The best energy crops are durable, their dry matter yields are high and constant and their production costs are low. A perennial growth habit, low agrochemical requirements, effective conversion of solar energy to biomass and ease of conversion back to useful energy guarantee that energy efficiency of production is high and environmental impacts are low. For combustible crops it is practical if the crop can be harvested when it is dry. Transportation, storage and combustion are easier and heating value is high. For biogas crops, a low moisture content is not as important because the crops are usually stored as silage. For biogas production the crops should be harvested when their gas production is at an optimum level.
Reed canary grass (RCG) - Reed canary grass is a winter hardy, highly productive and durable grass crop. Energy content of RCG is about 4.5 MWh/tdm. An RCG plant develops its root system during two summers and the first yield is harvested two years after sowing. The crop produces the best yields when it is older than three years. RCG grows well in all soil types, but the best yields have been recorded from moist mold and fine sand soils. Fields for RCG should be flat and stone-free so that is possible to cut the crop close to the soil surface, leaving a short stubble. The plantation can be broken up quite easily if the farmer wants to change a crop.
Willow - Willow (Salix spp.) includes several tree and shrub species, some of which grow fast. Willow has almost the same net heating value as the wood fuels, approximately 18.6 MJ/kgdm. The establishment period and intervals between harvests are 3 – 5 years and the yield can reach about 8 – 10 tones dry mater per hectare per annum. To date, willow chips for direct combustion are the most commonly used products on the market. Willow is usually supplied as 2 – 3 meter long branches that are cut between December and March when the buds are fully dormant. These can be planted immediately or carefully stored in cool conditions (-2 – -4°C) until they are used. It is necessary to protect the planting material from moisture loss and during storage prior to planting. Cuttings with burst buds should not be planted because they will not root easily.
Willow plantations are very demanding of water and nutrients, generally requiring 3 –5 mm of water per day during the growing season. The demand for nutrients varies according to age of the plantation and stage of crop development. Studies have suggested that an economic and environmental benefit may result from using waste water for irrigation, and sludge together with ash from biofuel combustion as fertilizer. Research has also demonstrated that willow can remediate soil contaminated by organic pollutants and heavy metals. Willow is preferably harvested during the winter when the ground is frozen and the moisture content in the biomass is at its lowest (ca. 50 %). Willow is harvested after 3 – 5 years of growth. Willow biomass is usually harvested directly by cutting and chipping in the field. The chips are then transported to district heating or CHP plants where they are stored and used. The same equipment can be used for producing and supplying willow chips for a CHP plant as is used to produce conventional wood chips. There is no major difference either in storage conditions used for willow or conventional wood chips. Willow can be stored in bundles over a longer time period without significant reduction in quality.
Hemp - Most of the hemp grown in Europe is used for fiber production. The fibers are used in the pulp and paper industry and the residual shivers are used as animal bedding. Energy production in the form of solid fuel from the whole hemp stem is a relatively new use for the crop.
To make full use of the growing season hemp should be sown as early in the spring as is possible. Seed is best sown at 2 – 3 cm depth and at a row distance of 25 cm. Although the seedlings will germinate and survive at temperatures just above freezing, soil temperatures of 8 – 10 °C are optimal. Good soil moisture is necessary for seed germination, and adequate rainfall is needed for good growth, especially during the first 6 weeks. For energy purposes a seeding rate of 20 – 40 kg/ha is recommended. Growing hemp may require addition of up to 110 kg/ha of nitrogen, and 40 – 90 kg/ha of potash.
Hemp is harvested for energy use as wilted, dry material in late winter or spring after the growing season. This means that the whole growing season is used, giving a high biomass yield. During winter the leaves fall off and the ash content is lowered. Some of the unwanted elements (N, S, Cl) are leached out of the material during winter, which produces better fuel than grass.
Hemp can be refined into briquettes or pellets to increase the energy density and to improve handling characteristics. This material can be used for heat production by the general public (burner, stoves and boilers) or in larger scale heating and power plants.