Bed Material Particles 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.

Influence of bed material on biomass gasification in fluidized beds via a TFM-DEM hybrid model

In the biomass gasification process, mixing and segregation behaviors of fuel particles and bed materials greatly influence the performance. In this work, the Two-Fluid Model-Discrete Element Method (TFM-DEM) hybrid model is employed to evaluate the impact of bed material on biomass gasification in a fluidized bed reactor, where fuel particles and bed material particles are described by the Lagrangian method and Eulerian method respectively. The biomass particle size distribution is taken into consideration. The kinetic theory of rough sphere (KTRS) is employed to consider the rotation impact of bed materials. Flow behaviors of binary mixture and gas composition distribution are analyzed. The results reveal that the bed materials with greater size can hinder the particle mixing behaviors and make the biomass particles concentrate on the bed surface so as to increase the non-uniformity of temperature distribution in the bed. A good fluidization with smaller bed material particles is beneficial to hydrogen production.

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.

Current velocity field in section of Sai Gon river during operation of flood control structures

The largest economic center in Vietnam - Ho Chi Minh City, is facing increasingly serious riverbank erosion, one of the main reasons being the flow. Therefore, in this study, the flow velocity field on the Saigon River section is accurately analyzed in different time intervals with the help of MIKE 11 and MIKE 21 models. The simulation results show that The number of flow velocities in the middle of the river is 3-4 times greater than that of the two banks. However, between high tide and low tide, the flow on both sides of the river is faster than the main flow, especially in the upper part of the winding banks, such as the section from Ben Nghe sluice to Tan Thuan, sewer and river section from An Loi Dong Ward police station to Thu Thiem Bridge 2. The velocity value on the studied river section, in most cases, exceeds the allowable value of the non-erosion velocity of the bed material particles. , riverbanks, as well as suspended sediment particles. Therefore, the erosion process on both sides of the river will occur regularly and continuously, so urgent measures are needed to protect the riverbank. During the operation of the anti-flood sluice, the flow velocity will decrease slightly before sluices also appear as whirlpools. The flow velocity on the Saigon River has a complex distribution and changes from time to time depending on the flood discharge from Dau Tieng Lake and the tidal currents of the East Sea.

Investigation on lateral dispersion characteristics of fuel particles in dense phase zone of a large-scale circulating fluidized bed boiler under combustion conditions

The dispersion characteristics of fuel particles in the dense phase zone in circulating fluidized bed (CFB) boilers have an important influence on bed temperature distribution and pollutant emissions. However, previous research in literature was mostly on small-scale apparatus, whose results could not be applied directly to large-scale CFB with multiple dispersion sources. To help solve this problem, we proposed a novel method to estimate the lateral dispersion coefficient (Dx) of fuel particles under partial coal cut-off condition in a 350 MW supercritical CFB boiler based on combustion and dispersion models. Meanwhile, we carried out experiments to obtain the Dx in the range of 0.1218–0.1406 m2/s. Numerical simulations were performed and the influence of operating conditions and furnace structure on fuel dispersion characteristics was investigated, the simulation value of Dx was validated against experimental data. Results revealed that the distribution of bed temperature caused by the fuel dispersion was mainly formed by char combustion. Because of the presence of intermediate water-cooled partition wall, the mixing and dispersion of fuel and bed material particles between the left and right sides of the furnace were hindered, increasing the non-uniformity of the bed temperature near furnace front wall.

Investigation on a single char particle movement behavior in a bubbling fluidized bed under high temperature

The movement behavior of fuel particles in a fluidized bed combustor plays a decisive role in heat and mass transfer, thus affecting the combustion process and pollutant emission. We presented a CFD-DEM framework to investigate the movement behavior of a single char particle and carried out a series of experiments in a visualized fluidized bed to validate the model reliability. The char particle movement behavior under high temperature was evaluated quantitatively by new metrics. The force on a single char particle was obtained under different bed heights. The relationship between the force and movement behavior was studied through theoretical calculation. Finally, the effect of operation parameters, such as superficial velocity and temperature on the movement behavior and combustion were investigated. The force on the char particle was observed to increase exponentially along the bed height. Gravity indicated a more remarkable influence on movement behavior in the splash zone, while the pressure gradient force and collision force were the governing factors in the dense phase. The char particle was dominated by all the forces once moving to the bed surface. The char particle movement behavior was effect under the joint action of bubbles and kinetic energy of bed material particles.

