There are several approaches for the invention of H2. It could be divided into two classes as traditional and non-traditional ways. In traditional methods, hydrogen is obtained from hydrocarbon reforming as natural gas (CH4) and hydrocarbon pyrolysis. In non-traditional methods or alternative methods that can use renewable sources (Nikolaiis and Poullikkas 2017).
Interestingly, methane is part of the biogas component, and biogas is generated from waste materials as adopted the renewable source. Therefore, biogas reforming seems to renewable energy from conventional techniques for the production of hydrogen (Nikolaiis and Poullikkas 2017).
To produce pure hydrogen, additional steps are included in any reforming process. A conversion reactor is utilized for converting the CO into CO2 by using shift reaction (Alves et al. 2013). The final process for hydrogen purification is done in a separate unit by pressure swing adsorption (PSA) (Nikolaiis and Poullikkas 2017). H2 could be obtained through the methane or reforming of biogas in various temperature limits (873–1273K) over reversible reactions and exothermic or endothermic. The reforming reactions are done by low pressure and high temperature. In several kinds of reforming methods, biogas responds along with representatives like oxygen or air and steam to generate syngas with H2 and other gases (Galvagno et al. 2013).
There are some purification techniques available for removing toxic and undesirable substances present in biogas: physiochemical and biological treatment (Alves et al. 2013). Figure 1 presents the important steps for the production of H2 from biogas.
Consequently, the biogas reforming reactions are parallel to natural gas (CH4) reforming. Table 2 illustrates the chemical reactions formed in reforming methods and its enthalpy of reaction at 298K and 1 atm. There are major reforming techniques for H2 production from biogas or methane as follows.
Table 2 Chemically reaction involved in reforming methods (Alves et al. 2013; Verma and Samanta 2016)
2.1 Steam Reforming (SR)
This process is done by methane and water (H2O) vapor with the catalyst that generates syngas (mixture of CO and H2, Eq. 1, Table 2). Methane is required a high temperature for reacting. The reaction of this process is extremely endothermic and needs a high temperature of nearly 650–850°C to attain a high yield of H2 in between 60 to 70% (Verma and Samanta 2016). The ratio H2/CO is three that means the higher yield of H2 nearly 75%. High temperature can also be reduced by using a suitable catalyst (Shanmugam et al. 2018).
This technique consists of a steam reformer, water gas shift reactor, and direct steam reforming reactions inside the reactor. Initially, methane responds with water in the steam reformer, and the syngas and CO and H2 are formed in the existence of a suitable catalyst. Then, the syngas is fed in the WGS reactor after cooling in a range of 300–500°C. CO reacts through the water to decrease the CO content of syngas and developed the H2 and CO2 (Eq. 2). High conversion of CO content and low kinetic reactions were done by high-temperature WGS reactor. While low CO conversion and high kinetic reactions are done by minimum temperature water-gas shift reactors (Minh et al. 2018). Fe, Cu, Fe-Pd and Mo alloys are mostly used catalysts in water gas shift (Verma and Samanta 2016).
In addition, combined reaction (Eq. (3)), also known as direct steam reforming reaction, is the addition of Eqs. 1 and 2. To yield pure hydrogen, need a separate unit for example, membrane or PSA (pressure swing adsorption), remove all gases as CO2, CO, and other gases except H2. Figure 2 shows the reactant and products for the steam reforming process.
The working conditions of steam reforming lead to corresponding reactions that reason carbon formation occur over the surface of catalytic as methane cracking reaction (Eq. (4)), Boudouard reaction (Eq. (5)), and reduction of CO reactions (Eq. (6)). When carbon development seems like the form of nanotubes, it offers a lower distribution of carbon at the surface of the catalyst. Consequently, the catalyst activity is conserved for a long duration (Gregorie et al. 2020).
