Recovery boilers, HRSGs and waste heat boilers

In July, Northgate's cogeneration plant shuts down one of its two trains for a week. Each train is a gas turbine driving a generator, with a heat recovery steam generator, or HRSG, on the turbine exhaust that makes steam for the upgrader. Ironbridge has the inspection support contract: opening access doors, building scaffolds inside the HRSG with the scaffold crew, cleaning areas for inspection and repairing whatever the inspectors find. Kiera spends the week inside the HRSG's casing, between tube rows a little wider than her shoulders. In August, she goes with Dale for two days to the Tamarack Pulp mill, where Ironbridge replaced recovery boiler floor tubes in the spring, to help with a follow-up inspection.

Not every boiler burns fuel to make steam for its own sake. Recovery boilers, HRSGs and waste heat boilers all make steam from heat or fuel that a process has already produced. They are built from the same parts as other water-tube boilers, but each has its own hazards and failure patterns.

The heat recovery steam generator

A gas turbine burns fuel and exhausts gas at about 560 °C (1,040 °F). Without heat recovery, that heat would go up the stack. An HRSG puts tube banks in the exhaust path to turn it into steam. Most HRSGs are horizontal: the gas flows horizontally through a long casing, past vertical tube banks, to a stack at the far end. The tube banks are made as modules called harps, rows of finned tubes welded between an upper and a lower header, which are shipped complete and lifted into the casing.

The gas meets the hottest section first. From the turbine end, a typical single-pressure HRSG has a superheater, then an evaporator connected to a steam drum on top of the casing, then an economizer. Larger HRSGs have two or three pressure levels, each with its own drum, evaporator and economizer, so they can recover heat down to a lower stack temperature. Many have a duct burner, a row of gas burners in the inlet duct, to add heat when the plant needs more steam than the turbine exhaust alone can make.

The temperature profile explains how an HRSG is designed. The gas cools steadily from inlet to stack. The water warms in the economizer, then stays at the saturation temperature in the evaporator while it boils, then heats again in the superheater. The narrowest gap between the two lines is usually at the cold end of the evaporator, and it is called the pinch. A smaller pinch means more steam but more tube surface, which costs money and adds back pressure on the turbine. The approach is the gap between the saturation temperature and the water leaving the economizer; it is kept positive so the water does not start to boil in the economizer, where it would cause flow problems and damage.

# Northgate cogeneration HRSG: steam made from gas turbine exhaust, set by the pinch (metric)
gas = 110                  # exhaust flow, kg/s
cp = 1.10                    # exhaust specific heat, kJ/kg K
t_in = 560                 # exhaust entering the HRSG, °C
t_sat = 253               # evaporator saturation temperature, °C
pinch = 10              # gas leaving the evaporator minus t_sat, °C
approach = 10             # t_sat minus water leaving the economizer, °C
h_sh = 3_214                 # superheated steam out, kJ/kg
h_econ_out = 1_052           # water at t_sat - approach, kJ/kg
h_fw = 440                 # feedwater in, kJ/kg

t_gas_pinch = t_sat + pinch
q_high = gas * cp * (t_in - t_gas_pinch)          # superheater + evaporator duty
steam = q_high / (h_sh - h_econ_out)
print(f"Gas leaving the evaporator: {t_gas_pinch:.0f} °C")
print(f"Superheater + evaporator duty: {q_high / 1000:,.1f} MW")
print(f"Steam flow: {steam * 3600:,.0f} kg/h")

q_econ = steam * (h_econ_out - h_fw)
t_stack = t_gas_pinch - q_econ / (gas * cp)
print(f"Economizer duty: {q_econ / 1000:,.1f} MW")
print(f"Stack temperature: {t_stack:.0f} °C")

# What a smaller pinch buys
for p in (5, 10, 15, 20):
    s = gas * cp * (t_in - t_sat - p) / (h_sh - h_econ_out)
    print(f"pinch {p:>3} °C -> steam {s * 3600:,.0f} kg/h")

Gas leaving the evaporator: 263 °C
Superheater + evaporator duty: 35.9 MW
Steam flow: 59,840 kg/h
Economizer duty: 10.2 MW
Stack temperature: 179 °C
pinch   5 °C -> steam 60,847 kg/h
pinch  10 °C -> steam 59,840 kg/h
pinch  15 °C -> steam 58,832 kg/h
pinch  20 °C -> steam 57,825 kg/h

The calculation shows how the pinch sets the steam flow. Every 5 °C (9 °F) of pinch is worth roughly 1,000 kg/h (2,200 lb/h) of steam here. Northgate's HRSG was designed with a pinch of about 10 °C (18 °F), and its performance tests compare actual pinch with design to see whether fouling of the fins is reducing heat transfer.

