Circulating Fluidized Bed Combustion Research Articles

Mechanical Activation of CaSO4 Enriched Petroleum Coke Fly Ash for Development of Green Pellets

Mechanical Activation of calcium sulphate enriched petroleum coke fly ash (PCFA) generated from circulation fluidized bed combustion (CFBC) boiler was investigated to improve the surface reactivity of PCFA. The physico- chemical, mineralogical (XRD), morphological (SEM) characteristics of PCFA, activated PCFA and developed green pellets were studied in detail. It was found that the milling time of 45 minutes is efficient for the activation of PCFA and results in fragmentation of coarse particle into fine porous particles. Mechanical activation results in enhancing physical properties and reactivity of PCFA without any further chemical treatment. The mean particle size of activated PCFA with 45 min milling time was 12.51-12.60µm and the porous morphology were seen through SEM. The hydration phases like ettringite, portlandite and calcium silicate hydrate mainly contribute to the development of mechanical strength in activated PCFA pellets which were also confirmed through morphology of needle-shaped crystallites. The study recommends that mechanically activated PCFA could be used as an adsorbent material for heavy metals removal from waste waters.

Effects of ammonia co-firing ratios and injection positions in the coal–ammonia co-firing process in a circulating fluidized bed combustion test rig

Ammonia (NH3) co-firing is a promising technology for reducing greenhouse gas emissions in coal-fired power plants. Prior to commercialization, an experimental study on coal–NH3 co-firing in a pilot-scale circulating fluidized bed (CFB) combustion test rig was conducted for technical verification. The comprehensive combustion characteristics, including pollutant emission, combustion efficiency, and ash properties, of NH3 co-firing with sub-bituminous coal in a CFB combustion test rig and the CO2 reduction according to NH3 co-firing ratios under two different injection positions (dense bed zone (DBZ) and wind box (WB) with primary air) were investigated. When NH3 was injected at the DBZ, NO emissions decreased as the NH3 co-firing ratio increased and CO emissions increased more rapidly than with only coal-fired combustion. Compared with only coal-fired combustion, a 25.4% NH3 co-firing ratio at the WB position simultaneously reduced NO and CO concentrations, achieving the highest combustion efficiency without ash-related problems. However, N2O emissions increased by > 1.5 times, indicating the formation of N intermediates during NH3 burning. Therefore, with minor retrofitting, coal–NH3 co-firing at the WB position is a feasible solution for simultaneously reducing CO2, NO, and CO emissions in commercial CFB combustion plants.

Recycled mineral admixtures based on recycled clay brick

The utilization of construction demolition waste as recycled mineral admixtures can simultaneously reduce cement consumption and the environmental impact on the construction industry. Thus, recycled mineral admixtures based on recycled clay brick powder (CBP) were prepared and circulating fluidized bed combustion (CFBC) fly ash was introduced as the activity compensation and sulfate excitation component. The effects of the hydration of recycled mineral admixtures on the mechanical performance and drying shrinkage of mortar were investigated. The results indicated that the incorporation of CFBC fly ash into the blended cement paste accelerated the hydration of C3S and shortened the induction period. The fly ash also promoted the secondary hydration of C3A and the conversion of ettringite to the monosulfate phase. The CBP consumed portlandite to form a calcium–silicate–hydrate gel at a later age, whereas CFBC fly ash promoted the pozzolanic reaction to occur earlier. The incorporation of the CBP and CFBC fly ash increased the porosity and pore volume of the blended paste.

NOx and N2O emissions from Ca-rich fuel conversion in oxyfuel circulating fluidized bed combustion

