[Stoves] a question about the Oxygen Bomb Calorimeter

[frank]

Gary,
I hope you don't mind if I copy this to the stove list serve.  I think 
this is important info when determining calories  in different types of 
wood based on the measurements quoted.  Mark Witt from the stoves group 
gave me your name.  Thank you Mark.

 If I conclude correctly from the info you provide the  moisture in the 
wood in the calorimeter makes a difference because there will be water 
droplets left in the cell that will hold in heat but the amount of heat 
held in the 'droplets' will not be the same as the heat needed and lost 
when converting the water to vapor. So one cannot use a calorimeter to 
determine the amount of energy in a dry wood sample compared to a wet 
wood sample. It is better to add a dry wood sample to the calorimeter 
and the one ml of water (to make sure the air is saturated in the cell) 
to determine the energy from the wood and then later calculate the 
energy loss from the moisture content; as is being done. I was wondering 
if the calorimeter could be used to show the difference. To do that the 
steam produced from the moisture in the wood would need be escaped from 
the cell along with the energy it contains, then the cell sealed and the 
remaining heat produced goes into the water jacket. It the unit could be 
made to allow the steam to escape and then the cell sealed as the steam 
pressure is reduced, I think the unit would be very useful to show the 
energy in wet wood vs dry wood.

Thanks

Frank


Bertrand, Gary Lane wrote:

>Hi Frank
>   Those are good questions, and they point out the differences that will be observed in different types of calorimeters.  The wood stove is best approximated by what I call the "peanut calorimeter".  A peanut or a piece of wood to be evaluated is held in a metal coil beneath a soda can containing a measured amount of water.  The temperature of the water is recorded, then the sample is burned, and the temperature rise of the water is measured.  The temperature increase can be related to the amount of heat the can and water receive, but this is obviously less than the amount released by the sample.  However, for similar samples, there should be a similar proportionality between the heat released and the heat received.  For a sample of damp wood (if you can get it to burn), the heat released will be less than if it were dry because of the energy required to vaporize the water.  There is another effect, in that the reaction products may not be the same for wet and dry wood because the flame temperature of the wet wood will be less than that of dry wood for at least part of the burn.  I suspect the ratio of CO/CO2 from the wet wood will be more than for the dry.
>   In a bomb calorimeter, the steam from the wet wood is condensed back to water, so that the net effect of evaporation/condensation is very small.  However, wet samples have a tendency to leave more soot as a reaction product because of the repression of the flame temperature, so a fraction of the reaction product is graphite rather than CO2.  You can really see this effect when you burn sugars, which have a low heat of combustion to start with.  When I was a TA at Tulane in New Orleans, we got terrible results for sugars because of the humidity.
>    And that is related to putting the water inside the bomb.  The amount is not important, but to standardize the operation, the amount should be reproducible between the calibrating runs and the measuring runs.  You want the vapor space in the bomb to be saturated with water vapor before and after the burn.  That way all of the water produced by the reaction ends up as liquid, rather than a tiny fraction ending up as vapor.  This simply avoids some tedious, and barely significant calculations.
>   I'm glad to hear you're enjoying the simulations.
> 
>Best regards,
>Gary
>
>________________________________
>
>From: frank [mailto:frank at compostlab.com]
>Sent: Wed 11/29/2006 12:54 PM
>To: Bertrand, Gary Lane
>Subject: a question about the Oxygen Bomb Calorimeter
>
>
>
>Dear Gary,
>I am thinking about how to interpret the Oxygen Bomb Calorimeter results
>and wonder if you would help.
>I have never used one nor do I have one available to use. I have looked
>at your great site on using the calorimeter.
>
>The problem is the difference when using wet wood vs dry wood in a wood
>stove used to heat a pot of water. All the wood fuels have values listed
>from a calorimeter.
>
>As I understand it:
>It takes some of the 'wood' to heat the water in the wet wood and turn
>it into steam. Then the rest of the wood can be used to heat the water.
>I am wondering if the Oxygen Bomb Calorimeter reflects this. If you were
>to add one gram of dry wood to the calorimeter and vary the amount of
>water you add to the sample will the results be different? That is, is
>the heat held in the water in the wood counted? In the procedure you add
>one ml of water? What is the purpose of this? and is it DI water? and
>does the one ml need be exact?
>
>
>I have been enjoying your web site and will now go back to it.
>
>Thanks for your time.
>
>Frank
>
>
>--
>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
>
>
>
>
>
>
>  
>

-- 
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