Moisture Content and Efficiency

[Nofossil]

Can you break out your calculation of the heat lost for to non water vapor flue gas for me?

Also, you're right not to inlucde any energy loss to vaporize water that's a byproduct of combustion, because it is created by the combustion in gaseous form. No latent heat required.

I used 6 pounds of air per pound of bone dry wood. As kuribo pointed out, that's the amount you need for a perfect stoichiometric ratio, and our boilers certainly provide more air than that.

65 pounds of wood + 390 pounds of air gives 455 pounds of flue gas. At .24 BTU/lb, raising it from 60 degrees to 600 degrees would take about 59,000 BTU. In my case, 600 is a bit high, but perhaps I should be using outdoor temp as the starting point. In any event, I think the differences are pretty minor in the grand scheme of things.

If I use air at 1.6 x stoichiometric, the loss jumps from 59,000 to 89,000.

 

[kuribo]

As a check, knowing the EKO 80 uses roughly 150 cfm (from their test data), assume the EKO 25 uses 100 cfm, then 480 lb of air are leaving per hour. That would work out to about 62,000 btu/hr loss.....

 

[Nofossil]

As a check, knowing the EKO 80 uses roughly 150 cfm (from their test data), assume the EKO 25 uses 100 cfm, then 480 lb of air are leaving per hour. That would work out to about 62,000 btu/hr loss.....

On the 80, is that 150 cfm mounted on the boiler with the shutter adjusted? If so, that's way more than what I've been led to believe is optimum. For the EKO 80, I calculate that at full output, I'd want 105 lbs/hr of air, or about 23 cfm.

Where is this test data, by the way? Don't tell me there's data I don't have!

 

[kuribo]

Volume of air measured through the flue.....

I received the report from Zenon. I can email to you if you wish...

 

[Nofossil]

Volume of air measured through the flue.....

I received the report from Zenon. I can email to you if you wish...

Volume after combustion? That would be very different, of course....

I'd love the report.

Thanks.

 

[kuribo]

Report sent...

By the way, I think I grabbed the wrong figure from the report. The correct one to use I believe is the airflow corrected to standard conditions. This yields 88 cfm.....

 

[Nofossil]

Report sent...

By the way, I think I grabbed the wrong figure from the report. The correct one to use I believe is the airflow corrected to standard conditions. This yields 88 cfm.....

If that's 88 cfm for the EKO 80 at 275 kBTU, that would translate to 27 cfm for the EKO 25 at 80 kBTU. Not too far from the 23 that I'd calculated, and pretty reasonable, though it suggests that cranking down the secondary inlets a bit might not hurt.

 

[kuribo]

Check your mail, the report should be there....Should be lots of useful info there!

 

[jebatty]

"Good question, other than the info states that 1 lb of bone dry wood is 8600 btu, which BTW is about the same as coal, peat, grass, etc. Rule of thumb for wood burned in a stove, air dry, is about 6500 btu/lb."

I think that is a good approximation, but if you look at the literature, the figure varies widely, all else constant, with species. Hickory/locust and the like are said to have 8600 btu/lb, while alder, poplar, etc., are around 5500 btu/lb.......Pine and some of the other softwoods are around 6000 btu/lb.

Chris - take a look at you info again, and forward me your source, as I think there is an error. Wood btu's vary by density, volume, moisture content, but not my weight where m.c. is the same, based on all info I have seen. One lb of dry oak has the same btu's as one pound of aspen, although the 1 lb of oak may be 1/2 the size (volume) of the aspen. See, for example: http://www.daviddarling.info/encyclopedia/W/AE_wood_heat_value_BTU.html

 

[kuribo]

See here:

http://www.hearthwise.builderspot.com/f/Heat_Comparison.doc

 

[Eric Johnson]

I'm going to expect a complete report on all this when you guys are done geeking it all out. Extra credit will be given for plain English.

