Reactor Energy Balance — From Diesel to Self-Sustaining
The reactor energy balance shows two phases — the first 1–2 hours run on diesel, then syngas from the condenser takes over as furnace fuel — making overall energy input approximately 15–20% of feedstock energy content, with 70–80% leaving as sellable products.
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How to read this sketch
This is an energy balance diagram where arrows represent energy flows, not material flows. Read it as follows:
- Reactor + furnace (centre): The core thermal system. Furnace wraps around the reactor and heats it.
- Red dashed arrow (left, early phase): Diesel startup energy with clock icon labelling the 1–2 hour startup window.
- Green dashed loop arrow (steady state): Syngas from condenser looping back to furnace burner — replaces diesel after startup.
- Energy output arrows (right): Three output energy flows — oil vapors (largest), char (medium), heat losses (smallest).
- Caption: 'Diesel for 1–2 hours, then the plant runs on its own gas.'
About this sketch
Every pyrolysis operator hears that the plant becomes self-sustaining after startup — but what does the energy balance actually look like? This diagram shows reactor energy flows at two operating phases.
Startup phase (first 1–2 hours): The reactor is cold. The furnace burns diesel or LPG to raise the reactor from ambient to operating temperature (350–550°C). For a typical 2–3 tonne batch reactor, startup fuel consumption is 15–40 litres of diesel.
Steady-state phase (after 1–2 hours): As the reactor reaches operating temperature, plastic begins cracking and vapors flow to the condenser. Non-condensable gas (NCG, syngas) builds up in the gas holder. The furnace switches from diesel to syngas — the green dashed loop arrow in the diagram. From this point, energy input equals syngas energy from the NCG loop; energy output equals pyrolysis oil vapors (primary product) plus char (secondary) plus heat losses through the reactor wall insulation.
The steady-state energy balance is approximately: 15–20% of the plastic's original energy content is consumed internally as syngas furnace fuel; 55–65% exits as pyrolysis oil; 8–12% exits as char; 5–10% is heat loss. Net energy efficiency — useful product energy out versus plastic energy in — is approximately 70–80%, significantly higher than incineration which captures only 20–30% as electricity through a steam cycle. This is the core commercial and environmental argument for pyrolysis: the plant extracts most of the energy locked in waste plastic as a sellable liquid fuel rather than burning it and losing most to flue gas.
Key insights
- The pyrolysis plant is energy self-sustaining after 1–2 hours of diesel startup — the syngas from the NCG loop provides all the furnace energy for the rest of the batch.
- Approximately 15–20% of the plastic's original energy content is consumed internally as syngas furnace fuel — the rest exits as oil (55–65%) and char (8–12%).
- Overall energy efficiency of 70–80% (useful products out versus plastic energy in) is significantly higher than incineration, which recovers only 20–30%.
- Diesel startup cost is a minor variable cost — typically 15–40 litres per batch for a 2–3 tonne reactor, representing 1–3% of the total batch revenue.
- Heat losses through the reactor wall (5–10%) are reduced by well-maintained refractory — cracked or thin refractory increases heat loss and diesel consumption.
