JP7155685B2 - SOx concentration control method for boiler exhaust gas - Google Patents

Description

The present invention relates to a method for controlling the _SOx concentration of boiler flue gas, and in particular, the _SOx concentration of flue gas discharged from a fluidized bed coal-fired boiler using an in-furnace desulfurization method in which limestone powder is added together with coal raw material for desulfurization. It relates to a _SOx concentration control method capable of economically suppressing the

Since coal contains about a few percent of sulfur, the flue gas emitted by burning coal in an oxidizing atmosphere contains sulfur oxides such as SO2, which is the main cause of air pollution ₍ hereinafter referred to as SO2). (also called _SOx ). Therefore, it is obligatory to measure the _SOX concentration of flue gas discharged from a coal-fired boiler before releasing it into the atmosphere. The measured _SOx concentration value is generally taken into a distributed control system (hereinafter referred to as DCS) and recorded, and whether the average value of the _SOx concentration in the flue gas over a 24-hour period is below a specified value. is being monitored. In many cases, the above DCS is designed to inform the operator of an abnormality by issuing an alarm or displaying a warning on the screen when the captured _SOx concentration value exceeds a preset threshold value. By storing measured values in the storage medium of DCS as trend records, past measured values can be checked.

Various desulfurization methods have been proposed in order to adjust the _SOx concentration value in the combustion exhaust gas to a specified value or less. For example, in a fluidized bed combustion boiler in which coal with a particle size of about several mm is fluidized in the presence of a fluidizing medium such as silica sand and burned at a combustion temperature of 800 to 1000 ° C., the SO 2 _X concentration is reduced as described in Patent Document 1. In order to keep it below a predetermined specified value, an in-furnace desulfurization method is used in which limestone powder is added as a desulfurization agent to the coal input portion of the fuel.

JP-A-10-267221

In the furnace desulfurization method described above, by adjusting the amount of limestone powder input based on the _SOX concentration value, the average value of the _SOX concentration for one day (24 hours) can be suppressed to, for example, the in-house control value of 200 ppm or less. becomes possible. At that time, it is possible to more reliably prevent the SO _X concentration from exceeding the specified value by adjusting the input amount of limestone powder to a large amount, but in this case, the limestone powder will be consumed more than necessary. It was not economical and it was sometimes difficult to find space to store the limestone powder. The present invention has been made in view of the problems of the conventional in-furnace desulfurization method described above, and can stably and reliably reduce the _SOx concentration in the flue gas discharged from the boiler without excessively adding limestone powder. The purpose of this study is to propose a method of suppressing it below the specified value.

In order to achieve the above object, the _SOX concentration control method according to the present invention is a method for controlling the _SOX concentration of exhaust gas from a coal-fired boiler that performs in-furnace desulfurization by adding limestone powder together with coal as fuel. Then, for each predetermined management time, the average value is calculated by dividing the integrated value of the _SOX concentration measured at regular time intervals from the start to the current time by the elapsed time from the start to the current time. Predicted integrated value of the _SOX concentration from the start time to the elapse of the control time when it is assumed that the _SOx concentration at the time is maintained until the elapse of the control time (based on the amount of fluctuation in the amount of sulfur in the fuel (excluding those to be predicted) is divided by the management time to calculate the predicted average value, and when these average values and predicted average values are each equal to or greater than a predetermined threshold, an alarm is issued or a warning is displayed on the screen, Assuming that the _SOX concentration of the exhaust gas is maintained at the regulation value for each control time throughout the control time, a target integrated value is obtained from the start of the control time until the control time elapses, and the integrated value is obtained. from the target integrated value is divided by the remaining time until the management time elapses from the current time to calculate the management target value, and the amount of limestone powder input is controlled based on the management target value . It is characterized by

Advantageous Effects of Invention According to the present invention, it is possible to stably and reliably suppress the _SOx concentration of flue gas discharged from a boiler without excessive charging of limestone powder.

