The efficiency of kiln drying firewood?

[Big J]

I'll start off by saying that I completely understand why anyone who is on an RHI tariff to dry firewood would do so. Had I been in the position at the time to take advantage of the proposterous amount of money the government were offering at the time, I'd have done the same. 

 

That being said, I fundamentally disagree with burning wood to dry fuel wood that would otherwise dry out in the air given sufficient time.

 

I am curious about the material efficiency of the process. In other words, if you were drying an artic load (25t) of split hardwood, what tonnage of fuel (at presumably 30% MC, or thereabouts) would be required to take it from green to sub 20% MC?

 

I have a feeling it's around 5t, but would be delighted to hear about other peoples experiences. 

 

The purpose of the thread is to make an environmental case for air drying firewood and to seek to undermine the governments recent policy pledges that discriminate against small scale, air drying firewood producers. So that my motives are clear for all to see ?

[Justme]

42 minutes ago, Big J said:

1, I fundamentally disagree with burning wood to dry fuel wood that would otherwise dry out in the air given sufficient time.

 

2, I am curious about the material efficiency of the process. In other words, if you were drying an artic load (25t) of split hardwood, what tonnage of fuel (at presumably 30% MC, or thereabouts) would be required to take it from green to sub 20% MC?

 

3, I have a feeling it's around 5t, but would be delighted to hear about other peoples experiences. 

 

1, I agree, its madness.

2,I think a better stat would be how many kWh can you get out of 26t of fresh felled compared to how much you can get out of that once kiln dried & how many kWh were used to dry it inc the extra handling. Then compare it to air drying.

3, I have asked this before but kilners dont seem to want to say.

 

For me the only justification is speed & space. 

[Peasgood]

It’s like getting grants, funding and subsidies for using tumble driers when there’s nowt wrong with your washing line.

[Big J]

1 minute ago, Peasgood said:

It’s like getting grants, funding and subsidies for using tumble driers when there’s nowt wrong with your washing line.

We had 26 days of rain last month - my washing line has been as much use as a chocolate fireguard this winter!

[Big J]

8 minutes ago, Justme said:

1, I agree, its madness.

2,I think a better stat would be how many kWh can you get out of 26t of fresh felled compared to how much you can get out of that once kiln dried & how many kWh were used to dry it inc the extra handling. Then compare it to air drying.

3, I have asked this before but kilners dont seem to want to say.

 

For me the only justification is speed & space. 

On point two, the only justifiable position for burning firewood is to burn it fully seasoned (ie, under 25%). I'm not sure that making the comparison between green and dry is relevent.

 

The reason I ask the question about fuel usage is that my back of fag packet calculations put the carbon footprint of UK produced kiln dried firewood at about 14 times higher than air dried, if my estimate of 5t fuel required to dry 25t of logs. I'd like to be proven wrong!

Edited March 2, 2020 by Big J

[Justme]

Power stations don't burn it seasoned. Its purely about how many useable kWh are gained.

[Big J]

5 minutes ago, Justme said:

Power stations don't burn it seasoned. Its purely about how many useable kWh are gained.

Very true, but I'm limiting my question to firewood for the domestic market, as that is the purview of the current legislative proposals.

[SbTVF]

I'll bite.

 

We use approx 350kg of sub 30% moisture softwood chip (seasoned in the round then chipped as demand dictates) to run the 150kw boiler for a day. 2 farm houses, 3 pig buildings and the kiln.

We estimate our boiler uses approx 25% more chip when the kiln is in use.

The kiln dries 16cube of logs in 7 days from fresh felled/processed. So less than a ton per cycle if you said the kiln ran 365days a year.

I prefer if at all possible, to season in IBC's for as long as possible but if we can get 4 months of good weather on them first it can reduce kiln time to 3 days.

 

We also dry using an AD plant on a Lauber Lenz system in 40yard skips. It's not a 'kiln' but it does a fantastic job and that heat would be wasted if we didn't use it. We use an insulated sheet and vents to save as much heat as we can and speed it up too whereas many just leave the top off on these systems because they're trying to keep the heat meter going round for RHI.

 

The 500mw plant is powered by grass silage, straw based cow manure and a small percentage of vegetable waste from chip/crisp manufacture.

 

We needed our kiln to be as efficient as possible so we didn't use heat we needed for the pigs. The boiler was put it for the pigs and saves 10k of electricity bills from heating a year as a result.

 

Monetarily speaking the outlay for a shed or storage big enough for the volume of dry wood we sell each winter would cost a considerable amount and take up space we do not currently have. Our farm is changing system this year though so hoping to be able to season and kiln where required once we have the space to do it. It'll take us to a level of output where it all makes a lot more sense and can be done more effectively too.

 

[Big J]

3 minutes ago, SbTVF said:

I'll bite.

We use approx 350kg of sub 30% moisture softwood chip (seasoned in the round then chipped as demand dictates) to run the 150kw boiler for a day. 2 farm houses, 3 pig buildings and the kiln.
We estimate our boiler uses approx 25% more chip when the kiln is in use.
The kiln dries 16cube of logs in 7 days from fresh felled/processed. So less than a ton per cycle if you said the kiln ran 365days a year.
I prefer if at all possible, to season in IBC's for as long as possible but if we can get 4 months of good weather it can reduce kiln time to 3 days.

We also dry using an AD plant on a Lauber Lenz system in 40yard skips. It's not a 'kiln' but it does a fantastic job and that heat would be wasted if we didn't use it. We use an insulated sheet and vents to save as much heat as we can and speed it up too whereas many just leave the top off on these systems because they're trying to keep the heat meter going round for RHI.

The 500mw plant is powered by grass silage, straw based cow manure and a small percentage of vegetable waste from chip/crisp manufacture.

We needed our kiln to be as efficient as possible so we didn't use heat we needed for the pigs. The boiler was put it for the pigs and saves 10k of electricity bills from heating a year as a result.

Thanks for that. Your imput is much appreciated! 

 

Do you think that you are gaining an efficiency saving by having the kiln as part of a larger system? You're operating on a roughly 8:1 ratio of timber dried to timber burned, but that's using sub 30% chip, which (if spruce) will be a little over half the weight for it's volume compared to fresh felled. So my notion of 5:1 holds (I think!).

[gdh]

My workings are a bit rough because we dry chip as well but in an average year (like the last one) we use 10% of our wood for drying. That's using softwood to dry hardwood so the percentage should be less for a direct comparison.

 

We use a heated (and insulated) shed with fans that we're constantly tweaking to improve things.