Life-Cycle Assessment and Systems Analysis of Hydrogen Production

Life-Cycle Assessment and Systems Analysis of Hydrogen Production

C H A P T E R 20 Life-Cycle Assessment and Systems Analysis of Hydrogen Production Abhijeet Pandurang Borole1, 2, Anne Landfield Greig3 1 The Univers...

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C H A P T E R

20 Life-Cycle Assessment and Systems Analysis of Hydrogen Production Abhijeet Pandurang Borole1, 2, Anne Landfield Greig3 1

The University of Tennessee, Knoxville, TN, United States; 2Oak Ridge National Laboratory, Oak Ridge, TN, United States; 3Four Elements Consulting, LLC, Seattle, WA, United States

1. INTRODUCTION This chapter aims to understand the environmental and related impacts of various hydrogen production technologies to obtain a comprehensive view of the hydrogen technology landscape. The primary goal is to evaluate biohydrogen technologies; however, most biological routes to hydrogen are not mature. A few key, mature, nonbiological technologies are included in this assessment, so the big picture on hydrogen technologies can be realized and critical drivers for advancing biohydrogen technologies can be identified. Included in the comparisons are steam reforming, water electrolysis, and solar and biomass thermochemical methods. Since a number of different tools have been used by various organizations to perform life-cycle assessments (LCAs) of the hydrogen production technologies, a direct comparison of the LCA results is difficult. The boundary conditions and modeling approaches for the individual analyses vary in some cases. When LCAs are calculated using different tools, the background data are often different, making precise comparisons almost impossible. Therefore any inferences from the comparative analyses should be taken with caution and an understanding of the differences in the LCA methods as well as boundary conditions used should be considered. As such, these differences are highlighted in the first few sections of the chapter and mentioned, wherever possible, during the discussion of the results. Following the LCA comparison, some deficiencies of the assessment of impacts reported using conventional LCA methods is described. A more holistic approach consisting of systems-level assessment is outlined, including economic and societal impacts in addition to the environmental impacts. Such an assessment has been reported in a few LCA studies already, as well as in a few independent studies, which provide a more comprehensive overview. Examples of such assessment include normalized ranking comparison, single point scores, etc., which allow quantification or visualization of multiple parameters in one figure

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

or chart. These methods offer an added dimension to the analysis needed to move the hydrogen production technologies towards more complete assessment of their socioeconomic and environmental impact. A total of seven technologies are assessed (Table 20.1). The various aspects of the analysis and methods used are listed in Table 20.1. The following sections give a brief overview of the technical aspects of the technology, followed by the major process parameters and the life cycle inventory (LCI) data, as well as the LCA boundaries of the referenced studies. In the following Section, the LCA results are presented, focusing on two main parameters: global warming potential (GWP) and global energy requirements (GERs) per unit of hydrogen mass produced. Additional LCA parameters, such as acidification potential (AP), etc., are also discussed, wherever such information is available.

TABLE 20.1

Hydrogen Production Technologies and Corresponding Life-Cycle Assessment Details LCA Calculation Tool

Major Process Components

Parameters Reporteda

Reference

1. Indirect biophotolysis

SimaPro 7.1 (2007)

Algal growth and fermentative H2 production

GWP and GER

[1]

2. Integrated fermentationMEC

GREET

Biomass pretreatment, hydrolysis, fermentation, and microbial electrolysis

GWP and GER

[2]

3. Biogas reforming

SimaPro 8 (v. 7.1.8)

Anaerobic digestion, autothermal reforming, and hydrogen purification

GWP, AP, ODP, POCP, EP, and GER

[3]

4. Natural gas reforming (with and without CCS)

SimaPro 7.1 and SETAC, 1997, 1998/NREL

Steam reforming and hydrogen purification (carbon capture and sequestrationdoptional)

GWP, AP, ODP, POCP, EP, and GER

[4e6]

5. Electrolysis

SETAC, 1997, 1998/NREL (no tool specified)

Wind turbine manufacture, installation, and electrolysis

GWP and AP

[4,5]

6. Solar splitting

SimaPro 7.1

Solar radiation capture, thermochemical cells, and catalyst manufacture

GWP, GER, and Renewability

[6]

7. Biomass gasification

ISO 14040b

Biomass production, transportation, gasification, and hydrogen separation and purification

GWP and GER

[7]

a

LCA parameters reported: GWP: global warming potential, E: energy used /kg-H2, GER: global energy requirement, AP: acidification potential, EP: eutrophication potential, ODP: ozone depletion potential, POCP: photochemical ozone depletion potential. b The researchers did not mention the specific software used, but the study followed ISO 14040 guidelines.

2. RENEWABLE METHODS OF HYDROGEN PRODUCTION

487

2. RENEWABLE METHODS OF HYDROGEN PRODUCTION 2.1 Photobiological Hydrogen Photolysis of water using sunlight or photons can result in capture of energy by biological systems in the form of biomass. This process is known as biophotolysis. This is the principle by which plants grow, but none of the plants directly generate hydrogen, since they do not possess the enzymes catalyzing hydrogen production. Microorganisms, on the other hand (algae), are capable of light harvesting, as well as hydrogen production, since they have proton-reducing enzymes to catalyze the hydrogen production. Growth of algae in specially designed photobioreactors can result in capture of the hydrogen due to the closed nature of the photobioreactor. Microalgae are most suitable for this purpose since they can be cultured in photobioreactors and show high hydrogen yields. Green algae and blueegreen algae are able to split water molecules and generate hydrogen ions and oxygen via direct and indirect biophotolysis, respectively. Hydrogenase enzymes catalyze proton reduction generating hydrogen in green algae. However, this enzyme is oxygen-sensitive, so hydrogen can be generated only when oxygen content is very low. Additionally, light-harvesting proteins are also not efficient and waste over 90% of the photons as heat. Significant work has been done to overcome the oxygen sensitivity and optimizing light-harvesting pigment concentration, but it still remains a challenge [8]. The largest advantage of the direct biophotolysis method is that hydrogen can be produced at standard temperature and pressure. This method has only been demonstrated at a laboratory-scale; therefore much more work needs to be done to bring this technology to commercialization. Indirect photolysis is a process where the light capture and hydrogen production processes are physically separated. The former process generates sugars and other biopolymers, which are stored in cells, followed by a second step, where the biopolymers and cellular biomass are broken down to generate hydrogen catalyzed by hydrogenase and/or nitrogenase enzymes. Dark fermentation uses anaerobic bacteria to degrade the organic material in the absence of light and is an attractive technology alternative, since it can also use other organic wastes as feedstock. One limitation of this kind of fermentation is the yield of hydrogen. A maximum theoretical yield of 4 is possible per mole of a 6-carbon sugar molecule via fermentation [9]. This is due to generation of carboxylic acids as coproducts, required to be generated to maintain redox balance in the cell. An integrated process based on indirect biophotolysis, consisting of growth of algal biomass using CO2 and sunlight and conversion of the biomass into hydrogen was reported by Ferreira et al. [10]. An LCA analysis was reported on this process using the method of indirect biophotolysis [1]. In the study reported by Ferreira et al., biomass growth was done in a 4500 L outdoor raceway pond (48 m2) to grow Scenedesmus obliquus. The circulation was based on the use of paddle wheels and the growth period was 55 days. The biomass was dried in an oven at 80 C for 16 h generating 4.7 kg of dry biomass. This biomass was fed into a dark fermentation system growing Clostridium butyricum under anoxic conditions. A yeast nitrogen base was used for growth and fermentation was carried out at 37 C for 48 h. The researchers used SimaPro 7.1 software for the LCA analysis [1]. Two scale-up scenarios were considered. One used a productivity of 2 g/m2-day (pilot-scale) and the other 25 g/m2-day (industrial-scale). Fig. 20.1 shows the flow diagram for this process, which includes production of the algae and its use for hydrogen production by C. butyricum.

