Patent Application
20230321598
The technology relates to the field of biogas conversion to biomethane for energy generation, and more particularly relates to the collection of biogas from a plurality of sources generating biogas from decomposition of organic waste, at a range of geographic locations, and routing the collected biogas to location(s) for processing into biomethane and/or generation of electricity, including generation of electricity in fuel cells.
Electricity is a necessary power source in a modern technology-based economy, and also in those economies that are developing into modern economies, which have exhibited a great demand for more electric power as standards of living improve. Initially, the basic source for electricity generation was combustion of coal (carbon), or oil (hydrocarbons), both of which are often called “fossil fuels.” Both coal and hydrocarbon oils generate carbon dioxide as a combustion byproduct, and carbon dioxide is a greenhouse gas that contributes to the phenomenon of global warming, or climate change.
Electricity has also been generated as hydro-electric power by turbines driven by flow of water, typically at dams when water is released through sluice gates to drive the turbines. More recently, there has been an upsurge in generation of electricity using solar power as the cost of solar cell panels have begun to decline thereby reducing the cost per KW-hr of electricity generated. In addition, the use of wind power turbines, especially in Texas and California, has caused electricity from wind power to become a significant part of the supply to the power grid. Solar and Wind both suffer the disadvantage of being intermittent (not schedulable) sources of electricity.
There also continues to be growing interest in both small as well as large scale generation of biogas (typically 60% methane) that can be purified into a “renewable natural gas” or RNG, a more purified combustible gas (generally at least 95% pure methane) from waste. Ordinarily, this biogas is a natural product of the decomposition of organic waste material, most often though an anaerobic process, and is released into the atmosphere. In the atmosphere, it is believed that biogas, especially the methane component, is a “greenhouse gas” that contributes to the phenomenon of global warming, or climate change. On the other hand, if the biogas could be captured and treated to generate electricity or processed into RNG, this methane gas could be used to generate power in methane-burning power plants or fuel compressed natural gas vehicles with the previously vented methane being destroyed in the process thus eliminating its greenhouse gas effect. Moreover, the substitution of bio-derived gas for transportation fuels, such as compressed natural gas (“CNG”) presents a further opportunity to back out fossil fuel combustion and thereby reduce greenhouse gas emissions.
Methane has a lower carbon to hydrogen ratio than coal or hydrocarbon oils. Thus, methane produces less carbon dioxide upon combustion. As a result the net effect of this capture of methane, and subsequent combustion to produce electricity, is to “back out” other fuel sources that would have been combusted in the power plants, such as coal or hydrocarbon oils. This is highly beneficial since the combustion of methane produces less carbon dioxide per KW-hr of electricity or less carbon dioxide per brake horsepower-hour of a vehicle engine, than the combustion of other hydrocarbon fuels because of its lower carbon to hydrogen ratio. Thus, the removal of the biogas preventing release into the atmosphere, and the combustion of methane from the biogas provides energy has a two-fold benefit in reducing greenhouse gas emissions: reduction of biogas into the atmosphere, and less combustion of carbon dioxide emissions in electricity generation by “backing out” coal or other hydrocarbons that have a higher carbon to hydrogen ratio. Moreover, organic waste generation and decomposition to produce biogas are continuously ongoing processes, so that biogas is, in that sense, a potentially continuously “renewable” energy supply.
The technology presented herein provides access to remote biogas sources (“digesters”) that are geographically widely distributed, predicts (based on instrumentation) the availability and quality of biogas from these remote sources typically 24 hours in advance, conditions and prepares the biogas to safely aggregate the biogas via a network of conduits and conveys the biogas to biogas consuming devises (“generators”) and/or to a single treatment facility where it is converted to RNG or biomethane and controlledly compressed into a natural gas pipeline for subsequent use as combustion fuel, compression to CNG or LNG (liquified natural gas) for vehicle fuel use, or use in fuel cells to create electricity directly. Electricity generated at the treatment facility, or at the remote biogas sources, can be used directly or indirectly (via a contract rather than physical delivery of electrons) to provide a renewable source of electrical energy for plug-in battery-powered electric vehicles (PEVs), thereby providing the desirable goal of converting biogas to clean transportation energy.
