Min Kuang

The dynamic behavior of high-speed craft navigating through variable sea states plays a pivotal role in ensuring maritime safety. However, many existing simulation approaches rely on linear or overly simplified representations of the marine environment, thereby limiting the fidelity of motion predictions. This study explores the motion characteristics of a 4.5-t high-speed vessel by conducting fully coupled numerical simulations using the STAR-CCM+ software. The analysis considers both calm and varying sea conditions, incorporating fluctuations in wave height, wavelength, and wind speed to reflect more realistic operating scenarios. Simulation results reveal that the vessel’s hydrodynamic response is highly sensitive to changes in sea state. As conditions deteriorate, the free surface becomes increasingly complex, with higher wave amplitudes and more pronounced interactions between the waves generated by the vessel and those imposed by the external environment. These effects lead to significant increases in roll, pitch, heave, and sway motions, thereby imposing greater demands on the vessel’s dynamic stability and operational safety. Furthermore, both hydrodynamic resistance and propulsive thrust exhibit notable dependence on sea state and vessel speed. Total resistance generally increases with rougher sea conditions, while thrust tends to rise with increasing forward speed. Under calm or mildly disturbed waters, a Froude number (Fr) of 0.5 appears to offer an optimal balance for initiating and controlling primary motions such as roll, pitch, heave, and sway. Conversely, in more challenging conditions—such as those represented by a Sea State 3—effective motion control is better achieved at a higher Froude number of approximately 1.0.

Within a 600 MW _e staged arch-firing furnace (SAF) having the staged hopper air (HA) and overfire air (OFA) as its last two air-staging layers, the present attention was focused on evaluating the impact of the HA:OFA distribution and determining an appropriate HA:OFA ratio used to improve the hopper environment and low-NO _x performance. Accordingly, by varying the HA:OFA distribution at a total air ratio of 31%, four HA:OFA ratio settings of 11:20, 13.5:17.5, 16:15, and 18.5:12.5 were formed to compare the in-furnace flow, coal combustion, and NO _x emissions. With increasing the HA flux to raise the HA:OFA ratio, all of combustion symmetry, the downward flame penetration, and the HA's combustion-aided effect on char firstly improved and then weakened, the OFA penetration became shallower, and the high-temperature flame in the upper furnace extended. Final performance indexes showed that both NO _x emissions and burnout loss first declined and then ascended, meaning a rise-to-drop trend in the low-NO _x performance as deepening the HA:OFA ratio. Among the four settings, the moderate HA:OFA = 13.5:17.5 achieved the optimal indexes with NO _x emission of 543 mg/m ³ (6% O ₂ ) and carbon in fly ash of 4.89%. Compared with its previous reference furnace (holding levels of 905 mg/m ³ and 5.1% respectively in the two index aspects), the SAF furnace using such a HA:OFA distribution mode reduced significantly NO _x by 40%, improved a little burnout, and dropped apparently the hopper temperatures by 500 K to eliminate its high overheating risk.

In today's call for environmental protection and sustainable development, it is urgent to solve the environmental pollution problems caused by industrial coal-fired boilers. Aiming at the ultra-low NO _x combustion against with poor burnout and hopper overheating of a 600 MW _e arch-firing furnace, this article developed the allocation scheme of hopper air (HA) as the third air-staging layer under a staged arch-firing framework (SAF). Investigations were devoted to the staged HA distribution between the upper and lower HA jets (called the UHA and LHA, respectively). The flow field, coal combustion, NO _x emissions, and hopper environment were compared comprehensively at five HA distribution setups of UHA:LHA = 4:6, 5:5, 6:4, 7:3, and 8:2. With increasing moderately UHA to raise the distribution ratio, the comprehensive benefits of the staged HA were obvious. That is, the UHA's function in promoting char combustion was strengthened and again, the roles of LHA to hold up the downward flame and protect hopper were guaranteed. The reductive atmosphere in the reburning zone was maintained at an optimal status added by the staged HA. While these positive improvements were reversed as raising excessively the distribution ratio. Trends of final performance indexes showed that with raising the distribution ratio, residual O ₂ and CO emission decreased in the whole, while both burnout loss in fly ash and NO output first descended and then ascended. Among the five setups, UHA:LHA = 6:4 gained the best low-NO _x combustion merits with NO _x output of 582 mg/m ³ (O ₂ = 6%) and unburnt matter in fly ash of 4.97%. Compared with its previous deep-air-staging low-NO _x furnace, the SAF incorporating with this optimal staged hopper-air distribution dropped further NO _x emissions by 36%, improved combustion, and raised burnout, thereby achieving the simultaneous NO _x reduction and burnout improvement. Again, the hopper environment was improved to be moderate-oxygen and low-temperature, alleviating the previous overheating problem to ensure the furnace's safe operations.

