For the first time, ammonia has been directly synthesised from wet air at intermediate temperature. Ce0.8Gd0.2O2−δ (CGO)–(Li,Na,K)2CO3 electrolyte together with a new perovskite oxide Pr0.6Ba0.4Fe0.8Cu0.2O3−δ were used for electrochemical synthesis of ammonia. An ammonia formation rate of 1.07 × 10−6 mol s−1 m−2 was obtained at 400 °C when applied a voltage of 1.4 V, while wet air was introduced to the single chamber reactor. This is just slightly lower than the value of 1.83 × 10−6 mol s−1 m−2 when wet N2 was fed under the same experimental conditions. These values are two to three orders of magnitude higher than the reported ammonia formation rates when synthesised from N2 and H2O at ∼600 °C. The perovskite catalysts are also low cost compared to the Ru/MgO and Pt/C catalysts in previous reports.
About 80 percent of ammonia produced is used as fertilizer and is applied either directly or converted to solid fertilizers before application (e.g. urea, ammonia nitrate). The remaining 20 percent is used in a variety of industrial applications, such as in the manufacture of plastics, fibers, explosives, amines, amides and other organic nitrogen compounds (IPTS/EC, 2007 p.35). Nitric acid, urea and sodium cyanide are the main products derived from ammonia. The fertilizer industry is a major energy consumer, responsible for about 2-3 percent of global energy consumption (IPTS/EC, 2007 p.3). Production of ammonia accounts for about 80-90 percent of the energy used in the manufacture of fertilizers (IEA, 2007 p.60).
Between 2000 and 2011, global ammonia production increased by 25 percent, from 131 million tonnes ammonia to 164 million tonnes. This represents an annual increase of about 2 percent on average (USGS, 2012). Over the past 15 years the largest (absolute) growth has been in China, where overall production increased by 65 percent. Trinidad and Tobago, India and Russia have also experienced large (absolute) growths in ammonia production over the past 15 years. In the United States, the production capacity of ammonia significantly decreased over this period by about 30 percent. Figure 1 shows the historical ammonia production trends since 1996.
Figure 1: Historical ammonia production trends (USGS, 2012)
Natural gas, naphta, heavy fuel oil, coal, coke oven gas and refinery gas can be used as feedstock in ammonia production. Globally, 72 percent of ammonia is produced using steam reforming of natural gas (IEA, 2012 p.329) and is the least energy intensive option. Coal is predominantly used in China and is generally characterized by high energy intensities. The table below provides the estimated distribution of feedstock use in selected countries.
a: Based on Table 3 under benchmarks section; b: China Chemical Energy Conservation Technology Association (CCECT), 2011; c: Fertilizer Association of India (2013)
Developments in Energy Use in Ammonia Plants
Increasing feedstock and energy prices in combination with increased competitiveness have forced many ammonia producers to revamp or modernize older and inefficient plants. Most of the revamp projects that have taken place were combined with a moderate increase in capacity (IPTS/EC, 2007 p.35).
The first single-train ammonia plant had an energy use of about 45 GJ/t NH3 (Ullmann’s, 2011 p.229). An energy efficient ammonia plant is characterized by an energy use of less than 29 GJ/t NH3. Significant developments that contributed to the major reduction in energy use were improvements in the steam reforming section, such as heat recovery from the primary reformer flue-gases, the installation of a pre-reformer, increased reformer operating pressure, lower steam to carbon ratios and shifting the primary reformer duty to the secondary reformer. Improvements have also taken place in other aspects of ammonia manufacture. For example, in shift conversion with the use of improved catalysts, in CO2 removal with the use of new solvents, and in ammonia synthesis with the use of improved catalysts and improved reactor designs. More developments consist of higher efficiency turbines and compressors, improved process control and process optimization.
The applicability rate of energy efficiency improving technologies for different countries is hard to determine as it strongly depends on the level of technology currently adopted. An estimate of the share of plants to which several revamps can be applied in the European Union, the United States and in India are given in the table below (Rafiqul et al., 2005).
|Type of Energy Efficiency Improvement||Applicability Rate (share of plants in percentage)|
|Euoropean Union||United States||India|
|Reforming (large improvements)||10||15||10|
|Reforming (moderate improvements)||20||25||20|
|CO2 removal section||30||30||30|
|Low pressure ammonia synthesis||90||90||90|
|Improved process control||30||50||30|
China is the largest and most energy intensive ammonia producer in the world. In 2011, China produced 51 Mt of ammonia, accounting for 31 percent of world production. In 2010, Chinese ammonia plants had an estimated average energy intensity of 49.1 t NH3. Although the Chinese gas-based ammonia production process is amongst the most energy efficient in the world, it only accounts for 24 percent of Chinese ammonia production. In 2010, about 75 percent of the ammonia in China was produced with the coal-based process. The average energy intensity of the Chinese coal-based ammonia producing plants is 54 t NH3. Oil-based production in China accounts for only 8 percent of total production.
The energy efficiency of ammonia production depends on the feedstock type that is being used and the plant production scale (IEA, 2007 p.84). Coal-based ammonia production in China takes place primarily in small-scale plants (90%). The remaining 10 percent is produced in medium-scale plants. The energy intensity of these medium-scale coal-based plants is about 55 GJ/t NH3, while small-scale coal-based plants consume about 53 GJ/t NH3. It is not clear why small-scale plants consume slightly less energy than medium-scale ones. In China, a shift to natural gas is not foreseeable due to recent major investments in coal-based processing plants (IEA, 2009b p.19). On the contrary, higher future natural gas prices may result in a wider uptake of coal-based processes (IEA, 2007 p.84).
India is currently the second-largest ammonia producer. Ammonia production is responsible for more than 50 percent of the Indian chemical and petrochemical industry's energy use. About 10 percent of ammonia is produced with oil-based processes (see Table 3). In 2010, the average energy use for Indian ammonia production was estimated at 37.7 GJ/tonne of ammonia, while the energy use for the best available technology using natural gas was 28 GJ/tonne of ammonia. The use of oil as feedstock is responsible for half of the gap in these energy intensities (IEA, 2011 p.43). In India, gas-based ammonia-producing plants consume on average 36.5 t NH3, while naphtha-based plants consume 39 t NH3 and fuel oil-based plants 48-87 t NH3 (IEA, 2007 p.85). Further information on the historical developments and future projections regarding energy intensity and CO2 emissions of Indian ammonia plants can be found here.
A shift to natural gas along with the implementation of the best available technology would result in around 25 percent reduction in energy use. In recent years, the increase in oil prices resulted in the rapid decline of oil shares in favor of natural gas. A switch from oil-based to natural-gas based ammonia production in India may also be favored in the future with the recent discoveries of offshore natural gas reserves (IEA, 2011 p.43).