Combustion Modeling In The Riser Of Fluidized Bed Combustor For Thar Coal And Its Blends For Emission Control

Abstract

Pakistan has faced a prolonged shortage of energy for the past several years. However, Pakistan is blessed with enormous coal reserves (185 billion tonnes), it is needed to utilize the latest technology for coal combustion. The research study aims to evaluate the physicochemical characterization of low-rank Thar coal to understand the combustion behavior and model the combustion performance in a circulating fluidized bed riser by using ANSYS FLUENT software. Computational fluid dynamics (CFD) analysis of Thar coal has been done to minimize emissions and improve combustion efficiency. This study also examines the physicochemical characteristics of rice husk biomass and its blends with Thar lignite coal for combustion and determines the influence of these blends’ proportions on pollution. Thermo-chemical characteristics of Thar coal were determined from a large number of Thar coal samples from Block II, and pyrolysis properties were determined with the chemical configuration of coal ashes as per standard methods. Using the Thermogravimetric (TGA) analysis, the pyrolytic heating rate and temperature were established, which have a significant effect on the pyrolysis of Thar coal. Kinetic constraints (frequency factor and activation energy) were obtained by curve-fitting the TGA data. Using these kinetic constraints, a one-step global model was used to forecast the pyrolytic transformation. In the co-combustion, it was found that rice husk contains a higher content of volatile matter, lowest in moisture and sulfur, and has higher ash contents. The blends of coal with rice husk in weight fractions of 90:10 (CRh-1), 80:20 (CRh-2), and 70:30 (CRh-3) were also characterized. The analysis revealed that 70:30 (CRh-3) contained the lowest elemental sulfur, NOx, and SOx emissions. A CFD model was developed to simulate the hydrodynamics of gas-solid flow in a circulating fluidized bed riser using the ANSYS FLUENT software. The effect of several exit shapes of the riser was studied using a mathematical parametric investigation of the two-phase gas-solid stream hydrodynamics of a CFB riser. The CFD model for the gas segment and the viscosity of static particles in the solids segment with a k-e turbulence model displayed virtuous mixing performance. These outcomes were found to be beneficial for the additional progress of gas-solid flow modeling in the riser. For combustion modeling, the FLIC code was found to be precise in simulating coal bed combustion, and the FLIC code's outcomes were imported into the FLUENT database. The maximum temperature inside the compartment, according to the FLUENT results, was around 1440K (1166oC), at the primary burning sector in the bed center. The peak value in the center-oriented riser/combustor was 3.3 m/s, as determined from velocity contours. The CO and CO2 mass fraction contours showed that it is concentrated in the center geometry, and a lower CO concentration was found in the parallel geometry. The contours indicate the amount of NOx at the highest level of around 31 ppm, while the parallel geometry establishes the lowest level at around 15 ppm. The Circulating Fluidized Bed Combustor is found to be the most advantageous and effective technology for producing power from Thar lignite coal while simultaneously reducing SOx and NOx emissions.