A mass-burn waste-to-energy (WtE) plant is a Rankine-cycle power plant fired by unsorted municipal solid waste burning on a moving grate instead of coal or gas. Three real, coupled effects drive its efficiency here:
Net calorific value (moisture penalty):
NCV ≈ Q_dry·(1−M) − 2.443·M [MJ/kg]
(2.443 MJ/kg = latent heat of vaporising water in the fuel)
Thermal input:
Q_in = ṁ_waste · NCV [MW]
Boiler efficiency (excess-air trade-off):
flue-gas loss ∝ (λ−1) — more excess air, more sensible
heat swept up the stack
unburned loss ∝ max(0, 1.3−λ) — too little air ⇒ incomplete
combustion, CO and char losses
η_boiler = 1 − flue-gas loss − unburned loss − radiation
Electrical output:
P_gross = Q_in · η_boiler · η_turbine
P_net = P_gross · (1 − parasitic load) (≈15%, flue-gas cleanup)
- Moisture — wetter waste wastes fuel energy evaporating water instead of raising steam, so NCV drops even though the plant burns the same tonnage.
- Excess-air ratio λ — grate combustion of heterogeneous solid fuel needs far more excess air than a gas or pulverised-coal burner (typically λ ≈ 1.6–2.2) to reach every particle, but each extra unit of air carries heat straight up the stack — there is a real efficiency sweet spot, not a monotonic "more air is better" curve.
- Turbine cycle efficiency — corrosive, chlorine-rich flue gas limits superheater steam temperature to roughly 400 °C (vs ~600 °C in a coal plant), which is why real WtE plants top out near 25–30% turbine-cycle efficiency and 18–27% net electrical efficiency overall, well below a modern coal or gas plant.
Real-world relevance: this is the same energy chain — grate, boiler, steam turbine, flue-gas treatment parasitic load — used in operating mass-burn WtE plants worldwide (e.g. Copenhagen's Amager Bakke, dozens of European and Japanese municipal incinerators) that turn residual waste into district heat and grid electricity while cutting landfill volume roughly 90%.