Abstract
The pulp industry in Taiwan discharges tons of wood waste and pulp sludge (i.e., wastewater-derived secondary sludge) per year. The mixture of these two bio-wastes, denoted as wood waste with pulp sludge (WPS), has been commonly converted to organic fertilizers for agriculture application or to soil conditioners. However, due to energy demand, the WPS can be utilized in a beneficial way to mitigate an energy shortage. This study elucidated the performance of applying torrefaction, a bio-waste to energy method, to transform the WPS into solid bio-fuel. Two batches of the tested WPS (i.e., WPS1 and WPS2) were generated from a virgin pulp factory in eastern Taiwan. The WPS1 and WPS2 samples contained a large amount of organics and had high heating values (HHV) on a dry-basis (HHD) of 18.30 and 15.72 MJ/kg, respectively, exhibiting a potential for their use as a solid bio-fuel. However, the wet WPS as received bears high water and volatile matter content and required de-watering, drying, and upgrading. After a 20 min torrefaction time (tT), the HHD of torrefied WPS1 (WPST1) can be enhanced to 27.49 MJ/kg at a torrefaction temperature (TT) of 573 K, while that of torrefied WPS2 (WPST2) increased to 19.74 MJ/kg at a TT of 593 K. The corresponding values of the energy densification ratio of torrefied solid bio-fuels of WPST1 and WPST2 can respectively rise to 1.50 and 1.25 times that of the raw bio-waste. The HHD of WPST1 of 27.49 MJ/kg is within the range of 24–35 MJ/kg for bituminous coal. In addition, the wet-basis HHV of WPST1 with an equilibrium moisture content of 5.91 wt % is 25.87 MJ/kg, which satisfies the Quality D coal specification of the Taiwan Power Co. requiring a value of above 20.92 MJ/kg.
Highlights
There has been an increasing demand for fossil fuels over the past decades
It is noted that the initial moisture content of the with pulp sludge (WPS) batches would vary depending on the source and pretreatment procedures
The results of this study showed two cases of WPS samples with high and low amounts of initial equilibrium moisture
Summary
There has been an increasing demand for fossil fuels over the past decades. With the consummation of fossil fuels, an increase in the emissions of greenhouse gases such as carbon dioxide has been confirmed to result in a severe global warming effect [1]. One of the foremost solutions for a substitute energy source for fossil fuels is to accelerate the deployment of renewable energy sources, which includes solar energy, biomass energy, hydroelectricity, wind energy, wave energy, tidal energy, geothermal energy, and waste-derived energy [5]. Among these types of renewable energy, biomass energy from vegetable-derived feedstock has a property known as carbon neutrality and may not contribute to an increase in CO2 in the atmosphere from a life cycle viewpoint [6]. The effective utilization of biomass energy is a preventive and strategic measure against global warming
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