Abstract:
A method for enhancing furnace efficiency of a fluidized bed boiler is provided. The fluidized bed boiler includes a boiler body for carrying out fuel combustion in a fluidized bed thereof; a fluidized gas inlet for inputting an oxygen-containing fluidized gas into the boiler body to fluidize a boiler bed and facilitate the fuel combustion; a steam outlet for outputting a steam resulting from the fuel combustion from the boiler body; and a flue-gas exhaust for emitting a flue gas resulting from the fuel combustion from the boiler body. The method includes steps of: detecting an oxygen concentration of the flue gas; feeding a portion of the flue gas back to the fluidized gas inlet; and dynamically adjusting a flow rate of the feeding fluidized gas according to oxygen concentrations of the flue gas and the feeding fluidized gas to achieve automatic control of the fluidized bed boiler.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates to a boiler, and particularly to a fluidized bed boiler. The present disclosure also relates to a method for enhancing furnace efficiency of the fluidized bed boiler. 
       BACKGROUND OF THE INVENTION 
       [0002]    A boiler is a closed vessel in which water or other fluid is heated by fuel combustion and converted into vapor. By means of the boiler, chemical energy of fuel is transformed into thermal energy for various heating applications. As known, combustion efficiency is determined by 3T—temperature, time and turbulence, which respectively represent temperature in the boiler, retention time of the fuel and turbulence of mixing fuel and air/oxygen. A fluidized bed boiler has the advantage of high turbulence. 
         [0003]    In recent years, bubbling fluidized bed boilers and circulating fluidized bed boilers are the most used fluidized bed boilers. In a bubbling fluidized bed boiler, air jets are provided from the bottom thereof, and heat pipes are placed in the sand bed. In a circulating fluidized bed boiler, sand and fuel from the bed are carried along with an upward air stream and then enter into a cyclone. In the cyclone, the heavier particles separate from the gas and return to the bed for recirculation. In both types of the fluidized bed boilers, gas flows upwards to pass and rush through solid particles so as to achieve efficient mixing. It assists in raising turbulence in the fluidized bed boilers and it is advantageous for increasing the combustion efficiency. 
         [0004]    However, during derating operation, fuel feeding decreases while the gas flow rate remains. In this condition, flue gases exiting to the atmosphere via a flue contain higher oxygen content so that furnace efficiency decreases. Furthermore, nitrogen oxides (NO x , e.g. NO or NO 2 ) in the flue gases after the combustion are hazardous to central nervous system (CNS). For example, chronic inhalation of the hazardous gases may cause cerebral palsy (CP) or limb atrophy. Thus, concentration of nitrogen oxides is an important index for describing air quality. 
         [0005]    To overcome the above-mentioned problem, a fluidized bed boiler with emitted flue gas containing low oxygen content and low nitrogen oxide emission is desired. 
       SUMMARY OF THE INVENTION 
       [0006]    A fluidized bed boiler is provided wherein oxygen content in a flue gas is lower than the statutory standard (7.2%) during derating operation. 
         [0007]    A method for enhancing furnace efficiency of a fluidized bed boiler is provided. The method takes advantage of recirculation of a portion of a flue gas to reduce oxygen content in the flue gas and nitrogen oxide emission. 
         [0008]    An aspect of the present disclosure provides a method for enhancing furnace efficiency of a fluidized bed boiler. The fluidized bed boiler includes a boiler body, a fluidized gas inlet, a steam outlet and a flue-gas exhaust. The boiler body carries out fuel combustion in a fluidized bed of the boiler body. The fluidized gas inlet inputs an oxygen-containing fluidized gas or a feeding fluidized gas into the boiler body to fluidize a boiler bed to form the fluidized bed and facilitate the fuel combustion. The steam outlet outputs a steam resulting from the fuel combustion from the boiler body. The flue-gas exhaust emits a flue gas resulting from the fuel combustion from the boiler body. The method includes steps of: detecting an oxygen concentration of the flue gas; feeding a portion of the flue gas back to the fluidized gas inlet to provide a recirculating flue gas which is mixed with the feeding fluidized gas to form the oxygen-containing fluidized gas; and dynamically adjusting a flow rate of the feeding fluidized gas according to the detected oxygen concentration of the flue gas and an oxygen concentration of the feeding fluidized gas to achieve automatic control of the fluidized bed boiler. 
