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Energy can be recovered from kiln exhaust gases in many ways, including the generation of hot water, the use of hot air for direct or indirect cooling, hot oil exchange, and combustion air heating for the burners on the kiln. The last option is the most interesting, because more energy is saved than recovered. Don’t believe me? Some of my clients don’t either, but let’s take a look at the math.
Simple ExampleLet’s say we have a periodic kiln operating at a soaking temperature of 2200ºF. We’re heating this kiln with natural gas and operating at a stoichiometric level. For the sake of argument, we’ll further assume that at this moment the net input to the kiln is 5,000,000 BTU/hour.
Net input is the heat required to maintain the temperature within the kiln, but the gross input (that is, what you pay for) is much higher. If we consider that the combustion air temperature in our example is 60ºF, for every BTU of energy that we purchase only 41% is available to heat the kiln. The rest of the energy goes toward heating the combustion air, fuel, products of combustion, and converting the water vapor combustion products to steam. This leaves us with a necessary gross input of 12,195,000 BTU (5,000,000 ⁄41%) under this condition.
Now let’s assume that we install a heat exchanger that allows us to heat up the combustion air to the burners of this kiln. It is a moderately effective system that provides our kiln with 800ºF combustion air by extracting some of the energy from the kiln exhaust. By heating up the combustion air, we reduce the amount of fuel used by our natural gas, and the heat available to the process jumps up to 53%. Our process still requires 5,000,000 BTU/hour, but in this case, our gross input to the kiln is only 9,434,000 BTU/hour (5,000,000/53%). Thus, our savings is 2,761,160 BTU/hour. The amount of energy that we have added to our combustion air is:
air mass in pounds x specific heat of air x ∆T of air, or
94,340 cfh x .0763 #/cubic foot x (800-60), or
Thus, we have recovered 1.3 MM BTU and have saved 2.8 MM BTU. Just like magic.
Other ConsiderationsOK, that’s an example for a preheated air periodic kiln, but what happens in a continuous kiln? More or less the same thing, but if we reduce the fuel input to the hot zone of a tunnel kiln, the volume of the products of combustion traveling into the preheating zones will decrease. This will dilute our savings a little. Even under this circumstance, though, the savings will be more than the heat recovered.
Of course, we can’t just apply a heat exchanger to an existing kiln and hope for the best. Hot air often requires larger air piping sizes, slightly larger burners, and perhaps a higher-pressure air supply due to the expansion of air at elevated temperatures. But heck, we’re engineers-these calculations are both simple and quick.
Few opportunities exist that provide energy savings that are higher than the energy we are able to recover. In the age of global warming, high fuel prices, curtailments and “green” thinking, this is a system to consider. Some European countries have laws that mandate heat recovery-maybe we should do the same.
SIDEBAR: Rules to Fire By
- The higher the preheated air temperature achieved, the higher the savings at a fixed process temperature.
- The higher the kiln temperature, the higher the savings with a fixed preheated air temperature.