Improving energy efficiency in an ammonia production plant

Authors:

D. VELÁZQUEZ, FRANCESCO ROSSI, J. RODRÍGUEZ, DVA Global Energy Services, Sevilla, Spain

F. GALINDO, FERTIBERIA, Madrid, Spain

This article was first published at Nitrogen and Syngas Conference, 5-8 March 2013, Berlin, Germany

Results are presented for an integral energy study carried out in an ammonia plant. The proposed projects were identified by carrying out an equipment audit as well as a pinch analysis linked to a thermal and power model aimed at reducing energy costs and CO2 emissions. A total of 34 projects were proposed, some of them with economic savings above 1 million Euros per year with payback times lower than 1 year.

INTRODUCTION

The directives 2003/87/EC and 2009/29/EC[1] [2] set strict CO2 emission limits for some chemical industries and in this way, CO2 costs strongly affect companies’ profits. This reason, as well as natural gas price increases during recent years, has forced many European ammonia producers to carry out important energy efficiency improvements, intended to maintain their competitiveness within the international market.

During recent years, the average specific consumption of ammonia production has been globally quantified in 36,6 GJ/tNH3 (LHV base). Performance indicators of ammonia plants located in Canada and western Europe how the best values. [3]

Within the most efficient regions, natural gas costs represent more than 80% of total ammonia production costs, reaching 90% in some cases.

The best available technologies for ammonia production from natural gas allow obtaining specific consumption levels of about 28 GJ/tNH3.[4]

Among the methodologies aimed at finding energy-saving opportunities, pinch analysis linked to power and steam modeling, has proved a powerful way for detecting projects and improving the overall energy efficiency of industrial sites. This procedure has been applied successfully in many industrial facilities, allowing optimal energy recovery in the process and hence reduction of fuel consumption. Pinch analysis has been carried out in some ammonia plants with excellent results [5].

This paper outlines the results of a comprehensive energy audit of the main equipment (furnaces, syngas, air and ammonia compressors, steam turbines, cooling towers, and refrigeration equipment), pinch analysis, and steam modeling for an ammonia production site located in Spain.

Obtained results for the proposed energy-saving measures are also shown, with maximum potential accumulated savings around €2.000.000 with payback times less than one year for some of them.

PLANT DESCRIPTION

Process description

The ammonia site for which energy audit has been carried out was designed for 1130 t NH3 per day, via catalytic steam reforming of natural gas, using Kellogg technology with ICI (Imperial Chemical Industries) license. Ammonia produced can be either exported or consumed by a urea plant located in the same facility and its utility services are also connected to an acid nitric factory. Figure 1 shows a simplified diagram of the process.

Fig. 1: Simplified diagram of the ammonia production site.

 Energy structure

The steam reforming process consists of an endothermic reaction and therefore it is operated at high temperatures. Thermal energy demand is supplied by a furnace located in the first reforming step. This furnace consumes more than 70% of the overall fuel supplied to the factory, being the main energy consumer. Combustion gases from this equipment are sent to a gases channel where energy is recovered through several coils that preheat process streams and produce steam at different pressure levels before leaving the stack. There is no requirement of thermal energy in the second stage of the reformer (secondary reformer) since it is itself supplied by the combustion reactions produced by introducing process air into the syngas stream. Downstream the reforming unit, heat recovery boilers generate high-pressure steam, as well as in the CO-to-CO2 conversion unit, where exothermic reactions take place.

Additional high-pressure steam is generated in an auxiliary boiler, whose combustion gases are sent to the same combustion exhaust gas channel of the reforming furnace. The auxiliary boiler is the second-largest energy consumer of the facility, representing over 20% of total energy consumption. The third-largest thermal energy consumption is the furnace upstream of the desulfurization reactor, where natural gas is heated.

Besides natural gas, the purge stream of the synthesis loop is used in the fuel feeding network as it contains high concentration levels of H2. Due to the exothermic reaction of ammonia production, a relevant amount of thermal energy is contained in the ammonia synthesis reactor outlet stream, which is used to preheat the boiler feed water. As shown in the diagram below, heat supplied to the process through the combustion of natural gas is subsequently recovered for steam production (power production) and heating combustion air and other process streams. Unrecovered heat is removed by cooling towers and air coolers. High-pressure steam is mainly consumed by turbo compressors for process air, syngas, ammonia cooling system, and steam fed to reforming process. An important amount of steam is also exported to the urea plant.

Fig. 2: Main inlet and outlet energy flows involved in the ammonia production process.

The table above in figure 2 shows energy percentages associated with the main inlet and outlet streams involved in the process. The natural gas consumption represents, for this site, more than 95% of the overall energy consumption of the factory. The main objective of the energy analysis project is to reduce natural gas fuel consumption, corresponding to 39,8% of the total energy inlet.

APPROACH

Equipment audit

The largest energy consumers were identified with the aim of performing an energy efficiency analysis and subsequent efficiency enhancement assessments for each one of them. Furnaces, boilers, compressors, and cooling systems of the facility were evaluated. A steam trap and insulation infrared thermography surveys were also carried out.

