Gas: heating and illuminating – Processes – Fuel mixtures
Reexamination Certificate
1998-07-24
2001-04-10
Beck, Shrive (Department: 1764)
Gas: heating and illuminating
Processes
Fuel mixtures
C422S145000, C422S147000
Reexamination Certificate
active
06214065
ABSTRACT:
The present invention refers to a method and system of operating a fluidized bed reactor system.
Fluidized bed reactors, particularly circulating fluidized bed (CFB) reactors, are extremely useful in practicing a wide variety of reactions, such as combustion and gasification of fuel material, in atmospheric or pressurized conditions. Gasification in a fluidized bed reactor is an attractive way to convert energy of fuel material into a more useful form, producing combustible gas. Combustion of fuel in a fluidized bed reactor may produce steam to drive a steam turbine. However, under many circumstances, the gas discharged from the reactor (e.g., fuel product gas) may contain undesirable substances such as extremely fine dust and tar-like condensable compounds. These substances tend to turn sticky especially below certain temperatures, and therefore deposit or accumulate on surrounding surfaces, in particular surfaces of gas cooling devices, having an adverse effect on the surfaces and heat transfer.
When the hot gas coming from the gasification/combustion reactor is introduced into a gas, the cooler above mentioned undesirable substances easily block the inlet of the gas cooler or heat transfer surfaces disposed therein. Especially, very fine carbon (soot), fine ash particles, alkali fumes, alkali oxides or liquid compounds tend to form deposits in the gas cooler.
In the gasification processes, the gas has to be cleaned before further use. The carbon particles (soot) contained in the gas is very fine, has typically a grain size of 0.1-5 &mgr;m, and is sticky. Such sticky fine material is difficult to separate by filtration. The gas can be filtrated by adding into the gas coarser non-sticky particles, having a grain size distribution of -200 &mgr;m. Those particles together with fine soot are able to form a filter cake on filter elements. Filtration properties will be further improved if the added particles are porous.
The problem of fouling of gas cooling surfaces has been addressed by using a direct heat transfer system, such as suggested in U.S. Pat. Nos. 4,412,848 and 4,936,872. In these patents, product gas is led into fluidized bed gas coolers, 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 to be 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.
It is, therefore, the primary object of the present invention to provide a method and system which minimize the problem of gas particles depositing on gas cooling surfaces.
It is also the object of the present invention to provide a method and system which minimize the fouling and corroding of cooling surfaces.
It is further the object of the present invention to provide a method and system which improve heat transfer from gas containing very fine particles or tar-like condensable compounds.
The above mentioned objects are achieved in accordance with the present invention by a method and system including the features recited in the pending claims.
The basic concept behind the invention thereby 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) in fluidized bed reactors to mechanically scrub the gas cooler's cooling surfaces so as to prevent accumulation of deposits thereon, and or to remove deposits therefrom.
It has earlier been suggested in applicant's co-pending patent application PCT/FI95/00438 that bed material is introduced from a separate bed material supply source into the gas cooler for cleaning the cooling surfaces. Alternatively, in circulating fluidized bed reactors where the main part of the solid bed material is separated from gases exhausted from the reactor chamber in a separator (e.g., a cyclone separator or similar device) before introducing the thus cleaned gas into the gas cooler, it was suggested to periodically decrease the efficiency of the separator (cyclone) and allow non-separated particles to flow with the gas into the gas cooler.
The present invention also solves the above mentioned problems of particles depositing on gas cooling surfaces, and it does so in a very simple and easily controllable manner. The present invention provides an alternative method to supply easily controlled amounts of bed particles, without the need to transport the particles from distant supplies.
The present invention is also applicable to all types of fluidized bed reactors and reactor systems, and 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 for reacting fuel is provided, said reactor system comprising:
a fluidized bed reactor chamber having a reactor chamber outlet for gas produced during fuel reaction (combustion, gasification, etc.)
a particle separator, such as a cyclone separator, connected to the reactor chamber outlet for separating solid material from gas exhausted from the reactor chamber, said particle separator having a solid particle outlet and a gas outlet, and
a gas cooler having cooling surfaces (heat transfer surfaces) and being connected to the gas outlet of the particle separator.
The method comprises the steps of:
(a) introducing solid material particles, fluidization medium and fuel into the reactor chamber to provide a fluidized bed therewithin;
(b) reacting the fuel material within the fluidized bed to produce exhaust gas and discharging the exhaust gas from the reactor chamber outlet;
(c) introducing the exhaust gas into the particle separator and separating solid particles from the gas in said particle separator;
(d) discharging from the separator a first flow of separated solid particles through the solid particle outlet and gas through the gas outlet and
(e) cooling the gas discharged from the separator in the gas cooler.
The method is characterized by the additional steps of:
(f) branching off from the first flow of solid particles, before or after discharging said first flow of solid particles from the particle separator, a second flow of solid particles;
(g) introducing said second flow of particles into the gas discharged from the separator during, or before step (e), so that the particles mechanically dislodge deposits from, and thereby clean, the cooling surfaces, and
(h) removing the particles from the gas after step (g).
Step (f) is practiced to provide a sufficient concentration and size of separated solid particles into the gas for cleaning the cooling surfaces or keeping the cooling surfaces clean.
Steps (f) to (g) are preferably practiced only at spaced intervals (e.g., intermittently or periodically, or in response to sensing of a decrease in cooling efficiency), but may be practiced continuously. Step (g) is typically practiced by introducing particles separated in step (c) into the gas just before the gas cooler.
Typically, step (b) is practiced to produce gas at a temperature above 600° C. and step (e) is practiced to cool the gas to about 40
Beck Shrive
Fitzpatrick ,Cella, Harper & Scinto
Foster Wheeler Energia OY
Varcoe Frederick
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