Hydrogen-Rich Syngas Production from Biodiesel-derived Glycerol: An Overview of the Modeling and Optimization Strategies

Bamidele Victor Ayodele Siti Indati Mustapa May AH Alsaffa

Introduction

The utilization of energy derived from fossil fuel is often associated with challenges of greenhouse gas emissions which have significantly result in global warming (Mardani et al., 2019). This phenomenon has increased the quest for alternative, cleaner, and renewable sources of energy in the past decades (Muhammad et al., 2018). Amongst the several renewable energy sources, biodiesel has been projected as a possible candidate to favorably compete with conventional petrodiesel (Mekhilef et al., 2011). Hence, the global production of biodiesel has increased drastically over

Biodiesel outlook from 2012 to 2021. (Source: OECD/FAO (2012)).

FIGURE 19.1

the years, as shown in Figure 19.1. However, biodiesel is produced concurrently with glycerol which has been a major challenge in the production line (Quispe et al., 2013). According to Yang et al. (2012), 10% (w/w) 1.05 pounds of glycerol is produced for an equivalent gallon of biodiesel. Hence, research focus has been on the sustainable ways of utilizing biodiesel-derived glycerol for the production of value-added chemicals (Ayodele et al., 2020).

Glycerol has been proven as a viable feedstock to produce chemical intermediates such as hydrogen-rich syngas by various thermo-catalytic and bioconversion processes (Seadira et al., 2017; Vaidya and Rodrigues, 2009). These chemical intermediates can be employed to produce other value-added chemicals (Tan et al., 2013). The conversion of glycerol to hydrogen-rich syngas can be achieved through the various thermo-catalytic process such as ‘steam reforming’, ‘dry reforming’, ‘autothermal reforming’, ‘supercritical water reforming’, ‘pyrolysis’, ‘photo-catalysis’, as well as ‘microbial bioconversion’ using a microorganism (Ayodele et al., 2019,2020).

Glycerol conversion to hydrogen-rich gas has been reported to be influenced by several parameters, such as the reaction temperature, pressure, water-to-glycerol molar feed ratio, and irradiation time (Estahbanati et al., 2017; Mangayil et al., 2015). The individual and interaction effects of these parameters on glycerol conversion and hydrogen-rich syngas production have been investigated using various modeling techniques (Adeniyi and Ighalo, 2019; Galera and Ortiz, 2015; Ghasemzadeh et al., 2018). In addition, the optimum conditions of these parameters that could maximize glycerol conversion and the formation of the desired product have been investigated using various optimization strategies (Estahbanati et al., 2017; Zamzuri et al., 2017).

The main objective of this chapter is to present an overview of the various modeling and optimization strategies that have been employed for hydrogen-rich syngas production from glycerol conversion.

Technological Routes for Hydrogen-Rich Syngas Production from Glycerol

Glycerol can be converted to hydrogen-rich syngas using various technological routes, such as reforming, pyrolysis, and fermentative processes, as depicted in Figure 19.2 (Monteiro et al., 2018). The reforming and pyrolysis of glycerol are thermo-catalytic processes that are performed under high temperature (> 500°C) using various types of catalysts (Dieuzeide et al., 2015; Shahirah et al., 2017). The details of each of these processes are discussed subsequently.

The reforming of glycerol using steam is one of the most established processes for producing hydrogen from hydrocarbon. The steam reforming of glycerol is a thermo-catalytic process which involves the reaction of glycerol and steam in the presence of a catalyst, as represented in Equation (19.1) (Dieuzeideh et al., 2015). Primarily, the steam reforming of the glycerol reaction in Equation (19.1) can be described as a summation of the glycerol decomposition reaction (Equation 19.2) and the water-gas-shift reaction (Equation 19.3) (Cheng et al., 2011).

