Gas analysis device, mercury removal system, gas analysis method, and removal method for mercury in flue gas

A gas analysis device according to the present invention includes a flue-gas extraction pipe for extracting flue gas from a flue gas duct to which flue gas including both of NH4Cl and SO3 is fed, a collector that is provided in the flue-gas extraction pipe, for removing soot dust contained in the extracted flue gas, a roll filter that is provided in the flue-gas extraction pipe, for depositing both of NH4Cl and SO3 contained in the flue gas, and a measurement device for measuring both of NH4Cl and SO3 contained in the flue gas by irradiating a sample including both of NH4Cl and SO3 deposited by the roll filter with X-rays and detecting fluorescent X-rays generated from the sample.

FIELD

The present invention relates to a gas analysis device, a mercury removal system, a gas analysis method, and a removal method for mercury in flue gas that enable to measure a concentration of ammonium chloride supplied into flue gas of a boiler.

BACKGROUND

Harmful substances such as soot dust, sulfur oxides (SOx), and nitrogen oxides (NOx) are contained in flue gas emitted from combustion facilities such as a boiler and a waste combustor and need to be removed by using a flue-gas treatment device. A typical flue-gas treatment device includes a denitrator that reduces NOx and a wet desulfurizer that uses an alkali absorbent as a SOx absorbing agent. The flue-gas treatment device treats harmful substances contained in flue gas by supplying ammonia (NH3) on an upstream side of the denitrator in a flue gas duct to reduce nitric oxide (NO) with a denitration catalyst of the denitrator to remove NOx as shown in the following formula (1) and absorbing SOx in the alkali absorbent by using the wet desulfurizer (see, for example, Patent Literature 1).
4NO+4NH3+O2+4N2+6H2O  (1)

SOx includes SO2and SO3. When a gas temperature of the flue gas reduces, SO2and SO3may change into ammonium hydrogen sulfate or ammonium sulfate as shown in the following formulas (2) and (3), and attach to the wall surface of the flue gas duct or devices that are installed in the flue gas duct. The ammonium hydrogen sulfate, the ammonium sulfate, and the like may cause clogging of an element of an air heater due to attachment thereto. Furthermore, the ammonium hydrogen sulfate and the ammonium sulfate are corrosive substances and thus the wall surface or the devices may corrode when these substances are attached thereto.
NH3+SO3+H20=NH4HSO4(2)
2NH3+SO3+H2O═(NH4)2SO4(3)

To reduce NOx, NH3is supplied in the upstream step of the denitrator. However, NH3is used for neutralization of SO3and thus the supply amount of NH3also needs to be adjusted.

Accordingly, gas analysis methods that enable to extract a part of flue gas, and perform a ultraviolet absorption analysis to analyze SO3and NH3in the flue gas and to measure concentrations of SO3and NH3in the flue gas have been conventionally proposed (see, for example, Patent Literature 2).

Coal-combustion flue gas or flue gas produced when heavy oil is combusted may contain metallic mercury (Hg0) in addition to soot dust, SOx, and NOx. Recently, methods or devices that enable to treat the metallic mercury (Hg0) by using a combination of the denitrator that reduces NOx and the wet desulfurizer that absorbs SOx have been variously developed.

As examples of the method that enables to treat the metallic mercury (Hg0) in flue gas, methods of spraying an NH4Cl solution in a liquid form on the upstream side of a reduction denitrator in a flue gas duct to supply the solution into the flue gas duct are proposed (see, for example, Patent Literatures 3 and 4). When the NH4Cl solution is sprayed in a liquid form into the flue gas duct, NH4Cl dissociates into ammonia (NH3) gas and hydrochloric acid (HCl) gas. The NH3gas acts as a reductant and the HCl gas acts as a mercury chlorinating agent. That is, on a denitration catalyst filled in the reduction denitrator, NH3has a reduction reaction proceeding with NOx in the flue gas as shown in the formula (1) and HCl has a reduction reaction proceeding with Hg0in the flue gas as shown in the following formula (4). After NH3is reductively denitrated on the denitration catalyst and the metallic mercury (Hg0) is oxidized to an aqueous mercury chloride (HgCl2), HgCl2is dissolved with water by a wet desulfurizer installed on the downstream side to remove mercury contained in the flue gas, and SOx contained in the flue gas is absorbed and removed.
Hg0+½O2+2HCl→HgCl2+H2O  (4)

CITATION LIST

Patent Literatures

SUMMARY

Technical Problem

However, when the NH4Cl solution is sprayed in a liquid form into the flue gas duct to oxidize Hg0contained in the flue gas to be treated in the desulfurizer, the conventional gas analysis method that enables to measure the concentrations of SO3and NH3in the flue gas as described in Patent Literature 2 cannot analyze chlorine ions (CL−) resulting from HCl generated by dissociation of NH4Cl. That is, when a device that supplies the NH4Cl solution to oxidize Hg0contained in the flue gas is added to a conventional flue-gas treatment device having a device that supplies NH3into the flue gas duct and when the concentration of NH3in the flue gas is measured as in the conventional technique as described in Patent Literature 2, whether a value of the NH3concentration obtained from analysis depends on the concentration of NH3supplied by the device that supplies NH3or on the concentration of NH3supplied by the device that supplies the NH4Cl solution cannot be determined.

