Steam ammonia power cycle

An integrated steam-ammonia power cycle is disclosed which achieves a close match to a glide heat source such as exhaust from a gas turbine, and which also eliminates sub-atmospheric pressure operation. With reference to FIG. 1, the exhaust heats in sequence steam superheater 107; steam boiler 105; feedwater preheater 104 plus ammonia superheater 103; and ammonia preheater 102. Steam is expanded to at least 17 psia in turbine 108, then condensed to boil ammonia in boiler 110. Superheated ammonia is expanded in turbine 112, and condensed in condenser 114. Feed ammonia is preheated in at least two parallel preheaters.

CROSS-REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING THE FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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BACKGROUND OF THE INVENTION

Gas turbines produce a relatively clean hot exhaust gas stream. Modern gas turbine combined cycle (GTCC) power plants have a steam-Rankine bottoming cycle which is heated by that exhaust which produces about half as much power as the gas turbine. In order for the steam cycle to more closely match the temperature glide of the exhaust, and hence be more efficient, there are typically two or three boiling pressures, and one or more reheats, especially in the larger plants. Even with that level of complexity, there remain problems and inefficiencies. The boilings are at constant temperature, so that part of the heat acceptance necessarily departs from the temperature glide of the heat source.

In the low temperature range, the boiling pressure of steam is on the order of one to five atmospheres. Any lower boiling pressure would require much larger flow passages and bulkier, costlier equipment, to mitigate serious pressure drop penalties. Similarly, the heat rejection is at deep vacuum. Much design effort is required to make the turbine final stages, exit passages, and condenser inlet passages very large and with low-pressure drop. It has been pointed out that this condenser design condition (typically 101° F. and 7 kPa absolute pressure) prevents steam plants from taking much advantage of colder-than-design conditions. Vacuum operation also allows air in-leakage, making de-aeration necessary, which adds to the thermal losses. The deep vacuum condenser must be bulky, with large flow passages, and has low transfer coefficients and high pressure drop losses. Those conditions also mitigate against air-cooled condensers.

The four inter-related factors of constant pressure boiling, vacuum, pressure loss, and cost make steam plants not very effective in the low temperature regime. Any tabulation of the loss mechanisms of multi-pressure steam plants is dominated by the low-pressure components. The capacity and efficiency of conventional steam bottoming cycles are critically dependent on using lots of cooling water to maintain low vacuum, and the low vacuum in turn makes it extremely difficult and costly to accomplish dry cooling.

Ammonia Rankine cycles are well known in the prior art. They have been applied in ocean thermal energy conversion (OTEC) applications, and elsewhere using low temperature heat sources. A 22 MW experimental prototype of an ammonia bottoming cycle for a nuclear-powered steam cycle has been tested. Steam under vacuum (about 0.5 bar absolute) boiled the ammonia, which was expanded without superheating. Ammonia extraction vapor was designated for feed heating. The objective was to overcome the limitations of the conventional vacuum steam condensation. A more recent study analytically investigated a triple power cycle wherein gas turbine exhaust heated a steam bottoming cycle, condensing steam boiled the ammonia, and exhaust superheated the ammonia vapor. That disclosed cycle has several disadvantages, including use of extraction steam for feedwater heating; no steam reheat; and no preheating of feed ammonia or feedwater by the exhaust. Other prior art power cycles incorporate ammonia turbines for impure ammonia, e.g., U.S. Pat. Nos. 6,058,695, 6,194,997, 5,950,433, and 6,269,644. Given the high condensing pressure of ammonia, air-cooling is more readily achieved.

What is needed, and included among the objects of this invention, is a bottoming cycle for a gas turbine, i.e. a power cycle for input glide heat above about 600° F., which achieves higher efficiency by achieving a better glide match with the heat source, and which also avoids the disadvantages associated with vacuum operation.

BRIEF SUMMARY OF THE INVENTION

The above and other useful advantages are provided by: an integrated steam-ammonia power cycle for a gas turbine combined cycle plant, said plant comprised of a gas turbine, an exhaust heated steam boiler, a steam superheater and a steam turbine, said power cycle additionally comprised of:a) a steam condenser/ammonia boiler;b) an ammonia superheater, and a feedwater preheater which are heated by said exhaust after said steam boiler;c) an ammonia turbine, condenser, and feed pump; andd) an ammonia feed preheater which is heated by said exhaust after said feedwater preheater and ammonia superheater.

Note that this combination is not the simple addition of an ammonia bottoming cycle to the turbine exhaust after the steam bottoming cycle. Were that to be done, the ammonia cycle would suffer the same losses due to constant pressure boiling that the steam cycle does. In the disclosed cycle, the ammonia boiling is provided by a constant temperature heat source—steam condensation. Hence, the losses are very minor—the temperature differential of the condenser/boiler—and much less than the avoided losses due to elimination of vacuum operation. The constant pressure boiling (from a glide source) penalty is only incurred once—in the steam portion of the cycle. The glide heat from the exhaust is only used in glide applications—superheating and feed liquid preheating—and hence can be matched almost perfectly.

