Double Combustion, as the name implies, splits the heat release into two separate reaction furnaces with cooling between.
In the first reaction furnace, all amine gas, sour water stripper gas and all the air (if required) are fed to the SURE™ burner together with the supplied oxygen, the level of which depends on plant throughput. The tip-mix burner allows for thorough mixing, giving excellent contaminant destruction efficiencies. Only oxygen is split between the first and second reactors.
There is no sulphur condenser between the first waste heat boiler (WHB), and the second reaction furnace. Also, there is no burner in the second reaction furnace. By design, the gases exiting the first waste heat boiler and entering the second reaction furnace are substantially above the auto-ignition temperature of hydrogen sulphide and sulphur vapour, under all normal and turn-down operation conditions. This system allows for low-pressure drop, which is easy to control and easy to install.
The result of this type of control is a temperature profile ideally suited to the Claus process.
Operating temperatures in the first reaction furnace are high enough to destroy ammonia and hydrocarbons, but remain well below refractory limitations.
The Double Combustion process illustrated here has been operating since 1990 at KOA Oil, Japan.
A novel approach has used the benefits of a multi-pass Waste Heat Boiler (WHB) for plants with restricted plot space. The zone between the first and second passes of the boiler is utilised as the second reaction furnace of the Double Combustion process.
In this situation, lances are installed in the channel head connecting the first and second pass of the WHB tube sheets (where the remaining oxygen can be added). For the optimum design and location of the SURE™ burner and oxygen lances, BOC uses a validated CFD model.
This particular approach has been operational at API Falconara, Italy, since 1996 and at Shell, Puget Sounds, General Chemicals Anacortes and Exxon Chalmette with 100% oxygen since 2000.
The change-out of the WHB can improve energy efficiency at a plant through the generation of valuable high-pressure steam.
Other energy efficiency benefits arise from the much-reduced process gas flow through the plant. This reduces the converter reheat and incinerator fuel gas requirements to a minimum. Reduced energy requirements significantly cut CO2 releases.