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cornish wood burner

Is wood ash benificial for trees

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For the last few years I have been planting a narrow strip of land between mature wooded hedges.

Once the trees I have planted become established I have been scattering wood ash around them. Species are the normal firewood types ie ash oak sycamore and hazel. All seem to be doing well but reading a recent post about farm yard manure for trees prompted me to ask if am I doing wrong?

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Difficult to find an evidence based answer. Below is the abstract from a 2005 paper that attempts to answer your question.

 

The use of wood fuel for energy production in the UK is set to increase in the near future as part of a government commitment to increase renewable sources to 10 per cent by 2010. The ash generated as a by-product of combustion, whether for heat or power generation, has potential use as a fertilizer in forest systems. This review assesses the available information on factors affecting the quality of the ash and environmental implications arising from its application. The key determinants of wood ash chemistry are the tree species combusted, the nature of the burn process and the conditions at the application site. Wood ash from hardwood species produces higher levels of macronutrients in their ash than conifers, and the silica content is frequently lower. A furnace temperature between 500 and 900°C is critical to the retention of nutrients, particularly potassium, and determines the concentrations of potentially toxic metals including aluminium in the ash. Fly ash, the lightest component that accumulates in the flue system, can contain high concentrations of cadmium, copper, chromium, lead and arsenic and this ash should not be used as fertilizer. The form of the ash at application is important, with loose ash releasing Ca, K and Na more rapidly than granulated ash. Heavy metal, radionuclide and dioxin contamination of wood ash-based fertilizers is minimal and unlikely to affect ecosystem function. The effects of wood ash are primarily governed by application rate and soil type. The benefits are maximized at low dose rates, with possible toxicity from applications in excess of 10 t ha−1. For most forest sites, a single wood ash application per rotation could replace all the nutrients lost after whole-tree harvesting (excepting N). Long-lasting positive effects on tree growth have been observed on shallow peats, in which the humus is slowly mineralized in response to elevated pH and increased nutrient availability. In contrast, wood ash application to podzols is only effective in enhancing tree growth when nitrogen availability is non-limiting. To date, published research of wood ash effects on trees growing in clays and loams is minimal. A lag time for positive tree responses to wood ash application is often observed, and may be the result of phosphorous limitation at higher soil pH. The greatest reported adverse ecological effects are to acidophilic ecosystems, particularly the constituent bryophyte, soil bacteria and ectomycorrhizal communities.

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Fly ash, the lightest component that accumulates in the flue system, can contain high concentrations of cadmium, copper, chromium, lead and arsenic and this ash should not be used as fertilizer.

 

I realise this was another's paper you were quoting: I cannot see concentrations of the mentioned heavy metals being even in the flyash unless treated wood or wood contaminated with fastenings is burnt.

 

However, if I remember rightly, fly ash is always considered a hazardous waste and should be disposed as such. In practice it seems to get mixed with bottom ash.

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Thanks for that info Woody guy.

I have been spreading ash from domestic fires but as part of my day job I run a couple of large wood chip boilers which it sounds like most of that article was aimed at. I was interested to hear about the fly ash. From the looks of it I would have put money on it being the best fertilizer, just shows take nothing for granted.

I will also keep an eye on our furnace temperatures but from memory they were very close to 900 deg especially interesting as a local farmer has been spreading some on his fields. Apparently grew a field of corn with it. I will find out how he got on. Cheers

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I realise this was another's paper you were quoting: I cannot see concentrations of the mentioned heavy metals being even in the flyash unless treated wood or wood contaminated with fastenings is burnt.

 

However, if I remember rightly, fly ash is always considered a hazardous waste and should be disposed as such. In practice it seems to get mixed with bottom ash.

 

Could it be the minerals and heavy metals accumulate in the bark and are then released by burning.

The fly ash certainly is hazardous to your lungs very fine and easily prompted to be airborne.

We do mix with the bottom ash on an outside tip so it is normally wet.

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Could it be the minerals and heavy metals accumulate in the bark and are then released by burning.

The fly ash certainly is hazardous to your lungs very fine and easily prompted to be airborne.

We do mix with the bottom ash on an outside tip so it is normally wet.

 

There is certainly a higher mineral content in needles, bark, buds and leaves than the wood in a tree, that's why for sustainability the fast grown hardwood plantations set the harvester to strip bark and leave all the lop and top to fertilise the next crop.

 

It must be a very complicated subject because fly ash is generally only collected in fan assisted burners. The particles are fine and they are mostly formed in the secondary combustion area (from small particles carried over from the fire bed) and apart from being mostly silica (SiO2) which itself is bad to inhale in fine particle (think silicosis and mesothelioma ) they contain alkali metals which lower the silica melting point, so they can form a droplet which being in the secondary combustion area forms a nucleus to attract products of incomplete combustion. These PICs can contain polycyclic aromatic compounds like benzo-a-pyrene the carcinogen know to cigarette smokers and dioxins. Any heavy metals that have species of oxides or carbonates which volatilise at combustion temperatures are likely to become concentrated in fly ash.

 

Most legislation about solid fuel burning comes from long experience with coal and fly ash from that (most ash is fly ash in pulverised coal burners I suspect) is definitely hazardous and cannot be disposed directly to land. I think the same rules apply to fly ash from biomass. I'm sure you also need an exemption from EA to apply any wood ash from a commercial biomass burner to land.

