Life cycle analysis of processes for hydrogen production

Life cycle analysis of processes for hydrogen production

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Life cycle analysis of processes for hydrogen production Miroslava Smitkova a,*, Frantisek Janı´cek a, Juri Riccardi b a b

Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia ENEL Produzione Ricerca, Via Andrea Pisano, Pisa, Italy

article info

abstract

Article history:

Hydrogen is considered to be an ideal energy carrier in the foreseeable future and can play

Received 24 April 2010

a very important role in the energy system. A variety of technologies can be used to

Received in revised form

produce hydrogen. One of the most remarkable methods for large-scale hydrogen

25 October 2010

production is thermo-chemical water decomposition using heat energy from nuclear, solar

Accepted 31 January 2011

and other sources. Detailed simulations of the two most promising water splitting thermo-

Available online 27 March 2011

chemical cycles (the Westinghouse cycle and the SulphureIodine cycle) were performed in Aspen Plus code and obtained results were used for life cycle analysis. They were

Keywords:

compared with two different processes for hydrogen production (coal gasification and coal

Hydrogen production

pyrolysis). Some of the results obtained from LCA are also reported in the paper.

Life cycle analysis

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Water splitting thermo-chemical

reserved.

cycles Westinghouse cycle Sulphureiodine cycle Solar energy

1.

Introduction

Energy is a key factor in economic development. In 2008, total worldwide energy consumption 11.3 Gtoe was representing a worldwide increase of 1.4% in comparison with 2007. While in European Union (EU) energy consumption decreases by 0.5%, China’s energy consumption increased by 7.2% and India’s energy consumption increased by 5.2%, with all figures in comparison with 2007 [3]. The worldwide question is how to meet increasing energy demands and moreover how to move towards clean energy systems. Present energy demand is met by fossil fuels; however hydrogen is considered to be an ideal energy carrier in the foreseeable future and can play a very important role in the clean energy system. Hydrogen can be produced from water using a variety of energy sources such as solar energy, nuclear

energy or fossil fuels. A hydrogen economy will need significant new primary sources of hydrogen. Nowadays hydrogen is produced mainly from fossil fuels. During recent years research has been focused on the development of technologies and processes that produce hydrogen without requiring energy sources which have a negative impact on human health and the environment [8,9,23,26]. Some of the more remarkable methods for hydrogen production are to split water using thermo-chemical cycles. Hydrogen can be produced through them in an environmentally attractive way, without using fossil fuels [8,16,19,23]. Two of the most promising of these water splitting thermochemical cycles were chosen for our simulation in Aspen Plus code: the Westinghouse cycle and the SulphureIodine cycle. Life Cycle Analysis (LCA) was performed using SimaPro code and

* Corresponding author. Tel.: þ421 2 602 91 783. ek), [email protected] E-mail addresses: [email protected] (M. Smitkova), [email protected] (F. Janı´c (J. Riccardi). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.177

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results were compared with the LCA from two other processes for hydrogen production (coal gasification and coal pyrolysis). Several papers, e.g. [4,6,22] describe chemical processes of water splitting thermo-chemical cycles but there is no practical industrial practice application of these cycles. Most of these studies only consider particular experimental aspects of the entire process and in particular they emphasize the use of small size laboratory reactors. In our work, for the selected thermo-chemical cycles, a complete and detailed simulation model of the entire process was performed in Aspen Plus and, moreover, their life cycle analysis was performed and compared with other industrial processes for hydrogen production, based on the gasification and pyrolysis of the coal. The outline of this paper is as follows: In the first section, water splitting thermo-chemical cycles processes for hydrogen production are briefly characterized. The main subsystems of both selected cycles are described. Life cycle analysis is briefly revealed in the next section and then input conditions for the simulation in SimaPro simulation are published. Main results and conclusions are reported.

2.