A part of Estonia’s power generation is based on oil shale combustion in thermal power plants, resulting in high CO2 and pollutant emissions. The presented paper is a part of long-term experimental work to investigate the combustion of Ca-rich oil shale for air and oxyfuel combustion environments. The experiments were performed in a 60 kWth circulating fluidized bed (CFB) test facility under air, O2/CO2, and O2 with the recycled flue gas (RFG) modes of inlet O2 as vol.% ranging from 21% to 52%, and with RFG of 50% and 87%. The influence of different combustion atmospheres, excess oxygen volumetric ratios in the primary oxidizer (λPr), and dense bed temperatures (TDB), on the total NOx and N2O formations, have been studied in detail.The results show that specific concentrations of NOx (per MJ) were reduced by 14% in O2/CO2 mode, and 22% in O2/RFG compared to air combustion. N2O emissions were increased in OXY21 mode and reduced significantly from 20 mg/MJ to 4 mg/MJ with increasing inlet O2% to 52% under O2/CO2. NOx and N2O emissions were the lowest of all combustion experiments at high inlet O2% with RFG application (OXY87 + RFG). Under all tested atmospheres, NOx emissions were increasing with increasing excess oxygen in the primary oxidizer. N2O enhanced with excess oxygen, accounted for the homogenous gas-phase reaction of volatile-N and the heterogenous reaction of char-N. NOx emissions at low operating TDB were decreasing with increasing bed temperatures and increased at higher TDB > 770 °C in air combustion and TDB > 850 °C in OXY30. N2O emissions were slightly decreased at higher bed temperatures under air and O2/CO2 modes. Combustion efficiency enhanced at higher inlet O2% resulting in lower CO concentrations with reduced fuel burnout, and increased NOx and N2O emissions.Overall, NOx and N2O formations were more affected by the condition of operating parameters under both air and oxyfuel combustion experiments, and the obtained results are giving confidence that oxyfuel technology does not influence the release of nitrogen emissions from oil shale combustion. To lower the cost of CO2 impurities removal, the optimal operating conditions during oil shale CFB combustion can be confirmed after considering the discussed parameters. However, further CO2 purification resulting from oil shale oxyfuel flue gas stream is possibly required.

Thermal–Hydraulic Calculation and Analysis on Water Wall System of a 700 MWe Ultra-Supercritical CFB Boiler

Ultra-supercritical circulating fluidized bed (CFB) boiler combustion technology has the advantages of environmental protection and good commercialization. As one of the key aspects in ultra-supercritical CFB combustion technology, the water wall system directly determines whether the CFB boiler can safely and effectively transform heat energy. This paper studies the thermal–hydraulic characteristics of the designed water wall system of a 700 MWe ultra-supercritical CFB boiler, the largest one that will be built next year. At four loads of the water wall system, hydrodynamic parameters are calculated based on the successfully verified mathematical models. The results show that the hydrodynamic characteristics of the system are good. The temperature distribution of the working fluid and metal of the water wall system is acceptable and safe at each load, which can support the technical development and improvement of the 700 MWe ultra-supercritical CFB boiler.

Experimental investigation of circulating fluidized bed combustion of dry powders of coal slime

Effective combustion of coal slime in circulating fluidized bed (CFB) boilers is essential for the sustainable development of clean coal technology. To overcome issues related to burning wet coal slime in CFBs, we propose to dry coal slime in a low-temperature drying system and burn the dried fine powders in a CFB with a highly efficient separator. This study reports a series of experiments conducted in a 4MWth pilot-scale CFB combustion test facility to assess the feasibility. The results demonstrate that the dried slime particles, which have a particle size distribution similar to that of circulating ash particles, can be burned stably, efficiently, and cleanly. The combustion of fuels with size distributions similar to that of circulating ash particles increases solids circulation rates, betters solids concentration distribution, enhances heat transfer combustion and improves operation and SO2 and NOx controls. Therefore, the study presents a potential opportunity to advance CFB combustion technology.

One-dimensional numerical simulation of CFB combustion: Improvement of hydrodynamic models for accurate prediction of pressure and particle size distributions

This study aims to improve the accuracy of one-dimensional numerical simulations of circulating fluidized bed (CFB) boilers by evaluating models related to solid properties, such as attrition, fragmentation, cyclone efficiency, and bed material discharge. The study validates models for estimating solids holdup and particle size distribution (PSD) using experimental results from a 12 MWth CFB boiler. The study found that accurate prediction of solid axial holdup and local PSD requires proper consideration of attrition and fragmentation, and the simulation results show that attrition has a more significant effect on numerical analysis performance than fragmentation. By using appropriate values for sand attrition constant (5E-7), char attrition constant (6E-6), and coal:1.5, sand:1.2, and char(ash):1.2 fragmentation constants, the pressure distribution measured from experiments was best predicted. The simulation successfully predicted the particle PSD distribution at a specific location in the CFB boiler and pressure change by height, similar to the experimental results In addition, this study investigated the accuracy of 3D computational fluid dynamics (CFD) simulations using solid properties obtained through 1D simulation as initial conditions compared to those based on experimental conditions. The results revealed that 3D CFD simulations using the solid properties from 1D numerical analysis are more accurate, implying that 1D-3D co-simulation approach is a good strategy to increase the simulation performance.