 

I've been out of dealing with commercial biomass boilers for over 5 years now but my view is that most nasties will be from "contraries" that come in with the chip. Toward the end I noticed bits of pallet, chipboard and even melamine resin in the biomass chip. The reason is simple; the gate fee for disposing treated timber at a licensed incinerator is between £50 and £80/tonne. The rules seem to allow a small % of contamination so if for instance you can get away with 1% contamination of an otherwise clean sample of wood chip and still sell it for £50 delivered in then for every 1000 tonne you gain up to £1300 by losing 10 tonnes of treated wood waste and this will contain the bulk of the copper, chrome, arsenic, cadmium, lead etc. not to mention pvc.

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There is certainly a higher mineral content in needles, bark, buds and leaves than the wood in a tree, that's why for sustainability the fast grown hardwood plantations set the harvester to strip bark and leave all the lop and top to fertilise the next crop.

 

It must be a very complicated subject because fly ash is generally only collected in fan assisted burners. The particles are fine and they are mostly formed in the secondary combustion area (from small particles carried over from the fire bed) and apart from being mostly silica (SiO2) which itself is bad to inhale in fine particle (think silicosis and mesothelioma ) they contain alkali metals which lower the silica melting point, so they can form a droplet which being in the secondary combustion area forms a nucleus to attract products of incomplete combustion. These PICs can contain polycyclic aromatic compounds like benzo-a-pyrene the carcinogen know to cigarette smokers and dioxins. Any heavy metals that have species of oxides or carbonates which volatilise at combustion temperatures are likely to become concentrated in fly ash.

 

Most legislation about solid fuel burning comes from long experience with coal and fly ash from that (most ash is fly ash in pulverised coal burners I suspect) is definitely hazardous and cannot be disposed directly to land. I think the same rules apply to fly ash from biomass. I'm sure you also need an exemption from EA to apply any wood ash from a commercial biomass burner to land.

 

I've been out of dealing with commercial biomass boilers for over 5 years now but my view is that most nasties will be from "contraries" that come in with the chip. Toward the end I noticed bits of pallet, chipboard and even melamine resin in the biomass chip. The reason is simple; the gate fee for disposing treated timber at a licensed incinerator is between £50 and £80/tonne. The rules seem to allow a small % of contamination so if for instance you can get away with 1% contamination of an otherwise clean sample of wood chip and still sell it for £50 delivered in then for every 1000 tonne you gain up to £1300 by losing 10 tonnes of treated wood waste and this will contain the bulk of the copper, chrome, arsenic, cadmium, lead etc. not to mention pvc.

As you say the fly ash is a complicated subject.Higher temperatures generated by the secondary burn might affect things we do not understand. Not sure if products of incomplete combustion are significant though. I thought the modern boilers were very efficient and had vertically complete combustion.

 

We did briefly use a little recycled wood but inspite of specifying clean wood only some plastic and wire crept in. I kicked that into touch several years ago before our farmer friend used it. Reason being I could see strange deposits on the boiler walls and in the ash which gave me concerns over the sensors.

We only use round wood chip now and I notice the difference. I don't normally go into the waste side but my experiences tie in with your post. Apparently the farmer analysed the ash before using and was happy with the results but I cannot say if he obtained a permit.I think I might investigate this further as we produce a significant pile of ash from the 3000 tons we burn.

For my patch I think I will keep to ash from my own domestic fires and do as Tony says (use sparingly)

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Not sure if products of incomplete combustion are significant though. I thought the modern boilers were very efficient and had vertically complete combustion.

 

Yes, PICS are generally in the soot so a standard smoke test will give an indication. I have a feeling that PICs will be related to CO:CO2, my home-made burner ran at 30ppm CO and seemed to be clean but I never had other analysis tools so never did find out how good it was. If there is any visible smoke you have PICs in abundance.

 

 

Dioxins are a different matter though and depend on how much halogenated hydrocarbon is in the feedstock. PVC is the obvious one and neoprene, if you burn it hot enough and quench the flue gas quickly they can be minimised but then you notice a cloud of vapour above the flue, this is the chlorine recombining with hydrogen and then sucking water vapour out of the air to rain as hydrochloric acid. This has interesting effects on stainless steel flues.

 

 

We did briefly use a little recycled wood but inspite of specifying clean wood only some plastic and wire crept in. I kicked that into touch several years ago before our farmer friend used it. Reason being I could see strange deposits on the boiler walls and in the ash which gave me concerns over the sensors.

 

As I said not only the sensors; I've seen a greenish deposit below a EGR pipe which was the first indication that acid had etched out the iron and micro perforated it, from the outside it looked fine. The cure should have been to only burn clean chip but insulating the pipe to prevent condensation solved it. I never did decide if it was an iron chloride or sulphate on the floor. The plastic wire you mention will almost certainly be PVC covered.

 

You need to look at exemptions U10 or U11

 

https://www.gov.uk/waste-exemption-u11-spreading-waste-to-benefit-non-agricultural-land

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If every farmer knew how to make charcoal and sourced the timber from their un managed hedgerows then put the end product back in the ground there would be no need for pesticides, but guess what the big companies have too much power and the chemicals keep their wallets full. the answer is yes it is beneficial

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