They say cracking occurs in there, Tars are cracked to methane (the monoxiding takes the oxygen and H2 are free, presume tars have some oxygens so CO2 will be in the outlet), however they forget a catalyst enhances the effect (this is why stills need to be of copper), is it feasible to add copper?
Exhaust may contain pre-heated water which can help with the gassification processes (methane), it will contain more water if the engine has diesel as a pilot fuel.
Here is my personal debate:
try a copper floor (the throat only?) or maybe a curved plate in the way of or bent tube the outlet gas, add steam? there will be steam in the exhaust if there are hydrocarbons also used. Does brass work too? may melt sooner? , may need to bend it into a flabby shape and hold it in that form, or just secure to an angled floor on which the gas is poured on. Probably could use more expensive more rigid metals from a catalytic converter. Some tars are described as acidic… (UNWGG says [coal] sulphur does damage both engine and G); Could consider using a HWS…
I think with enough heat, you can do without the catalyst. Fortunately, burning charcoal provides plenty of heat. The incandescent charcoal is also a fine catalyst. And cheap. What’s not to like?
I proposed this because tars are still present, and nothing should be remaining other than trace metals, so thinking that the tars were said to be in vapour form at some stage, i thought a catalyst must have some benefit.
Notes picked up:
There are processes available to cause tar to drip from the wood, leaving charcoal. tar components vary depending on pyrolytic process, method-duration-temperature. contains thousands of types, aromatic hydrocarbons, tar acids and tar bases. pine tar has turpentie, resin, guaiacol, creosol, methylcreosol, phenol,phlorol, toluene, xylene and other hydrocarbons. (resin content: rosin acids and retene). page 50: there are some hydrocarbons in producer gas too. (Probably from “producer gas: another fuel for motor transport” book)
In a well designed and well operated gasifier, tar is never a problem. There isnt any in the gas to begin with. Tar usualy occurs as a beginner problem, when you mess up. Load wet fuel, or wrong size, get voids in the charbed, underdraw the gasifier…
Two mechanisms get rid of tar for you. First, its oxidation. All the pirolisis gases (tar) have to pass trugh a hot, oxigen rich enviroment that is the oxidation zone. Here they burn in to CO2 and H2O wich can later be reduced down to CO and H2. But in case some tars do sneak trugh,
second is “cracking” and this proces will happen on the surface of carbon and is aided by a catalist. Luckly for us, the catalist comes prepackaged in the fuel. Its ash. At those temperatures ash on carbon surface will do the job just fine, and its self producing.
Now, again, if you did your job well first! If the gasifier runs cold or if you develop voids, wormholes, thugh wich tar can pass trugh the charbed without contacting hot charcoal, thats entirely personal fault.
Maybe i was (partly) mistaken, after another’s. Pyrolysis is the objective. But people are suggesting cracking, which is a lower temperature (which is unwanted) reaction. And gasifier people are saying cracking is happening as if it is contributive, hence why I said “use copper” (since copper is mandatory in stills as a catalyst). Pyrolysis should prevent (long) hydrocarbon exit, at all. Perhaps even methane would decompose if was hot enough.
So you’re right. Good design, Operate well, Properly feed.
Maybe a catalyst could be added to compensate if there are irrugularities in its operation, though.
It appears you have mistaken the gasification used to power engines on vehicles and stationary applications here with pyrolysis chambers used in industry. Pyrolysis gas from a pyrolysis vessel/reactor is generally nowhere near suitable to feed engines with, and is only feasible in industry with stationary engines specifically designed for burning pyrolysis gas that has been put through extensive and expensive heavy filtering that is simply not feasible for the average Joe.
Inside the downdraft wood gasifiers we use here, pyrolysis does take place in an upper zone between the stored solid fuel and the combustion zone. The pyrolysis product gases, vapors, liquids and tars then travel through this high temperature oxygen rich combustion zone and are oxidized into simpler, shorter/smaller forms of hydrocarbons. These lighter hydrocarbons are still not simple enough to burn cleanly in a piston engine so they enter the reduction zone, where the massive surface area of a high temperature carbon/ash medium cracks the hydrocarbons via reduction reactions. The result is the simplest possible reduction end product - carbon monoxide (CO) and hydrogen (H2). These will burn cleanly in an internal combustion engine, only ash and carbon soot needs to be filtered from the gas stream before entering the engine.
Pyrolysis is the raw wood above the firetube that is taking place in the hopper area. Moisture is first released with tar juices/sap and vapors cooking out of the wood. Then as the wood now partly into charcoal and brands moves down into the firetube nozzle area the temperature goes from 135 °f at the top of hopper to 800 °f at the bottom of the hopper to 2000 °f to 2500 °f in the firetube charcoal center lobe area above the restriction opening. The tars that made it to this spot is changed because of the very high intensity of the heat. This is what is called cracking the tar. The Charcoal and ash is all you need to do this and it becomes the solid by product of converting everthing to CO , H2 and methane gases. No tars should be able to make it through the hot charcoal bed of a well built gasifier running at the proper temperatures. No copper needed or anything else. There is plenty of water moisture passing through the charbed that has to be removed.