3. Carbon feed-based solar thermochemistry

#### 3.1 Methane reforming and decomposition

Nowadays, more than 95% of the hydrogen for refinery use is produced via hydrocarbon steam reforming [26]. Industrial hydrogen production through methane steam reforming exceeds 50 million tons annually and accounts for 2–5% of global energy consumption [27]. Methane steam reforming process for hydrogen production is usually described by the following reactions:

$$\text{CH}\_4(\text{g}) + \text{H}\_2\text{O}(\text{g}) \rightleftharpoons \text{CO}(\text{g}) + \text{3H}\_2(\text{g}), \Delta H^\circ \text{ ${}\_{25}$ °C} = \text{205.9 kJ/mol} \tag{30}$$

$$\text{CO(g)} + \text{H}\_2\text{O(g)} \rightleftharpoons \text{CO}\_2(\text{g}) + \text{H}\_2(\text{g}), \Delta H^\circ \text{C}\_2\text{C} = -41.1 \text{ kJ/mol} \tag{31}$$

where the reversible water-gas shift reaction Eq. (11) is sometimes considered as superimposed onto the methane reforming reaction Eq. (10) for conveniences of analysis on methane conversion:

$$\text{CH}\_4(\text{g}) + 2\text{H}\_2\text{O}(\text{g}) \rightleftharpoons \text{CO}\_2(\text{g}) + 4\text{H}\_2(\text{g}), \Delta H^\circ \text{C}\_2\text{C} = 164.8 \text{ kJ/mol} \tag{32}$$

The methane dry reforming is the reaction between methane and carbon dioxide for syngas generation, given as:

$$\text{CH}\_4(\text{g}) + \text{CO}\_2(\text{g}) \rightleftharpoons 2\text{CO}(\text{g}) + 2\text{H}\_2(\text{g}), \Delta H^\circ \text{ ${}\_{25}$ °C} = 247.0 \text{ kJ/mol} \tag{33}$$

3500. During the reaction, ZnO particles with an average particle diameter of 0.4 μm were sent into a cylindrical reaction chamber for reaction by methane. The products were Zn simple substance and synthesis gas (H2, CO). The reactor can

� accounts for the energy required to drive the reaction.

Methane decomposition is also an endothermic reaction at 600–1200°C,

Solar methane decomposition has been researched in both indirectly and directly heated reactors from solar thermal energy. A summary of experimental study has

Methanol reforming and decomposition also attracts lots of attention in the field of solar thermochemical fuel generation [44], as the reaction temperature is about 150–300°C, which is quite low and easy to be maintained by line-focusing solar collector (parabolic trough collector or linear Fresnel lens) with low cost. The reaction equations of methanol reforming and decomposition are given as:

CH3OH gð Þþ H2O gð Þ! CO2ð Þþ g 3H2ð Þg , ΔH°25°C ¼ 49:321 kJ=mol (35)

Both of the two reactions are endothermic, which can convert the low-level solar thermal energy (low temperature) into high-level chemical energy, and have been researched with combining with other systems, like PV cell module, and combined

Biomass is widespread and is often perceived as a carbon-neutral source of energy. Solar biomass gasification is a clean route to obtain fuels, which may also reach liquid fuel for vehicle or jet utilization. Detailed reviews on solar biomass

CH3OH gð Þ! CO gð Þþ 2H2ð Þg , ΔH°25°C ¼ 90:459 kJ=mol (36)

CH4ð Þg ⇌ C þ 2H2ð Þg , ΔH°25°C ¼ 74:6 kJ=mol (34)

reach 50% methane conversion at 1030°C.

3.2 Methanol reforming and decomposition

cooling heating and power in downstream.

3.3 Biomass gasification

153

given as:

been listed in Table 2.

Participant Qsolar

PROMES-CNRS

PROMES-CNRS

PROMES-CNRS

<sup>η</sup>chemical <sup>¼</sup> <sup>Δ</sup>Hreact@Treactor Qsolar

<sup>η</sup>thermal <sup>¼</sup> <sup>X</sup>reaction � <sup>m</sup>\_ CH4 � <sup>Δ</sup>Hreact@Treactor

a

b

Table 2.

(kW)

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

> Pressure (bar)

PSI 5 >1 1600 Vortex

PSI 5 >1 1600 Particle

NREL 6 1 2133 Aerosol

Qsolar

0.8 0.61 1700 Nozzle

Summary of experimental research on solar methane decomposition [36].

