Circulating Fluidized Bed Research Articles

Dynamic Simulation of MFT and BT Processes on a 660 MW Ultra-Supercritical Circulating Fluidized Bed Boiler

In order to study the dynamic characteristics of the 660 MW ultra-supercritical circulating fluidized bed (CFB) boiler when the main fuel trip (MFT) and boiler trip (BT) are triggered, a dynamic simulation model of the 660 MW ultra-supercritical circulating fluidized bed boiler was established on the Apros simulation platform. The model dynamically simulated the MFT and BT processes at 100% BMCR, 75% THA, and 50% THA conditions, respectively. The steady-state simulation results showed a high accuracy compared with the designed parameters. The dynamic simulation results showed that after triggering the MFT and BT, owing to the huge thermal inertia, the bed temperature and steam temperature decreased lowly. For 100% BMCR and 75% THA conditions, the moisture separator always worked in dry state during the MFT and BT processes. For the 50% THA condition, the moisture separator quickly switched from dry to wet operation after the boiler triggers MFT and BT and gradually switched from wet to dry operation after MFT and BT were reset.

Scale-up prediction of supercritical CO2 circulating fluidized bed boiler based on adaptive PSO-SVM

Supercritical CO2 (S-CO2) circulating fluidized bed (CFB) boiler has broad prospects in the field of coal-fired power generation because of its high combustion efficiency, compact structure, and low pollution emission. Scale-up regularity research based on combustion characteristics of S-CO2 CFB boiler is the key to make it a wide range of industrial applications. In order to simplify the complicated workload and save the huge time cost of numerical simulations, it is of great significance to accurately make the scale-up prediction according to the operating performance of the S-CO2 CFB boiler. This study proposed a scale-up prediction model corresponding to the combustion characteristics of S-CO2 CFB boiler based on adaptive particle swarm optimization support vector machine (APSO-SVM). The parameters of the particle swarm algorithm were processed adaptively firstly combined with the boiler characteristics, and then the adaptive particle swarm algorithm was integrated with the support vector machine to solve the precocious convergence problem of particles being easy to gather in a certain position in the process of pattern recognition. The novel method effectively predicts the boiler in the scaling process from the aspect of boiler capacity, optimizes the scale-up regularity expression by numerical simulations, greatly saves time cost and applicability of enlarge design by altering complex numerical simulations, and lays the application foundation of S-CO2 CFB boiler in the industrial field with acceptable operation accuracy.

Deep learning‐based tomographic imaging of ECT for characterizing particle distribution in circulating fluidized bed

AbstractThe gas and solids in a circulating fluidized bed (CFB) are heterogeneously dispersed and a multiscale flow regime may form both in time and space. Accurate measurement of the fluidizing process is significant for investigating the multiscale gas–solid flow characteristics and the design, optimization, and control of CFBs in various applications. This article develops a deep learning‐based tomographic imaging of electrical capacitance tomography (ECT) to characterize the particle concentration distribution in a CFB. The deep tomographic imaging approach is realized through training a well‐designed convolutional neural network (CNN) with the numerically built dataset. Simulation results demonstrate that the average values of the relative image errors reconstructed by CNN in the test set are 0.1110 and 0.1114 for the 60 and 100 mm pipes, respectively, which are better than the average values of 0.1819 and 0.2519 by the Landweber algorithm. With the verification of the trained model based on the prepared data can image the unseen typical flow patterns better than Landweber, it is further used to investigate the particle flow characteristics of a lab‐scale CFB. Experimental results reveal that the developed deep tomographic imaging of ECT can successfully measure the fluidized particle distribution in both the 60 and 100 mm pipes, showing good prediction and generality of the designed CNN model. A flow regime transformation from “annular” flow to “core‐annular” flow and pneumatic conveying is observed under the tested conditions. Besides, the flow regime would be highly affected by the fluidized gas flow rate and the initial bed height.

