Minnesota Power plans new scrubber for Boswell Unit 4

Minnesota Power wants to install a Circulating Dry Scrubber (CDS) on Unit 4 of its coal-fired Boswell Energy Center (BEC4) in order to control mercury and other emissions and keep this 585 MW (net) unit in its generating portfolio.

The utility, part of Allete (NYSE: ALE), on Aug. 31 filed an application with the Minnesota Public Utilities Commission for this project. “Retrofitting BEC4 to reduce mercury emissions and improving other aspects of its environmental performance as requested in this Petition will help to ensure BEC4 continues to deliver a large volume of essential energy to residents, communities and businesses in Northern Minnesota at a reasonable cost,” the utility told the commission. Boswell 4 is the single largest base load generator in Minnesota Power’s fleet, the utility said.

“Because much of Minnesota Power’s total energy supply is used by industrial customers that operate around the clock, the Company has a uniquely high load factor, requiring a power supply that is more steady than that of most utilities,” the utility said. Over half of the electricity Minnesota Power sells to retail customers is purchased by its 12 largest industrial customers. As well, Minnesota Power is anticipating significant customer growth in both energy and demand over the next decade.

Minnesota Power is required under the state’s Mercury Emission Reduction Act (MERA) to file a 90% mercury reduction plan for BEC4 by July 1, 2015, and implement the plan by Dec. 31, 2018. Under the project schedule outlined in the Aug. 31 petition, Minnesota Power would be in compliance with MERA more than two years in advance.

“Although several of the federal rulemakings affecting utility emissions are not yet final, Minnesota Power believes it now has sufficient information and access to proven environmental control technologies that will ensure Minnesota Power’s compliance with current and future environmental regulations for BEC4 over the long term,” the company said.

Utility says CDS allows a multi-pollutant approach

The BEC4 project is a reasonable and cost-effective multi-pollutant solution to reducing mercury, SO2, particulate matter (PM) and other hazardous air pollutants identified by EPA while also significantly reducing wastewater from BEC4 operations, the company said. Among other measures that will be undertaken to ensure successful construction, the company said it will obtain competitive quotations for major purchases and award contracts to the lowest evaluated bidders, secure a majority of the total cost of the project in fixed fee/lump sum contracts, utilize proven contractors with demonstrated bidding, construction management, and supplement its internal construction management team with a team from an external engineering/construction management company.

BEC4 is located in Cohasset, Minn., and was placed into service in 1980. Its boiler is a tangentially-fired steam generator. Until recently, it operated with a gross generation capability of 585 MW with 535 MW available as net output due to 50 MW of existing station service required to operate auxiliary equipment. In 2010, Minnesota Power replaced the original turbine with a more efficient design that is able to operate at over 635 MW gross capability and 585 MW net without consuming additional fuel.

From 1980 to 2011, Minnesota Power burned low-mercury, low-sulfur Montana Powder River Basin (PRB) coal at BEC. Based on testing done in recent years, Minnesota Power began burning a new blend of coal from Wyoming and Montana at BEC in January 2011.

BEC4 was originally constructed with first generation low NOx burners and close coupled over-fire air and, what was in 1980 a state-of-the-art wet spray tower absorber/particulate removal system. This system removes more than 85% of the SO2 and over 97.5% of the unit’s PM.

In late 2008, Minnesota Power installed Selective Non-Catalytic Reduction (SNCR) technology for the removal of NOx at BEC4. The SNCR system utilizes NALCO Mobotec’s Rotamix technology. In 2010, Minnesota Power increased its effectiveness in preventing the formation of NOx with the replacement of BEC4’s first generation low NOx burners with state-of-the-art low NOx burners and separated over-fire air technology that is widely used in coal-fired utility boilers to minimize the creation of NOx in the coal combustion process. These NOx controls provide about a 55% reduction in NOx emissions.

BEC4 currently utilizes a scrubber with a wet particulate removal system for PM control coupled with a spray tower absorber for SO2 control. Even though the existing scrubber is not designed for mercury control, Minnesota Power said it is receiving a limited amount of mercury emission reduction through the scrubber as a co-benefit.

A small portion of BEC4’s flue gas (2% to 5%) bypasses the scrubber. This bypass stream is treated by an electrostatic precipitator (ESP) for PM control before being blended with the remainder of the flue gas, where it acts to reheat the flue gas treated by the scrubber. This process results in keeping the flue gas dry as it exits the spray tower absorber and passes through the induced draft (ID) fans, duct work, and finally through the stack. Dry flue gas is critical because moist gas will adversely impact downstream equipment.

