Daniel Gauthier

Partitioning of trace elements in the flue gas from coal combustion

Abstract This study describes the partitioning of 16 harmful trace elements (TEs; As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, SE, Sn, Te, Tl, V, Zn) in the flue gases from burning coal at different temperatures (300–1800 K), with various atmospheres (oxidizing or reducing conditions) and types of coal (high- or low-ash coal). Interactions between 54 chemical elements and 3200 compounds are considered in one of the more complex thermodynamic systems so far. The 16 TEs may be classified chemically into two groups, according to their dominant species at low, intermediate and high temperatures, and to their various interactions. In all, 45 major species of the 16 TEs are identified, corresponding to two major kinds of interaction: 1) combination with oxygen; 2) combination with a metal. In the temperature range 400 to 1200 K, thermodynamic calculations show that the 16 TEs behave differently in competitive reactions with O, Cl, F, H, and S in a coal combustor.

Thermodynamic study of the behaviour of minor coal elements and their affinities to sulphur during coal combustion

The behaviours of ten minor coal elements (Al, Ca, Fe, K, Mg, Mn, Na, P, Si and Ti) during coal combustion in the temperature range 400–2000 K, under both oxidising and reducing conditions, have been studied in detail by a thermodynamic equilibrium analysis. The partitioning of these elements is calculated both in single minor element–coal–chlorine systems and in minor elements co-existing systems. Their vaporisation tendency is found in the order: (Si, Al)<(Fe, Ti)<(Ca, Mn)<(K, Na, P, Mg). Si, Ti, Al and P are present mostly as oxides and K and Na as chlorides, whatever the combustion conditions. Al, Ca, Fe, K, Mn and Na sulphates are dominant at low temperatures under oxidising conditions, whereas under reducing conditions most of them are sulphides and/or chlorides. Moreover, the interactions between these elements affect their major speciation: some species containing two elements among those studied are dominant in the minor elements co-existing systems. The affinities of minor coal elements to sulphur have been studied versus both temperature (400 or 800 K) and sulphur content (0.0062–6.20 wt.% in the coal), in order to find out their influence on the flue gas desulfurization. Two coal samples with different ash contents were considered, and it was found that the ash composition affects greatly the minor elements partitioning.

Lagrangian approach for simulating the gas-particle flow structure in a circulating fluidized bed riser

Lagrangian approach is used to simulate the realistic process of cluster formation in circulating fluidized bed riser. The influence of particle properties, porosity function and gas velocity on the global particle flow structure, and the formation and development of local regions of higher particle concentration are investigated. The collision parameters govern the formation of non-homogeneous flow structure because of the difference of kinetic energy and velocity distribution. Moreover, the porosity function has a significant effect on the clusters. The gas flows preferentially into the region of high porosity, which leads to non-uniform drag force on the particles and affects strongly the particle flow structure. Examination of the cluster microstructure shows that the particle velocity in the cluster is smaller than around it. Segregation of particles with different diameter and density is also predicted.

Volatilization behavior of Cd and Zn based on continuous emission measurement of flue gas from laboratory-scale coal combustion.

The accumulation of toxic metals generated by coal-fired power stations presents a serious threat to the environment. The volatilization behavior of two representative metals (Cd and Zn), and the influence of temperature were investigated during coal combustion. An inductively coupled plasma atomic emission spectrometric (ICP-AES) method was developed to continuously measure the heavy metal concentrations quantitatively in flue gas under combustion conditions in order to track the metal release process. This continuous heavy metal analysis system was implemented by coupling it to two types of high temperature reactors: a bubbling fluidized bed reactor and a fixed bed reactor with diameter of 0.1 m and 0.08 m respectively. For the two metals considered in this study (Cd and Zn), the experimental setup was successfully used to continuously monitor the metal vaporization process during coal combustion independent of reactor design, and at different temperatures. Cd is more easily vaporized than Zn during coal combustion. Temperature significantly influences the metal vaporization process. In general, the higher the temperature, the higher the metal vaporization, although the vaporization is not proportional to temperature. In addition to the experimental study, a thermodynamic calculation was carried out to simulate the heavy metal speciation during coal combustion process. The theoretical volatilization tendency is consistent with the experiment. The thermodynamic calculation identified the formation of binary oxides retarding heavy metal vaporization.

Possible interactions between As, Se, and Hg during coal combustion

Abstract A thermodynamic analysis has determined in detail the speciation of Hg, Se, and As, which are the most volatile trace elements in the flue gas from burning bituminous coal, depending on the temperature (400–1800 K), the atmosphere (reducing or oxidizing conditions), and the chlorine content. The amount of chlorine in the system greatly affects the identity of stable species; the relative affinities of the three elements to chlorine are in the order: Se > As > Hg under oxidizing conditions at both high (1100 K) and low (600 K) temperatures. Interactions between the three elements are investigated; AsSe(g) and HgSe(g) are found to be possibly present as the major species in the local CO-enriched zone in a boiler. The competing reactions of these three elements for oxygen, hydrogen, and chlorine are also discussed.

The effect of temperature and heating rate on char properties obtained from solar pyrolysis of beech wood

Char samples were produced from pyrolysis in a lab-scale solar reactor. The pyrolysis of beech wood was carried out at temperatures ranging from 600 to 2000°C, with heating rates from 5 to 450°C/s. CHNS, scanning electron microscopy analysis, X-ray diffractometry, Brunauer-Emmett-Teller adsorption were employed to investigate the effect of temperature and heating rate on char composition and structure. The results indicated that char structure was more and more ordered with temperature increase and heating rate decrease (higher than 50°C/s). The surface area and pore volume firstly increased with temperature and reached maximum at 1200°C then reduced significantly at 2000°C. Besides, they firstly increased with heating rate and then decreased slightly at heating rate of 450°C/s when final temperature was no lower than 1200°C. Char reactivity measured by TGA analysis was found to correlate with the evolution of char surface area and pore volume with temperature and heating rate.

Modelling of heavy metal vaporisation from a mineral matrix

This study deals with the fundamental aspects of the volatilisation of heavy metals (HM) during municipal solid waste (MSW) incineration. The thermal treatment of a model waste was theoretically and experimentally studied in a fluid-bed. A mathematical model was developed to predict the fate of metallic species according to the main phenomena controlling the process: heat and mass transfer (transport phenomena), chemical reactions involving HM, and mechanism of vapour metal species sorption inside the porous matrix. The model assumes local thermodynamic equilibrium between the vapour and the metal compound on the substrate in the pores of a particle. This approach permits to predict the extent of HM vaporisation from a mineral porous matrix when its physical properties are known. Experimental data concerning CdCl(2) release from an alumina matrix in a 850 degrees C fluidised bed are in good agreement with theoretical results.

Simulation of Coal Combustion in a Bubbling Fluidized Bed by Distinct Element Method

The distinct element method (DEM) is used to model combustion of coal particles in a bubbling fluidized bed. The gas phase is modeled as a continuum and the particle phase is modeled by DEM. The chemical reactions consist of the heterogeneous reactions of char with O 2 , CO, CO 2 , NO and N 2 O and in the homogeneous reactions involving CO, O 2 , NO and N 2 O. The colliding particle-particle heat transfer is based on the analysis of the elastic deformation of the spheres during their contact. The model predicts the particle heterogeneous flow structure, the thermal characteristics of burning coal particles, and the gaseous emissions from a fluidized binary mixture. Results show that the instantaneous contribution of the collision heat transfer ranges from 0 to about 1.0% of the total heat transfer during the coal combustion. The mean excess of coal temperature is approximately 140K, and the maximum excess of coal particle temperature is approximately 270 K after 2 s.

Numerical Simulation of the Turbulent Gas–Particle Flow in a Fluidized Bed by an LES-DPM Model

The turbulent gas–particle flow in a bubbling fluidized bed is numerically studied using large eddy simulation and discrete particle method (LES-DPM). The gas-phase model is based on locally averaged two-dimensional Navier–Stokes equations for two-phase flow with fluid turbulence calculated by LES, in which the particle effect on subgrid-scale (SGS) gas flow is taken into account. The particle motion is treated by a DPM, in which the particles are assumed to interact through binary, instantaneous and non-elastic collisions. The gas–particle turbulent flow structures are predicted. The distributions of the simulated particle anisotropic velocity show that the particles in the fluidized bed do not have any local equilibrium. The vertical components of both the gas and particle turbulent intensities are higher near the walls than in the centre, whereas these horizontal components are always low. The prediction results indicate that the profiles of the mean gas and particle velocities and of the turbulent intensities are rather insensitive to C k value, whereas the instantaneous particle structure and individual trajectories are sensitive to it in this turbulent model.

The kinetics of vaporization of a heavy metal from a fluidized waste by an inverse method

Abstract This study addresses the emission of heavy metals during the incineration of municipal solid waste. A global method was developed to determine the vaporization rate of the metal from the on-line analysis of exhaust gas. This method differs from direct models, which predict the time course of the metal concentration in the gas from the vaporization rate profile, but which are not practicable because this vaporization rate cannot be measured in real incinerators burning real wastes. The method is based on the determination of the global rate of release of heavy metal from the combustion of model wastes in a fluidized bed. It is an inverse method, which involves only the measured concentration of heavy metal in the exhaust gases and a model developed at the reactor scale. The thermal treatment of model wastes spiked with a metal was performed in a laboratory- scale fluidized bed. In fact, a solid matrix derived from real waste was dosed with Cd, Pb, or Zn and burned to simulate the metal’s release during the incineration of municipal solid waste. An on-line analysis system was linked to the gas outlet of the reactor, and the metal’s vaporization was tracked successfully by continuously measuring by inductively coupled plasma optical emission spectroscopy (ICP-OES) the relative concentration of the metal in exhaust gases. On the theoretical front, a bubbling bed model was developed and validated to calculate the metal’s vaporization rate from its concentration-time profile in the outlet gas. The inverse method consists in identifying these vaporization rates at the particle level from only the on-line diagnostic results and using the model, whatever the waste considered. The data obtained may be used in any process, in which wastes are heated rapidly (several hundreds of degrees per second), as in fluidized beds.