[Stoves] Crispin´s kiln. (was Re: Traditonal Charcola Making Process / retort

[AJH]

On Thu, 05 Jul 2007 00:54:57 -0500, Paul S. Anderson wrote:

>Subject changed to refect what we are actually discussing here!!

It would have been handy to snip out the non relevant portions of the
old thread too!

>
>1.  You say you need power of 35 KW.  But at is an electric kiln, right? 

Sounds about right, so as the electric elements deliver 100% of their
energy into the kiln then the kiln receives all 35kW as heat and no
mass leaves the kiln with sensible heat.  [1]

> Is
>that 35 kWe, and on a 10% cycle?  And can you translate that into total energy
>needed in terms of kWt (thermal kW is what we are after), 

Crispin specified a total electrical energy cost of 3.5kWhr per stove
body, he has subsequently said that the kiln holds 28 stove bodies.

Crispin describes a cycle that seemed to be 23.9 hours and also gave
the kiln cycle time of 18 hrs. I suspect the difference is the 6 or so
hours of slow cooling after the kiln is switched off.

>and also the time
>schedule for the delivery of the heat. 

OK this is the complicated bit, Crispin gives a period for drying at
120C and then various stages as the temperature is increased to a
final temperature of 1170. This is easily accommodated with an
electrical kiln by simply varying the time the element is switched on
compared with while it is off, the mark-space ratio, just like an
electric oven. Crispin mentions a maximum on/off period of 50%. It
looks like the power averaged over the period the kiln is powered (18
hrs) is only 5.4kW (3.5 times 28 divided by 18).

Ronal has figured out a ball park amount of wood required and char
produced for a kiln fired by offgas. I have mentioned the difficulty
of reaching the temperatures with offgas in a simple straight through
kiln. Tom Miles has advocated a downdraught gasifier with char removal
prior to the reduction zone and given some energy figures for the
reducer gas, I think he was using outputs on a cold gas, so missed out
any sensible heat the 850C producer gas could contribute.

All a bit of a muddle but the salient points I see are to do with heat
conservation and controllability. On controllability; one of the
attributes of a tlud device is the constant nature of its offgas
production, a downside of a retort set up is the peaky nature of
offgas production, so I suspect, if the poor quality char in produces
is acceptable, Tom Miles Downdraught gasifier has a lot going for it
in this use as it can be controlled easily.

I think there is scope for better energy conservation and I think some
of it can be done with natural circulation, I would suggest as
electricity is available there is little cost is fan powering the
gasifier. I proposed a 3 kiln system run sequentially, probably
mutually joined. I think the switch over could be simply arranged by
changing flues and filling or removing sand from internal passages.

> We want to determine the specifics of
>the power kWt/hr during the duration of the firing.  If it is on a 
>hourly basis
>then the kW energy and the kW/hr power are equal, right?. 

Yes if you run 1kW for one hour you have expended 1 kWhr of energy.


> But if you want ¨X¨
>of kWt energy delivered in 30 minutes, you need ¨2X¨ kWt/hr, right?  

No kW/hr is kW per hour, a rate of change of power. If you deliver XkW
in 30 minutes you have expended 0.5X kWhr of energy.

kW is to energy much as speed is to distance. kW(e) is different from
kW(t) only in its utility, when delivering heat energy  into this
system there is nothing between them apart from relative cost verses
utility.

>
>2.  With a professor/engineer here in Managua, we have been discussing a
>semi-continuous-operation kiln (for bricks) that does not need to cool down
>after each load of bricks.  Pretty simple design.  

Yes even a batch sequential system running counter flow to the bricks
conserves a lot of the energy wasted in a simple batch system, add to
this some natural convection cooling AND drying/preheating and there
is scope for decent energy saving. On top of this using the final flue
gas as a pre heater for the gas burner secondary air and you may raise
the final temperature over that normally attainable with wood
(especially if you also dry the wood, not normally though worthwhile
in simple wood heating systems).
>
>3.  Gasifiers are the answer.  At Chip Energy, my partner and I are 
>developing a
>biomass furnace that does about 60 kWt/hr (200,000 BTU/hr). 

Looks like you need a consultant, I make that just 58.56208kW(t) ;-).


Moving on: I have little experience with kilns, two in the early 70s,
the first was a beehive shaped brick kiln for firing expensive plates
and bowls with some metallic glazes, I suspect now it was similar to
"raku" glazing. This was fired with wood and propane, the wood was as
much to contribute a reducing atmosphere as well as energy, the
propane to reach adequate temperature. It was extremely wasteful of
energy but the plates cost a fortune so energy was not significant
cost. The second was a tile factory where I knew some chaps who hand
made tiles on piecework, it was victorian and not changed much. The
original wood fired kilns were still in use but the  wood burning
grates had been modified with Amal gas burners, again the losses were
high. The management had invested in a new gas fired kiln that was a
metal frame draped with cerablanket like pleats in a dress. the folds
protected the structure and added great depth of insulation. This
resulted in a thermally efficient kiln that had very little thermal
mass. The result I saw was some tiles that had vitrified and shrunk to
about 90% of the required dimension, whilst the efficiency was good
the low thermal mass coupled with their old control methods meant thy
couldn't maintain  quality as the temperature rose sharply at around
the critical point.

[1] Crispin says some combustion air is allowed in to burn char which
leaves pores in the stove body, so there is a massflow and energy
contribution from this.

AJH