PROCESSES FOR fertilizer -ammonia production

Fertilizer production involves chemical reactions, sometimes at high temperature and/or pressure,using natural raw materials. The transformation of these raw materials inevitably leads to process byproducts,emissions and wastes which can be solid, liquid or gaseous. The uncontrolled release of some of these substances could harm the environment. To avoid this and also to maximize process efficiency, a wide range of processes and waste recovery and abatement techniques is available. This chapter provides a general description of the main fertilizer production processes1, together with the best available techniques (BAT)2 for achieving emission limits and/or minimizing emissions to the environment. Typical plant inputs, outputs and achievable emission levels are given for the most significant types of emission, and for both new and existing plants3.

The products dealt with in this chapter are ammonia, nitric acid, urea, urea ammonium nitrate, ammonium

nitrate and calcium ammonium nitrate, sulphuric acid, phosphoric acid, single and triple superphosphate

and multi-nutrient fertilizers.

Ì 3.1 AMMONIA

More than 99% of world nitrogen fertilizer production is based on ammonia (NH3). Ammonia is basically produced from water, air and energy. The source of energy is normally coal or hydrocarbons which are reacted with water at high temperature and electricity to drive the compressors. Natural gas is generally the preferred hydrocarbon: some 77% of world ammonia production capacity is currently based on natural gas.

A modern ammonia plant has a typical capacity of 1000-2000 t/d, although new plants are now being commonly designed up to 3000 t/d. The process and energy systems are integrated to maximize energy efficiency, in such a way that compressors are driven by recuperation steam turbines and that surplus steam and/or energy, together with by-product carbon dioxide (CO2) may be exported. However, ammonia 

1 Potash and phosphate mining and their refining and beneficiation are not included.

2 BAT represent standards discussed within the EU, as a guide in setting emission limits. They include some

flexibility to permit local conditions to be taken into account. They should be relevant for most, if not all,

industrialized countries, but local conditions are very variable, especially in Central Europe, the FSU and the

developing countries. BAT generally incorporate the latest progress in plant processes and operation, without pre-determining any specific technology or methods of construction and operation. BAT techniques must be both technically and economically feasible.

3 Fugitive emissions, emissions due to rainwater, heat emissions, noise and visual impacts are not covered.

plants are commonly integrated with other plants, particularly with urea plants which make use of the

CO2. Ammonia plant battery limits generally include:

• – feedstock and fuel supply by pipelines at sufficient pressure for reforming,
• – untreated water and air,
• – ammonia product stored as liquid, either refrigerated at atmospheric pressure or non-refrigerated at ambient temperature and medium pressure.

Three main types of process are currently used for ammonia production
– steam reforming of natural gas or other light hydrocarbons (natural gas liquids, liquefied petroleum
gas, naphtha),– partial oxidation of heavy fuel oil or vacuum residue,– coal gasification.
.1.1 Processes
Steam Reforming of Natural Gas
About 85% of world ammonia production is based on steam reforming concepts.
þ Figure 3.1 Block diagram of steam reforming of natural gas

The reactions for ammonia synthesis gas production from natural gas are mainly:
– the reforming reaction: CH4 + H2O ———> CO + 3 H2 (endothermic) (1)
– the shift conversion reaction: CO + H2O ———> CO2 + H2 (exothermic) (2)
Reaction (1) takes place mainly in the primary and secondary reformer while reaction (2) takes place in
both reformers but mainly in shift conversion reactors (see fig. 3.1).
Globally approximate formulae for ammonia synthesis from natural gas are as follows:
(1) 0.88CH4 + 1.26 air + 1.24H2O ———> 0.88CO2 + N2 + 3H2
(2) N2 + 3H2 ————> 2NH3
The first reaction, producing synthesis gas, normally takes place at 25-45 bar pressure, whilst the ammonia
synthesis pressure ranges from 100-250 bar.

Natural gas contains sulphur compounds which would poison most of the process catalysts. Consequently,

these compounds are initially removed from the feed-gas by heating it to 350-400°C and then hydrogenating

the sulphur compounds to H2S in a desulphurisation vessel, typically using a cobalt molybdenum catalyst.The H2S is adsorbed on pelletized zinc oxide, forming zinc sulphide. The hydrogen is normally provided by the synthesis section, and the zinc sulphide remains in the adsorption bed. The resulting gas feed contains less than 0.1 ppm S and is then mixed with steam. This mixture is heated to 500-600°C and introduced to the primary reformer also called tubular reformer or fired reformer (or in some cases to an adiabatic pre-reformer preceding the primary reformer).

