Upgrading of Lignocellulosic Biomass to Hydrogen-Rich Gas

Due to limited fossil fuel reserves, the global political situation, and progressive environmental pollution, the development of new methods of hydrogen production is highly demanded. Higher sustainability in the H₂ formation process may be achieved by the use of renewable feedstock, such as lignocellulosic biomass. The benefits of the application of biomass are mainly related to its wide availability, relatively short reproducibility time, and the possibility of the limitation of carbon dioxide emission and waste disposal. H₂-rich gas can be produced through the use of various types of biomass, including straw, wood, grass, energy crops, and agricultural and forest waste. The literature shows that anaerobic digestion, fermentation, gasification, and pyrolysis are the most commonly used methods for this purpose (Figure 1).

Biochemical methods (anaerobic digestion and fermentation) are relatively slow and generate problems related to the decomposition of lignin-rich biomass. On the other hand, technologies based on thermal conversion (gasification or pyrolysis) allow for the more effective processing of lignocellulosic feedstock to more valuable products. However, some biomass properties, such as low bulk density, high oxygen content, and the presence of inorganic contaminants may affect the efficiency of lignocellulose decomposition. Moreover, the relatively low selectivity of the performed reactions and the formation of a wide range of intermediates results in the need for upgrading the composition of the final products (for example, upgrading biomass pyrolysis vapors, the steam reformation of tar, etc.). The mentioned problems should be solved before implementing the thermochemical conversion of biomass on an industrial scale. For this reason, this topic is still relevant, and the interest in thermochemically upgrading lignocellulose has increased in recent years. This is confirmed by the growing number of works devoted to the thermal processing of biomass in scientific journals (Figure 2).

Systematic information on achievements and challenges in the area of the upgrading of lignocellulosic biomass to hydrogen-rich gas can be found in several recently published review papers [1,2]. The Energies journal, aware of the importance of this subject, also published numerous manuscripts devoted to hydrogen production from lignocellulosic feedstock. Glushkov et al., meeting the needs of the scientific community, presented the current status of the research on the pyrolysis and gasification of biomass [3]. They described the successive stages of the thermal decomposition of lignocellulose, demonstrated the types of biomass used in pyrolysis or gasification, and discussed the significance of the factors influencing the efficiency of thermal treatment (biomass composition and structure, particle size, treatment temperature, and the type of gasifying agent in the case of gasification, among others).

Notably, reports regarding the conducted research on the thermal conversion of lignocellulosic feedstock to H₂-rich gas presented recently in Energies and other scientific journals are mainly related to various aspects associated with the application of new reaction systems, the development of methods of purifying the obtained products, the optimization of the process conditions, the upgrading of the primary products via the reforming process, and the effect of the use of catalysts.

Sarafraz et al. [4] described a supercritical water gasification system with a water–gas shift (WGS) module. It consisted of two reactors. In the first of them, biomass is gasified in the presence of water which results in the production of syngas. In the second one, the composition of the formed gaseous mixture can be substantially upgraded via a WGS reaction. The obtained results suggested that the addition of a water–gas shift module allowed for more effective control of the H₂:CO ratio and a decrease in the contribution of methane and carbon dioxide. Moreover, it was concluded that pressure does not have an influence on the performance of the WGS reactor; however, it may be helpful for the storage of produced syngas and reducing the cost of the post-compression of the obtained product. The studies performed by Chojnacki et al. [5] focused on the gasification of wood pellets with water steam using an electrically heated reactor equipped with a continuous biomass feed system. The conducted research exhibited the crucial impact of the flow rate of the steam and the temperature inside the reactor on the composition and calorific value of the obtained gas. It was demonstrated that the increase in the steam flow rate resulted in a drop in the hydrogen content and a growth in the contribution of carbon dioxide in the final product. The inverse relationship was observed in the case of the increase in the reactor temperature. Syngas with the highest calorific value was produced at 800 °C. The work of Stąsiek and Szkodo [6] on the high-temperature air and air/steam conversion process (HiTAG/HiTSG) is the next example of the investigations devoted to the development of new reaction systems for the gasification of biomass. In this case, preheated (up to 1600 °C) air or steam is used for biomass conversion. It is possible due to the application of a high-cycle regenerator that can provide extra energy for the reaction system. The authors suggested that the high temperature of the conducted process may contribute to a reduction in undesirable and toxic substances, such as tars, furans, or dioxins, which are observed in the case of processes performed at lower temperatures. Moreover, it was demonstrated that HiTAG/HiTSG technology enables the generation of cleaner gas as well as a product possessing higher calorific value than that obtained in low-temperature processes (for example, circulating or fluidized bed gasification). The final gaseous mixture can be applied for the production of energy (heat or electricity) as well as hydrogen, which requires its separation from other gas components.

