New laws prompt focus on low NOx options

Current proposed NOx emission limits in the EU, which could become law in 2001, include the requirement that new plant over 300 MWt must achieve 200 mg/Nm3 (@ 6 per cent O2), for example. The approach is somewhat different in the USA, but the thrust is the same. NOx emission budgets have been set by the US Environmental Protection Agency for 22 states, averaging a 25 per cent reduction across all sectors. The EPA ruling sets NOx budgets for the "Ozone season" (May-September), but the state decides the mix to meet the EPA allocations (this is known as the SIP Call, SIP standing for State Implementation Plan). The basic requirement is that by 2003 utility generating stations in the 22 states must achieve 190 mg/Nm3 (0.15 lb/MBtu) @ 6 per cent O2, May-September. EPA has however been challenged in the courts and there is likely to be a delay in implementation. Various compromises have been considered involving higher emissions in some states.

Nevertheless the message is clear in both the USA and Europe: there is going to be a growing demand for cost effective NOx reduction technologies. In-furnace measures include advanced burners, two-stage combustion, reburn (gas and coal) and pulveriser upgrades. These can be implemented on their own or in combination with post combustion techniques, SCR (selective catalytic reduction) or SNCR (Selective Non-Catalytic Reduction), in ways that minimise overall costs.

Low NOx burners are generally accepted as the primary technique for NOx reduction. In-flame NOx reduction is achieved in the near burner zone by alterations to the standard burner design, and in particular by ensuring initial combustion of the fuel in a fuel rich environment. The fuel rich environment is produced by control of the air and fuel mixing within the burner. Most burner manufacturers have been developing and applying low NOx burners over the past five to ten years, with varying degrees of success.

Another combustion modification technique available to the utility operator to reduce NOx emissions is Overfire Air (OFA). This involves furnace air staging, generally in combination with low NOx burners, with the burner zone run under fuel rich conditions. The balance of the combustion air required to complete the coal combustion is added through specially designed overfire air ports located above the burner zone. An additional NOx reduction of some 15 to 25 per cent (to the 0.30-0.35 lb/MBtu (400-470 mg/Nm3) region) can be obtained by adding an overfire air system to a low NOx burner system, with a significant increase in LOI levels unless the coal fineness is improved. Operating the burner zone fuel rich can cause significant water wall corrosion problems; the increased level of carbon in the fly ash can also lead to precipitator operation and ash disposal problems.

Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) are both post-combustion techniques involving the injection of ammonia or an ammonia-based compound into the combustion products. SCR involves injection into the cool flue gases downstream of the boiler, and the presence of a catalyst, on which NOx is reduced to molecular nitrogen and water. Large NOx reductions are achieved (> 80 per cent) but the capital and operating costs are high and there may be problems with catalyst disposal. With SNCR the injection is into the combustion products in the upper furnace or superheater region of the boiler without the presence of a catalyst. Temperatures here are typically around 1100 °C. The reduction efficiency is very temperature dependent, although it can be extended by using chemical additives. Reductions of the order of 40 to 50 per cent have been reported.

A major problem with SNCR, which to a lesser extent also effects SCR, is ammonia slip; that is unreacted ammonia passing through the system.

Reburning is a hybrid combustion modification technique. In gas reburning, natural gas is injected into the furnace after the primary, coal, combustion zone to produce a fuel-rich region where the NOx is reduced by reactions with hydrocarbon radicals. This is termed the reburn zone. Further downstream, in the so-called burnout zone, air is injected into the boiler to complete the combustion of any unburned coal from the primary zone and the carbon monoxide formed in the reburn zone. This air is also termed overfire air.

Although any hydrocarbon – including coal and even orimulsion – can theoretically be used as the reburn fuel, natural gas has distinct advantages which make it particularly attractive for large utility boilers. NOx reductions of between 50 and 60 per cent have been reported when around 20 per cent of the thermal input is replaced by natural gas.

Deciding on the most suitable low NOx option, or combination of options, is a complex task for the utility boiler operator, requiring the assessment of many conflicting technical and economic considerations. Generally, each power station, or groups of stations, must be treated individually as a specific case and the optimum option developed.

Advanced low NOx burner

Since its introduction in the late 1980s over 1600 Mitsui Babcock Mark III Low NOx Axial Swirl burners have been sold for retrofit or new boiler plant, giving one of the largest reference lists in the world for hard coal fired low NOx burners.

Air staging is achieved by splitting the combustion air into independently swirled secondary and tertiary air streams, the relative amounts of secondary and tertiary air mass flow rates being controlled by a damper incorporated into the burner design.

Swirl control of the combustion air streams is achieved by an adjustable axial swirl generator, a more efficient means of generating swirl compared to the standard radial swirl generator. Swirl levels can be adjusted independently of flow levels.

