Freeboard Gas Research Articles

Spatio-temporal prediction of temperature in fluidized bed biomass gasifier using dynamic recurrent neural network method

• Long-short term memory model is proposed to predict temperatures in fluidized bed gasifier. • The proposed model is validated with pilot-scale plant data resolved in space and time. • Multi-step ahead temperature progressions of freeboard, fluidized bed and outlet gas, are predicted. • Gated recurrent unit and long short-term memory models are compared. In this study, a long short-term memory (LSTM) based dynamic recurrent neural network model is proposed for multi-step ahead temperature predictions in a pilot-scale fluidized bed biomass gasifier (FBG). The LSTM model predicts not only the temporal but also the spatial distribution of temperature by considering the temperature of each region of the FBG (fluidized bed, freeboard and outlet gas) as a separate target parameter. The proposed model is validated by comparing simulation data with experimental observations acquired during operation of the FBG. The validation results reveal that the proposed LSTM model is capable of accurately (MAE<6) predicting 1-min-ahead temperature of all the FBG regions. The LSTM model is further challenged for temperature predictions at farther future points (3 min and 5 min ahead) to test the prediction limits of the LSTM model. For 5 min ahead predictions, the proposed LSTM-based prediction model is also compared with other state-of-the-art dynamic neural network methods that include the standard recurrent neural network (S-RNN) and its advanced variant, the gated recurrent unit (GRU). The comparative findings for far future predictions show that LSTM has the highest accuracy, and also exhibit that GRU does not have universally faster convergence than LSTM.

A two-fluid model simulation of an industrial moving grate waste incinerator

CFD modelling and simulation is an effective means of optimizing the design and operation of moving grate waste incinerators. Conventional approach models the grate combustion and the furnace combustion separately by using an in-bed/over-bed coupling procedure. In this paper, a comprehensive two-fluid reacting model that integrates the gas-solid grate incineration and the gas turbulent combustion in one scheme is developed for industrial incinerators. Realistic grate geometry and direct simultaneous coupling of the fuel bed and the freeboard gas phase are realized. According to different treatments of the solid phase, the whole incinerator is divided into three regions, namely the packed bed region, the fall region and the furnace region. The kinetic theory of granular flow (KTGF) is introduced to describe the rheological properties of waste particles, and the Ergun model is used for the gas-solid drag. Thermal conversion of wastes is characterized by the heterogeneous reactions of moisture evaporation, devolatilization, char-O2 combustion and the homogeneous reactions of hydrocarbons combustion. Distributions of temperatures and gas species are predicted and validated by measurements. Particle properties are calculated to reveal the grate incineration characteristics. Effects of waste throughput on the incineration are also investigated. Overall, the present model provides a new methodology of in-bed and over-bed integration for the moving grate incinerator simulation.

Wall-to-solid heat transfer coefficient in flighted rotary kilns: Experimental determination and modeling

A series of experiments were carried out on an indirectly heated pilot scale rotary kiln. These experiments aimed at recording, while the solids flow, the temperature profiles of the freeboard gas, the solid particle bulk and the wall, as well as the power supplied for heating, over a range of operating conditions. Based on these data, the experimental wall-to-solid heat transfer coefficient was determined through an energy balance. The effects of operating conditions, namely rotational speed, filling degree, lifter shape and controlled temperature, on the heat transfer coefficient are discussed. A model based on dimensional analysis is proposed to calculate the wall-to-solid heat transfer coefficient for low to medium heating temperatures (100–500 °C). The experimental and calculated results are in good agreement. The experimental results are also compared to the predictions of some existing models. While the predictions are within a reasonable order of magnitude with regard to the experimental results, these models fail to represent actual variations with operating conditions satisfactorily.

Impact of Using Alternative Fuels on Cement Rotary Kilns: Experimental Study and Modeling

A mathematical model was developed to predict heat transfer from the freeboard gas to the bed of a rotary kiln and to evaluate the fact of using olive pomace as an alternative fuel in cement manufacturing. Separate models were developed, viz: model for simulating reactions and heat transfer in the bed region, a model for simulating combustion and heat transfer in the freeboard region. The simulated results were compared with the available data from industrial kiln. The developed model and the presented results will be useful for predicting the influence of olive pomace on performance of rotary cement kilns.