Accordingly, a gas analysis device that enables to measure also a concentration of Cl−contained in the flue gas to determine a supply amount of the NH4Cl solution has been demanded.

The present invention has been achieved in view of the above problems and an object of the present invention is to provide a gas analysis device, a mercury removal system, a gas analysis method, and a removal method for mercury contained in flue gas that enable to measure a concentration of Cl−contained in the flue gas.

Solution to Problem

According to a first aspect of the present invention in order to solve the above problems, there is provided a gas analysis device including: a flue-gas extraction pipe that extracts, from a flue gas duct, flue gas that is emitted from a boiler and to which ammonium chloride is supplied; soot-dust removal unit that is provided in the flue-gas extraction pipe and removes soot dust contained in the extracted flue gas; a deposition unit that is provided in the flue-gas extraction pipe and deposits the ammonium chloride contained in the flue gas; and a measurement unit that measures the ammonium chloride contained the flue gas by detecting fluorescent X-rays generated by irradiation of the ammonium chloride deposited by the deposition unit with X-rays or laser beams.

According to a second aspect of the present invention, there is provided the gas analysis device according to the first aspect, wherein the flue gas further contains sulfurous acid, the deposition unit deposits sulfurous acid, and the measurement unit measures sulfurous acid.

According to a third aspect of the present invention, there is provided the mercury removal system that removes mercury contained in flue gas that is emitted from a boiler, the mercury removal system including: an ammonium-chloride supply unit that sprays a solution containing ammonium chloride into a flue gas duct of the boiler; a reduction denitrator that has a denitration catalyst reducing nitrogen oxides in the flue gas with ammonia and oxidizing mercury in coexistence of hydrogen chloride; a wet desulfurizer that removes the mercury oxidized in the reduction denitrator using an alkali absorbent; and an ammonium-chloride-concentration measurement unit that is provided on either one or both of upstream and downstream sides of the reduction denitrator and analyzes a concentration of the ammonium chloride contained in the flue gas, wherein the gas analysis device according to claim1is used as the ammonium-chloride-concentration measurement unit, and a spray amount of the solution containing the ammonium chloride is controlled according to the concentration of the ammonium chloride obtained by the ammonium-chloride-concentration measurement unit.

According to a fourth aspect of the present invention, there is provided the mercury removal system according to the third aspect, having a heat exchanger that is provided between the reduction denitrator and the wet desulfurizer and performs heat exchange with the flue gas having passed through the reduction denitrator for heat recovery, wherein a gas temperature of the flue gas that passes through the heat exchanger is controlled based on a relation between ammonium chloride concentrations and gas temperatures, which are obtained in advance.

According to a fifth aspect of the present invention, there is provided the mercury removal system according to the third aspect, having a heat exchanger that is provided between the reduction denitrator and the wet desulfurizer and performs heat exchange with the flue gas having passed through the reduction denitrator for heat recovery, wherein the gas analysis device according to claim2is used as the ammonium-chloride-concentration measurement unit, and a gas temperature of the flue gas passing through the heat exchanger is controlled based on either one or both of a relation between ammonium chloride concentrations and gas temperatures and a relation between sulfurous acid concentrations and gas temperatures, which are obtained in advance.

According to a sixth aspect of the present invention, there is provided a gas analysis method that enables to extract, from a flue gas duct, flue gas that is emitted from a boiler and to which ammonium chloride is supplied, remove soot dust contained in the flue gas, deposit the ammonium chloride contained in the flue gas, then cause the deposited ammonium chloride to be contained in analysis gas, extract the analysis gas, and measure the ammonium chloride contained in the analysis gas.

According to a seventh aspect of the present invention, there is provided the gas analysis method according to the sixth aspect, wherein the flue gas further contains sulfurous acid, the sulfurous acid is deposited in addition to the ammonium chloride, and the deposited sulfurous acid is measured.

According to an eighth aspect of the present invention, there is provided the mercury removal method that enables to remove mercury contained in flue gas emitted from a boiler, the removal method for mercury contained in flue gas including: an ammonium-chloride supply step of spraying a solution containing ammonium chloride into a flue gas duct of the boiler; a reduction denitration step of including a denitration catalyst that reduces nitrogen oxides in the flue gas with ammonia and oxidizes mercury in coexistence of hydrogen chloride; a wet desulfurization step of removing the mercury oxidized at the reduction denitration step using an alkali absorbent; and an ammonium-chloride-concentration measurement step of analyzing a concentration of the ammonium chloride contained in the flue gas on either one or both of upstream and downstream sides of the reduction denitrator, wherein the gas analysis method according to claim6is used at the ammonium-chloride-concentration measurement step, and a concentration of the ammonium chloride contained in the flue gas is obtained at the ammonium-chloride-concentration measurement step, and a spray amount of the solution containing the ammonium chloride is controlled according to the obtained concentration of the ammonium chloride.

