Satisfied with a plan to retrofit and then eventually shut the coal-fired Centralia power plant, the U.S. Environmental Protection Agency on May 23 proposed to approve the Best Available Retrofit Technology (BART) determination, under its regional haze rules, for the plant.
TransAlta Centralia Generation LLC, a unit of Canada’s TransAlta Corp. (TSX: TA and NYSE: TAC), operates the plant. The Washington State Department of Ecology (Ecology) submitted its Regional Haze State Implementation Plan (SIP) to EPA in December 2010. On Dec. 29, 2011, Ecology submitted an update to the SIP containing a revised and updated BART determination for TransAlta. EPA said it plans to act on the remaining Regional Haze SIP elements for Washington in the near future. Written comments on the BART approval for Centralia are being taken until June 22, said EPA in a May 23 Federal Register notice.
Centralia is a two-unit, coal plant, with each unit rated at 702 MW each when burning coal from the Centralia coalfield as originally designed. The units now burn sub-bituminous coal from the Powder River Basin and are rated at 670 MW each. The units were commissioned in 1971 and 1972.
In June 2003, EPA approved a revision to the Washington SIP for visibility (Visibility SIP) which included controls for NOX, SO2, and particulate matter (PM) for TransAlta. In the action approving these provisions, EPA determined the required controls to be BART for SO2 and PM. Concentric low NOX burners with overfire air was required to control NOX emissions with emission limits of 0.302 lb/mmBtu for Unit 1 and 0.306 lb/mmBtu for Unit 2. EPA found these controls did not represent BART for NOX in 2003 and the Federal Register notice accompanying that action stated that a BART determination for NOX was not being made at that time. Thus, to date there is not a SIP approved BART determination for NOX emissions at TransAlta.
The new TransAlta NOX BART determination was submitted to EPA in two separate actions. The first was included in the December 2010 Regional Haze SIP submittal. Washington subsequently reevaluated its determination for TransAlta and on Dec. 29, 2011, submitted an update to the Regional Haze SIP. This update included a revised NOX BART determination.
The revised BART determination and a Revised BART Compliance Order establish a NOX emission limit of 0.21 lb/mmbtu, and among other things, requires selective non-catalytic reduction (SNCR) to be installed by Jan. 1, 2013. The Revised BART compliance order also provides that one coal unit must cease burning coal by Dec. 31, 2020, and the other coal unit cease burning coal by Dec. 31, 2025, unless Ecology determines that state or federal law requires selective catalytic reduction (SCR) to be installed on either unit.
Washington law mandates phased Centralia shutdown
In May 2009, the Washington governor issued Executive Order 09–05 which contained provisions for TransAlta regarding compliance with Washington State’s greenhouse gas (GHG) emission performance standards. Subsequently, the Executive Order was superseded by Washington State Senate Bill 5769, which was signed by the governor in April 2011 and became effective in August 2011. The law requires that one of the TransAlta units comply with the GHG performance standards by Dec. 31, 2020, and the other by Dec. 31, 2025.
TransAlta is also required to install SNCR by Jan. 1, 2013, to control NOX emissions. Additionally, the law states that the requirement to meet the GHG performance standard does not apply if Ecology determines that state or federal law requires SCR to be installed on either coal-fired unit.
The Visibility SIP submittal for EPA’s June 2003 approval identified a long list of available NOX control technologies which were evaluated for technical feasibility at the TransAlta plant. That list was narrowed to the technically feasible controls which Ecology used as a starting point for the current BART determination. Ecology evaluated, or reevaluated, a number of the NOX control technologies for TransAlta including: low NOX burners with close coupled and over-fired air (LNC3); Flex Fuel; SCR; SNCR; Rotating over-fire air (ROFA)/Rota mix; neutral net technology; and natural gas re-burning.
Incidentally, Flex Fuel refers to the switch from local coal to coal railed in from the PRB. PRB coal has a higher heat content than the local coal from a now-shut mine, requiring less fuel for the same heat extraction, as well as a lower nitrogen and sulfur content than coal from Centralia. Flex Fuel also required changes to boiler design to accommodate PRB coal.
The state found that ROFA is infeasible because it has never been tested nor demonstrated in a large tangentially fired boiler of this size. The state also determined that “Neutral Net” technology likewise has not been guaranteed to perform and reduce emissions and there are comparable proven technologies available. The state also found that natural gas re-burning is not listed in an EPA roster for use in any coal fired boilers and that it would be less efficient at controlling NOX emissions than the Flex-Fuel plus SNCR as required by Washington’s Legislature.
Washington found that Flex Fuel alone will reduce NOX emissions by 3,139 tons per year (t/y) and will also reduce SO2 emissions by 1,287 t/y. Based on evaluation of installations at other large tangentially fired power plants, the state determined that SNCR plus Flex Fuel is expected to achieve a 20% to 25% reduction in NOX emissions. The state estimated capital costs for SNCR plus Flex Fuel at TransAlta to be $135m and annual operating costs of $17.3m based on an emission limit of 0.21 lb/mmbtu. The retrofit costs for TransAlta will be higher than other similarly sized power plants due to boiler design. The state also calculated the SNCR plus Flex Fuel cost effectiveness to be $2,162/ton based on a 25% control efficiency and a 8,022 t/y reduction in NOX emissions.
SCR deemed to be too expensive at this point
Washington also evaluated SCR, which would provide a 95% NOX control efficiency. The state considered two scenarios; one including SCR on only one unit and another scenario with SCR on both units. Using a presumptive BART emission limit of 0.15 lb NOX/mmbtu, they estimated the emission reductions for SCR on one unit to be 4,364 t/y and 7,855 t/y for SCR on both units. The capital cost for one unit was estimated at $290.12m and about double that for SCR on both units.
Washington estimated it would take four years to design and install SCR with a compliance date of late 2016. The cost effectiveness for SCR on only one unit was calculated at $8,205/ton. If SCR was to be installed on both units, the state calculated cost effectiveness for SCR on Unit 1 to be $14,800/ton and Unit 2 to be $8,400/ton. Washington determined SCR is not cost effective under either scenario and that it is not reasonable to require SCR for this facility.
Washington determined BART for NOX for the TransAlta plant is 0.21 lb/mmbtu based on installation and operation of SNCR plus Flex Fuel. The state’s BART determination also requires the use of sub-bituminous coal from the PRB, or other coal that will achieve similar emission rates, and a requirement to optimize SNCR for the lowest NOX emissions while minimizing ammonia slip. The BART determination allows for the NOX limit to be revised reflecting the optimization to a level no higher than 0.21 lb/mmbtu.
The plant currently employs wet limestone forced oxidation to control SO2 emissions, electrostatic precipitators followed by wet scrubbing systems to control particulate matter, and low NOX burners with close coupled overfire air to control NOX. These existing controls occupy space in the exhaust ducting, minimizing space for additional controls for NOX. Therefore, additional control equipment would require the redesign and installation of additional support structures, as well as the potential relocation of existing control equipment.