AU2020101347B4 - A waste heat recovery system - Google Patents

Abstract

A waste heat recovery system (10) for a power station delivering power to extractive industries infrastructure comprises at least one open cycle gas turbine (OCGT) including an exhaust system for discharging exhaust gas (1); and a waste heat 5 recovery unit (2) coupled with said exhaust system for recovering heat from the exhaust gas (1) wherein said waste heat recovery unit (2) provides heat to an organic rankine cycle based power generation system (20). The waste heat recovery system (10) efficiently recovers heat from a high efficiency OCGT delivering, for example, 40 45MW of power. 10

Description

A WASTE HEAT RECOVERY SYSTEM TECHNICAL FIELD

[0001] The present invention relates to a waste heat recovery system and, in particular, to a waste heat recovery system suitable for application in the extractive industries, such as the mining and oil and gas industries.

BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Significant waste heat is generated by power stations of the Australian mining and oil and gas industries. Open cycle gas turbines (OCGT) are commonly used in both sectors to generate power and to drive rotating equipment such as compressors and pumps. High value, medium temperature (450-600 0C) waste heat is vented to atmosphere via the exhaust gas produced by the OCGT, this providing an opportunity for heat recovery and utilisation, for example to generate zero-emission electricity. Such heat recovery potentially allows power generation at lower cost than from renewable resources such as solar and wind generation.

[0004] Typically, the quantity of heat generated by open cycle gas turbines, such as the GE LM 6000 family of OCGTs, currently including LM6000 PF and LM6000 PG variants, is adequate to support a traditional combined cycle power station, utilizing steam as the heat recovery medium (such as the South Hedland Power Station completed in 2017). Steam combined cycle is the "default" technology for waste recovery from OCGT systems, not least because of the achieved thermal efficiency. However, the operational characteristics of steam combined cycle makes this technology unsuitable for remote power generation. This is particularly so in remote and dry locations, such as in the mining regions of central and northern Australia, where the water losses and water balancing requirements of steam combined cycle systems act as significant constraints. Another constraint is the high level of maintenance activity associated with steam combined cycles and the manpower required to deliver ongoing maintenance in a remote location.

[0005] Given such constraints, the attraction of renewable resources, such as solar and wind, for meeting additional power requirements is significant. Indeed, there has been significant take up of power generation from solar and wind resources though this is still happening slowly in mining and energy applications with high demands for reliable baseload power generation at a competitive cost.

[0006] It is an object of the present invention to provide a waste heat recovery system which addresses the operational problems of the steam combined cycle when used for remote power generation, particularly in dry conditions.

SUMMARY OF INVENTION

[0007] With this object in view, the present invention provides - in one aspect - a waste heat recovery system for a power station delivering power to extractive industries infrastructure comprising:

at least one open cycle gas turbine (OCGT) including an exhaust system for discharging exhaust gas from the OCGT; and

a waste heat recovery unit coupled with said exhaust system for recovering heat from the exhaust gas

wherein said waste heat recovery unit provides heat to an organic rankine cycle based power generation system.

[0008] The at least one gas turbine is desirably an efficient aeroderivative gas turbine producing exhaust gas at a temperature below about 5500C with typical exhaust gas temperature below 5000C - such as the LM 6000 open cycle gas turbine available from General Electric which typically produces exhaust gas with temperature about 4610C. The at least one turbine may form part of a turbine system comprising two or more turbines.

