Acid Dew Point - Encyclopedia Article - Citizendium
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1 Chemistry and mechanism 1.1 Sulfuric acid dew point 1.2 Other acid dew points 1.2.1 Sulfurous acid 1.2.2 Nitric acid 1.2.3 Hydrochloric acid 2 Prediction of acid dew points 2.1 Predicting the sulfur trioxide content of flue gases 3 References From Citizendium, the Citizens' Compendium This article is about Acid dew point. For other uses of the term Dew point, please see Dew point (disambiguation). 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 condense into liquid acid. [1][2][3] 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 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. 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 2 ) and water vapor (H 2 O) as well as gaseous nitrogen (N 2 ) and excess oxygen (O 2 ) 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 oxides, and sulfur oxides in the form of gaseous sulfur dioxide (SO 2 ) and gaseous sulfur trioxide (SO 3 ). The SO 3 is present because a portion of the SO 2 formed in the combustion of the sulfur (S) compounds in the combustion fuel is further oxidized to SO 3 . The gas phase SO 3 then combines the vapor phase H 2 O to form gas phase sulfuric acid H 2 SO 4 : H 2 O + SO 3 → H 2 SO 4 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 °C (121 °F). The same flue gas with the addition of only 4 ppmv (0.0004 volume %) of SO 3 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 3 present in a flue gas affect the sulfuric acid dew point of the flue gas. Acid dew point - encyclopedia article - Citizendiu m http://en.citizendium.org/wiki/Acid_dew_poin t 1 of 4 11/15/2011 10:56 AM
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Transcript of Acid Dew Point - Encyclopedia Article - Citizendium
1 Chemistry and mechanism1.1 Sulfuric acid dew point1.2 Other acid dew points
1.2.1 Sulfurous acid1.2.2 Nitric acid1.2.3 Hydrochloric acid
2 Prediction of acid dew points2.1 Predicting the sulfur trioxide content of flue gases
3 References
From Citizendium, the Citizens' Compendium
This article is about Acid dew point. For other uses of the term Dew point, please see Dew point(disambiguation).
The acid dew point (also acid dewpoint) of aflue gas (i.e., a combustion product gas) is thetemperature, at a given pressure, at which anygaseous acid in the flue gas will start to condense
into liquid acid.[1][2][3]
The acid dew point of a flue gas, at a givenpressure, is often referred to as the point at whichthe flue gas is "saturated" with gaseous acid,meaning that the flue gas cannot hold any moregaseous acid.
In many industrial combustion processes, the fluegas is cooled by the recovery of heat from the hotflue gases before they are emitted to theatmosphere 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 theresulting liquid acid condensed from the flue gas can cause serious corrosion problems for the equipment used intransporting, cooling and emitting the flue gas.
Sulfuric acid dew point
As a broad generality, flue gases from the combustion of coal, fuel oil, natural gas, or biomass are primarily composed ofgaseous carbon dioxide (CO2) and water vapor (H2O) as well as gaseous nitrogen (N2) and excess oxygen (O2) remainingfrom the intake combustion air. Typically, more than two-thirds of the flue gas is nitrogen. The combustion flue gases mayalso contain small amounts of particulate matter, carbon monoxide, nitrogen oxides, and sulfur oxides in the form ofgaseous sulfur dioxide (SO2) and gaseous sulfur trioxide (SO3). The SO3 is present because a portion of the SO2 formed inthe combustion of the sulfur (S) compounds in the combustion fuel is further oxidized to SO3. The gas phase SO3 thencombines the vapor phase H2O to form gas phase sulfuric acid H2SO4:
H2O + SO3 → H2SO4
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 thewater dew point of the flue gases. For example, a flue gas with 12 volume % water vapor and containing no acid gases hasa water dew point of about 49.4 °C (121 °F). The same flue gas with the addition of only 4 ppmv (0.0004 volume %) ofSO3 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 theresultant composition of the flue gas. The adjacent graph depicts how the amounts of water vapor and gaseous SO3 presentin a flue gas affect the sulfuric acid dew point of the flue gas.
Acid dew point - encyclopedia article - Citizendium http://en.citizendium.org/wiki/Acid_dew_point
1 of 4 11/15/2011 10:56 AM
(PD) Graph: Milton Beychok
Calculated sulfuric acid dew points of typical combustion flue
gases, as a function of SO3 content, and water vapor content [4]
Given a flue gas composition, its acid dew point can bepredicted fairly closely. As an approximation, thesulfuric acid dew points of flue gases from thecombustion of fuels in thermal power plants range fromabout 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 alsocombine with water vapor in the flue gases and form gasphase sulfurous acid (H2SO3):
H2O + SO2 → H2SO3
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 thecombustion fuel. Some small amount of the nitrogen is oxidized into gaseous nitrogen dioxide (NO2) and some of that gasphase nitrogen oxide then combines with water vapor to form gas phase nitric acid (HNO3):
H2O + NO2 → H2NO3
water + nitrogen dioxide → nitric acid
Hydrochloric acid
Some flue gases may also contain gaseous hydrochloric acid (HCl) derived from chloride compounds in the combustionfuel. For example, municipal solid wastes contain chloride compounds and therefore the flue gases from municipal solidwaste incinerators may contain gaseous hydrochloric acid which will condense into liquid hydrochloric acid if those fluegases are cooled to a temperature below the acid dew point of hydrochloric acid.
