JP2781318B2 - Steam pressure control method for exhaust heat recovery steam generation system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust heat recovery steam generation system for generating steam such as steam and heat medium from an exhaust heat source of a high temperature fluid discharged from a gas turbine, a fuel cell, a factory of various industries, and the like. It relates to a pressure control method.

[0002]

2. Description of the Related Art Generally, in the above-described system for generating and recovering heat from exhaust heat, the flow rate of the heat source fluid is controlled by a valve, a damper, or the like, or an excessive amount of steam is always generated to generate excess steam. The steam pressure is controlled to a substantially constant value regardless of the steam generation load or the required amount of the steam demand by releasing the air to the air, the condenser, the regenerative heat exchanger, etc., other than the demand destination according to the pressure. ing.

That is, in the system as described above, when the demand for steam suddenly increases, the pressure tends to decrease in such a way that the amount of heat source fluid is increased by rapid control of valves and dampers, and the pressure is reduced. The width should be minimized, or the excess steam escape flow should be narrowed to ensure a flow to the steam demand side. If the pressure is going to increase when the demand for steam is suddenly reduced, the amount of exhaust heat source fluid is reduced by rapid control of valves and dampers to minimize the pressure increase width, or to release excess steam. It has been practiced to increase the flow rate and suppress the pressure rise.

However, there is no relation between the heat load of the system of the exhaust heat source and the steam demand of the exhaust heat recovery system, and they often change independently. Therefore, when the flow rate of the exhaust heat source fluid is low, the steam flow rate on the exhaust heat recovery system side may be high. Therefore, in such a case, the flow rate of the heat source fluid is secured by using the auxiliary combustor on the exhaust heat recovery system side.

[0005]

However, in general, when surplus heat is recovered from a waste heat source and discarded outside the system as surplus steam, water and chemicals for water treatment are also discarded at the same time. There are disadvantages in terms of economy and environment.
On the other hand, even if the flow rate on the heat source side is rapidly controlled, the evaporation amount does not change so rapidly, and a delay occurs with respect to the change in the steam load. Further, rapidly increasing the fuel flow rate of the auxiliary combustor temporarily but significantly increases the emission of NOx, carbon monoxide, and the like, and has a problem that the environment is adversely affected.

SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide a pressure control method for an exhaust heat recovery steam generation system which is highly economical, has little effect on the environment, and has a high follow-up ability to load changes. I do.

[0007]

SUMMARY OF THE INVENTION The present invention provides a hot exhaust gas,
In an exhaust heat recovery steam generation system having an evaporator that generates heat, steam, a heat medium vapor, etc. by recovering heat from hot waste water or the like, when the steam load is small, the steam pressure set value in the evaporator is shifted to a higher value, When the load is high, the steam pressure set value is shifted to a lower value.

[0008]

[Function] When the steam pressure set value is shifted to a higher value,
The saturation temperature of the medium liquid held inside the evaporator increases, and the heat storage amount increases. Therefore, the evaporation amount is reduced for a fixed heat source fluid flow rate, and it is possible to cope with a small steam load. On the other hand, when the set pressure value of the steam is shifted to the lower side, the heat capacity accumulated in the water retained in the evaporator is released, and the reduced-pressure boiling, so-called flash evaporation, occurs almost without delay with respect to the change in pressure. Therefore, the evaporation amount increases for a constant heat source fluid flow rate,
High steam load can be accommodated.

[0009]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing a schematic configuration of an exhaust heat recovery steam generation system. For example, an exhaust heat source fluid such as gas turbine exhaust gas is supplied to an evaporator 3 via an auxiliary combustor 1 and a flow control damper 2. is there. The above evaporator 3
Is supplied with water from a water supply tank 4 by a water supply pump 5, generates heat by exchanging heat with a steam exhaust heat source fluid in the evaporator 3, and generates steam through a steam flow control valve 6. It is delivered to a predetermined steam demand point. On the other hand, the exhaust heat source fluid that has exchanged heat with the feedwater is radiated from the chimney 7 into the atmosphere.

