ARTICLE pubs.acs.org/EF
Influence of Pyrolysis Operating Conditions on Bio-Oil Components: A Microscale Study in a Pyroprobe Suchithra Thangalazhy-Gopakumar,†,‡ Sushil Adhikari,*,‡ Ram B. Gupta,† and Sandun D. Fernando§ †
Department of Chemical Engineering and ‡Department of Biosystems Engineering, Auburn University, Auburn, Alabama 36849, United States § Department of Biological and Agricultural Engineering, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: A fast pyrolysis process has emerged as one of the techniques to produce transportation fuels using various biomass types that are regionally important. It is well understood that high heating rates, very small particles with low mass transfer limitations, and moderate operating temperatures are essential for obtaining high yield of liquid from the fast pyrolysis process. However, how the heating rates and operating temperatures would influence individual compounds in the liquid obtained from the fast pyrolysis process has not been studied in detail. Therefore, a microscale pyrolysis study was performed by changing different parameters (biomass type, filament heating rate, and final pyrolysis temperature) to understand the influence of these operating parameters on each compound formed during the process. Two biomass types (pine wood and switchgrass) were selected for this study: (i) pine wood was selected because of its availability in the southeastern region of the United States, and (ii) switchgrass was chosen because it has been identified as one of the bioenergy crops in the United States. Pyrolysis temperatures were changed from 450 to 750 °C in increments of 50 °C, whereas the heating rates selected were 50, 100, 500, 1000, and 2000 °C/s. Twenty-eight biooil compounds were quantified in each experimental condition. This study found that phenols and toluene concentrations increased with the increase in pyrolysis temperature irrespective to the biomass type. On the other hand, the change in the yield of ketones, furans, and guaiacols with the change in pyrolysis temperature depended on the type of biomass. The effect of filament heating rate on bio-oil yield was not statistically significant because the biomass heating rate was almost constant irrespective of different filament heating rates.
1. INTRODUCTION Biomass is considered to be one of the alternative resources to meet future energy demand. As a result, interest in converting biomass to valuable energy forms has been increased over several years. The “Energy Independence and Security Act of 2007” has mandated 36 billion gallons per year of biofuels by 2022 as transportation fuel. Among the biofuels, advanced biofuels (other than corn bioethanol and soybean biodiesel) account for 20 billion gallons.1 In order to achieve 20 billion gallons/year of biofuel by 2022, both biochemical and thermochemical processes have to be utilized. A fast pyrolysis (residence time of up to 2 s), a thermochemical process, can attain a high liquid yield from lignocellulosic biomass.2 The uniqueness of the fast pyrolysis process is the flexibility of feedstock and the high liquid yield, up to 70% of dry biomass.3-5 In the fast pyrolysis process, biomass is rapidly heated in the absence of oxygen.6 As a result, biomass is decomposed to char, vapors and aerosols, and gas. The vapors and aerosols are quickly condensed to a liquid called biooil which is a mixture of more than 300 compounds and has many applications. The major applications of bio-oil are as a feedstock for commodity chemicals and as an intermediate for fuel production. In addition, bio-oil has been tested for static applications such as boilers, furnaces, turbines, and diesel engines for heat, power, or electricity generation.7 However, the final applications of bio-oil are limited because of some of the negative attributes, such as high density, viscosity, acidity, water, oxygen, and low heating value. Some of these negative attributes can be removed partially through physiochemical treatments.8-10 Although there are different techniques to upgrade bio-oil as r 2011 American Chemical Society
transportation fuel, none of the processes have been demonstrated commercially.11 Current studies are being focused on optimizing pyrolysis conditions, upgrading bio-oil, and finding more applications for bio-oil.12 A detailed analysis of chemical composition in bio-oil is required to optimize the pyrolysis process for its final applications. Composition of bio-oil varies with the properties of biomass, pyrolysis temperature, heating rate, and the pyrolytic environment. Although a number of studies reported the influence of these parameters on bio-oil yield,13-18 only a handful of studies19-23 have analyzed bio-oil’s chemical composition. However, previous studies did not evaluate the significance of changing each pyrolysis process parameter on an individual species yield.24,25 To fill the knowledge gap in the field, this study systematically investigated the change in the yield of individual components of bio-oil by changing process parameters such as biomass type, pyrolysis temperature, and heating rate. Biomass types used in this study were pine wood and switchgrass. Pine wood is the primary forest woody biomass available in the southeastern region.26 Furthermore, the South’s market for forest products has weakened in recent years due to increasing global competition in the pulp and paper industry and mill closures due to aging facilities,27 and it is expected that biorefineries will rejuvenate this industry. Switchgrass can be grown on marginal lands that are not well suited for food crops and it has Received: August 6, 2010 Revised: December 13, 2010 Published: January 31, 2011 1191
dx.doi.org/10.1021/ef101032s | Energy Fuels 2011, 25, 1191–1199
Energy & Fuels a tolerance for cold temperatures.28,29 Also, it has been designated as an energy crop by the U.S. Department of Energy because of its high yield, excellent conservation attributes, and good compatibility with conventional farming practices.30,31
2. MATERIALS AND METHODS 2.1. Biomass Characterization. Pine wood chips were obtained from a local wood chipping plant in Opelika, Alabama, and a switchgrass (Alamo cultivar) sample was obtained from the E.V. Smith Research Center of Auburn University in Shorter, Alabama. Pine wood chips were dried in a furnace at 75 °C for 12 h prior to size reduction; whereas switchgrass was ground without drying (it was stored in a room for more than two years). A hammer mill (New Holland grinder model 358, New Holland, PA.) with 1.58-mm (1/16 in.) sieve size was used for particle size reduction. These biomass samples were further ground in a coffee grinder to make them fine powder (particle size
