Send your completed paper to Sandy Rutter at rutter@asabe.org by April 30, 2010 to be included in the ASABE Online Technical Library. Please have Word's AutoFormat features turned OFF and do not include live hyperlinks. For general information on writing style, please see http://www.asabe.org/pubs/authguide.html This page is for online indexing purposes. Author(s) First Name Middle Name Surname Role Email Jewel A Capunitan Presentor jcapunitan@ta mu.edu Affiliation Organization Address Country Texas A&M University 201 Scoates Hall, TAMU TX 77840-2117 US Author(s) repeat Author and Affiliation boxes as needed-- First Name Middle Name Surname Role Email Sergio C Capareda ASABE Member Affiliation scapareda@ta mu.edu Organization Address Country Texas A&M University 201 Scoates Hall, TAMU TX 77840-2117 Publication Information US Pub ID Pub Date 2010 ASABE Annual Meeting Paper The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2010. Title of Presentation. ASABE Paper No. 10----. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at rutter@asabe.org or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
An ASABE Meeting Presentation Paper Number: 1009881 Corn Stover Pyrolysis in a High-Pressure/High-Temperature Batch Reactor: Evaluation of Product Yields and Conversion Efficiencies Jewel A. Capunitan, Graduate Research Assistant Sergio C. Capareda, Ph.D., Assistant Professor Biological and Agricultural Engineering Dept., Texas A&M University, College Station, TX. Written for presentation at the 2010 ASABE Annual International Meeting Sponsored by ASABE David L. Lawrence Convention Center Pittsburgh, Pennsylvania June 20 June 23, 2010 Abstract. Pyrolysis of corn stover samples was carried out in a high pressure/high temperature reactor at varying temperatures of 400, 500 and 600 o C. The 1.8-L reactor is equipped with temperature controller and a condenser that was used to recover bio-oil from the gaseous product after pyrolysis. Maximum char yield of 38.3% was obtained at 400 o C while the gas yield was maximum at 600 O C, corresponding to a volume of 11.7 L. The bio-oil yield was also maximum at 400 o C, with a value of 29.6%. Mass and energy conversion efficiencies were evaluated, which indicated that around 66% of the energy contained in the feedstock was transferred to the char and only around 8% to the oil. The char and bio-oil were determined to have heating values of 27.9 MJ/kg and 33.8 MJ/kg, respectively. Keywords. pyrolysis, corn stover, bio-oil, char, synthesis gas The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2010. Title of Presentation. ASABE Paper No. 10----. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at rutter@asabe.org or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
Introduction Increasing energy consumption, depleting fossil fuel supplies, and worsening environmental conditions intensified researches on renewable energy, particularly on the utilization of biomass for the production of fuels that are relatively clean as compared to conventional fossil fuels. The thermo-chemical conversion pathway is one of the various ways by which energy and energy sources can be obtained from biomass. Various thermo-chemical processes have been developed for biofuel production, and one of these is pyrolysis, an old-age technology which involves thermal decomposition of biomass in the absence of oxygen to produce solid, liquid and gaseous products (Ioannidou, 2009). The pyrolysis process consists of a very complex set of primary and secondary reactions that involve the production of free radicals (Ioannidou, 2009). Depending on the temperature, heating rate and residence time, the process can be classified as flash, fast or slow. Flash pyrolysis involves rapid heating at temperatures ranging from 400 to 900 o C. Fast pyrolysis, on the other hand, occurs at temperatures lower than 600 o C and at not extremely high heating rates. Slow pyrolysis, as the term denotes, involves even lower heating rates at 450 to 700 o C. The main pyrolysis products include char, oil and gas, and each has its own particular use. Char, the solid product, can be used for combustion or as activated carbon for use in the food and beverage industry or waste treatment facilities. The gaseous product, on the other hand, is a combustible gas that can be further utilized for energy production. The liquid product, termed as bio-oil or pyrolysis oil, is a complex mixture of several hundreds of organic compounds that exhibit a wide range of chemical functionalities (Ioannidou, 2009). However, though the liquid product is termed as bio-oil, it actually consists of two phases, the organic phase and the aqueous phase. The aqueous phase contains low molecular weight oxygenated organic compounds while the organic phase contains mainly of aromatics, and has a potential to be used either directly as a fuel or as a source of high value chemicals. Aside from producing alternative fuels, the conversion