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45. Nitrogenous fertilisers (starting materials and end products)
2. Environmental impacts and protective measures
2.1 Nitric acid production
2.2 Sulphuric acid production
2.3 Ammonium nitrate production
2.4 Calcium-ammonium nitrate production
2.5 Ammonium sulphate
2.5.1 Production from coke-oven or coal gasification
2.5.2 Production from ammonia and sulphuric acid
2.5.3 As a by-product
2.5.4 Production from gypsum, ammonia and CO2
2.6 Calcium nitrate production
2.7 Production of nitrogen solutions
2.8 Ammonium chloride production
2.9 Ammonium bicarbonate
2.10 Transport, storage and bagging of solid fertilisers
3. Notes on the analysis and evaluation of environmental impacts
4. Interaction with other sectors
5. Summary assessment of environmental relevance
Nitrogenous fertilisers in the strict sense of the term include the following, which are considered in the context of this environmental brief:
- ammonium nitrate (abbreviation AN)
- calcium-ammonium nitrate (abbreviation CAN)
- ammonium sulphate (abbreviation AS)
- calcium nitrate (abbreviation CN)
- nitrogen solutions (abbreviation N solutions)
- ammonium chloride
- ammonium phosphates.
The nitrogenous fertilisers examined here are produced for agriculture in a granulated or prilled form with the exception of nitrogenous solutions, the use of which requires a system of mixing and distributor stations.
The primary products required for the manufacture of these fertilisers comprise:
- ammonia, covered by the environmental brief Nitrogenous Fertilisers (raw materials, ammonia and urea production)
- nitric acid
- sulphuric acid
- urea
- limestone.
The capacities of individual plants vary considerably; the upper limit for nitric acid, for example, is 2000 t HNO3/day, for sulphuric acid 3000 t H2SO4/day and for ammonium nitrate and calcium-ammonium nitrate 2000 t/day on one line.
2. Environmental impacts and protective measures
With the use of modern processes, environmental impacts can be confined to gaseous emissions in the overwhelming majority of cases. Any liquid emissions produced can usually be avoided by internal recycling, although in a few cases solid waste cannot be avoided, and noise emissions occur with most processes.
Figure 1 - Nitrogenous Fertiliser Production
2.1 Nitric acid production
Industrial production of nitric acid is based on the catalytic oxidation of ammonia and subsequent absorption of the nitric oxides, formed during oxidation, in water. The various processes used in industrial production differ mainly with regard to the pressure used in the burning or absorption stage and the efficiency of the heat recovery system. The acid produced for further processing into fertilisers is an aqueous solution containing up to about 60% HNO3.
· Pollutants produced and counter-measures
The process does not give rise to continuous liquid emission flows. Where liquid ammonia is used, an oily waste is produced intermittently depending on the oil content of the ammonia, which is collected and burnt in a suitable incineration plant. Gaseous emissions are the tail gas containing (NO + NO2) = NOx from the absorption column.
The higher the NO2 content, the more intense the brown colour of the waste gas, as is plain to see for miles around.
While the NOx content in older plants can be several thousand mg NO2/m3, modern facilities are designed for around 400 mg NO2/m3. There are a number of ways of removing nitrogen oxides completely, e.g. catalytic tail gas burning with hydrogen, ammonia or methane.
If neither fresh water nor seawater can be used as cooling water, blow-down water from the cooling water recycle arises which, in compliance with local provisions, cannot always be discharged directly as wastewater because of its increased salt concentration and other additives. In this case, it is conditioned in the wastewater treatment plant together with the other wastewater flows in the works. The residues must then be taken to a controlled dump or, in the case of biological wastewater purification, can be incinerated. Where fresh water is used for cooling purposes, the heat transferred to the river or lake must be taken into account; if necessary, measures are to be taken to cool it before it is discharged.
2.2 Sulphuric acid production
Today sulphuric acid is produced on an industrial scale almost exclusively using the contact process in which gases containing sulphur dioxide are channelled through a vanadium catalyst. The gases containing sulphur dioxide required as the primary product for sulphuric acid production come mainly from:
- the burning of elemental sulphur,
- roaster gases from pyrite,
- roaster gases from sulphide ores of non-ferrous metals.
