Conventional Circulating Fluidized Bed Research Articles

Application of post-combustion ultra-low NOx emissions technology on coal slime solid waste circulating fluidized bed boilers.

In order to achieve thermal treatment of coal slime and depth control of NOx, a 75t/h circulating fluidized bed (CFB) boiler arranging post-combustion air is constructed. In the experiment, the combustion atmosphere and the ratio of primary air and secondary air are adjusted to decrease the NOx emissions below 50mg/m3 (dry basis at 6% O2). Compared with conventional CFB combustion, the post-combustion technology decreases the NOx emissions from 92.9 to 65.4mg/m3 by adjusting the combustion atmosphere. Then, the primary air volume is adjusted to decrease the NOx emissions further. On the one hand, decreasing primary air volume contributes to inhibiting the NOx generation in the dense phase. On the other hand, it is proved that the combination of the cyclone and a post-combustion chamber plays a crucial role in the de-NOx process of post-combustion technology. More char particles are brought to the cyclone as the primary air volume decreases. The NOx reduction in the cyclone and the post-combustion chamber is promoted. Finally, the NOx emissions are decreased to 42.6mg/m3 when the ratio of primary air and secondary air is 50.0%. In addition, the SO2 emissions and the combustion efficiency during the ultra-low NOx condition are 23.2mg/m3 and 98.3%, respectively.

Computational fluid dynamics simulation of internally circulating fluidized bed reactor for dry reforming of methane

Internally circulating fluidized bed reactor (ICFB) is the reactor that combines the conventional circulating fluidized bed (CFB) reactor system, including riser, downer, loop seal and cyclones, into a single compact reactor column. The ICFB reactor column is separated into two sections (riser and downer) by baffles and is linked together via connecting ports. The solid particles are circulated in the ICFB reactor by unequal fluidizing velocities in the riser and downer sections. The solid particles in the downer section are operated with low fluidizing velocity in the bubbling fluidization regime. In the riser section, the solid particles are transferred to the downer section because of the high fluidizing velocity. In this study, the carbon dioxide reforming or dry reforming reaction of methane was investigated. Dry reforming is a reaction between carbon dioxide and methane, which are the major greenhouse gases, for producing syngas, hydrogen and carbon monoxide. The hydrodynamics and dry reforming reaction with different reactor designs were explored using two-dimensional Eulerian–Eulerian computational fluid dynamics simulation (CFD). The ICFB reactor with no elevation gas distributor, 90 cm of draft tube height, 2 cm of loop-seal length and 1 cm of gas outlet diameter gave the highest methane conversion of 15.4%. The analysis of variance (ANOVA) indicated that the length of loop-seal and the gas outlet diameter gave substantial negative and positive effects on methane conversion, respectively. The interaction between length of loop-seal and gas outlet diameter also had a significant effect on the methane conversion. Although the ICFB reactor in this study gave the methane conversion slightly lower than referenced literature conventional CFB reactor, the ICFB reactor is the reactor that can reduce construction cost and simplify complex reactor operation.

Life Cycle Assessment of Advanced Circulating Fluidized Bed Municipal Solid Waste Incineration System from an Environmental and Exergetic Perspective.

The production of clean and efficient energy from municipal solid waste (MSW) is extremely urgent matter due to an increasing energy demand and environmental concerns. In this study, a high steam parameter (520 °C, 7.9 MPa) circulating fluidized bed (CFB) MSW incineration system, equipped with a mechanical, biological treatment and external heat exchanger systems, was introduced and a comparative study with a typical mechanical grate (450 °C, 5.3 MPa) incineration system and conventional CFB (485 °C, 5.3 MPa) incineration system was carried out from a life-cycle, environmental and exergetic perspective which could assess different energy and material outputs based on real operating data. Moreover, the potential system optimization of this advanced CFB system was proposed. The results showed that the advanced CFB system was more environmentally friendly and resource-efficient than conventional MSW incineration systems. The recovery of material should be given priority over energy recovery. According to the assessment of the environment, and energy and material recovery, a process improvement with an incinerated refuse-derived fuel and a semi-compost produced by MBT as a soil conditioner was highly recommended.

