High-efficiency method for removing sulfur and mercury of coal-fired flue gas, and apparatus thereof

Abstract

A high-efficiency method for removing sulfur and mercury of coal-fired flue gas, and an apparatus thereof. The method comprises: activating, by using water vapor, lime or Ca(OH).sub.2 used as a sulfur removal and mercury removal absorbent and mixing the lime or Ca(OH).sub.2 with flue gas; conveying, by using water vapor, part of a by-product to a top of a reaction tower and mixing the part of the by-product with the flue gas, so as to strengthen the sulfur removal and mercury removal effect; the flue gas entering a bag type or electric bag compound dust remover after sulfur removal and mercury removal in the reaction tower, and conveying part of the collected by-product to the reaction tower for cycle use.

Claims

1. A method for removing sulfur and mercury of a coal-fired flue gas, comprising: water-spraying a flue gas to cool the flue gas to 72-78 C. in by humidifying and cooling the flue gas before the flue gas entering a reaction tower; spraying a Ca(OH)2 absorbent into the reaction tower through a water vapor conveyor to mix the Ca(OH)2 absorbent with the flue gas so that SO2 in the flue gas reacts with the Ca(OH)2 and, simultaneously, so that the Ca(OH)2 absorbent is activated into microporous fine particles by water vapor while conveyed by the water vapor to adsorb mercury vapor in the flue gas; collecting, by a dust remover, a fly-ash produced from sulfur removal and mercury removal from the flue gas; conveying a part of the flv ash collected by the dust remover, as a recycled flv ash, to an absorbent spraying inlet of the reaction tower through the water vapor conveyor; conveying outwards another part of the fly ash collected by the dust remover; feeding purified flue gas into a chimney through an induced draft fan to be discharged; mixing the recycled fly ash with the flue gas to produce a mixture that enters the reaction tower again for further sulfur removal and mercury removal from the flue gas; mixing the flue gas with the Ca(OH)2 absorbed and the recycled fly ash, and then fully mixing with the particles through a flue gas distributor, to produce a mixed and uniform distribution of the particles with SO2 and mercury vapor, wherein an effective height of the reaction tower is 20-24 meters, wherein a time for sulfur removal and mercury removal of the flue gas in the reaction tower is 4-6 seconds, wherein the water vapor used by the water vapor conveyor is waste water vapor from a boiler of 0.3-0.5 MPa and 250-300 C. and wherein a catalyst with MgO and Fe.sub.2O.sub.3 is added to the Ca(OH).sub.2 absorbent.

2. The method for removing sulfur and mercury of a coal-fired flue gas according to claim 1, wherein a weight ratio of the recycled fly ash to Ca(OH).sub.2 is 100:1-200:1, and wherein a conveying time is 2-5 seconds.

3. The method for removing sulfur and mercury of a coal-fired flue gas according to claim 1, wherein a molar ratio of Ca(OH).sub.2 to SO.sub.2 is 1.2-1.5, and wherein a conveying time of Ca(OH).sub.2 is 2-5 seconds.

4. The method for removing sulfur and mercury of a coal-fired flue gas according to claim 1, wherein a height of the flue gas distributor is 1/20 of a total height of the reaction tower.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a system diagram of the apparatus for removing sulfur and mercury of flue gas in the present invention.

(2) FIG. 2 is a schematic diagram of the structure of the reaction tower for removing sulfur and mercury in the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(3) The present invention will be described in further detail below with regard to a coal-fired unit boiler with reference to the accompanying drawings:

(4) The apparatus for removing sulfur and mercury in the present invention is structured as described below. As shown in FIGS. 1 and 2, an absorbent spraying inlet 13 and a gas flue inlet 3 are provided on the top of a reaction tower 4, a coal-fired boiler 1 is connected with the gas flue inlet 3 through a gas flue 2, and a water spraying inlet 19 is arranged on the upper part of the reaction tower 4; the water spraying inlet 19 is connected with a process water tank 18 through a water flow meter, a flue gas distributor 20 is arranged below the sulfur and mercury removal absorbent spraying inlet 13, and the lower end of the reaction tower 4 is provided with a flue gas outlet 5 connected with a dust remover outlet 6 of a bag type dust remover 7.

(5) Two by-product outlets 14, 14 are provided on the lower end of the bag type dust remover 7, One by-product outlet 14 is connected with a recycled absorbent inlet 15, the other by-product outlet 14 is connected with outward by-product conveying equipment 16, and the upper end of the bag type dust remover 7 is provided with an outlet 8 connected with a chimney 10 through an induced draft fan 9. An absorbent powder bin 11 loaded with lime and the recycled absorbent inlet 15 are connected with the absorbent spraying inlet 13 through a water vapor conveyor 12.

