Fluidized Bed Unit Research Articles

The Analysis of Influence of the Gangue in the Coal as Fired on the Coal Conveying System and the Boiler Operation of the Circulating Fluidized Bed Unit

The coal as fired, with unidentified characteristics of the coal gangue, was burned on a 300MW circulating fluidized bed unit. The equipment of the coal conveying system was damaged and the boiler operation was unstable. In response to the problems, the coal quality data and storage conditions of the coal were examined and the site was spot-checked to evaluate the coal quality characteristics. At the same time, the typical representative parameters of the coal handling system and boiler operation were selected. According to the analysis of coal quality and coal storage, the coal quality fluctuates greatly and the uniformity of particle size distribution is poor. There is actually the coal gangue with hard texture and hard to grind in the coal pile. The coal gangue will have adverse effects on the fine screening machine, fine crusher and other equipment. After burned this type of coal, the fluidized quality of the boiler bed is degraded to make an impact on the safe and stable operation of the boiler. It is recommended that the coal should be screened and then burned into the furnace to ensure safe and stable operation of the boiler.

Comparison of solid phase closure models in Eulerian-Eulerian simulations of a circulating fluidized bed riser

In this work, computational fluid dynamics with Eulerian-Eulerian modeling approach is used in the investigation of the effect of the solid phase closure models on the hydrodynamics of a circulating fluidized bed riser. The studied models are the kinetic theory of granular flow and the powder modulus model. Computational performance and model prediction accuracy are investigated by performing transient simulations for three different cases and for different mesh sizes. The modeling results are compared with measured vertical pressure profile, external circulation mass flow rates, and particle velocity and concentration profiles obtained from a laboratory scale circulating fluidized bed unit. Additionally the computational times are compared to estimate the computational performance and to investigate if a simpler set of closures is computationally more affordable. The results indicate that both closure models can give good predictions for some quantities, while lacking in other quantities. The results indicate that the closure models for the solid phase have a noticeable effect on the model predictions, such as the vertical pressure profile and external circulation rate of the particles.

Mechanism modelling on the coordinated control system of a coal-fired subcritical circulating fluidized bed unit

To improve the load following capacity of a coal-fired circulating fluidized bed (CFB) power generation unit, it is necessary to build a model for the design of the load control system, e.g. the coordinated control system (CCS). In this paper, a mechanism-based control model of the CCS with the balance between model complexity and prediction accuracy applicable for controller design is established based on the dynamic characteristics of a subcritical coal-fired CFB unit. The proposed model was applied on a 300 MW coal-fired subcritical CFB unit with the existing control system. The results showed that the model can reflect the main dynamic characteristic of power plant project with 300 MW CFB unit with the existing control system, has high precision, and can be used to design the controller of load control system. Moreover, the transfer function of coordinated control systems is derived to design the control algorithm.

Low NOx Emissions from Pulverized Coal Moderate or Intense Low-Oxygen Dilution Combustion in O2/CO2 Preheated by a Circulating Fluidized Bed

This work reports a newly preheated system with pulverized coal oxy-fuel moderate or intense low-oxygen dilution (MILD) combustion for low NOx emissions. During the experiment, high-temperature fuel preheated by a circulating fluidized bed unit would burn out in a down-fired combustor. This preheating process of pulverized coal tended to form a strong reducing atmosphere and promoted the fuel temperature above the ignition temperature, which contributed to NOx reduction and MILD combustion formation. During the oxy-fuel MILD combustion process, the CO2 concentration of flue gas reached approximately 95%, the NO2 and N2O emissions were nearly zero, and the NO emissions were 107.99 mg/MJ. The combustion efficiency was 98.21%, which indicates that the combustion efficiency would not decrease in MILD combustion. Also, NO emissions were further reduced to 37.25 mg/MJ through a rational arrangement of tertiary gas positions. For pulverized coal combustion, the accurate definition of MILD combustion should consider the disappearance of flame front besides the feature of temperature distribution.