C3H8O3 (g) + 3H2O(g) <-> 3CO, (g) + 7H, (g)

(19.1)

Catalyst

Reforming

¡team glycerol reforming

Dry glycerol reforming

Autothermal glycerol reforming Partial oxidation glycerol reforming Aqueous phase reforming Photocatalytic glycerol reforming yJSupercritical-water glycerol reforming J

H2, CO, CO2, CH4Bio-oil

FIGURE 19.2 The various technological pathways for conversion of glycerol to hydrogenrich syngas

C3H8O3(g)<->3CO(g) + 4H2(g) (19.2)

CO(g) + H2O(g)<->CO2(g) + H2(g) (19.3)

The steam reforming of the glycerol reaction is often influenced by several parameters. such as the feed composition (water/glycerol), the feed rate, the nature of the catalyst, the reaction temperature, and the gas hourly space velocity. Bobadilla et al. (2015) investigated the effect of the reaction temperature, water/glycerol molar ratio, and space velocity on the glycerol steam reforming of hydrogen. The study revealed that the hydrogen yield was strongly influenced by the reaction temperature. An increase in the hydrogen yield was observed with a corresponding increase in the reaction temperature up until 650°C. This is an expected trend for a temperaturedependent gas-phase reaction as stipulated by the Arrhenius theory. The authors also reported that the water to glycerol molar concentration in the feed significantly influenced hydrogen production. The hydrogen yield was reported to decrease with an increase in the glycerol concentration in the feed. The decrease in the hydrogen yield was attributed to the reduction in the activity of the catalysts as a result of coke deposition from the unconverted glycerol. Similarly, an increase in the space velocity reportedly results in a corresponding decrease in the hydrogen yield.

The nature of the catalysts used for steam reforming of glycerol has been reported to influence hydrogen production. As reported by Ayodele et al. (2020), the make-up of the catalysts significantly influences the yield of the hydrogen produced by the steam reforming reaction, as shown in Figure 19.3. However, the performance of the catalyst in terms of the product yield is not solely dependent on the chemical composition but also on parameters such as the preparation method, the activation temperature, and the reaction conditions (Ayodele et al., 2020).

The metal loading of the catalyst has also been reported to have a significant effect on catalytic performance. As reported by Senseni et al. (2016a), the variation of Ni

iiihlllllllLi

Hydrogen yield obtained for various catalysts used for steam reforming of glycerol

FIGURE 19.3 Hydrogen yield obtained for various catalysts used for steam reforming of glycerol.

loading from 5-20% has an impact on the performance of Ni/Al,0, during steam reforming of glycerol at 750°C. A decrease in the hydrogen yield was observed with an increase in Ni loading. An increase in the metal loading could result in sintering which is one of the major causes of Ni-based catalysts.

The reforming of glycerol using carbon dioxide (CO,) is often referred to as the 'dry reforming process’ (Arif et al., 2019). This process entails the utilization of CO, and glycerol for the production of hydrogen-rich syngas. The process offers the advantage of employing both CO,, a principal component of greenhouse gases, and glycerol, a by-product of biodiesel production, as feedstock for producing renewable hydrogen. The main reaction of the dry reforming of glycerol is represented in Equation 19.4. Besides the main reaction, other competing side reactions such as glycerol decomposition, water gas shift reaction, methanation, and carbon gasification play significant roles in the product formation (Rosian et al., 2020).

C3H8O3 (g) + CO, (g) 4CO(g) + 3H, (g) + H,O(g) (19.4)

Factors such as the nature of catalysts, the catalyst synthesis method, the type of reactor, and the reaction conditions have also been reported to significantly influence hydrogen production by the dry reforming of glycerol (Ayodele et al., 2020). Most of the studies conducted on the dry reforming of glycerol reported a temperature range of 700-850°C for hydrogen-rich gas production. One major advantage of the dry reforming of glycerol is that the hydrogemcarbon monoxide (H,:CO) ratio obtained from the reaction is often close to unity, making it suitable as a potential feedstock for the production of liquid hydrocarbon by ‘Fischer-Tropsch synthesis’ (Pakhare and Spivey, 2014). However, one of the major drawbacks of the process like other reforming reactions is catalyst deactivation by carbon deposition and sintering. Efforts are being made by researchers to develop various strategies that can mitigate the adverse effects of catalyst deactivation on the performance of the various dry reforming methods of glycerol catalysts.