What fails in an HRSG

HRSGs at plants that start and stop often see failures that steady utility boilers rarely do. Every start heats the tubes and headers quickly, and every stop cools them; the thick headers lag behind the thin tubes, so the tube-to-header welds are strained over and over. Over thousands of cycles, this thermal fatigue cracks the welds, usually at the toe on the tube side. In the low-temperature sections, flow-accelerated corrosion thins the tube walls where fast-moving water strips the protective oxide, especially at bends and at tube entries into headers. Condensate left standing in superheater tubes after a shutdown can be blasted into a hot header at the next start, quenching it. And finned tubes collect deposits from the gas, which reduce heat transfer.

At Northgate, the inspectors' plan targets those failure modes. Ironbridge removes insulation from the economizer's lower headers so Annika Sorensen can run ultrasonic thickness readings at the tube bends and phased array checks on a sample of the tube-to-header welds. The crew also opens the inlet duct liner, a set of stainless steel sheets on studs that protect the insulated casing from the hot gas, and finds three liner sheets with broken fasteners, flapping in the gas flow. Loose liner sheets let hot gas reach the casing and make hot spots that show up on the outside as burned paint. Kiera spends a day replacing the fasteners and the sheets with Dale.

The kraft recovery boiler

A kraft pulp mill cooks wood chips in a chemical solution to separate the fibres. The spent solution, called black liquor, contains the cooking chemicals and the dissolved lignin from the wood. A recovery boiler burns the black liquor. The lignin is the fuel; it makes steam and electricity for the mill. The chemicals collect in a molten pool at the bottom of the furnace called the smelt, which flows out through spouts into a dissolving tank, where it is turned back into cooking liquor. A recovery boiler is at the same time the mill's chemical recovery plant and its largest boiler.

The liquor is sprayed into the furnace through guns, dries and burns, and the char falls onto a bed on the furnace floor. The floor and lower walls are in contact with smelt and with a reducing, corrosive atmosphere, so they are built of special tubes, often composite tubes with a stainless steel outer layer bonded to a carbon steel inner tube, and they are studded or coated in some areas to hold a protective layer of frozen smelt. This is the floor that Ironbridge retubed at Tamarack.

The recovery boiler's special danger is the smelt-water reaction. If water from a leaking tube reaches the molten smelt, it can cause a violent explosion. For this reason, recovery boilers are covered by the recommendations of the Black Liquor Recovery Boiler Advisory Committee, BLRBAC, which include an emergency shutdown procedure: when a leak in the pressure parts is suspected, the operators stop firing, drain the boiler quickly to a set level and keep water away from the smelt. Tube welds in the lower furnace are examined thoroughly, and recovery boiler repairs are planned and inspected with more care than almost any other boiler work. When Kiera and Dale visit, the mill's inspector is examining the welds of the floor tubes after the first months of service; Dale shows her the welding procedure, the radiographs and the hydrostatic test record from the spring, all of which the mill keeps for the life of the boiler.

Waste heat boilers in refineries and plants

Many process plants have boilers that cool a hot process gas and make steam at the same time. A sulphur recovery unit in a refinery or gas plant burns hydrogen sulphide to make elemental sulphur, and the hot gas from its burner passes through a fire-tube waste heat boiler. A fluid catalytic cracker's regenerator gas may pass through a CO boiler. Ammonia, hydrogen and other chemical plants have process gas boilers and steam generators. These units are often fire-tube boilers on their sides, with the process gas through the tubes, and many run at temperatures and with gases that attack the tube sheet and the tube inlets. Ceramic ferrules, short ceramic sleeves pushed into the tube inlets, protect the hot tube sheet from the gas. Boilermakers replace ferrules and refractory at every turnaround and retube these boilers when the tubes fail.

Working inside an HRSG

An HRSG is a confined space with special features. Its tube banks are close together, often less than 300 mm (12 in) apart, so access is through the gaps between modules or through doors in the casing, and the scaffold is built in the narrow lanes. The finned tubes have sharp fin edges that cut gloves and coveralls, and the fins hold fine deposits that become airborne dust when disturbed, so the crew wears respiratory protection while cleaning. The casing is insulated on the inside and lined with thin sheet, which must not be walked on or punctured. Because the HRSG is connected to a gas turbine and its exhaust duct, the isolation covers the turbine and any duct burner fuel, and often a blanking plate or a closed and locked inlet damper. At Northgate, Imran Qureshi, Ironbridge's safety advisor, sets up the rescue plan before the first entry: a winch and tripod at the top access door, a second exit through the lower door at the stack end, and an attendant at each door while anyone is inside. Kiera learns to count the tube rows from the doors so that she can tell the attendant exactly where she is.