Title of Invention: HYDROGEN SULFIDE SCRUBBER

Inventor: Mark A. Moser

Oakland, California

DESCRIPTION

BACKGROUND OF THE INVENTION

This application claims priority of my prior, co-pending U.S. Provisional Patent Application, Serial Number 60/912,166, filed on April 16, 2007, entitled "Hydrogen Sulfide Scrubber," which is incorporated herein by reference.

Field of the Invention and Related Art

[0001] The production of methane from animal manures and other organic materials suitable for anaerobic digestion will always produce a biogas that contains mercaptan (RSH), of which the predominant form is hydrogen sulfide (H ₂ S). Natural gas can be recovered from said biogas, wherein said natural gas comprises a very high concentration of hydrogen sulfide, as much as 1500-3500 ppm, and typically 1600-2000 ppm.

[0002] The coiTOsive and toxic characteristics Of H ₂ S gas make its removal from the biogas stream important. H ₂ S in the biogas/natural gas results in acidic conditions on surfaces in contact with the biogas/natural gas. This acidic environment can destroy materials and components in a gas conveyance system (such as piping) that are not acid resistant and can accelerate the wear on materials and components of a gas conveyance system (such as piping) that are acid resistant. Hydrogen sulfide is often found in concentrations in biogas in excess of the human lethal exposure limit. However, hydrogen sulfide has rarely caused human injury. Most "manure gas" deaths, as they have been called, are attributed to suffocation from carbon dioxide. Still, there remain possible negative impacts on human health from H ₂ S in said biogas/natural gas.

[0003] The typical end-usage of biogas is combustion in boilers, engines, or flares. The same conditions that favor CH4 combustion concomitantly result in H ₂ S oxidation and release of SO _x compounds that have both unwanted odor and environmental impacts. SO _x can combine with water vapor to corrode boilers, flares, and exposed corrodible materials.

[0004] In view of these problems, larger digesters at municipal wastewater treatment plants have SO _x emission limits on their flares.

[0005] There are many techniques for FI ₂ S removal, including some biological options. The inventor of the present invention has developed a novel biological H ₂ S scrubber described herein. With the present invention, the concentration of hydrogen sulfide in the biogas is easily reduced to between about 100-500 ppm.

SUMMARY OF THE INVENTION

[0006] The present invention comprises a trickling filter system, and/or methods, for removing hydrogen sulfide from anaerobic digester gas, preferably using heated digester liquid effluent for nutrients and neutralization in the trickling filter system.

[0007] In preferred embodiments, biogas, produced from an anaerobic digester and containing H ₂ S, is pumped or sucked through a trickling filter tank of the invented system, which filter tank comprises at least a portion that is packed with media and sealed to prevent unplanned entry of air. In said system, gas flows up through the filter as heated digester liquid effluent solution trickles down through the filter. Said media may be natural or artificial media, preferably with a very large surface to volume ratio. The media preferably is installed on a support rack that also includes jet type nozzles for jetting gas or liquids into the media during filter cleaning/rejuvenation. Alternatively or optionally, the invented device may use a media that floats in water.

BRIEF DESCRIPTION OF THE DRAWING

[0008] Figure 1 is a schematic representation of one embodiment of the invented system for scrubbing H ₂ S from an anaerobic digester gas stream.

DETAILED DESRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Referring to the Figure, there is shown one, but not the only, embodiment of the invented system 10 comprising a biological scrubber or "filter" 1 1. The invention may comprise apparatus for, and/or methods of, scrubbing H ₂ S from a gas stream. In many embodiments, the gas stream being scrubbed results from processes involving the production of methane from animal manures and other organic materials suitable, for example, for an anaerobic digestion system that produces biogas containing H ₂ S.

[0010] Referring specifically to the embodiment of the biological H ₂ S scrubber system 10 of Figure 1 , the gas entry 12 is a pipe in the support rack area 13 of filter 11. A blower is used to introduce air (at air injection pipe 14), preferably at approximately 2- 10% of the biogas volume into the biogas flow at or near the gas entry point 16 into the scrubber.

[0011] The biogas and air mixture flows upward through a packed bed of media 20, and then out a top region 22 of the tank 24 to the biogas end use point 30. Said media 20 may be, for example, polyurethane foam blocks, or other media as may be understood by one of skill in the art after viewing and reading this disclosure. A bed of 3 m ³ of polyurethane foam blocks is one example of what is believed will be an effective filter bed; other inert support materials of high surface area may also be used.

