Char Combustion Model Research Articles

Global sensitivity analysis with limited data via sparsity-promoting D-MORPH regression: Application to char combustion

In uncertainty quantification, variance-based global sensitivity analysis quantitatively determines the effect of each input random variable on the output by partitioning the total output variance into contributions from each input. However, computing conditional expectations can be prohibitively costly when working with expensive-to-evaluate models. Surrogate models can accelerate this, yet their accuracy depends on the quality and quantity of training data, which is expensive to generate (experimentally or computationally) for complex engineering systems. Thus, methods that work with limited data are desirable. We propose a diffeomorphic modulation under observable response preserving homotopy (D-MORPH) regression to train a polynomial dimensional decomposition surrogate of the output that minimizes the number of training data. The new method first computes a sparse Lasso solution and uses it to define the cost function. A subsequent D-MORPH regression minimizes the difference between the D-MORPH and Lasso solution. The resulting D-MORPH based surrogate is more robust to input variations and more accurate with limited training data. We illustrate the accuracy and computational efficiency of the new surrogate for global sensitivity analysis using mathematical functions and an expensive-to-simulate model of char combustion. The new method is highly efficient, requiring only 15% of the training data compared to conventional regression.

Pulverized coal combustion computational modeling approach: A review

CFD (Computational fluid dynamics) is a tool, through which, the entire combustion process for pulverized coal combustion (PCC) can be predicted. The solid fuel combustion modeling is not as simple as gaseous or liquid fuel combustion modeling. In the solid fuel combustion process, various physical transformations and chemical reactions occur simultaneously. Therefore in the computational modeling approach for PCC, basic physical and chemical processes such as; turbulence, radiation, devolatilization, gas-phase combustion, char reaction and soot formation/reduction are integrated to obtain the desired results. However, in order to reach the desired accuracy, different computational models should be chosen carefully. In this review, various common models that are widely used in PCC simulation are discussed scientifically along with their modeling aspects. The basic advantages and disadvantages of these models are also discussed in this study. Moreover, different models preferred by various authors for pulverized coal combustion simulation process are also summarized. From the review, it has been observed that the eddy viscosity based two-equation turbulent models and Eulerian-Lagrangian multiphase approach are mostly preferred due to better computational economy. Two competing rate devolatilization model from various finite rate models and FG-DVC devolatilization model from various phenomenological models are mostly used for PCC. Diffusion kinetic control and Intrinsic char combustion models are mostly chosen to predict the char reaction. Finite reaction rate models are preferred when the computational economy is a concern, whereas phenomenological models provide results with better accuracy because in phenomenological models realistic inputs are given as input for simulation whereas, in finite rate models, finite rate constants are mostly empirical. Most of the gas phase models are given almost equal importance. Discrete ordinate (DO) is mostly used for PCC due to its ability of volumetric radiation calculation and the ability to evaluate radiation effect on the particle phase.

Experimental study and modelling of pressurized oxy-coal combustion: Effect of devolatilization conditions on the physical structure and combustion characteristics of char particles

Pressurized oxy-coal combustion has been regarded as a promising carbon capture and storage (CCS) technology, and the combustion characteristics of char particles under pressurized oxy-fuel combustion conditions have been extensively investigated. However, most previous studies used the char samples prepared under the atmospheric N2 condition rather than under the pressurized CO2 condition, which may lead to the misunderstanding of the combustion characteristics. In this study, the effects of devolatilization atmosphere and pressure on the physical structure of char particles were experimentally investigated. The char particles produced under different devolatilization conditions were then used for the oxy-char combustion experiments conducted in a fluidized bed with the operating pressure between 0.1 and 0.5 MPa. In view of the limitations of the experiment, an experimental verified particle-scale char combustion model was further used for predicting the char particle’s combustion reactivity and temperature, and the influence of physical structure on the combustion characteristics was analyzed. The results obtained from the experiments demonstrated that higher devolatilization pressure led to larger mean pore sizes and specific surface areas of char particles. The char particles obtained with CO2 atmosphere had larger specific surface areas but smaller mean pore sizes than those obtained with N2 atmosphere. Besides, increasing devolatilization pressure resulted in shorter burnout times of char particles. The simulation results showed that using the char samples prepared under the atmospheric N2 condition for the pressurized oxy-fuel combustion experiments led to the underestimations of char combustion reactivity and peak temperature.