According to a sixth aspect of the present invention, there is provided the removal method for mercury in flue gas according to the eighth aspect, including: a heat recovery step of performing heat exchange between the flue gas and a heating medium circulating in a heat exchanger between the reduction denitration step and the wet desulfurization step; and a reheat step of reheating cleaned gas emitted from the wet desulfurizer by performing heat exchange between the cleaned gas and the heating medium, wherein the gas analysis method according to claim6is used at the ammonium-chloride-concentration measurement step, and a gas temperature of the flue gas to be subjected to heat exchange with the heating medium at the heat recovery step is controlled based on a relation between ammonium chloride concentrations and gas temperatures, which are obtained in advance.

According to a tenth aspect of the present invention, there is provided the removal method for mercury in flue gas according to the eight aspect, including: a heat recovery step of performing heat exchange between the flue gas and a heating medium circulating in a heat exchanger between the reduction denitration step and the wet desulfurization step; and a reheat step of reheating cleaned gas emitted from the wet desulfurizer by performing heat exchange between the cleaned gas and the heating medium, wherein the gas analysis method according to the seventh aspect is used at the ammonium-chloride-concentration measurement step, and a gas temperature of the flue gas subjected to heat exchange with the heating medium at the heat recovery step is controlled based on either one or both of a relation between ammonium chloride concentrations and gas temperatures and a relation between sulfurous acid concentrations and gas temperatures, which are obtained in advance.

Advantageous Effects of Invention

According to the present invention, ammonium chloride contained in flue gas is deposited and then the deposited ammonium chloride is analyzed to measure a concentration of Cl−contained in the flue gas, thereby enabling to obtain a concentration of the ammonium chloride contained in the flue gas.

DESCRIPTION OF EMBODIMENTS

Modes for preferably carrying out the present invention (hereinafter, “embodiment”) will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the contents of the following embodiments. Constituent elements in the following embodiments include those that can be easily anticipated by persons skilled in the art, that are substantially identical, or that are in a so-called range of equivalents. Furthermore, constituent elements disclosed in the following embodiments can be combined as appropriate.

First Embodiment

A gas analysis device according to a first embodiment of the present invention is explained with reference to the drawings.FIG. 1is a schematic diagram of a gas analysis device according to the first embodiment, andFIG. 2depicts a configuration of a roll filter in a simplified manner. As shown inFIGS. 1 and 2, a gas analysis device10according to the present embodiment includes a flue-gas extraction pipe13that extracts flue gas11A from a flue gas duct12to which flue gas11containing both of ammonium chloride (NH4Cl) and sulfurous acid (SO3) is fed, a collector (soot-dust removal means)14that is provided in the flue-gas extraction pipe13and removes soot dust contained in the extracted flue gas11A, a roll filter (deposition means)15that is provided in the flue-gas extraction pipe13and deposits both of NH4Cl and SO3contained in the flue gas11A, and a measurement device (measurement means)19that measures both of NH4Cl and SO3contained in the flue gas11A by irradiating a sample16containing both of NH4Cl and SO3deposited by the roll filter15with X-rays17and detecting fluorescent X-rays18generated from the sample16.

The flue gas11is emitted from a boiler and the flue gas11contains SO3. Because an NH4Cl solution is supplied to the flue gas11within the flue gas duct12, the flue gas11contains NH4Cl. While gas components to be measured in the flue gas11contain both of NH4Cl and SO3, the present embodiment is not limited thereto and it suffices that the flue gas11is gas containing at least NH4Cl. The gas components contained in the flue gas11can contain also nitric oxide (NO), carbon monoxide (CO), water (H2O), nitrogen dioxide (NO2), methane (CH4), ammonia, benzene, or the like, in addition to the NH4Cl and SO3.

The flue-gas extraction pipe13is connected to the flue gas duct12and a part of the flue gas11flowing in the flue gas duct12is extracted through the flue-gas extraction pipe13. An adjustment valve V11is provided in the flue-gas extraction pipe13to adjust a flow rate of the flue gas11to be extracted from the flue gas dust12to the flue-gas extraction pipe13. Because the flue gas11can be continuously extracted through the flue-gas extraction pipe13, the gas components in the flue gas11can be semicontinuously measured.

The flue gas11A extracted to the flue-gas extraction pipe13is fed to the collector14through the flue-gas extraction pipe13. The collector14removes the soot dust contained in the flue gas11A. While a cyclone dust-collection device is used, for example, as the collector14, the present embodiment is not particularly limited thereto.

After the soot dust contained in the flue gas11A is removed by the collector14, the flue gas11A is fed to the roll filter15. The collector14has a soot-dust conveyance pipe21that emits the collected soot dust. The dust removed from the flue gas11A by the collector14is returned from the soot-dust conveyance pipe21to the flue gas duct12.

