The partial oxidation process is used in the production of ammonia from heavy feedstocks such as coal and heavy fuel oil. It differs from the steam reforming process on the way the synthesis gas is produced (IPTS/EC, 2007 p. 44). The process is flexible and capable of processing all kinds of hydrocarbon fuels, from natural gas to asphalt and plastic waste materials. Partial oxidation is mainly composed of two sub-processes; the feedstock gasification with air to produce the synthesis gas and the synthesis of ammonia.
As the amount of process generated heat in partial oxidation is limited, the degree of heat integration among different processes is lower than in the steam reforming process. Auxiliary boilers are used to generate the steam required to drive the mechanical equipment and electricity (IPTS/EC, 2007 p.47). Power needed to drive the compressors is in many cases purchased from the electricity grid.
Synthesis gas production using partial oxidation method includes the following steps:
Air Separation Unit (ASU):
The air separation unit (ASU) produces oxygen that is needed for partial oxidation, as well as pure nitrogen that is used in the ammonia synthesis reaction and in the liquid nitrogen wash that is applied to remove impurities from the synthesis gas (IPTS/EC, 2007 p.44).
In the gasifier, hydrocarbons and oxygen are introduced through nozzles into a vessel lined with heat resistant alumina bricks. Some steam is added for temperature moderation. The partial oxidation gasification is a non-catalytic reaction that takes place at elevated pressures up to 80 bar and temperatures of around 1,400oC. The reaction that takes place in the gasifier is:
2CHn + O2 → 2CO + nH2
The reaction gas (raw gas) is mainly comprised of CO and H2, but also contains about 3-5% CO2, 0.3% CH4, and 0.5% soot depending on the feedstock quality. The CO to H2 ratio depends on the feedstock composition and the quantity of the steam added to the process (IPTS/EC, 2007 p. 45). The hot raw gas exiting the gasifier is cooled either by water quench or with the use of a waste heat boiler.
There are two main processes used in ammonia plants for the partial oxidation of hydrocarbons; the Texaco Syngas Generation Process (TSGP) and the Shell Gasification Process (SGP). The TSGP and the SGP processes can handle fuel oil, vacuum residue and coal. In general, these two processes are very similar. The main differences are in the nozzle design, soot removal and recirculation and process gas cooling. The reaction pressures can be as high as 80 bars. Although there are no mechanical or material limitations of further increasing the pressure, when the overall ammonia production process is taken into consideration, a further increase of the pressure might be beyond the energy optimum as the energy demand for nitrogen and oxygen compression will increase (Ullmann’s, 2011).
The coal gasification processes used for ammonia production are classified based on the gasification method used into entrained flow, fixed or moving bed, and fluidized bed. Main gasifier types, and more detailed depictions of entrained flow and fixed bed gasifier examples are shown in the two figures below.
Figure: Different Gasification Types
Figure: Examples of entrained flow (GE Energy gasifier (formerly known as Texaco)) and fixed bed gasifiers (Lurgi Dry-Ash gasifier) (NETL, unknown date)
To recover and recycle the soot different methods can be applied. One of the processes uses naphtha to extract the soot. After separation from the water, the soot-naphtha suspension is mixed with the hydrocarbon feedstock and the naphtha is topped off in a distillation column. The naphtha is recycled to the extraction section and the mixture of carbon and heavy carbon is recycled to the partial oxidation reaction. Another method uses light gas oil to form agglomerates to extract the soot. The agglomerates can easily be separated from the water and recycled to the heavy hydrocarbon feed. To avoid the accumulation of compounds in the water circuit, some of the extracted water needs to be drained. With the application of settlers and/or filters the drained water is cleaned and after biological treatment disposed (IPTS/EC, 2007 p. 45).
Due to the presence of sulphur in the feedstock (up to 7%), raw gas contains sulphur, mainly in the form of H2S. Depending on the process configuration, the raw gas is cooled (with waste heat recovery) and scrubbed with a solvent, usually chilled methanol (-30oC) in order to separate a CO2/H2S fraction. This fraction is reprocessed to elementary sulphur in a Claus unit, where alumina catalysts are used to let the H2S react with air (IPTS/EC, 2007 p.46; EFMA, 2000 p.17).
In configurations without a separate sulphur removal step, the raw gas is sent directly to the shift conversion. The H2S is then removed after the shift conversion together with the CO2 formed in that process.