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Global warming potential of the sulfur–iodine process usinglife cycle assessment methodology

William C. Lattina, Vivek P. Utgikarb,*aDepartment of Environmental Sciences, University of Idaho, Idaho Falls, ID 83402, United StatesbDepartment of Chemical Engineering, University of Idaho, Idaho Falls, ID 83402, United States

a r t i c l e i n f o

Article history:

Received 10 June 2008

Received in revised form

7 October 2008

Accepted 22 October 2008

Available online 6 December 2008

Keywords:

Life cycle analysis

Sulfur–iodine cycle

Nuclear production of hydrogen

* Corresponding author. Tel.: þ1 208 282 772E-mail address: vutgikar@if.uidaho.edu (V

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.10.059

a b s t r a c t

A life cycle assessment (LCA) of one proposed method of hydrogen production – thermo-

chemical water-splitting using the sulfur–iodine cycle couple with a very high-temperature

nuclear reactor – is presented in this paper. Thermochemical water-splitting theoretically

offers a higher overall efficiency than high-temperature electrolysis of water because heat

from the nuclear reactor is provided directly to the hydrogen generation process, instead of

using the intermediate step of generating electricity. The primary heat source for the S–I

cycle is an advanced nuclear reactor operating at temperatures corresponding to those

required by the sulfur–iodine process. This LCA examines the environmental impact of the

combined advanced nuclear and hydrogen generation plants and focuses on quantifying

the emissions of carbon dioxide per kilogram of hydrogen produced. The results are pre-

sented in terms of global warming potential (GWP). The GWP of the system is 2500 g carbon

dioxide-equivalent (CO2-eq) per kilogram of hydrogen produced. The GWP of this process is

approximately one-sixth of that for hydrogen production by steam reforming of natural

gas, and is comparable to producing hydrogen from wind- or hydro-electric conventional

electrolysis.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction The goal of this life-cycle assessment is to evaluate the

Several thermochemical processes have been proposed for

large-scale production of hydrogen using heat from nuclear

reactors, including the sulfur–iodine cycle (S–I), UT-3 method

(University of Tokyo), hybrid sulfur, and Ispra Mark 9 process.

The sulfur–iodine cycle, combined with a new-generation

nuclear reactor as the source of heat for the process, is being

studied extensively for implementation and deployment in

the United States and throughout the world [1]. Life-cycle

assessments (LCAs) have been conducted for several of these

processes [2–4]. Because each uses different methodologies

and assumptions, comparison of results is difficult.

0; fax: þ1 208 282 7950..P. Utgikar).ational Association for H

environmental impacts of producing hydrogen using the

sulfur–iodine thermochemical cycle and a nuclear reactor

heat source. The LCA will identify and quantify significant

environmental aspects and assess their impacts. The assess-

ment can be used ‘‘stand-alone’’ or may be compared with

similar life-cycle assessments.

1.1. Thermochemical hydrogen production and thesulfur–iodine cycle

The decision to use the S–I cycle is based partially on a study

performed by General Atomics, University of Kentucky, and

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 7 3 7 – 7 4 4738

Sandia National Laboratories [1]. That study ranked twenty-

five thermochemical processes using qualitative parameters,

such as the number of chemical reactions involved, the

number of chemical separations necessary, number and

abundance of chemicals involved, process parameters (e.g.,

temperature and pressure), availability of data, and the

number of reports and studies published. Each parameter was

assigned a weighting-factor and assigned a numerical score.

The sum of the individually weighted values was summed for

a total process score. Two processes were selected for final

consideration: the UT-3 cycle and the sulfur–iodine cycle. The

U.S. Department of Energy’s Nuclear Hydrogen Initiative

funded research on the sulfur–iodine cycle, the hybrid sulfur

cycle, and the calcium–bromine cycle. The sulfur–iodine cycle

was selected for further development based on its higher

predicted efficiency. However, the study ‘‘neglects some

industrial-scale issues, like heat exchangers and the size of

equipment, and it tends to over-penalize the cycles lacking

relevant thermodynamic data’’ [17]. For example, the S–I cycle

requires a heat source capable of operating at over 1000 �C

reactor vessel outlet temperature, which results in peak fuel

temperatures of 1200 �C. Since current-generation light-water

reactors operate nominally at less than 350 �C, a new-gener-

ation advanced high-temperature reactor (AHTR) must be

designed [13].