 

[wdc1160]

Hehe

 

[kuribo]

Check this out-it goes into details with formula for making a lot of the calculations being discussed here:

http://www.ruraltech.org/projects/c...ggs_ch09/briggs_chapter09_complete.asp#higher

Topics covered:

Chapter 9. Energy
Proximate and Ultimate Analysis of Wood
Higher Heating Value (HHV)
Gross Heating Value (GHV)
Energy Losses
Recoverable Heat and Combustion Efficiency
Lower Heating Value
Heating Values per Unit Volume
Calculating Btus per Cubic Foot of Solid Wood
Calculating Btus per Cord
Calculating Btus of Chips and Hog Fuel
Working with Mixtures of Wood and Bark
Heating Values of Convetional Fuels

 

[Nofossil]

Check your mail, the report should be there....Should be lots of useful info there!

Wow - pretty good stuff. A few observations:

1) Pretty high water flow rate for these tests - 34 gpm (mine is around 8 gpm).
2) Low boiler inlet temp - 130 f
3) Low boiler outlet temp - 145 f
4) Really low flue temp compared to mine - 250 f

As I read it, they were running right around 1.6 x stoichiometric. I worked it out to about 111 cubic feet of air per pound of wood. Their definition of the stoichiometric ratio is 6.2 pounds of air per pound of wood, where I've been using 6.

EDITED: (Thanks, kuribo)
They were burning small birch chips - lots of surface area, 15% moisture content. Burn rate was 42 lbs/hr (36 lbs/hr dry fuel) as I read it. If I use 8600 BTU/lb that works out to 279,000 BTU/hr at 90% efficiency vs. 263,000 BTU/hr that the EKO 80 is rated for (77 kW).

 

[kuribo]

page 8 says 15% water in the birch chips...

I think you might be high with your BTU/lb wood figure....

 

[jebatty]

Chris -- I think your sources are saying nearly the same thing I said in general terms.

http://www.hearthwise.builderspot.com/f/Heat_Comparison.doc
The key to the info is in fn 3 and 4. I don't see why different "efficiency" ratings are given to different woods, and an explanation might be the key. But if all of the examples are given the same efficiency, then btu/lb is very close for each species.

http://www.ruraltech.org/projects/c...ggs_ch09/briggs_chapter09_complete.asp#higher
The hardwoods are all within about +/- 2.5% of btu/lb, while the softwoods have about a +/- 10% range. This data also is not in agreement with the prior reference.

For some a good working average does it; nofossil has great data with some precision, which I find very interesting. Chris, your points have been well-placed.

 

[kuribo]

I agree the differences are small and seem to be attributable to differences in btu/lb of the bark and sap....

 

[kuribo]

From the above source, I thought this might be of interest. The formula and example calcs for what follows are on the site: (http://www.ruraltech.org/projects/c...iggs_ch09/briggs_chapter09_complete.asp#gross)

Heat losses needed to calculate recoverable heat and combustion efficiency:

H1. Heat used to raise the temperature of water in the wood to the boiling point.

H2. Heat required to vaporize the water in the wood.

H3. Heat required to separate the bound water (water below fiber saturation point) from the cell walls.

H4. Heat required to raise the temperature of the vaporized water to the temperature of the exhaust gases.

H5. Heat required to evaporate water that forms when the hydrogen component of wood is combusted.

H6. Heat from combustion other than water vapor: dry gases.

H7. Heat required to raise the temperature of wood to the combustion temperature.

H8. Other heat losses.

Recoverable Heat and Combustion Efficiency

The recoverable heat (RH) is obtained by subtracting the sum of these eight losses from the gross heating value.

RH = Gross Heating Value - (H1 + H2 + H3 + H4 + H5 + H6 + H7 + H8).


Combustion efficiency (CE) is the ratio of recoverable heat to available potential heat:

CE (%) = (RH / GHV) * 100.

With current technology, combustion efficiency of wood fuels ranges from about 80% for dry fuels to

about 60% for wet fuels.

