Loop Seal Research Articles

Interconnected pyrolysis and gasification of typical biomass in a novel dual fluidized bed

A novel 50 kg/h dual fluidized bed (DFB) was designed and constructed, and pilot study of interconnected pyrolysis and gasification/activation were conducted. Four typical biomass materials were investigated and compared in DFB system. Pyrolyzer temperature and riser temperature were 480–550 °C and 800–850 °C, respectively. Dolomite were used as bed material, and superheated steam was adopted as the fluidizing gas in pyrolyzer and transport gas in loop seals. Biochars were comprehensively characterized and analyzed, and were potential matrix for carbon-based materials. Biochar from wood sawdust was the best carbon-based material matrix compared to other biochars with the highest adsorption capacities of iodine (397.08 mg/g) and methylene blue (600.15 mg/g). Phenols and benzenes (total content of 63.26–97.36%) were the richest components in all bio-oils. Non-condensable pyrolysis gas from corn cob had the highest heating value (10.14 MJ/Nm3) and largest H2 content (23.91 vol%). The total energy yield was 63.54–84.95%, while energy efficiency was 63.79–82.47%. The carbon fixation ratio was in the range of 38.61–53.27% and the highest carbon fixation ratio was 53.27% from peanut shells. The advantages and feasibility of interconnected pyrolysis and gasification/activation technologies by DFB for biomass utilization and carbon capture were assessed in this pilot study. DFB is a promising technology to achieve carbon negative economy, and deserves more research in the future.

Impact of Particle Size Polydispersity on Chemical Looping Combustion of Biomass under a Continuous Injection Mode

Particle size polydispersity considerably influences the chemical looping combustion (CLC) of solid fuels. In the present work, a three-dimensional multiphase particle-in-cell (MP-PIC) method was established and applied to study the full-loop CLC of biomass under a continuous injection mode. After model validation, the gas–solid flow characteristics, solid circulation rate (SCR), particle mixing in a fuel reactor (FR), and biomass leakage at different oxygen carrier (OC) particle size distribution (PSD) widths and biomass densities were systematically explored. The results showed that OC particles significantly stratified within the FR and both loop seals. Moreover, the fluctuation amplitude of the SCR decreased as the OC PSD width increased. Small biomass particles were more likely to leak. The number fraction of total leaked biomass particles first decreased and then increased as the OC PSD width increased. In the FR, small OC particles were primarily distributed in the upper part of the bed, and large particles were mainly distributed in the lower part. In addition, within the test range, the magnitude of the density difference between biomass and OC particles positively correlated with the likelihood of biomass leaking. These observations are all beneficial for the design and optimization of the CLC process of biomass.

On the comprehension of the gas split in loop seal devices

Gas distribution through the chambers of a loop seal in circulating fluidized bed (CFB) units is still difficult to predict and often misunderstood. Dedicated experiments were carried out in an isolated loop seal to assess how the gas fed into a loop seal is distributed under different aeration modes (through the recycle chamber, the supply chamber and both chambers), aeration flowrates and solids fluxes. Experimental results shed light on the non-ideality of the gas-solids flows occurring in these units and, in combination with theoretical considerations, it is demonstrated why semi-empirical models cannot effectively describe the performance of loop seals operating as non-mechanical valves. Recommendations are given for the optimization of the operation of a loop seal as a non-mechanical valve in a solids circulating loop. A method is proposed to characterize the performance of a loop seal in a CFB unit from measurements of that loop seal in isolation mode. • Gas splitting into the chambers of a loop seal is a key issue difficult to predict. • The gas distribution was experimentally assessed in an isolated loop seal. • Loop seal operation as non-mechanical valves leads to non-ideal gas-solids flows. • Non-ideal gas-solids flow pattern establishes the gas splitting into the chambers. • The study gives insights on how to control the solids circulation with a loop seal.

Comparing Analysis of Loss of Coolant Accident on Bethsy Facility with Apros 6.05 and 6.06.

In this paper, the comparison of analysis of international standardized problem ISP 27 using two versions of APROS process simulation software 6.05 and 6.06 is presented. Numerical simulation of experiment Bethsy 9.1b, also known as ISP 27 was performed on a scaled down model of a three loop, 900 MWe Framatome PWR. In the test a small LOCA, with 2-inch cold leg break, combined with High pressure Injection System (HPIS) failure is simulated. State oriented approach, which requires operators to start an Ultimate Procedure were used. Model was first built in APROS 6.05 using standard modules in order to describe the volumes, heat structures and regulation of the test facility and was then exported to APROS 6.06. The results from both versions showed all the processes such as loop seal clearing, core uncover and rise of cladding temperature and other processes taking place in the experiment were in a good agreement with experimental data. However even though results were similar some differences were noticeable. The differences in core cladding temperature, time integrated break mass flow, core liquid level and pressurized pressure were analysed in more detail in this paper.

