Impact from Sargassum macroalgae on energy content in biogas, Grenada

Sammanfattning: This study is part of the Minor Field Studies programme and includes a project carried out in Grenada in the spring of 2022. Grenada is one of several countries affected by the huge amounts of Sargassum macroalgae that has infested marine areas and coasts in the last decade. This has resulted in environmental, social and economic problems. The exploitation of Sargassum for production of biogas could therefore be a possible sustainable solution. To test this, a Homebiogas 7.0 was issued to the True Blue Bay Boutique Resort in Grenada. The aim of the project is to produce biogas in the Homebiogas with the use of Sargassum macroalgae and analysethe quality of the gas by approximating the methane content of the biogas. The biogas is produced by process of anaerobic digestion which is a complex process dependenton several variables. These in turn are affected by the biogas system and the input biomass. Therefore, a literature review of the subject was carried out. Two experimental setups were created to approximate the methane content of the produced biogas, one to approximate the gasflux and one for energy approximation. The input biomass in the Homebiogas consisted of three batches with different compositions of food waste and Sargassum. The feeding schedulelasted over a time period of 56 days. To approximate the methane content, calculations were made with the data from the experiments together with the literature study. A sensitivity analysis was also conducted on the variables with the largest impact on the result. The results of the calculations gave a gasflux of 0,136 L/s and average methane concentrations of49,4%, 39,2% and 47,0% during the three batches. Further, because of the retention time of the process, the batch with the most positive effect on the methane yields was the second batch with a composition of ⅔ Sargassum and ⅓ food waste. The sensitivity analysis showed that the possibility of error in the methane concentration was large due to the uncertainties of the gasflux and the thermal efficiency of the stove. It is also possible that higher yields of methane from Sargassum can be reached since the processis dependent on environmental variables and the design of the system. Further pretreatment to the Sargassum could be applied to increase yields since the only pretreatment used in this study was rinsing it from sand. Although the average methane yields obtained are uncertain and quitelow relative to the desired concentration of 60% for biogas, the Sargassum does not seem to have a negative impact on the overall process.