Abstract:
A system and method for converting otherwise wasted energy produced in the form of heated gases as a byproduct of an industrial process into electrical energy. At least some waste gases are diverted from a typical exhaust structure through a heat exchanger and back into the exhaust structure. The amount of gases flowing through the heat exchanger is monitored and regulated by a controller. A heat source liquid is simultaneously circulated under pressure through the heat exchanger and through an organic Rankine cycle system. The amount of heat source liquid being circulated is also monitored and regulated by the controller. The ORC system converts the heat from the heat source liquid into electricity.

Description:
TECHNICAL FIELD 
     The subject invention relates generally to an economical means for the conversion of otherwise wasted heat energy produced by industrial furnaces into electrical energy. More particularly, a system and method are disclosed for increasing the efficiency of a steel mill plant by recovering part of the wasted heat energy and transforming it into electrical energy that can be reused inside the same plant. 
     BACKGROUND OF THE INVENTION 
     Steel mills incorporate different types of furnaces. Slab reheating furnaces, annealing furnaces and other type of furnaces are typical in steel mill plants. The furnaces in general have a relatively low efficiency and an important portion of the heat produced as a result of the combustion of gas or other means, can&#39;t be transferred to the steel and is finally dissipated into the atmosphere. 
     Steel mills are major consumers of electrical energy. Most of the power plants in the world use fossil fuels that generate CO2 emissions. Therefore, it is important to reduce the electrical energy consumption to minimize CO2 emissions. 
     In some furnaces, a recuperator is included in the stack in order to heat the combustion air for the fuel or gas that is used to produce the heat required by the process. In other cases, the heat is used to heat water that is later used to heat buildings. A schematic diagram of a typical such system known in the art is presented in  FIG. 1  in which primary exhaust gases  5  from furnace  10  are fed into a recuperator  15  through which incoming air or liquids  20  are cycled so as to transfer and capture heat energy in output air or liquids  25 . The remaining secondary exhaust gases  30  are disposed of through stack  35 . Even though these systems normally recover a significant amount of heat, some significant portion of the heat is still wasted by releasing hot gases to the atmosphere. The temperature of these exhaust gases remain high enough to warrant efforts to transform that heat energy into electrical energy. 
     SUMMARY OF THE INVENTION 
     The invention relates to a system and method for recovering otherwise wasted energy generated in the form of waste gases as a byproduct of an industrial process. Waste gases are produced by a fuel-powered device and these gases are expelled into an exhaust structure. At least a part of these waste gases are diverted into a gas input of a heat exchanger which also includes a gas output, a heat source liquid input and a heat source liquid output. The input of an evaporator of an organic Rankine cycle (ORC) system is connected to the heat source liquid output of the heat exchanger while the output of the ORC is connected to the heat source liquid input of the heat exchanger. The amount of waste gases circulated through the heat exchanger and back into the exhaust structure through the gas output in the gas circuit is regulated by an exhaust fan connected to a first electric motor controlled by a first variable frequency drive (VFD). The amount of heat source liquid circulated through the heat exchanger and an evaporator in the ORC in a heat source liquid circuit is regulated by a pump connected to a second electric motor and a second variable frequency drive (VFD). The heat source liquid incorporates a pressurized expansion tank. A first controller which incorporates a Proportional-Integral regulator monitors the operation of both the gas circuit and the heat source liquid circuit and regulates the amount of gas and liquid, respectively, circulating through each circuit. A second controller connected to the fuel-powered device provides data to the first controller on the fuel consumption rate of the fuel-powered device. The fuel consumption data is used by the first controller to regulate gas and heat source liquid flows. An expander in the ORC is connected to a generator in the ORC and produces electricity which is measured by a transducer. 