Comparison of Blind and Open Calculation Results for Top-Slot Break LOCA in Fourth ATLAS Domestic Standard Problem

The advanced thermal-hydraulic test loop for accident simulation (ATLAS) was developed and operated at The Korea Atomic Energy Research Institute. The ATLAS is operated to simulate accidents in pressurized water reactors (PWRs). A domestic standard problem (DSP) using the ATLAS was proposed for transferring the database of the integral effect test to Korean nuclear researchers and developing the safety analysis methodology of PWRs. The fourth DSP (DSP-04) exercise was performed during 2015–2017 with 15 participants (13 organizations), that are universities, government, and nuclear industries. In DSP-04, a top-slot break at the cold leg was chosen as the target scenario to resolve an issue about the effect of loop seal clearing and the reformation on the peak cladding temperature during the cold leg top-slot break LOCA for APR1400. The participants performed a code calculation for the experimental simulation and sensitivity studies for the enhancement of the code. This paper includes brief information about the experimental and major code assessment results.

A Comparison Study on the Resistance Towards Material Degradation of Novel Fecral Alloys in the Loop Seal Superheater Position of a Waste Fired Cfb Boiler

A Comparison Study on the Resistance Towards Material Degradation of Novel Fecral Alloys in the Loop Seal Superheater Position of a Waste Fired Cfb Boiler

Minimizing gas leakages in a system of coupled fluidized bed reactors for chemical looping combustion

Chemical Looping Combustion (CLC) is a technology to generate a pure stream of CO2 during the combustion of fossil or renewable fuels for power generation. This is carried out in a system of interconnected fluidized bed reactors, at least one so-called air reactor and a fuel reactor. Gas leakages are a major problem in those CLC systems as undesired gases in the exhaust can make the later sequestration more expensive and might lead to further gas cleaning. Therefore, the off-gas of the fuel reactor, where the fuel is mainly converted, should be as pure as possible. In those interconnected fluidized beds, the loop seals or so-called siphons should prevent the gas leakage from one side to the other. The present work focuses on the pressure conditions inside an experimental 25 kWth pilot scale fluidized bed reactor system for CLC to minimize gas leakage between the single reactors. The system consists of a riser air reactor and a two-stage bubbling fluidized bed fuel reactor system. By using CO2 as tracer gas at different positions, the gas distribution from the two loop seals through the system as well as the leakage flow from the air reactor via the cyclone and loop seal to the fuel reactor is analyzed. From a detailed experimental and sensitivity analysis of the system behavior in different operation conditions, measures to minimize gas leakage are developed. A key factor for the understanding of the gas leakages in the system was derived from pressure drop analysis of 23 pressure ports. Each process unit is examined and design criteria for such systems of interconnected fluidized beds are derived.

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.

Fluid dynamic analysis of dual fluidized bed gasifier for solar applications

A hydrodynamic model of a dual fluidized bed gasifier (DFBG) is developed and its predictions are compared with measurements of solids flux and pressure profiles from a cold flow model (CFM). Then, the performance of a DFBG gasifier is theoretically analyzed in terms of solids circulation and solids distribution under changes in riser and loop seal aeration, solids inventory and particle size, and a sensitivity analysis is made to delimit the model prediction capability. Furthermore, the model is applied to analyze the effects of key design aspects of DFBG, such as the relative size of riser and gasifier, the connection between both units, the circulation rate of solids and their distribution around the system. The model is further used to extend the DFBG operation with external solar energy carried by heated solid particles, i.e. to design solar DFBG (SDFBG). The analysis is focused on the performance with high solids inventory in the gasifier to increase the char conversion (operation with a large solar share) and the control of solids circulation to meet the heat demand of the gasifier with the availability of solar energy. The operation with large solids inventory in the gasifier requires the size of the gasifier to increase considerably compared to that of the conventional DFBG. The substitution of the connection pipe between the riser and the bubbling bed (current design in commercial DFBG) by a lower loop seal enables better control of the solids circulation, thus, benefiting the solar design.

