1
Removal, Recovery, and Disposal of Carbon Dioxide
朱 信Hsin ChuProfessor
Dept. of Environmental EngineeringNational Cheng Kung University
2
1. Introduction Three potential control points
1) The atmosphere2) The surface waters of the oceans3) The stacks: high CO2 conc.
Next slide (Table 5.1)Practical energy required >> 10 ×(the thermodynamic min.)
Removal: non fossil fuel energy source – nuclear or solar The only current feasible method: gro
w biomass – plants or algae Disposal (storge) Reuse
4
2. Removal and Recovery of CO2 From Fossil-Fuel Combustion
Sources:The electrical power generation sector: large and relatively easy to remove CO2 The industrial and domestic thermal generation sector: small per unit The transportation power sector: tiny per unit
Next slide (Table 5.2)
methods for a coal-fired power plant
6
2.1 Absorption/Stripping Solvents (liquid)
Alkanolamines (monoethanolamine (MEA)): the lowest energy requiredAlcohols (methanol)Glycols
Absorption: lower tempStripping: heated by steam
2.2 Adsorption/Stripping Sorbents (solid)
charcoalMolecular sieves
Adsorption: higher pressureStripping: lower pressure
7
2.3 Refrigeration (Cryogenic)Gases are compressed → cooled down to a liquid or a solid
2.4 Membrane SeparationMembrane (different pore sizes)
PolymersMetalsRubber composites
Gas absorption membrane compositesAbsorbing liquid on one side of a porous membrane: providing a large surface-contacting area
8
2.5 Seawater Absorption Does not work: solubility! Alternative: pumping flue gas deep into
the ocean where the partial pressure of the dissolving CO2 is equal to the pressure of the ocean at that depth.
2.6 Oxygen/Coal-Fired Power Plant
Use pure O2 for combustion Pure CO2 in the flue gas → liquefying →
sequestering or reusing Next slide (Fig. 5.1)
Oxygen coal-fired plant flow chart
10
3. Disposal of CO2
1) Ocean disposal2) Depleted gas wells3) Active oil wells (enhanced oil
recovery) and depleted oil wells4) Coal beds and mines5) Salt domes6) Aquifers7) Natural minerals
11
3.1 Ocean Disposal The upper layer of the ocean is in equilibrium with the atmos
phere CO2.Thermocline: about 1000ft below sea surface, at which point the ocean temperature abruptly decreases.Below the thermocline: the concentration of dissolved CO2 is negligible. CO2 can be pumped down and readily dissolved at the ocean depths.
The capacity for dissolution of CO2 in the ocean is adequate to absorb all the CO2 from combustion of all the earth’s resources of fossil fuels. If liquid CO2 is pumped deep enough within the ocean, the density of liquid CO2 becomes greater than the density of seawater at that depth: liquid CO2 can sink to the bottom floor of the ocean and form a lake of clathrates (solid compounds of a CO2 molecule surrounded by about 5.75 molecules of water).
liquid CO2 is the most economical form to be disposed compared to gas or solid CO2 (dry ice)
12
3.2 Depleted Gas Wells Natural gas wells: high pressure without leakage
Up to several thousand pounds per square inchHundreds of depleted gas wells in the world: the capacity is limited
Can only sequester the CO2 from natural gas combustion (not enough for oil or coal): one volume of natural gas combustion produces one volume of CO2
3.3 active oil wells Primary oil production only removes about a third of the
oil from an active oil well. Various media, such as hot water, nitrogen, polymers, and
CO2 have been used for removal of the remaining two-thirds.
CO2 is preferred: in addition to displacement, CO2 dissolves in the oil and reduces its viscosity, making it easier to pump out.
Only a fraction of the oil combustion CO2 can be sequestered in oil wells: gaseous CO2 vol. >> liquid oil vol.
13
3.4 Coal Mines and Deep Beds Storage of CO2 in mined-out and abandoned
coal mine fields is not feasible: coal mines can’t be readily sealed to hold the pressure, gaseous CO2 vol. >> solid coal vol.
Deep coal deposits:CH4 coexists with coal.
Displacement of coal-bedded methane with CO2: production of CH4, twice the volume of CO2 can be absorbed on the surface of the coal than the natural gas originally present in the coal.
