Abstract
Grass from landscape management or from agricultural practices is currently destined mainly for composting, with the production of a valuable product; however, this process demands energy. Anaerobic digestion, instead, represents an energy-positive process that results in the production of fuel, biogas, and a fertilizer, namely digestate. Previous tests for the evaluation of biogas yield from freshly harvested grass gave promising results. However, for a practical exploitation of this resource, appropriate conservation is necessary in order to enable the daily load of digesters while reducing the loss of organic matter. The present work is focused on the evaluation of biogas and methane yield from dried and ensiled grass (without conditioning) in order to assess eventual biogas potential losses in comparison to digested fresh grass. Tests were performed with grass collected from riverbanks (Veneto, Northern Italy) in batch, lab scale digesters. Dry and ensiled grass showed a good potential for exploitation in the anaerobic digestion process, reaching biogas yields of 565.9 and 573.4 NL∙kgVS−1, respectively. Compared to the biogas yield of 639.7 NL∙kgVS−1 of the fresh grass, the conservation treatment determined yield reductions of 11.5% and 10.4% for dried and ensiled grass, respectively. However, considering the methane yields, conservation treatments showed lower reductions, amounting to 4.8% for dry grass and 0.5% for ensiled grass; presumably the higher concentration of organic acids in ensiled grass determined a higher methane content in biogas and the consequently lower reduction of methane yield.
Highlights
One of the most promising technologies for the management of fermentable organic wastes is represented by anaerobic digestion (AD) [7]: This energy-positive process provides high values products, such as biogas and digestate [8,9], but it gives a significant contribution to the reduction of the emissions of greenhouse gasses, methane (CH4 ) and carbon dioxide (CO2 ) in particular, and of odors deriving from uncontrolled fermentations [10,11]
Grass was mainly composed by Poaceae spp, with prevalence of Poa spp. and Festuca spp
Dry grass presented a higher content of total solids (TS), 89.08%, and a volatile solids (VS) content of 87.73%TS
Summary
The primary sector can play a significant role to reach the goals of sustainable development and circular economy by the exploitation of wastes or by-products for the production of renewable energy [1,2,3,4,5,6].One of the most promising technologies for the management of fermentable organic wastes is represented by anaerobic digestion (AD) [7]: This energy-positive process provides high values products, such as biogas and digestate [8,9], but it gives a significant contribution to the reduction of the emissions of greenhouse gasses, methane (CH4 ) and carbon dioxide (CO2 ) in particular, and of odors deriving from uncontrolled fermentations [10,11].Energies 2019, 12, 3244; doi:10.3390/en12173244 www.mdpi.com/journal/energiesAgricultural practices and landscape management produce relevant quantities of biomasses that could be successfully treated by the AD process but are not completely exploited [12,13].Composting still represents the main destination of these feedstocks: This mature process determines the production of a valuable soil amendment, compost, and requires energy [14]. One of the most promising technologies for the management of fermentable organic wastes is represented by anaerobic digestion (AD) [7]: This energy-positive process provides high values products, such as biogas and digestate [8,9], but it gives a significant contribution to the reduction of the emissions of greenhouse gasses, methane (CH4 ) and carbon dioxide (CO2 ) in particular, and of odors deriving from uncontrolled fermentations [10,11]. Grass from landscape management, instead, could represent a promising product to enhance the sustainability of the biogas field [15,16,17,18,19]. Studies focused on the definition of quantity and composition of grass are still missing, but these areas are characterized by a significant quantity of biomass [22,23]. Botanical composition is normally represented by Poaceae spp., Fabaceae spp., or Asteracee spp., but other species of grasses can be found depending on the area [24,25]
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