Velocity Bed Research Articles

Evaluation of ADCP Apparent Bed Load Velocity in a Large Sand-Bed River: Moving versus Stationary Boat Conditions

Detailed mapping of bathymetry and apparent bed load velocity using a boat-mounted acoustic Doppler current profiler (ADCP) was carried out along a 388-m section of the lower Missouri River near Columbia, Missouri. Sampling transects (moving boat) were completed at 5- and 20-m spacing along the study section. Stationary (fixed-boat) measurements were made by maintaining constant boat position over a target point where the position of the boat did not deviate more than 3 m in any direction. For each transect and stationary measurement, apparent bed load velocity (vb) was estimated using ADCP bottom tracking data and high precision real-time kinematic (RTK) global positioning system (GPS). The principal objectives of this research are to (1) determine whether boat motion introduces a bias in apparent bed load velocity measurements; and (2) evaluate the reliability of ADCP bed velocity measurements for a range of sediment transport environments. Results indicate that both high transport (v¯b>0.6 m/s) and mo...

Measurement and modeling of bed shear stress under solitary waves

Direct measurements of bed shear stresses (using a shear cell apparatus) generated by non-breaking solitary waves are presented. The measurements were carried out over a smooth bed in laminar and transitional flow regimes (~ 10 4 < R e < ~ 10 5). Measurements were carried out where the wave height to water depth (h/d) ratio varied between 0.12 and 0.68; maximum near bed velocity varied between 0.16 m/s and 0.51 m/s and the maximum total shear stress (sum of skin shear stress and Froude–Krylov force) varied between 0.386 Pa and 2.06 Pa. The total stress is important in determining the stability of submarine sediment and in sheet flow regimes. Analytical modeling was carried out to predict total and skin shear stresses using convolution integration methods forced with the free stream velocity and incorporating a range of eddy viscosity models. Wave friction factors were estimated from skin shear stress at different instances over the wave (viz., time of maximum positive total shear stress, maximum skin shear stress and at the time of maximum velocity) using both the maximum velocity and the instantaneous velocity at that phase of the wave cycle. Similarly, force coefficients obtained from total stress were estimated at time of maximum positive and negative total stress and at maximum velocity. Maximum positive total shear stress was approximately 1.5 times larger than minimum negative total stress. Modeled and measured positive bed shear stresses are well correlated using the best convolution model, but the model underestimates the data by about 4%. Friction factors are dependent on the choice of normalizing using the maximum velocity, as is conventional, or the instantaneous velocity. These differ because the stress is not in phase with the velocity in general. Friction factors are consistent with previous data for monochromatic waves, and vary inversely with the square-root of the Reynolds number. The total shear stress leads the free stream fluid velocity by approximately 50°, whereas the skin friction shear stress leads by about 30°, which is similar to that reported by earlier researchers.

Euler–Euler two-phase flow simulation of tunnel erosion beneath marine pipelines

In this study an Euler–Euler two-phase model was developed to investigate the tunnel erosion beneath a submarine pipeline exposed to unidirectional flow. Both of the fluid and sediment phases were described via the Navier–Stokes equations, i.e. the model was implemented using time-averaged continuity and momentum equations for the fluid and sediment phases and a modified k − ε turbulence closure for the fluid phase. The fluid and sediment phases were coupled by considering the drag and lift interaction forces. The model was employed to simulate the tunnel erosion around the pipeline laid on an erodible bed. Comparison between the numerical result and experimental measurement confirms that the numerical model successfully predicts the bed profile and velocity field during the tunnel erosion. It is evident that the sediments are transported as the sheet-flow mode in the tunnel erosion stage. Also the transport rate under the pipe increases rapidly at the early stage and then reduces gradually at the end of the tunnel erosion beneath pipelines.

