Inert Bed Material Research Articles

High-temperature potassium capture by ilmenite ore residue

In-situ alkali metals capture within the furnace is an effective method for mitigating issues such as fouling, slagging, agglomeration, and corrosion associated with the combustion of high-alkali fuels. Oxygen carrier aided combustion (OCAC) technology has been proposed to enhance the combustion efficiency of fluidized bed by replacing inert bed material with active oxygen carrier, demonstrating significant potential in preventing ash-related problems induced by alkali metals. From the perspective of coordinated disposal of solid waste and resource recycling, this study utilized ilmenite ore residue with a relatively low titanium grade as the active bed material in a fluidized bed. The high-temperature in-situ capture characteristics of potassium were investigated under both static and fluidized conditions. Characterization techniques such as ICP-OES, SEM-EDS, and TG-MS, were employed to unveil the behavior of potassium fixation in ilmenite. The results showed that ilmenite ore residue affects the decomposition temperature range and product distribution of K2CO3, thereby reducing the escape of potassium into the gas phase. Over 30 % of the injected potassium was captured in the first 6.5 h, and it remained at about 22.6 % after 19.5 h. Potassium tends to bond with titanium, forming stable KTi8O16 compounds. The high-temperature enrichment and migration mechanism of potassium in two primary mineral phases of ilmenite ore residue are proposed.

NOx and SO2 emission of oxygen carrier aided combustion in fluidized bed

Oxygen carrier aided combustion (OCAC) has been established as an innovative concept for improving fuel combustion in fluidized bed (FB) combustors. OCAC uses the particles of oxygen carrier (OC) to partly or completely replace the inert bed materials within the FB, which can make the oxygen distribution in the combustor more uniform and subsequently enhance combustion efficiency. However, the fundamental study on SO2 and NOx emissions during the OCAC is still not sufficient and controversial. In this work, the OCAC of coal and petroleum coke has been evaluated under different operating conditions on a lab-scale FB combustor with a special focus on the SO2 and NOx emissions. The conversion ratios of volatile-N (XV-N) and char-N (XC-N) have been quantitatively decoupled, and the influence of operating parameters on XC-N and XV-N has been revealed. Results showed that the OCAC concept has significantly reduced CO emission, enhanced NOx emission, and increased nitrogen conversion (XN). However, the NOx emission decreased by reducing the ilmenite ore proportion in the bed materials. This was attributed to the presence of ilmenite ore, which reduced the reducing regions and weakened the NOx reduction. Under OCAC operation, the XV-N increased significantly with the increase of O2 concentration and primary air ratio (A); and the variation of A shows a most decisive effect on XV-N. However, the XC-N was insensitive to the change of O2 concentration and A, when the XC-N was in the range of 31.01–34.89 % for all OCAC operations. The significant reduction of the SO2 emission during OCAC is mainly due to the employment of the ilmenite ore as bed materials that not only can absorb S element, but also lead to more uniform distribution of both oxygen and bed temperature, thereby improving the self-desulfurization of ash and reducing the sulfate decomposition.

Effect of time-dependent layer formation on the oxygen transport capacity of ilmenite during combustion of ash-rich woody biomass

Oxygen carrier aided combustion (OCAC) is a novel technology that aims to enhance combustion of heterogenous fuels by replacing the inert bed material with an active oxygen carrier. One of the promising oxygen carriers is natural ilmenite which shows decent oxygen transport capacity and mechanical stability under OCAC operating conditions. However, interactions between ilmenite and woody biomass ash lead to the formation of a calcium-rich ash layer, which affects the ability of the oxygen carrier (OC) to transfer oxygen throughout the boiler and subsequently decreases the combustion efficiency. This paper focuses on the time-dependent morphological and compositional changes in ilmenite bed particles and the consequence effects on the oxygen transport capacity and reactivity of ilmenite. Ilmenite utilized in this study was investigated in a 5 kW bubbling fluidized bed combustion unit, utilizing ash-rich bark pellets as fuel. A negative effect of iron migration on the oxygen transport capacity was observed in ilmenite bed particles after 6 h of operation in the bubbling fluidized bed reactor. The decrease in the oxygen transport capacity of ilmenite was found to correlate with the increased exposure time in the fluidized bed reactor and was caused by the migration and subsequent erosion of Fe from the ilmenite particles. On the other hand, the older bed particles show an increase in reaction rate, presumably due to the catalytic activity of the calcium-enriched outer layer on the bed particle surface.