Pressurized oxy-fuel combustion of a char particle in the fluidized bed combustor

Pressurized oxy-fuel combustion of coal in fluidized bed (FB) holds the potential to realize low-cost CO2 capture. However, the fundamental study in this manner is still rare due to the difficult access to the pressurized oxy-FB combustion tests. In this work, the experimental study of single char combustion was firstly conducted in a visualized pressurized FB combustor under various operating conditions. Then an experimentally verified particle-scale char combustion model was developed to reveal the dependence of char combustion on parameters. Results showed that the char conversion was accelerated with the increase of pressure, mainly due to the high oxygen diffusion and char gasification. The gasification played a non-negligible role in pressurized oxy-fuel combustion, especially under high oxygen concentration and bed temperature. Increasing oxygen concentration and bed temperature not only promotes the char oxidation rate and particle temperature, but also increases the gasification rate and the share of char conversion via gasification, resulting in shortening the burnout time of char. In addition, a higher fluidization number lowered both the burnout time and peak temperature of char particle, due to the simultaneous improvement of mass and heat transfer. The influences of char size and fluidization number on char gasification conversion ratio are very weak. In addition, the quantitative analysis of the influence of different operating parameters on the combustion process was obtained by model sensitivity analysis.

Effect of Swirl on Gasification Characteristics in an Entrained-flow Coal Gasifier

AbstractThe three-dimensional (3-D) comprehensive mathematical model was developed to simulate the coal gasification process in an entrained flow gasifier with a swirl burner. The models employed or developed includes the coal devolatilization model, the char combustion and gasification model, the gas homogeneous reaction model, the random-trajectory model, gas turbulence model, and the P-1 radiation model. The solution of models was executed based on the computational fluid dynamics (CFD). By qualitatively comparing the results at different swirl number, the significant influences of swirl on characteristics of coal gasification such as flow distributions, gas temperature and product composition including hydrogen (H2), carbon monoxide (CO), etc., and on the performance of coal gasification such as averaged exit product composition, carbon conversion rate and cold gas efficiency, were in detail discussed. Especially, a proper swirl number (S ≤ 0.65) in favor of gasification was found for the investigated gasifier in this paper.

Application of Bound-to-Bound Data Collaboration approach for development and uncertainty quantification of a reduced char combustion model

The development of efficient industrial oxy-coal boilers can be significantly aided by Computational Fluid Dynamics (CFD) tools, as far as fidelity in modeling coal combustion is also complemented by feasible computational costs. Reduced and predictive models are the most suitable for this application scale. Reduced models feature predictivity when they are validated against a broad range of experiments and targeted by Uncertainty Quantification (UQ) procedures. This work proposes a numerical procedure that uses Bound-to-Bound Data Collaboration (B2B-DC) to derive a reduced char combustion model describing transport phenomena and reactions between char carbon and O2, CO2 and H2O, in both conventional and oxy-conditions. The approach determines the consistency between a numerical model and an experimental dataset. The latter is made up of the experiments carried out in an optically accessible laminar entrained flow reactor, operated by Sandia National Laboratories. The procedure follows five steps towards predictive modeling capability, namely: quantification of the uncertainty in the experiments, via instrument verification and modeling; development of a physics model and continuous improvement of its fidelity, via model-form uncertainty; identification of the uncertain and most sensitive parameters and of their prior bounds; sampling of the initial uncertain parameter space and training of a surrogate model; validation of the physics model via inference from the data. The last step, also known as inverse problem, is performed by applying the Bound-to-Bound Data Collaboration approach. A char combustion model is found consistent with the experimental data and its validity stands for conventional and oxy-combustion conditions. It accounts for heterogeneous reactions at the particle surface, mass transport of species in the particle boundary layer, pore diffusion and surface area changes. The consistent reduced model overcomes the differences in mass transport and kinetics observed in the experimental campaign. A reduction of the initial degree of uncertainty in both model and experiments is achieved.

