Experiment and mathematical modeling of a bench-scale circulating fluidized bed gasifier

The gasification of two different coals and chars with CO₂ and CO₂/O₂ mixture in a 48-mm-i.d. circulating fluidized bed (CFB) gasifier is investigated. The effects of operation condition on gas composition, carbon conversion and gasification efficiency were studied. A simple CFB coal gasification district mathematical model has been set up. The effects of coal type and CFB operating conditions on CFB coal gasification are discussed based on the CFB gasification test and model simulation. The main operation parameters in CFB gasification system are coal type, gas superficial velocity, circulating rate of solids and reaction temperature. It is found that CO concentration and carbon conversion increase with increasing solids circulating rate and decreasing gas velocity due to the increase in gas residence time and solids holdup in the CFB. The carbon conversion increases with increasing temperature and O₂ concentration in the inlet gas. The experimental results prove that the CFB gasifier works well for high volatile, high reactivity coal.     

Hydrodynamics and reactor performance evaluation of a high flux gas‐solids circulating fluidized bed downer: Experimental study

Reactor performance of a high flux circulating fluidized bed (CFB) downer is studied under superficial gas velocities of 3–7 m/s with solids circulation rate up to 300 kg/m²s using ozone decomposition reaction. Results show that the reactant conversion in the downer is closely related to the hydrodynamics, with solids holdup being the most influential parameter on ozone decomposition. High degree of conversion is achieved at the downer entrance region due to strong gas‐solids interaction as well as higher solids holdup and reactant concentration. Ozone conversion increases with the increase of solids circulation rate and/or the decrease of superficial gas velocity. Overall conversion in the CFB downer is less than but very close to that in an ideal plug flow reactor indicating a good reactor performance in the downer because of the nearly “ideal” hydrodynamics in downer reactors. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3412–3423, 2014     

Three-dimensional numerical simulation of a circulating fluidized bed reactor for multi-pollutant control

Circulating fluidized bed adsorber (CFBA) technology is regarded as a potentially effective method for simultaneously controlling emissions of sulfur dioxide, fine particulate matter, and trace heavy metals, such as mercury vapor. In order to analyze CFBA systems in detail, a gas mixture/solids mixture model based on the three-dimensional Navier-Stokes equations is developed for particle flow, agglomeration, physical and chemical adsorption in a circulating fluidized bed. The solids mixture consists of two solids, one with components of CaO and CaSO₄, and the other being an activated carbon. The gas mixture is composed of fine particulate matter (PM), sulfur dioxide, mercury vapor, oxygen and inert gas. Source terms representing fine particulate matter agglomeration onto sorbent particles, sulfur dioxide removal through chemical adsorption onto calcined lime, and mercury vapor removal through physical adsorption onto activated carbon are formulated and included into the model. The governing equations are solved using high-resolution upwind-differencing methods, combined with a time-derivative preconditioning method for efficient time-integration. Numerical simulations of bench-scale operation of a prototype CFBA reactor for multi-pollutant control are described.     

Deactivation of a Ni-Based Reforming Catalyst During the Upgrading of the Producer Gas, from Simulated to Real Conditions

The deactivation of a nickel reforming catalyst during the upgrading of the producer gas obtained by gasification of lignocellulosic biomass was studied. The research involved several steps: the selective deactivation of the catalyst in a laboratory scale; the streaming of the catalyst with the producer gas of a downdraft and an oxygen/steam circulating fluidized bed (CFB) gasifier; and tests in a reformer placed in a slipstream of the CFB gasifier. The information obtained allowed to elucidate the catalyst deactivation mechanisms taking place during the reforming of the producer gas: physical deactivation by deposition of fine ashes, aerosol particulate or carbon; poisoning by H₂S and HCl present in the gas phase and thermal sintering because of the high operation temperatures required to avoid the chemical deactivation. These physical and chemical effects depended on the composition of the biomass fuel.     

A new correlation for predicting solids concentration in the fully developed zone of circulating fluidized bed risers

The present work focuses on developing a new comprehensive correlation for better prediction of the solids concentration in the fully developed region of co-current upward gas-solid flow in circulating fluidized bed (CFB) risers. Systematic experiments were carried out in two risers (15.1 m and 10.5 m high with the same 0.1 m i.d.) with FCC and sand particles. The results obtained from about 200 sets of operating conditions show that the average solids concentration in the fully developed region is more than just a function of the corresponding terminal solids concentration, as most previous correlations are based on. Operating conditions, particle properties and riser diameters also have significant effects on the solids concentrations in the fully developed region of CFB risers. Based on our experimental data and those reported in the open literature from CFB risers up to 0.4 m in diameter and 27 m in height with superficial gas velocities and solids circulation rates up to 11.5 m/s and 685 kg/m²·s, a new empirical correlation for predicting the average solids concentrations in the fully developed region of CFB risers is proposed. The correlation works well for a wide range of operating conditions, particle properties and riser diameters.     