A Study on the Co-Combustion Characteristics of Coal and Bio-SRF in CFBC

Bio-SRF based on livestock waste has low heating value and high moisture content. The concentration of toxic gases such as SOx, NOx, and HCl in the flue gas is changed according to the composition of fuel, and it has been reported. Therefore, the study of fuel combustion characteristics is necessary. In this study, we investigated combustion characteristics on the blended firing of coal and Bio-SRF (bio-solid refused fuel) made from livestock waste fuel in CFBC (circulating fluidized bed combustor). The raw materials for manufacturing Bio-SRF include agricultural waste, herbaceous plants, waste wood, and vegetable residues. Bio-SRF, which is formed from organic sludge, has a low heating value and a high moisture content. Bio-SRF of livestock waste fuel is blended with different ratios of coal based on heating values when coal is completely combusted in CFBC. In the result of experiment, the combustor efficiency of calculated unburned carbon concentration in the fly ash shows 98.87%, 99.04%, 99.64%, and 99.71% when the multi co-combustion ratio of livestock waste fuel increased from 100/0 (coal/livestock waste) to 70/30 (coal/livestock waste). In addition, the boiler efficiency is shown to be 86.23%, 86.30%, 87.24% and 87.27%. Through the experimental results, we have identified that co-combustion of livestock waste fuel does not affect boiler efficiency. We have systematically investigated and discussed the temperature changes of the internal combustor, compositions of flue gases, solid ash characteristics, and the efficiency of combustion and of the boiler during co-combustion of coal and Bio-SRF.

Operational optimization of air staging and flue gas recirculation for NOx reduction in biomass circulating fluidized bed combustion

Cost effective nitrogen oxide (NOx) reduction technology is crucial for industries and power plants owing to the stringent environmental regulations to which industries and power plants must adhere. Herein, the aim was to optimize the operational conditions for NOx reduction in forest biomass using circulating fluidized bed combustion (CFBC) combined with air staging and flue gas recirculation (FGR). The profiles of temperature and solid suspension density in the combustor, NO and CO emissions, fuel-N conversion, slagging and fouling tendencies, and combustion efficiency were comprehensively observed during combustion. The main operating parameters are the FGR ratios, optimal ratios of air staging [Primary air (PA):Secondary air (SA):Tertiary air (TA)] and FGR, and supply height of TA in the combustor. Notably, under the optimal operational conditions (air staging ratio; PA:SA:TA = 70:10:30, FGR ratio; 8%, and TA supply height; 8.1 m), the NO and CO emissions were simultaneously reduced to 25.9% and 18.2%, respectively, compared with that for the base scenario which excluded air staging and FGR. Furthermore, a high combustion efficiency of 99.72% was maintained without any ash-related problems at well-controlled bed temperatures owing to the constant solid circulation afforded by FGR. A thorough analysis of the this literature has been performed to ensure that the commercialization of the combination of air staging and FGR in biomass CFBC plants is feasible, which could be an inexpensive option for NO reduction.

A review on research and development of CFB combustion technology in China

The development history and present status of the circulating fluidized bed combustion (CFBC) technology in China are reviewed. This technology has been developed for nearly 40 years since the early 1980s in China, and its development history can be divided into five stages, including a learning stage, an improvement stage, a scale-up stage, a maturing stage, and an ultra-low emission and ultra-supercritical stage. There are currently >3500 units of CFB boilers operating in the country, with unit capacities covering the range of 35–2000 t/h in steam output, mainly used for coal firing. Among the units, the world's first 600 MW supercritical CFB boiler was successfully demonstrated in 2014 and was regarded as a milestone of CFBC technology in the world. CFB boilers are playing an important role in thermal and power generation in China. Several achievements in engineering application and fundamental research are achieved. The reviewed history and achievements could be used as references for researchers and engineers in future CFBC technology development. At the end of the paper, the prospects of CFBC technology in the area of carbon neutralization are discussed. Though the application in coal combustion is shrinking, CFBC technology will play an important role in CO 2 reduction and carbon neutralization. • Present status of circulating fluidized bed combustion (CFBC) technology in China is reported. • CFBC technology development history in China is introduced by five stages. • Major engineering applications of CFBC technology in each stage are reviewed. • Typical fundamental research and achievements on CFBC technology in China are introduced. • The prospects of CFBC technology for carbon neutralization are discussed.

