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and upto 30% higher for older units (Nitrex,1989). The SEC of modern natural gas steam reforming is 27 to 28 GJ/tonne (Appl,1994). Although, ammonia is used directly as a fertilizer in the U.S., most of the ammonia is converted to other compounds to be used as fertilizer. We give a short description of the main processes for fertilizer production. Urea is produced in two steps by the reaction of NH3 and CO2. The CO2 is produced in the NH3 synthesis. In the first step, carbamate (NH2CO2NH4) is synthesized. Feedstock is provided in an NH3/CO2 ratio varying from 2.5 to 3.5 to achieve a high conversion rate. In the second step, the carbamate is dehydrated to urea. The reaction is not complete. Therefore NH3 and CO2 are both stripped from the urea solution and recycled. Commercial processes differ in the dehydration step. The main processes are solution recycle (NH3 and CO2 are recycled to the synthesis reactor as an aqueous solution) and stripping process (the non-reacted carbamate is removed by partial pressure reduction, using one of the reactants). Several techniques are used to dry the urea solution. Normally the solution is treated in a prilling tower, to produce granulate. In Europe the urea plants consume between 3.2 and 4.6 GJ/tonne of primary energy (Worrell and Blok,1994). The CO2 is extracted from the NH3-process, so no energy is consumed. However in some NH3- processes there is a CO2 deficiency (depending on the production volumes of NH3 and urea) which is met by extra CO2 production (which consumes energy and emits carbon dioxide) in a separate plant. Nitric Acid (NA) is used mainly for the production of ammonium nitrate (NH4NO3). HNO3 is also used to produce non-fertilizer products. NH3 is burned over catalysts to produce nitrous oxides. The NOx are passed to an absorber column to produce HNO3 (reaction with water). The total process is highly exothermic, so waste heat boilers are installed to generate superheated high pressure steam. Two main types of processes can be identified: mono-pressure (oxidation and absorption pressures are the same) and dual-pressure (absorption pressure is higher than oxidation pressure). For both types a large number of different processes (with different operating pressures) have been developed. New designs have been developed which have a lower steam consumption (due to the incorporation of expansion turbines) or which have increased heat recovery. Ammonium nitrate (AN) is produced by the neutralization of HNO3 with NH3, in an exothermic reaction. The released heat can be used in the process internally (to evaporate the water of the HNO3 solution, or to preheat the HNO3, or evaporate the liquid NH3) and can produce steam, which can be exported. Whereas older plants work at atmospheric pressures (which require the import of steam), most modern processes work at elevated pressures (and export low pressure steam). Ammonium sulfate (AS) is produced as a byproduct of nylon 6,6 manufacture. In the synthesis of caprolactam ammonia is added to control the reaction. As much as three to five times as much ammonium sulfate may be produced than caprolactam (Lipinsky and Ingham,1994). After separation of the ammonium sulfate solution, the water is evaporated to produce a marketable product. 4.2 Characterization of the U.S. Ammonia Industry In the U.S. ammonia is produced in 41 plants (1996) which predominantly use natural gas as a feedstock, as can be seen from energy statistics (EIA,1997). Natural gas is the favorable feedstock for ammonia, as it has the highest hydrogen to carbon ratio, leading to reduced energy consumption, and carbon dioxide emissions. Natural gas based plants also have lower capital costs, although ammonia manufacture remains capital intensive. The capital costs for a new greenfield plant are estimated at roughly $300 per tonne annual capacity (Worrell and Blok,1994). Although currently available data makes it impossible to calculate the average age of 18 1PDF Image | Energy use and energy intensity
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