Effects of CO2 enriched atmosphere on chars from walnut shells pyrolysis in a drop tube reactor

A laminar drop tube reactor (DTR) was used to perform fast pyrolysis of walnut shells, a ligno-cellulosic biomass sample, in nitrogen and carbon dioxide atmospheres. The DTR reached the temperature of 1300 °C and the heating rate of 104–105 °C/s. Char samples collected at different residence times along the reactor were characterized by ultimate and proximate analysis and by SEM. Char combustion reactivity was then measured by non-isothermal thermogravimetric analysis (TGA) in air.The analyses show that at residence times of 66 ms pyrolysis in N2 is not complete, whereas it is complete in CO2. For residence times of 115 ms the differences between samples produced in N2 and CO2 atmospheres level off.The derivative thermogravimetric (DTG) curves of the char combustion show the existence of multiple peaks. Notably, early combustion peaks progressively fade in the chars collected at increasing reactor residence time, confirming the completion of pyrolysis. A kinetic model of char combustion is proposed which includes multiple parallel reactions.

Char particle burning behavior: Experimental investigation of char structure evolution during pulverized fuel conversion

The present work provides experimental results on structural changes of burning char particles derived in an entrained flow reactor. Changes in the char particle diameter, apparent and true density, specific surface area, and porosity are studied at different residence times using a bituminous coal with a moderate ash content of 11.8wt%. Results show the significance of mineral matter on structural properties of samples extracted along the combustion. The apparent density of a sample composed of mineral and organic matter increases progressively with proceeding char conversion, whereas the isolated char density decreases. The mean particle diameter decreases during conversion, indicating regime II conditions, with pore diffusion being the rate-controlling step. The decrease in char density and diameter follow well-known power law expressions, where the exponents are shown to be dependent on the effectiveness factor, and, thus the reaction regime. However, an ideal burning behavior in regime III, where the diffusion of oxidant through the boundary layer is rate-controlling, is not observed. Instead, large openings in the char structure enable the penetration of oxidant into the particle interior, in particular for highly porous cenospherical char particles. Hence, it is recommended to consider large openings and voids in the char structure and their changes, as well as mineral matter effects in advanced char combustion models.

Collaboration of simulations and experiments for development and uncertainty quantification of a reduced char combustion model

Computational fluid dynamics (CFD) plays a decisive role in the development of cost-effective oxy-coal combustion technologies to improve process efficiency and to decrease pollutant emissions. The implementation of detailed physical models describing coal devolatilization, char oxidation, gas-phase reactions and pollutant formation ensures accurate CFD simulations of coal combustion but is still challenging for large-scale combustors due to the significant computational efforts required, especially in the framework of Large Eddy Simulation (LES). The development of reduced physics model with quantified model-form uncertainty is needed to overcome the challenges of performing LES of industrial coal-fired boilers. Reduced models must reproduce the main features of the detailed models and the capability of bridging scales and being predictive. A tight coupling of simulation and experiments is necessary to ensure predictivity with uncertainty quantification for a reduced model. This work proposes a combined experimental/numerical methodology that uses global sensitivity analysis to rank fundamental input parameters of a reduced char oxidation and gasification model describing reactions between char carbon and O2, CO2 and H2O reagents in both air and oxy-coal conditions. A careful evaluation of uncertainty in the data, in the model form and in the model parameters is performed. The reference dataset, consisting of the experiments carried out in a laminar entrained flow reactor operated by Sandia National Laboratories, has been exploited. The methodology is based on the use of so-called instrument models to include all the physical sub-models and the sources of uncertainty considered in the experiments and in the numerical simulations and affecting the main quantities of interest, e.g. reaction rates. The quantified uncertainty in the instrument models provides the range of uncertainty for the reduced char combustion model. Then, the reduced model with quantified uncertainty will be validated against the experimental data. The reduced model capability to address heterogeneous reaction at the particle surface, mass transport of species in particle boundary layer, pore diffusion and internal surface area changes will be assessed.

