[Stoves] stoves and credits again

Fri Sep 22 07:36:42 CDT 2017

Dear Paul

The energy in a fuel can be estimated well using the following formula

HHV (in kJ/g) = 0.3491C + 1.41 H  - 0.1034 O + 0.1005 S - 0.0151 N - 0.0211 A

Where  C is the (mass) fraction of carbon, H of hydrogen, O of oxygen, S of sulfur, N of nitrogen, A of ash

There are other formulas in use - this one I believe comes from Tom Reed.

The conversion from HHV to LHV is standard and well known, accounting for water vapour generated by burning the hydrogen.
The compensation for moisture content is also well known involving a subtraction for the mass of water and then for heating and evaporating that moisture.

So using the above formula only you can get a pretty good estimate of the energy content of a wood or char fuel.

I developed a spreadsheet for determining the energy released during various portions of a Water Boiling Test to see how the assumption that the moisture left the fuel 'constantly' compared with the assumption that the fuel dried first and burned later. It is attached, and it uses the above formula.

You can enter fuels of your choice and burn and water loss choices to see how the energy released by the fire is accounted.

To use it, put in the dry fuel analysis, for example the Douglas Fir is entered in row 13. The moisture is entered in cell E12. That will calculate the wet basis ultimate analysis in row 12.

Do the same for the char remaining at the end in rows 15 and 16.

Rows 18-24 are the calculations provided by the WBT, at that time the UCB WBT 3.1 (uncorrected, maybe?).
Rows 28-32 are the calculations proposed where the fuel evolves during the burn. The point was to compare the energy released values on the right. The heat released calculation error is posted on the far right, in all cases the WBT over-estimates the energy released during a section of the test.

In the upper section the LHV is given for both the dry and moist fuels on a per kg basis.

You can enter any fuel, liquid or solid, just use the dry ultimate analysis as the raw data then enter the fuel moisture to get the current value. For ethanol and kerosene there might be no moisture at all.

Regards
Crispin

-----Original Message-----
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Andrew Heggie
Sent: 21-Sep-17 14:54
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] stoves and credits again

On 21 September 2017 at 18:17, Paul Anderson <psanders at ilstu.edu> wrote:
>
> 1.  Pyrolysis is around the 550 to 650 C range (unless burning very 
> hot with much forced air) 2.  The char yield is about 20% of the dry 
> weight of the biomass fuel (mainly think of wood).
> 3.  That char contains about 30%  (NOT 50%) of the ENERGY of the biomass.

Paul I have had a look at some of the early work on charcoal, I see graph by Pohl 1970 that show charcoal heated to 600C being composed 90% fixed carbon, now if you ignore the other 10% that contains carbon hydrogen oxygen and ash and you still have 20% of the original dry weight then you have 18% of the original dry weight as carbon. So if you start with dry hardwood at the 18.6MJ/kg Tom Reed allowed for it and most woody biomass and end up with 0.18 carbon at 33MJ/kg so about 32% in the fixed carbon plus a contribution from the 8% of tars etc.
less the 2-3% ash (more ash from leaves buds straw etc.) So I suspect you are more right to be at the lower end than the 50% I gave which is what I remember from charcoal made at lower temperatures for sale in barbecues.

>
> If writers wish to continue to say 50% of the ENERGY remains in TLUD-type
> charcoal, then let's resolve that here and now.   Otherwise there can  be no
> true discussion about the value of the TLUD char.

Agreed, let's see what figures others have.

Andrew

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