Basics of Top fired Reformer | Radiant and Convection section | Important Reactions taking place

1. Reformer Combustion and
Convection section
Presented by:
Soumya R Pradhan

2.  Introduction
 Important terms
 Reformer design specifications
 Combustion fundamentals
 Typical burner configuration
 Effect of potassium promotion
 Troubleshooting
Course contents:
Reformer convection section
 Alarm set points
 COD
 Combustion hazards
 Methods of control
 Reformer monitoring

3.  Steam reformer represents the core operating unit of most large scale hydrogen
production facilities. Therefore, its furnace combustion safety is fundamental to
overall safe operation of these plants. They can be operated with natural gas,
refinery-off gas, naphtha and other light-hydrocarbon streams.
 Radiant section is the portion of furnace in which the heat is transferred to the
tubes, primarily by radiation
 Convection section is that portion of reformer, downstream of the furnace, where
flue gas passes over heat exchangers and heat transfer occurs via radiation and
convection.
Introduction:

4. Reformer Combustion Section

5. 1. Burner: Device for the introduction of fuel and air into a combustion chamber
at a required velocity and concentration to maintain ignition and combustion
of fuel.
2. Burner Management System(BMS): Control system dedicated to
combustion safety and operator assistance in starting, stopping and
preventing mis operation of fuel preparation and combustion equipment
3. Casing: Metal plate used to enclose the fired heater.
4. Draft: Negative pressure(vacuum) measured at any point in the furnace,
typically expressed in mm of water column.
Important terms:

6. 5. Duct: Conduit for air or flue gas flow
6. Excess air: Air supplied for combustion in excess of theoretical air. Frequently
expressed as percentage above stochiometric requirements.
7. Flame detector: Device that senses the presence or absence of flame and
provides a useable signal.
8. Flue gas: Gaseous products of combustion including excess air.
9. Furnace: Portion of the reformer where the combustion process takes place.
10.Steam reformer: Processing unit where steam reacts with hydrocarbons over a
nickel catalyst at high temperature to produce hydrogen and carbon oxides.
Important terms:

7.  Potassium as a promoter to increase the rate of carbon gasification
 (1.5-2.5%)𝐾2𝑂 present in top 45% of our active nickel catalyst
 Ensure carbon removal rate is faster than carbon formation rate
 For a supported nickel catalyst, addition of dopant would increase the surface
basicity which aids in preventing carbon formation and reducing carbon formation to
a greater extent.
 Potassium forms hydroxide species in presence of steam that prevents any carbon
build-up on the surface
Effect of potassium promotion:

8. Combustion fundamentals:
Fuel, air and an ignition source are required for combustion to occur.
Combustion reactions involving organic compounds and oxygen takes place
according to stoichiometry combustion principles.
Steam reforming reactions are overall endothermic. As a result, a fired
heater type furnace provides the necessary heat of reaction. Heat transfer is
predominantly by infrared radiation. The furnace consists of radiant tubes
filled with nickel based reforming catalysts. Therefore steam reformer is a
complex combination of catalyst reactor and heat exchanger.

9. Ideal combustion reactions that can occur in a reformer furnace include:
Combustion fundamentals:

10. • Following are the reactions:
2CO = C + CO2 1
Heat
CO + H2 = C + H2O 2
CH4 = C + 2 H2 3
Heat
C2H6 = 2C + 3H2 4
Carbon formation:

11. • Following are the reactions:
C + 2H2O = CO2 + 2H2
Regeneration:

12. Typical burner configuration:
1. Top-fired steam reformers:
 Burners are located in the furnace ceiling and the flue gas is extracted
at the bottom of the furnace
 Coffin tunnels to ensure even flow of flue gas through the furnace.
 Co-current flow of process gas and flue gas

13. Typical burner configuration:
2. Side-fired steam reformers:
 Burners are located along the entire side wall of furnace and the flue gas
exhaust is at the top of the furnace
 Based on the concept of uniform heat flux by positioning the catalyst tubes
in the centre of furnace cell
 Cross-current flow of process gas and flue gas

14. Typical burner configuration:
3. Bottom-fired steam reformers:
 Burners are located in the floor level and the flue gas is extracted
From the top
 Firing arrangement is opposite of a top-fired unit
 Co/counter-current flow of process gas and flue gas

15. Combustion hazards:
Cause Hazard
1. Flame instability:
Fuel pressure or fuel mixing insufficient
Deviation of fuel composition from design
Uneven distribution of fuel gas in furnace
Incomplete combustion
Flame impingement that can damage
tubes, refractory etc
2. Flame lift-off:
Fuel pressure too high
Reformer draft is too high
Flame is extinguished
Uncombusted fuel accumulation in the
furnace
Severe potential damage and pulsation in
the furnace
3. Back burning:
Fuel pressure drops in fuel line below the
burner design parameters
Enables the flame to travel from burner
tip into the fuel supply in the line
Potential for unintended energy release

16. Combustion hazards:
Cause Hazard
4. After burning:
Lack of combustion air or insufficient
mixing of fuel and air
Incomplete combustion of fuel near
burner.
Fuel-rich flue gas reacts with oxygen
and burns, resulting combustion in
convection section
Damage to convection equipment and
refractory materials
5. Fuel accumulation:
Accumulation of combustible gases
upon mixing with sufficient air
Energy release result in total
destruction of furnace
Severity of energy release depending
on air to fuel ratio

17. Methods of control:
• Reformer draft: The reformer draft is controlled by modulating the stack damper (or ID fan damper if
installed), the more the damper is opened the higher the draft becomes (more negative).
•Plant capacity control: The plant capacity control scheme is configured to maintain a ratio between
steam and feed and to ensure an excess of steam when changing demand capacity of the reformer.
•Ratio controller: The ratio controller maintains a ratio between steam and feed flows. The lead/lag
system ensures that on-capacity demand increases, the steam flow increases first followed by an
increase in feed flow. On-capacity demand decreases the lead/lag system ensures that the feed flow
decreases first followed by a decrease in the steam flow.
•Reformer firing: The reformer firing is controlled by modulating the fuel gas pressure or flow control
valve from the DCS. In AUTOMATIC Mode, the reformer process outlet temperature and the reformer
crossover temperature controls the firing.
•PSA purge gas: PSA purge gas and supplemental fuel gas flows, with their heating values, are used
to calculate the total main burner firing rate.

