The Williams Group has also begun investigating two other advanced CCS concepts: CO2 slurryfed gasification and use of produced brine for plant cooling while enhancing geologic CO2 storage.


CO2-Coal slurry-feed gasification

The production of electricity, H2, synthetic liquid transportation fuels, and substitute natural gas via pressurized, O2-blown, entrained flow gasification with CCS is a proven approach for providing a wide range of low-carbon energy carriers from coal and biomass. But the high energy and economic costs of compressing gases compared to liquids has led to a class of commercial coal gasifiers (e.g. GE and Conocco-Philipps E-Gas) that employ a gasifier feed system employing a coal-water slurry that can be cheaply pumped to high pressure to enable injection into the gasifier. Such gasifiers are not well-suited for gasifying low-rank coals that account for about ½ of global coal reserves and whose high moisture contents render coal-water slurry feeding uneconomical—it consumes too much oxygen. Accordingly, for gasification of low-rank coals, dry-feed gasifiers (e.g., Shell and Siemens), which pressurize the feedstock via relatively complex and costly lockhoppers that require compressed gas, are preferred. But these 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 source 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 slurry feed systems (lower latent heat of vaporization) and dry feed systems (simpler, less costly pressurization). The concept appears to be particularly interesting for gasifying low-rank coals.

EPRI carried out preliminary investigations in the mid 1980’s but did not sustain this activity because of the absence of adequate CO2 supplies. During 2009 the concept of coal-CO2 slurry-fed gasification was investigated with Michiel Carbo, a visiting colleague from ECN. The pressure and temperature space of the slurry preparation, storage, transport and injection, were examined to try to understand the complex issues associated with mixing a supercritical CO2 with hot, pulverized coal and identify designs having notable thermodynamic and economic potential. Multiple Shell-based coal gasification plants (including feedstock pressurization, gasification, water-gas shift section, Rectisol acid gas removal, CO2 compression, syngas expansion and combined cycles) were constructed in Aspen Plus, operating either gas dry feed or CO2 slurry feeding—at multiple pressures. Particular attention was paid to detailed modeling of the Rectisol unit and optimization of the internal (flash drum) pressures, which vary with the gasifier pressure.

Preliminary results, presented at the Eighth Annual Conference on Carbon Capture and Sequestration, indicate that fuel pressurization with CO2-slurries results in slightly higher coldgas efficiencies and somewhat reduced energy penalties. However, the detailed mechanics and thermodynamics of slurry preparation have not yet been resolved, and the optimization protocol for the Rectisol unit is not complete. The researchers plan to finish the techno-economic analysis during 2010.


Using produced brine for plant cooling while enhancing CO2 storage

A 500 MWe coal fired power plant produces perhaps 100 million tonnes of CO2 over its 30 year lifetime. Ensuring safe and effective storage for such huge CO2 volumes poses major challenges. First, the individual CO2 well injection rates must be sufficiently low that the concomitant pressure rise within the storage formation remains below the fracture limit of the overlying cap rock. Second, the buoyant CO2 plume pressed against (and seeking fractures in) the cap rock can extend for tens of kilometers from the injection site. Third, high salinity brines are displaced from the reservoir by the supercritical CO2—on roughly an equal volume basis—for many decades after the injection ceases, posing a potential long-term environmental threat. At the same time, in many parts of the world, e.g. the western U.S. and in many parts of China, water scarcity makes siting large fossil energy conversion systems difficult.

Teaming with Ben Court and Mike Celia of the Storage Group, Williams and colleagues have begun research exploring the potential use for plant cooling of brine extracted from CO2 storage reservoirs. The basic idea is to produce brine from the reservoir in order to: 1) “make room” for the injected CO2, thus limiting the spatial extent of the pressure pulse, 2) “steer” the CO2 plume – via both brine production and injection wells – in order to more fully utilize the entire formation and to keep buoyant CO2 away from the cap rock, and 3) use desalinated brine for plant cooling, while pumping the lower volume residuum back into the formation.

Court is exploring the first two of these issues and the Williams Group is beginning to investigate the third, in conjunction with an extension of their models of thermochemical conversion facilities to include analyses of water requirements via wet and/or dry cooling and use of alternative water supplies, including desalinated produced brines. A techno-economic model is being constructed that enables examination of the tradeoffs associated with reservoir depth, brine salinity, desalination technology, and cost of conventional cooling water.