Investigation of green hydrogen production and development of waste heat recovery system in biogas power plant for sustainable energy applications

In this study, biogas power production and green hydrogen potential as an energy carrier are evaluated from biomass. Integrating an Organic Rankine Cycle (ORC) to benefit from the waste exhaust gases is considered. The power obtained from the ORC is used to produce hydrogen by water electrolysis, eliminate the H₂S generated during the biogas production process and store the excess electricity. Thermodynamic and thermoeconomic analyses and optimization of the designed Combined Heat and Power (CHP) system for this purpose have been performed. The proposed study contains originality about the sustainability and efficiency of renewable energy resources. System design and analysis are performed with Engineering Equation Solver (EES) and Aspen Plus software. According to the results of thermodynamic analysis, the energy and exergy efficiency of the existing power plant is 28.69% and 25.15%. The new integrated system's energy, exergy efficiencies, and power capacity are calculated as 41.55%, 36.42%, and 5792 kW. The total hydrogen production from the system is 0.12412 kg/s. According to the results of the thermoeconomic analysis, the unit cost of the electricity produced in the existing power plant is 0.04323 $/kWh. The cost of electricity and hydrogen produced in the new proposed system is determined as 0.03922 $/kWh and 0.181 $/kg H₂, respectively.     

An integrated system for ammonia production from renewable hydrogen: A case study

This study presents an analysis and assessment study of an integrated system which consists of cryogenic air separation unit, polymer electrolyte membrane electrolyzer and reactor to produce ammonia for a selected case study application in Istanbul, Turkey. A thermodynamic analysis of the proposed system illustrates that electricity consumption of PEM electrolyzer is 3410 kW while 585.4 kW heat is released from ammonia reactor. The maximum energy and exergy efficiencies of the ammonia production system which are observed at daily average irradiance of 200 W/m² are found as 26.08% and 30.17%, respectively. The parametric works are utilized to find out the impacts of inlet air conditions and solar radiation intensity on system performance. An increase in the solar radiation intensity results in a decrease of the efficiencies due to higher potential of solar influx. Moreover, the mass flow rate of inlet air has a substantial effect on ammonia production concerning the variation of generated nitrogen. The system has a capacity of 0.22 kg/s ammonia production which is synthesized by 0.04 kg/s H₂ from PEM electrolyzer and 0.18 kg/s N₂ from a cryogenic air separation unit. The highest exergy destruction rate belongs to PEM electrolyzer as 736.2 kW while the lowest destruction rate is calculated as 3.4 kW for the separation column.     

A flexible analytical model for operational investigation of solar hydrogen plants

Hydrogen will become a dominant energy carrier in the future and the efficiency and lifetime cost of its production through water electrolysis is a major research focus. Alongside efforts to offer optimum solutions through plant design and sizing, it is also necessary to develop a flexible virtualised replica of renewable hydrogen plants, that not only models compatibility with the “plug-and-play” nature of many facilities, but that also identifies key elements for optimisation of system operation. This study presents a model for a renewable hydrogen production plant based on real-time historical and present-day datasets of PV connected to a virtualised grid-connected AC microgrid comprising different technologies of batteries, electrolysers, and fuel cells. Mathematical models for each technology were developed from chemical and physical metrics of the plant. The virtualised replica is the first step toward the implementation of a digital twin of the system, and accurate validation of the system behaviour when updated with real-time data. As a case study, a solar hydrogen pilot plant consisting of a 60 kW Solar PV, a 40 kW PEM electrolyser, a 15 kW LIB battery and a 5 kW PEM fuel cell were simulated and analysed. Two effective operational factors on the plant's performance are defined: (i) electrolyser power settings to determine appropriate hydrogen production over twilight periods and/or overnight and (ii) a user-defined minimum threshold for battery state of charge to prevent charge depletion overnight if the electrolyser load is higher than its capacity. The objective of this modelling is to maximise hydrogen yield while both loss of power supply probability (LPSP) and microgrid excess power are minimised. This analysis determined: (i) a hydrogen yield of 38–39% from solar DC energy to hydrogen energy produced, (ii) an LPSP <2.6 × 10^?4 and (iii) < 2% renewable energy lost to the grid as excess electricity for the case study.     

