INDUSTRIAL AND THERMAL POWER PLANT.

Background of the Invention

Technical Field

The invention relates to heat recovery in general and more specifically a system and a method for recovering waste heat from an industrial process in pre heating combustion air and fuel.

Background Art

From prior art one should refer to general waste recovery technology wherein waste heat in a thermal power station in the flue is used to preheat air and/or fuel entering the thermal power station burner.

From prior art one should also refer to general waste recovery technology wherein waste heat in industrial processes such as aluminium electrolysis is recovered by heat exchangers and used for power generation, desalination and similar purposes.

At the same time such industrial processes, particularly electrochemical processes require large amounts of power and even with prior art the recovered power is low, complexities are high and temperature of the recovery system is low and thus overall efficiency is low. This also brings in associated problems such as pollution.

WO2012039624 and WO2013105867, both by the present applicant, relate both to systems and methods for control of side layer formation in an aluminium electrolysis cell. Disclosure of the Invention

Problems to be Solved by the Invention

Therefore, a main objective of the present invention is to provide a system and a method that overcomes the problems described above. Means for Solving the Problems

The objective is achieved according to the invention by an apparatus for recovering waste heat as defined in the preamble of claim 1 , having the features of the characterising portion of claim 1 and a method for recovering waste heat as defined in the preamble of claim 13, having the features of the characterising portion of claim 13.

A number of non-exhaustive embodiments, variants or alternatives of the invention are defined by the dependent claims.

The present invention attains the above-described objective by a plurality of heat exchangers extracting heat from flue gas from a power plant and waste heat from an industrial process and transferring the extracted heat to air and optionally fuel entering said power plant.

In a first aspect a system for recovering waste heat from a combined plant is provided, comprising a thermal power plant and an industrial plant, wherein the thermal power plant is distinct from the industrial plant, the system comprising a heat exchanger circuit having a working fluid, further comprising a heat extraction circuit comprising an industrial gas circuit comprising at least one heat exchanger for extracting waste heat from the industrial plant, and a burner circuit comprising at least one heat exchanger for heating at least one from a group comprising air in an air pre-heater heating air from an air inlet and fuel in a fuel pre-heater heating fuel from a fuel inlet of the thermal power plant, wherein the burner circuit uses heat extracted from the heat extraction circuit.

In one embodiment the heat extraction circuit further comprises a flue gas circuit comprising a flue gas heat exchanger for extracting waste heat from a flue gas in a flue of the thermal power plant.

In a further embodiment the heat extraction circuit further comprising a heat collection manifold wherein the flue gas circuit and the industrial gas circuit are connected in parallel to the heat collection manifold.

In a further embodiment the flue gas circuit and the industrial gas circuit are connected in series.

In one embodiment the heat extraction circuit further comprises a heat distribution manifold wherein the burner air inlet heat exchanger and burner fuel inlet heat exchanger are connected in parallel to the heat distribution manifold. In one embodiment the burner air inlet heat exchanger and burner fuel inlet heat exchanger are connected in series.

In one embodiment at least part of the exhaust gas in the exhaust conduit is fed into the air inlet of the thermal power plant.

In one embodiment the working fluid is air.

In another embodiment at least part of the working fluid is fed into the air inlet.

In yet another embodiment the working fluid is fuel.

In yet another embodiment at least part of the working fluid is fed into the fuel inlet. In one embodiment the industrial plant comprises an aluminium electrolysis cell and wherein the industrial plant circuit comprises an exhaust heat exchanger and a sidelining heat exchanger.

In a second aspect a method for recovering waste heat from a combined plant is provided wherein the flue gas circuit and the industrial gas circuit are connected in parallel to the heat collection manifold when temperature of the flue gas in the flue is above a threshold T _s , and wherein the flue gas circuit and the industrial gas circuit are connected in series when temperature of the flue gas in the flue is below a threshold T _p .

In one embodiment T _s is more than 150°C and T _p is less than 100°C.

Effects of the Invention

The technical differences over prior art is that feed in the form of air and/or fuel to the burner in a thermal power plant is pre heated in a sequence that provides optimum use of waste heat and delivering maximum pre heating to the burner.