Transient simulation of biomass combustion in a circulating fluidized bed riser

Interest in circulating fluidized bed (CFB) boilers as a power generation technology has sky-rocketed in recent years because of several advantages this technology offers over conventional boilers, such as increased gas-solid mixing, which results in higher combustion efficiency and the ability to use lower rank fuels. CFB combustors are operated at lower temperatures than conventional thermal power generation combustors, thus reducing NOx emissions, while SO2 emissions can be conveniently controlled through the addition of Ca-based sulfur sorbents within the combustor.This paper summarizes the modeling effort on a 50 kWth CFB combustor designed, built, and operated at CanmetENERGY in Ottawa, Canada. The numerical model employs the multiphase particle-in-cell (PIC) approach in the open-source Multiphase Flow with Interphase eXchanges (MFiX) Software Suite. The MFiX-PIC model parameters for the simulation are tuned against cold-flow experiments from CanmetENERGY using olivine sand as the inert bed material. It is shown that for the relatively coarse fluid meshes and large parcel sizes necessitated by the scale of the simulation, filter size dependent corrections to the drag law must be incorporated to ensure accuracy of the simulation results.The validated cold flow model is extended to simulate reacting flow with torrefied hardwood as the feedstock and to validate the combustion reaction scheme. The species concentrations at the riser outlet are compared against CanmetENERGY's experiments and show satisfactory agreement. The simulations demonstrate the ability of MFiX-PIC to accurately capture both the physics and chemistry of a CFB combustor at bench scales, which can be further extended to pilot- and industrial-scale systems.

The effect of different ages and water-binder ratios on the mechanical properties of circulating fluidized bed combustion desulfurization slag cement-soil

In this study, the effect of different ages and water-binder ratios on the mechanical properties of circulating fluidized bed combustion (CFBC) desulfurization slag cement-soil were discussed. Particle size distribution analysis, unconfined compressive strength test, scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were conducted to determine the outcome. The mechanical grinding treatment of CFBC desulfurization slag was found to improve its activity. The mechanical grinding time was controlled at 55 min. The mechanical grinding time did not exceed 75 min. With the increase in the total content of cementing materials, the unconfined compressive strength increased. The different cementing materials content of cement-soil corresponds to an optimal cement-binder ratio. At the age of 60d, the strength of test block which having 25 % and 30 % of total cementing materials respectively were determined to be 1.15 and 1.42 times higher than the cementing materials with 20 % content. Moreover, the strength was greatly improved. The total dosage of cementing materials and water-binder ratio must be controlled in the range of 25 % − 30 % and 1.0–1.3, respectively.

Theoretical Study on CO Participated N2O Reduction

ABSTRACT N2O is the main gaseous pollutant in circulating fluidized bed (CFB) combustion. Density functional theory (DFT) of quantum chemistry was used to theoretically study the role of CO interaction with N2O on the char surface in this paper. The rate parameters were calculated based on the transition state theory (TST) for kinetic analysis. The armchair configurations with six aromatic ring clusters were selected as the carbonaceous surfaces. This paper identifies a novel redox reaction pathway for N2O pre-adsorption on the char surface. By calculating the difference in activation energy between N2O-CO homogeneous and char catalyzed N2O-CO heterogeneous reduction, it is concluded that the char surface provides an active site for N2O reduction by CO and significantly reduces the activation energy for N2O abatement. In oxy-fuel CFB combustion, localized surface C-CO2 gasification of the char causes high concentrations of CO to enrich and defend the char surface, creating a Stefan flow that prevents O2 gas from approaching the char surface, which makes CO pre-adsorption possible. The reaction mechanism of the heterogeneous reduction on the char surface is discussed more comprehensively in terms of both N2O pre-adsorption and CO pre-adsorption conditions, which also enriches the mechanism of N2O abatement during CFB combustion.