[0009] The waste heat recovery unit could allow for direct heat transfer between turbine exhaust gas and ORC working fluid. Alternatively, the waste heat recovery unit conveniently allows heat exchange between exhaust gas and a first thermal fluid where direct heat exchange with an ORC working fluid is not practically possible due to excessive temperatures and the flammability of the ORC working fluid. A shell and tube heat exchanger is preferred, with tubes of such a heat exchanger being finned to increase heat transfer area. A finned design, by increasing heat transfer area, minimises the volume of the WHRU; and, also, the backpressure on the OCGT which is desirably maintained below about 2 kPa, preferably 1.5kPa. A heat exchange system, conveniently including a further set of heat exchangers, is then conveniently provided for exchanging heat between the first thermal fluid and a second thermal fluid. The second thermal fluid is the working fluid, conveniently cyclopentane though alternatives (including n-pentane, iso-pentane, n-butane, isobutane or siloxanes) are available, for the organic rankine cycle (ORC) power generation system. In the case of cyclopentane, and a range of other potential candidate second thermal fluids, it will be understood that these are flammable and cannot typically be used directly in a WHRU to capture waste heat from high temperature OCGT exhaust gas.

[0010] The first thermal fluid is conveniently a thermal oil. The use of thermal oils avoids issues with corrosion, water losses and balancing common to steam combined ?0 cycle operation.

[0011] The ORC system may include a single turbine though conveniently has the power generation duty split between a plurality of turbines, desirably two turbines, driving a common generator. Where two, or more, ORC turbines are used, these desirably share a common process system which consists of a common pre-heater, evaporator, superheater (if required), condenser and circulation pumps plus all associated pipework, valves and instruments. In this case, each ORC turbine discharges into its own dedicated regenerator which is situated upstream of the common condenser. The second thermal fluid of the ORC power generation system may be condensed (to remove unusable heat) following expansion by either an air cooled or water cooled condenser arrangement. Air-cooling may be preferred since this technology does not require water and may be more suitable for water constrained environments.

[0012] The waste heat recovery unit may include an external bypass stack to provide a direct route to atmosphere for exhaust gas when the thermal fluids are unable to accept heat inputs from the exhaust gas. In such case, combined cycle operation is not possible. However, an external bypass stack requires plot area which may not be available at some locations. Preferably, the waste heat recovery unit includes an integral bypass stack. An integral waste heat recovery unit typically includes a body comprising two separate compartments. One compartment houses a heat exchanger portion, such as the tubes of a shell and tube heat exchanger. The other compartment is an empty duct providing a bypass around the tubes. Conveniently, a diverter valve arrangement is located at an exhaust gas inlet to the waste heat recovery unit (WHRU), the diverter arrangement controlling exhaust gas flow to either the WHRU heat exchange portion compartment or the bypass compartment or duct.

[0013] Such an integral WHRU is common where aqueous solutions are used as the thermal fluid. Heating medium degradation and ignition is not a risk with aqueous solutions. However, where - as preferred - the thermal fluid is a thermal oil, the risks of degradation and ignition are relevant and an external bypass would typically be used. One reason for this is that the diverter valve arrangement does not completely isolate exhaust gas flow from the WHRU heat exchanger portion, for example the tubes of a shell and tube heat exchanger. Some diverter leakage is inevitable and heat cannot be ?0 completely isolated from the thermal oil system. In a situation where the thermal oil system is unable to safely receive and dissipate heat, diverter leakage will cause the thermal oil to heat up with the thermal oil temperature eventually approaching the exhaust gas temperature, for example around 4600 C. Most thermal oil products have an autoignition temperature of less than 4000 C. At its autoignition temperature, thermal oil can spontaneously ignite in the presence of oxygen without a source of ignition. Furthermore, thermal oils typically begin to degrade at temperatures around 3400 C, separating into low boiling point light molecules and high boiling point heavy molecules. The heavy molecules can deposit on the tube walls of the WHRU, impairing heat transfer performance whilst the light molecules can accumulate in high points of the system. These factors have also driven selection of an external bypass for a thermal oil WHRU in past practice. The Applicant has addressed this issue through selection of a thermal oil, at least, with an autoignition temperature greater than the maximum operating exhaust temperature, for example 5500C for an LM 6000 OCGT depending on operating load and ambient conditions. To this end, autoignition point for the thermal oil is preferably greater than 5000 C, more preferably greater than 5500C, optionally 6000 C. The waste heat recovery system desirably includes a shutdown cooler which allows cooling of the thermal oil and bypassing of the ORC power generation system when required due to inoperability. However, there may be situations where the shutdown cooler is itself inoperative. In this case, the waste heat recovery unit is coupled with a purging system which allows removal of thermal oil from the heat exchanger portion, such as the tubes of a shell and tube heat exchanger, particularly when the temperature of the thermal oil exceeds a temperature threshold indicative of heightened thermal cracking and/or autoignition risk. Such temperature may be set, for example, in the range of 350-400 0C dependent on the autoignition temperature and/or thermal cracking temperature of the thermal oil providing both safety and operational benefits. The purging system is coupled with an inert gas supply, such as a nitrogen supply, providing a purging flow directed to purge thermal oil from the heat exchanger portion. Nitrogen will typically be readily available at sites where large volumes of liquid fuel are stored. Purging flow may be maintained for a controlled duration, for example one minute, which is effective for purging a major portion of the thermal oil from the heat exchanger portion.

