Lignocellulosic crops for production of advanced biofuels
The term lignocellulosic covers a range of plant molecules/biomass containing cellulose, with varying amounts of lignin, chain length, and degrees of polymerization. This includes wood from forestry, short rotation coppice (SRC), and lignocellulosic energy crops, such as energy grasses and reeds. Specific felling of forestry wood for biofuels is generally not considered sustainable and is therefore not further discussed here. Wood wastes and forestry residues are, however, promising feedstocks for advanced biofuels.
Lignocellulosic materials have potential for use as a feedstock for advanced diesel and drop-in biofuels (via thermochemical conversion) and for production of cellulosic ethanol (via biochemical conversion). Lignocellulosic crops generally have a higher GHG efficiency then rotational arable crops since they have lower input requirements and the energy yield per hectare is much higher.
The degree of yield per hectare, lignification, and ability to tolerate environmental stresses (limited water, low nutrient levels, etc) varies from crop to crop. So different species are more suited to different types of marginal land, and to different types of conversion process (e.g. acid and enzymatic hydrolysis, pyrolysis, torefaction and gasification). Plant breeding and biotechnology is being used to optimise the traits of potential energy crops.
Examples of lignocellulosic energy crops
Miscanthus (above) has been trialled extensively in Europe and the US as an energy crop for biofuel production.
In Europe, there are presently an estimated 30,000 ha of miscanthus planted (see OPTIMISC, FP7 Project). Miscanthus x giganteus is best adapted to temperate climates but the traits of other Miscanthus species should also be considered. For example, Miscanthus sinensis genotypes are better adapted to more extreme climates.
Miscanthus is a promising lignocellulosic feedstock with various applications due to its rapid biomass accumulation in temperate climates. Trials indicate that Miscanthus provides relatively high yields (double that of corn), requires limited fertiliser, few other inputs and adds significant amounts of organic matter to the soil.
For example, a ten year trial of Miscanthus (2003-2013) by University of Illinois showed an average annual yield of 10.5 tons per acre (double that of a corresponding area planted with Switchgrass). The trial confirmed that Miscanthus grows well with little or no fertiliser input. After five years, the roots and rhizomes contribute 12 tons of biomass per acre to the soil (dry mass). The extensive root system of Miscanthus makes it suitable for stabilizing slopes or soils.
In March 2012, it was announced Mendel Biotechnology (Mendel Bioenergy Seeds) would carry out a 4-year field trial of PowerCane™ Miscanthus with BP Biofuels, as a potential feedstock for the cellulosic ethanol demonstration plant in Jennings.
Giant reedgrass (Arundo donax)
Giant reedgrass is adapted to a wide variety of ecological conditions, but is generally associated with riparian and wetland systems. Several field studies have highlighted the beneficial effect of the crop on the environment due to its minimal requirements on soil tillage, fertilizer and pesticide (e.g. Riffaldi et al, 2010). Moreover it offers protection against soil erosion, is well adapted to saline soils and saline water, and resistant to biotic and abiotic stresses. The fact that it can be cultivated fo between 20 to 25 years without replanting also makes it an interesting energy crop. Giant reedgrass (Spanish cane) is considered to be one of the most promising species for biomass production in Europe. It is being cultivated as a feedstock for the Beta Renewables commercial scale cellulosic ethanol plant in Crescentino.
Reed canary grass (Phalaris arundinaces)
Reed canary grass, Phalaris arundinacea, provides good yields on poor soils and contaminated land and is thus an interesting candidate for bioremediation of brownfield sites as well as a source of biomass for bioenergy (typically as briquettes) or pulp. Is also considered a suitable feedstock for cellulosic ethanol production [Source: Pahkala et al, VTI Finland, 2007].
Giant King Grass, Napier Grass, Elephant Grass (Pennisetum purpureum)
Pennisetum purpureum is a member of the Poaceae family and a perennial tropical grass native in Africa, where it is used as a fodder plant. It is an attractive energy crop because it reaches yields up to 40 tons/ha/yr and can be harvested 4–6 times a year. Furthermore water and nutrient requirements are low. In California, Viaspace Inc. is developing projects using Giant King Grass as a feedstock for advanced biofuels, and biomethane for energy production.
Switchgrass (Panicum virgatum)
Switchgrass has qualities that make it attractive as a biofuel source, including a seed that is easy to work with, adaptation across a wide geographic range, and a seed market already established for forage. Extensive research is being carried out into cultivation of Switchgrass as a biofuels feedstock in the US. The plant is a tall-growing, perennial grass that is native to North America.
