[Stoves] Metals in biofuels
tmiles at trmiles.com
Wed Jan 28 18:39:35 CST 2009
I agree that the concentrations should be expressed per unit mass. If we
know the energy then it is more meaningful to relate the concentrations to
The correlations are a challenge because the slagging and fouling result not
just from variations in fuel qualities but also variations in the design of
the combustor (Stove) and the operation (high-low fire) of the combustor.
Just to complicate things volatilization of the sodium and potassium goes up
with increasing temperature.
Stoves generally operate at such a low temperature, due to the high excess
air, that fouling is rarely experienced. Slagging and chemical attack on
refractory in a high temperature firepot are probably more common. Burning
high alkali fuels, like corn stover or straw, in combination with wood
reduces the slagging by dilution.
Philips apparently experienced high temperature corrosion in their firepot
which took them a couple of years to solve. I didn't get to their
presentation at ETHOS early enough to understand what caused it. In general
as we become more efficient in combustion problems like corrosion and
deposition begin to appear due to the higher operating temperatures.
From: frank [mailto:frank at compostlab.com]
Sent: Wednesday, January 28, 2009 3:43 PM
To: Tom Miles
Cc: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Metals in biofuels
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
>(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
>than you consume it, i.e. to make biochar, is one way to avoid slagging
>high alkali fuels.
>Chemical fractionation is very useful to quantitatively explain what
>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
>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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