126.96.36.199 Fertilizer manufacture
Swaminathan and Sukalac (2004) report that the fertilizer industry uses about 1.2% of world energy consumption and is responsible for about the same share of global GHG emissions. More than 90% of this energy is used in the production of ammonia (NH3). However, as the result of energy efficiency improvements, modern ammonia plants are designed to use about half the energy per tonne of product than those designed in 1960s, (see Figure 7.2), with design energy consumption dropping from over 60 GJ/t NH3 in the 1960s to 28 GJ/t NH3 in the latest design plants, approaching the thermodynamic limit of about 19 GJ/t NH3, and limiting scope for further efficiency increases. Benchmarking data indicate that the best-in-class performance of operating plants ranges from 28.0 to 29.3 GJ/t NH3 (Chaudhary, 2001; PSI, 2004).
Figure 7.2: Design energy consumption trends in world ammonia plants
Sources: Chaudhary, 2001; PSI, 2004.
The newest plants tend to have the best energy performance, and many of them are located in developing countries, which now account for 57% of nitrogen fertilizer production (IFA, 2004). Individual differences in energy performance are mostly determined by feedstock (natural gas compared with heavier hydrocarbons) and the age and size of the ammonia plant (PSI, 2004, Phylipsen et al., 2002). National and regional averages are strongly influenced by whether the sector has undergone restructuring, which tends to drive less efficient producers out of the market (Sukalac, 2005). Ammonia plants that use natural gas as a feed-stock have an energy efficiency advantage over plants that use heavier feedstock’s and a high percentage of global ammonia capacity already is based on natural gas. China is an exception in that 67% of its ammonia production is based on coal (CESP, 2004) and small-scale plants account for 90% of the coal-based production. The average energy intensity of Chinese coal-based production is about 53 GJ/t, compared with a global average of 41.4 GJ/t (Giehlen, 2006).
Retrofit of old plants is feasible and offers a potential for improved efficiency. Verduijn and de Wit (2001) concluded that the energy efficiency of large single train ammonia plants, the bulk of existing capacity, could be improved at reasonable cost to levels approaching newly designed plants, provided that the upgrading is accompanied by an increase in capacity. Significant reductions of CO2 emissions, below those achieved by state-of-the-art ammonia plants, could be achieved by using low-carbon or carbon-free hydrogen, which could be obtained through the application of CCS technology (see Section 7.3.7), biomass gasification, or electrolysis of water using electricity from nuclear or renewables. About half the ammonia produced for fertilizer is reacted with CO2 to form urea (UNIDO and IFDC, 1998), but the CO2 is released when the fertilizer is applied. However, this use of CO2 reduces the potential for applying CCS technology.