JPH0718528B2 - Once-through boiler - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a once-through boiler equipped with a steam separator and performing a variable pressure operation.

2. Related Art A system example of a conventional once-through boiler is shown in FIG. In FIG. 5, reference numeral 1 is a deaerator, 2 is a boiler feed pump (turbine driven), 3 is a boiler feed pump (motor driven), 4 is a feed water control valve, 5 is a high pressure feed heater, and 6 is a re-heater. Circulating water heat recovery feed water heater, 7 economizer, 8 furnace water cooling wall,
9 is an evaporator, 10 is a steam separator, 11 is a primary superheater, 12 is a primary superheat reducer, 13 is a secondary superheater, 14 is a secondary superheat reducer,
15 is a tertiary superheater, 16 is a primary superheat reducer spray water control valve, 17 is a secondary superheat reducer spray water control valve, 18 is a high pressure turbine, 19 is a reheater superheat reducer, 20 is a reheater, 21 Is a medium pressure turbine, 2 is a low pressure turbine, 23 is a condenser, 24 is a condensate pump, 25 is a filter, 26 is a condensate demineralizer, 27 is a low pressure feed water superheater, 28 is a reheat superheat reducer spray water A control valve and 29 are recirculation flow control valves, respectively.

FIG. 6 shows the relationship between the steam / water separator fluid pressure, the water cooling wall passage flow rate, and the boiler load. The example of FIG. 6 shows the fluid pressure in the steam-water separator (corresponding to the steam drum in the drum type boiler) of the boiler that performs the variable pressure operation from the supercritical pressure to the subcritical inlet fluid pressure, The fluid pressure becomes subcritical when the boiler load is less than about 76%. Even in a once-through boiler, even in the low load range, the flow stability of the in-pipe flow and the in-pipe flow velocity are insufficient only with the once-through flow rate, and the stability of the furnace water-cooled wall pipe cannot be ensured. The amount of water passing through the water cooling wall is controlled to be higher than the required minimum flow rate. In the example of FIG. 6, when the boiler load is about 30% or less, the boiler water supply pump is used to control the flow rate of the cooling water in the furnace water to a constant level (flow rate at 30% load). The surplus water supply amount (recirculation flow rate) is separated by a steam separator installed at the circulation system outlet including the water cooling wall of the furnace and recirculated. The method of recirculating the water supply is shown in FIG.

7, the same reference numerals as those in FIG. 5 indicate the same elements,
Reference numeral 30 indicates a boiler circulation pump.

In Fig. 7, (a) shows a system in which the boiler circulation pump 30 is arranged in the main water supply line, and the steam-water separator recirculation line is in contact with the suction side of the boiler circulation pump 30, and (b) shows the steam-water separation. A system in which the boiler circulation pump 30 is arranged in the device recirculation line is shown, and (c) shows, as in FIG. 5, after collecting the steam-water separator drain retained heat by the circulating water heat recovery feed water heater 6, The method of collecting the drain in the deaerator 1 is shown. In any of these circulation methods, at a load below the recirculation start point (load of about 30% in the example of FIG. 6), the water at the outlet of the reactor water cold wall becomes wet steam, so the steam-water separator 10 separates steam from water. The steam is sent to the primary superheater 11, and the water is recovered from the recirculation system to the main water supply system and recirculated. The load below the recirculation start point (when the water level appears in the steam separator 10) is in the wet operation range (recirculation operation range), and the load above the recirculation start point is (the furnace water cooling wall system outlet fluid becomes dry steam, The case where the water level does not appear in the water separator 10) is called the dry operation area (flow-through operation area), and the recirculation start point is called the wet / dry switching load.

Problems to be Solved by the Invention Here, the control of the boiler adopts a method of switching to a control method suitable for each of the wet operation area and the dry operation area with the switching load as a boundary. Varies depending on operating conditions. For example, when the fuel is changed, the heat absorption ratio between the evaporation section and the superheat section is different, so the switching load moves,
Further, even when the load changes, the switching load fluctuates due to heat transfer delay or fluctuation of the heat storage level in the evaporator, which may cause disturbance in the boiler control, resulting in control failure.

According to the present invention, as described above, in the conventional method, since the boiler control is the control method suitable for each area with the switching load as a boundary,
This was done to cause a control malfunction due to the fact that the switching load of the control system does not match the actual wet / dry transition load. A once-through boiler that allows smooth transition between wet / dry operation is provided. The purpose is to provide.

