Hydrogen Production from Biogas: Methods and Economic Analysis

In present days, H 2 is mainly produced via natural gas from thermochemical methods. The importance of renewable H 2 production has increased due to energy issues, global environmental, fossil fuel depletion, and pure hydrogen demand. Biogas is produced from anaerobic digestion or carbon-based waste fermentation methods. It might be a substitute resource to the creation of hydrogen. Methane (CH 4 ) and carbon dioxide (CO 2 ) are two signicant constituents of biogas, and these are multipurpose feedstock materials for the generation of valuable chemicals and fuels. This paper presents major hydrogen generation techniques from biogas reforming in detail and a short discussion of biogas reforming's economic analysis. The effective operating parameters such as reaction temperature, gas hourly space velocity and O 2 /CH 4 ratio are also discussed. Biogas has more potential; however, it needs more research on some issues such as deactivation of the catalyst, purication of biogas and removable of H 2 S. Various reforming methods can be nominated for H 2 production depend on biogas composition, the need for hydrogen purity, investment availability and quantity of the desired hydrogen.


Introduction
Growing concern about worldwide climate issues and diminution of fossil fuels, the utilization of renewable energy resources has been more focussed (Avraam et al. 2010). A large scale of H 2 production developed via reforming techniques, mainly natural gas (CH 4 ) and light hydrocarbons (HCs), used in the chemical industry eld (Alves et al. 2013). Hydrogen is abundant and is the most available in renewable energy. Furthermore, only water vapor is produced from the combustion of hydrogen. At present, biogas is an important group from renewable energy sources.
In the present scenario, hydrogen is produced from fossil fuels around 96% and 1% from biomass (Sinha and Pandey 2011). Currently, application H 2 as raw material is widely used in a combustion engine, fuel cell, fertilizer industries, chemical industry, pharmaceutical industry, hydrogenation process, Fischere-Tropsch synthesis, production of ammonia and ammonia, among others (Armor 1999).
Overall, biogas reforming is a favorable and attractive method to generate clean hydrogen gas. The main biogas bene ts for reforming purpose in H 2 production are decreased greenhouse gas emissions, local availability, reliability and better environment, and economic (Nalbant and Colpan 2020). The key motive of this paper is to express the numerous biogas reforming technique for H 2 production and its economic analysis.

Reforming Of Biogas For H2 Production
There are several approaches for the invention of H 2 . 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 (CH 4 ) 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 CO 2 by using shift reaction (Alves et al. 2013 There are some puri cation 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 H 2 from biogas.
Consequently, the biogas reforming reactions are parallel to natural gas (CH 4 ) 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 H 2 production from biogas or methane as follows.

Steam Reforming (SR)
This process is done by methane and water (H 2 O) vapor with the catalyst that generates syngas (mixture of CO and H 2 , 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 H 2 in between 60 to 70% (Verma and Samanta 2016). The ratio H 2 /CO is three that means the higher yield of H 2 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 H 2 and CO 2 (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 CO 2 , CO, and other gases except H 2 . 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 gasi cation 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 H 2 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 signi cant development in separation unit process through the selective membrane reactors or lters (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

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 di culty 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 CO 2, as denoted by Eq. 9 (Table 2 '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

Dry Reforming (DR)
In this approach, CH 4 reacts with CO 2 and obtains the mixture of CO and H 2 (Syngas) (Verma and Samanta 2016), refer Eq. 10 Table 2. This process requirements temperature more than 640 o C 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 CH 4  Based on the previous nding, the main reaction (Eq. 10 in Table 2) could modify by competing along with some parallel reactions, which shifts the CO 2 conversion in (CH 4 ) methane. These important reactions are mainly such as the reaction of reverse gas water shift (Eq. 2 reverse), Boudouard reaction (breakdown of CO 2 , Eq. 5 in Table 2), and decomposition of methane (Eq. 4, Table 2). DR method attains the ratio of CH 4 /CO 2 close to 1-1.

