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Layer Upon Layer

The protection of the environment is now high on the policy agenda the world over, reflected by increasingly stringent legislation. Like other industries, the power industry is responding to these changes through the application of technology as well as best practice management.

Conforming to legislation inevitably involves capital outlay, however, and conflicts with the need in the power industry to reduce costs and be competitive. Installing and running technology to reduce emissions of NOx and SOx will cost a utility millions of dollars.

The RJM-LT system uses a layered, flexible approach to NOx control, and can reduce the cost of NOx compliance by up to 40 per cent compared to SCR technology
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In response to these developments, Connecticut-based RJM Corporation has developed an innovative approach to NOx control that enables fossil fuel fired power plants to achieve a 90 per cent reduction in baseline NOx emissions at less than two-thirds of the cost of other NOx reduction technologies.

The system, RJM-LT, consists of five ‘layers’ of NOx reduction technology which can be applied to a power plant in various combinations to meet the needs of the operator. The advantage of the technology, says RJM, is that it allows power plants to comply with stringent emissions regulations without the expense of installing SCR (selective catalytic reduction) technology. In addition, the modular approach gives plant operators flexibility in terms of how emission reductions are met.

Cost savings

Compared to SCR, the installation of RJM-LT technology can result in savings of $5-15 million depending on boiler size. “At a cost that is 60 per cent of competing SCR technology, the RJM-LT is a viable alternative for the hundreds of power plants now contemplating NOx reduction systems,” said Richard J. Monro, President of RJM Corporation. “These cost savings can allow the generation companies to maintain their competitive production cost structure.”

John J. Halloran, RJM’s vice president, Combustion & Environmental Group, notes that, “With the RJM-LT, NOx reductions ranging from 25 to 90 per cent can be achieved on fossil fuel fired units. These layers can be selectively combined to achieve specific NOx reduction targets for any given boiler. The RJM-LT results in lower capital and operating costs compared to an SCR while requiring significantly shorter plant outage time. Moreover, the technology offers generating plants the option to buy the NOx reduction level they need. These savings can lead to significant cash flow reduction for power generators to achieve compliance.”

According to RJM, the market for NOx control technology is significant. In the USA alone, over 130 000 MW of generation capacity is expected to be retrofitted with NOx emission control technology before 2004. The USA is therefore a key market for RJM, but it is also pursuing other markets around the world.

Installing NOx reduction technology can cost millions. Cost savings and flexibility are particularly important in competitive markets
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The company recently opened up an office in the UK to handle its international operations, and Europe is one of its key markets.

Around the world, RJM has brought some 50 000 MW of capacity into compliance with its various NOx control technologies. In North America, the company says that its solutions have proved to be the least expensive route to complying with Environmental Protection Agency (EPA) and state mandated NOx regulations, typically saving customers 40-70 per cent in total installed cost when compared to alternative technologies like SCR or new low NOx burners.

Five layers

In November 2001, RJM announced a new ‘layered’ approach to NOx control. This incorporates its established emission control technologies, as well as a new, proprietary technology known as Absolute Compliance (RJM-AC). Five NOx reduction ‘layers’ can be applied in all: burner modifications; overfire air; NOx tempering; SNCR (selective non-catalytic reduction); and RJM-AC constitutes the fifth layer. Used together, the five layers can achieve a 90 per cent reduction in baseline NOx emissions.

The burner modification process includes four techniques, which together can achieve a NOx emission reduction of up to 70 per cent on baseline emissions:

  • Air Distribution Analysis (ADA): Air Distribution Analysis is a fast, accurate and cost-effective technique for balancing secondary air between burners. It uses actual data taken from burners, rather than information inferred from downstream data or drawn from simulated conditions.

    RJM pioneered the use of massive, airflow balancing datasets, collected from inside the burner throat, at the critical fuel-air interface to create definitive diagnostic performance results. Typically over 2400 individual data readings are taken for each burner. As a result, this proprietary technique provides reliable, accurate results to à‚±1.5 per cent.

    Results identify airflow differences between burners, differences within the burner itself and the precise nature and location of any inefficient windbox air distribution for each unit.