A modern sulphuric acid plant can be identified by optimum use of the reaction heat in the individual process stages. Most surplus steam is used for energy production, and in some plants, the low-temperature energy produced in the acid coolers is already being utilised.
The SO3 formed in catalytic SO2 oxidation is absorbed in 98% to 99% sulphuric acid, which yields H2SO4 in a reaction with water.
· Pollutants produced and counter-measures
There are no process-specific liquid emissions if sulphuric acid is produced by sulphur oxidation.
The tail gas from sulphuric acid facilities contains SO2 and SO3.
For sulphuric acid facilities, emissions of sulphur trioxide in the waste gas, at constant gas conditions, are limited to maximum 60 mg/m3. Moreover, the emissions can be further reduced by the use of the peracidox process, a fifth tray stage (5th catalyst level) or equivalent measures.
Where roasters are installed upstream, small quantities of contaminated sulphuric acid are produced in the form of washing acid, which, if it does not contain any harmful pollutants, can be concentrated and used, for example, in a fertiliser plant. If it contains harmful pollutants from the raw materials which have not been removed by the waste gas plant upstream, the acid must be neutralised and the residue dumped.
The slag may, depending on the feedstock analysis and possibly following an intermediate stage in which elements of any value are extracted, be passed to the steel industry or dumped. The remarks made in section 2.1 apply with regard to the cooling water problem.
In Germany, pure liquid sulphur is used almost exclusively. In the rare cases in which the sulphur contains arsenic or selenium, purification is essential and filtration residues must be dumped with care. Where the dumps are in the open, it must be ensured that the sulphurous acid formed by oxidation of the sulphur in the atmosphere does not percolate into the ground water with rainwater.
2.3 Ammonium nitrate production
Along with urea, ammonium nitrate is one of the most frequently used nitrogenous fertilisers worldwide. It is mainly produced by the neutralisation of 45 - 65% nitric acid with ammonia.
Ammonium nitrate is also a by-product of the nitrophosphate process in which NP or NPK fertilisers are made by the nitric acid decomposition of crude phosphates. The neutralisation reaction yields 95 to 97% solutions of ammonium nitrate.
The solution is processed further to obtain a marketable product by granulating or, after concentrating further to 99.5%, by prilling.
· Pollutants produced and counter-measures
Where the prilling process is used, the prilling tower in the dry part of the plant can give rise to serious emission problems, as the relatively large quantities of discharged air are extremely costly to purify. In time, ammonium nitrate dust kills vegetation in the surrounding area. Such problems can be dealt with far more easily in granulation installations. Thus, this aspect should be studied in depth before any new investment is made and before any decision is taken on the process to be used.
With granulation, the process gas flows must be purified in effective wet scrubbers before they are discharged into the atmosphere. The installation should be fitted with a dust extraction system to ensure the safety of operating personnel.
Waste fumes from neutralisation and evaporation also must be scrubbed if they are to be discharged into the atmosphere as vapour. The preferred solution is the condensation of purified fumes, which yields condensates polluted with ammonium nitrate and ammonia, some of which can be used as process water for an adjoining nitric acid plant. Condensate which contains small amounts of impurities can be fed through an ion exchanger installation and reprocessed to boiler feed water.
2.4 Calcium-ammonium nitrate production
While the ammonium nitrate considered in section 2.3 has an N content of 33.5 - 34.5%, the nitrogen content of calcium-ammonium nitrate is 20.5 - 28%, and EC regulations do not permit a nitrogen content of over 28%. The nitrogen content is reduced by the addition of crushed limestone. With the exception of this addition of crushed limestone and mixing with the ammonium nitrate melt immediately before the prilling or granulation process, calcium-ammonium nitrate is made in the same way as ammonium nitrate. For this reason, the comments made in section 2.3 regarding pollutants and counter-measures apply here too but, in addition, because of the crushing plant for the lime, increased noise emissions must be anticipated. An effective dedusting unit is to be provided for the crushing process. Where there is a constant electricity supply and the plant is maintained to West European standards, continuous dust removal to less than 50 mg/m³ can be achieved.
