Warm Syngas Clean-up Processes Applied in Synthetic Natural Gas (SNG) Production with Coal and Biomass

Abstract

In Taiwan, the Greenhouse Gas Reduction Act has been issued since 2015, which sets the target to reduce greenhouse gas (GHG) emissions to 50 % of the 2005 level by 2050. Natural gas is one of the options to reduce the GHG emissions due to lower CO₂ emission in electricity generation. The value discharged from NGCC (natural gas combined-cycle) is near half of that from coal-fired power plant. The price of natural gas in Taiwan is substantially higher than that of coal, which results in the situation that near 50 % of electricity is generated from coal and the capacity factor of NGCC units is relatively low. If cheaper gas fuel could be provided to domestic NGCC units, their capacity factor could be increased; then, the CO₂ emission in power sector could be decreased to help meet the GHG reducing target. Synthetic natural gas (SNG) from solid fuel via gasification is possible to provide a relatively lower price than that of natural gas to NGCC units in Taiwan. The price of SNG from coal has been studied in 2012, and the results showed that the mean price of SNG is US$ 12.27 /MMBtu, while the liquefied natural gas (LNG) is US$ 14.32 /MMBtu. It shows the possibility to decrease the CO₂ emission with relatively lower cost of electricity in Taiwan.
The SNG production processes have been built with the commercial chemical process simulator, Pro/II^® V8.1.1, to analyse the efficiency improvement with warm gas clean-up processes. The four major blocks, consisted of air separation unit (ASU), gasification island, gas clean-up unit, and methanation processes, were built in a previous study. Two different parts in the study, i.e., warm syngas clean-up processes and another kind of biomass that is possibly used in Taiwan. The warm gas clean-up process is implemented to keep the temperature of syngas in the range of 400 °C to increase the available energy which is compared with typical one with lower temperature. A series of sorbents are selected for the processes: e.g., Na₂CO₃-based sorbent to remove HCl and ZnO-based sorbent to deminish sulphur contained in syngas, while CaO-based sorbent for removal of CO₂ to enhanced the methanation processes. The results show that the energy penalty of CO₂ capture could be improved as warm gas clean-up processes are adopted in the system. Biomass could further reduce the CO₂ emission, due to the advantage of carbon neutral feature. The effect of biomass blended with coal shows the similar trend with previous study, i.e., the system performance is slightly decreased with the blend percentage of biomass.