- Nov 27, 2017 -
Flue gas emissions from coal-fired boilers are one of the major causes of foggy haze. Emissions of sulfur dioxide is also one of the air pollutants. It is the fundamental problem that causes acid rain and ozone layer loopholes. When sulfur dioxide is absorbed on the moist mucosal surface, it produces sulfite and sulfuric acid. Eye and respiratory mucosa have a strong stimulating effect. With the strict requirements of the state on flue gas emissions, domestic coal-fired boilers, such as thermal power plants and steel mills, must be equipped with desulphurization and denitrification devices. Currently flue gas desulfurization thermal power plant method.
First, desulfurization process profile
The wet limestone-gypsum method is a method in which limestone powder is slurried and the flue gas SO2 is removed in an absorption device to produce by-product gypsum. This method is currently the most widely used flue gas desulfurization (FGD) method. Desulfurization rate of up to 95%, reliable operation, the most mature technology, applicable to high, medium and low-sulfur coal.
Process System Introduction
Absorption tower is divided into three areas from top to bottom:
Circulation tank: slurry storage and reactor, mainly reflecting: limestone dissolved, sulfite oxidation into gypsum.
Washing area (spray layer): Arranged with a multilayer nozzle, according to the amount of flue gas treatment to determine the opening layer.
Gas zone: Spray top layer to the absorption tower outlet, the middle with a demister.
The system includes the following four main processes:
* Add fresh slurry to the circulation tank;
* SO2 absorption and reaction to generate calcium sulfite;
* Calcium sulfite oxidized gypsum (calcium sulfate dihydrate);
* Gypsum is separated from the circulation tank.
The overall reaction in the absorber is as follows:
CaCO3 + 2SO2 + H2O = Ca (HSO3) 2 + CO2
The slurry containing CaCO3 is injected from the upper part of the absorption tower to absorb the SO2 in the flue gas to generate Ca (HSO3) 2 and fall into the circulation tank and then oxidize to CaSO4 through the bubbling air Ca (HSO3) 2. The crystalline gypsum:
Ca (HSO3) 2 + O2 + CaCO3 + 3H2O = 2CaSO4.2H2O + CO2
When the circulating gypsum slurry gypsum reaches a certain degree of saturation (about 130%), part of the pumped out to the gypsum processing station, made of industrial gypsum. Simultaneously add fresh slurry to the circulation tank to maintain the pH of the absorbent slurry.
In this process, the measurement and control of the density of lime and gypsum slurry is a very crucial step, which will directly affect the desulfurization effect.
Second, desulfurization process need to control two important elements: PH value, slurry density.
Two Important Control Points in Power Plant Desulfurization and Denitrification Process
1. PH value; running PH value control in 5.4 ~ 5.8.
PH value is too low will cause equipment corrosion, PH value will cause corrosion of pipes and instruments. The higher PH value is beneficial to the dissolution of limestone and the increase of the capture rate of SO2. However, the high pH value will increase the consumption of limestone, which will increase the residual limestone in slurry and affect the quality of gypsum. According to the daily gypsum test results, the fuel sulfur rationally adjusted.
2. Slurry density: monitoring the density of the slurry (concentration)
Slurry density in 1050 ~ 1100Kg / m3. Concentration control in 10% to 15%.
Low slurry density reduces desulphurization efficiency. High slurry density causes equipment wear and pipeline blockage to maintain the solid concentration of the circulating slurry in the absorber tower at a high level (about 15%) to obtain the desired gypsum crystals. Solids content is too low will lead to precipitation appears, and too high will cause pump wear and other issues. Therefore, it is important to control the solids content within the specified limits (12-17%). According to the predetermined slurry density to decide whether to start absorption tower discharge pump discharge slurry.
Several ways to measure the density of a slurry:
Methods for measuring the solid content of the slurry capacitance density meter, mass flowmeter alternative density meter, differential pressure on-line density meter, nuclear radiation density meter.
① capacitance and differential pressure densimeter to measure the direct contact with the slurry, leading to serious corrosion and wear instruments, measurement results are not accurate, the control effect of non-compliance, easy to plug, short life.
② The use of mass flowmeter density meter barely, but also the mainstream products used in the desulfurization industry, but because lime slurry in the flow process will constantly flush flowmeter pipe, a long time will lead to flowmeter can not work properly. In general about 2 years or so need to replace this expensive mass flow meter, resulting in a substantial increase in equipment procurement costs.
③ nuclear radiation density meter: It is also used in engineering a more density meter.