EP2479491A1

EP2479491A1 - Method and burner for burning lean gas in a power plant boiler

Info

Publication number: EP2479491A1
Application number: EP11151532A
Authority: EP
Inventors: Pauli Dernjatin; Kati Savolainen
Original assignee: Fortum Oyj
Current assignee: Fortum Oyj
Priority date: 2011-01-20
Filing date: 2011-01-20
Publication date: 2012-07-25
Anticipated expiration: 2031-01-20
Also published as: WO2012098174A1; PL2479491T3; EP2479491B1

Abstract

To burn lean fuel gas in a furnace of a power plant boiler, a fuel gas flow is fed into the furnace via a fuel pipe (1), a primary air flow is fed centrally into the fuel gas flow discharging from an outlet (14) of the fuel pipe (1), and a secondary air flow is fed peripherally into the fuel gas flow discharging from the outlet (14) of the fuel pipe (1). The outlet (14) is provided with an inner stabilizing ring (4) and an outer stabilizing ring (8), both boosting flame ignition. The primary air channel (2) is provided with a first swirler (3) and the secondary air channel (6) is provided with a second swirler (7) to set the respective air flows into a rotation.

Description

FIELD OF THE INVENTION

The invention relates to a method for burning lean fuel gas, such as gas from gasification of biofuel or waste, in a furnace of a power plant boiler. The invention also concerns a burner for burning lean fuel gas.

