1. Introduction - Firenze University Press

Fig. 2. The exergy destruction of MEOH synthesis with the conversion ratio (circulation ratio)
Aiming at the lacks of the conventional single methanol production process, by proper adjustment
instead of full adjustment of the component of the syngas, the CO/H2 of the unreacted gas in
methanol synthesis unit can exactly satisfy the CO/H2 requirement for SNG synthesis and thus the
component adjustment for SNG synthesis can be avoided. On the other hand, part of the unreacted
gas, which is hard to be converted, will be sent to combustion for power. This concept of system
integration for polygeneration system is part of the principle “Cascade utilization of chemical and
thermal energy”, which is interrupted in Figure 3.
Fig. 3. The principle of system integration for polygeneration systems
In Figure 3, A on the vertical coordinate represents the energy level. The fossil fuel with high
energy level is first converted to chemicals according to its component in cascade. And then, the
unreacted, which is hard to be converted, is sent to generate high temperature by combustion for
high-efficiency combined cycle. By such cascade utilization of fossil fuels, energy systems with
high efficiency would be expected.
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