5. Hydrogen production from fossil fuels

ignition mixes ranges with air. The leak of hydrogen from its mixture with air will most probable cause an explosion, since the mixture of air and hydrogen form a broad explosive/ ignition when it gets contact with flame or spark. The utilization of hydrogen as a fuel is confined by this issue, particularly in non-open areas like underground parking or tunnels. The burning flames of pure hydrogen in the UV range are unseen; therefore flame detector is essential to sense the hydrogen leakage. Moreover, hydrogen can be detected by smelling since it is odorless. While the hydrogen economy is expected to make a smaller carbon footprint, there are lots complexities regarding the ecological matters of hydrogen manufacturing. Fossil fuel reforming represents the main source of hydrogen; nonetheless this technique eventually leads to larger carbon dioxide emissions when compared to fossil fuel used in an internal combustion engine. Other problems comprise hydrogen production through electrolysis entails a larger energy input than straight using renewable energy and the likelihood of other

Climate variation and fossil fuel exhaustion are the chief reasons leading to hydrogen technology. Various technologies are available for the production of hydrogen: These include electrical, thermal, hybrid and biological methods. Thermal conversion processes are the most utilized processes, with steam reforming becoming the best [34, 35]. Several reforming technologies are performed in industry. Hydrogen is also obtained from splitting water by electrolysis. Nevertheless, because of the strong bonds in water molecules, a 39.3 kWh of electrical energy is theoretically required to split 1 kg of water. Moreover, for hydrocarbon feedstock solar energy accomplishes most of the thermal conversion processes. Currently, in the refinery hydrogen is used as a raw material, rather than as an energy carrier. Often, hydrogen is transported via pipelines when it generated on-site. On the other hand, the procurement of an efficient, safe, and compact storage technology is vital for the transition from the fossil fuelbased energy carriers toward hydrogen. In recent days fuel cell applications are operated with liquefied hydrogen, kept in cryogenic tanks. Enormous research investigations are currently carried out in the arena of hydrogen solid-state storage. Nevertheless, the appropriate materials regarding the hydrogen storage capacity, cost, and thermodynamics are not yet enough. Lifetime, high storage density, prolonged cycle ability, satisfactory sorption kinetics and thermodynamics are the essential parameters for a hands-on storage material. No doubt hydrogen is a future energy carrier needed to have the proper the infrastructural technologies and logistics. Fossil fuel has the potential to balance between hydrogen as energy carrier of the future and the famous fossil fuel energy carriers. Hydrogen production processes are categorized as conventional and alternative energy resources like solar and wind, natural gas, coal, nuclear, biomass. There are several techniques for hydrogen generation from diverse raw materials and energy input selections. Having environmentally friendly properties, hydrogen became an eminent choice for an alternative fuel. The combustion techniques of fossil fuel destroy the environment and these days, less than 15% of total energy consumption of the world is not based on these techniques [36]. Consequently, hydrogen utilization as an alterna-

tive fuel is ideal since it is not an unsafe, poisonous or uncertain mode of production.

lateral outputs [33].

4. Hydrogen technology

44 Advances In Hydrogen Generation Technologies

Hydrogen is generated from fossil fuels using numerous technologies, the principal of which are pyrolysis and hydrocarbon reforming. These techniques are the most advanced and normally employed, recovering virtually the whole hydrogen needs. About 48% of hydrogen is obtained from natural gas, 18% from coal, and 30% from naphtha and heavy oils [37–39]. Currently, the fuels from the fossil possess the principal role in the world hydrogen resource. Membrane reactors are used in the chemical and biochemical industries, to produce H2 from traditional fuels. A membrane frame permits mass transfer by the influence of driving forces of pressure, concentration, electric potential, temperature, and other driving forces. Membranes are classified into biological and synthetic based on their nature. A high selectivity and permeability, excellent chemical and stability are the required characteristics of the efficient H2-production membrane. Consequently, for a composite membrane, indispensable parts include a permeable support permitting the gases crossing, blended with a barrier restrictive to the inter-diffusion in the metallic support.

