Numerical Investigation of Pressure Drop in Single Pellet String Reactors

In narrow fixed-bed reactors the influence of the confining wall on pressure drop cannot be neglected. Here, the pressure drop in single pellet string reactors, a limiting case of fixed-bed reactors with a cylinder-to-particle diameter ratio below 2, is studied using computational fluid dynamics simulations. Deviations to the Ergun, and more specifically Blake-Kozeny equation are evident though the general trend is met. A geometry-based weighting factor is introduced to scale the influence of the confining wall in an equivalent diameter expression. Agreement between numerical simulation and pressure drop predictions from correlation are thereby improved significantly.     

Flow through packed bed reactors: 1. Single-phase flow

Single-phase pressure drop was studied in a region of flow rates that is of particular interest to trickle bed reactors . Bed packings were made of uniformly sized spherical and non-spherical particles (cylinders, rings, trilobes, and quadralobes). Particles were packed by means of two methods: random close or dense packing (RCP) and random loose packing (RLP) obtaining bed porosities in the range of 0.37–0.52. It is shown that wall effects on pressure drop are negligible as long as the column-to-particle diameter ratio is above 10. Furthermore, the capillary model approach such as the Ergun equation is proven to be a sufficient approximation for typical values of bed porosities encountered in packed bed reactors. However, it is demonstrated that the original Ergun equation is only able to accurately predict the pressure drop of single-phase flow over spherical particles, whereas it systematically under predicts the pressure drop of single-phase flow over non-spherical particles. Special features of differently shaped non-spherical particles have been taken into account through phenomenological and empirical analyses in order to correct/upgrade the original Ergun equation. With the proposed upgraded Ergun equation one is able to predict single-phase pressure drop in a packed bed of arbitrary shaped particles to within ±10% on average. This approach has been shown to be far superior to any other available at this time.     

对干熄炉内压降的研究

测量了干熄炉内焦炭床层的阻力损失，研究了鼓风量、焦炭直径以及形状因子对阻力损失的影响。利用Ergun方程整理的实验常数，分析了与形状因子的关系。     

THE EFFECT OF SURFACE ROUGHNESS ON PRESSURE DROP IN A PACKED BED

Particle surface roughness is shown to have a significant effect on the pressure drop in a packed bed of adsorbent particles. The packed bed friction factor is determined using three spherical adsorbents of differing degree of surface roughness in the Reynolds number range 1-62. The results were successfully correlated using a correlation of the Ergun type. It is shown that surface roughness significantly increases the friction factor.     

PRESSURE DROP IN FLOW OF A NEARLY CRITICAL FLUID THROUGH PACKED BEDS OF SPHERES

The flow of nearly critical carbon dioxide through packed beds of glass beads and steel spheres has been investigated. Experiments have been carried out in the intermediate range of sphere Reynolds number. Results show that around the critical point effects of fluid compressibility are negligible, and, therefore, that the friction factor can be expressed as a function of Reynolds number only. Carman [1] and Dolejs and Lecjaks [2] equations show good agreement with the experimental data.     

Packing with Stamped Horizontal Lamellae Operating at Extremely Low Liquid Loads – III. Packing Pressure Drop

It has been discussed in previous papers [1–7] that the design of packings capable of operating at extremely low liquid superficial velocity allows the development of effective countercurrent flow packed bed columns even if the concentration of the absorbed component is very low, the absorption is an equilibrium process, and the gas is well soluble in the liquid phase. The construction of the new packing is described in [7]. The present paper reports the results of an investigation of the pressure drop for dry and irrigated packings up to a gas velocity equal to 3 m/s (F_G factor equal to 3.3 kg^1/2 m^–1/2 s^–1) as well as the equations for its calculation.     

Image based meshing of packed beds of cylinders at low aspect ratios using 3d MRI coupled with computational fluid dynamics

CFD is a valuable tool for understanding the flow and pressure drop in packed beds. However, determining the geometry can be complex. One possible method is to use a non-invasive imaging method such as MRI, however, problems occur in processing complex geometries when using traditional commercial meshing software. This work focuses on the use of image based meshing software originally developed for the field of computational biomechanics, to create geometries from 3d MRI scans of packed beds for use with computational dynamics. For this work we focus on disordered packed beds of cylinders at low aspect ratios and Reynolds numbers of Re = 1431-5074 (based on particle diameter and superficial velocity). We compare CFD studies with experimental data performed on the actual scanned beds and compare these with the correlation proposed by Eisfeld and Schnitzlein (2001). Computational data is shown to correlate well with experimental and theoretical results.     

