Patent Application
20090090799
The present invention generally relates to systems for grinding a substance into a powder and more particularly to systems for grinding coal into a powder that is suitable for combustion in a coal burner.
Combustion of coal is commonly used for industrial heating applications. For example, coal fired electrical power plants burn coal to generate the steam used to drive their electrical generators. Coal combustion is also often used in other industries having significant heating requirements, such as metal works, cement plants, and the like.
Industrial applications using coal-fired combustion generally require the coal to be pulverized into a finely ground powder before it is ignited in the combustion chamber of a coal burner. The finely ground coal powder burns more efficiently and tends to produce fewer noxious byproducts than other forms of coal. However, powdered coal presents a significant explosion risk. In order to minimize the risk of explosion, the coal is typically ground into a powder by an on site coal mill just before the coal is fed to the burner.
One prior mill system used to grind coal into a powder and meter powdered coal to a coal burner is referred to as a negative pressure mill system. In a typical negative pressure mill system, an exhauster downstream of a mill housing draws hot air into the mill housing through an air inlet. A milling unit in the mill housing includes one or more milling elements rotated by a drive system in a manner that grinds the coal into fine particles. Coal is fed to the milling unit through a coal inlet and ground by the milling unit in the housing into powdered coal. The exhauster draws air and finely ground coal particles from the housing through an outlet of the mill housing. A classifier in the mill housing causes larger coal particles that are entrained in the air flow in the housing to fall back into the milling unit for further grinding, while at the same time allowing finely ground coal particles to remain entrained in the air flow exiting the mill housing. The exhauster has an outlet connected to the coal burner so that the mixture of air and powdered coal is blown into the combustion chamber of the burner by the exhauster. The suction from the exhauster maintains the pressure in the housing at a relatively low pressure, which is why coal mills of this type are referred to as negative pressure mills.
One aspect of the invention is a system for grinding coal into a powder and supplying powdered coal to a burner. The system includes a mill housing having a coal inlet, an air inlet, and an outlet. A milling unit is in the mill housing. The milling unit is operable to receive coal that enters the housing through the coal inlet. The milling unit has a moveable milling member operable to grind the coal into a powder. The system also includes a drive system for moving the milling member. The drive system has a powered drive shaft and at least one gear for drivingly connecting the milling member to the drive shaft so that rotation of the drive shaft moves the milling member. A gear case defines a chamber. Said at least one gear is in the chamber. An exhauster is connected to the outlet of the mill housing. The exhauster is operable to draw air into the mill housing through the air inlet and further draw a mixture of air and powdered coal out of the mill housing through the outlet for delivery of the powdered coal to the burner. The system also has a gear case pressurization system operable to maintain a gear case pressure in the gear case at a higher pressure than an air inlet pressure at the air inlet.
Another aspect of the invention is a method of grinding coal into a powder. Coal is fed into a mill housing having a coal inlet, an air inlet, and an outlet, so that coal enters the mill housing through the coal inlet. A milling member in the mill housing is moved to thereby grind the coal into powdered coal by rotating a drive shaft drivingly connected to the milling member by at least one gear contained in a chamber defined by a gear case. Air is drawn into the housing through the air inlet and a mixture of air and powdered coal is drawn out of the housing through the outlet. Pressure in the gear case chamber in the gear case is maintained at a pressure higher than an air inlet pressure at the air inlet of the mill housing.
Still another aspect of the invention is a system for grinding coal into a powder and supplying powdered coal to a burner. The system includes a mill housing having a coal inlet, an air inlet, and an outlet. A milling unit is inside the mill housing. The milling unit is positioned to receive coal that enters the housing through the coal inlet. The milling unit has a moveable milling member operable to grind the coal into a powder when the milling member moves. The system also has a drive system for moving the milling member. The drive system has a powered drive shaft and at least one gear drivingly connecting the milling member to the drive shaft so that rotation of the drive shaft moves the milling member. A gear case defines a chamber. The at least one gear is in the chamber. An exhauster is connected to the outlet of the mill housing. The exhauster is operable to draw air into the mill housing through the air inlet and draw a mixture of air and powdered coal out of the housing through the outlet for delivery of the powdered coal to the burner. The system further includes a duct connected at one end to the air inlet of the mill housing so that the exhauster is operable to draw air from the duct into the mill housing through the air inlet. Another end of the duct is connected to a source of hot air. The hot air includes particulate matter. A duct pressurization system is operable to raise an air pressure in the duct upstream of the air inlet of the mill housing to be higher than ambient pressure.
Corresponding reference characters indicate corresponding parts throughout the drawings.
