CELLULOSE CHEMISTRY AND TECHNOLOGY CONTINUOUS ACTIVATION ENERGY REPRESENTATION OF THE ARRHENIUS EQUATION FOR THE PYROLYSIS OF CELLULOSIC MATERIALS: FEED CORN STOVER AND COCOA SHELL BIOMASS * *,** ABHISHEK S. PATNAIK and JILLIAN L. GOLDFARB *Division of Materials Science and Engineering, Boston University, 15 St. Mary’s St., Brookline, MA 02446 ** Department of Mechanical Engineering, Boston University, 110 Cummington Mall, Boston, MA 02215 ✉Corresponding author: Jillian L. Goldfarb, [email protected] Received January 22, 2015 Kinetics of lignocellulosic biomass pyrolysis – a pathway for conversion to renewable fuels/chemicals – is transient; discreet changes in reaction rate occur as biomass composition changes over time. There are regimes where activation energy computed via first order Arrhenius function yields a negative value due to a decreasing mass loss rate; this behavior is often neglected in the literature where analyses focus solely on the positive regimes. To probe this behavior feed corn stover and cocoa shells were pyrolyzed at 10K/min. The activation energies calculated for regimes with positive apparent activation energy for feed corn stover were between 15.3 to 63.2 kJ/mol and for cocoa shell from 39.9to 89.4 kJ/mol. The regimes with a positive slope (a “negative” activation energy) correlate with evolved concentration of CH 4 and C 2H2. Given the endothermic nature of pyrolysis, the process is not spontaneous, but the “negative” activation energies represent a decreased devolatilization rate corresponding to the transport of gases from the sample surface. Keywords : Arrhenius equation, biomass pyrolysis, evolved compounds, activation energy INTRODUCTION Fossil fuels comprise the majority of the total There are multiple methods used to analyze energy supply in the world today. 1One of the most the pyrolysis kinetics of solid carbonaceous fuels, critical areas to shift our dependence from fossil the overwhelming majority of which rely on the to renewable fuels is in energy for transportation, Arrhenius equation, expressed in the general form which accounts for well over half of the oil as: consumed in the United States. The Renewable Fuel Standard (RFS2) of the Energy (1 Independence and Security Act of 2007 mandates where A is the frequency (or pre-exponential) that 16 billion gallons of a cellulosic biofuel, factor, E the activation energy, T the absolute achieving a 60% reduction in greenhouse gas a temperature, R the universal gas constant, and k is emissions, be blended into transportation fuels by the is the reaction rate constant. It is common to 2022. A portion of this biofuel must be biodiesel assume that the pyrolytic decomposition of produced from biomass. 2 Many processes to biomass and other carbonaceous fuels proceeds convert biomass to liquid fuels rely on pyrolysis, according to an infinitely large set of first order the thermal decomposition of a solid fuel in the reactions, allowing for the calculation of an absence of oxygen. One of the challenges facing overall, or apparent activation energy assuming an the sustainable production of renewable energy overall first order reaction (or a series of reactions sources such as biomass is to develop an that sum to an overall first order reaction). understanding of the reaction kinetics when the Innumerable studies in the biomass pyrolysis biomass is thermochemically converted to literature calculate this activation energy using biofuel. Cellulose Chem. Technol., 50 (2), 311-320 (2016) ABHISHEK S. PATNAIK and JILLIAN L. GOLDFARB this assumption, also known as the Reaction Rate Arrhenius equation (ln( k) vs. 1/ T), we see Constant Method (RRCM). 3,4 Dozens of biomass multiple discreet changes in the slope of the curve pyrolysis studies in the literature show a reaction equating to changes in the reaction rates leading order of approximately unity. This assumption is to segmentation of the process into multiple commonly applied to account for the regimes of decompositions. However, there are simultaneous reactions occurring during the intermediate regimes wherein the slope of the pyrolysis of heterogeneous biomasses and is Arrhenius plot changes from negative to positive, considered a reasonable approximation given the equating to negative activation energy. It is not high degree of linearity of the Arrhenius plots. 2,5 clear that this represents a “spontaneous” Therefore, it provides a useful basis of pyrolysis reaction as one would associate with comparison for the pyrolysis of different negative activation energy. More likely, it is biomasses and other solid fuels in the literature. simply the decrease in reaction (mass loss) rate as However, despite the highly linear the biomass is depleted of one constituent at a relationships encountered for biomass pyrolysis, lower temperature, before the decomposition of a the RRCM fails to capture the entire range of more energy intensive component(s) begins at a decomposition. For biomass pyrolysis we higher temperature. This is perhaps an important commonly see multiple mass loss regimes over distinction often overlooked in the