Combustion system control

In a system for combustion of fuel to produce heat in which there are alternate primary and secondary means to supply combustion air and in which the fuel flow is ratioed to the total air flow, the flow of fuel to the combustion chamber is controlled on shutdown of operation of the primary means for supplying combustion air by activating the secondary means for supplying combustion air and relaying a signal to the ratio controller which falsely indicates sufficiently low flow of air to the combustion chamber to cause an immediate reduction of flow of fuel to the combustion chamber to be within safe limits.

BACKGROUND OF THE INVENTION 
This invention relates to the combustion of fuel to produce heat. In one of 
its aspects this invention relates to the controlled combustion of fuel. 
In another of its aspects this invention relates to supplying to a 
combustion chamber an amount of fuel ratioed to an amount of combustion 
air supplied. In still another of its aspects this invention relates to 
control of fuel flow during a period of shifting from a primary means of 
supplying combustion air to a secondary means for supplying combustion 
air. 
This invention has arisen from a specific situation in which hot exhaust 
gas from a combustion gas turbine is used as a source of heat and oxygen 
for burning supplemental fuel for a boiler or a heater. The invention is 
applicable, however, in any system in which combustion air is supplied 
from a primary air mover that has a backup, secondary air mover that is 
used in time of emergency shutdown of the primary means for supplying 
combustion air. Most combustion gas turbines supplying exhaust gas to 
waste heat boilers, for instance, have a fresh air blower that is started 
and supplies combustion air when the gas turbine is shut down or tripped 
off. 
The control of fuel and air supply in these combustion systems usually 
incorporates a conventional "lead-lag" fuel and air control system which, 
among other things, will detect a condition of insufficient combustion air 
and match it with a fuel rate that is ratioed to be safe regardless of the 
demand. Past experience has proven, however, that upon emergency shutdown 
of the primary means for supplying combustion air there is usually such 
rapid decay of air supply that the control limiting the fuel is unable to 
keep pace, causing the heating system to go through a dangerous period of 
excess fuel. Up to now it has been common practice to run the secondary 
means for supplying combustion air at all times so that it will be up to 
speed at any time it might be needed. Avoiding the use of energy required 
for running the standby system when it is not needed would, of course, 
improve the economics of the combustion system, but, more importantly, 
would eliminate a useless expenditure of energy. 
It is, therefore, an object of this invention to provide a safe and 
economical method for controlling the flow of fuel to a combustion chamber 
during the shifting of operation from a primary means for supplying 
combustion air to a secondary means for supplying combustion air. It is 
another object of this invention to provide apparatus for controlling the 
flow of fuel to a combustion chamber during the shifting from a primary 
means of supplying combustion air to a secondary means of supplying 
combustion air. 
Other aspects, objects, and the various advantages of this invention will 
become apparent upon study of this specification, the drawings, and the 
appended claims. 
STATEMENT OF THE INVENTION 
In a system for combustion of fuel to produce heat in which there are 
alternate primary and secondary means to supply combustion air and in 
which the fuel flow is ratioed to the total air flow, a method is provided 
for controlling the fuel flow to the combustion chamber on shutdown of 
operation of the primary means for supplying combustion air. In this 
method the secondary means for supplying combustion air is activated on 
shutdown of the primary means for supply of combustion air and a first 
signal falsely indicating low flow of air to the combustion chamber is 
relayed to a ratio controller controlling the fuel flow. This signal 
indicates sufficiently low flow of air immediately to reduce the flow of 
fuel to the combustion chamber so that it will be within safe limits. A 
falsely indicated low flow of air is continued for a time sufficient to 
allow the secondary means of supplying combustion air to come up to speed 
so the control of the fuel system can be safely shifted to the normal 
controlling system. In actual practice the false signal is continued for a 
time sufficient for flow from the secondary means of supply of combustion 
air to reach a predetermined level, the first signal is then increased 
automatically until it is greater than a second signal indicating actual 
flow of air and control of the fuel control valve is shifted to the second 
signal as the first signal becomes greater than the second signal. 
The control apparatus necessary for carrying out the method of the 
invention is made up of a valve for controlling the flow of fuel, a ratio 
controller generating a signal to operate the fuel control valve, a low 
signal selector relay switch which selects the input signal for the ratio 
controller, a device generating a signal to the low signal selector 
indicating the total combustion air flow, and a device actuated by the 
shutdown of the primary means of combustion air supply which generates a 
false signal sufficiently low to reduce fuel flow so that it is within 
safe limits during the actual period of low air flow. 
To accomplish the switch-over of control of the fuel flow to be dependent 
on the flow of combustion air supplied by the secondary means of supplying 
combustion air, the controls in the system must contain a means for 
continuing the original false signal for a time sufficient for flow from 
the secondary means of supply of combustion air to reach a predetermined 
level; a means for generating a second signal indicating the actual flow 
of air; a means for automatically increasing the first signal until it is 
greater than the second signal; and a means for shifting control of the 
fuel control valve to the second signal as the first signal becomes 
greater than the second signal.

