Plants are made up of four main elements, namely hydrogen (H₂),
oxygen(O₂), carbon(C) and nitrogen (N₂). Carbon, hydrogen and oxygen
are widely available as water and carbon dioxide and easily obtained from
plants from soil and air. Although nitrogen makes up 75 percent of the
atmosphere, it is in a form that unavailable to plants. Atmospheric nitrogen
is nutritionally unavailable because nitrogen molecules are held together by
strong triple bonds. In plants, nitrogen present in proteins, DNA and other
main component such as chlorophyll. It shows that nitrogen is the most
important fertilizer for the plant. To be nutritious, it must be in a fixed form
or converted into some bioavailable form, through natural or man-made
processes. Naturally, only some bacteria and their host plants can fix
atmospheric nitrogen (N₂) by converting it into ammonia.
Fertilizer is any material of natural or synthetic origin that is applied to
soils or plant tissues which normally the leaves. This is to supply one or
more plant nutrients essential to the growth of plants. It can be in the form
of solid, powder or liquids. Fertilizer helps and enhance the growth of plants
and this objective is met in two broad ways, the traditional one being
addictive that provide nutrients and second, enhancing the effectiveness of
the soil by modifying its water retention and aeration. Main micronutrients
Nitrogen (N), phosphorus (P), potassium (K) together with secondary
nutrients, calcium (Ca), magnesium (Mg), sulfur (S) and micronutrients
copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn) and
nickel (Ni) provided by the fertilizer are required by healthy plant life. A
compound containing the elements is the basis of fertilizers.
Fertilizer was used in the ancient time when the Egyptians, Romans,
Babylonians and early Germans are using minerals and or manure to
enhance the productivity of their farms. The use of wood ash as a field
treatment became widespread. The method in fertilizing the plants using
organic fertilizer such as animal matter, plant wastes from agriculture and
wastes from meat processing, peat, manure, slurry guano and sewage
sludge was then widely used around the world. Technological advances
during world war two (WW2) accelerated post-war innovation in all aspects
of agriculture, resulting in large advances in mechanization, fertilization and
pesticides. The development of chemical synthesized fertilizer started in
the 18th century and in the early decades of the 20th century, Carl Bosch
and Fritz Haber developed a process which utilized molecular nitrogen (N₂)
and methane (CH₄) gas in an economically sustainable synthesis of
ammonia (NH₃). The process was named Haber process and the ammonia
produced by this process has become the main raw material of the Ostwald
process in 1902, developed by Wilhelm Ostwald. It is a chemical process
for the production of nitric acid (HNO₃), a mainstay of the modern chemical
industry and provides the raw material for the most common type of
fertilizer production.
Classification of fertilizers is done in many ways. Fertilizer which
provides single nutrient is called as “Straight fertilizers” while fertilizer with
more nutrients is called as “Multinutrient fertilizer”. The most common
fertilizer used nowadays in agriculture is urea. Urea or carbamide is
an organic compound with the chemical formula CO (NH2) 2. The molecule
has two —NH2 groups joined by a carbonyl (C=O) functional group. Urea
serves
an
important
role
in
the metabolism of
nitrogen-containing
compounds by animals and is the main nitrogen-containing substance in
the urine of mammals. It is a colorless, odorless solid, highly soluble in
water
and
practically
non-toxic.
Dissolved
in
water,
it
is
neither acidic nor alkaline. The body uses it in many processes, the most
notable one being nitrogen excretion. Urea is widely used in fertilizers as a
convenient source of nitrogen. The advantages of urea fertilizer are as
follows; urea can be applied to soil as a solid or solution or to certain crops
as a foliar spray; urea usage involves little or no fire or explosion hazard;
urea's high analysis, 46% N, helps reduce handling, storage and
transportation costs over other dry N forms; urea manufacture releases few
pollutants to the environment; urea, when properly applied, results in crop
yield increases equal to other forms of nitrogen.
