wood gasifiers

I've never built one but have been peripherally involved in a couple of installations, none of which ever got properly commissioned. I know a chap from Redhill that has imported one of the GEK units and will run his CS on it (with the current veggie oil fuel being the ignition source I think).

The problem is wood is not really a very good fuel, just a heat source. So there are lots of probelms getting the temperatures, fuel feed and gas velocities right all the time to minimise tars in the gasifier output.

Far more straightforward to gasify a quality coal, charcoal or coke as there is more energy per massflow to complete all the reactions.

With wood one needs to go through the sequence drying->pyrolysis->oxidation->reduction->cooling before the gas gets to the engine with coke it's just oxidation->reduction->cooling and in fact there is scope for some thermal feedback to help with the cooling. The trouble is the resultant gas (CO+N2) has a low flame speed to the engine is more heavily derated than with proper producer gas which has hydrogen present to give a higher reaction speed.

At the entrance to Amberely there was a Tangye genset (IIRC) which is complete with coal/coke gasifyer' and hand powered start up blower, and was apparently used for mobile cinemas but I doubt it is a candidate for restoration.

Not that I've anything against wood gasification for power, we had a project which ran one of the Lucas APUs on pyrolysis gas for a couple of hours but it was the culmination of a long university project and when it finished the guy in Worcester that "looked after" the APUs said he had lost them.

AJH

Ken Boak? His sustainable suburbia blog and wastewatts yahoo group are well worth looking at if you are interested in this sort of thing and ISTR he has posted a few youtube vids of his wood gas / Lister CS activity.

NHH

Yes though I wasn't aware of the videos

AJH

Yes, I suspect lots of fine-tuning would be involved! I'm blessed with a good source of wood around here, and covered space to air-dry it (which is less than ideal, I realise - some form of "forced" drying would be better - but should still produce a functional fuel source).

I'm thinking along the lines of a downdraught unit, with filtration and cooling steps upstream of the engine. Engine power of around 66% (as compared to petrol / diesel) seems achievable - which isn't too bad (I'm only interested in stationary use anyway).

*if* it all works, there's lots of scope for tinkering, I think; e.g. running an alternator from the engine to provide electrical power to automate some of the process, adding forced induction to improve efficiency, scavenging some of the waste heat to help dry the fuel supply etc. - but while there's lots of theory online and a few photos to back things up, it's hard to find anyone who's actually built something successful.

Urgh, shame - sounds like it'd be an interesting thing to see running!

Sure - it's not exactly an ideal fuel, but it'd be a fun project to try and do, and result in something that could do some real work at the end of it (part of that work involving cutting and chipping its own fuel supply, of course ;-)

cheers

Jules

--One would think some woods are better than others for conversion to useable fuel. Is there a scale or chart somewhere? Is BTU/lb the best way to determine this or are there other factors, like amount of sap in the wood, etc? Reason for thinking this: Madrone (common in California) is about the best wood I've found for firing my steamboat boiler but it has virtually no sap. Eucalyptus is very sappy, lights quick, burns hot but it yields copious quantities of soot when burned, but maybe not when 'cooked'?

Hmm, I'd be interested to see one, too - I've been trawling around, but haven't found anything yet.

This doc seems quite comprehensive:

... and compares fuel types, but doesn't go into detail about different wood species, although it does say the following:

"Most wood species have ash contents below two percent and are therefore suitable fuels for fixed bed gasifiers. Because of the high volatile content of wood, updraught systems produce a tar-containing gas suitable mainly for direct burning. Cleaning of the gas to make it suitable for engines is rather difficult and capital and labour intensive. Downdraught systems can be designed to deliver a virtually tar- free product gas in a certain capacity range when fuelled by wood blocks or wood chips of low moisture content. After passing through a relatively simple clean-up train the gas can be used in internal combustion engines."

Maybe it doesn't matter all that much, far less than wood vs. a different fuel entirely (I'd be looking at direct burning from a downdraught system

- screwing around with gas storage in a DIY context seems like a good way to end up dead :-)

cheers

Jules

Ash wood burns hot and clean when dried for a few months & this old poem might be of some help.

Woods for Burning.

Logs to burn! Logs to burn! Logs to save the coal a turn! Here?s a word to make you wise When you hear the woodsman?s cries.

Beech wood fires burn bright and clear, Hornbeam blazes too? If the logs are kept a year To season through and through.

Oak logs will warm you well If they are old and dry. Larch logs of the pine smell But the sparks will fly.

Pine is good and so is Yew For warmth through winter days, But poplar and the willow too Take long to dry or blaze.

Birch logs will burn too fast, Alder scarce at all, Chestnut logs are good to last If cut in the fall

Holly logs will burn like wax ? You should burn them green. Elm logs like a smouldering flax, No flames to be seen.

Pear logs and Apple logs They will scent a room, Cheery logs across the dogs Smell like flowers in bloom.

But Ash logs all smooth and grey, Burn them green or old, Buy up all that come your way They?re worth their weight in Gold!

Un-known.

Elder, sweet chestnut, cedar, hemlock, balsam, spruce and the pines all spit hot cinders into the air.

There's a Dutch database called phyllis but as a rule of thumb for a given moisture content there's not much variation from 18.6MJ of energy per oven dry kg of wood less about 2.7MJ per kg of water in the sample. The big differences are in density (which impacts massflow for a given throat) and in how it shrinks in the process, hardwoods generally reduce without braking into dust.

That's the theory but still plenty of knights seeking the holy grail. In fact wood from the middle of a tree has a remarkably low ash content, which is why softwood pellets from a sawmill resaw line are so good in gasifiers.

If you want turnkey then I think the Ankur gasifiers run with old Listers plus the aforementioned GEK is a good demonstrator. Of more relevance is that getting it wrong, and most do, must be a sure way of fouling up an old engine.

the gasification list on bioenergylists.org is a good starting point.

AJH