Mediterranean Green Buildings & Renewable Energy by Ali Sayigh

Author:Ali Sayigh
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Statistical analysis indicates that an increase in the reaction time increases the yield to gas and drops the yield to liquid, regardless of the other operating conditions. In addition, the re-forming time does not have a significant effect on the solid yield. As an example, the effect of time is shown at the centre of variation in temperature and pressure (380 °C and 230 bar). The effect of temperature on global yields depends on both the pressure and catalyst/bio-oil ratio. In the absence of a catalyst, an increase in temperature from 310 to 450 °C at 200 bar increases the gas yield and cause a drop in the liquid yield. Between 310 and 380 °C the yield to solid increases with temperature, reaching its maximum. A further increase in temperature up to 450 °C slightly decreases the yield to solid. The temperature has a kinetic effect on cracking and re-forming reactions, thereby increasing the formation of gas from bio-oil. In addition, under this pressure, the increase in temperature changes the state of water from subcritical water to vapour at 366 °C. Steam can facilitate the gasification of char, which explains the experimentally observed reduction in the yield to solid. Conversely, at 260 bar an increase in temperature to between 310 and 380 °C slightly increases the yield to gas and causes a dramatic drop in the liquid yield. A further rise in the temperature up to 450 °C leads to a slight decrease in the gas yield and a sharp increase in the yield to liquid. At this pressure, this increase in temperature causes the water to change from a subcritical to supercritical state. Under subcritical conditions an increase in temperature increases gas production from bio-oil. Once supercritical conditions have been reached, cracking and re-forming reactions might be less favoured, thereby increasing and decreasing the yields to gas and liquid respectively. The yield to solid follows the same trend as described for 200 bar, indicating that the presence of supercritical water can partially reduce solid formation or favour its removal.

The effect of pressure also depends on temperature. While an increase in pressure from 200 to 260 bar increases gas formation and decreases the yield to liquid between 310 and 380 °C, it has the opposite effect from 380 to 450 °C. Between 310 and 367 °C water remains under subcritical conditions between 200 and 260 bar, and the pressure has a positive catalytic effect on the process, increasing the reaction rates of cracking and re-forming reactions. Conversely, between 367 and 450 °C the same increment in pressure changes the state of water from steam, where steam re-forming and cracking reactions are favoured, to supercritical water, reducing gas formation and increasing the yield to liquid. The effect of pressure on solid yield is relatively low. In general, an increase in pressure slightly increases the yield to solid, especially between 310 and 380 °C. The physicochemical properties of water changes at temperatures higher than 300 °C, and the solubility of bio-oil in water can be enhanced by pressure, decreasing char formation.

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