Ammonia as an energy carrier: evaluation of the energetic efficiency of the...

Ammonia as an energy carrier: evaluation of the energetic

efficiency of the production-transportation-utilization chain

E. Biagini1, L. Tognotti

2

1. D. Energy and Environment - C. Pisa Ricerche, Pisa - ITALY

2. Chemical Engineering Department - Università di Pisa - ITALY

1. Introduction

Some critical points in the hydrogen chain compromise the development of a hydrogen-based

economy. Problems in storage and distribution systems, material choice and safety procedures

can be overcome by the selection of ammonia as an energy carrier. Anhydrous liquid

ammonia contains more hydrogen than pressurized or liquified hydrogen, can be easily stored

and contains no carbon [1]. Ammonia produced from standard techniques (e.g., methane

reforming) can be easily coupled with CCS (CO2 capture and sequestration) option, while its

use as an energy carrier will avoid the decentrated emissions of CO2. Ammonia is already

distributed via pipelines, railroads, ships with standard safety procedures, while materials and

large scale distribution systems for hydrogen require further investigation. Ammonia can be

converted in energy directly (in dedicated engines) or indirectly (prior its dissociation into

hydrogen). In this latter case, fuel cells guarantee a high performance. Utilization systems for

ammonia, although studied since many decades require validation and demonstrative

applications.

The efficiency of the entire ammonia chain (production-transportation-utilization) should be

evaluated and compared with that of hydrogen. Detailed process models are developed in this

work considering the conventional production processes (methane reforming for hydrogen, an

updated Haber-Bosch process for ammonia), the transportation (under high pressure for

gaseous hydrogen, under moderate pressure for liquid ammonia) and the conversion-

utilization systems (PEMFC for hydrogen, and three systems for ammonia: cracking-PEMFC,

electrolysis-PEMFC, cracking-SOFC). The energetic efficiency of the ammonia-based chains

is optimized varying the configuration and the operating conditions. In case the efficiency is

comparable with that of hydrogen, the additional benefits (in storage and distribution, material

choice and safety procedures) can make the ammonia based chain a successful option for a

carbon free energy scenario.

2. Description of the processes

Different processes (Tab. 1) are studied to allow one to compare the configurations and

optimize the operating conditions. The feedstock for all processes is natural gas, so that

pretreatments (e.g., desulfurization) are not considered. In each process a first block is

represented by the production of the energy carrier, that is ammonia (processes 1 to 3) and

hydrogen (process 4). The transportation of liquid ammonia is operated under low pressure

(10-15 bar), while the gaseous hydrogen is transported under high pressure (700 bar).

Hydrogen can be used directly in a Proton Exchange Membrane Fuel Cell (PEMFC), while

ammonia have to be converted in a hydrogen stream by means of (process 1) electrolysis to be

used in a PEMFC, (process 2) thermal cracking to be used in a PEMFC or (process 3) in a

1

ragucci

Font monospazio

ISBN 978–88–88104–11-9 / doi : 10.4405/ptse2010.P2.16

Processes and Technologies for a Sustainable Energy

Solid Oxide Fuel Cell (SOFC). All units of the processes are modelled with a commercial

code (Aspen Plus® by Aspen Tech).

Efficiency parameters for the production process are adopted considering the large plant of

ammonia synthesis and natural gas reforming [2]. The total amount of carrier produced is

assumed to be transported over a 1000 km equivalent pipeline to a distribution site where n

utilization systems can be operated. Typical performance parameters for PEMFC and SOFC

are then considered for each system.

2.1. Ammonia-based processes

An updated Haber-Bosch process (Fig. 1) is studied for producing ammonia in processes 1 to

3. It consists of a first reforming process to convert the natural gas in a N2/H2 blend of

appropriate ratio (1/3), followed by the ammonia synthesis block. The integration of these two

parts improves the efficiency. A primary reformer (800°C) converts methane and steam to CO

and H2 with a conversion of 60%. In a secondary reformer also air is fed to convert the

residual methane and introduce the nitrogen. A couple of water gas shift reactors (WGSR)

converts CO to CO2 and water to H2, so that a condenser and a CO2 removal section

(absorption with amine solutions) are necessary. Residual CO has to be removed in a

methanation unit because CO is a poison for the following catalyst for the ammonia synthesis.

