Chilling economizer

Abstract

Chilling is produced from heat that is normally wasted in the economizer section of a steam boiler. A thermally-activated ammonia-water absorption chiller is powered by a heat recovery unit. The heat recovery unit supplies boiler exhaust heat to desorb the working fluid of the chiller. That can be directly, such that the heat recovery unit is a heat recovery vapor generator that can be colocated with an economizer, in parallel or series. The exhaust heat can alternatively be supplied to the AARC indirectly, via a heat transfer loop and a separate generator. The desorbed ammonia vapor is rectified, condensed, and then used to produce the chilling. The heat released in the chiller when low pressure ammonia vapor is re-absorbed is used to preheat the boiler feedwater.

Description

BACKGROUND

With steam boilers, it is good practice to include an economizer or feedwater heater to capture more useful energy from the exhaust. This increases boiler efficiency and reduces stack temperature. Thus it is surprising to learn that there is actually a lot of wasted availability that still remains even when economizing. The stack temperature is still well above ambient, and the economizing step employs very large temperature differentials, thus generating the entropy. The overall purpose of this disclosure is to make beneficial use of this presently wasted availability.

Many steam boilers are found in applications where refrigeration is also required (or would be useful). Examples include food processors, hospitals, laundries, hotels, and process industries. Additional examples are combined cycle plants and cogeneration plants where the boilers are heated by exhaust from the prime movers. On warm days those prime movers benefit markedly from chilling the inlet air. Hence one particular objective is to convert unused availability associated with current economizers into chilling.

PRIOR ART

It is known to chill the inlet air of a combustion turbine using refrigeration from an ammonia-water absorption refrigeration cycle (AARC). U.S. Pat. No. 2,322,717 to Nettel discloses that, using a fired combustion heater to power the AARC. More typically the AARC is powered by turbine exhaust heat. U.S. Pat. No. 2,548,508 to Wolfner describes one such embodiment. The exhaust first heats the compressed air in a recuperator, and then directly heats the aqua solution to desorb it. An alternative configuration for the AARC is presented in Malewski and Holldorff (1984). In that disclosure, the turbine exhaust heats a two-pressure steam bottoming cycle, then heats the aqua solution in a desorber (HRVG). Ammonia refrigerant is rectified in a rectification column, condensed, and then is supplied to a heat exchange coil for direct chilling of the inlet air.

The 1990 article by Ondryas et al also discloses exhaust powered ammonia absorption refrigeration as one option for chilling turbine inlet air.

U.S. Pat. No. 5,555,738 to Devault discloses chilling the turbine inlet air of a combined cycle configuration with an AARC that is powered by steam turbine exhaust or other waste heat source. The steam condenser 16 is also the aqua desorber, and the AARC does not have a rectification column or a refrigerant subcooler.

U.S. Pat. No. 6,058,695 to Ranasinghe et al discloses a gas turbine combined cycle wherein extraction ammonia-water fluid from a Kalina bottoming cycle is condensed to liquid, then is used to chill the turbine inlet air.

U.S. Pat. No. 6,173,563 to Vakil et al discloses an ammonia-water refrigeration cycle for a combined cycle plant. Exhaust heat in the LP economizer section of the HRSG directly partially vaporizes the aqua solution, which is then separated in a separator. The vapor fraction is partially condensed in an economizer that gives up heat to condensate from the gland seal condenser. Then it is fully condensed against cooling water, then depressurized to provide chilling to the inlet air. A small fraction of the liquid refrigerant is used in the ammonia subcooler to cool the liquid ammonia before expansion. Finally the low pressure vapor from the chilling coil and the liquid fraction from the separator are combined and absorbed against cooling water. This cycle is similar to the Devault and the Ranasinghe et al cycles in that there is no rectification column.