literature; a different temperature ranges accounting for the complete understanding of the pyrolysis process stage-wise decomposition of the primary biomass of biomasses may enable better temperature range constituents. That is, the Arrhenius plots have specifications for energy input, targeting the low discrete changes in slope that occur over specific activation energy regimes to maximize energy (though similar) temperature ranges for each savings and product yield. Given the push biomass. For example, using the RRCM, our towards lower energy consumption through the laboratory found three primary decomposition design of efficient industrial thermochemical regions for the pyrolysis of cabbage palm ( Sabal conversion processes, it becomes imperative to palmetto ), likely corresponding to devolatilization understand the decomposition kinetics of the of primarily hemicellulose, cellulose and lignin. 4 entire pyrolysis process. If there are regimes in However, we know that lignin can devolatilize which a mass loss rate decreases substantially over a broad temperature range, 7 which the despite additional heat input, such a process may limited “mass loss regime” approach of the require less energy to yield a similar product. This RRCM cannot differentiate. In addition, though paper presents an investigation of the pyrolysis the changes in slope in the ln( k)vs. 1/T Arrhenius kinetics of two common biomasses, feed corn plots are fairly straightforward to analyze, there stover and cocoa shell, applying a novel empirical can be substantial (between 2 and 20%) mass lost approach developed to analyze the transient between one “regime” and another depending on nature of pyrolysis through the changing slopes of the temperature ramp rate and the temperature the Arrhenius plot. range selected for analysis, which is not captured in the RRCM analysis. Selecting a temperature EXPERIMENTAL range that encapsulates the most mass loss will Materials decrease the linearity of the mass loss regime due Feed corn stover is a seasonally available to curvature of the line at tail ends, leading to the feedstock, grown across the United States. It is estimated that there is approximately 1 dry ton of inability to completely capture the entire 9 conversion in such an analysis. In their critical harvestable stover per acre of planted corn. According to the National Corn Grower’s Association, 97.2 review of the application of the Arrhenius million acres of corn were planted in 2012, almost 4 equation to solid-state kinetics, Galwey and million of which were in the Northeast and Mid- 8 Brown succinctly summarize why we Atlantic states. 10 Feed corn stover samples were continuously apply the Arrhenius equation to such collected early in October 2011 from the Coppal systems. “No realized alternative capable of House Farm in Lee, NH; the stalk and residual leaves expressing the form of the k-T relationship or were comingled. To prevent it from decomposing, the providing an explanation of this pattern of feed corn stover was dried in a laboratory oven behavior has gained acceptance.” immediately after collection and stored in airtight When analyzing the pyrolytic decomposition containers. process of a biomass across a broad According to the World Cocoa Foundation, total cocoa production was up 8.73% from 2008 to 2012, decomposition temperature range using the 312 Pyrolysis of cellulosic materials with 400 tons of cocoa beans ground and processed in samples obtained from Lindt Chocolate, Stratham, NH, the United States in 2011-2012, representing after removal of the cocoa nib were stored in airtight approximately 10% of the world’s grindings. 11 Cocoa containers as received. bean shells exit the chocolate process as a dry stream; Table 1 Ultimate and proximate analyses of feed corn stover and cocoa bean shell samples Wt% Cocoa Feed (Dry basis) bean shells corn stover C 47.04 46.55 H 5.43 5.66 N 2.79 0.95 S 0.16 0.13 O 35.59 39.59 Ash 9.00 7.12 Moisture 3.59 4.54 Both biomasses were ground and sieved to a 298 K at 10 K/min and held for 5 minutes. Then the particle size fraction of less than 125 µm. Table 1 gives sample was heated at 10 K/min up to 1020 K and held the ultimate and proximate analyses for each of these for 60 minutes to obtain a stable mass reading. The materials, performed by LECO Analytical. DSC (Differential Scanning Calorimeter) was calibrated with indium at a rate of 10 K/min. Each Activation energy calculation from sample was run a minimum of three times to insure thermogravimetric analysis reproducibility. The mass loss behavior of the two biomasses was The apparent activation energy of biomass characterized using thermogravimetric analysis (TGA). pyrolysis was calculated using the Arrhenius equation The feed corn and the cocoa shells were pyrolyzed in under the assumption of overall first order reaction high purity nitrogen at a flow of approximately 50 kinetics using the mass loss function. The rate of mL/min in a 70 µL alumina crucible in a Mettler material decomposition is given as: Toledo TGA/DSC1 with a 20 mL/min high purity nitrogen stream to protect the balance.