Char Conversion Research Articles

A Predictive Physico-chemical Model of Biochar Oxidation

Pyrolysis of solid fuels forms a solid carbon-rich fuel, also called char, whose physico-chemical description is rather complex. Heterogeneous oxidation reactions take place during thermochemical conversion of char. The present work proposes a predictive detailed kinetic model, opening a new path for a deeper understanding of the char conversion process. This model considers porosity, surface area, density of surface sites, and their evolution along the conversion process. The chemical aspects of char oxidation are modeled assuming a carbonaceous bulk structure, surrounded by a variety of surface sites which represent the chemical functionalities typically present in such materials. The heterogeneous chemical reactions and their kinetic parameters are defined based on previous studies in the literature and by analogy to homogeneous gas-phase reactions of aromatic species. A mathematical framework is proposed to couple physical and chemical descriptions of the oxidation process. Although the proposed model benefits from experimental information, it is able to comprehensively describe the conversion rate of a broad range of carbonaceous materials such as carbon nanotubes, graphite, and chars only on the basis of their elemental composition. The proposed model represents a first step in exploring the explicit and coupled treatment given to the physical and chemical evolution of the fuel throughout its conversion, allowing us to consistently describe the particle evolution, opening a path for reliable models to manage the chemistry of char conversion.

Experimental Analysis of Intrinsic Kinetics and Rate Profiles for Coal Char Conversion under Different Atmospheres

ABSTRACT This study focuses on determining intrinsic kinetic parameters and interpreting and simulating the rate profile. First, reactivity of coal char prepared at different temperatures was analyzed. Results showed that when coal pyrolysis temperature increases, coal char reactivity decreases; however, activation energy for coal char conversion does not always increase. Next, the weight loss process in non-isothermal conversion was discussed, and results illustrated that the weight loss mechanisms differ in each stage. The weight loss at the second stage was slightly influenced by residual volatile matter and gas diffusion, and the activation energy at this stage was close to an intrinsic value. Meanwhile, discussions on the effects of kinetic analysis methods in determining activation energy showed that Friedman and advanced isoconversional methods can obtain a true activation energy profile, whereas the Kissinger–Akahira–Sunose method was unsuitable. The rate profiles of isothermal conversion were then studied. The results demonstrated that occurrence of a maximum was an inherent feature but the “gas change” operation could shift or even cover up the true position of the maximum. Moreover, an increase in particle temperature could intensify the maximum value but slightly shift its position. Finally, an intrinsic reaction model that combines random pore model and the derived intrinsic kinetic parameters properly reproduced the rate profile of isothermal conversion (e.g., maximal conversion deviation was less than 5%).

Gasification Kinetics of Victorian Brown Coal-Derived Char in Fluidized Bed Reactor

Abstract In a chemical looping combustion (CLC) system, gasification kinetics of char holds immense importance being the rate-limiting reaction in the fuel reactor. This paper studied the gasification kinetics of char derived from Victorian brown coal (VBC) in a fluidized bed reactor which mimics the fuel reactor conditions of a chemical looping combustion process. Mass of char, char particle size, and gas flow conditions were optimized to ensure the gasification reaction free from mass transfer limitations. Effect of oxygen carrier (OC), hematite, being the bed material was also studied. The experiments were conducted in the temperature range of 800 °C–950 °C, which is a typical range for fuel reactor. The experimental results were modeled with the help of grain model (GM) and random pore model (RPM) to analyze the kinetic parameters. Activation energy was found to be around 177 kJ/mol in sand bed and 175.5 kJ/mol in the hematite bed. Reaction in hematite bed was found to be 42% faster on average compared with the reaction in a sand bed. Fastest total conversion of char took as low as 4.1 min in hematite bed at 950 °C. While catalytic effect of hematite was ruled out due to insignificant change in activation energy, it was concluded that increase in CO2 partial pressure at the vicinity of char particle enhanced the reaction rate in the case of hematite bed. This study has generated relevant information for the CLC of Victorian brown coal with hematite as the oxygen carrier.

