(203t) Process Simulation Study of Coal to Synthetic Natural Gas (SNG) With Gasification Technology

Taiwan is an isolated island with dense population and limited natural resources. In 2010, about 99.3% of Taiwan’s energy demand was dependent on import from abroad. The status of energy demand in Taiwan, by primary energy statistics, is described as follows: the percentages of crude oil, coal, natural gas, nuclear and others are 42.79%, 33.08%, 14.1%, 9.53% and 0.5%, respectively. The portfolio of electricity generation spreads over coal, gas, oil, nuclear, hydro and renewable (wind, solar, biomass and waste), with the portions of 49.91%, 24.61%, 3.83%, 16.85%, 2.94% and 1.86%, respectively. When energy prices rise, the cost of electricity and petroleum products will also be elevated; then, each sector will also be affected by the high energy cost, especially for the industrial sector.

It could be expected that the power generated from fossil plants will be increased to cover the shortage of electricity supply in Taiwan. The fluctuations in international energy prices and availability of imported energy supplies could deeply affect domestic socio-economic stability. This means that coal is still a major source for generating electricity in Taiwan. Since the price of coal is relatively lower than that of crude oil and natural gas, thus, it would be beneficial to employ gasification technology to convert solid fuel (coal) to alternative fuel, chemicals and electricity. The major advantages are lower cost of feedstock, higher reserve, easier import, etc.

The capacity factor of NGCC (natural gas combined-cycle) unit is lower in Taiwan due to the price of natural gas is relative higher than coal. It is beneficial to convert coal to synthetic natural gas (SNG), the SNG could be as fuel which the price lower than natural gas to the NGCC units. If the SNG is employed in Taiwan, the idled NGCC capacity could be activated. The electricity generation is increased without any new investment for power unit.

The system-level simulation model for synthetic natural gas production from coal gasification was performed in the study. The commercial chemical process simulator, Pro/II^® V8.1.1, is used in the study to build the analysis model. There are four major sections, i.e. air separation unit (ASU), gasification island, gas clean-up unit, and methanation processes included in the synthetic natural gas production model. Cryogenic air separation is employed in the ASU because of it is the most common technology to produce large quantities of oxygen and nitrogen in efficient and cost-effective means. The separation process of nitrogen and oxygen is based on two rectifying columns, i.e., high pressure (HP) and lowpressure (LP), respectively. The pressure levels of HP and LP were set to be 603.9 kPa and 162.5 kPa, respectively.

Gasification technology can convert solid fuel to gaseous fuel. Gasification reaction is a partial-oxidation reaction, of which the oxygen demand is in the range of 1/5 to 1/3 as the stoichiometric value with complete combustion; hence, it is sometimes called incomplete combustion reaction. Gasifier is operated at a high temperature in the range of 800 °C to 1,800 °C. The exact temperature depends on the characteristics of the feedstock and type of gasifier. For the entrained-bed gasifier, the gasification reactions are determined by the mass transfer phenomena with temperature above 1100 °C, and the time required for the gasification of a solid fuel is under 10 seconeds with particle size smaller than 0.1 mm. Thus, for chemical reactions at temperature above 1300 °C, equilibrium approach is adequate to calculate the thermodynamic state. GE slurry-fed entrained-bed gasification technology is used in the gasification island to convert solid fuel to synethesis gas (syngas).

The clean-up unit includes water-gas shift reaction, Selexol-based absorption process, and sulfur recovery processes. The water-gas shift reaction is adopted to adjust the specific ratio to meet the requirement from methanation processes. Selexol absorption process is adopted to remove sulfur compounds and CO₂ in the gas. The sulfur recovery process is used to produce elemental sulfur from H₂S stream which is stripped by steam. The process includes Claus process, Shell Claus off-gas treatment (SCOT) process and combustion of tail-gas.

The methanation process is adopted to convert syngas to methane. It consists of four methanation reactors with heat exchangers. The model of methanation process has been validated with the reference data. The methane content in the outlet stream is 96.56 mol% (dry) and the CO conversion is above 99.99%. The simulation results shown that the system efficiency of coal to SNG could be higher than 57% (HHV). In order to adjust the syngas content with specific ratio, 60.9% CO₂ is captured in the clean-up unit. It means the CO₂ emission could be lower than 450 g/kWh based on the CO₂ is captured in coal to SNG process. For Taiwan, it seems that coal to SNG is a beneficial clean coal technology option for electricity generation with lower CO₂ emission than pulverized coal-fired plant.