T (K) Reactor type

flow

flow

50 0.45 1928 Tubular 100 13.5<sup>a</sup>

flow

type

10 0.4 1773 Tubular 98 4.8<sup>b</sup> [39, 40]

Reactant conversion (%)

� accounts for the energy required to heat the reactants and effect the reaction.

64 15.1a

Efficiency (%)

99 16.1<sup>b</sup> [38]

90 2b [42]

95 5.9<sup>a</sup> [43]

Ref.

, 16.2<sup>b</sup> [37]

, 15.2<sup>b</sup> [41]

The reforming reactions, Eqs. (10), (12), and (13), are highly endothermic, and a large amount of heat is often provided by burning a supplemental amount of methane [28], which will decrease the heat value of fuel gas generated by 22% for the same amount of methane consumed and release large amounts of greenhouse gas CO2 [29]. In recent years, as the technologies of concentrated solar energy (CSE) and solar thermal utilization improve rapidly, methane reforming driven by CSE emerged as a promising method for hydrogen production [30], which derives heat from solar energy instead of fossil fuels. Besides, solar thermal energy with relatively low temperatures (compared with methane combustion) is absorbed by methane reforming reaction and upgraded to the chemical energy with higher energy level (ratio of exergy change ΔE to enthalpy change ΔH of a process [31]) in such process. Solar energy thus stored in hydrogen as chemical energy, and it could be converted into power with significantly greater efficiencies than that of solarthermal-only power generation in the same temperature range [32].

In order to achieve the high-efficient progress of the solar methane reforming reaction, research on solar methane reforming reactors has also continued. Klein et al. [33] proposed a schematic diagram of a fluidized bed reactor. This experiment is a methane dry reforming experiment, where gas reactants and carbon particles are mixed and passed into the reactor together. The reactor can achieve a concentration ratio of 3000 through primary and secondary light concentration, and the reaction temperature is between 950 and 1450°C with the ratio of carbon dioxide to methane changes from 1: 1 to 6: 1, which has a maximum methane conversion of 90%. Edwards et al. [34] studied methane steam reforming in a solar tubular reactor, which is condensed by a 107 m<sup>2</sup> dish condenser. The condensing temperature can reach 850°C, and the pressure can reach 20 bars. The reactor can stably produce hydrogen, but there is no detailed introduction on the conversion rate in the literature. The device for hydrogen production by metal oxide thermochemical cycling was proposed by Steinfeld et al. [35]. The system contains a 51.8 m<sup>2</sup> heliostat to focus the sunlight at first, and then the sunlight passed through a parabolic surface with an opening area of 2.7 m<sup>2</sup> to focus it again. The final focusing ratio was


a <sup>η</sup>chemical <sup>¼</sup> <sup>Δ</sup>Hreact@Treactor Qsolar � accounts for the energy required to drive the reaction.

b <sup>η</sup>thermal <sup>¼</sup> <sup>X</sup>reaction � <sup>m</sup>\_ CH4 � <sup>Δ</sup>Hreact@Treactor Qsolar � accounts for the energy required to heat the reactants and effect the reaction.

#### Table 2.

3. Carbon feed-based solar thermochemistry

production is usually described by the following reactions:

Nowadays, more than 95% of the hydrogen for refinery use is produced via hydrocarbon steam reforming [26]. Industrial hydrogen production through methane steam reforming exceeds 50 million tons annually and accounts for 2–5% of global energy consumption [27]. Methane steam reforming process for hydrogen

CH4ð Þþ g H2O gð Þ ⇌ CO gð Þþ 3H2ð Þg , ΔH°25°C ¼ 205:9 kJ=mol (30) CO gð Þþ H2O gð Þ ⇌ CO2ð Þþ g H2ð Þg , ΔH°25°C ¼ �41:1 kJ=mol (31)

where the reversible water-gas shift reaction Eq. (11) is sometimes considered as superimposed onto the methane reforming reaction Eq. (10) for conveniences of

CH4ð Þþ g 2H2O gð Þ ⇌ CO2ð Þþ g 4H2ð Þg , ΔH°25°C ¼ 164:8 kJ=mol (32)

