Event Report „Efficient Carbon Capture for Coal Power Plants“, 20th-22nd June, 2011, Frankfurt
Coal power plants will provide a significant share of Germany’s future energy generation. So-called CSS processes (carbon capture and storage) can help to reduce resulting CO2 emissions. CO2 is separated from the exhaust streams and subsequently stored or used for chemical reactions.
In order to separate the CO2 from the exhaust streams, additional energy is required, implying a substantial decrease in power plant efficiency. Currently power plants lose about 8-12 % of their efficiency degree due to CO2 separation. At a meeting in Frankfurt titled “Efficient Carbon Capture for Coal Power Plants”, June 20-22, 2011, new technologies were introduced that may help to reduce efficiency losses.
In order to scrub the flue gasses and capture the CO2 post combustion, the exhaust streams are channeled through an amine solution that binds CO2. In the following step the CO2 is reseperated from the washing agent. This process requires about 2.8 GJ per t of CO2 recovered, equaling an efficiency loss of about 10 % in the power plant. With improved amine solutions or other washing agents, e.g. ionic liquids, the efficiency loss can be limited to about 2 GJ per ton CO2 (8 % efficiency loss).
In this process, coal is combusted with pure oxygen instead of air. The process thus yields almost pure CO2 that can then be liquefied and separated. Rectification of oxygen from air requires energy, leading to an efficiency loss of about 8 %. Newly developed oxygen-transporting membranes allow for a reduction of losses to about 6 %.
Similar to the Oxyfuel process, the coal is combusted with pure oxygen and subsequently processed at high temperatures in the presence of water vapor. The resulting synthesis gas is converted completely to CO2 and hydrogen and the CO2 is washed out. The remaining hydrogen is then burned to water. The decisive step for an increase in efficiency is the optimization of the shift reaction. The efficiency losses are estimated at about 7%. By using combined water gas–shift-membrane reactors efficiency can be improved by 0.5-1%.
In the Chemical Looping process, metal oxides or limestone (carbonate looping) is used in order to recover pure CO2. In comparison to the abovementioned processes, chemical looping is yet at an early development stage and its application in solid fuel processes is limited. With the assistance of appropriate oxygen carriers or stable calcium carbonate modifications, efficiency losses in power plants are expected to be restricted to about 4% (chemical looping) and 7% (carbonate looping) respectively.
A detailed description of all processes and an overview on the technological status of CO2 sequestration in coal power plants can be found in Efficient Carbon Capture for Coal Power Plants, a book edited by the scientific organizers of the event, Prof. Detlef Stolten, Forschungszentrum Jülich, and Prof. Viktor Scherer, Universität Bochum. The contributions in this book are based on the presentations given at the event.