BACKGROUND OF THE INVENTION

Currently coal and oil are mainly combusted by means of dedicated burners in different kinds of boilers. As a result of burning coal and oil, carbon dioxide, which is a greenhouse gas, is emitted into the atmosphere. To reduce greenhouse gas emissions, renewable fuels of biological origin could be used to replace part of coal and oil. The method is then called co-firing of coal/oil and biofuels.
One practical way of carrying out co-firing comprises gasification of biofuels and combustion of the lean product gas thus produced in a boiler by separate burners. An advantage of this method is that gasification allows use of very heterogeneous fuels, such as wood, peat, waste, sludge, coal, etc., and it is still possible to produce fuel gas of uniform quality for burners. A drawback of this method is its high investment cost.
Typically, the calorific value of the product gas from gasification is from about 3 to 6 MJ/m³n, whereas the calorific value of natural gas is about 49 MJ/m³n. Furthermore, the product gas from gasification does not contain any oxygen, which is necessary for burning. The temperature of the product gas is usually from 600°C to 900°C, and the gas may contain from 100 to 200 ppm hydrogen sulphide. Stable combustion of this kind of product gas with low calorific value is much more difficult than combustion of natural gas, which is why specific burning apparatus is required. The high temperature and high hydrogen sulphide content together create a corrosion risk that should also be taken into consideration.
Conventionally, the product gas obtained from a separate gasifier has been fed into a furnace via burners. As the product gas has a low calorific value, the burners have been designed for a low axial velocity in order to assure reliable ignition, whereby the burner's dimensions have become very large. The gas volume flow becomes extremely large, which further complicates the fuel feed at a low velocity. Mounting large burners in an existing furnace is technically difficult, and adding new burners to an existing boiler cuts down the available heat transfer surface of the boiler. The additional burners may cause fouling and flow disturbances in the furnace, because introduction of new flows changes the flow patterns in the furnace, thus altering the fuel combustion process, the flame pattern, emissions and the heat transfer on the boiler's internal surfaces.
If the lean gas burners are placed on false grounds in an existing boiler, or if the flame ignition or burning is poor, the result may be deficient combustion degree of coal/oil, increased CO and NO_x emissions and increased boiler corrosion. Furthermore, in the placement of lean gas burners in the existing boiler, the total heat transfer of the boiler has to be taken into consideration so that it is not changed, because that would mean reduction of the combustion efficiency of the power plant.
In a prior-art lean gas burner that has been used in co-firing of lean gas in a powdered coal-fired boiler, the flame ignition has been poor because of the inferior aerodynamic design of the burner. In this prior-art burner, the velocity of the fuel gas in the fuel pipe is only 5 - 10 m/s, and the velocities of the primary air and the secondary air in the air channels are 10 - 15 m/s. The fuel gas is fed at the outer periphery of the burner very close to the refractory throat. The major part of the combustion air is supplied to the center of the burner, which is why a sufficient protective air curtain is not formed in between the fuel gas flow and the burner throat. As a result of the above aerodynamic design, slag is formed in the burner pipe, one of the reasons for this being the low velocity of the fuel gas. Slag is also accumulated on the refractory throat, because in practice the fuel gas gets into direct contact with the refractory throat as there is no sufficient protective air curtain.
Low velocities of the combustion airs and lack of stabilizing elements lead to poor flame ignition. Low velocities of both the fuel gas and the airs also lead to notably large burner sizes and high investment costs, with the specific drawback that it is necessary to make large holes in the boiler wall. If the boiler must be provided with new large holes, the water and steam circulation of the boiler may be disturbed. In practice, poor ignition and unstable flame cause boiler corrosion and material damages in the other burners of the furnace. Furthermore, the combustion degree of coal and the level of nitrogen oxides generated are not best possible.
Due to poor ignition of the prior-art fuel gas burners, they can only be started after the boiler has been pre-heated for several hours by heavy fuel oil fired burners. In other words, those lean gas burners cannot be used as ignition burners of the boiler. Furthermore, due to their poor ignition the burners cannot be staged, i.e. used with an air-fuel ratio of the flame in the range of 0.6 - 0.8. This will result in high nitrogen oxide emissions. To suppress NO_x generation during combustion of lean fuel gas, an atmosphere or a flame should be created that has a sufficiently low oxygen concentration and high temperature. In practice, the flame needs to be stable and short.
US 2008/0227040 A1 discloses a method and burner for combustion of lean fuel gas, comprising creating an inflammable pre-mixture containing air and fuel gas, ejecting the pre-mixture in rotation around a central axis, and ejecting a complementary air flow either in the center of the pre-mixture flow or around the pre-mixture flow as a peripheral complementary flow. The structure of the burner is very complicated.
EP 0 639 742 A2 discloses a method and device for low emission combustion of gaseous fuels with internal recirculation of flue gas, particularly in water tube boilers. A first part of combustion air is guided in a swirled manner as primary air coaxially with the fuel and essentially in rotational symmetry with respect to the longitudinal axis of the burner to a primary exit position. A second part of the combustion air is guided in two stages as secondary air to a secondary exit position. The secondary air is fed in the form of a number of free jets essentially in the direction of the primary air flow. These secondary air jets cause circulation of flue gases, which flue gases are cooler than the flame.
