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A large amount of primary energy stored in hydrocarbon-based solid, liquid, and gaseous fuels is consumed for electricity and heat generation in residential and industrial sectors and for internal-combustion-engine-based transportation. Worldwide, 7235 million t of oil equivalent (MTOE), or 75% of energy use, in 1997 was based on fossil fuels. Per capita CO2 emissions in that year amounted to 1.13 t of carbon [1]. In the 20 th century, the human population has quadrupled, while the

Fossilized Wood Atmospheric Gases

Wood

Nuclear

SolFus

Year

FIGURE 8.1 World primary energy substitution f = market share). (From Marchetti, C., Society of a Learning System: Discovery, Invention and Innovation Cycles Revisited, International Institute for Applied System Analysis, Pub. RR-81-29, Laxenburg, Austria, 1981. With permission.)

Year

Wood

Nuclear

SolFus

FIGURE 8.1 World primary energy substitution f = market share). (From Marchetti, C., Society of a Learning System: Discovery, Invention and Innovation Cycles Revisited, International Institute for Applied System Analysis, Pub. RR-81-29, Laxenburg, Austria, 1981. With permission.)

primary power consumption has increased 16-fold. The concentration of atmospheric CO2 has increased from about 275 to about 370 ppm. If nothing changes in our consumption pattern of energy, the CO2 concentration will pass 550 ppm this century [2]. However, atmospheric CO2 stabilization targets as low as 450 ppm could be needed to forestall coral reef bleaching, thermohaline circulation shutdown, and sea level rise from disintegration of the West Antarctic ice sheet [3]. These targets require immediate action on all levels of human life, especially in the field of research and development of new, environmentally benign, and highly efficient energy conversion technologies. There is but one extenuating circumstance in this seemingly suicidal human activity, which is, regrettably, not the result of reflection over the situation, but rather the result of thirst for fuels with ever-higher energy content. Figure 8.1 illustrates the data of primary energy substitution in the world since the time of the Industrial Revolution together with somewhat speculative prediction until the year 2050 [4]. A pattern in these fuel substitutions has been the replacement of fuels with low hydrogen-to-carbon (H/C) ratio with progressively higher H/C ratio fuels (see Table 8.1) [5]. Hidden behind this pattern is that technologies, not fuels, compete in the marketplace. Technologies win because they are better. "Better," a deceptively simple word, is a more encompassing concept than, for example, "cheaper." "Better," in this case, means technologies that can "squeeze" higher specific power and higher power density per unit mass and unit volume of primary energy source.

Any energy pathway can be represented by five links from sources to the services that require the input of energy. These five links are depicted on the upper part of Figure 8.2. To make the nomenclature of this five-link architecture clear, the lower part in Figure 8.2 gives some examples that are covered by the headings on the upper part. The real comparison between different energy pathways is not just simple comparison of production costs of energy vectors (e.g., hydrogen vs. gasoline), but a comparison between the life cycles of possible energy pathways needed to deliver certain services (e.g., transportation). A part of life-cycle analysis (LCA) for service

TABLE 8.1

Basic Characteristics of Fossil Fuels and Hydrogen

TABLE 8.1

Basic Characteristics of Fossil Fuels and Hydrogen

Hydrogen

Energy Content

Particulates

Carbon Dioxide

Fuel Type

(%)

(Btu/lb)

(lb/Btu)

(Rel. %)

Dry wood

5

6,900

5.22

100

Coal

50

10,000

5.00

31

Oil

67

19,000

0.18

21

Natural gas

80

22,500

<0.01

15

Hydrogen

100

61,000

0.0

0

Source: Cannon, J.S., Harnessing Hydrogen: The Key to Sustainable Transportation, New York: Inform, 1995; available on-line at http://www.ttcorp.com/nha. With permission.

Source: Cannon, J.S., Harnessing Hydrogen: The Key to Sustainable Transportation, New York: Inform, 1995; available on-line at http://www.ttcorp.com/nha. With permission.

FIGURE 8.2 General presentation of energy pathways.

like transportation is called, very indicatively, "well-to-wheel" analysis, and it comprehends the amount of energy required by certain traction system, e.g., for hydrogen-powered electric vehicles, versus the amount of energy required by another traction system, e.g., diesel-powered internal-combustion-engine vehicles, and all the costs associated with production and distribution.

The time sequence (I to IV) of competing technologies for electricity generation based on chemical fuels is a good illustration for the concept of "better" technology, and it simultaneously gives an idea about different possible energy pathways for grid electricity production from chemical fuels (Figure 8.3).

The "natural" tendency for new technologies to use fuels with higher energy content thus coincides with the fuels having higher H/C ratio. One can eventually arrive at the following conclusion: the most potent fuel among the "chemical" fuels is hydrogen. Fortunately, it is the most abundant element on Earth and in the universe, and it is also the cleanest fuel: the product of hydrogen combustion is water.

