by George J. Paulonis, Brian Hall, & Carl E. Frahme
An Introduction to Pulse Firing and How It Compares to Conventional Systems
One of the prime functions of any combustion control system is to vary the heat input to a process in response to the needs of the process. In pulse firing, the heat input is controlled by modulating the frequency of operating the combustion burner or burners. The burners are fired at high fire for controlled times and then cycled to either low fire or actually turned off. This cycle is repeated quite frequently, and the length of time the burner is at high fire and then at low fire or off is controlled by the process controller. Pulse firing can thus be accurately called frequency-modulated firing. Each burner is controlled independently of all other burners, resulting in a great deal of control flexibility and precision.
In conventional systems, the burners are modulated between high and low fire and can be at any setting between these. This is proportional firing, or amplitude-modulated firing. Generally, a number of burners, all hard-piped together, are fired and controlled as a group in this system. At first glance, it might seem that either system could offer good control and a great deal of flexibility. That is basically true, but in fact pulse or frequency modulated firing can be much more precisely controlled and does offer a distinctly higher level of performance. The advantages and benefits of pulse firing can be, and usually are, very significant and real.
Although pulse firing is relatively new to the United States, it has been in widespread use in Europe for about twelve years. The concept, and the required hardware, have been thoroughly developed, tested, and proven there by Kromschroder in a wide variety of industries. Kromschroder has been a pioneer in the pulse firing field and developed the idea of pulse firing individual burners for the industrial market. The idea was conceived by a university professor, while Kromschroder developed the controls and special valves that are the heart of successful pulse combustion.
The Benefits of Pulse Firing
The significant benefits that accrue from pulse firing are a direct result of the inherent nature of the control algorithm used in the process. These benefits deserve careful explanation and discussion. Pulse firing truly represents a major advance in combustion processing.
The inherent advantage offered by pulse firing is more precise and flexible control of the basic combustion process. Any combustion system needs to carefully control the natural gas and combustion air and deliver it safely to a combustion burner or burners. The ratio of air to gas must be reproducibly controlled, and the amount of air and gas at this ratio must be varied to allow the system heat input, and thus temperature, to be controlled. Both frequency modulated (pulse) and amplitude modulated (proportional) systems can do this, but pulse systems can do it demonstrably better. Let’s discuss the advantages offered by pulse firing in some detail, with emphasis on why these advantages exist.
More consistent and precise control of the air-gas ratio and of the energy input to the system can be achieved with pulse systems. In both pulse and proportional systems, a process controller determines when more or less heat input is required, usually in response to the output of a temperature sensor such as a thermocouple.
The limitations of proportional or amplitude-modulated systems can best be discussed by describing how such systems function. In a proportional system, the output of the controller is usually used to drive a butterfly valve to regulate the amount of combustion air. When maximum heat input is called for, the valve is fully opened. When less heat input is needed, the valve is partially closed, but it is never closed entirely. It can only be closed to a "low fire" setting that is consistent with the burner and the gas control system. The air-gas ratio is controlled by a master gas regulator that is "slaved" off of the air pressure downstream of the air butterfly valve. An air impulse line, usually of copper tubing, connects the air line to the gas regulator. As the air pressure drops, the regulator drops the gas pressure relatively proportionately, thus maintaining the air-gas ratio near to the desired setting. In general, it is possible to precisely set and control this ratio only at high and low fire, but between these settings, the precision is not nearly as good. This is inherent to the spring control systems used in the gas regulators. Since the needed heat input is somewhere between the maximum and minimum most of the time, a proportional system generally is delivering a gas-air mixture that is not on desired ratio most of the time. Another problem in proportional systems is that it is feasible to get a "turndown ratio" of only about six to one between high and low fire. That means that the heat input at low fire can not be less than one-sixth the input at high fire. That can be critical in many cases, especially where precise temperature control or large variations in heat input are required. A third problem in proportional systems relates to the mechanical aspects of piping. When a single air valve and gas regulator are used to control a number of burners, and this is the normal circumstance, the piping distance to each burner is somewhat different, as is the geometry of the piping and the number of elbows, etc. The flow properties to each burner are thus not identical, and thus each burner behaves a little differently. Often a gas regulator for each burner is required to minimize the setup problems at low fire.
