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Energy crops are low-cost and low-maintenance crops grown solely for energy production (not for food). The crops are processed into solid, liquid or gaseous fuels, such as pellets, bioethanol or biogas. The fuels are burned to generate power or heat.
The plants are generally categorized as woody or herbaceous. Woody plants include willow and poplar, herbaceous plants include Miscanthus x giganteus and Pennisetum purpureum (both known as elephant grass). Herbaceous crops, while physically smaller than trees, store roughly twice the amount of CO2 (in the form of carbon) below ground, compared to woody crops.
Through biotechnological procedures such as genetic modification plants can be manipulated to create higher yields. Relatively high yields can also be realized with existing cultivars.:250 However, some additional advantages such as reduced associated costs (i.e. costs during the manufacturing process ) and less water use can only be accomplished by using genetically modified crops.
The amount of carbon sequestrated and the amount of GHG (greenhouse gases) emitted will determine if the total GHG life cycle cost of a bio-energy project is positive, neutral or negative. Specifically, a GHG/carbon-negative life cycle is possible if the total below-ground carbon accumulation more than compensates for the above-ground total life-cycle GHG emissions. Whitaker et al. estimate that for Miscanthus x giganteus, carbon neutrality and even negativity is within reach. Basically, the yield and related carbon sequestration is so high that it more than compensates for both farm operations emissions, fuel conversion emissions and transport emissions. The graphic on the right displays two CO2 negative Miscanthus x giganteus production pathways, represented in gram CO2-equivalents per megajoule. The yellow diamonds represent mean values.
One should note that successful sequestration is dependent on planting sites, as the best soils for sequestration are those that are currently low in carbon. The varied results displayed in the graph highlights this fact. Milner et al. argue that for the UK, successful sequestration is expected for arable land over most of England and Wales, with unsuccessful sequestration expected in parts of Scotland, due to already carbon-rich soils (existing woodland). Also, for Scotland, the relatively lower yields in this colder climate make CO2 negativity harder to achieve. Soils already rich in carbon includes peatland and mature forest. Grassland can also be carbon-rich, however Milner et al. further argues that the most successful carbon sequestration in the UK takes place below improved grasslands. The bottom graphic displays the estimated yield necessary to achieve CO2 negativity for different levels of existing soil carbon saturation.
The perennial rather than annual nature of Miscanthus crops implies that the significant below-ground carbon accumulation each year is allowed to continue undisturbed. No annual plowing or digging means no increased carbon oxidation and no stimulation of the microbe populations in the soil, and therefore no accelerated carbon-to-CO2 conversion happening in the soil every spring.
Solid biomass, often pelletized, is used for combustion in thermal power stations, either alone or co-fired with other fuels. Alternatively it may be used for heat or combined heat and power (CHP) production.
In short rotation coppice (SRC) agriculture, fast growing tree species like willow and poplar are grown and harvested in short cycles of three to five years. These trees grow best in wet soil conditions. An influence on local water conditions can not be excluded. Establishment close to vulnerable wetland should be avoided.
Gas biomass (methane)Edit
Whole crops such as maize, Sudan grass, millet, white sweet clover, and many others can be made into silage and then converted into biogas.Anaerobic digesters or biogas plants can be directly supplemented with energy crops once they have been ensiled into silage. The fastest-growing sector of German biofarming has been in the area of "Renewable Energy Crops" on nearly 500,000 ha (1,200,000 acres) of land (2006). Energy crops can also be grown to boost gas yields where feedstocks have a low energy content, such as manures and spoiled grain. It is estimated that the energy yield presently of bioenergy crops converted via silage to methane is about 2 GWh/km2 (1.8×1010 BTU/sq mi) annually. Small mixed cropping enterprises with animals can use a portion of their acreage to grow and convert energy crops and sustain the entire farm's energy requirements with about one-fifth of the acreage. In Europe and especially Germany, however, this rapid growth has occurred only with substantial government support, as in the German bonus system for renewable energy. Similar developments of integrating crop farming and bioenergy production via silage-methane have been almost entirely overlooked in N. America, where political and structural issues and a huge continued push to centralize energy production has overshadowed positive developments.
European production of biodiesel from energy crops has grown steadily in the last decade, principally focused on rapeseed used for oil and energy. Production of oil/biodiesel from rape covers more than 12,000 km² in Germany alone, and has doubled in the past 15 years. Typical yield of oil as pure biodiesel may be is 100,000 L/km2 (68,000 US gal/sq mi; 57,000 imp gal/sq mi) or more, making biodiesel crops economically attractive, provided sustainable crop rotations exist that are nutrient-balanced and preventative of the spread of disease such as clubroot. Biodiesel yield of soybeans is significantly lower than that of rape.
