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Content archived on 2024-05-07

Development of improved combustion engineering models - applicationto nox reduction processes

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Computational fluid dynamics (CFD) models are increasingly applied as investigative and design methods in the power industry. However, it is recognised that the current generation of CFD codes are dependent on empirically-derived rate constants to describe many of the key processes and, as a result, they have only correlative rather than truly predictive capabilities. In particular, the treatment of gas-solid combustion reactions is considered to be unsatisfactory. The principal objectives of the project were to develop improved physical and numerical models of the coal particle combustion processes that occur in pulverised coal flames and to test these models in CFD simulations of full-scale burners. The description of coal particle devolatilisation, particle swelling/fragmentation and char burnout processes was addressed through a combined programme of experimental and computational work. Firing trials using two different coals were successfully completed in a fully instrumented combustion test facility, at a heat input of 35MW on a gross calorific value basis. The test coals were Kellingley, a high volatile bituminous coal from the north of England and Powder River Basin (PRB), a high volatile sub-bituminous coal from the western USA. In-flame gas analysis measurements were made and char samples were collected during both trials. In-flame temperature measurements were made during one trial. The char samples were characterised physically and chemically. Char particle samples were also collected from several locations in a full-scale multi-burner utility furnace. Gas analysis and temperature measurements were made at the locations from which the char samples were collected. The char samples were characterised by ultimate analysis and char burnout was calculated using ash as a tracer. Isothermal Plug Flow Reactor (IPFR) characterisations of the two coals used in the full-scale burner trials were successfully completed Devolatilisation tests, in a nitrogen atmosphere, were completed at several temperatures. Char burnout tests were completed at several temperatures and oxygen concentrations. Kinetic parameters were derived for the combustion of the two coals. CCSEM characterisation of the char samples collected during the full-scale burner trials and Kellingley IPFR tests were successfully completed. A quantified description of the size and composition (carbon, ash, open pores and closed pores) of many thousands of individual particles was produced for each char sample. The information was used to describe the range of char particle microstructures and to illustrate the changes in the distribution of char particle composition during the combustion of the two coals. Improved coal particle devolatilisation and char combustion numerical models were developed. They were validated against literature data and test results, plus characterisation data generated during the project. The Chemical Percolation Devolatilisation (CPD) model was used to describe the coal devolatilisation process. A model of the transient swelling of coal particles during devolatilisation was developed. Char combustion models using apparent kinetics, intrinsic kinetics and Langmuir kinetics were compared. An ash inhibition model was included to consider the influence of the ash layer on oxygen diffusion. The thermal annealing model was included for single step reaction kinetics to consider the variation in char reaction order due to variations in heat treatment. The improved coal particle models were integrated in a CFD based numerical model for pulverised coal combustion. A reasonable agreement was obtained between the axisymmetric simulation and the measurements made during one of the firing trials. The influence of the particle combustion model was assessed, but improvements in the overall performance of the CFD simulations could not be attributed to these models.

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