User:Milton Beychok/Sandbox

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 dewpoint 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 the flue gas.

Chemistry and mechanism

Sulfuric acid dewpoint

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

As a broad generality, flue gases from the combustion of coal, fuel oil, natural gas, or biomass are primarily composed of carbon dioxide (CO₂) and water vapor (H₂O) as well as 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:

    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 5 volume % water vapor and containing no acid gases has a water dew point of about 32 °C (90 °F). The same flue gas with the addition of only 0.01 volume percent of SO₃ will have a sulfuric acid dew point of about 118 °C (244 °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. 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 thermal power plants range from about 120 °C to about 150 °C (250 to 300 °F).

Prediction of acid dewpoints

These equations can be used to predict the acid dewpoints 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) dewpoint: 

(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) dewpoint: 

(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 dewpoint 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.