Emissions Control: Low cost low NOx

John Marion and Chris Smith, Alstom Power, Dr. Soung Kim, USA Department of Energy

As deregulation continues to take hold in the USA, existing coal-fired power plants continue to enjoy a particularly favourable role in baseload generation. This is due to their low cost of electricity resulting from low fuel costs, high availability, low remaining capital depreciation, and high capacity factor.

But while the federal Energy Information Agency projects that coal-fired generationà‚– which currently produces more than half of the nation’s electricityà‚– will continue to lead in the USA over all other fuel sources for decades to come, one area must be addressed: environmental performance.

Both power industry and government agencies agree that with coal-fired plants facing much tighter clean air standards in the coming years, new emissions control technologiesà‚– featuring low capital and operating costsà‚– must be developed to improve existing coal-fired unit emissions without negatively affecting the positive, overall plant economics.

With that goal in mind, the federal Department of Energy (DOE) has initiated a programme to develop new, advanced pollution control technologies designed to cut future environmental compliance costs. For nitrogen oxide (NOx) emissions control, the DOE’s National Energy Technology Laboratory (NETL) is working with a number of power industry suppliers to develop low cost, high efficiency technologies that can be retrofitted to existing coal-fired boilers.

Low NOx programme

Figure 1. NETL is working with several suppliers to develop low cost, high efficiency technologies
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Under the NOx control programme, the technologies under development will help the nation’s power producers comply with the Environmental Protection Agency’s (EPA) NOx State Implementation Plan (SIP) at costs at least 25 per cent lower than the best currently available technologyà‚– selective catalytic reduction (SCR).

The SIP call governs NOx emissions in 22 eastern states during the five-month summer ozone season (May through to September) with the objective to reduce NOx emissions by approximately 1219.2 million kg, starting in 2003. This legislation initially covers approximately 40 per cent of the USA utility boiler population and more than 880 coal-fired utility boilers.

Alstom Power in Windsor, Conn., USA, is working with the NETL to develop an in-furnace, Ultra Low NOx Integrated System that will address both present and anticipated NOx emissions control legislation for US coal-fired boilers. It expects the new system to be ready for commercial deployment by 2002.

Specifically, the programme targets existing tangentially-fired utility coal boilers subject to the NOx emission reduction requirements established by the EPA under Title IV of the 1990 Clean Air Act Amendment (CAAA), and future anticipated legislation for National Ambient Air Quality Standards for PM2.5 (air-borne particles less than 2.5 micron size).

The proposed Alstom Power system will be an aggressively air staged, in-furnace NOx reduction system designed to meet or exceed the DOE’s NOx target of 0.07 kg/MMBtu NOx for tangentially fired boilers, firing a wide range of coals (the project goals are 0.07 kg/MMBtu firing eastern bituminous coals; 0.05 kg/MMBtu when firing western sub-bituminous coals).

To meet project goals, in-furnace combustion modifications and a post-combustion Carbon Burn Out technology for non-reactive coals will be used. In addition, an intelligent control system incorporating neural network-based features with new, on-line sensor technologies will be used to maintain target NOx emissions over a wide range of boiler loads and operating conditions.

Figure 2. Pilot scale testing is now being carried out by Alstom
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The new system design will be based upon Alstom Power’s field-proven TFS 2000R low NOx firing system. Developed and commercially deployed throughout the 1990s, TFS 2000R low NOx firing system field retrofits have demonstrated NOx emissions at or below 0.11 kg/MMBtu when firing an eastern bituminous coal, and at or below 0.07 kg/MMBtu when firing a western sub-bituminous coal.

By calculating the known cost of its existing, in-kind technologies, Alstom Power anticipates the commercial cost of a new, more advanced system will be less than half that of a selective catalytic reduction (SCR) only installation, thereby exceeding the 25 per cent reduction mandated by the DOE.

New system development

To ensure that the new system meets end user requirements, an advisory committee consisting of the DOE, electric utility representatives and selected consultants from the Massachusetts Institute of Technology (MIT) has been formed to review project plans, provide input to the system design and analysis, and review project results.

