Biofuel and Bioenergy Technology

Wei-Hsin Chen 1,* , Keat Teong Lee 2,* and Hwai Chyuan Ong 3,* 1 Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan 2 School of Chemical Engineering, Universiti Sains Malaysia, Nibong Tebal 14300, Pulau Pinang, Malaysia 3 Department of Mechanical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia * Correspondence: chenwh@mail.ncku.edu.tw or weihsinchen@gmail.com (W.-H.C.); ktlee@usm.my (K.T.L.); onghc@um.edu.my (H.C.O.)


Introduction
Biomass is considered as a renewable resource because of its short life cycle, and biomass-derived biofuels are potential substitutes to fossil fuels.When biomass grows, all carbon in biomass comes from the atmosphere and is liberated into the environment when it is burned.Therefore, biomass is thought of as a carbon-neutral fuel.For these reasons, the development of bioenergy is an effective countermeasure to elongate fossil fuel reserves, lessen greenhouse gas (GHG) emissions, and mitigate global warming and climate change.Biomass can be converted into biofuels through a variety of routes such as physical, thermochemical, chemical, and biological methods.The common and important biofuels for bioenergy include charcoal, biochar, biodiesel, bioethanol, biobutanol, pyrolysis and liquefaction bio-oils, synthesis gas (syngas), biogas, and biohydrogen, etc.On account of the merit of bioenergy for environmental sustainability, biofuel and bioenergy technology plays a crucial role for renewable energy development.This Special Issue aims to publish high-quality review and research papers, addressing recent advances in biofuel and bioenergy.State-of-the-art studies of advanced techniques of biorefinery for biofuel production are also included.Research involving experimental studies, recent developments, and novel and emerging technologies in this field are covered.The particular topics of interest in the original call for papers included, but were not limited to:

Statistics of the Special Issue
The response to our call had the following statistics:   The Netherlands (1).
Published submissions are related to the most important techniques and analysis applied to the biofuel and bioenergy technology.In summary, the edition and selections of papers for this special issue are very inspiring and rewarding.We thank the editorial staff and reviewers for their efforts and help during the process.

Brief Overview of the Contributions to This Special Issue
Table 1 provides some of the key information, including the research type, field of study, final product as well as the key findings.As observed, a majority of the publications (twenty-three papers) focus on experimental work to improve or explore novel technologies for energy-products synthesis, while three papers focus on modelling studies and two papers focus on literature review studies.The following discussion highlights and groups the research findings in accordance to the corresponding research field or work.
As the initial step in most synthesis routes, biomass processing can enhance the substrate's quality for other synthesis processes.Thus, commonly, these are treated as pretreatment to enhance the characteristics of the biomass.In two research works [1,2], the combination of physical treatment (ball milling) and chemical treatment (ethanol organosolv) showed improved glucan digestibility.Three different biomasses such as giant miscanthus, corn stover and wheat straw were pretreated with ball milling and ethanol organosolv and the overall biomass size was reduced as a result of the prolonged pre-treatment [1].Due to the improved physicochemical characteristics resulting from the pre-treatment, a maximum of 91% glucan digestibility could be achieved.A parametric study on combined ball milling and organosolv was performed as well to optimize the glucan digestibility [2].It was determined that at 170 • C, with reaction time of 90 min and ethanol concentration of 40% and liquid/solid ratio of 10, the pretreatment process achieved the best results.Thus, the biomass processing method could be beneficial in generating desired products.

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Optimisation with response surface methodology.

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Oxidizer and fuel Lewis number were between 0.4 and 1, the maximum flame temperature was ~1860 K.

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Per unit of fuel Lewis number, the minimum thermophoretic force was −1.48 × 10 −8 N.

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These products can be used for bio-briquette fuel production.

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Occurrence of potentially harmful microorganisms such as Clostridium novyi was detected at higher ratio (65.63%) in the population of the bioreactor.

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First-Law Efficiency increased with increased reaction temperature for higher hydrogen and carbon monoxide yields.

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Second-Law Efficiency decreased with increased reaction temperature due to more complete chemical reaction.

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The review discussed the complexity of metabolic networks to obtain these bio-hydrocarbon products.

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Biogas composition was measured for 107 small-scale biogas plants, respectively.

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Different calcination temperatures (500-900 • C) of iron oxide (F e2 O 3 ) were tested to investigate their photocatalytic properties within the cathodic chambers.

