Esmail R. Monazam

Measurements of heat capacities, temperatures, and absorptivities of single particles in an electrodynamic balance

A rapid measurement technique to determine heat capacities, temperatures, and absorptivities of single submillimeter size (50–200 μm diam) particles in an electrodynamic balance (EDB) is described. Accurate estimates of heat capacities and absorptivities are made for single carbon particles at temperatures ranging from 800 to 1200 K with measurement times of less than 0.5 s. Measurements are made by isolating and holding charged particles at the null position in an EDB. Particles are heated using the output from a well‐characterized CO2 laser which is modulated to provide heating pulses of 3‐ms duration at a repetition rate of 100 Hz. The resulting periodic temperature oscillations are monitored continuously using a single wavelength high‐speed optical pyrometer. Heat capacities and absorptivities are determined based on comparisons of the recorded heating and cooling profiles with theoretical predictions obtained from numerical solution of the Fourier equation. Experimentally determined heat capacities f...

Measurements and analysis of temperature histories and size changes for single carbon and coal particles during the early stages of heating and devolatilization

Abstract A novel system is described for monitoring rapid changes in particle size and temperature during coal devolatilization at heating rates representative of high-intensity combustion environments. The system incorporates an electrodynamic balance and a pulsed radiation source to isolate and rapidly heat individual particles. A high-speed two-dimensional photodiode array and a single wavelength radiation pyrometer are applied to record changes in particle size and temperature. Dynamics of volatile evolution and particle swelling are also recorded using high-speed cinematography. Measurements of temperature and size changes are reported for carbon spheres and a HVA bituminous coal at heating rates on the order of 105 K/s. Measured temperature histories are compared with theoretical estimates of the temperature response of radiatively heated coal and carbon particles. The analysis considers the development of the transient temperature distribution inside the particles by numerical solution of the Fourier equation. Measurements and model predictions for 135-μm-diameter carbon spheres were in excellent agreement using property data correlations commonly applied in modeling coal devolatilization and combustion behavior. Model predictions for coal particles, however, significantly underestimated (on the order of 50%) the observed heating rates for 115-μm-diameter coal particles using the same property correlations. It is concluded that coal particles heat significantly faster than is predicted using commonly employed approaches to model heat transfer with assumptions routinely applied to coal. Potential reasons for this may include inadequate understanding of relevant coal thermodynamic and heat transfer properties as well as failure to account for particle shape factors. Heat transfer analyses employing spherical particle assumptions and commonly used coal property correlations can lead to large errors in predicted temperature histories and associated devolatilization rates.

Measurement and dynamic simulation of particle trajectories in an electrodynamic balance: Characterization of particle drag force coefficient/mass ratios

Complementary measurement and simulation methods are described that enable rapid characterization of drag force coefficient/mass (Cd/m) ratios of individual particles in the 20 to 200 μm size range. Individual particles are suspended in a charge trap known as an electro‐dynamic balance (EDB). A step change is applied to the EDB end cap voltage to stimulate a dynamic response of the particle from an initial steady state. The resulting transient response is measured by means of a high speed, two‐dimensional diode array and imaging system which provides an analog output indicating particle position along the EDB center axis at frame rates of 6200 per second. A particle dynamic model (PDM) is developed to simulate trajectories of particles in the EDB. The PDM is a force balance simulation which accounts for field forces, gravitational forces, and drag forces acting on a particle. Simulations are performed using particle Cd/m ratio as a fit parameter to match model outputs with measurements. Data are presented...

