A Theoretical And Experimental Investigation Of Coke Gasification In A Batch Reactor

A computer model has been developed to simulate the char-gasification process occurring in the gasification component of a crossflow coal gasifier. This process corresponds to the batch gasification process in a combustion pot. Temperature and concentration profiles along the gasifier were obtained by solving the material and energy balances. In order to obtain the data necessary for evaluating and improving the computer model, an experimental system was developed to obtain data for the char-gasification process in the combustion pot. The results show that the gasification rate strongly depends on the air flow rate and that the reactivity of the char has a strong effect on the output gas composition. The results from the computer model were compared with available literature data on the fixed-bed gasifier and the experimental data obtained from this study, and were found to be in good agreement. A sensitivity analysis was performed on the input parameters (heat transfer coefficient, void fraction, particle diameter, and reactivity factor coefficients) of the computer model. The results show the importance of the input parameters in predicting the desired gas composition and total process time

Transient Characterization of Type B Particles in a Transport Riser

Simple and rapid dynamic tests were used to evaluate fluid dynamic behavior of granular materials in the transport regime. Particles with densities ranging from 189 to 2,500 kg/m3 and Sauter mean size from 61 to 812 μm were tested in a 0.305 m diameter, 15.5 m height circulating fluidized bed (CFB) riser. The transient tests involved the abrupt stoppage of solids flow for each granular material over a wide range gas flow rates. The riser emptying time was linearly related to the Froude number in each of three different operating regimes. The flow structure along the height of the riser followed a distinct pattern as tracked through incremental pressures. These results are discussed to better understand the transformations that take place when operating over various regimes. During the transients the particle size distribution was measured. The effects of pressure, particle size, and density on test performance are also presented

Applications of tribology and fracture mechanics to determine wear and impact attrition of particulate solids in CFB systems

In recent years, much attention has been focused on the development of novel technologies for carbon capture and chemicals production that utilize a circulating fluidized bed configuration; examples include chemical looping combustion and circulation of temperature swing adsorbents in a CFB configuration for CO2 capture. A major uncertainty in determining the economic feasibility of these technologies is the required solids makeup rate, which, among other factors, is due to impact and wear attrition at various locations, including standpipes, cyclones, and the gas jets in fluid beds. While correlations have been developed that estimate the attrition rates at these areas, these correlations are dependent on constants that are an unknown function of the solid properties and system. Thus, it is difficult to determine the attrition rate a priori without performing extensive experiments on the materials or scaling up entirely. In this work, the authors apply knowledge of fundamental material properties from fields of tribology (the study of wear) and fracture mechanics to the knowledge of forces and sliding distances determined from hydrodynamic models to develop basic attrition models for novel CFB systems. The equations are derived for common equipment found in CFBs, and the equations are compared to experimental data of attrition in the literature

Flow Regime Study in a High Density Circulating Fluidized Bed Riser with an Abrupt Exit

Flow regime study was conducted in a 0.3 m diameter, 15.5 m height circulating fluidized bed (CFB) riser with an abrupt exit at the National Energy Technology Laboratory of the U. S. Department of Energy. Local particle velocities were measured at various radial positions and riser heights using an optical fiber probe. On-line measurement of solid circulating rate was continuously recorded by the Spiral. Glass beads of mean diameter 61 μm and particle density of 2,500 kg/m3 were used as bed material. The CFB riser was operated at various superficial gas velocities ranging from 3 to 7.6 m/s and solid mass flux from 20 to 550 kg/m2-s. At a constant riser gas velocity, transition from fast fluidization to dense suspension upflow (DSU) regime started at the bottom of the riser with increasing solid flux. Except at comparatively low riser gas velocity and solid flux, the apparent solid holdup at the top exit region was higher than the middle section of the riser. The solid fraction at this top region could be much higher than 7% under high riser gas velocity and solid mass flux. The local particle velocity showed downward flow near the wall at the top of the riser due to its abrupt exit. This abrupt geometry reflected the solids and, therefore, caused solid particles traveling downward along the wall. However, at location below, but near, the top of the riser the local particle velocities were observed flowing upward at the wall. Therefore, DSU was identified in the upper region of the riser with an abrupt exit while the fully developed region, lower in the riser, was still exhibiting core-annular flow structure. Our data were compared with the flow regime boundaries proposed by Kim et al. [1] for distinguishing the dilute pneumatic transport, fast fluidization, and DSU

Thermogravimetric Analysis of Modified Hematite by Methane (CH₄) for Chemical-Looping Combustion: A Global Kinetics Mechanism

Iron oxide (Fe₂O₃), known in its natural form as hematite, is potentially able to capture CO₂ through the chemical-looping combustion (CLC) process. Magnesium (Mg) is an effective methyl-cleaving catalyst, and as such it was combined with hematite to assess any possible enhancement to the kinetic rate of the reduction of Fe₂O₃ with methane. Therefore, in order to evaluate the effectiveness of Mg as a hematite promoter, the behaviors of Mg-modified hematite samples (hematite–5% Mg­(OH)₂) were assessed for any enhancement to the kinetic rate of the CLC process. The Mg-modified hematite was prepared by hydrothermal synthesis. The reactivity experiments were conducted in a thermogravimetric analyzer using a continuous stream of CH₄ (at concentrations of 5, 10, and 20%) at temperatures ranging from 700 to 825 °C over 10 oxidation–reduction cycles. The mass spectroscopic analysis of the product gas indicated the presence of CO₂, H₂O, H₂, and CO in the gaseous product. The kinetic data obtained by isothermal experiments at the reduction step are well fitted by two parallel rate equations. The modified hematite samples showed higher reactivity than the unmodified hematite samples during reduction at all the investigated temperatures