Removal of Siloxane and H2S From Biogas Using Microwave Energy
Siloxanes, hydrogen sulfide (H2S), and other contaminants in biogas damage combustion equipment and interfere with post-combustion emission systems for NOx, SOx, and hydrocarbon removal. A new biogas cleaning system removes both H2S and siloxanes from biogas using media regenerated in microwave reactors. The biogas cleaning system has three steps: contaminant collection, media regeneration, and contaminant destruction. In 2008 the California Energy Commission awarded a PIER Grant to the Sacramento Municipal Utility District (SMUD) (Grant # PNG-07-003) to demonstration the microwave regeneration of media saturated with H2S, siloxanes, and other biogas contaminants, and the microwave-induced destruction of these desorbed compounds. Working with SMUD CHA Corporation successfully completed a two-month field-demonstration in October 2009 at Carson Energy Group’s Elk Grove, CA power plant using a slip-stream of biogas produced from the Sacramento Regional Waste Water Treatment Plant’s anaerobic digesters.
An adsorber was fabricated and placed in the biogas supply stream. A mixture of activated carbon and silica gel was used to capture the contaminants. CHA Corporation’s trailer mounted microwave regeneration system was installed at Carson Energy to regenerate the saturated adsorbent. The microwave regenerator removed water, hydrocarbons (including siloxanes), and H2S from the media. The regenerated media was recycled back to the adsorber. The contaminants removed during regeneration were chilled to condense water and hydrocarbons. The non-condensable contaminants then flowed through a microwave carbon reactor where air was added so that H2S was converted to elemental sulfur and H2O. The sulfur was filtered out and the remaining contaminant gas flowed through a second catalytic microwave reactor where the remaining contaminants were oxidized. Oxidized siloxane forms solid silicon dioxide, which is removed with a filter. The gas from the catalytic microwave reactor flows through a final carbon adsorber to remove any undestroyed contaminants. There was no flare needed to burn waste gas.
Four adsorption-regeneration cycles were performed on two 75-lb batches of the mixed media. The Carbon Tetrachloride (CTC) number is commonly used to evaluate the capacity of adsorption medias. The CTC numbers for the regenerated media were equal to or greater than the fresh media CTC number, indicating that the microwave regeneration restored the media adsorption capacity. The average amount of liquid condensed during regeneration was 5% of virgin media weight. More than 70% of liquid recovered was hydrocarbons.
The analysis of gas samples taken from the regenerator outlet and GAC reactor outlet indicated that 99% of both H2S and siloxanes were destroyed in the GAC reactor. The microwave oxidizer destroyed 97% of the remaining siloxanes. The third stage carbon adsorber removed additional siloxanes, leading to a total removal efficiency of 99.98%.
Field Demonstration of Microwave Technology for Removal of Siloxanes and H2S from Landfill Biogas
The positive results from the demonstration on the anaerobic digester biogas from the wastewater treatment plant encouraged SMUD to awarded additional funding to CHA Corporation to conduct testing of the microwave technology to remove and destroy siloxanes, hydrogen sulfide and other contaminants from Landfill Gas (LFG) from the Yolo County California Landfill. A three-month demonstration was successfully completed in December 2009.
The tests used an adsorber fabricated from a standard 55-gallon drum and had two media beds. The upper bed contained 70 pounds of a mixture of silica gel and activated carbon to remove siloxanes, VOCs, and part of the H2S. The lower bed contained 30-lbs of Sulfusorb, a copper oxide impregnated carbon manufactured by Calgon Carbon, to remove trace levels of H2S from the LFG. 24-hr adsorption tests were performed at the gas residence times of 0.8 second (s), 1.0-s, 1.2-s, and 1.5-s. After 24 hours, the two beds were separately removed from the adsorber and transported to Carson Energy’ Elk Grove, CA facility for processing in the trailer mounted microwave carbon regeneration system.
The media from each bed in the adsorber was processed separately in the microwave regenerator. The mix from the upper bed was regenerated with nitrogen sweep gas at a 50-lb/hr regeneration rate. Carbon tetrachloride (CTC) were slightly lower than fresh media CTC number, indicating the regeneration rate was too high to remove all of the contaminants. Saturated Sulfusorb was regenerated using a mixed sweep gas of nitrogen and a small amount of steam. This method recovered a consistent adsorption capacity for the Sulfasorb.
The process flow diagram for the microwave system to remove and destroy siloxanes, VOCs, and H2S from the landfill biogas was revised based on this field-testing. The annualized costs were estimated at $0.56/lb and $0.32/lb, respectively for the mobile unit generating media onsite and the integrated microwave system for landfill biogas upgrading.
Removal of Hydrogen Sulfide from Biogas and Nitrogen Oxides from Engine Exhaust Using Microwave Technology at a Dairy Digester
In 2009 the California Air Resources Board awarded SMUD an ICAT grant (Contract #ICAT 08-3) to remove H2S from biogas and NOx from the exhaust of an engine running on biogas. The integrated emission control process that was demonstrated at the Tollenaar Dairy Farm consists of two systems; a sulfur removal system for the biogas and a NOx removal system on the engine exhaust. For this demonstration, the sulfur removal technology was a SulfaTreat system owned by Applied Filter Technology Inc. CHA Corporation supplied a microwave regenerated carbon adsorption system to remove NOx from the engine exhaust.
The NOx removal system consisted of an exhaust cooling heat exchanger, condensed water knockout pots, two carbon adsorbers with microwave regenerators and an NOx destruction reactor that uses microwaves to react NOx with carbon to form CO2 and N2. When the first carbon adsorber was saturated with NOx, engine exhaust was directed to the second adsorber. The saturated adsorber was then regenerated using microwaves. The two adsorbers were alternated in an adsorption mode and regeneration and cooling mode throughout the demonstration.
To regenerate the adsorber, it is first isolated from the exhaust and an inert, low volume sweep gas was passed through the carbon bed from top to bottom. Then microwaves are applied to heat up the carbon, causing desorption of the captured NOx into the sweep gas. The sweep gas containing concentrated NOx flows to the microwave NOx destruction reactor, where the NOx reacts with carbon to produce CO2 and N2, which were vented to the atmosphere.
During testing it was discovered the standard activated carbon did not adequately adsorb NOx at the system operating conditions. Research concluded that the NOx consists as an equilibrium mixture of NO and NO2 in the exhaust. The carbon adsorbs NO2, but not NO. The shallow carbon beds used in the system did not provide enough residence time for the equilibrium to shift to NO2 as the exhaust passed through the bed. To enhance the conversion of NO to NO2, magnesium oxide impregnated carbon was found effective, and substituted for the original activated carbon.
The NOx removal system was scheduled to operate for 5 months starting June 2011. However, during the test period the engine suffered numerous mechanical problems, limiting testing. We operate the NOx removal system whenever the engine was operated. The NOx removal system was able to meet new CARB emission standards of 5 ppm NOx required more frequent regeneration than anticipated due to the shallow adsorption beds used in the system.