OCR Text |
Show WASTE GAS FIRING IN PROCESS HEATERS AND BOILERS incomplete mixing creates zones that give a mixture that is flammable because in the zone, the percent by volume combustibles is greater than the irtinimum given by the chart. As shown by Figure I, when CO is mixed with the methane the limits of flammability approach each other. With a 50% methane and a 50% CO mixture the lower limit occurs at approximately 5.4% methane while the upper limit occurs at approximately 11.3% methane. The diagram indicates that there should be no problem burning the landfill gas, which is in fact true, as long as the burner design is capable of providing the increased stability required because of the reduced flame propagation speed and possible increased fuel discharge velocity. It is important to note that if preheated air and fuel are used, the lower limit of flammbility is decreased and the upper limit of flammability is increased. Figure I indicates a lower limit of approximately 4.4% by volume methane when 600°F preheated air is used. The upper limit increases but is usually not of as much concern since normal operation is with excess air. Just as additional heat added to the system via preheated air will give wider limits of flammability, elimination of heat via heat transfer will tend to narrow the limits of flammability. Figure II is the flammability diagram for shale oil retort off-gas. The gas contains 86.4% inert components. An operating line is shown for this composition along with an operating line for a mixture with 70% inerts. With 86.4% inerts the lower limit of flammability is 5.2% combustibles in the fuel-air mixture while 8.4% is the upper limit. It is very difficult if not impossible to design a combustion system that will mix the fuel and air in the proper proportions to maintain a mixture within these limits for all firing conditions. In addition, if any significant heat transfer occurs, the mixture is no longer flammable. Actual experience shows that operation of a system with this fuel requires a "burner" with a refractory lined combustion chamber to limit heat transfer and the use of preheated air to add additional heat to the system. An additional approach is to fire a supplementary fuel in combination with the off-gas to provide the required heat to maintain combustion. Figure III is the flammabiltiy diagram for the flexicoker off-gas. The off-gas, which is represented by the operating line for 59.2% inerts, has limits of flammability ranging from approximately 6.2% to 28% by volume of combustibles. The wide range of flammability limits indicates that the gas should be relatively easy to burn. As long as certain precautions are taken with the burner design, the flex gas can be an excellent fuel. However, it should be emphasized that severe stability problems have been encountered when a suitable burner design was not used. Since variations in composition of the flex-gas can exist, a second operating line for a mixture with 85% inerts is shown. As can be seen, this mixture is approaching the point of no longer being flammable under any excess air condition with 60°F air temperature. The addition of air preheat widens the flammability limits and tests conducted with 600°F air preheat in a conventional low air pressure drop burner, modified to burn the flex-gas, indicated stability can be achieved for this high inert case over a wide range of operating conditions. 1.1.4 |