Stockholm-based Vattenfall AB has ordered the first components for a 30 megawatt US$50 million (40 million euro) oxyfuel pilot plant, with construction to start next year. The plant will be built at Schwarze Pumpe, south-east of Berlin where they operate a conventional coal-fired power plant.
The plant will burn coal in a pure oxygen environment producing a flu gas that is mostly carbon dioxide and water. The water can be condensed and the remaining nearly pure carbon dioxide collected for sequestration. Nitrogen in the combustion air dilutes the oxygen in conventional coal furnaces, limiting the combustion temperatures, while In a oxyfuels furnace, the flu gases are recirculated to limit the oxygen concentration, producing the same effect. An integrated cryogenic oxygen plant will be built as part of the pilot plant.
Vattenfall's strategic director and oxyfuels project manager, says gasification, which is the main competing technology, is not ready to be a source of clean coal power. He cites mechanical difficulties that have plagued the gasification demonstration power plants built to date.
The development of advanced steam technology is not specifically linked to the CO2 capture field but more in general to the development of materials for extremely high pressures and temperatures, in combination with new boiler and turbine designs. During the 1990’s, power plants were built with very advanced steam data, such as Vattenfall’s lignite-fired units in Germany. Plant efficiencies (LHV) of over 49% have been obtained without CO2 capture.
One of the main opportunities with oxyfuel recycle combustion of coal in new plants is that the steam cycle is able to take advantage of the ongoing development to increase steam cycle efficiency through the use of advanced steam technology and lignite drying. The development of advanced steam technology is not specifically linked to the CO2 capture field but more in general to the development of materials for extremely high pressures and temperatures, in combination with new boiler and turbine designs
During the 1990’s, power plants were built with very advanced steam data, achieving efficiencies above 49%. For the ferritic materials used in these power plants the limit for the materials lies just above 600°C. Vattenfall is one of the 35 companies taking part the AD700 project which has a long-term target after year 2020 of a net efficiency above 55% (without CO2 capture) based on steam temperatures above 800°C. AD700 covers new materials (Ni-based superalloys, austenitic steels), new materials manufacturing methods and new welding methods.
Raw lignite contains roughly 50% of moisture, meaning that a non-negligible amount of the heat released during combustion is employed to evaporate water. Future lignite-fired plants will probably include lignite drying by using low-temperature heat from the steam power cycle or the flue gas.
The heat loss with the flue gas in a conventional air-fired boiler amounts to up to 10%. A significant part of this loss is the heat energy that leaves with the nitrogen in the flue gas. In the oxyfuel combustion boiler, there is no bulk nitrogen in the gas path, which in turn means that the heat losses with the flue gas can be significantly reduced.
Oxyfuel Combustion for Coal-Fired Power Generation with CO2 Capture - Opportunities and Challenges, Kristin Jordal et al, Vattenfall Utveckling AB
Simpler and Cheaper Clean Coal Technology, Technology Review, April 26,2006