CDR & carbon management
Biochar pyrolysis equipment
Why slow and fast pyrolysis are different machines rather than different setpoints, how temperature and residence time trade char yield against char stability, and where the process heat has to come from.
A pyrolysis reactor is not a container holding a reaction; it is a way of imposing three numbers on biomass — how fast it is heated, how hot it gets, and how long the vapours stay hot before they escape. Those numbers decide where the carbon ends up: in a solid, in a condensable liquid, or in permanent gas. The hardware follows from which of the three the operator is paid for.
Two products, two durabilities
Slow pyrolysis heats biomass gently — heating rates of at most tens of degrees per minute, solid residence of minutes to hours near 400–500 °C — and leaves roughly a quarter to a third of the dry mass as char. Fast pyrolysis is the opposite machine: heating rates in the hundreds of degrees per second, particles ground to millimetres, peak temperature near 500 °C and vapours quenched within about a second, which moves more than half of the dry mass into bio-oil. The two routes also differ in where durability comes from. Slow-pyrolysis char is durable in itself, because the aromatic condensation described on the biochar page is allowed to run far. Bio-oil is chemically unstable — in storage it ages, grows more acidic and phase-separates — and its permanence is borrowed entirely from the geologic formation it is injected into. One vessel cannot serve both chemistries: a fluidised bed that slams heat into sand-sized particles and strips vapours instantly is the wrong machine for holding wood warm for hours, and a kiln with long solids residence has no way to quench a vapour stream in one second. Architecture, not setpoint, separates the two routes.
The knobs pull against each other
Char yield falls monotonically with peak temperature as volatilisation runs toward completion. Vapour residence acts on the liquid: while hot, tars keep reacting, cracking to permanent gas or repolymerising onto the char — which is why the quench matters as much as the heating rate. Stability rises with severity: hotter, longer treatment strips more hydrogen and drives the H:C ratio down. So the settings that retain the most carbon in the solid are not the settings that make that solid most persistent, and neither coincides with the settings that maximise liquid. A kiln has one heating trajectory and one vapour path; it sits at a single point in this three-dimensional space, and optimising both products would mean occupying two points at once. Retrofits move the margins, not the architecture.
Where the heat comes from
Pyrolysis must be driven. Wet feed has to be dried below roughly 15 percent moisture — evaporating water alone costs on the order of 2.3 megajoules per kilogram — and then the solid must be raised to reaction temperature. The only fuel inside the fence is the plant’s own co-products: the non-condensable gas, the bio-oil or the char. Burning the gas is standard, and in many designs it leaves surplus heat to export as district heat, which is what makes a biochar plant an energy asset to its host. Burning char, by contrast, is burning the product: a tonne of char oxidised to keep the reactor warm is a tonne of removal undone. Heat integration is therefore not an economic nicety but the plant’s carbon balance. The limiting quantity is the share of co-product carbon that must be burned to hold the reactor at temperature, and a design is judged by how small it keeps that share.