Steam-Methane Reforming (SMR)

Among the three reforming routes, the SMR is the least expensive and most accepted one to generate hydrogen. It is a catalytic process which relies on a reaction between methane and steam, with the following reactions:

${\mathrm{CH}}_4{+\text{H}}_2\mathrm{O}\stackrel{\mathrm{catalytic}}{⟶}{\text{CO}+\text{3H}}_2{;\text{CH}}_4{+\text{2H}}_2\mathrm{O}\stackrel{\mathrm{catalytic}}{⟶}{\mathrm{CO}}_2{+\text{4H}}_2$ (2.i)

(2.i)

${\text{CO}+\text{H}}_2\mathrm{O}\stackrel{\mathrm{catalytic}}{⟶}{\mathrm{CO}}_2{+\text{H}}_2$ (2.ii)

(2.ii)

Fig. 2.2 displays the four basic steps of steam-methane reforming process. First, the feedstock has to be desulfurized since sulfur-contained compounds in natural gas will poison catalyst and damage equipments during the followed reforming reaction. Second, the desulfurized natural gas—mainly methane (CH₄)—is reformed by reacting with steam (H₂O) in furnaces loading with catalyst to form a gas mixture of hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), and unconverted methane (CH₄). The steam reforming reactions (Eq. 2.i) are highly endothermic, and the external energy has to be inputted by pre-heating the feedstock. Typically, the reactions are carried on at 800–1000°C under 1.4–2 Mpa with the help of high-efficiency nickel-based catalysts. Third, CO further reacts with steam (H₂O) in the presence of catalysts to produce CO₂ and more H₂. In this step, due to the exothermic characteristic of the water–gas shift reaction (Eq. 2.ii), a high-temperature-shift (HTS, 350°C) followed by a low-temperature-shift (LTS, 190–210°C) is often employed to achieve fast kinetics for H₂ production (Bhat and Sadhukhan, 2009). In the final step, in order to obtain the high-purity H₂ (≥99%), the physical separation process of pressure swing adsorption (PSA) is usually utilized to remove the considerable amounts of CO, CO₂, and CH₄ in the effluent gas after the water–gas shifting reaction.

Partial Oxidation (POX)

In the partial oxidation process, methane in natural gas reacts with oxygen of less than a stoichiometric amount in a partially oxidizing reaction to generate carbon monoxide and hydrogen. Unlike the endothermic reactions in SMR, the POX reaction releases enough heat to drive this process due to the exothermic nature of Eq. 2.iii or Eq. 2.iv.

${\mathrm{CH}}_4+\text{0}{\mathrm{.5O}}_2\to {\text{CO}+\text{2H}}_2$ (2.iii)

(2.iii)

${\mathrm{CH}}_4+\text{0}{\mathrm{.5O}}_2\stackrel{\mathrm{catalytic}}{⟶}{\text{CO}+\text{2H}}_2$ (2.iv)

(2.iv)

Fig. 2.3 shows the flowchart of the partial oxidation process, in which the desulfuration is also the first stage. Then, the desulfurized natural gas is mixed with a limited amount of air or oxygen to realize the partial oxidation reaction (Eq. 2.iii) in a high-pressure reactor. In this step, it is important to control the oxygen/carbon ratio for maximizing the yield of CO and H₂, as well as minimizing the coke formation. Meanwhile, a great amount of heat generated by the exothermic POX reaction is collected, and then utilized in the following water–gas shift reaction (Eq. 2.ii). Also, the purification process is necessary in the final step to obtain the high-quality hydrogen. The POX reaction can be carried out with/without the presence catalysts. The catalytic partial oxidation (Eq. 2.iv) is typically operated at the temperature ranging from 700°C to 900°C, which is lower than that of the corresponding noncatalytic partial oxidation reaction, with the temperature ranging from 1300°C to 1500°C (Enger et al., 2008). However, the reaction temperature of the catalytic partial oxidation is difficult to be controlled, which requires an expensive system for oxygen separation.