Demarcation on a new conventional circulating fluidization regime in liquid-solids fluidization via experimental and numerical studies

A new type of liquid-solid circulating fluidized bed, the conventional circulating fluidized bed (CCFB), which operates at superficial liquid velocities below the particle terminal velocity (Ut), was delineated through experimental and numerical studies, in both upward and inverse liquid-solid fluidization systems. The CCFB system was essentially operated in the conventional fluidization regime but with continuous feeding of particles into the liquid inlet end of the fluidization column and particles moving along with the liquid to the other end of the column to achieve solids circulation or circulating fluidization. An extended operating window of the CCFB was numerically studied using the Eulerian-Eulerian two-fluid model based on the existing experimental data. The effects of particle properties and operating conditions on the CCFB were studied. The hydrodynamics in the CCFB were also compared with that in a circulating fluidized bed (CFB). An inverse CCFB using particles with a density lower than the liquid was also investigated by simulations and compared with the upward CCFB. A new flow regime referring to the CCFB operating mode for the liquid-solids fluidization systems, both upwards and inverse, was proposed and situated in between the conventional and circulating fluidized beds.

Experimental study on ultra-low initial NOx emission characteristics of Shenmu coal and char in a high temperature CFB with post-combustion

The NOx emission of conventional circulating fluidized bed (CFB) boilers could hardly meet the increasing strict standards in China, so the initial NOx emission and control of Shenmu coal and char were investigated in a 0.5 MWth high temperature CFB test platform with a main combustion zone (MCZ) and a post-combustion zone (PCZ), the reducing atmosphere in MCZ suppressed the formation of NOx, high temperature in PCZ made the residual carbon and CO burnout, the technology of high temperature CFB with post-combustion had the advantage of decreasing the initial NOx emission while maintaining the combustion efficiency of fuel. The experimental results showed that as the injection position of post-combustion air (PCA) moving backward, the reducing zone in PCZ was enlarged so more NOx could be converted to N2. When PCA was injected from PCA1 and PCA3, NOx and CO emission were both relatively lower than other injection positions, the temperature in PCZ of Shenmu char was more uniform and its NOx emission was lower compared to Shenmu coal, the peak temperatures of MCZ and PCC were 968 °C and 936 °C respectively, the initial NOx emission of Shenmu coal was 46 mg/m3 (at 6% O2), meanwhile, the CO emission was 139 ppm. The initial NOx emission of Shenmu char could reach 41 mg/m3 when the temperatures of MCZ and PCZ were 931 °C and 913 °C respectively, the new ultra-low emission requirement (≤50 mg/m3) was successfully achieved without additional denitration equipment, so the technology of high temperature CFB with post-combustion is an effective ultra-low nitrogen combustion technology.

Effect of Different Nozzle Arrangements on Gas–solid Flow Characteristics of a New Air Distribution System Circulating Fluidized Bed

ABSTRACT The air distribution system was one of the key devices that affects solid flow characteristics in circulating fluidized bed (CFB). Different air distribution systems have an influence on gas–solid flow uniformity, solids velocity, and concentration. In the conventional CFB, the air distribution system which composed of air chamber and distributor was placed on the bottom of the reactor. In this paper, an atmospheric circulating fluidized bed gasifier (AT-CFBG) was examined. The chamber and distributor of CFB was canceled and which was replaced by six nozzles with asymmetrical and symmetrical arrangement that mounted on the bottom of the furnace. A cold test was established at a scale of 1:15, the optical fiber was used to measure solids concentrations and velocities in the furnace under the asymmetrical and the symmetrical arrangement. The results showed that the asymmetrical arrangement has a better uniform velocity profile and has a higher solids concentration than those of the symmetrical arrangement. And it also has a stable solids circulation and a higher solids circulation rate (0.37 kg/h) than those of the symmetrical arrangement (0.34 kg/h).