These equations can be used to predict the acid dew points of the four acids that most commonly occur in typicalcombustion product flue gases:
Sulfuric acid (H2SO4) dew point: [5][6]
(1)
or this equivalent form:[2][4][7]
(2)
Sulfurous acid (H2SO3) dew point: [2][7][8]
(3)
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Hydrochloric acid (HCl) dew point: [2][7][8]
(4)
Nitric acid (HNO3) dew point: [7][8]
(5)
where:
= The acid dew point temperature for the indicated acid, ( K )
= Partial pressure, ( atm for equation 1 and mmHg for equations 2, 3, 4 and 5 )
Compared with published measured data, the acid dew points predicted with equations 3, 4 and 5 are said to be within 6
kelvins, and within 9 kelvins for equations 1 and 2.[2]
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 inthe flue gas is required. That partial pressure can be readily determined given the total pressure of the flue gas and thevolume percent of sulfur trioxide in the flue gas, since the partial pressure of any component of a gaseous mixture may beobtained 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 moreof the sulfur in the combustion fuel will be oxidized into gaseous sulfur dioxide when the fuel is combusted. Then it iscommonly assumed that about 1 to 5 percent of the sulfur dioxide will be further oxidized into sulfur trioxide. In otherwords, if the sulfur dioxide in the flue gas is determined to be 0.3 volume percent and it is assumed that 3 percent of thatwill 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.
↑ David A. Lewandowski (2000). Design of Thermal Oxidation Systems for Volatile Organic Compounds, 1st Edition. CRC Press. ISBN1-56670-410-3. Available here (http://books.google.com/books?id=L-lKUWd-QOwC&printsec=frontcover&dq=intitle:Design+intitle:of+intitle:Thermal+intitle:Oxidation+intitle:Systems+intitle:for+intitle:Volatile+intitle:Organic+intitle:Compounds&hl=en&ei=wNOTTNWtEISdlgftvKmoCg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDMQ6AEwAA#v=onepage&q&f=false) inGoogle Books.
1.
↑ 2.0 2.1 2.2 2.3 2.4 John J. McKetta (Editor) (1997). Encyclopedia of Chemical Processing and Design, Volume 61, 1st Edition. CRC Press.ISBN 0-8247-2612-X. Available here (http://books.google.com/books?id=RNXyYP8EA2EC&printsec=frontcover&dq=0-8247-2612-x&hl=en&ei=z9GTTITsJoOglAect_2qCg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDUQ6AEwAA#v=onepage&q&f=false) inGoogle books.
2.
↑ W.M.M. Huijbregts and R. Leferink (2004). "Latest Advances in the Understanding of Acid Dewpoint Corrosion: Corrosion and StressCorrosion Cracking in Combustion Gas Condensates" (http://www.hbscc.nl/pdf/56%20ACMM%20Condensate%20%20SCC.pdf) .Anti-Corrosion Methods and Materials 51 (3): 173 - 188.
3.
↑ 4.0 4.1 Condensing Economizer Article (http://www.condexenergy.com/Condensing_Economizer_Article.pdf)4.↑ F.H. Verhoff and J.T. Banchero (1974). "Predicting Dew Points of Gases". Chemical Engineering progress 78 (8): 71 - 72.5.↑ R.R. Pierce (1977). "Estimating Acid Dewpoints in Stack Gases". Chemical Engineering 84 (8): 125 - 128.6.
↑ 7.0 7.1 7.2 7.3 V. Ganapathy (1993). Steam Plant Calculations Manual, 2nd Edition. CRC Press. ISBN 0-8247-9147-9. See Table 2.9 onpage 94. Available here (http://books.google.com/books?id=b_gTbFeHuiEC&printsec=frontcover&dq=intitle:Steam+intitle:plant+intitle:calculations+intitle:manual+inauthor:Ganapathy&hl=en&ei=ktKTTKuzIMLflge6h5mnCg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDMQ6AEwAA#v=onepage&q&f=false) in Google Books.
7.
↑ 8.0 8.1 8.2 Yen Hsiung Kiang (1981). "Predicting Dewpoints of Gases". Chemical Engineering 88 (3): 127.8.
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