LT3379B - System and method for reheat steam temperature control in circulating fluidized bed boilers - Google Patents

Abstract

A steam generator having a fluidized bed combustion system that includes a fluidized bed combustor and at least one hot separator, includes a superheater and a reheater. A first stage of reheater and a second stage or final stage of reheater are sequentially disposed in a common gas flue. Cold steam from a turbine is divided into selective first and second portions, the first portion being directed through the first stage of reheater and thereafter recombined with the second portion of cold steam. The recombined first and second portions of steam are directed through the second stage of reheater.

Description

The invention relates to a power plant comprising two stages of a steam turbine and an installation of a steam generator having a fluidized bed combustion system, at least one separator and a gas channel having a heater and a steam superheater.

The present invention also relates to a method for controlling the temperature of a superheater in a steam generator comprising a fluidized bed combustion system including a fluidized bed combustion chamber, at least one hot separator and a heater in a gas channel.

At present, several methods of controlling the temperature of the water vapor heating are known.

One way of controlling the temperature of the heater involves using a gas passage system through the heater in the convective duct of the boiler, providing two separate ducts (one for the gas superheater and one for the heater) with damper-like devices at each end of the gas duct. The water vapor temperature at the heater outlet can be controlled by varying the amount of flue gas flow between the convective ducts.

The main disadvantage of this system is that the valves are exposed to erosion and mechanical disintegration due to their placement in a dust filled tract at temperatures above 260-371 ° C. This type of system also limits the range of water vapor temperature control.

Another way of controlling the output temperature of the water vapor heater involves using an external heat exchanger. In this case, a portion of the recirculating solid particles contained in the circulating fluidized bed circulatory system is conveyed to an external heat exchanger with a fluidized bed, namely, an external heat exchanger containing the entire heater or section thereof. By changing the amount of solid particles flow to the external heat exchanger, the amount of heat transfer to the heater and the temperature of the water vapor heater outlet are controlled.

The main disadvantage of this system is that the solid particle flow control valve is a device that requires careful maintenance and that the outer surface of the heat exchanger tubes is exposed to erosion. This is important for the use of the device.

U.S. Pat. 4748940 proposes the heating surface of the first heater to be connected to a combustion chamber with a fluidized bed of flue gas to connect to the first heater a second installed in an external heat exchanger. An adjustable transition line is connected in parallel to the heat exchanger duct and heater to the heating surfaces. The output is controlled by regulating the flow of solid particles from the external heat exchanger and from the heater by regulating the water vapor flow in the two heaters through a transition pipe.

Further different designs of the first and second stages of the heater are proposed in European patent application no. This application discloses a power plant comprising a two-stage turbine and a gas generator unit comprising a fluidized bed chamber, particle separators, first and second heaters, or a final stage and steam superheater.

According to European patent application no. 0274637 The front and rear stages of the heaters are arranged in series. Two stages of heaters are located in separate external heat exchangers for cooling the ash to be removed. The rear stage of the heater is located in a gas passage connected to the gas outlet from the combustion chamber with a fluidized bed. The amount of heat transmitted in the first stages of the heaters and the temperature at the output of the heater are controlled by changing the flow of solid particles to the external heat exchangers. This scheme, as previously mentioned, is inappropriate due to the problem of servicing solid particle flow control valves.

Another way to control the temperature of the water vapor heater outlet is by using a spray cooler. In this case, water spraying is used for steam cooling and the water vapor temperature at the outlet of the heater is thereby controlled. This is a simple path but is not widely used due to the loss of cycle efficiency.

Another variant using excess air is still known. Excessive air supply to the boiler can be used to control the temperature of the water vapor heater. However, this option is not used because of the negative impact on boiler efficiency. In addition to the above, another option is known using gas recirculation. In this case, large amounts of smoke are recirculated to reach the target water vapor temperature at the heater outlet. This requires the use of a circulating gas fan to compress the hot, dust-saturated gas, which requires additional energy consumption, which makes this variant unusable.

The present invention is directed to an improved method and system for controlling the temperature of heating water vapor.

It is an object of the present invention to provide an improved method and system for controlling the temperature at the outlet of a boiler with a circulating fluidized bed of water vapor.

In order to solve this problem, the present invention provides a steam generator having a fluidized bed combustion chamber comprising a fluidized bed combustion chamber itself, at least one separator and a flue gas duct heater according to the invention comprising the first heater stage and the second rear heater stage successively in a common gas passage, means for distributing cold water vapor from the turbine to selectable first and second portions and diverting the first portion through the first heater stage and means for re-connecting and diverting the first and second portions through the second heater stage.

A gas generator is best provided with means for controlling the temperature of the second or end stage of the heater and means for passing a selected portion of the cold water vapor around said one stage heater directly to the second or end stage of said heater.

