These days, energy efficiency is not just a buzzword – it’s a fundamental requirement for a business looking to cut costs and optimize its operations. When looking to save money, a facility should evaluate the boiler. Older boilers can become less efficient over time, or they can be surpassed by newer technologies, in vessel construction, burner design, or controls.
Upgrading to a new industrial boiler presents significant upfront costs and can potentially delay a facility’s operation. Instead, individual parts can be upgraded. By assessing areas where a boiler underperforms, you can target individual upgrades that maximize the benefits – without investing the time and money to procure and install a new boiler.
If you’re looking to save on fuel costs or update your boiler, a burner retrofit is an option worth exploring.
What Is a Burner Retrofit?
A burner retrofit refers to the process of upgrading or modifying the existing boiler burner to improve its efficiency, reduce emissions, and ultimately lower boiler operating costs. The average level of efficiency for industrial boilers is only 75% to 77%. Most of the time, the loss of efficiency starts with the burner.
After assessing the conditions and identifying inefficiencies, a technician reviewing the burner can implement changes. These changes can range from minor adjustments and control upgrades to a complete burner replacement.
Burner retrofits are often considered when the burner struggles to maintain the correct air-to-fuel ratio, causing unburned fuel to deposit as soot in boiler tubes. This not only reduces the boiler’s efficiency but can also lead to damage over time.
Inefficient combustion in industrial boilers can be caused by hysteresis (an inability to repeat positions of fuel/ air controls) or by a burner’s inability to account for significant swings in supply air temperature or changes in draft conditions. Burner retrofits or control upgrades can address these issues.
Benefits of a Burner Retrofit
A burner retrofit improves a facility’s boiler system in several key ways:
- Reduced fuel consumption
- Advanced safety measures
- Enhanced reliability
Burners are the primary source of fuel usage in most facilities. A more efficient burner requires less fuel to produce the same heat output. A burner retrofit decreases emissions on top of reducing fuel consumption.
Modern safety controls improve safety by providing visibility for internal boiler functions (annunciation) and bringing the boiler’s safety systems into compliance with current codes. The service increases the burner’s reliability and reduces the risk of unscheduled downtime while extending the lifespan of the boiler.
If your boiler’s existing burner manufacturer is no longer in business, original factory parts will be less available, leading to extended downtime in the event of a failure while replacement parts are sourced.
Combustion is an act or instance of burning, typically a rapid chemical process (such as oxidation) that produces heat and usually light. For boilers and their burners, this typically involves carbon-based fuels, such as Natural Gas or Light Oil (#2).
During this process, the fuel (below comprised primarily of carbon and hydrogen) reacts with oxygen. This reaction produces heat (useful), light, and products of combustion and byproducts of combustion (these make up your flue gas).
There is a minimum amount of air needed to completely combust a given amount of fuel. This is the stoichiometric ratio, aka perfect combustion. This is only achievable in a laboratory because it requires precise control over the relative amounts of fuel and air molecules.
Since perfect combustion is not practical in real burner applications, excess air (any air not used in the combustion process – mostly comprised of Oxygen and Nitrogen) is introduced. Excess air is the result of complete combustion and eliminates the production of Carbon Monoxide (CO), which is dangerous and a marker of incomplete combustion.
Ultimately, the goal is to produce the minimum amount of excess air for complete combustion – no more or less.
Why not just add extra excess air to guarantee complete combustion? Too much of this air will carry heat away. As this air removes heat from the process, the stack temperature increases and reduces the boiler’s efficiency.
Excess air is primarily O2 and Nitrogen. When monitoring a boiler’s stack, just a 3% change in O2 present reduces a boiler’s efficiency by ~2%. For an 800HP boiler, loaded to an average of 80% of full rate, operating 4,000 hours per year, this 2% change in efficiency translates to roughly $15,000 per year.
Keeping the excess air to the minimum amount needed, over a wide turndown range (the ratio of a burner’s maximum and minimum fuel inputs) improves combustion efficiency. Modern burners can require less excess air, over a wider turndown range compared to older designs.