Urea is also an important raw material for the chemical industry. It
was discovered by a German, Friedrich Wöhler in 1828 that urea can be
produced from inorganic starting materials and was an important
conceptual milestone in chemistry, as it showed for the first time that a
substance previously known only as a byproduct of life could be
synthesized in the laboratory. Friedrich Wöhler’s first artificial aura was
obtained by treating silver cyanate with ammonium chloride.
(AgNCO + NH₄Cl → (NH₂)₂CO + AgCl)
It was the first time an organic compound was artificially synthesized from
inorganic starting materials, without the involvement of living organisms.
Nowadays, urea is produced on an industrial scale and used in many
purposes, mainly agriculture, chemical industry, explosive, automobile
systems, niche, laboratory, medical and analysis.
The basic process of artificial urea production was developed in 1922
and was named after its discoverers, Bosch-Meiser. In industry, urea is
produced from synthetic ammonia and carbon dioxide. The process starts
with ammonia production plant, where ammonia is produced from the raw
material, natural gas. The process is based on the Haber process, also
called the Haber-Bosch process. It is the industrial implementation of the
reaction of nitrogen gas and hydrogen gas and the main industrial
procedure to produce ammonia.
N₂ + 3H₂ → 2NH₃
In ammonia plant, ammonia production consists of steam reforming,
shift conversion, carbon dioxide removal, methanation, synthesis and
refrigeration. Hydrogen source is taken from methane and enriched by
steam reforming, where amount of medium pressure steam is mixed with
medium pressure process gas and flows through a series of catalyst and
heated in a furnace. Carbon monoxide is converted into carbon dioxide
over catalyst reaction in CO conversion and carbon dioxide later removed
from the process gas in the CO₂ removal unit. The carbon dioxide is
compressed and purified for urea production in urea plant. Passing the
methanator unit, the hydrogen gas is ready to be fed into the synthesis unit.
The source of nitrogen is taken from atmospheric air. Once compressed by
a compressor, it is called process air and fed into the secondary reformer to
aid the combustion through auto ignition. The oxygen in the compressed
process air is fully utilized, leaving nitrogen in the process gas and flows to
the downstream unit. In synthesis unit, nitrogen gas and hydrogen gas is
compressed into a high pressure gas and later introduced into ammonia
reactor, to produce ammonia with the aid of a catalyst. The ammonia gas
product is again compressed and refrigerated in refrigeration unit, before
the liquid ammonia is supplied to urea plant for urea production.
Urea is produced commercially by the dehydration of ammonium
carbomate
(NH₂COONH₄)
at
elevated
temperature
and
pressure.
Ammonium carbonate is obtained by direct reaction of ammonia with
carbon dioxide. These reactions are normally carried out simultaneously in
a high pressure reactor. In urea plant process, urea is formed by reacting
pressurized ammonia and carbon dioxide in urea reactor. This first form the
intermediate, ammonium carbomate, which then breaks down into urea and
water. Due to the reaction is relatively slow and incomplete, unreacted
feedstocks are removed from the reaction products after the reaction of
carbon dioxide stripping. The product has then undergone low pressure
recirculation, followed by evaporation process. After the evaporation
process, the product enters the granulation unit to shape the urea as
granules. Urea is mainly used as fertilizer urea and normally marketed in
solid form. In the earlier days, urea is produced as prills. The advantage of
prills is that, they can be produced more cheaply than granules and the
technique was firmly established in industrial practice long before
satisfactory urea granulation was commercialized. However, the inferiority
of urea prills to urea granules came into the picture when prills has limited
size of particles that can be produced with the desired degree of sphericity,
low crushing and impact strength, the performance of prills during bulk
storage, handling and use. For fertilizer use, granules are preferred over
prills because of their narrower particle size distribution, which is an
advantage of mechanical application. For many years, high-quality
compound
fertilizers
containing
nitrogen
co-granulated
with
other
components such as phosphates have been produced routinely since the
beginnings of the modern fertilizer industry, but on account of the low
melting point and hygroscopic nature of urea it took courage to apply the
same kind of technology to granulate urea on its own. But at the end of the
1970s, three companies, namely Nederlandse Stikstof Maatschappii, Toyo
Engineering Corporation and Stamicarbon began to develop fluidizedbed granulation. Today, urea fertilizer is commonly found and marketed as
urea granular.
