Mass loss and apparent kinetics of a thermally thick wood particle devolatilizing in a bubbling fluidized bed combustor

This work presents the results of experiments conducted to determine the mass loss characteristics of a cylindrical wood particle undergoing devolatilization under oxidation conditions in a bubbling fluidized bed combustor. Cylindrical wood particles having five different sizes ranging from 10 to 30 mm and aspect ratio (l/d = 1) have been used for the study. Experiments were conducted in a lab scale bubbling fluidized bed combustor having silica sand as the inert bed material and air as the fluidizing medium. Total devolatilization time and mass of wood/char at different stages of devolatilization have been measured. Studies have been carried out at three different bed temperatures (T_bed = 750, 850 and 950 °C), two inert bed material sizes (mean size d_p = 375 and 550 μm) and two fluidizing velocities (u = 5u_mf and u = 10u_mf). Devolatilization time is most influenced by the initial wood size and bed temperature. Most of the mass is lost during the first half of the devolatilization process. There was no clear influence of the fluidization velocity and bed particle size on the various parameters studied. The apparent kinetics estimated from the measured mass history show that the activation energy varied narrowly between 15 and 27 kJ/mol and the pre-exponential factor from 0.11 and 0.45 s⁻¹ for the wood sizes considered.     

Fragmentation of wood char in a laboratory scale fluidized bed combustor

Casuarina equisetifolia, a hard wood, and a popular energy crop in many tropical countries, was investigated experimentally for its char fragmentation in a laboratory scale atmospheric bubbling fluidized bed combustor. The effect of fuel shape and size on wood char fragmentation was studied. Wood particles of spherical, cylindrical (aspect ratio of 1), and cubical shapes of different sizes ranging from 10 to 25 mm were used in the experiments. Fragmentation of wood char was quantified in terms of various parameters, such as Number of Fragments (NF), Percentage of Fragmentation Events, Frequency of Fragmentation, Timing interval of Fragmentation, Size distribution of char and Fragmentation Index (FI). Also, qualitative observations on the evolution of char in terms of deformation, cracks and surface texture are discussed. It was observed that Casuarinaequisetifolia wood of sizes greater than 15 mm, of all shapes undergoes primary fragmentation during the devolatilization phase. Furthermore, chars fragment at the early stages (1st or 2nd quarter) of the char combustion phase, underscoring the significance of the phenomenon in fluidized bed combustion. For all the shapes of wood considered, there appears to be a cut-off size of the initial wood, below which its char certainly undergoes fragmentation. It was observed that the average char particle size at any instance during its combustion falls in a narrow range of 3.7–6.9 mm, 3–6.6 mm and 3–9.5 mm for spherical, cylindrical and cubical wood particles, respectively. Wood of initially cylindrical shape undergoes extensive fragmentation when compared with spherical and cubical shapes.     

Transient thermal behaviour of a cylindrical wood particle during devolatilization in a bubbling fluidized bed

This work proposes a transient heat transfer model to predict the thermal behaviour of wood in a heated bed of sand fluidized with nitrogen. The 2-D model in cylindrical coordinates considers wood anisotropy, variable fuel properties, fuel particle shrinkage, and heat generation due to drying and devolatilization. The influence of initial fuel moisture content, thermal diffusivity, particle geometry, shrinkage, external heat transfer coefficient, chemical reaction kinetics and heats of reaction on temperature rise is presented. The cylindrical wood particles chosen for the study have length (l) = 20 mm, diameter (d) = 4 mm and l = 50 mm and d = 10 mm, both having an aspect ratio (l/d) of 5. The bed temperature is 1123 K. The model prediction is validated using measurements obtained from literature. The temperature rise in the wood particle is found to be sensitive to changes in the moisture content and thermal diffusivity and heat of reaction (in larger particles) while it is less sensitive to the external heat transfer coefficient and chemical kinetics. Also shrinkage is found to have a compensating effect and it does not have any significant influence on the temperature rise. Beyond an aspect ratio of three, the wood particle behaves as a 1-D cylinder.     

Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor

Seven mixtures of coals, plastics and wood have been pelletized and fed into a pre-pilot scale fluidized bed gasifier in order to investigate the main aspects of the co-gasification of these materials. The main components of the obtained syngas (CO, H₂, CO₂, N₂, CH₄, C_nH_m) were measured by means of on-line analyzers and a gas cromatograph. The performance of the gasifier was evaluated on the basis of syngas composition, carbon conversion efficiency, energy content of syngas, cold gas efficiency and yield of undesired by-products (tar and soot-like particulate). The results of a first series of experimental tests showed the effect of gas fluidizing velocity and that of equivalence ratio on the main performance parameters for a specific coal-plastics mixture. A second series of tests has been carried out by changing the mixture composition keeping fixed the gas velocity and equivalence ratio. The presence of wood and coal in the mixture with plastics contributed to reduce the tar production even though it is accompanied by a lower syngas specific energy.     

Modelling wood combustion under fixed bed conditions

A computer model describing the conversion of wood under packed-bed conditions is presented. The packed bed is considered to be an arrangement of a finite number of particles, typically sized between 5 and 25 mm, with a void space left between them. Each particle is undergoing a thermal conversion process, which is described by a one-dimensional and transient model.Within the single-particle model, heating, drying, pyrolysis, gasification and combustion are considered, whereby each particle exchanges energy due to conduction and radiation with its neighbours. Because of the one-dimensional discretization of the particles, heat transfer and mass transfer is taken into account explicitly. Therefore, no macrokinetic data are needed within the model. For ease of implementation and access, kinetic data and property data are stored in a database. The global conversion of the packed bed is represented by the contributions of single particles, where each particle is coupled to the surrounding gas phase by heat and mass transfer. For gas phase flow through the porous bed, the conservation equations for mass, momentum and energy are solved on a Cartesian mesh by a Finite Volume method.Experiments have been performed to validate the single particle model for the conversion of beech wood during pyrolysis and char combustion. Agreement between experimental and predictions obtained by the model is very satisfactory. However, for wet wood, changes in structure seem to enhance the heat transfer to the solid which is not yet covered in the model.     

Model development and validation: Co-combustion of residual char, gases and volatile fuels in the fast fluidized combustion chamber of a dual fluidized bed biomass gasifier

A one-dimensional steady state model has been developed for the combustion reactor of a dual fluidized bed biomass steam gasification system. The combustion reactor is operated as fast fluidized bed (riser) with staged air introduction (bottom, primary and secondary air). The main fuel i.e., residual biomass char (from the gasifier), is introduced together with the circulating bed material at the bottom of the riser. The riser is divided into two zones: bottom zone (modelled according to modified two phase theory) and upper zone (modelled with core-annulus approach). The model consists of sub-model for bed hydrodynamic, conversion and conservation. Biomass char is assumed to be a homogeneous matrix of C, H and O and is modelled as partially volatile fuel. The exit gas composition and the temperature profile predicted by the model are in good agreement with the measured value.     

Mixing and combustion in a shallow coal-limestone fluidized bed combustor

A model based on the Monte Carlo approach was developed to simulate the mixing and combustion behavior of a shallow coal-limestone fluidized bed combustor. The model involved the coupling of two sub-models: a combustion sub-model based on the two-phase concept of fluidization and a mixing sub-model based on our previously developed dynamic mixing model. The combustion sub-model considered both the volatile and char combustion. It assumed that the combustor consisted of three distinct phases, i.e., jet, bubble and emulsion, with combustion occurring only in the emulsion phase. The mixing sub-model considered the upward or downward movement of a coal particle in the bed as being governed by certain probability laws; these laws were, in turn, affected by the bubbling hydrodynamics. In all, the combustor simulation model took into consideration the effects of coal feed rate, coal size distribution, limestone size, air flow rate and combustor temperature on the combustor behavior. The simulation results included the dynamic response of coal concentration profile, coal size distribution, coal particle elutriation rate as well as the mixing status between the coal and limestone particles.     

