[Stoves] Metals in biofuels

[Tom Miles]

Frank,

You can get slag in the firepot of a pellet stove which is only about 4-6 in
(100-150 mm) diameter and about 4-6 inches deep. A good high temperature in
the GEK will slag walnut shells. We measured concentrations of alkali,
primarily potassium, of 0.71-1.065 lb/MMBtu in walnut hulls and blows.
(Alkali Report Appendix C.8 Nut, Pits and Shells) Because the amount of ash
is small it takes time to accumulate it. Almond hulls are worse at 4
lb/MMBtu.

In a downdraft gasifier once you drop below the combustion zone (e.g. below
the nozzles) you have a significant drop in temperature and the ash
contained in the char is not likely to fuse. Withdrawing charcoal/ash faster
than you consume it, i.e. to make biochar, is one way to avoid slagging with
high alkali fuels.     

Chemical fractionation is very useful to quantitatively explain what happens
to inorganics in biomass fuels but it is very expensive and not generally
useful for common combustion problems. It is used primarily in modeling the
impact of burning high alkali biomass fuels with coal. We did some testing
for water soluble alkalis (Na2O, K2O). Water soluble alkali is a much
cheaper and faster test. A few boiler operators were able to correlate these
with slagging and fouling behavior in their particular boilers.    

Tom
 

-----Original Message-----
From: frank [mailto:frank at compostlab.com] 
Sent: Wednesday, January 28, 2009 1:13 PM
To: Tom Miles
Cc: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Metals in biofuels

Tom,

In the GEK we burned walnut shells and no problem with slagging yet the 
report mentions nuts and shells to be a problem due to the high 
potassium. I have not tested the potassium concentration in the shells 
so perhaps it has low potassium concentrations or the temperatures are 
not high enough or we did not burn enough to have a problem.  Can we get 
a slag problem in something as small as the GEK?
 
<snip>

>>I am thinking (hoping) that only the elements that dissolve in a hot acid
>>    
>>
>digest along with sulfate, chloride and phosphate is important for us and
>not the 'rock' material left behind in the acid treatment when predicting
>problems.
>
>The hot acid digest is similar to the chemical fractionation (leaching in
>water, dilute acid and strong acid) that we did in order to identify
>components that volatilize during combustion (water an dilute acid
soluble),
>reactor during combustion (Strong acid soluble) and pass through interest
>(residue from strong acid). We found that it is most useful to analyze for
>the total quantity of components, such as total chlorine, rather than just
>the soluble components because we found that in different fuels the
>components had different degrees of solubility. this reflects the many
forms
>(usually hydroxides) they take in the plant itself.  
>
>  
>
I found this procedure most interesting. Do you think the components of 
this procedure (1) water sol; 2) dil. acid sol. and 3) strong acid 
sol.4) non-acid sol.) correlated well when looking at slagging and 
make-up? Would you do this breakdown again in another study? Its a lot 
more work and I am just wondering if it was worth while.

Thanks
Frank

-- 
Frank Shields
Soil Control Lab
42 Hangar way
Watsonville, CA  95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com