How much inlet air preheat is really necessary?

I am starting to plan for a light weight wood gasifier with a ceramic insulated firetube on the order of Dustin Moore and Paul McCombie’s units. I want it to be easily disassembled but the hard part is connecting the air preheat to the nozzles in a small space (I need to keep the overall dimensions down). My question is this. Did anyone ever do a study on how much preheat is necessary before you come to a point of diminishing return? On my charcoal gasifiers there is no preheat at all and they work just fine. I would like to know what difference a scale of 50 or 100 degree increments in temperature would do to overall efficiency. Trucks with gasifiers seem to run just fine shortly after start up well before the unit gets up to high preheat temps. Maybe if you run all day long like Wayne does you would see increased efficiency in fuel use. I think I remember Carl saying that his hardly gets up to temp by the time he gets to work. Any thoughts?


Don I am tending to get away from air preheat for my system. Going to Argos when my thermocouples were working I had 300 degree F air going back into the gasifier. That was on a long drive and everything was heated up. Around town, the temp doesn’t get that high and I see no difference.

I have tried, and like Dustin’s way of having a double wall hopper and running the hot gas out of the reduction area right back into the hopper. That gives you about 600 degree F which theoretically is about what is needed to change the wood to charcoal. ( dam, I can’t think of the word for that. This old age is a beach ).

In my case, I would prefer to then send the gas to a cyclone AND possibly a bag filter. As Max has presented, put a container around the cyclone/ bag filter and run the “cooled” gas through the container to preheat it for its trip to the paper air filter in the TBI. TomC


Since you’re going ceramic I guess the firetube saving purpose is gone. Saving a few chunks might also have low priority. I guess we are left with turn down ratio. You mentioned:[quote=“don_mannes, post:1, topic:2792”]
Trucks with gasifiers seem to run just fine shortly after start up well before the unit gets up to high preheat temps
Leaves us with the upper end. Less air in to create the nessessary heat - less N - richer gas - more power. But how much more? That’s the question.
Very interesting topic.


Hello Mr. Don.

I don’t think I can help much with your question. It would take a controlled experiment with identical vehicles , weather conditions , road and , fuel to get some idea of improved performance or efficiency.

It is my thinking that any heat saved and put back in the gasifier will improve efficiency and or performance . The question is where will the point of diminishing returns kick in. The available space or complexity on any vehicle may have a lot to do with the point of diminishing returns .

But if you did have preheat on the above do you think it would run worse , the same or better ??


Hi, Don!
27. of October 2016
to mess.1

Probably based on GENGAS, Vesa Mikkonen has published a few curves covering the effect of preheating on product gas’ heating value ~as translated into motorpower. ~200 — 600*Celcius. (if remembering right ~15 – 20%)

In the same bunch curves of wood humidity affecting motorpower.

Some time ago I attempted to clarify the grounds of exchanging heat back to the primary air. (among messages)

If I remember right, it would be hard to come over 40%.
And this is because of the volume difference in/out (measured cold).

With max to no preheat the gasquality goes steadily down.

The air consumption will increase with less preheat.

As less than half of the exiting gas heat can be used for preheating the primary air, the rest goes to the crows, if not using it for “baking” the wood.

This will benefit already on short trips, as it dries the wood for a comfortable start up next time.

It’s not the normal heat leakage to the silo i am talking about, that is not enough, but by Imbert mantel (excluding condensation out).
Even that is questionable “halfways”, as the condense remains in the wood, so both active silo-center heating and periphery condensation is needed to have all benefits.

To get the best gas cleaning effect at the last end of purification, after cooling-condensing, the gas has to be reheated = dried before going through a large paperfilter which is also kept warm to be functional.
That is on the warm surface of the cyclone (same envelope for the cyclone and the paperfilter).
Fast warming, works on short trips.

Never place a paper filter for woodgas in the motor compartment! It will never be dry at start-up!
In the cyclone envelope it is guaranteed!


I think your comment on my build might be what you are refering to:

"0,7m2 as preheater is excellent as such; still the incoming airflow as cool as the (cooled) exhaustgas is only about 60 % of the outgoing gas by volume.

Tobler & Schläpfer 1937 Bern, Switzerland.

This means, that theoretically you cannot “reflow” more than about 37,5% of the outgoing gas’ heat content, however big the exchange surface.
In practice, less."

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Hi, Jan-Ola!

Thank you, that is the one, now needed, hopefully!

Just curious, how in practice? Imbert mantel + WK-tubes?

