DK172248B1 - Method of controlling combustion in a boiler with a vibration grate - Google Patents

Description

DK 172248 B1

The invention relates to a method for controlling the combustion in a boiler with a vibration grate which vibrates for a short period of time and is kept at rest for a substantially longer period, where primary air is supplied on the underside of the grate and flows through openings therein, where secondary air is supplied through nozzles disposed on at least one boiler wall and the fuel is thrown onto the grate through a feed-in opening into at least one boiler wall, where the supply of primary air flow and / or 10 fuel flow is determined depending on the result of continuous measurements of one or more of the boiler condition sizes and parameters, which are expressions of the excess air and heat absorption,

Known boilers of this kind are used, among other things. for 15 burning wood chips, wood waste, sawdust and similar materials or mixtures thereof, but can be used for burning virtually all solid fuels which can be spread on the grate.

The grid is periodically vibrated, typically for approx. 2-5 seconds with a frequency of 5-8 Hz and an amplitude of approx. 7-10 mm followed by a standstill period of 5-30 minutes. The grate can slope 7-10 ° towards a slag drop and vibration can typically occur in a direction inclined at an angle of 10-18 ° opposite the slope of the grate.

2 5 The intention of the vibrator nobody is partly that the fuel located on the grate must be distributed over its surface and partly that ashes originating from the combustion must be transported to the slag fall. Upon vibration of the grate, the instantaneous conversion of fuel to the grate is changed, and in the known boilers where high primary airflow is used, the combustion rate will be increasing. The impact on fuel turnover on the grate is significant for 2-5 minutes after shaking. Further, the vibration of the grate causes a part of the combustible material to swirl up from the grate and burn in the boiler room for a period of 3-10 seconds after the vibration.

Such sudden increases in the combustion rate 5 will, if no precautionary measure is taken, lead to a decrease in the oxygen content of the flue gas and thus to the discharge of large quantities of CO and other pollutants resulting from incomplete combustion. The control of the combustion in known boilers is based on the fact that the oxygen content of the flue gas and the value of one or more parameters which express the heat absorption in the boiler are continuously measured and that the amount of primary air, fuel and secondary air supplied is adjusted accordingly. This mode of control suffers from the lack of the 15 measurements reflecting the instantaneous conditions with a time delay of typically one minute, and it is therefore not possible for the control system to take sufficient account of the above rapid increase of the combustion rate which occurs with the vibration of the grate.

The increase in the combustion rate causes a rapid increase in the heat release and the oxygen demand, which results in a decrease in the oxygen content of the flue gas, and as a result of the discharge of large quantities of CO and 25 other pollutants resulting from incomplete combustion. To avoid this, such boilers typically use an air excess of about 30% over the stoichiometric required.

SUMMARY OF THE INVENTION It is an object of the invention to provide a process of the kind mentioned in the introduction, wherein the instantaneous heat release is kept approximately constant while avoiding sudden decreases in the oxygen content of the flue gas. This object is achieved according to the invention by reducing the supply of primary air quantity 35 and / or fuel, in connection with each vibration, in relation to the amount determined by the continuous measurement, that the reduction is started immediately before the vibration has reached its maximum frequency. and after the cessation of vibration is reduced to 5 substantially the level before the reduction, in such a way that the sum of the percentage reduction of the supply of primary air and the percentage reduction of the amount of fuel supplied are calculated separately as running average over five seconds in the main part. of 10, a period beginning when the vibration has reached its maximum frequency and ending ninety seconds later constitutes at least 15%. For example, if the percentage reduction of the supply of primary air, measured as indicated, is 50% and the percentage reduction of the amount of fuel added measured in the same way is 80%, the sum of the reductions is 130%.

The method of the invention utilizes the fact that two of the parameters that have a significant influence on combustion, namely the supply of primary air flow and the amount of fuel supplied, can be changed almost immediately. Further, the time at which the change of these parameters is to be determined from the knowledge of the time of onset of vibration is determined, thereby avoiding the previously mentioned 25 time delay in measuring the oxygen content of the flue gas. In the method according to the invention, the boiler is substantially controlled as in known boilers, but this regulation is overlaid by a regulation of the instantaneously supplied primary air quantity and / or fuel quantity which is controlled from the knowledge of the time of vibration.

This results in that variations in the oxygen content of the flue gas can be limited and that the average air excess can thus be reduced, while at the same time keeping the instantaneous heat release approximately constant.

4 DK 172248 B1

An overall effect is that a stable mode of operation is also achieved, which among other things. has the advantage that it allows for further combustion improvements which would not otherwise be possible.

The major advantages obtained by the process of the invention are that the excess air becomes smaller, which results in less production of NO, less energy consumption to pressurize combustion air and to extract flue gases through the boiler, and a smaller heat loss 10 with the boiler's flue gas.

