OCR Text |
Show boundaries of the flammable region is formed by the 100% hydrogen operating line and the line that drops down from the 100% hydrogen operating line at 4 % hydrogen to approximately 6 % oxygen and then proceeds to the right until it intersects the 100% hydrogen operating line at the upper flammability limit of 73 % hydrogen. If any mixture of hydrogen, air, and carbon dioxide falls outside the region described, it will not sustain combustion. However, there is no guarantee the mixture will provide stable combustion if it's composition falls inside of the region. The calculated adiabatic flame temperature for mixtures that fall on the flammable region boundary vary widely. For example, the calculated temperature for 100% hydrogen at stoichiometric air is well above 35OO°F, while the calculated temperature for mixtures that fall on the vertical line at 4% hydrogen are approximately 650°F. Mixtures along the latter line simply will not burn when injected through a burner nozzle into a furnace where heat transfer occurs. Firstly, if combustion could be initiated, the energy transferred from the flame would quickly reduce the temperature to a point below which combustion could be sustained. Secondly, because of the low temperature, the reaction rate is so slow that the flame front will simply move away from the burner. For stable combustion to occur, the concentrations of co~ponents in the mixture must be inside of the flammable region and, in addition, must be such that combustion will maintain some reasonable temperature in the combustion zone. Figure 3 illustrates the temperature profiles for various mixtures that fall in the flammable region and provides the temperatures for the mixtures that fall on the boundaries. Figure 3 provides the calculated adiabatic flame temperature for six mixtures of hydrogen and carbon dioxide. The maximum point on each curve represents approximately stoichiometric frring of the fuel mixture. The right end point for each curve corresponds to the adiabatic temperature at the upper flammability limit for the particular mixture, while the left end point is the calculated adiabatic flame temperature for the lower flammability limit. The maximum , ~)int for the 26 % H2 mixture is the adiabatic flame temperature at which the burner flamed \JUt. 10 |