From the narrowest to the broadest definition of combustion control, the goal is to achieve the best production results with a system that is safe and reliable.

Fig. 2. Indirect tube-fired burner system


What do you think of when you hear the terminology “combustion controls?” Maybe you would answer with any one or a combination of the following:
  • Flame-safeguard control – making sure there is flame present while gas is flowing to the burner
  • Temperature control – adjusting the firing rate to maintain the process heat
  • Air-fuel-ratio control – maintaining proper burner efficiency
  • Process control – adjusting multiple system components such as the burner firing rate, furnace pressure, circulation and exhaust fan rates, and product feed rates to achieve the best production results
All of these are important to consider for potential energy savings and reduced emissions. Collectively, they direct us to safely control combustion for effective and efficient heat generation and transfer, to maximize heat containment and to take advantage of heat recapture.

Before Considering a New Combustion Controller

Manufacturing owners and managers are continually looking for ways to reduce production costs and increase production rates. With the great technological advances in electronic controllers, the temptation may be to look to the mysterious and magical “black box” as a solution. Sometimes a salesman under pressure to make his quota may reinforce the idea of the fancy electronic control system as a panacea. Unfortunately, the most sophisticated controllers will not fix a poorly designed thermal process.

For example, an expensive air-fuel-ratio controller may provide some energy savings in the range of 2-10% over a conventional ratio-control system. The investment could be very large, however, making the payback excruciatingly long. Other actions to get even greater energy savings of 10-50% include:
  • Properly selecting heating equipment for the application
  • Maintaining and fixing worn-out thermal-process equipment
  • Replacing with newer burner designs having higher efficiencies and reduced emissions
We should investigate these first before considering retrofitting with an expensive control system.

Fig. 1. Line-style burner to fit air-duct cross section

Proper Equipment for the Application

Sometimes we accept previously designed equipment without questioning whether it is using the right type of heating package for the application. Whenever the opportunity arises, we should evaluate if there is a heating component that better matches the process and will save energy and reduce emissions. For example, many air heating applications need the heat distributed evenly over a broad area of the air stream. A cylindrically shaped flame in a large duct will not mix well, and any additional duct shapes to force mixing will require more energy. Also, the flame of the inappropriate burner may be quenched and produce high CO emissions. A better solution is a line-style burner geometrically arranged to fit the cross section of the air duct (Fig. 1). Its flame spreads over the airflow area for even heating.

Improving heat transfer means less energy goes out the exhaust stack. For metal treating, heat transfer is improved by changing from low to high velocities. The higher velocities also promote temperature uniformity by stirring the air across the volume of the furnace. Alternately, an indirect tube-fired system on a high-temperature furnace can provide heat through radiation that gives excellent heat transfer and uniformity (Fig. 2).

Fig. 3. Energy-loss curve

Maintaining Equipment

Not only must we properly select heating equipment for the application, but once we have it in place and running, we need to maintain it. Here the opportunities for saving energy are great. Many of the maintenance tasks deal with heat containment. For an analogy, think about your own living area. The most expensive home thermostat will not help if you leave the windows open. In high-temperature furnaces, we use radiation as an effective heat-transfer method. Unfortunately, it is also an effective heat-loss method when there are openings in the furnace. In Figure 3, you see the energy-loss curve rises rapidly as you approach the yellow-to-white-hot temperatures. Our goal is to prevent wasting energy by reducing wall losses from damaged insulation, by minimizing air or radiation leaks from damaged door seals and by adjusting dampers for proper furnace pressure. The furnace in Figure 4 shows excessive radiation leaks from a damaged door system. Energy is wasted, and it makes a hot and dangerous work area.

Fig. 4. Damaged furnace door - excessive radiation leaks

Tuning is as necessary for heating equipment as it is for musical instruments. The mechanical components drift over time – some due to temperature variations and others due to wear. For an example, suppose the excess air levels of the burner drifted from 10% up to 30%. You will save about 15% in energy costs when you tune the burner back to 10%.

Fig. 5. Fuel savings using preheated air

Replace Old Outdated Equipment

In many thermal-processing facilities, it is not uncommon to see combustion equipment that is over 15 years old. Newer burners offer substantial advances in nozzle mixing that can improve heat transfer, and they can reduce emissions that were not regulated or even considered years ago.

Higher burner turndown (maximum-to-minimum firing ratio) allows tighter temperature-control regulation. Low turndown systems often must shut off to prevent over-temperature conditions when the process load is at minimum. During the off time, the process may cool too much, requiring excess energy to get back to the original temperature.

A significant method for energy savings is recapturing the heat of the exhaust to preheat the combustion air. The chart in Figure 5 shows savings up to 51%. Air preheating is typically done by adding a heat exchanger to the system. Many new burner designs, however, now integrate the heat exchanger internally to preheat the combustion air.

Ratio Control Methods

Finally, after you have done all the bigger energy-saving steps, it is time to look at improving air-fuel-ratio control. We want to choose a method that suits the process. Why pay extra cost for the ability to characterize the ratio over the full firing rate of a burner if application requires the burner to be at full fire 99% of the time? We also want to consider the energy savings versus the cost. If the payback period is long, there has to be other benefits to justify the project. Finally, we want to make sure we are staying safe. In some cases, changing to lower excess air levels may not be appropriate for particular heating equipment. It could cause incomplete combustion and carbon buildup.