14
3.5 Salt Domes Pumping seawater from and to the ocean
for solution mining salt: the salt domes have been used to store oil, storing the CO2 is also possible.
3.6 Aquifers Shallow aquifers: water supply
Deep aquifers: usually saline, a significant capacity for sequestering CO2
Pressurized CO2 could displace the water as well as dissolve in the water of deep aquifers
15
3.7 Natural Minerals
Carbonate minerals: cannot be used Igneous rock: can react with CO2
Magnesium oxide bound to silica: MgSiO3 Alumina-forming aluminosilicates
16
4. Capacity for Sequestering CO2
Next slide (Table 5.3)
300 years: equivalent to the recoverable coal reserves
18
5. System StudyApplication of the absorption/stripping system and disposal of the CO2 Base year: 1980
5.1 CO2 Removal and Recovery System for Fossil-Fuel Power Plant Flue Gases
5.1.1 CO2 Emissions from Fossil Fuel
Next slide (Table 5.4)CO2 production by natural gas and fuel oil: 50% and 80% compared to coal
20
5.1.2 CO2 Removal and Recovery Using Improved Solvent Process MEA absorption/stripping: conventional
A newer alkanolamine-based solvent (DOW Gas/spec FS-1): more energy efficient
Next slide (Table 5.5)Energy required: DOW FS-1 < MEA
Following slide (Fig. 5.2)Flue gas > 250℉
Quenching → 120℉ before entering the absorber
23
DOW FS-1 Reaction
(steam) CO2 liquefying
Compressed to 2000 psia in a four-stage compression system Passed through a coolerLiquefied in a condenser at about 80℉Liquefying energy: 0.047 kwh(e)/lb of CO2 recovered
Nest slide (Table 5.6)Energy required: mainly removal and liquefaction
o
o
100 F
2 2 2 3 3300 F
R-NH +H O+CO R NH HCO
25
5.1.3 Integration of Power Plant and CO2 Recovery System
Next slide (Fig. 5.3)The extraction of latent heat from the low-pressure steam: otherwise would be lost in the condenser
Following slide (Table 5.7)Integrated plant: efficiency drops a little
28
5.1.4 CO2 Recovery Plant Costs100~1000 tons CO2/day 5.1 ~ 51 MWe power p≒lant 15 US$/ton of CO2
Next slide (Table 5.8)CO2 recovery and disposal may double the electricity cost Total power consumption for CO2 mitigation > 17%Plus conventional power plant in plant consumption < 8%
30
5.2 CO2 Disposal Systems6” pipeline from power plants to collection centers 36” pipeline from collection centers to the final sites
5.2.1 Ocean Disposal The density of liquid carbon dioxide < seawater
but liquid CO2 is much more compressible than seawater and has a much higher thermal coefficient of expansion.Therefore, the density of CO2 > seawater of similar temp. (37℉) at about 3000 m depth
Ocean depths of 3000 m: 4400 psiaabout 200 miles from the shoreline of most continents
Alternative: 2000 psia liquid CO2 pressure at a depth of 500 m already lower than thermoclineAbout 100 miles from the shoreline of most continents
Next slide (Fig. 5.4): 500 m depth disposal
32
An experiment is actually planned between the US nd Japan to inject CO2 in the ocean off the coast of Hawaii and monitor the conc. of CO2 at various ocean depth levels.
Next slide (Fig. 5.4A)Other injection methods (3, 4, 5)
34
5.2.2 Oil and Gas Wells DisposalThe US has 12,000 spent oil and gas wells Depleted wells: usually 100 ~ 500 psia
An increase of about 10℉ for every 1000 ft of depth 10,000ft-depth well: 180℉
Recovered 80℉ 2,000 psia CO2 → 180℉, 3,000 psia or more (10,000 psia)
5.2.3 Disposal in Salt CarvernsAgain, 3,000 psia or more
35
6. Comparison of Capture and Disposal Costs
Next slide (Table 5.16)Only for capture
Disposal cost US $ 15~50/tonne CO2 for 100 km distance
37
7. Problems Associated with Sequestering CO2 in the Ocean
Economic Capital and operating costs
EngineeringDeep CO2 pipelines: a challenging problem
Environmental effectsThe acidity of the ocean ↑pH↓to < 8Would kill marine organisms
Rapid release of sequestered CO2Thermal plumes and volcanic action in the ocean could suddly release the sequestered CO2