- Sources of emissions variation in biofuel cultivation:
Synthetic nitrogen fertlizer: temporal and spatial cultivation management variations occur depending on where and how mineral fertilizer is applied, with the mineral nitrogen fertlizer synthesis being responsible for ~5% of the global natural gas consumption (Butterworth, 2009). GHG emissions from biofuels due to fertlizer application can result in 14% higher emissions than using fossil fuels (Zamboni et al., 2011). Firstly, nitrous oxide emissions from soils vary sptially and temporally due to temperature, precipitation, pH and soil organic carbon (SOC) (Ogle et al., n/d). Secondly, emissions depend on the efficiency of fertilizer application (Butterworth, 2009). Additionally, emissions come from fertilizer manufacturing, which usually uses fossil fuels, and from the direct nitrous oxide emissions from the soil (Crutzen et al., 2008; Zamboni et al., 2011). Although nitrous oxide emissions are smaller by volume than carbon dioxide emissions, the former is 300 times more potent (Zamboni et al., 2011).
However, since nitrous oxide emissions increse with inefficient appliction, they can be minimized e.g. in his book ‘How bad are bananas: carbon footprint of everything’ (2010), Mike Berners-Lee shows that avoiding the common practice of excessive fertilizer application can decrease GHG emissions from rice cultivation by a third. This also applies to biofuel crops, where if fertilizer is only added according to demand, emissions can be significantly reduced (Butterworth, 2009). Other ways in which these emissions can be reduced are suggested by Butterworth (2009). He proposes using household organic waste material compost as fertlizer, estimating that 600 US households produce enough waste to fertlize 10 ha of cultivated land, which would result in 1 te of oil in just a 'single coldpress'. Although some emissions will still occur, these emissions would have occurred anyway as the waste would have decomposed in landfill. This means no extra emissions will result from compost fertlizer for biofuel cultivation in this way. Additionally, compost decomposes and releases nitrates slower making them available for crop consumption more slowly than synthetic fertlizer, meaning there will be less 'leakage' of nitrates, as the supply is more likely to meet demand (Butterworth, 2009). Other research is being done into using gypsum waste from construction sites as fertlizer, which may also reduce emissions (UNCC, 2010).
Zamboni et al. (2011) also found that there are other economically and environmentally undesirable effects of excessive fertilizer application which decrease GHG reduction efficiency of biofuels. While fertilizers increase crop yield, they increase the protein and thus decrease the starch content in crops like corn, thus reducing the efficiency of ethanol generation (which requires starch) and of GHG savings (Zamboni et al., 2011). However, not using fertilizer at all is unrealistic, due to the lack of economic sustainability. If the protein-rich by-product, DDGS (dry distillers grains with solubles), is utilized for energy generation on the other hand, the process becomes much more efficient in terms of emissions saving and economic sustainability, resulting in a 54-63% GHG saving for wheat and up to 80% for corn Zamboni et al., 2011).
Machinery use: the other emissions from cultivation come from machinery use, which usually operates on fossil fuels and thus depend on the intensity of this use and on the fuel type used in the machinery. For example, Butterworth (2009) provides an example of how these emissions have been minimised on Bate's farm in Lincolnshire by running machinery on 100% biofuel produced sustainably on the farm, where the biofuel is grown using organic waste from the farm. However, please note that so far i have only managed to find one published example of a farm where the sustainable practices suggested by scholars like Butterworth are extensively employed to date, meaning that in practice machinery use and fertilizer emissions still remain relatively important (DFT, 2010).
Zamboni et al. (2011) also found that there are other economically and environmentally undesirable effects of excessive fertilizer application which decrease GHG reduction efficiency of biofuels. While fertilizers increase crop yield, they increase the protein and thus decrease the starch content in crops like corn, thus reducing the efficiency of ethanol generation (which requires starch) and of GHG savings (Zamboni et al., 2011). However, not using fertilizer at all is unrealistic, due to the lack of economic sustainability. If the protein-rich by-product, DDGS (dry distillers grains with solubles), is utilized for energy generation on the other hand, the process becomes much more efficient in terms of emissions saving and economic sustainability, resulting in a 54-63% GHG saving for wheat and up to 80% for corn Zamboni et al., 2011).
