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.