Bioenergy systems for the future 11 advancements and confinements in hydrogen production technologies

Bioenergy systems for the future 11 advancements and confinements in hydrogen production technologies Bioenergy systems for the future 11 advancements and confinements in hydrogen production technologies Bioenergy systems for the future 11 advancements and confinements in hydrogen production technologies Bioenergy systems for the future 11 advancements and confinements in hydrogen production technologies Bioenergy systems for the future 11 advancements and confinements in hydrogen production technologies

Advancements and confinements

in hydrogen production

technologies

S Nanda*, K Li†, N Abatzoglou‡, A.K Dalai§, J.A Kozinski*

*York University, Toronto, ON, Canada,†Western Michigan University, Kalamazoo, MI,United States,‡Universit
e de Sherbrooke, Sherbrooke, QC, Canada,§University ofSaskatchewan, Saskatoon, SK, Canada

The fossil-fuel reserves are depleting across the world, thereby invigorating the movetoward renewable energy sources The instability in fuel prices, increasing greenhousegas emissions, and concerns over global warming are other factors contributing to thetransition toward bioenergy (Nanda et al., 2015b) The global primary energy demand

by 2050 is expected to be in the range of 600–1000 EJ (IEA, 2009) Today, the mary energy supply is derived from fossil fuels with nearly 80% of global energydemand being supplied from crude oil, natural gas, and coal (Balat and Kırtay,

pri-2010) The liquid fossil-fuel reserves are also estimated to be depleted in

<50 years at the present rate of consumption (Sheehan et al., 1998) There is an diate need for alternative energy sources, especially from renewables that couldreplace the dwindling fossil-fuel reserves and counteract the greenhouse effect.The trend of greenhouse gas emissions (e.g., CO2, CH4, and N2O) since 1750 isillustrated inFig 11.1 The recent atmospheric concentration of CO2is 403 ppm, indi-cating a 31% increase from its 1750 levels (278 ppm) (EEA, 2015) A secure and alter-native supply of energy is therefore indispensable for a sustainable future globaleconomy Over the years, there have been significant developments in alternativeenergy sources such as solar, wind, tidal, geothermal, and biomass Waste biomass,especially lignocellulosic materials and energy crops, are the feedstocks that cangenerate hydrocarbon biofuels to substitute gasoline and diesel (Nanda et al.,2014c) On the other hand, solar, wind, tidal, and geothermal resources are mostlyrelated to as energy carriers as they generate heat and electricity both for mobilityand stationary applications Hydrogen (H2) is a next-generation contender being anadvanced fuel, energy vector (fuel cells), and energy carrier (hydrocarbon fuel gen-eration) Hydrogen and fuel cells have tremendous prospects in the future’s sustain-able energy supply Out of the total renewable energy share of 36% by 2025, hydrogen

imme-is expected to constitute
11% (Balat and Kırtay, 2010) Similarly, it is also predictedthat hydrogen will contribute
34% of the share of renewable energy supply of
69%

by 2050 The diverse applications of hydrogen, represented inFig 11.2, are described

in subsequent sections

Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00011-9

© 2017 Elsevier Ltd All rights reserved.

Hydrogen in its monatomic form (H) is the most abundant chemical substance ring in the universe Hydrogen gas was artificially produced for the first time in the early16th century through the mixing of metals with acids Hydrogen gas is a clean fuel and

occur-an attractive alternative to carbon-based fuels, both renewable occur-and nonrenewable It duces water when burned, a property that defines its name, that is, hydro meaning

pro-“water” andgen meaning “generating.” It has a high heating value of 141.9 kJ/g that

Fig 11.1 Trend of worldwide greenhouse gas emissions over the years

Data fromEuropean Environment Agency (EEA), 2015 Atmospheric concentration of carbondioxide, methane and nitrous oxide Copenhagen, Denmark.http://www.eea.europa.eu

Accessed 10 March 2016

Electrochemical fuel cells

Fischer-Tropsch process

Liquification Aviation fuel Electricity

Syngas fermentation Ethanol

Microbial fuel cells

Hydrogen

Green diesel Hydrocarbons

Fig 11.2 Many industrial applications of hydrogen

is 2.6 times greater than natural gas (54 kJ/g), 3 times greater than gasoline (47 kJ/g),and 4.8 times greater than ethanol (29.7 kJ/g).Table 11.1highlights some of the fuelproperties of hydrogen relative to other commercial fuels and solvents The productionand utilization of hydrogen is recently gaining momentum as a next-generation fuel andenergy carrier for transportation and stationary applications.

Hydrogen can be produced from a variety of supply systems such as fossil fuels,biofuels, alcohols, nuclear reaction, biomass, and water Nearly 96% of hydrogen isproduced directly from fossil fuels and about 4% through other sources using electric-ity (Kothari et al., 2008) The dominant source for industrial hydrogen generation isvia steam reforming of natural gas or methane Although combustion of hydrogenresults in no CO2emission that makes it carbon neutral, its production from fossil-based resources imparts a large carbon footprint to the manufacturing process.Hydrogen has many potential applications, predominantly in chemical industry andenergy sector The predominant use of hydrogen is found in hydrogenation, hydro-treating, and hydrodesulfurization in petrochemical refineries The second significantuse of hydrogen is recognized in the production of ammonia Hydrogenation is a processwhere hydrogen is employed to reduce or saturate organic compounds In food indus-tries, hydrogenation converts liquid vegetable oil into semisolid fats (e.g., margarine) ordiesel components (Endisch et al., 2013) In petrochemical industries, hydrogenationconverts alkenes and aromatics into saturated alkanes (e.g., paraffins) and cycloalkanes(e.g., naphthenes) As an energy carrier, hydrogen also acts as a feedstock (synthesisgas) to produce synthetic chemicals and hydrocarbon fuels via Fischer-Tropsch catal-ysis (Tr
epanier et al., 2009) As a fuel, it can be used in combustion engines and fuel-cellelectric vehicles Hydrogen can be utilized in high-efficiency power generation systems,including fuel cells, for both vehicular transportation and distributed electricity gener-ation Fuel cells convert hydrogen or a hydrogen-rich fuel and an oxidant (usually pureoxygen or oxygen from air) directly into electricity using a low-temperature electro-chemical process Acting as an electron donor for reducing arsenate, chromate, nitrate,perchlorate, selenate, dibromochloropropane, and other oxidized pollutants, hydrogen isused to treat wastewater (Chung et al., 2007)

