Power Engineering

Acid Mist Causes Problems for FGD Systems

By: Wayne P. Buckley and Dr. Boris Altshuler,
Croll-Reynolds Clean Air Technologies, Inc.

Emissions of fine particulates, aerosols and acid gases are drawing significant attention from regulators, environmental groups, health agencies and the general public. Removing these species, particularly sulfuric acid mist from flue gas streams, is complicated. However, an understanding of their formation can assist in developing and applying effective emissions control systems.

SO2 emissions can be reduced using commercial flue gas desulfurization (FGD) technology and NOx emissions abated by selective catalytic reduction (SCR) or other processes. However, the removal of SO3 and the control of aerosol sulfuric acid and ammonia salts is not as straightforward.

Depending upon the type of FGD technology utilized, a considerable portion of these aerosols may exit the stack (30-60 percent) as respirable sub-micron fine particle emissions. This presents an extremely difficult air pollution control problem.1 Sulfur trioxide (SO3), when hydrated with moisture in the gas stream or in the atmosphere, forms sulfuric acid (H2SO4) which, if present in the flue gas, can violate local opacity regulations.

Sulfuric acid emissions are problematic in both wet and dry FGD processes. In wet processes, where the flue gas leaving the absorber is saturated with moisture, sulfuric acid mist can form instantly after the flue gas is saturated. When this occurs it immediately creates a stack opacity problem. Dry processes, where the flue gas is not saturated with moisture, typically remove fine particles such as (NH4)2SO4. However, the dry processes have low removal efficiency for SO3 vapors. At stack conditions, these vapors convert to sulfuric acid mist and produce significant visible emissions.

Sulfuric acid formation takes place through the oxidation of SO2 to SO3, followed by reaction with H2O to form H2SO4. During the combustion process, the sulfur in the fossil fuel reacts to form about 95-97 percent sulfur dioxide (SO2) and the remainder sulfur trioxide (SO3).2 Most of the SO3 in boiler flue gas likely forms during the several seconds when the combustion gas cools from 2900-3100 F to about 1830 F. SCR technologies will generate an additonal quantity of SO3 through catalytic conversion of SO2 to SO3 even at low temperatures.

Mist Formation Mechanisms

There are two primary mechanisms for sulfuric acid mist formation. The first mechanism is the reaction between H2O vapors and SO3 vapors that form liquid droplets. The second mechanism is sulfuric acid vapor condensation in the bulk gas phase when the gas stream temperature is lowered below the H2SO4 dew point.

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The sulfuric acid vapor dew point elevation can be determined from the ratio of the partial pressure of sulfuric acid to that of water vapor. Although the dew point of H2SO4 under typical conditions is 300-355 F, mist formation can occur at gas temperatures as high as 430 F because of uncertainties of bulk phase temperature differences, non-ideal conditions and wall effects.3

According to aerosol science, the saturation ratio "S" of a vapor in a gas, is the ratio of the partial pressure of the vapor in the gas to the saturation vapor pressure of the vapor over a plane of the liquid. When S>1, the gas is said to be supersaturated with vapor; when S=1, the gas is saturated; and when S<1, the gas is unsaturated with vapor.

For condensation to take place, sulfuric acid vapor must be supersaturated (S>1). However, the condensation of sulfuric acid can take place under conditions where it is thermodynamically impossible. For example, when the flue gas contains fly ash, the fly ash may act as a nucleus of condensation for the vapor. The sulfuric acid vapors condense on the fly ash as the flue gas flows through the cooler stages of the emissions control system.

Another mechanism for sulfuric acid condensation (when S<1) is condensation on colder surfaces which takes place if the temperature of the equipment walls is significantly lower than the temperature of the gas stream. This occurs in the very thin laminar boundary layer close to the wall surface. Because the thickness of the laminar layer is a small percentage of the total gas volume, even for heat exchanger surfaces, this condensation mechanism will not generally provide a considerable quantity of sulfuric acid mist.

However, the high saturation ratio in this laminar layer may be the mechanism that produces the largest portion of the finest sulfuric acid droplets. For example, downstream of an SCR system, where the flue gas temperature is about 480 F and the concentration of sulfuric acid vapor is 50 ppm, the sulfuric acid mist formation ratio is less than 1.0. At these conditions the gas phase condensation of sulfuric acid has already begun in the laminar layer.

