Abstract:
The present invention provides an in-line system for milling sorbent material to be used in a pneumatic conveying system. The system provides for optimal particle size in a duct injection system, regardless of the original sorbent particle size, and is designed to prevent clogging of the milled material through the system. Methods of operation for milling, as well as cleaning the mills while providing various sorbent material by-pass configurations to minimize system down-time while enhancing material throughput and that support the unique aspects of the system, are also described in detail below.

Description:
RELATED APPLICATION 
     The present application claims priority to U.S. provisional patent application No. 61/023,584, filed on Jan. 25, 2008; all of the foregoing patent-related document(s) are hereby incorporated by reference herein in their respective entirety(ies). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to the milling of product for use as a sorbent in a pneumatic conveying system. More specifically, the invention relates to an in-line system for milling (see DEFINITIONS section), and methods of operation for milling that support the unique aspects of the system. 
     2. Description of Prior Art 
     The market for air pollution control (APC) technology has grown tremendously since the Environmental Protection Agency (EPA) began to enforce more stringent standards via the Clean Air Act and its multiple rounds of amendments (most recently in 1990). According to BCC Research, “the total global air pollution mitigation market was worth $59.3 billion in 2006, and will reach $83.5 billion in 2007. By 2012, it is estimated that the global market will be worth over $138 billion, a compound annual growth rate (CAGR) of 10.6%.” Thus, air pollution control devices have become nearly a ubiquitous component of industrial facility design and operation, and technology improvements are constantly needed in this area. 
     Specific pollutants such as SOx and NOx which contribute to haze, acid rain, and ozone issues have become the focus of the Clean Air Interstate Rule (CAIR). State-enforced standards have been developed based on federal requirements defining: (i) reasonably available control technology (RACT); and (ii) maximum achievable control technology (MACT) standards. Coal-fired power plants, as well as industrial/institutional boilers and other fossil fuel fired systems, contribute a large volume of SOx and NOx emissions, if not properly controlled. Although flue gas desulphurization (FGD) is effective in controlling SO 2  to some degree, it is less effective upon SO 3  emissions, which create a visible haze via stack emissions from the plant. An APC technology that can control both SO 2  and SO 3  emissions effectively is needed. 
     Cost is another issue in reducing emissions from coal-fired power plants. Many existing plants have already been retrofitted with very expensive FGD systems. Further capital cost to meet the standards and remove the SO3 haze and gases is prohibitive for many plants. What is needed is a lower cost solution to achieve the additional reduction in SO 3  emissions and remove the stack emission haze and gases. 
     It has been found that injection of chemical sorbents, sulfur-absorbing materials, into the flue gas duct is an effective approach to reduce both SO 2  and SO 3  emissions from coal-fired power plants. Sodium bicarbonate, and hydrated lime can be used to absorb the gaseous sulfur and reduce SOx stack emissions. However, many sorbents are prone to binding and agglomeration making them difficult to work with. Another material found to be quite effective as an injected sorbent is trona/sodium bicarbonate, a naturally occurring sodium sesquicarbonate product. Trona/sodium bicarbonate and sodium bicarbonate can be quite expensive, but if the material is milled to a smaller particle size, the material becomes more cost-effective and more efficient in the process (milling the sorbent creates greater surface area for absorption). However, there are two technical challenges of milled trona/sodium bicarbonate or sodium bicarbonate. First, trona/sodium bicarbonate/sodium bicarbonate suppliers provide a wide variety of particle size in their specific product. This makes it difficult for a single duct injection system to accommodate multiple trona/sodium bicarbonate sources, and some particle diameter ranges may not even be usable in the system. Second, if the trona/sodium bicarbonate is pre-milled by the supplier, it is more difficult to handle and does not flow properly. Additionally, trona/sodium bicarbonate can agglomerate and clog delivery and piping systems. What is needed is a solution that accommodates a variety of trona/sodium bicarbonate particle sizes, and in addition does not cause additional problems with transport and injection of a milled trona/sodium bicarbonate product. 
     Some sorbent materials (see DEFINITIONS section) that have been proposed and/or used conventionally include: (i) sodium bicarbonate; (ii) dry hydrated lime; (iii) carbon; and/or (iv) trona. 