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

Gasification of woody biomass in a novel indirectly heated bubbling fluidized bed steam reformer

Within this work, a novel 50 kWth indirectly heated bubbling fluidized bed steam reformer (IHBFBSR) is presented, along with its commissioning experiments. In the IHBFBSR, heat is provided through two radiant tube natural gas burners in the bed and the freeboard area. The aim of this innovative design is sufficient heat provision for biomass steam reforming and cracking reactions and heat loss reduction, thus allowing the possibility of scaling-up to an industrial level. Experiments were performed with two woody biomass feedstocks and two bed material particle sizes under different operating conditions (steam to biomass ratio, lambda, temperature), in order to identify the setup's main characteristics. Product gas composition and quality, as well as the cold gas efficiency of the IHBFBSR were in reasonable agreement to similar systems, however carbon conversion prediction needs further improvement. H2 production and tar removal are favoured by small bed material particle sizes as well as by char accumulation in the bed area. Furthermore, air injection above the bed led to improved H2/CO ratios and lower tar yields compared to when air is used as a fluidization agent. Overall, it was shown that the IHBFBSR technology constitutes a promising development in the field of biomass allothermal gasification.

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.

Estimation of Bed Load Transport in River Osun, South-Western of Nigeria Using Grain Size Distribution Data

Sediment transport rate depends on bed composition, flow hydraulics and sediment supply. There is a paucity of information on bedload transport in River Osun. In this study, bedload in River Osun was estimated using grain size distribution data to predict channel migration and mitigate flooding. Grab sampler was used to collect sediment samples at the sampling point across the river designated as T1-T4. Sieve analysis was carried out in triplicate on sediment from sampling points using standard methods. Discharge and cross-sectional area were measured between December 2017 and December 2018 at sampling stations using standard methods. The seasonal and bedload were estimated using standards equations. The percentage of bed material particles above 5mm and less than or equal to 2mm were 50 and 22.49%, respectively. The average median grain (d50) size was 2.4mm. The discharge and cross-sectional area across River Osun ranged (0.53-17.46) m3/s and (3.83-47.46) m2. The seasonal suspended and bedload across the river were (206.43×103 kg/annum) and 2,538.77×103(kg/annum), respectively. The estimated sediment load of River Osun could be useful in determining the dredging period at any point across the river where deposition of sediment could be monitored.

Fluidised Bed Gasification of Diverse Biomass Feedstocks and Blends—An Overall Performance Study

The aim of this work is to investigate the fluidised bed gasification of several pure and blended feedstock prepared in the form of pellets: oak bark, two bark/wheat straw blends (85/15 and 50/50 wt%) and lignin residue remaining from bioethanol production. Gasification conditions were defined to be representative of dual fluidised bed ones (steam gasification at 850 °C, followed by air combustion of the char). The cold gas efficiency (77–81%), gas composition and tar content (0.9–2.3 g/kgdaf) are close for the gasification of bark and the two bark/wheat straw blends. For lignin residue, the cold gas efficiency is lower (71%), and the tar content is 9.1 g/kgdaf. The agglomeration propensity is much higher for lignin residue than for the other feedstock. This was put into evidence with in-bed temperature measurements at different levels, and confirmed with post-test size screening of the bed material particles. The 50/50 wt% bark/wheat straw blend seems to undergo defluidisation in combustion, however followed by refluidisation of the bed. These findings were also well correlated with a predictive model for defluidisation.

Surface characterization of ash-layered olivine from fluidized bed biomass gasification

The present study aims to present a comprehensive characterization of the surface of ash-layered olivine bed particles from dual fluidized bed gasification. It is well known from operation experience at industrial gasification plants that the bed material is activated during operation concerning its positive influence on gasification reactions. This is due to the built up of ash layers on the bed material particles; however, the chemical mechanisms are not well understood yet. Olivine samples from long-term operation in an industrial-scale gasification plant were investigated in comparison to fresh unused olivine. Changes of the surface morphology due to Ca-enrichment showed a significant increase of their surface area. Furthermore, the Ca-enrichment on the ash layer surface was distinctively associated to CaO being present. The presence of CaO on the surface was proven by adsorption tests of carbon monoxide as model compound. The detailed characterization contributes to a deeper understanding of the surface properties of ash layers and forms the basis for further investigations into their influence on gasification reactions.

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.