Hydrogen production

Hydrogen can be produced and converted into useful energy using a variety of energy sources such as renewable, nuclear and fossil fuels as well as by using variety of different technologies. Direct water dissociation is an impractical way to produce hydrogen, due to the relatively high temperatures required (as well as related material problems) and the small fraction of obtained hydrogen at thermodynamic equilibrium. It is possible to overcome the aforementioned problems via water splitting thermo-chemical cycles (WSTC), which are the processes used to decompose water into hydrogen and oxygen via chemical reactions that use intermediate substances which are recycled in the cycle [4,6,12]. These offer alternatives for producing hydrogen without using fossil fuels. The concept of WSTC was proposed in the 1960s and since then over one-hundred thermo-chemical cycles have been described; several of them have been successfully tested and evaluated including chemical processes and process engineering studies [10,22]. Some WSTC are purely chemical processes and others combine also electrochemical steps and are subsequently called hybrid cycles. The electrochemical step enables a decrease in the number of reactions required. WSTC consist of both endothermic and exothermic reactions. The main endothermic reactions take place at temperatures, typically in the range of 700e1200  C [1,10]. It is for this reason that high temperature sources alone should be chosen for this process, e.g. solar or nuclear energy. For our purposes, one pure thermo-chemical cycle with a relatively small number of reactions e the SulphureIodine cycle e and one hybrid cycle e the Westinghouse cycle e among the most promising cycles, were selected.

2.1.

[4,22]. The oxygen is available as by-product [12]. The decomposition voltage is lower than voltage used in direct water electrolysis. SO2 þ 2H2 O ¼ H2 þ H2 SO4

electrolysis; 25e100  C

1 H2 SO4 ¼ H2 OþSO2 þ O2 thermochemical; 800e850  C 2

(1)

(2)

There are four major sub-systems in the WH cycle: concentrator, decomposer, separator and electrolyser. The sub-systems are schematically shown in Fig. 1 and are briefly described below.

2.1.1.

Concentrator

The role of the concentrator is to remove water from sulphuric acid via heating and flashing [12,22]. They can be separated as they have different boiling points. The concentrated liquid mixture of sulphuric acid and water is sent to the decomposer and vaporized water is sent to the electrolyser.

2.1.2.

Decomposer

According to the obtained results from [12,22], operational conditions for solar reactor corresponding to pressure of 1 bar and temperature of 830  C were set. The reaction is endothermic and a high temperature is required for the sulphuric acid decomposition. Therefore only high temperature heat sources are usable for this process. In the decomposer H2SO4 is transformed into SO3, which is later decomposed at high temperature into SO2, and oxygen. The hot decomposed gas is sent to the cooler and then to a separator tank where the SO3/ SO2/O2 vapour mixture is separated. The SO2/O2 vapour mixture is transmitted the separator sub-system and the resulting liquid phase is sent to the electrolyser.

2.1.3.

Separator

The SO2/O2 vapour mixture is compressed using a compressor to achieve the high pressure necessary for efficient separation and then sent to the separation tank. A large fraction of SO2 liquid is transferred to the heater and then to the electrolyser. The remaining gas phase, constituted by oxygen and sulphur dioxide is transferred to a chiller for further separation step at a very low temperature. The SO2 obtained from the chiller is

Westinghouse cycle

The Westinghouse cycle (WH cycle) is a two-steps ((1) and (2)) thermo-chemical cycle to decompose water into hydrogen and oxygen and in which hydrogen is produced by electrolysis

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Fig. 1 e Schematic model of the WH cycle.

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sent to the electrolyser and the oxygen as a by-product can be stored for future use. This two-steps separation enables very pure oxygen to be obtained at the inlet as a by-product. The work during the separation phase can be one of the major units in overall cycle efficiency. Moreover amount of produced hydrogen depends on amount of SO2 sent to electrolyser. Therefore SO2/O2 separation sub-system was optimized to maximize oxygen production in gas phase and sulphur dioxide production in liquid phase. Using sensitivity analysis, were chosen settings in separation sub-system to optimize hydrogen production. For obtaining very pure oxygen stream at the outlet, a lower cooler temperature should be introduced. If some SO2 can remain in the oxygen outlet, separation process can be operated at a higher temperature. The sensitivity analysis was made by using different conditions for cooler e four different temperatures (85  C, 65  C, 45  C, 30  C) and three different pressures (10 bar, 20 bar, 30 bar). The best result was obtained at temperature T ¼ 85  C and pressure p ¼ 30 bar (3 MPa). Energy needed to achieve these conditions has a big impact on the overall efficiency of the thermo-chemical cycle. A good compromise in the operating conditions can be assumed setting T ¼ 45  C and p ¼ 20 bar (2 MPa). In these conditions the SO2 mass fraction at the outlet of the separation system is 98.8% and the purity of recycled sulphur dioxide is 99.4%.

