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Carbon Dioxide Capture by Adsorption:Traditional and Non-traditional Approaches

T. Golden, J. Hufton, R. Quinn

Air Products and Chemicals, Inc.

13th NIChE Conference

October 13, 2008

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Adsorption

• Interaction of gas with a solid surface

solid gas

S + CO2(g) = S-CO2

G = H - TS

• Adsorption is spontaneous if:

G<0 but S is <0 (negative)

H must be <0 (exothermic)

• Adsorption is driven by heat

of gas-solid interaction

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1. Physical adsorption (physisorption) - weak physical interactions -O=C=O

+

+

2. Chemical adsorption (chemisorption) - chemical bond formed

3. Bulk reaction: absorption - reaction at surface, diffusion into bulk

Types of Adsorption

O=C=O

OH

COH

O

O

=

CaO

CaCO3

CO2

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Types of Adsorption

Property Physisorption Chemisorption Absorption

interaction physical, weak chemical, strong chemical, strong

heats of adsorption low high high

reversible? yes, rapid maybe, slow maybe, slow

capacity low to medium low to medium high

temperature ambient ambient or higher ambient or higher

pressure high low OK low OK

example N2 on zeolite CO on CuCl/carbon

CO2/CaO

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The adsorption process

adsorptionstep

desorptionstep

CO2

others

CO2,others

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Pressure swing adsorption (PSA)

CO2 at P1

Adsorption Desorption

P1 > P2

CO2 at P2

• Feed exposure times are short (sec/min)• Modest working capacities• Best suited to bulk separation

purge

cap

ac

ity

gas partial pressure

P1P2

X1

X2

adsorption isotherm

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Temperature swing adsorption (TSA)

• Feed exposure times are long (hours) to minimize time for heating/cooling• Better suited to trace removal• Greater T – greater working capacity

CO2 at P1

T1 T2

Adsorption Desorption

T2 > T1

CO2

purge

X1

X2

cap

ac

ity T1

T2

P1

partial pressure

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CO2 sources

1. H2 synthesis by steam methane reforming - precombustion capture at relatively high pressure and CO2 concentrations

2. Gasification – precombustion capture

3. Coal fired power plant – postcombustion capture

How (or if) apply adsorption to CO2 capturedepends on the source of CO2 - gascomposition, flows, temperature, pressure

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H2 by Steam Methane Reforming

PSAhigh Tshift

CO + H2O = CO2 + H2

315-430ºC

CH4 + H2O = 3H2 + CO830-850ºC, 25-30 atm

naturalgas/air

naturalgas/steam

heatrecovery

>99.99%H2

PSA purgegas

73% H2, 12% CO2

8% CO, 7% CH4

(dry compositions)*

74% H2, 16% CO2

3% CO, 7% CH4

CH4 + 2H2O = 4H2 + CO2

~1.5 atm42% H2, 37% CO2

7% CO, 14% CH4

Flue gas:CO2, H2O, N2, O2

*Separation Technology R&D Needs for H2 Production in the Chemical and Petrochemical Industries, US DOE and Chemical Industry VISION 2020 Technology Partnership, December 2005.

reformer

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5 of 10 PSA vessels

• Largest third-party H2 producer, $1.5 billion revenue

• Operates over 70 plants – Americas, Europe, Asia

• 1.5 million ton/year produced

• 7 H2 pipeline systems (>350 miles)

Air ProductsCarson, CA

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Tarragona, Spain

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SMR H2

feed

high purity H2 product

activated carbon

zeolite

purge

lowerpressure

20-30 atm 37% CO2, rest CO, H2, H2O,CH4, N2

H2 PSA process

• Designed to give high purity H2

• CO2 “captured” at low pressure, low purity

- not suitable for sequestration

N2

CH4

CO

CO

CH4

CO2

H2O

N2

CH4

CO

CO

CH4

CO2

H2O

1.5 atm

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• Need to produce a high purity CO2 suitable for compression and sequestration without sacrificing H2 production.

