Kiln Drying Firewood - Yourself

[Duffryn]

Posted by Duffryn

 

"2) How much fuel do they use ? ( An 85% efficient 175kwh boiler should use circa 1 ton of 20% fuel a day)"

 

Must say I am shocked by this figure. So that's nearly 3m3 of dry logs a day just to run a kiln. Not quite sure how many days it takes to dry a load but I have seen talk of 5-7 days on here. So 15-21 m3 of logs are needed per cycle?

 

Clear this up for me Duffryn what is the conversion rate of fuel to finished product?

 

That depends on the boiler and the kiln. Logpro was drying 50 tons of firewood for circa 7.5 tons of fuel. A drying cycle at 10 degrees outside temperature was 2.5 days ( 50 to 20% m/c measured be weight). The GF boiler did this after being modified by GF to solve the excessive smoke and fuel use issues but the modification was not RHI compliant hence we could not roll the product out.

 

I have spoken to an arbtalk owner of a GF kiln who reported much higher fuel costs and significantly longer drying times using the GF kiln. There are very good reasons for this !

[Duffryn]

That depends on the boiler and the kiln. Logpro was drying 50 m3 of firewood for circa 7.5 tons of fuel. A drying cycle at 10 degrees outside temperature was 2.5 days ( 50 to 20% m/c measured be weight). The GF boiler did this after being modified by GF to solve the excessive smoke and fuel use issues but the modification was not RHI compliant hence we could not roll the product out.

 

I have spoken to an arbtalk owner of a GF kiln who reported much higher fuel costs and significantly longer drying times using the GF kiln. There are very good reasons for this !

 

corrected above dried 50m3 not 50 tons

[arboriculturist]

The Glen farrow boiler passed its emission test using a VERY small load of fuel. Yes the boiler passed the test but if you were to use a realistic fuel load (one that required refueling 3-4 times day) and the boiler were tested it would not pass the RHI emissions test. If you are claiming the RHI you would need to be happy with this.

 

It would be worth speaking to an owner of one of these systems and ask them :-

 

1) How many Mw a day does the boiler produce ( A 175kwh boiler should produce 3.5-4 mw a day in realistic use)

 

2) How much fuel do they use ? ( An 85% efficient 175kwh boiler should use circa 1 ton of 20% fuel a day)

 

3) How long do the logs take to dry in 20 degrees outside temperature ? Then you will need to factor in drying times when the weather is much colder. Also consider the fuel cost of these drying times.

 

As for the BSL sustainability question, this is largely a paper work / box ticking exercise and registering as a suppler producer is straight forward .

 

Many of the Forum posters will realise that they should take heed of what you post and if others they knew your track record I would suggest that very few would ignore your advice.

 

Prudent questions and many would find it difficult to provide answers.

 

1. With that design of Boiler it will not perform continuously at the claimed output without perfect low moisture content fuel.

 

2. Yes I agree 3m3 of timber sub 20% moisture will be required to get anywhere near 175kw/hr.

 

3. The will not meet the emissions test.

 

I would suggest it is a huge gamble using that type of boiler at this moment in time. How long before more regular emissions tests are brought in with boiler at full pelt, a bit like MOT test for emissions revving the guts out the vehicle!

 

D do you know the current emissions testing requirement for these. As far as I know only 1 initial test is required on commissioning? If they bring in more rigourous testing then many of the installations currently operating could come unstuck - badly!

[arboriculturist]

That depends on the boiler and the kiln. Logpro was drying 50 tons of firewood for circa 7.5 tons of fuel. A drying cycle at 10 degrees outside temperature was 2.5 days ( 50 to 20% m/c measured be weight). The GF boiler did this after being modified by GF to solve the excessive smoke and fuel use issues but the modification was not RHI compliant hence we could not roll the product out.

 

I have spoken to an arbtalk owner of a GF kiln who reported much higher fuel costs and significantly longer drying times using the GF kiln. There are very good reasons for this !

 

Please correct me if I am wrong - 50 tonnes for circa 7.5 tonnes of fuel. Is this not 50 m3 of loose fill containerised logs.