2.1.4.

Sulphureiodine cycle

The sulphureiodine cycle (SI cycle) is, as well as the WH cycle, one of the most promising candidates for thermo-chemical hydrogen production. The SI cycle consists of three pure thermo-chemical steps that sum to the dissociation of the water [4,6,16,25]. The SI cycle generates hydrogen in the three steps chemical reactions ((3)e(5)). 1 H2 SO4 ¼ H2 O þ SO2 þ O2 2

2HI ¼ H2 þ I2

(3)

endothermic reaction; 300e450  C

(4) (5)

The SI cycle can be divided into three major sub-systems, based on the three main reactions of the cycle: Gibbs reactor, Bunsen reactor and Equilibrium reactor. The sub-systems are schematically shown in Fig. 2 and briefly described below.

2.2.1.

Gibbs reactor

The role of the Gibbs rector section is to concentrate H2SO4, and for decomposition, similar as in the WH cycle. A small amount of SO3 is found in the outlet of the Gibbs reaction. Later, sulphur trioxide reacts with water and produces diluted H2SO4 which is recycled in the Bunsen reactor [3,15].

2.2.2.

Bunsen reactor

In the Bunsen reactor two immiscible acids aqueous are produced and separated. Separation occurs in the presence of a large excess of iodine via the formation of two immiscible liquids: a light liquid (H2SO4/H2O) which is the lower density phase, and a heavy liquid (HI/I2/H2O) called HIx. Hydrogen is then generated from the heavy phase. Reaction proceeds exothermically and iodine and water are later recycled in the cycle [4,6].

2.2.3.

Electrolyser

The role of the electrolyser is to produce hydrogen at the cathode and sulphuric acid at the anode. Sulphuric acid is circulated through a closed loop [12].

2.2.

H2 O þ SO2 þ I2 ¼ H2 SO4 þ 2HI Bunsen reaction; 120  C

Equilibrium reactor

In the equilibrium reactor, hydrogen iodine is concentrated and thermally decomposed at a moderate temperature of 450  C. The result from the equilibrium reactor is a split in a liquidegas separator. The hydrogen with trace of hydrogen iodine is separated from most of the iodine, which is returned to the main solution reaction in the Bunsen reactor. The gaseous hydrogen product is then separated from HI, which is sent back to the equilibrium reactor via a membrane and pure hydrogen is the resulting final product. For the SI cycle an optimization study of HI separation was carried out in order to maximize hydrogen production. Phosphoric acid was used for this separation to break up azeotrope between HI/H2O and maximize the amount of recycled iodine. Both cycles have problems with separation processes.

Fig. 2 e Schematic model of the SI cycle.

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2.3.

Coal gasification

The gasification is a process that converts carbonaceous materials (in this case coal) into carbon monoxide and hydrogen using the raw material at high temperatures with a controlled amount of oxygen. The resulting gas mixture is called syngas (synthesis gas e composed of carbon monoxide, hydrogen, carbon dioxide, other gases coming from the impurities presents in the fuel (mainly H2S) and ashes) and is itself a fuel. Gasification is a very efficient method for extracting energy from many different types of organic materials, and also has applications as a clean waste disposal technique. The auxiliary energy consumption in the gasification plant is higher respect to the pyrolysis plant because there are more sub-processes that need electricity (air separation unit and air compressor). After the gasifier the plant model has a syngas cleanup unit (syngas treatment) with amine (in the specific case MEA e monoethanolamine, often used in natural gas sweetening). The MEA is used in a water solution to clean the syngas from the compounds H2S and from a fraction of CO2. The capture of these compounds takes place at low temperature (35  C) therefore the syngas must be cooled before (syngas cooler). After the treatment the amine must be regenerated in a stripper through steam in reverse current, in which the gases are released and the regenerated solution water e MEA can be recycled in the plant [2,11]. The hydrogen production plant via coal gasification has been realized on the base of a Shell gasification power plant [20].