Langreck (2000) discloses an AARC for chilling inlet air to a combined cycle plant that uses direct exhaust heating of the desorber, but uses chilled water to cool the inlet air. The AARC includes a rectification column. Sigler et al (2001) calculate that a combined cycle plant enhanced by AARC shows improved warm weather performance even when the penalty of the driving steam for the AARC is accounted for.

U.S. Pat. Nos. 6,412,291 and 6,739,119 to Erickson disclose a gas turbine configuration including an exhaust heat powered AARC that delivers chilling to the turbine inlet air, plus additional advantageous features. U.S. Pat. No. 6,715,290 to Erickson discloses both simple cycle and combined cycle gas turbine plants having exhaust powered AARC for inlet air chilling, wherein the AARC has novel advantageous features, e.g. “glide heat”.

The general background of this field of invention is further defined in EPRI TR-102412 (1994) and in the following U.S. Pat. Nos. 3,796,045; 5,203,161; 5,632,148; 5,655,373; 5,782,093; 5,790,972; 6,321,552; 6,457,315; 6,460,360; 6,837,056; 7,178,348; 7,228,682; and 7,343,746. These references provide examples of alternative thermally-activated chillers, including LiBr absorption chillers; steam-driven mechanical compression chillers; and liquid desiccant dryer/chillers with thermal regeneration.

Included among the objects of this invention are to convert the waste associated with current economizers into useful chilling and additional feedwater heating. Both will improve the overall efficiency of the steam boiler.

DISCLOSURE OF INVENTION

The above and other useful objects are accomplished by providing a thermally activated chilling unit that is powered by a heat recovery unit (HRU) located in the boiler exhaust. The HRU can be directly supplied with working fluid from the thermally activated chiller, that is desorbed therein, normally referred to as a heat recovery vapor generator (HRVG). Preferably the HRVG is colocated with the economizer in the boiler exhaust. Alternatively, the HRU can be supplied a working fluid that in turn supplies heat to the thermally activated chiller. The preferred embodiment of thermally activated chiller is an ammonia-water absorption refrigeration cycle (AARC). An essential aspect of the invention is that the AARC reject heat is used to preheat the boiler feedwater and makeup feedwater, for example before it is sent to the economizer. That supplies three important benefits: it allows more of the exhaust heat to be sent to the AARC, thus increasing the amount of chilling; it reduces the amount of AARC heat that must be rejected to ambient, through a cooling tower or the like; and it raises the feedwater temperature above the exhaust dewpoint, so economizer corrosion is avoided.

Further to this disclosure, it is desirable to route the AARC working fluid in direct countercurrent contact with the exhaust. This allows it to extract more useful heat from the exhaust, i.e. down to a lower temperature, and also increases the temperature of the AARC reject heat, thus allowing more reject heat to be transferred to the feedwater.

In the combined cycle application, the chilling so produced is directed to chilling the inlet air of the prime mover that is producing the exhaust used to make the chilling. Thus the loop is closed to form an overall integrated cycle, and the chilling is produced without detracting from either the steam production or the feedwater heating, and also without the large parasitic electric load associated with mechanical compression chilling.

The HRVG can be colocated with the economizer in two basic configurations—either in parallel, or in series. The parallel arrangement provides the best thermodynamic results. That is because each heating load has a temperature glide, and hence the two in parallel can be sized to match the temperature glide of the exhaust, such that the driving temperature difference is essentially constant. However the drawback of the parallel configuration is that when there are times the AARC is not being used, e.g. at mild ambient conditions, and is turned off, part of the exhaust bypasses the economizer. Since the feedwater no longer receives preheat from the idle AARC, there will be a deficit in feedwater heating.

One way to solve that feedwater heat deficit problem is to leave the AARC always operating whenever the economizer (and boiler) are operating. That can be done by creating false load (vapor bypass) on mild days, or even by producing power with that idle capacity, via the “dual function” cycle. Another way to solve that problem is with a series arrangement, also referred to as the “split economizer” arrangement. The economizer is split into two or more segments, with a HRVG segment alternating with each economizer segment. The overall objective is to achieve a close match between the temperature glide of the exhaust and the temperature glide of heat acceptance. This is the same design objective as that used to design the entire HRSG of a combined cycle plant. With the series (split economizer) arrangement, all the exhaust flows through the economizer even when the AARC is turned off, so there is no deficit.

