Enhancement of hydrogen production by optimization of pH adjustment and separation conditions following dilute acid pretreatment of lignocellulosic biomass
Introduction
Renewable energy sources are currently of great interest due to increasing demand for energy, caused by development and increase in population. Due to non-renewability, fossil fuels are considered to be limited energy sources, which also pose adverse environmental effects, such as greenhouse gas emissions [1]. Biorefinery is the integration of various conversion and separation unit processes of biomass to fuel, chemicals, and energy. These processes enable the harnessing of renewable energy from carbohydrate-rich agricultural residues and biological matter by thermochemical conversion of biomass [2]. Thus, it is considered as one of the solutions to reduce carbon footprint and dependency on fossil fuel for sustainable development [3].
Among the currently studied renewable energy sources for biofuel production is lignocellulosic biomass, whose recalcitrant characteristic is due to its complex matrix structure composed of cellulose, hemicellulose, lignin and other components [4], [5], [6]. The use of raw lignocellulosic biomass yielded low production of biofuels and exhibited low production rates due to the low accessibility of the feedstock by microbial populations [7]. Pretreatment processes are needed to break the polysaccharide–lignin complex links, thereby increasing the accessibility of cellulose and hemicelluloses for biorefinery [8], [9], [10].
Dilute acid pretreatment is one of the most widely performed pretreatment methods for lignocellulosic biomass [4]. Dilute acid is normally preferred over concentrated acid since it is cost effective and environmentally-friendly [11]. During dilute acid pretreatment, the biomass is exposed to harsh physicochemical conditions to obtain a solution rich with monomeric sugars. The lignin-carbohydrate complexes are removed or disrupted, making the cellulose easily accessible to enzymes and microorganisms [12].
Downstream processes, such as pH adjustment and separation, accompany the chemical pretreatment prior to the biological conversion steps [13], [14]. pH adjustment is essential because biological conversion of biomass to biofuels is known to be significantly affected by pH, of which typical optimum values are near neutral [8], [15], [16], [17]. Moreover, pH of lignocellulosic hydrolyzate is sometimes adjusted to a mild basic pH range to facilitate precipitation of potential inhibitory compounds such as furfural and 5-hydroxymethylfurfural (5-HMF) [18]. Separation is also inevitable to fractionate the hydrolyzate from lignin and insoluble compounds. An ideal separation method of the aqueous and solid fractions of the hydrolyzate must recover as much sugar into aqueous phase as possible. The physicochemical unit processes between dilute acid hydrolysis and biological conversion may affect the sugar and byproduct contents of the hydrolyzate, although they have been regarded not to affect the recovery of the soluble compounds [19].
Dilute acid pretreatment of lignocellulosic biomass has been conducted in various studies; however, each study made use of different methods for neutralization and fractionation for the hydrolyzate. The study of the best neutralization and fractionation methods is important since the pH, state, and composition of the hydrolyzate are integral to the fermentation process taking place after pretreatment. After the downstream processes, the objective is to produce an energy-rich biomass hydrolyzate that can be used for biogas and biofuel production [20]. According to Sievers et al., solid–liquid separations are useful to provide a solids-free sugar stream for fermentation. Solids are known to interfere with fermentation of sugars [21]. Moreover, neutralization and fractionation must be optimized as sustainable and cost-effective treatment methods for lignocellulosic hydrolyzate. pH adjustment and separation downstream processes for hydrolyzates of lignocellulosic biomass are commonly performed in biorefinery processes. However, little attention is provided to these methods. Only a few separation and pH adjustment studies have been published for dilute acid hydrolyzate of lignocellulosic biomass. Moreover, no studies have shown the direct effect of these processes to H2 fermentation, according to the authors' knowledge.
Hydrogen is an environmentally friendly biofuel with high energy efficiency [22], [23], making it a promising alternative energy carrier. Due to the high energy consumption requirement of conventional physico-chemical H2 production methods, interest in biohydrogen has increased significantly in the recent years. Dark fermentation, or light-independent process, uses genera Clostridium, Enterobacter, or mixed culture dominated by these microorganisms to convert sugars into H2 [24]. Among these microbial species, Clostridium species were identified as the most dominant H2-producing bacteria for mesophilic dark fermentation at pH 5.5 [1]. These bacteria make use of monomeric sugars present in the dilute acid hydrolyzate of lignocellulosic biomass as substrate for dark fermentative process [24].
Pine tree wood, an agricultural feedstock of the wood pulp and paper industry, was used as the substrate for this study. This particular biomass was chosen since the paper industry is important in South Korean economy and this biomass is just disposed as waste or incinerated. Hydrolysis of lignocellulosic biomass, such as pine tree wood, leads production of a number of useful biofuels as compared to combustion, which primarily produces heat [25].
The objective of this study was to investigate resultant volume, sugar and byproduct contents, and biohydrogen production of dilute acid lignocellulosic hydrolyzate after pH adjustment and separation with various neutralizing agents (calcium hydroxide and sodium hydroxide), agent forms (powder and aqueous solution), separation methods (centrifugation and vacuum filtration) and the sequence of the unit processes (neutralization–separation and separation–neutralization). This study puts light to the significance and relevance of often overlooked yet important downstream process conditions, particularly, pH adjustment and separation, on biological conversion of biomass to bioenergy.
Section snippets
Dilute acid pretreatment of lignocellulosic biomass
Untreated pine tree wood pellet was acquired from a local paper production company and was the lignocellulosic biomass used in this study. The glucan, xylan, arabinan, lignin, ash, and extractives composition of the biomass were measured by following the NREL laboratory analytical procedure [26]. The biomass composition is presented, on dry basis, in Table 1.
The pine tree wood pellet was initially milled to reduce the particle size to 1–2 mm. Dilute acid pretreatment was performed with 10%
Neutralization of the dilute acid lignocellulosic hydrolyzate
The concentrations of sugar and byproducts present in the hydrolyzate solutions after neutralization using powder and aqueous solutions of Ca(OH)2 and NaOH were compared in Table 2, Fig. 2, Fig. 3. The data table and figures present the following terms, total sugar recovery, relative sugar recovery, and volume recovery, which were all calculated parameters in this study. Total sugar recovery of the hydrolyzate solution is the ratio of the total sugar content in the hydrolyzate solution and the
Conclusion
A series of pH and separation downstream processes were performed to determine the best option to obtain liquid feedstock for fermentation of dilute acid pretreated lignocellulosic biomass. The downstream unit processes and their sequence significantly affected sugar recovery. Filtration followed by pH adjustment using aqueous NaOH recovered 95.29% of sugars liberated from dilute acid pretreatment.
The dilute acid hydrolyzate solutions of pine tree wood were then used for dark hydrogen
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2A2A07000900).
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