Patent Application
20130186810
Not applicable
Not applicable
1. Field of the Invention
The present disclosure relates to a method and system of processing alternative fuel sources in the treatment of waste water, and more particularly, to a system having a gasifier, a gas cleaning and conditioning system, and a combustion system that utilizes synthesis gas and biogas to produce energy in an effort to improve the waste water treatment process.
2. Description of the Background of the Invention
Energy is typically provided to wastewater treatment plants and other industrial plants utilizing conventional forms of electrical power. Costs to operate a plant and the associated equipment are typically large. Further, numerous secondary or waste streams that are both toxic and non-toxic are typically emitted from the plant in solid, liquid, or gaseous form that must be treated and disposed of according to government regulations. Therefore, it is desirable to find ways to utilize and recycle the secondary streams in cost-effective and environmentally friendly manners.
One system and method disclosed herein combines a quantity of synthesis gas with a quantity of biogas and sends the resultant mixture to one or more boilers to fuel the boilers and/or other components of the facility. The boilers create steam, which can be used for a variety of purposes such as (1) turning a turbine to produce electrical energy, (2) sending the steam to a building for comfort heat, (3) sending the steam to a digester at a wastewater treatment plant to warm the digester, which thereby speeds the rate of digestion of biological material inside of the digester, and/or (4) any other desired usage of the boiler steam.
Both synthesis gas and biogas are fuels created from waste products. Synthesis gas generally comprises hydrogen, carbon monoxide, and carbon dioxide. Synthesis gas is typically made utilizing refuse derived fuel (“RDF”), which typically comprises various recycled materials such as waste wood, plastic, paper, cardboard, and/or other similar waste materials. Biogas is a type of biofuel and is derived from biogenic materials. Biogas is broadly characterized as any gas or combination of gases that are produced by the biological breakdown of organic materials in the absence of oxygen. Biogases (and/or biogas mixtures) may be made from biological materials such as waste water, pre-sludge, sludge, corn, and/or any other biological material. The sources of both the synthesis gas and the biogas are typically materials present in the environment (not mined from under the surface of the earth) and can therefore be argued to be carbon neutral.
Wastewater treatment generally involves separating solid organic materials, i.e., biosolids, from the water so that the water may be treated until it is sufficiently cleansed of contaminants to be returned to the environment. Biosolids material may either be disposed of in a landfill or, more preferably, processed to make fertilizer, a valuable commodity. Biosolids may also be subjected to anaerobic digestion/fermentation to yield combustible biogas, such as methane, a valuable energy source that can be used to ultimately fuel electrical turbines or other equipment. If biogas/biosolids are not used effectively, such use can contribute to detrimental greenhouse gases.
Numerous problems exist with respect to known prior art systems. For example, waste materials are often inefficiently disposed of rather than effectively processed to yield valuable, useable energy. In addition, various present methods of processing waste materials to render useable energy could be significantly improved. With regard to unused biogas, in some facilities, excess biogas generated during summer months may be burned/flared rather than used, which can be aesthetically and/or environmentally detrimental.
A further problem exists in that various prior art methods of processing biosolids to produce biogas and/or fertilizer are cost prohibitive because they typically must utilize significant amounts of outside energy to perform the processing steps. Additionally, prior art wastewater treatment plants may be inhibited from changing the plant's design for fear of failure and the associated expense of attempting such changes.
It is not uncommon for most treatment facilities to exhibit these types of inefficiencies. Further, these problems are exacerbated by the utilization of multiple parties for each phase of waste treatment. For example, in the treatment of wastewater, one party, such as a local government, may be responsible for pumping untreated wastewater to a treatment plant. A second party, perhaps a private contractor, may be responsible for one or more phases of the wastewater treatment process, such as, separating biosolids from the water, subjecting the biosolids to aerobic and/or anaerobic digestion, and/or subjecting the water or biosolids to one or more chemical or filtration processes prior to sending the treated biosolids to a landfill or to a fertilizer production service. A third party, such as a fertilizer production company, may collect treated biosolids from the wastewater treatment plant and subject same to further treatment in order to yield a commercially viable fertilizer. As may be seen, these prior art wastewater treatment operations that involve multiple parties performing different functions may lead to inefficiencies. One such inefficiency is that each waste processing function has energy requirements and the energy to fuel a particular process is often purchased from an off-site resource such as an electric or natural gas utility. Furthermore, many processes generate a significant amount of energy that is unutilized and wasted rather than captured and used.