The primary reformer consists of a radiant box containing the process tubes. These contain nickel catalyst and are externally heated by fuel gas. The combustion of this fuel gas provides the heat necessary for reaction (1). The amount of sulphur in natural gas is usually small enough to avoid the need for desulphurisation of the fuel-gas, but if this is necessary to meet local emission standards, the plant’sentire gas supply is desulphurised.

The flue gas leaving the radiant box has a temperature of about 1000°C. Only 50-60% of the fuel’s heat value is directly used in the process itself, but the enthalpy of the flue gas is used in the reformer convection section, for steam generation and other process requirements. In the usual reforming process,the consumption of fuel-gas energy is 40-50% of that of feed-gas energy. The flue gas leaves the convection section at 100-200°C and is the main source of emissions from the plant. They contain mainly CO2 and small amounts of NOx, CO and eventuallt SO2.

Only 40-50% of the feed-gas is reformed in the primary reformer according to reaction (1). To complete the

reforming reaction, the temperature is raised in a secondary reformer by internal combustion of part of the gas with the process air. Since the latter also provides the nitrogen for the synthesis gas, the extent of primary reforming is adjusted so that the air supplied to the secondary reformer meets both the heat balance and the stoichiometric synthesis gas requirement. The gas/air mixture is then passed over a nickel-containing catalyst which effects the remainder of the reaction (1): on emerging from the secondary reformer at a temperature of around 1000°C, about 99% of the original feed-gas has been converted. The process gas is then cooled to 350-400°C in a waste heat steam boiler or boiler/superheater.

This process gas contains 12-15% CO (dry gas basis). Most of this CO is converted to CO2 according to

reaction (2) firstly by passing the process gas through a bed of iron oxide/chromium oxide catalyst at around 400°C and then over a copper oxide/zinc oxide catalyst at about 200-220°C. The residual CO content of the gas is 0.2-0.4%. At this point, the gas consists mainly of H2, N2, CO2 and the excess process steam. Most of this steam is condensed by cooling the gas before it enters the CO2 removal system. The condensate normally contains 1500-2000 ppm of ammonia and 800-1200 ppm of methanol, as well as some CO2 and catalyst metals. These impurities should be stripped out and/or recycled.

In older plants, the process condensate is simply stripped in a column into which low-pressure steam is fed in at the bottom. A mixture of steam and gases is vented to the atmosphere, whilst the stripped condensate is cooled and discharged as effluent. Although this reduces the ammonia content of the condensate, it does not solve the environmental problem of the effluents and emissions. Two main solutions are available: – use of ion-exchange resins, passing the condensate through a strongly acid cation exchanger and then through a weakly basic anion-exchanger, and possibly through a mixed bed. This can enable reuse of the condensate as boiler feed water; – use of the condensate for natural gas saturation, thus reducing steam consumption and avoiding the need to treat the condensate. However, this requires special equipment and additional circuitry.Since ammonia plants are usually associated with nitrogen fertilizer plants, the ammonia process condensate can be mixed with the waste water of the fertilizer plants and treated for the recovery of useful substances. Nitric acid can be used to regenerate ion-exchange resins, and this can result in 20-28% ammonium nitrate solutions.

After the CO-CO2 shift conversion, the process gas usually contains about 18% CO2 which is then removed in a chemical or physical absorption process. The solvents used in chemical processes are mainly aqueous amine solutions or hot potassium carbonate solutions. Physical solvents include glycol dimethylethers and propylene carbonate. The following processes are currently recommended:

– AMDEA standard 2-stage process, or similar (amine solutions);

– GIAMMARCO-VETROVOKE process or BENFIELD process (HiPure, LoHeat), or similar (hot activatedpotassium carbonate solution);
– Selexol or similar (physical absorption processes);
– Pressure swing adsorption (this process is not used for large ammonia plants but for small hydrogen
production).

Absorption is followed by desorption through a pressure decrease and temperature increase. The resulting pure CO2 (1.3-1.4 t per t of NH3) is used for the manufacture of urea, dry ice, or in other applications.At this point, only small amounts of CO and CO2 still remain in the synthesis gas, but they must beremoved, because they would poison the ammonia synthesis catalyst. This removal is achieved by reaction of the CO and CO2 with a little amount of hydrogen out of the gas mixture and converting the mixture to methane (CH4) and water by passing it through a reactor filled with a nickel-containing catalyst at a temperature of around 300°C. The methane is an inert gas in the synthesis reaction, but the water must be removed; and this is achieved firstly by cooling and condensation and subsequently by condensation/absorption in the product ammonia.