#### 5.1. Methods for hydrocarbon reforming

The process by which the hydrocarbon fuel is changed to produce hydrogen via reforming systems is termed hydrocarbon reforming. During the hydrocarbon reforming other components are employed along with the hydrocarbon. These include carbon dioxide and the system is termed as CO2 reforming or dry reforming. Moreover steam may include as reactants in the reforming system of the hydrocarbon. This system is branded as steam reforming. Both dry and steam reforming reactions are endothermic, Therefore, it necessary to furnish energy. Reforming the hydrocarbon with oxygen is known as partial oxidation, and the reaction is exothermic. When the steam and partial oxidation reactions are combined the system is called autothermal reforming [40].

#### 5.1.1. Method of steam reforming

In the steam reforming process, the catalytic conversion of hydrocarbon into hydrogen and carbon monoxide is carried out in the presence of steam in the feed. The reforming procedure comprises gas purification, methanation, water-gas shift and synthesis gas production. Most feedstock contains natural gas, methane, and a mixture of light hydrocarbons, which include propane, butane, ethane, pentane, and both light and heavy naphtha. When the feed is contaminated with organic sulfur compounds, a desulphurization stage should precede the reforming step to circumvent the deactivation of the reforming catalyst which CO2 is seized and put in the ocean or geological reservoirs [41]. The primary chemical reaction that occurs during the steam reforming is:

$$\rm C\_nH\_m + nH\_2O \to \{n + 1/2m\}H\_2 + nCO \tag{3}$$

Depending on the values n and m dictate the hydrocarbon type. For instance, methane reforming n and m are equal to 1 and 4 respectively. The methane steam reforming is the best and well- advanced method employed for extensive hydrogen output. The conversion performance amounts to 74–85%. When natural gas and steam are reacted over a nickel-based catalyst to generate synthesis, the reaction temperature is usually set to 850–900C. About 30–35% of the entire amount of natural gas as a process fuel provides the needed energy of 63.3 kJ/mol of H2. To inhibit coke deposition on the catalyst and achieve a purified H2 output, the process operation is adjusted to 3.5 MPa pressure, steam-to-carbon ratios of 3.5, and high temperatures [42]. After the reformer, the mixture of gases goes through a heat recovery, and water gas shift reactor where an additional H2 is produced from the reaction between the steam and carbon monoxide then, the mixture of gases goes through either a pressure swing adsorption or through a CO2 removal and methanation producing virtually pure H2 [43]. Membrane reactors offer a remarkable solution. Since the topmost process of producing huge amounts of H2, SMR has been broadly evidenced by incorporating a delicate membrane which is operated right inside the reaction environs or downstream to reaction units (Figures 2 and 3).

Palladium-based membrane reactors of the second method provide considerable advantages by uniting the gas separation and chemical reaction in a unit. Generated H2 adsorbed and dissociated atomically on one side of the membrane in the reformer, diffuses, and lastly desorbs on the other side [44]. Similar reactant conversion is permitted by the Pb-based membrane reactors. Contrary to normal SMR which operate at high temperature 850–900C, the membrane reactors operate at a lower temperature of 450–550C and produce methane conversions up to 90–95% [45].

