The Ins and Outs of Low NOx Burner Retrofits

By Larry Berry

The one constant that all boiler owners and operators face is the ever-changing environmental regulations that are in effect one day and repealed the next. Being in compliance is continually subject to the whims of new political administrations and shifting environmental policies. Where Boiler MACT ultimately ends up or how the Clean Power Plan goes forward is purely speculation at this point-in-time. One criterion that won’t likely diminish in the near term is the ever-increasing need to implement low emission solutions. It’s a reality of the world in which we do business. The quest to be greener with even stricter emissions is here to stay.

The majority of these existing and unknown future air quality requirements for stationary combustion solutions will most likely require a burner retrofit in order to meet ever increasingly stringent air quality regulations.

Low NOx burner retrofits to an existing boiler, dryer or incinerator aren’t as simple as pulling out the old burner and bolting in a new one. There is so much more that needs to be considered as part of the engineering and purchase decision process.

Low NOx burners are often the go-to-strategy in reducing stack emissions based on several different compelling factors. Burners, in direct comparison to “back-end” solutions – Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR) − are usually less costly from a total installed cost standpoint, without having to assume the burden of recurring costs derived from reagent injection and catalyst replacement. Low NOx burners have the added advantage of not requiring additional space/ducting requirements that are often associated with SCR. When combined with SCR, low NOx burners reduce the size of the SCR-related equipment (reagent usage, catalyst quantity and ducting size), resulting in one of the most cost-effective NOx solutions.

When placed in head-to-head comparison with many existing burner designs, low NOx burners have significant differences – from different fuel/air mixing designs, internal dimensions, pressure drop requirements, flame geometries and control requirements. All of these need to be thoroughly reviewed and vetted when you’re budgeting, selecting and installing new burners.

When considering a burner retrofit project you need to look at not only the burner but also far beyond. Following are the best practices that need to be key steps in the decision-making process. Many times, these can be interrelated, where addressing one issue causes problems in unrelated areas.

Examining the Dynamics Surrounding Existing Fan Capacity

The majority of current low emissions burners require relatively high air side pressure drops to achieve the desired fuel/air staging within the burner itself. Based on this design consideration, the pressure drop may be far higher than what the original burner was designed for. The dynamic of pressure drop is commonly referred to as “register draft loss” or RDL. The new RDL requirements necessitate reviewing the existing forced draft fan to assure that the fan is able to provide the static pressure to accommodate new burner systems. The onus should be placed on the burner supplier to review and confirm the capability of the existing FD fan based on both review of subject fan curves and review of boiler operating data showing system pressure drops or through the performance of static pressure testing of existing fans.

Many low NOx burners include added flue gas recirculation (FGR) that further mitigate and minimize NOx emissions. Often times, FGR rates can range from 5 percent to 30 percent of the total boiler flue gas flow. The FGR can be induced into the FD fan (commonly referred to as IFGR) and mixed with combustion air prior to entry into the burner/windbox. The incorporation of IFGR adds the mass flow requirement of the FD (and ID) fans, while at the same time increasing the furnace and system pressure drop. It is paramount to review the existing FD fan (and ID fan where applicable) to assure that the existing combustion air and flue gas systems are able to accommodate the new equipment and performance requirements.

In applications where the existing fans in operation are insufficient to meet and exceed the new performance metrics, it is desirable to investigate using larger fans and motors, utilize a separate FGR fan, or to reduce the maximum boiler capacity.

Understanding the Implications of Furnace Dimensions and Burner Spacing

The ability to lower burner peak flame temperatures is an integral strategic direction in the quest to achieve low NOx emission levels. Many current low NOx burner designs incorporate fuel/air mixing strategies to spread out and enlarge the flame envelop which leads to minimized peak flame temperature zones and lower NOx.

A thorough review of the furnace geometry is a key requirement in the ability to assure that new, potentially larger low NOx flames won’t impinge on the boiler wall surfaces – either rear or the sidewalls.

Another dynamic to factor into retrofit burner decisions for multi-burner boiler applications is the fact that low NOx flames tend to have increased diameters when compared to previous designs where adjacent burner flames can overlap to a point where NOx reduction is limited and CO emissions increase. Many times, burner internals can be custom designed to be able to produce a flame pattern that fits the majority of furnace geometries and burner spacings. This may lead to a compromise of the degree of emissions reduction if the furnace/spacing dimensions are overly constrained. Usually, burner suppliers incorporate a CFD (computational fluid dynamics) model study of the combustion process to provide the user assurances that the burner design provides the specified performance metrics within the operating and physical constraints of the furnace and burner spacing. Flame dimensions are also reduced by increasing the burner RDL, and in turn, the mixing energy. However, this will trigger a review of the FD and ID fans as described in the fan capacity analysis mentioned previously. Flame shaping and fan capacity are very often interrelated and must be considered in that context.