Solid Phase Macromixing Study in a Pilot-Scale Geldart Group B Circulating Fluidized Bed Riser Using Single Particle RTD and RPT Measurements

Solid flow in a Geldart’s group B circulating fluidized bed (CFB) riser is complex, and it exhibits backflow and recirculation in the riser. A single radioactive tracer particle is used to measure the overall and sectional residence time distribution in a CFB riser at a gas velocity of 7.6–9.2 m/s and a solid flux of 100–200 kg/m2s. At the same time, radioactive particle tracking (RPT) data are used to measure the trajectories of the tracer particle and its length distribution at the bottom and middle sections of the riser. Both residence time distribution (RTD) and trajectory length distribution data obtained from RPT and RTD experiments are processed and compared. Results show that the bottom section has higher back mixing than the middle section. The results also show that back mixing in both the sections reduces with an increase in the gas inlet velocity and reduces marginally with an increase in the solid flux. Results confirm that RPT and RTD data are highly correlated and can be used with the same accuracy to quantify the macromixing behavior of any process vessel/reactor.

Magnetic properties of ilmenite used for oxygen carrier aided combustion

Oxygen carrier aided combustion is a combustion process that utilizes oxygen carrying particles in a fluidized bed to transport oxygen from oxygen-rich to oxygen-poor regions in the reactor. A commonly used oxygen-carrying material is ilmenite (FeTiO3) which is a naturally occurring mineral. At higher oxygen partial pressures ilmenite can react to pseudobrookite (Fe2TiO5) and thereby take up oxygen. Upon reduction of pseudobrookite in oxygen-lean locations the oxygen is released, which enhances the distribution of oxygen through the reactor.Ilmenite was used as bed material in an industrial 115 MWth circulating fluidized bed (CFB) boiler where recycled waste wood and wood chips were utilized as fuel. Bottom ash samples were extracted after one and two weeks and the samples were separated into two fractions by a magnetic separator. The magnetic fraction was expected to be enriched in iron-containing oxides and was therefore aimed to be recirculated into the boiler.The SEM-EDS analysis revealed that the non-magnetic fraction consists to the largest extent of feldspar (KAlSi3O8) particles. A significant amount of freshly introduced ilmenite particles was also classified as non-magnetic by the magnetic separator. Characteristic for these particles was a lack of ash layer, suggesting they had only recently been added to the system. In the magnetic fraction, several feldspar particles were found which were covered by a layer rich in Ca, Fe, Ti, and Si. Comparing the XRD analysis of the ash prior to magnetic separation with its magnetic fraction revealed a decrease of the peaks corresponding to feldspar. The removal of feldspar particles by magnetic separation was further corroborated by XRF analysis, where the concentration of K, Al and Si was significantly higher in the non-magnetic fraction, however, no changes were observed in the concentration of Fe. The present analyses shows that prolonged exposure time of ilmenite increases its magnetic susceptibility. Non-magnetic feldspar was shown to acquire significant magnetic susceptibility by formation of a surface layer containing Fe-rich attrition products from ilmenite.

Evaluation of mixing of a secondary solid phase in a circulating fluidized bed riser

Mixing of the secondary solids phase, i.e. fuel or sorbent is crucial in fluidized bed reactors due to its effect on the performance of the reactor. Fluctuations in the gas-particle flow affect mixing, thus modeling this is essential for the analysis of mixing. However, simplified mixing models are used for large circulating fluidized bed (CFB) reactors, which causes uncertainties. A novel averaging method is presented to obtain representative results of transient secondary solid phase mixing in CFBs, acknowledging the effect of fluctuations. A parametric study is performed to investigate the effect of particle size, density, and inlet velocity on mixing and for insight into the mixing process. Additionally, the presented approach is used to compare different steady-state mixing methods to evaluate their applicability for modeling mixing in CFB reactors. Initial momentum and elutriation tendency of fuel were found to affect the mixing behavior, with two distinct routes identified.

