Pretreatment of second and third generation feedstock for enhanced biohythane production: Challenges,recent trends and perspectives

In the context of biofuel production and achieving sustainable bioeconomy, the use of lignocellulosic and algae biomass in anaerobic fermentation processes yields biohythane that has a typical composition of 10–15% H₂, 50–55% CH₄ and 30–40% CO₂. Using organic biomass-based substrates has been shown to minimize environmental impacts due to the versatile production of high-value products under normal operating conditions that are practically achievable. However, the biohythane yield depends on different factors such as the biomass type, the organic loading rate, soluble metabolic products formed, the type of fermentation (single/dual stage) and the pretreatment strategy adopted for the biomass. Different pretreatment strategies based on physical, chemical and biological processes have been proposed in the literature. In this review, improvements in biohythane yield as a result of these pretreatment strategies, the need/effect of inoculum enrichment, the effects of pH, temperature, trace element addition and organic loading rate has been reviewed. Finally, the major developments of improving biohythane yield due to the addition of co-substrates and the current trends are discussed.     

Biohythane production in two-stage anaerobic digestion system

Hydrogen (H₂) and methane (CH₄) are the potential alternative energy carriers with autonomous extensive and viable importance. These fuels could complement the advantages, and discard the disadvantages of each other, if produced simultaneously. Considering their complementary properties, co-production of a mixture of H₂ and CH₄ in the form of biohythane in two-stage anaerobic digestion (AD) process is gaining more interest than their individual production. Biohythane is a better transportation fuel than compressed natural gas (CNG) in terms of high range of flammability, reduced ignition temperature as well as time, without nitrous oxide (NOx) emissions, improved engine performance without specific modification, etc. Other than production of biohythane, performing two-stage AD is advantageous over one-stage AD due to short HRT, high energy recovery, high COD removal, higher H₂ and CH₄ yields, and reduced carbon dioxide (CO₂) in biogas. For improved biohythane production, various aspects of two-stage AD need to be emphasized. Keeping the facts in mind, the process of two-stage AD along with microbial diversity in comparison to one-stage AD has been discussed in the previous sections of this review. For large scale commercial production, and utilization of biohythane in automobile sector, its execution needs evaluation of process parameters, and problems associated with two-stage AD. Hence, the later part of this review describes the production process of biohythane, concerned microbial diversity, operational process parameters, major challenges and their solutions, applications, and economic evaluation for enhanced production of biohythane.     

Techno-economic evaluation of biohydrogen production from wastewater and agricultural waste

The world is facing serious climate change caused in part by human consumption of fossil fuel. Therefore, developing a clean and environmentally friendly energy resource is necessary given the depletion of fossil fuels, the preservation of the earth's ecosystem and self-preservation of human life. Biological hydrogen production, using dark fermentation is being developed as a promising alternative and renewable energy source, using biomass feedstock. In this study, beverage wastewater and agricultural waste were examined as substrates for dark fermentation to produce clean biohydrogen energy.     

A review on process modeling and design of biohydrogen

The current energy supply depends on fossil fuels which have increased carbon dioxide emissions leading to global warming and depleted non-renewable fossil fuels resources. Hydrogen (H₂) fuel could be an eco-friendly alternative since H₂ consumption only produces water. However, the overall impacts of the H₂ economy depend on feedstock types, production technologies, and process routes. The existing process technologies for H₂ production used fossil fuels encounter the escalation of fossil fuel prices and long-term sustainability challenges. Therefore, biohydrogen production from renewable resources like biomass wastes and wastewaters has become the focal development of a sustainable global energy supply. Different from other biohydrogen production studies, this paper emphasizes biohydrogen fermentation processes using different renewable sources and microorganisms. Moreover, it gives an overview of the latest advancing research in different biohydrogen process designs, modeling, and optimization. It also presents the biohydrogen production routes and kinetic modeling for biohydrogenation.     

Effect of food to microorganisms (F/M) ratio on biohythane production via single-stage dark fermentation

Hythane is a mixture of hydrogen and methane gases which are generally produced in separate ways. This work studied mesophilic biohythane gas (H₂+CH₄+CO₂) production in a bioreactor via single-stage dark fermentation. The fermentation was conducted in batch mode using mixed anaerobic microflora and food waste and condensed molasses fermentation soluble to elucidate the effects of food to microorganisms (F/M) ratio (ranging from 0.2 to 38.2) on gas production, metabolite variation, kinetics and biohythane-composition indicator performances. The experimental results indicate that the F/M ratio and fermentation time affect biohythane production efficiency with values of peak maximum hydrogen production rate 9.60 L/L-d, maximum methane production rate 0.72 L/L-d, and hydrogen yield (HY) of 6.17 mol H₂/kg COD_added. Depending on the F/M ratios, the H₂, CH₄ and CO₂ biogas components were 10–60%, 5–20% and 35–70%, respectively. Prospects for the further real application for single-stage biohythane fermentation based on the experimental data are proposed. This work characterizes an important reactor operation factor F/M ratio for innovative single-stage dark fermentation.     

Biohythane production from swine manure and pineapple waste in a single-stage two-chamber digester using gel-entrapped anaerobic microorganisms

A mixture of swine manure and pineapple waste was used to check the feasibility of producing biohythane in a newly-developed single-stage anaerobic fermentation system that having immobilized H₂ and CH₄-producing microbes in a two-chamber digester. Tested hydraulic retention times (HRT) were from 96 h to 6 h. HRT 6 h resulted in peak gas production performance with hydrogen production rate 1240 and methane production rate 812 mL/L-d. Besides, the synergistic function of generation and consumption of volatile fatty acids in this hybrid biosystem had a significant impact on biohythane composition with acetate and butyrate being the dominant liquid metabolites. Chemical oxygen demand and ammonium removal efficiencies were 52.4 and 78.8%, respectively during steady-state conditions. Based on the experimental findings, prospects for field applications of single-stage biohythane fermentation were suggested.     

Enhancement of biohythane production from solid waste by co-digestion with palm oil mill effluent in two-stage thermophilic fermentation

Improvement of biohythane production from oil palm industry solid waste residues by co-digestion with palm oil mill effluent (POME) in two-stage thermophilic fermentation was investigated. A two-stage co-digestion of solid waste with POME has biohythane production of 26.5–34 m³/ton waste. The co-digestion of solid waste with POME increased biohythane production of 67–114% compared to digestion POME alone. Co-digestion of solid waste with POME enhanced hydrolysis constant (k_h) from 0.07 to 0.113 to 0.120–0.223 d⁻¹. The hydrolysis constant (k_h) of co-digestion was 10 times higher than the single digestion of solid waste. Clostridium sp. was predominated in the hydrogen stage, while Methanosphaera sp. was predominant in methane stage. The co-digestion of solid waste with readily biodegradable organic matter (POME) could significantly increase biohythane production with achieving the significant cost reduction for pretreatment of solid wastes.     

The current status and perspectives of biofuel production via catalytic cracking of edible and non-edible oils

Biofuel development has gained the attention of researchers in recent years owing to the rate of depletion of fossil fuels. Several processes are currently employed in the conventional production of different biofuels: the production of biodiesel is catalytically performed either through the transesterification of triglycerides using alcohol or the deoxygenative ecofining of triglycerides in a non-alcohol environment; bio-oil is produced by the pyrolysis of biomass; bio-ethanol is produced by the fermentation of sugars obtained from starch or cellulosic based biomass, while bio-gasoline is produced from the catalytic cracking of triglycerides. Owing to the enormous dependency of transport vehicles running on gasoline engines, the development of bio-gasoline may well reduced the dependence of the fuel market on fossil fuels. The present article summarizes recent progresses and future prospects of biofuel production via catalytic cracking technology. This technology can be implemented in current petroleum refineries with minor modifications. However, reactor design and catalyst choice are important issues and have to be addressed before successful implementation of this technology in commercial ventures.     

Renewable energy resources and technologies applicable to Ireland

The energy consumed in Ireland is primarily achieved by the combustion of fossil fuels. Ireland's only indigenous fossil fuel is peat; all other fossil fuels are imported. As fossil fuels continually become more expensive, their use as an energy source also has a negative impact on the environment. Ireland's energy consumption can be separated into three divisions: transportation, electricity generation and heat energy. Ireland however has a vast range of high quality renewable energy resources. Ireland has set a target that 33% of its electricity will be generated from renewable sources by 2020 [I. Government. Delivering a Sustainable Energy Future for Ireland; 2007.]. The use of biomass, wind and ocean energy technologies is expected to play a major part in meeting this target. The use of renewable energy technologies will assist sustainable development as well as being a solution to several energy related environmental problems. This paper presents the current state of renewable energy technologies and potential resources available in Ireland. Considering Ireland's present energy state, a future energy mix is proposed.     

A pilot study of biohythane production from cornstalk via two-stage anaerobic fermentation

This study aimed to study the feasibility and stability of biohythane production from cornstalk via two-stage anaerobic fermentation without hydrolysis step in a semi-continuous pilot scale system. The present study applied a 1 m³ continuous stirred tank reactor for biohydrogen production and a 0.5 m³ up-flow anaerobic sludge bed for biomethane production. During the entire operation, a hydrogen production yield of 25.02 L/kg TS and hydrogen production rate of 0.46 L/L/d was achieved in first-stage. In addition, a methane yield of 95.38 L/kg TS and methane production rate of 4.06 L/L/d was achieved in second-stage by using the liquid effluent after first-stage. The percentage of hydrogen in the biohythane gas was 18.47% which suitable for vehicle fuel. Moreover, it was feasible to use the solid residue as a growth medium in seedlings to improve energy and carbon recovery. The results suggest that biohythane production from cornstalk could be a promising biofuel avenue.     

Hydrogen production from agricultural waste by dark fermentation: A review

The degradation of the natural environment and the energy crisis are two vital issues for sustainable development worldwide. Hydrogen is considered as one of the most promising candidates as a substitute for fossil fuels. In this context, biological processes are considered as the most environmentally friendly alternatives for satisfying future hydrogen demands. In particular, biohydrogen production from agricultural waste is very advantageous since agri-wastes are abundant, cheap, renewable and highly biodegradable. Considering that such wastes are complex substrates and can be degraded biologically by complex microbial ecosystems, the present paper focuses on dark fermentation as a key technology for producing hydrogen from crop residues, livestock waste and food waste. In this review, recent findings on biohydrogen production from agricultural wastes by dark fermentation are reported. Key operational parameters such as pH, partial pressure, temperature and microbial actors are discussed to facilitate further research in this domain.     

Natural gas and the environmental results of life cycle assessment

Life cycle assessment (LCA) is a method aimed at identifying the environmental effects connected with a given product, process or activity along its life cycle. This paper presents results of the application of LCA method in order to evaluate the environmental advantages of natural gas over other fossil fuels and to have advanced techniques for analysing the environmental aspects of the gas industry. The evaluation of published studies and the application of the method to electricity production with fossil fuels, by using data from published databases and data collected by the gas industry, demonstrate the importance and difficulties of having reliable and updated data required for a significant LCA. Results show that the environmental advantages of natural gas over the other fossil fuels in the final use stage increase still further if the whole life cycle of the fuels, from production to final consumption, is taken into account.     

Biodiesel production and de-oiled seed cake nutritional values of a Mexican edible Jatropha curcas

The scarcity of fossil fuels, in addition to environmental damage due to fossil fuel use and exploration, promotes research into alternative energy sources such as biofuels. Biodiesel has attracted considerable attention in recent years as an alternative to fossil fuels, since it is renewable, biodegradable and non-toxic. Biodiesel can be obtained from animal fat, vegetable oils including cooking oil. In this work, a method of producing biodiesel from seed cake waste from the edible Jatropha curcas L. plant was developed. Oil extraction using hexane gave the best oil quality. Transesterifications of approximately 95% were obtained by alkali or acid catalysis, and the obtained biodiesel products were successfully corroborated with NMR techniques. Since J. curcas is a non-toxic plant, the remaining de-oiled cake was tested for its nutritional properties. Nutritional analysis showed a content of 43% and 33% of protein and carbohydrate, respectively; suggesting that this waste can be used as an attractive protein and carbohydrate source for fermentation processes and/or for formulations for animal feeding. In conclusion, this work provides evidence that the oil from an edible and non-toxic species of J. curcas is an attractive option for biodiesel production with nutritional applications and negligible wasting.     

Recent trends and prospects in biohythane research: An overview

Biohydrogen production from various organic wastes, wastewaters and biomass has been widely studied due to the higher production rates and fundamentals and technologies have also been well developed and heavily documented through diverse laboratory-scale bioreactors. Recently, research has been geared to the concomitant production of biohydrogen and methane which is so called “biohythane”. One-stage and two-stage (bio-H₂ + bio-CH₄) methods are the main biohythane production methods and this field of research for probing into green biofuels is gradually gaining ground. In this paper, the salient aspects of biohythane research at the present time are revisited and the research success and latent promise of biohythane are highlighted based on the findings of the relatively few publications in this area.     

Fermentative hydrogen production from wastewaters: A review and prognosis

Biohydrogen is a promising candidate which can replace a part of our fossil fuels need in day-to-day life due its perceived environmental benefits and availability through dark fermentation of organic substrates. Moreover, advances in biohydrogen production technologies based on organic wastewater conversion could solve the issues related to food security, climate change, energy security and clean development in the future. An evaluation of studies reported on biohydrogen production from different wastewaters will be of immense importance in economizing production technologies. Here we have reviewed biohydrogen production yields and rates from different wastewaters using sludges and microbial consortiums and evaluated the feasibility of biohydrogen production from unexplored wastewaters and development of integrated bioenergy process. Biohydrogen production has been observed in the range of substrate concentration 0.25–160 g COD/L, pH 4–8, temperature 23–60 °C, HRT 0.5–72 h with various types of reactor configuration. The most efficient hydrogen production has been obtained at an organic loading rate (OLR) 320 g COD/L/d, substrate concentration 40 g COD/L, HRT 3 h, pH 5.5–6.0, temperature 35 °C in a continuously-stirred tank reactor system using mixed cultures and fed with condensed molasses fermentation soluble wastewater. The net energy efficiency analysis showed vinasse wastewater has the highest positive net energy gain followed by glycerin wastewater and domestic sewage as 140.39, 68.65, 51.84 kJ/g COD feedstock with the hydrogen yield (HY) of 10 mmol/g COD respectively.     

An econometrics view of worldwide fossil fuel consumption and the role of US

Crude oil, coal and gas, known as fossil fuels, play a crucial role in the global economy. This paper proposes new econometrics modelling to demonstrate the trend of fossil fuels consumption. The main variables affecting consumption trends are: world reserves, the price of fossil fuels, US production and US net imports. All variables have been analysed individually for more than half a century. The research found that while the consumption of fossil fuels worldwide has increased trends in the US production and net imports have been dependent on the type of fossil fuels. Most of the US coal and gas production has been for domestic use, which is why it does not have a strong influence on worldwide fossil fuel prices. Moreover, the reserves of fossil fuels have not shown any diminution during the last couple of decades and predictions that they were about to run out are not substantiated. The nominal and real price of fossil fuels was found to change depending on the type. Finally, estimates of three econometric models for the consumption of fossil fuels from 1949 to 2006 are presented which identify the effects of significant variables.     

A comprehensive review on biohydrogen production pilot scale reactor technologies: Sustainable development and future prospects

The current study focuses on a comprehensive review of the pilot scale production of biohydrogen and various factors affecting the design experiments. Biohydrogen is a clean energy carrier that can be used as a potential alternative to fossil fuels. Biohydrogen as a fuel has several advantageous attributes, including; carbon-neutral or carbon-zero nature, easy renewability, eco-efficient productivity (via biological routes), eco-friendly conversion, and the highest energy content among all existing fuels. Pilot-scale production of biohydrogen is limited because it requires a better understanding of the possible interactions involved in the process. In this review, biohydrogen production on various types of reactors such as stirred tank reactors, packed bed reactors, fluidized bed reactors, trickling filter reactors, etc., have been discussed. However, biohydrogen production has been mostly studied on small scale, the most challenging issue concerning large-scale production of biohydrogen is its relatively high cost over fuels from fossil owing to high feedstock and manufacturing costs. Therefore, cost-effective and eco-friendly biohydrogen production technologies should be necessarily developed and continuously improved to make this biofuel more competitive over its counterpart. In comparison with fossil fuels, biohydrogen has a high energy yield and is highly renewable. It can fulfill the future demand as a transport fuel.     

Trace metals supplementation enhanced microbiota and biohythane production by two-stage thermophilic fermentation

The effect of trace metals supplementation into palm oil mill effluent on biohythane production and responsible microbial communities in thermophilic two-stage anaerobic fermentation was investigated. High biohythane yields were linked to Ni/Co/Fe supplementation (10, 6 and 20 mg L⁻¹, respectively) with maximum H₂ and CH₄ yields of 139 mL H₂ gVS⁻¹ and 454 mL CH₄ gVS⁻¹, respectively. The Ni/Co/Fe supplementation resulted in higher numbers of Bacillus sp., Clostridium sp. and Thermoanaerobacterium sp. together with increasing hydrogenase expression level leading to increasing hydrogen yields of 90.4%. The numbers of Methanosarcina, Methanomassiliicoccus, and Methanoculleus were enhanced by Ni/Co/Fe addition, accompanied by 21.7% higher methane yields. No correlation between methyl coenzyme-M reductase expression level and methane yields was observed. The Ni/Co/Fe supplementation improved gas production in the two-stage biohythane process via enhancing a number of viable hydrogen-producing bacteria together with hydrogenase activity in H₂ stage and enhancing number methanogens in the CH₄ stage.     

Biowaste energy potential in Kenya

Energy affects all aspects of national development. Hence the current global energy crisis demands greater attention to new initiatives on alternative energy sources that are renewable, economically feasible and sustainable. The agriculture-dependent developing countries in Africa can mitigate the energy crisis through innovative use of the available but underutilised biowaste such as organic residues from maize, barley, cotton, tea and sugarcane. Biogas technology is assumed to have the capacity to economically and sustainably convert these vast amounts of biowaste into renewable energy, thereby replacing the unsustainable fossil energy sources, and reducing dependency on fossil fuels. However, the total energy potential of biogas production from crop residues available in Kenya has never been evaluated and quantified. To this end, we selected five different types of residues (maize, barley, cotton, tea and sugarcane) from Kenya and evaluated their energy potential through biomethane potential analysis at 30 °C and a test time of 30 days.The specific methane yields for maize, barley, cotton, tea and sugarcane residues obtained under batch conditions were respectively 363, 271, 365, 67 and 177 m³ per tonne volatile solids. In terms of energy potential, maize, cotton and barley residues were found to be better substrates for methane production than tea and sugarcane residues and could be considered as potential substrates or supplements for methane production without compromising food security in the country. The evaluated residues have a combined national annual maximum potential of about 1313 million cubic meters of methane which represent about 3916 Gigawatt hour (GWh) of electricity and 5887 GWh of thermal energy. The combined electrical potential is equivalent to 73% of the country’s annual power production of 5307 GWh. Utilization of the residues that are readily available on a ‘free on site’ basis for energy production could substitute the fossil fuels that account for a third of the country’s total electricity generation. Besides, exploitation of the potential presented by the biowaste residues can spur an energy revolution in the country resulting in a major economic impact in the region.     

Impact of hydrogen on the environment

The present work considers the impact of hydrogen fuel on the environment within the cycles of its generation and combustion. Hydrogen has been portrayed by the media as a fuel that is environmentally clean because its combustion results in the formation of harmless water. However, hydrogen first must be generated. The effect of hydrogen generation on the environment depends on the production process and the related by-products. Hydrogen available on the market at present is mainly generated by using steam reforming of natural gas, which is a fossil fuel. Its by-product is CO₂, which is a greenhouse gas and its emission results in global warming and climate change. Therefore, hydrogen generated from fossil fuels is contributing to global warming to the similar extent as direct combustion of the fossil fuels. On the other hand hydrogen obtained from renewable energy, such solar energy, is environmentally clean during the cycles of its generation and combustion. Consequently, the introduction of hydrogen economy must be accompanied by the development of hydrogen that is environmentally friendly. The present work considers several aspects related to the generation and utilisation of hydrogen obtained by steam reforming and solar energy conversion (solar-hydrogen).