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Within one hour, oily water was best-degraded up to 99.3% with electrode material synthesised at 500 • C with maximum power density of 52.5 mW/m 2 .
Bio-oils can be synthesized from sewage sludge by using pyrolysis techniques [3].Taguchi optimization suggested the best pyrolysis was performed at 450 • C, 60 min and 10 • C/min, which also showed consistency with other research work.Nonetheless, under most conditions, pyrolytic oil/bio-oil requires further processing or upgrading for use as biodiesel.To maintain the stability of bio-oil, blending with emulsifier resulted in high solubility (58.83-70.96wt %) [4].These findings suggest that simple blending could improve the properties of biodiesel or bio-oil tremendously, which is worthy of further investigation.Aside from using bio-oil as a precursor for biodiesel [3], biodiesel could be directly synthesized using other oil materials such as Australian native stone fruit oil [6], rapeseed oil [7], Rhodotorula glutinis [8] and soursop seed oil [9] via transesterification techniques.Transesterification of Australian native stone biodiesel showed a high yield of 95.8% with the response surface methodology optimization and its quality fulfilled the ASTM D6751 and EN14214 requirements [6].Kuan et al. [8] investigate both direct acid and base-transesterification on Rhodotorula glutinis biomass which gave 111% yield of FAME and 102% yield of FAME, respectively, which were regarded as of good biodiesel quality.Another research work by Su et al. [9] used soursop seed to produce bio-oil which was eventually upgraded to biodiesel using a two-step acid catalyzed transesterification.The biodiesel produced met both EN14214 and D6751 standards.Encinar et al. [7] performed rapeseed transesterification with KOH catalyst as well as with the aid of ultrasound whereby the kinetic behavior obeyed a pseudo-first order trend.Liquid lipase-based esterification was attempted and optimized using RSM to enhance the usage of water removal agent in the system [11].In Anwar et al's, [5] work, it was found that by blending papaya oil biodiesel with varying contents (5-20%) with diesel could improve the engine testing properties.Microalgal biodiesel was generated using photobioreactor with coal-fired flue gas from three strains (M082, M134 and KR-1) [10].Among the strains, M082 generated high lipid value of 397 mg/g which was regarded to be a suitable feedstock for biodiesel production.
In recent years, gasification also garners high interest due to its rapid processing step and high yield of syngas which could be directly used for combustion.One of the main constitutes of syngas is hydrogen which usually provides high calorific value.Based on Chein and Hsu's [12] work, the tri-reforming process could produce good quality syngas.In addition, it was also found that at higher reaction temperatures, more hydrogen and carbon monoxide were produced.In pilot plant study, the gasifier which was embedded with specific solid oxide fuel cell system in an industrial scale was investigated in details [13].To further understanding the gasification process, a thermodynamic equilibrium constants derivation and modelling was performed for two cases, with and without tar.The simulated data were validated with experimental data [14].These findings could serve as good guides for future development of gasification process.
Bio-digester or bioreactor could also be used for biogas production.Černý et al. [15] discovered that microorganisms such as Clostridium novyi were detected at higher ratio (65.63%) in the population of the bioreactor for the biogas production.Such detection could serve as an important reminder to seek ways to inhibit these harmful microorganisms in the system.In an investigation and survey of 107 biogas plants, it was also found that the younger plant (<5 years) produced higher CH 4 (65.44%) and CO 2 (29.31%) [16].Addition of granular activated carbon (GAC) in the digestion system could also directly improve the methane production by more than 34% [17].Thermophilic anaerobic digestion is another interesting field of research.David et al. [18] found that by co-digestion under such conditions, high yield of methane (up to 305.45 kg/L) could be achieved.Musa's work critically reviewed some of the more critical findings on anaerobic membrane reactors for biogas recovery especially on membrane fouling and parameters of operation [19].
The studies on alcohol-based biofuels are increasing due to its high energy-content and suitability as fuel products.Dzieko ńska-Kubczak et al. [20] report that nitric acid as a form of chemical pretreatment could enhance the bioethanol production up to 30%.A Polymer Electrolyte Membrane Fuel Cell was used to produce isopropanol from acetone for use as a biofuel [21].The hydrogenation process consumes less energy and less chemical wastes compared to other techniques [21].Bio-hydrocarbons (alcohol and alkenes) produced from bacteria and their synthesis mechanisms are reviewed by Rahman et al. [22], as well as future challenges and complexity.
High pressure densification and torrefaction are currently attracting attention among the research community as these methods can produce potential solid-based fuels which require no further upgrade and can be use directly.Both methods are usually applied in mild synthesis conditions which differ from common thermochemical conversion techniques like slow pyrolysis and fast pyrolysis.For example, biomass with low ash content (1.16-8.62%)and good mechanical durability (97%) such as bamboo fibre and sugarcane skin could be directly densified as bio-briquette fuel without any energy processing [24].As for torrefaction, a form of mild pyrolysis, wood wastes could be converted into torrefied biomass as low as 300 • C [23].As observed, both methods consume relatively lower energy requirement and are simpler in term of synthesis process.In a modelling study, Lycopodium particles were also simulated and modelled as biofuel and burned in air environment [25].It was discovered that the particles of Lycopodium were greatly influenced by thermophoretic force.Microbial fuel cell studies were also being investigated thoroughly.The effect of the hydrodynamic layer thickness was found to be significant on the voltage and charged transfer resistance [26].In another study, the calcination temperature on the cathodic chambers was studied and it was found that the electrode synthesized at 500 • C could degrade oily wastewater up to 99.3% [27].Thus, the microbial fuel cell shows tremendous potential to be developed for other applications.

Table 1 .
Key Information of the Publications Submitted to Special Issue.