Containing carbon - carbon capture and storage
While there is a major drive both within the UK and globally to reduce carbon dioxide (CO2) emissions through renewable energy sources, fossil-based energy will remain a key player for many decades. The challenge taken up by equipment manufacturers and power utility companies is to develop technologies that produce cleaner electricity from fossil energy sources at affordable prices.
The UK Government announced a competition to build and demonstrate the country’s first full-scale carbon capture and storage (CCS) power plant to reduce CO2 emissions from fossil fuel plants by around 90%.
Carbon capture and storage removes CO2 from the gases produced when fossil fuels are burned or gasified, and compresses and transports the CO2 to locations suitable for long-term storage – for the UK, saline aquifers or exhausted oil/gas wells beneath the North Sea are options.
There are three main technologies available for the capture of CO2 from power generation and other industrial processes, all of which are being considered for use in the UK.
• Post-combustion – CO2 is separated from the process gas stream after the fuel has been burned.
• Oxy-combustion – fuel is burned in an oxygen enriched gas resulting in a flue gas with high levels of CO2 and steam.
• Pre-combustion – CO2 is removed from gasification product gas streams prior to combustion in gas turbines.
• The materials requirements depend on the technology used, but vary from changed boiler environments to new plant equipment where the conditions are more akin to those of a chemical plant.
A number of technology options exist for capturing CO2 downstream in both new plant and retrofit applications.
Low temperature liquid scrubbing, using amine-based solvents, is an established technology that is likely to be deployed in first generation systems. Scrubbing has been widely used in the oil/gas sector to separate CO2 from natural gas. The process environment is reducing (compared to oxidising) and at a much smaller scale than that required in a fossil fuel power plant.
Critical materials issues in amine scrubbing are –
• Durability of materials and joints.
• The performance of corrosion inhibitors.
• Surface treatments and coatings that offer protection and repair options.
• Lack of knowledge about corrosion mechanisms that may occur.
• The effects of different amines and their degradation products on corrosion.
Alternative post-combustion approaches include the use of ammonia, solid sorbents such as lime or alkali compounds, adsorption on molecular sieves or active carbons (using pressure, temperature or electrical swing systems), or cryogenics. All present materials challenges.
Oxy-combustion can also be deployed in new and existing pulverised coal plants using advanced steam conditions. The combustion of fossil fuels in an oxygen-enriched/low nitrogen environment leads to gaseous combustion products that are predominantly a mix of CO2 and steam. This can readily be separated using a condenser, leaving a high concentration CO2 stream for further clean-up, compression, transportation and storage.
The materials challenges are centred on the changes to the boiler environment as a result of the flue gas recycle required to replace the missing nitrogen to control flame temperatures. Other parts of the systems, such as the steam condenser/CO2 separator, could also provide unexpected materials problems in service.
In pulverised fuel boilers, the combustion chamber will experience significantly higher levels of CO2 and steam than in conventional air-firing, along with higher concentrations of contaminants, such as sulphur oxides (SOx), and changed ash deposition behaviour, depending on the type of recycle. The cleanest, and most expensive, option is to recycle after flue gas desulphurisation (FGD). This reduces SOx in the boiler, but increases the size of the FGD plant to handle the full flow of recycling flue gas. The simpler option is to recycle the flue gas with little pre-cleaning (after particle removal in the electrostatic precipitator), resulting in a dirty/high SOx flue gas which can lead to increased fouling and corrosion.
Water-wall and superheater corrosion in these systems are major research priorities, particularly when combined with advanced supercritical plant conditions leading to higher levels of contaminants and temperatures.
Integrated gasification combined cycle systems produce a combustible syngas which is burned directly in gas turbines. Pre-combustion capture of CO2 in the syngas can be used to produce a high hydrogen gas (H2) that will create only steam when burned. High pressure, oxygen-blown gasifiers are the most suitable for use with this CO2 capture approach.
To capture CO2, the syngas is fed through a shift reactor where most of the carbon monoxide is reacted with additional steam to give H2 and CO2. A number of approaches can be used to separate CO2 from the shifted flue gas. Variants include the use of physical solvents, solid sorbents or adsorbents, cryogenics and advanced gas separation membranes.
The membrane-based systems yield interesting materials development challenges. Membranes based on polymers, palladium-silver and ceramics have been developed and all have potential. The preferred systems should have high selectivity for H2 and a high flow capability in compact units, as power plants provide immense gas flows.