The system described herein may be used at a wastewater treatment plant or another comparable facility that produces or otherwise has biogas. For example, a wastewater treatment plant has digesters onsite and may also have buildings that require electricity and comfort heat. One or more boilers, or some other type of equipment, can be fueled by synthesis gas, biogas, and/or combinations thereof. Utilizing the system and method described herein to provide fuel to the facility in order to power lights, pumps, and/or other equipment may be cheaper than purchasing electricity from an offsite electric or gas utility or otherwise purchase oil or other fossil fuels to be used on-site to generate power.
Further, the method and system disclosed herein may also overcome other inefficiencies of the prior art. For example, the system efficiently leverages various waste streams to produce a significant amount of useable energy, which in turn, reduces the overall cost of processing such waste. One such example is that a first type of waste material, refuse derived fuel, can become a highly effective and practical fuel source for various components including, for example, boilers/turbines, when the RDF fuel is converted to synthesis gas and then combined with biogas prior to combustion in the boiler(s).
It is contemplated that different waste materials may be processed in one integrated system, and that one or more phases of fertilizer production may be integrated into such a system. Linking particular functions of the system together is highly advantageous from a commercial standpoint. The novel system disclosed herein combines (1) the process of converting RDF fuel to synthesis gas, (2) the process of converting biosolids to biogas, and (3) the process of producing fertilizer from biosolids. As a result of these synergistic combinations, certain phases of each process can complement each other, resulting in substantial processing efficiencies that achieve substantial cost savings. Therefore, utilizing these efficiencies makes each individual process step and the overall process more efficient than each individual process step may be on its own and reduces the need to purchase energy from off-site resources.
In one aspect of the present invention, an energy conserving wastewater treatment system capable of being fueled by alternate fuel sources comprises a synthesis gas generator that produces synthesis gas from a fuel and an organic waste digester that produces biogas. A combined synthesis gas and biogas storage reservoir is in communication with both the synthesis gas generator and the organic waste digester. At least one boiler is in communication with the combined synthesis gas and biogas storage reservoir.
In a different aspect of the present invention, an energy conserving wastewater treatment system capable of being fueled by alternate fuel sources includes a synthesis gas generator that produces synthesis gas from refuse derived fuel and gas cleaning equipment that cleans the synthesis gas. A first heat transfer apparatus transfers heat from the synthesis gas and sends the heat to a sludge dryer. An organic waste digester produces biogas from anaerobic digestion of biosolids. The system further includes biogas cleaning equipment that cleans the biogas. At least one boiler receives at least one of the synthesis gas or the biogas or a combination thereof. An electricity generating apparatus is in communication with the boiler.
In yet another aspect of the present invention, a method of conserving energy in a wastewater treatment plant comprises the steps of producing synthesis gas from a fuel using a synthesis gas generator and producing biogas from the anaerobic digestion of biosolids using an organic waste digester. Heat is captured from the synthesis gas production and sent to a biosolids dryer. At least one boiler is provided for combusting at least one of either the synthesis gas or biogas. Steam produced by the boiler is sent to both a turbine and to the organic waste digester.
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The alternate fuel may include contaminants, for example, such as trap metals and other non-conforming materials. The contaminants are normally present in an amount less than about 5%, and more preferably are present in an amount less than about 3%, and most preferably are present in an amount less than about 1% of the alternate fuel on a dry weight basis. The alternative fuel is preferably provided in a form wherein each particle's overall size is about 8 inches or less, more preferably about 6 inches or less, and most preferably about 4 inches or less. Further, it is preferable that the alternative fuel contain less than about 5% of metal content, more preferably less than about 3% of metal content, and most preferably less than about 1% of metal content.
