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Removal, Recovery, and Disposal of Carbon Dioxide

朱 信Hsin ChuProfessor

Dept. of Environmental EngineeringNational Cheng Kung University

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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

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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

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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

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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

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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

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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

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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)

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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.

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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.

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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

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3.7 Natural Minerals

Carbonate minerals: cannot be used Igneous rock: can react with CO2

Magnesium oxide bound to silica: MgSiO3 Alumina-forming aluminosilicates

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4. Capacity for Sequestering CO2

Next slide (Table 5.3)

300 years: equivalent to the recoverable coal reserves

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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

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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

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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

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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

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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%

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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

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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)

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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

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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

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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