The possibility of carbon formations is a serious issue since coke deposits and leading deactivation on catalyst surfaces such as Ni-catalyst surface and solid material of carbon; denoted by C. The basic arrangements can control the deposition of coke as if they promoted carbon gasification in water improving and its properties of adsorption (Nawfal 2015). The most popular catalyst is Ni-based on ceramic supports for this process due to its affordable, easily available, and high-quality catalytic properties. There are several catalysts established as Ni-based to improve system performance in the term of selectivity and reactivity (Nawfal 2015).
The Combine components to develop catalysts with oxides that are basic properties such as potassium, calcium and magnesium have been studied. Different catalysts such as pd and Pt-based catalyst have mere stability about coke formation (Alves et al. 2013). Ruthenium-based catalyst is also mostly used in the methane steam reforming method due to its more activity, the high selectivity of H2 and outstanding stability (Nawfal 2015). Another way to decrease the deposition of coke as a result of a reaction (Eq. 6) is to perform under the condition of high partial pressure of water to shift the equilibrium in the right direction and control the carbon formation (Noh et al. 2019). Some other studies explain significant development in separation unit process through the selective membrane reactors or filters (Mahecha-Botero et al. 2009).
All chemical reactions are done in a single vessel from a membrane reactor. To membrane system, both the reactions such as steam methane reforming reaction and shift reaction (Eqs. 1 and 2) co-occur inside the reactor, which comprises a catalyst bed. Additionally, membrane offers the good ability to shift the reaction towards a chemical equilibrium that improves hydrogen production and performs the temperature nearly below 500°C for reforming reaction in the direction of chemical equilibrium (Lin et al. 2003; Sato et al.2010).
2.2 Partial Oxidation Reforming (POR)
Partial oxidation reforming technique (Eq. 7, Table 2) is another alternative path to produce H2 from methane reforming. This technique reduces energy consumption/cost due to the exothermic nature and is contrary to the steam reforming process that endothermic nature (Verma and Samanta 2016).
A methane (CH4) molecule is incompletely oxidised to produce H2 and CO (known as syngas). The ratio of H2/CO is two that means yields of hydrogen around 67%. The process needs a temperature range (700-900oC) (Patinvoh et al. 2017). The molar ratio of O2/CH4 is a key parameter to control this process since the ratio of O2/CH4 more than 0.50 leads to perfect combustion of CH4 (Eq. 8, Table 2) (Lin et al. 2003).
Since CO selectivity slightly decreases, CH4 reacts with O2 and obtains CO2 that permits complete combustion and sudden temperature rise. That reason initiated a hot spot in the reactor bed that deposits coke formation over the surface of the catalyst (Patinvoh et al. 2017). Researchers have been studied the activity and stability of numerous catalysts with the purpose of POR of biogas. Temperature range more than 800°C, catalyst as a solid solution of (Ca–Sr–Ti–Ni), (Ni–Mg–Cr–La–O), (NiO–MgO), and mixed metal oxides and they are used as higher resistance of coke development (Verma and Samanta 2016; Nalbant and Colpan 2020; Gregorie et al. 2020).
2.3 Auto-Thermal Reforming (ATR)
This process is the union approach of SMR and POR and endothermic in nature. This process reduces steam reforming challenges due to its endothermic process and difficulty producing low yield of hydrogen of partial oxidation reforming (Gregorie et al. 2020).
The summation of reactions is expressed by Eqs. 3,7 and 8 (Table 2). It also happens in CO2, as denoted by Eq. 9 (Table 2) (Araki et al. 2009). When the steam and oxygen are provided into the reformer, reason reforming and oxidation reactions occur parallelly, become self-sustainable the whole process, and require less energy. (Nikolaiis and Poullikkas 2017; Verma and Samanta 2016). This process produces the maximum hydrogen yield of approximate 74% (about ratio of H2/CO is 2.8) (Nikolaiis and Poullikkas 2017).
‘Autothermal’ means that no need to supply from an external source for the process. ATR process stabled the heat requirement of the endothermic process through the heat released by the exothermic process and adopted it as an effective system (Nahar et al. 2017). ATR has more advantages, such as fast reactor processing, higher ratio of H2/O2 compared with POR, temperature control of processing, and minimize the hot spots that prevent the deactivation of catalysts (Verma and Samanta 2016; Patinvoh et al. 2017).
