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4.4.2.3 Reforming of Glycerol
ОглавлениеHydrogen has been recognized as “the fuel of the future” because it generates only water as a product after combustion. In addition to its utilization as an energy source, hydrogen is consumed as raw material to obtain formic acid, hydrochloric acid, cyclohexane, and other important solvents and reagents. Moreover, it is consumed as a reactant in the hydrogenation process carried out in food industries and the hydrocracking process of petrochemical industries. Methane from fossil resources is used as raw material for approximately 95% of worldwide hydrogen production. Syngas is a mixture of carbon monoxide (CO) and hydrogen (H2). It is a key reactant for several processes including ammonia synthesis, methanol production, Fischer–Tropsch synthesis, and H2 generation via fuel cells. Hydrogen/syngas obtained from renewable resources has the potential to be employed as an alternative within the contemporary fossil fuel-dependent energy landscape [8]. Given that global glycerol production continues to grow in the market, it can be used as an alternative for the large-scale production of hydrogen and syngas.
The thermochemical process is the most widely used route to generate the H2/syngas. Among these, steam reforming of methane is a popular, fully developed, and efficient technology to produce H2/syngas in industries [41]. Steam reforming has also been extensively explored for H2 generation from other hydrocarbons and alcohols (such as methanol and ethanol) [42]. Several reforming technologies, characterized as thermochemical routes, have been studied for hydrogen or syngas production from glycerol. The key technologies are steam reforming (SR), partial oxidation reforming (POR), autothermal reforming (ATR), and aqueous phase reforming (APR). Several important reactions associated with different reforming processes are as follow:
(4.9)
(4.10)
The gaseous product of the glycerol reforming process contains constituents such as CO, H2, and CO2 in major proportions. Gaseous products from these reforming processes also contain small fractions of compounds such as methane, ethylene, methanol, ethanol, acetic acid, and acetaldehyde. All of the glycerol reforming processes are generally performed in a fixed-bed reactor. Reforming processes are usually conducted with the help of a catalyst within the temperature range of 300–900 °C. The catalyst brings down the activation energy of the reforming process and favors the fast kinetic. To improve the suitability of the reforming processes for large-scale production, the catalyst must be highly active and stable, provides a high resistance for coke formation, exhibits the resistance for metal sintering, and suppresses undesirable side reactions such as methanation [41].
Steam reforming of glycerol has been identified as one of the most promising processes because the existing industrial plants of steam reforming of methane can be used for hydrogen or syngas production after slight modification. This process involves the glycerol reaction with steam on the surface of a catalyst and results in the generation of CO and H2. The overall reaction for glycerol steam reforming can be represented by stoichiometric Equation (4.1), where the molar ratio of hydrogen to glycerol (7/1) is quite interesting. The overall reaction may be viewed primarily as the combination of Equation (4.2) (glycerol decomposition) and Equation (4.3) (water-gas-shift reaction) [9, 12]. Water-gas shift reaction results in the generation of additional H2. The glycerol decomposition reaction is extremely endothermic, while the water-gas shift reaction is slightly exothermic, but the overall steam reforming is endothermic. Several studies have shown that this process shows the best result within the temperature range of 500–750 °C. Higher temperature increases the operating cost, energy requirement, and reactor cost. Lower temperature leads to a decrease in H2 selectivity due to the formation of CO2 and CH4 in higher amounts. The operating pressure generally used in this process is atmospheric. Lower pressure in the vacuum range facilitates the lower energy requirement, low reaction temperature, and reduce metal sintering in the catalyst.
The molar ratios of glycerol to water are commonly varied between 1/6 and 1/12. This molar ratio strongly affects the yield and selectivity of H2. According to Le Chatelier’s principle, at the higher water/glycerol molar ratio, the equilibrium shift in the direction of more water utilization, and consequently more H2 is produced. A high water/glycerol molar ratio also promotes the gasification of carbon and suppresses its accumulation as coke over the catalytic active sites. However, the operating cost becomes high due to the requirement of excessive energy to vaporize the reaction mixture of high water/glycerol molar ratio.
In addition, other thermodynamic feasible reactions include methane formation (Equations (4.4), (4.5)). The methanation reaction is favored at low temperatures and slightly influenced by pressure change. Coke formation also occurs in the steam reforming of glycerol (Equations (4.6)–(4.8)). The coke accumulates over the surface of the catalyst, blocks the active sites, and eventually deactivates them. This coke is mainly produced by Boudouard (Equation (4.7)) and methane cracking (Equation (4.8)) side reactions [41, 42].
In partial oxidation, the amount of oxygen is supplied below the stoichiometric requirement of complete combustion. The overall POR reaction is commonly expressed by Equation (4.11). Since POR is an exothermic process, the amount of oxygen directly controls the energy efficiency of this process. The complete oxidation of glycerol can be represented using Equation (4.12). POR process occurs under atmospheric pressure. POR is an energy-efficient process and enables the generation of syngas through varying oxygen amounts. The coke formation is very low under an oxidative environment.
