BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production of methane and other combustible gasses useful for generating power by converting non-toxic biodegradable organic materials and high solids such as sugar, starch and/or carbohydrates into a product gas. The gas is composed primarily of methane, carbon dioxide and hydrogen. Power may be produced by further burning the gas in a boiler or other suitable engine or generator to create electricity. The electricity may be used for operation of the gas-production plant itself, and any excess electricity made available for sale to others where it may be used in engines, cars, trucks, busses, etc. The gas may also be used as fuel for gas fired engine generators for peaker plants. Alternatively, the gas may be scrubbed and sold as clean gas to offset the use of natural gas.
2. Description of the Prior Art
The production of methane and other usable biogases by anaerobic digestion of various organic wastes, particularly sewage sludge organic waste, is well known. The organic feed mixture which provides the substrate for anaerobic biodegradation may comprise a wide variety of organic carbon sources. Many digester designs, feed stocks, mixtures and additives have been proposed to increase the methane yield and to provide greater conversion efficiency of organic materials to useful products.
The production of biogas was discovered in the seventeenth century. Today, biogases such as methane are commonly produced at municipal wastewater treatment facilities in a controlled environment using anaerobic digesters. Most of these digesters are completely mixed digesters. Treatment of wastewater in municipal facilities is very expensive, and the cost is covered by charging a fee to residents or businesses for disposing off their waste in the sewer. Anaerobic treatment requires substantial quantities of heat, but this heat may be compensated for by the methane gas produced. There are many variations of anaerobic digestion that are very successful and very basic. The conversion of organic materials into biogas is performed by the microorganisms in the digester. These microorganisms do the work so long as the organic content and temperature of the digester are maintained within certain ranges.
Anaerobic filter-type reactors promote the retention of bacteria in the digester by attaching bacteria to fixed inert materials in the digester. Anaerobic filter-type digesters are also limited to primarily liquid feedstocks containing less than about 1 percent (1%) solids since they become plugged when solids concentration in the digester increases due to higher solids loading or accumulation of solids over longer periods of operation.
Horizontal plug flow digester designs have been implemented, but horizontal plug flow reactors are limited to use of homogeneous solids feed materials (such as manure), which do not tend to settle by gravity. The horizontal plug flow reactor design encourages rapid disengagement of gas from the liquid phase. Horizontal plug flow reactors generally have poor conversion efficiencies of the biodegradable fraction, on the order of about 40 to 60 percent due to biologically unreactive zones within the digester, short circuiting of the feed material, and bacterial washout.
Anaerobic digestion of terrestrial plant material to produce methane gas been recognized as exemplified by D. L. Klass and S. Ghosh, “Methane Production by Anaerobic Digestion of Bermuda Grass,” presented at Symposium on Biomass as a Non-Fossil Fuel Source, ACS/Chem. Soc. of Japan Joint Chemical Congress, Honolulu, Hawaii, Apr. 1-6, 1979. Likewise, the anaerobic digestion of aquatic plant material to produce methane has been recognized as exemplified by R. P. Lecuier and J. H. Marten, “An Economic Assessment of Fuel Gas from Water Hyacinths,” Symposium papers, Clean Fuels from Biomass, Sewage, Urban Refuse, Agricultural Wastes, Orlando, Fla., Jan. 27-30, 1976.
U.S. Pat. No. 4,329,428 teaches production of methane gas in higher yields and at higher rates by thermophilic and mesophilic anaerobic digestion of a mixture of plant material of terrestrial or aquatic origin and organic waste. U.S. Pat. No. 4,424,064 teaches production of methane gas with higher yields and at higher rates by thermophilic or mesophilic anaerobic digestion of aquatic plant material, at least a portion or all of which has been grown in organically polluted water. U.S. Pat. No. 4,316,961 teaches higher yields of methane gas at higher rates by thermophilic or mesophilic anaerobic digestion of plant material and/or organic waste of normally low biodegradability in the presence of an extract of different plant material.