Pinch analysis

The aim of the pinch analysis is to improve the existing energy recovery from the process streams in order to reduce natural gas consumption. This analysis is closely linked to the steam model developed for the thermal and power system of the factory. The study also considered the identification of appropriate residual heat sources with the purpose of assessing the feasibility of implementing an absorption refrigeration system.

Thermal and power model

A model of the steam generation and power system was developed, in order to predict and economically assess the effects of the different energy savings projects identified in the previous stages of the study.The inputs to the model are properly linked to the results obtained from pinch analysis. All steam turbines, heat exchangers, boilers, steam generators, and consumers of the facility are included in the model. Besides this, it allows the specification of steam turbine performance curves, which are needed for predicting the performance of these equipment under different operating conditions. The model was developed in Excel format, which makes it easy to implement into any industrial facility, as well as allowing the availability of a fast and accurate calculating tool usable for economic assessment of changes introduced in the plant. The reliability of the model is based on the possibility of using real operating data to adjust the efficiencies and consumptions of each unit. Figure 3 shows the main interface of the model.

Fig. 3: Steam model

RESULTS

The following table shows a summary of the main energy saving projects identified and assessed in the analysis.

 

PINCH ANALYSIS

Pinch analysis allows the quantitative assessment of existing energy recovery, graphically represented by combined (hot and cold) composite curves which are shown in figure 4. The composite curves of the process represent the process energy demand (heating and cooling) versus their temperatures. These curves are obtained by adding the demand of hot streams, which need to be cooled, and cold ones, which oppositely need to be heated, for each temperature interval.

Fig. 4: Composite curves

 In spite of the proper level of existing energy recovery in the process, a big potential of improvement was detected by means of retrofitting of existing heat exchanger network. To do the analysis, a minimum approach of 20ºC between composite curves was chosen. Figure 4 shows the base case and the optimal energy recovery case, corresponding to the 20ºC minimum approach.

Potential savings

The table below shows potential savings and objectives of energy consumption for the target situation, in which the energy recovery is improved at 20ºC minimum approach. Although these values should not be considered as strict objectives to be fulfilled, their change provides useful guidelines for energy recovery.

 

 

For the same natural gas consumption in the auxiliary boiler and desulfurization furnace, the high-pressure steam generation could be increased by 23,3t/h. This fact allows reducing natural gas consumption in the boiler maintaining the same steam production.

Auxiliary boiler

Load reduction in the auxiliary boiler is required for achieving the savings shown in table 2 above. This equipment is a big energy consumer and at the same time, the load reduction can be compensated by recovering energy from the process with new heat exchangers and low payback projects.

Thermal and power effects in the channel gases of the primary reformer

All combustion gases of the plant are sent to the primary reformer channel gases, where energy recovery to different streams takes place. The stack of the desulphuration furnace is connected to the channel gases in the last section, affecting only to air that is heated. Due to this fact, a reduction of the natural gas burned in the boiler and/or the primary reformer will have important effects on the heat exchangers’ performance used for energy recovery in this duct. This channel was simulated and observed that the most affected exchangers were boiling feed water and combustion air heaters. As the steam generation is linked to the whole steam and power model, the general steam balance is affected once the natural gas burned is changed. This also affects the high-pressure steam passed through the turbines and therefore, the shaft work produced. For these reasons, the gases channel heat recovery was carefully modeled with the purpose of accurately quantifying these effects and link it to the general model.

Projects

The following table 3 summarizes the identified and proposed projects resulting from pinch analysis.

 

Nine different projects were identified, which causes important reductions in natural gas consumption in the site. Economic savings achieved by each project are between €200.000 and €1.200.000, with payback times lower than 1 year for most of them. The high profitability of one of the proposed projects is worth highlighting, which will be described in the following section. This project considers the installation of a new low-pressure steam generator in the gases channel.

Low-pressure steam generation in the gas channel

As previously mentioned, pinch analysis showed the possibility of increasing high-pressure steam generation reducing a 9% the natural gas consumption in the auxiliary boiler. A higher reduction of natural gas would affect the heat recovery through the exhaust gases channel, disabling the possibility of achieving a new thermal and power equilibrium in the whole plant unless major investments are carried out to revamp the current heat exchangers network.

This project proposes the installation of a new LPS generator in the channel gases with the aim of achieving 9% fuel consumption maintaining steam generation. The right amount of low-pressure steam generated predicted by the steam model was 7,2 t/h. The appropriate place for the installation of the LPS generator was in the gases channel, just before the energy recovery through combustion air preheaters.

The reduction of fuel consumption affects the energy recovery through the gases channel downstream the auxiliary boiler, altering the operating conditions of all the coils downstream. Figure 5 below shows these new conditions. In the proposed situation, combustion gases will be exhausted at a lower temperature (approximately 125ºC) than the current 154ºC. A total cost-saving value of €729.000 per year for this project has been calculated. With the addition of cost savings due to CO2 emissions, total savings rise up to €801.000 (CO2 emission tax: €15 /t).

