Carl’s questions pre-building a Charcoal gasifier for a 5-7hp engine

Hello everybody, I still have lots of interesting threads to catch up on, but I want to start floating a few questions of my own as I get caught up on all the ideas you all have already experimented with. I have to say I am very impressed with the work I have seen so far, and the cooperative spirit.

First off my goal is to build a small gasifier that runs on charcoal an can power a small lawn-mower type engine in the 5-7 hp range. Eventually I will make it do useful work, but that can come later. I am not terribly concerned with extended run times, but it would be nice to have 1 hour of fuel capacity. Also, since I am planning on using this as a generator - and since I have dump loads on my batteries, I will assume I am going to be running at a constant throttle setting, and not really needing to worry about fluctuating loads on the engine.

Once I get to construction, I have a metal shop with the basics: grinders, a chop saw, drill press, and an oxy-acetylene welder/cutting torch setup. I dont have any way to arc weld at the moment, so i am limited to welds of maybe 1/8" of thickness.

My first question is about the critical dimensions for the gasifier openings and the overall system ducting that delivers the gas to the engine. I found some interesting discussions in the thread on nozzles about calculating the nozzle size to get the desired “air blast velocity” - Bruce posted a chart:

My understanding of that chart was that higher velocity air in at the nozzle produces a more potent gas, (and much higher temperature).

So my questions are:

  1. Firt off, is using ducting of the same size as the carb/air intake of the engine adequate, or should it be oversized to account for friction loss in the pipe/filter/charcoal bed etc?
  2. Koen: Could you briefly explain your spreadsheet on calculating nozzle sizes (its a few posts down in that same thread)? I did not understand what all the fields were about.
    3)Anyone else who has played around with these dimensions: have people found things that work and things that dont by trial and error?

Thanks everyone!

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One inch inside diameter for “ducting” (gas hose) will work fine for 5-7 HP. Depending on your design, consider something heat resistant. Restriction at the engine (passing through a carburetor?) is more important than hose size. Adequate surface area for good flow through the filter is also more likely to affect flow than gas hose diameter. Controlling the gas /air mixture is also important, so use a gate valve for more accurate metering and better gas flow instead of ball or seat valve. An engine with electric start will make tuning in the gas/air mixture for starting a lot easier than guessing between rope pulls. Getting the right pressure to the flare burner is more important than the burner design, so use a PWM speed control on your startup fan. A fan/blower with too much pressure can push the gas through the burner so hard that it is hard to sustain a flare even with good gas.

The nozzle size is not very critical. Any size between 5/8" - 1" will work for a small engine.

Remember that the most common problem is air leaks, not duct size.

It has been said many times before, but build something simple to get experience starting and running an engine on charcoal. Give us some pictures or sketches and we can give better advice.

Charcoal gasification works with lots of different geometry. I’m looking forward to seeing which direction this black madness takes you.


You might make sure you read posting 58 in subject Questions on off grid systems By Steve Unruh. TomC

The black madness. I think that should be the official title of all charcoalers!

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I like using subpump hose for the gas line. Rather inexpensive and easy to work with. But it chafes easy so I try to protect it at wear points.

Hi Carl,

For the 5 to 7 Hp engines i use this "farmer style " setup

The nozzle size is 15 mm inside diameter

Charcoal size between 3 and 15 mm
Bottom pipe = 5 "
Top is propane tank
Filling cap is 4" pvc with Hdp cap

The piping i use is soft plastic draining pipe (under the sink or from waste water washing machine)
Piping always bigger then carburator, short lines as possible. ( i removed carburator for my purposes,) the diameter that works fine for me = 1,25"

The chart point out / works towards the desired nozzle speed at max velocity, calculating backwards from the engine demand / gas to air mixture / gasification process…
it indicates the calculated airspeed under certain conditions, to make you able to decide what nozzle size works best with your gasifier / charcoal.
My waterpump runs on full load 3200 rpm, so bigger nozzle
My generator runs at 3000 rpm but never full load… so smaller nozzle
my indicator= the molten slack formed just beyond the nozzle tip : to much slack = to hot, so larger nozzle

normal load for generator with normal carburator = between 1/2" and 3/4"
if you have very good charcoal, dry and fresh each start, then a 1" nozzle can work to


