A pyrolysis and gasification plant treating about 200 tonnes of municipal solid waste (MSW) a day can export roughly 2.9 MW of electricity, from about 3.4 MW gross. That was the design outcome in an outline proposal for a waste-to-energy plant planned for a coastal city in South-East Asia. The figures depend heavily on the waste’s calorific value and moisture, and on how the steam turbine is cooled.
The challenge: turning mixed municipal waste into steady power
Mixed household waste is variable, wet and bulky. A thermal plant needs a consistent, dry-enough feed, and a way of recovering heat reliably. A site developer wanted a modular plant that would take unsorted MSW, generate electricity for the plant and for export to the local network, and be delivered as one package.
Because the site had not yet been built, the proposal was outline only. It was based on indicative waste data and said so clearly: the figures were subject to a feasibility study.
The approach: prepare, pyrolyse, oxidise, recover
Design basis
| Parameter | Value | What it means |
|---|---|---|
| Waste feed | About 200 t/day wet; roughly 68,000 t/yr wet, 39,500 t/yr dry | The difference is water, which burns nothing and absorbs heat |
| Moisture | About 40% (wet basis) | Water in the waste has to be evaporated before it can burn |
| Calorific value | 14.4 MJ/kg (dry-weight basis) | Indicative figure, to be confirmed by sampling |
| Average thermal input | About 21 MW thermal | The energy in the waste, before conversion losses |
| Gross electrical output | About 3.4 MW | Roughly 16% of the thermal input |
| Parasitic load | About 0.5 MW | Plant power for fans, conveyors and pumps |
| Net export | About 2.9 MW | After parasitic load and an allowance for 85% availability |
The process train
- Feed preparation. Reception area, feed conveyor, a coarse-screen trommel with bag opener, a fine-screen conveyor and an RDF shredder. Its job is to give the thermal plant a consistent material.
- Feed system. A hopper and compactor form a secure seal so that air does not enter the retort.
- Pyrolyser and gasifier. A multi-lane retort thermally breaks down the organic fraction in a low-oxygen environment, producing gas. A separate gasifier converts the remaining carbon char. An ash removal system takes the residue.
- Thermal oxidiser. The gas is burned at high temperature, with a pre-heater burner (propane, diesel or natural gas) for start-up.
- Heat recovery. Waste-heat boilers capture about 85% of the thermal energy in the feedstock as high-pressure superheated steam.
- Power generation. A steam turbine drives a 400 V, 3-phase, 50 Hz alternator rated at 3.0 MW, with controls for export to a local network. Exhaust-gas clean-up completes the train.
Why it works, and the trade-offs
Moisture decides the output
Moisture is the single biggest factor in a thermal plant. The proposal states that if the waste is wetter than assumed, throughput is not reduced, but the net energy available for power falls. It is worth noting that the source quotes moisture on more than one basis. Any real project should fix one basis in the feasibility study and design to it.
Cooling the turbine
The 2.9 MW figure assumes condenser cooling water is available, for example from a river or large lake. Otherwise:
- A cooling tower is the intermediate option.
- A closed-circuit air-blast cooler needs fan power of about 0.7 MW, which would cut the export figure accordingly.
What the plant will and will not accept
The design specification sets limits, which any buyer should compare with their own waste:
- Waste passing a 100 mm sieve (after preparation).
- Density about 190 to 240 kg/m³.
- Chlorine and sulphur each below 3% by dry weight.
- No pressure vessels, and no volatile flammable liquids in quantities above 0.5 litres.
- A wide calorific range of 8 to 40 MJ/kg, with 14 MJ/kg as the design basis.
Chlorine and sulphur matter because they form acid gases that the exhaust clean-up has to handle. In the UK, a plant like this would be regulated under the environmental permitting regime; see the government’s guide to the energy-from-waste debate for background.
What to consider for your own project
- Sample the waste. Measure calorific value and moisture across seasons. The output of the whole plant hangs on them.
- Settle the cooling method early. It changes the net export by hundreds of kilowatts.
- Plan the front end. Screening, shredding and removal of unsuitable items are as important as the retort.
- Start with design. This proposal began with a front-end engineering design package (heat and mass balance, process flow diagram, P&ID, general arrangement and datasheets) of about four weeks, before detailed design.
- Budget for training. The proposal included a two-month training course for operators at a UK plant, followed by an operations engineer on site five days a week for three months, then a further three months of weekly visits.
- Expect to show a working plant. Some buyers want to see a running plant on similar feedstock. In this project, a visiting delegation toured an existing pyrolyser, which was not running that day, and then asked to return when it was.
The programme was about 74 weeks from order to handover: design, manufacture and procurement of about 30 weeks, then 12 weeks of installation and 12 weeks of commissioning and performance testing. It did not include time for third-party approvals.
FAQ
How much electricity can a tonne of municipal waste produce?
This design implies roughly 0.35 MWh per wet tonne (2.9 MW over 24 hours is about 70 MWh a day, from around 200 tonnes). That is a rough calculation from the quoted figures, not a guarantee, and it varies with waste quality.
What is the difference between pyrolysis and incineration?
Pyrolysis heats waste with little or no oxygen to break it into gas and char. Here the gas is then burned in a separate oxidiser, which allows better control of the combustion conditions.
Does the waste need sorting first?
Yes. Screening and shredding are included so the retort receives a consistent feed.
Why is the proposal described as outline?
The calorific value and site were not confirmed. A feasibility study and a site survey would fix those before the price and performance are finalised.
Talk to us about waste to energy
If you are weighing a pyrolysis plant for municipal waste, contact us with your tonnage, waste composition and cooling options, and we can help you set a realistic design basis.