Estimation of the solid circulation rate in circulating fluidized bed systems

The results of experiments performed in four different fluidized bed units (two pilot plants and two cold flow models, with a wide range of operating conditions) together with correlations from literature are used to establish methods to estimate the solid circulation rate in a circulating fluidized bed system. The estimation is based on the measured pressure drop in the upper part of the riser as well as particle properties and geometry of the riser exit. Investigations included smooth as well as abrupt riser exit configurations.In case of a smooth riser exit geometry (direct gas solids separation exit), the assumption of a uniformly upflow of solids in the riser is sufficient for the estimation of the solid circulation rate. Further, for an abrupt exit (L-shape exit), two methods were developed. Both methods are based on the assumption that the rate of reflected particles at the top of the riser increases with the so-called exit Froude number. One method is based on a correlation between interstitial gas velocity in the riser, slip velocity, mean particle velocity in the riser, and exit Froude number where it was possible to keep the difference between measured and estimated solid circulation rate in the range of approximately +75% and −40%. Further, this method is not able to consider the local mass flow distribution in the riser. For the other method, the output of a detailed mathematical model, based on information from literature, was used to establish a correlation between, exit Froude number, exit reflecting coefficient and local mass flow to estimate the solid circulation rate. Using this method it was possible to keep the difference between measured and estimated solid circulation rate in the range of ±40%. For both methods, the accuracy of the measurement of the pressure drop in the upper part of the riser is a significant parameter for the prediction of the solid circulation rate.

Evaluation of Meat and Bone Meal as a Secondary Fuel with Olive Byproducts in a Fluidized Bed Unit. Performance and Environmental Impact of Ashes

Cocombustion of olive byproducts with meat and bone meal (MBM) was investigated in a fluidized bed system. The performance of the blends in terms of efficiency and emissions as a function of operating conditions was examined. Fly and bottom ashes were characterized and their potential uses and environmental impact were assessed through leaching tests on alkaline and acidic soils. All fuels burned mostly within the bed with a high efficiency. CO emissions were low, and SO2 emissions were negligible, while NOx emissions were below legislation limits, except those of olive kernel/MBM 80:20. The optimum performance for the blends was achieved when the MBM percentage in the mixture was 10%, reactor loading was 0.6 kg/h, and excess air was 30%. Fly ashes were rich in Ca, P, K, and Si minerals and Cu, Zn, and Sr trace elements. MBM ash consisted of high melting point calcium phosphates. Heavy metal values leached through both soils were below legislation limits for ash disposal. Ash materials could be used for s...

Oxy-co-Firing in Fluidized Beds: Control of Sulfur Emissions and Assessment of Corrosion Issues

The main objective of this work is to experimentally analyse the effect of firing blends of coal and biomass in oxy-firing atmospheres on SO2 emissions and deposition/corrosion issues. The research has been conducted in a lab-scale fluidized bed unit, and chemical reactivity of deposited species has been simulated at bench scale.Two very different Spanish coals were selected for the oxy-combustion experiments: anthracite, with a very low volatile matter, and lignite with high sulphur content. As concerns the biomass, a short campaign was conducted with forest residues, but most effort was done firing corn stover (with three different chlorine levels). The experimental matrix was designed to analyse the effects of the following parameters: atmosphere (air vs. O2/CO2), coal-to-biomass ratio in the blend (90/10% vs. 80/20%) and bed temperature (850-900°C).Control of SO2 emissions was largely dependent on the governing desulfurization mechanism, while chlorine in the biomass plays a role in the condensed alkali sulfates. No significant deposition rates were detected even for the higher biomass shares in the blend (20%). Only when corn stover with the highest chlorine content (2%) was fired, a thick deposition scale was found. For the original corn stover, oxy-firing barely affects the deposition rate and slightly reduces the corrosion risk in comparison to conventional air-firing.