#### 5.1.2. Partial oxidation technique

Partial oxidation (POX) technique chiefly comprises the reaction transformation of hydrocarbons, oxygen, and steam, to synthesis gas which consists of hydrogen, carbon monoxide, and carbon dioxide. Feedstocks starting from methane to naphtha are often used in the catalytic process at about 950C, while the process operation takes place at 1150–1315C for noncatalytic systems [44]. Pure O2 is used to incompletely oxidize the hydrocarbon feedstock, after the elimination of sulfur content in the feed removal. The generated synthesis is additionally

purified and separated in a similar way as the output gas of the steam reforming method. The formidable price of the oxygen manufacturing and the extra expenses of desulphurization perform the process significantly expensive. In the process dealing with catalyst, the heat is delivered through the monitored combustion. Eq. (4), presents the catalytic reforming, while

Figure 3. Methane (•), ethane (◊), propane (♦), and butane (▴) conversion as function of temperature for S/C 2.5 in SR of

nO<sup>2</sup> ! <sup>1</sup> 2

Heavier feedstock like coal and heavy oil residues are suitable resources for the production of

In the process of autothermal reforming technique (ATR), the endothermic steam reforming receives heat from the combined exothermic partial oxidation to promote the production of hydrogen. Fundamentally, steam, air, and oxygen are fed to the reformer, starting the oxidation reactions as well as the reforming to happen simultaneously, as presented in Eq. (8).

mH<sup>2</sup> þ nCO (4)

Hydrogen Production from Light Hydrocarbons http://dx.doi.org/10.5772/intechopen.76813 47

CO þ H2O ! H<sup>2</sup> þ CO<sup>2</sup> (5)

3H<sup>2</sup> þ CO ! H2O þ CH<sup>4</sup> (6)

Eqs. (5) and (6) represent the chemical reactions of water gas shift and methanation.

1 2

CnHm þ

natural gas (catalyst with 900 cpsi); symbols: Experiment; lines: Model predictions [58].

hydrogen when partial oxidation technique is applied.

5.1.3. Autothermal reforming technique

Figure 2. Flow illustration of steam methane reforming unified-membrane process.

Figure 3. Methane (•), ethane (◊), propane (♦), and butane (▴) conversion as function of temperature for S/C 2.5 in SR of natural gas (catalyst with 900 cpsi); symbols: Experiment; lines: Model predictions [58].

purified and separated in a similar way as the output gas of the steam reforming method. The formidable price of the oxygen manufacturing and the extra expenses of desulphurization perform the process significantly expensive. In the process dealing with catalyst, the heat is delivered through the monitored combustion. Eq. (4), presents the catalytic reforming, while Eqs. (5) and (6) represent the chemical reactions of water gas shift and methanation.

$$\rm C\_nH\_m + \frac{1}{2}nO\_2 \rightarrow \frac{1}{2}mH\_2 + nCO \tag{4}$$

$$\text{CO} + \text{H}\_2\text{O} \rightarrow \text{H}\_2 + \text{CO}\_2\tag{5}$$

$$\text{CH}\_2 + \text{CO} \rightarrow \text{H}\_2\text{O} + \text{CH}\_4\tag{6}$$

Heavier feedstock like coal and heavy oil residues are suitable resources for the production of hydrogen when partial oxidation technique is applied.

#### 5.1.3. Autothermal reforming technique

to generate synthesis, the reaction temperature is usually set to 850–900C. About 30–35% of the entire amount of natural gas as a process fuel provides the needed energy of 63.3 kJ/mol of H2. To inhibit coke deposition on the catalyst and achieve a purified H2 output, the process operation is adjusted to 3.5 MPa pressure, steam-to-carbon ratios of 3.5, and high temperatures [42]. After the reformer, the mixture of gases goes through a heat recovery, and water gas shift reactor where an additional H2 is produced from the reaction between the steam and carbon monoxide then, the mixture of gases goes through either a pressure swing adsorption or through a CO2 removal and methanation producing virtually pure H2 [43]. Membrane reactors offer a remarkable solution. Since the topmost process of producing huge amounts of H2, SMR has been broadly evidenced by incorporating a delicate membrane which is operated right inside the

Palladium-based membrane reactors of the second method provide considerable advantages by uniting the gas separation and chemical reaction in a unit. Generated H2 adsorbed and dissociated atomically on one side of the membrane in the reformer, diffuses, and lastly desorbs on the other side [44]. Similar reactant conversion is permitted by the Pb-based membrane reactors. Contrary to normal SMR which operate at high temperature 850–900C, the membrane reactors operate at a lower temperature of 450–550C and produce methane

