Modeling of Mass Loss of Char Particles during Combustion and Interaction with Inert Material

The inspiration for this research was the numerical simulation of mass loss of char particles during its combustion in a CFB boiler. The mass loss was presented as the superposition of two separate processes i.e. combustion and surface erosion. The values obtained by the model and experimental data show a good level of conformity. INTODUCTION Until now many authors have attempted numerical simulations of the comminution of coal in the bed of the fluidized combustion (1,2,3). The main courses of numerical simulation were based on the system of combustion chamber or on the system of circulating fluidized bed in order to describe the intensity of the fuel comminution and the modification of the particle size distribution of a bed (4,5). However, none of the aforesaid attempts took the individual characteristics of coal into consideration, such as the chemical composition and petrographic structure, which both directly and indirectly affect the surface structure of the particle and its characteristics created during combustion. On the basis of tests carried out, as well as other research, it has been stated that the coal surface undergoes mechanical interaction with an inert material which changes the mechanism of combustion as well as accelerating the mass loss of the coal particle. However, the level of that PDF stworzony przez wersję demonstracyjną pdfFactory Pro www.pdffactory.pl/ 1 Pelka and Leszczynski: MODELING OF MASS LOSS OF CHAR PARTICLES DURING COMBUSTION AND INT Published by ECI Digital Archives, 2010 loss is different and this is a characteristic feature of coal. The research conducted by Pelka (6) of the mass loss during combustion for Polish coal in a flow of inert material with the same rate of flow as Gs= 2.5 kg/(ms) shows that the difference of time of overall mass loss for hard coal (d= 10 mm) is much bigger than in the atmosphere and can even amount to a few hundred seconds i.e. 30-40%. Thus, determining a parameter decisive of intensity for that loss and the related coal properties could be an important instrument facilitating modelling of the combustion process in fluidized bed conditions. Therefore, in this research an attempt at analyzing numerical simulations of the mass loss of a coal particle has been made while taking both processes into consideration. EXPERIMENTAL TESTS In order to determine the real coal particle mass loss in the process of its combustion in a flow of inert material a laboratory model of a fluidized bed combustor chamber was constructed. The schematic diagram of the test stand is shown in Fig. 1. The main elements of the test stand were as follows: an electric combustor (1), an accelerate pipe (2) and a vessel of inert material (4). Fig.1. Schematic diagram of the test apparatus: 1-combustion chamber, 2-acceleration pipe, 3-regulation valve, 4-upper vessel of inert material, 5-air heater, 6-rotameter, 7-air blower, 8-extensometer branch scale, 9-thermocouple, 10-control panel, 11-PCcomputer, 12-support, 13-lower vessel of inert material. PDF stworzony przez wersję demonstracyjną pdfFactory Pro www.pdffactory.pl/ 2 The 13th International Conference on Fluidization New Paradigm in Fluidization Engineering, Art. 82 [2010] http://dc.engconfintl.org/fluidization_xiii/82 The tests were carried out on Polish hard coal from Sobieski mine whose properties are presented in Table 1. In this test spherical particles were used that had been picked out from randomly chosen real coal particles and then polished manually. The diameter of the tested particles was 10 mm. At the beginning particles of the chosen coal type were burned in an atmospheric air without any inert material. Following this, the test was conducted with the inert material. From the recorded signal the course of the real mass loss of the particle was isolated by subtracting the mechanical interaction of inert material. Proximate analyses Coal type Mine Volatile matter Moisture Ash content Fixed coal Calorific value % % % % kJ/kg Hard Sobieski 27,9 12,4 16,7 48,4 21558 Table 1. Results of proximate analyses of tested coal. The first stage of the investigation was the combustion of coal particles without the influence of the inert material in an atmospheric air at temperature of 850C. For this reason the control valve of the solid flow was closed while only heated air was supplied to the combustion chamber. The tests were performed on seven samples in the same conditions. Following that, the tests of mass loss of coal particle were conducted in conditions of the flow of an inert material. The results obtained for different rates of flow of inert material Gs= {0,4,7,15} kg/(ms) are presented in Fig.2. 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0 100 200 300 400 500 600 700 800 90