Hybrid steam generating system and method

A hybrid steam generating system and method in which fluid is passed through the waterwall tubes of a furnace to transfer heat from the furnace to the fluid to convert at least a portion of the fluid to steam. Under certain operating conditions, the heated fluid is passed from the furnace to a separator for separating the steam from the heated fluid and the separated heated fluid is passed from the separator back to the furnace. The steam from the separator is passed to a steam utilization unit, and, under certain operating conditions, the heated fluid is passed from the furnace directly to the steam utilization unit.

BACKGROUND OF THE INVENTION 
The present invention relates to a steam generating system and method and, 
more particularly, to such a system and method which combines operating 
principles of both steam drum and once-through systems. 
Drum type steam generators, especially of the natural circulation type, are 
well known and usually incorporate a relatively large steam drum which 
contains the steam-water separators, saturated liquid inventory, and a dry 
steam space. These type of arrangements are relatively simple to startup, 
provide failsafe protection of the waterwall enclosure as long as the 
drum/water accumulator has water to a safe level, and do not require a 
boiler circulating pump if their circuitry is designed to provide 
circulation of the cooling water by natural circulation. However, these 
generators have several limitations, including: 
relatively thick walls which limit the rate of pressure increase due to 
thermal stress limits 
relatively large diameter waterwall tubes, which contain a relatively large 
inventory of water requiting a large overfiring rate in order to change 
load and pressure simultaneously 
a relatively low maximum permitted operating pressure (which is normally 
approximately 2850 psig), due to difficulties in separating steam and 
water above that pressure, which precludes operation to supercritical 
pressures, as is required for advanced cycles. 
Relatively large fixed available superheater surface area downstream of the 
location of saturated steam enthalpy which makes it difficult to achieve 
design main steam temperatures at low loads. 
The other main type of steam generator is a "once-through" unit which 
employs a boiler feed pump for pressurizing the system and forcing the 
liquid through the waterwall tubes. These systems are capable of operating 
to advanced, high pressures (5000 psig), and do not require large 
diameter, thick walled pressure vessels. As a result, the liquid inventory 
in the waterwalls, as well as the thermal stresses induced during fast 
temperature changes, are reduced. Also, the location at which saturated 
steam conditions exist over the load range is not fixed which permits main 
steam temperatures to be attained for all loads above the "once thru" 
load. Further, a once-through generator can take advantage of the combined 
oxygenated feedwater treatment method. However, these once-through systems 
are not without problems. For example their startup systems have generally 
been complicated to operate and expensive to install. 
SUMMARY OF THE INVENTION 
The present invention is a hybrid steam generator which combines the 
features of both a steam drum generator and a once-through generator while 
eliminating, or at least significantly reducing, the disadvantages 
thereof. To this end, fluid is passed through the waterwall tubes of a 
furnace to transfer heat from the furnace to the fluid to convert at least 
a portion of the fluid to steam. A separator is provided which, under 
certain operating conditions, receives the heated fluid from the furnace. 
The separator functions to separate the steam from the heated fluid and 
the remaining heated fluid is passed from the separator back to the 
furnace. A steam utilization unit receives the steam from the separator, 
and, under certain operating conditions, the heated fluid is passed from 
the furnace directly to the steam utilization unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to the drawing, the reference numeral 10 refers, in general, 
to a steam generator which includes a furnace 12 which may be of a 
conventional design and, as such, can be fired by oil, gas, or pulverized 
coal or by using a standard fluidized combustion process. The furnace 12 
is formed, in part, by four upright walls each of which is formed by a 
plurality of waterwall tubes 14. Although not shown in the drawing, it is 
understood that the tubes 14 are multilead, internally ribbed (rifled), 
and have continuous external fins extending outwardly from diametrically 
opposed portions thereof, with the fins of adjacent tubes being connected 
together to form a gas-tight structure. Since this type of tube design is 
conventional, it will not be described in any further detail. 
A heat recovery section, shown in general by the reference numeral 16, is 
located adjacent the furnace 12. The heat recovery section 16 includes a 
plurality of steam utilization units, such as superheaters, or the like 
(not shown), as well as an economizer 18 for supplying heated feedwater to 
the waterwall tubes 14, as will be explained. 
A plurality of inlet headers 19 (two of which are shown) are connected to 
the lower ends of the tubes 14 for receiving heated feedwater for passing 
through the lengths of the tubes, and a plurality of outlet headers 20 are 
connected to the upper ends of the tubes 14 for receiving the heated water 
from the tubes. The outlet headers 20 are connected, via a corresponding 
number of risers 22, to a separator inlet pipe 24 which, in turn, is 
connected to a separator 26. Although only one separator 26 is shown and 
will be described for the convenience of presentation, it is understood 
that a plurality of separators and associated flow circuitry would 
normally be provided. 
