In the earliest part of my career, I studied the use of plants for renewable energy applications. I learned about the need to supplant our use of dwindling reserves of fossil fuels that took eons to form but became increasingly depleted with industrialization. I began to understand the ramifications of not reversing our trajectory when it came to how we generate our energy. Today, the situation is even more bleak.
We considered plants like corn stover, kenaf, tall grasses (like switchgrass or pampas), and trees (like eucalyptus and pine). Two challenges to overcome in nominating useful plants for bioethanol production lay in how much cellulose they contain, and how easily the cellulose can be converted to sugars for fermentation to ethanol. While attending a conference in Australia, I heard one research group present their promising data on the use of hemp, which, at the time, was most illegal in the United States.
Then, I read Jack Herer’s The Emperor Wears No Clothes. I surely wasn’t oblivious to hemp’s industrial wingspan at this point, but Herer succinctly condenses the many uses of hemp to the point where it becomes stunningly optimistic and yet simultaneously tragic that such a useful plant was ostracized for so long.
In 2018, the United States government finally (FINALLY) legalized a form of Cannabis sativa, industrial hemp that contained less than 0.3% total tetrahydrocannabinol (THC). Zoom… and mountains of hemp were grown as many businesspeople saw fields of greenbacks just waiting for the picking. The rationale for growing the plants changed from targeting cellulosic fiber to predominantly secondary metabolites, and especially cannabidiol or CBD.
This created a conundrum because while there was cause to celebrate the reversal of hemp’s illegality, thereby releasing an imprisoned form of natural medicine, the fact that we were not recognizing all that the plant readily offered seemed like a vast waste. Farmers were “growing CBD,” one molecule amongst a panoply of others with medicinal or industrial merit.
Luckily, a review study recently revisited the prospect of industrial hemp for bioenergy.  Hemp has a fast-growing period (70-140 days)  and provides sizeable yields (15 tons) with water inputs (250-400 mm) that are lower than valued crops like alfalfa and corn . It can grow in mild temperatures (13-22°C) and due to its ability to protect itself (e.g., cannabinoids and terpenoids), it doesn’t need much outside help . These characteristics make hemp relatively inexpensive to farm.
Hemp has a high energy density due to the amount of carbon/cellulose contained within. It does not produce much ash when burned, resulting in less particulate released into our atmosphere. It also has much less lignin than many other plants. Lignin is like a glue that holds plants together and it inhibits the conversion of cellulose to sugars because it can bind precious enzymes used in the deconstruction of the biomass for bioethanol production.
Liquid, gaseous, and solid fuels can be produced from hemp biomass. Hempseed oil, for example, provides a strong candidate for biodiesel production . Hemp has a high fuel yield (784 L/ha), a low sulfur content (and hence, low sulfur oxide emissions), and a high flash point. These metrics rank it higher than other options like corn and oil palm. We’ve already discussed hemp’s viability for bioethanol. There’s also hemp bio-oil, a crude oil created by pyrolysis that can be used in liquid and gaseous fuels. Solid fuels could be briquettes, pellets, or biochar which can be created from waste hemp straw.
Hemp is a more profitable plant to cultivate for energy, “with an estimated revenue of $2632/ha, compared to $908/ha, $803/ha and $1725/ha for kenaf, switchgrass, and sorghum, respectively, with similar growing conditions.” 
Hemp cultivation mends our Earth [6-8] and offers an option for lowering greenhouse gases. Hemp-lime concrete can store carbon and hemp helps remove carbon dioxide from the environment, making it carbon-negative.
The beauty of the plant is that we can have it all by extracting its medicinal secondary metabolites to benefit us while utilizing the raffinate and unused plant waste for bioenergy production, benefiting both us and Mother Earth.
Unfortunately, many places still have silly laws that make it illegal to further utilize spent and waste cannabis/hemp biomass, despite what’s known about the wealth that we’re forced to treat like garbage. What’s the point of incinerating cannabis waste, for example, if the heat is not utilized by someone? Hopefully, lucidity seeps in, and those making up the rules as we go recognize that waste from the extraction industry and plant matter not used for cannabinoid-derived products still offer more value that’s worth investigation.
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Image Credit: Barbetorte, CC BY-SA 3.0 via Wikimedia Commons