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[welshboy]

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[Ivan]

Just thinking aloud - I remember doing a destructive distillation experiment in school - add a few chips of wood to test tube, wave in bunsen flame. There were definitely oily residues being produced in reasonably high proportion. I think the biggest difference might be that in a gasifier, you actually start a fire with plenty of air, and once it's good and hot, shut the air down...so you'd have glowing embers in there somewhere. With pyrolysis, you don't have the very hot bit to break down the organics as much.

Aren't the pyrolysis reactions carried out under pressure, too?

[renewablejohn]

Welshboy

If you want oil from woodchip I think the process is more akin to that used to extract oil from peat. Fill a steel barrel of woodchip heat over a flame and condense the gas given off which produces a tar oil. The peat process was very big in Ireland at the middle of the 19th century.

[KenB]

John, Ivan, Welshboy,

Destructive distillation of wood is possible in a closed container in the almost absence of air.

There's a Youtube showing woodgas flaring off a coffee-tin filled with sawdust and heated over a camping stove.

If gasification is not complete, some of the volatiles will be emitted as sticky tars, often seen when you put too much damp biomass on a bonfire and get an acrid brown smoke and a lot of steam, this condenses on cold surfaces to a sticky mix of tars.

The oily tars depend on the temperature of the distillation process, just like refining oil into different fractions.  The black foul smelling liquid is sometimes called bio-oil with the heaviest gloop known as Stockholm tar - very smelly and quite toxic - used as pesticide and wood preservative.

There is a company in the Netherlands doing "flash pyrolysis" of biomass which involves a rapid pyrolysis which maximises the liquid fractions of the bio-oil.

Tars and condensibles can be further cracked by reducing them over a bed of hot charcoal.  Tars are bad news for engine valves, so much of the development of gasifiers is to produce tar free gas, where tars are cracked to smaller chain hydrocarbons, eventually ending up as CO and H2 gas depending on the water content.


Ken

[Ivan]

..and I don't suppose tars are really what you want for bio-oil either. I remember the stuff we generated in testtubes in school was black and sticky - not the sort of thing you could replace your heating oil with. It sounds like the extra effort of cracking the oil is necessary, and the whole bio-oil process sounds suitable only for carefully controlled industrial set-ups rather than DIY, which is a shame.