Fluidized Bed Combustion Process Research Articles

Economical Operation and Hazardous Air Pollutant Emissions of Biodegradable Sludge Combustion Process in Commercial Fluidized Bed Plant

Waste sludge is characterized by high moisture, volatile compounds, toxic compounds, and ashes. The efficient operation of a commercial fluidized bed combustion (FBC) plant is important for reducing operational costs. We selected a commercial FBC plant for industrial waste sludge combustion to investigate the mass balance of the FBC process and the performance of the air pollution control device. Based on fuel analysis, the flow rate of incineration air was calculated as 4567 Nm3/h. After FBC combustion, the flow rate of the incineration gas increased to 8493.8 Nm3/h. Analysis of the heat balance showed that some heat potential was lost through leakage during the combustion process. The temperature of the incineration gas decreased to 200 °C at the inlet of the air pollution control device. According to the hazardous air pollutant emission testing of sampling points, the operation factors of lime slurry injection for SOx and HCl in the semi-dry reactor were 64.20 and 4.81 kg/h, respectively. In the wet scrubber, the operation factors of NaOH for SOx and HCl were 23.88 and 3.14 kg/h, respectively. At these operation factors, the available waste generation in the semi-dry reactor and wet scrubber was optimized to 76.6 and 42.57 kg/h, respectively.

Utilizing Sewage Sludge Slag, a By-Product of the Circulating Fluidized Bed Combustion Process, to Efficiently Remove Copper from Aquatic Environment

Currently, one of the greatest threats to the aquatic environment is industrial wastewater containing heavy metals and other toxic substances. Hence, it seems necessary to search for ecological and cheap technologies for removing metals from wastewater. In this research, slag was used as waste obtained in the circulating fluidized bed combustion technology (CFBC), which is considered to be a modern, clean, and very effective method of incineration of municipal sewage sludge. The physicochemical properties of the waste material were characterized using selected analytical techniques. Next, the processes of adsorption of Cu(II) ions on slag in aqueous solutions were investigated. The results showed a high metal removal efficiency of 98.8% at pH 1.8 and slag dosage 5 g/L. Numerous studies have demonstrated that high process efficiency at a level of at least 90% is attainable. Based on the Langmuir equation, the maximum adsorption capacity was calculated to be 70.3 mg/g. Kinetic analysis revealed that the process fits better into the pseudo-second-order reaction model and the Freundlich isotherm. The intraparticle diffusion model was considered as a rate-controlling step for Cu(II) adsorption. In summary, the slag waste produced in the CFBC technology seems to be a highly effective adsorbent for potential use in adsorption processes to remove heavy metals from the aquatic environment. This solution is in line with the current European ‘zero waste’ strategy and the assumptions of a sustainable development economy.

Particle scale study of heat transfer in the fluidized bed combustion process

Particle scale study of heat transfer in the fluidized bed combustion process

Enhancing oxygen savings and carbon dioxide purity in biomass oxy-circulating fluidized bed combustion with an oxygen carrier

A pure oxygen (O2) supply for oxy-fuel combustion causes excessive power consumption and decreased net plant efficiency. The oxygen carrier-aided oxy-circulating fluidized-bed combustion (OCA-Oxy-CFBC) process can facilitate reduction in the O2 supply requirement. Moreover, the use of an oxygen carrier, instead of silica sand, as bed materials may improve oxygen transport in the combustor, leading to a reduction in the pure O2 supply required. This study aimed to investigate the effect of oxygen carrier addition on Oxy-CFBC characteristics, including temperature, pressure, solid suspension density, pollutant emissions, combustion efficiency, and O2 reduction for stable operation. As an oxygen carrier, iron ore was introduced into the downcomer pipeline using a dedicated hopper during the biomass Oxy-CFBC process under low excess O2 conditions. Although the O2 concentration in the flue gas decreased by 1.7 vol%, the CO emissions decreased from 6.8 mg/MJ to 2.8 mg/MJ when the silica sand was replaced with 33 wt% iron ore. Furthermore, the CO2 purity in the flue gas improved from 94.3 vol% to 95.7–97.4 vol% under a lower equivalence ratio (1.01–1.07) during stable OCA-Oxy-CFBC operation. Consequently, oxygen can be transported and supplied by adding an oxygen carrier during the biomass Oxy-CFBC operation, indicating O2 savings of 4.86 % for the total amount of O2 required.

