Bed Defluidization Research Articles

Early Detection of Bed Defluidization in Steam–Oxygen Biomass-Pressurized Fluidized Bed Gasifiers

Early Detection of Bed Defluidization in Steam–Oxygen Biomass-Pressurized Fluidized Bed Gasifiers

An experimental investigation into the effect of sodium salts on quartz particle agglomeration and bed defluidisation in a bubbling fluidised bed

This work investigated systematically the effect of NaCl, Na2CO3 and Na2SO4 on the agglomeration of quartz particles and the bed defluidisation in a bubbling fluidised bed operating at high temperatures. Detailed chemical, mineralogical, morphological and cross-sectional characterisations on the bed particles and the agglomerates had been performed. With an interval feeding of NaCl or Na2CO3, bed defluidisation did not happen at 700 °C but occurred at 800–900 °C at NaCl(Na2CO3)/SiO2 ratios <1.32%, highlighting the role of Na and temperature in promoting particle agglomeration. This is respectively attributed to the NaCl melt and low melting point Na2SiO3 and Na2Si2O5 up to 155 μm for agglomeration initiation. As for Na2SO4, bed defluidisation did not occur at all the temperatures examined. In addition, pressure fluctuations of the bed as time were also analysed, with the release rate of these Na salts in 0.032–0.332 min−1 determined to comprehend the transitions behaviour of fluidisation.

The use of kaolin and dolomite bed additives as an agglomeration mitigation method for wheat straw and miscanthus biomass fuels in a pilot-scale fluidized bed combustor

Renewable biomass fuels are frequently used for power generation. Biomass ash causes bed agglomeration in fluidized bed boilers due to the formation of alkali silicate melts. Very few prior studies have tested dolomite and kaolin bed additives for agglomeration mitigation with agricultural biomasses. In this work, pelletized miscanthus and wheat straw were tested in a pilot-scale 65kWth fluidized bed combustor with varying dosages of dolomite and kaolin on a silica sand bed. Neither additive improved defluidization time with wheat straw, whereas additive use at all dosages prevented bed defluidization with miscanthus. Agglomerates were studied through a novel, detailed SEM/EDX analysis across structural features. SEM/EDX analysis presented evidence of chemical reaction between both additives and fuels. Potassium in ash migrated into kaolin particle at depths of up to 60 μm. With dolomite, calcium and magnesium raised melt temperatures. Thermochemical modelling of the ash and additive combinations predicted that additive use would substantially reduce ash melt formation. It is proposed that the wheat straw pellet acted as a “ready-made” agglomerate structure due to release of molten ash to the pellet surface which bed material then sticks to, hence the lack of change to defluidization time regardless of additive use. Future studies into this behaviour would improve additive use.

Coupling the biochemical and thermochemical biorefinery platforms to enhance energy and product recovery from Agave tequilana bagasse

The tequila industry is one of the pillar industries in many regions of Mexico, bringing wealth and economic development to rural areas. However, appropriate disposal and management of the tequila manufacturing residues such as the Agave tequilana bagasse (TB) represent not only an environmental challenge but also an economic opportunity. Previous works have unsuccessfully applied biochemical routes for complete TB valorization. Therefore, this study aimed to achieve the full conversion of TB to energy and products by coupling the biochemical and thermochemical biorefinery platforms. The biochemical platform included acid hydrolysis of TB, detoxification of the hydrolysates with activated carbon, and two-stage anaerobic digestion of the undetoxified and detoxified hydrolysates. The energy recovery (expressed as kJ of hydrogen and methane obtained) was optimized by applying a central composite design and using both hydrolysates as substrates. Results showed that hydrogen production with the detoxified hydrolysate outperformed 2 times the undetoxified one; however, the total energy recovery (hydrogen + methane) was not significantly favored with the detoxification process (35.04 and 37.15 kJ for the undetoxified and detoxified hydrolysate respectively), which was attributed to the robustness of the methanogenic process. Regarding the thermochemical platform, the acid hydrolysis of TB accelerated the microwave pyrolysis process and led to a higher oil yields (ca.1.5 times) with similar productions of furfural, phenol and their derivatives but significantly reduced the acetic acid formation compared to untreated TB. During the fluidized bed combustion tests, the Ca-rich TB’s ash mitigated the agglomeration formation and resulted in over 900 min operation with no sign of bed defluidization. This could offer great potentials in co-combustion with other problematic biomass fuels such as wheat straw, which had a defluidization time of 60 min. Overall, these results indicate that coupling the biochemical and thermochemical platforms for Agave tequilana bagasse can be a novel approach to refining gaseous and liquid products and extracting heat energy while disposing the bio-waste from tequila industry.

