[Gasification] Some fresh fuel !

[AJH]

On Sat, 24 Mar 2007 21:13:10 +1200, doug.williams wrote:

>Hi Andrew,
>
>Is the last part of this statement correct?

Yes but it is only the experience with one 25tonne load. The boiler is
200kW(t). I only got called in when the de ashing auger jammed, the
only way I could free it was to reverse its rotation and allow it to
wind its way out after the motor attachment bolts were removed. The
clinker was white concreted lumps. At the time I couldn't decide if
the fusion problem was from soil contamination during harvesting or
the high bark percentage, the ash did contain some small pebbles, I
suspect these were when the load was double handled from a stockpile
as they would have cause havoc with a chipper.
>
>> Just a late comment to say that I have found exactly the problems Doug
>> states for willow Arable Short Rotation Coppice chips in gasifiers
>> when used as a boiler fuel in a stepped grate staged combustion
>> boiler, to the extent the de ashing augers could not cope and the
>> maintenance interval for removing fines from firetubes decreased from
>> 14 days to 3.
>
>I know that RGL in N.Ireland are growing coppice for boiler use., but do you 
>mean that willow you used created problems?  This is how I read your 
>comment, and given that willow makes excellent charcoal if freshly dried, 
>should be as good as you can get for a boiler biomass fuel.

As I said I couldn't decide the reason for the ash fusion problem,
normally the boiler would use exhaust gas recirculation but this had
had to be switched off because the fuel was damp, yet the colour
indicates the grate had been hot, so some EGR may have been better but
would have cause more flyash.
>
>When I first gasified willow after trials in NZ, the stored willow at Long 
>Ashton had decomposed, and when it was gasified, collapsed into a shower of 
>soot in the reduction phase. The ash was not really a problem that I 
>noticed, but if willow has a problem in a combustion role, then I am pretty 
>sure you will find it to be caused by the need to dry the crop as it comes 
>off the field.  I doubt if the chip size would affect the outcome for boiler 
>fuel, but the drying is critical.

I've found chip size itself is not critical but fines content is,
especially if the fuel is damp, this is because the airflow rates have
to go up with wet fuel, fines then lift off and burn out in the
firetubes rather than on the moving grate. This means the cyclones in
the flue have to be emptied much more frequently. In this case the
flyash was also settled in the fire tubes (despite a compressed air
"sneezing" system and I removed about 50kg from the secondary
combustion chamber where it was seriously comprising the flame
conditions. These boilers will burn sawdust OK but it has to be low
moisture content and the EGR has to be off to prevent the primary
combustion fluidising. The EGR is largely to protect the grate.

The owners are unlikely to risk willow SRC again but it would be
interesting to try a clean, dry sample in case the problems were just
to do with soil contamination and too hot a primary combustion zone.
After combustion problems with forest residues and this willow the
source of fuel was changed to chipped hardwood joinery off cuts and
since then I have never been back!

One of the frustrations of just being called out when a problem has
occurred is that I never really get to monitor conditions in normal
use.

AJH