By supplementing improved burner designs, modern combustion controls, parallel position, or fully metered types, a burner’s air-to-fuel ratio can be tuned more precisely.
Instead of relying on a single motor, moving arms, and linkages for the fuel and air (aka jackshaft, or singlepoint), individual actuators are used to control the air and fuel independently. This allows the position of each actuator to be precisely defined through out a burner’s firing rate curve.
Linkage often leads to “bad spots” in the curve. These bad spots require sacrificing combustion at other points. For example, a burner performing well at High Fire, but too rich at Mid-Fire, may need to lean out the whole curve (add excess air). The added excess air makes the burner safer with complete combustion at Mid-Fire – but, sacrifice efficiency at High Fire.
Compared to singlepoint systems, the reduction in the amount of linkage used in parallel positioning systems reduces hysteresis. As linkages wear, they become loose, meaning the position of the air and fuel control won’t occur in the same position throughout the curve over time.
Combustion control and monitoring systems can be included in a retrofit. For example, an O2 trim is an oxygen sensor in the exhaust that will provide the boiler controller a real-time Oxygen reading. This enables the controller to “trim” the air damper to maintain an ideal air-to-fuel ratio, which allows the boiler to accommodate daily temperature changes.
In facilities with multiple boilers, a lead-lag system can sequence the boiler to meet a facility’s needs while optimizing the load on specific boilers. These systems will also rotate the boilers based on runtime (or schedule, number of cycles, etc.) to spread the wear and tear out, while ensuring no boiler sits unused for extended periods. Overall, this process will reduce cycling and limit downtime.
A burner retrofit can reduce emissions to meet current and future environmental regulations. Efficient combustion minimizes unburned fuel deposits – requiring less fuel to achieve the same output – which can significantly lower the emission of harmful pollutants.
One product of combustion is NOx or Oxides of Nitrogen. NOx is formed in all combustion processes. It is a general term that includes two forms; Nitric Oxide (NO) and Nitrogen Dioxide (NO2). NO2 is formed when NO is subjected to atmospheric Oxygen.
When NOx reacts with hydrocarbons and oxygen in the presence of sunlight, smog forms. Smog is a type of air pollution, originally named for the mixture of smoke and fog in the air. Classic smog results from large amounts of coal burning in an area and is caused by a mixture of smoke and sulfur dioxide.
In the 1950s a new type of smog, known as Photochemical Smog, was first described. Photochemical Smog is what you are most likely thinking of when you think of the hazy cloud that forms in modern, large cities. The resulting smog causes a light brownish coloration of the atmosphere, reduced visibility, plant damage, irritation of the eyes, and respiratory distress.
NOx can form in industrial boilers through three primary mechanisms:
- prompt formation
- thermal formation
- fuel formation
NOx formations can be reduced through the retrofit process.
Retrofitting, that targets NOx emissions, focuses on reducing the thermal and fuel NOx that result from combustion. Thermal NOx is the result of a reaction between oxygen from combustion reacting with nitrogen present in the air at elevated temperatures. Fuel NOx is the release of nitrogen present in the fuel source.
With coal or oil, fuel NOx can account for up to 80% of NOx emissions, whereas it hardly contributes to total NOx emissions when using natural gas.
Prompt NOx is created at the start of combustion, moment at the base of the flame when the process of combustion starts is the area of prompt NOx.
Low or Ultra Low NOx burners can be installed as part of a burner retrofit. Low NOx burners reduce NOx production to around 30PPM (versus uncontrolled NOx of ~60-70PPM).
These Low NOx burners satisfy Air Quality Requirements in effect in North and South Carolina. Ultra-low NOx burners can achieve NOx emissions below 9ppm. These burners are used primarily in California since they have the strictest NOx emission regulations.
