SMUD Digester Engine Project, ICAT 2008

This page updated March 3, 2009

Sacramento Municipal Utility District


Technology and Innovation

The integrated emission control process consists of two systems; a sulfur removal system for the biogas and a NOx removal system on the engine exhaust. For this demonstration, an existing chemical scrubber pilot system owned by Applied Filter Systems Inc. (AFT) will be modified for treatment of H2S in biogas of a dairy digester operation and used to reduce the H2S to less than 10 ppm. The byproduct of this H2S scrubber system is sulfur slurry or a sulfur cake that can be used as a soil amendment or disposed in the digester or a landfill. The innovation of this system is the ability to remove H2S from the very high concentrations in dairy digesters to levels necessary for post combustion NOx systems in a single process. The sulfur can be reused, sold or disposed safely. The operating costs are proportional to H2S removed and it can be used in combination with other H2S removal systems for treatment.

The NOx removal system is a pair of alternating carbon adsorption vessels installed on the engine exhaust pipe to capture nitrogen oxides (NOx). The efficient capture of NOx with carbon was demonstrated in Innovative Clean Air Technology (ICAT) grant 95-348 by the CHA Corporation (CHA). The carbon will capture cooled exhaust gas NOx, and is expected to pass through cleaned exhaust having NOx levels in the range of 1-5 ppm. The performance of this system is expected to reduce NOx to 5 ppm or less-below current BACT NOx levels of about 60 ppm and the more stringent levels of 9 ppm NOx proposed by some air districts. Each carbon adsorber is equipped with a microwave generator that desorbs the NOx and regenerates the carbon in place when it is isolated from the engine exhaust. Desorbed NOx is collected in a concentrated small volume sweep gas and reacted in a separate microwave activated reactor with consumable carbon to produce carbon dioxide (CO2) and nitrogen. The amount and cost of the carbon consumed is minimal. The innovation in NOx removal is in the recently developed microwave applicators that allow efficient regeneration of fixed carbon beds and the use of fixed  beds for adsorption and regeneration. The preferred system design uses a recently developed "leaky waveguide" applicator that allows the power flux to be distributed in a gradient along the length of the guide.

There are additional economic and technological advantages to the proposed integrated system. H2S into the engine is significantly reduced thereby significantly lowering engine wear and maintenance costs for oil change, plugs and rebuild. The NOx removal system uses low maintenance fixed bed carbon adsorbers and commercially available components that are easily repaired or replaced. This NOx removal system can used on engines operating on gas from landfill, municipal digesters, food digesters and waste gas engines in oilfields because the carbon will also capture VOC's in the engine exhaust and the microwave regeneration system will remove or destroy these contaminants. It can also be applied to natural gas ngines ,CHP systems and temporary use of back up generators in California.

Emission Benefit
Statewide, there are over 2,100 dairies in California with a potential to produce about 40 million cubic feet per day of biogas with a potential generation capacity of about 136-140 MW.  Assuming methane content of about 50%, this represents a potential reduction of about 20 million cubic feet per day of methane. The scope of this proposal is focused on engine systems using biogas at dairy digesters but the integrated system described here is applicable to other biogas systems with reciprocating engines. The undeveloped potential generation capacity at California landfills, wastewater digesters and food digesters is estimated at 600 MW which represents about 85 million cubic feet of methane.

Sulfur in the biogas precludes all commercially available post combustion controls using catalysts without reliable and robust H2S removal. The integrated H2S and NOx removal system will significantly reduce NOx and SOx emissions from biogas engines to meet the CARB 2007 emission standards.
Project Description
A pilot plant for H2S removal will be modified and installed at an existing dairy farm with a manure digester. A NOx removal system using alternating carbon adsorbers and microwave powered regeneration will be fabricated. A microwave NOx reactor will be used to destroy the NOx The system will be bench tested, shipped to the dairy site and installed on the exhaust of a 65 kW engine. After a 1 month start up period, the system will be demonstrated for and additional 5 months for continuous operation. The equipment will be removed from the dairy after the demonstration.

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