[Stoves] Chip drying without pyrolysing; Biocoal manufacture.

ajheggie at gmail.com ajheggie at gmail.com
Sat Jan 25 14:51:48 CST 2014


[Default] On Fri, 24 Jan 2014 08:27:58 -0500,Tom Reed
<tombreed2010 at gmail.com> wrote:

>The question of how to easily dry green wood chips addresses one of the most important problems in wood energy:  how to dry green chips!  Wood is much easier to chip wet, but a chip pile never dries of its own accord.  

If it doesn't re wet and moisture can continually leave the surface I
have found it does in large heaps, the problem is loss of dry matter
and the mycelium cause the chips to cake so it's not advisable. In UK
we have rather too much winter rain  to do this outside and I have
found even a small ingress of water into the fuel hopper of my boiler
upsets the combustion characteristics badly.

>If you are transporting the chips any distance by truck, 2/3 of the truck's fuel goes out the exhaust pipe as heat, enough to dry a load of chips the truck is carrying.  The temperature of the exhaust is closer to 600C as it leaves the engine, too hot for drying.  But if air is aspirated into a side stream of exhaust the ratio of exhaust heat to added air could be adjusted with a simple spring thermostat to 300 C (or other as required) to "cook or cool" the wet chips without overhearing them.  There is plenty of exhaust pressure available for the aspiration before the muffler, and the drying would muffle this side stream as well.


I've thought about this, actually I think the proportions of the fuel
going out the exhaust varies with load but we can make broad
assumptions. 

In UK a truck has a payload of about 29 tonnes and it is just about
economic to deliver chip 100km.

As a diesel with turbo is at best 40% efficient and possible averages
30% efficiency at converting heat energy to motion then assuming
radiative and other losses to be 10% we may be able to recover 50% of
the heat from the coolant and exhaust as low temperature water.

My old colleague Gavin Gulliver-Goodall developed a moving chain
woodchip dryer which took the pragmatic approach  of blowing up
through the heap but never letting the top layer become as dry to the
target dryness, he simply kept an eye on the interface between dry
chip and not so dry chip and skimmed the wetter layer back to the
input. This ensured the air left the chip heap saturated. For this
exercise I think we can assume the air will always leave the top
saturated as there is little hope of the top of the heap becoming dry
or warm, and hereby hangs a problem, saturated air will condense out
in the upper layers and there will be no net drying. Neglecting fan
power  IIRC (and this needs verifying) with air at about 40C Gavin
managed to remove 1kg of water for an energy input of 4MJ, the latent
heat of water vapour is 2.3MJ/kg.

A modern turbo charged diesel probably does not have much spare
pressure without affecting the turbo characteristics but I am aware of
some older dump trucks used to transport clay that did use the exhaust
to heat the sides of the body to ease the discharge of clay when
tipped so it is not unheard of.

Neglecting how we pump the heat through the chip and assuming we
capture all the 50% waste heat and it dries with the same coefficient
of performance of Gavin's device then we need to deliver 4MJ, which is
1.11kWh of heat for every 1kg of moisture removed.

Our trucks use about 47 litres/100km and there is approximately
10.15kWh/litre in diesel fuel, so we have half of 47*10.5=477.05Kwh of
heat available which will remove 215kg of moisture from 29 tonne of
woodchip which probably contains about 14 tonnes of free water.

Pre drying for direct combustion is seldom worthwhile but there are
very good reasons for pre drying in charcoal making and probably in
gasification for massflow considerations alone and in a static plant
with cold gas efficiencies around 70% there's much more low grade heat
to play with.

AJH





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