The potential of industrial hemp (Cannabis sativa L.) for biogas production
Sammanfattning: Biofuels are currently produced from agricultural crops, and an increasing use of crops for this application is expected in the EU in the years to come. The dominating crops cultivated in the EU for biofuel production today have a relatively large environmental impact. The European Energy Agency has identified several lignocellulosic crops, including industrial hemp, as more sustainable potential alternatives. However, the biofuel yield from industrial hemp was largely unexplored before the work presented in this thesis was initiated. In this thesis work, the focus was on the potential of using hemp for methane production through anaerobic digestion. The biomass yield per hectare and the specific methane yield were determined at four different hemp harvest times. The specific methane yield did not change with harvest time, the average yield was 234 ± 35 m3/t volatile solids. The most suitable harvest time was therefore at the time of highest biomass yield, in this study found in the beginning of September to the beginning of October. The biomass energy yield was 186 GJ/ha and the methane energy yield 88 GJ/ha. The effect of storing hemp as silage on the methane yield was investigated. It was found that ensiling conserved the energy efficiently as the same methane yield was achieved before and after more than 3 months storage. It was shown that the methane yield of ensiled crops could easily be overestimated when the dry matter was measured with a standard method. The standard method does not include correction for volatile organic compounds formed during ensiling and lost by evaporation during dry matter determination. A previously developed method for correcting dry matter was demonstrated to be useful in avoiding this error. The effect of chopping, grinding and using acid-catalysed steam pretreatment prior to methane production from hemp, and the effect of combining ethanol and methane production were investigated and compared to methane or ethanol production alone. Methane production or co-production of ethanol and methane gave twice the biofuel yield of ethanol production alone. The use of steam pretreatment gave a similar methane yield to that from ground hemp, but higher than that from chopped hemp. Addition of external cellulolytic enzymes in a separate hydrolysis step after steam pretreatment, prior methane production, did not give a higher methane yield, than direct anaerobic digestion after steam pretreatment. The experimental data on production of these biofuels was combined with heat and power production in techno-economical modelling of a large-scale plant. Methane production or co-production of ethanol and methane production together with combined heat and power production showed high energy efficiencies and similar economic performance. Chopped and steam-pretreated hemp performed similarly economically in biogas production when combined with heat and power production. The co-production of methane, heat and power satisfied the energy requirements of the process and yielded a surplus of sellable products such as methane, electricity and district heating corresponding to 71–75% of the energy of the biomass. Despite high energy efficiencies none of the processes analysed would be economically viable today. The cost of the feedstock accounted for more than half of the total process cost. For the co-production of biogas, heat and power to be economically viable, the total cost would have to be reduced by one third. Alternatively, the methane sale price would have to increase by more than 50% to 3.6 SEK/m3. The yields of methane and ethanol were found to influence the process economy considerably. The production of electricity and heat had a significant influence on the energy efficiency but less on the process economy.
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