[Stoves] Metals in biofuels
frank at compostlab.com
Wed Jan 28 17:42:46 CST 2009
The units of 'wt per energy' is something I have trouble thinking about.
Seems much better to report mg/kg than try to determine an accurate
measure of energy for me at this stage in the game..
Testing the many extractions are a lot easier now with the ICP replacing
the AA. So if there is good correlation of slag and foul with a
particular element extracted in one and not the other extraction
solution it might be worthwhile to look into doing it again. It is not
that costly if it is meaningful.
Tom Miles wrote:
>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
>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.
>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
>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?
>>>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
>>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
>>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
>>(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.
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