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Hi, Ola!
to mess. 8

Sounds like a very hypothetical question!

Who wants to “preserve” the steam with an Imbert mantel, especially now, as the “central heating” makes it possible to forcefully drive out the steam to the periphery for condensing out on a cool outer wall?

A WK lower part does not affect the arrangement in the silo.

My description went in 2 phases!

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Not to be nit-picky DonM
. . . but I think what you are asking is “how” beneficial is primary air heating in a woodgasifier?
. . . “where” are the diminishing returns points?

Then that leads to “beneficial” to the fuelgas out engine grade strength? Or: “beneficial” to the overall wood-in used to gas volume out conversion efficiency?

The easy answers are that no air-in heating is needed at all to make usable engine fuel gas.
But you can save wood-use; use wetter inputted wood; clean gas better, by using fuel-gas out to air-in AND hopper/filtering system output gas Heat Energy Recycling.

And that, too much air in heating will prematurely heat&erosion destroy things.
Steve Unruh


This is more or less what Max is referring to as “central heating” aka “the plank”:

Plank contains hot woodgas inside, heats fuel, steam moves towards cool (single-thick) hopper walls, for maximum condensing. Add WK style hopper tubes to increase the condensing effect.


I have put out all the air preheat assembly from my sistem, dont see any difference. Well, that isnt entirely true, since the whole gasifier-cyclone-hotgasfilter asembly is in an insulated alubox, and the air intake is in the alubox, this is allso a air preheat. But what Max is saying on hot hopper systems is definitly true. The wood left ower in the hopper at stop will continue to dry above water boiling point for about 4 hours. The steam goes thrugh the system and accumulates in the cooling rails. When l light a cold gasifier the wood in it is so dry a wood chunk can be lit with a match. People in WW2 were not stupid!

As for air preheat it self, l can only see advantige in it with a gasifier that does not slip char. Think of it that way:
Wood is a carbohydrate. Simply put, it is made out of carbon and watter. The carbon to water ratio is infact ideal for gasification as every carbon molecule in 100% dry wood can reduce every molecule of water (in a ideal world) to form CO and H2. So, if one wuld put 100% dry wood in a container and heat it to 1000c, the reaction wuld leave nothing but H2 and CO in the container (exept ash). No water vapor, no left over charcoal. But furthermore, there is allso enough energy in the carbon atoms to reduce all the chemical water, if we have no energy lost.
This is what we want to come close at wood gasification. Becouse in the real world we have 20% water in our wood, we are short of carbon atoms (charcoal). No matter what we do, there will always be a abbsence of charcoal. Eaven if we wuld burn 100% dry wood, becouse we loose some energy (heat) trugh the gasifier walls, up in the hopper and the outcomeing gas is hot when leaving the gasifier. This is where the air preheat comes in (or shuld l hay energy recirculation).
So, lets say we have no heat lost trugh gasifier walls and into hopper, and all the energy (heat) from the outcomeing gas is returned in to the gasifier heart, useing 100% dry wood, we wuld have 100% conversion, 0% water in the gas and 0% left over charcoal.
We want to come as close to this as possible, but no matther what we do, eaven with ideal conditions, with sliping char we ruin that equalibrium and get more water in the gas.
Hope l make sence


There are other advantages that seem to get overlooked with air preheating. Besides bringing back heat reclaim, you are also bringing in less inert gases as they expand when heated. This can counter overheating as well; when you are really getting into high temp ranges you are then bringing in less oxygen. In a well designed system the end result is a gas that is higher energy density. Air absorbs heat very rapidly this can be seen on any forced air heating system. Our newer systems are much less elaborate than systems of the past temps are more than our temp gun can read and that is plenty to meet our goals.


Hi Don,

Interesting observation/question, you did beat me to it…

My 2 cents:
1: don’t preheat the incoming air, you need the oxygen that comes with it. and preheating that air does a bad job with being able to control.
2: try to get the heat in the fuel, before the pyrolysing comes in effect, as stated by Max and Chris.
3: use an supplemental set of nozzles, or even a central nozzle , in the reduction zone, to keep that temperature up to the good level.
4: use the condensates to temper , in a controlled way, the reduction zone. ( same as dripping water in the charcoaling , even can preheat the condensate to steam )

Et Voila: the most efficient, do-able system… :grin:

Its actually to compare with a Don Mannes type charcoaling unit on top of a Gary Gilmore charcoal gasifier…
I think that should be the first choice for FEMA… ( i actually mean that )

Edit: to ad: more then 15% increase in efficiency compared with standard conditions… (input vs power output)


Tempering the reaction zone is done automaticly in a wood gasifier, with steam from the oxidation zone. If you put aditional water in the charbed you wuld risk tar formation due to to low temperatures, plus useing wood with 20% water already has exess of water wich doesent react with charcoal. This exess water shows up in the cooler. Kind of like puting too much water injection in the charcoal gasifier. The gas will be good, but water will start to acumulate in the cooler.

