Miscanthus x giganteus

Miscanthus × giganteus is propagated by cutting the rhizomes (its below-ground stems) into small pieces, and then re-planting those pieces 10 cm (4 in) below ground. One hectare (2.5 acres) of miscanthus rhizomes, cut into pieces, can be used to plant 10–30 hectares of new miscanthus fields (multiplication factor 10–30).[lower-alpha 5] Rhizome propagation is a labor-intensive way of planting new crops, but only happens once during a crop's lifetime. New and cheaper propagation techniques is underway, which seem to increase the multiplication factor from 10–30 to 1000–2000.[lower-alpha 6][lower-alpha 7] A halving of the cost is predicted.[lower-alpha 8]

A limited amount of herbicide should only be applied at the beginning of the first two seasons; after the second year the dense canopy and the mulch formed by dead leaves effectively reduces weed growth.[3] Other pesticides are not needed.[4] Because of miscanthus' high nitrogen use efficiency,[lower-alpha 9] fertilizer is also usually not needed.[lower-alpha 10] Mulch film, on the other hand, helps both M. x giganteus and various seed based hybrids to grow faster and taller, with a larger number of stems per plant, effectively reducing the establishment phase from three years to two.[lower-alpha 11] The reason seems to be that this plastic film keeps the humidity in the topsoil and increases the temperature.[lower-alpha 12]

Yield estimate for Miscanthus x giganteus in Europe (no irrigation).

Miscanthus is close to the theoretical maximum efficiency at turning solar radiation into biomass,[lower-alpha 13] and its water use efficiency is among the highest of any crop.[lower-alpha 14] It has twice the water use efficiency of its fellow C4 plant maize, twice the efficiency as the C3 energy crop willow (Salix viminalis), and four times the efficiency as the C3 plant wheat.[lower-alpha 15] This combined efficiency makes miscanthus fields energy dense. Since miscanthus has an energy content of 18 GJ per dry tonne, the typical UK dry yield (winter harvest) of 11–14 tonnes per hectare produce 200–250 gigajoules of energy per hectare per year in that particular region. This compares favorably to maize (98 GJ), oil seed rape (25 GJ), and wheat/sugar beet (7–15 GJ),[lower-alpha 16] underlining the differences between first and second generation bioenergy crops. In the USA, M. x giganteus has been shown to yield two times more than switchgrass.[5]

Hastings et al. note that "[f]ield trials have shown that for many locations in Europe M. x giganteus has the largest energy yield of all potential bioenergy crops in terms of net MJ ha ⁻¹ [megajoule per hectare], and the highest energy‐use efficiency (EUE), in terms of the energy cost of production, due to its relatively high yields and low inputs [...]".[6] The main competitors yield wise is willow and poplar, grown at short rotation coppice (SRC) or short rotation forestry (SRF) plantations. In the northern parts of Europe, willow and poplar approach and sometimes exceed miscanthus winter yields in the same location.[lower-alpha 17] FAO (Food and Agriculture Organization of the United Nations) estimate that forest plantation yields range from 1 to 25 m³ "green" (non-dried) wood per hectare per year globally, equivalent to 0.4–12.2 dry tonnes per hectare per year. Russian pine have the lowest yield (0.4–2 tonnes, or 1–5 m³), while eucalyptus in Argentina, Brazil, Chile and Uruguay, and poplar in France/Italy, have the highest (7.8–12.2 tonnes for eucalyptus and 2.7–8.4 tonnes for poplar.)[lower-alpha 18] For natural temperate mixed forests, Vaclav Smil estimates somewhat lower average sustainable yields (NAI: Net Annual Increment); 1.5–2 dry tonnes per hectare (2–2,5 m³ per hectare, ranging from 0.9 m3 in Greece to 6 m³ in France).[7] Forest Research estimate that UK forest stands typically yield 1 to 3 dry tonnes per hectare per year, with the exception of poplar stands, which yield 3 to 7 tonnes.[8] IPCC provides average net annual biomass growth data for natural forests globally. Net growth varies between 0.1 and 9.3 dry tonnes per hectare per year, with most natural forests producing between 1 and 4 tonnes, and with the global average at 2.3 tonnes. Average net growth for plantation forests varies between 0.4 and 25 tonnes, with most plantations producing between 5 and 15 tonnes, and with the global average at 9.1 tonnes.[9]

The miscanthus peak yield is reached at the end of summer but harvest is typically delayed until winter or early spring. Yield is roughly 33% lower at this point because of leaves drop, but the combustion quality is higher. Delayed harvest also allows nitrogen to move back into the rhizome for use by the plant in the following growing season.[lower-alpha 19]

