MULTI-FUNCTIONAL FECAL WASTE AND GARBAGE PROCESSOR AND ASSOCIATED METHODS
At least one aspect of the technology provides a self-contained processing facility configured to convert organic, high water-content waste, such as fecal sludge and garbage, into electricity while also generating and collecting potable water.
- 1-30. -30. (canceled)
- 31. A method of processing wet sludge with a multifunctional waste processing system for electricity and clean generation, comprising:
directing a flow of wet organic-based sludge to an inlet of a fuel path of a fuel dryer assembly, wherein the sludge comprises a mixture of water and solid fuel material, directing the flow of sludge along the fuel path through a heater portion of the fuel dryer assembly and boiling the wet fecal sludge and thermally separating the water from the solid fuel material to provide dried fuel, the fuel dryer having a first steam outlet, a dry fuel outlet; directing steam from the boiled sludge to a steam inlet of a fluid path of a condenser portion of the fuel dryer assembly, wherein the fluid path through the condenser is isolated from the fuel path and has a fluid outlet; condensing in a condenser steam liberated from the wet fecal sludge, wherein the condenser has a fresh water condenser assembly coupled to the first steam outlet and configured to condense the steam liberated from the wet fecal sludge for use as potable water; receiving in a dry fuel combustor assembly dried fuel the dry fuel outlet of the fuel dryer assembly, the dry fuel combustor assembly having a combustor portion and having a boiler configured to receive heat from the combustor portion, the boiler having a water inlet and a second steam outlet; combusting the dried fuel in the combustor portion to generate heat that boils water in the boiler to generate steam; generating electricity with a steam powered generator via the steam received from the second steam outlet of the boiler, the steam powered generator having a third steam outlet coupled to the steam inlet of the condenser portion; and pumping water with a water pump a flow of water from the fluid outlet of the condenser portion of the fuel dryer;
wherein the flow of water is pumped from a water outlet of the water pump to the water inlet of the boiler;
wherein the boiler converts a flow of water entering the boiler to a flow of steam to power the steam-powered generator.
- View Dependent Claims (32, 33, 34, 35, 36, 37, 38, 39)
This application is a continuation of U.S. patent application Ser. No. 15/629,642, filed Jun. 21, 2107, and titled MULTI-FUNCTIONAL FECAL WASTE AND GARBAGE PROCESSOR AND ASSOCIATED METHOD, which is a divisional of U.S. patent application Ser. No. 14/542,521, filed Nov. 14, 2014, and titled MULTI-FUNCTIONAL FECAL WASTE AND GARBAGE PROCESSOR AND ASSOCIATED METHODS, which is incorporated herein by reference in its entirety.
The present technology is directed to multi-functional fecal waste and garbage processing systems, equipment, and associated methods.
Many areas of the world utilize open sanitation systems for handling human waste and other garbage, while other areas utilize unsatisfactory septic systems or other systems that discharge raw sewage into open drains or surface waters. Such poor sanitation conditions contribute to significant health problems in these areas. Many of these areas with inadequate sanitation systems also struggle with maintaining clean drinking water, which further adds to potential health issues. These areas often have limited resources available for generating electricity, or the cost for generating electricity is prohibitively expensive. Accordingly, there is a need for adequate sanitation systems that keep waste out of the environment, for providing and maintaining access to clean potable water, and for generating inexpensive electricity.
The present technology provides multi-functional systems for processing waste while generating electricity and potable water in a manner that overcomes drawbacks experienced in the prior art and provides additional benefits. At least one aspect of the technology provides a self-contained processing facility configured to convert organic, high water-content waste, such as fecal sludge and garbage, into electricity while also generating and collecting potable water.
Many aspects of the present technology can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Instead, emphasis is placed on illustrating clearly the principles of the present technology. For ease of reference, throughout this disclosure identical reference numbers may be used to identify identical or at least generally similar or analogous components or features.
Appendix A includes additional information and calculations regarding aspects of the current technology.