Experimental investigations on novel electronic waste based oxygen carriers for direct coal fuelled chemical looping combustion process

Electronic waste is considered as a serious global threat to the environment due to the rapid technology development and population growth. Hence, these wastes have to be used in a best way to resolve soil and air pollution issues. The printed circuit board (PCB) found in computers contains mainly metals and polymers. Copper, iron and nickel are employed in PCB boards and these components could be used as potential oxygen carriers in the chemical looping combustion (CLC) process. The present study experimentally evaluated the feasibility of utilizing PCB based oxygen carriers in the CLC process. In the first step, a series of experiments such as pyrolysis, gasification and combustion process have been performed in a fixed bed reactor using a PCB board. During the pyrolysis under N2 atmosphere, the obtained syngas possesses a calorific value of 6 and 9 MJ/m3 at 500 °C and 700 °C, respectively. The pyrolyzed residue is then gasified to convert the polymers into syngas under CO2 atmosphere and the obtained syngas is estimated with an average calorific value of 6.3 MJ/m3. In the second step, the oxidized PCB metals under air atmosphere are used to combust the coal in the CLC process with various mass ratios (fuel:metal oxides) of 1:10, 1:20, 1:30 and 1:60. In the mass ratio of 1:60, a higher char conversion of 88.2% is achieved with more than 90% gas conversion and CO2 yield. Kinetic studies are performed using a thermogravimetric analyzer (TGA) under N2 and CO2 atmosphere with PCB based oxygen carriers by employing different kinetic models.

Microwave assisted steam - CO2 char gasification of oil palm shell

CO2 gasification of biomass is a promising candidate for the reduction of carbon footprint with the added benefit of improving syngas quality by eliminating N2 dilution. Further enhancement of syngas quality is proposed in this study by incorporating the water shift reaction to increase H2 concentration. Oil palm shell (OPS) as one of the major agriculture waste sources in Malaysia was investigated. Microwave assisted steam-CO2 gasification was characterized in the temperature range of 700–850 °C and steam/CO2 ratio in the range of 0.014–0.043. Surface composition and morphology of char were examined at different temperatures using scanning electron microscope and energy dispersive spectrometry (SEM/EDS). Optimum operation was achieved at steam/CO2 ratio of 0.028 and 850 °C with 92% and 85% CO2 and char conversions, respectively. Increasing steam flow at maximum temperature increased the H2/CO ratio by 22% and syngas heating value by 6%. Statement of noveltyThe effect of steam addition on Boudouard reaction and active sites on oil palm shell char micro-pore-structure, absorption of microwave irradiation and CO2-syngas conversion were characterized.

Evaluation of red mud as oxygen carrier for chemical looping combustion of methane and biomass in fluidized bed

The reaction performance of red mud, which generally contains high amount of Na, as an oxygen carrier for Chemical Looping Combustion (CLC) was evaluated using methane and biomass char as fuel respectively. Effects of reaction temperature and cycles on fuel conversion, particle agglomeration and Na distribution were investigated. Fluidized Bed Thermogravimetric test suggested a stable reactivity during successive redox cycles of the red mud with an oxygen transport capacity of 1.65%. The red mud demonstrates a low reactivity with methane, and a serious agglomeration was found during the fluidized bed experiments with methane due to the further reduction of Fe2O3 to its low state of FeO. It has enough reactivity to oxidize the char gasification productions, thus accelerating the char conversion. The average values of the instantaneous rate increase by 2.82 times at 1143 K, 3.88 times at 1163 K, 3.58 times at 1183 K and 2.0 times at 1203 K in comparison with inert gasification process. Almost no agglomerates formation was found for the CLC investigation using biomass char as fuel. Irrespective of the CLC process of methane or the biomass char, Na exists in a stable phase of NaAlSiO4 with a high melting point and has no influence on particle agglomeration.

