(169a) Comparison of Reduced Kinetic Mechanisms for Gas Phase Reactions in Fluidized Bed Biomass Gasification

Stark, A. K. - Presenter, Massachusetts Institute of Technology
Bates, R. B. - Presenter, Massachusetts Institute of Technology
Zhao, Z., Massachusetts Institute of Technology
Ghoniem, A. F., Massachusetts Institute of Technology

Kinetic study of the gas phase reactions is key to understanding fluidized bed biomass gasification (FBBG). Under typical operating conditions for FBBG (700-900℃), tars exist in the product gas in significant quantities (2-50 g/Nm3)[1].The formation and evolution of these condensable aromatic products is particularly important as they introduce several operational and cleanup challenges. A detailed gas-phase pyrolysis and oxidation kinetic mechanism has been proposed in the literature[2], including 327 species and over 10,000 reactions. However,owing to its high computational cost and the fact that the vast number of species cannot be experimentally measured, the direct application of the detailed mechanism is in intractable in most reacting flow simulations of biomass gasification. In particular, the development a reduced kinetic mechanism is necessary for detailed computational fluid dynamics (CFD) simulations of a fluidized-bed biomass gasifier.

In this work, reduced kinetic mechanisms based on the mechanism of Ranzi et al have been developed to modelthe secondary gas phase pyrolysis, partial oxidation, and combustion reactionsof the primary pyrolysis products of biomass.The reduced mechanism was developedusing the prediction from the detailed mechanismin a fluidized bed reactor modeled as a Continuously Stirred Reactor (CSTR) and a Plug Flow Reactor (PFR) to simulate the bed and free-board zone respectively. Several mechanism reduction approaches have been utilized to generate reduced kinetic mechanisms, including sensitivity and rate analysis, response-surface mapping methods, and reaction path analysis. The reduced mechanisms obtained in this study were then tested using a Reactor Network Model (RNM), which represents the transport and reaction processes within a typical fluidized bed reactor. The simulated outlet compositions and trends from the reduced mechanisms are compared to those predicted by the detailed kinetics as well as the experimental data for atmospheric air-blown beech gasification from van Paasen& Kiel (2004)[3]. The accuracies as well as the efficiencies are also compared among different reduced mechanisms.

[1]Milne, T.., Evans, R.J., Abatzoglou, N., 1998. Biomass gasifier “tars”: Their nature, formation and conversion ( No. TP-570-25357). National Energy Technology Laboratory (NETL), Boulder, CO.

[2]Ranzi, Eliseo et al., 2008. Chemical Kinetics of Biomass Pyrolysis.Energy& Fuels, 22(6), pp.4292–4300.

[3]vanPaasen, S.V.B., Kiel, J.H.A., 2004. Tar formation in a fluidised bed gasifier( No. ECN-C--04-013). Energy Research Center of the Netherlands.