Supercritical water fluidized bed (SCWFB) has been used to gasify biomass, coal and solid waste to produce gas fuel. Supercritical carbon dioxide fluidized bed (SCCO2FB) was applied in the coating industry. Both the two fluidized bed treats the supercritical fluids as fluidization medium. The fluidization behaviours of particles in the supercritical fluids are quite important issues for achieving the basic two phase flow pattern. Few research institutions have conducted experimental and numerical investigations on the fluidization in supercritical conditions. The authors try to establish a comprehensive insight of fluid dynamics of the supercritical fluidized bed. For the SCWFB, the fluidization transitions of fixed bed, homogeneous bed expansion and bubbling were demarcated by discrimination number Dn. A flow pattern map of Reynolds number vs. Archimedes number was available for describing the flow patterns and their boundaries of the SCCO2FB. Ergun equation was acceptable for calculating the fixed bed pressure drop for both SCWFB and SCCO2FB. Wei and Lu correlations of the minimum fluidization velocity, minimum bubbling velocity and homogeneous bed expansion rate are suggested to design the SCWFB. Wen and Yu equation of the minimum fluidization velocity, Vogt et al. correlation of the homogeneous bed expansion rate and their method for determining the minimum bubbling velocity, Nakajima et al. equation of transition velocity, and Bi and Fan correlation of turbulent velocity were recommended to calculate SSCO2FB.
Part of the book: Thermal Power Plants
Pyrolysis has profound implications for coal as a raw material to make phase change material (PCM). It is necessary to derive a pyrolysis kinetic model for predicting the yield of volatiles and reaction performance during pyrolysis of coal, which is of significant importance for its thermal processing. The devolatilization of coal is characterized by thermogravimetric analysis (TGA) at different heating rates, and many kinetic models can be achieved by analyzing the TGA data. This work was aimed to find an appropriate model to describe the pyrolysis of coal and took Zhundong coal as an example. Four types of isoconversion kinetic methods, that is, Friedman, Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), Miura-Maki method, and different distributed activation energy models (DAEM) were employed here to fit TGA data for pyrolysis of Zhundong coal. The pre-exponential factors and activation energies obtained from different kinetic models were analyzed. An m-nth-DAEM was developed by considering that m classes of reactions took place with the same pre-exponential factor k0 but different distribution activation energy following logistic distribution or Gaussian distribution. The results showed that the FWO model was better for description of pyrolysis process of Zhundong coal, and the 2-nth-DAEM assuming Gaussian distribution of activation energy gave the best fit for the TGA data of Zhundong coal. The research provides a valuable reference to the development of thermal utilization technology of Zhundong coal.
Part of the book: Phase Change Materials and Their Applications