Strategies for bioenergy production from agriculture and agrifood processing residues

Agriculture and agrifood processing activities worldwide generate huge amounts of by-products and waste. Such waste streams are only partially valorized at different value-added levels (biofuel production, to spread on land, animal feed, composting), whereas the main volumes are managed as waste of environmental concern, with significant negative effects on the global sustainability of the agriculture and agrifood sector. During the last few decades, concepts for waste management patterns and priorities have rapidly evolved. Priorities have been defined from the most preferred one, minimization/avoidance, to the least desirable, landfilling. In future waste management strategies and scenarios, materials recovery and energy production processes will play fundamental roles. In fact, the need for alternative energy resources is nowadays a strong and relevant issue worldwide. Therefore, proper waste management and treatment of agriculture and agrifood processing activities may allow for greenhouse gas (GHG) mitigation and give possibilities for bioenergy production. These constraints are a strong driving force toward developing new and affordable technologies, capable of combining proper and sustainable waste management with the production of clean and renewable energy.

Several technologies have been proposed for bioenergy production from agriculture and agrifood processing residues. These technologies can be classified as biochemical or thermochemical conversion processes. The selection of the most suitable conversion process depends strongly on the feedstock properties and the available pretreatment techniques and logistics.

Biochemical processes include the production of bioethanol, biodiesel and biogas through three different processes. Bioethanol can be produced from renewable biomass and adapted to existing fuel supply systems leading to cleaner combustion. Bioethanol is produced via pretreatment, hydrolysis and fermentation stages. Biodiesel is an alternative liquid fuel that has gained worldwide popularity in order to reduce the dependence on fossil fuels in the transport sector and to increase the renewable energy utilization. Biodiesel is produced through oil extraction and transesterifaction. Biogas is an alternative gaseous fuel that has attracted significant attention with the increasing interest in renewable and sustainable energy technology. Biogas production systems include different biological processes and energy conversion stages.

Thermochemical processes can be divided into two main groups based on the feedstock water content. Hence, combustion, gasification or pyrolysis are suitable when the feedstock has low water content. For example, several investigations have applied these processes to woody agricultural biomass waste or municipal solid wastes. When the feedstock presents with high water content (i.e. moisture higher than 60%), hydrothermal processes become more appropriate. These processes make use of hot pressurized water to convert wet substrates. Pressure is always held high enough to keep water in its liquid or, possibly, supercritical state. These processes are classified as hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL) and hydrothermal gasification (HTG) depending on the process temperature and the type of products obtained.

This special issue provides an overview of the development of economic and technological solutions for the recovery of various agriculture and agro-industrial wastes. This special issue presents the recent investigations into the appropriate waste treatment systems and conversion processes including feedstock characterization, experimental study and simulation of the biochemical and thermochemical conversion processes. Here is a short description of the nine papers that have been accepted in this special issue.

Nitos et al. have examined bioethanol production from olive pruning, vineyard pruning and almond shells [1]. The authors have focused on the effect of different pretreatment methods on the enzymatic saccharification efficiency of cellulose and the production of second-generation bioethanol. Hence, dilute acid, hydrothermal and steam explosion pretreatments were compared in order to remove hemicellulose and facilitate the subsequent enzymatic hydrolysis of the hemicellulose-deficient biomass to glucose. Enzymatic hydrolysis was performed in a free-fall mixing reactor enabling a high solids loading of 23% w/w. This allowed hydrolysis of up to 67% of available cellulose in almond shells and close to 50% in olive pruning samples, and facilitated high ethanol production in the subsequent fermentation step; the highest ethanol concentrations achieved were 47.8 g/l for almond shells after steam explosion and 42 g/l for hydrothermally pretreated olive pruning residue.

