1. Introduction - Firenze University Press

Not only at desined Rc (the ratio of chemical products to power) does the new polygeneration
system performance well, but also at other variable Rc. Results show that when Rc varies from 0.5 to
3, this new polygeneration system can always save primary energy compared with single product
systems. And an optimal Rc exists to make the thermal performance of the new polygeneration
system highest, as shown in Figure 6 and Figure 7. The results illustrated in figures 6 and 7 show
that the polygeneration system achieve best performance by adopting partial recycle instead of full
recycle of the unreacted gas.
To disclose the key process of energy saving and to reveal the high performance of the
polygeneration system, exergy analysis is applied and listed in Table 4 (at designed Rc). Compared
with the single product systems, the exergy destruction in the new polygeneration system can
decrease by 89.2MW assuming the same product output. In the new polygeneration system, the
internal power consumption, for example the work for compressors is supplied by steam turbines
and gas turbines instead of the captive power plant of single product systems, and this change can
decrease exergy destruction by 38.2MW. This is because coal-based steam system is adopted in
captive power plant, and the huge temperature difference between the combustion temperature of
coal, as high as 1600, and the 600 steam leads to large amount of exergy destruction of the
captive power plant. Whereas, in this polygeneration system, the difference between the inlet
temperature of gas turbine, as high as 1200, and the combustion temperature of fuel is much
smaller, and thus the exergy destruction is much less. By adopting proper component adjustment
instead of full adjustment of syngas and abolishing the component adjustment in SNG process, the
shift process can decrease exergy destruction by 3.1MW. Waste heat boiler instead of quench to
recover sensible heat of the raw syngas can decrease exergy destruction by 6.0MW. And recovery
of the chemical emissions to combustion for power can decrease exergy destruction by 15.6MW.
Other processes, like the ASU or gasification process, can also decrease the exergy destruction for
the less input of fuel in the new polygeneration system.
Table 4. The exergy analysis for polygeneration system and reference systems
Items
Polygeneration
system with CO2
MW
capture
%
11
Single product systems
Total of ref IGCC MEOH SNG
MW
MW
MW MW
Fuel exergy 739.1 100 828.3 206.8 241.3 380.2
Exergy output
SNG 215.9 29.2 215.9 215.9
Methanol 118.4 16.0 118.4 118.4
Electricity 89.5 12.1 89.5 89.5
Exergy
destruction
ASU 18.8 2.5 21.0 5.6 5.3 10.1