John Laswell, Notes on Burning Wood

It’s that time of year, and thoughts now turn to what’s in the wood shed.  John, a retired Engineer who has experimented with wood burning over the years has made three posts here.

You can find all three of his installments by entering Laswell into the search window in the upper right, and clicking on the posts..

One of the things I find interesting is John’s understanding of moisture in wood, and how we must use the BTUs in the wood to gas out the moisture, and how we can easily calculate how much energy we lose to the process of drying the wood before we can actually produce the gases that burn.

We see a few comments from followers of  the wood gas community about how this moisture can be a value, and the water can even become part of the fuel. Does that thought embrace scientific fact? We do know it could be fuel if we have temps like those inside our Sun.


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7 Responses to John Laswell, Notes on Burning Wood

  1. Russell says:

    Water can never be fuel. Even if the water is broken down to H2 and O2 that takes the same energy you get when it burns again.
    Unless you are talking fusion of the deuterium atoms hahaha
    I think the wood gas business is worth condsidering ( I have tons of chips) but needs critical thought

  2. Paul N says:


    I’m a long time reader but first time commenter here. I have been observing and researching the wood gas development for over a decade and think I can offer an informed comment here.

    In the discussion of wood gas versus wood burning, we need to keep in mind that our objectives are different, and so our treatment of the water might be different too.

    With wood burning for heat (which i do myself), there can be no dispute that the drier the wood, the better.

    With wood gas, the same rule applies, initially. The gasifier will indeed be more efficient -produce more btu’s per pound, with dry wood.

    But there’s more to the story than that…

    Producer gas is usually 20% H2 and about the same of CO. H2 is an excellent fuel for burning in an ICE. It has a high flame speed, it will burn across a higher range of air/fuel ratio than any other fuel gas. In the cylinder, the H2 flame is not “quenched” by the cylinder walls, as happens with gasoline/diesel (methane is less prone to this).
    CO, on the other hand, has a low flame velocity, burns over a lower range of air/fuel, gets quenched easier, and so on.

    In short, hydrogen combustion is more efficient, and results in more complete combustion and more shaft hp per btu, than CO.

    So, from the point of view of the engine, more H2 and less CO is better – and we can use the moisture to do this.

    The way you get more H2 and less CO is via the “water-gas shift” reaction. CO + H2O -> CO2 +H2 + 41kJ

    The reaction generally happens above 350C, but can happen at lower temperatures with special catalysts. Adjusting of the CO and H2 ratios is routinely done in steam reforming of natural gas for production of methanol, Fisher-Tropsch synthesis and so on.

    With woodgas, under the right conditions, you can the have hot steam reforming the CO, to produce a more hydrogen rich fuel.

    This moisture can either come from the fuel itself, or be added into the process as hot steam.

    If we are running an ICE, which sends out 1/3 of its energy as >400C exhaust, can we think of a source of >350C steam?

    Clearly, dry fuel + exhaust heated steam is the optimum way to go.

    But if we have moist fuel, what then?

    WE can have the moisture being evaporated, and going through the reactor, where it possibly cools the process and leads to excessive tar production.

    OR we can “evaporate” and remove that moisture first, to get dry wood, and then consider using that moisture to make steam for re-injection.

    The GEK gasifier pre dries the wood using the hot wood gas, a good solution.

    A more elegant one, in my opinion, is the “monorator” hopper, that was developed in WW2, and an oustanding paper on it is here (from the GEK archives)

    The monorator allows the fuel to evaporate the moisture in the hopper, and condense and collect it, rather than run it through the reactor. Thus, the reactor IS running on “dry” fuel.

    They pretty much did the tests you want, wet and dry fuel in the same gasifier. Dry fuel was always best, but monorator dried fuel was pretty close.

    Now, to get the thermodynamic advantage of higher H2 content, the evaporated moisture needs to be re-injected,a s steam, into the hot producer gas. At the cost of some complexity, you could then take the collected moisture, run it through an exhaust heated steam generator, and re-inject into the hot, producer gas, to get the water gas shift. This gas is then cooled as normal, which will condense out any remaining excess moisture from the now hydrogen rich fuel gas.

    While this would be practical for a stationary engine, at constant load, it would not be worth the trouble for a gasifier on a vehicle.

    So, shifting CO to H2 results in a better burning fuel, but certainly there is no magical production of btu’s. If heat can be supplied to this reforming process, by means of exhaust heat to make the hot steam, then we can “create” some fuel btu’s, but only about 5% more. Maybe worth it for an engine that runs 24/7, but 5% more wood is not a big deal either…

    Appreciate all your posts, and thoughts on engines. My father in Australia rebuilds old Lister and Southern Cross (an old Australian brand) diesel engines for a hobby – the things are damn near indestructible!

    Wish you were still making the PMG – I’d buy those for my micro hydro projects!



