[Stoves] Calculating th LHV for Biomass and Coal

[frank]

Dear Crispin,

Crispin Pemberton-Pigott wrote:

>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.
>
>  
>
I have tested a lot of biomass (I sent you a data pack) and I may 
suggest this be changed from biomass is 50% to 'Organic matter is 50% 
carbon' .
The amount of carbon is about 50% of the organic matter. Be it that a 
lot of woody material is mostly organic matter there are still other 
things like manure, dirty organic matter or material with a lot of 
silica that we will be working with in small fuels that the percentage 
of organic matter is much lower than woody materials.  Also; if you 
think it useful we could come up with several values for estimating 
carbon depending on the make-up of the fuel. I can help you with this 
from the data pack if you want. I think we have all the data info we need.

>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.
>  
>
What I have found from experience; when ashing a lot of samples of golf 
ball size that they turn white at different rates even when all are at 
the same temperature of 550 deg. C. Some the center of the sample stay 
black for a long time and it seems to help to take the samples out of 
the oven and let cool and re-ash to get all the carbon to turn white. 
The available O2 seems important. So I think the carbon in stove ash 
will vary depending on the stove design and air flow - and perhaps 
temperature difference.  But I noticed the high temperature does not 
necessarily mean faster  carbon reduction in the ash produced. Infarct 
I  think I have observed a temperature of 450 deg. C will produce good 
white ash faster than 550 deg. C.
Research: Is the rate ash is produced under the same design conditions 
differ with temperature.

Also: I have tested the 'plant available nutrients' in ash and found it 
to vary a lot from being a lot available to very little. I think this 
could be due to temperature of fire or type of fuel.
Research; What conditions cause some ash to be more soluble and 
extractable than other ash? (temperature, air flow, fuel type)

Also; some ash contains a lot of carbonates. Not sure if this is due to 
temperature, fuel or contaminates. But this carbon is measured in the 
Total Carbon analyzer but not likely in a calorimeter. and unlikely to 
be of any heat value. So it may need to be analyzed for carbonates and 
'calculated out' or removed before the Total Carbon is determined. The 
data pack has the carbonate values in addition to the total carbon.
Research: What causes high carbonates in ash? (temperature, fuel type, 
contaminates). Is the form of carbonates a 'waste' of energy?

So there is a lot of research that we need to do to determine the best 
condition to burn the small fuel.. And I think the stove designed for 
the small fuel is the direction we need to go. First we need to know the 
best conditions for burning the fuel type.

>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.
>  
>
I can pyrolyze some hardwood covered at 550 deg. C until I get as much 
black charcoal as possible. Then measure the Carbon content in the 
charcoal and compare to the original sample.
Will this work for you?

Frank




>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
>
>
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>
>  
>

-- 
Frank Shields
Soil Control Lab
42 Hangar way
Watsonville, CA  95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com