In the 1980s, Taiwan’s economy grew rapidly and its demand for electricity surged. New power plants were urgently needed. In order to reduce transportation costs of imported coal and to promote economic development in central Taiwan, a coal-fired power plant was constructed in Taichung. With state-of-the-art emissions reduction equipment, the site not only provides needed electricity, but has also proven to be a haven for fireflies, terns, and many other birds.
The Taichung Power Plant (TCPP) is located in the south of Taichung Harbor and in the north of Tatu River on land reclaimed by dredging sand, totaling 277.5 hectares. The station was established on July 18, 1989, and it was the first coal-fired power plant in Taiwan to pass the country’s Environmental Impact Assessment. About 45% of the site is filled with trees to minimize wind in the area.
The plant has eight GE and two Toshiba tandem compound reheat steam turbines with duplex flow at the low-pressure stage. Each utilizes a Foxboro distributed control system and a GE Mark VIe steam turbine controller. The generators have a water-cooled stator and hydrogen-cooled rotor.
Each unit also has selective catalytic reduction to reduce NOx, an electrostatic precipitator (EP) to remove particulate matter, and a limestone/gypsum wet scrubber flue gas desulfurization (FGD) system to limit sulfur oxides (SOx). The use of a wet FGD was driven by the need to conform to Taiwan’s environmental regulations and standards. At present, TCPP is equipped with 10 coal-fired units of 550 MW, four gas turbine units of 70 MW, and wind turbines with 22.2 MW of capacity. There are plans to add two 1,300-MW gas units by 2025.
In 1998, the spontaneous combustion of sub-bituminous coal was a challenge. However, TCPP learned to control the problem by mastering various coal-blending ratios, and continued to burn the fuel. TCPP effectively reduces dust from the coal and minimizes its escape, including from gravity drops during loading/discharging, mechanical power disturbance in stacking coal, wind erosion on conveyor belts and surface coal piles, and coal storage mainly in open air. The dust suppression practices and facilities of the site include 23-meter-high windproof grids around the area, sprinkler equipment at a certain distance, fixed hourly water sprinkling, sealed coal conveyor belt and transfer tower, and grab coal unloading.
The machine has been upgraded to a continuous coal unloader, compacted coal pile management, surrounding windbreak forests, and other commonly used best practices. The efficiency of dust suppression and control measures of the plant has reached 96.25%. TCPP is building a shed-type coal bunker that is designed with a windproof feature. In the future, the efficiency of dust suppression and control measures is expected to be as high as 98%. Approximately 857 tons of waste water produced per day in the reuse plant is properly treated and meets the standards announced by the Taiwan Environmental Protection Agency for applicable power plants, ensuring a better dust suppression effect.
The emission flue gas from the power generation process enters the air preheater through a primary air fan and forced draft fan, then enters the boiler, and mixes and burns the fuel to become fuel gas. After the heat exchange of the furnace tube in the furnace, it becomes flue gas and enters the selective catalytic reduction denitrification environmental protection equipment and EP dust removal. The booster fan and the gas-to-gas heat exchanger cool down the flue gas before it enters the FGD absorption tower.
Coal contains sulfur, and the flue gas produced after combustion contains SOx. TCPP’s wet FGD uses limestone powder slurry to absorb SOx in the flue gas. The slurry is sprayed from the top of the spray absorption tower, and the flue gas flows upward from the bottom of the tower. The FGD equipment can achieve a removal rate of more than 90% of the SOx in the flue gas, allowing the value of flue gas pollutants to comply with environmental protection regulations. It is finally discharged to the atmosphere through a 250-meter-high chimney.
The wastewater produced by the wet FGD process contains dissolved solids, chloride ions, calcium ions, sulfates, and nitrates, often in very high concentrations. The substances contained in coal such as heavy metals will also enter the flue gas during the combustion process, and later turn into water pollution through the desulfurization process.
FGD wastewater mainly comes from the concentrated filtrate of the secondary cyclone separator. It is an inorganic wastewater with high salinity and high hardness. The pH is about 4 to 6. The composition is quite complex, containing large amounts of suspended solids, chloride salts, heavy metals, and excessive boron.
In the past, the traditional chemical precipitation method was used for FGD wastewater treatment. Chemical coagulation precipitation is a standard solid-liquid separation procedure, which is widely used in various industrial wastewater applications and has a good removal effect on heavy metals in the water. The sources of boron in FGD wastewater include coal used for power generation and combustion, and limestone powder used in FGD equipment to remove SOx from flue gas.
The original FGD wastewater treatment process had extremely low removal rates for boron, which led to the release of boron in the water. The concentration was often much higher than the discharge water control standard of 5 milligrams/liter.