Modeling study on the dynamic characteristics in the full-loop of a 350 MW supercritical CFB boiler under load regulation

To improve the load regulation ability of the circulating fluidized bed (CFB) unit, investigating the hydrodynamic characteristics of the gas-solid flow in the CFB loop during such processes is of vital importance. In this work, based on the mass and pressure balance in the whole loop, a one-dimensional dynamic model on the gas-solid flow in the full-loop of an industrial CFB was established. In particular, the solid mass flow rate at the dust exit was correlated with the total inventory of the cyclone, and the re-circulating rate to the furnace was depicted by the pressure drop in the loop seal. The model was validated by the field test data of a 350 MW supercritical CFB boiler during a load increasing process. Subsequently, the impact of the load change rate and operating bed pressure on the hydrodynamic performance in the whole loop of the CFB boiler during the load reduction process were analyzed. The results show that the bed pressure in the furnace will fluctuate more violently when the load change rate increases, while the operating bed pressure has a weak effect on the load regulation process. These conclusions can provide theoretical guidance to the variable load operation of the CFB boiler.

Solid Circulation Study in a 1.5 MWth Cold Flow Model of Chemical Looping Combustion

Solid circulation in chemical looping combustion (CLC) is very important and affects the mass and heat balance and autothermal operation of a CLC system. A key task in developing CLC technology is to control the solid circulation. In this work, the solid circulation characteristic of a 1.5 MWₜₕ CLC cold flow model is reported. The solid circulation between the fuel reactor and the simplified air reactor riser is controlled by the overflow method. Three kinds of quartz sands are selected as fluidized particles, and their median particle diameters are 392, 249, and 122 μm, respectively. A reasonable pressure profile is obtained in the 1.5 MWₜₕ CLC cold flow model. The effects of operational parameters, including the fuel reactor gas velocity, loop seal gas velocity, simplified riser gas velocity, particle size, and static bed height, on the solid circulation and hydrodynamic characteristics are measured and analyzed. The maximum solid circulation rate can approach 130 kg/(m²·s), and this value satisfies the requirements of mass and heat balance in the CLC system. The static bed height in the fuel reactor should be higher than the overflow port to prevent it from becoming a constraint factor on the solid circulation rate. An overflow model is developed to predict the solid circulation rate, and the relative errors between the predicted result and the experimental data are within 25%.

CPFD simulation of a dual fluidized bed cold flow model

The present work was carried out to simulate a cold flow model of a biomass gasification plant. For the simulation, a Eulerian-Lagrangian approach, more specifically the multi-phase particle in cell (MP-PIC) method, was used to simulate particles with a defined particle size distribution. Therefore, Barracuda VR, a software tool with an implemented MP-PIC method specifically designed for computational particle fluid dynamics simulations, was the software of choice. The simulation results were verified with data from previous experiments conducted on a physical cold flow model. The cold flow model was operated with air and bronze particles. The simulations were conducted with different drag laws: an energy-minimization multi-scale (EMMS) approach, a blended Wen-Yu and Ergun drag law, and a drag law of Ganser. The fluid dynamic behavior depends heavily on the particles’ properties like the particle size distribution. Furthermore, a focus was placed on the normal particle stress (PS value variation), which is significant in close-packed regions, and the loop seals’ fluidization rate was varied to influence the particle circulation rate. The settings of the simulation were optimized, flooding behavior did not occur in advanced simulations, and the simulations reached a stable steady state behavior. The Ganser drag law combined with an adjusted PS value with (PS = 30 Pa) or without (PS = 50 Pa) increased loop seal fluidization rates provided the best simulation results.

Commissioning Analysis and Improvement measures for Biomass Circulating Fluidized Bed Gasifier

This paper presents the debugging test of biomass circulating fluidized bed gasifier in a rice mill factory of Jiangsu Province. The main debugging process of the gasifier system is reported. The typical problems of the gasifier system during the debugging period are analyzed, including the blockage of the primary cyclone separator, blockage of loop seal, bed inventory leakage of blast cap on air distribution plate, feeding tube coking, and large fluctuation of bed pressure. Reasonable improvement countermeasures and implementation plan is provided. The operation characteristics of the gasifier are preliminaries mastered through debugging, which provides reference for the operation and adjustment of the biomass circulating fluidized bed gasifier. These results provide certain guiding significance for the scale application of biomass circulating fluidized bed gasifier.