Combustion characteristics of a two-stage swirl-flow fluidized bed combustor

In order to investigate the combustion characteristics of a two-stage swirl-flow fluidized bed combustor, combustion experiments of low-grade anthracite coal were performed. Experimental parameters were the fluidizing air velocity, coal feed rates, bed temperature, stoichiometric air ratio, swirl nozzle diameter and rotational diameter. The experimental results showed that, due to the swirl flow, the elutriation rates of fines were lower than those of the single-stage fluidized bed combustor. The combustible contents of the ash in the outflow streams were also reduced. Therefore, the combustion efficiency of the two-stage swirl-flow fluidized bed combustor was 20% greater than that of the single-stage fluidized bed combustor under the same operating conditions.     

The fragmentation of coal particles during the coal combustion in a fluidized bed

This paper presents an experimental method for studying the fragmentation of coal particles during coal combustion in a fluidized bed and the quantitative fragmentation indexes of 10 typical Chinese coal ranks. The influences of a variety of factors such as the bed temperature, the size of coal particles, the coal rank and the fluidizing medium on the fragmentation index of coal particles are also studied. The research results show that the main reason for the fragmentation of coal particles is the primary fragmentation, and that the volatile matter can drastically influence the degree of fragmentation of coal particles.     

Modelling of circulating fluidized bed combustion of a char

The combustion of a char in the 41 mm ID riser of a laboratory circulating fluidized bed combustor has been investigated at different air excesses and rates of solids (char and sand) circulating in the loop. Riser performance was characterized by an axial oxygen concentration profile as well as by the overall carbon content and particle size distribution. The proposed model accounts for carbon surface reaction, intraparticle and external diffusion, and attrition. External diffusion effects were relevant in the riser dense region where char was potentially entrapped in large clusters of inert solids. Experimental data and results of the model calculations are in satisfactory agreement.     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.     

Devolatilization of coals of North-Eastern India under fluidized bed conditions in oxygen-enriched air

Oxygen-enriched air can increase the combustion efficiency, boiler efficiency, and sulfur absorption efficiency of atmospheric fluidized bed combustion (AFBC) boilers which use high-sulfur coal, and other combustion systems that use coal. Devolatilization is the first step in the gasification or combustion of coal. In this work, devolatilization characteristics of five run-of-mine (ROM) coals of North-Eastern India having particle-size between 4 mm and 9 mm are reported. The experiments were performed under fluidized bed conditions at 1123 K in enriched air containing 30% oxygen. The devolatilization time was correlated with the particle diameter by a power law correlation. The variation of mass with time was correlated by an exponential correlation. It was observed that the average ratio of yield of volatile matter to the proximate volatile matter decreased with the increase in volatile-content of the coals. A shrinking-core model was used to determine the role of film-diffusion, ash-diffusion and chemical reaction. The experimental results indicate the likelihood of film-diffusion to be the rate-controlling mechanism in presence of oxygen-enriched air. A cost-analysis was carried out to study the economy of the process.     

Effect of co-combustion of chicken litter and coal on emissions in a laboratory-scale fluidized bed combustor

Co-combustion of chicken litter (CL) with coal was performed in a laboratory-scale fluidized bed combustor to investigate the effect of CL combustion on pollutant emissions. The emissions of major gaseous pollutants including CO, SO₂, H₂S and NO and temperature distribution along the combustor were measured during the tests. Effects of CL fraction and secondary air on combustion characteristics were studied. The experimental results show that CL introduction increases CO emissions and reduces the levels of SO₂. The ratio of H₂S/SO₂ increases with increasing fraction of CL. NO emissions either increase or decrease depending on the percentage of CL in the mixed fuels. The temperature in the freeboard region increases with increasing the fraction of CL while the reverse is true for the bed temperature.     