In selecting a ratio control method, we first should understand the commonly used methods, their advantages and disadvantages, and learn about the more sophisticated control methods to determine if they are appropriate.

Fig. 6. Simple ratio control using linked valves

The Common Ratio Control Methods
The diagram of Figure 6 shows a simple method using linked valves for ratio control. The blower provides combustion air to a butterfly valve whose position is set by an actuator. A linkage rod connects to a characterized fuel-control valve. If the characterized valve is well built, you will be able to make a fairly good adjustment of the ratio over the entire firing rate. However, this method may require regular and seasonal adjustments to keep the air-fuel ratio optimized. Any changes in pressures will also affect the ratio. Advantages are that it is a simple, well-understood method with a low to moderate cost. Disadvantages are that it provides easy access to untrained employees, backlash in the linkage system can cause ratio variations and the system can fail in an unsafe mode.

Fig. 7. Pressure-control system for flow regulation

The system in Figure 7 uses the modulated air pressure downstream of the air butterfly valve to drive a diaphragm on the gas-proportioning valve. When the air butterfly valve opens, the pressure increases on the cross-connected sensing line, causing the diaphragm to push the fuel valve open until the fuel pressure balances against the air pressure on the diaphragm. It is basically a pressure-control system used to regulate flow. Advantages are that it is fail-safe, compensates for minor fuel pressure variations and has a low to moderate cost. Disadvantages are that it is not well understood, and it only allows adjustments at two points – high fire and low fire.

Fig. 8. Similar to Fig. 6 with electrically adjusted link

Electronic controllers (Fig. 8) enable characterized adjustments over the full firing range. In the simple form shown, it resembles the linked-valve method except instead of the mechanical link, it has an electrically adjusted link, and the characterization is done in the electronics. Advantages include precision over all firing rates, and many accept an oxygen probe input to fine tune the excess air levels. Disadvantages are that electronic skills are needed for commissioning, a coupling failure could lead to an unsafe condition, and it has a higher initial cost.

Fig. 9. Flow monitoring added to control

Mass-Flow Ratio Control
Most of the previous methods need well-regulated air and fuel pressures into the control valves. Also, variations of the chamber back pressure may cause shifts to the ratio. In cases where these conditions are not well regulated, then you must add flow monitoring to the control scheme (Fig. 9). The flow sensors provide feedback to the controller that will then make corrections in actuator positions to continuously maintain the correct ratio. Advantages are its precision overall firing rates, and it is a true flow-control method. Disadvantages are that electronic skills are needed for commissioning, a sensor failure could lead to an unsafe condition, and it has a higher cost.

The controller could be a dedicated unit specifically designed for the application or a generic programmable multi-loop unit. It should use cross-limiting for its control scheme or algorithm. This helps to prevent an unsafe burner firing condition by always keeping the fuel set point limited to the lowest value of either firing-rate demand or actual airflow. Likewise, the air set point is set to the highest value of either the demand or fuel flow. Therefore, cross-limiting prevents a fuel-rich ratio on rapid changes in the firing-rate demand signal.

A key factor in successful mass-flow implementation is the choice of the flow sensors. As with any sensor, it must be selected to match the process conditions. It must stand up to the ambient conditions and the pressure, temperatures and contaminants of the process gases. When comparing sensor types, the primary characteristics for good mass-flow ratio control are:
  • Turndown – The sensor must be able to handle the range of the burner. Some sensors will drop off to the minimum output signal when the flow drops below a certain range. In ratio control, if this happens on the air sensor before the gas sensor, it could lead to an unsafe gas-rich condition.
  • Repeatability – Although accuracy is always good, it is not as important as repeatability in ratio control. During commissioning, any accuracy problems are adjusted when the ratio is programmed at each firing position.
  • Response time – If the sensor response time is longer than the control actuators stroke, the system will be unstable and it will be difficult to tune the controller.
  • Drift – An electronic system is not maintenance free. The drift rating will determine how often the system will require calibration.


Fig. 10. Summary of sensors

Some sensors base their measurements on gas velocity or volume, so the mass flow must be calculated from additional inputs of temperature and pressure. These include the differential pressure, positive displacement, turbine, vortex shedding, ultrasonic and laser types. However, thermal dispersion and coriolis sensors base their measurements on true mass flow.

A brief summary of sensors is shown in Figure 10. The column V/M shows V if the sensor is velocity or volume-based and shows M for a true mass-flow sensor. In some types, the turndown shows a range to cover the variety of implementations from manufacturers. For example, an orifice meter with a simple differential-pressure transmitter may only provide a 3:1 turndown, but when the orifice is supplied with a matched “smart” microprocessor system, the turndown can increase to 10:1.

Summary

In the broadest sense, optimizing combustion can refer to controlling the complete thermal-process system. Narrowing down, it can refer to the design of the nozzle in a burner. From the narrowest to the broadest definition, our goal is to achieve the best production results with a system that is safe and reliable. The market directs us to improve efficiency for energy savings and to reduce emissions. It is accomplished best when we evaluate and prioritize the most beneficial actions, whether changes in heating equipment, implementing a maintenance program or adopting a new controller.IH

For more information:Dan Curry is an electronic products engineer for Eclipse, Inc., 1665 Elmwood Rd., Rockford, Ill. 61103; tel: 815-877-3031; fax: 815-637-7049; email: dcurry@eclipsenet.com; web: www.eclipsenet.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: emissions, heat transfer, radiation, heat exchanger, controller