1. Introduction - Firenze University Press

2.2. System integration and innovation
A conceptual flowsheet of the new polygeneration system is shown in Figure 4. The syngas
produced in a Texaco gasifier is sent to a Waste Heat Boiler (WHB) to recover its sensible heat
from 1346 to 231, in which the steam at 120bar will be superheated to 535 and then enters
the steam turbine. After heat recovery, the syngas together with a 230 steam at 30bar enters the
shift reactor for component adjustment, in which the mole ratio of H2/CO is adjusted to 2.65. Then
the syngas leaving the shift reactor is cooled down to 40 after two heat exchangers, in which
steams of 230 at 9.8bar and 180 at 5bar are generated and will be sent to the sequential cleanup
unit. In the clean-up unit, most of the sulfides and CO2 in the syngas will be removed through
the low-temperature methanol wash process. The CO2 from the clean-up unit is high concentrated
(mole fraction is typically over 99%). Then, the purified syngas, in which the mole content of H2S
is lower than 10 -6 , with CO and H2 will be sent to the methanol synthesis unit sequentially.
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1-raw syngas; 2-fresh gas; 3-crude methanol; 4-unrecycled gas for power; 5-unrecycled gas for
SNG synthesis; 6-cycled gas for SNG synthesis; 7-crude SNG; 8-recovered methanol; 9-distillation
waste; 10-inlet gas for gas turbine; 11- inlet gas for HRSG; 12- flue; 13-methanol product; 14-SNG
product; 15-CO2 product
Fig. 4. New polygeneration system adopting proper composition adjustment and partial-recycle
scheme with CO2 capture
Liquid Phase Methanol synthesis (LPMEOH) technology is adopted. The reaction occurs at 76bar,
and the temperature of the reactor is kept by the evaporation process of cooling water at about 200
.After the reactor, the crude products will be cooled down to 40 through two heat exchangers,
in which the steam for shift reaction and a steam saturated at 3.75bar are generated. Then, the crude
methanol and the unreacted gas will be separated by flash. Crude methanol is sent to a flash drum to
remove the dissolved gas, and then fed into a three-stage distillation unit. The unreacted gas will be
divided into three streams: the recycled gas, which will be recycled back to the methanol synthesis
reactor, the reacting gas for SNG synthesis, and the unrecycled gas, which will be sent to the
combined cycle subsystem for power.
High Pressure SNG synthesis technology is adopted in this polygeneration system. The reacting gas
for SNG synthesis from the methanol synthesis unit is sent to three sequential adiabatic reactors at
about 79bar. Nickel-based catalyst MCR-2X developed by Topsoe company is used for SNG
synthesis, which can keep high activity in the range of 300-700. Between each reactor,
exchangers are placed to recover the high-temperature heat of the products to avoid the
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