Firewood Kiln

[richy_B]

Hello all,

is there any on here who knows their stuff in regards using/building firewood kilns? I want to run some thoughts by someone but its a bit of a niche subject. I know the use of kilns for firewood is a debated topic when compared to air drying because of energy use etc but I am hoping to focus on kilns rather than the bigger debate.

 

What I am considering is the fairly common - insulated 20ft lorry container with several fans for air movement, a large vent and extractor/exhaust fan to remove/replace moist air. Track system for wheeling cages in and out. External heat source to produce hot water for heating. Moisture and temp meters for monitoring.

 

-Target of around 24 cubes capacity of split, rough filled hardwood logs.

-Temperature in mind of around 70-75C. Ideally keep it around that for the majority of the time.

-Method of heating is via hot water pump around the kiln. Energy for the heater is waste from another industrial process. The KW output should be enough to maintain the target temp.

-Use the exhaust fan to extract moist air and channel it via a heat exchanger to recapture of the energy.

-Target time of 5-7 days.

-Haven't included a dehumidifier.

-Series of fans working continuously to keep air moving.

 

My thoughts on the subject are that if we have 24 cubes of green firewood that equates to approximately 12t. Mix of hardwood but mainly ash, syc & oak. I am estimating a green moisture content of 50%. Our target MC is just under 20% so we need to reduce the moisture by 30%. This means we need to remove around 3.6t of water or 3600 litres. This is obviously a lot per day when look at over a 5-7 day cycle. Some reading from various sources suggest this is quite possible but I want to get some peoples opinions.

 

Any thoughts?

[Alycidon]

Ash wont be much higher than 35% fresh felled.

 

The guy that does the milling BigJ probably knows more about this than most. He would be worth talking to. A search or two may also throw up some answers.

 

Certainly the heat in a container in summer can be pretty hot, I was spraying a stove a special colour early this week in a container, paint did not take long between coats at all.

 

A

Edited September 5, 2013 by Alycidon

[richy_B]

Appreciate the info. I'll see if I can PM BigJ.

[openspaceman]

 

 

-Target of around 24 cubes capacity of split, rough filled hardwood logs.

6metre container 2.4 by 2.4 outside, what inside? Say a usable 28m3 at <50% stacking ratio as the stillages have worse edge effects, so at best 14 solid, in practice we only achieved 18 solid m3 in 12 metre container.

 

-Temperature in mind of around 70-75C. Ideally keep it around that for the majority of the time.

 

Why this choice?

 

-Method of heating is via hot water pump around the kiln. Energy for the heater is waste from another industrial process. The KW output should be enough to maintain the target temp.

 

OK but it means you have delta T losses across two heat exchange surfaces.

 

-Use the exhaust fan to extract moist air and channel it via a heat exchanger to recapture of the energy.

 

..and do what with this low grade heat, the humid air will be less than 70C and yet another delta T loss across the heat exchanger, you'll be lucky to recover it at more than a few degrees above ambient. Even power stations dumping heat to a cooling tower just vent it.

 

-Target time of 5-7 days.

 

 

How do you arrive at that?

 

-Haven't included a dehumidifier.

 

Sensible

 

I am estimating a green moisture content of 50%. Our target MC is just under 20% so we need to reduce the moisture by 30%. This means we need to remove around 3.6t of water or 3600 litres. This is obviously a lot per day when look at over a 5-7 day cycle. Some reading from various sources suggest this is quite possible but I want to get some peoples opinions.

 

You need to redo your sums, to reduce 12000kg of 50% mc wwb to 20% you need to take out 4500 kg of water and at 75 degrees this will need around 0.65kWh of heat per kg of water, best you might hope to achieve is a 50% of you heat input even optimistically you have to deliver 5.85MWh into the kiln plus losses through the kiln surfaces.

 

Any thoughts?

 

You need a premium market to justify the capital and O&M costs, can be well worth doing if you have a commercial bent.

