Methane Production From Industrial Hemp

. Due to the increasing shortage of fossil fuels, the use of alternative energy sources is becoming even more popular. In Latvia, maize is predominantly used for the production of biogas, and other crops are being studied for this purpose. ( Research purpose ) To study the productivity of industrial hemp varieties ( Cannabis sativa L.) and the possibility of obtaining biogas from hemp. ( Materials and methods ) Field experiments on hemp productivity were carried out on sod calcareous, heavy dusty sand clay soils in 2012-2014. Ten industrial varieties of hemp – 'Bialobrzeskie', 'Futura 75', 'Fedora 17', 'Santhica 27', 'Beniko', 'Ferimon', 'Epsilon 68', 'Tygra', 'Wojko', and 'Uso 31' – were sown with a seeding rate of 50 kilogram per hectare at the background of fertilizers: nitrogen – 120, phosphoric oxide – 90, potassium oxide – 150 kilogram per hectare. Hemp was sown on 10-square meter plots in mid-May, in triplicate. Hemp was harvested at the beginning of seed ripening phase. The whole crop of green mass was calculated on a completely dry matter. The fermentation process for the production of biogas, the average yield of methane, and other parameters were studied in the Laboratory of Bioenergetics of the Latvia University of Life Sciences and Technologies, using small-sized bioreactors. ( Results and discussion ) The dry matter yield of hemp obtained in the agro-climatic conditions of Latvia averaged 13.32-17.78 tons per hectare. For an average of three years (2012-2014), higher yields of dry matter were obtained from the varieties of 'Futura 75' (17.76 tons per hectare) and 'Tygra’ (16.31 tons per hectare). The average amount of methane obtained from the 'Uso 31' leaves was 0.365 litre from one gramme of dry organic matter, which is a very good result as compared to other energy crops, for example, corn silage (0.319-0.330 litre from one gramme of dry organic matter in Latvia). ( Conclusions ) The research has demonstrated that hemp can be successfully used to produce biogas, and hemp leaves are the most suitable starting material.


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NEW TECHNICS AND TECHNOLOGIES I ndustrial hemp (Cannabis sativa L.) is one of the oldest home-grown and most versatile plants, and it has been cultivated over a long time period. In 19th century, its cultivation thereof in Europe declined, while recently, it has attracted interest again [1]. Nowadays, industrial hemp has become important as a crop used for biomass production. Environmental concern and recent shortage of wood fibre have renewed utilisation of hemp for wide range of industrial products, including textiles, paper, and composite wood products [2].
Analyses of the latest trends in hemp cultivation and use as well as experimental results allow concluding that hemp cultivation and processing in Latvia are very perspective, since this plant is fast-growing and suitable for Latvia's agro-climatic conditions.
Recently in Latvia, the number of industrial hemp growers and the area of cultivated land both have increased, and, in line with the data provided by the Association of Industrial Hemp of Latvia, hemp plantations in 2018 occupied approximately 1100 ha, showing growing interest in this agricultural sphere.
The amount of biogas obtained from various raw materials differs [3,4]. Several researchers, when studying similar raw materials, have come to different conclusions, and the results depend upon substrate, conditions under which anaerobe process takes place, microorganisms content, and other factors [5,6]. One of the key factors influencing the production of biogas is the content of organic matter, especially the content of three main organics matter [7,8] groups: carbohydrates, proteins, and lipids.
Various kinds of straw can also be one of the raw materials used to produce biogas. Usually, they are used as bedding in livestock and poultry housings, and together with manure it is a cheap material.
Also freshly mown grass may be used for biogas production. Grasses vary, and the biogas yielded from them will differ as well. Theoretically, legumes should produce more biogas compared to grasses. Moreover, soil in which and climatic conditions under which grass has grown are of great significance. The average content of grass dry matter comprises 12-25%, the content of DOM -85-93%.
The research was conducted with industrial hemp (Cannabis sativa L.) that was taken from trial plots. The samples were tested with analyses necessary for successful anaerobic fermentation (AF) processcomplete dry matter, ash, and organic dry matter were measured. Basing on the results of the analyses, the necessary quantity of hemp was calculated (100 g), which, afterwards, was filled in the bioreactors B2, B3 and B4 together with yeast (2000 g) and water (1500 g). The remaining biogas potential of yeast was verified by fermenting it in the bioreactor B1. Substrate formation proportions were equal in all bioreactors: 2000 g of yeast, 100 g of hemp to be fermented, and 1500 g of warm water. Hemp was chopped -the length of pieces in coarse chopping was 1-2 cm, while fine chopping that was made with electric chopper produced 1-5 mm pieces. Each substance was carefully weighted before filling it in bioreactors. Fermentation took place within single filling regime till biogas was not forming anymore. Each day, all parameters necessary to control the process -gas amount and content, pH, pressure, temperature in room and bioreactors -were registered in a research journal. Also, digestate was weighted and the content thereof was analysed. Substrate in bioreactors was mixed with a specific devise operated by a mobile perforator. (Note: the quantity of yeast depended on the condition that such a quantity can be taken out of a continuously working bioreactor, thus ensuring equal conditions in all bioreactors. The condition that there was sufficiently much yeast, notably speeded up the start of a stable AF process.
The amount of biogas obtained was researched using a laboratory device consisting of four 5 l bioreactors (Fig. 2). Each bioreactor was equipped with the temperature maintenance and gas collection devices and an automatic registration apparatus. pH and pressure were controlled visually, afterwards registering parameters thereof in computer. Observations were made at 38±1°C temperature under a single filling regime.
With the aim to speed up the start of the process, the yeast for 5 l bioreactors was taken from a working bioreactor. Microbiological yeast was fermented with cattle manure that was added in each bioreactor (15% of total substrate). For substrate and dry matter of each substance, ash and organic dry matter were cleared