The role of gasification reactions during pulverized solid fuel combustion: A detailed char combustion model based on measurements of char structure and kinetics for coal and pre-treated biomass

A detailed char combustion model including gasification reactions with emphasis on char structural evolution is developed and validated using combustion experiments from a lab-scale entrained flow reactor burning a bituminous coal and torriefied wood. The char combustion model is a single film model based on nth order Arrhenius type equations for char reactions. Diffusion through the particle boundary layer and inside the particle interior is considered by an effectiveness factor approach. Char reactivity towards O2, H2O and CO2 is measured for five different char samples from four different parent fuels, being two bituminous coals and two pre-treated biomasses. Char samples are generated at high heating rates in an entrained flow reactor. Reaction rates and intrinsic reactivity values are quantified using a thermo-gravimetric analyzer. Reaction rates for pre-treated biomass samples are one to two orders of magnitude higher compared with the coal samples. The oxygen reaction is around four to five orders of magnitude faster than gasification reactions in the chemically-controlled reaction regime. The char combustion model is able to accurately describe char conversion and char structural changes. Model results reveal that there might be fragmentation effects in early stages of char conversion, presumably for cenospherical char particles. Particle reaction rates and reaction regimes are shown to vary strongly among different particle sizes and trajectories, and even for a single particle along its profile. Therefore, mean values calculated from CFD results have to be interpreted with care and can be misleading. A parameter study on temperature and stoichiometry shows the importance of gasification reactions. Depending on the conditions, gasification reactions are responsible for 2 up to 60 wt.-% of char conversion. They become relevant in regions of high temperatures and oxygen depletion, e.g. inside the flame. For typical furnace conditions of 1300 °C and a slightly over-stoichiometric flame, the Boudouard reaction accounted for approximately 10 wt.-% and the heterogeneous water-gas shift reaction for 4 wt.-% of char conversion. Therefore, gasification reactions cannot be neglected when predicting fuel conversion in PF systems, particularly in modern staged combustion systems with reducing furnace conditions.

The effect of model fidelity on prediction of char burnout for single-particle coal combustion

Practical simulation of industrial-scale coal combustion relies on the ability to accurately capture the dynamics of coal subprocesses while also ensuring the computational cost remains reasonable. The majority of the residence time occurs post-devolatilization, so it is of great importance that a balance between the computational efficiency and accuracy of char combustion models is carefully considered. In this work, we consider the importance of model fidelity during char combustion by comparing combinations of simple and complex gas and particle-phase chemistry models. Detailed kinetics based on the GRI 3.0 mechanism and infinitely-fast chemistry are considered in the gas-phase. The Char Conversion Kinetics model and nth-Order Langmuir–Hinshelwood model are considered for char consumption. For devolatilization, the Chemical Percolation and Devolatilization and Kobayashi-Sarofim models are employed. The relative importance of gasification versus oxidation reactions in air and oxyfuel environments is also examined for various coal types. Results are compared to previously published experimental data collected under laminar, single-particle conditions. Calculated particle temperature histories are strongly dependent on the choice of gas phase and char chemistry models, but only weakly dependent on the chosen devolatilization model. Particle mass calculations were found to be very sensitive to the choice of devolatilization model, but only somewhat sensitive to the choice of gas chemistry and char chemistry models. High-fidelity models for devolatilization generally resulted in particle temperature and mass calculations that were closer to experimentally observed values.

A comprehensive fractal char combustion model

The char combustion mechanisms were analyzed and a comprehensive fractal char combustion model was developed to give a better understanding and better predictions of the char combustion characteristics. Most of the complex factors affecting the char combustion were included, such as the coupling effects between the pore diffusion and the chemical reactions, the evolution of the char pore structures and the variation of the apparent reaction order during combustion, the CO/CO2 ratio in the combustion products and the correction for oxy-char combustion. Eleven different chars were then combusted in two drop tube furnaces with the conversions of the partly burned char samples measured by thermogravimetric analysis. The combustion processes of these chars were simulated with the predicted char conversions matching very well with the measured data which shows that this char combustion model has good accuracy. The apparent reaction order of the char combustion decreases, stabilizes and then increases during the combustion process. The combustion rates in the oxy-mode are generally slower than in the air-mode and the effect of the char-CO2 gasification reaction becomes obvious only when the temperature is relatively high and the O2 concentration is relatively low.