A novel methodology to construct compartment models for a circulating fluidized bed riser

During the integrated modeling of a chemical process, there always exists a conflict between model accuracy and computational speed for non-ideal reactors. In view of this, a novel, easy to use, methodology to construct compartment or ideal reactor network models is proposed for a circulating fluidized bed (CFB) riser based on time-series. The pressure drop time-series is sampled during the Multi-Phase Particle-in-Cell (MP-PIC) simulation of the riser. Detailed procedures for mapping the time-series to a directed weighted complex network and the determination of model parameters are presented. Effects of the sampling frequency and division scheme for the time-series on compartment models are also investigated. Findings indicate that the MP-PIC simulation is reliable to record the time-series of pressure drop in the CFB riser. The non-ideal flow in the riser can be equivalently reproduced by a compartment model consisting of eight CSTRs with various sizes and backflow rates. The appropriate sampling frequency for the time-series should be greater than 200 Hz. The optimum division scheme for the time-series is with the predefined threshold value of K = [4025, 4150, 4275, 4400, 4525, 4650, 4775]. It is believed that the proposed methodology based on the time-series can be generalized to develop compartment models for non-ideal flow. It is also expected that compartment models can be readily integrated into a process simulator, e.g., Aspen Plus, to resolve the conflict between model accuracy and computational speed in process simulation, on-line analysis and real-time optimization.

Evaluation of bed-to-tube heat transfer in a fluidized bed heat exchanger in a 75 MWth CFB boiler for municipal solid waste fuels

Bed-to-tube heat transfer has been investigated for a tertiary superheater in a 75 MWth Circulating Fluidized Bed (CFB) boiler in Norrköping, Sweden. The boiler is used for incineration of solid waste fuels. Two fluidized bed heat exchangers are located in loop seals, connecting the cyclones and the furnace. The heat exchangers are placed in series, with respect to the steam side, and in parallel, with respect to the particle side. The total heat transfer surface area is roughly 44 m2, distributed over 72 tubes. The total effect transferred most often is in the range 2–6 MW. The incoming steam temperature in the first superheater is 380–400 °C, while the exiting steam temperature from the second is around 450°, at 65 bar pressure. The bed temperature in the Fluidized Bed Heat Exchanger (FBHE) is 850–875 °C. The analysis is based on operational data from two time periods (2002–2005 and 2014–2021). The two periods use different heat exchanger designs, following a retrofit in 2005. The aim of the study is to establish the bed-to-tube heat transfer coefficient in an industrial FBHE unit and investigate how it varies over different time periods, for two different bed materials and for two different designs. Also, the experimentally determined heat transfer coefficients are compared with an established heat-transfer correlation, for prediction of heat transfer from bubbling fluidized bed to tubes. Operation with two bed materials were evaluated, namely silica sand and crushed and beneficiated ilmenite. Both materials are classified as Geldart B particles. Air is used as fluidization gas in the FBHE. The analysis show, with a few exceptions, comparably low heat-transfer coefficients from bed to tube of 100–150 W/(m2K). The results were similar for silica sand and ilmenite, but the highest measured heat transfer coefficient was for a period with ilmenite. The heat transfer was lower than expected based on literature data from FBHE units and fluidized bed boilers in general, and much lower than bed-to-tube heat transfer coefficients from lab-scale experiments and empirically derived predictive expressions. The difference could be related to one or more of several factors, such as the effect of very small tube spacing, unknown thermal conductivity of one of the layers in the tube bundle, the effect of lateral particle flow and the effect of fouling due to ash layers forming on the tube surfaces. It is suggested that it should be possible to significantly increase the bed-to-tube heat transfer by increasing the tube pitch, which is expected to improve bed mixing without increasing the risk of corrosion.