The alternate fuel is provided to the gasifier 110 and is converted to synthesis gas. In one embodiment, the gasifier 110 is a plasma arc gasifier, which is available from Westinghouse Plasma Corporation®. In a different embodiment, the gasifier 110 comprises a fluidized bed reactor, such as a reactor furnished by Frontline Bioenergy of Ames, Iowa. In yet another embodiment, the gasifier may be other types of gasifiers as known in the art. The gasifier 110 heats the alternate fuel to temperatures sufficient to convert a majority of the alternate fuel to synthesis gas. The preferred temperature varies according to the amount and exact composition of the alternate fuel, but the alternate fuel is preferably heated to a temperature between about 3,000° F. to about 15,000° F., and more preferably between about 7,000° F. to about 12,000° F., and most preferably about 10,000° F. The synthesis gas that is formed exits the gasifier 110 at a temperature lower than the heating temperature. For example, the synthesis gas typically exits the gasifier 110 at a temperature of between about 3,000° F. to about 3,500° F. The resultant synthesis gas comprises primarily carbon monoxide, hydrogen, carbon dioxide, methane, and other trace gases. The synthesis gas preferably contains less than about 10% residual materials when exiting the gasifier 110.
Still referring to
Optionally, a heat exchanger (not shown) as known in the art may be provided between the gasifier 110 and the gas cleaning and conditioning system 112 to cool the synthesis gas before it enters the gas cleaning and conditioning system 112. The gases may be cooled in the heat exchanger with exchange air forwarded as combustion air intake to the boilers 106a-106c and synthesis gas stripped of significant sentient energy sent to a gas scrubber (not shown). Any residual components may be water quenched and transported and disposed of in a landfill. Quench water may then be conveyed to the wastewater treatment plant inlet for treatment. In addition to cooling the synthesis gas, the heat exchanger may send heat/hot air to the intake of one or more of the boilers 106a-106c, thereby increasing the efficiency of the boilers.
After exiting the gas cleaning and conditioning system 112, the scrubbed synthesis gas is sent to a storage tank 114. In one embodiment, one storage tank is provided. In other embodiments, more than one storage tank 114 is utilized in manners consistent with this disclosure. The synthesis gas is combined with biogas in the storage tank 114 as described in more detail hereinbelow.
In a wastewater treatment facility, biogas is typically created when organic or biological material is contained within and allowed to ferment within one or more digesters 116 in the absence of air. The biogas that is formed is sent to a biogas cleaning system 118 in manners known to those in the art. The biogas cleaning system 118 removes sulfur compounds, siloxanes compounds, and any other undesirable components.
After exiting the biogas cleaning system 118, the resultant biogas is transported to the storage tank 114, wherein the biogas and the synthesis gas are combined to form fuel. The resultant biogas/synthesis gas has an increased heating value such that the resultant gas is easier to burn in “conventional” natural gas fired equipment as compared to synthesis gas alone, biogas alone, or other fuels. The synthetic gas is provided to the storage tank 114 at a rate of between about 50,000 lbs/hr to about 80,000 lbs/hr, and more preferably between about 65,000 lbs/hr to about 75,000 lbs/hr, and most preferably about 68,000 lbs/hr. The higher heating valve (HHV) of the synthesis gas is about 5,000 BTU/lb at about 68,000 lbs/hr. The biogas is provided to the storage tank 114 at a rate of between about 5,000 lbs/hr to about 25,000 lbs/hr, more preferably at a rate of between about 10,000 lbs/hr to about 20,000 lbs/hr, and most preferably at a rate of about 14,815 lb/hr. The higher heating valve of the biogas is about 10,125 BTU/lb at about 14,815 lb/hr.