(6) Wherein coal-fired flue gas enters the reaction tower 4 through the gas flue inlet 3 on the upper part of the reaction tower 4, and waste hot water vapor 17 from the boiler is fed into the absorbent spraying inlet 13 on the top of the reaction tower after mixing the absorbent lime or Ca(OH).sub.2 in the absorbent powder bin 11 with the recycled absorbent, such as fly ash, in the recycled absorbent inlet 15. The flue gas is mixed with the lime or Ca(OH).sub.2 and the recycled absorbent at the flue gas distributor 20, humidifying water is mixed with the flue gas through the water spraying inlet 2, the humidified flue gas is subjected to sulfur and mercury removal reactions in the reaction tower 4, the flue gas enters the flue gas inlet 6 of the bag type dust remover 7 through the flue gas outlet 5 of the reaction tower 4 after sulfur removal and mercury removal, and the flue gas enters the chimney 10 through the outlet 8 after passing through the bag type dust remover 7 and is then discharged. A part of the by-product collected by the bag type dust remover 7 passes through the recycled absorbent outlet 14 and the recycled absorbent inlet 15 and enters the absorbent spraying inlet 13 on the top of the reaction tower 4 through the water vapor conveyor 12, and the other part of the by-product is directly discharged through the by-product outlet 14.

(7) In the process of removing sulfur and mercury from flue gas, firstly, the flue gas is cooled to 70-80 C. and then fed into the reaction tower 4, and meanwhile, the boiler waste hot water vapor of about 250 C. is used as a conveying power source, the water vapor is controlled to be below 0.5 MPa using a water vapor flowmeter, absorbent lime powders in the absorbent powder bin 11 and a part of the recycled absorbent collected by the dust remover 7 are conveyed to the absorbent spraying inlet 13 in the reaction tower 4 through the water vapor conveyor 12, then mixing with the flue gas and entering into the reaction tower 4. The concentration of the particulate matters in the flue gas of the reaction tower 4 can reach 200-300 g/Nm.sup.3. The lime is activated and conveyed simultaneously in the process of being conveyed by the water vapor, with most of the lime being hydrated into strongly basic Ca(OH).sub.2. After Ca(OH).sub.2 reacts with SO.sub.2 in the flue gas, a part of the by-product collected by the bag type dust remover 7 serves as an auxiliary sulfur removal and mercury removal absorbent for cycle use. The auxiliary sulfur removal and mercury removal absorbent is mixed with the lime through the recycled absorbent inlet 15, the mixture enters the absorbent spraying inlet 13 through the water vapor conveyor 12 to be mixed with the flue gas, and the resulting mixture enters the reaction tower 4 for further removal of SO.sub.2 and mercury from the flue gas.

(8) The working principle of the apparatus for removing sulfur and mercury in the present invention is as follows: most of the lime, when being conveyed by the water vapor, is hydrated into strongly basic Ca(OH).sub.2, while at the same time completing activation and conveying of the lime. SO.sub.2 in the cooled flue gas within the reaction tower 4 reacts with Ca(OH).sub.2 in desulfurizer particles to generate CaSO.sub.2, which is finally oxidized into CaSO.sub.4. And meanwhile, microporous structures and large specific surface area generated in the process of conveying the absorbent can facilitate adsorption of SO.sub.2 and gaseous mercury. Gaseous divalent mercury is partially dissolved in water drops and adhered to the surface of the particles to form particulate mercury after collision with the sulfur removal absorbent; after being conveyed, the absorbent contains a large amount of free metal oxides, achieving catalytic adsorption of zerovalent mercury. Removal of different forms of mercury is accomplished with the help of the bag type dust remover 7.

Embodiment 1

(9) An experiment of this apparatus for removing sulfur and mercury is conducted in a 2t/h coal-fired experimental boiler 1, with the experimental system shown in FIG. 31. The reaction tower 4 for removing sulfur and mercury from flue gas has a dimension of 1200 mm7800 mm, the flue gas has a superficial velocity of 3.8 m/s, and the bag type dust remover 7 is arranged at the back of the reaction tower 4 for removing sulfur and mercury, with an aim of collecting the reaction by-product. Raw coals from Datong City, Shanxi Province are employed for experimental research. The part by percentage of sulfur in coal is 0.32%, and the content of mercury in coal is 0.35 mg/kg.

