CO₂-Coal Slurry-Feed Gasification

Carbon policy enactment might directly inspire new approaches to gasifier and energy system design—as illustrated by the idea, currently being investigated by the Williams Group, of using coal-CO₂ slurries for gasifier feed systems.

Pressurized, O₂-blown, entrained-flow gasification has proven to be a promising approach for converting coal and biomass to clean, low carbon energy carriers such as electricity, H₂, synthetic natural gas, and synthetic liquid transportation fuels. Pressurization confers many advantages, both thermodynamic and economic. However, it also exacts costs (thermodynamic and economic), especially for gases, which are much more energy-intensive and costly to compress than liquids. For this reason, an entire class of coal gasifiers (e.g. GE and Conoco-Phillips E-Gas technologies) employs a coal-water slurry that can be pumped at low energy penalty and cheaply to high pressure prior to injection into a high-pressure gasifier. Dry feed gasifiers (e.g. Shell and Siemens), in contrast, pressurize the feedstock via relatively costly and complex lockhoppers that require compressed gas. For this reason, dry-feed gasifiers are typically operated at lower pressures than their slurry-feed counterparts.

CCS at coal conversion facilities offers a potentially attractive alternative: an abundant supply of liquid (i.e. supercritical) CO₂ for preparation of a CO₂–coal slurry for coal pressurization and transport into the gasifier. Such a system may have advantages over both water-coal slurries (due to a lower latent heat of vaporization for CO₂) and dry-feed systems (simpler, less costly pressurization). This idea was investigated by EPRI in the mid 1980’s, but the relative advantages remain unclear.

An investigation of this concept is underway in an effort led Tom Kreutz of the Williams Group in collaboration with visiting colleague Michiel Carbo from ECN in The Netherlands. The investigation will explore the pressure and temperature space of the slurry preparation, storage, transport and injection, seeking to understand the complex issues associated with mixing supercritical CO₂ with hot, pulverized coal, and to see whether there are promising system designs offering notable thermodynamic and economic benefits.

While full-time at ECN, Carbo is also finishing his Ph.D. at Delft University of Technology. He has designed and analyzed in detail many important sub-systems at coal IGCC+CCS plants—including water-gas shift (WGS), H₂ separation membrane reactors, novel WGS designs, and off-design gas turbine operation.