Circulating Fluidized Bed Combustion Technology Research Articles

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.

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.

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.

High Efficiency of the Removal Process of Pb(II) and Cu(II) Ions with the Use of Fly Ash from Incineration of Sunflower and Wood Waste Using the CFBC Technology

In these research studies, fly ash (SW-FA) resulting from the incineration of sunflower (20%) and wood (80%) waste employing the circulating fluidized bed combustion (CFBC) technology was used to analyze the possibility of removing Pb(II) and Cu(II) ions in adsorption processes. Currently, great emphasis is placed on circular economy, zero waste or climate neutrality strategies. The use of low-cost SW-FA waste seems to fit well with pro-ecological, economic and energy-saving trends. Hence, this material was characterized by various techniques, such as granulation analysis, bulk density, SEM-EDX, XRD and XRF analysis, BET, BJH, thermogravimetry, zeta potential, SEM morphology and FT-IR spectrometry. As a result of the conducted research, the factors influencing the effectiveness of the adsorption process, such as adsorbent dosage, initial and equilibrium pH, initial metal concentration and contact time, were analyzed. The maximum removal efficiency were achieved at the level of 99.8% for Pb(II) and 99.6% for Cu(II), respectively. The kinetics analysis and isotherms showed that the pseudo-second-order equation and the Freundlich isotherm models better describe these processes. The experiments proved that SW-FA can act as an appropriate adsorbent for highly effective removal of lead and copper from wastewater and improvement of water quality.

A Review of the Research Results into the Technologies of Solid-Fuel Combustion in a Circulating Fluidized Bed Conducted Abroad and in Russia

The article provides an analysis of the current status and the development of the circulating fluidized bed (CFB) combustion technology. The main distinguishing features of the CFB combustion technology and the stages of its development worldwide are provided. The advantages and disadvantages of the CFB technology compared with traditional flame combustion are outlined. Examples of the largest power-generating units equipped with CFB boilers are provided. The latest studies of the specific features of the CFB boiler designs have been analyzed to increase the steam parameters and assure the highest efficiency of the power units equipped with material CFB boilers. The basic trends in the research into the hydrodynamics, separation, composition of the bed, etc. are considered. It is shown that the capturing efficiency of solid particles is the decisive factor in increasing their consumption and retention of fine calcinated particles in the circulating material. The high concentration in the disengaging space of the combustor results in an increase in the heat-transfer coefficient and ensures a uniform temperature distribution in the combustor, the required degree of reaction of limestone particles with SO2 for the most efficient binding of sulfur, and more complete combustion of the fuel (the highest possible residence time of the fuel particles in the combustor). Studies of Chinese researchers, their practical developments, and the introduction of energy-saving technology with a low volume of particles in the bed are analyzed. The results of the latest studies on the minimization of emissions of both conventional hazardous substances, such as nitrogen and sulfur oxides and other pollutants, including greenhouse gases, are provided. The problems that arise during the combustion of several kinds of fuel, including biomass, are investigated. Results of studying the processes of bed agglomeration, fouling of the heat-transfer surfaces, and corrosion of CFB boiler superheaters are presented.

Prospects for the Low Pollutant Emission Control of Circulating Fluidized Bed Combustion Technology

With the pollutant emission standards becoming increasingly stringent and considering the pressure of carbon neutral by 2060, the low pollutant emission potential of circulating fluidized bed (CFB) combustion technology needs to be further exploited, thus to promote the market competitiveness of CFB boilers; this is critical for the clean and efficient utilization of coal as well as for the energy transformation in China. In this article, we summarize the pollutant emission characteristics of CFB combustion, and review the development of major technologies for CFB boiler emission control. Based on the energy development strategies and corresponding policies in China, development suggestions are proposed for reducing pollutant emission of the CFB combustion technology. The most significant approach is to push the limits of original pollutant emission for CFB combustion by re-specifying the fluidization state and through in-furnace combustion adjustment, while the boiler thermal efficiency should be ensured. For the long-term development of coal energy, the new-generation CFB combustion technology with ultra-low emission should be researched and developed while combining with technologies such as supercritical/ultra-supercritical, intelligent operation, carbon capture, utilization, and storage, and energy storage technologies. The existing CFB boilers with small or medium capacity should also be upgraded. Considering the fuel flexibility of CFB combustion, biomass power generation should be promoted to realize low-cost and high-efficiency consumption of low-heat value fuels, urban refuse, industrial wastes, etc. The peak load regulation capacity and low pollutant emission property of the CFB boilers should be promoted to improve operation flexibility and renewable energy consumption. Moreover, the desulphurization ash produced in CFB combustion should be comprehensively utilized, and the N2O emission problem is also significant. The pollutant emission standards and related policies need to be formulated from an overall perspective to guide the healthy development of the energy industry.

