How a retort turns hemp hurd into biochar

In a pyrolysis retort, hemp hurd is heated with almost no oxygen, so it cannot burn. Instead it splits into a carbon rich solid (biochar) and hot combustible gases and vapors. Those gases are piped to a burner under the retort, which keeps the process hot, and the leftover heat is ducted to a dryer for the next batch of stalk.

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Feed with no airDry hurd fines and off grade stalk drop through a hopper with an airlock, so very little oxygen enters with them. Without oxygen the biomass cannot burn; it can only break down.
Auger through the hot zoneA slowly turning auger carries the material along a sealed steel tube sitting over a firebox. Moisture leaves first, then above roughly 250 °C the cellulose and lignin start to come apart.
Gases fuel the fireThe vapors and gases leaving the retort still carry most of the feed's energy. A pipe leads them to burners under the tube, so after startup the process can run on its own gas.
Hotter means less, purer charPush the temperature up and more of the solid turns into gas. You get fewer kilograms of biochar, but the char that remains holds a higher share of carbon.
Spare heat to the dryerExhaust from the burners is still several hundred degrees hot. Ducting it to a belt dryer dries incoming stalk, which is why char making and hemp processing pair well.

Yield and carbon versus temperature

Published hemp hurd tests show the same pattern: in one study of pellets made from hemp hurd blended with sawdust, solid yield fell from an estimated 79 percent near 360 °C to a measured 33 percent near 430 °C while carbon content rose from about 56 to 80 percent. Hemp stems held at 700 °C gave about 28 percent char with about 82 percent carbon.

The readouts use a simple fitted curve through those points: yield = 28% + 66% × e^(−(T − 330)/46) and carbon = 84% − 32% × e^(−(T − 330)/60). The data come from different reactors and hold times, so treat the curve as a teaching trend, not a design value.

Try it: set the feed to 100 kg/h and slide temperature from 350 to 700 °C. Char output drops from about 70 to about 28 kg/h, while spare heat for the dryer rises sharply and then levels off.

The energy balance, simplified

The heat readout assumes dry hurd holds about 18 MJ/kg and the char holds about 0.34 MJ/kg for each percent of carbon. Whatever energy is not locked in the char leaves as gas and vapor. The model burns that gas at 70 percent useful efficiency and subtracts about 1 MJ for each kilogram fed to heat and pyrolyze the feed. Power in kW is then energy per hour divided by 3.6.

All of these are rounded assumptions. Real kilns lose heat through walls, run with wet feed that needs more energy, and may vent some gas, so measured surplus heat is usually lower.

Sources

Assessment of hemp hurd derived biochar produced through different thermochemical processes and evaluation of its potential use as soil amendmentHeliyon (open access), via PubMed Central
Effect of thermochemical treatment on the physicochemical properties of fiber hemp (Cannabis sativa L.) by productBioResources, NC State University

How to cite this model

Avid Solutions International. (2026, November 11). How a retort turns hemp hurd into biochar [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/biochar-kiln

Last updated November 11, 2026

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