As shown in
Portions of the various streams of the process, such as the biogas and synthesis gas streams, may also be recycled or diverted to other areas of the facility as will be explained in more detail hereinbelow. For example, low pressure steam may be conveyed to the digesters 116 and used to assist with heating requirements for the wastewater treatment plant.
Referring to Tables 1 and 2 hereinbelow, various heat and materials data are provided as an example from a treatment plant utilizing various components of the present invention. In particular, the Tables 1 and 2 reflect the heat and materials data for the embodiment discussed with respect to
The data shown in Tables 1 and 2 provide a tabulation of the mass and energy entering, traveling through, and leaving the system of the system of this embodiment and the energy efficiency and viability of the system at a point in time. The numbers are calculated utilizing actual and “typical” chemical compositions for the feedstocks, synthesis gas, and biogas. Also incorporated into these calculations are system and equipment efficiencies that are typical for the type and size of the equipment used. In addition to the factors discussed hereinabove, the values in Tables 1 and 2 will vary based on operational levels of the system (e.g., 50 percent, 75 percent, or 100 percent of system capacity), the time of the year (there will be more heat loss from the system in the winter season as compared with the summer season and more biogas will be generated in the summer), and energy demand of the facility. During the operation of the system, operating parameters in the system such as temperatures, flow rates, and pressures are monitored and used to calculate real time operating values that are then incorporated into spreadsheets similar to Tables 1 and 2 and used to calculate the overall efficiency of the system and its unit operations.
Now turning to a different embodiment of the system for processing alternative fuel, a one boiler system is depicted in
As best seen in
Valuable fertilizer is created in a fertilizer production process within the system 200 for processing alternative fuel by subjecting the biosolids to various processing steps. In particular, when the biosolids in the dryer 218 reach a desired level of dryness, the biosolids are then transferred via a fourth conduit 222 to a storage silo 224 so that the biosolids therein may be picked up by a truck or other transport to a fertilizer manufacturer or supplier. Any suitable biosolids dryer 218 as known in the art may be implemented, such as a rotadisc dryer purchased from Haarslev Industries of Denmark. At any suitable point in the fertilizer production process, a dust control product such as Dustrol™ is optionally applied to the biosolids to reduce dust, as is generally known in the art. Also, the screening of the fertilizer to a desired particle size may be performed either on-site or off-site. Further details on the bisolids drying procedure and the screening of biosolids material are discussed hereinbelow with respect to
It should be noted that an additional conduit (not shown) may be optionally provided to route steam from the HRSG to any other desired location in the plant 200. For example, such steam could be sent to a boiler 226 to warm the water therein, or such steam could be sent to a biosolids waste digester 228 to circulate around the digester 228, thereby warming the biosolids therein to speed the rate of anaerobic digestion. The digester 228 may additionally or alternatively be heated by hot water that is recovered from the biosolids dryer 218. The dryer 218 produces steam vapors that are condensed with a small amount of water to make hot water from the latent heat within the vapors. This hot water may be circulated to the digester 228 heating system that heats the contents of the digester reactor to keep the sludge at a preferred temperature, which allows for the biological activity to break down the organic matter into digester gas. The HRSG may provide a second source of hot water for heating the digesters in addition to the turbine condenser hot water system that was previously discussed.
Although an HRSG is discussed with respect to this embodiment herein, it is contemplated that other types of heat exchangers as known in the art may be utilized in the embodiments disclosed herein.
