PFBC power plant

A power plant burning a fuel at a pressure exceeding the atmospheric pressure includes a combustor formed with cooled walls which form a feed water preheater. A by-pass conduit with a control valve for draining feed water is connected to a feed water conduit between the feed water preheater (the combustor walls) and an evaporator in the combustor. This by-pass conduit may be connected to a feed water tank. At a very low load and in the case of a gas turbine trip, the necessary water flow for cooling the walls of the combustor may exceed the requirement of feed water to the evaporator. Under these operating conditions feed water may be drained off ahead of the evaporator through the by-pass conduit.

BACKGROUND ART 
The invention relates to a PFBC power plant with a combustor operating at a 
pressure exceeding the atmospheric pressure and in which combustion gases 
drive a gas turbine which drives a compressor compressing combustion air. 
The walls of the combustor are water-cooled and form at least part of a 
feed water preheater for an evaporator and a superheater, placed in the 
combustor, for the generated steam (PFBC are the initial letters of the 
English expression Pressurized Fluidized Bed Combustion). 
In a PFBC power plant, an optimum dimensioning of the feed water preheater, 
the evaporator and the superheater entails special problems at a very low 
load. It is advantageous to utilize the cooled walls of the combustor for 
preheating the feed water. These walls may form the entire feed water 
preheater or a part thereof. At a very low load, the necessary water flow 
for cooling of the combustor walls may exceed the water requirement in the 
evaporator of the plant. This means that too small a proportion of the 
supplied water is evaporated in the evaporator. The steam flow through the 
superheater may become insufficient so that its boiler tubes reach too 
high a temperature and are damaged. Upon a load drop out and a GT (gas 
turbine) trip, the large heat contents in the bed material of the 
combustor entail special problems. The water flow required for cooling the 
walls of the combustor is so great that the same flow through a subsequent 
evaporator results in very little steam being generated and in the tubes 
of the superheater not receiving a steam flow necessary for the cooling 
thereof, with an ensuing risk of these tubes being damaged. 
SUMMARY OF THE INVENTION 
In a power plant in which the combustor walls form at least part of a feed 
water preheater, a by-pass conduit with a controllable by-pass valve for 
feed water is connected to the connection between the feed water preheater 
and the evaporator in the combustor. By means of the valve in the by-pass 
conduit, the water flow to the evaporator is controlled such that, in the 
case of a load drop out or a low load, suitable water flow is achieved in 
the evaporator and the superheater.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
In the drawing, 10 designates a pressure vessel. A combustor 12 with cooled 
panel walls 14 containing cooling tubes 16 is arranged in the pressure 
vessel 10. A distributor 18 for combustion air divides the combustor 12 
into a combustion space 20 and an ash chamber 22. The space 24 between the 
pressure vessel 10 and the combustor 12 contains compressed combustion air 
and communicates with the tubes 26 and the nozzles 28 of the distributor 
18. Through these nozzles 28, the combustion space 20 is supplied with air 
for fluidization of the material in the bed 30 and combustion of fuel 
supplied through the conduit 32 from a fuel storage (not shown). Fresh bed 
material can be supplied together with the fuel. Between the air 
distributor tubes 26 there are gaps 34 through which consumed material and 
formed ashes in the bed 30 are able to flow from the combustion space 20 
to the ash chamber 22. From the ash chamber 22, material is discharged 
through the conduit 36 and the rotary vane feeder 38. 
The combustion gases generated during the combustion are collected in the 
freeboard of the combustion space 20 and are led through the conduit 40 to 
a cleaning plant, symbolized by the cyclone 42. Cleaned gas is led from 
there a conduit 44 to the gas turbine 46 and is then forwarded through a 
conduit 48 to the economizer 50 and finally to a chimney (not shown). The 
gas turbine 46 drives the compressor 52, which through the conduit 54 
feeds the space 24 with compressed combustion air, and a generator 56 
which can also be used as starter motor. Between the conduits 44 and 54 
there is a short-circuit conduit 58. Valves 60, 62, 64 are provided in the 
conduits 44, 54, 58. In operation, the valves 60 and 62 are open and the 
valve 64 is closed. In the event of an operational disturbance resulting 
in a load drop out and a gas turbine trip, the valve 64 in the 
short-circuit conduit 58 is opened and the valves 60 and 62 are closed. 
The combustion space 20 of the combustor 12 comprises an evaporator 66 and 
a superheater 68. The evaporator 66 generates steam for a steam turbine 70 
and cools the bed 30. The superheater 68 superheats the steam. The turbine 
70 drives a generator 72. As shown by the block diagram in FIG. 2, the 
superheater 68 may be divided into a first part 68a and a second part 68b. 
A water injection device 75 for controlling the steam temperature may be 
provided between the parts 68a and 68b. 
Water from a feed water tank 74 is pumped by a pump 76 through the conduit 
78, the economizer 50 and the conduit 80 to the tubes 16 of the combustor 
wall 14, which tubes form a feed water preheater. The feed water, heated 
in the tubes 16 of the wall 14, is forwarded to the evaporator 66 through 
the conduit 82. Between the evaporator 66 and the superheater 68 there is 
a water separator 84. From the superheater 68, the steam is passed through 
the conduit 86 with the control valve 88 to the turbine 70. Steam from the 
turbine 70 is led to the condenser 90. The condensate is pumped by the 
pump 92 in the conduit 94 to the feed water tank 74. Between the steam 
conduit 86 and the condenser 90 there is a by-pass conduit 96 with a valve 
98 through which steam can be dumped to the condenser 90 upon drop out of 
the load of the generator 72 and closing of the steam control valve 88. 
The water separator 84 is connected to the feed water tank 74, by means of 
the conduit 100 with the control valve 102, for drainage of water that has 
been separated. A conduit 104 with a control valve 106 connects the 
connection conduit 82 for preheated feed water from the tubes 16 of the 
combustor wall 14 to the evaporator 66. A number of transducers for 
measuring temperatures, water flows, steam flows, and the like, and the 
operating devices of valves included in the plant are connected to signal 
processing and control equipment (not shown). 
In the event of operational disturbance resulting in a load drop out which 
causes a turbine trip, control measures are taken which reduce the energy 
development in the combustor 12. The fuel supply is interrupted, the bed 
depth is lowered, the air flow is reduced, and nitrogen gas can be 
supplied, etc. This means that the heat absorption in the evaporator 66 is 
reduced. The necessary water flow for cooling the combustor walls 14 is 
not reduced at the same rate and to the same extent. Water flow which 
prevents partial boiling and steam generation in the combustor wall 14 
results in the steam generation in the evaporator 66 ceasing. The 
necessary cooling of the combustor walls 14 and the sufficient steam 
generation in the evaporator 66 are obtained by draining feed water, which 
has been heated in the walls 14, from the connection conduit 82 through 
the by-pass conduit 104 with the control valve 106. Also, in case of low 
load operation, a suitable balance between the water flow for cooling the 
combustor walls 14 and the water flow in the evaporator 66 and the steam 
flow through the evaporator 68 can be attained by drainage of feed water 
through the conduit 104 and the valve 106 to the feed water tank 74. Upon 
a gas turbine trip, up to about 60% of the water flow in the combustor 
walls 14 is drained via the by-pass conduit 104.