[0012] Hydrogen-sulfide-oxidizing bacteria, either previously seeded on the media 20 or cultured from the digester effluent nutrient solution 40, colonize the media 20 and utilize the oxygen provided in the added air from air injection pipe 14 to oxidize the H ₂ S in the biogas stream to S ⁰ and SO ₄ ^"2 . Applicant has obtained and used a bacteria from a natural hot springs, which thrives in an acidic environment less than about 3 pH.

[0013] Preferably simultaneously with the processes described above, a digester effluent solution 40 is heated, balanced for water and nutrient content, which by experience, is typically 4/1 water to digester effluent flow, (see the equipment and steps generally noted as 50 in Figure I) ₅ and then re-circulated 51 to the top 52 of the trickling filter 11 and sprayed (54) in the short headspace 56 at the top of the tank onto the top of the media. Thus, the rising gas mixture and the downward trickling liquid flow counter- currently through the filter/tank. One may see the return/recycle 58 of nutrient effluent from the digester, to the heating and neutralization system of assembly 50, near the bottom of the tank 24.

[0014] Thus, in the preferred embodiments, digester liquid effluent, from a manure or other digester system, is used to provide a scrubber bacteria nutrient source. The method of using manure or any digester effluent in a H ₂ S scrubber, and the apparatus used to accomplish this method, are embodiments of the present invention. Further, the preferred embodiments of the invented methods and/or apparatus comprises heating a solution of digester effluent (preferably mixed with water in a ¹ A volume ratio as described above) to flow downward through the filter, providing nutrition to bacteria growing on the media. Said digester effluent may be filtered or unfiltered, full strength or diluted, and serves as nutrient source for the growth of H ₂ S-oxidizing bacteria. Preferably, the digester liquid effluent comprises about 1% suspended solids by weight, with a BOD of about 800 ppm, and a total nitrogen content of about 600 ppm. The digester liquid effluent also contains potassium and phosphorus nutrients, and cobalt, iron, nickel and other micro-nutrients.

[0015] Embodiments of the invention may comprise the method and/or apparatus for heating said nutrient solution using scrubber output gas. The source of the heat for heating the solution may be the combustion of the scrubbed biogas with the co-generated heat used directly or indirectly by means of solid, liquid, or gaseous transfer, or by means of generated electricity used to heat the re-circulating solution. Typically, the scrubber is preferably operated at about 100 ⁰ F. The supplied biogas at entry pipe 12 may be at 40°- 100 ⁰ F. Therefore, to raise the scrubber temperature, the recycle liquid stream 51 may be heated. Typically, this stream is controlled to be about 100 ⁰ F. Alternately, the inlet

biogas 12 and or air injection 14 may also be heated. The oxidation of the hydrogen sulfide in the biogas is slightly exothermic, but the volume of gas flow is high, so typically heat must be added to the scrubber system in order to operate at about 100 ⁰ F. Preferably, the biogas residence time in the scrubber is between about 10-40 seconds. Preferably also, the pressure drop across the bed of media is almost negligible, typically about 1 inch of water column. If the pressure drop is greater than this, it is usually recommended to clean and rejuvenate the media bed.

[0016] The bottom of the tank may serve as a water reservoir with a residence time, preferably about 0.5-5 days, for receiving the addition of makeup water and nutrient solution; the production of sulfate as H ₂ SO4 ultimately leads to the acidification of the circulating (recycle) fluid to less than about 1.5 pH. Preferably, the scrubber is operated at less than 3 pH, and typically, between about 1-2 pH.

[0017] Embodiments of the invention may comprise the methods and/or apparatus for using digested manure to neutralize scrubber liquid by-product, shown as "Neutral pH Effluent" in Figure 1. Siich neutralization of the liquid acidic effluent from the scrubber may be accomplished by mixing of the excess filter tank nutrient flow with digested manure in a small tank, for example, prior to its discharge from the scrubber, and return to the anaerobic digester outfall. The manure or digester effluent 40 is added to the neutralization tank to maintain the pH of the effluent from there at between about 6-6.5. The pH of the circulation (recycle) fluid in the scrubber is not controlled.

[0018] Embodiments of the invention may also comprise nitrifying the scrubber, as the scrubber may also provide a home for nitrifying bacteria according to conventional practices that will convert a portion of the circulating ammonia to nitrate.

