Liang-Shih Fan

Clean coal conversion processes – progress and challenges

Although the processing of coal is an ancient problem and has been practiced for centuries, the constraints posed on todays coal conversion processes are unprecedented, and utmost innovations are required for finding the solution to the problem.With a strong demand for an affordable energy supply which is compounded by the urgent need for a CO2 emission control, the clean and efficient utilization of coal presents both a challenge and an opportunity to the current global R&D efforts in this area. This paper provides a historical perspective on the utilization of coal as an energy source as well as describing the progress and challenges and the future prospect of clean coal conversion processes. It provides background on the historical utilization of coal as an energy source, along with particular emphasis on the constraints in current coal conversion technologies. It addresses the energy conversion efficiencies for current coal combustion and gasification processes and for the membrane and looping based novel processes which are currently under development at various stages of testing. The control technologies for pollutants including CO2 in flue gas or syngas are also discussed. The coal conversion process efficiencies under a CO2 constrained environment are illustrated based on data and ASPEN Plus® simulations. The challenges for future R&D efforts in novel coal conversion process development are also presented.

Chemical looping processes for CO2 capture and carbonaceous fuel conversion – prospect and opportunity

Chemical looping processes offer a compelling way for effective and viable carbonaceous fuel conversion into clean energy carriers. The uniqueness of chemical looping processes includes their capability of low cost in situ carbon capture, high efficiency energy conversion scheme, and advanced compatibility with state-of-the-art technologies. Based on the different functions of looping particles, two types of chemical looping technologies and associated processes have been developed. Type I chemical looping systems utilize oxygen carrier particles to perform the reduction–oxidation cycles, while Type II chemical looping systems utilize CO2 carrier particles to conduct carbonation–calcination cycles. The exergy analysis indicates that the chemical looping strategy has the potential to improve fossil fuel conversion schemes. Chemical looping particle performance and looping reactor engineering are the key drivers to the success of chemical looping process development. In this work, the desired particle characterization and recent progress in mechanism studies are generalized, which is followed by a discussion on the looping reactor design. This perspective also illustrates various chemical looping processes for combustion and gasification applications. It shows that both Type I and Type II looping processes have great potentials for flexible and efficient production of electricity, hydrogen and liquid fuels.

Neural network based multi-criteria optimization image reconstruction technique for imaging two- and three-phase flow systems using electrical capacitance tomography

A new image reconstruction technique for imaging two- and three-phase flows using electrical capacitance tomography (ECT) has been developed based on multi-criteria optimization using an analog neural network, hereafter referred to as Neural Network Multi-criteria Optimization Image Reconstruction (NN-MOIRT)). The reconstruction technique is a combination between multi-criteria optimization image reconstruction technique for linear tomography, and the so-called linear back projection (LBP) technique commonly used for capacitance tomography. The multi-criteria optimization image reconstruction problem is solved using Hopfield model dynamic neural-network computing. For three-component imaging, the single-step sigmoid function in the Hopfield networks is replaced by a double-step sigmoid function, allowing the neural computation to converge to three-distinct stable regions in the output space corresponding to the three components, enabling the differentiation among the single phases.

Characteristics of fluidization at high pressure

Abstract Experiments were conducted to investigate fluidization fundamentals at pressures up to 6485 kPa using nitrogen as the fluidizing gas. The particles under study were coal, char and Ballotini. Both a three-dimensional bed (10.16-cm-i.d.) and a two-dimensional bed (1.9x 10.16cm) were used in the experiments. The fundamentals of high pressure fluidization examined in this study include minimum fluidization velocity, bed voidage at minimum fluidization, bed expansion, and bubbling behavior. An empirical correlation was developed for determining minimum fluidization velocity. The effects of pressure upon bed voidage at minimum fluidization and expanded bed height were analyzed for several types of particles. High speed photographs were studied to describe bubbling behavior in a fluidized bed over a range of pressures.

Measurement of real-time flow structures in gas–liquid and gas–liquid–solid flow systems using electrical capacitance tomography (ECT)

Abstract The real-time cross-sectional distributions of the gas holdups in gas–liquid and gas–liquid–solid systems are measured using electrical capacitance tomography. For the gas–liquid system, air as the gas phase and both Norpar 15 (paraffin) and Paratherm as the liquid phases are used. Polystyrene beads whose permittivity is similar to that of Paratherm are used as the solid phase in the gas–liquid–solid system. The three-phase system is essentially a dielectrically two-phase system enabling measurement of the gas holdup in the gas–liquid–solid system independent of the other two phases. A new reconstruction algorithm based on a modified Hopfield dynamic neural network optimization technique developed by the authors is used to reconstruct the tomographic data to obtain the cross-sectional distribution of the gas holdup. The real-time flow structure and bubbles flow behavior in the two- and three-phase systems are discussed along with the effects of the gas velocity and the solid particles.