The roll filter15has a pair of rollers22, a conveyance belt23, a filter24, and a flue-gas feed pipe25. In the roll filter15, the pair of rollers22rotates to rotate also the conveyance belt23and move the filter24. The measurement device19analyzes concentrations of NH4Cl and SO3contained in the flue gas11A. For example, a fluorescent X-ray analysis device is used as the measurement device19. The measurement device19has an X-ray irradiation device26that irradiates the sample16with the X-rays17, and a detector27that detects fluorescent X-rays18generated from the sample16. The measurement device19has an opening19aon the side of a wall surface on which the filter24is placed, and the filter24is irradiated with the X-rays17from the X-ray irradiation device26therethrough. The flue gas11A fed from the flue-gas extraction pipe13to the flue-gas feed pipe25is fed to the filter24. When the flue gas11A passes through the filter24, NH4Cl and SO3contained in the flue gas11A is adsorbed by the filter24and a sample16acontaining both of NH4Cl and SO3is deposited on the filter24. The sample16adeposited on the roll filter15is conveyed to the measurement device19with movement of the filter24. A sample16bhaving moved near the opening19ais irradiated with the X-rays17from the X-ray irradiation device26in the measurement device19. NH4Cl and SO3contained in the sample16bare excited with irradiation of the X-rays17. The fluorescent X-rays18are generated from the excited NH4Cl and SO3. The generated fluorescent X-rays18are detected and analyzed by the detector27. The detector27analyzes NH4Cl and SO3contained in the flue gas11A based on energy of the fluorescent X-rays18that are emitted when NH4Cl and SO3in the sample16bis irradiated with the X-rays17.

The measurement device19is not limited to the fluorescent X-ray analysis device and another analysis device can be used as long as it can analyze NH4Cl and SO3contained in the flue gas11A.

Because the roll filter15deposits NH4Cl and SO3contained in the flue gas11A in the flue-gas extraction pipe13, it is preferable that the flue-gas feed pipe25through which the flue gas11A flows have a heater on an outer circumference of the flue-gas feed pipe25and heat the flue gas11A to prevent moisture contained in the flue gas11A from condensing and analysis accuracy in the measurement device19from reducing.

The gas analysis device10according to the present embodiment deposits NH4Cl contained in the flue gas11and then analyzes the fluorescent X-rays18generated from the deposited NH4Cl and SO3, thereby enabling the concentrations of ammonium ions (NH4+), chloride ions (Cl−), and SO3contained in the flue gas11to be stably and simultaneously analyzed. Accordingly, the concentrations of NH4Cl and SO3contained in the flue gas11can be stably and simultaneously measured. Therefore, when Hg contained in the flue gas11is to be oxidized, the concentration of the NH4Cl solution supplied into the flue gas duct12can be properly obtained even when the NH4Cl solution is supplied into the flue gas duct12in addition to NH3gas or NH3water.

While the solution including NH4Cl is used for the flue gas11in the gas analysis device10according to the present embodiment, the present embodiment is not limited thereto and any auxiliary agent can be used as long as it generates oxidizing gas to be used for oxidizing Hg when vaporized and reducing gas to be used for reducing NOx. Because the solution including NH4Cl is used in the present embodiment, the HCl gas is used as the oxidizing gas and the NH3gas is used as the reducing gas. Other than the solution including NH4Cl, a solution including an ammonium halide such as ammonium bromide (NH4Br) or ammonium iodide (NH4I) can be used.

Second Embodiment

An Hg removal system according to a second embodiment of the present invention is explained with reference to the drawings.FIG. 3is a schematic diagram of an Hg removal system according to the second embodiment. Because the Hg removal system according to the present embodiment uses the gas analysis device10according to the first embodiment shown inFIGS. 1 and 2as an NH4Cl measurement device (NH4Cl measurement means), explanations of the NH4Cl measurement device will be omitted.

As shown inFIG. 3, an Hg removal system30A according to the present embodiment is an Hg removal system that removes Hg contained in the flue gas11emitted from a boiler31, and has an NH4Cl-solution supply means32that sprays an NH4Cl solution41, a reduction denitrator (reduction denitration means)33that has a denitration catalyst for reducing NOx in the flue gas11with NH3gas and oxidizing Hg0in coexistence of HCl gas, an air heater (AH)34that performs heat exchange with the denitrated flue gas11, an electrostatic precipitator (ESP)35that removes soot dust in the denitrated flue gas11, a wet desulfurizer37that removes Hg oxidized in the reduction denitrator33by using a limestone-gypsum slurry (alkali absorbent), and NH4Cl measurement devices (NH4Cl measurement means)38-1and38-2that are provided on upstream and downstream sides of the reduction denitrator33to analyze a concentration of NH4Cl contained in the flue gas11, respectively, within the flue gas duct12downstream of the boiler31.

The NH4Cl solution41is supplied from the NH4Cl-solution supply means32to the flue gas11emitted from the boiler31. The NH4CL-solution supply means32has a spray nozzle42for oxidizing Hg0contained in the flue gas11, an ammonium chloride (NH4Cl)-solution supply pipe43that supplies the NH4Cl solution41in a liquid form to the spray nozzle42, and an air supply pipe45that supplies air44to the spray nozzle42to compress the NH4Cl solution41to be sprayed into the flue gas duct12.

