Proof of Concept of a Novel Oxy-Combustion Burner Design, 18-R8021Printer Friendly Version
Inclusive Dates: 01/01/09 06/30/10
Background - A new design for an oxy-burner used in the combustion of pulverized coal was developed to support further research in clean energy power production from fossil-fuel resources. Through a design review of other oxy-burner concepts, flow visualization techniques, and computational fluid dynamics, the project team developed a means of modeling and predicting combustion efficacy and suitability of a given burner design for the oxy-combustion process conditions. The methodology evaluates the oxy-burner design in terms of impingement of particles on the burner wall, combustible mass fraction at each point in the coal particle traverse, and residence of the coal particles within the burner.
Approach - The SwRI team's new swirl burner concept was optimized for coal particle size distribution, flow rate and heat output. The new down-flow oxy-burner design premixes the incoming oxygen and coal and further promotes an internal vortex to reduce wall impingement and extend the residence time in the burner. The design may be used with CO2 flue gas flow to maintain lower burner wall temperatures using CO2-assisted nozzles along the furnace perimeter. Cooling hole geometries for the combustor liner could be developed further using concepts from gas turbine engines where blade cooling is facilitated by compressor bleed air using internal convection or external film-cooling.
Using numerical CFD simulation to study the coal combustion was important in the design phase and in providing quantitative measures of the combustion process. Coal combustion is calculated by combining a particle transport calculation of the coal particles with an eddy dissipation calculation for the combustion of the volatile gases in the gas phase. To reduce computational time, the devolatilization is modeled by one reaction step using the generic Arrhenius multiphase reaction capability, although normally the process is represented by one or two reaction steps. The char reaction is determined by both the rate of diffusion to the surface and the rate of chemical reaction at the surface.
The oxy-combustion process is complicated by the need to maintain high burner temperatures to start combustion and assist in the coal’s gaseous transformation while keeping temperatures within the burner fairly uniform to reduce slag formation. An oxy-combustion burner design process should include methods of tuning the combustor performance to varying coal types, particle sizes, and intake rates. The combustion chamber design methodology explored herein provides a baseline tool for future design optimization opportunities.
Accomplishments - The research results found that the primary combustion characteristics (combustible mass fraction of the coal stream and residence time of the particles within the burner) can be improved by increasing residence time and by imparting the appropriate amount of swirl to the incoming oxygen gas and solid coal particles. From the four design cases studied, complete combustion occurred in only two of the median cases for combustion length-to-width ratios of 4.7 and 9.2. The combustion is incomplete for the other two cases, suggesting that the configuration is not an ideal one and the chamber is either too restrictive or oversized for the combustion process. As interpreted from the particle traces, one of the benefits of extending the width and length of the chamber was to increase residence time of the particles. The CFD results show that the particle residence time is greatest when the L/D is 4.7, when the particle residence time is extended by creation of an internal swirl within the burner. Further heat transfer analysis and design of the cooling passages is needed to complete the swirl oxy-burner design. The next recommended step is an experimental demonstration of the down-flow combustor design, comparing a coal slurry premix with direct mixing of oxygen and solid pulverized coal particles.