In the same context, Sivamani and Baskar have made an attempt to maximize bioethanol production from cassava stems using a statistical experimental design [2]. Therefore, organic acid catalysis was employed as an alternative pretreatment method to mineral acid catalysis. The effects of solid-to-liquid ratio (the ratio between the mass of the cassava stems and the volume of oxalic acid), reaction time and oxalic acid concentration were studied on the concentration of reducing sugars at 121 °C and 15 psi. An optimum yield of 1.564 g/l of reducing sugars was obtained at 0.08 g/ml solid-to-liquid ratio, 2.97% (w/v) oxalic acid concentration and 27 min reaction time with 5 g of cassava stems. Cassava stems were characterized using Fourier-transform infrared (FTIR) spectra and scanning electron microscopy (SEM) images before and after pretreatment. The residue was hydrolyzed with 0.5 ml of cellulase (16.8 FPU/g), and 1.889 g/l of glucose was obtained, after pretreatment. Finally, the fermentation was performed by co-culturing Kluyveromyces marixianus with Klebsiella oxytoca, Fusarium oxysporum, Zymomonas mobilis and Saccharomyces cerevisiae. A maximum ethanol concentration and yield of 1.622 g/l and 0.32 g ethanol/g cassava stem, respectively, were obtained for K. marixianus–K. oxytoca co-culture.

Biodiesel production from waste cooking oil (WCO) was examined for the first time in Vietnam by Nguyen et al. [3]. The authors performed their experimental tests in a static mixer reactor using sodium hydroxide as a catalyst. The influence of three parameters (methanol-to-oil molar ratio, catalyst concentration and reaction temperature) on fatty acid methyl ester (FAME) production was investigated. The optimization was determined by a response surface methodology (RSM) and the interaction of these parameters was described by a quadratic model. The results revealed that the catalyst concentration had the greatest influence on FAME yield among the investigated conditions. The highest content of FAME was achieved of up to 97.8 wt% at the optimized conditions (methanol-to-oil molar ratio of 6, reaction temperature of 60 °C and 1.0 wt% of catalyst). In addition, the obtained biodiesel product fulfilled the European and Vietnamese biodiesel standards.

Biogas production from lignocellulose biomass/lignocellulosic waste biomass (LCB/LCWB) is currently a major challenge. Although, these feedstocks are assumed to have a lower theoretical yield for biogas than waste material made of sugar or starch, they are free from the problems associated with other generations of biofuels. However, an inexpensive and efficient pretreatment method of LCB/LCWB is highly desirable so as to achieve an economical biogas production process. Ahmed et al. have reviewed the conventional, advanced and infant (i.e. under development) pretreatment methods that have been studied for the enhancement of biogas production [4]. In addition to various pretreatment methods, the authors have presented further aspects of the conventional, advanced and infant methods (nanotechnology) for pretreatment of LCB/LCWB. Thus, the review has provided information on the systematic technological strategies and new pretreatment approaches for sustainable bioprocessing of LCB/LCWB into a value-added product.

Biohydrogen is emerging as a suitable alternative to fossil fuels and has received worldwide popularity in recent years due to its economic, social and environmental benefits. Sekoai et al. have examined the effect of nitrogen gas sparging on dark fermentative biohydrogen production using suspended and immobilized cells of anaerobic mixed sludge [5]. A maximum biohydrogen fraction of 56.98%, which corresponded to a biohydrogen yield of 294.83 ml H₂/g total volatile solids (TVS) was achieved in a dark fermentation process using N₂-sparged cells that were immobilized in calcium alginate beads. The biohydrogen production yield from the N₂-sparged immobilized cells was 1.8 and 2.5 times higher than that of sparged suspended cells and non-sparged (control) suspended cells, respectively. Therefore, the synergistic effect of nitrogen gas sparging and cell immobilization was instrumental in inhibiting the biohydrogen-scavenging bacteria during the dark fermentation process, thereby enhancing the yield. These findings could pave the way for the development of a large-scale biohydrogen production process from biowaste feedstocks.