The proposed system will improve upon the existing NOx reduction technologies through advances that will address present constraints for achieving lower NOx emissions. The combination of improvements in both components and processes are based on fundamentally sound principles known to lower NOx formation and/or improve NOx destruction, while minimizing the impact on the balance of plant. These improvements include:

  • Milling system enhancements, both to the mill internals and coal particle size classification processes. For example, modifications will be made to the Alstom Power Dynamic Classifier to not only produce a finer coal product, but also provide more rigorous control over the particle size.

Figure 3. Modifications will be made to the Alstom Power Dynamic Classifier
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In addition, the amount of pulverized coal transport air will be decreased to facilitate a lower transport air to coal mass ratio. The fuel and air will be separated after exiting the mill, thus generating a fuel rich coal stream and a fuel lean transport air stream. These improvements will allow more rapid heating of the coal particles in the burner near field, resulting in greater fuel bound nitrogen release, and hence lower NOx, as well as lower unburned carbon at the exit of the upper furnace.

  • Low NOx oxidizing pyrolysis burners will be designed to promote a higher fuel nitrogen release through more rapid heating of coal particles. Operated with the fuel rich fraction of the transport air and coal mixture, these burners will minimize the formation of near field NOx by facilitating control over the local stoichiometry and mixing processes.
  • Additional near burner turbulence will be generated to create a more uniform, high intensity, fuel rich zone. Effective mixing of the combustion products in the fuel-rich lower furnace, coupled with longer residence times, will result in greater NOx destruction.

The fuel lean transport air, carrying the finer fraction of the pulverized coal stream, will be directed to a close coupled or intermediate elevation overfire air register in order to provide further NOx reduction through the reburn mechanism. Optionally, selective non-catalytic reduction (SNCR) may be employed in the reburn zone, or further downstream, if further trimming of NOx is required.

The company’s patented Concentric Firing System (CFS) will be used to provide additional, near field air staging and promote the formation of an oxidizing environment near the waterwall to mitigate against waterwall wastage.

  • For particularly un-reactive coals exhibiting higher levels of unburned carbon in the fly ash (which can increase operating costs by preventing the sale of the ash to cement manufacturers and/or introduce an ash disposal cost), a bubbling bed carbon burn out combustor developed by Progress Materials, Inc. may be used to oxidize the carbon to acceptable levels while also returning energy to the boiler.

Having the ability to reduce the carbon content of fly ash to commercially acceptable levels creates an additional degree of operational freedom for NOx control. Economics of the bubbling bed system are addressed through its operation as a heat recovery and saleable fly ash product device.

  • An intelligent control system incorporating advanced neural network features will be employed to maintain the target NOx emissions by controlling both local and global stoichiometries over the range of boiler operating loads and provide for fault tolerant operation as planned and unplanned system upsets occur.

The control system will diagnose the combustion process through existing instrumentation and employ advanced sensor technology including a carbon-in-ash analyser; coal mass flow measurement in transport air and fuel lines; and an advanced flame scanning spectrophotometer at each fuel injector.

Collectively these advanced control components are expected to help optimize system performance across a range of input parameter variations, providing for minimum NOx operation while simultaneously maintaining desired steam temperatures, carbon in ash levels and optimized plant heat rate.

Design and testing

After an initial design phase of the proposed system, pilot scale testing is now being carried out via Alstom Power’s Drop Tube Furnace System, Pulverizer Development Facility, and a 15 MWth Boiler Simulation Facility, all of which are located at the Windsor site.

Initial combustion tests began in October and are scheduled to continue through April 2001. Final integrated system design and economics will be reported by June 2001, with a commercialization plan completed by the following September.

By 2002, the new system will be ready for deployment on tangentially fired coal boilers. The longer term market is projected to be 131 000 MW or 40 per cent of the overall tangentially-fired boiler market based on expected changes to national Ambient Air Quality Standards, PM2.5 and the identification of pristine areas under future regulations.

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