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Title:
METHOD AND ARRANGEMENT FOR REDUCING NITROGEN OXIDE EMISSIONS FROMA FLUIDIZED BED COMBUSTION
Document Type and Number:
WIPO Patent Application WO/2002/090829
Kind Code:
A1
Abstract:
The invention relates to a method and arrangement for use in the combustion chamber (2) of a fluidized bed boiler (1) having a bubbling fluidized bed (5) operative in the bottom portion thereof, the method serving to reduce emissions of nitrogen oxides generated in fluidized bed combustion. In the method, primary air (4) is injected into the combustion chamber (2) upward from the bottom (3) of said combustion chamber (2), fuel (6) is fed into the fluidized bed (5) and secondary air (7) is injected into the combustion chamber (2) at a distance above the fluidized bed (5). Into the combustion chamber (2) is injected circulating gas into a region remaining between the top level of the fluidized bed (2) and the inlet point of the secondary air (7) or at the level of the secondary air inlet (7) separately from the inlet of the secondary air.

Inventors:
SALOKOSKI PIA (FI)
ALIN JARI (FI)
FABRITIUS MARKO (FI)
KILPINEN PIA (FI)
ZABETTA EDGARDO CODA (FI)
Application Number:
PCT/FI2001/000445
Publication Date:
November 14, 2002
Filing Date:
May 09, 2001
Export Citation:
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Assignee:
FORTUM OYJ (FI)
SALOKOSKI PIA (FI)
ALIN JARI (FI)
FABRITIUS MARKO (FI)
KILPINEN PIA (FI)
ZABETTA EDGARDO CODA (FI)
International Classes:
F23C9/00; F23C10/00; F23L7/00; (IPC1-7): F23C10/00
Domestic Patent References:
WO1993005340A11993-03-18
Foreign References:
JPS5664203A1981-06-01
DE3814314C11989-06-22
EP0237712A11987-09-23
US4766851A1988-08-30
JPS5952106A1984-03-26
Attorney, Agent or Firm:
SEPPO LAINE OY (Itämerenkatu 3 B Helsinki, FI)
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Claims:
What is claimed is:
1. Method for use in the combustion chamber (2) of a fluidized bed boiler (1) having a bubbling fluidized bed (5) operative in the bottom portion thereof, the method serving to reduce emissions of nitrogen oxides generated in fluidized bed combustion, and the method comprising the steps of injecting primary air (4) into the combustion chamber (2) upward from the bottom (3) of said combustion chamber (2), feeding fuel (6) into the fluidized bed (5), injecting secondary air (7) into the combustion chamber (2) at a distance above the fluidized bed (5), characterized in that into the combustion chamber (2) is injected circulating gas into a region remaining between the top level of the fluidized bed (5) and the inlet point of the secondary air (7) or, alternatively, at the level of the secondary air inlet (7) separately from the inlet of the secondary air.
2. Method according to claim 1, characterized in that the circulating gas is flue gas from the fluidized bed boiler (1).
3. Method according to claim 1, characterized in that the circulating gas is flue gas from a source external to the fluidized bed boiler (1).
4. Method according to claim 2, characterized in that the temperature of the circu lating gas is 120 to 200 °C.
5. Method according to claim 3, characterized in that the temperature of the circu lating gas is 120 to 600 °C.
6. Method according to claim 1, characterized in that the oxygen content of the circulating gas is 1.5 to 10 vol. %, most advantageously 2 to 5 vol. %.
7. Method according to claim 1, characterized in that the circulating gas comprises oxides of nitrogen.
8. Method according to claim 1, characterized in that the mass flow rate of the circulating gas is advantageously 5 to 40 %, most advantageously 10 to 30 % of the total air flow rate required for combustion in the combustion chamber (2).
9. Method according to claim 1, characterized in that into the combustion chamber is admitted tertiary air (8) at a distance above the inlet point of secondary air (7).
10. Method according to claim 1, characterized in that the circulating gas is flue gas of the boiler (1), taken to the combustion chamber (2) from a point downstream from a flue gas cleaning device (13).
11. Arrangement for reducing emissions of nitrogen oxides generated in combustion in the combustion chamber (2) of a fluidized bed boiler (1), the arrangement comprising a bubbling fluidized bed (5) operative in the bottom portion of a combustion chamber (2), primary air injection means (4) adapted to the bottom (3) of the combustion chamber (2) for injecting primary air upward from the bottom (3) into the fluidized bed (5), fuel feed means (6) for feeding fuel into the combustion chamber (2), and secondary air inlet means (7) for injecting secondary air into the combustion chamber (2), characterized by separate circulating gas injection means (14) for injecting circulating gas into the combustion chamber (2), whereby said means are adapted at a level between the top level of the fluidized bed (5) and the inlet point of the secondary air (7) or at the level of the secondary air inlet (7).
12. Assembly according to claim 9, characterized in that the circulating gas injection means (14) are directed tangentially.
Description:
Method and arrangement for reducing nitrogen oxide emissions from a fluidized bed combustion The invention relates to a method according to the preamble of claim 1 and to an arrangement according to the preamble of claim 11 for reducing nitrogen oxide emissions in fluidized bed combustion.

