Olumide Wesley Awe

Yaqian Zhao

Ange Nzihou

Doan Pham

Nathalie Lyczko

A Review of Biogas Utilisation, Purification and Upgrading Technologies

Keywords: Biogas upgrading, Biogas purification, Biogas utilization, Biomethane, CO 2 removal, H 2 S removal

of various technologies, recommendations are made on further research on the appropriate low cost technologies, especially using solid waste as low cost materials for biogas purification and upgrading.

Introduction

Biogas is a renewable energy resource that can be an alternative solution for the world insatiable energy demands and at the same time help in reducing waste and the greenhouse gases (GHG) emissions. It is also regarded as carbon neutral because the carbon in biogas comes from organic matter (feedstocks) that captured this carbon from atmospheric CO 2 over relative short timescale.

Biogas is produced from the methanation of biomass and organic wastes from sewage sludge anaerobic digestion, commercial composting, landfills, biomass gasification (thermos-chemical production process), animal farm manure anaerobic co-digestion with energy crops, agrofood industry digestion facilities in both mesophilic (35 °C) and thermophilic (55 °C) conditions. These activities produced biogas that is rich in methane (CH 4 ), with higher heating value range from 15 to 30 MJ/Nm 3 [START_REF]Swedish Gas Technology Centre: Basic data of biogas[END_REF][START_REF] Hosseini | Development of biogas combustion in combined heat and power generation[END_REF]. Raw biogases from anaerobic degradation of sewage sludge, livestock manure, and agro-industrial bio-waste are generally composed of methane (55-70%), and carbon dioxide (30-45%). Other gases (contaminants) present are nitrogen (0-15%), oxygen (0-3%), water (1-5%), hydrocarbons (0-200 mg m -3 ), hydrogen sulfide (0-10,000 ppm v ), ammonia (0-100 ppm v ), and siloxanes (0-41 mg Si m -3 ) Abstract Biogas is a valuable renewable energy and also a secondary energy carrier produced from biodegradable organic materials via anaerobic digestion. It can be used as a fuel or as starting material for the production of chemicals, hydrogen and/or synthesis gas etc. The main constituents of biogas are methane (CH 4 ) and carbon dioxide (CO 2 ), with various quantities of contaminants, such as ammonia (NH 3 ), water vapour (H 2 O), hydrogen sulfide (H 2 S), methyl siloxanes, nitrogen (N 2 ), oxygen (O 2 ), halogenated volatile organic compounds (VOCs), carbon monoxide (CO) and hydrocarbons. These contaminants presence and quantities depend largely on the biogas source, which could be anaerobic digestion of many substrates and landfill decompositions. The removal of these contaminants especially H 2 S and CO 2 will significantly improve the quality of the biogas for its further uses. In parallel, biogas upgrading market is facing challenges in term of operating costs and energy consumption. The selection of appropriate technology depends on the specific biogas requirements, site specific, local circumstances and is case sensitive. This paper reviews the present state-of-the-art of biogas cleaning and upgrading technologies, including its composition, upgrading efficiency, methane recovery and loss. In addition, biogas production, utilization and the corresponding requirements on gas quality for grid injection and vehicle usage are investigated. Based on the results of comparisons [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF][START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF]. Biogas produced from landfills is some complex mixtures, which composed of methane (35-65%), carbon dioxide (15-40%), hydrogen (0-3%), carbon monoxide (0-3%), nitrogen (5-40%), oxygen (0-5%), water (1-5%), halogenated hydrocarbons ppm v Cl -/F -), hydrogen sulfide (0-100 ppm v ), ammonia (0-5 ppm v ), volatile organic compound (0-4500 mg/m 3 ), and siloxanes (0-50 mg Si m -3 ) [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF][START_REF] Bailón Allegue | Biogas upgrading Evaluation of methods for H 2 S removal[END_REF][START_REF] Bailón Allegue | Biogas and bio-syngas upgrading[END_REF][START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF]. Typical compositions of different types of biogas which are comparable with natural gas and the possible impact of the contaminants are shown in Table 1.

Carbon dioxide (CO 2 ) is a recalcitrant gas that reduces the density and decreases the calorific value of the biogas, but it is not toxic and corrosive like H 2 S. This last one is harmful to the environment and corrosive to the metallic parts of engines, pumps, compressors, gas storage tanks, valves and reduce the lifespan of process equipment [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Huertas | Removal of H 2 S and CO 2 from biogas by amine absorption[END_REF]. Contaminative components in biogas have to be removed before any eventual utilization. Basically there are two steps involved in biogas treatment; cleaning (removal of harmful and toxic compounds such as H 2 S, N 2 , O 2 , Si, H, VOCs, CO, and NH 3 ), and upgrading (adjustment of CO 2 content, to increase the calorific value of the biogas to optimal level). Biomethane is the final product which composed of CH 4 (95-99%) and CO 2 (1-5%), with little or no trace of H 2 S [START_REF] Abatzoglou | A review of biogas purification processes[END_REF]. Many biogas upgrading technologies has been developed in recent years, and their main differences are related to the nature of the operation. These include; physical, chemical, and biological, their efficiency and operational conditions, investment and maintenance costs. These technologies are still highly energy and chemical intensive, which has prompted the rapid development of biogas upgrading based on biotechnologies, because of their superior economic and environmental sustainability [START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF]. Therefore, as biogas upgrading market and technologies are rapidly evolving, there is need for frequent evaluation of these state-of-the-art technologies. This paper reviews the current available technologies for the removal of biogas contaminants, with special focus on H 2 S, CO 2 , H 2 O, O 2 , N 2 , and siloxanes removal. The potentials and limitations of these technologies are also highlighted. In addition, new novel research by the authors on the valorization of calcium carbonate-based solid wastes, for removing the contaminants from the biogas stream, was also discussed.

Recall of Biogas Production and Utilization

Due to its increased usage of biogas in many ways, such as production of heat and steam, electricity production and co-generation, vehicle fuel, feedstock for the production of bio-based chemicals and substrate in fuel cells, starting reactants in chemical processes, substitute for natural gas for domestic and industrial use, and gas grid injections [START_REF]Swedish Gas Technology Centre: Basic data of biogas[END_REF], biogas production shows an increased trend in recent years. It should be noted that the final use of biogas is determined by its composition, the upgrading process required, national framework such as tax systems, subsidies, availability of heat and gas grids. Different countries have different defined standard and specifications for utilization as vehicle fuel or for grid injection of upgraded biogas (Table 2). In Europe it is called gas grid injection and outside Europe, it is SoCalGas (Southern California Gas Company) "Rule 30", which is stricter than European one and even Pacific Gas & Electric's "Rule 21". This is because of incremental specification for 17 "constituents of concerns" (CoCs) trace elements (it identified 17 for landfill gas, and 9 for digester gas) [START_REF] Svensson | Biomethane standards: Gas quality standardisation of biomethane, going from national to international level[END_REF][START_REF] Johnston | Breaking down renewable natural gas injection barriers[END_REF]. The energy content (calorific value) of biogas is that 1 Nm 3 (volume) of biomethane contains 10 kWh primary energy, equivalent to 36 MJ, while natural gas energy content is 11.0 kWh. Considering 1 L of petrol equals to 9.06 kWh, implying that 1 Nm 3 biogas corresponds to 1.1 L of petrol while 1 Nm 3 of natural gas corresponds to about 1.2 L of petrol. This is the energy content of biogas in relation to other fuels. Other biogas utilization technologies and H 2 S requirements are shown in Table 3.