Boron removal technology can be roughly divided into five types, namely chemical precipitation, ion exchange, adsorption, reverse osmosis, and extraction, but each method has its limitations. The method of reverse osmosis is currently the most widely commercialized technology, but it is limited to low-concentration boron-containing wastewater or seawater desalination.
Both the chemical precipitation method and the extraction method are very suitable for the treatment of high-concentration boron-containing wastewater. The solvents used in the extraction method are mostly diols, but the toxicity of the solvent and subsequent treatment are relatively tricky. The chemical precipitation method is the most widely used in the industry. It is an accepted technology, but the traditional coagulation precipitation method often requires high-temperature conditions to simulate the hydrothermal method to produce insoluble species.
The present invention relates to a method for treating high concentrations of boron-containing wastewater. The method includes one stage of pretreatment that uses hydrogen peroxides to react with borate ions and one stage of precipitation that uses precipitant (such as barium hydroxide and barium chloride) to extensively remove boron from boron-containing wastewater as perborate precipitants. The method of present invention is effective to high concentrations of boron-containing wastewater and capable of reducing boron level to less than 5 ppm, which is the average concentration of seawater.
TCPP engineers were so stressed in this situation that they were constantly seeking and eager to find an appropriate solution. They worked together with the Industrial Technology Research Institute (ITRI), which has professional experience in technology research and development. They used a patented fluidized bed to treat the boron-containing wastewater (Figure 1).
1. Taichung Power Plant uses fluidized-bed crystallization water treatment technology to effectively remove boron from wet flue gas desulfurization wastewater. Courtesy: Taipower
A fluidized-bed crystallization wastewater treatment device includes a reaction tank containing a carrier. The waste water to be treated enters from the bottom of the reaction tank and flows upward, and the reaction tank is connected with a first-rate water circuit. Part of the wastewater is pressurized and returned to fluidize the above-mentioned support, and part of it is brought into the reaction tank due to the siphon effect. The reaction tank is also connected with a medicament inlet for injecting medicament, so that the pollutants in the wastewater are on the fluidized support crystallization, and the treated wastewater flows out from the outlet on the top of the reaction tank.
According to a present invention, the method for treating high-concentration wastewater includes pre-treatment of boron-containing wastewater and a precipitation step of applying the pretreated wastewater to the wastewater. A proper ratio of hydrogen peroxide is stirred and reacted for a period of time. The precipitation step is to use a barium-containing compound to mix with the pretreated wastewater to produce perborate precipitation, and stir for a period of time. In a preferred embodiment, the method for treating high-concentration boron-containing wastewater further includes a pH control step, a mixing and stirring step to fully mix the solution, and the proper operating conditions of the pretreatment tank.
Fireflies appeared at the TCPP in the early days when larvae arrived with the planting earth when the facility was built. As TCPP maintains 50% of the land green and has a lot of trees planted around it, and there is low human interference in other areas, the environment has allowed the fireflies to gradually multiply for decades.
In the ecological pond water intake test and analysis, it is believed that the artificial repopulation technology will be stable, and the aquatic yellow firefly can grow up twice a year. The ecological pond was refurbished to build an isolated area with embankments, filled with treated water from the circulated fluidized bed. At the same time, Wu Guo fish, shrimp, and crabs that swallowed firefly larvae were removed. Native plants, such as Ficus and Jinxinmu ginger, create a suitable growth environment for the yellow-rimmed firefly. A black net was set up by the pool to block the street lights and car lights in order to reduce the damage to fireflies from incandescent.
Rehabilitation experts have said: “It takes at least three to five years for fireflies to repopulate to be stable. Of course, the environment must continue to maintain a good ecology to cultivate stable populations.”
The coal sinking pond and the forest on the southwest side of the factory are far away from the generator set and people. There are few cars passing by, there are not many street lights, and the green belt forms a suitable growth environment for Taiwanese window fireflies.
In addition, little terns (Figure 2) were observed circling, drinking water, and wetting their belly feathers around the coal sink, and then returning to the nest to cool their bird eggs. TCPP chose to build and maintain artificial breeding grounds for small terns near the wind turbines and on the roof of the warehouse, including laying gravel habitat, drainage design, vegetation and power grid maintenance, and installation of fake birds.
2. The ecological environment around the Taichung Power Plant supports fireflies and many types of bird species, including terns, such as the one pictured here. Courtesy: Taipower
After more than 15 status records, 73 species were found inside and outside the factory. Furthermore, more than 5,000 species of birds were active, carrying out breeding and nesting behaviors. TCPP intends to continue striking a delicate balance between plant development and ecological environment maintenance. ■
—Hsu ChiaHao is general manager and Liao ZiFeng is an environmental engineer at Taipower’s Taichung Power Plant.