Partial oxidation (POX) technique chiefly comprises the reaction transformation of hydrocarbons, oxygen, and steam, to synthesis gas which consists of hydrogen, carbon monoxide, and carbon dioxide. Feedstocks starting from methane to naphtha are often used in the catalytic process at about 950C, while the process operation takes place at 1150–1315C for noncatalytic systems [44]. Pure O2 is used to incompletely oxidize the hydrocarbon feedstock, after the elimination of sulfur content in the feed removal. The generated synthesis is additionally

reaction environs or downstream to reaction units (Figures 2 and 3).

Figure 2. Flow illustration of steam methane reforming unified-membrane process.

conversions up to 90–95% [45].

46 Advances In Hydrogen Generation Technologies

5.1.2. Partial oxidation technique

In the process of autothermal reforming technique (ATR), the endothermic steam reforming receives heat from the combined exothermic partial oxidation to promote the production of hydrogen. Fundamentally, steam, air, and oxygen are fed to the reformer, starting the oxidation reactions as well as the reforming to happen simultaneously, as presented in Eq. (8).

$$\rm C\_nH\_m + \frac{1}{2}nH\_2O + \frac{1}{4}nO\_2 \to nCO + \left(\frac{1}{2}n + \frac{1}{2}m\right)H\_2 \tag{7}$$

sequestration, which is energy intensive stage is replaced by carbon control that could be employed in the chemical industries and metallurgy. Therefore, the processes of partial oxidation or steam conversion are higher than investments for big plants causing 25–30% hydrogen production cost [45]. The price of hydrogen would be less if markets are found for the extensive amounts of carbon resulting from the natural gas decomposition. From the environmental perspective it would be more beneficial to dissociate catalytically natural gas to carbon and hydrogen, instead of H2 production by steam reforming of methane attached with CO2 sequestration [48]. For a specified temperature, the reduction of carbon content is increased by the constant elimination of hydrogen by membrane separation. For H2 separation, Pd-Ag alloys, which operate at lower temperatures and mitigate the carbon deposition, are normally used. The chief disadvantages of the present method are attributed to the very low hydrogen separation, which results from the membrane stability influenced by high temperatures required for the equilibrium of the reduction of the carbon content and the low H2 partial

Hydrogen Production from Light Hydrocarbons http://dx.doi.org/10.5772/intechopen.76813 49

Hydrocarbons, as their name denotes, are compounds of hydrogen and carbon. They represent one of the vital classes of organic chemistry. They exist in gaseous states such as propane and methane, a liquid state like benzene and hexane, and solid state as paraffin wax, polystyrene,

a. Saturated hydrocarbons which are formed completely of single bonds between carbon– carbon and are saturated with hydrogen. The compound formula with a linear structure, alkanes are CnH2nþ2. The universal formula of saturated hydrocarbons is CnH2nþ2 1ð Þ �<sup>r</sup> , where r represents the number of rings. One ring hydrocarbons are termed cycloalkanes. Linear and branched species of saturated hydrocarbons are the sources of petroleum. b. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with double bond are called alkenes. Those with one double bond and noncyclic structure have the formula CnH2n. Those having triple bonds are named alkyne.

c. Aromatic hydrocarbons are hydrocarbons that have at least one aromatic ring. The hydrocarbon is characterized by strong covalent (sigma) bonds and delocalized pi electrons between carbon atoms forming a circle. Some simple hydrocarbons and their variations

Hydrocarbons are a primary energy source for current civilizations. The predominant use of hydrocarbons is as a combustible fuel source. In their solid form, hydrocarbons take the form of asphalt (bitumen) [49]. Methane and ethane are gaseous at ambient temperatures and cannot be readily liquefied by pressure alone. Propane is however easily liquefied and exists in 'propane bottles' mostly as a liquid. Butane is so easily liquefied that it provides a safe,

Those with one triple bond have the formula CnH2n�<sup>2</sup>:

pressures in the reaction mixture [43].