Table 2 Vehicle and grid injection specification standard requirement in some countries [START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF][START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF][START_REF] Svensson | Biomethane standards: Gas quality standardisation of biomethane, going from national to international level[END_REF][START_REF] Petersson | Biogas upgrading technologies-developments and innovations. Task 37-Energy from biogas and landfill gas[END_REF] a France allows flexibility in O 2 and CO 2 under certain conditions b Nertharlands also allowed <10-10.3 for regional grid

Biogas Upgrading Technologies

Biogas purification and upgrading had been researched extensively in recent years. Hosseini and Wahid [START_REF] Hosseini | Development of biogas combustion in combined heat and power generation[END_REF] reviewed biogas purification processes with the focus on contaminants removal, while Ryckebosch et al. [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF] reported on various techniques for biogas transformation regarding their conditions, functioning, bottlenecks and efficiency. Bauer et al. [START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF] reviewed the commercial technologies on biogas upgrading. Most recently, Munoz et al. [START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF] provided a state of the art review on the biogas upgrading technologies with emphasis on biotechnologies for CO 2 , H 2 S, Siloxanes and halocarbon removal. Sun et al. [START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF] worked on appropriate biogas upgrading technology, focusing on product purity and impurities, methane recovery and loss, upgrading efficiency, investment and operating cost.

From the literature, the main techniques for biogas upgrading and purification are: water scrubbing, adsorption (physical and chemical), cryogenic separation, membrane technology, biological upgrading and in-situ upgrading methods.

CO 2 Removal from Gas Stream

Removal of CO 2 is necessary in order to increase the density and calorific value of the gas to meet Wobbe Index quality and specifications (Table 2). The current technologies are: pressure swing adsorption, physical absorption (water and organic solvent scrubbing), chemical absorption, cryogenic separation, membrane separation, biological methane enrichment. These are discussed in the order of old, current trends, and future technologies and ideas.

Physical Absorption (Water Scrubbing and Organic Solvent Scrubbing)

The separation principle of absorption is based on different solubility of various gas components in a liquid scrubbing solution. The absorbed gas components are physically bound to the scrubbing liquid. In this case water is a selective absorbent and it is widely used in water scrubbing of biogas at an industrial scale, with 41% share of the global biogas upgrading market because of its less sensitivity to biogas impurities [START_REF] Thrän | Biomethane status and factors affecting market development and trade[END_REF]. The solubility of CH 4 is 26 times lower at 25 °C than that of CO 2 [START_REF] Gavin | Chemical engineering design: principles, practice and economics of plant and process design[END_REF]. Also according to Persson et al. [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF], the different biding forces of the more polar CO 2 or H 2 S and the nonpolar methane are used to separate these compounds.

Therefore, H 2 S can be removed together with CO 2 in principle because the solubility of H 2 S in water is higher than that of CO 2 . It is however advisable to separate the H 2 S prior to CO 2 removal because the dissolved H 2 S is very corrosive and odour nuisance can cause operational problem. Biogas scrubbing is carried out in a column packed with Pall or Raschig to support an efficient gas-liquid mass transfer, in a countercurrent (compressed gas at 6-10 bar the bottom and water pressures from the top) [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF]. The used water is regenerated in desorption column with, either air or steam that release the CO 2 from the water at a decreased pressure (Fig. 1). It should be noted that N 2 and O 2 cannot be separated because they are non-condensable. Energy consumption is for gas compression, recirculation pumps and water regeneration. Constant purging of water is necessary to avoid H 2 S poisoning and fouling.

An organic solvent such as methanol and dimethyl ethers of polyethylene glycol (DMPEG) can be employed in CO 2 removal. The solvent can simultaneously absorb CO 2 , H 2 S and H 2 O since they have higher solubility in polyethylene glycol than CH 4 . The scrubbing and regeneration process is the same as water solvent (Fig. 1). The trade names for the solvents are Selexol ® and Genosorb ® and they exhibit higher affinity for CO 2 and H 2 S than water by five times, especially Selexol ® , which is a mixture of dimethyl ethers of polyethylene glycol [START_REF] Tock | Thermochemical production of liquid fuels from biomass: Thermo-economic modeling, process design and process integration analysis[END_REF]. As a result, smaller absorbent volume is needed with compact size and little pumping with the same quantity of biogas, thereby reducing the investment and operating cost. According to Bauer et al. and Sun et al. [START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF][START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF], a consistent biomethane content of 96-98.5% and less than 2% methane losses, can be achieved in an optimized full-scale plant, with the similar energy consumption as water scrubbing [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Huertas | Removal of H 2 S and CO 2 from biogas by amine absorption[END_REF][START_REF] Thrän | Biomethane status and factors affecting market development and trade[END_REF]. This process can produce high purity CO 2 , and additional drying of the upgraded gas is not necessary due to the fact that water and halogenated hydrocarbon (from landfill) are absorbed by glycol [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF].

Chemical Absorption

This is the same way as water/glycol scrubbing for biogas-liquid mass transfer principles, but chemical reaction takes place between the solvent and the absorbed substances. It relies on CO 2 reactive absorbents such as alkanol amines (mono ethanol amine (MEA)) or di-methyl ethanol amine (DMEA), and alkali aqueous solutions such as KOH, K 2 CO 3 , NaOH, Fe(OH) 3 , FeCl 2 [START_REF] Lasocki | Laboratoryscale investigation of biogas treatment by removal of hydrogen sulfide and Carbon Dioxide[END_REF]. The unit consists of a packed bed (random or structural) coupled with desorption unit equipped with reboiler to simplify process configuration, with no risk of biomass growth as result of high pH of the amine solutions [START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF]. Amine solution is widely used to absorbed CO 2 and there is little or no methane losses (0.1-1.2%), with methane recovery greater than 99% because the chemical solvent reacted selectively with CO 2 .

Although some reports [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Huertas | Removal of H 2 S and CO 2 from biogas by amine absorption[END_REF][START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF][START_REF] Lasocki | Laboratoryscale investigation of biogas treatment by removal of hydrogen sulfide and Carbon Dioxide[END_REF] reported methane losses up to 4% due to CH 4 dissolution in alkanolamine.

It also operated like water scrubbing countercurrent flow configuration, and H 2 S should be treated prior to chemical scrubbing because of amine poisoning (Fig. 2). The regeneration is usually accomplished with steam or heat and CO 2 with purity of 93% reportedly recovered in the process [START_REF] Deng | Techno-economic evaluation of biogas upgrading process using CO2 facilitated transport membrane[END_REF]. Despite its high CO 2 removal efficiency, energy consumption is relatively high (high regeneration heat), there is likely possibility of salt precipitation and foaming to occur coupled with O 2 poisoning of amine and other chemicals [START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF][START_REF] Deng | Techno-economic evaluation of biogas upgrading process using CO2 facilitated transport membrane[END_REF].

Pressure Swing Adsorption (PSA)

The process take place in vertical columns packed with absorbents under adsorption, depressurization, desorption and pressurization sequences, and the molecular sieves material is regenerated (Fig. 3). In the pressurized column methane-rich gas passed through while CO 2 is adsorbed. The most commonly used adsorbents are zeolite, activated carbon, activated charcoal, silica gel and synthetic resins. This can be used to separate CO 2 , N 2 , O 2 and H 2 S from the gas streams by selectively adsorption of CO 2 over CH 4 onto the porous adsorbents with a high specific area, since CH 4 molecule is larger than the other gas molecules [START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF]. It is also advisable to dry (cool) the gas and also remove H 2 S prior to CO 2 adsorption because H 2 S will adsorbed irreversibly to the molecular sieves [START_REF] Bailón Allegue | Biogas and bio-syngas upgrading[END_REF].

After saturation with CO 2 , biogas is led to a new column, while CO 2 saturated column is stepwise depressurized (almost atmospheric pressure), to release CO 2 /CH 4 mixture with high methane content, which is vacuum and recycle back to the PSA inlet. According to Bauer et al. [START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF], several columns were linked together to create a continuous operation and to reduce energy need for gas compression. The CH 4 recovery rate of 96-98%, with 2-4% methane losses had been reported [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF]. The off-gas must be treated (burnt in a flox burner) to avoid release methane to the atmosphere, or use for combustion.

