Tree surgery waste as heating fuel

[KenB]

List,

The ability to exploit unusual fuels is paramount to obtaining independence from fossil fuels.
 
If you find a way to utilise a particular fuel, it puts you one step ahead of your neighbour. 
 
Waste vegetable oil is already becoming short in supply, because more and more people are discovering how to utilise it.
 
This is why I am casting a wider net, and looking at fuels that may appear to be of marginal use.
 
My wife's nephew is a tree surgeon, and it is common practice to take all the prunings and run them through a diesel powered wood chipper, and blow them into the back of the truck.
 
There is currently no market for this waste woodchip product, other than mulch - thousands of tonnes of it are dumped locally. 
 
A tonne of this assorted green tree waste takes up about 5 cubic metres, and has a calorific value of between 2 and 4 kWh per kilogramme.
 
If you can sun dry this material to reduce the moisture content, and then burn it in an adapted stove, then you are looking at almost a free fuel supply.

A couple of days ago,  I collected 200 litres of this green wood waste from the tree surgery dump.   I specifically collected the poorest quality, because if I could burn that successfully then anything else would be easy.

The sample I had was clearly oak and still full of green leaves, twigs, bark, acorns and little chips (10mm to 20mm) of wood.  It was about as rough as it can get.
 
I did some burn tests last night using this green tree chips in my Sunstove, using a small 12V electric blower fan (about 20W) , and can report that I  achieved a smokeless burn, intense heat (about 10kW)  and very little ash residue.

The "fireball" within the combustion zone was so intense that it was too bright to look at, and caused the outside of the stove to glow orange hot.  At one stage I has crimson woodgas flames roaring out of the top of the 6' chimney pipe.

The key to burning any fuel is to supply sufficient oxygen,  get the quantity of fuel correct,  dry the fuel and pre-heat the air.  Achieve these and you can burn any old forestry waste efficiently and cleanly.

I am now working on an automatic fuel feed system with the intention of being able to convert a standard woodstove to burn this low grade fuel.  Think of windscreen wiper motors,  post boring augers and hot air guns and you will have the basic concept.



Ken

[Panda_Badger]

Hi, Ken,

Very interesting, but wouldn't it be more productive to make a large compost pile with these off-cuts and produce heated water and methane? Afterwards the wood goes back to the ground as compost and there is no burning involved to produce a fuel.  

All the best,

The Panda.

[lightfoot]

Hi Ken,

Be wary about burning too much oak, as it is very acidic, especially when green and will rot your wood stove if your not careful. Put a piece of clean mild steel in or even next to a pile of damp oak for a day or two and your see what I mean    If you must burn it, then try and mix it in with some other woods and make sure it's well seasoned.

Good luck with your windscreen wipers etc....your going to need a bigger shed soon


Lightfoot.


PS, I know it wouldn't be as much fun for you, but maybe you should just bite the bullet and invest in a proper high efficiency, clean burning wood boiler, with plenty of thermal storage... http://www.baxi.co.uk/products/biomass.htm   

[damnager]

Ken,
      As Panda Badger said, did you see this thread http://www.navitron.org.uk/forum/index.php/topic,4025.0.html and the youtube videos that went with it.

Regards

Keith

[KenB]

Lightfoot,

You miss the point slightly.   If I were to use logs, I would end up paying the market price - and the Baxi is far too big for my humble needs.

The purpose of this exercise is to use the woodchips that few can use, straight off the pile.

If you've ever seen a firestorm in miniature, then that's how these chips burn with sufficient draught.   Theoxygen was being used so quickly, that the smoke and fumes from the neighbouring chips was being drawn through the fireball  and consumed - after dark, you can see it happen.

My little Sunstove will burn about 10 litres of these chips per hour. All I need now is an automatic feed mechanism. 



Ken.

[lightfoot]

Hi Ken,

Sorry, I've just noticed that the link I originally posted was to the wrong page on the Baxi website.  It was the likes of the hopper fed Multiheat I was on about, which can burn wood chips, pellets, grain etc.....
http://www.wolseleysbc.co.uk/active/download/PROD_DATA.pdf?type=DIRECTORY&id=131

I appreciate what you are saying with regards to their output if your needs are very small, but that's where the thermal storage comes in handy, especially with the batch boilers......and with the way things are going, maybe you could talk your neighbors into a small scale district heating scheme....or maybe they could just pop around for a bath 

Cheer's

Lightfoot.

[renewablejohn]

Ken

I wish you would not tell everybody about this energy supply as it is my cheap fuel for CHP projects.

Picking up on what Lightfoot said about corrosion you may want to look at the woodgun design which now has a stainless steel option due to the fact the original mild steel design corroded very badly.

http://www.alternateheatingsystems.com/woodboilers.htm

We have asked for the company to modify a woodgun so that we can use thermal oil instead of water as the heat transfer medium. This will then enable us to use a standard thermal oil steam evaporator to convert into steam for our steam engine.

[guydewdney]

why not use the blown air, with a venturi, as the transport mechanism?

Hopper full of chips. small hole in bottom, with 'waggle stick' in it. pipe runs from fan to pipe at bottom of hole. chips fall into pipe, get blown along pipe into fire.

[lightfoot]

Ken

I wish you would not tell everybody about this energy supply as it is my cheap fuel for CHP projects.

Picking up on what Lightfoot said about corrosion you may want to look at the woodgun design which now has a stainless steel option due to the fact the original mild steel design corroded very badly.