[County 764]

I think there was a video of a container kiln on hear

[richy_B]

Cheers guys. I like to 'try' and understand a topic/process rather than just go on bits and pieces I've read so please excuse any assumptions I've made that are inaccurate. Openspaceman, in regards your reply - most of this info is based on my own reading up on the subject. I'n obviously no expert! In reply:

 

14m3 sounds more realistic. So assuming this would be a total weight of around 7 tons. Sound about right?

 

Temperature. I read a US study that advised 82C was an optimum temperature and keeping below 100C was advisable as 'steam' was going to create issues with pressure and new h&s issues. About 80C is what I have from my heating source so I figured on losing 5-10C in the kiln. What do you suggest here? What's an ideal temp in your mind?

 

I only considered a heat exchanger as I thought some heat could be captured. If, in practise, it is not worthwhile I would just vent it. I did consider a secondary chamber where the heat was sent to via a heat exchanger. Perhaps to heat a storage area where nets were kept etc. I don't know the best option here I just felt it would be better to try and use all the energy.

 

Target time is just what I'd like to achieve.

 

In regards your equation how do you arrive at 4500kg? I don't doubt it but I am not clear. Based on what you are saying though if you needed 5850KWH at 75C and you had a theoretical input of 50KW per hour from your heat source this would equate to around 117 hours or just under 5 days. Of courses losses would occur and this time scale would increase but it seems possible in the 7 day turnaround. Have I understood that correctly?

 

Thanks for the input, this is exactly what I wanted. I've done quite a bit of searching on the forum as well as google but there is really contrasting information. I assume commercial operators keep this info to themselves.

[openspaceman]

 

14m3 sounds more realistic. So assuming this would be a total weight of around 7 tons. Sound about right?

 

I think 9 probably allows for air space, trolleys and passages.

 

Temperature. I read a US study that advised 82C was an optimum temperature and keeping below 100C was advisable as 'steam' was going to create issues with pressure and new h&s issues. About 80C is what I have from my heating source so I figured on losing 5-10C in the kiln. What do you suggest here? What's an ideal temp in your mind?

 

There is no ideal, it's down to heat source, air movement costs, time. I think your figure is to do with kilning lumber without degrade, this does not matter with firewood, although sweet chestnut does shatter at higher temperatures.

 

 

Target time is just what I'd like to achieve.

 

 

 

Easy way to check is with an electric oven, set it to your desired temperature and put one log in, you'll need to get it out and weigh it frequently which will aslo ensure enough air changes but my guess is that if the oven is set to 70C to allow loss of 10C from your heat source you will barely do it. Consider that you have to do two things, deliver enough hot dry air to the surface to evaporate water and get heat into the middle of the log to get the water there to move, it's this last bit that is the problem. A sycamore woodchip will dry in 2 hours at 40C in dry air but a 6 inch log will take a week.

 

 

In regards your equation how do you arrive at 4500kg? I don't doubt it but I am not clear.

 

 

12000 @ 50% means 6000kg water and 6000kg oven dry wood

7500kg wood at 20% contains 1500kg water you need to remove the difference.

 

Based on what you are saying though if you needed 5850KWH at 75C and you had a theoretical input of 50KW per hour from your heat source this would equate to around 117 hours or just under 5 days. Of courses losses would occur and this time scale would increase but it seems possible in the 7 day turnaround. Have I understood that correctly?

 

50kW is already a rate of energy use so the per hour would be a rate of change of power. Your numbers are OK but you haven't allowed for the time taken between delivery of heat to the wood surface and that heat getting to the wet core and migrating liquid to the surface.

 

I assume commercial operators keep this info to themselves.

 

They would wouldn't they (paraphrasing Mandy R-D). Commercial operators won't concern themselves with the workings, they'll look at the "box", cost the inputs and price the outputs and see if they can sell that at a profit.

[broonie]

on the study I have done 220 - 270 Fahrenheit is the optimum temp for drying firewood fast and if you have cheap or free wood, a forced air wood furnace, approx 175kwh for 24 cm3 iirc, revesible circulating fans with 20000cfm capacity