     SUMMARY OF THE INVENTION 
     As the inventor(s) have recognized, in a high volume throughput system for sorbent delivery and duct injection, several other challenges need to be addressed. The high volume of material being processed, and the need for continuous delivery of sorbent into the system increases the need to minimize downtime required for cleaning. Cleaning the sorbent material routinely from the system is needed to ensure that good system performance is maintained. However whenever the sorbent is not being delivered to the duct for emission control, no treatment is being applied to the emissions from the facility, which is not preferable. Therefore there is a need to not only minimize the downtime of the sorbent milling and delivery system, but also a need to design the system such that sorbent delivery never stops as long as the facility is operating. 
     A further challenge is the cleaning method itself. Water proves to be a preferred substance for flushing residual sorbent from the mill; however, introducing water risks additional moisture and humidity to be conveyed downstream of the mill and can cause unwanted agglomeration of sorbent material in the system. The conveying system needs to be free of water for proper operation. Therefore there is a need to thoroughly dry the mill after flushing with water in order to maintain good system performance after a cleaning cycle. Further, if heat is used to dry the mill after a cleaning cycle, the temperature must be controlled as high temperatures can degrade the sorbent material chemistry and reduce its efficacy. Therefore there is a need to design the cleaning and drying cycles to avoid negative sorbent performance impacts. 
     According to the present invention, an in-line system for milling sorbent material to be used in a pneumatic conveying system is provided. The system provides for optimal particle size in a duct injection system, regardless of the original sorbent particle size. The system is designed to prevent clogging of the milled material through the system. Inventive methods of operation for milling are described in detail below. Inventive methods of cleaning the mills while providing various sorbent material by-pass configurations to minimize system down-time while enhancing material throughput are described in detail below. 
     Various embodiments of the present invention may exhibit one or more of the following objects, features and/or advantages: 
     (i) it is an object of the invention to provide an improved APC system; 
     (ii) it is another object of the invention to provide an APC technology that can control both SO 2  and SO 3  emissions effectively; 
     (iii) it is another object of the invention to further ensure the capital expenditure for the APC technology is sufficiently low to implement in plants that have already invested in large APC systems, such as FGD systems or to implement in smaller/older plants that cannot economically employ higher capital cost technology; 
     (iv) it is another object of the invention to increase the utilization of costly trona/sodium bicarbonate sorbents, therefore reducing operating costs for the APC system; 
     (v) it is another object of the invention to accommodate a variety of trona/sodium bicarbonate particle sizes, and in addition avoid problems with clogging, relative to transport and injection of a milled trona/sodium bicarbonate product; 
     (vi) it is another object of the invention to minimize the downtime of the sorbent milling and delivery system; 
     (vii) it is another object of the invention to provide a design that supports continuous sorbent delivery as long as the facility is operating; 
     (viii) a further object of the invention to provide a system to thoroughly dry the mill after flushing with water in order to maintain good system performance after a cleaning cycle; 
     (ix) it is yet a further objects of the invention to incorporate design features to enable: (1) cleaning the system without introducing water downstream to minimize downtime for the system during cleaning, and (2) allowing for a variety of material bypass options to provide the most flexibility in milling, while increasing the material throughput, and minimizing system downtime; 
     (x) it is another object of the invention to design the cleaning and drying cycles to avoid negative sorbent performance impacts. 
     According to the present invention, a system for processing sorbent material includes a set of non-fully-milled sorbent material supply sub-system(s), a first mill, a second mill and a duct. The set of non-fully-milled sorbent material supply sub-system(s) include a first non-fully-milled sorbent material supply sub-system. Each non-fully-milled sorbent material supply sub-system is structured to supply sorbent material in a non-fully-milled state. The first mill is structured and located to receive non-fully-milled sorbent material from the set of non-fully-milled sorbent material supply sub-system(s) and to mill the non-fully-milled sorbent material to make milled sorbent material. The second mill is structured and located to receive non-fully-milled sorbent material from the set of non-fully-milled sorbent material supply sub-system(s) and to mill the non-fully-milled sorbent material to make milled sorbent material. The duct structured and located to receive milled sorbent material from at least the first mill and the second mill. 
