[Stoves] [biochar] Heat from combustion of TLUD gases with char remaining

[rongretlarson at comcast.net]

Two lists, Paul, Crispin , Hugh 

Below is one way to answer Paul's question more fully on the energy content of the pyrolysis gas stream. 

To Crispin's example, I add two more important (measurable) variables: X, the char yield (%w/w), and MC, the moisture content (%) 

Paul is being generous in urging use of no more than two significant figures. We may be lucky to get one with confidence. As Tom Miles has just pointed out, the energy content of the char can vary a lot - especially if there is a lot of ash content (which depends on the species as well as the max pyrolysis temperature. But you see 30 MJ/kg a lot for low ash material - and that is assumed here. 

Rather than using moisture content MC as a useful variable, in order to see the trends most easily, I assume unit increments in the initial (now moist) biomass fuel input energy content (Y in MJ/kg), starting with 18 MJ/kg for bone dry material. It would have been just as easy to vary moisture content by 5% or so for each row - but then it would be harder to see the smooth variations that exist when you start with the "nice" numbers 30 and 18 MJ/kg.. 

The basic equation for the upper matrix entries is Z1= Y - 30 * X(%); That for the lower char energy entries is Z2 = 30 * X(%). This last is (perhaps surprising until you think about it) independent of the input moisture content. 

Obviously, the energy difference from the initial 18 MJ/kg for various moisture contents comes from the moisture in wet fuel not providing any energy. 

I have not shown entries in the upper left and lower right - as they are not realistic. Since we mostly/often don't know either the moisture content or the yield very accurately, this below is only to help one get in the right ballpark. 

For sure, we should not use any single constant number for all fuels and pyrolysis processes for the energy content in the pyrolysis gases coming from one kg of fuel. I show in bold below, in the middle of the upper matrix, gas energy densities that are probably often seen. Repeating, for emphasis: these are values for an initial 1 kg of (generally not dry, and not much ash content) fuel with two major simplifying (but pretty accurate) assumptions of 18 and 30 MJ/kg for the fuel and char - and both are assumed bone dry. 

Ron 



	Matrix of Pyrolysis Gas Energy (MJ/kg wet biomass) for six possible moisture (%, in rows) and five possible char yields (%, in columns) 
	
	
	Char yield (%W); char energy of 30MJ/kg 
	Moisture % 	Fuel Energy MJ/kg 	15% 	20% 	25% 	30% 	35% 
	0.0% 	18 	
	
	10.5 	9 	7.5 
	5.6% 	17 	
	11 	9.5 	8 	6.5 
	11.1% 	16 	11.5 	10 	8.5 	7 	5.5 
	16.7% 	15 	10.5 	9 	7.5 	6 	4.5 
	22.2% 	14 	9.5 	8 	6.5 	5 	

	27.8% 	13 	8.5 	7 	5.5 	
	

	
	
	
	
	
	
	

	Matrix of Char Energy (MJ/kg wet biomass) for six possible moisture (%, in rows) and five possible char yields (%, in columns) 
	
	
	Char yield (%W); char energy of 30MJ/kg 
	Moisture % 	Fuel Energy MJ/kg 	15% 	20% 	25% 	30% 	35% 
	0.0% 	18 	
	
	7.5 	9 	10.5 
	5.6% 	17 	
	6 	7.5 	9 	10.5 
	11.1% 	16 	4.5 	6 	7.5 	9 	10.5 
	16.7% 	15 	4.5 	6 	7.5 	9 	10.5 
	22.2% 	14 	4.5 	6 	7.5 	9 	

	27.8% 	13 	4.5 	6 	7.5 	
	


----- Original Message -----
From: "Paul Anderson" <psanders at ilstu.edu> 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Cc: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>, "Hugh McLaughlin" <wastemin1 at verizon.net>, biochar at yahoogroups.com, "Doc Anderson" <psanders at ilstu.edu> 
Sent: Sunday, November 4, 2012 7:27:11 AM 
Subject: [biochar] Heat from combustion of TLUD gases with char remaining 






Stovers and Biochar-ites, 

Crispin has made a very useful comment that should not be buried under a 
Subject/Thread called Re: [Stoves] Smoke-free biomass pellet fueled stove 

So I have started a new thread and invite comments from those who know 
much more about this than I do. Can Aprovecho or CSU or ISU (Iowa) or 
U of Dayton or others confirm, adjust or reject the calculations? 

Even if the numbers change a little, I believe this could be 
important. We do want to see how it is useful. 

Paul 

Paul S. Anderson, PhD aka "Dr TLUD" 
Email: psanders at ilstu.edu Skype: paultlud Phone: +1-309-452-7072 
Website: www.drtlud.com 

On 11/4/2012 4:59 AM, Crispin Pemberton-Pigott wrote: 
> Dear Paul 
> 
> Have you decided on a favourite value for the heat of combustion from the 
> gases developed from a wood fire in the TLUD's? If you have a wood at 18.4 
> MJ/kg dry and you have (say) 20% char left at the end, what would you 
> consider the heat value per missing kg? Obviously it is not the same as the 
> initial wood. 
> 
> So I wanted to explore this with you. Let's set the moisture at 15% to be 
> typical of air-dried hot-country fuel wood. 
> 
> Starting with an LHV for the dry wood, the net heat LHV is 15.25 MJ/kg. If 
> there is 200 g of char remaining, and we treat it as being the same as 
> regular hardwood charcoal, we can assign a heat value of 29.5 * 0.2 = 5.9 MJ 
> for the remaining char. 
> 
> What remains as heat available is 9.35 MJ. This is from 800 g of missing 
> fuel so the energy average per missing gram is 9.35/0.8 = 11.69 MK/kg or 
> 11.69 Joules per gram. 
> 
> Do you agree with this approach? 
Paul Anderson inserts: I like the approach. But at best we are dealing 
with 2 digit accuracy. And it is easier to remember 11 to 12 MJ/kg = 
the pyrolytic heat from TLUD making biochar. 

Crispin continues: 
> 
> What it means is that if you put a char-making stove on a scale and run it 
> you have a value of heat per g missing from the scale. The missing mass is 
> moisture, wood gas and some of the carbon (about 1/2). 
> 
> Using this approach you can determine the net thermal efficiency of the 
> flame-to-pot+water. As the amount of char remaining is variable and not 
> known until after the test is completed, it is hard to know what the 
> performance is during the test but that is a detail. 
> 
> You can determine if the thermal efficiency has been improved should you 
> make changes to the stove body. 
> 
> For those who like details, the heat value of the char is usually unknown. 
> It would have to be homogenised and a sample tested in a bomb calorimeter to 
> know what it is exactly. As Penn Taylor pointed out, the value can range 
> from about 12 to 33 MJ/kg so the real value is going to vary from test to 
> test. 
> 
> Regards 
> Crispin 
> 
> 
> 
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