Referring to the drawings, and first to
The bowl shaft 119 is drivingly connected by at least one gear to a drive shaft 133 powered by a suitable power source, such as a motor 135 as illustrated in
A coal feed mechanism, such a volumetric feeder (not shown), is connected to the coal inlet 107 so that the coal feed mechanism meters coal into the mill housing 103 through the inlet. In the embodiment shown in
An exhauster 161 is connected to the outlet 109 of the mill housing 103 and is operable to draw air into the mill housing through the air inlet 105 by drawing air out of the housing through the outlet. Such an exhauster 161 is suitably conventional and need not be described in further detail. The air inlet 105 is suitably positioned at the base of the mill housing 103 and the outlet 109 is suitably at the top of the mill housing so that overall air flow through the mill housing is generally upward. Pulverized coal is entrained in the air flowing through the mill housing 103 toward the outlet 109. Before the air and pulverized coal reach the outlet 109, they flow through a classifier 165 (e.g., at the top of the mill housing 103) that allows finely ground coal particles to remain entrained in the air flow and exit the housing through the outlet, while causing larger coal particles that are entrained in the air flow to fall back onto the mill bowl 117 for further grinding.
A conduit 171 suitably connects the outlet 109 of the mill housing 103 to an impeller housing 173 of the exhauster 161 to convey the mixture of air and powdered coal from the mill housing to the impeller housing. An impeller (not shown) in the impeller housing 173 generates the suction that sustains air flow through the mill housing 103. The impeller also blows the mixture of air and powdered coal into a coal burner 175 through an exhauster outlet 177. As indicated schematically in
The air inlet 105 of the mill housing 103 is connected to a duct 181 extending away from the air inlet so that the exhauster 161 draws air from the duct into the air inlet of the mill housing. Upstream of the mill housing 103, the duct branches into a hot air supply duct 183 and a cold air supply duct 185. The hot air supply duct 183 is suitably connected to a source of relatively hotter air and the cold air supply duct 185 is connected to a source of relatively cooler air so that the air drawn into the mill housing 103 can be a mixture of hot and cold air. The words “hot”, “heated”, “cold”, “cooler” and the like are used in reference to a difference between the relative temperatures of the air in the respective ducts 183, 185. The cold or relatively cooler air referred to herein may have a temperature high enough to be considered hot in other contexts and may be heated to some degree (e.g., by proximity to the coal burner or its exhaust). However, the temperature of the cold or relatively cooler air is less than the temperature of the hot air despite any such heating.
As indicated in
In one embodiment, the heat exchanger 189 provides a common air supply supplying air to each of the gear cases of each of plurality of the mill grinding systems 101. A controller 204 monitors a position of each air inlet dampers 203, 205 providing air to the gear case of each mill grinding system 101 and controls the common air supply (e.g., duct pressure control 206 in
In one embodiment, the set point position for the dampers 203 should not greater than 75% open so that the controller 204 increases the air inlet pressure when any one of the damper positions of any one of the mill systems 101 needs to be greater than 75% open in order to maintain the hot air dampers 205 within a range of operability.
Damper 205 controls the cold ambient air flow into duct 181 and is mixed with the hot air from 183, both of which are controlled by the various controllers to optimize total air flow through the mill at various loads and to control mill air-fuel mixture discharge temperatures exiting the mill at 109. In one embodiment, a secondary air system 190 source of air comes from the same forced draft (FD) fan 191 and air heater system from which the coal mills or pulverizers receive their hot air (hot air going to a mill is also referred to as primary air). An optional perforation plate 208 in a duct supplying the secondary air may be used to meter the secondary air flow. In one embodiment, each of corners of the coal burner 175 may be supplied with secondary air. This allows finer control of the pressure within the gear case.
A hot air blast gate 201 is positioned in the hot air supply duct 183 and allows the hot air supply duct to be selectively opened and closed. The blast gate 201 is typically open when the system 101 is in operation and closed when the system is idle. A hot air damper 203 is positioned in the hot air supply duct 183 between the blast gate 203 and the intersection 195 of the hot air supply duct and the cold air supply duct 185. A cold air damper 205 is positioned in the cold air supply duct 185 upstream of the intersection 195 of the hot air supply duct 183 and the cold air supply duct. An exhauster damper 211 is positioned in the conduit 171 between the mill housing outlet 109 and the impeller housing 173 of the exhauster 161.