     According to a further aspect of the present invention, a system for processing sorbent material includes a set of non-fully-milled sorbent material supply sub-system(s), a first mill, a second mill, a duct and an electronic controller. The set of non-fully-milled sorbent material supply sub-system(s) includes at least a first non-fully-milled sorbent material supply sub-system. Each non-fully-milled sorbent material supply sub-system(s) is structured to supply sorbent material in a non-fully-milled state. The first mill is structured and located to receive non-fully-milled sorbent material from the set of non-fully-milled sorbent material supply sub-system(s) and to mill the non-fully-milled sorbent material to make milled sorbent material. The second mill is structured and located to receive non-fully-milled sorbent material from the set of non-fully-milled sorbent material supply sub-system(s) and to mill the non-fully-milled sorbent material to make milled sorbent material. The duct is structured and located to receive milled sorbent material from at least the first mill and the second mill. The electronic controller is structured, connected and/or programmed to control the milling operation of the first mill and the milling operation of the second mill. 
     According to a further aspect of the present invention, a method for processing sorbent material includes the following steps (but not necessarily in the following order or in a serial manner at all): (a) supplying not-fully-milled sorbent material to a first mill; (b) supplying not-fully-milled sorbent material to a second mill; (c) milling, by the first mill, the not-fully-milled sorbent material supplied at step (a) to form milled sorbent material; (d) milling, by the second mill, the not-fully-milled sorbent material supplied at step (b) to form milled sorbent material; (e) injecting, into a duct, the milled sorbent material formed at step (c); and (f) injecting, into the duct, the milled sorbent material formed at step (d). These steps should only be considered to be in order, performed serially and/or performed concurrently to the extent: (i) explicitly set forth; or (ii) as necessarily implied by the subject matter of the method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic view that illustrates a first embodiment of an in-line system for milling sorbent material to be used in a pneumatic conveying system, according to a first embodiment. 
         FIG. 2  is a schematic view that illustrates a second embodiment of an in-line system for milling sorbent material to be used in a pneumatic conveying system, according to a second embodiment of the present invention. 
         FIG. 3  is a schematic view that illustrates a third embodiment of an in-line system for milling sorbent material to be used in a pneumatic conveying system, according to a third embodiment of the present invention. 
         FIG. 4  is a schematic view that illustrates a fourth embodiment of an in-line system for milling sorbent material to be used in a pneumatic conveying system, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present preferred embodiments of the invention, wherein like reference numerals refer to like components, examples of which are illustrated in the accompanying drawings. 
     Turning to  FIG. 1 , a schematic view that illustrates an in-line system  100  for milling sorbent material to be used in a pneumatic conveying system according to a first embodiment of the present invention is shown. Sorbent material (not shown) enters system  100  via scalping screener  101 , passes through metal separator  102  into loss-in-weight feeder  103 . From loss-in-weight feeder  103 , sorbent material passes through transitional vent hopper  104  into a two-way diverter assembly  110 . A conditioned air stream (not shown) is provided to two-way diverter assembly  110  via dryer/chiller  105 , PD blower  106 , and air to air heat exchanger  107 . 
     Within two-way diverter assembly  110 , “A” diverter  111  allows sorbent material to flow toward mill  125 , or diverts it as a bypass line toward “D” diverter  128  which then passes milled sorbent material through toward the duct injection site  145 . “B” diverter  112  directs sorbent material into mill  125 , but also acts as a converging diverter to allow air into mill  125  via the assembly of atmospheric air inlet  114  and air heater  115 , and first butterfly valve  113 . 
     Sorbent material enters mill  125 , which processes the sorbent material to a milled sorbent material having a reduced diameter. Milled sorbent material exits mill  125  and flows into “C” diverter  126 , where the milled sorbent material is then routed into “D” diverter  128 . “D” diverter  128  passes milled sorbent material through toward the duct injection site (not shown). In the case when “A” diverter  111  is in bypass mode, unmilled sorbent material passes through “D” diverter  128  and into the duct, to provide a continuous path for sorbent to flow through the system  100  even during cleaning cycles. To clean mill  125 , water is supplied through line  121  and delivery is controlled via ball valve  135 . 
