Many industrial manufacturers are learning that pulsing burners — from high fire to low fire — is a quick way to reduce costs, improve safety and meet emission-level targets for industrial furnaces and ovens. Upgraded systems enhance efficiencies and production processes and help users gain the edge on the competition and generate greater revenue and profits.
Pulse-firing benefits include:
- Reduced emissions.
- Temperature uniformity.
- Improved process control.
- Enhanced output.
- Extended equipment life.
- Improved product quality.
- Increased safety.
- Fuel savings.
So, how does it work? Pulse firing is meant for multiple-burner combustion systems. In pulse firing, the heat input is controlled on each individual burner by modulating the frequency of the high and low firing ranges (figure 1). This results in specific control, flexibility and precision. In effect, the burners are fired at high fire for set periods of time and then cycled to low fire or turned off. This firing cycle is repeated throughout the course of the production period and is monitored by the process controller via the control algorithm within a programmable logic controller (PLC) or pulse-control module.
Unlike the typical linkage controls that modulate air valves — often found on combustion systems within the United States — pulse-firing technique requires an air solenoid valve at each burner as well as a ratio regulator on each burner’s gas train for individual burner air/fuel adjustments. The burners are reliable and designed for high duty — more than 10,000 cycles.
Furnace chamber dimensions also must be analyzed. Sometimes, burners are added or relocated to optimize turbulence and minimize cold spots. (If the burner chamber is too large, the pulse-firing method would be ineffective, and the appropriate temperatures, mixing and uniformity may not be achieved.)
Modifications to pulse-fired systems are completed electronically via the PLC or pulse-control module. The computer control ensures automated, safe and simplistic startup. Complicated piping design becomes a non-factor, and commissioning time can be reduced for pulse-fired systems compared to cross-connected combustion systems.
Pulse-fired systems are efficient because they operate at high fire and on ratio. The high turbulent chamber mixing shortens temperature ramp-up times and increases uniformity. This helps create the ideal conditions for higher product quality and production rates.
Equipment itself also may see extended useful life from intermittent operation at its peak firing rate. Specific burner types like radiant-tube burners encounter extended tube life from a more uniform tube temperature. Flat-flame burners maintain their intended flame shape from the high velocity design of pulse firing, reducing the possibility of flame impingement or rogue flames that damage refractory.
Another attractive attribute of pulse-firing systems is the innate reduction of burner emissions. When there is low excess air and more gas being consumed, NOX levels are low, helping to meet regulatory emission targets.
The flexibility of being able to tune and control each burner is, by far, the most substantial benefit of pulse firing. Typically, the burners are tuned to one rate: high fire, which allows for straightforward maintenance and troubleshooting as well as sustained on-ratio air/fuel mixtures. Pulse-firing systems can see up to 30 percent in fuel savings because they are not constantly firing or adjusting from low to high fire, which can reduce the accuracy of the air/fuel ratios over the firing ranges. Also, when the pulse-fired system is firing, the fuel used is benefitting production rather than being wasted by lingering in the combustion chamber and escaping through small leaks or cracks or accumulating to produce a potentially dangerous situation.
Case in Point: Stress-Relieving Furnace
A Fortune 500 metals corporation recently upgraded to a two-zone, pulse-fired control system on a car-bottom, stress-relieving furnace that was operating as a Class 2 furnace at temperatures up to 1050°F (565°C). The previous control used an obsolete pulse-firing system that required an increasing amount of maintenance and could not achieve the desired, lower temperature setpoints.
In the furnace, each zone on the system contained six direct-fired, medium-velocity burners, which were reused during the upgrade. The new on/off control system operates the burners only at their most efficient high fire rate. Code-compliant burner safety-shutoff valves allow for individual burner shutdown. As heat demand increases or decreases, the control algorithm turns on or shuts off select burners, resulting in optimum fuel usage for those select burners.
Originally, the furnace was pulse fired with optional impulse-bleed control for high excess air operation. The pulse controller was an obsolete packaged unit that is no longer available, so it was removed. The existing PLC was replaced with a safety PLC compliant with National Fire Protection Association (NFPA). The PLC provides advanced, configurable safety logic, a human machine interface (HMI) touchscreen and troubleshooting capabilities such as control overview and equipment permissive screens. Modern pulse-firing controllers with rack-mounted burner management system (BMS) units on the control-panel door and new gas valve-train components completed the install.
The temperature control-loop logic in the new PLC maintains up to three sets of proportional-integral-derivative (PID) constants. The PLC varies the active PID settings and setpoint ramp rates depending upon temperature difference (ΔT) from setpoint. This provides the ability to use more aggressive PID constants and ramp rates when further from the target setpoint and less aggressive settings when closer to setpoint to prevent overshooting. The control-loop setup screen (figure 2) allows the operator to modify PID settings and provides a pictorial view of the current switch points. It also identifies which set of PID parameters is in use at the moment (figure 3).
The operator uses the system control screen to ascertain control and status during normal operation. For example, the HMI screens display which burners are at high fire, low fire or off. If an equipment or control problem arises, the control system alerts the operator of all inputs that the PLC code is expecting for a certain function. These tools point maintenance personnel to the exact location of a given problem and reduce the time it takes for troubleshooting (figure 4).
For the metals company, the result was a furnace that is operator friendly, has Class 1 (±5°F) uniformity with accurate pressure control, excellent turndown via shorter pulse times and low fire settings while high/low pulsing. With a flick of a switch, the system can be modified to pulse high/off if additional turndown is required.
The project took a total of four weeks (three weeks for installation and one week for commissioning). The initial investment is expected to be offset by reduced maintenance times, lower fuel usage, improved temperature uniformity and compliance with NFPA 86 requirements. Already, the metals company experienced an immediate decrease in fuel usage of 5 percent.
In conclusion, the benefits of implementing better process control via upgraded combustion control systems are vast and impactful. Depending upon the system’s initial setup — from air/fuel ratio accuracy to equipment conditions and in-house maintenance abilities — savings on fuel and production efficiency can be well beyond the standard projections of 20 to 25 percent.
The transition to a pulse-fired system from current combustion designs can be a beneficial change for an organization. The pulse-firing techniques help secure enhanced product quality, extended equipment longevity and simplified burner maintenance and procedures. These benefits can generate greater savings and in turn, greater revenue, for manufacturers. In the ever-changing manufacturing world with tighter environmental and safety restrictions, internal process enhancements can help ensure a sustainable future.