By the same token that adding an economizer to a boiler increases the backpressure, and hence requires adjustments to the emissions and other boiler controls, colocating the HRVG with the economizer will further increase the backpressure, and hence require further adjustment.

The terms “economizer” and “feedwater heater” are used in their broad sense, i.e.

including functions such as condensate heating, makeup feedwater heating, etc.

With single pressure steam boilers, e.g. for steam pressures in the range of 50 to 100 psig, the exhaust temperature after the economizer is typically close to 300° F. With multi-pressure steam plants, more typically encountered in cogen and combined cycle applications, and very large applications, the exhaust temperature after the economizer is more typically about 200° F. or lower. Especially in the latter case, it is desirable to use a more advanced AARC that can beneficially use the exhaust to lower temperatures. In particular, this would entail use of a three-pressure absorption refrigeration cycle, with or without GAX (generator-absorber heat exchange). The three-pressure cycle is able to extract useful driving energy from the exhaust down to about 170° F.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional prior art steam boiler with a fired burner, a feedwater pump, and a feedwater economizer in the exhaust stack.

FIG. 2 illustrates one way to convert the FIG. 1 economizer to a chilling economizer. Additional heat exchange surface is colocated in the economizer (parallel in this instance) for a heat recovery vapor generator that is part of an ammonia absorption refrigeration cycle, said HRVG receiving pumped liquid from a solution heat exchanger and sending two phase partially desorbed fluid to a rectifier. The AARC also has a condenser, a refrigerant heat exchanger, an evaporator that produces the chilling, and an absorber that is used to preheat feedwater enroute to the economizer.

FIG. 3 illustrates a gas turbine combined cycle plant with exhaust powered AARC to supply inlet air chilling and feedwater preheating, where the HRVG is series colocated with the LP economizer. The AARC is a three-pressure version with exhaust heating at both pressure levels. The AARC is also adapted to provide turbine inlet air heating when necessary, and to expand otherwise unused ammonia vapor for power production when available. Note that part of the HRVG is at the cold end of the exhaust flowpath.

FIG. 4 is a simplified flowsheet for an alternative three-pressure AARC adapted for firing from a HRVG that is colocated with a low-pressure economizer. In this case, the HRVG is shown being in parallel with the feedwater economizer (FWE). The AARC has internal latent heat exchange in the form of an IP GAX, part of the “vapor exchange” configuration.

FIG. 5 illustrates a chilling economizer on a fired steam boiler wherein a heat transfer loop transfers boiler exhaust heat to the AARC, and the feedwater is heated by both condenser reject heat and absorber reject heat from the AARC.

BEST MODE FOR CARRYING OUT THE INVENTION

It would be possible to mount the HRVG downstream of the economizer, i.e. at its cold end, and just use the final waste heat of the exhaust to make chilling. However that would waste about half of the actual potential of that exhaust to produce chilling. The features desirable to achieve high levels of chilling are: to colocate the HRVG of the AARC with the economizer, such that higher driving temperatures are available to the HRVG; to achieve maximum useful temperature glide in the HRVG by supplying it directly with pumped and preheated solution; to rectify the desorbed solution to higher ammonia purity so as to get more useful chilling from a given amount of desorbed fluid; and to preheat feedwater with absorber reject heat. The latter step has two benefits: more of the exhaust heat becomes available to the HRVG; and the economizer feed is warm enough (e.g. above 140° F.) that acid condensation of the exhaust gas is not a concern. Note that this requires that absorber heat be used, as were the condenser pressure to be increased high enough to produce that temperature of reject heat in sufficient amounts to do that heating, the pressure would be unreasonable and less chilling could be produced. The four critical features listed above are illustrated in the FIG. 2 flowsheet.

