[Stoves] Comparing the fate of fuel energy between stoves -- a simple table?

Fri Jul 19 22:26:38 CDT 2013

List and Julien 

This mainly to respond to Julien below re char-making stoves for especially Bangaldesh, but of course to endorse Dr. Paul Olivier's remarks immediately below.. 

1. Assuming Bangladesh is big in rice, I would check out more of Paul O's work and justification for using rice husks with a char co-product. Of course Paul is also eloquent on the value of placing char in the ground for climate (CO2 sequestration) reasons. That should resonate more in Bangladesh than most any other country 

2. Your timing is a little off for getting help from the stove camp starting in three days at Aprovecho. But Dean Still and Dr. Paul Anderson have both got publcations answering the energy questions you have asked below (look at the Phillips stove for one). They would both recommend publications by EPA's Jim Jetter. 

3. All three would probably say that the available stove testing data says that char-making stoves will be getting about 30-40% of the energy in both the cookpot and the char, and the remainder (20-40%) is lost. This latter number is low in the stove testing world. I think all three would agree with Paul O. on the value in not combusting the char. He also emphasizes the importance of health impacts, soil improvement, carbon dioxide removal (CDR), and the good economics of saving the char, and spending less time fire-tending, etc I do also. 

4 However, it sounds like you don't need much help along those lines. Best of luck. Please let us know how your talks go. 

Ron 

----- Original Message -----
From: "Paul Olivier" <paul.olivier at esrla.com> 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Sent: Friday, July 19, 2013 5:32:24 PM 
Subject: Re: [Stoves] Comparing the fate of fuel energy between stoves -- a simple table? 

Crispin, 

We have measured the temperature of the syngas as it exits the reactor, and as the reaction proceeds from top to bottom in a TLUD, the temperature of the gas right before combustion drops significantly. Of course one could insulate the reactor, but this would put a lot of stress on the metal used to construct the reactor. I think therefore that it is safe to conclude that a lot of the heat generated as char is combusted would also be lost through the wall of the reactor. Also when char is consumed, the temperature within the reactor rises to dangerously high levels. The bottom of the reactor turns red hot, and at this high heat, metal quickly degrades. 

Suppose that the reactor is not insulated and that char begins to burn 50 cm below the pot. It is hard to imagine anything more inefficient than this. 

One of the reasons that I moved in the direction of reducing the height of the reactor was to minimize heat loss. As the reaction proceeds from top to bottom, the temperature of the syngas drops by as much as 250 C. At this lower temperature, the syngas does not combust in the same manner as at the higher temperatures. The flame is not as rich, it becomes more ethereal, and the speed at which water boils is impacted. 

So I tried to reduce the height of the reactor as much as I could, while still maintaining a burn time on loose biomass of at least 40 minutes. This led me to choose a reactor height of 50 cm for loose biomass such as rice hulls and coffee husks. But when I switched to pellets, I discovered that I could have a net reactor height of only eight inches with a burn time as long as 1.5 hours. 
https://dl.dropboxusercontent.com/u/22013094/150%20Gasifier/Short/Photos/IMG_1568.JPG 
This gives a very hot gas throughout the burn, and the reaction zone itself is much closer to the pot. But for normal cooking over a period of about 40 minutes, the net reactor height could be reduced even further to about four inches. 

With pellets we can design TLUDs that are lightweight and compact. But one thing that I have learned in this regard. Do not try to burn the biochar pellets when the burn comes to an end. The heat generated here is absolutely horrendous. These are temperatures at which a blacksmith works. 

Thanks. 
Paul 

On Fri, Jul 19, 2013 at 8:23 PM, Crispin Pemberton-Pigott < crispinpigott at gmail.com > wrote: 

Dear Julien 

I think it is a mistake (or a poor assumption) to assume that a 'small char 
fire' is a) at the bottom, b) is not an effective heat source and c) 
represents a loss of some kind. 

The distance between a heat source and a pot is not as indicative of 
anything significant. 

My rule is: Never Assume Anything. 

Calculate first, then observe, as appropriate. 

The simmering that is sometimes done in a cooking event is often well 
supplied with heat with a low power char fire at the end. 

Convective heat transfer can be very efficient. Basically the argument that 
distance matters is an assumption that most heat transfer from burning char 
is from radiant. This is nearly never true in a real fire. If the convective 
heat transfer is done with a low excess air level, it can be very good. 

Regards 
Crispin 

-----Original Message----- 
From: Stoves [mailto: stoves-bounces at lists.bioenergylists.org ] On Behalf Of 
Julien Winter 
Sent: Friday, July 19, 2013 9:08 PM 
To: Discussion of biomass cooking stoves 
Subject: [Stoves] Comparing the fate of fuel energy between stoves -- a 
simple table? 

Hello Stovers; 

I am in Dhaka, Bangladesh, and I am going into a meeting in a couple of days 
to talk about biochar production through cookstoves. I have already been 
asked a question about whether biochar-making is a cost (in fuel) that 
stove-user may not want to make. For example, "why not burn the fuel to ash 
and not save the biochar?" ... to which I reply that for a ND-TLUD, 70% of 
the fuel energy was in the gas fire, and the carbon fire is a long way from 
the pot, and a TLUD is a great deal more efficient that a traditional stove, 
and who would want a high temperature carbon fire at the bottom of one's 
cherished stove anyway. 

I realize that there are some huge ceteris paribus assumptions to be made 
about standard conditions for comparing different stove types and fuels, but 
can anybody have a crack at filling in this "simple" table? 
I just want some ballpark figures to help justify biocharmakery. 

FATE OF THE ENERGY CONTAINED IN A WOOD FUEL 

STOVE_TYPE COOKING BIOCHAR LOST 
============================================ 
----------------------- (%) 
----------------------------- 
1) ND-TLUD 
2) 3-stone 
3) Anila 
4) Chula 
5) etc. 

I have just put in some stove names as examples. The value for %energy in 
biochar is zero for many stoves, so many comparisons amounts to a comparison 
of stove efficiencies. 

As must be the case for many less well off countries, the most ecologically 
sound method of making biochar in Bangladesh will be through their 
cookstoves, and the biggest, most immediate impact of biochar on people's 
lives will be to improve the fertility of homestead gardens. That is easy 
to say; it will be a lot harder to do. 

Thanks, 
Julien. 

-- 
Julien Winter 
Cobourg, ON, CANADA 

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-- 
Paul A. Olivier PhD 
26/5 Phu Dong Thien Vuong 
Dalat 
Vietnam 

Louisiana telephone: 1-337-447-4124 (rings Vietnam) 
Mobile: 090-694-1573 (in Vietnam) 
Skype address: Xpolivier 
http://www.esrla.com/ 
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