Gas-based reforming for low-emission ammonia production: ATR, POX, and two-step reforming

Around three quarters of ammonia produced globally each year is based on gas feedstock, mainly utilizing steam methane reforming (SMR) as a primary reformer, combined with an air-fired autothermal reformer (ATR) as a secondary reformer to produce syngas, which is further purified to obtain a stoichiometric mixture of hydrogen and nitrogen.

We previously discussed various technology options for low-emission ammonia production from gas, including steam methane reforming (SMR), autothermal reforming (ATR), 8RH₂ with convective reforming, electrified steam methane reforming (eSMR), and methane pyrolysis. In all cases, except for methane pyrolysis (which yields solid carbon as a co-product next to hydrogen), CO₂ is captured and stored permanently (in mineralized form) to yield low-emission ammonia. In our 2023 workshop on decarbonizing existing, SMR-based ammonia plants, we also discussed the use of electrical pre-heaters to decrease gas consumption.

Here, we discuss some other technology improvements for gas-based, low-emission ammonia production, such as autothermal reforming and partial oxidation (POX) technology, as well as revamping and adding waste heat reformers in newbuild and existing ammonia plants.

Some reformer basics

The traditional two-step reforming process (steam methane reforming as a primary reformer and autothermal reforming as a secondary reformer) requires more gas than either the autothermal reforming process or the partial oxidation process. However, an Air Separation Unit (ASU) is required in case of the autothermal reforming process and the partial oxidation process, requiring additional electricity or steam as export from the ammonia plant or from a fired boiler, which would increase the gas consumption.

The chemistry of reformers is favored by lower pressures, higher steam-to-carbon ratios, and higher temperatures. In industry, compromises are found, with reformers typically operated at 30-50 bar, reflecting an optimum between the chemistry, the size of the equipment (higher pressure means smaller equipment), and the overall pressure-drop profile. The steam-to-carbon ratio in reformers at ammonia production locations is typically around 2.5, to drive full conversion of natural gas to hydrogen. The temperature is limited by the possible operating conditions for the catalyst and the metallurgy of the equipment, with typical temperatures of 950-1050°C for autothermal reformers.

Autothermal reforming

Autothermal reforming (ATR) technology combines the endothermic steam methane reforming reactions (requiring heat) with a gas burner at the inlet of the reactor, rather than an external burner like in steam methane reforming. The more direct heating in an autothermal reformer compared to a steam methane reformer (or a two-step reforming process) usually results in a lower overall gas requirement for autothermal reforming-based ammonia, and especially when high CO₂ capture rates are required. However, the capital investment for autothermal reforming is usually significantly higher than for a two-step reforming process for plants below 1 million tons of ammonia per year. This makes autothermal reforming-based ammonia plants mainly applicable for large-scale projects, typically above 1 million tons of ammonia per year.

The pressure of an autothermal reformer is chosen to achieve the lowest cost of production, with typical operations at 30-50 bar in ammonia processes. LCA (Leading Concept Ammonia) plants operated by ICI in the 1980s used autothermal reformers with an operating pressure of around 35 bar. More recent autothermal reformers have continued to operate around 30-40 bar, as this has provided the overall most effective pressure-drop profile over the front-end of the flowsheet. High-pressure operations may allow for easier CO₂ capture, owing to the higher partial pressure of CO₂ in the process, but potentially at the cost of lower equilibrium conversions.

Click to enlarge. Effect of carbon capture rate on the carbon intensity split between the plant boundaries (scope 1), power (scope 2), and upstream emissions (scope 3). Source: Linde (presented at Nitrogen+Syngas 2025).

The specific energy demand and the plant configuration for an autothermal reforming-based ammonia plant strongly depends on the overall carbon capture rate required. The energy consumption increases as the carbon capture rate increases, especially above 90% carbon capture rates. Thus, higher carbon capture rates within the ammonia plant boundaries will result in higher upstream emissions from gas extraction and transportation.

Click to enlarge. Differences between Topsoe’s ammonia plants based on a two-stage reforming and autothermal reforming (SynCOR Ammonia™). Source: Topsoe.

