[L16] Carbon capture & storage

Carbon capture

CO2 can be captured from a process or the atmosphere and permanently stored geologically

Carbon separation plant

• extracts CO2 from gases

• CO2 capture & separation → CO2 compression unit → transport via pipeline → CO2 compression unit → CO2 injection & storage

Necessity of carbon capture & applicable sectors

• fossil fuels will likely remain a major source of energy globally as is cheapest option

• can be applied to electricity and industry sectors (main sources of CO2 emissions)

CSS demand

Total UK emissions in 2023 ~380 MtCO2

IEA (International Energy Agency) projects ~120 GtCO2 sequestered total by 2050 (mainly coal power)

IPCC (Intergovernmental Panel on Climate Change) up to 20 GtCO2/a of GGR in 2100 → ideal case

GGR

greenhouse gas removal

Current CSS state

~ 50 MtCO2/a operational 

~ 310 MtCO2/a in various stages of development and construction 

➔ <1 % of projected demand

Conventional combustion

Post-combustion capture

Pre-combustion capture

Oxy-fuel combustion

Separation can happen on

• flue gas

• fuel

• air

CSS current technologies

• large variety

• must advance through series of scale-up steps

• trouble at TRL 3 (=proof of concept), TRL 6 (=pilot plant), and TRL 7 (demonstration)

• technical challenges or insufficient funding

Chemical absorption post combustion capture

• chemical absorption

• separation using liquid solvent

• Aamines such as Monoethanolamine (MEA) and Piperazine typically used as solvents

1. Lean solvent (blue) is contacted with the flue gas and absorbs some of the CO2

2. Heat is applied to the rich solvent (red) to reverse the reaction and recycle the solvent ➔ takes steam away from plant 

• absorption based on the fact that CO2 is slightly acidic

• undertaken at low pressure (1 – 2 bar_a)

Chemical absorption post combustion capture advantages

• “End of pipe” technology ➔ can be retrofitted to existing plant = retrofittable

• Mature technology, experience with large-scale projects in the O&G industry 

• Flexibility → range of operating conditions

Chemical absorption post combustion capture disadvantages

• High CapEx → large gas volumes 

• large equipment 

• Parasitic energy ➔ reduced plant efficiency → parasitic energy for solvent regeneration: ~20 % of steam from plant

• Solvent losses & solvent disposal

→ Amine degradation

Solvent susceptible to chemical degradation in O2, SO2, CO2, high temperatures

Solvent losses to environment 

Measured amine loss of ~ 0.35 – 2.0 kgsolvent/tCO2 from first pilot plants (much improved now)

Degradation products could present health risks, need to be monitored

Adsorption post-combustion capture

• Adhesion of species/molecules to a solid surface 

• Porous solids are used in a cyclic process to separate gas mixtures → pores give more surface area per volume 

• Absorption depends on pressure, concentration, and temperature → cycling via pressure changes (PSA, VSA) or temperature changes (TSA)

Sorbent regeneration in adsorption post-combustion capture

Research typically focused on VSA 

• Fast cycle times (minutes), greater throughput 

TSA not considered traditionally 

• Cycle times >6-12 hours 

• Better CO2 recovery as compared to VSA

New ‘rapid-TSA’ process developed 

• Cycle time ~2 minutes

Rotary absorbers for commercial application in adsorption post-combustion capture

Rotary wheel adsorber (Svante VeloxoTherm )

