Deep Dive 1

Model-based energy and emission analysis of ammonia production for improved process sustainability

Banafsheh Jabarivelisdeh
Enze Jin
Phillip Christopher
Eric Masanet

1 Bren School of Environmental Science and Management,

University of California Santa Barbara, CA, USA

2 Department of Chemical Engineering,

University of California Santa Barbara, CA, USA

2 Department of Mechanical Engineering,

University of California Santa Barbara, CA, USA

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Ammonia production is highly unsustainable. Our model-based analysis reveals that in the US, emissions from the ammonia industry could be reduced by 10% by taking measures to enhance the efficiency of existing plants. To reduce emissions to near-zero by 2050, more fundamental changes to production pathways are necessary. Low-carbon technologies such as water electrolysis and carbon capture and storage will need to be developed and deployed to achieve this near-zero target with minimum cost.

Fig. 4 GHG emissions trajectories under different scenarios

the problem

Ammonia is a key primary chemical whose production underpins much of the chemical industry, especially the nitrogen fertiliser supply chain. However, ammonia production is currently highly energy- and emissions-intensive, responsible for 35% of CO2 emissions from across the chemical sector. Other research from the C-THRU project considers the whole life cycle of nitrogen fertilisers; in this analysis, we focus specifically on the ammonia production phase and consider a wider range of novel production technologies.

To improve the sustainability of the production industry, we need to understand the energy efficiency and emissions intensity of current production assets and identify targets for emissions savings. We explore the pressing questions facing the industry:

  • What level of emissions reduction can be achieved by retrofitting existing facilities?
  • To what extent are substantial changes to production pathways and the deployment of low-carbon technologies required to reach near-zero emissions?

To answer these questions and assess the potential for emissions mitigation, we have developed a unit-process systems model of the US ammonia production industry. We simulate Steam Methane Reforming (SMR), which is currently the dominant ammonia production technology, to quantify the volume of emissions that could be reduced through improvements to existing ammonia plants. We explore two key decarbonisation scenarios for the industry: 73% emissions reduction by 2050 and 96% emissions reduction by 2050. Using the TIMES modelling platform, we show how these scenarios might be achieved at the minimum costs.

Key Results
  • Applying efficiency improvements to existing ammonia plants could reduce US emissions from ammonia production by ~10%, or 3.2 million tonnes of CO2 per year.
  • Measures to retrofit existing Steam Methane Reforming (SMR) facilities are therefore important, but insufficient to achieve near-zero emissions. Production pathways must ultimately shift away from SMR altogether.
  • In the 73% emissions reduction scenario, Autothermal Reforming (ATR) fitted with Carbon Capture and Storage (CCS) technology dominates production by 2050.
  • In the 96% emissions reduction scenario, ATR is also largely phased out and instead water electrolysis becomes the predominant production pathway by 2050.
  • Biomass gasification barely features in any of the scenarios due to its high cost.
  • Our results indicate that in the US, the deployment of carbon capture and water electrolysis will be key to creating a minimum-cost, sustainable ammonia production industry by 2050.

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Model-based energy and emission analysis of ammonia production for improved process sustainability

Ammonia production is highly unsustainable. Our model-based analysis reveals that in the US, emissions from the ammonia industry could be reduced by 10% by taking measures to enhance the efficiency of existing plants. To reduce emissions to near-zero by 2050, more fundamental changes to production pathways are necessary. Low-carbon technologies such as water electrolysis and carbon capture and storage will need to be developed and deployed to achieve this near-zero target with minimum cost.

