Article pubs.acs.org/EF
Production of Deoxygenated Biomass Fast Pyrolysis Oils via Product Gas Recycling Charles A. Mullen,* Akwasi A. Boateng, and Neil M. Goldberg USDA-ARS, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States ABSTRACT: A bench scale fluidized bed system was modified to recycle and utilize the gaseous products of biomass fast pyrolysis as fluidization gas and to create a reactive gas atmosphere to replace, in part or in full, the added nitrogen gas. The effect of the presence of the reducing atmosphere on the pyrolysis oils produced was studied for three different biomass feedstocks, white oak, switchgrass, and pennycress presscake, and compared with those produced under an inert N2 atmosphere as the control. The reductive atmosphere, consisting primarily of CO, CO2, H2, and light hydrocarbons, had a significant deoxygenation effect on the pyrolysis oil produced from oak and switchgrass. The effect was significantly smaller for the pyrolysis of pennycress presscake, which inherently produces a lower oxygen product. When oak and switchgrass were used as feedstocks, pyrolysis oils with molar C/O ratios of 9.1 and 8.5 were achieved, respectively, which is increased from about 2.1 for the control experiments for each feedstock. The pyrolysis oils produced under the reducing atmosphere were richer in aromatic hydrocarbons and nonmethoxylated phenolics and had lesser concentrations of levoglucosan and acids than were the pyrolysis oils in the control experiments. These pyrolysis oils had lower total acid numbers (TAN) and higher energy content than those produced in the control experiments. These results compare favorably to zeolite catalyzed fast pyrolysis but without the added catalyst cost and concern for catalyst deactivation or need for regeneration.
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INTRODUCTION Fast pyrolysis has become the most promising method for the production of liquid fuel intermediates from lignocellulosic biomass.1,2 The pyrolysis process holds promise for utilization in small on-the-farm systems because of its smaller footprints and the logistical advantage of transporting dense liquids over bulky biomass.3,4 However, it is well documented that biomass fast pyrolysis oils have compatibility issues with the current infrastructure. Whether they are to be used for stationary boiler fuels or to be upgraded into hydrocarbon transportation fuels, problems with pyrolysis oils are due to their high acidity and instability, which are mostly associated with high oxygen content. For that reason, it has been the goal of many pyrolysis researchers to produce deoxygenated pyrolysis oils resulting in better characteristics for direct combustion and an easier path to “drop in” transportation fuels. To produce the desired deoxygenated fuel intermediates, many have focused on adding an oxygen-rejecting catalyst to the pyrolysis process. Most of the reports on catalytic pyrolysis involve the use of solid acid catalysts such as zeolites to promote cracking type reactions.5−10 The general mechanism by which these catalysts work are through protonation of oxygenates and generation of carbocations through dehydration. These reactions produce olefins which aromatize under the reaction conditions. The removal of hydrogen via these types of reactions from already hydrogen deficient feedstocks results in coke formation; this reduces the carbon conversion to the liquid product and also deposits coke on the catalyst, thereby deactivating it. Therefore, reactor design for catalytic pyrolysis systems must provide for continual regeneration of catalysts, which results in a more complex system than one for thermal pyrolysis alone. These systems may require additional footprints, control, expertise, and expense to run and complicates the process of deploying an on-the-farm or mobile © XXXX American Chemical Society
system of this type. It is therefore desirable for the scale of interest to produce partially deoxygenated stable fuel intermediates without the use of limited lifetime catalysts. Herein, a process is described, where recycled volatile pyrolysis products are used as fluidizing gas and tuned to provide a reducing reaction atmosphere, resulting in a change in chemical pathways. The result is that partially deoxygenated pyrolysis oils are produced without the use of externally added catalysts. In this study, the fluidized bed fast pyrolysis of three different types of biomass was examined. The gas stream produced in the pyrolysis process was used in varying amounts as a fluidizing gas and reaction atmosphere, and the effects were examined. The three feedstocks studied were white oak, switchgrass, and pennycress presscake. Oak and switchgrass have a mostly lignocellulosic composition and are known to yield highly oxygenated, acidic, unstable pyrolysis oils upon traditional inert atmosphere pyrolysis.1,11 They are also relatively abundant feedstocks. Pennycress presscake is material left over after mechanical extraction of vegetable oils from the seeds of the pennycress plant, a member of the mustard family. The presscake is a highly proteinaceous material and is different in composition and pyrolysis behavior than switchgrass or oak. Like some other types of proteinaceous biomass, pyrolysis of pennycress presscake yields a pH neutral, stable, partially deoxygenated but nitrogen rich liquid product.12,13 In this study, it was found that while the recycle gas atmosphere had a dramatic deoxygenation effect on the pyrolysis oils produced from switchgrass and white oak, little effect on the pyrolysis of pennycress presscake was observed. Others have studied the Received: April 23, 2013 Revised: June 6, 2013
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dx.doi.org/10.1021/ef400739u | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 1. Feedstock Elemental Composition (wt %) oak switchgrass pennycress presscake a
as is dry ash free as is dry ash free as is dry ash free
C
H
N
S
Oa
moisture
ash
50.12 51.23 46.55 49.41 44.79 52.37
6.29 6.43 5.75 6.06 5.32 6.22
0.51 0.52 0.48 0.51 5.66 6.62
