Processing and storage of CO₂ is carried out at a Central Processing Facility (CPF) at Krechba. Piped gas from two wells 120 kilometres to the south, together with gas from the Krechba field, is mixed. Then the CO₂ is separated out because the natural gas contains 1-10% CO₂, which has to be removed to meet purity standards for sale. The CO₂ is condensed to 185 bar pressure and transported 14 kilometres by pipeline to the injection wells. Once it is injected, the CO₂ turns naturally under pressure to a liquefied form.
The CO₂ is separated from the natural gas in a multi-stage, proprietary aMDEA (activated methyl diethylamine) chemical engineering process licensed by BASF. The acid feed-gas, defined as any gas that contains significant amounts of acidic gases such as carbon dioxide (CO₂) or hydrogen sulfide (H₂S), makes contact with an activated amine solution in an absorber column by passing through two parallel process trains. The treatment with the amine, a type of organic chemical compound related to ammonia, removes virtually all the CO₂. The resulting methane gas stream requires further dehydration in a glycol contactor prior to export.
The amine solution is stripped from the CO₂ and sent back to the absorber to begin the next process cycle. This regeneration process produces a CO₂ stream of over 98% purity at low pressure, around 1.4 bar.
About 1.4 million standard cubic metres per day of CO₂ are produced from the gas processing facility at Krechba. Before being re-injected into the Krechba reservoir this is compressed to a very high pressure of 185 bar to force it into the reservoir’s low permeability sandstone.
Two high-volume, centrifugal compressor trains were purpose-built to meet the In Salah reservoir’s unusually demanding requirements. Each comprises four stages of compression and is driven by electric motors that together require 24 megawatts of power, two thirds of the site’s total power generation output. Because CO₂ is heavier than natural gas, significant amounts of energy are needed to compress it, resulting in high discharge temperatures of more than 250ºC.
Each stage of compression is preceded by an inlet suction scrubber and followed by an air cooler to regulate temperature. Between the third and fourth stages, the CO₂ is dried by glycol dehydration because wet CO₂ is mildly acidic and can react with ferrous metals to erode the carbon steel flow lines transporting the gas to the injection wells up to 14 kilometres away.
At the discharge pressure of 185 bar, the CO₂ is in its dense or supercritical phase where it behaves more like a liquid. It can quickly shift between the gaseous, liquid and solid states, so it must be kept hot and at high pressure. If it cools and depressurises, it becomes ‘dry ice’. For this reason some areas of the plant are designed to withstand temperatures below -80ºC.
Energy penalty of capturing CO₂
The extra energy required by Krechba’s two high power centrifugal compressors creates additional emissions but most of these are offset through efficient heat recovery. In turn, this leads to a corresponding decrease in the fuel needed for the plant’s process heating.
The net increase of around 45,000 tonnes of CO₂ per year emitted by power generation is far outweighed by the positive benefits of storing up to one million tonnes of CO₂ each year in the Krechba reservoir.