Electrical Capacitance Volume Tomography

A dynamic volume imaging based on the principle of electrical capacitance tomography (ECT), namely, electrical capacitance volume tomography (ECVT), has been developed in this study. The technique generates, from the measured capacitance, a whole volumetric image of the region enclosed by the geometrically three-dimensional capacitance sensor. This development enables a real-time, 3-D imaging of a moving object or a real-time volume imaging (4-D) to be realized. Moreover, it allows total interrogation of the whole volume within the domain (vessel or conduit) of an arbitrary shape or geometry. The development of the ECVT imaging technique primarily encloses the 3-D capacitance sensor design and the volume image reconstruction technique. The electrical field variation in three-dimensional space forms a basis for volume imaging through different shapes and configurations of ECT sensor electrodes. The image reconstruction scheme is established by implementing the neural-network multicriterion optimization image reconstruction (NN-MOIRT), developed earlier by the authors for the 2-D ECT. The image reconstruction technique is modified by introducing into the algorithm a 3-D sensitivity matrix to replace the 2-D sensitivity matrix in conventional 2-D ECT, and providing additional network constraints including 3-to-2-D image matching function. The additional constraints further enhance the accuracy of the image reconstruction algorithm. The technique has been successfully verified over actual objects in the experimental conditions

Some aspects of high-pressure phenomena of bubbles in liquids and liquid–solid suspensions

Abstract Some aspects of bubble dynamics and macroscopic hydrodynamic properties in high-pressure bubble columns and three-phase fluidization systems are discussed. Experimental results along with discrete-phase simulations of a single bubble rising in liquids and liquid–solid suspensions at high pressures are presented. A mechanistic model is described, which accounts for the initial size of bubble from a single orifice in liquid–solid suspensions. The mechanism for bubble breakup at high pressures is illustrated by considering bubble instability induced by internal gas circulation inside a bubble, and an analytical expression is obtained to quantify the maximum stable bubble size. Experimental examinations on the roles of bubbles of different sizes indicate the importance of large bubbles in dictating the macroscopic hydrodynamics of slurry bubble columns. Further, extensive studies are made of the key macroscopic hydrodynamic properties, including moving packed bed phenomena, flow regime transition, overall gas holdup, mean bubble size, and bubble size distribution. An empirical correlation is introduced which predicts the gas holdup in slurry bubble columns of different scales. A similarity rule is revealed for the overall hydrodynamics of high-pressure slurry bubble columns, which takes into account the operating conditions, the maximum stable bubble size, and the physical properties of the gas, liquid, and solids. The heat transfer characteristics under high pressures are also investigated. A consecutive film and surface renewal model is used to characterize the heat transfer mechanism.

Numerical simulation of gas–liquid–solid fluidization systems using a combined CFD-VOF-DPM method: bubble wake behavior

Abstract A new approach that can predict the characteristics of discrete phases of three-phase flows is provided in this study. In this model, the gas–liquid–solid flow in a fluidized bed is simulated by a combined method of the computational fluid dynamics (CFD) with the discrete particle method (DPM) and a volume-tracking represented by the volume-of-fluid (VOF) method. A bubble induced force model, a continuum surface force model and Newtons third law are, respectively, applied to account for the couplings of particle–bubble (gas), gas–liquid and particle–liquid interactions. A close-distance interaction model is included in the particle–particle collision model, which considers liquid interstitial effects among particles. Simulations of gas bubble rising in water in a small two-dimensional bed (height 0.1 m, width 0.06 m) with 1000 particles (glass beads, d p =0.8 mm, ρ p =2500 kg m −3 ) show that the model can capture the bubble wake behavior such as the wake structure and the shedding frequency. The simulation results are in good agreement with the experimental findings.

Particle image velocimetry for characterizing the flow structure in three-dimensional gas-liquid-solid fluidized beds

Abstract A particle image velocimetry (PIV) system is developed to measure local flow properties of a three-dimensional gas-liquid-solid fluidized bed. Both qualitative visualization and quantitative results on a flow plane including instantaneous velocity distribution of different phases, velocity fluctuations, accelerations, gas and solids holdups, bubble sizes and their distributions, and statistical flow information are obtained with this method. Sample results concerning the freeboard phenomena are presented. The use of PIV system in conjunction with the refractive index matching technique for the measurement of flow structural under high solids holdup conditions is discussed.

Hydrodynamic characteristics of inverse fluidization in liquid—solid and gas—liquid—solid systems

Abstract The flow characteristics of inverse fluidization utilizing low density spherical particles are experimentally investigated for both the liquid—solid and gas—liquid—solid systems. The experimental data for the bed expansion in the liquid-solid system are correlated, both empirically and semi-empirically. In the gas—liquid—solid system in which the gas and liquid flows are countercurrent, two modes of fluidization are examined. They are fluidization with the liquid as a continuous phase and fluidization with the gas as a continuous phase; the former characterizes the inverse gas—liquid—solid fluidized bed, while the latter characterizes the turbulent contacting bed. A flow regime diagram which portrays these modes of fluidization is presented. Correlations of the bed expansion and gas hold-up are proposed for the inverse gas—liquid—solid fluidization.