(82h) Combined Transport and Kinetic Modeling of Downdraft Biomass Gasifier

Bio-energy is now accepted as having the potential to provide the major part of the projected renewable energy provisions of the future. Gasification is one of the efficient technologies to convert the energy embedded in the biomass (Babu and Sheth, 2004). The understanding of the chemical and physical mechanisms during gasification is of fundamental importance for the optimal design of biomass gasifier (Babu and Chaurasia, 2004). Modeling of biomass gasification implies the representation of chemical and physical phenomena constituting drying, pyrolysis, combustion, and reduction in the mathematical form (Babu and Sheth, 2005). Kinetics-free equilibrium models can predict the exit gas composition, given the solid composition and the equilibrium temperature, but they cannot be used for reactor design (Babu and Sheth, 2009). A transient one-dimensional model for the throated close-top downdraft biomass gasifier has been developed by Sheth and Babu (2009). The model takes into account of the pyrolysis, secondary tar reactions, homogeneous gas reactions and heterogeneous combustion/gasification reactions. The drying and pyrolysis phenomena are modeled together as one zone in their studies. The drying zone is indirectly incorporated in the pyrolysis zone model. The pyrolysis model is divided into two subsystems, i.e., gas phase inside the bed and the individual particles. The model for single particle, developed by Babu and Chaurasia (2004), is used in the complete gasifier model (Sheth and Babu, 2009). The composition of volatiles is found using the experimental data of Boroson et al. (1989), which predicts the release of mainly water vapor from the pyrolyzing particle below 120 °C. Di Blasi (1998) also reported that at low temperatures the degradation rates are much slower than the drying rate. The presence of moisture is seen to delay wood pyrolysis and ignition. Therefore, drying and pyrolysis can be seen as two separate (sequential) processes in the biomass gasification (Di Blasi, 1998).

In the present study, the drying zone of the biomass gasifier is modeled independently and incorporated into the model proposed by Babu and Sheth (2009). A model of moisture evaporation and transport phenomena is presented to account for the drying. The drying model is divided into two subsystems, i.e., the bulk phase in the bed and the individual particles. Bulk phase modeling is carried out by a transient mass balance for water vapor. Single particle model incorporates the conservation of moisture, water vapor and energy. The kinetic model of drying developed by Chan et al. (1985) for wood pyrolysis is used in the present study. The simulated model can predict the performance, a priori, of the biomass gasifier, which is useful in the design of a downdraft biomass gasifier.

References

Babu, B.V., Chaurasia, A.S., 2004. Heat Transfer and kinetics in the pyrolysis of shrinking biomass particle, Chemical Engineering Science, 59, 1999-201.

Babu, B.V., Sheth, P.N. 2004. Modeling and Simulation of Downdraft Biomass Gasifier, Proceedings of International Symposium & 57th Annual Session of IIChE in association with AIChE (CHEMCON-2004), Mumbai, December 27-30, (2004)

Babu, B.V., Sheth, P.N. 2005. Parametric Study of Factors affecting the Composition of Producer Gas, Proceedings of CHEMCON-2005, New Delhi.

Boroson, M.L., Howard, J.B., Longwell, J.P., Peters, W.A., 1989. Product Yields and Kinetics from the Vapor Phase Cracking of Wood Pyrolysis Tars. American Institute of Chemical Engineers Journal, 35, 120-128. Chan W.R., Kelbon M., Krieger B.B. 1985. Modeling and experimental verification of physical and chemical processes during pyrolysis of a large biomass particle, Fuel 64, 1505?13.

Di Blasi C., 1998. Multi-phase moisture transfer in the high temperature drying of wood particles, Chemical Engineering Science, 53, 353-366.

Sheth, P.N., Babu, B.V., 2009. Modeling and Simulation of Downdraft Biomass Gasifier. Proceedings of 2009 Annual Meeting of AIChE, Gaylord Opryland Hotel, Nashville, TN, USA, November 8-13, (2009)