Pure hydrogen as the strongest chemical fuel gives a possibility to suppress the CO2 and particulate emissions almost completely (depending on the process of hydrogen production) and to lower the NOx emissions (depending on the energy conversion system used).

FIGURE 8.3 Competing technologies for electricity generation. Their development and market penetration through time (I-IV) dictated the type of fuel that these technologies preferably use. (From Song, C., Catal. Today, 77, 17, 2002. With permission.)

However, hydrogen is not only the strongest "chemical" fuel, it also serves as an energy carrier (vector). Namely, one can produce hydrogen in a "thousand and one" ways and then use it for transfer of energy over short and long distances or for on-site transformation of energy into electricity and heat. Among the methods for hydrogen production, we are obviously interested in those that are sustainable or, even better, renewable. In the remainder of this chapter, we shall describe some technologies for hydrogen production from sustainable and from renewable primary energy sources, with the emphasis on hydrogen cleaning processes.

Since our civilization depends to a large degree on the power delivered by heat engines and, in transportation, specifically by internal-combustion engines (ICE), the primary energy sources are predominantly "chemical" fuels that can burn. The efficiencies of the current thermal electric power generation plants are about 35 to 40% [6], and the well-to-wheels efficiency of the current four-passengers car is between 14 and 22% [7]. These numbers reveal that we are using our natural resources, fossil fuels, extremely inefficiently; most of the chemical energy stored in fossil fuels is used to produce waste heat and proportionally large amounts of greenhouse gases.

Among the energy converters, fuel cells are unique direct-energy-conversion devices capable of converting the energy of chemical reactions into electricity with the highest maximum feasible efficiency of 90% [8]. The value of deriving electric current directly from the chemical reactions of fuels was recognized well before electricity became a commodity sold by power utilities. The first investigations go back to 1839 and Sir William Grove. It was not until the 1960s, however, that fuel cells were employed in a practical capacity. NASA had used them first to provide electric power on board the Gemini space mission. Afterward, the fuel cells were used on board nearly every space mission, regardless of the country having space technology. Steady progress over the last 40 years has made it possible for fuel cells to start displacing combustion from its central technological role. Nowadays, we have experienced the use of fuel cell technology in demonstration projects encompassing electric cars and buses and mobile and stationary use in the span of power from a few tens of watts to a few megawatts. Fuel cell technology is now at the dawn of commercialization [6].

TABLE 8.2

Fuel Cells Ordered According to Operating Temperature and Type of Electrolyte

Type Electrolyte Operating Temperature (oC)

AFC KOH solution 50-90

PEMFC Solid polymer 50-95

PAFC H3PO4 190-210

MCFC Li2CO3/K2CO3 630-700

SOFC Y-stabilized ZrO2 900-1000

Most fuel cells being developed consume either hydrogen or fuels that have been preprocessed into a suitable hydrogen-rich form. Some fuel cells can directly consume sufficiently reactive fuels such as methane, methanol, carbon monoxide, or ammonia, or they can process such fuels internally. Different types of fuel cells are most suitably characterized by the electrolyte that they use to transport the electric charge and by the temperature at which they operate. This broad classification is presented in Table 8.2.

Further, we shall concentrate on the low-temperature proton-exchange-membrane fuel cells (PEMFC) as a most representative H2/O2 or H2/air fuel cell. We shall do this deliberately, since PEMFCs, working at low temperature, have high thermodynamic equilibrium potential, and therefore they can reach high open-circuit voltage and potentially high efficiency in energy conversion. Low working temperature also poses fewer restrictions on the construction materials.

A PEM fuel cell consists of a negatively charged electrode (cathode), a positively charged electrode (anode), and a thin proton-conducting polymer electrolyte membrane. Hydrogen is oxidized on the anode, and oxygen is reduced on the cathode. Protons are transported from the anode to the cathode through the electrolyte membrane, and electrons are carried to the cathode over an external circuit. On the cathode, oxygen reacts with protons and electrons, forming water and producing heat. Both the anode and the cathode contain a catalyst to speed up the electrochemical processes. The schematic construction and both half-cell reactions are depicted in Figure 8.4.

The electricity and heat are produced by the cathode reaction. Theoretically, the Gibbs energy of the reaction is available as electrical energy, and the rest of the reaction enthalpy is released as heat. In practice, a part of the Gibbs energy is also converted into heat via the loss mechanisms.

One unit cell produces, roughly, 2 kA/m2 at a cell voltage of 0.8 V. This implies that we have to couple several cells in series to increase the voltage to usable levels. This is done in a fuel cell stack, where bipolar plates serve both as separators and current conductors between adjacent anodes and cathodes. These bipolar plates also serve as gas distributors to the electrodes through channels in their structure. In addition, the edges of the plates serve as manifolds for the fuel cell stack. Figure 8.5 shows the principle behind a bipolar stack.

