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1. Introduction - Firenze University Press

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4. DISCUSSION<br />

The graphic exergy analysis (EUD methodology) for the key processes in the polygeneration<br />

system is adopted to reveal the internal phenomena of high performance of the key processes. A<br />

represents for the energy level, and H represents for the enthalpy change. Figure 8(a) illustrates<br />

the Energy Utilization Diagram for combustion of coal in boiler of captive power plant of single<br />

product systems. The oxidation of coal (curve Aed1) and the heat recovery in coal economizer (curve<br />

Aed2) act as the energy donor. The energy acceptors include preheating of air (curve Aea1), the<br />

preheating of fuel (curve Aea2), preheating (from 90 to 35<strong>1.</strong>6), evaporation, superheating (from<br />

35<strong>1.</strong>6 to 540) of water in boiler (curve Aea3), and the water preheating in boiler economizer<br />

(from 50 to 90, curve Aea4). The shaded area between energy donor and acceptor represents for<br />

the exergy destruction of the process. The area 1 in Figure 8(a) represents the exergy destruction<br />

in boiler, and 2 for exergy destruction in coal economizer. The average energy level of coal is<br />

about <strong>1.</strong>03, but the average energy level of superheating steam is no more than 0.65. This indicates<br />

that huge difference exists between the energy level of energy donor and acceptor, which causes<br />

large amount of exergy destruction in captive power plant of single product systems.<br />

Fig. 8. (a) EUD for combustion of coal in boiler of captive power plant of single product systems;<br />

and (b) EUD for the combined cycle in the new polygeneration system<br />

13<br />

(b)<br />

(a)

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