     In accordance with an embodiment of the present invention, a method of operation of the in-line milling system  100  is provided. In operation, the mill system  100  processes sorbent material as follows. Scalping screener  101  protects the mill  125  by removing large debris, which may consist of wood, pieces of metal or other debris, that could damage the mill or other downstream apparatus if left in the incoming sorbent material stream. Scalping screener  101  also acts as a silo to feed the system  100 . Examples of such equipment are made by Russell Finex and Eriez. In a second screening step, finer pieces of metal are magnetically removed by metal separator  102 , such as those made by Eriez, to protect the overall system and specifically the mill  125 . The loss-in-weight hopper  103  helps to manage the overall feed rate into the system  100 , by measuring material via load cells and an air lock controlled by a variable frequency drive. The sorbent material passes through a small transitional vent hopper  104 , using a rotary valve to adjust the rate at which material passes into the convey line leading to the 2-way diverter assembly  110 . Dryer/chiller  105  (manufactured by companies such as Munters) dries the air provided to the pressure differential (PD) blower  106  (such as Gardner Denver&#39;s DuroFlow line), but also chills the air that has been heated in the drying process, prior to passing to the inlet of PD blower  106 . The discharge of air from PD blower  106  is further conditioned using an air to air heat exchanger  107 , such as those manufactured by Xchanger. As previously noted, the air temperature must be controlled in order to avoid physical degradation of the sorbent (e.g. via calcining). It should be noted that the screener, magnet, heat exchanger, and dehumidifier are preferred components but are not required for the system to operate. 
     The conditioned air from air to air heat exchanger  107  and the sorbent material from transitional vent hopper  104  enter the 2-way diverter assembly  110 . Each diverter ( 111 ,  112 ,  127 , and  128 ) in 2-way diverter assembly  110  was designed by Nol-Tec specifically for use in system  100  to selectively control flow direction of liquid and/or entrained sorbent materials, and is constructed of urethane designed and tested to withstand 80 psi of water pressure. Following the flow of sorbent material, “A” diverter  111  can either pass sorbent material into “B” diverter  112 , or if a cleaning and drying cycle is taking place, “A” diverter  111  can bypass mill  125  and flows directly to the duct via “D” diverter  128  as unmilled sorbent. 
     “B” diverter  112  either delivers sorbent material into mill  125 , or if the flow has been diverted for a cleaning and drying cycle, “B” diverter  112  delivers air via the assembly of atmospheric air inlet  114  and air heater  115 , and first butterfly valve  113 . 
     During normal milling operations, sorbent material passes through mill  125 , through “C” diverter  126  into “D” diverter  128  and into the duct system. In a cleaning and drying cycle, water is injected into mill  125  via line  121  and ball valve  135 , and in this case “C” diverter  126  directs the sorbent material and water slurry into receiving tank  130  for reclamation or proper disposal. The contents of receiving tank  130  may be removed for treatment, or may further be routed to an on-site treatment system (not shown). Potential dust emissions from receiving tank  130  are controlled via bin vent  131 . 
     Finally “D” diverter  128  either delivers milled sorbent material to the duct system from mill  125  via “C” diverter  126 , or alternatively delivers unmilled trona/sodium bicarbonate to the duct system when mill  125  is not operational. While this may not be the most ideal use of the sorbent material, and likely requires an increase in flow rate to meet emissions requirements, it is still an improvement in emissions reductions compared to not supplying sorbent to the duct at all during cleaning and drying cycles. 
     In accordance with an embodiment of the present invention, a method of cleaning the mill  125  is provided. The method of cleaning mill  125  uniquely addresses the problems of minimizing downtime, removing adhered sorbent from the interior of mill  125 , and drying mill  125  after cleaning so no additional moisture is introduced to the system. This method is critical to maintain best operation of the mill by ensuring (1) the milling is able to achieve the desired milled sorbent particle diameter, (2) energy costs are minimized (accumulated sorbent in mill  125  elevates the energy demand to run the apparatus) and (3) that in turn the milled sorbent will perform as expected in reducing SO x  emissions. 
     Several approaches may be used to determine frequency and schedule for cleaning. The power draw to mill  125  increases with material buildup and reduces efficiency and cost-effectiveness. Therefore the cleaning should be sufficiently frequent to avoid unnecessary use of power, but limited to when needed due to the downtime for sorbent milling. In one example, empirical evidence is used to measure the length of time from when mill  125  begins operation “clean”, to the point at which energy demand to run mill  125  is prohibitively high. This length of time can be used to set a schedule, or a timer, for initiating cleaning cycles. 
