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Unfortunately, the thermodynamics of combustion ensure that you never get all of the fuel that you pay for. Generally, a cubic foot of natural gas contains about 1000 BTUs, but the heat available to do what you want it to -- which is to heat the kilns -- is much less, depending on a variety of factors.
If you use excess air (XSA), you pay a penalty to heat the air in excess of the combustion needs. And as the temperature of your process rises, the available heat also declines drastically. Table 1 indicates the drastic differences in available heat when firing with a cubic foot of natural gas at varying rates of XSA and preheated air temperature.
You can often reduce XSA, but you are stuck with your process temperature. So what can you do? The table shows that preheating the combustion air has a major impact, but this can be very costly, difficult and cumbersome, with oversized combustion air piping, insulation, recuperator installation in the exhaust flue, and ratio inaccuracies resulting from changing air properties. Because of a tunnel kiln's recuperative properties, higher-temperature zones exhaust into cooler zones; therefore, the chart overstates the effective savings. But in a periodic kiln, where the kiln exhausts at process temperature, the savings can be spectacular. In addition, if there is incineration on the periodic kiln, the reduction in exhaust gas flow due to preheated air results in savings there as well.
One option for preheated air is to use oxygen enrichment as a means of increasing the effective BTU value/efficiency, and this solution can also be effective. Unfortunately, oxygen is expensive. Several projects that I have worked on net similar savings when the combustion air is enriched to 25% O2, but about half of that savings ends up being lost by paying for the oxygen use.
Therefore, preheated air is a desirable solution, but one that quite often requires investment in the recuperator, piping, controls, etc. Another interesting option is available, however, and I admit that I need your help to explore it.
Self-Recuperating BurnersSome companies manufacture burners that "self recuperate" -- they withdraw a fraction of the kiln exhaust gases around the periphery of the burner nozzle. This hot exhaust air heats the incoming combustion air inside the burner with a mini-heat exchanger, generating preheated air without the complication of normal methods.
This concept is simply elegant, and though it needs a burner exhaust collection system, the piping can be relatively simple. I do have some questions, however, including:
- What is the impact on the process temperature uniformity when exhaust gases are exhausted from many different sites?
- What means are used to control ratio accuracy in this system, with widely variable combustion air temperatures?
- What is the burner life (any impact on the burner maintenance, etc.)?