• Conventional H2 PSA will not give pure CO2

• Modification of commercial PSA method required for CO2 capture

• Air Products developed such two processes: Gemini and CO2 VSA in 1980s – CO2 for urea market

Application of PSA-based process to CO2 capture

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PSAhigh Tshift

naturalgas/air

naturalgas/steam

heatrecovery

H2

1.5 atmPSA purgegas, 37% CO2

Current PSA Process

flue gas

20-25 atm74% H2, 16% CO2

3% CO, 7% CH4

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Gemini Process 2 products: H2 and CO2

H2 PSAfluegas

naturalgas/airnatural gas/

steam

H2

PSA purgegas, 6% CO2

97+% CO2

vacuumpump

More equipment, capital, energy costs so need mandated sequestration, sale or use of CO2 product to justify implementation

$$$$$$$$

high Tshift

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• CO2 removal rate about 7 lb CO2/h/ft3 carbon

• Typical 100 MM SCFD plant – 250 ton activated carbon (~14,000 ft3), 1 train of 10 beds, 14x25 ft

• Worldwide estimated H2 usage 50 MM tons/year

• 80% purified by PSA = 800 million tons CO2/yr

• Requires ~100,000 tons of both activated

carbon and zeolite

BUT…BUT…

Current scale of CO2 removal via PSA is big

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• Very high flows and quantities of CO2

• A gasification plant requires 2,000 to 2,500 tons carbon adsorbent (>100,000 ft3), 10 trains of 10 beds! Coal-fired power plant still more

• Huge technical challenge

• Need higher capacities, faster cycles, or both• Are there nontraditionalnontraditional adsorption based

alternatives?• Numerous efforts in nontraditional adsorption

The scale of CO2 capture is immense

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CO2 sources

1. H2 synthesis by steam methane reforming - precombustion capture at relatively high pressure and CO2 concentrations

2. Gasification – precombustion capture

3. Coal fired power plant – postcombustion capture

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Precombustion capture: CO2 removal in gasification

Gasification

fuel

O2

H2O

CO2

RemovalCombustion

Turbine/generator

CO2

kW

H2O, minor CO2600 psi, 1340ºCmostly H2, CO, H2O CO2

H2, CO

air

• CO2 at high pressure, concentration, temperature• Capture is “easy”

• Ideally high T capture, high CO2 capacity, high H2

recovery• Adsorbent must be OK cycled at high T, high steam• The adsorbent is the key…

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• Mg6Al2(OH)16[CO3]·4H2O calcined to a Mg-Al oxide

• K2CO3 promotion required for fast adsorption

• Operating temperature 400ºC (adsorb/regen)

• Mode of CO2 adsorption unclear – speculate

combination of physical/chemical adsorption;

acid/base chemistry• Capacity of 1.5 mmol/g (~6.5 wt%) at 5 atm and 400ºC• No negative impact of

steam on adsorption• Good cycling stability

K2CO3 promoted hydrotalcite for

precombustion capture

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• WGS catalyst + high temperature CO2 adsorbent - HTC

• Removes CO2 from hot feed gas (400-500ºC), drives CO towards zero, increases conversion to H2

• Cyclic process - reaction/adsorption and regeneration

Sorption Enhanced Water Gas Shift Process

air

Gasification

fuel

O2

H2O

CO2 forcapture

57% H2, 16%CO10% CO2, 16% H2O

CombustionTurbine/

generator

kW

N2, H2O, <1% CO2

87% H2, 0.5%CO2% CO2, 8% H2O

WGS reaction: CO + H2O = CO2 + H2

WGS catalyst/HTC adsorbent

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Metal oxides for precombustion capture

mm

ol

CO

2/g

CaO + CO2(g) = CaCO3

700-750C

>800C

• Very high capacity, 78.5 wt% (17.8 mmol/g!)• Cheap• High regeneration temperatures• Poor cycling stability• Large volume, phase changes • Efforts to stabilize capacity vs time

CaO , 750C, 30 min CO2, 30 min N2 cycles

5

10

15

0 10 20 30 40 50 60cycle number

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Complex metal oxides for precombustion capture

