logwood gasifying boiler or woodchip boiler?

[openspaceman]

The other rub is wood is known to burn at very high temps, normally relying on a bed of ash to insulate the metal grate, or in the case of a logwood gasifier, the "grate" is ceramic and can sustain the high temps.

I am not sure how they get around this in wood-chip burners.

Do they rely on the damp (30% for domestic?) incoming chip keeping the grate cool?

 

It's not so much the wood that's burning hot, it's that the wood at the grate has turned to char, this them reacts with primary air to get the high temperature which heats the grate. Even then the grate doesn't melt, it gradually oxidises. That's why a cast iron grate lasts longer, the sites for oxidation are filled with little graphite crystals which protect the iron.

 

The grate is cooled by incoming primary air. As you say, dry wood doesn't need much primary air because it has a far higher volatiles content than a coal fire. Little primary air means little cooling effect.

[StephF]

''We could discuss why this unsuitability for stokers is the case, I guess most will realise that arb arisings contain leaves and needles that push up the moisture content but there are other things about them that need to be considered when used in a chip stoker. ''

 

I'm no scientific expert, but a local forest manager told me large power stations that burn wood do not accept green waste: eg: needles and leaves due to toxins released during burning that are not nice! I presumed this was why brash that contained leaves/needles was the reason no good for burning on boilers.

[farmer rod]

''We could discuss why this unsuitability for stokers is the case, I guess most will realise that arb arisings contain leaves and needles that push up the moisture content but there are other things about them that need to be considered when used in a chip stoker. ''

 

I'm no scientific expert, but a local forest manager told me large power stations that burn wood do not accept green waste: eg: needles and leaves due to toxins released during burning that are not nice! I presumed this was why brash that contained leaves/needles was the reason no good for burning on boilers.

 

Steph

 

I am prepared to be corrected, but I think that the green elements contain more chlorine, which can attack the insides of boilers and hence why generally unwelcome in the chip.

 

My boiler has a metal grate with primary air drawn up through holes in it, whilst Im not entirely sure, I think it was a combination of dry wood with too many fines that would slow the cooling air, but some have said it could be moist chips with an increased quantity in the hearth. Once the things alight its a bit difficult to examine whats going on. The problem manifests itself with the tipping grate sticking on a bit of slag as it opens or closes. For the last 6 months it has behaved very well (touch wood)

 

Ive just had my wood chipped for this coming year. All of the trailer loads that I have tested work out at below 36%, but in the shed the pile is steaming. I hope that this is the excess moisture rising up and the pile will cool down and be dry at the end of the process. If Ive got it wrong then Ive got 50 tonne pile of compost.

 

A lot of the wood is given to me, (collected from Arb jobs) some I pay for and I cut some on the farm, it is valuable, but more or less valuable depending where it is and how wet!

[skyhuck]

Steph

 

I am prepared to be corrected, but I think that the green elements contain more chlorine, which can attack the insides of boilers and hence why generally unwelcome in the chip.

 

My boiler has a metal grate with primary air drawn up through holes in it, whilst Im not entirely sure, I think it was a combination of dry wood with too many fines that would slow the cooling air, but some have said it could be moist chips with an increased quantity in the hearth. Once the things alight its a bit difficult to examine whats going on. The problem manifests itself with the tipping grate sticking on a bit of slag as it opens or closes. For the last 6 months it has behaved very well (touch wood)

 

Ive just had my wood chipped for this coming year. All of the trailer loads that I have tested work out at below 36%, but in the shed the pile is steaming. I hope that this is the excess moisture rising up and the pile will cool down and be dry at the end of the process. If Ive got it wrong then Ive got 50 tonne pile of compost.

 

A lot of the wood is given to me, (collected from Arb jobs) some I pay for and I cut some on the farm, it is valuable, but more or less valuable depending where it is and how wet!

 

I would be very concerned by this!!

 

If I were you I'd dig into the pile and check they are not getting hot, they could easily self combust!!

 

You may be able to stop the composting by simply turning the piles with a tractor front loader.

[farmer rod]

I would be very concerned by this!!

 

If I were you I'd dig into the pile and check they are not getting hot, they could easily self combust!!

 

You may be able to stop the composting by simply turning the piles with a tractor front loader.

 

Skyhuck , thanks I am quietly c...pping myself . I have been keeping a close eye, and things are actually calming down now, but there was a few loads that we chipped from the brash pile that got quite warm after only one day. I dug a great hole out of the pile with the loader and it cooled off straight away. Fingers crossed its all behaving at the moment

 

Rod

[openspaceman]

'

I'm no scientific expert, but a local forest manager told me large power stations that burn wood do not accept green waste: eg: needles and leaves due to toxins released during burning that are not nice! I presumed this was why brash that contained leaves/needles was the reason no good for burning on boilers.

 

Steph, I doubt many toxins survive the high temperature of a clean burning furnace, Products of Incomplete Combustion ( soot and smoke) do tend to have carcinogenic properties and are implicated in acute repiratory infections (the major cause of infant mortality in households cooking with wood in the undeveloped world). This is how smokers contract lung cancer, the sooty particles are small enough to embed in the lung tissue.

 

Also woody biomass has a fairly low mineral content and little heavy metals. So whole tree woodchip is cleaner than most coals to burn.

 

What is true of bark needles, leaves and buds is that they have higher moisture content and higher ash. Therein lies the problem for a hot furnace, because 50% of the ash is species of compounds derived from the alkali metals, Potassium, calcium and some phosphorus these end up on the grate or in the flyash ( most phosphorous compounds leave as a gas so I doubt they feature in this problem), they have a strange property in that they lower the melting point of the remaining silica in the ash. This is similar to glass making where soda is added to sand to make it melt earlier. Now this fused ash causes a few problems, one it oozes over the grate and blocks air passages as clinker and the other is that it is carried as droplets onto the heat exchanger surface where it sticks to them and cuts heat transfer.