Means for Solving the Problems According to the present invention, in a once-through boiler that has a steam separator and extracts spray water for a superheat reducer from the inlet of a water wall of a furnace water reactor or from its upstream side, a certain load of wet motion Switch between the wet operation control and the dry operation control of the control system. Under the same load, the separated water of the steam separator is collected by the recirculation system (wet operation), and the water cooling wall outlet fluid wet flow through operation area above the same load. Then, the spray water control valve controls the spray water so that the water level of the steam separator does not exceed a predetermined level, and the wet / dry switching of the control system is matched with the wet / dry transition of the steam separator fluid. The boiler controllability is improved, and the water cooling wall outlet fluid is maintained at a substantially saturated temperature until the load is high enough to ensure the required flow stability of the water cooling wall pipe flow, and the fluid temperature inside each water cooling wall pipe is made uniform.

Example FIG. 1 shows an example of a control system for a once-through boiler according to the present invention. In FIG. 1, reference numeral 1 is a deaerator, 3 is a boiler feed water pump, 4 is a feed water control valve, 5 is a high pressure feed water heater, 6 is a recirculating water heat recovery feed water heater, 7 is a economizer, and 8 is Furnace water cooling wall, 9 evaporator, 10 steam separator, 11 primary superheater, 12
Is a primary superheat reducer, 13 is a secondary superheater, 14 is a secondary superheat reducer, 15 is a tertiary superheater, 16 is a primary superheat reducer spray water control valve, 17 is a secondary superheat reducer spray water control valve, 29 is a recirculation flow control valve, 31 is a feed water flow meter, and 32 and 33 are water level detectors.

The example of FIG. 1 is a system including a feed water superheater 6 for recovering the heat of recirculated water retained by the steam separator 10.

The low-pressure feed water enters the deaerator 1, the deaerated feed water is boosted by the boiler feed water pump 3, passes through the feed water control valve 4, the high pressure feed water heater 5, the recirculated water-holding heat recovery feed water heater 6, and the economizer. 7 is supplied. The water supply flow rate is measured by the water supply flow meter 31 installed on the inlet side of the economizer 7, and is controlled by the water supply control valve 4.

A part of the water supplied from the economizer 7 is supplied to the primary overheat reducer 12 and the secondary overheat reducer 14 as spray water, and the rest of the supplied water passes through the furnace water cooling wall 8 and the evaporator 9 and is separated into a steam separator. Ten
Reach. In the wet operation area, the steam / water separator 10 separates steam / water, and the separated water is recovered as recirculated water to the deaerator 1 via the recirculated retained heat recovery feedwater heater 6. The separated steam is sent to the high-pressure turbine via the primary superheater 11, the primary superheat reducer 12, the secondary superheater 13, the secondary superheat reducer 14 and the tertiary superheater 15. Fig. 2 shows the relationship between the flow rate through the water cooling wall and the boiler load, Fig. 3 shows the relationship between the enthalpy of the fluid inside the boiler and the pressure, and Fig. 4 shows the relationship between the flow rate through the water cooling wall and the dryness against the boiler load. It shows the relationship. In this example, the wet / dry switching load of the control system is set to 25%, the load below this load is set as the recirculation operation range A, and the load above that load is set as the once-through operation range, and the required water flow stability of the water cooling wall pipe flow is ensured. Wall outlet fluid wet / dry load is set to 35%, and below this load is water cooling wall outlet fluid wet operating area B, and 25% to 35% load area is water cooling pipe outlet fluid wet operating area C, and 35%
The load or more is set as the water-cooled wall outlet fluid dry once-through operation region D.

The water supply flow rate to the furnace water cooling wall 8 is measured by the water supply flow meter 31, and the water supply control valve 4 controls the water supply flow rate to the furnace water cooling wall to the water supply flow rate required in relation to the boiler unit output.
That is, the minimum feed water flow rate is controlled in the wet operation area (recirculation operation area A), and the flow rate is adjusted to match the load in the dry operation area (flow-through operation area).

In the wet operation area, the recirculation system (recirculation water heat recovery feed water heater 6
And the recirculation flow control valve 29 in the recirculation flow control valve 29) is controlled by the water level signal detected by the water level detector 32 to collect the separated water.