Agglomeration-influenced transformation of heavy metals in gas-solid phases during simulated sewage sludge co-incineration: Effects of phosphorus and operating temperature

Phosphorus and operating temperature not only affect the agglomeration behavior but also the transformation and migration of heavy metals. Accordingly, this study examined the effect of temperature and phosphorus in a fluidized bed combustion process to understand the emission and distribution of heavy metals by both experimental and thermodynamic calculations. The experimental results indicated that the sodium-phosphate reactions occur before the sodium-silicate reaction in the solid phase when the ratio of P/Na was 1/2. A low-melting-point sodium phosphate component, such as NaPO3, leads to easier particle agglomeration than Na2O-SiO2. In terms of the emissions of heavy metals, Pb and Cd show a similar trend: both the amount of emission smaller than that without adding phosphorus and the amount of emission share an upward trend with the operating time increased during MSS fluidized bed combustion. However, with the presence of phosphorus, the emission of Cr shows slightly decreased, and then sharply dropped, after that, increasing with operating time increased. Generally speaking, the maximum amount of Pb and Cd emitted was at 900 °C, followed by 800 °C and 700 °C. The higher temperature would promote the volatilization of Pb and Cd to emit. On the other hand, Cr emitted at the beginning tended to increase but later decreases when the temperatures were 700 and 900 °C, which may be due to the emission of Cr being influenced by the different affinities of both Al and Cr, reacting with Na in a fluidized bed incinerator. As for the distribution of heavy metals in the solid phase, a higher concentration of heavy metals was found in both the coarsest and finest particles during the process of agglomeration/defluidization.

Prediction of biomass corrosiveness over different coatings in fluidized bed combustion

Energy production in biomass fired boilers is increasing rapidly due to the advantages of CO2 neutrality and renewability, however damaging agents present in biomass composition accelerates power plant components corrosion. This study evaluates the influence of the biomass burned in fluidized bed combustion processes on high-temperature corrosion, by means of thermodynamic equilibrium modelling, considering those reactions occurring between the combustion atmosphere and different protective coatings (isFeAl, isNiAl and isSiCrAl). Fuels composition and operating conditions from a 10 kW BFB boiler were introduced as input data to improve the performance of the model. Representative samples from agricultural waste, industrial wood and forestry wood waste were selected for evaluation. Results showed industrial wood waste as highly damaging for most coatings studied, with high risk of salt stickiness, deposits formation and release of acidic gases. The elevated volatiles percentage together with significant ash content determined might lead to a major ash components release to the gas phase, available to later condense in the metals surfaces. Silication of alkali and deposited alkali chlorides were the dominant corrosion mechanisms observed for most cases. An increase in alloys corrosion resistance was detected through the model when nickel or chromium was present, showing isSiCrAl as the most resistant. However, alloys protection exhibited significant variations depending upon the biomass burned, thus materials selection should consider the compatibility with conditions for its final use. Thermodynamic modelling, based on real conditions and fuels composition, provides a useful tool to identify key factors for protective coatings design when employing new waste fuels.

Review of Biomass Agglomeration for Fluidized-Bed Gasification or Combustion Processes with a Focus on the Effect of Alkali Salts

Review of Biomass Agglomeration for Fluidized-Bed Gasification or Combustion Processes with a Focus on the Effect of Alkali Salts

Effect of Natural Ilmenite on the Solid Biomass Conversion of Inhomogeneous Fuels in Small-Scale Bubbling Fluidized Beds

The application of oxygen carriers as alternative bed material in fluidized bed combustion originates from chemical lopping processes. They serve as oxygen transport agents undergoing consecutive redox cycles. Thereby, oxygen carriers can provide surplus oxygen in oxygen-lean areas of fluidized bed combustion processes. In turn, re-oxidation takes place in oxygen-rich reactor parts. A more homogeneous combustion and reduced CO emissions follow during steady-state operation. However, especially regarding solid biomass conversion, inhomogeneous fuel qualities result in transient combustion conditions. Therefore, this research deals with the influence of the oxygen carrier ilmenite on solid biomass conversion. Separated batch experiments with methane (volatile), char and wood pellets took place in a laboratory bubbling fluidized bed reactor. They reveal that ilmenite enhances the in-bed CO2 yield by up to 63% during methane combustion. Batch char experiments confirm that solid–solid reactions with ilmenite are negligible. However, heterogeneous gas–solid reactions reduce the O2 partial pressure and limit the char conversion rate. The batch wood pellet experiments show that the ilmenite oxygen buffering effect is mitigated due to high local oxygen demand around the pellets and limited pellet distribution in the bed. Finally, the continuous operation in a 100 kWth BFB with inhomogeneous fuel input indicates a higher in-bed fuel conversion and confirms lower CO emissions and less fluctuation in the flue gas during inhomogeneous fuel supply.

