Doing it through the application of advanced combustion engineering with minimal investment and no loss of flexibility and efficiency
By James Dennis, John Goldring, and Lawrence Berg
Conventional fossil fired boilers across the globe are being subjected to ever more stringent emissions limitations, despite the majority of those installations having been built during an era when the requirement to control such emissions was not a factor in their design. The combination of this departure from the original design intent of the firing system and the aging of associated plant systems, creates unique challenges when these boilers are modified for emissions reduction to allow continued operation.
Careful consideration of the capacity and performance of each plant system is required to ensure that a retrofitted firing system will not impose new operational limitations on the boiler. For example, a new burner must be designed to allow sufficient air-flow through the secondary air register, at the correct velocity to maintain both momentum balance and stoichiometry in the flame.
A detailed analysis is required of each individual unit to consider both the original design and limitations arising due to age related plant deterioration. Failure to complete this detailed analysis and incorporate these factors into the design of an emissions reduction retrofit can lead to critical failures, such as destruction of boiler tube walls and pendants due to increased flame length, limited ramp rates due to steam temperature excursions, large amounts of unburnts leaving the lower furnace or, in the most extreme cases, unstable and potentially unsafe combustion.
This paper explores the considerations taken by RJM combustion engineers during the design phase of a NOx reduction project, giving examples of some of the challenges seen and the methods employed to successfully overcome those challenges.
Combustion Emissions Reduction
Emissions of nitrogen oxides and carbon monoxide can be managed through proper control of combustion conditions inside the furnace; unlike coal ash particulate matter and sulphur dioxide, which require post combustion flue gas treatment such as precipitators or FGD. We describe these systems of emissions control during combustion as “primary measures”, with “secondary measures” referring to the flue gas treatment plant. NOx reduction through primary measures, when properly executed, has the potential to cut the cost of NOx compliance ten-fold when compared to the capital and operating costs of secondary measures.
Primary and secondary measures can be used in combination, to achieve the most stringent emissions requirements. For example, a low NOx firing system can be used in series with an SNCR system. The success of this combination relies on the two systems being designed such that they do not compete. If the low NOx firing system drives for the lowest possible NOX emissions, this can create CO concentrations at the SNCR level, which inhibit the reagent efficiency. The two systems must therefore be designed to complement each other, such that minimal reagent is used.
Primary measures for NOx and CO control include staging of combustion air, by reducing the air available for combustion at the burner, introduction of flue gas to reduce peak combustion temperatures and controlling the peak temperatures within the combustion chamber. Recognising the importance of these parameters relies of an understanding of the NOx formation mechanisms at work within the combustion chamber.
Nitrogen oxide formation occurs via 3 independent mechanisms: fuel NOx, thermal NOx and Prompt NOx.
In the case of coal combustion, the majority of NOx produced is from nitrogen, stored in the fuel, which is liberated during devolatilization (the process by which the particle heats and the volatile content is released into the furnace). Once these nitrogen radicals are released into the combustion chamber, they will react with available oxygen atoms to form NOx. However, where oxygen is unavailable, these nitrogen radicals are forced to revert to their diatomic state, which can be safely released to atmosphere. NOx formation can therefore be controlled by limiting the oxygen available whilst these volatiles are reacting. This is the reason that air is staged in low NOx coal firing systems; to reduce the oxygen available during the first stages of the combustion. Over Fired Air is then employed to offset this low stoichiometry by injecting large amounts of oxygen post-devolatilization, to oxidise the remaining carbon and thereby preserving combustion efficiency, though the extent of that preservation relies on the quality of the design.
When considering gas combustion, thermal NOx takes the most significant role. This occurs where nitrogen in the combustion air is broken down, to create radicals for reaction with the available oxygen. This formation rate is therefore controlled by the energy available to deconstruct nitrogen molecules in the air, making the temperature of the combustion the key parameter to control where fuel-bound nitrogen is not present. Therefore, the firing system must be designed to distribute the heat release evenly across the furnace, as demonstrated in Figure 1.
Prompt NOx occurs early on during the combustion process. Partial oxidation of hydrocarbons leads to highly reactive radicals, which react with atmospheric nitrogen.
These nitrogenated compounds then react with oxygen in a similar manner as fuel NOx. This pathway is typically only considered at very low (less than 0.03 lbs/MMBtu NOx) NOx emissions on gas burners.
Air Flow Limitations
When designing a staged firing system, the first limitation that must be considered is the limitations of the existing draft plant. Re-distributing the air vertically across the boiler can create additional pressure drops within the system, which the FD fan may be unable to overcome (not least the high pressure drops required to achieve penetration from a Over Fire Air (OFA) system). Similarly, the ID fan may not be capable of accommodating an increase in the excess oxygen level, measured at the exit of the boiler. The impacts of these limitations can be numerous and severe.
Reducing the stoichiometry at the burner requires installation of OFA to maintain combustion efficiency. This creates an additional pressure drop in the secondary air supply system given the OFA is likely to require a much higher velocity than a typical burner, if it is to achieve penetration across the entire furnace. This additional pressure demand must be accommodated by the FD fan, unless additional booster fans are fitted at significant cost.