The present disclosure describes multi-functional waste processing systems configured for generating electricity and potable water in accordance with certain embodiments of the present technology. Several specific details of the technology are set forth in the following description and in
The sludge 12 flows through a sludge dryer assembly 14 that evaporates water from the sludge to generate steam, such that the solid materials are sufficiently dried to provide combustible solid fuel material. For purposes of this description, the steam evaporated from the sludge is referred to as sludge vapor. The liberated sludge vapor is very hot for a sufficient duration, so that the sludge vapor is sterile (i.e., free of pathogens). The system 10 contains and condenses the sterile sludge vapor in a water treatment system 16 to provide clean potable water. The system 10 also burns the dried solid fuel material, in a combustor, such as a fluidized bed combustor 18. In some embodiments, other dried fuels, such as coal, wood pellets, garbage or other organic material can be added if necessary to provide additional fuel to the combustor 18. The system 10 of the illustrated embodiment is configured to continually produce up to approximately 150 kW (approximately 200 hp) of electricity and to process approximately 8500 kg or 8.5 m3 of fecal sludge and 1100 kg of garbage or more per day.
Heat from the fuel combustion in the combustor 18 is used to heat a boiler 20, which pressurizes water in a substantially closed primary water circuit 21 to generate steam for use by a steam-driven power plant 22 that produces electricity. The water in the primary water circuit 21 is referred to as primary water, which may be primary steam or primary liquid water, depending upon the location within the primary water circuit. Primary steam exhausted from the power plant 22 which includes a steam engine 26 and a generator 25 is used as a heat source by the fuel dryer assembly 14 before the primary steam flows through a condenser 24 and is converted back to primary liquid water and pumped back to the boiler 20. A portion of the electricity from the power plant 22 powers electrical components of the system 10, and the remaining electricity can be provided to a power grid or otherwise used locally, such as to power external electrical items.
The processing system 10 of the illustrated embodiment is a self-contained system that requires substantially no outside electricity, water or drainage to process the wet sludge and generate electricity and potable water. In one embodiment, the illustrated system 10 can be configured to occupy a volume with a footprint of approximately 15 m×3 m, which corresponds to a typical shipping container, such that the system 10 may be transportable. Accordingly, the system 10 is well suited for use in a wide range of geographic locations, such as under developed urban locations that may have inadequate sewage systems, and that could benefit from additional sources of electricity and clean, fresh potable water. The components of the system 10 of the illustrated embodiment are discussed in greater detail below.
Sludge Holding and Delivery System
The system 10 of the illustrated embodiment shown in
Sludge Dryer Assembly
The sludge dryer assembly 14 of the illustrated embodiment includes two enclosed large diameter pipes that each form a shell 42 that houses a small diameter pipe forming a hollow sludge carrier 44. Each sludge carrier 44 contains a rotatable, hollow auger 46, and the sludge carrier 44 receives the sludge through the inlet 40 such that the sludge at least partially surrounds the hollow auger 46. In the illustrated embodiment, each shell 42 includes a steam inlet 48 that receives the exhausted primary steam from the steam engine 26 (
The two sludge carriers 44 are interconnected at their ends by transfer housings 50 that each have sludge passageways therethrough that allow the sludge to flow axially through one sludge carrier 44 in one direction, through the sludge passageway in the transfer housing 50, and axially through the other sludge carrier 44 in the other direction.
In operation, the sludge level within the sludge dryer assembly 14 increases as additional wet sludge is delivered into the sludge carrier 44 by the transition auger 52 (
This recirculation of the drying sludge also prevents the sludge from reaching a condition referred to as the “sticky” phase, wherein the sludge moisture content is about 0.3523 kg H2O per kilogram of dry matter or 25% to 75% dry solid. Unlike in the “wet” or “paste” zones where the sludge displays fluid-like properties, in the “sticky phase” the contact between the sludge and heated wall of the sludge carrier 44 decreases dramatically, which negatively affects the evaporation rate. When the sludge is dried past the “sticky” phase to the “granular” phase, the drying sludge increasingly maintains homogeneous contact with the heated wall of the sludge carrier 44, which allows the evaporation rate to return back to its original value. In addition to decreased heat transfer effectiveness, material in the “sticky” zone exhibits considerable shear strength, such that the sludge material is more likely to adhere to the rotating auger 46 rather than being conveyed by it. Recirculation of some dry sludge material helps to ensure that the contents of the sludge dryer assembly always remain within or close to the “granular” zone, thereby avoiding the “sticky” zone.