Further, in the dry operation range (the load range in which the control system is in the dry operation), the recirculation flow rate control valve 29 is fully closed. At this time,
In the wet-flow once-through operation area C until it reliably reaches the dry operation area,
The water-cooled wall outlet fluid is moist steam, so that the water level appears in the steam separator 10. Therefore, the water level is detected by the water level detector 33 provided in the steam / water separator 10 at a position higher than the water level detector 32, and the water level of the steam / water separator 10 is equal to or higher than a predetermined water level higher than the control water level in the wet operation region. When this happens, the primary superheat reducer spray water control valve 16 increases the amount of spray water to reduce the flow rate through the water cooling wall, making the water cooling wall outlet fluid dry steam so that the water level does not rise any further.

Effect of the Invention When the outlet water of the furnace water cooling wall is wet steam, the control mode is switched to dry operation / wet operation, and in the dry operation region, the recirculation system from the steam separator is fully closed and the steam separator The following effects are achieved by controlling the superheat reducer with the spray water drawn from the inlet of the furnace water cooling wall or the upstream side thereof so that the water level does not exceed the predetermined water level.

(A) In the dry operation region, the recirculation from the steam separator 10 is completely stopped, the boiler is in the once-through operation, the control mode and the operating state of the boiler are the same, and it is possible to prevent disturbance to the control system. Absent.

(B) In the load range above the control mode switching load, since the water level of the steam separator 10 does not exceed the predetermined water level, there is no water overflow from the steam separator 10 to the superheater, and only one phase of steam Because of the flow, the flow stability in the superheater is maintained and the reliability of the superheater is improved.

(C) Even in the load range above the control mode switching load, the primary superheat reducer spray water is zero or the minimum amount necessary for steam temperature controllability, and the steam / water separator inlet fluid is completely dry steam (dryness 100). %), The outlet fluid of the furnace water cooling wall 8 or the evaporator 9 is maintained at a substantially saturated temperature, so that the temperature difference between the water cooling wall tubes or the evaporation tubes is small, and the reliability of the furnace water cooling wall 8 or the evaporator 9 is reduced. Is improved. In this example, as shown in FIG. 4, in the operating range C, the control mode is switched at 25% load, and the outlet fluid of the furnace water cooling wall 8 is on the saturated vapor line up to 35% load.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a control system of a once-through boiler according to the present invention, FIG. 2 is a diagram showing a relation between a water cooling wall passage flow rate and a boiler load, and FIG. 3 is a relation between an enthalpy of a fluid in a boiler and a pressure. Fig. 4, Fig. 4 is a diagram showing the relationship between the flow rate and dryness through the water cooling wall and the boiler load, Fig. 5 is a diagram illustrating the system of a conventional variable pressure operation once-through boiler, and Fig. 6 is a steam-water separator fluid. The figure which shows the relationship between pressure, the water cooling wall passage flow rate, and boiler load, FIG. 7 (a), (b) and (c) is a figure which illustrated the recirculation system of various boilers. 1 ... Deaerator, 2 ... Turbine driven boiler feed pump, 3 ... Motor driven boiler feed pump, 4 ... Water feed control valve, 5 ... High pressure feed water heater, 6 ... Recirculated water Heat recovery water heater, 7 ... coal saver, 8 ... furnace water cooling wall, 9 ...
… Evaporator, 10 …… Steam-water separator, 11 …… Primary superheater, 12…
… Primary overheat reducer, 13 …… Secondary overheater, 14 …… Secondary overheat reducer, 15 …… Tertiary overheater, 16 …… Primary overheat reducer Spray water control valve, 17 …… Secondary overheat reducer Spray water control valve, 18 …… High pressure turbine, 19 …… Reheater overheat reducer, 20
...... Reheater, 21 …… Medium pressure turbine, 22 …… Low pressure turbine, 23 …… Condenser, 24 …… Condensate pump, 25 …… Filter, 26 …… Condensate demineralizer, 27 …… Low pressure water heater, 28 ...
… Reheat superheat reducer Spray water control valve, 29 …… Recirculation flow control valve, 30 …… Boiler circulation pump, 31 …… Feed water flow meter,
32,33 …… Water level detector.

Claims (1)

[Claims] 1. In a once-through boiler equipped with a steam separator, which is equipped with a low-temperature equipment for extracting spray water for a superheat reducer from a furnace water cooling wall inlet or an upstream side thereof, a control mode switching between dry operation and wet operation is performed. This is done when the outlet water of the furnace water cooling wall is in a wet steam state, and in the dry operation region, the recirculation of separated water from the steam separator is completely stopped and the water level in the steam separator is at a predetermined water level. In the above case, the once-through boiler is characterized in that the amount of spray water for the superheat reducer is increased so that the water cooling wall outlet fluid becomes dry steam.