Fast pyrolysis of sweet sorghum bagasse in a fluidized bed reactor: Product characterization and comparison with vapors generated in analytical pyrolysis

Lignocellulosic biomass can be used as an alternative for the production of fuels and chemical feedstocks using thermochemical conversion processes, such as pyrolysis. This study aims to perform the fast pyrolysis of sweet sorghum bagasse in a fluidized bed unit, comparing the results obtained with the products from analytical pyrolysis. Furthermore, the apparent activation energy of thermal decomposition of biomass was determined using two global reaction models. The estimated values of apparent activation energy ranged from 106.2 to 203.3 kJ/mol. The main compounds identified in the vapors generated in the analytical pyrolysis were acetic acid, isoprene, methyl pyruvate, furfural, 2,3-dihydrobenzofuran, 4-hydroxy-3-methylacetophenone and 5-hydroxymethylfurfural. In the bio-oil produced in the fluidized bed unit, the main compounds identified were benzene, acetic acid, isoprene, 3-methoxypropanal, toluene, furfural, 1,1-dimethoxycyclohexane and phenol. The difference between the identified compounds could be attributed to the solvent used, the efficiency of the condensation system, the occurrence of secondary reactions during the fast pyrolysis process in the fluidized bed reactor and the polymerization after the condensation of the vapors produced.

A conversion-class model for describing fuel conversion in large-scale fluidized bed units

Solid fuel conversion in fluidized beds is often modelled with the use of population balances, where the fuel conversion process is divided into a number of classes based on for example fuel particle size. The present work investigates and evaluates different methods for the discretisation of the fuel conversion process into classes, as well as the number of classes necessary to yield a satisfactory accuracy. A discretisation method, which defines classes based on the conversion degree (rather than size or density) and that is valid for all conversion regimes, is proposed. The results show that application of the proposed class division method for modelling biomass gasification in a fluidized bed gives an accuracy that is up to ten times higher than that given by a distribution with equally large classes. For all three conversion processes of biomass gasification (drying, pyrolysis, and char gasification), discretisation into 6 classes is sufficient to yield errors of around 1%, when compared to the continuous conversion curves given as input to the conversion class discretisation model (generated by a particle model in the present work). In line with this, when the conversion class model is used in a semi-empirical 1D model of indirect biomass gasification, the resulting char conversion in the gasifier does not change significantly when more than 3–6 char conversion classes are used.

Continuous testing of carbonate loop in the fluidized bed unit with closed circulation

Continuous testing of carbonate loop in the fluidized bed unit with closed circulation

The role of limestone during fluidized bed oxy-combustion of coal and biomass

The interest in bio-CCS technologies is growing due to their potential to reduce CO2 emission in power generation. Oxy-co-firing in fluidized-bed units is one of the available techniques to develop bio-CCS, offering wide fuel flexibility and low SO2 and NOx emissions. This paper discusses the results of an experimental campaign carried out in a lab-scale fluidized bed reactor. The work focuses on the influence of limestone when oxy-firing blends of lignite and corn stover. Two different types of limestone with two Ca:S molar ratios were tested, and operational conditions were selected to compare the mechanisms governing desulphurization. Emissions of SO2, NO and HCl, together with deposition rates and ash mineralogy are studied in the paper. SO2 capture increases with the Ca:S ratio and bed temperature, but to a different extent depending on the limestone fragmentation. The amount of NO emitted rises with the Ca:S ratio and the presence of calcined limestone (indirect desulphurization). The HCl concentration in the gas phase is dominated by alkali sulfation. Finally, the conditions for the highest desulphurization efficiency diminished the deposition rates, but increased the risk for chlorine-induced corrosion.

Investigation of the Combustion Performance of Residues from Vineyards and Processing Industry via Fluidized Bed Experiments, Factorial Design, and Modeling

ABSTRACTResidues from vineyards and wine/spirits industry, as being abundant in South European countries, were investigated for their application in thermal processes for energy production. The thermal behavior and the reactivity of the fuels and their blends were determined by thermogravimetric analysis tests. The combustion performance of the residues in terms of efficiency and emissions was studied by carrying out experiments in a fluidized bed unit. The effects of excess air ratio, fuel loading, and blending ratio were examined and a factorial design and modeling was developed in order to provide optimal values of process variables for minimizing emissions. The results showed that CO emissions were low, SO2 emissions were negligible, while NOx emissions from grape husks were high. A mitigation of CO and NOx levels could be achieved by decreasing excess air from 50% to 30% or reducing reactor loading. By increasing the percentage of grape husks in the mixtures, flue gas emissions were increased, however, combustion efficiency was maintained at high levels. Factorial design analysis and modeling indicated that the optimum combustion and emission performance was accomplished when the combustor was fed with vine shoots at reduced loading and excess air below 40%.