[0014] The purging system may allow for nitrogen, or other inert gas, to remain pressurised within the heat exchanger portion until the waste heat recovery system is ?0 ready to resume normal operation. Thermal oil is desirably purged to a storage vessel where it is blanketed with inert gas. A pressure relief valve is conveniently provided on the WHRU thermal oil discharge to allow for thermal expansion of the inert gas and other overpressure events.

[0015] The waste heat recovery system of the present invention operates efficiently and may be included in new power stations, such as those including GE LM 6000 or comparable gas turbines, or retro-fitted to existing plant used in the extractive industries, particularly for energy industry and mining infrastructure. The waste heat recovery system may supplement base load capability of a turbine power station at a cost lower than for renewable resources such as solar and wind.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Further features of the present invention are more fully described in the following description of preferred non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a schematic flow diagram of a portion of a power station being a waste heat recovery system including an organic rankine cycle power generation system in a first operating mode according to a preferred embodiment of the present invention.

Figure 2 is a schematic flow diagram of one embodiment of the organic rankine cycle power generation system of Figure 1.

Figure 3 is a schematic flow diagram of a second embodiment of the organic rankine cycle power generation system of Figure 1.

Figure 4 is a schematic flow diagram similar to Figure 1 but in a second operating mode of a further preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0017] Figure 1 schematically shows a portion 10 of a power station for providing electricity to a mining facility with a hot dry climate, rarely dropping below 200C at night and often approaching 500 C daytime temperature during summer months. The power station includes a bank of two aeroderivative open cycle gas turbines (OCGT) of the high efficiency General Electric LM 6000 type (such as LM 6000 PF or LM 6000 PG with LM 6000 PF being used in this embodiment) which are commonly used in the mining and oil and gas industries, each delivering up to about 40MW of power. The gas turbines each include a combustor (not shown) which ignites the gas fuel, with the resulting thermal expansion of the fuel/air mixture driving the turbine and producing exhaust gas 1 at a normal operating temperature of about 461°C but which can produce exhaust gas up to a temperature of about 5500C dependent on load.

[0018] The turbine exhaust gas 1 therefore contains significant waste heat which can be recovered to supplement the baseload capacity of the power station. Typically, the waste heat would be recovered through a steam based combined cycle plant. However, there are constraints with steam based combined cycle plants as follows:

• Superheated steam is required to avoid condensation across steam turbine and limit blade erosion;

• Constant bleeding of steam is required to maintain boiler feedwater (BFW) quality.

• Water consumption (noting the dry climate) is unavoidable even with air-cooled solutions.

• High operational complexity due to BFW management and superheating.

• Constant attendance by licensed personnel.

" Thermal efficiency is significantly lower at partial load (< 70% design load).

• Not grid compatible with intermittent renewable sources such as wind and solar.

