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In CI's October 2003 issue (p. 29), we discussed fuel consumption in tunnel kilns. Following the efficiency recommendations provided in that column should have saved your company some fuel. This month, we will review a combustion system investment that you should consider to save even more energy.
Types of Firing SystemsThe early adopters of pulse firing have been accruing savings while other manufacturers have sat on the sidelines watching, but pulse firing must be considered in light of your existing system. Let's review why it has so much potential. First, we must consider our firing system. There are typically three types: excess air, proportional and pulse firing. Let's look at the hot zone of a tunnel kiln and consider the impact of each system on fuel consumption.
XSA ModelThe simple cross section shown in Figure 1 illustrates the typical arrangement of lower burners in a tunnel kiln. Let's assume that the firing temperature is 2500°F and that our example uses 10 burners, each rated at 500,000 BTU/
hour. Further, let's presume that the excess air system input is set up with rated air input (5000 SCFH/burner), and that under normal control circumstances the fuel flow is 400 SCFH per burner. Thus, the total fuel flow input to the hot zone is 10 burners x 400 SCFH gas/burner = 4000 SCFH of gas, and this is the equivalent of 20% XSA.
Many kilns are fired in this manner-fixed air/variable fuel-and if you have one of them today, it is time to get rid of it. It is a design that works well when the cost of fuel is relatively unimportant. But at today's average of $6-10 per MM BTUs, this system is unacceptable.
Proportional vs. Pulse FiringLet's compare the XSA fuel use to proportional firing and pulse firing, operating under stoichiometric conditions. Straightforward calculations show that both systems will provide the same fuel savings-about 30% for the burners in the hot zone. But because the pulse system operates only at high velocity, unlike proportional, it can provide better temperature uniformity. Pulse firing is not inherently more efficient; after all, stoichiometric fuel and air mixtures possess a fixed quantity of BTUs, regardless of the firing system. But the manner in which pulse firing delivers this input-and the attendant operational performance-makes the selection of this system imperative. You can see this clearly by looking at how each burner operates.
What's Your Burner Doing?The high-velocity burner provides the obvious advantage of penetrating velocity within a kiln, but even greater gains are achieved by the entrainment of the gases in the ware space. This is vital, and it is a principal reason why pulse firing works so well. Consider the proportional and pulsing systems shown in Figure 2 (p. 21) with an output of 50% of maximum firing into our 2500°F hot zone.
The pulse system entrains much more air than the proportional system and, as a result, the gaseous mix that is injected into the ware space is much closer to the ware temperature-translating to better temperature uniformity with no hot spots. Note that as the proportional system is run at lower outputs, its condition worsens, and vice versa.
As with all systems, the details of design are critical. Some key points:
- Install the burner so that the outlet of the burner refractory is flush with the interior wall of the kiln. For kiln gases to be entrained, they must be available to the burner jet. Installing the burner in a recessed port is bad design.
- Carefully consider the pulse sequence and burner firing strategy. Reject systems that allow for a long (more than 10-15 seconds) time between burner high fire operation. The best systems have a short cycle time and variable burner high fire time.
- Watch what components are selected-high cycle valves must be used. And remember that the cyclical action of the air and fuel input often leads to adjustment variation. Know what you are buying!
- Burner firing sequence can have a major impact on performance. Analyze the sequence with consideration to temperature uniformity.