Effect of sediment pulse grain size on sediment transport rates and bed mobility in gravel bed rivers

Sediment supply to gravel bed river channels often takes the form of episodic sediment pulses, and there is considerable interest in introducing sediment pulses in stream restorations to alter bed surface grain size distributions and bed mobility. A series of laboratory experiments was conducted in order to examine how sediment pulse grain size and volume affects the mobility of bed material in gravel bed channels. Pulses used in the experiments were composed of either the fine tail or the median of the subsurface bed material grain size distribution. Bed material refers to sediment in the channel prior to the pulse introduction exclusively. Both types of pulse were finer than the bed material surface median. Two pulse sizes were used, which were either equivalent to the volume of sediment required to cover the entire bed one median subsurface bed material grain diameter deep (full unit) or 1/4 of this volume (1/4 unit). The latter was designed to produce a transitory pulse. With the exception of the 1/4 unit coarse pulse, introduction of the sediment pulses to the channel caused dramatic increases in the bed load flux. The coarse sediment pulses fine the bed surface and coarsen the bed load. Finer pulses also fine the surface, but the bed load fines while the bed material load (that excludes pulse material) coarsens. The greatest effects on the fractional transport occurred during the full unit fine pulse where the pulse covered the greatest bed surface and effectively smoothed the bed, increasing near bed velocity and mobilizing the coarse particles. Overall, the coarse pulses were not very effective at mobilizing bed material. The large fine pulse mobilized ∼35% of the bed material surface (∼35% of its input weight) and was most effective at mobilizing the surface. However, the small fine pulse mobilized 50% of its input weight as it passed through the channel, making it the most efficient at mobilizing the bed material.

Coherent structures in wave boundary layers. Part 2. Solitary motion

This study continues the investigation of wave boundary layers reported by Carstensen, Sumer &amp; Fredsøe (J. Fluid Mech., 2010, part 1 of this paper). The present paper summarizes the results of an experimental investigation of turbulent solitary wave boundary layers, simulated by solitary motion in an oscillating water tunnel. Two kinds of measurements were made: bed shear stress measurements and velocity measurements. The experiments show that the solitary-motion boundary layer experiences three kinds of flow regimes as the Reynolds number is increased: (i) laminar regime; (ii) laminar regime where the boundary-layer flow experiences a regular array of vortex tubes near the bed over a short period of time during the deceleration stage; and (iii) transitional regime characterized with turbulent spots, revealed by single/multiple, or, sometimes, quite dense spikes in the bed shear stress traces. Supplementary synchronized flow visualization tests confirmed the presence of the previously mentioned flow features. Information related to flow resistance are also given in the paper.

Dynamics of gas–solid fluidised beds with non-spherical particle geometry

Fluidised beds play an important role in physical and chemical engineering processing. Understanding the granular motion within these beds is essential for design, optimisation and control of such processes. Motion on the particle scale is difficult to measure experimentally, making computational simulations invaluable for determining the dynamics within such systems. Computational models which have had the greatest success at capturing the full range of dynamics are coupled discrete element model and Navier–Stokes solvers, based on a pressure-gradient-force formulation. However, most discrete element models assume spherical geometry for the particles. Particle shape in many important industrial processes, such as catalysis and pyrolysis, is often non-spherical. We present a re-formulation of the pressure-gradient force model, based on a modified pressure correction method, coupled to a discrete element model with non-spherical grains. The drag relations for the coupling are modified to take into account the grain shape and cross-sectional area relative to the local gas flow. We show that grain shape has a significant effect on the dynamics of the fluidised bed, including increased pressure gradients within the bed and lower fluidisation velocities when compared to beds of spherical particles. A model is presented to explain these effects, showing that they are due to both decreased porosity within the bed as well as the relative particle cross-sectional area creating a greater net drag over the bed. Our findings will be of interest from an applied standpoint as well as showing fundamental effects of particle shape on coupled fluid and granular flow.

Determination of laminar and turbulent flow ranges through vertical packed beds in terms of particle friction factors