Distribution of PCDD/F and PCB at Different Positions of Circulating Fluidized Bed Municipal Solid Waste Incinerators

Because of a high moisture content and a low heating value of Chinese municipal solid waste (MSW), circulating fluidized bed (CFB) incinerators have been widely adopted in China for incinerating MSW since 1998. In this study, two typical CFB incineration plants (A and B) were investigated for contents and fingerprints of PCDD/F and PCB at different positions downstream their post-combustion zone, aiming to draw a full picture of formation and distribution of these organochlorinated pollutants. Both flue gas and ashes were sampled at five different positions of Plant A, from high-temperature superheater to outlet of baghouse filter, representing a huge range of flue gas temperatures; for Plant B, five ash samples were collected at different positions of the waste heat boiler (from high-temperature superheater to lower economizer). A continuous increase in contents of PCDD/F and PCB in flue gas was observed from superheater to inlet of air pollution control system (APCS) in Plant A, with the most significant rise noticed at air preheater. The load of PCDD/F and dioxin-like PCB in ashes also amplified steadily along the cooling path of flue gas in both plants. Changes in formation pathways are discussed based on homologue and isomer distribution patterns.

Ash characterisation formed under different oxy-fuel circulating fluidized bed conditions

The CO2-intensive oil shale power industry accounts for more than 70% of the Estonian energy sector, producing large amounts of ash that are mainly landfilled. Adopting oxy-fuel technology in the Estonian energy sector, in conjunction with CO2 storage, can be a critical tool for reducing carbon footprint. Nevertheless, several differences in ash formation under oxy-fuel conditions can be expected, which can lead to additional ash handling/utilization and related environmental concerns. In this context, the composition of oil shale ash was obtained from a series of combustion experiments in a 60 kWth circulating fluidized bed (CFB) test unit under both air and oxy-fuel combustion regimes studied by means of physical, chemical analysis as quantitative X-ray diffraction and element analysis for ashes from different separation ports. During the experimental work, special attention was given to understanding the impacts of the boiler temperature, recycled flue gas (RFG), and elevated oxygen concentration. As a result, the primary goal of this research is to characterise the ash produced by oxy-fuel combustion of oil shale in order to gain a better understanding of its chemical and mineral compositions, including Ca-Mg silicates and clay minerals, as well as to investigate the behaviour of the formation of free lime and anhydrite, which are the major compounds influencing potential ash utilisation routes. The results show that although the decomposition of calcium carbonates was limited during oxy-fuel combustion because of the high CO2 partial pressure, the contents of secondary silicates and anhydrite were slightly higher. The increased particle temperature and the higher SO2 partial pressure (with RFG) can explain this. The particle size distributions (PSDs) were wider under oxy combustion, and the specific surface area (SSA) in Filter ashes were much higher under elevated inlet O2% atmosphere. Oxy-fuel combustion had no significant impact on the concentration of elements in the ash. Overall, the mineralogy of the ashes formed in air and oxy-fuel combustion environments is similar, yet slight differences exist in the formation of clay minerals in the case of increased oxygen concentrations and with the application of RFG as a result of the increased residence time of the particles.

Particle Size Distribution and Enrichment of Alkali and Heavy Metals in Fly Ash on Air and Oxy-Fuel Conditions from Sludge Combustion

Comparative tests in air and oxy-fuel combustion were conducted in a 30 kWth circulating fluidized bed (CFB) pilot plant for waste sludge combustion. General combustion characteristics of the CFB, such as pressure profiles, temperatures along the bed, and flue gas composition, were different under the air and oxy-fuel conditions. At the bottom and in the fly ash, alkali and heavy metals had different distributions under the air and oxy-fuel combustion conditions. The particle size distribution in fly ash from air combustion was dominated by coarse particles, over 2.5 μm in size, whereas with oxy-fuel combustion, most particles were submicron in size, approximately 0.1 μm, and a smaller quantity of coarse particles, over 2.5 μm in size, formed than with air combustion. Mass fractions of Al, Ca, and K, below 2.5 μm in size, were found in the ashes from oxy-fuel combustion and in higher quantity than those found in air combustion. Submicron particle formation from Cr, Ni, Cu, and Zn in the fly ash occurred more during oxy-fuel combustion than it did in air combustion.

