1. Introduction
Thermal power generation is the primary mode of power generation in China. The installed capacity and power generation of thermal power plants in China account for approximately 80% and 78.72% of the total installed capacity and power generation in the country, respectively [
1].
Thermal power plants consume large amounts of fresh water during the power generation process [
2]. It has been reported that the water consumption of thermal power plants accounts for 11% of the total industrial water consumption in the country, and the circulating cooling water system of thermal power plants itself accounts for 84% of the water consumption of thermal power plants [
3,
4]. Therefore, reducing the consumption of circulating cooling water is of great significance for realizing zero liquid discharge in power plants in accordance with the national zero liquid discharge policy. The amount of circulating cooling water used can be reduced by changing the water source and reducing freshwater consumption [
1]. For example, Yi et al. proposed using reclaimed water as circulating cooling water to reduce freshwater consumption and improve the utilization efficiency of water resources [
5]. Reductions in the amount of circulating cooling water used can also be realized by improving the concentration ratio and utilization rate of the circulating water. For example, in a study carried out by Rahmani et al., the authors demonstrated that increasing the concentration ratio of circulating water from 6.5 to 9 can reduce annual freshwater consumption by 1.1 × 10
6 m
3. Such an approach can greatly reduce the amount of water used for replenishing circulating water as well as that of wastewater discharge [
6].
Fouling is the first problem that needs to be solved to improve the concentration ratio of circulating water. Currently, fouling issues are primarily resolved by either adding chemical agents or using sulfuric acid as a scale inhibitor. Ochoa et al. experimentally studied the performance of a specific type of corrosion and scale inhibitor by adding these chemical inhibitors to a circulating water system [
7]; it is common to use scaling inhibitors like phosphates, polyphosphates, and organophosphonates, as well as corrosion inhibitors such as zinc sulphate and azoles [
8]. Rahmani et al. analyzed the scale-inhibiting effect and the associated working principle of adding sulfuric acid as a scale inhibitor in the fluid during the pretreatment process for circulating water replenishment [
8]. Sulfuric acid has normally been used for carbonate calcium scale control in cooling water systems, as acid treatment converts calcium bicarbonate to more stable and soluble calcium sulfate. Although these two methods can effectively prevent the formation of calcium carbonate scales, failing to remove calcium carbonate from the circulating water results in very high hardness in the wastewater generated in subsequent processes. Therefore, it becomes more challenging to fully treat wastewater in later stages. Adding lime or sodium carbonate directly is an effective way to soften water, which is a different method from those mentioned above and can reduce the hardness of water. However, this method will produce a lot of sludge, and the operation is complex and equipment fouling occurs easily. Thus, this pretreatment method is not suitable for softening of circulating water [
9,
10].
The chemical crystallization circulating pellet fluidized bed (CPFB) softening method is a highly efficient and environmentally friendly softening technology that can be used to reduce water hardness during the pretreatment process of the circulating water. For example, Hu et al. applied CPFBs to soften groundwater and determined the operating parameters under specific water quality conditions [
11]. They further studied the growth rate of the particles in Amsterdam’s crystallization fluidized bed reactor, as well as the relationship between the growth rate of the bed, rising flow rate, particle size, and supersaturation conditions on a pilot scale. A mathematical model for calculating the corresponding parameters is also provided in their study [
12]. Tang et al. applied chemical-crystallization CPFBs to treat wastewater in a thermal power plant and verified their water-softening effect [
13].
In summary, CPFBs can be used in the softening pretreatment of circulating water to remove calcium carbonate and prevent scale formation, as well as to simplify subsequent wastewater treatment processes. As an engineering case of circulating-water saving technology applied in a large-scale power plant, this paper provides important engineering data and information for the subsequent application of the proposed technology in industrial water circulation systems. In this study, the softening performance of CPFBs was studied under production testing conditions on the circulating water used in the Guohua Dingzhou Power Generation Co., Ltd., Hebei, China. Additionally, the impact of water quality was explored on improving the concentration ratio of circulating water after softening pretreatment under laboratory conditions.