From Mechanical to Electronic Control
Similarities Between Automobile Engines and Industrial Burners. Automobile engines and industrial burners both convert fuel to energy; both employ combustion techniques; and both emit pollutants. But, the similarities do not stop there: The auto engine has become a high performance engine, more efficient and less polluting. Relays, coils and distributors have been supplemented by engine management systems. Carburetors have given way to fuel-injection systems. Lambda control has improved economy and reduced pollution while exhaust gas recirculation has further reduced emissions.
Controls for industrial burners also have changed. Mechanical cam fuel-to-air ratio control, the equivalent to the carburetor, has been replaced by self-checking fuel-to-air ratio electronic control. Oxygen trim, the burner's equivalent to lambda control, has been added, and flue gas recirculation reduces emissions as it does in the auto. Performance has been improved by increasing turndown ratios through more flexible control strategies and fan speed control and by better matching of load and demand from embedded three-term PID circuits. Software options such as boiler lead/lag control improve boiler and burner utilization, and communications software greatly enhances information flow.
Most improvements to burner controls have been introduced in the last few years. These controls can be specified on new burners or retrofitted to existing plant equipment. It is now possible to reduce energy costs, lower emissions and obtain trending information for management decisions. Self-checking controls with automatic logging and dial-out can release boiler house manpower for other process applications
Mechanical Cams and LinkagesConsider a typical industrial boiler with mechanical cam control, and assume that the burner has recently been serviced. At high fire, the oxygen level in the flue gas would have been set at a safe level - slightly above optimum - to allow for changes in conditions that affect combustion.
Then, say the ambient temperature swing is 30 to 40°F in a 24 hr period and accept that as the air temperature increases, air expands and becomes less dense, resulting in less oxygen per cubic yard being delivered to the burner. If moisture is present in the air, it will cause displacement, again reducing the amount of oxygen being delivered to the burner. When burning oil, viscosity, calorific value and filter condition all will cause variations in combustion, and when firing on gas, the gas supply pressure and calorific value of the gas will cause variations in combustion.
The engineer will make allowances for any backlash in the linkages associated with the cam. Taking the already noted variables into account, he will set the cam to ensure that the oxygen cannot fall too low. After all, if the oxygen level in the flue gas is too low, the risk of explosion is increased. In addition, the risk of emissions containing unburned fuel also will be increased, and fuel could be wasted. When oxygen is low, the flame from the burner lengthens, which can cause damage to the tubes in packaged fire-tube boilers. However, if the engineer sets the oxygen level too high, there will be an increase in excess air and heat will be lost up the stack. So, the engineer sets the cam's high-fire point at a safe position above optimum and wastes heat up the stack.
The engineer's next aim is to achieve a maximum turndown ratio for the burner. (Turndown is the ratio of high fire to low fire. For example, a turndown of 7:1 would mean that the low fire fuel flow would be 1/7th of the high fire fuel flow.) With mechanical cam controls, the ignition point determines and is the same as the low fire point. For essential safety reasons, each time the burner starts up, the boiler is purged with ambient air, which cools the boiler. Purge cycle duration typically is 3 min. By maximizing the turndown ratio:
- Burner on/off cycles are minimized.
- Heat is not taken from the vessel and wasted up the stack.
- The boiler can more readily respond to increases in load.
- Expansion/contraction cycling, which increases boiler plant downtime, is minimized.
At low fire, fuel flow is at its minimum and efficiency is less important than at high fire, so the engineer concentrates on acceptable combustion consistent with achieving maximum turndown ratio and a reliable startup. Again, he sets oxygen level high.
The other points on the cam are set to give a smooth curve between low and high fire with emphasis on achieving best practical efficiencies at mid-fire and above.
Mechanical cams and linkages have been around for so long that their limitations have been forgotten and everyone, including the plant manager, believes that they have their burners set and operating at maximum efficiency. However, it would be more accurate to say that a burner has been set to give the best possible result within the scope of the controls available.
Electronic Controls can Reduce the Energy BillWhen an electronic fuel-to-air ratio control is retrofitted, the existing PID control, modulation motor, and cams and linkages are removed and servo motors (actuators) are fitted to the air vanes and fuel valves. Likewise, when electronic control is specified on a new burner, the burner arrives with its new control and its servo motors factory fitted.
Using electronic controls, plant personnel can expect several benefits.
Increased Turndown. On an electronic control, the low fire point can be set lower than the ignition point, which means that the turndown ratio can be increased. In addition, burner on/off cycles and their associated cold air purges also can be reduced, which will provide energy savings. While savings from reduced on/off cycles will vary with boiler utilization, savings of 5% have been reported on a burner that prior to conversion had an on/off frequency of approximately once every 10 min.
A Second PID Control. Some electronic fuel-to-air ratio controls have two internal PID modulation circuits. If a plant does not run continuously, then this second PID control's setpoint can be used to switch the boiler to a lower steam pressure or hot water temperature during periods of reduced activity. One manufacturer employing this approach is Land Rover cars in England. The company uses hot water for paint drying in their paint shop but the process is held on standby at night. Using a second boiler setpoint provides energy savings of approximately 10% pa.
Oxygen Trim. Oxygen trim is a closed loop system available as an option on some electronic fuel-to-air ratio controls. Oxygen trim was employed on industrial boilers/burners 15 years ago - about the same time that lambda control first was used on automobiles. But, the oxygen sensors on some of the early trim systems suffered from short operating lifetimes and were expensive to replace. More robust sensors now are available.
Oxygen trim automatically and continuously compensates for the variables that affect efficient combustion. You will recall that on mechanical cam controls, the engineer set the oxygen level high to retain a margin of safety. When oxygen trim is included, the oxygen levels can be set at their optimum level. In addition, if the trim control is adaptive, it will contribute energy savings of approximately 2 to 3%.
Fan Speed Control. With mechanical cam control and basic electronic fuel-to-air ratio controls, processors sacrifice combustion efficiency at low fire to achieve an improvement in burner turndown. Most air vanes leak, and even when fully closed, the airflow can be significant. In effect, processors can reduce the fuel valve setting but cannot reduce the air to match. Recent advances in air damper vane design are overcoming this shortcoming but combustion efficiency can be improved at low fire with old vane designs if the fan speed is reduced.
Fan speed control is an easy-to-add option on some electronic controls. By adding fan speed control, burner turndown can be increased without compromising efficiency, and additional fuel savings can be achieved. The benefits of variable speed control do not end here: When an variable frequency drive is used to slow the speed of an AC electric motor, electrical energy savings result. For example, when a fan motor is slowed to 30 Hz (that is, to half speed), an 80% electrical energy saving is achieved.
Lead/Lag Boiler Control and Communication Software. Some electronic fuel-to-air ratio controls incorporate boiler lead/lag and communications software. Lead/lag boiler control enables the plant operator to achieve better utilization while the communications software provides vastly improved information. Depending on the software package, the PC can display a dynamic plant mimic with live values, commissioning data and operating parameters. Flue gas temperature trend data can indicate when boiler servicing is due and oxygen trim trend data can indicate when burner servicing is due. An auto dial-out feature can alert the engineering staff to potential problems.
As you can see, it is possible to reduce unit cost by reviewing your burner control strategy.