Topsoe licenses its autothermal reforming-based ammonia process as SynCOR Ammonia™. The low steam-to-carbon ratio of 0.6 (versus about 3.0 in a two-stage reforming process) decreases steam throughput, which results in cost savings at large scale. Topsoe’s SynCOR autothermal reforming technology is used by Linde to feed Woodside’s low-emission ammonia plant in Beaumont from 2026 onward. Topsoe’s SynCOR Ammonia™ process including autothermal technology and ammonia synthesis loop was also chosen for the upcoming 4,000 ton ammonia per day plant of CF Industries, JERA, and Mitsui in Donaldsonville, Louisiana, also known as the Blue Point Number One ATR Project.

Click to enlarge. Air Liquide’s and KBR’s autothermal reforming-based ammonia synthesis technology. Source: KBR.

Air Liquide and KBR collaborate on autothermal reforming-based ammonia synthesis technology, with Air Liquide providing the autothermal reforming technology and KBR providing the ammonia synthesis technology. Single train ammonia production with capacities in excess of 6,000 tons per day is possible, with KBR announcing a possible design for up to 10,000 tons production per day. Air Liquide will demonstrate low-emission ammonia production at the Kashiwazaki Clean Hydrogen/Ammonia Project Niigata demonstration in Japan, which has a capacity of 500 tons of ammonia per year, with CO₂ used for enhanced gas recovery (EGR) of depleted gas fields.

Johnson Matthey and thyssenKrupp Uhde have a collaboration for a fully integrated low-carbon ammonia flowsheet, based on Johnson Matthey’s autothermal reformer and thyssenKrupp Uhde’s ammonia synthesis loop, with numerous operational ammonia references (albeit not low-carbon). The largest references for ammonia production have over 3,500 tons of ammonia per day production capacity. Future scale-up is possible for plants above 6,000 tons of ammonia per day. References up to the equivalent of 4,700 tons of ammonia per day are operational for methanol production, with similar autothermal reforming conditions. In this low-carbon ammonia flowsheet, the reforming section is integrated with amine solvent for CO₂ removal, such as also deployed for the process gas CO₂ in two-stage reforming processes.

Click to enlarge. Johnson Matthey’s and ThyssenKrupp Uhde’s low-carbon ammonia flowsheet. Source: Johnson Matthey.

Johnson Matthey’s longneck design of the autothermal reformer allows space for (1) the oxidant to entrain both the hot combustion product gas and fresh feed gas in a fuel-rich combustion process, (2) a high velocity turbulent gas mixing zone, generating a well-mixed and defined process gas flow, upstream of the catalyst, and (3) a high velocity, which provides significant convective cooling of the burner tip. Typical operating pressures are 30-40 bar, and CO₂ capture rates of over 99% can be achieved (if desired).

The relatively simple autothermal reformer design results in lifetimes that are multiples of typical 5-year plant shut down cycles, as demonstrated by the record for the longest life of such a burner nozzle (27 years), in an air-fired secondary reformer in an LCA (Leading Concept Ammonia) designed by ICI in the United Kingdom. This indicates near-zero maintenance over a long period.

Click to enlarge. Casale’s HyPure-N process. Source: Casale.

Casale has operational references for high-pressure (≥30 bar), oxygen-blown autothermal reformers in ammonia plants for over 20 years, including a 1,500 tons per day ammonia plant in China. Operational pressures of 60+ bar can be achieved. Casale utilizes a water-cooled type burner, achieving burner lifetimes of more than 10 years in some cases. Casale uses the autothermal reforming technology as the base of the HyPure processes of the FLEXIBLUE® family. The natural gas is reformed in an autothermal reformer, whereafter the produced mixture of gases enters the water gas shift (WGS) section. The WGS catalysts convert carbon monoxide and steam to CO₂ and hydrogen. Thereafter the CO₂ is separated, and a liquid nitrogen wash (LNW) is removed to remove inerts from the feed gas for the ammonia synthesis loop.

Click to enlarge. NextChem’s NX AdWinHydrogen® combined with Stamicarbon’s NX Stami Ammonia^TM synthesis loop. Source: Stamicarbon.