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

FIGURE 20.1 Schematic of the indirect biophotolysis process for hydrogen production. (A) Scenedesmus obliquus biomass production, (B) fermentative medium preparation, (C) fermentation. Extracted with permission from A.F. Ferreira, J. Ortigueira, L. Alves, L. Gouveia, P. Moura, C. Silva, Biohydrogen production from microalgal biomass: energy requirement, CO2 emissions and scale-up scenarios, Bioresour. Technol. 144 (2013) 156e164.

The LCA included the pilot-scale and industrial-scale scenarios; however, due to the early stage of this technology, the LCI data was based on fermentation experiments conducted in the laboratory. The inventory included components of microalgal growth to hydrogen production, such as nutrient medium, preinoculum preparation, fermentation unit operation, and hydrogen purification. Hydrogen storage or distribution was not included. The electrical input was derived from a local grid based in Portugal, where the study was conducted. It consisted of 56% nonrenewable and 44% renewable energy. Transmission losses of 8% were included. The raceway pond used 0.1 kW of energy and was covered with canvas. The algal recovery process included decantation and centrifugation, followed by drying for 16 h. The nutrient usage was included in the LCI, as presented in Table 20.2. Many of the unit operations used in this study for LCA were extrapolated from laboratory conditions, although the scale-up scenarios did include simulation based on process equipment, such as flocculators/electrocoagulators, solar ovens, etc. A factor of 1.83 kg CO2/kg-algal biomass

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2. RENEWABLE METHODS OF HYDROGEN PRODUCTION

TABLE 20.2

Material Input and Process Output Data for Indirect Biophotolysis Inputs/Outputs Data in Hydrogen Production

Process (Outputs)

Inputs

Amount Requirement

Microalga biomass production (1 g of Scenedesmus)

Bristol medium culture Nutrients (g/L) NaNO3

0.250

K2HPO4

0.075

CaCl2$H2O

0.033

MgSO4$7H2O

0.075

NaCl

0.025

Fe-EDTA

0.060

KH2PO4

0.175

H3BO3

2.86 3 103

MnSO4$4H2O

2.03 3 103

ZnSO4$7H2O

0.22 3 103

Na2MoO4$2H2O

0.06 3 103

CuSO4

0.05 3 103

COC12$7H2O

0.09 3 103

Water (L)

1.000

Electricity (MJ) Preinoculum (Clostridium butyricum)

Basal medium (20 mL BM1)

3

0.050

N2 gas (cm )

1956

Electricity (MJ)

0.011

Nutrients Trypticase Soya Broth w/o Dextrose (g)

0.100

Cysteine-HCl (g)

0.010

Salt solution A (mL)a

0.200

Salt solution B (mL)b

0.040

Salt solution C (mL)c

0.040

Salt solution D (mL)d

0.200

Phosphate buffer 50 mM (pH 6.8) (mL)

20.00

N2 gas (cm3)

1956

Electricity (MJ)

0.012 (Continued)

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

TABLE 20.2

Material Input and Process Output Data for Indirect Biophotolysisdcont'd Inputs/Outputs Data in Hydrogen Production

Process (Outputs)

Inputs

Stock solutions (20 mL of stock solution)

Yeast nitrogen base (YNB)

Fermentation (3.47 mmol H2/g biomass)

Amount Requirement

N2 gas (cm3)

391

Electricity (MJ)

0.004

Electricity (MJ)

0.005

a

Salt solution A: NH4Cl (1 g/L), MgCl-6H2O (0.1 g/L), CaCl2H2O (0.1 g/L), distilled water (1000 mL). Salt solution B: K2HPO4$3H2O (0.4 g/L), distilled water (1000 mL). c Salt solution C: Resazurin (1 mg/L), distilled water (1000 mL). d Salt solution D: Na2$EDTA-2H2O (5 mg/L), CoCl2-6H2O (1.5 mg/L), MnCl24H2O (1 mg/L), FeSO4$7H2O (1 mg/L), ZnCl2 (1 mg/L), AlCl3$6H2O (0.4 mg/L), Na2WO4$2H2O (0.3 mg/L), CuCl2$2H2O(0.2 mg/L), NiSO4$6H2O (0.2 mg/L), H2SeO3 (0.1 mg/L), H3BO3 (0.10 mg/L), Na2MoO4-2H2O (0.1 mg/L), distilled water (1000 mL). Table reprinted with permission from A.F. Ferreira, J. Ortigueira, L. Alves, L. Gouveia, P. Moura, C. Silva, Biohydrogen production from microalgal biomass: energy requirement, CO2 emissions and scale-up scenarios, Bioresour. Technol. 144 (2013) 156e164. b

was used to estimate CO2 absorption. The system was designed for the end use of hydrogen in a regional fuel cell vehicle fleet in the city of Lisbon. The hydrogen need was based on 9.5 g-H2/km, corresponding to 1.96 kg-H2/taxi [1].

2.2 Fermentative Hydrogen Besides generation of hydrogen from algal biomass under the indirect photolysis pathway, it can also be produced from nonalgal biomass, such as that derived from plant biomass. This is typically referred to as the fermentative hydrogen pathway. Any kind of organic material sourced from plants, as well as animals, wastes, microbes (nonphototrophic), etc., falls under this category. Glucose is a model substrate for this kind of fermentation, and similar to the fermentation pathway used in indirect biophotolysis, the maximum theoretical yield of hydrogen is 4 mol/mol of hexose. Recent developments have tried to overcome some of the limitations of the original process [11,12]. The advantage of this method is that hydrogen can be produced 24 h a day, since there is no light requirement and it does not have land requirement either, in terms of the biomass conversion process. The biggest limitation is the availability of the fermentable substrate of the biomass source. Glucose is an ideal substrate; however, production of glucose from biomass is a difficult and expensive task. Additionally, transport of biomass to one location to achieve economies of scale can be costly, which is a limitation of this pathway, but further innovation can alleviate some of these issues.