Cryogenic Separation of CO 2

This new technology uses temperature difference to separate the gases. CO 2 has a boiling point of -78 °C, while methane is -160 °C, resulting in CO 2 separation from the biogas by cooling the gas mixtures at elevated pressure (Fig. 4). This difference in condensation temperatures can be exploited to separate other gases like N 2 , O 2 and siloxanes from the biogas through condensation and distillation. According to Hosseini and Wahid [START_REF] Hosseini | Development of biogas combustion in combined heat and power generation[END_REF], the process is of specific value when the final product is liquid biomethane (LBM), which is equivalent to liquid natural gas (LNG). The process is more advantages for treating landfill gas, for pure CO 2 production (98%), high-purity methane with less than 1% losses and can be utilized directly for vehicles or injected to grid as gas. The energy consumption is however high with raw biogas compressed at 200 bar, which can amount to 5-10% of the biomethane produced [START_REF] Bailón Allegue | Biogas and bio-syngas upgrading[END_REF][START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF][START_REF] Johnston | Breaking down renewable natural gas injection barriers[END_REF].

Membrane Separation

The separation is based on the selective permeability property of the membranes, which can be gas-gas separation (gas phase on both side of the membranes) or gas-liquid separation (liquid absorbs the H 2 S and CO 2 molecules diffusing through the membranes). The liquid solution can be amine and the system is highly selectively compared to the solid membrane systems, and operated well at low pressure [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF]. The material used is micro porous hydrophobic membrane, which separates the gaseous molecules stream flowing in one direction with the liquid flowing in countercurrent flow from the other side and diffused through the membrane. The amine solution can be regenerated with heating to release the CO 2 , which can be collected separately (Fig. 5).

On the other hands, the gas-gas separation works at high pressure greater than 20-40 bar or at lower pressure of 8-10 bar, resulting in 92-97% methane production [START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF]. This technology allows CO 2 , H 2 S, H 2 O, O 2 pass through the membrane to the permeate side while retaining CH 4 on the inlet side. Some methane losses are possible since the CH 4 may also pass through the membrane in an effort to achieve higher purity [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF][START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF]. Higher purity of CH 4 can be achieved with larger size or several membranes in series. Therefore, there is need to balance the desire for high purity methane and low methane loss. Recirculation of off-gases can help in reducing this loss. Membrane is based on different molecules of different sizes with different permeability through the membrane, and is also driven by pressure difference between the two sides of the membrane and the biogas temperature [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF]. Deng and Hagg [START_REF] Deng | Techno-economic evaluation of biogas upgrading process using CO2 facilitated transport membrane[END_REF] worked on the efficiency of CO 2 -selective polyvinylamine/polyvinylachohol blend membrane and reported that the optimal process can deliver 98% CH 4 purity. Scholz et al. [START_REF] Scholz | Transforming biogas into biomethane using membrane technology[END_REF] reviewed membrane based technologies used for commercial biogas upgrading. He reported that single stage permeation process are not able to produce high CH4 purity and simultaneously obtain a high CH4 recovery. He concluded that, multistage concepts are mandatory.

Hydrate Formation

The process was firstly proposed by Yoon and Lee [START_REF] Yoon | Clathrate phase equilibria for the water-phenol-carbon dioxide system[END_REF] who investigated clathrate phase equilibrium for the water-phenol-carbon dioxide system based on the equilibrium partition of the components between gaseous phase and the hydrate phase. Kang et al. [START_REF] Kang | Gas hydrate process for recovery of CO 2 from fuel gas[END_REF] used this principle to work on gas hydrate process of recovery of CO 2 from fuel gas. According to Tajima et al. [START_REF] Tajima | Energy consumption estimation for greenhouse gas separation processes by clathrate hydrate formation[END_REF], the basic mechanism of the separation process is a selective partition of the target component between the hydrate phase and the gaseous phase. Generally, the hydrate phase is stable under high pressure-low temperature conditions. This has attracted other researchers to look at the possibility of employing this technology to reduce CO 2 , H 2 S, and other contaminants from biogas and syngas streams. For example, Wang et al. [START_REF] Wang | Study on the recovery of hydrogen from refinery (hydrogen + methane) gas mixtures using hydrate technology[END_REF], investigated the recovery of hydrogen from refinery (hydrogen + methane) gas mixtures using hydrate technology, while Yang et al. [START_REF] Yang | Progress and perspectives in converting biogas to transportation fuels[END_REF] worked on progress and perspectives in converting biogas to transportation fuels. The process has also been successfully utilized to remove CO 2 from contaminated natural gas. According to [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF], there was 16% reduction in the CO 2 content from CH 4 /CO 2 ratio of 75/25%, but methane loss was still relatively high. It was also reported that the CO 2 capture by hydrate formation consumes a large amount of energy, as a result of high pressure required for hydrate formation [START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF][START_REF] Tajima | Energy consumption estimation for greenhouse gas separation processes by clathrate hydrate formation[END_REF][START_REF] Wang | Study on the recovery of hydrogen from refinery (hydrogen + methane) gas mixtures using hydrate technology[END_REF][START_REF] Yang | Progress and perspectives in converting biogas to transportation fuels[END_REF]. This technology can be improved upon to reduce energy consumption, methane loss and simplify the separation process for adaptable for future use.

Biological Methane Enrichment (Biotechnological)

Strevett et al. [START_REF] Strevett | Chemo-autotrophic biogas purification for methane enrichment: mechanism and kinetics[END_REF] reported the microbial conversion of CO 2 and H 2 to CH 4 based on the ability of hydrogenotropic methanogens capacity to use CO 2 as their carbon source and electron acceptor, and H 2 as electron donor in the energy yielding reaction.

Other studies of CO 2 conversion using H 2 as electron donor were also reported in recent years. [START_REF] Kim | Reduction in carbon dioxide and production of methane by biological reaction in the electronics industry[END_REF] used CO 2 off-gases from electronic production waste to produce methane under mesophilic and thermophilic conditions. Luo et al. [START_REF] Luo | Simultaneous hydrogen utilization and in situ biogas upgrading in an anaerobic reactor[END_REF] worked on simultaneous H 2 utilisation in in-situ biogas upgrading with CH 4 content of 95% under various conditions, while Bauer et al. [START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF] investigated process performance and microbial activities during codigestion of manure with whey. Several microorganisms' species from Archaeal family such as Methanobacterium sp., Methanothermobacter sp., Methanococcus ap., Methanosarcina sp., Methanosaeta sp., Methanoculleus sp. and Methanospirillums sp. were reported in stand-alone bioreactors and anaerobic digesters upgrading CO 2 to CH 4 by H 2 injection [START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF][START_REF] Kim | Reduction in carbon dioxide and production of methane by biological reaction in the electronics industry[END_REF][START_REF] Luo | Simultaneous hydrogen utilization and in situ biogas upgrading in an anaerobic reactor[END_REF].

It is equally possible to upgrade syngas to methane since biomass gasification process contained CO, CO 2 , H 2 , based on the ability of methanogens to convert CO to CH 4 and CO 2 , with the following reaction [START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF]: Also, Yan and Zheng [START_REF] Yan | Performance of mixed LED light wavelengths on biogas upgrade and biogas fluid removal by microalga Chlorella sp[END_REF] employed microalga photosynthesis to sequester anthropogenic CO 2 and reported that CH 4 content of the upgraded gas could reach 90-95%.

(1)

4H 2 + CO 2 → CH 4 + 2H 2 O (ΔG o = -131KJ) (2) 4CO + 2H 2 O → CH 4 + 3CO 2

Perspectives on CO 2 Removal

Table 4 summaries the advantages and disadvantages of the technical alternatives and features of CO 2 removal from biogas streams. The chemical and physical absorption technologies are feasible and had been used widely in practice. Water as absorbent is cheap, efficient and environmentally friendly, but required intensive energy for pressurization and regeneration. More effective absorbents are very expensive and usually have negative environmental impacts. Yet, these technologies can be improved, especially their efficiency and reduction in their energy demand by looking at possibility of upgrading biogas at low temperature and pressure to save cost. Ideally, biogas produced exit digesters at minimum 35 °C, heat exchanger can be used to increase or decrease the heat need for various upgrading systems. For example, the saved heat can be used for water regenerations in water scrubbing, PSA, steam generation during the stripping of CO 2 for water and solvent recirculation.

