Patent Application
20050061001
The present invention generally relates to a process for producing high purity methane gas from biogas or a mixed gas stream and further relates to a streamlined high purity methane gas generation system that operates in accordance with the inventive process.
High purity methane gas production by digestion or composting of organic materials offers promise for solving alternative energy problems. For example, in agricultural settings this technology could be used to offset energy expenses, to control odor, and to produce a marketable product.
Digestion occurs when bacteria produce biogas by decomposing organic matter in the presence of air for aerobic digestion, or in an environment that is devoid of air for anaerobic digestion. Interest in on-farm biogas generation was prompted by the energy crisis of the 1970s, with several farm operations throughout the country experimenting with anaerobic digesters. This experimentation, however, was frequently marked by dissatisfaction on the part of farm owners due to high system cost and/or system failure. The lack of economic feasibility and technological failure appear responsible for the fact that anaerobic digestion has not been widely used in agricultural settings.
Thus, a need exists for a more reliable and cost effective system for generating renewable sources of high purity methane gas.
It is therefore an object of the present invention to provide such a system.
It is a more particular object to provide a high purity methane gas generation system that is streamlined in design and-that reliably and cost-effectively converts organic materials such as animal waste into the desired methane product gas.
It is another more particular object to provide a reliable, cost-effective and streamlined high purity methane gas generation system that is self-sustaining during normal operation, requiring no outside sources of energy (e.g., electric, gas).
The present invention therefore provides a process for producing high purity methane gas from digested or composted organic materials, which comprises:
The present invention further provides a streamlined, high purity methane gas generation system. The inventive system uses biogas to fuel one or more mixed gas-driven compressors employed therein, and basically comprises:
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and accompanying drawings.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Particular features of the disclosed invention are illustrated by reference to the accompanying drawings in which:
The methane gas generation system of the present invention is capable of delivering at least one (1) cubic foot per hour at standard conditions (1 scfh) or 0.5 standard liters per minute (0.5 slpm) of the desired methane gas product stream. The inventive system is streamlined in design and provides a more reliable and cost-effective source of renewable methane gas. Further, the inventive system depends upon digested or composted organic materials and not fossil fuels to produce high purity methane gas. Moreover, in a preferred embodiment, the invention system is self-sustaining during normal operation, requiring no outside sources of energy (e.g., electric, natural gas).
Referring now to
The biogas or mixed gas stream emanating from digester or composter 12 is passed to a first mixed gas purification device 14 for removing at least a portion of the hydrogen sulfide from the stream to maximize the life of, for example, downstream molecular sieves or adsorbents. As is well known to those skilled in the art, hydrogen sulfide may be removed using conventional hydrogen sulfide scrubbers, absorbents or adsorbents such as activated charcoal and bituminous coal, zinc oxide, and mixtures thereof.
In a preferred embodiment, hydrogen sulfide is removed by catalytic carbon. More specifically, activated bituminous coal is employed in the first mixed gas purification device 14 and a small amount of air is added to the stream prior to entering device 14 so as to facilitate the following reaction on the activated bituminous coal:
2H2S+O2→2 H2O+2S
In a more preferred embodiment, the first mixed gas purification device 14 is a carbon adsorber, which is available from USFilter Westates Carbon, 10 Technology Drive, Lowell, Mass. 01851, under the product designation VENT-SCRUB™ VSC-Series VSC1000 carbon adsorber, preloaded with Midas OCM odor control media.
A portion of the desulfurized, mixed gas stream is then passed to at least one mixed gas-driven engine 16, which powers at least one compressor 18, while remaining portions are passed to the compressor 18, which compresses the gas to a pressure exceeding the operating pressure of a second mixed gas purification device 20. In a preferred embodiment, the gas is compressed to a pressure of up to about 1.18 megapascals (MPa), more preferably up to about 1.38 MPa.
A portion of the mixed gas stream (either before or after hydrogen sulfide removal) may also be passed to one or more mixed gas-driven engines (not shown) for powering one or more generators (also not shown). The generator(s) would produce electric energy for on site use to offset energy expenses and/or for export to electricity grids. One or more batteries may be used in conjunction with the generator(s).
Upon leaving compressor 18, the compressed, desulfurized, mixed gas stream is directed to the second mixed gas purification device 20 for removing at least a portion of the carbon dioxide and any excess nitrogen (i.e., nitrogen levels exceeding 0.5% by volume in the mixed gas stream) contained therein.
The second mixed gas purification device 20 is not limited. In a preferred embodiment, device 20 is a membrane-based separation device or system that employs at least one membrane having carbon dioxide selectivity and optionally, also employs at least one membrane having nitrogen selectivity. In a more preferred embodiment, the second mixed gas purification device 20 is a pressure swing absorption (PSA) device or system comprised of at least two molecular sieve chambers. The PSA device or system may be used alone or in combination with the membrane-based separation device or system.
