General Pathways For Hydrogen Production

Free Power Secrets

Making Your Own Fuel

Get Instant Access

A simple general scheme for hydrogen production is given in Figure 8.7, while Table 8.4 lists the major hydrogen production processes from various feedstocks, convenient energy sources to drive these processes, and the emissions that each process produces [11]. It can be quickly realized that the only feedstock from which hydrogen can be produced without emissions is water. However, one has quite a number of possible processes to produce hydrogen from water. Further on, we shall focus on the thermochemical processes, since these are of immediate interest due to our technological heritage based on exploitation of fossil fuels, which will continue for several decades from now.

FIGURE 8.7 General scheme for hydrogen production from fossil primary energy sources (shadowed area) and from renewable and sustainable sources.

TABLE 8.4

Major Hydrogen Production Processes

Primary Method

Process

Feedstock

Energy

Emissions

Reforming

Natural gas

HT steam

CO2

HT water

Water

HT heat from gas

none

splitting

cooled nuclear

Thermochemical

Gasification

Coal,

reactors HT&P steam and

CO2, SO2,

Biomass

O2

NOx

Pyrolysis

Biomass

Moderate HT steam

CO2

Electrolysis

Water

Electricity from wind, solar, hydro or nuclear

None

Electrochemical

Electrolysis

Water

Electricity from coal or gas

CO2, SO2, NOX

Photoelectroche

Water

Direct sunlight

None

mical

Photobiological

Water in algae strains

Direct sunlight

None

Biological

Anaerobic digestion

Biomass

Ht heat

CH4, NH3

Fermentative

Biomass

HT heat

CO2

microorganisms

Today, hydrogen is produced as a chemical reactant in processes like ammonia synthesis, hydrogen peroxide synthesis, and various hydrogenation processes in the petrochemical industry, pharmaceutical industry, fine organics synthesis, etc. The exact quantities of hydrogen production are difficult to estimate, since hydrogen represents an intermediate reactant in the vast majority of cases. There are, however, some estimates regarding the resources from which hydrogen is produced. Table 8.5 gives one such estimate. Obviously, over 95% of hydrogen is currently derived from fossil fuels, and only about 5% is generated from sustainable or renewable resources. This situation will not change overnight, of course, since the processes (technology)

TABLE 8.5

Hydrogen Sources Today and Their Percentages

H2 source

Percentage

Natural gas

48

Oil

30

Coal

18

Water splitting

4

Biomass

low

FIGURE 8.8 Fuel processing scheme for a PEMFC system.

for hydrogen production are based on fossil feedstock. Substantial research and development efforts in the near future must be engaged to develop new technologies adapted on sustainable and renewable feedstock.

8.2.1 Thermochemical Pathways for Hydrogen Production

We shall discuss in this section only conventional processes and catalysts for hydrogen generation and their limitations with respect to PEMFC technology to demonstrate how different are specific technical demands for hydrogen production and cleaning in this case.

Fuel processing system for a PEMFC system is illustrated in Figure 8.8. The major components of the system are:

Catalytic reformer Shift reactor PrOx reactor Fuel cell stack

Turbocompressor to provide compressed air to the fuel cell stack, the reformer, and PrOx reactor Condenser/radiator to recover water Water-storage tank

The catalytic reformer in Figure 8.8 should be understood in a broader sense, including all the options: partial oxidation, steam reforming, and the combination of these — autothermal reforming.

Partial oxidation is based on extreme rich fuel combustion (low air/fuel ratios), where the following reactions may occur:

The process can be performed in both a catalytic and a noncatalytic manner. If a catalytic system is used, the reformer can be operated at a much lower temperature and the heat can be supplied directly into the catalyst bed. The advantage with this process is that it is rather insensitive to contaminants and that it is rather independent of fuel. The biggest drawback is the risk for carbon formation if the gas composition exceeds the equilibrium constant of any of the following carbon-forming reactions :

CO(g) + H2(g) ^ H2O(g) + C(s) CO2 (g) + 2H2 (g) ^ 2H2O(g) + C(s)

The first reaction (Boudouard equilibrium) will favor carbon formation at lower temperatures than encountered in partial oxidation. The hydrogen concentrations that can be theoretically attained when using different fuels in a partial oxidation process are represented in Table 8.6, together with values obtained experimentally.

Steam reforming can be represented with the following equation:

CxHyOz + (2x - z)H2O ^ (y/2 + 2x - z)H2 + xCO2 (8.3)

The biggest advantage with steam reforming is the high concentrations of hydrogen that can be achieved. For instance, with methanol as feedstock, 75 vol% hydrogen can be obtained at stoichiometric conditions and total conversion. However, steam reforming is an endothermic process, which lowers otherwise high system efficiency.