NEXTCHEM (Maire Group) has developed the NX AdWinHydrogen® technology, which is a high-pressure autothermal reformer operating at 60 bar and above. High-pressure operation decreases the size of equipment, piping and valves, resulting in lower capital expenditure. An additional benefit of high-pressure operation is that a physical wash system with a cold methanol loop (CML) can be used to remove CO₂ before the ammonia synthesis loop. The CML is more compact and has lower operating expenses, eliminating the need for hot regeneration and proprietary solvents. It should be noted that proprietary solvents are the industry norm, and this does not cause significant issues. A pressure swing adsorption (PSA) step is added after the cold methanol loop to further purity the hydrogen to 99.99+ mol.%, eliminating the need for a continuous purge in the ammonia synthesis loop. Also, the higher-pressure autothermal reformer decreases the requirement for feedstock compression for the ammonia synthesis loop. CO₂ capture rates of up to 98% are possible, without external power input, and without constant operation of an auxiliary boiler.

Partial oxidation

Click to enlarge. Non-Catalytic Partial Oxidation with dual syngas coolers to generate high-pressure steam. Fig 6-2 from Blue Hydrogen Production Technology Review (Progressive Energy, Sept 2022).

Partial oxidation or POX is a process in which gas and potentially steam are partially oxidized with an oxygen stream to form syngas (a mixture of hydrogen, carbon monoxide, and carbon dioxide). Note that some process configurations do not require the presence of steam. Non-catalytic partial oxidation or thermal partial oxidation (TPOX) typically operates at temperatures of 1200°C and above at an operating pressure of around 40-85 bar. Catalytic partial oxidation (CPOX) is also available, decreasing the operational temperature (of the catalyst bed) to 800-900°C.

Click to enlarge. Experimental results for methane slip and soot formation in POX reactors. From Hot Oxygen Technology: Supporting Decarbonization, Resource Efficiency, and Circular Economy Development (Linde, Oct 2023).

A benefit of the non-catalytic POX technology is the high tolerance for impurities in the feed gas, such as Sulphur, and the feedstock flexibility. Also, the non-catalytic partial oxidation reactions are not significantly impacted by the operational pressure due to very high operating temperatures in the POX reactor with fast quenching, resulting in low methane slip. Shell claims that partial oxidation has a lower capital investment than autothermal reforming, due to less equipment items, thereby reducing the risk for unplanned downtime.

Click to enlarge. The Shell Blue Hydrogen Process (SBHP). Source: Shell.

Shell licenses its non-catalytic partial oxidation process as the Shell Blue Hydrogen Process, with over 100 references for POX reactors (mostly for other applications than ammonia production). Shell has 11 references for its non-catalytic partial oxidation process in ammonia plants, often using heavy fractions from refineries such as vacuum residue, bunker crude oil, and asphalt residue. Two operational references for ammonia production use natural gas as feedstock. The CO₂ is captured using Shell’s proprietary ADIP ULTRA CO₂ removal unit, using a blend of the amines methyl diethanolamine (MDEA) and piperazine, dissolved in water.

It should be noted that steam needs to be added downstream the partial oxidation reactor (or is added in the partial oxidation reactor), as the water gas shift (WGS) catalysts cannot cope with steam to carbon ratios far below 2. This is also true for autothermal reforming (with low steam-to-carbon ratios), but not for the two-step reforming process. The WGS catalysts convert carbon monoxide and steam to CO₂ and hydrogen.

The Shell Blue Hydrogen Process was selected for its own Blue Horizons project in Oman, and for the ammonia project of Trillium H2 Power in Central Appalachia in the United States.

Air Products has designed and licensed over 55 non-catalytic partial oxidation plants. Currently, around 24 of these non-catalytic partial oxidation plants are in operation, with various end applications, including hydrogen, carbon monoxide, ammonia, methanol, and oxo chemicals. Approximately half of the Air Products licensed partial oxidation plants use liquid waste streams and offgases next to  natural gas as primary feedstock.

Click to enlarge. The Air Products POX process, with waste heat boiler (WHB) design (Top) and with Quench design (Bottom). Source: Air Products.