Plant microbial fuel cells (PMFCs) have emerged as a renewable source of energy that can produce concurrent bioelectricity and biomass continuously in a clean, sustainable and efficient manner. The core idea lies in the conversion of the solar energy trapped by plants into electricity with the aid of bacterial actions in the rhizosphere of plants. PMFC research is still in an early phase. However, many stakeholders including private organizations, universities and individuals are currently experimenting with and building their own PMFC prototypes to improve the power delivered while exploiting complementary advantages such as the treatment of wastewater. PMFC is a new technology and involves multidisciplinary fields ranging from the study of microbes to electrochemistry, electrical engineering, chemical engineering and plant science itself. The science of the relationship that exists among these aspects in terms of system performance is still not clarified. PMFC relies on a biological process and can be operated under mild operating conditions. After proof of the principle in 2008 on rhizosphere-mediated electricity production, many advancements have been made. Regmi et al. have provided a concise update on PMFC research [6]. Some important breakthroughs are mentioned, along with a discussion of the present scenario and future directions.

During the last decade, hydrothermal processes have received increasing attention. Aykaç et al. have employed hydrothermal processing to produce crude bio-oil and biochar from waste jujube stones by deploying metal carbonates as catalysts [7]. The effects of metal carbonate catalysts (K₂CO₃, Na₂CO₃ and SrCO₃) in different concentrations on the product yields and properties were tested for the conversion studies. The use of metal carbonates (K₂CO₃, Na₂CO₃ and SrCO₃) increased the yield of bio-oil almost twofold, with the highest yield (18.66 wt%) obtained using K₂CO₃ at a concentration of 10 wt%. (E)-9-octadecenoic acid (elaidic acid), 2,6-dimethoxyphenol (syringol) and 2-methoxyphenol (guaiacol) were some of the prominent compounds found in these crude bio-oils. The process also resulted in an efficient conversion of jujube stones to biochar with a high content of oxygenated functional groups, which makes it an effective precursor for various applications. The heating values of the bio-oils and biochars were significantly improved compared to those of the feedstock.

Among the various available hydrothermal processes, the hydrothermal gasification process consists of bringing a wet biomass beyond the supercritical conditions of temperature and pressure for syngas production. Houcinat et al. have developed a kinetic mathematical model of glycerol supercritical gasification [8]. This model was resolved with Mathcad 14 and assessed by comparison with experimental results from previous work reported in the literature. The effect of three parameters, namely temperature, residence time and glycerol concentration, on the efficiency of gasification, the production of gases and the lower calorific value were investigated. Using surface response methodology, three quadratic models which correlated the gasification efficiency, the gas yields and the lower calorific value (LCV), in terms of the considered factors, were also obtained and assessed using analysis of variance. Thus, optimal operating conditions were obtained and the results showed that maximum gasification efficiency, H₂, CO production and LCV responses were obtained at a temperature of 809.36 K, a residence time of 10 s and an initial glycerol concentration of 5 wt%.

Catalytic steam gasification is a promising technique for hydrogen production. Hussein et al. have developed a simulation model for the catalytic steam gasification of palm kernel shells in an atmospheric dual fluidized bed gasifier using an Aspen Plus® simulator [9]. The catalytic adsorbent-based steam gasification of palm kernel shells is studied in a pilot-scale dual fluidized bed reactor using coal bottom ash as a catalyst for hydrogen and syngas production. The use of a catalyst along with the adsorbent improved tar cracking and enhanced the hydrogen content of the syngas. The effect of temperature and the steam–biomass ratio on hydrogen yield, syngas composition and lower and higher heating values was studied. An increase in steam–biomass ratio enhanced the hydrogen content from 60 to 72 mol%. The maximum value of hydrogen production, i.e. 72 vol%, was achieved at a steam–biomass ratio of 1.7. The use of adsorbent and coal bottom ash had a significant effect on hydrogen and syngas yield. A maximum of 80.1 vol% hydrogen was achieved at a temperature of 650 °C with a 1.25 steam–biomass ratio and 0.07 wt% coal bottom ash.

The Guest Editors are grateful to the Editor-in-Chief of Biofuels Journal, Professor Marc Rosen, for providing the opportunity to organize this special issue. Thanks also go to the entire production team of the journal for their valuable support in bringing out this issue. Last but not least, we express our sincere appreciation to all the reviewers for their invaluable and critical review comments on the manuscripts that were submitted for this special issue.

Disclosure statement

No potential conflict of interest was reported by the author(s).