Where a landfill gas power station is already running, its waste heat can evaporate leachate and leave only a small volume of dry solids to dispose of. For a 200 m³/day duty at a southern European landfill, three routes were compared: direct contact with engine exhaust gas, heat exchangers using engine hot water and steam, and superheated steam. This post sets out how they differ and why the cheapest was not automatically the right choice.
The challenge: leachate with no easy discharge route
The site was fitting a leachate evaporation system rated at 200 m³/day, alongside a gas-engine power station of eleven engines. Evaporation suits sites where leachate cannot be discharged to sewer or river, because it removes the water and leaves a concentrate. The question was where the heat would come from and how to avoid fouling the equipment.
The three options
1. Exhaust gas evaporation
Engine exhaust (around 350°C at the evaporator inlet and 425°C at the dryer) is brought into direct contact with the leachate, so no heat exchange surfaces are needed to foul. A hot gas fan draws exhaust from a new common chimney. The evaporator turns the leachate into a pumpable slurry. A cyclone dryer/granulator then forms pellets of about 10 mm. Vapour leaves at about 100°C with a dew point near 60°C, passes through catalytic oxidation to destroy ammonia and VOCs (leaving water, nitrogen and carbon dioxide), and then a condenser and a 15 m stack. Incoming leachate does the first stage of cooling and is pre-heated in the process.
The evaporator and dryer use a high-alloy stainless steel (Avesta 254 SMO) chosen for hot, corrosive, saline duty. Electrical demand was around 150 kW with up to 7.5 MW of thermal input.
- For: lowest capital cost, no fouling heat exchangers, no chemicals other than anti-foam.
- Against: modifying engine exhausts is non-standard and worries plant engineers, and a standby heat source must be an air heater, not a packaged boiler.
2. Heat-exchanger evaporation
Here engine hot water (about 150°C at 10 bar) drives a forced-circulation evaporator under vacuum, followed by a wiped-film evaporator-drier that produces a dry powder for bagging. Sulphuric acid and anti-foam are dosed first to stabilise ammonia.
- For: uses existing engine heat recovery, can have extra capacity added, and a standard packaged boiler can provide backup.
- Against: by far the most expensive of the three, and the heat exchanger surfaces will foul unless the leachate is pre-treated, so regular cleaning is needed.
3. Superheated steam evaporation
Low-pressure steam is superheated and put in direct contact with the leachate. It avoids engine exhaust modifications and fouling surfaces, and costs less than option 2. The limit is steam saturation: the amount of water the steam can absorb is capped by the boiler used.
Standby fuel
When the engines are off, the plant needs a backup heat source, and fuel becomes the main running cost. The proposal listed the hourly fuel quantity for diesel, natural gas, heavy fuel oil, propane and iso-butane, with propane and iso-butane needing the least mass. Burners were optional extras.
What the concentrate becomes
The solids can be returned to the landfill in containers, encapsulated in concrete, or vitrified in a high-temperature furnace. Vitrification and encapsulation were options outside the base scope. Disposal of concentrate is a decision that should be settled before the plant is chosen.
Evidence from operating plants
The scope cited four operating leachate evaporation plants in Hong Kong, ranging from 480 to 1,800 m³/day, using flare exhaust heat, package boilers or steam from leachate vaporisation. One included a precipitation unit that contacts exhaust vapours with incoming leachate to drop out carbonates where they are easy to deal with.
What to consider for your own project
- Audit your waste heat. Exhaust, hot water and steam are different grades of heat and suit different processes.
- Plan for engine shutdowns. Decide what provides heat when the engines stop.
- Decide on pre-treatment. Fouling is the main operating risk with heat exchangers.
- Settle where the solids go before you commit.
- Check the permit position. See the Environment Agency’s landfill permit guidance.
FAQ
Can landfill gas engines heat a leachate evaporator?
Yes. Exhaust gas, hot water or steam from the engines can each drive evaporation, with different trade-offs.
What is left after evaporation?
Dry solids, as pellets or powder, which can be returned to landfill, encapsulated or vitrified.
Why use direct contact?
It avoids heat exchanger surfaces that foul with leachate.
What treats the ammonia in the vapour?
Catalytic oxidation converts ammonia and VOCs to water, nitrogen and carbon dioxide.
Looking at leachate evaporation?
Tell us your leachate volume and the heat you have available and we will compare routes. See also dissolved methane in leachate.