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United States Department of Agriculture

Agricultural Research Service

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Research Project: Torrefaction of Specialty Crops Pomace to Produce High-Energy Density Fuels

Location: Bioproducts Research

2013 Annual Report

1a. Objectives (from AD-416):
Producers of apple juice, wine, olive oil, and tomato paste and juice in California generate hundreds of thousands of tons of pomace each year. Nut producers, specifically almond and walnut, also generate tons of hulls and shells. In the next decades, some specialty crop industries are expected to grow at a rapid pace, leading to generation of even more byproducts. For instance, the olive oil industry is expected to produce ten times more oil within the next decade. These byproducts are mostly converted to low-value compost or animal feed and have to be used quickly because they can be highly perishable. Consequently, growers have to find ways to transport or stabilize the byproducts and this can be costly. Torrefaction of pomace, hulls, and shells can provide added value by converting these byproducts to stable, high-energy density fuels. Torrefaction involves heating biomass under inert atmosphere between 200-300C. This removes most moisture and volatile components to produce a fuel comparable to low-rank coal. The torrefied material is hydrophobic and stable to biological activity and moisture. Torrefaction decreases mass of the material, resulting in reduced shipping costs. The objectives of this project are to 1) Optimize torrefaction conditions for apple, grape, olive, and tomato pomaces as well as almond and walnut hulls and shells, 2) Examine alternative methods for torrefaction, such as microwave heating, 3) Simulate the effects of torrefaction on material properties using modeling software, 4) Feed byproducts into continuous torrefaction reactor owned by a collaborator, Renewable Fuel Technologies (RFT), to determine their potential as biomass sources, and 5) Examine torrefied biomass as hydrophobic fillers, binders, and adhesives to produce polymer composites with superior material properties.

1b. Approach (from AD-416):
We first characterize physical properties of different pomaces and nut shells. We then optimize the torrefaction process in a muffle furnace by varying moisture contents of pomace and nut shells, treatment temperatures, and treatment times. We measure energy values of final products and examine effects of torrefaction on mass, moisture, lignocellulose, and ash contents. We examine sample stability by measuring moisture sorption at different relative humidity. Also, we use alternative heating methods for torrefaction, such as microwave heating. We then compare final properties of torrefied samples from the different heating methods. In addition, we use COMSOL Multiphysics software to model the torrefaction process. In this way, we determine the energy required to produce torrefied samples at different processing scales. We also incorporate torrefied biomass as fillers into polymers to produce composites with superior material properties.

3. Progress Report:
Design of experiments was used to optimize torrefaction conditions for each by-product. The torrefaction temperature was varied from 200°C to 260°C for apple pomace and 230°C to 290°C for all other samples. In addition, the torrefaction time was varied from 20 to 60 minutes for all samples. Proximate analysis was completed for all raw and torrefied samples. From the torrefaction runs and proximate analysis results, response surface methodology was used to determine mass and energy yields for each by-product as a function of torrefaction temperature and time. Grape pomace had the largest mass and energy yields at torrefaction temperatures of 260°C and 290°C. In comparison, apple pomace had the lowest mass and energy yields at the torrefaction temperature of 260°C. These results might be explained in part by raw grape pomace having high lignin content and raw apple pomace having low lignin content. At the experimental torrefaction temperatures, lignin does not degrade and samples with high lignin content should not lose much mass and should retain high energy values, resulting in large mass and energy yields. The project collaborator, Renewable Fuel Technologies, has torrefied almond shells at different temperatures and times in their torrefaction reactor. ARS scientists measured energy values of these torrefied samples. This research addresses objective 1b: "Develop novel commercially viable composite materials for agricultural applications and consumer goods" and 3: "Develop commercially viable biobased polymers and polymer blends with improved functionality" of the parent project.

4. Accomplishments

Last Modified: 05/25/2017
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