It is a further, substantial advantage that the method of the invention allows to shake the load, relative to known boilers, to be substantially increased. It has been found that in known boilers 15, there is in practice an upper limit of the load at about 2MW / m. If the load is to be increased further, it is necessary either to increase the amount of primary air or to utilize the primary air volume better than before.

If it is chosen to increase the primary air volume, 20 other problems arise, first of all possible. unburned particles are inflated in the boiler room.

If, on the other hand, it is chosen to increase the thickness of the fuel layer, thereby creating an intimate contact between the primary air and the fuel, in order to avoid increasing the primary air volume, each vibration will cause such a sharp decrease in the oxygen content of the flue gas and of the heat release that the combustion cannot be kept under control of the usual control system of the boiler, which results in unstable operation.

According to the invention, there is now provided a method for controlling the combustion in a boiler which causes such decreases in oxygen content and heat release to be substantially reduced or completely eliminated. This allows the operation of boilers with a shaking load that can be 10-15% higher than previously used.

A preferred embodiment of the invention is characterized in that the secondary air supply 5 is increased simultaneously with the reduction of the primary air supply and / or the amount of fuel supplied.

By counteracting the increased combustion rate during and after the vibration of the grate by reducing the supply of new fuel, the total amount of flue gas will decrease, with a decrease in the evolution of water vapor from drying of the supplied fuel. Similarly, reduced primary air flow will reduce the total flue gas flow. By increasing the secondary air flow, a constant amount of flue gas can be maintained substantially. Hereby, variations in heat absorption in the boiler heater surfaces are reduced and variations in pressure and temperatures in the boiler become limited.

Another method according to the invention is characterized in that the determination of the size 20 of said reduction of the applied primary air flow rate and / or fuel flow rate during a given vibration includes the result of measuring at least one of the boiler condition sizes.

The intention is to ensure that the reduction of the 25 supply of primary air and / or fuel quantity is the correct size in relation to the current operating state. The result of the boiler condition size measurements by prior vibration is included in an algorithm for determining the magnitude of the reduction 30 by a subsequent vibration.

A third method according to the invention is characterized in that said state size is the oxygen content of the flue gas and that the magnitude of the reduction of the applied primary air quantity and / or fuel quantity is increased when the measurement indicates a decrease in the oxygen content of the flue gas immediately after the vibration of the grate and that the magnitude of the reduction decreases when the result of the measurement indicates an increase in the oxygen content of the flue gas.

5 When measuring the oxygen content of the flue gas, the algorithm may consist of a comparison of the oxygen content of the flue gas immediately before, and 1-2 minutes after the vibration of the grate, where a decrease in the oxygen content of more than 0.2% points causes the reduction of the feed the primary air volume and / or the amount of fuel added is increased by 20% and where an increase in the oxygen content of the flue gas of more than 0.2% points causes that reduction to be reduced by 20%.

A fourth embodiment is characterized in that the reduction of the supply of primary air at the beginning of the vibration is 30% of the initial value, that the reduction of the applied fuel quantity is reduced to 30% of the initial value five seconds after the cessation of the vibration, and that each of the said quantities deviate from the percentage. from the initial value is gradually reduced, so that they are halved each time two minutes have elapsed.

This gives a smooth, asymptotic transition to the output value before vibration, which helps to keep the oxygen content of the flue gas constant. Whether it is the 25 primary air supply or the amount of fuel or a combination of these that is reduced is not crucial to achieving the desired effect. If there is a reduction of only one parameter, this reduction must be more severe.

In a fifth embodiment of the method, an air flow can be created over the grate in the direction from the outlet edge of the grate at the slag drop to the opposite end of the grate, which in a sixth embodiment can be created by supplying secondary air through several secondary air nozzles placed in the boiler wall above the outlet edge of the grate 7 and below the level of the fuel inlet opening, which nozzles may be directed to the surface of the grate in the area at the rear edge of the grate. Further, the center line of each secondary air nozzle can be placed within a 5 cone surface with a vertex in the nozzle and with a vertex of approx. 20 °, the center axis of the cone is directed to a point of intersection between the surface of the grate and an imaginary vertical line starting from one at the same level as the nozzle and opposite this point of the surface of a part of a boiler wall.

Hereby, a portion of the combustible material which, due to the vibration of the grate, swirls up from it is brought to the area at the rear of the grate where, due to the deflection of the flue gas at the boiler wall 15 at the rear of the grate, it will be separated from the stream and fall back on the grate where it is part of the fuel located on the grate and burn with it, instead of burning in the boiler room above the grate. At the same time, the gas stream collects and mixes primary air and degassing products from the grate, thereby ensuring a strong combustion in the area above the grate, which ensures radiant radiation to the grate.

