A versatile solution? Growing Miscanthus for bioenergy

The need for a rapid reduction in greenhouse gases is well known, and bioenergy is one of the possible solutions. The new perennial grass crop Miscanthus is particularly promising for bioenergy, as it is hardy, fast growing and efficient in its use of water. Jonathan Harvey reviews the technical and commercial progress, and future prospects, for the crop.

Miscanthus is the name of a group of perennial grasses native to Asia and Africa, and first introduced into the UK as an ornamental plant in the 19th century. The type most commonly grown for biomass is a sterile hybrid (called Miscanthus x giganteus), which, once established, grows 3-metre woody canes each summer.

The canes themselves sprout from underground rhizomes, which are perennial and tough – able to survive in all but the coldest areas of Europe. The cane senesces in the autumn and winter, with most of the nutrients and salts being returned from the canes to the rhizomes below ground for next year’s growth. The standing cane’s dry matter content will increase over winter, reaching 60%-90% by the time it is ready to be harvested in February to April.

Yields of Miscanthus cane from mature crops are typically 12-20 tonnes dry matter per hectare per year in temperate Europe. Yields in the USA (Illinois) have been recorded at over 40 tonnes.

The energy content of the cane is typically 17 MJ per kg of dry matter, similar to wood but with higher silicon content.

Miscanthus compared with other crops

Miscanthus and maize are the only agricultural crops grown in northern Europe that possess the C4 photosynthetic pathway, all others being C3 pathway (C3 and C4 are the two main types of photosynthesis). The C4 system requires more energy (light and heat) to drive it than C3, but in favourable conditions it can operate with greater photosynthetic efficiency, and with more efficient use of water and nitrogen. Most C4 plants grow in warm temperate or tropical climates, and few have evolved to be frost hardy. Miscanthus is an exception, with some varieties able to survive in countries with much colder winters than the UK, such as Japan. In total, C4 plants comprise only 5% of global biomass, but they include some of the highest-yielding crops in the world (C4 crops include sugar cane, sorghum and maize). When harvested, Miscanthus has a higher dry matter content than trees or SRC willow. Despite high efficiency in water use, the crop is still rainfall dependent to achieve highest yields.

Environmental benefits of Miscanthus

When the potential for specific energy crops was examined, one of the key issues was that the energy expended in growing them should be less than for conventional arable crops. This is helped if the energy crops have low requirements for fertilizers, pesticides and other agrochemicals, as these consume large amounts of fossil energy in their manufacture. Low machinery costs are also desirable, and perennial crops have an advantage as annual soil cultivations are not needed. The energy and carbon savings of growing perennial energy crops in countries such as the UK can be considerable.

The old and the new? Miscanthus bales outside a coal-fired power station in the UK supergen

By contrast, there is no certainty of similar savings from co-firing of imported biomass unless the sustainability of each product and source is carefully assessed, including transportation from source to power station.

Miscanthus energy balance
The key benefit of Miscanthus is that the energy expended in growing it (planting, chemicals, harvesting, drying etc) is much less than that released when it is used for fuel. This energy ratio has been calculated (by ADAS Consultancy Ltd working for ETSU/UK Department of Trade and Industry) for Miscanthus and other crops (see Table 1). This table show that planting Miscanthus in place of typical arable crops can greatly reduce energy inputs whilst increasing energy output in the form of mainly cellulosic material, well suited to use as fuel. Similar calculations have been published by DTI (in URN 01/797) and put the energy ratio for Miscanthus at 36:1.

Carbon balance and emissions ‘saved’
The high energy balance with Miscanthus is associated with a high carbon balance for the crop. Calculations published by DTI (publication URN 01/797) put the Miscanthus carbon ratio at 53:1. This means that, of the carbon contained in the fuel, for every one part of man-made carbon inputs needed to grow and harvest it, 53 parts are absorbed by the crop from the environment. Miscanthus biomass contains 47% carbon; therefore a crop of 15 tonnes dry matter per hectare would fix, in a single year, 7 tonnes of carbon – equivalent to 25.7 tonnes of CO2. Harvested for co-firing in late winter, the crop is potentially carbon neutral, because only the carbon fixed by photosynthesis in the previous growing season is released when the crop is co-fired with coal, saving an equivalent amount of fossil fuel carbon from coal. Thus a significant quantity of emissions can be prevented by co-firing. Dr Mortimer, in the DTI publication Climate Change and the European Countryside, has calculated that co-firing Miscanthus pellets could save 16.4 tonnes of CO2 equivalent in greenhouse gases per hectare per year of a high yielding Miscanthus crop.

