Hemp hurds are the woody residues left over after the fibers have been removed from hemp plants. They are produced in large amounts but are generally classed as waste even though they do have a few uses, such as garden mulch. However, processing hemp hurds generates liquids that have been shown to contain potentially valuable ingredients. Until now, researchers have not identified the range of these compounds in large, real-world samples, or measured their quantities.
In this study, the team uses a technique called slow pyrolysis to convert four types of industrial hemp hurds into liquid distillates and biochar (a type of carbon-rich charcoal used to enrich soils or sequester carbon). This was done at relatively low process temperatures from room temperature up to the maximum operating temperature of 350 ̊C. The researchers collected raw distillates at three stages of the slow pyrolysis process (drying, torrefaction and pyrolysis).
Detailed chemical composition of the distillates allowed the team to identify possible valuable molecules or molecule groups. They used various analytical techniques to study the sample, including Fourier transform infrared (FTIR) spectroscopy performed 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.
For the first time, some potentially valuable molecules were identified and quantified in hurd liquid distillates. The team showed that the concentration of compounds varied hugely in different distillates. The compounds of interest appeared mainly in three different hemp hurd samples, especially from the torrefaction and pyrolysis phase distillates condensed below 100 ̊C.
The main constituent of all samples was acetic acid, but other potentially valuable compounds included guaiacol and syringol derivatives such as 2,6- Dimethoxyphenol, guaiacol (2-Methoxyphenol), vanillin, and eugenol. Vanillin and eugenol, for example, are used as ingredients by the functional food and pharmaceutical sectors. Many of these compounds are expensive to produce because they occur in distillates in low concentrations. Then they have to be separated and purified.
The authors carried out rough calculations to judge the economics of the process. They calculated that it would generate about 300 kg of biochar (worth around €400 at current prices) from one ton of hurds (€200 at current prices). It would also produce about 40 kg of acetic acid, worth around €100 as a bulk product. In particular, they considered 1-hydroxybutan-2-one, the most expensive of the minor distillate compounds, often used as a flavor or fragrance agent. They estimated that one ton of hurds would generate around 1.3 kg of 1-hydroxybutan-2-one, which could then be purified before being sold, raising between €1300–6500.
In conclusion, the team has produced novel data on the chemical composition of hemp hurd liquid distillates. They also demonstrated how slow pyrolysis has the potential to generate useful and valuable products. The researchers recommend further research to evaluate the economic potential in detail, for example by considering the purification process costs, as well as the market value and volume of high-value chemicals, and the business potential in general.
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Salami, A. et al, (2020) Industrial Crops and Products
Volume 155, 1 November 2020, 112760