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The ammonia production industry is currently highly unsustainable, responsible for ~20% of total energy consumption and ~35% of CO2 emissions from the whole chemical sector. In this work, we present a model-based assessment of low-carbon technology pathways for ammonia production. We first develop a unit process systems model to simulate the current major ammonia production technology (natural gas steam reforming). The model is founded on thermodynamic/material flow analysis of the unit processes involved, allowing us to understand existing plants’ energy use and CO2 emissions and identify targets for reducing their emissions through energy efficiency improvements. The model outputs from the first step are then used as a basis for exploring further decarbonisation scenarios for deep emission reductions. To do so, we consider different low-carbon technology options including water-electrolysis, carbon capture and sequestration (CCS) technologies, and bio-based feedstocks, including their performance and economic data. The options are evaluated using the TIMES modelling platform to identify which viable decarbonisation pathway carries the minimum cost for each of the two target emission reduction scenarios (73% and 96% reductions in emissions by 2050). The major output is the quantified share of different low-carbon technologies (with their associated CO2 reduction trajectory) needed to meet regional ammonia demand based on each emission scenario.

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Ammonia (NH3) is one of the largest volume products in the chemical sector, with a global production of approximately 185 million metric tons in 2020.1 Demand for ammonia comes mainly from the agricultural sector; more than 80% of the ammonia produced worldwide is utilised for fertiliser manufacturing.2 By 2050, demand for ammonia is projected to increase by up to 40% relative to today’s levels, driven by the growing size and wealth of the global population.1 Ammonia production is energy and emissions-intensive and currently relies heavily on fossil fuels. In 2020, global ammonia production was responsible for ~8.6 EJ final energy consumption and ~450 million tons of CO2 emissions (corresponding to ~20% of total energy use and ~35% of CO2 emissions from the whole chemical sector).1,3 Ammonia is principally manufactured from hydrogen (H2) and nitrogen (N2), which are then reacted to produce ammonia through the Haber–Bosch process.4 Currently close to 100% of the required hydrogen for ammonia production comes from fossil fuel feedstocks, mainly through the steam reforming of natural gas (NG) and the gasification of coal.1 In 2020, ammonia production accounted for 50% and 44% of the global chemical industry’s use of NG and coal, respectively, for feedstocks and process energy.1 Therefore, significant efforts are being dedicated to improving the sustainability of ammonia production, with the hopes of making it compatible with climate change mitigation goals.5

Achieving this will require substantial changes in current production pathways through emerging near-zero-emission technologies (such as green hydrogen electrolysis and carbon capture and storage (CCS) primarily for H2 production). The modification of existing production assets (which will not be retired in the near future) for reduced emissions must also be a high priority, as the global average energy intensity of ammonia production is currently estimated to be 50% higher than best available technology energy performance levels. This suggests that strategies to improve the performance of the existing capital stock will be an important part of the industry’s energy transition.1

In this work, we perform model-based assessments to quantify the potential for improving the sustainability of the ammonia production industry. The assessment has two main steps. First, we derive a unit process systems model to simulate NG steam reforming-based ammonia (as the major production technology), founded on thermodynamic/material flow analysis of the involved unit processes. Accordingly, the model is used to estimate energy use and emissions from current ammonia production plants and quantify the level of mitigation potentials. The purpose of this step is to explore possibilities for shifting the overall process fuel intensity to its lowest value through energy efficiency improvements. Second, the unit process model outputs are used as a basis for exploring two major decarbonisation scenarios (73% and 96% reductions in emissions from ammonia industry by 2050) in a cost-efficient way. To do so, we consider the performance and economic data of a range of low-carbon technology options, including water-electrolysis, carbon capture and sequestration (CCS), methane pyrolysis, and bio-based feedstocks.

The options are then implemented through the TIMES platform6 to identify the viable decarbonisation pathway with the minimum production cost. The major output of the second step is the quantified share of different low-carbon technologies (with their associated CO2 reduction trajectory) used to meet regional ammonia demand based on the target decarbonisation scenario. As a case study for the model-based assessments performed in this study, we consider the ammonia production industry in the United States. The US ammonia industry currently produces 9% of global ammonia, predominantly through NG steam reforming.1

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Unit process systems modelling of current US ammonia production

