Particle Size Of Bed Material Research Articles

Eulerian investigation of the mechanism of Ultra-low NOx emissions from CFB boilers with finer bed material

Industrial practice has confirmed that decreasing the bed material size substantially diminishes nitrogen oxide (NOx) emissions in coal-fired circulating fluidized bed (CFB) boilers. This reduction allows for compliance with the stringent ultra-low emissions standard (NOx < 50 mg/Nm3@6%O₂, dry) proposed by the Chinese government, all without the need for supplementary denitrification devices. Consequently, it becomes essential to delve into the mechanisms through which particle size, as a gas–solid flow characteristic, directly affects the chemical reaction dynamics within the coal combustion process. Due to the difficulty of field tests, this paper employs the Eulerian-Eulerian numerical method suitable for wide particle size distributions to investigate the impact of bed material particle size on gas–solid flow and NOx concentration. Furthermore, the generalized QC-EMMS drag model is utilized to predict the heterogeneous flow across varying particle size conditions. Industrial test data validated the simulation accuracy, with prediction errors of bed pressure drop, temperature, and NOx emission concentration ranging from 5 % to 8.1 %. The analysis revealed that the particle concentration distribution heterogeneity decreases as the particle size diminishes. The variation trends of NOx with particle size exhibit distinct differences between the lower dense phase region and the upper dilute phase region within the bed. For finer bed material particles, the high NOx concentration generated in the dense phase region experiences a more thorough reduction in the dilute phase region. Through the analysis of material concentration and reaction rate, it is believed that this phenomenon is related to the increased catalytic effect of fine ash particles in the dilute phase region, that is, the reduction in particle size leads to an increase in catalytic specific surface area, enhancing NOx reduction, thus achieving ultra-low emissions at the outlet. These research results provide theoretical support for further development of ultra-low emission coal-fired CFB boilers.

Investigation on solid dispersion in a CFB dense region with Bluetooth tracking method and MP-PIC simulation

Solid dispersion behavior in a dense region of a six-cyclone pant-leg CFB was investigated experimentally and numerically. The Bluetooth tracking technique was used to measure the solid dispersion characteristics. The numerical simulation was done by MP-PIC model and it was extended to study the influence of tracer particle size on solid dispersion characteristics after it was verified by the tests results. Effects of superficial velocity (Ug), initial bed inventory (Hm) and bed material particle size (dpi) were tested and compared. The results showed that wall constraint in width (x) direction is smaller than depth (y) direction and the dispersion coefficient Dx can be determined by analyzing X2 and dX2/2dt respect to time. The average particle displacement X/X0 increased with Ug, Hm and decreased with dpi. The Dx, in the range of 0.0021m2/s ∼ 0.0108 m2/s, has a linear relationship respect to Ug and Hm. A correlation formula for lateral dispersion coefficient was summarized and its results were compared with the present and literatures' work.

Application of imaging techniques for the characterization of lumps behaviour in gas–solid fluidized-bed reactors

Gas-solid fluidized-bed reactors are often used in waste pyrolysis and gasification processes thanks to their excellent mixing properties, which guarantee temperature uniformity. However, this latter property can fail when large objects, such as lumps, are introduced or form in the system. Understanding the motion characteristics and thermal behaviour of lumps in a high temperature fluidized-bed reactor can help determining how the presence of lumps impact reactors’ performance. This was the object of this study. In particular, this work aims to assess how process variables and physical properties impact the segregation behaviour, dispersion coefficients and heat transfer coefficients of these lumps during operation. The system used in this work is a down-scaled pseudo-2D fluidized bed operated at ambient temperature and at fluidization velocities ranging between 1 Umf and 10 Umf. Rutile sand with four different mean particle sizes (60 μm, 100 μm, 153 μm and 215 μm) was used as bed material. Fabricated lumps were introduced in the fluidized bed to reproduce realistic conditions, as when lumps form in a high-temperature fluid bed. The density ratio between the lump and the bed material particle was varied between 0.32 and 0.55 to account for different lump compositions. X-ray digital radiography and infrared thermography were used respectively to track the fabricated lumps and to obtain their temperature time evolution. The lump density was found not to have a significant effect on the lump dispersion coefficients or on the heat transfer coefficient. Optimal values of fluidization velocities that guarantee proper lump mixing and maximum heat transfer coefficient were obtained. This latter increases by up to 10 times if the optimal fluidization velocity is selected. An increase in the bed material particle size was found to cause an increase in the dispersion coefficients and a decrease in the heat transfer coefficient. The trend of the heat transfer coefficient as a function of the fluidization velocity was found to vary significantly between different bed material particle sizes. A new correlation for the Nusselt number as a function of the object Reynolds number and of the size ratio between lump and bed material was obtained. This correlation applies to cases where particle convection is the dominant mechanism of heat transfer. The results of this work provide important knowledge to minimize the impact of lumps on fluidized-bed reactors and to optimize their operation.

