Method of operating a denitration apparatus for a coal-fired boiler

Abstract: An apparatus installed between a fuel economizer and an air preheater for the denitration of exhaust gas from a coal-fired boiler includes at least one vertical flue (5) situated upstream of a reactor (3) containing a denitration catalyst (3') to cause an upward flow of the gas. A hopper (6) is located below the vertical flue (5) for collecting ash separated from the gas. A baffle (7) is preferably provided in the flue (5) to ensure the still more effective separation of ash from the gas.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to a method of operating a denitration apparatus for a coal-fired boiler as defined in the preamble of claims 1 and 2. 2. Description of the Prior Art: A known denitration apparatus for a coal-fired boiler is usually installed between the exit of a fuel economizer and an air preheater, and comprises a denitration reactor 3 containing a denitration catalyst 3' and connected at its top to a horizontal flue 4 connected to the exit of the fuel economizer in the boiler 1, as shown in Figure 4. The fuel economizer has a hopper 2 at its exit. The exhaust gas of a coal-fired boiler contains several tens of grams of ash per Nm 3 , but there has not been any clogging of the catalyst 3' in the denitration reactor 3 by any such ash, since the ash particles usually have a diameter not exceeding 1 mm, and even the largest particles have a diameter of, say, only 2 mm. The exhaust gas of a coal-fired wet-bottom boiler in which a molten slag is dropped from a combustion furnace to be discharged, however, contains a large amount of coarse ash, as shown in Figure 6 and Table 1. The dry-bottom boiler which also appears in Figure 6 and Table 1 is of the type in which combustion takes place at a high temperature and all the slag and ash, as well as combustion gases, are discharged from the combustion furnace. The catalyst employed in the denitration reactor is usually of the honeycomb or plate type, though three types of catalyst are shown in Figure 7. In the denitration apparatus, ammonia is injected into the exhaust gas to convert nitrogen oxide to harmless water and nitrogen in accordance with the following reaction with the aid of the catalyst: 4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O If the exhaust gas contains a large amount of coarse ash, however, the coarse particles leaving the boiler 1 shown in Figure 4 flow horizontally through the flue 4 into the denitration reactor 3, though they are partly trapped in the hopper 2 at the exit of the fuel economizer, as shown in Figure 5 showing the result of simulation of gas flow in the apparatus shown in Figure 4. The coarse ash particles reaching the denitration reactor 3 gather on the upstream side of the catalyst 3', as shown in Figure 8(a), and prevent the exhaust gas from flowing smoothly into the denitration reactor 3, thereby lowering the performance of the apparatus, particularly when the boiler is of the wet-bottom type. A method and apparatus for cleaning hot flue or process gases are described in EP-A-0600440. Exhaust gas from a combustor is first cleaned in a particle separator and then guided through a horizontal inlet duct connecting the top section of the particle separator with a top section of a vertically extending gas inlet space. From the inlet space the gas enters a filer module and a reactor where a catalytic reaction is performed with the gas. A hopper is provided at the bottom portion of the vertically extending inlet space to collect fly ash and other particles separated in the filter module. A similar apparatus is also described in DE-A-4135817. The document JP-A-2724176 describes a process according to claims 1 and 2, where the gas from the fuel economizer is passed through a horizontal and then upwardly through a vertical passage at a flow rate of 10 m/s. SUMMARY OF THE INVENTION It is the object of the present invention to provide a method of operating a denitration apparatus for with a coil-fired boiler which enables the reliable and effective denitration of exhaust gas from the boiler by collecting coarse ash particles effectively to prevent them from flowing in to a denitration reactor according to independant claims 1 and 2. The coarse particles of ash which the exhaust gas of the boiler contains are effectively removed from the gas flowing up the vertical passage at a rate of preferably 10 to 15 meters per second, and are collected in the hopper. Therefore, there is no fear of coarse particles of ash entering the denitration reactor, but the denitration apparatus always remains effective and reliable. The baffle ensures the still more effective removal of ash. If it is a porous plate having a pore diameter which is smaller than the diameter of ash particles to be removed, the baffle ensures the effective separation of any ash particles coarser than its pores. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view showing the layout of an apparatus embodying this invention; Figure 2 is a view showing the result of simulation of the flow of gas containing coarse ash particles in the apparatus shown in Figure 1; Figure 3 is a view similar to Figure 1, but showing a modified form of apparatus; Figure 4 is a view showing the layout of a known denitration apparatus for a coal-fired boiler; Figure 5 is a view similar to Figure 2, but showing the simulation of flow in the apparatus shown in Figure 4; Figure 6 is a graph comparing the particle diameter distributions of ash in the exhaust gases of a wet-bottom boiler and a dry-bottom boiler; Figure 7 is a view showing different forms of denitration catalyst; Figure 8(a) is a view showing the deposition of ash in the denitration reactor in the known apparatus; Figure 8(b) is an enlarged view of part A of Figure 8(a); and Figure 9 is a graph comparing the separation of coarse ash particles having a diameter of 2 mm or more. DETAILED DESCRIPTION OF THE INVENTION An apparatus used in embodying this invention is shown in Figure 1. It has a vertical flue 5 having a lower end connected to the downstream end of a horizontal flue 4 and an upper end connected to the top of a denitration reactor 3, and thereby defining a vertical passage for an upward flow of gas from the horizontal flue to the reactor, while a hopper 6 is provided below the vertical flue 5. In any other portion thereof, the apparatus is identical in construction to the known apparatus shown in Figure 4 Therefore, like numerals are used to indicate like parts in Figures 1 and 4, and no repeated description is made of any common parts. The exhaust gas of the boiler 1 is caused to flow at a rate of 10 to 15 meters per second through the horizontal and vertical flues 4 and 5. Despite its high flow rate as stated, the vertical flue 5 in which the gas is directed to flow upwardly makes it possible to separate coarse particles of ash from the gas and thereby prevent any clogging of the catalyst 3' in the denitration reactor 3 that would give rise to a reduction in efficiency of the denitration reaction. The minimum flow rate of 10 meters per second is highly desirable to prevent the deposition of ash in the denitration reactor 3, while the maximum flow rate not exceeding 15 meters per second is recommended to avoid any wear that ash might cause to the walls of the flues 4 and 5. Figure 2 shows a simulated flow of gas containing coarse ash particles as discharged from the boiler when it is of the wet-bottom type. As is obvious from Figure 2, coarse ash particles are effectively collected in the hopper 2 at the exit of the fuel economizer and in the hopper 6 below the vertical flue 5, and only the gas from which the coarse particles have thoroughly been removed enters the denitration reactor 3. Although the flow rate of the gas in the vertical flue 5 is be as high as 10 to 15 meters per second, the upward flow of the gas therein enables the removal of at least 80% of coarse ash particles having a diameter of 2 mm or more, which is a drastic improvement over the separation of coarse ash particles from a horizontal flow of gas, as is obvious from Figure 9. Moreover, the high flow rate of gas in the vertical flue 5 makes it possible to reduce its cross section and thereby the cost of its construction A modified form of apparatus used on embodying this invention is shown in Figure 3. It is substantially identical to the apparatus shown in Figure 1, but further includes a baffle 7 provided across the vertical flue 5 for stopping ash. The baffle 7 comprises a porous plate having a porosity of at least 60% to ensure a small pressure loss of the gas flowing therethrough. It has a pore diameter which is smaller than the diameter of ash particles to be removed. Thus, the baffle 7 can separate any and all ash particles larger than its pores from the gas flowing through the flue 5. Tests have been conducted by employing baffles having different porosities to determine the pressure loss caused by those baffles. The results are shown in Table 2 below. Porosity (%) Pressure loss 70 90 60 100 (standard) 50 120 40 150 30 200 As is obvious from Table 2, the baffles having a porosity lower than 60% cause a sharply increasing pressure loss. Thus, the baffle 7 according to this invention preferably has a porosity of at least 60%. It is also to be understood that, though the apparatus shown in Figures 1 and 3 have both been shown as having a single vertical flue, they may alternatively have a plurality of vertical flues.