Removal of lithium and uranium from seawater using fly ash and slag generated in the CFBC technology.

Fly ash and slag were produced as a result of the incineration of municipal sewage sludge using the circulating fluidized bed combustion (CFBC) technology and were examined for the simultaneous recovery of lithium and uranium from seawater in batch adsorption experiments. These waste materials have been characterized in terms of their physicochemical properties using several research methods including particle size distribution, bulk density, SEM-EDS analysis, thermogravimetry, SEM and TEM morphology, BET, specific surface area, pore volume distribution by the BJH method, ATR FT-IR, and zeta potential. The fly ash and slag waste materials showed the following research results for Li-ion recovery: adsorption efficiency 12.1% and 6.8%, adsorption capacity 0.55 mg g−1 and 0.15 mg g−1, respectively. Better results were reported for U ion recovery: adsorption efficiency 98.4% and 99.9%, adsorption capacity 21.3 mg g−1 and 56.7 mg g−1 for fly ash and slag, respectively. In conclusion, the conducted research revealed that CFBC fly ash and slag are promising low-cost adsorbents for the effective recovery of Li and U ions from seawater.

A Technical Analysis of Oil Shale Firing Power Units Retrofitting for Carbon Capture and Storage (CCS)

In Estonia the significant part of power generation is still based on firing oil shale fuel in thermal power plants despite of the growth of the share of the green energy in recent years. Electricity generation is based on power units utilizing different combustion technologies. A significant part still comprises old units with capacity 185-195 MWe commissioned in early 70s of last century. These pulverized combustion (PC) based units utilize oil shale as a fuel and are operating with subcritical steam parameters, characterizing by relatively low efficiency 26 - 30% (net, LHV based). Due to their low performance, operational issues (intensive fouling of heating surfaces), and remarkable environmental impact, these units were decommissioned or retrofitted with boilers implementing circulating fluidized bed combustion (CFBC) technology in 2004-2005. Implementation of CFBC technology for oil shale combustion significantly reduced both operational issues and environmental impact. With some additional modifications in turbine island allowing to increase rated capacity up to 215 MWe and after increasing steam parameters, the net efficiency of up to 37% was achieved. Consequently, the specific CO2 emissions in the range of 0.99 – 1.05 t/MWhe depending on quality of oil-shale used, was achieved. Recently, the new 300 MWe CFBC based power unit was commissioned. It is operating with subcritical steam parameters: live steam temperature 565 °C and pressure 17 MPa. Net efficiency (LHV based) of that unit is 40 % and specific CO2 emissions up to 0.9 t/MWhe in case of firing only oil shale. In this study the technical viability of retrofitting oil shale based power generation units with CC is described. In the analysis two CC methods were considered: post-combustion (assuming conventional amine process) and oxy-fuel combustion. Among other CC technologies these processes are at the present time characterized by higher technical readiness level, can be applied for existing units with minimal modifications and are applicable for oil shale fuel with it specifics. Besides power generation, Estonia has for almost 100 years of experience in production of shale oil through thermal decomposition of oil shale kerogen in retorts. Currently, solid heat carrier (SHC) method is the main method used for producing shale oil. In this method, direct CO2 emission comes from the preparation of SHC by combustion of solid retorting residue (semi-coke). Mainly two combustion methods are presently applied: lift pipe combustion and CFBC which was recently successfully integrated into shale oil production process. The possibility of utilizing CC technologies in shale oil production units and their technical analysis is described in this study.