Partitioning strategy for investigating the prediction capability of bed load transport under varied hydraulic conditions: Application of robust GWO-kernel-based ELM approach

In this study, modeling capability of bed load sediment transport was investigated under varied hydraulic conditions through Grey Wolf Optimization based Kernel Extreme Learning Machine (GWO-KELM) and Standard Gradient Descent based Gaussian Process Regression (SGD-GPR) models. 966 sets of data, covering wide range of gravel-bed rivers and streams were utilized to test the developed methodology. The partitioning strategy was used in which, the available data was categorized into different quantitative intervals based on the hydraulic and sediment characteristics. Some specific quantitative intervals of the bed load transport rate, flow discharge, median particle size of bed material, the Shear Reynolds number and the ratio of hydraulic radius to median particle size were examined. The results demonstrated the great consistent performance of the proposed hybrid GWO-KELM model under different hydraulic conditions.

Experimental Study on the Effect of Bed Material and Fuel Particle Size on Load Change of Circulating Fluidized Bed

Experimental Study on the Effect of Bed Material and Fuel Particle Size on Load Change of Circulating Fluidized Bed

Investigation of the Oxygen Supply and Distribution in a Bubbling Fluidized Bed by Using Natural Ilmenite for Oxygen Carrier Aided Combustion

The application of oxygen carrier materials has one of its origins in the chemical looping combustion processes, which aim at inherent CO2 sequestration by realizing fuel oxidation in the absence of combustion air. This concept can be transferred to conventional fluidized bed combustion processes, the so-called oxygen carrier aided combustion. The replacement of the conventionally applied bed material with an oxygen carrier focuses mainly on large-scale circulating fluidized bed combustion plants. The main stated performance improvements include the reduction of carbon monoxide emissions even at lower excess air ratios and the enhancement of the combustion efficiency. Nevertheless, the ability of shifting oxygen in space and time is of certain interest especially in small-scale bubbling fluidized bed systems. Imperfect fuel mixing, limited residence time in the dense bed zone, as well as limited system complexity often mitigate their combustion performance. Process conditions in bubbling fluidized beds differ from those prevalent in circulating beds regarding bed material particle size distribution, fluidization velocities, or particle volume fraction in the bed. Within this study, the widely used oxygen carrier ilmenite serves as bed material in a laboratory bubbling fluidized bed combustion. The focus was on the oxygen distribution profiles and their shift by ilmenite into the bed during methane combustion. The variation of the excess air ratio, fluidization velocity, and bed height reveal a significant increase of the in-bed CO2 yield (max. 55%) within ilmenite experiments. Moreover, the oxygen conversion profile confirms the shift of the oxygen supply by ilmenite from the lower reactor part to the reduction dominated zone. The amount of shifted oxygen increases at lower excess air ratios. Thus, full conversion is not achieved as kinetics and limited residence time in the bed inhibit the fuel conversion.

