Char Conversion Research Articles

Modelling pulverized coal combustion using air and [formula omitted] recirculated flue gas as oxidant

This work uses a numerical model to analyse the flame structure and the radiations heat transfers of pulverized coal firing with Air and in oxyfuel conditions. The numerical model considers an axisymmetric approximation for the flow and coal simulation based on a Lagrangian method. The combustion in the gas phase is handled by the eddy dissipation model combined with global kinetics. The coal conversion is based on the CPD volatilisation model and the char conversion by a first order kinetic model. Radiation properties are calculated based on a spectral line-based weighted-sum-of-grey-gases (SLW) model that was found appropriate for this situation. The radiation properties of particles are considered through the use of approximate functions for the absorption and scattering efficiencies. Calculations are presented for tests carried out in the RWEn power's 0.5MWth combustion test facility (CTF) with a Russian coal fired in a burner with 3% O2 in the flue gases, for air firing and simulated recirculated flue gases (RFG) with oxygen injection. The predicted incident radiation heat fluxes to the furnace walls are compared with experimental data. The calculations indicate that the flow structure is modified by the operating conditions leading to similar results when using the higher recirculation ratio and air firing. The computed wall incident heat fluxes present greater dependency of the wall temperatures than the CO2 level or the recirculation ratio.

Lattice Monte Carlo simulation of single coal char particle combustion under oxy–fuel conditions

A descriptive model for isolated char particle combustion under conventional and oxy–fuel conditions was developed. Suggested model is based on the percolation theory and Monte Carlo simulation technique. Char particle was modeled as a square lattice consisting of a large number of small sites. Sites correspond either to fixed carbon, ash, or pore, and they were distributed randomly inside char lattice using percolation concept, at the start of simulation. Random walk model was used to represent gaseous species diffusion through particle pores. Char combustion was modeled using power law Arrhenius model which assumes that reaction rate depends of particle temperature and oxygen partial pressure on particle surface. The main aim of the proposed model was to take into account influence of heterogeneous char particle structure to randomness of the char combustion process. The suggested model’s behavior was validated by qualitative comparison with experimental data obtained in single particle reactor. It was found that simulated combustion time, char burnout and particle temperature values are in good agreement with experimentally determined data. Special emphasis was given to the CO2 gasification reaction influence on char conversion and particle temperature values. Further development of the proposed model with appropriate simplifications would enable its inclusion in comprehensive CFD codes.

The Fate of Char Nitrogen in Black Liquor Combustion—Cyanate Formation and Decomposition

The main gaseous nitrogen emissions from a Kraft pulp mill are in the form of NOx and NH3. In black liquor combustion roughly 60% of the black liquor nitrogen is released during the devolatilization phase and finally forms N2 or NO. The remaining 40% is retained in the char and forms sodium cyanate during char conversion. A part of the sodium cyanate decomposes in the lower furnace, while the rest exits the recovery boiler with the smelt. Sodium cyanate decomposes to NH3 in green and white liquor, which can contribute to the overall nitrogen emissions of a pulp mill. The purpose of this work is to better understand the formation and decomposition of cyanate during thermal conversion of black liquor. The formation and stability of cyanate was studied during thermal conversion of black liquor droplets in different gas compositions, and the formation was also studied by controlled char gasification in CO2 atmosphere. The decomposition was studied by exposing laboratory made black liquor smelt to various gas atmospheres at different temperatures. The results show that gas atmosphere and temperature play a role in how much cyanate is formed as well as the rate of decomposition. The share of black liquor nitrogen converted to cyanate is clearly higher if the gas composition contains CO2 or O2 mixed with N2. Less cyanate is formed if the surrounding atmosphere is pure N2 gas or contains water vapor. The formed smelt cyanate is stable at 800 °C in atmospheres containing only CO2 and N2, whereas it decomposes slowly at 900 °C in atmospheres containing only CO2 and N2. The cyanate in smelt decomposes quickly in oxygen and water vapor containing atmosphere compared to pure N2 atmosphere. Also, importantly, cyanate leads to very little NO formation (less than 5% of the original black liquor N) in all of the conditions tested. This information will be utilized in the future to develop a simplified char-N model for implementation in computational fluid dynamics (CFD) with the ultimate goal of finding new ways of further lowering NO emissions from recovery boilers without downstream NOx reduction technologies.

