Solid as a rock: mineralising carbon dioxide

Carbon capture and storage (CCS) is proving to be a thorny technological problem. The various steps in the process purifying and concentrating carbon dioxide from a combustion source, transporting it to a storage site and injecting it underground have all been demonstrated separately and at different scales, but putting it all together without fatally distorting the economics of the process it’s being applied to is another matter.

However, not all CCS requires that the CO2 should be in a pure gaseous form. Reacting the gas chemically to convert it into mineral form is another option; this, of course, is the way that carbon is sequestered in nature. The massive deposits of chalk and limestone all around the world are testament that turning carbon and oxygen into rock is a very stable way of storing it; after all, nobody’s expecting the North or South Downs, the White Cliffs of Dover, or for that matter the Himalayas — all made from limestone minerals — to dissolve into gas.

28 White Cliffs.jpg
Mineralisation is the way that nature deals with CO2, locking it up in stable minerals such as chalk and limestone – but this takes millions of years

It’s not necessarily an easy option, but a variety of projects in Europe and the US are looking at methods to mineralise CO2. Pilot studies in the UK commissioned by the Energy Technologies Institute and in the Netherlands are looking at ways of locking CO2 into solid forms and possible ways of using it. In Texas, a company is building demonstration plants that, it claims, will not only mineralise CO2, but will turn a healthy profit while they do it.

While the ETI prepares a map of sites in the North Sea suitable for CO2 storage, chief executive David Clarke pointed out that not all carbon sources are close enough for transport to an offshore storage site to be practical or economical. This applies particularly to non-power station sources, he added. ’Point sources of CO2 from steel works, cement works and glass works are the next biggest after power plants and we’re looking at mineralisation using materials that they’ll have on site already.’

Limestone deposits all around the world prove that turning carbon and oxygen into rock is a stable way of storing it

These would be naturally alkaline chemicals that react with acidic CO2 to form a neutral solid, in the form of calcium carbonate (chalk). This is easy and safe to store, or can be used in aggregates, bricks, or as a filler in concrete.

The ETI is funding a £1m consortium project led by Caterpillar and including Shell, the British Geological Survey and the Centre for Innovation in CCS at the University of Nottingham to look into possibilities for mineralisation, distribution of suitable materials in the UK and the economics of the process. ’Much of the research in this field has concentrated on the chemistry involved,’ Clarke said. ’The ETI is looking to develop system solutions and identify the necessary technologies.’

US company Skyonic, meanwhile, has already commercialised a two-step process that turns the flue gases from combustion -even of a relatively dirty fuel such as lignite coal into calcium bicarbonate. ’It’s high-school chemistry,’ explained chief executive Joe Jones. ’You use a strong base to neutral-ise a weak acid. We use electrolysis or heat to split common salt, sodium chloride, into hydrochloric acid and caustic soda, then react the soda with the carbon dioxide to make bicarbonate.’

Both the of these are produced at food grade, Jones said, and there are markets for both. ’They can displace products that are made using energy,’ he said. ’The HCl will go into the starch industry it’s used to make starch from grain. The bicarbonate will go into the animal-feed sector. Yard-raised cattle get a condition called acidosis, because they’re fed on grain rather than the grass they’ve evolved for; bicarbonate is used as a feed additive. It displaces bicarbonate mined from natural deposits or made by the Solvay process from brine and limestone, which uses a lot of energy.’

Skyonic’s process, as applied to smaller plants that don’t generate much waste heat, uses a small chlor-alkali plant operating at low energy to electrolyse brine. Available low-grade heat from flue gases is used to warm the brine, which reduces the voltage needed to split the salt. ’This also cools the flue gas to below its dewpoint, which knocks the moisture out of it,’ Jones said. ’That water is slightly acidic and it also brings out any mercuric oxide, lead, selenium, antimony and other toxic trace metals that you don’t want in your products; we’ve been able to make solids from those and recycle the water into the process.’

It has to be a good idea – in the end it’s the only type of technology that will survive

JOE JONES, SKYONIC

The cooled flue-gas stream, which now contains carbon dioxide and oxides of sulphur and nitrogen in trace quantities, then mixes with the sodium hydroxide caustic soda from the chlor-alkali plant. ’That forms sodium sulphate, sodium nitrate and sodium carbonate, which we convert into bicarbonate. The sulphate and nitrate are present in very small quantities and we don’t need to separate them out. We recover all the water using a patented process that uses molecular sieves rather than heat.’

Set in stone:cement works are a major CO2 source
Set in stone: cement works are a major point source of CO2 emissions, and prime candidates for a mineralisation process

All carbon capture incurs an energy penalty and chemical mineralisation is no exception, said Jones. ’The standard methods of scrubbing, with capture using an amine to absorb the CO2, compression, transport to site and pumping it underground gives you a lifecycle energy penalty of the mid-40s in percentage terms. We’re achieving numbers more in the 30s. And if we use the process on a plant that generates a lot of waste heat and we can split the salt thermolytically rather than electrolytically, we can get into the 20s or even the teens.’

Jones is currently assessing a thermolytic version of the process for a UK 500MW-plus power station, where he claimed it is able to remove up to two-thirds of the CO2 from the flue gases.

Another promising market for the bicarbonate is in growing algae and Skyonic has been working with the University of Texas on projects to grow algae for biofuels for the past three years. ’If you just bubble CO2 into an open algae pit it doesn’t work very well it doesn’t dissolve readily and isn’t incorporated into the algae,’ said Jones. ’But using a solid form, we can transport it easily [and have] found that it accelerates algae growth by 300 per cent compared with a gaseous CO2 feed, so it matures in four days rather than three weeks and it also gives algae with a very high lipid count; normally you’d trade off growth speed with lipid production, so that’s a double benefit.’

Jones admits that there currently isn’t a big market for algae biofuels, but algae are grown in very large volumes for commercial aquaculture.
Skyonic has built three demonstration plants so far two on coal-fired power stations and one on a cement plant. It has received a $25m (£15m) grant from the US Department of Energy and is building its first commercial plant, at a cement works Capitol Aggregates in San Antonio, Texas slated for start-up next year. The plant will capture 75,000 tonnes per year of CO2 and sell the mineralised products at a profit, Jones said. ’It has to be a good idea to develop a process that will support itself commercially,’ he said. ’In the end, it’s the only type of technology that will survive.’

in depth – shellfish scheme

Mineralisation method mimics natural process used by sealife Another approach to mineralisation is being developed at the Lawrence Berkeley National Laboratory in California. A team led by Ronald Zuckerman is attempting to mimic the natural process used by shellfish to mineralise CO2 in a crystalline form called calcite and has found a method to accelerate this process up to 40 times.

The process uses a type of synthetic polymer called a peptoid as a catalyst to speed up the growth of calcite crystals. Peptoids, also known as poly-N-substituted glycines, mimic the shape and functionality of natural proteins and peptides, but are more stable and can be tailored for specific application, the team explained.

The peptoids are effective even at very low concentrations of CO2 and are reusable, the team reports. While previous attempts to catalyse calcite could only achieve a 150 per cent acceleration, the peptoids can accelerate the process 20 to 40 times, they claim.