FIELD OF THE INVENTION
The present invention relate to a system for isolation of external heat transfer surfaces of Superheater and Reheater bundles kept inside Fluidised Bed Heat Exchangers (FBHE) during maintenance of FBHE enabling part load operation of the Circulating Fluidised bed Combustion (CFBC) boiler without shutting down the boiler. More particularly, the invention relates to a method for isolation of external heat transfer surfaces of Superheater and Reheater constituting tube-bundles immersed inside Fluidised Bed Heat Exchanger (FBHE) in a Circulating Fluidised bed Combustion (CFBC) boiler.
BACKGROUND OF THE INVENTION
In Circulating Fluidised bed Combustion (CFBC) boilers, the main heat transfer surfaces are a Combustor, a back pass and an external Fluidised Bed Heat Exchanger (FBHE). Heat transfer surfaces such as the tube-bundles of Superheater (SH) and Reheater (RH), are accommodated both inside said back pass in the flue gas path and in said external Fluidised Bed Heat Exchangers (FBHE) where they are kept immersed inside an inert bed material to recover the heat of the solid particles available in the fluidised bed. The SH surfaces are divided into three stages for example, a Low temperature SH, an intermediate SH and a final SH. The RH surfaces are divided into two stages namely, a first stage RH, and a final RH. The Low temperature SH, the final SH and the final RH tube-bundles are kept in the back pass the first stage RH and the intermediate SH bundles are kept in the external Fluidised Bed Heat Exchangers (FBHE).
In the FBHE, the tube bundles are kept immersed inside the fluidised bed in a bundle chamber and their inlet and outlet headers are kept outside of the bundle chamber and connected through a plurality of tubes penetrating the walls of the bundle chamber. The tube-bundles are either suspended from top or supported from bottom based on the coil penetration location with respect to walls of the

bundle chamber. There are a number of tube rows constituting parallel flow paths with defined spacing in between the tube elements. Just below the tube bundle, multiple air nozzles are provided in a grate for proper fluidisation of the bed material. The walls of the FBHE chambers are completely refractory lined The operating environment inside the fluidised bed is much severe when compared to the environment in which the back pass bundles are kept in the flue gas path. The tube-bundles are subjected to dynamic forces of fluidisation. Due to their compact configuration and refractory content, the maintenance cycle time for the FBHE is quite high when compared to the back pass.
The heat transfer bundles in the back pass and the FBHE are connected in series according to the heat duty available at each location. If there is a tube failure in upstream part of the formed steam circuit, it severely affects the performance of the subsequent downstream heat transfer tube-bundles as the continuity of the steam circuit is getting affected.
For taking up maintenance work at FBHE, the boiler needs to be shut down and kept idle during the entire maintenance period as there is no provision in the prior art for isolating the heat transfer surfaces inside the FBHE. Thus, when the FBHE is shut-down for maintenance, the boiler is capable of meeting 40% to 50% of the rated capacity subject to establishment of a continuity in steam circuit This disadvantage of prior art, necessitated development of a system for isolating the heat transfer surfaces inside the FBHE to allow at least a part load operation of the boiler without a complete shut down of the boiler during FBHE maintenance.
Patent US 4920751 describes system and method for reheat Steam temperature control in Circulating fluidized bed boilers. The system has a cold steam by-pass system for controlling the temperature of the second or final stage reheater by dividing reheat steam into selective first and second portions and directing the

first portion to the first stage reheater and recombining said first and second portions and directing them through the second or final stage reheater.
US 2013 / 0305722 A1 teaches a method for operating a steam power plant at low load comprising the extraction of live steam and/or reheated steam before the last superheater and / or the last reheater and using the thermal energy of this extracted steam in other heat sinks. Thus, nearly constant steam parameters of the live steam are achieved and the overall efficiency of the steam power plant remains at a high level.
US 4357803 discloses a control system for a steam turbine operated with a steam bypass system. In the control system, a bypass valve control is implemented according to set points generated as a function of a combined flow reference signal which is representative of boiler outlet flow under all turbine operating phases. An actual load demand signal indicative of the turbine demand for steam is produced and used for intercept valve control. Excessive steam flow in the lower pressure bypass subsystem is prevented by providing an override for the normal control to prevent high heat impact to the condenser and latter stages of the turbine high pressure section.
US 4693086A describes a reheat steam turbine power plant having a boiler with a superheater and reheater disposed therein, a high pressure steam turbine, an intermediate pressure steam turbine, a low pressure turbine, and a condenser. The condenser condensing a steam exhausted through the low pressure turbine to a condensate. A turbine bypass pipe is provided with a bypass valve. A branch pipe branches off a cold reheater pipe between the check valve and the reheater, which is connected to the condenser for introducing steam flowing through the turbine bypass pipe. A control valve is arranged in the branch pipe and a controller controls the bypass valve and the control valve. When the turbine bypass line is operational in one of a start-up or an auxiliary operation of the power plant, the control valve and the bypass valve are controlled by the

