214
continue unchecked. Nearly all emissions (97%) in the U.S. come from three primary sources: electric power plants (55% of emissions), transportation (23%), and
industrial processes (19%). The two worst industrial processes for GHG emissions
are cement manufacturing facilities and petrochemical processing plants. The
worldwide cement industry alone contributes about 5% of the total global emissions
of anthropogenic CO 2 (Songolzadeh et al. 2014).
Dealing with the GHG emitted from these major sources requires capturing carbon dioxide and isolating it from the atmosphere. There are two approaches: the first
captures the GHG out of the smoke stack directly from combustion products, and
the second attempts to reduce the levels of CO 2 already in the atmosphere. The first
approach uses a process known as carbon capture and storage, or CCS to capture
carbon dioxide from fossil fuel combustion gases before it is emitted into the atmosphere. This operates directly on combustion products in the stack like the capture
of fly ash or the removal of sulfur dioxide from flue gases to prevent acid rain. The
captured CO 2 is stored underground in isolation from the atmosphere, or as a solid
mineral phase like calcite. The second approach for removing CO 2 already in the
atmosphere is a process called direct air capture or DAC. This uses biological techniques like planting trees, or mechanical techniques that remove carbon dioxide
from large volumes of air. Most scientists and engineers think CCS is more practical
because it is applied to concentrated sources of CO 2 , whereas DAC is forced to work
with very dilute amounts (even the current sky-high atmospheric concentration of
more than 410 ppm is still considered to be quite dilute from a chemical engineering
perspective).
In the early days of the technology, the captured carbon was said to be “sequestered,” but this term is no longer in common use. In English it implies separation, or
being kept apart, as in a sequestered jury. The meaning in French culture is more
sinister, equivalent to being held against one’s will or kidnapped. For the sake of
international harmony, the USDOE has adopted carbon “storage,” although some
have argued that what is really meant is “disposal.” However, DOE is trying to convince people that carbon dioxide can actually be a useful commodity instead of just
a waste product, so the term storage is considered more desirable.
There are two basic processes for carbon capture: chemical and cryogenic
(Songolzadeh et al. 2014). Membrane separation is a third potential approach, but is
largely experimental at present. Chemical methods use carbon dioxide-absorbing
materials like amines to grab onto CO 2 in the flue gas and then release it for storage
using either a pressure drop or a change in temperature. Cryogenic techniques take
advantage of the fact that carbon dioxide “freezes” out of the air at temperatures that
are cold (−109.3 °F or − 78.5 °C), but still much warmer than the liquefaction point
of other gases like nitrogen or oxygen. The cryogenic method essentially turns the
CO 2 into “dry ice,” which can then be taken away for storage.
These techniques have an energy cost of 15% or more of a power plant’s output,
and either one will raise the price of fossil fuel electricity if implemented (Kramer
2018). As such, DOE and others have been looking for various processes that can
utilize the captured gas to help improve the economics of CCS (USDOE 2012). So
11 Balancing Energy, Environment, and Economics
continue unchecked. Nearly all emissions (97%) in the U.S. come from three primary sources: electric power plants (55% of emissions), transportation (23%), and
industrial processes (19%). The two worst industrial processes for GHG emissions
are cement manufacturing facilities and petrochemical processing plants. The
worldwide cement industry alone contributes about 5% of the total global emissions
of anthropogenic CO 2 (Songolzadeh et al. 2014).
Dealing with the GHG emitted from these major sources requires capturing carbon dioxide and isolating it from the atmosphere. There are two approaches: the first
captures the GHG out of the smoke stack directly from combustion products, and
the second attempts to reduce the levels of CO 2 already in the atmosphere. The first
approach uses a process known as carbon capture and storage, or CCS to capture
carbon dioxide from fossil fuel combustion gases before it is emitted into the atmosphere. This operates directly on combustion products in the stack like the capture
of fly ash or the removal of sulfur dioxide from flue gases to prevent acid rain. The
captured CO 2 is stored underground in isolation from the atmosphere, or as a solid
mineral phase like calcite. The second approach for removing CO 2 already in the
atmosphere is a process called direct air capture or DAC. This uses biological techniques like planting trees, or mechanical techniques that remove carbon dioxide
from large volumes of air. Most scientists and engineers think CCS is more practical
because it is applied to concentrated sources of CO 2 , whereas DAC is forced to work
with very dilute amounts (even the current sky-high atmospheric concentration of
more than 410 ppm is still considered to be quite dilute from a chemical engineering
perspective).
In the early days of the technology, the captured carbon was said to be “sequestered,” but this term is no longer in common use. In English it implies separation, or
being kept apart, as in a sequestered jury. The meaning in French culture is more
sinister, equivalent to being held against one’s will or kidnapped. For the sake of
international harmony, the USDOE has adopted carbon “storage,” although some
have argued that what is really meant is “disposal.” However, DOE is trying to convince people that carbon dioxide can actually be a useful commodity instead of just
a waste product, so the term storage is considered more desirable.
There are two basic processes for carbon capture: chemical and cryogenic
(Songolzadeh et al. 2014). Membrane separation is a third potential approach, but is
largely experimental at present. Chemical methods use carbon dioxide-absorbing
materials like amines to grab onto CO 2 in the flue gas and then release it for storage
using either a pressure drop or a change in temperature. Cryogenic techniques take
advantage of the fact that carbon dioxide “freezes” out of the air at temperatures that
are cold (−109.3 °F or − 78.5 °C), but still much warmer than the liquefaction point
of other gases like nitrogen or oxygen. The cryogenic method essentially turns the
CO 2 into “dry ice,” which can then be taken away for storage.
These techniques have an energy cost of 15% or more of a power plant’s output,
and either one will raise the price of fossil fuel electricity if implemented (Kramer
2018). As such, DOE and others have been looking for various processes that can
utilize the captured gas to help improve the economics of CCS (USDOE 2012). So
11 Balancing Energy, Environment, and Economics