The synthesis gas must then be compressed, since ammonia synthesis takes place under pressures which are normally in the range of 100-250 bar. Modern plants use centrifugal compressors, usually driven by steam turbines with the steam from the ammonia plant. The reaction uses an iron catalyst at temperatures of 350-550°C and is exothermic. Consequently, extensive heat exchange is required. Moreover, only 20-30% of the synthesis gas is converted with each pass through the converter, thus necessitating a loop arrangement, with the ammonia being separated off by cooling/condensation and fresh synthesis gas making up the difference in the loop. Inert gases left in the synthesis gas are taken out in a purge stream which is designed to leave the level of inerts in the loop at about 10-15%. The purge gas is scrubbed with water to remove ammonia and is then used as fuel or sent for hydrogen recovery.

The cooling and condensation of the product ammonia is achieved thanks to an auxiliary frigorific cycle whose refrigeration compressor is normally driven by a steam turbine. If cooling is with water or air,condensation is largely incomplete. Consequently, vaporized ammonia is used as a refrigerant in most plants, in order to achieve satisfactorily low ammonia concentrations in the gas recycled to the converter.

The liquefied product ammonia is either used directly in downstream plants or stored in tanks. These
tanks are of three types:
– fully refrigerated, with a typical capacity of 10000 - 30000 t,
– pressurized storage spheres up to about 1700 t,
– semi-refrigerated tanks.
Steam reforming ammonia processes offer sufficient waste heat for steam production and electricity, but
usually it is more convenient to export small amounts of steam and import electricity.
Future developments are expected to increase efficiency and decrease emissions by:
– lowering the steam to carbon ratio,
– increasing the conversion share of the secondary reformer,
– improving the purification of the synthesis gas,
– increasing the efficiency of the synthesis loop,
– improving the power energy system.
Recent improvements tending in these directions include:
– excess air secondary reforming,
– O2 enriched air to the secondary reformer,
– heat exchange autothermal reforming.
Excess Air Secondary Reforming
The low marginal efficiency of the primary reformer has prompted the design of processes which shift
more of the reaction on to the secondary reformer. These processes feature:

• – Decreased firing in the primary reformer, lowering the outlet temperature to about 700°C. This increases the firing efficiency, reduces the size and cost of the primary reformer, and prolongs the life of the catalyst, the catalyst tube and the outlet header;
• – Increased process air supply to the secondary reformer. This increases the internal firing to achieve the same degree of total reforming. It also necessitates increased compression capacity;
• – Cryogenic final purification. This removes all the methane and excess nitrogen from the synthesis gas, as well as some argon. With almost no impurities in the synthesis gas, higher ammonia conversion rates can be achieved with a lower purge flow. Since cooling is produced by depressurization, no external coolant is needed. The net result is a significantly more efficient process.

Heat Exchange Autothermal Reforming

Reformer exit gases are at such high levels that it is thermodynamically wasteful to use them solely to raise steam. This heat can be recycled to the process itself by using the heat content of the secondary reformed gas in a primary reformer based on heat exchange, thus eliminating the fired furnace. In this case, surplus air, or oxygen-enriched air, is needed in the secondary reformer to meet the heat balance-requirements. The elimination of flue-gas from the primary reformer obviously reduces atmospheric emissions. For example, compared to conventional steam reforming, NOx emissions may be reduced by 50% or more.

Partial Oxidation of Hydrocarbons or Coal by Steam or Ain-Oxygen Mixtres

Partial oxidation processes using heavy fuel oil, residual oils or coal offer an alternative path for ammonia

production, which depends on the relative availability and/or cost of these various feedstocks and

oxygen in relation to investment costs and other factors such as the environmental need to use waste

materials. In this respect, extremely viscous hydrocarbons and plastic wastes can be used as part of the

feed. Typical processes using oil and coal are shown in figures 3.2 and 3.3. There are several variations.