The furnace 12 has a roof 28, which is shown in dashed lines for the 
convenience of presentation, and which has an inlet header 28a disposed at 
one end thereof. The roof extends to, and is in fluid flow communication 
with the heat recovery area for passing the fluid to the latter area for 
further processing. A bypass pipe 29 extends from the separator inlet pipe 
24 to the roof inlet header 28a and a control valve 30 is interposed 
therein. An outlet pipe 31 extends from the separator 26 to the roof inlet 
header 28a and a header 32 is interposed in the pipe 31. 
A drain pipe 36 extends from the separator 26 to a downcomer 38 which 
extends to a furnace feed pipe 40. A check valve 42 is interposed in the 
downcomer 38 along with a mixing tee 44 disposed downstream from the check 
valve. A conduit 46 connects the outlet of the economizer 18 to the mixing 
tee 44 for supplying feedwater to the tubes 14 in a manner to be 
described, and a monitoring device 48 is interposed in the pipe 40 for 
monitoring the flow of fluid through the latter pipe for reasons to be 
described. It is understood that the check valve 42 is operable by 
external circuitry which respond to various load conditions and other 
parameters to control its position, in a conventional manner. 
A vent pipe 50 extends from the drain pipe 36 to the header 32 and a 
plurality of accumulators 52 are provided in the pipe 50 to increase the 
liquid inventory available for emergency use during transients. The 
accumulators 52 are approximately the same diameter and wall thickness as 
the separators(s) 26 and, although not clear from the drawing, are 
inclined with respect to the horizontal to provide continuity of liquid 
surface area of volume vs liquid height. The accumulators 52 are designed 
to emulate the function of a steam drum, without imposing the same thermal 
stress limits. 
A bypass pipe 54 extends from the downcomer 38 and has a control valve 56 
disposed therein for controlling bypass flow from the separator, as will 
be described. Although not shown in the drawings, it is understood that 
the bypass pipe 54 extends to a blowdown pipe, or the like (not shown). 
In operation, from approximately 0 to 25% of the maximum continuous rated 
load (hereinafter referred to as "MCR load"), the steam generator 10 
operates as a natural circulation drum unit. To this end, the valve 30 is 
closed, the valve 42 is open and the feedwater flows from the economizer 
18 to the tee 44 and is passed to the headers 19 for passage upwardly 
through the waterwalls of the furnace 12 where it is heated from a 
temperature below saturated liquid conditions to form a two-phase mixture. 
The mixture is collected in the waterwall outlet headers 20 and is routed, 
via the risers 22 and the separator inlet pipe 24, to the separator 26. 
The separator 26 is designed for the full design pressure of the high 
pressure circuitry, and functions to separate the two-phase mixture into a 
saturated liquid stream and a wet steam stream at these low loads. The 
stream of wet steam leaving the separator 26 is routed through the pipe 
31, the header 32 and to the roof inlet header 28a of the roof 28 for 
passage onto one or more downstream heat utilization units, such as 
superheaters, or the like (not shown), in the heat recovery area 16, with 
the final steam outlet temperature being controlled by the use of 
attemporator sprays in the heat recovery area 16. The separated saturated 
liquid discharging from the separator 26 passes through the drain pipe 36 
and the downcomer 38 and mixes with the feedwater from the economizer 18 
in the tee 44 before being passed to the inlet headers 19 for 
recirculation. During this operation, the feedwater flow is regulated in a 
manner to maintain a water level in the separator 26 sufficient to insure 
this recirculation of liquid from the separator. The flow rate of the 
recirculated liquid flow from the separator 26 is governed by the heat 
absorption of the furnace waterwalls, the sizing of the drain pipe 36 and 
the downcomer 38, and the pressure drop through the system of heated and 
unheated risers. To the extent necessary, steam temperature is controlled 
by attemporator sprays in the heat recovery section 16, in a conventional 
manner. 
From approximately 25% to 50% MCR load, the unit operates both as a natural 
circulation unit and a once-through unit. As such, the rate of the fluid 
entering the separator 26, and therefore the fluid level in the separator, 
is controlled by opening the valve 30 to the extent that a portion of the 
two-phase mixture from the risers 22 and the separator inlet pipe 24 
bypasses the separator and rather is circulated directly to the roof inlet 
header 28a. Thus, the mixture mixes with the steam received directly from 
the separator 26 in the header 28a before passing downstream through the 
roof 28 to the heat recovery area 16, as described above. The feedwater 
from the economizer 18 continues to mix with the recirculated saturated 
liquid from the separator 26 in the tee 44 before being passed to the 
inlet headers 19 for recirculation. During this operation, and beginning 
at approximately 33% MCR, the operating pressure in the furnace 12 
increases in proportion to increases in load up to and including 
approximately 95% MCR. The feedwater flow rate is varied in parallel with 
the firing rate to control the temperature of the steam output in a "once 
through" control mode for all loads above 25% MCR. 
