Бурение грунтовых зондов, установка энергетических колодцев

Switchgrass (Panicum virgatum) is a perennial warm-season bunch — grass native to North America, where it occurs naturally from 55°N latitude in Canada southwards into the United States and Mexico. Other common names for switchgrass include tall panic grass, Wobsqua grass, black bent, tall prairie grass, wild redtop, and Virginia switchgrass. Switchgrass is one of the dominant species of the Central North American tall grass prairie and can be found in remnant prairies, in native grass pastures, and naturalized along roadsides. It is used primarily for soil conservation, forage produc­tion, game cover and as an ornamental grass. More recently it has been used as a biomass crop for ethanol in phytoremidiation proj­ects, fiber, electricity, heat production, and for biosequestration of atmospheric carbon dioxide.

Switchgrass is a hardy, deep-rooted, perennial rhizomatous grass that begins growth in late spring. It can grow up to 2.7 m high, but is typically shorter than big bluestem grass or indiangrass. The leaves are 30-90 cm long, with a prominent midrib. Switchgrass uses C4 carbon fixation, giving it an advantage in conditions of drought and high temperature. Its flowers have a well-developed panicle, often up to 60 cm long, and it bears a good crop of seeds. The seeds are 3-6 mm long and up to 1.5 mm wide, and are devel­oped from a single-flowered spikelet. Both glumes are present and well developed. When ripe, the seeds sometimes take on a pink or dull-purple tinge, and turn golden brown with the foliage of the plant in the fall. Switchgrass is both a perennial and self-seeding crop, which means farmers do not have to plant and reseed after annual harvesting. Once established, a switchgrass stand can sur­vive for ten years or longer. Unlike a number of food crops like corn, switchgrass can grow on marginal lands and requires rela­tively modest levels of chemical fertilizers. Overall, it is considered a resource-efficient, low-input crop with tremendous potential as an energy crop. A detailed chemical analysis of switchgrass is shown in Table 3.12.

Ken Vogel’s group from the USDA has analyzed the net energy efficiency and economic feasibility of switchgrass and similar crops. In large-scale field experiments, they managed switchgrass as a biomass energy crop in field trials of 3-9 ha (1 ha = 10,000 m2) on marginal cropland on 10 farms across a wide precipitation and temperature gradient in the Midcontinental United States [120]. Their objective was to determine net energy and economic costs based on known farm inputs and harvested yields. In this report, they summarized the agricultural energy input costs, biomass yield, estimated ethanol output, greenhouse gas emissions and net energy results. Annual biomass yields of established fields aver­aged 5.2-11.1 Mg-ha-1 with a resulting average estimated net energy yield (NEY) of 60 GJ-ha-1-y -1 [120]. Switchgrass produced 540% more renewable energy than nonrenewable energy consumed. Switchgrass monocultures produced 93% more biomass yield, and an equivalent estimated NEY than previous estimates from human — made prairies that received low agricultural inputs. Estimated average greenhouse gas (GHG) emissions from cellulosic ethanol derived from switchgrass were 94% lower than estimated GHG from gasoline. This is a very encouraging baseline study from the USDA that represents the genetic material and agronomic technol­ogy available for switchgrass production in 2000 and 2001, when the fields were planted. It is generally believed that improved genetics and agronomics may further enhance energy sustainabil­ity and biofuel yield of switchgrass [120].

Switchgrass has been researched as a renewable bioenergy crop since the mid 1980s, particularly in the United States, in a 10 year US Department of Energy (USDOE)-sponsored research program designed to evaluate and develop switchgrass. The programmatic objectives were to identify the best varieties and management prac­tices to optimize productivity, while developing an understanding of the basis for long-term improvement of switchgrass through breeding and sustainable production in conventional agro ecosys­tems. This research has reduced the projected production cost of switchgrass by about 25% through yield increases of about 50% achieved through selection of the best regionally-adapted varieties, through optimizing cutting frequency and timing, and by reducing the nitrogen fertilization level in about 40%. Breeding research has made further gains in productivity of switchgrass that exceed the historical rate of yield improvement of corn. Studies of soil carbon storage under switchgrass indicate significant carbon sequestration will occur in soils that will improve soil productivity and nutrient cycling, and can substantially augment greenhouse gas reductions associated with substituting renewable energy for fossil energy.

Switchgrass is now being considered for cellulosic ethanol as well as in several bioenergy conversion processes, including production of biogas, bio-oil, and direct combustion for energy applications. The main agronomic advantages of switchgrass as a bioenergy crop are:

1. High yield per hectare [119],

2. Drought and flooding tolerance,

3. Relatively low herbicide and fertilizer input requirements,

4. Widespread adaptability in temperate climates; it can grow and even thrive in many weather conditions, dif­ferent soil types, and land conditions,

5. Ease of management.

Switchgrass is a high yielding grass, and in some warm, humid southern zones in the United States it has the ability to produce up to 25 oven-dry tonnes per hectare (ODT/ha). In 13 research trial sites in the United States the top two cultivars produced 9.4 to 22.9 t/ha, with an average yield of 14.6 ODT/ha [121]. Some of the results of long-term switchgrass plantations in the United States are shown in Table 3.13.

However, these yields were recorded on small plot trials, and commercial field sites could be expected to be at least 20% lower than these results. In the United States, switchgrass yields appear to be highest in warm, humid regions with long growing seasons such as the US Southeast, and lowest in dry, short season areas of the northern Great Plains [121].

The energy inputs required to grow switchgrass are favorable when compared with annual seed-bearing crops such as corn, canola or soybean, which can require relatively high energy inputs for field operations, crop drying, and fertilization. Whole plant her­baceous perennial C4 grass feedstocks are desirable biomass energy feedstocks, as they require fewer fossil energy inputs to grow, and effectively capture solar energy because of their C4 photosynthetic system and perennial nature. One study cites that it takes from 0.97 to 1.34 GJ to produce 1 tonne of switchgrass, compared with 1.99 to 2.66 GJ to produce 1 tonne of corn. [122]. Another study found that switchgrass uses 0.8 GJ/ODT of fossil energy compared to grain

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corn’s 2.9 GJ/ODT [123]. Given that switchgrass contains approxi­mately 18.8 GJ/ODT of biomass, the energy output-to-input ratio for the crop can be up to 20:1. This highly favorable ratio is attribut­able to its relatively high-energy output per hectare and low-energy inputs for production.

Considerable effort is presently being expended in developing switchgrass as a cellulosic ethanol crop in the United States. In 2006, President George W. Bush in his State of the Union address proposed using switchgrass for ethanol; since then, over US $100 million has been invested into researching switchgrass as a potential biofuel source. Switchgrass has the potential to produce up to 380 liters of ethanol per tonne harvested. However, current technology for her­baceous biomass conversion to ethanol is about 340 liters per tonne.

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