Pulse or frequency-modulated systems offer offer a number of benefits in comparison to proportional systems. Pulse firing allows unprecedented burner control. Rather than control the amplitude of the fuel input, the pulse system controls the frequency and timing of firing of each burner at predetermined and precisely regulated fuel inputs. Each burner has its own air valve and its own gas regulator. When an individual burner is not being "pulsed" at high or maximum fire, it is either left firing at a preset and carefully regulated low fire or is completely turned off. If a low fire method is used, overall turndown ratios of twenty to one can be achieved. If the burner is cycled off and then restarted under control, an infinite turndown ratio can be achieved. There are no in-between firing settings with their inherent off-ratio characteristics. The nature of the frequency modulation control is interesting. Although each burner is individually controlled, burners are often set up in zones. The controller varies the high-low or on-off time in relation to the input control signal, again usually from a thermocouple. Each burner in a zone is fired sequentially, and the time on is varied as the heat input to the process changes. The process is described in much greater detail in another section below.
Better temperature uniformity in the kiln or furnace is assured for a number of reasons. The basic reason is that each burner is individually controlled. Zoning of burners can be done more flexibly. It is much easier and less expensive to have lots of zones of control. The combustion system can be designed to maximize gas circulation in the furnace, thus enhancing temperature uniformity.
Greater process control flexibility can be achieved. The process lends itself to complete computer control. Mechanical control and adjustments are eliminated or minimized. In periodic kilns, there is more latitude in using different firing schedules from one firing to the next. Greater turndown ratios can allow faster heating rates without sacrificing control at peak soak temperatures. A pulse system can not only be operated in a heating-only mode, but it can also be operated in a heating and cooling mode. The kiln temperature can be rapidly lowered if desired or needed by using a cooling mode in which the gas solenoid valve closes and only the air valves are controlled. Only air is injected. If the temperature controller again calls for heat, the burners are re-ignited and a heating mode resumes. In continuous or tunnel kilns, greater numbers of burner zones can allow more flexibility in varying the temperature-time profile along the length of the kiln. Since zoning is not "hard-piped" into the kiln, it is feasible to provide more zones and to change the zoning much more easily than would otherwise be possible. Drastic changes require mostly wiring and controller changes, not expensive and time-consuming changes in piping and hardware.
Improved productivity results from the ability to tailor the combustion process more closely to the nature of the product and in providing better control. Throughput can be enhanced. The greater turndown capability of pulse firing can allow a faster cycle, assuming the product can be fired faster, since a larger burner size can be used to increase heat input without sacrificing control at low fire.
Higher product quality and lower losses are the direct result of better temperature uniformity and tighter temperature-time control. This savings, although hard to predict ahead of time, can be very significant.
Significant fuel savings are very common when pulse systems replace or are compared to proportional systems. One of the prime reasons for this is the maintenance of precise air-gas ratios during the entire firing process. The air-gas ratio can be set at or very close to "perfect" combustion (a 10:1 ratio) at the peak firing temperature in both continuous and periodic kilns. In the pre-heat zones of a tunnel or continuous kiln, excess air can be used. In a periodic kiln, excess air can be used at early firing stages to burn off organics and enhance air circulation. At the peak firing temperatures, more economical "perfect combustion" ratio settings can be used. This requires additional control valves and control programming, but it is entirely feasible with pulse firing. Because of these advantages, energy savings of 20-25% are not at all unusual. In comparison to less efficient systems, the savings can be substantially higher.
Reduced air pollution is another result of the more precise control offered by pulse systems. Levels of NOx can be kept much lower, an important consideration in this day of tighter air pollution regulation. Actual levels that can be attained depend on a number of factors such as peak firing temperature, but significant improvements over conventional systems are possible.
Enhanced safety can easily be built into the combustion system. Computer, electronic control and individual flame monitors built into each burner allow greater safety in operation. The loss of one flame monitor signal for whatever reason does not interfere with the operation of the other burners in the system. When heat is demanded, the pulse control system simply ignores any burner in the sequence without a safe flame monitor signal, and the other burners take up the slack until the cause of the defective signal can be corrected. Computer control also assures automated, safe, and simple furnace start-up procedures. Insurance costs can potentially be lowered. Because the flame in a Kromschroder burner is very stable, flame rods can be used with a high degree of reliability. It is not necessary to use more finicky ultraviolet detectors, as is often the case when less stable burners are used.
