Patent Application
20080223267
The present disclosure relates generally to circulating fluidized bed (CFB) combustion systems and, more particularly, to an improved direct sorbent preparation/feed apparatus and method for a CFB boiler system.
Fluidized bed combustion (FBC) is a combustion technology used in power plants primarily to burn solid fuels. FBC plants are more flexible than conventional plants in that they can be fired on coal, coal waste or biomass, among other fuels. The term FBC covers a range of fluidized bed processes which include Circulating Fluidized Bed (CFB) boilers, Bubbling Fluidized Bed (BFB) boilers and other variants. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process, resulting in a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides a means for more effective chemical reactions and heat transfer.
During the combustion of fuels that have a sulfur containing constitutent, coal for example, sulfur is oxidized to form primarily gasous SO2. In particular, FBC reduces the amount of sulfur emitted in the form of SO2 by a desulfurization process. A suitable sorbent, such as limestone containing CaCO3, for example, is used to absorb SO2 from the flue gas during combustion. In order to promote both combustion of the fuel and the capture of sulfur, FBC combustion operates at temperatures lower than conventional combustion systems. FBC systems operate in a range typically between about 780° C. and about 1000° C. Since this allows coal to combust at cooler temperatures, NOx production during combustion is lower than other coal combustion processes. Fluidized-bed boilers evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers).
CFB boiler systems are generally associated with limestone feed systems for sulfur capture. Processed limestone fed to a boiler is typically conditioned by means of size reduction machines to specific size ranges to allow for the desulfurization process to proceed efficiently. If the particles are too large, the desulfurization process will not be efficient because there is insufficient limestone particle surface area to react with the flue gas. On the other hand, if the particles are too small, the limestone will be carried out of the desulfurization zone with the flue gas before it can react to remove the sulfur. Typically, limestone is fed to the boiler with a median particle diameter in the range of (as an example, but not limited to) about 100 to about 400 microns. In order to achieve this particle size range, unprocessed, raw limestone is reduced in both size and moisture content by size reducing machines. Presently, there are various machines available for crushing limestone, including for example, hammer mills, roll crushers and roller mills. Regardless of the type of equipment used for limestone crushing, the particles are dried either before or during crushing in order to produce a freely flowing material.
Traditionally, limestone is prepared separately from the boiler system, either on-site or by the limestone supplier. Prepared limestone is conveyed to a storage system in the boiler house from which it is thereafter metered and injected into the boiler. Experience has shown that the cost of prepared limestone using separate on-site systems or supplied from off-site vendors is expensive. In the case of separate, on-site systems a separate building and auxiliary fuel is used to dry the limestone. On the other hand, a limestone preparation and feed system may also be integrated with the boiler system itself, resulting in a significant reduction in capital and operating costs. Specifically, CFB boilers may be equipped with an integrated limestone preparation and feed system that resides in the boiler building. Such a system that dries and prepares limestone as needed is also referred to a Just-In-Time (JIT) limestone system.
Systems using roller mills have also been designed and installed on CFB boilers. Roller mill systems utilize hot air (mill air) for drying of limestone and transportation to the CFB. Mill air is typically obtained from the primary air stream as the mills utilize air at elevated pressures. For most CFB boilers, hot primary air in the range of about 400° F. to about 600° F. is typically available. The hot air is used to dry the entering limestone, sweep the limestone out of the mill, and convey it to the boiler. However, in order to protect the mechanical components of roller mills, the operating temperatures thereof are kept relatively low, for example in the range of about 170° F. to about 200° F.
Furthermore, limestone generally contains low quantities of moisture (e.g., less than 5%). For proper system operation, the air temperature entering the roller mills is a function of the acceptable mill mechanical operating temperature, the limestone moisture content, and the ratio of air to limestone. For a JIT system as described above, air temperatures entering the mill will generally be less than about 250° F. Therefore, given the available primary air temperature in the range of about 500° F., the mill air obtained from the primary air stream is therefore tempered in order to produce a mill exit air temperature, typically in the range of about 180° F. to 225° F.