 

This is more common on furnaces burning agricultural residues, like straw, which have much higher ash content. There are a number of fixes for this but the pragmatic one is to use crude technology and put up with the problems. This is why in the rush for NFFOs 20 years ago Fibrowatt invested in 1940s steam ship boiler designs for their power stations. Apparently one could see the slag dripping off the superheater tubes.

 

I think it is also the reason the advanced fluidised bed design at Slough Heat and Power was often broken.

 

Steph

 

I am prepared to be corrected, but I think that the green elements contain more chlorine, which can attack the insides of boilers and hence why generally unwelcome in the chip.

 

As I said the green bits do contain the trace elements but chlorine is not significant (other than in certain agri residues like beet pulp IIRC). Whilst it sounds like chlorophyll contains chlorine it doesn't. The words both drive from the Greek for green. Chlorophyll is an organic compound whose structure is similar to haemoglobin ( the compound which carries oxygen in our blood) but with a magnesium content rather than the iron in blood.

 

Whilst burning biomass is a major source of dioxin in the air I think this is from open fires and particularly seasonal burning of grassland rather than burning woodchip. Generally I don't think stoves and boilers emit much and they don't get hot enough to synthesise dioxins from mineral salt on the timber i.e. dioxins are most likely formed by incompletely burning an organic compound containing chlorine, PVC and neoprene being common culprits.

 

My boiler has a metal grate with primary air drawn up through holes in it, whilst Im not entirely sure, I think it was a combination of dry wood with too many fines that would slow the cooling air, but some have said it could be moist chips with an increased quantity in the hearth. Once the things alight its a bit difficult to examine whats going on. The problem manifests itself with the tipping grate sticking on a bit of slag as it opens or closes. For the last 6 months it has behaved very well (touch wood)

 

 

This sounds like slagging but you have also identified one of the problem of fines in the woodchip, it blocks the interstitial spaces which the primary air needs to get through to pyrolyse and gasify the wood on the grate. This also causes hot spots where newly formed char then burns out at 2000C rather than gasifying to CO at 1100C. So the grate gets hot enough to oxidise and the ash liquefies producing the slag that interferes with the grate.

 

 

Ive just had my wood chipped for this coming year. All of the trailer loads that I have tested work out at below 36%, but in the shed the pile is steaming. I hope that this is the excess moisture rising up and the pile will cool down and be dry at the end of the process. If Ive got it wrong then Ive got 50 tonne pile of compost.

 

Yes it's composting which is a form of slow combustion, as John says the spores can be a health hazard plus of course you are losing calorific value. In the days when we had big chip stockpiles for paper making it would be compacted by dozers in layers, like silage in a clamp, to squeeze out the air spaces.

 

You'll need to get the chip down below 25% and probably as low as 17% to stop these moulds working. To some extent the heat from the composting will carry other moisture away with it ( aerobic composting turns the biomass to CO2 and water vapour which rises as a saturated gas, I've never done the sums but this CO2+H2O can probably accommodate more water as it rises through the heap. Some of the conditions will be anaerobic and methane will be given off. This is why some wood chip heaps are covered in a fleece, it keeps the surface warm, so water doesn't condense and run back into the heap and allows vapour through but is not permeable to rain or dew. From the little I hear now they have limited success.

 

Apart from forced drying the best bet is to chips after a summer drying in the round. Arb arisings have to be chipped and transported green and my first move would be to screen it to remove fines <2mm and the longer >100mm if I could figure the economic means to do so.

 

Arb waste ship also tends to be poorer quality with jagged edges and this combined with the mould means the angle of repose in a silo can be negative, the silo looks full but there's an empty cavern over the sweep auger. My virtual friend Alex addresses this problem with innovative reciprocating wall and travelling augers for his feed system.

[difflock]

WoW, Openspaceman,

All good interesting very relevant stuff, apparently a bit mare:lol: to burning wood-chip than meets the eye.

m

[Odysseus3]

Hi

 

I'm new to arbtalk so hope I'm posting in right section.

 

I have just installed a 40 Kw log gasifier with a 2200 l accumalator tank. It is heating a 2200 sq ft house with 13 radiators. Was wondering what kind of length of time I should get out of my accumulator tank if I get it to 85 degrees C.

 

At the moment I get approx 4 hours where tank temp dosen't drop very much when I have just the downstairs heating on. But tank could drop to about 40-45 within 1 1/2 hours when I put on upstairs radiators as well. Would be grateful to hear other peoples experiences.

 

Thanks

[openspaceman]

Hi

 

I have just installed a 40 Kw log gasifier with a 2200 l accumalator tank. It is heating a 2200 sq ft house with 13 radiators. Was wondering what kind of length of time I should get out of my accumulator tank if I get it to 85 degrees C.

 

 

Between 85C and 45C your tank has about 100kWh of energy stored in it, so if you extract that heat over 4 hours you are taking an average of 25kW(thermal).

[difflock]

Who designed/installed your system?

What is your heat loss/heat demand?

How well insulated is your house?

How long, or how often were you told you would need to burn the boiler btw?

I am heating about 3750 sq ft of 1990's build with exactly the same set-up.

40kw/2200l tank.

It is lit once per day, at quitting time, drives underfloor for a couple of hours, and brings the accumulator tank up to temp.

Which then provides heat to drive all the heating for a couple of hours in the morning.

Cept we also run a wood burning stove in the front room.

marcus