6. Light hydrocarbons (LHs)

naphthalene, and polyethylene. Hydrocarbons are classified into:

are given in Table 1.

When methane is autothermally reformed at 700�C inlet temperature and the proper ratios of steam to carbon and oxygen to carbon, a thermal efficiency of 60–75% and maximum hydrogen yield of about 2.8 are achieved [42]. The investment cost is about 50% lower than coal gasification. A small improvement is reported when the ATR reactor is combined with a Pd membrane. The high operating temperature of 900�C needed by the system ruins the efficiency improvement brought by the membrane [43].

#### 5.1.4. Dry reforming

The dry reforming is a chemical process that consists of converting hydrocarbon and carbon dioxide, considered as one of the world's most abundant greenhouse gases to synthesis gas with a proper H2/CO molar ratio [46]. As a result, this process has the potentials to alleviate the environmental challenges related to greenhouse gases emissions and to transform biogas and natural gas to synthesis gas. Furthermore, the produced H2/CO ratio synthesis gas is suitable for the production of hydrocarbons via Fischer-Tropsch synthesis, in addition to the synthesis of oxygenated chemicals [47]. The dry reforming reaction is:

$$\rm C\_nH\_m + n\ CO\_2 \rightarrow \left\{\frac{1}{2}m\right\}H\_2 + 2nCO \tag{8}$$

Being an extremely endothermic reaction, dry reforming requires high operating temperatures, usually in the range of 900–1273 K, to achieve the desired conversion levels. The forward reaction is favored at low pressures as dictated by stoichiometry. Additionally, it has been observed that an nCO2/CnHm molar ratio higher than the stoichiometric requirement of unity can also provide high synthesis gas yields. The positive effects of high reaction temperatures, low pressures, and high nCO2/CnHm molar.

#### 5.2. Pyrolysis of hydrocarbon

The pyrolysis of hydrocarbon is a famous method where hydrogen solely comes from the hydrocarbon subjected to thermal decomposition via the following universal reaction:

$$\rm{C}\_{n}H\_{m} \rightarrow n\rm{C} + \frac{1}{2}mH\_{2} \tag{9}$$

Light liquid hydrocarbons having boiling points 200�C are decomposed thermo-catalytically to generate elemental carbon and hydrogen, however, dealing with residual fractions having boiling temperatures above 350�C, the production of hydrogen requires hydrogasification and cracking of methane. The direct reduction of carbon content in the natural gas known as often de-carbonization, which constitutes mainly methane, is performed at 980�C temperature and atmospheric pressure in the environment, where there is no water and air. Furthermore, pyrolysis does not involve CO2 removal steps and water gas shift. Carbon control and sequestration, which is energy intensive stage is replaced by carbon control that could be employed in the chemical industries and metallurgy. Therefore, the processes of partial oxidation or steam conversion are higher than investments for big plants causing 25–30% hydrogen production cost [45]. The price of hydrogen would be less if markets are found for the extensive amounts of carbon resulting from the natural gas decomposition. From the environmental perspective it would be more beneficial to dissociate catalytically natural gas to carbon and hydrogen, instead of H2 production by steam reforming of methane attached with CO2 sequestration [48]. For a specified temperature, the reduction of carbon content is increased by the constant elimination of hydrogen by membrane separation. For H2 separation, Pd-Ag alloys, which operate at lower temperatures and mitigate the carbon deposition, are normally used. The chief disadvantages of the present method are attributed to the very low hydrogen separation, which results from the membrane stability influenced by high temperatures required for the equilibrium of the reduction of the carbon content and the low H2 partial pressures in the reaction mixture [43].
