LV12242B

LV12242B - Method and arrangement for burning gas in a furnace

Info

Publication number: LV12242B
Application number: LVP-98-285A
Authority: LV
Inventors: Markku Raiko; Tommy Jacobson; Kari J��SKEL�INEN
Original assignee: Imatran Voima Oy
Priority date: 1996-06-28
Filing date: 1998-12-23
Publication date: 1999-06-20
Also published as: AU3264397A; CN1228830A; HUP0101129A2; WO1998000675A1; EP0907870A1; EE9800442A; HUP0101129A3; FI962669A0; LV12242A; PL330818A1; CZ430998A3; FI100355B; RO117982B1; SK177498A3

Description

1 LV 12242

Method and arrangement for burning gas in a furnace

The present invention relates to a method according to the preamble of claim 1 for burning fuel gas in a fur-nace, particularly for burning gases of low heating value in a boiler principally fired with another main fuel.

The invention also concerns an arrangement suitable for implementing said method.

The invention is particularly suited for firing with gases produced by air gasification, blast-furnace gas and other gases of low heating value. The invention is utilized in, e.g., performing fuel changes in the firing of a boiler and it can also be used for firing gases of high heating value such as natūrai gas.

Conventionally, the product gas obtained from a separate gasifier has been fed into the furnace via burners. If the product gas has a low heating value, the burners must be designea for a low axial velocity in order to assure reliable ignition, whereby the burner dimensions become very large. When firing with low-caloric-value gases such as gases produced by air gasification, the gas feed volume rāte becomes extremely large, which complicates fuel feed at a low velocity and makes the burner size larger. Furthermore, the adaptation of larger burners in an existing furnace is technically difficult, and additionally, revamping an old boiler with more burners cuts down the availabie heat transfer surface by a substantial amount. Difficulties in achieving a sufficient residence time in product-gas firing and construction constraints may also cause complications in the arrangement of additional burners. While product gases of high heating value can be fed at higher axial velocities, the adaptation of new burners in an existing boiler is nonetheless difficult. As boilers are generally designed for a specific fuel and air flow rāte, addi-tional burners can cause fouling problems and flow dis-turbances, because the introduction of new flow rātes in the boiler change the flow patterns in the furnace thus 5 altering the fuel combustion process, the flame pattern, stack gas composition and heat transfer on the boiler internai surfaces.

It is an object of the present invention to provide a 10 method suited for burning low-caloric-value gas particularly in existing boilers designed for other type of main fuel without any need for major construction changes in the boiler. '5 The goal of the invention is achieved by feeding the pro-duct gas of a gasifier or other gas to be burned into the lower part of the boiler under oxygen-depleted conditions and then burning the gas in the upper part of the boiler at desired Ievels of combustion air infeed or at a 20 greater number of combustion air infeed Ievels.

More specifically, the method according to the invention is characterized by what is statea in the characterizing part of claim I. 25

Furthermore, the arrangeraent according to the invention is characterized by what is stated in the characterizing part of claim 7. 30 The invention provides significant benefits.

By virtue of the invention, existing boiler constructions can use verv advantageously gases of rather low heating value with minor modifications in the boiler. The heat transfer surface area of the steam generator need not be reduced and boilers equipped with combustion gas re-circulation can use the existing recirculation system for 35 LV 12242 *ϊ feeding the supplementary fuel gas into the lower part of the boiler. The invention achieves stable combustion and causes only minor changes in the internai flow pattern of the furnace. In the method, the flame radiation stabil-5 izes the ignition and burning of the supplementary fuel gas. Furthermore, the novel infeed arrangement offers simpler revamping in an existing boiler, because gas inlet openings are easier to adapt in the furnace than large burners. 10

The novel method is more flexible with respect to the fuel type and facilitates wider possibilities over the prior art to reduce the amount of nitrogen oxides in stack gas by combustion control means. As the method “5 achieves strong staging in the furnace by means of a correctly timed air feed scheme, the boiler can be operated in the same fashion as staged low-NOx burners.

The benefits of the method will be most pronounced when 20 the combustion of a product gas is connected as a supplementary fuel infeed to an existing boiler. Bv virtue of feeding the product gas via nozzles adapted to the lower part of the furnace or via openings made to a possible combustion gas recirculation system, instead of 25 using burners, changes reguired on the heat-transfer surfaces remain essentially smaller than when equipping the boiler for product-gas firing by means of burners. Additionally, the above-described combustion process benefits are gained. Furthermore, if a new boiler is 30 particularly designed for mixed fuel use, particularly advantageous conditions depending on the product gas com-position can be attained for the reburning effect, which reduces the NOx emissions. Herein, the hydrocarbon radicals formed from the fuel reduce nitrogen monoxide back to molecular nitrogen thus reducing the total amount of NOx emissions. 35 4

The method according to the invention can be applied to the combusrion of ali types of gaseous fuels. However, the benefits of the method related to boiler construction are best utilized in the combustion of low-caloric-value gases such as gasification gas produced by air gasification, blast-furnace gas and waste gases. Nevertheless, the above-described benefits of improved combustion may also be exploited when burning gases of high caloric value gases such as natūrai gas.

