Burners Meet High Efficiency, Low Emissions Demands
Higher turndown ratios, lower excess air levels and reduced NOX are among the benefits of today’s burners.
In recent years, burner technology advancements have led to the development of higher efficiency, lower emissions products. Burner engineers have made great strides in increasing burner turndown and lowering excess air levels even at the lower firing rates. At the same time, they have engineered ways to reduce NOX to single-digit levels.
Burner Turndown. Through an ongoing commitment to research and development, burner manufacturers have increased burner turndown. All burners have a specific turndown, ranging from 4:1 to 10:1 or higher, depending on the burner design. As the demand for steam or hot water decreases, the burner “turns down” to meet the (lower) required demand. If a burner only has a 4:1 turndown, that means it can only operate at 25 percent of firing capacity (100 percent divided by four is 25 percent). As the process’s demand continues to decrease, the firing rate must decrease. If the demand falls below the minimum turndown, the boiler quickly will meet the minimum process and then “cycle off” until more demand is needed. This negatively impacts efficiency and increases costs because of the pre- and post-purge requirements that are essentially robbing heat from the boiler.
Several years ago, the typical turndown ratio on an industrial burner was 4:1 or 5:1. With today’s technological innovations, 10:1 easily is achieved with good repeatability and excellent control of the fuel and air ratios.
Excess Air Levels. Advancements also have been made to lower excess air levels. Higher excess air translates to lower combustion efficiency. With boiler combustion, if the proper amount of air is not added during the oxidation process, unburned fuel, soot and carbon monoxide will exhaust from the stack, polluting the environment while fouling the heat transfer surfaces of the boiler. Conversely, another problem occurs when too much excess air is allowed in the combustion process: It lowers the efficiency because too much of the burner’s heat is being used to heat this additional air.
The industry standard for excess air is 4 to 6 percent O2, and the goal is to be between 3 and 4 percent O2, thereby creating a more turbulent mixture and yielding a more efficient burner. Many modern burners achieve the optimal 3 percent O2 throughout most of their firing range, which facilitates high-efficiency operation.
Operating with too much excess air reduces efficiency. For instance, suppose 3 percent O2 is achievable, but it is not being met when operating in the lower end of the firing range. If instead, the O2 level increases to 7 percent O2 — the equivalent to 45 percent excess air — there is a 2 percent loss in efficiency because for every 2 percent increase in O2 there is a 1 percent loss in efficiency.
Advanced controls and burner designs allow a burner to modulate from low to high fire while maintaining safe but low excess air levels throughout the turndown. This includes those burners with high turndown capabilities in the 10:1 range.
It should be noted that some installed burners can be field-retrofitted to achieve higher turndown, avoiding excessive cycling losses while providing better and repeatable fuel-to-air ratio control. However, if a burner has too many worn-out parts or outmoded technology, the optimum solution is not new controls. Instead, the best option is to replace the burner with one with features such as low excess air, optimum fuel/air control and high turndown capability. Depending on several factors unique to a boiler system, replacing an old burner can reduce fuel use by 5 to 10 percent.
Engineered to Meet Emissions Regulations
While a burner’s efficiency is important, given today’s tougher regulations, its emissions reduction potential is equally important. The greatest pollutant boiler stacks emit today is NOX, which is generated during combustion. Nitrogen is a harmless element in the air until it is oxidized, and then it becomes a pollutant. When NOX exits a boiler stack and mixes with sunlight and other VOCs, it forms ground-level smog.
There are three types of NOX: prompt, thermal and fuel-bound. Thermal and prompt NOX are temperature and duration dependent, which means they are propagated during the combustion process. By contrast, fuel-bound NOX, which accounts for 0.3 percent of the total NOX generated, is a component of fuel. The only way to eliminate it is to remove it from the fuel prior to combustion. Given this, a facility should focus its efforts on eliminating thermal NOX because it can be readily addressed.
During combustion, natural gas generates approximately 120 ppm NOX. Some state regulations require NOX to be below 30 ppm, and the most stringent areas require levels below 5 ppm. Fortunately, there are low NOX burners available that can reduce NOX from natural gas from 120 ppm to 60 ppm. A few burners even can decrease the NOX level to 30 ppm without any auxiliary devices post-combustion.
When a facility is trying to achieve a NOX level below 30 ppm, flue gas recirculation (FGR) typically is recommended. In a firetube or watertube boiler with FGR, exhausting flue gas is metered and mixed with the combustion-supporting (secondary) air while, at the same time, cooling the flame without quenching it. Excessive cooling of the flame will cause it to rumble, create soot and form carbon monoxide (CO), another hazardous and dangerous pollutant. For this reason, incorporating quality controls and employing a qualified technician in setup are important.
It should be noted that FGR is brought in from the stack and mixed with the secondary air supply. This combustion-supporting mix will be higher in temperature than ambient air. Based on the amount of mass flow and temperature, it may impact the size of the fan and motor horsepower requirements.
The amount of FGR needed depends upon the NOX reduction goal. The lower the NOX requirement, the more FGR will be introduced into the secondary air supply.
Typically, to achieve a NOX level less than 15 ppm, boiler turndown is affected as well. If a burner is capable of 10:1 turndown, and a 15 to 20 percent recirculation rate is required to get below 15 ppm NOX, the turndown on that burner will probably fall to 8:1. With the lower turndown, if the boiler has not been matched to the load very well, it will cycle more, which leads to pre- and post-purge losses and higher operating costs.
In a few years, the industry likely will see a burner that can achieve 10 ppm NOX without FGR and can modulate from low to high fire while maintaining 3 percent O2.
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