Referring now to FIG. 1, hot exhaust air from a gas turbine is normally 
transferred through line 1 to a boiler combustion chamber 3. Fuel, in this 
case fuel oil, is transferred through line 5 which contains fuel control 
valve 7 into the combustion chamber 3. A secondary supply of combustion 
air can also be provided through line 9, force draft fan 11, and line 13 
into the combustion chamber 3. In the combustion chamber the fuel is mixed 
with the air supply and burned to produce heat for generating steam in a 
boiler. Flue gas from the combustion is removed from the system in line 15 
and the produced steam is removed through line 17. 
The basic control system upon which the improvement of this invention 
operates is set out in FIG. 1. 
The control system is based on flow measurements from means 19 for 
generating a signal indicating the flow of hot exhaust air from the gas 
turbine through line 1 and means 21 for generating a signal indicating the 
flow of fresh air induced through line 9 by forced draft fan 11. These 
signals are transmitted through lines 23 and 25, respectively, to a means 
for summing the signals 27 which transmits the summation of the signals 
through line 29 to a ratio controller 31. 
On the fuel inlet line 5 is a means 33 for generating a signal indicating 
the flow of fuel through the fuel inlet line 5. This signal is transmitted 
through line 35 to a means 37, such as a flow indicator controller, for 
generating a signal 37 which is transmitted through line 39 to control 
valve 7 which controls the flow of fuel to boiler combustion chamber 3. 
The signal indicating the flow of fuel through line 5 is also transmitted 
through line 41 to a means 43, such as a flow indicator controller, for 
generating a signal which is transmitted through line 45 to control the 
dampers on forced draft fan 11. Both of the means 37, 43 for generating 
control signals are controlled through the ratio set of the signal 
generated from the ratio controller 31 through line 47. It can be seen 
that the basic control of the fuel to the boiler combustion chamber is 
responsively ratioed to the total flow of air to the boiler combustion 
chamber. 
The basic control system shown in this application is commonly called 
"parallel control" in which the same signal, line 47, sets both fuel and 
air flow rates. A widely used system commonly called "lead-lag" can also 
be used as control system. This invention would have identical results 
when used in a "lead-lag" system. Lead-lag is not described since the 
system is well understood by those skilled in the art. 
Operating with the control system as described above with the combustion 
air being supplied entirely by turbine exhaust gas through line 1 and with 
forced draft fan 11 controlled by the gas turbine tripout 49 so that the 
fan 11 immediately becomes operative on the failure of the gas turbine, 
there is sufficient lag in the control system to allow a serious condition 
of fuel overfeed as the supply of air from the turbine exhaust rapidly 
decays before the air supply from the forced draft fan becomes sufficient 
to supply combustion air. 
This condition is graphically illustrated in FIG. 2. At zero seconds the 
turbine trips out. All of the functions dependent upon the air flow from 
the turbine immediately begin to decay. A signal is sent to the forced 
draft fan to begin operation, but there is an approximate six second lag 
before air is supplied through this means (line B). In the meantime the 
supply of air from the turbine exhaust gas flow (line A) decays rapidly. 
The ratio control cuts back on the flow of fuel oil (line C), but there is 
sufficient lag in this control that there is a serious overfeed of fuel 
for a few seconds as indicated by the molar oxygen content of the flue gas 
(line D) which becomes zero for the period noted as Z on the graph. To 
eliminate the period of excessive fuel flow, means and method are herein 
set forth by which the flow of fuel (line C) can be more rapidly decreased 
so that the flue gas molar oxygen content (line D) will not reach zero. 
Referring now to the inset in FIG. 1, which describes a pnematic system, 
but which can be accomplished equally well with readily available 
electronic systems, the same signal that the gas turbine has tripped that 
starts the motor of forced draft fan 11 also deenergizes solenoid valve 51 
which shuts off the air supply through line 53 and threeway valve 55 to 
low signal selector 57 which is inserted in line 29 between the means 27 
for summing the signals to indicate total air flow to the boiler 
combustion chamber and ratio controller 31. The pressure from the air 
supply in line 53 is always set higher than the maximum signal generated 
in response to the total air flow to the boiler combustion chamber. The 
operation of three-way valve 55 substitutes a low flow signal 59 at a 
sufficiently low pressure rapidly to drain the system's air to a pressure 
below the signal indicating the summation of air flow to the boiler 
combustion chamber. The fuel flow through the control valve 7 is rapidly 
cut back in ratio to this false low flow signal. 
The remainder of the system illustrated in FIG. 1 discloses means by which 
control can be automatically reassumed in response to the signal 
indicating the flow of combustion air into the boiler combustion chamber. 
In operation, after the switch in three-way valve 55 and the draining of 
air pressure from the control system to match the low pressure signal 
through line 59, an adjustable timer 61 which has been preset to a desired 
lag time, for this system about six seconds, and which has been activated 
by the gas turbine trip, reenergizes solenoid valve 51 to switch the 
three-way valve 55 so that the air supply is again admitted into the 
system through line 53. This air supply flows through line 63 and 
restriction valve 65, line 67 and into volume chamber 69 which is 
connected to the low signal selector 57 by line 71. The restricted flow of 
higher pressure air into a system with built-in volume capacity chamber 
69, allows the slow building of pressure against the low signal selector 
57, thus allowing time for the volume of combustion air transported by 
forced draft fan 11 to increase sufficiently that the signal generated by 
summation means 27 is sufficiently high to facilitate change of control of 
the fuel inlet valve as the pressure through line 71 on the low selector 
switch 57 exceeds that of the signal generated by summation means 27 so 
that control is passed again to the signal generated by summation means 
27.