Kiln Connection: Burner Control Systems, Part 3
This method does come with certain liabilities, including high equipment costs for cyclical control components, sophisticated control software and additional training requirements for personnel. It is not a panacea for all kilns, and careful design and component selection are critical. The best pulse systems are remarkably accurate and efficient; the worst pulse systems have poor performance, compromised safety and extremely high equipment maintenance costs.
Why Use it?All firing systems involve a compromise of one type or another. If we look at high-velocity burners firing proportionally, and in excess air mode, we can see how pulse firing overcomes some of the problems associated with the traditional firing means.
High-velocity burners are well accepted. The high-velocity jet promotes temperature uniformity due to its ability to create strong, internal circulation inside of the kiln. The high kinetic energy of the burner jet creates excellent convection, but only when the burner is operating at high output. As the burner output decreases, the level of circulation is diminished, and the degree of uniformity is decreased. With decreased entrainment of kiln gases, the temperature of the burner jet increases, which promotes NOx formation. This is always the case in proportional firing situations, since the burners should always operate below maximum input to maintain control.
To offset these problems, the high-velocity burner is often used in excess air mode, rather than proportional mode. The burner input is controlled by throttling the fuel, while the air volume input is fixed at a high level. In this manner, the burner’s jet velocity is maintained and circulation is excellent. Operation in excess air mode consumes more fuel, and the extra available oxygen promotes NOx.
Unlike proportional control, where a burner group’s output is gradually modulated to achieve temperature control, the pulse system is quite the opposite; burners are fired at full output and then returned to minimum, or even shut off, until the next pulse is required. At first glance, this seems a strange way to fire ceramics, but in reality, pulse firing can apply very gentle heating rates to the kiln and products. When the burner input is required, it is applied at the highest output, providing the high degree of circulation desired.
Important ConsiderationsHaving a pulsing system does not guarantee results. Design of the system’s hardware and software is critical in achieving high performance. As you explore different pulsing systems, ask your supplier the following questions:
- What is the life of the pulse components? This is important; pulse valves may have to cycle two million or more times per year.
- How is the input controlled? Are the pulses of constant duration, with variable pauses between pulses, or is the pulse time variable, within a constant cycle? The latter provides a more regular pulse frequency, which means that burners operate consistently.
- Is the system high fire/low fire, or high fire/off? High fire/off means that the burner will have to be relighted a few million times per year—not a very attractive proposition.
- Does the system have a pilot? If so, are the main flame and pilot burner separately monitored for flame failure? Some systems only sense the pilot flame, and as a result are potentially unsafe.
- How many zones of control are there? The best systems have a single zone per burner.
- How is the sequence of pulsing controlled? The worst condition is a random pulsing sequence; a well-controlled sequence of pulsing can enhance uniformity.
- How is minimum firing rate controlled? Are there adjustable bypass valves for air and gas?
- Does constant actuation of the pulsing valves cause gradual degradation of system setup? Some systems actually have to be adjusted weekly because of their design.
- Can the system be run in different modes—for example, excess air mode (when oxygen is required for the process)? Is the mode of firing software selectable?
- Does the burner have a wide range—at least 25:1 turndown? It is very desirable for the minimum firing rate to be as low as possible, so that almost all of the burner input takes place at high firing rate where the circulation is as high as possible.