From the perspective of the heat recovery train for the exhaust from the boiler, this enhancement entails adding from two to four more transfer units (NTUs) of heat exchange.

Beyond the key features recited above, various other additional features will be found advantageous in particular applications. For example, when the steam boiler is part of the HRSG of a gas turbine combined cycle plant, the chilling is advantageously applied to chill the turbine inlet air. When even lower exhaust temperatures are desired, the AARC can be a three-pressure version. This yields more chilling, and can have other beneficial effects, such as making it easier to recover water and/or CO2 from the exhaust. The three-pressure cycle can have external heating at both pressure levels, as in FIG. 3, or can have internal GAX heating as in FIG. 4. One suitable method of beneficially applying the third pressure level in the AARC is via the “vapor exchange” cycle, as disclosed in U.S. Pat. No. 5,097,676, to Erickson. FIGS. 3 and 4 present variants of that.

Whereas FIG. 3 presents a two pressure steam bottoming cycle in the combined cycle, it will be recognized that any other bottoming configuration applies equally well, e.g. three pressure steam cycle, with or without reheat, etc. Similarly the gas turbine can optionally have intercooling, recuperation, STIG, and the like. The bottoming portion of the cycle can be simply for steam production, i.e. cogeneration, instead of or in addition to power production.

With the three-pressure AARC, the exhaust temperature can readily and beneficially be brought down to the 160° F. to 190° F. range, depending upon ambient temperature. When using the absorber for feedwater preheating, the feedwater can be preheated to at least 140° F., and also warmed by at least 30° F. Note that the heat can beneficially be applied to other loads as well. When the heating is to be applied to cold water or supply water, the colder condenser reject heat from the AARC can also be used, in series with the hotter absorber heat. Note that it is frequently desirable to have more than one absorber, with the colder one cooled by cooling water, and the warmer one doing the feedwater heating, as shown in FIGS. 3 and 4. That makes more chilling and further reduces the exhaust temperature.

FIG. 3 illustrates the “split economizer” or series configuration—there are two sections of LP Econ and two sections of HP HRVG in alternating sequence. There are also two LP absorbers—one for heat recovery, and the other rejecting to ambient. The rectifier has internal heat recovery in both the stripping section and rectifying section.

The disclosed chilling economizer can produce approximately 90 tons of energy-free chilling from the exhaust of a 20 million BTU/hour boiler. In the combined cycle application, it can produce about 800 tons of energy-free chilling in a 120 MW plant using a two-pressure steam bottoming cycle. As another example, with a 520 MW combined cycle with three pressure plus reheat bottoming steam cycle, the chilling economizer can produce up to 3000 tons of energy-free chilling. That chilling can beneficially be applied to turbine inlet air chilling.

FIG. 5 illustrates two alternative features relative to the FIG. 2 embodiment. First, the boiler exhaust heat is recovered into a heat transfer fluid, which in turn heats the generator of the absorption cycle. This variant of the HRU provides some advantages, such as being able to supply both AARC heating duty and economizing duty with only a single exchanger in the exhaust path. The downside is the need for the additional pump to circulate the heat transfer fluid, plus an expansion tank. Secondly, the feedwater is supplied heat from both the condenser and the absorber of the AARC, in that order. This is particularly valuable when the steam boiler requires a lot of cold makeup water. FIG. 5 also illustrates conventional features in the feedwater system, such as steam trap, condensate tank, condensate pump, deaerating feed tank, and feedwater pump.

Claims

1. An apparatus for producing chilling comprised of:

a. A steam boiler;

b. An economizer in the exhaust from said steam boiler;

c. A thermally-activated chiller;

d. A heat recovery vapor generator (HRVG) for said chiller that is colocated with said economizer in said exhaust stream; and

e. A thermal heating load that receives reject heat from the absorber of said chiller.