An Upflow Anaerobic Sludge Blanket (UASB) process has been developed for bioconversion of feedstocks which contain primarily soluble organic waste wherein small amounts of solids, ordinarily less than 1 percent of the feedstock. The bacterial mass are allowed to settle in the reactor. The Upflow Anaerobic Sludge Blanket process and reactor are described in the following publications: G. Lettinga, et al, “Anaerobic Treatment of Methanolic Wastes,” Water Research, Vol. 33, pp. 725-737, Pergamon Press Ltd. 1979; and G. Lettinga, et al, “Upflow Sludge Blanket Processes,” 3rd International Symposium on Anaerobic Digestion, 1983, Cambridge Mass.; and G. Lettinga, et al, “Anaerobic Treatment of Raw Domestic Sewage” at Ambient Temperatures Using a Granular Bed .UASB Reactor, Biotechnology and Bioengineering, Vol. XXV, pp. 1701-1723, 1983. This reactor design is limited to liquid feedstocks containing less than about 1 percent solids, and it requires effective gas/liquid separators, recycle for bed expansion, and means for distributing the feed over the bottom of the reactor.
Continuous flow fluidized bed fermenters embodying a tower design or a supported film reactor are described in G. F. Andrews, “Fluidized-Bed Fermenters: A Steady-State Analysis,” Biotechnology and Bioengineering, Vol. XXIV, pp. 2013-2030, 1982. This article teaches that stratification tends to occur in tower fermenters, and solids concentration varies along the height of the tower fermenter, with a low cell concentration in the upper parts of the tower fermenter leading to a low volumetric productivity.
U.S. Pat. No. 4,208,279 teaches anaerobic digestion of animal waste which is fed to the top and one end of an unstirred digestion volume which is about five times as wide as it is high. Effluent sludge is removed at the opposite end of the reactor. Solids movement in the digester is essentially horizontal and the liquid volume is not agitated, except by gas formation. Suitable solids residence times are one month and over, and the solids feed concentration is about 5 percent.
U.S. Pat. No. 4,311,593 teaches anaerobic digestion of waste water in a digester volume which is about four times as wide as it is high. Microorganisms are stabilized on high surface area media. Agitation of the microorganism biomass on the media is provided by gas formation bubbling up through the reactor liquid. U.S. Pat. No. 4,388,186 teaches mechanical condensation of sludge prior to anaerobic digestion of sludge in a vertically elongated, stirred digester tank. The '186 patent also teaches conducting an acid fermentation stage and an acid regression stage separately prior to carrying put an alkaline fermentation stage in the elongated, stirred digester tank. U.S. Pat. No. 1,806,698 teaches a sludge digester wherein solids collect at the bottom. Supernatant liquid accumulating in the upper portion of the digester is recycled to the surface of the digester contents to reduce foam scum. U.S. Pat. No. 1,880,773 also teaches anaerobic digestion of sewage sludge in a digester wherein solids settle to the bottom of the tank. Liquid supernatant from the upper portion of the digester is recirculated to prevent the accumulation of scum or foam at the top surface of the digester contents.
SUMMARY OF THE INVENTION
The present invention provides both a process and apparatus for supplying one or more digesters with an appropriate feedstock for the production of usable biogas. The feedstock is a sugar, starch and/or other appropriate carbohydrate material (hereafter “feedstock”) which is 10 to 100 percent (10-100%) biodegradable solids, and may be provided either as entirely new feedstock, or in combination with a waste stream. The invention uses a feedstock with a high percentage of solids in which ninety-five percent (95%) or more of the solids are biodegradable and converted into biogas. Pure sugar is an excellent feedstock as it contains over ninety-nine percent (99+%) solids and is ninety-nine percent (99+%) biodegradable. Beet molasses has a sugar base of between about forty-five percent (45%) and about forty-seven percent (47%). Such molasses will convert about ninety-five percent (95%) by weight of its sugar to biogas. Ninety-five percent (95%) of starch by weight will be converted to biogas. The balance of the sugar, starch or carbohydrate feedstock is mostly water.
Other available feedstocks include without limitation such materials as starches, high sugars (a.k.a., polysaccharides), beet sugar, sugar beet molasses, sugar beet syrup, sugar beet juice, sugar cane molasses, cane sugar, sugar cane syrup, sugar cane juice, corn syrup, glucose, cereal grains, corn flour, wheat flour, rice flour, potato juice, potato pulp, soybean sorghums, and other similar materials, and/or combinations thereof which are all feedstocks having 10 to 100 percent (10-100%) biodegradable solids. The pure sugar example above will convert the solids to 100% biogas.
It is an object of this invention with its process using anaerobic digesters will have extremely small amount of liquid waste to dispose of when not used with a wastewater treatment plant. This process converts 98 to 100 percent of its feedstock into product gas, which is a main object to this invention.