Processing of low-density waste in fluidized bed made out of lightweight expanded clay aggregate

The main aim of the study is to obtain a new, innovative method for the organization of combustion processes in fluidized bed reactors. A key aspect of the change in the organization of the fluidized bed combustion process is the creation of a fluidized bed made out of lightweight expanded clay aggregate (LECA). The new fluidized bed has a density of 0.5 g/cm3, therefore, allowing for thermal processing of low-density solid and liquid industrial wastes, as well as waste biomass. Polyolefins (polyethylene, linear low-density polyethylene, high-density polyethylene, polypropylene, and polypropylene homopolymer) were selected as examples of low-density solid wastes. The use of an innovative fluidized bed for liquid waste degradation was studied on the example of glycerol combustion. Waste wood chips were chosen to represent low-density biomass. Thermal degradation of selected materials in the fluidized bed made out of LECA was carried out at a temperature range of 500–900 °C with an interval of 100 °C. Video analysis of the fluidized bed surface proved the sinking of waste materials into the bed and significant elimination of diffusion flames in the freeboard. Qualitative and quantitative flue gas analysis was carried out using the FTIR spectrometer. More than 95% conversion of the carbon contained in the polyolefins to CO2 was achieved already at 700 °C. At 500 °C, more than 30% of the polymeric carbon left the reactor as aromatic compounds. To avoid the emission of harmful aldehydes during the thermal processing of glycerol, the bed should be heated to at least 800 °C. Waste wood chips as an alternative fuel can be thermally incinerated in the fluidized bed made out of LECA. The disappearance of volatile organic compounds and CO emissions occurred at 700 °C and 800 °C.

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.

Prioritized multi‐objective model predictive control without terminal constraints and its applications to nonlinear processes

AbstractMany process control problems encapsulate multiple and often conflicting objective criteria spanning different levels of relative importance. In this paper, we consider a class of multi‐objective receding horizon optimal control problems and propose a novel multi‐objective model predictive control (MO‐MPC) scheme for nonlinear systems subject to constraints and several prioritized (economic) criteria. Combining the lexicographic optimization and the receding horizon principle, a prioritized MO‐MPC scheme without terminal constraints is presented to solve economically optimal control problems of the constrained nonlinear system. The results on recursive feasibility and stability of the MO‐MPC are established in the context of economy optimization and no terminal constraints. Particularly, for the systems without state constraints, the computational burden of the MO‐MPC is reduced due to the removal of terminal constraints. Using an intuitive optimizaiton, the feasible set of initial states is offline estimated to move out the initial feasibility condition. The proposed MO‐MPC strategy is verified by the multiple control problems of a coupled‐tank system and a six‐order fluidized bed combustion process.

3D simulation on pressurized oxy-fuel combustion of coal in fluidized bed

Pressurized oxy-fuel combustion technology is considered as a perspective carbon capture technology in industrial process. A computational fluid dynamics (CFD) model based on Multi-Phase Particle-In-Cell (MP–PIC) method was developed to predict pressurized oxy-coal combustion process in fluidized bed. The heterogeneous and homogeneous combustion reactions of coal were considered in this model. The predicted results were validated the accuracy of this model with experimental data from a 15 kWth pressurized fluidized bed combustor in terms of the gas component and temperature characteristics. The characteristics of gas–solid flow and combustion under different pressure (0.1–2 MPa) and oxygen atmosphere were studied in this work. The predicted results show that the intensity of particle motion and the expansion degree in the fluidized bed was gradually decreased with an increase in pressure. A correlation was proposed based on the simulation results to maintain suitable fluidization conditions in pressurized circulating fluidized bed at different pressures. The temperature of particle phase region gradually increased with combustion pressure and inlet O2 concentration increased. In addition, the CO2 concentration in outlet increased while the emission of CO and NOx decreased as the combustion pressure increased.

