WO2002039018A1 - Method for firing an oil firing or gas firing boiler with a dust-like fuel - Google Patents

Abstract

A boiler designed for combusting gas or oil should be fired using dust without requiring modifications to the boiler apart from replacing the devices used for firing the boiler. The coal dust is introduced in a fluidized state into a combustor (8) and is partially combusted therein, whereby the burning air/fuel mixture is accelerated to a flame (11), which has a high velocity and which is blown into the firing chamber (1) of the boiler. At least 30 % of the fuel is combusted inside the combustor (8) under a pressure that is at least 200 Pa greater than the pressure inside the firing chamber (1). The flame (11) is directed into the firing chamber (1) in such a manner as to effect a more rapid circulation of the flue gases therein. This results in compensating for the reduction in the corner temperature by increasing the heat transmission coefficient, and in achieving a complete burn-out of the fuel without modifying the design of the boiler.

Description

 Process for firing a boiler designed for oil or gas firing with a dust-like fuel

description

The invention relates to a method for firing a boiler designed for oil or gas firing with a dust-like fuel fluidized with the aid of air.

The need to convert existing boiler systems from oil or gas firing to the increase in the cost of dusty fuels arose in the summer of 2000, when oil and gas prices rose by more than 100% within a short period of time. The heat price of heavy fuel oil reached about 2.5 times the heat price of boiler coal; the gas price usually follows the heating oil price with a certain time delay.

Oil or gas fired water tube boilers for generating z. B. of steam are so constructed today that

the combustion chamber is sufficient to burn out the oil or gas flame, and

the temperature of the flue gases in the area of the combustion chamber outlet / superheater inlet (also called corner temperature t _E ) is usually between 1,050 and 1,150 ° C.

A simple replacement of the oil or gas burner with coal dust burner does not achieve the goal for the following three reasons:

1) The dust flame requires 2 to 3 times the burnout volume compared to an oil or gas flame. If the combustion chamber volume is dimensioned for gas or oil firing, it is therefore not sufficient for the burnout of dust. With the same firing capacity as with oil or gas, the burnout of the dust would extend far into the downstream units, such as the superheater, and cause severe slagging and corrosion there.

So the first problem is to accelerate the burnout of the dust by 100 or 200%.

2) At the corner temperatures of 1,050 to 1,150 ° C usual for oil or gas firing Slag formation come. The corner temperature t _E must therefore be reduced, depending on the fuel used:

a) for hard coal, the Ruhrkohle-Handbuch (7th edition, 1987, page 160, chapter 3.4 "ash melting behavior") specifies the permissible t _E value with a maximum of 950 ° C;

b) for brown coal, the Rheinbraun data sheet from March 1987 states the ash melting behavior: "sintering temperature> 900 ° C"

The applicant's own investigations with lignite revealed:

t _E - 940 ° C ... heavy slagging t _E = 920 ° C ... beginning of slagging t _E = 880 to 900 ° C ... no slagging.

The figures therefore correspond to the values given in the Rheinbraun data sheet.

The second problem is to take measures that lower the corner temperature from 1,050 to 1,150 ° C to approx. 900 ° C.

3) If the corner temperature t _{E is} reduced as above, the temperature gradient from the flue gas (temperature t _E ) to the superheater pipe wall (mostly 550 to 600 ° C) decreases

so far z. B. 1,100 ° C - 600 ° C = 500 ° C

on now z. B. 900 ° C - 600 ° C = 300 ° C,

So about 60% of the previous temperature difference. The steam superheat is reduced accordingly, which is not permissible if, for example, a turbine is connected downstream of the boiler.

The third problem therefore arises in increasing the superheater output despite the reduced corner temperature to the old value that would result if oil or gas were fired. Conventional solutions, with which the three aforementioned problems could be solved, would lead to a very large construction effort:

1) For firing with dust-like fuel, an enlargement of the combustion chamber is necessary according to the state of the art.

2) To lower the corner temperature, the heating surface in the combustion chamber must be enlarged according to the state of the art.

3) If the superheater output is to be maintained despite the lower corner temperature, the superheater must be increased in accordance with the prior art.

The invention has for its object to provide a method of the type mentioned that can be carried out without changing the existing boiler dimensions.

This object is achieved by the features specified in claim 1. Advantageous embodiments of the invention are the subject of the dependent claims.