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NEW TECHNICS AND TECHNOLOGIES out before filling them in bioreactors. Measurement precision was ±0.02 for pH, ±0.0025l for gas, and ±0.1°C for temperature. Biogas content was measured periodically by finding out the content of CH 4 , CO 2 , O 2 , and H 2 S. Complete dry matter was measured with the dry matter weights "Shimazu" at the temperature of 120°C, the content of organic dry matter was determined by drying the samples under a particular programme at 550°C in the oven "Nabertherm". Gas content was measured with gas analyser "GA 2000", thus finding out the content of methane, oxygen, carbon dioxide, and hydrogen sulphide in biogas, as well as the pressure and normal gas volume. Weighting was made with the weights "Kern FKB 16KO2"; pH was measured with the pH meter "PP-50" with accessories (stationery).
A mean hemp sample was taken, and its content was studied basing on standardised methodologies in compliance with ISO 6496:1999 in the Bioenergetics Laboratory of Latvia University of Life Sciences and Technologies. Mean sample and yeast of each raw material group were investigated to determine full dry matter, organic dry matter, and the content of main elements; moreover, each sample was weighted carefully and the yeast mixed with the rest of the mass. The same yeast was used for all four samples -digestate from the continuously working bioreactor. The bioreactors of 0.7l were filled with 20 g of hemp and 0.5 l of yeast (weight was registered with a 0.2 g precision). All bioreactors (altered standard containers) were connected with gas accumulation bags and taps, placed in the oven under the temperature of 38±0.5°C. The amount and content of gas produced were measured every day; bioreactors were shaken with the same frequency thus mixing the substrate and reducing the floating layer.
Statistical assessment showed that meteorological conditions present during the growing season influenced the total volume of dry biomass yield.
The measurements obtained were summarised in tables and served as a base to calculate the potential for producing biogas and methane in each bioreactor. Calculations were made bearing in mind also the volume of gas produced in the control bioreactor (the one from which yeast was taken) ( Table 2).
Hemp cultivar 'Uso 31' was taken from the trial field on 24 August 2013, on a very dry and sunny day, and on 4 September 2013, it was filled in a bioreactor. This condition as well as the condition that this hemp was stored in suitable premises may explain the comparatively high content of dry matter and dry organic matter. Anaerobe fermentation lasted for 53 days. The results of digestate analyses are shown in Table 3.
In all bioreactors, gas was appearing evenly, already within the first days, except the control bioreactor, which contained only yeast and water. In this reactor,

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only a small amount of gas was produced, which means that yeast contained a very small quantity of not decomposed organic matter that is consumed by bacteria. Biogas and methane yields produced in all bioreactors are shown in Table 4.
The highest amount of biogas and methane was produced in the bioreactor B1-4, which fermented chopped leaves. This may be explained by the fact that hemp stems contain comparatively more cellulose and lignin, which is difficult to degrade by bacteria.
When calculating the volumes of biogas and methane obtained, also the biogas and methane produced from control yeast were taken into account (they were subtracted from the total volume acquired in each bioreactor). Also, the average indicators of each group were calculated. The results of raw material analyses were summarised in Table 5.
As it can be seen in Table 5, hemp has high contents of dry matter (41.62-62.96%) and organic dry matter. This may be explained by the fact that hemp was harvested in dry weather and kept in dry premises before chopping. The results of digestate analyses are presented in Table 6.
The biogas and methane volumes yielded from the coarse-chopped 'Futura 75' are presented in Table 7.
The biogas and methane volumes yielded from the fine-chopped hemp cultivar 'Futura 75' are shown in Table 8.
The biogas and methane yields obtained from the fine-chopped 'Uso 31' are summarised in Table 9.
The biogas and methane yields obtained from chopped 'Uso 31' leaves are shown in Table 10.
The average biogas and methane volumes yielded from various hemps are summarised in     The average amount of methane obtained from 'Uso 31' leaves (0.365 0.010 l/g DOM) is a very good result as compared to other energy crops, for example, maize silage (0.319-0.330 l/g DOM in Latvia). Studies on 'Futura 75' resulted in 0.234-0.290 l/g DOM of methane. Research on the influence of harvesting time on the methane output allowed concluding that this influence is insignificant -only slightly smaller than that on the hemp harvested in October [14]. A more notable effect was left by pre-processing, and, if samples were chopped into 1-2 mm pieces, a total of 0.290 l/g DOM of methane was obtained. CONCLUSIONS 1. Under agro-climatic conditions of Latvia, varieties of industrial hemp provide on average 15.0 t/ha of dry matter yield. The highest biomass yield during trial years was obtained from the cultivars 'Futura 75' and 'Tygra': 17.76 t/ha and 16.31 t/ha respectively. According to the data, a conclusion can be drawn that the growing season and the selected industrial hemp variety had a significant (p<0.05) effect on hemp yield.
2. The research suggests that the biomass of hemp grown in Latvia provides high methane extraction; therefore it can be used for biogas production.
3. A larger methane outcome was obtained from finely chopped hemp stalks and leaves.
4. Influence of hemp harvesting time of two weeks on the methane output is insignificant.

BIOGAS AND METHANE YIELDS FROM THE COARSE-CHOPPED HEMP CULTIVAR 'FUTURA 75
Raw/digester Biogas, l Biogas, l/g DOM    Table 11