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The race is on Biodiesel is big and getting bigger, but is it any better than its petroleum-derived equivalent in terms of global warming? Eric Johnson and Russell Heinen find out

Biodiesel is on a roll, a subsidy-led roll. Thanks to tax-breaks for retailers and quota exemptions for farmers, world production has soared from about 300 000t in 1995 to nearly 7m t in 2006 – an annualised growth of 33%. Over the coming decade, output is expected to climb perhaps another ten-fold, which surely makes it the world’s fastest-growing bulk chemical product.

Ironically, biodiesel’s rise comes a century later than originally planned. When inventor Rudolf Diesel showed his eponymous compression engine at the 1900 World’s Fair in Paris, France, it ran on peanut oil. Although he did not name it a peanut engine, Mr Diesel did intend his motors to run on vegetable oils, which are only one process step removed from biodiesel. Despite his intentions, petroleum derivatives ended up cornering the motor fuels markets, and for one very good reason: even at today’s $60-ish per barrel crude prices, they cost less.

This inconvenient truth has not stopped governments around the world from pushing biofuels; indeed it spurs them on. The latest move was from the European Commission, which in January 2007 proposed to peg biofuels’ 2020 share of all transport fuels at 10%. This comes on top of the 2003 EU Biofuels Directive that aims for a 5.75% share in 2010. Either target is well above current levels of about 2%.

Why such interest in biofuels? For one, many analysts believe that crude’s price party is over: that cheap oil has permanently disappeared. This ties to another biofuels hot button: the desire for greater energy security. Better to have the local farmer growing at least some of your fuel instead of counting on supplies from notoriously unreliable exporters in, say, Russia, the Middle East or Venezuela. And then there are the farmers themselves. What better way to subsidise this numerically small yet politically mighty group than to hand them the keys to the nation’s petrol tank.

Powerful argument?
These seem to be solid reasons in themselves, yet the most powerful argument for boosting biofuels, the EU says, is to combat global warming. As the 10% by 2020 target was announced, European Commission president José Barroso crowed: ‘We can say to the rest of the world – Europe is taking the lead. You should join us in fighting climate change.’

But do biofuels really reduce global warming? To answer this crucial question, SRI Consulting (SRIC) recently sized up biodiesel made from rape oil – Europe’s and the world’s predominant feedstock – versus petroleum diesel refined from crude oil – and came to answers that, although politically incorrect, are worth considering.

We inventoried the lifetime greenhouse-gas emissions of RME (rape methyl ester biodiesel) and petroleum diesel, as commonly produced in Europe, from production on through to combustion in an automobile. For RME, that lifetime starts with growing rapeseed on a farm, which is then crushed to extract oil, which is chemically processed into biodiesel, which is burnt in an engine. For petroleum diesel, the lifetime begins as crude oil in a well, which is produced, refined and then also burnt. We used emissions data for these steps from a variety of public sources, including SRIC’s own process models of biodiesel synthesis.

The resulting inventories show that some two-thirds of RME’s greenhouse gas emissions occur during the farming of rapeseed, where cropland emits N₂O that is 200 to 300 times as potent in its globalwarming potential as CO₂ itself. N₂O emissions have been researched heavily in recent years, and they appear to be a function of four main factors (see below). Fertiliser production and tilling also generate significant carbon emissions, while everything else in the life cycle, including electricity generation, accounts for only about 15% of the CO₂-equivalent (CO₂e) total.

Petroleum diesel, by contrast, emits some 85% of its greenhouse gas emissions in the final use stage, from being burnt in the engine.

Based on the results of our analysis, it turns out that RME and petroleum diesel are almost equal on global warming contribution per unit of energy delivered. If rapeseed is grown on dedicated farmland, which over time is likely to be the case, then the contest is a draw: RME accounts for nearly the same amount of CO₂e per kilometre driven. However, if rapeseed is grown on land that otherwise would be set aside temporarily, which means they will emit significant quantities of N₂O whether fallow or planted, RME wins – emitting about 25% less CO₂e per kilometre driven.

Useful comparison
An even more useful comparison, however, involves comparing greenhouse gas emissions normalised by land use, either to grow rapeseed or trees. If petroleum diesel were substituted for biodiesel, land would be freed up to grow some other crop, including a forest that would function as a carbon dioxide sink. What would this do to the greenhouse gas balance? To answer that question, we used figures from the well-known EcoInvent database for the production of air-dried, sawn hardwood. Plugging these data into our inventory model gives a hands-down win by a factor of almost 2:1 for petroleum diesel.

For minimum greenhouse gas emissions, set-aside arable land should therefore be used as forest and not for growing biodiesel. To answer the question posed at the start of this article: no, the trade-off of substituting biofuels for fossil fuels – at least in the case of RME versus petroleum diesel – does not give a payoff with respect to global warming.

Broader picture
But before policymakers rush to ditch biofuels altogether, they might want to consider a rather broader picture. In addition to our study of global warming, we also compared the two fuels in terms of a range of other environmental impacts – from eco and biological toxicity to ozone layer depletion and acidification. To do so, we weighted the complete emissions inventories of each system, not just greenhouse gases, by using a commonly used impact assessment method. The answer is equivocal: petroleum diesel comes out ahead in five categories; biodiesel comes out ahead in the other five.

Eric Johnson, editor of Environmental Impact Assessment Review, is based in Zurich, Switzerland

Russell Heinen, vice president of SRI Consulting and manager of its Process Economics Program, is based in Houston, Texas, US

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N₂O laughing matter

Because emissions of N₂O (laughing gas) are a hot topic of research and crucially important to the study’s conclusions, we looked very carefully at four sensitivities:

Yield of rapeseed: We used a figure – sourced from the LCA Food Database, published by Denmark’s Ministry of Food, Agriculture and Fisheries – that is slightly below the average for Germany, which has the EU’s highest yields. Thanks to improved farming methods, these have nearly doubled from their levels of the 1970s. At the same time, these improvements require more fertiliser.

Quantity of nitrogenous fertiliser used: Our figure, again from the LCA Food Database, squares with those estimated by the United Nations’ Intergovernmental Panel on Climate Change (IPCC) for rotated crops.

N₂O emissions: These emissions are critical and also subject to ongoing debate by, among others, the European Council for Automotive R&D, the European Commission¹ and the IPCC. We have used a figure recently estimated by a group of UK researchers² that accounts for soil effects as well as whether fields are cultivated or fallow.

Organic farming: Emissions of N₂O are substantially lower from organic farms that use less fertiliser, but so are rapeseed yields.

References

1 Well-to-Wheels analysis of future automotive fuels and powertrains in the European context WELL-TO-TANK report version 2b, May 2006. European Council for Automotive R&D, CONCAWE and European Commission Joint Research Centre. Available at http://ies.jrc.ec.europa.eu/WTW

2 L Brown et al, Atmos. Environ. 2002, 36 (6), 917.

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