Optimization of Fluidization State of a Circulating Fluidized Bed Boiler for Economical Operation

To reduce the auxiliary power consumption and improve the reliability of large-scale circulating fluidized bed (CFB) boilers, we developed energy-saving CFB combustion technology based on the fluidization state re-specification. A calculation model of coal comminution energy consumption was used to analyze the change in comminution energy consumption, and a 1D CFB combustion model was modified to predict the operation parameters under the fluidization state optimization conditions. With a CFB boiler of 480 t/h, the effect of fluidization state optimization on the economical operation was analyzed using the above two models. We found that combustion efficiency presents a nonmonotonic trend with the change in the bed pressure drop and feeding coal size. There are an optimal bed pressure drop and a corresponding feeding coal size distribution, under which the net coal consumption is the lowest. Low bed pressure drop operation achieved by reducing the coal particle size is not beneficial to SO2 and NOx emission control, and the pollutant control cost increases. The effect of fluidization state optimization on the gross cost of power supply can be calculated, and the optimal bed pressure drop can be obtained.

Comparative studies on the adsorption of Pb(II) ions by fly ash and slag obtained from CFBC technology

Abstract Fly ash and slag were examined for the removal processes of Pb(II) ions from water in batch experiments under different conditions of adsorbent dosage, initial concentration, pH and contact time. The materials are industrial waste generated from the high temperature treatment of sewage sludge by the circulating fluidized bed combustion (CFBC) technology. Physical and chemical properties, as well as adsorption efficiency and calculated maximum adsorption capacity of Pb(II) ions were determined using a variety of methods. The kinetic analysis revealed that the adsorption process is better described by the pseudo-second order equation and it is well fitted to the Freundlich model.

Leaching Characteristics of Low Concentration Rare Earth Elements in Korean (Samcheok) CFBC Bottom Ash Samples

Coal-derived power comprises over 39% of the world’s power production. Therefore, a mass volume of coal combustion byproducts are generated and shifted the extra burden onto the economy and environment. Circulating fluidized bed combustion (CFBC) has been found to be a clean and ultimate technology for Korea’s coal-fired power plants to have effective power generation from low-grade imported coal with reduced emissions. Efforts have been made to broaden the utilization of CFBC coal ash, and to promote sustainable development of CFBC technology. Investigations provided numerous evidences for coal ash to be a potential deposit for rare earths reclamation. However, the basic characteristics and the methods of rare earth mining from the CFBC bottom ash lack detailed understanding and are poorly reported. This study highlighted an insight of the CBFC bottom ash with respect to REEs concentration. Moreover, agents were tested as a means for leaching REEs from Samcheok CFBC bottom ash. The leaching tests were performed in relation to variations in concentration, time and temperature. The results were applied to identify suitable processes to leach REEs from the ash and clarify the potential valuation of CFBC bottom ash. The leaching conditions attained by ANOVA analysis for hydrochloric concentration, temperature, and time of 2 mol L−1, 80 °C, and 12 h, were found to provide a maximum extraction of yttrium, neodymium and dysprosium of 62.1%, 55.5% and 65.2%, respectively.

Thermal-hydraulic calculation and analysis on evaporator system of a 660 MWe ultra-supercritical CFB boiler

Ultra-supercritical circulating fluidized bed (CFB) boilers have great commercial potential in large-scale clean utilization of low-quality fuels. Evaporator system is one of the keys in the combination of ultra-supercritical steam cycle with CFB combustion technology. Aiming at examining the thermal-hydraulic characteristic of the evaporator system of the conceptual design of a 660 MWe ultra-supercritical CFB boiler, a mathematical model was developed in this paper. A semi-empirical bed-to-wall heat flux model and a set of empirical correlations available for heat transfer and hydraulic resistance calculation were embedded in the thermal-hydraulic model. By iteratively solving the model, the thermal-hydraulic parameters of the evaporator system at different operating loads, including total pressure drops, mass flux distribution, and metal temperature were obtained. The results exhibit good flow distribution characteristic and low temperature deviations in the evaporator system. The outlet fluid temperature deviation and metal temperature in the evaporator system are both permissible, implying that the conceptual design of the evaporator system of the 660 MWe ultra-supercritical CFB boiler is acceptable.

