Stanford team proposes method for electricity production from oil shale with in situ carbon capture to provide transportation energy with reduced CO2 emissions

System diagram of EPICC process. Electricity generation in subsurface provides the heat requirement for retorting. Separation of gaseous from liquid HCs can occur in the subsurface at a condensation front or in above ground separation with reinjection. Credit: ACS, Mulchandani and Brandt. Click to enlarge.

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.

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.

Resources

• 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