Catalytic pyrolysis of sugarcane bagasse and pinewood in a pilot scale unit

Recently, many studies have been published on the catalytic pyrolysis of biomass, to improve the quality of the liquid product from fast pyrolysis, especially with ZSM-5 catalysts. However, only a few studies in the literature were not limited to the use of woody biomass as feed.This paper describes a catalytic pyrolysis study with two different biomass types (sugarcane bagasse and pinewood) using a commercial ZSM-5 catalyst and silica as a reference in a pilot- scale circulating fluidized bed unit. The chemical characterization of the liquid products was performed by comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOFMS).The gas yields increased with temperature and the presence of ZSM-5 contributed to greater quantities of gaseous deoxygenation products. The coke yields decreased with reaction temperature, but were not influenced by the bed reactor material. Bagasse produced more gas and less coke than pinewood. The liquid yields passed through a maximum at 500°C.The catalytic pyrolysis with ZSM-5 generated a significantly greater amount of valuable aromatic compounds, such as toluene and C2-benzene isomers (xylenes and ethyl-benzene), which are petrochemical intermediates with high added values. The source of biomass proved to be relevant to define bio-oil chemical composition. Hydrocarbon and ketone yields were higher for pyrolysis of pinewood, while carboxylic acids, cycloketones, and O-heterocyclic compounds increased when bagasse was pyrolyzed.

Release behavior of Hg, Se, Cr and As during oxy-fuel combustion using Loy Yang brown coal in a bench-scale fluidized bed unit

This experimental study investigates, for the first time, the fate of trace elements (Hg, Se, Cr and As) during combustion of Loy Yang brown coal in a large bench-scale oxy-fuel fluidized bed. The results indicated that up to 70%, 28%, 7% and 2% of total As, Cr, Se, and Hg respectively were retained in the collected ash at 15–30% oxygen concentration in the oxy-fuel combustion atmosphere at around 850°C. With the increase in oxygen concentration in oxidant, the Hg adsorption in ash was found to decrease. Compared to bed ashes, higher Hg adsorption was observed in fly ashes containing higher carbon content. Therefore, a higher level of Hg capture is expected in oxy-fuel circulating fluidized bed combustion due to the circulating materials having higher carbon content.

Thermal Valorization of an Animal Sludge for Energy Recovery via Co-Combustion with Olive Kernel in a Fluidized Bed Unit: Optimization of Emissions

The thermal valorisation of an animal sludge was investigated through combustion and co-combustion experiments with an agricultural residue, olive kernel, in a fluidized bed unit. The performance of the fuels and their blends in terms of efficiency and emissions was tested under different operating conditions. The thermal behavior and the reactivity of the fuels as well as the distribution of product gases were provided by thermogravimetric–mass spectrometric experiments. The effects of excess air ratio, fuel loading, and blending ratio were examined, and a model was applied in order to provide optimal values of process variables for minimizing emission costs. During fluidized bed combustion, both fuels burned mostly within the bed with a high efficiency. CO emissions were low, SO2 emissions were negligible, and NOx emissions of animal sludge were similar or lower than those of olive kernel. An increase in excess air or high fuel feeding increased CO and NOx levels. Co-combustion showed an additive behavi...

CPFD modeling and experimental validation of gas–solid flow in a down flow reactor

This manuscript reports the fluid dynamics in the developed flow region of a cocurrent gas–solid down flow fluidized bed unit. Gas–solid flows are simulated using a Computational Particle Fluid Dynamics (CPFD) numerical scheme and experimental data from the CREC-GS-Optiprobes. This model represents clusters in a downer unit. It is hypothesized that in downers, clusters are formed via a Random Particle-Selection Method (RPSM) ensuring cluster dynamic stability. To accomplish this, a statistical particle-selection of clusters (SPSC) method is developed. This hybrid model is validated with experimental data obtained in a 2m height and 2.57cm diameter column. Observed time-averaged axial and radial velocities and solid concentration profiles are successfully simulated by the Hybrid CPFD/CREC-GS-Optiprobes Data Model. These findings support: (a) a narrow distribution of particle cluster residences, (b) the relatively flat radial solid concentrations and solid cluster velocities, and (c) a valuable approach for establishing slip velocities in downer units.