Capital investment costs can be reduced in multi-zoned kilns with a pulse system. Considering the better performance, one might not expect this to be the case. However, only one air and gas manifold is required.
Pulse Firing Concepts for Ceramic Kilns
As is obvious from the above discussions, pulse firing systems offer a great deal of design latitude that other systems cannot. It is now economic to design with a large number of burner zones. Each zone requires its own controller, but PLC controllers are so modestly priced that this is no longer prohibitive. In fact, each burner can be an individual zone if desired. This is seldom necessary or practical, but no longer must one think in terms of control zone limitations. The limiting factors now become the ability to locate a thermocouple or other sensor appropriately to provide meaningful control of any given zone. The geometry of the kiln, including the distribution of the ceramic load, flue locations, car or deck design, and similar factors, dictate the zoning in a furnace or kiln.
For example, the lower burners in a cross-fired kiln can be treated as a separate zone relative to the upper burners firing across the load in the opposite direction. Assuming thermocouples can be appropriately located, the lower burners can be asked to fire harder than the upper burners to help minimize top-to-bottom temperature differences, which are all too common in kilns. This approach will not necessarily completely eliminate such differences, but it can reduce them. This concept can be used in both continuous and periodic kilns.
In continuous or tunnel kilns, it is entirely feasible to have many more control zones along the length of the kiln. This makes it possible to develop a temperature profile along the length of the kiln that is more closely in tune with the ceramic product being fired and to assure that this profile is actually attained. A programmer-controlled cooling zone can be provided for critical applications, such as firing ware with a matte glaze. If different products are fired at different times, the profile can be changed at will to match each product. If the loading of ware through the kiln changes, which is not at all uncommon, the system can react to maintain the proper temperature profile. The profile can even be changed when the loading is changed. Such flexibility in operation is simply not possible or practical using more conventional systems. Concepts that were unimaginable previously are now feasible and practical, and relatively easy to attain. Pulse firing has the capability of revolutionizing ceramic firing and kiln design!!
Advanced Combustion Safety Systems
Flame monitoring and combustion safety systems are integral parts of Kromschroder pulse firing. Again, because of the individual control of each burner, the highest levels of safety management are relatively easy to attain with Kromschroder pulse firing. Kromschroder recommends they be built into every system. Quite frankly, it is somewhat disturbing to see the lack of safety systems that is all too common in existing equipment. Kromschroder can add safety systems to existing equipment, even if it is not converted to pulse firing. However, conversion to a Kromschroder pulse system is the best way to upgrade the safety aspects of the kiln or furnace.
Computer-based Process Control Systems Integrated with Pulse Firing
All of us are aware of the impact that computers have had on every aspect of our lives, and control of ceramic firing processes are no exception. The advent of PLC and PC computer technology has allowed dramatic improvements in the level of control that can be exercised over the firing process. In addition, it is now convenient to include a great deal of process reporting that can be invaluable in monitoring and improving product quality and process yields.
Kromschroder pulse firing makes maximum use of such capabilities. Kromschroder uses a systems approach to integrate all aspects of hardware and software to give a package offering significant benefits. Each system is essentially custom engineered for each customer and application. Whether it be a retrofit on an existing kiln or the design of an entirely new kiln, the pulse combustion systems will be carefully integrated by Kromschroder with the latest appropriate computer control system. Depending on customer needs and expectations, this may range from a relatively simple system with a basic temperature programmer to a very sophisticated PC-based system with extensive data logging and reporting. Costs do go up with higher levels of sophistication, but those increases are no longer prohibitive.
The Details of How Pulse Firing Works
This section is included for those who want to understand in greater detail just how pulse firing works. With such an understanding it is easier to see just how potent this technology really is and how it might be most fully utilized in any given circumstance. If questions still remain in your mind after reading this section, Kromschroder will be pleased to address them.
