Fundamentals of biochar production

Biochar begins where biomass meets heat. The transformation from organic matter to a stable, carbon-rich material doesn’t happen by accident—it happens through a specific set of chemical and physical changes, triggered by heat and shaped by time, pressure, and the properties of the feedstock itself. This chapter lays out those fundamentals: what happens when biomass is thermally decomposed, what determines the yield and quality of the resulting biochar, and how those factors can be managed or optimized in real-world settings.

What is thermal decomposition?

At the heart of biochar production is pyrolysis—a process in which organic material is decomposed by heat in the absence or near absence of oxygen. The purpose is to avoid combustion, which would release carbon as gas, and instead produce solid biochar by breaking down complex biopolymers like cellulose, hemicellulose, and lignin.

The composition of the original biomass—its carbon, hydrogen, oxygen, nitrogen, and mineral content—plays a central role in determining how it breaks down and what’s left behind. But it’s not just what the biomass is made of. How it is heated matters just as much.

Heating biomass drives a cascade of thermal decomposition reactions. These can be grouped into three dominant pathways:

1. A biochar- and gas-forming pathway,

2. A liquid- and tar-forming pathway, and

3. A gasification and carbonization pathway.

The balance between these is controlled by process parameters like maximum temperature (often referred to as highest treatment temperature, or HTT), residence time, and how fast volatile vapors are removed from the reactor.

At lower HTTs—generally under 300°C—the decomposition of lignocellulosic biomass is dominated by free-radical reactions. These reactions are initiated by the breaking of chemical bonds in the presence of moisture, structural oxygen, or trace minerals. This stage yields the most char per unit of biomass, though the char at this point still retains much of its original structure and is chemically less stable.

The role of temperature and time

Temperature is the single most important variable in determining what kind of biochar you’ll get—and how much of it. At HTTs of around 300°C to 600°C, the production of biochar starts to fall, and the system begins to yield more liquids and vapors. The chemistry gets more complex, as sugars and fragments of cellulose begin to form tars or volatilize entirely. But if these products remain in the reactor, they can further break down into char. The balance between vapor escape and secondary degradation plays a big role in determining overall yield.

At higher temperatures, more free radicals are formed. These recombine to form condensed aromatic structures—the backbone of stable biochar. This is also the temperature range (typically around 500°C to 600°C) where biochars become highly reactive toward oxygen. Many are pyrophoric, meaning they can spontaneously ignite when exposed to air if not cooled properly.

Residence time—the duration the biomass spends under heat—also matters, especially when it comes to forming stable carbon structures. Short times may leave volatile compounds trapped in the char; longer times allow those volatiles to escape and let more carbon restructuring occur. But beyond a certain point, extended time brings diminishing returns unless matched with a specific process goal.

How different feedstocks behave

Biomass is not a uniform material. It’s made up of a mix of cellulose, hemicellulose, lignin, and smaller amounts of extractives (such as waxes or resins) and inorganic minerals (which become ash). Each of these components behaves differently under heat.

Hemicellulose starts to break down first, from around 220°C, and is usually gone by 315°C. It produces mostly gases and low-molecular-weight organics like acetic acid and furfural. These are valuable in industrial contexts but don’t contribute much to biochar yield.

Cellulose follows, decomposing at around 315°C and peaking around 360°C. In slow pyrolysis, it can contribute substantially to biochar if volatiles are retained long enough to condense and re-form solid carbon. Otherwise, it tends to form condensable vapors and aerosols. These products can also be broken down under heat, leading to more biochar—but that depends on how fast they’re removed from the hot zone and what catalysts or minerals are present.

Lignin is the most stubborn. It starts to degrade at 160°C and continues steadily up to 900°C. This long, slow breakdown yields a high proportion of solid residue—up to 40% of the original lignin mass. Its contribution to biochar yield is significant, and its complex aromatic structure forms the core of stable biochar.

What affects yield—and why it matters

Yield isn’t just a number—it’s an expression of the balance between decomposition pathways. Higher yields often mean lower energy input and greater carbon retention. But depending on the application, yield might be traded for surface area, porosity, or functional chemical groups.

For example, a high-temperature biochar made at 700°C may have a lower yield but much higher stability and surface area, making it ideal for carbon sequestration or water filtration. A low-temperature biochar made at 350°C might have more volatile compounds and functional groups that are beneficial for soil microbial activity or nutrient exchange.

The heating rate also plays a role. In general, slower heating rates favor solid char formation, while faster heating promotes the formation of tars and gases. Traditional charcoal kilns and muffle furnaces operate at slow heating rates and begin cellulose decomposition at temperatures as low as 250°C. At these conditions, biochar yields are maximized, but the products are less chemically altered. High heating rates (as in fast pyrolysis) produce less biochar but yield valuable liquid products and highly structured carbon residues.

Even particle size matters. Smaller biomass particles heat faster and more uniformly, which can favor liquid production if volatiles escape quickly. Larger particles have internal gradients of heat and moisture, which encourage more secondary reactions and char formation.

Inside the reaction zone

Thermal decomposition is not just chemistry—it’s fluid dynamics, too. The environment inside a pyrolysis reactor affects how products form and leave the system. If vapors are swept away quickly, less time is available for them to re-condense or decompose. If they’re allowed to linger, they may form secondary chars or tars, changing both the yield and the properties of the biochar.

This is where reactor design overlaps with the fundamentals. Indirect heating, sweep gas flow, and system pressure all influence whether you end up with a porous, high-surface-area char or a dense, tarry residue. Systems with poor vapor management may clog or produce sticky products that are difficult to handle. Those with efficient vapor removal and secondary cracking zones can improve both product quality and reliability.

The chemistry of what stays behind

Biochar is not a single compound. It’s a complex matrix of carbon structures, formed mostly from aromatic rings—some fused, some free, some interrupted by oxygen, hydrogen, or minerals. The distribution of these structures, and the presence of functional groups like carboxyl or phenol groups, determines how biochar behaves in soil, water, or air.

Some biochars are highly reactive and oxidize easily, while others are remarkably inert. Some can retain nutrients or adsorb contaminants, while others act more as structural fillers. Understanding the fundamentals of production helps explain these differences—and helps producers tailor biochars to specific needs.

The fundamentals of biochar production are not just academic. They’re the keys to unlocking performance, consistency, and value—whether you're heating a barrel in a field or running a continuous reactor on the edge of town. What happens in the hot zone defines what biochar becomes. And knowing that gives us the power to do it better.