The safe disposal of waste or contaminating materials has been recognized as a significant health and economic issue for many years. The ability to merely dump raw materials into the oceans or landfills is no longer an acceptable mechanism for disposal of the waste. Waste organic matter including that found in raw wastewater (i.e., sewage), sludge from sewage treatment facilities, farm waste, organic industrial waste, leachate, and so forth is a principle cause of water pollution. Therefore, waste organic matter from these and other sources ideally is treated before release into the environment in order to reduce or eliminate the presence of environmentally harmful organic compounds.
One method of treating waste organic matter, especially in wastewater treatment plants and concentrated animal farms, is through anaerobic digestion. Anaerobic digestion is the biological degradation of organic material without oxygen present in which bacteria degrade or digest or decompose the organic matter fed into the system. The anaerobic digestion process has been utilized to treat and remove organic compounds from waste products such as sewage, sewage sludge, chemical wastes, food processing wastes, agricultural residues, animal wastes, including manure and other organic waste and material. As is well known, organic waste materials are fed into an anaerobic digestion reactor or tank which is sealed to prevent entrance of oxygen and in these airfree or “anoxic” conditions, anaerobic bacteria digests the waste. Anaerobic digestion may be carried out in a single reactor or in multiple reactors of the two-stage or two-phase configuration. Heat is normally added to the reactor or reactors to maintain adequate temperatures for thermophilic or mesophilic bacteria which accomplish the breakdown of the organic material. Mixing of the wastes by either mechanical or gas recirculation is can be provided to accelerate digestion.
The products or effluent from anaerobic digestion consist of: (1) a gas phase containing methane, carbon dioxide, and trace amounts of other gases, such as hydrogen sulfide, which in total comprise what is commonly called biogas; (2) a liquid phase containing water, dissolved ammonia nitrogen, nutrients, organic and inorganic chemicals; and (3) a colloidal or suspended solids phase containing undigested organic and inorganic compounds, and synthesized biomass or bacterial cells within the effluent liquid. The gas phase (biogas), if captured, can be utilized as a valuable clean fuel for heat and power generation or transportation. By capturing the biogas, the use of anaerobic digestion can produce valuable energy from waste streams of natural materials or to lower the pollution potential of a waste stream.
The biogas generated from anaerobic digesters in wastewater treatment plants (WWTPs) and concentrated animal farms usually comprises a mixture of several gases and vapors, mainly methane and carbon dioxide. This biogas is rich in methane, typically containing 50-70% methane with the balance being carbon dioxide, hydrogen sulfide, water, and siloxanes. The methane in the biogas contains the bulk of the energy value of the biogas, and thus, this methane in the biogas allows the biogas to be burned for heat or used to fuel an electric generator among other uses. Most WWTPs and farms use a portion of this gas to provide fuel to their digester and process boiler. Unfortunately, using the biogas as a fuel for the digester and process boiler can consume over 50% of the total gas generated.
In many cases, the portion of the biogas not used as fuel for the digester and/or process boiler is flared. Flaring the gas ejects millions of BTUs to the atmosphere that might otherwise be put to good use. In addition to wasting the BTUs that this portion of biogas could provide, flaring of the excess biogas also results in release of significant quantities of so-called “greenhouse gases” such as carbon dioxide. The atmospheric levels of greenhouse gases, which include carbon dioxide (CO2) are rising rapidly and are believed to be a significant factor in the rise in global warming and its potential impact on the earth's climate, ocean levels and human lifestyles. Carbon dioxide levels in the earth's atmosphere are at historic high levels. Although the greenhouse warming potential of carbon dioxide is small compared to some of the other greenhouse gases, due to the sheer mass of carbon dioxide emitted into the atmosphere, carbon dioxide presently has a significant impact as a greenhouse gas in the atmosphere.
It is estimated that globally, over 24 billion metric tons of carbon dioxide were emitted into the earth's atmosphere in 2001 as a result of burning fossil fuels. Some predict that by the year 2025, global emissions of carbon dioxide may reach 35 billion tons. If it is possible to capture and use a significant amount of the biogas that would otherwise be flared and released as greenhouse gases into the atmosphere, the potential impact of carbon dioxide on global warming may be limited. Thus, in addition to the economic waste caused by flaring the excess biogas that could otherwise be used as fuel, flaring has become an unacceptable disposal method of the biogas because flaring the biogas wastes a diminishing hydrocarbon resource and is also a source of air pollution.
In an attempt to be more energy efficient and utilize a portion of the biogas obtained from anaerobic digestion, many WWTPs have installed on-site electric generators to make use of the digester gas energy. If the gas composition of the biogas contains a minimum amount of methane required to provide adequate energy upon combustion, a fuel stream is fed from the biogas stream to an engine driving an electrical generator capable of being powered by the combustion of the methane. The electrical generator is provided to generate a minimum amount of electricity to provide power to the facility and any excess can be exported from the facility. Electrical generation, however, is costly and maintenance is intensive and, in most regions of the country, does not provide the WWTP facility with any savings over purchased electricity. In fact, the high maintenance and operating costs involved in using an electrical generator have compelled many wastewater treatment plants to abandon the use of electric generators, and return to flaring the gas. The recent focus on self-sustainability, energy conservation, and the need to reduce greenhouse gases has prompted these plants to seek new alternatives for utilizing the energy in the flared gas.