The process of mist formation in the laminar layer may also occur when slow gas quenching takes place, such as through the use of an indirect contact condenser. With a slow cooling process the presence of fly ash is an important factor because it provides a large surface area on which the vapors can condense.4 The advantage of this type of cooling is that most of the acid will be condensed on the existing fly ash particles and the condenser walls. On the other hand, most FGD systems use rapid quenching technologies with direct contact cooling of the hot flue gas with water. This process inherently generates a large number of sulfuric acid droplets by homogeneous nucleation.

Removal Efficiency Impacts

The sulfuric acid mist removal efficiency of a conventional FGD system is estimated at 40-70 percent.1 Figure 1 shows the opacity of the outlet gas for different FGD scrubbing efficiencies. Using this graph it is possible to calculate the total efficiency of an FGD system and the additional cleaning equipment necessary to get to near zero visible emissions.

Visible emissions (greater than 5 percent opacity) can be generated at 7 ppm of H2SO4 or higher. In other words, if the inlet to the FGD unit has a concentration of H2SO4 of 50 ppm and the FGD has a 50 percent removal efficiency of H2SO4, to get less than 5 percent opacity at the stack, the additional opacity abatement equipment system should have an H2SO4 removal efficiency of about 70 percent. However, this value of efficiency is minimum because at least two other processes take place, which increase opacity.

First, when the flue gas enters the FGD scrubber, coagulation occurs. The initial high concentration of sulfuric acid mist begins to absorb water from the saturated gas, which in turn increases the droplet size and the weight of the mist loading. In addition, the final sulfuric acid concentration can differ for various droplet sizes.

As an example, a 0.5 micron droplet can absorb about 50 percent of its weight of water, which doubles the mist loading weight. This means that the opacity abatement equipment after the FGD unit should have an efficiency of at least 85 percent for an inlet concentration of 50 ppm H2SO4.

The second consideration is that opacity is a function not only of the concentration of sulfuric acid mist but also of the dust or solid particulate loading. If the flue gas contains solid particulates, a synergetic effect takes place and the opacity increase is greater than the proportional increase in combined weight loading of sulfuric acid mist and solid particles.

Flue gas that has passed through a selective catalytic reduction (SCR) system, an air pre-heater and a dry electrostatic precipitator (DESP) is normally at 285-320 F. After scrubbing in a wet FGD process, the gas temperature usually drops to 130-140 F. To minimize condensation of acidic liquor, which causes stack corrosion and in some cases acid rain, the flue gas should be reheated to about 175 F. In addition, to eliminate the formation of any visible plume the flue gas temperature should be about 285 F.1

Obviously, for large gas flow rates, such as a utility boiler exhaust, gas reheating is very costly. Although reheating can minimize the visible plume caused by sulfuric acid and water vapor it cannot remove solid particulates or reduce existing acid emissions to satisfy environmental emission limits.

If reduction of mass emissions and/or stack opacity are required, it is necessary to use a technology such as wet electrostatic precipitation (WESP) that will simultaneously remove the sulfuric acid mist and the solid particulate material from the flue gas. WESP technology also has the added potential for abatement of heavy metals (including mercury) and water mist carryover from an FGD scrubber system while minimizing both the capital and operating costs.5


  1. J. Ando, SO2 Abatement for Stationary Sources in Japan, EPA-600/7-78-210, Nov. 1978, p. 82.
  2. P.N. Cheremisinoff, Clean Air Pollution Engineering, June 1990, p. 66.
  3. D.R. Duros, E.D. Kennedy, "Acid Mist Control," CEP, Sept. 1978, p. 70
  4. A.S. Damie, D.S. Ensor, L.E Sparks, "Options for Controlling Condensation Aerosols to Meet Opacity Standards," JAPCA, Aug. 1987, 37, No. 8, p. 925.
  5. R. Altman, W. Buckley, I. Ray, "Wet Electrostatic Precipitation Demonstrating Promise for Fine Particle Control," Power Engineering, Jan. 2001, p. 37.


Wayne Buckley is Vice President and General Manager of Croll-Reynolds Clean Air Technologies. He holds a B.S. in mechanical engineering from the City University of New York, and has more than 30 years of experience in the incineration and air-pollution control (APC) industries. Buckley holds several U.S. patents for APC devices and is the author of numerous technical papers and articles.

Boris Altshuler is a Senior Process Engineer with Croll Reynolds Clean Air Technologies. He holds an M.S. in chemical engineering from Mendelcev University of Chemical Technology (Moscow), and a Ph.D. in chemical technology from AU-Union Research Institute of Natural Gasses (Moscow). He was awarded the "Best Inventor on the Gas Industry" by the former Soviet Union, and is a member of ACS.

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