Wood and pellets: a “burning” fine particulate problem.

28 Oct 2015: new link added at the end of the blog
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The heating season is about to start here in Luxembourg. I heat my home with an oil driven central heating, one of my neighbors only burns wood (in cords). The quantities of wood he uses are breathtaking, but probably he choose wood burning as “climate friendly” . Indeed, the carbon dioxide released has been gobbled up by the tree during its 30 to 60 year life from the atmosphere, and returning it to the air will be, at least at a first glance, be “carbon neutral”. At a second thought, the problem is more complicated: his CO2 release is a spike that would not have occurred if the wood had been left to rot (the release would have been over many years ), so that at least on a short period, there is not much gain by switching, say, from gas to wood.

There is much talk during the last years about the dangers of fine particles (the smaller than 2.4 micron PM2.5), be they released by Diesel engines or other energy providers. In Europe all new Diesels have particulate filters which should solve this problem. Burning wood is a very big PM2.5 emitter, and I will discuss this in the next chapters.

1. CO2 emissions

CO2_emissions_of_fuelsThis figure shows that the CO2 emissions per KWh energy from burning wood are about the double of those from natural gas. So say if a state installs a CO2 emission measuring system (using perhaps a satellite like the OCO-2), wood burners would be in a delicate position.

If we look at the composition of the exhaust, we have this:

exhaust

There is a 1% per weight emission of NOx, which is about the same for gas or oil; there are not negligible VOC (volatile organic compound) and particles emissions. Clearly the exhaust from a wood stove is very different from clean air!

2. The PM problem

Fine particles are the crux of burning wood:

PM25_pollution_annual
This figure from the www.treehugger.com web site shows the tremendous difference between an uncertified wood-stove and a usual gas furnace. Things become much better if you use a pellet system, but nevertheless remain 162 times higher than gas (the uncertified wood-stove emits 1464 time more than gas!).

Now very often the discussion on particulate emission puts the blame also on agriculture. But the picture is much different, as agriculture does not emit the same percentage of very small particles (the PM 2.5), which are thought to be the most dangerous, being able to transit to the lung, the heart and even the brain. The next figure compares the two emission sources:

PM_size_distribution

3. Hourly emissions

The next figure shows the emissions of particles in g/h for burning oak (the 3rd most frequent wood in Luxembourg): note that the emissions of the larger PM10 are about the same as the dangerous PM2.5; fire logs are possibly wax-wood mixtures, and so have about 4 times less emissions.

PM_g_per_hour

These numbers apply to about a burning rate of about 3 kg dry wood per hour (see here).


4. Conclusion

If you burn wood (cord or pellets), you environmental impact may not be what you intend: your immediate CO2 emissions are comparable to those of other fossil fuels, and your particulate pollution is much much worse! That is the reason why for instance the city of Parish forbids burning wood in open fires. Switching to gas (or nuclear powered electrical heating!) would be more environmentally friendly.

 

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28 Oct 2015: see also this article by Scilogs: “Unterschätzte Gesundheitsgefahr durch Holzrauch“.

2 Responses to “Wood and pellets: a “burning” fine particulate problem.”

  1. eder23eder Says:

    I think it is future for wood fuel pellets. It is ecological, not expensive and pellets have the high calorific value. For example, Oak pellet has the following characteristics :
    Moisture content – less than 8%.
    Diameter – 6 mm.
    Length – not more than 45 mm.
    Very low ash content – 0,48-0,52%.
    The calorific value – 4377-4680 kkal / kg
    And price is 170 $ per tonne or 160 E per tonne

    • fmassen Says:

      Hi Eder,
      you do not reply to the subject of this post: the fine particles emitted by wood and pellet burning….why that?

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