Comparison of biofuels produced by two different conversion methods: Torrefaction and hydrothermal carbonization

Olive pomace is a by-product which is obtained during the process of olive oil production mostly in Mediterranean countries such as Turkey. This product is mainly eliminated by direct combustion. However, due to its organic structure, it has the potential to be a lignite-like solid biofuel. In this study, olive pomace was treated by two different biomass conversion methods and the products were compared by their structural characteristics. The conversion methods were determined as hydrothermal carbonization and torrefaction. Torrefaction experiments were carried under the temperature values of 250, 275, 300 and 350°C; reaction times of 10, 20 and 30 minutes. The experiment conditions at the hydrothermal carbonization were performed as temperature values of 180, 200 and 220°C and with the reaction time of 2, 3 and 4 h. The proximate and elemental analyses of the products were made. The higher heating value of the raw biomass was measured as 20,34 MJ/kg. Hydrothermal carbonization products' HHV were between 22,14 – 26,94 MJ/kg and HHV of torrefaction products' were between 20,58 – 29,41 MJ/kg. HHV for lignite were calculated to be approximately 26,7 MJ/kg. Despite the high energy contents, the energy yields dramatically decreased for higher torrefaction temperatures (300 - 350°C). Energy densities for torrefaction were calculated between 1,01 and 1,45. On ther other hand, energy densities of the hydrothermal carbonization products were measured between 1.09 and 1.32. The higher heating values of torrefaction at 275°C for 20 minutes and the hydrothermal carbonization at 220°C for 2 h were measured 23,73 and 25,20 MJ/kg respectively, for the same energy yield value of 62 %. Atomic O/C and H/C ratios were also obtained as 0,46 and 1,33 for the torrefaction; 0,39 and 1,31 for hydrothermal carbonization. In contrast of hydrothermal carbonization products, the ash contents of torrefied biomass increased correspondingly with the higher temperature values and reaction times. It may be concluded that it is possible to produce chars from olive pomace through hydrothermal carbonization process with low temperature values, without a drying process and obtain hydrochars which have approximate high heating values, energy yields and atomic O/C and H/C ratios as torrefaction products.