The problem of the presence of contaminants in the gaseous mixture obtained during biomass gasification was raised by Marcantonio et al. [7]. They reported that chlorine, mainly in the form of HCl, is one of the major pollutants of formed syngas. The presence of HCl may lead to the corrosion of the metallic parts of equipment, cause health problems, pollute the environment, or even deactivate catalysts used in the further processing of gaseous products. Therefore, before being used, raw gas obtained during biomass gasification should be cleaned. The data presented by the authors demonstrated that two types of gas cleaning methods can be used for this purpose—low- and high-temperature clean-up. It was suggested that the high-temperature process is more suitable for syngas cleaning due to the improved thermal efficiency of the process, among other reasons. Additionally, the use of CaO as a HCl sorbent allowed for the effective binding of hydrogen chloride; however, this sorbent decomposed at a higher temperature. This resulted in a decrease in its adsorption capacity and the release of the previously adsorbed HCl. An application of NaAlO₂ and Na₂CO₃ proved to be more promising. Nevertheless, their adsorption efficiency decreased in the presence of H₂S. That is why the separated adsorption of these two substances in two different reactors has been proposed.

The gasification of lignocellulosic biomass is a very complex process depending on numerous factors. Because of this, understanding its mechanism is not an easy task. One of the solutions to this problem is the implementation of mathematical modeling. It allows for the prediction of the influence of various factors (such as operating parameters (temperature, steam to biomass ratio, etc.), reactor designs, the composition of biomass, or the application of gas cleaning systems) on the efficiency of the analyzed process. The advantage of using mathematical models is related to the limitations of the high cost and time of the experimental procedures. Moreover, theoretical modeling enables the analysis of various scenarios with different complexity levels [8]. These benefits undoubtedly increase the popularity of this topic in the literature [9,10]. Recent studies mainly focus on the development of stoichiometric and non-stoichiometric thermodynamic equilibrium models and the introduction of correction factors minimizing errors arising during calculations [11,12,13].

Pyrolysis is the next method of production of hydrogen-rich gas from lignocellulosic feedstock described in the literature. The current research in this area is mainly focused on the development of new reaction systems and the design of catalysts allowing for the increase in the efficiency of biomass decomposition and a rise in the selectivity of hydrogen production. Sieradzka et al. [14] described a novel approach to the production of syngas via the thermochemical conversion of biomass integrated with carbon dioxide capture. This idea was based on the application of calcium oxide as a CO₂ sorbent. It was demonstrated that the presence of CaO allowed for a decrease in the CO₂ content in the produced gas. The concentration of carbon dioxide decreased with the increase in the pyrolysis temperature. The captured CO₂ reacted with the sorbent to form calcium carbonate. Moreover, the presence of CaO promoted the cracking of tar. All of this contributed to the increase in the efficiency of H₂ production.

Due to the insufficient selectivity of the biomass pyrolysis process, it is necessary to apply catalysts to enhance hydrogen-rich gas production [15]. However, the activity and stability of the catalyst are crucial for the successful thermal decomposition of lignocellulosic feedstock to value-added products [16]. Commercial catalysts usually do not have satisfactory stability; therefore, research has been carried out to increase their catalytic performance. Ni-based materials are the most used for the thermochemical conversion of biomass [17]. However, their catalytic properties strongly depend on the type of support used. Therefore, it is possible to enhance the catalytic performance of nickel by designing the most suitable supports for increasing the activity and lifetime of the catalyst. Metal oxides, zeolites, and silica-based mesoporous materials were used for this purpose [18]. The performed studies demonstrated that Ni/ZrO₂ was the most active in biomass upgrading to H₂-rich gas among metal oxide-supported catalysts. Recent research showed that the modification of zirconia by alkali, alkaline earth, and rare earth metals allowed for a further increase in the efficiency of the thermal decomposition of biomass [19,20]. This effect was related to the improvement of its oxygen storage capacity, an increase in the number of oxygen vacancies, the stabilization of the support structure, and the higher reducibility of nickel oxide, among others [21]. On the other hand, it was presented that the high catalytic performance of silica-based materials is associated with the possibility of strict control of the porosity and acidity of the formed materials [22]. The development of catalysts for biomass upgrading to hydrogen-rich gas includes also the recently popular trend of using waste materials as metal supports (for example, fly ash or slag). This increases the sustainability of the entire process [23].

The final studies devoted to upgrading lignocellulosic biomass and the formation of H₂-rich gas focus on the steam reformation of volatiles derived from the pyrolysis of lignocellulosic feedstock. Santamaria et al. [24,25] demonstrated that the whole process can be conducted in two steps. In the first study, a conical spouted bed reactor was used for the pyrolysis of biomass. In the second one, volatiles that formed in the first step were subjected to reforming in a fluidized bed reactor. It was suggested that the steam reforming of pyrolysis intermediates can be performed in lower temperatures than steam gasification and may contribute to the limitation of tar production. However, in the case of the use of catalysts for the reforming step, its deactivation by carbon deposition was still observed. The stability of the catalyst can be improved by the optimization of the conditions of the performed process, but this does not fully solve this problem.

It is worth emphasizing that the interest in the upgrading of lignocellulosic biomass to H₂-rich gas is constantly growing. The results of the studies published in recent years in the literature are promising and should contribute to the increase in the competitiveness of the process in focus and the more efficient production of hydrogen from renewable feedstock.