The circuit of the flow control damper 2 and the evaporator 3 is provided with a bypass conduit 8 connecting the upstream side of the flow control damper 2 and the downstream side of the evaporator 3. By controlling the opening and closing of the bypass damper 9, the amount of the exhaust heat source fluid discharged directly from the chimney 7 bypassing the evaporator 3 can be controlled. The auxiliary combustor 1 can be supplied with auxiliary fuel and combustion air via a fuel flow control valve 10 and an air flow control valve 11, and when the flow rate of the exhaust heat source fluid is small, The auxiliary combustor 1 is used to ensure the flow rate of the heat source fluid.

The evaporator 3 is provided with an evaporator pressure sensor 12, and a steam flow sensor 13 is provided downstream of the steam flow control valve 6. The steam pressure set value Ps is calculated from the steam flow rate F and the maximum steam flow rate Fmax, etc., and a deviation signal between the steam pressure set value Ps and the evaporator pressure detected by the evaporator pressure sensor 12, and the time of the evaporator pressure The opening of each damper 2, 9 or the control valve 10 is controlled by the sum of the change rate and the function (see FIG. 2).

In the present embodiment, the pressure set value Ps in the evaporator 3 is changed in proportion to the difference between the maximum steam flow rate Fmax and the current steam flow rate F by the arithmetic unit of the control device as shown in FIG. Can be

That is, the current steam flow rate F is fetched by the steam flow rate sensor 13 into the arithmetic unit of the control device, and the fetched flow rate signal and a preset maximum flow rate Fmax
Using the maximum set pressure Pmax and the minimum set pressure Pmin, the evaporator pressure set value Ps is calculated by the following calculation.

Ps = Pmin + (Pmax-Pmin) (Fma
x-F) / Fmax Also, as described above, the auxiliary fuel flow control valve 10 or the dampers 2 and 9 perform proportional control so as to minimize the sum of the function of the deviation with respect to the pressure set value and the time rate of change of the pressure. And so on. That is, the operation amount is ｛−f (P−Ps) −
dp / dt ・ Δt｝.

Here, P: actual pressure t: time Δt: time constant of the device Next, a change in state is shown by specific numerical values.

Here, assuming that Pmax = 20 bar, Pmin = 15 bar, Fmax = 1 kg / s, the capacity G of the water retained in the evaporator is G = 2 ton, if the steam demand rapidly changes to the maximum flow rate Fmax, The flow control valve 6 is quickly controlled to open. When the steam flow control valve 6 is controlled in the opening direction in this manner, the pressure in the evaporator 3 decreases in accordance with the opening operation of the valve, and flash evaporation occurs due to the pressure. The steam flow rate changes up to a maximum flow rate of 1 kg / s almost in accordance with the opening speed of the steam. The rate of change of the opening degree of the steam flow control valve 6 is normally about 2 seconds from fully closed to fully opened, and the steam flow rate also changes at this rate.

On the other hand, since the set value Ps of the pressure changes according to the steam flow rate as described above, when the steam flow rate changes to the maximum steam flow rate, the set pressure after the change becomes the minimum 15 bar.
Becomes However, the actual pressure does not decrease rapidly due to the increase in the steam flow rate due to flash evaporation, but gradually decreases. In such a case, since the current actual pressure initially has a positive deviation from the set pressure, the fuel flow control valve 1 of the auxiliary combustor 1 is reduced so that the deviation from the set value is reduced.
0 and the flow control damper 2 for the exhaust heat source fluid are controlled in the opening direction, and the flow control damper 9 is controlled in the closing direction.

Thus, the pressure in the evaporator gradually decreases and the flow rate of the heat source fluid gradually increases.