Hydrogen production and storage analysis of a system by using TRNSYS

In this study, hydrogen production and storage were investigated. The Transient System Simulation Program (TRNSYS) and Generic Optimization Program (GenOpt) packages were combined for the design and optimization of a system that produces hydrogen from water and stores the hydrogen it produced in the compressed gas tank. The system design is based on the electricity grid. Electrical energy produced in photovoltaic (PV) panels was used to electrolyze water. The systems for Izmir, Istanbul and Ankara provinces which are in different climate zones of Turkey were optimized and the annual system performances based on the optimum angles were analyzed. For the mentioned provinces, the PV tilt angles which minimize electricity drawn from the grid at the electrolyzer are also investigated. The electrical energy produced in the photovoltaic panels, the hydrogen and oxygen amounts produced, the efficiency of the electrolyzer, the gas and pressure levels in the hydrogen tank were compared. According to the results of the analysis, the annual total power produced in photovoltaic panels is 42803.66 kW in İzmir, 42573.74 kW in Istanbul and 44613.95 kW in Ankara. Hydrogen levels produced in the system are calculated as 10488.39 m³ year⁻¹ in Izmir, 9824.70 m³ year⁻¹ in Istanbul, and 10368.65 m³ year⁻¹ in Ankara.     

Sulphonated (PVDF-co-HFP)-graphene oxide composite polymer electrolyte membrane for HI decomposition by electrolysis in thermochemical iodine-sulphur cycle for hydrogen production

In overall iodine-sulphur (I-S) cycle (Bunsen reaction), HI decomposition is a serious challenge for improvement in H₂ production efficiency. Herein, we are reporting an electrochemical process for HI decomposition and simultaneous H₂ and I₂ production. Commercial Nafion 117 membrane has been generally utilized as a separator, which also showed huge water transport (electro-osmosis), and deterioration in conductivity due to dehydration. We report sulphonated poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) (SCP) and sulphonated graphene oxide (SGO) composite stable and efficient polymer electrolyte membrane (PEM) for HI electrolysis and H₂ production. Different SCP/SGO composite PEMs were prepared and extensively characterized for water content, ion-exchange capacity (IEC), conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with commercial Nafion117 membrane. Most suitable optimized SCP/SGO-30 composite PEM exhibited 6.78 × 10^?2 S cm^?1 conductivity in comparison with 9.60 × 10^?2 S cm^?1 for Nafion® 117. The electro-osmotic flux ofSCP/SGO-30 composite PEM (2.53 × 10^?4 cm s^?1) was also comparatively lower than Nafion® 117 membrane (2.75 × 10^?4 cm s^?1). For HI electrolysis experiments, SCP/SGO-30 composite PEM showed good performance such as 93.4% current efficiency (η), and 0.043 kWh/mol-H₂ power consumption (Ψ). Further, intelligent architecture of SCP/SGO composite PEM, in which hydrophilic SGO was introduced between fluorinated polymer by strong hydrogen bonding, high efficiency and performance make them suitable candidate for electrochemical HI decomposition, and other diversified electrochemical processes.     

Hydrogen generation by alcohol reforming in a tandem cell consisting of a coupled fuel cell and electrolyser

The performance of a novel electro-reformer for the production of hydrogen by electro-reforming alcohols (methanol, ethanol and glycerol) without an external electrical energy input is described. This tandem cell consists of an alcohol fuel cell coupled directly to an alcohol reformer, negating the requirement for external electricity supply and thus reducing the cost of operation and installation. The tandem cell uses a polymer electrolyte membrane (PEM) based fuel cell and electrolyser. At 80 °C, hydrogen was generated from methanol, by the tandem PEM cell, at current densities above 200 mA cm⁻², without using an external electricity supply. At this condition the electro-reformer voltage was 0.32 V at an energy input (supplied by the fuel cell component) of 0.91 kWh/Nm³; i.e. less than 20% of the theoretical value for hydrogen generation by water electrolysis (4.7 kWh/Nm³) with zero electrical energy input from any external power source. The hydrogen generation rate was 6.2 × 10⁻⁴ mol (H₂) h⁻¹. The hydrogen production rate of the tandem cell with ethanol and glycerol was approximately an order of magnitude lower, than that with methanol.     

