1. The importance of the energy efficiency of fertiliser production
2. Improving energy efficiency of nitrogenous fertiliser production
3. Ammonia plants
4. Energy saving from integrating ammonia plants with fertiliser production plants
5. Other factors that affect plant efficiency
6. Measuring ammonia plant efficiency and greenhouse gas emissions

The importance of the energy efficiency of fertiliser production

Life cycle analyses that have been carried out for agricultural crops consistently show that the single major environmental impact of the production process relates to emissions of greenhouse gases, and that the energy involved in the production of fertilisers is a significant component of this. Figure 1 shows that N₂O emissions from nitrogen (N) fertiliser production and from the agricultural field are predominant sources of GHG.

GHG emissions from N fertiliser production are mainly from two sources, which are CO₂ from the use of fossil energy sources (mainly natural gas) as feedstock and fuel in ammonia synthesis, and the N₂O emitted from nitric acid production. Approximately 70% (feedstock) of the natural gas (methane) used to make ammonia provides 60% of the hydrogen (H₂) required for the reaction with nitrogen (N₂) from the air to synthesise ammonia. The other 40% of the H₂ is derived from water in the modern steam-reforming ammonia plants.  These reactions are almost at their maximum theoretical efficiency.  The other 30% of the methane used is as fuel to heat the processes.  Figure 2 gives the levels of GHG emissions from ammonium nitrate (AN) production at different levels of technology.

Improving energy efficiency of nitrogenous fertiliser production

For the reasons given above, the improvement of energy efficiency is the main target of developments in N fertiliser production technology. Such developments are put into practice both through improvements in the design of new plants, and the retro-fitting of established plants.

Ammonia plants are intensive energy consumers.  Part of that energy is used as electric power and fuel gas, but the major part is consumed as feed gas.  This feed gas, usually natural gas, is converted in the front-end of an ammonia plant with steam to hydrogen.  Air is the source for nitrogen.

An overview of the historical developments and future trends in the production of ammonia is provided by the FertInform page on Energy Efficient Ammonia Production.

Urea plants are also energy consumers.  They require medium pressure steam supplied from an external source, usually an ammonia plant.  On the other hand, urea plants themselves are producers of low pressure steam.  Steam from these two sources, together with the required electric power, should be used in the urea manufacturing process as energy-efficiently as possible.  Revamp techniques have been developed to improve the energy efficiency of urea plants on a ‘stand alone’ basis, while efficiencies can also be achieved through better energy integration between ammonia and urea plants.

Rather than being energy consumers, nitric acid plants are energy producers.  The steam generated in the reactor section is used to drive the air compressor.  But this is not sufficient by itself: usually the major part of the power required to drive the compressor is delivered by the tail gas expansion turbine of the plant.

Ammonia plants

The main process steps of all ammonia plant designs are similar: process air compression, feed treatment, primary reforming, secondary reforming, steam generation, shift conversion, CO₂ removal, methanation, further syngas purification, compression, ammonia synthesis, refrigeration, and storage of ammonia.  The main differences between the designs are globally as follows:

• type of process air compressor drive: steam turbine or gas turbine.
• type of preheat of the process air and of the so-called mixed feed (mixture of natural gas and steam).
• type of waste-heat recovery from the flue gas in the primary reformer convection section, and from the secondary reformer effluent.
• pressure and temperature of the generated steam.
• type of CO₂ removal system.
• operating pressure in the ammonia synthesis loop.
• type of waste-heat recovery from the ammonia synthesis loop.

Feed gas consumption rate per tonne of ammonia is similar in all ammonia plant designs, and there is little scope to affect this. Rather, the main focus of efficiency savings is to reduce consumption of fuel gas, which is consumed in the radiant burners of the primary reformer, in a gas turbine if applied, and also firing for steam superheater burners or auxiliary boiler burners.

Techniques that have been developed to improve the efficiency of the production process include:

• Use rotating equipment with higher compression and expansion efficiency, resulting in savings in driving power requirement, particularly for the two main rotating energy consumers: the process air compressor and the syngas make-up compressor.
• Use a variable drive instead of a fixed speed driver to avoid recycling in the compressor train at low load and also to further improve the anti-surge control system to avoid fixed opening of the antisurge at lower loads.
• Improvements in the CO₂ removal section, through measures such as the application of better catalysts and/or installation of a so-called rich-reflux re-boiler.
• Redesigning the radiant burners of the primary reformer, such as by sending the mixed feed gas at a higher temperature to the radiant tubes, by adding a pre-reformer to the system, or the application of a lower steam-to-carbon ratio in the mixed feed gas.
• Preheat fuel and combustion air to reduce fuel consumption by the heat exchanger in process streams or flue gas exhaust, wherever margin is available.
• Add a nitrogen gas saturator coil in the convection section where the Boiler Feed Water / process condensate is added.
• Use new improved catalysts in the shift converter section to use less steam and to operate at lower temperature. Operating a high temperature shift catalyst at lower temperature, so more heat will be available for steam generation in the Waste Heat Boiler upstream of the high temperature shift.
• Increase the process air preheat temperature.
• Increase the pressure of the steam generated in the front-end (this is more easily achieved as part of the installation of a new plant, rather than the revamp of an existing one).
• Pre-heat the high pressure boiler feed water by using heat sources such as the inlet and outlet streams of the low temperature shift converter, or by convection section heat, instead of the effluent stream of the ammonia converter, which can more efficiently be used for HP steam generation.
• If not already installed, changing to use aMDEA system for CO₂ removal.
• Minimise the methane slip from the secondary reformer, thus keeping the synthesis make-up gas pure and reducing the need for gas purging.
• Minimise the front-end pressure drop. Use of pilot operated valves to increase operating pressures where economically favourable, for instance in the steam grid.
• Use a low temperature methanator catalyst, avoiding the use of high pressure steam for preheating the methanator feed.
• Reduce the energy required by the compression of synthesis make-up gas stage through changes such as chilling the make-up gas at the suction of the syngas compressor, lowering the operating pressure of the synthesis loop by changing the converter basket design, or by adding a third catalyst bed to the ammonia synthesis loop.
• Change the synloop from a wet loop to a dry loop with the use of a dryer or ammonia wash.
• Recover waste heat from the synloop, preferably by high pressure steam generation, or by a combination of high pressure steam generation and boiler feed water preheat.
• High pressure steam superheating to a higher degree to reduce consumption in the turbine.
• Improved interstage cooling of compressors to reduce power consumption; e.g. chilling of gas with an ammonia refrigeration compressor if margin available.
• Use a VAM (Vapour absorption machine) to utilise low grade heat; e.g. to reduce the inlet temperature of the process air compressor.
• Use an online ball cleaning system of surface condensers for sites where fouling in clean water exchangers is frequent.
• Use an expander instead of simply letting down fluid from high pressure to low pressure to integrate energy with other streams, such as producing power through the cold box expander.

Energy saving from integrating ammonia plants with fertiliser production plants

Where an ammonia plant is integrated with a fertiliser production plant, a number of additional ways exist to increase energy efficiency:

• Deliver the ammonia as a low pressure gas, or as warm liquid ammonia to a urea plant.
• Use offsite boilers and integrated steam systems, doing away with an auxiliary boiler.
• Use excess steam produced in the nitric acid plant in the ammonia plant.
• Supply carbon dioxide from the ammonia plant to a urea plant.

Other factors that affect plant efficiency

In addition to design and technology changes, there are other factors that can affect plant efficiency, not all of which are under managerial control. Several climatic conditions, for instance, can do this.

Other factors which influence plant efficiency include:

• Number of plant start-ups and shutdowns (energy consumed whilst no product being made).
• An ammonia plant start-up can increase annual energy consumption by 0.1 GJ/te.
• A back end (partial) trip of an ammonia plant lasting 24 hours has the effect of increasing annual energy consumption by around 0.1 GJ/te.
• One full plant start-up and 4 days worth of back end trips can therefore increase annual energy consumption by 0.5 GJ/te.
• Catalyst reductions – when certain catalysts are changed out, an energy consuming catalyst reduction is required.
•  Equipment and catalyst performance (ammonia plants are large single stream plants with overhauls typically on a 2 to 4 year frequency). Equipment and catalyst performance will deteriorate with time. Whilst much performance will be restored at overhaul, complete restoration may not be practicable.
• Throughput – periods of reduced rate operation will usually have a detrimental effect on plant efficiency.

Measuring ammonia plant efficiency and greenhouse gas emissions

Ammonia plant efficiencies are calculated and quoted for a number of different reasons some of which include:

• Vendors publicity material demonstrate the efficacy of plant and technology.
• Internal control and monitoring highlight the importance of efficiency and track changes over time.
• Plant surveys compare current to historic performance and highlight opportunities for improvement.
• Benchmarking surveys (Williams et al., 2007):
• past performance – a comparison of current versus historical performance;
• industry average – based on an established performance metric, such as the recognised average performance of a peer group;
• best in class – benchmarking against the best in the industry and not the average;
• best practices – a qualitative comparison against certain, established practices considered to be the best in the industry.

With some notable exceptions, the basis for calculation of efficiency is often unclear. There are several different ways of calculating efficiency and there are no right or wrong ways for performing the calculation. A full thermodynamic calculation of plant efficiency can be time consuming; as a result various conventions have arisen to simplify the calculations.  Whilst most efficiency data is unquestionably fit for the purpose for which it was originally envisaged, it can be misleading if existing data is applied to a new purpose without full knowledge of the basis on which is was calculated. The same can be true if data from different sources is mixed and matched or if overall plant efficiency data is used without knowing its breakdown.