Despite the presence of a variety of studies dealing with the magnitude of particle Reynolds number, Re p defining transition from laminar to turbulent regime for flow through packed beds, the manner is still one of the unknowns. An approach based on the experimental data concerning upward airflow through fixed cylindrical packed beds is presented in this paper. The utilized packed beds had the following ranges of; sphericity, Φ, 0.55 ⩽ Φ ⩽ 1.00, packing material diameter to bed length ratio, D p/ L, 0.04 ⩽ D p/ L ⩽ 0.72, and bed porosity, ε, 0.36 ⩽ ε ⩽ 0.56. The test cases covered the ranges of particle Reynolds number, Re p 708 ⩽ Re p ⩽ 7772 and particle Froude number; Fr p 2.86 ⩽ Fr p ⩽ 10.39. The measurements of pressure drop through packed bed; Δ P Bed and superficial mean exit velocity; U are used to determine bed frictional effects in reference to the available literature on particle friction factors, f p. The magnitude of Re p defining transition is assumed to be 2000 with particular emphasis to the flow dynamics. The definitions of Bird et al. [R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena , John Wiley and Sons, NY, 1960] are used to calculate f p. The calculated f p for the covered test cases are given as a function of pressure coefficient, Δ P* and Re p, Fr p, Φ, ε, D p/ L in the approximate ranges of laminar and turbulent flow for Re p < 2000 and Re p > 2000, respectively. The proposed separate equations of f p = f p(Δ P*, Re p, Fr p, Φ, ε, D p/ L) are satisfied for laminar and turbulent flows with corresponding average error margins of ±7.6% and ±18%. Furthermore ranges of transitional and fully rough flow through packed beds are estimated as 2000 ⩽ Re p ⩽ 4000 and Re p > 5000 with an analogy to the well-known Moody Chart in pipe flows.

The relations between ‘standard’ fluvial habitat variables and turbulent flow at multiple scales in morphological units of a gravel‐bed river

AbstractFluvial fish habitat is often characterized by highly turbulent flow conditions. Several laboratory experiments suggest that unpredictable turbulent fluctuations can increase the swimming energy costs of fish. At the scale of fish habitat models, it can be hypothesized that turbulence can be captured by the combined effects of the standard habitat variables: depth, velocity and substrate. However, recent studies conducted at the reach scale suggest that turbulent properties are more controlled by the large‐scale bed morphology than by individual roughness elements. In this study, we investigate the spatial structure of turbulent flow and the potential relationships between ‘standard’ habitat variables and turbulent flow properties in pools and riffles of a shallow gravel‐bed river. The study explores these relations at multiple spatial scales. Mean turbulent properties and turbulent flow structures statistics were computed from 1932 near bed velocity time series sampled with acoustic Doppler velocimeters on a regular grid in four morphological units (two pools and two riffles) presenting a gradient of complexity. We used a novel multivariate variation partitioning analysis involving principal coordinates of neighbour matrices (PCNM) to partition turbulent flow properties into six significant spatial scales (VF: 0.35, F: 0.75, M: 1.25, L: 2, XL: 2.5 and XXL: 3 m). Between 45 and 70% of the variance of the turbulent flow properties were explained by the spatial PCNM. In the four units, turbulent properties exhibited a spatial dependence across the entire range of scales. However, the proportion of variation explained by the larger‐scaled PCNMs was higher in the most homogeneous units. In general, the spatial dependence of turbulent flow was lower in the riffles than in the pools, where the mean flow velocity was slower. The capacity of ‘standard’ fish habitat variables to explain turbulent properties was relatively low, especially in the smaller scales, but varied greatly between the units. From a practical point of view, this level of complexity suggests that turbulence should be considered as a ‘distinct’ ecological variable within the range of spatial scales included in this study. Further research should attempt to link the spatial scales of turbulent flow variability to benthic organism patchiness and fish habitat use. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Bottom friction effects on linear wave propagation

Bottom boundary layer effects on the linear wave propagation over mild slope bottoms are analyzed. A modified WKB approximation is presented including boundary layer effects. Within the boundary layer, two cases are considered: laminar (constant viscosity) and turbulent. Boundary layer effects are introduced by coupling the velocity inside the boundary layer to the irrotational velocity in the core region through the bottom boundary condition. This formulation properly accounts for the phase between near bed velocity and bed shear stress. The resulting differential equation for the energy conservation introduces a new term accounting for the energy losses due to the boundary layer effects.

Analytical Model for the Throughput and Drive Power Calculation in the Melting Section of Single Screw Plasticizing Units considering Wall-Slippage

Abstract A two-part article presents a process model developed at the KTP for describing the pressure/throughput and drive power behaviour of wall-slipping polymers in the melting section of single-screw plasticizing units. After a new model for describing the flow behaviour in the melt film was developed in the first part of the paper, this second part now covers the adaptation of an existing process model for pure melt conveyance to allow for the processes that take place in the melt pool, and the extension of the model to include the drive power calculation. Following this, the two individual models are then coupled through the assumption of a constant pressure gradient in the conveying direction. Using the overall model that is generated, the pressure/throughput and power behaviour during melting are investigated on the basis of the influencing dimensionless variables. This requires the geometry of the solid bed and the solid bed velocity to be specified as input variables. Apart from this, the other material parameters that need to be defined are the viscosity of the polymer melt, the critical shear stress, a material-specific parameter for describing the slipping-velocity-dependent shear stress and the operating point of the process that is taken as a basis.

