[Stoves] Calculation help

Wed Jan 18 23:40:46 CST 2012

If you start with ice you could define the enthalpy being 0 at 0 C. You 
always work with enthalpy differences so you should choose a convenient 
zero condition.

Peter

On 19/01/2012 04:32, Frank Shields wrote:
>
> Dear Crispin,
>
> Not that this has anything to do with stoves, but if water has 0 
> enthalpy at 0 deg C and increases 4.186 j/c/g what would be the 
> enthalpy of ice? And ice at -10 deg C.? And water being the densest at 
> 4 deg C does this 4.186 j/c/g constant through the 4 deg C down to 0 
> deg C?  Just wondering.
>
> And is the LHV ever calculated using the temperature of the stack gas? 
> It seems it would not be because some water may have condensed on the 
> cold surface meaning the total water would not all be in the gas phase.
>
> Thanks for your patience.
>
> Frank
>
> Frank Shields
>
> 42 Hangar Way
>
> Watsonville,  CA  95076
>
> (831) 724-5244 tel
>
> (831) 724-3188 fax
>
> frank at bioCharlab.com
>
> *From:*stoves-bounces at lists.bioenergylists.org 
> [mailto:stoves-bounces at lists.bioenergylists.org] *On Behalf Of 
> *Crispin Pemberton-Pigott
> *Sent:* Tuesday, January 10, 2012 2:20 PM
> *To:* Stoves
> *Subject:* Re: [Stoves] Calculation help
>
> Dear Frank
>
> There is no need to over-complicate the issues for HHV.
>
> There are three stages of calculation involved:
>
> From liquid to boiling,
>
> From boiling to evaporated water at boiling
>
> From boiled vapour to hotter vapour
>
> >I realize for biomass the difference between J used in evaporation (2256
>
> j/g) and energy in water at 450c (2854 j/g) is well within the 'noise' 
> of biomass fuel so doesn't matter which one we use.
>
> The energy 'at 450' includes parts of all three segments of the 
> calculation. The number 2854 is the sum of three things. I was showing 
> how to calculate them separately.
>
> > And if we all are using the 2256 value that is what I will do.
>
> 2257 is probably the most accurate. Often people use 2260 for 
> convenience being a rounder number.
>
> >But it seems since the hydrogen in biomass never is in a liquid state 
> or  never even water vapor before going from solid state to water 
> vapor at ~450  that we should be using 2854 ((2256
>
> +(1.72 X 350)).
>
> Hydrogen is in the solid state in the fuel (not the H2 in the 
> moisture, the H2 in the biomass). It is liberated as a gas and most of 
> it burns to water, but not all. There can be quite a lot of H2 in the 
> exhaust. See this graph.  The lower line is free H2(EF) which means it 
> is the measured concentration multiplied by (Excess Air +100%). You 
> can see there is nearly a fixed ratio between the CO and the H2. This 
> is common though not universal. Even after 3 hours, there is still 
> lots of free floating H2.
>
> >For the LHV calculation of methane;  I see wiki says the HHV is 
> product of water in liquid form [and cooled to 0 degrees C] and LLV is 
> product of water in vapor form - same way you calculate biomass. [It 
> has nothing to do with the fuel type, it is how to calculate the heat 
> content of combustion products] But, of course water is never in the 
> liquid form (until it completely condenses at below 100c.). Hydrogen 
> is held by carbon or then water vapor at the combustion temp. so I am 
> surprised they calculate (estimate) it the way they do.
>
> In fact it is pretty reasonable. Lower heating value is what heating 
> can be done by the fire. That is a pretty real world attitude. Yes, it 
> has not been done exactly. But it is easy to calculate.
>
> >So I will not bother anyone with more questions, will calculate as 
> everyone does (subtract energy from water liquid to water vapor)
>
> Yes. The HHV from a bomb calorimeter minus the latent heat of 
> evaporation of the moisture from combustion of Hydrogen gives the 
> useable dry fuel heat called LHV. So far no one has come up with a 
> difference between the latent heat of evaporation and latent heat of 
> condensation. If you can create one, you will have a perpetual motion 
> heat engine!
>
> Regards
>
> Crispin
>
>
>
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