Gas and steam electrical power generating system

An electrical power generation plant has a gas turbine subsystem free of a compressor. A compressor subsystem remote from the gas turbine subsystem has an inlet receiving air and an outlet furnishing compressed air. A compressed air line interlinks the outlets and gas turbine subsystem. An aspect or the invention includes combusting sufficient fuel having sulphur content with sufficient limestone to establish a ratio of Ca/S greater than 3.4.

Many power generating plants utilize a gas turbine having a compressor 
driven by the turbine to drive an electrical generator. Air enters the 
compressor where it is compressed. The compressed air is mixed with fuel 
which is ignited in a burn chamber. The burning fuel/air mixture then 
expands through an expander to rotate the turbine that drives the 
generator and the compressor. 
In combined cycle gas turbine electrical generators, after the hot gas 
expands through the expander, it is used to heat water to produce steam. 
The steam drives a steam turbine, which also drives the generator. 
A search of the prior art revealed the following U.S. Patents: 
U.S. Pat. No. 2,663,145 (Waeselynck) 
U.S. Pat. No. 2,663,144 (Nordstrom) 
U.S. Pat. No. 2,605,610 (Hermitte) 
U.S. Pat. No. 2,540,598 (Ruiz) 
U.S. Pat. No. 2,486,291 (Karrer) 
U.S. Pat. No. 2,424,387 (Lysholm) 
According to the invention, an electrical power generation plant includes a 
gas turbine generating plant; a separate compressor subsystem; and a 
compressed air line which delivers compressed air from the compressor 
subsystem to the burn chamber for mixing with the fuel. 
According to one aspect of the invention, the power generating plant 
comprises a conventional gas turbine generator with the compressor 
removed. A plenum replaces the compressor and receives compressed air from 
the compressor subsystem. Hot exhaust gas heats water to create steam 
which drives a steam turbine which adds power to the generator. Blowoff 
valves near the plenum blow off compressed air being delivered from the 
compressor subsystem in the event of sudden loss of electrical load on the 
generator, thereby preventing system destruction. 
The compressor subsystem includes a boiler which heats water to generate 
steam, and the steam drives a compressor to provide compressed air to the 
plenum of the gas turbine generator. The compressed air is heated by being 
passed through a coil within the boiler before being delivered to the 
plenum. 
The gas turbine generator burns clean, relatively expensive, premium fuels, 
such as natural gas. The boiler of the compressor subsystem burns 
relatively cheap fuel, such as hydrogen deficient solids, petroleum cokes, 
chars, coals, refinery bottoms, vacuum bottoms, residual oils, or any 
combination thereof. 
According to an aspect of the invention, sufficient fuel having sulphur is 
combusted with sufficient limestone to reduce SO2 emission and establish a 
Ca/S ratio greater than 3.4, preferably within the range of 5.1 to 10.2 
and typically about 8.5, to produce an ash with significant free CaO.

Referring to FIG. 1, in a typical combined cycle combustion turbine 
generator plant, air is drawn into compressor 1 where it is compressed 
significantly. One commercially available engine ingests about 2,500,000 
pounds of air per hour at sea level and compresses it to about 12.5 
atmospheres pressure. The temperature rises to about 700.degree. F. 
Most of the compressed air is mixed with fuel and ignited in burn chamber 
2. Although only one burn chamber 2 is shown, many commercially available 
machines have more. The fuel should be gaseous to achieve long hot section 
life; particulates and certain gases contained in the combustion products 
of many liquid fuels, and especially in solid fuels, can be extremely 
damaging to the stator and rotor blades of expander 3. Natural gas 
(methane) is preferred, as it is both clean burning and available 
nationwide. 