Miscanthus is a hardy crop and can survive winter in all but the coldest of European climates supergen

Soil carbon sequestration
In addition to its positive energy balance (particularly when compared with other energy crops such as rapeseed), Miscanthus can sequester large amounts of carbon underground for a long period (over 100 years). Miscanthus cane is harvested annually but no soil cultivation is carried out after the first year, so that in the first four years Miscanthus may sequester 7-9 tonnes of carbon per hectare in the soil. The long-term benefit of this will depend on yield, soil type and climate, but in some circumstances will be significant. By contrast, long-term annual cropping reduces soil carbon, and arable soils typically contain less than 2% carbon. Non-cultivation of arable land typically increases soil carbon. Land typically contains 130 to 650 tonnes of carbon per hectare, so the potential for carbon sequestration with Miscanthus can be considerable. R.B. Mathews and P. Grogan, in a paper published by the Association of Applied Biologists (AAB 65:303-312), have compared long-term potential carbon sequestration rates for SRC willow, woodland and Miscanthus. High rates of carbon sequestration are predicted for all three crops, with the highest rate for Miscanthus at 0.93 tonnes of carbon/ha/year.

A perennial crop, Miscanthus can produce canes up to 3 metres high each summer bical

Impact on biodiversity
The expansion of perennial biomass crops in the UK is potentially more environmentally friendly than the use of other crops. The leaf litter from the previous harvest remains; this prevents weed growth, preserves soil moisture, reduces soil erosion and represents a good environment for insects and other fauna. Miscanthus can support a greater diversity of species than annual crops, and creates a number of ecological niches in which populations of organisms are increased. Pheasants have been observed, and deer are commonly seen in UK plantations. A review for MAFF (the then UK Dept of Agriculture) (Brent K.J 1998) reports greater species diversity with Miscanthus than for rye. It also reports that in Germany a Miscanthus plantation gave winter harbour to deer, hares, quail and partridge. Later, nesting reed warblers and linnets were observed.

A more recent study funded by the DTI (Semere, T. & Slater, F. 2004) stated: ‘In conclusion, because perennial rhizomatous grasses require a single initial planting, and because the crops were harvested in March and not disturbed by cultivation every year, the fields were used as over-wintering sites for birds, small mammals and invertebrates, suggesting immediate benefits to biodiversity. Further trials are currently being funded by DEFRA and BICAL Ltd.’

Miscanthus for atmospheric carbon reduction
By efficient growth and conversion of biomass to power, including sequestration of power station stack emissions of CO2 by burial, or conversion by algae to biomass, it is in theory possible to generate electricity, and simultaneously reduce GHG levels in the atmosphere. Clearly the technology is not ready, but the potential is there already.

Miscanthus as fuel

Development of UK co-firing market
Currently, the main use of Miscanthus in the UK is in co-firing with coal in existing power stations, in line with the Renewable Obligation. There are 15 large co-firing locations registered in the UK, creating an annual requirement for over 3 million tonnes of biomass fuel. Several are interested in Miscanthus supply contracts.

Of the Miscanthus supply companies, Biomass Industrial Crops Ltd (trading as BICAL) appears to be dominant, having the major EU supply of rhizome for planting, and having invested heavily in planting and cane handling technology. Current estimates suggest it has a 7% share of this market for contracts. The Miscanthus fuel is supplied to the power stations as field-compacted bales, or compacted products (pellets or cubes).

BICAL has secured large-scale fuel contracts with substantial power companies, including Drax, and is currently aiming to secure contracts for 32,500 ha of Miscanthus crops (so far around 7000 ha of Miscanthus have been established).

To help supply these substantial end users, BICAL Ltd has set up four wholly owned subsidiary fuel producer groups. The core product of these groups will be Miscanthus for co-firing, but products such as pellets for the heat market and animal bedding products will be developed.

Planting machines in the UK. Over the next few years, Miscanthus growing could expand from a relatively small business into a huge global industry bical

UK dedicated crop- fuelled power stations
Around the UK, there are a number of small power stations built or under construction which will use Miscanthus as feedstock.

The EPR crop-fuelled power station at Ely has been operating successfully for several years burning mainly cereal straw bales on a grated bed system. Some modifications have been made, and now enable Miscanthus bales to be handled. Commercial trials with Miscanthus were successful, and it is now considered a valid alternative fuel. However the present owners do not currently burn Miscanthus, and have no plans to do so. The power station is rated at 38 MWe and conversion efficiency is well over 30%.

