Effect of pretreatment with carbonic acid on "HyperCoal" (ash-free coal) production from low-rank coals

The use of "HyperCoal" (ash-free coal) as feedstock for gas turbines results in higher net power output with lower CO2 emissions. HyperCoal can be produced by thermal extraction from low-rank coals with industrial organic solvents in an inert atmosphere, providing raw materials. The pretreatment of low-rank coals with carbonic acid (CO2 dissolved in water-CO2/H2O) produced a strong increase in HyperCoal yields at relatively lower CO2 pressures of 0.1-0.5 MPa; the thermal extraction yields at 360 degrees C increased by 7%-15% with extraction yields of 52% and 45% obtained for Wyodak sub-bituminous coal and Beulah-Zap lignite, respectively. In the range of 320-360 degrees C, crude methylnaphthalene oil (CMNO) extraction yields of pretreated Wyodak coal increased significantly (by 4%-11%) over those of raw coal. The enhanced extraction yields of these low-rank coals are attributed to disruption of cation-bridging crosslinks on acid pretreatment, and the release of the hydrogen bonds by CMNO extraction.