Co-production of furfural and acetic acid from corncob using ZnCl₂ through fast pyrolysis in a fluidized bed reactor

Highlights

• Corncob with ZnCl₂ was successfully pyrolyzed in a bench-scale fluidized bed.

• Relative response factor was applied for quantification of bio-oil components.

• The highest furfural yield was 8.2 wt% of the product.

• The highest acetic acid yield was 13.1 wt% of the product.

Abstract

Corncob was pyrolyzed using ZnCl₂ in a pyrolysis plant equipped with a fluidized bed reactor to co-produce furfural and acetic acid. The effects of reaction conditions, the ZnCl₂ content and contacting method of ZnCl₂ with corncob on the yields of furfural and acetic acid were investigated. The pyrolysis was performed within the temperature range between 310 and 410 °C, and the bio-oil yield were 30–60 wt% of the product. The furfural yield increased up to 8.2 wt%. The acetic acid yield was maximized with a value of 13.1 wt%. A lower feed rate in the presence of ZnCl₂ was advantageous for the production of acetic acid. The fast pyrolysis of a smaller corncob sample mechanically mixed with 20 wt% of ZnCl₂ gave rise to a distinct increase in furfural. A high selectivity for furfural and acetic acid in bio-oil would make the pyrolysis of corncob with ZnCl₂ very economically attractive.

Introduction

Recently, the demand of fine chemicals has been increasing due to the rapid development of chemical industry. One of these fine chemicals is furfural which a platform chemical with a growing market. It has been widely used as a solvent, food additive and fungicide and also in the manufacture of pharmaceutical products, resins, and etc. (Lu et al., 2011a). In contrast to most conventional fine chemicals which are generally petroleum-based, furfural is exclusively produced from renewable biomass resources. Industrial furfural production is presently conducted by acid catalytic dehydration of pentosan-containing lignocellulosic materials in a batch or continuous reactor (Di Blasi et al., 2010a). Since this type of production method brings about tons of acid waste water, eco-friendly production methods have been passionately being researched. A promising alternative to the acid dehydration for the production of furfural is the pyrolysis of lignocellulosic biomasses consisting of cellulose, hemicelluloses, and lignin. Pyrolysis is a thermal process in the absence of oxygen. Recently, biomass pyrolysis focuses on the production of hydrocarbons by catalytic upgrading (Stefanidis et al., 2011) and it has also potential to produce value-added chemicals. It is well-known that furfural is produced from both of cellulose and hemicelluloses (Lu et al., 2011b). Corn residues such as corn stover and corncob have been usually used for the furfural production by pyrolysis (Ioannidou et al., 2009). In particular, corncob is the widely used feedstock for the furfural production due to its rich contents of pentosans and cellulose (Branca et al., 2010), although it can also be used for the production of bio-oil as a green fuel (Zheng et al., 2013). In a study on the non-catalytic pyrolysis of feedstocks with significant contents of cellulose/pentoses, furfural yields were in the range 2–0.8 wt%. (Di Blasi et al., 2010a). Demiral et al. conducted a series of experiments on a sample of corncob to determine the effects of pyrolysis parameters in the temperature ranges of 400–550 °C. In the experiments, the maximum oil yield of 26.4 wt% was obtained at a pyrolysis temperature of 500 °C, heating rate of 40 °C/min and sweeping gas flow rate of 100 cm³/min (Demiral et al., 2012). In studies focusing on the production of furfural by pyrolysis, however, ZnCl₂ as a catalyst has been mostly applied. Encinar et al. reported in an earlier study on the pyrolysis of grape and olive bagasse with ZnCl₂ that furfural concentration increased with temperature up to 600 °C, and then it decreased when temperature is further increased (Encinar et al., 1997). Recent researches continued to investigate the effect of ZnCl₂ on the furfural formation during pyrolysis. A study done by Di Blasi et al. revealed that ZnCl₂ was a particularly effective catalyst (concentrations of 1–6% and temperatures of 427–527 °C) to maximize the yield of furfural which was augmented by a factor of 5 and that a higher ZnCl₂ content, however, would cause the decreasing of the furfural formation (Di Blasi et al., 2008). Branca et al. conducted the pyrolysis of corncobs impregnated with variable amounts of ZnCl₂ and concluded that ZnCl₂ catalyzes the primary paths of furfural formation via dehydration of pentosyl and glucosyl residues (Branca et al., 2010). Amarasekara et al. investigated the effect of ZnCl₂ on the degradation of cellulose. The major non-gaseous products of the degradation of cellulose containing 0.5 mol of ZnCl₂/mol of glucose unit were furfural, 5-hydroxymethylfurfural and levulinic acid. The maximum yield for furfural was 8%, based on glucose unit of cellulose (Amarasekara and Ebede, 2009).

Meanwhile, some studies focusing on the production of acetic acid by pyrolysis were recently performed. Qi et al. performed the pyrolysis of bamboo over zeolite NaY and found that the content of acetic acid was the main component of bio-oil and its content is more than two times higher than that from a non-catalytic process (Qi et al., 2006). Lu et al. produced acetic acid of around 4 wt% by fast pyrolysis of biomass materials impregnated with ZnCl₂ in a small lab-scale equipment (Lu et al., 2011b).

Till now, most researches on the production of furfural and acetic acid were performed with Py-GC/MS or in a small scale apparatus. This study reports experimental results on the co-production of furfural and acetic acid from corncob with ZnCl₂ in a bench-scale fast pyrolysis plant equipped with a fluidized bed reactor having a capacity of up to 0.5 kg/h, which will give more practical aspects on the furfural production than ever. Main aim of this study is to find the effects of reaction conditions, the ZnCl₂ content and contacting method of ZnCl₂ with corncob on the yields of furfural and acetic acid. For a more reliable GC analysis, relative response factors (RRFs) for each component were calculated using the effective carbon number (ECN) and reference compounds. In addition, the quantification of furfural and acetic acid in the bio-oil was performed with the GC external standard method.