I am in the process of building a pyrolysis chamber producing wood gas for use in a generator.

The process involves heating a batch of biomass (wood) in absence of oxygen (air) in a closed vessel, and separating carbon monoxide, hydrogen, and other products gaseous in NTP conditions from heavier molecules in a heat exchanger.

The output will be further filtered for particulates, but that is not important for this question:

The heat exchanger stage produces two end products: The set of volatile flammable gases, and a liquid phase containing indeterminate combination of complex hydrocarbons and a large fraction of water.

This liquid is toxic, and harmful to the environment in its initial form. What would be the easiest way to dispose of this liquid waste?

I am building this system in a remote location, so I don't have access to municipal waste water system, and I cannot transport large amounts of waste to be processed elsewhere. I can use heat to decompose the waste.

I suspect the best (and only) way is to burn the toxic hydrocarbons in a furnace. What makes this hard is the water suspension.

If injected into a hot furnace as is, I suspect the water would flash boil, cooling its surroundings a lot, and produce a gas cloud of vapor where the nasty hydrocarbons are mostly left untouched. This could probably be mitigated through careful engineering of the furnace.

If I try to boil the water off first, I will certainly also evaporate many volatile hydrocarbons, producing equally toxic gas fumes.

  • $\begingroup$ Can you use a parallel process to make some sort of charcoal to which the hydrocarbon would adsorb and the burn that? $\endgroup$
    – Andrew
    Commented Feb 4, 2022 at 1:23
  • $\begingroup$ Yes. In fact as the biomass is I intend to use is wood, bark, and leaves, I am left with biochar in the primary process. Hovever, for this method to work, I would need to trust the charcoal scrubs the liquid so completely, it is essentially clean enough to be released to nature. How could I verify that if I tried? $\endgroup$
    – Elmore
    Commented Feb 4, 2022 at 2:20
  • $\begingroup$ Test the process in a lab? Use gas or liquid chromatography to check for the contamination before and after charcoal treatment $\endgroup$
    – Andrew
    Commented Feb 4, 2022 at 23:51
  • $\begingroup$ It might help to provide more context regarding constraints such as budget, location, volume of production, applicable ordinances, etc The following gives ideas: DOI:10.3390/en13102594 $\endgroup$
    – Buck Thorn
    Commented Feb 5, 2022 at 14:33

1 Answer 1


What you are describing is an experiment similar to one which I did when I was about 12 years old. I took an old sweet tin and packed it with bits of wood (I think pine) and I had made a hole (about 5 mm) in the lid. I put the lid on as tightly as I could and then I heated it up by putting it on the red hot coals formed towards the end stage of a bonfire of garden waste. What happened was that the wood was pyrolyised and converted into a mixture of steam, hydrocarbons, some hydrogen, tars and charcoal. The gases came out of the hole and I ignited them with a flame and watched the flame burning.

What we were both doing was making something similar to coal gas but by heating wood instead.

Now there is your problem, the heat exchanger can be used to recover heat from the hot gases from the degradation of the wood, in the heat exchanger the water and the tars will condense away from the gases. The thing to be aware is that the tars from the degradation of coal tend to contain a lot of nasty carcinogens. Coal tar is well known to contain things like anilines, polycyclic aromatic hydrocarbons, benzene, thiophenes, phenols and other things.

Tars and oils from wood will be a bit different.

If you read Characterization of biomass-based flash pyrolysis oils by Kai Sipilä, Eeva Kuoppala, Leena Fagernäs and Anja Oasmaa in Biomass and Bioenergy, Volume 14, Issue 2, 23 March 1998, Pages 13-113

Then you will see how the degradation of pine wood forms formic and acetic acids along with smaller amounts of other oils. The water insoluble oil from pine was reported to be 69.3 % C, 6.4 % H, 0.2 % N and 24.1 % O (all are weight / weight %). The gas chromatography suggests that the oil was complex and packed with lots of oxygenated compounds including phenols. Some of these phenols are toxic.

I do not want to pour cold water on your ideas for renewable energy, but I worry that you might start to make a toxic mess. If you start trying to process the liquid which condenses from the heating of wood then you could expose yourself to a range of toxic substances. From reading the paper I think that most of the water soluble parts of the oil are things which would degrade with ease in soil. But the water insoluble part of the oil will be more toxic and I suspect that burning it on a fire at your site would not be a good method of dealing with it.

I think that the best way to make renewable energy from wood waste would be to do the processing on the small industrial scale where the plant can be professionally designed and a process devised which will take care of the tar. I think that the best small scale production of biofuel for a internal combustion engine would be biodiesel (FAME = Fatty Acid Methyl Esters) from waste cooking oil. Sadly that it is a bit harder than it is from unused cooking oil. But my thoughts on renewable / bio fuels are drifting a bit off topic from the question.

Sorry if my answer is a bit of a "mind dump".

  • 1
    $\begingroup$ Hmm. I should probably ask the pros in question. Oasmaa and some others mentioned in the paper you referred are still working at research center VTT that happens to be like 800m from my workplace in Espoo Finland. Small world. :D $\endgroup$
    – Elmore
    Commented Feb 5, 2022 at 8:57

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