In an exemplary embodiment there is provided a biogas production, conditioning, collection, electrical generation, purification and delivery system. The system aggregates biogas from a plurality of remote sources and treats the biogas to produce electricity and/or biomethane. The system comprises: a network of conduits configured to convey biogas from the plurality of remote sources of the biogas based on a monitored or automatically detected availability and quality of biogas at each respective remote source. A biogas compressor at each of the remote locations is controlled by a central controller that utilizes data that includes biogas availability and quality data. The central controller is configured to instruct the remote biogas compressor(s) to supply compressed biogas to several potential processes, as well as to a central biogas processing facility, depending upon input data. Thus, the biogas may be compressed to a fuel cell or to an internal combustion engine powered generator for direct conversion of the biogas to electricity to power at least some of the equipment at the remote source or at the centralized facility, or the electricity can be transmitted to a charging station for PEVs to charge vehicle batteries. Compressed biogas can also be supplied to a biogas header that conveys the biogas to the central processing facility. Here, the received biogas from many remote sources in the network of linked-together plurality of remote sources is treated and processed from biogas into biomethane and/or also converted to electricity pre (as biogas fuel) or post upgrading (as biomethane fuel). The central processing facility has an input compressor that is controlled by the central controller that operates based on data including the data from the remote sources about the availability of biogas at the sources. The central treatment facility includes several biogas treatment operations, including but not limited to a biogas hydrogen sulfide removal stage; an activated carbon scrubber (which may be downstream from the hydrogen sulfide removal stage); a carbon dioxide removal stage (which may be) downstream from the hydrogen sulfide removal stage for purifying the biogas into biomethane by removal of carbon dioxide. And the central processing facility also has a biomethane gas compressor for compressing the produced biomethane, also under control of the central controller. Thus, the biomethane may be charged to a generator such as a fuel cell for direct conversion of the biomethane to electricity to power at least some of the equipment at the treatment facility, or the electricity and/or its environmental attributes can be transmitted (e.g. directly or virtually via a contract) to a charging station for PEVs to charge vehicle batteries. The biomethane can also be charged to a system for compression to renewable compressed natural gas (R-CNG) for use as a vehicle fuel. Or, the biomethane can be compressed to a natural gas pipeline, as explained in more detail here below. The decision (and control) of the biomethane disposition to the natural gas pipeline or to CNG or to fuel cells to create electricity for PEVs is based on several control variables including biogas availability, projected biogas availability, quality, quantity, moisture content, presence or absence of contaminants, hydrogen sulfide levels, oxygen levels, nitrogen levels, historical production rates, projected generator or upgrading plant demand rates, upcoming system maintenance inputs, digester status, digester feedstock availably, ambient temperature and digester efficiency factors, current biogas storage levels and remaining biogas storage levels.
The sources of biogas may include organic animal waste anaerobic digesters, such as for example dairy farm animal waste, and/or captured biogas from waste water treatment plant digesters, special purpose organic waste or mixed substrate digesters, landfills and or combinations of these sources.
The plurality of sources of biogas of the exemplary embodiment may include biogas sources located remote from each other, and the network of conduits enable fluid communication between the remote sources and a central biogas header that is the portion of the network carrying biogas to be received at the central processing facility.
A consideration to bear in mind is that to safely and efficiently convey biogas in a conduit it is (likely) necessary to “condition” the biogas to remove most of its hydrogen sulfide and water prior and in some cases its carbon dioxide prior to its insertion into the gathering/collection line. This conditioning of the biogas will occur at or close to the digester source and prior to compression into the gathering or collection line. If the biogas is utilized by an internal combustion engine or fuel cell generator directly from the gathering line it may need further polishing to remove all traces of contaminants such as hydrogen sulfide or siloxanes.
The biogas hydrogen sulfide removal stage of the central processing facility of the exemplary embodiment may include a sodium hydroxide gas scrubber or an iron sponge column.
At least some of the sources of biogas of the exemplary embodiment may include a gas quality sensor (measuring and logging for example the percent of methane, oxygen, carbon dioxide, moisture, hydrogen sulfide) and a gas quantity flow meter. The quantity and quality of the energy exported form each digester feeds the control system and also the accounting system for allocation of payments back to the owner or landlord hosting the digester. A hydrogen sulfide sensor located to confirm quality is acceptable and/or to detect release of hydrogen sulfide into a surrounding vicinity of the at least some of the sources and activate a central controller to shut a valve to cut off flow of biogas, as necessary, to eliminate or minimize leakage upon detection of hydrogen sulfide gas. It is also important to detect hydrogen sulfide for other reasons: hydrogen sulfide poisons upgrader membranes or pressure swing absorbents, poisons fuel cells, and damages internal combustion engines. So, the sensor should supply input to the control system to confirm acceptable quality. [Currently in California the collection line, by permit and safety considerations, cannot have>10 ppm H2S (sometimes up to 100 ppm H2S is OK, typical biogas is 2000-5000 ppm H2S.]
The remote sources of biogas may each include a flow sensor configured to detect a flow of biogas from the source. The flow sensor data is transmitted to a central controller that controls a valve, such as a three-way valve, to recycle a portion of the flow back to the respective source of biogas. The remote sources (digesters) may include a gas chromtograph or other form of gas analyzer supplying data to the control system to ensure the quality of the biogas is acceptable for delivery to the collection system and delivery ads a fed gas to the generators and/or to the upgrading plant(s). The remote source may include a flow meter also providing data to the control system to measure the flow quantity of gas for control purposes and also for accounting of the digester production of energy and its quality into the system.)