2.3 Microbial Electrolysis An upcoming process for generation of hydrogen from waste is microbial electrolysis [13]. This method employs microbial catalysis and electrocatalysis to convert organic waste into hydrogen. Most of the early development of the microbial electrolysis cell (MEC) technology was done using acetic acid as the carbon source, although many more complex substrates have been evaluated recently [14e16]. Electroactive biofilms growing on anode generate electrons from the organic substrate, which are then transferred to the cathode via an external circuit (Fig. 20.2). Additionally, protons are also generated, which have to be transferred

2. RENEWABLE METHODS OF HYDROGEN PRODUCTION

491

FIGURE 20.2 A schematic of an MEC. Reproduced from A.P. Borole, G. Reguera, B. Ringeisen, Z-W. Wang, Y. Feng, B.H. Kim, Electroactive biofilms: current status and future research needs, Energy Environ. Sci. 4 (2011) 4813e4834, with permission from The Royal Society of Chemistry.

from the anode to the cathode. Alternately, hydroxide ions may be transferred to the anode from the cathode, depending on the type of membrane separator used. MEC is a relatively new technology; however, significant progress has been made in the last decade [17,18]. Its use has been demonstrated for conversion of wastewaters from municipal streams, breweries, biorefineries, etc. [19e22]. Technoeconomic and life-cycle analyses have not been conducted for standalone MECs but those for integrated processes have been performed as discussed in Section 2.4.

2.4 Integrated FermentationeMEC An integrated process consisting of fermentation and microbial electrolysis has been suggested to increase the yield of hydrogen from the carbon sources, since only 4 mol can be generated from 1 mol of hexose. There is potential to generate 12 mol of hydrogen per mole of hexose, theoretically; however, there are thermodynamic limitations. Use of an MEC can allow one to overcome those limitations via the use of a small external electrical potential of about 0.3e1.0 V. A preliminary LCA has been reported on this integrated process [2,24]. The process was based on the use of corn stover as the feedstock and consisted of pretreatment and hydrolysis of the corn stover to generate a hydrolysate [2]. A schematic of the integrated process is shown in Fig. 20.3. The scheme included multiple unit operations from biomass handling to hydrogen purification. Simultaneous conversion of glucose and xylose was assumed to generate two primary products: hydrogen and acetic acid. The LCI data for the process is presented in Table 20.3. The water requirement for the process included that for pretreatment, hydrolysis, and enzyme production steps but not for the MEC process. Several assumptions were made for enabling process simulation including: 1. 80 wt% conversion of glucose and xylose to hydrogen in fermenter, 2. 90% conversion of acetic acid to hydrogen in MEC,

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

FIGURE 20.3

Process schematic for integrated fermentationeMEC process. Figure exacted with permission from A. Elgowainy, Q. Dai, J. Han, M. Wang, Life Cycle Analysis of Emerging Hydrogen Production Technologies, 2016. https://www. hydrogen.energy.gov/pdfs/review16/sa057_elgowainy_2016_o.pdf.

TABLE 20.3

Materials Inventory for Integrated FermentationeMEC Process [2]

Corn Stover (kg/kg H2)

23

Ammonia (kg/kg H2)

0.102

NaOH (kg/kg H2)

0.389

H2SO4 (kg/kg H2)

0.207

Glucose (kg/kg H2)

0.335

CSL (kg/kg H2)

0.008

DAP (kg/kg H2)

0.015

Process water (gal/kg H2)

27.5

Energy inputs, electricity (kWh/kg-H2)

6.03

2. RENEWABLE METHODS OF HYDROGEN PRODUCTION

493

3. MEC electricity requirement: 15 kWh/kg H2, 4. 80 wt% H2 recovery for pressure-swing adsorption (PSA), and 5. 300 miles H2 transportation and distribution distance.

2.5 Biogas Reforming Much of organic waste generated in the western countries end up in landfills. In addition to the loss of an energy resource, this also poses an environmental problem. Landfills generate methane, which is released into the environment, which could be a safety hazard for buildings nearby. Anaerobic digestion is a century old technique to generate energy from waste, if the biogas thus produced can be captured. This technique can be employed to solve both the problems of waste to landfill and minimizing greenhouse gas (GHG) emissions and reducing GWP. Furthermore, the methane from the biogas can be converted into hydrogen via steam reforming, resulting in further benefits. An LCA was reported by Battista et al. for a biogas autothermal reforming (ATR) process, called BioRobur, is included here [3]. Compared to steam reforming, the ATR process has been reported to be more efficient [25], since it operates mostly in an adiabatic state, without much loss of heat, which is common in the steam reforming process. The process is shown in Fig. 20.4. Oxygen is added to the inlet gas to obtain a ratio of 1:1 versus carbon, as well as steam to get a steam:carbon ratio of 2:1. The biogas sourced by Battista et al. came from agro-food waste, which resulted in approximately 60% methane and 40% carbon dioxide with small amounts of NOx and other hydrocarbons. The CO2 from the biogas did not require separation and was directly used in the ATR reactor. A conversion factor of 250 m3/ton of agro-food waste was used. A 65% efficiency of conversion of methane to hydrogen was assumed. The process included a power plant operated at a temperature of 800e870 C. A soot trap followed the reformer, followed by high- and low-temperature wateregas shift reactor and a PSA system for hydrogen

FIGURE 20.4 Schematic of the biogas reforming process: BioRobur. Figure extracted with permission from F. Battista, Y.M. Camacho, S. Hernández, S. Bensaid, A. Herrmann, H. Krause, D. Trimis, D. Fino, LCA evaluation for the hydrogen production from biogas through the innovative BioRobur project concept, Int. J. Hydrogen Energy 42(19) (2017) 14030e14043.

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

TABLE 20.4

Details of Biogas Autothermal Reforming Process [3]

INLET STREAMS Biogas (m3)

3,111,768

Water (kg)

2,722,104

Air (kg)

5,708,736

POWER (W) PSA compressor

18,454

Air compressor

837

Water pump

29

ENERGY CONSUMPTION (TJ) PSA compressor

5.26

Air compressor

0.24

Water pump

0.00827

purification and production at 1.5 bar and 40 C. They used SimaPro 8 software 7.1.8 to do the analysis. The study targeted production of 4.5 kg H2/h. The details of the process and catalysts (Scenario 1 and 2) are presented in Table 20.4. In addition, the LCI for the process included materials, such as steel, cast iron, copper, synthetics, aluminum oxide, fiber and ceramic materials, plus other metals [3].

