User:Milton Beychok/Sandbox

The water dew point of air or any other gas containing water vapor usually refers to the temperature (for a given pressure) at which the air or gas is saturated with water vapor. That means that the air or gas is at the point where the water vapor will start to condense into liquid water if the temperature is lowered beyond that point.

As a broad generality, the combustion flue gases from steam generators fueled by coal, fuel oil, natural gas, or biomass are composed of carbon dioxide (CO₂) and water vapor (H₂O) as well as nitrogen and excess oxygen 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 percentages of air pollutants such as 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 compounds in the steam generator's furnace fuel is further oxidized to SO₃ as the flue gas travels through the superheater and reheater sections of the steam generator (see the above diagram of a steam generator). The gas phase SO₃ then combines the vapor phase H₂O to form gas phase sulfuric acid H₂SO₄:

H₂O + SO₃ → H₂SO₄

Because of the presence of gaseous sulfuric acid, the dew point of most flue gases is much higher than the water dew point of air and the flue gas dew point is referred to as the acid dew point. That is the flue gas temperature at which acid will begin to condense out of the flue gas if the temperature is lowered beyond that point. 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 an 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 acid dew points of flue gases from thermal power plants range from about 120 °C to about 150 °C (250 to 300 °F).

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