ECN publication
Hogere koolstofconversie in CFB-biomassavergassers
Published by: Publication date:
ECN 1-6-2003
ECN report number: Document type:
ECN-C--03-053 ECN publication
Number of pages: Full text:
44 Download PDF  

Circulating fluidised beds (CFB) for the gasification of biomass arevery attractive because of its suitability for both small and large scale as well as its flexibility towards fuel type and size. However, in practice CFB gasifiers turn out to have only approximately 90% carbon conversion. This not only results in sub-optimal efficiencies, it also means that the ashes contain large quantities of carbon. This can have negative effect on ash use and costs involved. Furthermore, carbon rich ashes bring along risks of dust explosions. In conclusion it can be stated that the increase of carbon conversion of CFB gasifiers can have great positive effects.

In the present project, several methods have been tested and evaluated. Tests were performed in the 500 kWth CFB gasifier at ECN in the Netherlands. From the evaluation it can be concluded that the application of the so-called HSD-CFB principle is the most attractive option. The HSD-CFB principle divides the riser into two zones by a local narrowing. Biomass entering the upper zone cannot fall down into the bottom zone. With this method, it is achieved that the oxygen/air added to the bottom zone can only react with the circulating char. This increases the carbon conversion considerably and even 96% has been measured. At the same time the cold gas efficiency increases with 1%point. Technically, the HSD-principle has no real draw backs.

Three other methods can have economical benefits as well. These are:

  • no air preheat
  • gasification at higher temperature
  • cooling the reactor
All three methods are based on the relation between carbon conversion and air/fuel ratio. It turns out that the methods can be combined as long as the total heat loss does not exceed 5% (based on the heat input of the solid fuel). Up to this, the carbon conversion linearly increases with heat loss so that the cold gas efficiency remains unchanged.

Of all tested methods, only the addition of steam has a significant effect on tar formation. The calculated tar dewpoint reduces from 200 to 175°C. This however is generally still too high. Adding steam also reduces economical benefits due to a reduction of cold gas efficiency.

As a final remark it might be even more attractive to implement the HSD-principle in a CFB gasifier by enlarging the circulation siphon rather than having a narrowing ring in the riser. This might even create the possibility to separate the low carbon ashes that remain in the siphon when operated under oxidising conditions.

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