Effect of pre-treatment on catalytic coal gasification characteristics of sub-bituminous coal

Sintering or agglomeration is formed in the process of the gasification of coal when fluxing K2CO3, which leading to the entire bed defluidization. In order to study the catalytic coal gasification reactivity and agglomeration properties in pressurized fluidized bed, Wangjiata raw coal, demineralized coal with HCl-HF treatment, and raw coal with different amounts of clay mineral addition were selected. The catalytic coal gasification reactivity, the agglomeration and sintering properties were studied. As well as water-soluble catalyst recovery from the gasification residue was also determined. The results show that for potassium-catalyzed steam gasification, not only coal ash content but also composition affected the gasification reactivity, agglomeration, and catalyst recovery. The carbon conversion increased varying degrees with increasing K2CO3 loadings, K2CO3 impregnated coal showed a high degree of agglomeration, and agglomeration increased with increasing K2CO3 amount. Demineralized coal couldn't improve the agglomeration property. The addition of Clay mineral, was found to be very efficient in anti-sintering. However, compared with demineralized coal, the carbon conversion and water washing catalyst recoveries decreased obviously for those samples with the addition of clay minerals. The test verifies that part of potassium catalyst irreversibly reacted with coal ash to form relatively water-insoluble potassium compounds such as kaliophilite and sanidine.

Agglomeration and the effect of process conditions on fluidized bed combustion of biomasses with olivine and silica sand as bed materials: Pilot-scale investigation

Bubbling fluidized bed combustion of biomass has benefits of fuel flexibility, high combustion efficiency, and lower emissions. Bed agglomeration is where bed particles adhere together with alkali silicate melts and can lead to unscheduled plant shutdown. This pilot-scale study investigates performance and agglomeration when varying fuel (white wood, oat hull waste, miscanthus, wheat straw), bed height, bed material, and includes a novel spatial analysis of agglomerates from different bed locations. White wood was the best performing fuel and did not undergo bed defluidization due to its low ash content (0.5% mass as received), whereas wheat straw (6.67% mass as received ash) performed worst (defluidization times <25 min). Olivine was a superior bed material to silica sand, with 25%+ longer defluidization times with the worst performing fuel (wheat straw). Calcium-rich layers formed at olivine particle surfaces within wheat straw ash melts, and capillary action drew potassium silicate melt fractions into olivine particle fractures. An analysis of agglomerate samples from different bed spatial locations following tests with oat hull waste showed that ash layers on agglomerates retrieved further from the landing point of fuel onto the bed had reduced potassium and elevated calcium, likely due to reduced availability of fresh fuel ash for reaction with bed material.

Bed Particle Agglomeration and Defluidization in the Rubber Wood and Coir-Fired Fluidized Bed

The behaviors of the bed particle agglomeration and the bed defluidization during the fluidized bed combustion of the rubber wood and the coir were experimentally studied. This work focused on (1) the effects of the bed operating variables on the bed agglomeration tendency and (2) the behaviors of the fuel inorganic elements and the governing mechanisms associated the bed agglomeration. During the combustion, the fluidized bed underwent the defluidization from partial to complete by the growth of the bed particles and the accumulation of the agglomerates. The fuel composition was the significant effect. The defluidization was promoted by the increasing bed temperature and bed particle size, and the decreasing air velocity. The morphological and chemical analyses revealed that the molten K-silicates formed via the reactions between the fuel’s potassium and silicon of the bed particle contributed towards the bed agglomeration. The migration of the inorganic compounds from the fuel particle to the bed particle was likely dominated by the deposition of the released gaseous/aerosol inorganic species. The thermodynamic examination corroborated that the K–Si adhesive materials which presented as the liquid phase at the observed bed conditions caused the agglomeration. The large melt fractions in the materials were shown.

