Integrated biogas cleaning system to remove water, siloxanes, sulfur, oxygen, chlorides and volatile organic compounds

Abstract

An integrated biogas cleaning system is provided to clean biogas from sources such as landfills and digesters for heat and power generation systems such as boilers, engines, turbines, or fuel cells. Siloxanes, chlorine, oxygen and sulfur are removed to parts per billion levels as well as removing the majority of water and some volatile organic compounds. The biogas system cools a biogas stream to partially remove contaminants, blends in a small concentration of hydrogen gas and then combusts the remaining oxygen to heat the biogas and leave sufficient hydrogen suitable for a downstream sequence of further contaminant conversion and removal in stages using a hydrodesulfurization bed and adsorbent media beds. Heat exchange arrangements provide efficient recycling of waste heat and compensation for varying levels of oxygen in the incoming biogas waste stream, suitable for use in a wide range of biogas generating sources. The resulting biogas fuel product is suitable for use in combustion generators, fuel cell systems, greenhouse boilers, or as feedstock for further upgrading to renewable natural gas.

What is claimed is: 
     
         1 . A biogas cleaning system for purifying a biogas waste stream containing methane, carbon dioxide and water and impurities that may include sulfur, halides, siloxanes, and volatile organic compounds, to form a combustible clean biofuel, the biogas cleaningcleaning system comprising;
 (a) a gas control system for blending hydrogen gas into a biogas waste stream, having a hydrogen flow controller and hydrogen port,   (b) a de-oxidizer catalyst bed fluidly coupled downstream of the gas control system for receiving and catalytically combusting the blended hydrogen with the remaining oxygen in the biogas such that oxygen is substantially removed from the biogas and the biogas stream is heated,   (c) an hydrodesulphurization catalyst bed fluidly coupled downstream of the de-oxidizer catalyst bed that receives the heated biogas stream for the purpose of hydrogenating sulfur species to substantially hydrogen sulfide and hydrogenating chlorine species substantially to hydrogen chloride   (d) a first hydrogen sulfide removal adsorption bed fluidly coupled downstream of the hydrodesulphurization catalyst bed for the purpose of substantially removing sulfur from the biogas   (e) a thermal sensor controller for measuring biogas stream temperature and in communication with the gas control system,   whereby when operable and receiving a biogas stream, hydrogen is mixed with the biogas, then the oxygen is substantially removed, the biogas stream is heated sufficiently by the exothermic combustion such that the sulfur species are hydrogenated by the hydrodesulphurization catalyst bed being converted primarily to hydrogen sulfide, then the converted hydrogen sulfide is substantially removed the adsorbent bed, and further the rate of hydrogen blending is controlled in response to the thermal sensor controller.   
     
     
         2 . The biogas cleaning system of  claim 1 , wherein said gas control system provides added hydrogen concentration greater than 2%, and rate of combustion of the biogas stream at the deoxidizer bed provides a biogas temperature between 250° C. and 400° C., prior to the hydrodesulphurization catalyst bed. 
     
     
         3 . The biogas cleaning system of  claim 1 , further comprising:
 (f) a sulfur polisher adsorption bed fluidly coupled downstream of the first hydrogen sulfide removal adsorption bed in order to remove trace sulfur levels from the biogas, consisting in part of copper or nickel in a reduced or oxide state.   
     
     
         4 . The biogas cleaning system of  claim 1 , further comprising a heat exchanger loop,
 (g) an economizer heat exchanger fluidly coupled upstream of de-oxidizer catalyst bed and to heated biogas stream exiting the hydrogen sulfide removal adsorption bed, wherein the cool heat exchange side includes the incoming biogas stream and outgoing cooled clean biogas, and the hot heat exchange loop is formed of outgoing clean heated biogas stream and the biogas stream upstream of de-oxidizer catalyst bed   (h) supplementary heater within the hot heat exchange loop for heating the biogas stream upstream of the de-oxidizer catalyst bed between 250° C. and 400° C. sufficient for substantial oxygen removal when there is insufficient exothermic temperature rise in the de-oxidizer catalyst bed.   (i) aupplementary cooler within the hot heat exchange loop for cooling the biogas steam, the cooler selected on of a second heat exchanger to reduce the temperature of the clean heated biogas stream and a biogas flow controller fluidly coupled to the sulfur adsorption bed, heat exchanger and outgoing cooled clean biogas stream and operable to distribute the hot clean biogas stream between the hot or cool exchange loops and heat exchanger in response to thermal sensor indicating there is excessive exothermic temperature rise within the de-oxidizer catalyst bed,   whereby in operation the flow and temperature of the hot biogas returning to the economizer heat exchanger, is adjustable to maintain the biogas temperature downstream of the de-oxidizer catalyst bed in a range 250° C. to no higher than 400° C.   
     