In fluidised bed combustion, the fuel is combusted and partially gasified in a fluidized bed which resides above a diffuser located in the bottom portion of the combustion chamber of the fluidised bed boiler and is formed by particulate matter bed material and the fuel mixed therewith. Conventionally, the bed material is sand.

The bed is maintained in a fluidized state by way of injecting fluidizing air upwards into the bed from nozzles located in the grid. Boilers utilizing fluidised bed combustion may be categorized on the basis of their bed type into two main types: boilers with a bubbling fluidized bed and boilers with a circulating fluidized bed. In a bubbling fluidized bed, the velocity of the fluidizing air injected into the fluidized bed is so low as to allow the fluidized bed to form in the bottom portion of the combustion chamber. In contrast, a circulating fluidized bed uses a much higher velocity of the fluidizing air, whereby a fraction of the bed material can rise with the fluidizing air to the upper portion of the combustion chamber, wherefrom it is circulated back to the bottom portion of the combustion chamber. A bubbling fluidized bed generally also uses a coarser bed material than that used in a circulating fluidized bed. In a circulating fluidized bed boiler, the fuel, combustion air and bed material become effectively mixed with each other, whereby the combustion chamber is operated at an equalized temperature and the occurrence of local temperature peaks is avoided.

It is an object of the invention to provide a novel method and system for reducing nitrogen oxide emissions in bubbling fluidized bed combustion.

The goal of the invention is achieved by means of feeding circulating gas to the bottom portion of the combustion chamber of a fluidized bed boiler, to above the fluidized bed. The circulating gas is fed into the region remaining between the inlet point of the secondary air and the top level of the fluidized bed or, alternatively, at the level of the secondary air inlet; however, so as to prevent the circulating gas from becoming mixed with the secondary air either before being discharged into the combustion chamber or immediately after entering the same. Such a mixing would be detrimental because it counteracts the object of establishing reactions in which nitrogen oxides are reduced. The circulating gas can be flue gas taken from the fluidized bed boiler proper or from a combusting system operating external to the boiler. The oxygen and nitrogen oxides contained in the circulating gas react with nitrogen compounds emerging from the fluidized bed thus forming molecular nitrogen N2. Hence, the invention is only suited for use in fluidized bed boilers operating with a bubbling fluidized bed. In contrast, the arrangement according to the invention is suited not for use in circulating fluidized bed boilers because in these the boiler construction, flow conditions and temperature distributions are essentially different from those prevailing in boilers operating with a bubbling fluidized bed.

In the prior art, circulating gas has been admitted into the combustion chamber of a fluidized bed boiler in order to cool the bed. For instance, patent publication EP 0,603,262 teaches injection of circulating gas into the upper portion of the combus- tion chamber of a circulating fluidized bed boiler in order to cool the combustion chamber. Also known in the art is the injection of circulating gas into the fluidized bed so as to cool the bed. However, none of these embodiments can provide similar conditions and chemical reactions as the arrangement according to the invention for reducing nitrogen oxide emissions.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the assembly according to the invention is characterized by what is stated in the characterizing part of claim 11.