Laminated gas channels

• Adsorbent coated on laminations 

• Negligible pressure drop 

• Higher gas throughput 

1tpd plant applied at a cement plant 

• 30 tpd demonstrator plant in operation

• 500 tpd and 2,000 tpd plants designed

Advantages of adsorption post-combustion capture

• Retrofittable 

• Range of operating conditions, many potential materials available 

• Particularly well suited for low concentrations ➔ direct air capture 

• Potentially cheaper and more environmentally friendly than amine absorption

Disadvantages of adsorption post-combustion capture

• Challenging material selection 

• Need for cyclic processes 

• Energy requirements to generate vacuum 

• So far only used for small volumes, expensive to scale up

Pre-combustion capture

Hydrogen production

1. React hydrocarbon fuel (typically natural gas) with steam and oxygen

- Produces ‘syngas’ – CO2, CO, CH4, H2

- Syngas further reacted to produce CO2 and H2

2. CO2 is separated from H2 and stored 

3. H2 is used as a clean fuel 

First part of process identical to conventional H2 production process

Hydrogen / CO2 separation in pre-combustion capture

Conventional technologies from industrial hydrogen production process 

• Pressure swing adsorption 

• Cryogenic separation 

➔  fundamentally based on boiling point of species

• Physical absorption 

• Chemical absorption

Advantages of pre-combustion capture

• Uses processes that are already commercially used 

• Lower energy penalty than post-combustion capture 

• Overall hydrogen production process can be very efficient (60 – 65%) – does not include efficiency of power plant

Disadvantages of pre-combustion capture

• High CapEx (capital expenditure = upfront money)

• Complex process – low flexibility 

• No commercial scale demonstration of pre-combustion capture for power generation (hydrogen turbine is issue currently)

Oxy-fuel combustion

• O2 separated from air

• Fuel is combusted in pure oxygen 

• Flue gas (ideally) only contains H2O and CO2 

• H2O can be condensed out

• No further separation of CO2 required

Air separation in oxy-fuel combustion

Cryogenic distillation

• Mature, very high capacity, very high purity, high energy requirements

Vacuum swing adsorption 

• Mature, medium capacity, high purity, low energy requirements 

Membranes 

• High purity, very low capacity

For a 500 MWe power plant, ≈10,000 tpdO2 (tonnes per day) is required 

• Only cryogenic distillation can provide this quantity

Innovation in CCS for cement

• Two-thirds of CO2 come directly from the limestone 

• Indirect heating of calciner results in pure CO2 stream, no further separation required

• Gas turbine CCS technology can be applied to flue gas from furnaces

CO2 transport technologies

Available & mature:

• CO2 pipelines already exist for EOR 

• Ship transport already exists for food grade CO2

Large scale CCS: pipeline, ship

Small scale CCS: trucks, rail 

• Unlikely to be economically viable

CO2 transport challenges

• Transported under pressure (≈100 bar, 30 °C) or liquefied (-37 °C, 11 bar) 

• Significant energy requirement for compression

• Purification required before transport and storage

CO2 storage

• enhanced oil recovery

• saline aquifers

• carbonation

Injection and storage of CO2 over 1 MMtpa CO2 is technically viable 

• >10 industrial scale demonstration projects 

• Enhanced Oil Recovery (CO2 injected into oil reservoir)

>98 % of properly stored CO2 will remain over 10,000 year

CO2 storage research

• not concerned with actual storage of CO2

Focused on improving:

• Site characterisation 

• Plume migration 

• Managing risks of leaking 

• Detecting leaks with certainty

Captured versus avoided CO2

• additional energy required to operate the CCS plant can have associated emissions (ex: additional boiler to generate steam for regeneration) 

→ CCS processes could have more emissions than CO2 they capture 

Captured CO2 = gross CO2 to storage 

Avoided CO2 = net CO2 (emissions of original process – emissions of new process) 

Avoided CO2 always < captured CO2

➔ Cost of avoided CO2 > cost of captured CO2 (per unit of CO2)

Factors that impact cost of CCS

Location in the world

• Available resources (e.g. land, water) 

Technology 

• Brownfield vs greenfield 

• Technology maturity 

Labour 

• Rates 

• Unionised 

Commercial 

• Risks

• Contingencies 

• Warranties and insurances 

• Price of CO2

Capture process 

• Technology choices 

• Chemicals and fuel cost 

Transport 

• Mode of transport 

• Route distance 

• Flow rate through pipeline

• Pressures

Storage

• “Finding costs” / exploration 

• Capacity

•  Injectivity

• Containment

Evolution of cost number of plants in operation

• learning curve

• costs will reduce as more plants are built

Costs versus CO2 concentration

• lower concentration = more expensive = more difficult to separate a dilute stream