Innovative approach to solvent based CO 2 absorption is required, and this must be economically viable with minimum environmental impacts. This will also depend on the selection of appropriate absorbents, the separation process, the intensity of the energy requirement, effective integration of biogas purification, and the upgrading process to save cost.

Cryogenic is a promising upgrading technology for CO 2 removal, especially with capability to produce liquid CO 2 , which can be utilized in technologies that requires pure CO 2 . It can also produce liquid biomethane (LBM) which is equivalent of liquid natural gas (LNG) for transportation fuel [START_REF]Swedish Gas Technology Centre: Basic data of biogas[END_REF]. The energy consumption is very high for separating the gases into various components, but with recent interest in LBM production, the solution to the existing problem can be overcoming through research and development.

Hybrid systems involving the integration of two or more purifications and upgrading methods in order to exploit their advantages and also bridge their drawbacks is another possible option for CH 4 /CO 2 separation. The integration of different technologies into one unit will lead to reduction in CH 4 losses, minimising capital cost and recurrent cost, reduce footprint of the conditioning systems. Innovative solution can overcome other impurities such as H 2 S degradation or contamination of the absorbent, and thereby allow the absorbent to be easily and cheaply regenerated with minimum of gases concentration.

It is worthwhile noting that the valorization or the storage of CO 2 recovered is also an option in biogas upgrading. However, it is out of the scope of the review.

H 2 S Removal from Biogas Stream

Hydrogen sulfide along with other S bearing compounds (mercaptans etc.) are the most common contaminants in biogas and their quantity, which can vary from 100 to 10,000 ppm, depends largely on the composition of the organic matter (but, mostly protein-rich). They must be removed before any utilization because they are highly corrosive to pipes, pumps and engines and they have environmental concerns due to their conversion to sulfur dioxide (SO 2 ) and sulfuric acid (H 2 SO 4 ) [START_REF] Hosseini | Development of biogas combustion in combined heat and power generation[END_REF]. Ammonia, like H 2 S, is highly corrosive when combusted and can transformed into nitrogen oxides (NO x ), which are part of greenhouse gases that are causing climate change and polluting the environment. As such, H 2 S and NH 3 need to be removed early in the upgrading process, preferably in the digestion chambers or in the gas stream of the upgrading process.

Physical and Chemical Absorption

The upgrading process takes place in conventional gas-liquid contactors (packed bed or spray towers) using either water or organic solvent in the physical absorption process or using aqueous chemical solutions to convert H 2 S to elemental sulfur or metal sulfide [START_REF] Schiavon | Purification of biogas for energy use[END_REF]. Single pass absorption and regenerative absorption are commonly used, but high water consumption is needed without the regeneration step. Therefore, Selexol (mixture of dimethyl ethers of polyethylene glycol), which exhibits higher affinity for H 2 S than water by five times can be used, resulting in smaller absorbent volume with compact size and require regeneration step [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF]. This is suitable for the removal of low concentration of H 2 S or in combination with CO 2 removal (Fig. 6).

To achieve an efficient H 2 S mass transfer, the addition of chemical reagents, such as NaOH, FeCl 2 , Fe 3+ /MgO, Fe(OH) 3 , Fe 3+ /CuSO 4 and Fe 3+ /EDTA (ethylenediaminetetraacetate), will reduce liquid to gas ratio because of the quick chemical reactions with these solvent [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF]. Petersson and Wellinger [START_REF] Petersson | Biogas upgrading technologies-developments and innovations. Task 37-Energy from biogas and landfill gas[END_REF] reported the reaction of H 2 S with NaOH and produced soluble salt of sodium sulfide (Na 2 S) and sodium hydrogen sulfide (NaHS), where the precipitate is not regenerated with consequence of disposal problem. The dilution of iron salt (FeCl 2 ) solvent with H 2 S is based on the formation of insoluble FeS that also needs to removed and disposed, while Fe(OH) 3 addition can remove H 2 S, resulting in formation of Fe 2 S 3 and regeneration is possible with oxygen or air [START_REF] Schiavon | Purification of biogas for energy use[END_REF].

Regarding the chemical absorption of H 2 S, an ironchelated Fe 3+ /EDTA solution (0.2 mol) is the most popular catalyst to be used, resulting in production of elemental sulfur (S) during the reduction of Fe 3+ to Fe 2+ according to the following reaction:

The regeneration of iron-chelated Fe 3+ /EDTA solution occurs by its oxygenation according to Eq. 4, followed by conversion of pseudo-catalyst into its active form Fe 3+ in Eq. 5:

Large consumption of chemical is avoided due to the regeneration Fe 3+ /EDTA solution and it is H 2 S specific, with H 2 S removal rate of 90-100% at a gas flow rate of 1 dm 3 /min with catalytic solution flowing rate of 83.6 cm 3 /min [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF]. The iron-chelated based technologies were also reported to remove 50-90% of the mercaptans presented in the biomethane, without any significant reduction in the concentration of CO 2 with operation cost of 0.2-0.3 €kgS -1 [START_REF] Hosseini | Development of biogas combustion in combined heat and power generation[END_REF]. The available materials for

(3) S 2-+ 2F 3+ → S + 2Fe 2+ (4) 0.5O 2 (g) + H 2 O(l) → 0.5O 2 (aq) (5) 0.5O 2 (aq) + 2Fe 2+ ↔ 2Fe 3+ + 2OH -

Dosing with Air/Oxygen into Biogas System (In-situ)

Microorganisms can be employed for desulphurization of biogas. This technique is based on the conversion of H 2 S to elemental sulfur by a group of specialized microorganisms through microbiological oxidation process. The microorganisms employed are Thiobacillus family and they use the CO 2 from the digester as their carbon source. Depending on the H 2 S concentration, small amount of oxygen (2-6% of air in biogas) is needed for the biological reaction to take place (Eq. 6), by injecting air directly into the digester.

H 2 S concentration can be reduced by 95% to less than 50 ppm, depending on the temperature, the reaction time, and the amount of air added. However, safety measures must be taken to avoid overdosing of air, as biogas air in the range of 6-12% is highly explosive [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF][START_REF] Petersson | Biogas upgrading technologies-developments and innovations. Task 37-Energy from biogas and landfill gas[END_REF]. However, it is worthwhile noting that this approach is not suitable for upgrading biogas to natural gas quality since excessive oxygen and inert gas use have to be remove from the treated gas [START_REF] Bailón Allegue | Biogas upgrading Evaluation of methods for H 2 S removal[END_REF].

Iron Chloride (FeCl 2 ) Dosing into the Digester (In-situ H 2 S Precipitation)

H 2 S can be reduced by adding Fe 2+ or Fe 3+ in the form of FeCl 2 , FeCl 3 and FeSO 4 2 into the digester or the influent mixing tank. According to Persson et al. [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF], FeCl 2 is the most regularly used as the Fe 2+ reacts with S 2-, leading to insoluble iron sulfide (FeS) formation (Eq. 7).

The resultant FeS can be removed from the system with discharged solids. When it is spread as fertilizer, it is oxidized by atmospheric oxygen, forming soluble salt which acts as nutrients to the plants. Although this method is effective in reducing H 2 S, but less effective to reduce it up to the level of vehicle and injection into the gas grid specifications. H 2 S reduction to 100-200 ppm has been reported, depending on the amount of iron chloride added [START_REF] Bailón Allegue | Biogas upgrading Evaluation of methods for H 2 S removal[END_REF].