    • George B. says:

      HI Paul,

      Sorry for the delay, I just got back from a stay at Cowiche.. a good time. Thank you for your post..

      As for the wood gasifier and the wood stove, I see them as very similiar devices, we use wood for fuel, and we extract energy from both of them.

      The wood stove runs well unattended, it’s trouble free and not so expensive.. The gasifiers? Seems they are a bit more trouble to operate, and do they ever pay for themselves… that is a question, and has been for years.

      • Paul N says:

        Hi George,

        I guess I’ll have to agree to disagree that they are similar devices, as I see them for different purposes.

        Certainly burning wood for heat is easy, cheap and reliable.

        Gasifying wood for engine fuel is, by comparison, difficult, expensive and unreliable.

        So why would you do it?

        For an off-grid situation, I wouldn’t. The cost of a GEK gasifier would buy you a very good solar+small wind setup, and an engine backup.

        Where can it pay for itself? Those (relatively rare) situations when fuel is extremely expensive -e.g. remote places where it has to be flown in, brought on ice road, etc .

        The other way it can pay for itself, which is my interest, is to generate electricity for sale back to the grid. This would allow for waste wood resources – abundant in my area of BC – to be put to a profitable use. For a woodgas engine operation to produce enough electricity to be a viable business, it would need to put out 50+ kW, and ideally >100. This is greater than the current systems available, but GEK are working on a 100kW unit.

        In such a situation – an engine set up for continuous running at continuous load – then it is worth tweaking the gas quality for max efficiency, and the “steam reforming” of CO to H2 would be worth the trouble.

        There are currently no commercial systems for doing wood to electricity under 2MW(and $10m), so this would open up opportunities for for individuals and small co-ops to make a living by producing electricity.

        And I am all for the ability of people and communities to do that.



        • George B. says:

          Similiar? Carnot and all who followed him would know them that way. And they might laugh at those who buy the gassifier without asking the most basic questions. We are DIYers, so I focus on what we do, not on producer gas plants larger than houses and as old or older than our dying republic.

          We disagree you say? I quote you: “Gasifying wood for engine fuel is, by comparison, difficult, expensive and unreliable.

          So why would you do it?” Seems we have identical questions.

          I put energy into this device, and what do I get out? If we are using the GEK to run an engine, we need know how long the engine might run, and certainly how much labor is involved? If we need three hands to feed it, to change filters, and to look after it the same way a new mother looks after her baby, we need ask. If we put the same effort into turning a bell crank with our feet, will we render as much work done, and feel less tired at the days end?

          Carnot inspired Diesel to know it on paper first.. what a great mind Carnot had. Even though he had it wrong, he was mindful to measue >all< that went in, and the useful work that came out. Today I see the Chevy Volt advertisements have returned! Thoughtful people realize that each one rolled off the assembly line is paid for NOT by the owner, but from a direct transfer from our savings and pension funds. The car is just an example of all the free stuff given away to secure the vote for a party that has no plan other than to stay in power at any cost. Our Socialist Neighbor Canada knows better, they manage their debts and measure twice before they invest. The party who holds the reins here has already built their legacy, we have machines scattered across the landscape, they call them green, we tendered the payment or were forced to borrow in our name.. and the figures as to what goes in, and the work done purposely hidden from us. We sat by without a complaint! Our Media too married to the powers that be to ever ask... what went in, and what comes out, and why is it that it's kept from the public? We Americans down here need be nice to our Canadian Cousins, they will be added to the list of Landlords we make payment to... Thanks for your comments Paul, I'll continue to consider what our disagreement really is???

          • Paul N says:

            Hi George,

            In its simplest sense, our disagreement is merely about the fate of moisture in wood fuel – for a gasifier.

            I am saying that the water can be manipulated – with some effort – to actually create a better fuel. I want the moist stuff (the wood) made dry, and the dry stuff (the producer gas) made moist.

            Where we are in agreement is that gasifiers for engine fuel are not worth the trouble/time for diy-ers, in which case the discussion of moisture in the fuel/gas is purely academic.

            As with many of these things, the effort/attention needed is not that much different whether you are making 5kW or 105kW – but 105kW might just be enough for a viable business, and need not be as big as a house.

            As for the Chevy Volt, I couldn’t agree more that it is a monumental waste of effort. Better mileage could be had from a Euro style diesel vehicle. The Volt and stuff like that are ways to ease the guilt of middle class liberals – at everyones expense.

            Not sure Canada will become a landlord, but we might just become an “oil-lord”, but that would at least be better than the present ones.



  3. George B. says:

    A real interesting conversation Paul, and I remain unconvinced that moisture can help create even a BTUs worth of hydrogen over dry. I know I know.. it could be my lack of chemistry courses.. Dr. Rice or maybe Quinn Farnes might be along to break it down into simple terms so I can understand.

    As for your oil.. it’s nice to buy stuff from friends.


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