Prerequisites for sustainable energy production are high biomass production and low inputs of fossil fuels in agricultural operations, transportation and storage of energy raw material. In addition to high biomass production, quality of the biomass is also a significant factor. Existing bioenergy technologies have different quality requirements for biomass. Biological fermentation methods benefit from high concentrations of soluble sugars and starch, in lignocellulotic methods different pre-treatments are needed to convert cellulose and hemicellulose approachable to hydrolysing enzymes. High alkali metal (K, Na) and silicon (Si) concentrations in biomass can cause slagging and corrosion in combusting process. In addition to plant species, management such as harvesting and fertilization, weather and soil condition can influence biomass quality and its usefulness. Similarly different plant parts can vary in their chemical composition due to divergent translocation of different elements and partitioning of soluble sugars and storage carbohydrates in growing plants.
We have grown hemp (Cannabis sativa L.) and maize (Zea mays L.) in 2007-2009 at Viikki Research Farm, University of Helsinki, Finland. Maize was grown on 20 m-2 and hemp in 10 m-2 plots. Maize was fertilized with 120 kg N ha-1 and hemp with 60 kg N ha-1. Samples of 1 m-2 per plot were harvested twice in the autumn during the growing seasons and once in the following spring for quantity and quality of biomass produced. The chemical composition (Al, B, C, Ca, Cu, Fe, K, Mg, Mn, N, Na, P, S, Si and Zn; Cl; ash; sugars) and energy content (MJ ha-1) of the samples were analysed from dried, ground subsamples.
Maize dry matter yield was in average 30 t ha-1 in 2007, 15 t ha-1 in 2008 and 26 t ha-1 in 2009. Hemp dry matter yield was approximately 14 t ha-1. Early in the autumn harvested yield with rather high water content is most suitable for methane and ethanol production, whereas late autum and spring harvested yield with low water content is suitable for combustion and pyrolysis.