2. Life cycle assessment (LCA)

The LCA process is defined in the ISO 14040 series of standards

and includes goal and scope definition (defining the system

under consideration), inventory analysis (identifying and

quantifying system input and output), impact assessment

(assessing the effects of the activities), and interpretation

(evaluating the results) [5].

2.1. System definition

Definition of system boundaries has significant impact on the

outcome of an LCA. This LCA defines the boundaries of

the hydrogen production system as the nuclear reactor and

the hydrogen plant subsystems. The system is analyzed for

a functional unit of production of 1 kg of hydrogen. For the

system being considered, one or more 600 MW(th) AHTR

reactor modules are coupled to a hydrogen production plant

[30]. The analysis of the nuclear reactor includes mining,

milling, conversion and enrichment of uranium ore; fabrica-

tion and transportation of nuclear fuel; construction, opera-

tion and decommissioning of the nuclear power plant; and

nuclear waste disposal [6]. Some studies do not include

disposal of radioactive waste and spent nuclear fuel in the

analysis; however, operation of one nuclear reactor for 20

years results in over 35 ton of heavy metal for disposal or

reprocessing, not a trivial amount when the large number of

nuclear reactors required to support a hydrogen economy is

considered. All of the thermal energy from the nuclear reactor

is transferred to the thermochemical process through an

intermediate heat exchanger (IHX). Additional energy is

necessary to operate the reactor and hydrogen plant auxiliary

systems [7].

The boundary of the hydrogen plant subsystem includes

construction and operation of the physical plant, acquisition

of raw materials for the thermochemical process (i.e., water,

iodine and sulfuric acid), including the method of production

for each chemical used in the process, the energy required for

extraction and refining, and its relative abundance. The

interface between the two subsystems is a heat transfer loop

consisting of an intermediate heat exchanger (IHX) and heat

transfer medium (helium gas). This LCA does not include

liquefaction, storage and distribution of hydrogen product in

the analysis since those operations are independent of the

method of production and depend upon the intended use of

the product. End-use of the hydrogen product is also excluded

from the study since a single purpose would be presumed for

the product (e.g., transportation using fuel cells or internal

combustion engines) and would not represent the current

uses of hydrogen (production of ammonia fertilizer and

hydrogenation of petrochemicals).

2.2. Life cycle inventory

Once the system boundaries are established, inputs to the

system (i.e., inventory) must be defined. Major inputs to

the system being studied include materials of construction for

the reactor and hydrogen plant, such as concrete, structural

steel, stainless steel, and other materials; nuclear fuel; reactor

coolant; feed material for the process (iodine, sulfuric acid, and

water); fossil fuels and electricity necessary for construction;

and energy and materials needed to operate the facility

(replacement nuclear fuel, electricity, fossil fuels and elec-

tricity, make-up helium gas, process materials, and water).

2.2.1. Nuclear reactor inventorySince the advanced high-temperature nuclear reactor (VHTR)

is currently in the conceptual stage of design, it is assumed

that advanced nuclear plants are equivalent to existing

nuclear plants with regard to quantities and types of materials

of construction, although there are differences in construction

(e.g., containment and confinement structures, shielding, etc.)

[3]. This assumption is conservative and provides an upper

bound for calculations. Therefore, emissions from materials

of construction and construction activities are assumed to

be similar. Concrete and steel represent greater than 95%

of the materials used for construction of a nuclear reactor

plant. Anigstein et al. estimate a 1970-vintage 1000 MW(e)

(3000 MW(t)) pressurized water reactor nuclear power plant

contains 34,811 metric tons of steel [8]. The reactor and asso-

ciated systems account for 18,364 ton, with the remainder in

the turbine building and electrical generation equipment.