[0019] While these issues could be addressed, in theory, by an organic rankine cycle (ORC) - based waste heat recovery system, there have previously been design constraints. First, steam combined cycle plants typically achieve higher overall thermal efficiencies than ORC solutions. Second, a power station - such as here disclosed having two LM 6000 OCGTs would support a 14 MW ORC power generation system whilst the maximum capacity (10 MW) of a single ORC turbine produced by a market leading manufacturer of ORC technology is not suitable for LM 6000 derived waste heat recovery. In this regard, ORC technology studies have previously been focussed upon smaller reciprocating engine power stations typically used to power gold mines or off grid townships with ORC power generation capacities in the range 0.5MW to 3MW.

[0020] Turbine exhaust gas 1 is directed to a waste heat recovery unit (WHRU) 2 containing a heat exchanger 3 comprised by the finned tubes of a shell and tube heat exchanger. WHRU 2 is of integral design, with heat exchanger 3 being included in a first compartment 2a of the body of the WHRU 2, other than as described below, and a second compartment, separated from first compartment 2a by partition 41, forming bypass duct 4. A diverter valve arrangement 5 comprising banks of diverter valves 5a and 5b controls whether exhaust gas 1 flows through the heat exchanger compartment 2a or bypass duct 4 in response to commands from a control system, such as a PLC system which is conveniently integrated with the power station control system, for the waste heat recovery system 10. As shown in Figure 1, WHRU 2 is in normal operation with diverter valves 5a closing the bypass duct 4 and the diverter valves 5b being open allowing exhaust gas 1 to flow through heat exchanger compartment 2a for heat exchange with thermal oil in heat exchanger 3. No external bypass stack is required in this preferred embodiment with savings in terms of plot area and cost.

[0021] Tubes 3 allow exchange of heat between exhaust gas 1 and a first thermal fluid, a thermal oil. Given the application, in this case, to an LM 6000 PF OCGT, the thermal oil temperature is unexpectedly high (about 3150C against 2800C) in comparison to current WHRU designs. The first thermal oil is described further below.

[0022] Thermal oil is directed through a thermal oil sub-system 110 comprising a ?0 flow line 13 connecting WHRU 2 (and specifically tubes 3) to ORC power generation system 20. ORC power generation system 20 is connected to WHRU 2 by a return flow line 12a including an expansion vessel 11 and pump 12. An alternative flow line 12b connects return flow line 12a to flow line 13 through a valve 12c, the purpose of this flow line 12b being described below. Tubes 3 are connected to flow line 13 and return flow line 12a through WHRU isolation valves 2b and 2c. A drain line 2d is connected to return flow line 12a downstream of WHRU isolation valve 2c. Thermal oil flow through drain line 2d is controlled by isolation valve 2e. Valves 2b, 2c and 2e are control valves, such as solenoid valves, which are controlled by the control system for the waste heat recovery system 10.

[0023] The hot thermal oil carries heat through flow line 13 to ORC power generation system 20, where heat is transferred to the second thermal fluid, conveniently cyclopentane in this embodiment, the working fluid of the ORC power generation system 20. Cool thermal oil from the ORC power generation system 20 is returned to WHRU 2, and more specifically tubes 3, via expansion vessel 11 by pump 12 through flowline 12a. Alternative flow line 12b for thermal oil is closed off by valve 12c. The rate of heat transferred into the thermal oil in the WHRU 2 is equal to the rate of heat transferred out of the thermal oil in the ORC power generation system 20. The design and operation of ORC power generation system 20 is further described below.