 

[Nofossil]

I've read this paper a few times in the past year. I haven't been able to find an answer to a fundamental question: Is the widely held value of 8600 BTU/lb for dry wood the heat energy that you would get if you could burn it completely, or is it the theoretical energy based in the number of carbon and hydrogen atoms available to be combined with oxygen? Everything I've read infers the former, in which case losses H3 and H5 would seem to be invalid since combustion is not possible without incurring those losses. I crunched the numbers on the rest as part of the exercise of launching this thread.

I think I'll grab some quality time with my friend Jack D. and work up a loss budget based on my 20% and 30% cases as well as the EKO lab test that kuribo sent me.

Interesting point on the European EKO 80 test - they list the fuel value of birch at 15% as two different values:

1) 'Heat of combustion' at 7474 BTU/lb - scales to 8793 BTU/lb of dry wood
2) 'Caloric value' at 6742 BTU/lb - scales to 7932 BTU/lb of dry wood

Any idea what these terms mean in the context of this thread? Neither one scales up to 8600 BTU/lb for bone dry wood.

By the way, they calculate an efficiency of 90%. Losses as I interpret them:

Heat up the flue - 8% or so, presumably including water vapor
Incomplete combustion (solid and gas) - .5%
Other (heat from boiler to environment) - 1%

 

[kuribo]

Based on this:

"Proximate and Ultimate Analysis of Wood

Wood is usually converted into energy by burning. Combustion commences by evaporating the water present in the wood structure. Then combustible and noncombustible components are driven off at temperatures from 100˚ to 600˚ C. Table 9-1 gives the proximate analysis of wood and bark, showingthat about 75 to 85% of the wood can be volatilized. Carbon is oxidized in the final stage of combustion.


Combustion of wood involves two reactions:
(1) combining carbon from the wood with oxygen to form carbon dioxide and (2) combining hydrogen from the wood with oxygen to form water.

Table 9-1 also gives the chemical analysis (termed ultimate analysis) of wood and bark, showing average percentage composition of carbon, hydro­gen, and oxygen. Neither ultimate nor proximate analyses vary greatly between species.

Part of the oxygen in these reactions comes from the oxygen already present in the wood, and the rest that is needed comes from the air. The minimum total amount of oxygen and hence air needed from these two sources can be theoretically calculated, but in practice more air than the theoretical amount is admitted to ensure complete combustion. This additional air is called excess air and the amount is carefully controlled in modern combustion systems.

Higher Heating Value (HHV)

Higher heating value (HHV) is a laboratory measurement of the stored chemical energy in a fuel and is expressed in British thermal units per pound (Btu/lb), joules per kilogram (J/kg), or megajoules per kilogram (MJ/kg). Calculations in this chapter are in Imperial units. See Appendix 1 for conver­sions between energy measures.

Table 9-2 presents average higher heating values for northwestern species. Wilson et al. (1987) have provided an extensive compilation of ultimate and proximate analysis and higher heating values for northwestern species. Note that HHV is measured for oven-dry wood (zero percent moisture content).

Differences between species, between wood and bark, and variations within a species reflect differ­ences in chemical composition, especially extrac­tives. In engineering calculations, it is common to use an average HHV of 9,000 Btu/dry lb for the more resinous conifers and 8,300 Btu/dry lb for the less resinous hardwoods.

Gross Heating Value (GHV)

Since a pound of fuelwood is generally not received in the oven-dry condition, HHV does not correctly represent the available potential heat. "




I would say that the figure you are using is the HHV, which is a laboratory measurement of complete combustion of 0% moisture wood aimed at determining the total chemical energy available. So to answer your question:

"Is the widely held value of 8600 BTU/lb for dry wood the heat energy that you would get if you could burn it completely, or is it the theoretical energy based in the number of carbon and hydrogen atoms available to be combined with oxygen?"

It is both!


Your other question, about the test values not scaling to 8600BTU/LB can be answered by the possibility that they are not using 8600 as their HHV. This value moves around a bit depending on the source and species as I have indicated....It seems they are using 8793.