In the illustrated embodiment shown in
In addition to removing the dried solid fuel material from the sludge carriers 44, the sludge vapor liberated from the sludge is removed from the sludge dryer assembly 14 through vapor outlet ports 66 in communication with the interior area of each sludge carrier 44. The sludge vapor flows from the vapor outlet ports 66 through conduits to the water treatment system 16 (
As heat from the primary steam is transferred to the sludge, the primary steam cools, such that the sludge dryer assembly 14 acts as a condenser, wherein the primary steam condenses within the shells 42 to primary liquid water. The condensate remains isolated from the sludge and is removed from the shells 42 by a condensate siphon tube assembly that extracts the primary liquid water and directs it into one or more primary water lines 62 that carry the primary liquid water away from the sludge dryer assembly 14 along the primary water circuit 21 (
The auger 74 is configured to rotate within the trough 72 so that the curved steam pipes 86 pass through the spaces between the steam pipes 78 in the trough 72. The auger'"'"'s curved steam pipes 86 can be slightly angled relative to the central shaft 84 so as to act as propulsion members that engage and push the sludge axially through the trough over the curved steam pipes 78, thereby heating and boiling the sludge. The hot primary steam in the central shaft 84 and in the curved steam pipes 86 also heats the sludge, which results in the primary steam condensing within the auger 74. One end of the auger'"'"'s central shaft 84 has a condensate outlet that directs the condensate out of the auger and along the primary water circuit 21 (
The auger 90 has a substantially hollow central shaft 94 connected to a plurality of hollow, straight finger pipes 96 that project radially from the central shaft 94. Each of the finger pipes 96 includes a support web 98 secured to the central shaft 94 to provide additional strength and rigidity to the respective finger pipe 96 as the auger 90 rotates and the steam-heated finger pipes 96 move through the sludge and slowly move the drying sludge axially toward the dry fuel outlet. In one embodiment, the support webs 98 can also be angled relative to the central shaft'"'"'s longitudinal axis, and the support webs 98 may engage a portion of the sludge to facilitate mixing and/or to incrementally move the drying sludge along the length of the trough 72.
For purposes of an example, the central shaft 94 of the auger 90 is a rigid, 24-inch-diameter pipe operatively connected to approximately 140 protruding 5-inch finger pipes 96 distributed around the pipe along its length. The finger pipes 96 extend internally into the steam-filled central shaft 94 to ensure proper condensate removal upon condensation of the primary steam during operation. Each of the finger pipes 96 and associated support web 98 are configured to accommodate the force of the drive motor'"'"'s full torque if that torque was fully applied to the end of a single one of the finger pipes 96, while maintaining an actual material stress below the material allowable stress for the auger'"'"'s designed pressure and temperature, such as approximately up to 100 psig and 328° F. In one embodiment, the finger pipes 96 are oriented in a generally helically arranged pattern down the length of the central shaft 94 in a configuration so no two finger pipes 96 initially engage the sludge material at precisely the same moment, thereby evenly distributing the impact loads throughout the auger'"'"'s full rotation. In addition, neighboring planar finger pipe groupings are rotationally offset by approximately 45° to facilitate the sludge flow through the trough 72 during the drying process.
As indicated above, the sludge vapor generated within the trough 72 is extracted through a vapor outlet. In one embodiment, the vapor outlet is positioned adjacent to the trough'"'"'s end panel that the sludge moves toward during the drying process. The sludge vapor removed from the trough 72 flows into a water treatment system 16 where the sludge vapor is cleaned and collected, as discussed in greater detail below.
In one embodiment wherein the system 10 is used to process very wet sludge (e.g., sludge having a solid content of approximately 15% solid materials or less). The system 10 dries the wet sludge utilizing a two-stage sludge dryer system that includes a high pressure first-stage dryer assembly 200 and a low pressure second-stage dryer assembly 220.