Autothermal Reforming (ATR)

As stated above, the SMR reactions are endothermic, while the POX reaction is exothermic. The ATR process combines the SMR reactions and POX reaction to provide a (nearly) thermodynamically neutral reaction, by utilizing the heat generated in the POX to meet the heat required by the SMR. The integrated reaction of the ATR process is proposed as Eq. 2.v:

${\mathrm{CH}}_4+\text{0}{\mathrm{.5H}}_2\text{O}+\text{0}{\mathrm{.25O}}_2\stackrel{\mathrm{catalytic}}{⟶}\text{CO}+\text{2}{\mathrm{.5H}}_2$ (2.v)

(2.v)

Fig. 2.4 illustrates that to achieve the thermodynamical neutralization in the ATR process, a unique reactor with both a combustion zone and a catalytic SMR zone has to be used to treat the desulfurized natural gas with the concurrently flowing oxygen and steam. To be specific, the heat generated by the exothermic POX reaction in the combustion zone is directly transferred to the catalytic zone via the flowing reaction gases, and then driving the downstream endothermic SMR reactions. In the unique reactor, methane reacts with both steam and oxygen to produce syngas at a temperature ranging from 900°C to 1100°C in the presence of the catalyst bed (Aasberg-Petersen et al., 2011). The following two stages of the ATR process are same as those of the SMR and POX processes, namely the WSG reaction and product purification.

Gasification Technologies

At present, a variety of coal gasification technologies are commercially available for hydrogen production, which can primarily be classified into three main categories according to the type of gasifier: Fluidized bed, fixed bed, and entrained flow gasifiers. Table 2.1 compares the operating conditions of the three types of gasifier such as particle size of coal feedstock, residence times, operating temperature and pressure, as well as feeding and contact mode.

Underground Coal Gasification

The above-mentioned three coal gasification technologies all rely on a surface reactor to gasify the pulverized coal, while the underground coal gasification (UCG) technology supplies an alternative pathway for coal conversion, wherein, the nonmined coal is converted to syngas in-situ by reacting with the injected oxidants. The process of UCG can be depicted briefly as follows (Fig. 2.6).

Firstly, a pair of vertical wells are drilled from the surface to the coal seam, one well, namely the injection well, is utilized to supply air or oxygen. While another, known as production well, is used to extract the generated syngas (containing hydrogen), moreover, a horizontal connection for enhancing the permeability of coal seam between the two wells is created. In the gasification step, the coal stem is ignited, and air or oxygen is pumped into the permeable bed through the injection well, for supporting the chemical reaction between the air/oxygen with the coal. The generated syngas flows through the horizontal connection and the production well, and then it is collected at the surface for the subsequent purification and utilization.

Solid Polymer Electrolyzer

In the solid polymer electrolyzer, a solid sulfonated polystyrene membrane is employed as the electrolyte. This concept also refers to proton exchange membrane and polymer electrolyte membrane (both abbreviated as PEM) water electrolyzer. In such an electrolyzer (Fig. 2.10), hydrogen ions pass through the membrane and get to the cathode chamber, where they recombine with electrons to form hydrogen gas, and then be captured as products. While in the anode chamber, oxygen gas accumulated in the water can also be collected.

Liquid Electrolyzer

The electrolyzer using liquid electrolyte (most commonly KOH) has been the most accepted commercial electrolytic technology for a long history. Equipped with two electrodes immersed in the liquid alkaline electrolyte (typically a caustic solution with 20%–30% concentration of KOH), the liquid electrolyzer performs similar functions as the PEM system. As described in Fig. 2.10, a diaphragm is used in the electrolyzer to separate the cathode and anode, keeping hydrogen and oxygen apart from each other. In such a system, hydroxyl ions migrate through the electrolytic material to produce oxygen in the anode chamber; while in the cathode chamber, the generated hydrogen is extracted readily from the water stream.

Conventional Thermolysis

The process of conventional water thermolysis just needs a single-step (Fig. 2.13), in which, a solar-based heat source of above 2227°C is required to achieve a reasonable degree of water dissociation (9% at 1 bar, 25% at 0.05 bar). Moreover, such a high temperature can also prevent explosions by separating the generated hydrogen and oxygen—an explosive mixture (Kogan et al., 2000).