Estimation of the Monthly Average Ratios of Organic Mass to Organic Carbon for Fine Particulate Matter at an Urban Site

Two independent methods are used to estimate the seasonality of the ratio of fine particulate organic matter (OM) to fine particulate organic carbon (OC) for atmospheric particulate matter collected at the St. Louis—Midwest Supersite. The first method assumes that all of the fine particulate matter mass that cannot be attributed to sulfate ion, nitrate ion, ammonium ion, elemental carbon and metal oxides is organic matter. Using this method, 98 daily samples were used to estimate the annual average fine particulate matter OM/OC ratio to be 1.81 ± 0.07 with a summer average of 1.95 ± 0.17 and a winter average of 1.77 ± 0.13. The second approach to estimating fine particle OM/OC employed OC source apportionment results and estimates of source specific OM/OC, including primary sources and secondary organic aerosol. The OM/OC estimate that was based on 98 daily source apportionment calculations over a two year period yielded an annual average ratio of 1.96 ± 0.03. Methods used in the study yielded a relatively stable annual average estimate of the OM/OC ratio for fine particulate matter in the St. Louis area. The source apportionment results indicate that the similar OM/OC ratio for St. Louis in the summer and winter results from an increased relative contribution of secondary organic aerosol in the summer months that is balanced by the higher woodsmoke in the winter. Although the estimated OM/OC ratios that were determined for St. Louis cannot be directly applied to other locations, the methodologies used to estimate OM/OC can be broadly applied given the necessary data for these calculations.     

下行气固两相流与管内壁间的摩擦压降

在较宽的操作条件范围内系统测试了下行床床层压力降,获得气固两相流与管内壁间的摩擦压降,提出了下行气固两相流与管壁间摩擦压降的计算模型。结果表明,在下行床的充分发展段,气固两相流与管壁间的摩擦导致表观颗粒浓度显著小于真实颗粒浓度;当表观气速大于8 m·s^-1时,气固两相流与管壁间的摩擦压降接近甚至超过气固两相流重力产生的静压降。在采用压差法测试下行床中的平均颗粒浓度时,如忽略气固两相流与管壁间的摩擦,则可能导致显著的偏差。下行气固两相流与管内壁间的摩擦压降主要来自于颗粒与管壁间的摩擦。颗粒直径对气固两相流与管壁间摩擦压降的影响随着操作气速的提高逐渐减弱。采用提出的摩擦压降模型对表观颗粒浓度进行修正后,预测值与实验值吻合较好。     

New closure models for CFD modeling of high-density circulating fluidized beds

Experimental results from a high-density circulating fluidized bed (CFB) riser have been used to develop new closure models for the drag coefficient and the gas-solid mixture viscosity. The models predict a rapid increase in both viscosity and drag associated with the high solids concentrations near the riser wall. These new models have been incorporated into the commercial Computational Fluid Dynamics software FLUENT and the predictions of FLUENT have been compared with experimental data from the literature. With the inclusion of the new closure models, FLUENT was able to predict the radial distribution of solids concentration and solids mass flux found experimentally in three different cold-flow CFB risers operated at solids mass fluxes between 148 kg/m²·s and 455 kg/m²·s and superficial gas velocities between 4.7 and 10 m/s. These conditions lead to average solid concentrations in excess of 10 vol%, which corresponds to high-density CFB operation.     

Catalytic Ozone Decomposition in a Gas‐Solids Circulating Fluidized‐Bed Riser

Computational fluid dynamics (CFD) modeling of the catalytic ozone decomposition reaction in a circulating fluidized‐bed (CFB) riser, using iron‐impregnated fluid catalytic cracking particles as catalyst, is carried out. The catalytic reaction is defined as a one‐step reaction, and the reaction equation is modified by with respect to the particle surface area, A_p, and an empirical coefficient. The Eularian‐Eularian method with the kinetic theory of granular flow is used to solve the gas‐solids two‐phase flow in the CFB riser. The simulation results are compared with experimental data, and the reaction rate is modified by using an empirical coefficient, to provide better simulation results than the original reaction rate. Moreover, the particle size has great effects on the reaction rate. The generality of the CFD model is further validated under different operating conditions of the riser.     