Study of air-steam gasification of spherocylindrical biomass particles in a fluidized bed by using CFD-DEM coupling with the multi-sphere model

In this work, a reactive CFD-DEM coupling with the multi-sphere model has been established and used to investigate the air-steam gasification of spherocylindrical biomass particles in a fluidized bed. Specifically, biomass particles are assumed to have a spherocylindrical shape while small inert bed materials (i.e., sand particles) have a spherical shape. Both kinds of particles are characterized by the multi-sphere model and each particle is individually tracked. Moreover, particle-particle collisions, heat convection, heat conduction, radiation, and homogeneous and heterogenous reactions during biomass gasification are all considered. After model validation, the effect of biomass particle aspect ratio on five different indicators (e.g., fluidization behavior, gas production, particle thermochemical conversion, particle height, and particle orientation) are systematically explored. Results show that the gas porosity near the fuel injection location is high due to the gas release of the injected biomass. Enlarging the particle aspect ratio can accelerate biomass conversion processes. In addition, more particles prefer to show a nearly vertical orientation in the fluidization region, and this orientation behavior can be enhanced by increasing the particle aspect ratio.

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.

Transient simulation of biomass combustion in a circulating fluidized bed riser

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

Iron based oxygen-carrier-aided oxy-fuel combustion of coal char: Reactivity and oxygen transfer mechanism

Adding an active oxygen carrier to the inert bed material may favor the oxygen distribution in fluidized bed (FB) combustors. This so-called method “Oxygen-carrier-aided combustion, OCAC” has been applied recently to air combustion in FB boilers, but could also be potentially interesting in oxy-fuel combustion. Here we analyze OCAC in oxy-fuel combustion by experimental determination of the reactivity of char from three coals (lignite, bituminous and anthracite coal) using three oxygen carriers (OC, analytical reagent Fe2O3, hematite, and steel slag). The tests were conducted in a micro thermo-gravimetric analyzer (micro-TGA) and the combustion kinetics was determined for the nine combinations of char and OC. Additional tests were conducted in a macro-TGA i.e., much sample in a big ceramic crucible, to evaluate the phase composition and crystal quality using X-ray diffraction (XRD) to the combustion residues collected at various degrees of carbon conversion (0.2, 0.5, and 0.8). The results show that the burnout temperature decreases with OCs as the combustion rate increases, while the ignition temperature hardly changes. The enhancement of the rate of char combustion by OCs is in the following order: steel slag < hematite < AR-Fe2O3. The lower the fuel rank, the stronger the effect of the oxygen carrier. The combustion kinetics of chars using OC has a higher activation energy and pre-exponential factor than pure char. The enlarged lattice constant of Fe2O3 measured by XRD during the char combustion indicates that the redox reactions of Fe2O3 accelerate the oxidization of char by enhancing the rate of oxygen transport to the carbon surface.

Process Analysis of Chemical Looping Gasification of Biomass for Fischer–Tropsch Crude Production with Net-Negative CO2 Emissions: Part 1