TABLE 8.6

Concentration of Hydrogen in the Product Gas Obtained with Different Fuels

H2 Concentration, % (dry)

TABLE 8.6

Concentration of Hydrogen in the Product Gas Obtained with Different Fuels

H2 Concentration, % (dry)

Fuel

Theory

Experiment

H2 selectivity a (%)

Temperature

Methanol

70

64

91

450

Ethanol

71

62

88

580

i-Octane

68

60

88

630

Cyclohexane

67

61

91

700

2-Pentene

67

58

88

670

Toluene

61

50

82

660

a Selectivity = (% H2 in product gas) X 100/(% H2 theoretically possible). For example, with C8H18 and x = 4, the theoretical hydrogen percentage is 68%. Thus, 60% hydrogen in the product is equal to a selectivity of 88%.

a Selectivity = (% H2 in product gas) X 100/(% H2 theoretically possible). For example, with C8H18 and x = 4, the theoretical hydrogen percentage is 68%. Thus, 60% hydrogen in the product is equal to a selectivity of 88%.

Copper-based catalysts have been mostly used for ethanol or methanol steam reforming. For methanol, CuO/ZnO/Al2O3 catalyst is usually used at temperature between 180 and 250°C. When unreacted methanol is present, the rate of the water-gas-shift (WGS) reaction is negligible. As the conversion of methanol approaches 100%, the rate of the WGS reaction becomes significant, and the CO concentration in the reformer product gas approaches equilibrium. The reforming of ethanol proceeds better over CuO/ZnO catalyst at temperatures above 300°C.

Autothermal reforming is a combination of steam reforming and partial oxidation that, in theory, can be totally heat balanced. The reaction can be represented by the following equation:

where m is an oxygen/fuel ratio. The concentration of hydrogen in the product gas is {(2 x - 2m - z + y/2) / (x + (2 x - 2m - z + y/2) + 3.76m )}l00 (8.5) and the reaction enthalpy is calculated as

Hr = x • HfCO, - (2x - 2m - z) • Hf^ - Hf,>d (8.6)

Table 8.7 gives the ratio of oxygen/fuel at which the reaction is heat balanced (thermoneutral).

TABLE 8.7

Calculated Thermoneutral Ratios for Oxygen/Fuel (mO2) and Theoretical Yield

TABLE 8.7

Calculated Thermoneutral Ratios for Oxygen/Fuel (mO2) and Theoretical Yield

AHr, fuel

M02

Efficiency

CxHyOz

x

y

z

(kcal/mol)

y/2z

AHr = 0

(%)

Methanol

1

4

1

-57,1

2

0,230

96,3

CH3OH

Methane

1

4

0

-17,9

2

0,443

93,9

CH4

Iso-Octane

8

18

0

-62,0

1,125

2,947

91,2

C8H18

Gasoline

7,3

14,8

0,1

-52,0

1,014

2,613

C7,3H14,8O0,1

Power density, specific power, dynamic response, cost, and consumer safety are obviously critical considerations when developing a fuel processor [12]. New technologies have to be developed to meet these requirements, since the traditional ones have several limitations. Some of them are [13]:

• Current steam reforming catalysts based on Ni are extremely sulfur sensitive and deactivate considerably in the presence of traces of sulfur.

• The hydrodesulfurization (HDS) process operates at pressure highly exceeding the pressure of natural gas available in existing infrastructure.

• Ni-based reforming catalysts are pyrophoric; they will sinter if exposed to air, and they represent a fire hazard for consumers.

• Steam reforming is an endothermic process that requires complicated heat management of the system.

• High- and low-temperature water-gas-shift (WGS) catalysts based on Fe and Cu, respectively, require slow and carefully controlled activation procedure. After reduction, they are highly reactive toward air and can be a fire hazard to the consumer.

• Methanation of CO requires removal of CO2 due to the highly exothermic competitive methanation.

• Ni-based methanation catalysts are also pyrophoric.

• Pressure-swing adsorption requires high pressure.

• Industrial H2 production plants operate at steady state. They were not designed for numerous start-ups and shutdowns nor for the cycling in load. The catalyst and other reformer materials would be chemically and physically damaged.