循环型高通量输送床的建立与控制

密相输送床气化和双流化床气化是基于循环型流化床反应器发展起来的两种新型煤和生物质气化技术,根据这两种技术对流动的要求,提出了在循环流化床的下行床底部耦合一段移动床,为输送床内的流动提供足够高的驱动压力而提高颗粒循环量的技术思想。在根据该思想而建立的直径90 mm的输送床实验装置上的实验研究表明,利用所提出的床型构造可在表观气速9.6 m•s^-1下实现400 kg•m^-2•s^-1的颗粒循环量。输送床的一次风速和移动床松动风速是影响颗粒循环量和输送床内颗粒浓度的主要因素,但循环量随输送床一次风速的增大而增加的走势弱于普通循环流化床。移动床松动风速在小于颗粒最小流化速度的范围内轻微变动即可显著改变颗粒循环量和输送床内颗粒浓度。在保持输送床总气速不变的前提下,通过二次风可在40%的比例范围内调节颗粒循环量,且调节作用随二次风位置的增高而减弱。     

上行气固两相流充分发展段的颗粒浓度

在Ф100mm× 16m循环床提升管实验装置上测试了134组操作条件下提升管 12个截面位置的平均颗粒浓度 ,其中 113组操作条件下提升管中的气固两相流展现出明确的充分发展段 .结果表明 ,充分发展段的颗粒浓度εs 随颗粒循环量G_s 的增加而增大且具有显著的线性关系 ,ε_s 随表观气速U_g 的增加以幂函数关系减小 .所提出的实验关联式不仅较好地拟合了本文所获得的充分发展段的平均颗粒浓度数据 ,而且诠释了以往有关提升管稀相段出口颗粒浓度预测结果之间的差异 ,更明确地反映了充分发展段颗粒浓度与操作参数之间的定量关系     

Study of gas–solids flow in a short CFB riser by statistical and chaotic deterministic analysis of optical fibre probe signals

In this work, a short CFB riser with a height of 2.42 m and an ID of 82 mm was operated under different dilute operating conditions to study the fluid dynamics of FCC catalyst particles (d_p = 80 µm, ρ_p = 902 kg/m³) in air. The electrical signals from the optical fibre probe were sampled at a frequency of 1000 Hz for a period of 30 s and were obtained at different positions along the radius and height of the riser. Data were analysed using both statistical methods (time average, standard deviation and frequency distribution) and chaos methods (construction of attractors; correlation dimension, D_ML; and Kolmogorov entropy, K_ML). Some results on solids holdup deduced from the electrical signal are also presented for the developed zone of the riser. It was verified that for very dilute conditions, increasing gas velocity produces more complex and less predictable fluctuations in solids concentration, while increasing solids flux generally reduces complexity and increases predictability. However, results for the most dilute condition used shows that for the radial position where solids holdup is higher (near the wall), the increase in solids concentration does not affect the mean free path of the particles, resulting in higher values of D_ML than in the dilute region (core). © 2012 Canadian Society for Chemical Engineering     

The circulating fluid bed reactor — its main features and applications

After a brief review of the development of the circulating fluid bed (CFB) reactor principle, its main features as an efficient tool in performing reactions between gases and finely grained solids are discussed. Today, the circulating fluid bed reactor is used in numerous industrial processes or has passed the pilot or demonstration stage in many others. Large-scale CFB calciners have replaced the rotary kiln in the alumina industry. Combustion of carbon-containing fossil fuels and process residues for energy supply with accompanying low levels of noxious emissions is one of the most promising fields of future applications, especially in the power industry and municipal combined heat and power generation. Dry-scrubbing of process and power plant waste gases is also promising. Many more industrial applications of CFB reactors can be expected.     

Particle velocities and their residence time distribution in the riser of a CFB

The riser of a Circulating Fluidised Bed (CFB) is the key-component where gas-solid or gas-catalytic reactions occur. Both types of reactions require different conditions of operating velocities (U), solids circulation fluxes (G), overall hydrodynamics and residence times of solids and gas. The solids hydrodynamics and their residence time distribution in the riser are the focal points of this paper. The riser of a CFB can operate in different hydrodynamic regimes, each with a pronounced impact on the solids motion. These regimes are firstly reviewed to define their distinct characteristics as a function of the combined parameters, U and G.Experiments were carried out, using Positron Emission Particle Tracking of single radio-actively labelled tracer particles. Results on the particle velocity are assessed for operation in the different regimes. Design equations are proposed.The particle velocities and overall solids mixing are closely linked. The solid mixing has been previously studied by mostly tracer response techniques, and different approaches have been proposed. None of the previous approaches unambiguously fits the mixing patterns throughout the different operating regimes of the riser. The measured average particle velocity and the velocity distribution offer an alternative approach to determine the solids residence time distribution (RTD) for a given riser geometry. Findings are transformed into design equations.The overall approach is finally illustrated for a riser of known geometry and operating within the different hydrodynamic regimes.     

