Abstract:
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.

Description:
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 &#34;once-through&#34; 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 &#34;once thru&#34; 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above brief description, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings which is a schematic view of the system of the present invention. 
    
    
     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 &#34;MCR load&#34;), 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 &#34;once through&#34; 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 THROUGHSEPARATOR    NONE        THROTTLED    FULLY OPENBYPASSFURNACE  CONSTANT    CHANGES      CHANGESPRESSURE             WITH LOAD    WITH LOADSEPARATOR    FEEDWATER   CONTROL      NONEFLUID LEVEL    CONTROL     OF VALVE 30CONTROL______________________________________ 
    
     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 &#34;once thru&#34; 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.