In agriculture, more than 90 percent of world urea production is
destined for use as nitrogen release fertilizer. It has the highest nitrogen
content of all solid nitrogenous fertilizers in common use. Therefore, it has
the lowest transportation costs per unit of nitrogen nutrient. It makes urea
fertilizer in granules a popular choice of countries around the globe. Based
on a report by IFA Global Fertilizers and Raw Materials Supply and
Supply/Demand Balances 2013 – 2017, for the year 2013, estimated
277,486,000 metric nutrient tones per year is produced in the world, with
estimated 173,184,000 metric nutrient tones goes to nitrogen based and
the statistic is expected to increase in the year 2014.
Beside its valuable function in agriculture and specifically farming,
urea fertilizer also may pose effects and threats to the environment. This
comes from both upstream process, where the urea fertilizer is produced
and at the downstream, where the urea fertilizer is used. Urea fertilizer is
basically a naturally occurring product. It is not toxic, not radioactive and
not harmful to the environment. Major environmental impacts normally
come from ammonia/urea production plant liquid discharge, waste water
system and emission from the stack. Although most of the ammonia/urea
production complex runs the state-of-the art system, some unavoidable
incidents does exists. Natural gas is the raw material to produce ammonia
and carbon dioxide. Not all of the components are fully utilized in the
process and the excess gas is vented to flare stack and burnt while furnace
operations produce flue gas. Continuous process of both produces carbon
monoxide and other greenhouse gases which affecting global temperature
and ozone depletion. In the other hands, water treatment is crucial. Excess
ammonia and urea in waste water contributes to excess Chemical Oxygen
Demand (COD).
At the downstream the environmental impact may be caused by
nature, management system, human error and the urea fertilizer itself. In
countries where heavy rains pose danger of flood, agricultural run-off is a
major contributor to the eutrophication of fresh water bodies. For example,
in the US, about half of all the lakes are eutrophic. The nitrogen-rich
compounds found in fertilizer runoff are the primary cause of serious
oxygen depletion in many parts of oceans, especially in coastal zones,
lakes and rivers. Nitrogen-containing fertilizers can cause soil acidification
when added. This may lead to decreases in nutrient availability which may
be offset by liming. High levels of fertilizer may cause the breakdown of the
symbiotic relationships between plant roots and mycorrhizal fungi.
The biggest issue facing the use of chemical fertilizers is groundwater
contamination. Nitrogen fertilizers break down into nitrates and travel easily
through the soil. Because it is water-soluble and can remain in groundwater
for decades, the addition of more nitrogen over the years has an
accumulative effect. The NPK number for urea fertilizer is 46-0-0 which
means, urea fertilizer is basically 45 percent of ammonia and the rest is
inert. Excess application will reverse the benefits of urea fertilizer. In
addition to their providing the nutrition of plants, excess fertilizers can be
poisonous to the same plant. The excess application also contributes to
increased accumulation of nitrates in the soil. An anonymous reports
describes urea fertilizer is causing rapid growth pushes plants to grow too
fast, promotes stress to plants, destroying soil organisms, increasing pest
activities, increasing disease activities, urea breakdown into various
compounds some of which can inhibit plant growth, decreasing plant
production, decreasing nutritional values of plants to humans while
increases nutritional value to pests and the carbon in urea based fertilizers
is chemically converted to CO₂ and lost to the atmosphere. In general, the
said implications seem to be caused by urea fertilizer itself. However, in
most cases and situations, it is more related to humans, such as excess
application, awareness and knowledge limitation.
Some report claims that urea has adverse effects on seed
germination, seedling growth, and early plant growth in soil. Because there
is evidence that these adverse effects are caused largely, if not entirely, by
ammonia produced through hydrolysis of urea fertilizer by soil urease.
