1. North Carolina Agricultural and Technical State University
Extreme Science and Engineering Discovery Environment 2015 (XSEDE15) Poster Competition
Computational Fluid Dynamics (CFD) modeling of Biomass Gasification in a Bubbling Fluidized Bed Gasifier (BFBG); Examining the Effects
of Equivalence Ratio, Gasifier Temperature, and Particle-Particle Heat Transfer on Synthesis Gas Quality
Samuel Asomaning Agyemang (Computational Science and Engineering)
Dr. Lijun Wang, Dr. Abolghasem Shahbazi and Dr. Yevgenii Rastigejev
ABSTRACT
INTRODUCTION
OBJECTIVES
GASIFICATION SIMULATION METHODOLOGY
RESULTS AND ANALYSIS
Gasification has been identified as an energy-efficient, environmentally friendly and
economically feasible technology to partially oxidize biomass into a gaseous mixture of
syngas consisting of H2, CO, CH4 and CO2. High quality syngas can be further used to
catalytically synthesize liquid fuels and produce hydrogen. There is a need to fully understand
the gasification phenomenon, which involves a series of complicated gas-particle
hydrodynamics resulting in an intimate mixing of gasification components which leads to
chemical reactions with heat and mass transfers. Computational Fluid Dynamics (CFD) is
proven to be an effective strategy to examine the physics of a gasification system and also as
a means to estimate or measure internal parameters of the gasifier which otherwise will be
indeterminate through costly and time consuming experiments. In this research, an Eulerian–
Eulerian computational fluid dynamics (CFD) model of the gasification processes in a
bubbling fluidized bed gasifier (BFBG) is presented. In this presentation, the gasification
model considers separate phases for the three interacting components of the bubbling
fluidized bed gasifier, i.e. biomass, sand, and gas. This model includes gasification reaction
kinetics such as moisture evaporation, devolatilization or primary pyrolysis, gas-solid
heterogeneous reactions and gas-gas homogeneous reactions. These reaction kinetics,
written as subroutines in C-programing language, are implemented as User Defined
Functions (UDF’s) on the commercial ANSYS Fluent CFD platform. The complicated effects of
heat transfer due to the stochastic interactions of biomass particles and the heated sand bed
material is also modeled via C-programing language and implemented on the CFD platform as
a UDF. The simulation is carried out for different experimental conditions as set by the
operations of the biomass gasifier situated at the North Carolina A & T State University Farm.
The simulations are conducted to examine the effects of gasification input parameters such
as gasifier temperature, Equivalence Ratio (ER), and sand-biomass particle-particle heat
transfer on the quality of synthesis gas produced at the gasifier exit. The hydrodynamic
behaviors as well as species and reaction distribution within the gasifier are presented.
Results from varying equivalence ratio while maintaining the gasifier temperature at
1073.15K show that gas yields from gasification at an ER of 0.4 was higher (2.70 Nm3/kg) than
that of gasification at an ER of 0.2 (1.55 Nm3/kg). Also gasification conducted at 0.2 ER
generated higher H2 and CO concentrations (17.66 vol.% and 20.05 vol.% respectively) than
gasification conducted at 0.4 ER, which produced 12.92 vol.% H2 and 14.71 vol.% CO. It was
also found that gasification at an ER of 0.4 resulted in higher carbon conversion of 72.21%
than gasification at 0.2 ER, which resulted in a 68.75% carbon conversion. Including particle-
particle heat transfer models in the gasification simulation resulted in increments in
volumetric concentrations of CH4 and CO2 by 17.6% and 19.3%; 31.8% and 51.9%; 19.0% and
6.9% for gasification conducted at 0.2, 0.3 and 0.4 ER’s respectively.
 Model biomass gasification kinetics and incorporate them into the CFD
model under the ANSYS Fluent simulation platform via User Defined
Functions (UDF)
 Model interactions between bed materials and solid biomass particles
and incorporate them into the CFD model via UDFs
 Model the dynamic changes of biomass particles during gasification in
the CFD model i.e. communition, particle weight loss and diminishing
particle size
 Validate the CFD model through experiments on the existing fluidized
bed gasifier
 Advance the design and operation of fluidized bed gasifiers using the
results from the CFD model
Schematic of Biomass Gasification Process
Two Dimensional Gasifier Model and Operating Conditions
Disclaimer: This Project was fully supported by funds provided by NSF-CREST Project (Award number 1242152) and USDOE Project (Award number:
EE0003138). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the funding agencies
Continuity/Mass Conservation Equation
Momentum Conservation Equation
Realizable K- ε Turbulence Equation
Energy Conservation Equation
Particle-Particle Heat Transfer
Coefficient Model
Species Conservation Equation
Table 1 Gasification Kinetics
Conclusions
Varying the equivalence ratio while maintaining the gasifier temperature at
1073.15K show that gas yields from gasification at an ER of 0.4 was higher (2.70
Nm3/kg) than that of gasification at an ER of 0.2 (1.55 Nm3/kg). Also gasification
conducted at 0.2 ER generated higher H2 and CO concentrations (17.66 vol.% and
20.05 vol.% respectively) than gasification conducted at 0.4 ER, which produced
12.92 vol.% H2 and 14.71 vol.% CO. The higher H2 concentration led to the higher
energy content of the synthesis gas produced from gasification conducted at 0.2
ER (i.e. HHV of 4.85 and 6.09 MJ/Nm3 for ER 0.4 and 0.2 respectively). It was also
found that gasification at an ER of 0.4 resulted in higher carbon conversion of
72.21% than gasification at 0.2 ER, which resulted in a 68.75% carbon conversion.
Gasification is the thermal conversion of organic materials at elevated
temperatures (700 – 1200oC) and incomplete oxidation to produce primarily
permanent gases, with traceable char, water and condensibles.
Sand and Gas Volume Hydrodynamic Distribution
Synthesis Gas Species Distribution in Gasifier
Notables
1. Red areas indicate regions of highest particle-
particle collisions and heat transfer coefficients
2. Particle-particle collisions and recorded heat
transfer coefficients are restricted to the
bottom half of the gasifier
3. Time Scales of biomass existence in the
gasification domain
4. Gasification is a very fast process, taking 1 to 2
s to convert the biomass particle
5. Recorded heat transfer coefficient decreases
with the conversion of biomass particles