An RDF pyrolysis and gasification plant can be built from repeatable modules, each handling about 4 tonnes of fuel an hour and supporting roughly 2.5 MW of electricity. In a UK proposal sized for about 68 dry tonnes of refuse-derived fuel a day, the modules share one feed system and one steam turbine, so capacity grows by adding modules rather than redesigning the plant.
The challenge: turning variable fuel into steady power
Refuse-derived fuel (RDF) is a processed fuel made from non-recyclable waste. Its calorific value and moisture vary. A conventional boiler responds to changes in steam demand by feeding more or less fuel. That approach struggles with variable fuels, and it can push a thermal plant away from its best operating point.
The approach: a self-heating pyrolysis module
The design basis was RDF at 20% moisture entering the pyrolyser, with a dry calorific value of 18.75 MJ/kg. Each 2.5 MW module has the following key components:
- three feed compactors;
- three pyrolysis tubes with screws;
- one gasifier and one ash removal system;
- one thermal oxidiser and a heated chamber (the hotbox);
- one boiler and one bag filter;
- two induced-draught fans.
The fuel feed, the steam turbine, the condenser and the cooling system are shared by all modules. A 7.5 MW plant would therefore be three modules and three boilers, combining steam into one 7.5 MW turbine.
Feeding
RDF drops from the feed hoppers into a screw compactor. The compactor squeezes the waste into a plug, or bung, which seals the process from outside air. Keeping oxygen out matters because the pyrolysis stage is meant to run without combustion. The hoppers need near-vertical sides, with a valley angle of at least 70°, since this kind of fuel bridges across shallow slopes. The screws are variable-speed and reversible, and they must be able to start under a hopper that has been full and settling for 24 hours.
Pyrolysis and gasification
Pyrolysis tubes pass through a hot chamber. Indirect heat first drives off moisture. Above about 250°C the organic material breaks down into a medium-calorific gas, leaving char and inert solids. Each tube takes up to about 1.3 tonnes an hour, which is why a module tops out at around 4 tonnes an hour: the limit is heat transfer into the waste.
The char falls into a deep-bed gasifier, where it reacts with steam to give more fuel gas and minimal particulate carry-over. A hot cyclone then catches fine particles before the gas goes to the oxidiser. Ash is about 10% of the fuel by weight and is discharged through a sealed system into bins.
Oxidation and heat recovery
The thermal oxidiser burns the fuel gas at a controlled high temperature. A range of 850 to 1,050°C has been shown to burn out the gases from the intended wastes, and the residence time is at least two seconds. The design limits for the exhaust are:
- carbon monoxide: 30 mg/Nm³;
- volatile organic compounds (as carbon): 7 mg/Nm³, on a dry gas, 11% oxygen basis.
Hot exhaust leaves the oxidiser at about 1,100°C and is cooled to about 1,000°C before it heats the pyrolysis tubes. That recirculated heat makes the module self-sustaining once running, so the burners are mostly needed for start-up, shutdown and very low calorific fuel below 8 MJ/kg. The remaining heat raises superheated steam in a water-tube boiler, and a turbine with a single extraction point generates the power.
Why the control philosophy is unusual
The plant does not follow the electrical load. Fuel is fed at a constant, pre-determined rate and the pyrolysis screw speed sets the thermal control. The stated aim is to maximise thermal destruction of waste, with steady power as a by-product. At maximum electricity production, the design target for net electrical efficiency is better than 17%.
The whole plant runs at negative pressure, held by the induced-draught fans, so leaks draw air in and do not push gas out.
Design margins worth noting
| Item | Margin or rule |
|---|---|
| Boiler heat-transfer area | 10% margin |
| Turbine bypass capacity | at least 110% of turbine capacity |
| Ash handling equipment | at least 20% capacity margin |
| Equipment design life | 25 years for the boiler package |
| Programme | about 12 months to commercial operation |
What to consider for your own project
- Fuel specification first. Moisture and calorific value drive the net output. Settle them before choosing equipment.
- Design for modularity. Shared feed and turbine make later expansion cheaper.
- Plan for permitting. In the UK, thermal treatment requires an environmental permit from the Environment Agency.
- Decide where the heat goes. The steam extraction point makes combined heat and power possible.
FAQ
What is the difference between pyrolysis and gasification?
Pyrolysis heats fuel without air to produce gas and char. Gasification reacts the char with steam to produce more gas.
Why use a thermal oxidiser?
It burns the fuel gas at high temperature with enough residence time to destroy organic compounds before heat is recovered.
How many tonnes of RDF does one module take?
Up to about 4 tonnes an hour, limited by heat transfer in the pyrolysis tubes.
Can the plant be expanded?
Yes. Modules share the feed and turbine, so a 7.5 MW plant is three 2.5 MW modules.
Looking at RDF to power?
Contact our engineering team with your fuel data and we will outline a module configuration.