Large-scale biofuel production plants require an efficient gasification process that generates syngas of high quality (with minimal gas contaminants and inert gases) to minimize the extent of the syngas cleaning processes required for liquid biofuel production. This work presents process modeling of the chemical looping gasification (CLG) process for syngas production. The CLG process is integrated with a Fischer–Tropsch synthesis (FTS) process to produce Fischer–Tropsch (FT) crude with net-negative CO2 emissions, enabling process and system-level analyses of this novel biomass-to-liquid process. CLG resembles indirect gasification in an interconnected circulating fluidized bed reactor, where instead of inert bed material, a solid-oxygen carrier, such as mineral ores rich in iron or manganese oxides, is used. The oxygen carrier particles undergo oxidation and reduction in the air reactor and fuel reactor, respectively, thereby providing heat and oxygen for gasification. This work uses data from CLG experiments performed with steel converter slag as the oxygen carrier and investigates its potential when integrated with different downstream gas cleaning trains and the subsequent fuel synthesis process with the primary objective of quantifying and evaluating the performance of the integrated CLG–FT process plant. Syngas with a high energy content of 12 MJ/Nm3 (lower heating value basis) is predicted with a cold gas efficiency of 73%. CO2/CO ratios, higher than indirect biomass gasification, are also predicted in the raw syngas produced; thus, there exists an opportunity to capture biogenic CO2 with a relatively lower energy penalty in the subsequent gas cleaning stages. This work quantifies other key performance indicators, such as heat recovery potential, negative CO2 emission capacity, and FT crude production efficiency of the CLG–FT plant. A 100 MWth CLG plant produces roughly 677–696 barrels per day of FT crude, with net-negative emissions of roughly 180 kilotonnes of CO2 annually.

Solar-driven gasification in an indirectly-irradiated thermochemical reactor with a clapboard-type internally-circulating fluidized bed

A 7-kWth indirectly irradiated solar gasifier with a clapboard-type internally circulating fluidized bed was developed and conducted experimentations under concentrated solar radiation. The solar CO2 gasification of charcoal produced by low-temperature pyrolysis was examined under a 28-kWe high-flux solar simulator considering different operating conditions. The effects of CO2:C and feedstock mass on the key performance indicators e.g. the carbon conversion, energetic upgrade factor, and solar conversion efficiency were investigated by using alumina granules as an inert bed material. The first-hand, reproducible experimental data indicated the highest values of the average carbon conversion of 0.53 ± 0.04, energetic upgrade factor of 0.883 ± 0.03, solar conversion efficiency of 5.3 ± 0.6% were achieved under the operating conditions of 80 g charcoal, 1.5 kg alumina particles, and 2 L/min CO2. The feedstock mass significantly affected the solar conversion efficiency but no notable effect on the energetic upgrade factor was observed. The solar conversion efficiency can be significantly improved via fully replacing inert alumina particles by feedstock, adding a continuous feeder, and optimizing the superficial velocity to minimize particle entrainment.

Small Scale Bubbling Fluidized Bed Gasifier for Syngas Extraction from Napier grass

For some farmers, Napier grass is non-beneficial since it may compete for growth area and nutrients for common agricultural crops. However, this grass has the potential to become biomass energy source. It is known that Napier grass is a candidate source of syngas or synthetic gas which can be utilized in different applications. To extract synthetic gas, the Napier grass must undergo gasification process. This study focuses on the design and testing of a small-scale bubbling fluidized bed gasifier for Napier grass. The reactor chamber is made up of low carbon steel while the inert bed material used is silica. The materials for the gasifier were chosen by using the quantitative method of material selection. The temperatures at the inlet, sections of the reactor and exhaust of the reactor were monitored. The maximum furnace temperature of the gasifier was 473°C and observed after 170 minutes due to increased furnace and reactor heating surface area. The syngas extracted from the gasifier was tested and analyzed that there is 21.23 ppm of Carbon Monoxide (CO) and low Volatile Organic Compound (VOC) content. The composition of syngas conformed with the standards and can be used for cooking applications

Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas

Oxygen carrier aided combustion (OCAC) is utilized to promote the combustion of relatively stable fuels already in the dense bed of bubbling fluidized beds by adding a new mechanism of fuel conversion, i.e., direct gas–solid reaction between the metal oxide and the fuel. Methane and a fuel gas mixture (PSA off-gas) consisting of H2, CH4 and CO were used as fuel. Two oxygen carrier bed materials—ilmenite and synthetic particles of calcium manganate—were investigated and compared to silica sand, an in this context inert bed material. The results with methane show that the fuel conversion is significantly higher inside the bed when using oxygen carrier particles, where the calcium manganate material displayed the highest conversion. In total, 99.3–99.7% of the methane was converted at 900 °C with ilmenite and calcium manganate as a bed material at the measurement point 9 cm above the distribution plate, whereas the bed with sand resulted in a gas conversion of 86.7%. Operation with PSA off-gas as fuel showed an overall high gas conversion at moderate temperatures (600–750 °C) and only minor differences were observed for the different bed materials. NO emissions were generally low, apart from the cases where a significant part of the fuel conversion took place above the bed, essentially causing flame combustion. The NO concentration was low in the bed with both fuels and especially low with PSA off-gas as fuel. No more than 11 ppm was detected at any height in the reactor, with any of the bed materials, in the bed temperature range of 700–750 °C.

Experimental and CFD investigation of inert bed materials effects in a high-temperature conical cavity-type reactor for continuous solar-driven steam gasification of biomass

Solar biomass gasification was performed in a high-temperature conical bed reactor for thermochemical syngas production, which represents an efficient route to promote biomass valorization while storing intermittent solar energy into carbon–neutral solar fuels. The developed solar reactor is based on a conical cavity-type receiver enabling continuous biomass feedstock injection under real concentrated solar irradiation. In this study, the use of inert bed materials is considered to directly absorb a portion of the entering solar power and to transmit thermal energy to the gas and reactive particulate phase by radiation, convection and particle-to-particle interactions, with the aim of homogenizing the reactor temperature. A fluid dynamics study of the gas/particle flow was first performed to provide insights into the reactor hydrodynamics in the case of empty cavity and cavity containing inert particles in the forms of packed-bed and spouted-bed. The beneficial effect of the inert particles bed was emphasized. The spouted bed particles play a role in retaining the reacting biomass particles longer inside the reaction zone and promote phase mixing, as also confirmed by cold tests on a replicated transparent mockup cavity. Different inert particle bed materials were then considered and experimentally tested to unravel their impact on the biomass conversion, syngas composition and yield, and gasification performance in the solar-heated reactor operated at 1200 °C and 1300 °C.

A Methodology for Measuring the Heat Release Efficiency in Bubbling Fluidised Bed Combustors

Differences in the densities of bed material and—especially biogenic—solid fuels prevent an ideal mixture within bubbling fluidised bed (BFB) combustors. So, the presence of fuel particles is usually observed mainly near the surface of the fluidised bed. During their thermal conversion, this leads to a release of unburnt pyrolysis products to the freeboard of the combustion chamber. Within the further oxidation, these species will not transfer their heat-of-reaction to the inert bed material in the way of a convective heat transfer, but rather increase the gas phase temperature providing probably some additional radiative heat transfer to the dense bed. In this case, the so-called heat release efficiency to the fluidised bed, being the ratio of transferred heat to the fuel input, will be reduced. This paper presents a methodology to quantify this heat release efficiency with lab-scale experiments and the observed effects of common operating parameters like bed temperature, fluidisation ratio and fuel-to-air ratio. Experimental results show that the air-to-fuel ratio dominates the heat release efficiency, while bed temperature and fluidisation ratio have minor influences.

Chemical‐looping gasification of coal with CuFe2O4 oxygen carriers: The reaction characteristics and structural evolution

AbstractIn this study, coal gasification to produce synthesis gas by chemical looping was investigated with CuFe2O4 oxygen carriers (OCs), including the reaction characteristics and structural evolution process. It was found that the presence of a CuFe2O4 OC increases the gasification reaction rate of coal in comparison to the use of inert bed material (silica sand). The CuFe2O4 OC enhanced the performance of hematite and improved the ability to produce syngas of CuO OCs. During the chemical looping process, CuFe2O4 was reduced to Fe3O4 and Cu. Furthermore, N2 adsorption/desorption and scanning electron microscopy images verified that the released O2 and the generated CO2 by the CuFe2O4 OCs improved the carbon conversion and facilitated the pore opening and expansion shown by the increase in specific surface area. This can be used as a theoretical reference that can be used to gain a better understanding of the microscopic mechanism for how CuFe2O4 OCs can accelerate the chemical looping gasification of coal.