At least one of the plurality of biogas sources of the exemplary embodiment may include a flexible roof over the waste digester. The flexible roof expands upward when an amount of generated biogas in the digester increases and biogas pressure increases under the flexible roof. The biogas source may have an automatic detection device that provides data about the amount of biogas available from the digester, such as but not limited to, a laser deflection measurement apparatus. The apparatus may be positioned and configured to measure a degree of deflection of the flexible roof as it moves under biogas pressure. The measured laser deflection data is transmitted to the central controller that controls a compressor that draws biogas from the digester of the respective biogas source based on the data that includes data about biogas availability. Alternatively, or in addition, biogas availability can also be estimated by periodic human inspection. These inspections could be weekly, daily or several times per day depending on the production rate and remaining storage capacity of the digester. Thus, from observation, if the roof is “highly deflected upward,” more biogas is available. Conversely, when the roof deflection is lower and/or declining, less biogas is available.
There is also presented an exemplary biogas purification apparatus and method used in conjunction with a fuel cell for directly converting biogas to electricity. The biogas supply to the biogas purification system may be from the central biogas processing facility also described herein where some purification of raw biogas has already taken place and where the exiting biogas has a reduced concentration of compounds such as H2S and other sulfur compounds, as well as reduced moisture content. This biogas is the source gas for further purification by removal of deleterious compounds to provide a biogas that is a suitable fuel cell feedstock. Typically, but not necessarily, the biogas may contain about 60% or more methane, along with CO2 as a large component.
The fuel cells useful include those fuel cells adapted to generation of electricity directly from methane. Solid Oxide (“SOFC”) fuel cells, for example, are able to convert an input of methane gas to electricity that could be input to the electrical grid or used to charge electrical vehicles (EVs) or batteries, or for any other purpose, such as driving compressors, pumps and other equipment.
Fuel cells typically require a methane feedstock free of chemicals that would be deleterious to the cell and also prefer a continuous methane feed varying between 20% and 100% of the fuel cell capacity for efficient operation and to maintain the required operating temperature. Accordingly, while the flow rate of the biogas supply to the fuel cell might be variable over time, care should be taken to avoid a “low flow” condition that drops below the minimum turndown rate of the cell that might cause an interruption in cell operation. The exemplary biogas purification apparatus provided has an input of biogas (raw biogas that has already been pretreated to remove the bulk of the hydrogen sulfide) and further treats (“polishes”) the biogas to remove sulfurous compounds, including carbonyl sulfides (COS), which are deleterious to the fuel cell.
There is provided a continuous system of producing electricity directly from biogas. The system includes at least one, and in some cases a plurality of animal manure digester(s). The digesters produce a raw digester biogas comprising methane, carbon dioxide and sulfur compounds, and also usually contain varying amounts of the gas components of air or oxygen, either accidentally drawn into the digester, or deliberately added to the digester for purposes such as to assist aerobic bacteria to remove H2S and other sulfur compounds from the digester gas, while the biogas is still in the digester. The digester gas may be fed directly to a biogas treatment system for removal of the bulk of the H2S and then to a biogas purification system before being charged directly to a fuel cell. The biogas purification apparatus is configured to continuously treat the biogas to remove residual H2S as well as other sulfur compounds, such as COS, that are potential fuel cell poisons, from the biogas or the biomethane as the case may be. It should be noted that the fuel cell can also be charged with biogas from the central processing facility, described elsewhere herein) bypassing the gas purification system, provided that the biogas is substantially free of contaminants such as COS and H2S that might poison the fuel cell.
The biogas purification and electricity generation system optionally includes apparatus (or a method if this is achieved by “a book and trade” of the renewable energy credits (RECs) and electricity from grid injection) to utilize the fuel cell-produced electricity to charge electrical vehicles; and in that event the system is configured to measure, record and transmit data about the amount of electrical energy used to charge electrical vehicles. This can be transmitted in real time to a cloud-based computer system so that a determination can be made from the data of units of electrical charge produced per unit of biogas, and units of electrical charge received by EVs and other users. From this data, an amount of renewable energy credits attributable to conversion of biogas to electricity can be calculated and apportioned.
The exemplary biogas purification system can operate using a variety of sources of biogas, and the digesters at the plurality of remote sources of biogas can be digesters of any suitable animal manure, including without limitation, cow manure chicken manure, or pig manure or horse manure. Advantageously, when sourced from cow manure, the biogas is free of siloxanes, which is deleterious to fuel cells. The biogas purification treatment process provided herein provides a treated biogas that is substantially free of carbonyl sulfides or has such low trace amounts of carbonyl sulfide that these are not deleterious to operation of the fuel cell. In an exemplary embodiment, the system uses a biogas treatment apparatus, such as a packed columns, comprising copper oxide or potassium permanganate, and an activated carbon component configured to treat biogas by polishing it to remove carbonyl sulfides and deleterious trace residues, such as hydrogen sulfide, that were not removed by any prior bulk H2S removal steps. In addition, a gas demister may be deployed in the biogas purification apparatus to remove any entrained fine oil droplets in the biogas in order to protect any components of the biogas polishing apparatus and the fuel cell from the deleterious effects of oil incursion.