(10) The temperature of the flue gas entering the reaction tower 4 is 128 C., the concentration of SO.sub.2 is 578 mg/Nm.sup.3, and the concentration of mercury vapor is 21.6 g/Nm.sup.3. The temperature of the flue gas exiting the reaction tower is kept at 75 C. by spraying water to the reaction tower, and the concentration of the particulate matters inside the reaction tower 4 is 250 g/Nm.sup.3 after sulfur removal and mercury removal. The results show that the sulfur removal efficiency is up to 89.6% when the molar ratio of Ca/S is less than 1.2, the efficiency of removing gaseous divalent mercury is up to 91.6%, the efficiency of removing gaseous zerovalent mercury is up to 86.8%, the efficiency of removing gaseous total mercury is up to 89.2%, and the solid mercury trapping rate is up to 99.3%.

Embodiment 2

(11) An 85t/h coal-fired boiler 1 is employed. The content of sulfur in coal is 0.5%, and the content of mercury in coal is 0.11 mg/kg. The temperature of the flue gas at outlet is 139.9 C., with the amount thereof being 234000-236000 Nm.sup.3/h and the concentration of SO.sub.2 in the flue gas being 920 mg/Nm.sup.3. By adopting the apparatus for removing sulfur and mercury from flue gas in the present application as shown in FIG. 1, physical and chemical properties of the by-product have been changed significantly after the by-product is conveyed by the water vapor. CaO is partially converted into Ca(OH).sub.2 of a porous structure, which improves the adsorption capability of the absorbent to mercury. The temperature of the flue gas after sulfur removal and dust removal is 76-79 C. When the molar ratio of CaO/SO.sub.2 is less than 1.2, the concentration of SO.sub.2 in the processed flue gas is 89 mg/Nm.sup.3, and the sulfur removal efficiency is up to 89.3%. About 98.6% of mercury vapor in the flue gas can be removed by this apparatus in combination with the bag type dust remover 7, and the content of mercury in the by-product can reach 1.58 mg/kg. The performance in removing various forms of mercury is comparable to the effect of removing mercury with activated carbon reported in foreign countries, and the operating cost is significantly lower than those of special technologies for removing mercury through activated carbon injection.

INDUSTRIAL APPLICABILITY

(12) Compared with the existing methods and apparatuses for removing sulfur and mercury, the method for removing sulfur and mercury from flue gas and the apparatus thereof in the present invention have advantages described below.

(13) (1) In the method of the present invention, lime/Ca(OH).sub.2 is used as a sulfur removal and mercury removal absorbent and a recycled absorbent is used as an auxiliary absorbent. The flue gas is cooled by water-spraying the flue gas in advance, and this is favorable for conversion of mercury vapor into solid mercury, which is collected by the dust remover in the back; most of divalent mercury is dissolved in water drops, and then adhered to the surface of and inner side of the micropores of the particles of the absorbent and the recycled absorbent after collision therewith; the absorbent and the recycled absorbent both have larger specific surface areas and high porosities and therefore have a strong adsorption capability for mercury vapor and also outstanding absorptive effects for various forms of mercury; meanwhile, it is advantageous for the zerovalent mercury absorbed by the recycled absorbent to be converted into divalent mercury under the catalytic oxidation effect of MgO and Fe.sub.2O.sub.3, and finally, the gaseous zerovalent mercury in the flue gas is converted into solution-state or granular mercury and then collected by the bag type or electric bag compound dust remover.

(14) (2) Micropores on the surface of the recycled absorbent tend to adsorb finer particles to block off micropore channels. When the recycled absorbent is conveyed by the water vapor, the water vapor can activate the recycled absorbent so that these fine particles are released to generate more micropores; meanwhile, Ca, Si and Al that exist in the recycled absorbent will undergo a pozzolanic reaction under the effect of water vapor, so as to generate calcium silicate hydrate (CaO.SiO.sub.2.H.sub.2O), dicalcium silicate hydrate (2CaO.SiO.sub.2.H.sub.2O) and tetracalcium aluminate hydrate (4CaO.Al.sub.2O.sub.3.13H.sub.2O). All these materials have high specific surface areas. And also, when the hydrates come into contact with hot flue gas, water in these hydrates is released and more pores are generated to increase the specific surface area of the recycled absorbent, endowing the recycled absorbent with outstanding mercury removal capability.

(15) (3) The specific surface area and micropores of the particles are increased greatly after the recycled absorbent is mixed with lime. These microporous particles are conducive to absorption of SO.sub.2 and mercury vapor in the flue gas and the efficiency of sulfur removal and mercury removal is improved accordingly.