Patent Application
20150330622
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.
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.
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.
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 (NOx, e.g. NO or NO2) 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.
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.
A fluidized bed boiler is provided wherein oxygen content in a flue gas is lower than the statutory standard (7.2%) during derating operation.
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.
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.
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.
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:
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.
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.
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
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 5A connected between the flue-gas exhaust 4 and the boiler body 1 (
Please refer to
QT=Qa+Qf (1)
QO=Qa×Ca+Qf×Cf (2)
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.
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.
Please refer to
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
In the embodiment, the boiler body 100 further includes, but not limited to, a fuel feeding tank T1, an evaporator E1, an economizer E2, an air preheater E3, a freeboard heat pipe E4, a bed heat pipe E5 and a boiler feedwater pump P1 (for boiler feed water BFW), as shown in
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 B3 and the fluidized gas inlet device B1. The detector 12 is connected to the flue-gas exhausting device B2 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 C1 and C3 to control the fluidized gas inlet device B1 and the flue-gas recirculating device B3, 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.
In the embodiment, the detector 12 is disposed between the flue-gas exhausting device B2 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 B3 and the fluidized gas inlet device B1. 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 B3 to adjust the flue-gas recirculation ratio.
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 B1 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 B2 is an induced draft fan; and the flue-gas recirculating device B3 is a blower with an inverter duty motor whose rotational speed is varied to adjust the flow rate of the recirculating flue gas.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103117378 | May 2014 | TW | national |
