Study On Coal Char Characteristics From Rapid Devolatilization Under Pressurized Condition

This dissertation investigated characteristics of resulting coal char from rapid devolatilization under pressurized condition, both char pore structure and gasification reactivity were studied. Methodologies used in this dissertation contained:(1) coal char acquiring experiment using Pressurized Drop Tube Furnace( PDTF in the following) experimental system,(2) char sample analysis using general instrument,(3) pore structure formation model based on bubble behavior simulation, and(4) calculations on parameters of reaction kinetics and diffusion coefficient. Main conclusions of this dissertation are as follows. First, complexity of coal composition and external factors such as heating rate, peak temperature and pressure, which affected bubble diffusion, coalescence and escape behaviors in devolatilization process, could eventually determine char pore size distribution. Further, by affecting char pore size distribution, these factors influenced ratio of surface area that stayed in the ―kinetic zone‖, which affected char apparent reactivity.Liugou bituminous( a typical softening coal) char samples were acquired under different peak temperature( 1000-1400 ℃) and gauge pressure( 0.0-1.7 MPa) conditions by the PDTF experimental system developed by Tsinghua University. Pore structure and gasification reactivity of the resulting char samples were then tested by general instrument, reaching conclusions as follows.For the Liugou coal char at 1100 ℃ conditions, when the gauge pressure increased from atmospheric pressure to 1.7 MPa, a smaller amount of larger pore structure was formed; the nitrogen adsorption specific surface area decreased in general; the porosity had an increasing trend; the swelling ratio of the particles increased to the maximum at 1.0 MPa and then slowly decreased; the weight loss decreased, the true density decreased; and H/C increased. Both apparent and intrinsic reactivity were enhanced when the gauge pressure became higher. The mechanism was that, the extent for a declining area surface to inhibit reactivity was weaker than that for an increasing H/C to improve reactivity.For the Liugou coal char at atmospheric conditions, when the peak temperature increased from 1000 ℃ to 1400 ℃, a larger amount of smaller pore structure was formed; the nitrogen adsorption specific surface area increased in general; the porosity slightly increased; the swelling ratio of the particles slightly increased; the weight loss increased; the true density increased; and H/C decreased. Both apparent and intrinsic reactivity were reduced when the peak temperature became higher. The mechanism was that, the extent for an increasing area surface to improve reactivity was weaker than that for a declining H/C to inhibit reactivity.This dissertation improved the existing volatile bubble model to analyze pressure and temperature effects on bubble diffusion, coalescence and escape process. According to the calculation, this dissertation suggested that a higher pressure enhanced diffusion and coalescence process, which led to a greater amount of larger pore structure; while a higher peak temperature inhibited diffusion and coalescence process, which led to a smaller amount of larger pore structure. Forecasts in this dissertation were consistent with the experiment phenomena, which could explain the mechanism of pressure and temperature effects on the pore structure formation of resulting char.This dissertation calculated intrinsic reaction rate by thermal gravity analysis( TGA) results, and characterized the relative strength between reaction and diffusion by Thiele modulus. The calculation indicated that for Liugou coal char TGA test under low temperature( 900 ℃), pores of all sizes were in the ―reaction kinetics control zone‖. It was found that as gauge pressure rose,(1) different size of hole would enter the ―transition zone‖ under different temperature conditions,(2) the calculation of micro-pore could inform the starting temperature of diffusion effect on apparent reaction,(3) apparent reaction rate estimation should consider three key factors, including the active site distribution concentration which affected intrinsic reaction rate, the total area of pore structure, and effective area ratio under the diffusion effect, thus a method of quantitative estimation of pore structure influence degree on the apparent reaction degree was proposed, by calculating the effective surface area according to pore size section.