2.5 Ammonium sulphate
In view of the popularity of more highly concentrated nitrogenous fertilisers, the consumption of ammonium sulphate with just 20.5% N is constantly declining and now, worldwide, accounts for just 6% of nitrogenous fertiliser consumption. The strong physiologically acidic effect of this fertiliser is also to blame for the decline in its use.
The main industrial-scale production methods are:
- from coke-oven or coal gasification;
- from ammonia and sulphuric acid;
- as a by-product of organic syntheses, e.g. caprolactam manufacture;
- from gypsum, either from natural deposits or as a by-product of other processes, by reaction with ammonia and carbon dioxide.
2.5.1 Production from coke-oven or coal gasification
In both dry distillation and pressure gasification, some of the nitrogen in the coal forms ammonia. This ammonia is also found in the aqueous and carbon dioxide-rich condensate produced when the gas is cooled. The gas condensate also contains tar, phenols, pyridins, hydrogen sulphide, hydrocyanic acid etc., which cause serious problems when it comes to ammonia recovery and wastewater purification. When the tar has been separated and the phenols
removed, the volatile components of the gas condensate are stripped in a column by steam injection. The fumes from the stripper are scrubbed with sulphuric acid in coking plants, and the acidic gases remaining after sulphuric acid scrubbing are either processed to sulphur in a Claus plant or converted directly to sulphuric acid in a wet catalysis installation. Fume burning could well be an option for consideration where only small quantities are produced, but this must be in line with sulphur emission regulations.
The wastewater must undergo a biological treatment as it contains various sulphur compounds, phenol and other organic compounds.
· Pollutants produced and counter-measures
The problems arising from ammonia production have already been examined in the previous section and should be the topic of a separate study - on coal. The dust needs to be removed from waste gases produced by ammonium sulphate drying before they can be discharged into the atmosphere, as otherwise they lead to overfertilization with the associated negative consequences for soil and water quality.
2.5.2 Production from ammonia and sulphuric acid
Neutralisation and crystallisation are carried out under vacuum or at atmospheric pressure. Crystallised ammonium sulphate is removed from the resulting mash in centrifuges and then dried.
· Pollutants produced and counter-measures
The fumes produced by the exothermic reaction of sulphuric acid and ammonia, in particular the ammonia in the waste gas which can cause caustic burns to man, animals and plants, may contain impurities depending on the process used, and should be fed though a scrubber before being discharged into the atmosphere.
Dedusting systems are needed to remove the dust content from drying plant waste gases before they are released into the atmosphere.
2.5.3 As a by-product
Ammonium sulphate is obtained from the liquid waste of some organic processes, e.g. the production of caprolactam or acrylonitrile which yields a dilute ammonium sulphate solution, by evaporation, crystallization, centrifuging and drying.
For information on pollutants and counter-measures, see section 2.5.2.
2.5.4 Production from gypsum, ammonia and CO2
The feedstock is finely ground natural gypsum or anhydrite, or alternatively calcium sulphate - a by-product, for example, of phosphoric acid production - which is converted with ammonia and carbon dioxide. The calcium carbonate obtained from the reaction is filtered off and the ammonium sulphate solution evaporated, crystallised and treated as described in section 2.5.3.
· Pollutants produced and counter-measures
In principle, the same factors as stated in 2.5.2 need to be considered. Where natural gypsum is used, there is the added nuisance of noise from the grinding plant. The details given in 2.4 apply with regard to the dust produced in the grinding process.
2.6 Calcium nitrate production
Ca(NO3)2 is produced either directly via the reaction of nitric acid with limestone or, alternatively, produced as a by-product of the nitrophosphate process.
In direct manufacture, limestone is dissolved in dilute nitric acid and granulated or prilled after evaporation of the dilute calcium nitrate solution.
In the nitrophosphate process, in which crude phosphate is decomposed with nitric acid, the calcium nitrate is crystallised by cooling, separated and, after appropriate treatment, granulated or prilled.
· Pollutants produced and counter-measures
In direct manufacture, the dissolution process yields gases which contain NOx and need to be extracted and absorbed, mainly to protect the health of operating personnel, although the gases are also responsible for corrosion of equipment and buildings.
Either appropriate precautions have to be taken at the design stage, or a scrubber installation is to be provided to reduce the pollutant content of the fumes produced during evaporation. Any purification stage installed after dissolving generates a moist waste which - depending on its composition - can be used in other plants or must be dumped.