Characterization of sand particles in a bubbling fluidized bed with diverging riser

The utilization of a diverging riser in fluidized bed technology has resulted in inherent hydrodynamic behavioural outcomes. Smooth fluidization of coarser particles and avoidance of elutriation of finer particles from the bed are some benefits over other conventional fluidized bed risers. For the same inventory height, the lesser pressure drops along the conical riser have resulted in better mixing of particles and gases than in columnar riser. The sound mixture of gas and solid particles demonstrates excellent hydrodynamic behaviour. The characterization with operating parameters of different hydrodynamic characteristics has been discussed in recent decades. Extensive research on the heat transfer phenomenon in diverging riser has not yet been done. This article investigates the gas-solid flow along with the heat transfer behaviour using numerical simulation. The article also describes the influence of working parameters on the above features. The findings indicate that the bed expansion ratio is inversely proportional to the bed material height and particle size. However, the bed is seen to expand more for the higher value of air velocity. Higher bed height and superficial velocity are also observed to be favourable for higher heat transfer coefficient.

Laboratory investigation of sand and water inflow into a hole of under-waterway tunnels

The effects of a cracked tunnel underneath a canal bed on water and sand inflow through the tunnel hole and distortion of the canal bed were experimentally examined. A circular hole in the centre of the base plate of the flume was used to simulate the cracked tunnel. In total, 108 tests were conducted to investigate four factors that affect canal bed morphology (four water depths, three bed material sizes, four hole diameters and three bed material heights above the tunnel hole). The water and sand discharges into the tunnel hole were measured. The scour bed geometry due to the cracked tunnel, including the erosion volume and scour hole dimensions, was also measured and analysed. The results indicated that the sand height, tunnel hole diameter and bed material particle size were the important factors affecting the sand flux, while Froude number and canal water depth were the parameters governing the water flow rate. The tunnel hole became blocked when the particle size was larger than 0·2 times the tunnel hole diameter, indicating that this could be used to temporarily block a cracked tunnel. Relationships were developed to correlate the investigated parameters and to estimate the flow rate and the volumes of water and sand passing through the tunnel hole.

Evaluation of bed load equations using field measured bed load and bed material load

ABSTRACT Bed load transport can be determined by direct measurement of bed load being transported by a stream. In India, measurement of bed load is rarely done. In the present study, bed load has been measured by unequal width incremental method using United States Geological Survey (USGS) bed load sampler (Modified-Helley–Smith bed load sampler) and bed material has been measured using scoop type sampler for the Ver stream in Mandvi region of Surat, Gujarat, India. From the field measurements and analysis, it is seen that the Ver stream has dominance of bed load transport with a Rouse number higher than 2.5 for all the selected cross sections. The measured average bed load transport rate is found to be 7.77 t/day. The measured bed load transport rate has been tested using two prominent bed load models. In 1950 Brown proposed the reach averaged approach which was augmented and supported by Recking in 2013. Both Brown's and Recking's reach averaged approaches have been used for computing bed load of Ver stream using particle size of bed load and bed material. Bed load computed using Recking's approach matches well with the observed bed load while Brown's model over-predicted bed load transport rate using bed load particle size. Overall, Recking's reach-averaged approach performs well using bed load particle size and bed material particle size.

Particle size distribution of bed materials in the sandy river bed of alluvial rivers

The particle size distribution of bed materials in the sandy river bed of alluvial rivers is important in the study of topics such as friction, river bed evolution, erosion, and siltation. It also can reflect the dependency relation between river bed sediment and flow intensity. In this paper, the critical pattern of sediment movement in the near-wall region of a sandy river bed was analyzed. According to the principle of momentum balance, the critical settling-rising condition of bed material in a sandy river bed was found to be instantaneous turbulent velocity equal to 2.7 times the sediment settling velocity in quiescent water. Based on a vertical instantaneous turbulent velocity with a Gaussian distribution, a theoretical relation for calculating the particle size distribution of bed materials in a sandy river bed without pre-known characteristic grain sizes was developed by solving a stochastic equation. The formula is verified using measured data, and the results show that the proposed formula was in accordance with the measured data. This study has theoretical significance and practical value for determining the bed material particle size distribution of the sandy bed of alluvial rivers.

