Article pubs.acs.org/EF
Catalytic Fast Pyrolysis of Prairie Cordgrass Lignin and Quantification of Products by Pyrolysis−Gas Chromatography−Mass Spectrometry Min Zhang and Alex Moutsoglou* Department of Mechanical Engineering, South Dakota State University, Brookings, South Dakota 57007, United States ABSTRACT: Organosolv lignin extracted from prairie cordgrass (PCG) was pyrolyzed in the presence of HZSM-5 and analyzed using pyrolysis−gas chromatography−mass spectrometry (Py−GC/MS). The pyrolysis products detected are primarily aromatic hydrocarbons and phenolic compounds. The effects of the HZSM-5 catalyst and pyroprobe temperature on the production of aromatic hydrocarbons and phenolic compounds were assessed quantitatively. HZSM-5 mixed with PCG lignin at a ratio of 5:1 and a pyroprobe temperature of 650 °C maximized the production of the detected aromatic hydrocarbons, resulting in about a 13 wt % yield of PCG lignin. At these conditions, the products detected are estimated to have a total oxygen content of about 1.4% and a higher heating value of 49 MJ/kg, although the detected compounds may constitute less than a quarter of the volatiles released during the pyrolysis of PCG lignin. Toluene and p-xylene were the two most abundant hydrocarbons formed in the presence of HZSM-5, with yields of 3.6 and 3.2% of the pyrolyzed lignin, respectively. This provides great potential for replacing a sizable portion of production for the two aromatic hydrocarbons from fossil sources worldwide. In the presence of HZSM-5, the largest total yield of the aromatic hydrocarbons from the pyrolysis of organosolv lignin extracted from aspen trees is almost twice as much as that produced from the pyrolysis of lignin extracted from PCG, a perennial grass. On the other hand, in the presence of catalyst, the volatile compounds detected from the pyrolysis of PCG lignin contain less oxygen than those for aspen lignin, thus resulting in a better quality product with enhanced higher heating value (HHV).
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INTRODUCTION A recent review of the existing literature on catalytic fast pyrolysis of lignocellulosic biomass is presented by Dickerson and Soria.1 Lignin is one of the three major components of lignocellulosic biomass. The use of lignin as a pyrolysis feedstock provides several advantages over lignocellulosic feedstock. The lower oxygen content of lignin results in pyrolysis oil that is less acidic and has a greater higher heating value (HHV). Lignin is a significant byproduct of pulp mills and forest biorefineries and can be extracted via the Kraft, sulfite, organosolv, and soda pulping processes. Catalytic fast pyrolysis of lignin extracted from hardwoods and softwoods, as well as grasses and straws, has been the subject of several studies.2−13 Of all of the catalysts studied, HZSM-5 is found to be the most effective catalyst in producing aromatic hydrocarbons from the pyrolytic vapors. Kraft lignin was pyrolyzed in the presence of several catalysts, including HZSM-5.2−5 Ben and Ragauskas2−4 report the near complete loss of aliphatic hydroxyl and carboxyl groups as well as the significant decrease of methoxyl groups in the bio-oil from the fast pyrolysis of softwood Kraft lignin in the presence of ZSM-5. The effects of the Si/Al ratio and pore size of ZSM-5 on the production of aromatic hydrocarbons from the catalytic fast pyrolysis of lignin have also been documented.4,6,7 Lee et al.8 used a mesoporous Y zeolite catalyst in a secondary step to upgrade the vapors from fast pyrolysis of Kraft lignin. Lignin extracted from wheat straw and sarkanda grass using the soda pulp process (marketed as Asian and Granit) has been pyrolyzed in the presence of several catalysts, including HZSM-5.7,9−11 Thring et al.12 studied the catalytic fast pyrolysis of Alcell lignin using HZSM-5. Alcell lignin is extracted from hardwoods using an organosolv process. © 2014 American Chemical Society
Prairie cordgrass (PCG) has a great potential as a bioenergy crop because of its ability to thrive in marginal soils that are unsuitable for conventional crop cultivation. Recent results from the non-catalytic pyrolysis of three lignins14 showed that lignin extracted from aspen trees (hardwood) via an organosolv process produced significantly higher amounts of phenol-, guaiacol-, and syringol-substituted compounds (H/G/S) than PCG lignin. Kraft lignin produced by far the lowest amounts of all three types. On the other hand, PCG lignin was found to release the most light volatile products (