     In a second example, a more continuous measurement approach is used. A control system (not shown) detects the condition of mill  125  and reports on this condition, so that the frequency of cleaning can be determined (and possibly even initiated automatically). Power draw to mill  125  may be measured and a threshold level determined for triggering the need to clean mill  125 . Alternatively, the threshold of accumulated buildup can be related to the volume of sorbent material processed, such that the loss-in-weight feeder  103  load cells may be used to measure and trigger the need to clean mill  125 . 
     In general, one process for cleaning mill  125  consists of injecting water into mill  125  via water injection line  121  and ball valve  135 . The water is circulated through mill  125 , drained via “C” diverter  126  and routed to receiving tank  130 . However, due to the high volume throughput of the system  100  and the need to coordinate all equipment in the system, the actual method is more complex. A method  200  of cleaning mill  125  consists of the following steps: 
     Step  201 : Stopping sorbent material feed and air system—The flow of sorbent material will be stopped by stopping the loss-in-weight rotary airlock, and then stopping the vent hopper rotary airlock. The vent butterfly valve at the discharge of the heat exchanger will be opened to permit PD blower air to discharge to atmosphere. The convey line butterfly valve will then be closed to isolate the PD blower air from the remainder of the convey line. The air operated butterfly valve at the discharge of the diverter “C” can be opened to permit rinseate to drain to receiving tank. (Note: it should be understood that although the two butterfly valves are not illustrated as air operated valves which would be controlled by the PLC  198 , but they could be so operated.) PLC  198  is in the form of an electronic controller and, more preferably in the for of a computer-based controller. The four diverters “A” “B”: “C” and “D” can be operated simultaneously to make the bypass of unmilled trona/sodium bicarbonate and liquid fill of the mill. The dryer/chiller, PD Blower, and heat exchanger can remain operational to minimize trona/sodium bicarbonate convey down time during the switching of diverters to bypass unmilled trona/sodium bicarbonate during the cleaning cycle. 
     Step  202 : Verifying a “no-flow” condition—Via visual, mechanical, or electrical detection means, an operator confirms that both sorbent material and air flow has stopped. 
     Step  203 : Opening air inlet  114 , toggling “B” diverter  112 , and toggling “C” diverter  126 ; confirming toggled condition—Mill  125  will require continuous air flow to conduct a cleaning cycle. Therefore atmospheric air is provided to mill  125  by air inlet  114  but requires “B” diverter  112  to be switched to allow flow from air input  114  but not from “A” diverter  111 . “C” diverter  126  is also required to be toggled to pass water used for cleaning to receiving tank  130  after flushing mill  125 , instead of passing material through to “D” diverter  128  and on to the duct. AS an option, off-leg of “C” diverter air operated butterfly valve must be closed during the mill cleaning cycle to retain water solution in the mill. This is an important step that provides the ability to clean mill  125  without introducing moisture to “D” diverter  128  and downstream in the system. Via visual, mechanical, or electrical detection means, an operator confirms that the flow through both “B” diverter  112  and “C” diverter  126  has been successfully switched as described. 
     Step  204 : Toggling “A” diverter  111  and “D” diverter  128 ; confirming toggled condition—To divert the flow of sorbent away from the mill and directly to the duct as unmilled sorbent, “A” diverter  111  is switched to divert sorbent material that flows from the rotary airlock vent hopper  104  to “D” diverter  128 , where the switched position of “D” diverter  128  accepts flow from “A” diverter  111  and delivers the unmilled sorbent to the duct (not shown). Via visual, mechanical, or electrical detection means, an operator confirms that the flow through both “A” diverter  111  and “D” diverter  128  has been successfully switched as described. 
     As stated above, the blower system operates continuously and will not have to be restarted. This will minimize switch over time from milled materials injection to unmilled trona/sodium bicarbonate injection and vice versa. 
     Step  206 : Restarting sorbent material feed and verifying a “flow” condition—Similarly, the sorbent material flow is restarted and sorbent flow is verified via visual, mechanical or electrical means. 