Combustion modelling of coal volatiles in the freeboard of a calorimetric fluidized bed combustor

A simplified kinetic approach, based on functional groups of the parent coal, was coupled with the bed hydrodynamics and a volatiles evolution region within the bed to conduct a parametric study with the experimental results obtained from a calorimetric fluidized bed combustor (FBC). The model results revealed that, for high-volatile coals with particle diameters of 1–3 mm, the fraction of the original volatiles burnt above the bed may be as high as 0.44-0.20, 0.36-0.09 and 0.30-0.02 for excess air levels ranging from 0 to 40% and bed temperatures of 800, 850 and 900°C respectively. For a low-volatile coal, the computed fractions were found to be in the ranges 0.35-0.08, 0.29-0.01 and 0.25-0.00 for similar operating conditions to the above. Good agreement between the model and experimental data suggests that the evolution of volatiles for coal particle diameters <5 mm is mechanistically controlled by both diffusion and chemical kinetics, while their combustion is largely governed by the mixing of volatiles and oxygen in the bed region.     

Formation of bed agglomeration in a fluidized multi-waste incinerator

A case study was carried out to investigate the bed agglomeration observed in a fluidized bed incinerator when burning blends of three wastes (carbon soot, biosludge and fuel oil). Several instrumental approaches were employed (i.e. XRF, SEM, XRD, and ICP-AES) to identify the bed materials (fresh sand and degrader sand) and clinkers formed in the full-scale incinerator tests. Several elements (V, Al, S, Na, Fe, Ni, P, and Cl), which normally are associated with the formation of low melting point compounds, were found in the waste blends at high content levels. The clinker bridges were identified to be associated with Al, Fe, V, K, Na, S, Ni, and Si elements.The effects of temperature and blending ratio were investigated in a muffle furnace. Carbon soot is believed to be more susceptible to the clinker formation than the other two fuels. Thermodynamic multi-phase multi-component equilibrium calculations predict that the main low melting point species could be Al₂(SO₄)₃, Fe₂(SO₄)₃, Na₂SO₄, NaCl, Na₂SiO₃ and V₂O₅. This information is useful to understand the chemistry of clinker formation. Also, it helps to develop methods for the control and possible elimination of the agglomeration problem for the design fuels.     

Experimental investigation of a rotating fluidized bed in a static geometry

Rotating fluidized beds in a static geometry are based on the new concept of injecting the fluidization gas tangentially in the fluidization chamber, via multiple gas inlet slots in its cylindrical outer wall. The tangential injection of the fluidization gas fluidizes the particles tangentially and induces a rotating motion, generating a centrifugal field. Radial fluidization of the particle bed is created by introducing a radially inwards motion of the fluidization gas, towards a centrally positioned chimney. Correctly balancing the centrifugal force and the radial gas-solid drag force requires an optimization of the fluidization chamber design for each given type of particles. Solids feeding and removal can be continuous, via one of the end plates of the fluidization chamber.The fluidization behavior of both large diameter, low density polymer particles and small diameter, higher density salt particles is investigated at different solids loadings in a 24 cm diameter, 13.5 cm long non-optimized fluidization chamber. Scale-up to a 36 cm diameter fluidization chamber is illustrated.Provided that the solids loading is sufficiently high, a stable rotating fluidized bed in a static geometry is obtained. This requires to minimize the solids losses via the chimney. With the polymer particles, a dense and uniform bed is observed, whereas with the salt particles a less dense and less uniform bubbling bed is observed. Solids losses via the chimney are much more pronounced with the salt than with the polymer particles.Slugging and channeling occur at too low solids loadings. The hydrostatic gas phase pressure profiles along the outer cylindrical wall of the fluidization chamber are a good indicator of the particle bed uniformity and of channeling and slugging. The fluidization gas flow rate has only a minor effect on the occurrence of channeling and slugging, the solids loading in the fluidization chamber being the determining factor for obtaining a stable and uniform rotating fluidized bed in a static geometry.     