Thermochemical Water Splitting Cycles

As the utilization of nuclear energy (discussed in Section 3), the radiant light and heat energy emitted from the Sun can also be employed to drive the thermochemical water splitting cycles. Compared to the conventional water thermolysis, the thermochemical cycles allow for realizing the same reactions and separating the explosive mixture of hydrogen and oxygen at much lower operation temperature.

Pyrolysis

Biomass pyrolysis refers to the process of incomplete thermal degradation of biomass feedstock (contents of C, H, and O) into useful fuels like char, condensable liquids (bio-oil) and noncondensable gases (H₂, CO, and CH₄). In the process, the complex biomass molecules are first dissociated into simple units (Eq. 2.xxv) by being rapidly heated to 380–530°C at 0.1–0.5 MPa in the absence of oxygen, or by being partially combusted with a limited amount of oxygen supply. Then, the catalytic steam reforming reaction (Eq. 2.xxvi) at 750–850°C over a nickel-based catalyst coupled with the water–gas shift reaction (Eq. 2.ii) is used to convert the bio-oil into (more) hydrogen (Balat, 2009).

$\mathrm{Biomass}\stackrel{\mathrm{Energy}}{⟶}{\text{Bio}-\text{oil}+\text{Char}+\text{H}}_2{+\text{CO}+\text{CH}}_4+\mathrm{\dots }$ (2.xxv)

(2.xxv)

${\text{Bio}-\text{oil}+\text{H}}_2\mathrm{O}\stackrel{\mathrm{Catalytic}}{⟶}{\text{CO}+\text{H}}_2$ (2.xxvi)

(2.xxvi)

Gasification

A variety of agricultural and forest product residues can be taken as feedstock to produce hydrogen by conventional gasification. Among all the available bio-based hydrogen technologies, gasification (Eq. 2.xxvii) coupled with water–gas shift (Eq. 2.ii) is the most practical process, in which the biomass is firstly dried to the moisture content of less than 35%. Subsequently, thermal gasification of the solid feedstock is carried out in a gasifier at the temperature ranging from 600°C to 1000°C (Kalinci et al., 2009). Similar with the process of coal gasification, the gasification of biomass also generates useful gas products including H₂, CO, CO₂, etc.

${\text{Biomass}+\text{H}}_2\mathrm{O}\to {\text{CO}+\text{H}}_2{+\text{CO}}_2$ (2.xxvii)

(2.xxvii)

Supercritical Water Gasification (SCWG)

The critical point for pure water is 374°C and 22.1 MPa, beyond which, supercritical water (SCW) with gas-like viscosity and liquid-like density can be formed. The dramatic change in its physical properties makes water to behave as a homogeneous fluid as well as a catalyst; in other words, SCW has good solvation capacity as well as excellent mass transfer rate. In the SCWG process, the undried biomass as the feedstock can be directly gasified in the supercritical water. Under the supercritical condition, oxygen in water transfers to the carbon atoms of the biomass to generate CO, which further reacts with steam to generate the gas products of H₂, CO, and CO₂ by the water–gas shift reaction (Reddy et al., 2014).

Biophotolysis Routes

By absorbing and then utilizing the solar energy, some phototropic organisms, e.g., purple bacteria, green algae, and Cyanobacteria, can produce hydrogen. The biophotolysis can be fulfilled by the direct and indirect pathways with a similar mechanism: Microalgae adsorb solar energy and generate electrons. The generated electrons is transfered to ferredoxin, which in turn acts on the hydrogen production enzymes to release H₂ from water or organic.

Direct Biophotolysis. In the direct biophotolysis pathway, photosynthetic microalgae, such as green algae or Cyanobacteria, is used to directly generate hydrogen via the photosynthetic reaction (Eq. 2.xxviii), in which the feedstock (usually water) is dissociated into H₂ and O₂ with the aid of hydrogenase enzyme. Since the activity of hydrogenase enzyme is sensitive to oxygen, the O₂ partial pressures should be kept below 0.1% to maintain the hydrogen production process (Hallenbeck and Benemann, 2002).