Prediction of SO2 and NOx emissions for low-grade Turkish lignites in CFB combustors

Abstract The CFB technology is widely used for combustion of coal because of its unique ability to handle low quality, high ash, high sulphur and low heating value coals. This paper presents a modeling study of pollutant emissions such as sulphur dioxide (SO 2 ) and nitric oxide (NO x ) resulting from coal combustion in a CFBC. Using this model, overall SO 2 and NO x emissions are predicted for the combustion of three different kinds of low-grade Turkish lignites. The contents of these lignites are as follows: ash from 23.70% to 45.31%, sulphur from 1.81% to 8.40% and calorific values (LHV) from 10,283 to 15,215 kJ/kg. The data is obtained from two pilot scale CFBCs (50 and 80 kW) and an industrial scale CFBC (160 MW). The present study proves that CFB combustion demonstrated by both experimental data and model predictions produces low and acceptable level of SO 2 and NO x emissions resulting from the combustion of low-grade lignites. Developed model can also investigate the effects of different operational parameters on overall SO 2 and NO x emissions. As a result of this investigation, it is observed that increase of excess air decreases SO 2 and NO x emissions. However, NO x emission increases with the operational bed velocity while SO 2 emission decreases. A bigger inlet bed pressure value results in lower emissions of SO 2 and NO x if other parameters are kept unchanged.

A simplified correlation for fixed bed pressure drop

An experimental study was conducted on the pressure drop characteristics of a variety of vertical packed beds in turbulent flow of air. The materials of different particle diameter, D p, with a range of sphericity Φ, 0.55 ≤ Φ ≤ 1.00 were used in random loose packing to produce beds of different lengths, L, with a range of porosity, ε, 0.36 ≤ ε ≤ 0.56. In the covered test cases the cross-sectional velocity distribution at the exit plane of the packed beds and the pressure drop Δ P Bed were measured in a particle Reynolds number range of Re p, 675 ≤ Re p ≤ 7772. The particular emphasis of the study was given to determine the influence of ε, Φ, D p, L, Re p on Δ P Bed. In this respect the measurements of Δ P Bed were compared with the well-known Ergun's Equation and the data were expressed in terms of correlations through introduced dimensionless parameters of pressure coefficient, Δ P ⁎ and exit Reynolds number Re exit. The proposed correlations of Δ P ⁎ = Δ P ⁎( εRe p D p / L) and Re exit = Re exit( Re p D p / L) are found to be appropriate for the determination of Δ P Bed and mean exit velocity, U, respectively with an acceptable fit of experimental data in an error margin less than ± 20%. The methodology is presented in this paper as an alternative approach to the available literature on packed beds.

Field Assessment of Alternative Bed-Load Transport Estimators

Measurement of near-bed sediment velocities with acoustic Doppler current profilers (ADCPs) is an emerging approach for quantifying bed-load sediment fluxes in rivers. Previous investigations of the technique have relied on conventional physical bed-load sampling to provide reference transport information with which to validate the ADCP measurements. However, physical samples are subject to substantial errors, especially under field conditions in which surrogate methods are most needed. Comparisons between ADCP bed velocity measurements with bed-load transport rates estimated from bed-form migration rates in the lower Missouri River show a strong correlation between the two surrogate measures over a wide range of mild to moderately intense sediment transporting conditions. The correlation between the ADCP measurements and physical bed-load samples is comparatively poor, suggesting that physical bed-load sampling is ineffective for ground-truthing alternative techniques in large sand-bed rivers. Bed velocities measured in this study became more variable with increasing bed-form wavelength at higher shear stresses. Under these conditions, bed-form dimensions greatly exceed the region of the bed ensonified by the ADCP, and the magnitude of the acoustic measurements depends on instrument location with respect to bed-form crests and troughs. Alternative algorithms for estimating bed-load transport from paired longitudinal profiles of bed topography were evaluated. An algorithm based on the routing of local erosion and deposition volumes that eliminates the need to identify individual bed forms was found to give results similar to those of more conventional dune-tracking methods. This method is particularly useful in cases where complex bed-form morphology makes delineation of individual bed forms difficult.

