Pyrolysis of Biomass Particles Using Circulating Fluidized Bed reactor with Heat loop of the Heat Carrier Particles

S. Azizi, D. Mowla^*

Environmental research center in petroleum and petrochemical industries, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345-1789, IRAN

dmowla@shirazu.ac.ir

Introduction

Conversion of biomass into fuels and chemicals has received considerable attentions in recent years for reducingof greenhouse gas emissions and global energy security and sustainability [1]. One of the potential alternatives is thermal decomposition of biomass (400-600 ⁰C) at inert atmospheric conditions, leading to the release of volatiles components and remaining of non-volatiles as char. The produced volatiles after condensation, called bio-oil, could be converted into hydrogen by steam reforming or directly combusted in boilers for generation of heat and electricity. Syngas (CO + H₂) from biomass pyrolysis could also be converted into liquid fuels by Fisher-Tropsch processes or combusted directly for power generation [2].The design of the pyrolysis process has an important role on obtaining higher rates of bio-oil products. In this process, a key factor in increasing the bio-oil to char ratio, is increasing the rate of heat transferto the biomass particles (fast pyrolysis) and fast removal of the volatiles [3].

Although the fluidized bed reactors provide higher rate of heat transfer compared to conventional pyrolysis reactors, the heat transfer rate is limited by the convection to the particles surfaces and the conduction inside the particles. Xue et al. developed a numerical method to simulate fast pyrolysis of the biomass in a fluidized bed of sand particles to enhance heat transfer from heated walls of the reactor to the biomass particles [4]. The added inert particles to the fluidized bed improve the rate of heat transfer and fluidization properties of the biomass to reach desired fluidization velocity and good mixing of the particles. In this work, instead of using heated wall, performance of a circulating fluidized bed reactor with the heating loop of inert particles at the circulation path is investigated for pyrolysis of biomass.

Modeling

The inert and biomass particles are supposed to form two separate granular phases. In the circulating fluidized bed reactor, an Eulerian approach, based on granular temperature energy of the particles was used to model the hydrodynamics of the particulate phases coupled with the continuum gas phase. Considering the reaction kinetics, the heat transfer rate and chemical species conservation applied to thermo-chemical behavior of the pyrolysis reactor, a transient numerical simulation has been done for the fluidized bed reactor for a period of 10 seconds with time step of 10^-4 second. The numerical domain consists of gas phase (nitrogen), biomass inlet, the heating loop of the inert particles, and outlet of volatiles. A 2D domain due to less computational costs was used to solve numerical model with appropriate mesh sizes.

Results

In the CFD simulation, the distribution of granule phases, gas temperature, particles temperature, and pyrolysis productsconcentrations were studied to show the performance of the fluidized bed reactor. The effects of the particles diameter were investigated on the segregation of particles, heat transfer rate and yield of the pyrolysis products. The efficiency of the heat transfer rate and the yield of the pyrolysis products were compared to the conventional fluidized bed with heated wall boundary condition. In order to reach the high heat transfer efficiency and yield of the products, desired operating conditions of the pyrolysis reactor were illustrated. By using of thecirculating fluidized bed with the heating loopingof inert particles, impressive progress of the biomass pyrolysis was observed.

References

[1] Renewables 2011: Global Status Report (2011) 17-18.

[2] Iliuta I, L.A., Larachi F, Allothermal steam gasification of biomass in cyclic multi-compartment bubbling fluidized-bed gasifier/combustor-New reactor concept. Bioresource Technology 101(2010), 3194-3208.

[3] ChungenYin, Microwave-assisted pyrolysis of biomass for liquid biofuels production, Bioresource Technology 120(2012),273–284.

[4] Q. Xue, T.J. Heindel, R.O. Fox, ACFDmodelforbiomassfastpyrolysisin fluidized-bedreactors, ChemicalEngineeringScience66(2011),2440–2452.