     Step  207 : Providing pre-allocated amount of water to mill  125 ; washing mill  125  with water—Once the system  100  is ready for the washing cycle, a pre-allocated amount (for example by volume or by duration of flow) is provided through water injection line  121  by opening ball valve  122 . The water is agitated in mill  125  (it maintains operation throughout the entire cleaning cycle) and rinses accumulated sorbent material from the inside of mill  125 . The air operated butterfly valve at the discharge of diverter “C” can be operated to permit reinstate to drain to receiving tank. The rinseate (water and sorbent material) flows out of mill  125  through “C” diverter  126  into receiving tank  130 . 
     Step  208 : Stopping water flow—After the cleaning cycle is complete (for example, a pre-determined length of time based on empirical tests and size of mill), ball valve  122  is closed to stop water flow, and atmospheric vent  114  remains open to permit air flow to continue through the mill. 
     To determine when the drying cycle is complete, the humidity (moisture) within mill  125  may be measured. Alternatively, a specific length of drying time can be empirically derived and used as a measurement of when to complete the drying cycle. 
     Step  210 : Stopping sorbent material feed; operation of dryer/chiller  105 , PD blower  106 , and heat exchanger  107 —The entire system  100  is to be switched back to in-line milling operation after mill  125  is clean and dry. The flow of sorbent material must be stopped before diverting flow back to the mill. 
     Step  211 : Verifying a “no-flow” condition—Via visual, mechanical, or electrical detection means, an operator confirms that both sorbent material and air flow has stopped. 
     Step  212 : Closing air inlet  114 , toggling “B” diverter  112 , and toggling “C” diverter  126 —The flow of atmospheric air is stopped by closing air inlet  114  but requires “B” diverter  112  to be switched to allow flow from “A” diverter  112 . “C” diverter  126  is also required to be toggled to “D” diverter  128  and on to the duct. This is an important step that provides the ability to quickly switch back to full milling operation with minimal downtime and waste of sorbent material. If mill  125  is very briefly starved for air (e.g., less than one minute) this will not harm operation of mill  125 . Via visual, mechanical, or electrical detection means, an operator confirms that the flow through both “B” diverter  112  and “C” diverter  126  has been successfully switched as described. 
     Step  213 : Toggling “A” diverter  111  and “D” diverter  128 ; confirming toggled condition—To divert the flow of sorbent back to the mill and away from the duct, “A” diverter  111  is switched to divert sorbent material that flows from the rotary airlock vent hopper  104  to “B” diverter  112 , where the switched position of “B” diverter  112  accepts flow from “A” diverter  111  and delivers the sorbent to mill  125 . Similarly, “D” diverter  128  is switched from accepting sorbent material from “A” diverter  111  to accepting milled sorbent from “C” diverter  126 . Via visual, mechanical, or electrical detection means, an operator confirms that the flow through both “A” diverter  111  and “D” diverter  128  has been successfully switched as described. 
     Step  214 : Restarting Operation of air system—Open the convey line butterfly valve and close the atmospheric vent to restart operation of the air system. 
     Step  215 : Restarting sorbent material feed and verifying a “flow” condition—Now that all four diverters ( 111 ,  112 ,  126 , and  128 ) have been confirmed as switched, the system  100  operations can restart, and milling of sorbent can proceed. 
     Step  216 : Method  200  ends—Method  200  demonstrates how the invention provides for minimal downtime, supports continuous sorbent delivery, thoroughly dries the mill after cleaning to maintain good system performance, and avoids negative sorbent performance impacts. 
     In accordance with an embodiment of the present invention, a method of supplying water and disposal or reclamation of rinseate is provided. As described in Step  207  of Method  200 , a pre-allocated amount (for example by volume or by duration of flow) of water is provided through water injection line  121 . The water can be provided on-site at the facility, or alternatively can be provided via a mobile tank unit placed on-site. The on-site tank may also optionally have capacity for storing rinseate for disposal. After the cleaning cycle is complete, rinseate (water and sorbent material) flows out of mill  125  through “C” diverter  126  into receiving tank  130 . The rinseate may be either disposed or treated and reused in the process. 
     In one example, the rinseate may be discharged directly to a sewer (as long as the liquid meets disposal and applicable permit requirements), or may require some treatment prior to discharge to a sewer. 