Here i use a straight 6" pipe with a 7 hp engine and a 16 mm nozzle

Hello everyone, thanks for all the good info. I am going to have to start rounding up some parts and putting together a prototype…

Is the hose you guys use rated for high temperature, or does it matter much? How hot is the gas coming out of the top of the reactor?

Bruce, this leads right into my next question:

What have people done to make an airtight seal where the nozzle tube runs through the side of the reactor? My understanding is that the nozzle is likely to end up being a consumable part, so it makes sense to me that it should be at least somewhat removable (i.e. not welded in place). It seems to me that the consensus around here is that the air jet should be oriented vertically, but I see a lot of pictures with horizontal pipes with a drilled hole for the nozzle. Any thoughts on the merits of that system vs a vertical pipe? Have people tried inlets that come through the bottom of the reactor container?

Hi Carl,

If you keep a layer of charcoal above the nozzle , about 20", then the gas outlet would be cool…anything above that layer can be consumed.

If I understand you correctly, you are saying that the charcoal fuel level has to stay about 20" above the nozzle to keep the outlet gas properly cooled? Is that the reasoning behind the long tube on the bottom of your reactor below? The propane tank on top is acting as a fuel hopper that keeps the tube filled up? What sort of run-time did you get with that quantity of charcoal?

Is it hard to clean it out once it starts to fill with ash and clinker?

My plastic hose is not rated for high temp. I tend to have some kind of cooling before plastic just to be safe.

For my 6.5hp I use a grate no wider than two inch located at bottom. I like to run at 2400rpm, sounds nice at that point.


Gary puts a pipe coupling through a piece of metal plate that is curved to match the reactor wall. The coupling is welded to the plate so that threaded nozzles can be replaced on the inside and the outside threads receive plumbing for water drip and shutoff. This curved plate is bolted to the reactor wall and sealed with stove gasket. Jeff puts a couple of inches of refractory cement in the bottom of the reactor with a vertical hole for a nozzle. I like to use 2 pipe nipples opposite one another big enough to receive my pass through copper nozzle. I bore a hole through pipe caps, slide over the nozzle ends and fill the space around the nozzle with high temp putty. As the pipe caps tighten, the putty is compressed and squeezes out a bit around the nozzle to seal. The pipe nipples can be held in place with electrical connector lock nuts and high temp sealant or welded into place. My brother’s rig has a piece of pipe welded through the side of his reactor. A fire brick with a hole bored through it slides over the pipe and serves as a long lasting nozzle. Lots of ways to skin this cat.


Hello Carl , simple fire, and simple build , you said you dont mind the run time as long as you can get 1 hour , so my suggestion would be to grab a hold of a small drum , i started with a 25 litre drum with a lid then i went to hardware store and bought some 25mm galvanased pipe fittings with nuts , i then drilled a hole in side of drum and inserted 1 of the threaded couplings and with a large 25mm nut tightened it down onto the fitting against the drum , applied some high temp silicone around the outside , i did the same to the lid as that was my gas outlet , and just used some 1 inch hose pushed down over the thread of the fitting with a clamp to get good seal and then onto a filter bucket and onto the engine , back down on the nozzle end i cast a nozzle in a tin can with a piece of pipe in the middle that was a push fit into the galv fitting screwed onto my barrel and thats it , that is all there was to making gas and running an engine for roughly 1 and a half hours hours , no welding at all and as long as you dont let it get too hot the pipe stayed ok , the lid seal i just used high temp silicone that had gone hard around the lid and pulled tight with long springs from top of drum to bottom .

