Hi Peter and Colleagues, I think we should chang the Subject line for any further discussion.
I'm pleased you were able to qualify some of the details relating to the pH questions, but more importantly the circumstances in which you experienced the ash formations. > I don't claim to understand the chemistry, only that observations don't > always match expectations. This is why I would like to see others more > qualified than myself do the char analysis and research. A chemist will always argue that their explanations are correct, because chemistry is just a bunch of equations I guess(:-) As you say though, we are surrounded by people more qualified than ourselves who understand more about what "we" choose to do to make it, one would hope! > In some cases however with a little more information and the benefit of > hindsight this can be partly explained. In regard assumptions, firstly > you need to be careful that all the fine ash is in fact being caught > with and included in the main char and not elsewhere in the system such > as the cyclones. This would be virtually impossible, as ash is not going to drop out of a turbulent moving gas flow. When present, only the larger or heavier ash will stay in the char, but depends a lot, on how you separate the char in the bottom end from the gas flow. At a guess, anything under about 15-20 micron will entrain in the gas, if the gas exits with the char, rather than it be mechanically dropped through the grate into a dead space. >During our recent pilot trial we were presented with > firstly fresh eucalyptus wood chips (35-40% mc) then a load of fine > planer shavings from a nearby dry mill (8%mc). Using a initial mix of > (by volume) of 1:1 of this fine material with the wood chips we > collected 12 litres of carbon/mineral dust in <4 hours, noticing this > only when the fan started to growl and surge because the cyclones were > allowing this material to bypass as their collectors were full, and it > began to build up in other parts of the system before letting go in > lumps, giving the fan indigestion. When you explain it like this, it does change the issue of ash formation and pH, because blended fuels will cause a change in the way the packed beds behave, and the way the ash content can increase. Cyclones at best, work proportionally down to 5 micron, becoming more efficient as their size is reduced, hence single cyclones cannot remove fines as well as multi-cyclone systems. The fan is well proven to create the right phenomena, where the turbulence and changing pressures agglomerate these fines. > Something worth looking at when we start up again in the new year is to > see if the PH of the cyclone dust is similar to the char bin. One might assume that as the cyclone collects a visibly greater amount of ash, it's pH may be high. I haven't seen any pH readings higher than 8.6, but that figure relates to ash removed in a condensing/cooling phase. > The main problem though with the original assumption is that the ash > does not concentrate inside the remaining charcoal fraction beyond the > effective % change from mass reduction through initial loss of moisture > and lighter volatiles. The woody particles first lose their volatile > fraction and then ablate as the outer carbon surface is oxidised, > becoming smaller to the point of passing through the grate, the > inorganic ash freed as the outer layers of the particle oxidise forms a > separate very fine particulate with different characteristics and > mineral concentration to adjoining char particles and is easily sieved > out of the charcoal fraction. While this description fits most bed activity, the mixed fuel changes the game and the amount of ash formed by combustion. The shavings being the finer char, it's large surface area reacts faster than the larger chip. It's rapid consumption opens up the interstitial space between the larger chip, and this allows more free oxygen to become available for combustion. The swept surface of the larger chip changes and you can watch the ash form on it's surface as the interstitial space increases in size. This can affect the gas analysis, pressure drops across the beds, and temperatures of the exiting gas. You need all these parameters recorded as it happens to really see what is actually happening. > It is the larger (>3mm) screened material we mainly use as biochar, so > not all the inorganic ash is contained or therefore being measured as > part of this material. So from feed stocks with the same original ash > content char with seemingly different characteristics can be collected, > quite independent of process yield. The selective separation from a relatively small char production doesn't appear to have any economic sense, unless "there is gold in that there char". What it does show however, is that the remaining larger char is the slowest to pass through the bed, thanks to the exothermic heat provided from the shaving char. > In contrast as Doug rightly pointed out Pyrolysis chars tend to retain > all the original ash content, however as not all of inorganic ash is > kept within the gasifier char the result as you can see from above is > counter intuitive to the original assumption (as we are not looking at a > closed system as the assumption requires)...and certainly offsets the > yield difference. As we cannot always see how these differing circumstances within any process originate, some of these discussions are difficult to articulate. It always helps to stimulate discussion when you provide more specific detail as in this reply. > Doug has relayed as reported to him by others the suggestion that some > of this fine mineral ash generated embeds within the pores of the > charcoal. I have not seen this with our system, even looking at the > chars under a powerful lab microscope. This was a simplistic description of how ash will entrain and coat the char porosity, rather than fill it as it transports through the system, so long as ash is present, the char will have a positive pH factor. >Though when operating in fixed > bed mode they can get a light external coating. This does not > necessarily mean this is always the case but I can't readily imagine a > mechanism for this to occur to any great degree as the char within the > pyrolysis/gasification/reduction zones would be experiencing varying > degrees of outgassing, so these pores as they occur would be under > positive internal pressure resisting plugging for most of their > gasification experience. As you describe it here, it can be understood why the ash doesn't enter the porosity of the char. <Snip> > Perhaps even as our steel research experience > indicated, that blended chars may give optimum results. Not sure why your steel research crosses over to soil applications, but interested to hear more if you have the time. >Which brings me > back to my original concern with the research bias against gasifier > chars we have experienced. Simple. You need a lot of gasifiers to create commercially viable char quantities, and Australian politics isn't interested in gasifiers as a working technology. At best, you might find a guy looking for a paper to write to get funds to do more research so that he can have an academic career(:-) > When this is fully overcome, then we might all move another step forward. Sounds like wishful thinking for the majority of places that should be actively supporting development programmes, but yes, we plod on regardless, but I am not sure I will see the day within my lifetime. Happy New Year folks. Doug Williams Fluidyne----
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