Project Cluster

CO₂ capture in oxyfuel coal-fired power plants

Research approach

Oxyfuel pilot plant in Schwarze Pumpe in the Lusatia region in eastern Germany ©Vattenfall

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.

Research goals

Oxyfuel

Combustion technology

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 NO_x, SO_x and CO
Gradual addition of oxygen to improve the combustion degree and reduce the formation of pollutant gases

Steam generator

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)

CO₂ capture

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 (SO_x, NO_x, CO) during dehumidification
Pumps for transporting liquefied CO₂
Minimising internal power requirement
Long-term stability of materials for flue gas dehumidification

Overall process

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 MW_el gross power plant capacity)
Specification of the denitrification and desulphurisation plants that may be necessary

Pilot plant

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

Outlook

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.

Background

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 Nm^³/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 m^³/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.