Project investment considering new heat exchanger steam generator, piping, and insulation results in €155.000, which means a payback time of less than 4 months.

Fig. 5: Simulation of gases channel and steam generation

 

EQUIPMENT AUDIT

Compressors

Two energy efficiency enhancement projects resulted from compressors assessment, both of them looked at reducing the consumption of their driving steam turbines.

–  Surge control improvement for the ammonia refrigeration centrifugal compressor.

During cold weather periods (5 months per year) this compressor works at reduced loads, which causes the surge control to act to prevent it from malfunctions. It was proposed the installation of an automatic surge control system that minimizes operating hours at recycling valve opened. This system adjusts the set-point of these valves as a function of the operating conditions and the dynamic surge curves.

–  Recovering residual heat to produce cold glycoled water in Libr/H2O or NH3/H2O absorption chillers. Results from economic assessment of the latter projects showed high payback times (>7 ue to the investment required for heat recovery equipment, absorption chillers, and dedicated cooling towers.

Cooling water

The cooling water system was properly operated. Nevertheless, it was proposed the replacement of existing cooling tower fans for more efficient and newer designs. This thereby results in a reduction of the electric consumption of their motors. The estimated saving was €63.000/year with a payback time of less than 1 year.

The cooling network designed also presented an improvement in the way it is configured.

Combustion control

The control of combustion systems was done manually. Combustion optimization is not of priority and thereby not performed on a continuous basis, being the main efforts of operators intended to maintain the production of the plant. Therefore an automatic control system was proposed for the combustion equipment in the auxiliary boiler and primary reformer. The installation of an automatic control system could be difficult considering the age of the primary reformer and the number of burners. So this project may require a complete revamp of the combustion system. This issue will be studied by the company. Savings of €200.000/year and were assessed for both equipments, with payback time lower than 1 year for the auxiliary boiler.

Natural gas saturator

Saturation of natural gas using hot condensates upstream the primary reformer was simulated and assessed. This measure leads to a reduction of steam injection demanded by steam reforming and thus the possibility of reducing the steam generation in the auxiliary boiler. Estimated savings for this project are €437.000, with a payback time less than 1 year.

Natural gas expander

Natural gas fuel is currently expanded from a 45-50 barg pressure to 2 barg through a valve system. Therefore, it was evaluated the possibility of using this expansion to produce electric power. The nominal power of this equipment is 450 kW, which produces an economic savings of €285.000/year. The investment associated with the acquisition and installation of the expander results in a payback time of 2 years. The feasibility of the project depends on the speed and reliability of the control system to open the relief bypass in case of failure of the expander.

Maintenance

Two main areas were assessed:

–  Thermal losses caused by insulation conditions, mainly located in the reformer furnace and main pipelines, were evaluated. The economic losses correspond to €17.000/year.

–  Steam traps do not usually draw the attention of maintenance personnel, either because being insufficiently accessible or because being considered equipment of only a slight influence on energy efficiency. Through an exhaustive steam trap ultrasound analysis,amongthe 300 steam traps of the facility, 20 were found to be working improperly which were valued in economic losses of €78.000 per year

CONCLUSIONS

This paper highlights the results of the methodology used by DVA in its energy analysis of an ammonia site, using a systematic approach to any possible source of inefficiency in main equipment and energy recovery using pinch analysis linked to detailed simulation of thermal generation and power. The development of a steam model, versatile and capable to carefully represent the steam and power system of the facility, provides a powerful calculation instrument for assessing the thermal and economic behavior of the plant. Finally, it is shown that a thorough study of the process and the appropriate techniques and tools, provides the opportunity to discover new projects and ideas whereby important energy reductions with attractive paybacks can be achieved.

FUTURE DEVELOPMENTS

In order to maintain the achieved savings predicted in the study, it will be necessary to install an energy management system. The aim of this system is to keep track of the energy efficiency of the whole site, main equipments and areas, defining achievable objectives of energy efficiency, and acting to correct the deviation from those defined targets.

The most important item in an energy management system is to define properly the main energy efficiency indicators (EEI) and their variables of influence (VI). In that sense, the company developed a powerful tool, iManergyTM, that allows the definition of dynamic references for the EEI correlated to its variables of influence. This software allows the client to compare its performance with the best historical values under homogeneous conditions, which represents an important help to improve the daily operational performance of the plant from the energy point of view.

The ammonia producer, considering the need for continuous improvement in its operation will consider the installation of this energy manager software.

Fig. 6: iManergy™ energy optimization of operation.

References

[1] Directive 2003/87/EC

[2] Directive 2009/29/EC

[3] Benchmarking energy efficiency and carbon dioxide emissions. Canadian ammonia producers.

[4] Methodology for the free allocation of emission allowances in the EU ETS post 2012. Ecofys, Fraunhofer ISI, Öko-Institut

[5] PINCH ANALYSIS: For the efficient use of energy, water and hydrogen. Natural resources Canada.

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