THE EFFECT OF FLOW CONDITIONS AND GEOMETRIC PARAMETERS ON THE SCOUR VALUE DOWNSTREAM COMPOSITE STRUCTURES OF WEIR AND GATE

In this research, a study was conducted experimentally to investigate the scour hole dimensions downstream the combined structures which consist of weir and gate. Twelve models have been designed and every model is formed from composite weir consists of two geometric shapes and three types of gates which are rectangular, semi-circular and triangular in shape, where multi factors were studied to find out the effect of changing geometry for both weir and gate, discharge flowing in the flume and particle size of bed material on the dimensions of scour hole. The experiments was conducted in a laboratory channel was constructed from blocks and concrete with length of 18 m, 1 m width and 1 m depth. The laboratory models were installed after 7 m from the main gate which is controlling the passage of water from the main reservoir into the flume.At the beginning, the calibration process was conducted to identify the actual discharge values that pass in the flume, then experiments were conducted to calculate the discharge coefficient for each model, which represents one of the studied factors within the dimensional analysis of the variables to derive the empirical formulas to calculate the dimensions of scour hole.Then the experiments were conducted in order to derive formulas to investigate the depth and length of the scour hole which formed in the sand floor spreading as a layer of 30 cm in thickness for a distance 4 m downstream combined structure. Two samples of sand were used in the experiments with different median size of particles ( d50), the first of 0.7 mm and the second of 1 mm.Using the dimensional analysis by π theorem and IBM SPSS 21 program, Four nonlinear relationships were derived to calculate the dimensionless scour depth (SD / d50) and another four nonlinear relationships calculates the dimensionless length of scour (SL / d50)depending on the laboratory results for each of the relative discharge (Qr), Froude number in terms of mean size of particle of bed material (Frd), non-dimensional difference head between upstream and downstream of combined structure (HD / d50), dimensionless distance between the lower edge of the weir and the upper edge of the gate (y3 / d50), dimensionless head over the crest of compound weir (h/d50) and the discharge coefficient (Cd), where the resulted determination coefficients (R2) from these relationships were good.

Three-zonal engineering method of heat calculation for fluidized bed furnaces based on data on commercial investigations of heat generation distribution during biomass combustion

A three-zonal method of heat calculation of furnaces for combustion of biomass and low-caloric fuel in the fluidized bed is described. The method is based on equations of thermal and material balances that account for heat generation by fuel in the zone, heat-and-mass transfer heat exchange between the furnace media and surfaces that bound the zone, and heat-and-mass transfer between furnace zones. The calculation procedure for heat generation by fuel in the fluidized bed (FB) using the heat generation portion by the fuel is proposed. Based on commercial investigations, the main factors that affect the average temperature in the FB and the portion of fuel heat that is released in the FB are determined. Results of commercial investigations showed that the airflow coefficient in the FB should be recognized as the main operation parameter that affects the average temperature in the FB and, consequently, heat generation in the FB. The gas flow rate in the FB can be marked out as the second factor that affects the consumption degree of oxidizer supplied in the FB. Commercial investigations revealed that mixing is affected by the gas flow rate in the FB and the bed material particle size, which may be changed during the boiler operation because of the agglomeration of particles of sand and ash. The calculation processing of commercial investigations on a KM-75-40M boiler of a CHP-3 of an Arkhangelsk Pulp and Paper Mill (APPM), which was carried out using the inverse problem procedure by means of a developed computer program, determined the range of the fuel heat release share in the FB, which was 0.26–0.45 at an excess air factor of 0.59–0.93 in the bed, and the heat release share in the maximum temperature zone in the total heat release in the superbed space. The heat release share in the bed is determined as an approximating function of the excess air factor in the bed and the fluidization number. The research results can be used during designing boilers with the fluidized bed on biomass.

Hydrodynamics of a hybrid circulating fluidized bed reactor with a partitioned loop seal system

A circulating fluidized bed (CFB) with a hybrid design has been developed and optimized for steam hydrogasification. The hybrid CFB is composed of a bubbling fluidized bed (BFB) type combustor and a fast fluidized bed (FB) type gasifier. Char is burnt in the combustor and the generated heat is supplied to the gasifier along with the bed materials. Two different types of fluidized beds are connected to each other with a newly developed partitioned loop seal to avoid direct contact between two separate gas streams flowing in each fluidized bed. Gas mixing tests were carried out with Air and Argon in a cold model hybrid CFB to test the loop seal efficiency. Increase in solid inventory in the loop seal can improve the gas separation efficiency. It can be realized at higher gas velocity in fast bed and with higher solid inventory in the loop seal system. In addition, bed hydrodynamics was investigated with varying gas flow conditions and particle sizes in order to obtain a full understanding of changes of solid holdup in the FB. The solid holdup in the FB increased with increasing gas velocity in the BFB. Conversely, increase in gas velocity in the FB contributed to reducing the solid holdup in the FB. It was observed that changing the particle size of bed material does not have a big impact on hydrodynamic parameters.

