1. Introduction - Firenze University Press

ineffectiveness of the catalyst, in which steam at 535 and 120bar is generated and sent to
combined cycle subsystem for power. After the three reactors, the products is cooled down to 25
to remove water and then sent to a flash drum to separate a spot of methanol which is mixed in the
SNG. SNG of over 93% CH4 (mole basis) is produced and the separated mixed methanol is sent for
combustion in the combined cycle subsystem for power.
As shown in Figure 4, the new system applies the serial connection between the chemical
production process and power generation system: all of the syngas produced by gasifier enters the
chemical production process at first, and after methanol production and SNG production, the
unreacted gas is then sent to power generation subsystem.
Compared with the single methanol and SNG synthesis production process, the new system has the
following key features: (1) Adopting partial adjustment of composition of the fresh gas instead of
full adjustment to satisfy the H2/CO for both methanol synthesis and SNG synthesis, and thus
avoiding greater exergy destruction for individual adjustment of composition of fresh gas in single
production systems; (2) Instead of total recycle of the unreacted gas in methanol synthesis unit,
partial-recycle scheme is adopted in the polygeneration system, avoiding the sharp increase of
energy consumption for methanol synthesis, and at the same time realizing the SNG synthesis with
no adjustment of the composition and efficient power generation; (3) recovery of the sensible heat
of the syngas and recovery of gases released in chemical synthesis process for power.
2.3. Description of the reference systems
For performance comparison, a system of integrated gasification combined cycle (IGCC) for power
generation is selected as a reference for the application of power generation. A coal-based methanol
production process which adopts the low-pressure Lurgi methanol synthesis technology is selected
as the reference for single methanol production system. And a coal-based SNG synthesis process, in
which High Pressure SNG synthesis technology is applied, is selected as the reference system for
SNG production.
Figure 5(a) shows the flowsheet of IGCC system, including the air separation unit (ASU), a Texaco
slurry-feed and O2-blown gasification unit, clean up unit adopting Selexol process to remove
sulfids, and a combined cycle unit.
Figure 5(b) shows the flow sheet of single methanol product process, which can be identified into
fresh gas preparation subsystem and methanol synthesis subsystem. An ASU unit, a Texaco
gasifier, and a cleanup unit are included in fresh gas preparation unit. The composition adjustment
unit for producing fresh gas with proper CO/H2, which can meet the synthesis of methanol, is also
included in fresh gas preparation subsystem. The methanol synthesis subsystem includes the
synthesis unit and distillation unit. In methanol synthesis unit, nearly all of the unreacted gas is sent
back to the reactor to pursue highest conversion of materials. In the distillation unit, the crude
methanol will be refined as the final product. The steam and work requirements in single methanol
product process are supplied by a captive power plant with coal-fired boiler, and by recovering the
surplus heat of the chemical reaction.
The single SNG product process is shown in Figure 5(c), which can also be divided into fresh gas
preparation subsystem and SNG synthesis subsystem. Different from the single methanol process,
in the cleanup unit CO2 and sulfids will be removed by low-temperature methanol wash method. In
SNG synthesis unit, three sequential reactors adopting TREMP process designed by Haldor Topsoe
are placed, in which synthesis reaction occurs at about 80bar, above 300. Between reactors, steam
with high parameters, usually 120bar, 535, will be generated to recover heat from the products in
case the catalyst was burnt. The superheated steam will supply the work requirement of the single
SNG product process, the insufficient of which will be provided by a captive power plant with coalfired
boiler if necessary.
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