A small portion of the air compressed by compressor 1 bypasses burn chamber 
2 and is used to cool the vanes and blades of expander 3. The bypass air 
and products of combustion within burn chamber 2 are mixed, with an 
average temperature of about 1900.degree. F., and passed through expander 
3. The hot gases expand, and work is extracted. When the gases have almost 
fully expanded, the temperature is about 980.degree. F., and the pressure 
is slightly above the pressure at the inlet to compressor 1. The expansion 
of the hot gases through expander 3 produces about 205,000 KW of shaft 
work; compressor 1, on the same shaft as expander 3, requires about 
120,000 KW to perform its function. This loss leaves a net of 85,000 KW 
applied to shaft 4, which drives generator 5 to generate electricity. For 
simplicity, only one generator 5 is shown; most commercially available 
combined cycle plants have two or more independent generators. 
The pressure at the expander exit is sufficient to drive the hot, expanded 
gas through steam generator 6. The hot gas heats water circulating through 
steam generator 6, converting the water to steam which passes through and 
drives steam turbine 7. This produces about 40,000 KW additional power, 
which is applied to generator 5 via shaft 8. Thus, a total of about 
125,000 KW is applied to generator 5. 
The water supplying steam generator 6 is circulated through the system by 
feedwater pump 10. After passing through steam turbine 7, the steam is 
condensed back to water by condenser 11. Cooling circuit CC and cooling 
tower 12 are used to eject the heat of condensation. 
The mass flow, temperatures, pressures, and power output described above 
are for a generating plant operating at sea level, with an inlet 
temperature of 60.degree. F. and a relative humidity of 20%. If ambient 
temperature increases, power output decreases. If the relative humidity 
increases, the heat rate increases. If the plant is located above sea 
level, power output decreases. If the plant is operated at less than full 
load, the heat rate increases. All combined cycle generating plants 
exhibit these characteristics. 
Referring now to FIG. 2, an electrical power generation plant according to 
the invention is disclosed. The invention makes use of a combined cycle 
combustion turbine generator plant, as shown in FIG. 1, from which 
compressor 1 has been removed. Plenum 9 replaces compressor 1 to receive 
compressed air from the compressor subsystem, to be described below, via 
compressed air line AL. Preferably, the air delivered to plenum 9 is 
characterized by the conditions of temperature, pressure, and flow rate 
produced by compressor 1 in the system of FIG. 1 to take advantage of the 
design of the combined cycle plant being modified. If these conditions are 
met, and fuel burn rate in combustor 2 is maintained, combustor 2 and 
expander 3 performs as if compressor 1 were still present. With compressor 
1 removed from the system, the 205,000 KW produced by the expansion of hot 
gas through expander 3 is all applied to generator 5 through shaft 4. The 
total power applied to generator 5 is now 245,000 KW (205,000 KW+40,000 KW 
produced by steam turbine 7). 
The compressed air may be supplied to plenum 9 by driving compressor 1, 
which has been removed, with a steam turbine capable of producing 120,000 
KW. Alternatively, the compressed air may be supplied by a compressor 
subsystem as shown in FIG. 2. 
Fluid bed boiler 13 is fired with relatively inexpensive fuels. Any one of, 
or combination of, the following are equally acceptable: hydrogen 
deficient solids such as petroleum cokes, chars, coals; very heavy 
refinery bottoms; vacuum bottoms; and/or residual oils. Water circulating 
within coil 21 passes through fluid bed boiler 13 and is converted to 
steam. The steam passes through and drives steam turbine 14, which drives 
high-efficiency compressor 1'. Although only one steam turbine 14 and 
high-efficiency compressor 1' are shown, several turbine/compressor 
assemblies may be provided. 
With a currently commercially available high efficiency compressor 1' about 
95,000 KW is required from steam turbine 14 to obtain the same flow rate 
and pressure obtained using replaced compressor 1 (2.5 million 
pounds/hour; 12.5 atmospheres). High-efficiency compressor 1' does not, 
however, raise the temperature of the compressed air to the same level as 
compressor 1 (700.degree. F.). By passing the compressed air through 
heating coil 16, located within boiler 13, the compressed air receives 
enough heat to raise its temperature to the appropriate level. The 
compressed air, now at the levels of mass flow rate, temperature, and 
pressure for the system of FIG. 1, is supplied to plenum 9 via compressed 
air line AL. 
