Gas-turbine and steam-turbine based electric power generation system with an additional auxiliary steam turbine to compensate load fluctuations

An electric power generating system employing a gas turbine (1) and a steam turbine (5) and a gasifier (9) supplying fuel-gas to the gas turbine (1) and to a steam generating fluidized bed combustor (37), the latter providing steam for the steam turbine (5). An auxiliary steam turbine (21), driving an electric generator (22), is also supplied by the circulating fluidized bed combustor steam generator (37). Since the gasifier (9) cannot be reversibly turned down easily, on a fall in load on the main steam turbine (5), the excess steam generated is diverted to the auxiliary steam generator (21) and the gasifier (9) kept running efficiently.

BACKGROUND OF THE INVENTION 
This invention relates to an electric power generation system of the kind 
employing both gas and steam turbines and their associated electric 
generators. It is known to supply the gas turbine with gas from a gasifier 
of the fluidized bed type in which low grade solid fuel is granulated, 
heated with refractory particles and `fluidized` by forcing air through 
the bed of material. The heating is controlled so that low grade fuel gas 
is produced which, after suitable cleaning and cooling, is supplied to the 
gas turbine where it is burnt with excess air and the combustion products 
used to power the turbine. 
The gas turbine exhaust gases contain useful heat which can be extracted in 
a waste heat recovery boiler. Steam produced by this boiler is used to 
drive a steam turbine and its associated electric generator. 
Further steam for the steam turbine can be produced by a circulating 
fluidized bed combustor which may be fed by char from the gasifier, 
supplementary coal, surplus gas produced by the gasifier, or any 
combination of these. 
A problem arises in systems such as the above when sudden load variations 
occur on the electrical system fed by the turbogenerators. It is 
undesirable to run down the gas turbine in such circumstances since its 
efficiency suffers significantly with load variation. Load variations are 
therefore absorbed as far as possible by control of the steam turbine. The 
steam turbine can be run down rapidly but this results in surplus steam 
supply. A considerable portion of the available steam, say 30%, can, 
initially at least, be diverted to a steam condenser, bypassing the steam 
turbine in so doing. This is wasteful however, and further off-loading of 
the turbine cannot in any case be dealt with in this way. Reduction of 
steam generation can be achieved by diverting the gas turbine exhaust 
directly to atmosphere (via the `blast stack`) so bypassing the waste heat 
recovery boiler, or by reducing the gas supply to the circulating 
fluidized bed combustor. The former alternative is wasteful and the latter 
causes a gas surplus in the gasifier output. 
It is difficult to turn the gasifier down to any significant extent without 
losing the ability to turn it up again rapidly. Turning down can be 
achieved by `slumping the spouting bed`, i.e. cutting off the air supply 
so that the bed is no longer fluidized. This effectively shuts down the 
gasifier with the result that resumption of electric power generation will 
take up to 6 hours. Such a situation is clearly to be avoided if at all 
possible. 
SUMMARY OF THE INVENTION 
It is an object of the present invention therefore to provide a power 
generation system using gas and steam turbines and gasification means in 
which the gasification means, of whatever kind, can be kept running at or 
approaching its rated output despite variations of considerable extent in 
the electrical loading of the steam turbine. 
It is emphasized that the invention is not limited to any particular system 
of steam generation, such as that described above, nor to any particular 
gasifier, since advantages obtain with a system having only the following 
essential features. 
According to the present invention, in an electric power generation system 
comprising a gas turbine and a main steam turbine adapted to drive 
respective electric generators, and gasification means for producing 
fuel-gas for driving the gas turbine and for supply to steam generating 
means for driving the main steam turbine, in which the gasification means 
is unable to follow variations of load on the main steam turbine as 
quickly as the main steam turbine itself, an auxiliary steam turbine and 
associated electric generator are provided and steam from the steam 
generating means is diverted from the main steam turbine to the auxiliary 
steam turbine to accommodate a reduction in load on the main steam 
turbine. 
The steam generating means preferably includes a solid fuel combustor 
adapted to burn the fuel-gas additionally. 
The steam generating means preferably includes first heat exchange means 
for recovering heat from the gas turbine exhaust gases, and second heat 
exchange means for extracting heat from raw fuel-gas output from the 
gasification means. 
There are preferably included control means for controlling the relative 
supply of steam to the main steam turbine and to the auxiliary steam 
turbine and, independently for controlling a supply of steam to the 
gasification means particularly for starting up the gasification means. 
