Statistical data from the past five years shows that China's current power generation structure is still dominated by thermal power generation, accounting for about 70%; followed by hydropower, accounting for about 14%; nuclear power accounts for 5%, and wind power and solar power account for a smaller proportion.

Data source: National Bureau of Statistics

Thermal power generation is mainly dominated by coal-fired power generation, and the combustion of coal produces a large amount of pollutants such as sulfur dioxide, nitrogen oxides, and dust. In order to reduce air pollution emissions, coal-fired power plants generally have a complete flue gas treatment system, of which the desulfurization system is an important component.

Limestone-gypsum wet flue gas desulfurization technology has become the mainstream treatment process for flue gas desulfurization in China's current power plants due to its advantages such as high desulfurization efficiency, mature technology, compact equipment layout, and strong adaptability to water quality changes. In the wet flue gas desulfurization process, as the absorption reaction continues, the Cl- concentration in the slurry gradually increases. Excessive Cl- concentration, on the one hand, affects the desulfurization efficiency; on the other hand, it affects the quality of gypsum and can also cause pipe corrosion. In order to maintain the stable operation of the absorption system and ensure the quality of gypsum products and desulfurization efficiency, it is necessary to control the Cl- concentration in the slurry, generally requiring it to be below 20000 ppm, so it is necessary to continuously discharge part of the slurry. Although the amount of desulfurization wastewater is a small proportion of the wastewater in the power plant, it is characterized by high salt content, many types of pollutants, large water quality fluctuations, high turbidity, and high hardness, making it the most complex and difficult wastewater to treat in coal-fired power plants.

Project Overview

The power plant project is a newly built 2×1000MW ultra-supercritical coal-fired power generation unit, with the simultaneous construction of two sets of limestone-gypsum wet flue gas desulfurization units and their public systems.

This project requires zero discharge of desulfurization wastewater. The design treatment capacity is 25m³/h, and the total TDS value of desulfurization wastewater under harsh conditions is 40000-60000mg/L. The overall zero-discharge system mainly includes two parts: low-temperature flash evaporation and concentration + bypass flue gas spray drying.

Hezhongsi (Beijing) Environmental Engineering Co., Ltd. (hereinafter referred to as Hezhongsi) is responsible for the design of the process package (Hezhongsi®HLTC) for the low-temperature flash evaporation and concentration process section, the supply of complete sets of equipment, and technical services.

Design Objectives

01

Desulfurization wastewater zero-discharge system processing capacity ≥25m³/h

02

Annual equivalent availability factor of the entire unit [(available hours - derated output equivalent downtime hours) / statistical period hours]: not less than 95%

03

Annual operating time under the principle of orderly power use: 6000 hours

04

Ensure that the measured noise level at a distance of 1m from the equipment (including insulation) and outer casing is no more than 85dB(A)

05

The system output loss shall not exceed 20% during the equipment life cycle.

06

The quality of the produced water meets the water replenishment requirements of the desulfurization process.

Process Description

3D model of the low-temperature flash evaporation and concentration unit

The wet desulfurization wastewater low-temperature flash evaporation and concentration treatment technology (Hezhongsi®HLTC) is based on the principle of gypsum crystal seed technology. Hezhongsi has made a large number of creative improvements, such as adjusting process parameters and optimizing equipment structure, to maximize the delay or even eliminate the occurrence of scaling and salt accumulation inside the evaporator.

Internal condition of the evaporation heat exchanger after three months of operation

Core Equipment: The core equipment of the low-temperature concentration system is a three-effect forced circulation evaporation unit, operating under vacuum conditions, with a water recovery rate of up to 90%.

Heat Source The heat source for low-temperature flash evaporation and concentration is taken from the waste heat of the flue gas after the induced draft fan, without increasing new energy consumption. Superheated steam is used as a backup heat source.

Cooling Water The system has its own complete set of cooling circulating water equipment, requiring only a small amount of water replenishment, without drawing circulating water from the plant system, and does not affect the operation of the main unit.

Wastewater Process The supernatant liquid from the wastewater hydrocyclone enters the buffer tank, and is directly pumped into the first-effect evaporator by the wastewater feed pump. Under the action of the forced circulation pump, the wastewater is heated by the first-effect heat exchanger and then enters the first-effect separation chamber to complete gas-liquid separation. The preliminary concentrated liquid enters the second effect for further concentration, and the final concentrated liquid is discharged from the third effect, and is pumped into the bypass flue gas spray drying system by the discharge pump.

Steam/Condensate Process The heating steam enters the first-effect heating chamber, releases heat after condensation, and is pumped out of the system for reuse by the front-end condensate pump. The secondary steam generated by wastewater evaporation is finally condensed and collected into the tail-end condensate tank, and is sent to the desulfurization process water tank or used as supplementary water for circulating cooling water by the tail-end condensate pump.

Process Characteristics

01

The concentration system uses negative pressure and low-temperature operation, which can effectively prevent or reduce equipment corrosion.

02

No pretreatment is required, there is no reagent cost consumption, no solid salt is produced, and the operating cost is low.

03

The heat source for low-temperature flash evaporation and concentration is flue gas waste heat, without increasing new energy consumption.

04

The entire system has few equipment, short process, and low investment cost.

On-site Situation

Location of the flash evaporation system

System side

System front

Details

Summary

The National Energy Power Plant's 2×1000MW unit desulfurization wastewater zero-discharge project is currently in operation. The low-temperature flash evaporation process section (Hezhongsi®HLTC) undertaken by Hezhongsi has been operating stably since commissioning and has been fully affirmed by the owner and general contractor.