Article

Effect of Moisture Content on Devolatilization Times of Pine Wood Particles in a Fluidized Bed

Authors:
  • Spanish National Research Council (CSIC) , Instituto de Carboquímica (ICB)
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Abstract

This work analyzes the effect of moisture content on devolatilization times of pine wood particles in a fluidized bed combustor. The devolatilization process was followed by measuring the CO2 and O2 concentrations obtained after the complete combustion of the volatiles. The devolatilization rate decreased and was more uniform along the devolatilization time as the moisture content of the wood particles increased. The devolatilization times increased almost linearly with moisture, and the slope slightly increased when the bed temperature decreased. The devolatilization times were correlated by a power-law relationship, which related the devolatilization time to the fuel particle diameter and shape factor [tv = a(dp,eqφ)n]. The values of exponent n were between 1.5 and 1.7 and were almost unaffected by the bed temperature or the moisture content. The values of the constant a decreased with increasing the bed temperature and with decreasing the moisture content of the wood particles. To predict the devolatilization times of wood particles as a function of their moisture content, a modification of the power-law relationship is proposed.

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... It is reported in the literature that the pyrolysis of the biomass is directly influenced by the heating rate and that -from an idealised point of view -no other reactions take place during the pyrolysis process [31][32][33]. However, deDiego et al. observed the formation of a char layer on the surface of the decomposing fuel particles (in this case, pine wood was used as feed) during devolatilisation, [34] and [35]. Similar phenomena were reported by Hagge and Bryden as well as Babu and Chaurasia, who modelled the pyrolysis of solid biomass particles. ...
... Their model depends on several physical and kinetic parameters that are averaged for different bed temperatures. In order to describe the devolatilisation process of non-spherical woody biomass with different water contents, de Diego et al. proposed the following model with a reduced number of input parameters [34,35]. ...
... The pre-exponential parameter a ref depends on the bed temperature, Table 4. A reduction of a ref for increasing temperatures has been reported in [35]. In the case of constant fuel properties (in particular particle size, shape, and water content), higher temperatures provide shortened overall devolatilisation times due to higher heating rates, and therefore, more rapid volitales release. ...
... de Diego et. al. [5] have experimentally investigated the influence wood moisture on the devolatilization time. Pinus Sylvestris wood cuboids were used at sizes 15×15×15 mm, 20×9×20 mm, 10×16×15 mm, 10×10×10 mm and 20×4×20 mm. ...
... The size of the cylindrical wood particles is represented by the size of a sphere having the same volume to surface area ratio. The structure of the equation is similar to that given by de Diego et al. [5]. ...
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This paper presents a detailed review of experimental and modeling work carried out on devolatilization of different kinds of wood under fluidized bed combustion/pyrolysis conditions. Laboratory scale experimental studies as well as analytical, phenomenological and numerical modeling works have been reviewed and presented. It has been found that attempts to determine the kinetics in actual fluidized bed conditions have been carried out. There is no single devolatilization model that incorporates all the physical and chemical phenomena occurring during devolatilization. Moreover, the physical phenomenon of primary fragmentation has not been adequately incorporated in the models. Also, non-intrusive temperature measurement techniques need to be developed and demonstrated.
... During heterogeneous combustion of char the density decreases by about 10-25%, indicating the possibility of internal reactions with oxygen diffusing from the outer surface through the porous structure. Combustion of small-sized (maximum sizes of about 15-20mm) wood particles in fludized-bed reactors (temperatures) is investigated by de Diego et al. (2002 de Diego et al. ( ,2003), who also report a two-stage process where devolatilization is followed by combustion. For the small-scale reactor, batch-wise operated, it appears that most of the volatiles generated from the pyrolysis of wood particles burn in the free-board of the reactor and not around the particle. ...
... These conclusions can also be drawn from the experiments carried out by other researchers (for instance, Winter et al. (1996), Leckner et al. (1999). The most probable explanation for this behavior is that as long as volatiles are continuosly released with sufficiently fast rates, oxygen is prevented from reaching the surface of the fuel particle (De Diego et al. (2003)). Another consideration to be made is that, when the particle size is small, devolatilization is completed before the particle surface attains temperatures high enough for the heterogeneous reactions to become active. ...