[0019] The filter of most or all embodiments will eventually clog with bacteria and sulfur unless it is cleaned out occasionally. A preferred cleaning technique comprises filling the filter 11 with water, allowing floatable media, when, that is, media 20 is floatable, to float, and then violently agitating the media by pumping a liquid or gas through nozzles 70 mounted in the tank. This agitation will loosen bacteria and sulfur. The loosened materials will tend to flow out the bottom of the filter when the added water is drained out. Alternative embodiments may comprise media that is less likely to float,

or unlikely to float, but preferably the force of the jet nozzles of other agitation that is supplied during cleaning steps will fluidize all or a portion of the media bed to loosen material(s) that needs to be loosened, displaced, or removed in order to rejuvenate the filter 11.

[0020] One may note that the schematic diagram of Figure 1 does not necessarily contain all piping, ports, tanks, sensors, and control systems that may be necessary or useful in the invented methods, but, upon viewing and reading this disclosure, one of skill in the art may readily provide said piping, ports, tanks, sensors, and control systems.

EXAMPLE: Summary of Inputs and Outputs. [0021 ] Inputs

Upflow:

1. Biogas with H2S - about 100 scfm

2. Air proportionally mixed into biogas - about 10 scfm Downflow trickling liquids:

1. Recirculated heated nutrient solution

2. Digester effluent for bacteria nutrition — about 50 mil/min

3. Digester effluent for neutralization - about 400 mil/min

4. Fresh water - about 200 mil/min

[0022] Outputs

1. Biogas with reduced H2S - about 110 scfm

2. Neutralized, diluted, digested manure solution - about 650 mil/min

[0023] Optionally, the filter 1 1 (housed in tank 24) may comprise multiple media types, including media of alternative shapes, compositions, and other features that provide the fluid-media contact and fluid flow characteristics that are desired. For example, as shown in Figure I ₅ an additional, preferably-different media 120 may be provided. Layering of different media may be beneficial for creating a more turbulent flow inside at least portions of the filter, for example. Further, media for specific treatment steps may be included in the filter 11 /tank 24, for example, a layer of

chemically-treated media, such as iron sponge, may be used as a top layer inside the filter (see position of 120) for final chemical recovery of H ₂ S if the HTS is not sufficiently captured biologically. Such additional media or other post-treatment (after the main filter bed of media 20) may be beneficial for applications wherein a very high percentage of removal of HoS must be achieved (for example, greater than or equal to 99% removal) . Also, different media may be mixed, if desired.

[0024] In summary, preferred embodiments may comprise apparatus and/or methods that include a trickling filter for removing hydrogen sulfide from gas, preferably using heated digester effluent for nutrients and neutralization. Biogas produced from an anaerobic digester, and containing H ₂ S, may be pumped or sucked through the trickling filter tank, which comprises at least a portion that is packed with media(s) and sealed to prevent unplanned entry of air. In said system, gas flows up through the filter as a heated water and nutrient solution trickles down. Said media(s) may be natural or artificial media, preferably with a very large surface to volume ratio. Jet type nozzles or other agitation or fluidization may be used for filter cleanout.

[0025] In especially-preferred embodiments, digester effluent solution, from a manure or other digester system, may be used as (or as a supplement to) the downward- flowing liquid stream, in order to provide H ₂ S scrubber bacteria nutrient source(s). This way, a solution of digester effluent and water may be heated and trickled downward through the filter, providing nutrition to bacteria growing on the otherwise insert media and/or on the layers or mixtures of media. Said solution may be heated using scrubber output gas, for example, by means of combustion of the scrubbed biogas with the co- generated heat used directly or indirectly by means of solid, liquid or gaseous transfer, or generated electricity used to heat the re-circulating solution. A preferred cleaning technique comprises filling the filter/tank with water, allowing floatable media to float, and aggressively/rigorously agitating the media, for example, by injecting liquid or gas through nozzles mounted in the tank to loosen bacteria and sulfur.

[0026] A further feature of the preferred embodiments may be that digested manure is used to neutralize acidic scrubber by-product, for example, by mixing of excess filter tank nutrient flow with digested manure prior to its discharge from the

scrubber and return to the anaerobic digester outfall. The preferred embodiments may comprise nitrifying the scrubber, as the scrubber may also provide a home for nitrifying bacteria that will convert a portion of the circulating ammonia to nitrate.

[0027] Although this invention has been described above with reference to particular means, materials and embodiments, it is to be understood that the invention is not limited to these disclosed particulars, but extends instead to all equivalents within the scope of the following claims.