Hi Mark, Kevin, Doug & list
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.
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. 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.
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.
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.
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.
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.
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. 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.
Now with pyrolysis chars, particularly at lower process temperatures,
this outgassing will be patchy and incomplete and the retention of some
new thermo-chemically transformed fractions can alter the
characteristics of the char. A good proportion of the resulting carbon
as a result is labile, that is easily mobilised in the environment quite
apart from normal soil chemical reactions, particularly through the
actions of bacteria and other soil organisms.
Remember wildfires naturally generate a wide range of chars, and
ecosystems have evolved to utilise these, however rarely are chars
naturally produced in large volumes that are all uniform in their
characteristics, when we add volumes of crushed uniform biochar which
has a large labile fraction to a soil we may well be introducing
monoculture effects at the micro level! This can consume soil nitrogen
as part of this process, and together with other soil biota and chemical
changes can have initially a detrimental effect on productivity, as
recorded in a number of studies. It also makes the higher "yield" of
pyrolysis chars a bit illusory.
The problem is "biochar" can be blamed for such poor results as a
generalisation, when in fact it may well have been the process used for
making the char and how it is applied that led to the sub optimal
result, not strictly the addition of thermally treated organic carbon.
So I agree with Doug, not all chars are created equal and more work
remains to be done. Perhaps even as our steel research experience
indicated, that blended chars may give optimum results. Which brings me
back to my original concern with the research bias against gasifier
chars we have experienced.
When this is fully overcome, then we might all move another step forward.
Best wishes to all,
Peter & Kerry Davies
On 29/12/2012 7:00 AM, [email protected] wrote:
Dear Peter
"Observations trump intuitive beliefs.":-)
# Assuming that it is the "alkaline ash" that drives the pH of
biochar, then intuitively, a "gasifier char" with lower yield, and
consequently higher residual ash content, should yield a higher pH in
a biochar application. Equally, one could intuitively reason that with
a pyrolysis char, containing more tars, the tars would "coat the ash
and hid the ash activity", also giving a lower pH for chars with
higher weight based yields.
# Your actual observations suggest different mechanisms at play. Is it
possible that there are organic compounds produced in pyrolysis char
making that are strongly alkaline in nature, and that they are absent
in gasifier char that was produced at higher temperature?
Best wishes,
Kevin
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