Toward an effective use of microalgae biomass from Uasb reactors: from harvesting to small gasification

Abstract

Sanitary sewage, traditionally seen as a source of expenses and problems, has come to be seen as an opportunity and source of funds. This is because of the three major current demands of modern society, two can be extracted directly from sewage (water and energy), and one (food) can benefit from the recovery of nutrients to agriculture. Therefore, Wastewater Treatment Plants (WWTP) which go beyond the treatment itself and reuse by-products to improve their energy and economic performance is being increasingly studied. This thesis discussed the reuse of microalgae biomass produced in WWTP as a source of energy. Based on the data from the literature survey, a conceptual scenario for the use of microalgae biomass for microgeneration in WWTP was built. Thermochemical gasification was the chosen conversion process since it is one of the most promising for the microgeneration of electricity. The results showed a production potential of 0.167 kWh/m3 of treated sewage, and investments financially returned after five years. After this theoretical approach, an experimental investigation was carried out using the microalgae produced in a WWTP pilot, constructed within the area of Companhia Espírito Santense de Saneamento (CESAN), with resources from Financiadora de Estudos e Projetos (FINEP), and in partnership with the company Fluir Engenharia Ambiental. The microalgae were cultivated in two high-rate algal ponds (HRAP), fed with the effluent obtained after wastewater treatment in the UASB reactor (Up-flow Anaerobic Sludge Blanket). The biomass was then harvested in a coagulation-flocculation system, dried, characterized in terms of its calorific value, ultimate, proximate, ash, thermogravimetric, and differential thermal analysis. The effects of seven commercial coagulants on the thermochemical conversion of microalgae were evaluated and the results revealed that coagulants could affect the energy recovery. Some coagulants showed catalytic effects and were beneficial to the gasification process, while others impaired the energy recovery of biomass. Lastly, experimental microalgae gasification was evaluated in a pilot-scale downdraft gasifier. Unlike other studies reported in the literature, which simulate microalgae gasification in laboratory equipment, the gasifier used in this study is a commercial technology, already widespread in the market and present in over 40 countries. Another important difference of this work in relation to the published ones concerns the microalgae type. While the literature generally reports the gasification of pure microalgae species (monoculture), obtained in a controlled manner and free of chemical coagulants used in the harvest stage, this study presents the gasification of biomass composed of different species of microalgae, bacteria and other organisms present in the HRAP, besides chemical coagulants. The effects of air-fuel equivalence ratio (ER) on the produced gas composition, higher heating value (HHV), cold gas efficiency (CGE), and production rate were presented. An increasing and then decreasing trend with ER with a peak was seen, indicating that there is an optimum ER of 0.23 for the best performance of the process. The cold gas efficiency, syngas composition, HHV, and production rate were 87%, 11.86% H2, 19.45% CO, 8.5% CH4, 9.82% CO2, 6.23 MJ/Nm3, and 2.79 Nm3/kg biomass dry, respectively. The tests demonstrated the possibility to use wastewater microalgae as fuel in downdraft gasifier. The energy recovery could help drive the WWTP to a more economical and sustainable process