18. Methods of control:
1. Effective fire control:
 To maintain flue and process temperatures within the desired operating
range and equipment design limits. The steam reformer firing control scheme is used to
achieve appropriate operating temperatures by balancing the firing rate with the heat
demand of the steam reforming reaction.
 Typically PSA tail gas provides the majority of the firing duty requirement. An auxiliary
fuel(naphtha) gas is adjusted to make up the balance of the firing duty required to
maintain the appropriate operating temperature.
 The use of feed-forward controllers or model predictive controllers can adjust firing to
respond to changes in variables such as process feed rate, combustion air flow and
temperature and PSA tail gas flow and heating value.

19. Methods of control:
2. Over firing protection:
 Over firing, particularly at startup or during rate changes, can result in overheating and
potential damage to tubes. Alarms and/or interlocks may be used to mitigate the risk.
There are multiple parameters that may be used to set overfiring protection limits,
including:
 FGT/ROT rate of rise
 Temp diff between ROT & FGT
 Min steam flow during s/u
 Comparison of calculated energy requirements with theoretical reforming requirements
 FGT based on system mechanical limitations

20. 1. Tube skin temperature(TST):
 An optical/infrared pyrometer to be used as a device to read the tube wall temp with
emissivity set to1.0
 Increase in temp could be result of excessive firing, swing in feed/fuel, burner issue,
catalyst deactivation, improper flow distribution through the reformer
 Tube discoloration or hotspots could represent carbon formation or sulfur poisoning of
catalyst or flame impingement on the tubes
 Especially brighter areas of tubes should be watched closely to ensure they don’t exceed
their design temp
 An external inspection to ensure uniform expansion of tubes
REFORMER MONITORING:

21. 2. Excess Oxygen:
 To ensure complete combustion of fuel streams
 Incomplete combustion can lead to further ignition in case of oxygen ingress in flue gas
duct or in stack with potential of damage to equipment
 Oxygen analyzer at the exit of radiant section to measure the oxygen levels and
minimize the possible effect of air ingress
 Typical set points are 1.2-2.0%, approx. corresponding to excess air values of 10%
 Flue gas oxygen controlled though manipulation of comb air flow via FD fan damper
position and lead lag arrangement
REFORMER MONITORING:

22. 3. Draft control:
 ID fan maintains box pressure and pressure sensing device directly connected to box
 To protect personnel from exposure to hot combustion gases
 To ensure uniform flame pattern and heat distribution to tubes
4. Fuel gas pressure:
 Controlled within apt range as defined by burner design so as to avoid low pressure
flame instability or loss of flame or high pressure over firing protection
 Measurement should be made by pressure sensing device
REFORMER MONITORING:

23. Reformer Convection Section

24. • Depending on the design, the convection section can contain
numerous coils that provide a different service. The following
are typically provided and are listed in the order that the flue gas
comes into contact with them.
– Mixed Feed Preheat Coil
– Steam Superheat Coil
– Steam Generating Coil
– Air Pre-Heaters
Major Components:

25. Faults Consequences Remedies
Draft is high (to negative) Possible burner flame out caused
by flame lifting off the burner tip
Adjust ID fan damper to bring the
draft down.
Draft is low (to positive) Possible injury to personnel who
are on the reformer caused by hot
flue gases
Adjust ID fan damper to increase
the draft .
Reformer Inlet Temp too High Thermal cracking of feed gas on
the inlet system resulting in carbon
deposition on the catalyst
Remove the feed gas and steam
catalyst at elevated temperature.
Fuel gas pressure too high High reformer tube metal
temperatures
Reduce the fuel gas pressure.
Fuel gas pressure too low Poor reaction and conversion Increase the fuel gas pressure.
Consequence of deviation:

26. Faults Consequences Remedies
Steam-to-Carbon ratio
too low
Possible carbon deposition on to the
catalyst
Increase the process steam flow or
reduce feed gas flow.
Steam-to-Carbon ratio
too high
Reduction in methane slip leading to high
tube metal temperatures (TMTs)
Reduce the process steam flow or
increase the feed gas flow.
Low excess O2 in the flue
gas
Possible afterburning If reformer box draft is low, open the
ID fan damper a little. If reformer
box draft is high, open the FD fan
damper.
High excess O2 in the flue
gas
Possible high TMTs wasting fuel If reformer box draft is low, close the
FD fan damper a little. If reformer
box draft is high, close the ID fan
damper a little.
Consequence of deviation:

27. Faults Consequences Remedies
Burner flame is impinging
on the radiant tubes
Overheating of the tubes,
leading to tube failure
Remove the burner and
check that the tip it is not
partially plugged.
Burner flame is lazy and
yellow
Inefficient mode of
operation, low heat
transfer to the process
side
Check draft and excess
O2, make adjustments
accordingly
Consequence of deviation:

28. Thank you!!