Computational Fluid Dynamics (CFD) Modeling Study of Thermal Performance for Multipurpose Solar Heating System

The 3-D numerical simulations of the thermal collectors in solar heating systems were conducted to simulate the conventional solar heating system, multipurpose solar water heater (MPSWH), and multipurpose solar air heater (MPSAH).  The commercial computational fluid dynamics (CFD), AVL Fire ver. 2009.2 was used to solve and investigate the temperature distributions in the absorber plate and riser tube of both solar water and air heater during summer and winter seasons. The RNG k - turbulence model was employed for this CFD study.  The present paper was to provide a good understanding of thermal performance for the solar collector at different operating conditions. The experimental setup and physical data of   Venkatesh, R. and Christraj, W. [15] were employed as geometric parameters and initial boundary conditions to model and to validate the predicted numerical values. Comparing to the values of temperatures for the conventional SWH and SAH, the predicted results of the MPSWH and the MPSAH showed a good improvement on the thermal performance. These enhancements on the temperature may have been due to the new design adopted in the multipurpose solar heating systems by using riser tubes and headers to the original design of the thermal systems. Additionally, the thermal performance of solar collectors increases with increasing the mass flow rates and thermal conductivity of absorber plate. For validation aspect, the predicted results of all cases examined showed a good agreement against the measured results in terms of temperature distribution levels and thermal efficiencies

Computational fluid dynamic modelling of a 550 MW tangentially-fired furnace under different operating conditions

In the present paper, a computational fluid dynamics (CFD) modelling study was performed for the combustion of the brown coal in a large-scale tangentially-fired furnace (550 MW) under different operating conditions. The AVL Fire CFD code has been used to model the furnace. The mathematical models of coal combustion with the appropriate kinetic parameters were written and added to the code as user defined functions. It consists of pulverised coal (PC) devolatilization, char burnout, heat and mass transfer, and nitric oxide formation. The simulation of the PC combustion was carried out using multi-step reaction chemistry schemes. The level of confidence of this numerical model was based on the previous validations of the lignite combustion in a lab-scale furnace, as well as the validation parameters of the present furnace at the standard existing conditions in terms of temperature values and species concentrations. Performance of the boiler under different operating conditions was investigated, from which the effects of air and coal mass flow rates were considered at full load with different operating schemes of coal mills (out-of-service operations). The validated model was used to perform the following investigation parameters: furnace gas temperatures, species concentrations (CO and CO2), and velocity distributions. This study provides good information to optimize the operations of the utility tangentially-fired boiler with less emissions production

Effect of chemical reaction mechanisms and NOx modeling on air-fired and oxy-fuel combustion of lignite in a 100-kW furnace

In the present paper, a three-dimensional numerical investigation of pulverized dry lignite was undertaken, integrating the combustion of four different scenarios adopted experimentally in a 100-kW Chalmers laboratory-scale furnace. A hybrid unstructured grid computational fluid dynamics (CFD) code was used to model and analyze: an air-fired, oxy-fuel OF25 (25 vol % O2 concentration), oxy-fuel OF27 (27 vol % O2 concentration), and oxy-fuel OF29 (29 vol % O2 concentration). The appropriate mathematical models with the related kinetics parameters were implemented to calculate the temperature distributions, species concentrations (O2, CO2, CO, H2O, and H2), NOx emission concentrations, and the radiation heat transfer. The multistep chemical reaction mechanisms were conducted on the gas phase and solid phase of coal reaction in one-, two-, and three-step reaction schemes. The predicted results showed reasonably good agreement against the measured data for all combustion cases; however, in the three-step scheme, the results were highly improved, particularly in the flame envelope zone. For the NOx calculations, the obvious differences between the air-fired and oxy-fuel (OF27 and OF29) cases were evident. In the OF27 and OF29 cases, the expected increase in the flame temperatures and CO2 and H2O concentrations led to a slight increase in the radiative heat fluxes on the furnace wall, with respect to the air-fired case. As a continuation of improvement to the oxy-fuel combustion model, this numerical investigation might probably provide important information toward future modeling of a 550-MW, large-scale, brown coal oxyfuel tangentially fired furnace

Numerical study of one air-fired and two oxy-fuel combustion cases of propane in a 100 kW furnace

A computational fluid dynamics (CFD) modeling study has been carried out. The study involved gaseous fuel combustion with associated chemical reactions, radiative heat transfer, and turbulence. The three different combustion environments that were adopted experimentally in a 100 kW drop-tube firing unit were examined. One air-fired and two oxy-fuel-fired cases [21 vol % O2 for one combustion case (OF21) and 27 vol % O2 for the other combustion case (OF27)] were investigated. A swirl injection system was used to achieve the flame stability of the turbulent non-premixed combustible gases. A modified eddy breakup (EBU) model was used with appropriate empirical coefficients for propane combustion reactions. The irreversible single-step and reversible multi-step reaction mechanisms were considered. The overall agreement of the CFD results with the available measured data was reasonable. The data compared were the temperature distributions and the species concentrations (CO2, CO, and O2) at the most intensive combustion locations in the furnace. The luminous appearance and temperature levels of the OF27 flame were relatively close to the reference (air-fired) flame. This was due to a reduced volumetric flow rate and an increase in the O2 concentration in the gas mixture. The carbon dioxide concentrations for both oxy-fuel-fired scenarios were around 8 times higher than that of the air-fired combustion case. The results obtained with the multi-step chemistry mechanism showed improved agreement, particularly in the flame zone. The concentration of CO was lower in the OF21 case. The unburnt fuel in the air-fired and OF27 cases was less than that of the OF21 case because of the low oxygen concentration used in the latter combustion case. This study can provide a basis for the future investigation of combustion characteristics in a large-scale furnace under oxy-fuel-firing conditions

Numerical simulation of brown coal combustion in a 550 MW tangentially-fired furnace under different operating conditions

In the present paper, a computational fluid dynamics (CFD) modeling study was performed for the combustion of the brown coal in a large-scale tangentially-fired furnace (550 MW) under different operating conditions. The AVL Fire CFD code has been used to model the combustion processes. The mathematical models of coal combustion with the appropriate kinetic parameters were written and incorporated to the code as user defined functions. These models consist of pulverised coal (PC) devolatilization, char burnout, and heat and mass transfer. The simulation of the PC combustion was carried out using multi-step reaction chemistry mechanisms. The level of confidence of this numerical model was based on the previous validations of the lignite combustion in a lab-scale furnace, as well as the validation parameters of the present furnace at the standard existing conditions in terms of temperature values and species concentrations. Performance of the boiler under ten different operating conditions was investigated. The strategy of operation schemes for the first six combustion scenarios were based on the change of the out-of-service (turned off) burners under full load operation, while the rest cases were carried out at 20% lower and 20% higher loads than the standard operating conditions. The validated model was used to perform the following investigation parameters: furnace gas temperatures, species concentrations (O2, CO and CO2), velocity distributions, and char consumption. The predictions demonstrated that there are good temperature distributions in the furnace when the turned off burners are set in the opposite direction under full load operation. For higher aerodynamic effect, the numerical results showed improvements on the combustion characteristics in terms of species concentrations and char burnout rates in comparison with the standard operating case. The findings of this study provide good information to optimize the operations of the utility tangentially coal-fired boiler with less emission

CFD modelling of air-fired and oxy-fuel combustion of lignite in a 100 KW furnace

In this paper, a comprehensive computational fluid dynamics (CFD) modelling study was undertaken by integrating the combustion of pulverized dry lignite in several combustion environments. Four different cases were investigated: an air-fired and three different oxy-fuel combustion environments (25 vol.% O2 concentration (OF25), 27 vol.% O2 concentration (OF27), and 29 vol.% O2 concentration (OF29) were considered. The chemical reactions (devolatilization and char burnout), convective and radiative heat transfer, fluid and particle flow fields (homogenous and heterogenous processes), and turbulent models were employed in 3-D hybrid unstructured grid CFD simulations. The available experimental results from a lab-scale 100 KW firing lignite unit (Chalmer's furnace) were selected for the validation of these simulations. The aerodynamic effect of primary and secondary registers of the burner was included through swirl at the burner inlet in order to achieve the flame stability inside the furnace. Validation and comparison of all the combustion cases with the experimental data were made by using the temperature distribution profiles and species concentration (O2, CO2, and H2O) profiles at the most intense combustion locations of the furnace. The overall visualization of the flame temperature distributions and oxygen concentrations were presented in the upper part of the furnace. The numerical results showed that the flame temperature distributions and O2 consumptions of the OF25 case were approximately similar to the reference combustion case. In contrast, in the OF27 and OF29 combustion cases, the flame temperatures were higher and more confined in the closest region of the burner exit plane. This was a result of the quick consumption of oxygen that led to improve the ignition conditions in the latter combustion cases. Therefore, it is concluded that the resident time, stoichiometry, and recycled flue gas rates are relevant parameters to optimize the design of oxy-fuel furnaces. The findings showed reasonable agreement with the qualitative and quantitative measurements of temperature distribution profiles and species concentration profiles at the most intense combustion locations inside the furnace. These numerical results can provide useful information towards future modelling of the behaviour of pulverized brown coal in a large-scale oxy-fuel furnace/boiler in order to optimize the burner's and combustor's design

CFD modelling of air-fired and oxy-fuel combustion in a large-scale furnace at Loy Yang A brown coal power station

Oxy-fuel combustion technique is a viable option to reduce several types of greenhouse gases (GHGs) emissions from the pulverized coal (PC) combustion systems. In this paper, a computational fluid dynamics (CFD) modelling study has been developed in order to investigate the Victorian brown coal combustion in a 550 MW utility boiler under the air-fired (reference case) and three oxy-fuel-fired scenarios. The reference firing case was modelled based on the operating conditions of Loy Yang A power plant located in the state of Victoria, Australia. While Chalmers' oxy-fuel combustion approach was selected for the present oxy-fuel combustion simulations, which referred to as OF25 (25 vol.% O 2), OF27 (27 vol.% O 2), and OF29 (29 vol.% O 2). User-defined functions (UDFs) were written and incorporated into the CFD code to calculate the following mathematical models: the PC devolatilization, char burnout, multi-step chemical reactions, mass and heat transfer, carbon in fly-ash, and NO x formation/destruction. A level of confidence of the CFD model was achieved validating four different parameters of the conventional combustion case, as well as the previous preliminary CFD studies that conducted on a 100 kW unit firing propane and lignite under oxy-fuel combustion environments. The numerical results of OF29 combustion condition were considerably similar to the reference firing results in terms of gas temperature levels and radiative heat transfer relative to the OF25 and OF27 combustion cases. This similarity was due to increasing the residence time of PC in the combustion zone and O 2-enriched in feed oxidizer gases. A significant increase in the CO 2 concentrations and a noticeable decrease in the NO x formation were observed under all oxy-fuel combustion scenarios. The combustion chemistry was adopted in these investigations in order to capture the effects of O 2 concentrations and gas temperatures on the CO/CO 2 production rate and equilibrium between H 2 and H 2O in the combustion zone. Also, the use of O 2-enriched atmospheres during oxy-fuel-fired cases was slightly enhanced the carbon burnout rate. These predicted results were reasonably consistent with the experimental investigations and numerical modelling found in the literature. This study of Victorian brown coal oxy-fuel combustion in a large-scale tangentially-fired boiler is important prior to its implementation in real-life

Numerical investigation of pyrolysis of a Loy Yang coal in a lab-scale furnace at elevated pressures

A computational fluid dynamics (CFD) model of the pyrolysis of a Loy Yang low-rank coal in a pressurised drop tube furnace (pdtf) was undertaken evaluating Arrhenius reaction rate constants. The paper also presents predictions of an isothermal flow through the drop tube furnace. In this study, a pdtf reactor operated at pressures up to 15 bar and at a temperature of 1,173 K with particle heating rates of approximately 10(5) K s(-1) was used. The CFD model consists of two geometrical sections; flow straightner and injector. The single reaction and two competing reaction models were employed for this numerical investigation of the pyrolysis process. The results are validated against the available experimental data in terms of velocity profiles for the drop tube furnace and the particle mass loss versus particle residence times. The isothermal flow results showed reasonable agreement with the available experimental data at different locations from the injector tip. The predicted results of both the single reaction and competing reaction modes showed slightly different results. In addition, several reaction rate constants were tested and validated against the available experimental data. The most accurate results were being Badzioch and Hawksley (Ind Eng Chem Process Des Dev 9:521-530, 1970) with a single reaction model and Ubhayakar et al. (Symp (Int) Combust 16:427-436, 1977) for two competing reactions. These numerical results can provide useful information towards future modelling of the behaviour of Loy Yang coal in a full scale tangentially-fired furnace

Computational fluid dynamics modelling of chemistry reaction schemes in a lab-scale oxy-fuel furnace

This paper presents a three-dimensional numerical investigation of pulverized dry lignite in a 100 kW oxyfuel furnace. A computational fluid dynamics (CFD) code was used to model four different combustion scenarios. One air-fired combustion case and three oxy-fuel-fired cases, known as OF25 (25 vol. % O2 concentration), OF27 (27 vol. % O2 concentration), and OF29 (29 vol. % O2 concentration), were modelled. User-defined functions (UDFs) for the multistep reaction schemes were written and incorporated to the CFD code. Under oxy-fuel combustion, the appropriate mathematical models were implemented to calculate the flame temperature distributions and species concentrations (O2 and CO2). The multi-step chemical reaction schemes were used for the gas-phase and solid-phase coal particle reactions. In addition to the one-step (reference) reaction scheme, twostep and three-step reaction schemes were considered in this numerical study. Compared to the one-step and twostep reactions, the three-step reaction results showed a reasonably good agreement against the experiments for all combustion cases. This numerical investigation of the oxyfuel combustion scenarios might probably provide significant information towards modelling of large-scale oxy-fuel-fired coal tangentially furnaces/boilers