Energy, environmental and operation aspects of a SRF-fired fluidized bed waste-to-energy plant

A methodology based on the ISO 14031:2013 guideline has been developed and applied to a full-scale fluidized bed waste to energy plant (WtE) burning solid recovered fuel (SRF). With reference to 3years of operation, the data on energy and environmental performance, on raw materials consumptions such as sand and diesel fuel, accidental reasons of plant shutdown, have been acquired and analyzed. The obtained results have allowed to quantify the energy and environmental performance of the WtE plant under investigation by varying the amount and mixings of the inlet waste, available in form of thickened and fluff (similar to coriander) SRF. In terms of the energy performance, the fluidized bed technology applied to the SRF was able to guarantee an adequate production of electricity (satisfying the market demands), showing a relative flexibility with respect to the inlet waste. In terms of net energy production efficiency, the plant showed values in the range of 13.8–14.9% in line with similar installations. In terms of the environmental performance, the adoption of a cleaning system based on SNCR (Selective Non Catalitic Reduction)+semi-dry scrubbing+Fabric filter generated emissions usually well below the limits set by the EU Directive 2000/76/EC as well as the Italian Law 46/2014 (more restrictive) with reference to all the key parameters. In terms of the plant shutdown, the majority of problems focused on the combustion chamber and boiler due to the erosion of the refractory material of the furnace as well as to the breaking of the superheaters of the boiler. In contrast, the mechanical and electrical causes, along with those related to the control and instrumentation system, were of secondary importance. The sand bed de-fluidization was also among the leading causes of a frequent plant shutdown. In particular, results showed how although the SRF presents standard characteristics, the use of different mixtures may affect the number of plant shutdowns. The full-scale data highlighted how the lower the rate of fluff in the mixture was, the greater the number of plant shutdown due to sand bed de-fluidization was. Finally, the aspects in terms of the energy, environmental protection and raw material consumption have been discussed with reference to similar WtE plants such as Robbins (Chicago, USA), Lidköping (Sweden), Toshima (Tokyo, Japan), Madrid (Spain), Dundee (Scotland, UK) and Valene (Mantes la Jolie, France).

Fluidised bed gasification and chemical exergy analysis of pelletised oil palm empty fruit bunches

The National Biomass Strategy was envisioned to foster the efficient valorisation and management of Oil Palm Waste (OPW) in Malaysia. However, the proposed Circular Energy Economy is hampered by poor OPW fuel properties, inefficient conversion techniques, and process design. This study explored the valorisation of Oil Palm Empty Fruit Bunches (OPEFB) Briquettes through fluidised bed gasification with the aim of exploiting the superior qualities of pelletised biomass and excellent reactor dynamics of fluidised beds. Gasification of OPEFB Briquettes was examined from 600 – 800 °C and equivalence ratio, ER is 0.20 – 0.25 under atmospheric pressure. The fuel properties and chemical exergy of OPEFB briquettes were characterised. The gasification of OPEFB briquettes produced high biochar yield and bio syngas with higher heating value from 1.15 – 3.05 MJ/m3 whereas the Cold Gas Efficiency (CGE) and Carbon Conversion Efficiency (CCE) ranged from 6.54 – 17.34 % and 43.37 – 78.16 %. Bed agglomeration and defluidisation typically encountered in pulverised OPEFB gasification were minimal during the gasification of OPEFB briquettes. In conclusion, the results demonstrated that OPEFB Briquettes gasification is a practical route for valorising OPW into renewable energy and sustainable fuels.

