Miscanthus - the future biomass fuel that’s available today?

We are getting a lot of enquiries about biomass boilers that can burn miscanthus, so we thought that some information about its benefits may be useful.

Miscanthus as biomass fuel

Miscanthus is growing in popularity as a biomass fuel for use in some wood burning boilers (also known as biomass boilers). This feature looks at the reasons behind its growth in popularity, some of the issues relating to biomass heating with miscanthus and the type of wood burning boiler you need to effectively use miscanthus as a biomass fuel.

Read up on Treco’s Guntamatic Powerchip range of multi-fuel biomass boilers which run on wood chips, wood pellets, grain and chopped or pelleted miscanthus. For details of other systems in our range please refer to the biomass boilers page.

What is miscanthus?

Miscanthus (commonly known as elephant grass) is a high yielding, perennial energy crop that grows over three metres tall and resembles bamboo. The rapid growth, low mineral content, and high biomass yield of miscanthus make it an emerging biomass fuel.

Facts:

• The density of chopped miscanthus is around 150kg/M³ but when pelleted this rises to 600kg/M³.
• 1 tonne of miscanthus can produce 4 MWh of heat which is about the same as that generated from 400 litres of heating oil
• In terms of price, miscanthus offers large fuel cost savings compared to conventional fossil fuel heating sources, such as oil. In chopped form, miscanthus saves up to 87% compared to oil and, in pelleted form, savings of up to 50% are not uncommon.
• Biomass heating could contribute to as much as 3% of the of the total UK energy requirements

What are the benefits of miscanthus?

With higher photosynthetic efficiency and lower water use and nutritional requirements than other kinds of plants, miscanthus grows well on less fertile land without the aid of heavy fertilization. Research shows that it can sequester carbon and, as a sterile hybrid, miscanthus is completely non invasive.

Energy crop

Miscanthus is growing in popularity in Europe as a commercial biomass energy crop for heat and electricity generation as it emits significantly less CO₂ than fossil fuels. For example, miscanthus can generate as little CO₂ as 5.4g per kWh, whereas LPG produces 323g, oil produces 350g, coal produces 484g and electricity 530g per kWh. This means every tonne of miscanthus that replaces coal prevents up to two tonnes of CO₂ being emitted. (Source, Biomass Energy Centre)

As well as emitting less CO₂, miscanthus also offers large cost savings versus fossil fuels. Currently LPG costs around 7p per kWh, oil is around 6p and electricity is 13p per kWh. Chopped miscanthus costs around 1.4p per kWh, cheaper than wood chips at 2.7p. In pelleted form, you can expect to pay 3.5p per kWh, on a par with wood pellets. Drax Power Station to Reduce CO₂ by 17% with miscanthus.

The massive saving in CO₂ versus fossil fuels has led to miscanthus being used as an alternative fuel for heat and power. Its use is most notable at Drax power station, the largest, cleanest and most efficient coal-fired power station in the UK. Drax now co-fires coal with biomass produced from miscanthus and hopes to produce 12.5% of its electricity from renewable energy and reduce its carbon emissions by over 17%. The use of miscanthus as biomass fuel has been so successful that the power station have been working with local and potential growers to plant more of it.

Solid biomass heating with miscanthus

Just one tonne of miscanthus can produce the equivalent amount of heat of 400 litres of heating oil. This makes miscanthus a great solid biomass fuel for some wood burning boilers in pelleted or chopped form. However, miscanthus has a low fluid ash fusion temperature, whereby ash melts at high temperatures and deposits, known as 'clinker”' With miscanthus, clinker occurs at 650-700°C whereas wood chips don’t form clinker until they are burned at much higher temperatures.

'Clinker' deposits stick to the bottom and side walls of the combustion zone. It can not only cause blockage in the system’s inner workings, but prevent heat from reaching the heat exchangers and lower the boiler’s heating efficiency. Although adding 0.3% of limestone hydrate to the total weight of chopped fuel before combustion can minimise clinker forming, it remains a common problem in fixed grate boilers.

The trick with burning miscanthus is therefore keeping the temperature on the grate low, but burning the gases at high temperatures.

Guntamatic’s Powerchip, Biocom and Powercorn biomass boilers are some of the handful of biomass boilers available in the UK that can handle miscanthus. These sophisticated, fully automated systems achieve low temperatures of 700°C on the grate by the introduction of primary and secondary air to degass the fuel. The hot gases are then burned to reach higher temperatures in the boiler of around 1,200°C.

Combined with the self cleaning, moving step grate on the Powerchip, Biocom and Powercorn, this enables very high combustion efficiency, even at a low 26% of output. A stainless steel liner is also supplied as an option in these boilers, to combat the acidic nature of miscanthus when burned.

This more complete burn generates low volumes of ash from miscanthus to the volume of fuel in Guntamatic’s boilers. The ash that is generated is broken up by the moving step grate and removed into the ash box, preventing the build up of clinker before it can cause any combustion or blockage problems. An auger in the ash box ensures that ash is levelled as it builds up in the bin and the maximum amount of ash can be stored, minimising the frequency the ash box needs to be emptied.

To protect these multi fuel wood burning boilers further, the self cleaning heat exchanger’s tubulators also continually move, meaning the heat exchanger is continually cleaned.

Conclusion

Miscanthus can offer viable biomass heating, saving CO₂, fuel costs and making good use of poor quality land. Burning it as biomass fuel presents a range of challenges, so selecting the right boiler for the job is paramount in order to enjoy the full benefits of this emerging biomass fuel.