Hard wood vs softwood wood chip

[gdh]

If you can use 350 kW or more then you can burn wet wood in an auto fed boiler. The fuel is introduced then dried as it approaches the fire. Step grate boilers are more expensive and generally more common in the larger systems but they are available at a cost.

That's interesting, we've got a 199kw chip boiler and we did look at a way of drying the chip as it goes in but couldn't get anything to work.

[cornish wood burner]

That's interesting, we've got a 199kw chip boiler and we did look at a way of drying the chip as it goes in but couldn't get anything to work.

Probably all of the bigger boiler manufactures do a step grate system. Just had a look at a couple and are available in smaller sizes. I'm familiar with the uniconfort global but many others out there. Cost will of course be higher as there will be the complexity of the grate and drive. Nice bonus will be automatic de-ashing that normally comes with it.

[openspaceman]

Probably all of the bigger boiler manufactures do a step grate system. Just had a look at a couple and are available in smaller sizes. I'm familiar with the uniconfort global but many others out there. Cost will of course be higher as there will be the complexity of the grate and drive. Nice bonus will be automatic de-ashing that normally comes with it.

 

Ian I take it you mean a moving or reciprocating grate? My boiler has a simple static stepped grate and it is as crude as that, the moving grate keeps the bed more homogeneous and thus avoids ratholing where jets of primary air escape the bed without doing their work.

 

As you say with bigger burners one can burn wetter wood cleanly but there gets the stage where the thermal losses in the steam and sizing of the heat exchange to accommodate lower furnace temperatures make it of dubious value.

 

The idea is to keep the firebox temperature up and this can be done by having the primary fire under a hot ceramic arch over which the secondary combustion takes place, the theory being this arch transmits the some heat from the secondary area back down to the incoming chip by radiation from the arch, this pre dries the chip but the steam co-mingles with the offgas. Also the secondary air can be preheated by passing over the secondary area before entering the flame. I have never seen either of these techniques used in UK.

 

Seasoning in the round and chipping at below 30% mc wwb seems to be the way the market is gone.

 

Alvan Blanch the grain drier people offered a gasoil powered dryer for green woodchip which was used at Tilbury and possibly Aberthaw but his was a bit of a cheat IMO.

[Duffryn]

With chip wood at circa £30 a tonne and hardwood at circa £50 a tonne is doesn't really make economic sense to chip hardwood. Takes longer to chip hardwood as well which knocks chipping cost up. For most installations unless you are earning RHI out of drying the chip then much better off chipping seasoned wood. On my boiler 20% m/c gives the best return, though I guess this will be boiler specific. As others have said boiler settings will need changing if you switch fuels and mixing wont give the best results unless the mix is consistent.

[gdh]

I'm not sure on whether the extra cost of hardwood is worth it in terms of kws produced (similar argument for firewood) but we only chip 4 inch down hardwood as opposed to up to to 12 inch softwood so it's more to save the labour of cutting small stuff for firewood.

I might be missing something but as far as I can remember we've never adjusted our boiler for different types of chip, I was under the impression that everything's automatic - it basically keeps filling and burning until the buffer tank reaches a set temperature,in our case 70c. It's an evoworld 199kw boiler.

[cornish wood burner]

Good morning Andrew

I was meaning a full moving or reciprocating grate that carries the wood forward. We considered this when we installed our big Binder but the extra cost (pre rhi)and increase in height intruding on our neighbours view steered us away from that route. As you say better to let the sun and wind dry the wood rather than reduce the efficiency. I've just chipped and even in this weather the lower part of the slab wood stacks are coming out at 18%. Top is not so good though. Even adding in the wetter round wood I am sure we have made the right decision.

Depends on what's available so in a wet part of Scotland or if you are considering tree surgeon waste perhaps then it might have a place. Fire box temp is very important as we

know so a step grate might be necessary in some cases.

As Duffryn says it doesn't make economic sense to chip hard wood, purely from a cost/ tonne point of view.

[Duffryn]

I'm not sure on whether the extra cost of hardwood is worth it in terms of kws produced (similar argument for firewood) but we only chip 4 inch down hardwood as opposed to up to to 12 inch softwood so it's more to save the labour of cutting small stuff for firewood.

I might be missing something but as far as I can remember we've never adjusted our boiler for different types of chip, I was under the impression that everything's automatic - it basically keeps filling and burning until the buffer tank reaches a set temperature,in our case 70c. It's an evoworld 199kw boiler.

 

Most woodchip boilers work on using 8 or so key settings to arrive at the optimum efficiency for the fuel type. Yes the boiler will still work using hard wood on a softwood setting but you will waste a lot of wood , especially when the boiler is in standby. Been there, done that !

[cornish wood burner]

I'm not sure on whether the extra cost of hardwood is worth it in terms of kws produced (similar argument for firewood) but we only chip 4 inch down hardwood as opposed to up to to 12 inch softwood so it's more to save the labour of cutting small stuff for firewood.

I might be missing something but as far as I can remember we've never adjusted our boiler for different types of chip, I was under the impression that everything's automatic - it basically keeps filling and burning until the buffer tank reaches a set temperature,in our case 70c. It's an evoworld 199kw boiler.

 

kWhr/ kg is not much different between hard or soft so soft wood will be much cheaper. Biggest difference is the density. If your boiler is fed a certain volume of soft wood /hour running at its max output and you change to hard wood then you will be inputting more energy so exceeding rated output and probably wasting energy as a result.

[openspaceman]

As you say better to let the sun and wind dry the wood rather than reduce the efficiency. I've just chipped and even in this weather the lower part of the slab wood stacks are coming out at 18%. Top is not so good though. Even adding in the wetter round wood I am sure we have made the right decision.

 

On your scale with stacks 4 metre high (IIRC??) I doubt rain penetrates very far before it runs off and drips from the ends. On the small scale and and especially with softwood logs I think rain penetration is more significant than people think. Part of my sheeting blew off my store at home and the logs seem much wetter than those where it remained covered.

 

Yes in your case that low moisture content is easy to burn clean so a more sophisticated boiler isn't warranted. The Kobs would burn 40% mc and that was just as well as I never tested the allegedly G30W30 chip and found mc in spec. I think a case can be made for drying high moisture content chip on the fly but probably with screening in order to upgrade it from a power station product to a G30W30 premium for a smaller boiler.

[cornish wood burner]

I used to reckon the top half of the stack had a 5% increase chipping in or after rain. Most of that would be in or just under the bark assuming it's not going over. Stacks were 30% MC top and 18 to 20% bottom to middle after all the rain we've had. Probably some stacks are close to 5 metres high now so contain more dry wood. That's the beauty of 100% mechanical handling.

Lower stacks obviously would be worse as you have found. Roundwood seems a little better than slab but the trend is the same.