Performance evaluation of different approaches for early detection of defluidization

Identifying the onset of an agglomeration phenomenon at an early stage in processes utilizing gas-solid fluidized beds that operate under the influence of cohesive interparticle forces affords enough time to apply counteractive strategies and avoid a disastrous agglomeration of particles potentially leading to complete bed defluidization. In this paper, we compare the performance of different leading approaches proposed in the open literature for the advanced detection of defluidization. The approaches include the single-signal-monitoring of evolutions of total bed pressure drop, standard deviation of pressure signals, or S-value from the attractor comparison analysis as well as the simultaneous-monitoring of temperature and in-bed differential pressure signals during the process. The results show that the simultaneous-monitoring of temperature and in-bed differential pressure signals provided the best prediction of the onset of agglomeration while it demonstrated the least sensitivity to the changes in gas velocity, operating temperature, and bed inventory.

Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier: Results with Miscanthus and Olivine

This article presents a novel air-blown bubbling fluidized bed device that has the ability to sample gas and bed materials at various axial positions during the gasification experiments. The reactor was operated with olivine, as bed material, and Miscanthus, a biomass rich in potassium and silica, and thus prone to bed agglomeration. The comparison of gas and char axial profiles along the bed allows a better understanding of the biomass gasification: it shows in particular that O2 consumption and CO2 production at the bottom of the bed are mainly due to char oxidation, even if few pyrolysis gases may also be produced and oxidized near the grid. Regarding bed defluidization, the agglomerate fraction is followed by taking bed samples during the operation: it is shown that the rate of agglomeration is linear while defluidization signs appear when the agglomerate fraction reaches 6% near the grid. Small agglomerates are observed on the top of the bed, whereas big agglomerates are segregated near the grid. The...

Behavior of Alkali Metals and Ash in a Low-Temperature Circulating Fluidized Bed (LTCFB) Gasifier

A low-temperature circulating fluidized bed system (LTCFB) gasifier allows for pyrolysis and gasification to occur at low temperatures, thereby improving the retention of alkali and other inorganic elements within the system and minimizing the amount of ash species in the product gas. In addition, the low reactor temperature ensures that high-alkali biomass fuels can be used without risk of bed defluidization. This paper presents the first investigation of the fate of alkali metals and ash in low-temperature gasifiers. Measurements on bed material and product gas dust samples were made on a 100 kW and a 6 MW LTCFB gasifier. Of the total fuel ash entering the system, the largest fraction (40–50%) was retained in the secondary cyclone bottoms, while a lower amount (8–10%) was released as dust in the exit gas. Most of the alkali and alkaline earth metals were retained in the solid ash, along with Si and a minor fraction of Cl. Most Cl and S were released in gaseous form, with chlorine partly as methyl chlori...

Fluidized‐Bed Chemical Vapor Deposition of Silicon on Very Dense Tungsten Powder

AbstractA very dense tungsten powder was coated by silicon from silane using the fluidized‐bed chemical vapor deposition process. A reactor of reduced diameter was developed in order to decrease the weight of powders treated. Results show that the fluidization of this powder is possible in this reactor, but with non‐optimal gas solid contact induced by the high powder density. Important disturbances of the fluidized‐bed temperatures appeared in the presence of silane, due to a partial bed defluidization related to the cohesive nature of the deposit. These disturbances are clearly exacerbated by the exceptional density of the particles. This is probably why a powder agglomeration has been unavoidable when the bed temperature was too low and/or the silane inlet concentration too high. However, a set of operating parameters could be found, allowing a uniform and continuous coating of the whole powder surface.

