Oak Creek air quality control project

We have a long-standing commitment to environmental performance. Since the late 1980s, we have steadily reduced sulfur dioxide (SO2), nitrogen oxide (NOx) and particulate emissions. As emission control technology advanced, we have implemented new equipment at our generation facilities to meet new state and federal air quality requirements while maintaining system reliability and cost-effectiveness.

The air quality control project completed in 2012 at Oak Creek Power Plant upgraded the environmental controls to further reduce SO2 and NOx emissions. The new system cut SO2 emissions by more than 90 percent and NOx emissions by more than 70 percent.

How we produce electricity while reducing emissions

The process of generating electricity occurs in several steps. First, coal is burned in a large boiler to make high-pressure, high-temperature steam. A turbine converts the steam's thermal energy to mechanical energy. The spinning turbine then drives the generator to produce electricity.

Specialized equipment is used at different points throughout the generation process to clean the flue gas. The first step in reducing emissions occurs by using a low-NOx burner to burn the coal. This equipment regulates the coal's combustion rate by controlling the amount of air available at different elevations within the boiler to complete the combustion. This results in lower NOx emissions.

After exiting the boiler, the flue gas goes through an electrostatic precipitator (ESP). In the ESP, a series of positively charged collection plates remove negatively charged particulate matter from the flue gas. The ESP captures more than 99 percent of the particulate matter in the flue gas.

To reduce NOx emissions even further, a selective catalytic reduction system (SCR) is employed. Within this system, an ammonia solution is injected into the flue gas stream. The NOx in the flue gas reacts with the injected ammonia, in the presence of a catalyst, producing nitrogen and water vapor.

Next, the wet flue gas desulfurization (FGD) system, also known as the wet FGD or “scrubber,” is used to control SO2. As the flue gas passes through the scrubber, it is mixed with limestone slurry. The limestone reacts with the SO2 in the flue gas, forming calcium sulfite. Air (oxygen) is blown into the absorber tank, causing a chemical reaction that converts the calcium sulfite into calcium sulfate, or synthetic gypsum. The gypsum slurry is then dewatered and can be used as a valuable product to make wallboard for construction projects.