Effect of waste incineration and gasification processes on heavy metal distribution

This study investigates the distribution of heavy metals during incineration and gasification. Because heavy metals (Cr, Pb, Cd) become volatile at high temperatures, their concentrations in fly ash increase as the operating temperature of incineration/gasification increases. By contrast, the efficiency of the bed-material mixing and the contact between the bed material and the heavy metals inside the fluidized bed increase when the operating-gas velocity increases; hence, the heavy-metal content in the bed material also increases. Our results show that the heavy-metal content in the fly ash increases with the bed-material particle size. The probability of contact between the heavy metals and the bed material is reduced at higher bed-material particle sizes; hence, the emission concentration of heavy metals increases. The heavy-metal concentration in the fly ash after gasification is approximately 1/10–1/30 of that after incineration. However, the proportion of the heavy metals trapped in the bottom ash after gasification is higher than that after incineration under any of the operating conditions, regardless of the volatility of the heavy metals. Overall, gasification lowers the emissions of heavy metals, but traps more heavy metals in the bottom ash than incineration.

Gasification of lignite in a dual fluidized bed gasifier — Influence of bed material particle size and the amount of steam

The dual fluidized bed steam gasification system, developed by the Institute of Chemical Engineering at Vienna University of Technology, was originally developed for the utilization of woody biomass as feedstock, but during practical operation of the industrial sized plants of this system it turned out that fuel flexibility is the key issue for economic breakthrough. Therefore, gasification tests were carried out with lignite at different operating conditions at the dual fluidized bed pilot plant at Vienna University of Technology. Tests were performed with an input fuel power of 90kWth. Olivine was used as bed material and the applied particle size was 370 and 510μm. The steam-to-carbon ratio was varied between 1.3 and 2.1kgH2O/kgcarbon. In addition to standard online measurements of the permanent gas components of the product gas, impurities like NH3, H2S and tar were also measured. It turned out that a lower amount of steam for fluidization caused a better performance of the gasification reactor in terms of product gas yield, carbon conversion and water conversion. The high catalytic activity of the lignite ash was also a reason for the high product gas quality producing low amounts of condensable hydrocarbons, like tar.

Experimental results of gasification of waste tire with air in a bubbling fluidized bed gasifier

Waste tire can be thermochemically disposed to solve environmental problems and to produce energy. Gasification is one of the methods to obtain a product gas with a high calorific value. The product gas is a potential resource for electrical energy production. This paper presents the experimental results of gasification of waste tire in a fluidized bed under air atmosphere. The effects of equivalence ratio (ER), gasification temperature and bed material particle size on the composition of the product gas are investigated. The equivalence ratio is varied in the range of 0.15–0.45. The composition of the product gas is determined with an online gas analyzer which measures CO, CO2, CH4, H2 and O2 components. The lower heating value (LHV) of the product gas is calculated by using the gas composition measurements. Lower ER values result in lower CO2 concentrations, higher CH4 and H2 concentrations and higher LHVs. This study shows the importance of gasification temperature at low ER values, produces data points for four bed material particle sizes to verify the relation between LHV and ER, and proposes generalized correlations for different ER ranges.

Experimental results of gasification of waste tire with air&CO2, air&steam and steam in a bubbling fluidized bed gasifier

Gasification is one of the thermochemical conversion methods to produce both product gas and solid char. The quality of the product gas widely differs depending on the type of the gasification agent used in the gasification process. Therefore, one of the important parameters in the design phase of the gasification system is the gasification agent. In the present study, a laboratory scale bubbling fluidized bed gasifier was used to gasify the waste tire with the gasification agents of air&CO2, air&steam and steam. Within this frame, the effects of gasification agents, bed material particle size, CO2 to air ratio, steam to air ratio, steam temperature and steam to fuel ratio on the quality of the product gas were investigated. In order to determine the composition of the product gas, an online gas analyzer, which can measure CO, CO2, CH4, H2 and O2 components, was used. By using the measured gas compositions, the lower heating value (LHV) was calculated to represent the quality of the product gas. Comparing the gasification agents of air&CO2, air&steam and steam, the LHV of the product gas was obtained 9.59, 7.34 and 15.21MJ/Nm3, respectively. The repetition and uncertainty calculation of the tests were performed.

