Electrically Assisted Thermal Tar Removal

I am concerned about ruining my 20hp generator from tars and acids within the woodgas flow due the downdraft flow not fully breaking them down at the beginning of the cycle or if conditions in the gasifier briefly shifted to do the same. And I would prefer to have the option of using less than ideally dry wood in larger sizes and still maintain a clean flow. The library says that temperatures above 1100C are needed to thermally break tars down. Could a thermally insulated stainless steel pipe electrically heated to 1500C guarantee a clean gas flow? If a low voltage high amperage transformer (soldering gun?) heated it, and it was held in a vertical position for easy cleaning, wouldn’t that be worth it if it worked? Thoughts?

Someone please correct me if I am wrong. I think that the tars and oils have to be run through carbon/charcoal that is at that temp to be broken down because they bind with the carbon after they are broken down.


I think various heat sources could theoretically do the job of tar cracking. But given the very high energy requirements, if this was derived from an electrical source, it would significantly reduce the system efficiency/ total energy output.

A soldering iron won’t even touch 1% of the energy requirements to crack the flow of a 20hp engine.

And even if the kilowatts of power pumped in were successful, then that excess power would have to be expelled from the system somehow, a waste.

That is why people aim to use the natural combustion process to efficiently crack the tars.


My generator is gasoline rated for 13,000 watts. Even at a bit lower output for wood gas is assumed , if 3000 of those watts were used to heat a rockwool well insulated 3" OD x 5 foot stainless pipe to 1500 C, most definitely some percentage of the tar in the gas flow through it is going to be broken down. Its just a matter whether the surface area is sufficient. Are you able to calculate the surface area at 1500 C needed for the approximate input gas flow of a 20hp generator? If not, then you’re really just guessing. I would be very willing to accept a lower overall efficiency if the lifespan of the generator would be protected.

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It would be more efficient if you just preheat the gas before entering the engine. Not to crack the tar, just get the gas above atmospheric due point. You can use the exhaust as a heat reclaim here. You will lose some power as the gas / air, fuel mixture will be expanded but the tars will be in a gas state. Much simpler to do and more efficient than adding a massive amount of heat to heat the gas to this point. This is also more efficient than adding restricting filtering systems that only rob power and will result in the gasifier crashing creating more tar production clogging up the filter even more. Filter for dust, not tar, crack the tar so you dont have to filter it.

A 500 cc engine is going to pull in around 250 cfm of just gas @ 1800 rpms. You would be better off building the reactor and reduction with better insulation to ensure you are achieving tar cracking. If you are good you can build a machine that will go beyond and fully crack all the water. If efforts will be better spent building a machine that can do this.

Use dry fuel, build a machine that can run your fuel, and use this heating method and you will have a pretty bullet proof machine I think.


Also applying heat alone I dont believe will do anything. As mentioned you need a carbon bed at 1100c to crack the tars. Its the carbon bed that needs to be 1100C not the gas.


Additionally, you should not be afraid of hurting your engine. You can completely rebuild these V Twins in a matter of hours. Regardless of fuel, these small engines are only good for a year of daily use. Generally they will just need a re ring job. Nothing to it and a full rebuild kit for most of these small engines are not very much. Worth your time and money to learn to do this once a year. That engine will last you a life time if refreshed every year.


Even if I preheated the tars to a temperature that kept them in a gaseous state, they would immediately cool when meeting the pistons, then contaminate the insides. I don’t feel good about working on the engine each year. The rate of tar breakdown is proportional to surface area, residence time, and temperature. Looking at Google Scholar, I found the following link: https://backend.orbit.dtu.dk/ws/portalfiles/portal/3427636/sevilla2000_v8-67.pdf that demonstrates no carbon is needed for tar breakdown. Their experient yielded that 1290 C for 1/2 second residence time using a low surface area tube was sufficient to removal pretty much all of the tar. Soot was the only remaining result, which could be removed afterward by a low wattage electrostatic precipitator before feeding to an engine. A vertical pipe would allow later cleaning out of the insides of the pipe. However after a bit of research, I did find that a downdraft gasifier was far better than I had guessed for tar removal, probably due to the large surface area of the bottom hot carbon/ash layer. I do still feel uneasy about relying solely on an ash layer that starts out cold before every burn. I only intend to run the generator for an hour or two a day to recharge batteries feeding an inverter. I have solar panels, but they don’t output much in winter.

