SULFURIC ACID IN GAS SAMPLES
Sulfuric acid is very commonly found in gas samples. This note will discuss the problems created
by sulfuric acid in gas samples, sources of the acid in gas samples, and ways to remove it while
minimizing losses of other components of the sample.
Problems Created by Sulfuric Acid
Pure sulfuric acid (H2SO4) is a clear, colorless, oily liquid at room temperature. It is a very
strong acid and is highly corrosive. It has a great affinity for water, and will absorb it from
surrounding air. It boils at about 290°C. At 340°C it decomposes into sulfur trioxide (SO3) and
water, so at the high temperatures present during combustion processes, sulfuric acid is not
present, only sulfur trioxide. When gases are cooled, however, sulfur trioxide will react very
readily with any available water to form sulfuric acid. At low concentrations and elevated
temperatures this sulfuric acid is present as vapor only, but as the gases cool mists (tiny
droplets) of sulfuric acid form. These will coalesce on components of an analysis system, causing
damage due to corrosion or clogging.
Since sulfuric acid only forms at lower temperatures, the acid is not removed by filters in stack
probes where the gases are still quite hot. It often forms too slowly even to be removed
adequately by chiller/condenser systems when they are used to remove water from the sample. When
this happens, sulfuric acid corrosion occurs downstream of the chiller, damaging analyzers or other
components of the sampling system.
Dew points of complex gas mixtures are not dependent purely upon the water concentration; the final
dew point actually depends upon not only on the water content but also upon the content of other
compounds that condense into liquids at ambient temperature. The higher the boiling point of the
other compound, the more effect it has on the dew point. Because sulfuric has a very high boiling
point (290°C) it can have a significant effect on the final sample dew point. Other acids such as
hydrochloric or nitric acid have much less effect because they have much lower boiling points.
Because of its extremely corrosive nature, its tendency to form acid mists at temperatures above
the dew point predicted by merely the water content of the sample, and the difficulty in removing
it, sulfuric acid is a common source of problems with stack gas sample analysis.
Sources of Sulfuric Acid
Sulfuric acid is produced commercially primarily by two methods:
1. Contact Process
2. Cascade Process
In the Contact Process of sulfuric acid synthesis sulfur dioxide forms sulfuric acid in the
presence of oxygen and water by a two-step reaction:
2SO2 + O2 2SO3 SO3 + H2O
The first reaction in this process has a reaction constant such that the sulfur trioxide level is
normally about 10% of the sulfur dioxide level, given the oxygen levels present in stack gas
samples. The second reaction proceeds very rapidly to form sulfuric acid whenever the temperature
is below 340°C. Consequently when the sulfur dioxide level in a gas sample is high, sulfuric acid
problems develop.
In the Chamber Process of sulfuric acid synthesis sulfur dioxide forms sulfuric acid in the
presence of nitrogen oxides, oxygen, and water by a somewhat different two-step reaction:
2NO + O2 2NO2
NO2 + SO2 + H2O H2SO4 + NO
These second reactions imply that if the NOx levels are high and sufficient sulfur dioxide is
present, sulfuric acid problems develop.
When fuels containing sulfur are burned, sulfur dioxide is produced. The Chamber Process and the
Cascade Process are two examples of ways in which sulfur dioxide in turn can react with other
components in the combustion gases (oxygen, water, nitrogen oxides) to form sulfuric acid.
Elimination of Sulfuric Acid Problems
Even if sulfuric acid forms in a gas sample, as long as it remains in the vapor phase it generally
causes little or no problem. When the concentrations of water and sulfuric acid are sufficiently
high to form acid mists at ambient temperature, corrosion problems will likely occur.
The total dew point of a gas sample containing water and sulfuric acid can be calculated from the
following equation (taken from “Predicting Dew Points of Acid Gases”, by Yen- Hsiung Kiang):
1000/TDP = 2.276 – 0.0294 ln (PH2O) – 0.0858 ln (PH2SO4)
+ 0.0062 ln (PH2O) ln (PH2SO4)
where: TDP = dew point temperature in degrees Kelvin P = partial pressure in mm Hg
Although accurate, this equation is tedious to use. It does, however, show that both water and
acid contribute to the total gas dew point. As a practical matter, most samples of gases from
combustion processes where coal or oil is the fuel, contain about 10%-12% water. Gases from
combustion processes where natural gas is the fuel contain about 22% water because the fuel has
more hydrogen to form water. Gases from wet scrubber systems have much higher water contents,
typically 40-60%.
When sulfur dioxide is present in gas samples at concentrations of a few hundred ppm or less,
formation of acid mist is not normally a problem unless the ambient temperature is quite low. If
the sulfur dioxide concentration in a gas sample is more than 1000 ppm, acid mist problems are more
likely, especially if the nitrogen oxides level is also high.
One answer to this problem is simply to lower the water content. Since the acid dew point depends
upon the water content as well as the acid, lowering the water content sufficiently will prevent
the formation of acid mists unless the acid content is quite high.
Nafion® gas dryers from Perma Pure can reduce the water content of a sample down to a water dew
point as low as -45°C (about 75ppm of water), although -10°C to -25°C is typical. Several models of GASS™ Gas Analysis Sampling Systems are available from Perma Pure that incorporate these dryers into complete sample conditioning systems to prepare a gas sample for analysis.
Unfortunately, sometimes the sulfuric acid content of a sample is so high that merely removing more
water is not enough. From the equation above one can see that even if the water content is zero
sulfuric acid alone contributes its own dew point. If the sulfuric acid concentration is quite
high it is necessary to reduce both the sulfuric acid concentration and the water concentration to
prevent the formation of acid mists.
The GASS-2040 Gas Analysis Sampling System is designed specifically to address this problem. In this
system, a coalescing filter is installed upstream of the gas dryer. The temperature of this filter
and an accompanying Hastelloy heat exchanger is controlled separately from the rest of the system.
By lowering the temperature of the gas sample sufficiently (typically down to 60°-75°C) while it
still contains significant amounts of water, sulfuric acid can be forced to condense. The
coalescing filter will then remove the acid from the gas stream. An accompanying automatic drain
will periodically remove the condensed acid. This process will remove very little of other gases,
even water-soluble ones such as sulfur dioxide or nitrogen oxides, because only sulfuric acid is
being condensed not water. The presence of the heat exchanger gives the sample sufficient time to
cool down so that sulfuric acid mist droplets will reach a size where they can be efficiently
removed by the filter.
After sufficient sulfuric acid has been removed, the sample is reheated and passed to a Nafion gas
dryer to selectively remove the water. After exiting the GASS system, the sample has a much lower
concentration of both sulfuric acid and of water, so acid mists do not form again downstream. Chiller/condenser systems on the other hand often fail to remove the very fine acid mist formed in
the short time the sample is within the cool zone of these systems.