Torrefaction of almond shell as a renewable reinforcing agent for plastics: techno-economic analyses and comparison to bioethanol process
The U.S. state of California alone produced nearly 3.5 billion kilograms of almonds accounting for approximately 84% of the world's almond production. This generates about 2.58 million metric tons of almond residues. The almond shells are currently either burned to generate power or disposed of in the landfill. Valorizing almond shells and hulls provides an opportunity to replace petroleum-derived products and divert organic material from landfills. Here we demonstrate a detailed techno-economic analysis of an almond shell torrefaction process capable of utilizing the 520,000 metric tons of almond shells produced annually in California. Our process also includes preprocessing the torrefied biomass to exploit it as a reinforcing agent for plastics. We further compared the revenue generated from the torrefied biomass and bioethanol derived from the same amount of almond shells. We considered three different torrefaction facility scales to evaluate trade-offs between economies of scale at the facility and trucking costs to deliver almond shells. A facility that takes in 200,000 metric tons (MT)/year of almond shells results in lower per-unit-output basis capital and operating cost relative to other smaller scales of torrefaction facility, including 10,000 MT/year and 50,000 MT/year, considered for analysis in this study. The large-sale facility results in the minimum selling price of the torrefied biomass of $311.4/MT. An analogous techno-economic analysis on converting almond residues into bioethanol is also investigated. The minimum selling price of almond shells derived ethanol ($1.71/kg) is higher than corn ($0.48/kg) or cellulosic biomass ($0.88/kg) derived ethanol. Compared to the bioethanol route, the torrefied almond shells result in 3 times more revenue if it is utilized as a reinforcing agent for plastics.