The N2/H2 mixture (with traces of CH4 and Ar) is compressed to 250 bar and heated to 400 °C

before the double ammonia reactors. The intermediate cooling is used to produce steam for

the previous reforming. The gas conversion to ammonia is limited to 25%, so separation (in a

refrigeration section) and recirculation are operated. Purge gas is used to provide heat for the

other parts of the process. Liquid ammonia is obtained under 15 bar.

The energy loss due to the transportation is calculated considering the pressure drop along a

pipeline and maintaining the ammonia in the liquid state.

Ammonia utilization systems are studied under 1.1, 2 and 3 bar, and the energy needs for

external inputs (e.g., water and air) are quantified.

The ammonia electrolysis (process 1) separates it into H2 and N2 and is modelled according to

[2] to estimate the energy need for this step. The thermal cracking (processes 2 and 3) is

performed in a catalytic reactor at 700 or 800 °C [3].

The PEMFC (processes 1, 2 and 4) is modelled according to [5]. Two temperatures are

studied (60 and 90 °C). Water is added to maintain the 100% moisture in the cell. The

performance is related to the concentration of hydrogen in the inlet stream. Unconverted

hydrogen at the exit of the PEMFC is burnt to heat the feed streams.

SOFC (process 3) is modelled according to [6]. Two temperatures are fixed (800 and 900 °C).

The unconverted hydrogen is burnt to preheat the feed. Also heat recovery can be converted

to further energy.

Tab. 1. Characteristics of the processes studied Process Carrier Transportation Transformation Utilization Heat recovery

1 Ammonia liquid/low P electrolysis PEMFC -

2 Ammonia liquid/low P cracking PEMFC possible

3 Ammonia liquid/low P cracking SOFC possible

4 Hydrogen gas/high P - PEMFC -

2

Ischia, June, 27-30 - 2010

Primaryreformer

Steam

Natural gas

Reforming air

Secondaryreformer

HT COshifter

LT COshifter

CO2absorption

Condensateseparator

CO2desorption

CO2

Methanationreactor

Refrigerationsection

Ammoniareactor1

Ammoniareactor2

Evaporator

Water

Flash1

Flash2 Ammonia

Furnace

Combustion air

AmmoniaCracking

N2, H2

PEMFC

electricity

Electrolyzer

N2H2

SOFC

electricity

PEMFC

electricity

Ammonia

storage

and

trans-

portation

Ammoniacracking

N2, H2

Fig. 1 Simplified scheme of the ammonia production process and the utilization options.

Steam

Natural gas

Methanereformer

HT COshifter

LT COshifter

CO2absorption

Condensateseparator

CO2desorption

CO2Evaporator

Water

Furnace

Combustion air

H2

PEMFC

electricity

Hyrogen

storage

and

trans-

portation

Hydrogencompression

Fig. 2 Simplified scheme of the hydrogen production/utilization process.

2.2. Hydrogen-based process

The hydrogen (process 4) is produced via methane reforming by using a part of the inlet fuel

in a furnace to produce steam and heat. High and low temperature WGSRs convert CO in

CO2 and water in H2. CO2 can be separated for an optional CCS unit (not studied here). A

multi-stage compression section raises the pressure of hydrogen to 700 bar. The energy loss

due to the transportation is calculated considering the pressure drop along a pipeline and

maintaining the hydrogen under high pressure. The PEMFC is modelled as described above.

3. Results

Based on 1 kg of methane, 1.88 kg of ammonia or 0.28 kg of hydrogen can be obtained. Mass

and energy streams are shown in the graphs of Fig. 3 (for utilization pressure of 1.1 bar) and 4

(for utilization pressure of 3 bar). Transportation, transformation and utilization needs are

quantified and global efficiency parameters are calculated:

44

.1

CHin LHVCH

powernetGE

44

..2

CHin LHVCH

needstransportpowernetGE

44

...3

CHin LHVCH

energyrecoevredneedstransportpowernetGE

3


Natural gas pretreatment/ transportation

(1 kg CH4)

