The greenhouse effect of carbon dioxide (CO2) is one of the critical factors that provide our planet with a suitable temperature to support life. However, since the industrial revolution, the concentration of CO2 in the atmosphere has risen from levels of about 280 parts per million 200 years ago to more than 380ppm today, and the current rate of increase is faster than has ever previously occurred.
Increased CO2 levels are the primary reason for the generally accepted phenomenon of global climate change, and the main culprit in this is the burning of fossil carbon supplies to meet our ever-increasing energy requirements.
So what steps are being taken to mitigate these high levels of CO2? The UK is a signatory to the Kyoto Protocol, which requires us to reduce our overall greenhouse gas emissions by 5.2 per cent below the 1990 levels by 2012. One approach to achieving this is to use biofuels derived from plants, the principle being that plants capture CO2 from the atmosphere in the process of photosynthesis, which is then returned back to the atmosphere when the plants are used as fuel - thus the process has the potential to be carbon neutral. However, in order to maximise the benefits of using biofuels, it is necessary to maximise the energy balance of the crop or, in other words, we need to keep energy inputs associated with their production to a minimum to make sure we are not using more energy to produce the biofuel than we save.
Biofuels can be divided into two categories - the so-called first and second generations. First-generation biofuels are represented by biodiesel and bioethanol, derived from plant oils or the fermentation of starches and sugars respectively, and by biomass fuels, which are burnt directly to generate heat or electricity. Second-generation biofuels are instead produced by converting the bulk lignocellulose component of plants, such as that sourced from biomass crops like short-rotation coppice willow and Miscanthus, or crop waste such as wheat straw, into a range of derived liquid fuels.
While the latter promises greater returns for reducing carbon emissions without competing with food supplies, the technology for its wider application is still under development. However, first-generation biofuels are here and now, and new legislation is driving their increasing utilisation.
In the UK, transport fuels account for about a quarter of national CO2 emissions. The UK government, acting on requirements imposed by the EU Biofuels Directive 20/30/EC, has instigated the Renewable Transport Fuel Obligation programme (RTFO). This sets out a stepwise process that obligates fuel providers to incorporate a minimum five per cent renewable component into fuel sold from UK forecourts by 2010. This can be achieved by blending biodiesel in diesel and bioethanol in petrol, and five per cent blends will run in standard car engines without any modifications. Achieving these targets would mean a decrease in emissions equivalent to about one million tonnes of CO2 per year.
Biodiesel is produced by refining used vegetable oil and from plant oils such as palm, corn, soya, sunflower and rapeseed oil, following a chemical process called
de-esterification which turns the oil to biodiesel and glycerol. While palm oil is currently the cheapest and most abundant source of plant oil, there are, however, a number of critical environmental issues such as the clearing of rainforests to make way for plantations, together with transport miles, that make this unfavourable for use in the UK and which would potentially tarnish the environmental credentials of a producer. The major UK oil crop is oilseed rape; this could provide the feedstock for UK production of biodiesel and will be the focus for the remainder of this article.
Oilseed rape is grown as a break crop in cereal rotations, with the oil historically being used primarily to produce culinary oils for cooking or margarine production. However, it turns out that rapeseed oil is also suitable, amongst other alternative end uses, for biodiesel production. According to EurObserv’ER, the UK produced 129,000t of biodiesel in 2005.
Defra statistics show that oilseed rape covered 519,0000 hectares in the UK in 2005. However, in order to meet the 2010 requirements of the RTFO, it is estimated that more than an additional 700,000ha would be required, with the current UK average yields of just over 3t/ha.
In order to deliver sufficient home-grown rapeseed oil, there is a need to increase yields. Expanding the area grown is one approach, but it is unrealistic to expect to meet the targets by this alone. Alternative strategies include increasing the oil content, which is currently 40-45 per cent of the seed and a priority trait for breeders, and to increase the yield per hectare, which is also another priority trait. The average UK annual yield has stagnated over the past 20 years, despite an ongoing increase in yields reported in HGCA recommended list trials, where top varieties can yield 5t/ha or more. Thus, a potential 50 per cent increase in yield could be obtained by improved agronomic practices.
It is also informative to compare oilseed rape with wheat, which has been the subject of intensive breeding efforts for more than 100 years, and can be considered as a highly developed crop.
Oilseed rape, on the other hand, has an intensive breeding history of only 40 years or so. This is reflected by the relative harvest indices of the two crops, i.e. the proportion of the entire crop that is harvested, which is around 50 per cent for wheat, while only 30-35 per cent for oilseed rape. This suggests that there is considerable progress still to be made in this area.
It is a general property of crop breeding that repeated selection for certain crop traits, with the elimination of undesirable characters, has the effect of reducing the genetic variation for the further improvement of the crop, as other useful genes will also be inadvertently removed in the process. Warwick HRI is addressing this issue with Defra-funded research by capturing the more extensive genetic diversity available in Brassica napus, the species to which oilseed rape belongs, and other closely related brassica species (cabbages and cauliflowers which have never been bred for their oils and show variation which may be of interest for industrial uses such as biodiesel), in a form that is suitable for breeders and researchers. These experimental lines will be a valuable source of useful properties that can be incorporated into future breeding programmes for the benefit of biodiesel production and a variety of other end uses.
Although maximising yield is crucial to the viability of using oilseed rape for biodiesel production, the promised reduction in greenhouse gas emissions offered by many biofuels is also offset to a considerable extent by the concomitant energy used in their production and distribution. UK oilseed rape has been bred to perform optimally under high-input conditions, and hence high yields are dependent on high nitrogen fertiliser inputs which are typically about
200kg per hectare. Analysis of the energy used in oilseed rape production, in terms of relative greenhouse gas emissions, reveals that 70 per cent can be attributed to the manufacture of the fertiliser, which relies on the very energy-demanding Harber-Bosch process, used to convert atmospheric nitrogen into ammonia.
In fact, according to the HGCA Greenhouse Gas Converter
(www.hgca.com), this process produces equivalent to 6.69kg of CO2 per kg of nitrogen. The adverse affect of nitrogen fertilisers is not restricted to their manufacture and application, either. Despite the high fertiliser nitrogen requirement, relatively little nitrogen is incorporated into the seed of oilseed rape, and the residues from the straw and soil can be broken down into pollutants such as ammonia and nitrous oxide (N2O).
N2O is also known as laughing gas, but environmentally it is no laughing matter, as it is a potent greenhouse gas with a global warming potential of over 300 times greater than that of CO2. Thus, in order to maximise the mitigation of atmospheric CO2 levels by using oilseed rape in biodiesel, it is necessary to find ways to reduce the amount of nitrogen fertiliser used. Warwick HRI is collaborating with other organisations, including ADAS, Rothamsted Research, other academic institutions, a number of breeders and other crucial players in the supply chain, on projects which aim to reduce the amount of nitrogen fertiliser applied to oilseed rape. This work will involve the development of new breeder-friendly screening technologies, advancement in our understanding of the genes involved in nitrogen usage and the identification of new plant lines with low nitrogen-requiring properties for breeders to use.
It is estimated that if the nitrogen requirement of oilseed rape can be halved, the reduction of greenhouse gas emissions would be the equivalent of more than 70,000t of CO2 a year.
The Defra projects referred to in this article are IF0125 and HH3723 and Defra-LINK project LK0979. Details of these and the range of other climate change related projects at Warwick HRI can be found at: http://www.go.warwick.ac.uk/climatechange/
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