As mentioned earlier, hydrogen can be generated from both renewable resources(e.g., biomass) and nonrenewable resources (e.g., coal, natural gas, and fossil fuels).This chapter gives an overview of both the commercial and underdeveloped hydrogengeneration technologies along with the advantages and challenges associated witheach of them The hydrogen production technologies discussed in this chapter includehydrocarbon reforming, gasification, pyrolysis, fermentative pathways, and electrol-ysis.Table 11.2summarizes some common hydrogen production technologies withtheir advantages and disadvantages

There are three primary routes for hydrogen production from hydrocarbon and fossilfuels, namely, steam reforming (SR), alkaline-enhanced reforming (AER), partial oxi-dation (POX), and autothermal reforming (ATR) As a result of the reforming process,

Table 11.1 Comparison of the fuel properties of hydrogen with other fuels

Properties Hydrogen Methane Propane Methanol Ethanol Gasoline

Molecular weight (kg/kmol) 2.02 16.04 44.1 32.04 46.07 100–105

Density at 20°C and 1 atm (kg/m3

) 0.0838 0.668 1.87 791 789 751Viscosity density at 20°C and 1 atm

(g/cm/s)

8.81105 1.10104 8.012105 9.18103 0.0119 0.0037–0.0044Boiling point (°C) 253 162 42.1 64.5 78.5 27–225

Vapor specific heat at 20°C and 1 atm 0.0696 0.555 1.55 — — 3.66

Flash point (°C) < 253 188 104 11 13 43

Autoignition temperature in air (°C) 585 540 490 385 423 230–900

Based on U.S Department of Energy (USDE), 2016 Comparative properties of hydrogen and fuels Hydrogen Analysis Resource Center http://hydrogen.pnl.gov Accessed 10 March 2016.

Technology Advantages Disadvantages

Hydrocarbon-based processes

Alkaline-enhanced

reforming

(i) Higher yield of H2

(ii) Less CO and CO2formation due to carbonprecipitation

(iii) Lower-temperature requirement than steamreforming

(i) Precipitation of carbon as salts in the reactor(ii) Chances of reactor corrosion due to salt formation

Partial oxidation (i) No feedstock desulfurization requirement

(ii) No catalyst required(iii) Low CH4and CO2generation

(i) Low syngas ratio (H2/CO
2:0)(ii) High temperatures required and released(iii) Soot formation adds to process complexity(iv) Requires oxygen

Steam methane

reforming

(i) Tremendous industrial progress(ii) Low-temperature requirement(iii) Best syngas ratio (i.e., H2/CO
3:1) achieved(iv) Viable approach for near term hydrogen market

(i) High operation and maintenance costs(ii) High off-gas emissions

(iii) Causes pollution and global warming

(iii) Flexibility in products range, for example, H2,syngas, bio-oil, ethanol, butanol, and biodiesel

(i) Seasonal and geographical availability(ii) Expenses involved in biomass pretreatment(iii) Instances of heat and mass-transfer limitations,pressure resistance, and undesired thermal cracking(iv) Valorization of by-products is necessary

Pyrolysis (i) Carbon-neutral process

(ii) Less CO and CO2yield(iii) Energy-dense bio-oil recovered(iv) Simplicity and compact reactor

(i) Requires biomass drying and pulverizing(ii) Gas yield is less compared to bio-oil and char(iii) Catalyst deactivation due to fouling of carbon(iv) Valorization of by-products is necessary

Technology Advantages Disadvantages

Gasification (i) High operating pressure can reduce the cost of H2

compression(ii) High reaction rate and better heat transfer(iii) High H2concentration in gas product(iv) No interphase mass-transfer resistance(v) No biomass drying saves energy and costs(vi) Results in low-cost synthetic fuel in addition tohydrogen

(i) High reactor costs(ii) Low thermal efficiency due to moisture in gasproducts

(iii) Reactor plugging by tar formation at hightemperatures and longer reaction time(iv) High energy input

(v) Chances of reactor corrosion due to salt formation(vi) Feedstock impurities (high mineral matter) influencegasification

Dark fermentation (i) No light source required

(ii) Wide selection of carbon source as feedstock(iii) Valuable metabolites recovered (e.g., acetate,butyrate, lactate, and formate)

(iv) Self-sustaining technology

(i) Oxygen sensitivity to hydrogenase enzyme(ii) Low H2yields

(iii) CO2yield in gas products requires separation

Photofermentation (i) Wide spectral light rays used by bacteria

(ii) Flexibility in feedstock selection(iii) No biomass drying saves energy and costs

(i) Contaminants in feedstock might inhibitmicroorganisms

(ii) Oxygen sensitivity to nitrogenase enzyme(iii) Low conversion efficiency (1%–5%)Photolysis (i) Produces H2from water and sunlight

(ii) Cost-effective process(iii) Causes no pollution(iv) Can use fuel-cell technologies(v) Integration with other biorefining technologies torecycle wastewater

(i) Requires high light intensity(ii) Lower photochemical efficiency(iii) High oxygen yield (
30%) in gas products(iv) High capital costs

Based on Reddy, S.N., Nanda, S., Dalai, A.K., Kozinski, J.A., 2014 Supercritical water gasification of biomass for hydrogen production Int J Hydrogen Energy 39, 6912 –6926; Nanda, S., Mohammad, J., Reddy, S.N., Kozinski, J.A., Dalai, A.K., 2014c Pathways of lignocellulosic biomass conversion to renewable fuels Biomass Convers Biorefin 4, 157 –191;

K ırtay, E., 2011 Recent advances in production of hydrogen from biomass Energy Convers Manage 52, 1778–1789; Guo, Y., Wang, S.Z., Xu, D.H., Gong, Y.M., Ma, H.H., Tang, X.Y.,

2010 Review of catalytic supercritical water gasification for hydrogen production from biomass Renew Sustain Energy Rev 14, 334 –343; Balat, M., Balat, M., 2009 Political, economic and environmental impacts of biomass-based hydrogen Int J Hydrogen Energy 34, 3589 –3603; Holladay, J.D., Hu, J., King, D.L., Wang, Y., 2009 An overview of hydrogen production technologies Catal Today 139, 244 –260; Das, D., Veziroglu, T.N., 2008 Advances in biological hydrogen production processes Int J Hydrogen Energy 33,