Generally speaking, in the contemplated PSA device or system, the mixed gas stream would be passed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb carbon dioxide and any excess nitrogen (i.e., the more strongly adsorbed components), while at least methane would pass through (i.e., the less strongly adsorbed component(s)). At a defined time, the passing of the mixed gas stream to the PSA device or system would be terminated and the adsorption zone(s) would be depressurized by one or more concurrent depressurization steps where the pressure would be reduced to a defined level which would permit the separated, less strongly adsorbed methane remaining in the adsorption zone(s) to be drawn off. Then, the adsorption zone(s) would be depressurized by a counter-current depressurization step where the pressure in the adsorption zone(s) would be further reduced by withdrawing desorbed gas counter-currently to the direction of the mixed gas stream. Finally, the adsorption zone(s) would be purged and re-pressurized. As is well known to those skilled in the art, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA device or system.
In a more preferred embodiment, the PSA device or system 20 is comprised of a housing and at least two molecular sieve chambers (preferably, from about 5 to about 10 molecular sieve chambers) contained within the housing for receiving a molecular sieve or adsorbent for separating carbon dioxide and any excess nitrogen from the mixed gas stream.
Molecular sieves or adsorbents suitable for use in the present invention are not limited and include carbon fiber composite molecular sieves, zeolite molecular sieves, as well as, other molecularly selective media.
In yet a more preferred embodiment, the second mixed gas purification device 20 is a rotary valve driven nine bed PSA device or system, which is available from QuestAir Technologies Inc., 6961 Russell Avenue, Burnaby, BC V5J 4R8, under the product designation QuestAir M-3200 PSA gas separator.
The process for producing high purity methane gas embodied within system 10, as shown in
The inventive system 10 produces up to about 3000 slpm of methane gas at >90% purity, at pressures ranging from about 1.0 to about 1.4 MPa. The methane gas product stream exiting the second mixed gas purification device 20 meets the Society of Automotive Engineers (SAE) standards for compressed natural gas (CNG) and may be used for low-pressure applications such as fuel for natural gas engines and any other devices that use pipeline natural gas.
In a more preferred embodiment and as best shown in
Means 22 for removing at least a portion of any water contained in the mixed gas stream is not limited and, in a preferred embodiment, comprises a coalescing filter in which small water droplets present in the mixed gas stream combine to form larger droplets that are of sufficient size to accumulate or collect in the filter housing. In a more preferred embodiment, coalescing filter 22 serves to reduce the relative humidity in the mixed gas stream to a level ranging from about 70 to about 95%.
Means 24 for introducing air into the mixed gas stream to facilitate hydrogen sulfide removal in the first mixed gas purification device 14 is also not limited and, in a preferred embodiment, comprises an air injection system capable of providing air to the mixed gas stream in an amount ranging from about 10 to about 500 times stoichiometric. At low flow conditions (i.e., 20% rated) the air injection system 24 provides no more than about 0.5% by volume, based on the total volume of the mixed gas stream, of air to the mixed gas stream, while at high flow conditions (i.e., 100% rated) system 24 provides an amount of air to the mixed gas stream equal to at least about 10 times the amount of total sulfur in the gas stream.
Means 26 for heating the mixed gas stream to prevent water “drop out” and to facilitate hydrogen sulfide removal in the first mixed gas purification device 14 is also not limited and, in a preferred embodiment, comprises a heating device capable of heating the mixed gas stream to a temperature ranging from about 15 to about 33° C. before the mixed gas stream enters the first mixed gas purification device 14.
Means 28 for neutralizing sudden pressure surges in gas streams is known and includes, but are not limited to, surge tanks capable of holding one to five times the volume of high purity methane gas emanating from the second mixed gas purification device 20.
The process for producing high purity methane gas, which is embodied within the more preferred embodiment of device 10, as shown in
The more preferred embodiment of inventive system 10 produces from about 200 to about 1000 slpm of methane gas at >98.5% purity, at a pressure of about 1.4 MPa.
Although this invention has been shown and described with respect to detailed embodiments thereof, it will be apparent to those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the invention.
This application is a continuation-in-part of U.S. patent application Ser. No. 10/766,552, filed Jan. 27, 2004, which claims priority from U.S. Provisional Patent Application Ser. No. 60/443,410, filed Jan. 29, 2003.
| Number | Date | Country | |
|---|---|---|---|
| 60443410 | Jan 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10766552 | Jan 2004 | US |
| Child | 10956803 | Oct 2004 | US |