2.4.

Coal pyrolysis

The pyrolysis is chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam. Pyrolysis is a special case of thermolysis. The products of this hot no-oxygen reaction are a stream of char and a stream of syngas with TAR (heavy hydrocarbons). The char obtained after the pyrolysis is entirely composed of carbon, so after the combustion the only output products of the combustor are ashes and CO2. Through the cracking the complex organic molecules such as TAR are broken down into simpler molecules (light hydrocarbons). The stream of syngas obtained is sent to the treatment with amine and subsequently to the reformer where the syngas reacts through the heat coming from the combustor with steam and a catalyst to obtain a syngas rich in H2. The stream of H2 is then separated from the overall stream through the PSA (Pressure Swing Adsorption) [2]. The hydrogen production plant via coal pyrolysis has been realized on the base of a model done with the software Aspen Plus, considering the same inlet of coal of the Shell gasification plant [20], on the base of internal reports of ENEL and also from assumptions made to obtain power plant for hydrogen production.

3.

Life cycle analysis

Life Cycle Analysis (LCA) is the assessment of the environmental impact of a given product or service throughout its

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lifespan [4,6,14]. All energy, material, and waste flows released into the environment are evaluated and accounted [4,6]. The LCA evaluation process covers the whole life cycle and considers three main phases: construction, operation, and dismantling. Analysis includes extraction of raw materials, fabrication processes, transport, distribution, utilization/ production, re-use, internal recycling and final disposal. The goal of life cycle analysis is to better understand of the critical points, to compare environmental impact and to select of the least burdensome process or product. LCA is an environmental quality standard and based on ISO 14044, which consists of following part [21]: Goal and scope definition (ISO 14041): means definition of the case study and the reasons behind it. Inventory analysis: during this phase the flows of energy and materials throughout the production process are assessed, reconstructing thus the transformation from raw materials to final product. The issue is an ordered list of all inputs and outputs, which is actually a model of the real system. Life Cycle Impact Assessment (LCIA, ISO 14042): this phase allows passing from data collected during the inventory analysis to the assessment of the environmental impact. The purpose is environmental determination of all effluents and raw material consumptions documented in the inventory analysis. It is necessary in this part to select the impact categories, category indicators and characterization models [15,18,24]. Interpretation (ISO 14043): is the last phase of the LCA study, its aim is to suggest the changes necessary to reduce the environmental impact of the processes or activity considered, evaluate them in order to improve the process itself [17,24].

3.1.

LCA of processes for hydrogen production

3.1.1.

Goal and scope definition

A hydrogen economy will need significant new sources of hydrogen. A variety of process technologies can be used for hydrogen production, including chemical, biological, electrolytic, photolytic and thermo-chemical. Hydrogen is considered as an environmentally friendly energy carrier but its impact to the environment depends on the way how it is produced. Hydrogen production through thermo-chemical water splitting cycles represents an environmentally attractive way to produce hydrogen without using fossil fuels. On the other hand the technologies of WSTC need to be improved while technology for gasification and pyrolysis are on the good level but the emission reduction can play the negative role. There were two main goals for performing LCA: 1. assess the relative environmental impact of four different processes for producing hydrogen e coal gasification, coal pyrolysis, the WH cycle and the SI cycle, 2. determine the most critical part in each cycle which needs to be improved. Life cycle analysis can give us answers to important decision criteria such as the impact of the resources that are used, land use, emissions, the impact to human health, etc. However there is one more important criterion e hydrogen

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cost. This question is not included in this study, but is one of the most important criteria for hydrogen economy. System boundaries for studied processes are shown in Fig. 3. Inlet and outlet streams for processes simulation consist of the raw material flows and waste/emission flows of the operation phase. A critical issue is the definition of the functional unit. All measurement will be referred to it during LCA. This is particularly relevant if two or more different products or processes which should perform the same function are being compared. The functional unit states the reference unit to which all inlet and outlet flows will be referred. For our case as a functional unit 1 kg of produced hydrogen was chosen. There are more possibilities for choosing the functional unit e.g. 1 MJ of produced hydrogen. Our functional unit was reflecting the results of the WSTC simulations which were performed in Aspen Plus and which refer to the weight category.

