BCR: Managing Gas Flow for Optimum Firing Control
In today’s difficult economy, just about everyone is trying to find new ways to cut costs while maintaining—or improving—product quality. For some plants, achieving this goal will require the purchase of a new kiln. For many others, however, some minor modifications to their existing kiln(s) can often help improve product quality and maximize energy efficiency.
One of the easiest ways to optimize the firing process is by managing the flow of gases through the kiln. With a few simple tools, you can turn your “old” kiln into a state-of-the-art, fuel-efficient system.
Preventing Cold Air InfiltrationMany tunnel kilns use a thermal turndown system to control kiln temperature. In this method, the air introduced through the burners remains constant, and the fuel is varied to provide the required energy input. The amount of excess air being introduced into the system varies constantly based on the demands of the process and the amount of fuel flowing through the burners. In this type of system, a large flow of gases moves through the tunnel kiln. The products of combustion (P/C) exhaust fan must remove these gases, and a relatively high negative pressure must be developed at the charging end of the kiln to accomplish this task.
Converting the kiln to an on-ratio pulsing or stepping method of temperature control can provide increased fuel efficiency because it enables the burners to run at or close to stoichiometric ratios. However, the flow of gases through the kiln is considerably reduced—normally, about 20 to 40% of the total gases used.
Good furnace pressure control will maintain a pressure setting at some point in the kiln that has been selected as the control point. However, the front end of the kiln, which is between this control point and the exhaust off-takes to the P/C fan, remains largely negative. As a result, some additional means of control is required to prevent massive cold air infiltration into this part of the kiln.
An oxygen analyzer can be added somewhere in the high heat sections of the kiln to work in conjunction with the P/C exhaust fan. The purpose of this unit is to control the amount of air that is moved from the cooling end of the kiln into the burner section. The analyzer corrects for the amount of excess fuel in the last zone of burners (which are normally operated fuel-rich) to provide the necessary oxidation levels in the high heat zones. This is achieved by measuring the [net] oxygen in that zone and adjusting a damper or a variable frequency drive on the waste heat fan to balance the flows of cooling gases as required. That is, if the oxygen level in the burner section rises above set point, the speed on the waste heat fan will increase to divert air to the dryer (or the damper on the waste heat fan to the dryer will open accordingly). The reverse action is true as the oxygen level in the firing section drops below set point.
Increasing EfficiencyAdding a variable frequency drive (VFD)—or a damper control in the exhaust duct, which can be placed before or after the fan—will enable the P/C fan to automatically react to an upset in the furnace pressure with an increase or reduction in speed. However, if no compensation is made to allow for the great negative pressures generated by the P/C fan to move the gases, cold air will continue to be a problem in the front end of the kiln.
The ideal situation is to shift the neutral pressure points in the charging end of the kiln forward (toward the charging doors) as the volume of gases entering the kiln is reduced. This will help reduce the cold air infiltration and will allow the kiln to run at its optimum conditions. The process would be as follows:
• The P/C fan reduces speed as the volume of gases through the burners is reduced.
• The waste heat fan system reacts accordingly, dumping additional gases to the dryer from the cooling end of the kiln, since these gases are not needed by the burners at a lower input rate.
• A supervisory computer (controller) looks at the output to the waste heat exhaust fan and provides an offset (cascaded) signal to the set point of the P/C exhaust fan system, raising the set point. This shifts the neutral pressure point forward toward the charging doors and reduces the amount of negatives that must be generated by the P/C exhaust fan to move the gases through the kiln. As a result, the amount of cold air infiltration to the front end of the kiln is reduced, allowing the product to heat more uniformly.
• As the furnace pressure in the charging end of the kiln increases, the amount of power required to generate the driving force to move the exhaust gases is reduced, thereby reducing power
• As the amount of cold air drawn into the kiln is reduced, the temperature differential from the top of the kiln to the bottom of the kiln is lowered, allowing faster heating of the bottom products. This speeds the oxidation process in the burner section of the kiln and allows the kiln to be pushed at a faster schedule.
Figure 1 shows an example of a tunnel kiln incorporating the above-mentioned systems. These systems work together to achieve the optimum control of the flow of gases through the kiln, and to keep the amount of oxygen in the burner sections at the most economical levels.