In Europe the peak (autumn) dry mass yield has been measured to 10–40 tonnes per hectare per year (4–16 tonnes per acre per year), depending on location, with a mean peak dry mass yield of 22 tonnes.[lower-alpha 20] Yields are highest in southern Europe; Roncucci et al. quote dry mass yields of 25–30 tonnes generally for that area under rainfed conditions. With irrigation, trials in Portugal yielded 36 tonnes, Italy 34–38 tonnes, and Greece 38–44 tonnes.[10] Trials in Illinois, USA, yielded 10–15 tonnes per acre (25–37 t/ha). Like in Europe, yields increase as you move south. In general, Vaclav Smil estimates roughly a doubling of net primary production (NPP) of biomass in the tropics compared to the temperate regions of the world.[11] For Micanthus x giganteus specifically, there are no scientific field trials available yet regarding yields in the tropics. However, Sheperd et al. state that Micanthus x giganteus «downregulates assimilate production above 28°C».[12] Consequently, they predict that yields in the tropics will be low. No average estimate is provided, although worldwide they expect an average yield of 9 tonnes (this includes very cold regions).[13] The authors note that other miscanthus genotypes have a higher tolerance for heat, e.g. Miscanthus Sinensis that does not start to downregulate photosynthesis until the temperature reach 35 °C.[12] Other elephant grass types more suited to high temperatures (different napier variants) have been shown to yield up to 80 tonnes per hectare,[lower-alpha 21][lower-alpha 22][lower-alpha 23] and commercial napier grass developers advertise yields of around 100 dry tonnes per hectare per year, provided there is an adequate amount of rain or irrigation available (100 mm per month).[lower-alpha 24][lower-alpha 25]

Felten et al. found a mean winter/spring yield of 15 tonnes per hectare per year (6.1 tonnes per acre per year) during a 16-year trial on arable land in Germany.[lower-alpha 26] McCalmont et al. estimate a mean UK yield of 10–15 tonnes if harvested in the spring,[14] while Hastings et al. estimate a "pessimistic" UK mean yield of 10.5 tonnes.[lower-alpha 27] Nsanganwimana et al. summarize several trials, and give these numbers:

• Austria: Autumn harvest 17–30. Winter harvest 22.
• Denmark: Autumn harvest 17. Winter harvest 10.
• Italy: Winter harvest 20–25.
• France: Autumn harvest 42–49. Winter harvest 30.
• Germany: Autumn harvest 17–30. Winter harvest 10–20.
• Portugal: Autumn harvest 39. Winter harvest 26–30.
• The Netherlands: Autumn harvest 25. Winter harvest 16–17.
• Spain: Winter harvest 14.
• UK: Winter harvest 11–17.[1]

Steep, marginal land.

Marginal land is land with issues that limits growth, for instance low water and nutrient storage capacity, high salinity, toxic elements, poor texture, shallow soil depth, poor drainage, low fertility, or steep terrain. Depending on how the term is defined, between 1.1 and 6.7 billion hectares of marginal land exists in the world.[lower-alpha 28] For comparison, Europe consists of roughly 1 billion hectares (10 million km2, or 3.9 million square miles), and Asia 4.5 billion hectares (45 million km², or 17 million square miles). The IPCC estimates that there is between 0.32 and 1.4 billion hectares of marginal land suitable for bioenergy in the world.[lower-alpha 29]

Quinn et al. identified Miscanthus x giganteus as a crop that is moderately or highly tolerant of multiple environmental stressors, specifically, heat, drought, flooding, salinity (below 100 mM), and cool soil temperatures (down to −3.4 °C, or 25 °F).[lower-alpha 30] This robustness makes it possible to establish relatively high-yielding miscanthus fields on marginal land, Nsanganwimana et al. mention wastelands, coastal areas, damp habitats, grasslands, abandoned milling sites, forest edges, streamsides, foothills and mountain slopes as viable locations.[15] Likewise, Stavridou et al. concluded that 99% of Europe's saline, marginal lands can be used for M. x giganteus plantations, with only an expected maximum yield loss of 11%.[lower-alpha 31] Since salinity up to 200 mM does not affect roots and rhizomes, carbon sequestration carry on unaffected.[lower-alpha 32] Lewandowski et al. found a yield loss of 36% on a marginal site limited by low temperatures (Moscow), compared to maximum yield on arable land in central Europe. The authors also found a yield loss of 21% on a marginal site limited by drought (Turkey), compared to maximum yields on arable soil in central Europe.[lower-alpha 33] Using yield prediction software Miscanfor, Zhang et al. predicts an average yield of 14.6 dry tonnes per hectare per year for miscanthus on marginal land in China, 12.6% below the expected average yield on arable land. The authors calculate that miscanthus on marginal land in China can produce 31.7 EJ (exajoule) of energy annually,[16] an amount equivalent to 39% of the country's 2019 coal consumption.[lower-alpha 34]

Miscanfor predicts that 30 days of soil dryness is the mean maximum amount of time a miscanthus crop can endure before wilting, while 60 days is the maximum before its rhizomes are killed and the crop has to be replanted.[lower-alpha 35] In addition to adequate rainfall, soil water holding capacity is important for high yields, especially in dry periods—in fact Roncucci et al. reports approximately two times better yield for miscanthus planted in silty clay loam compared to sandy loam soil (Italy) after a relatively normal growing season precipitation wise, and approximately six times better yield after a growing season containing severe drought.[lower-alpha 36] The authors note that in soils with poor water holding capacity, irrigation in the establishment season is important because it allows the roots to reach far deeper underground, thereby increasing the plants' ability to collect water.[lower-alpha 37] Irrigation can also increase yield if applied during dry growing seasons (defined as 150–300 mm rainfall). The authors argue however that in soils with good water holding capacity, irrigation can potentially be avoided if rainfall exceeds 420 mm.[lower-alpha 38] Stričević et al. make a similar point for crops in Serbia. The soil in this area is generally well wetted at the start of the growing season because of snow melt. If the roots go deep (2–3 m) and the soil has good water holding capacity, 300–400 mm rainfall during the season is enough for good yields (20–25 tonnes per hectare per year).[lower-alpha 39] The authors note however that if there are no water constraints at all, that is, if the crops are irrigated, you can actually expect twice the yield (42 tonnes per hectare per year).[lower-alpha 40]

