Py-GC/MS based analysis of the influence of citric acid leaching of sugarcane residues as a pretreatment to fast pyrolysis

Highlights

• Citric acid-leaching slightly changed biomass elemental composition.

• Analytical pyrolysis of citric acid-leached sugarcane trash and bagasse is reported.

• Levoglucosan yields from citric acid leaching are close to that with mineral acids.

• Sugarcane trash can be considered a valuable feedstock for fast pyrolysis.

Abstract

Sugarcane trash (SCT) and sugarcane bagasse (SCB) are prospective feedstock materials for fast pyrolysis. However, their relatively high inorganic content, especially alkali and alkaline earth metals (AAEMs), affects the quality of the produced bio-oil. This study evaluates the effect of SCT and SCB leaching by citric acid (CA) on the chemical composition of pyrolysis vapors, viz. by applying micro-pyrolysis (Py-GC/MS). Comparison was made between CA and well-known leaching agents including H₂SO₄, HCl and water. Accordingly, biomass samples were previously leached at different temperatures and leaching times. The results indicate that leaching with either citric acid or well-known inorganic acids causes the yields of levoglucosan in the pyrolysis vapors to increase by 5–8 fold. The mild structural changes in the lignocellulosic matrix, caused by hydrolysis during leaching, demonstrated to be as important for subsequent thermal degradation behavior (and thus increased anhydrosugars production) as the suppression of the intrinsic catalytic effect of the inorganic constituents by their removal during leaching. CA treatment generally favored the reduction in the total production of ketones and furans independently of leaching conditions (temperature, time). Differences in the range of leaching conditions tested (T = 25–50 °C; t = 1–12 h) had only minor influence on the composition of the pyrolysis vapors derived from CA pretreated sugarcane residues.

Introduction

The versatility of sugarcane residues (mainly bagasse and trash) for the production of paper, energy and biofuels has been widely illustrated in literature [1]. Indeed, sugarcane bagasse and to a lesser extent, sugarcane trash continue to receive attention as potential feedstock for producing bio-oil through fast pyrolysis [1,2]. Sugarcane bagasse (SCB) is the fibrous residue of sugarcane after crushing and extraction of its juice and yields about 140 kg (dry basis) of material per 1000 kg of processed (wet) cane [3]. Sugarcane trash (SCT) is a substantial portion of crop residues, composed of plant tops and leaves [4], and represents one third of the sugarcane plant mass (140 kg dry SCT per 1000 kg of sugarcane d.b.) [3]. The energy content in SCT ranges between 17.4 and 17.7 MJ kg⁻¹ [5,6]. Despite this potentially usable calorific value, most common practices for sugar production do not consider the SCT as an energy source; instead, it is burned on the field, posing both environmental and technical challenges.

SCT and SCB are composed of hemicellulose (23–32 wt.%, d.b), cellulose (32–48 wt.%, d.b), lignin (13–24 wt.%, d.b) and ash (1.7–8 wt.%, d.b) [4,7]. The ash in SCB and SCT are different in mass and composition, but generally they contain several inorganic elements, mainly alkali and alkaline earth metals (AAEMs). In terms of specific metal ions, Si, Na, K, Mg and Ca are the major inorganic components in sugarcane [8,9]. A fraction of the AAEMs exist as salts deposited in cells or pores after drying, while another fraction is ionically bound to lignin or hemicellulose via the free electron pair of the oxygen atoms in carboxyl and/or hydroxyl functionalities [10].

The content and nature of the inorganic species in lignocellulosic biomass strongly affect the pyrolysis process and influence the composition and stability of the liquid product (bio-oil). According to previous studies, the AAEMs can act as catalysts during thermal decomposition affecting both the devolatilization and charring reactions, potentially reducing the overall bio-oil yield [[11], [12], [13], [14]]. Additionally, the presence of AAEMs in the biomass alters the physicochemical properties of the resulting bio-oil, including viscosity, heating value, and water content [9,15]. Most notably, AAEMs appear to suppress the pyrolytic production of levoglucosan (an anhydrosugar) out of cellulose and instead, favor the production of lighter oxygenates (including glycolaldehyde and hydroxyacetone) [11]. Levoglucosan (1,6-anhydro-β-D-glucopyranose) is a major constituent present in bio-oil [16,17], which has a demonstrated potential for application in chemical synthesis of oligosaccharides, alkyl glycosides and chiral complex molecules, an example of the latter being the chemical synthesis of macrolide antibiotics using levoglucosan as precursor [18]. Also, levoglucosan can be used as feedstock in fermentation to ethanol or be upgraded to transportation fuels or to additives in conventional fuels, if it can be produced cheaply enough [19]. Therefore, the advantage of biomass as a promising source of renewable fuels or platform chemicals would be greatly enhanced by the effective production of levoglucosan [20].