The invention solves the complex task consisting of three individual problems by a single measure. This consists of replacing the previous oil or gas burners, which are positioned horizontally at the bottom of the combustion chamber, with combustors, which are arranged in the area of the combustion chamber cover or on them and fire as far down as possible so that the flame jet has the largest possible expansion volume in the combustion chamber and slag formation on combustion chamber walls is avoided.

Combustors are to be understood as small, very heavy-duty special combustion chambers in which at least 30%, preferably 60%, of the fuel heat is converted, and which end in a flame acceleration nozzle, which generate a flame jet of at least 40 m / s, preferably 120 m / s speed , The rest of the fuel burns out in the flame jet. The flame jet of a combustor has such a high speed that the flue gases in the combustion chamber are set in motion by the flame jet, which in addition to the heat transfer from the flame to the combustion chamber walls by radiation also generates heat transfer by convection and thus leads to a lowering of the corner temperature ,

It depends on the suitable selection of the device with which the dust-like The invention is explained in more detail below with reference to an oil or gas boiler shown schematically in FIG. 1, which is converted to firing with coal dust.

The drawing shows a combustion chamber 1 delimited by water pipes O with a superheater 2 and a flue gas outlet 3. Downstream of this there are secondary heating surfaces 4. All water pipes O and heating surfaces are connected at the top to a boiler drum 5 and at the bottom to a water collector 6. In the lower area, dashed line 7 indicates where an oil or gas burner was previously flanged.

The above-mentioned corner temperature t _{E of} the flue gases prevails in the area of exit from combustion chamber 1 / entry into superheater 2 and is between 1,050 and 1,150 ° C. in modern boilers.

To change the firing of the water tube boiler shown to coal dust firing, the oil or gas burner 7 is removed or stopped. Instead it is on the head, i.e. At the top of the boiler, a combustor 8 is arranged which fires downwards from above. A combustor 8 consists of an arbitrarily designed combustion chamber 9, in which at least 30% of the fuel heat is converted, and a flame acceleration nozzle 10 adjoining the combustion chamber 9, which generates a flame jet 11 which has a speed of approximately 100 m / s. The water pipes O are heated by the flame jet 11 and the flue gases generated by the combustion.

The combustor 8 is designed in the present case so that about 50% of the fuel heat is converted in it. The remaining 50% burn in the flame jet.

The combustor 8 is attached to the boiler in such a way that the flame jet blown into the combustion chamber generates the fastest possible circulation of the flue gases.

The operation of the device shown will now be explained. The dust-like fuel, for example coal dust, is fed to the combustor 8 in a fluidized state. It burns at least 30%, preferably 50% of the fuel at a pressure which is at least 200 Pa, preferably 1,000 Pa higher than the pressure in the combustion chamber 1. The burning air / fuel mixture is in the flame acceleration nozzle 10 to at least 40 m / s, preferably 100 m / s accelerated flame jet speed. The one from the flame beam the buoyant forces of the hot flue gases are negligible. Only the flame jet 11 determines the flow pattern in the combustion chamber 1.

The remaining fuel heat burns out in the flame jet. If this burnout component per heat quantity has twice the space requirement as the oil or gas flame before, the combustion chamber designed for oil or gas firing is therefore sufficient for coal dust firing. This solves the problem mentioned in the first place above.

This also solves the second problem. This can be seen when you look at the numbers using a typical industrial water tube boiler:

Boiler output 40 t / h, total burner output 38 MW.

The smoke gas content of combustion chamber 1 is approx. 28 kg.

At 90 m / s flame jet speed, the combustor has an impulse or jet thrust of approx. 1.66 kN, or 170 kg.

The pulse of 170 kg is large compared to the amount of flue gas of 28 kg and thus determines the speed of the flue gases in the combustion chamber 1. The upward speed on the rear wall 12 of the combustion chamber is approximately 50 m / s.

The flue gases circulate several times in the combustion chamber 1 before they leave it through the superheater 2 and the outlet 3. In this way, in addition to flame and gas radiation, heat transfer by convection is generated, the corner temperature t _E of z. B. lowers 1,100 ° C to 900 ° C.

This also solves the second problem.