Each scraper disc 204 has a plurality of apertures 206 that axially align with the apertures 206 in the other scraper discs 204. A plurality of steam tubes 208 extend substantially along the length of the outer pipe 202 and through the aligned apertures 206 in the scraper discs 204. The scraper discs 204 also include bearings 209 that engage the inside surface of the outer pipe. The ends of the outer pipe 202 are connected to manifold portions 210 that communicate with the interior of the steam tubes 208. One of the manifold portions 210 (i.e., an inlet manifold 210a) has a steam inlet port 212 connected to the primary water circuit and configured to receive high temperature primary steam exhausted from the steam engine 26 (
The outer pipe 202 has a sludge inlet port 211 that directs a flow of very wet sludge into the pipe'"'"'s interior area such that the wet sludge directly engages the high temperature steam tubes 208. The structurally interconnected scraper discs 204 are connected to a reciprocating drive shaft 212 that sealably extends through the inlet manifold 210a and connects to an actuator 213, such as a hydraulic cylinder. The actuator 213 is operable to push and pull the drive shaft 212, thereby moving the scraper discs 204 as a unit axially back and forth within the outer pipe 202 and through the wet sludge. The high temperature primary steam in the steam tubes 208 boils the water in the sludge to generate sludge steam, thereby decreasing the water content of the sludge.
An elongate auger assembly 214 sealably extends through the inlet manifold 210a and into the interior area of the outer pipe for engagement with the sludge. As the sludge thickens due to the water evaporation, the auger assembly 214 helps move the thickened sludge through the outer pipe 202 to a sludge outlet port 215 at the end of the outer pipe 202 opposite the inlet port 211 of the dryer assembly 200. The extracted thickened sludge is then passed through a throttle 220 to decrease the pressure and directed into the second-stage dryer assembly 220 (
As the primary steam in the steam tubes 208 heats and boils the wet sludge, the primary steam condenses and the resulting primary liquid water flows out of the steam tubes 208 into a collection area in the outlet manifold 210b. The primary liquid water flows out of the collection area through a primary water outlet port and into a conduit coupled to a radiator 190 (discussed below) that cools the liquid water in the primary water circuit 21. The sludge vapor liberated from the sludge is heated and maintained a high temperature during the drying process, which results sterilizing the sludge vapor while in the outer pipe 202. As seen in
In the illustrated embodiment, the second-stage dryer assembly 220 is substantially identical to the sludge dryer assembly of
After the heated, pressurized sludge vapor flows through the curve pipes 78 and/or the auger 74/90, and the sludge vapor condenses. The resulting condensate extracted from the return manifold pipe 82 and from the auger'"'"'s hollow central shaft 84 flows to the water treatment system 16. In addition, the drying process within the second-stage dryer assembly 220 boils water out of the drying fecal sludge, and that sludge vapor exits the trough 72 of the dryer assembly 70 and flows to the water treatment system 16 (
Water Treatment System
The sterile water is then purified by an aeration process, then a bleaching process, and then a filtration process through selected purification filter, such as one or more charcoal filters. The purified, clean, potable water is then captured in a clean water storage tank 108, from which the clean water can be dispensed.
Dried Solid Fuel Handling System
Returning now to the dried solid fuel material, as it exits the sludge dryer assembly 14/70/200/220 as discussed above, dried solid fuel material enters the dry fuel hopper 56.
In the event that water or moisture somehow get into the hopper 56 and soaks the dried solid fuel material, or if that the dried fuel solid material is too wet to efficiently burn, then the hopper 56 will need to be emptied. Accordingly, the hopper 56 includes a wet fuel out-feed auger 115 that will direct the wet fuel back to the wet sludge holding tank 32 (
As seen in
In one embodiment, the waste processing system 10 (
As shown in
The air distribution grate 130 includes an insulated air distribution pipe 140 with an air inlet 142 and a plurality of sparger-type air manifold tubes 144 connected to the air distribution pipe 140 downstream of the air inlet 142. The manifold tubes 144 are parallel and spaced fairly close to each other to allow ash and small sand particles to easily fall between the manifold tubes 144 for removal by the discharge auger 128 to the discharge bin 126 (
In the illustrated embodiment shown in
The combustor assembly 18 is positioned within the boiler 20, and the heat generated upon burning the dried solid fuel material provides a continuous flow of heated exhaust gas that flows through the boiler 20 along an exhaust gas path 158 (
Primary Water Circuit Prior to Boiler
Turning now to the primary water path 160, the flow of primary water enters the boiler 20 in the liquid phase. As discussed above in connection with the sludge dryer assembly 14, the primary water flow from the steam engine 26 is condensed in the sludge dryer assembly to the liquid phase. In the illustrated embodiment shown in
As the primary water (sometimes referred to as “feedwater”) moves through the primary water circuit 21 in the steam/vapor and liquid phases, some of the primary water may be lost. For example, some primary water may be lost by steam blowing by in the steam engine 26 wherein steam blows past the piston along the cylinder walls in the engine. In addition, some of the primary water may be removed from the system 10 and discarded at the lowest point in the system 10 to remove any used chemicals or minerals that may have precipitated out of the primary water, which is referred to as blowdown. Depending upon the water quality and the system 10, blowdown can constitute up to approximately 5% of the total flow of the primary water. Accordingly, makeup water can be added to the primary water circuit 21 via a water conditioner 192 located downstream from the radiator 190.