3.1.2.

Inventory analysis

During inventory analysis, preparation and collection of relevant data of four studied processes were performed. At first the process were divided into the sub-processes. A detailed procedure for data collection is prepared, to be used by the operators. In the study only construction and operation phase were taken into account. Future task is an implementation of dismantling phase of the plants to the LCA. A critical point is often data allocation. Usually different products are produced and it is thus necessary to allocate mass flows to each production line separating as far as possible input and outputs from common stock sources. In our case only mass allocation was taken into account as a percentage of the particular output to the total mass flow.

Table 1 e Hydrogen production and the input of fuel for compared processes.

H2O SO2 Electric power Coal equivalent

Input

Input scaled to the functional unit

Unit

3473.47 70.35 4873.11 e

9.17 0.19 12.86 4.29

kg kg kW kg

The plants are not really exist, but grown out from the real power plants modification. Data for operation phase of the WH cycle and the SI cycle were obtained from Aspen Plus simulation. They were scaled to the functional unit and output steams were allocated according to six principal categories. Oxygen was considered as an avoided product in the WH cycle and the SI cycle because it is possible to re-use it e.g. in fuel cells. Data used for the operation phase of coal gasification were obtained from literature [20,24]. The operation phase’s data of coal pyrolysis were gain from process simulation in Aspen Plus code. Sulphur was considered as an avoided product in the gasification and pyrolysis process. Data used for the construction phase of both cycles were taken from literature and were adopted and modified to our conditions from existed power plants [20]. Data for the operational phase of the WH cycle and the SI cycle were obtained from Aspen Plus simulation and are summarized in Tables 2 and 3. One of the inputs in the operation phase of the cycles is the electric power (Table 1) needed to supply pumps, the compressor, cooler and electrolyser. In the gasification and purification system coal is used as the input (a raw material).

Fig. 3 e Definition of system boundaries for a) the WH cycle, b) the SI cycle, c) gasification, and d) pyrolysis.

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100

Table 2 e Operation phase inputs e the WH cycle. Hydrogen production (kg/h)

Specific production (H2/kg of coal)

112 400 112 400 1624 1682

4190 14 660 379 373

0.04 0.13 0.23 0.22

80 70 60 %

Pyrolysis Gasification WH cycle* SI cycle*

90

Coal/coal equivalent* (kg/h)

50 40 30 20

* For WH and SI cycle were recalculated coal equivalent.

10 0

Human Health

In order to compare all the mentioned processes, required electrical power was recalculated to a coal equivalent (CO2 emissions were also considered). Conservative approach was assumed for this recalculation, i.e. coal was taken into account as the source for all the electricity production.

3.1.3.

Pyrolysis

WH cycle

Resources SI cycle

Fig. 4 e Comparison of processes for hydrogen production e construction phase.

LCA software e SimaPro 7.0

LCA tools were used to investigate hydrogen production impacts from the WH cycle, the SI cycle, coal gasification and coal pyrolysis. Simulations were performed in SimaPro 7.0, which is basically a database able to reconstruct a “history” of several processes and materials and to aggregate an inventory of elemental pollutants inventory in order to obtain values for the selected environmental impact indicators [7].

3.1.4.

Gasification

Ecosystem quality

Life cycle impact assessment

Several widely accepted assessment methods have been determined. There are large differences among these methods when defining categories (they consider different amounts of impact categories) [13]. Concerning our goals the “damageoriented” method is profitable for our analysis. There are more possible methods proposed by SimaPro software, e.g. Eco-Indicator 95, Eco-Indicator 99, Impact2002þ. In this simulation EcoIndicator 99 was chosen because grouping into tree damage categories is considered to be ideal mix of impacts for your goals. All types of impacts in Eco-Indicator 99 are reduced to the following damage categories: Human health, Ecosystem quality and Resources [5], which are shortly described below:

3.1.5.

