Absolute combustion defined

Operating a process heater simply to achieve a minimum excess oxygen target in the flue gas can waste a great deal of energy. The proper way to adjust O2 to a heater is to target for the point of absolute combustion, as shown in Figure 15-5. The point of absolute combustion is defined as that air rate that maximizes heat recovery to the process. That is, either a decrease or an increase in the combustion air supply will reduce heat absorbed in the heater. 

The heater process outlet temperature declines as air flow is reduced past the point (f absolute combustion as defined in the next section. In this situation we have products of incomplete or partial combustion such as aldehydes, ketones, and carbon monoxide going up the stack. This sets the heater up for afterbum in the stack, and the heating value of the fuel is also effectively reduced. 

The emissions from combustion processes may be predicted to some extent if the variables of the processes are completely defined. Figure 6-7 indicates how the emissions from a combustion source would be expected to vary with the temperature of the reaction. No absolute values are shown, as these will vary greatly with fuel type, independent variables of the combustion process, etc. 

In this equation (2) t, is the time spent necessary for the isothermal combustion of the deactivating compounds, b the heating rate, T the absolute temperature, R the ideal gas constant, p the pressure and E the activation energy for an oxidative regeneration. The temperature 7] is the temperature at which a defined percentage of the residuals is burned off, and can be determined with the help of thermogravimetric analysis. 

AH is the enthalpy of the reaction W, is the mechanical work produced and r define the absolute temperatures corresponding to a cold and a hot reservoir, respectively. The efficiency ethemai is the ratio of the work done by the engine to the heat drawn out of the hot reservoir. This efficiency is lower than 40% for gas turbines and lower than 30% for internal combustion engines. 

A minimum export steam plant is defined as a plant that optimizes heat recovery in the plant to the maximum extent possible. This is often done when the value of steam is essentially zero or the price of the feedstock and fuel are exceptionally high. Minimum steam export is often achieved by first increasing the SMR process gas inlet temperature and then the combustion air preheat temperature, both of whieh reduce the fired duty of the SMR. Typical temperatures are 1150°F for the process gas and 900°F for the air preheat. These changes reduce the export steam to a low level, but typically not completely to zero. Additional modifications are required to reduce the export steam to an absolute minimum. 

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