In the ATR process, SR and POR occur simultaneously in the same reactor. The reaction mixture contains glycerol vapors, steam, and oxygen. ATR has been identified as a promising process because of its energy self-sufficiency. This feature of ATR attributes to the endothermic nature of SR and the exothermic behavior of POR reactions. Oxygen delivers the required heat via the oxidation reactions, which is the major reason for ATR to be energy efficient. This reaction does not need an external power supply, which reduces the operating cost. ATR is different from POR because of the supply of water vapor, which increases hydrogen production. Another advantage of ATR is that small volume units are possible to fabricate for decentralized small-scale production due to the high efficiency and compactness of the reaction system. The optimized reaction condition for hydrogen production was reported at T = 600–750 °C, steam/glycerol molar ratios 9/1–12/1, and oxygen in proportion 0–0.4 [9, 13]. Some major side reactions, for example, methanation and coke deposition can also be reduced significantly under this condition.
APR is a process of transforming glycerol in the aqueous medium without pre-vaporization. APR is usually conducted at moderate temperature (~250 °C) and high pressure (60 atm) in a continuous fixed-bed reactor. The overall reaction of APR is expressed in Equation (4.1). First, glycerol is decomposed into syngas (Equation (4.2)), and then syngas converts into CO2 and H2 through a water-gas shift reaction (Equation (4.3)). The generated H2 is utilized by reacting with intermediates such as CO and hydroxides and in dehydration reactions. This process takes into account the scission of the C–C, C–H, and C–O bonds [41]. To achieve significant selectivity for H2 production, the catalyst must break the C–C bond and expedite the reaction of CO with water vapor on its active sites. However, it should not favor the scission of the C–O bond or dehydration reaction, which is responsible for the formation of alkanes.
APR exhibits several advantages in comparison to the steam reforming method, e.g., (a) low reaction temperature; (b) greater energy efficiency because of the liquid/liquid phase; (c) lower cost of reactor system because of the liquid system; and (d) lower energy cost because of no need of the steam-generating system. The main advantage of this process is that it is a liquid-phase process, unlike other existing technologies which occur in the gas phase. It is also important for other biomass-derived liquids which vaporize at high temperature. The main disadvantages of this process in comparison to SR are its low selectivity towards H2 because of low operating temperature, and the high tendency of the formation of alkanes.
Rahman et al. [43] have used multiwalled carbon nanotubes supported by bimetallic Pt–Ni and Cu–Ni composites for the APR of glycerol in a continuous fixed-bed reactor. Multiwalled carbon nanotubes were selected as support because of mesoporous structure, low mass transfer resistance, high surface area, and high thermal stability. The catalyst with 1 wt% Pt and 3 wt% Ni exhibited the best performance with glycerol conversion >99%, selectivity for hydrogen = 90%, and rate of formation = 21.2 mmol H2 gcat–1 h–1 at T = 250 °C and P = 40 bar. The catalyst showed higher activity and selectivity for the production of hydrogen as compared to the monometallic catalyst. This catalyst retains its activity up to 100 h test cycle and thus, suitable for long-term operations [43]. The incorporation of Re over Pt/C increases the turnover frequency for the formation of synthesis gas from glycerol [44]. Table 4.3 summarizes the performance of different catalysts for reforming.
Dumesic and co-workers reported several studies for the APR of glycerol using carbon-based materials as supports [44, 45]. Carbon-supported platinum–rhenium catalysts (Pt–Re/C) have been employed for the transformation of glycerol into hydrogen or synthesis gas. It was observed that a Pt–Re/C catalyst with an atomic ratio Pt/Re =1 showed high catalytic activity, selectivity to syngas, and long-term stability [45]. The carbon was used as support owing to its long-term hydrothermal stability [44].
The pyrolysis of glycerol with the support of carbonaceous catalyst is an important method for the formation of syngas with a higher H2/CO ratio. Other methods used for the formation of synthesis gas include dry and steam reforming of glycerol. Fernandez et al. [46] have compared the above three methods (steam reforming, dry reforming, and pyrolysis) for the synthesis of syngas using commercially available activated carbon as a catalyst. Carbon-base catalyst was favorable for generating syngas with H2/CO ratio not far from 1, reduce the CO2 fraction in the gaseous product. The reforming of glycerol uses CO2 (dry reforming) or H2O (steam reforming) as an oxidizing agent which encourages the higher conversion of glycerol as compared to pyrolysis. The dry reforming generates the lowest amount of hydrogen and syngas, and the highest amounts gas fraction, whereas the reverse occurs in the steam reforming process. They have also compared the microwave-assisted process with the conventional heating process. The microwave-assisted method promotes more conversion of glycerol as compared to the conventional method [46].