At Eccleshall in Staffordshire, a small Miscanthus-fuelled power station is currently under construction, supplied by a growers’ group organized by BiEcc Ltd, a subsidiary of BICAL Ltd. It has a Talbott boiler system and power output is rated at 2.5 MWe.

In another project, John Amos & Co Ltd is supplying Miscanthus fuel for the first on-farm combined heat and power station in UK. The plant is powered by a Talbott BG 100 generator producing 100 kW and 200 kWth of heat. There are further plans to install five new power stations with a combined capacity of up to 10 MWe.

UK domestic and commercial heat market
Domestic heaters and boilers fuelled by pellets or cubes are beginning to emerge as a substantial long-term market, albeit one that Miscanthus is not currently exploiting. However the market is strong, with pellets selling at £130 ($250) per tonne being competitive with fossil fuels. Both BICAL and John Amos & Co are planning to produce Miscanthus pellets to tap into this lucrative sector.

Biomass heating has been identified by the UK DTI as the most efficient way to utilize local biomass. A recent report suggests agricultural biomass could contribute up to 3% of UK energy supply in this way.

Pembrokeshire Bio-energy is a Miscanthus growers’ group which now has 100 ha of crop planted, sufficient to fuel the entire Bluestone Holiday Village Project. To run the operation they have set up an energy services supply company, which supplies the heat requirements of the village, charged to users per kWh. In addition they plan to produce biomass pellets for domestic and commercial use.

Future markets for Miscanthus

Whilst combustion in UK power stations, mainly in co-firing with coal, is the main current use of Miscanthus, other developments show great promise in the longer term, including paper pulp and liquid biofuels.

Cellulosic bioethanol and biodiesel
Bioethanol and biodiesel are currently produced mainly from seed crops by processes that give net energy (output/input) balances of 1.5:1 or less. In some of the older processes the balance is less than one, so that there is a net energy loss in the process.

However companies in USA, Canada and Spain are just starting to build the first commercial plants to make bioethanol from cellulosic crops and crop residues. These will give much higher energy balances, producing about twice the amount of ethanol per hectare of crop than is possible by fermentation of maize grain. Initially, corn stover and other cereal straws will be used for the cellulosic conversions, but one company in Louisiana has selected sugar cane bagasse as raw material, which is similar to Miscanthus cane. The United States Department of Energy (DOE) has estimated that cellulosic processes could potentially yield one million litres of ethanol for every 150-300 hectares of crop.

There is considerable interest in the use of switchgrass and Miscanthus for ethanol production, particularly in the United States. Both crops are C4 pathway perennial grasses, and although switchgrass is native to the North American prairies, Miscanthus appears to be higher yielding, at least in some of the northern states including Illinois, according to work done at the University of Illinois.

According to Michael Wang of Argonne National Laboratories USA, these types of cellulosic ethanol are estimated to reduce greenhouse gas emissions (compared to gasoline) by 80% (compared with 20%-30% for grain ethanol), and so could produce major environmental benefits.

It is also technically possible to make synthetic diesel from Miscanthus by fast pyrolysis or the Fischer-Tropsch process. Despite this there have so far been no announcements on the commercial use of Miscanthus for biofuels.

Potential for Miscanthus across Europe
Clifton-Brown et al., (in Global Change Biology April 2004) estimate that if Miscanthus was grown on 10% of suitable land in the EU-15 countries it could produce electricity equal to 9% of the gross electricity output in 2000. Total carbon mitigation could be about 9% of the EU carbon emissions (at 1990 levels), which could allow the union to meet its Kyoto obligations from Miscanthus alone. (Note that the figure of 9% of total carbon emissions includes the displacement of coal and the potential carbon sequestered to the soil by the Miscanthus crop).

To date the UK appears to have the main stocks of biomass-quality Miscanthus rhizomes in Europe. To capitalize, BICAL has set up subsidiary companies in France and Ireland with trading partners, operating under the BICAL identity. The company is also active in Germany, Austria, Italy, Spain and Poland. Other companies, including ADAS Consulting Ltd are also active in some countries in a consultancy or supply role.

In France, INRA, the government research organization, has set up a development programme for cellulosic biomass ethanol crops, and Miscanthus plays a major role in this. Meanwhile BICAL France is actively recruiting growers for a range of end uses, including co-firing, over most of the country.

