A significant proportion of carbon dioxide emissions come from power generation from fossil fuels. While nuclear and renewable energy sources could reduce emissions, neither are currently able to meet world energy demand and carbon capture and storage (CCS) is required as the transition is made.
Status of CCS
There are a number of CCS demonstration projects planned, which aim to advance the technologies and prove technical viability at a realistic scale. The high cost of CCS coupled with the technical risks of the developing technology makes investment in such demonstration projects unattractive without government funding or incentives.
The UK government is to provide funding for up to four CCS demonstration projects for coal fired power stations each with a capture capability of 300 MW or greater net capacity and has currently budgeted £ 90 million for design and development work for the first project in 2009 – 2010. Similar government support is being given for projects worldwide, including Australia, Canada, Europe and the USA.
Post combustion capture
Power generation with post combustion carbon capture involves fuel being burned in air with carbon dioxide captured from the resultant flue gas. This method has the advantage that it could be retrofitted to existing plants although the additional plot area and utilities required could make this difficult unless considered in the initial design, hence the emergence of ‘capture ready’ power plants.
Power generation with precombustion carbon capture involves production of synthesis gas (H2 and CO), either from reforming of natural gas or gasification of coal or biomass. The hydrogen yield is maximised by shift reactors operating at successively lower temperatures. The hydrogen is then conditioned and burnt in a combined cycle gas turbine; a process known as integrated gasification combined cycle (IGCC). The advantage of this technology is that the synthesis gas, prior to combustion, is rich in carbon dioxide and at high pressure, making separation of the carbon dioxide less energy intensive and the separation equipment more compact.
Oxyfuel combustion of fuels in oxygen rather than air results in flue gas with very high carbon dioxide concentration, typically 75 - 85 mol.% or higher on a dry basis, compared with 15 mol.% from conventional combustion of coal. The balance of the flue gas is made up mainly of nitrogen, with some oxygen and argon together with SOx and NOx impurities. Feed oxygen purity and air ingress are the main factors influencing flue gas composition.
To avoid multi phase flow, pipeline transport will be at supercritical pressure in the dense phase, where the carbon dioxide has a density similar to liquid and a compressibility and viscosity similar to gas. Delivery in the dense phase can be achieved by compression or by liquefaction and pumping.
Carbon dioxide transport by ship is relatively uncommon, with the current fleet comprising a few small ships transporting food grade carbon dioxide. The design of carbon dioxide carriers is similar to LPG carriers and it is thought that such ships will be built in the same yards.
There is potential to store carbon dioxide in geological formations such as depleted oil or gas fields, deep saline formations or unmineable coal seams.
Storage of carbon dioxide deep in the sea has also attracted some interest, through creation of carbon dioxide lakes or through dissolution.
The potential for mineral carbon dioxide storage is being investigated, where carbon dioxide would be reacted with metal oxide bearing materials to form a solid product, providing the potential for long term storage and low risk of future release.
A number of CCS projects are planned to demonstrate the scale up of the technologies in the transition prior to adoption at a commercial scale. This is largely dependant on government support and funding. Once proven, an appropriate financial and regulatory framework would be needed if CCS technologies are to be more widely implemented by 2020.
Costain Energy & Process, UK
You can read the full article in the January issue of Hydrocarbon Engineering.