As the first step for increasing the sustainability of the ammonia industry, we evaluate strategies for minimising the impacts of existing plants before they are replaced. To this aim, we develop a unit process systems model to understand the energy performance of the current production pathways and identify the best targets for process improvements. The model covers ammonia production based on NG steam reforming. The typical process flowsheet is shown in Figure 1, and includes three major stages: synthesis gas (syngas) production, syngas purification, and ammonia synthesis.4 The first stage involves mixing and reacting the feed (methane derived from NG) with process steam to produce syngas containing CO, CO2, and H2 in the primary reformer (through (R-1): CH4 + H2O ↔ CO + 3H2 and (R-2): CO + H2O ↔ CO2 + H2) with ~60% of feed conversion. The overall reaction is highly endothermic and additional heat is required to raise the temperature to 780-830°C at the reformer outlet. The feed conversion is then completed in the secondary reformer, where the reformed mixture is partially burned with air to supply both the required reaction heat and the stoichiometric N2 to H2 ratio (N2/H2=3).4 The reformed existing gas (at ~1000°C) is then cooled and enters the water-gas shift reactors (operating at ~400 and 250°C) to increase the content of H2 through reaction (R-2). In the purification section, first the CO2 content of the syngas is scrubbed, usually using amine-based chemical absorptions.1,7 Then, any residual CO and CO2 remaining in the syngas is removed, most commonly by the methanation process, through conversion to CH4 (inert gas). The purified syngas (containing H2 and N2) is then compressed to high pressures (~200 bar) and enters the ammonia synthesis reactor, in which the production of ammonia (through (R-3): N2 + 3H2 ↔ 2NH3) occurs on iron-based catalysts at ~350-550 °C. Due to the chemical equilibrium, the conversion is only 20-30% of N2 per pass. The produced ammonia is then separated from the unreacted gas through cooling/refrigeration and ammonia condensation, while the unreacted gas is recycled to the synthesis reactor. For process details refer to Ullmann.4

Accordingly, we model the production process with three main subsections: Syngas production, purification, and the ammonia synthesis. The unit operations implemented for the modelling of ammonia production subsections are presented in Table 1. Based on the energy input/output model for each unit operation presented in this Table, we calculate the process primary energy (fossil fuel input) consumption and the corresponding CO2 emissions (based on fuel-specific emission factors).8

TIMES modelling of decarbonisation scenarios for US ammonia production

The second step to decarbonise the ammonia industry is to implement low-carbon technologies that focus on feedstock substitution, fuel switch, and carbon sequestration and storage (CCS). To simulate the decarbonisation pathways for the US ammonia industry, the TIMES (The Integrated MARKAL-EFOM System) model is used. TIMES is a technology rich, bottom-up model generator that uses linear-programming to optimise the energy system with a least-cost solution according to user constraints over medium to long-term time horizons. TIMES models include all the processes for primary resource generation, energy commodity transformation, distribution, and conversion into the supply of energy service to fulfil the demand by energy consumers.12

Low carbon technologies for ammonia

In the US, the dominant ammonia production technology is methane steam reforming (SMR) from natural gas with the Haber Bosch process. Auto-thermal reforming (ATR) is an advanced SMR technology that requires more methane input and less energy input than conventional SMR. Both SMR and ATR can be retrofitted with CCS equipment to further reduce carbon emissions. A potential alternative is water electrolysis, a promising low-carbon technology to produce hydrogen for ammonia synthesis. Carbon emissions could be net zero if electrolysis were powered by renewable electricity. Methane pyrolysis is another potential route to produce near-zero-emission ammonia, but the technology is still under development. The process uses the very high-temperature provided by electrical plasma to split methane into its constituent hydrogen and carbon atoms without burning it.1 Biomass gasification is also considered as a low carbon technology for ammonia production since it provides a source of biogenic carbon feedstock. Table 2 provides detailed information of the technology dataset used in the TIMES model for US ammonia scenarios.