In an eighth embodiment of the method according to the invention, the applied primary air quantity 25 can be distributed over the grate so that the primary air quantity per unit area supplied through most of the third part of the grating active area closest to the slag drop is greater than the primary air quantity per unit area supplied. the entire active area of the grate, where the active area is defined as the area of the smallest reactor which encloses all the openings of the grate.

By adding one primary air quantity per area unit as indicated above on the bottom third of the grate, a strong combustion is achieved at the bottom of the grate, with a high temperature especially in the zone where the fuel ceases to cover the primary air openings of the grate. This ensures a stable ignition in the flue gas flow across the grate.

The supply of secondary air to the secondary air nozzles 5 can be between 1/4 and 1/8 of the primary air flow at a pressure, which at the maximum output of the boiler is typically 6000 Pa. The fuel injection must be adjusted so that approx. 2/3 of the fuel can be thrown on the middle third of the grate, while 1/3 is thrown on the top third-10 of the grate in such a way that the amount of fuel reaching the rear of the grate is near zero.

In the above operating mode, it is possible to use very low primary air volumes typically 30% primary air and 70% secondary air when burning wood chips with up to 15 55% moisture content. At such low primary air proportions, by the above primary air distribution and the above gas movement across the grate, it is possible to reduce the amount of fuel particles blown out of the boiler room, to reduce the associated losses, and to reduce the amount of CO and other environmentally harmful substances resulting from incomplete combustion of these particles during the cooling of the flue gases.

The invention will now be explained in more detail with reference to the drawing, in which FIG. 1 is a side view of a vertical section of the boiler room of a boiler for use in the practice of the invention; FIG. 2-5 are graphs showing operating parameters and condition sizes of a known boiler depending on the time, namely the vibration frequency of the grate, the amount of fuel supplied, the amount of primary air supplied, and the O2 content of the flue gas; 6-9 graphs showing parameters and state sizes as in FIG. 2-5, but for a boiler for carrying out the method of the invention.

9 DK 172248 B1

The boiler has a boiler room 1 which is restricted by a front wall 2 and a rear wall 3 and below it has a vibration grate 4. Below the grate there is ash fall 5, and at the end of the grate closest to the front wall a slag drop 6. The fuel 5 is supplied by means of a snail conveyor not shown to a shaft 7, from which it slides to a spreader sticker which throws it onto the grate 4 through an opening in the front wall 8 by means of an inlet nozzle 9 which is supplied with air from an air duct 10. The inlet of the fuel must be adjusted so that approx. 2/3 of the fuel can be thrown on the middle third of the grate, while 1/3 is thrown on the top third of the grate in such a way that the amount of fuel reaching the rear of the grate is close to zero. Under the aperture 8 in the front wall are arranged 15 nozzles 15 which are supplied with air from an air duct 16. The nozzles 15 are directed to the rear edge of the grate and serve to mix degassing products and primary air from the fuel on the vibration grate 4.

For supplying secondary air, the boiler has several 20 groups of secondary air nozzles. At the rear wall above the grate there are nozzles 17 and 19 which are supplied with air from air ducts 18 and 20 respectively. On the front wall and the rear wall are located opposite projections 25 and 26, which carry secondary nozzles 30,31, which are supplied with air from 25 an air duct 32 and 33.34 supplied to air from an air duct 35. The supply of secondary air to the secondary air nozzles can be between 1/4 and 1/8 of the primary air flow at a pressure, which at the maximum output of the boiler is typically 6000 Pa.

30 The vibration grate 4 is positioned so that it inclines 7 ° with respect to the horizontal and with the highest point at the rear wall 3. For the flow of primary air, the grate typically has 1000 holes per year. m with a diameter of 3-4 mm. The grate is vibrated by a vibration unit 11 having a frequency of approx. 9 Hz and an amplitude of ± 5 mm in a direction which forms an angle of 13 ° with the horizontal and is directed opposite to the inclination of the grate. The vibration is performed periodically, the grating for each period being vibrated for approx.

2-5 s followed by a standstill period of 5-20 min.

5 To illustrate the effect obtained by the process according to the invention, the combustion conditions of a known boiler, as illustrated in FIG. 2-5 and then the corresponding conditions for a boiler for carrying out the method 10 according to the invention as illustrated in FIG. 6-9.

FIG. 2 shows how the frequency of the vibration of the grating changes with time. In the left side of the figure, the frequency sequence is shown over an abscissa axis divided into 10 second intervals, while the abscissa axis in the right side of the figure is divided into 1 minute intervals. The same classification of the axes is used in all the following figures and will therefore not be discussed in connection therewith.

It can be seen from FIG. 3 and 4, that the amount of fuel supplied and the amount of primary air supplied is regulated down within the first few minutes after the cessation of vibration of the grate, after which the boiler control system slowly increases said quantities to the values they had before the vibration started. The reason for this slow down regulation is shown in FIG. 5, which shows that the actual value of the oxygen content of the flue gas (curve denoted by reference a) decreases abruptly as a result of the vibration of the grate, while the value measured by the conventional C> 2 meters, shown by the curve 30 b, decreases more slowly. It is as a result of the latter decrease that the primary air quantity and the amount of fuel are adjusted down and that the oxygen content of the flue gas rises asymptotically to the level before the vibration.

