While post-combustion NOx control techniques are proven and effective, avoiding the formation of NOx rather than removing it afterwards is a preferred route, writes Paul Kazalski
|Coal-fired plants should assess the entire range of NOx reduction techniques|
Tightening NOx emissions limits around the world are making it more difficult for coal-fired power plants to meet compliance levels.
Existing and pending NOx regulations in the US, Europe, and Asia, for example, impose NOx emission limits in the 30-200 mg/Nm3 range.
Plant owners often must decide among multiple undesirable outcomes: shut the plant down, invest in expensive emissions control equipment to comply with regulations, or change the operating mission of the plant while still meeting applicable emission requirements.
For NOx control, while technologies such as selective catalytic reduction can typically provide the reductions necessary, the cost is quite high. To optimize plant value, facility owners should consider other operational changes and equipment upgrades that can provide substantial NOx reduction at lower cost.
In developing an effective NOx reduction strategy, plant owners must carefully evaluate and balance three elements:
Compliance approach: The NOx reductions strategy should take into account the emissions limits for each plant in accordance with both current and expected future regulations. This usually includes provisions for buying or selling NOx credits if a market is available.
Operational strategies: Owners should evaluate the likely operational modes for the plant. For example, running baseload versus intermittent or load following will affect NOx emissions, as will shifting to a cleaner coal.
Technology selection: Plant owners must compare the procurement and installation of new equipment with the enhancing or upgrading of existing equipment. Such a comparison will take into account, for example, the trade-offs between capital/operating costs and site conditions (such as space limitations).
The plant’s compliance and operational strategies will dictate the NOx targets that must be achieved by the chosen NOx control technologies. Therefore, compliance and operational strategies should be fully evaluated before focusing on technology selection. In some situations, the best approach may be to specify and procure new equipment. In other cases, however, a dispassionate analysis may identify lower-cost opportunities that can provide significant NOx reductions.
Specify and procure
If the NOx reduction strategy indicates that the best approach involves new equipment, coal-fired power plants typically consider four types of equipment modifications, upgrades and additions:
1. Low-NOx burners (LNBs) use fuel staging, air staging, balancing, and air/fuel ratio control to reduce flame temperatures and thereby convert fuel nitrogen into nitrogen gas. Proper air and fuel flow distribution between burners is critical to achieving the desired NOx reduction. LNB side effects include higher levels of CO and unburned carbon (UBC), which can impact boiler efficiency.
2. Overfire air (OFA) is a NOx reduction technique that removes a portion of the combustion air from the burner windbox and injects it into the upper furnace through OFA ports above the burners. This technique provides an additional layer of air staging to the furnace zone. By decreasing the burner’s air/fuel ratio, the oxygen concentration at the flame is reduced, thereby reducing thermal NOx. As with LNBs, side effects include higher levels of both CO and UBC.
3. Selective non-catalytic reduction (SNCR) is a post-combustion process involving the injection of either ammonia or urea into the firebox of the boiler at a location where the flue gas is between 1,400 and 2,000 °F (760 and 1,090 °C). The ammonia reacts with NOx formed during combustion to form nitrogen, CO2 and water. If urea is used as the reagent, it is converted at these temperatures to ammonia and CO2. The major side effect of SNCR is unreacted ammonia remaining in the flue gas, or ammonia slip. Note that SNCR may be difficult to apply to variable load units because the flue gas temperature must be between 1400 and 2,000 °F.
4. Selective catalytic reduction (SCR) is the most powerful post-combustion NOx reduction process for coal-fired power plants. Ammonia or urea is injected into the flue gas upstream of a catalyst bed, and NOx is then converted to nitrogen and water on the catalyst bed. If urea is used as the reagent, CO2 will form as a byproduct. As with SNCR, the major side effect for SCR is ammonia slip.
These NOx reduction techniques can be used alone or in combination with one another to achieve specific levels of NOx reduction, as shown in Figure 1 (above). SCR typically can achieve 90 per cent or greater NOx reduction on its own, while combinations of the other techniques can exceed 70 per cent with proper tuning and operation. Figure 1 (above) provides estimated NOx reduction levels that are achieved with combinations of different NOx-reduction technologies.
