Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.3, No.11, 2013 29 Analysis of Biomass Pyrolysis Product Yield Distribution in Thermally Thin Regime at Different Heating Rates Pious O. Okekunle 1* , Hirotatsu Watanabe 2 , Ken Okazaki 2 1. Department of Mechanical Engineering, Ladoke Akintola University of Technology, P.M.B. 4000, Ogbomoso, Oyo state, Nigeria. 2. Department of Mechanical and Control Engineering, Graduate School of Engineering, Tokyo Institute of Technology, 2-12-1-I6-7, Ookayama, Meguro-ku, Tokyo, Japan. *E-mail of the corresponding author: pookekunle@lautech.edu.ng Abstract A better understanding of biomass pyrolysis process at various thermal regimes is fundamental to the optimization of biomass thermochemical conversion processes. In this research work, the behaviour of biomass pyrolysis in thermally thin regime was numerically investigated at different heating rates (1, 5, 10 and 20 K/s). A kinetic model, consisting of five ordinary differential equations, was used to simulate the pyrolysis process. The model equations were coupled and simultaneously solved by using fourth-order Runge-Kutta method. The concentrations of the biomass sample (Maple wood) and product species per time were simulated. Findings revealed that tar yield increased with increase in heating rate. Char yield, however, decreased with increase in heating rate. Results also showed that the extent of secondary reactions, which influenced gas yield concentration, is a function of residence time and temperature. This model can be adopted for any biomass material when the kinetic parameters of the material are known. Keywords: Biomass, pyrolysis, kinetic model, thermally thin regime 1. Introduction Ever increasing energy demand and the problem of greenhouse gases emissions from combustion of fossil fuels have resulted in seeking alternative, environmentally friendly and renewable energy sources [1]. For quite some decades, biomass energy has been attracting attention as one of the possible alternatives for fossils. Biomass is a versatile renewable source of energy that can be readily stored and transformed into electricity and heat. Developing countries have a greater interest in biomass because their economies are largely based on agriculture and forestry [2]. Although there are several methods of converting biomass into energy, thermochemical processes are often used due to the possibility of converting the feedstock into three constituents; solid (char or carbon), liquid (tar and other heavy hydrocarbons) and gas (CO 2 , CO, H 2 , C 2 H 4 and H 2 O et c.) through these processes. Pyrolysis, being a precursor of other thermochemical processes (gasification and combustion) plays a vital role in determining the eventual product yield distribution. Many research works have been done on pyrolysis process [3-7] but little attention is paid to results interpretations relative to the thermal regimes under study. This, in turn, has led to some misconceptions about the effect of various process parameters on pyrolysis. This work is therefore set out to numerically investigate the effect of different heating rates on biomass pyrolysis product distribution in thermally thin regime. In this regime, the rate of heat transfer to and within the particle is very fast compared to the reaction rate. Therefore, the solid temperature will be essentially the same as that of the reactor environment and the overall controlling factor is the intrinsic kinetics [22]. Hence, biomass decomposition reaction takes place isothermally and the process progresses under conditions of pure kinetic control. In other words, pyrolysis occurs throughout the particle and there is no char insulating the unreacted core [23]. Over the years, many reaction mechanisms have been devised for the study of biomass pyrolysis. These are either one-step global models [8-12] or one- or two-stage multiple reaction models [13-19]. Prakash and Karunanithi [20] have reviewed some advances in modeling and simulation of biomass pyrolysis. In this study, a two-stage multiple reaction model was used. This is because models as such consider intra-particle secondary reactions of primary products of pyrolysis. 2. Modeling 2.1 Pyrolysis mechanism The pyrolysis mechanism adopted for this study was proposed by Park et al. [21]. As shown in Figure 1, the virgin biomass (wood) primarily decomposes by three competitive endothermic reactions to yield gas, tar and intermediate solid. The tar generated from primary pyrolysis participates in secondary reactions yielding more gas and char. The intermediate solid, however, further decomposes through exothermic secondary reactions into char. A detailed description of this model has been given by Park et al. [21].