The fuel cell system also may include apparatus to convert a direct electrical current produced by the solid state fuel cell to alternating current and apparatus to feed alternating current to an electrical grid.
The fuel cell of system can be any of a variety of fuel cells suitable to convert methane to electricity. An example is the solid oxide fuel cell that can be configured to process biogas to carbon dioxide and water while producing electricity. The condensate formed from conversion of the methane of the biogas to electricity in the fuel cell can be recycled (or disposed of) on the farm that is providing the manure. The (clean) CO2 waste from the fuel cell could be vented, or used in a green house, or other plant growing operation, to enhance the growth of plant, algae or other CO2 consuming photosynthesis-based life form.
The biogas purification system may include a flow control apparatus that includes a biogas holdup tank configured to buffer variations in biogas flow rate into the system or to retain a buffer so that if there is any interruption in supply, the buffer can be used to smooth out the biogas delivery rate to the fuel cell, maintain the flow rate above the minimum turndown rate, and maintain fuel cell operating temperature. These variations in biogas flow rate may reflect variations in digester gas flow rate from the digester. Preferably, the digesters have a bladder configuration, as described herein. The system has a compressor which may be used to compress biogas into the holdup tank, and an electronic controller is configured to maintain a continuous steady biogas flow to the solid state fuel cell, even as flow of raw digester gas from the digester varies over time. The controller may be configured to make step changes in biogas flow rate to the solid state fuel cell based on data that may include detected biogas availability.
The biogas purification system may be configured such that the solid state fuel cell operates at 80 to 100% of capacity, where it is most efficient. The treated biogas charged to the fuel cell comprises up to 40 mol. % carbon dioxide and is free of sulfurous compounds, meaning that the level of sulfur and sulfurous compounds is so low as to not adversely affect the operation of the fuel cell during a normal operational cycle. The system is configured such that the solid state fuel cell operates at temperatures in the range about 500 to about 1000° C., preferably in the range about 500 to about 750° C., or less than about 750° C.
The system is configured to deliver a continuous supply of fuel within the range of 205 to 80% of the capacity of the fuel cell such that the fuel cell is always on, remains hot and does not have to be turned off, restarted, or lose its minimum operating temperature.
The foregoing presents a brief and non-comprehensive summary of some aspects of the technology that is further explained in more detail, here below.
The foregoing aspects and many of the attendant advantages of the present technology will be more readily appreciated by reference to the following Detailed Description, when taken in conjunction with the accompanying simplified drawings of exemplary embodiments. The drawings, briefly described here below, are not to scale, are presented for ease of explanation and do not limit the scope of the inventions recited in the accompanying patent claims.
The following non-limiting detailed descriptions of examples of embodiments of the invention may refer to appended Figure drawings and are not limited to the drawings, which are merely presented for enhancing explanations of features of the technology. In addition, the detailed descriptions may refer to particular terms of art, some of which are defined herein, as appropriate and necessary for clarity.
In the specification and claims, the term “remote,” as it pertains to the geographic location of biogas sources that are linked by the network of conduits to a portion of the network (a common biogas header pipe) that conveys and feeds biogas to the central treatment/processing facility, means that the biogas sources are geographically distant from each other (i.e. not located on the same farm, or waste disposal site, for example). Absent the network of conduits, the biogas supply from these sources would have been processed separately at each of the biogas sources, if at all.
A “biogas source,” as the term is used in the specification and claims, means a source of biogas such as, but not limited to, an organic waste digester that digests animal waste (e.g. manure from a dairy or a waste disposal site) or a landfill, for example, and may include several digesters/landfills at the same geographic location that are linked together and may be counted as a single remote biogas source. The term “biogas” means a gas containing methane that originates from animal waste or landfills. Biogas contains several components such a hydrogen sulfide and carbon dioxide, aside from methane. When purified to standards of methane concentration suitable for injection into pipelines, it is referred to as biomethane.
A remote location may include a biogas conditioning plant that not only removes hydrogen sulfide and water but also removes carbon dioxide thus reducing the quantity of gas that is needed to be collected and transported to the generators and/or to the centralized upgrading plant. This could be a membrane-based biogas conditioning package that selectively separates methane form H20, H2S and CO2. In an exemplary embodiment, the centralized upgrading plant could receive biomethane (as opposed to biogas) and thus its primary purpose would be, under control of the central processor to accept this feed gas, remove any residual contaminants in order to meet pertinent applicable utility specifications, and then compressing and injecting the purified biomethane into the pipeline.