Conversion of methanol into light olefins over ZSM-11 catalyst in a circulating fluidized-bed unit

Methanol conversion and the reaction pathway were investigated in a pilot-scale circulating fluidized-bed (CFB) unit over hierarchical ZSM-11 catalyst. Experimental results indicated that ZSM-11 catalyst was highly resistant to external coke due to the formation of mesopores. Elevated temperatures favored the production of propylene and butylene and decreased the yield of ethylene. Additionally, no direct relations were shown between the formation of ethylene and other products under different pressures, suggesting that ethylene was a primary product produced at the initial of the reaction. Methylation-cracking and oligomerization were verified as the main reaction pathway for the formation of C 3 + alkenes., Methylation and oligomerization of olefins were dominated under high methanol partial pressure and consequently responsible for the production of higher olefins, while the b-scission of C 7 = for propene and butylene, and C 8 = for butylene were enhanced at low methanol partial pressure.

Utilization of exhausted dust from FCC flue gas as material for FCC catalyst preparation

In this paper, new matrix and corresponding fluidized catalytic cracking (FCC) catalysts were prepared by reutilizing the exhausted dust which was derived from fluidized catalytic cracking flue gas. The results showed that the new matrix has wider mesopore size distribution than pseudo-boehmite binder. With the increase of the additional amount of this new matrix, FCC catalyst possessed increased micro-activity test conversion of vacuum gas oil (VGO) from 68 % to 71 %. The catalytic cracking performance on a fixed fluidized bed unit showed that compared with CAT-1, the conversion of VGO on CAT-5 increased by 1.19 %, heavy oil yield and coke yield decreased by 2.16 and 1.65 %, respectively. Besides, the total liquid yield and light oil yield increased by 2.27 and 2.26 %, respectively. The reaction performance of CAT-3 and CAT-5 is obviously superior to CAT-1. The appropriate pore structures and crystallized Y zeolite component on this new matrix improve the cracking ability of FCC catalysts.

Experimental Study on Hydrogen Direct Reduction of Hematite in a Lab Scale Fluidized Bed Reactor by Estimating the Gas Consumption Rate

Hematite reduction using hydrogen was conducted and the various process parameters were closely observed. A lab scale fluidized bed unit was designed especially for this study. The optimal values of the gas velocity, reduction time and temperature were evaluated. The values which indicated the highest reduction rate were set as fixed parameters for the following tests starting with the reduction time of 30 minutes and 750 ℃ of temperature. Among these variables the one with the highest interest was the gas specific consumption. It will tell the amount of the gas which is required to achieve a reduction rate of over 90% at the optimal conditions. This parameter is important for the scale up of the lab scale unit. 1,500 Nm 3 /ton-ore was found to

Fuel Nitrogen Conversion in Chemical Looping with Oxygen Uncoupling of Coal with a CuO-Based Oxygen Carrier

The interest in chemical looping with oxygen uncoupling (CLOU) of coal as a method for CO2 enrichment has increased drastically during recent years. The objective of this work was to experimentally investigate fuel nitrogen conversion in the CLOU process of coal with a CuO-based oxygen carrier in a batch fluidized-bed unit. Three coals of different rank (anthracite, sub-bituminous, and bituminous) and their corresponding chars were used as fuels. Furthermore, experiments of coal combustion under air-fired conditions were performed to recognize the unique pathway of fuel nitrogen conversion in CLOU. Experimental results indicated that NO was formed in the largest amount of total NOx. For the Shenhua bituminous coal that was used, most of the NO, NO2, and N2O from fuel nitrogen was formed during the devolatilization stage of the coal. However, with Xuzhou sub-bituminous coal or Huaibei anthracite as fuels, a certain portion of NO was derived from char-N. Under the current experimental conditions (temperatur...