The spray nozzle42is a two-fluid nozzle that is provided to be inserted into the flue gas duct12and simultaneously sprays the NH4Cl solution41and the air44into the flue gas duct12.FIG. 4schematically depicts a configuration of the spray nozzle. As shown inFIG. 4, the spray nozzle42is formed of a double pipe48including an inner pipe46and an outer pipe47, and a nozzle head49provided at the head of the double pipe48. The inner pipe46is used for feeding the NH4Cl solution41. The outer pipe47is provided to cover an outer circumference of the inner pipe46and is used for feeding the air44into a space formed with the inner pipe46. The spray nozzle42sprays the NH4Cl solution41into the flue gas duct12(seeFIG. 3) and also sprays the air44into the flue gas duct12, from the nozzle head49.

As shown inFIG. 3, the NH4Cl solution41is fed from an NH4Cl solution tank51to the spray nozzle42through the NH4Cl-solution supply pipe43. A flow rate of the NH4Cl solution41supplied from the NH4Cl-solution supply pipe43is adjusted by an adjustment valve V21. The NH4Cl solution41is adjusted in the NH4Cl solution tank51to have a predetermined concentration. The NH4Cl solution41can be generated by dissolving ammonia chloride (NH4Cl) powder in water. The predetermined concentration of the NH4Cl solution41can be adjusted by adjusting supply amounts of the NH4Cl powder and the water. Alternatively, the NH4Cl solution41can be generated by mixing an HCl solution and an NH3solution in a predetermined proportion in concentration.

The air44is fed from an air supply unit52to the spray nozzle42through the air supply pipe45and is used as compression air when the NH4Cl solution41is sprayed from the nozzle head49. By atomizing the NH4Cl solution41with an air stream of the air44, the NH4Cl solution41to be sprayed from the nozzle head49can be sprayed as fine liquid droplets into the flue gas duct12. A flow rate of the air44supplied from the air supply pipe45is adjusted by an adjustment valve V22.

As shown inFIG. 4, the liquid droplets of the NH4Cl solution41sprayed from the nozzle head49into the flue gas duct12evaporate due to a high ambient temperature of the flue gas11to generate fine solid particles of NH4Cl, and decompose into HCl and NH3and sublimate as shown in the following formula (5). Accordingly, the NH4Cl solution41sprayed from the spray nozzle42is decomposed to generate HCl and NH3, and NH3gas and HCl gas can be supplied into the flue gas duct12.
NH4Cl→NH3+HCl  (5)

As shown inFIG. 3, the size of the liquid droplets of the NH4Cl solution41sprayed from nozzle holes of the nozzle head49can be adjusted by using the flow rate of the air44supplied from the air supply pipe45. The flow rate of the air44sprayed from the nozzle head49is preferably an air/water ratio of 100 or higher and 10000 or lower (in volume), for example. This is to spray the NH4Cl solution41from the nozzle head49into the flue gas duct12as fine liquid droplets.

Because the air44flows in the space between the inner pipe46and the outer pipe47as shown inFIG. 4, the air44acts for cooling the NH4Cl solution41and can suppress heat of the flue gas11in the flue gas duct12from being transmitted to the NH4Cl solution41via the air44as shown inFIG. 3. Because heating of the NH4Cl solution41by the heat of the flue gas11can be suppressed, the NH4Cl solution41can be kept in a liquid state until immediately before it is sprayed.

As shown inFIG. 3, after being caused to contain the HCl gas and the NH3gas that is generated from the liquid droplets of the NH4Cl solution41sprayed from the NH4Cl-solution supply means32into the flue gas duct12, the flue gas11is fed to the reduction denitrator33. The reduction denitrator33uses the NH3gas generated by decomposition of NH4Cl for reduction denitration of NOx and uses the HCl gas for oxidation of Hg, thereby removing NOx and Hg from the flue gas11.

That is, on the denitration catalyst filled in the reduction denitrator33, the NH3gas reductively denitrates NOx as shown in the following formula (6) and the HCl gas performs mercury oxidation of Hg as shown in the following formula (7).
4NO+4NH3+O2→4N2+6H2O  (6)
Hg+½O2+2HCl→HgCl2+H2O  (7)

While the reduction denitrator33has one denitration catalyst layer53, the present embodiment is not limited thereto and the number of denitration catalyst layers53in the reduction denitrator33can be appropriately changed according to denitration performance.

After reduction of NOx and oxidation of Hg in the flue gas11is performed in the reduction denitrator33, the flue gas11passes through the air heater34and the precipitator (ESP)35and then is fed to the wet desulfurizer37.

In the wet desulfurizer37, the flue gas11is fed from the wall surface side of a bottom portion in a device body55, and a limestone-gypsum slurry36to be used as the alkali absorbent is fed into the device body55through an absorbent feed line54to be jetted from a nozzle56toward a top portion. The flue gas11rising from the bottom portion of the device body55and the limestone-gypsum slurry36jetted from the nozzle56to flow down is caused to face each other to be in gas-liquid contact, and HgCl2and sulfur oxides (SOx) in the flue gas11are absorbed in the limestone-gypsum slurry36to be separated and removed from the flue gas11, thereby cleaning the flue gas11. The flue gas11cleaned by the limestone-gypsum slurry36is emitted from the top portion as cleaned gas57and discharged from a stack58to outside of the system.