‘Aqueous phase reforming’ is an emerging technological route to produce hydro-gen-rich syngas from glycerol (Manfro et al., 2013). This process involves the conversion of glycerol in water using a suitable catalyst as shown in Equation 19.5. Unlike the other reforming processes, aqueous phase reforming does not require high thermal energy. Studies have shown that aqueous phase reforming of glycerol often occurs at a pressure and temperature range of 15-50 bar and 2OO-3OO°C, respectively (Ayodele et al., 2020). These reaction conditions do not allow the occurrence of catalyst deactivation. Besides the main reaction in Equation 19.5, a water-gas shift reaction also occurs as a side reaction. However, the reaction conditions of aqueous phase reforming often favor the production of hydrogen with low CO content from the water-gas shift reaction.

C3H8O3(g) + H,O(g)<->3CO(g) + 7H2(g) (19.5)

Aqueous phase reforming of glycerol in a batch reactor over a Pt/Al,O, catalyst has been reported by Seretis and Tsiakaras (2016). The effect of the reaction temperature (200-240°C), the reaction time (30-240 min), the glycerol concentration (1 and 10%), and the Pt loading (0.5-5.0%) on the product distribution was investigated. The analysis of the gaseous products after each reaction run revealed the presence of H2, CO, CO,, and CH4 in the outlet stream. The findings showed that maximum yields of the gaseous products were obtained using 1 g of 5% Pt/Al,O„ 1 wt% of glycerol solution, 240°C, and a 4 h reaction time. Optimization strategies such as the response surface methodology can be employed to determine the interaction effects of all the process parameters and the optimum conditions at which the gaseous products and the glycerol conversion can be maximized.

‘Partial oxidation reforming’ is a process that involves the use of an appropriate amount of oxygen for reforming glycerol into hydrogen-rich syngas (Rennard et al., 2009). Unlike the steam and dry reforming of glycerol, which are endothermic reactions, the partial oxidation reforming of the glycerol reaction occurs exothermically. However, a hot zone is usually created within the reactor as a result of the excessive heat generated from the exothermic reaction. Hence, due to the sensitivity of the partial oxidation reaction, an appropriate control measure is often put in place during the experimental phase. Moreover, materials that could withstand high temperatures are often considered for building a reactor for the partial oxidation reforming process. One major limitation of the partial oxidation reforming of glycerol is the low hydrogen yield that is usually obtained from this process.

Partial oxidation reforming of glycerol can also be conducted under autothermal conditions using oxygen and steam. Liu and Lin (2014) demonstrated the ‘autothermal partial oxidation'-based glycerol reforming using LaMnO,- and LaNiO,-coated monolith perovskite catalysts. The study revealed that the hydrogen-rich syngas was produced by the autothermal partial oxidation reaction over the LaMnO, and LaNiO, catalysts. However. LaMnO, showed superior activity in the autothermal reforming compared with the LaNiO, catalyst.

Studies have shown that parameters such as the reaction temperature, stearmcarbon ratio, oxygemcarbon ratio, and the gas hourly space velocity significantly influence the yield of the hydrogen-rich syngas during the partial oxidation autothermal reforming of glycerol. Liu and Lin (2014) reported an operational range of 300-700°C, 0.4-1.5, and 0.1-0.3 for inlet temperature, stearmcarbon ratio, and oxygemcarbon ratio, respectively. The study revealed that the gaseous products which include H,. CO. CO,, and CH4 increased with increased inlet temperature. This trend was attributed to the high rate of conversion of the glycerol as the temperature increases. On the other hand, the stearmcarbon ratio was found to affect the yield negatively. The hydrogen yield obtained from the partial autothermal reforming of glycerol was observed to decrease with an increase in the stearmcarbon ratio. The yield of hydrogen was however found to increase with an increase in the oxygemcarbon ratio. Applying appropriate optimization strategies could help to determine the optimum conditions to obtain the maximum yields of the hydrogenrich syngas from autothermal reforming.