When the feedstock is heavy fuel oil with high sulphur content or coal, partial oxidation gasification is

non-catalytic and occurs at high pressure and high temperature. Pressures exceed 50 bar, and temperatures

are around 1400°C. Some steam is added to moderate the temperature. A preliminary air separation unit is
needed to supply oxygen and nitrogen. Typical processes, using oil and coal, are shown in figs. 3.2 and

The reaction of the hydrocarbon with oxygen produces CO and hydrogen, as well as some CO2, CH4 and
soot. Soot removal and recycling is difficult. Sulphur compounds in the feed are converted to H2S. This
is then separated from the process gas, using a selective absorption agent, and is then converted to
elemental sulphur or sulphuric acid in a separate unit. The CO is converted to CO2 by passing through
two high temperature catalyst beds with intermediate cooling. The CO2 is then removed by an absorption
agent, which could be the same as in the sulphur removal. After this, residual traces of the absorption
agent and CO2 are removed, before final purification of the gas by a liquid nitrogen wash. This results in
a virtually pure synthesis gas, to which some nitrogen is added to provide the stoichiometric H/N ratio
for ammonia. This avoids the need for a purge in the ammonia synthesis loop and improves its efficiency,
compared with the steam reforming process. On the other hand, auxiliary boilers are needed, if the
compressors are steam driven, and boiler fuels result in emissions of SO2, NOx and CO2, unless electricity
is used.

 Inputs, Outputs and Emission Levels

Typical production inputs, outputs and atmospheric emission levels for modern ammonia plants, based on BAT, are shown in table 3.1. These figures are purely indicative: they are not intended to be comprehensive, and circumstances can vary considerably. The use of by-product CO2, for example, reduces the total CO2 emissions very considerably, but this depends entirely on the needs of downstream facilities. Fugitive emissions of light hydrocarbons, hydrogen, NH3, CO and CO2 can occur due to leaks from flanges, stuffing boxes, maintenance work, and catalyst handling. Moreover, temporary situations, such as plant start-up, involve the flaring of synthesis gas, and in this case NOx emissions may amount to an additional 10-20 kg/hr. Obviously, much depends on standards of operation and maintenance, as well as on the process and raw materials used.

We have not included sulphur as an output. As indicated above, sulphur compounds are present in very small quantities in natural gas (up to 5 mg S/Nm3) and are almost entirely recovered from the feed gas by a zinc oxide adsorber. In steam reforming, sulphur emissions from the fuel gas are almost negligible.

However, larger amounts of sulphur are present in heavy oils and coal. Partial oxidation processes extract up to 95% of this in a Claus sulphur recovery unit. Even so, the amount of sulphur emissions in this case is much more than in the case of natural gas. Depending on recovery equipment, up to 3 kg SO2/t NH3 could be emitted from coal- and fuel oil-based plants, compared with less than 0.01 kg in gas-based plants.

Table 3.1 clearly indicates the superiority of steam reforming over partial oxidation processes, both with
regard to inputs and emissions. It also shows the wide range of achievable performances. Excess air
reforming and autothermal reforming provide significant reductions in emissions, and autothermal
reforming can also reduce total energy consumption by necessitating (increased) power importation and
decreasing net heat loss. Energy consumption has decreased continuously in new plants over the last 25
years, and the optimal energy consumption of new plants may soon be 27-29 GJ/t NH3, the theoretical
minimum being around 25 GJ (HHV).
þ Table 3.1 Typical inputs, outputs and atmospheric emission levels in modern ammonia plants




Without treatment, emissions to water can arise from process condensates or waste gas scrubbing.
Condensates arise mainly from cooling the process gas prior to CO2 removal. About 1m3 condensate/t
NH3 is produced, containing up to 1 kg NH3 and 1 kg methanol (CH3OH). About 95% of these substances can be recovered and recycled by stripping with process steam. The stripped condensate can be further purified by ion-exchange and then re-used as boiler feedwater. Similarly, NH3 from purge and flash gases can be recovered in a closed loop. Thus, emissions to water can be almost fully avoided, although, in the non catalytic partial oxidation process, traces of soot and coal slag occur.
BAT ammonia processes do not normally produce solid wastes. Spent catalysts and molecular sieves are
removed, and their valuable metals are recovered; and, as indicated above, solid sulphur is recovered in

partial oxidation processes.

The economically achievable minimum emission levels vary between new and existing plants. Assuming steady-state production, NOx can be limited to 0.45 kg NO2/t NH3 (75 ppmv or 150 mg/Nm3) in new plants, and about twice this in existing plants. Total energy consumption can be reduced to about 32 GJ/t NH3 in new reforming plants. NH3 in waste water can be reduced to 0.1 kg/t NH3 produced. Spent catalysts can be limited to about 0.2 kg/t NH3. Environmental investment and operating costs are obviously variable from plant to plant, according to emission standards or targets, process design, integration with other facilities, raw materials, revamping requirements, etc.