Interpretation

Interpretation is the final step of LCA, where the results are checking and evaluating, comparing them with the goal, and establishing the limits and completeness of the LCA. Results of the performed life cycle analysis for the compared hydrogen production are shown in Figs. 4e6. The pyrolysis construction phase has very small impact (1.3%) to overall LCA (considering the construction and operation phase, dismantling phase was not consider yet), see Fig. 5 the major impact (98.7%) has operation phase due to big amount of coal used in the process. The impact of the WH cycle is inverse in comparison with pyrolysis – impact of the constriction phase is large (83%) and the rest 17% is operation phase. During operation phase (Fig. 5) the worst results are obtained from pyrolysis, which reflect a high amount of coal used during the process. The coal usage has impact to deletion of fossil fuels, acid rains and coal mining, which is also taking into account in the analysis, has adverse environmental effects. Production of carbon dioxide and other substances that influence climate change have negative impact to human 100 90 80 70 60 %

Human health: typical substances contributing to this category are those which affect respiration or produce cancer, but also those which cause global climate changes and ozone depletion. Ecosystem quality: is calculated by an estimate of how many vegetable and animal species will disappear from the reference area as a consequence of climate changes evidently linked to the product/process.

Resources: include extracted raw materials, and the fuels and minerals used.

50 40 30

Table 3 e Operation phase inputs e the SI cycle.

H 2O I2 Electric power HI H3PO4 CO2

20

Input

Input scaled to the functional unit

Unit

4313 26 4770 1443 1764 e

12 106 101 14 5 11

kg kg kW kg kg kg

10 0

Human Health Gasification

Ecosystem quality Pyrolysis

WH cycle

Resources SI cycle

Fig. 5 e Comparison of processes for hydrogen production e operation phase.

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100 90 80 70

%

60 50 40 30 20 10 0

Human Health Gasification

Ecosystem quality Pyrolysis

WH cycle

Resources SI cycle

Fig. 6 e Comparison of processes for hydrogen production e LCA (construction and operation phase).

health and also to ecosystem quality. Usage of coal effects release of CO2, which is one of the greenhouse gases and causes also climate change and global warning. Pyrolysis has the largest impact in all LCA categories (Fig. 6). The impact of consumed coal is much higher during the pyrolysis process in comparison with gasification. This is mainly due to the fact that the amount of hydrogen produced from gasification is nearly 3.5 times higher than from pyrolysis. For coal gasification and coal pyrolysis the main impact to ecosystem quality is due to climate changes related to the emission of CO2 and other substances that influence the climate change. The higher impact of the SI cycle is because of problems with recycled iodine used during the simulation of the SI cycle in Aspen Plus. A future improvement of the SI cycle simulation is necessary and the subsequent LCA should be completed. When the overall LCA for the four processes is considered, the disadvantages of the construction phase are negligible in comparison with the operational phase. Therefore the main attention for cycles improvement is focused on operation phase.

4.

Conclusions

Based on Aspen Plus simulation, life cycle analysis for the WH cycle and the SI cycle was performed and was compared with two other processes for hydrogen production e coal gasification and coal pyrolysis. The LCA results confirm that the water splitting thermo-chemical cycles are attractive methods for hydrogen production due to their low environmental impact. Furthermore, the utilization of solar energy as a heat source decreases detrimental environmental impact during hydrogen production. The SI cycle and the WH cycle use solar energy as a heat source and similar results for both cycles were expected. Problems with the HI separation process and imperfection in oxygen separation have a big influence on LCA results; the use of H3PO4 also has a negative impact. Let us assume that after improvement in the HI separation process for the SI cycle the LCA results will be more similar to WH cycle results. All results are recalculated to the functional unit (1 kg of produced hydrogen) so the impact of each process also depends on overall hydrogen production. In the future could be also

performed LCA with different functional unit, e.g. 1 MJ of produced hydrogen. There is still place for improving research activities for of both WSTC simulations by optimizing of particular components (e.g. the separation of H2SO4/HI mixture e the most critical part in the SI cycle) in order to improve the cycle’s efficiency and maximize the hydrogen production; both of these factors influence life cycle analysis results. New results from simulation in Aspen Plus code could be applied for the life cycle analysis and also a dismantling phase could be considered in the future in order to realize a complete LCA study.

Acknowledgments This contribution is the result of the project implementation VEGA under num. 1/0687/09 and the Marie Curie research training network ‘Inspire’.

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