Successful co-firing of lean product gas with coal or oil requires good ignition and flame stability of the gas burner. The burner should remain free of slag and it should not cause corrosion of the boiler walls. In co-firing one should also take notice of the other burners and the performance of the over-fire air system. If the change in the boiler performance caused by addition of lean gas burners is not considered in the operation of the over-fire air system, the result may be deterioration of the combustion degree of coal and increase of carbon monoxide emissions.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved method and burner for burning lean fuel gas in a power plant boiler.
The method according to the present invention is characterized by what is stated in claim 1.
Furthermore, the burner according to the present invention is characterized by what is stated in claim 8.
In the method, a fuel gas flow is fed into a furnace via a fuel pipe, a primary air flow is fed centrally into the fuel gas flow discharging from the fuel pipe, and a secondary air flow is fed peripherally into the fuel gas flow discharging from the fuel pipe. A small amount of air may be introduced into the fuel gas flow in the fuel pipe. The outlet of the fuel pipe is provided with an inner and an outer stabilizing ring that contribute to the flame ignition. The primary and the secondary air channels are provided with swirlers that set the respective air flows into rotation.
The product gas received from a biofuel gasifier is oxygen-free and its temperature is typically in the range of 600°C to 800°C. This being the case, producing a stable gas flame requires efficient mixing of air with the fuel gas to create a combustible mixture. The novel gas burner supports mixing the air and the fuel gas in several significant ways. Firstly, part of the combustion air - from 20 to 40% of the total amount - is supplied as primary air via a primary air channel in the center of the fuel gas flow. The primary air flow is given a drastic tangential acceleration at the outlet of the primary air channel, thus boosting the mixing of the air with the surrounding fuel gas flow. Part of the secondary air may be introduced into the fuel pipe through separate nozzles arranged in the separating wall to increase the oxygen content of the fuel gas already in the fuel pipe. The secondary air flow is given a high velocity, preferably in the range of 40 to 60 m/s. The share of secondary air is preferably from 60 to 80% of the total amount of combustion air. Mixing of combustion air with fuel gas improves as the velocity difference increases. The velocity of the fuel gas can be maintained in the range of 15 - 30 m/s, preferably 20 to 25 m/s. Furthermore, by providing the refractory throat with a small burner throat angle, e.g. in the range of 0° - 10°, the secondary air flow can be forced to efficiently mix with the fuel gas.
Rapid ignition of fuel gas can be secured by means of flame stabilizing rings fitted in connection with the outlet of the fuel pipe. Rapid ignition also requires intense swirl numbers of the primary and secondary air flows. The swirl numbers should preferably be in the range of 0.6 - 1.0. Cutting down the swirl number would impair ignition.
Advantageously, the fuel pipe is made of corrosion resistant stainless steel, which preferably contains more than 15% chromium. Calculations have shown that in the novel burner a chromium oxide layer is formed on the metal surface, limiting the corrosion rate to an acceptable level.
In the new lean gas burner the flame ignites rapidly and bums stably. This is achieved by utilizing specific flame stabilizing rings and by setting the velocities of the combustion airs and the fuel gas to an optimal level. As the flame ignition is simple, the furnace may be started up directly by the new fuel gas burners. In that case, the heavy fuel oil fired start-up burners that have previously been used for ignition can be eliminated. Heavy fuel oil is expensive and tends to cause detrimental emissions.
Slag-formation can be prevented by guiding a large amount of protective secondary air close to the refractory burner throat and by increasing the velocities of air flows. By the above measures, oxygen-rich conditions will be generated in the vicinity of the refractory throat and the burner throat will remain clean. Slagging of the fuel pipe can be further prevented by increasing the velocity of the fuel gas flow.
Due to the good ignition and the stability of the flame, the novel burner enables very low air-fuel ratio of the flame, such as 0.6 - 0.8, combined with the over-fire system of the boiler. This makes it possible to reach very low level of nitrogen oxide emissions.
Furthermore, because of the good ignition and the stability of the flame, the new burner produces a short flame, which is easy to fit in the furnace in such a way that the flame does not cause detrimental corrosion of the boiler walls and/or deteriorate the other burners of the furnace.
The novel lean gas burner can be installed in existing boilers or in new boilers. When renovating an existing power plant boiler, care should be taken of the accompanied changes in the over-fire space. Furthermore, the boiler may be operated solely with lean fuel gas or it may be a boiler co-firing e.g. coal or oil and the product gas.
Due to high air and gas velocities, the novel lean gas burner has smaller dimensions than the prior-art lean gas burners. That is why the holes needed in the water wall of the furnace are not as large as when using the prior-art gas burners.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention is explained in more detail by reference to illustrative embodiments represented in the drawings.