• not suitable for small scales (house boilers for ex)

GGR technologies

• greenhouse gas removal necessary for 1.5C target

• Bioenergy with carbon capture & storage (BECCS)

• Direct air capture (DACCS)

Bioenergy with CSS

• Feasible technology 

• Social aspects & social acceptance 

• Life cycle analysis needed to calculate net carbon removals 

• Drax bioenergy plant to be fitted with post-combustion CCS

• controversial ➔ burning trees and biomass then capturing CO2

• must replant for it to be effective

• energy created by burning biomass ➔ CO2 emitted and trapped

= two steps of removal (biological + geological)

Direct air capture

• Adsorption based (Climeworks & others) (solid materials)

• Absorption based (Carbon Engineering) (liquid chemicals)

• Technically feasible, but needs large energy input as CO2 is very dilute

• Economically questionable 

• Socially acceptable 

• No large-scale demonstrations yet

DACCS vs BECCS

DACCS 

• Lower land requirement 

• Higher social acceptance 

• Energy consumer

• Very expensive

BECCS 

• Higher land requirement 

• Lower social acceptance 

• Energy producer 

• Cheaper (but still expensive)

Both have their place:

• Region specific requirements

• Life cycle assessment to calculate net removals ➔ make sure CO2 removed >> CO2 caused by operating plant

CO2 utilisation

Convert the captured CO2 to valuable products 

• Enhanced oil recovery 

• Chemicals 

• Plastics 

• Food/beverage

Main sectors using CO2 as feedstock:

• fertilizer (urea)

• methanol (fuel) → can use carbon to make it → recycling carbon

• inorganic carbonates (cement-like materials, minerals)

• organic carbonates / polyurethanes

• technological use (supercritical CO2, solvents, extraction)

• food & drink (carbonated drinks, modified atmosphere packaging, dry ice)

CCS barriers

Every element required for CCS is already proven but is not deployed

Economics 

• No inherent value: not doing CCS will always be cheaper 

• Product (CO2) has no value 

• Transport & storage infrastructure not available / too expensive 

Policy 

• Regulations, difficult permitting processes 

• No mandates for CCS 

• Carbon prices not high enough

Making it a commercial reality

• Any current CCS examples involves some amount of public funding → reduces capital costs for developer

• Incentives for carbon capture (e.g., US: 85$/tCO2 stored) 

• Carbon pricing to make unabated fossil fuels less attractive 

• Mandates (e.g., emission standards) 

• Manage risks associated with CCS projects 

• Pick the right application

Service providers and users

• Power companies do not want to start operating CO2 stores 

• Clear transfer of liability needed

CCS criticism

Plenty of points of criticism 

• Unreliable / technically infeasible

• Increases emissions 

• Prolongs use of fossil fuels 

• Distraction / Diverts investment from other “clean” solutions 

• Waste of taxpayer money 

• Greenwashing 

• Not zero emissions ➔ Some are justified, others not

CCS conclusion

Required to achieve climate targets, but should not compete with other low carbon technologies (e.g., renewables)

• Provide different services & value in the economy

• CCS technologies are well understood and considered mature

• Improvements are possible, but current technology is sufficient to deploy CCS 

• Need to consider the whole energy system 

• can provide value with power system resilience (grid balancing)

• combination of technologies are required to achieve emissions targets 

• can provide emission reductions for industry 

• some processes do not have CO2-free alternatives at all, or that will be available at large scale by 2050

• CCS required in the short-term (~30 years) while transitioning to renewables and new industrial processes 

• GGR is no longer optional

• Key challenge is to find a ‘business case’ 

• Specific for each scenario 

• Supportive policy framework and clear financial models needed 

• Transparency essential for public acceptance, need to show successes