No they wouldn’t, the piston is never cool, combustion ICE exhaust temps are in excess of 1200* F. The only thing cooling the piston is the air fuel mixture. If you are heating the air fuel mixture than it would be impossible for the piston to be colder than the air fuel mixture. You dont need to heat it very much, 140 to 160* F should be enough, but the engine could be subject to over heating additional cooling maybe needed


You are going to rebuild that engine regardless of fuel used. They are only good for a year full time off grid.

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They may be only good for a year full time. But if by full time you mean 24/7, over a year equals 8760 hours. If I run it for an hour a day to recharge batteries and do high power tasks, then those 8760 hours aren’t over for 8760 days, which is 24 years.

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No I mean off grid using a battery bank. My generator runs 4 to 6 hours a day, I have to rebuild them every year.


Once you learn you can have one of those engines apart and back together in a few hours. Its really not that hard or difficult to do.

You really dont want to wait for it to need the rebuild. Do this before and it will last longer, not expensive either. A $1000.00 or better generator is worth $50 bucks and a few hours of your time once year to protect that investment.


I would value that skill. Need someone to sit down and do it with me to really get it. At the moment I’m puttering around with making a small (one cubic foot) stirling engine to generate minimal amounts of electricity. Hopefully outputting 150 watts.


It is interesting they were able to crack the tars with high temps alone. In the gasification process, the chains that make up the tar are generally complex hydrocarbon molecule chains. Where these chains are simplified is at restriction just bellow the primary oxidation process. This is basically where all the heat is; as the restriction is there to basically focus as much heat generated to a local area or point of process for tar cracking or in better terms simplifying the complex chains.

The reduction zone (carbon bed) is where we take those chains and break them down thru oxygen to carbon transfer. So we may have misspoke about the process of cracking tars. In the reduction process, we are stripping out the oxygen in the chains combining with the carbon produced that makes up the carbon bed. The oxidation process produces Co2 while also liberating chemically bound H2O (steam). This process strips out the “O” making and leaving CO in the case of Co2 and making CO while leaving H2 with the hydrocarbon chains and H2O.

It would be interesting to see an induction heater. All you need to heat is the walls of the tube. Gas when flowing thru a tube does not flow in a static straight line. It flows thru rolling over and over again. Like blowing a smoke ring but much faster at these velocities.


I’ve been down this rabbit hole. Jump on Youtube and check out the videos showing repairs and rebuilds of riding lawn mower/tractor engines. There are a bunch and after watching them you will feel a lot more confident about being able to service these engines yourself.

You might also get try buying a couple used engines on the cheap for practice. If I had to rely on a backup generator running on woodgas I would want at least two running engines of the same type at any given time and extra parts / gaskets / rebuild kits for them. A third “donor” engine isn’t a bad idea either. You don’t want to ruin a brand new, expensive engine with a newly built and perhaps tarry gasifier. Better to buy used (and more than one) to learn on.

Mass market, cheap engines with well known “issues” are actually an interesting way to go. There are lots of used ones available on EBay, working or otherwise and lots of videos on youtube detailing the common problems and how to fix them. B&S Intek and Honda GX clones such as HF Predators come to mind, both commonly found on lawn tractors.

Once everything is nailed down and running well it may make sense to buy a shiny new Honda GX670 v-twin.


That makes a lot of sense. Ideally, I’d like to be able to use logs in a downdraft gasifier, followed by a 1300 C stainless pipe, followed by a low watt electrostatic precipitator, followed by a limewater solution (acid neutralization and cooling), followed by a loose furnace filter. If all that were done, hopefully the generator would last as long as it would on propane. My Honda generator is a tri-fuel model which I’ve only run on propane.

IC engine life can be counted in the number of from-cold to fully warmed up running cycles. Only a fully warmed up engine will have the design metals clearances; the lubrication distribution established; the stabilized heat zones for clean fuels combustion.
Until then . . . greater internal metals wear, and combustion products build-ups (piston top , rings lands and oil carbons deposites).
The maths of this are not as simple/transferable as you make them.
You proposal will be a sludge’d up engine in a year.
MattR’s longer-use cycles have the running time to vaporized and clear out the engine cooling down internal moisture’s deposits (crankcase acids!)

Master Auto Tech Steve Unruh


It baffles me that people are willing to spend $1000.00 to $6000.00 bucks spend weeks, months or even years building and perfecting a system as to save a $200.00 engine

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For long life and high reliability, fuel tolerance, I’d consider the Rider Ericsson hot air engines. They were best used pumping water which maximized the temperature difference, but that could be addressed. The original units are pretty rare, but once obtained could provide shaft power for a century with basic maintenance…

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