Bryan and Dudley, as cited in Peterson, estimate 190 cubic

yards of concrete per megawatt of capacity for the same plant

[9,10]. Although the sizes of individual reactor components

may vary differently (i.e., geometrically for vessels and piping),

for conservatism, a linear relationship is assumed between

plant size and power capacity. Therefore, the reference

600 MW(t) advanced nuclear plant contains roughly 1/5 of the

material as a 3000 MW(th) plant. This results in approximately

3675 metric tons of steel, and 114,000 cubic yards (209,760 ton)

of concrete. At the end of the plant’s 30-year life it will be

decontaminated, decommissioned and disposed as waste.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 7 3 7 – 7 4 4 739

The core of a 1000 MW(th) reactor contains 75 ton of

enriched uranium fuel [13]. Assuming the core loading is

proportional to rated power, the 600 MW(t) reference plant

contains 45 ton of enriched uranium. To operate at elevated

temperatures necessary to support the S–I cycle, a new type of

nuclear fuel with high-temperature coating is being developed

[39]. Advanced nuclear reactors will be designed to achieve

higher fuel burn-up which will reduce the quantity of nuclear

fuel required, and the quantity of spent fuel to be disposed.

Newer fuel enrichment technology (gas centrifuge) requires

forty times less energy than the current method (gaseous

diffusion) [6]. In 2002, 55% of fuel produced worldwide used

gaseous diffusion, and 45% used gas centrifuge for enrich-

ment, whereas all of the fuel produced in the U.S. is by

gaseous diffusion [4]. The worldwide trend is to use gas

centrifuge technology for all enrichment [6]. Consequently,

the emissions associated with future nuclear fuel production,

consumption and disposal should be lower for the advanced

nuclear plant. To maintain efficient operation, about one-

third of the spent nuclear fuel will be removed from the

reactor every year and replaced with fresh fuel [13].

Helium is used as a reactor coolant and heat transfer

medium in the nuclear reactor and the hydrogen production

plant. It is chemically stable, has a relatively high specific heat

capacity, and has negligible cross-section for neutron

absorption and capture [10]. It is recovered commercially from

natural gas deposits by low-temperature cryogenic fractional

distillation. It may be present in natural gas at concentrations

of up to 7% by volume [31]. In 2006, 170 million cubic meters of

helium were produced worldwide [32]. For the purpose of this

study, the VHTR is assumed to contain 3.685 tons of helium as

reactor coolant, based on similarity to the Fort St. Vrain high-

temperature gas reactor [11]. Alternative designs may use

different coolant systems. For example, Forsberg proposes to

use molten fluoride salt coolant, based on higher specific heat

capacity and smaller component piping [12]. Large pipe size

increases heat loss and cost.

In addition to the heat produced by the reactor, approxi-

mately 100 MW(e) additional power is required to operate

pumps, compressors, and auxiliary equipment associated

with the nuclear reactor and the hydrogen generation plant

[26]. This study assumes that power is obtained from the U. S.

distribution grid, and is generated using the mix of sources

identified in the Mid-Western United States [21]. However, if

this power would be provided by a nuclear reactor, the

resultant GHG emissions would be significantly lower.

2.2.2. Hydrogen generation plant inventoryThe reference hydrogen plant produces 200 ton of hydrogen

per day of operation. Assuming a capacity factor of 0.9, about

1.97Eþ 9 kg of hydrogen are produced over the 30-year life of

the plant. Hydrogen product is available at the plant gate as

a compressed gas at 346 psia and 99.6% purity [35].

Spath and Mann have estimated types and quantities of

materials of construction for a hydrogen generation plant using

steam reforming of natural gas [28]. Assuming similar

construction for the S–I cycle, the plant requires 3272 ton of

steel, and approximately 10,242 ton of concrete (cement with

aggregate).They furtherestimate that materials ofconstruction

and decommissioning account for 0.4% of total GHG emissions.

A significant difference among thermochemical processes

involves the chemicals used in the processes. For example, the

UT-3 cycle uses bromine, calcium, and iron. Ispra Mark 9 uses

iron and chlorine. Hybrid sulfur uses sulfuric acid coupled

with electrolysis, whereas the S–I process uses sulfuric acid

and iodine. Since energy is required to circulate the material

through the plant, processes using liquid or gas are preferred

over those using solid materials.