     The invention also relates to a method for regulating the generation of electrical power from heated waste gases emitted from a fuel-powered industrial device using the system described above. The optimum target temperature for the heat source liquid is calculated based on a function having as input variables the temperature feedback of heated gases entering the gas input of the heat exchanger and the device fuel consumption as indicated by the second controller, if such data is available, added to the heat source liquid initial target temperature. Then, a desired speed feed forward command for the first VFD is further calculated based on a function having as input variables the optimal heat source liquid temperature and a target speed reference for the second variable frequency drive. Yet a further calculation is then made of a speed adjustment for the exhaust gases fan based on the measured temperature of the heat source liquid and the proportional and integral gains of the Proportional-Integral regulator incorporated into the first controller. The target speed of the first VFD is then set along with its calculated maximum allowable speed. If the fan speed target exceeds the maximum allowable speed, it is clamped to the maximum allowable speed. Next, the target speed for the second VFD is subsequently calculated based on a function having as input variables the heat source liquid target temperature at the heat source liquid outlet of the heat exchanger and the temperature of the ORC system cooling fluid based on a feedback signal from a temperature sensor. The maximum allowable speed for the second VFD is determined based on a function having as an input variable the power output of the ORC as measured by the transducer. If the maximum allowable speed for the second VFD is exceeded, its target speed is clamped to the allowed level. Until the fuel-powered device is shut down, the method returns to the point where a determination is made whether fuel consumption data is available. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of the invention with reference to the drawings, in which 
         FIG. 1  is a schematic diagram of a gas heat recuperator system known in the art. 
         FIG. 2  is a schematic diagram showing the main elements of an industrial energy recovery system. 
         FIG. 3  is a block diagram of the method used to implement the industrial energy recovery system of this invention. 
         FIG. 4  is a schematic diagram of an alternative arrangement showing the main elements of an industrial energy recovery system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates in schematic diagram form the functional elements of the system of this invention. The same elements are present as shown in  FIG. 1  but, in addition, furnace controller  40  is required to monitor the operation of furnace  10  and to provide data concerning furnace fuel consumption to controller  90 , as discussed below. Tap  45  is added to divert at least a portion of the secondary exhaust gases  30  prior to their evacuation through an exhaust structure such as stack  35  into a tertiary exhaust gas stream  50 . Tap  45  feeds tertiary exhaust gas stream  50  into first heat exchanger  55 . This heat exchanger is designed based on the temperature range of the exhaust gases, the acceptable temperature range for the heat source liquid, the amount of heat to be transferred to the heat source liquid and the acceptable pressure drop on both circuits that will provide an economic solution based on the cost of the heat exchanger and the energy to be consumed by exhaust gases fan and the heat source liquid circulating pump. The material of the heat exchanger has to be suitable for the chemical composition of the exhaust gases. Tertiary exhaust gas stream  50  is circulated through first heat exchanger  55  by using exhaust gases fan  60  which is driven by first electric motor  65  controlled by first variable frequency drive (VFD)  70 . Exhaust gases fan  60  is sized to overcome the pressure drop introduced by first heat exchanger  55  under the maximum capacity (maximum flow) of the system and for the suction of the exhaust gases from stack  35 . In case of a shut down, exhaust gases fan  60  is stopped so that the gases stop circulating through first heat exchanger  55 . The heat source medium used by heat exchanger  55  is a liquid, such as water, water and glycol mix, thermal oil or equivalent, since these types of fluids have a larger thermal capacity than exhaust gases and allow efficient transfer of heat to Organic Rankine Cycle (ORC) system  130  within its acceptable working temperature range. First temperature sensor and transmitter  75  is located at the input of tertiary exhaust gas stream  50  into first heat exchanger  55  and measures the temperature of entering hot gases. Second temperature sensor and transmitter  80  monitors the temperature of liquid exiting first heat exchanger  55 . The temperature data measured by the two sensors is transmitted to controller  90  which may be a commercially available programmable logic controller (PLC) or similar device and is used to regulate the temperature and flow of the heat source liquid by changing the speed target of first VFD  70  which controls first electric motor  65 . 
     The heat source liquid circuit incorporates heat source liquid circulating pump  95  which maintains the proper flow of liquid through ORC system  130  and may be of either a fixed or variable speed type. Second electric motor  100 , which may be either a fixed or variable speed electric motor, is coupled to liquid circulating pump  95  and is controlled by second VFD  105  in the case of a variable speed pump. Second VFD  105  is, in turn, regulated by controller  90 . This system is properly sized to overcome the maximum pressure drop expected under the maximum possible flow of the heat source fluid. The heat source liquid circuit incorporates heat source liquid expansion tank  115  which is pressurized with inert gas  120  such as is typically available at a steel mill in which this invention may be used and includes pressure relief valve  110  connected to the expansion tank  115 . Third sensor  125  is a pressure sensor located in the high temperature side of the heat source circuit and functions to monitor the pressure. Evaporator  132 , which is part of ORC system  130 , completes the heat source liquid circuit. 
