4 Grasses as an energy source

There is increasing interest in using grasses as a biofuel source. Grasses grow quickly, produce a large amount of biomass per unit growing area and leave only small amounts of residue when they are burned. Typically, grasses are burned to produce heat and steam to power turbines in conventional power plants. The largest power station in the UK, Drax, currently burns 300,000 tonnes of Miscanthus x giganteus annually, alongside its usual coal supplies. Another advantage of using grasses is that many of them multiply by underground rhizomes, which means they spread quickly and generate new shoots easily. However, this can also make them invasive and difficult to control and eradicate, so they are considered by some to be weeds.

Figure 9 Canary grass (Phalaris canariensis) originated in the Mediterranean region but has been cultivated and grown in the Middle East, Europe and parts of Argentina. It was formerly cultivated primarily for the production of bird seed but is now being considered as a biofuel due to its high yields and ability to thrive in temperate regions. Breeding programmes have selected varieties which have a particularly high carbon content in their cell walls, giving a potentially greater energy output when burned.

Maximise

Figure 10 Miscanthus x giganteus is a large perennial grass that can grow to a height of over four metres. It is grown in the UK and elsewhere in Europe and for several years there has been intense research into its potential as a biomass crop. It reproduces by underground stems (rhizomes) and is considered to be an environmentally friendly crop due to its large root system, which captures nutrients, and its stems, which provide wildlife cover.

There has been interest in using mixtures of a number of plant species, including grasses, together to produce biofuel. These mixtures allow the maximum interception of light as the different plants grow at different rates. They are considered to be carbon neutral as they are perennials, which do not require re-sowing each year, and, most importantly, they can be sown on degraded agricultural land so food production is not jeopardised.

Figure 11 Switch grass (Panicum virgatum) is a dominant species of the tallgrass prairies in North America and is commonly used as an ornamental grass in gardens. It thrives in harsh conditions, needing only poor soil with a low nutrient content, and undergoes rapid growth. These characteristics make it a strong candidate for biomass production.

Grasses which grow year on year are called perennials. Harvesting of perennial grasses for biofuel can take place anytime from late November to April. The moisture content of the leaves is highest in November meaning that the grass cannot be baled and needs to be chopped into smaller pieces which dry more easily. As time passes from November to April the grass loses leaves, thus reducing the harvestable biomass, but harvesting in April means it can be baled which costs less than chopping the grass up during harvest. Many burners used to generate heat or electricity from wood fuel can also combust bales of grass or straw. Various studies have revealed that it becomes uneconomic to transport grasses for burning as biofuels more than about 80 kilometres.