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Carbon Dioxide (CO2) Capture atCoal-Fired Power Plants

Presented ByBrian McNamara

bmcnamara@kentlaw.edu

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What is CO2 Capture? A process consisting of separating CO2 from

energy-related sources before it enters theatmosphere

The current goal is to capture approximately90% of CO2 emissions from these sources CO2 capture is most applicable to large,

centralized sources like power plants

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Reasons to Consider CO2 Capture Coal is the largest contributor to global CO2 emissions

from energy use (41%), and its share is projected toincrease 50% of the electricity generated in the U.S. is from coal In 2006, coal-fired power plants produced

approximately 36% of the total U.S. CO2 emissions The U.S. produces about 1.5 billion tons per year of

CO2 from coal-burning power plants In the U.S., electricity demand is expected to nearly

double over the next 30 years China and India are rapidly expanding their use of coal

for energy generation

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Why is the Future Use of Coal Likely toIncrease?

Coal is cheap Coal is abundant The United States, Russia, China and India

have immense coal reserves The economics and security of supply are

significant incentives for the continuing use of coal

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CO2 Separation Technologies

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CO2 Separation withSorbents/Solvents

Separation occurs whenCO2-containing gas comesin contact with asorbent/solvent that iscapable of capturing theCO2

Regeneration of thesorbent/solvent after beingheated, after pressuredecrease or other change incondition releases the CO2

Recycled and make-upsorbent/solvent is sent backto capture more CO2

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CO2 Separation with Membranes Membranes are

manufactured materials(polymeric, metallic,ceramic) that allow theselective permeation of

CO2 through them The selective permeation

of CO2 is usually drivenby a pressure differenceacross the membrane(high pressure is usuallypreferred)

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CO2 Separation by CryogenicDistillation

CO2 can be separated

from other gasesthrough a series of compression, cooling

and expansion steps Once in liquid form, the

components of the gas

can be separated in adistillation column

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CO2 Capture Systems

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Post-Combustion CO2 CaptureSystems

Defined as the separation of CO2 from theflue gases produced after burning coal in air

The CO2 separation and recovery from theflue gas occur at low concentration and lowpartial pressure

The most common separation method used

are chemical solvents (amines)

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Post-Combustion CO2 Capture System:Chemical Absorption with Amines

The CO2 is captured from the flue gas stream by

absorption into an amine solution in anabsorption tower The absorbed CO2 must then be stripped from

the amine solution via a temperature increase,regenerating the solution for use again in theabsorption tower

The recovered CO2 is cooled, dried, andcompressed to a supercritical fluid The CO2 is then ready to be piped to storage

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Post-Combustion CO2 Capture System:Chemical Absorption with Amines

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Challenges of Post-Combustion CO2Capture

Low pressure and dilute CO2 concentration of the flue

gas requires a high volume of gas to be treated CO2 removal from the flue gas requires a lot of energy Compression of the captured CO2 requires a lot of

energy The low-pressure steam used for the regeneration of

the amine solution reduces steam to the turbine whichreduces the net power output of the generating plant

For existing plants, the coal input must be increasedand the plant size expanded to maintain constant netpower generation

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Pre-Combustion CO2 Capture Systems Defined as the separation of coal into CO2 and hydrogen

before combustion The capture process involves the following stages:

First, the coal is processed in a reactor with steam and air oroxygen to produce a mixture consisting mainly of carbonmonoxide and hydrogen (synthesis gas)

Second, in another reactor (a shift reactor) the synthesis gas isreacted with steam to produce an additional mix of CO2 andhydrogen

Third, the resulting mixture of CO2 and hydrogen can then beseparated into separate gas streams

Fourth, the CO2 can be captured and the hydrogen used as fuelto generate a gas turbine

Pre-combustion would be used at power plants thatemploy integrated gasification combined cycle (IGCC)technology

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Pre-Combustion CO2 Capture System:

Integrated Gasification CombinedCycle System (IGCC) A gasifier transforms the coal to a synthesis gas The synthesis gas goes through several stages of

cleanup (shift reactors) where the CO2 can be captured A gas turbine burns the cleaned synthesis gas to

produce electricity and exhaust heat Exhaust heat from the gas turbine is recovered to

produce steam to power a steam turbine

The system produces energy through the two turbines gas turbine and steam turbine

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Pre-Combustion CO2 Capture System:IGCC with CO2 Capture

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Challenges of Pre-Combustion CO2Capture for IGCC Plants

Perception of poor availability and operability

Current demonstration plants required between 3-5 yearsto reach 70 80 % availability Operating issues

No single process component is responsible for themajority of shutdowns

Operational performance has not typically exceeded 80% Cost

Requires more maintenance than a PC unit Spare gasifier ($$) provides better availability

Maturity IGCC system is not mature enough for coal-based electric

generation

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Oxy-Fuel Combustion CO2 CaptureSystems

The system uses oxygen rather than air for the

combustion of coal Combustion produces flue gas that is mainlywater vapor and CO2

Flue gas has a relatively high CO2 concentration(greater than 80% by volume)

CO2 is separated from the water vapor by coolingand compressing the flue gas

Flue gas may be further treated to remove airpollutants and non-condensed gases before theCO2 is captured

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Oxy-Fuel Combustion CO2 CaptureSystem

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Challenges for Oxy-Fuel CombustionCO2 Capture

The technology is only at a development stage

(no commercial experience to rely upon) The air-separation unit that supplies the

oxygen consumes a lot of energy and reducesthe plants efficiency

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Retrofit or Rebuild the Existing U.S.Coal-Based Generating Plants?

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U.S. Coal-Based Generating Plants Average age of the fleet is over 35 years old There are more than 1,000 boilers in the U.S.

fleet Average generating efficiency of the fleet is

about 33% Current life-extension capabilities could keep

some units in operation for another 30+ years More than 100 new coal-based power plants

are being considered for construction

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Post-Combustion CO2 CaptureRetrofits

Retrofit refers to installing CO2 capture

systems to existing units Retrofit with CO2 capture systems seem

unlikely due to: Reductions in unit efficiency and output Increased on-site space requirements

Unit downtime

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Post-Combustion CO2 CaptureRebuilds

Rebuild refers to rebuilding the core of an

existing unit by installing higher efficiencytechnology along with CO2 capture

Rebuilds appear more attractive than retrofitsbecause

Cost of both are about the same

Units with rebuilds have higher efficiency

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Pre-Combustion CO2 Capture Retrofits& Rebuilds

Retrofitting an IGCC unit would appear to be

less expensive than retrofitting a PC unit Retrofitting would not make the unit an

optimum CO2 capturing unit Rebuilding would involve significant changes

in most components of the unit to optimize

CO2 capture

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Oxy-Fuel Retrofits and Rebuilds Good option for retrofitting PC units because

the boiler and steam cycle are less affected The major impact is the increased energy

requirement of the air-separation unit Rebuilds to improve efficiency appear to be

competitive with post-combustion rebuilds

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Retrofit or Rebuild the Existing U.S.Coal-Based Generating Plants?

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Retrofit or Rebuild the Existing U.S.Coal-Based Generating Plants?

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What will the Application of CO2Capture Depend on?

Technical maturity Costs Diffusion and transfer of the technology to

developing countries and their capacity toapply the technology Regulatory aspects Environmental issues Public perception