[Stoves] Calculating th LHV for Biomass and Coal

[Crispin Pemberton-Pigott]

Dear Franks

That is a very useful offer.  The importance is to determine the HHV of the
charcoal, and then the LHV based on the hydrogen content and the temperature
of the rejected gases.

If you look at a lot if charts, you will find that nearly all biomass is 50%
carbon give or take a couple of percent.  50% is a good figure for an
'average biomass' calculation.

Charcoal produced in a fire might not be made at 550 and as Dr Tom points
out, when you get is determined by the temperature and there is probably
more than one type of charcoal in a fire, even in the depth of charcoal on a
single log.  So...being realistic and practical, we need to know the carbon
content of the charcoal produced in an ordinary fire.  It has to be real
wood, not a charcoal briquette made from other things.

I managed to get a 400 degree figure for gum tree logs and it was 82.1%.
That is probably realistic, representative and liveable because the total
heat from the wood in the log is close to 'average wood'.  The money is in
the carbon not the volatiles so the heat not given to the pot by the
left-over charcoal is mostly in the carbon.

If it turns out that charcoal made in a fire from falling off pieces that do
not burn completely is about 82 or 85% carbon, then we know what the heat
content is.  I worked out 29.477 MJ/Kg LHV using a process that seems
reasonable to me.

The reason it is important is that the heat value for different woods is
known and varies, but the heat value for charcoal produce from them is
pretty much the same.  The total mass made from the wood is variable I
agree, but the heat content of the charcoal per KG is pretty constant, far
more constant than the heat from different wood species.

So...as you change from softwood to hardwood, the heat applied to the pot
changes, but the value of heat in the charcoal is about the same per kg.
Thus there is a correction to be made for this.  Actually there are a couple
of corrections to be made.

Until now people have been taking the wood burned to be the initial dry mass
minus the charcoal mass, then either giving the charcoal a certain heat
value (usually the HHV) or treating the charcoal as if it has 1.5 times the
heat content of the wood, both of which are incorrect.  As the charcoal
yield goes up, the error gets greater and greater until the reported heat
applied is several times the actual heat applied, particularly for fuels
with a low heat content like rice husks and dung. This has the effect of
reducing the apparent efficiency of the stove by anything from 3 to 70%.

If you can burn some hardwood, get some charcoal and tell me what the carbon
content is, that would be great.  I can use that figure until someone finds
a better number.

Best regards
Crispin


-----Original Message-----
From: frank [mailto:frank at compostlab.com] 
Sent: October 3, 2007 1:30 PM
To: crispin at newdawn.sz; Discussion of biomass cooking stoves
Subject: Re: [Stoves] Calculating th LHV for Biomass and Coal

Dear Crispin,

I would be glad to do what I can but you are way over my head with all 
these calculations. I have yet to take the time to work then out. And I 
still have problems with the definition of 'volatiles' in charcoal.

If it will help I could run a total carbon analysis on charcoal 
briquettes using a Leco. That heats the sample to 1100 deg. C in pure 
oxygen stream and measures the CO2.
Also we do ash values on all types natural organic materials  at 550 deg 
C in air and I could run a total carbon on samples after ashing if that 
would help.
I could ash samples covered (lack of air) at 550 deg. C for X amount of 
time then measure the total carbon.

Also; I notice when I do an acid digest and wash (to get silica value) 
of the ash (550 deg. C) there is often a lot of carbonates - the carbon 
in this inorganic fraction will show up in the total carbon analyzer 
unless you think i should acid wash the ash before to remove the 
inorganic fraction.

To help me understand the volatiles in charcoal: As I understand it 
Charcoal is heated to 550 deg. without O2. Left is carbon along with 
'contaminants' of hydrogen and some oxygen that turn into 'tars' at 
higher temperatures. Depending on what the higher temperature is 
determines the type of tar that forms. (am I close so far??) It seems 
the only way tar can be determined is to heat charcoal to 1000 deg. C 
without O2 and determine the residue  that forms. Not an easy task. We 
have a furnace that will go to the temperature but I would need special 
equipment to put the sample in and vent to the outside to cool and 
collect the condensed tars and weigh then as they come out. (volatiles 
are lost) Or I could measure the ash at 550 deg. C (covered) and heat to 
1000 deg. C (covered) and determine the loss in weight as 'tar'.
I could ash covered (flush with nitrogen) at 550 deg. C then dilute acid 
wash (remove carbonates and salts) and cover, flush with nitrogen and 
heat to 1000 deg. C.  ??? Determine weight difference.

Frank