1. Introduction - Firenze University Press

Figure 9(a) illustrates the energy utilization of quench process in single product systems, in which
cooling water of about 30 will be jet to mix with the hot syngas continuously to lower the
temperature of the syngas very quickly. Figure 9(b) illustrates the energy utilization of recovery of
sensible heat of syngas in the novel polygeneration system. In both Figure 9(a) and 9(b), the cooling
down of syngas (curve Aed1) acts as the energy donor and the heat absorption of water acts as the
acceptor (curve Aeas). It can be found that the average energy level of the acceptor in the novel
polygeneration system has been increased for just one jet of water in WHB is adopted, whereas the
average energy level of the acceptor in single product systems has been decreased for the adding of
the cold water continuously, and thus the exergy destruction in the novel system has been decreased
by 6.0MW for such process.
Fig. 10. EUD for recovery of chemical emissions
Figure 10 discloses exergy utilization in the process of recovery of chemical emissions. Curve Aed
represents for the oxidation of the recovery chemical emissions. Curve Aea1 represents for the fuel
heating process, and cure Aea2 represents for the air heating process. In single product systems, the
chemical emissions will be emitted to atmosphere directly after combustion, whose exergy
destruction is represented by the area between curve Aed and the horizontal ordinate. Whereas, in
the novel polygeneration system, the chemical emissions will be recovered for power, and the
shaded area 1 represents for the exergy destruction of this process, which is much smaller than
that in single product systems.
5. CONCLUSIONS
In this paper, a novel coal-based polygeneration system with CO2 capture, which cogenerates
power, natural gas and liquid fuel, has been proposed. With 54.9% of thermal efficiency and 62% of
carbon captured, the primary energy saving ratio of this novel polygeneration system can reach as
high as 10.8 percent compared with the single product systems. Based on the graphical exergy
analysis, it is disclosed that abolishing captive power plant, converting the unreacted gas from
methanol synthesis unit into SNG without composition adjustment, and recovering chemical
emissions for power play an important role in decreasing exergy destruction in the novel system.
This novel system has realized “the cascade utilization of chemical and thermal energy of the coal”
and realized the CO2 separation with low energy penalty. The promissing results obtained in this
coal-based polygeneration system can realize both the coal decarbonization with low energy penalty
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