A Chemical Engineering Perspective on Delivering CCS at Commercial Scale

The Institute of Chemical Engineers (IChemE), UK, has recently released a report on the challenges and opportunities for delivering carbon capture and storage (CCS) technologies at commercial scale to mitigate the efforts of climate change. The professional body sees a significant role for the profession of chemical engineering in tackling what is arguably one of the largest, most complex and daunting problems facing the world today.

Warming planet

Most environmental models developed by scientists point to a warming planet, a sizeable portion of which can be attributed to manmade activity. There is also broad agreement that any chance of limiting the temperature rise to 2°C, as agreed in Paris, will require a range of CCS technologies to be deployed commercially, as they represent the lowest-cost way to transition to a low-carbon economy.

Several studies also indicate that carbon dioxide utilisation (CDU) strategies are unlikely to make a major contribution to decreases in anthropogenic carbon dioxide (CO2) emissions at the rate required. The global CDU industry currently utilises just about 0.2-Gt of CO2 per annum, of which only 25% is sequestered for the long term. While it will have a role in promoting the wider principle of resource efficiency and developing a circular economy, its impact for lowering CO2 levels will be negligible. In short, it will be up to CCS to deliver.

Early stage of CCS

As of now CCS technologies have barely made an impact. There are just 37 projects worldwide, with only 18 operational and the balance in early stages of implementation. All told, they sequester 31-mt of CO2 annually – a tiny fraction of the ~10-Gt (Gigatonnes) needed to be sequestered by 2050.

Of the several technologies at varying stages of development, most are at the pilot plant stage or above. This, however, does not necessarily imply that they are commercially relevant and will make it to the next stage. A number of technologies are at a Technology Readiness Level (TRL) of nine, implying they are now deployable at a commercial scale, i.e. capable of capturing upwards of 0.4-mtpa of CO2. The list includes post-combustion capture using solvents; pre-combustion natural gas processing; sequestering in saline formations; and use for enhanced oil recovery (EOR).

If these ready technologies have not yet been commercialised beyond a limited scale, it is due commercial realities. Capturing CO2 from exhaust gases from power plants burning fossil fuels typically increases costs by 45-70%, depending on the type of solvent system used, the region, the plant fuel and design. These costs will come down with wider adoption, larger scale and technological evolution, but the key to wider adoption will be a tax on carbon or other efforts to monetise the CO2 produced, such as deployment for EOR. Indeed, this how the majority of CCS projects – mostly in the US – utilise the CO2.

Carbon capture

The costs of carbon capture – the first stage of CCS – account for up to 80% of total CCS chain costs for most ready technologies. This is, hence, the target for innovations leading to cost reductions.

The capture technologies can be broadly classified into three: post-combustion capture, where CO2 is removed from flue gases, leaving behind mostly nitrogen; oxy-fuel combustion, wherein a fuel is burnt in a stream of pure oxygen (instead of air), making CO2 capture easier; and pre-combustion capture, as in an integrated gasification combined cycle (IGCC) power plant in which the fuel is first gasified in a steam methane reformer (SMR) to produce carbon monoxide, CO2, and hydrogen, and the CO2 is captured prior to combustion of the other two gases in a gas turbine. The hydrogen can alternatively be used in fuel cells for power generation, or used for decarbonised heating.

Post-combustion capture, assessed at TRL9, has been practiced for long in the chemical industry – albeit at a smaller scale – including for the production of methanol and ammonia. Chemical absorption using solvents such as monoethanolamine & its derivatives has also been used to remove CO2 from natural gas for decades and is also used in two commercial scale post-combustion capture facilities. Membrane separation of CO2 is one notch lower in TRL, and significant research is currently being carried out to move it further. Physical adsorption systems are also an option, and have the potential to be lower cost and more energy efficient than solvent based methods.

Pre-combustion capture enables more energy efficient separation of CO2 and more compact plants at reduced costs, but some of these gains are offset by the additional energy consumption (about 15% more) associated with conversion of the fuel to syngas, compared to burning for power directly.

Carbon transport

Carbon transport and injection has been practised at industrial scale for EOR since the 1970s, and pipeline transfer poses few technical challenges that prevent it from being deployed at regional or national levels. Water content needs to be maintained below 50-ppm to avoid gas hydrate formation and ensure integrity of the equipment metallurgy. The CO2 should ideally be in a dense or supercritical phase, and improving costs here will have an impact.

Chemical engineers will have a role to play in the design and specifications of optimised CO2 transport networks, into which a wide range of diverse CO2 generators feed. Optimising connectivity between CO2 sources and sinks is a systems challenge, and a key element of reducing costs and improving process efficiencies.

Carbon storage

The most established storage option is sequestration in sub-surface geological formations, and studies have revealed a potential availability to store about 11,000-Gt of CO2 – four times the maximum cumulative storage likely to be required at the end of the century. The characterisation of storage potential is, however, poor in the Asia-Pacific, though it is fairly well done in the UK, EU and North America. Deep saline aquifers have been used in Canada, USA and Norway for storage, but this requires extensive and costly sub-surface characterisation studies. In contrast, depleted hydrocarbon reservoirs are well understood and pose a cheaper opportunity. One interesting development is injection of CO2 into deep basalt rock formations.

Chemical engineering expertise in reactive flow will be important in the appraisal of injection and storage potential of water-bearing carbonate aquifers and depleted oil & gas reservoirs, where the acidic CO2 fluid will react with limestone to modify the pore space during the storage process. Better models are required for the design and optimisation of storage in these systems, which represent about half of the potential storage sites globally.

Multi-disciplinary approach

The challenges associated with successful deployment of CCS require a multi-disciplinary approach, with chemical engineering playing a major role. Chemical engineers are currently limited to the development and application of several CCS technologies, but besides providing technical analyses, they can also define the role of CCS in the overall energy system by applying a systems approach.

They can also assist in creating support for the technology by engaging – directly and indirectly – with policy planners, NGOs and the public, and so ensure the funding needed to deliver this initiative on a scale at which it will make an impact.