Decarbonisation scenario design and key assumptions

The baseline scenario (BAT) follows the current technology development with the best available technology but with no strict mitigation actions taken by the industry. Two decarbonisation scenarios are considered in comparison to the baseline scenario. The first decarbonisation scenario (DS-73%) presents a pathway with a 73% carbon reduction goal for ammonia production by 2050. The second decarbonisation scenario (DS-96%) has a more ambitious carbon reduction goal of 96%. These two decarbonisation scenarios are designed based on the Sustainable Development Scenario (SDS) and Net Zero Emissions by 2050 Scenario (NZE) in the IEA ammonia technology roadmap report.1 Future demand for ammonia production in the US is assumed to be constant for all scenarios, estimated at 16.5 million metric tons per year. The remaining asset lifespan of current ammonia plants is assumed to be 15 years. Under this assumption, all existing ammonia plants will be retired by 2035. All scenarios consider future changes in energy prices (e.g., natural gas and renewable electricity prices), the carbon emission intensities of technologies, and capital investment for low-carbon technologies.

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Fig. 1 Model structure following typical process flowsheets for ammonia synthesis through NG steam reforming

Table 1 Main unit operations and energy models used for ammonia production process9–11

Table 2 Characteristics of conventional and decarbonisation technologies for ammonia production in the US1,13–15

Results & Discussion

Unit process systems modelling of current US ammonia production

To proceed with our model-based assessments, we first validate our unit process model by comparing its outputs to the available data on US ammonia production. To evaluate the model’s accuracy, we define credible ranges for the main model parameters that dictate fuel consumption, which are established based on extensive literature review (Table 3).

Using the average value of each parameter, we estimate the average process energy consumption (feedstock and fuel) and corresponding CO2 emissions (using NG emission factor of 50.3 kg-CO2/MJ), presented in Table 4.8 The estimations of energy and emissions intensity are consistent with those available for the US ammonia industry. The average energy intensity estimated and reported by the industry is 37 MJ/kg-NH3, with a corresponding emissions-intensity of ~2 kg-CO2/kg-NH3.22,24 Our model estimated these figures as 36.5 MJ/kg-NH3 and 1.91 kg-CO2/kg-NH3, respectively. Our results indicate that ~60% of process energy is used as the feedstock, while the rest is used as the fuel to meet process energy (heating/power) requirements. Around 62% of the estimated CO2 emissions arise from feedstock conversion to syngas, while the rest is the diluted CO2 stream results from fuel combustion. For the whole production process, a total of 1.8 MJ/kg-NH3 electricity is required to supply process power requirements (~46% for syngas compression, 32% for air compression, and 22% for ammonia separation), of which 0.7 MJ/kg-NH3 electricity is supplied by the waste heat recovery, and the rest from onsite electricity generation by the steam turbines.

Next, we implement the model to gain insights into the main drivers of the overall process energy consumption and corresponding emissions. To this aim, we conduct a sensitivity analysis to identify the parameters that contribute the most to the variances in the estimated fuel energy intensities, excluding the feedstock energy. Accordingly, it is identified that heat recovery efficiency has the greatest impact on the process fuel consumption, followed by reformer efficiency. Below, we demonstrate the application of the model for identifying specific energy efficiency upgrades and quantifying their savings potentials. To do so, parameters are set to their best practical value (based on their already reported performance range) one by one according to the order of their selection from the sensitivity analysis. The corresponding energy use is then calculated. Figure 2 shows the degree of reduction in the fuel (primary energy) and net energy use of the NG steam reforming-based process estimated by the model. This was determined through the step-by-step adjustment of key model parameters, and corresponds to the estimated remaining energy efficiency potential for US ammonia plants. The net energy basis considers the amount of surplus steam and electricity produced in the plant (usually considered for export purposes) and subtracts it from the fuel energy consumption. The fuel energy requirement could be reduced by 28%, from 14.2 to 10.2 MJ/kg-NH3 by changing the parameter values to best practice values. The net energy reduction could be even greater, from 14.2 to 8.7 MJ/kg-NH3 (with about 1.5 MJ/kg-NH3 of the excess steam). The reduced fuel consumption would also reduce the combustion-related CO2 emissions of the process from 0.71 to 0.51 kg-CO2/kg-NH3, which corresponds to 10% total process emissions reduction. Although this emissions reduction from retrofitting existing plants is critical for charting the pathway toward more sustainable practices, these results indicate that it is insufficient; a deep decarbonisation of the ammonia industry will require the widespread deployment of alternative low-carbon production technologies.

