Doug All good comments.
Certainly getting rid of all tars in the gasifier is the "Holy Grail" of gasification. Community Power Corporation has done this, but at considerable cost. I'm not privy to their cost figures, but if $2000/kWhr of generation capacity is a low target figure, they would be at least double that.What cost figures do you think are current today? With all the charcoal that is coming and going during gasification it should be possible to filter the gas. Get your thinking caps on guys. Tom Reed Aka Dr Thomas B Reed President, The Biomass Energy Foundation www.Woodgas.com On Feb 1, 2011, at 11:36 PM, "doug.williams" <[email protected]> wrote: > Hi Tom, > > This reads like a trick question, as you obviously have something specific in > mind. > > To what extent can you clean today's gas by passing it (cooled to about 150 > C) through a barrel of tomorrows REASONABLY DRY fuel, thus permitting you to > burn all the tars AND finish drying the fuel? > > Firstly, I wouldn't do it, as once gas is made, I don't like to bring it > anywhere near filter material that can continue to discharge humidity into > the gas, just to dry the next days fuel. Yes the fuel might collect some of > the tars, but how to keep the operational temperatures of the filter mass > stable, can be severely affected by environmental and gas output conditions. > Gas humidity also carries sub-micron particulates, plus being inherently > acetic in the presence of tar, the condensate ingested into the engine will > shorten the oil and engine life. > > Of the gasifiers I have seen using incoming fuel as the filter for dirty tar > laden gas: > > 1. Thailand using rice husk in large tanks, needing to be dug out by hand. > > 2. Wood blocks used to catch tars in the first Biomass Engineering gasifier > they inherited as a project in N.Ireland as a first stage filter, but quickly > replaced as their own engineering expertise recognised it's short comings for > later projects. > > 3.Then one you already know, wood chips are used as a filter for the FEMA > emergency gasifier. > > 4. Fluidynes own early experimental filtration systems used wood shavings and > wood blocks, very nasty to clean out . Good reason to learn how to make the > gas tar free ):-) > ----------- > > I don't recommend anything that filters tar, which in any way might bring the > operators into handling and breathing vapours from tar contaminated wood. > > While a moisture content less than 20% is desirable for gasifier fuel, I > suspect it should not be bone dry for long runs because it becomes > autopyrolytic and won't retain a level pyrolysis zone. > > I will show my ignorance as never seen written or used the term > "autopyrolytic". All I can say is, that what ever it means to you, I have > never seen any evidence that oven dried fuel is detrimental to gas making in > the way that I understand in downdraft gasification. The fact that you refer > to a level pyrolisis zone, suggests that you are linking into either stoves > or open core gasifiers as the ones quoted affected by this terminology. > > The only thing that affects the zones in packed bed downdraft gasifiers with > fixed air nozzles using appropriately dried fuel, is excessive gas output > rate, which can see the pyrolysis zone shallow out as more endothemic heat is > consumed in reduction, and less radiant exothermic heat/time is available to > the incoming fuel for pyrolysis. Even using oven dry, roughly 20% of the > fuel weight is bound moisture, and H2 levels remain fairly constant, but less > or no condensate results. > > Hope this may be of assistance, and look forward to any other contributions > on the subject as you describe. > > Doug Williams. > Fluidyne.
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