[0024] Returning to the first thermal fluid, or thermal oil, the maximum exhaust gas temperature is approximately 5500 C depending on operating load and ambient conditions. This presents an autoignition risk that the Applicant has addressed through selecting a thermal oil with an autoignition temperature greater than the maximum exhaust gas temperature. Therminol 72, available from Eastman and described in material data sheets incorporated herein by reference, despite slightly inferior heat transfer performance to more typical products such as Therminol 66 (approximately 5% less heat recovery) has a high temperature stability in terms of ability to withstand high operating temperatures without cracking, with a maximum operating temperature of 3800 C. The autoignition temperature is 6030 C. This property is not sufficient as there are alternative products to Therminol 72 but these alternatives have significantly higher freezing points and higher viscosities at low temperatures (less than 200 C). These alternatives would present operational issues when attempting to start up from ambient ?0 temperature, requiring heat input at start-up to liquefy the thermal oil or sufficiently reduce the viscosity so that the thermal oil can be pumped. Therminol 72 is more expensive than lower temperature alternatives; however, the cost savings achieved by eliminating the external bypass stack and the benefits associated with a more compact installation outweigh the additional product cost.

[0025] Therminol 72 will thermally crack at temperatures greater than 3800C, resulting in fouling of the WHRU internal surface area (heat exchanger tubes 3). However, this presents a performance risk rather than a safety risk and can be addressed as described below.

[0026] Referring now to Figures 2 and 3, the ORC power generation system 20 will be described. ORC system 20 embodies a closed-loop thermal process that converts waste heat, as recovered from turbine exhaust gas 1 in the first thermal fluid, to electricity. The ORC fluid, cyclopentane in this case, undergoes a series of phase, temperature and volume changes through the various stages of the thermal process (i.e. the organic rankine cycle) and ORC system 20 which includes, in series, a pre heater/evaporator/superheater 22, turbine generator 23, regenerator 24 and condenser 25 (which is air cooled in Figure 2 and water cooled in Figure 3). In addition, the condenser 25 is connected to the regenerator 24 through a pump 26. The ORC power generation system 20 operates as follows.

[0027] (1) High pressure, medium temperature cyclopentane in the liquid phase flows through line 218 to the pre-heater/evaporator 22 where heat is exchanged from the Therminol 72 thermal oil in inlet line 210 into the cyclopentane, raising the temperature of the cyclopentane and evaporating it to produce high pressure, high temperature vapour in line 212. Cooled Therminol 72 flows back to thermal oil sub system 110 and the WHRU through return line 12a.

[0028] (2) High pressure, high temperature vapour flows through line 212 to the turbine generator 23 where it expands and generates rotational motion in the turbine which drives the turbine generator 23 and produces electrical power. Cyclopentane working fluid leaves the turbine generator 23 through line 214 as a low pressure, medium temperature vapour.

[0029] (3) Low pressure, medium temperature cyclopentane vapour flows through line 214 to the regenerator 24 where heat is transferred from the medium temperature vapour to the low temperature liquid flowing through line 216 from the pump 26. Low pressure, low temperature cyclopentane vapour leaves the regenerator 24 through line 240.

[0030] (4) Low pressure, low temperature vapour flows through line 240 to the condenser 25 where the vapour is cooled further and condenses into the liquid phase. Low pressure, low temperature liquid leaves condenser 25 through line 250.

[0031] (5) Low pressure, low temperature cyclopentane liquid flows through line 250 to the pump 26 where its pressure is raised. High pressure, low temperature cyclopentane liquid leaves the pump 26 through line 216.

[0032] (6) High pressure, low temperature cyclopentane liquid flows through line 216 to the regenerator 24 where heat is transferred to the liquid cyclopentane from the medium temperature cyclopentane vapour flowing from the turbine (23) exhaust. High pressure, medium temperature cyclopentane liquid leaves the regenerator 24 and flows through line 218 to the pre-heater/evaporator/superheater 22, completing the cycle.

[0033] The cycle of thermal process stages (1)-(6) continues for as long as the waste heat recovery unit 2 and thermal oil sub-system 110 is coupled with the ORC system 20 to generate power.