The methane dry reforming is the reaction between methane and carbon dioxide

CH4ð Þþ g CO2ð Þg ⇌ 2CO gð Þþ 2H2ð Þg , ΔH°25°C ¼ 247:0 kJ=mol (33)

The reforming reactions, Eqs. (10), (12), and (13), are highly endothermic, and

In order to achieve the high-efficient progress of the solar methane reforming reaction, research on solar methane reforming reactors has also continued. Klein et al. [33] proposed a schematic diagram of a fluidized bed reactor. This experiment is a methane dry reforming experiment, where gas reactants and carbon particles are mixed and passed into the reactor together. The reactor can achieve a concentration ratio of 3000 through primary and secondary light concentration, and the reaction temperature is between 950 and 1450°C with the ratio of carbon dioxide to methane changes from 1: 1 to 6: 1, which has a maximum methane conversion of 90%. Edwards et al. [34] studied methane steam reforming in a solar tubular reactor, which is condensed by a 107 m<sup>2</sup> dish condenser. The condensing temperature can reach 850°C, and the pressure can reach 20 bars. The reactor can stably produce hydrogen, but there is no detailed introduction on the conversion rate in the literature. The device for hydrogen production by metal oxide thermochemical cycling was proposed by Steinfeld et al. [35]. The system contains a 51.8 m<sup>2</sup> heliostat to focus the sunlight at first, and then the sunlight passed through a parabolic surface with an opening area of 2.7 m<sup>2</sup> to focus it again. The final focusing ratio was

a large amount of heat is often provided by burning a supplemental amount of methane [28], which will decrease the heat value of fuel gas generated by 22% for the same amount of methane consumed and release large amounts of greenhouse gas CO2 [29]. In recent years, as the technologies of concentrated solar energy (CSE) and solar thermal utilization improve rapidly, methane reforming driven by CSE emerged as a promising method for hydrogen production [30], which derives heat from solar energy instead of fossil fuels. Besides, solar thermal energy with relatively low temperatures (compared with methane combustion) is absorbed by methane reforming reaction and upgraded to the chemical energy with higher energy level (ratio of exergy change ΔE to enthalpy change ΔH of a process [31]) in such process. Solar energy thus stored in hydrogen as chemical energy, and it could be converted into power with significantly greater efficiencies than that of solar-

thermal-only power generation in the same temperature range [32].

3.1 Methane reforming and decomposition

Wind Solar Hybrid Renewable Energy System

analysis on methane conversion:

for syngas generation, given as:

152

Summary of experimental research on solar methane decomposition [36].

3500. During the reaction, ZnO particles with an average particle diameter of 0.4 μm were sent into a cylindrical reaction chamber for reaction by methane. The products were Zn simple substance and synthesis gas (H2, CO). The reactor can reach 50% methane conversion at 1030°C.

Methane decomposition is also an endothermic reaction at 600–1200°C, given as:

$$\text{CH}\_4(\text{g}) \rightleftharpoons \text{C} + 2\text{H}\_2(\text{g}),\\\Delta H^\circ \text{O}\_{2\text{S}^\circ \text{C}} = 74.6 \text{ kJ/mol} \tag{34}$$

Solar methane decomposition has been researched in both indirectly and directly heated reactors from solar thermal energy. A summary of experimental study has been listed in Table 2.

#### 3.2 Methanol reforming and decomposition

Methanol reforming and decomposition also attracts lots of attention in the field of solar thermochemical fuel generation [44], as the reaction temperature is about 150–300°C, which is quite low and easy to be maintained by line-focusing solar collector (parabolic trough collector or linear Fresnel lens) with low cost. The reaction equations of methanol reforming and decomposition are given as:

$$\text{CH}\_3\text{OH}(\text{g}) + \text{H}\_2\text{O}(\text{g}) \rightarrow \text{CO}\_2(\text{g}) + \text{3H}\_2(\text{g}), \Delta H^\circ \text{C}\_{25^\circ \text{C}} = 49.321 \text{ kJ/mol} \tag{35}$$

$$\text{CH}\_3\text{OH}(\text{g}) \rightarrow \text{CO}(\text{g}) + 2\text{H}\_2(\text{g}), \Delta H^\circ \text{e}\_{25^\circ \text{C}} = 90.459 \text{ kJ/mol} \tag{36}$$

Both of the two reactions are endothermic, which can convert the low-level solar thermal energy (low temperature) into high-level chemical energy, and have been researched with combining with other systems, like PV cell module, and combined cooling heating and power in downstream.