Emissions Reduction by Combustion Modeling in the Riser of Fluidized Bed Combustor for Thar Coal Pakistan

Pakistan has experienced a protracted electricity shortage for the past few years. However, despite Pakistan’s abundant coal deposits, modern coal combustion technology is still required to reduce emissions. Pakistan is struggling to utilize its energy resources and currently experiencing an electrical shortage of more than 8000 MW. The research study models the combustion performance in a fluidized bed riser using ANSYS FLUENT software to understand the combustion behavior of low-rank Thar coal. A simple circulating fluidized bed (CFB) combustion riser was modeled for computational fluid dynamics (CFD) to study the hydrodynamics of gas-solid flow in a circulating fluidized bed riser to reduce emissions and operating costs. Three different types of risers/combustors geometries were used center flow, counter flow, and parallel flow. The CFD model for the solids segment with a k-e turbulence model and the viscosity of static particles in the gas segment both showed excellent mixing performance. According to the FLUENT data, the riser/combustor maximum temperature is around 1400 K or 1130 o C at the primary burning sector in the bed center. According to velocity contours, the greatest velocity in the center-oriented riser/combustor peaks at 3.3 m/s. The CO and CO2 both mass fraction counters show maximum concentration in the center geometry, whereas lower CO concentration is found in parallel geometry. The lowest level of NOx is established in the parallel geometry at around 15 ppm, whereas the counter contours establish the maximum level of NOx at about 31 ppm. Circulating Fluidized Bed Combustor is found to be the most advantageous and effective technology for producing power from Thar lignite coal and reducing emissions.

Issues in deep peak regulation for circulating fluidized bed combustion: Variation of NOx emissions with boiler load

In order to improve the consumption of renewable energies, certain amounts of circulating fluidized bed (CFB) boiler units should assume the task of deep peak regulation. However, the effect of CFB boiler load on pollutant emissions such as NOx still needs to be clarified. In this paper, the NOx emission characteristics of two industrial-scale CFB boilers within a wide load range (35%–100%) were further analyzed by using a comprehensive one-dimensional, two-phase CFB mathematical model. Simulation results reveal that, when the load ratio decreases, the NOx emission decreases first and then increases. The non-monotonic variation trend is also confirmed by the operational data collected from the SC-350 boiler. However, for different boilers, the load ratio corresponding to the turning point of NOx emission may be different, e.g., for the 135 MWe super high steam pressure boiler, it is about 40%, while for the 350 MWe supercritical boiler is 50%. On the one hand, the decrease in boiler load leads to a decline in the furnace temperature, which contributes to reducing NOx emission due to the decrease of volatile yields, the lower conversion rate of Vol-N to NOx, and the enhancement of the overall NO reduction on chars. On the other hand, at low loads, the excess air coefficient is generally set to high values, and air staging is weakened, resulting in adverse effects on the NOx emission control. In addition, when the CFB boiler operates at low loads, the solid circulation loop performance usually worsens, and the heat loss caused by incomplete combustion may increase. This study points out that high NOx emission is an unavoidable issue in the process of deep peak regulation for CFB boilers.

Simulation of sorption-enhanced gasification: H[formula omitted]O staging to a circulating fluidised bed gasifier to tailor the producer gas composition

Sorption-enhanced gasification (SEG) is an advanced gasification technology for biofuel and biochemical synthesis processes. The SEG process allows tailoring of the producer gas composition for different synthesis processes by adjusting the temperature of the gasifier. Conventionally, the solids circulation rate from the combustor to the gasifier has been used to tailor producer gas composition. In this study, H2O staging is investigated in a circulating fluidised bed (CFB) gasifier as an alternative approach. The H2O staging can be used to decrease the riser temperature locally to enhance the main chemical reactions. The H2O staging as gas and liquid phases were considered in this study. Also, reactor cooling was combined with the staging. A validated one-dimensional CFB gasifier model was used to investigate the process. The simulation results indicate that H2O staging and cooling enhance the gasifier’s limestone carbonation and water-gas shift reactions. Thus, allowing to tailor the producer gas composition directly by adjusting the gasifier’s operating parameters. The methods investigated increase flexibility for operating the SEG CFB gasifier and the reactor system. Understanding different operation methods of the process and their impact on the system’s operation enables more flexible use and design of the gasification process.

