Stack loss

In Fig. 6.27, the flue gas is cooled to pinch temperature before being released to the atmosphere. The heat releaised from the flue gas between pinch and ambient temperature is the stack loss. Thus, in Fig. 6.27, for a given grand composite curve and theoretical flcune temperature, the heat from fuel amd stack loss can be determined. 

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

As with the steam turbine, if there was no stack loss to the atmosphere (i.e., if Qloss was zero), then W heat would he turned into W shaftwork. The stack losses in Fig. 6.34 reduce the efficiency of conversion of heat to work. The overall efficiency of conversion of heat to power depends on the turbine exhaust profile, the pinch temperature, and the shape of the process grand composite. 

Qloss stack loss from furnace, boiler, or gas turbine (kJ s )... 

Figure 1. Stack loss vs. excess oxygen and stack temperature.

The profile shown in Figure 15.21 represents the furnace efficiency, if the casing heat losses are neglected. Making this assumption, the process duty plus the stack loss represents the heat released by the fuel. 

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 Figures 16.27 and 16.28, the flue gas is capable of being cooled to pinch temperature before being released to atmosphere. This is not always the case. Figure 16.29a shows a situation in which the flue gas is released to atmosphere above pinch temperature for practical reasons. There is a practical minimum, the acid dew point, to which a flue gas can be cooled without condensation causing corrosion in the stack (see Chapter 15). The minimum stack temperature in Figure 16.29a is fixed by acid dew point. Another case is shown in Figure 16.29b where the process away from the pinch limits the slope of the flue gas line and hence the stack loss. 

Example 16.4 The process in Figure 16.2 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 A7 = 10°C for process-to-process heat transfer but A7 = 30°C for flue gas to process heat transfer. A high value for A 7 mln for flue gas to process heat transfer has been chosen because of poor heat-transfer coefficients in the convection bank of the furnace. Calculate the fuel required, stack loss and furnace efficiency. 

It is necessary practically to use more than Ihe theoretical air requirements to assure sufficient oxygen fur complete combustion. Excess air would not be required if it were possible to have an ideally perfect union of air and fuel. It is necessary, however, to keep the excess al a minimum in order to hold down Ihe stack loss. The excess air that is not used in the combustion of the fuel leaves the unit at stack temperature. The beat required to heat this air from room temperature to stack temperature serves no purpose and is lost heat. Table 5 gives realistic values of excess air lor the fuel-burning equipment which experience has shown is required to assure complete combustion for various fuels and methods of firing. 

The stack losses, while not insignificant, represent only 5.5% of the exergy in the coal in contrast, they represent nearly 15% of the coal s energy content. 

One last point to be noted pertains to a comparison between the steam-reforming reaction (Case 4) and the methanol cracking reaction (Case 5). From the exergy ratio calculations, the reforming reaction appears to be superior in its ability to produce lower quality fuel. However, the overall efficiency calculations show a lower value for Case 4 than that for Case 5. The main reason for this reversal is due to the fact that nearly 25% of the recuperated energy for Case 4 is in the form of the heat of evaporation of H 0 and is not recovered from the exhaust gases. The result 1s an increase in the stack losses. 

We will determine the dependence of the P.O.P. stack loss (to be denoted by y) upon the cooling water temperature, Xw, and the air temperature, Xa. 

It was assumed that the stack loss was to a first approximation related linearly to the tw o independent variables., so that it could be represented by an equation of the form... 

From prior calibrations we found that pH of the solution denoted the sulfite/bisulfite salt ratio and conductivity data were related to the concentration of salts in solution. From these data the quantity of ammonia that was regenerated could be calculated. A calculated ammonia balance over the first 83 hr of operation showed 4.23 gram-moles ammonia loss from all causes, which is representative of a maximum stack loss of 3.53 ft of ammonia. Since a total of 41,500 ft of gas was processed, ammonia loss was about 85 ppm. On a once-through basis this represents about... 

Absorption trains of early ammonia oxidation processes to nitric acid were constructed of chemical stoneware or acid-proof brick, which restricted acid production to near ambient atmospheric pressure because of the low strength of the structural materials. The discovery that Duriron (silicon-iron) or high chrome stainless steels could tolerate these corrosive conditions well allowed the adoption of pressure absorption. This measure markedly decreased the size of the absorbers required and reduced nitrogen oxide stack losses. Pressure operation was easiest to achieve by compression of the feed gases at the front end of the process. In this way improved acid production is obtained at comparable capital costs per unit of product by operation at atmospheric pressure. 

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