#### 3.3 Biomass gasification

Biomass is widespread and is often perceived as a carbon-neutral source of energy. Solar biomass gasification is a clean route to obtain fuels, which may also reach liquid fuel for vehicle or jet utilization. Detailed reviews on solar biomass


which requires high concentration ratio and large mirror area, and the system will be more complex and expensive. In situ separation by a permeable membrane for a target product shifts thermodynamic equilibrium of chemical reactions in favor of reactants conversion, which equivalently lowers solar collection temperature. Combination of membrane reactor and solar thermal collection offers unique advantages in many respects, such as the increment of conversion rate, decrease of reaction temperature, and emission reduction, which are otherwise unattainable by either alone. Besides, the all-solid-state feature and isothermal operation enable compact design of solar fuel reactors with minimized thermal stress. Now, the selective permeation membrane for gas species in high temperature is mainly oxygen permeation membrane, hydrogen permeation membrane, and carbon dioxide permeation membrane, which have been researched for solar thermochemical fuel

Perovskites, ZrO2 and CeO2 (or doped ZrO2 and CeO2), usually constitute the selective oxygen permeation membrane utilized in high temperature (>600°C). Wang et al. [59] proposed a theoretical framework for the thermodynamic analysis of solar oxygen permeation membrane reactor, and the solar-to-fuel efficiency (ratio of the higher heating value of products to the total energy input) can reach as high as 89% in methane-assisted membrane reactor. Zhu et al. [60] brought up a thermodynamic model of ceria dense membrane for CO2 splitting, and the energy efficiency is above 10% at 1800 K without heat recovery. Steinfeld et al. [61, 62] have done a lot of experimental researches about solar CO2 splitting for CO generation by oxygen permeation membrane with 100% selectivity (e.g., La0.6Sr0.4Co0.2 Fe0.8O3-<sup>δ</sup> at 1030°C [61], CeO2 at 1600°C [62]), and Ozin [63] said the research of Steinfeld is an elegant demonstration and an exciting breakthrough for continuous

CO2 splitting in a single step, at a single temperature, in a single reactor.

The materials of the hydrogen permeation membrane are various, such as metal (e.g., palladium, nickel), perovskites, pyrochlores, fluorites, polymers, which are usually used in the reaction of reforming, splitting, partial oxidation of hydrocarbon, splitting of other hydrogen carriers (e.g., NH3), and water-gas shift reaction. Li et al. [30] first presented an innovative solar-assisted hybrid power system integrated with methane steam reforming in membrane reactor, and the simulation results showed that capture ratio of CO2 is 91% and exergy efficiency and thermal efficiency are 58 and 51.6% (10.2 and 2.2% points higher than the CO2 capture from exhaust cycle), respectively. Said et al. [64] simulated a CFD model about solar molten salt-heated H2-selective membrane reformer for methane upgrading and hydrogen generation, and the results showed the fuel heating value upgrade of 40% with methane conversion rate of 99% and hydrogen recovery of 87% at 600°C. Wang et al. [65] put forward a novel reactor, which realized direct methane steam reforming in parabolic trough collector integrated with hydrogen permeation membrane reactor, and the system can perform high and stable efficiency (above 80%) at 400°C. Mallapragada et al. [66] proposed a novel system that consists of oxygen permeation membrane and hydrogen permeation membrane for solar water splitting, and the solar-to-H2 efficiency (ratio of the lower heating value of hydrogen to the reversible work input for Gibbs free energy change of water splitting) is 72.4–80.1% at the concentration ratios of 2000–10,000. Sui et al. [67] reported an exploration on an efficient solar thermochemical water-splitting system enhanced

4.2 Hydrogen permeation membrane for hydrogen generation

4.1 Oxygen permeation membrane for H2O/CO2 splitting

generation.

Solar Thermochemical Fuel Generation DOI: http://dx.doi.org/10.5772/intechopen.90767

155

Table 3.

Summary of experimental research published in gasification of solid hydrocarbon feed [36].

gasification have been conducted by Epstein et al. [45], Lédé [46], Nzihou et al. [47], and Puig-Arnavat et al. [48], which will not be discussed here. A summary of experimental work published in gasification of solid hydrocarbon feed has been listed in Table 3.