After passing through and driving steam turbine 14, the steam passes to 
condenser 22 where it is condensed into condensate which is circulated 
back to boiler 13 by feedwater pump 15. Water circulated through cooling 
circuit CC' by pump 19 cools condenser 22, and cooling tower 17 ejects 
heat from the steam turbine cycle. Cooling tower 18 ejects heat ejected by 
compressor 1'. In practice, a single cooling tower may perform the 
functions of cooling towers 12, 17, and 18. 
Blowoff valve 23, or a series of valves, is located either on plenum 9, or 
somewhere along compressed air line AL very near to plenum 9. This safety 
valve structure prevents destruction of expander 3 in the event of a 
sudden removal of electrical load from the system. There is then an 
immediate cutoff of fuel to both burn chamber 2 and boiler 13. In a 
conventional gas turbine generator as shown in FIG. 1, compressor 1, 
located on the same shaft as expander 3, provides a braking force which 
prevents turbine shaft overspeed. In the present invention, there is 
nothing to brake expander 3. Although steam flowing to steam turbine 14 
could be vented before entering it, compressor 1' would still be 
generating compressed air as it spins down. This air would generate power 
on shaft 4 with no load, rapidly increasing shaft speed until the unit 
generator and/or turbine were destroyed. By opening blowoff valve 23 
cutting all fuel supplies and venting steam in the compressor subassembly, 
the invention avoids this destruction. 
Blowoff valve 23 also functions as a control device during generator 
startup. Conventional power generators, as shown in FIG. 1, require a 
separate starter to initiate spin-up of expander 3 because compressor 1 is 
physically coupled to expander 3. In the present invention, because the 
compressor subassembly is decoupled from expander 3, spin-up of the gas 
turbine is accomplished simply by firing boiler 13. Steam begins to flow, 
and compressor 1' begins to deliver air to plenum 9. Because generator 5 
is not loaded until shaft 4 reaches synchronization speed, there is no 
load on the system to prevent overspeed, which can be catastrophic as 
described above. Blowoff valve 23 allows an operator to regulate the 
quantity and pressure of air being delivered to plenum 9, thereby 
preventing shaft overspeed and destruction. 
Having described a novel electrical power generation plant according to the 
invention, other advantages and benefits will be described. 
By separating the compressor from the gas turbine, the work required to 
compress the air is no longer provided by the burning of relatively 
expensive gas turbine fuel; rather, it is provided by the relatively 
inexpensive fuel used to drive the steam cycle driving the remote 
compressor. Cost savings are readily obtainable. Fuel costs per KWH, which 
ordinarily increase with decreased electrical load on the generator, tend 
to remain constant, if not improve, with reduced electrical load. An 
electrical generating power plant according to the invention is 
essentially flat-rated. Changes in air density, whether effected by 
changes in temperature, altitude, or humidity, have much less impact on 
system performance than in a conventional, combined cycle gas turbine 
generator. 
In a generating plant employing a conventional steam cycle--such as the 
steam cycle used to drive steam turbine 14--the boilers typically burn 
relatively inexpensive fuels, including hydrogen deficient solids such as 
cokes, chars, coals, and very heavy refinery bottoms. The construction 
costs of steam plants are high, and as they generate less than their 
nominal maximum power output, their heat rate and fuel cost per KWH rise. 
Furthermore, ramping rates on solid fuel-, coke-, coal- and char-fed 
boiler cycles are low, as compared to gas turbine engines. Nevertheless, 
considering their relatively low fuel costs, steam cycle-based plants may 
be economically justified if they are base loaded for their operating 
lifetime and not cycled extensively. 
