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Renewable natural gas from food waste offers negative carbon intensity as LCFS pathway: -80 gCO2e/MJ

South San Francisco Scavenger Company (SSFSC) has submitted an application to the California Air Resources Board (ARB) seeking certification of Tier 2 pathways for biomethane (Bio-CNG) from anaerobic digestion (AD) of food waste and urban landscaping waste at their facility in South San Francisco, California. The proposed carbon intensities for the two pathways are -79.91 gCO2e/MJ and 0.28 gCO2e/MJ, respectively.

The fuel production facility, which is co-located with the Blue Line Transfer facility, receives pre-separated food scraps and urban landscaping waste (ULW), also known as green waste, from households and commercial generators, and sends them to an anaerobic digestion unit to produce raw biogas. Blue Line Transfer is a public disposal and recycling facility located in the City of South San Francisco.

The ULW is collected from residential locations and bins. The food scraps are collected from grocery stores, restaurants and food service locations. The ULW is weighed on the SSFS facility scales and placed directly into the anaerobic digester (AD) receiving bay with no further processing. The food scraps are weighed on the onsite scales after processing through the Scott THOR Turbo Separator but immediately prior to being placed into the AD receiving bay.

The enclosed vessel digester uses the high solids anaerobic digestion (HSAD) technology to produce biogas.

The biogas production occurs in batches and the digestion phase is terminated by an aeration process where fresh air is pumped through the digested waste from the digester floor and purged biogas is collected. Biogas is collected on an ongoing basis as it is produced.

Once the methane concentration of the biogas falls below 22%, the purged biogas (lean gas) is diverted to a boiler as auxiliary fuel and combined with pipeline natural gas and tail gas from the biogas upgrading treatment system. The gas-fueled boiler is used to generate heat, which maintains optimal temperature in the digester and heats up the percolate. The percolate is the liquid that is sprayed on the organics. The organisms that produce the biomethane reside in the percolate. Electricity is used to run the upgrading unit.

The raw biogas is collected from the digester and is upgraded onsite to pipeline-quality Bio-CNG (RNG). The pipeline quality RNG is stored on-site in high pressure tanks and is subsequently used for fueling of CNG trucks. The digestate is sent to an offsite facility for composting. Emissions and credits associated with composting are kept outside the system boundary and hence are not included in the CI calculations.

The carbon intensity (CI) value is calculated based on life cycle analysis using a modified version of the CARB-approved Tier 1 Simplified CI Calculator for Biomethane from Anaerobic Digestion of Organic Waste.


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