6046 –6057; Huber, G.W., Iborra, S., Corma, A., 2006 Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering Chem Rev 106, 4044–4098.

a gas stream containing H2, CO, and CO2is produced Hydrogen can be produced

by steam or aqueous reforming of oxygenated hydrocarbons such as methanol, nol, and glycerol and carbohydrates such as glucose The catalytic reforming processresults in the cleavage of bonds in the hydrocarbons such as those of CdC, CdH, andOdH (Levin and Chahine, 2010) The catalysts used for hydrocarbon reforming can

etha-be classified into two types, namely, nonprecious metals (e.g., Ni) and precious metalsfrom group VIII elements (e.g., Pt or Rh) (Holladay et al., 2009) Eqs.(11.1)–(11.9)describe the steam reforming reaction of a few hydrocarbon fuels

Steam reforming of hydrocarbons is given by

2010) Several metal catalysts that have been thoroughly investigated to produce

hydrogen via steam reforming of oxygenated hydrocarbons include Ir, Co, Ni, Pt, and

Rh (Zhang et al., 2007a; Huber et al., 2003) Other catalytic formulations based onmetals such as Ni, Cu, and Zn along with some promoters (e.g., Cr and Zr) are used

at high temperatures for methanol reforming to produce hydrogen (Gadhe and Gupta,

2005) There is tremendous potential for hydrogen generation using the steamreforming process of hydrocarbon fuels For instance, about 43,860 tons of crude glyc-erol is generated every year from 500 million liters of biodiesel produced in Canada.This could be used to produce
6700 tons of hydrogen that could fuel 26,800 vehiclesfor 1 year (Levin and Chahine, 2010)

Bio-oil produced via biomass pyrolysis and liquefaction can also serve as an tive feedstock for hydrogen production through catalytic steam reforming (Czernik

attrac-et al., 2007; Vagia and Lemonidou, 2007; Galda´mez attrac-et al., 2005) Bio-oil has a umetric energy density up to 10 times greater than that of biomass that makes it easierfor transportation to the refineries It consists of many oxygenated compounds such asacids, aldehydes, ketones, alcohols, phenols, esters, guaiacols, and aromatics (Nanda

vol-et al., 2014a) Although the steam reforming of bio-oil inclines toward carbon ity, it is not the case for the steam reforming of fossil fuels (Eqs.11.8, 11.9) However,the intricate compositions of different bio-oils (requiring a case-by-case processoptimization) and carbon deposition on catalyst surface during reforming reactionsare significant hurdles on the way toward their large-scale applications Ni-, Ru-,and Rh-based catalysts have shown good performance for bio-oil conversion to hydro-gen, but their poor stability over longer periods of operation (>100 h) due to carbondeposition is a major challenge (Trane et al., 2012)

neutral-Alkaline-enhanced reforming is a new approach to convert aqueous organics tohydrogen at relatively lower temperatures (<220°C) and pressures However, the pro-cess operates with the use of a base (mostly NaOH) at an alkaline pH that leads to afaster reaction kinetics than conventional steam reforming (Levin and Chahine, 2010).Alkaline-enhanced reforming process requires significantly lower temperatures thanthe conventional steam reforming However, CO2generated by alkaline-enhancedreforming reaction precipitates as alkali salt (e.g., Na2CO3) leading to its lower con-centrations in the gas products This makes alkaline-enhanced reforming beneficialcompared with steam reforming, although there might be some chances of reactor cor-rosion due to salt formation and precipitation Eqs (11.10)–(11.14) represent thealkaline-enhanced reforming of methane, methanol, ethanol, glycerol, and glucose,respectively

Alkaline-enhanced reforming of methane [ΔH¼54 kJ/mol] is given by

Alkaline-enhanced reforming of glycerol [ΔH¼10.9 kJ/mol] is given by

Partial oxidation of hydrocarbons is given by

up (Wilhelm et al., 2001) Autothermal reforming of hydrocarbon fuel, methane, andmethanol is shown in Eqs.(11.19)–(11.24)

Autothermal reforming of hydrocarbons is given by

CmHn+ 0:5mH2O + 0:25mO2! mCO + 0:5m + 0:5nð ÞH2 (11.19)

Autothermal reforming of methane (without steam or with CO2) is given by

as Pd/ZnO, Pt/ZnO, and Cu/ZnO (Huber et al., 2006)

Gasification is a biomass-to-gas (BTG) technology applied for organic wasteconversion to H2, CO, CO2, and CH4 through high-temperature (300–700°C)reactions with controlled amounts of oxygen, steam, or supercritical water Syngas(primarily H2and CO) is the major product of gasification The syngas obtainedcan be converted to green diesel and other hydrocarbons through an integratedcatalytic gas-to-liquid (GTL) technology called Fischer-Tropsch process (Nanda

et al., 2014c; Tr
epanier et al., 2009) The oxygenated fuels such as methanoland ethanol are also generated via the synthesis of alcohols in GTL technologies.Gasification of coal and biomass is a mature and well-known technology for hydro-gen production The advantage of gasification of biomass is its high-pressurehydrogen generation that cuts down the compression costs during its storage(Demirbas, 2009)

In the primary phase of gasification, biomass components start to devolatilize torelease moisture, CO2, and oxygenated vapors As the temperature increases above

500°C, the primary vapors generate gaseous olefins, CO, CO2, H2, and condensed oilssuch as phenols and aromatics (Huber et al., 2006) In most cases, high temperaturesresult in the formation of tar and other heavy aromatic compounds The operatingpressure and the selection of oxidant (e.g., oxygen, air, or steam) affect the biomassgasification reactions (Roddy and Whitton, 2012) The syngas produced from gasifi-cation with air results in low hydrogen concentration (8–14 vol%) and trivial lowerheating value of 4–6 MJ/m3(Delgado et al., 1997) The use of air as an oxidant duringsteam gasification produces syngas with hydrogen levels of 30–60 vol% and heatingvalues of 10–16 MJ/m3(Lv et al., 2004) The introduction of steam or superheatedcompressed water (or supercritical water) as gasifying agent improves the hydrogencontent and reduces the formation of tar and char.

