A pyrolysis and gasification plant heats waste in the absence of air to turn it into a fuel gas, burns that gas in a high-temperature oxidiser, and uses the heat to raise steam for a turbine. The plant described here was proposed to a North African client in two sizes: a 15 MW scheme for fresh municipal waste, and a modular 2.5 MW building block for refuse-derived fuel. This article explains each step and the reasoning behind the design.
The challenge: waste that is variable and wet
Municipal and refuse-derived waste varies in calorific value, moisture and ash. The design range in the proposal was 8 to 30 MJ/kg dry, ash from 0 to 40%, with a design point of 18 MJ/kg dry. Moisture matters most. The proposal assumed no more than about 33% moisture on a wet basis; above that, throughput holds up but less energy is available for power.
The scale options
| Large scheme | Single module | |
|---|---|---|
| Feed | 560 wet tonnes/day of fresh municipal waste | 90 wet tonnes/day of refuse-derived fuel |
| Thermal input | About 98 MW | About 14 MW |
| Gross electricity | About 15 MW | About 2.5 MW |
| Parasitic load | About 4.5 MW | About 0.7 MW with air-cooling |
| Net export | About 10.5 MW | About 1.8 MW |
About 85% of the thermal energy in the feed is captured as high-pressure superheated steam. The parasitic load depends heavily on the turbine cooling method. Where clean water from a river or lake is available it falls; with an air-blast cooler it rises.
How the process works, step by step
1. Feeding and sealing
Waste is metered from near-vertical-sided hoppers into screw compactors. The compactors squeeze the waste into a dense plug, which acts as a gas seal and keeps oxygen out of the process. Fuel is fed at a constant rate rather than varied with steam demand, which is unlike conventional boilers. The plant is run to maximise destruction of waste, not to hold a steady electrical output.
2. Pyrolysis
Each compactor feeds a heated tube with a screw conveyor. Heat applied from outside first dries the waste, then above about 250°C the organic material breaks down into a medium-calorific gas (pyrogas). The rotation speed of the screw is the main thermal control. Each tube handles up to 1.3 tonnes an hour, so a three-tube module has a maximum of about 4 tonnes an hour.
3. Gasification
The solid residue, mainly carbon char and inerts, falls into a deep-bed gasifier, where it reacts with steam to give further fuel gas. The chamber is sealed to stop air entering. Ash is extracted mechanically through a sealed arrangement into bins, and design assumed ash at about 10% of the feed by weight.
4. High-temperature oxidation
The gases pass through a hot-gas cyclone and into a thermal oxidiser held at a minimum of 1,000°C for at least two seconds. Temperatures between 850°C and 1,050°C were stated to give complete combustion of the gases, and the combination of high temperature and residence time is intended to destroy dioxin and furan precursors. The oxidiser was specified to keep carbon monoxide to 30 mg/Nm³ and volatile organic compounds to 7 mg/Nm³ (dry, 11% oxygen). Two auxiliary burners heat the plant from cold and hold temperature if the feed is very low in calorific value or stops.
5. Using the heat twice
The hot exhaust leaves the oxidiser at about 1,100°C. Cooled to about 1,000°C, it is passed around the pyrolysis tubes, which is why the process is self-sustaining once running. It then goes to a water-tube packaged boiler, designed for continuous operation, with a margin of 10% on heat transfer area. If grid power fails, the boiler must keep operating safely for 30 minutes so the heat in the system can decay in a controlled way.
6. Steam turbine
The design steam conditions are 45 barg and 450°C. A condensing extraction turbine produces power, with steam bled off for process uses such as gasification and de-aeration. The condenser is cooled by a closed circuit.
7. Flue gas treatment
Acid gases are removed with dry sodium bicarbonate injected ahead of a bag filter, and a continuous emissions monitoring system measures the stack gas. The whole plant runs at negative pressure, held by induced-draught fans at the end of the line.
Staged delivery
The proposal was built in stages: a front-end feasibility study (confirming waste characterisation, quantities, power, cost to within ±10% and programme), then front-end engineering design (heat and mass balance, process flow and P&ID diagrams, general arrangement), then equipment supply. This reduces the risk of committing capital before the waste data is confirmed.
What to consider for your own project
- Measure the waste. Calorific value, moisture and ash drive everything.
- Choose the cooling method early. It changes net export by a large margin.
- Plan the ash route. Ash storage and disposal sit outside the equipment scope.
- Check permitting. In the UK, thermal treatment needs an environmental permit.
- Think modular. Modules can be added and combined into one turbine.
FAQ
What is the difference between pyrolysis and incineration?
Pyrolysis and gasification heat waste in an oxygen-starved environment, producing a fuel gas that is then burned. Incineration burns waste directly with excess air.
How much electricity does a tonne of waste make?
In the module example, 90 wet tonnes a day gave about 2.5 MW gross. The result depends on calorific value and moisture.
Is auxiliary fuel needed?
For start-up and shut-down, and for very low calorific feed (under about 8 MJ/kg).
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