Consultants deliver largely similar findings

In 2007, Minnesota Power contracted with the engineering firm Burns & McDonnell to conduct a feasibility study to initially look at the options available to reduce NOx, SO2, PM and mercury at BEC4. Minnesota Power again contracted with Burns & McDonnell in 2008 to conduct a cost estimate study that provided more detail and insight on available options for retrofitting the existing environmental controls on BEC4.

Through these investigations, Burns & McDonnell evaluated combining use of the existing wet scrubber with the use of a fabric filter and a powdered activated carbon injection system because that combination showed the best potential of achieving the required mercury emission reduction. The study results indicated, however, that installation of this equipment would compromise performance of the existing BEC4 absorber tower used for SO2 removal. The alkaline fly ash that is currently captured and subsequently utilized in the absorber towers for SO2 emission reduction on BEC4 today would no longer be available once a fabric filter is added for PM and mercury control.

In 2009, Minnesota Power contracted with the engineering firm Black & Veatch to obtain a second opinion of available options and costs associated with an environmental retrofit on BEC4. The results of this work identified very similar costs to the earlier Burns & McDonnell study. However, Black & Veatch identified Circulating Dry Scrubber (CDS) technology as an additional available option. Benefits associated with installation of CDS include: high mercury and SO2 removal efficiency; integral sulfur trioxide, hydrochloric acid (HCl), hydrofluoric acid, mercury, heavy metals, dioxins and furans and PM2.5 reduction; and lower capital cost, and lower operational and maintenance costs, when compared to wet FGD systems.

Based on the outcome of these studies, Minnesota Power proposes to install the CDS system for the removal of SO2, PM and mercury as part of the BEC4 project. The CDS technology will also further reduce emissions of acid gases, including HCl and trace metals. And, similar to the BEC3 Environmental Improvement Plan, Minnesota Power proposes to install a powdered activated carbon (PAC) injection system to capture flue gas mercury, and a fabric filter incorporated with the CDS to control PM and help optimize mercury removal performance.

CDS would replace some existing emissions systems

CDS is a type of semi-dry flue gas desulfurization system. In a CDS system, flue gas enters a vertical reactor tower before exiting to a fabric filter where additional emission capture and collection takes place.

Flue gas enters at the base of the vertical reactor tower and flows upward through a “venturi,” mixing with the fluidized bed which is comprised of a mixture of dry lime and fly ash. The intensive gas-solid mixing occurring at this point in the CDS process promotes reaction of sulfur oxides in the flue gas with the dry lime particles. Water is introduced separately above the venturi section for flue gas humidification to enhance the reactivity of the lime and physical absorption for more effective SO2 removal. PAC is injected into the vertical reactor tower for the purpose of capturing mercury and is collected along with the PM in the fabric filter. Introducing the PAC prior to the flue gas entering the fabric filter allows enough reaction time to maximize mercury removal, the utility noted.

The start-up process for this new system is projected to occur between April 2015 and October 2015.

BEC4 is jointly owned by Minnesota Power and WPPI Energy (formerly Wisconsin Public Power). Since 1990, WPPI Energy has owned 20% of BEC4 which currently equates to 117 MW. As co-owner of BEC4, WPPI Energy will pay a proportionate share of the required capital and operations and maintenance (O&M) associated with the BEC4 project. The total capital cost of the project is about $350m, which includes the WPPI share.

The CDS system will completely replace the existing BEC4 wet FGD and ESP systems (which will be abandoned in place). The existing systems are unable to be incorporated with new equipment needed to meet MATS regulations and therefore new equipment needs to be installed. Because the new system will be semi-dry, the existing stack and ID fans will be utilized. However, new variable frequency drives (VFD) will be installed on the ID fans to improve efficiency. The existing low NOx burners, SOFA and SNCR equipment will not be affected by the new CDS system.

Minnesota Power plans on using two different sorbents. Pebble lime will be delivered by truck to the site and placed into a storage silo. The pebble lime will be hydrated and used in the removal of SO2. BEC4 will also have the capability to receive lime in hydrated form directly from suppliers by truck or rail.

The powdered activated carbon will be delivered by truck to a silo and injected into the flue gas via blowers. PAC can be delivered either brominated or nonbrominated. BEC4 is expected to use brominated PAC. The benefit in using brominated PAC is that it helps in the oxidation of mercury.

About Barry Cassell 20414 Articles
Barry Cassell is Chief Analyst for GenerationHub covering coal and emission controls issues, projects and policy. He has covered the coal and power generation industry for more than 24 years, beginning in November 2011 at GenerationHub and prior to that as editor of SNL Energy’s Coal Report. He was formerly with Coal Outlook for 15 years as the publication’s editor and contributing writer, and prior to that he was editor of Coal & Synfuels Technology and associate editor of The Energy Report. He has a bachelor’s degree from Central Michigan University.