Coating and melt induced agglomeration in a poultry litter fired fluidized bed combustor

The combustion of poultry litter, which is rich in phosphorus, in a fluidized bed combustor (FBC) is associated with agglomeration problems, which can lead to bed defluidization and consequent shutdown of the installation. Whereas earlier research indicated coating induced agglomeration as the dominant mechanism for bed material agglomeration, it is shown experimentally in this paper that both coating and melt induced agglomeration occur. Coating induced agglomeration mainly takes place at the walls of the FBC, in the freeboard above the fluidized bed, where at the prevailing temperature the bed particles are partially molten and hence agglomerate. In the ash, P2O5 forms together with CaO thermodynamically stable Ca3(PO4)2, thus reducing the amount of calcium silicates in the ash. This results in K/Ca silicate mixtures with lower melting points. On the other hand, in-bed agglomeration is caused by thermodynamically unstable, low melting HPO42− and H2PO4− salts present in the fuel. In the hot FBC these salts may melt, may cause bed particles to stick together and may subsequently react with Ca salts from the bed ash, forming a solid bridge of the stable Ca3(PO4)2 between multiple particles.

Agglomeration/defluidization in a fluidized bed reduction of Fe2O3 particles by CO: Influences of iron precipitation on particle cohesiveness

Influences of iron precipitation on particle cohesiveness related to agglomeration and defluidization in Fe2O3 reduction were investigated in a visual fluidized bed reactor. A new method of the controlled defluidization tests was employed to examine the evolution of bed agglomeration. The real-time agglomeration tendency was represented by the defluidization temperature. Results showed that defluidization potential increased by increasing metallization degree. According to thermo-mechanical analysis, the precipitation of metallic iron significantly reduced the sintering temperature and surface viscosity of Fe2O3 particles, resulting in the evolution of the bed materials from cohesiveless to sticky. The SEM analysis showed that fine iron nuclei with nano/micro size on the surface acted as necks to form agglomerates by sintering and enhanced particle cohesiveness. Thus, the metallized bed particles tended to adhere and resulted in bed defluidization. The calculated adhesive force between particles depended on both metallic iron content and surface viscosity, which was in good agreement with the experiment results for defluidization.

Polyethylene Cracking on a Spent FCC Catalyst in a Conical Spouted Bed

The catalytic cracking of HDPE (high density polyethylene) at 500 °C using a spent FCC catalyst agglomerated with bentonite (50 wt %) has been studied in a conical spouted bed reactor. The reaction is carried out in continuous regime (1 g min–1 of HDPE is fed) with no bed defluidization problems. The results obtained, namely, total conversion, and high yields of gasoline (C5–C11 fraction) (50 wt %) and C2–C4 olefins (28 wt %), are explained by favorable reactor conditions and good catalyst properties. These results are compared with those for a catalyst prepared in the laboratory by agglomerating a commercial HY zeolite (SiO2/Al2O3 = 5.2). The conical spouted bed is a suitable reactor for enhancing the physical steps of melting the polymer and coating the catalyst with the melted polymer. Furthermore, high heat and mass transfer rates promote devolatilization, and short residence times minimize secondary reactions from olefins by enhancing primary cracking products. The meso- and macroporous structure of the spent FCC catalyst matrix enhances the diffusion of long polymer chains, whereas the zeolite crystals have micropores that give a proper shape selectivity to form the lumps of gasoline and light olefins. Because of long use in reaction–regeneration cycles, the moderate acidity of the spent FCC catalyst minimizes the secondary reactions of hydrogen transfer, and so restricts the formation of aromatics and paraffins, as well as the reactions of overcracking and condensation and, therefore, the coke formation. The spent FCC catalyst exhibits a low deactivation rate and is regenerable by coke combustion with air at 550 °C. Consequently, the use of a catalyst with the sole cost of a simple agglomeration and the production of value added product streams make the process of polyolefin catalytic cracking a promising option for refinery integration.

