A hospital producing around 25 tonnes of clinical waste a day can treat it on site in a pyrolysis and gasification plant: shred and sterilise, convert to gas, burn the gas in a high-temperature oxidiser and clean the exhaust. This article walks through the design basis of a proposal for a Caribbean hospital, including why this version dumps its heat instead of making power.
The challenge: waste that is hazardous and variable
Clinical waste includes items soiled with blood or body fluids, human tissue, sharps, pharmaceuticals and cytotoxic waste. Poor segregation is costly and risky. Studies cited in the source material suggest that as much as half of waste sent for incineration as clinical waste is in fact general waste, which raises costs needlessly. Any treatment plant must therefore accept a wide range of material and cope with variable moisture and ash.
The design basis
| Parameter | Value |
|---|---|
| Waste | About 25 tonnes a day, bagged or boxed |
| Feed rate | About 1,040 kg/h wet, 730 kg/h dry |
| Calorific value | 19.5 MJ/kg dry (design range 8 to 40 MJ/kg) |
| Moisture | Up to 30% wet basis |
| Thermal input | About 3.9 MW |
| Turndown | 4:1 before auxiliary fuel is needed |
| Potential power | About 700 kW, not included in the offer |
Limits on the waste: chlorine and sulphur below 3% dry, density 190 to 240 kg/m³, able to pass a 100 mm sieve, no volatile flammable liquids in volumes above 0.5 litres and no pressure vessels.
The treatment train
- Feed: a tippler, hopper and a sterilising shredder, then a compactor that seals the feed against air.
- Pyrolyser and gasifier: a two-lane retort, with a gasifier for the carbon char and ash removal.
- Oxidiser: burns the gas, with a pre-heater burner (propane, diesel or natural gas) for start-up.
- Heat dump: a hot oil heat exchanger rejects heat to cooling towers. This replaces the boiler and turbine to keep the first phase simple.
- Flue gas treatment: bicarbonate injection, a bag filter and NOx inhibitor.
Heat is dumped because the offer was for waste destruction. A later steam turbine could recover about 700 kW, but turbine specifications strongly affect boiler design, so they were left for a second stage.
Precedent and regulation
The proposal points to a UK plant that received an Environment Agency authorisation under the Integrated Pollution Prevention and Control regime in 2001 and has run commercially on clinical and special wastes since. In the UK, plants like this require an environmental permit.
Delivery programme
The delivery programme totalled 60 weeks: 4 weeks front-end design, 12 weeks detailed design, 20 weeks manufacture and procurement, 12 weeks installation and 8 weeks commissioning and performance testing, plus client approval periods. Training for 20 staff in operation and maintenance was included.
What to consider for your own project
- Segregate at source. Cleaner segregation reduces cost.
- Define the waste. The type and moisture decide the design.
- Decide on energy recovery early. Boiler design depends on turbine choice.
- Plan the interfaces. Waste transport, storage and feed ahead of the hoppers were excluded.
FAQ
What is a sterilising shredder?
A shredder that also disinfects the waste as it is reduced in size.
Is this the same as incineration?
No. Pyrolysis heats waste without air, then burns the resulting gas under control.
Can it make electricity?
Potentially about 700 kW, but this was excluded.