Carbon capture and storage (CCS) (or carbon capture and sequestration or carbon control and sequestration) is the process of capturing waste carbon dioxide (CO 2) usually from large point sources, such as a cement factory or biomass power plant, transporting it to a storage site, and depositing it where it will not enter the atmosphere, normally an underground geological formation. The aim is to prevent the release of large quantities of CO 2 into the atmosphere from heavy industry. It is a potential means of mitigating the contribution to global warming and ocean acidification of carbon dioxide emissions from industry and heating. Although CO 2 has been injected into geological formations for several decades for various purposes, including enhanced oil recovery, the long term storage of CO 2 is a relatively new concept. Direct air capture is a type of CCS which scrubs CO 2 from ambient air rather than a point source. Carbon dioxide can be captured directly from the air or from an industrial source (such as power plant flue gas) using a variety of technologies, including absorption, adsorption, chemical looping, or membrane gas separation technologies. Amines are used as solvents in the leading carbon scrubbing technology. CCS applied to a modern conventional power plant could reduce CO 2 emissions to the atmosphere by approximately 80–90% compared to a plant without CCS. If used on a power plant capturing and compressing CO 2, other system costs are estimated to increase the cost per watt-hour of energy produced by 21–91% for fossil fuel power plants; and applying the technology to existing plants would be even more expensive, especially if they are far from a sequestration site. As of 2019[update] there are 17 operating CCS projects in the world, capturing 31.5Mt of CO 2 per year, of which 3.7 is stored geologically. Most are industrial not power plants. It is possible for CCS, when combined with biomass, to result in net negative emissions. A trial of bio-energy with carbon capture and storage (BECCS) at a wood-fired unit in Drax power station in the UK started in 2019: if successful this could remove a tiny amount of CO 2 from the atmosphere. Storage of the CO 2 is envisaged either in deep geological formations, or in the form of mineral carbonates. And pyrogenic carbon capture and storage (PyCCS) is being researched. Deep ocean storage is not used, because it could acidify the ocean. Geological formations are currently considered the most promising sequestration sites. The US National Energy Technology Laboratory (NETL) reported that North America has enough storage capacity for more than 900 years worth of carbon dioxide at current production rates. A general problem is that long term predictions about submarine or underground storage security are very difficult and uncertain, and there is still the risk that some CO 2 might leak into the atmosphere.