Today, SACE released a new report on Dry Sorbent Injection (DSI) technology – a troubling new sulfur dioxide control method recently touted by some utilities as a cheap, quick answer to comply with EPA’s new mercury and air toxics (MATS) regulations. SACE commissioned the report from Dr. Ranajit (Ron) Sahu, a leading environmental engineering consultant, after learning that the Tennessee Valley Authority is considering installing DSI on some of its coal-fired generation units. The report sheds light on the potentially negative environmental impacts of DSI implementation, including concerning implications for water quality.
As explained by Dr. Sahu in the report, DSI was originally used as a means of controlling acid gases, and not sulfur dioxide, from the air pollution stream of electric generating units. Reduction of sulfur dioxide emissions from a unit’s air pollution stream is a concurrent side effect of DSI’s reduction of acid gases. Historically, utilities have installed flue gas desulfurization (FGD) systems, also known as scrubbers, to reduce sulfur dioxide emissions. Installing an FGD system on a coal plant, however, is extremely expensive (for example, TVA is currently moving ahead with a $1 billion scrubber on its Gallatin Plant in middle Tennessee – an action that has recently garnered litigation from environmental groups). In contrast, the capital costs of installing DSI are much less, making it a more attractive pollution control option for utilities.
Both the design of particular DSI installations and the possible environmental impacts of DSI implementation vary greatly across coal-fired units. There are, however, common environmental concerns that must be considered with any DSI installation. For example, increased sulfur dioxide removal using the most common sodium-based sorbent used in DSI, trona, could result in the increase of other known air pollutants from the unit, including carbon dioxide and nitrogen oxides.
The most troubling aspect of DSI installation and implementation arises from the fact that DSI can alter the chemical make up of coal 
ash in such a way that will make it more likely for coal ash to leach out of ash impoundments and into groundwater and surrounding water bodies. Greater toxicity of coal ash means that any unlined ash impoundments would ideally need to also be retrofitted with liners or a utility would have to construct a new lined ash impoundment – both actions would result in significant additional costs. Unfortunately, however, EPA does not currently require impoundments to be lined, so increased toxicity from use of DSI would simply threaten nearby soil and water without proper corresponding environmental requirements.
As more utilities begin to explore DSI as a compliance solution for EPA’s MATS regulations, it will become increasingly important for the environmental community to understand the myriad of cost considerations and potential environmental impacts associated with this type of pollution control system.
In addition to the report, you can also access a recent SACE webinar featuring Dr. Ron Sahu – Understanding Concerns with Dry Sorbent Injection as a Coal Plant Pollution Control.