Furthermore, product gas can be burned as the main fuel.

In the following the invention will be examined in greater detail by making reference to the appended drawing in which an embodiment of the invention is diagrammatically illustrated.

Referring to the diagram, the product gas is fed into an air-deficient (reducing) zone in the lower part 10 of the furnace 2, thus assuring sufficient residence time and avoiding furnace explosions. As no combustion air is fed into this zone, the gas is igmted and burned only in the combustion air flows which are fed into the upper zones of the furnace. Air can be fed into the boiler via burners 8 feeding other fuels and/or via separate air infeed nozzles 9. The ločus of combustion and thereby further the heat-transfer distribution within the steam-generating Circuit of the boiler 1 can be effectively controlled by means of the vertical distribution of air infeed, which determinēs the location of the different zones of the combustion process in the furnace 2. The air feed distribution also offers efficient means for con-trolling the formation Chain of nitrogen oxides in the furnace 2, whereby a correctly staged air infeed scheme can achieve essential reduction of obnoxious emissions.

The boiler 1 shown in the diagram is a conventional boiler designed, e.g., for burning coal or peat. Burners LV 12242 3 used for the infeed of the solid fuel are arranged in rows on the wall of the furnace 2, wherebv the combustion air required by the burners is fed into the furnace via air infeed nozzles adapted to the burners 3, and when required, also via auxiliary air infeed nozzles located in the vicinity thereof. In addition to the burner air infeed nozzles, the boiler 1 is equipped with at least one top auxiliary air infeed nozzle 9 for feeding air into the upper part of the furnace 2. Conventionally, the upper part of the boiler 1 includes superheaters for high-temperature heating of the steam exiting the steam generator, arranged to surround the furnace 2, and an exit connection of the stack gas duct.

The producr gas is taken to the lower part 10 of the furnace 2, underneath the burners 8, into a zone of no air infeed thus creating strongly reducing conditions therein. The gas is produced in a gasifier 5, which may be an air gasifier, blast furnace or any other unit in which is produced or is formed a combustible gas. The invention makes it possible to utilizē gases of extremely low heating value, thereby exploiting their heat value and simultaneously achieving efficient and cost-effective reduction of obnoxious emissions as the advanced combustion process of a large boiler with the stack gas clean-ing equipment connected thereto also performs effective combustion/cleaning of the gas produced in a gasifier. From the gasifier, the gas to be burned is taken via a large-diameter duct 6 to nozzles 7 located in the lower part 10 of the furnace, wherefrom the gas can flow upward in the furnace. Although the gas will not ignite in the air-deficient lower part of the furnace, it will be heated and readily ignited in the upper zones under flame radiation of the furnace 2 (that is, the thermal radia-tion of the combustion process) and oxygen introduced via the air nozzles of the burners 8. The combustion process in the furnace is further staged by feeding the rest of 6 the required combustion air to the upper part of the furnace via the top auxiliary air nozzles 9.

In addition to the above-described example, alternative 5 embodiments of the invention may be contemplated.

The method is also applicable to boilers adapted for burning product gas only, whereby separate burners are not necessarily required, but instead, the burners may be 10 replaced by air infeed nozzles alone. However, in this case it may be advisable to assure gas ignition and com-bustion with the help of, e.g., a natūrai gas or oil burner. Further, a single boiler can be used for burning product gases from several different gasifiers, which xnay 15 find advantageous use in the gasification of wastes into fuel.

Abstract

The invention relatcs to a method and ar-rangemem for buming fuel gas in a furnace (2). particulariy for buming gases of !ow heating value in a boiler (1) principally fired with a main fuel. The invention is based on feeding the product gas of a gasifier (S) or other combustible gas into air-deficient conditions of the lower pait (10) of the fumace (2) and the gas is combusted upper in the furnace (2) at a desired Ievel of combustion air infeed, or aitematively, at multiple infeed leveis of the combustion air. LV 12242

