How much Carbon and GHG emissions is biofuel expected to save?

I would like to start with perhaps the most optimistic view of the potential carbon saving impact of biofuels to achieve juxtaposition of the theory underlying the concept of biofuels with the reality of their effects in practice.

In his work, Bill Butterworth uses the Carboniferous Era as a model for biofuels. He suggests that approximately 350mya, the death of the vegetation that later formed our contemporary fossil fuels acted as a carbon sink, where the amount of GHGs that was taken up by the plants during their life exceeded the amount that was released. The idea of biofuels thus also parallels with that of farming, Butterworth points out, where the crops uptake carbon dioxide and release oxygen during growth. While GHGs are emitted on death, this happens at such a slow rate that it makes the process sustainable, enabling farming practices to take place for over 10,000 years (Butterworth, 2009 and Butterworth, 2009).

‘So why on Earth has the issue of climate warming not been resolved yet?’, was the question that left me unconvinced after I first read his book.

One only has to look a little closer to realize that the answer lies within using the processes of the Carboniferous 350mya and farming practices 10kya as analogues for the present. The first limitation to this approach is that machinery and synthetic fertilizers were not used to promote vegetation growth during those times. These usually operate on fossil fuels, decreasing the carbon saving efficiency of biofuels in practice. Additionally, in reality there are other factors that are not taken into account by Butterworth’s model. These are reflected in the following estimates of GHG reduction potential calculated by the Renewable Fuels Agency and the Department for Energy and Climate Change (RFA & DEC, 2009).

For example, synthesising ethanol from European sugar beat in the UK was calculated to result in either ~30% carbon saving or ~30% carbon increase in comparison with fossil diesel. Similarly, using tallow as biofuel in the UK may produce 56% less carbon in the best-case scenario, but may amount to releasing 13% more carbon than fossil fuel diesel in the worst-case scenario. 

On the other hand, the calculations are highly optimistic for other types of material. For example, the greatest carbon saving results from MSW (municipal solid waste), which produces a ‘carbon saving’ of 193% compared to fossil fuel (RFA & DECC, 2009). Utilizing wheat straw in the UK was also calculated to result in a high carbon saving of 80% compared to fossil diesel (RFA & DECC, 2009).

The question that this raises is why are there such large uncertainty levels for some biofuel materials and not for others, and why some biofuel materials result in significantly higher carbon savings than others, and what does this show about how ‘green’ biofuels are overall.