To obtain the H2 through the biogas, the various the ratios range for H2O/CH4, O2/CH4, and H2/CO are taken as 1.0–2.5, 0.25–0.55, and 2.0–3.5, respectively (Verma and Samanta 2016; Patinvoh et al. 2017; Mosayebi et al. 2012; Cai et al.2006).
The maximum efficiencies of plant to biogas reforming with ATR method are about 75%, range of temperature 500 -700oC and the ratio of O/C close to 0.8–0.9 (Rau et al. 2019).
2.4 Dry Reforming (DR)
In this approach, CH4 reacts with CO2 and obtains the mixture of CO and H2 (Syngas) (Verma and Samanta 2016), refer Eq. 10 Table 2. This process requirements temperature more than 640oC and work on the endothermic process so need the heat of external source. Biogas is very useful for this process because it has two basic compounds available as CH4 and CO2 (Gregorie et al. 2020). This reaction of this process can be attractive an environmental point of view because they are strong greenhouse gases (CH4 and CO2), but this process can consume it. Contrarily, the rise in CO2 emissions due to the proven external heat (other burned fuels) to endothermic reaction (Nalbant and Colpan 2020). Dry reforming uses the ratio of H2/CO nearly 1 and attaining the yield of H2 about 50%. The H2/CO ratio built this reaction attractive about requirements to further use synthesis gas as Fischer–Tropsch synthesis (Gregorie et al. 2020).
Based on the previous finding, the main reaction (Eq. 10 in Table 2) could modify by competing along with some parallel reactions, which shifts the CO2 conversion in (CH4) methane. These important reactions are mainly such as the reaction of reverse gas water shift (Eq. 2 reverse), Boudouard reaction (breakdown of CO2, Eq. 5 in Table 2), and decomposition of methane (Eq. 4, Table 2). DR method attains the ratio of CH4/CO2 close to 1-1.5 (Verma and Samanta 2016; Serrano-Lotina et al. 2012). The essential issue of DR method is the coke establishment that causes the deactivation of catalyst and blockage of the pipes and reactors (Alves et al. 2013). Pt-, Ru- and Rh-based catalysts improve the H2 generation rate, but they are more expensive, very limited availability, and not established at an industrial scale. Generally, common catalysts such as Co- and Ni- based catalysts are more used (Cai et al. 2006).
2.5 Dry Oxidation Reforming (DOR)
This method has been established for governing the congestion of carbon formation over catalyst surface. It is a joined approach of DR and POR process. This process decreases the energy demand of reaction because the exothermic for partial oxidation releases high temperature that is beneficial to the endothermal and dry oxidation reforming process. The ratio of H2/CO also increases and achieving yield of H2 approximately 60% (Gregorie et al. 2020).
There are additional advantages for this process where parallel feeding of CH4 with CO2 and O2 such as the improved conversion of methane, decreased total energy of the process, higher catalyst activity, improved H2 yield at lower temperatures, and enhanced resistance of deactivation (Chen et al. 2010). Eq. 11 represents the DOR process where β is stoichiometric ratio of CO2 fed with the traditional DR (Table 2). If β value is 1, the reaction would be non-oxidative type DR reaction; now, if β value is 0, the reaction would be partial oxidation hence, as the β value must be in between 0–1(Alves et al. 2013; Verma and Samanta 2016; Nalbant and Colpan 2020).
In the DOR process, the process of changing parameters like reaction temperature and O2 concentration feed supports the control of the ratio of H2/CO and the appearance of reaction such as exothermic or endothermic. At a specific temperature, the nature of exothermic is increased by the increasing concentration of O2. As known, the reaction of POR (Eq. 7) is exothermic and reaction of DR (Eq. 10) is endothermic by nature. Therefore, DOR method can be used as a preferable substitute over the convention DR method using various molar ratios of oxygen. Thus, the production of syngas from biogas through this method has improved energy performance and requires less exterior energy (Avraam et al.2010; Alves et al. 2013).