Retrofiting an economical, ecological solution
If your boiler/burner package is like the more than 200,000 commercial- and industrial-boiler packages currently in operation, it probably is more than 20 years old. It most likely is only 60- to 70-percent efficient, and large amounts of fuel and associated cash are wasted with every minute the equipment is operated.
Burning money unnecessarily in the current economy simply is unacceptable. While a new boiler package would fix the problem, such a significant investment (usually $100,000 or more) may be difficult to justify during these challenging economic times.
Fortunately, less costly retrofits and upgrades can be made to an existing boiler package, increasing efficiency and reducing fuel usage. Assuming a boiler vessel is in good shape, the most immediate opportunities to cut boiler-room fuel use and electricity costs involve burners, burner components, and controls. This article will examine various burner retrofit options.
Replacing a Burner
The burner is the true driver of a boiler system's fuel use and costs. After about 20 years of service, a typical burner simply gets tired. Linkage joints, cams, and other moving parts wear out, and a burner's ability to keep tight control of its air-to-fuel ratio degrades. The result commonly is referred to as “hysteresis,” which, in turn, results in a burner's inability to maintain desired excess-air levels across the firing range for optimum combustion. Higher excess air means lower combustion efficiency. A legacy burner can suffer from a host of other issues, including plugged or deteriorated nozzles and gas orifices and deterioration of other combustion-head components responsible for proper fuel and air mixing. All of this results in unburned fuel (carbon monoxide, hydrocarbon, etc.) and higher-than-required excess-air levels, leading to poor performance, overall efficiency reduction, and unnecessary spending.
If a burner has too many worn-out parts or is based on outmoded technology, adding new controls probably is not going to fix the underlying issues. Instead, the old burner can be replaced with a new one that features advanced controls, higher turndown capability, and lower excess-air requirements. Depending on a variety of factors unique to each system, replacing a burner can reduce fuel usage by 5 to 10 percent.
Older burners also are out of step with modern, more stringent emissions regulations established by the U.S. Environmental Protection Agency (EPA) and related air-quality-management districts. For systems that include a legacy burner, the looming threat of financial penalties for violations of EPA standards more than likely outweighs the upfront costs of replacing old burners with newer models that emit significantly less pollutants.
Replacing a legacy burner with a new burner that incorporates high-turndown (HTD) capability is one of the most beneficial upgrades available. While older burners typically operate in a narrow turndown range, HTD burners continue operating at even lower firing rates to meet lower loads.
With greater turndown capability, a burner experiences fewer cycles. Before each firing cycle, a burner must perform an automatically controlled pre-purge, during which fresh air is blown through flue-gas passages for a specified amount of time. Depending on several factors, such as agency and insurance approvals, a burner also may be required to perform a post-purge sequence. The purges rid the furnace of any unburned fuel that may have pocketed and, therefore, could have ignited in an uncontrolled condition. In many applications, a post-purge must commence once a load is satisfied, after which the burner proceeds to shutdown or standby mode. Any time cooler air is blowing through a hot boiler or heat exchanger, heat loss occurs. Therefore, on subsequent firing cycles, the burner must raise the temperature of the water or fluid to be heated to make up for the heat losses associated with pre- and post-purging. The fan motor must be activated for each purge, adding to unnecessary energy costs.
HTD burners can reduce these cycling occurrences and related expenses dramatically. When an HTD burner is used, the boiler is hotter when a new cycle begins, reducing the amount of energy required to return to set point. Additionally, an HTD burner is less likely to overshoot the desired set point and waste more fuel as the burner oscillates (over- and undershooting) while trying to meet the desired pressure or temperature.
A reduction in cycles also means a decrease in failures brought on by cyclic fatigue, reducing ongoing maintenance costs.
Upgrading an Existing Burner
If a burner meets emissions standards and retains a satisfactory ability to track at good excess-air levels throughout its firing range, it may be a candidate for various retrofit-technology upgrades.
New burner controls incorporating a programmable logic controller (PLC) or microprocessor-based technology offer additional opportunities to improve a boiler package's performance and efficiency through precise and customized control.
While traditional controls use “single-point” positioning, incorporating shafts and linkages for air-to-fuel-ratio control over a firing range, parallel positioning allows independent control of air and fuel flows. Because desired and exact air-to-fuel ratios can be set at each point in the range, proper oxidation is ensured.
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Traditional single-point-positioning systems typically compromise excess-air levels at firing rates between low and high fire because of the inherent difficulty of setting up linkages tied to a single positioning point, or “modulation” motor. Parallel-positioning systems drive air, fuel, and possibly low-nitrogen-oxide flue-gas-recirculation (FGR) metering valves independently via a preprogrammed control system for specified air-fuel-FGR ratio curves set up by the burner commissioner.
Able to provide accuracy within one-tenth of one angular degree, parallel-positioning motors, or “servos,” are set to predetermined positions throughout the firing range to provide optimal air-fuel-FGR conditions, matching burner capability and load conditions. Because keeping excess air to a minimum throughout a firing range is crucial to combustion efficiency, parallel-positioning capability can improve repeatability and reduce fuel consumption by up to 5 percentage points.
