Technology Zone
Incineration, pyrolysis, gasification and drying recover energy and materials from waste that cannot be recycled mechanically. This zone covers thermal processing technology, emission control and the plants that run it.
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What this zone covers
Thermal treatment recovers energy or materials from waste that cannot be recycled mechanically, and reduces its volume and hazard. Incineration burns waste with excess air and recovers heat as steam or power; gasification converts it with limited oxygen into a syngas; pyrolysis heats it without oxygen into oil, gas and char; drying removes water from sludges and organics before further processing. Each route has a different feed requirement, product and emission profile.
Grate furnaces take unsorted municipal and commercial waste as received; fluidised-bed furnaces need a prepared, shredded feed and burn at lower temperatures with better control; rotary kilns handle hazardous and liquid waste. The boiler recovers heat for electricity, district heating or process steam; the flue-gas train removes acid gases, dust, heavy metals, dioxins and nitrogen oxides to the emission limits; bottom ash is treated to recover metals and produce aggregate.
These processes aim at chemicals and fuels rather than heat: pyrolysis oil from plastics and tyres, syngas from wood and refuse-derived fuel, char from biomass. They are sensitive to feed quality — moisture, chlorine, inert content, particle size — and most failed plants failed on feed preparation and on the cleaning of the product gas or oil, not on the reactor. Tyre pyrolysis and chemical recycling of mixed plastics are the growth areas.
Sludge, organics and wet fractions must be dried before they can be burned efficiently or processed further. Belt, drum and contact dryers use waste heat or flue gas; the exhaust needs odour and dust treatment. Refuse-derived fuel (RDF) and solid recovered fuel (SRF) production — shredding, sorting, drying, pelletising — turns mixed waste into a specified fuel for cement kilns and industrial boilers.
Emission limits (dust, HCl, SO2, NOx, dioxins, mercury) drive the design and the operating cost of every thermal plant; continuous emission monitoring is mandatory. Residues — bottom ash, fly ash, flue-gas cleaning residues — need treatment and a disposal or recovery route. The permit sets what can be accepted, at which rate, and what must be measured; acceptance control at the gate is the first line of defence.
| Parameter | Typical range | Rule of thumb |
|---|---|---|
| Grate incinerator temperature | 850–1,100 °C; flue gas ≥ 850 °C for 2 s | The 2-second rule is the legal minimum for destroying organics |
| Lower heating value of feed | Municipal waste 8–12 MJ/kg; RDF/SRF 12–20 MJ/kg | Below ~6 MJ/kg support fuel is needed |
| Energy recovery | Electricity 20–25 % of input; combined heat and power up to 80 % | Heat off-take decides the economics more than the boiler |
| Pyrolysis temperature | 400–600 °C plastics and tyres; 500–800 °C biomass | Temperature and residence time set the oil-gas-char split |
| Gasification temperature | 800–1,200 °C; higher for slagging | Tar in the syngas is the classic operating problem |
| Feed moisture for combustion | < 40–50 % for self-sustaining burn | Drying with waste heat pays back quickly |
| Bottom ash | 20–25 % of input by mass; 5–10 % of volume | Metals recovery from bottom ash is a revenue stream |
| Emission limits (EU BAT) | Dust < 5 mg/Nm³; HCl < 8; NOx 80–150; dioxins 0.06 ng TEQ/Nm³ | Continuous monitoring on dust, HCl, SO2, NOx, CO, TOC |
| Symptom | Likely causes | What to do |
|---|---|---|
| Furnace temperature unstable | Feed heating value varies; moisture swings; poor mixing on the grate | Bunker crane mixing, feed control on flue-gas temperature, drying of wet fractions |
| Corrosion in boiler tubes | Chlorine and sulphur in the waste; high tube temperatures; deposits | Lower steam parameters, cladding on tubes, soot blowing, feed control on chlorine |
| Slagging and fouling | Low-melting ash (alkalis); high temperature; glass and fines in feed | Remove glass and fines upstream, temperature control, additives |
| Emission limit exceedances | Reagent dosing lag; filter damage; feed spikes (PVC, mercury); poor mixing | Predictive dosing on raw-gas measurement, filter leak test, acceptance control |
| Tar and blockages in gasifier gas line | Temperature too low; feed too wet; poor gas cleaning | Higher reactor temperature, drier feed, tar cracking or scrubbing |
| Pyrolysis oil off-specification | Feed contamination (PVC, PET, inerts); wrong temperature; long residence | Feed sorting and dechlorination, temperature control, product upgrading |
| Odour and dust complaints from the dryer | Exhaust not treated; leaks; product residue | Biofilter or thermal oxidiser on exhaust, negative pressure, cleaning |
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Incineration is the mature route for mixed waste: it accepts almost anything, recovers heat and power reliably and has a proven emission train. Gasification suits prepared, uniform feeds where a syngas has a use nearby. Pyrolysis aims at oils and chemicals from specific streams — tyres, mixed plastics, biomass — and depends on strict feed preparation. For most municipal waste the answer is still incineration; for sorted plastics and tyres the newer routes are becoming competitive.
Dry (usually below 10 % moisture), low in chlorine (PVC out), low in inerts and metals, and shredded to a consistent size. Most plants that underperformed did so because the feed was dirtier and more variable than the design assumed; the sorting and drying steps before the reactor deserve as much attention as the reactor itself.
The furnace heats a boiler that raises steam; the steam drives a turbine for electricity and, in combined heat and power plants, feeds district heating or industrial process heat. Electricity alone recovers 20–25 % of the waste's energy; with a heat off-take total efficiency reaches 70–80 %. The heat customer is the key to the economics.
Refuse-derived fuel is shredded, dried and sorted mixed waste with a defined heating value; solid recovered fuel is the standardised version (EN ISO 21640 classes). Cement kilns, industrial boilers and dedicated power plants burn it as a coal substitute. Producing it requires shredding, sorting, drying and often pelletising — a plant in itself.
A chain of steps after the boiler: dust removal (electrostatic precipitator, fabric filter), acid gas removal (dry, semi-dry or wet scrubbing with lime or sodium bicarbonate), activated carbon for dioxins and mercury, and selective catalytic or non-catalytic reduction for NOx. Continuous emission monitoring reports every value to the authority; the flue-gas train is a large share of both capital and operating cost.
Bottom ash, about a quarter of the input mass, is cooled, aged, screened and stripped of ferrous and non-ferrous metals — a valuable recovery — and the mineral fraction is used as aggregate where regulations allow. Fly ash and flue-gas cleaning residues are hazardous and go to treatment or secure landfill.
Water carries no energy but takes energy to evaporate: a sludge at 75 % moisture cannot burn on its own. Drying it to 30–40 % with waste heat or flue gas makes it a fuel, reduces the mass to transport and stabilises it. The dryer's exhaust needs odour and dust treatment, and the dried product can self-heat in storage.
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