Study on the effect of stepwise warming on two-phase anaerobic fermentation of cow manure
Authors: Wang Zejia , Yin Xuefeng , Menggen Tuya , Hu Ying , Chang He , Feng Xueyan 
Published: March 2021

Abstract
In order to explore the influence of different stepwise heating methods for the acid-producing phase on the gas production of the methane-producing phase, the stepwise heating method with the best acidification, the highest gas production and the best energy-saving effect was obtained. Using cow dung as the raw material, the acid-producing phase is used for stepwise warming, the temperature increase range is 20–35 °C, set three different step-by-step heating methods respectively: the first group is heated by 5 °C/2 d, and the second group is heated by 2.5 °C/d; the third group was heated by 2 °C, 8 °C and 5 °C respectively once every other day; constant temperature fermentation after heating to 35 °C; and a set of acid-producing phase constant temperature 35 °C fermentation; the methane-producing phase is always fermented at a constant temperature of 38 °C; analyze the changes of different parameters. The results show that, in the first group, the total mass concentration of volatile fatty acids is 9372.46 mg/L; the proportion of the total concentration of acetic acid butyric acid to the mass concentration of volatile fatty acids is 77 %; the cumulative gas production volume is 6253 mL, and the cumulative methane production volume is 3741 mL, the total energy consumption is 638.32 kJ, and the heat generated by biogas combustion is 156.33 kJ; the difference between system energy consumption and production capacity is 481.99 kJ. As for the second group, the total mass concentration of volatile fatty acids is 7810.49 mg/L, and the proportion of the total concentration of acetic acid butyric acid to the mass concentration of volatile fatty acids is 74 %. The cumulative gas production volume is 4583 mL, and the cumulative methane production volume is 2521 mL. The total energy consumption is 629.46 kJ, and the heat generated by biogas combustion is 114.58 kJ. Moreover, the difference between system energy consumption and production capacity is 514.88 kJ. In the third group, the total mass concentration of volatile fatty acids is 7664.71 mg/L, the proportion of the total concentration of acetic acid butyric acid to the mass concentration of volatile fatty acids is 75 %; the cumulative gas production volume is 4606 mL, the cumulative methane production volume is 2579 mL. The total energy consumption is 624.18 kJ, and the heat generated by biogas combustion is 115.15 kJ. The difference between system energy consumption and production capacity is 509.03 kJ. In the constant temperature 35 °C group, by contrast, the cumulative gas production volume is 6565 mL, the cumulative methane production volume is 3939 mL. The total energy consumption is 765.74 kJ, and the heat generated by biogas combustion is 164.13 kJ. The difference between system energy consumption and production capacity is 601.61 kJ. According to the comparative analysis of the three groups, the first group has the best acidification effect and highest gas production, and the difference in energy consumption compared with the constant temperature group is also lower, which is more economical. Therefore, the step-by-step heating method of 5 °C/2 d is the best.