For purposes of discussion, Figure 1 shows a schematic of a typical amplitude-modulated, or proportional, combustion system. Note that there is one combustion air control valve and one air-gas regulator valve for the entire burner zone. As the process requires more heat input, the temperature controller signals the air control motor to open. The air pressure thus increases at the burner (higher air flow) and an impulse line located downstream of the air valve leads air into the lower chamber of the air-gas ratio regulator. The air pressure forces the diaphragm up, thereby opening the valve seat to allow more gas flow to the burner. As the gas pressure builds downstream of the regulator, this pressure acts on the regulator diaphragm to create a balance of pressure between the air
Figure 1: Typical Amplitude-Modulated Combustion System
and gas. The gas outlet pressure is approximately equal to the air impulse pressure. The regulator is fitted with a factory set spring to compensate for the weight of the diaphragm assembly. This spring is also used to set low fire or minimum gas flow for the burner. In such a system, when the controller calls for full output, all burners are at full output. If the controller calls for 50% output, all burners are at 50% output. Although this has been an accepted and standard method of burner control in the industry for some time, it has definite drawbacks. It is possible to accurately set the air-gas ratio only at high and low fire. The fuel flow as a function of air impulse pressure, which in essence is the air-gas ratio, is dependent on the regulator characteristics. They are not truly linear, and thus the ratio in between high and low fire settings will vary somewhat from the desired value. Another drawback mentioned earlier is the limited turndown ratio possible with this system, in the range of 6:1. Often in multiple burner zones it is necessary to have a gas regulator for each burner to minimize low fire set-up problems.
Figure 2 shows a schematic of a typical frequency-modulated or pulse firing system.
Figure 2: Typical Frequency-modulated (pulse) Combustion System
Please note that the layout differs only slightly from the layout shown in Figure 1 for the amplitude-modulated system. In the pulse system an air solenoid valve is used at each burner, replacing the large butterfly control valve for each zone. Also, an air-gas regulator is located at each burner, replacing the single regulator shown in Figure 1 for each zone. This regulator has an internal bypass for setting low fire rates. The air solenoid valve is hydraulically dampened in order to allow a smooth opening and closing action. When the control system demands more input, the air valve opens, and the air pressure downstream of the air valve increases. This pressure increase is transferred via the impulse line to open the air-gas ratio regulator. This regulator performs the same function as that used in an amplitude-modulated system except that it controls flow to only one burner. When the air solenoid valve closes, the impulse pressure to the regulator is reduced, and the valve closes. Low fire gas to the burner is now directed through the bypass orifice in the air-gas ratio regulator. Pulse control systems will operate reliably at a 20:1 turndown ratio, compared to approximately 6:1 for amplitude-modulated systems. The above description is for a high-low control system. A pulse system can also be set up for on-off operation by closing the gas solenoid valve at the same time the air valve closes.
A pulse control system operates in a manner quite different from amplitude-modulated systems. There is quite a bit of variation possible in the actual nature of the pulse control system hardware, and this can and will be varied to meet the specific needs of the customer. For purposes of discussion, one of the original control concepts, which is still in use today, will be described. However, due to advancements in electronic and computer technology, it is likely that Kromschroder will use even more state-of-the-art controls in actual proposals. The purpose of this discussion is to make clear the basic nature of the pulse control concept. The basis of all pulse control systems is the principle of programming "on" and "off" time in relationship to an input control signal from a controller. The concepts discussed below are thus basic to the operation of a pulse system.
In a pulse combustion system, a signal converter system controls the operation of the solenoid valves via a flame management system specifically designed for pulse firing. The pulse controller receives a 4-20 mA signal from the temperature controller and, through a converter, changes this analog signal into a sawtooth voltage signal. The frequency of this
Figure 3: Example of trigger voltage assigned to a specific burner
Figure 4: Burner period as a function of required output
voltage signal varies proportionally with the temperature controller input. As the output voltage from the pulse converter increases with time, each burner in the zone is sequentially activated at a different voltage level. The firing time is fixed and the next firing cycle for a given burner is only triggered when the output voltage again increases to the same level on the next cycle. When the controller calls for greater than 95% output, all burners remain on 100% of the time. Within a control loop the burner control takes over the function of a continuous proportional control. A proportional dependency is obtained between the controller output and the burner capacity. In this example, a 4 mA signal represents zero output, a 12 mA output represents 50% output, and a 20 mA output represents 100% output.
At this point, an example will be helpful in demonstrating pulse operation. We first need to define some terminology.
S/I = Seconds per impulse
I/S = Impulses per second
I/H = Impulses per hour
For our example, let us set S/I = 5 sec I/S = 0.2 I/H = 720
At 100% output, the burner is always on.
At 75% burner capacity,
S/I = 5/0.75 = 6.667 sec., I/S = 1/6.667 = 0.15 I/H = 0.15 x 3600 = 540
In this case the burner will re-ignite every 6.6 seconds, with on "off" time of 1.6 seconds.
