Working Principle of Biomass Combustion and Its CFD Modelling

Principle of  Biomass Combustion

• Biomass combustion is a complex process that consists of consecutive heterogeneous and
  homogeneous reactions.
• The main process steps are
  • Drying
  • Devolatilization
  • Gasification
  • Char combustion,
  • Gas-phase oxidation.
  • Residence  time

• The residence time of each reaction depends on the fuel size and properties, on temperature, and on combustion conditions.
• Biomass combustion takes place when biomass (solid material like a pellet) reacts with the oxygen in the air to produce heat.
• The heat created by the burning of biomass is used in the operation of equipment such as boilers, furnaces, kilns, and engines.
• Along with heat, carbon dioxide and water vapor are created as byproducts of the chemical reaction

Design and Modeling of Biomass combustion

• Biomass combustion modeling is a multidisciplinary subject that combines principles from various fields of science and engineering.
• The basic subjects involved in biomass combustion modeling include:

1. Thermodynamics:
   • Thermodynamics is the study of heat transfer and energy conversion processes.
   • In biomass combustion modeling, thermodynamics principles are used to understand heat generation, heat transfer, and the energy balance within the combustion system.
2. Fluid Mechanics:
   • Fluid mechanics deals with the behavior of fluids (liquids and gases) and their interactions with solid boundaries.
   • In biomass combustion modeling, fluid mechanics principles are used to study the flow of air and combustion products within the combustion chamber.
3. Combustion Chemistry:
   • Understanding the combustion of solid state is essential for engineers for designing biomass combustion and stove.
   • Fuel characteristics, Chemical kinetics, and pyrolysis (devolatization) need to be understood while selecting the fuel types.
   • Combustion chemistry is the study of chemical reactions that take place during the burning of fuels.
   • In biomass combustion modeling, knowledge of combustion chemistry is essential to predict reaction rates, species concentrations, and emissions.
   • Turbulent multiphase combustion is presented in the previous post.
4. Heat Transfer:
   • Heat transfer is the study of the movement of heat from one place to another.
   • In biomass combustion modeling, understanding heat transfer mechanisms is crucial for predicting temperature distributions and heating rates within the combustion system.
5. Reaction Kinetics:
   1. Reaction kinetics deals with the rates at which chemical reactions occur. In biomass combustion modeling, reaction kinetics is used to model the combustion process and the time-dependent evolution of species concentrations.
6. Turbulence:
   • Turbulence is the chaotic and unsteady motion of fluids.
   • In biomass combustion modeling, turbulence plays a significant role in mixing air and fuel, affecting combustion efficiency and emissions.
7. Computational Fluid Dynamics (CFD):
   • CFD is a numerical method used to solve fluid flow and heat transfer problems. In biomass combustion modeling, CFD is employed to simulate and visualize the complex flow and combustion phenomena within the combustion chamber.
8. Radiation Heat Transfer:
   • Radiation heat transfer deals with the transfer of thermal energy in the form of electromagnetic radiation. In biomass combustion modeling, radiation heat transfer is considered in cases where high-temperature flames emit significant amounts of thermal radiation.
9. Emission Control:
   • Emission control principles are employed to understand and minimize the formation of pollutants during biomass combustion.
   • Techniques to reduce emissions, such as optimizing combustion conditions and using emission control technologies, are studied.
10. Experiment and Testing of Biomass Stove
    • Experimental techniques are used to validate the results of biomass combustion modeling.
    • Data from experimental studies provide crucial information for improving model accuracy and reliability.

• Overall, biomass combustion modeling is an interdisciplinary field that draws upon various branches of science and engineering.
• By combining these basic subjects, researchers can develop comprehensive models to simulate and optimize biomass combustion processes for better energy efficiency and reduced environmental impact.

CFD Modeling of Biomass Combustion

• (Computational Fluid Dynamics) modeling of biomass combustion is a powerful simulation technique used to study and analyze the complex fluid flow, heat transfer, and chemical reactions occurring during the combustion process of biomass in various systems, including stoves, boilers, and biomass power plants.
• CFD simulations enable researchers and engineers to gain insights into the behavior of biomass combustion, optimize system designs, and improve combustion efficiency while minimizing emissions.

Key aspects of CFD modeling of biomass combustion include:

Governing Equations:

• CFD models use fundamental conservation equations, such as the Navier-Stokes equations for fluid flow, the energy equation for heat transfer, and species transport equations for combustion products like carbon dioxide, water vapor, and other gases.
• These equations are solved numerically to simulate the behavior of the fluid and combustion process.

Combustion Chemistry:

• Biomass combustion involves complex chemical reactions, including pyrolysis (thermal decomposition of biomass), devolatilization (release of volatile gases), char combustion (burning of solid carbon), and gas-phase combustion reactions.
• CFD models incorporate detailed chemical reaction mechanisms to accurately represent these processes.

Turbulence Modeling:

• Turbulent flow is common in combustion systems.
• Various turbulence models, such as Reynolds-Averaged Navier-Stokes (RANS) or Large Eddy Simulation (LES), are employed to predict turbulence characteristics and their effects on combustion.

Boundary Conditions:

• Accurate specification of boundary conditions, such as the inlet air velocity, temperature, and species concentrations, is crucial for realistic simulations.
• These conditions can vary depending on the specific biomass combustion system being modeled.
• Inlet condition and Fuel composition
  • Properties of biomass materials based on ultimate and proximate analysis are input for CFD solver
  • Enter the mass flow rate of air and fuel at the inlet of the combustion chamber
  • Mole fraction of biomass fuel and air stream
  • The inlet temperature of biomass and air
• Particle Tracking:
  • In cases where solid particles are present, such as char particles in biomass combustion
  • Particle tracking models like DPM are used to study their behavior, including residence times and combustion rates.
• Validation: C
  • FD models are validated against experimental data to ensure their accuracy and reliability.
  • Validation involves comparing simulation results with real-world measurements to verify the model’s predictive capabilities.

CFD Results and Analysis 

Applications of CFD modeling of biomass combustion include:

• Understanding the flow patterns, temperature distributions, and gas concentrations in combustion chambers or furnaces.
• Investigating the impact of design modifications on combustion efficiency and emissions.
• Assessing the influence of fuel properties, such as moisture content and particle size, on combustion performance.
• Optimizing burner designs for better mixing and heat transfer during combustion.
• Predicting pollutant formation and emissions during biomass combustion.