CO2 capture in oxyfuel coal-fired power plants
The oxyfuel process was developed in order to avoid the energy disadvantage, which was considerable initially, of post-combustion capture processes when capturing CO2. This process, too, is based on the steam power process that has been known for decades, which is used in all coal-fired plants for power generation. The only difference is that combustion takes place here with highly concentrated oxygen and without nitrogen. The cryogenic air separation (using extreme refrigeration) that is necessary here involves high energy consumption and therefore requires significant further development of this process, on the one hand, and the development of alternative methods of oxygen production such as membrane technology or the chemical-looping process, on the other hand.
A further research goal here is the optimisation of the combustion process with oxygen and recirculated flue gas. In addition, research is required on the influence of impurities in CO2-rich flue gas. These impurities have to be considered as regards the compatibility of the storage and barrier rocks in the ground. Process integration has an important role in the oxyfuel process, too, in order to deliver technology that can be used on a large scale.
- Determination of optimal values of oxygen excess and oxygen fraction for combustion
- Burning behaviour of coal in atmospheres containing CO2, H2O and O2 with oxygen excesses and oxygen fractions typically found in service
- Mechanisms of formation of the pollutant gases NOx, SOx and CO
- Gradual addition of oxygen to improve the combustion degree and reduce the formation of pollutant gases
- Effect of changed flue gas composition on heat transfer, particularly on radiation heat transfer
- Operationally reliable mixing of oxygen with the recirculated flue gas
- Possibilities of using low-temperature flue gas heat at the steam generator outlet
- Optimal temperature of recirculated flue gas
- Alternative steam generator designs (fluidised bed, melting chamber)
- Determination of the phase equilibria of flue gas mixtures as a basis for the design of liquefaction plants
- Influence of kinetics on concentrations in liquid CO2
- Distribution of the pollutant gases (SOx, NOx, CO) during dehumidification
- Pumps for transporting liquefied CO2
- Minimising internal power requirement
- Long-term stability of materials for flue gas dehumidification
- Increasing efficiency by integrating CO2 capture and air separation as key components into the overall process
- Performance during and suitability for partial-load operation
- Feasibility of air separation plants for the required output classes (> 400 MWel gross power plant capacity)
- Specification of the denitrification and desulphurisation plants that may be necessary
- Optimal distribution of oxygen and recirculated flue gas
- Pollution and corrosion behaviour under oxyfuel conditions
- Capture behaviour of flue gas purification plants
- Heat transfer in the combustion chamber and close to contact heating surfaces
- Start-up behaviour
- Dynamic interplay of individual components
- Reduction of costs of investment and operations in future larger-scale plants
Oxycoal (refer also to gas separation with membranes)
- Development of suitable membrane materials with sufficient long-term stability
- Development of membrane modules that are as leak-free as possible
- Engineering and energy-related limitations with regard to the integration of the membrane module into the flow path of the flue gas
- Development of hot-gas purification for the required output class
- Design of the overall process
Chemical looping (refer also to looping processes)
- Design of the overall process
- Suitable carrier materials with sufficient long-term stability
- Reactor system
- Control and dynamics of the reactor system
- Capture processes for ash and carrier material particles
The oxyfuel process for power plants has been realised in a number of test plants on laboratory scale so far. In September 2008, Vattenfall commissioned the largest oxyfuel plant in the world with a combustion heating capacity of 30 MW. The test plant includes a steam generator, an air separation plant, flue gas purification and equipment for CO2 capture. Depending on the success of the current R&D work, a demonstration plant (250 MW) was planned that should go into service in 2015. The plan are on hold now due to legal and financial restrictions. The test plant operated by Vattenfall AG is one of the largest current oxyfuel projects worldwide. A range of scientific institutes are also involved here (Technical University of Hamburg-Harburg, Technical University of Dresden, etc.). The R&D work is part of the ADECOS joint project (Advanced development of the coal-fired oxyfuel process with CO2 separation) within the framework of the COORETEC research initiative.
Oxyfuel processes represent an important research focus within the EU's ENCAP project (Enhanced capture of CO2). A number of industrial companies (e.g. ALSTOM, Siemens, Air Liquide, Vattenfall) are also involved as part of the ENCAP programme. In addition, a range of oxyfuel activities are integrated into the IEA's Oxyfuel Combustion Network.