Samuel Roberts Noble Foundation has developed novel strains of switchgrass that contain lower amounts of lignin and hence boost biofuel yields by over a third [Source: Proceedings of the National Academy of Sciences]. Following a $5m grant from the DOE in 2009, University of Tennessee and Genera Energy have developed a new feedstock logistics systems using chopped switchgrass, which aims to bridge the gap between growers and biofuel producers.
Short Rotation Coppice of Willow and Poplar (Salix spp. and Populus spp.)
Short Rotation Coppice SRC - where species such as willow and poplar are grown on marginal land typically over 3-5 year cycles - has potential for providing feedstocks for advanced biodiesel and drop-in biofuels (via thermochemical conversion) and for production of cellulosic ethanol (via biochemical conversion).
"Willow is planted as rods or cuttings in spring using specialist equipment at a density of 15,000 per hectare. The willow stools readily develop multiple shoots when coppiced and several varieties have been specifically bred with characteristics well suited for use as energy crops. During the first year it can grow up to 4m in height, and is then cut back to ground level in its first winter to encourage it to grow multiple stems. The first harvest is in winter, typically three years after cut back, again using specialist equipment, however a cycle of 2 or 4 to 5 years is also common. In fertile sites growth can be very strong during the first two years after coppicing, giving rapid site capture, reducing thereafter and so a 2 year cutting cycle may be more appropriate.
Poplar displays more apical dominance than willow and is therefore less ready to develop multiple stems following coppicing. Shoots can reach up to 8m by the end of the first rotation. It therefore tends to develop fewer, thicker stems than willow, and consequently has a lower bark to wood ratio. Individual shoots can reach up to 8m by the end of the first 3 year rotation. Poplar is planted in spring, from cuttings. These cuttings must have an apical bud within 1 cm of the top of the cutting. Because of this it is difficult to use poplar in equipment developed for planting willow short rotation coppice" [Source: Bioenergy Centre, UK © DEFRA].
Planting density for poplar is lower than that for willow, typically 10-12,000 per ha. Cut back takes place late in the following winter.
Improving performance and availablity of Short Rotation Plantations is one of the aims of the FP7 project, ROKWOOD, which supports cooperation between six European research-driven clusters in order to improve RTD, market uptake and to increase investments in wooden biomass production and utilisation schemes at regional level.
The EC project ENERGYPOPLAR (2008-2012) aimed to develop energy poplar trees with both desirable cell-wall traits and high biomass yield under sustainable low-input conditions to be used as a source of cellulosic feedstock for bioethanol production.
Willow and poplar may be grown and harvested in 2-5 year cycles as an energy crop (Short Rotation Coppice).
See the sugar crops page for development of energy cane varieties.
Rusby or Virginia mallow (Sida hermaphrodita)
Sida belongs to the mallow family and is native to North America. It can grow 1 – 3 m tall, has hairy stems and leaves with toothed lobes. Plantations can be used up to 25 years whereby the dry woody parts are harvested from late autumn to spring with high and stable yields. It is an easy to handle crop that can be cultivated with conventional farming methods and machinery.
Halophytes (Various species)
Halophytes as feedstock for bioethanol production were explored by Abideen et al (2011). Their study shows that species such as Halopyrum mucronatum, Desmostachya bipinnata, Phragmites karka, Typha domingensis and Panicum turgidum, have potential as bio-ethanol crops. These perennial grasses are salt tolerant with high growth rates and produce lignocellulosic biomass of "good" quality as a feedsdtock.
Recent Research on Lignocellulosic Crops
Research funded by the BBSRC, UK has identified natural variants of 'straw plants' that are easier to break down but are not otherwise weaker or smaller. Researchers in the Centre for Novel Agricultural Products at the University of York, working with colleagues in France, screened a large collection of variants of the model grass species Brachypodium for digestibility. Reseachers also found that the genes responsible for the digestibility of the cell wall can be identified [Source: BBSRC 2014].
See also the page on Plant Biotechnology.
References and Links
Cascading of woody biomass: definitions, policies and effects on international trade
IEA Bioenergy Task 40 Working Paper.
Ollson et al. (2016)
Halophytes: Potential source of ligno-cellulosic biomass for ethanol production
Abideen et al (2011)
Bioenergy – a Sustainable and Reliable Energy Source – Main Report
IEA Bioenergy ExCo (2009)
Comparison of Soil Organic-Matter Characteristics under the Energy Crop Giant Reed, Cropping Sequence and Natural Grass.
Communications in Soil Science and Plant Analysis, 41:173–180
Riffaldi, R et al (2010)