Iodine is one of the essential elements necessary in the S–I

cycle. In the United States, iodine is extracted from subsurface

brine associated with natural gas and oil deposits [20]. Energy

must be supplied to pump the brine, compress air for the

blowout process, to purify and crystallize the product. The

current cost of iodine is $17.03 per kg [20]. Although the stoi-

chiometric amount of iodine required for the S–I cycle is

2120 ton, bench-scale experiments indicate a need for

approximately 10,000 ton of material [17]. Worldwide

production of iodine is roughly 18,000 ton per year [33].

Assuming nominal process losses of iodine (lifetime 10%),

a significant percentage of the current world’s production of

iodine is involved in the inventory of one hydrogen plant.

The other substance required in the S–I cycle is sulfuric acid.

The reference plant will contain about 100 ton of high-purity

sulfuric acid. Sulfuric acid is one of the most widely used

chemicals in industry. Although much of the sulfuric acid in use

is recovered from industrial processes, high-purity sulfuric acid

is produced from oxidation of sulfur and sulfur dioxide [40].

For every mole of hydrogen produced, one mole of high-

purity water must be consumed in a stoichiometric reaction.

For the reference plant producing 200 ton of hydrogen per day,

1.77Eþ 7 metric tons of water are consumed over the 30 year

life of the plant. Bench-scale experiments demonstrate that

water must also be supplied in excess up to eight times the

required amount for the reaction to proceed [27].

2.2.3. Intermediate heat exchanger (IHX) inventoryDesign of an IHX capable of operating at the required

temperature (1000 �C) poses an engineering challenge,

requiring advanced design and materials. Since designs for

the IHX are still conceptual, the type of heat exchanger (e.g.,

blade, printed circuit), types and quantities of materials are

not known at this time. For reference, the Calder-Hall reactor

has four Inconel heat exchangers, each 18 feet in diameter, 70-

feet high, and each weighing 200 ton. Some advanced, high-

temperature resistant materials proposed for the IHX include

SiC and Inconel 600H [34]. Silicon carbide requires more

energy to produce (180 MJ/kg), therefore the contribution from

the IHX to the total emissions would increase over more

conventional materials [22].

2.2.4. Emissions inventoryThe total emissions from the proposed system will be the sum

of the emissions from the nuclear power plant and the S–I

hydrogen generation plant subsystems. A summary of life-

cycle emissions from several different studies of nuclear

power plants are shown in Table 1. Primary focus is on carbon

dioxide emissions and global warming potential (GWP) since

they have global impacts. Acidification, measured as grams of

SO2-equivalent, is a regional effect and results must be

interpreted for the specific geographic area of concern [42].

Table 1 – Life cycle emissions inventory from nuclearpower plants

Author CO2

(g/kWh)SO2

(mg/kWh)NOx

(mg/kWh)

Koch [23] 2–59 3–50 2–100

Meier [24] 17 Not reported Not reported

Krewitt [25] 19.7 32 70

CRIEPI [43] 22 Not reported Not reported

British Energy [6] 5.05 10 20

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 7 3 7 – 7 4 4740

2.3. Life cycle inventory assessment

Life cycle assessment of each subsystem uses data such as

specific energy consumption (MJ/kg) or specific emission

factors (grams of emissions per kilogram of product). Those

quantities are then summed to yield total emissions for the

nuclear hydrogen generation system.

2.3.1. Nuclear reactor plant inventory assessmentTotal energy used in the production of steel is estimated to

range from 25.5 GJ/ton for mild steel, to 100 GJ/ton required for

specialty and stainless steel [15,22]. Mild steel can be prepared

from secondary scrap steel in a modern, efficient electric arc

furnace. Nuclear-grade steel (meeting nuclear quality assur-

ance standards) requires primary steel produced from

primary sources. Using a mid-point value of 60 GJ/ton results

in 2.21Eþ 5 GJ required for the 3675 metric tons of steel in the

nuclear plant. Coking coal, used for primary steel production,

emits 0.111 kg CO2/MJ [21]. This results in emissions of

8.17Eþ 6 kg CO2-eq to produce the steel necessary for

a nuclear plant. Since 1.97Eþ 9 kg of hydrogen are produced

over the 30-year life of the plant, the specific carbon emission

for steel is 4.15 g CO2-eq/kg of hydrogen produced. Alterna-

tively, Worrell estimates emission of 0.5–0.82 ton CO2/ton

steel. Using the higher estimate results in 1.53 g CO2/kg H2,

about one-half of the calculated value.