Channel evolution of Des Moines Lobe till drainage ditches in southern Minnesota (USA)

Some drainage ditches in the intensively managed row-crop agricultural region of southern Minnesota evolved from a trapezoidal form to multi-staged channel forms similar to natural streams. Older ditches constructed in cohesive sediment of the Des Moines Lobe till tend to follow a channel evolution model developed by Simon and Hupp. Site cross sections, longitudinal water and bed profiles and bed material particle size were determined according to Harrelson and others at 24 older ditch reaches, 5 newly constructed ditch reaches and 13 natural stream reaches. Morphological features were hypothesized to change from trapezoidal form to flat bench banks, similar to benches found in natural stream channels. All data were statistically analyzed with respect to drainage area using regression, because channel form is directly related to drainage area for a given climate, geology and land use. Results show similar regression slope and intercept for bankfull channel width and bankfull cross-sectional area (CSA) of older ditches and natural streams compared to typical trapezoidal designed ditches. Evolved ditches developed a small floodplain bench above the ditch bed and adjusted their bankfull widths similar to natural stream channels with respect to drainage area. Old ditches showed a relatively strong R 2 (0.82, 0.68) for channel CSA and width, a weaker R 2 (0.45) for water surface slope, and little to no correlation with bed particle size. Channel form appears to have adjusted more quickly than bed facets and/or bed particle size distribution. However, stepwise regression determined that D84, width/depth ratio and mean bankfull depth explained 83 % of the variability of channel features across varying drainage areas. Findings suggest a possible reduction of long-term maintenance costs if older ditches are allowed to evolve over time. A stable ditch form similar to natural streams is typically self-sustaining, suggesting that prior to a scheduled clean-out, the ditch should be examined for hydraulic capacity, sediment transport and bank stability.

만곡하천의 자갈하상재료 분포에 따른 한계수류력 평가

The distribution of gravel-bed materials in mountainous river is formed by the process of deposition and transportation of sediment responding to stream power of the latest flood that is over the certain scale. The particle size of bed material was surveyed in the longitudinal points of river and detail points of a specific meandering section and used to estimate the critical velocity and stream power. Yang`s critical unit stream power and Bagnold`s critical stream power for gravel-bed materials increased with the distance from downstream to upstream. Dimensionless shear stress based on the designed flood discharge in Shields diagram was evaluated that the gravel-bed materials in most survey points may be transported as form of bedload. The mean diameter in the meandering section was the biggest size in first water impingement point of inflow water from upstream and the second big size in second water impingement point by reflection flow. The mean diameters were relatively the small sizes in points right after water impingement. The range of mean critical velocity was 0.77~2.60 m/s and critical unit stream power was big greatly in first water impingement point. The distribution of critical stream power, range of 7~171 , was shown that variation in longitudinal section was more obvious than that of cross section and estimated that critical stream power may be affected greatly in first and second water impingement point.

Evaluation of heavy metal distribution and biological toxicity in agglomeration bed material during artificial waste incineration in fluidized bed

This study discusses the impact of different operating parameters on the bed material particle size and heavy metal distribution, and evaluates the impact of bed material heavy metal on the environment through TCLP and Vibrio fischeri test. The experimental results show that the bed material particle size distribution inclines to smaller particle sizes as the operating temperature increases. When there is Na, the particle size increases due to the agglomeration of eutectic. As for the heavy metal distribution, the combination of the fine particle sizes ( 0.84 mm) and fine (<0.59 mm) particle sizes have the maximum leaching concentration. As for the biological toxicity, when the temperature is 700 °C or Na concentration is 0.3%, the biological toxicity is at its maximum, which may due to a high accumulation of heavy metals.