Successful study and experiment has proved that urea fertilizer’s adverse
effect urea fertilizer on seed germination, seedling growth, and early plant
growth in soil could be eliminated or markedly reduced by amending the
fertilizer with as little as 0.01% (wt/wt) of N-(n-butyl) thiophosphoric
triamide. While in another report describes that increased agricultural
productivity over the past 50-100 years has led to increase of atmospheric
concentration of CO₂, CH₄, N₂O and H₂O. These gases, along with
additional trace gas species are referred to as ‘greenhouse gases’ and
maybe causing an increase in global temperature and ozone depletion
(NASA 1988; Rowland 1989). As an example, methane emissions from
crop fields such as rice paddy fields are increased by the application of
ammonium-based fertilizers.
In year 2014, the world population has reached 7.2 billion people.
The statistic came together with high number of hunger and malnutrition.
As reported by the World Food Program (WFP) in the year 2014,
some 805 million people in the world do not have enough food to lead a
healthy active life. That's about one in nine people on earth. What does the
number show? It is not only a statistic, but also shows a sign of significant
increase in the needs of food supply, industrial development and services
by the population. More demand for food puts pressure on agriculture as
the main source of foods. Ammonia/urea production plant will increase the
production, hence release more greenhouse gases and more population
will contribute to increasing numbers of vehicles on the roads.
The rapidly increasing importance of urea fertilizer in world agriculture
has stimulated research to find methods of reducing the problems
associated with the use of this fertilizer. The environmental effects can be
eliminated or minimize by changing processes and procedures. Issues
related to human can be minimized by continuous broadcast of information,
awareness, knowledge and training. It may ensure correct methods are in
place when dealing with urea fertilizer.
For many years, studies on how to produce a slow-release formula of
urea fertilizer has been a continuous effort while some of it shows a
success. These formulas contain larger molecules that are coated, helping
them to break down slowly in the soil. A typical slow-release fertilizer
releases nutrients over a period of 50 days in a year, reducing the chance
of burning the plant or root system. Specially formulated inorganic fertilizers
are those that are created for a specific type of plant. As of the year 1995,
Slow- and controlled-release involve only 0.15% (562,000 tons) of the
fertilizer market. This includes the currently used urea formaldehyde. It is
used as a controlled release source of nitrogen fertilizer. Urea
formaldehyde’s rate of decomposition into CO₂ and NH₃ is determined by
the action of microbes found naturally in most soils. The activity of these
microbes, and, therefore, the rate of nitrogen release, is temperature
dependent. The other identified method is Isobutylidenediurea (IBDU).
IBDU
is
a
single
compound
with
the
formula
(CH3)2CHCH(NHC(O)NH2)2 whereas the urea-formaldehydes consist of
mixtures of the approximate formula (HOCH2NHC(O)NH)nCH2. Sulfurcoated urea or SCU is another example of slow-release fertilizer. In sulfur
coated urea, nitrogen is released via water penetration through cracks and
micropores in the coating. Once water penetrates through the coating,
nitrogen release is rapid. The particles of fertilizer may in turn be sealed
with wax to slow release further still, making microbial degradation
necessary to permit water penetration.
In another field of use, Urea solution, made with 32.5 percent high
purity urea and 67.5 percent deionized water which is called as ‘Adblue’, is
a method used to convert harmful NOx pollutants in exhaust gases from
combustion from diesel, dual fuel, and lean burn natural gas engines into
harmless nitrogen and steam. It therefore considerably reduces the
emissions of nitrogen oxides that are major source of atmospheric pollution
and that lead to smog in urban centers.
It is true that urea fertilizer has both the good and the bad. With
continuous study and effort, the disadvantages of urea fertilizer may be
brought to the minimum level. As worldwide accepted, fertilizer generated
from ammonia produced by the Haber process is estimated to be
responsible for sustaining one-third of the Earth's population and through
time, it will continue to serve its purpose.