Li4SiO4 + CO2(g) = Li2CO3 + Li2SiO3 705

CaO + CO2(g) = CaCO3 892

• Desorption can be achieved at lower temperature but still high capacities – theoretical 8.3 mmol/g (36.7 wt%)

• A solid absorbent from Toshiba Corp has some very promising properties for precombustion CO2 capture

T, ºC for equil.P(CO2) = 1 atm

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• Li4SiO4 with <10 mole% K2CO3; Li2TiO3 binder; 5 mm

spheres

• Very large capacity, 5.4 mmol/g, 650C, 1 atm CO2

• Fast absorption/desorption, fully reversible

• Excellent physical, cycling stability

• OK in the presence of steam

• Absorption chemistry (confirmed by XRD)

Li4SiO4 + CO2(g) = Li2CO3 + Li2SiO3

equil. P(CO2) = 0.15 atm at 650C

• K2CO3 lowers mp of Li2CO3 (723C) product; molten

phase improves absorption properties

Toshiba absorbent

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Toshiba absorbent – TGA studies

650C

500C

Adsorption is very fast at 650C1 atm CO2

700C10 min CO2,10 min N2

• Excellent cycling stability inspite of large phase changes• No loss of crush strength or physical integrity

2.0

4.0

6.0

0 5 10 15 20 25 30

time, min

mm

ol

CO

2/g

4.0

4.5

5.0

5.5

0 10 20 30 40 50 60cycle number

mm

ol

CO

2/g

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CO2 sources

1. H2 synthesis by steam methane reforming - precombustion capture at relatively high pressure and CO2 concentrations

2. Gasification – precombustion capture

3. Coal fired power plant – postcombustion capture

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Postcombustion capture:500 MWe Coal Fired Power

Plant*

2 rail cars/henough carbon/yr to make 10% of methanolfor the entire planet!

air 2,450 mt/h

coal 208 mt/h

flue gas cleanup

flue gas2,770 mt/h;466 mt CO2/h

boiler/superheater

steam turbine/generator

500 MWelectricity

SOX, NOX,Hg, PM

*”The Future of Coal” (MIT, 2007)

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Retrofit for postcombustion CO2 capture & sequestration

flue gas with 47 mt CO2/h

CO2 capture

419 mt CO2/h

compress

undergroundsequestration

air 2,450 mt/h

coal 208 mt/h

flue gas cleanup

boiler/superheater

steam turbine/generator

500 MWelectricity

SOX, NOX,Hg, PM

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Postcombustion capture is not easy…

*500 MW plant, “The Future of Coal” (MIT, 2007)

• Very high flows* – 2.0x106 Nm3/h

• Very large quantities of CO2 to be captured* –

420 mt/h for 90% capture

• Ambient pressure, only ~0.15 atm CO2

• Flue gas contaminants – O2, SOx, NOx, Hg

• Water vapor – deleterious effect on most adsorbents• Need process that’s fast, high capacity, inert vs water

and flue gas contaminants, low regeneration energy• Adsorption options are limited• May need a creative solution beyond conventional

fixed bed technology

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Alkali carbonates forpostcombustion capture

K2CO3 + H2O(g) + CO2(g) = 2KHCO3

•Maximum capacity - 7.2 mmol/g (~32 wt%)•Infinite selectivity vs nonreactive gases•Absorb at 60ºC, regenerate at >150ºC

•Water required for CO2 adsorption

•High heats, 34.5 kcal/mol CO2

•Capture process

- difficult with fixed bed adsorption process

- likely need fluidized bed, “entrained bed reactor”

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RTI Entrained Bed Reactor

• Bench scale demo: 50 lb Na2CO3, >90% capture, 2-10 lb CO2/h

Issues:- physical attrition- phase/volume changes- high SO2 affinity

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Conclusions• Adsorption based CO2 capture from H2 plants is achievable but requires additional unit operations beyond currently existing ones. A driver for CO2 capture is required – mandated capture, sale or use of CO2 product.

• Larger scale capture such as gasification, power plants presents a significant challenge for an adsorption process.

• Nontraditional adsorbents and processes will likely be required, especially for postcombustion capture.