Supercritical water gasification uses supercritical water (374°C and 22.1 MPa) asthe reaction medium for biomass conversion Two types of reaction mechanisms arefound in supercritical water gasification, namely, ionic reaction mechanism andfree-radical mechanism The ionic reaction mechanisms are favored at near-criticalpressures (
22.1 MPa) and lower temperatures (<550°C), whereas free-radicalmechanisms exist at supercritical pressures (>22.1 MPa) and higher temperatures(600°C) (B€uhler et al., 2002) A decrease in the density and dielectric constant ofwater above its critical point leads to the inhibition of ionic mechanisms, thereby pro-moting free-radical mechanisms At near-critical conditions (i.e., higher water den-sity), the concentration of H+ and OH ions are greater, providing an effectivemedium for acid- and base-catalyzed reactions (Kruse and Dinjus, 2007) The gener-ation of H+and OHions during supercritical water gasification creates an optimumenvironment for hydrolysis and pyrolysis reactions On the contrary, at supercriticalconditions (i.e., lower water density), supercritical water behaves as both catalyst andsolvent by releasing free radicals (Hland OHl) that actively take part in the decom-position of organic components (Nanda et al., 2015c)

Supercritical water gasification has been widely studied for lignocellulosic biomass.Typically, lignocellulosic biomass consists of 35%–55% cellulose, 20%–40% hemicel-lulose, and 10%–25% lignin (Nanda et al., 2013) While cellulose and hemicellulosebreak down to simple sugars, lignin in biomass converts to phenolic compounds,ultimately being reformed to H2, CO, CO2, and CH4 (Fang et al., 2008a,b) TheEqs (11.25)–(11.29) represent some crucial reactions occurring during supercriticalwater gasification of biomass

Gasification of organic biomass is given by

CHmOn+ 2ð  nÞ H2O! CO2+ 2ð  n + 0:5 mÞ H2 (11.25)

CHmOn+ 1ð  nÞ H2O! CO + 1  n + 0:5 mð Þ H2 (11.26)Glucose reforming reaction is given by

Steam reforming reaction of phenol is given by

Water-gas shift reaction [ΔH¼41.1 kJ/mol] is given by

Steam reforming reaction is a reaction where the biomass degradation products such asphenolics or phenol react in supercritical water to produce CO and H2 Water-gas shiftreaction increases the amount of H2in the product gas at the expense of CO (Reddy et al.,2014; Nanda et al., 2015a) Water-gas shift reaction typically requires high temperature

in order to achieve fast kinetics, but at equilibrium, high CO is obtained with low H2selectivity Hence, water-gas shift reactor is often followed by a low-temperature reactor

to decrease CO content to<1% (Holladay et al., 2009) Industrial hydrogen productionvia water-gas shift reaction consists of two sequential reactors: (i) high-temperature reac-tor at 350–500°C with Fe-based catalyst and (ii) low-temperature reactor at
200°C withCu-based catalyst (Huber et al., 2006) The CO concentration in the first reactordecreases to about 2%–3% and further to
0.2% in the second reactor

Some catalysts that have been investigated for supercritical water gasification

of biomass and phenol reforming include alkali carbonates and hydroxides (e.g.,NaOH, KOH, Na2CO3, and K2CO3), metal oxides (e.g., Al2O3, CeO2, and ZrO2),supported-metal oxides (e.g., Ni, Rh, Pt, Pd, and Ir), and activated carbon (Azadiand Farnood, 2011; Guo et al., 2010; Constantinou et al., 2009) The choice of catalyst

to gasify biomass depends on its ability to cleave CdC bonds, promote water-gas shiftreaction, and lower the CdO bond cleavage activity

Pyrolysis is often considered as an indirect hydrogen production technology It isused to thermally convert biomass to bio-oil, biochar, and gases From all these pyrol-ysis products, H2can be produced using the explicit technologies reported previously.Specific to the process temperature, heating rate, and residence time, pyrolysis can beclassified into slow, fast, and flash pyrolysis While fast and flash pyrolysis results inhigh amount of bio-oil, slow pyrolysis produces considerable amount of biochar(Mohanty et al., 2013) In any case, a substantial fraction of noncondensable gases(e.g., H2, CO, CO2, CH4, C2H4, and C2H6) are generated in all the pyrolysis processes.Eqs.(11.30), (11.31)illustrate typical reactions in biomass pyrolysis

Pyrolysis of biomass is given by

CxHyOz! 1  xð ÞCO + y  4½ð Þ=2H2+ CH4 (11.31)Pyrolysis is performed under inert atmosphere with no added oxidants Drying the bio-mass to remove moisture could result in high-quality bio-oil and reduce the formation

of carbon oxides (e.g., CO or CO2), thus eliminating the need for secondary reactors(e.g., water-gas shift or partial oxidation reaction) (Holladay et al., 2009)

11.2.3 Electrolysis

Hydrogen synthesis by electrolysis of water (or water splitting) relies on passing anelectric current through a conductive electrolyte in water (mostly alkaline or poly-meric) This results in splitting of water molecules into hydrogen and oxygen

(Eq.11.32) Hydrogen produced through water splitting is of relatively high quality

as no carbon, nitrogen, or sulfur-containing compounds are generated This icantly reduces the purification costs for fuel-cell grade hydrogen compared withhydrogen generated from steam reforming technologies (Levin and Chahine,

signif-2010) Electrolysis is emission free, but its life-cycle assessment studies alsoaccount for the greenhouse gas emissions at the original source of electricity used

The technological maturity has been achieved for alkaline water electrolysis, based

on its electrolyte solution used (e.g., KOH) and conversion efficiencies (64%–70%)(Mueller-Langer et al., 2007) Some commercial low-temperature electrolyzers haveefficiencies of 56%–73% (70.1–53.4 kWh/kg H2) (Turner et al., 2008) However,NaOH, NaCl, and other electrolytes have also been examined The electrolyte solutionenables the ionic conduction between the electrodes efficiently Although the electro-lyte solution is not consumed during the reaction, yet it needs to be replenished peri-odically to prevent any losses Typically, 25%–30% of KOH is used with currentdensities in the range of 100–300 mA/cm2 in commercial systems (Turner et al.,

2008) Ni with a catalytic coating of Pt is the most common cathode, whereas, foranode, Ni or Cu coated with metal oxides (e.g., Mn, W, or Ru) is used (Holladay

et al., 2009)

Most alkaline water electrolyzers operate at 80–90°C It is important to note that asthe temperature increases, the equilibrium voltage decreases The type and concentra-tion of electrolyte also influence the electrolysis efficiency due to the ionic transfermechanisms A well-conductive electrolyte favors the ionic transfer in the solution