Combined cycle generators have very high cycle efficiencies, but with 
higher fuel costs than conventional solid fuel boiler plants. Unlike 
boiler-based cycles, combined cycles can be ramped rapidly. Like 
boiler-based cycles, heat rate and fuel cost per KWH increase with 
decreased loading. 
Changes in ambient air density affect combined cycle net production of 
power. Because the turbine spins at a set generator synchronization speed, 
the compressor delivers essentially a constant volume of air. Given a 
maximum turbine inlet temperature, however, power output depends upon the 
mass of air flowing through the turbine. Therefore, as ambient air density 
decreases, power output decreases. Furthermore, the compression work is 
supplied by the burning expansion of air into which an expensive, clean 
fuel has been injected to provide enthalpy above that provided by the 
compression itself. With a given mass flow rate of air through the 
compressor and a given compressor efficiency, to achieve greater turbine 
power output requires increasing the enthalpy of the burned air; i.e., 
burning more of the expensive fuel. 
Conversely, to reduce power, fuel input is reduced, reducing enthalpy to 
and expansion work from the expander. The compressor work remains 
constant, however. Therefore, heat rate rises sharply as power output 
decreases. 
By separating the compressor from the turbine according to the invention, 
essentially all of the work created by the combustion of the expensive, 
premium gas turbine fuel is converted to electrical energy by the 
generator. The energy for driving the remote compressor is supplied by the 
cheaper fuels used to fire the steam boiler to effect significant cost 
savings. 
Furthermore, boiler 13 may be fired at a design firing rate which allows 
steam turbine 14 to furnish essentially constant power. As ambient 
temperature and/or humidity change, affecting air density, steam turbine 
14 driving compressor 1' changes speed to maintain design mass flow to 
plenum 9 and gas turbine 3. With constant mass flow, the combustion 
turbine is essentially flat rated, regardless of changes in the ambient 
conditions. 
With boiler 14 operating at design heat input rate and delivering air at 
the proper mass, pressure, and temperature to plenum 9, steam turbine 7 
generates about 30% of the rated shaft work to generator 5, even without 
any fuel input to burn chamber 2. The heat rate is then high, but with 
very inexpensive fuel in boiler 13, the cost of generation per KWH is 
lower than even the best combined cycle plant as long as the differential 
between the cost of gas and the cost of boiler fuel is about $1.00 per MM 
BTU. As power increases are needed, gas is added to combustion chamber 2. 
At full power, the cost of power production is less than that of the best 
combined cycle at a fuel cost differential of about $0.90 per MM BTU. 
Another feature of the invention is a turndown capability of from 100% to 
about 30% that is stable and controllable over the entire range. It has a 
ramping rate equal to that of a combined cycle and exhibits a constant, if 
not declining, cost per KWH over the entire turndown ratio. Power output 
is relatively unaffected by air density changes. 
Still another feature of the invention is the addition of more limestone to 
the normal beds in fluid bed boiler 13 than is needed to reduce SO2 
discharge adequately. Adding limestone (sometimes referred to as stone 
hereafter) to the normal beds controls SO2 discharge. The stone first 
calcines to CaO, and the CaO reacts with oxygen in the furnace air and 
then further reacts with the SO2 formed when the S in the coke burns. 
Disposal of ash material from burning fuel, typically coke, has been a 
problem, and the typical prior art approach adds just enough limestone to 
the fuel bed to get the desired SO2 capture, thereby minimizing ash 
production. This typical approach uses just enough stone to get a Ca (in 
the stone) to S (in the coke, coal or other fuel having S) atomic ratio of 
1.7 Ca/S. This ratio results in capturing sufficient SO2 to meet pollution 
regulations. 
It has been discovered that by increasing this ratio and adding more stone, 
thus putting free CaO in the material, results in creating a series of new 
products of value that is a function of free CaO content. 