Additionally, in the case of a coal burner where the stoichiometry at the burner is lowered, the secondary air flow rate is reduced. In contrast, primary air to fuel ratio, and therefore primary air flow rate, must be maintained for safe conveying of the required mass of fuel to achieve full load. Therefore, in order to maintain the ratio of combustion air to primary air/fuel momentum within the flame, a higher velocity is required in the secondary air register, to compensate for the reduced mass flow of air. This additional pressure demand must be accommodated by the FD fan, unless additional booster fans are fitted at significant cost.
In the case of a recent RJM project, a 200 MW forced draft unit, which had been designed for a turbulent combustion system (without emissions control), and operated at 2.5 percent excess oxygen at full load. Based on a 34 percent unit efficiency and a standard fuel specification, this unit would require approximately 430,000 CFM of combustion air. Assuming the unit is served by two identical FD fans, the required flow per fan would be 215,000 CFM. This analysis assumes that the FD fan is operating as per its design and has not deteriorated.
A typical burner might require 5” wg of pressure drop to maintain design flame dynamics, adding an assumed system resistance through the secondary air supply duct work, associated air heaters and control surfaces produces a total system resistance of 25” wg at full load. This is plotted as point 1 in Figure 2.
Based on typical furnace dimensions for this size of unit, an estimate of 20” wg is made for the required OFA duct to furnace differential pressure to provide the OFA jets with sufficient velocity to penetrate the flue gas stream and achieve the required level of flue gas / air mixing. The magnitude of this pressure requirement can be affected by the depth of the combustion chamber, the internal cross-section of the furnace and subsequent flue gas stream velocity and the accessibility to install OFA ports on multiple wall of the boiler. Where access is only available at the front wall, the velocity (and therefore pressure) demand may be much higher.
The addition of the OFA therefore, moves the system resistance curve from the orange, to the yellow position as shown on Figure 2. This is because, with the modification of the flow areas into the boiler and the increased air velocity required to achieve that flow rate through the system, a proportionately higher amount of pressure is required to realize a given flow rate.
Assuming no change in unit efficiency, the amount of fuel required for full load remains constant and therefore the required air flow for 2.5% excess oxygen at full load remains at 215000 CFM. Referring back to the fan performance curve in Figure 2, we see that the required pressure to achieve this flow rate (point 2) is now above the pressure limit of the fan for that flow rate and the fan is therefore incapable of meeting the operational requirement and instead, the maximum air flow rate is reduced to approximately 205000 CFM. This is a 4% reduction on boiler air flow, and would limit the full load excess oxygen to 1.7%; which is unlikely to be enough to maintain complete combustion.
Role of Secondary Air in Flame Dynamics
Not only would the limitation on pressure and flow affect the penetration of the OFA across the flue gas stream, as shown in Figure 3d, but it would also affect the burner’s flame dynamics. As outlined above, the momentum balance in the flame is essential for maintaining flame stability, structure and emissions performance.
Correct flame dynamics position the combustion within the depth of the boiler and retain the visible flame within the combustion chamber, or below the furnace nose, as shown in Figure 3a and Figure 3c. A weaker secondary air momentum resulting from reduced flow or velocity will produce a longer, thinner flame as the axial component of the burners momentum ratio, which is contributed mainly be the primary air, will dominate; as shown in Figure 3b. The thermal profile of these older furnaces, originally designed for turbulent burners, is centred on a large heat release near the front wall of the furnace. Longer, thinner flames push that heat release onto the rear wall of the boiler and ultimately carry flame out of the combustion chamber and up into the pendants, as shown in Figure 3d. As well as elevating temperatures at the pendants, this tube wall impingement rapidly degrades the interior tube walls of the combustion chamber.
Back to the Future
This weakening of the secondary air momentum is exactly the case in a project resolved by RJM International in Europe. Here, a low NOx firing system had been installed, with limited fan capacity and space for only one bank of OFA ports on the front wall. Failure to account for this impact on flame dynamics had resulted in long flames, with combustion carrying into the convective passes of the boiler (Figure 4). The heat release amongst the superheater pendants raised peak superheater temperatures by 75° above design; causing repeated tube failures, significantly lowering plant availability in a market where load factor would otherwise have been high.
Limited superheater temperature control also inhibited the unit’s ability to respond rapidly to changes in load demand, given such transient conditions would result in further deterioration in flame structure. In the absence of strong flame dynamics, the ignition front and fuel/air mixing are compromised when the momentum ratio is varied, such as during a load ramp when air and fuel velocities may vary independently of one another; thus exacerbating the firing systems limitations.
Through intelligent burner design and utilising advanced CFD techniques, RJM were able to custom-engineer a solution which returns flame structure and actually improves combustion efficiency within the limitations of the existing draught plant (Figure 5). Rebalancing the furnace thermal profile in this way brought superheater temperatures back to design levels and subsequently ceased tube failures. In addition, RJM delivered a 45 percent reduction in NOx performance through primary measures, improved combustion efficiency through a 1 percent LOI reduction and enhanced load flexibility as flame structure was made stable even through aggressive load ramps.