Gas yields from coal devolatilization in a bench-scale fluidized bed reactor

Devolatilization behavior of Australian bituminous coal-gasification was determined in a 0.1 m diameter fluidized bed at 650-900 °C. To predict gas yields from devolatilization, several correlations reported for gas yields were evaluated with the present experimental data. The correlation of Goyal and Rehmat [1993] was found to be good one. Also, a correlation for the product gas yields has been proposed from devolatilization of bituminous coals as a linear function of temperature with constants from the experiment. The experimental yields of product gas show good agreement with the values calculated by the proposed correlation. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8-10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Combustion of liquid petroleum gas in a fluidized bed furnace with a jetting-mixing distributor

Liquid petroleum gas (LPG) fluidized beds have potential applications in metal heating or workpiece heat treatments. The combustion of LPG and the controls of the atmosphere inside the bed and the bed temperature are very concerned. The combustion of LPG has been investigated in a pilot-scale bubbling fluidized bed with a jetting-mixing nozzle distributor and hollow corundum sphere particles of 0.867-1.212 mm in diameter and 386-870 kg/m³ in bulk density at 800-1100°C. Experiments were carried out for fuel-rich mixtures to explore the possibility to obtain mild oxidizing, non-oxidizing or reducing atmosphere in the bed. Air factor (the ratio of the volume of air actually fed into the bed to that in a stoichiometric mixture) is in between 0.3 and 1.0 and U/U_mf 1.3-3.0. The distributor brings LPG and air into an intense contact sufficient to permit in-bed combustion without backfire problems. The experimental results show that the fluidized bed furnace offers excellent thermal uniformity and temperature control. The size of the combustion zone is usually larger than that of the temperature variation zone. Particle properties, initial bed height, air factor and U/U_mf all affect the bed temperature profile, whereas only the air factor and U/U_mf have significant effects on the combustion in the bed. The bed temperature can be adjusted by separate or combined adjusting of air factor and U/U_mf.     

Flow pattern in a fluidized bed with a non-fluidized zone

The flow pattern of a fluidized bed with non-fluidized zones is investigated both experimentally and theoretically. Experiments were carried out in such a way that air was introduced only through part of the distributor. The results show a significant amount of air flowing to the zone where no air is introduced. However, once the gas velocity exceeds the minimum fluidization velocity in the zone where the air is introduced, the cross-flow hardly changes upon further increase of the gas velocity. A continuity equation and Ergun's equation are used to describe the flow pattern and pressure distribution over the bed. Very good agreement between the experimental and calculated results is achieved without any fitting parameter. The results are relevant to the understanding of heat transfer behaviour of a fluidized bed combustor (FBC) that is only partly fluidized to control its load.     

Limestone fragmentation and attrition during fluidized bed oxyfiring

Attrition/fragmentation of limestone under simulated fluidized bed oxyfiring conditions was investigated by means of an experimental protocol that had been previously developed for characterization of attrition/fragmentation of sorbents in air-blown atmospheric fluidized bed combustors. The protocol was based on the use of different and mutually complementary techniques. The extent and pattern of attrition by surface wear in the dense phase of a fluidized bed were assessed in experiments carried out with a bench scale fluidized bed combustor under simulated oxyfiring conditions. Sorbent samples generated during simulated oxyfiring tests were further characterized from the standpoint of fragmentation upon high velocity impact by means of a purposely designed particle impactor. Results showed that under calcination-hindered conditions attrition and fragmentation patterns are much different from those occurring under air-blown atmospheric combustion conditions. Noteworthy, attrition/fragmentation enhanced particle sulfation by continuously regenerating the exposed particle surface.