Referring to FIG. 3, there is shown a graphical representation of the 
incremental value in S/hour of product created as a function of the Ca/S 
ratio. The incremental value curve shows that adding enough limestone to 
double the typical ratio from 1.7 to 3.4 results in no increase in 
incremental value. It has been discovered that until the free lime, CaO, 
exceeded 50%, the resultant product would still be undesired ash. However, 
increasing this ratio above 3.4 progressively increases the incremental 
value/hour of useful product produced. Balancing the value of the useful 
product produced against the cost of the limestone and the fuel required 
to convert the limestone into free lime, CaO, there is an economic benefit 
above about Ca/S ratios of 4.5/1, and more meaningful economic benefit 
above 5.5/1. 
Thus, by increasing limestone feed by a factor of 4, 5, 6 or even 7, there 
is a surprising result. 
Those skilled in the art of the design and sale of fluid bed boilers 
regarded ash captured in baghouse and the bed ash as probably a hazardous 
waste because the ash would contain certain heavy metals from the crude 
oil typically used to make the coke. Accordingly, economic evaluations 
typically included a line item of cost reflecting the disposal cost of 
such ash as would be produced. Thus, the typical prior art approach 
involved minimizing the quantity of ash produced while meeting the license 
requirements for meeting pollution standards. Fluid bed ash from coke 
burning can be utilized as a soil stabilization material in road 
construction. The value of such ash for that application is slightly above 
that of the limestone fed to the furnace due in part to the gypsum content 
of the ash. 
By radically increasing the Ca/S ratio according to the invention and 
examining both the free lime in the resultant ash product and potential 
value of such new products, it has been discovered that tripling or 
quadrupling this ratio markedly increased product value. In fact, 
departing from minimizing ash production to increasing ash production to 5 
or 6 times current practice produced a significant beneficial result. 
Above the 3.4 Ca/S ratio, there is a gradual increase in free lime shown 
in FIG. 3 in value dollars per hour. It can be shown that by 
differentially combining the incremental value of the cost of added stone 
plus added fuel and the increased useful product, there is an economic 
benefit from very high Ca/S ratios. At a ratio of 8.5, a credit as high as 
$985/hour can be achieved according to the invention. 
Practicing the invention with commercially available fluid bed boilers 
involves adding a larger component of limestone to the boiler with its air 
feeder, involving increasing the bed volume sufficiently to accommodate 
the increased limestone, increasing the limestone feed capacity, and 
increasing the fuel feeder capacity and ash recovery capability. 
With these changes and with a reasonable range of commercial coke and stone 
prices, the invention results in creating a product competitive with 
commercial lime, significantly reducing the heavy metals problem in ash 
and the possibility of producing power at negative cost. With no gas in 
the compressorless combustion turbine, the expander could generate 
electrical power at cost ranging from -$0.003 to $0.0015 per kilowatt 
hour. Without any value for the decrease in SO2, the cycle would produce 
60 pounds/hour of SO2 in a plant burning 1,800,000,000 BTU/hr. Operation 
of fluid bed boilers with Ca/S ratios according to the invention, 
especially within the range of 5.1-10.2, will result in generating power 
at significantly lower costs than operating at a ratio of less than 5.1. 
This advantageous result is available also when using the boilers to 
operate steam turbines to directly produce power or to create an 
intermediate product hot compressed air, used to generate electric power 
from any coal, petroleum coke or other fuel with sulphur. 
The novel ash product so produced has value in certain traditional burned 
lime markets that significantly offsets the cost of the limestone and 
fuel. 
Referring to FIG. 4, there is shown a typical expected value in dollars/ton 
as a function of the Ca/S ratio with shipping allowance deducted. 
It is evident that those skilled in the art may now make numerous uses and 
modifications of and departures from the specific apparatus and techniques 
described herein without departing from the inventive concepts. 
Consequently, the invention is to be construed as embracing each and every 
novel feature and novel combination of features present in or possessed by 
the apparatus herein described and limited solely by the spirit and scope 
of the following claims.