Effective adsorption of lead ions using fly ash obtained in the novel circulating fluidized bed combustion technology

Fly ash was examined for the separation efficiency of Pb2+ ions from water in batch experiments. Fly ash is industrial waste produced from the incineration of sewage sludge by the novel circulating fluidized bed combustion (CFBC) technology and was taken into the experiments. Physical and chemical properties of the material were characterized using several methods, such as fraction analysis, particle size distribution, bulk density, composition, morphology, surface and internal structure by elemental analysis, FT-IR, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction analysis, comprehensive thermal analysis, fly ash texture by BET adsorption isotherms, specific surface area, volume of pores, average pore diameter and pore volume distribution by the BJH method. The influence of pH values on zeta potential of the fly ash surface before and after the adsorption process was conducted. The calculated maximum adsorption capacity of Pb2+ by fly ash reached 51.98 mg/g. The kinetic analysis revealed that the adsorption process is better described by the pseudo-second order equation and it is well fitted to the Freundlich model. In conclusion, fly ash obtained in the CFBC technology is a promising low cost adsorbent for the highly effective recovery of Pb2+ from wastewater.

Development of a supercritical and an ultra-supercritical circulating fluidized bed boiler

The supercritical circulating fluidized bed (CFB) boiler, which combines the advantages of CFB combustion with low cost emission control and supercritical steam cycle with high efficiency of coal energy, is believed to be the future of CFB combustion technology. It is also of greatest importance for low rank coal utilization in China. Different from the supercritical pulverized coal boiler that has been developed more than 50 years, the supercritical CFB boiler is still a new one which requires further investigation.Without any precedentor engineering reference, Chinese researchers have conducted fundamental research, development, design of the supercritical CFB boilers independently. The design theory and key technology for supercritical CFB boiler were proposed. Key components and novel structures were invented. The first 600 MWe supercritical CFB boiler and its auxiliaries were successfully developed and demonstrated in Baima Power Plant, Shenhua Group as well as the simulator, control technology, installation technology, commissioning technology, system integration and operation technology. Compared with the 460 MWe supercritical CFB in Poland, developed in the same period and the only other supercritical one of commercial running in the word beside Baima, the 600 MWe one in Baima has a better performance. Besides, supercritical CFB boilers of 350 MWe have been developed and widely commercialized in China. In this paper, the updated progress of 660 MWe ultra-supercritical CFB boilers under development is introduced.

Carbon dioxide emission factors for oxy-fuel CFBC and aqueous carbonation of the Ca-rich oil shale ash

Oil shale (OS) is a low-calorific-value fossil fuel. Today, Estonia’s OS usage is the largest in the world. Approximately 76 % of the electricity is produced from Ca-rich OS. Yearly, approximately 12 million tons of OS is used for power generation utilizing pulverized combustion (PC) and circulating fluidized bed combustion (CFBC) technologies that produce nearly 6 million tons of Ca-rich ash.Estonian kukersite OS consists about one third of carbonate minerals, mainly calcite. Therefore, in addition to the combustion of organic carbon, the carbonaceous minerals decompose and release additional CO2. The extent of decomposition of carbonaceous minerals depends on combustion technology. Using oxy-fuel CFBC technology alters ash properties, including decomposition of carbonate minerals. By means of aqueous carbonation of Ca-rich ash, CO2 can be stored safely and leakage-free for very long time.In order to understand the changing CO2 sequestration potential, oil shale ashes were produced at a 60 kWth CFBC facility in oxy-fuel mode. The ash was treated with water and CO2 in order to mimic the ash treatment technology currently in use in the industry. The ash bound 81 kgCO2/tash it means that 6 % of the CO2 emitted would be bound at the ash fields. Due to decreased decomposition of carbonates, when using oxy-fuel CFBC, the CO2 specific emission of combustion would decrease by 5 % compared to regular CFBC and 19 % compared to PF. Decreased CO2 production would result in reduced CO2 transportation and further utilization or storage cost.