FIG. 6 shows in the same way as in FIG. 2 shows the frequency 35 of the vibration of the grate, but for a boiler for practicing the method according to the invention. FIG. 7 and 8 show that the amount of fuel supplied or the amount of primary air supplied, respectively, decreases abruptly by the vibration, after which these quantities increase in a controlled manner. In the case of the supply of the primary air quantity, in FIG. 8 indicates that the initial reduction of the 30% primary air flow rate, indicated by curve e, is approximately halved for every 2 minutes elapsed. The curves shown in FIG. 7 and 8 labeled 10 and d, respectively, denote the value of the fuel amounts and primary air quantities, respectively, set by the boiler's control system based on conventional 02 measurement of the flue gas. The curves c and e, respectively, show the actual set fuel amounts and primary air quantities obtained by the additional control that occurs from the knowledge of the onset of vibration.

In FIG. 9, curve g shows the actual value of the oxygen content in the boiler room, while curve h shows the value measured by a conventional 02_meter.

As can be seen, a practically constant oxygen content in the flue gas has been obtained as a result of the regulation of the supplied primary air volumes and fuel quantities, however, as indicated by the reference numeral i, a brief increase in this, in a conventional manner measured O .

Claims (9)

12 DK 172248 B1 A method for controlling the combustion of a boiler with a vibration grate which vibrates for a short period of time and is kept at rest for a substantially longer period of time, where primary air is supplied on the underside of the grate and flows through openings therein, where secondary air is supplied through nozzles. disposed on at least one boiler wall and the fuel is thrown onto the grate through a feed-in opening into at least one boiler wall, and where the supplied primary air quantity and / or fuel quantity is determined depending on the result of continuous measurements of one or more of the boiler condition sizes and parameters which is the expression of the excess air and heat absorption, characterized in that the applied primary air quantity 15 and / or fuel quantity, in connection with each vibration, is reduced relative to the quantity determined by the continuous measurement, that the reduction is started immediately before the vibration has reached its maximum frequency and decreases after the cessation of vibration to 20 substantially the level before the reduction, in such a way that the sum of the percentage reduction of the supply of primary air and the percentage of reduction of the amount of fuel supplied are calculated separately as running average over five seconds for the majority of 25 for a period which begins when the vibration has reached its maximum frequency and ends ninety seconds later, at least 15%. Process according to claim 1, characterized in that the secondary air supply is increased simultaneously with the reduction of the primary air supply and / or the amount of fuel supplied. Method according to claim 1 or 2, characterized in that in determining the magnitude of said reduction of the primary air supply 35 and / or fuel flow during a given vibration, the result of measuring at least one of the boiler condition sizes is included. . Method according to claim 3, characterized in that said state size is the oxygen content of the flue gas 5 and that the magnitude of the reduction of the supplied primary air quantity and / or fuel quantity is increased when the measurement indicates a decrease in the oxygen content of the flue gas in the period immediately after the vibration of the grate. that the magnitude of the reduction is reduced when the result of the measurement indicates an increase in the oxygen content of the flue gas. Process according to any one of the preceding claims, characterized in that the reduction of the applied primary air quantity at the beginning of the vibration is 30% of the initial value, that the reduction of the applied fuel quantity is reduced to 30% of the initial value five seconds after the cessation of the vibration, and that each percent of said quantity deviation from the starting value is gradually reduced so that they are halved 20 each time two minutes have elapsed. Method according to any one of the preceding claims, characterized in that an air flow is created over the grate in the direction from the outlet edge of the grate at the slag drop to the opposite end of the grate. Method according to claim 6, characterized in that the air flow is created by supplying secondary air through several secondary air nozzles which are placed in the boiler wall above the outlet edge of the grate and below the level of the fuel inlet opening, and that the nozzles are directed to the surface of the grate in the area at the rear edge of the grate. Method according to claim 7, characterized in that the center line of each secondary air nozzle is placed within a cone surface having a vertex in the nozzle and having a vertex of approx. 20 °, the center axis of the cone is directed toward an intersection between the surface of the grate and an imaginary vertical line starting from a plane at the same level as the nozzle and opposite this point of the surface of part of a boiler wall. Method according to claims 6-8, characterized in that the applied primary air flow is distributed over the grate such that the primary air flow per unit area supplied through most of the third part of the active area of the grate located 10 closest to the impact drop is greater. than the primary air quantity per unit area supplied throughout the active area of the grate, where the active area is defined as the area of the smallest reactor which encloses all the grate openings.