1 is a schematic flow diagram of a coal-fired boiler facility to which the _SOx concentration control method of the present invention can be preferably applied; FIG. 4 is a flow chart of a _SOX concentration control method according to an embodiment of the present invention. In the control method of the embodiment of the present invention, the average value of the _SOX concentration calculated by DCS for the input amount and the trend of the _SOX concentration in the flue gas when the coal raw material having a high sulfur content is input, It is the graph shown with the trend of the prediction average value and the control target value. In the control method of the comparative example of the present invention, the average value of the _SOX concentration calculated by DCS for the input amount and the trend of the _SOX concentration in the flue gas when the coal raw material having a high sulfur content is input, It is the graph shown with the trend of the prediction average value and the control target value. In the control method of the embodiment of the present invention, the average value of the _SOX concentration calculated by DCS for the input amount and the trend of the _SOX concentration in the flue gas when the coal raw material with a low sulfur content is input, It is the graph shown with the trend of the prediction average value and the control target value. In the control method of the comparative example of the present invention, the average value of the _SOX concentration calculated by DCS for the input amount and the trend of the _SOX concentration in the combustion exhaust gas when the coal raw material with a low sulfur content is input, It is the graph shown with the trend of the prediction average value and the control target value.

Hereinafter, the _SOx concentration control method of the embodiment of the present invention will be described in detail with reference to the drawings. First, coal-fired boiler equipment to which the _SOx concentration control method of the embodiment of the present invention is suitably applied will be described. As shown in FIG. 1, a fluidized-bed coal-fired boiler (hereinafter also referred to as a boiler) 1 has a vertical cylindrical shape with a tapered bottom, and coal as fuel fed into the bottom is Combustion is performed while being fluidized by air introduced from below the boiler 1 together with limestone powder similarly introduced as a desulfurizing agent for removing sulfur oxides. The boiler water is warmed by the radiation heat transfer of the combustion gas generated thereby, and steam is generated from the boiler drum 1a provided in the upper part of the boiler 1 .

The combustion gas whose heat is recovered by the above-mentioned radiation heat transfer exits the boiler 1, and after the solid content is removed by the cyclone 2, it is introduced into the heat recovery section 3 equipped with a superheater and an economizer, where the convective heat transfer is performed. Further heat recovery is achieved by That is, in the heat recovery unit 3, the boiler water supplied from the boiler water tank 4 through the degassing device 5 is preheated by exchanging heat with the combustion gas, and the saturated steam generated from the boiler drum 1a is heated. It is heated to make superheated steam for in-house power generation.

After the superheated steam is introduced into the turbine generator 6 and used to rotate the turbine, it is condensed and recovered in the boiler water tank 4 . On the other hand, the combustion gas heat-recovered by the heat recovery unit 3 is introduced into the electric dust collector 7 as the combustion exhaust gas to be dust-removed, and then released to the atmosphere through the chimney 8 . The stack 8 is provided with a _SOx concentration meter 9 so that the _SOx concentration of the flue gas flowing through the stack can be measured. The SO _x concentration value measured by this SO _x concentration meter 9 is output to DCS 10 . The DCS 10 performs a calculation using the _SOx concentration value as a parameter in accordance with a predetermined algorithm, and based on the calculation result, for example, by increasing or decreasing the motor rotation speed of the limestone powder supply feeder 11, the amount of limestone powder input can be controlled. done.

Methods for removing _SOx gas contained in combustion gas generated by burning fossil fuels such as coal are broadly classified into wet methods and dry methods. Since limestone powder having an action can be used as a fluid medium, a dry in-furnace desulfurization method can be adopted. In addition, since the fluidized boiler efficiently transfers heat in the fluidized bed, low-temperature combustion of about 860° C. is possible, so that the amount of _NOx generated can be suppressed. That is, when limestone (CaCO ₃ ) powder is mixed with coal at a furnace temperature of about 860° C., SO ₂ can be removed by the reactions of Equations 1 and 2 below.

[Formula 1]
_CaCO3 →CaO+ _CO2
[Formula 2]
CaO ₊ SO2→ _CaSO3

By the way, the amount of _SOx gas generated in the combustion gas varies depending on disturbances such as the type and amount of coal used as fuel and the load on the boiler. It is preferable for the operator to monitor the operation of the boiler, since there are cases where the suppression of _SOx gas emissions into the atmosphere cannot keep up with the quantity. However, monitoring by an operator cannot avoid oversight of data, delay in response to an abnormality, and the like, and there is a risk that exhaust gas with a high _SOx concentration will continue to be discharged into the atmosphere. As a result, there was a possibility that the above-mentioned 24-hour average value would exceed the default value.