${\mathrm{2H}}_2\text{O}+\text{Solar}\mathrm{energy}\stackrel{\mathrm{Enzymes}}{⟶}{\mathrm{2H}}_2{+\text{O}}_2$ (2.xxviii)

(2.xxviii)

Indirect Biophotolysis. The indirect biomass route is a two-stage light driven process, it usually utilizes Cyanobacteria to produce hydrogen and oxygen in separated phases, respectively (Manish and Banerjee, 2008). The first stage refers to the photosynthetic reaction (CO₂-fixation and O₂-generation) in which the reduced substrates such as carbohydrates (i.e., starch in microalgae or glycogen in cyanobacteria) are accumulated and O₂ is generated simultaneously (Eq. 2.xxix). While in the second stage (CO₂-evolution and H₂ generation), the carbohydrates are subsequently dissociated into H₂ and CO₂ under anaerobic conditions (Eq. 2.xxx).

${\mathrm{12H}}_2{\text{O}+\text{6CO}}_2+\mathrm{Solar}\mathrm{energy}\stackrel{\mathrm{Enzymes}}{⟶}{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6{+\text{6O}}_2$ (2.xxix)

(2.xxix)

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6{+\text{12H}}_2\text{O}+\mathrm{Solar}\mathrm{energy}\stackrel{Enzymes}{⟶}{\mathrm{12H}}_2{+\text{6CO}}_2$ (2.xxx)

(2.xxx)

Fermentation Routes

Fermentation routes provide environmentally-friendly bio-approaches for the fermentative conversion of organic substrates into hydrogen. The processes can be realized by photo-fermentation under light condition and dark-fermentation without the sunlight. Moreover, the utilization of combined dark- and photo-fermentation processes could achieve a higher hydrogen production.

Photo-Fermentation. In the photo-fermentation process, photosynthetic microorganisms are employed as biological converters to produce hydrogen through their nitrogenase action under nitrogen-limited conditions. The process uses sunlight as the energy resource, while organic acids or biomass as the feedstock. In the absence of oxygen, the fermentative microorganisms can take simple organic acids as donors to provide electrons. When the nitrogenase enzymes receive the electrons with the aid of ferredoxin, protons can be reduced into hydrogen under the nitrogen-deficient conditions. The overall reaction for hydrogen production through the photo-fermentation process is given in Eq. 2.xxxi.

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6{+\text{12H}}_2\text{O}+\mathrm{Solar}\mathrm{energy}\stackrel{\mathrm{Enzymes}}{⟶}{\mathrm{12H}}_2{+\text{6CO}}_2$ (2.xxxi)

(2.xxxi)

Dark-Fermentation. In dark-fermentation, the heterotrophic bacteria and microalgae have the ability to convert carbohydrate-rich substrates to hydrogen, volatile fatty acids (VFAs), and carbon dioxide under anaerobic conditions without the sunlight. In this route, the hydrogenase enzyme also plays an important role in the catalyzed formation of H₂ by combining protons and electrons. Meanwhile, the pH value of the process should be maintained at 5–6 for achieving the optimum hydrogen production. In a typical dark fermentative process, the model substrate of glucose is converted to H₂, CO₂, and acetic acid or butyrate under different reaction conditions as shown in Eq. 2.xxxii (Argun and Kargi, 2011):

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6{+\text{2H}}_2\mathrm{O}\stackrel{\mathrm{Enzymes}}{⟶}{\mathrm{2CH}}_3{\text{COOH}+\text{4H}}_2{+\text{2CO}}_2\mathrm{or}{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6{+\text{2H}}_2\mathrm{O}\stackrel{\mathrm{Enzymes}}{⟶}{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\text{OOH}+\text{2H}}_2{+\text{2CO}}_2$ (2.xxxii)

(2.xxxii)