In response to the high NO _x emissions and hopper overheating problem of a 600-MW _e arch-fired furnace, a staged arch-firing framework (SAF) with an upper/lower staged hopper-air pattern was proposed to replace the original low-NO _x technique. With implementing such a staged hopper-air configuration to enhance air staging and reduce the downward flame penetration, it was required to (i) affirm the SAF's effectiveness related to the mentioned problems and (ii) assess the upper hopper-air (UHA) angle effect for obtaining a reasonable UHA angle. Accordingly, the in-furnace flow, combustion, and NO _x generation were respectively investigated under five UHA angles of θ  = 0°, 20°, 30°, 35°, and 40°. Meanwhile, real-furnace tests and simulations in the original furnace were conducted to make comparisons with the SAF furnace. As θ increased, the flame penetration progressively deepened. The primary hopper-air function in favoring char combustion and overall combustion performance first strengthened and then weakened, while the production of CO and NO and combustible matter in fly ash first dropped and then ascended. The setup with θ  = 35° demonstrated the most favorable low-NO _x performance, exhibiting NO _x levels of about 600 mg/m ³ (O ₂ = 6%) and combustible matter of about 5% in fly ash. A comparison before and after the SAF application indicates that, despite reducing downward flame penetration, the SAF bolstered primary combustion and increased air utilization to facilitate char combustion in the hopper and the burnout zone. Consequently, a further 33.5% NO _x reduction was achieved, along with slight burnout improvement and notably reduced hopper temperatures.

Upon the background of China's dual-carbon energy and environment strategies and the requirements of green and sustainable development in the new era, how to gradually reduce coal consumption while at the same time enhance the efficient and clean use of coal and reduce pollutant emissions is attracting more and more attention. For a 600-MW _e arch-fired furnace facing persistent challenges of high NO _x output and an overheating risk in hopper as firing anthracite, a cascade-arched low-NO _x and high-efficiency configuration (CLHC) was taken as an alternative to the existing multiple-injection and multiple-staging combustion technique (i.e., the MIMSCT, denoted as the reference furnace or technique in this study). In particular, along the furnace height the CLHC's overfire air (OFA) position in the burnout zone has an important influence on the low-NO _x performance due to the shrunk furnace-arch space and a short upper furnace. Aiming at evaluating the OFA-location effect and confirming the CLHC in resolving the above problems, industrial-scale experiments and modeling were performed in the reference furnace and thereafter, the low-NO _x characteristics with the CLHC was simulated considering three different OFA locations of the upper-furnace OFA, throat OFA, and arch OFA. In the OFA-location elevated order, the blending position of OFA and the main upward gas first lowered and then elevated, while the OFA penetration, overall combustion performance, and major low-NO _x accomplishment indexes related to NO _x yield and burnout loss initially improved but then deteriorated. As a result, the medium throat OFA presented the optimal low-NO _x merit among the three setups, with the unburnt combustible of 5.3% in fly ash alongside NO _x yield of 660 mg/m ³ (O ₂ = 6%), respectively. By comparison to the reference technique, the CLHC gained a 30% NO _x reduction ratio without affecting burnout and greatly relieved the hopper overheating issue via reducing sharply its temperatures by 400 K, thereby confirming the CLHC's viability. This study provided guidance on the safe furnace operations and reduction of pollutant emissions, benefiting the efficient and environmentally friendly usage of low-quality coals in industrial-scale furnaces.

To address the persistently high NO _x production and the heightened overheating risk in the hopper of a 600-MW _e , deep-air-staging, arch-fired boiler furnace (i.e., the reference furnace), a solution was devised with a staged arch-firing framework (SAF) and flue gas recirculation (FGR). This required establishing an appropriate position for the burner-FGR and confirming the viability of the SAF for the furnace. Comprehensive industrial-scale physical tests and computer simulations were conducted using the reference furnace. Subsequently, the furnace with SAF was examined with FGR introduced sequentially, first using a fuel-rich mixture, then using an inner secondary-air flow, and finally using an outer secondary-air flow (i.e., denoted in turn as FGR-FR, FGR-IS, and FGR-OS). Given this FGR-location order, the FGR’s functions about combustion degradation and NO inhibition weakened, resulting in increased NO _x emissions and continuously decreasing burnout loss. Considering the satisfactory burnout levels across all three configurations, the FGR-FR configuration demonstrated the best reduction in NO _x emissions, achieving NO _x output of about 600 mg/m ³ (O ₂ = 6%) and an unburnt combustible rate in fly ash of about 5%. Comparing the conditions before and after implementing the SAF indicated that the SAF enhanced combustion intensity and improved the utilisation of overfire air and hopper air, resulting in a further 33.3% reduction in NO _x emissions without compromising burnout efficiency. Additionally, the SAF effectively mitigated the overheating risk in the hopper by significantly lowering local temperature levels by 400 K.

In order to establish an efficient air‐regulating solution as varying overfire air (OFA) and recommend an appropriate OFA ratio for a 600‐MW _e W‐shaped flame furnace equipped with the proposed cascade‐arch‐firing low‐NO _x and high‐burnout configuration (CLHC), four air‐regulating solutions were established by respectively adjusting the air into secondary air, tertiary air, and hopper air or proportionally varying the air into the three jets in a uniform distribution form, which were labeled in turn as SA, TA, HA, and UD, respectively. After the efficient air‐regulating solution fixed, four OFA ratios of 15%, 17.5%, 20%, and 22.5% were evaluated. Among the four air‐regulating solutions, only UD develops symmetrical combustion. An order of TA < HA < SA < UD in the final performance indexes enables UD as the efficient air‐regulating solution as varying OFA. A symmetrical flow field and combustion pattern develop at the former three OFA ratios. The excessive OFA ratio of 22.5% achieves a severely deflected flow field and asymmetric combustion. Increasing the OFA ratio generates a decrease‐to‐increase trend in burnout and a continuous decrease trend in NO _x emissions. Consequently, the 20% setting is preferred as an appropriate OFA ratio, showing the best low‐NO _x and high‐burnout accomplishment. The improvement in both burnout and NO _x emissions as opening OFA to deepen air staging is attributed to the improved symmetrical combustion and the strengthened OFA penetration in the upper furnace.