Solid Velocity in Shallow Spouted Beds Consisting of Solids of Varying Density

The vertical component of solid velocity has been measured in shallow spouted beds by means of an optical fibre, for different geometric factors of the contactor (angle and inlet diameter) and under different experimental conditions (height of the stagnant bed, particle diameter and air velocity). The study has been carried out with glass beads and materials of lower density (high- and low-density polyethylene, polypropylene and extruded and expanded polystyrene) in the spout zone of shallow spouted beds. The calculation of velocity along the axis has been carried out by solving a force balance rising along the spout and the core of the fountain.

On the determination of the stable bed slope of a channel using mathematical model

The paper deals with an analysis of selected equations used for the determination of a stable longwise slope calculation of torrential rivers. Irregularity of the gradient, accompanied by heavy bed-load experiencing abrupt changes of the flow as a result of heavy rainfalls of short duration and high intensity, these are typical features impacting the behaviour and characteristics of torrential rivers. The determination of the stable bottom slope, when the river bed is kept unpaved but still provides resistence against harmfull effects of rapids, becomes an essential objective of the study. Three methods are used to determine the stable slope: the first is based on tangent tension (shear stress theory), the second observes a (critical) non-scouring cross-sectional velocity (critical mean channel velocities), and the third applies the bottom layer velocity, (the critical bed velocities). The mathematical hydraulic model HEC-RAS v. 3.1.3 has been used for the verification of the methods in the Jindřichovick&amp;yacute; creek case study in the Kru&amp;scaron;n&amp;eacute; hory.

When is stream invertebrate drift catastrophic? The role of hydraulics and sediment transport in initiating drift during flood events

Summary1. The term ‘catastrophic drift’ is used to describe the large‐scale displacement of invertebrates that occurs during periods of increased river discharge. However, the physical processes that lead to animals entering the water column at such times remain poorly understood. Specifically, the hypothesis that the movement of bed sediments during floods triggers a large increase in drift has lacked a rigorous field test.2. Using a portable flume, the hydraulic conditions and rates of bedload transport associated with small, frequent floods were createdin situwithin a reach of a gravel bed river. Experiments focussed on the patches of fine sediment which are the dominant source of bed material transported during small floods. The flume produced near bed velocities of up to 2 m s−1over the patches, increasing shear stress, initiating sediment transport and causing invertebrates to enter the drift.3. The total number of individuals lost from the bed, as well as the taxonomic composition of the drift, were influenced strongly by shear stress and bedload. The rate of loss from the bed was low at shear stresses &lt;9 dynes cm−2(0–4 individuals min−1from the 0.5 m2flume bed area). Once shear stress exceeded 9 dynes cm−2, the threshold that resulted in consistent bedload transport from the patches, the rate of loss of animals increased to a maximum of 56 individuals min−1. When bedload transport rates were at their highest, the taxonomic composition of the drift was more similar to the benthos than it was to the drift observed when bed material was stable.4. Absolute rates of bedload transport created by the manipulations were extremely low (&lt;7 g m−1 s−1) and typical of those measured during small, frequent floods. Events of this magnitude do not break up the armour layer across the reach as a whole and so exposed patches of fine sediment are the principal source of bedload material. Consequently, discharge events not considered as disturbances in geomorphic terms may initiate frequent episodes of so‐called ‘catastrophic drift’ from patches of stream bed.

Quantifying fluid and bed dynamics for characterizing benthic physical habitat in large rivers

Summary Sturgeon use benthic habitats in and adjacent to main channels where environmental conditions can include bedload sediment transport and high near-bed flow velocities. Bed velocity measurements obtained with acoustic Doppler instruments provide a means to assess the concentration and velocity of sediment moving near the streambed, and are thus indicative of the bedload sediment transport rate, the near-bed flow velocity, and the stability of the substrate. Acoustic assessments of benthic conditions in the Missouri River were conducted at scales ranging from the stream reach to individual bedforms. Reach-scale results show that spatially-averaged bed velocities in excess of 0.5 m s−1 frequently occur in the navigation channel. At the local scale, bed velocities are highest near bedform crests, and lowest in the troughs. Low-velocity zones can persist in areas with extremely high mean bed velocities. Use of these low-velocity zones may allow sturgeon to make use of portions of the channel where the average conditions near the bed are severe. To obtain bed velocity measurements of the highest possible quality, it is necessary to extract bottom-track and GPS velocity information from the raw ADCP data files on a ping-by-ping basis. However, bed velocity measured from a point can also be estimated using a simplified method that is more easily implemented in the context of routine monitoring. The method requires only the transect distance and direction data displayed in standard ADCP data-logging software. Bed velocity estimates obtained using this method are usually within 5–10% of estimates obtained from ping-by-ping processing.

