Pyrolysis Systems and Temperatures
Chapter on reactor modes, temperature classes, residence time, and heat management—the controllable engineering variables that set biochar yield and chemistry. Report context: 2026-09-16.
Related notes: Production · Biochar Quality from Process · Gasification Residues · Feedstocks · Woolf IPCC Fperm · PAHs · Production Hub · Key Sources
1. Temperature classes used in inventories
models soil GHG from biochar / estimates soil C stock change from biochar permanence and FC tables use:
| Class | Peak temperature | Typical role |
|---|---|---|
| Low | 350–450 °C | Higher yield, higher H/C_org, lower (F_\mathrm{perm}) |
| Medium | 450–600 °C | Common agronomic / CDR operating window |
| High | ≥600 °C | More condensed aromatics; often lower mass yield |
(Woolf DOI; IPCC PDF). Materials <350 °C excluded from these permanence methods (Woolf §2.1.1).
EBC batch rule of thumb: declared pyrolysis temperature may fluctuate by up to ±20% (e.g. 600 °C → short-term 480–720 °C) without breaking the batch, if documented (EBC 10.5E §5.5). EBC-FeedPlus / Swiss-style agricultural rules often require ≥500 °C for ≥10 min, proxied by H/C_org < 0.4 (EBC §10.4).
2. Heating rate and residence time
| Mode | Heating rate / solids residence | Product emphasis |
|---|---|---|
| Slow pyrolysis | Low °C/min; minutes–hours | Maximize biochar |
| Fast / flash | Very high heating; seconds | Maximize liquids |
| Gasification | Limited oxidant; often >700–800 °C bed | Maximize syngas; solid = residue |
Residence time at peak T matters as much as the peak number for condensation (Keiluweit et al. 2010; Woolf notes T is a weaker condensation proxy than H/C_org because time also matters — DOI).
3. Reactor families (descriptive)
Commercial and farm-scale systems span:
- Continuous auger / rotary / screw pyrolyzers — steady T profiles, good batch documentation for MRV.
- Fixed / moving bed and fluidized bed — industrial throughput; fluid beds favor fast heat transfer (often liquids if configured that way).
- Kilns / retorts / flame-curtain (e.g. Kon-Tiki class) — farmer-scale; EBC cites flame-curtain examples as capable of low PAH when vapor handling is correct (EBC 10.5E §7.11; Cornelissen et al. 2016 emissions/char quality study cited therein · DOI).
- Gasifiers (downdraft, updraft, fluidized) — see Gasification Residues.
EBC requires each pyrolysis unit certified individually (with limited exceptions for identical parallel lines sharing feedstock/output) (EBC §8.6).
4. Energy and emissions engineering
From Woolf life-cycle framing and EBC:
- ~35–60% of feedstock energy often ends in pyrolysis gas that is burned in-plant (EBC §8.5).
- Modern plants should fully combust organics to CO₂; simple kilns may emit CH₄ and VOCs that erase climate benefit if unmanaged (Woolf 2021 §2.1.2 item 7; Woolf et al. 2017 open-source reactor).
- EBC: no uncombusted pyrolysis-gas release; fossil fuel banned for reactor heat except preheat; electric heat must be renewable/surplus (EBC §8.1–8.4).
5. Link to permanence and contaminants
| Higher peak T / longer residence | Typical effect | Notes |
|---|---|---|
| H/C_org, O/C_org | ↓ | H-Corg and O-Corg |
| (F_\mathrm{perm}) | ↑ | Woolf IPCC Fperm |
| Mass yield | Often ↓ | Trade-off vs C per tonne feedstock |
| BET surface area | ↑ then may ↓ at very high T | Properties |
| PAH | Not a simple T function—vapor condensation dominates | PAHs |
6. Practice takeaway
- Log peak T + residence + feedstock blend for every batch (EBC Biochar Tool culture).
- Prefer measured H/C_org over T-class alone for CDR math (Woolf IPCC Fperm).
- Design vapor–solid separation before cooling to control PAHs (PAHs).