A team at Stanford University is proposing using solid oxide fuel cells as the basis for a method for electricity production from oil shale with in situ carbon capture (EPICC) as a means to provide transportation services from oil shale with greatly reduced CO2 emissions.
In a paper published in the ACS journal Energy & Fuels, Hiren Mulchandani and Adam Brandt note that oil shale contains large amounts of stored chemical energy—more than 1 trillion barrels of oil equivalent is present in the Green River formation of the United States alone. However, liquid hydrocarbon (HC) fuels derived from oil shale have ~1.2-1.75 times the GHG emissions of HC fuels produced from conventional oil on a full-fuel-cycle (well-to-wheels) basis. These emissions consist almost entirely of carbon dioxide, with minor emissions of methane.
These emissions estimates raise a question: is the energy content of shale effectively “off limits” in a GHG constrained world, or is there a way to extract the stored chemical energy from oil shale with greatly reduced CO2 emissions?—Mulchandani and Brandt
In EPICC, waste heat from a solid oxide fuel cell (SOFC) placed in the subsurface is transferred via conduction to a geologic formation containing oil shale. The heat causes kerogen and bitumen to decompose into liquid and gaseous HCs (retorting). As vaporized liquid HCs move toward producer wells and contact cool shale, some condensation occurs. Upon further heating, liquid HCs crack into low-molecular weight gases (CH4, H2, CO, CO2) and char. The process would convert much of the organic carbon in oil shale to the char which is left in the subsurface.
Produced HC gases are then fed back into the fuel cell to generate electricity and to provide more waste heat for retorting. Excess gas can be sent to a combined cycle gas turbine to produce additional electricity.
|Conceptual Differences between Traditional Oil Shale Retorting Process and EPICC|
|Trad. Liquids production||EPICC|
|Resource||Chemical energy in shale|
|Heat source||direct thermal or electrical energy||secondary use of waste heat|
|Energy carrier to consumer||liquid HC fuel||electricity|
|Conversion to work via||internal combustion engine||in situ fuel cell|
|Scale of conversion to work||decentralized||centralized|
|Waste heat from conversion||to atmosphere||to shale|
EPICC is designed to maintain, to the extent possible, a bulk shale temperature below that at which significant carbonate mineral decomposition to CO2 occurs, since this would render it impossible to produce low-CO2 electricity. Because of the volumes of produced gases generated during retorting and cracking of HCs, EPICC is in most cases a self-fueled process.
A key factor in the efficiency and low-CO2 nature of EPICC is the “rearrangement” of the order of the conventional transportation fuel cycle. The chemical energy contained in the shale is converted to work (e.g., electricity) in the subsurface, rather than in distributed internal combustion engines, allowing waste heat from work conversion to supply the heat of retorting. In other words, retorting thermal demands are provided by the waste heat that is unavoidably generated with any conversion of chemical energy to work. Also, conversion to work occurs in a centralized location, enabling easier control of resulting CO2 emissions (although this is not explored here).
EPICC should not be viewed as a method to produce natural gas from shale: such methods would consume significant amounts of primary energy and result in a lower value product than the oil that was destroyed to produce gas. Instead, EPICC produces gas as an intermediate product, and the waste heat output from conversion of this gas to electricity provides the driving heat for kerogen decomposition and cracking of hydrocarbons. Thus, the heat integration with electricity production is a fundamental part of the EPICC concept.—Mulchandani and Brandt
Based on a modeling study, Mulchandani and Brandt found that the resulting life cycle GHG emissions from EPICC amount to ~110 g of CO2 per km, ~0.5 times those of conventional fuel cycles or ~0.33 times those from other proposed oil shale conversion processes.
They suggest that in high carbon tax scenarios, EPICC could represent an economic improvement over traditional in situ retorting as well as an environmental improvement.
What is more questionable is whether EPICC would be economically valuable in a high carbon tax scenario compared to low-carbon renewable energy technologies (e.g., the higher carbon prices that tip the balance toward EPICC and away from the ICP would also disfavor carbon-based fuels in general). Also, EPICC should be considered relative to the variety of other low- CO2 oil shale technologies that have been proposed. These technologies include nuclear powered oil shale retorting, oil shale retorting with intermittent wind power, and oil shale retorting with CCS. These technologies will have CO2 emissions similar to that of conventional oil and could be less costly than EPICC.—Mulchandani and Brandt
They also note that other potential drawbacks of EPICC include uncertain operation of subsurface fuel cells; potential geophysical impacts without pressure management; and economic concerns associated with the value of stranded energy left in the formation and the long time period of retorting.
Hiren Mulchandani and Adam R. Brandt (2011) Oil Shale as an Energy Resource in a CO2 Constrained World: The Concept of Electricity Production with in Situ Carbon Capture. Energy Fuels, Article ASAP doi: 10.1021/ef101714x