Experimental study and theoretical analysis on fluidized activation of coal gasification fly ash from an industrial CFB gasifier

The gasification fly ash (GFA), a bulk industrial solid waste from coal gasification process, urgently needs to be effectively disposed. In order to use the GFA as porous carbon materials, fluidized activation experiments of the GFA from an industrial circulating fluidized bed (CFB) gasifier were conducted in a bench-scale CFB test rig, as well as steam activation experiments of GFA in a vertical tube furnace and theoretical analysis on the activation process. Due to the ultrafine particle size, the GFA faces a fluidization problem and auxiliary particles are needed to stabilize its fluidization. In the fluidized activation, the pore structure of GFA particles becomes developed in a seconds-level time (about 1.5 s). The specific surface area (SBET) of activated GFA increases with temperature, maximally increasing by 48.9 % and reaching 204 m2/g. Steam activation experiments show that the GFA has an activation potential of 362 m2/g (SBET) and the pore structure evolution of GFA can be quantified by carbon conversion ratio. Based on this, the fluidized activation of GFA is found in the stage of pore development. By appropriately increasing the carbon conversion ratio (below 40 %), the fluidized activation effect of GFA is expected to be improved. Theoretical analysis indicates for the GFA the features of ultra-fine particle size and well-developed pore structure greatly enhance the diffusion rate of active component into the particles. Under the strong diffusion effect, increasing temperature is a critical means to realize the rapid and effective activation of GFA in a finite time.

Evaluation of Fly Ash from Co-Combustion of Paper Mill Wastes and Coal as Supplementary Cementitious Materials.

The applications of waste-derived fuel from paper mills in industrial boilers benefit the reduction of carbon emissions. However, the co-combustion of waste-derived fuel and coal causes significant changes in the characteristics of the ash and brings about the need to find possible means of the utilization of the ash produced. In this work fly, ash samples were collected from circulating fluidized bed (CFB) boilers co-combusting paper mill wastes with coal and analyzed in detail. The chemical, physical, and thermal characteristics of two different co-combustion fly ashes (CCFA) were investigated using X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscope (SEM). The chemical composition of CCFA is largely affected by the fuel source type. Thermal analyses of CCFA show that the type of desulfurization system used by the boiler influences the form of sulfate present in the fly ash. The presence of calcium sulfite hemihydrate can cause a high loss in the ignition of CCFA. By comparing the physical requirements specified in the ASTM standard for coal fly ash to be used in concrete, the CCFA produced from paper mill wastes was found to show good potential as supplementary cementitious materials.

Particle Residence Time Determination with More Than 400 Tracer Particles in a Circulating Fluidized‐Bed Full‐Loop System

AbstractThe residence time of particles in a circulating fluidized‐bed (CFB) reactor is the essential parameter for design and performance of a CFB system. In this work, a noninvasive measurement method of positing tracer particles in a full‐loop CFB system is proposed. Based on the radio frequency identification technology, more than 400 tracer particles with individual code were made and moved freely in the CFB full loop system. With detecting the tracer particles' trajectory by high‐frequency readers located outside of the system, the effects of operating parameters including fluidization number and solid inventory on the movement and residence time of the tracer particles in the CFB system were investigated. The particle residence time is distributed in a certain range, influenced by superficial gas velocity and solid inventory.