Each of the dampers 203, 205, 211, which are suitably conventional dampers known to those skilled in the art, is selectively moveable to various positions between an open position in which the damper provides relatively less resistance to air flow through the damper and a closed position in which the damper provides relatively more resistance to air flow through the damper. The dampers 203, 205, 211 allow air flow through the mill housing 103 and the temperature in the mill housing to be regulated, as will be discussed in greater detail below. A temperature sensor (not shown) is positioned to measure a temperature of the mill housing 103 (e.g., by being positioned to measure the temperature of the mixture of air and coal at the outlet 109 of the mill housing) to provide feedback that may be used to regulate the temperature in the mill housing. The dampers 203, 205, 211 are suitably controlled automatically by a controller 215 (e.g., electronic processor), as illustrated in
Referring to
The gear case chamber pressurization system 231 is suitably operable to raise the pressure in the gear case chamber 143 in response to an increase of air pressure in the mill housing 103. Likewise, the gear case chamber pressurization system 231 is suitably operable to reduce the pressure in the gear case chamber 143 in response to a decrease of pressure in the mill housing 103. In one embodiment of the invention, the gear case pressurization system 231 is suitably operable to maintain pressure of the gear case chamber 143 in a predetermined range from about 0.5 to about 1.5 inches of water gauge higher than the pressure in the mill housing 103.
In the particular embodiment illustrated schematically in
In one embodiment, the pressure controller 241 comprises both a computerized control algorithm within the mill digital control system and a field I/P (electrical current to pneumatic pressure) controller. The pressure controller 241 outputs a relatively small volume air stream that is directed by a conduit 255 to the eductor 245. The pressure controller 241 is operable to vary the rate at which air is directed to the eductor 245, as will be discussed in more detail later.
The eductor 245 in the illustrated embodiment has a venturi 261 having a discharge outlet 263 connected to the gear case chamber 143 and a suction inlet 265 for receiving air into the eductor. The suction inlet 265 of the eductor 245 is suitably positioned to receive air from the exterior of the mill housing 103 into the venturi 2611. A filter (not shown) is suitably positioned to filter air entering the suction inlet 265 of the eductor 245 to filter debris from the air before it reaches the gear case 141. The terminal end of the conduit 255 from the pressure controller 241 defines a port 267 in the venturi. Thus, the air stream from the pressure controller 241 is delivered into the venturi 261 through the port 267. Movement of the air delivered by the pressure controller 241 through the venturi 261 toward the gear case 141 creates a low pressure in the venturi that draws additional air into the eductor 245 through the suction inlet 265. The eductor 245 is designed to amplify the air flow from the pressure controller 241 so that the relatively small variable rate air flow delivered into the eductor by the pressure controller controls a larger volume of air flow from the eductor 245 into the gear case chamber 143.
The differential pressure sensor 243 senses a difference between the pressure in the mill housing 103 (e.g., as measured in the duct 181 adjacent the air inlet 105 to the mill housing) and the gear case chamber 143. As indicated in
The pressure controller 241 is responsive to the signal from the differential pressure sensor 243 to raise the gear case pressure PGC (e.g., by increasing the rate at which air in the variable rate air flow is delivered to the eductor through the conduit 255 and port 267) when the signal from the differential pressure sensor 243 adjusted by a proportional-integral-derivative (PID) control loop 244 indicates an amount by which the gear case pressure PGC (143) exceeds the air inlet pressure PD (181) decreases below a specified minimum pressure (i.e., increase air flow rate to eductor 245 when PGC−PD<minimum). The amount the gear case pressure PGC is increased is controlled by the PID control loop 244. Thus, the air inlet pressure is increased when a difference between the gear case pressure less the air inlet pressure is less than a minimum.
Likewise, the pressure controller 241 is responsive to the signal from the differential pressure sensor 243 to lower the pressure PGC in the gear case chamber 134 (e.g., by reducing the rate at which air in the variable rate air flow is delivered to the eductor 245 through the conduit 255 and port 267) if the signal from the differential pressure sensor 243 as adjusted by the PID control loop 244 indicates the amount by which the pressure PGC in the gear case chamber exceeds the pressure PD at the air inlet increases above a specified maximum pressure (i.e., decrease air flow rate to eductor 245 when PGC−PD>maximum). The amount the gear case pressure PGC is decreased is controlled by the PID control loop 244. Thus, the air inlet pressure is decreased when a difference between the gear case pressure less the air inlet pressure is greater than a maximum.
In addition, the pressure controller 241 is responsive to the bowl pressure sensor 117S signal adjusted by a PID control loop 118 to increase the air flow provided to the eductor 245 as the bowl pressure sensor 117S indicates that the bowl pressure, which is usually negative, is approaching zero and will become a positive pressure. In particular, the hot air damper 203 is decreased as the above bowl case pressure 117 approaches a positive pressure and the rate of decrease is controlled by the mills PID damper controllers.
In one embodiment, the pressure controller 241 is a low select pressure controller meaning that is maintains the pressure in the gear case at the lowest of the pressure value indicated to maintain a negative above bowl pressure per the above bowl pressure PID control loop 118 and the pressure value indicated to maintain the differential pressure per the differential PID control loop 244. Thus, the air inlet pressure is decreased based on the lower of the indication from the bowl pressure PID control loop and the differential pressure PID control loop.
