Numerical Modelling for Process Investigation of a Single Coal Particle Combustion and Gasification

Combustion and Gasification are commercial processes of coal utilization, and therefore continuous improvement is needed for these applications. The difference between these processes is the reaction mechanism, in the case of combustion the reaction products are CO2 and H2O, whereas in the case of gasification the products are CO, H2 and CH4. In order to investigate these processes further, a single coal particle model has been developed. The definition of the chemical reactions for each process is key for model development. The developed numerical model simulation uses CFD (Computational Fluid Dynamic) techniques with an Eddy Break Up (EBU) model and a kinetics parameter for controlling the process reaction. The combustion model has been validated and extended to model the gasification process by inclusion of an additional chemical reaction. Finally, it is shown that the single coal particle model could describe single coal particle combustion and gasification. From the result, the difference between single coal particle combustion and gasification can clearly be seen. This simulation model can be considered for further investigation of coal combustion and gasification application processes

On the effects of exothermicity and endothermicity upon the temperature fields in a partially-filled porous channel

Forced convection of heat in a two-dimensional channel, partially filled by a porous insert is considered. This system is assumed under fully developed conditions and constant wall heat flux. Further, the fluid and solid phases can feature internal heat generation (exothermicity) and consumption (endothermicity). Analytical solutions are developed for the solid and fluid temperature fields by applying local thermal non-equilibrium (LTNE) conditions and the Darcy-Brinkman model of momentum transport. Two existing interface models (Models A and B) are employed to describe the thermal boundary conditions at the porous-fluid interface. The developed solutions for the temperature fields are compared to those found by applying the local thermal equilibrium (LTE) assumption and, therefore, the validity of the LTE is examined. This is done for a wide range of pertinent parameters including Biot number, conductivity ratio, Darcy number and thickness of the porous insert. It is found that the thermal behaviour of the investigated partially filled system is influenced by the heat sources in both solid and fluid phase. It is further shown that the LTE approach remains an acceptable assumption only for some specific regions of the parametric space. Furthermore, the occurrence of temperature gradient bifurcation on the surface of the porous-fluid interface is examined. It is demonstrated that this effect is highly sensitive to the intensity of the energy sources

Three-dimensional numerical simulations of free convection in a layered porous enclosure

Three-dimensional numerical simulations are carried out for the study of free convection in a layered porous enclosure heated from below and cooled from the top. The system is defined as a cubic porous enclosure comprising three layers, of which the external ones share constant physical properties and the internal layer is allowed to vary in both permeability and thermal conductivity. The model is based on Darcy's law and the Boussinesq approximation. A parametric study to evaluate the sensitivity of the Nusselt number to a decrease in the permeability of the internal layer shows that strong permeability contrasts are required to observe an appreciable drop in the Nusselt number. If additionally the thickness of the internal layer is increased, a further decrease in the Nusselt number is observed as long as the convective modes remain the same, if the convective modes change the Nusselt number may increase. Decreasing the thermal conductivity of the middle layer causes first an increment in the Nusselt number and then a drop. On the other hand, the Nusselt number decreases in an approximately linear trend when the thermal conductivity of the layer is increased

Novel real-time PCR assays for the specific detection of human infective Cryptosporidium species

Cryptosporidium is a protozoan parasite causing gastrointestinal illness. Drinking waterborne outbreaks have been caused by C. hominis, C. parvum and C. cuniculus. Molecular detection techniques already exist for Cryptosporidium and usually target housekeeping genes. We set ourselves the task to identify species-specific genes. These genes are likely to be involved in host parasite interaction and virulence. Three subtelomeric species-specific putative virulence factor genes (Cops-2, Chos-1 and Chos-2) were identified in silico and used to develop novel real-time PCR assays. Our results show that Chos-2 is a suitable target for probe-based assays for the specific detection of C. hominis and C. cuniculus (two very closely related species) and that Cops-2 is a suitable target for specific detection of C. parvum

Investigation of coal particle gasification processes with application leading to underground coal gasification

A coal particle model is developed to investigate the thermochemical processes of gasification for underground coal applications. The chemical reactions are defined with an Eddy Break up (EBU) model for controlling the reaction mechanisms and the study is particularly focused on identification of the important kinetic parameters, which control the consumption rate of coal mass. As an initial validation, the coal particle oxidation based on the experimental results is used for comparison. The gasification reactions are subsequently applied for the thermochemical process investigation, and the results show that the best agreement of coal oxidation is achieved by the pre-exponent factor (A) of 0.002 and 85500, for the reactions, R2 (C + O2 = CO2) and R3 (C + 0.5O2 = CO), respectively. The kinetic parameters for the gasification process of coal particle leading to the syngas production are also optimised. The results show that the production of H2 and CO is controlled significantly by the level of oxygen concentration in the char reactions. However, their chemical rates are strongly dependent upon the reaction zones. For example, CO is produced in both oxidation and reduction reaction zones, while H2 production is dominated in the reduction zone. Spatio-temporal distributions of the gas species along with the coal particle temperature provide additional information for further development of UCG modelling. Ultimately, the model gives a good guideline with the associated thermochemical processes that can help developing advanced coal gasification technology and lead to improved syngas quality

The Lick-Carnegie Exoplanet Survey: A Saturn-Mass Planet in the Habitable Zone of the Nearby M4V Star HIP 57050

Precision radial velocities from Keck/HIRES reveal a Saturn-mass planet orbiting the nearby M4V star HIP 57050. The planet has a minimum mass of 0.3 Jupiter-mass, an orbital period of 41.4 days, and an orbital eccentricity of 0.31. V-band photometry reveals a clear stellar rotation signature of the host star with a period of 98 days, well separated from the period of the radial velocity variations and reinforcing a Keplerian origin for the observed velocity variations. The orbital period of this planet corresponds to an orbit in the habitable zone of HIP 57050, with an expected planetary temperature of approximately 230 K. The star has a metallicity of [Fe/H] = 0.32+/-0.06 dex, of order twice solar and among the highest metallicity stars in the immediate solar neighborhood. This newly discovered planet provides further support that the well-known planet-metallicity correlation for F, G, and K stars also extends down into the M-dwarf regime. The a priori geometric probability for transits of this planet is only about 1%. However, the expected eclipse depth is ~7%, considerably larger than that yet observed for any transiting planet. Though long on the odds, such a transit is worth pursuing as it would allow for high quality studies of the atmosphere via transmission spectroscopy with HST. At the expected planetary effective temperature, the atmosphere may contain water clouds.Comment: 20 pages, 5 figures, 3 tables, to appear in the May 20 issue of ApJ

Combustion characteristics and pollutant emissions in transient oxy-combustion of a single biomass particle: a numerical study

Oxy-combustion of biomass is a potentially attractive, and yet largely unexplored, technology facilitating the negative generation of CO2. In this paper, numerical simulations are conducted to investigate the transient combustion process of a single biomass particle in O2/N2 and O2/CO2 atmospheres and the results are validated against the existing experimental data. Oxygen concentration varies from 27% to 100% in the investigated gaseous atmospheres. The spatiotemporal evolutions of the gas-phase temperature and species concentration fields are explored to further understand the transient combustion characteristics of biomass particles in oxygenated atmospheres. The results show considerably different burning behaviours under carbon dioxide and nitrogen containing atmospheres. Simultaneous and sequential combustion of the volatiles and char are distinguished from the numerical simulations. Further, NOx and SOx emissions are predicted on the basis of the validated numerical combustion model. A qualitative analysis is then performed to investigate the influences of oxygen concentration and carbon dioxide atmosphere upon the pollutant emissions. It is shown that CO2 has a significant inhibitory effect on NOx formation, while it promotes SO2 emissions. As oxygen concentration increases, the NO and SO2 emission rates decrease under both types of gas atmospheres. Nonetheless, the overall NOx and SOx emissions feature different trends

Numerical simulation approaches for modelling a single coal particle combustion and gasification

Combustion and gasification are the fundamental process es of coal utilization, and t he research of these applications has been continuously progressing. Numerical modelling is one of the methodologies that also has significant advancement , due to the progress of computational engineering and also considering economic impact. This paper is a part of the numerical developments on the coal combustion and gasification that introduces a new approach by which a single coal particle model has been developed and used to investigate those process es. CFD (Computational Fluid Dynamics ) techniques with an Eddy Break Up (EBU) model and also with a set of kinetics parameter reactions are used in the study . However, defining the chemical reactions is crucial for the model development . Seven reactions for coal combustion and additional six reactions for ga sification are investigated . It is identified that the best fit kinetic parameter value for the pre- exponent factor ( A ) of R2 and R3 , while comparing with the experimental results, is 20 and 1000, respectively . Finally, these values are implemented in to the model of both coal particle combustion and gasification for investigation. The results of the simulation show that the H 2 and CH 4 products from the gasification are significantly higher than those from the combustion. The maximum mole fraction value of CO products in combustion is ~ 1.5 times higher than in gasification at an air condition, which is unexpected. However, CO production lasted longer than ~ 200 ms at O 2 condition below than 21% in the coal gasification, which resulted in more CO production. The se results clear ly identify the process of coal combustion and gasification. Th is particle model can thus be considered for further investigation for various coal combustion and gasification applications

Numerical modelling of unsteady transport and entropy generation in oxy-combustion of single coal particles with varying flow velocities and oxygen concentrations

Unsteady generation of entropy and transfer of heat and chemical species in the transient oxy-combustion of a single coal particle are investigated numerically. The burning process takes place in oxygen and nitrogen atmospheres with varying chemical compositions and under either quiescent or active flows. The combustion simulations are validated against the existing experimental data on a single coal particle burning in a drop-tube reactor. The spatio-temporal evolutions of the gas-phase temperature and major gaseous species concentration fields as well as that of entropy generation are investigated for the two types of gas flow. It is shown that the rates of production and transport of chemical species reach their maximum level during the homogenous combustion of volatiles and decay subsequently. Yet, the transient transfer of heat of combustion continues for a relatively long time after the termination of particle life time. This results in the generation of a large amount of thermal entropy at post-combustion stage. The analyses further indicate that the entropy generated by the chemical reactions is the most significant source of unsteady irreversibilities. Most importantly, it is demonstrated that a slight oxygenation of the atmosphere leads to major increases in the total chemical entropy generation and, thus it significantly intensifies the global irreversibilities of the process. However, upon exceeding a certain mole fraction of oxygen in the atmosphere, further addition of oxygen only causes minor increases in entropy generation. This trend is observed consistently in both quiescent and active flow cases