Category: Analysis

 

Edit:  This post is obsolete.  See Post #1893.  The most recent estimate for the impact of black carbon is much lower than it was when I wrote this post.  (But still within the error bars of that prior analysis).  From a global warming perspective, if you have a modern air-tight wood stove, the drawbacks of wood burning are much less than I estimated for this post.

Original post follows:

I bought my wood-burning fireplace insert circa 2007.  The idea — consistent with scientific thinking at the time — was that I could reduce my global warming impact by burning wood (“biomass”) instead of fossil fuels. 

And, as far as C02 emissions goes, that’s right.  Over a ten-year time frame, the wood I burn is essentially a carbon-neutral fuel source.  I’ll show the math below.

But that was before the global warming impact of black carbon (airborne soot) had become well-known.  Over the time since I bought that wood stove, the estimated global warming impact of soot had grown significantly.

This year, as I get ready to have the chimney cleaned and purchase my usual two cords of wood, I’ve decided to take a good, hard look at that issue.  And, roughly speaking, the tiny amount of soot that my wood stove puts out completely offset the benefits of using a carbon-neutral fuel.

Bottom line:  With the grid getting cleaner every year, it now looks like my most carbon-sparing heating option, by far, is just to run my ground-source heat pump. 

I guess I should have seen that coming.  This is really part-and-parcel with the decision to go to electrically-powered miles via a Prius Prime.  The de-carbonization of the electric grid makes electricity the preferred fuel from a global warming standpoint.  Not just for the car, but now also for the home. 

Sometimes the tree of knowledge is not the tree of happiness.  Not even if it’s cut down, split up, kiln dried, and turned into a nice, cozy fire.

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Why burn wood?

Firewood is very nearly carbon-neutral within a roughly one-decade timeframe.  For me, at least.  Sure, burning wood generates C02, just like burning any other carbon-based fuel.  The difference is that my firewood was atmospheric C02 just ten years ago, on average.  Ten years ago, C02 was deposited out of the atmosphere in the form of wood.  Now it goes back into the atmosphere as C02.  The net impact on atmospheric C02, for that ten-year period, is zero.

It may surprise some to hear that my firewood is just ten years old, on average.  That’s because the age of the wood is not the same as the age of the tree.  The typical piece of wood I’m burning has about 30 annual rings on it.  It is, if you will, a “30 year old tree”.  Or tree limb.  But:

• Only the very center of the wood is 30 years old.
• The outside edge of the wood is a year old.
• Trees get bigger as they grow.

You can work out the math any number of ways — from calculus to setting up a spreadsheet — but the average age of the wood will always work out to be somewhere around one-third of the age of the tree.  So, by weight, the wood in my “30 year old trees” is, on average, just ten years old.

The upshot is that if you think in terms of the impact over ten years, burning wood added nothing to atmospheric C02.  It just re-injected C02 into the air that had been extracted (by a tree) a decade earlier.

I bet some of you thought I burned firewood because I’m cheap.  I am cheap.  But in this locality, firewood at (say) at $350 a cord (stacked!) is a more expensive heat source than natural gas at around $1.50 a therm.  Just wanted to make that clear.

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A few ifs, ands, and buts

There are some nuances to the simple carbon-neutral argument.

First, some fossil fuel is consumed in harvesting and transporting the wood in the first place.  But by any account, that amount is negligible compared to the energy value of the wood.  The heat value of cord wood varies by species and condition, but a good round-numbers value for a typical hardwood is that a cord contains about 20 million BTUs of energy, and weighs about a ton and a half.  Even if that has to be trucked a total of 50 miles from source to delivery (including deadheading), in a typical small dump truck (roughly 7 MPG), the roughly 7 gallons of diesel consumed contain less than a million BTUs of energy.  In the absolute worst case, the fuel required to truck the wood around eats up 5 percent of the energy value of the wood.