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This review critically examines char conversion kinetics in oxidizing or reducing at-mosphere. In kinetic analysis the devolatilization of biomass and the conversion of char are usually investigated by means of separate experiments, although some kinetic mod-els are also available of biomass combustion. The large majority of the char conversion kinetics consists of a global reaction with activation energies of 196-200kJ/mol (carbon dioxide gasification), 138-271kJ/mol (steam gasification), 75-229kJ/mol (char oxidation) and a power law dependence on the partial pressures of the gasifiying agents. The rate ex-pressions usually also incorporate a structural contribution. The wide range of variation of kinetic parameters is due to the different char properties (biomass characteristics and pyrolysis conditions) and mainly to the simplifications introduced in the mathematical treatment of the data.
... Besides the detailed 1D models, empirical correlations have also been developed to estimate the devolatilization time of biomass particles [7,[21][22][23][24][25]. Most of the existing correlations primarily use a power law to correlate the devolatilization time with particle size with a with relatively low temperatures (< 1000°C) [21][22][23][24][25]. Empirical correlations are rarely reported for biomass devolatilization under pulverized flame conditions. ...
... Besides the detailed 1D models, empirical correlations have also been developed to estimate the devolatilization time of biomass particles [7,[21][22][23][24][25]. Most of the existing correlations primarily use a power law to correlate the devolatilization time with particle size with a with relatively low temperatures (< 1000°C) [21][22][23][24][25]. Empirical correlations are rarely reported for biomass devolatilization under pulverized flame conditions. ...
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... On the other hand, devolatilization of coarse fuel particles, of size larger than a few millimeters, can also be directly characterized by periodic sampling of fuel particles from the fluidized bed at regular time intervals, followed by the analytical determination of the residual volatile matter in the samples. 16,32,49 Techniques based on the analysis of the time-resolved concentration of gases released during devolatilization 30,42,[50][51][52][53][54][55][56][57] are almost as simple as the flame methods. When pyrolysis occurs in inert conditions, devolatilization times corresponding to either 50 or 95% conversion degrees are usually calculated by the analysis of the time-resolved methane concentration measured at the exhaust. ...
... [50][51] When pyrolysis is carried out under oxidizing conditions, the devolatilization rate and time may be assessed by consideration of the time series of carbon dioxide and oxygen concentrations at the exhaust. 30,42,[52][53][54][55][56] The time/temperature history of the fuel particle can be further analyzed since its injection in the fluidized bed to assess the progress of drying, devolatilization and char burn-out. [26][27]46,[58][59][60] Ross et al. 59 proposed to take the time interval between fuel particle injection and the time at which the center of the particle reaches the bed tempera-ture as a measure of the devolatilization time. ...
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... where t (devolatilization time) is in seconds, d p (particle diameter) is in millimetres and T (temperature) is in Kelvin. A knowledge of wood devolatilization kinetics is of great importance for the efficient design of combined gasification/ combustion processes [7,[17][18][19], in particular for circulating fluidized bed systems: sufficient biomass particle residence time is required in the gasifier to allow for total devolatilization of the biomass, before it enters the combustion zone, where only char should be present. ...
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... According to de Diego et al (2003), the effects of moisture can be estimated by multiplying the result from 8a by the factor = 1 + 1.7 × 10 −2 ...
... If the effect of heat of pyrolysis is assumed insignificant, Eq. (4) is becomes (sphericity) φ s , size d and ambient temperature on devolatilization and burning of wood particles have been studied [25,26]. Equations of the form ...
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A detailed mathematical model is presented for the temporal and spatial accurate modeling of solid-fluid reactions in porous particles for which volumetric reaction rate data is known a priori and both the porosity and the permeability of the particle are large enough to allow for continuous gas phase flow. The methodology is applied to the pyrolysis of spherically symmetric biomass particles by considering previously published kinetics schemes for both cellulose and wood. A parametric study is performed in order !o illustrate the effects of reactor temperature, heating rate, porosity, initial particle size and initial temperature on char yields and conversion times. It is observed that while high temperatures and fast heating rates minimize the production of char in both reactions, practical limits exist due to endothermic reactions, heat capacity and thermal diffusion. Three pyrolysis regimes are identified: 1) initial heating, 2) primary reaction at the effective pyrolysis temperature and 3) final heating. The relative durations of each regime are independent of the reactor temperature and are approximately 20%, 60% and 20% of the total conversion time, respectively. The results show that models which neglect the thermal and species boundary layers exterior to the particle will generally over predict both the pyrolysis rates and experimentally obtainable tar yields. An evaluation of the simulation results through comparisons with experimental data indicates that the wood pyrolysis kinetics is not accurate; particularly at high reactor temperatures.
