[Stoves] Calculating th LHV for Biomass and Coal

Mon Oct 1 06:59:21 EDT 2007

Dear Crispin and All:

It is an embarrassment that we (in the US) use the high heating value 
for wood and coal and low heating value for liquid fuels and natural gas!

To calculate the LHV you must subtract the heat of vaporization of H2O 
from the HHV. 

Long ago the heat of vaporization of water was 550 cal/g or (X the Mole 
wt of 18) 9900 cal/mole. Now it is (multiplying by 4.184 J/cal) 
41,422J/mole or 41.4  kJ/mole (thanks to Mr. Metric). 

In order to apply this accurately to a particular sample it is necessary 
to know the ultimate analysis values of C, H and O in order to determine 
how much water is made during combustion.  This could cost you $100-$200 
at a laboratory or $10,000 if you want to set up your own lab. 
------------------------------------------------------------------------
Fortunately, most biomass has the C-ratio formula CH1.4O0.6 and burns 
according to

CH1.4O0.6 + 1.05 (O2 + 3.76N2) ==> CO2 + 0.7 H2O + 4 N2

It has a heating value of 21 kJ/g (bone dry, and minus the ash,  
Dry-Ashfree, DAF) and a molecular weight of 23.  The hydrogen fraction 
of DAF biomass is 1.4/23 = 0.061,  so the heat remaining in the 
H2O(steam) will be (.061X41.4) 2.52 kJ/mole.

The LHV of biomass is then ~ 21-2.5 = 18.5 kJ/mole or 1.03 kJ/g, ~ 12% 
less than the HHV. good enough for a general rule and warning. 
------------------------------------------------------------------------

In Europe they use the LHV for all fuels.  As a consequence Swedish wood 
stoves have a 12% higher rating in Europe than in the US.  They are also 
more energy conscious, and sometime condense and use the water during 
biomass combustion.  So sometimes their efficiency exceeds 100% 
calculated on a LHV basis!

I hope this keeps us all honest here at stoves/gasification/REPP.

Yours truly,

TOM REED                    THE BIOMASS ENERGY FOUNDATION

------------------------------------------------------------------------

Crispin Pemberton-Pigott wrote:
> Dear Friends
>
> I have been looking unsuccessfully for a method of calculating the Lower
> Heating Value for charcoal.  As it has little hydrogen in it, there will be
> little water formed and therefore not as much heat lost 'not condensing it'
> compared with burning wood.
>
> The reason for this search is our attempt to get an accurate method of
> calculating the heat yielded by a wood fire.
>
> The progress so far is as follows:
>
> The useful heat yielded by burning ordinary biofuels has these main
> considerations:
>
> Heat yielded by burning the actual dry wood content of the fuel consumed
> Heat absorbed by heating and boiling off the moisture in the fuel
> Heat lost by not burning the charcoal which remains in the fire
> Latent heat lost by not condensing the combustion products
> Heat lost by not considering the cooling of combustion products to ambient
> temperature
>
> This will give us the Lower Heating Value (LHV) of the fuel.  From that the
> dry wood equivalent can be calculated, and the actual efficiency of any
> portion of a stove test calculated accurately.
>
> There is more than one way to calculate the LHV.  One method is to subtract
> 1.32 MegaJoules from the HHV.  This approach compensates for the latent heat
> lost when the hot exhaust gases pass away from the pot without cooling below
> 100 degrees C.  This method is used in the UCB Water Boiling Test.  
>
> The definition provided at http://en.wikipedia.org/wiki/Lower_heating_value
> says that the combustion products are cooled to 150 C which seems to be the
> lowest useful temperature in a non-condensing application such as a stove.
>
> The HHV assumes the original starting temperature is also the ending
> temperature and all heat liberated during combustion is counted.
> Subtracting only 1.32 MJ does not represent the entire loss so it is more
> accurate to subtract whatever heat is not really available to a pot during
> cooking.  This will give an LHV that represents '100% of the available
> heat'. A stove can then be tested against that figure.  
>
> A method seeking to achieve this is to subtract 1.39 MJ from the HHV to
> account the heat content of water vapour at 100 C.  There is a description
> of this approach in the appendix of the full text of the UCB-WBT as
> recommended by Baldwin. It appears in footnote 5, page 21 of
> http://ceihd.berkeley.edu/Docs/WBT_Version_3.0_Jan2007.pdf . It seems this
> approach is attempting to reflect in the LHV the actual heat available to a
> pot.  It is not the standard approach to calculating LHV, however it is more
> realistic and I like it for that reason.  The use of the temperature 150 C
> as done by the American Petroleum Institute will revise the 1.39 figure
> upwards.
>
> Dry Wood Equivalent
> Calculating the dry wood equivalent of a given mass of fuel  looks like
> this:
>
> Total mass of fuel minus the water content
> Times
> The lower heating value (LHV) for that type of fuel
> Minus
> Energy needed to heat the moisture in the fuel to the local boiling point
> Minus
> Energy needed to boil the moisture off
> Minus
> The energy in the remaining charcoal, if any
> All divided by
> The LHV for that type of fuel
>
> The answer is a fraction which represents the dry wood equivalent mass of
> the initial moist fuel mass.
>
> HHV or LHV?
> The 'heat value for that type fuel' (above) should be the LHV calculated in
> a fashion that represents reality.
>
> The energy remaining in the charcoal is the HHV, because that is how much
> energy is in it, but it isn't really fair to use that because the HHV is not
> attainable in a stove.  Thus began a search for an LHV for charcoal.
>
> The Higher Heating Value of charcoal is about 29.5 KG/Kg. Matthew Owen from
> Chardust, a knowledgeable man, says charcoal is 'over 30 MJ/Kg'.  The number
> 29.5 is used in CDM calculations such as the Arvind Textiles project
> http://cdm.unfccc.int/UserManagement/FileStorage/3QU14POMQ9R8Q1LSC8QX7C0U55H
> ZHC 
>
> Another value is 29.77 sourced from
> http://www.extension.iastate.edu/AGDM/wholefarm/html/c6-86.html
>
> LHV for stove charcoal is clearly not 29.77-1.39 MJ/Kg because it does not
> have the same chemical composition as wood.  As we will have to use an
> 'average charcoal' produced in a fire as the figure for a practical general
> case formula, the composition should be based on whatever that chemistry is
> on average.
>
> So..how does one calculate the LHV for average charcoal for a cooking to 150
> C?
>
> Thanks
> Crispin
> _______________________________________________
> Stoves mailing list
> Stoves at listserv.repp.org
> http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org
> http://stoves.bioenergylists.org
> http://info.bioenergylists.org
>
>
>   

-- 
ÐÏà¡±á