Influences of secondary gas injection pattern on fluidized bed combustion process: A CFD-DEM study

Computational fluid dynamics coupled with the discrete element method (CFD-DEM) is applied to study coal combustion in a lab-scale bubbling fluidized bed. The predictions in terms of outlet gas compositions and temperature profiles are compared with the experimental measurements in the literature, and a reasonable agreement is achieved. The temporal and spatial properties of the coal combustion process are then investigated from the particle level, and the influences of the secondary gas injection parameters, including the secondary gas ratio, the jetting velocity, and the injection height are demonstrated. The results indicate that each coal particle has different entrainment behaviors, heating process, and chemical reactions, and the secondary gas injection pattern significantly influences the mixing between the oxygen and coal particles. The increase of the secondary gas ratio enhances the gas temperature fluctuation, resulting in severe hot spots which are undesirable in fluidized bed combustors. The typical formation process of a hot spot is further captured and analyzed. It is revealed that the hot spot is mainly caused by the violent combustion of volatile gas in the gas bubbles. These results provide valuable insights regarding the influence of the secondary gas injection and the bubbles on chemical reactions.

Simultaneous Measurement of Turbulence and Particle Kinematics Using Flow Imaging Techniques.

Numerous problems in scientific and engineering fields involve understanding the kinematics of particles in turbulent flows, such as contaminants, marine micro-organisms, and/or sediments in the ocean, or fluidized bed reactors and combustion processes in engineered systems. In order to study the effect of turbulence on the kinematics of particles in such flows, simultaneous measurements of both the flow and particle kinematics are required. Non-intrusive, optical flow measurement techniques for measuring turbulence, or for tracking particles, exist but measuring both simultaneously can be challenging due to interference between the techniques. The method presented herein provides a low cost and relatively simple method to make simultaneous measurements of the flow and particle kinematics. A cross section of the flow is measured using a particle image velocimetry (PIV) technique, which provides two components of velocity in the measurement plane. This technique utilizes a pulsed-laser for illumination of the seeded flow field that is imaged by a digital camera. The particle kinematics are simultaneously imaged using a light emitting diode (LED) line light that illuminates a planar cross section of the flow that overlaps with the PIV field-of-view (FOV). The line light is of low enough power that it does not affect the PIV measurements, but powerful enough to illuminate the larger particles of interest imaged using the high-speed camera. High-speed images that contain the laser pulses from the PIV technique are easily filtered by examining the summed intensity level of each high-speed image. By making the frame rate of the high-speed camera incommensurate with that of the PIV camera frame rate, the number of contaminated frames in the high-speed time series can be minimized. The technique is suitable for mean flows that are predominantly two-dimensional, contain particles that are at least 5 times the mean diameter of the PIV seeding tracers, and are low in concentration.

Fluidized bed combustion coal fly ash: comparative evaluation for potential use in alkali-activated binders

ABSTRACT The physical, chemical, mineralogical and toxicity attributes of two fluidized bed combustion (FBC) coal fly ashes were evaluated, and were compared against that of a conventional coal fly ash. The potential use of the FBC ash in alkali-activated binders was also evaluated. Alkali-activated mortars were prepared and tested for compressive strength at different ages. The alkali-activated binders were also evaluated using SEM, TGA and FTIR methods. This investigation accounted for: (i) the differences in the combustion temperatures of FBC versus conventional methods of coal combustion; (ii) capture of sulfur from combustion emissions in the FBC method, and its incorporation into fly ash as crystalline solids; and (iii) the type and particle size of the sorbent used in the FBC process for capture of sulfur from combustion emissions. FBC ashes were found to differ from the conventional coal fly ash in terms of chemical composition, crystalline phases and carbon content. The higher crystallinity and carbon content and the larger particle size of FBC ashes were noted to challenge their potential use in alkali-activated binders. Suggestions are made based on the experimental results towards effective use of FBC ashes in alkali-activated binders.Abbreviations: FBC: Fluidized Bed Combustion; SEM: Scanning Electron Microscope; TGA: Thermogravimetric Analysis; DTA: Differential Thermal Analysis; XRF: X-Ray Fluorescence; XRD: X-Ray Diffraction; LOI: Loss-On-Ignition; TDS: Total Dissolved Solids; EPA: Environmental Protection AgencyI; CP-OES: inductively couple plasma spectroscopy; DI: Deionized; CFB: Circulating Fluidized Bed; ASTM: American Society for Testing and Materials

Carbon dioxide purity and combustion characteristics of oxy firing compared to air firing in a pilot-scale circulating fluidized bed

This study aims to optimize the oxy-circulating fluidized bed combustion (oxy-CFBC) process by reducing the amount of flue gas with high-purity carbon dioxide. To achieve this, the stable transition from air mode to oxy mode is tested and validated in a 0.1-MW oxy-CFBC test rig. The results prove that flue gas carbon dioxide separation can achieve 96 vol% (dry) through a stable transition from air mode to oxy mode. Moreover, flue gas production emitted in oxy mode is reduced to one fifth compared to air mode. The proven technologies and oxy-fuel combustion database from this test rig operating experience can be used as empirical operating parameters for the next steps such as demonstration and commercial-scale operation.

