To help mitigate the effects of global warming and fossil fuel depletion caused by human use of fossil fuels, solid fuel pellets were developed from a mixture of spent coffee grounds (SCG) and pine sawdust (PS). The feasibility of SCG-PS pellets as biofuel was also verified by evaluating its fuel quality. An increase in the proportion of SCG in the pellet led to an increase in its calorific value, owing to the high C, H, and oil contents, and increases in the ash and S contents, owing to the high S content in SCG. Analysis of the feedstock particle size distribution revealed that SCG particles are smaller than PS particles; thus, the durability of the pellet decreases as the proportion of SCG increases. Accordingly, the samples with higher SCG proportions (70 and 90 wt.%) did not meet the moisture content standards for biomass solid refuse fuel (bio-SRF) set by the Korea Ministry of Environment, whereas the samples with lower SCG proportions did. In particular, CP10 (10 wt.% SCG + 90 wt.% PS) satisfied the quality standards of Grade 1 wood pellets, demonstrating the feasibility of using SCG as a raw material for biofuel pellet production.

The optimum conditions for producing biodiesel by combining beef tallow, a waste resource with high saturated fatty acid content, and soybean oil, which has high unsaturated fatty acid content, were investigated. Furthermore, the kinematic viscosity reduction effects of biodiesel by using heating and ultrasonic irradiation were verified, and their impacts on engine performance and exhaust emissions were evaluated. The result shows that the optimum production conditions are a blend ratio of TASO3 (soybean oil to tallow blend ratio of 7:3) and a methanol to oil molar ratio of 12:1. Kinematic viscosity reduction experiment results showed that the kinematic viscosity reduction effects of ultrasonic irradiation and heating were similar, but the heating device is considered more effective because it is simpler and cheaper than the ultrasonic device. Experiment results on output performance and exhaust performance showed that the engine performance and exhaust performance of the fuel with reduced kinematic viscosity were higher than those of the untreated fuel. However, the output of biodiesel was lower than those of conventional diesel (CD) due to the low calorific value of biodiesel; the NOx and CO2 emissions of biodiesel were higher than those of CD, but the CO and HC emissions of biodiesel were lower.

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Spent coffee grounds (SCG), the residue after brewing coffee beverage, is a promising biodiesel feedstock due to its high oil contents (15‒20%). However, SCG should be pretreated to reduce the high free fatty acid content, which hampers transesterification reaction. To overcome this, we explored a direct transesterification reaction of SCG using ultrasound irradiation and identified the optimal sonication parameters. A high fatty acid methyl ester (FAME) content, up to 97.2%, could be achieved with ultrasound amplitude of 99.2 <i>μ</i>m, irradiation time of 10 min, and methanol to oil ratio of 7:1 in the presence of potassium hydroxide concentration of 1.25 wt.%. In addition, we demonstrated that ultrasound irradiation is an efficient method to produce biodiesel from untreated SCG in a short time with less energy than the conventional mechanical stirring method. The physical and chemical properties of the SCG biodiesel met the requirements for an alternative fuel to the current commercial biodiesel.

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