A numerical simulation study of ash deposition in a circulating fluidized bed during Zhundong lignite combustion

In the present work, a modified critical viscosity model considering condensation, thermophoresis and inertia impaction has been established to simulate ash deposition on a probe perpendicularly placed in the flue path in a circulating fluidized bed (CFB) burning high-sodium (Na) Zhundong lignite (ZD). The key influencing factors including flue gas velocity (3 – 15 m/s) and temperature (1023 – 1123 K), probe surface temperature (773 – 873 K), particle temperature (1073 – 1173 K), particle size (5 – 127 µm), and particle chemistry were systematically simulated and also experimental observed. The simulation results show that the thickness of the deposit on the windward side reached 3.4 mm over 6 h, which agreed well with the experimental observation. The deposit incurred an increase in the surface temperature to 1000 K, reducing the heat flux by 62.5 %. The mean size of the deposited ash at the stagnant point was found to be 25 – 30 µm, signifying its role in the deposition initiation. This is attributed to the synergistic effect of particle impaction and particle sticking, since particles with a higher Stokes number (0 – 8) would have a higher impaction probability (0 – 0.3) but a lower sticking probability once their kinetic energy exceeds 11.4 × 10-9 J. Moreover, ash deposition is also found to be affected by flue gas temperature, followed by particle temperature and probe temperature to a less degree. In addition, particles with smaller sizes rich in Na tended to deposit on the probe than those particles with larger sizes and abundant in Si and Al, highlighting the role of Na in initiating ash deposition during ZD CFB combustion.

The morphological and mineralogical characteristics and thermal conductivity of ash deposits in a 220 MW CFBB firing Zhundong lignite

Formation of ash deposit on heat transfer surfaces during Zhundong lignite (ZD) combustion in circulating fluidized bed (CFB) boiler has affected boiler operation and its thermal efficiency. This work therefore characterised the morphology, mineralogy and thermal conductivity of the ash deposits in a 220 MW CFB boiler firing ZD for an improved understanding of deposit characteristics. By using XRF, SEM-EDS, XRD, and laser conductometer, results revealed that deposits on superheaters and reheaters were as thick as 3.0–4.5 cm. Moreover, the inner layer of the deposits consisted of fine ash particles enriching in Na, Ca and S in the forms of Na2SO4, CaSO4, and Na2Ca(SO4)2, whereas the outer layer comprising CaSO4, Na6Ca2Al6Si6O24(SO4)2, Fe2O3, SiO2 and Mg2SiO4 with more SiO2 and Al2O3 presented. Besides, thermal conductivity of the inner layer was ca. 1.83 W/(m · K), and that of the middle layer with an increased pore size was ca. 1.50 W/(m · K), both of them were independent of temperature at 550–900 °C and overweighed the out layer being 0.65–0.82 W/(m · K). This would lead to an increase in coal consumption of ca. 12.9%, highlighting the severity of deposit build-up in adversely impacting the heat transfers and deteriorating the fuel economics during ZD CFB combustion.

Numerical simulation method of a circulating fluidized bed reactor using a modified MP-PIC solver of OpenFOAM

There are some previous works for simulating a bubbling fluidized bed (BFB) reactor using OpenFOAM's MPPICFoam solver, but those for circulating fluidized bed (CFB) reactors are few. In this study, the MPPICFoam solver was verified with a BFB reactor by comparing the solver's simulation results with the experiment and simulation results from the literature. By introducing the inter-boundary particle circulation function and a new module for setting sand particle size distribution, simulations were performed for a 0.1 MW th pilot-scale CFB combustor under isothermal conditions. The modified solver for the CFB reactor gives similar results as the experiment and commercial computational fluid dynamics (CFD) tool, well-predicting the pressure distribution and solid volume fraction in the riser. • MPPICFoam solver reliability for lab-scale bubbling fluidized-bed reactor verified. • Accurate solver prediction of solid volume fraction (horizontal) distribution. • Model used for numerical simulation to study riser hydrodynamics of a CFB combustor. • Particle size distribution module and new circulating boundary conditions added. • Improved solver shows superior predicted experimental results in dense bed region.