The primary fuel for production of cement is coal. Portland

cement production requires 4.89–6.33 MJ per kilogram of

cement [14]. With the addition of sand and aggregate,

production of concrete requires a total of 0.893 MJ/kg [15,16].

For 209,760 ton of concrete contained in the nuclear plant, this

represents 1.87Eþ 8 MJ, which results in emission of

4.76Eþ 7 kg CO2-eq. For 1.97Eþ 9 kg of hydrogen produced

over the life of the plant, the specific carbon emission for

concrete is 24.2 g CO2-eq/kg of hydrogen produced.

The total calculated emissions from steel and concrete are

24.2 g CO2-eq/kg H2. Since this represents about 95% of the

emissions from construction, the total emissions are 25.5 g

CO2-eq/kg H2. Emissions from decontamination and decom-

missioning are assumed to be 10% of that from construction

[35]. This adds 2.6 g CO2-eq/kg H2, for total emission of 28.1 g

CO2-eq/kg H2 from plant construction and decommissioning.

Mining and milling of uranium ore result in 1.85 g CO2/kWh

[6]. The contribution to emissions from enrichment is

dependent upon the type of process used. Enrichment by gas

centrifuge adds 0.43 g CO2/kWh, whereas gaseous diffusion

uses forty times the energy, resulting in 17.2 g CO2/kWh [19].

Using these values, the contribution of emissions from

nuclear fuel is either 1372.18 g CO2/kg H2 if gaseous diffusion

is used, or 164.23 g CO2/kg H2 if a gas centrifuge is used. This

reactor is assumed to be constructed in the U.S. Since gaseous

diffusion is used to produce all of the uranium fuel in the U.S.,

the higher value is used in this LCA.

Information regarding specific energy consumption and

emissions for helium is not reported in the literature, and

existing life cycle inventory databases are not sufficiently

mature to include all substances of interest. Lacking specific

data, an alternative method of calculating GHG emissions is

necessary. It is reasonable to assume that for commonly

available substances, the cost of the commodity (less

a reasonable amount for profit and overhead) is directly

proportional to the energy required for production.

The cost of helium in theU.S. is regulated by statute at $1.965

per m3 for government supply. Commercial prices range $2.42–

2.63 per m3 [20]. With a density of 0.0001785 g/cm3, each cubic

meter of helium contains 0.1785 kg. Thus, the regulated price of

helium is $11.00/kg, and the commercial price ranges from

$13.56 to 14.73 per kilogram. Natural gas is the primary energy

for production of helium andsells for $0.60 per therm (105.5 MJ).

Using the regulated price of helium and CO2 emissions for

natural gas results in 29 MJ/kg He, and 0.5 kg CO2-eq/kg He.

Assuming the reactor inventory is 3.685 ton (3.69Eþ 3 kg), the

gross energy requirement for the helium coolant inventory is

1.06Eþ 5 MJ, which results in emission of 1.84Eþ 6 g CO2-eq.

For 1.97Eþ 9 kg of hydrogen produced over the life of the plant,

the specific carbon emission from helium is 9.34E� 4 g CO2-eq/

kg H2. Assuming the hydrogen plant has the same volume of

helium as the reactor, and assuming 10% loss per year from the

system, the total specific carbon emission for helium is 0.002 g

CO2-eq/kg of hydrogen produced.

Operation of the nuclear and hydrogen plants requires an

additional 100 MW(e) of electricity [26]. If this power is

obtained from the grid in the U.S., which primarily uses coal

for electrical generation, this would result in an additional

7804 g CO2-eq/kg H2. In Europe, or U.K., electricity is generated

primarily from nuclear power. The same 100 MW(e) would

result in only 60.6 g CO2-eq/kg of hydrogen.

2.3.2. Hydrogen plant inventory assessmentSpath and Mann estimate construction of a plant for steam

reforming of natural gas requires 3272 ton of steel, and

10,242 ton of concrete [28]. They have calculated total emis-

sions from this plant at 11,888 g CO2/kg H2. Construction of an

S–I cycle plant and the steam reforming plant is assumed

similar for the purpose of this LCA. A value of 0.4% is provided

as the contribution to emissions from plant construction and

decommissioning, yielding 47.55 g CO2/kg H2.