FIG. 1 is a front view of a lean gas burner.
FIG. 2 is a cross-sectional view along the line A-A of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 illustrate a gas burner suitable for burning lean fuel gas, such as product gas from gasification of various kinds of biofuel or waste material.
The burner comprises a fuel pipe 1 for feeding fuel gas into a furnace, a primary air channel 2 arranged centrally in the fuel pipe 1, and a secondary air channel 6 arranged circumferentially around the fuel pipe 1. In the center of the primary air channel 2 there is an auxiliary fuel firing tube 5, which may provide a jet of liquid fuel, such as light fuel oil, when the combustion furnace is started up. Furthermore, there may be a gas igniter 11 arranged in the fuel pipe 1. The fuel firing tube 5 and the gas igniter 11 are alternative means for ignition of the flame. It would be sufficient to have either of them.
The interior of the fuel pipe 1 is provided with deflectors 12 to guide the fuel gas flow from the inlet 13 of the fuel pipe 1 to the outlet 14 through which the fuel gas is ejected into the furnace.
The outlet 14 of the annular fuel pipe 1 is provided with two annular stabilizing rings 4, 8. The inner stabilizing ring 4 is fitted at the end of the inner wall of the fuel tube 1. The inner ring 4 comprises a plurality of tooth-like projections 15 extending radially outwards into the fuel pipe 1. The outer stabilizing ring 8 is fitted at the end of the outer wall of the fuel tube 1. The outer ring 8 comprises a plurality of tooth-like projections 16 extending radially inwards into the fuel tube 1. In addition to the inwards extending projections 16, the second stabilizing ring 8 comprises an annular section 17 extending outwards into the secondary air channel 6.
The two stabilizers 4 and 8 are arranged such that they generate turbulence in the fuel gas flow ejecting from the fuel pipe 1, which makes it easier to mix the turbulent fuel gas flow with the tangential primary air flow injected from the primary air channel 2 and the tangential secondary air flow injected from the secondary air channel 6. The stabilizing rings 4 and 8 form a two-part flame holder.
The primary air channel 2 is provided with a first swirler 3 that sets the primary air flow in a rotation close to the outlet of the primary air channel 2. Here the swirler 3 is fixed to the auxiliary fuel firing tube 5. The secondary air channel 6 is provided with a second swirler 7 that sets the secondary air flow in rotation close to the outlet of the secondary air channel 6. The angle of the blades of the second swirler 7 may be adjustable in order to control the rotation of the secondary air flow discharging from the secondary air channel 6.
Furthermore, the outer wall of the fuel pipe 1 is provided with a plurality of nozzles 9 that allow introduction of air from the secondary air channel 6 into the fuel pipe 1. The number of the nozzles 9 in the outer wall of the fuel pipe 1 may be 5 - 10. The amount of air introduced into the fuel pipe 1 through the nozzles 9 may be adjustable in the range of 10 - 20% of the total amount of secondary air.
The fuel gas is injected to the furnace via the fuel pipe 1, which is advantageously made of high-chromium stainless steel. The velocity of the fuel gas flow is adjustable in the range of 15 - 30 m/s, preferably 20 - 25 m/s. The fuel gas is mixed with a small amount of secondary air introduced through nozzles 9 in the fuel pipe 1.The primary air channel 2 injects a tangential primary air flow into the fuel gas flow discharging from the fuel pipe 1, and the secondary air channel 6 injects a tangential secondary air flow around the fuel gas flow discharging from the fuel pipe 1. The velocity of the primary air flow may be adjustable in the range of 15 - 40 m/s. The velocity of the secondary air flow may be adjustable in the range of 40 - 60 m/s. The two stabilizing rings 4 and 8 assist in mixing the fuel gas with the tangential air flows.
The burner is fixed in the wall of the furnace via a refractory throat 10 that surrounds the outlet of the secondary air channel 6. The refractory throat 10 has a burner throat angle in the range of 0 - 10°. The air flow ejecting from the secondary air channel 6 is arranged to flush the surface of the refractory throat 10, thus cooling the surface of the burner throat 10 and preventing slagging. The annular section 17 of the outer stabilizing ring 8 helps in guiding the secondary air flow to flush the surface.
Burning may be carried out with an air-fuel ratio of 0.6 - 0.8, which ensures that the generation of nitrogen oxides is minimized. The flame provided by the burner is relatively short and hot. Thus the flame does not disturb the operation of any other burners in the furnace.
The air supply is advantageously divided such that 20 - 40% of the total amount of burner air is supplied to the flame via the primary air channel 2 and 60 - 80% of the total amount of burner air is supplied to the flame via the secondary air channel 6. The relationship between the two air flows can be adjusted by control dampers arranged in the pipes supplying air into the primary air channel 2 and the secondary air channel 6.