Ebullated bed fluid dynamics relevant to industrial hydroprocessing

This study investigates the overall fluid dynamics of an ebullated bed operating at high gas holdup conditions to provide relevant observations for industrial residue hydroprocessors. Scaling approaches for three-phase fluidized beds were compared specifically for the scale-down of the industrially observed high gas holdup conditions. Five dimensionless groups, a binary approach for bubble coalescence behaviour in multi-component liquids, and geometric considerations are proposed to achieve dynamic similitude. Experiments were carried out in a 101.6mm diameter co-current gas–liquid–solid fluidized bed operating at 0.1 and 6.5MPa with liquids that do (e.g., 0.5wt% aqueous ethanol) and do not (e.g., tap water) significantly inhibit bubble coalescence. A comparison of the overall phase holdups for two sizes of cylindrical particles (dSV of 1.6 and 3.9mm) at matching dimensionless groups provided a preliminary verification of the proposed scaling approach when bubble coalescence was sufficiently and consistently inhibited. The impacts of increased liquid viscosity (e.g., greater vacuum distillation tower residue feed fraction), varying superficial gas velocity (e.g., inlet gas flow rate and gas entrainment in the liquid recycle line), and varying superficial liquid velocity (e.g., liquid recycle pump speed) were experimentally studied due to their relevance for industrial ebullated bed hydroprocessors. For the studied operating conditions, gas holdups in the ebullated bed were much greater when bubble coalescence was sufficiently inhibited (up to approx. 45vol%) compared to a coalescing system at equivalent conditions (up to approx. 25 and 35vol% for 0.1 and 6.5MPa, respectively). When increasing the liquid viscosity of the 0.5wt% aqueous ethanol, a fraction of the gas was entrained in the liquid recirculation, increasing gas holdups and exhibiting operational similarities to industrial hydroprocessors. After adjusting the gas and liquid flow rates based on the gas recirculation, the increased liquid viscosity mainly reduced the solid holdups while gas holdups were approximately unchanged. Enhanced bubble break-up or bubble coalescence due to particles resulted in an overestimation or underestimation, respectively, of the freeboard gas holdups from a solids-free estimate based on the bed region phase holdups, where this model could not capture the complex bubble–particle interactions. Experimental results at high gas holdups conditions were used to correlate the bed and freeboard phase holdups based on the proposed dimensionless groups.

Decoupled numerical simulation of a solid fuel fired retort boiler

The paper deals with numerical simulation of the retort boiler fired with solid fuel. Such constructions are very popular for heating systems and their development is mostly based on the designer experience. The simulations have been done in ANSYS/Fluent package and involved two numerical models. The former deals with a fixed-bed combustion of the solid fuel and free-board gas combustion. Solid fuel combustion is based on the coal kinetic parameters. This model encompasses chemical reactions, radiative heat transfer and turbulence. Coal properties have been defined with user defined functions. The latter model describes flow of water inside a water jacked that surrounds the combustion chamber and flue gas ducts. The novelty of the proposed approach is separating of the combustion simulation from the water flow. Such approach allows for reducing the number of degrees of freedom and thus lowering the necessary numerical effort. Decoupling combustion from water flow requires defining interface boundary condition. As this boundary condition is unknown it is adjusted iteratively. The results of the numerical simulation have been successfully validated against measurement data.

Temperature distribution within the moving bed of rotary kilns: Measurement and analysis

Inadequacies in the temperature measurement within the moving bed have hindered a thorough understanding of the processes occurring within rotary kilns. A new measuring system, consisting of thermocouple arrays, a radio-transmitter, a radio-receiver and a computer monitor is introduced in this paper. With it, the 3D temperatures within the moving bed, as well as the temperatures of the freeboard gas and the kiln wall, can be measured and saved automatically. Experiments with sand on a co-current pilot kiln demonstrated that, in the passive layer of the moving bed, the temperatures were approximately constant in the circumferential direction. In the radial direction, however, large temperature difference was observed within the bed near the feed end of the kiln, and the difference became smaller as the bed went progressed through the kiln. This temperature measuring system can be used to obtain data over a wide range of operating conditions for use in engineering design. The obtained results may give new thoughts in theoretical modeling of heat transfer within the moving bed of rotary kilns.

Laser Diagnostic Investigation of the Bubble Eruption Patterns in the Freeboard of Fluidized Beds. 1. Optimization of Acetone Planar Laser Induced Fluorescence Measurements

Here, the mixing of the freeboard gas with the gas of single bubbles and a continuous stream of bubbles is investigated using acetone planar laser induced fluorescence (PLIF), a technique only recently introduced to fluidized bed research [Solimene et al. Chem. Eng. Sci. 2007, 62, 94]. Various improvements with respect to laser diagnostics are presented leading to optimal signals from acetone PLIF. The results obtained are compared with models presented in the literature. The observed bubble eruption pattern generally corresponds to the models proposed by Levy and Lockwood [AIChE J. 1983, 29, 889], Yórquez-Ramirez and Duursma [Powder Technol. 2001, 116, 76], and Solimene et al. [Chem. Eng. Sci. 2007, 62, 94]. No difference was observed between the eruption patterns of a single bubble compared to a continuous stream of bubbles. In contrast to the models of Yórquez-Ramirez and Duursma and Solimene et al., five different patterns of the release of gas at a dome formed by a rising bubble are observed. An explanation for the formation of a thin layer of acetone after the eruption of a bubbles is given.

