The steam reforming of natural gas to produce syngas consists of two integrated parts: the primary and secondary reformer.
The primary reformer is a furnace type reactor, in which catalyst tubes are exposed to radiant heat from the burner flames.
The secondary reformer is an adiabatic reactor where air is added to the gas. The nitrogen is needed for the ammonia synthesis, while the oxygen is burnt releasing more heat to complete the reforming reaction.
Before introduction to the secondary reformer, the air is compressed by a compressor, driven by a gas turbine. The gas turbine exhaust gases are used as preheated combustion air for the primary reformer furnace.
As the total fuel gas consumption is an important factor in the ammonia production cost, efforts were directed to savings in fuel gas consumption.
Steam reforming of hydrocarbon feeds is the principal process for the production of hydrogen gas and hydrogen containing gas mixtures. The desulphurized feedstock is diluted with large quantities of steam, then preheated to elevated temperatures and finally contacted with nickel catalysts.
Following reactions are involved in the generation of hydrogen:
CH4 + H2O -----> CO + 3 H2 Reforming reaction
CO + H2O -----> CO2 + H2 Shift reaction
Since several decades, this process takes place inside catalyst filled tubes, which are suspended in the radiant section of a furnace.
Unfortunately, only 30 - 35 % of the heat released by the burners is effectively absorbed by the reformer tubes. The balance of the heat carried by the flue gases that leave the radiant section is recovered in various convection coils. Once the temperature is reduced sufficiently, the flue gases are discharged into the atmosphere by induced draft fans.
The heat requirements of the radiant section determine the total fuel gas consumption of the furnace , as this heat is needed at that highest temperature level.
To reduce the fuel gas consumption, the duty of the radiant section was decreased by extended preheating of the hydrocarbon/steam mixture before it reaches the catalyst tubes. This was done by installing a new highly alloyed mixed feed preheat coil in the convection section of the furnace. In this way, radiant heat is substituted by heat available in the convection section at a sufficiently high temperature level.
A second key factor in achieving the fuel gas savings was the installation of a second generation gas turbine. The amount of oxygen available in the exhaust gases from this machine matches closely the oxygen requirements of the primary reformer furnace.
In this way, the flow through the radiant box of the furnace is minimised allowing to reach the high temperatures required in the fire box with a lower fuel gas consumption.
Due to the low oxygen surplus, the furnace burners have required modifications to assure an adequate distribution of gas turbine exhaust over the burners. This is important to obtain complete combustion of the furnace fuel and to assure a uniform heat release in the fire box. This last issue is critical for the lifetime of the catalyst tubes, as local overheating of the tubes might cause premature tube failure.
Low oxygen surplus gives the benefit of reduced NOx emissions.
Fuel gas consumption of the gas turbine is decreased as well because of the high efficiency of this new machine.
Because of the reduced fired duty and because a larger share of the total heat is absorbed in the reforming process (Mixed feed preheat coil + radiant tubes) a lower heat load is available for the rest of the convection section.
For this reason, all convection coils in the heat recovery section were rechecked for the revamped process requirements and, where necessary, additional surface was added.
The rearrangement of the convection coils which is an integral part of the proposed furnace revamp, also serves another objective: the heat conservation is further optimised by reducing the stack losses.
The furnace was coupled to a gas turbine, which generates 7MW mechanical power and was delivering its exhaust gases as combustion air to the main and auxiliary burners. Gas turbine exhaust was at 425 oC and contains 17% of oxygen. Part of the gas turbine exhaust was sent to the radiant section, the balance was dumped in the convection section to recover its residual heat.
In the previous situation, the gas turbine power was not sufficient to drive the process air compressor. Therefore, a helper steam turbine was necessary to supply extra power to match the power absorbed by the process air compressor.
After revamp, the power of the new gas turbine is sufficient to drive the process air compressor, so the helper steam turbine was eliminated. The gas turbine exhaust gases are at around 520 oC and all exhaust is sent through the radiant section to provide sufficient oxygen for combustion in the furnace.