[Stoves] Traditonal Charcola Making Process / retort

[AJH]

On Fri, 29 Jun 2007 10:15:22 +0000, crispinpigott at gmail.com wrote:

>"How many kg of 15% mc (wet basis) is needed to generate sufficient offgas, when pyrolysed to 500C (typical charcoal kiln temperature or
>TLUD yielding 80+% fixed carbon), which when burned to fire the kiln will provide sufficient heat and temperature to be equivalent to an
>electrci furnace using 3.5kWhr of electrcial energy to fire a clay stove body?"
>
>Yeah that will do.

A modicum of agreement on a start point then.
>
>
>>The variables you haven't >given are the  temprature and >times of
>>firing.
>
>Well, that's not important.

Of course it is, firstly can an offgas flame achieve the temperature
required and you even reference the second reason in later posts,
replies to Kevin and Jeff. The offgas will have a lower energy content
than the wood it is produced from would have if completely combusted.

The electric input delivers 100% of its power into the kiln. The only
places this heat can go is into raising the temperature of the
pottery, contributing to any processes taking place in the pottery
(drying or vitrification) and heat losses through the kiln walls.

The gas flame has to do exactly the same but has to reject it's flue
gases also. These flue gases have to be at the temperature currently
achieved within the kiln, unless some means is available to make use
of the sensible heat in these flue gases.

Therefore we need to know the temperature of the kiln in order to
calculate how much energy is being rejected in the sensible heat of
the flue gases.

You said that the natural gas fired kiln would have 50% efficiency,
i.e. would use twice as much energy to fire the stove body as the
3.5kWhr per stove. Even this does not readily convert to a figure we
can use for pyrolysis offgas in the absence of a temperature (or
temperature profile) because the pyrolysis offgas has a much higher
massflow than natural gas for the same energy output.
>
>> Although you gave a figure of >50% efficiency for the gas kiln
>>(presumably compared with >100% delivery of heat into the >kiln by the
>>electric elements).
>
>Good so far.
>
>If I could run a thousand degrees of preheat that would be nice...

Yes and the simplest way of doing this is by running a counter flow
heat exchanger or recuperator in the kiln exhaust. As the secondary
air massflow is by far the biggest part of the massflow and a TLUD
cannot have preheated primary air then we could reasonably expect to
preheat to 500C+ without compromising materials.

Possibly also in a batch loaded kiln the initial drying and preheating
could take place in an ante chamber (in practice a separate but
attached kiln) and heated from energy from the cooling process again
in a separate but attached kiln.

AJH