The invention offers significant benefits.

An essential reduction in the nitrogen oxide emissions of a fluidized bed boiler can be attained by way of admitting circulating gas into the bottom portion of the combustion chamber of the boiler, at a level above the fluidized bed. The arrange- ment according to the invention has a very simple construction inasmuch it only needs piping for the circulating gas with feed means connected thereto for passing the circulating gas into the combustion chamber of the boiler, as well as control devices connected thereto for adjusting the flow of the circulating gas. Accordingly, the assembly according to the invention is easy to adapt to existing boilers, too.

In the following, the invention is examined in more detail by way of referring to the attached drawing, wherein is schematically illustrated a fluidized bed boiler operating with a bubbling fluidized bed.

The walls of the combustion chamber 2 of a fluidized bed boiler 1 are in a conven- tional manner lined with boiler tubes containing flowing water or steam into which the heat released by the combustion of the fuel is transferred. The bottom portion of the combustion chamber 2 is formed by a air grid 3 via which ash formed in the combustion is removed from the combustion chamber 2. The air distribution is pro- vided with nozzles wherefrom fluidizing air 4 is injected upward into the combustion chamber 2. The fluidizing air 4 also serves as the primary combustion air for the fluidized bed 5 formed by particulate matter. The velocity of the fluidizing air 4 is adjusted such that the fluidized bed 5 is formed in the bottom portion of the combustion chamber 2 and no substantial loss of the bed material from the combustion chamber 2 along with the gas flow will take place, thus generally disposing with the need for circulating the bed material particles back to the bed 5.

This type of fluidized bed is also known as a bubbling fluidized bed. Although a fraction of the bed material particles can rise with the flow of the primary air 4 up to the middle portion of the combustion chamber 2, a bubbling fluidized bed 5 has a clearly discernible top level. The infeed rate of the primary air 4 is about 35 % of the total air feed required for complete combustion. The fuel is introduced at a distance above the fluidized bed 5 via fuel feed means 6 such as openings provided on the walls of the combustion chamber 2.

To above the fuel feed means 6 into the combustion chamber 2 is fed secondary air via secondary air inlet means 7 at an infeed rate typically representing about 35 % of the total air feed into the combustion chamber 2. The secondary air inlet means 7 are adapted on the walls of the combustion chamber 2 displaced at a distance above the top level of the bubbling fluidized bed 5. Additionally, tertiary air is admitted into the combustion chamber 2 at an infeed rate of about 30 % of the total combustion air feed into the boiler. The tertiary air inlet means 8 are adapted on the walls of the combustion chamber 2, at a distance above the secondary air inlet means 7. The arrangement according to the invention may also be used in such fluidized bed boilers in which the fuel feed means 6 are located above the secondary air inlet means 7.

Fuel fed to above the fluidized bed 5 falls into the bed material wherein it is dried and partially gasified. Pyrolysis gases formed in the gasification of the fuel react with the oxygen of the primary air and simultaneously rise upward in the combustion chamber 2. The ungasified fraction of the fuel is combusted both in the fluidized bed 5 and, with a continuous diminishing size of fuel particles, also above the bed 5.

A fraction of the pyrolysis gases rising from the fluidized bed 5, as well as the ungasified portion of the fuel introduced into the bed, typically coke, and the carbon monoxide generated in the combustion reactions of coke are combusted in a burning zone created at the level of the secondary air inlet means 7. The possibly still combustable fraction of the flue gases after passing the secondary air inlet means 7 is brought to complete combustion with the help of the tertiary air 8.

As the flue gases rise above the tertiary air inlet means 8, they meet a superheater 9 located in the upper portion of the combustion chamber 2. After passing the super- heater 9, the flue gases flow past heat recovery panels comprising a feed water preheater 10 and a combustion air preheater 11. Prior to the entry of the flue gases into a smoke stack 12, the particulate matter traveling with the flue gases is removed by means of a cleaning device such as an electrostatic filter 13.

