Fertilizers energy requirements

The effect that nitrogen-containing synthetic fertilizers can have on yields is illustrated in Figure 15.13. It requires a lot of energy to mine and refine synthetic fertilizers, however, and so they are expensive. For example, of the total energy required to produce corn in the United States, at least one-third is needed to produce, transport, and apply the fertilizer. Nevertheless, synthetic fertilizers are widely used, and our present food supply depends on them. 

Concentration of acid The phosphoric acid produced in several of the above-mentioned processes may, depending upon the application (currently mainly fertilizer manufacture), have to be concentrated. Fertilizer production requires acids containing between 40 and 54% diphos-phorus(V) oxide. For transportation purposes further concentration to 52 to 72% diphosphorus(V) oxide is required. The use of certain evaporation processes, e.g. submerged burner, vacuum evaporation etc., to concentrate the phosphoric acid is problematical due to the high corrosiveness of the acid, the formation of precipitates and the release of acid-containing gases (fluorine compounds and phosphoric acid mist). Furthermore, they are very energy intensive. 

Space and energy requirements as well as investment costs are frequently factors that render an otherwise perfectly feasible and desirable process uneconomical and kill a project. Sometimes they also incorrectly direct the interest toward methods which, after superficial investigation, seem to offer cheaper alternatives. In this context it should be stressed that the entire process must always be considered. For example, for the granulation of fertilizers a granulation drum may seem to be the cheaper solution in comparison with a roller compactor if only investment costs are considered this may even be true if the entire system is investigated. However, if space requirements and energy (operating) costs are compared, a different conclusion may be obtained (see Section 5.4.4). 

Table II shows the evolution of energy requirements for the production of corn in the United States between 1950 and 1975. Total energy consumption per acre including the energy for fuel, fertilizer, pesticides, herbicides, manpower, etc., but not including energy for the manufacture of the farm machinery, increased from 3.8 million Btu s per acre in 1975 to 10.51 million Btu s per acre in 1975 (a factor of 2.8). The yield per acre in turn increased from 38 bushels per acre to 86 bushels per acre or a factor of about 2.3. During this same period, the energy consumed per bushel of corn increased only 22.2%. In all cases, the energy consumption expressed as equivalent gallons of oil consumed per bushel of corn was less than 1.

Table II contains a summary of energy requirements for 1975 corn production in the United States. The individual energy components in 1975 corn production are listed in Table A-1. This table shows that the three largest sources of energy consumption are fuel, fertilizer, and irrigation. Table A-2 contains the energy content of the corn grain and associated biomass material (stover and cobs). The energy production (810,000 Btu/bu corn) is about 6.6 times the energy used in producing the corn.

Field emissions from fertilization. Solar energy requirement. Electricity. 

Raw materials. Energy use to extract or grow the raw materials. For polyester this could include the energy required to extract and refine the crude oil used to produce ethylene, one of the base molecules for polyester fiber. Or it could include the energy consumption for the production of nitrogen fertilizers used in cotton cultivation. 

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