Cost-effective sizing of a hybrid Regenerative Hydrogen Fuel Cell energy storage system for remote & off-grid telecom towers

There is an urgent need to provide cost-effective, clean, distributed electricity to ensure reliability for mobile network operators in Sub-Saharan Africa. A comprehensive semi-empirical MATLAB/Simulink model of a novel low-pressure, solid-hydrogen based energy storage system combined with Solar PV and battery energy storage including dynamic losses of the power conditioning equipment is built. Levenburg-Marquardt least square algorithm is used for semi-empirical parameterisation of the metal-hydride and fuel cell models, simulations are performed using experimentally obtained telecom tower load data. The results show the overall system efficiency of the energy system drop from 21.05% for a Solar/Battery system to 17.43% of the most cost-effective hybridised system, which consists of 16.2 kW Solar PV coupled to a 10kW/40 kWh Li-Ion battery, and a Regenerative Hydrogen Fuel Cell (consisting of a 10 kW PEM Electrolyser, 1,000 kWh Ti-based AB2 Solid-Hydrogen Storage Cell, and 5 kW PEM Fuel Cell). This system achieves a Levelised Cost of Electricity of 17.16 ¢/kWh compared to 73.40 ¢/kWh for a Diesel Genset, with a Net Present Value of $109,236 and an Internal Rate of Return of 15.15%.     

Sustainable hydrogen production: Technological advancements and economic analysis

Hydrogen (H₂) is pivotal to phasing out fossil fuel-based energy systems. It can be produced from different sources and using different technologies. Very few studies comprehensively discuss all available state-of-the-art technologies for H₂ production, the challenges facing each process, and their economic feasibility and sustainability. The current study thus addresses these gaps to effectively direct future research towards improving H₂ production techniques. Many conventional methods contribute to large greenhouse gas footprints, with high production costs and low efficiency. Steam methane reforming and coal gasification dominate the supply side of H₂, due to their low production costs (<$3.50/kg). Water-splitting offers one of the most environmentally benign production methods when integrated with renewable energy sources. However, it is considerably expensive and ridden with the flaw of production of harmful by-products that affect efficiency. Fossil fuel processing technologies remain one of the most efficient forms of H₂ production sources, with yields exceeding 80% and reaching up to 100%, with the lowest cost despite their high reliance on expensive catalysts. Whereas solar-driven power systems cost slightly less than $10 kg^?1, coal gasification and steam reforming cost below $3.05 kg^?1. Future research thus needs to be directed towards cost reduction of renewable energy-based H₂ production systems, as well as in their decarbonization and designing more robust H₂ storage systems that are compatible with long-distance distribution networks with adequate fuelling stations.     

On the reduction of electric energy consumption in electrolysis: A thermodynamic study

Electricity is the major component in the cost of hydrogen production via electrolysis. This study aims to reduce electric energy consumption in electrolysis and replace it with lower cost thermal energy. An idealized thermodynamic analysis of hydrogen production by electrolysis at higher temperatures is presented, with particular emphasis on isolating the work and thermal components of the required energy. There is significant advantage in using thermal energy from another complementary process to overheat the inlet steam to meet this need. The electricity demand for electrolysis, under reasonable conditions, can thus be reduced to 26.63 kWh kg^?1. Introducing multiple stages can further reduce this to 25.22 kWh kg^?1 and more significantly, greatly reduce the temperature of the thermal energy needed. At lower utilizations, it is possible to reduce the electrical requirement to below 20 kWh kg^?1, which is less than half the most aggressive targets for electrolyzer improvements.     

Assessment of power and hydrogen production performance of an integrated system based on middle-grade geothermal source and solar energy

In this study, power and hydrogen production performance of an integrated system is investigated. The system consists of an organic Rankine cycle (ORC), parabolic trough solar collectors (PTSCs) having a surface area of 545 m², middle-grade geothermal source (MGGS), cooling tower and proton exchange membrane (PEM). The final product of this system is hydrogen that produced via PEM. For this purpose, the fluid temperature of the geothermal source is upgraded by the solar collectors to drive the ORC. To improve the electricity generation efficiency, four working fluids namely n-butane, n-pentane, n-hexane, and cyclohexane are tried in the ORC. The mass flow rate of each working fluid is set as 0.1, 0.2, 0.3, 0.4 kg/s and calculations are made for 16 different situations (four types of working fluids and four different mass flow rates for each). As a result, n-butane with a mass flow rate of 0.4 kg/s is found to be the best option. The average electricity generation is 66.02 kW between the hours of 11⁰⁰-13⁰⁰. The total hydrogen production is 9807.1 g for a day. The energy and exergy efficiency is calculated to be 5.85% and 8.27%, respectively.     

Thermoeconomic analysis and artificial neural network based genetic algorithm optimization of geothermal and solar energy assisted hydrogen and power generation

The study aims to optimize the geothermal and solar-assisted sustainable energy and hydrogen production system by considering the genetic algorithm. The study will be useful by integrating hydrogen as an energy storage unit to bring sustainability to smart grid systems. Using the Artificial Neural Network (ANN) based Genetic Algorithm (GA) optimization technique in the study will ensure that the system is constantly studied in the most suitable under different climatic and operating conditions, including unit product cost and the plant's power output. The water temperature of the Afyon Geothermal Power Plant varies between 70 and 130 °C, and its mass flow rate varies between 70 and 150 kg/s. In addition, the solar radiation varies between 300 and 1000 W/m² for different periods. The net power generated from the region's geothermal and solar energy-supported system is calculated as 2900 kW. If all of this produced power is used for hydrogen production in the electrolysis unit, 0.0185 kg/s hydrogen can be produced. The results indicated that the overall energy and exergy efficiencies of the integrated system are 4.97% and 16.0%, respectively. The cost of electricity generated in the combined geothermal and solar power plant is 0.027 $/kWh if the electricity is directly supplied to the grid and used. The optimized cost of hydrogen produced using the electricity produced in geothermal and solar power plants in the electrolysis unit is calculated as 1.576 $/kg H₂. The optimized unit cost of electricity produced due to hydrogen in the fuel cell is calculated as 0.091 $/kWh.     

Hydrogen production from wind energy in Western Canada for upgrading bitumen from oil sands

Hydrogen is produced via steam methane reforming (SMR) for bitumen upgrading which results in significant greenhouse gas (GHG) emissions. Wind energy based hydrogen can reduce the GHG footprint of the bitumen upgrading industry. This paper is aimed at developing a detailed data-intensive techno-economic model for assessment of hydrogen production from wind energy via the electrolysis of water. The proposed wind/hydrogen plant is based on an expansion of an existing wind farm with unit wind turbine size of 1.8 MW and with a dual functionality of hydrogen production and electricity generation. An electrolyser size of 240 kW (50 Nm³ H₂/h) and 360 kW (90 Nm³ H₂/h) proved to be the optimal sizes for constant and variable flow rate electrolysers, respectively. The electrolyser sizes aforementioned yielded a minimum hydrogen production price at base case conditions of $10.15/kg H₂ and $7.55/kg H₂. The inclusion of a Feed-in-Tariff (FIT) of $0.13/kWh renders the production price of hydrogen equal to SMR i.e. $0.96/kg H_2, with an internal rate of return (IRR) of 24%. The minimum hydrogen delivery cost was $4.96/kg H₂ at base case conditions. The life cycle CO₂ emissions is 6.35 kg CO₂/kg H₂ including hydrogen delivery to the upgrader via compressed gas trucks.     

Design of 3 kW integrated power generation system from solar and biogas

Energy/Power is an important element of social and economic development. Without availability of energy at a reasonable price, there is little prospect of developing the country's economy and people's living conditions. A quarter of Pakistani population have no access to electricity, and currently a shortage of 5.0–7.0 GW power supply causes load shedding/blackout problems. In Pakistan, most of the power is generated from non-renewable sources like natural gas etc. Pakistan receives almost 15.525 × 10¹⁴ kW-hour of solar energy per year and sunshine duration is normally 8.0–10.0 h per day. Animal/farm wastes are readily available in Pakistan. Residues from these sources produce 103.0 billion m³ of biogas per year that is equivalent to 63.20 TWh. The objective of this study is energy production by using easily available indigenous resources. In this study a 3.0 kW integrated solar/biogas power generation system consist of 2.84 kW solar system and 4.0 m³ biogas system is designed and installed. This paper also present simulation model of system. A hybrid inverter is used to convert DC power of photovoltaic modules and the battery bank in to AC power and combines with the output power of biogas generator. Performance of the hybrid system are analysed from May 1, 2018 to June 15, 2018. During the test maximum power produce by the integrated system is 1.10 kW in morning, 2.14 kW in noon and 1.16 kW in afternoon.     

An analysis of hydrogen production from renewable electricity sources

Three aspects of producing hydrogen via renewable electricity sources are analyzed to determine the potential for solar and wind hydrogen production pathways: a renewable hydrogen resource assessment, a cost analysis of hydrogen production via electrolysis, and the annual energy requirements of producing hydrogen for refueling. The results indicate that ample resources exist to produce transportation fuel from wind and solar power. However, hydrogen prices are highly dependent on electricity prices. For renewables to produce hydrogen at $2 kg⁻¹, using electrolyzers available in 2004, electricity prices would have to be less than $0.01 kWh⁻¹. Additionally, energy requirements for hydrogen refueling stations are in excess of 20 GWh/year. It may be challenging for dedicated renewable systems at the filling station to meet such requirements. Therefore, while plentiful resources exist to provide clean electricity for the production of hydrogen for transportation fuel, challenges remain to identify optimum economic and technical configurations to provide renewable energy to distributed hydrogen refueling stations.     

The feasibility of renewable energies at an off-grid community in Canada

Three renewable energy technologies (RETs) were analyzed for their feasibility for a small off-grid research facility dependent on diesel for power and propane for heat. Presently, the electrical load for this facility is 115 kW but a demand side management (DSM) energy audit revealed that 15–20% reduction was possible. Downsizing RETs and diesel engines by 15 kW to 100 kW reduces capital costs by $27 000 for biomass, $49 500 for wind and $136 500 for solar.The RET Screen International 4.0^® model compared the economical and environmental costs of generating 100 kW of electricity for three RETs compared to the current diesel engine (0 cost) and a replacement ($160/kW) diesel equipment. At all costs from $0.80 to $2.00/l, biomass combined heat and power (CHP) was the most competitive. At $0.80 per liter, biomass’ payback period was 4.1 years with a capital cost of $1800/kW compared to wind's 6.1 years due to its higher initial cost of $3300/kW and solar's 13.5 years due to its high initial cost of $9100/kW. A biomass system would reduce annual energy costs by $63 729 per year, and mitigate GHG emissions by over 98% to 10 t CO₂ from 507 t CO₂. Diesel price increases to $1.20 or $2.00/l will decrease the payback period in years dramatically to 1.8 and 0.9 for CHP, 3.6 and 1.8 for wind, and 6.7 and 3.2 years for solar, respectively.     

Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology

Results related to the development and testing of a unitized regenerative fuel cell (URFC) based on proton-exchange membrane (PEM) technology are reported. A URFC is an electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode) or as a H₂/O₂ fuel cell for the production of electricity and heat (fuel cell mode). The URFC stack described in this paper is made of seven electrochemical cells (256 cм² active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm⁻² (85% efficiency based on the thermoneutral voltage of the water splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term.     

Operation of first solar-hydrogen system in China

The first solar-hydrogen (S-H) system in China, which consists a 2 kW PV cell array, a 48 V/300Ah lead-acid battery bank, an 0.5 Nm³/h hydrogen production capacity alkaline water electrolyzer, a 10 Nm³ LaNi₅ alloy hydrogen storage tank and a 200 W H₂/air PEM fuel cell, was installed in the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University and has been operated for several months. The goal of the system was to study the technical and economical feasibility of using such a system to produce hydrogen in large scale for the future hydrogen energy society. With two months operation, experimental results reveal 40.68% energy transformed to hydrogen with 7.21 kWh/Nm³ H₂ electricity consumption. Economic analysis results illustrate that the present system is not cost-efficient and the energy conversion efficiencies of PV panel and electrolyzer are suggested to increase in technology improvement to cut down cost.     

