Energy & Fuels 2003, 17, 549-558
549
Kinetic Modeling of Coal/Agricultural By-Product Blends D. Vamvuka,* N. Pasadakis, and E. Kastanaki Department of Mineral Resources Engineering, Technical University of Crete, Greece
P. Grammelis and E. Kakaras Mechanical Engineering Department, National Technical University of Athens, Athens, Greece Received August 12, 2002
There is renewed interest worldwide in the production of sustainable energy from renewable sources, such as biomass, aimed mainly at the decrease of fossil fuels use. Except for its potential to be CO2 “neutral” during combustion, biomass has a low sulfur content and a high volatile concentration: these characteristics favor clean combustion conditions. The knowledge of the kinetics of biomass pyrolysis and combustion is important for the control of such thermochemical processes. The main objective of this work was to determine the kinetic parameters of thermal decomposition of two biomass materials, olive kernel and straw, and of their mixtures with a high- and a low-rank coal. The study was carried out using a thermogravimetric analyzer (TGA) in nitrogen atmosphere, at a heating rate of 10 °C/min. A kinetic model, involving first-order independent parallel reactions, was used. Activation energies and frequency factors were determined for two different particle sizes. The analysis indicated that the pyrolysis of the coals and the biomass samples could be modeled successfully via the independent reactions models, the pyrolysis of biomass being described by reactions corresponding to hemicellulose, cellulose, and lignin decomposition. The results showed that the chemical composition of each biomass type plays a fundamental role in the kinetics determination. Smaller conversion times and increased devolatilization rates were obtained, when biomass was added in the fuel blend with coal. The additive properties of coal and biomass, pyrolyzed in blends, were examined. It was proven that the mass loss vs time during thermal conversion of coal/biomass blends is welldescribed by the sum of each individual coal and biomass decomposition.
Introduction There is renewed interest worldwide in the production of sustainable energy from renewable sources, such as biomass, stimulated by increasing fossil and nuclear fuel use. Energy from biomass can be most conveniently used in places where it is found naturally, or where it is produced as a process refuse. In countries of the Mediterranean basin, such as Greece, there is a large amount of biomass in the form of agricultural or agroindustrial residues, such as olive kernel and straw. The conversion of these materials by thermochemical processes, such as pyrolysis, combustion, and gasification, can significantly and immediately reduce the mass and volume of wastes, allowing for energy recovery. Furthermore, their coutilization with coals in operating power plants provides technical, economical, and environmental benefits. Knowledge of the kinetics of biomass and biomass/coal blends’ pyrolysis is also important during combustion, providing useful data for the process control. Thermoanalytical techniques, in particular thermogravimetric analysis (TGA), allow this information to be obtained in a relatively simple and straightforward manner. Pyrolysis studies, using thermal analysis methods, have been previously performed with wood/cellulosic, * To whom correspondence should be addressed. Tel: +30 821 37603. Fax: +30 821 69554. E-mail:
[email protected].
fossil fuels, plastics/resins/foam, and biomass/municipal solid waste materials.1-6 There is a diversity of opinions in the specialized literature, dealing with the thermal decomposition of lignocellulosic materials, due to the complexity of the process.7,8 The large majority of the available kinetic studies points to a poor fit of simple reaction models in the whole range of conversions, and a break point is frequently included in the calculations, to obtain satisfactory fits to the experimental results.9,10 Alternative solutions consist of calculating distribution functions for the activation energies,9 or using models of consecutive or independent reactions.4,11-14 A first(1) Williams, P. T.; Besler, S. Renewable Energy 1996, 7, 233. (2) Wu, C. H.; Chang, C. Y.; Lin, J. P. J. Chem. Technol. Biotechnol. 1997, 68, 1. (3) Saade, R. G.; Kozinski, J. A. J. Anal. Appl. Pyrolysis 1998, 45, 9. (4) Orfao, J. J. M.; Antunes, F. J. A.; Figueiredo, J. L. Fuel 1999, 78, 349. (5) Zheng, G.; Kozinski, J. A. Fuel 2000, 79, 181. (6) Stenseng, M.; Jensen, A.; Dam-Johansen, K. J. Anal. Appl. Pyrolysis 2001, 58-59, 765. (7) Simkovic, I.; Varhegyi, G.; Antal, M. J.; Ebringerova, A.; Szekely, T.; Szabo, P. J. Appl. Polym. Sci. 1988, 36, 721. (8) Jakab, E.; Faix, O.; Till, F.; Szekely, T. J. Anal. Appl. Pyrolysis 1995, 35, 167. (9) Cordero, T.; Maroto, J. M. R.; Mirasol, J. R.; Rodriguez, J. J. Thermochim. Acta 1990, 164, 135. (10) Cordero, T.; Rodriguez-Maroto, J. M.; Garcia, F.; Rodriguez, J. J. Thermochim. Acta 1991, 191, 161. (11) Varhegyi, G.; Antal, M. J.; Szekely, T.; Szabo, P. Energy Fuels 1989, 3, 329.
10.1021/ef020179u CCC: $25.00 © 2003 American Chemical Society Published on Web 04/03/2003
550
Energy & Fuels, Vol. 17, No. 3, 2003
Vamvuka et al.
Table 1. Proximate Analysis (wt %), Ultimate Analysis (wt %), and Calorific Values on a Dry Basisa sample
V. M.
F. C.
ash
C
H
N
O
S
HHV (MJ/kg)
Megalopolis lignite Go¨ttelborn lignite olive kernel straw
47.30 33.60 70.66 77.97
16.07 56.90 19.39 16.27
36.63 9.50 10.06 5.76
31.85 66.16 46.86 41.37
3.28 4.16 5.43 6.03
1.37 1.54 3.23 0.47
22.73 17.38 33.90 45.23
4.07 0.81 0.58 0.19
10.0 30.1 18.1 16.5
a
V. M.: volatile matter. F. C.: fixed carbon. HHV: high heating value.
order reaction has been widely considered for the decomposition of the whole biomass, or for the formation of some products, or for the the scheme of the two or three series of reactions proposed,8,11,15-17 although other reaction orders have been also referred to in the literature.11,17,18 Several authors4,15,19 have investigated the kinetics of biomass materials; however, only a few20 have determined the kinetics of their blends with coals. The aim of this work was to determine and compare the kinetic parameters, as well as the devolatilization characteristics of a high-rank coal, a low-rank coal, and two biomass components (olive kernel and straw) and their blends, under various experimental conditions, by carrying out TGA experiments. The application of TG analysis, combined with kinetic modeling described in this paper, can provide quite accurate kinetic data to be used for boilers’ furnaces modeling purposes. The obtained information would be useful in the development of biomass utilization processes, as well as in the evaluation of the coprocessing of these fuels for power generation. Experimental Section Materials. The materials selected for this study were one high-rank coal (Gottelborn), one low-rank coal (Megalopolis Greek lignite), olive kernel, and straw. Also, blends of each coal with each biomass sample were prepared, in biomass percentages of 10% and 20 wt %. These percentages were chosen, since they are typical of cofiring applications used in the European industry. It is worth mentioning that similar percentages were used during the cofiring demonstration of olive kernel and lignite at the Megalopolis power plant, in Greece. After air-drying, the samples were milled and sieved to (0.7-1) mm and
