California Air Resources Board (ARB) staff has posted thirteen new Low Carbon Fuel Standard (LCFS) fuel pathway applications to the LCFS public comments website. Among them are two pathways for cellulosic ethanol produced by Abengoa Bioenergy at its Hugoton, Kansas plant. One pathway uses corn stover for feedstock, the other uses wheat straw. The carbon intensity (CI) for corn stover cellulosic ethanol is estimated to be 29.52 g CO2e/MJ; for wheat straw ethanol, the CI is estimated to be 23.36 g CO2e/MJ. The 2015 LCFS compliance schedule target for gasoline and fuels used as a substitue for gasoline is 96.48 g CO2e/MJ.
Corn stover comprises the dried leaves, stalks, husk, and cobs left on the ground after the harvest of the corn crop. Wheat straw is the dried stalk of the wheat crop after the grain and chaff have been removed. The carbon intensities of both pathways include the GHG emissions impacts associated with the nutrients that must be applied to fields from which stover and straw have been removed to make up for the nutrients lost when residues are removed.
Hugoton Process. The stover and straw is screened to remove the fines and cobs; these components are used as fuel in the biomass boiler. The remaining feedstocks undergo acid digestion and enzymatic treatment steps to convert the cellulose and hemi-cellulose components into sugars.
Sugars produced by pre-treatment/acid digestion, and subsequent enzymatic hydrolysis are then fermented into ethanol using yeast. The fermented beer is then distilled to produce near-pure ethanol at the top of the column, and stillage at the distillation bottoms. The stillage is fed to the biomass boiler as process fuel. Two additional byproducts—biogas and wastewater sludge—are also used as fuel in the biomass boiler.
The ethanol produced is denatured and loaded onto rail car tankers destined for California. Once the ethanol arrives in California, it is assumed to be transported by pipeline (1 mile) to a bulk terminal, and then by heavy-duty diesel tanker trucks to another bulk terminal for blending with California reformulated gasoline blendstock for oxygenate blending (CARBOB). The blended reformulated gasoline is distributed to fuel dispensing stations.
The crop residues are left on the ground by mechanized harvesting equipment after harvest of the corn and wheat crops. Residue is collected using windrowing machines, baled in the fields, netted, and then stacked. Only diesel fuel energy is expended in the feedstock collection process. Abengoa also assessed the GHG emissions associated with the polypropylene (PPE) twine used to secure the bales.
Although agricultural phase GHG emissions do not accrue to the residues, their removal of the straw from the ground deprives the soil of essential nutrients, including nitrogen (N), potassium (K), and phosphorus (P). For the lifecycle analysis, Abengoa assumed that all N, P, and K nutrients displaced from the fields by residue removal would be replenished by application of synthetically produced fertilizer.
Abengoa calculated that the cogeneration of electricity at Hugoton from process-based biomass combustion will produce 2.46 kilowatt-hours of surplus electricity per gallon of ethanol produced for the corn stover pathway, and 2.84 kilowatt-hours of surplus electricity per gallon of ethanol produced for the wheat straw pathway.
This excess electricity would be exported to the public grid where it displaces average grid-based electricity (net of 6.5% transmission and distribution losses). This displacement of electricity earns a co-product credit in the pathway equal to the GHG emissions associated with the production, transmission, and distribution of average electricity.
The newly constructed plant in Hugoton is expected to produce more than 19 million gallons per year of ethanol using both feedstocks.