![]() ![]() H 2O + NO 2 → H 2NO 3 water + nitrogen dioxide → nitric acid Hydrochloric acid ![]() Some small amount of the nitrogen is oxidized into gaseous nitrogen dioxide (NO 2) and some of that gas phase nitrogen oxide then combines with water vapor to form gas phase nitric acid (HNO 3): The nitrogen in flues gases is derived from the combustion air as well as from nitrogen compounds contained in the combustion fuel. H 2O + SO 2 → H 2SO 3 water + sulfur dioxide → sulfurous acid Nitric 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 2SO 3): As an approximation, the sulfuric acid dew points of flue gases from the combustion of fuels in thermal power plants range from about 120 ☌ to about 150 ☌ (250 to 300 ☏). ![]() Given a flue gas composition, its acid dew point can be predicted fairly closely. 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. 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 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 ☌ (267 ☏). 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 gas phase SO 3 then combines the vapor phase H 2O to form gas phase sulfuric acid H 2SO 4:Ĭalculated sulfuric acid dew points of typical combustion flue gases, as a function of SO 3 content, and water vapor content H 2O + SO 3 → H 2SO 4 water + sulfur trioxide → sulfuric acidīecause 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. 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 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). ![]() Typically, more than two-thirds of the flue gas is nitrogen. 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.Ĭhemistry and mechanism Sulfuric acid dew pointĪs 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 2O) as well as gaseous nitrogen (N 2) and excess oxygen (O 2) remaining from the intake combustion air. 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). 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. 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. 2.1 Predicting the sulfur trioxide content of flue gases. ![]()
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