Second, no trees were harmed in making this firewood.  By that I mean that in this urbanized area, firewood is mostly a waste byproduct of tree trimming services.  Trees weren’t cut down to make firewood. Instead, my firewood comes primarily from trees that were going to come down in any case.  Frequently, it’s from trees that had already died.

One way or the other, once a tree is dead, it begins the process of returning to C02.  It’s just a question of speed.  Smaller pieces can end up chipped into wood mulch, which then becomes C02 in a few years as that mulch rots.  The larger pieces of wood can’t be disposed of that easily.  If left above-ground, and not turned into lumber, they’ll rot in a few years to a few decades.  And they can always end up in the landfill.  There, they might last longer, but their anaerobic decomposition produces methane (which, if not captured and burned, results in global warming).  If nothing else, processing the downed trees into firewood keeps them from ending up in the landfill.

Because urban firewood is largely a byproduct of tree-trimming, the fossil-fuel inputs are lower than for trees purpose-cut for firewood.  These trees were going to get trucked away from where they fell, no matter what. The only incremental transportation fuel used in the firewood process is for the delivery trip.)

The upshot is that production of urban firewood is the exact opposite of clear-cutting a forest.  If you take standing biomass of a forest and purposefully cut that down and burn it, that definitely increases atmospheric C02.  (E.g., the ongoing destruction of tropical rainforests is accounted for in estimates of man-made C02 production.)  Urban firewood is more like waste disposal than like forestry clear-cutting.  In the main, you’re only using trees that were coming down anyway.

Third, in my house, I obtain a secondary environmental benefit from burning wood.  Burning wood makes my ground-source heat pump more efficient.

The main heating/cooling system for my house is a ground-source heat pump.  Using about 6000 feet of plastic pipe buried in my back yard, this system extracts heat from the ground in winter, and disposes of heat into the ground in summer.

Heat travels slowly through the ground.  (That’s why this system needs so much pipe — it needs all that surface area.)  Ideally, the injections and withdrawals of heat should be matched.  If not, excess heat withdrawals during the winter will cool the ground field and so reduce the wintertime efficiency of the heat pump.

This is not a small matter.  In my area, heating demand is about five times cooling demand, when measured in heating and cooling degree-days.  Thus, I need a significant additional heat source if I’m going to keep that ground-source heat pump operating at peak efficiency.  Which is supplied nicely by burning a couple of cords of wood each year.

Fourth, wood burning raises real health concerns.  No matter how hard you try, a wood burning stove will pollute your indoor air to some degree.  Every time you open it up to add wood, you’re letting something out into your house that you’d really be better off not breathing. And, to some degree — typically less than the indoor air, due to dilution — you do the same to nearby outdoor air.

Those problems tend to be more significant:

• In areas where many people use wood as the sole or principal source of heat.
• In areas where older, higher-polluting stoves are in use.
• In tighter home construction, with fewer air changes per hour.

I use a new, relatively clean stove (rated at under two grams of soot per hour), in an old leaky home that probably gets a complete air change every half-hour.  I’m also the only person within at least a half-mile radius who routinely heats with wood.

In short, I just ignored the health-related issues.  I don’t think they are a huge concern in my situation.

Fifth, on the positive side, heating your home with wood teaches you a lesson in just how damned much fuel a modest suburban home consumes.  Every year, I buy and burn roughly three tons of wood.  Every piece of it gets picked up and moved three times.  By the end of the heating season every year, I’m right tired of firewood.

The mass of that firewood is obvious.  But it’s only modestly greater than the mass of the fuel I would burn if I heated entirely with natural gas.  Two cords of wood weighs about three tons and generates about 40 million BTUs.  That’s the heat value of 400 therms of natural gas, which would weigh (in round numbers) about one ton.  With firewood, you are constantly reminded of the enormous mass of the fuel being used.  With natural gas, it’s still an enormous mass of fuel, but you are completely blind to the order of magnitude.

So chalk that one up in favor of firewood.  It never lets you forget that you’re burning tons of fuel to heat your home each year.