The limestone-gypsum slurry36used to desulfurize the flue gas11is generated by mixing limestone slurry CaCO3, which is obtained by dissolving limestone powder in water, gypsum slurry CaSO4, which is obtained by causing limestone and SOx in the flue gas11to react with each other and to be oxidized, and water. The limestone-gypsum slurry36is used by pumping the fluid stored in a bottom portion59of the device body55of the wet desulfurizer37, for example. SOx in the flue gas11reacts with the limestone-gypsum slurry36in the device body55as shown in the following formula (8).
CaCO3+SO2+0.5H2O→CaSO3.0.5H2O+CO2(8)

Meanwhile, the limestone-gypsum slurry36that has absorbed SOx in the flue gas11is mixed with water61supplied into the device body55and is oxidized by air62supplied into the bottom portion59of the device body55.

At that time, the limestone-gypsum slurry36having flowed down in the device body55reacts with the water61and the air62as shown in the following formula (9).
CaSO3.0.5H2O+0.5O2+1.5H2O→CaSO4.2H2O  (9)

The limestone-gypsum slurry36being stored in the bottom portion59of the wet desulfurizer37and having been used for desulfurization is oxidized, then drawn off from the bottom portion59, fed to a dewaterer63, and then discharged to outside of the system as dewatered cake (gypsum)64including mercury chloride (HgCl). For example, a belt filter is used as the dewaterer63. Filtrate obtained by dewatering (post-dewatering filtrate) is subjected to effluent treatment such as removal of suspended solids and heavy metals in the post-dewatering filtrate and pH adjustment of the post-dewatering filtrate. A part of the post-dewatering filtrate subjected to the effluent treatment is returned to the wet desulfurizer37and the remaining part of the post-dewatering filtrate is treated as water discharge.

While the limestone-gypsum slurry36is used as the alkali absorbent, any solution can be used as the alkali absorbent as long as it can absorb HgCl2in the flue gas11.

The limestone-gypsum slurry36does not always need to be jetted toward the top portion from the nozzle56and can be flowed down from the nozzle56to face the flue gas11, for example.

(Control of Spray Amount of NH4Cl Solution)

The NH4Cl measurement device38-1is provided on the upstream side of the reduction denitrator33, and the NH4Cl measurement device38-2is provided on the downstream side of the reduction denitrator33. The NH4Cl measurement devices38-1and38-2use the gas analysis device10according to the first embodiment shown inFIGS. 1 and 2, as mentioned above. Therefore, the NH4Cl measurement devices38-1and38-2can analyze the concentration of NH4Cl supplied from the spray nozzle42into the flue gas11. For example, when the boiler31is a coal combustion boiler31, the flue gas11contains also SO3. The NH4Cl measurement devices38-1and38-2can measure also the concentration of SO3contained in the flue gas11and accordingly the NH4Cl measurement devices38-1and38-2can simultaneously measure the concentrations of NH4Cl and SO3contained in the flue gas11.

Measurement results of the concentration of NH4Cl contained in the flue gas11, measured by the NH4Cl measurement devices38-1and38-2, are transmitted to a controller70. A map indicating a relation between NH4Cl concentrations and gas temperatures at which NH4Cl deposits and a map indicating a relation between SO3concentrations and gas temperatures at which SO3deposits, which are obtained in advance, are recorded in the controller70. For example, the gas temperature at which NH4Cl deposits increases as the NH4Cl concentration increases, and the gas temperature at which SO3deposits increases as the SO3concentration increases. When the map indicating the relation between NH4Cl concentrations and gas temperatures at which NH4Cl deposits and the map indicating the relation between SO3concentrations and gas temperatures at which SO3deposits are obtained in advance, the gas temperature can be adjusted according to the NH4Cl concentration or the SO3concentration to prevent deposition of NH4Cl or SO3.

The controller70can obtain the concentration of NH4Cl contained in the flue gas11by analyzing the concentration of Cl−contained in the flue gas11based on the map indicating the relation between NH4Cl concentrations and gas temperatures at which NH4Cl deposits, which is obtained in advance from the measurement results of the concentration of NH4Cl contained in the flue gas11, measured by the NH4Cl measurement devices38-1and38-2. By obtaining the concentration of NH4Cl contained in the flue gas11, the controller70can control the spray amount of the NH4Cl solution, so that the NH4Cl solution can be sprayed from the spray nozzle42in an appropriate spray amount.

Because the NH4Cl measurement devices38-1and38-2can measure also the concentration of SO3in addition to the concentration of NH4Cl contained in the flue gas11, the NH4Cl measurement devices38-1and38-2transmit the concentration of SO3contained in the flue gas11to the controller70. The controller70can obtain the concentration of SO3contained in the flue gas11by analyzing the concentration of SO3contained in the flue gas11based on the map indicating the relation between SO3concentrations and gas temperatures at which SO3deposits, which is obtained in advance from measurement results of the concentration of SO3contained in the flue gas11, measured by the NH4Cl measurement devices38-1and38-2. By obtaining the concentration of SO3contained in the flue gas11, the controller70can control the spray amount of the NH4Cl solution, so that the NH4Cl solution can be sprayed from the spray nozzle42in an appropriate spray amount.

