Dry sorbent injection (DSI), which is a cheaper though somewhat less effective emissions control option for coal-fired power plants than the common wet scrubber, may play a key role in compliance with the U.S. Environmental Protection Agency’s Mercury and Air Toxics Standards (MATS).
The U.S. Energy Information Administration noted that fact in a brief “Today in Energy” news item posted to its website on March 16.
EPA finalized the MATS rule in December 2011. The rule requires that all U.S. coal- and oil-fired power plants greater than 25 MW meet emission limits consistent with the average performance of the top 12% of existing units —known as maximum achievable control technology (MACT). The rule applies to three pollutants: mercury (Hg), hydrochloric acid (HCl), and filterable particulate matter (fPM) and has a compliance deadline in 2015, though that can be extended on a case-by-case basis.
While DSI systems do not control for mercury, they can, when combined with a particulate control filter, meet this standard for two of the three controlled pollutants, EIA noted. DSI systems remove hydrogen chloride (HCl) and other acid gases through two basic steps.
- Step one. A powdered sorbent is injected into the flue gas—combustion exhaust gas exiting a power plant—where it reacts with the HCl. The sorbents most commonly associated with DSI are trona (sodium sesquicarbonate, a naturally occurring mineral mined in Wyoming), sodium bicarbonate, and hydrated lime.
- Step two. The compound is removed by a downstream particulate matter control device such as an electrostatic precipitator (ESP) or a fabric filter. Fabric filters are generally more effective (when combined with DSI) than ESPs with respect to overall HCl reduction, EIA said. For modeling purposes, EPA estimates that a DSI system with a fabric filter is expected to achieve 90% removal of HCl, while an ESP only achieves 60% removal, although actual performance will vary by individual plant.
DSI and flue gas desulfurization (FGD) scrubbers (both wet and dry) are technologies that will allow plants to meet the MATS for HCl and other acid gases. As of 2010, 54% of U.S. electric generating capacity already have FGDs installed, EIA pointed out. A number of the remaining, uncontrolled plants will need to determine the effectiveness of installing FGD scrubber or DSI to comply with MATS.
Economic and engineering tradeoffs exist between the two technologies, EIA pointed out. FGD systems are large capital projects that require a significant upfront investment, but have relatively lower operating costs. DSI systems generally do not require significant capital expenses, but may rely on significant quantities of sorbent to operate effectively, which increases operating costs. Waste disposal for DSI may also be a significant variable cost, while the waste products from an FGD system can be sold as feedstock for industrial processes, like the making of wallboard. Also, DSI’s potential effectiveness is limited to certain types of plants. Because of the amount of sorbent needed, DSI will likely be implemented most often at plants that are 300 MW or less and burn low-sulfur coal. It is very common for power generators to consider DSI for power plants already burning low-sulfur Powder River Basin coal.
DSI systems can also significantly reduce SO2 emissions through the same process as HCl removal. While the MATS rule does not specifically address SO2, it has similar qualities to HCl and other acid gases that enable it to respond similarly in a DSI system, EIA aid. SO2 is also regulated under EPA’s Cross State Air Pollution Rule (CSAPR), which is currently on hold due to court appeals and was due to take effect Jan. 1. Therefore, installing a DSI or FGD system to comply with MATS may also help plants meet or even exceed their CSAPR emission limits, EIA added.
While compliance with the MATS rule was not assumed in EIA’s Annual Energy Outlook 2012 (AEO2012) Early Release, it will be assumed in the AEO2012 full Reference case to be released this spring, and DSI will be included as a compliance option for coal plants, the agency said.
Midwest Generation is one DSI tester
One of the power generators looking heavily lately at DSI is Midwest Generation, which has several coal-fired power plants in Illinois. In December 2006, Midwest Generation worked out a deal with the Illinois EPA to reduce mercury, NOx and SO2 emissions at its plants. Under the SO2 part of the agreement, Midwest Generation must comply with an SO2 annual emission rate beginning with 0.44 lbs/MMBtu in 2013 and decreasing annually until it reaches 0.11 lbs/MMBtu in 2019 and thereafter.
“Testing of dry scrubbing using Trona on select Midwest Generation units has demonstrated significant reductions in SO2 emissions,” said Midwest Generation’s Feb. 29 annual Form 10-K report. “Use of dry sorbent injection technology in conjunction with low sulfur coal is expected to require substantially less capital and time to construct than the use of spray dryer absorber technology, but would likely result in higher ongoing operating costs and may consequently result in lower dispatch rates and competitiveness of Midwest Generation’s plants, depending on competitors’ costs.”
Detroit Edison also looking at DSI
In September 2011 testimony filed at the Michigan Public Service Commission about long-range capacity forecasting, a Detroit Edison official outlined Detroit Edison’s DSI evaluations. The official was Angela Wojtowicz, Manager of the Wholesale Power group in the Generation Optimization department of the Regulated Marketing Organization at Detroit Edison.
Wojtowicz said Detroit Edison has been testing DSI to determine if it could be technologically and economically feasible on some of the company’s coal-fired units. If feasible, it is expected to be implemented on some units as a viable alternative to retirement. The company in the September 2011 filing assumed the use of DSI on St. Clair Units 1, 2, 3, 4, and 6 and Belle River Units 1-2.
As for MATS compliance for Detroit Edison, she said: “These emission limits would require additional emission reduction technologies. Several technically and commercially proven technologies are available to power plants to meet the emission limits, including wet and dry scrubbers (FGD), activated carbon injection (ACI) systems, and fabric filters (FF). The company has a plan in place for compliance with the Michigan Mercury Rule that utilizes FGD on Monroe Units 1-4, and ACI on all remaining units, several with FF for additional particulate control. This compliance plan will reduce mercury to EGU MACT levels, but additional control is needed for acid gases on any non-scrubbed units and some units will require additional particulate controls to meet the proposed PM limits. FGD has been proven to meet the acid gas limits in the EGU MACT. FGD, while an effective acid gas technology, is a technology that requires a significant capital investment. This larger capital investment may not be justified on some units in the portfolio, particularly some of the older, smaller units. Those units may be retired if that capital investment is required.”