Effect of CO2 on oxy-fuel combustion of coal-char particles in a fluidized bed: Modeling and comparison with the conventional mode of combustion

A char combustion model is developed to study the effect of CO2 on the combustion of coarse char particles under oxy-fuel conditions in a fluidized bed (FB). It is a transient one-dimensional model, taking into account the heat and mass transfer from the bed to the particle and the heterogeneous combustion and gasification of char. The model shows good ability to predict the char temperature history measured in our previous work for different combinations of O2/CO2 and O2/N2 with various coal types. Simulations are carried out to establish the role of CO2 in oxy-fuel conversion at different O2 levels, particle sizes, and bed temperatures. The model is used to analyze the relative contribution of carbon in the char consumed by CO2 (gasification) and O2 (combustion), as well as the differences of the peak temperatures and the burnout times in O2/CO2 and O2/N2 for char particles in a commercial FB combustor. The results indicate that the conversion of coarse (mm size) char particles in an oxy-FB is controlled by the diffusion of O2 both in the O2/CO2 and O2/N2 case. The burn-out time decreases with the bed temperature also in both cases. The lower O2 diffusion rate in CO2 compared to N2, is the main reason for the longer burnout time and lower peak temperature found using O2/CO2 at bed temperatures of 1073–1173K. In that temperature window, the contribution of the CO2-char gasification is limited, being notable only at high bed temperature in O2/CO2, e.g. 1223K. In such high temperature conditions (rarely expected to be found in commercial coal FBC) the predicted burnout time of a lignite char-particle becomes shorter in O2/CO2 than in O2/N2.

Influence of temperature on oxygen permeation through ion transport membrane to feed a biomass gasifier

Oxygen-permeable perovskite membranes with mixed ionic-electronic conducting properties can play an important role in the high temperature separation of oxygen from air. A detailed design of a membrane test module is presented, useful to test mechanical resistance and structural stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) capillary membrane in the reactor environment. Preliminary experimental results of membrane permeation tests highlight the positive effect of temperature on perovskite materials. This behaviour is also confirmed by a computational model of char combustion with oxygen permeated through the membrane module, when it is placed inside a gasifier reactor to provide the necessary input of heat to the gasification endothermic process. The results show that the temperature affects the oxygen permeation of the BSCF membrane remarkably.

Variation of Apparent Reaction Order in Char Combustion and Its Effect on a Fractal Char Combustion Model

The variation of the apparent reaction order, , during char combustion is studied by simulating the combustion process inside the char particle pores. An expression for is obtained as a function of the particle temperature, the O2 partial pressure, and the pore structural parameters. A modulus, , is shown to represent the pore diffusion resistance, with a sufficient condition for given based on . becomes lower as the particle temperature and the O2 partial pressure increase and the pore structure has higher diffusion resistance. The expression for is used to improve the accuracy of a previous fractal char combustion model. The effect of on the char combustion rate and the regularity of how changes in the char combustion process are also studied.

On the carbon monoxide formation in oxy-fuel combustion—Contribution by homogenous and heterogeneous reactions

This work investigates CO formation mechanisms under oxy-fuel combustion conditions. The importance of the possible explanations for increased inflame CO concentrations in oxy-fuel flames compared to air-firing are discussed. A model based on a detailed gas-phase reaction mechanism is combined with a lignite char combustion model, including apparent surface kinetics for oxidation as well as carbon dioxide and steam gasification and implication of diffusion limitation.In agreement with other authors work, it is concluded that in gas-fired oxy-fuel flames the CO formation is promoted by a homogenous reaction between hydrogen radicals and CO2. Additionally, this work concludes that in lignite-fired oxy-fuel combustion, this gaseous reaction route is of less importance. In oxy-lignite flames, CO2 gasification is the largest contributor to the increased CO formation compared to air firing. The substitution of CO2 with steam in the oxidizer during wet oxy-fuel combustion has moderate influence on the CO2 gasification, whereas the homogenous CO formation is strongly reduced.

