Biofuels - can they reduce carbon emissions ?

Currently, the most common types of biofuels are Bioethanol and Biodiesel. Bioethanol is generally derived through fermentation from crops such as sugar beet, cane or corn while Biodiesel is generally derived through esterification of vegetable oils from crops such as rapeseed, jatropha, soy or palm oil. The theory for using biofuels to reduce the CO2 emissions from transport is compelling: CO2 emitted during the combustion of biofuels does not contribute to net emissions of carbon dioxide because these emissions have already been absorbed by plants during growth.

While this theory is not wrong per se, it covers only a small part of the whole puzzle. Unfortunately it does not reflect the fact that there can be large CO2 and other greenhouse gas emissions in the biofuel production process. These emissions stem mainly from the use of farm machinery for planting and harvesting, use of fertilizers, and energy requirements for conversion to a liquid biofuel and in some cases they can be in fact larger than the CO2 savings from displacing fossil fuels. Fertilizers have attracted special attention since their use increases the release of nitrous oxides (N2O) from soil and N2O has a Global Warming Potential which is 298 times larger than CO2 (IPCC 2007). Moreover, a recent study* found that N2O emissions from soil can in fact be much larger than previously thought. Life Cycle Analyses and Well-to-Wheel studies** of various biomass sources and production paths showed that in some cases there can be significant reductions in greenhouse emissions compared to the fossil alternative, especially in case of biogenic wastes ranging from grass to wood.

However, new biofuel plantations can lead to land use changes (e.g. deforestation) that release large carbon emissions from loss of above ground stocks of biomass and from soil carbon. For example, converting wet rainforest to plantation would require 70 to 300 years of using palm oil to offset fossil fuels to compensate for the losses in the carbon stocks.***

Probably the most important drawback for biofuels is that land is a finite resource. Using all cultivable land for energy crops would not supply even current energy demand and land used for energy crops is not available for food.  

Food vs. Fuel

Probably the most important drawback for current biofuels is that land is a finite resource. Using all cultivable land for energy crops would not supply even current energy demand and land used for energy crops is not available for food. Competition for land has already affected food prices, which have gone up at an unprecedented rate in recent months leading to public outcry, for example in Mexico over tortillas and in Italy over pasta. Some of the price rise is due to abnormal weather, but at least in part results from biofuel demand.*** For instance, in 2004 the US produced 3.4 billion gallons of ethanol, consuming 12% of all corn grown that year in the US. However, this ethanol accounted for less than 1.5% of US motor vehicle fuel use (on an energy basis).****** Another study concluded that a 5% displacement of current diesel use would require about 15% of available cropland in the EU to produce biodiesel.***

This issue could be partly mitigated by high yield perennial ligno-cellulosic (ie. woody) crops such as willow and switch-grass (2nd generation biofuels). These crops do not compete with food and they achieve a higher fuel yield per hectare while minimising the processing needs at the same time. Despite these promising prospects, it still makes more sense to use these biomass sources directly as fuels for heat or combined heat and power instead of processing them into liquids.***

Sources:

* Crutzen P. J. et al., “N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels” Atmos. Chem. Phys. Discuss., 7, 11191-11205, 2007

** Zah R. Et al., Life Cycle Assessment of Energy Products: Environmental Assessment of Biofuels - Executive Summary. Bern, 2007 and CONCAWE/EUCAR/JCR, Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains in the European Context, Well-to-Wheels Report Version 2c, 2007.

*** Clift R. Sustainable fuel or folly? Freight Transport Review, Issue 15, 2007 and references therein

**** Volvo

***** International Energy Agency, Biofuels for transport - An international perspective, 2004.

****** Larson E.D. A review of life-cycle analysis studies on liquid biofuel systems for the transport sector, Energy for Sustainable Development, Vol X no 2, 2006