There mustnt be any oxigen in the reduction zone. Perhaps you ment to put nozzles ABOVE reduction zone?
I have found this sistem is great in terms of shorter startup time, smaller chance of tar formation, but there isnt any more water cracked. Keep in mind at the top of the reduction, there already is a considerable amount of hydrogen formed from water cracking in the oxi zone. When air is introducet, along with charcoal, hydrogen also burns.


Hi Kristijan,

Let me elaborate on my words:
In order to consume the charbed, to reduce volatiles into usefull gasses, to crack tar,
you need oxydiser’s…

Steam from the oxidation zone can be an oxydiser if its been cracked into H and O2…
Thus; more moister to crack = more oxygen needed in the reduction zone to keep up the temperature

Today, actually after typing this, i will walk to my oven , where i did put different samples of charcoal in.
Processing it to 1200 degree celcius, in absence of oxygen, using Argon as shielding gas.

The relevance to gasifiers is obvious… how to connect C with O to make CO

I will come back with some more data, to give my first impressions on the results…

btw, you’r up early ?

Footnote: The amount of water in the cooler is also related to the amount of Co2 in your gas produced…
edit: I forgot, its not to ad extra water in the reduction zone, its to use the condensate to temper the reduction zone in a controlled way…
Its all about making a controllable balance
The same great stuff you’r building, with some more controls to keep it in balance…
Running a small “batch” system with some controlled “continious gizmo’s”

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“The amount of water in the cooler is also related to the amount of Co2 in your gas produced…”

A correct statement! It shuldnt be hard to calculate this corelation, given the temperature and reaction constants of hydrogen and carbon oxidation. If l remember right, hydrogen has stronger affinity to oxigen thain carbon, so there shuld be more H2O thain CO2, but the ratio shuld be constant. But only when useing 100% dry wood!

Ha, you culd say early or late… night shift

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Hi Kristijan,

Also, The chemical reaction that generates CH4, CO +H + minerals in the ashes, generates water and Co2,

If there is room for oxidation gasses to pass the reduction zone, on a thin surface, there is more heat and more moister…

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I just had a thod, do you think it wuld be possible to perform the “wood in the container” experiment l mentioned earlyer in your fancy oven? Put a peace of 100% dry wood in a sealed steel contayner in a inert atmosphere and heated to cracking temperatures?

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from my understanding of reading Vesa Mikkonen’s book there are two options that lead to the same results i.e stronger woodgas.

  1. you can add heat to the hopper to dry the wood before it goes into your hearth area
  2. preheat your air before entering the nozzles.
  3. already have very dry wood near 100% and there is no need for preheat/heated hoppers
    all contribute to the same ends.

summarized from his book

"Heating primary air up to 400C increases engine power output by 10% at the same time fuel consumption is cut by 10% "

from his graph in the book

wood being 100% dry (0% moisture content)

0 Celsius preheat = 55% effective cylinder pressure compared to petrol
200C = 57%
400C = 58%
600C = 59%

now wood being 70% dry
0C = 48%
600C = 53%

From the graph I don’t see the “10% increase for 400C preheating of air” but Mikkonen states that "power loss tends to be even more than {just the} reduction in explosion pressure.

he also states

“at the upper end of fuel solids content, engine power generation is reduced about 0.5% at every increased moisture percentage”(Mikkonen, V. Wood Gas for Mobile Applications. 2012)

So my thoughts are if you can dry and store your wood to around 10% moisture content or greater (90% dry substance content) or achieve that level of moisture in the hopper before entering the hearth you would achieve roughly the same power output as preheating your incoming air to 600C using 30% moisture content wood. Or at least that is how I interpret the graph. The furnace insulation does a very good job of insulating and protecting the metal housing. So far I have not seem any deterioration and the firetube (made out of 18gauge sheet metal shows no heat related issues. My heated hopper was a good start but by only heating the exterior we loose a lot of heat and also condensing area. Chris has it right by running the gas through the hopper.

Great topic, look forward to seeing what you come up with.