A networked collection system may also include a portion of the system where the biogas or biomethane is conveyed from the remote source location to the centralized location, or generators, via tank cars. The transported biogas may be off loaded at the centralized treatment plant for purification, quality control and delivery into the pipeline. The control and reporting of this “virtual” pipeline would also provide data to the central controller so that the entire system is coordinated and controlled.
In general, the processing of biogas to biomethane at each location where biogas is produced means that equipment and labor has to be expended at each location. Often, this is not economically feasible. According to exemplary embodiments of the invention, biogas from a geographically widely spread out plurality of biogas sources can be accumulated in a network of conduits (pipelines), often linked into a common header pipe, and carried in these conduits to a central gas treatment/processing facility. In addition, in order to deliver the produced biomethane (RNG) it is necessary to bring it to a centralized location where the local gas utility is ready willing and able to receive the gas into their natural gas distribution or transmission system for delivery downstream to customers. This minimizes the outlay of capital in equipment but increases the cost of the conduits. However, advantageously, these conduits may be of inexpensive polymer materials that are impervious to attack by chemicals found in biogas, such as water and hydrogen sulfide, which has a highly offensive odor (“rotten egg stench”) and is readily detected. However, the collection, or aggregation, of the biogas from far-ranging farms, landfills and other sources and processing of the aggregated volume of biogas to biomethane presents several challenges that must be met to produce methane that is of a purity acceptable for combustion in power plants, and for use in producing CNG, while maintaining standards of safety.
In addition to the challenges of biogas aggregation, transport and processing, there is also the issue of distribution of the produced biomethane into natural gas pipelines. In general, pipelines have a system of requiring a gas supplier to contract in advance (by about 24 hours) the volume of approved quality methane it will be able to supply to the pipeline. Failure to meet the contracted supply results in penalties. Accordingly, there must be an accurate determination made in advance (about 24 to 36 hours in advance) of the amount of biogas that will be available to convert to biomethane for charging to a natural gas pipeline, and the remote sources that will supply this biogas. In addition, the biogas supply to the central treatment facility must be controlled. This presents significant challenges. While natural gas produced from a geological formation has relatively predictable rates of production or can be turned off an don as needed as it is stored in an existing geological formation, in a biogas supply system, the volume production of biogas is dependent upon many factors, including weather, ambient temperature, conditions at the waste digesters, remaining storage capacity, and the like that are not readily and accurately predictable. In addition, biogas may be routed at the remote source to a generator or fuel cell for direct conversion to electricity into the grid and/or to a PEV charging station, or to operate equipment at the remote source. Accordingly, the central controller should take into account the amount of biogas available at the remote sources (whether from manual data input or from automated measurements), and the potential alternative disposition of the biogas and address the challenge of being able to predict at least 24-36 hours in advance the availability of biogas to be charged to the treatment facility that will produce a predictable supply of biomethane.
The present technology uses data collected at each of the remote sources of the system that are able to supply biogas to assess biogas availability at each source at least about 24-36 hours in advance and uses this data to be able to commit to inject biomethane from this available biogas supply into the natural gas pipeline. The collected data is collected automatically or manually estimated at each remote source and input to the central controller.
As explained in more detail here below, in some examples the waste digesters have expandable roofs that deflect upward as gas accumulates under the roof. The deflection may be measured, by laser, for example, and the gas volume available can be estimated from the deflection. This provides a means for estimation of the total volume of biogas that could be continuously processed to biomethane and injected into a pipeline. Of course, other methods than roof deflection measurements may also be used. For example, human observation, or measurement of biogas pressure under the roof.
Several exemplary embodiments of systems that treat biogas to produce biomethane for natural gas pipeline injection (or other disposition as described herein) are set forth here below. As already pointed out here above, some of the biogas can be directly converted to electricity in fuel cells. And, as pointed out above, some of the biomethane may be used to make CNG. However, for the sake of simplicity, the explanations may focus mainly on biomethane for natural gas pipeline injection. There are some common technologies among the provided examples, and some variations between them as to apparatus. Nonetheless, each has in common the detection of the volume of biogas available at the remote sources either in real time or periodically. In addition, the connecting network of conduits has a biogas “hold up” volume that is also available to be processed as part of the biomethane that is contracted to be delivered. The technology presented herein provides access to remote biogas sources that are geographically widely distributed, predicts (based on instrumentation and/or human data input) the availability of biogas from these remote sources at least 24-36 hours in advance, safely aggregates the biogas via a network of conduits, and conveys the biogas to a single conversion facility to produce biomethane where it is converted to biomethane and controlledly compressed into a natural gas pipeline and/or supplied to a CNG facility, and/or used in a fuel cell to directly make electricity.
