Acid dew point

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.

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.

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 (CO₂) and water vapor (H₂O) as well as gaseous nitrogen (N₂) and excess oxygen (O₂) 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 (SO₂) and gaseous sulfur trioxide (SO₃). The SO₃ is present because a portion of the SO₂ formed in the combustion of the sulfur (S) compounds in the combustion fuel is further oxidized to SO₃. The gas phase SO₃ then combines the vapor phase H₂O to form gas phase sulfuric acid H₂SO₄:

(PD) Graph: Milton Beychok
Calculated sulfuric acid dew points of typical combustion flue gases, as a function of SO₃ content, and water vapor content

    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 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 SO₃ 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 SO₃ 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 plants 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 (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 the combustion fuel. Some small amount of the nitrogen is oxidized into gaseous nitrogen dioxide (NO₂) and some of that gas phase 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 combustion fuel. For example, municipal solid wastes contain chloride compounds and therefore the flue gases from municipal solid waste incinerators 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 (H₂SO₄) dew point: 

(1)   $1000/T=1.7842\,-\,0.0269\,\log _{10}\,(P_{\mathrm {H_{2}O} })\,-\,0.1029\,\log _{10}\,(P_{\mathrm {SO_{3}} })\,+\,0.0329\,\log _{10}\,(P_{\mathrm {H_{2}O} })\,\log _{10}\,(P_{\mathrm {SO_{3}} })$

or this equivalent form:

(2)   $1000/T=2.276\,-\,0.02943\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,-\,0.0858\,\log _{e}\,(P_{\mathrm {SO_{3}} })\,+\,0.0062\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,\log _{e}\,(P_{\mathrm {SO_{3}} })$

Sulfurous acid (H₂SO₃) dew point: 

(3)   $1000/T=3.9526\,-\,0.1863\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,+\,0.000867\,\log _{e}\,(P_{\mathrm {SO_{2}} })\,+\,0.000913\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,\log _{e}\,(P_{\mathrm {SO_{2}} })$

Hydrochloric acid (HCl) dew point: 

(4)   $1000/T=3.7368\,-\,0.1591\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,-\,0.0326\,\log _{e}\,(P_{\mathrm {HCl} })\,+\,0.00269\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,\log _{e}\,(P_{\mathrm {HCl} })$

Nitric acid (HNO₃) dew point: 

(5)   $1000/T=3.6614\,-\,0.1446\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,-\,0.0827\,\log _{e}\,(P_{\mathrm {HNO_{3}} })\,+\,0.00756\,\log _{e}\,(P_{\mathrm {H_{2}O} })\,\log _{e}\,(P_{\mathrm {HNO_{3}} })$

where:

$T$	= The acid dew point temperature for the indicated acid, ( K )
$P$	= 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.

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.