[Stoves] H2O produce burning ethanol

[IPC]

Dear Crispin

I feel your "problem" arises from the fact that the LHV has the water as
vapour, NOT liquid.  So what is missing if you use the LHV and the water
vapour escapes at 100 (or 150 deg C)is the heat needed to raise the water
vapour from 20 deg C to 100 (or 150).

At 20 deg C water vapour has a heat content of 2538.2kJ/kg; at 100 deg C it
has 2676.1 kJ/kg.  So you lose out on 137.9kJ/kg or 0.14kJ/g water

1g ethanol will give rise to 54/46 = 1.174g water, so the loss you seek for
1g ethanol is 0.16kJ/g ethanol.  You will get that much back if you stir the
porridge well!

You can do the same calculation for the CO2, and you come to same sort of
answer.  You can take it up to 150 deg C, and it goes up by about 50% - but
it is still small in comparison with the LHV.  As I said, the sensible heat
of gases is small!

Here endeth Thermo 102!

Philip

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Crispin
Pemberton-Pigott
Sent: 04 November 2007 07:07
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] H2O produce burning ethanol

Dear Philip

>...Or are you using the word "exclude" in the sense of "reduced by"? 
>In which case, yes, you are correct.

That is indeed what I meant,

>That is close enough to your 1.32kJ to suggest, again, we are talking 
>the same basic language, and including the heat from condensing the 
>water produced in combustion.

Yes.

>So the heat in to the pot should be the LHV, NOT the HHV, period.  

Agreed.  What I wanted to see was a worked calculation to see if it includes
the heat 'lost' by not cooling the moisture and gases from (say) 100 to 20,
the initial ambient temperature use to calculate the HHV.

I offered the three 'reduced by' numbers I can think of:

HHV - condensing heat - hot gases - hot water, or
29.67 - 2.65 - 0.070 - 0.393 = 26.557 kJ/g of Ethanol or 20.953 kJ/ml

Including the latter two increases the 'discount' by 17.5%.

For natural gas water production would be quite a bit higher so not
including the cooling of the water would make a bigger difference.

I have read and read about how LHV's are calculated and no one ever seems to
include the heat of the water between 20 and 100 degrees.  Perhaps it is
just easier that way.

Dr Reed, Dr Bond, Dr Verhaart... anyone? Can you please comment on the issue
of whether the heat offered to the pot should be based on a 100 or 150
degree rejection temperature?

Thanks
Crispin


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