Nsanganwimana et al. found that M. x giganteus grows well in soils contaminated by metals, or by industrial activities in general.[17] For instance, in one trial, it was found that M. x giganteus absorbed 52% of the lead content and 19% of the arsenic content in the soil after three months.[18] The absorption stabilizes the pollutants so they don't travel into the air (as dust), into ground water, neighbouring surface waters, or neighbouring areas used for food production.[lower-alpha 41] If contaminated miscanthus is used as fuel, the combustion site need to install the appropriate equipment to handle this situation.[19] On the whole though, "[…] Miscanthus is [a] suitable crop for combining biomass production and ecological restoration of contaminated and marginal land."[20] Because of miscanthus' ability to be "[…] productive on lower grade agricultural land, including heavy metal contaminated and saline soils […]" Clifton-Brown et al. conclude that miscanthus can "[…] contribute to the sustainable intensification of agriculture, allowing farmers to diversify and provide biomass for an expanding market without compromising food security."[21]

To calculate land use requirements for different kinds of energy production, it is essential to know the relevant area-specific power densities. Smil estimates that the average area-specific power densities for modern biofuels, wind, hydro and solar power production are 0.30 W/m², 1 W/m², 3 W/m² and 5 W/m², respectively (power in the form of heat for biofuels, and electricity for wind, hydro and solar).[22] The average human power consumption on ice-free land is 0.125 W/m² (heat and electricity combined),[23] although rising to 20 W/m² in urban and industrial areas.[24]

The reason for the low area-specific power density for biofuels is a combination of low yields and only partial utilization of the plant (for instance, ethanol is typically made from sugarcane's sugar content or corn's starch content, while biodiesel is often made from rapeseed and soybean's oil content).

Regarding ethanol production, Smil estimates that Miscanthus x giganteus fields generate 0.40 W/m² when utilized for this purpose (yield 15 t/ha).[25] Corn fields generates 0.26 W/m² (yield 10 t/ha).[26] In Brazil sugarcane fields typically generate 0.41 W/m².[26] With the highest large-scale plantation yields in the industry (roughly 80 t/ha wet), sugarcane fields can generate 0.50 W/m².[27] Winter wheat (USA) generates 0.08 W/m² and German wheat generates 0.30 W/m².[28] When grown for jet fuel, soybean generates 0.06 W/m², while palm oil generates a healthier 0.65 W/m².[27] Jathropa grown on marginal land generate 0.20 W/m².[27] When grown for biodiesel, rapeseed generate 0.12 W/m² (EU average).[29] In contrast to miscanthus cultivation and solid fuel production, typical liquid biofuel feedstocks and fuel production require large energy inputs. When these inputs are compensated for, power density drops further down: Rapeseed based biodiesel production in the Netherlands have the highest energy efficiency in the EU with an adjusted power density of 0.08 W/m², while sugar beets based bioethanol produced in Spain have the lowest, at only 0.02 W/m².[30]

Combusting solid biomass is more energy efficient than combusting liquids, as the whole plant is utilized. For instance, corn plantations producing solid biomass for combustion generate more than double the amount of power per square metre compared to corn plantations producing for ethanol, when the yield is the same: 10 t/ha generates 0.60 W/m² and 0.26 W/m² respectively (even without compensating for energy input).[31] For large-scale plantations with pines, acacias, poplars and willows in temperate regions, Smil estimates yields of 5–15 t/ha, equivalent to 0.30–0.90 W/m².[32] For similarly large plantations, with eucalyptus, acacia, leucaena, pinus and dalbergia in tropical and subtropical regions, his estimate is 20–25 t/ha, equivalent to 1.20–1.50 W/m² (a somewhat higher yield estimate than the FAO estimate above, and a yield that put these plantations' area-specific power densities in-between the densities of wind and hydro).[32] In Brazil, the average yield for eucalyptus is 21 t/ha, but in Africa, India and Southeast Asia, typical eucalyptus yields are below 10 t/ha.[33]

Oven dry biomass in general, including wood, miscanthus[34] and napier[35] grass, have a calorific content of roughly 18 GJ/t.[36] When calculating power production per square metre, every t/ha of dry biomass yield increases a plantation's power production by 0.06 W/m².[37] As mentioned above, Smil estimates that the world average for wind, hydro and solar power production is 1 W/m², 3 W/m² and 5 W/m² respectively. In order to match these power densities, plantation yields must reach 17 t/ha, 50 t/ha and 83 t/ha for wind, hydro and solar respectively. This seems achievable based on the yield data in the preceding sections. To match the world average for biofuels (0.3 W/m²), plantations need only to produce 5 tonnes of dry mass per hectare per year.