Early work in Germany, particularly in Bavaria, gave very high yields, but progress was set back by winter hardiness problems with small tissue-cultured plants not surviving the first winter. Now however a number of farmers and businesses are actively developing the crop. Germany has always had considerable expertise in the development of liquid fuel alternatives to gasoline, and seems likely to continue to lead in this area.

Figure 1. Map showing the predicted yields of Miscanthus across Europe. Source: Kassel University

In Ireland the government has recently announced its intention to set up an energy crop establishment support scheme, with EU funding for willow SRC and Miscanthus planting. Possible uses would be the conversion of the current peat burning fluidized bed power stations to biomass, at least in part, and there are also possibilities to co-fire Miscanthus with coal in large modern plants. Both uses would improve carbon mitigation. Ireland’s pastureland produces heavy emissions of methane from the ruminant livestock. There are possibilities for profitable conversion of this land to Miscanthus for energy cropping with concurrent reductions in GHG emissions, and good soil carbon sequestration potential.

In the Ukraine, Miscanthus is being planted on a 5000 ha energy park, but little other information is currently available.

Back in the UK, the DTI and the Carbon Trust have forecast that energy crops could supply 5%-6% of electricity demand by 2020, through a combination of combustion and gasification technologies. This assumes the planting of 350,000 ha (35% of the DEFRA estimated available space) of energy crops, and could save emissions up to 8 million tonnes of CO2/year. The DTI estimates suggest that Miscanthus (22-27 g/kWh net CO2) gives lower emission rates than SRC willow or straw.

European Union Biomass Plan
Considering the EU-25 as a whole, the European Biomass Plan (published in December 2005) shows very high requirements for energy crops from agriculture and biomass in total across the EU-25 from 2010 to 2030 (Table 2). This suggests colossal rates of increase in biomass cropping for electricity, transport fuels and heating. In total, the biomass plan would reduce emissions by 209 million tonnes CO2 equivalent per year, and would provide employment for 250,000-300,000 people, mainly in rural areas. As much as one quarter of the cropping could be provided by Miscanthus, if sufficient rhizome stock were available.

Potential for Miscanthus in the USA
Trial yields of Miscanthus in the USA have been very high, reaching over 40 tonnes dry matter/hectare in Illinois. Indeed, Illinois used to be called ‘The Prairie State’ with over 60% of the state covered in grassland, until it was ploughed up for arable farming. Illinois University (Steve Long and Emily Heaton) has led this work on Miscanthus, and commercial uses are being developed. These could include co-firing in local coal power stations, and use as pellets for pellet stoves. Pellet stoves that handle grain, corn stover and switchgrass are available and being further developed for Miscanthus. It is known that there is significant potential, particularly for cellulosic bioethanol and biodiesel production, in the USA, from a range of technologies, including gasification and pyrolysis conversions as well as novel enzymatic systems. Illinois farm management budgets suggest much higher net margins from Miscanthus than from corn and soya bean rotations.

CERES, a California-based biotechnology company, is involved in breeding new varieties or multiplication (increasing plant stocks) of switchgrass and Miscanthus, while BICAL is working with Illinois University, and has farming partners establishing commercial Miscanthus production. BICAL has also formed a partnership with Environmentally Correct Concepts Inc in Illinois, to utilize their patented technology for carbon sequestration quantification.

USDOE energy crop national forecasts
The US Department of Energy, along with its Oak Ridge National Laboratory (ORNL), forecasts the requirement for energy crops by 2008 at 188 million tonnes (at less than $50 per tonne dry matter). Other ORNL data suggests that if perennial energy crop yields are high (as they have been so far for Miscanthus) the requirement for all uses could be 377 million tonnes dry matter, and with moderate yields 150 million tonnes of dry matter.

USDOE forecasts total potential biomass production (all sources) of 1 billion tonnes dry matter per year by 2030. These quantities are of similar orders of magnitude to those forecast by the EU over the same period.

Achievement of these requirements, with Miscanthus, will be difficult, due to the shortage of Miscanthus rhizome for planting currently available in the USA. However with sufficient government determination towards liquid fuel self- sufficiency, and increasing awareness of the need for carbon mitigation, these targets could be reached.

Jonathan Harvey is an agronomist with experience in the introduction of novel crops to UK agriculture. He operates a consultancy organization, Crops for Industry, specializing in agricultural energy crops and, recently, algaeculture.
e-mail: JONHATKENN@aol.com

The author wishes to thank Dr Paul Carver, technical director of BICAL Ltd, for information provided for this article.

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