Flame Dynamics for Emissions Performance
Controlling flame dynamics in this way and ensuring complete combustion within the furnace, even at low stoichiometries, not only balances the thermal profile, but also raises the CO breakpoint; accommodating the NOx performance demanded by modern emissions legislation. As the stoichiometry in the flame reduces, flame dynamics become increasingly important as the fuel particle must be retained in the flame for as long as possible, maximising exposure to the limited oxygen available. Figure 6 illustrates this relationship between NOx performance and combustion efficiency. The effect of advanced burner dynamics is to move the CO curve to the left and to slide the Ideal Operating Range further down the NOx emissions curve.
As outlined above, though stoichiometry is the key parameter to control for coal combustion, in the case of gas combustion, the critical parameter to consider is the flame temperature. Assuming no change in the heat liberation per megawatt generated, the total thermal heat release for full load must also remain constant. Temperature is a function of the heat released per unit volume. Therefore, to reduce the flame temperature, the heat release must be spread as evenly as possible across the available space. RJM achieves this through advanced computational modelling, looking for hot spots in the combustion which are associated with localised elevations in NOx formation.
For gas combustion, Flue Gas Recirculation can also be employed to significantly reduce flame temperature, whilst maintaining the flame dynamics required for stable combustion. This is because of the reduced oxygen concentration of the flue gas. When atmospheric composition combustion air is replaced with low oxygen flue gas, the effect is to average the oxygen content proportionately. For example, if 90,000 cubic feet of atmospheric air (at 20.9% oxygen) is mixed with 10,000 cubic feet of flue gas with only 2.5 percent oxygen, the result mixture will be 100,000 cubic feet of gas with an oxygen concentration of 19 percent. Reducing the available oxygen has the same effect as a reduction in fuel; it limits the reaction rate. However, flue gas recirculation has further benefits.
The flue gas serves to temper the combustion, as the higher volume of inert gas absorbs heat from the flame.
It is using these mechanisms that RJM has been able to deliver up to 60 percent NOx reduction with the incorporation of flue gas recirculation. RJM’s CleanAir Burners are now achieving emissions performance as low as 0.02 lb/MMBTU incorporating FGR with advanced combustion dynamics.
Enhanced Flame Dynamics for Minimum Load Reduction
The same principles of flame dynamics can also allow for enhanced stability at low load. The critical parameter with regard to flame stability is maintaining the flame front at the burner, to ensure fuel entering the furnace is ignited on entry. Attention to near burner dynamics will allow for enhanced stability ranges and in a recent project has allowed a reduction in minimum stable generation on a 165MWe front wall fired gas unit; from an original MSG of 35 MWe, down to 13 MWe.
Operationally, though the plant was required to stay online, acting as spinning reserve to provide back-up for wind generation. However, economics dictated that generation be as minimal as possible to conserve fuel consumption. To accomplish the reduction in MSG, among the issues that required resolution, was ensuring that the burners would remain stable as fuel pressure was lowered. Near burner flames are compromised when either the local mixture falls outside stability limits, or when the fuel/air velocity exceeds local flame speeds.
“Combustion control in conventionally fired boilers is paramount for maintaining combustion efficiency, emissions performance and boiler integrity.”
For this project, a particularly stable near-burner aerodynamic region was engineered. This was accomplished using RJM’s proprietary technology; which allows for a constant aerodynamic stability region to be created. This region is of nearly constant relative strength across a significant variation of secondary air velocities. The distribution of fuel and air at the burner face actually accommodates an emissions reduction across the load range.
Combustion control in conventionally fired boilers is paramount for maintaining combustion efficiency, emissions performance and boiler integrity. In the absence of comprehensive consideration of all parameters which can affect and be affected by the combustions system, all of these performance parameters are jeopardised. Equally, advanced combustion control can offer emissions reduction at significantly reduced cost over more expensive flue gas treatment processes.
A key consideration in the design of a staged firing system, is the availability of pressure and flow from the existing draught plant. Modifications to burner hardware and the additional pressure required for OFA jet penetration of the flue gas can quickly increase the required pressure beyond that capacity of the existing fans.
As well as the requirement for OFA jet penetration, pressure and flow must be maintained at the burner to ensure flame structure is held at all times. This is particularly difficult during transient conditions when variation in air and fuel flows can move independently of one another. Failure to maintain flame structure produces longer flames and can allow combustion to carry into the convective section, causing irreparable damage to the heat transfer surfaces.
RJM International has demonstrated through multiple successful projects that firing systems can be designed to meet even the most challenging performance requirements, under some of the most challenging conditions. Key to that success is a detailed, comprehensive analysis of the plant and intelligent design to mitigate those limitations.
James Dennis is lead combustion engineer at RJM International. John Goldring is managing director at RJM International. Lawrence Berg is vice president of Engineering at Systems Analyses and Solutions Inc.