CO2 emissions of wood burning.

[EbS-P]

I have been meaning to run some CO2 per1k btu numbers for wood vs heat pump. This is what I have so far.

1kg of wood nets 10k btus. Emissions of burning that let’s call 1.5kg?? I need a decent source. https://www.kaltimber.com/blog/2017/6/19/how-much-co2-is-stored-in-1-kg-of-wood

So .15kg/1k btus.

Im going to assume .33kg per Kwh and a heatpump COP of 3
https://app.electricitymaps.com/

So 1kwh yields about 9k btus

So 0.04 kg per 1k btus.

This is really surprising. Electricity is 4 times less co2 than wood.

Of course wood is renewable. But does it matter from an emissions only point of view? Is my estimate correct?

This says from an emission perspective it’s always cleaner to use electricity (for me). I’m trying to figure out in my head what paying for carbon credits on the electricity usage would do for costs. I will post back if when I get that worked out.

 

[woodgeek]

I would've expected more CO2 per kg of wood. 1kg of Carbon emits >3 kg of CO2.

The usual argument is that the wood will decay and release the CO2 even if you don't burn it. And might release some methane too. The first factor makes you net zero, the latter factor makes you net negative for woodburning.

 

[stoveliker]

At least 1/6th of the weight of wood is water, so that's a order 10% error (overestimating CO2 production) in your wood numbers.
(Of course the comparison also goes with 10,000 btu/kg, when I thought dry wood is 8600 BTU per lbs, so almost 19,000 BTU per kg, so you'd also have to dial up the kWhs needed from electricity - though some of those BTUs are going out the door with stove efficiency, and boiling off the water above.)

 

[stoveliker]

Also, I think for folks using a heat pump and a stove (as I am), the COP of the minisplit is no longer 3 at the times I burn most of my wood.
Near the cross over it may be, but when it's cold it won't be.

 

[EbS-P]

All good points. I would agree my literal napkin math could have an error of 100%.

keeping the tree growing has some positive carbon capture. Any guess what that would be on a per kg (1kbtu) basis.

I think it’s at least reasonable to say they could have equivalent emissions. I still find that surprising. Why cut down a tree for firewood if the grid has the same emissions. (Yeah yeah cost I know. ) I think all of this points to a steady decrease in reliance on wood heat.

 

[woodgeek]

All good points. I would agree my literal napkin math could have an error of 100%.

keeping the tree growing has some positive carbon capture. Any guess what that would be on a per kg (1kbtu) basis.

I think it’s at least reasonable to say they could have equivalent emissions. I still find that surprising. Why cut down a tree for firewood if the grid has the same emissions. (Yeah yeah cost I know. ) I think all of this points to a steady decrease in reliance on wood heat.

A mature forest doesn't sink carbon, so cutting and pulling out carbon (sustainably) forces the forest to sequester more.

 

[stoveliker]

Yes, but pulling out and putting the carbon back in the air would only create a cycle rather than net sequestration.
I don't think the goal of all this (wood heat) was net sequestration.

Of course some have the argument that the cycle is the principle of sustainability.

One other argument is that I don't cut down trees - well, I do, but they are dead already.
Most of my wood, however, was already cut down by tree services, and would (literally) go in the landfill. I have the strong impression that
1. the carbon would end up in the atmosphere from there too (though slower), but
2. the carbon from there would end up in the atmosphere with a much higher concentration of methane (as it's going to be mostly anoxic) - which very quickly outdamages the geo-system as compared to CO2.

Therefore, given that someone made the decision to have trees cut down, I use them to offset mostly oil heat, and my emissions are CO2 when burning versus significant (but slower) methane otherwise, I feel (..., no numbers here...) good about it.

Of course I could go to a true hyper heat heat pump for cold days. But then I'd have to add the methane emissions to my footprint given that I made the decision to change my ways with more methane emissions as a result. I admit that's a bit convoluted thinking, but I don't think it's without merit.