     
         5 . The biogas cleaning system of  claim 1 , further comprising;
 (j) a chlorine removal adsorbent bed upstream of the first sulfur removal adsorbent bed.   
     
     
         6 . The biogas cleaning system of  claim 4 , further comprising:
 (k) a siloxane removal adsorption bed upstream of the heat exchanger for the purpose of removing siloxanes from the biogas.   
     
     
         7 . The biogas cleaning system of  claim 4 , further comprising a biogas pre-cooler fluidly coupled upstream of hydrogen port, one selected from the group of refrigerant loop providing less than approximately −10° F. to condense water and contaminants, a refrigerant loop providing between −10° F. and 32° F. to condense water and some contaminants, and a water condenser operable between 32° F. and 50° F. to condense out water content substantially. 
     
     
         8 . The biogas cleaning system of  claim 4 , further comprising a biogas pre-cooler fluidly coupled upstream of hydrogen port, formed of a water sprayer and water separator for the purpose of pre-cooling the biogas and to absorb some of the impurities such as sulfur dioxide and hydrogen sulfide in the water. 
     
     
         9 . The biogas cleaning system of  claim 6  wherein the siloxane adsorption bed is formed one selected from the group of activated carbon, silica get, molecular sieve, or zeolite. 
     
     
         10 . The biogas cleaning system of  claim 1  wherein the de-oxidizer catalyst bed includes noble metal catalysts selected from the group of platinum, palladium, rhodium, and metals in a reduced state. 
     
     
         11 . The biogas cleaning system of  claim 1  wherein the hydrodesulfurization catalyst bed includes metals selected from the group of cobalt, molybdenum and nickel. 
     
     
         12 . The biogas cleaning system of  claim 5  wherein the hydrogen chloride removal adsorption bed comprises in part disodium oxide. 
     
     
         13 . The biogas cleaning system of  claim 1  wherein the sulfur adsorption bed comprises active compound selected from the group of zinc oxide, iron oxide, and activated carbon. 
     
     
         14 . The biogas cleaning system of  claim 7 , whereby the clean biogas output by the system downstream has sufficiently low levels of contaminants such that the biogas can be reliably and repeatably burned as fuel in a generator selected one from the group of internal combustion engine, turbine, fuel cell system and boiler, with reduced corrosion or damage to the generator. 
     
     
         15 . The biogas cleaning system of  claim 1 , further comprising a plurality of sulfur adsorbent beds with each is arranged in a lead and lag arrangement such that during operation one of said plurality of sulfur adsorbent beds is removed from biogas stream offline and changed while at least one other adsorbent bed is operable inline to the biogas stream. 
     
     
         16 . A biogas cleaning system for purifying a biogas waste stream containing methane, carbon dioxide and water and impurities that may include sulfur, halides, siloxanes, and volatile organic compounds, to form a combustible clean biofuel, the biogas cleaning system comprising;
 (a) a biogas pre-cooler for reducing volatile organic compounds and water in the biogas waste stream,   (b) a siloxane removal adsorption bed fluidly coupled downstream of the biogas pre-cooler for substantially removing siloxanes from the biogas   (c) a gas control system for blending hydrogen gas into a biogas waste stream, having a hydrogen flow controller and hydrogen port downstream of the siloxane removal adsorption bed,   (d) a de-oxidizer catalyst bed fluidly coupled downstream of the gas control system for receiving and catalytically combusting the blended hydrogen with the remaining oxygen in the biogas such that oxygen is substantially removed from the biogas and the biogas stream is heated,   (e) an hydrodesulphurization catalyst bed fluidly coupled downstream of the de-oxidizer catalyst bed that receives the heated biogas stream for the purpose of hydrogenating sulfur species to substantially hydrogen sulfide and hydrogenating chlorine species to hydrogen chloride   (f) a chlorine removal adsorbent bed downstream of the hydrodesulfurization catalyst and upstream of the first sulfur removal adsorbent bed.   (g) a first hydrogen sulfide removal adsorption bed fluidly coupled downstream of the hydrodesulphurization catalyst bed for the purpose of substantially removing sulfur from the biogas   (h) a sulfur polisher adsorption bed fluidly coupled downstream of the first hydrogen sulfide removal adsorption bed in order to remove trace sulfur levels from the biogas   (i) a thermal sensor controller for measuring biogas stream temperature and in communication with the gas control system,   (j) a heat exchanger fluidly coupled downstream of hydrogen port and upstream of de-oxidizer catalyst bed and to heated biogas stream exiting the hydrogen sulfide removal adsorption bed, wherein the cool heat exchange loop is formed of incoming biogas stream and outgoing cooled clean biogas, and the hot heat exchange loop is formed of outgoing clean heated biogas stream and the biogas stream upstream of de-oxidizer catalyst bed further including a biogas flow controller fluidly coupled to sulfur polisher adsorption bed, heat exchanger and outgoing cooled clean biogas stream and operable to distribute the hot clean biogas stream between the hot or cool exchange loops and heat exchanger   whereby when operable and receiving a biogas stream, siloxanes, VOC's and water is removed, then hydrogen is mixed with the biogas, flow and temperature of the hot biogas returning to the economizer heat exchanger is adjusted to maintain the biogas temperature downstream of the de-oxidizer catalyst bed in a range 250° C. to no higher than 400° C. such that remaining oxygen is removed and the biogas stream is heated such that the sulfur species are hydrogenated by the hydrodesulphurization catalyst bed being converted primarily to H2S, then halides are removed prior to absorbing sulfides to produce a clean biogas fuel.   
     