Designing a new bell-type primary air nozzle for large-scale circulating fluidized bed boilers

The design of energy efficient engineering systems is crucial for sustainable operation when economic and environmental consequences are considered. Circulating Fluidized Bed (CFB) boilers, which are among the major contributors to world electricity production, are still increasing in numbers and unit sizes. Primary air nozzles are key components of CFB boilers that may decrease energy consumption and increase energy efficiency, and they need to be carefully designed. There are certain types of nozzles commonly used in the air distribution grate, but even minor design improvements on the nozzle can significantly decrease the pressure loss. This work is about optimizing the bell-type primary air nozzle used in the Turkish lignite-fired ÇAN Thermal Power Plant (CTPP), which has two 160 MWe CFB boilers, through computational fluid dynamics (CFD). Initially, the bell-type nozzle was designed newly by changing the inner head holes geometry. After that, the nozzle geometry was optimized by changing the orifice size and angle to decrease the pressure drop, increase the orifice velocity outlet, and flow uniformity through CFD simulations. With the optimum nozzle geometry, the velocity at the outlet orifices was increased, and a decrease of 2.86 kPa was achieved in the total pressure loss. Furthermore, when the nozzle orifices were designed downwardly with an angle of 105°, pressure drop across the nozzle decreased by 7.6 %, and the uniformity index increased by 2 % at the outlet orifices. Using the bell-type primary air nozzle, which is newly designed, in the CTPP boiler not only will save 2.26 GWh/year of energy consumption but also minimize the backflow risk in the boiler operation.

Evaluation of effects of ammonia co-firing on the thermal performances of supercritical pulverized coal and circulating fluidized bed boilers

Ammonia is an effective fuel in reducing carbon dioxide due to a carbon-free fuel. Accordingly, the use of ammonia co-firing technology for carbon neutrality has emerged in the power generation industry. It is necessary to determine the optimal operating conditions by analyzing boiler performance during ammonia co-firing for optimal power plant operations. In this study, a process simulation was performed to determine the effect of ammonia co-firing on the thermal performances of supercritical pulverized coal (PC) and circulating fluidized bed (CFB) boilers. The target boilers were 870 and 550 MWe supercritical PC and CFB boilers, respectively. The process simulations were conducted using co-firing ratios of 5, 10, 15, 20, and 30%, as well as various coal grades and load conditions at an ammonia co-firing ratio of 20%. Although the amount of CO2 reduction during ammonia co-firing was confirmed, the radiation and convective heat transfer rates decreased due to changes in the flue gas composition and due to increased moisture loss. Accordingly, the plant efficiency decreased due to lower main and reheat steam temperatures. These findings can be used to establish optimal operating conditions for CO2 reduction and improve plant efficiency, which are operational trade-offs during ammonia co-firing.

Experimental study on coal combustion by using the ilmenite ore as active bed material in a 0.3 MWth circulating fluidized bed

The oxygen-carrier aided combustion (OCAC) technology which improves the uniformity of oxygen distribution in the fluidized bed and subsequently enhances the combustion efficiency has attracted more and more attention. However, the existing studies on OCAC mainly focused on the combustion of methane or biomass. There were few studies on low-volatile fuels (such as coal), especially pilot-scale studies that have not been reported. In this work, the effectiveness of OCAC technology on coal combustion has been comprehensively evaluated in a 0.3 MWth pilot-scale circulating fluidized bed (CFB) combustor. The experimental parameters including oxygen carrier (OC) proportion, primary air ratio (A) and oxygen concentration have been carefully investigated. The CO, NOx, SO2 and particulate matter (PM) were measured in real-time from the outlet of the cyclone separator by a gas analyzer and electrical low pressure impactor (ELPI). The results showed that the CO emission from the exit of the cyclone was reduced when the inert bed materials were replaced with the ilmenite ore, especially under the conditions of low oxygen concentration and A. The OCAC technology exhibited good thermal buffering capacity and oxygen buffering capacity. Compared to sand as bed material, the SO2 and NOx emission in flue gas under OCAC decreased and increased, respectively, which is attributed to the more uniform oxygen distribution with more available oxygen in the combustor. Such properties also promoted the self-desulfurization reaction of coal ash. However, it also reduces the reducibility area in the combustor and thus weakens the reduction of NOx. Compared to the sand case, the PM emission under OCAC case is more diminutive, especially in the size of 0–0.4 μm, the PM concentration under OCAC case is much lower than that under sand case.