Modeling of Limestone Injection for SO2 Capture in a Coal Fired Utility Boiler

A comprehensive model for the simulation of sorbent injection in the superheater section of TransAlta Utility's Keephills 400 MWe, split furnace, tangentially fired boiler has been developed and demonstrated using CFX-TASCflow software. The model consists of a computational grid of the superheater section of a tangentially fired utility boiler. The inlet boundary conditions were taken from previous numerical simulations of the coal combustion in the firebox of the same boiler. These previous simulations were performed by Dr. E. Chui of the Canada Centre for Mineral and Energy Technology (CANMET) in Ottawa. To accurately simulate the particle temperature and residence time distributions in the boiler the simulations were three-dimensional. The computational grid consisted of eleven individual grid blocks, each representing different sections of the boiler convection pass. In order to make the computational grid tractable the tube banks were represented as porous blocks. The pressure drops and heat transfer processes occurring in the various tube sections of the boiler were simulated with user supplied momentum and energy source terms. These terms were based on actual boiler operation data, where available. The predicted pressure drop of 1280 Pa was in good qualitative agreement with plant data. The predicted temperatures were within 5°C of the reported values. The sub-models to account for the sulphation, calcination, pore plugging and particle sintering of the sorbent particles were developed based on previous work by Bortz and Dolignier, 1997 and Flament, 1993. These models were validated using literature data (Flament, 1993). The sorbent particles were injected at the inlet of the domain and tracked using a Lagrangian tracking option in the CFD code. All flow rates and gas compositions were based on furnace outlet data from the CANMET simulations using actual coal properties and boiler operating parameters. In all the simulations the sorbent was assumed to enter the boiler gas stream perfectly distributed at the superheater inlet plane. The sulphur dioxide concentration at the inlet plane was specified assuming 100% conversion of coal sulphur to SO2. For the boiler simulations based on Highvale coal, the average inlet SO2 concentration was about 460 ppm by weight. The effects of gas inlet SO2 concentration and calcium to sulphur ratio were investigated in the full-scale simulations. The simulation results for the full scale simulations showed that the predicted sulphur capture increased with both increased SO2 concentration (or coal sulphur content) and increased Ca/S. For the Keephills boiler burning Highvale coal, sulphur captures of 16% and 28% were predicted for the swirling inlet with Ca/S ratios of 2.0 and 4.0 respectively. The predicted sulphur capture was within the range of values measured in previous 150 MW scale tests of sorbent injection performed by Saskatchewan Power. In this work a uniform sorbent distribution was assumed at the inlet plane. More realistic sorbent injection options should be modelled to investigate the effects of sorbent inlet distribution and mixing on sulphur capture. The model should be extended to include the firebox and the rest of the boiler back pass (to the air heater) so that all practical injection options and operational effects could be investigated. The model developed here could be extended to consider current erosion control strategies and possible future boiler erosion impacts of sorbent injection.