     Steel plants typically have a plant water supply kept at a controlled temperature for cooling purposes. Part of this water supply  160  can be diverted and incorporated into heat sink circuit  134  which is part of ORC  130 . In the event that a variable speed heat source liquid circulation pump  95  is used rather than a fixed speed one, additional temperature sensor and transmitter, such as fourth sensor  165 , is required to measure the temperature of the cooling medium. This temperature is required to calculate a reference for second VFD  105  to regulate the speed of heat source liquid circulating pump  95 . This additional sensor can be included as a part of the ORC system or added externally. Based on the values of this temperature variable and the heat source liquid target temperature, controller  90  modifies the pump speed reference in order to maintain the maximum possible output power and efficiency of the system. When the temperature of the ORC system  130  cooling media and/or the target temperature for the heat source liquid changes, the system will modify the flow of the heat source liquid in an attempt to maintain the power generated and the ORC efficiency at the maximum possible values. 
     ORC system  130  used in this invention can be any one of several presently commercially available ORC systems. Expander  135  of such a system is coupled to generator  140  which is itself connected to the steel mill plant electrical distribution system through properly sized electrical feeder  145  and corresponding circuit breaker  150 . The electrical power output of ORC system  130  is monitored by electrical active power transducer  155  and the resulting data is transmitted to controller  90 . The purpose of power transducer  155  is to function as a protective device. Different protection levels can be set. For example, in case of excessive power being generated by the system, controller  90  can be programmed to reduce the speed of exhaust gases fan  60  in order to reduce the heat transferred or to stop the operation of exhaust gases fan  60  completely under pre-designated circumstances. Some commercially available ORC systems also incorporate a by-pass valve for the heat source fluid as a protection. In the event an upstream electrical interruption occurs, such as through tripping of a circuit breaker, and generator  140  is disconnected from the distribution network, protection would also be required. In this case, active power transducer  155  will indicate zero power and a stop exhaust gases fan  60  sequence will also be initiated. If the liquid pressure exceeds a predetermined certain value, detected by third sensor  125 , the target reference of first VFD  70  for exhaust gases fan  60  will be reduced as a measure to slow down the heat transfer that could be contributing to high pressure. In the event of sensing of a predetermined greatly excessive pressure, pressure relief valve  110  will actuate and the corresponding signal will be used to shut down the system, by reducing the speed target of first VFD  70  for exhaust gases fan  60  to zero. 
       FIG. 3  is a block diagram of the method used to implement an industrial energy recovery system. The system uses software code stored in controller  90  to calculate speed targets of first VFD  70  for exhaust gases fan  60  and of second VFD  105  for heat source liquid circulating pump  95  which will maximize the generated power and maintain the process temperatures and flows within the design parameters of the components of the system. The temperature of the exhaust gases and the corresponding flow are a direct result of the fuel consumption of furnace  10 . When the furnace changes from idle to full load operation or vice versa, there is a time delay before the temperature of the exhaust gases reaches the steady state temperature. This information is included in the model that calculates the temperature target T* for the heat source liquid. This temperature target T*, the corresponding temperature feedback of the heat source liquid Tho obtained from temperature sensor  80  located at the outlet of heat exchanger  55  and the flow of the heat source liquid, which is calculated from the speed target n* of second VFD  105  for heat source liquid circulating pump  95 , are used to calculate the speed target of first VFD  70  for exhaust gases fan  60 . Furnace controller  40  can provide furnace fuel consumption data, Fuel_C. If so, that data is retrieved and transmitted to controller  90  at  305 . A determination is made at  300  whether furnace  10  has been operating for a sufficiently long period of time. This data along with the initial target temperature T 1 * (a parameter stored in controller  90 ) are used to calculate the optimum heat source liquid target temperature at  310  using the formula T*=K 0 (Thg,Fuel_C)+T 1 * in which T* is the optimum target temperature for the heat source liquid, K 0 (Thg.Fuel_C) is an interpolation block or a function having as input variables the temperature feedback of hot gases entering the system (Thg.) and the furnace fuel consumption (Fuel_C) which may or may not be available, and T 1  * is the heat source liquid initial target temperature T 1 * stored as a parameter in controller  90 . As the temperature of the gases or fuel consumption rise, KO will assume higher values until it reaches a preset limit. If