Claims

Method for burning lean fuel gas in a furnace of a power plant boiler, comprising the steps of:
- feeding a fuel gas flow into the furnace via a fuel pipe (1),

- feeding a primary air flow centrally into the fuel gas flow discharging from an outlet (14) of the fuel pipe (1),

- feeding a secondary air flow peripherally into the fuel gas flow discharging from the outlet (14) of the fuel pipe (1),

- providing the outlet (14) of the fuel pipe (1) with an inner stabilizing ring (4) and an outer stabilizing ring (8) to boost flame ignition,

- providing the primary air channel (2) with a first swirler (3) to set the primary air flow into a rotation before the outlet,

- providing the secondary air channel (6) with a second swirler (7) to set the secondary air flow into a rotation before the outlet.
Method according to claim 1, comprising introducing some air from the secondary air channel (6) to the fuel gas flowing in the fuel pipe (1).
Method according to claim 1 or 2, wherein the velocity of the fuel gas discharging from the fuel pipe (1) is in the range of 15 to 30 m/s, preferably 20 to 25 m/s.
Method according to any one of the preceding claims, wherein the velocity of the primary air is in the range of 15 to 40 m/s and the velocity of the secondary air is in the range of 40 to 60 m/s.
Method according to any one of the preceding claims, wherein 20 - 40 % of the air fed to the fuel gas flow is supplied as primary air and 60 - 80 % of the air is supplied as secondary air.
Method according to any one of the preceding claims, wherein the air-fuel ratio of the flame is 0.6 - 0.8.
Method according to any one of the preceding claims, comprising the step of is starting up the boiler by lean fuel gas burners, thus eliminating the need of specific start-up burners.
Burner for burning lean fuel gas in a furnace of a power plant boiler, comprising:
- a fuel pipe (1) for feeding the fuel gas into the furnace,

- a primary air channel (2) arranged centrally in the fuel pipe (1) for feeding primary air into the fuel gas flow discharging from the fuel pipe (1),

- a first swirler (3) provided in the primary air channel (2) to set the primary air flow into rotation,

- a secondary air channel (6) arranged circumferentially around the fuel pipe (1) for feeding secondary air into the fuel gas flow discharging from the fuel pipe (1),

- a second swirler (7) provided in the secondary air channel (6) to set the secondary air flow into rotation,

- an inner stabilizing ring (4) provided at the edge of the inner wall of the fuel pipe (1), and

- an outer stabilizing ring (8) provided at the edge of the outer wall of the fuel pipe (1).
Burner according to claim 8, wherein the outer wall of the annular fuel pipe (1) is provided with nozzles (9) to allow introduction of secondary air into the fuel pipe (1).
Burner according to claim 8 or 9, wherein the fuel pipe (1) is made of corrosion resistant stainless steel.
Burner according to any one of claims 8 to 10, wherein the velocity of the fuel gas flow is adjusted to be 15 - 30 m/s, preferably 20 - 25 m/s.
Burner according to any one of claims 8 to 11, wherein the velocity of the primary air flow is adjustable in the range of 15 to 40 m/s and the velocity of the secondary air flows is adjustable in the range of 40 - 60 m/s.
Burner according to any one of claims 8 to 12, wherein the percentage of the primary air is 20 - 40% and the percentage of the secondary air is 60 - 80% of the total amount of combustion air supplied to the burner.
Burner according to any one of claims 8 to 13, which is arranged to operate at an air-fuel ratio of 0.6 - 0.8.
Burner according to any one of claims 8 to 15, comprising a refractory throat (10) provided with a burner throat angle in the range of 0° - 10°.