Thus, while the actual pressure changes from the maximum set value Pmax to the minimum set value Pmin, the change in the saturation temperature of the internal retained water is about 15 ° C., and the change in specific enthalpy ΔH is about 6.8 kj / kg. . When this heat amount is converted into a steam amount Gs at a pressure of 15 bar, the latent heat L of evaporation is about 1946 kj /
Gs = ΔH · G / L = 68 × 2000/1946 = 69.9 (kg) By dividing this by the steam flow rate Fmax, the time during which this flow rate can be maintained is obtained as follows.

T = Gs / Fmax = 69.9 / 1 = 69.9 (sec) That is, according to the above embodiment of the present invention, 0% of the steam flow rate
Steam can be generated without delay for about 70 seconds without the addition of a heat source fluid for abrupt changes from to 100%. During this time, the evaporation flow rate due to the original heating can be increased instead of decreasing the flash evaporation flow rate due to the gradual increase in the heat source fluid.

On the other hand, when the steam flow rate suddenly decreases from 100% to 0%, the same principle as that when the steam flow rate increases suddenly is used.
The pressure set value is set to Pmax. Thereafter, the pressure increases, and reaches the set pressure after about 70 seconds if no control is performed. However, the change is more gradual because the heat source fluid flow is actually controlled.

However, in the present invention, no excess steam is generated in any case, and the steam is not discharged into the environment.

Further, in the above embodiment, the pressure set value can be continuously changed with respect to the steam flow rate.
In this case, the set value of the evaporator pressure does not change with a slight change in the steam flow rate, but the increase in the auxiliary fuel flow rate or the exhaust heat source fluid flow rate follows such a small change with a slight delay. So there is no problem. On the other hand, when the steam flow rate changes significantly, the same state change as in the first embodiment can be realized.

An accumulator tank and a pressure control valve may be provided between the evaporator 3 and the steam flow control valve 6 so that the pressure on the steam demand side does not change. In this case, the set pressure of the accumulator tank is set to be equal to or less than the minimum value of the pressure set value of the evaporator.

[0025]

As described above, in the present invention, when the steam load is small, the steam pressure set value in the evaporator is shifted to a higher value, and when the steam load is high, the steam pressure set value is shifted to a lower value. As a result, when the steam flow rate sharply increases, the pressure set value suddenly decreases, and a decrease in the pressure inside the evaporator is accepted, and the increase in demand can be covered by the steam flow rate by flash evaporation. Therefore, it is possible to sufficiently follow a change in load. Further, as described above, the increase in demand is temporarily covered by flash evaporation, so that a rapid increase in the auxiliary fuel flow rate is not required, and combustion can be maintained in a state close to normal operation. Can be suppressed.

When the steam flow rate drops sharply, the pressure set value rises suddenly, and the pressure rise in the evaporator is allowed. Therefore, the amount of evaporation is reduced, and the surplus amount of heat received from the heat source can be absorbed by increasing the temperature of the internally held liquid. Therefore, the load following ability can be made very high as in the case of increasing the load. Also, without giving a sudden disturbance to the combustion state of the auxiliary combustor,
Stable combustion can be performed.

Further, since steam is not released into the environment for pressure control, environmental protection is not impaired.
The economy can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic system diagram of an exhaust heat recovery steam generation system.

FIG. 2 is a control block diagram showing a steam pressure control method of the present invention.

FIG. 3 is a graph showing an example of a method of determining a set value of steam pressure in the exhaust heat recovery steam generation system of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Auxiliary combustor 2 Flow control damper 3 Evaporator 4 Supply tank 5 Supply pump 6 Steam flow control valve 7 Chimney 9 Bypass damper 10 Fuel flow control valve

Claims (1)

(57) [Claims] 1. A method of controlling steam pressure in an exhaust heat recovery steam generation system having an evaporator for recovering heat from high-temperature exhaust gas, hot waste water and the like to generate steam, heat medium steam, and the like. A steam pressure control method, wherein the steam pressure set value is shifted to a higher value and the steam pressure set value is shifted to a lower value when the steam load is high.