In summary, one embodiment of the invention comprises a duct pressurization system operable to raise an air pressure in the duct upstream of the air inlet of the mill housing to be higher than ambient pressure, to maintain a gear case pressure as negative, to increase the upstream air pressure when a difference between the gear case pressure less the upstream air pressure is less than a minimum, and to decrease the upstream air pressure when a difference between the gear case pressure less the upstream air pressure is greater than a maximum.
In one embodiment of a method of grinding coal into a powder according to the present invention, the coal feed mechanism is used to feed coal into the mill housing 103 through the coal inlet 107 and convey the coal to the milling unit 115. The coal falls from the feed pipe 155 onto the mill bowl 117. The drive shaft 133 is rotated (e.g., using the motor 135) to rotate the gears 137, 139 and thereby move (e.g., rotate) the mill bowl 117. As the mill bowl 117 rotates, coal spreads out and up the inclined peripheral portion 121 of the mill bowl. Coal on the peripheral portion 121 of the mill bowl 117 is ground by the cooperative action of the roller(s) 131 and the mill bowl. The exhauster 161 draws air into the housing 103 through the air inlet 105 and draws a mixture of air and powdered coal out of the mill housing through the outlet 109. Then the exhauster 161 blows the mixture of air and powdered coal into the coal burner 175.
The hot air damper 203 is used to regulate the temperature in the mill housing 103 (e.g., as measured by the temperature sensor (not shown) at the mill housing outlet 109) to maintain the temperature in a range that is hot enough to remove moisture from the coal and to prevent condensation from interfering with the milling operation, but cool enough to limit the risk of premature ignition of the coal. To increase the temperature in the mill housing 103, the hot air damper 203 is moved farther toward its open position to allow more hot air into the mill housing. Conversely, to decrease the temperature in the mill housing 103, the hot air damper 203 is moved toward its closed position to allow less hot air into the mill housing. The amount of heating that is required to maintain the temperature in the desired range will depend on various factors, including the rate at which coal is fed into the mill housing 103, the moisture content of the coal, etc. Generally, when the mill system 101 operates under low loading, the amount of coal that needs to be heated and dried per unit of time is less than when the mill operates under heavier loading. Thus, when operating the mill system 101 under low loading conditions, the hot air damper 203 is typically closer to its closed position than it is under heavy loading conditions to restrict flow of hot air into the housing 103.
The cold air damper 205 is used to regulate pressure in the mill housing 103 (e.g., at the air inlet 105) to help minimize the impact adjustment to the hot air damper 203 has on air flow through the mill housing. The cold air damper 205 is moved farther toward its open position to increase pressure in the mill housing 103 (e.g., in response to movement of the hot air damper 203 toward its closed position). Conversely, the cold air damper 205 is moved farther toward its closed position to decrease the pressure in the mill housing 103 (e.g., in response to movement of the hot air damper toward its open position).
When the mill system 101 is operating under high loading conditions, the pressure in the hot air supply duct 183 upstream of the hot air damper 203 is suitably pressurized to a pressure above the ambient pressure (e.g., to a pressure that is from about 3 to about 8 and preferably including 6.5 inches of water gauge to increase flow of hot air from the hot air supply to the mill housing 103. The dampers 203, 205, 211 are suitably positioned and the duct pressurization system 191 operated so that the pressure at the air inlet 105 of the housing 103 is at no less than about 1.5 inches of water gauge below the ambient pressure, more suitably no less than about 0.75 inches of water gauge below the ambient pressure, still more suitably at least about equal to the ambient pressure, and even more suitably above the ambient pressure. When the load on the mill system 101 decreases, the dampers 201, 203, 211 and/or duct pressurization system 191 are suitably adjusted to reduce the pressure at the air inlet to the mill housing (i.e., increase the suction pressure at the air inlet). In addition to the FD fan 191 on the front end, an ID (induced draft) outlet fan on the back end may be used to draw air from the gear case to maintain a negative pressure.
The gear case chamber pressurization system 231 maintains the gear case pressure at a pressure that is higher than the air inlet pressure of the mill housing 103 (broadly, the pressure inside the mill housing). For instance, when the pressure at the air inlet 105 is increased (e.g., as indicated in the sequence illustrated in
When the gear case pressurization system 231 illustrated in
Although the embodiments described in detail herein refer to flow of “air” through various parts of the mill grinding system 101, it is understood that the term air is used in a generic manner to refer to any gas or mixture of gases suitable for moving powdered coal out of the mill housing. In particular, it is understood that it may be desirable in some instances to use air in one or more parts of the system that is substantially devoid of oxygen to reduce the risk of premature combustion of the coal.
When introducing elements of the present invention or the preferred embodiments thereof, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
| Number | Date | Country | |
|---|---|---|---|
| 60978318 | Oct 2007 | US |