I know it's a lot to read, but you wouldn't want your bread to be grown on fossil fuels, would you? So we need to evaluate whether biofuels can be our saviour. To do this, keep on reading!
Machinery use: the other emissions from cultivation come from machinery use, which usually operates on fossil fuels and thus depend on the intensity of this use and on the fuel type used in the machinery. For example, Butterworth (2009) provides an example of how these emissions have been minimised on Bate's farm in Lincolnshire by running machinery on 100% biofuel produced sustainably on the farm, where the biofuel is grown using organic waste from the farm. However, please note that so far i have only managed to find one published example of a farm where the sustainable practices suggested by scholars like Butterworth are extensively employed to date, meaning that in practice machinery use and fertilizer emissions still remain relatively important (DFT, 2010).
Crop type used: this also affects GHG emissions and savings of biofuels, with crops that are not nitrogen-intensive, such as switchgrass, elephant grass and palm oil resulting in larger GHG savings, while nitrogen-intensive crops, like rapeseed may even result in a 1-1.5 times higher warming impact than fossil fuels (Crutzen et al., 2008). * This may be worrying as at present over 80% of biodiesel contains rapeseed (Crutzen et al., 2008). This reflects on the greater GHG saving efficiency of 2nd generation biofuels compared to 1st generation e.g. wsitchgrass and poplar result in 3 times greater GHG rediction than soybean-corn rotation (Adler et al., 2007).
Cultivation techniques: other aspects of cultivation management variations include the tilling method used ('tillage' is the preparation of soil for crop planting, through activities such as ploughing: PSU, 1996) e.g. the use of plough tillage reduces the soil organic carbon (SOC) content by 30% in 100 years for grassland soils, while if tillage is not practiced and winter cover crops are used, the SOC increases by 35%, thus indicating a major change towards net carbon uptake (Kim et al,, 2008). The SOC content of soils also differs, producing different emissions savings e.g. a forest soil has a higher SOC than a grassland soil and will therefore result in larger emissions and smaller GHG savings (Kim et al,, 2008). This land-use change effect will be discussed further in later posts.
Summary of the last two posts: assuming no major land-use change emissions from biofuels, cultivation may account for ~45% of total GHG emissions from biofuel production (Zamboni et al., 2011). Cultivation emissions arise mainly from synthetic nitrogen fertilizer application, machinery use and management practices such as ploughing; what is done with the by-products such as DDGS may also be crucial. Most of these emissions can theoretically be minimised significantly using appropriate management techniques, such as the monitoring of fertilizer application to make sure supply meets crop demand. However, this does not mean that such management has been extensively implemented in practice to date.
*Note: this figure may be considered an over-estimate, as the study included the emissions from manure. However, manure is a side-product of cattle-farming and the emissions are therefore only indirectly related to biofuels (Ogle et al., n/d). The debate into whether indirect emissions from biofuels should be incorporated into emissions calculations will be mentioned in later posts.
Summary of the last two posts: assuming no major land-use change emissions from biofuels, cultivation may account for ~45% of total GHG emissions from biofuel production (Zamboni et al., 2011). Cultivation emissions arise mainly from synthetic nitrogen fertilizer application, machinery use and management practices such as ploughing; what is done with the by-products such as DDGS may also be crucial. Most of these emissions can theoretically be minimised significantly using appropriate management techniques, such as the monitoring of fertilizer application to make sure supply meets crop demand. However, this does not mean that such management has been extensively implemented in practice to date.
*Note: this figure may be considered an over-estimate, as the study included the emissions from manure. However, manure is a side-product of cattle-farming and the emissions are therefore only indirectly related to biofuels (Ogle et al., n/d). The debate into whether indirect emissions from biofuels should be incorporated into emissions calculations will be mentioned in later posts.
No comments:
Post a Comment