Synergetic effects of cement bonded copper ore and red mud as oxygen carrier during in-situ gasification chemical looping combustion of coal char

Oxygen carrier (OC), which should be both highly-reactive and thermal-resistant, plays an important role in chemical looping combustion (CLC). In this work, fine copper ore bonded with red mud by cement is used as OC for in-situ gasification chemical looping combustion (iG-CLC) of coal char in a batch fluidized bed reactor. 8.7 wt% of copper ore is designed to mix with 71.3 wt% of red mud and the weight percent of cement added into the OC is 20 wt% (denoted as Cu10.9Red89.1@C). The results indicate that this OC exhibits better combustion performance than those of single material OCs, due to its excellent reactivity towards char gasification products. Subsequently, synergetic effect between red mud and copper ore is observed and then extensively evaluated. It is found that the designed OC shows a higher synergetic level in comparison to Cu20Fe80@C (16 wt% of copper ore bonded with 64 wt% of iron ore by 20 wt% of cement) during the iG-CLC reaction with both anthracite and lignite chars. Due to the synergetic effect, the average instantaneous carbon conversion rate of both chars is increased by ca. 6%. Both alkali metal salts and copper ore (capable of oxygen uncoupling) contribute to the promotion of char gasification and oxygen transfer rate. Furthermore, the Cu10.9Red89.1@C OC maintains good and stable combustion performance during 16 redox cycles with anthracite char as fuel, regardless of the gradual loss of sodium contents. Overall, the synergetic effect of Cu10.9Red89.1@C on char conversion is attained and quantified by comparative analysis, and the proposed Cu10.9Red89.1@C appears as a competitive OC for industrial implementation of iG-CLC process.

Activation of rice hull char with steam to improve lithium storage performance of SiO2/C

Rice hull-derived SiO2/C composites are promising anode materials for Li-ion battery, but it is a challenge to improve their lithium storage performance by a simple method. In this research, SiO2/C composites were prepared by heating rice hull char in steam flow for different time to study effects of steam activation on lithium storage performance of SiO2/C. As the activation time increased, the composites’ reversible capacity rose, because of the increase in silica content and micropore volume and the decrease in graphitization degree. Besides, when the activation time rose, the composites’ predominant electrochemical impedance (Rct &Zw) decreased whereas the initial coulombic efficiency firstly decreased and then increased. By activation of rice hull char with steam for 2 h, the reversible capacity of SiO2/C at 0.2 A/g increased by 170 %–691 mAh/g. This value exceeded the corresponding values for most reported SiO2/C composites. The research revealed that steam activation was an effective strategy for improving lithium storage performance of SiO2/C and value-added conversion of rice hull char.

A novel developed weighted exponential sum model for char cloud conversion behavior under air- and oxy-combustion: Experiments and kinetics modeling analysis

Comprehension of the fossil fuel chemical reaction conversion behavior benefits to insight micro physico-chemistry mechanism of burning chemical kinetics especially for conventional air-fired combustion and oxyfuel combustion. Detailed combustion reaction experiments and a novel developed kinetics model were proposed to investigate coal char conversion behavior under enriched-oxygen and high carbon dioxide dilution environment compared with air environment in present work. Primary characteristic parameters, i.e., combustion conversion rate, conversion ratio, burnout time, etc., mainly affected by limited chemical interactions between char oxidation reaction and gasification reaction, were studied under typical environments, i.e., O2/CO2, O2/Ar, CO2/Ar, O2/N2, using detailed experiments and a novel developed kinetics model, i.e., the weighted exponential sum model (WESM). Present kinetics predictions were also compared with previous typical kinetics models, i.e., the homogeneous reaction model (HRM), the shrinking core model (SCM), the modified volumetric reaction model (MVRM). Results showed that present kinetics model gave relatively more coherent predictions of the char conversion rate, conversion ratio, the maximum conversion rate and burnout time compared with those predicted by HRM, SCM and MVRM models, respectively. Meanwhile, the contribution ratio from endothermal gasification chemical reaction, i.e., C+CO2→2CO, on char maximum conversion rate was 41.1% and 45.2% in 1473 K and 1663 K, respectively. Correspondingly, the maximum and average interaction contribution ratios (χinteramax,χinteraave) decreased as 9.0% and 1.7%. The maximum rate ratio for oxyfuel combustion increased from 52.7% to 61.7%, and the average rate ratio slightly increased from 52.5% to 54.2% in these combustion reactions.