GE2 - 8.0%

3.2 kg H2Oreforming

0.85 kg CH4feedstock

0.15 kg CH4fuel

4.8 MJenergy needs

1.88 kg NH3

Heat generationand recovery

0.26 MJenergy needs

NH3transportation

16.3 MJenergy needs

NH3electrolysis

0.4 MJenergy needs

PEMFC

17.8 MJenergy production

NH3 electrolysis / PEMFC

NH3 syntesis and transportation

air

H2

GE2 13.5%

heat generation

NH3cracking

0.25 MJenergy needs

PEMFC


NH3 cracking / PEMFC

air

H2 - N20.3

0.7

GE3 21.2 %

4.1 MJthermal energy

GE2 4.6%

heat generation

NH3cracking

0.36 MJenergy needs

SOFC


NH3 cracking / SOFC

air

H2 / N2

GE3 18.7%


N2

2.3 kg H2O

reforming

1.2 MJenergy needs

0.28 kg H2


3.69 MJenergy needs

H2transportation

H2 syntesis and transportation

air

H2 compressionto 700 bar

2.15 MJenergy needs

GE2 17.9%

0.3 MJenergy needs

PEMFC


PEMFC

air

H2

residual gas

Utilization assumptions:- NH3 electrolysis / PEMFC (1.1, 90, 60)- NH3 cracking / PEMFC (1.1, 700, 60)- NH3 cracking / SOFC (1.1, 700, 800)

- H2 / PEMFC (1.1, - , 60)

where (P, Tpr, Tfc) represents the pressure of the utilization line (atm), the temperature of the ammonia pretreatment (°C) and the temperature of the fuel cell (°C), respectively.

Fig. 3 Detailed results of process models: mass and energy balance along the production-

transportation-utilization chains (case of pressure 1.1 bar in the utilization systems).


(1 kg CH4)

GE2 1.3%

3.2 kg H2Oreforming

0.85 kg CH4feedstock

0.15 kg CH4fuel

4.8 MJenergy needs

1.88 kg NH3


0.26 MJenergy needs

NH3transportation

16.3 MJenergy needs

NH3electrolysis

3.3 MJenergy needs

PEMFC



NH3 syntesis and transportation

air

H2

GE2 13.5%

heat generation

NH3cracking

1.1 MJenergy needs

PEMFC


NH3 cracking / PEMFC

air

H2 - N20.3

0.7

GE3 21.2 %


GE2 13.9%

heat generation

NH3cracking

2.6 MJenergy needs

SOFC


NH3 cracking / SOFC

air

H2 / N2

GE3 18.8%


N2

2.3 kg H2O

reforming

1.2 MJenergy needs

0.28 kg H2


3.69 MJenergy needs

H2transportation

H2 syntesis and transportation

air

H2 compressionto 700 bar

2.15 MJenergy needs

GE2 28.5%

1.65 MJenergy needs

PEMFC


PEMFC

air

H2

residual gas

Utilization assumptions:- NH3 electrolysis / PEMFC (3, 90, 90)- NH3 cracking / PEMFC (3, 700, 90)- NH3 cracking / SOFC (3, 700, 900)

- H2 / PEMFC (3, - , 90)

where (P, Tpr, Tfc) represents the pressure of the utilization line (atm), the temperature of the ammonia pretreatment (°C) and the temperature of the fuel cell (°C), respectively.

Fig. 4 Detailed results of process models: mass and energy balance along the production-

transportation-utilization chains (case of pressure 3 bar in the utilization systems).

4

Ischia, June, 27-30 - 2010

The above parameters are used to remark the importance of the transportation step and the

energy obtainable from the heat recovery at high temperature in processes 2 and 3. All cases

are compared in Fig. 5. In general, the higher the pressure of the utilization systems, the

higher the efficiency of the process. This is verified even if the energy needs due to

compression of the inlet streams are accounted for in all cases. The increase in the efficiency

is due to the higher power obtained in the fuel cells for a higher partial pressure of the

hydrogen. Also the temperature of the transformation and utilization units influences the

efficiency, because of the dependence of the power production on the temperature in the

various contributions (electromotive force, activation, ohmic and concentration overvoltages).

The energy needs for transportation reduce the global efficiency more significantly for H2

based systems than those based on ammonia (see GE2 in Fig. 5). However, comparing only

the values of GE2, the ammonia based systems are still less efficient than the hydrogen based

systems for all conditions. Obviously further benefits will have to be considered for this step

(e.g., safety issues, material choice).

