Eric Wisniewski's gasifier

Got this question from Eric, and reposting it here so he can get some intelligent responses.


I have a couple of questions and would like for you to help me out. First off I would like to explain a little of my gasifier setup. I have a 420cc ~12hp generator running on the wood gas. The Gasifier has 5x 3/8" nozzles with a 4" throat. The system consist of the gasifier, a cylone, a tubular heat exchanger, a 9"x9" 30" tall filter box, silicone hose to ball valves for throttle/gas regulation, a condensation catch can, and a smaller 6"x6" filter box. The primary filter box has wood chips in it and the secondary has shredded paper. The wood I’m burning are small wood chips that typically come from a chipper shredder of which is laid out to dry before burned. The air intake enters at the top, on the side of the gasifier and travels down to a chamber which feeds the nozzles. I’m only measuring metal temperatures of the gasifer from the outside and this is what I have.

At throat area on outside shell: 350-400 deg
Gas outlet to cyclone: 140-160 deg
Cyclone out to heat exchanger: 100-120 deg
Heat exchanger out to primary filter box: 85-105 deg
Filter box out: 60-80 deg
Catch can:40-60 deg
Secondary filter box: 45-65 deg
Ambient Air Temp: 35-55 deg

Right now the biggest problem I have is tar build up on the intake valve and making it stick pretty bad. As well, I have a lot of moisture throughout the system with different amounts at each drain point, about a quart of water total after a couple of hours of run time. I also have to change out the wood in the primary filter box every couple of days because of moisture and tar build up. I’m ultimately wondering if you can help me figure out what could be improved. Here’s just some of my simple questions:

Is my throat too big?
Is my cyclone/heat exchanger not taking out enough moisture?
Should my heat exchanger be taking out more heat?
Removal/Reduction of moisture help reduce tar turning it into soot?
Is soot easier to clean/filter than tar?

One theory I have is if the throat is too big, then there’s not enough time to allow the water to convert to good wood gas. Also, I’m taking out a lot of moisture now, but I feel like I have more than I should in the first place and I can’t remove it fast enough which is why I have tar throughout the system. So, should my heat exchanger be cooling the wood gas down more than what it’s doing. These are just some of my problems I have and I much appreciate your input. I’m sorry for making this long but I want to make sure I provided enough information for you to help me.

First impression…
Ya need some restriction, I’d start with 2"
The Nozzles are to big… I would start with 3/16th and go larger from there.
Pictures and a drawing with measurements would really make it much easier to help.
All the Best

Hi EricW.
I am assuming that the engine generator will start up and run on your woodgas fuel mix? If yes. Congratulations.
Read this: then MicroGasifiers
Almost all of your answers are here.

Now what I do see from the info you’ve given is the system core is operating way too cold. In Fahrenheit you would want to see a reactor/outlet gas temperature always ABOVE 250F and reallty want 450-550F here. This will mean you are maintaining a hot enough ending gasses passed through the hot char zone gas temperature. Lots of variables temp measuring like this. Conductence of the types of metals in the housings, surface areas, outside temeratures, how much heat they are radiating, etc. Only true internal measurments could be compared.
Only maintaining actual core temperatures above min 1400-1500F will the gasification making process occur. Actual water conversion takes 2200-2400F and LOTS of heat energy. Yes you can too get this. But in a small system like this you are only ever going to have enough surplus heat to do a Very Small percentage of any the THREE possible systems internal waters.

So as tries to see what directions to rebuild to try these Operator steps first:

Always Electrically LOAD that generator in 1000’s of watts so the engine will suck more fuelgas which will in turn suck/pull the gasifier hotter. Hotter is better right to the point of internal metals destruction. Hotter WILL always mean less tars made, and those made, are broken down. Metals damage IS good. Means you were finally getting it hot enough. You can then redesign to handle this high of heat. Advice available for this.

Oven dry your fuel wood chips to reduce the free fuel moisture to a minimum fir some try runs. In Imperial/American for every 5 pounds of chips at 20% moisture will be 1 pound of water you are forcing your system to try to eat and handle. You will never have enough heat to do this well. Hint - heres your quart collected and more. When you find this gives an improvement then you can Systems set up to really pre-dry and warm ALL of your fuel wood just before dumping in. NOT that hard when you have all of that lovely nice high energy heats blowing off of the engine, engine exhaust and even the hot loaded gen head. AND even redesign your hopper area to condense out and remove the excess moisture and non-volatile heavy tars out eanrgy wise cheaply UP There instead of forcing them down through the whole system to be robbing expensive valuable core heat and later just clogging in the filters and cooling train.

ALL produced gases will be 100% humidiy wet for the temperature. ANYTIME they are cooled even in plactic piping and tubing at all, condensates will drop out. Expect at Every down area consensate will collect and pool. Need collection point drains and system design with sloping to get all of this down to the minimun number of maintanece points. AND these always with enough gasses flow bypass capability that when the condensate do low point flow and collect they do not block gases flow.
Get rid of the actual tars properly in the hot reactor and these condensates should at the most be wet and sooty at the cyclone, at the cooler but clearish condensates after the primary filter.

You will find it the most benefitual to Insulate the cyclone even if it IS a heat exchanger to keep it always above 250F. Hot and “dry” will remove more particulates. Only cool below 250F later where you can collect condensates and suspended soots readily.

Immediately ditch out of the shreded paper. Your housing will cool just enough to condense out and wet it causing gasses flow clogging. Stuff that housing for now with open cell foam or my favorite now, loosly woven/knitted true sheeps wool. Banket, sweater, whatever is actual loose real natural fiber wool. Best abilty to shingled surfaced fiber catch and hold moistuure and soots. Good heat resistance. Washable. Reusable.

Your primary filter area is going to basicly be a labyrinth impingement filering area. Means the actual filtering is gone by making the gases to twist, turn, slow down, cool and condensate drop out carrying soots and any remaining ash with them. ANYTHING porous in here will quickly suck up moistures and work no better than a non pourous material. PLAN on cooling this housing. Expect lots of soot blackened consensate wash out collecting in the bottom. Coarse wood chips if you got them for free will need to be air dried, and burn later. Small stones would work just as well. I think hard surfaced oat or wheat straw here has now proven to be superior. This housing has to be large enough NOT to add actual gases flow restriction when the filter media is wetted and at normal range of gasses flow.

Dutch John writes this ALL so-o-o-o much clearer and simpler.

Steve Unruh