(P_\mathrm{HCl})</math>
'''''Nitric acid (HNO<sub>3</sub>) dew point:'''''&thinsp;<ref name=Kiang/>
:(5) &nbsp; <math>1000/T = 3.6614\, -\, 0.1446\, \log_e\,(P_\mathrm{H_2O})\, -\, 0.0827\, \log_e\,(P_\mathrm{HNO_3})\, +\, 0.00756\, \log_e\,(P_\mathrm{H_2O})\, \log_e\,(P_\mathrm{HNO_3})</math>
where:
:{|border="0" cellpadding="2"
|-
|align=right|<math>T</math>
|align=left|= The acid dew point temperature for the indicated acid, (&thinsp;K&thinsp;)
|-
|align=right|<math>P</math>
|align=left|= [[Partial pressure]], (&thinsp;[[Atmosphere (unit)|atm]] for equation 1 and [[torr|mmHg]] for equations 2, 3, 4 and 5&thinsp;)
|}
Compared with published measured data, the acid dew points predicted with equations 3, 4 and 5 are said to be within 6 [[Kelvin (unit)|kelvins]], and within 9 kelvins for equations 1 and 2.<ref>{{cite book|author=John J. McKetta (Editor)|title=Encyclopedia of Chemical Processing and Design, Volume 61|edition=1st Edition|publisher=CRC Press|year=1997|id=ISBN 0-8247-2612-X}}</ref>
===Predicting the sulfur trioxide content of flue gases===
As can be seen in the above equation for the sulfuric acid dew point of a flue gas, the partial pressure of sulfur trioxide in the flue gas is required. That partial pressure can be readily determined given the total pressure of the flue gas and the volume percent of sulfur trioxide in the flue gas, since the partial pressure of any component of a gaseous mixture may be obtained by simply multiplying the total gas pressure by the component's volume fraction of the gaseous mixture.
Determining the volume percent of sulfur trioxide in a flue gas by theoretical calculations is quite difficult and unreliable. However, the volume fraction of the sulfur dioxide in the flue gas can be determined by assuming that 90 percent or more of the sulfur in the combustion fuel will be oxidized into gaseous sulfur dioxide when the fuel is combusted. Then it is commonly assumed that about 1 to 5 percent of the sulfur dioxide will be further oxidized into sulfur trioxide. In other words, if the sulfur dioxide in the flue gas is determined to be 0.3 volume percent and it is assumed that 3 percent of that will be further oxidized to sulfur trioxide, the volume fraction of sulfur trioxide in the flue gas will be (0.003)(0.03) = 0.00009 and, if the flue gas pressure is essentially 1 atm (760 mmHg), the  partial pressure of the sulfur trioxide will be (0.00009)(760) = 0.0684 mmHg.
==References==
{{reflist}}

Revision as of 02:50, 23 May 2010

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