Wood chips flow in a rotary kiln: Experiments and modeling (original) (raw)
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Experimental Study of Wood Chips Torrefaction in a Pilot-scale Rotary Kiln
Chemical Engineering Transactions, 2014
This work aims to study the beech chips torrefaction in a continuous pilot-scale rotary kiln. The effects of operating parameters – temperature, residence time and solid hold-up – on the temperature profile of the solid bed along the kiln and the process mass yield are evaluated. It has been verified that an increase of the temperature level or residence time leads to a decrease of the process mass yield.. Furthermore, it has been evidenced that an increase of the solid hold-up tends to decrease the mass yield too. Studying the temperature profile along the kiln has enabled to locate the drying and torrefaction zones. Torrefaction only begins in the last third of the heating zone. It has also been observed that an important solid hold-up induces a slower heating of the biomass. On the other side, for high temperatures and/or solid hold-up, the biomass temperature exceeded the set-point temperature, which evidences the occurrence of exothermic reactions.
Residence time distribution and material flow studies in a rotary kiln
Experiments were conducted in a rotary kiln containing ilmenite particles to study the residence time distribution (RTD) of low-density particles, holdup, and bed depth profile. The variables include feed rate of solids, slope and rotational speed of the kiln, type and size of the tracer, and dam height. Correlations are presented for mean residence time, dispersion number, holdup, and steady-state throughput of solids in terms of the process variables. A simple method is proposed to estimate the dam height that gives rise to a flat profile of solids bed along the length of the kiln.
Kinetic model for torrefaction of wood chips in a pilot-scale continuous reactor
Journal of Analytical and Applied Pyrolysis, 2014
Torrefaction is a mild thermal treatment (200-300 • C) in an inert atmosphere, known to increase the energy density of biomass by evaporation of water and a proportion of the volatiles. In this work a "twostep reaction in series" model was used to describe the thermal degradation kinetics of pine wood. The kinetic parameters were determined using a thermogravimetric analyzer (TGA) and the mass loss during the initial heating period was taken into account when deriving the kinetic parameters. It was shown that the experimental results at different heating rates (10-50 • C min −1) are in good accordance with the model data. In an additional step a continuous, pilot scale reactor was built to produce torrefied wood chips in large quantities. The "two-step reaction in series" model was applied to predict the mass yield of the torrefaction reaction. Parameters used for the calculation were the temperature along the reactor and the biomass feeding rate in combination with the kinetic parameters obtained from the tests in the TGA. Together with results from a laboratory scale, batch torrefaction reactor that was used to determine the higher heating value (HHV) and mass loss (y) of the same material at different torrefaction temperatures, it was possible to predict the HHV of torrefied wood chips from the pilot reactor. The results from this study and the presented modeling approach can be used to predict the product quality from pilot scale torrefaction reactors based on small scale experiments and could be used to improve the homogeneity of torrefied products, which still is a problem for most operational torrefaction pilot plants today.
Residence time and mass flow rate of particles in carbon rotary kilns
Chemical Engineering and Processing: Process Intensification, 2009
The influences of operational and structural parameters on the mean residence time (MRT) and the mass flow rate (MFR) of solids are presented on an experimental rotary kiln. Experimental results show that the increase of rotation speed and kiln slope reduces MRT and increases MFR. Also, MFR increases with increasing charge dam height. MRT increases when the rotation speed is lower than 2.0 rpm but decreases when the rotation speed is higher than 2.0 rpm with increasing dam height. Installation position of the feed pipe also has a significant effect on MFR. The product of MRT and MFR increases approximately linearly with MFR per kiln rotation at the same kiln slope.
Bioresources, 2014
The torrefaction of red oak (Quercus rubra) was performed in a pilot rotary kiln reactor, and the apparent kinetic results were compared with the results of torrefaction performed in a bench-scale fluidized reactor. Mass loss, gross calorific analyses, ultimate analyses, and proximate analyses were applied to the final torrefied material. The experimental torrefaction temperatures were 250, 275, 300, and 325 °C, and the experimental total torrefaction times were 20, 35, 50, and 80 min. A significant variation of the energy content occurred in the range of temperature between 275 and 300 °C, with the energy yield changing from 97.5% to 83.6%, respectively. The molar ratios H:C:O for the torrefied red oak presented a behavior independent of the experimental equipment when the temperature ranged between 250 and 325 °C. For the torrefaction process of red oak in the pilot rotary kiln reactor, a first-order reaction and one-step kinetic model were fitted with a maximum error of about 7.5% at 325 °C. The observed reaction rate constant (k) for the rotary reactor was 0.072 min −1 at 300 °C, which was 71% lower than the reaction rate constant for torrefied red oak in a bench-scale fluidized reactor. Arrhenius analysis determined an activation energy of 20.4 kJ/mol and a frequency factor of 5.22 min −1. The results suggest significant external heat and mass-transfer resistances in the rotary system.
