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

This 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:

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:

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:

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.

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“.

Electricity generation: very different capacity factors!

The US Energy Information Administration (EIA) has an interesting post on the huge differences between countries and origins of electricity generation efficiency, or more precisely the capacity factors.

1. Definition of the capacity factor and the “Volllaststunden”.

Let me recall that the capacity factor is simply the yearly energy produced divided by a hypothetical maximum which would have been produced if the generator had functioned 8760 hours at its the name plate capacity. An example: suppose a wind turbine has a name plate capacity of 2.5 MW; if it would deliver this power during the whole year (what is clearly impossible!), it would produce an energy of 2.5*8760 = 21900 MWh. Now its real production has been only 4380 MWh. So the capacity factor is:

CF = 4380/21900 = 0.20  which is often given as a percentage by multiplying by 100, i.e. here CF%= 20%.

In Germany one uses mostly the term “Volllaststunden” (yes, you can write this using 3 letter l). The VS would be equal in our example  to VS = (CF%)*8760/100, i.e. 1752 hours.

2. Capacity factors vary with type of electricity generation and country

The IEA report has several interesting statistics which give the capacity factors of different countries and regions for the period 2008 to 2012. I modified the first table by discarding 4 countries or regions: Russia, because its quasi nonexistent wind/solar production, Japan, because its shutdown of all its nuclear reactors after the Fukushima accident, the Middle East because it has only negligible nuclear electricity production, and Australia/New Zealand for the same  reason. That leaves 12 countries or regions with the following statistics:

The vertical red lines give the average capacity factors  of the different types of production: nuclear is the absolute champion with 79.8%, fossil and hydro are close at 45.9% and 41.9%, and solar/wind come out very low at 21.9%. If we call “renewables” the last two categories, clearly hydro is the only one delivering acceptable capacity factors. Now lets separate the last category into solar and wind. This time one can keep 13 countries or regions, omitting only Brazil, Central/South America and Russia for not having (or having communicated) any solar production.

Solar comes out at an abyssal low CF of 11.9%, whereas wind practically doubles with CF=23.6%

Our first conclusion comes as no surprise: nuclear really shines when it comes to availability and stability; both solar and wind can not deliver (at least for the moment) a reliable electricity production!

3. Why the big differences?

In most countries, solar and wind have an absolute priority to deliver their electricity into the grid, penalizing the non-renewables which must be turned down to adapt production to demand. If that political decision would not exist, and the free market rules would apply, solar and wind production would have still lower capacity factors. Regarding hydro, one clearly sees that countries like Canada and Brazil have a clear advantage in disposing of enormous hydro potential, which may have reached its peak for many reasons. The OECD hydro electricity production is practically at its maximum, so the CF of 40% will be impossible to increase in the future.

Fossil producers suffer the most from the prioritizing of solar and wind: nuclear facilities are difficult to rapidly ramp down or up, but gas turbines (and even some of the latest coal power stations) can do this quite easily, and so are often used to deliver peak load. Often a certain type of generation is put on hold for commercial reasons, so the capacity factors must be taken with a grain of salt: they not only reflect technical deficiencies or for instance lower wind resource, but also ramp down/up decisions taken at the big electricity exchanges (as the EEX at Leipzig) for monetary reasons.

Now the 100 billion dollar question: if you want carbon free electricity, which type of generation would you choose?

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added 14 Oct 2015:

Read also this comment on declining wind capacity factors on the US West Coast