[0034] There may be fault or maintenance situations in which the ORC system 20 is unable to accept heat from the first thermal fluid, i.e. Therminol 72 in this embodiment. The temperature of the Therminol 72 may also reach a critical level. The waste heat recovery system 10 is adapted to address such situations as described below.

[0035] As shown in Figure 1, the waste heat recovery system 10 includes a shutdown cooler 16 and isolation valves 16a and 16b controlling Therminol 72 flow through or around the shutdown cooler 16. Similarly, the ORC power generation system 20 includes isolation valves 20a and 20b controlling Therminol 72 flow through or around ORC power generation system 20. During an ORC power generation system 20 shutdown, the ORC system 20 is automatically isolated from a flow of Therminol 72 by valves 20a and 20b. Simultaneously, the WHRU diverter valves 5a and 5b move to divert exhaust gas through the WHRU integral bypass 4. This is a bypass mode. Due to diverter leakage, approximately 5% of normal heat input will continue even in this bypass mode. Pump 12 will maintain a minimum flow of Therminol 72 through the waste heat recovery system 10 and the shutdown cooler 16 will automatically come online via valves 16a and 16b to safely dissipate heat and maintain Therminol 72 temperature within the design temperature limits (-14C-380C). This provides a first level of safety.

[0036] A second level of safety is provided, in this embodiment, by a purging system 18 operable when the shutdown cooler 16 also fails or is inoperative leading to an increase in Therminol 72 temperature. The purging system 18 includes a vent line 19, with a pressure safety valve 19a and a vent control valve 19b, communicating Therminol 72 flow-line 13 with a storage vessel (not shown). Flow line 13 is also communicated with a nitrogen gas supply 50 through a purge line 52 including a nitrogen purge valve 53 which controls flow of nitrogen through purge line 52. Flow line 13 includes a temperature sensor 13a and valve 2b for controlling Therminol 72 flow towards ORC power generation system 20.

[0037] If Therminol 72 temperature measured by temperature sensor 13a exceeds 375 0C, indicating approach towards the thermal cracking limit of 3800 C, a control system for waste heat recovery system 10 will automatically initiate the following actions:

(1) Open WHRU diverter 5a and close diverter 5b so that exhaust gas bypasses compartment 2a and flows directly to atmosphere via duct 4.

(2) open WHRU bypass valve 12c (allowing Therminol 72 flow through flow line 12b).

(3) isolate WHRU 2 from thermal oil system by closing isolation valves at WHRU inlet and outlet (valves 2b and 2c).

(4) open WHRU drain valve 2e to open drain line 2d.

(5) open nitrogen purge valve 53 for a duration, as set by the control system of the waste heat recovery system 10, of one minute. Purge valve 53 closes at the end of the one minute duration.

Figure 4 shows the open/closed positions of the valves during nitrogen purging.

[0038] Purging system 18, when operated in the above described automated response, isolates the WHRU 2 from the thermal oil sub-system 110 and evacuates thermal oil (Therminol 72 in this embodiment) from the WHRU tubes 3. A thin film of Therminol 72 will remain on the inside of the tubes 3 and will crack and cause fouling, however, bulk degradation of the Therminol 72 is avoided.

[0039] At the end of the nitrogen purge, the bypass valve 12c (open), isolation valves 2b and 2c (both closed), drain valve 2e (open) and nitrogen purge valve 53 (closed) remain in their final positions until the waste heat recovery system 10 is ready to resume normal operation. The WHRU tubes 3 remain filled with nitrogen which will heat up to the exhaust gas 1 temperature. Thermal expansion of the nitrogen is accommodated by the pressure relief valve 19a located on the WHRU thermal oil outlet 3a.

[0040] Drain valve 2e provides an open connection to a thermal oil drain vessel (not shown) which is blanketed by nitrogen via a separate supply (not shown). Therefore, if the nitrogen remaining in the WHRU tubes 3 cools and contracts, negative pressure in the tubes 3 will be avoided.

