All Text from: Environmental Protection Agency (2016). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2014.
Emissions of carbon dioxide (CO2) occur during the production of synthetic ammonia, primarily through the use of natural gas, petroleum coke, or naphtha as a feedstock. The natural gas-, naphtha-, and petroleum coke-based processes produce CO2 and hydrogen (H2), the latter of which is used in the production of ammonia. The brine electrolysis process for production of ammonia does not lead to process-based CO2 emissions. Emissions from fuels consumed for energy purposes during the production of ammonia are accounted for in the Energy chapter.
In the United States, the majority of ammonia is produced using a natural gas feedstock; however one synthetic ammonia production plant located in Kansas is producing ammonia from petroleum coke feedstock. In some U.S. plants, some of the CO2 produced by the process is captured and used to produce urea rather than being emitted to the atmosphere. There are approximately 13 companies operating 26 ammonia producing facilities in 17 states. More than 56 percent of domestic ammonia production capacity is concentrated in the states of Louisiana (29 percent), Oklahoma (21 percent), and Texas (6 percent) (USGS 2015). There are five principal process steps in synthetic ammonia production from natural gas feedstock. The primary reforming step converts methane (CH4) to CO2, carbon monoxide (CO), and H2 in the presence of a catalyst. Only 30 to 40 percent of the CH4 feedstock to the primary reformer is converted to CO and CO2 in this step of the process. The secondary reforming step converts the remaining CH4 feedstock to CO and CO2. The CO in the process gas from the secondary reforming step (representing approximately 15 percent of the process gas) is converted to CO2 in the presence of a catalyst, water, and air in the shift conversion step. Carbon dioxide is removed from the process gas by the shift conversion process, and the hydrogen gas is combined with the nitrogen (N2) gas in the process gas during the ammonia synthesis step to produce ammonia. The CO2 is included in a waste gas stream with other process impurities and is absorbed by a scrubber solution. In regenerating the scrubber solution, CO2 is released from the solution. The conversion process for conventional steam reforming of CH4, including the primary and secondary reforming and the shift conversion processes, is approximately as follows:
0.88CH4 + 1.26Air + 1.24H2O -> 0.88CO2 + N2 + 3H2
N2 + 3H2 -> 2NH3
To produce synthetic ammonia from petroleum coke, the petroleum coke is gasified and converted to CO2 and H2. These gases are separated, and the H2 is used as a feedstock to the ammonia production process, where it is reacted with N2 to form ammonia. Not all of the CO2 produced during the production of ammonia is emitted directly to the atmosphere. Some of the ammonia and some of the CO2 produced by the synthetic ammonia process are used as raw materials in the production of urea [CO(NH2)2], which has a variety of agricultural and industrial applications.
The chemical reaction that produces urea is:
2NH3 + CO2 -> NH2COONH4 -> CO(NH2)2 + H2O
Only the CO2 emitted directly to the atmosphere from the synthetic ammonia production process is accounted for in determining emissions from ammonia production. The CO2 that is captured during the ammonia production process and used to produce urea does not contribute to the CO2 emission estimates for ammonia production presented in this section. Instead, CO2 emissions resulting from the consumption of urea are attributed to the urea consumption or urea application source category (under the assumption that the carbon stored in the urea during its manufacture is released into the environment during its consumption or application). Emissions of CO2 resulting from agricultural applications of urea are accounted for in the Cropland Remaining Cropland section of the Land Use, Land-Use Change, and Forestry chapter. Emissions of CO2 resulting from non-agricultural applications of urea (e.g., use as a feedstock in chemical production processes) are accounted for in the Urea Consumption for Non-Agricultural Purposes section of this chapter.
Total emissions of CO2 from ammonia production in 2014 were 9.4 MMT CO2 Eq. (9,436 kt), and are summarized in Table 4-18 and Table 4-19. Ammonia production relies on natural gas as both a feedstock and a fuel, and as such, market fluctuations and volatility in natural gas prices affect the production of ammonia. Since 1990, emissions from ammonia production have decreased by 28 percent. Emissions in 2014 have decreased by approximately 5 percent from the 2013 levels (pp. 4-21, 4-22).
Profile of the largest emitter in the industry: CF Industries in Donaldsonville, Louisiana.