How pyrolysis actually works

Most explanations of pyrolysis stop at "heating biomass without oxygen." That is true and almost useless. It does not tell you why two plants running the same feedstock at the same temperature produce different amounts of biochar, of different quality, and it does not tell you which knob to turn when yours is underperforming.

The part that matters is what happens to the vapours after the solid has decomposed. That is where most of the yield is won or lost.

Three product streams, always

Heat biomass in the absence of oxygen and it separates into three things:

• Char. The solid carbon skeleton left behind. This is the product.
• Condensable vapours. Tars and bio-oil, liquid at room temperature and gas at reaction temperature. The troublesome stream.
• Permanent gases. Mostly CO, CO₂, CH₄ and H₂. These stay gaseous and, in a well-designed plant, are burned to supply process heat.

Every design decision in a pyrolysis unit is, one way or another, a decision about how those three streams are split and what happens to the second one.

What happens as the temperature climbs

Biomass is not one material. Its three main polymers decompose at different temperatures, which is why the process is gradual rather than a single event.

That last row is doing more work than it appears. The char is not finished when the volatiles stop coming off. It keeps changing structurally, and that structural change is what determines how long its carbon stays put.

Primary and secondary reactions

Most accounts stop before this part, which is a pity.

Primary reactions

The solid decomposes. Polymer chains break, and the fragments either stay behind as char or leave as vapour. If this were the whole story, char yield would be a simple function of feedstock and peak temperature.

Secondary reactions

It is not the whole story, because those vapours are still inside a very hot vessel, in contact with hot char and hot metal. They react again, along two competing routes:

• Cracking. Heavy tar molecules break into lighter permanent gases. Favoured by high temperature and long residence in the gas phase. Carbon that would have been char leaves as gas.
• Secondary char formation. Vapours repolymerise and deposit carbon back onto the existing char surface. Favoured by vapours staying in contact with the hot solid. Carbon that would have left as vapour stays as char.

Both are happening in every pyrolysis unit. Which one dominates is largely a design and operating choice, and it is the biggest lever on yield that most operators never knowingly touch.

What actually controls char yield

Temperature is the variable everyone watches. It matters, but it is not the one that separates a 15% plant from a 30% plant.

Vapour residence time and contact with the solid

If vapours are swept out of the reactor the instant they form, whether by high sweep-gas flow, aggressive extraction or a bed too thin to hold them, secondary char formation barely happens and the carbon leaves as oil and gas. Let them percolate through the hot char bed on the way out and a meaningful fraction of that carbon is recovered as additional char.

This is the whole difference between fast and slow pyrolysis. Fast pyrolysis holds vapours for under two seconds and is designed to maximise bio-oil. Slow pyrolysis holds them far longer and is designed to maximise char. Same feedstock, same peak temperature, very different products.

Heating rate

Slow heating gives the solid time to rearrange and form stable char structures. Fast heating blows the material apart into volatiles. Slower is better for char.

Particle size

A large particle is its own secondary reactor: vapours generated at the centre must travel out through hot solid, and some of that carbon deposits on the way. Fine material heats through instantly and releases its vapours straight into the free gas space. Consistency of size matters as much as the size itself: a mixed feed pyrolyses unevenly, giving you part char, part torrefied material and part ash in the same discharge.

Pressure

Elevated pressure keeps vapours in contact with the solid for longer and raises char yield significantly. Most rural-scale equipment operates at or near atmospheric pressure, so this is a design consideration more than an operating one. It does explain why some published yields are unreachable on an ordinary plant.

Feedstock moisture

Water does not just cost energy to evaporate. It changes the thermal profile of the whole reactor, slows the material's arrival at reaction temperature, and on a plant running near its thermal limit it can drag the reaction zone into the torrefaction range without anyone noticing. Inconsistent moisture is the most common cause of inconsistent output we see.

Yield against permanence

This is where the chemistry becomes commercially awkward. Push the temperature up and the char becomes more aromatic and more condensed. Its hydrogen-to-organic-carbon ratio falls, and that ratio is the standard proxy for how long the carbon will persist; certification schemes set thresholds on it because it tracks stability.

Higher temperature also means less char by mass. So:

• Higher peak temperature → more stable, more permanent carbon, at a lower mass yield
• Lower peak temperature → more char by mass, in a less condensed structure, more of it potentially outside certification thresholds

The two things you are selling pull in opposite directions. Optimise purely for tonnes and you may produce material that will not certify. Optimise purely for permanence and you leave yield on the table.

There is a third axis, too. Char made at lower temperatures retains more oxygen-containing surface groups, which is useful agronomically for holding and exchanging nutrients. Very high-temperature char has enormous surface area and high pH but fewer of those functional groups. The biochar that is best for a carbon credit is not automatically the biochar that is best for a farmer's soil.

So "what temperature should I run at?" has no single technical answer. It is a commercial question about which of the two products you are selling, and it wants settling before the plant is specified rather than after.

What this means when your plant misbehaves

Yield is low, temperature is right

Suspect vapour handling before anything else: extraction rate, sweep gas flow, bed depth, anything pulling vapours out of the hot zone early. Then particle size, then heating rate.

Yield swings run to run

Almost always feedstock, and usually moisture rather than species. Measure incoming moisture before you change any process setting.

Char quality is inconsistent within a batch

Two decomposition ranges running side by side in the same charge, because heat or particle size is uneven. Sample from several points across the discharge before concluding anything about the process.

Persistent tar fouling

Condensable vapours are finding cold surfaces before they are burned. Tar is a design or operating problem in the gas path; cleaning it out on a schedule treats the symptom and nothing else.

The plant will not sustain its own heat

The permanent-gas fraction is what should be supplying the process heat. If external fuel is needed continuously, either water is absorbing the energy or those gases are not reaching the burner.

Why we bother writing this down

Pyrolysis is often sold as a black box: put biomass in one end, take biochar and carbon credits out of the other. Run as a black box it underperforms, and the reasons stop being mysterious as soon as somebody measures the right things.

Nothing above is proprietary; any competent process engineer would tell you the same. What is scarce is somebody standing in front of the machine who can apply it.

Temperature ranges given here are general characteristics of biomass pyrolysis reported in the literature. They are not measurements from any particular installation, and actual behaviour depends heavily on feedstock, reactor design and operating conditions.