From a CFB reactor to a CFB boiler – The review of R&D progress of CFB coal combustion technology in China

Circulating fluidized bed (CFB) technology was applied for coal combustion after CFB chemical reactors have been widely used and studied in chemical engineering. However, there were still many special phenomena found in the CFB boiler. In this paper, the main differences between a conventional CFB reactor and a CFB boiler are summarized. A CFB boiler is a special CFB reactor. It is an open circulating system with a wide size distribution feedstock and a low circulating solid flow rate. The CFB boiler behaves as particle selection machine, retaining particles within a certain size range in the furnace. Its fluidization regime is distinguished from a conventional CFB reactor, composing of a bubble or turbulent dense bed in the lower furnace and a fast dilute bed in the upper furnace. A large portion of heat is released in the dilute bed, and there is an oxygen-deficient combustion zone in the center of the upper furnace of a CFB boiler. Based on the theoretical studies and engineering practice, the principles and a guidance map of fluidization state specification for CFB boiler design are presented. In addition, the paper reviews the research and development (R&D) history and important achievements of the Chinese CFB combustion technology.

APPLICATION OF HIGH-LOW BED CFB COMBUSTION TECHNOLOGY TO OIL SHALE COMBUSTION

A 75 t/h pulverized coal-fired boiler has been successfully revamped to a 65 t/h oil shale-fired circulating fluidized bed (CFB) boiler employed by the high-low bed CFB combustion technology. The 65 t/h boiler was installed in Suixi cement factory, Guangdong province, China, and put into operation in 2006. The designed fuel is oil shale and comes from Maoming deposit, Guangdong province. The average low heating value is about 4.1 MJ/kg. Boiler performance tests showed that the main technical and economic parameters meet the designed requirements, which can provide a theoretical and technical basis for the design and operation of a large-scale oil shale-fired circulating fluidized bed boiler.

Comparative study on air and oxy combustion in a pilot CFB boiler

In the present work, measurement experience of oxy combustion in a pilot scale circulating fluidized bed (CFB) combustor is summarized. The aim was to study combustion dynamics under air-and oxygen-firing conditions in order to develop and validated combustion controls for Flexi-Burn™ CFB combustion technology. Dynamic CFB-pilot combustion tests were carried out including various step and ramp tests in both air and oxy combustion and different operation strategies for oxy combustion.

Design assessment of a 150 kWt CFBC Test Unit

For clean and efficient energy generation from coal, the most suitable technology known to date is ‘Fluidized Bed Combustion’ technology. Applications of circulating fluidized bed (CFB) combustion technology have been steadily increasing in both capacity and number over the past decade. Designs of these units have been based on the combustion tests carried out in pilot scale facilities to determine the combustion and desulfurization characteristics of coal and limestone reserves in CFB conditions. Similarly, utilization of Turkish lignites in CFB boilers necessitates adaptation of CFB combustion technology to these resources. However, the design of these test units are not based on firing coals with high ash, volatile matter and sulfur contents like Turkish lignites. For this purpose, a 150 kWt CFB combustor test unit is designed and constructed in Chemical Engineering Department of Middle East Technical University, based on the extensive experience acquired at the existing 0.3 MWt Bubbling Atmospheric Fluidized Bed Combustor (AFBC) Test Rig. Following the commissioning tests, a combustion test is carried out for investigation of combustion characteristics of Çan lignite in CFB conditions and for assessment of the design of test unit. Comparison of the design outputs with experimental results reveals that most of the predictions and assumptions have acceptable agreement with the operating conditions. In conclusion, the performance of 150 kWt CFBC Test Unit is found to be satisfactory to be utilized for the long term research studies on combustion and desulfurization characteristics of indigenous lignite reserves in circulating fluidized bed combustors.