Desulfurization characteristics of slaked lime and regulation optimization of circulating fluidized bed flue gas desulfurization process—A combined experimental and numerical simulation study

Abstract

Circulating fluidized bed flue gas desulfurization (CFB-FGD) process has been widely applied in recent years. However, high cost caused by the use of high-quality slaked lime and difficult operation due to the complex flow field are two issues which have received great attention. Accordingly, a laboratory-scale fluidized bed reactor was constructed to investigate the effects of physical properties and external conditions on desulfurization performance of slaked lime, and the conclusions were tried out in an industrial-scale CFB-FGD tower. After that, a numerical model of the tower was established based on computational particle fluid dynamics (CPFD) and two-film theory. After comparison and validation with actual operation data, the effects of operating parameters on gas-solid distribution and desulfurization characteristics were investigated. The results of experiments and industrial trials showed that the use of slaked lime with a calcium hydroxide content of approximately 80% and particle size greater than 40 mm could significantly reduce the cost of desulfurizer. Simulation results showed that the flow field in the desulfurization tower was skewed under the influence of circulating ash. We obtained optimal operating conditions of 7.5 kg·s−1 for the atomized water flow, 70 kg·s−1 for circulating ash flow, and 0.56 kg·s−1 for slaked lime flow, with desulfurization efficiency reaching 98.19% and the exit flue gas meeting the ultraclean emission and safety requirements. All parameters selected in the simulation were based on engineering examples and had certain application reference significance.

1. Introduction

Circulating fluidized bed flue gas desulfurization (CFB-FGD) has been widely applied and developed both domestically and internationally for decades. Compared with the more widespread wet FGD technology, CFB-FGD offers the advantages of zero wastewater discharge or gypsum rain in the exit flue gas [1,2]. However, the desulfurization efficiency of CFB-FGD is relatively low [3], and the procedure is considered to be generally applicable to flue gas with low SO2 content, such as sintering flue gas. Many iron and steel mills have lowered the quality of sintered ore to reduce costs, which results in increased SO2 concentration. To achieve emission standards, it is common practice to directly increase the amount of slaked lime as well as use slaked lime with high calcium content; however, this leads to a significant increase in the cost of desulfurization.

To minimize this cost, many studies have focused on finding cheaper alternatives to desulfurizers such as limestone, magnesium hydroxide, and concrete pile sludge [4-6]. However, their desulfurization effect cannot be guaranteed under increasingly stringent emission standards and often requires medium particle actuation [7] and a large amount of engineering modifications. In contrast, industrial wastes, such as calcium carbide slag, fly ash, calcined slag, etc., modified and treated for semi-dry desulfurization have been reported to exhibit favorable desulfurization results [8-10]. There are also studies on adding additives to desulfurizers or improving the preparation methods of desulfurizers [11,12]. While the above strategies can be effective in reducing costs, the source of waste slag is not extensive, its composition is complex, and the effective desulfurization ingredients fluctuate depending on the process. Consequently, the risk of substandard flue gas emissions increases, and the generated desulfurization products become difficult to handle.

Previous studies have ignored the possibility of reducing the cost of slaked lime. This cost depends primarily on the calcium hydroxide content, with higher content requiring a higher unit price. Taking data from a steel plant as an example, a decrease in calcium hydroxide content from 92% to 82% resulted in a decrease in price of nearly 31% per ton. The use of low-quality slaked lime implies the need to increase the desulfurizer consumption under the same loading conditions; however, the relationship between the cost savings and the increased consumption has not been systematically investigated. This study aims to fill that gap by experimentally and numerically analyzing the desulfurization characteristics of slaked lime with varying properties and optimizing the CFB-FGD process parameters to achieve both cost reduction and high efficiency.