SNCR and SCR, while powerful NOx reduction tools, are ‘big ticket’ items compared to LNBs and OFA. Both SNCR and SCR require the use of ammonia or urea, and ammonia slip must be managed, resulting in increased maintenance costs and a higher risk of higher forced outage. Moreover, with SCR, catalyst management plans must be developed and followed because catalyst deactivation occurs over time. Table 1 compares the relative costs of the four techniques, with SCR representing the baseline at a cost of 1. To achieve higher levels of NOx reduction, costs increase as additional techniques are added.
It is always less costly to prevent the formation of NOx than it is to eliminate NOx later on. Before specifying and purchasing NOx reduction equipment, therefore, plants should examine cost-effective ways to get the most benefit from existing equipment for the targeted fuel range. Selection of such ‘smart’ strategies will depend on the performance needs and expected operation of the plant.
An objective assessment of the boiler’s current condition (incorporating testing and validation techniques) will help quantify improvement opportunities. For instance, recording and analyzing the results of adjustments in OFA flow, burner vanes, air-to-coal ratios, and mill classifier speeds can provide data to inform the identification of “low-hanging fruit” that can produce large NOx reductions.
A variety of minor equipment upgrades on both the fuel and air side can provide low-cost NOx reductions. Table 2 (below) summarizes how several coal-fired power plants around the world have applied these techniques and the successful results achieved.
• Fuel Side Upgrades: Typically, it is possible to reduce NOx by improving coal balance and coal fineness. Better fuel balance creates more consistent flames across the boiler, resulting in better combustion that eliminates ‘pockets’ of CO and UBC. Balancing coal flow also minimizes peak furnace temperatures to keep NOx low. Finer coal particles do not directly produce less NOx, but they produce less UBC and CO, which allows operators to take greater advantage of air-staging in order to reduce NOx further. Common methods of upgrading the fuel side of the combustion system are outlined below:
• Retrofit mills with dynamic classifiers to increase coal fineness and/or throughput. Compared to static classifiers, dynamic classifier offers improved fuel fineness and provides more even distribution among the outlet coal pipes.
• Install adjustable riffle distributors to balance air and coal between coal pipes exiting a junction.
• Install coal distribution control vanes over a section of a mill exit port to control air and coal flow to specific ports. These types of control vanes are ideal for addressing persistent imbalance issues.
• Air Side Upgrades: Air imbalances, such as those shown on the CFD image in Figure 2, cause oxygen imbalance in the furnace, increase pressure drop, and generate high levels of UBC and CO. In contrast, Figure 3 shows the same windbox with vastly improved air flow characteristics. Methods of improving the combustion air flow are as follows:
• Install inexpensive windbox vanes to achieve proper windbox balancing.
• Adjust windbox control dampers to equalize flow among ducts and account for changing operating conditions. Balancing air flow enables operators to lower excess air without forming CO pockets.
• Position duct turning vanes to reduce pressure drop during turns and to direct secondary air into hard-to-reach areas. Well-positioned vanes can equalize flow between ducts.
• Adjust perforated plates to ensure secondary air enters from all sides of the burner register.
Left: Figure 2. Velocity distribution without windbox vanes.
Right: Figure 3. Velocity distribution after windbox vanes installed
While post-combustion NOx control techniques are proven and effective, avoiding the formation of NOx rather than removing it afterwards is typically the preferred route.
Moreover, boiler NOx reductions through effective tuning, controlling fuel fineness, and balancing air and fuel often coincide with good engineering practices, smart testing, and smart tuning.
Coal-fired power plants should assess the entire range of available NOx reduction techniques – in light of overall emission compliance and operational strategies – prior to making investment choices.
Retrofit MBF vertical spindle mills with new Aeroport design
Amec Foster Wheeler’s MBF prior art mill technology uses a two-piece airport design, consisting of upper and lower components seated against the stationary seal ring attached to the mill housing. This design gives rise to an uneven velocity profile across the port opening, which can result in ultra-high velocity flow streams, increased pressure drop, and accelerated wear. Further, as the airport wears, its cross-sectional area increases, resulting in reduced air velocity through the airport.
Amec Foster Wheeler’s latest design, the single-piece Aeroport, features a contoured profile to improve air flow velocity distribution. The Aeroport design provides a balanced exit velocity from the port, reduced pressure drop, and improved wear life properties. Also, lower primary air is required for fuel entrainment, supporting staged combustion.
Paul Kazalski is a combustion engineer and is responsible for designing the low-NOx burners and overfire air system at AMEC Foster Wheeler