Environmental Engineering Reference
In-Depth Information
plants down to just above 30 GJ/t NH
3
. Another effi-
ciency gain came when M. W. Kellogg Co. and Krupp
Uhde began offering new designs requiring less than
28.9 GJ/t NH
3
(fig. 10.6). Kellogg's latest KRES design
saves another 1 GJ/t NH
3
, and Topsøe's heat exchange
reformer cuts the need for reformer fuel by 70% and total
natural gas consumption by 15%. The best designs now
need about 30% less energy than in the early 1970s (M.
W. Kellogg Co. 1998; Haldor Topsøe 1999). By the
year 2000 the best operating plants needed only about
27 GJ/t NH
3
, and rates of 25-26 GJ/t appeared to be
technically feasible. The lower rate would be less than
20% above the stoichiometric requirements. Typical new
plants need 30 GJ/t NH
3
with natural gas-based steam
reforming, about 20% more when using partial oxidation
and heavy fuel oil, and up to about 48 GJ/t NH
3
for
coal-based installations (Rafiqul et al. 2005).
The average worldwide energy efficiency of ammonia
synthesis, lowered by energy-intensive reforming of
heavier hydrocarbons and coal, has improved as well. Be-
fore 1955 the global mean for all ammonia plants (natu-
ral gas, oil, and coal-based) was at least 80 GJ/t NH
3
;by
1965 it had declined to just over 60 GJ/t; and by 1980
it was about 50 GJ/t. The best estimate for the year
2000 would be about 40 GJ/t NH
3
, or roughly 48
GJ/t N. Ammonia is a gas under normal pressure and
that is why urea, a fertilizer solid with the highest share
of nitrogen (45%), emerged as the dominant source of
N, particularly in all rice-growing countries. Ammonium
nitrate (35% N) is the second most popular choice. Con-
version of ammonia to urea needs 9-10 GJ/t N, which
means that the total cost of this solid fertilizer (including
prilling and bagging for distribution) is typically 55-58
GJ/t N.
Nitrogenous fertilizers remain energy-intensive, even
with huge improvements in the synthesis of ammonia,
particularly when compared with the sources of other
two macronutrients. Potassium is diffused overwhelm-
ingly as potash (KCl), whose production (nearly 25 Mt/
year by 2005) involves either conventional shaft mining
or solution extraction. The former process dominates
worldwide, costing as little as 4-5 GJ/t; the latter needs
between 15-20 GJ/t; the North American average is
about 7 GJ/t. Mining of phosphate rock (about 140 Mt
in 2005) costs no more than mining potash (4-5 GJ/t),
but its subsequent treatment with acids multiplies the en-
ergy cost. Single superphosphate (8-9% P) is produced
using H
2
SO
4
, nitrophosphate using nitric acid, triple
superphosphate (averaging about 20% P) using phos-
phoric acid, and ammonium phosphates (up to 23% P)
by reacting phosphoric acid and NH
3
. For single super-
phosphate, costs are 18-20 GJ/t P, for concentrated
superphosphate at least 25 GJ/t P, and for diammonium
phosphate 28-33 GJ/t P.
An approximate global summation of energy costs of
N, P, and K inorganic fertilizers (using averages of 55
GJ/t N, 20 GJ/t P, and 10 GJ/t K) yields about 5 EJ
in the year 2000: for comparison, Kongshaug (1998)
estimated 4.4 EJ for global use in 1996. The total was
dominated (about 90%) by the energy cost of synthetic
nitrogenous fertilizers and hence by the consumption of
natural gas, and it amounted to less than 1.5% of global
TPES in 2000. Few energy uses produce such a critical
payoff as this feedstock and fuel, particularly in the syn-
thesis of nitrogenous fertilizers. Higher yields resulting
from their application now produce an adequate food
supply for close to 40% of the world's population (Smil
2001).