We have asked for the company to modify a woodgun so that we can use thermal oil instead of water as the heat transfer medium. This will then enable us to use a standard thermal oil steam evaporator to convert into steam for our steam engine.

The way things are going, it won't be long before others do a bit of joined up thinking and make the connection between what's considered a waste product in this country and a potential useful energy source.  I just hope people make best use of this resource and burn/consume it, as efficiently as possible, as any form of biomass is only sustainable as long as it can grow faster than it's used.....let's hope it doesn't get squandered like the oil and gas has, maybe the powers that be should of made the likes of high efficiency, condensing gas/oil boilers compulsory many years before they did....but then again, it's a shame we need a nanny state to hold our hands !.....sorry for the rant, I don't mean to preach to the converted and I'm not suggesting everyone should rush out and/or struggle to retro fit the latest all singing, all dancing, biomass system, but if space/funds permit, then investing in a proper biomass boiler maybe the way to go.  It would be good to see more, well thought out, district heating/CHP schemes, especially in small rural developments.

As for the corrosion issue, I think the likes of the Baxi Multiheat also uses stainless steel for some parts of the boiler too. 

John, just out of interest, why are you going the thermal oil route, is it just to keep the system working pressure down  ? .......strike that from the record, the penny has just dropped 

Cheer's

Lightfoot.

[renewablejohn]

Lightfoot

We are using thermal oil because as you say we do not want to squander the useful resource of woodchip so we will be using solar thermal as part of the duel fuel system.

[lightfoot]

Lightfoot

We are using thermal oil because as you say we do not want to squander the useful resource of woodchip so we will be using solar thermal as part of the duel fuel system.

Hi John,

I see, so are you also using solar to heat/pre-heat the water in the steam evaporator and am I right in assuming the reason you are using the thermal oil, is so you can achieve higher temps with a lower primary system pressure?  Sounds like fun, out of interest, what sort of scale are you talking about, boiler size and how many panels etc....what sort of temperatures will they have to handle ?

Cheer's

Lightfoot.


PS, Sorry Ken, I don't mean to hijack your thread 

[Ivan]

So many interesting things on this thread, where do I start?

If you are set up to use biomass (woodchips), then I guess there's no reason why you couldn't add grass clippings, hedge clippings etc. to the mix

[lightfoot]

Hi Ivan,

I guess if it was dried out enough, you could burn it in the same way as straw bales etc, but I'm not sure a standard hopper/auger fed wood chip boiler could handle it, some struggle if the wood chips are too wet or the wrong size/shape etc and there maybe possible burn back issues too.....but they do come in many different flavors.

lightfoot.


http://www.ala-talkkari.fi/?lang=3&yks=1&kax=0

[renewablejohn]

Lightfoot

The water in the steam evaporator is preheated from the steam engine condensate return. You are right the thermal oil allows a lower primary system pressure and quite frankly the thought of a conventional steam boiler system scares me.
The steam evaporator will produce 5 tonnes of steam per hour @ 150 PSI to feed a Bellis & Morcom steam engine which will generate 300kw of electric. Thermal oil temp range 250C to 300C with combined boiler output of approx 4MW and solar array of 4000 sq mtrs

Ivan

The problem with very green material is how to dry it without it turning into compost. My preferred method has been to use roofing tins as benches covered in a thin layer of green waste and kept dry in the polytunnel. The polytunnel is on a slight incline so any moisture is automatically vented out at the higher end.

[KenB]

Guy, ist

why not use the blown air, with a venturi, as the transport mechanism?

Hopper full of chips. small hole in bottom, with 'waggle stick' in it. pipe runs from fan to pipe at bottom of hole. chips fall into pipe, get blown along pipe into fire.

It would be interesting to try an air delivery system (vacuum cleaner on blow?) , but I suspect that the mass of air needed to blow the assorted woodchips into the combustion zone would exceed the optimum amount needed by the correct air fuel ratio.

It takes between 5.5 and 6.5 kg of air to burn 1kg of woody biomass.  If you can deliver this amount of air, plus the finely divided fuel into the burner, all will be well, and the science of combustion will take over.  Too much air and I think that you are going to dump too much cold nitrogen into the reaction which will carry away the heat and possibly chill the combustion zone.

If you look at a modern condensing boiler, the combustion chamber is very small - about the size of a 5 litre paint can, and that includes the primary heat exchanger and the insulation.  These combustion chambers are blown by an electronically controlled fan - capable of supplying about 130 m3 of air per hour, and modulating down to about 1/3 power.  Its the advent of reliable electronics into condensing boilers that helps them gain their overall efficiency.

If woodchip/woodgas is to gain more in efficiency terms, it too will need to employ electronic control of the air fuel mixture in order to achieve the correct stoichiometric ratio.  Fortunately you can just use the lambda sensor from a vehicle exhaust manifold to tell the system when the oxygen ratio is correct.

Woodchips are a currently underexploited fuelsource. (so is scrap wood from skips). I suggest a 2 pronged attack:

If disposal of chips is a problem, then I suggest that the tree surgeons form co-operatives and develop grid tied CHP stations - possibly of around 300 to 500kW, to which they can dispose of their green waste.   Alternatively they should also look at the work of Jean Pain, and learn how they can recover energy from their compost piles - and then still have the compost to sell.  A 40 tonne compost pile could be worth 100,000 kWh, which at 2p per kWh (thermal) is certainly worth considering.

Secondly, there is an opportunity for small sized efficient woodchip burners, similar in principle to corn/pellet stoves.



Ken