Just like the aqueous phase reforming of glycerol, another emerging reforming process is the ‘supercritical water reforming’ of glycerol which employs the conditions of supercritical water (pressure > 221 bar and temperature > 374°C) for the conversion of glycerol to hydrogen-rich syngas (Markocic et al.. 2013). Parameters such as inlet concentration of the feed, the nature of the catalysts, the pressure, temperature, reaction time, and type of reactor are vital in obtaining the desired product distribution in the supercritical water reforming of glycerol. However, these parameters need to be optimized using various strategies that can help to determine the optimum conditions that could maximize hydrogen-rich gas production.

Other than using thermo-catalytic means for converting glycerol to hydrogen-rich syngas, which require high thermal energy to initiate the reaction, researchers are tapping into the abundance of solar energy and readily available photocatalysts to effect this conversion (Vaiano et al., 2018). The ‘photocatalytic reforming’ of glycerol employs a photo-induced hole, which facilitates the conversion of the glycerol, and induced electrons that reduce the H+ to H2. In comparison with the thermo-catalytic routes, there is no issue of deactivation with the photocatalysts since the reaction readily occurs at low temperatures. According to Li et al. (2009), factors such as irradiation time, initial concentration of glycerol in the feed, and pH have been reported to significantly influence the rate of hydrogen production from the photocatalytic reforming of glycerol. The rate of hydrogen production was found to increase with an increase in the irradiation time and glycerol concentration in the feed. Similarly, the rate was found to increase as the pH increases from 4 to 8 and subsequently to decrease from a pH greater than 8. Optimum conditions for the photocatalytic reforming of glycerol can be obtained for hydrogen production using the appropriate optimization strategies.

Besides reforming, ‘pyrolysis’ is another thermal process that can be employed to convert glycerol to hydrogen-rich syngas (Shahirah et al., 2017). Typically, pyrolysis of glycerol often produced char, biooil, and gaseous products which are mainly hydrogen-rich syngas. The formation of the various categories of products depends on the reaction conditions. The formation of biooil is favored at a temperature range of 400-600°C while a temperature above 75O°C favors the formation of gaseous products. The gaseous product distribution is dependent on whether the pyrolysis process is catalyzed or not. Non-catalyzed glycerol pyrolysis often depends on parameters such as temperature, the flow rate of the carrier gas, and the particle diameter of the materials used for packing. Studies have shown that the yield and selectivity of gaseous products obtained in the non-catalytic glycerol pyrolysis are often lower compared to the catalytic pyrolysis of glycerol. The low yield of the gaseous products obtained for non-catalytic glycerol pyrolysis can be attributed to the higher rate of converting glycerol to biooil. Just like other thermo-catalytic processes, the catalytic pyrolysis of glycerol is also constrained by catalyst deactivation and reactor blockage. Applying the appropriate optimization of the catalyst synthesis and the pyrolysis reaction conditions for the glycerol conversion could mitigate these challenges.

The challenges of high thermal energy requirement and catalyst deactivations could be mitigated using the method of converting glycerol through biological means. The bioconversion process entails the use of microorganisms to metabolize glycerol to hydrogen-rich syngas (Garlapati et al., 2016). The bioconversion process is usually performed in a batch reactor using microorganisms such as Citrobacter, Clostridium. Enterobacter, Klebsiella. Thermotoga, and Bacillus spp. Although the yield of hydrogen obtained from the bioconversion of glycerol is far lower compared with that obtained from the thermo-catalytic process, bioconversion is more environmentally friendly and does not require high thermal energy. The gaseous product distributions in the bioconversion of glycerol are strongly dependent on parameters such as the N source, glycerol concentration, and pH of the medium. The yield of hydrogen from the bioconversion of glycerol can be improved using appropriate optimization strategies.

The various technological routes for converting glycerol to hydrogen-rich syngas can be improved by applying appropriate modeling and optimization strategies. Thosemodeling that have been employed are discussed in the subsequent section.

 
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