either the furnace fuel consumption data or temperature feedback of hot gases entering the system (Thg) or both are not available, K 0  will be simplified accordingly. If furnace  10  fuel consumption data is not available, then the optimal temperature T* is calculated at  320  based on the formula T*=K 0 (Thg)+T 1 *. Using the calculated optimal temperature, T*, the desired speed feed forward command of first VFD  70  for exhaust gases fan  60  is further calculated at  325  using the formula F*ff=K 1 (T*,n*) where F*ff is the exhaust gases fan speed expressed as a feed-forward command and K 1 (T*,n*) is obtained from an interpolation block or function having as input variables the calculated optimal heat source liquid temperature, T* and the target speed reference n* for second VFD  105  of heat source liquid circulating pump  95 . The amount of the speed adjustment is calculated at  335  according to the formula F*c=(Kp+Ki/s).(T*−Tho) where F*c is the exhaust gases fan speed target compensation, Tho is the heat source liquid temperature as measured by second sensor  80  of liquid leaving heat exchanger  55  and Kp and Ki are the proportional and integral gains of the exhaust gases fan speed regulator which correspond to a typical proportional and integral (PI) regulator although other types of regulators may also be used for this purpose. The term 1/s is an operator known in the art that corresponds to an integrator and is derived from applying the Laplace transformation to the solution of differential equations. After the compensation F*c is calculated, the target speed F* of first VFD  70  for exhaust gases fan  60  is set at  340  according to the formula F*=F*ff+F*c, and the maximum allowable speed F*max of first VFD  70  for exhaust gases fan  60  is calculated according to the formula F*max=K 2 (T*,P,Pr) where F*max is the maximum allowable fan speed reference of the VFD  70  and K 2 (T*,P,Pr) is an interpolation block or function having as input variables the heat source liquid target temperature T*, the output power feedback of the ORC system in kilowatts P as measured by transducer  155  and a feedback signal from third sensor  125  representing the pressure Pr of the heat source liquid. The function K 2  can be simplified in case the P or Pr variables are not available. It is desirable to know F*max in order to avoid running the exhaust gases fan at an excessive speed and to prevent excessive heat source liquid pressure in the system. A comparison of F* with F*max at  350  establishes whether the exhaust fan speed target is too high. If so, the exhaust gas fan speed target is adjusted at  355  so that F*=F*max. Afterwards, processing continues at  360  where the speed target n* of second VFD  105  for heat source liquid circulating pump  95  is calculated according to the formula n*=K 3 (T*,Tc)+n 1 * where K 3 (T*,Tc) is obtained from an interpolation block or a function based on the input variables T*, for heat source liquid target temperature at the outlet of the heat exchanger as calculated at  310  and Tc for the temperature of the ORC system cooling fluid based on a feedback signal from fourth sensor  165  and where n 1 * is the base speed target of second VFD  105  for heat source liquid circulating pump  95 . When the ORC cooling fluid temperature and/or the target temperature of the liquid heat source liquid change, K 3  will change in order to maintain the power generated and the efficiency of the ORC system  130  at the maximum possible values. The maximum allowed speed target of second VFD  105  for heat source liquid circulating pump  95  is calculated at  365  according to the formula n*max=K 4 (P) where K 4 (P) is an interpolation block or a function for which the only input variable is the output power of ORC  130  as measured at transducer  155 . If the target pump speed n* exceeds the maximum permissible pump speed n*max as determined at  370 , a limit is imposed on the speed target of second VFD  105  for circulating pump  95  at  375  to reduce that speed. This method represents a control loop which is in constant use when the furnace is running. 
     In  FIG. 4 , an alternative arrangement of the functional elements of the system of this invention is presented in a schematic diagram form. In this arrangement, exhaust gasses fan  60 , first electric motor  65  and first variable frequency drive  70  are eliminated. Instead, valve  170  is incorporated at exhaust gas tap  45  where a portion of the exhaust gases exiting recuperator  15  are first diverted into the energy recovery system, heat exchanger  55 . Valve  170  is regulated by controller  90  so as to change the flow of exhaust gases into the energy recovery system in a manner similar to that described above for providing a fan speed target of VFD  70  for exhaust gases fan  60 . 
     The foregoing invention has been described in terms of a preferred embodiment. However, it will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed apparatus and method without departing from the scope or spirit of the invention and that this invention has applicability to many other industrial processes besides steel manufacturing in which hot exhaust gases are produced, such as, for example, cement plants and power generation. The specification and examples are exemplary only, while the true scope of the invention is defined by the following claims.