Therefore, in the _SOx concentration control method of the embodiment of the present invention, the _SOx concentration averaged for each predetermined management time such as 24 hours becomes a value as close as possible to the target value within a range that does not exceed the predetermined target value. The amount of limestone powder input is controlled as follows. For example, when managing every 24 hours as a predetermined management time, the _SOX concentration is measured and integrated at regular intervals, for example, every minute as a fixed time interval from 0:00, which is the start time. The calculated integrated value is divided by the elapsed time from _0:00 to the current time to calculate the average value up to the current time. It is possible to confirm whether or not the change is within the specified range. Furthermore, by setting a predetermined threshold value for this average value, it is possible to inform the operator that the _SOx concentration may exceed the specified value by means of an alarm or, for example, a warning display on the display screen for operation management. more secure management.

Furthermore, the method for controlling the _SOX concentration of the embodiment of the present invention is a method for controlling the SOX concentration for 24 hours, assuming that the _SOX concentration is maintained for 24 hours at a predetermined regulation value such as the voluntary control value of each factory. The integrated value up to the present time is subtracted from the integrated value, and the value is divided by the remaining time until 24 hours from the current time (that is, the time obtained by subtracting the time from 0:00 to the present time from 24 hours). Thus, a new _SOX management target value can be set for the remaining time until 24 hours have elapsed from the current time.

By reflecting this _SOx management target value as a control target value in the input amount of limestone powder, the consumption of limestone powder can be suppressed. That is, the amount of limestone powder input for removing SOx gas is generally adjusted according to the deviation of the measured value of _SOx concentration from the target value by feedback control typified by PID control. By using the _SOX management target value calculated based on the accumulated value of the past _SOX concentration up to the present time as the target value of the feedback control, the amount of limestone powder to be fed can be optimized.

Note that in the _SOx concentration control method of the embodiment of the present invention, a predicted average value from 0:00 to 24:00 is calculated assuming that the current _SOx concentration measurement value is maintained for the remaining time. Also, if the difference between the calculation result and the specified value falls below a specified value, an alarm other than the above, or a warning display on the monitor, etc. will be issued, so that _SOx gas emissions will exceed the specified value. It is possible to more reliably prevent it from being done.

In the SO _X concentration control method of the embodiment of the present invention, it is preferable that the series of steps described above be arithmetically processed by the DCS. For example, FIG. 2 shows a flow chart in the case of processing the above steps by DCS. Specifically, first, in step A, the measured values _{x 0} , _{x 1} , . A value A _t is calculated, and in step B, the integrated value A _t calculated in the above step A is divided by the elapsed time t from the start time 0 o'clock to the current time t minutes to obtain an average value A _t / t [unit: ppm] Calculate. Note that “A _t =A _t +x _t ” in step A is a notation based on programming languages such as BASIC and C language, and the operation result on the right side of the symbol “=” is It means to substitute for "At". The same applies to step H1.

Next, in step C, the average value A _t /t calculated in step B is compared with a predetermined threshold α, and if the average value A _t /t exceeds the threshold α, an alarm or warning is issued in step C1. do. Note that the threshold for issuing a warning and the threshold for issuing a warning in step C1 may be set to different values.

Next, in process D, the target integrated value 1440 is assumed to be maintained at the regulation value β such as the voluntary control value from the start time _0:00 to 24:00 when the control time elapses.・Calculate β, and subtract the integrated value A _t from the start time 0:00 calculated in step A above to the current time from this target integrated value (1440 β-A _t ), the remaining time until the control time elapses. Divide by (1440-t) to obtain the SO _x concentration control target value (1440·β-A _t )/(1440-t). Based on this management target value [unit: ppm], in step E, the amount of limestone input to the boiler is controlled as described above.

Next, assuming that the currently measured SO _x concentration value x _t in process F is maintained for the remaining time 1440−t until the control time elapses, the predicted integrated value {A _t +x _t ·( 1440-t)}, and this predicted integrated value is divided by the control time of 24 hours to obtain the predicted average value {A _t +x _t ·(1440-t)}/1440. Then, in step G, the predicted average value [unit ppm] calculated in step F is compared with a predetermined threshold value γ, and if the predicted average value exceeds the threshold value γ, an alarm or warning is issued in step G1. In this case as well, the threshold for issuing an alarm and the threshold for issuing a warning may be set to different values as in C1 above. After that, the above steps A to G are repeated until 24 hours pass through steps H1 to H3. As a result, in a fluidized-bed coal-fueled boiler that performs in-furnace desulfurization by adding limestone powder together with coal as a fuel, SO _{The x} concentration can be suppressed stably and reliably.

