Boiler Exhaust Regeneration
The discussion of losses in combustion driven systems in the last chapter has shown that regeneration, i.e., use of exhaust energy by means of heat exchange within the system, can yield dramatic improvement of engine efficiency. In direct continuation of the argument, we first discuss regeneration in steam cycles, which rely on external combustion. For this, we need to consider not only the steam cycle, but also its heat source, which is hot combustion air.
Figure 12.1 shows a heat engine (HE) which is driven by heat exchange with a hot combustion product. Air at T0, flowing at rate m˙ , is mixed with fuel and burned so that the combustion product has the temperature TF . The heat supplied to the air from the combustion is (air standard approximation, i.e., fuel mass ignored)
The hot gas runs through the heat exchanger which it leaves at temperature TX , so the heat supplied to the heat engine is
remains unused; this is just the heat added to the environment when the exhaust equilibrates.
Earlier, we have discussed this set up when the heat engine is a Carnot engine, and have found the exhaust temperature TX for optimum work out- put, see Sec. 11.7. The discussion showed that the simplest way to utilize the exhaust heat Q˙ E is regeneration by preheating the air before combustion.
Figure 12.2 shows the system with an added regenerator for preheating the air. The heat exchange in the regenerator is, from the first law,
where TE is the final exhaust temperature, and TR the preheat temperature.
In a perfect regenerator the preheat temperature would be TX , and the exhaust would leave at T0. Accordingly, the regenerator effectiveness is defined as the ratio between the heat used for preheating, (hR − h0), and the heat available for preheating, (hX − h0), that is
where Q˙ H = m˙ (hF − hX ) is the heat supplied to the heat engine as before. Thus, with a perfect regenerator (ηreg = 1), we have Q˙ H = Q˙ F , i.e., all the heat provided from the fuel arrives in the engine.
A realistic regenerator has effectiveness of about 80%, and still leads to a much better fuel usage compared to direct exhaust into the environment. It must be noted that for several reasons a somewhat elevated exhaust temperature TE is beneficial: The combustion of fossil fuels generates water and sulfur oxides; the exhaust temperature must be high enough to avoid water condensation and subsequent formation of sulfuric acid. Also, the combustion air must be moved through the system, either by means of fans, or by natural draught chimneys, which rely on the buoyancy of warm air (Sec. 13.8). Since effective natural draught requires relatively warm exhaust, there is a marked loss. Therefore, modern power plants use fans.
From our previous discussion of heat engines we know that efficiency is high when heat is added at larger temperatures. Thus, for the heat engine one will aim at having the average temperature for heat addition TH as high as possible. The temperature TH is limited by the temperature-pressure characteristics of the working fluid and the materials used for construction. The maximum steam temperature in steam cycles using steel pipes in the steam generator is 560 ◦C. The regenerative steam cycles discussed below aim at raising the average temperature for heat addition, and thus increasing efficiency.
External (to the heat engine) irreversibilities occur in the combustion chamber, and in heat transfer to the heat engine. Our discussion of combustion processes in Chapter 25 will show that combustion irreversibility decreases with increasing flame temperature TF . On the other hand, heat transfer irreversibility grows with the temperature difference between combustion product (TF ) and the heat engine (TH ). If TH is limited, as is the case in steam power plants, reduction of TF decreases heat transfer irreversibility, but increases combustion irreversibility, with the total irreversibility staying relatively constant. Heat is transferred more easily at larger temperature dif- ferences, and one can adjust TF for efficient heat transfer. More efficient use of the fuel is made when the heat engine temperature TH is increased, as in the combined cycle of Sec. 13.6.