3. HYDROGEN FROM FOSSIL RESOURCES/NONBIOLOGICAL METHODS 3.1 Steam Reforming Steam methane reforming (SMR) is a fully developed commercial process for generating hydrogen; however, it uses fossil fuel resources, such as natural gas [26]. As such, it can generate high amounts of CO2, leading to high GHG emissions. The SMR process includes a high-temperature chemical reaction between steam and natural gas resulting in the production of a gas mixture containing hydrogen, carbon monoxide, and carbon dioxide. The reaction is endothermic, so an external source of heat is required. It does not use oxygen directly; however, combustion of natural gas with air is typically used to generate the heat in a separate combustion reactor. A wateregas shift reaction is a follow-on step to convert the carbon monoxide to hydrogen. Many researchers have reported life-cycle analysis of the steam reforming process [5,6,27]. Due to the high volume of CO2 produced, carbon capture and sequestration (CCS) methods have been investigated as an add-on operation. Here, both

3. HYDROGEN FROM FOSSIL RESOURCES/NONBIOLOGICAL METHODS

TABLE 20.5

495

Energy and Material Balance for the SMR-CCS Process [6] Parameter

Overall conversion

85%

Reaction temperature

850 C

Natural gas consumption as reagent, kg/kg-H2

3.1

Natural gas consumption as heat, MJ/kg-H2

13.7

Steam consumption, kg/kg-H2

14.8

Steam production, kg/kg-H2

10.2

Catalysts consumption, g/kg-H2

0.524

Electricity consumption, MJ/kg-H2

14.8

options, with as well as without CCS, are included to compare with the renewable methods of hydrogen production. The results obtained by Dufour et al. for LCA analysis of the steam reforming process are included here [6]. Their results were based on two previous studies that included detailed LCI and process details [5,27]. The process is operated at 850 C and assumes a total conversion of 85%. Table 20.5 presents the other process details. In addition to the primary natural gas reforming reaction, low- and high-temperature wateregas shift reactions are also used to generate hydrogen. Hydrogen purification is carried via PSA. A simplified process block diagram is shown in Fig. 20.5. The CCS-based process uses an amine to concentrate CO2, followed by an underground injection. The excess heat is recovered to make steam required for the reforming reaction.

3.2 Electrolysis Water is the cleanest source of protons for hydrogen production; however, it requires an energy source to split the water. Water splitting is carried out in an electrochemical cell,

FIGURE 20.5 Process block diagram for hydrogen production via natural gas steam reforming. Reproduced with permission from P.L. Spath, M.K. Mann, Life Cycle Assessment of Hydrogen Production Via Natural Gas Steam Reforming, National Renewable Energy Lab., Golden, CO (US), 2000.

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

which partitions the hydrogen and oxygen into two different chambers. The cells are arranged in series or parallel resulting in modular systems. The water used has to be of high-purity and the hydrogen generated is cooled, purified, compressed, and stored. Renewable, as well as fossil, resources have been used to power electrolysis; however, here we will primarily discuss the renewable technologies. Electricity derived from wind, solar, hydropower, etc., is on the rise and can lead to substantial amounts of hydrogen production. A number of researchers have investigated the environmental effects of renewable energybased electrolysis; however, Spath and Mann provided the first complete picture of these effects [28]. While electrolyzers have existed for many decades, they are still not economical compared to fossil-based hydrogen technologies. Many different types of electrolyzers exist, such as alkaline, polymer electrolyte membrane, solid oxide, etc. [4]. The capacity of electrolyzer for producing hydrogen is one parameter that differs among these technologies. Alkaline electrolyzers are the most developed and are commercially available with capacities up to 760 Nm3/h. The analysis of environmental impacts has shown that the electrolyzer itself is less of a contributor compared to the source of electricity; therefore most LCA studies classify the technologies based on electricity source rather than the electrolyzer type. For instance, contribution of materials to make the electrolyzer contributed 1%e15% versus 53%e96% coming from the electricity generation component of the lifecycle database [4]. In this chapter, wind electrolysis is used as the primary electrolysis reference method, since it has the most favorable LCA impacts among all electrolysis methods [4]. The major source of the LCI data for wind electrolysis used in LCA studies reported in the literature is Spath and Mann [28]. Their system used three 50 kW Atlantic Orient Corporation wind turbines with a 30 Nm3/h Stuart Energy electrolyzer. The analysis included transportation of turbine blades from a manufacturer in Europe to Midwest via ships and then rails from a US port. Their electrolyzer was sized for 75% of maximum wind speed and included multiple turbines connected to the electrolyzer, which had an efficiency of 85%. The transmission losses were assumed to be 7.03%. The LCA system boundary included hydrogen compression to a pressure of 20 MPa and storage (Fig. 20.6). Dispensing of the hydrogen at a fueling station for supporting 36 vehicles at 3 kg of H2 per week was also included.

3.3 Solar Technologies A number of solar technologies have evolved for hydrogen production due to the ubiquitous nature of this energy source. The major ones include photovoltaic, photosplitting, and two-step thermochemical. Photovoltaic refers to use of sunlight for producing electricity, followed by traditional electrolysis to make the hydrogen. Photosplitting refers to the use of photons to split water directly using a photosensitive catalyst and thermochemical refers to a method where solar heat and chemicals are used to decompose water into hydrogen and oxygen. Assessments comparing these methods have reported that photosplitting process results in lowest GWP [4,6]. This method is included in this chapter for comparison with other methods. Photosplitting or photocatalytic decomposition generates electrons and holes, which oxidize/reduce water into oxygen/hydrogen, respectively. Titanium oxide is a commonly used photosensitive material, but its yield is quite low in the visible spectrum,

3. HYDROGEN FROM FOSSIL RESOURCES/NONBIOLOGICAL METHODS

497

FIGURE 20.6 Block diagram for hydrogen production via wind electrolysis. Reproduced with permission from R. Bhandari, C.A. Trudewind, P. Zapp, Life cycle assessment of hydrogen production via electrolysisea review, J. Clean. Prod. 85 (2014) 151e163.

which is not good for a process aspiring to have high efficiency and durability. Some of the new materials reported include CdSeCdOeZnO or Cd1xZnxS, where x ¼ 0.2e0.4 [6]. The process for hydrogen production reported by Dufour et al. requires the use of two sacrificial chemical reagents: Na2S and Na2SO3 (Fig. 20.7). These agents prevent photocorrosion of the catalyst due to oxygen and degradation of the catalytic material. The process parameters are presented in Table 20.6. The LCA of this process assumed a lifespan of 5 years or approximately 10,000 working hours for the equipment.

FIGURE 20.7

Flowchart for water photosplitting. Extracted with permission from J. Dufour, D.P. Serrano, J.L. Gálvez, A. González, E. Soria, J.L. Fierro, Life cycle assessment of alternatives for hydrogen production from renewable and fossil sources, Int. J. Hydrogen Energy 37(2) (2012) 1173e1183.