Air Products’ has various design options for its non-catalytic partial oxidation plants. Heat recovery can be maximized by steam generation via a waste heat boiler (WHB) configuration. Alternatively, a quench configuration can be used, which eliminates the need for steam addition for the WGS reaction, which has a lower investment than the waste heat boiler configuration.

The latter design is also chosen for Air Products’ Louisiana Clean Energy Project, which is set to become the largest partial oxidation-based low-emission hydrogen project, designed to produce 750 MMSCFD (million standard cubic feet per day) of hydrogen. Air Products also owns and operates a partial oxidation plant located in La Porte, Texas, producing hydrogen and carbon monoxide since 1996.

Next to Air Products, other industrial gas suppliers Air Liquide and Linde also have non-catalytic POX technologies. Linde is developing its POX technology for syngas production as feedstock for steel production via the MIDREX DRI (direct reduction of iron) process. A wide range of feedstocks can be utilized, such as natural gas, coke oven gas, plastics, biomass, pyrolysis oil, and solid municipal waste. POX can be used in the Linde Ammonia Concept (LAC^TM).

Casale’s non-catalytic partial oxidation technology was first used in 1998, with the newest plant producing around 105 tons of hydrogen per day. The non-catalytic partial oxidation reactor utilizes a water-cooled burner design. The burner comprises a body and an oxygen lance, fashioned in austenitic stainless steel. Every surface exposed to the flame (and thus subject to thermal stresses) is cooled with demineralized water to improve the burner lifetime and to ensure safe operations. Casale’s operational POX references operate at a pressure of around 30 bar, but operation at higher pressures is not limited by the technology. Casale’s POX burner operates inside the reactor, which is a pressure vessel lined with refractory material. The combustible fuel and oxygen exit the burner separately, forming a diffusion flame as they come into contact.

Click to enlarge. Casale’s HyPOX process. Blue shaded blocks indicate proprietary equipment from Casale. Source: Casale.

Casale’s HyPOX process consists of the non-catalytic partial oxidation section, followed by the water gas shift section, whereafter CO₂ is removed and the hydrogen is purified. The addition of nitrogen from an ASU is required thereafter for the production of ammonia.

Air Liquide, Air Products, Casale, Linde, and Shell all claim a lower methane slip in the non-catalytic partial oxidation process compared to other reforming technologies, due to the higher operating temperature in non-catalytic partial oxidation. Lower methane slip results in less accumulation of methane in the ammonia synthesis loop, thereby decreasing the purge and ammonia recovery system, improving the conversion rate to ammonia. Additional methane (and Argon) removal before the ammonia synthesis loop is possible via a cryogenic separation step such as the KBR Purifier, a liquid nitrogen wash (LNW), or a pressure swing adsorber (PSA), eliminating the need for a purge and ammonia recovery system.

NEXTCHEM has a catalytic partial oxidation technology NX CPO™, which uses a controlled partial oxidation of feedstock with oxygen or air to produce hydrogen-rich syngas, to produce carbon monoxide and hydrogen at around 30-40 bar. The resulting syngas can be upgraded to pure hydrogen via the WGS reaction and pressure swing adsorption (PSA) purification, with pre-combustion CO₂ capture with amine sorbents integrated into the process. The compact and modular reactor design makes the catalytic partial oxidation technology a feasible option for small-scale ammonia plants with a capacity of around 500 tons of ammonia per day. CO₂ capture rates of up to 98% are possible, which is similar to high-pressure ATR.

Note that a catalytic partial oxidation reactor does not use a burner (as opposed to a non-catalytic partial oxidation reactor), but rather converts the hydrocarbon feedstock with oxygen to the products via a catalyst. The catalyst is relatively flexible toward the hydrocarbon feedstock, albeit less flexible toward impurities such as Sulphur compared to non-catalytic partial oxidation (a Sulphur removal step would be required prior to the catalytic partial oxidation reactor reactor). The feed gas is pre-heated before entering the catalytic partial oxidation reactor. As opposed to the ATR process scheme, the partial oxidation processes do not have a pre-reformer.