Adsorption Using Iron Oxides/Hydroxides Fe 2 O 3 / (Fe(OH) 3 )

This is a process based on two parallel adsorbent columns packed with either activated carbon, iron oxide (Fe 2 O 3 ), iron hydroxide (Fe(OH) 3 ) or zinc oxide (ZnO), configured in adsorption and regeneration mode. The principle is that H 2 S reacts easily with Fe 2 O 3 , Fe(OH) 3 or zinc oxide

(6) 2H 2 S + O 2 → 2S + 2H 2 O (7) Fe 2+ + S 2- → FeS
to form insoluble FeS or ZnS, respectively. The materials used to immobilize the chemical reagents are steel wool (cover with rust), wood chips covered with iron oxide, or pellets made of red mud (aluminum manufacture waste) [START_REF] Hosseini | Development of biogas combustion in combined heat and power generation[END_REF]. However, wood chips impregnated with iron oxide are mostly used, because it has a larger surface to volume ratio than plain steel and inexpensive [START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF].

It was reported that at H 2 S concentration of 1,000-4,000 (ppm), 100 g of pellets can bind 50 g of sulfide. The conventional available materials (iron sponge) grades are; 100, 140, 190, 240 and 320 kg Fe 2 O 3 , with density of 800 kg/ m 3 . However, recently, a numbers of proprietary iron oxide materials have been offered as improved alternatives to the conventional ones, such as Sulphur-Rite ® , SulfaTreat ® , SOXSIA ® , Meda-G2 ® , and Sulfa-Bind ® [START_REF] Bailón Allegue | Biogas upgrading Evaluation of methods for H 2 S removal[END_REF][START_REF] Petersson | Biogas upgrading technologies-developments and innovations. Task 37-Energy from biogas and landfill gas[END_REF].

Adsorption on Activated Carbon

H 2 S can be removed using adsorption into non-impregnated (virgin), catalytic-impregnated, and impregnated activated carbons. The catalytic-impregnated and impregnated activated carbons will catalyze H 2 S oxidation to elemental sulfur and water at higher rate than the virgin activated carbon (AC) [START_REF] Hosseini | Development of biogas combustion in combined heat and power generation[END_REF]. As in biological desulphurization, oxygen (4-6%) can be added (catalytically) to convert H 2 S to elemental sulfur and water (Eq. 8) [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF]:

The required pressure for this reaction to occur is between 7 and 8 bar and temperature 50-70 °C. The formed elemental sulfur is adsorbed by the AC (Fig. 7). In other to improve the rate of reaction, chemical adsorption is done by impregnating the AC with alkaline or oxide such as sodium carbonate (Na 2 CO 3 ), potassium iodine (KI), sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium bicarbonate (NaHCO 3 ), and potassium permanganate (KMnO 4 ). This can enhance the removal capacity from a normal 10-20 kg H 2 S/m 3 (virgin carbon) to 120-140 kg H 2 S/m 3 (impregnated carbon) [START_REF] Bailón Allegue | Biogas and bio-syngas upgrading[END_REF]. According to Petersson and Welinger [START_REF] Petersson | Biogas upgrading technologies-developments and innovations. Task 37-Energy from biogas and landfill gas[END_REF] only KI or KMnO 4 impregnation supports the partial oxidation of H 2 S without any added oxygen. As a result, they are more preferred option for any desulphurization that will lead to biomethane injection to national grid or for vehicle fuel utilization. Two connected columns were used for continuous operation for adsorption and desorption, with heat (steam injection) and depressurization usually employed for regeneration. The drawback of this technology is the replacement of the activated carbon instead of regeneration when the solid is saturated with sulfur, and there is a growing environmental concerns over appropriate disposal methods. In addition, dust and water must be removed prior to AC treatment [START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF][START_REF] Cosoli | Hydrogen sulphide removal from biogas by zeolite adsorption. Part I. GCMC molecular simulations[END_REF].

(8) 2H 2 S + O 2 → 2S + 2H 2 O

Biological Desulphurization and Biofiltration of H 2 S

This technology employed specialized microorganisms to reduce the level of H 2 S in biogas by converting it to elemental sulfur and some sulphates, similar to the technique of addition of air or oxygen into the digestion tank. The technique is on the basis of lithautotrophic bacteria to use H 2 S as electron donor and CO 2 as carbon source, and also for the development of end-of-pipe solutions for biogas upgrading (Montebello [36]; Mora et al. [START_REF] Iovane | Experimental test with polymeric membrane for the biogas purification from CO 2 and H 2[END_REF]). About 4-6% of air/oxygen was used as electron acceptor and provided the energy needed for lithotrophic growth in order to oxidize H 2 S (Eqs. 9 & 10) [START_REF] Mora | Kinetic and stoichiometric characterization of anoxic sulfide oxidation by SO-NR mixed cultures from anoxic biotrickling filters[END_REF].

Bailon and Hinge [START_REF] Bailón Allegue | Biogas upgrading Evaluation of methods for H 2 S removal[END_REF] had proved that it is possible to use NO 3 -(NO 2 -) as electron acceptor instead of using O 2 in the biofiltration unit for the oxidation of H 2 S (Eqs. 11, 12) due to the fact that at the low O 2 /S and NO 3 -/S ratio, elemental sulfur is produced.

The sulfur oxidizing microorganisms used are mainly from the family of Thiobacillus, Thiomonas, Paracoccus, Acidithiobacillus, Sulfurimonas or Halothiobacillus [START_REF] Montebello | Aerobic biotrickling filtration for Andrea Monzón Montebello[END_REF]. The optimum temperature is in the range of 28-35 °C, with pH of 6-8. However, Montebello [START_REF] Montebello | Aerobic biotrickling filtration for Andrea Monzón Montebello[END_REF] discovered that some extremophile species such as Acidithiobacillus ferrooxidants or Acidithiobacillus thioxidants, present an optimum biocatalytic activity in the low pH range of 2-4. This technology has been implemented mainly in biotrickling filter (BTF), due to its efficient gas-liquid mass transfer, cost effectiveness, and easy control of other variables (9)

H 2 S + 0.5O 2 → S + H 2 O (10) H 2 S + 2O 2 → SO 2- 4 + 2H + (11) 3H 2 S + NO - 3 ← 3S + 0.5N 2 + 3H 2 O (12) 3H 2 S + 4NO - 3 → 3SO 2- 4 + 2N 2 + 6H +
like nutrients supply, pH and temperature [START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF][START_REF] Montebello | Aerobic biotrickling filtration for Andrea Monzón Montebello[END_REF][START_REF] Mora | Kinetic and stoichiometric characterization of anoxic sulfide oxidation by SO-NR mixed cultures from anoxic biotrickling filters[END_REF]. H 2 S can be reduced from 3000-5000 ppm to 50-100 ppm, and ammonia (NH 3 ) is also jointly removed [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF]. The packed bed columns materials for supporting biofilm growth in BTF desulphurization are pall rings, HD-QPAC or polyurethane foam etc.

Membrane Separation

This is similar with CO 2 removal process by membrane separation. The separation is based on the selective permeability property of the membranes, which can be gas-gas separation or gas-liquid separation. This allows the permeability of H 2 S while retaining the CH 4 on the other side of the membranes. In the gas-liquid separation, alkaline liquid is used on the microporous hydrophobic membrane, which can support H 2 S removal by 98% during desulphurization, leaving only 2% in the upgraded gas [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Deng | Techno-economic evaluation of biogas upgrading process using CO2 facilitated transport membrane[END_REF][START_REF] Scholz | Transforming biogas into biomethane using membrane technology[END_REF]. The gas stream molecules flow in one direction diffusing through the membrane, while H 2 S is absorbed by the liquid on the other side. Iovane et al. ( 2014) recently worked on experimental test with polymeric membrane (150 × 1210 mm) for the biogas purification from CO 2 and H 2 S and reported the H 2 S removal efficiency of 54-94% at pressures of 25-41 (N/cm 2 ).

Perspectives on H 2 S Removal

Table 5 shows the summary of the alternatives and technical features of H 2 S and other contaminants removal from gas streams. It should be note that H 2 S is poisonous and corrosive in nature, which make regeneration almost impossible, and very expensive to manage, where possible. The possible solution is to focus on research and development that can produce a system for sulfur recovery at less cost, while minimizing its impact on equipment and the environment. One possible approach is to develop a hybrid system removal along with other contaminants from biogas stream.

