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288 PJ in fuels and 13.7 PJ electricity in 1994. Consumption of energy sources for feedstock is estimated at 368 PJ (EIA,1997). The specific feedstock consumption is estimated to be 22.6 GJ/tonne ammonia (HHV). Carbon emissions can be estimated on the basis of total fuel consumption of the sector, accounting for the carbon dioxide that is used as feedstock in urea manufacture,10 and carbon dioxide recovered for other purposes. Urea production in 1994 is estimated at 8,067,000 tons (CMA,1996). On a molecular basis, 20% of the urea is carbon. The amount of carbon exported in urea is estimated at 1.46 MtC.11 There are no data available on the amount of carbon dioxide recovered for other purposes in the U.S. ammonia industry. The total carbon emission in the U.S. (1994) is estimated to be minimal 9.07 MtC.12 This is relatively low due to the high use of natural gas as the main feedstock and energy carrier (99% of fuels used). Non-CO2 Greenhouse Gas Emissions The production of nitric acid is one of the industrial emission sources of nitrous oxide (N2O), besides the production of adipic acid (for Nylon 6,6).13 In the combustion of ammonia to produce nitric acid, some nitrous oxide may be formed as an accidental co-product. Off-gas measurements at DuPont showed a wide range in emissions, varying from 2 to 9 g N2O/kg HNO3 (IPCC,1995). It is not known if the DuPont emission factors are representative for all processes used, or for the whole industry. The total emission of the U.S. fertilizer industry is estimated at 16-72 Gg N2O, equivalent to 1.4-6.1 Mt C (using a GWP of 310 for a 100 year time period (IPCC,1996)). US DOE estimates the 1994 nitrous oxide emission from nitric acid production at 47 ktonnes (EIA,1996), equivalent to 3.9 Mtonne C. At high concentrations (as with adipic acid production) the N2O emissions can be reduced by oxidation in a flame. It is not clear if this technology can also be used in the production of nitric acid. However, the wide range shows that good process control may influence the N2O emission. Future emission controls may be based on catalytic processes, but no technologies are commercially available yet. Energy Intensity and Carbon Dioxide Emissions U.S. energy statistics do not give direct information on the energy consumption for ammonia manufacture. As 80-90% of the primary energy in SIC 2873 is consumed in ammonia manufacture, the total consumption gives some rough information on the energy consumption (see above). According to a survey of the Fertilizer Institute among 77% of North American ammonia producers (incl. Canada) the energy intensity ammonia making is estimated at 39.3 GJ/tonne (HHV) of natural gas and 140 kWh per tonne of ammonia (Vroomen, 1998). This is equivalent to a primary energy consumption of 40.9 GJ/tonne (HHV) or 37.1 GJ/tonne (LHV)).14 10 Carbon dioxide is actually released to the atmosphere as soon as the urea is applied. 11 This assumes that all carbon dioxide is recovered from ammonia manufacture, and that no extra carbon dioxide is produced for urea manufacture. However, back-up carbon dioxide generators are often available to meet peak CO2 demands. In that case, the actual CO2 will be higher than the figure mentioned above. 12 The following emission factors are used for the carbon calculation: coal: 27.0 kg/GJ; natural gas: 15.3 kg/GJ, and for electricity: 50.5 kg/GJe (EIA,1996). 13 Note that the industrial emissions of nitrous oxide are relatively small compared to that of other sources, i.e. mobile sources. Also note that the application of fertilizers may lead to the emission of nitrous oxides, depending on the application method, soil type, groundwater level and fertilizer type. 14 Lipinsky and Ingham (1994) estimated the 1992 total energy consumption for ammonia manufacture at 42.2 Mbtu/ton ammonia (16.1 Mbtu/ton for energy and 26.1 Mbtu/ton for feedstock). This is equivalent to 49.1 GJ/tonne (HHV). This seems to be too high, compared to the information from the industry’s survey, and to intensities in other countries (Worrell et al.,1994b). 20 1PDF Image | Energy use and energy intensity
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