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 natural 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 designed 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 rate 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 available 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 rate, addi¬ tional burners can cause fouling problems and flow dis¬ turbances, because the introduction of new flow rates in the boiler change the flow patterns in the furnace thus altering the fuel combustion process, the flame pattern, stack gas composition and heat transfer on the boiler internal surfaces.

It is an object of the present invention to provide a 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.

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 levels of combustion air infeed or at a greater number of combustion air infeed levels.

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

Furthermore, the arrangement according to the invention is characterized by what is stated in the characterizing part of claim 7.

The invention provides significant benefits.

By virtue of the invention, existing boiler constructions can use very 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

feeding the supplementary fuel gas into the lower part of the boiler. The invention achieves stable combustion and causes only minor changes in the internal flow pattern of the furnace. In the method, the flame radiation stabil¬ 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.

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 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 the combustion of a product gas is connected as a supplementary fuel infeed to an existing boiler. By 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 using burners, changes required 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 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.

The method according to the invention can be applied to the combustion of all 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 natural 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 ignited 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 locus 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 determines 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

8 used for the infeed of the solid fuel are arranged in rows on the wall of the furnace 2, whereby the combustion air required by the burners is fed into the furnace via air infeed nozzles adapted to the burners 8, 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 product 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 utilize 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 combus- tion 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

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 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 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 natural gas or oil burner. Further, a single boiler can be used for burning product gases from several different gasifiers, which may find advantageous use in the gasification of wastes into fuel.