OPERATIONAL INFLUENCE OF THE MONO-CHAMBER AERATION MODE IN THE LOOP SEAL OF A CIRCULATING FLUIDIZED BED

Fluidization numbers varying from 0.84 to 1.68 were used in the loop seal valve of a bench-scale circulating fluidized bed (CFB) system to analyze the influence of the mono-chamber aeration mode on both the solids circulation rate and the static pressure drop inside the solids recycle device. Runs were carried out using 4 kg of overall solids inventory and particles of 183 µm in Sauter mean diameter, which were kept under fast fluidization regime at superficial gas velocity of 4 m/s. Results showed that the choice of the chamber to be aerated can noticeably affect the gas-solid hydrodynamics. In this sense, the analysis of variance applied on the experimental data indicated that the aeration into the recycle chamber of the loop seal offers lower levels of solids circulation rate but also allows to control it within a wider range of fluidization numbers and with less pressure drop or energy demand.

Simulation of co-gasification of coal and wood in a dual fluidized bed system

A comprehensive three-dimensional (3D) model based on multiphase particle-in-cell (MP-PIC) approach is developed to simulate the co-gasification process of coal and wood in a dual fluidized bed (DFB) system. The pyrolysis of coal and wood, formation and conversion of tar and gaseous pollutants, as well as gasification and combustion reactions are integrated with dense gas-solid flow and heat transfer. The pilot DFB system comprises a bubbling bed gasifier and a fast fluidized bed combustor connected by loop seals. The model correctly predicts profiles of pressure and temperature, the yield and components of the product gas. The gas composition distributions, allocations of particle mass, and solid residence time inside the reactors are explored. The effects of various fuel blend ratios and particle sizes on gasification performances are also investigated. The results show that increasing coal ratio accelerates the steam gasification due to higher char content, which results in the increment of H2 and CO concentrations. The tar content in the product gas continues to decline, while the emissions of NH3 and H2S increase. The size variation of feedstock is not enough to dramatically affect product gas components. The tar content and product gas yield appear a slight upward trend with the smaller size. The variations of NH3 and H2S concentrations are consistent with those of bed temperature.

Investigation on influences of loose gas on gas-solid flows in a circulating fluidized bed (CFB) reactor using full-loop numerical simulation

Abstract The influences of loose gas on gas-solid flows in a large-scale circulating fluidized bed (CFB) gasification reactor were investigated using full-loop numerical simulation. The two-fluid model was coupled with the QC-energy minimization in multi-scale theory (EMMS) gas-solid drag model to simulate the fluidization in the CFB reactor. Effects of the loose gas flow rate, Q, on the solid mass circulation rate and the cyclone separation efficiency were analyzed. The study found different effects depending on Q: First, the particles in the loop seal and the standpipe tended to become more densely packed with decreasing loose gas flow rate, leading to the reduction in the overall circulation rate. The minimum Q that can affect the solid mass circulation rate is about 2.5% of the fluidized gas flow rate. Second, the sealing gas capability of the particles is enhanced as the loose gas flow rate decreases, which reduces the gas leakage into the cyclones and improves their separation efficiency. The best loose gas flow rates are equal to 2.5% of the fluidized gas flow rate at the various supply positions. In addition, the cyclone separation efficiency is correlated with the gas leakage to predict the separation efficiency during industrial operation.

Measurement of bed-to-tube surface heat transfer coefficient to a vertically immersed u-tube in bubbling loop seal of a CFB boiler

Experimental measurements of bed-to-tube surface heat transfer coefficient were conducted in a 12 MWth circulating fluidized bed boiler/gasifier where a vertical U-tube was immersed into the bed. The aim was to study the influence of superficial gas velocity, solid flux of bed material, direction of the tube relative to the circulating particle flow, choice of fluidization gas, bed temperature and to compare the results with predicted values based on heat transfer correlations.The experiments displayed high heat transfer coefficients of around 550 W/(m2K) with sand and with ilmenite approximately 850 W/(m2K) when operating with air at a superficial gas velocity of 0.25 m/s. Replacing air with steam and recirculated flue gas as fluidization gas increased the heat transfer by more than 40% with both steam and flue gas. Having the U-tube oriented parallel to the circulating particle flow reduced the heat transfer by around 8% compared to a perpendicular orientation.

