Environmental Engineering Reference
In-Depth Information
Hydrogen is then combined with atmospheric nitrogen to produce anhydrous ammonia
by compressing the gases at pressures between 14 and 35 MPa and passing the mixture
over an iron catalyzer held at temperatures ranging from 400 to 500°C (Appl, 1997;
EPA, 1995).
N (gas)
+
3H (gas)
↔
2NH (gas)
[3.3]
2
2
3
The ammonia is then purified and liquefied by cooling it down under pressure. Estimations
of the energy intensity to produce anhydrous ammonia vary. According to Worrell et al. (2000),
it takes 39.3 GJ to manufacture 1 metric ton of ammonia, and Gellings and Parmenter (2004)
estimates it takes 69,530 kJ of energy and 1.25 m
3
of natural gas to produce 1 kg of nitrogen
fertilizer.
Anhydrous ammonia can be applied directly to the soil by injection; however, more
frequently it is transformed into other chemical species to improve its handling and
application. Ammonia combined with sulfate, nitrate, calcium and nitrate, and carbon dioxide
produces ammonium sulphate, ammonium nitrate, calcium ammonium nitrate, and urea,
respectively.
Urea is a solid granular compound easily dispersible on the fields. The chemical reaction
for urea production is a two-step process. In the first reaction, ammonia and carbon dioxide
are converted into ammonium carbamate:
2NH
+↔
CO
NH COONH
[3.4]
3
2
2
4
In a second reaction, the ammonium carbamate is transformed into urea:
NH COONH
↔
NH CONH
+
H O
[3.5]
2
4
2
2
2
Both of these reactions are reversible, so when urea is applied to the ground it gets converted
back to ammonia that then is transformed into nitrate ions, which are absorbable by plants.
Ammonium nitrate is the product of combining ammonia with nitric acid:
NH
+
HNO
↔
NH NO
[3.6]
3
3
4
3
A frequent method to synthesize nitric acid (HNO
3
) is by ammonia oxidation, followed by
nitric oxide oxidation and absorption (EPA, 1995). In the first step, ammonia mixed with air
at a 1:9 ratio is passed through a catalytic converter heated to a temperature between 750 and
800°C. The reaction that describes this step is as follows:
4NH
+→ +
5O
4NO
6H O
[3.7]
3
2
2
Higher temperatures favor the formation of nitric oxide while lower ones promotes the
formation of molecular nitrogen and nitrous oxide (EPA, 1995).
In a second step, nitric oxide is transformed into nitrogen dioxide (NO
2
) (and its dimer
nitrogen tetroxide, N
2
O
4
) by addition of extra oxygen to the nitric oxide, according to the fol-
lowing reaction:
[3.8]
2NO
+→
O
2NO
↔
N O
2
2
2
4