Data regarding specific energy consumption and specific

emissions for the production of iodine are lacking in the liter-

ature. The same method used to determine emissions from

helium can be applied to the iodine inventory. The cost of

iodine is reported as $17.03 per kilogram [20]. If energy is

supplied totally by natural gas, production of 1 kg of iodine

requires 97 MJ. Heat from recycled brine could conceivably

supply 50% of the process energy, which would reduce the

specific energy requirement range to 49 MJ/kg of iodine.

Resultant CO2 emission would be 1.8/kg CO2-eq/kg I2. The

hydrogen plant inventory of iodine is 2120 ton, based on the

stoichiometric reaction. Using 49 MJ/kg yields 1.04Eþ 8 MJ, and

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 7 3 7 – 7 4 4 741

5.77Eþ 9 g CO2-eq for the life of the plant. Adding the contri-

bution from 10% process loss over the entire life of the plant

results in 0.33 g CO2-eq per kilogram of hydrogen.

The U.S. Environmental Protection Agency reports an

emission factor of 4.05 kg carbon dioxide emitted per metric

ton of sulfuric acid produced [40]. The initial inventory of

H2SO4 in the hydrogen plant is 100 ton. The assumed loss rate

is 1% per year of operation, therefore 130 ton of sulfuric acid is

necessary to operate the process. This yields 526 kg of CO2

over the life of the plant, or 5.26E-4 g CO2-eq/kg H2.

In order to supply high purity water as the source of

hydrogen requires 3–15 kWh/m3. Assuming the density of

water is 1 g/cm3, 1.77Eþ 7 m3 of water are used over the life of

the plant. Reverse osmosis purification requires 3–15 kWh/m3

(1830–9150 g CO2/m3) resulting in 16.4–82.18 g CO2/kg H2.

Because of their relative size and importance, the contri-

butions from intermediate heat exchangers are calculated

separately from the nuclear and hydrogen plants. Based on

the size of the Calder-Hall reactor heat exchangers, 800-tons

of Inconel alloy are required for fabrication. Using a value of

60 GJ/ton to produce steel results in 4.8Eþ 7 MJ required, and

emission of 1.76Eþ 9 g CO2-eq. The production of 1.97Eþ 9 kg

H2 yields a contribution of 0.89 g CO2-eq/kg H2.

2.3.3. Resource depletionThere is consensus that resource depletion should be

considered in life cycle analyses. The impacts of resource

management (i.e., extraction and processing) and depletion

may surpass other aspects of the life cycle [37]. In order to

produce 1 ton of hydrogen requires circulation of large quan-

tities of material, from 500 to 10,000 ton depending upon the

material [17]. Iron, chlorine, and calcium are relatively abun-

dant and easily produced in quantities required. Bromine and

iodine, on the other hand, are less abundant and require

energy-intensive separation techniques.

Scarcity is defined as a change in the availability of

a resource over time [38]. The availability of a resource may

depend upon other factors. For example, production of helium

is tied to the production of natural gas. As natural gas

becomes scarcer, the supply of helium may be less stable and

the cost may increase. Similarly, iodine production in the U.S.

is extracted from subsurface brines, often associated with oil

production. As oil becomes scarcer, the supply of iodine may

be affected. As discussed earlier, production of iodine world-

wide is 18,000 ton per year. One sulfur–iodine hydrogen

production plant would require 56% of the current annual

world production capacity of iodine.

Steel and concrete, on the other hand, are relatively

abundant, as is sulfuric acid. Global steel production in 1997

was 773 million tons. Worldwide production of cement totaled

1.25 billion tons in 1991 [14]. In 1995, 35.6 million tons of

sulfuric acid was produced in the United States [41].

Construction and operation of the reference plant have

a negligible effect on the supply of these resources.

3. Sensitivity analysis

Due to differing assumptions in calculations, variability in

data, and differences in reporting, uncertainty exists in the

calculation of emissions. In particular, the chemical industry

is restrictive concerning information on certain production

processes [29]. Therefore, specific data is lacking regarding key

materials in the S–I process, such as specific energy

consumption and GHG emissions in the production of helium

and iodine. Sensitivity analyses were performed to evaluate

the effects of variability of data on the calculation of overall

life cycle emissions.