     In another example, the rinseate is treated with known methods to reclaim the sorbent material (e.g. through settling and/or membrane treatment) and the water may either be discharged, or can be reused for repeated mill  125  cleaning cycles. 
     In a third example, the rinseate in receiving tank  130  is transferred to a mobile tank (not shown) and shipped off-site for disposal. 
     While cleaning mills with water is a preferred mode for practicing the invention, a dry cleaning agent could also be employed in a manner similar to that of using water. In addition, the mills could also be cleaned by taking the mill to be cleaned off-line, and reversing its direction. The agglomerated sorbent would then be ejected from the mill through this reversal of mill direction. 
     Note that system  100  and method  200  apply to a configuration where one mill  125  is included in the design for milling sorbent material. It can easily be seen by those skilled in the art that there exist numerous other arrangements of mill units (e.g. including a plurality of mills  125 ) in series or in parallel to achieve specific overall desired operational set points. 
     In one example, an identical mill  125  is placed downstream of PD blower  106  with no other changes to system  100  required. Two mill units  125  in parallel ( FIG. 2 ) or series ( FIG. 3 ) would allow for continuous milling of sorbent without the need to divert unmilled sorbent to the duct. The arrangement of diverters (such as  111 ,  112 ,  126 , and  128 ) would also be changed to support the same functions of system  100 , namely that continuous or nearly continuous milling operation is possible, with proper particle diameter of sorbent being delivered to the duct, and minimal downtime or minimal negative impact of the cleaning cycle observed. 
     In another example illustrated in  FIG. 4 , larger particle size sorbent is routed through a two-step milling process to reduce the sorbent to a usable diameter (Note: The goal of all particle-sizing techniques is to provide a single number that is indicative of the particle size. However, particles are three-dimensional objects for which at least three parameters (length, breadth and height) are required in order to provide a complete description. Most sizing techniques therefore assume that the material being measured is spherical, and report the particle size as the diameter of the “equivalent sphere” which would give the same response as the particle being measured.) In this example, a “pre-mill” is designed to provide a rough cut (for example, reducing particle diameter from about 300 microns to 50-70 microns). However this modified pre-mill can handle four times the capacity of mill  125 , and thus can be used directly upon delivery to the site from rail car or storage without slowing transport. The pre-mill is designed to both optimize in-flow rate (order of magnitude higher than in-process) and achieve correct particle diameters. Due to the higher flow rate, an increased, conditioned atmospheric air flow is required for delivery through this step. After the pre-milling step is conducted, the remainder of system  100  and method  200  remains the same. 
     It should be noted that the present system has been designed to work with a variety of mill types, including turbine and cutter mills; turbine and screen mills; and pin mills. 
     While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims. 
     DEFINITIONS 
     Receive/provide/send/input/output: unless otherwise explicitly specified, these words should not be taken to imply: (i) any particular degree of directness with respect to the relationship between their objects and subjects; and/or (ii) absence of intermediate components, actions and/or things interposed between their objects and subjects. 
     sorbent material: any material now known or to be developed in the future in the form of solid particles suitable to absorb pollutant(s) out of an airstream, including sorbent material that absorb pollutant(s) by chemical action and sorbent materials that absorb pollutant(s) by physical action. 
     mill: any device now known or to be developed in the future that makes particles of sorbent material smaller by any kind of mechanical action, without regard to: input particle dimensions, output particle dimensions, uniformity of input particle dimensions, uniformity of output particle dimensions, degree of reduction in particle size and/or throughput; mills include, but are not limited to turbine and cutter mills, turbine and screen mills, and pin mills. 
     non-fully-milled sorbent material: sorbent material that is unmilled or pre-milled, but that should optimally be further milled before delivery into a duct. 
     sorbent material supply sub-system: any sub-system now known or to be developed in the future for supplying sorbent material at a fixed on-site location in an at least substantially continuous manner; a sorbent material supply sub-system may include, but does not necessarily include one or more of the following components: a dryer; a chiller; a dryer/chiller; a PD blower; a heat exchanger; a scalping screener; a metal separator; a feeder (for example, a loss-in-weight feeder); a vent; a diverter; a valve; a hopper; and/or sorbent material conveying passages.