Fluid dynamic analysis of dual fluidized bed gasifier for solar applications

A hydrodynamic model of a dual fluidized bed gasifier (DFBG) is developed and its predictions are compared with measurements of solids flux and pressure profiles from a cold flow model (CFM). Then, the performance of a DFBG gasifier is theoretically analyzed in terms of solids circulation and solids distribution under changes in riser and loop seal aeration, solids inventory and particle size, and a sensitivity analysis is made to delimit the model prediction capability. Furthermore, the model is applied to analyze the effects of key design aspects of DFBG, such as the relative size of riser and gasifier, the connection between both units, the circulation rate of solids and their distribution around the system. The model is further used to extend the DFBG operation with external solar energy carried by heated solid particles, i.e. to design solar DFBG (SDFBG). The analysis is focused on the performance with high solids inventory in the gasifier to increase the char conversion (operation with a large solar share) and the control of solids circulation to meet the heat demand of the gasifier with the availability of solar energy. The operation with large solids inventory in the gasifier requires the size of the gasifier to increase considerably compared to that of the conventional DFBG. The substitution of the connection pipe between the riser and the bubbling bed (current design in commercial DFBG) by a lower loop seal enables better control of the solids circulation, thus, benefiting the solar design.

Influence of gas and solid phase thermo-physical and transport properties on the thermo-chemical conversion of char in reacting media: intra-particle, microscopic and temporal mass loss-based sensitivity analysis

The article addresses the implications of adopting (a) non-kinetic theory-based correlations for the estimation of thermodynamic and transport properties and (b) over-simplifying assumptions, as reported in literature, in the numerical simulation of thermo-chemical conversion of char. A one-dimensional char thermo-chemical conversion model with discretisation to capture intra-particle gradients of temperature and species is developed. Species and mixture property estimations are based on kinetic theory approaches, and all extraneous assumptions are eliminated. Simulation results indicate char conversion in air and to be near diffusion and near kinetic limit, respectively, while char conversion in has a competitive influence of both diffusion and kinetics . Literature-specific correlations yield an order of magnitude deviation (up to 800%) in the properties (thermal conductivity, specific heat, mass diffusivity). Adopting mole/mass fraction weighted averaging for mixture transport properties results in variations to the extent of 85%. Unity Lewis number assumption introduces one order magnitude deviation in presence of . It is observed that the deviations introduced in conversion process can be surmounted by using correction factor in the kinetic rate parameters, a prevalent practice. Such an approach is however thermodynamically untenable.

Modelling fuel flexibility in fixed-bed biomass conversion with a low primary air ratio in an updraft configuration

Fixed-bed biomass conversion with a low primary air ratio and a counter-current configuration has a high feedstock flexibility, as it resembles updraft gasification, and the potential to reduce emissions when integrated in biomass combustion systems. A 1D bed model was validated with experimental results from a biomass combustion boiler with such a bed conversion system, predicting with a good accuracy the temperatures in the reactor and producer gas composition. The model was applied for different cases to investigate the fuel flexibility of this combustion system, including the influence of moisture content and the maximum temperatures achieved in the bed. It was shown that with variations in fuel moisture content from 8 to 30% mass w.b. the producer gas composition, char reduction to CO or maximum temperatures at the grate were not affected due to the separation of the char conversion and pyrolysis/drying zones. Flue gas recirculation was the only possible measure with the tested configuration to reduce the maximum temperatures close to the grate, which is beneficial e.g. to avoid slagging with complicated fuels. A higher tar content was obtained than in conventional updraft gasifiers, which is attributed to the absence of tar condensation in the bed due to the limited height of the reactor and the integration in the combustion chamber. The presented model can support the development of such combustion technologies and is a relevant basis for detailed CFD simulations of the bed or gas phase conversion.

Entrained-Flow Coal Gasification Process Simulation with the Emphasis on Empirical Char Conversion Models Optimization Procedure

Computational fluid dynamics (CFD) modeling of an entrained-flow reactor is demonstrated and compared with experimental data. The study is focused on char conversion modeling and its impact on gasification simulation results. An innovative procedure of optimizing input data to empirical char conversion kinetic-diffusion model is investigated, based on the complex carbon burnout kinetic model for oxidation (CBK/E) and gasification (CBK/G). The kinetics of the CBK/G model is determined using the data from char gasification experiments in a drop tube reactor. CFD simulations are performed for the laboratory-scale entrained-flow reactor at Brigham Young University for the bituminous coal. A substantial impact of applied kinetic parameters on the in-reactor gas composition and char conversion factor was observed. The effect was most considerable for the reduction zone, where gasification reactions dominate, although a non-negligible impact could also be observed in the flame zone. Based on the quantitative assessment of the incorporated optimization procedure, its application allowed to obtain one of the lowest errors of CO, H2, CO2, and H2O axial distribution with respect to the experimental data. The maximum errors for these species were equal to 18.48, 7.95, 10.15, and 20.22%, respectively, whereas the average errors were equal to 4.82, 5.47, 4.72, and 9.58%, respectively.

Identifying the contribution of rich-CO2/H2O gasification on the char conversion in typical atmospheres of chemical looping with oxygen uncoupling via single particle simulation

Char conversion is an important but also complicated step during the combustion of solid fuels (i.e. coal and biomass) in chemical looping with oxygen uncoupling (CLOU). Potentially, both the lean-O2 oxidation and rich-CO2/H2O gasification reactions of char contribute to the char conversion. However, the contribution degree of char gasification reaction has rarely been quantified in typical CLOU atmosphere. In this work, the single particle simulation is conducted to theoretically understand the effects of different operation conditions on the char conversion, and to identify the contribution degree of rich-CO2/H2O gasification in CLOU. A 10-step heterogeneous reaction mechanism is used for the gas-char reactions, and the random pore model is adopted to account for the evolution of porous char structure as well as its impact on reaction rates. The detailed conversion processes of chars with different sizes are simulated in various concentrations of gasification/oxidation agents and at different temperatures. The amounts of char consumed by lean-O2 combustion, rich-CO2 gasification, and rich-H2O gasification are assumed to be proportional to the amounts of surface adsorbed species generated by char reacting with O2, CO2, and H2O, and a formula to quantitatively calculate the relative contributions of the gasification reactions (both with CO2 and steam) is finally proposed. The results indicate that the gasification reactions (occurring in the inner core of the char particle) hardly influence the oxidation reaction (dominating the heterogeneous reaction in the external layer of the char particle) under CLOU conditions. The CO2-char gasification contributes less than 3% on the char conversion, while the contribution of H2O-char gasification can even reach 2–18%. The char-H2O gasification contributes more significantly (18% for 0.5 vol.% O2, and 2% for 4 vol.% O2) under the O2-deficient conditions. Generally, the overall contribution of gasification reactions on the char conversion is between 1 and 20% in CLOU, which depends on the reaction temperature, gas composition, and char particle size.

Ash transformation during single-pellet gasification of agricultural biomass with focus on potassium and phosphorus

Agricultural biomasses and residues can play an important role in the global bioenergy system but their potential is limited by the risk of several ash-related problems such as deposit formation, slagging, and particle emissions during their thermal conversion. Therefore, a thorough understanding of the ash transformation reactions is required for this type of fuels. The present work investigates ash transformation reactions and the release of critical ash-forming elements with a special focus on K and P during the single-pellet gasification of different types of agricultural biomass fuels, namely, poplar, grass, and wheat grain residues. Each fuel was gasified as a single pellet at three different temperatures (600, 800, and 950 °C) in a Macro-TGA reactor. The residues from different stages of fuel conversion were collected to study the gradual ash transformation. Characterization of the residual char and ash was performed employing SEM-EDS, XRD, and ICP with the support of thermodynamic equilibrium calculations (TECs). The results showed that the K and P present in the fuels were primarily found in the residual char and ash in all cases for all studied fuels. While the main part of the K release occurred during the char conversion stage, the main part of the P release occurred during the devolatilization stage. The highest releases – less than 18% of P and 35% of K – were observed at the highest studied temperature for all fuels. These elements were present in the residual ashes as K2Ca(CO3)2 and Ca5(PO4)3OH for poplar; K-Ca-rich silicates and phosphosilicates in mainly amorphous ash for grass; and an amorphous phase rich in K-Mg-phosphates for wheat grain residues.

Primary fragmentation behavior of refuse derived fuel pellets during rapid pyrolysis

Primary fragmentation during rapid pyrolysis is considered to be the critical size reduction mechanism as it determines the particle size distribution and char conversion rates in the fixed bed gasifier. This study aims to investigate the primary fragmentation behavior of Refuse Derived Fuel (RDF) pellets during a rapid pyrolysis process. RDF pellets consisted of different blending ratios of cardboard and polyethylene (PE) were produced by a single-pellet densification process. Pyrolysis tests at temperature ranges of 500–700 °C were performed for each pellet type, and the number and particle size of the fragmented particles were analyzed. The results demonstrate that the different composition of RDF causes different particle binding mechanisms that behave differently during pyrolysis. Densification of cardboard particles can maintain the structure of the pellet char at higher temperatures compared to PE, due to the more stable lignin binding mechanism. A modified Fragmentation Ratio (FR) is introduced to quantify the degree of fragmentation. It is shown that a raise of the concentration of PE from 25 to 75 wt% increases the FR value by up to 4.2 times, whereas an increase in pyrolysis temperature from 500 to 700 °C only slightly changes the FR values. It can be concluded that the type of materials in the RDF pellet has a more dominating effect on the fragmentation compared to the pyrolysis temperature. Further, the volatile matter content of pellets shows a linear correlation with the FR value, whereas no clear relation is found in the case of the pellet's mechanical strength.

Insitu gasification – chemical looping combustion of large coal and biomass particles: Char conversion and comminution

Utilization of large solid fuel particles in fluidized bed – Chemical Looping Combustion (CLC) has the benefits of reduced energy penalty related to carbon capture. When large fuel particles (mm-sized) are used, comminution plays a vital role in the fuel conversion rate, characteristics of ash, inventory loading and thus in the effective design of CLC systems. The present work deals with the conversion of char of large fuel particles and char fragmentation phenomenon in the insitu-gasification CLC environment. Three different coals and a woody biomass in the size range of + 8–25 mm are tested at three different bed temperatures (800 to 950 °C) in a hematite-based batch CLC unit, using steam as the fluidizing/gasification agent. The char conversion time is found to increase by 60 to 170% when particle size changes from 8 to 25 mm and reduced by 42 to 86% with the increase in bed temperature. Regardless of fuel type and feed size, the inception of char fragmentation is noticed in the very first quarter of conversion indicating its significant influence on the char burnout time. A minimum critical char size exists below which char weakening does not yield breakage, whose values vary between 4.4 and 14.2 mm depending on fuel type and feed size. Fuel type is found to be the prime influencer of char conversion time and fragmentation phenomena. This study recommends the use of large particles of all fuels up to 25 mm in CLC systems without any prior size reduction, except the high-ash coals.

Effect of different pre-treatment methods on gasification properties of grass biomass

The effect of different pre-treatments method on the gasification efficiency of grass biomass have not previously been evaluated. In this study, the effect of three different pre-treatment methods on gasification properties of grass biomass was investigated under CO2 conditions. The pre-treatment methods were dry torrefaction, wet torrefaction, and leaching (chemical). The results obtained showed that the heating values increased by 2,77% in the leached grass, 8,3% in the dry torrefied grass and 13,5% in the wet torrefied grass. However, the wet torrefaction had the highest reactivity index of 0,25 followed by dry torrefaction 0,182, then leaching 0,156. The effect of the different pre-treatment on activation energy showed that the activation energy of raw grass biomass was reduced from 161,7 kJ/mol to 141.5 kJ/mol for leached grass, 124.3 kJ/mol for dry torrefied and 86.97 kJ/mol for wet torrefied grass. These results show that wet torrefaction can improve gasification properties significantly when compared to dry torrefaction and leaching. The pore structure and pore volume effect of treated biomass was likely the predominant reason for the better char reactivity and conversion during gasification of wet torrefied sample. The research supplied an insight into the effect of different pre-treatment methods on grass biomass gasification.

