Solar furnace for lime and bricks

[Sourdois]

Hello everybody

I represent an association in Burkina Faso that works on bioclimatic habitat in the Sahel. As part of our activities, we must find an alternative to the use of wood for the production of lime and the production of terracotta bricks.

We are planning to launch a feasibility study for the construction of a solar oven suitable for this type of production (minimum 900 ° c).

We are looking for all the skills and useful information to launch this project. (feasibility study to be carried out in 2012).

Thanks,

[RedGuff]

Bonjour.
There are two main ways to focus the rays: lenses (Fresnel) and mirrors. Mirrors are cheaper, stronger and easier to handle.

[Sourdois]

Bonjour.
There are two main ways to focus the rays: lenses (Fresnel) and mirrors. Mirrors are cheaper, stronger and easier to handle.

Thank you for the answer.
Yes, this is the technology we are targeting. We are looking for contacts for the implementation of the oven (technical and financial study, import of mirrors and necessary equipment, technical monitoring of construction, training ...)

[chatelot16]

for lime it is quite easy because it is enough to heat, and if the limestone to be cooked is in small pieces it is fast enough

the temperature does not need to be precise

in large lime kilns the stone takes more than 24 hours to pass through the whole oven, but it's very large stones, more than 20cm ... smaller stones (1cm) are cooked in 1 hour, I have some experience with a small wood oven

it is therefore possible to cook a good quantity continuously throughout the day

for bricks it will be more difficult because the bricks must be cooked well gradually without sudden temperature variation: it cannot be baked in one day: it will therefore need a well thermally insulated oven to not cool at night, and ca will make a batch in one week

not to get cold at night you will need a huge oven: at least a hundred tonnes

to make it smaller you have to be more reasonable: oven heated in the sun during the day and in fuel at night

if the sun brings half the energy it will be fine

but what is the current fuel consumption?

do not forget that a good tunnel oven consumes 10 times less than an archaic oven: I am talking about a tunnel where the bricks advance on wagons so that the hot baked bricks that come out give heat to the air cold entering to power the fireplace

in France these tunel furnaces are very big, but I have a little trick to make them smaller

alas it has no profitability in france next to the big tunnel kilns of the current brickyard

it would be interesting to build in africa

[Sourdois]

Thank you for this long development.

To move forward, what information do you need?

Lime has been produced for several decades with old wood ovens. I have pictures of the current installations. I can also provide you with information on current production and consumption of wood.

This project is of great importance and should mobilize several partners including international cooperation.

The project will be presented in April to launch the feasibility study.
Funding for this study has not yet been completed.

If you are interested in this project, we are open to study your technical and financial proposal.

Sincerely.

for lime it is quite easy because it is enough to heat, and if the limestone to be cooked is in small pieces it is fast enough

the temperature does not need to be precise

in large lime kilns the stone takes more than 24 hours to pass through the whole oven, but it's very large stones, more than 20cm ... smaller stones (1cm) are cooked in 1 hour, I have some experience with a small wood oven

it is therefore possible to cook a good quantity continuously throughout the day

for bricks it will be more difficult because the bricks must be cooked well gradually without sudden temperature variation: it cannot be baked in one day: it will therefore need a well thermally insulated oven to not cool at night, and ca will make a batch in one week

not to get cold at night you will need a huge oven: at least a hundred tonnes

to make it smaller you have to be more reasonable: oven heated in the sun during the day and in fuel at night

if the sun brings half the energy it will be fine

but what is the current fuel consumption?

do not forget that a good tunnel oven consumes 10 times less than an archaic oven: I am talking about a tunnel where the bricks advance on wagons so that the hot baked bricks that come out give heat to the air cold entering to power the fireplace

in France these tunel furnaces are very big, but I have a little trick to make them smaller

alas it has no profitability in france next to the big tunnel kilns of the current brickyard

it would be interesting to build in africa

[Rabbit]

Wouldn't it be interesting to use a hawker fluid?

Like fluorine salts? As in nuclear.

[chatelot16]

you can give information on the furnaces currently used and especially on the consumption of wood and the production of lime (or brick)

the mirror surface is expensive and the amount of scrap to build a large dish also

the solar project deserves to be studied but we must also consider progress simpler

for lime: manufacture of charcoal by pyrolysis, and continuous lime kiln where burnt charcoal mixed with limestone to be cooked: excellent thermal efficiency, consumption 10 times lower than a separate wood fireplace

making charcoal by pyrolysis produces gas, methanol and acetone, instead of smoking as it is unfortunately everywhere

the construction of a wood pyrolysis plant not only reduces the consumption of wood in the lime kiln, but also reduces the amount of wood needed for all charcoal users in Africa!

the use of charcoal is often criticized: it is however the best fuel ... it is only the (old) archaic manufacturing method which is bad

I replace old with archaic because pyrolysis is old too: we called it wood distillation in 1850 when it produced the spirit of wood, old name of the alcol to burn

in 1850 this method was not profitable for charcoal because of the high price of the necessary material

today the material is cheaper and energy becomes more expensive, it changes everything

pyrolysis of wood is also interesting to do by solar method: instead of wasting part of the wood to heat it itself, we heat by the sun and we keep all the combustible gases produced for something else: for example heating a brick at night when the sun is down

the advantage of the gas produced by the pyrolysis of wood is that it can be stored to supply the oven continuously

I tried 15 years ago to start this kind of manufacturing of charcoal in France ... but impossible to build something in France ... steeped in regulations that bite the tail

difficult to build also in africa where everything is more expensive because of the lack of everything

[chatelot16]

Wouldn't it be interesting to use a hawker fluid?