The large liquid preheating requirement for ammonia makes it necessary to use both exhaust heat and ammonia vapor de-superheat to supply it. Even so, it extracts more heat from the exhaust than the conventional steam cycle. The exhaust is cooled well into the condensation range. This necessitates appropriate measures for handing acidic condensate. However, it provides compensating advantages—it very effectively reduces emissions, and provides a ready source of water which can be used for cycle enhancement (e.g., fog injection) or for cooling. When necessary, some of that ammonia preheat duty can be supplied by steam condensation, and/or by ammonia extraction vapor, with some reduction in cycle efficiency.

It is preferred to operate the steam condenser/ammonia boiler above atmospheric pressure on the steam side. That way, no air in-leakage is possible, and no de-aerating feed tank is necessary during normal operation. Thus, more glide heat is available to ammonia superheating, and cycle efficiency improves. The practical lower limit is about 17 psia. Clearly, there will be certain applications where other considerations dictate some degree of vacuum for the steam pressure, down as low as 10 psia.

It is beneficial to maintain at least about 20 parts per million to 5000 parts per million steam in the ammonia vapor, to inhibit corrosion and thermal decomposition. That is done by maintaining approximately 0.1% to five percent water in the ammonia boiler. The water in turn should be inhibited by NaOH or equivalent to a pH in the approximate range of 10 to 10.5.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG.1. heat recovery unit (HRU)101is comprised of ammonia economizer (preheater)102, ammonia superheater103, feedwater economizer104, steam boiler105and associated steam drum106and steam superheater107. The superheated steam is expanded in steam turbine108to produce work, which powers electrical generator109. Exhaust steam is condensed in steam condenser/ammonia boiler110. The condensed steam is pressurized to feedwater pressure by feedwater pump111, and routed to economizer104. Ammonia vapor from boiler110is routed to ammonia superheater103, then expanded in ammonia turbine112, producing additional work. Expanded ammonia vapor is desuperheated in ammonia economizer113, and then condensed in ammonia condenser114. Condensed ammonia is pressurized to feed pressure by ammonia feed pump115, and then proportioned between exhaust ammonia economizer102and desuperheating ammonia economizer113by splitter116. The preheated ammonia feed is then sent to ammonia boiler110, completing the cycle.

As example operating conditions for theFIG. 1flowsheet, consider a 900° F. exhaust gas. Steam is boiled at 600 psia and superheated to 850° F., then expanded to 17 psia and 220° F. in turbine108. Each pound of steam condensed in condenser110produces 3.09 pounds of ammonia vapor at 890 psia, and each pound of steam expanded produces 300 BTU of work in an 84% isentropic efficiency turbine. The 3.09 pounds of ammonia vapor is superheated to 460° F., then expanded to 196 psia, producing 105 BTU/lb work in an 84% turbine (324.5 BTU for the 309 pounds). The ammonia is then desuperheated to preheat approximately half the feed ammonia, and then condensed at 95° F. The other half of the feed ammonia is preheated in economizer102.

InFIG. 2, like-numbered200series components have the same description as theirFIG. 1counterparts. The added features in theFIG. 2flowsheet are steam reheater217, reheat steam turbine218(intermediate pressure turbine), steam desuperheater/feedwater preheater219, feedwater splitter220, plus the gas turbine221. InFIG. 3, the steam boiler322is depicted as once through type, i.e., no steam drum, such as would be used for near-critical or super-critical steam generation. Also, a second ammonia turbine323is depicted. It operates over the same pressure range as primary ammonia turbine312, but at a lower temperature. The ammonia vapor expanded in it receives lower temperature superheat from any available source, e.g., from desuperheater/superheater324. This turbine is used to help balance the latent duties of condenser/boiler310, while at the same time maximizing the superheat temperature in303.

Dependent upon the exhaust temperature entering HRU301, it may be desirable to add a second reheater plus second reheat turbine, at lower pressure. In general, the turbine exhaust temperature and the plant size will determine how many turbines are used, e.g., choice ofFIGS. 1,2, or3. Larger plants and higher exhaust temperatures justify the added number of turbines, as higher efficiencies are achieved. The disclosed eonomizers (or preheaters) are key to achieving high cycle efficiency, i.e., good glide matching down to low temperature.

Various other features commonly encountered in GTCC plants would be present when appropriate: attemperators, rotor air coolers, fuel (gas) preheaters, etc.

The ammonia turbine(s) operate in a favorable pressure ratio range between 3 and 10, with little or no condensation in the exhaust. The same applies to the steam turbines when more than one (i.e., reheat) is used. The single steam reheat adds markedly to cycle performance.

Any of the figures or disclosed cycle variants adapt readily to dry cooling (air only) or to evaporative cooling, with either saturated or unsaturated exhaust air.