Energy crops for biobutanol are grasses. Two leading non-food crops for the production of cellulosic bioethanol are switchgrass and giant miscanthus. There has been a preoccupation with cellulosic bioethanol in America as the agricultural structure supporting biomethane is absent in many regions, with no credits or bonus system in place. Consequently, a lot of private money and investor hopes are being pinned on marketable and patentable innovations in enzyme hydrolysis and the like.
Bioethanol also refers to the technology of using principally corn (maize seed) to make ethanol directly through fermentation, a process that under certain field and process conditions can consume as much energy as is the energy value of the ethanol it produces, therefore being non-sustainable. New developments in converting grain stillage (referred to as distillers grain stillage or DGS) into biogas energy looks promising as a means to improve the poor energy ratio of this type of bioethanol process.
Energy crop use in various countriesEdit
In Sweden, willow and hemp are often used.
Energy crop use in thermal power stationsEdit
Several methods exist to reduce pollution and reduce or eliminate carbon emissions of fossil fuel power plants. A frequently used and cost-efficient method is to convert a plant to run on a different fuel (such as energy crops/biomass). In some instances, torrefaction of biomass may benefit the power plant if energy crops/biomass is the material the converted fossil fuel power plant will be using. Also, when using energy crops as the fuel, and if implementing biochar production, the thermal power plant can even become carbon negative rather than just carbon neutral. Improving the energy efficiency of a coal-fired power plant can also reduce emissions.
- Mola-Yudego, Blas & Aronsson, Pär. (2008). Yield models for commercial willow biomass plantations in Sweden. Biomass and Bioenergy. 32. 829-837. https://doi.org/10.1016/j.biombioe.2008.01.002
- «In conclusion, the annual net SOC [soil organic carbon] storage change exceeds the minimum mitigation requirement (0.25 Mg C ha−1 year−1)[0.25 metric tons carbon per hectare per year] under herbaceous and woody perennials by far (1.14 to 1.88 and 0.63 to 0.72 Mg C ha−1 year−1, respectively).» Agostini F, Gregory AS, Richter GM. Carbon Sequestration by Perennial Energy Crops: Is the Jury Still Out?. Bioenergy Res. 2015;8:1057–1080, page 1057. doi:10.1007/s12155-014-9571-0 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4732603/ This article incorporates text available under the CC BY 4.0 license. (The CC BY 4.0 licence means that everyone has the right to reuse the text that is quoted here, or other parts of the original article itself if they credit the authors. More info: https://en.wikipedia.org/wiki/Creative_Commons_license)
- Ara Kirakosyan; Peter B. Kaufman (2009-08-15). Recent Advances in Plant Biotechnology. p. 169. ISBN 9781441901934. Retrieved 14 February 2013.
- An example here is [https://biotechnologyforbiofuels.biomedcentral.com/articles/10.1186/s13068-017-0793-1 CINNAMOYL-CoA REDUCTASE maize
- «A life‐cycle perspective of the relative contributions and variability of soil carbon stock change and nitrogen‐related emissions to the net GHG intensity (g CO2‐eq MJ−1) [gram CO2-equivalents per megajoule] of biofuel production via select production pathways (feedstock/prior land‐use/fertilizer/conversion type). Positive and negative contributions to life‐cycle GHG emissions are plotted sequentially and summed as the net GHG intensity for each biofuel scenario, relative to the GHG intensity of conventional gasoline (brown line) and the 50% and 60% GHG savings thresholds (US Renewable Fuel Standard and Council Directive 2015/1513); orange and red lines, respectively. Default life‐cycle GHG source estimates are taken from Wang et al. (2012) and Dunn et al. (2013); direct N2O emissions from Fig. 1; and soil carbon stock change (0–100 cm depth) from Qin et al. (2016). See Appendix S1 for detailed methods.» Whitaker, J. , Field, J. L., Bernacchi, C. J., Cerri, C. E., Ceulemans, R. , Davies, C. A., DeLucia, E. H., Donnison, I. S., McCalmont, J. P., Paustian, K. , Rowe, R. L., Smith, P. , Thornley, P. and McNamara, N. P. (2018), Consensus, uncertainties and challenges for perennial bioenergy crops and land use. GCB Bioenergy, 10: 150-164. https://doi.org/10.1111/gcbb.12488 This article incorporates text available under the CC BY 4.0 license. (The CC BY 4.0 licence means that everyone has the right to reuse the text that is quoted here or other parts of the original article itself if they credit the authors. More info: https://en.wikipedia.org/wiki/Creative_Commons_license)