498

20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

TABLE 20.6

Energy and Material Balances for Solar-Based Water Splitting Process [6]

Parameter

Catalyst 1

Catalyst 2

Energy efficiency

0.5%

0.5%

Photocatalyst

Cd1xZnxS, where x ¼ 0.3

CdSeCdOeZnO

Photocatalyst consumption, kg/kg-H2

0.019

0.027

Na2S consumption, kg/kg-H2

0.179

0.179

Na2SO3 consumption, kg/kg-H2

0.115

0.115

Land use, m3a/kg-H2

70,595

70,595

Deionized water, kg/kg-H2

9.7

9.7

Heat as natural gas, associated with catalyst manufacturing, MJ/kg-H2

8.8

9.2

3.4 Biomass Gasification Another way of generating hydrogen from the sun’s radiation is using the energy content captured from the sun via photosynthesis. Use of biomass to make hydrogen is, however, classified under a different set of technologies tied to the use of biomass resource. Here, one of the thermochemical methods to convert biomass to hydrogen via gasification is included. In this process, biomass is gasified with air and steam, and the resulting gas stream is cleaned and used for hydrogen production via wateregas shift reaction. Many variations of the process exist, depending on the type of gasifier used, ratio of oxygen in air, pressurization and downstream separation, etc. [29,30]. Here, the life-cycle analysis reported for a downdraft gasifier-based process is included [7,31]. As shown in Fig. 20.8, the gas is then used for power generation in addition to hydrogen production. These operations require different pressures (20 and 35 bar, respectively), which is carried out using individual compressors. The hydrogen is then purified using PSA equipment to 25 C and 32 bar. In the study reported by Kalinci et al. they sectioned the process into pretreatment of biomass, hydrogen production, and the use of hydrogen [7]. Fig. 20.8 shows the first two components of the process they studied and the respective LCA boundaries. The LCI data for the gasification plant is presented in Table 20.7. Pinewood with 22.6% moisture was used as the biomass at a mass flow rate of 2531 ton/year. The biomass was dried to 8% before use in the gasifier. Their process targeted a hydrogen production rate of 38,094 kg/day.

4. LIFE-CYCLE ASSESSMENT 4.1 Methodology The first published guidelines on LCA were developed by SETAC in the 1990’s, which eventually led to the creation of the International Standardization Organization (ISO) 14040 standard on LCA in 1997 [32]. ISO 14040 and earlier versions of the LCA standards

4. LIFE-CYCLE ASSESSMENT

499

FIGURE 20.8 Biomass gasification process for hydrogen production with LCA boundaries. Figure extracted with permission from Y. Kalinci, A. Hepbasli, I. Dincer, Life cycle assessment of hydrogen production from biomass gasification systems, Int. J. Hydrogen Energy 37(19) (2012) 14026e14039.

TABLE 20.7

Lifetime Material Usage for Downdraft Gasifier-Based Hydrogen Production. Plant Life was Used as 15 years [7]

Material

Amount Required, mg/kg-H2

Concrete

138

Steel

44

Aluminum

0.36

Iron

0.54

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

evolved into more precise requirements and guidelines.1 They continue to be revised, and newer LCA-related guidelines, based on these core standards, are continuing to be developed.2 While the ISO 14040 [33] and 14044 standards [34] offer adequate guidance for a properly executed LCA and lend credibility to a study, the flexibility that is given around study parameters (including the system boundaries, allocation decisions, impact categories evaluated, etc.) make it difficult to interpret results of separate studies. In other words, even though many of the studies presented in Table 20.1 are based on the ISO guidelines, they may still vary considerably in many respects. Valente et al. have given a comprehensive overview of LCA studies pertaining to hydrogen production technologies [35]. The main components of these LCA studies include resource extraction, production, and supply of the hydrogen to end users. The use of hydrogen (i.e., fuel for transportation, etc.) is not always included in the analysis. Another important factor is the physical state of delivered hydrogen, which can vary from low pressure to as much as 200 bar or even as liquefied hydrogen. A few studies have reported assessments using the hydrogen in fuel cell vehicles compared with gasoline-powered vehicles [36,37]. The resource data was identified in earlier sections as the LCI data set. For fossil-based technologies, inclusion of resource extraction is very important, while for biohydrogen technologies, their contribution is much less, so it is less important; however, any contribution no matter how small, has to be included in the analysis. Carbon dioxide capture and sequestration (CCS) is important for natural gas reforming, so results may include the comparison with and without CCS. Impact assessment has been reported in the literature using different methods, including CML 2001, Eco-indicator 95, and a European Union-based method FCHyGuide. The impact parameters commonly reported in LCA studies include GWP, GER, AP, ozone layer depletion potential, photochemical oxidation potential, also called photochemical ozone creation potential, and eutrophication potential (EP). The functional unit of hydrogen production is an important parameter in comparison of different technologies. A kilogram of hydrogen has been used as the unit of measure, while some researchers have reported results based on Nm3 or MJ of hydrogen produced. For this chapter, all the results have been converted to use a kg of H2 as the functional unit for comparative purposes. Another important factor in reporting of the LCA is the purity of hydrogen. Usually, the end use for fuel cells requires 99.999% purity; however, not all studies report it to this purity. The hydrogen pressure or physical state (gas or liquid) is a related measure and is important because significant energy is needed to compress or liquefy hydrogen. Storage tanks can contribute a good amount to the LCA results as well. Table 20.1 also lists the major conversion/process components of various LCA studies included in the comparison here. 1

ISO 14040:2006, the International Standard of the International Standardization Organization, Environmental management: Life cycle assessment. Principles and framework. This is the second edition of ISO 14040; together with ISO 14044:2006 Environmental managementdLife cycle assessmentdRequirements and guidelines, these cancel and replace ISO 14040:1997 [32] ISO, ISO 14040: The International Standard of the International Standardization Organization, Environmental management. Life cycle assessment. Principles and framework, 1997 [31]. ISO, ISO 14040: The International Standard of the International Standardization Organization, Environmental management. Life cycle assessment. Principles and framework, 1997. ISO 14041:1998, ISO 14042:2000, and ISO 14043:2000. 2

For example, ISO has standards and guidance to produce Type III Ecolabels and carbon footprints of products based on the LCA framework.

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4.2 LCA Comparisons 4.2.1 Global Warming Potential 4.2.1.1 BIOHYDROGEN TECHNOLOGIES

The results from LCA analyses for the seven hydrogen production processes were compiled and their impact on global warming is plotted in Fig. 20.9. The results show that the GWP ranges from 0.97 kg/kg-H2 to as high as 67 kg/kg-H2. The three biological processes are considered first. For the indirect biophotolysis process, the GWP has been reported with an upper and lower bound [10]. The researchers reported values with and without inclusion of the CO2 absorption related to algal growth. A high GWP of 67 kg/kg-H2 was reported for the process when CO2 absorption by the algae is not included. However, since CO2 is removed from the atmosphere during algal growth in the raceway ponds used in the study, the net GWP should include the CO2 absorption. If this is included, the GWP comes out to be a negative value (94.2 kg/kg-H2) indicating that a new removal of the CO2 from the environment is possible. This process is an integration of the algal biomass production and the hydrogen-producing fermentation process. The two processes have been reported in such an integrated fashion relatively recently [1] and not many studies exist on LCA studies of the combined process besides that reported by Ferreira et al. Many assumptions were made in the estimation of the GWP, so the real value is likely to be in between the two ranges reported. The high upper range of the GWP is due to a number of factors including the low hydrogen productivity of 7.3 g-H2/kg of the algal biomass (S. obliquus) used in the analysis. This was based on experimental data and could potentially be improved to improve the LCA, as well as the economics of the process.

