Title: Hydrothermal carbonization of animal manures into hydrochar and its soil and environmental applications Author
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 10, 2014
Publication Date: N/A
Technical Abstract: Hydrothermal carbonization (HTC) is a wet carbonization technique using subcritical water under autogenic pressure and temperatures. HTC process conditions are at the lower region of liquefaction process. HTC is also referred as wet pyrolysis or hydrous pyrolysis. HTC can convert wet feedstock biomass into carbonaceous solids at relatively high yields without the need for an energy-intensive drying before or during the process. Potential HTC feedstocks include wet animal manures, sewage sludges, and municipal solid waste streams, as well as aquaculture residues. While these feedstocks represent large, renewable residual streams that require some degree of treatment to protect environment, HTC may be used to eliminate or at least reduce the burden of the treatment while producing value-added hydrochar, a carbon-rich, coal-like solid product of HTC. Hydrochar may be used in a variety of environmental applications including as a soil amendment, energy source, environmental sorbent and a material for energy storage. Hydrothermally carbonizing surplus animal manures from concentrated animal feeding operations (CAFOs) may become a viable manure management alternative. HTC not only provides environmentally acceptable manure treatment, but it may also bring potential income revenues to farmers from producing nutrient-rich hydrochar. Emerging compounds, such as pharmaceuticals, personal care products, and endocrine disrupting compounds posing significant environmental concerns in animal wastes, landfills, and wastewater may be thermally degraded/transformed under autoclaving conditions of HTC. Although there exist numerous research studies in the literature on soil and environmental applications using plant-based biochar made from traditional dry pyrolysis (hereafter referred as pyrochar), a very few such studies exist for hydrochar made from hydrothermally carbonizing animal manures. The objectives of this study are to 1) compare physic-chemical and thermal characteristics of swine manure-based hydrochar and pyrochar, and 2) investigate greenhouse gas emission and groundwater pollution potentials of the swine hydrochar when used as a soil amendment. Dewatered swine solids were obtained from a 5,600-head finishing swine operation in Sampson County, North Carolina, which were further dried and stored in a refrigerator until experiments. Swine pyrochar was prepared using a skid-mounted pyrolysis system which heated the dried swine solids to 620 degree Celsius (oC) for two hours. Swine hydrochar was prepared by hydrothermally carbonizing the swine solids at 250 oC for 20 hours. Some of the hydrochars were washed with 200 millilitre acetone for 2 hours in order to remove labile compounds accumulated on the hydrochar surface. Two soil pot incubation experiments were conducted approximately 4 months apart. Hydrochar or pyrochar was mixed at a rate of 20 gram per kilogram (g kg-1). Sufficient deionized H20 was added periodically so that each pot would be maintained at 10 % moisture content. Triplicate pots containing soil without biochar served as controls. During the incubation periods of 42 to 127 days, greenhouse gas (carbon dioxide CO2 and nitrous oxide N2O) emission fluxes were measured by nonlinearly regressing time-series headspace gas concentrations. Higher heating values (HHV) of raw swine solids, hydrochar, and pyrochar were 19.5 million Joule per kg dry basis (MJ/kgdb), 21.7 MJ/kgdb, and 18.6 MJ/kgdb, respectively. The heating value of pyrochar was slightly lower than the raw swine solids due to higher ash contents. The ash content increased significantly from 21.2% in raw swine solids to 44.7% in pyrochar. Volatile matter (VM) of the raw swine solids (69.5%) decreased to 60.3% (hydrochar) and 14.1% (pyrochar). When the hydrochar was washed with acetone, its VM further decreased from 60.3% to 49.9%. The opposite trends were observed for fixed carbon and ash contents. When the swine solids were pyrolyzed at 620 oC, the oxygen content was decreased to negligible amounts. Mehlich 1 extraction of the initial soils amended with both pyrochar and hydrochar showed significant increase in nutrients such as Potassium (K), Phosphorous (P), Calcium (Ca), Magnesium (Mg), Zinc (Zn) and Manganese (Mn). The soils amended with pyrochar leached significantly higher amounts of nutrients than control soils, suggesting the potential for groundwater pollution from the pyrochar-amended soils. Surprisingly, the hyrochar-amended soils leached very little of these nutrients probably due to complex surface functionalities binding these nutrients. Addition of swine solid hydrochar dramatically increased soil CO2 emission; however, N2O emission was repressed. Throughout the incubation period, the control soils emitted 0.16 g CO2 square meter per day (m-2d-1). The hydrochar-amended soils, however, emitted 3.0 g CO2 m-2d-1, much higher than 0.32 g CO2 m-2d-1 from the pyrochar-amended soils. Washing the hydrochar with acetone reduced CO2 emission rate to 1.1 g CO2 m-2d-1. The control soils produced 0.31 mg N2O m-2d-1, while the pyrochar-amended soils did not produce any N2O. The hydrochar-amended soils produced 0.24 mg N2O m-2d-1; however, the acetone-washed hydrochar amended soils did not product any N2O.