Extracting High-value Products from Hemp Waste
Hemp (Cannabis sativa) is cultivated for its fiber (bast fiber) and its edible seeds as well as some medicinal products . When bast fibers are separated from the hemp stem, what’s left is called hemp hurds. These woody residues are the least valuable part of the hemp stem and are treated as a by-product of fiber production, even though they represent the largest fraction of the hemp plant. Hemp hurds can be used in a range of applications such as animal bedding, construction materials, and garden mulch , but they are still generally considered as waste.
However, thermochemical processing of hemp hurds can produce some high-value products. One particular thermochemical process called slow pyrolysis can be used to convert hemp hurds into biochar, liquids (distillates), and gases . These are produced in approximately equal amounts, although the process conditions can be adjusted to maximize the yield of a particular fraction.
Slow pyrolysis is usually used to convert biomass into biochar, a type of carbon-rich charcoal that is used as a soil improver or to store carbon. Liquid distillates are also produced, but they are considered a by-product and are often burned or dumped. However, these liquid distillates contain bioactive compounds and could be collected to generate additional income.
In this study, four types of industrial hemp hurds were thermally processed and converted into liquid distillates by slow pyrolysis at different temperatures . The team investigated the chemical composition of the distillates to identify possible valuable molecules or molecule groups. They believe this is the first time that large samples (kilograms) have been studied in this way. Previous studies have focused on small, lab-scale samples (grams) .
The authors processed the hurds using slow pyrolysis at relatively low process temperatures from room temperature up to the maximum operating temperature of 350 ̊C. They collected raw distillates at three stages of the slow pyrolysis process (drying, torrefaction, and pyrolysis).
Detailed Analysis of Samples
Various analytical techniques were employed to study the samples. These included Fourier transform infrared (FTIR) spectroscopy, one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy, liquid chromatography–high-resolution mass spectrometry (LC–HRMS), and two-dimensional gas chromatography-mass spectrometry (2D GC-MS). For example, FTIR spectroscopy was used to obtain information regarding functional groups of all the hemp hurd distillate fractions. All spectra were measured using Bruker’s Alpha FTIR spectrometer equipped with attenuated total reflection diamond, a sensitive 2 × 2 mm diamond crystal surface, and sample compartment RT-DLaTGS.
The team identified and measured some potentially valuable molecules for the first time. The analyses showed remarkable differences in the concentration of compounds in different distillates. The relevant compounds came from three different hemp hurd samples, especially from torrefaction and pyrolysis phase distillates condensed below 100 ̊C.
Acetic acid was the main component of all samples. Other interesting compounds included guaiacol and syringol derivatives such as 2,6- Dimethoxyphenol, guaiacol (2-Methoxyphenol), vanillin, and eugenol.
Most of these compounds are expensive to make because they appear in low concentrations in distillates, which means they must be separated and purified (although several modern scalable techniques are available). Such compounds could be used as purified products for nutritional, pharmaceutical, and agricultural purposes. Vanillin and eugenol, for example, are used as ingredients by the functional food and pharmaceutical sectors.
The authors estimate that one ton of hurds (€200 at current prices) would produce about 300 kg biochar (worth around €400 at current prices). It would also produce about 40 kg of acetic acid, the main compound in the distillates, worth around €100 as a bulk product. One ton of hurds would generate around 1.3 kg of 1-hydroxybutan-2-one, the most expensive of the minor distillate compounds. In principle, this could be purified to higher than 95% purity and sold for €1300–6500. 1-hydroxybutan-2-one is often used as a flavor or fragrance agent.
This study provides useful baseline data for chemical profiling of wood distillates, especially hemp hurd distillates. It also shows clear potential to generate high-value products from hemp hurds by utilizing slow pyrolysis to generate biochar and distillates that contain potentially useful ingredients. The whole process can be optimized to generate the most valuable products, varying factors such as temperature, heating rates, and residence times. Further processing of distillates would involve separation and purification procedures such as short path distillation and centrifugal partition chromatography. The researchers recommend further research to evaluate the economic potential in detail, for example, by considering the purification process costs and the market value and volume of high-value chemicals, and the business potential in general.
Bruker offers the broadest range of analytical techniques used in the emerging global Cannabis Industry already today. The portfolio includes benchtop and floor-standing NMR, optical methods like FTIR and Raman spectroscopy and mass spectrometry. This makes Bruker the only end-to-end solution provider with applications tagging into every stage of the Cannabis value chain. We enable our customers to generate new revenue streams and reduce waste.
Bruker does not support, encourage, or intend that its products or services be used in connection with any illegal use, cultivation or trade of cannabis or cannabis products. Bruker products are intended to be used only in compliance with all applicable laws in a manner that promotes public safety and in connection with any lawful and approved scientific or medical research activities.
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