Air quality controls at Longview Power
By Charles Huguenard, Vice President and General Manager, Longview Power.
|Longview Power’s wet flue gas desulfurization process will achieve 98 percent SO2 removal efficiency. Photo courtesy Longview Power.|
A new, technologically advanced 695 MW(net) supercritical pulverized coal-fired power plant nearing completion in Maidsville, W. Va., is receiving attention for its design efficiency and environmental profile.
The $2 billion Longview Power project is expected to be 17 percent more efficient than the average existing U.S. coal plant, resulting in a proportionate reduction of CO2 emissions. Longview is already evaluating further emission controls—plant owner GenPower Holdings LP launched a study in early 2010 to assess the applicability of Siemens’ second generation amino acid salt post-combustion CO2 capture (POSTCAP) process. Siemens will work to optimize the air quality control system (AQCS) it has provided to accommodate POSTCAP, which could help Longview further reduce its carbon footprint.
With Longview’s advanced supercritical combustion technology, the net plant heat rate of 8,728 Btu/kWh will be one of the lowest of all the coal plants in the PJM Interconnection, the regional transmission organization that serves 51 million people in 13 states. On a national level, Longview’s heat rate will be among the lowest 0.3 percent in the U.S.
Longview Power’s air emissions will be lower than those from existing regional coal plants. According to the West Virginia Department of Environmental Protection, Longview’s emission limits are the strictest issued by the state to a pulverized coal plant to date. Emission limits include 0.095 lb/MMBtu for SO2 and 0.065 lb/MMBtu for NOX. The completed Longview Power project will include more than $500 million in back-end environmental control systems and technologies.
Built by a consortium of Aker Construction and Siemens, Longview Power is one of the largest private investments in the history of West Virginia. More than $1.5 billion has been spent to date on the plant, which will be more than 85 percent complete before the summer of 2010. Longview is scheduled to reach commercial operations in early 2011.
Advanced Supercritical Boiler
The centerpiece of the Longview plant is the 695 MW (net) advanced supercritical pulverized coal (SCPC) boiler. More efficient than older sub-critical boilers and the first generation of supercritical boilers, Longview’s low mass flux vertical tube Benson boiler will consume less fuel per megawatt hour while producing lower air emissions, before the post-combustion air quality control systems. Longview’s SCPC boiler was provided by Foster Wheeler and incorporates Siemens’ Benson low mass flux design principle, developed for coal-fired steam generators.
According to Siemens, a coal-fired power plant with the Benson design technology can sustain load transients of 4 percent to 6 percent per minute over a wide load range and in sliding pressure mode, a condition essential to reducing thermal stresses in the turbine.
Emission Control Systems
After combusting fuel with low NOX burners and over-fire air, the Longview plant provides a series of back-end air pollution controls to further reduce boiler air emissions. The systems include selective catalytic reduction (SCR) for NOX control, dry sorbent injection (DSI) for acid mist control, pulse jet fabric filter (PJFF) technology for particulate matter control and wet flue gas desulfurization (FGD) for SO2 removal.
Each of these control systems meets or exceeds best available control technologies (BACT) performance standards and the combined systems will also provide for significant mercury removal. Longview’s air permit requires the plant to routinely achieve removal efficiencies of 98 percent for SO2 and more than 99 percent for particulates. Plant throughput and stack emissions will be monitored closely through a series of continuous emission monitors (CEMs), fuel analyses, periodic stack testing and visible emissions observations.
Selective catalytic reduction (SCR) is a proven technology for removing NOX from the flue gas streams of coal-fired power plants. The SCR process forces the chemical reduction of NOX to nitrogen (N2) and water (H2O) by allowing the NOX and ammonia (NH3) to react in the presence of a catalyst. Before the flue gas reaches the catalyst, a proportionate quantity of aqueous (19.5 percent) ammonia is injected.
The Longview SCR will utilize a Haldor Topsoe catalyst consisting of active ingredients vanadium pentoxide (V2O5) and tungsten trioxide (WO3) in a titanium dioxide matrix to achieve Longview’s permitted NOX emission rate of .065 lb/MMBtu. Catalyst plates are arranged in blocks with a down-flow orientation. The blocks are installed in layers in the SCR. Two layers, each containing 315 cubic meters of catalyst blocks, will be installed.
The catalyst is designed for an operating temperature range of 630 F to 800 F. Twenty-four sonic horns are provided to remove ash deposits from the catalyst on a 10-minute cycle. Two static mixers are provided inside the SCR inlet flue, immediately downstream of the ammonia injection grid, to allow for more complete mixing of ammonia with flue gas before reaching the catalyst. The system is designed to produce a maximum ammonia slip of 2 ppmvd at 3 percent O2.
Following the SCR, Longview provides DSI in which hydrated lime (calcium hydroxide, Ca(OH)2) is injected to react with acidic combustion products.This reaction reduces the emissions of hydrochloric acid (HCI), hydrofluoric acid (HF) and sulfuric acid (H2SO4), the primary hazardous air pollutants (HAPs) produced from coal combustion.