Bed agglomeration characteristics of palm shell and corncob combustion in fluidized bed

Abstract Bed particle agglomeration was studied experimentally in an atmospheric laboratory scale fluidized bed combustor using quartz sand as bed material. Palm shell and corncob were tested. The objectives of the study were (i) to describe the contributions of the biomass ash properties and the operating conditions on the bed agglomeration tendency in term of the bed defluidization time (tdef) and the extent of potassium accumulation in the bed (K/Bed) and (ii) to further elucidate the ash inorganic behaviors and the governing bed agglomeration mechanisms. Defluidization caused by the bed agglomeration was experienced in all experiments during combustion of these biomasses, as a consequence of the presence of potassium in biomass. The experimental results indicated that biomass ash characteristics were the significant influence on the bed agglomeration. The increasing bed temperature, bed particle size and static bed height and the decreasing fluidizing air velocity enhanced the bed agglomeration tendency. The SEM/EDS analyses on the agglomerates confirmed that the agglomeration was attributed to the formation of potassium silicate liquid enriched on the surface of quartz sand particles in conjunction with the high surface temperature of the burning biomass char particles. Thermodynamic examination based on the phase diagram analysis confirmed that the molten phase formation was responsible for the agglomeration. In this study, the high molten ash fraction resulting from the high potassium content in biomass promoted the agglomeration and thus defluidization.

Identification of defluidization in fluidized bed coating using the Gaussian spectral pressure distribution

The partial or complete bed defluidization is an undesired phenomenon in industrial application involving fluidized bed operations. In the process of particle coating, the defluidization can be prevented by increasing the gas velocity and/or decreasing the coating suspension flow rate, if the changes in the hydrodynamics of the fluidization are early detected. For industrial applications, the use of a technique that can quickly identify the defluidization region is very important, avoiding loss of efficiency or even the necessity of shutting down the process. The purpose of the present work is to identify the defluidization region in a fluidized bed coating process using the Gaussian mean frequency, which is an alternative methodology based on pressure fluctuation analysis. Experiments were carried out in a Plexiglas column with a 0.143 m in inner diameter and 0.71 m in height, at a temperature of 70 °C. Microcrystalline cellulose (MCC) of 325 μm in diameter and an apparent density of 980 kg/m 3 was used as fluidizing particles, which was coated by a suspension based on Eudragit®. All raw data sets of pressure in the plenum were collected at a sampling rate of 400 Hz, considering 2048 data points. LabView® 7.1 software was used for all data acquisition and signal processing. The results showed that the defluidization region can be clearly identified using the methodology employed. It also could have a high potential in industrial applications, especially for on-line control in order to avoid the defluidization region and consequently the efficiency loss of a coating process.

Identification of defluidization region in a gas-solid fluidized bed using a method based on pressure fluctuation measurements

Industrial applications that involve fluidized bed operations must prevent the undesirable phenomenon of partial or complete bed defluidization. Defluidization can be avoided by increasing the gas velocity and/or, in some cases, changing the solid feed conditions in the system, provided that the changes in the hydrodynamics of the flow are detected early enough. The use of a technique that can perform an early detection of the defluidization condition in industrial applications is important, in order to avoid the loss of efficiency or even an undesirable shutting down of the process. The objective of this work is to show the application of a method for early detection of the condition where the bed is tending to the defluidization, in a gas-solid fluidized bed flow. The method is based on pressure fluctuation measurements. Experimental tests are carried out using two solid particles: microcrystalline cellulose and sand. Results show that the proposed method is efficient in detecting the fluidization condition in a conventional bubbling bed regime. The potential of application of the technique is also shown for the control of the defluidization phenomenon in industry.

The role of ash particles in the bed agglomeration during the fluidized bed combustion of rice straw

In this paper, the effects of fluidization velocity, bed temperature and fuel feeding rate on the defluidization time and agglomerate fraction in the fluidized bed combustion of rice straw were studied. The fuel ash, necks in agglomerates and coating layers of bed particles were studied by means of the scanning electron microscope, coupled with energy-dispersive spectroscopy (SEM/EDS). Results showed that the stickiness of bed particles induced by coating layers is the direct reason for bed defluidization. The alkali metals such as K and Na mainly exist in the outer layer of rice straw particles. During combustion the high temperature can cause the alkali species melting and coating the surfaces of ash particles. Consequently, ash particles become sticky and tend to adhere to the surfaces of bed particles. The large-sized ash particles may act as the necks in the formation of agglomerates. The small-sized ash particles may contribute to the formation of coating layers.