Comparing the structural development of sand and rock ilmenite during long-term exposure in a biomass fired 12 MWth CFB-boiler

Oxygen Carrier Aided Combustion (OCAC) is a novel combustion concept with the purpose to increase the overall efficiency in conventional circulating fluidized bed (CFB) boilers. By replacing the commonly used bed material with an oxygen carrier (OC), the conceptual idea is to utilize the fluid dynamics in a CFB and the inherent oxygen transport supported by the OC to increase the oxygen distribution within the furnace in time and space. The OCAC concept has been successfully validated and further reached long-term demonstration in full scale operation (75-MWth).This work presents a first evaluation of how ilmenite particles are affected in regard to mechanical resistance during long-term exposure to OCAC conditions in Chalmers 12-MWth CFB-boiler. A sand and a rock ilmenite are evaluated with regard to their mechanical stability. For evaluation, samples of the fresh materials and samples collected during operation in the Chalmers boiler are investigated. The study shows that the two materials differ in how the mechanical degradation occurs with exposure time. The sand ilmenite form cavities which are held together by an ash layer before they are shattered into numerous pieces, whereas the rock ilmenite develops distinct cracks that cause splitting of the particles.

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.

Investigation of gas–solids flow characteristics in a circulating fluidized bed with annular combustion chamber by pressure measurements and CPFD simulation

For large scale circulating fluidized bed (CFB) boilers, a new type of annular combustion chamber was proposed to improve the secondary air penetration and provide adequate space to arrange the heat transfer surface. To investigate the gas–solids flow characteristics and optimize the structure of the annular combustion chamber, a “cold” test rig was built with a cross sectional area of 1.18m2 and a height of 8.0m. High frequency pressure transducers were used to obtain the pressure fluctuations characteristics. CFD simulation based on computational particle fluid dynamic (CPFD) was used to obtain pressure fluctuations and particles concentration. The time domain analysis, power spectrum analysis and Hilbert–Huang transform were applied to the experimental and simulation data. Different flow regimes including single bubble regime, exploding bubble regime and turbulent fluidization regime were identified. The flow characteristics were investigated in terms of standard deviation and higher-order moments, auto-correlation, power spectrum, and EMD energy entropy. The results show that the gas–solids flow in the bottom region of the annular combustion chamber was relatively uniform. The gas–solids flow in annular combustion chamber was proved similar to that in a conventional CFB riser.

Simulation of a Lignite‐Based Polygeneration System Coproducing Electricity and Tar with Carbon Capture

AbstractLignite‐based polygeneration systems for coproducing tar and electricity with and without carbon capture and storage (CCS) were proposed and simulated. Predried lignite was pyrolyzed into coal gas, tar, and char. Coal gas was fired in a gas turbine after the cleanup process, while char was combusted in circulating fluidized‐bed (CFB) boilers. The polygeneration plant without CCS turned out to be more efficient than the conventional CFB power plant, suggesting that the former is a promising and efficient option to utilize lignite resources. Moreover, the performance and emissions of polygeneration plants with and without CCS were compared. It was shown that the more CO2 is captured, the larger energy penalty it will cost. Therefore, a trade‐off should be made between low emissions and high efficiency.

