Apparatus for generating electric energy using high temperature fumes

Field of Invention

0001This invention concerns in general a system for producing electric energy starting from high temperature fumes or gas coming from any type of heat source. In particular, the invention concerns an apparatus for generating electric energy with a turbogenerator operating according to the Rankine cycle with organic work fluid (ORC).

State of the Technique

0002A system for the production of electric energy of the type taken into consideration basically comprises: a source of fumes or gas at a high temperature, a heat exchanger between the fumes and a thermovector fluid circulating in an intermediate circuit, a heat exchanger between the intermediate thermovector fluid and an organic work fluid for the evaporation of the latter, a turbine (hereafter named the main turbine) fed by the work fluid vapour and connected to an electric generator, a regenerator for the recovery of the thermal content of the work fluid vapour, a condenser of the working fluid before it is recycled.

0003Usually, diathermic oil is used as an intermediate thermovector fluid and silicone oil as an operating fluid. The diathermic oil is made to circulate in a coil around which circulate the fumes or gasses at high temperature. Then it heats the operating fluid so as to generate vapour which feeds the turbo-generator.

0004In the Rankine cycles with organic operating fluid (ORC) with a high temperature difference between the hot and cold source, the volume flow rate of the vapour which passes through the turbine is much higher at discharge than at input.

0005This difference in volume flow rate has a negative effect on the efficiency of the turbine.

0006For example, in the particular case, but of great practical importance, of a system for the combustion of biomass, feeding an ORC cycle through an intermediate diathermic oil circuit with oil temperatures of 310°C at input and 240°C at output, the fluid vapour temperature is usually around 270°C and that of the condensation about 100°C.

0007Even though the input temperature of the diathermic oil would allow a higher evaporation temperature to be used, the output temperature specified is not normally exceeded.

0008The aim of this is to limit the expansion ratio of the turbine, which is often equipped with a small number of stages (for example two) and to avoid too great a difference in length of the blades in the passage from the first to the last stage.

0009In fact a rapid increase in the length of the blades in passing from the first stator to the last rotor involves a number of problems in the mechanical and fluid dynamic design of the turbine, such as the sharp deviation of the flow downstream of the rotors, the non-optimal length/diameter ratio of the blades, etc.

0010On the other hand, the use of higher input temperatures into the turbine would allow higher efficiency of the thermo dynamic cycle and consequently higher production of electric power in relation to the thermal power introduced.

0011The same considerations are valid for the lowering of the condensation temperature, which also involves an increase in the expansion ratio, besides an improvement in the cycle efficiency.

Objectives of the Invention

0012Starting from this preamble, one objective of this invention is to create the conditions for lowering the feed pressure of the main turbine compared to the evaporation pressure or otherwise to increase the evaporation pressure to noticeably improve the cycle efficiency.

0013The objective is reached according to the present invention by the adoption on the work fluid path, upstream of the main turbine, of at least one auxiliary turbine in which a pre-expansion of the work fluid is realised and an additional coaxial electric generator, assembled on the auxiliary turbine drive with a high speed rotation shaft aimed at optimising the power extraction from the pre-expansion of the work fluid.

Brief Description of the Drawings

0014The invention will however be described in greater detail made in reference to the enclosed indicative and not limiting drawings, in which; <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> shows a circuit diagram of the system including an auxiliary turbine;</li><li><figref idref="f0002">Fig. 2</figref> shows the same diagram as in <figref idref="f0001">Fig. 1</figref>, but implemented with a secondary evaporator;</li><li><figref idref="f0003">Fig. 3</figref> shows a cross section of a possible configuration of the auxiliary turbine; and</li><li><figref idref="f0003">Figs. 4 and 5</figref> shows the views of a main turbine and an auxiliary turbine integrated in different ways.</li></ul>

Detailed Description of the Invention

0015The system here proposed basically comprises, in association with a source of high temperature fumes - not shown, a turbogenerator group 10 using organic work fluid running in a relative circuit 11 and, between the fumes source and the turbogenerator, an intermediate circuit for a thermal carrier fluid 12.

0016The turbogenerator group 10 comprises a main turbine 13 with a respective electric generator 13'. The turbine is downstream of the heat exchanger group that comprises a pre-heater 17, an evaporator 14 the work fluid and a possible superheater 15 of the feed vapour of said turbine.

0017The fluid on exiting the main turbine 13 is directed into a work fluid condenser 16 and at least one regenerator 16', using the heat of the vapour to preheat the work fluid.

0018The high temperature fumes, by means of a primary heat exchanger - not shown, heat the thermal carrier fluid circulating in the intermediate circuit 12 and are directed immediately to a chimney or through a pre-heater used to pre-heat the comburent air to be fed to the combustor and a possible economizer used to heat a liquid for various purposes - not shown.

0019On exiting the intermediate heat exchanger, the heated thermal carrier fluid is made to circulate, in the direction of the arrows F in <figref idref="f0001">Fig. 1</figref>, in the superheater 15, if provided, and in the evaporator 14 to produce the feed vapour of the main turbine 13 before returning in cycle in the intermediate circuit through the pre-heater 17 for the work fluid.

0020In the intermediate circuit of the thermal carrier fluid 12 and in the work fluid circuit 11 pumps will be provided in the usual way and control valves 18, 18' to control the circulation of the respective thermal carrier and work fluids, plus other useful components.

