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Energy crop | Wikipedia audio article


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. 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.===CO2 neutrality===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. estimates 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. argues 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 makes
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.==Types=====Solid biomass===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)===
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 farms 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.===Liquid biomass=======
Biodiesel====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.====Bioethanol====
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 countries==
In Sweden, willow and hemp are often used. In Finland, Reed Canary Grass is a popular
energy crop.==Energy crop use in thermal power stations
==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 then just carbon neutral.
Improving the energy efficiency of a coal-fired power plant can also reduce emissions.==See also

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