The Effect of Iron‐ and Manganese‐Based Oxygen Carriers as Bed Materials in Oxygen Carrier Aided Combustion

Combustion of organic materials in fluidized bed combustion is generally performed using an over‐stoichiometric air‐to‐fuel ratio. Despite excess air in the system, sub‐stoichiometric combustion regions are present in the fluidized bed because of nonperfect mixing of reactants in the system. These regions with oxygen‐deficient combustion contribute to increased levels of nonoxidized or partially oxidized carbon species. To enhance heat transfer and uniform heat distribution in the incineration chamber, an inert fluidized bed material such as silica sand is generally used. Substitution of silica sand in favor of an oxygen carrier could potentially be used to promote oxygen distribution in the incineration chamber. This is referred to as oxygen carrier‐aided combustion. In this work, three alternative bed materials, previously investigated in chemical looping combustion, are investigated and compared with silica sand in over‐ and sub‐stoichiometric combustion. The materials investigated as bed materials are a manganese ore, the mineral ilmenite, and a synthetic material mixture of Fe2O3 on a ZrO2 support. Results show that during combustion using sub‐stoichiometric air‐to‐fuel ratios, the amount of CO in the effluent gases can be reduced using an active bed material compared to inert silica sand.

An experimental approach to thermochemical conversion of a fuel particle in a fluidized bed

Measuring the temperature of a fuel particle during the thermochemical process inside the fluidized bed is of great importance in order to obtain detailed knowledge of the conversion behavior of the fuel particle and so optimize the combustion performance. In this study a number of experiments were done in order to register the temperature and hydrodynamics of a biomass fuel particle in a fluidized bed during the devolatilization process of a biomass particle. The experiments were done in a 2D fluidized bed with a front transparent window in order to use the particle image velocimetry (PIV) method to obtain information on the hydrodynamics of the fuel particle and inert bed material inside the bed. A thermocouple was also used to measure the temperature of the particle during the conversion. Experiments were done at a fluidized bed temperature and fluidization velocity in the range of 350–450 °C and 0.2–0.6 m/s, respectively. The effect of the bed’s temperature and fluidization velocity on the drying and devolatilization process of the biomass fuel particle was investigated. The results indicate that the bed’s temperature and fluidization velocity have a significant effect on the mass and heat transfer between the fuel particle and the bed during the conversion process.

CFD-DEM investigation of the fluidization of binary mixtures containing rod-like particles and spherical particles in a fluidized bed

In the energy conversion process, the inert bed material such as sand with spherical shape is usually added to the fluidized bed for the proper fluidization of the non-spherical biomass particles. Understanding fluidization of the binary mixtures containing the biomass and the bed material is therefore significant for better utilization of the biomass. To this end, CFD-DEM model is used to simulate the fluidization of the binary mixtures containing the rod-like particles that represent the light and large biomass particles and the spherical particles that represent the dense and small bed material, in which all particles are described by super-ellipsoid model. Five granular systems with different volume fractions of the rod-like particles but the same particle volume are simulated in this paper. The simulation results demonstrate that the minimum fluidization velocity would increase with the volume fraction of the rod-like particles. The addition of the spherical particles is beneficial to the fluidization of the rod-like particles. The mixing rate in binary granular system and the distribution of the coordinate number would be affected by the gas superficial velocity and the volume fraction of the rod-like particles. For the orientation of the rod-like particles, its distribution in binary granular system is different from that in monodisperse rod-like particle system. But whether in binary granular system or monodisperse rod-like particle system, there is apparent wall effect on the particle orientation.