Note however that yields need to be adjusted to compensate for the amount of moisture in the biomass (evaporating moisture in order to reach the ignition point is usually wasted energy). The moisture of biomass straw or bales varies with the surrounding air humidity and eventual pre-drying measures, while pellets have a standardized (ISO-defined) moisture content of below 10% (wood pellets)[lower-alpha 42] and below 15% (other pellets).[lower-alpha 43] Likewise, for wind, hydro and solar, power line transmission losses amounts to roughly 8% globally and should be accounted for.[lower-alpha 44] If biomass is to be utilized for electricity production rather than heat production, note that yields has to be roughly tripled in order to compete with wind, hydro and solar, as the current heat to electricity conversion efficiency is only 30-40%.[38] When simply comparing area-specific power density without regard for cost, this low heat to electricity conversion efficiency effectively pushes at least solar parks out of reach of even the highest yielding biomass plantations, power density wise.[lower-alpha 45]

At the end of each season, the plant pulls the nutrients to the ground. The color shifts from green to yellow/brown.

Plants sequester carbon through photosynthesis, a sunlight-driven process where CO₂ and water are absorbed and then combined to form carbohydrates. The absorbed carbon is released back to the atmosphere as CO₂ when the harvested biomass is combusted, but the belowground parts of the plant (roots and rhizomes) remain in the soil and can potentially add substantial amounts of carbon to the soil over the years. Belowground carbon does not stay below ground forever however; "[…] soil carbon is a balance between the decay of the initial soil carbon and the rate of input […]."[39][lower-alpha 46] Plant derived soil carbon is a continuum, ranging from living biomass to humus,[40] and it decays in different stages, ranging from months (decomposable plant material; DPM) to hundreds of years (humus). The rate of decay depends on many factors, for instance plant species, soil, temperature and humidity,[41] but as long as fresh new carbon is input, a certain amount of carbon stays in the ground—in fact Poeplau et al. did not find any "[…] indication of decreasing SOC [soil organic carbon ] accumulation with age of the plantation indicating no SOC saturation within 15–20 years."[42] Harris et al. estimate 30–50 years of SOC change following a land use change between annual and perennial crops before a new SOC equilibrium is reached.[43] The amount of carbon in the ground under miscanthus fields is thus seen to increase during the entire life of the crop, albeit with a slow start because of the initial tilling (plowing, digging) and the relatively low amounts of carbon input in the establishment phase.[lower-alpha 47][lower-alpha 48] (Tilling helps the soil microbe populations to decompose the available carbon, producing CO₂.[lower-alpha 49][lower-alpha 50]) Felten et al. argue that high proportions of pre- and direct-harvest residues (e.g. dead leaves), direct humus accumulation, the well-developed and deep-reaching root system, the low decomposition rates of plant residues due to a high C:N ratio (carbon to nitrogen ratio), and the absence of tillage and subsequently less soil aeration are the reasons for the high carbon sequestration rates.[44]

A number of studies try to quantify the net amount of miscanthus-induced below-ground carbon accumulation each year, after decay is accounted for, in various locations and under various circumstances.

Dondini et al. found 32 tonnes more carbon per hectare (13 tonnes per acre) under a 14 year old miscanthus field than in the control site, suggesting a combined (C3 plus C4) mean carbon accumulation rate of 2.29 tonnes per hectare per year (1 tonne per acre per year), or 38% of total harvested carbon per year.[lower-alpha 51] Likewise, Milner et al. suggest a mean carbon accumulation rate for the whole of the UK of 2.28 tonnes per hectare per year (also 38% of total harvested carbon per year), given that some unprofitable land (0.4% of total) is excluded.[lower-alpha 52] Nakajima et al. found an accumulation rate of 1.96  (± 0.82) tonnes per hectare per year below a university test site in Sapporo, Japan (0.79 per acre), equivalent to 16% of total harvested carbon per year. The test was shorter though, only 6 years.[lower-alpha 53] Hansen et al. found an accumulation rate of 0.97 tonne per hectare per year (0.39 tonnes per acre per year) over 16 years under a test site in Hornum, Denmark, equivalent to 28% of total harvested carbon per year.[lower-alpha 54] McCalmont et al. compared a number of individual European reports, and found accumulation rates ranging from 0.42 to 3.8 tonnes per hectare per year,[lower-alpha 55] with a mean accumulation rate of 1.84 tonne (0.74 tonnes per acre per year),[lower-alpha 56] or 25% of total harvested carbon per year.[lower-alpha 57] The IPCC argues that increased soil carbon content is an important option for both climate mitigation and adaptation.[lower-alpha 58]

The development of the torrefaction process began as research into coffee roasting, in the late 19th century.[45]

Biomass in general, including miscanthus, have different properties compared to coal, for instance when it comes to handling and transport, grinding, and combustion.[46] This makes sharing the same logistics, grinding and combustion infrastructure difficult. Often new biomass handling facilities have to be built instead, which increases cost.[lower-alpha 59] Together with the relatively high cost of feedstock, this often leads to the well-known situation where biomass projects have to receive subsidies to be economically viable.[lower-alpha 60] A number of fuel upgrading technologies are currently being explored, however, that make biomass more compatible with the existing infrastructure. The most mature of these is torrefaction, basically an advanced roasting technique which—when combined with pelleting or briquetting—significantly influences handling and transport properties, grindability and combustion efficiency.