of biomass, particularly agricultural residues, alleviates the problem of waste disposal. Being one of the world s staple food, corn is a significant crop around the world and its worldwide annual production amounts to 520 x 10 9 kg, with North America leading the major producing regions at 42% (Ioannidou, 2009). Thus, the leaves and stalks that are usually left in the field after harvest, also known as corn stover, are also of considerable amount and may be converted into biofuels. According to Nielsen (1995), the amount of corn stover that can be harvested would range from 6.7 to 10.1 dry t/ha (3-4.5 t/ac) in fields with estimated grain harvest ranging from 100 to 150 bu/ac. Various studies have been made on pyrolysis of corn stover using different reactor configurations and reaction conditions. Cao, et.al. (2004) investigated the pyrolysis of corn cob in a tube-type reactor. A study on the kinetics of corn straw pyrolysis was also done by Lanetta and Di Blasi (1998). Ioannidou, et.al. (2009) studied both non-catalytic and catalytic pyrolysis of corn residues in two reactor configurations: (1) fast pyrolysis in a captive sampler reactor and (2) non-catalytic slow pyrolysis and catalytic pyrolysis in a fixed-bed reactor. Results show that liquid was the major product from catalytic pyrolysis (about 40-44 wt% on biomass) and that the gas products from the captive sampler reactor gave higher percentage of syngas and lower carbon dioxide content compared to that from the fixed-bed reactor. In 2010, Mullen, et.al. investigated the fast pyrolysis of corn cobs and corn stover for bio-oil and bio-char production in a fluidized bed reactor and characterized such products for energy and soil amendment properties. Bio-oil yield of 60% was obtained from both feedstocks while char yield amounted to 2
around 18.9% and 17% from corn cobs and corn stover, respectively. The bio-oil was found to have a heating value of ~20 MJ/kg. The previous studies focused on quantifying product yields and assessing their qualities as potential energy sources using various reactor configurations and conditions. In this study, corn stover pyrolysis was investigated using a laboratory scale high-pressure/high-temperature batch reactor at varying temperatures and at high pressure. The general objective is to evaluate the product yields at varying pyrolysis temperatures. Specifically, the study aims to: (a) determine the effect of temperature on the yield of pyrolysis products and (b) determine the overall energy and mass conversion efficiencies of the process. Methodology Sample Preparation Ground corn stover samples were used in the experiment, with particle sizes passing through 3 mm-screens using Wiley Laboratory Mill Model #4 distributed by Arthur Thomas Company, Philadelphia, PA, USA. The moisture content of the samples was ensured to be less than 10% prior to subjecting them to the process. Moisture content of the sample was determined following ASTM D3173. Proximate analysis of the sample was also done in accordance with ASTM standards (E 1755 and E 3175). The heating value of the sample was also determined according to ASTM D 2015 using Parr isoperibol bomb calorimeter (Model 6200, Parr Instrument Company, Moline, IL). Experimental Set-up Pyrolysis runs were done in a batch pressure reactor with automatic temperature control equipped with a condenser. Water displacement method was used to measure the volume of gas produced. Figure 1 shows the set-up used in the experiment. Pyrolysis Runs The pyrolysis runs were performed following a one-factor, three-level, completely randomized experimental design. The temperature values that were tested include 400, 500 and 600 o C. The runs were made in three replicates for each temperature. Approximately 100 g of biomass sample was put into the reactor. The reactor was then sealed by placing the flanges in the reactor head and tightening the bolts. Prior to starting each run, the reactor was purged with nitrogen gas to ensure that the process will proceed in the absence of oxygen. Nitrogen was then introduced into the reactor at less than 10 psi pressure for 20 minutes. The reactor controller was then turned on and the temperature set point was specified. The reactor was allowed to be heated until the desired temperature was attained. The valve going to the condenser was closed until the reactor pressure attains 100 psi. As pressure increased beyond 100 psi due to further gas production, the valve was slightly opened to maintain the pressure at 100 psi. When the desired temperature was attained, the process is allowed to proceed for around 20 minutes. Gas samples were also obtained during the run. After this, the heater was shut down and the reactor allowed to cool down. Bio-oil was collected from the vessel below the condenser and weighed. The reactor was opened and the char was collected and weighed. 3