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Drying and devolatilization from coal particles and the mass transfer processes in the immediate vicinity of burning char particles in a fluidized bed consisting of predominantly smaller non-combusting particles are reviewed. Single particle studies with well defined external heat/mass transfer conditions are thought necessary to refine the current understanding of drying, devolatilization and volatile matter combustion local to the coal particle. An empirical model and a theoretical model are considered to most accurately reflect the mass transfer data. It is considered important to include in theoretical models dealing with various aspects of coal combustion (drying, devolatilization, heat/mass transfer, char combustion) the two-phase nature of the bed and the possibility that the burning particle spends a significant proportion of its 'life' in the bubble phase. Further careful experimentation is necessary to refine mechanistic understanding.
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Release of volatiles of non-spherical pine wood particles was analysed by means of continuous measurements of the CO2 and O2 concentrations obtained after the complete combustion of the volatiles and from flame extinction times. The effect of the atmosphere used for devolatilisation was tested. The volatiles' evolution was nearly identical using air or N2 as fluidising gas. The devolatilisation times increased with increasing the equivalent particle diameter, but there was an important scattering in the results. The data dispersion greatly decreased when the shape factor of the wood particles was considered. The devolatilisation times were fitted to a power-law relation replacing the particle diameter by the equivalent particle diameter multiplied by the shape factor. The effect of the moisture content was studied by analysis of the devolatilisation process of pine wood particles of the same size and different moisture contents (0–50%). As the moisture content of the wood particles increased the devolatilisation rate of combustible volatiles decreased and was more uniform along the devolatilisation time.
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A non-interference technique has been developed for measuring devolatilization rates of large coal particles in a cylindrical fluidized bed at 850 °C by on-line analysis of oxygen and carbon dioxide concentrations in the flue gas. The experiments were conducted over a range of inlet oxygen concentrations (3–10 vol%). Oxygen concentrations were analysed by a zirconia probe, and carbon dioxide concentrations were analysed by an on-line infrared gas analyser. Associated problems and suitability of the measuring technique are discussed. A bituminous coal (d = 9–16.3 mm) was used and devolatilization periods >130 s were observed. The devolatilization times showed a dependence on the inlet oxygen concentration, particularly at the lower (3 vol%) inlet oxygen concentration. A simple calculation to demonstrate the need for considering devolatilization rates, ‘coal’ dispersion coefficients, excess air levels and combustor geometry in FBC modelling is also given.
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The behaviour of very wet Victorian brown coal was examined in a bed of sand fluidized, at temperatures around 1000 K, with either air or nitrogen. Small batches of coal with a narrow particle size range were added to the 76 mm diameter bed and the times required for devolatilization and total combustion were recorded. Changes in particle water content, volatiles level and particle size distribution were also measured. All the particles tested, up to 8.4 mm in diameter, dried rapidly and remained substantially intact throughout carbonization and combustion. Devolatilization was complete after about 60 s but extensive freeboard combustion of volatiles was evident. The water content of the coal had very little influence on burnout time. Char combustion dominated the overall combustion process and took place under kinetic control with significant pore burning.
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In a two-dimensional (15 × 200 × 400 mm) high-temperature fluidized bed, devolatilization ignition and combustion phenomena of single coal particles have been studied. The particles, with diameters of 4–9 mm, were selected from three coal types of widely different rank: brown coal, bituminous coal, and anthracite. The bed consisted in most cases of porous alumina particles (0.6 mm diameter), and was fluidized by O2/N2 gas mixtures. At constant bed temperatures ranging from 200 to 850°C, the various stages prior to the eventual combustion of the residual char particle were recorded on videotape. This paper gives an account of visual observations on the release, ignition, and combustion of volatiles as well as on the ignition of char. Results of measurements of the temperature and delay time of both volatiles and char ignition are also reported. Finally, the period over which flames of volatiles are visible in the bed has been measured for each coal particle; at sufficiently high bed temperature they are indicative for the total devolatilization time.