Claims: 1. A method of burning gas in a furnace, c h a r a c -terized in that - the gas to be burned is fed into a reducing zone in the lower part (10) of said furnace (2} , and - combustion air is fed into the furnace (2) in at least one zone located above the infeed zone (10) of the gas to be burned. 2. A method as defined in claim 1, in which method gas is burned in a boiler equipped with at least one burner (8) for burning some other fuel than the gas to be burned, characterized in that the gas to be burned is fed into the furnace (2) underneath the burners (8) . 3. A method as defined in claim 2, characterized in that a portion of the combustion air is fed into the furnace (2) via the air nozzles of said burners (8) and another portion via at least one air infeed nozzle (9) adapted above said burners. 4. A method as defined in claim 1, characterized in that a portion of the combustion air is fed into the furnace (2) in a first stage adapted above the gas infeed point and auxiliary air is fed into the furnace in at least one stage adapted above the first air infeed point. 5. A method as defined in any foregoing claim, characterized in that the gas to be burned is fed into the furnace (2) via a combustion gas recirculation system. ο Ο 6. Α method as defined in any foregoing claim, characterized in that the gas to be burned is gas produced by air gasification, blast-furnace gas, waste gas or other gas of low heating value. 7. An arrangement for burning gas in a furnace (2), :haracterized by - openings (7) adapted to the reducing zone in the lower part (10) of said furnace (2), via which openings the gas is fed into said furnace (2), and - at least one air infeed nozzle for feeding combustion air into said furnace (2) in at least one zone located above the infeed zone (10) of the gas to be burned. 8. An arrangement as defined in claim 7, said arrangement comprising in its furnace at least one burner (8) for burning other fuel than the gas to be burned, characterized in that the openings (7) for feeding the gas to be burned into the furnace (2) are adapted beiow said burners.(8). 9. An arrangement as defined in claim 8, charac-terized by air nozzles of said burners (8), through which nozzles a portion of the combustion air can be fed into the furnace and by at least one air nozzle (9) located above said burners (8) for feeding auxiliary air into the furnace (2). 10. An arrangement as defined in claim 7, characterized by air nozzles through which a portion of the combustion air can be fed into the furnace (2) in the first stage adapted above the infeed point of the fuel gas and by at least one auxiliary air nozzle (9) located LV 12242 9 above the said first air infeed point for feeding auxiliary air in at least one stage into the furnace (2). 11. An arrangement as defined in any of foregoing claims 5 7-10, characterized in that the gas to be burned is fed into the furnace (2) via stack gas recirculating system. 12. An arrangement as defined in any of foregoing claims 7-11, characterized in that said arrangement comprises an apparatus (5) for generating the fuel gas and at least one nozzle (6) for feeding the fuel gas into said furnace (2) for combustion therein. 10

Claims

A method for incinerating a gas in a furnace, characterized in that: - the combustible gas is introduced into the reducing zone at the lower part (10) of said furnace (2) and - the combustion air is introduced into the furnace (2) in at least one zone above the combustion plant; gas supply areas (10).

2. The method of claim 1, wherein the gas is burned by a boiler equipped with at least one burner (8) for combustion of another fuel, with the exception of combustible gas, characterized in that the combustible gas is introduced into the furnace (2) below the burners (8). .

3. A method according to claim 2, characterized in that part of the combustion air is fed into the furnace (2) through said air burners (8) of said burner (8) and the other part is fed through at least one air supply nozzle (9) located above mentioned burners.

4. A method according to claim 1, characterized in that a portion of the air for combustion is introduced into the furnace (2) in the first zone formed above the gas supply point and additional air is introduced into the furnace in at least one zone above the first air supply site. .

A method according to any one of the preceding claims, characterized in that the combustible gas is introduced into the furnace (2) via a gas-gas recirculation system.

6. A method according to any one of the preceding claims, characterized in that the gas to be incinerated is a gas obtained by gasification by air, blast furnace gas and waste gas or other low calorific gas.

A gas combustion plant in the furnace (2), characterized in that it is provided with apertures (7) adapted to the reducing zone in the lower part (10) of said furnace (2) through which gas is introduced into said furnace (2), and - at least one air supply nozzle for supplying air for combustion in said furnace (2) in at least one zone located above the combustion air supply zone (10).

8. An apparatus according to claim 7, wherein said furnace comprises at least one burner (8) for the combustion of other fuels, with the exception of combustible gas. characterized in that the openings (7) for supplying the combustible gas to the furnace (2) are below said burners (8).

9. An apparatus according to claim 8, characterized in that said burners (8) have air nozzles through which parts of the combustion air can be introduced into the furnace and have at least one air nozzle (9) located above said burners (9). 8), in addition to supplying air to the oven (2).

10. Apparatus according to claim 7, characterized in that it is equipped with air nozzles through which parts of the combustion air can be introduced into the furnace (2) in the first zone above the flue gas supply and at least one additional air nozzle (9). ), located above the first air supply site, in addition to supplying air at least at one stage in the furnace (2).

Apparatus according to any one of the preceding claims 7 to 10, characterized in that the combustible gas is introduced into the furnace (2) via a combustion gas recirculation system.

Apparatus according to any one of claims 7 to 11, characterized in that said apparatus comprises apparatus (5) for generating combustible gas and at least one nozzle (6) for supplying combustible gas to said furnace (2) for its incineration.