Dylan Ryan, Lecturer in Mechanical & Energy Engineering, writes for The Conversation
In Ireland, there has recently been some controversy over a proposal to transition a number of the country’s dirtiest power stations away from burning peat bogs, which emits even more carbon than coal. Instead, the plan is to burn 'biomass' – that is, wood. However, because Ireland has relatively little forestry, there is not enough wood available to meet demand. That’s why Bord na Mona, a semi-state body that manages several peat burning power plants, proposed to source the wood from Australia.
This angered conservation groups, who pointed to the very high carbon footprint of hauling timber all the way from the other side of the world, just to burn it for electricity. And over the summer Irish planning authorities refused permission for one peat-burning power plant in County Offaly to be converted to biomass, putting the plans on hold.
Burning Australian wood in Ireland does indeed sound daft, at first. But the true carbon footprint isn’t always as straight forwards as it would seem at first glance (just look at how, for example, cutting plastic packaging can sometimes lead to more food spoiling and thus higher carbon emissions, or how cotton or paper bags can sometimes work out worse than a plastic bag). Therefore, since Bord Na Mona has been slow to release details on the potential carbon emissions, I thought it would be useful to try and estimate them myself.
First I want to clear up one thing: burning trees doesn’t necessarily count as emissions. Though trees are made of carbon, if at least one is planted for every one cut down then the overall amount of carbon in the atmosphere should remain roughly neutral.
There are many other sources of carbon emissions related to forestry though, including land use changes, forest management or processing of the wood after harvest. But in this particular case, the main source of carbon emissions would be transport.
Calculating the footprint
To make the calculations simple, let’s assume a shipment of exactly 1,000 tonnes of logs from Australia to Ireland, a distance of about 21,000km by sea. We’ll also assume another 500km by lorry to and from the port. The carbon footprint of a cargo ship depends on the type of ship, fuel used, route, speed, and so on, but for a bulk carrier it works out to about 8 grams of CO2 per km per tonne of cargo.
For trucks it varies between 40 to 90 grams, but 55 grams per km per tonne would be a reasonable estimate. Do the maths and that works out at 168 tonnes of CO2 emitted by the ship, and 27.5 tonnes by truck, giving a combined total of 195.5 tonnes of CO2.
Whether these emissions are worthwhile depends on how much energy the timber contains, and that depends on the type of wood and its moisture content (wood absorbs water, making it heavier and less energy dense). A fast growing and moderately wet hardwood such as eucalyptus has an energy content of 3,500 kilowatt hours per tonne. We then have to assume the power plant will lose around 70% of all that energy (mostly as heat) when burning it to make electricity.
What this means is 1,000 tonnes of eucalyptus will yield around 1.05m kilowatt hours of electricity (the full calculation is at the end of the article). And when you take the total carbon emitted in transporting those logs to Ireland, and divide it by that total electricity generated, you get a carbon footprint of 186 grams of CO₂ per kilowatt hour.
It is worth emphasising that there is considerable sensitivity in these estimates. If any of the key variables change – if the distance to port increases, if we use a different type of wood with less moisture, and so on – it can have a big impact.
By comparison, the carbon footprint of importing biomass from North America to the UK has been estimated at 122 gCO2/kWh. One 2014 study found that a more conventional biomass operation using locally sourced timber would have a footprint of 30 gCO2/kWh, compared to 34 gCO₂/kWh for wind and 50 gCO₂/kWh for solar.
So hauling biomass such a long distance doesn’t look like a great idea. However, the carbon footprint of peat is at least 1,100 gCO2/kWh, nearly five times higher, and coal is very similar. And even these figures ignore the enormous environmental destruction that comes from peat extraction or coal mining.