Outside of the EU, further important research work on oxyfuel is currently being carried out in Canada (CANMET project, 300 kW reactor) and Japan (1.2 MW plant). The Callide Oxyfuel Project (funding volume of A$50 million) was also started in Australia in 2006. The goal is to retrofit oxyfuel technology to an existing coal-fired power plant. This project is supported by public funding and is being conducted by industrial companies and research institutes.
The name 'oxyfuel process' refers to the combustion of carbon-containing fuels with pure oxygen. The CO2 content in the flue gas from oxyfuel plants is around 89% by volume, in contrast with a value of between 12 and 15% by volume for conventional power plants. After flue gas purification and scrubbing, the flue gas consists principally of a carbon dioxide-steam mixture. Once the steam has been removed by condensation, the result is CO2-rich flue gas that can be compressed and then transported by pipeline to the storage location. Oxygen for combustion is provided by cryogenic air separation plants that extract oxygen from air by condensation at low temperatures (< -182 °C). This process is being used today on a large scale in the steel industry and, more recently, in gas-to-liquid plants (production of power and fuel from natural gas).
The oxygen capacities of the largest plants currently planned (e.g. for synthesis gas production) are approximately 800,000 Nm³/h. To put this in perspective, a black-coal power plant block with a power output of 500 MW and a degree of utilisation of 43% requires around 270,000 m³/h of oxygen for stoichiometric combustion. Combustion with excess oxygen (typical air-fuel ratios in large-scale plants are currently around 1.15) increases the amount of O2 required accordingly. Even with these air-fuel ratios, the size of the cryogenic plant required is not problematic.
If fuel is burned with pure oxygen, the combustion temperature is significantly higher than in the case of conventional combustion. A modification of the steam generator and measures to limit the combustion temperature then become necessary because of other heat- and flow-related constraints. For this reason, a fraction of the CO2-rich combustion gas (around two thirds of the volumetric flow of flue gas) is recirculated back to the combustion chamber in order to reduce the combustion temperature, as required by the limited temperature-resistance of the materials of construction used. In addition, unreacted oxygen is returned to the oxidation process again and the residual oxygen content of the flue gas is reduced.
Combustion with pure oxygen leads to significantly reduced amounts of flue gas and to a change in radiation heat transfer in the flue and combustion gases (because of the change in the CO2 and H2O concentrations). Redesign of the heat exchange surfaces and combustion chamber geometries and the implementation of an optimised flue gas duct system also become necessary here. There are also significant problems and issues with regard to coal combustion because of the modified amount of excess air. As the oxygen excess is lower than that in conventional combustion processes, problems with combustion degree and corrosion on the combustion chamber walls can occur. Another important issue is the optimal thermodynamic integration of carbon dioxide treatment into the actual power plant process, which has the potential to further increase efficiency.
3 current research projects
ADECOS-ZWSF Further development and investigation of the oxyfuel process with circulating fluidised-bed combustion with regard to engineering feasibility and economic viability
Organisations carrying out research: Technische Universität Hamburg-Harburg - Maschinenbau - Institut für Energietechnik
Technische Universität Dresden - Fakultät Maschinenwesen - Institut für Energietechnik - Professur Verbrennung, Wärme- und Stoffübertragung
Universität Stuttgart - Fakultät 4 Energie-, Verfahrens- und Biotechnik - Institut für Feuerungs- und Kraftwerkstechnik (IFK)
Project numbers: 0327872A, 0327872B, 0327872C
CLOCK Chemical looping combustion of coal for CO2 capture in atmospheric fluidised-bed reactors for steam power process
Organisations carrying out research: Universität Stuttgart - Fakultät 4 Energie-, Verfahrens- und Biotechnik - Institut für Feuerungs- und Kraftwerkstechnik (IFK)
Technische Universität Hamburg-Harburg - Institut für Feststoffverfahrenstechnik und Partikeltechnologie
Technische Universität Hamburg-Harburg - Institut für Energietechnik (IET)
Project numbers: 0327844A, 0327844B, 0327844C
OXYCOAL Development of a coal combustion process with no CO2 emissions for power generation, Project phase 2: Pilot plant
Organisations carrying out research: Rheinisch-Westfälische Technische Hochschule Aachen
Fakultät 4 – Maschinenwesen
Lehrstuhl für Wärme- und Stoffübertragung
Lehrstuhl und Institut für Regelungstechnik
Institut für Werkstoffanwendungen im Maschinenbau
Lehrstuhl und Institut für Strahlantriebe und Turboarbeitsmaschinen
Institut für Technische Verbrennung
Project numbers: 0326890O