But if i were to build one now i would always make it so the nozzle is pointing upwards ,even if you go in on the side try and get a 90 deg bend so that it points upwards and it will increase the lengh of time your nozzle lasts .
have lots of fun and show us all the photo’s


Hi Carl,

Yes the propane tank is the hopper.
Runtime about 1 kg per hour at full load, refill each 3 kg

Emptying: check this video :

How it glows inside :

You can start without a hopper and keep the pipe straight for your first tests, runtime about an hour or so, but same principle.
here a clip running the straight with a 2,5 kwh generator (7hp)


Hello everybody, thanks for the useful info. I am starting to get a picture in my head of what I would like to build. I will make some sketches when I get some free time - the grape harvest is not far off, so I am going to get pretty busy here for a while. I did pick up a steel ammo can (50 cal square box with gasket) that I am thinking of using as a filter. A quick search of the forum made it clear that I was not the first one to think of it. I will poke around and see if i can figure out what has been tried. I was thinking of maybe brazing/welding a cyclone filter to the bottom of it, though, and then plumbing the cyclone straight to the top of the reactor with a thin walled steel tube. I am going to have to go down to the steel yard and look for a section of 5" pipe for the hearth section of my reactor.

Koen: I was thinking about this design you posted:

Instead of wrapping the hearth section in pipe, what about wrapping it in a slightly larger sleeve? You would leave enough of a gap so that the cross section (seen from above) between the hearth pipe and the sleeve was about the same area as your nozzle. The sleeve would be sealed at the bottom, so your draft air would have to be sucked down along the hot sides - preheating your reaction air and also cooling your reactor vessel. You could plumb in a port for EGR in the bottom, but I do see that adding water might be hard to implement. What do you think of that idea? For maximum efficiency is adding steam a must?

Thanks guys, I will be in touch again as soon I can!

Hi Carl,

Any idea will work, but only will work if you have your char glowing… ( starting point)
I love not to preheat my air, since i want as much heat going in the glowing for charcoal as i can get, colder air gets more oxygen in a smaller airflow, and not blow away my charcoal with hot air and no burning…

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It seems I maybe didnt understand the function of the stainless pipe that is wrapped around the base of the RET gasifier - is that only creating steam, while cool reaction air is drawn in from directly below the nozzle?

Hi Carl,

Indeed, your last observation is correct.
For your generator however , you can do without the steam.
I used the steam to drive a trike engine. My pumps and generators run without extra steam


Okay, I made it through another grape harvest - so now I should have a little more free time. I have been working on the dump loads for my photovoltaic system, with modest success. I have made enough hot water on sunny days to take a hot shower without burning any propane, but I have had to babysit it. I want to have my electronics do all the switching so that I can simply feed power into my batteries, and not worry about overcharging them. I have some parts on order to set up a switching relay to direct power to an air heater once the hot water heater thermostat opens. We will see how that all works out…

Anyhow, I did get a little bit of work done in the shop during a rainy day, and tried building a hand-cranked blower fan out of an old gearbox that I had (off an old label gluing machine). It has about a 10.5x gear ratio

I mounted a shaft on some bearings where the motor originally sat, and fabricated up a radial bladed fan. It is about 4 1/2 inches in diameter, and the blades are maybe 1" tall. I only put on 4 out of laziness, 8 might have been better.

I made a shroud, and brazed that to the plate with the inlet, then bolted the whole thing together. It should probably be sealed with some sort of caulking, but the performance does not seem to be good enough to merit putting more work into it - it just doesnt blow very much air.

I might play around with it some more, but I also found a couple of fans at Goodwill for inflating air mattresses or whatnot, which seems like a much easier approach. I got a 12 volt model and also a more powerful 130Watt AC unit. I remember reading that it is important to be able to adjust the airflow of the blower to match the engine, can that be accomplished by reducing/throttling the opening or exit of the blower, or is it easier to adjust the speed of the fan? On DC I would think fan speed would be proportional to voltage, but can you slow down an AC fan?

I am looking forward to a couple of rainy days to read some more posts.