At 50% burner capacity,
S/I = 5/0.5 = 10 sec., I/S = 1/10 = 0.10 I/H = 0.10 x 3600 = 360
The burner will re-ignite every 10 seconds, with an "off" time of 5 seconds.
Figure 5 shows a four burner system with a burner firing sequence for 50% and 25% output. The burner on time is 6 seconds.
Figure 5: Four burner system with the burner firing sequences for 50% and 25% output
At the heart of a pulse firing system are solenoid operated control valves on the air and gas lines to each burner. These valves are designed to operate at a high enough frequency to respond to precise control inputs. At a frequency of ten cycles per minute, twelve hours per day, five days per week, the valves will be subject to 36,000 cycles per week. Typical solenoid valves are designed for one million cycles to failure, and at the frequency required for the pulse system just mentioned, the valves will last about six months. This is obviously not acceptable. Therefore, valves have been designed specifically for pulse firing applications. They are available and have been fully proven in service over many years. Valves operating on standard 120 vac with full wave rectified dc coils are supplied with the capacity to run twenty million cycles before failure. That is equivalent to about ten years of continuous operation at the above frequency. The life expectancy of these valves is known not only from laboratory life cycle testing, but also from actual kiln operation over at least twelve years. The fact is that without such high cycle capacity valves, pulse firing would only be an interesting theory. Kromschroder has proven the practicality of the theory in actual production environments.
The actual details of operation and the design of the optimum system for any application can be somewhat complex. The basic principles are relatively straightforward, but the experts at Kromschroder are highly qualified to extract the greatest benefit out of the technology for any given application. The result will be a combustion system that offers a major improvement in overall performance from a number of viewpoints, including fuel savings, flexibility of operation, pollution control, product throughput, and product quality. These are not idle claims but are substantiated by years of experience in the ceramic and other industries.
Conversion of Existing Systems to Pulse Firing
There are large numbers of ceramic kilns in operation that could benefit from conversion to pulse firing, and such conversions are not as difficult as one might think. Many of the components of the existing system can be used if they are in good condition and of appropriate type, size, capacity, etc. Blowers and burners are examples. Any burner that can be or is fitted with appropriate flame monitoring can be used with Kromschroder pulse firing. In fact, well known companies offering pulse firing often use Kromschroder systems with their own burners. Of course, Kromschroder burners have been specifically designed for pulse firing operation, so burner conversion will need to be considered in a conversion package.
In any conversion, the air and gas control valves and the associated gas train components will almost certainly need to be changed. Since a pulse firing system cycles from high fire so much more frequently than does a conventional proportional system, the air and gas valves are critical to the long term operation of the system. Kromschroder valves are specifically designed for this application and have been proven over years of service. To speed the conversion and assure a well engineered and installed system, Kromschroder can help arrange for pre-assembled gas trains for each burner. These will contain all the components in place ready for quick installation and wiring. The computer control package, which is an integral part of the Kromschroder system, would similarly be pre-assembled ready for fast hook-up to the combustion components. Using this approach, a conversion can be accomplished with minimum downtime and interruption of production. A key to accomplishing this occurs in the early planning stages, where a little imagination can go a long way in the design of a new system that takes advantage of all of the attributes and benefits that pulse can incorporate.
Designing Pulse Firing Systems for New Equipment
Designing pulse firing systems for new kilns is, of course, somewhat more straightforward and less restricted. The design imagination can be given full swing, encumbered only by practicality and an appreciation of economics. Kromschroder is capable of assisting in all stages of the design and engineering process, from concept to development of the detailed component layout drawings and wiring diagrams for the final system. Kromschroder can provide the computer control systems ready for shipment to the end user, with programming in place. Kromschroder prides itself on its systems approach to designing the most appropriate system for each customer and application. This includes ceramic technology capabilities to be integrated with all aspects of the pulse firing technologies.
In Summary
Pulse firing represents a whole new level of combustion technology. It offers the ceramic industry a major expansion of its firing process horizons. Levels of process control and flexibility that were not practical previously are now easily within reach. With an understanding of what pulse firing is, how it operates, and what it can achieve, the only missing ingredient is a dash of imagination. It is hoped that this paper has stimulated your imagination.