At least some of the electric power for sustaining gasification means and 
auxiliary drives associated with it may be available from the electric 
generator driven by the auxiliary steam turbine. 
There may be provided means for directing the fuel-gas output of the 
gasification means to an application independent of the gas turbine and 
the main steam turbine to permit independent operation of the gasification 
means. 
The auxiliary steam turbine may have a power rating in the range 1/25th to 
1/10th of the overall rating of the system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawing, this shows a dual turbine system, a gas turbine 1 
driving an electric generator 3 as a turbogenerator set, and a steam 
turbine 5 driving its generator 7 similarly. Typically, both 
turbogenerator sets are connected to feed an electricity supply system 
e.g. the grid system. 
The gas turbine 1 is driven in known manner by combustion gases resulting 
from the combustion of fuel-gas, the latter being produced by gasification 
means comprising a fluidized bed 9 which heats and extracts fuel-gas from 
low grade solid fuel granules. Starting up the gasifier 9 is a major 
operation in view of the quantity of material involved and generally 
requires a fuel oil input 13 in addition to the normal pressurized air 
input 11 which fluidizes the bed. In normal operation steam is also input, 
as indicated at 15. 
The residue of the gasifier bed, after de-gasification, is a char which is 
used elsewhere as will be explained. 
Fuel-gas output from the gasifier 9 is first filtered in a cyclone filter 
17, which produces more char, and then passed to a gas cooler 19 which 
acts as a main boiler to feed an auxiliary steam turbogenerator set 21. A 
steam turbine such as this is essential to the inventive aspect of this 
system. The steam turbine 21 in this example has a rating of 30 MW against 
the overall rating of the system of 450 MW. A suitable rating for this 
auxiliary steam turbine 21 is in the approximate range 1/25th to 1/10th of 
the overall rating, i.e. 18 MW to 45 MW. 
The gas cooler/boiler 19 is followed by a `candle filter` 23 comprising 
candle-like porous ceramic tubes and producing gas having negligible solid 
content. This cleaned gas is fed mainly to the gas turbine 1 for 
combustion, compression, and driving the turbine. The turbine exhaust 
gases, at 25, are at a fairly high temperature, and stored heat is 
extracted by a waste heat recovery boiler 27 to produce steam. This steam 
is fed to a controlled distribution unit 29 from which it may be supplied 
to the steam turbine 5 on path 31, or to a further distribution unit 33 on 
path 35, or to both in chosen proportion. The unit 33 can be controlled to 
feed steam from the gas-cooler/boiler 19 to the auxiliary steam turbine 21 
or to the gasifier 9 on path 34 or, on path 35 to the unit 29 and the main 
steam turbine 5. In other circumstances, when it is required to off-load 
the main steam turbine 5, steam is fed from the unit 29 and from the 
cooler/boiler 19 to the unit 33 for joint supply to the auxiliary steam 
turbine 21. The units 29 and 33 have their exits and their entrances and 
one unit in its time plies many ports. 
A solid fuel combustor in the form of a circulating fluidized bed combustor 
37 is supplied with char from the gasifier 9, the cyclone filter 17 and 
the candle filter 23. It may also be supplemented by coal. The gasifier 
normally produces more fuel-gas than is required by the gas turbine, and 
the surplus is fed to the combustor 37 to supplement the solid fuel input. 
The unit 29 receives steam from the boiler 27, the combustor 37, and in 
some circumstances from the cooler/boiler 19 on path 35, and supplies it 
to the main steam turbine 5. 
Compressed air for the gasifier 9 is provided by a boost air compressor 39 
which receives air bled from the gas turbine (1) compressor by way of an 
air/air heat exchanger 43 and an air/water cooler 41. 
In the basic topping cycle as proposed hitherto, the low calorific value 
gas produced by the gasification process 9 is burnt in the gas turbine 1 
as fuel. The exhaust gas from the turbine is passed through a divertor 45 
routing it normally to the boiler 27 or to the blast stack 47 (bypass) 
during start-up and certain fault conditions. 
In addition to the two boilers 27 and 37, heat is extracted from the raw 
fuel-gas in the steam boiler 19 and low grade heat is extracted from 
additional coolers (not shown) in the gasifier char `letdown` and the 
booster compressor 19 delivery lines. 
All the heat produced at various points in the thermodynamic cycle is 
collected together and the resultant steam passed through the main (and 
only) steam turbine 5. 
