|Minnesota Power’s 1,025 MW Boswell Energy Center received new air quality control equipment, including wet flue gas desulfurization, low NOx burners, selective catalytic reduction, baghouse and a carbon injection system. Photo courtesy Alstom Power.|
Many U.S. generating companies operating aging coal-fired plants are being forced to meet increasingly strict emission control standards. Retrofitting steam turbines is a solution that will not only reduce emissions by increasing plant efficiency, but will also allow power output to be maintained at plants that have implemented new pollution control equipment.
Minnesota Power is an investor-owned utility (IOU) that provides electricity in a 26,000-square-mile electric service territory in northeast Minnesota. More than half of the electricity it generates is for energy-intensive industrial users, which include some of the nation’s largest taconite (an iron-bearing rock) mining and paper production companies.
Its largest generating asset is the Boswell Energy Center, a coal-fired plant in Cohasset, Minn., which has a total power output of just over 1,025 MW from four units. Units 1 and 2, each with an output of 70 MW, began operating in 1958 and 1960, respectively. Unit 3 has an output of 350 MW and began operation in 1973. Unit 4 has an output of 535 MW and began operation in 1980.
To meet the requirements of the Minnesota Mercury Emissions Reduction Act of 2006 and comply with other recently established and future state and federal emission requirements, Minnesota Power has made significant reductions in mercury, oxides of nitrogen (NOx), sulphur dioxide (SO2) and particulate matter (PM) emissions at the Boswell Energy Center.
Under the “Boswell 3 Plan,” Minnesota Power obtained approval to install new air quality control equipment, including wet flue gas desulfurization, low NOx burners, selective catalytic reduction, baghouse and a carbon injection system. The company’s goal was to maintain Boswell 3’s net output capability while powering the additional environmental control equipment.
Minnesota Power helped pioneer the use of Powder River Basin coal on Boswell 3. In 1986, then-president of Minnesota Power and Light, Axel Herbert, announced a three-way agreement among the utility, Peabody Coal Co., Great Northern Railway and Northern Pacific Railway to open the western coal reserves to Midwestern states. The Boswell 3 boiler was supplied by Combustion Engineering, a predecessor to Alstom. The original generator was built by General Electric. Unit 3 also originally included a first-generation particulate wet scrubber by Krebs-Elbair.
In 2007, Minnesota Power contracted with Alstom to perform an optimized plant retrofit (OPR) study for Unit 3. As part of the study Alstom evaluated the boiler, auxiliary components and turbine in an effort to determine the necessary modifications to achieve the desired megawatt uprate to drive the additional “scrubbing” equipment within the customer’s specific operating boundaries and future fuel blends.
The Boswell 3 OPR resulted in recommended modifications to boiler pressure parts, firing system, HP/IP turbine as well other smaller auxiliary components. Ultimately, as a result of the OPR, its role as the boiler OEM, and its turbine retrofit solutions for these GE and Westinghouse units, Alstom secured the contracts for the components tied to the overall unit operating performance. This resulted in Alstom securing a contract to retrofit Unit 3 HP/IP turbine in December 2007. Alstom was also awarded a contract for a new low-temperature superheater and a new low NOx burner system with over-fire air for Unit 3.
Reasons to Retrofit
Retrofitting steam turbines is often the best solution to maintaining plant output while providing additional station service to drive new environmental equipment. Although installing completely new generating units is always an option, it can sometimes result in higher rates to customers. Also, sometimes it may be unnecessary to replace all the components associated with a new unit. With the existing units at Minnesota Power already more than 30 years old, retrofitting made more economic sense.
Besides covering the power needs of the emissions reduction equipment, Alstom was able to retrofit the steam turbine to deliver a few extra megawatts of power. The units could also be retrofitted without losing much generating time. The steam turbines were being retrofitted during an outage schedule that was largely dictated by the demands of a customer base that has a large number of industrial users.
Unit 3 Retrofit
The contract for the Unit 3 retrofit was signed in December 2007 and Alstom began with a baseline program, whereby engineering drawings were checked and produced to make sure the parts could be delivered according to the contractual date. A design review was then performed and a customer kick-off meeting held to fix the design parameters. Once the design was finalized, the procurement procedure was initiated.
First equipment was delivered to the site in early August 2009 and the outage began on August 15th. The outage was scheduled for 12 weeks to accommodate the work in the boiler and the installation of environmental equipment (for details on that project, see the June 2010 issue of Power Engineering magazine).