[0041] When the waste heat recovery system 10 is ready to resume normal operation, the drain valve 2e is closed and the vent control valve 19b is opened. Flow to the tubes 3 of WHRU 2 is re-established by opening the inlet and outlet isolation valves 2b and 2c and closing the bypass valve 12c. Flow through the vent control valve 19b is observed via a sight-glass until all vapour has been displaced, at which point the vent valve 19b is closed and normal operation is resumed.

[0042] The combination of an LM 6000 gas turbine plant with a waste heat recovery system including a ORC power generation system offers a high-value zero-emission alternative to intermittent solar and wind generation. Cost comparison for such a system as here described indicates that the ORC power generation system option delivers a levelized cost of energy (LCOE) about 30-35% less than a comparable PV solar option when calculated using a discount rate of 8% over a project lifespan of 25 years.

[0043] This is primarily due to the comparatively high energy density of waste heat resources compared to the dilute nature of solar and wind resources. In that regard, with currently existing renewable energy technology, hydrocarbon-fuelled dispatchable capacity is required to provide a stable and constant source of power. Therefore, gas fueled generation will be required by mining and oil and gas operators into the medium term. The high degree of flexibility of the waste heat recovery system makes it attractive to operators planning to integrate renewable power generation into primarily gas-fuelled power systems, whilst seeking to further reduce the emissions of those power systems.

[0044] Further, the use of an ORC power generation system 20 avoids the above described disadvantages of a steam combined cycle, advantages of the ORC system 20 including:

(1) "non-condensing" ORC working fluid eliminates turbine blade erosion;

(2) water-free solution, working fluid is contained within closed loop with negligible fluid losses;

(3) low operational complexity, saturated working fluid and no boiler feed water (BFW) management issues;

(4) autonomous operation;

(5) maintains high thermal efficiency, even at 50% design load; and

(6) grid compatible with intermittent renewable resources.

[0045] Those skilled in the art will appreciate that the waste heat recovery system described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

Claims (5)

CLAIMS:

1. A waste heat recovery system for a power station delivering power to extractive industries infrastructure comprising:

at least one open cycle gas turbine (OCGT) including an exhaust system for discharging exhaust gas from the OCGT;

a waste heat recovery unit coupled with said exhaust system for recovering heat from said exhaust gas, said waste heat recovery unit including a turbine exhaust gas inlet at the bottom of the waste heat recovery unit and a heat exchanger allowing heat exchange between turbine exhaust gas and a first thermal fluid being a thermal oil; and

an organic rankine cycle based power generation system including a heat exchange system for exchanging heat between the first thermal fluid and a second thermal fluid being the working fluid for the organic rankine cycle based power generation system, said organic rankine cycle based power generation system including at least one turbine and at least one regenerator

wherein said at least one OCGT produces exhaust gas at an exhaust gas temperature below 500 0C dependent on load, having a direct path to atmosphere via the waste heat recovery unit; and wherein the backpressure of the waste heat recovery unit on said at least one OCGT is maintained below 2 kPa, preferably below 1.5 kPa.

2. The waste heat recovery system of claim 1, wherein said waste recovery unit includes an integral bypass stack or an external bypass stack.

3. The waste heat recovery system of claim 1 or 2, wherein said thermal oil has autoignition temperature above 5000 C, preferably 5500 C, optionally 6000 C.

4. The waste heat recovery system of any one of the preceding claims, wherein said waste heat recovery unit is coupled with a purging system which allows purging of thermal oil from the heat exchanger of the waste heat recovery unit when the temperature of the thermal oil exceeds a temperature threshold, preferably in the range of 350-4000 C.

5. The waste heat recovery system of claim 4, wherein the purging system is coupled with an inert gas supply, such as a nitrogen supply, providing a purging flow directed to purge thermal oil from the heat exchanger of the waste heat recovery system; and wherein said purging flow is maintained for a controlled duration.