Investigation of Turbulent Pulverized Solid Fuel Combustion with Detailed Homogeneous and Heterogeneous Kinetics

A comprehensive Euler–Lagrange framework for pulverized coal combustion using detailed multi-step heterogeneous kinetics is presented. The heterogeneous kinetics employ the POLIMI model that involves 37 species (22 solid species and 15 gas species) and 49 reactions to describe detailed pyrolysis as well as char oxidation, gasification, and annealing for a wide range of coals. The porous structure of the coal particles is considered, and the heterogeneous reactions are assumed to occur throughout the entire particle in a volume-based approach. The ordinary differential equations of the heterogeneous kinetics are integrated on each Lagrangian coal particle and predict the conversion of the raw coal components to light volatile hydrocarbons, heavy tar species, and char off-gases. Hence, the composition of the solid fuel components and the released gas changes dynamically in space and time, providing high-fidelity predictions of solid fuel combustion. The chemical conversion of the released species in the gas phase is described by a homogeneous kinetic mechanism with 76 species and 973 reactions that was reduced from the comprehensive CRECK-G-1407 kinetic mechanism. The new modeling framework is employed within carrier-phase direct numerical simulations (CP-DNS) of pulverized coal combustion in a three-dimensional turbulent mixing layer. This configuration includes the additional physics of turbulence and particle group combustion by mixing solid fuel particles suspended in a primary oxidizer stream with the products from lean volatile combustion in a secondary stream. The CP-DNS results are analyzed with and without the available set of 14 char conversion reactions, and a low degree of char conversion indicated by an increased rate of CO production is captured for particles with temperatures higher than 1800 K. The CP-DNS results from the detailed POLIMI approach feature a distinct bimodal shape of the volatile release curve and multi-regime combustion. The POLIMI data are used to evaluate the predictive capability of simpler pyrolysis models. The original competing two-step model (C2SM) by Kobayashi is investigated and shown to predict heavily delayed ignition. A new competing two-step devolatilization approach is proposed as an alternative model reduction suitable for fitting bimodal volatile release rates, such as that predicted by POLIMI. The CP-DNS using the alternative pyrolysis model faithfully captures the onset of ignition and multi-regime flame branches. Differences arise in the local tar species compositions in the gas phase as a result of the time-varying (POLIMI) and fixed (new C2SM) volatile compositions for the respective models. The flame structure is further analyzed by chemical explosive mode analysis (CEMA), and the occurrence of premixed and non-premixed flames zones is confirmed, whereas a simpler flame index analysis fails to correctly indicate the multi-regime nature of the flame. This recognition of multi-regime combustion serves as a guidance for selecting suitable conditioning variables for flamelet and other combustion submodels in large eddy simulation.

On the mathematical model simplification using constant Lewis number – Impact assessment on heterogeneous char conversion process

Reactive systems in a thermochemical conversion domain are modelled considering N-specie, 1-energy and 2-mass conservation equations assuming negligible pressure gradient resulting in N+3 non-linear coupled PDE system with dependency on thermodynamic and transport properties. Typically, simplistic temperature-dependent polynomials are chosen for estimating thermal conductivity and specific heat, however, the estimation of mass diffusion coefficient (Di;mix) follows a complicated procedure involving kinetic theory culminating in Chapman-Enskog equation. This renders the solution computationally intensive. The complexity is simplified by assuming a constant Lewis (Le) number, a standard practice in the analytical solution for conventional reactive systems. In fixing Le, (Di;mix) is equated to thermal diffusivity (a ratio of thermodynamic properties) resulting in the specie and energy equation yielding a similar solution and collapse of N+3 system of simultaneous equations to 3 equations. The current article explores the validity and limitation of assuming constant Le in the simulation of char conversion process in air and steam. Results of char conversion are compared for fixed Le and D estimated with Chapman{Enskog expresion. The analysis suggests that Le remains invariant only under a severely restricted set of conditions. Fixing Le influences, the conversion process either over-/under-predicting the conversion time scales and the product gas composition.