In an alkaline electrolyzer, water is introduced in the cathode where it is decomposedinto H2and OH The OHion travels through the electrolyte to the anode where O2

is formed H2, generated in the alkaline solution, is separated using a gas-liquidseparation system The reactions involved in alkaline electrolysis are shown inEqs.(11.33)–(11.35)

Reaction at cathode is given by

Reaction at anode is given by

Overall alkaline electrolysis [ΔH¼–288 kJ/mol] is given by

at the cathode to form hydrogen The oxygen gas is left behind with the unreactedwater Eqs.(11.36), (11.37)represent the reactions occurring during proton exchangemembrane electrolysis

Reaction at anode is given by

Solid oxide electrolysis cells are the solid oxide fuel cells operating in reverseorder These systems substitute a portion of the electric energy required to split waterwith thermal energy Analogous to alkaline water electrolysis, solid oxide electrolysiscauses an oxygen ion to travel through the electrolyte leaving the hydrogen

in unreacted medium (Holladay et al., 2009) The Eqs.(11.34), (11.37)explain thesolid oxide electrolysis An increase in temperature from 102°C to 777°C lowersthe combined electric and thermal energy requirements by
35% (Utgikar andThiesen, 2006) Hence, high temperatures often increase the electrolysis efficiency.The materials used for solid oxide electrolysis include yttria-stabilized zirconia asthe electrolyte, nickel-containing yttria-stabilized zirconia as the anode, and metal-doped lanthanum metal oxides as cathode (Holladay et al., 2009) Solid oxide elec-trolysis integrated with high-temperature nuclear reactors, combustors, or solar power

could achieve high efficiencies Nevertheless, it is also a promising technology to duce oxygen in CO2-rich atmosphere in Mars for human missions (Sridhar andVaniman, 1997).

Hydrogen produced from water, renewable organic wastes, or biomass with theinvolvement of biological agents (e.g., microorganisms, enzymes, or plants) isreferred to as biohydrogen Biohydrogen production is an environmentally friendlyand less energy-intensive but time-consuming process Different biological hydrogenproduction processes include (i) direct and indirect photolysis, (ii) hybrid dark andphotofermentation, (iii) dark fermentation, (iv) photofermentation, (v) biologicalwater-gas shift reaction, and (vi) microbial electrolysis cells

Photosynthesis in plants and photosynthetic microorganisms uses solar energy to vert CO2and water to carbohydrates and O2 For some photosynthetic organisms, excesssolar energy is released with the production of hydrogen through direct photolysis ofwater In biological systems, photolysis causes the dissociation of a substrate (usuallywater) into molecular hydrogen and oxygen in the presence of light (Eq.11.37) Photo-autotrophic algae and cyanobacteria use sunlight and CO2as the sole source for energyand carbon, respectively As green algae possess genetic, metabolic, and electron-transport machinery, they have been the most preferred microbial factories forphotoconverting water to hydrogen.Levin et al (2004)have reviewed a few algae capable

con-of producing biohydrogen such asAnabaena variabilis IAM M-58 (4.2μmol H2/mg chla/h), Aphanocapsa montana (0.4μmol H2/mg chl a/h), Nostoc linckia IAM M-14(0.17μmol H2/mg chla/h), and Synechococcus PCC 6307 (0.02μmol H2/mg chla/h).Under anaerobic conditions, green algae can either use hydrogen as an electrondonor in CO2-fixation process or release H2(Levin et al., 2004) Green microalgaeupon anaerobic incubation in the dark synthesize and activate hydrogenase enzyme.Hydrogenase syndicates protons (H+) in the medium with electrons donated byreduced ferredoxin to produce hydrogen Hydrogenase is highly sensitive to oxygen;hence, photosynthetic production of hydrogen and oxygen should be temporally spa-tially separated (Lee et al., 2010) The synthesis of hydrogen allows the sustained elec-tron flow through the electron-transport chain, which supports the synthesis ofadenosine triphosphate (ATP)

Algal photosynthesis oxidizes water and releases oxygen The process occurs inalgae’s thylakoid membrane where two photosystems are located, that is, photosystem

I or PSI (plastocyanin-ferredoxin oxidoreductase) and photosystem II or PSII plastoquinone oxidoreductase) Solar energy absorbed by PSII generates electronsfrom water oxidation, which are transferred to ferredoxin by PSI (Yu andTakahashi, 2007) Hydrogenase accepts the electrons directly from reduced ferre-doxin to generate hydrogen As a result of direct photolysis, oxygen is evolved fromthe oxidizing side of PSII, and hydrogen is produced from the reducing side of PSI.The maximum H2/O2 ratio (mol/mol) obtained is 2:1 during direct photolysis Ashydrogenase is sensitive to oxygen, the level of the latter should be<0.1% to sustainhydrogen production (Melis et al., 2000)

(water-A green algae Chlamydomonas reinhardtii is found to deplete oxygen levelduring oxidative respiration to enhance hydrogen production (Hemschemeier

et al., 2008) Among the several green algae studied for hydrogen production,maximum hydrogenase activity has been identified forChlamydomonas moewusii(460 nmolH2/(gChl a/h)) followed by Chlamydomonas reinhardtii (200 nmolH2/(gChl a/h)), Scenedesmus vacuolatus (155 nmolH2/(gChla/h)), and Scenedesmusobliquus (150 nmolH2/(gChl a/h)) (Winkler et al., 2002)

The problem of oxygen sensitivity toward hydrogenase is circumvented by rating oxygen and hydrogen via indirect photolysis According toKırtay (2011), indi-rect photolysis involves four steps: (i) biomass production through photosynthesis,(ii) biomass concentration or densification, (iii) aerobic dark fermentation generating

sepa-4 mol of H2per mol of glucose in algal cell along with 2 mol of acetic acid, and(iv) conversion of 2 mol of acetic acid into hydrogen Cyanobacteria can synthesizehydrogen through photosynthesis using indirect photolysis as shown in Eqs.(11.38),(11.39):

Indirect photolysis of water is given by

12H2O + 6CO2Light energy! C6H12O6+ 6O2 (11.38)

