(357b) The Composition of Industrial Hemp and Its Economic Potential As a Raw Material for Ethanol and Biodiesel Co-Production

Hemp is traditionally grown for its seeds and fiber, finding applications in textile, food, and essential oil production. Hemp is a drought-tolerant plant; it can grow on infertile soil and overgrow weeds. Hemp yield high biomass per land area and its cellulose content is higher than many other lignocellulosic feedstocks. The fibers contain cellulose (55%), hemicellulose (16%), pectic polysaccharides (18%), and lignin (4%) (Rehman et al., 2013). The high carbohydrate content in hemp biomass makes it a potential candidate as a bioenergy crop to produce lignocellulosic biofuels (Gunnarsson et al., 2015; Rehman et al., 2013).

Lignocellulosic crops can produce higher ethanol yield per land area than 1^st generation feedstock (Zatta and Venturi, 2006). It is estimated that 4500 liters of lignocellulosic ethanol can be produced from a hectare of hemp cultivation (Zatta and Venturi, 2006). Utilizing an entire hemp plant can yield 3000 liters of ethanol when co-produced with methane (Kreuger et al., 2011). The net carbon dioxide released while producing biofuels is equivalent to that absorbed during hemp plant growth. Ecological assessments on biofuel production from hemp indicate the possibility of zero environmental impact. These traits make hemp a promising raw material to produce fuel ethanol.

Plant oils have high energy density and are catalytically upgraded to a wide range of products, including chemicals, and biofuels (Lligadas et al., 2010; Salimon et al., 2012). After ethanol, biodiesel is a promising biofuel that is derived from lignocellulosic feedstock. Hemp seeds contain 35% oil, and it is more attractive than soybean (21% oil) (Rehman et al., 2013). Furthermore, hemp oil is rich in TAG (triacylglycerol), a key ingredient in producing biodiesel. The high oil content has encouraged investigating its biodiesel production feasibility (Li et al., 2010). Investigators reported very high conversion (> 99.5%) of hemp oil and 97% biodiesel yield through transesterification with a base catalyst (Li et al., 2010). Such high conversion and yield, together with high oil content in hemp seeds reflect higher biodiesel production per cultivation land area.

Biodiesel production from current oil crops, including soybean, face agricultural land use challenges and high production costs. With current feedstock, it is rather unfeasible to contribute to the plant oil market to make biodiesel and replace fossil energy (Vanhercke et al., 2019). Therefore, a new resource needs to be identified that discounts major drawbacks of traditional oil resources. One approach to new oil sources is to increase TAG accumulation in plant leaves and stems (Long et al., 2015). TAG functions as a storage lipid in plants, and recent achievements show that plants can be genetically modified to increase TAG content (James et al., 2010). For instance, by expressing the genes responsible for TAG biosynthesis, a 20-fold increase in TAG concentration was observed in tobacco leaves (Andrianov et al., 2010). Huang and others characterized transgenic lipid producing sugarcane (lipid-cane) (Huang et al., 2017) and then investigated the feasibility of utilizing plant oils as a platform chemical to make biobased products. Furthermore, a detailed economic analysis of biodiesel and ethanol co-production from lipid-cane (Huang et al., 2016) was performed. They showed that transgenic lipid-cane can produce 6700 liters of biodiesel from a hectare of land while soybeans can produce 500 liters. This analysis is a shred of evidence showing the technology and economic feasibility of developing transgenic plant biomass that can be derived from both sugar and lipid-based bioproducts.

Industrial hemp contains small quantities of oil in plant parts other than seeds. Accumulating or increasing lipid concentration in the entire plant will make transgenic hemp a dual crop to produce both ethanol and biodiesel. Our proof of concept assessment indicates that 67.5 gallons of biodiesel can be produced from an acre of transgenic hemp biomass assumed to contain 5% oil. Further increasing biomass oil content to 10% increases biodiesel production by 2 fold. Lipid containing hemp (5% lipids) can produce 1.3 times more biodiesel from an acre of land, compared to biodiesel produced from soybean (51 gal/acre).

Hemp biomass can be pretreated and then enzymatically hydrolyzed to produce sugars for ethanol production while the lipid portion is extracted separately to make biodiesel. Dilute acid pretreatment (DA) is a well studied method that has a high potential for industrial scale up (Jönsson and Martín, 2016). However, neutralization and detoxification in DA pretreatment can deteriorate the lipids in the biomass. On the other hand, mild pretreatment with liquid hot water (LHW) can avoid such degradation (Geddes et al., 2010) due to low severity. During LHW pretreatment, water molecules hydrate and partially remove complex sugars (Jönsson and Martín, 2016). Yet, sugar release from LHW pretreatment is low and this leads to poor enzyme hydrolysis. The hydrolysis requires high enzyme loading and residence time for better yields (Yang et al., 2018). Post-pretreatment mechanical refining of biomass increases pretreatment severity favoring high sugar yields (Kim et al., 2016). The plant cell matrix is disrupted during mechanical disk milling enhancing cellulose fiber accessibility. As fibers are exposed, enzymes are able to hydrolyze more sugars and improve their yield.

In this work, the composition of five hemp hybrids is characterized to determine structural carbohydrates and lipid composition. The hybrids were found to contain more than 30% (w/w%) glucan and at least 10% xylan. Later, the biomass is pretreated with Liquid Hot Water (LHW) at 180C° and then enzymatically hydrolyzed to produce monosaccharide sugars. After 72 h, glucose and xylose concentration reported are between 30 to 42% and 12 to 18%, respectively. Lipids present in all hybrids are extracted and TAG is separated from an aliquot of the whole lipid sample. The TAG and Total samples are transesterified to Fatty acid methyl esters (FAME/Biodiesel). The ratio of TAG obtained from TAG to Total lipids was between 1.76 to 17.8%. Using the experimental data, detailed technoeconomic analysis was performed for the co-production of ethanol and biodiesel from hemp. A plant consuming hemp biomass at an hourly rate of 85 tons can produce 114 kilotons of ethanol and 13 kilotons of biodiesel annually. The process plant requires a total capital investment of $382 MM. The total operating cost including raw materials, facility-dependent costs, utilities and other cost factors is estimated to be $141 MM.

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