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Today’s focus on reducing greenhouse gas emissions creates challenges for operators of natural-draft fired heaters. For hundreds of years, natural-draft heaters have provided the necessary heat for process applications. They often are chosen for installations that do not have access to grid power, and they continue to have a significant presence in industry today.

Natural-draft heaters utilize convection created by a heated stack to draw in the air required for combustion. This draft is set manually during commissioning and cannot be adjusted automatically to adapt to changes in load, weather or other factors that may impact heater operation and efficiency. For this reason, a safety margin is added to ensure that the burner is not running rich and creating both an environmental and safety hazard. This safety margin has the unfortunate side effect of reducing the heater’s combustion efficiency.

Maximizing combustion efficiency in natural-draft heaters is key to reducing associated greenhouse gas emissions. A typical 1 million BTU/hr natural gas-fired burner emits an average of 1.3 metric tons of carbon dioxide (CO2) per day. This emission is equivalent to burning 1,454 lb of coal or traveling 3,306 miles in a passenger car. The CO2 emissions are directly proportional to the number of BTUs produced by a heater.

The following information relates to indirect gas-fired natural-draft process heaters. Multiple factors — including burner tube design, secondary air control (SAC), fuel train operation and preventive maintenance — can significantly influence the overall combustion efficiency.

Properly set up natural-draft systems can reach 80 percent combustion efficiency utilizing current technology. This efficiency, along with the correct burner control and more advanced combustion management, can significantly reduce greenhouse gas emissions in natural-draft systems. Photo credit: Profire Energy Inc. (Click on the image to enlarge.)

Burner tube designs must be sized to allow adequate heat flux to transfer the required heat to the process. Improper tube design can lead to excessive heat loss through the burner stack and premature burner tube failure. The heater tube stack height and flame arrestor must be designed properly to draw the required air into the burner tube to support clean combustion.

Innovations in gas-fired burners allow burner technicians to make adjustments that maximize usable heat. These improvements can offer emission savings on new equipment as well as older systems already in operation. Burners need to be rated and sized to provide the gross heat input required to meet the net heating requirements of the process.

The use of a stack combustion analyzer provides the information needed to set the primary air mixer on a burner properly. The primary air mixer controls the amount of premix air and fuel combusted at the burner nozzle. The primary air mixer will allow a clean, stable flame at the burner tip when adjusted properly.

Burners that include a secondary air control, or SAC, provide the burner technician  the ability to balance secondary airflow through the burner tube. Controlling secondary air is critical to maximizing the system’s combustion efficiency. Data from the combustion analysis allows the technician to adjust the shutter on the SAC to a level that supports clean combustion and limits the draft velocity, retaining more usable heat. Combustion analysis has shown gains of 3 to 15 percent in combustion efficiency on heaters that include a secondary air control device. This gain in efficiency directly reduces the amount of fuel combusted and the resulting CO2 emissions.

Fuel train selection and type of control can affect overall emissions from a heater. Fuel trains that operate based upon on/off main burner control operate at a lower efficiency than systems that offer consistent modulating fuel control. The lower efficiency in on/off control is caused by the cooling in the burner tube during the off cycle. When a heater reaches the desired setpoint and the main burner is shut down, the burner tube continues to draw outside air through the system. The outside air that is naturally pulled through the burner tube creates a cooling effect. This cooling must be overcome in the following main burner on cycle.

Use of a stack combustion analyzer provides the information needed to set the primary air mixer on a burner properly. The primary air mixer controls the amount of premix air and fuel combusted at the burner nozzle. Photo credit: Profire Energy Inc. (Click on the image to enlarge.)

By contrast, modulating fuel train control provides an efficient approach in natural-draft heaters. A temperature control valve (TCV) modulates the fuel gas to the main burner based on heat demand. Temperature control valves can be pneumatically (I/P) or electrically (4 to 20 mA) actuated. The temperature control valve throttles the fuel gas flow to the main burner based on a signal provided by the burner management controller. The modulated flow increases the main burner on time but limits the burner’s firing rate percentage. A system using a TCV may operate the main burner constantly at a 40 percent firing rate versus on/off control with a 100 percent firing rate. The constant heat introduced to the burner tube eliminates the cooling effect created during on/off operation. The modulated control approach uses less fuel gas, leading to a reduction in overall emissions.

Modulating control still has limitations, however. Natural-draft burners are optimized for a 100 percent firing rate during the initial setup. As the fuel input is reduced, the burner requires less air for combustion, and more of the air slips around the burner through the SAC, which is optimized for maximum combustion air. With the push to reduce greenhouse gas emissions further, operators of natural-draft burners will require more advanced methods of managing the combustion to optimize efficiency regardless of changes to external conditions, load, fuel composition and other similar factors.

This typical natural gas fuel train design includes an electrically actuated TCV for modulating burner operation. Photo credit: Profire Energy Inc. (Click on the image to enlarge.)

Utilizing an intermittent pilot burner also can lead to emissions savings in natural-draft heaters. Numerous older natural-draft heater systems operate with a continuous pilot. These pilot burners operate continuously whether a system requires heat or not. Many modern control systems allow the pilot burner to shut down at the desired temperature above a process temperature setpoint. When the heater requires an increase in process temperature, the pilot is ignited and establishes adequate flame quality. The main burner then is fired to produce the required heat. Using intermittent pilot options helps maximize fuel savings and minimize the environmental impact.

When properly tuned, maintained and managed, a natural-draft burner can come close to matching the combustion efficiency that could be seen with a forced-draft burner. To push either a natural-draft burner or a forced-draft burner significantly above 80 percent combustion efficiency, the latent heat trapped in the flue gas water vapor needs to be recovered. Large efficiency improvements can be achieved by condensing the water vapor, which is a primary product of hydrocarbon combustion. Natural-draft burners face challenges in recovering the latent heat from the flue gas water vapor. Maintaining proper heat in the stack while removing the water vapor would be a significant technical challenge; however, the potential efficiency benefits will offset much of the cost and complexity of a latent heat recovery system.

A well-designed and maintained system will deliver the heat required with minimal environmental impact. Photo credit: Profire Energy Inc. (Click on the image to enlarge.)

Preventive maintenance is essential in all heater systems and can impact greenhouse gas emissions. In many cases, natural-draft burners are being over-fueled to increase heat output. Unfortunately, this leads to increased emissions without the desired increase in process temperature and excessive carbon buildup in the heater. Regular inspection and cleaning of natural gas burners along with tuning burners to changing site conditions is beneficial. Fuel-train regulators and valves need to be inspected regularly for proper setpoints and operation. A well-designed and maintained system will deliver the heat required with minimal environmental impact.

Natural-draft heaters present a significant challenge for operators when dealing with increasing environmental pressure. High BTU applications with limited availability to grid power may require that many operators continue to rely on natural-draft systems. Properly set up natural-draft systems can reach 80 percent combustion efficiency utilizing current technology. This efficiency, along with the advanced burner control and combustion management, can reduce greenhouse gas emissions in natural-draft systems.