Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2002-09-30
2004-11-16
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S039511, C060S039530
Reexamination Certificate
active
06817185
ABSTRACT:
The present invention relates to an engine comprising a positive displacement isothermal air compressor provided with a liquid spray to spray liquid into the air as it is compressed so that the compression is substantially isothermal; a combustion chamber in which the compressed air is expanded to generate power; means to feed the compressed air from the isothermal compressor to the combustion chamber; a separator to remove liquid from the compressed air upstream of the combustion chamber; a primary heat exchanger for transferring heat from exhaust gas from the combustion chamber to compressed air upstream of the combustion chamber; a precompressor to compress the air upstream of the isothermal compressor; and means to feed the air from the precompressor to the isothermal compressor.
Such an engine, which will subsequently be referred to as “of the kind described”, is disclosed in WO 94/12785.
A different heat engine is disclosed in U.S. Pat. No. 5,839,270. In this engine, ambient air is fed directly to the sliding-blade compressor. Water is sprayed into the air during compression to ensure that the process is generally isothermal. The mixture of water and compressed air is then fed to a separator where the water and air are separated. Some of the air is then fed to a combustion chamber where fuel is injected and the fuel/air mixture is burned. Combustion gas is then fed to a separate expander where it is adiabatically expanded to perform useful work and also to drive the compressor. The exhaust gas from the expander is fed to a recuperator in which it preheats the remainder of the cool compressed air from the separator. The preheated compressed air mixes with the cool compressed air in the combustion chamber. In one example, the expander is cooled by water from the separator. The warmed water leaving the expander is then fed to a boiler which is used to generate steam to provide useful work in a steam expander. Thus, this document does not disclose a combustion chamber in which the compressed air is expanded, nor does it disclose a precompressor, or a means to feed air from the precompressor to the isothermal compressor. This is therefore not an engine of the kind described.
In order to recover heat from the combustion chamber in WO 94/12785 a fraction of the compressed air from the separator is heated by passing it through a cooling jacket surrounding the combustion chamber. This heated compressed air is then expanded in a cylinder in order to do useful work. However, the temperature of the coolant is necessarily limited to that which the materials and/or the lubrication oil of the combustor can withstand. The amount of heat that can be absorbed in the cooling jacket per unit mass of compressed air is therefore quite small. In order to recover this heat, a considerable amount of extra compression work is required from the isothermal compressor, thereby reducing the benefit of the heat recovery.
According to a first aspect of the present invention, an engine of the kind described is characterised by a secondary heat exchanger provided to transfer heat recovered from a part of the engine to the compressed air from the isothermal compressor upstream of the primary heat exchanger.
The present invention therefore offers a more efficient heat recovery process than that disclosed in WO 94/12785 because it makes more economical use of the air that is compressed. This saves compression work and increases the overall efficiency. The present invention recovers heat from a part of the engine and uses this to preheat the compressed air produced by the isothermal compressor. Unlike the invention described in WO 94/12785, no additional air needs to be compressed in order to recover this heat. Furthermore, unlike WO/12785, the present invention does not need to have a separate expansion chamber for the expansion of air used to recover the additional heat.
The heat recovered by the secondary heat exchanger may be recovered from any part of the engine including the exhaust gas. In this latter case, if the precompressor is arranged to be driven by the exhaust gas, for example by means of a turbine or other form of expander, the temperature of the exhaust gasses which have been used to drive the precompressor may still be high enough that heat can be recovered at the secondary heat exchanger from these gasses. However, it is primarily intended to recover heat from the combustion chamber, which passes into a combustion chamber cooling system, and/or from the air which is compressed by the precompressor.
For the most efficient engine cycle, heat should be recovered from both the combustion chamber and from the air from the precompressor. In this case, the secondary heat exchanger comprises a precompressor heat exchanger and a combustion chamber heat exchanger connected in parallel, and wherein the compressed air from the isothermal compressor is split into two streams, one stream being fed to the precompressor heat exchanger to receive heat from air from the precompressor, and the other stream being fed to the combustion chamber heat exchanger to receive heat from the combustion chamber; the engine further comprising means to control the split of the flow of air from the isothermal compressor into the two streams.
The advantage of providing two heat exchangers in parallel, each of which recovers heat from a separate component of the engine, lies in the fact that the range of temperature of the heat available from the separate components is broadly similar, but can vary according to engine load. A series arrangement presupposes that one heat source is consistently at a significantly higher temperature than the other. This is not the case with the heat from the precompressor and the heat from the engine cooling system, since both produce heat over overlapping temperature ranges. A parallel arrangement is more flexible in this situation.
Also, in spite of the reduced air flow in each of the parallel flow paths, it is still possible to provide sufficient capacity to absorb most of the available heat by adding water to the compressed air entering the secondary heat exchanger in a quantity which will be vaporised in the secondary heat exchanger. The high latent heat evaporation of water allows a considerable amount of heat to be absorbed by a small amount of water. The benefit of the evaporated water is that it provides additional working fluid to the engine, which is gained without any additional compression work. This results in a higher engine efficiency and a higher power output. It is generally found that the optimum cycle efficiency is achieved if the system is configured to maximise the total enthalpy of water vapour which is produced upstream of the primary heat exchanger. Carryover of excess liquid water into the primary heat exchanger is generally found to be detrimental to the cycle efficiency, although it may benefit the power output.
Adding the liquid to the compressed air creates a two-phase stream. It is difficult to control the composition of such a stream when the stream is distributed amongst a number of heat exchange elements within the secondary heat exchanger. Therefore, preferably, when the secondary heat exchanger has a number of heat exchange elements carrying the compressed air, the liquid is directly injected into each element. This allows direct control of the phase composition throughout the heat exchanger.
The liquid may be taken from any source. However, it is most conveniently taken from the separator.
When heat is recovered from both the air from the precompressor and the combustion chamber, liquid is preferably supplied to each of the compressed air streams in the precompressor heat exchanger and combustion chamber heat exchanger respectively. In this case, the engine further comprises means to control the flow of liquid to the precompressor heat exchanger and combustion chamber heat exchanger.
The streams from the precompressor and combustion chamber heat exchangers may be fed separately into the primary heat exchanger. However, they are preferably combined upstream o
Abdallah Hicham Salah
Coney Michael Willboughby Essex
Richards Roger
Birch & Stewart Kolasch & Birch, LLP
Freay Charles G.
Innogy PLC
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