         [0009]    Another aspect of the present disclosure provides a fluidized bed boiler. It includes a boiler body, a fluidized gas inlet device, a steam outlet device, a flue-gas exhausting device, a flue-gas recirculating device and a detector. The boiler body carries out fuel combustion in a fluidized bed of the boiler body. The fluidized gas inlet device is connected to the boiler body for inputting a feeding fluidized gas into the boiler body to fluidize a boiler bed to form the fluidized bed and facilitate the fuel combustion. The steam outlet device is connected to the boiler body for outputting a steam resulting from the fuel combustion from the boiler body. The flue-gas exhausting device is connected to the boiler body for emitting a flue gas resulting from the fuel combustion from the boiler body. The flue-gas recirculating device is connected between the flue-gas exhausting device and the boiler body for feeding a portion of the flue gas back to the boiler body to provide a recirculating flue gas which is mixed with the feeding fluidized gas to provide an oxygen-containing fluidized gas to the boiler body. The detector is connected to the flue-gas exhausting device for detecting an oxygen concentration of the flue gas to dynamically adjust a flow rate of the feeding fluidized gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The advantages of the present disclosure will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
           [0011]      FIG. 1A  is a functional block diagram illustrating a method for enhancing furnace efficiency of a fluidized bed boiler according to an embodiment of the present invention; 
           [0012]      FIG. 1B  is a functional block diagram illustrating a method for enhancing furnace efficiency of a fluidized bed boiler according to another embodiment of the present invention; 
           [0013]      FIG. 2A  is a functional block diagram illustrating a method for enhancing furnace efficiency of a fluidized bed boiler according to a further embodiment of the present invention; 
           [0014]      FIG. 2B  is a functional block diagram illustrating a method for enhancing furnace efficiency of a fluidized bed boiler according to a further embodiment of the present invention; and 
           [0015]      FIG. 3  is a schematic diagram illustrating a structure of a fluidized bed boiler according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]    The present disclosure will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
         [0017]    In order to overcome the problem of high oxygen content in a flue gas and high nitrogen oxide emission, a portion of the flue gas is fed back for recirculation and mixed with a feeding fluidized gas to maintain a gas flow rate in the fluidized bed boiler. By this way, oxygen supply is lowered so that the oxygen content in the flue gas decreases. In addition, the recirculation of the flue gas also decreases nitrogen oxide emission. 
         [0018]    Concretely, the method for enhancing furnace efficiency of the fluidized bed boiler involves replacing a portion of primary air with the recirculating flue gas. The ratio of the recirculating flue gas to the primary air is controlled by an automatic control system to ensure that a flow rate of the primary air and oxygen supply are in a steady state. The detailed description is given below with reference to  FIG. 1A  and  FIG. 1B . 
         [0019]    A fluidized bed boiler usually includes a boiler body  1 , a fluidized gas inlet  2 , a steam outlet  3  and a flue-gas exhaust  4 . The boiler body  1  carries out fuel combustion in a fluidized bed of the boiler body  1 . The fluidized gas inlet  2  inputs an oxygen-containing fluidized gas or a feeding fluidized gas into the boiler body  1  to fluidize a boiler bed of the boiler body  1  to form the fluidized bed and facilitate the fuel combustion. Steam resulting from the fuel combustion exits the boiler body  1  through the steam outlet  3 . The flue-gas exhaust  4  emits a flue gas resulting from the fuel combustion from the boiler body  1 . According to the present disclosure, a portion of the flue gas exiting through the flue-gas exhaust  4  is fed back to the boiler body  1  for recirculation. In an embodiment, the recirculation of the flue gas is implemented by a recirculating duct  5 A connected between the flue-gas exhaust  4  and the boiler body  1  ( FIG. 1A ). In another embodiment, the recirculation of the flue gas is implanted by a recirculating duct  5 B connected between the flue-gas exhaust  4  and the fluidized gas inlet  2  ( FIG. 1B ). The recirculating flue gas is combined/mixed with the feeding fluidized gas to form the oxygen-containing fluidized gas to be provided to the boiler body  1  to fluidize the fluidized bed and facilitate the fuel combustion. Therefore, the flow rate of the feeding fluidized gas decreases, and it leads to reduction of the oxygen content in the flue gas and the nitrogen oxide emission. Nevertheless, amount of the outputted steam is still maintained so that overall furnace efficiency is improved. 
