Willow

 

Willows offers great potential as a source of renewable energy, that does not add to production of greenhouse gases or acid rain.

Producing willow as an energy crop contributes to sustainable development :

• putting the land, and the farmers' skills and equipment, to good use;
• securing jobs in rural areas;
• helping rural communities to remain viable.

Willow grows throughout the northern hemisphere, mainly in cold and wet areas, and a few species are native to the southern hemisphere. Willows :

• produce a lot of biomass in a short period, and are among the fastest growing woody species in northern Europe;
• can be grown with low inputs of agro-chemicals;
• are easily established from un-rooted cuttings;
• re-sprout vigorously after each harvest;
• offer large potential for genetic improvement;
• have an energy balance in the region of 20:1 (ie  the energy obtained can be 20 times as much as the energy used to grow the crop);
• can be used as a vegetation filter during "bio-remediation" of waste water or contaminated land.

 

Willow for energy is normally grown as coppice. The plants are cut back at intervals near ground level, and allowed to re-grow as multiple shoots (a coppice "stool") rather than a single stem. Willow coppice might be harvested up to six times, typically at intervals of 3 - 5 years. At the end of that time (perhaps 25 years), the stumps can be removed, and the land re-planted with agricultural crops or more coppice.

 

Why coppice ?

Some plants accumulate a lot of dry matter in their roots, others in leaves and stems  (where it can be harvested repeatedly in a coppice system). Willows tend to have a shallow and sparsely spread rooting system in the early years, but allocate more dry matter to the roots in later years. Harvesting at 3 - 5 year intervals ensures that the vigorously growing juvenile stages are maintained, and dry matter continues to accumulate in the stems.

 

Willow can be used to produce heat or electricity. Electricity is either used on site, or sold through a distribution grid. An engine or turbine driving a generator converts only 25 - 33% of the energy content of the fuel into electricity, the remainder is emitted as heat. Where this heat energy can be utilised in a combined-heat-and-power (CHP) system, the total efficiency can be increased to 85% or more.

There are three methods for converting willow into energy:

• combustion is used for heating water or to raise steam for a turbine;
• gasification produces a combustible gas that can be burned in a boiler, or used as fuel for an engine or gas turbine;
• pyrolysis can be used to convert the crop into gas, oil or charcoal fuels.

 One hectare of a well-managed willow plantation can yield 10 - 12 tonnes of dry matter per year, with energy equivalent to about 5 000 litres of oil.

As a rough guide, 1 kg of willow will yield about 1 kWh of electrical output. A district heating scheme for a development of 100 houses would require about 25 hectares of willow coppice. A combined heat and power system with 100 kW electrical output will use 50 ha of willow coppice harvested on a three year cycle. A power station generating 5 MW of electricity would need around 2 500 ha of willow.

Most users need chipped fuel. The chipping operation, sometimes referred to as "comminution", often takes place during harvesting. Alternatively the crop can be harvested as long sticks, and chipped later.

Willow is harvested in winter. This coincides with the main demand for space heating, but electricity and industrial process heat are generally required all year round. Storage requirement can be minimised by using willow as it is harvested during winter, and other fuels (usually forestry residue) over the remainder of the year.

 Storage and transport need careful management. Freshly harvested chips can deteriorate in store unless they are dried; full length willow sticks are easier to store, but more difficult to transport.