The water conditioner 192 can also add chemicals or additives to the primary water while in liquid phase. In some embodiments, the chemicals and/or additives are added to the makeup water introduced into the primary water circuit 21. For example, the makeup water may be softened via chemical additives prior to entering the primary water circuit to reduce scaling of the pipes in the boiler 20. Chemical additives may also be used to minimize impurities and corrosion products, which can negatively impact heating efficiencies or can potentially shorten the operational life of the conduits through which the primary water flows in the primary water circuit 21. In addition, the water conditioner 192 can be used to treat incoming water, which may be hard public water, prior to the makeup water being added into the primary water circuit 21.
The primary water flows from the water conditioner 192 and is collected in a feedwater tank 194 before the primary liquid water is introduced into the boiler 20. The feedwater tank 194 can include a level switch so that after the primary liquid water is returned, the system has a way of measuring and adding the appropriate quantity of makeup water and chemicals to account for any losses in the primary water circuit 21. The primary liquid water is drawn from the feedwater tank 194 by a feedwater pump 196 that pumps the primary liquid water into the boiler 20.
Primary Water Path in Boiler
Returning now to the boiler 20,
The primary water flows from the secondary economizer 170 through the primary economizer 168, wherein the primary water is heated to its boiling point. The primary water flows out of the primary economizer 168 as steam and into a steam drum 199, wherein the dry, saturated steam is separated from any saturated liquid. Any saturated liquid in the steam drum 199 is returned and reintroduced into the evaporator 162. The dry primary steam flows out of the steam drum 199 and sequentially through the secondary and primary superheaters 166 and 164. The primary steam exits the primary superheater 164 as high-temperature, superheated steam, which flows out of the boiler 20, along the downstream portion of the primary water path 160 to the steam engine 26.
Although the boiler 20 illustrated in
Each frame structure 242 and its respective boiler components (i.e., superheater 224, economizer 226, and/or evaporator 228) is movable as a unit relative to the housing 241 in a translatable manner analogous to a drawer motion between an open, exposed position (
In another embodiment, the boiler 20 can be a concentric boiler having a central combustion chamber and fluidized bed. A generally cylindrical evaporator is coaxially arranged with the combustion chamber, and the superheater and the economizer are concentrically disposed radially outward of the evaporator. Other embodiments can utilize boilers with other configurations and/or components and/or component arrangements.
The steam engine 26 driving the generator 28 receives the superheated primary steam from the boiler 20 (
The reciprocating steam cycle of the steam engine 26 consists of four distinct events taking place over two strokes of the engine'"'"'s piston within its cylinder. Starting at Top Dead Center (TDC), the cylinder'"'"'s intake valve 306a opens and the superheated, high-pressure steam (received from the boiler) flows through the steam inlet port 302 and into the cylinder while the piston moves downwardly toward Bottom Dead Center (BDC). At a specified cut-off volume of steam, the intake valve 306a closes and the piston completes the power stroke to BDC. At BDC the exhaust valve 306b opens, and the exhaust stroke begins as the piston moves upwardly toward TDC. At a specified time before TDC, the exhaust valve 306b closes so the cylinder pressure rises close to the boiler pressure. This minimizes the throttling losses when the intake valve 306a opens.
As the steam engine 26 of the illustrated embodiment is operating with steam based on a boiler pressure of approximately 4130 kPa (600 psia), the intake and exhaust valves 306a and 306b must be carefully controlled via precise cam profiles and valve train arrangement to maximize the engine'"'"'s efficiency and power for the given boiler pressure and the engine torque limits. In the illustrated embodiment, at a boiler pressure of approximately 4130 kPa (600 psia), the cut off ratio for each cylinder (i.e., the ratio of the cutoff volume to the total volume of the cylinder) is approximately 11%. Accordingly, the intake valve 306a must be opened just long enough to fill 11% of the cylinder with the high-pressure primary steam. The steam engine 26 (
As shown in
The illustrated cylinder head configuration is such that the hot, high-pressure steam is on top of the cylinder head, and the inlet valve 306a needs to be on the same side as the high-pressure steam, otherwise the inlet valve 306a would open by the steam pressure. As the inlet valve'"'"'s position is on the top of the head below the steam inlet port 402, the high-pressure steam holds the intake valve 306a closed. In the illustrated embodiment, the intake valve 306a is connected to a spring 328 that provides additional forces to help lift and open the intake valve to let steam into the cylinder as the piston moves from TDC until the achieving the cutoff volume (˜11%).