These effects provide in turn several further advantageous effects:

· it reduces fuel consumption and thus C02 emission

• it simplifies energy recovery since steam raising is located in one place only it improves safety since steam is removed from certain industrial processes where water can cause explosion in liquid metal

it makes it possible to increase burn temperature

increased temperature increases Carnot cycle efficiency

increased temperature allows for combusting and destroying harmful substances such as PCB

Brief Description of the Drawings

The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an [exemplary] embodiment of the invention given with reference to the accompanying drawings.

The invention will be further described below in connection with exemplary embodiments which are schematically shown in the drawings, wherein:

Fig. 1 shows an embodiment with a parallel heating of air and fuel using heat in a flue and in an industrial process, and

Fig. 2 shows an embodiment with a series heating of air and fuel using heat in a flue and then in an industrial process.

Description of the Reference Signs

The following reference numbers and signs refer to the drawings:

100 Combined plant

1 10 Thermal power plant

1 1 1 Air inlet

1 12 Air pre-heater

1 13 Fuel inlet

1 14 Fuel pre-heater

1 15 Burner

1 16 Boiler

1 18 Turbine system

1 19 Flue

120 Industrial plant

122 Electrolysis cell

124 Sidelining

126 Exhaust conduit 140 Heat exchange circuit

142 Heat collection manifold

144 Heat distribution manifold

150 Flue gas circuit

152 Flue gas heat exchanger

160 Industrial plant circuit

162 Exhaust heat exchanger

164 Sidelining heat exchanger

170 Burner circuit

172 Burner air inlet heat exchanger

174 Burner fuel inlet heat exchanger

Detailed Description

Various aspects of the disclosure are described more fully hereinafter with reference to the accompanying drawings. This disclosure may, however, be embodied in many different forms and should not be construed as limited to any specific structure or function presented throughout this disclosure. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Based on the teachings herein one skilled in the art should appreciate that the scope of the disclosure is intended to cover any aspect of the disclosure disclosed herein, whether implemented independently of or combined with any other aspect of the disclosure. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the disclosure is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to or other than the various aspects of the disclosure set forth herein. It should be understood that any aspect of the disclosure disclosed herein may be embodied by one or more elements of a claim. The invention will be further described in connection with exemplary embodiments which are schematically shown in the drawings, wherein Fig. 1 shows an

embodiment with a heating of air and fuel using heat in a flue gas and in an industrial process. Principles forming the basis of the invention

The underlying principle is that heat is recovered in a heat extraction circuit at different stages in a power station and/or in an industrial plant. Preferably the order of heat recovery is in increasing order of temperature. Heat recovered is then used to heat air and/or fuel entering a burner in the power station in a burner circuit.

Best Modes of Carrying Out the Invention

The embodiment of the apparatus according to the invention shown in Fig. 1 and 2 comprises a combined plant 100 comprising a thermal power plant 1 10 co located with an industrial plant 120 operating an industrial process. This is illustrated with an aluminium electrolysis cell 122 surrounded by a sidelining 124 that is cooled to prevent liquid metal from eroding and destroying the cell. Gases from the process leave through an exhaust conduit 126. Such gases typically comprise fluorides from the melt and carbon dioxide and carbon monoxide resulting from consumption of a carbon anode.

The thermal power plant 1 10 burns fuel from a fuel inlet 1 13 through a fuel pre- heater 1 14 wherein the fuel is burned in a burner 1 15 in air from an air inlet 1 1 1 through an air pre-heater 1 12. The burner heats a boiler 1 16 feeding a turbine system 1 18 with steam, preferably superheated steam. The boiler is a complex system of heat exchangers operating at different temperatures similar to what is known in prior art but adapted to handle the efficiency of the embodiments. The remaining heat then passes through the flue 1 19.

The industrial plant 120 can be any energy demanding process that generates a lot of waste heat. One such example is metal processing such as aluminium

electrolysis. A plant typically comprises a plurality of electrolysis cells 122 but for simplicity a single electrolysis cell is shown. The cell is shown in simplified form with a sidelining 124 that is instrumental in containing liquid electrolyte including aluminium and is kept cooled to maintain a sidelayer, thus avoiding destruction of the electrolysis cell. Exhaust gases are ventilated through an exhaust conduit 126.

