Research paperEnergy self-sufficient production of bioethanol from a mixture of hemp straw and triticale seeds: Life-cycle analysis
Introduction
Feedstock costs dominate total costs in ethanol production [1]. Many investigations are based on low-cost substrates, such as wheat straw, so-called waste materials or co-products. After the building up of a plant that uses these materials, market prices for the substrates increased hugely [2]. Hence, the profitability of such a plant decreases. A more modest approach consists of the use of an agricultural crop with a high glucan content and high yield per hectare, e.g. fibre hemp Cannabis sativa L. Industrial hemp is characterised by glucan contents of more than 60% in dry matter (DM) [3] and a yield ranging from 5.6 t ha−1 DM in the United Kingdom to 25 t ha−1 DM in Italy [4]. Application of genetically modified yeasts that ferment xylose and arabinose increases ethanol yield. The data of Pakarinen et al. [5] predict 33% higher ethanol yields.
Hemp production improves the soil structure with its enormous root growth [6], especially in landscapes that are suitable for grain production. A low demand for fertilizers of 70–90 g m−2 marks hemp as an environmentally friendly crop [7]. Fast plant growth causes fast soil coverage that inhibits the sprouting of pest plants and secondary growth. Its high durability against plant diseases decreases the application of pesticides to a minimum [8].
Hemp dominated the fibre market worldwide in earlier times and was used especially for sailcloth and cordage [9]. Nowadays, hemp cultivation has been revived in many countries throughout the world. Hemp fibres are still under investigation and requested as tissue [10], composite [3], [11] and paper [12]. Hemp hurds, also called shives, serve as an energy source in power plants, animal and pet bedding, garden mulch, and are used in light-weight concrete or for ethanol production [13]. Oil extracts from hemp seeds are widely demanded for the cosmetics [14] and pharmaceutical industry [15], [16] and as a high-value food oil [17]. Casas and Rieradevall i Pons [18] analysed the use of hemp oil diesel. The whole hemp plant can be used for ethanol and methane production [19], [20] and combustion [21], [22].
A continuous ethanol plant requires the biomass to be storable after harvest. Hemp straw achieves DM contents >0.9 g g−1 after field retting and does not need any further treatment in storage. Frost-retting has been shown to avoid mould growth, especially compared to unretted hemp straw [23]. Ensiling of fresh substrate presents another storage possibility, however, it greatly increases the transport weight. Furthermore, storage requires anaerobic conditions. Bacteria will usually build up in an acidic environment by the fermentation of free sugars into organic acids. Due to the lack of free sugars, cut hemp needs to be pre-hydrolysed by enzymes or treated with acids for preservation [24].
Hemp substrate needs to be cut or chopped prior to cellulose digestion. Zhang et al. [25] have related particle size to the cellulose conversion grade of steam-exploded substrates. However, the pretreatment conditions considered were more extreme than in this work. Considering other publications, steam-explosion treatments with or without acid or alkaline addition have been carried out between 170 and 250 °C [26], [27], [28], [29], [30].
Furthermore, Zhang et al. [31] predicate synergies between cellulases and pectinases in steam-exploded and ensiled hemp. Both the very common dew retting in the fields and water retting cause pectin degradation by bacterial pectinases [32]. Pakarinen et al. [20] confirmed enhanced enzymatic accessibility to the biomass after enzymatic or chemical pectin removal. Similarly, cellulases interact synergistically with xylanases. Hu et al. [33] reported enhanced hydrolysis yields if xylan is degraded. A huge challenge arises from enzyme binding to the surface of lignin molecules, as it makes them unavailable for cellulose degradation [34]. Digestion of lignin to phenols solves that problem; however, those hydrolysates lead to toxic fermentation brews and complicate yeast fermentation. Furthermore, several cellulases are very susceptible to phenols [35].
Different strategies for the production of fossil fuels have to be compared to lignocellulosic ethanol production. Life cycle analyses (LCAs) take account of material, energy and pollution flows. Although calculations of flows for the use of the same energy crop under the same conditions and application should not differ very much between different studies, assessment methodologies for the impact of biofuels do differ [36], [37], [38], [39]. Differences in feedstock, country, scope of approach, system boundaries, land use change, by- and co-products and costs are the key issues [39]. The kind of conversion technology applied especially influences the yield, costs and environmental impacts [40]. Lee [41] described the same challenges in conversion technology in 1997 as today. Furthermore, available arable land limits the profitability of ethanol plants by increasing transport distances [42].
The potential of one kind of renewable energy can be determined regarding the yield per hectare, environmental impact or monetary efforts. The CO2 abatement costs estimate monetary input per saved ton of CO2 emissions using bioenergy. An advanced calculation also has to take external costs into account. These costs are not part of the market price of a product, but have to be paid as social costs [43]. The German Federal Ministry of the Environment proposes a calculation of costs of 20, 70 and 280 € t−1 CO2 equivalent. Indeed, costs and especially savings of renewable energies receive only a little attention in publications [44].
Section snippets
Goal and scope
This LCA presents an ethanol biorefinery combined with biogas combined heat and power (CHP) plant. An essential part of the ethanol production is the co-fermentation of the non-food cereal triticale and cellulose from hemp. The biogas fermenter is powered by slurry enriched with triticale straw. Furthermore, cultivation of the energy crops is included. The residues from the biogas fermenter return to the fields and decrease the consumption of fertilizers.
The analysis includes the flows for
Hydrolysis
Chopped and steam-exploded hemp straw mashes with a DM content of 0.083 g g−1 were hydrolysed in combination with two different enzyme dosages [Table 1]. Batch 1 was supplied with 13.6 mg enzyme per g DM and batch 2 with 27.2 mg enzyme protein per g DM. In each case, more than 0.5 g g−1 of cellulose was hydrolysed after 24 h. Mashes with an addition of 13.6 mg enzyme per g DM achieved 35.4 g glucose in mash (0.67 g g−1 hydrolysed cellulose) and mashes with 27.2 mg g−1 enzyme in DM achieved
Conclusion
Hemp appears to be the perfect plant for producing cellulosic ethanol concerning yields and glucan content. The low hemicellulose content in hemp straw makes genetically modified yeasts dispensable. The low lignin content saves input of high enzyme concentration. No pesticides are necessary because the fast and tall growth and durability of hemp make them redundant.
A total of 0.3 m3 ha−1 of high-value food grade hemp oil emphasises the importance of by-products. With good marketing, the selling
Acknowledgments
We gratefully acknowledge German Hasselbeck (Erbslöh, Geisenheim, Germany) for enzyme supply, Bastian Fleitz (Institute for food science and biotechnology, Universität Hohenheim, Germany) for construction of an applicable enzyme activity test, and Christof Wetter and Daniel Baumkötter (University of applied sciences Münster, Germany) for biogas investigation. Thanks to the DBFZ and BMU for financial support. Many thanks to Sandra Schläfle (Institute for Food Science and Biotechnology,
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