The Mapping of Flow Regimes for a Light Material: Cork

A series of experiments was conducted in the 0.3-meter diameter circulating fluidized bed test facility at the U.S. Department of Energy’s National Energy Technology Laboratory (NETL). Cork, the bed material used in this study, is a coarse, light material, with a particle density of 189 kg/m3 and a mean diameter of 1007 μm. Fluidizing this material in ambient air provides approximately the same gas to solids density ratio as coal and coal char in a pressurized gasifier. Furthermore, the density ratio of cork to air under ambient conditions is similar to the density ratio of coal to gas at the gasification and pressurized fluidized bed combustion environment. The purpose of this study is to generate reliable data to validate the mathematical models currently under development at NETL. Using such coarse, light material can greatly facilitate the computation of these mathematical models. This paper presents and discusses data for the operating flow regimes of dilute-phase, fast-fluidization, and dense-phase transport by varying the solid flux (Gs) at a constant gas velocity (Ug). Data are presented by mapping the flow regime for coarse cork particles in a ΔP/ ΔL-Gs-Ug plot. The coarse cork particles exhibited different behavior than the measurements on heavier materials found in published literature, such as alumina, sand, FCC, and silica gel. Stable operation can be obtained at a fixed riser gas velocity that is higher than the transport velocity (e.g. at Ug = 3.2 m/sec), even though the riser is operating within the fast fluidization flow regime. Depending upon the solid influx, the riser can also be operated at dilute-phase or dense-phase flow regimes. Experimental data were compared to empirical correlations in published literature for flow regime boundaries, and solid fractions in the upper-dilute and the lower-dense regions of a fast fluidization flow regime. Comparisons of measured data show rather poor agreement with these empirical correlations. Xu et al. (2000) have observed this lack of agreement in their study of the effect of bed diameter on the saturation carrying capacity. The basis of empirical correlations depends on bed diameter and particle type, and are generally not well understood.Copyright

Experimental Investigation of Gas-Solid Disengagement in Bubbling Fluidized Bed Cold Model

The aim of this experimental study is to investigate the separation performances of a new gas-solid disengagment (GSD) device in a 10 cm bubbling fluidized bed cold model. An impactor separator is designed to be fitted internally onto the loop seal and install in the bubbling fluidized bed based on the impingement separation concept. The effects of operating parameters, such as static bed height, the length of the GSD device’s dip leg, and effect of dip leg porting were examined. The results indicated that a higher static bed height increases the mass of elutriated particles from the bed column. The length of the GSD device’s dip leg has a small effect on its separation ability. A longer dip leg can reduce the amount of particles elutriated due to the reduced pressure it experiences on the solids exit as compared to a shorter dip leg. The addition of fluidization ports to the dip leg has a negative effect on preventing particles from escaping through the gas outlet. The fluidizing gas will bypass the gas inlet of the GSD device through the dip leg and no particles will impact the baffle to be separated from the gas flow. Increasing the diameter of the solid exit for the dip leg increases the efficiency of the GSD device. The higher rate of particles passing through the dip leg reduces the chance of gas flow blockage by particles accumulating in the GSD device. The research concludes that with an effective design for a gas-solid disengagement device will reduce up to 75% of the particles that enter the filtration system that elutriate from the bubbling fluidized bed.

Hydrodynamics of a Transport Reactor Operating in Dense Suspension Upflow Conditions for Coal Combustion Applications

A series of experiments were conducted in the 0.3-meter diameter, 15.45-m high cold flow circulating fluid bed (CFB) test facility at the National Energy Technology Laboratory (NETL) of the U. S. Department of Energy. Operation of the CFB demonstrated that high density conditions can be achieved throughout the entire riser with sufficiently high solid fluxes in a riser taller than what has been previously reported in the literature. Tests were conducted on Geldart type B, 60 μm diameter, glass beads at two different gas velocities (5.1 and 7.8 m/s). The riser’s axial solids fraction profile provided distinct characteristics that enabled us to differentiate between dense suspension upflow (DSU) and core annular flow regimes. The apparent solids holdup in the riser exceeded 7% when operating in DSU. A fiber optic probe was used to measure particle velocities near the wall 8.5 m above the solids entry. These measurements did not always record upward particle velocities when in DSU conditions. A number of possible reasons are identified and discussed. Solid fluxes greater than 250 kg/m2 -s for 5.1 m/s and 350 kg/m2 -s at 7.8 m/s appeared to be sufficient to achieve DSU conditions. The trend in the measured particle velocities near the wall was also consistent with these transitions. The transition from core annular conditions to DSU operations depended upon both gas velocity and solids flux and was in good agreement with an existing correlation found in the literature.Copyright