In summary, as of 2007, on balance, wood heating:

• reduced my carbon footprint,
• increased the efficiency of my ground-source heat pump,
• made me aware of the large amounts of fuel I was using, and
• posed seemingly small heath risks.

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The evolution of thinking on black carbon a.k.a. soot.

When I bought my wood stove, the importance of black carbon (soot) emissions for global warming was only starting to be realized.  I believe that the 2007 report of the Intergovernmental Panel on Climate Change was the first time that black carbon made it into the summary for policy makers.  If I look back at an earlier (2001) report, it wasn’t even clear at that time that all soot much mattered.  They just didn’t know one way or the other.  Just read that 2001 chart.  The level of understanding about black carbon was very low, and the estimated impact was quite low as well.

Source:  IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp.

By the time of the IPCC fifth assessment report (2014), they had enough understanding of the issue to start suggesting that reducing soot emissions was a good way to slow global warming.  This AR5 report showed this history of the estimate of radiative forcing (the impact on global warming) of black carbon.  It’s only a modest exaggeration to say that every time they reconsidered it, the estimated impact on warming doubled.

Source:  Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza,
T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United
Kingdom and New York, NY, USA.

Currently, the U.S. EPA uses a value of just over 0.6 (watts per square meter) as its estimate of the impact of black carbon.  So it’s a pretty good guess that the IPCC’s estimate is going to go up in its next (Winter 2022) summary report.

Source:  US EPA

In short, between the time I bought my stove and today, the estimated importance of soot as a source of global warming has increased dramatically.    At present, black carbon (soot) is now reckoned to be the third most important contributor to global warming.  Only C02 and methane are judge to be more important.

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A little soot goes a long way.

But can this possibly be that big a problem?  For me, I mean?

Rather than fuzzy-think my way to avoiding the issue, I decided to do a quick calculation.  If I shut down my wood stove and fired up the equivalent amount of natural gas heat, would that increase or decrease my contribution to global warming?

I figured this had to be a no brainer. Just look at the facts.

Properly run, my wood stove produces under 2.0 grams of soot an hour.  Some of that gets trapped in the flue.  The rest of it stays in the atmosphere for (on average) just a couple of weeks. And my black carbon gets carried out over the Atlantic, and so doesn’t cause the darkening of snow surfaces that’s an important part of the overall impact of black carbon.

Seriously, how bad can that be?

Turns out, a crude calculation shows that it’s uncomfortably close to a wash.  Based on the published estimates of the 100-year “global warming potential” of black carbon, the grams of soot emitted by my wood stove, hanging in the atmosphere for a couple of weeks, matter just about as much as the kilograms of C02 put out by my natural gas furnace, hanging out in the atmosphere for the next century.  All that, for producing the same amount of heating.

What I thought was a no-brainer is nothing of the sort.

Here’s the crude calculation, comparing the soot from my stove, to the C02 from my gas heater, for an equivalent amount of heat energy.

First, I need to be able to compare the impact of soot to the impact of C02.  That comparison is defined as the global warming potential (GWP) of soot.  The GWP shows how much global warming you would expect from emitting a gram of any given substance, compare to the warming you’d get from emitting a gram of C02.  The GWP is generally cited over a given period of time, say 20 years or 100 years.

I simply have to accept somebody’s reasonable estimate of that.  Estimates vary, but a reasonable mid-range value for the 100-year GWP of black carbon is 900.  That is, each gram of black carbon emitted causes as much warming, over the next century, as 900 grams of carbon dioxide emitted.

(I will note in passing that this is truly a mind-blowing number, given that the black carbon only stays in the atmosphere an average of about two weeks.  While the C02 is, for all intents and purposes, practically forever.  Apparently, a black carbon particle is just about a perfect absorber of sunlight, and so provides an enormous amount of heating to the atmosphere during its brief lifetime).

Next, let me produce 100,000 BTUs of heat with either my wood stove, or a natural gas furnace.  And compare the global warming impact of the soot from the wood stove and the C02 from the gas furnace.