0021The heat exchangers indicated may be set up as separate bodies or may be integrated in a single unit fulfilling the indicated functions.

0022According to an innovative aspect of the system, in the organic work fluid path, upstream of the main turbine 13, between the latter and the evaporator 14 or superheater 15, where provided, is inserted at least one auxiliary turbine 19, which creates a first expansion, or pre-expansion, of the fluid and the output of which is connected to the input of the main turbine.

0023Said auxiliary turbine 19 is equipped with a respective coaxial electric generator 19', assembled on the shaft 20 of the turbine, with a high rotation speed sufficient to optimise the power extraction from the pre-expansion.

0024The electric generator 19' connected to the auxiliary turbine 19 is preferably the permanent magnets type and with a rated capacity sufficiently high to enable operating without the intervention of adjustment devices in all the function field of the main turbine.

0025The auxiliary turbine 19 is preferably equipped with variable section nozzles to optimise its operating to the different loads that is in the presence of varying vapour flow-rate.

0026Furthermore it will have inlet and discharge volutes 21, 21' -<figref idref="f0003">Fig. 3</figref>- designed to keep their volume as small as possible so that the fluid content of these volutes does not increase the overspeed of the main turbine in the case of sudden lack of load, with consequent rapid closure of the input valves to the turbines.

0027In this way it is possible to avoid having to insert a cutoff valve, which would be very large and costly, between the discharge of the auxiliary turbine 19 and input of the main turbine 13.

0028The auxiliary turbine 19, besides, must be equipped with a sealing system so as to avoid access of high temperature work fluid into the area the coaxial electric generator turns in. For this purpose and in conjunction with another innovative aspect, around the shaft 20 of the auxiliary turbine 19, an annular chamber 22 is provided which is isolated with regards to the chamber in which the rotor turns by a labyrinth seal 23 made according to the known techniques and maintained at a pressure close to the one of the condenser 16 thanks to a duct 24 connected to the condenser itself. The cross section of the duct 24 must be such so as to enable the fluid leaking through the labyrinth seal to be transferred to the condenser at an acceptable loss of pressure.

0029The sealing system on the shaft will then have, according to the known technique, a mechanical seal, a further labyrinth seal.

0030A further innovative aspect consists in the injection in the zone of said ring shaped chamber 23, by means of another duct 25, of a small amount of liquid work fluid, correctly filtered, coming from circuit 11 and which evaporating at a pressure close to that of the condenser, guarantees to cool the shaft 20 and adjacent devices.

0031Furthermore, an interesting aspect deriving from the use of the auxiliary turbine, is the possibility of using an auxiliary turbine with a few robust blades, for example made by milling from a solid piece in the rotor disk, or by casting, maintaining between the length of the blade and axial chord of the blade a low ratio, for example less than a unit.

0032In this way the auxiliary turbine becomes a robust device capable to smooth the flow of vapour fed to the main turbine, above all in relation to the risk of dragging "plugs" of liquid during transient periods.

0033To reduce the volume of its load and discharge volutes 21, 21', the construction and dimension costs, the auxiliary turbine 19 can be made with the discharge volute integrated with the input volute of the main turbine according to the illustrative diagrams in <figref idref="f0003">Figs. 4 and 5</figref>.

0034It is also possible to foresee - <figref idref="f0001">Fig. 1</figref>- an additional duct 26 with feed valve 27 of the auxiliary turbine 19, which is opened when the main input valves 18, 18' are rapidly closed. Through this duct 26 the auxiliary turbine can be fed with a fluid delivery sufficient to maintain the respective electric generator 19' at a generating level equal to the power absorbed by the auxiliaries and to keep the electric generator at no-load at the rated speed , which will be maintained in rotation at a controlled speed.

0035This avoids having to stop the plant should there be a short time interruption in the mains.

0036Furthermore, the work fluid circuit, downstream of the evaporator and possible superheater, can be equipped with a shunted line 28 with a control valve 28' to bypass the auxiliary turbine 19, both in the case of a breakdown of the latter and for its maintenance, and to feed the main turbine 13 with an increase in delivery.

0037Furthermore, it will be possible to feed the main turbine 13 with a shunted delivery of vapour product in an exchanger/secondary evaporator 29 - <figref idref="f0002">Fig. 2</figref>- that receives heat from the intermediate circuit of the thermal carrier fluid 12, downstream compared with the main evaporator 14.

0038This delivery has a lower pressure compared to the evaporation temperature of the remaining delivery crossing through the main evaporator. Therefore it will be possible to produce a shunted delivery with an exchanger/secondary evaporator with a smaller surface compared to the surface which would be needed for a counter requirement to evaporate the same delivery of work fluid at the pressure of the main evaporator.

0039Said exchanger/secondary evaporator 29 can be fed by a separate pump and pre-heater, positioned in parallel compared with the main evaporator 14, but as an alternative and preferably, as shown in <figref idref="f0002">Fig.2</figref>, the secondary evaporator 29 will be fed with the liquid work fluid collected downstream of the pre-heater 17 and appropriately reduced in pressure in a throttle valve 30 upstream of the secondary evaporator.

0040Downstream of the secondary evaporator will b e inserted an interceptor valve 31 to avoid over-speed of the main turbine in case of lack of load.