Obviously case dependent. Wood laying in a forest, decaying, also supports diversity in ecosystems, which is a valuable thing in itself.

Many facets to this...

 

[GG Woody]

I only cut dead trees, I like to think that I help clean up the forest floor. The CO2 produced in a forest fire is massive, the Co2 produced in fighting a fire is also massive.
Don't mind me and my good deeds, cleaning up is all I'm up to.
My home insurance should be paying me as well...

 

[woodgeek]

I did a little googling. Looks like cellulose is 44% C by weight, so assuming perfect dry wood is cellulose (lignans are probably similar) gets 1.65 kg CO2/kg. Add in 10% water and you are at ~1.5, as assumed.

I forgot about the heavy oxygen atoms in cellulose.

I mostly burn on very cold days when my (20 year old) heat pump can't keep up and I'm running strip heat (a giant hair dryer). This amounts to about a face cord in the 6 weeks from Jan 1 to Feb 15th.

 

[EbS-P]

Cold enough and enough clouds today ran the heatpump as soon as it was over 45. (Didn’t have any kindling split or wood hauled this morning when I got up so we didn’t have a morning fire even though it was 37). So the battery didn’t get a full charge today.

It’s been interesting watching the production and consumption rates. Winter time is definitely not a net zero season even down here.

So that’s what started my thinking on the topic.

 

[stoveliker]

Yesterday (first rain since sometime in August...) I made 1.6 kWh ...
I think I used about 10 times as much... (my usage is about 15 kWh per day averaged over the year including heat pump (10.5 kWh per day without heat pump); I did run the minisplit until 7.30 ish when I lit the stove).

 

[ericm979]

Of course wood is renewable. But does it matter from an emissions only point of view?

Where the carbon comes from matters a lot. There are two carbon cycles- the short term cycle where carbon in the air gets taken up by plants and then returned to the air when the plants die. The long term cycle takes carbon, usually from plants, and sticks it far underground where it becomes oil, gas or coal. Eventually geologic processes puts some of that carbon back into the air via volcanoes but it's on a much longer time scale than the short term cycle.

If you harvest some wood and burn it, you're shortening the short term cycle a bit for that carbon. But you're not adding carbon to the cycle. As long as we keep growing plants, that CO2 will get taken up and cycled into plants.

Where if you're running the mini split on electricity generated by burning fossil fuels, the energy to run it came out of the long term cycle and adds carbon to the short term cycle.

I used to be against bio fuels because it takes more energy to grow, harvest and then refine the bio fuel than it does to pump oil and refine that. But that's looking at it the wrong way. The bio fuel is made from carbon from the short term cycle, so burning it is not adding carbon to that cycle. Where oil pumped from the ground is coming from the long term cycle and being added to the short term cycle.

 

[stoveliker]

The question is whether the processing of liquid biofuel requires more BTUs per gallon produced than that it costs to make.
(I don't know at this point, and don't have the time to search for solid reliable info - too much junk out there.)

Otherwise it ends up into a rocket equation.

 

[bogieb]

I have friends who harvest wood from their property as part of the ecosystem maintenance of their 35 acres (mature forests do not have the diversity for wildlife as do forests in various stages - from glades to young growth to mature. Literally, their only method of heating is wood. If they were to buy and install a heating system, how much CO2 would be released in manufacturing the metals and plastics and other materials needed? Oh, and they would have to upgrade their whole electrical system and panel too.

I run pellets stoves. Pellets are mainly made from leavings of other wood harvesting activities. Or, sawdust created from furniture making, or even old pallets. So I'm thinking the net there isn't much more than would be if the trash was left on the forest floor or the sawdust/pallets were disposed of. Of course, there are manufacturing considerations for wood pellets too.

For either type of burning, the wood is sourced fairly locally, even if you buy firewood instead of cutting your own. There is a pellet plant within 25 miles of me. So there is little movement of materials as there is for transport of oils/propane/NG into this state to create electricity. So all of the conjecture may very well be related to where you live too.