California ARB posts LCFS pathway for biomethane from dry fermentation; -15.29 g CO2e/MJ, compared to +95.86 and +94.71 for gasoline and diesel
05 July 2012
The California Air Resources Board (ARB) has posted a new Low Carbon Fuel Standard (LCFS) internal pathway for biomethane produced from the high solids anaerobic digestion (HSAD) (dry fermentation) of organic (food and green) wastes—with accommodations for small proportions of food-contaminated, non-recyclable (soiled) paper, and fats, oils, and greases (FOG) in the feedstock. The HSAD fuel carbon intensity is -15.29 g CO2e/MJ, compared to gasoline and diesel carbon intensities of 95.86 and 94.71g CO2e/MJ respectively.
The pathway, developed in partnership with CalRecycle, is based on a multi-stage, mesophilic process to produce biomethane for use as a vehicle fuel. The feedstock for this pathway is 40% food waste and 60% green waste. The carbon intensity analysis takes into account well-to-wheel emissions including avoided landfill and composting emissions and compost co-product credits for the displacement of synthetically produced fertilizer.
ARB staff assumed for the purposes of this analysis that the HSAD facility would be sited adjacent to a landfill, or local transfer station, minimizing any transportation distance differentials between feedstocks delivered to the HSAD facility, and wastes delivered for disposal, transfer, or recovery.
Staff also assumed that grid-based electricity would power the anaerobic digestion, and biogas purification and compression processes. The electricity consumed would be generated from an energy portfolio mix consisting of 78.7% natural gas, and 21.3% renewables (excluding large hydro-electric and biomass based generation).
Biogas produced from the anaerobic digestion of the organic matter (mostly methane (CH4) and carbon dioxide (CO2) in equimolar proportions) would be purified and made available on-site at the facility to fuel transit buses and other CNG-fueled vehicles, or would be refined to pipeline quality natural gas standards.
Process solid residue (digestate) would be composted using either in-vessel composting (IVC), covered aerated static pile (CASP), or open windrow composting processes. The result would be a high-quality compost co-product that could be marketed as a fertilizer or soil amendment.
Other criteria for this HSAD pathway include:
The pathway applies only to fuel produced by a multi-staged, mesophilic, dry fermentation (high solids anaerobic digestion or HSAD) process. It cannot be used by producers using a wet fermentation (wet AD or low solids) process.
The annual organic waste throughput of the HSAD process must be equal to or greater than 30,000 tons.
The biomethane produced must conform to prevailing California pipeline-quality compositional and performance standards, including any specifications imposed by the regulated utility and transmission companies on parameters such as the Wobbe Index, and trace impurity levels of compounds such as hydrogen sulfide (H2S).
The product gas discharge pressure must be no greater than 800 psig for tie-into the utility company’s transmission system.
The facility must employ biofilters during the active phase of the digestate composting process.
LCFS credits from the production of biomethane via the HSAD pathway have the potential to generate a new stream of revenue for Anaerobic Digestion facilities which will produce low carbon fuels while diverting food and green waste from landfills.
The Low Carbon Fuel Standard is a regulation to reduce the carbon intensity of fuels sold in California by 10% by 2020.
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