As described above, the Hg removal system30A to which a spray device is applied according to the present embodiment can stably and simultaneously analyze the concentrations of NH4+, CL−, and SO3contained in the flue gas11and thus can stably and simultaneously measure the concentrations of NH4Cl and SO3contained in the flue gas11. Therefore, the NH4Cl solution41can be sprayed from the spray nozzle42into the flue gas duct12in an appropriate amount and accordingly Hg removal performance and NOx reduction performance can be stably maintained in the reduction denitrator33. Corrosion of spray facilities such as the outer pipe47of the spray nozzle42can be avoided, which realizes a stable operation and also enables lives of devices such as the spray nozzle42to be extended and costs required for maintenance of the devices to be reduced. Furthermore, when an NH3-water supply means that supplies NH3water into the flue gas duct12is installed, the concentration of the NH4Cl solution supplied into the flue gas duct12can be appropriately obtained even when the NH4Cl-solution supply means32is newly installed in the flue gas duct12.

A flowmeter71that measures a flow rate of the flue gas11is provided on the upstream side of the spray nozzle42. The flow rate of the flue gas11is measured by the flowmeter71. The value of the flow rate of the flue gas11measured by the flowmeter71is transmitted to the controller70and the flow rate, angle, initial velocity, and the like, at which the NH4Cl solution41is to be sprayed from the nozzle head49can be adjusted based on the flow rate value of the flue gas11.

An NOx concentration meter72is provided on the side of an outlet of the reduction denitrator33. The value of the concentration of NOx in the flue gas11measured by the NOx concentration meter72is transmitted to the controller70. The controller70can check a NOx reduction ratio in the reduction denitrator33based on the concentration value of NOx in the flue gas11measured by the NOx concentration meter72. Therefore, based on the value of the concentration of NOx in the flue gas11measured by the NOx concentration meter72, the NH4Cl concentration and the supply amount of the NH4Cl solution41sprayed from the spray nozzle42can be adjusted and also the supply amount of the NH3water separately supplied into the flue gas11can be adjusted to adjust an NH3mixture ratio. Accordingly, NOx in the flue gas11can be reduced in the reduction denitrator33and the reduction denitrator33can meet predetermined denitration performance.

Hg concentration meters73-1to73-3that measure content of Hg in the flue gas11emitted from the boiler31are provided in the flue gas duct12. The Hg concentration meter73-1is provided in the flue gas duct12between the boiler31and the nozzle head49, the Hg concentration meter73-2is provided between the reduction denitrator33and the air heater34, and the Hg concentration meter73-3is provided on the downstream side of the wet desulfurizer37. Values of the concentration of Hg in the flue gas11measured by the Hg concentration meters73-1to73-3are transmitted to the controller70. The controller70can check the content of Hg contained in the flue gas11from the values of the concentration of Hg in the flue gas11measured by the Hg concentration meters73-1to73-3. Specifically, the Hg concentration meters73-1to73-3each can optionally measure metallic mercury Hg0, mercury oxide Hg2+, and total mercury (an amount of mercury including the metallic mercury Hg0and the mercury oxide Hg2+). When a ratio of the mercury oxide Hg2+to the total mercury is known by using the Hg concentration meters73-2and73-3, a mercury oxidation rate of Hg contained in the flue gas11can be obtained. By controlling the NH4Cl concentration and the supply flow rate of the NH4Cl solution41based on the values of the concentration of Hg in the flue gas11measured by the Hg concentration meters73-1to73-3and the mercury oxidation rate, the NH4Cl concentration and the supply flow rate of the NH4Cl solution41sprayed from the nozzle head49can be controlled to meet predetermined denitration performance and keep Hg oxidation performance.

An oxidation-reduction-potential measurement controller (ORP controller)74that measures an oxidation-reduction potential of the limestone-gypsum slurry36is provided in the bottom portion59of the wet desulfurizer37. The value of the oxidation-reduction potential of the limestone-gypsum slurry36is measured by the ORP controller74. The supply amount of the air62to be supplied into the bottom portion59of the wet desulfurizer37is adjusted based on the measured value of the oxidation-reduction potential. By adjusting the supply amount of the air62to be supplied into the bottom portion59, reduction of oxidized Hg collected in the limestone-gypsum slurry36that is stored in the bottom portion59of the wet desulfurizer37and diffusion thereof from the stack58can be prevented.

The oxidation-reduction potential of the limestone-gypsum slurry36in the wet desulfurizer37is preferably in a range of no less than 0 millivolt and no larger than 600 millivolts, for example, to prevent re-entrainment of Hg from the limestone-gypsum slurry36. When the oxidation-reduction potential is in this range, Hg collected as HgCl2in the limestone-gypsum slurry36is stabilized and re-entrainment into air can be prevented.

While the solution including NH4Cl is used to oxidize Hg and reduce NOx in the Hg removal system30A according to the present embodiment, the present embodiment is not limited thereto and a solution including ammonium halide such as NH4Br or NH41can be used other than the solution including NH4Cl, as mentioned above.

Third Embodiment

An Hg removal system to which a spray device is applied according to a third embodiment of the present invention is explained with reference to the drawings.FIG. 5depicts a configuration of the Hg removal system according to the third embodiment, andFIG. 6depicts a configuration of a heat exchanger in a simplified manner. Members of the Hg removal system that are redundant to constituent elements in the Hg removal system according to the second embodiment of the present invention are denoted by like reference signs and explanations thereof will be omitted.