  • Fuel Balancing: Balancing the fuel flow from burner-to-burner (coupled with airflow balancing ADA) ensures that the minimum furnace excess oxygen level is achieved. Fuel balancing and reduced furnace excess oxygen are beneficial to unit heat rate, boiler thermal efficiency, superheater temperature profiles, flame stability as well as NOx reduction.

    For coal firing, RJM utilizes a RotorProbe and Dirty Air Pitot tube to measure the existing burner primary air and coal flow deviations. The RotorProbe is an effective measurement technique of coal flow distribution deviations and ‘roping’ in the coal pipe runs, prior to the burner. This enables RJM to recommend changes to existing pipe orifices to correct the fuel distribution to à‚±10 per cent.

    Alternatively, to limit outage time, RJM supplies balancing dampers for each coal pipe, which facilitates the on-line adjustment of the fuel balance. The balance of fuel and air is confirmed by measuring LOI, percentage O2 and CO across a sampling grid at the boiler or economizer outlet flue. RJM also supplies a coal distribution device to eliminate the coal ‘roping’ in the burner.

  • CFD Modelling: RJM uses CFD combustion modelling to verify the baseline burner NOx and CO emissions and to design the burner modifications. The final burner configuration, including the flame stabilizer is modelled to ensure complete burnout of the fuel with low CO and NOx emissions. The difference between the baseline and modified burner emissions determine the percentage of NOx reduction from the burner modifications.
  • Flame Stabilizer: RJM adds a patented flame stabilizer to each burner to stabilize the combustion process and allow the unit to be operated at lower excess O2. In addition, it radially and circumferentially stages the secondary air zone of the burners to reduce NOx emissions. This design creates the minimum swirl necessary to maintain a stable fire.

The remaining secondary air is injected in a low or non-swirl mode outside the primary combustion zone. The application of the flame stabilizer allows the air doors to be set in a full open or nearly full open position, removing any inconsistencies between burners caused by the air doors. The quantity of air is effectively controlled in the primary combustion zone where the majority of the NOx emissions are formed.

To enhance the NOx reduction capabilities of the burner modifications, the flame stabilizer is designed with internal air/fuel staging. This sets up fuel rich and lean zones downstream of the stabilizer in the primary combustion zone. This provides additional staging, flame stability and lower NOx emissions.

Further reductions

Overfire Air (up to 25 per cent incremental NOx reduction): NOx emissions can be reduced an additional 25 per cent through the use of RJM’s advanced OFA ports. An OFA system diverts secondary air above the top burner rows. The ports are designed to inject air at the proper velocity to complete combustion prior to the furnace exit.

NOx Tempering System (15-30 per cent incremental NOx reduction): The patented NOx tempering technology is another layer that can be added to incrementally reduce NOx. This technology injects micronized water droplets into high NOx production zones in the near burner region, and can achieve up to an additional 30 per cent NOx reduction.

SNCR System (up to 40 per cent incremental NOx reduction): In this process, an aqueous solution containing a reagent (urea-based with chemical enhancers) is injected into the lean fuel zone above the furnace. The reagent reacts chemically with the NOx in the combustion gas to form nitrogen, a harmless gas.

Absolute compliance

RJM-AC System (30-60 per cent incremental NOx reduction): The final layer, RJM’s Absolute Compliance, involves amine reagent injection in the primary combustion zone. The reagent uses combustion turbulence for complete dispersion and targets specific zones where optimum chemical kinetics achieve high NOx reduction.

RJM-AC is based on Rich Reagent Injection (RRI) technology, which was developed by the Electric Power Research Institute and a Utah, USA-based modelling company called Reaction Engineering International (REI).

RRI technology was successfully demonstrated in 2001 by RJM, REI and EPRI on cyclone furnaces at Conectiv’s BL England power station in the USA, where an 80 per cent reduction in NOx emissions was achieved. The technology is now used by RJM under license from REI in its Absolute Compliance layer.

One of the key benefits of the Absolute Compliance system is that its running costs are low compared to SCR systems as there is no catalyst to replace. In addition, the layered solution requires less plant downtime for installation à‚— usually one week à‚— compared to six to eight weeks for an SCR system.

RJM has yet to install its Absolute Compliance system in a commercial application but says that it is in the advanced stages of negotiation with several utilities in the USA to use the technology.

“The layers can be selectively combined to achieve specific NOx reduction targets for any given boiler”