Abstract
This paper describes the role of ambient pressure in self-heating torrefaction of livestock manure. We explored the initiating temperatures required to cause self-heating of wet dairy cattle manure at different ambient pressures (0.1, 0.4, 0.7, and 1.0 MPa). Then, we conducted proximate, elemental, and calorific analyses of biochar torrefied at 210, 250, and 290°C. The results showed that self-heating was induced at 155°C or higher for 0.1 MPa and at 115°C or lower for 0.4 MPa or higher. The decrease of the initiating temperature at elevated pressure was due not only to more oxygen, but also to the retention of moisture that can promote chemical oxidation of manure. Biochar yields decreased with increasing torrefaction temperature and pressure, and the yield difference at 0.1 and 1.0 MPa was more substantial at lower temperatures: a 29.8, 16.4, and 9.4% difference at 210, 250, and 290°C, respectively. Proximate and elemental analyses showed that elevated pressure promotes devolatilization, deoxygenation, and coalification compared to atmospheric pressure; its impact, however, was less at higher temperatures as the torrefaction temperature became more dominant. Calorific analysis revealed that elevated pressure can increase the higher heating value (HHV) on a dry and ash-free basis at 210°C because of the increase in carbon content, but its impact is limited at 250 and 290°C. Meanwhile, the HHV on a dry basis exhibited the opposite trend due primarily to an enlargement of ash content. The present study revealed that ambient pressure considerably affects the initiating temperature of self-heating and the chemical properties of biochar at a low torrefaction temperature.
Highlights
Due to the growth of intensive farming, the thermochemical treatment of livestock manure is gathering attention in the livestock sector
The liquid remaining after wet torrefaction contains organic compounds including furfural and its derivatives, organic acids, and phenol and phenolic derivatives [16], and chemical or biological treatment would be needed before discharging it into the environment [17,18,19]. To overcome this practical difficulty, we recently reported a self-heating torrefaction system that uses the heat generated from the chemical oxidation of manure as a heat source for drying and torrefaction [20]
The initiating temperatures decreased with increasing pressure; atmospheric pressure (0.1 MPa) required a preheating temperature of 155 ̊C or higher to induce the self-heating of manure, while an elevated pressure of 1.0 MPa caused the process to proceed from 100 ̊C or lower
Summary
Due to the growth of intensive farming, the thermochemical treatment of livestock manure is gathering attention in the livestock sector. Manure can be used as an organic fertilizer and soil amendment, its overapplication may cause some environmental issues as it can be a primary source of greenhouse gas emissions, water pollution, and acidification [1,2,3,4]. In this context, the conversion of manure into biochar is an attractive option. Japan Society for the Promotion of Science (JSPS) KAKENHI (No JP262692130 to K.I. and No JP17J00272 to T.I.), https://www.jsps.go.jp/ english/. The specific roles of these authors are articulated in the ‘author contributions’ section
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