Bulk coarse particle arching phenomena in a moving bed with fine particle presence

Bridging or arching of flowing solids particles is a serious hazard in the operation of moving bed systems. The mechanics of the arching has been extensively analyzed in the context of particle discharge from a hopper with conical geometry by considering the particulate layer stress distribution. However, bridging can also occur in a moving bed system with cylindrical geometry during the continuous mass flow of solids particles. Experimental work conducted in this study reveals that the appearance of solids bridging is normally accompanied by the presence of fine particles in the coarse moving particles as well as by the countercurrent interstitial gas flow. In this study, a stress analysis of the layered particles distributed in a cylindrical, vertical moving bed that flows downward opposing to upward flow of the interstitial gas is developed to quantify the bridging phenomenon. The analysis takes into account of the effects of presence of fine powder in the coarse particle flows and properties, such as particle‐size distribution, bed voidage, and interstitial gas flow rate. The experimental validation of the present stress analysis for moving bed systems with varied fine and coarse particle concentration distributions, and interstitial gas velocities is also conducted. The stress distributions of the particles under flowing and arching conditions are obtained. An arching criterion is formulated, which indicates that the critical radius of the standpipe to avoid arching phenomenon is only related to the property of the bulk solids in the present geometric configuration of the flow system. © 2014 American Institute of Chemical Engineers AIChE J, 60: 881–892, 2014     

Improvement of the Uniformity of Radial Solids Concentration Profiles in Circulating Fluidized‐Bed Risers

In order to enhance the uniformity of the radial solids distribution and thereby the performance of industrial circulating fluidized‐bed (CFB) risers, an approach by using the air jet from the riser circumference is proposed. The Eulerian‐Eulerian computational fluid dynamics (CFD) model with the kinetic theory of granular flow is adopted to simulate the gas‐solids two‐phase flow in a CFB riser with fluid catalytic cracking (FCC) particles. The numerical results indicate that by employing the circumferential air jet approach under appropriate jet velocities, the maximum solids concentration in the near‐wall region can be greatly reduced, the entrance region can be shortened, and the uniformity of the flow structure can be significantly improved.     

Solids concentration in the fully developed region of circulating fluidized bed downers

To investigate solids concentration in the fully developed region of co-current downward gas-solid flow, actual solids concentrations were measured in a circulating fluidized bed (CFB) downer with 9.3 m in height and 0.1 m in diameter using a fiber optical probe. The results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB downers is not only a function of the corresponding terminal solids concentration, but the operating conditions and particle properties also have influences on the average solids concentration in the fully developed region of the CFB downers. Particle diameter and density affect the solids concentrations differently under different operating conditions. Downer diameters almost have no influence on the solids concentrations. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation to predict the solids concentrations in the fully developed region of CFB downers is proposed. The predictions of the correlation are in good agreement with the experimental data of this work and in the literature.     

Numerical study of the effect of the gas and solids distributors on the uniformity of the radial solids concentration distribution in CFB risers

The non-uniform radial solids distribution usually has a negative effect on the performance of the circulating fluidized bed (CFB) riser since it may greatly decrease the reactor efficiency and controllability. In order to improve the performance of industrial CFB risers, the numerical study of the effects of the gas distributor and solids distributor at the CFB riser inlet on the uniformity of the radial solids distribution was carried out in this study. Two potential approaches to improve the uniformity of radial solids concentration profile were proposed: (1) the use of the center-sparse side-dense air jets arrangement for the gas distributor and (2) the use of the side-covered arrangement for the solids distributor. The Eulerian-Eulerian computational fluid dynamics (CFD) model with kinetic theory of granular flow was adopted to simulate the gas-solids two-phase flow in a CFB riser with FCC particles. The numerical results show that the patterns of the inlet gas distributor and solids distributor have significant effect on the flow structure in both the entrance region and the fully-developed region in the riser. The gas distributor with center-sparse side-dense air jet arrangement improves the uniformity of the radial solids distribution, while the center-dense side-sparse air jet arrangement steepens the non-uniformity of the solids radial profile. The core-annulus structure can be greatly flattened by applying a side-covered solids distributor, while it can be heavily steepened by employing the center-covered solids distributor.     

循环床提升管中粗重颗粒浓度的轴向分布

在10m高提升管中对空气-沙子体系的压力梯度进行系统测试,研究了粗重颗粒平均颗粒浓度云的轴向分布及操作条件对它的影响。结果表明,粗重颗粒的^εs在相同操作条件下显著低于FCC颗粒;随操作条件的不同,沙子颗粒表现出与FCC显著不同的轴向分布形态。高气速下粗重颗粒^εs的轴向分布与FCC相似表现为单调下降或直线形关系;但在表观气速Ug降低至某一临界值后,粗重颗粒^εs的轴向分布呈现出波动形式,表明沙子颗粒在提升管中的流动是一个加速-减速-再加速直至充分发展的过程。随Ug减小或Gs增大,提升管各截面上云升高;当^εs的轴向分布为波动形式时,提升管底部截面和中部颗粒聚集截面上^εs的变化较其它截面更为显著。