Figure 1. Experimental set-up during corn stover pyrolysis. Analytical Methods The yields of char and bio-oil were expressed as percentage of the amount of biomass pyrolyzed. These products are therefore weighed after each run. The amount of gas produced, on the other hand, was measured in terms of the gas volume. For the energy and mass balance calculations, the char and bio-oil products were analyzed for their heating values using Parr bomb calorimeter. The gas was analyzed for its composition using an SRI Multiple Gas Analyzer #1 (MG#1) gas chromatograph (GC) equipped with an on-column injection system and two detectors: helium ionization detector (HID) and thermal conductivity detector (TCD). The columns used were 6 Molecular Sieve 13X and 6 Silica Gel, with helium as the carrier gas. The calibration gas standard mixture used consisted of H 2, N 2, O 2, CO, CH 4, CO 2 and C 2 H 6 (Praxair Specialty Gases, Austin, TX) with analytical accuracy of ±5 %. The data was statistically analyzed using One-way Analysis of Variance (ANOVA) at 95% level of significance. Results and Discussion Biomass Characterization The proximate analysis of the corn stover samples are presented in Table 1. The heating value is also presented. 4
Table 1. Corn stover characteristics. Biomass Characteristics Experimental Value Corn Stalk (Zabaniotou, 2008) Proximate Analysis, d.b. Moisture Content, % 6.18 ± 0.14 5.78 Ash, % 6.62 ± 0.27 2.3 Volatile Combustible Matter (V.C.M.), % 78.7 ± 1.4 83.49 Fixed Carbon (F.C.), % 14.7 ± 1.65 8.43 Heating value, MJ kg -1 17.0 ± 0.7 -- Approximately 79% of the biomass consists of volatile combustible matter while the fixed carbon content amounts to almost 15%. These constituents are partitioned into the product gas, liquid and solid during the onset of the different pyrolysis reactions. The values were comparable to those obtained by Zabaniotou and Ioannidou (2008). The experimentally determined heating value of the biomass is close to that obtained by Mullen, et.al. (2010), which had a value of 18.3 MJ kg -1. According to Edens, et. al. (2002), a rough estimate of the moisture content of corn stover can be obtained by doubling the grain moisture content, provided that grain moisture is between 18 and 31% w.b. Also, they observed that the corn stover on the ground after grain harvest had a higher moisture content than the standing stover. This indicates that drying the corn stover prior to using them as pyrolysis feedstock is necessary. Effect of temperature on product yields Figure 2 shows the plot of %char yield at varying reaction temperatures. 50 40 Char yield, % 30 20 10 0 300 400 500 600 700 Temperature, o C Figure 2. Char yield (%) as a function of reaction temperature. 5
The plot indicates that the char yield decreased with an increase in temperature. ANOVA results at 95% significance level indicated that temperature has an effect on the char yield. Comparison among the treatment means indicated that the char yield at 400 o C is significantly different from the char yields at 500- and 600 o C, whereas the char yields at 500- and 600 o C are not significantly different. Thus, there was as initial decrease in char yield but further increase in temperature resulted in insignificant char yield changes. The decrease in char yield with an increase in temperature was observed since as the reaction temperature was increased, volatiles are released from the biomass particles, thus the decrease in the weight of the solid products. A maximum char yield of 38.3% was achieved at the lowest temperature. Figure 3 shows the gas yield as a function of temperature. The plot shows an increase in gas yield as the pyrolysis temperature was increased. ANOVA results proved that temperature indeed has an effect on gas yield. However, comparison of the treatment means indicated that the yields at 500 and 600 o C are not significantly different while those for the 400-500 o C and 400-600 o C pairs are significantly different. Thus, there was as an initial increase in gas yield with an increase in temperature. However, further increasing the temperature beyond 500 o C resulted in insignificant gas yield changes. These observations are due to the release of more volatiles at higher temperatures as caused by secondary reactions of pyrolysis vapors. These results are consistent with pyrolysis studies done by Ioannidou, et.al. (2009) and Cao, et.al. (2004), wherein they reported maximum char yield and lower gas yield at lower temperatures. Ioannidou, et.al. (2009) observed higher yields of char mostly at low temperatures (360 o C), while higher yields of gas were observed mostly at temperatures above 500 o C. 40 30 Gas volume, L 20 10 0 300 400 500 600 700 Temperature, o C Figure 3. Gas yield as a function of reaction temperature. Figure 4 shows the yield of bio-oil as a function of temperature. The plot indicates that there is a decrease in the oil yield with an increase in temperature. ANOVA results indicate that these differences are significant. 6