Since the AHTR and the hydrogen plants are currently in

the conceptual stage, the types and quantities of materials

have been estimated. Projects at the conceptual stage typically

exhibit uncertainty in the range from �10 to þ25% [18]. As

seen in Table 2, variability in data for materials of construc-

tion has little effect on overall GHG emissions. Even when that

data are adjusted upward by 50%, the total emissions are

affected by only 1.6% (6.7% versus 5.1% of total). When the

relatively small contribution to total emissions from these

materials is considered, a large error in calculated values

results in very small changes to the results of the LCA. The

same is true for chemical inventory of the hydrogen plant, i.e.,

helium, iodine and sulfuric acid.

Assumptions regarding plant life and capacity factor affect

specific emissions and unit cost of a process. For example,

General Atomics assumes a 60-year plant life, with a 90%

availability factor, resulting in 54 effective-years of operation.

For ISPRA Mark 9, the corresponding values are 30-years, 80%,

and 24 effective-years. Uranium Information Center uses a 40-

year life and 80% capacity factor, resulting in 32 effective-years.

This LCA uses a 30-year plant life and 90% availability factor.

Since the S–I process requires an AHTR, the shorter plant life is

assumed to account for high-temperature corrosion, and

fatigue due to thermal cycles in both the reactor and hydrogen

plants. The 90% availability factor is based on actual experience

in operating nuclear reactors gained over the past 40 years.

The largest single contributor to greenhouse gas emissions

from the nuclear fuel cycle is operations associated with

mining, milling and enrichment of nuclear fuel. According to

British Energy, about 37% of the total carbon footprint results

from extraction, conversion, enrichment and fabrication of

nuclear fuel [6]. The amount of emissions is proportional to

the concentration of uranium present in the ore, as well as the

method of enrichment. The remainder of the emissions

results from nuclear plant operations; construction and

decommissioning; fuel reprocessing; and construction and

operation of radioactive waste facilities.

For this LCA, the largest potential contributor to green-

house gas emissions is the electrical power required to oper-

ate the nuclear and hydrogen plants’ process and auxiliary

equipment. That power is assumed to originate from the

power grid in the U.S., generating electricity from the

combustion of coal, oil, natural gas, and about one-fifth from

nuclear and renewable sources. Emissions from that power

alone are 7804 g CO2/kg H2. These emissions would be reduced

by 99% if that energy could be supplied from nuclear power,

either from a separate power plant or a hybrid nuclear plant.

The use of a hybrid 1000 MW(th) nuclear plant could supply

both 600 MW(th) nuclear heat to the S–I thermochemical cycle

and 100 MW(e) electrical power for nuclear plant operations

and the S–I process equipment. This would increase emis-

sions due to construction of the turbine building and power

Table 2 – Greenhouse gas emissions for sulfur–iodine cycle

Component/material Inventory Specific CO2 emissions(g CO2-eq/kg)

Total GHG emissions(g CO2-eq/kg H2)

Helium 3.7 ton 0.5 0.002

Sulfuric acid 100 ton 4.1E� 6 0.000526

Iodine 2120 ton 2.0 0.33

Water (high purity) 40 ton 82.18 82.18

Hydrogen plant

construction [28]

(11,888) –

Steel 3272 ton 0.4% of total [28] 45.77

CementþAggregate 10,242 ton

Decommissioning (10% of construction)

Heat exchanger 800 ton 0.89 0.9

Subtotal hydrogen

plant subsystem

129.2

Nuclear plant Construction –

Steel 3675 ton 4.15 32.3 (36.7)

Concrete 209,760 ton 24.2

Remainder (5% of total) 1.3

Decommissioning (10% of construction) 2.6

Nuclear fuel 45 ton 2277.5 (1372.2) 2277.5 (1372.18)

Operating electrical 100 MW(e) 60.6 (7804) 60.6 (7804)

Subtotal nuclear

plant subsystem

2370.4 (9212)

Total 2499.6 (9341.9)

Note: Values in parentheses for nuclear plant subsystem assume electrical power obtained from the U.S. grid. Other values assume power from

hybrid nuclear plant or other nuclear source.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 7 3 7 – 7 4 4742

generation equipment by a factor of two. However, as

demonstrated in this LCA, construction is a relatively small

contributor to overall emissions. The reactor, however, would

require 66% more nuclear fuel, increasing the GHG contribu-

tion to 2277.5 g CO2/kg H2. Both of these together would result

in a total GWP of for the system of 2675 g CO2/kg H2. This is still

a factor of 3–4 less than production of hydrogen by steam

reforming of natural gas.