The catalytic steam-reforming process operates at low space velocity (gas hourly space velocity [GHSV] between 3000 and 8000 h-1) owing to slow kinetics. Although these conditions are not desirable for transient duty cycles, this process gives the highest yield of hydrogen compared with partial oxidation and autothermal reforming processes. Ni-based catalysts are cost effective and commercially available, but besides being pyrophoric, they are prone to coke formation at lower H2O/C ratios. Commercial Cu-based methanol steam-reforming catalysts deactivate when exposed to liquid water during shutdown mode, and they are also pyrophoric. Recently, the researchers from Tsukuba Research Center in Japan have tested a new nanostructured cerium oxide-based Cu catalyst in steam reforming of methanol. They have found that this catalyst, containing 3.8 wt% Cu, gives higher methanol conversion (53.9%) than the Cu/ZnO (37.9%), Cu/Zn(Al)O (32.3%), and Cu/Al2O3 (11.2%) catalysts containing the same amount of Cu [14, 15].

For mobile applications, the most suitable reforming technology appears to be autothermal reforming (ATR) because of the adiabatic design permitting a compact smaller reactor, which has a small pressure drop. The design combines a highly exothermic partial-oxidation reaction, which provides the energy for the endothermic steam reforming. A new generation of natural-gas ATR reactor design is based on the overlapped reaction-zone concept: the bottom wash-coat layer with Pt/Rh steam-reforming (SR) catalyst is covered with the Pt/Pd partial oxidation (CPO) catalyst.

The heat released in the CPO layer is consumed by the SR reactions immediately without going through any heat-transfer barriers [13]. Another, efficient radial-flow ATR reactor, which uses Cu/SiO2 and Pd/SiO2 for CPO and SR reactions, was used for methanol reforming [16].

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be further converted to H2 through the water-gas-shift reaction (WGSR). The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% below 200°C. However, the kinetics are very slow, requiring space velocities less than a few thousand per hour. The commercial Fe-Cr high-temperature-shift (HTS) and Cu-Zn low-temperature-shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeO2 catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [17]. However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur.

The gas at the outlet of the WGSR still contains from 0.1 to 1.0% of CO, depending on operating conditions. In the last step of hydrogen production for low-temperature fuel cells, the CO concentration has to be reduced to a minimum. The most effective mechanism for CO removal in PEMFC-grade H2 production is selective oxidation. Because of the high ratio of H2 to CO (>>100:1) at the outlet from the LTS reactor, the oxidation catalyst has to be highly selective. The process is therefore called selective oxidation or preferential oxidation (PrOx). The process runs in the temperature window between 80°C, the PEMFC working temperature, and about 200°C, the LTS reactor working temperature. The PrOx reactor must run at the varying flow. This poses additional demand on catalyst selectivity. Pt-based PrOx catalysts, for instance, will produce CO by the reverse WGSR at longer residence time, since the oxygen is consumed in the first part of the catalyst bed. Besides, the cost of Pt is high. The CO oxidation over this catalyst is a multistep process, commonly obeying Langmuir-Hinshelwood kinetics for a single-site-competitive mechanism between CO and O2. An optimum range of O2/CO ratio is required to obtain proper balance of adsorbed CO and adsorbed O2 on adjacent sites. However, pure precious metals lack the selectivity that is required for PrOx. Recently, we have developed a nanostructured CuxCe1-xO2-y catalyst that is highly selective, active, and stable at given reaction conditions [18]. We shall describe the performance of this catalyst in the last section of this chapter.

8.2.2 Hydrogen Production from Sustainable and Renewable Feedstock

Although hydrogen is currently derived from nonrenewable coal, oil, and natural gas, it could in principle be generated from sustainable and renewable sources such as biomass and water.

Biomass has the potential to accelerate the realization of hydrogen as a major fuel of the future. Since biomass is sustainable and consumes atmospheric CO2 during growth, it can have a small net CO2 impact compared with fossil fuels. However, hydrogen from biomass has major challenges. There are no completed

TABLE 8.8

Composition of Common Fuels

Coal Oil Methane Wood

technology demonstrations. The yield of hydrogen is low from biomass, since the hydrogen content in biomass is low to begin with (Table 8.8), and the energy content is low due to the 40% oxygen content of biomass.

In steam reforming of biomass, the energy content of the feedstock is an inherent limitation of the process, since over half of the hydrogen from biomass comes from spitting water in the steam reforming reaction. The yield of hydrogen in a steam reforming process as a function of oxygen content is shown in Figure 8.9. The low yield of hydrogen on a weight basis is misleading, since the energy conversion efficiency is high. For example, the steam reforming of bio-oil at 825°C with a five-fold excess of steam demonstrated in the laboratory has an energy efficiency of 56% [19].

However, the cost of growing, harvesting, and transporting biomass is high. Thus, even with reasonable energy efficiencies, it is not presently economically competitive with natural gas steam reforming for stand-alone hydrogen without the advantage of high-value coproducts. Additionally, as with all sources of hydrogen, production from biomass will require appropriate hydrogen storage and utilization systems to be developed and deployed.

Was this article helpful?

0 0
Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment