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This study reviews the innovations and optimizations in biogas production from the biological perspective reported by recently published patents and research works. The proposed biological strategies can be categorized into three different groups, i.e., upstream, mainstream, and downstream approaches. In the first part of this review, upstream strategies, including pretreatments by fungal, microbial consortium, and enzymatic as well as some other biological methods including microaeration, composting, ensiling, and genetic and metabolic engineering are discussed in detail. The impacts of upstream strategies on biogas production as well as their potentials in further improving the biogas industry are comprehensively scrutinized. Despite their promising impacts on biogas production, such biological innovations are time-consuming and require extra equipment and facilities that should be addressed in future studies. Overall, most information on biogas production has been generated through lab-scale investigations and not by commercial plants, undermining the commercial value of these data for the right decision-making. Pilot data would be necessary for techno-economic analyses with acceptable accuracies. Therefore, the future efforts should be directed toward providing the missing data for re-engineering designs, calculations, and life cycle assessment (LCA) of the newly designed biogas plants.

This review is aimed at detailing and discussing biological innovations and optimizations including upstream, mainstream, and downstream approaches in biogas production elaborated in the recently published patents and research papers. In the first part of this review, upstream strategies including pretreatments by fungal, microbial consortium, and enzymatic as well as some other biological methods including microaeration, composting, ensiling, and genetic and metabolic engineering were comprehensively presented and duly discussed. Despite the fact that these approaches hold great promise for improving the quantity and quality of the evolved biogas, the need for more processing time and extra equipment are the main drawbacks of such strategies. In the second part of this review, mainstream and downstream strategies are reviewed, and their merits and limitations are outlined. Bioaugmentation, anaerobic co-digestion, and integrated biogas production are evaluated as mainstream strategies. Finally, downstream biological approaches used for removal of CO2, H2S, and other impurities are reviewed and discussed. Among the mainstream biological techniques, bioaugmentation is highly advised to accelerate start-up periods at commercial scale while anaerobic co-digestion is recommended to not only enhance biogas yield throughout the operation but also to take advantage of different waste streams. Such mainstream strategies could contribute to improving the economic facets of biogas plants. Despite their currently underdeveloped status, downstream techniques for biological removal of biogas impurities are expected to replace their physiochemical counterparts at industrial scale in future owing to increasingly stringent environmental regulations.

It is obvious that Iran agricultural industry, unlike Brazil and USA, cannot afford to provide conventional biomass, i.e. sugary or starchy biomass for bioethanol production, mainly due to climatic and geographic conditions. With some exception of date (fruit), first-generation ethanol production triggers food vs. fuel debates in Iran and put nation to hunger. Agricultural products including apple, barley, carrot, corn, grape, orange, potato, rice, sugar beet, sugarcane, and wheat are consumed domestically, exported, or even lost because of poor harvesting and processing conditions such as transportation or packaging. These products may alone generate 21.56 million ton per annum green wastes upon processing in the food industry. Every year about 5.4 billion liters of bioethanol can be produced by establishing second-generation ethanol plants next to the food processing sectors. Seventy-seven-percent of this amount of bioethanol can easily support 5% ethanol (E5) policy to phase out the consumption of 4.2 billion liters methyl tert-butyl ether (MTBE) for raising the octane number of gasoline in the country. If more comprehensive policy is adopted, larger quantities of lignocellulosic feedstocks can be gathered from agro as well as forestry practices. Second-generation bioethanol technology can help Iran to tackle air pollution in its big cities and to address the adverse effects of MTBE on its populations and ecosystem. The other advantages are improvement of fuel security, mitigation of climate change, and development of economy. The motivation can be created through passing a framework policy to cut fossil fuel subsidies, to mandate bioethanol blends in gasoline, and to impose carbon taxes. Development of coherent socially and environmentally relevant strategies and facilitation of investment in bioethanol industry are also necessary.

Appropriate bioprocessing of lignocellulosic materials into ethanol could address the world’s insatiable appetite for energy while mitigating greenhouse gases. Bioethanol is an ideal gasoline extender and is widely used in many countries in blended form with gasoline at specific ratios to improve fuel characteristics and engine performance. Although the bioethanol production industry has long been operational, finding a suitable microbial agent for the efficient conversion of lignocelluloses is still an active field of study. Among available microbial candidates, engineered bacteria may be promising ethanol producers while may show other desired traits such as thermophilic nature and high ethanol tolerance. This review provides the current knowledge on the introduction, overexpression, and deletion of the genes that have been performed in bacterial hosts to achieve higher ethanol yield, production rate and titer, and tolerance. The constraints and possible solutions and economic feasibility of the processes utilizing such engineered strains are also discussed.

Liquid transportation biofuel production is a promising strategy to reduce greenhouse gas emissions. Hydrothermal gasification (HTG) has shown great potential as an effective method for valorizing wet biomass. The high-quality syngas produced using the HTG process can be chemically/biochemically converted to liquid biofuels. Therefore, this paper aims to comprehensively review and critically discuss syngas production from biomass using the HTG process and its conversion into liquid biofuels. The basics and mechanisms of biomass HTG processing are first detailed to provide a comprehensive and deep understanding of the process. Second, the effects of the main operating parameters on the performance of the HTG process are numerically analyzed and mechanistically discussed. The syngas cleaning/conditioning and Fischer-Tropsch (FT) synthesis are then detailed, aiming to produce liquid biofuels. The economic performance and environmental impacts of liquid biofuels using the HTG-FT route are evaluated. Finally, the challenges and prospects for future development in this field are presented. Overall, the maximum total gas yield in the HTG process is obtained at temperature, pressure, and residence time in the range of 450–500 °C, 28–30 MPa, and 30–60 min, respectively. The highest C5+ liquid hydrocarbon selectivity in the FT process is achieved at temperatures between 200 and 240 °C. Generally, effective conversion of biomass to syngas using the HTG process and its successful upgrading using the FT process can offer a viable route for producing liquid biofuels. Future studies should use HTG technology in the biorefinery context to maximize biomass valorization and minimize waste generation.

Environmental deterioration, global climate change, and consequent increases in pollution-related health problems among populations have been attributed to growing consumption of fossil fuels in particular by the transportation sector. Hence, replacing these energy carriers, also known as major contributors of greenhouse gas emissions, with biofuels have been regarded as a solution to mitigate the above-mentioned challenges. On the other hand, efforts have been put into limiting the utilization of edible feedstocks for biofuels production, i.e., first generation biofuels, by promoting higher generations of these eco-friendly alternatives. In light of that, the present review is aimed at comprehensively assessing the role and importance of microorganisms such as bacteria and yeasts as catalysts for sustainable production of liquid biofuels including bioethanol, biomethanol, biobutanol, bio-ammonia, biokerosene, and bioglycerol. Various aspects of these biofuels, i.e., background, chemical synthesis, microbial production (including exploitation of wild and metabolically-engineered species), and product recovery as well as the derivatives produced from these biofuels which are used as fuel additives are thoroughly covered and critically discussed. Furthermore, the industrial features of these green liquid fuels including the industrial practices reported in the literature and the challenges faced as well as possible approaches to enhance these practices are presented.

Finding renewable alternative energy resources for fossil fuels substitution has become very vital due to the serious challenges faced by humankind at present such as environmental pollution, greenhouse gas emissions, climate change, crude oil price volatility, and fossil fuels exhaustion. Macroalgae (seaweeds) are fast-growing marine plants, providing several harvests per year without the need for arable land, fertilizer, and fresh water. Various types of ecosystems like coral reefs, mangrove forests, and rocky shores can efficiently host the seaweeds production systems. These characteristics have made them highly suitable feedstocks for third-generation bioethanol production. Iran has a huge potential in renewable energy resources owing to its unique geographical location and climatic features. The country borders with the Caspian Sea in the north and with the Persian Gulf and the Gulf of Oman in the south. Seaweeds farming can also play a key role in mitigating air pollution, increasing employment rate, sustaining fossil fuel resources, bioremediating contaminated water, and improving marine ecosystem in the Persian Gulf and the Gulf of Oman. In the present article, macroalgae diversity, cultivation, and their conversion and upgrading technologies into bioethanol in Iran are scrutinized and discussed. Finally, the potential of Bushehr (the Persian Gulf) and Chabahar (the Gulf of Oman) coastlines for macroalgae cultivation is investigated. These locations receive the annual solar radiation in the range of 1680‒1753 kWh/m2 and the photosynthetically active radiation (PAR) in the range of 2.6‒2.71 GJ/m2/year with 3051‒3311.9 h sunshine per annum. Furthermore, the nutrient-rich and calm water with relatively stable pH, salinity, and temperature make these coasts suitable for macroalgae farming. A potential yield up to 147‒153 t/ha/year can be obtained if proper native/engineered species, well-situated sites, and compatible cultivation techniques are selected.

Agricultural and livestock/slaughterhouse wastes are produced in huge quantities and could be promising sources for electricity generation to partially reduce dependence on fossil-oriented electricity. In light of that, the state of the production for these wastes in different parts of the globe was first presented. Subsequently, various waste-to-energy technologies and their electricity generation feasibility from both environmental and economic perspectives were reviewed. Finally, electricity generation and greenhouse gas (GHG) reduction potentials through anaerobic digestion of the above-mentioned wastes generated in each province of Iran in 2016 were investigated as a case study. Accordingly, Khuzestan province with the highest amount of agro-wastes (7.61 million tons) and Sistan and Baluchestan province with the highest amount of livestock/slaughterhouse wastes (10.69 million tons) had the maximum electricity generation potentials of 82.83 MW and 164.23 MW, respectively. Moreover, the country's total potentials for electricity generation from these wastes was determined at 2848.26 MW. Based on the life cycle assessment (LCA) results, a minimum of 10,693.5 thousand tons CO2eq/yr could be avoided by valorizing these wastes into electricity in Iran. This would be translated into 1.5% reduction of Iran's annual GHG emissions; a promising achievement given Iran's international GHGs reduction commitment. Moreover, if a short-term time horizon would be considered for electricity generation from these wastes, the reduction rate could reach 24,153 thousand tons CO2eq/yr, i.e., 3.39% reduction of Iran's annual GHG emissions. To the best of our knowledge, this is the first report scrutinizing electricity generation potentials from both agricultural and livestock/slaughterhouse wastes in Iran while assessing the consequent GHG emission reduction potentials. The outcomes of the study could assist policy makers with decisions aimed at taking advantage of these resources while the platform introduced pave the way for other researchers to conduct similar studies in different parts of the world.

Application of acid-activated bentonite and SO3H-functionlized multiwall carbon nanotubes (SO3H-MWCNTs) for lowering free fatty acids (FFAs) content of low-quality residual olive oil, prior to alkali-catalyzed transesterification was investigated. The used bentonite was first characterized by Scanning Electron Microscopy (SEM), Inductively Coupled Plasma mass spectrometry (ICP-MS), and X-ray fluorescence (XRF), and was subsequently activated by different concentrations of H2SO4 (3, 5, and 10 N). Specific surface area of the original bentonite was measured by Brunauer, Emmett, and Teller (BET) method at 45 m2/g and was best improved after 5 N-acid activation (95–98°C, 2 h) reaching 68 m2/g. MWCNTs was synthesized through methane decomposition (Co-Mo/MgO catalyst, 900°C) during the chemical vapor deposition (CVD) process. After two acid-purification (HCl, HNO3) and two deionized-water-neutralization steps, SO3H was grafted on MWCNTs (concentrated H2SO4, 110°C for 3 h) and again neutralized with deionized water and then dried. The synthesized SO3H-MWCNTs were analyzed using Fourier-Transform Infrared Spectroscopy (FTIR) and Transmission Electron Microscopy (TEM). The activated bentonite and SO3H-MWCNTs were utilized (5 wt.% and 3 wt.%, respectively), as solid catalysts in esterification reaction (62°C, 450 rpm; 15:1 and 12:1 methanol-to-oil molar ratio, 27 h and 8 h, respectively), to convert FFAs to their corresponding methyl esters. The results obtained revealed an FFA to methyl ester conversion of about 67% for the activated bentonite and 65% for the SO3H-MWCNTs. More specifically, the acid value of the residual olive oil was decreased significantly from 2.5 to 0.85 and 0.89 mg KOH/g using activated bentonite and SO3H-MWCNTs, respectively. The total FFAs in the residual olive oil after esterification was below 0.5%, which was appropriate for efficient alkaline-transesterification reaction. Both catalysts can effectively pretreat low-quality oil feedstock for sustainable biodiesel production under a biorefinery scheme. Overall, the acid-activate bentonite was found more convenient, cost-effective, and environment-friendly than the SO3H-MWCNTs.

A sustainable management of environment and agriculture is crucial to protect soil, water, and air during intensified agriculture practices as well as huge industrial and transportation activities. A promising tool to address these challenges could be the application of biochar, a carbonaceous product of biomass pyrolysis. The efficiency of biochar could be improved through physical, chemical and microbial procedures. Engineered biochar could then be applied for various applications ranging from sustainable agriculture to pollution remediation and catalytic reactions. Biochar engineering allows achieving biochar properties which are optimum for specific applications and/or under specific conditions. This would lead to harnessing the favorable features of biochar and to enhance its efficiency while simultaneously minimizing the existing tradeoffs. This review covers the production and applications of engineered biochar by summarizing great deals of research and knowledge on the field. Unlike previous reviews, herein biochar physical and chemical properties and the factors affecting them (i.e., biomass nature and pyrolysis conditions) have been discussed in detail. Moreover, the contributions of each physical and chemical activation/modification method to improving biochar characteristics with respect to environmental applications have been specifically scrutinized. By providing the state-of-the-art knowledge about engineered biochar production, properties, and applications, this review aims to help research in this field for identification of the culprits that must be addressed in future experiments.

Although currently microalgae biomass is not considered as a sustainable feedstock for biofuel production, future developments of microalgae cultivation and harvest could make the commercial application of such fast-growing photosynthetic biomass economically and environmentally feasible. This article aims at reviewing thermochemical conversion of microalgae into bio-crude oil through pyrolysis and hydrothermal liquefaction technologies. Subsequently, possible solutions to overcome the constraints to achieve the sustainable conversion of microalgae biomass are discussed in detail. The drawbacks of bio-crude oil as a transportation fuel and the technologies required for its upgrading are highlighted. Currently, microalgae-derived bio-crude oil is inferior to biodiesel and diesel in terms of quality, thus cannot be used as a transportation or jet fuel. It requires catalytic upgrading steps and further processing, including durable and cost-effective catalysts with strong regenerative capabilities.