     
         17 . The biogas cleaning system of  claim 16 , wherein when operable, the resulting clean biogas fuel has reduced contaminant concentrations, including sulfur less than 50 ppb, halides less than 50 ppb and siloxanes less than 50 ppb. 
     
     
         18 . A biogas cleaning method, for purifying a biogas waste stream containing methane, carbon dioxide and water and impurities that may include sulfur, halides, siloxanes, and volatile organic compounds, to form a combustible clean biofuel, the biogas cleaning method comprising
 (a) blending hydrogen with a biogas waste stream using a gas control system,   (b) combusting the blended hydrogen and biogas stream to remove oxygen and heat the biogas to a range 250° C. to no higher than 400° C.,   (c) then hydrogenating the resulting heated biogas stream, converting sulfides to substantially hydrogen sulfide,   (d) then absorbing hydrogen sulfide,   (e) controlling the hydrogen concentration in response to temperature following step b) whereby a substantially clean biogas fuel is provided.   
     
     
         19 . A biogas cleaning method for purifying a biogas waste stream containing methane, carbon dioxide and water and impurities that may include sulfur, halides, siloxanes, and volatile organic compounds, to form a combustible clean biofuel, the biogas cleaning method comprising;
 (a) pre-cooling a biogas waste stream reducing volatile organic compounds and water,   (b) adsorbing siloxanes substantially removing siloxanes from the biogas waste stream   (c) then adding hydrogen gas in the biogas stream,   (d) catalytically combusting the blended hydrogen with the remaining oxygen in the biogas such that oxygen is substantially removed from the biogas and the biogas stream is heated,   (e) then hydrogenating sulfur species to substantially hydrogen sulfide and simultaneously hydrogenating chlorine species to hydrogen chloride   (f) then adsorbing hydrogen chloride,   (g) then adsorbing hydrogen sulfides using a sulfur adsorbent,   (h) then further adsorbing remaining hydrogen sulfides using a sulfur polisher   (i) adjusting flow and temperature of the hot biogas returning to the economizer heat exchanger to maintain the biogas temperature downstream of the de-oxidizer catalyst bed in a range 250° C. to no higher than 400° C., whereby a substantially clean biogas fuel is provided.   
     
     
         20 . The biogas cleaning method of  claim 19 , wherein, the resulting clean biogas fuel has reduced contaminant concentrations, including sulfur less than 50 ppb, halides less than 50 ppb and siloxanes less than 50 ppb. 
     
     
         21 . The biogas cleaning system of  claim 16  wherein said biogas pre-cooler further comprises;
 a flow controller coupled to a first fluid conduit output of the precooler heat exchanger and a bypass fluid conduit, to blend the biogas streams from the conduits to reduce the temperature of biogas leaving the pre-cooling system in response to a present target temperature. 
 
     
     
         22 . The biogas cleaning system of  claim 16 , wherein said biogas pre-cooler further comprises an adsorption chiller coupled through a closed fluid loop thermally coupled to a second heat exchanger downstream of the sulfur polisher adsorption bed whereby the heat required using the adsorption chiller is provided by the second heat exchanger for efficient energy use. 
     
     
         23 . The biogas cleaning system of  claim 1  further comprising an air inlet having a air flow controller fluidly coupled downstream of the siloxane removal adsorption bed, whereby the air flow controller is in communication with and responsive to the thermal sensor controller such that additional air can be added to the biogas in a controlled manner in order to increase the exothermic heating of the biogas during combustion in the de-oxidizer catalyst bed.