Gas as fertilizer feedstock

Natural gas is a key source of fertilizers in the form of ammonia and urea.

Ammonia and urea

Ammonia is the second largest chemical product produced in the world, behind sulfuric acid. The demand for ammonia is driven by the demand for fertilizers. Of the world’s nitrogen demand, 85% is for fertilizer primarily derived from ammonia in the form of:

• Urea
• Ammonium nitrate
• Phosphate
• Sulfate

Other uses of ammonia include fibers, resins, refrigeration, and pulp and paper industries.

Ammonia can be produced from different hydrocarbon feedstocks such as natural gas, coal, and oil. Natural gas accounts for more than 95% of ammonia tonnage. Natural gas is the preferred feedstock primarily because:

• It is intrinsically the most hydrogen rich and, therefore, contributes more hydrogen compared with other feedstocks on a unit weight basis.
• The heavier feedstocks, like coal and oil, are more complex to process; therefore, the capital costs are higher compared to natural gas.

A relatively small volume (10%) of ammonia that is produced is traded as ammonia. This is a result of the difficulty of using ammonia directly as a fertilizer. Most farmers prefer a solid fertilizer. These factors drive the producers of ammonia to either develop regional markets for ammonia or convert the ammonia to urea, a dry solid that can be stored and moved relatively easily and cheaply. For stranded gas, away from the regional markets, the integration of the ammonia and urea plants makes commercial sense. It should be noted that the production of urea requires CO₂ (in addition to ammonia), which is a byproduct of ammonia production.

In year 2000, ammonia was a 131 million metric tons per year industry. Worldwide annual growth is anticipated to be approximately 1.6 to 2%. Urea is a 107 million metric tons per year industry directly derived from ammonia. The end uses for urea are:

• Fertilizers
• Formaldehyde-urea resins
• Plastics
• Fibers

History

The first commercial ammonia plant was commissioned in the early 20th century on the basis of the fundamental research work of Haber. Bosch and his engineering team developed the ammonia-synthesis process with a promoted iron-based catalyst. Since then, there has been no fundamental change in the ammonia-synthesis reaction itself. A mixture of hydrogen and nitrogen reacts on the iron catalyst at elevated temperatures in the range of 400 to 500°C operating at pressure above 100 bar.

3H₂ + N₂ ⇄ 2NH₃

The unconverted part of the synthesis gas is recirculated (after the removal of ammonia) and supplemented with fresh synthesis gas to compensate for the amount of nitrogen and hydrogen converted to ammonia. The production of ammonia synthesis gas, consisting of pure hydrogen and nitrogen, is the largest single contributor of the production cost of ammonia. Hence, in contrast to the ammonia-synthesis section, dramatic changes have been made over the years in the technology for the generation of synthesis gas. Net energy consumption has been reduced progressively, from approximately 88 GJ/ton ammonia in the days of coke-based water-gas generators to approximately 28 GJ/ton ammonia today with the use of natural gas in a steam reforming unit.

Ammonia process

Fig. 1 shows the three principal steps in the production of ammonia from natural gas.

• 

  Fig. 1—Simplified block-flow diagram for the production of ammonia.

Syngas generation

A synthesis gas with a 3:1 final H₂:N₂ mole ratio is required for the synthesis of ammonia. This syngas is generated by steam reforming of natural gas under pressure. Sulfur compounds, if any, in the feed gas have to be removed before the reforming process. The basic reactions involved in the steam reforming of methane, which is the main constituent of natural gas, are represented by the following reactions:

CH4+H2O&rlarr2;CO+3H2and CO+H2O&rlarr2;CO2+H2. 

The required stoichiometric hydrogen-to-nitrogen ratio is achieved by introducing air into the process. It is typically done by splitting the reforming into two steps:

• Primary reforming
• Secondary reforming

In primary reforming, the natural gas is reformed with steam in furnace tubes packed with nickel catalyst. Natural gas burners in the furnace radiation box supply the intense heat needed for the endothermic reaction. The reaction is controlled to achieve only a partial conversion, leaving approximately 14% methane in the effluent gas (dry basis) at temperatures of approximately 750 to 800°C.

The effluent gas is then introduced into a secondary reformer, a refractory-lined vessel filled with nickel catalyst, in which it is mixed with a controlled amount of air introduced through a burner. This raises the temperature of the gas sufficiently to complete (as much as possible) the reforming of the residual methane without any further addition of heat. It also introduces the nitrogen needed for the synthesis of ammonia. The gas usually leaves the secondary reformer at a temperature of approximately 850 to 1,000°C, depending on the process technology.