FIGURE 20.9 Comparison of global warming potential (GWP) for renewable- and fossil-based hydrogen production processes.

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

The integrated fermentationeMEC process has an overall large GWP (9.8 kg/kg-H2) [2]. The highest GWP contributions to the process come from electricity requirement (34%) and biomass growth (24%), followed by PSA (21%) for compression of hydrogen. The electricity requirement includes that required for MEC, which is in addition to that produced internally via waste burner/boiler turbogenerator. The process requires a total of 21 kWh/kg of H2, which is a large requirement. Reduction in this requirement can be achieved with the use of renewable electricity, since MEC only requires a low voltage (below 1.0 V), which can be easily obtained using distributed production. The second largest contributor is the biomass itself. The process described by Dai et al. envisions a biorefinery processing biomass at large volumes on the order of several tons per day, which requires its transportation to a central facility. While the MEC process is modular, the fermentation process requires large reactors and is not modular, requiring centralized production to achieve economies of scale. One alternative that is available is to modify the MEC to process the biomass hydrolysate directly [14]. Such a process has multiple benefits including modular, distributed production, as well as the potential for utilization of the lignin-derived phenolic compounds, which have been shown to contribute to hydrogen production in an MEC [38e40]. The biogas reforming process is related to the natural gas reforming process and is primarily considered due to the potential for GHG reduction of the reforming process via use of a biogenic source of the methane: biogas. The one specific case included here is called the BioRobur process. Battista et al. reported LCA for this process using a unit of 1 Nm3; however, the results were converted obtaining a GWP of 6.7 kg/kg-H2 for the process. As seen from Fig. 20.9, this is lower than the GWP reported for SMR. Other processes for biogas to hydrogen have also been reported with similar estimates. For instance, the conventional steam reforming of biogas was reported to yield 5.59 kg/kg-H2 of GWP [41]. The BioRobur process emits most CO2 in the compression and preheating component (Educt processing) part of the process. This is due to the high-temperature operation of the ATR process (700 C), which is still lower than conventional steam reforming, resulting in lower emissions. Batista et al. also compared this process to use of biogas in an internal-combustion engine, followed by an electrolyzer. The latter system was reported to result in 72% higher GWP compared to the BioRobur process. The feedstock used in the analysis report does not generate high concentration of contaminants, such as hydrogen sulfide and siloxanes, since it was sourced from agro-food waste compared to other waste sources. The presence of these pollutants at higher levels can result in greater impacts on the environment due to the requirement for their removal prior to the use in the ATR reactor due to the potential of catalyst poisoning. Additionally, the release of methane from digesters operated under less controlled conditions (farms and fields) or from the source of waste itself, e.g., animal manure, can further increase GWP, due to its high potency compared to CO2. 4.2.1.2 NONBIOLOGICAL TECHNOLOGIES

The commercial process that produces most of the hydrogen used in the world today is SMR. The average GWP of this technology is 10.4 kg/kg-H2 [6,42]. Based on the comparison shown in Fig. 20.9, if the CO2 absorption in the biophotolysis process is taken into account, SMR ends up being the process with the largest emissions of all processes considered here. Since there is significant interest in continuing use of SMR, capture of CO2 produced in the process is of much interest. The SMR-CCS process can result in a reduction in the emissions

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by more than half; however, it comes at a cost. Nevertheless, this is an option that needs to be considered, since so many units based on this technology are in existence today. Among the solar energy-harvesting processes, the solar splitting process has been reported to be the lowest in terms of the GWP, which is 1.12 kg/kg-H2 [4,6]. The other solar processes, such as thermochemical cycling and electrolysis, have 4e5 times higher GWP than the solar splitting process. This is primarily due to the higher utilization of steel and construction materials for the thermochemical system and the need for heat energy in the production of solar photovoltaic panels. Compared to the other processes considered in this chapter, solar splitting has the second lowest GWP value. This is due to the simplicity of the process and the assumed durability of the photocatalysts, which does need to be confirmed via long-term use. The form of electrolysis included for comparison with other hydrogen technologies is wind electrolysis. Use of wind energy for water splitting was reported to have the lowest GWP impact amongst the electrolysis technologies compared in an analysis by Bhandari et al. [4]. Some of the differences from other methods, such as PV and hydrothermal, come from specific aspects of these processes, such as the emissions-related PV module manufacturing or the massive civil works associated with hydropower. One other form of electrolysis of potential interest is grid-based electrolysis. This is directly related to the makeup of the grid electricity in different countries and regions. The majority of the countries still rely on fossil fuels for electricity, so this option typically leads to high GWP. In the technologies considered in this chapter, wind electrolysis ranks the lowest even in comparison to other nonsolar technologies. Thus, at present, this is the least impact option available for hydrogen production in terms of the global warming effect. This is primarily due to the low fossil energy requirement for wind electrolysis. Looking at the resources used for harvesting wind energy, the contribution of iron and limestone is large compared to the oil and natural gas, since large amounts of materials are needed for wind towers and turbines, but still the total GWP is lower than any of the other technologies. Biomass gasification ranked third among the technologies evaluated. Kalinci et al. [7], who described the downdraft gasifier technology used as the reference biomass technology here, reported lower values than those used here from Bhandari et al. [4]. Bhandari et al. used data from Kalinci et al. and other references and recalculated the LCA parameters. The GWP estimates for the bio-based technologies for hydrogen production are expected to evolve as the technologies become more mature. At present, most of the data used in the analyses are based on laboratory experimentation and little information exists on pilot or demo units. However, the results also show that the GWP is on the same order as the SMR for fermentationeMEC and the biomass reforming process. So, further innovation is needed to integrate renewable energy and reduce fossil energy use in the production of hydrogen via these methods. 4.2.2 Global Energy Requirements This is the total energy demand for the process including the whole value chain from resource extraction to the point of end use. It includes the gross heating value of all raw materials derived from fossil sources, as well as the nonrenewable energy required during processing of all materials. A related factor that has been reported in the literature is “Renewability,” which is the ratio of the renewable energy use to the total energy use, including the solar and fossil energy used [43].

