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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #208974

Title: Determination of Thermal Properties of Composting Bulking Materials

item AHN HEE, KWON - Illinois State University
item Sauer, Thomas

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2008
Publication Date: 9/1/2009
Citation: Ahn Hee, K., Sauer, T.J., Richard, T.L., Glanville, T.D. 2009. Determination of Thermal Properties of Composting Bulking Materials. Bioresource Technology. 100(17):3974-3981.

Interpretive Summary: Composting is a process that takes organic materials like leaves, manure, crop residues and allows them to break down into a more stable material that can be used for fertilizer, mulch, or a soil amendment. The composting process can be controlled to limit the amount of decomposition by adjusting the length of time the materials are composted and/or the temperature and moisture content during composting. This study involved measuring the thermal properties of materials often used in compost, called compost feedstocks. A series of laboratory experiments were completed using three different methods and both bulk and ground feedstocks. Measured heat transfer through the feedstocks under these controlled conditions was used to calculate their thermal properties. These measurements were made with varying moisture content and density within ranges known to occur in compost piles. The results indicate that the different feedstock materials did have different thermal properties that changed significantly with moisture content and density. A relationship was developed to show that these relationships could be expressed as a function of degree of saturation, which is the amount of air-filled pore space in the compost. This expression for each feedstock enables scientists to more accurately predict temperatures and moisture content during composting with these materials. This reseach is important to scientists, regulators, and practitioners as it will enable better control of the composting process to ensure that sufficient temperatures are reached to destroy harmful pathogens present in compost feedstocks.

Technical Abstract: Thermal properties of compost bulking materials affect temperature and biodegradation during the composting process. Well determined thermal properties of compost feedstocks will therefore contribute to practical thermodynamic approaches. Thermal conductivity, thermal diffusivity, and volumetric heat capacity of 12 compost bulking materials were determined in this study. Thermal properties were determined at varying bulk densities (1, 1.3, 1.7, 2.5, and 5 times uncompacted bulk density), particle sizes (ground and bulk), and water contents (0, 20, 50, 80% of water holding capacity and saturated condition). For the water content at 80% of water holding capacity, saw dust, soil compost blend, beef manure, and turkey litter showed the highest thermal conductivity (K) and volumetric heat capacity (C) (K: 0.12-0.81W/m deg C and C: 1.39-2.58 MJ/m^-3 deg C). Silage showed medium values at the same water content (K: 0.09-0.47 W/m degC and C: 0.93-1.76MJ/m^-3 deg C). Wheat straw, oat straw, soybean straw, cornstalks, alfalfa hay, and wood shavings produced the lowest K and C values (K: 0.03-0.30 W/m deg C and C: 0.28-1.54MJ/m^-3 degC). Thermal conductivity and volumetric heat capacity showed a linear relationship with moisture content and bulk density, while thermal diffusivity showed a nonlinear relationship. Since the water, air, and solid materials have their own specific thermal property values, thermal properties of compost bulking materials vary with the rate of those three components by changing water content, bulk density, and particle size. The degree of saturation was used to represent the interaction between volumes of water, air, and solids under the various combinations of moisture content, bulk density, and particle size. The first order regression models developed in this paper represent the relationship between degree of saturation and thermal conductivity (r^2=0.77-0.93) and volumetric heat capacity (r^2=0.7-0.91) well. Improved knowledge of the thermal properties of compost bulking materials can contribute to improved thermodynamic modeling and heat management of composting processes.