In summary, in an exemplary embodiment there is provided a biogas collection, purification and biomethane delivery system. The system aggregates biogas from a plurality of remote sources, deducts remote uses such as for generation or other biogas take-off, adds virtual deliveries and treats the biogas to produce biomethane. The system comprises: a network of conduits configured to convey biogas from the plurality of remote sources of the biogas based on a monitored or automatically detected availability of biogas at each respective remote source. A biogas compressor at each of the remote locations is controlled by a central controller that utilizes data that includes biogas availability data. The central controller is configured to instruct the biogas compressor to supply compressed biogas to several potential processes, as well as to a central biogas processing facility, depending upon input data. Thus, the biogas may be compressed to a fuel cell for direct conversion of the biogas to electricity to power at least some of the equipment at the remote source, or the electricity can be transmitted to a charging station for PEVs to charge vehicle batteries. Compressed biogas can also be supplied to a biogas header that conveys the biogas to the central processing facility. Here, the received biogas from many remote sources in the network of linked-together plurality of remote sources is treated and processed from biogas into biomethane. The central processing facility has an input compressor that is controlled by the central controller that operates based on data including the data from the remote sources about the availability of biogas at the sources. The central treatment facility includes several biogas treatment operations, including but not limited to a biogas hydrogen sulfide removal stage; an activated carbon scrubber (which may be downstream from the hydrogen sulfide removal stage); a carbon dioxide removal stage (which may be) downstream from the hydrogen sulfide removal stage for purifying the biogas into biomethane by removal of carbon dioxide. And the central processing facility also has a biomethane gas compressor for compressing the produced biomethane, also under control of the central controller. If the biogas to biomethane upgrading is handled at some or each remote location then the central treatment plant still remains as the control hub and RNG product quality station to ensure the RNG meets the utility gas specifications (e.g. Rule 30 for Sempra and Rule 21 for PG&E) Thus, the biomethane may be charged to a fuel cell for direct conversion of the biomethane to electricity to power at least some of the equipment at the treatment facility, or the electricity can be transmitted to a charging station for PEVs to charge vehicle batteries. The biomethane can also be charged to a system for compression to CNG. Or, the biomethane can be compressed to a natural gas pipeline, as explained in more detail here below. The decision (and control) of the biomethane disposition to the natural gas pipeline or to CNG or to fuel cells to create electricity for PEVs is based on several control variables including biogas availability, projected biogas availability and (use same language as previously)
In general, in a simplified explanation of an exemplary embodiment, the central processing facility has several processing units. The processing units may be skid-mounted. Among the processing units are a hydrogen sulfide removal stage, which may include, but is not limited to, a sodium hydroxide scrubber or iron sponge columns. Further, an activated carbon scrubber is deployed downstream from the hydrogen sulfide removal stage to remove further residual amounts of hydrogen sulfide and other contaminants susceptible to activated carbon removal. To remove water vapor, a drier may be located downstream from the activated carbon scrubber. At this stage, the biogas still has significant amounts of carbon dioxide. Thus, a carbon dioxide removal stage, which may include, but is not limited to, a membrane separator that separates out the biomethane from the carbon dioxide, which may be vented. There may be a gas chiller downstream from the carbon dioxide removal stage to chill the biomethane which is at this point high purity combustible methane. A system compressor is configured to compress the biomethane at a controlled rate of compression that is based on data that may include data received from the instruments measuring biogas accumulation at biogas remote sources in the network, but that is also based on the amount of gas being metered into the pipeline. The gas hold-up in the conduit network of the system may also be used as part of the control data for the compressor.
The remote biogas sources may not be of uniform size and are generally not producing biogas at the same rates. Accordingly, with biogas availability being variable within the system, controls are needed so that the system can operate continuously with reasonable predictability of biomethane supply capability, as far as possible based on the availability and expected availability of biogas from the sources. Of course, even if all biogas supply from the biogas sources were shut in, there is still an amount of biogas held up in the volume of the conduits, and in the volumes within the central processing facility that could be available to process biomethane. Depending upon the rate of compression of gas, this gas hold-up in the system represents a “time-lag” within the system that a controller can take into account.
In the exemplary, simplified overview illustration of
Biomethane gas to the natural gas pipeline upgrader must be at acceptable purity and quality (H2S, O2, inerts, water, etc.). The utility operator requires delivery into its point of interconnect (POI) at a flow rate between the minimum and the maximum rates as described in an interconnection agreement so that thee revenue metering equipment can remain within calibration. This min/max flow must be uniform and must be communicated to the utility prior to delivery including a duration of flow.
In an exemplary embodiment, the flow rate from the local biogas compressor 220 is measured by flow detector 224, which controls a control valve 226. This allows the recycling of a proportion of the biogas back into the digester through valve 226, which can have the benefit of reducing the hydrogen sulfide concentration in the biogas. In general, an amount of recycle of from about 2 to about 8 percent by measured volume of biogas can be helpful in this regard.