Numerical simulation of waste tyres gasification.

Gasification is a thermochemical pathway used to convert carbonaceous feedstock into syngas (CO and H2) in a deprived oxygen environment. The process can accommodate conventional feedstock such as coal, discarded waste including plastics, rubber, and mixed waste owing to the high reactor temperature (1000 °C-1600 °C). Pyrolysis is another conversion pathway, yet it is more selective to the feedstock owing to the low process temperature (350 °C-550 °C). Discarded tyres can be subjected to pyrolysis, however, the yield involves the formation of intermediate radicals additional to unconverted char. Gasification, however, owing to the higher temperature and shorter residence time, is more opted to follow quasi-equilibrium and being predictive. In this work, tyre crumbs are subjected to two levels of gasification modelling, i.e. equilibrium zero dimension and reactive multi-dimensional flow. The objective is to investigate the effect of the amount of oxidising agent on the conversion of tyre granules and syngas composition in a small 20 kW cylindrical gasifier. Initially the chemical compositions of several tyre samples are measured following the ASTM procedures for proximate and ultimate analysis as well as the heating value. The measured data are used to carry out equilibrium-based and reactive flow gasification. The result shows that both models are reasonably predictive averaging 50% gasification efficiency, the devolatilisation is less sensitive than the char conversion to the equivalence ratio as devolatilisation is always complete. In view of the high attained efficiency, it is suggested that the investigated tyre gasification system is economically viable.

Effect of operating parameters on performance of an integrated biomass gasifier, solid oxide fuel cells and micro gas turbine system

An integrated power system of biomass gasification with solid oxide fuel cells (SOFC) and micro gas turbine has been investigated by thermodynamic model. A zero-dimensional electrochemical model of SOFC and one-dimensional chemical kinetics model of downdraft biomass gasifier have been developed to analyze overall performance of the power system. Effects of various parameters such as moisture content in biomass, equivalence ratio and mass flow rate of dry biomass on the overall performance of system have been studied by energy analysis.It is found that char in the biomass tends to be converted with decreasing of moisture content and increasing of equivalence ratio due to higher temperature in reduction zone of gasifier. Electric and combined heat and power efficiencies of the power system increase with decreasing of moisture content and increasing of equivalence ratio, the electrical efficiency of this system could reach a level of approximately 56%.Regarding entire conversion of char in gasifier and acceptable electrical efficiency above 45%, operating condition in this study is suggested to be in the range of moisture content less than 0.2, equivalence ratio more than 0.46 and mass flow rate of biomass less than 20 kg h−1.

Brown coal char CO2-gasification kinetics with respect to the char structure. Part I: Char structure development

German lusatian brown coal was gasified in a laboratory scale fluidized-bed type reactor using CO2 as gasification agent. The experiments were carried out at temperatures of 800, 850, 900 and 950°C. The raw brown coal was first pyrolyzed in a way to minimize the volatile matter content in the resulting char. Then the char was used to study the Boudouard reaction (C+CO2=2CO) with respect to the char structure. Temperature range as well as char particle size fraction (200–315μm) were specifically chosen in order to approach the chemically controlled kinetic regime. Chars were gasified completely as well as partially up to certain carbon conversion degrees. Structure characteristics of partially gasified chars such as surface area and porosity were determined using the N2 adsorption technique and mercury porosimetry respectively. Surface area values were obtained from adsorption isotherms using several methods: Brunauer–Emmett–Teller (BET), Langmuir, t-plot and Barrett–Joyner–Halenda (BJH). The choice of the methods was discussed. The results show that the development of char surface area with char conversion at lower temperature (800°C) slightly differs from those at higher temperatures (850–950°C). It was found, that the heterogeneous char-CO2 reaction mostly takes place on mesopore surface area, which presumably is developed from micropore surface area in the beginning of char conversion. This was also confirmed by mercury porosimetry analysis. Total char porosity increases linearly during char conversion at all studied temperatures. The change of char particle size during carbon conversion showed no obvious change in char particle size until 50% of carbon conversion; however, the size decreased beyond this carbon conversion from particle fragmentation.