Pressure drop prediction for horizontal dense-phase pneumatic conveying of pulverized coal associated with feeding to gasifier

Based on extensive bench-scale data derived from the horizontal dense-phase pneumatic conveying of pulverized coal, a correlation of solids friction factor λ_z was proposed in an effort to establish a model to predict the pressure drop when coal fed to the gasifier. Further, it was also an attempt to modify some public models to verify their availabilities. Then, based on the data collected from an industrial-scale horizontal pipeline under the high pressure up to 2.0 MPa, the proposed model was found to be possibly among the best ones for predicting the pressure drops of the dense flow of pulverized coal. The modified Mallick and Wypych model can also provide satisfying predictions. The results suggest that the two models are both suitable for scale-up of dense-phase pneumatic conveying of pulverized coal at high pressures.     

Volatile trace element behaviour in the Sasol fixed-bed dry-bottom (FBDB)™ gasifier treating coals of different rank

Trace element simulation and validation of model predictions for the elements Hg, As, Se, Cd and Pb have recently been undertaken for the Sasol^® FBDB™ gasification process operating on lump coal. The validation was conducted by interpolating the residual trace elements content remaining behind in the solid coal/char/ash fractions after sequential mining of a quenched commercial-scale gasifier operating on low rank grade C bituminous Highveld coal used for gasification in South Africa. This paper extends the research understanding by comparing the volatile trace element behaviour of these same elements, using the same gasification technology, but operating on North Dakota lignite. The focus will be on the behaviour of the volatile Class III trace elements: Hg, As, Se, Cd and Pb within the Sasol^® FBDB™ gasifier as function of coal rank. This study excludes the downstream gas cleaning partitioning and speciation behaviour of these elements.Findings indicate that although the feed concentration and mode of occurrence of these elements differ quite substantially between the two coal types studied, that the volatilization profiles of the elements are indeed quite similar; being within 0.1%-15% lower in the case of the lignite when compared to the bituminous coal. In both cases, Hg was found to be the most volatile and As the least; with the volatility order varying slightly for the metals Se, Cd and Pb for the two coal types. The differences observed in the trace element volatilization rate are supported by the temperature profile which was inferred from the reflectance of vitrinite (RoV) measurements of the dissected fuel bed material. The highly reactive lignite, is successfully gasified at a lower temperature than is the case for bituminous coal using the Sasol^® FBDB™ gasification process. Speciation predictions have earlier shown that: H₂ Se, CdS, PbS/Pb/PbCl, and AsH₃ species possibly exist in the gas phase. In reality, organically-associated trace elements will also be volatilized into the gas phase, but due to a lack of thermodynamic data for the lignite organo-metallic species at this stage only inorganic associations could be modelled.     

Three‐stage well‐mixed reactor model for a pressurized coal gasifier

Three Canadian coals of different rank were gasified with air‐steam mixtures in a 0.1 m diameter spouted bed reactor at pressures to 292 kPa, average bed temperatures varying between 840 and 960°C, and steam‐to‐coal feed ratios between 0.0 and 2.88. In order to analyze gasifier performance and correlate data, a three‐stage model has been developed incorporating instantaneous devolatilization of coal, instantaneous combustion of carbon at the bottom of the bed, and steam/carbon gasification and water gas shift reaction in a single well mixed isothermal stage. The capture of H₂S by limestone sorbent injection is also treated. The effects of various assumptions and model parameters on the predictions were investigated. The present model indicates that gasifier performance is mainly controlled by the fast coal devolatilization and char combustion reactions, and the contribution to carbon conversion of the slow char gasification reactions is comparatively small. The incorporation of tar decomposition into the model provides significantly closer predictions of experimental gas composition than is obtained otherwise.     