A hydrogen sulfide detector 230, or several of these, may be located in the vicinity of the digester 210 and in the vicinity of the network pipelines. When the detected level in the atmosphere increases above a preset threshold, the detector shuts down the cutoff valve 232 thereby preventing further flow of the gas through any leak that might have arisen. In addition, a signal may be sent to the central controller 70 to indicate an alarm condition and initiate appropriate action.
For further biogas purification to meet combustion standards, the gas is charged to an activated carbon gas purifying unit that removes residual hydrogen sulfide such that the purified biomethane meets standards for combustion, as to residual hydrogen sulfide. As illustrated in the example, a pair of activated carbon columns 321, 322 are used in tandem so that one is in use, while the other is being regenerated or refilled, as the activated carbon becomes spent. The packed columns may be equipped with sensors 323, 324 to detect hydrogen sulfide breakthrough in the packed beds to facilitate the switchover from one packed bed to the other and maintain treated gas quality as the biogas exit in conduit 314.
After the gas has been purified to remove hydrogen sulfide, the gas may contain moisture. Thus, the purified biogas is now charged to a dryer 330 where residual moisture is removed. Gas drying may be achieved with a desiccant packed into the dryer, or by other means. The dried gas in conduit 314 is charged to a carbon dioxide removal unit 340. A non-limiting example of such a unit is a membrane gas separator, where the membrane is selected to separate the methane gas in the purified biogas from the much larger carbon dioxide molecules also present in biogas. The methane-rich gas exiting from this unit 340 in conduit 318 has significantly reduced carbon dioxide content and is then routed to a chiller 350 for cooling prior to controlled compression in the system biomethane compressor 36 into a natural gas pipeline.
Summarizing, there is provided an exemplary method of aggregating and treating biogas from a plurality of remote sources to convert the biogas to a processed combustible biomethane gas for compression to a natural gas pipeline, or storage or processing to CNG or conversion to electricity via a fuel cell. The method includes the steps of detecting the availability of biogas at remote sources to permit prediction of biogas availability about 24-36 hours in advance, coupling the plurality of remote biogas sources to a network of conduits and delivering the biogas from the remote sources to a central processing facility. Treating the delivered gas by removing hydrogen sulfide in the biogas at the central processing facility. The treatment may include contacting, in counter-current flow, with a solution of sodium hydroxide to react with the hydrogen sulfide. The method further includes removing trace residual hydrogen sulfide and other contaminants by flowing the gas through activated carbon packed beds. The treated gas is charged to a membrane separator to separate out carbon dioxide from the desired biomethane in the biogas.
Other exemplary method steps may include measuring or observing a deflection of flexible roofs of remote sources and using the deflection measurements or input observations via a central controller configured to control the individual biogas compressors at the remote sources, the biogas charge compressor at the central treatment facility and the processed gas compressor. Further, the methods may include measuring hydrogen sulfide concentration in the atmospheric environment at the remote biogas sources and along the network of conduits and using the measured concentration to control cut-off valves when a predetermined concentration is detected indicating a leak.
The exemplary process flow diagrams of
As to
By recycling a selected appropriate portion of the methane-rich biogas from the second stage membrane 556, the concentration of methane in the system upstream from the membrane stages 554, 556 is increased and the concentration in the gas exiting the membrane separation step is increased. Clearly, the higher the proportion recycled, the higher the methane concentration at the exit of the separation stages 554, 556 will be. Thus, the amount of recycle is set to a level that ensures the exiting biomethane for compression to the natural gas pipeline meets specifications.
Methane-rich biogas exiting the second stage separator 556 is analyzed 561, and gas meeting methane specifications (hereafter biomethane) is charged to a compressor system mounted on skid 560. The compressor system includes a compressor suction scrubber 562 to remove water from the biomethane and route the water to recycling. The biomethane is then charged to compressor 564 and the exiting compressed biomethane is cooled in a compressor cooler 566. The cooled biomethane is charged to a compressor discharge scrubber to remove condensed water for recycling, and the biomethane is charged to a chiller skid 580 that includes further gas cooling apparatus. The biomethane enters a gas/gas precooler 582 where it is cooled against chilled biomethane. Then, the pre-cooled biomethane enters a water-cooled exchanger 584 where it is further cooled (to around 40° F.) against cold water from water chiller 588. Any separated condensate is separated out in the chiller separator 586 and routed to recycle. The chilled biomethane is routed to the pre-cooler 582 to cool incoming biomethane. The warmed biomethane then flows through a gas analyzer 587 and a gas meter 589 and can then be routed at 590 to a gas pipeline or other transport means. Gas analyzers and gas flow meters at each remote location measure and provide data to the central controller. The central controller receives as inputs variables including but not limited to pressures, gas analyses, humidity, oxygen %, inert %, H2S ppm, data about the presence of other contaminants (e.g. siloxanes for waste water and landfills), and the like that are or become necessary under particular circumstances to control the entire system.
