Stacks temperature

Figure 6.29 Furnace stack temperature can be limited by other factors than pinch temperature.

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 ...

Fired Heater as a Heat-Exchangee System. Improved efficiency in fired heaters has tended to focus on heat lost with the stack gases. When stack temperatures exceed 150°C, such attention is proper, but other losses can be much bigger when viewed from a lost-work perspective. For example, a reformer lost-work analysis by Monsanto gave the breakdown shown in Table 2. 

Equipment Tests. Procedures for rigorous, detailed efficiency determination are available (ASME Test Codes) but are rarely used. For the objective of defining conservation potentials, relatively simple measurements are adequate. For fired heaters, stack temperature and excess O2 ia stack should be measured for turbiaes, pressures (ia and out) and temperatures (ia and out) are needed. 

Checking Against Optimum Design. This attempts to answer the question whether a balance needs to be as it is. The first thing to compare against is the best current practice. Information is available ia the Hterature (13) for large-volume chemicals such as NH, CH OH, urea, and ethylene. The second step is to look for obvious violations of good practice on iadividual pieces of equipment. Examples of violations are stack temperatures > 150° C process streams > 120° C, cooled by air or water process streams > 65° C, heated by steam t/ urbine 65% reflux ratio > 1.15 times minimum and excess air > 10% on clean fuels. 

High stack temperature can be the result of an improper air to fuel mixture. A leak of combustible material from the process side to the firetube is also a cause. It can also be the result of excessive soot deposition in the firetube. 

When lowering stack temperature, be aware of the dew point as discussed in the next topic. 

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

The efficiency for a boiler or heater is improved by lowering its stack temperature. The stack minimum temperature is frequently limited by SO3 gas dew point. References 6, 7, and 8 discuss this important subject. A stack as hot as 400°F (or perhaps higher) can have problems if the SO3 concentration is high enough. Reference 9 states that SO, condensation will produce a blue-gray haze when viewed against a clear blue sky. 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

Gnu)cg is usually limited by the allowable stack temperature Ts. As a fraction of the heat supplied to the cogeneration plant it remains constant in this application. 

Zinc oxide loading heavy during zinc additions. Stack temperatures high. 

With the incorporation of an economizer into a boiler system, typically a 1% gain in boiler efficiency can be achieved for every 40 to 50 °F reduction in stack temperature. 

Where stack temperatures are low and sulfur gases are present, problems of dry soot emissions give way to wet acid smutting. 

Natural draft is a low capital cost option, but the draft requirements tend to lead to high stack temperature with a low efficiency for the furnace. Forced draft and induced draft are higher capital cost options than natural draft but tend to lead to higher furnace efficiencies as the stack temperature can be lowered significantly. 

Example 15.6 For the three cases in Example 15.5, determine the furnace efficiency for an assumed stack temperature of 100°C. 

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. 

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. 

A fuel cell system for automobile application is shown in Figure 1.5 [41]. At the rated power, the PEMFC stack operates at 2.5 atm. and 80°C to yield an overall system efficiency of 50% (based on lower heating value of hydrogen). Compressed hydrogen and air are humidified to 90% relative humidity at the stack temperature using process water and heat from the stack coolant. A lower system pressure is at part load and is determined by the operating map of the compressor-expander module. Process water is recovered from spent air in an inertial separator just downstream of the stack in a condenser and a demister at the turbine exhaust. 

Since each system achieves the same total fuel utilization (90%) across the same total area, each stack has the same average current density. Irreversible voltage loss is mainly a function of current density and stack temperature. Since these parameters are equivalent in each stack, it is assumed that the Nemst potential of each stack would be reduced by the same amount. 

Between stacks networked in series, heat can be removed from the reactant streams to assist in controlling stack temperatures. The heat in a network reactant stream can be transferred to a cooler process stream in a heat exchanger or hot and cold reactant streams can be mixed directly. The recovered heat may be utilized in a combined cycle or for cogeneration. 

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