Revamping of two-step gas reforming-based ammonia plants

Click to enlarge. Mitsubishi’s KS-21™ Post-combustion CO₂ Capture Unit. Source: Mitsubishi Heavy Industries.

Today, the majority of ammonia production plants utilize two-step reforming, via steam methane reforming (SMR) as a primary reformer combined with an air-fired autothermal reformer (ATR) as a secondary reformer to produce syngas, which is further purified to obtain a stoichiometric mixture of hydrogen and nitrogen. In these plants, the CO₂ is captured from the process gas, before entering the ammonia synthesis loop. This represents about two thirds of total CO₂ produced, with the remaining one third produced as flue gas CO₂ in the burners of the steam methane reformer. The flue gas CO₂ can be captured with a post-combustion CO₂ capture unit (PCCU), with 90+% carbon capture rates. Mitsubishi Heavy Industries has 18 operational references for PCCU as of 2024, mostly for CO₂ utilization for urea production. ThyssenKrupp Uhde will install a carbon capture unit for flue gas CO₂ at MOPCO’s ammonia and urea complex in Damietta, Egypt, using its own amine-based technology. The flue gas CO₂ will be used for urea production.

Methane slip from the secondary reformer and subsequent steps increases upon increasing the operating pressure, which potentially negatively affects the operations of the ammonia synthesis loop due to high purge rates. However, high-pressure operation in a two-step reforming process operating at pressures up to typically 45 bar (and potentially up to 50-55 bar) can be facilitated by purification of the hydrogen and nitrogen gas prior to the ammonia synthesis loop, such as a KBR Purifier, a liquid nitrogen wash (LNW), or a pressure swing adsorber (PSA).

Click to enlarge. A combination of a Gas Heated Reformer (GHR) and an Autothermal Reformer (ATR). Fig 6-1 from Blue Hydrogen Production Technology Review (Progressive Energy, Sept 2022).

Additional energy savings in the process can be achieved through waste gas reformers, also known as gas heated reformers (GHRs). Various ammonia licensors offer such configurations for ammonia plants based on steam methane reforming (SMR), and for ammonia plants based on autothermal reforming. In case of autothermal reforming, such gas heated reformers are used as pre-reformers before the autothermal reformer, utilizing exhaust heat of the autothermal reformer as the heat source for the waste gas reformer.

In case of steam methane reformers, such gas heated reformers are used as an add on retrofit. The gas heated reformer is placed after the secondary reformer, utilizing high-grade heat from the secondary reformer. This configuration allows for milder operating conditions in the primary reformer, resulting in around 10% less natural gas firing duty. This reduces the amount of natural gas requirement per amount of ammonia produced, as well as the flue gas CO₂. This means that the size of the potential post-combustion CO₂ capture unit (PCCU) can be reduced. This potentially results in around 5-10% total CO₂ savings at the ammonia production location. It should be noted that there are no operational ammonia plants yet with this configuration.

Click to enlarge. The KRES™ (KBR Reforming Exchanger System). Source: KBR.

KBR claims that its KRES™ (KBR Reforming Exchange System) can increase production capacity by up to 30% in existing ammonia plants. Such a revamp also necessitates revamps at other locations in the ammonia plant to increase the ammonia synthesis loop capacity, for example via inert removal prior to the ammonia synthesis loop, and via an add-on ammonia synthesis reactor.

Similarly, Casale and Technip Energies offer revamping schemes with an exchanger reformer known as the Technip Parallel Reformer (TPR). This allows for a capacity increase by up to 25-30%, while also reducing the carbon footprint of the produced ammonia. The TPR has an axial symmetrical design, limiting potential metal dusting issues relevant for such exchanger reformers.

Click to enlarge. Johnson Matthey’s LCH™ technology. Source: Johnson Matthey.

Johnson Matthey offers a Gas Heated Reformer (GHR) as a retrofit after the secondary reformer in an ammonia production site, while also offering GHR as an add-on to autothermal reforming as part of its LCH™ technology for blue hydrogen production (MAXERGYTM design).

Topsoe offers the HTER (Heat Exchange Reformer), claiming a capacity increase by up to 30%. Furthermore, Topsoe claims more than 15 years of operational experience for its HTER.