Thermodynamic and economic analysis of polygeneration system integrating atmospheric pressure coal pyrolysis technology with circulating fluidized bed power plant

Lignite-based polygeneration system has been considered as a feasible technology to realize clean and efficient utilization of coal resources. A newly polygeneration system has been proposed, featuring the combination of a 2×300MW circulating fluidized bed (CFB) power plant and atmospheric pressure fluidized bed pyrolyzers. Xiaolongtan lignite is pyrolyzed in pyrolyzers. Pyrolyzed volatiles are further utilized for the co-generation of methanol, oil, and electricity, while char residues are fired in CFB boilers to maintain the full load condition of boilers. Detailed system models were built, and the optimum operation parameters of the polygeneration plant were sought. Technical and economic performances of optimum design of the polygeneration plant were analyzed and compared with those of the conventional CFB power plant based on the evaluation of energy and exergy efficiency, internal rate of return (IRR), and payback period. Results revealed that system efficiency and the IRR of the polygeneration plant are ca. 9% and 14% points higher than those of the power plant, respectively. The study also analyzed the effects of market fluctuations on the economic condition of the polygeneration plant, and found that prices of fuel, material, and products have great impacts on the economic characteristics of the polygeneration plant. Polygeneration plant is more economic than CFB power plant even when prices fluctuate within a wide range. This paper provides a thorough evaluation of the polygeneration plant, and the study indicates that the proposed polygeneration plant has a bright prospect.

Effect of Bed Pressure Drop on Performance of a CFB Boiler

The effect of bed pressure drop and bed inventory on the performances of a circulating fluidized bed (CFB) boiler was studied. By using the state specification design theory, the fluidization state of the gas−solids flow in the furnace of conventional CFB boilers was reconstructed to operate at a much lower bed pressure drop by reducing bed inventory and control bed quality. Through theoretical analysis, it was suggested that there would exist a theoretical optimal value of bed pressure drop, around which the boiler operation can achieve the maximal combustion efficiency and with significant reduction of the wear of the heating surface and fan energy consumption. The analysis was validated by field tests carried out in a 75 t/h CFB boiler. At full boiler load, when bed pressure drop was reduced from 7.3 to 3.2 kPa, the height of the dense zone in the lower furnace decreased, but the solid suspension density profile in the upper furnace and solid flow rate were barely influenced. Consequently, the average heat transfer coefficient in the furnace was kept nearly the same and the furnace temperature increment was less than 17 °C. It was also found that the carbon content in the fly ash decreased first with decreasing bed pressure drop and then increased with further increasing bed pressure drop. The turning point with minimal carbon content was referred to as the point with optimal bed pressure drop. For this boiler, at the optimum point the bed pressure was around 5.7 kPa with the overall excess air ratio of 1.06. When the boiler was operated around this optimal point, not only the combustion efficiency was improved, but also fan energy consumption and wear of heating surface were reduced.

Demarcation of a new circulating turbulent fluidization regime

AbstractTransient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Hydrodynamic Modeling of CFB Risers and Downers

Making best use of the Circulating Fluidized Bed (CFB) hydrodynamics is crucial in order to attain optimal yield and selectivity of desirable products in such reactors. While the conventional CFB riser has been proven to be highly effective for fast gas-solid reaction systems, the CFB downer promises further improvements over the CFB riser, due to its more uniform flow and better controlled fluid-solid contact times. To obtain a better understanding of both reactors, individual hydrodynamic models are developed for a downer and a riser. Both hydrodynamic models incorporate parameters that have not been investigated in depth so far. For example, empirical correlations have been developed to estimate the agglomeration within downers and the slip velocity in risers.The simulation results of both models are verified using experimental data that has been collected using a state-of-the-art riser/downer pair. The comparison of downer and riser model predictions reveals that downers operate at higher particle velocities and under more dilute conditions than risers and may therefore be better suited for fast chemical reactions that are run under dilute conditions. The benefit of downers may be extended if the higher solids holdups observed in risers can be attained through improvements in recycling and feeding particles into the downer. Most of the catalyst particles that the feed gas contacts in the downer are at their highest activity levels since the particles move slightly faster than the gas phase due to gravitational effects. Also, in case of catalytic reactions where over-cracking is an issue, the narrow residence time distribution in downers allows for better reaction control that leads to higher rates of desirable products. Therefore, at similar solids concentrations, downers are expected to outperform risers for catalytic reactions.