Acid Gas

What is Acid Gas?

Acid gas is a mixture of hydrogen sulfide and carbon dioxide, with small amounts of hydrocarbon gases and usually water vapor (according to Science Direct).

Sulfur Dioxide

One of the components in the flue gas produced in a coal-fired boiler is sulfur dioxide or SO₂. Mitigation of SO₂ release is critical to maintaining emissions within federal, state, and local limits. In a recent blog, Alex Filar discussed various flue gas desulfurization systems. These systems use a sorbent, calcium, or sodium sulfate is used to absorb SO₂ from the flue gas.

The concern with SO₂ emissions is producing sulfuric acid (H₂SO₄) when the gaseous SO₂ reacts with moisture in the atmosphere, creating “acid rain”.  However, by capturing the SO₂ and controlling its reaction with water, high-purity sulfuric acid can be produced and sold for other industrial processes.

AC (Activated Coke)

In one such system, a flowing bed of activated coke is used to absorb the SO₂.  Saturated coke is then moved to a heater, where the SO₂ is forced out of the coke.  The acid gas is cleaned of other impurities, then mixed with water to produce sulfuric acid. The desired concentration can be achieved by controlling the ratio of acid gas and water. The acid is then stored on-site for sale and transportation to the customer. An overview of this process is shown below.

In this system, activated coke (AC) flows through the Adsorber, where it traps SO₂ as well as other contaminants, such as mercury. Hot, saturated SO₂ then travels to the Regenerator, where it is heated. This results in the SO₂ gas being released from the AC; this allows the AC to be cooled and returned to the Adsorber to continue the process.

AC Breakdown

Over time, the movement of AC through the system (as well as the absorption process) causes the AC to break down, creating fine particles. These particles are removed in the AC Classifier; the Classifier also takes AC from the Makeup Storage Tank as needed to maintain proper AC inventory.  Flue gas exits the Adsorber through a separate polishing filter to remove any AC particles carried out of the Adsorber; the cleaned flue gas is then released to the Exhaust Stack.

SO₂ and other contaminant gases are released by the heating of the AC in the Regenerator. This gas is cleaned in a wet scrubber and wet electrostatic precipitator (WESP) to remove any particulate matter. It then passes through a Selenium Filter, where mercury contaminants are removed. The wet SO₂ gas is dried in the Drying Tower; clean, dry SO₂ gas is oxidized in the Converter to produce SO₃ gas.  This is then mixed with pure water in the Absorbing tower to produce sulfuric acid. Clean, high-purity sulfuric acid is then stored for sale and distribution.

Minimizing the release of contaminants into the environment is critical to the operation of any fossil-fueled power boiler. Precisely, sulfur gases must be controlled to prevent the generation of sulfuric acid in the environment. However, if the generation of sulfuric acid is done in a controlled manner, plants can both minimize the release of sulfur gases AND produce a byproduct that can be sold for use in other industrial processes.