The present invention provides for the injection of large amounts of non toxic feedstock to provide uniformly high rates of bioconversion and increased process stability and profitability. Increased anaerobic digestion system stability results in production of higher quality product gas having a higher methane content, and more efficient utilization of feedstock. Valves and/or pumps utilized in the practice of the present invention are located externally of the digester contents, and may be replaced if needed, requiring little or no digester downtime. The present invention provides substantially complete bioconversion of biodegradable components of feedstocks and a higher bioconversion of wastewater biosolids in standard municipal wastewater treatment plants due to the extremely high concentration of thermophilic microorganisms and methanogenic organisms that are developed and retained in the digesters. The present invention provides substantially complete bioconversion of biodegradable components of feedstocks at enhanced bioconversion rates.
The process of the present invention includes the basic steps of determining the appropriate feedstock(s) to be used, supplying one or more such feedstocks to at least one digester either as entirely new feedstock or mixed with waste water to form an input stream, and recirculating the stream through a thermophilic anaerobic digester and then a mesophilic anaerobic digester operated in a series. The content of the input stream as well as the temperature of each of the digesters must be carefully established and monitored during the conversion process. The process may include the additional steps of preheating the input stream (particularly when entirely new feedstock is used), and scrubbing the biogas produced from the reaction for more refined uses.
The process will be able to work a single digester or a series of several digesters of the complete mixed digesters type. The process and apparatus work in a variety of shaped tanks. The inside of the digester tanks contain no moving parts. The anaerobic digestion process and apparatus of the present invention allow two to ten times greater solid loads than conventionally used continuously stirred tank digesters, thus providing lower digestion volume requirements per pound of solids converted to useful gas. Conventional stirred tank digesters operating under mesophilic condition accommodate solids loading of about 0.05 to about 0.1 lbs. organic matter/ft3/day. The solids concentrating thermophilic anaerobic digestion process and apparatus of the present invention operates a much higher solids loading in excess of 3.0 lbs. organic matter/ft3/day under loading of sugar, starch or carbohydrates. The less sludge entering the digester the more sugar that may be applied and the more gas produced. The advantage of sugar-fed anaerobic digesters is the temperature, which will be controlled at 70° C. or higher when used with municipal sewage waste water treatment plants, and will retain greater system stability. The hydrolysis fermentation microbial population is relatively resistant to contaminants and toxic components while the methane-producing microorganisms have a lower resistance to toxic components.
The apparatus includes at least one, but preferably two, anaerobic digesters operated in series, an appropriate feedstock collection and conveyance system such as a drop tank or the like, piping, tubing, valves, temperature controls, heaters and scrubbers. In particular, the apparatus includes piping and holding tanks for injecting metered, controlled feedstocks such as sugar beet molasses or sugar cane molasses into the mixing line of the thermophilic digester so that the input comes out at the top liquid level in the thermophilic digester and mesophilic digester. The apparatus reduces or eliminates scum formation by recycling a portion of the digester contents continually and returning the contents to the top of the liquid level of the digester.
In particular, the mixing/recirculating line of the digester has an external pump that draws the slurry from the bottom of the digester and pumps the slurry to the top of the slurry level of the digester near the top, this pumping method makes the digester completely mixed at all times. New feedstock having a suspended solids concentration of between about 10 to 100 percent biodegradable solids is introduced into the recirculating mixing line, and is diluted immediately with the digesters slurry and enters the digester at the top. Normal wastewater treatment plants have very low solids coming into the plant. The primary and secondary sludge is controlled as to the desired solids content of the digester. Said control of desired solids allows the operator to add an appropriate amount of sugar, starch and/or carbohydrate feedstock in order to create very active thermophilic and mesophilic digesters.
At least two very different systems are provided by the present invention. The first is a stand-alone system for production of biogas that utilizes an input stream containing a carefully selected and closely monitored feedstock mixed only with water and/or the existing digester slurry. The second is an add-on (retrofit) system to an existing waste stream (such as is found in a municipal waste water treatment facility) for improving the quality of the existing waste stream for better production of biogas and higher percentage of conversion of biodegradable solids in the waste stream.
The primary object of this invention is to efficiently create the maximum amount of biogas for the least amount of cost.