Like fluorine salts? As in nuclear.

horror and putrefaction ... eviton the useless pigs

if you had to heat an oven only in the sun and store heat for the night, I would put simple graphite crucible full of cast iron, which will melt at around 1000 ° C in the sun and restore the heat by solidifying at night

but I think you have to start by making a solar lime oven, which can be of reasonable size before thinking of a necessarily huge brick oven ... and which can also operate a cast iron or bronze foundry!

[dedeleco]

All the advice from chatelot16 is to be listened to, with great care, to be weighed, given the scope and importance of the project, in particular, start with simple tests with simple mirrors (same plans, 20 of 1m2 are 20KW on 1m2) , on small dimensions, then, the improvement of the yield by good thermal insulation, recovery of the heat between entry and exit, enormous economy with what comes out warming what enters, and in addition, test of heat conservation at night, which requires an enormous volume in refractory rocks such as wools or pulverulent lavas, with thermal insulation, especially by the slow thermal diffusion, over approximately 0,2m in one night from 8 to 10h.

It all depends on the volume used, but from a few meters in diameter, we must be able to keep most of the temperature at night and therefore save a lot of time heating or even cooking for lime or brick, and save a lot of expenses.

So in a ball a few meters in diameter, isolated and with earth walls or rocks with low thermal conductivity (pumice, sand, etc.?) Supporting inside 900 ° C, it should be possible to keep on one night most of the heat.

So the characteristic length of thermal diffusion increases as the square root of time :
http://fr.wikipedia.org/wiki/Diffusivit%C3%A9_thermique
http://fr.wikipedia.org/wiki/Conduction_thermique
for clay coefficient of thermal diffusion approximately 1mm2 / s gives for 10h of night or 36000s a length of 1xrac (36000) = 189mm and therefore the heat only enters the wall on a few multiples of this length,
For a large hot tank put, suddenly for the night, in a large bunker made of almost cold clay or brick at the start, the heat will only go out by diffusion on a few multiples of this length of 18,9cm.

This animated figure shows this slowing down, in square root of time, which allows to keep a high proportion of the heat over a finite time, by the slow diffusion of heat and T, in a cold wall, brutally brought into contact with the hot :

There is a loss of heat and of T as the ratio of the thermal capacities of the hot tank to that of the wall over its diffusive thickness.
All the basic equations are on and the wikipedia links in it:
http://fr.wikipedia.org/wiki/Conduction_thermique
A very large tank, almost sphere or cubic cylinder of 5m in diameter, will thus keep most of its temperature.


It’s often overlooked, but even allows to keep the heat underground between summer and winter as realized at www.dlsc.ca to heat the winter.

See on ggogle all the theories and realizations of solar power plants with planar mirrors with tracking or parabolic.
What limits the T max is the infrared radiation from the heated surface like T ^ 4, which is therefore very strong at 900 ° C., and therefore very difficult to obtain (surface very red to white), requiring a large number of mirrors.
Basis for this radiation not black at all at 900 ° C:
http://fr.wikipedia.org/wiki/Corps_noir
and all the links in
http://fr.wikipedia.org/wiki/%C3%89nerg ... _thermique

Also perhaps, it is better to see it as a means of greatly reducing the consumption of wood or fuel oil.

[chatelot16]

the solar oven can be a brick cavity with a small entry hole in the hearth of a parabola: it is the hole which must be in the hearth not the material to be heated

the light passed through the hole diverges a little before lighting the bricks in the oven, but the heat will not be distributed: the bricks directly lit will overheat and be destroyed, and the others will wait their turn

in traditional brick ovens the flames are made directly in the oven between the bricks: it is naturally distributed ... and again it takes a certain know-how to develop and make good quality bricks

good insulation is not enough to reduce the consumption of a simple brick oven: at each furnace, the entire thickness of the brick and insulation must be allowed to cool before being able to enter and take out the fired bricks ... and all the mass of brick from the oven and to heat for the next batch

there were long ago the hofman ovens: a large oval tunnel with doors delimiting a dozen rooms

one of the rooms is cold and we can take out the baked bricks and put the new ones

then we open the door which communicates with the room next to where the combustion gases arrive which we have already heated from other room, and an opening towards the chimney: the smoke is therefore completely cooled by passing over the cold bricks

the other adjoining room contains finished brick: it sends cold air which heats up by recovering the heat from the cooked bricks and the tunnel walls

the air heats up by passing in increasingly hot rooms before arriving at the room in fires or one balances coal by a hole in the ceiling

and in the next rooms the combustion gases gradually heat the bricks

this therefore like the tunnel oven with the bricks advancing except that there is no mechanics advancing: it is the fires that advance from room to room: it recovers both the heat contained in the brick and the heat contained in the oven walls

unlike the tunnel oven where the bricks are advancing to progressively approach the fires, here it is the position of the burning chamber which advances one space twice a day ... it goes around in about a week

the hofman oven has disappeared because we know how to make lighter materials than old refractory bricks and it is enough to recover the heat from the baking bricks