[Stoves] Request for technology proposals - Clean Stove Initiative, Indonesia

Fri Feb 21 13:16:37 CST 2014

Dear Ron

I think there is an assumption which is not always true: that a TLUD makes
char. It is true that stoves can be designed to produce char from the fuel
placed in them - many types - but it is not the case that all TLUD's have to
make char. 

Where there is a fuel consumption metric based on the total fuel used per
cycle, whether the stove makes char or not is up to the producer. A water
boiler could have an heat transfer efficiency that was very high, make some
char, and still meet the minimum 1-Star target of an overall thermal
efficiency of 45%. 

You will perhaps recall the discussions we have had in the past where the
calculations were laid out in such a way that the energy accounting is
plainly stated so that each portion of the process is identified and the
overall raw fuel needed to achieve the result also given.

If you have a 10 litre water boiler, it will require about 3.5 MJ to be
delivered to the water container. At a fire heat to water container
efficiency of 65% it means a fire generating a bit more than 5.2 MJ. Suppose
that is 1/3 of a kg of wood pellets. 

The baseline device has an efficiency (based on raw fuel consumed) of about
17% which means it would take 1.3 kg of those same pellets to do the same
thing. 

1/3 x (65/17) = 1.3

I am not saying there is a local baseline pellet stove, just using biomass
fuel with the same energy per kg.

So if you were to achieve an overall thermal efficiency of 40%, while
actually having a heat transfer efficiency of 65%, the difference can be
produced as unburned charcoal. In fact a dedicated water boiler should be
able to achieve a heat transfer efficiency of 75% or better so this is not a
stretch.

Take a mass of raw fuel, calculate the energy content. Suppose it is 20 MJ
from 1 kg.

Transfer 40% of that to the water vessel, that is, 8 MJ to boil 22 litres of
water.

Actual heat transfer efficiency is 65%, i.e. 13 MJ available were all the
fuel to be burned.

The difference, 5 MJ, could instead be char. At a heat content of 172 g of
char or 17.2% char yield.

In this example, the water boiler would complete the boiling task, it would
receive 1 Star for overall thermal efficiency and it would also produce 172
g of char.

If the overall thermal efficiency was lower (not for this programme however)
say, 25%, the char producible would quite a bit higher: an additional 138 g
for a total of 310 (31%). I think at the moment no TLUD is produce 31% char
yield.

I have encouraged everyone for several years to make this sort of
calculation because if you have a char production target, say 18-20%, and
you have a fuel efficiency target, there is a heat transfer efficiency
target that will produce all the results simultaneously. 

I am confident that with the larger boilers, 20 litres, the heat transfer
efficiency will be in the high 80's. It is, after all, a gas flame burning
in constant, perfect circumstances each time with no need for control.

To calculate that, let's use 85% HT eff and 3.5 MJ/10 litres:

1 kg fuel @ 17MJ/kg AR (As received) total 17 MJ

2 Stars needs 55% overall thermal efficiency (Heat in the fuel v.s. heat in
the pot)

20 litres needs 7 MJ in the pot.  7/0.85 = 8.235 MJ of fire heat

2 Stars means using up to 7/.55 = 12.727 MJ of heat

Difference is 4.49 <J or 155 g of char

Mass of fuel used including making the char = 749 g with 155 g left as char
(21%)

This is technically possible.

For reference: Baseline 17% eff = 1.211 kg fuel per 10 litres boiled

Someone who built a boiler that burned the great majority of the char would
of course use far less raw fuel. Customers may prefer that. 

You were correct in your earlier comment that I proposed a 10% efficiency
gift to the char makers but that was not accepted by the programme. Maybe
next year, if there is judged to be a benefit from having the char, and
people are willing to deal with handling it - something not yet proven.

I hope this clarifies everything.

Regards
Crispn

From: Ronal W. Larson [mailto:rongretlarson at comcast.net] 
Sent: Friday, February 21, 2014 1:22 PM
To: Discussion of biomass; Crispin Pemberton-Pigott
Subject: Re: [Stoves] Request for technology proposals - Clean Stove
Initiative, Indonesia

Crispin:

            I think TLUD companies might find it interesting to enter on the
water-boiling side of this RFTP.   But it is not clear how charcoal will be
counted.  I can see at least three formulas for calculating the efficiency;
one ignoring the char, by which no TLUD could ever win if they wanted to
save the char.  The two formulae that count char as a useful co-product
could either (or both) place the char in the numerator or the denominator.
One of these formulas will make the proscribed efficiencies quite
achievable.  I can't find the intended formula(e) in what you have sent.

Ron

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