Examining the Relationship of Windbox Design and Dimensions

When entering into a retrofit burner situation, it is usually desirable to keep and reuse the existing burner windbox. Removal and replacement of the windbox, most notably in multi-burner applications, is a costly proposition. If the windbox requires deepening or replacement there are many downstream issues to consider ranging from space availability and component withdrawal distances on through to platform modifications and dismantling and reinstallation of windbox mounted equipment – most notably instrumentation and valves. The cost implications are substantial. With insulated windboxes, the issue of asbestos enters into the equation.

Typically, low NOx burners have larger internal length to diameter ratios when compared to conventional (register) style burners. This requires close review of the existing windbox depth. Since precise air/fuel control is essential to achieving lower NOx emissions, having equal air distribution to and around each burner is key to achieving peak burner performance.

Smaller windboxes have reduced air residence times which may result in improper air distribution between burners. This maldistribution can increase the ducting and burner pressure drops which can impact fan capacity. Adding FGR to an existing windbox further reduces the residence time in the windbox and negatively impacts air distribution.

The majority of the time, issues with windbox dimensions and distribution are overcome by performing a CFD flow model of the combustion air system (and FGR when applicable) – preferably from the fan outlet to and through the windbox. The results of the modeling often result in the need to add baffles/vanes in the ductwork and/or the windbox to assure the optimum distribution of air (and FGR) to the burners.

The Pros and Cons of Eliminating Air Pre-Heat

Many large industrial and utility boilers operating today incorporate preheating of the combustion air to help maximize boiler efficiency. Despite the advantage of better efficiency metrics delivery from the boiler, the preheating of the combustion air increases the burner flame temperature. The result is greater levels of thermal NOx emissions.

Through the elimination of burner air pre-heat, significant reduction in NOx reductions are achieved in operating conditions. Any efficiency losses are able to be offset through the addition of a boiler economizer surface to preheat the boiler feedwater rather than heating combustion air.

Although the required mass flow of air for combustion remains the same, the reduction of the air temperature in the windbox greatly reduces the volume of air entering the windbox which ultimately results in very low pressure drops across existing burners. Sufficient pressure drop (RDL) is needed to assure sufficient mixing of the fuel and air.

The burners become significantly oversized for applications that apply the elimination of air pre-heat. The associated very low throat velocities and resultant low mixing “energy” negatively impacts the ability of the burner to provide the fuel air mixing for proper operation and emissions. It also severely impacts the ability of the burner to turndown. To increase the RDL and associated mixing energy, many times new smaller burner throats or larger burner internals are required. When in multi-burner applications, an option to offset the low burner RDL is to reduce the number of operating burners per boiler.

Thorough Analysis of Furnace Waterwall Openings

Many times, low NOx burners incorporate a degree of internal staging (fuel and/or air) to achieve low NOx emissions. This may increase the throat diameter in relationship to the existing non-low NOx burners that are in place. If furnace burner walls contain boiler generating tubes, a review of the tube bending diameters around the burner openings is warranted. This exercise assures that the new burner throat diameter is not larger than the available waterwall openings and doesn’t require (expensive) modifications to the pressure components of the boiler.

Usually, the burner throat diameter can be reduced if there is interference with the furnace waterwall. This reduced throat diameter increases the burner pressure which results in the review requirement of the existing fan system − which again brings up the need to understand interrelated dynamics.

The Implication of Fuel Supply and Control Modifications

There are some low NOx burner designs that require specific fuel supply and/or control requirements. These range from multiple control valves and flame scanners all the way through to reduced boiler ramp rates, special fuel/air ratio curves and fuel and air flow measurements. It is imperative that you identify any changes required of existing fuel trains, the Burner Management System (BMS) or the Combustion Control System (CCS) and factor them into the overall assessment. FGR systems add another layer of control along with the associated additional dampers and control elements.

The Overall Performance Attributes of the Boiler

If the emissions reduction application includes FGR, additional mass flow through the boiler has the potential to impact the boiler superheat temperature to a point where superheater modifications or additional attemperation is required. This operating dynamic may require thoroughly reviewing the impact on the superheater. This is also pertinent to project applications where a new fuel is added to the boiler to assure compatibility between the new fuel’s characteristics and the installed superheater. The burner supplier should be able to perform a boiler impact study to assess the capability of the boiler internals with regard to the new burners/mass flow/temperatures/fuels.

What to Look For In a Combustion Solution Supplier

There are so many interrelated elements to factor in when considering a comprehensive burner retrofit project. It is imperative to have a close working relationship with the combustion solution supplier as so much is at stake from an operating and overall business standpoint. The match has to be right on all sides, from the burner itself to the people working the engagement.

Your burner supplier needs to reach far beyond the simple product solution. They need to be an integral partner in helping you to work through all burner project dynamics. The overall recommended product solution must go beyond the product itself to encompass all of the existing boiler ancillaries that may affect the success of the retrofit project.

Larry Berry is Director of Combustion Solutions for Victory Energy Operations.