In this system, water scrubbing can be used to remove both H 2 S and CO 2 , and the scrubbing wastewater can be circulated and regenerated in an air stripping unit (Fig. 8). This waste and air from the scrubber can then be treated in biological disulphurisation unit (Bio-Trickling Filter). Cooling and drying of the biogas in the same process will enable the removal of water, remaining H 2 S and other remaining contaminants, such as silicon organic compounds. This process can be controlled, optimized and monitored with Programmable Logic Controller (PLC) control system, thereby ensuring that H 2 S, CH 4 , H 2 O, CO 2 and Wobbe Index are measured in real time situation.

Removal of Water

The biogas that leaves digesters is always saturated with water and the absolute water content depends on the temperature (at 35 °C, the water content of the biogas is usually 5%). Generally, the lower the temperature, the lower the water content in the raw biogas. This water must be dried if the biogas is to be used for grid injection or vehicle fuel, and even gas turbines and combined heat and power (CHP) (Tables 1,2). Water can be removed by physical separation (condensation) and chemical drying (adsorption). The physical drying methods by condensation are demisters (liquid particles are separated by wired mesh, with microspores 0.5-2 nm that can attain dew point of 2-20 °C), cyclone separators (utilizing centrifugal force to separate water droplets), moisture traps (by expansion, causing low temperature to condense water), water traps (design with biogas pipe to collect and remove water) [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF][START_REF] Bauer | Biogas upgrading-technology overview, comparison and perspectives for the future[END_REF][START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF]. However, they are less efficient in separating water since it can only decrease the biomethane dew-point to 0.5 °C, and this will lead to operational problem such as freezing the heat exchanger's surface [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF].

The chemical drying is basically absorption of water in glycol (drying agent), which has a binding component that can reduce the dew point from -5 to -15 °C, and can be regenerated at temperature of 200 °C [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF]. Water can also be dried using silica gel, magnesium oxide, activated carbon, alumina and other chemical agents that have binding components, and decrease the dew point to -40 °C under 6-10 bar [START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF]. For continuous operation, two columns of packed bed (with drying agents) are required for operation and regeneration mode in parallel. Raw biogas is passed through them at high pressures to dry it and, regenerated by evaporation through decompression and heating. Hygroscopic salts that dissolved to absorb water from the biogas can be used, but they need to be replaced when saturated [START_REF] Bailón Allegue | Biogas and bio-syngas upgrading[END_REF][START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF].

General Discussion and Suggestions for Future Development

The main purpose of biogas upgrading is aimed at: (i) purification process which is targeted specifically to remove the trace components that can affect the end-users, grid transmission, machineries, storage facilities, (ii) CO 2 removal in order to adjust (increase) the heating value and the relative density of the biogas that conforms with Wobbe Index requirements and, (iii) to a lesser extent, upgrading using dry reforming for the production of syngas (CO + H 2 ). These are used for the production of chemical via Fisher-Tropsch Process. While the biogas purification is compulsory, the biogas upgrading is optional. However, it will be beneficial if these two processes (purification and upgrading) can be integrated. This is because the contaminants removal can be carried out together with CO 2 removal simultaneously. No doubt, this can offer significant cost reduction of biogas conditioning. The purification and upgrading methods discussed extensively in this paper have the ability and capacity to simultaneously remove both the contaminants and the CO 2 . For examples, membrane separation, adsorption, cryogenic, biofiltration and biological methods can all use multi-step operation, advanced multifunctional materials, biogas recycling, contaminants conversions and other techniques to increase the efficiency of the integrated biogas conditioning system.

It may be necessary for synergy, cost saving and practical to consider siting anaerobic digestion (AD) biogas plants together with biorefineries to be able to produce biomethane for transportation fuel, biobased chemicals and other biobased materials. It will also be possible to link biogas produced from the landfills to the biorefineries complex, thereby reducing, or avoiding, the separate cost of biogas conditioning, were it is stand-alone operation. This will at the same time enable the AD biogas plants to play its roles in waste management and sustainable energy supply by utilizing on-site wastes, CO 2 waste and surplus heat and electricity for its operation.

There is also the need for further research on the use of locally available waste materials as sorbents for the removal of H 2 S, CO 2 and other biogas impurities. Materials such as anaerobic digested sludge, water treatment sludge, and waste from industrial processes are promising and cheap alternatives for biogas conditioning as the current treatment technologies are energy intensive and relatively expensive. The authors are currently working on the valorization of calcium carbonate-based solid wastes, for removing the contaminants such as H 2 S from the biogas stream. Solid wastes containing mainly calcium carbonate and various inorganic elements including Mg, AL, Fe, Si, Na were found reactive for the selective removal of H 2 S from the biogas matrix. Furthermore, the coupling of a solid waste and a commercial activated carbon (AC) allowed improving the sorption performance compared to pure commercial calcium carbonate (CaCO 3 ) and AC. The results obtained open a new promising way for the valorization of calcium carbonate-based wastes for the removal of H 2 S from the gas phase, in particular for the purification of biogas.

Another possible pathway for biogas utilization without wasting the heating value of the biogas due to flaring as a means of disposal is catalytic reforming of the biogas (Fig. 9). This has the potential to fully utilise the energy in the biogas. This option is very attractive because the availability of CO 2 and CH 4 is relatively inexpensive. Most large plants emit large amount of CO 2 and methane is being flared at many gas plants, while landfill gas contained 50% CO 2 and 50% CH 4 . Therefore, the conversion of biogas into higher-value compounds is very attractive. It is possible to explore these three catalytic reforming methods; dry reforming, steam reforming and auto-thermal for the utilization of biogas.

Dry reforming of biogas is an endothermic reaction that can directly convert CO 2 and CH 4 into H 2 and CO, known as syngas, which is a valuable gas mixture and can be used as a combustion enhancer due to its high reactivity to improve the combustion efficiency, thereby reducing the engine emission during combustion [START_REF] Bailón Allegue | Biogas and bio-syngas upgrading[END_REF]. Alternatively, the resultant syngas (low CO 2 /H 2 ratios) can be further upgraded to produce H 2 -rich feed for use in fuel cells, or in the alternative converting to liquid fuels (gasoline, kerosene, aldehydes, and alcohols) [START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF]. The only technical challenge of dry reforming relates with the deactivation of catalyst due to the carbon deposit and high temperature and metal and support sintering. This has limited its application in biogas reforming, but getting the right catalyst will eliminate the carbon deposit and make it more acceptable, especially because of its applicability in the areas where there is water shortage. More research work in this area is ongoing, especially with the work of Rego de Vasconcelos [START_REF] Vasconelos | Phosphates-based catalyst for synthetic gas (syngas) production using CO 2 and CH 4[END_REF], who is focusing on using phosphates-based catalyst for synthetic gas (syngas) production using CO 2 and CH 4 to reduce carbon deposition on the catalyst. Up-to-date, there has not been industrial application of DRM yet.

On the other hand, steam reforming is the most widely used process to convert natural gas to syngas and it is responsible for over 50% hydrogen production in the world [START_REF] Boyano | Conventional and advanced exergoenvironmental analysis of a steam methane reforming reactor for hydrogen production[END_REF]. Therefore, biogas can be an alternative raw material to conventional steam reforming technology. This is because biogas is similar to natural, and also has additional benefits, such as, it is a renewable resource; it reduces methane emission to the atmosphere; it is commercially produced in large quantities and helps in diverting waste from the landfill and produces nutrient-rich fertilizer as by-product. Steam reforming consists of two reactors (reform reactor and shift reactor). Catalytic reforming reaction of methane, hydrocarbons, naptha or ethanol with water (pre-vaporised by steam generator) takes place in the reform reactor. Three H 2 moles per CH 4 and H 2 O mole fed (main objective of the reform) and one CO mole are the products of this reactions. The CO produced in the reform reactor reacts with water steam in the shift reactor (water-gas-shift reaction), to produce H 2 and CO 2 . The end products of these two reactors are a mixture of H 2 and CO 2 [START_REF] Martelli | Design criteria and optimization of heat recovery steam cycles for integrated reforming combined cycles with CO 2 capture[END_REF][START_REF] Braga | Hydrogen production by biogas steam reforming: a technical, economic and ecological analysis[END_REF].

