Attrition of lignite char in a spouted bed combustor

The generation of carbon fines by attrition during burning of Thai lignite char has been studied experimentally by means of a 92 mm i.d. continuous spouted bed combustor at different values of spouting gas velocity, bed temperature, and char feed size. Both inert particle size and static bed depth were fixed for all experimental runs. The collected data were used to analyse size distributions of both in-bed particles and elutriated fines, and to generate the suitable correlations for carbon attrition rate. Results obtained showed that attrition rate in the spouted bed is proportional to the excess of gas velocity above the minimum spouting gas velocity and the overall bed carbon surface exposed to attrition. The attrition rate constant is slightly dependent on operating bed temperature. Its values for the char studied were 1.6511 × 10^?6 for 707°C operating bed temperature, and 1.1222 × 10^?6 for 850°C, with the average for all tested runs being 1.224 × 10^?6.     

Combustion kinetic parameters of thai lignite char

Kinetic parameters, the pre-exponential factor and the activation energy, of two Thai lignite char of different ash contents and electrode carbon have been experimentally determined based on the Semenov's thermal ignition theory adapted to single particle ignition. Combustion rate constants obtained have been comparatively plotted with those from Field's correlation (1969). Results indicated that the reaction rate coefficient for lignite char approaches value estimated from Field's equation at temperature about 850°C.     

Spouted BED combustion of paddy husk

This communication reports the results of an experimental study of spouted bed combustion of paddy husk. The results indicate that husk combustion intensity of the order of 350 kg/h — m² of reactor area is possible with a combustion efficiency of over 85 per cent. Selection of sand size and bed-depth for the spouted bed have emerged to be important parameters in view of pressure drop across bed and entrainment from combustor. The changes in efficiency of combustion were insignificant for different particle size and different bed depths. However there seems to be an optimum combustion intensity i.e. 350 kg/h — m² below and above which combustion efficiency decreases.     

Turbulent flow with combustion in a moving bed

This paper presents a mathematical model for treating turbulent combusting flows in a moving porous bed, which might be useful to design and analysis of modern and advanced biomass gasification systems. Here, one explicitly considers the intra-pore levels of turbulent kinetic energy and the movement of the rigid solid matrix is considered to occur at a steady speed. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. Results indicate that fixing the gas speed and increasing the speed of the solid matrix pushes the flame front towards the end of the reactor. Also, since the rate of production of turbulence is dependent on the relative velocity between phases, as the solid velocity approaches that of the gas stream, the level of turbulence in the flow is reduced.     

Spouted and spout-fluid bed combustors. 2: Batch combustion of carbon

A batch combustion model of carbon particles in a spouted bed has been developed based on the concept of carbon residence time in each region of the spouted bed (annulus, spout and fountain). Both isothermal and nonisothermal particles are considered, with the assumption of no temperature gradient inside the particle. Model predictions in terms of carbon conversion and burnout time were in good agreement with experimental data obtained from a laboratory scale spouted bed combustor. For comparison, an experimental study on a spout-fluid mode was also carried out.     

Equilibrium modeling of gasification: Gibbs free energy minimization approach and its application to spouted bed and spout-fluid bed gasifiers

Spouted beds have been found in many applications, one of which is gasification. In this paper, the gasification processes of conventional and modified spouted bed gasifiers were considered. The conventional spouted bed is a central jet spouted bed, while the modified spouted beds are circular split spouted bed and spout-fluid bed. The Gibbs free energy minimization method was used to predict the composition of the producer gas. The major six components, CO, CO₂, CH₄, H₂O, H₂ and N₂, were determined in the mixture of the producer gas. The results showed that the carbon conversion in the gasification process plays an important role in the model. A modified model was developed by considering the carbon conversion in the constraint equations and in the energy balance calculation. The results from the modified model showed improvements. The higher heating values (HHV) were also calculated and compared with the ones from experiments. The agreements of the calculated and experimental values of HHV, especially in the case of the circular split spouted bed and the spout-fluid bed were observed.     