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 bene cial to the endothermal and dry oxidation reforming process. The ratio of H 2 /CO also increases and achieving yield of H 2 approximately 60% (Gregorie et al. 2020).
There are additional advantages for this process where parallel feeding of CH 4 with CO 2 and O 2 such as the improved conversion of methane, decreased total energy of the process, higher catalyst activity, improved H 2 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 CO 2 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 O 2 concentration feed supports the control of the ratio of H 2 /CO and the appearance of reaction such as exothermic or endothermic. At a speci c temperature, the nature of exothermic is increased by the increasing concentration of O 2 . 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 (  More current results in order to stream reforming connecting membrane reactors con rm that more than 20% increases the system e ciency by considering the membrane reactor. Indeed, the system e ciencies are informed closely to 25-28% along with a fraction of S/C = 3 for a traditional ATR method and between 46-52% and the fraction of S/C = 4 to traditional SR. At the same time, the membrane reactor Carbon Capture and Storage (CCS), which is the method to decrease carbon emissions from the sources of natural gas. Hence, biogas and decentralized generation seem fascinating renewable sources and carbon-free hydrogen production, either if few additions are expensive about impurities removal for biogas reforming like sulphur and siloxanes. Additionally, the dry reforming technique seems to be interesting for biogas reforming and mainly depends on biogas compounds as a reactant.

Role Of Operating Parameters
The advancement conversion of biogas needs to address thermodynamic problems such as the need for temperature and high pressure to raise the cost of energy intake. Biogas conversion to syngas requires high temperature (500-800°C) due to endothermic reaction (Chen et al. 2017).

Reaction Temperature
Reaction temperature has found more in uencing parameters on catalyst activity in biogas conversion to syngas. Ni-based catalysts are broadly utilized for biogas conversion to syngas due to their low prices and more potential of catalytic activity than other metals ( Over the catalyst, low variance between the renovation of CH 4 and CO 2 at a temperature less than 450°C or higher than 750°C, RWGS was unfavorable due to endothermic reaction. Conversion of CO 2 was appeared low due to the favorable condition of Boudourad equation. At a temperature of more than 750°C, methane decomposition, methane steam reforming could occur; thus, methane conversion may improve, approaching the transformation of CO 2 . A rise in ratio of H 2 /CO was detected as the temperature is increased. However, there was below unity in all cases what recommended that reaction of RWGS was always occur but in low range when temperature is raised (Chen et al.2005;Yasyerli et al.2011). A detail of the effective terms in catalytic conversion of methane to syngas using Ni-based catalysts is offered in Table 3. Table 3 The impact of operating parameters on methane conversion to syngas at 1 atm.

Conclusion
Reforming biogas is a promising method to generate green H 2 and minimize the overload on natural gas (CH 4 ). The main di culties of biogas reforming methods are raised associated with coke establishment over the catalyst surface done by most researchers and specialists. It also reviewed related to toxic conditions on account of sulfur content which causes catalyst inactivation and diminishes the production of H 2 . Ni-based catalysts are mostly used in miscellaneous reforming techniques such as natural gas reforming and biogas reforming. A huge quantity of examining works is also attentive to the effectiveness of catalyst support and some additional promoter elements to reduce the issues of coke formation.
The utilization of catalyst in the reaction of H 2 generation must be advanced to a longer usage life cycle to avoid coke formation over the active catalyst surface. The ideal catalysts are Ni-based in reforming processes because of their lower cost. DR and DOR methods are appeared more attractive techniques for H 2 generation from biogas. However, these methods are not favorable as carbon development on the surface of catalysts and extra oxidant is needed.
ATR process could be attractive because of higher conversion e ciency and H2 yields; however, this technique is a highly complex control system. At present, mostly H 2 generation form biogas is attained in steam reforming method though conventional reformer. However, it is possible to obtain H2 from biogas with advanced membrane reactors (only in one step) and at low temperatures (i.e., reforming process and separation happens at the same location).
GHSV is an essential parameter to design and optimization purpose in steam reformers and has more effect on conversion on methane distribution of temperature inside the reactor. Important operating parameters such as temperature mostly utilized the range from 700-800 o C for maximum conversion of CH4 and CO2 and different catalysts as reviewed by this present paper.
The economic analysis point of view that more current results to stream reforming connecting membrane reactors con rm that more than 20% increases the system e ciency by considering the membrane reactor. The cost of H 2 production from biogas must depend on methane content in biogas. Hydrogen generation cost from biogas (0.07 €/ kWh) is higher than natural gas (0.03 €/kWh).
Various reforming methods can be nominated for H 2 production depend on biogas composition, the need for hydrogen purity, investment availability, and quantity of the desired hydrogen.

Declarations
Ethics approval and consent to participate: Not applicable.
Consent for publication: Not applicable.
Availability of data and materials: The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Funding: There is no funding.
Competing interests: The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper. The ow process of hydrogen generation from biogas (Nalbant and Colpan 2020) Figure 2