Research on desulfurization wastewater evaporation: Present and future perspectives
Introduction
Chinese energy structure dominated by coal will not be changed in short term. With the improvement of domestic economy, the electricity demand grows rapidly, thus leading to the increase in the coal construction of power plants, which increases the emissions of dust and SO2. As one of main precursors of acid rain, excessive SO2 has caused severe acid precipitating problem and smog in many areas of China. Therefore, Chinese government has made it particularly strict for flue gas treatment in coal-fired power plants in recent years. The power plants are facing more stringent environmental restrictions, especially after the implementation of new “air pollutant emission standards in coal-fired power plant” (GB13223-2011) in 2012 [1].
At present, in most coal-fired power plants in China, wet limestone–gypsum flue gas desulfurization (FGD) is most widely used. This is a mature technology with high desulfurization efficiency and availability [2], which is suitable to various kinds of coal and adaptable to boiler load changing [3]. Chnia has rich limestone resources and the desulfurization byproducts (gypsum) can be comprehensively utilized as additives in cement and other construction materials effectively. The temperature of flue gas entering desulfurization system through the electrostatic precipitator (ESP) is often very high (100–130 °C), causing the high evaporation water consumption in desulfurization tower, and affecting the life span of the absorber. Thus, flue gas reheating system was most widely employed in the early wet FGD systems, i.e., gas–gas heater (GGH), to reduce the inlet temperature of the flue gas entering into the scrubber and improve the discharging temperature of wet flue gas from stack, which is conducive to wet flue gas proliferation in atmosphere. But in actual operation in many power plants of China, lots of problems occurred due to the application of GGH, such as clogging and increasing pressure loss [4], which leads to the increases of investment and operation cost.
How to solve a series of problems caused by GGH? Based on years of researches on flue gas pollutants control and desulfurization wastewater treatment in thermal power plants, the authors put forward the idea that flue gas desulfurization wastewater is evaporated to reduce temperature of entering the absorber. This technology not only solves the problem of cooling the flue gas before entering desulfurization tower, but also provides a new way for desulfurization wastewater treatment. But the researches about desulfurization wastewater evaporation (DWE) have barely mentioned and the micro-mechanism about the technology also was not clear. There are still some questions about the technology applications in some aspects. Whether the evaporation was complete in flue duct? What influences aiming to electrostatic precipitator (ESP) and FGD system will be produced? Whether secondary pollutions will exist when desulfurization wastewater is injected into high temperature flue duct? Above questions should be answered before the wastewater evaporation technology in flue duct was used. So, this paper covers the negative impacts due to the application and omission of gas–gas heater (GGH) and the difficulties in desulfurization wastewater treatment in current power plants, and points out that desulfurization wastewater evaporation technology is an important way to achieve zero emission of power plants and solves the problems related to GGH.
Section snippets
The development and problems of GGH currently
Original desulfurization process used in Germany installed GGH at the entrance and exit of absorber, the gas–gas heat exchanger was designed for three intentions [5]:
- (1)
German environmental laws required that the discharge temperature of flue gas should not be lower than 72 °C in 1980s–1990s.
- (2)
Reduce the temperature of flue gas entering desulfurization absorber, in order to protect the corrosion protection layer, such as rubber interior liner in the absorber.
- (3)
Raise the temperature of flue gas at the
Research and problems of desulfurization wastewater treatment currently
Desulfurization equipment is required to remove the sulfur dioxides from the power plant stack gases. This is done in absorption towers and scrubbers, which requires significant amounts of water. In the scrubbing process sodium or calcium based alkaline reagent is injected in the spray tower to absorb the SO2, neutralize into a solid compound as either sodium or calcium sulfate. In US, 85% of the systems are wet scrubbers [10], [11]. There are still some F− and Cl− left in flue gas, after
Zero emissions of power plant wastewater
Besides desulfurization wastewater, there are a lot of other high-salinity wastewater that needs treatment, such as concentrated water discharged from reverse osmosis, drainage of circulating cooling water in high circulating rate, part of the boiler blowdown water, and neutralization water of the acid and alkaline solutions for resin regeneration in power plants. Taking an example for wastewater emissions in one power plant with 2 generating units of 330 MW in Inner Mongolia, China , the
New flue gas cooling technology
Taking all the studies on the GGH and desulfurization wastewater treatment above into considerations, a set of desulfurization wastewater evaporation system is designed, using desulfurization wastewater as the cooling water of flue gas. After mixed with compressed air, the pressurized wastewater is sprayed into the flue duct between the air pre-heater and electrostatic precipitator. The wastewater is evaporated instantaneously due to the waste heat of flue gas, and at the same time flue gas
Current researches and problems on desulfurization wastewater evaporation
Currently, the desulfurization wastewater evaporation technology by waste heat of the flue gas is rarely used in coal-fired power plants, and theoretical research is relatively scarce, only mentioned in few literatures [25], [26], [27].
However, the spray drying technology, similar to the desulfurization wastewater evaporation system, has a history of more than 100 years, and is widely applied in industrial production [28], [29]. Since the spray drying is more and more popular, researches on the
Advantages and current problems of desulfurization wastewater evaporation
To sum up, the desulfurization wastewater evaporation achieves the temperature reduction of flue gas and the treatment of desulfurization wastewater at the same time. It not only solves the problem of cooling the flue gas, but also removes F− , Cl− and heavy metal ions etc. effectively. Most worthy of mention is that water is considered as a scarce resource in the power plants, the application of this technology can bring a favorable effect to water saving. According to the water use survey
Technical application prospects
Although not always desired, FGD wastewater treatment is increasingly concerned due to the stricter environmental standard. The current and the foreseeable trends indicate that water resource and energy development of the future will continue to become more complicated, and intertwined with each other. So achieving a wastewater evaporation unit in individual system as a terminal device is an attractive endeavor for environmental researcher.
In recent years, many areas in China has suffered from
Acknowledgments
The author is grateful to the Beijing Natural Science Foundation (3142017) for supporting this work.
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