Combustion and Emission Characteristics of Egyptian Sugarcane Bagasse and Cotton Stalks Powders in a Bubbling Fluidized Bed Combustor

Combustion and emission characteristics of Egyptian sugarcane bagasse and cotton stalks powders were investigated in a bubbling fluidized bed combustor (BFBC) using silica sand as an inert bed material. Mixing and fluidizing characteristics of the two biomass materials and bed material of a certain size distribution were studied. It was found that, the feed rate ranged from 70 to 113 g/min and feed rate of 51 g/min for cotton stalks and sugarcane bagasse, respectively may be suitable for better combustion characteristics in the BFBC. The effect of different operating parameters on the axial temperature and gas concentration profiles (O2, CO2, CO, H2S, SO2 and NOx) was investigated. It was found that the formation of NOX during combustion doesn’t represent a problem as the nitrogen content of fuel is low, the bed temperature of fluidized bed is below 900 °C, and the particle size of the biomass is small. It was found that combustion efficiency ranged from 90 to 100% and from 93 to 98.8% for sugarcane bagasse and cotton stalks, respectively. Thermal efficiency was found to be in the range of (33.7–41%) and (32.5–48.7%) for sugarcane bagasse and cotton stalks, respectively. A verification model for mass and heat balance of combustion experiments was constructed to confirm that the measured values of different parameters were measured accurately with acceptable percent of error.

Enhancements in Biomass-to-Liquid processes: Gasification aiming at high hydrogen/carbon monoxide ratios for direct Fischer-Tropsch synthesis applications

The Fischer-Tropsch process is one of the possible routes to produce synthetic liquid fuels and chemicals employed on large scale in Gas-to-Liquid and Coal-to-Liquid processes, after appropriate transformation of respective fossil sources into syngas. However, syngas can be produced by biomass gasification as well. One of the main problems is the need to ensure high H2/CO ratio to allow the Fischer-Tropsch reaction to occur, operation commonly achieved by resorting to a Water-gas shift step. The present work reports an experimental study on biomass gasification. The aim was to set up a more efficient Biomass-to-Liquid process, which ensures a high H2/CO ratio syngas suitable for Fischer-Tropsch synthesis directly from biomass gasification. In this way, it is possible to remove the Water-gas shift section and consequently make biofuels production more attractive, reducing costs and plant complexity. Two types of biomass have been tested: softwood from forest residues and fast growing crops. Moreover, a bubbling fluidized bed reactor and an indirect gasifier with internal circulating fluidized bed have been used. Gasification tests have been conducted by running gasifiers with both inert and catalytic bed materials. The results have shown that, using the direct gasifier with catalytic bed and a proper configuration, H2/CO molar ratio of 2 can been obtained even at low temperature and low steam/biomass ratio, whereas this is not the case in the indirect one. However, tests suggest that an inverted configuration would make it possible to obtain values near two also with indirect gasifiers, resulting in a new interesting approach.

Control of the solids retention time by multi-staging a fluidized bed reactor

Fluidized bed reactors are often operated with particles of different size and density present simultaneously (e.g. inert bed material and fuel particles in fluidized bed combustion, or catalyst particles and polymer particles in fluidized bed polymerization). This paper presents a general concept to separate a single fluidized bed reactor into two reaction zones by introducing a primary chamber in the bed where flotsam particles are fed and spend a certain initial residence time. The feasibility of the concept is experimentally proven in a fluid-dynamically down-scaled unit, both in terms of functionality (ability to maintain two separate reaction regions and to avoid backflow from the secondary to the primary region of the solids fed) and of operability (ability to control the residence time distribution of the solids fed in the primary chamber through operational parameters such as pressure and bed height). Numerical simulations show that the residence time distribution is sensitive to the degree of segregation in the primary chamber, and that the heat transfer between the two reaction regions by means of bulk solids mixing is still sufficient to sustain an endothermic process in one zone by an exothermic process in the other.