[Gasification] Ash to land?

[Max Kennedy]

I am treating this as on topic as the responsible disposition of process waste is integral to gasification.  If the moderators disagree please let me know privately and I will not post again on the topic.  

The question of returning ash to the land from which the tree's were taken is a good one.  Chemical testing does give elevated readings of heavy metals.  It needs to be recognised that in this concentrated form this is indeed toxic and needs to be handled as such.  The unfortuneate thing is though concentrated relative to the natural environment they are not sufficiently concentrated to be processed for the metals.  Returning the materials to the land is viable however what needs to considered is area, timescale and bioavailability.  A tree took in these componenets from the environment usually through its root structure which generally extends under a significant area of land, for arguements sake lets call it 10 m2 ,  the tree did this over a time of 50 years, for the sake of arguement again, so the ash from the 1 tree could be safely spread over a similar area during a similar time period.  Unfortuneately this is unrealistic.  Another poster brought up that returning it
 to the land was the same as the tree falling and rotting (paraphrased, if I've changed your intent apologies).  This is untrue in 2 ways, time scale and bioavailability.  The rotting of a tree takes a significant time.  A few years ago I returned to a forest in BC where a tree had fallen over 40 years ago, though slumped, encroached by roots and covered in moss I could still recognise that tree.  The mound had been disturbed in 1 area, mountain bikes, and though rotted there was still recogniseable wood structures in the interior.  The slow process of recycling does not make a sudden surge of the toxic elements available all at 1 time as does the application of ash.  Additionally, many heavy metals though present are sequestered by being combined with other compounds and so is not available to be incorporated into an organism.  One such mechanism is chelating.  Think of it like lead shot.  If your swallow lead shot you will get lead poisoning.  The processes I am talking
 about, such as chelating, are like taking the lead shot and coating it with plastic then swallowing it.  Unless your do something to damage the coating, like chew on it, the plastic is inert and you do not get lead poisoning, the pellets go through you.  The rotting tree vs ash is the same.  in the rotting tree many compounds reduce/eliminate the activity of the heavy metals so even though it is present it isn't poisonous.  Burning the tree is like chewing on the plastic coated lead pellets in that it destroys the protecting compunds and makes the heavy metals available to do their nastiness.  Please accept this as a very simplified explanation, I know there is a great deal more to it but this should be sufficient for our purposes here.  Having said all this back to the question of what to do with the ash.

For small scale individual producers there should not be a problem in disposing of the ash on the land if it can be spread out over a relatively large area, by which I mean roughly equal to the landspace from which the tree's/other biomass was harvested, or made relatively unavailable through burying.  I would recommend against garden use as the toxic materials will concentrate fairly quickly.  For large scale producers landfill or treatment is necessary.  Sequestering by incorporation into other materials such as bricks should be an appropriate method of treatment.

Max K.



Dear Leland.

You raised a very good point. We've always been concerned about how to use
the carbon-ash streaming out from the gasification process. We've been
asking question about how properly it would be returning such gasification
waste to the wood where raw material come from for being gasified. Some
people say it is OK, some other people express doubts about the correct
amount to be returned because, according to them, returning all such waste
to the forest does not mean we are returning to the soil those components we
took out from it when getting the raw material for being gasified.
Please, do you have some real lead, according to your experience, on this
issue which can help me in correctly addressing this problem....?????

Thanking you in advance and at the same time looking forward to receive your
kind answer.

Kindest regards.

Oscar.


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