It is often assumed that there is a relationship between efficiency and CO₂ emissions. Whilst this can certainly exist, it is not always the case, especially with efficient plants which export significant quantities of steam.

Energy efficiency can be defined as (CEFIC, 1998):

The matter is somewhat confused when a fuel is also used as a feedstock as is the case with ammonia production and there are valid arguments for and against including the feedstock element. The convention adopted by the ammonia industry is to include the feedstock element with the result that ammonia plant efficiency is usually calculated as follows:

where

‘Feed’ is the amount of feedstock admitted to the plant (i.e. which ultimately passes through the reformers)

‘Fuel’ is the amount of fuel used on the primary reformer and other fired heaters. Although this definition excludes internal recycles such as loop and refrigeration purge gases which are often used as fuels, their energy value is incorporated into the feed term.

‘Other’ includes steam, electricity and feedstock (some plants can, for example, import or export hydrogen, syngas and nitrogen), corrections for product state, and energy consumption by associated utilities.

The definition is logical in that the boundaries between feed and fuel can be blurred in some designs of ammonia plant, and because the combustion reactions which take place in the secondary reformer using ‘feedstock’ are a source of energy rather than hydrogen (and hence ammonia).

In many ammonia plants, feed is the dominant term and it is important to recognise that the feed requirement can never be reduced to levels below that required by reaction stoichiometry.

On ‘typical’ plants with average energy efficiency, other energy credits and debits are small in comparison to ‘feed + fuel’ and ammonia plant efficiency can then be calculated, with little error, as

On such plants, there is a strong correlation between ammonia plant efficiency and CO₂ emissions for plants which use similar feedstock and fuel. On other plants, a term which includes many of the more efficient and less efficient plants, ‘other’ can be significant and can weaken the link between ammonia plant efficiency and CO₂ emissions.

References

Dybkjaer, I. (1984).  ‘Energy Consumption in Ammonia Production: Influence of External Conditions and Key Process Factors’ IFA, Paris.

Jenssen, T.K. and Kongshaug, G. (2003). Energy consumption and greenhouse gas emissions in fertiliser production. Proceedings International Fertiliser Society, 509.

Williams, G.P. and Al-Ansari, F. (2007). IFA Benchmarking of Global energy Efficiency in Ammonia Production, , IFA Technical Committee Meeting, Ho Chi Minh City, Vietnam. IFA, Paris.  http://www.fertilizer.org/

Links to Related IFS Proceedings

479, (2001, Energy Conservation: Key to Survival for Fertiliser Producers, W D Verduijn, J J de Wit

484, (2001), Energy Audits of Fertiliser Production Plants, I R Barton, J Hunns

509, (2003), Energy Consumption and Greenhouse Gas Emissions in Fertiliser Production, T K Jenssen, G Kongshaug

601, (2007), Ammonia Production: Energy Efficiency, CO2 Balances and Environmental Impact, J D Pach

602, (2007), Ammonia Plant Energy-saving Project: Design and Implementation, W T Nobel, R B J Waggeveld, M J Walton, P A Sharp

639, (2008), GHG Emissions and Energy Efficiency in European Nitrogen Fertiliser Production and Use, F Brentrup, C Pallière

643, (2009), EU Climate Policy and Emission Trading: Challenges for the European Fertiliser Industry, R Zwiers, J A M van Balken, E Y E Härmälä, M Cryans, C Pallière

747, (2014), Ammonia Technology Development from Haber-Bosch to Current Times, J G Reuvers, J R Brightling, D T Sheldon

770, (2015), World-Wide Trends in Urea Process Technologies, J M G Eijkenboom, M J Brouwer

787, (2016), Targeting Improving Performance and Conversion Efficiency in Nitric Acid Plants, O Kay and T Buennagel

788, (2016), Improvements in Nitrogen Addition to the Fertiliser Production Flowsheet, I Blazsek and M J Cousins

803, (2017), Changes, challenges, and opportunities in fertiliser-manufacturing processes: A personal review and outlook, J G Reuvers

818, (2018), Dual pressure nitric acid technology with high energy recovery, P Muñoz

853, (2021), Reducing Emissions from Ammonium Nitrate Based Fertiliser Operations, G. Cousland, M. Dean, R. Peddie

854, (2021), New Developments in Emissions Control for Ammonium Nitrate Based Plants, A. Gullà, S. Spreafico

856, (2021), Progress in Using Artificial Intelligence in Process Control to Reduce Energy Usage, S. Rademakers

860, (2021), Identifying and Resolving Root Causes of Poor Performance in Nitric Acid Plants, J Ashcroft