Some aspects of heat transfer in rolling mode rotating drums operated at low to medium temperatures

Rotating drums in industries are often operated at elevated temperatures. Heat transfer in such devices therefore plays a prime role. However, the controlling steps in heat transfer have not been fully clarified though a considerable amount of work has been carried out. In this paper, heat transfer in rotating drums operated in a rolling mode is assessed. The focus is on direct heating operations at low to medium temperatures, where thermal radiation is unimportant. It is shown that heat transfer from the covered wall to the particle bed is the dominant mechanism in supplying heat to the bed. Heat transfer between the freeboard gas and the exposed surface of the free cascading layer accounts for only a small portion, in contradiction to our previously held opinion. The heat transfer rate between the freeboard gas and the uncovered drum wall may be comparable to that between the covered drum wall and the particle bed indicating that both steps could be controlling. If heat is supplied to the boundaries of the particle bed rapidly, and macroscopic particle mixing is rapid, then heat transfer within the bed may be controlled by either or both of gas–solid heat transfer and heat conduction within individual particles depending upon the physical properties of both interstitial gas and particles. Although theoretical analyses are principally used in this work, comparison with experiments is also made where appropriate.

Insights into the instantaneous freeboard flow above a bubbling fluidised bed

The freeboard of a fluidised bed reactor, that is, the space above the bed material where the fluidising gas disengages from solids before leaving the fluidisation vessel, can be a place where unexpected additional reaction can take place. In this study particle image velocimetry (PIV), implemented by image shifting, has been used to provide instantaneous gas velocity vector maps of the central vertical plane of a bubbling fluidised bed freeboard in order to study the flow pattern in a cold model of a fluidised bed used for polymer recycling. The velocity vector maps obtained show that the freeboard gas flow is characterised by the presence of eddies and vortices generated by bubble eruption at the bed surface.

ON FLOW-INDUCED KINETIC DIFFUSION AND ROTARY KILN BED BURDEN HEAT TRANSPORT

The cross-section of a partially-filled cylindrical kiln rotating on its horizontal axis and processing granular solids produces a shear zone (active layer) at the free surface which grows with the kiln's rotational rate. The active layer, although relatively thin, compared with the rest of the bed burden, drives all physical/chemical reactions. This is because of the high rate of surface renewal which, in turn, promotes heat exchange between the exposed surface and the higher temperature freeboard gas. Unlike packed beds, particulate diffusion induced by the flow of granules, adds a significant component to the overall heat transfer in the bed. Problem formulation and modeling of heat conduction using flow fields derived from experiments suggest that at slow kiln speeds the diffusion effect may not be recognized due to long term duration of particle contacts and hence packed-bed heat conduction models may provide adequate characterization. However, at moderate and high kiln speeds particle collisions are short-termed and kinetic diffusion contributes to the effective thermal conductivity by as much as tenfold thereby resulting in well-mixed conditions and a homogeneous bed temperature. Industrial processing ramifications such as kiln speed control and product quality are discussed hereafter.

A thermal model for the rotary kiln including heat transfer within the bed

A mathematical model was developed to predict heat transfer from the freeboard gas to the bed of a rotary kiln. The thermal model incorporates a two-dimensional representation of the bed's transverse plane into a conventional one-dimensional, plug flow model for rotary kilns. The result, a quasi-three-dimensional rotary kiln model, significantly improves the ability to simulate conditions within the bed without the necessity of rigorously accounting for the complex flow and combustion phenomena of the freeboard. The combined model is capable of predicting the temperature distribution within the bed and the refractory wall at any axial position of the kiln.

