In this study, the devolatilisation process of Miscanthus particles inside a pulverised coal combustion chamber is characterised with the aim of finding conditions for which the devolatilisation rate of coal and Miscanthus is similar. However, choosing a power station as an experimental set-up for research is awkward because of the scale of operation (> 500 MWel). Therefore, BTG has designed and constructed a droptube reactor for well-controlled Miscanthus devolatilisation experiments with operational conditions that resemble those of a pulverised coal combustor. The droptube reactor has an internal diameter of 0.050 m and a maximum heated length of 1.6 m. Parameters which have been varied are: the droptube temperature (1000°, 1200°, 1300°, 1400°C); the heated droptube length (0.4, 0.8, 1.2, 1.6 m); and the particle size or sieve fraction (0.6¿1, 1¿2, 2¿2.8 mm). For a droptube length of 1.6 m, this results in a particle residence time of approximately 1 s. The experimental study on high-temperature Miscanthus decomposition in the droptube showed that Miscanthus particles which belong to the smallest sieve fraction (0.6¿1 mm) could be devolatilised completely in a 1.6 m long droptube. Apart from the experimental investigation, a numerical model has been developed. Samples of Miscanthus particles, representing grass-/straw-like crops, have been characterised in detail with respect to their size distribution. These data have been used to validate the numerical model with the results from the droptube experiments. The validation was successful. The model was then applied to predict the Miscanthus devolatilisation behaviour in a pulverised coal power station. The model predicts full conversion of Miscanthus particles for particles with a diameter smaller than 3 mm, in the core of the coal flame. Feeding of Miscanthus particles with a diameter up to 3 mm can therefore be recommended. Miscanthus particles with a diameter larger than 3 mm contribute to a geometrical extension of the coal flame in the upward direction. This should be avoided and firing of such large particles in a pulverised coal combustor is discouraged.
Wagenaar, B. M., & van den Heuvel, E. J. M. T. (1997). Co-combustion of Miscanthus in a pulverised coal combustor: Experiments in a droptube furnace. Biomass & bioenergy, 12(3), 185-197. https://doi.org/10.1016/S0961-9534(97)00009-3