Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 22.4.4 Revamp: increase capacity and energy efficiencyDescriptionThe revamp of a 20 year old reduced primary reforming ammonia plant (1100 tonnes/day) isaimed to improve the efficiency of the primary reformer furnace/gas turbine combination byextensive preheating of the mixed feed going to the furnace and by the installation of a highlyefficient gas turbine of which the operating conditions can be adapted to suit the oxygenrequirements of the furnace. Table 2.11 gives an overview of the implemented measures.MeasureExtended preheating of thehydrocarbon/steam feedNew gas turbineModifications of the burnersRearrangement of theconvection coils and addadditional surfaceDescriptionTo reduce the fuel gas consumption, the duty of the radiant sectionwas decreased by extended preheating of the hydrocarbon/steammixture before it reaches the catalyst tubes. This was done byinstalling a new highly alloyed mixed feed preheat coil in theconvection section of the furnace. In this way, radiant heat issubstituted by heat available in the convection section at asufficiently high temperature level.A second key factor in achieving the fuel gas savings was theinstallation of a second generation gas turbine. The amount ofoxygen available in the exhaust gases from this machine matchesclosely the oxygen requirements of the primary reformer furnace. Inthis way, the flow through the radiant box of the furnace isminimised and allows the high temperatures required in the fire boxwith less fuel gas consumption.Due to the low oxygen surplus, the furnace burners have requiredmodifications to assure an adequate distribution of gas turbineexhaust over the burners. This is important to obtain completecombustion of the furnace fuel and to assure a uniform heat releasein the fire box. This last issue is critical for the lifetime of thecatalyst tubes, as local overheating of the tubes might causepremature tube failure. After revamp, the power of the new gasturbine is sufficient to drive the process air compressor, so thehelper steam turbine was eliminated. The gas turbine exhaust gasesshow temperatures of about 520 °C and all exhaust is sent throughthe radiant section to provide sufficient oxygen for combustion inthe furnace.Because of the reduced fired duty and because a larger share of thetotal heat is absorbed in the reforming process (mixed feed preheatcoil + radiant tubes), a lower heat load is available for the rest ofthe convection section.For this reason, all convection coils in the heat recovery sectionwere rechecked for the revamped process requirements and, wherenecessary, additional surface was added.The rearrangement of the convection coils which is an integral partof the proposed furnace revamp, also serves another objective: theheat conservation is further optimised by reducing the stack losses.MaintenanceAbout 50 % of the efficiency increase is achieved by re-establishingthe original state of the plant, e.g. closing leaks.Table 2.11: Some implemented measures to revamp the 20 year old plant64 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers