Sangmin Choi

The combustion of simulated waste particles in a fixed bed

Abstract Stable combustion of waste in an incinerator is necessary for optimal operation. However, complicated reactions, as well as heat and mass transfer, make understanding the process difficult. An experimental bed reactor is utilized to investigate the combustion of simulated waste particles, and a computational model is introduced to predict the phenomena. When the bed is brought into a radiating environment, an apparent flame zone is formed at the bed’s top surface after a certain time delay. Subsequently, the flame front moves downwards into the bed of fuel. The flame propagation speed, i.e., the rate of progress of the apparent flame zone, is dependent on the air supply rate, the calorific value of the solid fuel, and the particle size. Based on the availability of oxygen, two distinct reaction modes are identified: the oxygen-limited mode and the reaction-limited mode. An increase in the flow rate of air eventually leads to flame extinction, as a result of excessive cooling by convection. The numerical results show good agreement with the experimental observations. The transient behavior of the local temperature and the rate of oxygen consumption are adequately reproduced. The effect of the combustion parameters on combustion in the bed is also discussed further.

Combined Simulation of Combustion and Gas Flow in a Grate-Type Incinerator

Abstract Computational fluid dynamic (CFD) analysis of the thermal flow in the combustion chamber of a solid waste incinerator provides crucial insight into the incinerator’s performance. However, the interrelation of the gas flow with the burning waste has not been adequately treated in many CFD models. A strategy for a combined simulation of the waste combustion and the gas flow in the furnace is introduced here. When coupled with CFD, a model of the waste combustion in the bed provides the inlet conditions for the gas flow field and receives the radiative heat flux onto the bed from the furnace wall and gaseous species. An unsteady one-dimensional bed model was used for the test simulation, in which the moving bed was treated as a packed bed of homogeneous fuel particles. The simulation results show the physical processes of the waste combustion and its interaction with the gas flow for various operational parameters.

Comparative evaluation of municipal solid waste incinerator designs by flow simulation

Flow simulations have been carried out to evaluate the effects of combustion chamber design and air/combustion gas flow configuration on the overall performance of municipal solid waste incinerators. Computational results show velocity and temperature fields in the entire region of flow passage. Local recirculations and uneven distributions of flow velocity and temperature should be minimized and mixing is to be enhanced. Two parameters are proposed to help quantify the overall flow condition. The degree of mixing of different species, which enter the incinerator from the air and combustion gas inlets, is represented by the mixing parameter α. Here, α is calculated on the nodal points. The probability distribution of α in the entire computational domain is used for comparative evaluation of incinerator designs. The thermal decomposition parameter β is calculated by integrating the kinetic rates along the trajectory of a fluid element. This parameter represents the portion of the unreacted materials among the total pollutants released from the bed. By employing these parameters, various incinerator design alternatives can be quantitatively analyzed from two principal viewpoints, i.e., the effectiveness in mixing and the thermal decomposition of pollutants.

Pressurized drop tube furnace tests of global coal gasification characteristics

Pressurized drop tube furnace (PDTF) tests were performed with an Indonesian sub-bituminous coal while temperature, oxygen/coal ratio, steam/coal ratio and pressure were systematically varied. The tests were designed to investigate the effects of these experimental parameters on the pulverized coal gasification characteristics at elevated pressure. The results showed that the gasification at elevated pressure is more productive than that at atmospheric pressure, considering the carbon conversion and cold gas efficiency. The oxygen/coal ratio at the maximum cold gas efficiency ranged between 0.5 and 0.7 g/g. Only when the temperature was sufficiently high, did the increase of steam/coal ratio result in the improvement of cold gas efficiency. As the pressure increased, the contribution of carbon conversion by heterogeneous reactions increased while the conversion by pyrolysis decreased. Copyright

Effect of fuel layer mixing in waste bed combustion

Abstract A computational model was introduced to study the effect of fuel mixing in waste bed combustion in municipal solid waste incinerators. One-dimensional progress of the apparent flame zone into the bed was simulated by accommodating the heat and mass transfer, along with heterogeneous combustion and gaseous reactions in the waste bed. Mixing of the waste bed was modeled as an exchange of certain sections in the fuel layer from a cold region over a flame region. Combustion characteristics of the waste bed based on the model prediction are discussed, along with the experimental measurement results from a batch-type bed reactor. The computational model adequately described the heat transfer of the unburned fuel section over the existing flame zone. Improvement of the combustion intensity by fuel mixing was observed, as evidenced by a decrease in the required time for complete burn-out. Control of the primary air supply could further enhance the waste combustion, which is a common practice in commercial incinerators.