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

It should be noted that the SMR-CCS process has a higher energy requirement compared to the SMR process, due to the need for the extra energy for carbon sequestration. The SMR process requires fossil fuels for not only heat production but also as a reagent for hydrogen production. In the SMR-CCS process, the use of natural gas as a reagent is about 75% of the total energy used. Thus replacement of this fossil resource with biogenic resource can improve the environmental impact of the process. This is what is done in the biogas reforming process. Thus a 75% reduction in GER is observed. The renewable technologies based on solar and wind have the lowest GER as expected, due to the majority of the energy needed for hydrogen energy generation coming from renewable sources. The indirect biophotolysis process shows the highest GER among all technologies evaluated (Fig. 20.10). This is unusual, since solar energy is used as a source; however, the high GWP reported by Ferreira for this process is likely related to the origin of the data used for chemicals and nutrients for algal growth and fermentation (Table 20.2). The analysis was based on upscaling nutrient requirements from that used in the laboratory, so this apparently contributed significant energy towards the LCA. Another parameter that most likely affected the GER is the low efficiency of conversion of the algal biomass into hydrogen, which was reported to be 7.3 g/ kg of biomass. Thus many opportunities exist for obtaining correct data for the LCI, which can reduce the energy requirements for the indirect biophotolysis process. The integrated fermentationeMEC process reported by Dai et al. only included the electricity requirements for the process. No energy data was given for the biomass growth and transportation. Therefore the estimate shown in Fig. 20.10 of 22 MJ/kg-H2 is likely not representative of the GERs. In case of the biomass gasification, the data reported by Kalinci et al. yielded a GER of 24.7 MJ/kg-H2 [7]. Their analysis included the energy requirement for biomass chipping, transportation, embodied energy in materials, as well as that for hydrogen compression and transportation. Thus the GER estimate for the biomass gasification is likely to

FIGURE 20.10 Comparison of global energy requirement (GER) for hydrogen production from biological and nonbiological methods.

5. SYSTEMS ANALYSIS

505

be close to the overall energy requirements. Additionally, in comparing the other technologies, this alternative represents a highly favorable GER, since most of the other technologies do not include hydrogen transportation costs, which are close to 40% of the overall energy requirements. Thus biomass gasification energy needs are similar to the wind and solar technologies. 4.2.3 Acidification Potential The negative effect of a process/technology on the environment resulting from acid rain and reduced vegetation is called AP. It is due to chemicals, such as sulfur oxides, nitrogen oxides, and ammonia. This parameter has been reported by only a few researchers, and not available for the majority of the biotechnologies explored here. Bhandari et al. reported the AP for some of the established technologies. Among the technologies considered here, wind electrolysis had the lowest AP of 2.6 g SO2-eq/kg H2 [4]. This was followed by SMR at 15 g SO2-eq/kg-H2. Plant biomass-based hydrogen technologies, such as fermentationeMEC, may rank high in this category due to the need for fertilizers, plantation, biomass transportation, etc. 4.2.4 Other Categories Categories, such as EP, POCP and ODP, are not always reported by researchers. However, a recent comparison of fossil, as well as renewable, technologies by Bhandari et al. reported wind electrolysis to have a lower impact among others including hydroelectrolysis, solar electrolysis, SMR, in that order [4].

5. SYSTEMS ANALYSIS When the concept of LCA was originally developed, it was the only way of capturing the potential impacts of a given technology on the human condition. Over the last few years, the evaluation of human impact has evolved beyond environmental sustainability. It is not only important to look at environmental impacts, but also on social and economic impacts beyond the boundaries of the immediate process and the region. A global perspective has emerged resulting in the need to assess the impacts of a technology on all living systems on the planet. Thus a more comprehensive “Systems Analysis” concept has emerged to enable a better understanding of the global economic, social, and environmental impacts of a technology, as well as the interrelationships between them. Fig. 20.11 shows the parameters that need to be considered for a comprehensive analysis of the various factors impacting sustainability from environmental, economic, and social perspectives. GHGs and energy needs are the basic parameters, but in addition, impacts related to water, land, soil, and air quality; efficiency, as well as social factors, such as job growth, physical and mental health, education, and security; and economic factors, such as local economy, minimum wages, and percent employment, should be considered. In this section, the concept of systems analysis will be explored for bringing together the multiple impacts related to the hydrogen production processes. Fig. 20.11 identified the high-level framework for integrated assessment of the various impacts [44]. Such a concept is not new and has been developed by many researchers, although its application to the hydrogen production process is relatively new. A single score comparison method based

506

20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

FIGURE 20.11 Systems analysis via determination of overall impact and sustainability parameters. Reproduced with permission from A.P. Borole, Sustainable and efficient pathways for bioenergy recovery from low-value process streams via bioelectrochemical systems in biorefineries, Sustainability 7(9) (2015) 11713e11726.

on the Eco-indicator 95 method can be considered as the first step towards this approach, integrating all the environmental impacts into one. This score is reported in “m Pt/Nm3H2,” which is based on a formula to combine all points. The upper and lower values reported to date have ranged from 0.005 for wind electrolysis to 10.3 for biomass gasification-based electrolysis [4]. For SMR, this value is 0.4e0.45; however, that for SMR-CCS is slightly more, apparently due to the use of grid electricity for CCS, which results in NOx production leading to higher acidification and smog impacts. The only way to reduce it would be to use renewable technologies. Koroneos et al. have also reported this score for a few hydrogen production technologies [45]; however, the values do not compare well with those reported by Bhandari et al. This was reported to be due to inclusion of less number of parameters in the single score analysis by Koroneos et al. Thus the single score method of comparison is quite limited in obtaining the big picture encompassing multiple impacts.

5.1 Social Considerations Quantifying the impact of social and economic factors related to a hydrogen technology is important for determining the best paths forward for the society. Recently, a new method

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FIGURE 20.12 SCC comparing the socioeconomic impact of various hydrogen production technologies.

was developed by a group in the United Kingdom focused around quantifying social impacts. They defined a parameter called social cost of carbon (SCC) using an integrated assessment framework. It is based on the relationship between emissions and resulting temperature changes, which were then linked to economic damages. It defines a baseline reference and a perturbed scenario based on the introduction of a new technology and calculates the difference. The input parameters include population, rate of energy production, and related emissions. The normalized difference between the baseline and perturbed scenario gives the SCC. Parry et al. reported SCC for selected hydrogen production methods using an average of $160 per tonne of CO2 emissions. Fig. 20.12 shows the SCC for various hydrogen production technologies considered here. It should be noted that the SCC analysis here is primarily based on this proposed cost of carbon emissions. So, even this parameter does not capture the impacts of the technologies on the society completely. Based on current estimates of carbon emissions, the SCC results indicate that the biohydrogen technologies need to be improved to reduce their impact on the society.