Spouted and spout-fluid bed combustors 1: Devolatilization and combustion of coal volatiles

The devolatilization and volatile combustion of a single coal particle in spouted and spout-fluid beds have been studied. The results showed that the flame extinction time increases with the particle diameter, and decreases with the bed temperature. When the bed temperature and the air flow rate were fixed, the operation modes (spouted or spout-fluid bed) showed less effect on the mean flame extinction time. A mathematical model of the spouted bed mode for preignition and postignition periods has also been developed assuming the devolatilization rate to be controlled by heat transfer and multireaction pyrolysis kinetics based on volatile products. Ignition, heat transfer back from the volatile flame to the particle surface, variation in flame temperature, and the hydrodynamics of SB are taken into account. The model predictions, with some adjusting parameters, were in good agreement with experimental results.     

CFB combustion of low-grade lignites: Operating stability and emissions

This work presents the results of a comprehensive experimental investigation on the combustion of the low grade Turkish lignites in a 30 kW_th circulating fluidized bed combustor. This is the first study of this kind has ever been undertaken on these coals. Eighteen lignite samples procured from various lignite sites of Turkey have been burned under similar combustion conditions in order to access to their combustion stability and to determine the emissions of the major pollutants such as CO, NO_X and SO₂ in the flue gas from combustor. The qualities of lignites were evaluated based on van Krevelen graph which was highly scattered and diverse in respect to the degree of ageing. A steady and stable combustion was observed in the temperature range of 725–950 °C with an average operating temperature of around 850 ± 50 °C for all lignites. Under the operating condition applied in the study, CO, NO_X and SO₂ emissions varied mostly in the ranges of 120–600 mg/Nm³, 90–420 mg/Nm³ and 1100 mg/Nm³ - 18000 mg/Nm³, respectively. From the experimental results it seems that the most challenging problem may be faced during the CFB combustion of most of these lignites will be SO₂ emissions.     

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.     

Comparison of bituminous coal and lignite during combustion: Combustion performance,coking and slagging characteristics

The combustion characteristics such as combustion performance, coking, and slagging—at high temperatures (700–1300 °C) of bituminous coal and lignite were investigated and compared. The results show that the ignition temperature and the activation energy of lignite are lower than those of bituminous coal, and the combustion index and the burnout index are less than those of bituminous coal. Lignite has almost no coking while bituminous coal tends to coke at high temperatures. The larger the content and reflectivity of the vitrinite, the more severe the degree of coking. In the range of 700–1300 °C, the increase of temperature has little influence on the coking characteristics of lignite and bituminous coal. The low-rank lignite has larger amounts of mineral content which tend to form low-fusion-temperature eutectics. Furthermore, there is a connection between the combustion performance, coking and slagging characteristics through the maceral compositions: the coal which is hard to ignite but easy to burn out is more likely to have strong coking ability. Meanwhile, coking tends to keep alkaline oxides stay in the char and reinforce slagging at high temperatures.     

Migration and transformation of sodium and chlorine in high-sodium high-chlorine Xinjiang lignite during circulating fluidized bed combustion

The Xinjiang lignite mined from Shaerhu coalfield (SEHc) easily causes severe fouling and corrosion because of its high sodium and chlorine contents. Therefore, it is necessary to study the migration and transformation behavior of sodium and chlorine during combustion in order to reveal the mechanisms of fouling and corrosion, and propose the effective solutions of above problems. In this study, based on the 0.4 T/D circulating fluidized bed (CFB) test system, the migration and transformation behavior of sodium and chlorine in SEHc during combustion at 950 °C was explored. The migration and transformation paths of sodium and chlorine were proposed through the chemical characterization of ash samples along the flue gas flow direction, as well as the thermodynamic equilibrium calculation by the software of Factsage 6.1. The experimental studies show the sodium and chlorine mainly in the form of NaCl crystal in raw coal underwent a series of physical and chemical changes during combustion, and subsequently distributed in bottom ash/circulating ash, fly ash and gas phase in various forms including sodium aluminosilicates, chlorides and sodium oxides. Sodium was more inclined to be resided in ash in the form of aluminosilicates through the reactions with other minerals (SiO₂ and Al₂O₃), while chlorine was easily released into the flue gas in forms of HCl, Cl₂, NaCl, etc. The Cl-based species might result in the corrosion of metal heating surfaces because of the presence of corrosion products (metal chlorides) in fly ash. As temperature decreased, the sodium or chlorine vapors would successively deposit in fly ash via physical condensation or chemical reaction. At 840～570 °C, the sodium-based species (Na₂O and NaCl) would first deposit in fly ash, then gaseous chlorine species (NaCl, FeCl₃ and so on) primarily deposited at 570～180 °C.     

Influence of pyrolysis pressure on structure and combustion reactivity of Zhundong demineralized coal char

The investigation on evolution of coal char structure during pressurized pyrolysis can reveal the combustion reactivity of coal char in thermal utilization at elevated pressure. In this study, Zhundong subbituminous coal was demineralized and a pressurized drop tube reactor (PDTR) was used to prepare coal char under different temperature and pressure conditions. The physicochemical structures of raw and demineralized coal chars were characterized by the application of nitrogen adsorption analyzer, automatic mercury porosimeter, and Fourier transform infrared spectroscopy (FTIR). The change mechanism of char infrared structure with pyrolysis pressure is revealed on the molecular level in this paper. The results show that the N₂ adsorption quantity of raw coal char increases with the increase of pyrolysis temperature, while that of demineralized coal char decreases. Because of the difference in molecular volume and steric hindrance between aliphatic and aromatic structure in char, the increasing pressure has less inhibition effect on the escape of the former than the latter. With the increase of pyrolysis pressure, the combustion reactivity of char is related to the infrared structure at 700 and 800 °C while to macropore structure at 900 and 1000 °C.     

Forest biomass waste combustion in a pilot-scale bubbling fluidised bed combustor

Combustion experiments of forest biomass waste in a pilot-scale bubbling fluidised bed combustor were performed under the following conditions: i) bed temperature in the range 750-800 °C, ii) excess air in the range 10-100%, and iii) air staging (80% primary air and 20% secondary air). Longitudinal pressure, temperature and gas composition profiles along the reactor were obtained.The combustion progress along the reactor, here defined as the biomass carbon conversion to CO₂, was calculated based on the measured CO₂ concentration at several locations. It was found that 75-80% of the biomass carbon was converted to CO₂ in the region located below the freeboard first centimetres, that is, the region that includes the bed and the splash zone.Based on the CO₂ and NO concentrations in the exit flue gas, it was found that the overall biomass carbon conversion to CO₂ was in the range 97.2-99.3%, indicating high combustion efficiency, whereas the biomass nitrogen conversion to NO was lower than 8%.Concerning the Portuguese regulation about gaseous emissions from industrial biomass combustion, namely, the accomplishment of CO, NO and volatile organic compounds (VOC) (expressed as carbon) emission limits, the set of adequate operating conditions includes bed temperatures in the range 750°C-800 °C, excess air levels in the range 20%-60%, and air staging with secondary air accounting for 20% of total combustion air.     

Numerical studies on the combustion properties of char particle clusters

Particle clustering is an important phenomenon in dense particle–gas two-phase flow. One of the key problems worth studying is the reacting properties of particle clusters in coal particle combustion process in the dense particle region. In this paper, a two-dimensional mathematical model for the char cluster combustion in airflow field is established. This char cluster consists of several individual particles. The comprehensive model includes mass, momentum, and energy conservation equations for both gas and particle phases. Detailed results regarding velocity vector, mass component, and temperature distributions inside and around the cluster are obtained. The micro-scale mass and heat transfer occurred inside and around the char cluster are revealed. By contrastively studying the stable combustion of char particle clusters consisting of different particles, the combustion properties of char clusters in various particle concentrations are presented and discussed.     

Description,applications and numerical modelling of bubbling fluidized bed combustion in waste-to-energy plants

The use of the fluidized bed combustor (FBC) has increased. It began in the 20th century as coal combustion and gasification, which then developed into catalytic reactions. Only recently, the application field has been extended to the incineration of biomass and pre-treated waste, for either power generation or waste disposal. The success of fluidized bed combustion is due to high combustion efficiency, great flexibility when it comes to the heating value of the fuel and reduction in pollutants emitted with the flue gas.     

Experimental Study and Modelling of Char Combustion under Fluidized Bed Conditions

INTRODUCTIONMailyexperimentshavebeendonetostudytheconversionbehaviorofdifferentcoalsundervariousreactorsandconditions.Someauthorshaveproposeddetailedmodelsfromobservedresults,orfromthealreticalaspects.Twotypemodelsaredevelopedtopredicttheconcelltrationprofilesforthegaseousspeciesaroundaburningcharparticled].In'single--film'models,thecarbonreactdirectlywithoxygentoformCOorCOb.In'double--film'modelsitisassumedthattheoxygendiffusingtowardsthepaxticleisconsumedbeforeitreachesthesuxface.Thep…     

Experimental studies on cotton stalk combustion in a fluidized bed

The present work reports studies on the mixing and combustion characteristics of cotton stalk (CS) with 10–100 mm in length in a fluidized bed. Effects of length and initial weight percentage of CS, diameter of alumina bed material as well as gas velocity on the mixing characteristics of CS with alumina were investigated. CS can mix well with 0.6–1 mm alumina at fluidization number N=3–8.     

Mutual effects of porosity and reactivity in char oxidation

The motivation for this review is the need to understand the interdependence of porous structure and reactivity of highly porous carbonaceous materials during oxidation. These materials can be oxidized in three regimes: regime I, kinetically controlled conditions; regime II, partial diffusion-controlled conditions; regime III, diffusion controlled conditions. Since the emphasis here is on the porous structure and its influence on reactivity, conditions where transport processes are dominant were not included for they mask the view of interest. Therefore, the review discusses only physicochemical processes occurring during oxidation of highly porous chars in regime I. Furthermore, reactivity is influenced by many factors, such as catalysis, volatile matter, and water content. To avoid the effect of these factors, highly porous synthetic chars with nothing but elemental carbon and residual hydrogen and oxygen was chosen. Mainly, we discuss a commercial product known as Spherocarb which consists of spherical particles with specific surface area of about 1000 m² g⁻¹ and porosity of about 0.6. These particles are well defined and reproducible in their properties. They serve well as model materials for various synthetic chars, coal chars, and other carbonaceous materials. The review presents in a systematic manner macroscopic properties and processes that shed light on different aspects of porosity and reactivity. These are presented both from experimental observation as well as modeling view. An attempt was made to present a porous structure model that can reconstruct all available experimental data on these particles during oxidation. In the review the following processes and properties are discussed: shrinkage, fragmentation, and porosity. All are directly connected to porous structure and reactivity.     

Numerical study on solar spouted bed reactor for conversion of biomass into hydrogen-rich gas by steam gasification

A solar-powered biomass steam gasification system was developed, in which heat transfer model, flow model and chemical model were constructed to predict the distributions of temperature, pressure, mole fraction of syngas, and solar incident flux. Several key parameters of gasifier were designed to ensure the fluidization stability. Based on the model validation, gasifier performance simulations in the design working conditions were obtained. The effects of the key variable parameters, including the rim angle of the dish collector, steam-to-biomass mass flow ratio, biomass feeding rate and the solar irradiance in the different operation working conditions on the composition of syngas, lower heating value, and efficiencies were investigated. The results reveal that the coupled system implements the best gasification performance in the design conditions which the rim angle, steam-to-biomass mass flow ratio, and biomass feeding rate are set at 60°, 0.4, and 2.5 g/min, while the LHV, carbon conversion, and gasification energy efficiencies are 11.51 MJ/m³, 78.17%, and 93.01%, respectively. The overall energy efficiency considering solar energy is 30.79%.