Still referring to
Once the synthesis gas reaches a desired reduced temperature in the HRSG, such as about 700° F., the reduced temperature synthesis gas travels through a fifth conduit 232 (see
Similarly, the biosolids waste digester 228 sends biogas produced therein to biogas cleaning equipment, such as a conventional biogas scrubber 240, via conduit 242. Once the biogas is cleaned in the biogas scrubber 240, the biogas is sent via conduit 244 to the reservoir 236 for storage. Combining the cleaned biogas with the cleaned synthesis gas in the reservoir 236 is advantageous because, as noted previously, the synthesis gas by itself has lower energy than the biogas. By adding higher energy biogas to the synthesis gas, the resultant gas mixture has a higher energy than the synthesis gas itself, thereby making the gas mixture a more desirable fuel for a boiler 226. In addition, as noted previously, the rate of biogas production might be slower in some plants than the rate of synthesis gas production. Therefore, combining these gases in some instances creates a useable quantity of fuel for the boiler 226 than might otherwise be available.
After the biogas and synthesis gas are combined, the boiler 226 draws the mixed gas from the reservoir 236 via conduit 246 and sends steam to an electricity generating apparatus provided in the form of a steam turbine 250 via conduit 252. The steam turbine 250 is operably connected to an electric generator 254 to produce electricity. The electricity may be sent to a plant building 256, to onsite pumps or lights (not shown), to other system components, and/or may be sent offsite of the plant 200 for sale or other use. A suitable turbine 250 and generator 254 may be used as known in the art.
As best seen in
A condenser 270 may be provided in the alternative fuel processing system 200 to supply hot water to various components within the system 200. Low pressure steam from the turbine 250 travels under vacuum to the condenser 270 where it is condensed to hot water. The hot water is then sent to storage tank 260 where it can then be routed to one or more components including the dryer 218, the boiler 226, the digester 228, a centrifuge (not shown), building 256, and/or any other components within any of the systems described herein.
Additionally, the condenser 270 generates a vacuum to increase the turbine 250 efficiency. By condensing the water, wasted water is minimized and the turbine runs more efficiently. The condenser 270 is typically a two-stage condensing system that utilizes hot water to capture the heat from the latent heat of the steam into sensible heat in the hot water. Additional cooling water is provided by clean effluent sewage water that is used to complete the condensing process and to discharge heat to the sewage water. Condensing steam from the turbine provides numerous beneficial uses as opposed to wasting the heat to the sewage water. The amount of hot water that is created by changing the latent heat to sensible heat provides heat to the digesters and centrifuge feed by converting the steam back to water.
Referring again to
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Any centrifuge as known in the art may be used, such as, for example a decanter centrifuge manufactured by Alfa Laval. Further, the thickening tank 308 is preferably a gravity thickening tank or a dissolved air flotation tank, but other thickening tanks as known in the art may be used. The thickening tank 308 sends treated centrate to a sewer (not shown) via a suitable conduit 324.
A suitable control/starter panel 326 is used to start/stop or otherwise control all equipment in the subsystem 300. The panel 326 may optionally be incorporated into the control 272 of
Returning again to the dryer 304 operation, if the biosolids entering the dryer 304 are too wet, the biosolids are generally too difficult to process. Specifically, a roughly 45% solids composition exists in a gummy or sticky phase that is difficult to process using a conventional drying apparatus. Therefore, a portion of the dryer's 304 dried biosolid output is recycled back into the centrifuge 316 via conduit 328. Dried biosolid output from the dryer 304 is typically comprised of about 90% to about 92% dry solids composition. Sludge exiting the thickening tank 308 comprises about 25% dry solids composition. The dried output from the dryer 304 is mixed with the sludge from the tank 308 in a ratio of about 2:1, dried-output-to-sludge ratio (e.g., two pound to one pound), to achieve a preferred approximately 68% dry particle composition entering the centrifuge 316. A paddle mixer (not shown), well known in the art, may be optionally used to mix the dried output from the dryer 304 with the sludge from the thickening tank 308 prior to entering the centrifuge 316.
