3.5 Combustion Systems: Burners, Stokers, Pulverizers, Fans & Draft Control

Combustion Systems: Burners, Stokers, Pulverizers, Fans & Draft Control

Core Trade Concept: Everything on the fire side of a boiler exists to deliver fuel and air to the furnace in the right proportion, at the right place, and to move the products of combustion out again. Boilermakers build and repair the hardware that does it — windboxes, burner throats, mill housings, ductwork, dampers and fan casings — and they open it all up during outages. Knowing how the system is supposed to run tells you what the damage you are looking at means.

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1. The Combustion Requirement

Complete combustion needs three things in balance: fuel, oxygen, and ignition temperature, with enough turbulence, time and temperature (the "three Ts") for the reaction to finish before the gas leaves the furnace.

• Excess air is supplied above the theoretical (stoichiometric) requirement because mixing is never perfect. Too little and you get carbon monoxide, soot and unburned fuel — a fuel-rich furnace is also an explosion hazard. Too much and you carry heat straight up the stack and raise NOx.
• Excess air is monitored as oxygen in the flue gas, trimmed lower on clean gaseous fuels and higher on solid fuels because solid-fuel mixing is far worse.

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2. Fuel-Firing Equipment

Oil and gas burners

• A register burner admits combustion air through adjustable vanes around a central fuel gun, imparting swirl that anchors and shapes the flame.
• Oil must be atomized before it will burn: steam atomizing, air atomizing, mechanical (pressure) atomizing, and rotary cup designs. Heavy fuel oil must additionally be heated to reach the required viscosity at the tip.
• Low-NOx burners stage the air and fuel to lower peak flame temperature; staging lengthens the flame, which is why replacing burners frequently drives burner throat refractory and waterwall changes — a boilermaker scope item.
• The windbox distributes air to the burner registers; the burner throat is a refractory-lined transition into the furnace, and its condition governs flame shape.

Solid fuel: stokers

Solid fuel: pulverized coal

• A pulverizer (mill) — bowl mill, ball-and-race, or ball tube design — grinds coal to a powder. Primary air both dries the coal in the mill and transports it through the coal pipes to the burners.
• A classifier at the mill outlet returns oversize particles for regrinding. Typical fineness targets are roughly 70–75% passing 200 mesh with very little retained on 50 mesh; coarse coal burns late, lengthening the flame into the superheater and causing slagging and tube erosion.
• Mill maintenance — grinding rolls, bull rings, classifier vanes and journal assemblies — is heavy boilermaker outage work.

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3. Fans and Draft Systems

        DRAFT ARRANGEMENTS

  NATURAL        FORCED (pressurized)     BALANCED
  stack effect   FD fan only              FD + ID fans
  only           furnace ABOVE            furnace held slightly
                 atmospheric              BELOW atmospheric
                 (gas-tight casing        (leaks pull air IN,
                  is mandatory)            not gas out)

• A balanced-draft unit is held slightly negative in the furnace — commonly on the order of a few tenths of an inch of water column — so any casing leak draws air inward rather than blowing hot flue gas and ash into the boiler house.
• Furnace implosion is the hazard specific to balanced draft. A rapid loss of firing with the ID fan still pulling can collapse the furnace inward, which is why buckstay systems (Section 4.2) are designed to a transient negative pressure far beyond normal operating draft, and why NFPA 85 requires implosion protection interlocks.
• Fan control is by inlet vane, variable-speed drive, or outlet damper; outlet damper control is the least efficient because it throttles against the fan.

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4. Burner Management and Furnace Purge

Boilermakers do not operate the burner management system (BMS), but they work inside the furnace it protects, and the interlocks are what stand between them and a fuel-air explosion.

• NFPA 85, Boiler and Combustion Systems Hazards Code, governs furnace purge, flame supervision, trips and fuel-valve proving.
• Purge before every light-off: the furnace and gas passes must be swept with at least five volume changes of air before an igniter is permitted, so that any accumulated fuel is removed.
• Flame scanners supervise each burner; loss of flame with fuel admitted is a master fuel trip.
• Never trust the BMS alone for entry. Furnace or duct entry requires the full lockout/tagout and blinding regime from Section 1.3 — positive isolation of every fuel line, not just a closed automatic valve.

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5. What Goes Wrong, and What the Boilermaker Repairs

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6. Realistic Trade Scenario: Restoring Furnace Draft After a Casing Repair

A balanced-draft industrial boiler is running with visible flue gas puffing out of the observation doors, and the ID fan is at maximum vane position while furnace pressure still drifts positive.

Investigation during the outage finds the cause is on the boilermaker's side of the fence:

1. Casing leakage. A prior repair patched skin casing with unsealed lap joints and left three unsealed penetrations where sootblower wall boxes were relocated. Air in-leakage through those openings loads the ID fan with cold tramp air it must move alongside the real flue gas.
2. Expansion joint failure. A fabric expansion joint in the ID inlet duct has torn, admitting more tramp air ahead of the fan.
3. Consequence. Excess air rises without any fuel benefit, ID fan capacity is consumed by air the burners never saw, and furnace pressure control loses margin.

The repair scope is straightforward boilermaker work — seal-weld the casing laps, install proper wall box seal plates, replace the fabric joint and remove its shipping restraints (Section 4.2) — and it restores draft control without touching a single control setting.