2. The apparatus according to claim 1 wherein said thermally-activated chiller is an ammonia-water absorption refrigeration cycle (AARC), and wherein said thermal heating load is a preheater for the feedwater supplied to said economizer.

3. The apparatus according to claim 2 wherein said economizer is comprised of at least two sections, with a section of HRVG between adjoining economizer sections.

4. The apparatus according to claim 2 wherein said economizer and said HRVG are in parallel with respect to the exhaust flow direction.

5. The apparatus according to claim 2 additionally comprised of a gas turbine that discharges combustion exhaust to heat said boiler, and a turbine inlet chilling coil that is supplied chilling from said AARC.

6. The apparatus according to claim 5 wherein said AARC is adapted to supply heating to said chilling coil in cold ambient conditions.

7. The apparatus according to claim 5 additionally comprised of a second generator for said AARC that is supplied steam from said boiler.

8. The apparatus according to claim 2 wherein working fluid from said AARC is supplied directly to said HRVG and has countercurrent flowpath to said exhaust flow.

9. The apparatus according to claim 2 wherein a manufactured product is heated by steam from said boiler and is chilled by said chiller.

10. The apparatus according to claim 2 wherein a conditioned space is heated by steam from said boiler and cooled by said chilling.

11. The apparatus according to claim 2 wherein said feedwater preheater is the absorber in said AARC.

12. The apparatus according to claim 11 additionally comprised of a second absorber for said AARC that is cooled by heat rejection to ambient, e.g. via cooling water.

13. An apparatus for heating boiler feedwater and for producing chilling comprised of:

a. A steam boiler;

b. A thermally-activated chiller that includes a rectifier;

c. A heat recovery unit (HRU) for said chiller that is located in the exhaust stream of said boiler, and that transfers exhaust heat to said chiller; and

d. An absorber in said chiller that transfers reject heat to said feedwater such that the feedwater is heated to at least about 140° F.

14. The apparatus according to claim 13 wherein the chiller working fluid is an ammonia-water mixture and wherein the feedwater is also heated by the condenser of said chiller.

15. The apparatus according to claim 14 wherein said HRU supplies heat to the generator in said AARC.

16. An ammonia-water absorption refrigeration cycle (AARC) that is powered by waste heat associated with the exhaust of a steam boiler, comprised of:

a. A solution pump and solution heat exchanger (SHX);

b. A rectifier;

c. A heat recovery unit that: i. Supplies heat to the generator of said AARC, ii. Wherein said generator is supplied pumped solution after heating in said SHX; and iii. Said generator supplies partially desorbed solution to said rectifier;

d. At least one low pressure absorber that supplies heat to a heat load; and

e. An evaporator that supplies chilling.

17. The apparatus according to claim 16 wherein at least part of said heat load is preheating of the feedwater and/or makeup water for said economizer.

18. The apparatus according to claim 16 additionally comprised of an intermediate pressure absorber and an intermediate pressure generator that supplies it vapor.

19. The apparatus according to claim 17 additionally comprised of a gas turbine combined cycle that includes said boiler, and an inlet air chilling coil for said turbine that utilizes said chilling, and wherein said AARC is adapted to provide heating to said inlet air coil when needed.

20. The apparatus according to claim 16 additionally comprised of an ammonia expansion turbine that produces work from high pressure ammonia vapor by expanding it to low pressure.

21. A method for producing chilling and feedwater heating from steam boiler exhaust comprising:

a. Providing an AARC;

b. Transferring boiler exhaust heat to said AARC;

c. Desorbing pumped AARC liquid with said heat;

d. Rectifying the desorbed vapor;

e. Condensing the rectified vapor;

f. Evaporating the condensed vapor to produce said chilling; and

g. Preheating boiler feedwater to at least 140° F. with heat rejected from the absorber of the AARC.

22. The method according to claim 21 additionally comprising using a three-pressure AARC to cool the steam boiler exhaust to below 190° F.