Toward Stable Operation of Coal Combustion Plants: The Use of Alumina Nanoparticles To Prevent Adhesion of Fly Ash

Abstract

In pulverized coal combustion plants, low-grade sub-bituminous coal is known to cause adhesion of fly ash and its growth on the surface of the super heater, heat exchanger, and furnace walls, even when cocombusted with high-grade bituminous coal. In this study, six different fly ash samples collected from commercial pulverized coal combustion plants were investigated to find if the porosity and the liquid phase would have a significant impact on the tensile strength of the ash powder beds. Herein, we demonstrate that the use of alumina (Al2O3) nanoparticles as an additive is highly effective in preventing such troublesome events, which would be a great aid to facilitate stable and long-term operation of coal combustion plants. The function of Al2O3 nanoparticles is found to be 2-fold: retaining the porosity and perturbing the chemical composition of the ash samples. This is rationally supported by experimental and theoretical results. The key to our study is a split-type tensile strength tester that can directly measure the tensile strength of a powder bed being studied at various temperatures.

Role of Phosphorus and Iron in Particle Adhesiveness at High Temperatures Using Synthetic Ashes

“…Thus, porosity of the powder beds was increased in the presence of nanoparticles with poor packing ability. This effect had been demonstrated in previous studies. ,,, A previous study demonstrated that the porosity increase upon addition of nanoparticles induced a decrease in the tensile strength of a cohesive powder bed at high temperatures. ,, This phenomenon can be explained by the following Rumpf theory equation: where σ is tensile strength, ε is porosity, x is particle size, and F is attractive force between particles (mainly liquid bridge forces) at high temperatures. The degree of the decrease in tensile strength induced by the porosity increase in the Fe-added P-SiO 2 system was estimated by the theory (calculation details are shown in the Supporting Information).…”

“…In contrast, no significant difference in porosity of the powder beds of the synthetic ashes was observed. Previously, the porosity of powder beds affected the tensile strength at high temperatures, which made the mechanism of adhesion induced by chemicals difficult to understand. , In the present research with synthetic ashes, a system was constructed to determine only the effect of chemicals (presence of P) on the increase of adhesiveness.…”

“…Previously, the porosity of powder beds affected the tensile strength at high temperatures, which made the mechanism of adhesion induced by chemicals difficult to understand. 39,41 In the present research with synthetic ashes, a system was constructed to determine only the effect of chemicals (presence of P) on the increase of adhesiveness. Tensile strengths of the synthetic ashes were measured at 900 °C (Figure 2), and the proportional relation between concentration of P and tensile strength was determined.…”

“…A device to evaluate the adhesiveness of particles at high temperatures was developed previously and could measure the adhesiveness of many types of particulate materials, including combustion and incineration ashes. − An analysis of various ashes revealed that P, Na, and K in ashes increased adhesiveness at high temperatures. − For sewage sludge incinerated ashes, which contained P at high concentrations, adhesiveness began to increase near 600 °C, a temperature that is lower than that of coal ashes. , This observation indicated that the adhesiveness of ashes that contain P easily adhere inside the incineration machinery. These provided a promising method for quantifying the adhesiveness of P-containing ashes at high temperatures.…”

Experimental investigation of Zhundong high‐calcium coal ash adhesion characteristics on metal surfaces at high temperatures

“…Many studies had shown that the ash adhesion strength was mainly caused by the liquid phase produced by partial particle melting. [27][28][29] The liquid phase connects the particles with other solid surfaces (heating surface or other particles) to generate liquid bridging force and promote the adhesion strength of ash. In order to discuss the particle melting behavior at different temperatures, a HSML was used to test and obtain the sample shrinkage curves at high temperatures.…”