As shown inFIGS. 5 and 6, an Hg removal system30B according to the present embodiment has a heat exchanger80installed between the air heater34and the precipitator35to perform heat exchange with the flue gas11having passed through the reduction denitrator33for heat recovery. The heat exchanger80includes a heat recovery unit81and a reheater82. The heat recovery unit81is provided between the air heater34and the precipitator35and performs heat exchange between the flue gas11emitted from the boiler31and a heating medium83circulating in the heat exchanger80. The gas temperature of the flue gas11emitted from the boiler31is in a range between 130° C. to 150° C., for example, and the gas temperature of the flue gas11emitted from the heat recovery unit81falls in a range between 80° C. to 100° C., for example, by the heat exchange of the flue gas11with the heating medium83circulating in the heat exchanger80. The reheater82is provided on the downstream side of the wet desulfurizer37and performs heat exchange between the cleaned gas57emitted from the wet desulfurizer37and the heating medium83to reheat the cleaned gas57.

The heat exchanger80has a heating-medium circulating passage84for the heating medium83to circulate through the heat recovery unit81and the reheater82. The heating medium83circulates between the heat recovery unit81and the reheater82via the heating-medium circulating passage84. A plurality of finned tubes85are provided on the surface of the heating-medium circulating passage84located within the heat recovery unit81and the reheater82. A heat exchange unit86is provided on the heating-medium circulating passage84and heat exchange of the heating medium83with steam87is performed to adjust a medium temperature of the heating medium83.

Because the concentration of NH4Cl and the concentration of SO3contained in the flue gas11can be measured by the NH4Cl measurement devices38-1and38-2, the controller70increases the medium temperature of the heating medium83by causing the heat exchange unit86to perform heat exchange of the heating medium83with the steam87based on either one or both of the map indicating the relation between NH4Cl concentrations and gas temperatures at which NH4Cl deposits or the map indicating the relation between SO3concentrations and gas temperatures at which SO3deposits, which are obtained in advance. When the gas temperature of the flue gas11on the outlet side of the heat recovery unit81is set to be equal to or higher than the gas temperature at which the NH4Cl and SO3deposit, deposition of SO3on the finned tubes85of the heat recovery unit81can be suppressed. Accordingly, corrosion of the finned tubes85of the heat recovery unit81can be suppressed.

When the amount of the heating medium83flowing in the heat recovery unit81is reduced, a heat recovery amount recovered by the heating medium83in the heat recovery unit81is reduced and thus the heat recovery unit81keeps a high outlet gas temperature. Because an amount of heat of the heating medium83flowing in the reheater82is small in this case, the temperature of the cleaned gas57entering the reheater82cannot be increased. Accordingly, to increase the temperature of the cleaned gas57having passed through the reheater82, an amount of the steam87to be added is increased to increase the heat amount of the heating medium83flowing in the reheater82, so that the temperature of the cleaned gas57passing through the reheater82can be increased.

When the amount of the heating medium83flowing in the heat recovery unit81is increased, the heat recovery amount recovered by the heating medium83in the heat recovery unit81is increased. Accordingly, the outlet gas temperature of the flue gas11exiting the heat recovery unit81is lowered and the temperature of the heating medium83flowing in the reheater82is increased, which increases the temperature of the cleaned gas57entering the reheater82. Therefore, the supply amount of the steam87supplied for heat exchange with the heating medium83can be reduced.

The heating medium83is supplied to the heating-medium circulating passage84from a heating medium tank88. The heating medium83is circulated through the heating-medium circulating passage84by a heating-medium feed pump89. The supply amount of the steam87is adjusted by an adjustment valve V31according to the gas temperature of the cleaned gas57, and the heating medium83to be fed to the reheater82is supplied to the heat recovery unit81by an adjustment valve V32according to the gas temperature of the flue gas11emitted from the heat recovery unit81, thereby adjusting the supply amount of the heating medium83to be fed to the reheater82.

Therefore, the concentration of NH4Cl and the concentration of SO3contained in the flue gas11are measured by the NH4Cl measurement devices38-1and38-2, the medium temperature of the heating medium83is increased based on either one or both of the map indicating the relation between NH4Cl concentrations and gas temperatures at which NH4Cl deposits and the map indicating the relation between SO3concentrations and gas temperatures at which SO3deposits, which are obtained in advance, and the gas temperature of the flue gas11on the outlet side of the heat recovery unit81is set to be equal to or higher than the gas temperature at which NH4Cl and SO3deposits. Accordingly, deposition of NH4Cl and SO3on facilities installed within the flue gas duct12, such as the finned tubes85of the heat recovery unit81, can be suppressed and corrosion of the finned tubes85of the heat recovery unit81and the like can be suppressed.

While the heat exchanger80is provided between the air heater34and the precipitator35in the present embodiment, the present embodiment is not limited thereto and it suffices to provide the heat exchanger80between the reduction denitrator33and the wet desulfurizer37.

REFERENCE SIGNS LIST