Biomass Torrefaction in a Two-Stage Rotary Reactor: Modeling and Experimental Validation
Energy & Fuels, 2017
A mechanistic model of torrefaction was developed for a two-stage rotary reactor and it was verified with experimental results. Mass and energy balances for each phase considered in the model. A kinetic model, which considers the progressive decomposition of biomass into volatiles and char released simultaneously from the raw biomass was coupled to balances. Mathematical expressions for residence time, heat transfer coefficient, and bed height inside the kiln were taken from literature for model calculations. Release of condensable and non-condensable volatiles from biomass during the process was considered in the gas phase, while the solid phase included raw and torrefied biomass. The model can predict different output parameters of torrefaction in a rotary continuous torrefier like final amounts of solids and gas yields, and temperatures for different operational conditions. Properties for torrefied solid such as high heating value, fixed carbon, and volatile matter, can also be predicted by the model through mathematical correlations obtained in a previous experimental work. Results obtained in the model were compared with experimental data and a good agreement was found.
2014
Experiments on the residence time distribution (RTD) and axial dispersion for the continuous ow of sand and broken rice, through a pilot scale rotary kiln equipped with lifters, are reported. Factors such as the rotational speed, the kiln slope, the materials ow rate and the exit dam height have been studied. Furthermore, two proles of lifters were used: straight lifters (SL) and rectangular lifters (RL). Thus, under varying conditions the RTDs were obtained by the typical stimulus response test using a tracer and the corresponding axial dispersion coecients were determined. The validity of the axial dispersion model was assessed in this study, and the model was found to match well with the experimental data. A large number of experiments was conducted, so that, a mathematical model could be developed to predict the axial dispersion coecient of the solid particles within the kiln. Comparisons with reported models are also discussed. The second part of this study will be concerned with the experimental kiln holdup and the mean residence time (MRT) of solid particles.
Experimental study of residence time, particle movement and bed depth profile in rotary kilns
The Canadian Journal of Chemical Engineering, 1995
Experiments were performed in a pilot scale rotary kiln with coal and coke particles to study their mean residence time, residence time distribution, bed depth profile and time spent at the bed surface. The influence of filling ratio on residence time was studied with a uniform bed depth in the kiln. Residence time distribution and bed depth profile measurements were performed in a kiln without end constriction, at different rotational speeds and different solid inputs.
Numerical prediction of the mean residence time of solid materials in a pilot-scale rotary kiln
Powder Technology, 2019
Five models that predict the Mean Residence Time (MRT) of solids in a rotary kiln are tested on three materials and validated experimentally. Furthermore, the influence of the kiln rotational speed and incline on the MRT was investigated. Determination and modelling of the MRT in pilot-scale reactors (length/diameter = 10.5) without a discharge dam, has not been studied yet. The prediction of the MRT with existing models gave poor results, therefore adaptions were necessary. The Saeman's model that was corrected with a new boundary condition decreased the mean absolute error on the experimental results from 54.5% to 15.3%. While the empirically corrected models of Saeman, Sullivan, Chatterjee and U.S. geological survey predicted the solid's MRT with an error b10% for kiln inclinations b1°. It was concluded that the MRT and the kiln's rotation relate inversely proportional, while the kiln's inclination relates logarithmically to the MRT.
A two dimensional model for torrefaction of large biomass particles
Journal of Analytical and Applied Pyrolysis, 2016
Torrefaction is defined as a thermal pre-treatment process performed within a temperature range of 200-300 • C, at low-heating rates (<20 • C/min) and for residence times between 15-60 min in inert environments. A phenomenological model of the torrefaction process of large biomass particles is developed in this work. Mass and energy balance coupled to a kinetic model take into account two steps of the biomass decomposition. First of the two steps, considers simultaneous production of vapor and solids from raw biomass. The vapor phase comprises a mixture of condensable and non-condensable gases, while the solid phase consists of torrefied biomass. The second step involves decomposition of volatiles into gases and secondary char. The model analyzes torrefaction behavior of both large and small biomass particles, predicting their final solid and gas yields, temperatures distribution, internal pressure and velocity of the gas phase within the particles. The model also predicts maximum conversions for given particle sizes and temperatures during the process. For given set of conditions small particles showed higher (∼77%) than that (∼52%) for large particles. Maximum interstitial gas velocities inside the large particle (25 mm in diameter and 65 mm in length) was about 1.2 mm/s and pressure gradients of about 2000 kPa and it occurred after 20 min in the process.