Utah Clean Coal Program
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Large Eddy Simulations of Oxy-Coal and Gasification Flames

Oxy-coal flame simulation

Oxy-coal combustion, in which an O2/CO2 mixture replaces air, is one of the few possible capture technologies to enable CO2 sequestration for existing coal-fired boilers. Burning coal with relatively pure oxygen, together with recycled flue gases, can produce a highly concentrated (up to 95% CO2) flue gas stream, which makes carbon sequestration more economical. One issue of interest in rapidly implementing a strategy to retrofit existing air-fired burners is to understand how replacing air by a O2/CO2 mixture affects the kinetics, aerodynamics and flame ignition in the near burner region. In a turbulent coal flame, accurate predictions of the particle size and velocity distributions are critical for predicting the flame characteristics. LES combined with DQMOM has the potential to predict oxy-coal flame characteristics and to be an important tool in the retrofitting process [1].

Oxy-coal flame stability was experimentally studied by Zhang et al. [2,3] using optical measurements of the flame stand-off distance in a 40 kW pilot facility at the University of Utah. Experiments suggest that flame stability is affected by primary PO2 , secondary preheat temperature, secondary PO2 , and the transport medium. Simulations of the facility were performed with ARHCES and provide additional insight into the experimentally observed data. The importance of factors such as heterogeneous reactions, radiation or wall temperature can be better understood thanks to simulations.

The following paragraphs discuss the simulation of the near-burner section of the oxy-coal furnace at the University of Utah. A coaxial burner is used. The center pipe is fed with a O2/CO2 mixture at room temperature which carries coal particles (Utah bituminous coal). The secondary stream is a preheated (544K) O2/CO2 mixture. The coal particles were modeled using the DQMOM approach. The simulation includes the following models:

  • Fully coupled mass, momentum and energy equations between the gas phase and particle phase
  • Discrete ordinate method for radiation
  • Particle devolatilization and oxidation models

A more detailed description of the DQMOM approach and of the coal particles models can be found in [6].

In a coal flame, particles exhibit very different behavior depending on their size, velocity or temperature. DQMOM offers the ability to track multi-dimensional particle distributions using a set of internal coordinates and quadrature nodes. For this simulation, 7 internal coordinates were used to describe coal particles:

  • Particle diameter,
  • Particle velocity (3 components),
  • Particle temperature,
  • Particle raw coal mass,
  • Particle char mass

The distribution was represented by 3 quadrature nodes corresponding to 3 particle sizes:

Quadrature Node Particle size (um) Weight % Feed rate (g/s)
1 40 11.2 0.15
2 63 58.6 0.82
3 100 30.2 0.42