Fuel staging is achieved within the burner, by subdividing the primary air/fuel mixture into several discrete streams, with a resultant controlled variation of the fuel/air ratio around the primary air annulus. Coupled with appropriate aerodynamic flow patterns produced by the swirling combustion air and the bluff body device on the end of the primary air tube, high temperature devolatilisation of the fuel in a reducing atmosphere occurs.

NOx reductions achieved with the Mk III burner are in the range 40 to 70 per cent with many coal types, but for certain coals, eg South African, projected lower NOx emission limits would not be met with this technology. Recognising that many plants are moving to using world traded coals with lower fuel ratios, that NOx legislation will tighten in the future, that improved burners are the most cost-effective NOx reduction technique and that there was a need for advanced low NOx burners as a "plug-in" retrofit, Mitsui Babcock has developed a new high performance low NOx axial swirl burner, the Mk V.

Recently this strategy was rewarded with the winning of an order for a Mk V burner retrofit project in the USA, at Detroit Edison's River Rouge plant. This plant aims to achieve the lowest emissions possible from a "plug-in" burner.

The Mk V design has been developed using a combination of computational fluid dynamic models and full scale combustion testing in the company's new 90 MWt Multi-fuel Burner Test Rig at Renfrew.

Having validated the performance of the computer based mathematical models against both test facility and plant performance data for the Mark III burner, the computer model was used to explore variations in the burner geometry and process flow conditions which could possibly lead to increased NOx reductions.

The computer modelling indicated that the residence time of the pulverised fuel particles in the near burner recirculation zone could be increased by reducing the primary air velocity, and the size of the near burner recirculation zone could be increased by the incorporation of an additional bluff body into the burner design and by an increase in the tertiary air swirl level. Such a larger recirculation zone and an increased residence time of the fuel in the recirculation zone is conducive to NOx reduction by promoting the release of nitrogenous species from the fuel and subjecting these species to reduction reactions through hydrocarbon radicals and other intermediate species.

As a result of the computer modelling, an advanced low NOx burner design was manufactured and installed in the test facility. Test results showed that the desired near burner flow characteristics were achieved, NOx levels some 20 to 25 per cent lower than those of the Mark III burner being obtained. Carbon in fly ash levels achieved with the advanced burner design appeared to be no higher than those produced by the Mark III. A Mk V burner has been installed and extensively tested at the Cockenzie power station in Scotland.

Reburn

If natural gas is available on site, reburning is a viable NOx control option for pulverised coal fired boilers (see left). The Thermie-supported Longannet retrofit project (see Modern Power Systems, May 1998) has successfully demonstrated that gas-over-coal reburn can achieve NOx emissions of around 300 mg/Nm3 (0.25 lb/MBtu) on a 600 MWe coal, wall-fired, boiler and has developed and validated engineering and process design tools to enable the technology to be replicated on similar boilers throughout the world.

Where gas is not available, or expensive, coal-over-coal reburn technology may be appropriate. The largest demonstration yet of this technology is in start-up mode at ENEL's 320 MWe Vado Ligure plant in Italy, where the NOx emissions target is 420 mg/Nm3.

Also a Thermie project, utility partners are ENEL, ESB (Ireland), EDF, Powergen and Electricidade de Portugal PROET, while equipment suppliers include Ansaldo, Mitsui Babcock and James Howden.

Future markets in the USA

In the USA, gas-over-coal reburn has has been implemented on a number of smaller units, mainly in the northeast Ozone Transport Region, where the substitution of coal with gas has additional political benefit.

Gas reburn is expected to grow as a clean coal option in the USA, but even though reburn and the other in-furnace NOx control measures (low NOx burners and overfire air) are effective, they cannot achieve the target of 190 mg/Nm3 (0.15 lb/MBtu) @ 6 per cent O2 with the majority of US boilers. To achieve this some post-combustion gas cleaning is needed for all but the most volatile US coals. Therefore US utilities are planning to install SCR gas cleaning on their larger units. The cost of operating SCR is however directly related to furnace-exit NOx concentrations, so this creates a continuing market for in-furnace NOx control.

Meanwhile, many US utility generators are switching to cleaner sub-bituminous western coals for emissions compliance and to take advantage of lower coal prices. This is creating a requirement for coal pulveriser upgrades to process the increased coal flow resulting from the fuel switch. Mitsui Babcock has recently installed the first in a series of such upgrades at US coal plants.

The Mitsui Bacock approach is therefore to develop and offer a portfolio of low NOx technologies – an approach that ties in with the possibility of NOx allowances being traded between sources and third parties. This trading system will probably result in supercompliance on some units and undercompliance elsewhere – providing a future marketplace for a broad range of technologies.