         [0020]    Please refer to  FIG. 2A , a functional block diagram illustrating an embodiment of a method for enhancing furnace efficiency of a fluidized bed boiler with automatic control. Flow rates of the recirculating portion of the flue gas and/or the feeding fluidized gas are dynamically adjusted and automatically controlled. Thick arrows in the diagram show flowing directions of respective material in tubes or pipes, while thin arrows show signal transmission of control or detect signals. Compared with the embodiment with reference to  FIG. 1A , a detector  6  and a controller  7  are further provided in this example. The detector  6  is disposed between the flue-gas exhaust  4  and the boiler body  1  and configured to detect oxygen concentration of the exiting flue gas. The controller  7  dynamically adjusts the flow rate of the feeding fluidized gas which is fed into the boiler body  1  through the fluidized gas inlet  2 , and dynamically adjusts the flow rate of the recirculating flue gas which is fed back to the boiler body  1  through the recirculating duct  5 A. The flow rates of the feeding fluidized gas and the recirculating flue gas have relations of: 
         [0000]        QT=Qa+Qf   (1)
 
         [0000]        QO=Qa×Ca+Qf×Cf   (2)
 
         [0000]    wherein QT is an overall flow rate of the oxygen-containing fluidized gas, Qa is the flow rate of the feeding fluidized gas, Qf is the flow rate of the recirculating flue gas, QO is overall oxygen content in the oxygen-containing fluidized gas, Ca is an oxygen concentration of the feeding fluidized gas and Cf is an oxygen concentration of the (recirculating) flue gas. 
         [0021]    In this embodiment, the feeding fluidized gas is air and Ca is a constant or nearly constant value about 0.21. It should be noted that gas mixture with other oxygen content is applicable to the feeding fluidized gas, for example, pure oxygen with Ca=1. For several cases, the overall flow rate of the oxygen-containing fluidized gas QT and the overall oxygen content in the oxygen-containing fluidized gas QO are predetermined constant values or almost constant values. Hence, the flow rate of the feeding fluidized gas Qa and the flow rate of the recirculating flue gas Qf can be calculated from equations (1) and (2) according to the detected oxygen concentration of the flue gas Cf and the oxygen concentration of the feeding fluidized gas Ca. 
         [0022]    Please refer to  FIG. 2B , a functional block diagram illustrating another embodiment of the method for enhancing furnace efficiency of a fluidized bed boiler with automatic control. Compared with the embodiment with reference to  FIG. 1B , a detector  6  and a controller  7  are further provided in this example. As described in the above embodiment, the controller  7  in cooperation with the detector  6  can dynamically adjust the flow rates of the feeding fluidized gas and the recirculating flue gas. 
         [0023]    An example of a fluidized bed boiler structure is given here to realize the above-described embodiments of the methods for enhancing furnace efficiency of the fluidized bed boiler. Please refer to  FIG. 3 , a schematic diagram illustrating a structure of the fluidized bed boiler. The fluidized bed boiler  10  includes: a boiler body  100  configured to carry out fuel combustion in a fluidized bed  101  of the boiler body  100 ; a fluidized gas inlet device B 1  connected to the boiler body  100  and configured to input a feeding fluidized gas into the boiler body  100  to fluidize a boiler bed  101  and facilitate the fuel combustion; a steam outlet device  102  connected to the boiler body  100  through pipes or tubes and configured to output a steam resulting from the fuel combustion from the boiler body  100 ; a flue-gas exhausting device B 2  connected to the boiler body  100  through a pipe  13  and configured to emit a flue gas resulting from the fuel combustion from the boiler body  100  to a flue  103 ; and a flue-gas recirculating device B 3  connected between the flue-gas exhausting device B 2  and the boiler body  100  through a pipe  14  and configured to feed a portion of the flue gas back to the boiler body  100  for recirculation so that the recirculating flue gas is combined and mixed with the feeding fluidized gas to provide the oxygen-containing fluidized gas to be provided for the boiler body  100 . Other portion of the flue gas exits the fluidized bed boiler  10  through the flue  103 . The oxygen-containing fluidized gas may enter the boiler bed  101  through the bottom portion only. Alternatively, the oxygen-containing fluidized gas is divided into a primary input and a secondary input to enter the boiler bed  101  through different entrances. 