Experience has shown that the use of tempering air effectively reduces heat recovery from the flue gas. In turn, the reduced heat recovery lowers boiler efficiency, as an example, in the range of about 0.5% to about 1.5%, thereby increasing equipment size and operating costs. Although the installation of a JIT limestone system using roller mills on a CFB boiler provides an initial benefit with respect to reduced capital cost for the CFB boiler, there is a tradeoff with respect to an increase in long term operating costs (e.g., as high as 15% to 30% compared to the cost of the CFB boiler). Accordingly, it would be desirable to be able to improve heat recovery and thus increase boiler efficiency for a CFB boiler equipped with a JIT limestone system.
According to aspects illustrated herein, a sorbent conditioning and feed apparatus for a circulating fluidized bed combustion system includes a dryer apparatus configured to dry unprocessed sorbent material transported thereto from a raw storage container; a sorbent crushing device configured to reduce the particle size of dried sorbent material discharged from the dryer apparatus; and a circulating fluidized bed (CFB) boiler configured to receive processed sorbent material conveyed from the sorbent crushing device; wherein the dryer apparatus utilizes an untempered hot air source diverted directly from a primary air stream input to the circulating fluidized bed boiler.
According to other aspects illustrated herein, a direct limestone conditioning and feed apparatus for a circulating fluidized bed combustion system includes a dryer apparatus configured to dry unprocessed limestone material transported thereto from a raw storage container; a roll crushing device configured to reduce the particle size of dried limestone material discharged from the dryer apparatus; and a circulating fluidized bed (CFB) boiler configured to receive processed limestone material conveyed from the roll crushing device; wherein the dryer apparatus utilizes an untempered hot air source diverted directly from a primary air stream input to the circulating fluidized bed boiler.
According to other aspects illustrated herein, a method of conditioning and feeding sorbent material for a circulating fluidized bed combustion system includes transporting raw, unprocessed sorbent material from a raw storage container to a dryer apparatus; diverting untempered hot air directly from a primary air stream input to the circulating fluidized bed boiler, the untempered hot air used to dry the raw sorbent material; directing dried raw sorbent material from the dryer apparatus to a crushing device configured to reduce the particle size of the dried raw sorbent material discharged from the dryer apparatus; and conveying processed sorbent material from the crushing device to the CFB boiler.
The above described and other features are exemplified by the following figures and detailed description.
Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike:
Disclosed herein is an improved direct sorbent preparation/feed apparatus and method for a CFB system. Briefly stated, a sorbent preparation/feed apparatus utilizes an alternate crushing device (i.e., one that does not need the use of tempered air) with a separate, high temperature dryer apparatus in order to supply dry, sized limestone on a just-in-time basis to a CFB boiler without reducing boiler efficiency.
As indicated above, CFB boilers equipped with roller mill JIT limestone systems employ tempering air in order to control the entering air temperature, which results in reduced heat recovery from the flue gas. The reduced heat recovery in turn lowers boiler efficiency (e.g., on the order of about 1%) and thus increases equipment size and operating costs. As will be illustrated from the embodiments discussed herein, a JIT system incorporating one or more roll crushers and utilizing a separate limestone drying apparatus eliminates the use of tempering air, thus allowing for maximum heat recovery from a boiler air heater.
As used herein, “Primary Air” (PA), in the context of FBC boilers, refers to combustion air delivered to the bed fluidization grate at the bottom of the furnace. Primary air is generated at relatively high pressures. In addition, some primary air may be diverted to other equipment, as well as enter the furnace at locations other than the grate. Furthermore, in the context of FBC boilers, “Secondary Air” (SA) refers to combustion air delivered through openings in the furnace walls. Secondary air is generated at pressures lower than primary air, and is used to fluidize the bed and stage combustion for emission control. “Tempered Air” refers to hot air that is cooled (i.e., tempered) with relatively cold air, such as ambient air for example. “Untempered hot air” refers to hot air leaving an air heater of the FBC system. Both primary air and secondary air leaving the air heater are considered untempered hot air streams. “Unprocessed Sorbent” refers to raw sorbent delivered to the FBC boiler conditioning (size reduction & drying) system. Unprocessed sorbent has not been conditioned to the proper size and moisture content required for feeding to the FBC boiler.
