• Energy Research

Abstract A natural draft dry cooling tower (NDDCT) is demanded to save water for power generation in arid area, but its performance could be degraded greatly by the crosswind. To overcome the degradation, this paper proposes the installation of a windbox, which consists of windbreaks, an enclosure, a back barricade and a wind top to improve the pressure distribution outside the heat exchanger bundle of a NDDCT, so as to increase the ventilation rate under crosswind condition. Full dimensional computational fluid dynamics (CFD) modelling was conducted for the windbox installed around the NDDCT of a large scale coal-fired power plant. Different configurations of the windbox were studied. The flow characteristics along the streamline in the NDDCT were analyzed and quantified by adopting the concept of flow loss factor (FLF). Based on simulation and experiments, the installation of the windbox is shown to be effective. The windbox with a 120 m radius enclosure and a full size louver-type top could improve the ventilation rate of a NDDCT by ∼60% in gale crosswind condition, and keep high performance in breeze crosswind condition. Consequently, an annual reduction of coal consumption of 30,000–45,000 tons could be achieved on a 1000 MW unit, which is ∼3000,000–4500,000 $/a.

Abstract Swirl burner is widely used in coal-fired boilers. Coal particle size is an important factor affecting the two-phase flow as well as the combustion performances of a swirl burner. A bench-scale experiment with CFD simulation was conducted with full burner complexity. The results show that asymmetry of the particle phase is gradually weakened as the particle size increases, opposite to the variation trend of the continuous flow field in a hot state. The asymmetry of the particle phase is much more affected by the asymmetry of the secondary air jet flow than that of the primary air jet flow. As the particle size increases within a certain range, the combustibility of pulverized coal decreases, which enhances the asymmetry of the continuous flow field. When the particle size increases more than 50 μm, an increase in the concentration of incomplete combustion particles in the side recirculation zone around the burner occurs, intensifying the tendency of slagging. Thus, controlling the pulverized coal particle size below 50 μm is a considerable choice to improve combustion efficiency and avoid slagging.

Abstract The natural draft dry cooling tower (NDDCT) is a critical facility for an indirect dry cooling power plant in arid area for its merit of excellent water-saving. While crosswind degrades the performance of a NDDCT by changing the flow field inside and outside. In order to quantitatively study the influence of different flow characteristics on the performance of a NDDCT, hence to grasp the affecting mechanism of crosswind, a half-cylindrical computational fluid dynamics (CFD) model of a Heller type 660 MW NDDCT is developed and validated by a hot state modelling test rig. A flow loss factor (FLF) is derived and verified to linearly describe the effect of local flow field changing on the overall performance of a NDDCT. Based on the conjoint studies of the local FLF variation trends and the changing processes of correspondent flow characteristics in each specific flow segments, the critical factors influencing the performance of a NDDCT are identified under different crosswind conditions.

Abstract With the wide utilization of a natural draft dry cooling tower (NDDCT) in power generation in arid areas, the degradation of its performance under crosswind conditions is increasingly concerned. Based on the influencing mechanisms of the crosswind, the paper reconstructs the destructed inlet flow field with a labyrinth structure. The effect of the labyrinth structure is firstly assessed by means of a verified computational fluid dynamics (CFD) model. Then, on the basis of CFD results, the labyrinth structure is further optimised by adopting a quantification method using a flow loss factor (FLF). Numerical results revealed that the proposed flow field reconstruction approach could increase the ventilation rate of a NDDCT by ∼62% under high speed crosswind condition, correspondently reducing the overall coal consumption by 23,100–33,000 t annually for a 660 MW coal-fired unit. Moreover, the negative effect of the crosswind on the performance of a NDDCT could be reversed to a positive one. The numerical results are well validated by the modelling experiments conducted in a wind tunnel.

To ensure carbon neutrality goal, coal-fired power plants will have to take responsibility of deep peak regulation, which is essential to the development of renewable energy power. The penalty for deep peak-regulating operation of coal-fired power units could be more than 40 g/kW h of the net coal consumption rate (NCCR), which is contracted to the adoption of renewable energy power. Hence, in order to identify promising energy saving directions, a distribution of exergy losses in thermodynamic systems of several typical large coal-fired power units is studied in this work. For reference, systematic investigations into related conventional energy saving technologies are conducted, which consider thermodynamic interactions among the components. As a result, three novel technologies are proposed to minimize the exergy loss of the units on the deep peak-shaving condition. Based on the performed feasibility study of these technologies, it is found that: (1) using novel steam governing method on the part-load conditions offers ∼5.5 g/kW h energy saving potential; (2) in the winter season, reducing the back pressure of a dry cooling unit from 15 to 4 kPa at partial load can reduce the NCCR by ∼15 g/kW h; and (3) adopting the efficient heat-supply technology based on pressure matching would provide a NCCR reduction potential of ∼6.2 g/kW h. These new solutions, due to their energy saving effect, positive environmental impact, and cost-effectiveness, forge a synergetic development path of coal-fired and renewable energy power generations under the carbon neutrality target.

In this study, the steam reforming of naphthalene and pyrene as heavy tar model compounds was investigated experimentally in a horizontal tube reactor and theoretically using the CHEMKIN simulation. The experimental results revealed that the reactivity of naphthalene was higher than that of pyrene in the presence of steam. The carbon content converted to soot is a little more than that converted to light gas during tar steam reforming. The kinetic parameters of the overall reaction were determined, and the pre-exponential factor and activation energy were calculated using the experimental data. The comparison of the numerical simulation with the experimental findings exhibited an excellent agreement for the prediction of the light gas products and soot after eliminating the influence of the water–gas shift. Further, the reaction schemes including the reaction pathway and associated kinetics were determined for the steam reforming of these model compounds. Both naphthalene and pyrene exhibited a similar performance during the reaction in the presence of steam. Benzene and naphthalene, representing the precursors of the light gas product, were confirmed to be the dominant intermediate components of naphthalene and pyrene, respectively. The consecutive reactions of these intermediates subsequently resulted in the generation of the light gaseous products.

Abstract Crosswind degrades the cooling performance of a natural draft dry cooling tower (NDDCT) by affecting the air flow field at the inlet and outlet and inducing complex vortices inside and outside the tower. The distribution of the vortices along the flow streams is found to be a key factor for the ventilation rate. The parameter of flow loss factor (FLF) is proposed to quantitatively identify the effect of the vortices and unbalanced flow on the ventilation rate. Approaches of the installations of windbreaks and enclosure on the cooling performance of the NDDCT are numerically studied. It is found that both approaches can individually reduce the size of the inner wall vortex, improving the flow field characteristics. However, they have different strengths in breaking up the side low pressure areas and reducing the swirling intensity of the mainstream vortices. Results show that the approaches of windbreaks and enclosure can effectively prevent the degradation of the cooling performance for the NDDCT in a wide crosswind velocity range, and their combination could nearly eliminates the negative effect of the crosswind.