[Stoves] Is there a role for combining torrefaction and char-making stoves?

Wed Feb 29 12:43:27 CST 2012

Frank, Andrew, List 

Note this (below) is a double response - to both Frank and Andrew. 
I had short conversation today with Nat Mulcahy - who disfavors torrefaction - so I may be changing my TBD views after longer talk tomorrow. 

----- Original Message -----
From: "Frank Shields" <frank at compostlab.com> 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Sent: Tuesday, February 28, 2012 4:51:56 PM 
Subject: Re: [Stoves] Is there a role for combining torrefaction and char-making stoves? 

Stovers, 

So we have 'Wood and Biomass and we have biochar. Isn't everything in 
between torrefied biomass? 
[RWL: I don't believe the (rapidly growing) torrefaction community would want that. Nor the charcoal nor Biochar communities. I have not read any torrification or char literature that has sich a wide range. The largest range I have seen is 200 - 320 oC.; generally tighter. 

And the definition of it starts when wood gasses start leaving the biomass 
and ends when the weight loss becomes stable (to a point)? 
[RWL: I think you are probably correct on the start of the range (I don't recall seeing that but now will look more closely). The top might be when you are predominantly exiting exothermic gases (beginning a runaway process). This is near 280 or so apparently - probably dependent on species.) See Andrew's comments below - also not wanting 450 to be part of TBM. 

If not we need 
more terms for fractions not filled in by torrefied biomass. I prefer having 
several degrees of torrefaction. That because the curve is steep for weight 
loss and structural changes between 250c and 450c. Stable below 250 and 
above 450c. 
[RWL: Agree with all. ] 

Not sure how to group the lower, middle and upper values of torrefied 
biomass. Lower could be up to when the biomass snaps meaning structure is 
broken, Upper is when the volume is 25% reduced to biochar. 
[RWL: A said above, I only can agree on the "lower" being torrefaction. I think/hope the definition includes the concept of hydrophobicity. ] 

The TD sticks start at 0.69 cm wide and 15 cm long ( 10.35 cm sq)and end up 
at 650c to be 0.42 wide and 13 cm long (5.46 cm sq). 

[RWL: It would be very helpful to have other reduction numbers at (any) other T's. At 650 0C, the two dimensional changes are to 87 and 61% (not surprising - the literature is full of references to shrinkage being different with and against the grain). Your area change is to about 53%. Assuming thickness varies as width, the volume change is to 32%. I have never measured in this way, but guess that my previous tests were not so extreme - and therefore unlikely that my char-beds reached 650 oC. Any others have any similar data? Needed because I presume this is wood species dependent . I have seen shrinkage data before - and will go look for it again. In your case, they probably strive hard to keep the raw wood material the same - but there could be some additional start-off dimensional changes in rooms with different humidity. 

Just suggestions 

Controlling the process of making the torrefied material we want to be sent 
for char making seems real tricky IMO. So much happens in such a small 
temperature-time change in this range. 
[RWL: But I think it not so bad when we are striving for temperatures like 250 oC (and you have some measure of temperatures and can stop the process). Certainly there is a lot of TBM development going on for the electric utility market. With charcoaling, it is also not bad since we are (almost always??) taking (not demanding) what we get on temperature - which is at the end of the exothermic gas release period. 

Thanks for the added input. 

See few comments also back to Andrew below. Ron] 

Frank 

Frank Shields 
42 Hangar Way 
Watsonville, CA 95076 
(831) 724-5244 tel 
(831) 724-3188 fax 
frank at bioCharlab.com 

-----Original Message----- 
From: stoves-bounces at lists.bioenergylists.org 
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of 
ajheggie at gmail.com 
Sent: Tuesday, February 28, 2012 1:29 PM 
To: Discussion of biomass cooking stoves 
Subject: Re: [Stoves] Is there a role for combining torrefaction and 
char-making stoves? 

I' not sure about the attribution of the bit below but anyway: 
[RWL: Me either. I was responding, as above, to Frank. 

On Tue, 28 Feb 2012 00:32:13 +0000 (UTC), rongretlarson at comcast.net 
wrote: 

>I don't think that 450 is the right upper limit for the term "torrefaction" 
- I think even 280 oC might be too high an upper limit. Saying this because 
I think we want to keep as much energy gas in the product as possible. Some 
of the first useful gases coming out are not exothermic - and so we don't 
mind losing them. We certainly shouldn't ever mind the big water weight 
loss. 

450C is definitely the realms of charcoal, over half the dry matter has gone 
by this stage. 
[RWL: I quoted you above on this point.] 

Just because it has been given off by an endothermic process doesn't mean 
the offgas species are non combustible. 
[RWL: I didn't mean to imply it was, I believe every gas given off is combustible (except for water) . But I think we are much more willing to give up gases which are endothermic, over those that are exothermic. What I don't know enough about is the temperature regime where we can achieve hydrophobicity. 

I think anhydrous acetic acid will 
burn but not in combination with the other early species of pyrolysis, 
mostly water. 
[RWL: I can't recall where I saw it or the material, but think it was at 280 oC, where the weight loss was 89% water, and almost 5% acetic acid and nothing else above 1%. Anyway , you are correct that these gases aren't of (any or much??) help in the early stages of combusting or pyrolyzing anything. This is behind my question on whether torrefied material can be a boost for char-making stoves. I repeat that the answer is still up in the air -and I am sure to have new issues after talking tomorrow to Nat. Maybe not clear enough yet - is that I see such a push for TBM for replacing coal (meaning huge amounts likely to be in the market soon) - that the TBM may have such a good price and availability that we can also use it in char-making stoves to replace char-using stoves. With Biochar benefits for atmosphere and soils. 

Here is another possible (not sure) rationale for TBM (and would like your/anyone's opinion on this - still poorly thought out idea). It is that we have many countries where char-production is illegal - but still continues. If we are making char in stoves with the intent to use as Biochar - how can/will that char be distinguished from char made illegally? Can the shape of the TBM be enough to distinguish from illegal char? Would those illegally making char try to hide that fact by making charred pellets (still useful in char-using stoves)? Or might there be greater profit (larger output and less effort) in switching to making (legal) TBM. Should there be licensing or certification? (This to ensure the maker was not using someone else's raw material).. Any thoughts would appreciated on how the illegal (and highly wasteful) manfacture of char can be stamped out would be appreciated. 

Andrew - I still owe you additional answers, but this probably covers a lot. Ron] 

AJH 

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