C6H12O6+ 12H2OLight energy! 12H2+ 6CO2 (11.39)The hybrid photo and dark fermentation occurs in two phases The first phase occurs

in the absence of light by facultative anaerobes where CO2 is fixed into H2-richsubstrates (e.g., acetic acid and butyric acid) (Kırtay, 2011) In the second phase, pho-tosynthetic bacteria in the presence of light convert acetic or butyric acid to hydrogen.The following Eqs (11.40), (11.44) represent the two phases involved in hybriddark and photofermentation

Phase 1, dark fermentation is given by

dark fermentation can be performed at different temperatures such as mesophilic(25–40°C), thermophilic (45–65°C), extremely thermophilic (65–80°C), and hyper-thermophilic (>80°C) Compared with mesophilic bacteria, thermophiles cantheoretically produce 60%–80% of hydrogen through dark fermentation (Kova´cs

et al., 2006) Concentrated H2is produced from direct and indirect photolysis systems,whereas dark fermentation results in H2, CO2, and traces of CH4, CO, and H2S Hence,

a separation step is required to purify hydrogen from the gas mixture Theoreticalyields of about 4 mol of H2per mol of glucose can be obtained with acetic acid asthe end product, while
2 mol of H2per mol of glucose can be produced with butyricacid as the end product (Ren et al., 2006) Eqs.(11.41), (11.42)represent dark fermen-tation by anaerobic bacteria The complete oxidation of glucose can have a stoichio-metric yield of 12 mol of H2per mol of glucose, but in such a scenario, no energy will

be gained for the bacterial metabolism (Kova´cs et al., 2006)

Clostridium pasteurianum, Clostridium butyricum, and Clostridium beijerinckiiare high biohydrogen producers (Holladay et al., 2009) However, Clostridiumpropionicum is an inefficient biohydrogen-producing strain The partial pressure ofhydrogen is a limiting factor in dark fermentation because an increase in hydrogenpressure decreases its production efficiency (Levin et al., 2004) In order to preventthis issue, hydrogen has to be continually separated as it is generated The production

of acetic, butyric, and other organic acids such as propionic acid can also suppresshydrogen productivity by diverting the metabolic pathway toward organic chemicalproduction This challenge is analogous to the biphasic butanol production fromorganic substrates using Clostridium spp Butanol bioproduction is composed oftwo phases: (i) acidogenic phase producing acetic acid, butyric acid, H2, and CO2and (ii) solventogenic phase producing acetone, butanol, and ethanol from the degra-dation of previously generated organic acids (Nanda et al., 2014b)

Photofermentation depends on the functionality of nitrogenase enzyme by purplenonsulfur bacteria These bacteria can evolve molecular hydrogen catalyzed by nitro-genase under nitrogen-deficient conditions using light as the energy source andreduced compounds (organic acids) as the carbon source (Fedorov et al., 1998) Inphotofermentation, light-harvesting pigments such as chlorophylls, carotenoids,and phycobilins harvest solar energy similar to photolytic algae While sunlight dis-sociates water into protons, electrons, and oxygen, the nitrogenase catalyzes the reac-tion of protons and electrons with nitrogen and ATP to generate ammonia, hydrogen,adenosine triphosphate (ADP), and inorganic phosphate (Pi) (Sørensen, 2005).The main components of nitrogenase are heterotetrameric MoFe protein andhomodimeric Fe protein Hydrogen production through nitrogenase has an order ofmagnitude lower than [NiFe]-hydrogenase, for example, 1.3 and 2.4μmol mg/pro-tein/min for [Mo]-nitrogenase and [Fe]-nitrogenase, respectively (McKinlay andHarwood, 2010) Nitrogenase can use magnesium adenosine triphosphate (MgATP)and electrons to fix nitrogen and reduce a variety of substrates including protons(Ni et al., 2006) The green sulfur bacteriumChloroherpeton thalassium possesses[Fe]-nitrogenase, while the oxygenic cyanobacterium Anabaena variabilis retains[V]-nitrogenase (McKinlay and Harwood, 2010) The [V]-nitrogenase and [Fe]-nitrogenase favor hydrogen production, having H2/NH3ratio approximately threefold

and ninefold higher than that of [Mo]-nitrogenase, respectively Hydrogen production

by different nitrogenase enzymes is empirically shown in Eqs.(11.45)–(11.48).Hydrogen production by [Mo]-nitrogenase enzyme is given as

N2+ 8H++ 8e+ 16ATP! 2NH3+ H2+ 16ADP (11.45)Hydrogen production by [V]-nitrogenase enzyme is given as

N2+ 12H++ 12e+ 24ATP! 2NH3+ 3H2+ 24ADP (11.46)Hydrogen production by [Fe]-nitrogenase enzyme is given as

N2+ 24H++ 24e+ 48ATP! 2NH3+ 9H2+ 48ADP (11.47)Absence of nitrogen is given as

Under anaerobic conditions, the purple nonsulfur bacteria are able to use simpleorganic acids (e.g., acetic acid and butyric acid) or hydrogen disulfide as electrondonors (Kirtay, 2011) These electrons are transported to nitrogenase by ferredoxinusing ATP as the energy source In the absence of nitrogen, nitrogenase can reduceproton into hydrogen using energy in the form of ATP The following Eq.(11.49)illustrates the photofermentation reaction

Photofermentation is given by

C6H12O6+ 12H2OLight energy! 12H2+ 6CO2 (11.49)Alike hydrogenase, nitrogenase is also sensitive to oxygen Hence, for an efficient andcontrolled productivity of hydrogen, the nitrogen fixation and oxygen generation should

be spatially and temporally separated in a cyanobacterial biosystem Rhodobactersphaeroides and Rhodobacter capsulatus have hydrogen production efficiencies inthe range of 30%–86% from a variety of substrates such as lactic acid and sugar refinerywastewater (Ni et al., 2006).Rhodopseudomonas capsulate, Rhodobacter sphaeroides,andRhodospirillum rubrum have been reported to produce hydrogen at rates up to 50,

100, and 180 mL of H2/L of culture/h, respectively (Levin et al., 2004) Among all thebiohydrogen production technologies, photofermentation is most favored due torelatively higher substrate-to-hydrogen yields (
80%) and ability to scavenge lightenergy under a wide range of light spectrum (Gadhamshetty et al., 2008; Manish andBanerjee, 2008)

Hydrogen can also be produced by biologically mediated water-gas shift reaction(similar to Eq 11.29) Some photoheterotrophic bacteria, for example, Rhodospi-rillum rubrum can use CO as the sole carbon source in the absence of light and gene-rate ATP by coupling the oxidation of CO with the reduction of H+to H2(Kerby et al.,