The method of the invention, wherein the heater is divided into first and second or end stages in a common gas passage, separates the cooled water vapor returning to the heater optionally to the first and second portions and diverting the first portion through the first heater stage and through the second or rear stage of the heater.

The above other objectives and advantages will be apparent from the following description and the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a typical system of a circulating fluidized bed boiler;

FIG. 2 is a schematic diagram illustrating a second embodiment of the present invention;

FIG. 3 is a schematic diagram illustrating an installation with two typical boilers connected to one turbine.

FIG. The power plant shown in Figure 1 has a typical boiler with a circulating fluidized bed with steam superheater and heater, with a system implemented in accordance with the preferred embodiment of the present invention. The boiler installation 1 comprises a combustion chamber unit 2 with a fluidized bed having a combustion chamber 3 in which flammable material, non-combustible material, primary air and secondary air are introduced. In the combustion chamber, the layer is maintained in a fluidized state providing an accurate amount of material layer and airflow. The combustion chamber has a base 4 with a lattice structure through which the fluidized air is introduced. The walls of the combustion chamber are preferably membrane-type tubular walls with or without a refractory coating.

The first stage 5 and the second stage of the steam superheat are placed inside the combustion chamber. The materials contained in the combustion chamber are transferred from the combustion chamber by gas channels 7 to a hot separator 8 where the solid particles are separated from the smoke by the solid particles recirculation system 9, 10, 11 for return to the combustion chamber base for recirculation. Before being returned to the combustion chamber, they may be passed through pseudo-liquefied refrigerators or similar facilities.

The details of the drinking water circuit and the primary steam superheaters are not shown as they are not essential to the present invention.

The flue gas from the hot separator enters the convection conduit 13 through the gas conduit 12, and the single-stage steam superheater 14 is housed in the convection conduit by installing heaters 15 and 16 behind the vapor condenser 14 above the surface of the economizer 17. The heater shown in two stages, the first stage being numbered 16 and the second or rear stage being numbered 15. These two stages are arranged as a counterflow heat exchanger, directing the flow of gas from the top down and the heated water vapor from the bottom up. Placing the superheater 14 inside this duct facilitates maintaining the gas flow temperature in the heater 15 below the critical. This circuit with the surrounding path, as will be explained, provides the heater with unique and effective temperature control inside the sections.

When the temperature of the water vapor leaving the particular section (in the counter flow heat exchanger scheme) is close to the temperature of the gas entering this section, reducing the water vapor flow significantly reduces heat absorption. When the water vapor temperature equates to the gas temperature, the useful thermal effect suitable for heat transfer is reduced. This forms the basis of the principle used in the invention in the temperature control system.

The generating system shown in FIG. 1, consisting of water vapor injection into a two-stage turbine. In the illustrated scheme, water vapor from the steam superheater 14 passes through the outlet manifold 18 and the connected conduit 19 via a tap 20 to the inlet side of the high pressure turbine 21. The cooled water vapor exiting the turbine 21 returns to the first and second stages 16 and 15 of the heater via a return line 22 and is connected to the return line 22 at point 24 in the heater and shoots the cooled vapor portion through the differential control valve to the first stage 16 of the heater. the entrance manifold as part of the remaining water vapor.

The water vapor passing through the heater 16 is supported by the manifold 27 and is recombined or mixed with the cooled water vapor portion of the circumferential path at point 28. The bypass regulator 29 is provided for flow in the bypass pipe 23 between the elbow of the heater first stage 16 Water vapor 28 enters second or final stage 15 of heater, inlet manifold 30 where it is further heated and passes through outlet manifold 31, feed pipe 32 and tap 33 to second stage or lower stage of medium pressure turbine 34. The selective dosing of the cooled water vapor between the surrounding path pipeline 23 and the heater second stage 16 enables the creation of an effective and useful temperature control means at the heater stages. The location in the gas channel 16 of the first stage heater is selected to cool the necessary portion of the water vapor directly to the second stage 15 of the heater. Shunting may not increase the water vapor from the first stage heater above the temperature allowed for the heater tubing material.

There will be a limit for protecting the first stage heater materials from exceeding their allowed metal temperature.

The temperature value of 566 ° C is a typical temperature and may vary depending on the specific design conditions. The purpose of this system is to keep the maximum temperature of the outer surface of the tubes within the allowable temperature range of the selected material.

The setting of the control valves 25 and 29 is chosen in such a way that the regulation possibility is guaranteed by the range of regulation of the water vapor temperature and allows to change all heating surfaces in the convective duct of the boiler, limiting the need

It also allows for a simpler start-up scheme, such as having more than one boiler connected to a common turbine unit. In this scheme, the valve group provides equalization of the water vapor flow preheating under various operating conditions.