Dust-laden gases must be cleaned before discharge into the atmosphere. Any washing solutions produced by these cleaning operations are to be concentrated and recirculated.
2.7 Production of nitrogen solutions
The following are used as liquid nitrogenous fertilisers:
- liquid ammonia;
- aqueous ammonia solutions (e.g. 25%);
- solutions which contain free ammonia together with either ammonium nitrate or urea, or both;
- solutions of ammonium nitrate or urea, or both.
Liquid ammonia is used directly as a fertiliser principally in the United States, where it is injected 15 - 25 cm deep into the soil with special equipment.
Where applied in this way, storage, transport and transfer equipment are the basic essentials, and the precautionary measures stated in the first section with regard to ammonia are to be observed.
These same precautionary measures are also to be taken in a somewhat diluted form for other nitrogenous solutions containing free ammonia.
The long-term implications - especially on soil microorganisms and the humus layer - should be examined for the particular soil type concerned before liquid ammonia or nitrogenous solutions containing free ammonia are used.
2.8 Ammonium chloride production
This salt, which - at 26% N - has a somewhat higher nitrogen content than ammonium sulphate, is not used alone as a nitrogenous fertiliser in Germany. Its main areas of use are China, Japan and India, principally in rice paddies as an alternative to ammonium sulphate, which decomposes into toxic sulphides where rice is attacked by fungus. The use of ammonium chloride is now on the decline as soils become overchlorinated if chloride is used for prolonged periods.
By far the largest share of ammonium chloride made for use as fertiliser is produced in solvay plants modified for soda production. After separating the sodium bicarbonate, ammonium chloride is crystallised out of the remaining solution by additional process stages, thus obviating the need for the usual ammonia recovery with its attendant yield of relatively useless calcium chloride, and instead ammonium chloride fertiliser is obtained as a by-product.
· Environmental impacts and counter-measures
As facilities of this kind yield ammonium chloride as a by-product of soda manufacture, the main measures applicable are those relating to soda works. The additional equipment required for ammonium chloride production must be fitted with efficient dedusting systems, especially for waste gases from driers.
2.9 Ammonium bicarbonate
To complete the picture, mention must also be made of this nitrogenous fertiliser, which is only produced and used in China. According to statistics, of the 11.1 million tonnes made in China in 1983, 6.4 million tonnes went to the fertiliser market in the form of ammonium bicarbonate. The reason for this one-off development lies in the rapid establishment of nitrogenous fertiliser production from 1960 on, with the creation of a large number of small facilities for ammonia production using carbon gasification. The CO2 obtained as a by-product is used directly for neutralisation of the ammonia produced.
Please refer to the section on ammonia synthesis using coal gasification for information on environmental impacts and counter-measures.
2.10 Transport, storage and bagging of solid fertilisers
Because they are water soluble, and in view of their hygroscopicity, fertilisers must be stored in bulk goods stores which are roofed and enclosed on all sides and then transferred to a bagging and transfer station in the immediate vicinity for dispatch. The delivery, removal and transfer points are to be of an as dust-tight as possible design, and - as in production plants - at critical points, where enclosure is not feasible, dust-laden waste gases must be collected and transferred to a dedusting installation.
3. Notes on the analysis and evaluation of environmental impacts
The basic regulations to be considered for this environmental brief are found, in Germany, in the 1. Allgemeinen Verwaltungsvorschrift [1st General Administrative Regulation] to the Bundes-Immissionsschutzgesetz [Federal Immission Control Act] (Technische Anleitung zur Reinhaltung der Luft [Technical Instructions on Air Quality Control] - TA-Luft) of 27.02.1986.
It is often the case in countries without firm regulations that the relevant German provisions are used when designing such facilities.
The NOx emission for new nitric acid facilities is now restricted to 0.45 mg/m3, expressed as nitrogen dioxide, and waste gases must be colourless before discharge. NOx is determined analytically by titration or photometry.
