Environmental Engineering Reference
In-Depth Information
Although global energy is currently used more efficiently, the demand for oil will continue to increase with the increase of
the population and the economy. by 2030, it is estimated that oil will contribute to 29% of global energy consumption and solar
power to 1%. It is estimated that for the same period, the transport sector will have the largest increase in oil demand [7].
In the industry, CO
2
is used in processes such as the synthesis of urea, methanol, carbonated beverages, in greenhouses [8],
supercritical extraction [9], and in extinguishers, among others. The volumes generated worldwide far exceed the requirements
of this gas in these processes.
The urgent need for accessible and affordable energy, based on the burning of fossil fuels and consumption projections
in the coming decades, requires the development of various technologies to mitigate or minimize CO
2
emissions to the
atmosphere.
However, these technologies cannot be used in all sectors. For example, emissions from houses and transportation vehicles,
which are important due to the volume they generate, can present significant economic challenges for successful implementa-
tion, because these effluents enter the air directly and it is not practical to separate greenhouse gas streams containing a few
parts per million. This does not happen in the industrial sector, where it is more feasible to capture CO
2
because of the huge
volumes and high concentrations that are generated.
The questions that arise are (i) How can we remove or capture CO
2
? and (ii) What do we do with the large amounts of CO
2
,
once captured? To answer the first question, there are several proven technologies for removing CO
2
; examples of such processes
in industrial application are precombustion, postcombustion, and oxycombustion. Once removed, greenhouse gases can be
disposed of through various proposals, the most studied being the geological sequestration and storage options at the bottom of
the sea and mineral carbonation or sequestration.
21.2 Co
2
storaGe aND sequestratioN
21.2.1 Geological sequestration
The most readily available method of sequestration is underground containment. Capturing CO
2
from major stationary sources,
transporting it usually by pipeline, and injecting it into suitable deep rock formations for geological storage provide a way of
avoiding the emission of CO
2
into the atmosphere. There are a number of oil and gas wells, coal mines, and abandoned salt
domes or low production yields. These sites have huge open spaces where it is feasible to store CO
2
at high pressure (Fig. 21.5).
The engineered injection of CO
2
into subsurface geological formations was first undertaken in Texas in the United States in the
early 1970s, as part of enhanced oil recovery (EOR) projects [10]. In 1996, the world's first large-scale storage project was ini-
tiated by Statoil and its partners at the Sleipner Gas Field in the North Sea. The world's first large-scale CO
2
storage project in
a gas reservoir was conducted in Salah, Algeria [11].
To geologically store CO
2
, it must first be compressed, usually to a dense fluid state known as “supercritical.” Depending on
the rate at which temperature increases with depth (the geothermal gradient), the density of CO
2
will increase with depth, until
at about 800 m or greater, the injected CO
2
will be in a dense supercritical state (see Fig. 21.6).
The most effective storage sites are those where CO
2
is immobile because it is trapped permanently under a thick, low-
permeability seal or is converted into solid minerals (e.g., carbonates) or adsorbed onto surfaces of coal micropores or through
a combination of physical and chemical trapping mechanisms.
CO
2
has different water solubilities depending on the temperature, pressure, and dissolved salts. Once dissolved in water, it can
form carbonic acid or other insoluble carbonates depending on the rock mineralogy. Underground coal is also an adsorbent of
many substances, including CO
2
, H
2
S, and SO
2
, among others. The carbon affinity for CO
2
is 2-8 times greater than for methane.
by contrast, the H
2
S found in gas and sulfur oxides (SO
x
), which are found in the flue gases, have higher affinity for carbon as CO
2
.
The geological disposal of CO
2
therefore needs to meet three requirements:
1. Capacity: the disposal unit has to have sufficient capacity to receive and retain the intended volume of CO
2
.
2. Injectivity: the ability to inject CO
2
deep into the ground at the rate that it is supplied from the CO
2
source.
3. Confinement: if CO
2
is not confined, then, due to its buoyancy (being lighter than water), it will flow upward, ultimately
entering the shallow hydrosphere (including potable groundwater), the biosphere, and the atmosphere.
CO
2
disposal in geological media has not yet been implemented as a mitigation measure for climate change, although CO
2
injection and disposal have occurred for different reasons in the last three decades. There are other challenges facing the
large-scale development of the geological disposal of CO
2
, but they are of an economic, financial, legal, and regulatory nature
and are also likely to be linked to public attitude toward such developments [14].
