Waste Heat Boiler
• General design – The SMR effluent steam generator or waste heat boiler is
normally a large, horizontal heat exchanger. See Fig. 16 for a sketch of a typical
WHB exchanger.
The inlet to the process side of the exchanger is connected to the SMR furnace
with the refractory-line transition header. The WHB inlet is normally refractory
lined also, with high alloy inserts into each of the boiler tubes.
There is normally a single, larger, lined tube that passes through the boiler to act
as a hot gas bypass duct. In some units, this may be an actual bypass around the
WHB, rather than through it.
The hot gas bypass is equipped with a damper to control the final process outlet
temperature from the WHB. The damper is generally arranged to act like a threeway valve – restricting the flow through the boiler tubes as it opens to allow hot
gas bypassing. Sometimes, the bypass is automatically controlled, but manual
control is also used.
On the water side of the exchanger, the flow is generally by thermosiphon, so
water from the overhead steam drum is distributed to the WHB at several points
along the bottom of the shell. Mixed steam/BFW returns to the steam drum from
outlets distributed along the top of the shell. Intermittent blowdown is taken
from the bottom of the shell. The steam side of the WHB is hard to inspect
normally.
• Metal dusting – The process conditions in the WHB can result in metal dusting.
This is where the process gases cause migration of carbon to the grain boundary
metals, and the metal crystallites essentially turn to dust. It is promoted by
reducing gases (H 2 , CO, methane) in the 900–1,500
F range. While a hydrogen
plant normally has an oxidizing atmosphere, the reformer outlet is much closer
to reducing conditions. Metal dusting is seen more at low steam/carbon ratios.
The metal tubes and the hot gas bypass in a waste heat boiler are especially
susceptible to this damage. The inserts on the tube inlets and selection of
metallurgy help control this phenomenon.
Outlet to
HTSC
Refractory-lined
Inlet Channel & Pipe
Temperature
Control Damper
Mixed Phase Risers to Steam Drum
Hot SMR
Process
Outlet
Hot Internal Bypass
Duct
Boiler Tubes
- Refractory-faced Tubesheet
- Inlet Ferrules
Outlet Channel May
Also Be Refractory-lined
Boiler Water In
Boiler Water In
Intermittent Blowdown
Fig. 16 Sketch of a typical SMR furnace process outlet waste heat boiler or steam generator
Hydrogen Production and Management for Petroleum Processing
599
• General design – The SMR effluent steam generator or waste heat boiler is
normally a large, horizontal heat exchanger. See Fig. 16 for a sketch of a typical
WHB exchanger.
The inlet to the process side of the exchanger is connected to the SMR furnace
with the refractory-line transition header. The WHB inlet is normally refractory
lined also, with high alloy inserts into each of the boiler tubes.
There is normally a single, larger, lined tube that passes through the boiler to act
as a hot gas bypass duct. In some units, this may be an actual bypass around the
WHB, rather than through it.
The hot gas bypass is equipped with a damper to control the final process outlet
temperature from the WHB. The damper is generally arranged to act like a threeway valve – restricting the flow through the boiler tubes as it opens to allow hot
gas bypassing. Sometimes, the bypass is automatically controlled, but manual
control is also used.
On the water side of the exchanger, the flow is generally by thermosiphon, so
water from the overhead steam drum is distributed to the WHB at several points
along the bottom of the shell. Mixed steam/BFW returns to the steam drum from
outlets distributed along the top of the shell. Intermittent blowdown is taken
from the bottom of the shell. The steam side of the WHB is hard to inspect
normally.
• Metal dusting – The process conditions in the WHB can result in metal dusting.
This is where the process gases cause migration of carbon to the grain boundary
metals, and the metal crystallites essentially turn to dust. It is promoted by
reducing gases (H 2 , CO, methane) in the 900–1,500
F range. While a hydrogen
plant normally has an oxidizing atmosphere, the reformer outlet is much closer
to reducing conditions. Metal dusting is seen more at low steam/carbon ratios.
The metal tubes and the hot gas bypass in a waste heat boiler are especially
susceptible to this damage. The inserts on the tube inlets and selection of
metallurgy help control this phenomenon.
Outlet to
HTSC
Refractory-lined
Inlet Channel & Pipe
Temperature
Control Damper
Mixed Phase Risers to Steam Drum
Hot SMR
Process
Outlet
Hot Internal Bypass
Duct
Boiler Tubes
- Refractory-faced Tubesheet
- Inlet Ferrules
Outlet Channel May
Also Be Refractory-lined
Boiler Water In
Boiler Water In
Intermittent Blowdown
Fig. 16 Sketch of a typical SMR furnace process outlet waste heat boiler or steam generator
Hydrogen Production and Management for Petroleum Processing
599