In sulphuric acid plants, sulphur trioxide emissions in waste gas, at constant gas conditions, are restricted to 60 mg/m3 maximum. The sulphur dioxide content of the tail gas is determined by the conversion level, which must be at least 99.6% in the double-contact process, with a minimum sulphur dioxide volume content of 8% in the input gas and at constant gas conditions. Furthermore, emissions are to be further reduced by the use of the peracidox process, a fifth tray stage or equivalent measures. Sulphur dioxide can be determined iodometrically, titrimetrically, gravimetrically or colorimetrically. For continuous measurement, recording analyzers are used, working on the basis of optical absorption in the infrared or ultraviolet spectral range or the electrical conductivity of the sulphur dioxide.
For fertiliser plants, dust emissions from granulation and drying installations for multinutrient fertilisers with an ammonium nitrate content of over 50% or a sulphate content of over 10% are restricted to 75 mg/m3 maximum. This category includes, for example, the following fertilisers: ammonium nitrate, calcium-ammonium nitrate and ammonium sulphate. For other fertiliser plants, the dust emission is to be kept at no more than 50 mg/m3. Operating licenses set values of 35 mg/m3 maximum for the free ammonia content of waste gases. Dust is analyzed gravimetrically with filter head equipment. Compliance with sampling technique rules is of utmost importance for the reliability of analyses and thus compliance with statutory limits. Free ammonia is determined by titration.
4. Interaction with other sectors
Today, it is frequently the case that complexes are not confined solely to the production of nitrogenous fertilisers but make NP and NPK fertilisers, too. In this case, the sulphuric acid obtained is used for phosphoric acid production. The phosphoric acid is then neutralised with ammonia to ammonium phosphates which are processed in granulation operations to DAP fertilisers or, after adding potassium salts and micronutrients as necessary, to NPK fertilisers. This sort of combined economic management is characterised by a high level of flexibility with regard to fertiliser type. Furthermore, individual plants, including any ammonia synthesis upstream, can have increased capacities and thus manufacture their products economically; finally, a complex of this kind is self-sufficient in electricity because of the extra energy provided by the sulphuric acid installation. A further possibility is that of using the Müller-Kühne process or a modern variation of it to reconvert into sulphuric acid the gypsum produced in the phosphoric acid plant, which in many instances represents a major dumping problem.
The slag from a roaster plant can be a raw material for non-ferrous metal and/or steel works.
Use of the nitrophosphate process obviates the need for sulphuric acid, in which case calcium nitrate is a by-product that can be converted to ammonium nitrate and fertiliser lime or calcium-ammonium nitrate where cheap carbon dioxide is available, e.g. from an adjoining ammonia synthesis plant.
The special variant of the solvay process for soda production practised in the Far East, of which ammonium chloride is a by-product, has already been mentioned.
For high-capacity nitrogenous fertiliser facilities, having ammonia synthesis close by is always worthwhile unless the plant enjoys an excellent transport infrastructure (e.g. ports and harbours, cf. environmental brief) and can also conclude favourable long-term supply contracts.
References are given in the relevant environmental briefs.
5. Summary assessment of environmental relevance
In nitrogenous fertiliser production facilities, the implications for the environment concern in the main gaseous waste (dust, ammonia, nitrous gases, sulphur dioxide), and noise, plus, in the case of roaster installations, process-specific by-products and residues.
Nitric acid installations can be operated such that gaseous emissions are practically colourless, i.e. NOx-free, by the use of catalytic tail gas treatment where the NOx design value is not sufficient.
In sulphuric acid plants, the officially prescribed emission values listed in section 3 are to be further reduced by the installation of a fifth tray stage, the use of the peracidox process or equivalent measures. Where roasters are installed upstream, the slag, if it cannot be further used, must be dumped, the washing acid neutralised and residues dumped if further utilisation is not possible in view of the impurities they contain.
In plants for the production of salt, prilled or granulated fertilisers, an efficient dedusting system is of prime importance. This requires the separate treatment of the individual waste gas flows in specific dedusting installations. As stated, liquid waste from gas scrubbers is returned to the process. With modern technology, the harm to the environment can be kept low in the processes described here.