CO2 gasification rate analysis of coal char in entrained flow coal gasifier

Coal chars of four coal types were gasified with carbon dioxide using a PDTF or TGA at high temperature and pressure. Test conditions of temperature and partial pressure of the gasifying agent were determined to simulate the conditions in air-blown or oxygen-blown entrained flow coal gasifiers. Coal chars were produced by rapid pyrolysis of pulverized bituminous coals using a DTF with a nitrogen gas flow at 1670 K. In gasification tests with the PDTF, gasification temperatures were 1670 K or below and partial pressures of carbon dioxide were 0.7 MPa or below. Carbon monoxide of 0.6 MPa or below was supplied for the gasification tests with the TGA.As a result, coal types showed a large difference in the char gasification rate with carbon dioxide, and this difference remained large without decreasing even in the high-temperature area when the gasification rate was controlled by pore diffusion the same as in entrained flow gasifiers. Inhibition of the gasification reaction by carbon monoxide was also observed. Reaction rate equations of both the nth order and Langmuir-Hinshelwood type were applied to the char gasification reaction with the random pore model and the effectiveness factor, and the applicability of these rate equations to air-blown and oxygen-blown entrained flow gasifiers evaluated. Gasification rate equations and kinetic parameters applicable to a pore diffusion zone at high temperature were obtained for each coal.     

A mathematical model of a spouted bed gasifier

A mathematical model of the spouted bed gasifier has been constructed based on simplified first order reaction kinetics for the gasification reactions and the stream tube hydrodynamic model of Mathur and Lim. This two region model treats the spout as an isothermal plug flow reactor with cross flow into a series of streamtubes forming the annulus. Each streamtube is considered as a plug flow reactor. The effects of kinetic and hydrodynamic parameters on model predictions are illustrated, and a comparison made with experimental gas composition profiles obtained in a 0.30-m dia. gasifier.     

Simulated moving bed technology applied to coal gasification

An alternative technology to conventional coal gasification is discussed in order to improve the performances of the existing processes. The concept of simulated moving bed (SMB) is applied to coal gasification: a number of fixed-bed reactors are connected in series and a suitable switching policy is applied so that each fixed-bed is cyclically operated both as a combustion and a gasification reactor.     

Flow of a gas-solid two-phase mixture through a packed bed

This paper is concerned with an upward co-current flow of a gas-solid two-phase mixture through a packed bed, a system employed in a number of industrial processes. Experimental work was carried out by using glass balls for packed bed, and both glass beads and FCC as suspended particles. The effects of solids loading and gas velocity on the pressure drop as well as the static and dynamic solid hold-ups within packed bed were examined. Experimental results showed different behaviour of the FCC from glass beads. At a given gas velocity, pressure drop increased approximately linearly with solids loading with a slope for FCC much higher than that for glass beads. The static hold-up of glass beads was much lower than corresponding dynamic hold-up at a given gas velocity, and it did not seem to change much with solids loading under the conditions of this work. At a given gas velocity, the static hold-up of FCC, however, was found to be comparable with the corresponding dynamic hold-up. An analysis was conducted on the pressure drop using a modified version of the Ergun equation by taking into account the effects of suspended particles on the viscosity and density, as well as the gravitational force. It was found that the modified Ergun equation agreed well with the experimental results of both this work and those reported in the literature. Effort was also made to develop relationships for the dynamic hold-up and the interaction coefficient between the suspended and the packed particles, the so-called solid-phase friction factor in the literature. The dynamic hold-up was found to increase with an increase in the product of velocity ratio of the solid to gas phases and the square root of the diameter ratio of the suspended to packed particles, whereas the interaction coefficient increased in general with increasing Froude number but with significant scattering.     

Development of an innovative 3-stage steady-bed gasifier for municipal solid waste and biomass

Gasification of biomass, municipal solid waste, waste-derived fuels and residues has been lately gaining in attractiveness as an alternative thermal treatment method to produce power and heat. Presented in this paper is a new, recently patented, 3-stage gasification scheme, designed for all aforementioned types of fuels and for producing a synthesis gas free of tar and dioxins. The proposed 3-stage gasification scheme comprises of three stages: i) pyrolysis, ii) combustion and iii) gasification. The proposed 3-stage gasification scheme is valid for municipal solid waste and any type of biomass despite differences in chemical composition. The innovation of this 3-stage gasification scheme is based on the fact that the transition between normal and reverse operation and vice versa is achieved only by the proper rotation settings of four air blowers, thus creating a new model of gaseous flow management between the three aforementioned stages. The presented model can achieve a safe industrial-scale operation while producing a gas free of harmful components. The proposed gasification model is validated as suitable for small-to-medium scale capacities, achieving an overall electrical efficiency of 30% and minimum environmental impacts well below all existing thresholds, including those set by the Directive 2000/76/EC on solid waste incineration.     