Referring initially to
One reason for requiring tempered air in a conventional roller mill preparation/feed apparatus 100 relates to the temperature limits of the crushing device (roller mill) itself. In addition, the velocity of the air conveying the conditioned limestone from the roller mill to the CFB boiler is controlled. If, for example, the velocity of the conveying air is too low, sorbent particles may settle out in the pipe, leading to blockage.
Accordingly,
In an exemplary embodiment, the sorbent dryer 202 is configured to discharge warm air, which carries sorbent fines (fine particles) directly to the boiler, thus bypassing the roll crusher device 204. In this sense, the sorbent dryer 202 has a particle separation capability. The remaining dry, raw sorbent that is not already of a fine particulate size is input from the sorbent dryer 202 to the roll crusher 204. As such a device does not have the same temperature restrictions as a roller mill, the need to temper the air used to dry the particles is eliminated. Once the dry, raw sorbent particles are conditioned (i.e., reduced to the desired size), they are conveyed (e.g., pneumatically or mechanically) to the boiler (not shown in
As indicted above, the dryer 202 may include a particle separation capability so as to remove sorbent fines with exiting warm air to the boiler, thereby preventing an unnecessary feeding of sorbent fines into the roll crusher 204. One exemplary type of dryer in this regard may be a fluidized bed dryer. However, other types of high-temperature dryers may also be used. In any event, warm air containing evaporated water and limestone dust (sorbent fines) is conveyed directly to the boiler 302 as part of the combustion air. On the other hand, limestone discharged from the dryer 202 is conveyed to one or more roll type crushers 204 arranged in series where it is crushed to a desired size and discharged. Downstream of the roll crushers 204, the dried, sized limestone is conveyed (e.g., pneumatically or mechanically) to the CFB boiler 302. Although not specifically depicted in
Through the use of the above described CFB boiler system and improved direct sorbent preparation/feed apparatus embodiments, the total amount of pressurized primary air in the system may be reduced. Because the elimination of tempering air reduces the amount of primary air needed to dry the sorbent, the total primary air is reduced and replaced by lower pressure secondary air.
Moreover, the above exemplary embodiments further improve particle size distribution of the conditioned limestone to the CFB boiler. As discussed, limestone (sorbent) to the boiler requires a specific range of sizes for optimum sulfur capture. If the size distribution becomes coarser, the quantity of unused sorbent rises. Larger particles are not sufficiently broken down in the furnace before removal, thus resulting in a greater amount of unused sorbent. On the other hand, if the size distribution becomes too fine, the quantity of unused sorbent rises due to reduced residence time for the sulfur capture reactions.
Accordingly, improved particle size distribution is achieved when small particles are removed (classified) prior to entering the roll crusher. If allowed to enter the crusher, many initially small particles are made even smaller, thus increasing the amount of unused sorbent. By using a separate sorbent dryer with direct contact of hot air (e.g., a counter flow flash dryer), entrainment of the smaller particles can be achieved thus avoiding additional size reduction in the crusher.
Further, it will be appreciated that the disclosed embodiments reduce the amount of high pressure air (primary and tempering) in the CFB system. This reduction is obtained primarily by using higher temperature air to dry the sorbent. The reduction lowers total power consumption of the air fans, which in turn improves boiler economics. For example, the use of primary air at a maximum air heater outlet temperature maximizes energy recovery from the flue gas, thus increasing boiler efficiency with respect to a JIT system using tempering air. As the boiler efficiency is significantly increased, a long-term savings in plant operating costs is realized. By way of example, the boiler efficiency may be increased in the range of about 0.5% to about 1.5%.
While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