40 30 % Bio-oil yield 20 10 0 300 400 500 600 700 Temperature, o C Figure 4. Bio-oil yield(%) as a function of reaction temperature. The decrease in liquid yield occurred due to an increase in the rate of secondary cracking reactions of the volatiles at higher temperatures. Studies done by Ioannidou et.al. (2009) also showed a decrease in liquid yield as the reaction temperature increases. Material and Energy Balance Figure 5 shows the percentage distribution of the products in terms of mass and energy terms, during corn stover pyrolysis at 400 o C. According to Onay and Kockar (2003), the proportion of gas, liquid and solid products depends very much on the pyrolysis technique used and on the reaction parameters. Maximum yield of char is attained with slow heating processes over long period of time, with moderate amounts of tar by-products. High liquid yields are achieved at high heating rates and short reaction times. In this study, the heating rate is very slow, and thus, the char yield was observed to be high. With an assumed corn stover yield of 10.1 dry tons/ha (Nielsen, 1995), the amount of products can be roughly estimated from the material balance calculations. Char yield in terms of per hectare of land amounts to 4.10 tons/ha. Oil yield corresponds to 3.72 tons/ha while gas yield (in terms of volume) is approximately 1542 m 3 /ha. 7
LOSSES 4.29% GAS 23.15% CHAR 37.88% BIO OIL 34.68% Figure 5. Product distribution at 400 o C (% by weight based on biomass input) The energy content of each product was also determined. The heating values of char and bio-oil heating values were determined to be approximately 27.9 MJ/kg and 33.8 MJ/kg, respectively, using a bomb calorimeter. Figure 6 shows the energy conversion efficiencies, calculated based on the amount of energy contained in the feedstock. GAS 11.9% LOSSES 13.1% CHAR 66.7% BIO OIL 8.3% Figure 6. Distribution of energy from feedstock at 400 o C (% energy contained in products) Due to the high yield of char, it is obvious that most of the energy in the feedstock is captured in this product. The organic phase of the bio-oil, which has a higher value of the energy content than the char, contained only around 8.18% of the energy content of the feedstock, since the portion of organic phase in the oil is very minimal as compared to the aqueous phase, which has no heating value at all. The product gas, on the other hand, contained 25.64% of the energy from the feedstock. 8
Conclusion Pyrolysis of corn stover at different temperatures in a batch high-pressure/hightemperature reactor resulted in different distributions of product (char, bio-oil and gas) yields. Increasing the reaction temperature lead to a decrease in char yield and an increase in gas yield. Maximum char yield of approximately 38% was obtained at the lowest temperature (400 o C). Mass and energy conversion efficiencies were also determined, analyzing the distribution of both mass and energy among the products. Due to the very slow heating rate of the process, char yield was considerably high as compared to the liquid and gas portions. The liquid phase, on the other hand, consisted mostly of the aqueous phase. This caused the bio-oil to contain only approximately 8% of the energy from the feedstock. The char contained majority of such energy, amounting to approximately 66%. References Cao, Q., Xie KC, Bao WR, Shen SG. 2004. Pyrolytic behavior of waste corn cob. Bioresour. Tech. 94 : 83 89. Edens, W.C., L.O. Pordesimo, and S. Sokhansanj. 2002. Field drying characteristics and mass relationships of corn stover fractions. ASAE Paper No. 026015. St.Joseph, MI.: ASABE. Iannidou, O., A. Zabaniotou, E.V. Antonakou, K.M. Papazisi, A.A. Lappas, and C. Atahnassiou. 2009. Investigating the potential for energy, fuel, materials and chemical production from corn residues (cob and stalks) by non-catalytic and catalytic pyrolysis in two reactor configurations. Renewable and Sustainable Energy Reviews. 13: 750 762. Lanzetta M, Di Blasi C. 1998. Pyrolysis kinetics of wheat and corn straw. J. Anal. Appl. Pyrolysis. 44: 181 192. Mullen, C.A., A.A. Boateng, N.M. Goldberg, I.M. Lima, D.A. Laird, and K.B. Hicks. 2010. Bio-oil and Bio-char production from corn cobs and stover by fast pyrolysis. Biomass and Bioenergy. 34: 67-74. Nielsen, R. L. 1995. Questions Relative to Harvesting and Storing Corn Stover. AGRY-95-09. West Lafayette, Indiana: Purdue University. Available at: http://www.agry.purdue.edu/ext/corn/pubs/agry9509.htm. Accessed 24 March 2010. Onay, Ozlem and O.M. Kockar. 2003. Slow, fast and flash pyrolysis of rapeseed. Renewable Energy. 28:2417-2433. Xu, R., L. Ferrante, C. Briens, and F. Berruti. 2009. Flash pyrolysis of grape residues into biofuel in a bubbling fluid bed. J. Anal. Appl. Pyrolysis. 86: 58 65. Zabaniotou, A. and O. Ioannidou. 2008. Evaluation of utilization of corn stalks for energy and carbon material production by using rapid pyrolysis at high temperature. Fuel. 87: 834-843. 9