Transport of bulky, water absorbing miscanthus bales in England.

Miscanthus chips have a bulk density of only 50–130 kg/m³,[lower-alpha 61] bales 120–160 kg/m³,[lower-alpha 62] while pellets and briquettes have a bulk density of 500 and 600 kg/m³ respectively.[47] Torrefaction works hand in hand with this trend towards a denser and therefore cheaper to transport product, specifically by increasing the product's energy density. Torrefaction removes (by gasification) the parts of the biomass that has the lowest energy content, while the parts with the highest energy content remain. That is, approximately 30% of the biomass is converted to gas during the torrefaction process (and potentially used to power the process), while 70% remains, usually in the form of compacted pellets or briquettes. This solid product contains approximately 85% of the original biomass energy however.[48] Basically the mass part has shrunk more than the energy part, and the consequence is that the calorific value of torrefied biomass increases significantly, to the extent that it can compete with energy dense coals used for electricity generation (steam/thermal coals). Vaclav Smil states that the energy density of the most common steam coals today is 22–26 GJ/t.[49]

The higher energy density means lower transport costs, and a decrease in transport-related GHG emittance.[50] The IEA (International Energy Agency) has calculated energy and GHG costs for regular and torrefied pellets/briquettes. When making pellets and shipping them from Indonesia to Japan, a minimum of 6.7% energy savings or 14% GHG savings is expected when switching from regular to torrefied. This number increases to 10.3% energy savings and 33% GHG savings when making and shipping minimum 50mm briquettes instead of pellets (briquette production requires less energy).[lower-alpha 63] The longer the route, the bigger the savings. The relatively short supply route from Russia to the UK equals energy savings of 1.8%, while the longer supply route from southeast USA to the Amsterdam-Rotterdam-Antwerp (ARA) area is 7.1%. From southwest Canada to ARA 10.6%, southwest USA to Japan 11%, and Brazil to Japan 11.7% (all these savings are for pellets only.)[51]

Torrefaction also converts the biomass from a hydrophilic (water absorbing) to a hydrophobic (water repelling) state. Water repelling briquettes can be transported and stored outside, which simplifies the logistics operation and decreases cost.[lower-alpha 64] All biological activity is stopped, reducing the risk of fire and stopping biological decomposition like rotting.[50]

Generally, torrefaction is seen as a gateway for converting a range of very diverse feedstocks into a uniform and therefore easier to deal with fuel.[50] The fuel's parameters can be changed to meet customers demands, for instance type of feedstock, torrefaction degree, geometrical form, durability, water resistance, and ash composition.[52] The possibility to use different types of feedstock improves the fuel's availability and supply reliability.[50]

Coal grinders

Unprocessed M. x giganteus has strong fibers, making grinding into equally sized, very small particles (below 75 µm / 0.075 mm) difficult to achieve. Coal chunks are typically ground to that size because such small, even particles combust stabler and more efficient.[53][54] While coal has a score on the Hardgrove Grindability Index (HGI) of 30–100 (higher numbers mean it is easier to grind), unprocessed miscanthus has a score of 0.[lower-alpha 65] During torrefaction however, "[…] the hemi-cellulose fraction which is responsible for the fibrous nature of biomass is degraded, thereby improving its grindability."[55] Bridgeman et al. measured a HGI of 79 for torrefied miscanthus,[lower-alpha 66] while the IEA estimates a HGI of 23–53 for torrefied biomass in general.[56] UK coal scores between 40 and 60 on the HGI scale.[lower-alpha 67] The IEA estimates an 80–90% drop in energy use required to grind biomass that has been torrefied.[57]

The relatively easy grinding of torrefied miscanthus makes a cost-effective conversion to fine particles possible, which subsequently makes efficient combustion with a stable flame possible. Ndibe et al. found that the level of unburnt carbon "[…] decreased with the introduction of torrefied biomass", and that the torrefied biomass flames "[…] were stable during 50% cofiring and for the 100% case as a result of sufficient fuel particle fineness."[58]

Raw miscanthus biomass has a relatively high chlorine amount, which is problematic in a combustion scenario because, as Ren et al. explains, the "[…] likelihood of corrosion depends significantly on the content of chlorine in the fuel […]."[59] Likewise, Johansen et al. state that "[…] the release of Cl-associated [chlorine-associated] species during combustion is the main cause of the induced active corrosion in the grate combustion of biomass."[60] Chlorine in different forms, in particular combined with potassium as potassium chloride, condensates on relatively cooler surfaces inside the boiler and creates a corrosive deposit layer. The corrosion damages the boiler, and in addition the physical deposit layer itself reduce heat transfer efficiency, most critically inside the heat exchange mechanism.[lower-alpha 68] Chlorine and potassium also lowers the ash melting point considerably compared to coal. Melted ash, known as slag or clinker, sticks to the bottom of the boiler, and increase maintenance costs.[lower-alpha 69][lower-alpha 70]

In order to reduce chlorine (and moisture) content, M. x giganteus is usually harvested dry, in early spring, but this late harvest practice is still not enough of a countermeasure to achieve corrosion-free combustion.[lower-alpha 71]

