Optimisation of Solid Fuel In-furnace Blending for an Opposed-firing Utility Boiler: A Numerical Analysis

Abstract

Continuous research on the clean and effective use of coal is still necessary as coal will continue to play a key role in global energy supply for the foreseeable future. Hence, in the current study, the optimisation of in-furnace coal blending for one of Malaysia's opposed-firing utility boilers was numerically executed on the basis of hydrodynamics and combustion characteristics. The predicted FEGT from the numerical model was validated against the actual FEGT from the coal-fired power plant, revealing a difference of less than 10 %. Four (4) coal blended cases were tested, which included both bituminous (bit) and sub-bituminous (sub-bit) coals. The findings demonstrate that due to the difference in density between bit and sub-bit coals, the hydrodynamic performance is predicted to significantly improve when sub-bit coal is injected at the bottom burner as opposed to the upper burner. In terms of kinetics, the higher volatile matter (VM) of sub-bit coal in contrast to bit coal has been postulated to release a substantial amount of volatiles and improve the combustibility of bit coal. Furthermore, enhanced oxygen release from sub-bit coal volatiles can aggravate the gas-solid heterogeneous reaction during bit coal char combustion. As a result of the bottom burner's high temperature, it has been discovered that introducing sub-bit coals into those burners speeds up VM release and char combustion, which increases the rate of combustion. Thus, when combustibility rises, the peak temperature position moves downward, reducing the likelihood of delayed combustion and, consequently, the risk of heat exchanger pendant failure and ash deposition. In a furnace with a relatively long coal residence time, a considerable fraction (>20 %) of high gross calorific value (GCV) sub-bit coal (>5800 kcal/kg) is predicted to produce two peak flame temperatures exceeding 1600°C owing to the likelihood of enhanced char which created delayed combustion. Therefore, a furnace condition with a comparatively shorter coal residence time may aid in the rapid evacuation of residual char from the combustion/burner zone and minimise the potential for delayed combustion. Nonetheless, residual char escape may exacerbate the emission problem by releasing considerable unburned carbon. Overall, the current numerical model has the potential to be a reliable and cost-effective tool for investigating the combustion characteristics of coal blends in a power plant boiler.

Oil Palm Wastes Co-firing in an Opposed Firing 500 MW Utility Boiler: A Numerical Analysis

“…The detailed chemical reaction modelling schemes of solid fuel combustion used in this research work are similar to those demonstrated in our previous research [35][36]. The chemical kinetics and combustion models used in this study account for the three main stages of solid fuel combustion: devolatilisation, char conversion/reaction, and volatiles reactions.…”

“…The three-dimensional (3D) configuration of a coal-fired boiler as shown in Figure 1 was created using as-built dimensions from the actual coal-fired power plant under consideration. The boiler has a wall-firing design with 36 coal burners that are also the primary air (PA) inlets, with 36 secondary air (SA) inlets circumferentially positioned at each burner-PA inlet to enable better mixing of the incoming coal and air, as well as to provide a dry low NOx region closer to the burner area where the incoming SA creates a recirculation zone for the incoming coal and air [35][36]. Each side of a boiler (front and rear walls) has each 18 burner-PA inlets and 18 SA inlets located at three different heights at a burner zone due to the wall-firing layout.…”

“…Each side of a boiler (front and rear walls) has each 18 burner-PA inlets and 18 SA inlets located at three different heights at a burner zone due to the wall-firing layout. To reduce computational costs, the tube bundles of superheaters and reheaters were simplified to a few thin walls [35][36]. The accuracy of computational works considered, heat transfer models' implementation to thin walls to replicate the heat transport process between the flue gas and the steam within the heat exchanger bundles.…”

“…Mills 1-6, as shown in Figure 1, supply coal and PA for each burner-PA inlet elevation (A to F). In a common utility coal-fired boiler framework, the coal-fired boiler integrates the piping structure and the furnace as one system, with all mills connected to discharge pipes that deliver coal particles and PA into the boiler [35][36]. However, in order to save computational costs, the piping structure is not part of the computational domain for this study.…”

“…To assure that the spatial convergence accuracy is sufficient, the grid independent test is performed. Meshes (elements) are built with orthogonal quality and skewness in mind to reflect mesh attributes that influence the level of spatial discretisation errors [35][36]. To ensure that acceptable mesh qualities could be constructed, the orthogonal and skewness features of all generated meshes in the grid independent test were controlled.…”