[openspaceman]

Posted by Duffryn

 

"2) How much fuel do they use ? ( An 85% efficient 175kwh boiler should use circa 1 ton of 20% fuel a day)"

 

Must say I am shocked by this figure. So that's nearly 3m3 of dry logs a day just to run a kiln. Not quite sure how many days it takes to dry a load but I have seen talk of 5-7 days on here. So 15-21 m3 of logs are needed per cycle?

 

Clear this up for me Duffryn what is the conversion rate of fuel to finished product?

 

I cannot see it worth burning 20% mc fuel in a boiler as large as this, nor would I expect to get to 85% of the wood energy into water but the figures are fairly straightforward, first need to clear up the differece between power and energy, power is the amount of heat the boiler will give out in a unit of time, so we'll say a 175kW(thermal) boiler. Run this flat out for an hour and it will have released 175kWh(t) of heat, some of this heat will be lost up the flue or out of the sides but accepting the 85% into water figure that means the water receives 3.57MWh of heat over the 24 hours running, in practice the boiler has to modulate to an average figure less than this or something tends to blow off.

 

Of the 20% mc (wet weight basis) wood that needs to be burned each kg burned perfectly yields about 4kWh of heat and we need 4200kWh so fractionally over a tonne as Duffryn says. Logs are far to valuable to burn doing this.

 

Next you must consider the conversion of energy in the heated water to drying the logs. This will vary enormously with many factors like insulation, saturation of air and temperature of moisture laden air leaving the kiln. We aimed for a coefficient of performance of 1.11kWh per kg of water removed from wood on a simple system. One could do better but with much increase capital cost.

 

Short cycle times are entirely dependent on temperature as once you pass enough air over the log surface to evapourate moisture it's the time that takes for the water to move out of the log that limits you.

[arboriculturist]

I cannot see it worth burning 20% mc fuel in a boiler as large as this, nor would I expect to get to 85% of the wood energy into water but the figures are fairly straightforward, first need to clear up the differece between power and energy, power is the amount of heat the boiler will give out in a unit of time, so we'll say a 175kW(thermal) boiler. Run this flat out for an hour and it will have released 175kWh(t) of heat, some of this heat will be lost up the flue or out of the sides but accepting the 85% into water figure that means the water receives 3.57MWh of heat over the 24 hours running, in practice the boiler has to modulate to an average figure less than this or something tends to blow off.

 

Of the 20% mc (wet weight basis) wood that needs to be burned each kg burned perfectly yields about 4kWh of heat and we need 4200kWh so fractionally over a tonne as Duffryn says. Logs are far to valuable to burn doing this.

 

Next you must consider the conversion of energy in the heated water to drying the logs. This will vary enormously with many factors like insulation, saturation of air and temperature of moisture laden air leaving the kiln. We aimed for a coefficient of performance of 1.11kWh per kg of water removed from wood on a simple system. One could do better but with much increase capital cost.

 

Short cycle times are entirely dependent on temperature as once you pass enough air over the log surface to evapourate moisture it's the time that takes for the water to move out of the log that limits you.

 

 

We have all suffered from 'delusions' from time to time and no disrespect, but I would suggest that many firewood producers who have now installed a KIln Drying setup, will never attain the RHI returns their initial 'desktop study' predicted.

 

I think it is accepted that in a 40ft container you can dry 30m3 of green split logs (e.g. S.Chestnut 60% moisture Ash 37%) in stillages down to 20% moisture average, providing it is insulated in 3.5 days.

You would need to burn 10.5 m3 of timber below 20% moisture to do this. The big boys can attain this production in 2.5 days, but small setups will never achieve the 2.5 day threshold.

 

Anyone like to crunch the numbers on this one!

[openspaceman]

We have all suffered from 'delusions' from time to time and no disrespect, but I would suggest that many firewood producers who have now installed a KIln Drying setup, will never attain the RHI returns their initial 'desktop study' predicted.

 

I think it is accepted that in a 40ft container you can dry 30m3 of green split logs (e.g. S.Chestnut 60% moisture Ash 37%) in stillages down to 20% moisture average, providing it is insulated in 3.5 days.

