High-Pressure Boilers and Combustion Systems
This domain examines the high-pressure boiler as a system: circulation, heat absorption, drum level control, combustion air/fuel management, draft, and the fouling and corrosion that follow poor control. Calculations usually involve excess air, flue-gas oxygen or stack loss.
Circulation and heat absorption
- Natural circulation depends on the density difference between water in the downcomers and the steam-water mixture in the risers. It weakens at low pressure and low load, so circulation is verified during start-up.
- Once-through and controlled-circulation designs have no steam drum and are far less forgiving of flow or chemistry upsets.
- Superheat is controlled by attemperation spray, tilting burners or gas recirculation. Spray water must be high purity, and every kilogram of spray becomes extra steam.
Combustion
- The three T's - time, temperature, turbulence - plus correct air/fuel ratio.
- Stoichiometric air burns all fuel; excess air completes combustion. Too little gives CO and smoke; too much raises stack loss and lowers efficiency.
- Typical flue-gas O2 targets (dry): gas 2-3%, oil 3-4%, coal 4-5%. O2 trim adjusts air flow to the measured oxygen.
- Furnace draft is normally slightly negative (about -0.05 to -0.1 kPa) to keep products in and prevent air ingress. A purge at 25-30% of full-load airflow for at least five minutes is mandatory before light-off.
- Cold-end air heater corrosion is controlled by keeping metal temperature above the acid dew point, which depends on fuel sulphur. Sootblowers control slagging and fouling.
Worked example
Flue gas reads 4.0% O2 dry. Excess air = O2/(20.9 - O2) x 100 = 4.0/16.9 x 100 = 23.7%. If the unit has drifted to 7% O2 (excess air 50%) and efficiency falls from 84% to 82%, returning toward 4% saves roughly two points of fuel. Confirm with a CO reading first: low O2 with rising CO means incomplete combustion and unburnt fuel loss, which is worse than the stack loss you were chasing.
Traps
- Treating oxygen percentage and excess-air percentage as the same number.
- Expecting drum level to behave simply on load swings; shrink and swell disguise true level, which is why three-element control uses steam flow, feedwater flow and level together.
- Sizing safety valve capacity to normal load instead of maximum evaporating capacity.
- Sootblowing without watching drum level and furnace pressure.