One alternative to the use of electric generators for utilizing a portion of biogas not used as fuel is using a pressure swing adsorption (PSA) system to upgrade the biogas to pipeline quality which would provide a clean gas for on-site use, for sales to the pipeline, or for compressed natural gas. Pressure swing adsorption (PSA) technology has recently found application for upgrading the biogas from anaerobic digesters to produce a valuable high-BTU fuel that can be sold directly to the pipeline or converted to compressed natural gas (CNG) or liquid natural gas (LNG).
Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing undesirable impurities. Gas separations by pressure swing adsorption (PSA) are achieved by coordinated pressure cycling of a bed of adsorbent material which preferentially adsorbs at least one or more readily adsorbable components present in a feed gas mixture relative to at least one less readily adsorbable component present in the feed gas mixture. That is, the bed of adsorbent material is contacted with a ready supply of a feed gas mixture. During intervals while the bed of adsorbent material is subjected to the ready supply of feed gas mixture and the bed is at or above a given feed pressure, a supply of gas depleted in the at least one more readily adsorbable component may be withdrawn from the bed. Eventually, the adsorbent material in the bed becomes saturated with the at least one more readily adsorbable component and must be regenerated. At which point, the bed is isolated from the ready supply of feed gas mixture and a gas enriched in the at least one more readily adsorbable component is withdrawn from the bed, regenerating the adsorbent material. In some instances, the bed may be subjected to a purge with depleted gas to facilitate the regeneration process. Once the adsorbent material is sufficiently regenerated, the bed is again subjected to the ready supply of feed gas mixture and depleted gas can once again be withdrawn from the bed once the pressure on the bed is at or above the given feed pressure. This cycle may be performed repeatedly as required.
The adsorbent material selected for use in the pressure swing adsorption units depends on the component to be separated from the feed stream. Adsorbent materials suitable for use in the pressure swing adsorption apparatus include, but are by no means limited to, activated carbon; carbon molecular sieve (CMS) adsorbents; activated alumina; silica gels; zeolites; and the titanium silicates. One skilled in the art is able to select a given adsorbent material or mixtures thereof, for use with a given feed gas mixture and desired product materials.
Numerous patents describe PSA processes for separating carbon dioxide from methane or other gases. One of the earlier patents in this area is U.S. Pat. No. 3,751,878, which describes a PSA system using a zeolite molecular sieve that selectively adsorbs CO2 from a low quality natural gas stream operating at a pressure of 1000 psia, and a temperature of 300° F. The system uses carbon dioxide as a purge to remove some adsorbed methane from the zeolite and to purge methane from the void space in the column. U.S. Pat. No. 4,077,779, describes the use of a carbon molecular sieve that adsorbs CO2 selectively over hydrogen or methane. After the adsorption step, a high pressure purge with CO2 is followed by pressure reduction and desorption of CO2 followed by a rinse at an intermediate pressure with an extraneous gas such as air. The column is then subjected to vacuum to remove the extraneous gas and any remaining CO2.
The use of PSA technology for upgrading digester biogas is relatively new and only a few PSA systems have been installed on digesters. PSA, and other purification methods such as membranes, separate the compounds in the biogas into two streams, one rich in methane (referred to as the product) and the other rich in contaminants (referred to as the tail gas). Irrespective of how efficient the separation process is, the PSA tail gas still contains some methane, on the order of 11 to 20%. The low methane level of the tail gas and the presence of contaminants, such as siloxanes, render the tail gas unusable as an engine fuel to drive pumps, blowers, or generators. To date, the tail gas has been disposed in a thermal oxidizer or flare. These disposal systems for the tail gas can be expensive especially on gases below 200 BTU/cf and can require supplemental high-purity gas. This minimum requirement on the BTUs of the gases required by the disposal systems compromises the economic viability of a gas upgrade project. Most likely due to the recent emergence of gas separation systems for upgrading digester gas, to date, there is no evidence to indicate that any strategies have been advanced to address this problem.
The low-BTU tail gas stream produced by the PSA system is too low in heating value to be directly useful as a fuel and requires disposal in a flare or thermal oxidizer. The low-BTU tail gas stream consists largely of carbon dioxide, moisture, hydrogen sulfide, other contaminants, and low levels of methane. The composition of the tail gas stream does not contain a sufficient methane concentration to provide adequate energy upon combustion. While this tail gas is too low in heating value to be useful as a fuel, the amount of tail gas is significantly large in volume and therefore represents a considerable loss of valuable methane. Moreover, flares are capital intensive and can be difficult to operate on very low quality fuels. Furthermore, using a PSA system still requires gas being flared, which means that PSA upgrading represents only a partial solution to the recovery of digester gas. What is needed is an environmentally and economically sound strategy for eliminating flaring and disposal of the tail gas produced by the biogas upgrading process.