Effect of char generation method on steam, CO2 and blended mixture gasification of high ash Turkish coals

Char particles of high ash Turkish coal of two sizes were produced by pyrolysis in an atmospheric pressure thermo-gravimetric apparatus using 3 heating rates (100K/min, 500K/min and 800K/min). It is observed that the particle size (for particles between 0.8 and 3mm) did not change the pyrolysis process results. Char gasification rates in CO2, steam and mixture of CO2+steam are investigated in the same TGA system over the temperature range of 850–950°C. The pyrolysis heating rate for char formation is observed to have a marked influence on the subsequent gasification reactivity of the char. The results indicate that the char produced from high heating rates show improved gasification rates. Smaller particles exhibit higher char–CO2 and char–steam gasification rates. Increasing the temperature from 850 to 950°C, leads to a significant reduction of the time required for 50% char conversion both for small and large particles. The maximum reaction rate is shifted to higher conversion degrees when the chars are produced from high heating rates. The gasification rate of char–H2O strongly depends on the H2O partial pressure. Gasification rates in CO2 ambiance are much lower than those in steam. For CO2–steam blended ambiance, no obvious CO2 inhibition effect is observed. But this topic requires more investigation. Kinetic parameters of the char particles are estimated using three different kinetic models. The obtained activation energies for char gasification essentially depend on the particle size and the three kinetic models give very close activation energy values for all the tested conditions.

Using Mathematica software for coal gasification simulations – Selected kinetic model application

Abstract Coal gasification is recognized as a one of promising Clean Coal Technologies. As the process itself is complicated and technologically demanding, it is subject of many research. In the paper a problem of using volumetric, non-reactive core and Johnson model for coal gasification and underground coal gasification is considered. The usage of Mathematica software for models' equations solving and analysis is presented. Coal parameters were estimated for five Polish mines: Piast, Ziemowit, Janina, Szczyglowice and Bobrek. For each coal the models' parameters were determined. The determination of parameters was based on reactivity assessment for 50% char conversion. The calculations show relatively small differences between conversion predicted by volumetric and non reactive core model. More significant differences were observed for Johnson model, but they do not exceeded 10% for final char conversion. The conceptual model for underground coal gasification was presented.

Effect of Torrefaction on Physical Properties and Conversion Behavior of High Heating Rate Char of Forest Residue

In the work reported here, both forest residue (FR) and torrefied forest residue (TFR) were devolatilized in a drop tube reactor at 1473 K at a heating rate greater than 104 K/s. The physical properties of parent fuel particles and their corresponding char particles were examined by using a scanning electron microscope and a granulometer. After the same milling and sieving process, the TFR particles had a smaller size and smaller aspect ratio than the FR particles. The char particles consisted of two types of particles with different sizes and morphologies: a small particle mode (presumably char fragments) and a large particle mode. The volume fraction of char fragments in the TFR char was considerably less than for the FR char. Both types of char were converted in a drop tube reactor under oxidation and gasification conditions at 1473 and 1573 K, respectively. The total organic mass loss and release of individual organic elements during char conversion were determined using a tracer method. Calcium, manganese, barium, and magnesium were found to be suitable for use as tracers. The fractional carbon conversion rate of TFR char was found to be slower than that of FR char under both oxidation and gasification conditions. The fractional release rate of hydrogen was much higher than that of total organic mass loss, while the corresponding oxygen release was lower for both types of char and for different reactive environments.

Three-dimensional calculation of a chemically reacting porous particle moving in a hot O2/CO2 atmosphere

This work is devoted to the three-dimensional numerical investigation of a chemical reacting porous carbon particle moving in a hot O2/CO2 gas. The porous particle was represented by an agglomerate of small monodisperse spherical particles distributed inside a sphere. The ambient temperature was systematically varied in a range between 1200 and 2500K. To solve the Navier–Stokes equations for the flow field coupled with the energy and species conservation equations, a finite volume solver was applied. In addition to the solid carbon the model incorporates six gaseous chemical species (O2, CO, CO2, H2, H2O, and N2). The reaction mechanism includes semi-global carbon monoxide oxidation, the forward and backward water–gas-shift reaction, and four heterogeneous reactions. The ambient medium was assumed to be nearly dry. The main objective of this work was to understand the char conversion processes inside a porous carbon particle. In particular, the change of regimes depending on the ambient gas temperature and the influence of the flow velocity on the processes inside the porous particle were studied in detail. As a reference case the oxidation of a solid non-permeable sphere moving in a hot O2/CO2 gas was considered, and the results for the porous particle are discussed and compared against the reference case.

Effects of detailed operating parameters on combustion in two 500-MWe coal-fired boilers of an identical design

In a large coal-fired power plant, it is common for the multiple boilers to be of an identical design in order to reduce construction and operating costs. Despite the design and operating conditions being the same, the boilers often exhibit different combustion characteristics after being in operation for an extended period. Using computational fluid dynamics (CFD), this study diagnosed the combustion in two identical coal-fired boilers with a capacity of 500 MWe, but which exhibited different trends in their temperature and heat distribution. Based on a survey on the detailed operation parameters, individual windboxes with different damper settings were analyzed to determine the air flow distribution between the burners and the overfire air ports. Then, the swirl burners were investigated in an attempt to identify a correlation with the swirl strength for a range of vane angles and air flow rates. The results were introduced into the CFD simulations for the boilers, for which we adopted advanced models for the coal devolatilization and char conversion. The predicted temperatures and heat transfer rates were in reasonable agreement with the measured data. The main reason for the higher gas temperatures and heat transfer rates in boiler A was found to be the higher air flow rate in the burner zone. Individual burners produced different gas flow paths and, therefore, the low-O2 pockets downstream could be traced to particular burners. Those burners with narrower vane gaps produced stronger swirls in the secondary air, which led to wider-angled flames closer to the burners, together with strong internal recirculation zones. This increased the heat flux on the adjacent wall. This study shows that CFD simulations for existing boilers require careful consideration of the operating parameters that may lead to significant differences in the combustion characteristics.

A comprehensive model for char particle conversion in environments containing O2 and CO2

In this work, conversion of char is studied in a simulation code that includes a simplified heterogeneous adsorption–desorption reaction mechanism for char chemical reactivity and uses GRI-Mech 3.0 as the chemical kinetic mechanism that describes the impact of homogeneous reactions. Besides sub-models for the consequences of chemical reactions, a mode of char particle conversion sub-model is included that describes how particle size and apparent density vary with mass loss as well as a radiation sub-model that describes the radiant exchange of energy between the char particle and surrounding particles and walls. The code is transient and zero dimensional in space, and designed to be used both as a stand-alone gasification/combustion code and as a sub-model for heterogeneous reactions of solid particles in a CFD code.

Investigation of coal properties to avoid lump coal fines generation in COREX process

Fines generation of lump coal is a most important limiting factor for efficient production in the COREX process. In this paper, the fines generation of lump coal has been examined based on simulation experiments of the relationships of coal char conversion with temperature and heat absorption. According to the results, in the early stage of coke formation in the melter gasifier, lump coal absorbs large amounts of heat but without significant increase in its high temperature strength. Strength was only improved at later stages of the coke forming process. Residual coal was also found in most tuyere coke samples illustrating that the lump coals added to the melter gasifier were not completely transformed into coke when they arrived at tuyere region. An improvement in the high temperature performance of lump coal is speculated be limited in the melting gasifier due to the low coke forming rate which could become the core factor of fines generation of lump coal in the melting gasifier. Thus, we suggest preheating the lump coal before it is added to the melting gasifier as a solution.

Gasification of pyrolysis chars from sewage sludge

Gasification of char derived from sewage sludge was studied by using the TG–MS system. Experiments were carried out at different temperatures and steam concentrations. The temperatures of 700–900°C were necessary to complete the conversion in reasonable time. Three reaction rate models for gas–solid reaction were applied to describe the effect of char conversion on the reaction rate. In comparison to the shrinking core model for reaction-controlled regime and random pore distribution models, simple pseudo-homogeneous first-order kinetics was found to be the best for predicting the rate of char steam gasification. Kinetic parameters estimated from the experimental data are in accordance with the literature for lignocellulosic char gasification and are the first ones published for sewage sludge char gasification.

Analysis of Hydrogen Generation through Thermochemical Gasification of Coconut Shell Using Thermodynamic Equilibrium Model Considering Char and Tar.

This work investigates the potential of coconut shell for air-steam gasification using thermodynamic equilibrium model. A thermodynamic equilibrium model considering tar and realistic char conversion was developed using MATLAB software to predict the product gas composition. After comparing it with experimental results the prediction capability of the model is enhanced by multiplying equilibrium constants with suitable coefficients. The modified model is used to study the effect of key process parameters like temperature, steam to biomass ratio, and equivalence ratio on product gas yield, composition, and heating value of syngas along with gasification efficiency. For a steam to biomass ratio of unity, the maximum mole fraction of hydrogen in the product gas is found to be 36.14% with a lower heating value of 7.49 MJ/Nm3 at a gasification temperature of 1500 K and equivalence ratio of 0.15.

On a Highly Reactive Fe2O3/Al2O3 Oxygen Carrier for in Situ Gasification Chemical Looping Combustion

Interest in the direct use of solid fuel in chemical looping combustion (CLC) technology makes the in situ gasification chemical looping combustion (iG-CLC) an attractive approach for the low-cost capture of CO2. Highly reactive material is required in iG-CLC in order to achieve a fast reaction between the fuel and oxygen carrier. In this work, a material, Fe2O3/Al2O3 synthesized by sol–gel, was evaluated in a fluidized-bed reactor by reaction with lignite. This is the first time sol–gel-derived Fe2O3/Al2O3 material has been used in an iG-CLC process. Operation conditions, including steam content in the fluidization gas, temperature, and potential oxygen ratio, were investigated to explore their influence on combustion and char conversion. The results show that a higher steam concentration can greatly enhance the rate of char gasification and hence the reaction between the lignite and the oxygen carrier, whereas a negligible effect of the steam content was noticed on volatile combustion. In addition, the use of the highly reactive Fe-based material prepared by the sol–gel method significantly increased the char gasification rate as compared to other previously evaluated materials. Moreover, the combustion efficiencies of volatiles and char from the lignite, respectively, were studied. Using the Fe2O3/Al2O3 material enabled a low oxygen carrier inventory of 600 kg/MWth to be reached in order to achieve 99% char combustion, which is much lower than that reported in other works. These findings suggest that Fe2O3/Al2O3 prepared by sol–gel is a highly reactive oxygen carrier for iG-CLC.

Effects of Steam on the Release of Potassium, Chlorine, and Sulfur during Char Conversion, Investigated under Dual-Fluidized-Bed Gasification Conditions

The corrosion and fouling of heat-transfer surfaces and the agglomeration of bed materials in fluidized beds are some of the ash-related problems caused by the transformation and release to the gas phase of ash-forming elements from biomass during thermochemical conversion processes. The magnitudes of these problems are largely dependent upon the release of potassium (K), chlorine (Cl), and sulfur (S) from the biomass. We investigated the effects of steam on the release of K, Cl, and S during char conversion, under conditions relevant for dual-fluidized-bed gasification (DFBG). The study was carried out with wheat straw in a laboratory-scale bubbling fluidized-bed reactor in the temperature range of 800–900 °C. The release of K, Cl, and S from wheat straw during devolatilization, char gasification, and char combustion was quantified with a mass balance that linked the masses of these elements in the wheat straw to the mass of the solid residue obtained at the end of each experiment. To facilitate analyses of the experimental results, leaching and the Brunauer–Emmett–Teller surface area measurement of the wheat straw and some of the solid residues were carried out. The results show that, during devolatilization, the release of volatile salts, e.g., KCl, is significantly limited by intraparticle diffusion resistance, owing to a compact char matrix (i.e., negligible porosity). However, during char gasification, steam renders the char less compact by expanding and/or creating new pores in the char. As a result, intraparticle diffusion resistance decreases, thereby facilitating the evaporation of volatile salts of K and S from the char matrix. The conversion of the char is also conducive to the release of char-bound K and S, especially at 900 °C. At temperatures of >800 °C, the relative proportions of the elements released and char gasified indicate that the release of K can somewhat be decoupled from the release of S and Cl by maximizing the extent of char conversion in the gasification chamber. The results also show that, during char combustion, the proportions of the char that can be combusted and the extent of the release of the elements are influenced by the extent to which the char is gasified in the gasification chamber.

Co-gasification of biosolids with biomass: Thermogravimetric analysis and pilot scale study in a bubbling fluidized bed reactor

This work studied the feasibility of co-gasification of biosolids with biomass as a means of disposal with energy recovery. The kinetics study at 800°C showed that biomass, such as switchgrass, could catalyze the reactions because switchgrass ash contained a high proportion of potassium, an excellent catalyst for gasification. However, biosolids could also inhibit gasification due to interaction between biomass alkali/alkaline earth metals and biosolids clay minerals. In the pilot scale experiments, increasing the proportion of biosolids in the feedstock affected gasification performance negatively. Syngas yield and char conversion decreased from 1.38 to 0.47m3/kg and 82–36% respectively as the biosolids proportion in the fuel increased from 0% to 100%. Over the same range, the tar content increased from 10.3 to 200g/m3, while the ammonia concentration increased from 1660 to 19,200ppmv. No more than 25% biosolids in the fuel feed is recommended to maintain a reasonable gasification.

Changes in char reactivity due to char–oxygen and char–steam reactions using victorian brown coal in a fixed-bed reactor

This study was to examine the influence of reactions of char–O2 and char–steam on the char reactivity evolution. A newly-designed fixed-bed reactor was used to conduct gasification experiments using Victorian brown coal at 800°C. The chars prepared from the gasification experiments were then collected and subjected to reactivity characterisation (ex-situ reactivity) using TGA (thermogravimetric analyser) in air. The results indicate that the char reactivity from TGA was generally high when the char experienced intensive gasification reactions in 0.3% O2 in the fixed-bed reactor. The addition of steam into the gasification not only enhanced the char conversion significantly but also reduced the char reactivity dramatically. The curve shapes of the char reactivity with involvement of steam were very different from that with O2 gasification, implying the importance of gasifying agents to char properties.

Modeling Biomass Conversion during Char Gasification, Pyrolysis, and Torrefaction by Applying Constrained Local Thermodynamic Equilibrium

The char and biomass conversions during gasification, pyrolysis, and torrefaction were studied using the constrained free energy (CFE) method. The Gibbs free energy minimization method is extended by implementing immaterial constraints for describing partial equilibria in the gaseous phase and for kinetically controlling slow reactions associated with the conversion of char and biomass. The connection between immaterial constraints and the affinities of slow chemical reactions are illustrated. The method presented allows for the Arrhenius type of kinetic model to be incorporated into the calculation of local constrained thermodynamic equilibrium. Thus, the kinetically constrained chemical reactions, equilibrium reactions, and reaction enthalpies can be solved simultaneously by applying CFE methodology. A conceivable approach for introducing pseudo-biomass components into the thermochemical system is evaluated. The technique applies to statistical estimates of standard enthalpy and standard entropy based on assumed molecular compositions. When incorporated into a thermodynamic model, the pseudo-components allow for estimating the composition of biomass during the process and the fast volatilization of oxygen- and hydrogen-containing species at the beginning of the processes. The CFE method was successfully used for modeling the char conversion. The high operating temperature of the gasification process justifies the assumption of local equilibrium in the gas phase. The immaterial constraint can be used for controlling the release of carbon to the gas phase as the reaction proceeds. When pyrolysis and torrefaction were studied, the immaterial constraints could be successfully used for describing biomass conversion in solid phases. However, for these processes, the assumption of local equilibrium in the gas phase is not valid, because no equilibrium reactions occur in the low-temperature conditions.