Power generation obtainable with the heat available at high temperature in processes 2 and 3

makes the efficiency of NH3 based systems to be comparable (and in some cases higher) to

that of H2 based systems. As a matter of fact this possibility makes more complex the

utilization systems, so further consdieration should be discusses (cost analysis).

GE1:

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

1.1 2 3

Pfc atm

Glo

bal

Eff

icie

ncy

NH3el-pem NH3cr-pem NH3cr-sofc H2-pem

T fc: 60 °C 60 °C 800 °C 60 °C

no transportationno heat recovery

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

1.1 2 3

Pfc atm

Glo

bal

Eff

icie

ncy


T fc: 90 °C 90 °C 900 °C 90 °C

no transportationno heat recovery

GE2:

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

1.1 2 3

Pfc atm

Glo

bal

Eff

icie

ncy


T fc: 60 °C 60 °C 800 °C 60 °C

including transportationno heat recovery

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

1.1 2 3

Pfc atm

Glo

bal

Eff

icie

ncy


T fc: 90 °C 90 °C 900 °C 90 °C

including transportationno heat recovery

GE3:

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

1.1 2 3

Pfc atm

Glo

bal

Eff

icie

ncy


T fc: 60 °C 60 °C 800 °C 60 °C

including transportationincluding heat recovery

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

1.1 2 3

Pfc atm

Glo

bal

Eff

icie

ncy


T fc: 90 °C 90 °C 900 °C 90 °C

including transportationincluding heat recovery

Fig. 5 Results of the process models: comparison of the global efficiency under different

operating conditions (of temperature and pressure in the utilization system) and

assumptions (with or without transportation and heat recovery).

5



(1 kg CH4)

Ammonia synthesisand transportation

(1.88 kg NH3)

NH3 cracking/ PEMFC

NH3 cracking/ SOFC


global efficiency +9.7 / 26.6%

global efficiency -14.4 / +1.8%

global efficiency+ 4.6 / 23.0%

Natural gas

Hydrogen productionand transportation

(0.28 kg H2)

PEMFC

global efficiency + 12.3 / 35.9%

studied systems

Fig. 6 Summary of results of the process models: range of efficiency obtained in all cases.

4. Conclusion

Considering all configurations, all the operating conditions and the different definitions of the

global efficiency, the range of efficiency obtainable in the four processes studied are

summarized in Fig. 6. The global efficiency of the hydrogen-based process (4) varies between

12% and 36% in the range of conditions studied. The highest values are obtained when

neglecting the transportation needs. Among the ammonia-based processes, the electrolysis

step of process 1 is extremely costly, so that the global efficiency is mostly negative, that is

the energy needs are bigger than the power produced in the utilization system. The efficiency

of process 1 is slightly positive only in few cases (for pressure of 3 bar in the PEMFC). Also

processes 2 and 3 give lower values of the global efficiency with respect to the hydrogen base

process if transportation needs and heat recovery are not considered. Comparable values to

those of the hydrogen based process when including the transportation needs and the energy

obtainable from the heat recovery. Therefore, from an energetic point of view, the use of

ammonia as an energy carrier can be competitive with the use of hydrogen. The optimization

of the configuration and the operating conditions is indeed necessary.

Further benefits of ammonia as an energy carrier derive from the easy transportability (safety

standards, choice of materials, existing distribution systems) of ammonia with respect to

hydrogen. More confirmations are needed for the ammonia utilization systems.

Future work will be devoted to optimize alternative processes (for instance, starting from

renewable feedstocks, like the gasification of biomass fuels) for ammonia production, even

coupled with the CCS option.

5. References

1. Zamfirescu C., Dincer I. Fuel Processing Technology, 90:729 (2009).

2. Ullmann's Encyclopedia of Industrial Chemistry

3. Boggs B. K., Botte G. G. Journal of Power Sources, 192:573 (2009).

4. Choudhary T.V., Goodman D.W. Catalysis Today, 77:65 (2002).

5. Caux S., Hankache W., Fadel M., Hissel D. Energy Convers. Manage., 51:320 (2010).

6. Campanari S., Iora P. J. Power Sources, 132:113 (2004)

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