The present invention relates generally to fluidized bed combustion and, more particularly, to certain new and useful improvements in fluidized bed combustion devices and methods for carrying out fluidized bed combustion.
Fluidized bed combustion is currently one of the most promising technologies for advances in the field of combustion furnaces, boilers, incinerators, etc., particularly in these days of exhorbitant energy costs and restrictive environmental protection regulations. A significant reason that fluidized bed combustion holds such promise is my previous invention described in detail in my U.S. Pat. No. 4,240,377 granted Dec. 23, 1980 the disclosure of which is hereby incorporated by reference herein.
Briefly, fluidized bed combustion permits the use of a wide range of solid, liquid and gaseous fuels, while at the same time reducing pollutants such as nitrous oxide and sulfur dioxide, and providing high combustion and heat exchange efficiencies when compared with conventional combustion techniques. This invention of my '377 patent enables higher combustion and heat exchange efficiencies than previously known fluidized bed combustion devices. Furthermore, it provides several control points to permit immediate variation of steam generating capacity.
Although numerous significant advantages are realized by my invention described in said Pat. No. 4,240,377 ("my '377 patent") over the prior state-of-the-art fluidized bed combustion techniques, there are certain aspects which can be improved to provide even further enhanced performance and improved efficiency. For example, the use of the solids circulation system coupled with the deletion of heat exchange tubes in the combustion bed resulted in much higher controlled heat transfer from the bed to the heat exchange surfaces. However, greater efficiency of combustion and heat exchange will enhance such controlled heat transfer.
In addition, despite the dramatically low production of sulfur dioxide, there is an economic need to minimize the amount of extraneous sulfur dioxide removing agent (e.g., limestone) added to the combustion chamber while still maximizing removal of the sulfur gas. Furthermore, despite the presence of cyclone gas-solids separators and/or various superheater tubes located above the combustion bed, it is important to retard the loss of fine particulate matter with the flue gas. The best way of doing so is by providing additional means for returning these fine particles to the combustion bed.
A significant drawback to conventional fluidized bed combustion devices is that limestone and ash must be removed from the combustion bed to maintain an equilibrium of reactivity within the bed. Such removal is usually done by "purging" the bed--i.e., by a continuous process of physically removing bed constitutents from the bed for disposal. This results in loss of the heat from the combustion zone as well as unreacted fuel and limestone particles. It also presents a significant disposal problem since dumping can contaminate the aquifer.
Finally, the efficiency of combustion can be enhanced by increasing the amount of movement, or circulation, of the bed particles. Thus, the greater the circulation of bed constituents within the burning zone, the more efficient the combustion of fuel particles and the greater the reaction to reduce sulfur dioxide.
Accordingly, it is an object of the present invention to further improve the overall efficiency of fluidized bed combustion and heat exchange in fluidized bed combustion, particularly in a boiler or like apparatus. It is a concommitant object of the invention to provide new and improved fluidized bed combustion devices and methods of carrying out fluidized bed combustion.
It is also an object of the present invention to provide a new and improved fluidized bed combustion apparatus and method of carrying out fluidized bed combustion providing heat exchange contact between constituents from the combustion bed and physically separated heat exchange means so as to ensure that the rate of heat exchange may be varied while at the same time constant temperature is maintained within the burning zone.
It is a further object of the invention to improve fluidized bed combustion by continuously reducing the size of limestone and ash particles in the combustion bed. As a result, final removal of spent particles in the form of fine dust-like ash and other similarly sized particulate matter collected at the flue gas exit. By the same token, it is also intended that the entrainment of the remaining unspent limestone, fuel particles and other bed constituents with the flue gases be restrained.
It is still another object of the invention to improve fluidized bed combustion by generating increased and, therefore, thorough circulation of bed constituents in the combustion bed. At the same time, it is intended to cause even distribution of the bed constituents not only to increase efficiency of combustion but also to enhance reactions in the bed such as the reaction of sulfur dioxide with limestone.
Various objects and advantages of the invention have been set forth above and are described in greater detail below. While such improved performance characteristics are specifically described herein, other improvements in various aspects of fluidized bed combustion and heat exchange will be apparent to those skilled in the art either by reason of this description or from practice with the invention as embodied in the novel and improved structures, instrumentalities, methodologies, operational steps and combinations thereof disclosed herein.