If for any reason the steam turbine loses load, the heat is dissipated by 
allowing up to 30% of the normal steam flow to flow direct to the 
condenser. 
The heat input has now to be reduced to match the output. 
In order to do this the gas turbine heat recovery system generator (HRSG) 
27 has to be reduced in output. The gas turbine can be off-loaded quickly 
and the exhaust may be diverted to the blast stack 47 using the divertor 
45. The circulating fluidized bed combustor (CFBC) 37 and gasifier 9 have 
to be turned down in order to match the 30% water flow (i.e. condensed 
bypass steam). 
The timescale for this is very short and can only be met by `slumping the 
spouting bed` of the CFBC as mentioned previously. 
Thus loss of steam turbine load leads to shutdown of the gasifier: 
resumption of station electric power generation will then take up to 6 
hours. 
The embodiment described above breaks this loop of dependency on the main 
steam turbine. It achieves this by the addition of the small, auxiliary, 
steam turbine 21 which will utilize all the heat generated in the 
gasification part of the process. 
Heat from the boost compressor cooler 43 and the char cooler (not shown) 
totalling about 18 (MTW) is used as low temperature feed heating. 
The raw gas cooler 19 is used as the main boiler and the output is high 
pressure up to 160 Bar (2300 psi ) superheated steam, as in the similar 
"syn-gas" boilers in the process industry. 
For a topping cycle plant of 450 MW, the proposed auxiliary steam turbine 
21 would be about 30 MW and require steam inlet pressure of about 40 bar 
(580 psig) and 400.degree. C. (750.degree. F.) for optimal thermal 
efficiency. 
The high pressure steam is de-temperated and reduced in pressure as 
necessary to match the auxiliary steam turbine requirements. 
The auxiliary steam turbine system includes full feed heating, gland steam 
condensers, condensate pump and boiler feed pump and is capable of full 
operation independent of the main steam turbine. 
The advantages of this are as follows: 
1. The auxiliary steam turbine provides all the electrical power for 
sustaining the gasification system and all other gasification system 
auxiliary drives. 
2. The gasification system can be run independent of the gas turbine 1 or 
the main steam turbine 5 providing the gas produced can be used elsewhere 
(e.g. as shown at 49). 
3. The gas turbine 1 can be operated separately. 
4. The main steam turbine 5 can be operated over a wide load range without 
altering the gas fuel flow to the gas turbine. 
5. When the station is operating at a high load factor any sudden load 
changes can be done on the much faster response 30 MW steam turbine. The 
load then can be re-adjusted on the main gas (1) and steam turbine (5) at 
a slower rate. This is an improvement as it reduces the effect of thermal 
cycling on the main gas turbine. 
6. Steam can be extracted from the cooler/boiler 19 and used in the 
gasifier 9. (It should be noted that, in the known system this gasifier 
steam is extracted from the input to the main steam HP turbine. Reduction 
of steam turbine load reduces the steam pressure requirement and at lower 
loads this requirement becomes lower than the gasifier pressure. So the 
main steam turbine cannot then be used as the sole source of steam for the 
gasifier). 
7. The steam supply to the auxiliary turbine 21 can be used for warming the 
main steam turbine and maintaining gland sealing. 
8. Under high load conditions steam can be transferred from the 
cooler/boiler 19 to the main steam turbine 5 along path 35 or vice versa 
for optimum cycle efficiency. 
9. Under abnormal low frequency conditions on the electricity supply 
network that normally determines the speed of auxiliary drives in the 
system, (as in the year of 1963 on the national grid) the independent 
output generated by the steam turbine 21 enables all the auxiliary drives 
to operate at full speed instead of grid frequency. Boiler feed pumps are 
electric motor driven, and cannot sustain full pressure at such reduced 
speed. 
10. For `black start` conditions the starting of the gasifier on oil can 
produce steam in the cooler 19 which is then supplied on path 34 and used 
to start the full gasification. As more heat is produced the steam turbine 
21 can be used to provide some electrical power until there is sufficient 
to start the main gas turbine. 
11. In the event of grid failure and consequent loss of load, the auxiliary 
drives are still kept going and the gasification system maintained at 
operational temperature so that full load can be resumed quickly once 
demand is re-established. 
12. Fitting the auxiliary steam turbine as part of the gasifier system 
fundamentally improves the versatility of the gas steam combined cycle and 
its safety and operational integrity.