An integrated system development including PEM fuel cell/biogas purification during acidogenic biohydrogen production from dairy wastewater

Biohydrogen production from dairy wastewater with subsequent biogas purification by hollow fiber membrane module was investigated in this study. The purified and not purified (raw) biohydrogen were used as fuel in polymer electrolyte membrane (PEM) fuel cell. Furthermore, the effect of CO₂ on the performance of PEM fuel cell was evaluated considering cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and polarization curves. The maximum H₂ production rate was 0.015 mmol H₂/mol glucose and the biohydrogen concentration in biogas was ranged 33%–60% (v/v). CO₂/H₂ selectivity decreased with increasing pressure and maximum selectivity was obtained as 4.4 at feed pressure of 1.5 bar. The electrochemical active surface (EASA) areas were decreased with increasing CO₂ ratio. The maximum power densities were 0.2, 0.08 and 0.045 W cm⁻² for 100%, 80% and 60% (v/v) H₂, respectively. The results indicated that integrated PEM fuel cell/biogas purification system can be used as a potential clean energy sources during acidogenic biohydrogen production from dairy wastewater.     

Techno-economic evaluation of a grid-connected PV-trigeneration-hydrogen production hybrid system on a university campus

Many universities have plans to reduce campus energy consumption with developed energy efficiency strategies, supply the energy needs of the university campus with renewable energy and create a green campus. In order to serve this purpose, this study focuses on the simulation of the installation of an on-grid photovoltaic (PV) power system at the Vocational Colleges Campus, Hitit University. On the other hand, the integration of the simulated PV system with a gas fired-trigeneration system is discussed. Moreover, the study explores opportunities for solar hydrogen generation without energy storage on campus. For the PV system simulation, three different scenarios were created by using web-based PV system design software (HelioScope). Installed powers in the simulation were found as 94.2 kWe, 123.9 kWe, and 157.5 kWe for the low scenario (on the rooftop), high scenario (on the rooftop), and the high + PV canopy arrays scenario (on the rooftop and an outdoor parking area), respectively. The levelized cost of electricity (LCOE) values were 0.061 $/kWh, 0.065 $/kWh, and 0.063 $/kWh for the low scenario, high scenario, and the scenario including PV canopy, respectively. The energy payback time is found to be 6.47–6.94 years for the 20–25 years lifetime of the PV plant. The simulation results showed that the PV system could support it by generating additional electrical energy up to 25% of the existing system. The campus can reduce GHG emissions of 1546–2272 tonnes-CO₂eq, which is equivalent to 142–209 ha of forest-absorbing carbon unused during the life of the PV system. Depending on the production and consumption methods utilized on campus, which is a location with relatively large solar potential, the levelized cost of hydrogen (LCOH) of hydrogen generation ranged from 0.054 $/kWhH₂ (1.78 $/kgH₂) to 0.103 $/kWhH₂ (3.4 $/kgH₂). Consequently, with proper planning and design, a grid-connected PV-trigeneration-hydrogen generation hybrid system on a university campus may operate successfully.     

Biogas production in low-cost household digesters at the Peruvian Andes

Low-cost tubular digesters originally developed in tropical regions have been adapted to the extreme weather conditions of the Andean Plateau (3000-4000 m.a.s.l.). The aim of this study was to characterise biogas production in household digesters located at high altitude, operating under psychrophilic conditions. To this end, two pilot digesters were monitored and field campaigns were carried out in two representative digesters of rural communities. Digesters’ useful volume ranged between 2.4 and 7.5 m³, and hydraulic residence time (HRT) between 60 and 90 days. The temperature inside the digester’s greenhouse ranged between 20 and 25 °C. Treating cow manure, a specific biogas production around 0.35 m³ kg_VS⁻¹ was obtained, with some 65% CH₄ in biogas. In order to fulfil daily requirements for cooking and lighting, biogas production should be enhanced without increasing implementation costs as not to impede the expansion of this technology at household scale. In this sense, HRT below 60 days and OLR above 1 kg_VS m⁻³ day⁻¹ should be investigated to decrease digesters’ volume (i.e. costs) and increase biogas production rate. The adaptation of conventional gas burners to biogas characteristics can also contribute in improving the efficiency of the system.