Agriculture Reference
In-Depth Information
Urea fertilizers are produced by reacting liquid ammonia with carbon dioxide at high
pressure (European Commission, 2006; EFMA, 2000b):
2NH
3
+ CO
2
NH
2
COONH
4
CO(NH
2
)
2
+ H
2
O.
The steps of the process include (Figure 4):
•
solution synthesis where ammonia and carbon dioxide react to form ammonium
carbamate which is dehydrated to form urea with a yield on the order of 50-80%;
•
solution concentration by vacuum, crystallization, or evaporation to produce a melt;
solids formation by prilling (converting a material into a granular free-flowing form)
or granulating;
•
solids cooling and screening;
•
coating the solids; and bagging and/or bulk loading.
The carbon dioxide from urea manufacture is produced as a by-product from the
ammonia plant. The design of urea production processes has involved the separation of urea
from the other constituents, the recovery of excess NH
3
and the decomposition of carbamate
for recycling. The simplest way to decompose the carbamate to form CO
2
and NH
3
requires
the reactor effluent to be depressurized and heated. Recycling techniques were developed to
recover and recycle some of the NH
3
and CO
2
to the process. The recovery of the gases for
recycling was essential to the urea synthesis, in order to optimize raw material utilization
since recompression was too expensive, an alternative method was developed. This involved
cooling the gases and re-combining them to form a carbamate liquor which was then pumped
back into the reactor. A series of loops involving carbamate decomposers at progressively
lower pressures and carbamate condensers was used (Total Recycle Process). A consequence
of recycling the gases was that their molar ratio in the reactor increased, thereby increasing
the urea yield.
Production of urea is usually linked to an ammonia plant, where by-product CO
2
from
ammonia synthesis is used as a primary input in urea production. Atmospheric emissions are
mainly NH
3
and urea dust. Both arise from the prilling or granulation process. From the
prilling tower, emissions should range from 0.5 to 1 kg NH
3
/ton and 0.5-1.5 kg urea dust/ton.
With granulation, the granulator exit gas is scrubbed and losses can thereby limited to 0.25-
0.8 kg NH
3
/ton and 0.25-0.4 kg urea dust/ton. NH
3
also escapes from the process absorption
vents within a range of 0.2-0.75 kg/ton urea. Liquid effluents are mainly NH
3
, CO
2
and urea
in a well-managed BAT plant, emissions to water can be limited to 0.0025 kg NH
3
and 0.0005
kg urea per ton of product. From a life cycle point of view there are discrepancies among
different authors in the accounting for CO
2
consumption. Kongshaug (1998) and Kuesters and
Jenssen (1998) consider that the consumption of CO
2
derived from ammonia production
constitutes a net reduction in by-product CO
2
emissions, Davis and Haglund (1999),
considering that the CO
2
is only stored for a short time and released upon application of urea
fertilizers in the field, did not include this net credit. Data average for Greenhouse Gas
Emission Factors for urea production in Europe vary from 420.0 g kg
-1
of product to 1,848.7 g
kg
-1
(Wood & Cowie, 2004).
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