Many such plants require substantial amounts of power. In many cases aluminium plants were built next to major hydroelectric power stations. Where this is not possible dedicated thermal power plants have been co located with the aluminium plant. One such example is Quatalum, an aluminium smelter plant powered by a 1350 MW natural gas power plant. Electricity from the thermal power plant is fed to the aluminium electrolysis cell (not shown). In addition the two are also connected by a heat exchanger circuit 140 comprising a flue gas circuit 150 and an industrial plant circuit 160 collecting heat from the processes. The heat is then brought into a burner circuit 170. The heat exchanger circuit uses a working medium for transferring heat.

The flue gas circuit 150 comprises a flue gas heat exchanger 152. The flue gas heat exchanger is able to raise the temperature of the working medium.

The industrial plant circuit 160 comprises an exhaust gas heat exchanger 162 and a sidelining heat exchanger 164. By passing the working medium first through the exhaust gas heat exchanger 162 and then the sidelining heat exchanger 164 the temperature of the working medium is able to absorb heat efficiently. Typically the temperature is raised to a temperature near the flue gas temperature and then to an even higher temperature in the next stage.

Fig. 1 shows a first embodiment where working fluid enters the flue gas circuit and the industrial plant circuit in parallel and the working medium of both branches are collected in a heat collection manifold 142. From there the working medium passes to a heat distribution manifold 144 to the burner circuit 170 where a first part passes working medium to a burner air inlet heat exchanger 172 heating air in the air pre- heater 1 12, and a second part passes working medium to a burner fuel inlet heat exchanger 174 heating fuel in the fuel pre-heater 1 14.

Fig. 2 shows a second embodiment where the working fluid enters the flue gas circuit and the industrial plant circuit in series. This provides a series heating of air in flue and then in an industrial process. A heat collection manifold is therefore not required. From there the working medium passes to a heat distribution manifold 144 to the burner circuit 170 where a first part passes working medium to a burner air inlet heat exchanger 172 heating air in the air pre-heater 1 12, and a second part passes working medium to a burner fuel inlet heat exchanger 174 heating fuel in the fuel pre-heater 1 14.

Using valves (not shown) it is possible to switch between the parallel heating shown in Fig. 1 and the series heating shown in Fig. 2.

If the temperature of the flue gas is too low to increase the efficiency of the thermal plant, the working medium will flow first through the exchanger in the flue gas (152) before flowing through the heat recovery heat exchangers in the industrial plant circuit (160), i.e series.

If the temperature of the flue gas is high enough to increase the efficiency of the thermal plant (compare to the series one), the parallel process will be used.

Alternative Embodiments

A number of variations on the above can be envisaged. For instance the burner circuit 170 can be changed.

In one embodiment the burner circuit is a series connection between burner air inlet heat exchanger 172 and burner fuel inlet heat exchanger 174, in this order or opposite, optionally switchable between the two. In these embodiments a heat distribution manifold is not required.

It is also possible to heat only one of the air and the fuel. In a variation of the parallel embodiment shown in Fig. 1 the exhaust gas of the industrial plant can be provided with a secondary circuit comprising further exhaust gas heat exchanger to recover heat remaining after passing through the exhaust gas heat exchanger 162. The heat in the secondary circuit can then be used to provide further pre heating of fuel or air.

The working medium can be a type of oil that can withstand the temperatures present in the industrial process. Alternatively air can be used and enter the air inlet, directly or via the burner circuit. While heat capacity of air is low it can be used to handle very high temperature stages without problems of fluids reaching due to high temperature.

Alternatively a fuel can be used as a working medium and enter the fuel inlet, directly or via the burner circuit. This can be beneficial for working temperature below the decomposition point for a fuel. One example is the use in the secondary circuit. In certain industrial processes the exhaust gases comprise oxygen and/or flammable gases. For instances in the exhaust gas from aluminium electrolysis there is some CO present, a gas that is both flammable and also present a health risk. In one embodiment the exhaust gases are separated using membrane filters and

combustible bases are fed into the burner 1 15 providing extra energy while rendering dangerous gases harmless. Industrial Applicability

The invention according to the application finds use in co located systems comprising a power plant providing power to an industrial process. Alternatively energy could also be sent to the grid.