A heat-transfer model for the rotary kiln: Part I. pilot kiln trials

A model for rotary kiln heat transfer, which accounts for the interaction of all the transport paths and processes, is presented in a three-part series. In this first paper, the pilot kiln facility is described, and the significant results from the heat-transfer trials are identified. Limestone, Ottawa sand, and pctroleum coke were heated using a range of firing rates, while other operating variables were held nearly constant. Measurements were made to obtain the net rates of heat transfer for the bed material, freeboard gas, refractory wall, and, unique to the study, the radial heat flux at the inside refractory surface as a function of circumferential position. High rates of net heat input to the bed material, occurring very near the feed end, were found to decline quickly with distance, and for an inert bed, leveled out at a value well below the rate of loss through the kiln wall. The onset of an endothermic bed reaction resulted in sharp increases in both the temperature cycling at the inside refractory surface and the net heat input to the bed, but no corresponding jump in the kiln wall heat loss. The temperatures of the bed material and inside refractory surface always were coupled closely, even in the presence of bed reaction. Regenerative heat transfer from the covered wall to the contacting bed material was not a major component of the net input to the bed, and for the inert bed, negative regeneration was en-countered beyond the kiln midpoint.

Elutriation from fluidized beds—II. Disengagement of particles from gas in the freeboard

The freeboard above a fluidized bed is the dilute phase region in which the gas and particles disengage. The freeboard container is normally cylindrical and usually of the same diameter as the bed but sometimes larger. Theory is given to describe horizontal turbulent diffusion of fine particles towards the freeboard walls. On reaching the walls, the fine particles descend as a falling film which may drag down gas and thus generate a gas circulation current in the freeboard. This theory gives predictions of (1) upward flux of fine particles and (2) particle concentration, both as functions of distance above the bed surface. These predictions are in reasonable agreement with (1) measurements, using isokinetic sampling, of upward particle flux above an 0.6 m diameter bed of polymer particles of mean diameter 760 μm, and (2) measurements of particle concentration at five levels above a 0.3 m square bed containing a mixture of 73 and 370 μm particles, using entrapment of particles between horizontal shutters in the freeboard. The theory gives a working formula to predict the transport disengaging height (TDH) in reasonable agreement with published data. The theory predicts that the TDH increases with freeboard diameter. The theory predicts, and experiments confirm, that the TDH can be reduced by inserting vertical baffles into the freeboard. The circulation of freeboard gas, generated by the fine particle motion, may explain the published observations (Geldart, D., Cullinan, J., Georghiades, S., Gilvray, D. and Pope, D.J., 1979, Trans. Inst. Chem. Engrs 57, 269) that adding fine particles increases the elutriation of coarse particles.

Radiative heat transfer in rotary kilns

Radiative heat transfer between a nongray freeboard gas and the interior surfaces of a rotary kiln has been studied by evaluating the fundamental radiative exchange integrals using numerical methods. Direct gas-to-surface exchange, reflection of the gas radiation by the kiln wall, and kiln wall-to-solids exchange have been considered. Graphical representations of the results have been developed which facilitate the determination of the gas mean beamlength and the total heat flux to the wall and to the solids. These charts can be used to account for both kiln size and solids fill ratio as well as composition and temperature of the gas. Calculations using these charts and an equimolar CO2−H2O mixture at 1110 K indicate that gas-to-surface exchange is a very localized phenomenon. Radiation to a surface element from gas more than half a kiln diameter away is quite small and, as a result, even large axial gas temperature gradients have a negligible effect on total heat flux. Results are also presented which show that the radiant energy either reflected or emitted by a surface element is limited to regions less than 0.75 kiln diameters away. The radiative exchange integrals have been used, together with a modified reflection method, to develop a model for the net heat flux to the solids and to the kiln wall from a nongray gas. This model is compared to a simple resistive network/gray-gas model and it is shown that substantial errors may be incurred by the use of the simple models.

Mathematical model of the SL/RN direct reduction process

A mathematical model has been developed to predict the operating behavior of an SL/RN direct reduction kiln from a knowledge of the main process variables. The model is based on steady state principles and is capable of quantitatively describing the different chemical reactions in the kiln such as reduction, Boudouard reaction, coal devolatilization, combustion in the freeboard together with the mass and heat flows. Results of the model, which are in the form of axial temperature profiles in the freeboard gas, solids bed and wall, and axial concentration profiles in the gas and solids, are in good agreement with measurements made on a 200 ton per day pilot plant at the Steel Company of Canada. The model has been used both to investigate the influence of process variables on the kiln performance and to elucidate operating features of the process. The model has shown that highly reactive coals such as subbituminous and lignite, and highly reducible pellets are most suitable for the SL/RN process. It has also demonstrated that the most important process variable from the standpoint of control is the air profile in the kiln; and consequently that heat transfer is the rate controlling step. The model has also been employed to examine scale-up of the SL/RN process by calculating the operating behavior of a large commercial kiln. It has shown that the large kiln should run cooler than the pilot-size unit so that accretion problems may be significantly reduced.