There are several variations to the conventional syngas generation scheme defined here in an attempt to improve energy efficiency and reduce cost. These include the use of:

• Prereformers
• Heat exchange reforming
• Fully autothermal reforming

Syngas purification

This is the second key step in the ammonia production process. The syngas from the secondary reformer contains CO and CO₂, which must be removed before syngas is sent to the ammonia-synthesis section to avoid damaging the ammonia-synthesis catalyst. Reformed gas is typically purified with high and low temperature shift of CO to CO₂, CO₂ removal by solvent absorption, or methanation. There are several alternative routes for the purification of syngas, which include pressure-swing adsorption and cryogenic methods.

Ammonia synthesis

The final key step in ammonia production is ammonia synthesis. In this step, the purified syngas mixture of hydrogen and nitrogen is compressed and synthesized to produce ammonia.

Various technology licensors offer technology for the production of ammonia. Haldor Topsoe and Uhde typically use conventional two-stage reforming, primary tubular reforming followed by air-blown secondary reformer. Both use conventional magnetite catalysts for ammonia synthesis. KBR offers different technology options including conventional, KBR Advanced Ammonia Process (KAAP™), KAAP™ with purifier, and KAAPplus™.9,10 The last three use processes based on ruthenium catalyst.

Urea process

There are several process/technology options for producing urea. Fig. 2 shows a simplified block-flow diagram for the production of urea from ammonia. Only a brief outline of the generic technology options is given here. Urea (NH₂CONH₂) is produced from liquid ammonia and carbon dioxide gas through a rapid exothermic reaction that leads to the formation of an intermediate liquid product called ammonium carbamate (NH₂COONH₄). This intermediate product dehydrates into urea and water through a slow and slightly endothermic reaction. Unreacted feed components and the intermediate product are recovered to maximize the product yield by stripping, recirculation, or recycling. Vacuum evaporation is used to concentrate the urea product and remove water to create a high-weight-percentage “melt.” The melt can be used to produce either prilled or granular products.

2NH3+CO2&rlarr2;NH2COONH4.NH2COONH4&rlarr2;NH2CONH2+H2O. 

Different urea technologies use somewhat different process steps to maximize product yield and energy efficiency. Major licensors of process technologies are Snamprogetti, Stamicarbon, and Toyo Engineering Corp.

• 

  Fig. 2—Simplified block-flow diagram for urea production.

Screening criteria

The capacities of ammonia and urea plants are generally not limited by gas availability. The single-train plant capacities of currently operating ammonia plants are approximately 2000 metric tons per day (mTPD). The gas consumption for a stand-alone ammonia plant of this size is approximately 29 million Btu per metric ton of ammonia product based on the lower heating value (LHV) of the feed gas. For a feed gas with LHV of 917 Btu/scf, a 2000-mTPD ammonia plant requires approximately 63 MMscf/D of feed gas. If the entire ammonia product is converted to urea, the gas consumption will increase to approximately 36 million Btu per metric ton of ammonia product. 3500 mTPD of urea will be produced. Hence, for an ammonia/urea complex with a capacity of 2000 mTPD of ammonia, the feed-gas consumption will be approximately 79 MMscf/D. A gas field of at least 0.7 Tcf is required to support this gas consumption over a project life of 20 years. The ammonia/urea complex is typically self-sufficient in utilities, depending on the choice of the plant cooling medium.

Key considerations

Ammonia and urea have been produced in large quantities from natural gas since approximately 1950. It is a mature technology with minimal technology risk. The ammonia/urea industry is characterized as a commodity producer in a mature market. The demand for fertilizers is driven by population growth; however, economics as well as politics can drive fertilizer projects.

The quantity of gas required for a single train is small compared with LNG, GTL, and even large-scale methanol plants. The use of this option, by itself, is not suitable for large gas fields. However, for large gas fields, this could be an appropriate option in combination with other options. The production of ammonia in conjunction with other gas utilization options may offer synergies that could result in reducing the cost, as well as the product market risk, of the total project.

The combined costs of feedstock and energy for a steam-reforming plant are the principal determinant of the overall production cost of the plant. Capital cost of the plant is another significant factor that needs to be considered. The supply and demand of ammonia play a critical role in determining ammonia prices. When supplies are tight, prices rise dramatically. Fertilizers are a relatively small cost component in agriculture and cannot be avoided. The cost of ammonia production is somewhat determined by the cost of feedstock, which for the majority of the ammonia plants is natural gas. New ammonia projects tend to be cyclical, driven by product demand and positioned where feedstock prices are low.

The future trend in ammonia plants is clearly toward larger plants (capacities ranging from 3000 to 4000 mTPD) and locations with low-cost gas supplies.

Noteworthy papers in OnePetro

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See also

Gas utilization options

Monetizing stranded gas

Stranded gas

Transporting stranded gas as hydrates

PEH:Monetizing_Stranded_Gas