The Basics of Pulse Firing (Part 2)
Benefits of Pulse FiringIn certain applications, pulse systems can offer a number of benefits in comparison to amplitude-modulated systems.
When an individual burner is not being pulsed at high fire, it is either left firing at a precisely set low-fire position or it is completely turned off. If a high/low fire method with a precision bypass is used, overall turndown ratios of 20-to-1 can be achieved. If the burner is cycled on-off, then an infinite turndown ratio can be achieved. Figure 1 compares the turndown of a zone of burners controlled with amplitude modulation to pulse firing.
Better temperature uniformity is possible because the combustion system allows flexible individual burner control to maximize circulation of the products of combustion with the furnace atmosphere. Since burners operate at high fire during the on cycle, a burner always operates at the highest velocity possible. This ensures that the maximum volume of gas from the furnace atmosphere is entrained with the products of combustion. This mixture brings the gas temperature closer to the furnace temperature, resulting in fewer hot spots. Furnace uniformity-survey adjustments are quicker than with amplitude-modulating systems because the burners can be grouped and their timings individually biased up or down to achieve uniform temperatures across varying furnace or load conditions.
Fuel savings of 20-30% are claimed for pulse-fired systems when compared to amplitude-modulating systems. One reason is the excess air that either comes from the burner or is purposefully injected into the furnace in order to stir the atmosphere and produce uniform temperature throughout the furnace. Excess air acts as an additional heat load because it must also be heated. Pulse systems provide easier control of the air/fuel ratios since the burner has only two states: high and low or high and off. Also, air is not injected in pulse systems because the burner outlet velocity provides the stirring and uniformity.
The ability to precisely control each burner’s air/fuel ratios reduces the air pollution this burner causes. Figure 2 shows a typical emissions chart for a burner. Most burners emit the lowest NOx emission levels when operating at high fire (the chart trend-line drops), so pulse-firing burners always operate at their cleanest conditions.
Pulse systems achieve greater process-control flexibility because the process lends itself to programmable control systems that can provide different firing schedules from one firing to the next. The greater turndown ratios for the on-off pulse mode can allow faster heating rates without sacrificing control at soak temperatures. This could result in shorter process times and higher production rates. A pulse system can switch operation from a heating mode to a cooling mode where the furnace temperature can be rapidly lowered by closing the gas valve and pulsing only the air valves.
In long continuous furnaces, a greater number of burner zones can allow more flexibility in varying the temperature-time profile along the length of the furnace. In pulse systems with individual burner control and programmable systems, therefore, it is more feasible to change the zoning compared to zones that are hard piped and wired together as a group.
Problems of Pulse FiringCosts and Maintenance
Because pulse-fired systems include more components than typical amplitude-modulated systems, as shown in Figure 3, the initial capital investment is higher. The increased cycling of the valves also requires more frequent maintenance. On-off pulse control puts stress on the ignition components and reduces operation reliability.
The company responsible for developing the pulse sequencing must rely on programmable logic controllers (PLCs). The developers must have software writing and testing skills. An in-house software expert or the developer himself is required to address any problems that arise or any changes that are required in the code. If access to the software is not properly restricted, changes to critical parameters might lead to unsafe conditions.
Furnaces used for highly versatile load types, load sizes and a wide range of soak temperatures could present challenges to pulse systems that do not have a cooling mode or an excess-air mode. Similarly, systems designed for higher temperatures may have uniformity problems when kept at a low temperature for a long time because the burner may not pulse frequently enough to keep the furnace atmosphere stirred.
To prevent cold-air infiltration, the pressure inside the furnace should be slightly above neutral but typically under 0.2-inch w.c. (0.5 mbar). The rapid pressure changes caused by pulse-firing burners can complicate furnace pressure control.
Many amplitude-modulating systems use circulation blowers and excess air to stir the atmosphere. Compared to amplitude modulating, the area directly in front of a pulsed-burner outlet will always be much hotter since the burner is at its maximum during the high or on pulse, as illustrated in Figure 4. Limiting the individual burner high firing rate on pulse systems requires more effort because adjustments must be made at each burner. Amplitude-modulating systems can limit the heat-demand signal, which drives the actuator motor on the modulating valve.
Some sensitive products are adversely affected by the rapid switching between high and low (or high and off) firing rates of pulsed burners. Since an amplitude-modulating system can control the speed of the actuator that varies the firing rate, this system can be gentler on sensitive products in the surrounding area.