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Fig. 1: 100 kW OxyFuel pilot plant

OxyFuel is a combustion technology where the combustion air is an artificial mixture of recycled flue gas and oxygen. The output of the combustion is a CO2-rich flue gas which is best suited for capture and storage. Due to the mixing of flue gas and oxygen, no nitrogen is introduced and capture of carbon dioxide is facilitated. 

Application of pure oxygen in industrial furnaces has the potential to provide concentrated CO2 exhaust streams for carbon capture and, additionally, adds a relevant degree of freedom to the combustion process because of the variable oxygen content of the synthetic air mixture (combustion gas). The main advantage of fluidized bed boilers is the relatively isothermal combustion chamber due to the heat capacity of the bed material. It is possible to extract heat directly out of the fluidized bed material. This means that the oxygen content in the combustion gas may be increased without increasing the combustion temperature. Less exhaust gas needs to be recycled. The OxyFuel process offers a supplementary degree of freedom, in the terms of O2 concentration in the feed which allows improved operation especially for low calorific fuels.

Oxygen enrichment of combustion air has been applied for low heating value fuels like sewage sludge in bubbling fluidized bed combustors.

The present project is to some extent a combination of the above approaches and paves the way for capture ready waste incineration with immediate economic benefit. Such benefit comes e.g. from the avoidance of fossil fuel co-firing when converting low calorific fuels.

In a final stage of development with infrastructure for CO2 compression, transport and storage, the technology offers the potential of below zero emission spots if the fuel mix has a non-fossil carbon content.

The pilot rig (Fig 1.) is located at the site of an air separation facility where technical gases are readily available. The plant is designed for hard coal operation but with additional feeding devices for alternative solid fuels such as sewage sludge and wood chips (Höltl, ECM4 2009). The nominal operating point is 100 kWth for hard coal at 21 v‑% O2 in the combustion gas. Excess heat can be withdrawn from the bed material return loop in a controlled way. For higher O2 content in the combustion gas the necessary fuel power may be higher than 100 kWth to keep the fluidization conditions in the CFB riser. The main part of the new project O2fuel is a change in the fluidization design. Now three different operation modes are possible: air-mode, air-oxygen-enriched-mode and oxyfuel mode. With these new capabilities, comparative emission investigations are feasible.

The variable O2 content in the combustion gas allows for operation of low calorific fuels such as mechanically dewatered sewage sludge.

The most important results of the OxyFuel activities are summarized in the PhD-thesis of Gregor Tondl (2013) which can be downloaded at the library of Vienna University of Technology (written in german).



Industrial partner & financial support:
Messer Austria GmbH



Projectass. Dipl.-Ing. Dr.techn. Stefan Penthor