Detection of the minimum gas velocity region using Gaussian spectral pressure distribution in a gas–solid fluidized bed

The undesired phenomenon of partial or complete defluidizing can occur in industrial applications involving fluidized beds. That could be prevented by increasing the gas velocity, provided an adequate observation of changes in the hydrodynamic of the fluidized bed is detected early enough. Since the value of gas velocity on defluidization of bed is near the gas velocity at minimum fluidization condition, it is very important to use a methodology that can promptly detect the minimum fluidization velocity region, so that shutting down the process can be avoided. The literature shows that the methods to detect the region of such a velocity are limited, and in practice, the traditional method for determination of minimum fluidization velocity (pressure drop across the bed versus superficial gas velocity) is still the most used. In the present work, a methodology is proposed to detect the region where the minimum velocity takes place in a gas–solid fluidized bed, based on the spectral analysis of pressure fluctuation measurements using a Gaussian distribution curve. Results indicate that the method is satisfactory, and it could be used as a more practical indicator of the region where the defluidization phenomenon occurs.

Cross‐shore sediment transport on natural beaches and its relation to sandbar migration patterns: 1. Field observations and derivation of a transport parameterization

Cross‐shore sediment transport processes are investigated with measurements of horizontal velocity and sediment suspension obtained with electromagnetic current meters and optical backscatter sensors on five different beaches across Europe. Data were gathered under a wide variety of hydrodynamic and morphological conditions spanning the swash, surf, and shoaling zones. Results show that the near bed velocity moments, normalized by the local energy level (〈ut2〉n), have consistent shapes (shape functions) when plotted against normalized cross‐shore depth (h/hb). According to the energetics approach, near bed velocity moments are good descriptors of sediment transport processes, consequently, the shape functions describe and quantify the cross‐shore distribution of the most relevant cross‐shore sediment transport processes. The pattern consists of net onshore transport in the swash zone, offshore transport in the surf zone, and onshore transport in the shoaling zone thereby producing divergence of sediment in the inner surf/swash zone and convergence in the breaker zone. In consequence, the shape function supports the breakpoint hypothesis for sandbar generation. This behavior is a product of the balance between multiple opposing mechanisms including undertow, coupling between mean flows and short (long) wave stirring, short (long) wave skewness, and long wave coupling with short wave variance. The cross‐shore structure of the measured and normalized cross‐shore sediment fluxes is consistent with the velocity moment shape functions, showing the robustness of the approach. Apart from the shortcomings bound to the energetics approach, the present parameterization is also limited due to the difficulty of defining with confidence a breaker depth (hb) in bar‐trough systems.

Two-dimensional coal combustion modeling of CFB

In this study, a dynamic 2D model for a CFB combustor has been developed which integrates and simultaneously predicts the hydrodynamics, heat transfer and combustion aspects. Hydrodynamic model used in this study has been developed in our previous studies. Simulation model calculates the axial and radial distribution of voidage, velocity, particle size distribution, pressure drop, gas emissions and temperature at each time interval for gas and solid phase both for dense bed and for riser. The model has been validated against the data from a pilot-scale 50 kW CFB combustor and an industrial-scale 160 MW CFB combustor. A sensitivity analysis is carried out using the model to examine the effect of different operational parameters and coal properties on bed temperature and the overall CO, NO x and SO 2 emissions from the combustor. As a result of parametric study, it is observed that by increasing bed operational velocity or excess air ratio, bed temperature decreases and CO emission increases. Bed operational velocity has a more significant effect on CO emission than to bed temperature. Another effect of increasing excess air is the decrease of SO 2 and NO x emissions. However, NO x emission increases with the operational bed velocity while SO 2 emission decreases.