The first aspect of the invention is the stand-alone system which utilizes both a thermophilic anaerobic digester and a mesophilic anaerobic digester that operate with a controlled feedstock of 10% to 100% organic biodegradable solids. The feedstock must be highly biodegradable and use only enough water to form a slurry for mixing purposes. The stand-alone system does not use any waste products nor is it used in conjunction with any wastewater or sewage treatment plant of any kind. The digesters in the system each have an active, producing microbial anaerobic digestion population comprising hydrolysis fermentation organisms suspended in liquid. In the stand-alone system, the feedstock may be sugar, starch, carbohydrates, or any combination of them, and should preferably contain between ninety and one hundred percent (90%-100%) solids that are substantially fully biodegradable (i.e. approaching ninety-nine percent 99% biodegradable). Once the digesters are started, the feedstock is mixed and then carefully introduced into the digesters in series where it is digested and becomes biogas.
The stand-alone system provides virtually complete (i.e. approaching 100%) bioconversion of biodegradable solids. As a result, some non-contaminated water added to dilute the feedstock must be used to keep the digester at an acceptable level for the process to continue. The biogas produced is wet as it rises and is drawn off the top of the inside of each digester. Because the stand-alone system uses non-toxic, organic, highly biodegradable feedstock with low water content, there is little, if any, washout of feedstocks and microorganisms, thus resulting in increased conversion efficiency and more stable digester condition.
It is therefore an important object of the stand-alone system to create very little effluent to dispose of, make higher biogas production and have no biosolids to dispose.
It is also an important object of the stand-alone system to provide unlimited retention time as the feedstock stays in each digester until it comes out as biogas.
It is also an object of the stand-alone system to provide a process and apparatus for anaerobic digestion of feedstocks of sugar, starch and carbohydrates in a totally newly constructed, free standing facility built for the sole purpose of creating biogas from a renewable product and not using a waste product or being used in conjunction with a waste water treatment plant receiving sewage from a city.
It is an object of the stand-alone system to use dry feedstocks such as sugar, starches and carbohydrates that will be mixed with either non-contaminated water or digester liquid to create a slurry needed for ease of pumping.
It is an object of the stand-alone system to avoid the need for higher temperatures otherwise necessary to pasteurize the pathogens and other contaminants found in a waste stream. In a stand-alone digester/biogas plant, after all of the solids have been converted to gas there will be little if any remaining waste to dispose of.
In another aspect of the invention, an appropriately selected feedstock concentration is mixed with a waste stream and then introduced into the digester(s) for conversion into biogas. Combining the system with a municipal or other waste water treatment facility will help to create a high volume of biogas which is profitable to sell, while also helping the facility to attain the Class A biosolids 40 C.F.R. Part 503 Standard by pasteurizing sewage sludge.
One of the main costs in producing methane gas is the cost of building the digester. In the case of municipal wastewater treatment plants, many thousands of sites the world over are existing, and have been built at a cost of hundreds of thousands and many in the hundreds of millions of dollars. With the treatment plants in place and no dollar return ever, all are non-profit due to the fact that they were never built to create an income. They were built solely to treat wastewater for the safety of the environment. With the process and apparatus of this invention, the digesters will produce very large amounts of methane, which will return a profit on additional money invested. The gas produced can be used in many ways. The gas can be used straight as biogas in the boiler for steam or electric cogeneration. Scrubbing the biogas to clean methane gas and pressurizing the clean gas and pumping to storage tanks allows for sales to the public in place of natural gas.
It is an object of the process and apparatus of the present invention to create an economical and profitable anaerobic digester from very large amounts of gas generated by the controlled feeding of a non-toxic biodegradable sugar or starch feedstock such as starches, high sugars, (a.k.a., polysaccharides), beet sugar, sugar beet molasses, sugar beet syrup, sugar beet juice, sugar cane molasses, cane sugar, sugar cane syrup, sugar cane juice, corn syrup, glucose, the starch group of cereal grains, oat flour, rice flour, corn flour, wheat flour, potato juice, potato pulp, sorghum, and other similar materials, and/or combinations thereof, all of which are hereinafter referred to as “feedstocks”.
It is an object of this invention to enhance existing municipal wastewater sewage treatment plants and digesters and to provide substantially complete bioconversion of biodegradable components of biomass and organic waste materials to usable product gas.