controller so that the quantity of reheat steam introduced into the reheater is controlled and excess or surplus steam is discharged to the condenser through the branch pipe.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a method for isolation of external heat transfer surfaces of Superheater and Reheater constituting tube-bundles immersed inside Fluidised Bed Heat Exchanger (FBHE) in a Circulating Fluidised bed Combustion (CFBC) boiler.
Another object of the invention is to propose a method for isolation of external heat transfer surfaces of Superheater and Reheater constituting tube-bundles immersed inside Fluidised Bed Heat Exchanger (FBHE) in a Circulating Fluidised bed Combustion (CFBC) boiler, which allows a part load operation of the boiler during maintenance of FBHE without shutting down the boiler.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method for isolation of external heat transfer surfaces of Superheater and Reheater constituting tube-bundles immersed inside Fluidised Bed Heat Exchanger (FBHE) in a Circulating Fluidised bed Combustion (CFBC) boiler. According to the invention, for isolating the tube-bundles of the superheaters in the fluidized bed heat exchangers, a bypass line has been introduced. During maintenance of the tube-bundles of superheater in the FBHE, the steam flow through these tube-bundles according to the invention can be closed and diverted through said bypass line, and after completion of repair maintenance of the FBHE, the said line is connected to the main line. This bypassing establishes a continuity in steam circuit and the boiler can be operated at reduced loads without a complete shut down of the boiler.

Similarly, for isolating the tube- bundles of re-heaters immersed in the FBHE, a second bypass line has been introduced. During maintenance of the FBHE RH bundles, the steam flow through these tube-bundles as per the invention, can be closed and diverted through the FBHE RH bypass line (second bypass line). This dual provision of bypassing establishes a continuity in the reheater steam circuit and the boiler can be operated at reduced loads without shut down of the boiler.
BRIEF DESCRIPTION OF A ACCOMPANYING DRAWINGS
Figure 1 - shows a Schematic diagram of a typical Circulating fluidised bed boiler system.
Figure 2 - shows a schematic diagram of a Superheater circuit and FBHE SH bypassing process.
Figure 3 - shows a schematic diagram of Reheater circuit and FBHE RH bypassing process.
DETAILED DESCRIPTION OF THE INVENTION
The accompanying Figures 1,2 and 3 may be referred for better understanding of the invention with assigned reference numerals of the constituent components. In the CFBC boiler, a Combustor (1), a cyclone (2) and a plurality of FBHE (4,5,6) forms part of a solids circulation circuit through which hot solids remain in circulation for complete combustion of the fuel. The solids are captured in the cyclone (2) and the flue gas exits through the top of cyclone (2) which passes through a back pass (3) in which the heat transfer surfaces as described below, are arranged. Low temperature super heater (9), Final Re-heater (10) and Final Super-heater (11) are arranged inside the back pass (3). The first intermediate

