Ammonia Production Technologies

Date Published:   01/07/2020

Competition between Ammonia technology suppliers is quite fierce. Three technology licensors — KBR (Kellogg Brown and Root), Haldor Topsøe, and ThyssenKrupp Industrial Solutions (TKIS) — currently dominate the market. Ammonia Casale, which offers an axial-radial catalyst bed design, is a market leader in revamps of existing plants.

Most of the ammonia plants recently designed by KBR utilize its Purifier process, which combines low-severity reforming in the primary reformer, a liquid N2 wash purifier downstream of the methanator to remove impurities and adjust the H2:N2 ratio, a proprietary waste-heat boiler design, a unitized chiller, and a horizontal ammonia synthesis converter.

Depending on the configuration of the plant, energy consumption can be as low as 28 GJ/m.t. Because the secondary reformer uses excess air, the primary reformer can be smaller than in conventional designs. The cryogenic purifier, which consists of an expander, condenser, feed/effluent exchanger, and rectifier column, removes impurities such as CO, CH4, and argon from the synthesis gas while adjusting the H2:N2 ratio of the makeup gas in the ammonia loop to the optimum level. The ammonia concentration exiting the low-pressure-drop horizontal converter is 20–21%, which reduces energy requirements for the recycle compressor.

The syngas generation section (or front end) of a Haldor Topsøe-designed plant is quite traditional with the exception of its proprietary side-fired reformer, which uses radiant burners to supply heat for the reforming reaction. Haldor Topsøe also offers a proprietary iron-based synthesis catalyst, radial-flow converters consisting of one, two, or three beds, and a proprietary bayonet-tube waste-heat boiler. More recent developments include the S-300 and S-350 converter designs. The S-300 converter is a three-bed radial-flow configuration with internal heat exchangers, while the S-350 design combines an S-300 converter with an S-50 single-bed design with waste-heat recovery between converters to maximize ammonia conversion.

ThyssenKrupp offers a conventional plant with a unique secondary reformer design, a proprietary waste-heat boiler, radial-flow converters, and a dual-pressure ammonia synthesis loop. Today, a production rate of 3,300 m.t./day can be achieved using the TKIS dual-pressure process.

ThyssenKrupp’s dual-pressure synthesis loop design features a once-through reactor between syngas compressors.

The Linde Ammonia Concept (LAC) is an established technology process scheme with over 25 years of operating experience in plants with capacities from 200 m.t./day to over 1,750 m.t./day. The LAC process scheme replaces the costly and complex front end of a conventional ammonia plant with two well-proven, reliable process units:

a) production of ultra-high-purity hydrogen from a steam-methane reformer with PSA purification;

b) production of ultra-high-purity nitrogen by a cryogenic nitrogen generation unit, also known as an air separation unit (ASU).

Also featured is a pressure-swing adsorption unit for high-purity hydrogen production and an air separation unit for high-purity nitrogen production.

Ammonia Casale’s plant design has a production rate of 2,000 m.t./day. One of the key features of this design is axial-radial technology in the catalyst bed. In an axial-radial catalyst bed, most of the synthesis gas passes through the catalyst bed in a radial direction, creating a very low pressure drop. The rest of the gas passes down through a top layer of catalyst in an axial direction, eliminating the need for a top cover on the catalyst bed. Casale’s axial-radial catalyst bed technology is used in both high-temperature and low-temperature shift converters, as well as in the synthesis converter. Ammonia Casale’s process employs a catalyst bed that harnesses axial-radial technology, which has a lower pressure drop and higher efficiency than standard catalyst beds.

Ammonia from coal

The basic processing units in a coal-based ammonia plant are the ASU for the separation of O2 and N2 from air, the gasifier, the sour gas shift (SGS) unit, the acid gas removal unit (AGRU), and the ammonia synthesis unit. Oxygen from the ASU is fed to the gasifier to convert coal into synthesis gas (H2, CO, CO2) and CH4. There are many gasifier designs, but most modern gasifiers are based on fluidized beds that operate above atmospheric pressure and have the ability to utilize different coal feeds. Depending on the design, CO levels of 30–60% by volume may be produced.

After gasification, any particulate matter in the synthesis gas is removed and steam is added to the SGS unit. The SGS process typically utilizes a cobalt and molybdenum (CoMo) catalyst specially designed for operation in a sulfur environment.

After reducing the CO concentration in the synthesis gas to less than 1 vol%, the syngas is fed to an AGRU, where a chilled methanol scrubbing solution (e.g., Rectisol) removes CO2 and sulfur from the synthesis gas. The CO2 overhead is either vented or fed to a urea plant. The sulfur outlet stream is fed to a sulfur recover unit (SRU).

Syngas that passes through the AGRU is typically purified by one of two methods:

a) a nitrogen wash unit to remove residual CO and CH4 from the syngas before it is fed to the synthesis loop;

b) a PSA system for CO and CH4 removal.

Cost of Ammonia Plants

There have been several recent cases of greenfield new ammonia plants that have taken 36 to 48 months for mechanical completion and have cost in the billions. While full cost reports are not in, the expected final costs are in the $1,500 to $2,200 per annualized ton range (Total capital cost divided by the expected total annual tons).

A recent used plant for relocation project was a 1,000 ton per day Kellogg Design that was removed, relocated, rebuilt and upgraded. This project was in the range of $500 million and took 25 months to mechanically complete from the groundbreaking (after an environmental permit was issued). The ground breaking occurred in December 2013 and the first drop of ammonia was in April 2016. The full costs were less than $1,100 per annualized ton.

References
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