I use a Lopi Revere fireplace insert.  At a typical burn rate, that produces about 30,000 BTUH, and is certified to emit less than 2 grams of soot per hour.  Let me assume all that soot goes into the atmosphere (as opposed to being deposited in the flue).  In round numbers, to produce 100,000 BTUs of heat, my stove emits 6 grams of black carbon.

When I take my six grams, multiply by the GWP factor of 900, and convert to pounds, I find that six grams of soot, from my wood stove, produces warming that is equivalent to 12 pounds of C02.

But here’s the punchline.  To produce 100,000 BTUs of heat with natural gas, you (by definition) burn a therm of gas.  Burning a therm of natural gas, to create the same amount of heat, releases just under 12 pounds of C02.

In other words, as a rough cut, and with a lot of uncertainty, heating with wood has no benefit from a global warming standpoint.  Those six stinking grams of soot, from burning my carbon-neutral firewood, have as much global warming potential as the C02 from the natural gas that the wood replaces.

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When the facts change, I change my mind.  What do you do?

The above is a quote from John Maynard Keynes, arguably the most influential economist of the 20th century.  I believe it has general application outside the field of economics.

There’s surely a lot of uncertainty in this calculation.  Mostly, it’s uncertainty about that figure of 900 for the global warming potential of soot.  At the minimum, I should try to remove the portion of that attributable to the darkening of snowpack from soot.  Where I’m located, that just doesn’t apply to any material degree.

Beyond that:

On the one hand, a lot of the soot from the stove ends up stuck to the flue pipe.  Exactly what fraction is hard (maybe impossible) to determine.  For sure, that’s not a question that gets asked every day.  Best I can figure, based on one reported experiment, I should expect half the soot to end up in the flue and not in the air.  But clearly that has to depend on the (e.g.) the length of the flue.

On the other hand, I believe that two-grams-per-hour figure is for a perfectly maintained burn.  It doesn’t include the warm-up time.  It doesn’t include any time the fire is smouldering (no visible flames).  It doesn’t account for imperfectly cured (high-moisture-content) firewood.

All of those factors — cold operating temperature, smouldering, wet wood — result in higher soot emissions.  The two grams figure just generally doesn’t account for all the forms of operator error that can result in a smoky (high-soot) fire.

If nothing else, this has convinced me to switch entirely to kiln-dried wood.  In general, the drier the wood, the less soot it produces.  I have bought air-dried wood from a trusted supplier for years.   But two years ago I was forced to buy some kiln-dried, due to a badly cured batch of wood.  Not only does kiln-dried burn much more readily and with less soot, my best guess is that the increased heat output from the kiln-dried wood more than pays for the fossil fuels used in the kiln drying process.

I’m also going to re-evaluate my heating mix in light of the ever-cleaner U.S. grid.  Years ago, I compared emissions from my gas furnace to estimated emissions from my ground source heat pump.  I decided that they produced more-or-less equivalent C02 emissions for a given amount of heat.  But as the grid has gotten cleaner, it’s probably smarter to use the heat pump more, and other sources less, even if that reduces the efficiency of the heat pump somewhat.

(Best guess, my heat pump nets out to a coefficient-of-performance of about 3.0.  That is, I get about three KWH of usable heat energy for every KWH used to run the equipment.  You’d think it would be higher, but you spend a lot of energy pumping water through pipes, and a lot of energy moving air through crappy old ductwork.)

And when I do that calculation, the now-cleaner Virginia grid makes this almost a no-brainer.  With cleaner electricity, the heat pump beats the gas furnace hands-down.

The heat that would have generated 12 pounds of C02 when generated by my (efficient!) natural gas furnace would only generate a little over six pounds of C02 if generated by my heat pump.

It’s starting to look like the right answer for me is just to run the heat pump all the time.   That’s about as un-romantic a solution as you can get.  But the numbers are what they are.