Flue-gas stack temperature

The heat of combustion is a product of the amount of fuel consumed and the net heating value of the fuel. The heater s efficiency is a function of the flue-gas stack temperature, the excess air or oxygen, and the ambient-heat losses from the firebox and the convective-section structures. 

Reduction of the flue gas stack temperature to reduce heat losses to the atmosphere [623]... 

High preheat temperatures for feed and combustion air lead to reduced fuel consumption and thereby to savings, especially when the steam production in the plant must be minimized. Reduction of the flue gas stack temperature reduces the heat loss to the atmosphere. A similar effect has, perhaps more importantly, been obtained by improved insulation in the reformer. 

A2. Because fuel costs are much higher in high-temperature furnaces than in lower temperature furnaces as a result of the higher flue gas exit temperature causing higher stack loss. 

The loss caused by sensible heat in the flue gases (stack loss) can be evaluated as the %net heating value (90% for natural gas) minus the %available heat at the flue gas exit temperature, from Figure 5.1. At high temperature, the loss becomes excessive, especially with high excess air thus, such cases give payback by using heat recovery. 

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 6.30 shows the grand composite curve plotted from the problem table cascade in Fig. 6.186. The starting point for the flue gas is an actual temperature of 1800 C, which corresponds to a shifl ed temperature of (1800 — 25) = mS C on the grand composite curve. The flue gas profile is not restricted above the pinch and can be cooled to pinch temperature corresponding to a shifted temperature of 145 C before venting to the atmosphere. The actual stack temperature is thus 145 + 25= 170°C. This is just above the acid dew point of 160 C. Now calculate the fuel consumption ...

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

A further complication is that as exit-gas temperatures rise, the volume of air and flue gas passing through the stack increases, and this can cause load limitations and a further reduction in efficiency. 

Fired heaters radiant rate, 12,000 Btu/(hr)(sqft) convection rate, 4000 cold oil tube velocity, 6 ft/sec approx equal transfers of heat in the two sections thermal efficiency 70-75% flue gas temperature 250-350°F above feed inlet stack gas temperature 650-950°F. 

The split between the radiant and convection section heat varies according to the design. Casing losses are usually between 1 and 3% of the heat release from combustion. The heat loss from the stack is constrained by the desire to avoid any condensation of water vapor in the convection section. If there is any sulfur present in the fuel, then the condensate will be corrosive. The temperature at which the flue gas starts to condense is the acid dew point. For sulfurbearing fuels, the temperature of the flue gas is normally... 

Obviously, the lower the stack temperature, the higher the furnace efficiency. As already noted, it is desirable to avoid condensation in the convection section and the stack. If there is any sulfur in the fuel, the condensate will be corrosive. There is thus a practical minimum to which a flue gas can be cooled without condensation causing corrosion in the stack, known as the acid dew point. If there is any sulfur in the fuel, the stack temperature is normally kept above 150 to 160°C. Natural gas can normally be cooled to... 

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. 

Selection of Oxides. At Amoco, previous studies in the literature on SO2 removal from flue gas have been used to guide the selection of oxides for the UltraCat process but they have been of limited direct usefulness. This was true because of the peculiar requirements of the UltraCat process of high adsorption temperature, low regeneration temperature, and non-interference with the cracking reactions. The previous literature studies generally assumed that SO2 would be adsorbed at temperatures close to a stack gas temperature of 600 E, and desorb at either the same temperature or higher. The conditions of these studies was set. 

The elemental sulfur is removed by conventional technology. The gases are purified by the Lurgi Rectisol process which uses a low temperature methanol wash to remove H2S, COS and CO2. The acid gas stream is then passed to a Stretford unit which is preferred to the Claus unit because of the high percentage of carbon dioxide in the stream. Sulfur in the stack gas would be removed by conventional flue gas desulfurization techniques and the sulfur would then remain as sulphite sludge and not be recovered as elemental sulfur. 

It is also doubtful that the industry will be in a position for many years to come to undertake sulfur removal from residual fuels solely to improve product quality. A number of consumer industries demand low sulfur fuel oils, but these special requirements can at present be met more appropriately by selection of crude rather than by adoption of desulfurization processes. In general industrial use, it is corrosion and atmospheric pollution that are the main disadvantages of high sulfur content. But there is no sign yet of the development of a cheap desulfurization process, the cost of which can be substantially offset by the gain in efficiency resulting from permissible lower stack temperatures or by the elimination of flue gas scrubbing equipment previously necessary for reduction of sulfur dioxide content. 

I have noted that the stack, flue-gas temperature of the boiler is 680°F. Let s assume that we have around 3 percent oxygen in the flue gas (or about 15 percent excess air). We can then apply the following rule of thumb For every 300°F that the flue-gas temperature exceeds the ambient air temperature, the boiler efficiency drops by 10 percent. 

What will happen is that flames and black smoke will pour out of the heater s stack. It looks very bad, very dangerous, but it is really not. There is not enough excess oxygen in the firebox to cause a very high temperature. The combustion gases, or flue gas, are too fuel-rich to explode. We say that the flue gas is above the explosive region. 

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