In a boiler with a circulating fluidized bed, combustion takes place in a fluidized bed of an inert material. The pseudosized fluid material exiting the combustion chamber is returned by a hot collector, such as a hot cyclone, through a suitable compactor. During operation, air and fuel are introduced into the combustion chamber 3, where the material layer is maintained in a fluidized state, precisely maintaining the air flow and material layer. Pseudo-liquefied air is introduced through a grid-shaped grate or through a structure 4 at the bottom of the chamber. The flue gas and combustion products together with the transported solid particles first transfer the heat to the steam superheaters 5 and 6 via a gas channel 7 to a hot separator 8 where the solid particles are separated and returned to the combustion chamber by circulating loops 9, 10 and 11. The hot flue gas from the hot separator (s) 8 through the gas passage 12 to the section of the convective passage 13 which incorporates the end stages 14 and 16 of the steam superheater.

The three stages of the gas superheater, which are mounted in the system described and are marked with positions 5, 6 and 14, furthermore, the stage 14 is mounted in the convection channel of the flue gas. Steam coolers may be located between the stages of the steam superheater for water vapor control if necessary. The two heater stages 15 and 16, which are integrated in the convection channel 13, complete with control valves and connecting piping, allow precise control of the water vapor temperature at the heater outlet. The piping system is such that the cooled water vapor, which is fed back to the system by pipeline 22, is optionally split into two streams at its junction 24 with the surrounding path pipeline. One flow passes to the first stage heater and is distributed by the inlet manifold 26. The second flow enters the second stage heater through the valve 29 and the inlet manifold 30. The optional flow division will be proportional to the temperature control performed by the valves 25 and 29.

The hot flow exiting the heater first stage through the outlet manifold 27 is mixed in the bypass pipe 23 with cooled water vapor, entering the second stage heater through the inlet manifold 30. The flow rate of the heater leaving the flow control valve 29 and the mixed first stage flow is controlled by two control valves 25 and 29 which regulate the temperature of the water vapor exiting the heater second stage 15. The hot water vapor from the second or final stage of the heater is directed back through the hot water vapor pipeline to the turbine.

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The pressure drop control unit controls the set pressure of the valve 25 for the pressure change control, which is suitable for the control valve 29.

Block 35 responds to a pressure drop between the cooled water vapor return conduit 22 and the surrounding path conduit 23 with an outlet from the heater 16 junction 28 as shown in FIG. Dotted Line 1 36. Valve 25 is adjustable to the boiler load function by setting the control unit. The valve 29 in the bypass conduit 23 is controlled by a temperature control unit 37 which responds to the temperature of the vapor exiting the second or final stage of the heater 15 as shown in FIG. Dotted Line 1 38. In the illustrated embodiment, for example, heater 15 maintains a temperature within the range of about 538 ° C to 10 ° C. When the temperature of the steam leaving the heater 15 rises above 543 ° C, the valve 29 opens for additional shunting of the cooled water vapor to the heater 15. When the temperature drops below 532 ° C, the valve 29 is closed to reduce the flow of the shunt cooled water vapor to the second stage 15.

FIG. 2 is identical to the system of FIG. 1, only here is a steam superheater 14 between heater stages 15 and 16. The convective channel is provided with a single stage steam superheater 14 with a heater second stage 15 located above the steam superheater and a first stage heater below a superheater 14. The economizer 17 mounted below 14. This is the opposite of what was shown in FIG. 1. The arrangement of the second stage heater 15 above the steam superheater 14 allows it to absorb more heat at lower loads. This allows him to extend his range of control of water vapor temperatures while giving little effect on the range of the steam superheater, if at all

effect. This possible expansion of the range of water vapor preheating temperature range makes it easier to combine two units into one turbine in terms of temperature matching capacity.

the critical temperature of the heater 15 gas

By placing the heater stage 15 above the steam superheater 14, this system provides even greater temperature control in the heater stages. Since the gas now passes through the heater 15 before it enters the steam superheater 14, it cannot have a lower critical temperature for the heater 15 just before a certain boiler load. Thus, when the steam superheater is positioned in channel 14, the temperature will be lower than the heater 15 only until a load of about 2530% is reached. At this point, the cooled water vapor is suitable for temperature control according to the present invention. If a higher loading point is required, the pipe metal materials can be upgraded to a maximum load of 35-40%. Another advantage of the present invention is the circumstance that it is not necessary to pass the heater until the block is 25-40% full.

FIG. 3 is identical to the system illustrated in FIG. 1 but with identical boiler. In this system, the elements of the first boiler plant are marked with the same numerals as in FIG. 1, and the installation of the second boiler room is marked with the same initial digits. Thus, this scheme describes a turbine boiler installation in which two boilers inject water vapor into one turbine. One essential feature required for this type of system is that a means to control the flow of water vapor heated to each boiler must be provided such that the water vapor heating temperature at the outlet is within the set limits under all operating conditions. In the illustrated system, two boilers, both boilers are provided with identical regulators and piping.

