Albeit some distinctive limitations, such as the poor permeability of coal which reduces the amount of CO2 that can be injected, coal seams that are not meant to be exploited also can become storage sites.
The interest of coal seams lie in the fact that after CO2 has been injected, it sticks to the walls of coal and locks in, due to the phenomenon of Adsorption.
Coal seams often contain a vast amount of gas trapped in the internal structure of the mineral. At the selected sites, the gas found is methane, itself an important source of energy. Coal has a greater affinity with CO2 than methane, therefore in the presence of both it will release methane in order to adsorb CO2.
Another particularity of coal is that it can contain twice as much CO2 as methane. Consequently, it would be possible to store CO2 in coal while recovering the methane thus freed and commercialize it – this technique is called enhanced coalbed methane (ECBM).
A favorable geographic distribution
Coal seams are common geological structures found on every continent. Heavy industries installations, major sources of CO2, are often built on coalbeds, which limits transportation costs.
Reselling methane could repay part of the CO2 injection costs
The methane that is recovered can be commercialized: it represents a vast energetic resource not really exploited today and which could be used more largely in the future thanks to ECBM technique.
Caption: Concept of enhanced coalbed methane (ECBM) enabling exploitation of the natural gas (methane) found in coal
Several tests were run to assess the feasibility of large-scale CO2 storage in coal layers with recovery of methane. The permeability of coal seams as well as CO2 and methane adsorption/desorption mechanisms in coal pores still needs to be investigated further – studies are currently being carried out in laboratories and on site to progress in these areas.