The frequency, time, and depth of peak shaving are increasing. Deep peak shaving not only tests the reliability and economy of unit operation, but also affects the stable operation of environmental protection facilities and the compliance of pollutant emissions. For flexibility transformation technology, a lot of research has been conducted in boiler, steam turbine, auxiliary equipment, control system, and flue gas denitrification, but for flue gas desulfurization
There is very little research on how to quickly adapt to deep peak shaving (especially rapid load increase) in the FGD system. Previously, the flue gas desulfurization (FGD) equipment in coal-fired power plants in China mainly used the limestone/gypsum wet desulfurization process, where the absorbent was limestone (mainly composed of CaCO3). This desulfurizer was relatively inexpensive and had decent desulfurization capacity, but due to its low solubility and weak alkalinity, there was a bottleneck in further improving desulfurization efficiency after reaching a certain level.
The current coal supply is becoming increasingly tight and the types of coal are complex and varied. High sulfur coal (hereinafter referred to as "high sulfur coal") or uneven coal blending pose significant challenges to SO2 emissions. Conventional methods such as optimizing the operation of slurry circulation pumps, adjusting the frequency of circulation pumps, regulating the pH value of absorption tower slurry, and using desulfurization additives for rapid and deep adjustment have insufficient performance and many drawbacks. During the rapid and frequent load increase process of the unit, the existing design capacity and response speed of the FGD cannot meet the demand, and the SO2 emission concentration (in this article, SO2 "concentration" refers to mass concentration) often exceeds the standard, which brings great pressure to the power plant and operating personnel; Therefore, it is very necessary to develop corresponding new technologies to address this issue and improve the adaptability and emergency response capabilities of existing FGD systems. This article takes a 600MW unit as an example and conducts experiments using a new type of high-efficiency desulfurization absorbent (hereinafter referred to as the "new desulfurizer") to solve the problem of excessive SO2 emissions during deep peak shaving and rapid load increase of the unit, while improving the adaptability of the FGD system to sulfur-containing coal.
New desulfurizer and its desulfurization principle
The new desulfurizer is a calcium based absorbent, mainly composed of very fine Ca (OH) 2. It uses calcium as the substrate and some alkaline metals and carbonate ions as additives. After activation and modification of various functional components, it achieves strong alkaline performance and can efficiently absorb SO2. The preparation process is as follows: Carefully selected and calcined high-purity lime blocks (mainly composed of CaO) are fed into a digester by a feeder, and water and a special catalyst are added to digest the lime blocks into mature lime Ca (OH) 2. Then, after multi-stage filtration and purification, impurities such as inert insoluble substances are removed, and finally a Ca (OH) 2 suspension with a mass fraction of about 25% (containing a small amount of alkaline soluble substances such as Na2O and MgO contained in the original lime) is obtained. The particle size of the new desulfurizer is about 200nm analyzed using a Malvern laser particle size analyzer. The main indicators of this product are: milky white liquid, with a solid mass fraction of (25 ± 1)%; The mass fraction of acidic insoluble substances shall not exceed 0.3%; The pH value is not less than 13. Compared with ordinary hydrated lime powder on the market, this product has high purity, small fineness, and high reactivity, but the cost difference is not significant.
The total reaction equation for the absorption of SO2 by the new desulfurizer is
It can be seen that the reaction of Ca (OH) 2 and limestone absorbing SO2 is basically the same, and the by-products are also the same. Therefore, the addition of the new desulfurizer will not have adverse effects on the operation of the existing limestone/gypsum wet flue gas desulfurization system. The advantages of the new desulfurizer are as follows:
a) SO2 is an acidic oxide, and the new desulfurizer Ca (OH) 2 is strongly alkaline. Therefore, when the two meet, a strong and irreversible acid-base neutralization reaction will occur, with a much higher reaction rate and activity than the series of weak acid and weak alkali salt reactions between limestone and SO2; Therefore, the reaction rate and surface activity of the new desulfurizer are much higher than those of the conventional absorbent limestone.
b) The particle size of the new desulfurizer is relatively concentrated, mainly around 200nm. Compared with conventional limestone powder (particle size less than 44 μ m), the particle size difference between the two is more than 200 times; Therefore, the new desulfurizer has a larger specific surface area, higher reactivity, and can react fully and rapidly with SO2 without residue. Due to the small surface area of limestone, weak alkali salt CaCO3, about 10% impurities in limestone, and the operation process of ball mills, 10% to 20% of limestone powder will not react and remain in the absorption tower, causing sedimentation and even scaling inside the tower.
c) The new desulfurizer is a strong alkali and CaCO3 is a weak alkali salt. The former has a much higher effect and speed in improving the pH value of the absorption tower slurry than the latter; Therefore, the new desulfurizer can quickly increase the pH value of the absorption tower slurry and improve the desulfurization effect of the absorption tower.
d) The new desulfurizer has a large surface area and strong activity, so impurities such as fly ash and limestone that affect the quality of the slurry, as well as chloride ions, cannot affect its reaction. It has a good governance effect on the slurry that has been deteriorated by impurities.
e) The hardness of the new desulfurizer is much lower than that of limestone, and Ca (OH) 2 is a strong alkali, which reduces the wear, blockage, and acid corrosion of equipment and pipelines in the FGD system. It can improve the service life of equipment and pipelines and reduce equipment maintenance rates.
