End-of-life treatments for cannabis plant residues : a techno-socio-economic and environmental assessment for a New Zealand case study : a dissertation presented in partial fulfilment of the requirements for the degree of Master of Engineering in Chemical and Bioprocess Engineering Subject at Massey University, Manawatū, New Zealand
Loading...
Date
2022
DOI
Open Access Location
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Massey University
Rights
The Author
Abstract
The cannabis plant and its products have been utilised for thousands of years in many industries but, over the last century, legislation and regulations have been introduced in many parts of the world to limit its cultivation and use. Nevertheless, the medicinal/recreational cannabis and hemp seed and fibre industries are rapidly growing around the world, and this is associated with increasing quantities of waste plant residues; up to 45% of the plant is considered waste, not including the root systems. For end-of-life (EoL) management of cannabis plant residues, current EoL treatments include landfilling, composting, anaerobic digestion, and incineration. A potential alternative treatment is pyrolysis which produces biochar or charcoal, a high-carbon content product. Biochar can decrease the soil bulk density, increase soil porosity, increase soil-specific surface area, and increase soil water retention/water-holding capacity. Another benefit of biochar is its potential for carbon sequestration when applied to soil. In this study, a mass and energy balance of each EoL treatment was undertaken, and then a Life Cycle Assessment (LCA) of each EoL treatment. The ReCiPe 2016 v1.1 method was used to assess 15 impact categories: climate change, fine particulate matter formation, fossil depletion, freshwater consumption (inventory indicator), freshwater ecotoxicity, freshwater eutrophication, human toxicity (cancer and non-cancer), ionizing radiation, marine ecotoxicity, marine eutrophication, photochemical ozone formation (ecosystems and human health), terrestrial acidification, and terrestrial ecotoxicity. In general, the ranking across all assessed impact categories for the alternative treatments was composting (best), anaerobic digestion, pyrolysis, incineration, and landfill (worst). Although pyrolysis was in the middle of the ranking, it had the only net negative climate change impact (i.e. mitigating climate change) when accounting for the contribution of carbon storage in biochar. Cannabis plant residues consist of leaves, stalks, and roots. Samples of both hemp and medicinal cannabis plant residues were collected at selected hemp and medicinal cannabis cultivation facilities. A pyrolysis unit fabricated by the workshop staff at Massey University was used to complete experimental trials on the residues. A pyrolysis temperature of 420-450℃ was used, with a biochar yield of 37-53% and a condensate (oil and water phases) yield of 10-20% being obtained. The biochar and condensate were analysed in the Py-GCMS to find if the compounds of interest, cannabinoids, were present. This would be a concern if the cannabinoids remained in the biochar due to the risk of cannabinoids entering the human food chain (e.g. from cows grazing on pasture which has been treated with biochar). These results show that pyrolysis is a suitable EoL treatment for cannabis plant residues because it removes all the cannabinoids from the biochar; this means the biochar can be subsequently spread onto land. Cannabinoids were found to be present in the condensate which is not a problem because typically it would be flared, or the cannabis company would have stringent protocols in place to ensure no cross contamination with the biochar. The cannabinoids found were Cannabicyclol (CBL), Cannabidiol (CBD), Cannabinol (CBN), and Dronabinol (Δ9-THC) in all condensate samples. As well as the environmental aspects, a preliminary assessment of the economic, social, and cultural aspects of the alternative EoL treatments were also undertaken. A basic economic analysis was completed, and it was found that the composting EoL treatment had the lowest capital costs, operating costs, and a better net present value (NPV) after 15 years across alternative EoL treatments. While many social indicators can be used, the one selected for this study was the number of hours worked by employees per year to process 30 tonnes of cannabis plant residue. The landfill and composting EoL treatments would provide 132 hours for four employees, the anaerobic digestion EoL treatment would provide 780 hours of work for four employees, and the incineration and pyrolysis would provide 420 hours of work for six employees. Through a cultural assessment, it was found that there were seven significant headings (sets of values) that represent the values of the Māori people in relation to the technologies assessed in this study. The cultural assessment found that the EoL treatment rankings were: pyrolysis (most preferred), anaerobic digestion, composting, landfill, and incineration (least preferred). When the environmental, economic, social, and cultural aspects are considered, the ranking of the alternative EoL treatments is pyrolysis (most preferred), composting, anaerobic digestion, landfill, and incineration (least preferred). Several challenges arose during the study, including the LCA software and database specificity, site selection and sampling due to Covid lockdowns, licencing and regulation restrictions, time and funding, experimental limitations and trade-offs, and participant acquisition for cultural interviews. Multiple LCA software options are available with access to different databases; the one licensed by Massey University was used, and a comparison was completed on different databases. A licence is required for any location where cannabis plant material is stored or used in experiments. After many telephone calls and hours of discussion, the Ministry of Health decided that this project would not be granted a licence; this meant that testing had to be completed on-site at cultivation facilities; this also limited the availability of material for testing due to having to work around Covid lockdowns and cultivation cycles. Overall, one medicinal cannabis and one hemp plant could be tested, which meant that selection bias occurred due to material availability. Time was constrained by the degree requirements with a maximum of three years, and funding was constrained based on the support provided by Massey University, Callaghan Innovation, and the Dick and Mary Earle Technology Scholarship Grant. Finally, the cultural assessment was completed with the help of this project's original funding organisation, Zymbl Ltd, who had contacts in the local Rotorua iwi. Members were asked to join the discussion; however, only two were available. Despite these limitations, the study is still valuable for both the New Zealand and global industry as it is the first study evaluating alternative EoL treatments for cannabis plant residues.
Description
Figures 1 & 9 and Tables 4 & 5 are re-used with permission.