Page 407 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

386 12 Carbon Capture and Storage

All these species are grouped as dissolved inorganic carbon (DIC). H 2 CO 3 also
reacts with dissolved limestone in the ocean, which is a result of weathering the
earth surface, and the simplified chemical reaction is

CaCO 3 þ H 2 CO 3 ! Ca 2þ þ 2HCO ð12:66Þ
3
Besides the inorganic carbons, CO 2 is also converted into particulate organic
carbon (POC) and fertilizer by photosynthesis. For example,

6CO 2 þ 6H 2 O þ uv ! C 6 H 12 O 6 þ 6O 2 ð12:67Þ

106CO 2 þ 122H 2 O þ 16HCO þ H 3 PO 4
3
ð12:68Þ
! C 106 H 263 O 110 N 16 P þ 138O 2
One way or another, CO 2 is not saturated in deep ocean as a result of these
chemical reactions and biological conversions of CO 2 into organic and inorganic
carbon compounds. Therefore, more CO 2 can be stored in deep ocean. However,
natural absorption of CO 2 into ocean is a slow process and it cannot catch up with
the increase rate of anthropogenic CO 2 in the atmosphere.
This process can be expedited by injecting CO 2 into deep ocean. Based on the
properties of liquid CO 2 in ocean, engineered deep ocean CO 2 storage can be
achieved by
• direct CO 2 dissolution, and
• liquid CO 2 isolation

The fate of the injected CO 2 depends on the injection depth in the ocean. At a
depth of about 500 m (corresponding to the pressure of 4–5 MPa and temperatures
in the range of 0–10 °C), CO 2 starts to liquefy and the liquid CO 2 has a density of
3
860–920 kg/m . The deeper into the ocean, the greater liquid CO 2 density as a
result of the greater pressure.
According to the liquid CO 2 density relative to the seawater density, the depths
can be divided into three zones where CO 2 , floats, suspends, and sinks, respec-
tively. As shown in Fig. 12.11, liquid CO 2 density is close to that of seawater at
depths of about 2,500–3,000 m. This is also referred to as thermocline. Above this
zone, CO 2 density is less than the surrounding seawater. Therefore, the liquid CO 2
droplets rise as a plume. In the transition zone, CO 2 density is nearly close to that of
seawater and it suspends in seawater due to neutral buoyance. Further deep into the
ocean, CO 2 density surpasses that of the seawater resulting in sinking CO 2 plume.
For any one of the three scenarios, most of the released CO 2 will eventually
dissolve in the ocean.
The sinking CO 2 droplets eventually reach the bottom of the ocean enabling
liquid CO 2 isolation as a liquid lake in an ocean floor depression. Such a CO 2 lake
can be made by injecting liquid CO 2 to the ocean floor depression or releasing CO 2
at a depth that is close to the target lake. Settled liquid CO 2 in the lake mixes with