Although PEMFCs have been in use for more than 40 years in diverse advanced applications (space, military), further accelerated development of this technology is

Gas impermeable Diffusion layer membrane Catalyst Gas distributor layer

Air or oxygen (humidified)

Air or oxygen in excess + product water

Hydrogen (humidified)

Current load

Hydrogen in excess

FIGURE 8.4 Schematic representation of the PEMFC cross-section.

Cooler plate

FIGURE 8.4 Schematic representation of the PEMFC cross-section.

Cooler plate

Bipolar Plate

Bipolar plate

Membrane & Electrode Assembly

Bipolar plate

Membrane & Electrode Assembly

FIGURE 8.5 Bipolar configuration of the stack.

yet to come within different areas of its application. According to a recent report, the U.S. market for stationary use of fuel cells in the electric power generation (e.g., remote supply, grid support, cogeneration) will commercialize first, followed by mobile use of fuel cells in portable power (e.g., cellular phones, handheld computers, camcorders) and in automobiles (not before 2011) [9]. In all these areas of application, the PEMFC has an outstanding role. Obviously, the most demanding market

TABLE 8.3

Well-to-Wheel Analysis of Vehicle Traction Systems Efficiency

Gasoline

System Performance a ICE _Fuel Cell System Type

TABLE 8.3

Well-to-Wheel Analysis of Vehicle Traction Systems Efficiency

Gasoline

System Performance a ICE _Fuel Cell System Type

Gasoline

Ethanol

Methanol

Hydrogen

ATR

ATR

SR

cH2

Weight, W/kg

560

320

290

290

420

Efficiency at 100% load,

35%

39%

40%

46%

58%

LHV

Efficiency at 20% load,

25%

36%

37%

43%

62%

Note: ICE = internal combustion engine; ATR = autothermal reforming; SR = steam reforming; cH2 = compressed H2; LHV = lower heating value.

Note: ICE = internal combustion engine; ATR = autothermal reforming; SR = steam reforming; cH2 = compressed H2; LHV = lower heating value.

a Includes engine/fuel cell stack, radiator, fuel processor, fuel, and fuel tank.

for fuel cells is their use in transportation. The technology of the internal-combustion engine (ICE) is very sophisticated, developed through more than 100 years. To penetrate the market with a new fuel-cell-based electric engine is a challenging but enormously complicated task. This is at the same time the application area where most of the future advanced research and development efforts will be concentrated. Table 8.3 illustrates the well-to-wheel analysis of efficiencies for differently fueled polymer-electrolyte-membrane fuel cell (PEMFC) vehicle traction systems in comparison with the gasoline-fueled internal-combustion engine [10].

Of the fuel cell systems, direct hydrogen has the highest power density and efficiency. This system has 1.5 to 2 times higher efficiency than the PEMFC systems fueled with reformate fuels.

PEMFC is, in fact, a superb example of a catalytic membrane reactor performing a variety of reactions and separations. To fuel it, we need hydrogen. Distributed combined heat and power (CHP) generation based on today's common (logistic) fuels demands that hydrogen be produced either on-site (for stationary applications) or onboard (for mobile applications). A general scheme for PEMFC-grade hydrogen production from sustainable energy sources (biomass, organic waste, ethanol) or from fossil fuels (coal, oil, natural gas) is represented in Figure 8.6. As can be seen, nearly all processes in the chain are catalytic.

On-site and onboard hydrogen production processes are viewed as a transitional solution until new H2 production and H2 storage technologies are developed. On the other hand, the catalytic processes for hydrogen production from hydrocarbons have been practiced in the chemical industry for many years. Therefore, every step forward in the formulation of the new catalysts and the design of new reactors and processes are very demanding because of pressure for time to enable fuel cell technology for market penetration and because of the relatively long and rich history of each and all of the catalytic processes involved in this technology. Some of the classical processes for hydrogen production and cleaning are discussed elsewhere in this book (Chapter 11 by N. Hickey, P. Fornasiero, and M. Graziani). In the present chapter,

Optional H2S capture

Fuel-

Optional H2S capture

Fuel-

Organic

Fuel

Water

Selective

S removal

reforming

Gas shift

CO removal

Options: HDS or Steam reforming High-temp. Selective oxid.

S adsorpt. Partial oxidation Low-temp. O2-bleeding Autothermal ref. Methanation

Catal. decompos. Catal. pyrolysis

Options: HDS or Steam reforming High-temp. Selective oxid.

S adsorpt. Partial oxidation Low-temp. O2-bleeding Autothermal ref. Methanation

Catal. decompos. Catal. pyrolysis

FIGURE 8.6 Steps and options for on-site and onboard processing of renewable and fossil fuels.

the discussion will therefore be limited on some possible routes for hydrogen production from sustainable and renewable feedstock and on some new catalysts for hydrogen cleaning in downstream processes.

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