The presence of water has reduced the possibility of carbon generation and deposition on the catalyst surface, hence it is more acceptable and widely used in natural gas reforming for the production of hydrogen. This is a better path way for biogas utilization.

Lastly, auto-thermal reforming (ATR) is another promising catalytic method that can be used to convert biogas into syngas, using air as a co-reactant. The added air combusts with a portion of CH 4 within the catalytic reactor, thereby producing CO 2 , H 2 O and heat, that can drive the endothermic dry reforming reactions [START_REF] Kohn | Catalytic reforming of biogas for syngas production[END_REF]. The ATR also maintains the catalyst activity by providing additional oxidant that reduces the potential for carbon formation in the reactor, especially for biogas mixtures with high CH 4 content, thereby preventing carbon deposition on the catalyst surface.

Cryogenic separation can produce pure CO 2 , CH 4 , N 2 , and O 2 and are very useful especially, CO 2 . The utilisation of the recovered CO 2 , can further lower the cost of biogas upgrading as it can be use or sold for enhanced oil recovery (EOR), algae production and mineralization, carbon sequestrations, methane and syngas productions. Finally, there is need for biogas conversion standardization globally and clearer information provided for specific application of biogas upgrade quality. This has become necessary due to confusions, because different countries, equipment manufacturers, and utility company give different specifications for the same thing.

Conclusions

The production of biogas from waste and renewable resources is a promising answer to the environmental and energy challenges facing world. The upgraded biogas (biomethane) can be used as a replacement of fossil fuels such as production of heat and stream, electricity production and co-generation, vehicle fuel, and feedstock for the production of bio-based chemicals and substrate in fuel cells, starting reactants in chemical processes, substitute for natural gas for domestic and industrial use, gas grid injections. This paper reviewed the state-of-the-art of biogas purification and upgrading technologies. Gas quality requirements supersede costs as criterion for selection of appropriate technology. This is because the selection of appropriate technology depends on the specific biogas requirements, site specific, local circumstances and are case sensitive. The physical and chemical technologies based on adsorption, absorption, chemical reaction, cryogenic and membrane separations are mature and capable of providing biomethane that meet Wobbe index for grid injection and vehicle utilization. However, they are still very expensive and this high costs are discouraging more upgrading plants implementations.

Some novel technologies such hydrate separation, biotechnologies (Biofilter / Biotrickling filter and insitu upgrading), cryogenic separation, and chemolitotroph-based bioreactors (which can convert CO 2 from the biogas into methane). This is particularly possible by the conversion of excess electricity grid power during the night into H 2 to serves as electron donor for the chemolitotroph-based bioreactors. This will support the new concept of power to gas initiatives. It had so far been evaluated only at laboratory and pilot scale, therefore industrial scale testing and optimization are still needed to show their full potential for biogas upgrading. Biotrickling filter has shown H 2 S removal efficiency more than 99% and its operation costs are lower than that of chemical precipitation, chemical scrubbing or adsorption. Efforts should be directed for more research into these new areas to stimulate its developments. Lastly, dry reforming (DRM) is a potential option that needs to be considered, for biogas purification and upgrading as many efforts are concentrated in research and development (R&D) nowadays.

Fig. 1

 1 Fig. 1 Biogas upgrading by water scrubbing and organic solvent scrubbing to remove CO 2

Fig. 2 Fig. 3

 23 Fig. 2 Biogas upgrading by chemical absorption (Amine scrubbing) of CO 2

Fig. 4 Fig. 5

 45 Fig. 4 Flowsheet of cryogenic separation of biogas (removal of CO 2 )

Fig. 6

 6 Fig. 6 Biogas upgrading by water scrubbing of H 2 S

Fig. 7

 7 Fig. 7 Pressure swing adsorption for H 2 S removal

Fig. 8

 8 Fig. 8 Hybrid solution for the removal of CO 2 , H 2 S and other contaminants

Table 1

 1 Parameters and composition of gases from different origins, impurities and their consequences[START_REF]Swedish Gas Technology Centre: Basic data of biogas[END_REF][START_REF] Sun | Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation[END_REF][START_REF] Petersson | Biogas upgrading technologies-developments and innovations. Task 37-Energy from biogas and landfill gas[END_REF]

	Parameters	Unit	Biogas from AD Landfill gas North Sea gas natural	Dutch natural gas Impact on biogas utilization
	MJ/Nm 3 Lower heating value KWh/Nm 3 6.5 23	16 4.4	40 11	31.6 8.8
	Density	MJ/kg Kg/Nm 3	20 1.1	12.3 1.3	47 0.84	38 0.8
	Relative density Wobbe index, upper MJ/Nm 3 -	0.9 27	1.1 18	0.63 55	0.6 43.7
	Methane number	-	>135	>130	73	-
	Vol% Heavy hydrocarbons Vol% Methane (CH 4 )	60-70 0	35-65 0	85-92 9	80-90 9
	Water vapour (H 2 O) Vol% Hydrogen Vol%	1-5 0	1-5 0	-0	--	Corrosion in compressors, gas storage tanks and engines due to reaction with H 2 S, NH 3 , CO 2 to form acids
	Carbon dioxide	Vol%	30-40	15-40	0.2-1.5	0.2-1.5	Decreasing calorific value, anti-knock prop-erties of engines and corrosion
	Nitrogen, range	Vol%	0-0.5	15	0.3-1.0	14	Decreasing calorific value, anti-knock prop-erties of engines and corrosion
	Oxygen	Vol%	0	1	-	-	Corrosion, fooling in cavern storage, risk of explosion
	Hydrogen sulphide	Ppm	0-4000	0-100	1.1-5.9	-	Corrosion, catalytic converter poison, emis-sion and health hazards. SO 2 , SO 3 are form
	Ammonia (NH 3 ) Total chlorine as Cl -Mg/Mm 3 0-5 Ppm 100	5 20-200	0 -	--	Emission, anti-knock properties of engines and corrosion when dissolved Corrosion in engines

Table 3

 3 Biogas utilization technologies and H 2 S requirements[START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF][START_REF] Bauer | Biogas upgrading-review of commercial technologies[END_REF][START_REF] Petersson | Biogas upgrading technologies-developments and innovations. Task 37-Energy from biogas and landfill gas[END_REF]

	Technology	H2S tolerance (ppm)
	Heating (boilers) and stirling engines	<1000
	Kitchen stoves	<10
	Internal combustion engines	<500 ppm (depending on the kind of engine; it can be <50 ppm)
	Turbines	<10,000
	Micro-turbines	<70,000
	Fuel cells
	Proton exchange membrane (PEM)	<1
	Phosphoric acid fuel cell (PAFC)	<20
	Molten carbonate fuel cell (MCFC)	<10 in fuel (<0.1-0.5 at the anode)
	Solid oxide fuel cell (SOFC)	<1
	Natural gas upgrade	<4 (variations among countries)