The configuration of the steam engine 26 of the illustrated embodiment also provides improved temperature control of the engine during operation, particularly at high RPMs (i.e., ˜1850) over very long time periods. Unlike conventional steam engines that use double acting cylinders with steam pressure applied alternately to either side of the piston and exhausted on either side of the piston, the steam engine 26 of the illustrated embodiment has single acting cylinders. To avoid steam leaking around the piston particularly at low operating temperatures (i.e., during start up), the current engine 26 utilizes liquid coolant built into the engine with both a radiator and heater to control the temperature of the engine. When the engine 26 is starting and not yet warmed up, the heater keeps the engine'"'"'s cylinders well above water'"'"'s boiling temperature, so the steam will not condense. Because the high-pressure steam is hot, once the engine is running, the temperature control system is in a cooling mode. Accordingly, the temperature control system carefully controls the engine temperature and prevents the engine 26 from getting too hot, which would damage the oil, and from getting too cold (i.e., below approximately 160° F.), wherein the oil in the crank case and any water that gets past the piston via blow-by would mix and form an emulsion that would be impossible to separate.
The fecal sludge waste processing system 10 of the illustrated embodiment also includes a plurality of automated, integrated, computerized controls interconnected and configured for control of the entire system 10 with only minimal supervision from an operator, during normal operation. Control and monitoring of the equipment and processes are accomplished primarily through a central programmable logic controller (PLC) that collects inputs from sensors and sets output levels for the control devices, such as the valves and motors. The PLC is also configured to control operation of specialty controls for the electric generator system and propane burner used during startup. The PLC is also configured to divide the overall system into manageable subsystems, such as clean water/steam, combustion, fuel handling, and power generation. Control inputs are provided to decouple subsystems from each other to the extent desired. The subsystems can be further divided into control loops to provide set points for individual outputs.
The clean water/steam subsystem is configured to provide steam at a constant temperature and pressure to power plant 22, and to provide heat (in the form of steam) to the sludge dryer assembly 14 for generating sufficiently dry solid fuel. Control loops are used to regulate the quantity of makeup water entering the system, the condensate quantity entering the evaporator, the quantity of steam bypassing the steam engine, and the heat applied to the sludge drying assembly. The clean water/steam system is also configured to monitor and treat any external water entering the system, such as city water, and to control the total dissolved solids content of the boiler water through a blowdown system.
The combustion subsystem is configured to provide sufficient heat to keep the clean water/steam system producing the correct amount and temperature of steam. Control loops are provided that regulate the air flow through the fluidized bed, to operate the propane burner during startup, and to control the air pressure in the combustion chamber. This system will also monitor combustion emissions and exhaust gas handling and maintenance tasks, such as removal and fluidized bed material replacement.
The fuel handling subsystem is configured to provide the correct quantity of dried fuel to the combustion process and handle the waste water generated from the drying process. Control loops are used to provide the correct quantity of wet fuel, to regulate the dwell time of the solid fuel material in the sludge dryer assembly, to meter the dried solid fuel material into the combustor, and to handle the water condensation and treatment process.
The power generation subsystem is configured to provide power to the grid when available. This subsystem has control loops that regulate the electrical power output and regulate the engine speed and torque through modulation of the engine throttle. The control subsystems and low level loops can be integrated into a higher level controller to handle startup and shutdown sequences and to handle emergency and alarm situations appropriately.
From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the invention. Additionally, aspects of the invention described in the context of particular embodiments or examples may be combined or eliminated in other embodiments. Although advantages associated with certain embodiments of the invention have been described in the context of those embodiments, other embodiments may also exhibit such advantages. Additionally, not all embodiments need necessarily exhibit such advantages to fall within the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.