5.2 Economic Considerations The cost of production of hydrogen is probably the most important factor, since most decisions made by industries undertaking the massive job of implementing these new technologies will use economic gain as the primary determinant of which technologies to implement. Even this simplistic idea to decide what should be done is not free of challenges. This is because many hydrogen technologies are not yet fully developed, and the true costs of the technologies are not known. Nevertheless, it is important to understand the potential of various technologies using the best estimates available. Table 20.8 gives a comparison of

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20. LIFE-CYCLE ASSESSMENT AND SYSTEMS ANALYSIS OF HYDROGEN PRODUCTION

Comparison of Hydrogen Production Costs for Various Technologiesa

TABLE 20.8EQ.

Hydrogen Production Technology

Capital Costs

Estimated Production Cost, $/kg-H2

Indirect biophotolysis

$135/m2

$1.42

SMR

$180.7 M

$2.08

SMR-CCS

$226.4

$2.27

e

$2.57

Biogas reforming

NA

NA

ATR of methane with CCS

$183.8

$1.48

Biomass gasification

$149.3 M

$1.77e2.05

Wind electrolysis

$504.8 M

$5.89e6.03

Solar splitting

$421 M

$5.10e10.49

Fermentation b

$6.61

FermentationeMEC

a

The cost data presented in the table was obtained from Nikolaides et al. except as indicated below [46]. This cost estimate was obtained from James et al. [47].

b

the current hydrogen production costs of the relevant technologies. The comparison shows that indirect biophotolysis may be competitive with existing commercial technologies; however, it has a high capital cost, which needs further research to increase biomass and hydrogen yields. FermentationeMEC is an upcoming technology, but improvements in costs are needed. A recent study has shown the integration of the two components into one, where the whole biomass can be used to generate hydrogen in an MEC [48]. This can potentially reduce costs via process intensification and utilization of all biopolymers of biomass to make the hydrogen versus just the sugars. The MEC technology, however, is one of the youngest and includes integration of microbial and electrochemical disciplines, which in itself poses new challenges. However, it may be worth the risks due to the significant rewards encompassing productivity, efficiency, and better resource utilization [22,49].

5.3 Energy and Exergy Efficiency Analysis Energy efficiency is primarily the ratio of the amount of energy extracted into hydrogen and the total energy spent in its production. Another parameter that is useful in the analysis of technologies is exergy, which is the rate of energy flow in and out of the system. It is determined by multiplying the mass flow with the energy content of the stream. Exergy efficiency can be calculated similarly to the energy efficiency, which is a ratio of the exergy of hydrogen to the total exergy input. Holladay et al. have reported these parameters for several hydrogen technologies [26]. Fossil fuel and biomass gasification technologies have favorable energy and exergy efficiency; however, the solar methods do not [46].

5. SYSTEMS ANALYSIS

509

5.4 Overall Assessment A comprehensive assessment of new hydrogen-producing technologies requires tools to integrate the environmental impacts determined via LCA methods with economic and social factors. One such method has been developed by Dincer and Acar to assess the technologies via visualization of the impacts in a single chart [50]. They came up with a hexagonal chart to compare six different factors: GWP, AP, SCC, cost of production, energy efficiency, and exergy efficiency. Fig. 20.13 shows a comparison of a few hydrogen production approaches classified by the method of production [50]. Only some of the parameters used by them are not available for the seven specific technologies we have considered in this chapter, so such a chart could not be created for these specific integrated/hybrid technologies. However, the charting approach provides a step forward in performing a comprehensive assessment of the socioeconomiceenvironmental impacts of the complete system. The approach uses a 0e10 point scale to assign to each of the six parameters in a normalized fashion to make such a comparison possible. The ideal system will be one, which has 10 points for all parameters. Based on the analysis shown in Fig. 20.13, each of the methods has some pros and cons, so no technology can be perfect in all measures; however, this chart does give us a visual tool to determine to make some decisions. If we know which factors are most important for a given region or a community, then depending on the population, resource availability, and financial status, one can identify the most suitable alternatives.

FIGURE 20.13 Normalized ranking of hydrogen production technologies based on primary energy sources. Figure extracted with permission from I. Dincer, C. Acar, Review and evaluation of hydrogen production methods for better sustainability, Int. J. Hydrogen Energy 40(34) (2015) 11094e11111.

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6. CONCLUSIONS AND PERSPECTIVES Biohydrogen technologies were evaluated using LCA and overall systems analysis to understand the nontechnical factors important for consideration in understanding the impact of technologies and identification of areas of further research and development. Biological technologies evaluated include indirect biophotolysis, integrated fermentationeMEC, and biogas ATR. These technologies were selected based on the latest advancements in the technologies, potential for higher productivity, efficiency and availability of the data on the environmental, and societal and economic parameters. These technologies were compared with existing, nonbiological technologies including SMR, biomass thermal conversion, solar splitting, and wind electrolysis. Among the LCA parameters, GWP, AP, and GERs were available for most technologies and are compared. Among the three biohydrogen technologies, indirect biophotolysis has the potential to have the lowest impact as far as global warming is concerned. This is due to the potential for CO2 capture by algal biomass, of which not all gets converted into hydrogen. Biogas reforming and integrated fermentationemicrobial electrolysis have a GWP of 6.7 and 9.8 kg/kg-H2, respectively, based on current estimates, which puts them closer to the GWP for SMR (10.4 kg/kg-H2). Thus improvements in these two biotechnologies may be necessary for reducing their impacts or better methods of estimation may be necessary to determine true impacts. One approach for the fermentationeMEC technology alternative is the integration of the two processes via microbial electrolysis of whole biomass, with or without pretreatment. This has been shown to yield high productivity of hydrogen and can lead to reducing environmental impact and better utilization of the biomass resource. Among the nonbiological technologies, wind electrolysis has the most favorable outcomes in terms of environmental impacts. An overall assessment of the technologies considering social and economic impacts was also discussed. In addition to the LCA parameters, economic and social factors, such as production cost and SCC, were included. New tools, such as normalized ranking of combined environmental, social, and economic parameters via visual charting, can provide better assessment of alternatives, which can then be weighed versus regional resource availability, economy, etc., to help in making decisions for further research, as well as investments, for better conditions for existence of human beings on the planet in the globally competitive 21st century.

Acknowledgments The manuscript has been coauthored by UT-Battelle, LLC, under Contract No. DEAC05-00OR22725 with the US Department of Energy. We acknowledge the comments from Dr. Barbara Evans in revising this manuscript.

References [1] A.F. Ferreira, J. Ortigueira, L. Alves, L. Gouveia, P. Moura, C. Silva, Biohydrogen production from microalgal biomass: energy requirement, CO2 emissions and scale-up scenarios, Bioresour. Technol. 144 (2013) 156e164. [2] Q. Dai, A. Elgowainy, J. Kelly, J. Han, M. Wang, Life Cycle Analysis of Hydrogen Production from Non-Fossil Sources, Argonne National Laboratory, 2016. https://greet.es.anl.gov/publication-h2-nonfoss-2016.

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