TIMES modelling of decarbonisation scenarios for US ammonia production

The scenario results for ammonia production via different process routes are shown in Figure 3. In the baseline scenario, all ammonia production is produced by ATR process route by 2050 since existing SMR-based ammonia plants are retired after 2035. In the DS-73% scenario, 80% of ammonia is produced from low-carbon process routes by 2050. ATR is the major production technology and most ATR-based ammonia plants are coupled with CCS. Methane pyrolysis only accounts for 10% share of total ammonia production due to its low TRL. In the DS-96% scenario, ATR with CCS installation accounts for a greater share of production by 2040 compared to the DS-73% case. By 2050, the water electrolysis process contributes 60% of total ammonia production and all ATR-based ammonia plants implement CCS technologies to reach near-zero emissions. The GHG emission trajectories under different scenarios are depicted in Figure 4.

According to the decarbonisation pathways under the different scenarios, the ATR process route will play an important role in decarbonising the US ammonia production in the short term. ATR technology has a relatively low total production cost (see Figure 5) and emits lower CO2 than SMR technology. Therefore, new ammonia plants should be built with ATR installation after 2035. In the intermediate decarbonisation scenario (DS-73%), ATR coupled with CCS accounts for the majority of total ammonia production. Although a small proportion of ammonia will be produced by methane pyrolysis, it is only economically viable if there is a market for solid carbon as a byproduct. To reach the near-zero emission target, the water electrolysis process route should be deployed so that it accounts for around 60% of total ammonia production by 2050. Therefore, some existing ATR plants will be retired from ammonia production by 2050. In the considered scenarios, there is no deployment of biomass gasification technology, mainly due to its high production costs.

This study only considers the ammonia demand for fertiliser use in the US. Ammonia could also be used as an energy carrier in the application of power generation and marine fuel cells in the future. The IEA project that global demand for ammonia could more than double by 2050, driven by its growing use for both fertilisers and energy storage.1 In future work, we will therefore conduct an additional scenario to explore how the decarbonisation pathways change with the increasing demand for ammonia production. In addition, some policy interventions (e.g. incentives for low-carbon technologies and carbon prices) will be further integrated into the decarbonisation scenario analysis.


Improving the ammonia industry’s sustainability requires not only the management and retrofitting of existing assets for improved performance, but also the deployment of innovative zero-carbon technologies. In this work, we perform model-based analysis, firstly to identify levers for energy/emission reductions in the current major production pathway (NG steam reforming) through process efficiency improvements, and secondly to evaluate the implementation of near-zero emission technologies (water electrolysis, CCS, methane pyrolysis, and biomass gasification) for deeper emission reductions. Our results indicate that investing in efficiency improvements for US ammonia production would lead to ~10% lower emissions than simply waiting for plant retirements (equivalent to a total annual emission saving of 3.2 Mt-CO2 based on the current domestic production). The share of near-zero emission technologies, and thus ammonia production costs, differ depending on the emission reduction scenario. Our results indicate that for the US, carbon capture technology and water electrolysis will be key to achieving minimum-cost, sustainable ammonia production by 2050.

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Table 3 Defined ranges of main process parameters16–23

Table 4 Energy/emission intensities resulted from the modelling of US ammonia production

Fig. 2 Cumulative energy intensity improvement through adjusting the key modelling parameters to their best practical values for the NG steam reforming pathway for US

Fig. 3 US ammonia production by process route and scenario

Fig. 4 GHG emissions trajectories under different scenarios

Fig. 5 Levelised cost of US ammonia production by process routes in 2020

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