You would need to burn 10.5 m3 of timber below 20% moisture to do this. The big boys can attain this production in 2.5 days, but small setups will never achieve the 2.5 day threshold.

 

Anyone like to crunch the numbers on this one!

 

We got 18 wet tonnes into our 40ft container and it came out 24 hours later at 11 tonnes.

 

Burning 10.5 m3 to dry 30m3 is probably fair assessment with a overall COP of 2 as the energy in evaporating water from wood is about 14% of the wood energy availble and you'll need double to allow for wastage. Spend more money on bits and you can do better. You also have to allow extra for the energy needed to circulate air and this becomes a trade off with temperature, the hotter things are the less air circulation required.

 

I think your figure for ash is a little low and sweet chestnut considerably too high.

 

Personally I don't think it's worth going to 20% mc as that is getting too close to the equilibrium moisture content outside in most of England so some of the logs may soak up air from the atmosphere as the moisture in the logs settles averages out.

 

I know nothing about RHI payments but do realise that generally desktop studies and spreadsheets are too optimistic in practice.

[arboriculturist]

We got 18 wet tonnes into our 40ft container and it came out 24 hours later at 11 tonnes.

 

Burning 10.5 m3 to dry 30m3 is probably fair assessment with a overall COP of 2 as the energy in evaporating water from wood is about 14% of the wood energy availble and you'll need double to allow for wastage. Spend more money on bits and you can do better. You also have to allow extra for the energy needed to circulate air and this becomes a trade off with temperature, the hotter things are the less air circulation required.

 

I think your figure for ash is a little low and sweet chestnut considerably too high.

 

Personally I don't think it's worth going to 20% mc as that is getting too close to the equilibrium moisture content outside in most of England so some of the logs may soak up air from the atmosphere as the moisture in the logs settles averages out.

 

I know nothing about RHI payments but do realise that generally desktop studies and spreadsheets are too optimistic in practice.

 

Ah thanks for the reply. The timber I tested was fresh felled, crosscut, split and centre of log tested. However I know internal moisture levels can vary a lot.

 

You must use use the full 40ft of container to get in 36M3 in stillages. Are you running a chip/pellet boiler?

[arboriculturist]

Ah thanks for the reply. The timber I tested was fresh felled, crosscut, split and centre of log tested. However I know internal moisture levels can vary a lot.

 

You must use use the full 40ft of container to get in 36M3 in stillages. Are you running a chip/pellet boiler?

 

Are your powered vents linked to a RH stat, so vents open and dump the moisture laiden air every 10-30 mins depending on at what point in time you are in the drying cycle?

 

Do you agree with 3.5 days to get 36m3 down to 20-25%?

[Duffryn]

Many of the Forum posters will realise that they should take heed of what you post and if others they knew your track record I would suggest that very few would ignore your advice.

 

Prudent questions and many would find it difficult to provide answers.

 

1. With that design of Boiler it will not perform continuously at the claimed output without perfect low moisture content fuel.

 

2. Yes I agree 3m3 of timber sub 20% moisture will be required to get anywhere near 175kw/hr.

 

3. The will not meet the emissions test.

 

I would suggest it is a huge gamble using that type of boiler at this moment in time. How long before more regular emissions tests are brought in with boiler at full pelt, a bit like MOT test for emissions revving the guts out the vehicle!

 

D do you know the current emissions testing requirement for these. As far as I know only 1 initial test is required on commissioning? If they bring in more rigourous testing then many of the installations currently operating could come unstuck - badly!

 

Absolutely spot on ! In Germany there are annual tests on biomass boilers and it would not surprise me if the UK followed.

 

There is a significant correlation between emissions and fuel load. On a batch fed boiler the emissions test is difficult to pass and so the temptation to use a totally unrealistically low fuel load is high. Whilst the boiler may have "passed" the manufacturers RHI emissions test owners will need to be happy of the potential risk they run in using a boiler that will not be RHI compliant in every day use. Your comparison to the MOT emissions test is a great analogy.

 

The test itself is carried out by the manufacturer and an independent test house. It is not currently a requirement for the boiler to be tested in use or after commissioning.