On the process management side of such plants, all waste gas purification installations must be systematically monitored and maintained. In particular, regular maintenance - which includes the cleaning of machines, motors and plant - is a major determining factor in the operating efficiency of such systems. Another important factor is the timely provision of the necessary spare parts. Monitoring also includes regular analyses by an efficient laboratory so that appropriate measures can be taken promptly when values drift out of the permitted range. Works environmental safety officers should also be appointed; they should have the appropriate powers and should be responsible for the training and upgrading of personnel and for raising their awareness with regard to environmental matters.
Retention basins are also to be provided so that, if there should be any process incident resulting in an unforeseen production of wastewater, the plant does not have to be immediately shut down.
Although the dusts and gases produced are fertilizing substances, attention must be paid to compliance with prescribed emissions as, in the long-term, excessive immissions can be harmful to plant crops or trees in the surrounding area.
The affected population should be involved at the planning stage, and access to medical care must be guaranteed.
Abfallbeseitigungsgesetz, 04.03.1982.
31. Abwasser VwV Wasseraufbereitung, Kühlsysteme, 13.09.1983.
American National Standards Institute Safety Requirements for storage and handling of anhydrous ammonia, ANSI K 61.1., 1972.
1. Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz (Technische Anleitung zur Reinhaltung der Luft - TA-Luft), 27.02.1986.
44. Allgemeine Verwaltungsvorschrift über Mindestanforderungen an das Einleiten von Abwasser in Gewässer, Herstellung von mineralischen Düngemitteln außer Kali, 44. Abwasser VwV, 05.09.1984.
Arbeitsstätten-Richtlinien (ASR).
The relevant accident prevention regulations of the employers' liability insurance associations (Berufsgenossenschaften) relating to the handling of hazardous materials.
Gesetz zur Ordnung des Wasserhaushalts, Wasserhaushaltsgesetz, 16.10.1976.
Gesetz zum Schutz vor schädlichen Umwelteinwirkungen durch Luftverunreinigungen, Geräusche, Erschütterungen und ähnliche Vorgänge, Bundes-Immissionsschutzgesetz BImSchG, 04.10.85, and the associated enforcement ordinances and general administrative provisions.
Katalog wassergefährdender Stoffe, German Federal Ministry of the Interior (BMI) publication, 01.03.1985.
Merkblätter Gefährliche Arbeitsstoffe (codes of practice for hazardous materials), e.g.:
Blatt S 24 Nitrogen dioxide (Stickstoffdioxyd)
Blatt S 33 Nitrogen oxide (Stickstoffoxyd)
Blatt S 03 Nitric acid (Salpetersäure)
Blatt A 64 Ammonium nitrate (Ammoniumnitrat)
Blatt A 59 Ammonia solution (Ammoniaklösung)
etc.
Technische Anleitung zum Schutz gegen Lärm (TA-Lärm), 16.07.1968.
Technische Regeln zur Arbeitstoffverordnung TRgA 511, Ammoniumnitrat, September 1983.
TRgA 951 Ausnahmeempfehlung nach 12 Abs.2 in Verbindung mit Anhang II, Nr. 11 of the ArbStoffV für die Lagerung von Ammoniumnitrat und ammoniumnitrathaltigen Zubereitungen, October 1982.
Ullmanns Enzyclopädie der technischen Chemie, 4. Auflage.
VDI guidelines, e.g.:
VDI-2066 Staubmesssungen in strömenden Gasen, pages 1 (10.75), 2 (6.81), 4 (5.80)
VDI-2456 Messung gasförmiger Emissionen; Messen der Summe von Stickstoffmonoxyd und Stickstoffdioxyd, pages 1 + 2 (12.73).
Messen von Stickstoffmonoxyd, Infrarot-absorptionsgeräte URAS, UNOR, BECKMANN, Modell 315, page 3 (4.75).
Messen von Stickstoffdioxydgehalten, Ultraviolettabsorptionsgerät -LIMAS G, page 4 (5.76).
Analytische Bestimmung der Summe von Stickstoffmonoxyd und Stickstoffdioxid, Natriumsalicylatverfahren, page 8 (11.83).
VDI-2298 Emissionsminderung in Schwefelsäureanlagen.
Verordnung über gefährliche Arbeitsstoffe, Arbeitsstoffverordnung - Arbstoff V., 11.02.1982.
Verordnung über Arbeitsstätten, Arbeitsstättenverordnung, ArbStätt V, 01.08.1983.