灰熔聚流化床粉煤气化技术0.6MPa工业炉运行概况

介绍了中科院山西煤炭化学研究所开发的灰熔聚流化床粉煤气化技术及其适用范围、工业应用情况。灰熔聚流化床粉煤气化技术具有气化温度适中、氧耗量较低、煤种适应性宽、产品气不含焦油、气化炉耐火材料要求低等优点。同时,对0.6 MPa工业装置的运行情况进行了介绍,总结了0.6 MPa工业装置运行结果,并指出当前需要改进和完善之处。     

Carbon particle type characterization of the carbon behaviour impacting on a commercial-scale Sasol-Lurgi FBDB gasifier

Char-form analysis, whilst not yet an ISO standard, is a relatively common characterization method applied to pulverized coal samples used by power utilities globally. Fixed-bed gasification coal feeds differ from pulverized fuel combustion feeds by nature of the initial particle size (+6 mm, −75 mm). Hence it is unlikely that combustion char morphological characterization schemes can be directly applied to fixed-bed gasifier chars. In this study, a unique carbon particle type analysis was developed to characterize the physical (and inferred chemical) changes occurring in the particles during gasification based on coal petrography and combustion char morphology. A range of samples sequentially sampled from a quenched commercial-scale Sasol-Lurgi fixed-bed dry-bottom (FBDB) Gasifier were thus analysed.It was determined that maceral type (specifically vitrinite and inertinite) plays a pivotal role in the changes experienced by carbon particles when exposed to increasing temperature within the gasifier. Whole vitrinite particles and vitrinite bands within particles devolatilized first, followed at higher temperatures by reactive inertinite types. By the end of the pyrolysis zone, all the coal particles were converted to char, becoming consumed in the oxidation/combustion zone as the charge further descended within the gasifier.The carbon particle type results showed that both the porous and carbominerite char types follow similar burn-out profiles. These char types formed in the slower pyrolysis region within the pyrolysis zone, increasing to around 10% by volume within the reduction zone, where 53% carbon conversion occurred. Both of these char forms were consumed by the time the charge reached the ash-grate at the base of the reactor, and therefore did not contribute to the carbon loss in the ash discharge. It would appear as if the dense char and intermediate char types are responsible for the few percent carbon loss that is consistently obtained at the gasification operations.The carbon particle type analysis developed for coarse coal to the gasification process was shown to provide a significant insight into the behaviour of the carbon particles during gasification, both as a stand alone analysis and in conjunction with the other chemical and physical analyses performed on the fixed-bed gasifier samples.     

Pore-scale derivation of the Ergun equation to enhance its adaptability and generalization

The empirical nature of the well-known Ergun equation for prediction of the permeability of granular materials inhibits the straightforward generalization to other geometries of the pore space and non-Newtonian effects of traversing fluids. In this paper the results are discussed of a pore-scale model that can be regarded as qualitative and quantitative proof of the Ergun equation. The pore-scale model has superior adaptive capabilities and also allows investigation of the porosity dependence of the empirical coefficients of the Ergun equation. Some advantages, based on physical grounds, of the pore-scale model are outlined.     

Changes in char reactivity due to char-oxygen and char-steam reactions using Victorian brown coal in a fixed-bed reactor

This study was to examine the influence of reactions of char–O2 and char–steam on the char reactivity evolution. A newly-designed fixed-bed reactor was used to conduct gasification experiments using Victorian brown coal at 800 °C. The chars prepared from the gasification experiments were then collected and subjected to reactivity characterisation (ex-situ reactivity) using TGA (thermogravimetric analyser) in air. The results indicate that the char reactivity from TGA was generally high when the char experienced intensive gasification reactions in 0.3%O2 in the fixed-bed reactor. The addition of steam into the gasification not only enhanced the char conversion sig-nificantly but also reduced the char reactivity dramatical y. The curve shapes of the char reactivity with involve-ment of steam were very different from that with O2 gasification, implying the importance of gasifying agents to char properties.