1. Introduction - Firenze University Press

studied by Möllersten et.al, Obersteiner et.al [17-19], and chemical-looping combustion systems
have been investigated by Jin and Ishida [20].
However, great energy penalty must be paid for CO2 capture in most of the above systems except
for few polygeneration systems or chemical combustion systems et.al, which usually lead to an
overall thermal efficiency decrease by nearly 7.0~15.0 percent points. Thus, how to reduce the
energy penalty for CO2 capture in coal-based energy systems is of significant importance.
Meanwhile, for countries with abundant coal but lack of natural gas and oil, energy security is
another big issue that needs to be considered. For these counties, production of SNG or alternative
liquid fuel from coal may be a more wise strategy instead of increasing import increasingly. For
example, in China, with the rapid economic growth, the liquid fuel demand is increasing sharply as
a result of the up burst of the number of cars. It is predicted that the demand for liquid fuel will soar
to 0.45~0.61 billion tons [21-22] whereas the supply of liquid fuel can only keep at about 0.2 billion
per year [22]. Huge gap exists between liquid fuel demand and supply in China.
Aiming at the two big issues including CO2 abatement and energy security, the purpose of this
paper is (1) to integrate a novel coal-based polygeneration system in which power, natural gas and
liquid fuel are cogenerated and CO2 is captured with low energy penalty; (2) to reveal the internal
phenomena of key processes in the new system by exergy analysis; (3) and to provide an option for
production of SNG and alternative liquid fuel from coal to enforce energy security.
2. PROPOSAL OF THE NEW POLYGENERATION SYSTEM
2.1. Basic concept of system integration
Fig. 1. Single methanol production process
For single methanol production process, it is the main target to convert the raw material into
products to the maximum extent. To achieve this target, the H2/CO in the syngas must be adjusted
to be 2:1 and the unreacted gas should be fully recycled. Whereas, the component adjustment of the
syngas will lead to great portion of exergy destruction and the full recycle of the unreacted gas will
also requires large amount of work. When the conversion ratio exceeds a certain value, the exergy
destruction for methanol synthesis will increase sharply if we purely pursue higher conversion of
the raw material, as shown in Figure 2. This concept that “exhaustion of the active composition of
the material” causes great energy consumption for methanol production in some way, which is
about 45GJ/t.
2