1995) A few gram-negative bacteria (e.g.,Rhodospirillum rubrum and Rubrivivax

gelatinosus) and gram-positive bacteria (e.g., Carboxydothermus hydrogenoformans)possess the ability for the biological water-gas shift reaction (Ni et al., 2006).Water-gas shift reaction is mediated in these organisms using proteins coordinated

in an enzymatic pathway under anaerobic conditions Initially, CO induces the thesis of several proteins including CO dehydrogenase, Fe-S protein, andCO-tolerant hydrogenase The electrons generated from the oxidation of CO are con-veyed via the Fe-S protein to the CO-tolerant hydrogenase for hydrogen production(Ensign and Ludden, 1991) The CO oxidation is thermodynamically favored at ambi-ent temperature and pressure as a result of which stoichiometric amounts of CO2and

syn-H2are produced (Levin et al., 2004)

Microbial electrolysis cells are microbial bioreactor systems that useelectrohydrogenesis to convert biodegradable material into hydrogen (Call andLogan, 2008) In a microbial fuel cell, microorganisms termed as exoelectrogensoxidize organic matter and transfer electrons to the anode The electrons reach thecathode after traveling through an external resistance and combine with protonsand oxygen to produce water The microbial fuel cell is deprived of oxygen at the cath-ode The anode used in microbial electrolysis cells is chemically inert with a lowelectronic resistance (e.g., graphite)

In the anodic compartment, exoelectrogens (typically in the form of microbial film) are present that oxidize an organic compound and transfer the released electrons

bio-to the anode (Geelhoed et al., 2010) The combination of electrode andexoelectrogenic microorganisms is often referred to as bioanode An external voltage

is applied to the cell because energy is required for hydrogen formation from substrate(i.e., acetate, butyrate, lactate, propionate, and ethanol) decomposition (Lee et al.,

2010) The pH at the anode surface is frequently decreased as one proton is producedper electron transferred (Geelhoed et al., 2010) However, bioanodes operate favor-ably at neutral pH (Biffinger et al., 2008); hence, the medium should be well buffered

or replenished regularly to maintain a neutral pH (Sleutels et al., 2009) Eqs.(11.50),(11.51)show the generation of electrons from acetate (at anode) and formation ofhydrogen (at cathode) in a microbial electrolysis cell

Oxidation of acetate at anode is give by

in biofilms A few microorganisms that are useful in microbial electrolysis cellsare Acidiphilium sp., Aeromonas hydrophila, Clostridium butyricum, Desulfovibriodesulfuricans, Desulfobulbus propionicus, Desulfuromonas acetoxidans, Escherichia

coli, Geobacter metallireducens, Geobacter sulfurreducens, Geopsychrobacterelectrodiphilus, Geothrix fermentans, Klebsiella pneumoniae, Ochrobactrumanthropi, Pichia anomala, Pseudomonas aeruginosa, Rhodoferax ferrireducens,Rhodopseudomonas palustris, Shewanella oneidensis, Shewanella putrefaciens, andThermincola sp (Logan, 2009).

technologies

11.3.1 Fuel cells

The development in fuel cell is a major advancement in hydrogen market today Fuelcells are emerging clean alternative technology to the conventional internal combus-tion engines With the rapid development in technology and mobile devices, thedemand for portable electric power supplies (e.g., batteries) is also skyrocketing

A fuel cell is similar to a constantly recharging battery that generates electricity bylow-temperature electrochemical reactions between hydrogen and oxygen from air

It converts a fuel’s chemical energy into electricity through a reaction of positivelycharged H+ions with oxygen or another oxidizing agent

In modern-day batteries, the chemicals present in their chambers react with eachother to generate an electromotive force (emf ) In contrast, a fuel cell requires a con-tinuous source of fuel and oxygen (or air as the oxidant) to sustain the chemical reac-tion and generate electricity Another difference is that batteries can store the electricenergy, whereas fuel cells can produce electricity continuously as long as there is aconstant supply of the fuel and air (or oxygen) As the fuel cells use hydrogen as areactant and operate at temperatures much lower than internal combustion engines,they emit water as the end product, virtually causing no pollution Any hydrogen-richfuel can be used in fuel cells (for fuel-reforming process), but using a hydrocarbon fuelmay lead to CO2emission Fuel cells can convert hydrogen into electricity at morethan twice the efficiency of internal combustion engines as they overrule the limita-tions of the Carnot cycle (Edwards et al., 2008)

While the conventional gasoline-driven vehicle engines have an efficiency of

<20%, engines operating on hydrogen fuel cell have an efficiency of up to 60%(USDEHP, 2006) Moreover, hydrogen fuel cells also surpass the efficiency of mostcombustion-based power plants that typically generates electricity at efficiencies ofabout 35% An overall efficiency of 85% can be achieved when the heat generated

in fuel cells is also utilized in combined heat and power (CHP) systems The vehiclesequipped with electric motors powered by hydrogen fuel cells are energy-efficient

by using 40%–60% of the fuel’s energy that corresponds to about 50% reduction infuel consumption compared with gasoline-powered internal combustion engines(USDEHP, 2006) About 1 ton of hydrogen can power from two to four fuel-cell vehi-cles for 1 year or one urban transit bus for about 1.5 months (Levin and Chahine, 2010).Although there are different fuel cells dedicated for diverse energy applications, yetthey have the basic design of the electrolyte (solid, liquid, or membrane) sandwichedbetween two electrodes (anode and cathode) While hydrogen (or fuel containing

hydrogen) is fed into the anode, oxygen (or air) is fed into the cathode of the fuel cell.The electrochemical reactions occur at the electrodes that are assisted by the catalysts.Sometimes, bipolar plates are used on either side of the electrodes to help distribute thegases and serve as current collectors The electrolyte enables the transfer of ionsbetween electrodes, while the excess electrons flow through an external circuit to gen-erate electricity.