Fly ash and residuals from DSI are captured by the downstream PJFF baghouse, which contains 12 bag compartments. Compressed air expands the bags in the baghouse at high speed, dislodging most of the dust collected on the outside of the bag when it stops at its outer position. Nozzles direct this compressed air downward into the bags, distributing the pulse pressure evenly throughout the array. This drops the fly ash into the hopper below. The total cloth area of the Longview PJFF is approximately 600,000 square feet. Collected fly ash is forwarded from the baghouse to two silos, where trucks are loaded for removal.
Flue gas desulfurization (FGD) is the standard technology to scrub SO2 from coal-fired power plant emissions. FGDs scrub sulfur compounds from the flue gas with a slurry that contains calcium (lime (CaO) or limestone (CaCO3)) to create calcium sulfate dehydrate (gypsum, CaSO4-2H2O). Longview will use limestone slurry produced with local limestone (82 percent CaCO3) as processed by two integrated ball mills. Cooling tower blowdown will be used as the water source to make the slurry and dibasic acid (DBA) will be added to buffer the slurry pH and improve SO2 absorption. TMT 15 (a 15 percent solution of the trisodium salt of trimercapto-s-triazine) will also be added to minimize mercury emissions.
The slurry solution will be sprayed from multiple spray headers in the FGD absorber vessel where SO2 is absorbed from the flue gas into the liquid slurry. The absorber is also equipped with a dual flow tray that further enhances SO2 absorption from the flue gas. The scrubbed flue gas continues upward through two chevron-style mist eliminators, which remove entrained droplets before the gas enters the stack. The slurry falls into an absorber/recycle tank at the bottom of the absorber where the SO2 forms both calcium sulfite and calcium sulfate. A forced oxidation blower then supplies air to a sparger in the absorber/recycle tank, forming air bubbles that flow up through the slurry. This causes the predominant reaction byproduct to be gypsum, which is then removed from solution by a series of hydroclones and vacuum filters. Filtrate is returned for re-use by the FGD.
The suite of back-end controls at Longview Power—integrating SCR, DSI, PJFF and FGD—will reduce mercury emissions. An October 2009 report to the United States Senate Subcommittee on Clean Air and Nuclear Safety confirmed this co-benefit for plants with similar air pollution control systems.
Longview’s SCPC boiler emissions will be monitored by an extensive series of CEM systems. Longview will maintain CEMs for mercury (Hg) and particulate matter (PM), as well as for NOX, SOX, CO, CO2 and stack flow. Longview CEMs will use Spectrum Systems’ hardware and the latest version of SpectraView software.
Integrated Project Efficiencies
Longview Power was designed to integrate optimal process and environmental efficiencies. As a mine-mouth project, local bituminous coal arrives by overland conveyors. The conveyors increase project efficiency, reducing overall carbon footprint as well as the impact of coal trucks on local public roads. Longview’s boiler is designed to burn local coal without prior washing or other preparation (“run-of-mine”). This reduces water consumption and fuel cost. Fly ash, bottom ash and gypsum will be transported to an engineered fill on adjoining property, where it will be compacted in a dry state. The close proximity minimizes transportation costs. Longview will also use locally mined limestone in the wet FGD scrubber, further reducing transportation and related environmental impacts.
When coupled with the efficient boiler and control devices, Longview’s many process efficiencies allow for one of the lowest dispatch costs of any pulverized coal plant in the region.
Broader Environmental Benefits
During the initial permitting process, Longview Power made commitments to further reduce the project’s environmental footprint. Longview Power has committed to provide additional mitigation to protect pristine federal lands in the region including the Shenandoah National Park, Dolly Sods Wilderness Area, Otter Creek Wilderness Area and the James River Face Wilderness Area. For each ton of SO2 emissions, Longview will buy and retire an additional 1.1 SO2 allowances from other regional facilities beyond Longview’s federal acid rain allowance requirements.
Longview Power also committed to fund efforts by a non-profit organization to establish a carbon dioxide (CO2) sequestration program and a stream mitigation program. Longview will provide $500,000 a year to this organization for the first 10 years of operation and then $300,000 per year for the balance of the plant’s life. For the CO2 sequestration program, funding preference will be given to projects in West Virginia. For the stream mitigation program, preference will be given to projects and programs taking place in or benefiting the Class I federal lands noted above.
As a new coal facility being built in a time of environmental uncertainty, Longview will be a high-profile project. Longview is positioned to meet expectations and deliver a new generation of power, while providing added benefits to the coal-producing state of West Virginia.
Author: Charles Huguenard has 30 years of experience in the power generation industry in plant asset management, operations and maintenance, technical support, environmental permits and labor agreements. He oversees development of the Longview Power project and of operations teams for both Longview and its developer, GenPower Holdings LP. Huguenard holds a Bachelor of Science in mechanical engineering from Auburn University.
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