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In this paper, the effect of water content on combustion characteristics of watery waste material, such as kitchen garbage, was studied experimentally. In order to clarify the water content quantitatively, a permeable balsa wood was chosen as the sample material. Combustion tests of the small balsa pieces (0.1–0.5 g) containing water up to 70 wt% were conducted in a thermogravimetric furnace. Mass reduction during pre-heating, volatile matter combustion and char combustion were measured with a microbalance. Also, the variations of flame temperature and center temperature of the test piece during the combustion process were measured by means of R- and K-type thermocouples, respectively. In the case of the combustion of balsa with high water-content, it was found that the ignition started before the water was removed completely, namely the simultaneous vaporization of the remaining water and devolatilization of volatiles occurred during volatile matter combustion. The ignition delay and the retention time of volatile matter combustion increased with increasing water content. The flame temperature decreased when the water content of wet balsa exceeded 50 wt%. The combustion rate of the volatile matter was drastically reduced in proportion to the water content. On the other hand, the char combustion rate increased slightly with increasing water content since part of the char is burned during the long volatile matter combustion. It was found from this study that excess moisture remarkably affected the volatile matter combustion with a flame.
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Radiative pyrolysis of thermally thick beech wood has been investigated through a comparison between dry and moist [11% dry basis (db)] particles, for heat fluxes in the range 27.5−80 kW/m2. The initial moisture content has also been varied from 0 to 50% (db) for two radiative fluxes, 27.5 and 49 kW/m2, corresponding to slow and fast external heat-transfer rates, as steady surface temperatures are about 625 and 800 K, respectively. For very slow heating, moisture evaporation precedes wood pyrolysis. As the external heating conditions are made more severe and/or the initial moisture content is increased, the two processes take place simultaneously, associated with the propagation of separate fronts along the particle radius. Spatial gradients also increase, while apparent weight loss kinetics from a single-peak rate turn into a two-peak rate. The conversion times increase almost linearly with the initial moisture content, but differences in primary product (char, gas, and liquids) yields and gas composition are negligible.
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The coupled effects of particle size and external heating conditions (reactor heating rate and final temperature) on cellulose pyrolysis are investigated by means of a computer model accounting for all main transport phenomena, variable thermophysical properties and primary, and secondary reaction processes. The dynamics of particle conversion are predicted, and final product distributions are favorably compared with experimental measurements. A map is constructed, in terms of particle size as a function of the reactor temperature, to identify the transition from a kinetically controlled conversion to a heat transfer controlled conversion (thermally thin and thermally thick regimes) and from flash to slow-conventional pyrolysis. Conditions for maximizing oil, gas, or char yields are also discussed.
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Experimental and modeling work on pyrolysis of wood under regimes controlled by heat and mass transfer are presented. In a single-particle, bell-shaped Pyrex reactor, one face of a uniform and well-characterized cylinder (D = 20 mm, L = 30 mm) prepared from Norwegian spruce has been one-dimensionally heated by using a Xenon-arc lamp as a radiant heat source. The effect of applied heat flux on the product yield distributions (char, tar, and gas yield) and converted fraction have been investigated. The experiments show that heat flux alters the pyrolysis products as well as the intraparticle temperatures to a great extent. A comprehensive mathematical model that can simulate pyrolysis of wood is presented. The thermal degradation of wood involves the interaction in a porous media of heat, mass, and momentum transfer with chemical reactions. Heat is transported by conduction, convection, and radiation, and mass transfer is driven by pressure and concentration gradients. The modeling of these processes involves the simultaneous solution of the conservation equations for mass and energy together with Darcy's law for velocity and kinetic expressions describing the rate of reaction. By using three parallel competitive reactions to account for primary production of gas, tar, and char, and a consecutive reaction for the secondary cracking of tar, the predicted intraparticle temperature profiles, ultimate product yield distributions, and converted fraction agreed well with the experimental results.