Shortly before the superheater 2, the flue gas speed is still 30 to 40 m / s. The flow is thus very turbulent, whereas previously the rising flame gases only reached the superheater 2 at 6 to 10 m / s in the oil or gas burners 7.

Since the heat transfer coefficient α goes with the 0.7th power of the speed:

α ° - ^j v 0.7 the heat transfer coefficient of the flue gases in the superheater increases accordingly. The reduced temperature gradient between flue gas and superheater due to the cooler flue gases (t _E = 900 ° C instead of earlier 1,100 ° C) is more than compensated for by the increased heat transfer coefficient α.

This also solves the third-mentioned problem.

The increase in heat transfer in the combustion chamber 1 achieved by the measures according to the invention is also still energy-saving, as the following consideration shows:

the air is supplied to the combustor 8 at a pressure which is above the pressure in the combustion chamber 1, at least 200 Pa, preferably 1,000 Pa above the pressure in the combustion chamber;

in the combustion chamber 9 of the combustor 8, part of the fuel burns at this higher pressure; '

In the flame acceleration nozzle 10, the pressure drop from the combustion chamber 9 to the combustion chamber 1 is converted into speed.

So the air becomes:

compressed to the pressure of the combustion chamber 9,

then heated by partial combustion, and

finally relaxed to the pressure in the combustion chamber 1.

This is a heat engine process that frees up mechanical work. In the present case, this work appears as an increase in energy, i.e. Acceleration of the flame beam. Its energy comes mainly from the combustion and only has to be applied to a small extent by the blower with which the fluidized fuel is introduced into the combustor 8. The power requirement of this air blower is hardly greater than that of a conventional oil burner.

You get these economic effects not only when converting oil or gas boilers to coal dust firing according to the invention, but even more so when you designs a boiler from the start as a coal dust boiler. You can then make the combustion chamber just as small from the outset as you would with an oil-fired boiler.

The same considerations apply to fuel dusts other than coal.

The combustor can fire in any direction depending on the boiler design, i.e. downwards, upwards, across or at an angle. This depends on practical considerations such as B. the achievement of the fastest possible circulation of the flue gases in the combustion chamber or the blowing out of impurities. It is important that the flame jet emerging from it has a sufficient free volume into which it can spread without hitting the boiler walls and producing an ash deposit which leads to slagging. The combustor is horizontal when the boiler is lying, i.e. attached essentially horizontally.

In the exemplary embodiment shown, the combustor 8 is arranged eccentrically, because this gives the greatest flue gas circulation speeds under the conditions of the example. If other conditions require, the combustor can also be arranged symmetrically to the firebox, whereby the circulation speeds become somewhat lower.

In combustors of larger dimensions perpendicular to the plane of the drawing, several combustors can be arranged side by side as a battery.

It should finally be mentioned that one can set up an ash extraction on the boiler, if necessary, in the area of the floor where the water collector 6 runs, if fuel with a high ash content should be used. The applicant's experience has shown, however, that most types of coal can dispense with such an ash extraction, since the ash as fly ash can leave the combustion chamber together with the flue gases due to the violent movement of the flue gas and can be filtered out of the flue gases outside the boiler.

Claims

Expectations 1. Method for firing a boiler designed for oil or gas firing with a dust-like fuel fluidized with the aid of air, which is fed to a combustor and at least 30% of the fuel is burned therein, the pressure in the combustor being at least 200 Pa above that There is pressure in the combustion chamber of the boiler, the burning air / fuel mixture is accelerated to a flame jet at a speed of at least 40 m / s, which is blown into the combustion chamber, and the flame jet is directed into the combustion chamber in such a way that there is a faster Circulation of the flue gases results. 2. The method according to claim 1, characterized in that the pressure during combustion in the combustor is 1,000 Pa higher than the pressure in the combustion chamber. 3. The method according to any one of claims 1 or 2, characterized in that the burning air / fuel mixture is accelerated to 100 m / s. 4. The method according to any one of the preceding claims, characterized in that the accelerated flame jet in the upper region of the combustion chamber is introduced into this. 5. The method according to claim 4, characterized in that the flame jet is directed vertically from above into the combustion chamber. 6. The method according to any one of the preceding claims, characterized in that the flame jet is directed off-center in the combustion chamber. 7. The method according to any one of the preceding claims, characterized in that 60% of the fuel is burned in the combustor.