However, the chlorine amount in miscanthus reduces by approximately 95% when it is torrefied at 350 degrees Celsius.[lower-alpha 72] Chlorine release during the torrefaction process itself is more manageable than chlorine release during combustion, because "[…] the prevailing temperatures during the former process are below the melting and vaporization temperatures of the alkali salts of chlorine, thus minimizing their risks of slagging, fouling and corrosion in furnaces."[61]

For potassium, Kambo et al. found a 30% reduction for torrefied miscanthus.[62] However, potassium is dependent on chlorine to form potassium chloride; with a low level of chlorine, the potassium chloride deposits reduce proportionally.[lower-alpha 73]

Li et al. conclude that the "[…] process of torrefaction transforms the chemical and physical properties of raw biomass into those similar to coal, which enables utilization with high substitution ratios of biomass in existing coal-fired boilers without any major modifications."[63] Likewise, Bridgeman et al. state that since torrefaction removes moisture, creates a grindable, hydrophobic and solid product with an increased energy density, torrefied fuel no longer requires "[…] separate handling facilities when co-fired with coal in existing power stations."[46] Smith et al. makes a similar point in regard to hydrothermal carbonization, sometimes called "wet" torrefaction.[lower-alpha 74]

Ribeiro et al. note that "[…] torrefaction is a more complex process than initially anticipated" and state that "[…] torrefaction of biomass is still an experimental technology […]."[64] Michael Wild, president of the International Biomass Torrefaction Council, stated in 2015 that the torrefaction sector is "[…] in its optimisation phase […]", i.e. it is maturing. He mentions process integration, energy and mass efficiency, mechanical compression and product quality as the variables most important to master at this point in the sector's development.[52]

GHG / CO2 / carbon negativity for Miscanthus × giganteus production pathways.

Relationship between above-ground yield (diagonal lines), soil organic carbon (X axis), and soil's potential for successful/unsuccessful carbon sequestration (Y axis). Basically, the higher the yield, the more land become a CO2 negative climate mitigation tool (including relatively carbon rich land.)

The amount of carbon sequestrated and the amount of GHG (greenhouse gases) emitted 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. The authors argue that a miscanthus crop with a yield of 10 tonnes per hectare per year sequesters so much carbon that the crop more than compensates for both farm operations emissions and transport emissions. The chart on the right displays two CO₂ negative miscanthus production pathways, represented in gram CO₂-equivalents per megajoule. The bars are sequential and move up and down as atmospheric CO₂ is estimated to increase and decrease. The green bars represents soil carbon change, the yellow diamonds represent mean values.[lower-alpha 75]

Emmerling et al. make the same point for miscanthus in Germany (yield 15 t/ha/yr, carbon sequestration 1.1 t/ha/yr): "Miscanthus is one of the very few crops worldwide that reaches true CO₂ neutrality and may function as a CO₂ sink. [...] Related to the combustion of fuel oil, the direct and indirect greenhouse gas emissions can be reduced by a minimum of 96% through the combustion of Miscanthus straw (emissions: 0.08 kg CO₂‐eq MJ⁻¹ (fuel oil) vs. 0.0032 kg CO₂‐eq MJ⁻¹ (Miscanthus straw)). Due to the C‐sequestration during Miscanthus growth, this results in a CO₂‐eq mitigation potential of 117%".[lower-alpha 76]

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 chart highlights this fact.[lower-alpha 77] 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 makes CO₂ negativity harder to achieve. Soils already rich in carbon include peatland and mature forest. Milner et al. further argue that the most successful carbon sequestration in the UK takes place below improved grassland.[lower-alpha 78] However, Harris et al. notes that since the carbon content of grasslands vary considerably, so does the success rate of land use changes from grasslands to perennial.[lower-alpha 79] 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.

Fundamentally, the below-ground carbon accumulation works as a GHG mitigation tool because it removes carbon from the above-ground carbon circulation (the circulation from plant to atmosphere and back into plant.) The above-ground circulation is driven by photosynthesis and combustion—first, the miscanthus fields absorb CO₂ and assimilates it as carbon in its tissue both above and below ground. When the above-ground carbon is harvested and then burned, the CO₂ molecule is formed yet again and released back into the atmosphere. However, an equivalent amount of CO₂ (and possibly more, if the biomass is expanding) is absorbed back by next season's growth, and the cycle repeats. This above-ground cycle has the potential to be carbon neutral, but of course the human involvement in operating and guiding the above-ground CO₂ circulation means additional energy input, often coming from fossil sources. If the fossil energy spent on the operation is high compared to the amount of produced energy, the total CO₂ footprint (CO₂ emissions from operation plus plant burning, minus carbon sequestration) can approach, match or even exceed the CO₂ footprint originating from burning fossil fuels exclusively, as has been shown to be the case for several first-generation biofuel projects.[lower-alpha 80][lower-alpha 81][lower-alpha 82] Transport fuels might be worse than solid fuels in this regard.[lower-alpha 83]

The problem can be dealt with both from the perspective of increasing the amount of carbon that is moved below ground (see Carbon sequestration, above), and from the perspective of decreasing fossil fuel input to the above-ground operation. If enough carbon is moved below ground, it can compensate for the total lifecycle emissions of a particular biofuel. Further, if the above-ground emissions decreases, less below-ground carbon storage is needed for the biofuel to become CO₂ neutral or negative. To sum up, a GHG negative life cycle is possible if the below-ground carbon accumulation more than compensates for the above-ground lifecycle GHG emissions.

