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New Wet FGD Process Using Granular Limestone Hirofumi Kikkawa* Kure Research Laboratory, Babcock Hitachi K.K., Kure-Shi, Hiroshima-Ken, 737-8508, Japan
Takanori Nakamoto Kure Division, Babcock Hitachi K.K., Kure-Shi, Hiroshima-Ken 737-8508 Japan
Masanori Morishita and Kazuyuki Yamada Electric Power Development Co., Ltd., Chuo-Ku, Tokyo 104-8165 Japan
A new wet limestone-gypsum process for flue gas desulfurization (FGD) has been developed. The main difference compared to a conventional wet FGD process is the ability of the new process to utilize granular limestone directly as a desulfurizing reagent. Thus, the pulverizing of limestone, which causes power consumption, can be avoided. The performance of the wet FGD process using granular limestone was confirmed to be equal to or higher than that of a conventional process in various tests. Tests were also performed in a 2000 m3Normal/h demonstration plant introducing actual flue gas of various types of coals at Matsuura Thermal Power Station of the Electric Power Development Co., Ltd. (EPDC) in Japan. It was also confirmed that the desulfurization performance of the granular limestone FGD process was not decreased by aluminum-fluorine-containing compounds in the absorbing slurry as much as in the pulverized limestone case. In this new wet FGD process, gypsum quality was also higher than that in a conventional wet FGD process. Introduction The wet limestone-gypsum flue gas desulfurization (FGD) process is the most widely used because of its high desulfurization performance, reliability and low utility consumption. The absorbers for wet limestonegypsum FGD processes are generally classified as spraying, packed-perforated plate, and bubbling systems, depending on the gas-liquid contacting method.1-6 Although each type has outstanding characteristic features, the spraying type is considerably popular and reliable and is thus the most widely used worldwide. The conventional spray-type FGD system once comprised three towers, i.e., a prescrubber for cooling and dust-removal treatment of the exhaust gas, an absorber for spraying absorbent into the exhaust gas for reaction with SO2, and an oxidation tower for the oxidation of calcium sulfite formed in the absorber. We have developed an advanced FGD process called the “Single-Tower Desulfurization System” in which the absorber provides both prescrubbing and oxidation functions.1-5 This process has achieved a considerable reduction in utility consumption and installation costs, and it exhibits an excellent response to load changes. In the conventional wet limestone-gypsum FGD process, limestone is usually used after being pulverized to an average size of 5-20 µm, which consumes a great deal of power. The absorbing liquid contains not only limestone, which contributes to SO2 absorption, but also a considerable amount of gypsum, which makes no contribution to SO2 removal efficiency. When the proportion of the limestone in the absorbing liquid is increased so as to improve the SO2 removal efficiency, * To whom correspondence should be addressed. Tel.: 81846-45-4744. Fax: 81-846-45-4537. E-mail:
[email protected].
the quality of gypsum is decreased to a useless level. Thus, in conventional wet limestone-gypsum FGD plants, it is necessary for the concentration of limestone in the absorbent liquid to be at a level under a predetermined value. It seems to be possible to treat the absorbent liquid containing gypsum and limestone by means of a classifier such as a hydrocyclone to selectively collect gypsum in the absorbing liquid. Such a classification, however, is not effective because the difference in diameter between limestone and gypsum particles in the conventional FGD plants is too small. In the new wet FGD process using granular limestone, the separation of limestone particles from gypsum particles can be expected without any classifiers because of the large difference in size between the limestone and gypsum particles. This enables an improvement in the desulfurization performance and a decrease in the power consumption for grinding while maintaining the quality of the gypsum. In a coal-fired power plant, most of the combustion ash (fly ash) is removed by electrostatic precipitator (ESP) upstream of the FGD plant, but some is passed into an absorber and captured by the sprayed droplets. The fly ash contains an aluminum component, a part of which dissolves in the absorbent liquid when SO2 is absorbed therein to decrease its pH value. On the other hand, the hydrogen fluoride (HF) contained in the flue gas is captured by the sprayed droplets, and in the presence of the Al component, it reacts with limestone to form aluminum-fluorine-containing compounds, typically represented by the chemical formula CaAlF3(OH)2CaF2. Such compounds are deposited on the surfaces of the limestone particles to decrease the reactivity of the limestone.7-10 In this new FGD process, however, fluoride in the absorbing slurry does not affect the desulfurization performance as much as in the conven-
10.1021/ie0109760 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/17/2002
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Figure 1. Schematic diagram of the fundamental test facility. Table 1. Chemical Analysis of Limestone CaCO3 (wt %) MgO (wt %) Al2O3 (wt %) SiO2 (wt %)
Ikura limestone
Taiyuan limestone
99.4 0.17