Mathematical model of thermal processes in an iron ore sintering bed

An unsteady one-dimensional model of an iron ore sintering bed with multiple solid phases was proposed. The proposed model confers a phase on each solid material. The present model was established with a series of conservation equations in the form of a partial differential equation for each solid phase and gas phase. Coke combustion, limestone decomposition, gaseous reaction, heat transfers in/between each phase, and geometric changes of the solid particles are reflected by each term of the governing equations. Simulation results are compared with the limited experimental data set of sintering pot tests. Parametric studies for various initial water contents and coke diameters have also been performed. The simulation results predict the experimental results well and show physically reasonable trends for various parameters.

Computational Fluid Dynamics Evaluation of Good Combustion Performance in Waste Incinerators

ABSTRACT Combustion control techniques have become a legal requirement to minimize pollution in municipal solid waste incinerators. In-furnace destruction of pollutants is achieved when 2-second gas residence time at 850 oC and 6% O2 are guaranteed. Incinerator performance is analyzed numerically to validate good combustion performance. Computational Fluid Dynamics (CFD) modeling of gas flow inside the furnace chamber provides three-dimensional temperature, concentration, and velocity vectors. General flow patterns and the presence of recirculation pockets are traditionally observed. Local temperature and oxygen concentration can also be checked. The CFD results are analyzed further in terms of residence time, mixing, and thermal decomposition of potential pollutants. The residence time needs to be carefully determined based on the gas inlet position. The statistical variation requires evaluation of the average and minimum (or shortest) residence time. Mixing is quantified by defining a local mixedness pa...

Unsteady one-dimensional model for a bed combustion of solid fuels

Abstract A transient one-dimensional combustion model of a solid fuel bed that can be applied to various types of combustors employing the solid bed is suggested. Configuration of solid beds can be characterized by combinations of solid and gas flow directions, ignition methods, and dominant reactions. These features can be reflected numerically to the boundary and initial conditions of the model, while the governing mechanism in each type of bed is similar. The mechanism is expressed as forms of conservation equations for the solid and the gas phase. Solid-gas reactions, gaseous reactions, heat transfer, geometrical changes of the particles, and physical properties are modelled and reflected to each term of the governing equations. Simulation results are shown for two cases: a packed bed combustor and an iron ore sintering bed. Two cases have distinctively different bed configurations and ignition methods. The propagation rates of the combustion zone are quantitatively discussed in terms of the flame propagation speed. Future development of the model is discussed for possible application to more complicated simulation cases.

Application of Intra-Particle Combustion Model for Iron Ore Sintering Bed

In order to quantitatively predict the behavior of the material in the packed bed, a single particle model is developed to describe the combustion and sintering process inside an individual particle composed of multiple solid material fines, including iron ore, coke and limestone, and is applied to the combustion modeling of an iron ore sintering. By analyzing three typical fuel distribution cases using the developed single particle combustion model, the effects of temperature and oxygen concentration gradient inside the particle on heat and mass transfer and the combustion behavior of the iron ore sintering process are investigated. Considering the various combustion rates which are highly dependent on the fuel distribution methods, correction factor for single particle model is also introduced and systematically analyzed. The aim of this research is to supplement particle technology to conventional approach and it is found that the oxygen concentration gradient inside the particle is significantly affected from the mixing method thereby changing the completion times of sintering process.

Cold-flow simulation of municipal waste incinerators

Configurations of combustion chamber and air distribution are major parameters that influence theperformance of incinerators. Cold-flow simulations have been performed experimentally and numerically for selected designs of grate-type municipal waste incinerators. Counterflow and parallel-flow arrangements are considered for the combustion chamber configuration. Two-dimensional water table models of 1/20 scale were made for cold-flow tests, and numerical models have been developed. Flow visualizations using water table flow models and computational results using the numerical model are in good agreement. Velocity vector fields and concentrations of tracer gases are evaluated, and the effects of combustion chamber geometry, amount of overfire air, and jet direction have been investigated. Mixing characteristics in the combustion chamber are examined by adopting a newly defined mixing parameter, which has value of zero when gases are mixed completely and whose departure from zero indicates the degree of nonmixing. In representing the degree of mixing over the entire chamber, probability density function of this mixing parameter is more adequate. It is useful in comparative assessments of incinerator chambers and flow designs. Interpretation of flow field predictions in terms of residence time of combustion gases and velocity uniformity is also presented. It is suggested that understanding of local flow characteristics should be preceded in assessing the incinerator performance.