Claims: A method of operating a denitration apparatus for a coal-fired boiler arrangement, the denitration apparatus being installed between the exit of a fuel economizer and an air preheater, said method comprising the steps of: passing gas from the fuel economizer through a horizontal passage (4) and then upwardly through at least one vertical passage (5) to a top thereof, the at least one vertical passage having a hopper (6) located therebelow; passing the gas from the top of the at least one vertical passage to an inlet of a denitration reactor (3); wherein the gas is passed horizontally through the horizontal passage (4) at a flow rate of 10 to 15 meters per second and then upwardly through the at least one vertical passage (5) at a flow rate of 10 to 15 meters per second, whereby ash is separated from the gas and collected in the hopper (6); and blocking ash, in the vertical passage, from reaching the denitration reactor (3) with a porous member (7) having a pore diameter which is smaller than a predetermined particle diameter of the ash. A method of operating a denitration apparatus with a coal-fired boiler arrangement, comprising: combusting coal and passing the combustion gas through a coal fired boiler (1) so as to produce an exhaust gas; passing the exhaust gas through a fuel economizer connected with said coal fired boiler (1) and then to an exit of said fuel economizer; passing the exhaust gas through a horizontal passage (4) having an upstream end connected at said exit of said fuel economizer and a downstream end, at a flow rate of 10 to 15 meters per second; passing the exhaust gas from the horizontal passage (4) to an exhaust gas denitration apparatus installed between said downstream end and an air preheater, said denitration apparatus including a denitration reactor (3), the exhaust gas passing upwardly through a vertical passage (5) provided upstream of and preceding said denitration reactor (3) at a flow rate of 10 to 15 meters per second so as to cause a flow of the exhaust gas to the top of the vertical passage (5) and then to an inlet of the denitration reactor (3); and passing the exhaust gas through a porous member (7) in the vertical passage and blocking ash from reaching the denitration reactor (3), said porous member (7) having a pore diameter which is smaller than a predetermined particle diameter of the ash, and separating the ash from the exhaust gas and collecting the ash in a hopper (6) located below said vertical passage (5) and upstream of said porous member (7), wherein said vertical passage (5) extends upwardly from said downstream end of said horizontal passage (4) to said denitration reactor (3). The method of claim 2, wherein said porous member (7) is a porous plate. The method of claim 2, wherein said combusting comprises combusting coal in a combustion furnace and dropping molten slag into a coal fired wet-bottom boiler. The method of claim 1 or 4, further comprising trapping ash in a further hopper (2) located at the exit of the fuel economizer. The method of any one of claims 1 to 5, wherein said porous member (7) has a porosity of at least 60%. The method of claim 1 or 2, wherein at least 80% of coarse ash particles having a diameter of two millimeters or more are removed from the gas between the horizontal passage (4) and the denitration reactor (3) by passing the gas vertically through the at least one vertical passage (5) at the rate of 10 to 15 meters per second. The method of claim 1 or 2, wherein the flow rate of 10 meters per second through the horizontal passage (4) and through the at least one vertical passage (5) is a minimum flow rate, and the flow rate of 15 meters per second through the horizontal passage (4) and through the at least one vertical passage (5) is a maximum flow rate.

Inventors: KANEKO SHOZO, KOBAYASHI NORIHISA, MORII ATSUSHI, SUYAMA KOUHEI, KANEKO, SHOZO, KOBAYASHI, NORIHISA, MORII, ATSUSHI, SUYAMA, KOUHEI, Kaneko, Shozo, c/o Mitsubishi Jukogyo K.K., KOBAYASHI, NORIHISA, C/O MITSUBISHI JUKOGYO K.K., Morii, Atsushi, c/o Nagasaki Shipyard &amp; Mach.Works, Suyama, Kouhei, c/o Nagasaki Shipyard &amp; Mach.Works
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1995
Main classification: B01D53
IPC codes: B01D53, F23J15