From earlier studies of Pan and Richards [21], it is known that during the pyrolysis process, K and Ca can catalyze biomass conversion and char forming reactions. Although, Lang and Neavel [22] found that the catalytic activity of K under pyrolysis conditions is considerably greater than that of Ca. Moreover, Oasmaa et al. [23] found that K was associated with additional water and gas formation, leading to a decrease in organic liquid yields. Nik-Azar et al. [24] studied the effect of these metals on beech wood pyrolysis and concluded that Na and K are stronger cracking catalysts than Ca. Müller-Hagedorn et al. [25] demonstrated that K and Na ions have a strong influence on the decomposition temperature in pyrolysis and on the product distribution. The differential thermogravimetric (DTG) curve is shifted to lower temperatures and the yield of levoglucosan is reduced to less than half [24,25]. An additional problem posed by the minerals in the feedstock with respect to fast pyrolysis of SCB and SCT, is that a portion of the minerals induce lower stability of the bio-oil due to some minerals being carried over in fine char particles in the bio-oil [9,11,21].

In order to reduce the negative effect of AAEM on the yield and quality of the bio-oil, solutions have been proposed. On one hand, the fines carryover could be substantially reduced by using alternative reactor configurations which do not rely upon high gas flow rates (i.e., in fluidization), such as vacuum pyrolysis reactors and rotating screw or rotating cone reactors [9,11,26,27]. However, these designs do not remediate the negative catalytic effect of ash during pyrolysis itself. The latter problem could be mitigated by demineralizing the feedstock prior to pyrolysis, in order to decrease its ash content, thus reducing the catalytic effect of AAEMs [8,9]. The demineralization process can be carried out by leaching the biomass with several solutions (e.g., water, acids or solvents), which necessarily leads to changes in the textural properties of the biomass by increasing the surface area and porosity. Moreover, leaching could also modify the composition and structure of the fibers by partially hydrolyzing the polysaccharides and lignin, and by reducing the crystallinity of cellulose [28]. The typical quality improvements in bio-oil from acid-washed biomass are: high levoglucosan concentration (e.g. 50% yield rise by pyrolysis of 1% H₂SO₄-heated cottonwood [29]), reduced water content (from 12.0% to 8.3% in case of raw and HCl-treated SCB, respectively [9]) and reduced concentration of lighter oxygenates [30].

Generally, the removal efficiency of AAEMs during leaching depends on several factors including the type and concentration of the leaching solution, temperature and contact time [31]. Furthermore, as minerals in biomass can be bound physically or chemically, the leaching process can vary greatly from one lignocellulosic material to another [32]. Among the most used methods for the pretreatment of sugarcane bagasse prior to pyrolysis are leaching with water, dilute acid and organic solvents [8,31,[33], [34], [35], [36]]. Deng et al. [37] suggested that water washing is a prospective treatment to remove a large part of troublesome elements from biomass fuels. However, sulfuric acid solutions are the most used liquids for sugarcane bagasse leaching [15,34,36]. Nevertheless, this acid is corrosive and leads to the formation of biomass degradation products. Other compounds such as hydrochloric acid (HCl) [9,36,38], hydrogen fluoride (HF) [9,15,39], nitric (HNO₃) [39,40] and phosphoric (H₃PO₄) [38,41] acids have also been studied.

Recently, Rodríguez-Machín et al. [36,42] have demonstrated the effectiveness of citric acid (CA, C₆H₈O₇) as a leaching agent for pretreating SCB and SCT. The authors compared their results with the pretreatment of sugarcane residues with water, HCl and H₂SO₄, and demonstrated that pretreatment of SCB and SCT with citric acid is as effective in removing the AAEMs as the mineral acids. Furthermore, the CA pretreatment did not induce extensive hydrolysis, as was confirmed by a minimal shift of the TG (thermogravimetric) curves for CA–leached SCT and SCB compared to the raw biomasses. Despite the extensive literature on biomass leaching, there is scarce information on the effect of SCT and SCB leaching at different conditions (temperature, time) by well-known leaching agents including H₂SO₄, HCl and water on the physicochemical composition of the bio-oil upon pyrolyzing the pretreated feedstock. Especially, with respect to CA as leaching agent no information was found in the literature reviewed.

Analytical pyrolysis (sometimes also referred to as micro-pyrolysis) is a well-known technique to analyze high-molecular weight compounds. The micro-pyrolysis coupled to gas chromatography mass spectrometry (Py-GC/MS) technique has been considered a rapid method for obtaining information on the thermochemical decomposition of lignocellulosic materials and plastics [40,43,44]. This technique has been used along with some pretreatments (e.g. leaching [40], torrefaction [45]) for analyzing their effect on the composition of pyrolysis vapors and the subsequent quality of bio-oil.

Therefore, this study aims to evaluate the effect of leaching both sugarcane trash and sugarcane bagasse with citric acid – being an organic, less characterized pretreatment agent – compared to well-known leaching agents including H₂SO₄, HCl and water on the chemical composition of pyrolysis vapors. The experiments are carried out at analytical level in Py-GC/MS as a reliable and rapid method for gathering information. The effect of operational variables of the leaching process on the ash and thus AAEM removal is assessed and correlated to the variations in the composition profile of pyrolysis vapors.