There are a few types of Low and Ultra Low NOx burners:
- Flue gas recirculators (FGR) work by recirculating exhaust back into the combustion chamber, effectively reducing NOx concentrations to sub 30ppm. This well-known method uses cooled flue gases from the boiler stack as a source of dilution gas. They are very low in oxygen content and are composed of inert compounds like nitrogen, water vapor, and carbon dioxide, which are excellent heat sinks. These inerts reduce the peak flame temperature in the combustion chamber and reduce NOx formation. There is an increase in blower motor size with these systems, required to handle the additional mass of the Flue Gases being recirculated, as well as some additional cost in components such as FGR duct, insulation, FGR damper, and actuator.
- Premix Surface Burners incorporate surface burning, radiant technology to spread the flame out into a thin film and reduce peak flame temperatures. Fully premixed air and gas are diffused through a matrix-style combustion head, usually constructed of metal, ceramic, or fibrous materials. In our industry, these combustion heads are now constructed predominantly of special alloyed metals for durability and long life. CO emissions are extremely low and these burners provide for reliable, smooth operation. Besides the ultra-low NOx to sub 7ppm and CO emissions throughout the firing range that these burners exhibit, FGR and associated flue gas piping are not required, therefore providing lower cost and easy installation. Premix Surface burners generally require a combustion air intake filter to prevent particulates from clogging the surface burn element.
- Premix Non-Surface Burners are conceptually similar to Premix Surface Burners in that the fuel and air are fully pre-mixed, allowing ultra-low NOx and CO emissions can be achieved. The primary difference between the Premix Non-surface and Premix Surface burners is that the flame of the Premix Non-surface burner produces a more traditional, nozzle mix burner type flame in appearance, generally not requiring combustion air intake filters. These burners can be combined with FGR to achieve ultra low NOx emissions down to sub 5PPM.
As environmental regulations around NOx emissions continue to tighten, burner retrofits offer a cost-effective solution for industrial boiler operators to be prepared. Under the Federal Clean Air Act Amendments of 1990 (CAAA), states have been required to submit to the EPA for approval, then enforce, State Implementation Plans (SIP’s) to attain and maintain National Ambient Air Quality Standards (NAAQS).
NOx emissions regulation can be required in geographical areas that are in non-compliance with applicable NAAQS standards. States determine the emission requirements for boilers. Learn more about North Carolina’s boiler emission standards here, or South Carolina’s here.
By investing in advanced low-NOx burners and implementing strategies like flue gas recirculation or premix systems, you can significantly reduce your boiler’s NOx emissions, increase efficiency, and ultimately lower your boiler operating costs.
Check out our other blog to learn more about NOx.
When Should I Replace My Boiler’s Burner
Replacing a boiler’s burner is a critical decision that can impact your operations’ efficiency and cost. Boiler burners, despite their robustness, are not immortal. Depending on factors such as the type of load and conditions at your site, the practical life of the burner could range from 10-20+ years.
Beyond the lifespan of the part, many older burners (and even some newer models) are operating on the high/low/off principle, or utilizing simple modulating single-point positioning systems. This limits the adjustability and limits your control over the air-to-fuel ratio. This ultimately can lead to increased boiler cycling, high excess air, or sooting – increasing the risk of boiler failure, fuel costs, and higher emission rates.
In contrast, today’s advanced burners can offer more control. With the ability to adapt to varying operational demands, modern burners can mitigate energy waste, leading to greater efficiency and reduced costs. Ultimately, however, the right solution depends on your facility’s unique needs and your boiler system’s ability to meet them.
During a burner retrofit, a technician will identify where the system can improve – whether that’s with a new burner or with updated controls to improve your burner’s efficiency, safety, and reliability.
An important part of reviewing the efficacy of a burner retrofit is the review of the estimated payback of any efficiency gains versus the cost of the upgrade. See our blog on Life Cycle Cost Analysis for some details on what such an analysis should entail.
A burner retrofit can dramatically increase your boiler’s efficiency, resulting in cost savings over time. Moreover, it’s a proactive step towards reducing your facility’s environmental footprint. And if you’re looking to update your boiler system with a burner retrofit, connect with the WC Rouse team to find the best fit for your system.