- «Whilst these values represent the extremes, they demonstrate that site selection for bioenergy crop cultivation can make the difference between large GHG savings or losses, shifting life‐cycle GHG [green house gas] emissions above or below mandated thresholds. Reducing uncertainties in ∆C [carbon increase or decrease] following LUC [land use change] is, therefore, more important than refining N2O [nitrous oxide] emission estimates (Berhongaray et al., 2017). Knowledge of initial soil carbon stocks could improve GHG savings achieved through targeted deployment of perennial bioenergy crops on low carbon soils (see section 2). […] The assumption that annual cropland provides greater potential for soil carbon sequestration than grassland appears to be over‐simplistic, but there is an opportunity to improve predictions of soil carbon sequestration potential using information on the initial soil carbon stock as a stronger predictor of ∆C [change in carbon amount] than prior land use.» Whitaker, J. , Field, J. L., Bernacchi, C. J., Cerri, C. E., Ceulemans, R. , Davies, C. A., DeLucia, E. H., Donnison, I. S., McCalmont, J. P., Paustian, K. , Rowe, R. L., Smith, P. , Thornley, P. and McNamara, N. P. (2018), Consensus, uncertainties and challenges for perennial bioenergy crops and land use. GCB Bioenergy, 10: 150-164. https://doi.org/10.1111/gcbb.12488 This article incorporates text available under the CC BY 4.0 license. (The CC BY 4.0 licence means that everyone has the right to reuse the text that is quoted here or other parts of the original article itself, if they credit the authors. More info: https://en.wikipedia.org/wiki/Creative_Commons_license)
- «Fig. 3 confirmed either no change or a gain of SOC [soil organic carbon] (positive) through planting Miscanthus on arable land across England and Wales and only a loss of SOC (negative) in parts of Scotland. The total annual SOC change across GB in the transition from arable to Miscanthus if all nonconstrained land was planted with would be 3.3 Tg C yr−1 [3.3 million tonnes carbon per year]. The mean changes for SOC for the different land uses were all positive when histosols were excluded, with improved grasslands yielding the highest Mg C ha−1 yr−1 [tonnes carbon per hectare per year] at 1.49, followed by arable lands at 1.28 and forest at 1. Separating this SOC change by original land use (Fig. 4) reveals that there are large regions of improved grasslands which, if planted with bioenergy crops, are predicted to result in an increase in SOC. A similar result was found when considering the transition from arable land; however, for central-eastern England, there was a predicted neutral effect on SOC. Scotland, however, is predicted to have a decrease for all land uses, particularly for woodland due mainly to higher SOC and lower Miscanthus yields and hence less input.» Milner, S., Holland, R. A., Lovett, A., Sunnenberg, G., Hastings, A., Smith, P., Wang, S. and Taylor, G. (2016), Potential impacts on ecosystem services of land-use transitions to second‐generation bioenergy crops in GB. GCB Bioenergy, 8: 317-333. https://doi.org/10.1111/gcbb.12263 This article incorporates text available under the CC BY 4.0 license. (The CC BY 4.0 licence means that everyone has the right to reuse the text that is quoted here or other parts of the original article itself if they credit the authors. More info: https://en.wikipedia.org/wiki/Creative_Commons_license)
- Hartwich, Jens (2017). "Assessment of the regional suitability of short rotation coppice in Germany (PDF Download Available)". Doctoral Thesis. Freie Universität Berlin. Institut für Geographische Wissenschaften. doi:10.13140/rg.2.2.17825.20326 – via ResearchGate.
- Hartwich, Jens; Bölscher, Jens; Schulte, Achim (2014-09-19). "Impact of short-rotation coppice on water and land resources". Water International. 39 (6): 813–825. doi:10.1080/02508060.2014.959870. ISSN 0250-8060.
- Hartwich, Jens; Schmidt, Markus; Bölscher, Jens; Reinhardt-Imjela, Christian; Murach, Dieter; Schulte, Achim (2016-07-11). "Hydrological modelling of changes in the water balance due to the impact of woody biomass production in the North German Plain". Environmental Earth Sciences. 75 (14): 1–17. doi:10.1007/s12665-016-5870-4. ISSN 1866-6280.
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