In the dryer 304, steam flow, residence time, and temperature can be regulated as necessary to achieve an appropriate evaporation rate. Further details of the dryer 304 operation are discussed below in relation to
The centrifuge 316 is also fed by the anaerobic digester 302 via conduit 330. As discussed previously, any digester disclosed herein, such as digester 116 (
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Turning now to
Biogas 422 is sent to biogas cleaning equipment 424 and, once cleaned, preferably travels to a biogas storage tank 426. The biogas is then sent to a biogas boiler intake 428. The boiler 402 also includes an ambient air intake 430. The biogas and synthesis gas may be combusted together in the boiler 402 using one burner, or alternatively the gases could be combusted with separate burners. Flue gas may exit the boiler 402 at a suitable port 432. An additional heat exchanger (not shown) is optionally added to the system 400 to capture heat from the flue gas. Low pressure steam may travel from the turbine 414 through a first pathway 434 to a hot water heat exchanger 436. The heat exchanger 436 preferably includes a process water inlet 438 and a condensate outlet 440. Hot water travels via conduit 442 from the heat exchanger 436 to a sludge processing pathway 444.
Low pressure steam from the turbine 414 may also follow a second pathway 446 from the turbine 414 to a divergence point 448. Such low pressure steam may then diverge into an HVAC pathway 450, that routes the steam to the plant building 256 (
As best seen in
The biosolids material is sent from the separation bin 448 by conveyor or other suitable means to a sizing and cooling apparatus 452. The screened granules are cooled in the cooling apparatus 452 to allow for the proper temperature prior to storage in a silo 454 before being sent to the market as fertilizer. The cooling apparatus 452 may employ an air system, which mixes cooling air with the dried hot granules. Effluent cooling water may be used in a direct Venturi contact system as known in the art. The cooling of the hot dried granules is accomplished by mixing cold air with the hot granules. The hot air is cooled in a direct Venturi water device (not shown) that reduces the heat content of the air that is recycled back to cool the granules. The water that is heated is sent to the sewer and new cold water is added to the device. From the cooling apparatus 452, the biosolids material is sent to the fertilizer silo 454 for storage.
Still referring to
Water vapor and organic vapor may also travel from the biosolids dryer 446 into a condenser 464. The vapors that are created from the dryer 446 have the dust removed by the use of a suitable dry cyclone separator (not shown), which is well known in the art that recycles the dust back to the drying process. The dust-free vapors are sent to the condenser 464 to be condensed by Venturi water scrubbing as known in the art, using plant effluent. The water mixture can be used to generate hot water, or further cooled with water to be discharged to the main drain of the plant. The non-condensable vapors are treated in a thermal oxidizer 466 to destroy the odors. It should be noted that thermal oxidizer 466 operates at a high temperature of about 1800° F. for about two seconds to incinerate vapors and is therefore expensive to operate. It is therefore preferable to minimize usage of the oxidizer 466 by condensing a maximum amount of vapor from the dryer 446 and using the condenser 464 so that condensate can be discharged to a main sewer drain of the plant rather than sent to the oxidizer 466.
The centrifuge 460 is also in communication with digested sludge from a digester 468. The digester 468 is in communication with the condenser 464, similar to the HRSG shown in
It should be noted that additional components may be included as appropriate throughout the system. For example, heat exchangers could be positioned at other desired locations within any of the foregoing systems, such as any of the conduits illustrated. It should be further noted that in any of the illustrated embodiments, additional standby boilers (not shown) could be provided that are set up to run exclusively on fuel purchased offsite, such as natural gas. In addition, one or more boilers could be incorporated into any of the embodiments that run on biogas in combination with such natural gas. Furthermore, it should be evident that one or more boilers could be set up to run exclusively on synthesis gas or biogas. Such standby boilers could be advantageous during periods in which RDF fuel or biosolids fuel is less available, and these standby boilers could also be useful in instances when other boilers are shut down for maintenance.
Numerous modifications will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is presented for the purpose of enabling those skilled in the art to make and use what is herein disclosed and to teach the best mode of carrying out same. The exclusive rights to all modifications which come within the scope of this disclosure are reserved.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/505,950, filed on Jul. 8, 2011.
| Number | Date | Country | |
|---|---|---|---|
| 61505950 | Jul 2011 | US |