It is an object of this invention to be able to add this process to any existing municipal wastewater treatment plant or other similar facility, and to create a return on the investment therein resulting from the cost of such things as tanks, pumps, compressors and feedstock required by the addition.
It is an object of this invention to aid municipal wastewater treatment plants in meeting the fecal coliform requirements of the recent 40 C.F.R. Part 503 Standards for Class A biosolids through the pasteurization capability of the thermophilic digester.
It is an object of the present invention to produce all the electric and gas needs of the treatment plant and have excess power and gas to sell to outside markets.
It is an object of the present invention to teach municipal wastewater treatment plants how to produce methane gas in higher yields and at higher rates by thermophilic and mesophilic anaerobic digestion by adding extra feedstock, such as sugars, starches and carbohydrates, in existing treatment plant digesters.
It is an object of the present invention to teach the production of clean burning gas which is made from renewable non toxic organic materials like corn syrup, corn flour, wheat flour, potatoes, sugar beet syrup, sugar beet molasses, sugar cane juice, syrup and molasses.
It is an object of the present invention to teach the production of clean burning gas which is made from renewable non toxic organic materials like sugar, starch and carbohydrates.
It is an object of the present invention to teach the use of scrubbing biogas to create a salable gas product (i.e. methane) that may be used to operate any existing truck, bus, auto or farm engine which is now running on natural gas with little or no adjustment.
It is another object of the present invention to utilize any unscrubbed biogas by burning it in a gas fired boiler to create the steam needed in the wastewater treatment plant for such diverse uses as heating digesters, molasses tanks and pipes; for steam turbines to generate electricity; for heat exchangers for the plant use, and/or to generate and sell excess power to the local power company in the area.
It is another object of the present invention to create a large demand for sugar beet molasses and cane molasses or sugar beet juice, cane juice, corn flour and wheat flour in order to create a larger demand so as to enable the farmers to produce and market a larger supply of feedstock.
It is an object of the invention to improve most wastewater treatment plants through a two-stage digester system, one a thermophilic and the other a mesophilic digester.
It is yet another object of the invention to successfully use high solid concentrations of sugar, starch and/or carbohydrates in a thermophilic or a mesophilic anaerobic digester apart from any wastewater treatment facility in a profitable manner.
It is an object to this invention to use a process of high biodegradable solids such as sugar or sugar beet molasses and cane molasses and starches.
Beet molasses is one example of the type of feedstock that is described in this invention. A brief analysis from BSK Analytical Laboratories, Fresno, Calif., of a solid sample of beet molasses is shown in the following tables:
|TABLE 1 |
|Analysis of Beet Molasses (solid sample) (sampled 9/1/2000) |
| || || || || || || ||Prep ||Analysis || |
|Analyte ||Method ||Result ||Units ||PQL ||Dilution ||DLR ||Date ||Date |
|BOD ||SM 5210-B ||380000 ||Mg/Kg ||50 ||1000 ||50000 ||09/07/2000 ||09/12/2000 ||H |
|COD ||SM 5220-D ||690000 ||Mg/Kg ||50 ||1000 ||50000 ||09/07/2000 ||09/07/2000 |
|pH - DI ||EPA 9040 ||9.1 ||Std. Unit ||— ||1 ||N/A ||09/07/2000 ||09/07/2000 |
|Solids ||SM 2540-B ||98 ||% ||0.1 ||1 ||0.1 ||09/08/2000 ||09/11/2000 |
|Volatile ||SM 2540-B ||100 ||% ||0.1 ||1 ||0.1 ||09/18/2000 ||09/20/2000 ||H |
|TABLE 2 |
|Analysis of Sugar (Solid Sample) (sampled 1/12/2001) |
| || || || || || || ||Prep ||Analysis |
|Analyte ||Method ||Result ||Units ||PQL ||Dilution ||DLR ||Date ||Date |
|BOD ||SM 5210-B ||520000 ||Mg/Kg ||50 ||2000 ||100000 ||1/12/2001 ||01/17/2001 |
|pH - DI ||EPA 9040 ||4.7 ||Std.Unit ||— ||1 ||N/A ||1/15/2001 ||01/15/2001 |
|Solids ||SM 2540-B ||100 ||% ||0.1 ||1 ||0.1 ||1/16/2001 ||01/19/2001 |
|Volatile ||SM 2540-B ||100 ||% ||0.1 ||1 ||0.1 ||1/16/2001 ||01/19/2001 |