Analysis and modeling of char particle combustion with heat and multicomponent mass transfer

A char combustion model suitable for a large-scale boiler/gasifier simulation, which considers the variation of physical quantities in the radial direction of char particles, is developed and examined. The structural evolution within particles is formulated using the basic concept of the random pore model while simultaneously considering particle shrinkage. To reduce the computational cost, a new approximate analytical boundary condition is applied to the particle surface, which is approximately derived from the Stefan–Maxwell equations. The boundary condition showed reasonably good agreement with direct numerical integration with a fine grid resolution by the finite difference method under arbitrary conditions. The model was applied to combustion in a drop tube furnace and showed qualitatively good agreement with experiments, including for the burnout behavior in the late stages. It is revealed that the profiles of the oxygen mole fraction, conversion, and combustion rate have considerably different characteristics in small and large particles. This means that a model that considers one total conversion for each particle is insufficient to describe the state of particles. Since our char combustion model requires only one fitting parameter, which is determined from information on the internal geometry of char particles, it is useful for performing numerical simulations.

Combustion Rate for Char with Fractal Pore Characteristics

A char combustion model was derived by assuming constant intrinsic activation energy for the different chars. The char combustion rates are affected by various diffusion processes within the different char pores. The fractal pore diffusion effects were characterized by three pore structure parameters (porosity, specific surface area, and fractal dimension) and the mean particle diameter. Three-dimensional numerical results for various char particles burned in drop tube furnaces obtained using the model compare well with experimental results to verify the model. The comparisons show that the model has good accuracy for a wide range of char, from lignite to anthracite.

Oxy-coal combustion in an entrained flow reactor: Application of specific char and volatile combustion and radiation models for oxy-firing conditions

The deployment of oxy-fuel combustion in utility boilers is one of the major options for CO2 capture. However, combustion under oxy-firing conditions differs from conventional air-firing combustion, e.g., in the aspect of radiative heat transfer, coal conversion and pollutants formation. In this work, a numerical study on pulverised coal combustion was conducted to verify the applicability and accuracy of several sub-models refined for oxy-fuel conditions, e.g., gaseous radiative property model, gas-phase combustion mechanism and heterogeneous char reaction model. The sub-models were implemented in CFD (Computational Fluid Dynamics) simulations of combustion of three coals under air-firing and various oxy-firing (21–35% vol O2 in O2/CO2 mixture) conditions in an EFR (entrained flow reactor). The predicted coal burnouts and gaseous emissions were compared against experimental results. A good agreement between the simulations and experiments was achieved, indicating a good applicability and reliability of the refined sub-models and suitability of use of the experimentally derived kinetic data in coal devolatilisation and char oxidation sub-models. The sub-models and the practices implemented in this work can be used in large-scale oxy-fuel combustion processes for reliable design and optimization.

Analysis of the errors associated with typical pulverized coal char combustion modeling assumptions for oxy-fuel combustion

In CFD models of pulverized coal combustion, which often have complex, turbulent flows with millions of coal particles reacting, the char combustion sub-model needs to be computationally efficient. There are several common assumptions that are made in char combustion models that allow for a compact, computationally efficient model. In this work, oft used single- and double-film simplified models are described, and the temperature and carbon combustion rates predicted from these models are compared against a more accurate continuous-film model. Both the single- and double-film models include a description of the heterogeneous reactions of carbon with O2, CO2, and H2O, along with a Thiele based description of reactant penetration. As compared to the continuous-film model, the double-film model predicts higher temperatures and carbon consumption rates, while the single-film model gives more accurate results. A single-film model is therefore preferred to a double-film model for a simplified, yet fairly accurate description of char combustion. For particles from 65 to 135μm, in O2 concentrations ranging from 12 to 60vol.%, with either CO2 or N2 as a diluent, particle temperatures from the single-film model are expected to be accurate within 270K, and carbon consumption rate predictions should be within 16%, with greater accuracies for a CO2 diluent and at lower bulk oxygen concentrations. A single-film model that accounts for reactant penetration and both oxidation and gasification reactions is suggested as a computationally efficient sub-model for coal char combustion that is reasonably accurate over a wide range of gas environments.