By controlling the water vapor preheat temperature, the control valves 25 and 29 can be used to maintain the flow equilibrium temperature within the output of the heater under near-normal as well as abnormal conditions. In this arrangement, the pressure-reducing valves 35 and 37, together with the steam coolers 39 and 40, provide flexibility during cold start, hot start, and also for starting the second unit while the first unit is in operation. This simple system is limited by the complex water vapor mixing system required. It provides a simple and efficient system and a way to control the water vapor preheat temperature at the outlet under changing load conditions.

During a cold start procedure, combustion in the combustion chamber 3 begins by supplying fuel and air entering the combustion chamber. Because of the heat generated by the combustion, the hot combustion gas moves vertically upwardly into the combustion chamber, transferring heat to the water within the combustion chamber and to the steam superheaters 5 and 6. Hot gas, combustion products and solid particles pass from the combustion chamber along the gas channel 7 to the hot separator 8, in which solid particles are separated for return to the combustion chamber. The hot flue gas passes through the gas conduit 12 to the convective conduit 13, where heat is sequentially supplied to the gas superheater 14, the second or final heater stage 15, and the first heater stage 16. Hot air flow through the system begins to cool the cold steam stream. The boiler is fired and the fuel burns for a period of time, providing hot gas up to the water vapor and starting the turbine. The cooled water vapor does not pass until the turbine is started.

As hot gas releases its heat to water and steam in the water walls of heaters and steam superheaters, the temperature drops so that it decreases with each successive step. It should be noted that the temperature of the gas leaving the combustion chamber at full load is between 843 and 927 ° C. The greater the temperature difference between gas and water, the higher the heat transfer and the colder the gas leaving the respective heater.

Accordingly, when the gas passes the steam superheater 14, its temperature is not lower than the critical temperature of the heater 15, including a slight boiler load. In this way, with the steam superheater 14 in the gas channel upstream of the heater 15, the gas temperature will be below the critical heater temperature until a load of about 40-50% is reached thereafter. At this point, the cooled water vapor is suitable for temperature control according to the present invention. Another advantage of the present invention is the circumstance that does not require flow through the heater up to 50% block load.

The standard systems themselves require a flow through the heater, not higher in the early stages (hot or cold) to protect them from sunburn. In this way, it is necessary to use an expensive bypass system. However, physically installing this system eliminates the need for a bypass path and can shorten system startup times.

Other modifications and changes are possible in the foregoing description of the invention, and in some cases some of the features may be used without other features of the corresponding use. Accordingly, although the present invention is illustrated by way of specific embodiment, it will be appreciated that many changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims (8)

DEFINITION OF INVENTION A power plant consisting of a two-stage steam turbine and a boiler ignition system comprising a combustion chamber (2) and a convective transition in which the first and second or rear stages of the steam superheater (14) are arranged in series in an exhaust gas conduit (13). characterized in that it has a fluidized bed boiler chamber system in which at least one separator (8) is arranged between the radiative and convective parts of the boiler and the vapor separating from the high pressure section of the two stage turbine into selected first and second parts and said first means for directing a selected first vapor portion to a first superheat stage and a selected second vapor portion for recombining means (28) mounted in the second superheat stage and connected to the first stage heater outlet. 2. Power plant according to claim 1, characterized in that the steam superheater (14) is arranged in the direction of movement below the second stage (15) of the heater. 3. Power plant according to claim 1, characterized in that the cold steam separating means (23,29) comprises a bypass pipe (23) with at least one flow control valve (29) selected for transferring a cold steam portion from the turbine around the first stage (16) of the heater. to the final stage of the heater (15). 4. Power plant according to claim 1, characterized in that at least one pressure control valve (25) is provided at the inlet of the first stage heater (16), which is sensitive to the difference between the back flow pressure and the outflow pressure from the first stage heater. 5. Power plant according to claim 1, characterized in that it has cold steam separating means (29) sensitive to the temperature of the heater. in a second stage (15) output generator in a steam stage having two stages of steam temperature 6. A method of adjusting the superheater, the turbine and the first and second heater stages in a common convective transition section dividing the cold vapor at the junction of the first heater inlet at two junctions, the first directed to the first heater stage and the second to the bypass line. characterized in that the first flow portion is controlled by a first flow control valve (25) disposed between the connection point and the first stage of the heater, and the second flow portion is controlled by a second flow control valve (29) located in the bypass line. 7. The method of claim 6, wherein the second flow control valve is controlled by a temperature control device (37) responsive to the temperature effect of the flow leaving the heater in the second stage. The method according to claim 6, wherein the first flow rate regulates the difference between the back pressure of the automatic pressurizer and the pressure of the outflow stage. releasing a control valve (38) responsive to the first of the heater