Carbon Dioxide (CO2) Capture at

Coal-Fired Power Plants

Presented By

Brian McNamara

bmcnamara@kentlaw.edu

What is CO2 Capture?

• A process consisting of separating CO2 from

energy-related sources before it enters the

atmosphere

• The current goal is to capture approximately

90% of CO2 emissions from these sources

• CO2 capture is most applicable to large,

centralized sources like power plants

Reasons to Consider CO2 Capture

• Coal is the largest contributor to global CO2 emissions from energy use (41%), and its share is projected to increase

• 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

Why is the Future Use of Coal Likely to

Increase?

• 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

CO2 Separation Technologies

CO2 Separation with

Sorbents/Solvents• Separation occurs when

CO2-containing gas comes in contact with a sorbent/solvent that is capable of capturing the CO2

• Regeneration of the sorbent/solvent after being heated, after pressure decrease or other change in condition releases the CO2

• Recycled and make-up sorbent/solvent is sent back to capture more CO2

CO2 Separation with Membranes

• Membranes are manufactured materials (polymeric, metallic, ceramic) that allow the selective permeation of CO2 through them

• The selective permeation of CO2 is usually driven by a pressure difference across the membrane (high pressure is usually preferred)

CO2 Separation by Cryogenic

Distillation

• CO2 can be separated

from other gases

through a series of

compression, cooling

and expansion steps

• Once in liquid form, the

components of the gas

can be separated in a

distillation column

CO2 Capture Systems

Post-Combustion CO2 Capture

Systems

• Defined as the separation of CO2 from the

flue gases produced after burning coal in air

• The CO2 separation and recovery from the

flue gas occur at low concentration and low

partial pressure

• The most common separation method used

are chemical solvents (amines)

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 an absorption tower

• The absorbed CO2 must then be stripped from the amine solution via a temperature increase, regenerating the solution for use again in the absorption tower

• The recovered CO2 is cooled, dried, and compressed to a supercritical fluid

• The CO2 is then ready to be piped to storage

Post-Combustion CO2 Capture System:

Chemical Absorption with Amines

Challenges of Post-Combustion CO2

Capture

• 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 which reduces the net power output of the generating plant

• For existing plants, the coal input must be increased and the plant size expanded to maintain constant net power generation

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 or

oxygen to produce a mixture consisting mainly of carbon monoxide and hydrogen (“synthesis gas”)

– Second, in another reactor (a “shift reactor”) the synthesis gas is reacted with steam to produce an additional mix of CO2 and hydrogen

– Third, the resulting mixture of CO2 and hydrogen can then be separated into separate gas streams

– Fourth, the CO2 can be captured and the hydrogen used as fuel to generate a gas turbine

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

Pre-Combustion CO2 Capture System:

Integrated Gasification Combined

Cycle 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

Pre-Combustion CO2 Capture System:

IGCC with CO2 Capture

Challenges of Pre-Combustion CO2

Capture for IGCC Plants• Perception of poor availability and operability

– Current demonstration plants required between 3-5 years to reach 70 – 80 % availability

• Operating issues– No single process component is responsible for the

majority 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

Oxy-Fuel Combustion CO2 Capture

Systems

• The system uses oxygen rather than air for the combustion of coal

• Combustion produces flue gas that is mainly water vapor and CO2

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

• CO2 is separated from the water vapor by cooling and compressing the flue gas

• Flue gas may be further treated to remove air pollutants and non-condensed gases before the CO2 is captured

Oxy-Fuel Combustion CO2 Capture

System

Challenges for Oxy-Fuel Combustion

CO2 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 reduces

the plant’s efficiency

Retrofit or Rebuild the Existing U.S.

Coal-Based Generating Plants?

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

Post-Combustion CO2 Capture

Retrofits

• 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

Post-Combustion CO2 Capture

Rebuilds

• Rebuild refers to rebuilding the core of an

existing unit by installing higher efficiency

technology along with CO2 capture

• Rebuilds appear more attractive than retrofits

because

– Cost of both are about the same

– Units with rebuilds have higher efficiency

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

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

Retrofit or Rebuild the Existing U.S.

Coal-Based Generating Plants?

Retrofit or Rebuild the Existing U.S.

Coal-Based Generating Plants?

What will the Application of CO2

Capture Depend on?

• Technical maturity

• Costs

• Diffusion and transfer of the technology to

developing countries and their capacity to

apply the technology

• Regulatory aspects

• Environmental issues

• Public perception