Fluidized bed combustion bottom ash: A better and alternative geo-material resource for construction.

Though the majority of research on fly ash has proved its worth as a construction material, the utility of bottom ash is yet questionable due to its generation during the pulverized combustion process. The bottom ash produced during the fluidized bed combustion (FBC) process is attracting more attention due to the novelty of coal combustion technology. But, to establish its suitability as construction material, it is necessary to characterize it thoroughly with respect to the geotechnical as well as mineralogical points of view. For fulfilling these objectives, the present study mainly aims at characterizing the FBC bottom ash and its comparison with pulverized coal combustion (PCC) bottom ash, collected from the same origin of coal. Suitability of FBC bottom ash as a dike filter material in contrast to PCC bottom ash in replacing traditional filter material such as sand was also studied. The suitability criteria for utilization of both bottom ash and river sand as filter material on pond ash as a base material were evaluated, and both river sand and FBC bottom ash were found to be satisfactory. The study shows that FBC bottom ash is a better geo-material than PCC bottom ash, and it could be highly recommended as an alternative suitable filter material for constructing ash dikes in place of conventional sand.

Effects of gas-solid drag model on Eulerian-Eulerian CFD simulation of coal combustion in a circulating fluidized bed

The Eulerian-Eulerian two fluid model (TFM) has been widely used to simulate the fluidized bed combustion/gasification process, where the dense gas-solid flow, heat and mass transfer, and various chemical reactions are coupled. Significance of the drag coefficient in determining the gas-solid flow characteristics is well known. However, effects of the drag coefficient on the overall performance of the fluidized bed combustion/gasification are not emphasized. In the present work, two widely used drag models, namely the EMMS model and the Gidaspow model, are applied to simulate the coal combustion in a circulating fluidized bed combustor. Performance of the hydrodynamics, temperature, and species distribution by the two models is compared in details. It is found that the EMMS result has a better agreement with the experimental data compared to the Gidaspow result in this work. This work indicates that reliability of the gas-solid drag coefficient lays a solid foundation for the comprehensive simulation of the fluidized bed combustion process, not only for the gas-solid flow characteristics.

Effect of Chemical Compositions on Ash Fusibility Characterization of a Jincheng Anthracite during Combustion and Gasification

The ash melting temperature of coal ash has an important effect during the fluidized bed combustion and gasification process, which affects the slagging and deposition characteristics of the boiler. Experiments on the effects of chemical components on the ash fusion behaviors have been completed on the ash fusion temperatures (AFTs) analyzer under typical gasification and combustion atmospheres. Meanwhile, calculations on the variation of minerals in ash with ash composition were conducted using the FactSage software. The results indicated that the AFTs under gasification were a little higher than those under the combustion atmosphere. On increasing the Fe2O3, CaO, and Na2O contents under the combustion and gasification atmospheres, the four temperatures deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) decreased dramatically and the generation and transformation of minerals occurred. The iron-containing minerals, such as hercynite and fayalite, formed with increase in the content of Fe2O3; the Ca-bearing feldspar minerals, like gehlenite and anorthite, started appearing on increasing the CaO content, and the Na-containing feldspar minerals, like carnegieite, were detected as the Na2O was increased. These three minerals can form low-temperature eutectics, decreasing the fusion temperature.

Numerical simulation of soot combustion process in a bubbling fluidized bed with segregation behavior

AbstractAlthough only about 1 wt.% soot is formed after asphalt gasification, it is generally recognized that handling of the soot places a considerable financial burden on the integrated deasphalting–gasification process, because the disposal of the filter soot to landfill or other destinations causes serious environment problems. The bubbling fluidized bed combustion process, which takes the advantage of segregation behavior, was proposed to solve soot handling problem. A multi‐fluid model based on an Eulerian–Eulerian approach was developed to reveal the characteristics of segregation and combustion behavior. The distributions of flow, temperature, and species concentration were obtained, and the simulation results demonstrate that the segregation behavior is beneficial to discharge particle mixtures with low soot content at the bottom of the bed, which will favor the following vanadium recovering process. The effects of several key operating conditions, such as excess air coefficient, superficial gas velocity and initial static bed height were also discussed. © 2015 Curtin University of Technology and John Wiley & Sons, Ltd.