Boswell Unit 3’s original steam-turbine and generator was supplied by GE. Limitations on the generator output meant only the HP/IP module in the Unit 3 steam turbine would be retrofitted.
Alstom’s scope of supply included the design, manufacture and delivery of new rotating parts; that is, rotor and new blades as well as stationary parts such as a new inner casing, diaphragms and other assembly parts.
The new HP section is a 9-stage HP steam path configured for partial arc admission. It features a new HP inner casing, new balance piston gland carrier, new inlet sleeves and seal kits and eight HP controlled flow diaphragm stages of platform construction.
The new IP section is a 5-stage IP steam path design. There are five stages of IP diaphragms. The existing IP carrier has been re-used.
The retrofit design includes features intended to increase thermodynamic performance and long-term mechanical integrity. The diaphragms are of platform construction that allows the inclusion of 3D aerofoil sections. This “controlled flow” design of fixed blade minimizes aerodynamic losses at root and tip. The diaphragm and moving blades include “masked axial gaps”, which have the same effect as very small clearances, resulting in reduced leakage at root and tip.
HP Stage 1 moving blade is designed for partial arc admission (2+1+1) and consists of a 4-finger root and uses alternate torsion design, resulting in a robust design. The other moving blade stages are all 3D aerofoils and use integral pre-twisted shrouds and pinned roots. The gland seals of some stages include swirl breaks to increase the margin on rotor stability.
At the site, Minnesota Power removed the old HP/IP module so that only the turbine outer casing remained. The new parts were then re-assembled and installed by Minnesota Power’s engineers, under the supervision of Alstom. Performance tests were completed on Dec. 8, 2009. The next major milestone will be the end of the warranty period in October 2011.
Unit 4 Retrofit
Alstom also was awarded the contract to retrofit the Unit 4 steam turbine, which was signed in February 2008. Work is still ongoing. The steam turbine has a completely different design compared to Unit 3. It is a typical BB44+BB271 Westinghouse design, where the HP and IP are separate modules. Unlike Unit 3, the Unit 4 turbine generator has sufficient capacity to allow the HP/IP and two LP modules to be retrofitted.
Alstom’s scope of supply includes the design, manufacture and delivery of an HP-IP rotor with an 8-stage HP and 5-stage IP; HP/IP inner casing with a horizontal joint bolting for the new inner casing HP-IP; centre gland carrier and sealing segments and HP rear gland carrier and sealing segments.
For each LP cylinder Alstom will design, manufacture and deliver a fully bladed low-pressure double flow rotor (7 stages per flow) of welded design and inner casings with fully bladed carriers. The outer casing will be re-used.
The manufacturing and process for the new parts for Unit 4 will be different from Unit 3. The new HP/IP module— the HP/IP rotor and stationary parts—is being manufactured and pre-assembled in Morelia, Mexico. The two LP modules are being manufactured and pre-assembled in Elblag, Poland. This is due to the physical size of the LP modules. Elblag has the capacity needed for these larger rotors. It also has large enough facilities to carry out the balancing and over-speed tests that are necessary before shipment.
The new modules are due to arrive on site in summer 2010. Installation will last approximately six weeks, with the plant scheduled to resume operation by fall 2010. Performance testing will then last two or three days. Final acceptance is scheduled for October 2010.
Alstom also provided the low NOx burner system for Unit 3. Boswell Unit 3 uses a tangentially-fired boiler supplied by Combustion Engineering burning low sulfur, low mercury sub-bituminous coal. Particulate emissions were controlled by a wet particulate scrubber, which also removed approximately 30 per cent of boiler SO2 emissions.
Minnesota Power is also committed to using the most mature, commercially available technology to significantly reduce emissions of mercury. Additionally, the use of well-established air quality control technologies systemwide will significantly reduce NOx, SO2 and particulate matter.
Minnesota Power began on-site construction of the environmental equipment in spring 2007. With installation of the environmental system complete, Minnesota Power is continuing to work to optimize the system.
The environmental project will result in considerable reductions in key pollutant emissions from the Boswell electric power generating station. Emissions of SO2 were reduced by 99 percent and nitrogen oxides (NOx) were reduced by 92 percent from Unit 3. Emissions of mercury will also be reduced by more than 90 percent from Unit 3.
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