A comparative evaluation of recarbonated CaCO3 derived from limestone under oxy-fuel circulating fluidized bed conditions

SO2 emissions from coal-fired boilers are air pollutants and a source of acid rain, causing extensive environmental pollution. Limestone (CaCO3) is a Ca-based sorbent which is injected into circulating fluidized bed (CFB) boilers, where it combines with SO2 to produce calcium sulfate (CaSO4). As a result, SO2 emissions from a power plant are reduced. In this study, CaCO3 addition was proposed and the desulfurization efficiency improved. The direct desulfurization reaction is dominant in a commercial CFB boiler due to the high CO2 partial pressure, but CaO is formed at a fast reaction rate by calcination in the high temperature or in the low CO2 partial pressure region. When CaO remains in the loop seal, it is exposed to a high CO2 partial pressure condition moving through the recirculation section for an extended period and re-injected into the furnace as recarbonated CaCO3. To analyze the direct desulfurization reaction kinetics, a shrink core model in which the reaction proceeds inside the particle was adopted. Surface observations through FE-SEM of CaSO4 produced by the 180 minute long desulfurization experiment using TGA suggest that the CaSO4 crystal growth rate increased after the pre-treatment (recarbonation) of limestone. Recarbonation lowered the limestone crystallinity, causing a faster reaction. The CaCO3 recarbonation increased the Ca utilization by more than 20% when the direct desulfurization reaction occurred. The TGA experiments show that recarbonation contributes to CaSO4 conversion. Increasing the desulfurization efficiency using recarbonation can reduce the fixed investment and operating costs of oxy-fuel CFB plants because only desulfurization in the furnace is able to meet SO2 emission regulations or lower the flue gas desulfurization (FGD) dependence. Accordingly, the desulfurization conversions of recarbonated CaCO3 and limestone were compared in this study. Morphological changes in the limestone were also evaluated using XRD, FE-SEM, and other analysis methods.

Counterpart test for LSTF 1% cold-leg break LOCA (SB-CL-32) utilizing ATLAS test facility

The counterpart test of the OECD-ATLAS project named A5.1 was performed using a thermal–hydraulic integral effect test facility, ATLAS. The target scenario for the A5.1 test was the LSTF SB-CL-32 test. The SB-CL-32 test is a 1% horizontal small break loss of coolant accident (SBLOCA) at the cold leg with cold leg injection of the emergency core cooling system (ECCS) and accident management (AM) action. The test objective is to investigate the thermal hydraulic phenomena during a cold leg SBLOCA such as core heat-up, loop seal clearing (LSC), and the effect of AM actions. To achieve the test objectives, scaling analysis between LSTF and ATLAS was conducted first. The detailed test conditions such as initial and boundary conditions and sequence of events were determined based on the scaling analysis result. The experimental result showed that overall thermal hydraulic behavior which can be expected in this kind of SBLOCA scenario was observed. However, significant differences were found in the A5.1 test result compared with the LSTF SB-CL-32 test result in the view point of the loop seal clearing phenomenon and the maximum heater rod surface temperature behavior. These differences result from the different design between two test facilities which are refer to the different prototype nuclear power plant. Thus, several test condition parameters were analyzed as the sensitivity study utilizing the thermal hydraulic system analysis code to check their effect on the system response. From the sensitivity analysis result, it was confirmed that the A5.1 test was well modeled as a counterpart test of LSTF and realized the thermal hydraulic behavior during SBLOCA transient. This test result can be utilized to evaluate the prediction capability of thermal hydraulic system analysis codes and to validate the scale-up capability of integrated effect test (IET) test data to the nuclear power plant condition.

Numerical study of biomass gasification in a 0.3 MWth full-loop circulating fluidized bed gasifier

Biomass gasification process in a three-dimensional pilot-scale circulating fluidized bed (CFB) gasifier is numerically simulated via the multiphase particle-in-cell approach. The physical and thermochemical properties of gas and solid phases (i.e., temperature, density, and viscosity, pressure, and species) in the gasifier under high temperature are comprehensively explored. The results show that the size- and density-induced segregation causes the accumulation of biomass particles in the standpipe. Elutriation of biomass particles leads to the presence of low-temperature endothermic zones and high-temperature exothermic regions in the riser. The large contents of combustible gas species exist in the left part of the riser. Continuous decrease of O2 is due to gas species oxidation, while the reduction of N2 results from the release of gas species. The dense region has a large content of N2 but a small content of combustible gas species. Large turbulent gas viscosity appears in the riser. Enlarging the gas velocity decreases the solid concentration but increases the gas temperature. Gasification temperature has an insignificant influence on the solid concentration and the contents of combustible gas species. Based on the relationship between the solid transportation and gas–solid thermochemical properties, the current work provides a guideline for the optimization of CFB gasifiers, for example, the loop seal is advised to be optimized to enhance the gasification performance of this kind of apparatus.