Firewood kilns

[openspaceman]

3,200kwh in a tonne of 20% m/c wood. It doesn’t matter whether this is softwood or hardwood there is roughly the same energy per tonne. Softwood will take up more space and need more refueling than hardwood

 

I reckon on nearer 4kWh/kg with hardwood at 20% WWB and that allows for dumping the flue heat at about 150C, Cornishwoodburner manages to extract more heat as he runs a low flue temperature, softwood can be a bit more because it has a higher lignin content.

[Duffryn]

I reckon on nearer 4kWh/kg with hardwood at 20% WWB and that allows for dumping the flue heat at about 150C, Cornishwoodburner manages to extract more heat as he runs a low flue temperature, softwood can be a bit more because it has a higher lignin content.

 

Perhaps but i I was speaking in headlines to get the general point across. Point remains return on investment isn't there any more

[County 764]

Not done it but wouldn't advise it other than low temperature lumber drying.

 

The thing is that heat pumps have a Coefficient of Performance which varies inversely with the difference in temperature you are pumping through.

 

So assuming your heat source can stand the heat removal ( and the one I remember couldn't as the pipes under the lawn froze the soil and could be seen as a network of ridges ) then it's good for underfloor heating as you only have to pump up to about 25C and the COP will be better than 3, that is 1kW(electrical) in will deliver 3+kW(thermal) out.

 

However as has been discussed log drying benefits from higher temperatures than 25C and the COP would drop if you could find a pump capable of running at 60C out.

 

Thanks, wasn't too sure how the ground source thing worked. Would of been good if you could get the correct temperature for firewood as the RHI is good for it.

[County 764]

Forgetting the RHI would something like this be feasible connected to an insulated container?

 

100kw WOOD SPACE HEATER WASTE WOODCHIP SAWDUST CARDBOARD COAL LOG PELLET met man | eBay

 

Or would it eat-up to much fuel?

[matelot]

Does anyone use a dehumidifier in a kiln? I can't help thinking that heating a kiln then venting the humid air out loses a lot of heat.... Why not heat the kiln and have a dehumidifier inside to remove the moisture?

 

A little known fact is that a dehumidifier produces more heat than the electricity put into it.... You should get at least twice the heat from a dehumidifier as the electricity put into it.

[Duffryn]

You would be loading it every 15 mins

[Duffryn]

Does anyone use a dehumidifier in a kiln? I can't help thinking that heating a kiln then venting the humid air out loses a lot of heat.... Why not heat the kiln and have a dehumidifier inside to remove the moisture?

 

A little known fact is that a dehumidifier produces more heat than the electricity put into it.... You should get at least twice the heat from a dehumidifier as the electricity put into it.

 

Yes but powering by electricity is "ouch" expensive and trying to recirculate that heat a major challenge

[openspaceman]

Does anyone use a dehumidifier in a kiln? I can't help thinking that heating a kiln then venting the humid air out loses a lot of heat.... Why not heat the kiln and have a dehumidifier inside to remove the moisture?

 

A little known fact is that a dehumidifier produces more heat than the electricity put into it.... You should get at least twice the heat from a dehumidifier as the electricity put into it.

 

BigJ is the expert on this I think.

 

A dehumidifier is a heat pump it compresses a refrigerant back to a liquid, which releases heat and is blown into the container then it allows the liquid to evaporate, this requires heat so a cold spot develops which is below the dewpoint of air in the container so water condenses and runs into a container, all the electricity is turned into heat and there is no net change in the container as water has left the wood and then drained out still as water but again it's a low temperature method, great for drying wet clothes.

 

In practice it's likely to be cheaper to add a bit of cheap heat and still recover the moisture as liquid via a heat exchanger, there is a thread where this was discussed.

[openspaceman]

Forgetting the RHI would something like this be feasible connected to an insulated container?

 

100kw WOOD SPACE HEATER WASTE WOODCHIP SAWDUST CARDBOARD COAL LOG PELLET met man | eBay

 

Or would it eat-up to much fuel?

 

Forgetting RHI is probably good

 

The amount of wood needed depends on its efficiency, no matter what device you use. It does look a bit expensive and would need lots of manual refuelling. The efficiency here would depend on the flue temperature as that is the only wasted heat.

 

One thing it has got going for it is that it avoids the need to heat water so only one heat exchange surface but what temperature would you run it at? It looks like it's for a workshop at room temperature rather than 60C.

 

If you refer back to Duffryn's first post today in this thread you will see his first option is a straight through system. If we were using natural gas to dry say paper slurry then there would be no need of heat exchange surfaces. Consider there is a loss at each heat exchange. So one could run a flame direct onto the wettest wood with enough bypass air to prevent scorching. The wood would have to be moving into the container consistently and probably require more than 12m to make sure you got maximum heat into the wood. If you had a clean enough wood chip stoker burner it would be an interesting project.

 

Back in the 80s it was possible to buy a cheap 150kW(t) Veto stoker-burner which would be ideal as long as you had some dry arb wood chip and maybe some dunlop coal to get rid of. Having said that there's nothing difficult about building a chip stoker: A feed auger, rotary valve, stoking auger, fire brick and fans plus a couple of inverters and Programmable Integrated Circuits.

 

In practice it would pay to jig the basic concept about a bit to make it workable as the solely concurrent flow doesn't make best use of the heat.

 

It could make a nice retirement project.

[cornish wood burner]

Couple of things not mentioned yet.

Consider using saw mill waste as a fuel source. We buy slabwood at around £10 less than round. If anyone has a mill near then it could be even cheaper. It also has an advantage that it comes in partially dry so you get more wood / tonne. Roundwood chip costs us no more than £70/ tonne, some less. Slabwood takes longer to chip but maintains the £10 difference due to the lower mc when bought. We do have economies of scale but the principle should apply to smaller use.

Also you need to maintain a reasonable return temperature otherwise your boiler will suffer badly from condensation. Not good for the boiler or combustion efficiency.