For first generation bio-energy crops, the greenhouse gas footprints were often large, but second generation bio-energy crops like miscanthus reduces its CO₂ footprint drastically. Hastings et al. found that miscanthus crops "[…] almost always has a smaller environmental footprint than first generation annual bioenergy ones [...]."[lower-alpha 84] A large meta-study of 138 individual studies, done by Harris et al., revealed that second generation perennial grasses (miscanthus and switchgrass) planted on arable land on average store five times more carbon in the ground than short rotation coppice or short rotation forestry plantations (poplar and willow).[lower-alpha 85] Compared to fossil fuels, the GHG savings are large—even without considering carbon sequestration, miscanthus fuel has a GHG cost of 0.4–1.6 grams CO₂-equivalents per megajoule, compared to 33 grams for coal, 22 for liquefied natural gas, 16 for North Sea gas, and 4 for wood chips imported to Britain from the USA.[lower-alpha 86]

Confirming the above numbers, McCalmont et al. found that the mean energy input/output ratios for miscanthus is 10 times better than for annual crops, while GHG costs are 20-30 times better than for fossil fuels.[lower-alpha 87] For instance, miscanthus chips for heating saved 22.3 tonnes of CO₂ emissions per hectare per year in the UK (9 tonnes per acre), while maize for heating and power saved 6.3 (2.5 per acre). Rapeseed for biodiesel saved only 3.2 (1.3 per acre).[lower-alpha 88] Lewandowski et al. found that each hectare (2.47 acres) of Central European arable land planted with miscanthus can reduce the atmospheric CO₂ level with up to 30.6 tonnes per year, save 429 GJ of fossil energy used each year, with 78 euros earned per tonne reduced CO₂ (2387 euros earned per hectare per year)—given that the biomass is produced and used locally (within 500 km / 310 miles).[lower-alpha 89] For miscanthus planted on marginal land limited by cold temperatures (Moscow), the reduction in atmospheric CO₂ is estimated to be 19.2 tonnes per hectare per year (7.7 tonnes per acre), with fossil energy savings of 273 GJ per hectare per year (110 GJ per acre). For marginal land limited by drought (Turkey), the atmospheric CO₂ level can potentially be reduced with 24 tonnes per hectare per year (9.7 tonnes per acre), with fossil energy savings of 338 GJ per hectare per year (137 tonnes per acre).[lower-alpha 90] Based on similar numbers, Poeplau and Don expect miscanthus plantations to grow large in Europe in the coming decades.[65] Whitaker et al. state that after some discussion, there is now (2018) consensus in the scientific community that "[…] the GHG balance of perennial bioenergy crop cultivation will often be favourable […]", also when considering the implicit direct and indirect land use changes.[lower-alpha 91]

Felten and Emmerling argue that Miscanthus fields may facilitate a diverse earthworm community even in intensive agricultural landscapes.

Haughton et al. found breeding skylarks in miscanthus crops.

Below ground, Felten and Emmerling found that the number of earthworm species per square meter was 5.1 for miscanthus, 3 for maize, and 6.4 for fallow (totally unattended land), and state that "[…] it was clearly found that land-use intensity was the dominant regressor for earthworm abundance and total number of species." Because the extensive leaf litter on the ground helps the soil to stay moist, and also protect from predators, they conclude that "[…] Miscanthus had quite positive effects on earthworm communities […]" and recommend that "[…] Miscanthus may facilitate a diverse earthworm community even in intensive agricultural landscapes."[66][lower-alpha 92]

Nsanganwimana et al. found that the bacterial activity of certain bacteria belonging to the proteobacteria group almost doubles in the presence of M. x giganteus root exudates.[18]

Above ground, Lewandowski et al. found that young miscanthus stands sustain high plant species diversity, but as the miscanthus stands mature, the canopy closes, and less sunlight reach the competing weeds. In this situation it gets harder for the weeds to survive. After canopy closure, Lewandowski et al. found 16 different weed species per 25 m² plot. The dense canopy works as protection for other life-forms though; Lewandowski et al. notes that "[…] Miscanthus stands are usually reported to support farm biodiversity, providing habitat for birds, insects, and small mammals […]."[lower-alpha 93] Supporting this view, Caslin et al. argue that the flora below the canopy provides food for butterflies, other insects and their predators, and 40 species of birds.[lower-alpha 94]

Both Haughton et al.[lower-alpha 95] and Bellamy et al. found that the miscanthus overwinter vegetative structure provided an important cover and habitat resource, with high levels of diversity in comparison with annual crops. This effect was particularly evident for beetles, flies, and birds, with breeding skylarks and lapwings being recorded in the crop itself. The miscanthus crop offers a different ecological niche for each season—the authors attribute this to the continually evolving structural heterogeneity of a miscanthus crop, with different species finding shelter at different times during its development—woodland birds found shelter in the winter and farmland birds in the summer. For birds, 0.92 breeding pairs species per hectare (0.37 per acre) was found in the miscanthus field, compared to 0.28 (0.11) in the wheat field. The authors note that due to the high carbon to nitrogen ratio, it is in the field's margins and interspersed woodlands that the majority of the food resources are to be found. Miscanthus fields work as barriers against chemical leaching into these key habitats however.[lower-alpha 96]

