Temperature theoretical flame

Figure 6.28 Increasing the theoretical flame temperature by reducing excess air or combustion air preheat reduces the stack loss.

Example 6.4 The process in Fig. 6.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C, and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume = 10°C for process-to-process heat transfer but = 30°C for flue-gas-to-process heat transfer. A high value for for flue-gas-to-process heat... 

The fuel consumption is now calculated by taking the flue gas from theoretical flame temperature to ambient temperature ... 

Flame Temperature. The adiabatic flame temperature, or theoretical flame temperature, is the maximum temperature attained by the products when the reaction goes to completion and the heat fiberated during the reaction is used to raise the temperature of the products. Flame temperatures, as a function of the equivalence ratio, are usually calculated from thermodynamic data when a fuel is burned adiabaticaHy with air. To calculate the adiabatic flame temperature (AFT) without dissociation, for lean to stoichiometric mixtures, complete combustion is assumed. This implies that the products of combustion contain only carbon dioxide, water, nitrogen, oxygen, and sulfur dioxide. 

Actual temperatures in practical flames are lower than calculated values as a result of the heat losses by radiation, thermal conduction, and diffusion. At high temperatures, dissociation of products of combustion into species such as OH, O, and H reduces the theoretical flame temperature (7). Increasing the pressure tends to suppress dissociation of the products and thus generally raises the adiabatic flame temperature (4). 

Theoretical flame temperatures assuming adiabatic and stoichiometric air. 

Example 15.5 A gas, which can be considered to be pure methane, is to be used as fuel in a furnace. Both the fuel gas and combustion air are both at 25°C. Calculate the theoretical flame temperature if the methane is burnt in ... 

Again, estimate the theoretical flame temperature to be 2000°C. The mean heat capacities are as before and the heat balance is now ... 

It can be seen that excess air and humidity in the combustion air both act to reduce the theoretical flame temperature. However, the excess air has the more significant effect. In some combustion processes, steam is injected into the combustion process to decrease the flame temperature to decrease NOx formation. This will be discussed later in Chapter 25. 

In these calculations, the fuel and combustion air were both at the standard temperature of 25°C. If the temperature of either had been below 25°C, then AllK would have acted to decrease the theoretical flame temperature. If either had been above 25°C, the effect would have been to increase the theoretical flame temperature. One energy conservation technique sometimes used in furnace design is to use waste heat to preheat the combustion air. This has the effect of increasing the theoretical flame temperature, and as will be seen later, increases the fuel efficiency. 

It should be emphasized that the theoretical and real flame temperatures will be significantly different. The real flame temperature will be lower than the theoretical flame temperature because, in practice, heat is lost from the flame (mainly due to radiation). Also, part of the heat released provides heat for a variety of endothermic dissociation reactions, which occur at high temperatures, such as ... 

However, as the temperature of the flue gas decreases, as heat is extracted, the dissociation reactions reverse and the heat is released. Thus, although theoretical flame temperature does not reflect the true flame temperature, it does provide a convenient reference to indicate how much heat is actually released by combustion as the flue gas is cooled. Figure 15.21 shows the flue gas starting from the theoretical flame temperature. This is cooled... 

For a given stack temperature, the higher the theoretical flame temperature, the higher the furnace efficiency. However there is a minimum excess air required to ensure that the combustion is itself efficient. 

As excess air is reduced, theoretical flame temperature increases. This has the effect of reducing the stack loss and increasing the thermal efficiency of the furnace for a given process heating duty. Alternatively, if the combustion air is preheated (e.g. by heat recovery), then again the theoretical flame temperature increases, reducing the stack loss. 

In Figure 16.27, the flue gas is cooled to pinch temperature before being released to atmosphere. The heat released from the flue gas between pinch temperature and ambient is the stack loss. Thus in Figure 16.27, for a given grand composite curve and theoretical flame temperature, the heat from fuel and stack loss can be determined. 

Fig. 15.20 Theoretical flame temperature versus air-to-fuel ratio of a boron-containing pyrolantwith and without boron combustion.

The adiabatic flame temperature may now be read from the graph shown in Figure 2.10. In this example, -1688kJmor corresponds to a theoretical flame temperature of about 3070 K. 

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