Fluidized bed reactors, particularly circulating fluidized bed reactors, are extremely useful in practicing a wide variety of reactions, such as combustion and gasification of fuel material. Gasification is an attractive way to convert energy of fuel material into a more useful form, producing combustible gas; or combustion of the fuel in the reactor may produce steam to drive a steam turbine. However under many circumstances, the gas is discharged from the reactor (e.g. fuel product gas) may contain undesirable substances such as tar-like condensable compounds. These substances tend to turn sticky below certain temperatures, and therefore deposit or accumulate on surrounding surfaces, particular surfaces of gas cooling devices.
Gasification of solid fuel material in fluidized bed has been discussed in U.S. Pat. Nos. 4,017,272 and 4,057,402. The gasification process is generally stated to be characterized by following reactions:
______________________________________ C + O.sub.2 = CO.sub.2 (exothermic) C + 0.5 O.sub.2 = CO (exothermic) C + CO.sub.2 = 2 CO (endothermic) C + H.sub.2 O = CO + H.sub.2 (endothermic) C + 2 H.sub.2 O = CO.sub.2 + 2H.sub.2 (endothermic) C + 2 H.sub.2 = CH.sub.4 (exothermic) ______________________________________
The product gas usually contains substances which cause difficulties when the gas is cooled, i.e. compounds (e.g. tars) which turn sticky when cooled. The existence of these cause problems in cooling the product gas by depositing or accumulating on heat transfer surfaces of gas cooler. The problem of fouling of gas cooling surfaces has been addressed by using a direct heat transfer system, such as in U.S. Pat. Nos. 4,412,848 and 4,936,872. In these patents the product gas is led into fluidized bed gas cooler, and the fouling components are captured by particles of the fluidized bed.
The use of a separate fluidized bed--as described above--is hardly an ideal solution to the problem, however, since the additional bed consumes space and requires construction and maintenance of different components, which can make costs prohibitive. Using indirect recuperator heat exchangers has also been found unacceptable, however, due to exhaust fouling difficulties. The fouling problem described above is particularly acute under pressurized conditions, e.g. superatmospheric pressure of about 2-50 bars. Under such pressurized conditions conventional steam soot blowers do not work properly. The problems as indicated above do not exist solely during gasification, but also during combustion of a number of different types of fuel in a fluidized bed. For example when brown coal is burned the flue gases contain alkali species which condense on cooling surfaces, accumulating on the surfaces, fouling them, and causing corrosion of surrounding surfaces. Difficulties also occur particularly in the combustion of municipal waste or sludge.
According to the present invention a method and system are provided which overcome the problem of gas particles depositing on (and thereby fouling and perhaps corroding) gas cooling surfaces. The invention solves this problem in a simple yet effective manner. The basic concept behind the invention is to utilize the very same solids which are used as bed material (e.g. inert bed material such as sand and/or reactive bed material such as limestone) to mechanically scrub the gas cooler cooling surfaces so as to prevent accumulation of deposits, and/or remove deposits, therefrom. The invention is applicable to all types of fluidized bed reactors and reactor systems, but is particularly applicable to circulating fluidized bed reactors, and to pressurized systems (that is operating at a pressure of about 2-50 bar, preferably, 2-30 bar).
According to one aspect of the present invention, a method of operating a fluidized bed reactor system comprising a fluidized bed reactor containing solid material particles and for reacting fuel, and a reactor outlet for gas produced during fuel reaction (combustion, gasification, etc.), and a gas cooler having cooling surfaces and connected to the reactor outlet. The method comprises the steps off: (a) Introducing solid material particles, fluidization medium, and fuel into the reactor to provide a fluidized bed therewithin. (b) Reacting the fuel material within the bed to produce exhaust gas and discharging the gas from the reactor outlet. (c) Cooling the gas from the reactor outlet in the gas cooler. (d) Cleaning the cooling surfaces of the gas cooler by introducing a sufficient concentration and size of bed particles into the gas during, or before, step (c), so that the particles mechanically dislodge deposits from, and thereby clean, the cooling surfaces. And, (e) removing the particles from the gas after step (d).
Step (d) is preferably practiced only at spaced time intervals (e.g. intermittently or periodically, or in response to sensing of a decrease in cooling efficiency), but may be practiced continuously. Step (d) is typically practiced by introducing particles separated in step (e) into the gas just before the gas cooler, and/or by introducing so particles from a bed particle supply into the gas just before the gas cooler. Alternatively, or additionally, where the reactor is a CFB reactor having a particle separator between the reactor gas outlet and the gas cooler which normally operates at a first efficiency (which does not allow passage of a sufficient number or size of bed particles therethrough to effect gas cooler cleaning) step (d) is practiced by interrupting operation of the particle separator so that it operates at significantly less than the first efficiency, so that a sufficient number and size of bed particles pass therethrough to effect cleaning of the gas cooler surfaces. This may be done--where the particle separator is a cyclone separator which produces a vortex--by introducing a fluid stream into the vortex to disrupt the vortex flow and thereby reduce separation efficiency, or by introducing a solid object into the vortex, having the same affect. The stream may be steam, air, inert gas, or a liquid. Typically step (b) is practiced to produce gas at a temperature above 600 degrees C., and step (c) is practiced to cool the gas to about 400 degrees C.
According to another aspect of the present invention a circulating fluidized bed reactor system is provided, comprising the following elements: A fluidized bed rector having a bed material inlet, an exhaust gas outlet, and a fluidizing gas inlet. A cyclone separator normally operating at a first efficiency and connected to the reactor gas outlet, and having a gas outlet, and a particle outlet for returning separated bed material to the reactor. A gas cooler connected to the separator gas outlet and having cooling surfaces. And, means for affecting the operating efficiency of the separator so that it is significantly less than the first efficiency so that sufficient bed particles pass through the separator to effect mechanical dislodgement of deposits which form on the cooling surfaces.
The system preferably further comprises pressure vessels surrounding the reactor, separator, and cooler for maintaining them at superatmospheric pressure (e.g. 2-50 bar), and a second separator is preferably provided downstream of the gas cooler for separating bed particles from gas discharged from the cooler, and at spaced time intervals introducing at least some of the separated bed particles into the gas at or just before the cooler.
The means for affecting operating efficiency of the cyclone separator may be means for introducing a fluid stream into the cyclone separator vortex, or for introducing and removing a solid object.
According to yet another aspect of the present invention a circulating fluidized bed reactor system is provided comprising the following elements: A fluidized bed rector having a bed material inlet, an exhaust gas outlet, and a fluidizing gas inlet. A cyclone separator connected to the reactor gas outlet, having a gas outlet, and a particle outlet for returning separated bed material to the reactor. A gas cooler connected to the separator gas outlet and having cooling surfaces. A second cyclone separator downstream of the gas cooler for separating bed particles from gas discharged from the cooler, and including a particle discharge. And, a conduit extending from the second cyclone particle discharge to the gas cooler or to between the first separator and the gas cooler.
It is the primary object of the present invention to avoid the problem of gas cooler surface fouling in fluidized bed reactors in a simple yet effective manner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.