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

Thu Jul 5 21:20:55 EDT 2007

Andrew,

Assume:
Kiln 			0.7 m^3 (24 ft3)
Stove bodies 	28
Power 		3.5 kW electric 
Temperature		ramp 600C to 1150 C (1112 F - 2102F) Cone 5
			Desired 1250 C (2282 F) Cone 10

Selecting a Kiln
Minnesota Flat Top Kiln by Nils Lou 22.5 ft3 (.64 m3)
Inside dimensions 31.5 in wide x 36 in deep x 34 in high (800mm w  x 914 mm
l x 864 mm h)
http://www.potterymaking.org/FlattopKiln_Complete.pdf
3 burners propane or natural gas.

Selecting a burner. This kiln is designed for three propane or natural gas
burners. Using a burner selection chart from a burner supplier for a
firebrick kiln of 24 ft3 rated to 1300C I get 205 kW, 739 MJ/hr, 176,400
kcal, 700,000 Btuh. "More energy is expended (proportionally) in the last
300C than at other stages. If firing only to 1080C or less, you may deduct
approximately 30% from the maximum capacity needed." So I might use a 205 kW
gasifier with three burners each capable of generating 68.4 kW, 246 MJ,
58,800 kcal 233,333 Btuh.    

Select a 205 kW gasifier, 700,000 Btuh, 176,400 kcal gas out. This is a
similar application to Agni Energy India that has been heating kilns and
furnaces to 1200 C. 
http://gasifiers.bioenergylists.org/gasdoc/AGNI/agni_2005.html
Other similar heating systems are by:
NARI, India,  http://gasifiers.bioenergylists.org/?q=naribagassegas
IISc, India http://gasifiers.bioenergylists.org/?q=iiscgasifiers
TERI, India http://gasifiers.bioenergylists.org/?q=teriiea06
Ankur Scientific, India WBG 80 200 Nm3/hr, 206,000 kCal/hr, 64-80 Kg/hr
http://www.ankurscientific.com/thermalapplication.htm
Carbo Consult SA, 180 Nm3 http://www.carboconsult.com/installations.asp

There are clearly several updraft ad downdraft gasifiers capable of
generating the required heat. I don't know which, if any, have heated kilns
of a similar size. 

Tom 

> -----Original Message-----
> From: stoves-bounces at listserv.repp.org [mailto:stoves-
> bounces at listserv.repp.org] On Behalf Of AJH
> Sent: Thursday, July 05, 2007 2:04 PM
> To: Discussion of biomass cooking stoves
> Subject: Re: [Stoves] Crispin´s kiln. (was Re: Traditonal Charcola
> Making Process / retort
> 
> 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
> 
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