From approximately 50% to 100% MCR load, the valve 30 is completely opened 
to partially bypass the separator and thus reduce the pressure drop across 
the separator at high loads. There will be two flow paths of the two phase 
fluid--one through the separator 26 and the other through the bypass 
conduit 29, with the flow distribution through each being related to their 
relative flow resistance. The valve 42 is closed, thus terminating 
recirculation of the saturated liquid from the separator 26 to the tee 44 
and to the inlet headers 19. Thus, the water level in the separator 26 is 
not controlled at loads above 50% MCR and there is no recirculated flow of 
the liquid from the separator back to the waterwalls of the furnace 12. 
The feedwater flow rate continues to be varied in parallel with the firing 
rate to control the temperature of the steam output. Thus, this phase of 
the operation is essentially the same as that for a once-through system. 
Thus, the key operating parameters for the various load conditions are as 
follows with the understanding that the MCR percentages set forth are 
approximate: 
______________________________________ 
0-25% MCR 25-50% MCR 50-100% MCR 
LOAD LOAD LOAD 
______________________________________ 
TYPE OF NATURAL NATURAL CIR./ 
ONCE- 
OPERATION 
CIRCULATION ONCE-THROUGH THROUGH 
SEATOR 
NONE THROTTLED FULLY OPEN 
BYPASS 
FURNACE CONSTANT CHANGES CHANGES 
PRESSURE WITH LOAD WITH LOAD 
SEATOR 
FEEDWATER CONTROL NONE 
FLUID LEVEL 
CONTROL OF VALVE 30 
CONTROL 
______________________________________ 
During emergencies, such as when transients occur during operation, the 
accumulators 52 receive liquid from, or discharge liquid to, the drain 
pipe 36. Since the accumulators 52 are designed to emulate the function of 
the steam drum without imposing the same thermal stress limits, disruption 
of waterwall circulation and possible distress of the heated waterwall 
tubes in response to routine transients in the feedwater flow or firing 
rate is avoided. 
The present invention enjoys several advantages, examples of which are as 
follows: 
1. The steam generator 10 is relatively simple to start up, provides fail 
safe protection of the waterwall enclosure as long as the separator 26 or 
the water accumulator 52 has water to a safe level, and does not require a 
boiler recirculating pump. 
2. The diameter and wall thickness of the separator(s) 26 is limited to 
moderate values, thus reducing the thermal stresses generated during fast 
changes in fluid temperature. 
3. The bypass pipe 54 and the control valve 56 can also be used to help 
ensure a steady minimum feedwater flow rate during low load operations, 
since the valve could be programmed to control to a high separator water 
level. 
4. The monitoring device 48 can provide an indication that feedwater is 
bypassing the generator 10 and flowing into and through the downcomer 38 
and that the valve 42 should be closed. 
5. The steam generator can operate at relatively high pressures without the 
necessity of maintaining a relatively large liquid inventory in the 
waterwalls. 
6. The location at which saturated steam conditions exist over the load 
range is not fixed which permits main steam temperatures to be designed 
for all loads above the "once thru" load. 
It is understood that several variations may be made in the foregoing 
without departing from the scope of the invention. For example, although, 
in the example set forth above, the roof 28 is located immediately 
downstream of the separator 26, a upper furnace steam-cooled enclosure 
wall can be interposed between the outlet of the separator 26 and the 
roof. Thus, the wet steam from the separator 26 would be fed to the latter 
enclosure wall prior to passing to the roof 28. In this case the upper 
furnace enclosure wall would utilize two distinct passes: a two-phase pass 
which is a continuation of the lower furnace pass, and a wet steam-cooled 
pass. 
Further, it is understood that the present invention is not limited to the 
use of vertical waterwall tubes and the particular operating conditions 
set forth above including the specific ranges set forth in the table. For 
example, the waterwalls can be formed by spiral wound tubes as disclosed 
in U.S. Pat. No. 4,191,133 and No. 4,344,388 both of which are assigned to 
the assignee of the present invention and both of which are hereby 
incorporated by reference. According to this arrangement, the pressure in 
the steam generator 10 is held constant during relative low loads, is 
varied linearly during intermediate loads and is held a relatively high 
constant pressure in the relatively high load range. Also, the two-pass 
upper furnace circuit described above could be used. 
It is further understood that the present invention is not limited to the 
use of the control valve 30 to bypass the separator 26 during the 
conditions described above. Rather, the suction inlet of a relatively 
small spray water pump 60 can be connected to the downcomer 38 upstream of 
the valve 42. In the above described load range of 25-50% MCR, while the 
check valve 42 is open, the pump 60 would control the fluid level in the 
separator 26 by spraying the excess separator liquid into a superheater, 
or the like, located in the heat recovery section 16 based on the water 
level in the separator 26. 
It is understood that other modifications, changes and substitutions are 
intended in the foregoing disclosure and in some instances some features 
of the invention will be employed without a corresponding use of other 
features. Accordingly, it is appropriate that the appended claims be 
construed broadly and in a manner consistent with the scope of the 
invention.