In the arrangement according to the invention, circulating gas is injected to the bottom portion of the combustion chamber 2. Separate means 14 for injecting the circulating gas are adapted on the walls of the combustion chamber 2 or on the boiler walls at a point between the secondary air inlet means 7 and the top level of the bubbling fluidized bed 5. Alternatively, the circulating gas injection means 14 and the discharge point of the circulating gas may be located at the same level with the secondary air inlet means 7 with the provision that the circulating gas is prevented from becoming mixed with the secondary air either before being discharged into the combustion chamber 2 or immediately after entering the combustion chamber 2.

Such a mixing of the circulating gas with the secondary air before the injection of the circulating gas into the combustion chamber 2, at the injection point of the gas into the combustion chamber 2 or in a close vicinity of the same does not allow enough time for the reactions required for the reduction of nitrogen oxide emissions to take place, thus nullifying the goal of the invention. The circulating gas can be generally taken from the flue gases formed by the combustion process in the combustion chamber 2 by way of passing a fraction of the flue gases, advantageously taken via a duct connected to a point between a flue gas blower 15 and the smoke stack 12, with the help of a blower 16 back to the combustion chamber 2. The dust content of the circulating gas is low, since the gas is first purified in a cleaning device 13.

The oxygen content of the circulating gas is typically 1.5 to 10 vol. %, most advanta- geously 2 to 5 vol. % and the gas temperature is 120 to 200 °C. The mass flow rate of the circulating gas is advantageously 5 to 40 %, most advantageously 10 to 30 % of the total air flow rate required in combustion. To achieve an efficient mixing of the circulating gas with the process gas rising from the fluidized bed 5, the circulating gas injection means 14 may be placed, e. g., on opposite walls of the combustion chamber 2. In this manner, the circulating gas can be injected tangentially into the combustion chamber 2.

The oxygen and nitrogen oxides contained in the circulating gas injected into the combustion chamber 2 react with the nitrogen compounds rising from the fluidized bed 5 so as to form molecular nitrogen N2. Herein, the fluidized bed 5 is operated in a reducing state. The temperature and steam content of the circulating gas affect the reaction rates and paths of the chemical processes taking place. The reactions are sensitive to both of the above-mentioned parameters.

The injection of the circulation gas lowers the temperature of flue gases in the combustion chamber 2 in the region between the secondary air inlet means 7 and the tertiary air inlet means 8 to about 1-100 °C. Without the injection of the circulating gas, this temperature would typically be in the order of 1200-1300 °C. The circulating gas injection also improves the mixing of flue gases in the region above the fluidized bed 5 thus preventing bundling of the gas flows rising from bed.

Resultingly, a homogeneous gas flow is created in the secondary air inlet zone. The oxygen imported by the circulating gas promotes combustion in the bottom portion of the combustion chamber 2 thus compensating for the temperature decrease due to the imported circulating gas in the circulating gas injection zone.

The fuel to be combusted in the fluidized bed boiler 1 must be relatively dry and have a high heat value in order to obtain the desired outcome from the injection of the circulating gas. The fuel shall advantageously have a heat value greater than 6 MJ/kg, most advantageously greater than 7 MJ/kg.

In the context of the present invention, the term circulating gas refers to any type of flue gas formed in conjunction with combustion so that the gas has a sufficiently low oxygen content and contains nitrogen oxides. In lieu of or in addition to the flue gases of a fluidized bed boiler, the circulating gas may be taken from an external combustion process, such as that of a combustion engine, if the exhaust gas delivered by the same has a sufficiently low oxygen content. Herein, the temperature of the admitted gas may be higher than that mentioned above, even up to 600 °C. Typically, the exhaust gas of a combustion engine contains a plurality of uncombusted compo- nents: hydrocarbons, soot and carbon monoxide. Additionally, the exhaust gas is rich with nitrogen oxides and, if the engine is run using a sulfur-containing fuel, oxides of sulfur. The uncombusted components of the exhaust gas will burn in the combustion chamber 2 and the nitrogen oxides can thereby interact with the above-mentioned chemical reactions. The oxides of sulfur are removed in the sulfur removal system of the fluidized bed boiler 1. The exhaust gas of a gas turbine, however, is not suitable for use as the circulating gas inasmuch its oxygen content due to the high air/fuel ratio is greater than 10 vol. %.