The Characteristics of the After-Combustion in a Commercial CFBC Boiler Using the Solid Waste Fuel

A CFBC (Circulating Fluidized Bed Combustor) boiler for combusted SRF (Solid Refused Fuel) is designed for solid waste combustion and power generation. The boiler consumes about 200 tons/day of SRF and generates 60 ton/h of steam or 10 MWe in electricity. The boiler is designed to burn pelletized waste fuel made of municipal solid waste collected from a town with a population of 400,000. Heat and mass balance calculations over the combustor and at each boiler section were performed and compared between the designed and measured data to analyze the boiler’s performance. After-combustion, the most significant phenomenon in low-density waste-derived fuel combustion in a CFBC boiler was monitored. The heat and mass balance were the most appropriate tools to analyze the boiler performance. The flow rate of spray water at the de-superheater was a reliable indicator to quantify the after-combustion. The design modification of the boiler unit for after-combustion control in the existing boiler was based on the quantification of spray water. The load distribution of the de-superheater decreases from 1.76% to 0.87% in 89% MCR before the installation of the evaporator and 82* % MCR load distribution of each boiler part after installation. The result was effective for the control of after-combustion in the existing boiler.

Accuracy improvement of CFD modeling of a hydraulics with wood pellet combustion operating in a full-loop circulating fluidized bed combustor (CFBC)

Accuracy improvement of CFD modeling of a hydraulics with wood pellet combustion operating in a full-loop circulating fluidized bed combustor (CFBC)

Effect of carbonation curing on physicochemical properties of mineral admixture blended belite-rich cement

The present study investigates the effect of CO2 curing on carbonation degree, CO2 uptake, mechanical and physicochemical properties of mineral admixtures blended with belite-rich cement (BRC). Pulverized coal combustion (PCC) ash, circulating fluidized bed combustion (CFBC) ash, and blast furnace slag (SG) were utilized as mineral admixtures. The results revealed that the PCC ash blended with BRC attained the highest compressive strength under CO2 curing. CFBC ash was comparable to PCC ash in terms of the ability to improve the mechanical and physicochemical properties. TG analysis revealed that CFBC ash blended BRC was excellent for promoting carbonation and CO2 uptake. TG, MIP, XRD, and 29Si NMR results showed that the SG was the least reactive mineral admixture with BRC under CO2 curing and negatively affects the strength attainment outcome, carbonation degree, and CO2 uptake. 29Si NMR confirmed the potential of PCC ash and CFBC ash in enhancing the reactivity of BRC under CO2 curing than the SG. Therefore, it is concluded that the blending of PCC ash and CFBC ash is beneficial in promoting the CO2 sequestration characteristics and compressive strength of BRC.

Hydrochloric Acid Leaching of Philippine Coal Fly Ash: Investigation and Optimisation of Leaching Parameters by Response Surface Methodology (RSM)

Coal Fly Ash (CFA) is a hazardous waste from coal-fired power plants, but has increasingly become a popular supplementary cementitious material for cement in the construction industry. As a secondary resource of REE, its main advantage lies in its fine particle size that eliminates the need for costly and energy-intensive comminution. In this study, the potential of CFA from the Philippines as a secondary REE resource was investigated by direct leaching of REEs with hydrochloric acid (HCl). The CFA sample came from a coal-fired power plant with a circulating fluidized bed combustion (CFBC) technology. For the leaching tests, the effects of HCl concentration, leaching time and leaching temperature on REE extractions were elucidate optimized via Response Surface Methodology (RSM). The RSM results showed that the optimum leaching occurred at 3M HCl, 65 °C and 270 min with extractions of Nd, Er, Eu, Tb and Dy at 70.8%, 76.34%, 88.02%, 90.01% and 73.38%, respectively. According to these results, the CFA from the Philippines is a promising secondary resource of REE and the extraction method employed was effective in achieving a relatively high REE dissolution. Moreover, the empirical model that was established accurately predicted the dissolution of REE with an accuracy of 98.20%, 96.66%, 97.09%, 98.17% and 97.78% for Nd, Er, Eu, Tb and Dy, respectively.