In the series of processes by the DCS, it is preferable to display the calculation results on, for example, the boiler monitoring screen. That is, in addition to the above measured values of _SOx concentration, the current average value calculated based on the above 24-hour control time, the control target value, and the trend of the predicted average value are displayed. A warning is preferably displayed when thresholds are exceeded for values and predicted averages. As a result, the operation can be supported so that the _SOx gas does not exceed the specified value. The regulation value for the SO _x concentration is generally about 200 ppm, but a more safe regulation value, for example, about 150 ppm, may be set, and SO _x gas emissions may be controlled at that regulation value.

The _SOx concentration in the flue gas was controlled according to the flow chart shown in FIG. At that time, the regulation value of the _SOx concentration was controlled on the safe side at 150 ppm obtained by subtracting 50 ppm as a margin value from the average value of 200 ppm averaged over 24 hours of control time. As a result, as shown in the _DCS trend graph in FIG. However, the amount of limestone powder put into the boiler was controlled based on the control target value calculated in Process D, so the time on the horizontal axis turned to decrease at around 1:50. In the end it was almost back to normal.

On the other hand, FIG. 5 shows a graph of the trend of DCS when the _SOX concentration is lowered by charging a coal raw material with a low sulfur content. In this case, it can be seen that the limestone powder consumption is suppressed while the _SOx concentration is kept low by controlling the amount of limestone powder input in accordance with the change in the management target value.

On the other hand, for comparison, when the boiler was operated under the same conditions as above, except that the amount of limestone powder input to the boiler was not controlled based on the management target value and was supplied at almost a fixed amount, the sulfur content was reduced. When a high coal raw material was fed, as shown in FIG. 4, there was a delay in suppressing the _SOx concentration, and the average value increased steadily. In addition, when a coal raw material with a low sulfur content is charged, as shown in FIG. 6, there is a delay in suppressing the amount of limestone powder charged, and an excessive amount of limestone powder is charged more than necessary, resulting in economical operation. did not become Table 1 shows the average value and standard deviation of the _SOx concentration during the periods shown in FIGS. Table 2 shows the average input amount of limestone powder, the standard deviation, and the average _SOx concentration during the periods shown in FIGS.

Thus, it can be seen that by controlling the _SOx concentration according to the flowchart of FIG. 2, the standard deviation of the _SOx concentration can be reduced, control stability can be improved, and limestone powder consumption can be reduced.

REFERENCE SIGNS LIST 1 coal-fired boiler 1a boiler drum 2 cyclone 3 heat recovery section 4 boiler water tank 5 degassing device 6 turbine generator 7 electrostatic precipitator 8 chimney 9 _SOx concentration meter 10 DCS
11 Limestone powder supply feeder

Claims (2)

A method for controlling the _SOx concentration of flue gas from a coal-fired boiler that performs in-furnace desulfurization by adding limestone powder together with coal as fuel, wherein the control method comprises a constant time interval from the start to the present time at each predetermined management time. When the average value is calculated by dividing the integrated value of the _SOX concentration measured by the elapsed time from the start to the present time, and assuming that the _SOX concentration at the present time is maintained until the above-mentioned control time elapses Calculate the predicted average value by dividing the predicted integrated value of the _SOX concentration from the start of the control until the elapse of the control time (excluding the value predicted based on the amount of fluctuation in the amount of sulfur in the fuel) by the control time. Then, when each of these average values and predicted average values is equal to or greater than a predetermined threshold value, an alarm is issued or a warning screen is displayed, and the SOX concentration of the exhaust gas is _the regulation value for each control time, and the total control time Obtain the target integrated value from the start time to the time when the control time has elapsed, and subtract the target integrated value from the target integrated value when the control time has passed from the current time A control method for _SOx concentration, comprising: calculating a control target value by dividing by the remaining time until the control target value, and controlling the input amount of the limestone powder based on the control target value . When the predetermined management time is 24 hours, the elapsed time from the start to the present time is t, and the SO measured at the present time _X x _t , the SO _X A _t When the above SO _X The predicted integrated value of the concentration is "A _t + x _t (1440-t)”, SO according to claim 1, _X Concentration control method.

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