4. Discussion of results

Results of various energy and life-cycle analyses show that

nuclear-based processes for production of hydrogen result in

significantly lower usage of fossil fuels and lower greenhouse

gas emissions than steam methane reforming, which is the

predominant methodofhydrogenproduction.Steamreforming

of natural gas results in 9000–11,888 g CO2-eq/kg H2. Coal gasi-

fication produces 12,400 g CO2-eq/kg H2 [35,36]. On the other

hand, high-temperature electrolysis of water using a very high-

temperature gas-cooled nuclear reactor (VHTR) results in 2000 g

CO2-eq/kg H2 [3]. Utgikar and Bradley estimate greenhouse gas

emissions of 2515 g CO2-eq/kg H2 using Ispra Mark 9 process

coupled with VHTR [2]. For each process, nuclear reactor

construction and operation contribute 1250 g CO2-eg/kg H2.

Wu et al. performed a life cycle assessment for generic

thermochemical processes resulting in 25–30 g CO2-eq/km,

which includes end-use in hydrogen fuel-cell vehicles.

Assuming 95 km/kg H2 results in w2700 g CO2-eq/kg H2. They

conclude that hydrogen production in a central plant using

a thermochemical process coupled with a nuclear heat source

reduces total energy usage by 21–26%, and reduces green-

house gas emissions by 74–80%.

For comparison, production of hydrogen from solar

(photovoltaic), solar (thermal), wind and hydro-electric energy

results in 2124, 800, 860, and 584 g CO2-eq/kg H2, respectively

[44,45]. From a strict comparison of greenhouse gas emissions,

these technologies appear competitive with nuclear-power

based production methods. Further analysis of these renew-

able energy technologies is necessary to determine their

overall competitiveness with respect to cost, spatial require-

ments, and other environmental impacts. Such analysis is

beyond the scope of this study.

The results of this LCA compare favorably with previous

studies. The total global warming potential for the sulfur–

iodine cycle coupled with an advanced high-temperature

reactor is 2500 g CO2-eq/kg H2, assuming electrical power to

operate the reactor pumps and process equipment is supplied

from a nuclear reactor. If that power is supplied from the grid

in the U.S., the carbon footprint of this process is nominally

only 20% better than the current method of hydrogen

production, with emissions of 9477 g CO2-eq/kg H2.

5. Conclusion

It is concluded that production of hydrogen from the sulfur–

iodine thermochemical cycle coupled with a nuclear reactor

results in approximately one-fifth to one-sixth the green-

house gas emissions from steam reforming of natural gas. It is

further shown that relatively little difference in greenhouse

gas emissions exists between several hydrogen production

processes using a nuclear reactor as the heat source.

Life-cycle assessments should be performed which

examine each process objectively, consistently, and equitably.

Further work needs to be done to standardize life-cycle

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 7 3 7 – 7 4 4 743

analyses for the various available nuclear options to enable

valid comparisons. However, this environmental LCA

provides only one input to the decision-making process. The

decision to use a specific thermochemical cycle should be

based on objective criteria, as well as technical feasibility and

life-cycle environmental impacts. Other factors to be consid-

ered would include socio-economic impacts, resource usage

and depletion, politics, and national strategy. Sensitivity

analyses should be performed to determine which factors

have the greatest effect on results.

Ultimately, the results of a life-cycle analysis must be

examined in the context of its stated purpose. If the intent of

hydrogen generation is to replace hydrocarbon-based fuels in

automobiles and enhance energy independence, then higher

greenhouse gas emissions may be acceptable from the

process. On the other hand, if reducing GHG emissions is the

ultimate goal, various LCAs can be compared and the appro-

priate technology selected.

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