Heat exchange – With adjustor for heat – or exchange material – flow
Reexamination Certificate
1997-12-22
2002-01-08
Atkinson, Christopher (Department: 3743)
Heat exchange
With adjustor for heat, or exchange material, flow
C165S104140, C422S145000, C422S146000, C422S147000, C122S00400R
Reexamination Certificate
active
06336500
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of and an apparatus for controlling heat transfer in a fluidized bed reactor, according to the preambles of appended independent claims.
The present invention particularly relates to a method and an apparatus for recovering heat from solid particles in a fluidized bed reactor comprising a processing chamber, having a fluidized bed of solid particles therein, and a heat transfer chamber, being in solid particle communication with the processing chamber and having heat transfer surfaces disposed therein. The heat transfer chamber may be connected in various ways to the processing chamber so that there is solid particle exchange between the chambers. The heat transfer chamber may in some special case even be formed within the processing chamber itself.
The present invention relates to a method and apparatus applicable in atmospheric, as well as, pressurized fluidized bed reactor systems.
BACKGROUND OF THE INVENTION
Fluidized bed reactors, such as circulating fluidized bed reactors, are used in a variety of different combustion, heat transfer, chemical or metallurgical processes. Typically heat, originating from combustion or other exothermic processes, is recovered from the solid particles of the fluidized bed by using heat transfer surfaces. Heat transfer surfaces conduct the recovered heat to a medium, such as water or steam, which transports the heat from the reactor.
Said heat transfer surfaces are usually located in the. processing chamber or within a convection section arranged in the gas pass after the processing chamber or, in circulating fluidized bed reactors, within a particle separator. Additional heat transfer surfaces may be arranged in separate heat transfer chambers (HTC), which may be a part of the processing chamber, a separate chamber adjacent to the processing chamber or, in circulating fluidized bed reactors, part of the solid particles recycling system.
In a heat transfer chamber (HTC), heat is typically recovered by continuously introducing hot solid particles from e.g. the processing chamber into the HTC, recovering heat from said solid particles in the HTC, and continuously discharging said solid particles from the HTC into the processing chamber. Said heat recovery takes place by using heat transfer surfaces disposed in the HTC.
The HTC thereby comprises inlet means for introducing a continuous flow of hot solid particles from the processing chamber into the HTC, heat transfer surfaces and means for transporting the heat recovered from the hot solid particles out from the HTC, and outlet means for continuously recycling solid particles discharged from the HTC into the processing chamber.
Accurate and fast controllability of the heat transfer is an important consideration in many applications of fluidized bed reactors, such as where maintaining constant steam temperature may require rapid and accurate adjustments of heat transfer. The reason for the need of controlling action may be a changing demand of the produced steam or abnormality in the fuel quality or fuel feed or some other abnormality in the system. Also there may be a need to adjust the system to proper operating state. In steam boilers, additional requirements to adjust the heat transfer arise from the fact that heat is usually recovered in many stages, i.e. in evaporators, superheaters, economizers and reheaters, which may need independent control,
From the point of view of the processes in a fluidized bed reactor, the aim of the heat transfer control is to maintain optimum performance, especially taking into account the harmful emissions or combustion efficiency. Usually, this implies that the temperature of the reactor should stay constant, even in conditions of varying heat recovery and fuel input rates.
In circulating fluidized bed reactors, the rate of heat recovery in the upper parts of the furnace can be varied by changing the bed density. This can be realized by collecting part of the bed material to a storage, as shown in U.S. Pat. No. 4,823,739 or, more simply and quickly, by changing the fluidizing gas velocity. However, the fluidizing gas is an important factor in reactions taking place in the processing chamber of the circulating fluidized bed reactor. To maintain an economically and ecologically favorable operation, changes in the fluidizing gas require other simultaneous changes, such as changes in the fuel feed rate. Thus, this method of heat transfer control effects all heat transfer surfaces of the system and can be favorably put into effect only in the time scale of the thermal time constant of the whole system.
Due to the large heat capacitances involved, the thermal time constant of a fluidized bed reactor, i.e. the time when, after a step-wise stimulus, approximately two thirds of its temperature change has taken place, can be very long, e.g. 25 minutes. Thus, the heat transfer from a fluidized bed based on heat transfer surfaces having an invariable thermal contact to the bed is not fast enough for many applications of fluidized bed reactors.
To render possible a fast control of the heat transfer from fluidized bed reactors, with time constant of e.g. some tens of seconds, different constructions utilizing separate HTCs have been developed. Because also in a HTC the temperature of the solid particles can vary only slowly, different techniques have been developed, which do not depend on varying the temperature of said solid particles to control the heat transfer.
The simplest means for such control is to vary the amount of hot material in contact with the heat transfer surfaces in the HTC so that only a variable part of the heat transfer surfaces are covered by the solid particles. This kind of construction was disclosed e.g. in U.S. Pat. No. 4,813,479. However, to control the level of solid particles at least one additional flow duct and a controlling valve is needed, which increases the complexity and costs of the system.
Another approach, which has been used in circulating fluid bed reactors, is to divide the flow of hot solid particles after the particle separator to two channels, of which only one has heat transfer surfaces. Thus, when varying the division ratio of solid particles flowing through said two channels, the rate of heat transfer is varied. In order to function properly, this technique also requires a rather complicated construction, such as that disclosed in U.S. Pat. No. 5,140,950, in which many compartments and channels are used.
HTCs are normally bubbling fluidized beds with low gas flow velocities, e.g. from 0.1 to 0.5 m/s. The transport of solid particles through a HTC or through its different channels can be controlled by mechanical valves or by varying the fluidizing gas velocity, and thereby the bed height, in different portions of the HTC.
It is known that the heat transfer coefficient of a heat transfer surface in a fluidized bed can to some extent be varied by changing the velocity of fluidizing gas flow. (The heat transfer coefficient refers to the amount of thermal energy transferred across one square meter of the heat transfer surface per one degree temperature difference between the bed and the medium transporting the heat away.) This is due to the fact that with higher velocities of fluidizing gas flow, the solid particle movements are more intense and give a more uniform temperature distribution in the fluid bed, and, thus, the heat transfer on the heat transfer surfaces is enhanced.
Because, in typical HTC constructions, the gas flow velocities are related to the particle flow, they cannot be independently varied. U.S. Pat. No. 5,425,412 discloses an arrangement in the return duct of a circulating fluidized bed reactor, where the HTC contains a separate heat transfer section where the gas flow velocity can be varied independently from the particle flow. Moreover, U.S. Pat. No. 5,406,914 discloses another arrangement with a separate heat transfer section which has also an additional passage for particles directly from the processing chamber to the HTC. With a similar princ
Atkinson Christopher
Foster Wheeler Energia Oy
LandOfFree
Method and apparatus for controlling heat transfer from... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for controlling heat transfer from..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for controlling heat transfer from... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2873235