SH-I (4), the second intermediate SH-II (5), and the first Reheater-1 (6) are arranged inside the fluidized bed heat exchanger (FBHE).
Figure -2 shows that during the normal operation, the formed Superheater circuit operates as under-Saturated steam from the drum passes through the Low temperature SH (9), a first De-superheater-l (12), the first intermediate SH-I (4), the second intermediate SH-11 (5), a second De-superheater-l I (13), and the Final Superheater (11). The superheated steam passes through then main steam line to the steam turbine. The first De-superheater-l (12) and the second De-superheater-l I (13), are used for controlling the temperature of steam in the circuit.
For isolating the FBHE Intermediate SH bundles (4 and 5), a FBHE SH bypass line (19) has been introduced which branches between the first De-superheater-l (12) and the first Intermediate superheater SH-I (4) and is connected between the second Intermediate superheater SH-ll (5) and the second De-superheater-l I (13). A first gate valve (15) is introduced in the main line for closing and a second gate valve (16) is provided in the bypass line (19) for opening and diverting the steam flow through the bypass line (19) and vice versa during restoration. Similarly, a first non-return valve (17) is introduced in the main line and a second non-return valve (18) is provided in the bypass line (19) for avoiding a reverse flow. For enhancing safety of the system, a safety valve (25) is provided just before the FBHE SH bypass line (19).
If there is any problem during maintenance of the FBHE Intermediate SH bundles (4 & 5), the steam flow through these bundles (4,5) can be closed by the first Gate valve (15) and diverted through the FBHE SH bypass line (19) and again connected to the main line. This bypassing establishes a continuity in steam

circuit and the boiler can be operated at reduced loads without shut down of the boiler until the problem in FBHE is rectified.
For the Reheater circuit as shown in Figure-3, the circuit formed during normal operation of the boiler, operates as under:- From the steam turbine, steam passes through a Cold reheat line, Reheater-I (6), a third RH De-superheater (14), a Final Reheater (10) and passes through hot reheat line to steam turbine. RH De-superheater (14) is used for controlling the temperature of steam in the circuit.
For isolating the first FBHE Reheater-I (6), a FBHE RH bypass line (20) has been introduced which branches from said Cold reheat line before the first Reheater-I (6) and connected between the first Reheater-I (6) and RH De-superheater (14). A first gate valve (21) is introduced in main line for closing, and a second gate valve (22) is provided in the bypass line (20) for opening and diverting the steam flow through the bypass line (20) and vice versa during restoration. Similarly, a first non-return valve (23) is introduced in the main line and a second non-return valve (24) is provided in the bypass line (20) for avoiding reverse flow. For enhancing safety of the system, a safety valve (26) is provided just before FBHE RH bypass line (20).
If there is any problem during maintenance of the first FBHE Reheater-l (6), the steam flow through these bundles can be closed by the first gate valve (21) and diverted through the FBHE RH bypass line (20) and again connected to the main line. This bypassing establishes a continuity in the Reheater steam circuit and the boiler can be operated at reduced loads without a complete shut down of the boiler during maintenance of the first FBHE Reheater-I (6).
For isolating the FBHE from the solid circulation circuit, a first solids flow control valve (7) between the cyclone (2) and the FBHE, and a second solids flow control valve (8) between the FBHE and the Combustor (1) are provided.

WE CLAIM
1. A method for isolation of external heat transfer surfaces of Superheater and Reheater constituting tube-bundles immersed inside Fluidised Bed Heat Exchanger (FBHE) in a Circulating Fluidised bed Combustion (CFBC) boiler, the method comprising the steps of:-
- introducing a first bypass line (19) for isolating FBHE Intermediate SH bundles (4,5) which branches between a first De-superheater-1 (12) and a first intermediate SH-I (4), the bypass line (19) being connected between a second Intermediate SH-II (5), and a second DE-superheater-ll (13);
- introducing a first gate valve (15) in the main line for closing the main line, and a second gate valve (16) provided in the bypass line for opening and diverting the steam flow through the bypass line (19) and vice versa during restoration;
- providing a first non-return valve (17) in the main line, and a second non-return valve (18) in the bypass line (19) for avoiding a reverse flow;

- providing a FBHE RH bypass line (20) for isolating the FBHE Reheater (6) which branches from a Cold reheat line before said Reheater-l (6) and connected between said Reheater-l (6) and a RH De-superheater (14);
- introducing a third gate valve (21) in the main line for closing the main line, and a fourth gate valve (22) provided in the bypass line (20) for

opening and diverting the steam flow through the bypass line (20) and vice versa during restoration;
- providing for the reheater circuit, a first Non-return valve (23) in the main line and a second non-return valve (24) in the bypass line (20) for avoiding a reverse flow; and
- providing a first safety valve (25) before said FBHE SH bypass line (19) and a second safety valve (26) before the FBHE RH bypass line (20).