Fuel cells are also attractive for their modularity generating electric power rangingfrom microscale (<1 kW) to large-scale (>10 MW) systems (Buonomano et al.,

2015) The scalability of fuel cells make them ideal for a broad range of applications,especially in laptop computers (50–100 W), powering homes (1–5 kW), and vehicles(50–125 kW) as well as central power generation infrastructures (>200 MW)(USDEHP, 2006) The global market potential for fuel cells is expected to reach

50 GW by 2020 (Prabhu, 2013) Fuel cells mostly vary in terms of their electrolyteand operating temperature, which determines their specific electricity output, effi-ciency, and applications The electrolyte used in fuel cells can be a base, acid, salt,ceramic, or polymeric membrane that conducts ions Some of the widely investigatedfuel cells include alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), directmethanol fuel cells (DMFCs), molten carbonate fuel cells (MCFCs), proton exchangemembrane or polymer electrolyte membrane fuel cell (PEM), and solid oxide fuelcells (SOFCs).Table 11.3 summarizes the chief features, electric efficiencies, andapplications of various fuel cells

In particular, solid oxide fuel cells are the most attractive of all fuel cells for possiblesystem hybridization due to their high operating temperatures up to 1000°C (McPhail

et al., 2011) The high-temperature solid oxide fuel cells and molten carbonate fuelcells are being used for distributed energy supply using natural gas The natural gas

is first reformed to hydrogen-rich syngas in an external reformer before passingthrough the internal reforming section of the solid oxide fuel cells (Lee et al.,

2015) However, developing new facilities or retrofitting the existing reforming structures for solid oxide fuel cells to use hydrogen directly can help boost the hydro-gen economy Nevertheless, this could provide many economic and environmentalbenefits over today’s commercially available hydrocarbon fuel processing techno-logies that impart some carbon footprint Using hydrogen directly in the fuel cellscould also reduce the costs associated with adding an external reformer to the system.The fuel cells have potentials for use as supplementary power generators for vehiclesand off-grid applications to replace small diesel generators (Edwards et al., 2008)

infra-It is worth noting that the electrochemical reaction in the fuel cells is highly thermic liberating considerable amount of heat energy that can be profitably used forcombined heat and power and cogeneration purposes (e.g., domestic hot water, spaceheating, street lighting, and steam production) In the case of high-temperature fuelcells, the exhaust gases can be also used to drive a bottomed thermodynamic cyclesuch as Rankine cycle and Brayton cycle (Buonomano et al., 2015) Brayton cycle

exo-is a thermodynamic cycle describing the working of a heat engine at a constant sure On the other hand, Rankine cycle is a thermodynamic cycle of a heat enginedescribing the conversion of heat energy into mechanical work Organic Rankinecycle is one of its subsystems that can be used in trigeneration plants

pres-In a hybrid scenario, solid oxide fuel cells have achieved high efficiency of over70% with regeneration (Mahato et al., 2015) They are particularly well suited forcogeneration (combined heat and power) applications as they produce high-gradewaste heat (or cooling) and electric power Integration of fuel cells has also provedsignificant in trigeneration (combined cooling, heat, and power, CCHP) Tri-generation is a type of plant operation where the power generation, heating, andcooling are from the same source In trigeneration plants, a portion of the waste heat

Table 11.3 Type of fuel cells with their technical features

and key applications

Electrolyte

Operatingtemperature(°C)

Electricefficiency(%)

Electricpower output

fuel cell

supplyMolten

fuel cell

(80–85with CHP)

large-scalepowerdistribution,CHPProton exchange

Based on Edwards, P.P., Kuznetsov, V.L., David, W.I.F., Brandon, N.P., 2008 Hydrogen and fuel cells: Towards a sustainable energy future Energy Policy 36, 4356 –4362; U.S Department of Energy Hydrogen Power (USDEHP),

2006 Hydrogen fuel cells DOE Hydrogen Program www.hydrogen.energy.gov Accessed 7 March 2016.

from the prime mover is used for heating (i.e., producing steam), while the remainingportion is used for cooling (i.e., cooling air).Al-Sulaiman et al (2010)have performedthe energy analysis of a trigeneration plant based on solid oxide fuel cell and organicRankine cycle They found that the efficiency of solid oxide fuel cells decreased as thecurrent density increased Furthermore, as the inlet flow temperature of fuel cellincreased, the trigeneration efficiency also improved.

Low-temperature proton exchange membrane fuel cells and alkaline fuel cells offerhigher power density compared with other fuel-cell systems (Edwards et al., 2008).However, a major limitation is that they require costly platinum catalyst and ultrapurehydrogen On the contrary, phosphoric acid fuel cells are more tolerant to impurities inhydrogen.Qingfeng et al (2001)reported that phosphoric acid when doped onto poly-benzimidazole polymer membranes enhanced fast electrode kinetics and renderedhigh tolerance to fuel impurities in proton exchange membrane fuel cells Protonexchange membrane fuel cells were developed since the 1960s and used in NASAspace programs such as the Gemini and Apollo space missions to provide drinkingwater for the crew during space mission (Sopian and Daud, 2006) With a global effortbeing invested to develop commercial systems, proton exchange membrane fuel cellshave been the most favored for automotive and small-scale CHP applications Phos-phoric acid fuel cells are currently used for small-scale stationary and mobile (auto-motive) power generation systems Although commercially available today, these fuelcells have restricted market consumers due to their high cost

Direct methanol fuel cells are powered by methanol and are considered as an native to batteries for portable electronic devices and auxiliary power units The mainadvantages include low-temperature liquid-fed fuel cells, high energy density ofmethanol, and high energy efficiency (Sebastia´n et al., 2016) However, the Nafionmembranes used in direct methanol fuel cells suffer from high methanol permeabilityand high material cost, which directs the interest on synthesizing efficient polymerelectrolyte membrane materials to overcome this issue and realize better cell perfor-mance Recently, Dutta et al (2015)have synthesized a low-cost effective protonexchange membrane polymer upon partially sulfonating polyaniline as a coconstituentalong with partially sulfonated poly(vinylidene fluoride-co-hexafluoropropylene).This polymeric material exhibited extremely low methanol permeability with highmembrane selectivity

11.3.2.1 Fischer-Tropsch process

The gas-to-liquid (GTL) technologies can convert syngas (H2and CO) obtained fromgasification or reforming reactions to liquid hydrocarbon fuels The GTL technologiescan address the problems of remote gas utilization, hydrogen storage, and producingbiofuels that are compatible for use in existing power-generating infrastructures andvehicles The most widely known GTL technology is Fischer-Tropsch process thatconverts syngas into liquid fuels with the application of catalyst, heat and pressure.The syngas obtained after reforming reaction or gasification is required to be adjusted