         [0024]    In the embodiment, the boiler body  100  further includes, but not limited to, a fuel feeding tank T 1 , an evaporator E 1 , an economizer E 2 , an air preheater E 3 , a freeboard heat pipe E 4 , a bed heat pipe E 5  and a boiler feedwater pump P 1  (for boiler feed water BFW), as shown in  FIG. 3 . These elements are known for persons skilled in the arts and detailed description related to the elements is not given here. In addition, a lime/limestone feeding tank T 2  is provided for carry out flue-gas desulfurization in the fluidized bed. It should be noted that sulfur dioxide (SO 2 ) may be removed from the flue gases by a variety of methods, and the flue-gas desulfurization is not limited as mentioned in the embodiment. Furthermore, a baghouse  15  may be provided for filtering out particles in the flue gas in advance. 
         [0025]    In the embodiment, the fluidized bed boiler  10  further includes a controller  11  and a detector  12 . The controller  11  is in communication with the detector  12 , the flue-gas recirculating device B 3  and the fluidized gas inlet device B 1 . The detector  12  is connected to the flue-gas exhausting device B 2  or the flue  103 . The detector  12  detects the oxygen concentration Cf of the flue gas and issues a detect signal D to the controller  11 . The controller  11  calculates the optimal flow rates of the feeding fluidized gas Qa and the recirculating flue gas Qf in response to the detect signal D based on the equations (1) and (2). Then, the controller  11  issues control signals C 1  and C 3  to control the fluidized gas inlet device B 1  and the flue-gas recirculating device B 3 , respectively to adjust the flow rate of the feeding fluidized gas Qa and the flow rate of the recirculating flue gas Qf. Thus, the feeding fluidized gas and the recirculating flue gas with adjusted flow rates Qa and Qf are continuously fed to the boiler body  100 . 
         [0026]    In the embodiment, the detector  12  is disposed between the flue-gas exhausting device B 2  and the flue  103 . However, the position of the detector  12  can be arranged in view of pipe and/or circuit layout. For example, the detector  12  may be disposed downstream the flue  103  to detect the oxygen concentration of the flue gas Cf. The controller  11  is in communication with the detector  12 , the flue-gas recirculating device B 3  and the fluidized gas inlet device B 1 . On condition that the overall flow rate of the oxygen-containing fluidized gas QT and the overall oxygen content in the oxygen-containing fluidized gas QO are predetermined values with or without slight fluctuations, the controller  11  dynamically adjusts the flow rate of the feeding fluidized gas Qa and the flow rate of the recirculating flue gas Qf. It should be noted that the control basis is not limited to the above description. For example, on condition that the ratio of the recirculating flue gas to the overall flue gas is fixed, the controller  11  needs not be in communication with the flue-gas recirculating device B 3  to adjust the flue-gas recirculation ratio. 
         [0027]    An example is given that the detector  12  is an oxygen analyzer; the controller  11  is a computer system or a microcontroller; the fluidized gas inlet device B 1  is a blower with an inverter duty motor whose rotational speed is varied to adjust the air flow rate; the feeding fluidized gas is air; the steam outlet device  102  is a steam drum; the flue-gas exhausting device B 2  is an induced draft fan; and the flue-gas recirculating device B 3  is a blower with an inverter duty motor whose rotational speed is varied to adjust the flow rate of the recirculating flue gas. 
         [0028]    The present disclosure takes advantage of flue-gas recirculation to lower oxygen content in the flue gas and decrease nitrogen oxide emission. In particular, the oxygen content in the flue gas during the derating operation is lower than the statutory standard (7.2%) by using the fluidized bed boiler of the present disclosure which is advantageous over conventional boilers. The present disclosure may be implemented by modifying pipe layout in the present factory for supporting recirculation of the flue gas and disposing the detector at a proper position to dynamically adjust the flow rates of the recirculating flue gas and the feeding fluidized gas (or other gas such as primary air) to achieve automatic control. Thus, the furnace efficiency of the fluidized bed boiler is enhanced in a much convenient manner. 
         [0029]    While the disclosure has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.