Table 4

 4 Alternatives and technical features of CO 2 removal from gas streams[3-5, 7, 8, 10, 30]

	Disadvantages	Expensive investment and operation Cost: 0.105 € m -3 (250 Nm 3 /h) and 0.052 € m -3 (2000 Nm 3 /h) Power cost: 0.4-0.5 kWh m -3 produce gas Clogging due to bacterial growth Possible foaming, low flexibility toward variation of input gas	Expensive investment and operation Power cost: 0.21 kWh m -3 of gas Difficult operation Incomplete operation when stripping/vacuum (boiling required), reduced operation when dilution of glycol with water	Expensive investment, heat required for regeneration, corrosion, decomposition and poisoning of the amines by O 2 or other chemi-cals, precipitation of salts, foaming possible	Expensive investment and operation, extensive process control needed, CH 4 losses when malfunctioning of valves	Low membrane selectivity: compromise between purity of CH 4 and amount of upgraded biogas, multiple steps required (modular system) to reach high purity, CH 4 losses Cost: 50 Nm 3 /h (0.432 ct Nm -3 ); 200 Nm 3 /h (0.211 ct Nm -3 ) Power cost: 0.22 kWh m -3 produce gas CH 4 losses <10%	Expensive investment and operation CO 2 can remain in the CH 4 , High energy required for cooling Addition of H2, still on experimental basis (not on large-scale)
	Advantages	High efficiency (>97% CH 4 ) Simultaneous removal of H 2 S (when H 2 S <300 cm 3 /m 3 ) Capacity is adjustable by changing temperature or pressure Low CH 4 losses (<2%), tolerant to impurities, Regeneration pos-sible	High efficiency (>97% CH 4 ) Simultaneous removal of organic components, H 2 S, NH 3 , HCN and H 2 O Energetic more favorable than water Low CH 4 losses, regeneration possible with low-temperature waste heat	High efficiency (>99% CH 4 ) Cheap operation, regenerative, more CO2 dissolved per unit of volume compared with water, very low CH 4 losses (<0.1%) Power cost: 0.05-0.25 kWh m -3 of gas	Highly efficient (95-98% CH 4 ), H 2 S is removed, low energy used: high pressure but regenerative, compact technique, also for small capacities, tolerant to impurities	H 2 S and H 2 O are removed, simple construction, simple operation, high reliability, small gas flows treated without proportional increase of costs Gas/gas removal efficiency <92% CH 4 (1 step) or >96% CH 4 , H 2 O is removed Gas/liquid removal efficiency >96% CH 4 , cheap investment and operation, Pure CO 2 can be produced	90-98% CH 4 can be reached, CO 2 and CH 4 in high purity, low extra energy cost to reach liquid biomethane (LBM)	/h and No unwanted end products Enrichment of CH Removal of H 2 S and CO 2 4 DMT filter ® 100,000 m 3 can handle air flow rates of between 10 m 3 /h without any problem
	Technology/alternatives	Absorption (water scrubbing)	Absorption (organic solvent-polyethylene glycol)	Chemical absorption (Amines)	PSA/VSA (Carbon Molecular sieves, Zeolites Molecular sieves Alumina silicates)	Membrane technology (Gas/gas or gas/liquid)	Cryogenic separation	Biological removal Air/oxygen dosing digester slurry , Biofilter/Biotrickling filter (BiogasCleaner ® Biopuric ® ) , DMT filter ® Bioscrubber (Thiopaq™)

Table 5

 5 Alternatives and technical features of H 2 S and other contaminants removal from gas streams[START_REF] Persson | Biogas upgrading to vehicle fuel standards and grid[END_REF][START_REF] Bailón Allegue | Biogas upgrading Evaluation of methods for H 2 S removal[END_REF][START_REF] Ryckebosch | Techniques for transformation of biogas to biomethane[END_REF][START_REF] Huertas | Removal of H 2 S and CO 2 from biogas by amine absorption[END_REF][START_REF] Muñoz | A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading[END_REF][START_REF] Schiavon | Purification of biogas for energy use[END_REF]

	Reactions and advantages Disadvantages	H 2 S <15 cm 3 m -3 , Cheap when water is available, CO perature, difficult technique Low efficiency (100-150 cm 3 m -3 ), expensive operation (iron salt), changes in pH and temperature not beneficial for digestion process, correct dossing is difficult, required specialized super-vision, high pressure problems 2 is also removed [2] Not regenerative, expensive operation: High pressure, low tem-2Fe 3+ + 3S 2-= 2FeS Fe 2+ + S 2-= FeS Elemental S is formed, cheap investment: storage tank and dosing pump, low electricity and heat requirement, simple operation and maintenance, compact technique, H2S not in biogas wire, no air in biogas. Operation cost: 0.024 € m -3 of biogas	Sensitive for water, expensive operational costs, regeneration is exothermic: Risk of ignition of chips, reaction surface reduced each cycle, released dust can be toxic, reagent disposal Fe 2 O 3 + 3H 2 S = Fe 2 S 3 + 3H 2 O 2Fe(OH) 3 + 3H 2 S = Fe 2 S 3 + H 2 O 2Fe 2 S 3 + 3O 2 = 2Fe 2 O 3 + 6 S High removal efficiency: >99%, Mercaptanes are also removed, Cheap investment, Simple Can handle 0.3-500 kg/d oh H2S	Difficult technique, regenration through oxygenation, CO 2 =H 2 CO 3 (using EDTA) leads to precipitation Build-up of thiosulfates from chelates + H 2 S (using EDTA), Required specialized supervision, High pressure problems [8] S 2-+ 2Fe 3+ = S + 2Fe 2+ 0.5O 2 + 2Fe 2+ = 2Fe 3+ + 2OH -High removal efficiency: 95-100%, cheap operation, small volume required, regenrative, low CH 4 losses, can remove 50-90% mercaptans, cost of 0.24-0.3 € kgS -1 Can handle 0.5-15 ton/day of H 2 S Capita cost: 3-120 € (m 3 /h) -1	Expensive investment and operation, More difficult technique, Not regenerative Low electricity requirements, Smaller volume, less pumping (compare with water absorption), Low CH4 losses Operating cost: 0.03 € m -3 2 S still high (100-300 cm 3 Concentration of H m -3 ) Excess O 2 /N 2 in biogas (means difficult upgrading or required addition cleaning), Overdosing air will result in explosive mixture Require strict control of bacterial conditions Optimization is still an issue of biogas 2H 2 S + O 2 = 2 S + 2H 2 O H 2 S + 2O 2 = SO 4 2-+ 2H + High removal possible: >97% Elemental S is formed, used bacteria or microorganisms at room tempera-ture, Cheap investment and operation: low electricity and heat require-ments, no extra chemicals or equipment required, Simple operation and maintenance BiogasCleaner ® is automated bio-trickling filters for H2S removal Biotrickling filter has >99% H 2 S removal efficiency. Commercially avail-able in full-scale plants Can handle 50-20,000 kg/d of H 2 S. Operating cost: 0.013-0.016 € m -3 Expensive investment and operation, CH 4 losses (it absorbs CH 4 ), H 2 O and O 2 prior treatment before removing H 2 S, H 2 O can occupy the binding places needed by H 2 S, Regeneration at 450 °C, Residue present till 850 °C 2H 2 S + O 2 = 2 S + 2H 2 O High efficiency (H 2 S <3 cm -3 m 3 ) High purification rate Low operating temperature Compact technique High loading capacity	Removal of H 2 S >98% is possible, CO
	Technology/alternatives	Absorption in water	Adidition of iron chloride in digester (FeCl 3 / FeCl 2 /FeSO 4 )	Iron oxide/iron hydroxide (Fe(OH)3/Fe 2 O 3 ) -bed Rust steel wool, impregnated wood chips or pellets	Chemical absorption Fe(OH) 3 Fe-EDTA Cooab™	, Air/oxygen dosing digester slurry Biological/Biotechnological 3 FeCl NaOH Chemical absorption Biofilter/biotrickling filter (BiogasCleaner ® Biopuric ® , DMT filter ® ) Bioscrubber (Thiopaq™)	Adsorption with activated carbon (Impregnated with KI 1-5%)	Membranes

is also removed Expensive operation and maintenance, highly complex solution

Acknowledgements

The first author acknowledges the support from School of Civil Engineering, University College Dublin, scholarship support from Student Universal Support Ireland (SUSI) and Université de Toulouse, Mines Albi, CNRS UMR 5302, Centre RAP-SODEE, Campus Jarlard, Albi, F-81013 cedex 09, France. Fig. 9 Catalytic reforming option for biogas utilization