Landfill mining for waste to energy works because waste that has sat in a loosely built landfill for around nine months is drier, lower in organics and easier to process than fresh waste. In a proposal for a 1 MW pyrolysis plant in South-East Asia, that stabilised waste was mined, screened, shredded and dried into a solid refuse fuel (SRF) of about 36 dry tonnes a day. The fuel then drove a steam turbine rated at 1.2 MW gross.
The challenge: fresh municipal waste is a poor fuel
Fresh municipal solid waste is wet, rich in organic matter and full of plastic bags. Its high organic fraction holds a lot of water and contributes little calorific value to a pyrolysis process, so a plant fed directly from collection vehicles needs heavy front-end separation and drying.
The proposed concept turned that around by using the landfill itself as a stabilisation buffer.
The approach: stabilise first, then mine
Step 1: build the landfill to stabilise
Waste is received on a large flat area sized for two days of buffering, then landfilled. The structure matters a great deal. Dense compaction can triple the volume you fit into a site, but a loose, shallow layer of about one metre lets air flow through and stabilises the waste faster. The right balance depends on the planned retention time, which in this design was at least nine months.
Stabilisation brings three benefits:
- Moisture: as organics degrade into soil-like material, the water they held is much easier to remove.
- Organic content: the wet, low-calorific organic fraction decomposes and can be screened out.
- Plastics: plastic does not decompose, but the bags degrade, so a bladed trommel can open them without a heavy bag opener.
Step 2: mine, screen and transport
Mined waste is wet, at about 30% moisture, with a high share of inerts such as metal, glass and soil. Excavators each mine 10 to 12 tonnes an hour, and a 2 to 3 hour buffer feeds the first-stage trommel. The trommel removes particles larger than 60 mm and returns the rejects to the landfill, so diesel haulage is kept short. Each trommel handles about 10 tonnes an hour.
The front end is split across two locations for this reason. The first sits in the middle of the landfill, where mining and separation happen and rejects can be tipped back easily. The second is a covered reception hall beside the shredders. The primary stage is open to the weather and runs ten hours a day. The secondary stage runs 24 hours a day on three shifts.
Step 3: store, shred and dry
The two stages run on different hours, so a covered SRF store is sized for 24 hours of buffer. At an assumed density of 500 kg/m³ and a maximum depth of 1 m, that needs about 50 m² for every tonne an hour fed to the shredder, before access space is added. Shredders cut the SRF to below 50 mm. Thermal-screw dryers then take it from up to 40% moisture to 20 to 25%, which is the standard feed requirement for the pyrolyser. A final hopper holds around six hours of fuel, because stopping a pyrolyser is costly and the hopper has to ride through short breakdowns upstream.
The conversion plant
| Item | Value |
|---|---|
| Dry feed | about 1,500 kg/h (36 tonnes a day) |
| Calorific value (dry) | 16.67 MJ/kg |
| Thermal input | about 7.8 MW |
| Gross electricity | about 1.2 MW |
| Net export | about 1.0 MW |
The process is pyrolysis and gasification, followed by a thermal oxidiser and a heat-recovery boiler feeding a steam turbine. About 85% of the energy in the fuel is captured as superheated steam. The gap between 1.2 MW gross and 1.0 MW net is the plant’s own consumption, which depends largely on how the turbine exhaust is cooled. With a river or lake, the parasitic load is lower. A closed-circuit air cooler can use about 0.3 MW, and a cooling tower sits in between.
The moisture of the fuel matters. If it rises above the design value, throughput is not reduced but the net energy available for power is.
Why this route can make sense
- It reduces the intensity of front-end separation, because the landfill has already done part of the work.
- It keeps diesel use low by siting the first stage inside the landfill.
- It puts rejects straight back where they came from.
The trade-off is time and space. You need a landfill area able to hold at least nine months of waste, and the timeline from tipping to power is long.
What to consider for your own project
- Characterise the waste. Moisture, calorific value, ash and chlorine determine the fuel, and a front-end feasibility study confirms them.
- Size every buffer. Two days at the tip, 24 hours in the SRF store and six hours at the hopper were all deliberate.
- Decide your cooling method early. It changes net export by around 0.15 MW in this case.
- Check permitting. In the UK, the Environment Agency regulates thermal treatment through environmental permits.
FAQ
What is landfill mining?
Excavating previously landfilled waste to recover materials or fuel, here after a deliberate stabilisation period.
Why wait nine months?
Organics degrade over time, which lowers moisture and improves the fuel. The nine-month period was the design minimum.
What size must SRF be for pyrolysis?
Under 50 mm and below 25% moisture in this design.
How much electricity does 36 tonnes a day of SRF make?
About 1.2 MW gross and 1.0 MW net, at a calorific value of 16.67 MJ/kg dry.
Planning a waste-to-energy scheme?
Talk to our engineers about your waste and the fuel it can make.