As pointed out here above, the treated biogas, now meeting natural gas specifications, can be used in power plants as fuel, and can also be used as a substitute for fossil-fuel methane in production of CNG for transport fuel. It can also be charged to fuel cells and converted directly to electricity. The overall effect of the systems proposed herein is to reduce greenhouse gas emissions.
In another exemplary system and apparatus, the biogas produced by the plurality of digesters and collected from the remote digesters, as described above, and treated is treated locally, on or near the dairy farm for example, to remove the bulk of the hydrogen sulfide (H2S) in the gas, and to remove moisture. This biogas having about 60% methane content and Carbon dioxide (CO2) along with residual H2S and other sulfurous compounds, is further treated in a biogas purification system in order to produce a biogas that is suitable as feedstock to a fuel cell for generation of electricity. The biogas purification system removes sulfurous compounds from the treated biogas, such as carbonyl sulfide, that are fuel cell poisons. In the case of dairy manure, the produced biogas is free of siloxanes, that are often found in landfill-sourced biogas. Siloxanes, if present, must also be removed. The biogas purification system removes sulfurous compound so that the treated biogas is “substantially free of sulfur,” meaning that any traces of residual sulfur do not have a deleterious effect on the operation and cycle life of the fuel cell. Depending upon a range of factors, including the specific type of solid oxide fuel cell, sulfur (total) at less than about 1.5 ppmv or 1.6 ppmv (peak) is desirable. Fuel cells are more sensitive to COS where less than 1 ppmv is generally necessary.
The apparatus 620 has the following operations in series: a biogas “polisher” 630 for removal of residual H2S; a compression apparatus 632; an oil filter 634 for removal of any oil that may have entered the biogas stream through the compression or other operating processes; a dehydration step 636 that removes water and moisture from the biogas and directs the water to a collector 637 for re-use on the farm (for example in the waste lagoons); a re-heater 638 that heats the biogas (which was chilled for dehumidification); and a treatment process that removes COS and H2S residue from the biogas such that the biogas can be charged to the fuel cell 650. The fuel cell produces electricity 652, and CO2 (654) that may be directed to the farm for use in plant photosynthesis, and water (656) that is redirected to the farm for suitable use.
Aside from the removal of sulfurous compounds and other possible deleterious compounds from the biogas, oil removal is required to remove any fine mist of droplets of oil that during enter the biogas and may originate from equipment and this oil may subsequently have a deleterious effect on fuel cell performance and/or life.
Fuel cell 650 receives a substantially purified biogas stream, that has been much purified compared to the biogas exiting from the local processing facilities 602. The fuel cell 650 may be, without limitation, a solid oxide fuel cell, or any other type capable of generating electricity directly from methane, with limited waste products, such as water and carbon dioxide.
The electricity from the fuel cell 650 may be utilized in any of several ways, for example including but not limited to being sent to the grid, directed to an EV charging facility, whether for private cars or government/municipal use, and at least some of it might be directed to power the system as a whole, or a portion of it.
There are advantages to sending the electrical power to an EV charging facility based in the renewable energy credits that can be obtained in some states of the United States. The biogas-sourced electricity can be readily measured as it is used to charge EV batteries, the amount of energy for each battery pack can be tracked and accumulated in a cloud-based database. This information can be used to calculate renewable energy credits (RECs) and how to apportion these between biogas electricity supplier and biogas electrical energy consumer.
The high purity biogas, now free of contaminants that can harm the fuel cell, is routed the fuel cell 740 for generation of electricity. As pointed out above, the waste water and CO2 can be reused on the premises. The electricity can be directed at many uses including the grid, battery powering, EV powering, use in municipal or other government facilities as power, and the like.
The foregoing are descriptions of examples of the type of apparatus and the nature of the process flows useful for systems for aggregating and processing of biogas to biomethane. While examples of embodiments of the technology have been presented and described in text, and some examples, by way of illustration, it will be appreciated that various changes and modifications may be made in the described technology without departing from the scope of the inventions, which are set forth in, and only limited by, the scope of the appended patent claims, as properly interpreted and construed.
The present disclosure is a continuation of U.S. patent application Ser. No. 17/689,332, filed on Mar. 8, 2022, which is a continuation of U.S. patent application Ser. No. 16/783,940, filed on Feb. 6, 2020 which claims priority to U.S. Provisional Patent Application No. 62/802,502, filed Feb. 7, 2019. The present disclosure is also a continuation-in-part of U.S. patent application Ser. No. 16/783,683, filed on Feb. 6, 2020 which claims priority to U.S. Provisional Patent Application No. 62/802,502, filed Feb. 7, 2019. all of which is All of these disclosures are incorporated herein by reference in their entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 62802502 | Feb 2019 | US | |
| 62802502 | Feb 2019 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17689332 | Mar 2022 | US |
| Child | 18208555 | US | |
| Parent | 16783940 | Feb 2020 | US |
| Child | 17689332 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16783683 | Feb 2020 | US |
| Child | 16783940 | US |