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A mathematical model of wood pyrolysis is presented that is in satisfactory agreement with experimental reaction product distributions over a range of conditions of practical importance for gasification and combustion. Both chemical and physical processes are described, using fundamental principles. Inclusion of water release, tar cracking and char deposition chemical reactions results in predictions of ultimate product distributions (gas, tar and char yields) that are in good agreement with experiment and can aid in optimization of processes to maximize or minimize tar production. Predictions of the rate at which products are instantaneously released from a single wood pellet are also in agreement with experiment. This capability is important for combustion modelling and gasifier simulation. The study provides both extensive data and a fundamental modelling approach that will enhance understanding of the effects of physical properties and processes on the chemistry of devolatilizing biomass. The work emphasizes the need for information both on char deposition rates during carbonization of a range of fuel types and on char thermal properties over a wide range of temperatures and porosities.
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Times required for devolatilisation of large coal particles in a fluidised-bed operating at 750, 850 and 950°C and in gas environments simulating pyrolysis, combustion and gasification conditions were investigated. The effect of coal type and coal moisture content was also investigated at a bed temperature of 850°C under fluidised-bed combustion conditions. The devolatilisation time was defined as the time taken from immersion into the bed until the centre temperature of the coal particle equalled the bed temperature. Six coals of varying rank in the size range from 6 to 17 mm, were used in this present study. The devolatilisation time was found to be influenced by the bed temperature, gas environment, coal moisture content and coal type which is in agreement with data reported in the literature. The results were correlated with the classic empirical particle diameter power law relation (tv=Adpn) where it was observed that the devolatilisation time was directly proportional to the particle diameter. A new definition to distinguish between heat transfer and chemical-kinetically controlled regimes during devolatilisation validates present method and explains experimental observations relating to the influence of bed temperature and gas environment.
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A general model of the pyrolysis of a wood slab is presented and validated with a set of heat release data. The model is applied to particle half-thicknesses from to , temperatures from 800 to , and moisture contents from 0% to 30%. Internal temperatures, pyrolysis rates and yields of tar, hydrocarbons and char are presented. Four pyrolysis regimes are identified, depending on external temperature and particle size: thermally thin—kinetically limited, thermally thin—heat transfer limited, thermally thick, and thermal wave regimes.
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The burning times of the volatiles from twelve Turkish coals ranging from bituminous to lignite was experimentally investigated in a bench-scale atmospheric fluidized bed combustor. Devolatilization time was determined by visual observation of the plume-like volatiles flame. The time lapse between the appearance and disappearance of this flame was recorded as flame extinction time. The effects of particle size (1.0–11.2 mm) and bed temperature (650–920 °C) were investigated, and the effect of moisture was determined by measurements on oven-dried samples. Fragmentation was observed for a few coals after devolatilization was virtually complete, but the majority preserved their original particle shape. Flame extinction times were related to initial particle diameter by a power law; numerical constants for this relation were determined for each coal by regression analysis. Two parameters — volatile matter/fixed carbon ratio and volatile matter heating value — are proposed to characterize the effect of coal type on devolatilization time. The observed effects of particle size, bed temperature, moisture pore structure and coal type on flame extinction time are consistent with heat transfer to and in the coal particle as the rate-controlling step.
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The formation of different gases in the thermal decomposition of cellulose and pine sawdust has been studied. The kinetic constants of CO{sub 2} and H{sub 2} formation in cellulose decomposition have been determined from results obtained in isothermal experiments. These kinetic constants have been taken as representative of pine sawdust decomposition at T > 292 C, and values for lower temperatures have bene obtained from isothermal experiments performed with pine sawdust. For both materials, a simple model without adjustable parameters has been applied that allows one to calculate the local temperature, solid conversion, and yield of each gas. The results obtained in dynamic experiments with heating rates ranging between 2 and 53 C/min have been compared with the theoretical results, and an acceptable agreement has been achieved.
Article
The thermal decomposition of relatively large particles of pine wood has been studied. By use of spherical particles of different sizes (2-5.6 cm in diameter), the experimental solid conversion values and the temperature at different points of the solid are analyzed for different heating rates of the system. The experimental results have been compared with those calculated with a simple model, which involves the solution of the heat and mass balances and the kinetic equations, using experimental values as boundary conditions. A good agreement has been obtained for particles up to 4 cm in diameter. For larger particles the mass transfer resistance inside the solid and the existence of secondary reactions may acquire an appreciable importance.
Momentum and Mass Transport through a Shrinking Biomass Particle Exposed to Thermal Radiation
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