Microscopic investigation of layer growth during olivine bed material aging during indirect gasification of biomass

Olivine bed material used in an aging experiment for indirect gasification in the Chalmers 2–4-MWth DFB gasifier was investigated with the aim to determine the mechanism of layer formation around the particles upon exposure to gasification conditions. The collected samples were exposed for 1, 2 and 4 days. The development of ash layer around the bed material particles was studied with different analysis methods. Formation of Ca3Mg(SiO4)2 and MgO was confirmed by X-ray diffraction (XRD). Cross-sections of the bed material samples were prepared using Broad Ion Beam (BIB) milling and were further analyzed with Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM-EDS). The analysis of the produced cross-sections revealed the formation of a Mg-rich surface layer on top of the Ca-rich ash layer. Minor amounts of K were also found near the sample surface. Based on the results from the characterization techniques, a reaction mechanism involving the transition of Mg2SiO4 with CaO to MgO and Ca3Mg(SiO4)2 was suggested which was confirmed by equilibrium calculations. This mechanism was supported by Transmission Electron Microscopy (TEM) analysis where diffraction patterns corresponding to MgO were found. TEM-EDS line-scan revealed the presence of ash components in the ash layer such as P and Ti at locations coinciding with high levels of Ca which indicates the formation of Ca3(PO4)2 and CaTiO3. The results presented provide detailed information on the composition of the ash layer which can be used to fully understand the mechanism responsible for the formation of catalytically active ash layers.

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.

Dual fluidized bed steam gasification: Change of product gas quality along the reactor height

The impact of the counter-current column of the gasification reactor of a 100 kWth dual fluidized bed steam gasification pilot plant on the product gas quality was investigated. Through the advanced design of the gasification reactor by operating the lower part as bubbling bed and the upper part as counter-current column, the gas-solid interactions between downward flowing hot bed material particles with upwards flowing product gas could be enhanced. This was realized by equipping the counter-current column with constrictions, which increase the residence time and the bed material hold-up. Thus, the conversion efficiency of the fuel including the tar was improved. For the investigations three different experimental campaigns converting softwood pellets using a mixture of olivine and limestone (50/50 wt.-%), a mixture of feldspar and limestone (50/50 wt.-%), and 100 wt.-% quartz as bed materials were conducted. Higher H2 contents and lower contents of higher hydrocarbons could be detected along the height of the counter-current column. Especially heavy tar compounds could be reduced significantly. These two effects are explained by enhanced water gas shift and steam reforming reactions. In case of catalytically inactive quartz, only thermal effects are available and therefore lower effects on tar reduction could be obtained.

Hydrodynamic Study of a Circulating Fluidized Bed at High Temperatures: Application to Biomass Gasification

Experimental data on the hydrodynamic behavior of dense and circulating fluidized beds at high temperatures are scarce in the literature. This work deals with the hydrodynamic study of a Fast Internally Circulating Fluidized Bed (FICFB) used for biomass gasification. The first part of this study investigates the influence of the bed temperature (between 20 and 950 °C) and the nature of fluidizing gas (air or steam) on the hydrodynamic parameters of a dense fluidized bed of olivine particles (i.e. minimum fluidization velocity and voidage as well as average voidage). Three olivine batches are used with a mean Sauter diameter of 282, 464 and 689 μm, respectively. Experimental results are compared with different empirical correlations from the literature to evaluate their validity under elevated temperature conditions. Besides, two dimensionless correlations calculating the minimum fluidization velocity and average bed voidage are proposed. The second part of this study focuses on the hydrodynamic behavior of an FICFB operating between 20 and 850 °C. The effect of different process parameters (i.e. bed material nature, air velocity, solids inventory, bed temperature) on the solids circulation flow rate is investigated. It was found that the transport velocity Utr is not affected by the bed temperature and the bed material inventory. It mainly depends on the terminal settling velocity Ut of bed material particles. Besides, key parameters controlling solids flow rate are the combustor gas velocity and the solids inventory. An increase in these parameters leads to a higher circulation flow rate.

Bed material as a catalyst for char gasification: The case of ash-coated olivine activated by K and S addition

In this paper, the ability of an ash-coated olivine to catalyze the steam gasification of biomass-derived char is investigated in a laboratory reactor. The olivine investigated is a sample from the Chalmers dual fluidized bed gasifier and it has been activated by the in-bed addition of S and K2CO3. The char and bed material samples were analyzed by Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS). It is shown that the ash layer coating of the olivine can catalyze the steam gasification of char by transferring catalytic potassium (K) to the char particles. The mobilities of the catalytic species from the olivine ash-layer are discussed. This work furthers the current understanding of the catalytic activities of ash-coated bed material particles during the thermochemical conversion of carbonaceous feedstocks in fluidized beds. In addition, it complements the existing literature on catalytic bed materials, which to date have focused on tar removal and improving gas quality.