The gas reacts with calcium, iron and other minerals in the rock to form a whitish, chalky material

Greenhouse gas excesses converted into chalk blocks

Greenhouse gases have been transformed into blocks of chalk in Iceland, a development which could allow for the easy removal of carbon dioxide (CO2) from the Earth’s atmosphere.

The technique has been successfully tested by a British-led team at the world's largest geothermal power plant in Iceland.

Scientists conducting a pilot study injected water containing thousands of tons of CO2 into deep wells sunk into volcanic basalt rock.

In less than two years, the gas reacted with calcium, iron and other minerals in the rock to form a whitish, chalky material.

Turning CO2 into a chalky mineral is a better option than pumping large volumes of the gas into porous rock underground, the traditional method of geological carbon capture, it is claimed.

Mass carbon storage programmes have been cited as a promising way to remove the build-up of CO2 in the earth’s atmosphere resulting from human activities.

The UK’s Drax power plant was trialling such a system last year which involved storing its CO2 output in underground reservoirs before the government pulled its funding as part of cost-saving measures.

Experts have warned that this method, which generally stores the CO2 in a volatile liquid or gas form, is at high risk of rupture which would result in CO2 escaping back to the surface through fractures, or underground reservoirs being damaged by earthquakes or human activity.

Prior to the new study, it was thought that a major obstacle in the way of mineralising carbon was time, with previous studies suggesting the process could take hundreds or even thousands of years.

However, at the Hellisheidi plant, which uses volcanically heated water to provide power for Iceland's capital, Reykjavik, it took less than two years for more than 95 per cent of the injected carbon to solidify.

Lead scientist Dr Juerg Matter, associate professor in geoengineering at the University of Southampton, described this as "amazingly fast".

"Carbonate minerals do not leak out of the ground, thus our newly developed method results in permanent and environmentally friendly storage of CO2 emissions,” he added.

"On the other hand, basalt is one of the most common rock types on Earth, potentially providing one of the largest CO2 storage capacity.

"Storing CO2 as carbonate minerals significantly enhances storage security which should improve public acceptance of carbon capture and storage as a climate change mitigation technology.

"We need to deal with rising carbon emissions. This is the ultimate permanent storage - turn them back to stone."

The pilot project, known as Carbfix, was started in 2012 with the aim of removing Hellisheidi's CO2 emissions along with foul-smelling hydrogen sulphide, another byproduct of geothermal power generation. The plant produces 36,287 tonnes of CO2 per year.

The team initially piped 227 tonnes of CO2 mixed with water and hydrogen sulphide to a depth of 400 to 800 metres.

In 2014, buoyed by indications that much of the carbon had mineralised, Reyjavik Energy started injecting CO2 into the wells at a rate of 4,536 tonnes per year.

Monitoring shows that mineralisation has continued at the same pace and this summer the energy company plans to double the injection rate.

One drawback of the process, is that it requires approximately 21 tonnes of water to condense one tonne of CO2. This could be solved by placing carbon mineralisation plants in or near the sea, scientists have said.

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