In addition, oxygen (O2) or O2-trim systems can work with parallel positioning to manage excess-air levels even more precisely. Changes in barometric pressure, temperature, fuel heating value, and other environmental conditions can drive excess-air levels up and down throughout the heating season. When O2 trim is utilized, an O2 sensor measures the flue-gas O2 concentration and compares it with the set-point value at each position in the firing range. If not on set point, the fuel or air motors or servos driving valves and dampers are “trimmed” to return air-fuel-FGR ratios to set point. This feature also can improve system fuel utilization by up to 5 percentage points.
Adding a variable-speed drive (VSD) or a variable-frequency drive (VFD) also can improve a combustion system's efficiency. With older burners, combustion-air or blower/motors are set at a constant speed, regardless of a system's load. Combustion-air flow rates are metered by dampers. Therefore, while the heating load may be 30 percent of full load, the electrical load on a blower motor may be much greater, 70 percent or more.
Because it is useless to have a boiler that fails to deliver enough hot water or steam when needed, boilers typically are oversized for their applications. To that end, inherent capital and installation costs preclude the installation of undersized boilers. Therefore, in many boiler installations, the burner operates for significant periods at low fire while pulling electrical loads on the blower motor much closer to high-fire usage, resulting in wasteful electricity consumption and higher-than-necessary electrical bills.
VSD pull less excess electrical current because they slow fans to lower firing rates. According to “fan laws,” this results in lower motor-horsepower requirements and electricity usage. Many boilers are oversized from the outset and, therefore, operate at lower loads nearly all of the time. When VSD are incorporated, electricity savings add up quickly, often paying for the upgrade in less than two years.
Because they operate at lower fan speeds, incorporating VSD/VFD also can reduce noise levels. In general, the faster a fan rotates, the more noise it emits. Boilers typically operate at lower-than-maximum firing rates and, therefore, function more quietly when VSD/VFD are added. In addition to creating a quieter environment, this can decrease maintenance costs because the stress on moving parts is reduced.
PLC- and microprocessor-based controls systems are particularly useful for boiler and burner sequencing in complex HVAC systems that include two or more boilers. Rather than sizing each boiler to handle the full load requirement — a peak load that rarely is reached — multiple boilers can be sized to meet part of the load and fired/modulated as needed. Over the life of each boiler, sequencing helps reduce energy use and wear associated with the cycling of boilers and associated boiler/burner components. Sequencing also provides the insurance of redundant equipment, as well as flexibility for scheduled maintenance.
Proper Burner Integration
Proper integration of a burner to a boiler and ancillary boiler-room systems provides optimal control to meet plant demand as well as integration with advanced building-management and communications systems.
Together, many of these upgrades and additions, if integrated properly, can yield a 15- to 20-percent energy savings over time.
A boiler system often incorporates equipment from a variety of suppliers following different communication protocols. Therefore, part of burner integration is accommodating diverse equipment, varying communication protocols, and getting everything to work together to produce optimal results. Only knowledgeable and experienced boiler/burner personnel and combustion controls engineers are qualified to address these issues. To do their jobs correctly, they must have a full grasp of what is required by a particular installation and the technology that is available to meet those needs.
The importance of matching a burner to a boiler cannot be understated. Perhaps the most important factor is the characteristics of the burner's flame. The flame's shape and length, or flame envelope, must be matched to the furnace or combustion chamber to transfer as much heat as possible without impinging on the vessel's walls in a manner that could be detrimental to the vessel or fire-side materials.
Another key aspect of matching a burner to a boiler or heat exchanger deals with a phenomenon known as combustion noise, combustion vibration, or “combustion rumble.” Every boiler assembly has its own resonant frequency, so a burner's combustion characteristics must integrate well with a boiler's acoustical nature. Because most burners are not custom designed for each application, a burner must be flexible in its design so that any undesirable combustion noise can be “tuned out” for smooth operation throughout the firing range.
Additionally, a burner should be constructed of castings or heavy-gauge materials and spinnings. These materials afford strength to burner surfaces, reducing unwanted high-and low-frequency vibrations. Without heavy-duty construction, the whole package may vibrate at certain loads, yielding unacceptable noise and undue system wear and tear. Burners constructed of light-gauge sheet metals and large, flimsy surfaces may be less expensive up front, but they are noisier, have a much shorter life expectancy, and generally end up being more expensive over time.
Lastly, a new burner should be designed to be easy to inspect and maintain on a regular basis. Easy access for inspection and component replacement is a key element of good burner design. Service personnel should not have to pull a burner off of a vessel or — worse yet — crawl through a vessel to get to a burner for maintenance.
In the final analysis, even with all of the latest technology incorporated into a state-of-the-art burner, regular maintenance is necessary to ensure an optimal air-to-fuel ratio throughout a boiler's firing range and equipment life. If you keep a close eye on your equipment, you will see efficiency improvements and lower fuel costs for years to come.
The president of Industrial Combustion LLC, Michael Valentino is a 30-year industry veteran. His expertise in steam generation, hot water, and thermal-fluid heating spans many disciplines, including combustion, thermodynamics, heat transfer, fluid flow, research and development, burner design, and boiler-room installation.