Caslin et al. further argue that miscanthus crops provides better biodiversity than cereal crops, with three times as many spiders and earthworms as cereal.[lower-alpha 97] Brown hare, stoat, mice, vole, shrew, fox and rabbit are some of the species that are observed in miscanthus crops. The crop act as both a nesting habitat and a wildlife corridor connecting different habitats.[lower-alpha 98]

McCalmont et al. claim that miscanthus fields leads to significantly improved water quality because of significantly less nitrate leaching.[lower-alpha 99] Likewise, Whitaker et al. claim that there is drastically reduced nitrate leaching from miscanthus fields compared to the typical maize/soy rotation because of low or zero fertilizer requirements, the continuous presence of a plant root sink for nitrogen, and the efficient internal recycling of nutrients by perennial grass species. For instance, a recent meta-study concluded that miscanthus had nine times less subsurface loss of nitrate compared to maize or maize grown in rotation with soya bean.[lower-alpha 100]

The fibrous, extensive miscanthus rooting system and the lack of tillage disturbance improves infiltration, hydraulic conductivity and water storage compared to annual row crops, and results in the porous and low bulk density soil typical under perennial grasses, with water holding capabilities expected to increase by 100–150 mm.[lower-alpha 101] Nsanganwimana et al. argue that miscanthus improves carbon input to the soil, and promote microorganism activity and diversity, which are important for soil particle aggregation and rehabilitation processes. On a former fly ash deposit site, with alkaline pH, nutrient deficiency, and little water-holding capacity, a miscanthus crop was successfully established—in the sense that the roots and rhizomes grew quite well, supporting and enhancing nitrification processes, although the above-ground dry weight yield was low because of the conditions. The authors argue that M. x giganteus' ability to improve soil quality even on contaminated land is a useful feature especially in a situation where organic amendments can be added. For instance, there is a great potential to increase yield on contaminated marginal land low in nutrients by fertilizing it with nutrient-rich sewage sludge or wastewater. The authors claim that this practice offer the three-fold advantage of improving soil productivity, increasing biomass yields, and reducing costs for treatment and disposal of sewage sludge in line with the specific legislation in each country.[3]

Miscanthus × giganteus parents on both sides, M. sinensis and M. sacchariflorus, are both potentially invasive species, because they both produce viable seeds. M. x giganteus does not produce viable seeds however, and Nsanganwimana et al. claim that "[...] there has been no report on the threat of invasion due to rhizome growth extension from long-term commercial plantations to neighbouring arable land."[20]

Young miscanthus test crop in England.

There seem to be agreement in the scientific community that a shift from annual to perennial crops have environmental benefits. For instance, Lewandowski et al. conclude that analyses "[...] of the environmental impacts of miscanthus cultivation on a range of factors, including greenhouse gas mitigation, show that the benefits outweigh the costs in most cases."[67] McCalmont et al. argue that although there is room for more research, "[...] clear indications of environmental sustainability do emerge."[lower-alpha 102] In addition to the GHG mitigation potential, miscanthus' "[…] perennial nature and belowground biomass improves soil structure, increases water-holding capacity (up by 100–150 mm), and reduces run-off and erosion. Overwinter ripening increases landscape structural resources for wildlife. Reduced management intensity promotes earthworm diversity and abundance although poor litter palatability may reduce individual biomass. Chemical leaching into field boundaries is lower than comparable agriculture, improving soil and water habitat quality."[68] Milner et al. argue that a change from first generation to second generation energy crops like miscanthus is environmentally beneficial because of improved farm-scale biodiversity, predation and a net positive GHG mitigation effect. The benefits are primarily a consequence of low inputs and the longer management cycles associated with second generation (2G) crops.[lower-alpha 103] The authors identifies 293247 hectares of arable land and grassland in the UK (equivalent to 1.3% of the total land area) where both the economical and environmental consequences of planting miscanthus is seen as positive.[lower-alpha 104] Whitaker et al. argue that if land use tensions are mitigated, reasonable yields obtained, and low carbon soils targeted, there are many cases where low-input perennial crops like miscanthus "[...] can provide significant GHG savings compared to fossil fuel alternatives [...]."[lower-alpha 105] In contrast to annual crops, miscanthus have low nitrogen input requirements, low GHG emissions, sequesters soil carbon due to reduced tillage, and can be economically viable on marginal land.[lower-alpha 106] The authors agree that in recent years, "[...] a more nuanced understanding of the environmental benefits and risks of bioenergy has emerged, and it has become clear that perennial bioenergy crops have far greater potential to deliver significant GHG savings than the conventional crops currently being grown for biofuel production around the world (e.g. corn, palm oil and oilseed rape)."[lower-alpha 107] The authors conclude that "[...] the direct impacts of dedicated perennial bioenergy crops on soil carbon and N2O are increasingly well understood, and are often consistent with significant lifecycle GHG mitigation from bioenergy relative to conventional energy sources."[69]