Secrets of Reducing Your Carbon Footprint

Can we grow willow and bury it?

This carbon footprint article from The Independent reckons that the average Briton’s carbon footprint is 10.92 tons of CO₂.

This article about phytoremediation in Sweden suggests short rotatation coppicing gives a yield of about 6-12 tonnes of oven-dried willow per hectare per year. Similar yields in England are suggested by the survey results that I got Ian Tubby of the Forestry Commission’s Biomass Energy Centre to e-mail me. Optomistic rule of thumb: 10 tonnes of willow per hectare per year.

Willow is about 50% carbon. So from one hectare we can sequester 3-6 tons of carbon.

That seems like a long way off the 10.92 tons of CO₂ that we’re each responsible for producing. But hold onto your apples and oranges there. A little bit of chemistry reveals that 1 ton of carbon is equivalent to 3.67 tons of CO₂. That’s because carbon has atomic weight 12, but CO₂ has molecular weight 44 (12+16+16), so every 44 tons of CO₂ has only 12 tons of carbon in it.

So our 10.92 tons of CO₂ per year is only 2.98 tons of carbon. Which we can easily offset with 0.6 hectares of willow or so. Of course to actually offset the carbon we need to bury the willow. In a hole in the ground. Like maybe a coal mine. (And how do we replace the P, N, and K that I’ve just buried?)

Of course as well as sequestering carbon to offset my footprint I could displace carbon. Instead of burning coal (geological carbon) I could burn willow. This leaflet from some random consultants suggests that 0.7 hectare of willow is sufficient to heat a 3 bedroom house.

Anyone know the average number of KWh per year it takes to heat a house in the UK? It’s surprisingly hard to find the answer in anything approaching an SI unit. This so obviously transient page (referenced on 2007-04-20) links to dataset ST341114 from the Office of National Statistics. That gives a 2001 figure of 1210 for space heating and 450 for water heating. Per household. The units? Why, kilograms of oil equivalent of course (haven’t these guys heard of SI?). Which Wikipedia suggests is a bit of a variable quantity; 1 kilogram of oil equivalent could be 42, 41.868, or perhaps 41.85 MJ. Really I only need a rough guide. Call it 42. That’s 69.72 GJ per jear.

Plausibility check: I burn about 1.2 tonnes of anthracite a year, plus some electricity to heat water in the summer. Anthracite has calorific value of 36 MJ kg^-1 so that’s 43.2 GJ plus the electricity. So we’re in the right ball park. An even cruder check would be that 1.2 tonnes of coal is surely about the same amount of heat as 1.2 tonnes of crude. This time I’m thankful that the ONS uses silly non-SI units.

Seasoned wood has a calorific value of 16 MJ kg^-1. The average household will need 69.72/16 = 4358 Kg of seasoned willow. About 0.5 hectare then (and a 5-year lead time (3 for growing up to the first harvest, 2 for seasoning), eek!). So the random consultants are in the same ball park with 0.7 hectare. There are 2.4 people per household so we only need 0.2 or 0.3 of a hectare per person to displace the carbon we were using for heating (which, according the The Independent article, is .40 tonnes). The lower calorific value of willow (compared to anthracite or crude) means we need to grow more willow, but we end up burying less.

So by swallowing a few assumptions, I personally could make myself carbon neutral with less than a hectare of willow. Unfortunately the UK has only 24 million hectares of land (and no, we’re not going to be growing willow on all of it). What are the rest of you going to do?

Still, it was a nice thought experiment.