Optimal processtemperatur vid mesofil samrötningsprocess

Sammanfattning: The purpose of this master thesis has been to investigate how a change in process temperature can affect the stability and production of biogas. The plant which is operated and administrated by OX2 Bio Produktion AB located in Helsingborg produces biogas from primarily household food waste and waste from food industry. An ideal temperature to produce biogas at the plant is 37 °C but there is a fluctuation throughout the year depending on season. During summer the process temperature within the anaerobic digesters can rise close to 42 °C. How this can affect the production of biogas and process stability was investigated and evaluated in this master thesis. Main focus throughout the work has been to investigate the different temperatures 37 °C, 39 °C, 44 °C and 49 °C in terms of amount produced biogas, concentration of free fatty acids,concentration of ammonium, pH, temperature, dry matter content and the gas composition. Initially four reactors of 4.5 liters each were operated for 103 days. A start-up period during day 1-34 and a temperature change period during day 35-49 led up to the intended temperatures. At intended temperatures the reactors were operated during day 50-103. To evaluate if it was viable to raise the temperature from an energy perspective calculations were made for this purpose. These calculations were made to investigate if more energy in form of methane could be produced than needed to heat the process to a higher temperature. During start-up it was shown that the reactors produced equivalent amount of biogas during equivalent conditions. After the temperature change the microorganisms in reactors with a larger temperature change were most affected. This resulted in reduced amount of produced biogas. The concentrations of free fatty acids also increased in those reactors with a major temperature change. At intended temperatures only three of the initial four reactors were used because one reactor which was intended to have a temperature of 49 °C cracked at day 52. As the microorganisms customized to the new condition it was shown that a higher amount of biogas was produced at 44 °C compared to lower temperatures. Concentrations of free fatty acids were initially increased but decreased as the microorganisms had adapted. Concentration of free fatty acids remained higher in the reactor with higher temperature throughout the experiment. Concentration of ammonium was slightly higher for a higher temperature. The fact that the microorganisms produced higher amounts of biogas at higher temperature indicates that the concentrations of ammonium did not cause inhibition. pH was stable throughout the entire experiment for all reactors. pH-values was slightly higher at higher temperatures. Energy calculations showed that more energy was produced in the form of methane at higher temperature than needed to raise the temperature to the higher temperature. This means that it is energetically beneficial to produce biogas at a higher temperature. A variation in temperature is not favorable for the microorganisms since it contributes to instability. It furthermore leads to lower amount of produced biogas and higher concentrations of free fatty acids since the microorganisms is not effective enough during the transition phase to decompose all substrate that is added to the reactor. At 44 °C the seasonal variations in reactor temperature would be eliminated. Taken together results from this master thesis show that it would be favorable to produce biogas at 44 °C compared to 37 °C.