OPPORTUNITIES OF ADVANCED BIOMETHANE PRODUCTION FROM MICROALGAE GROWN ON BIOGAS PLANT DIGESTATE. Part 2.

Abstract

The aim of the work is to continue the review given in part 1 of the article and to provide a more complete analysis of technologies and equipment for growing microalgae (MA), technical and economic indicators of the technology of cultivating microalgae in biogas plants with additional biomethane yield and utilization of the liquid fraction of digestate.

The design features of flat-panel photobioreactors and their influence on the productivity of growing MA are considered. It is shown that to achieve the productivity of growing MA at the level of 5 g/(L·day) the ratio of the volume of the photobioreactor to its area should not exceed 10-12 L/m². The physiological strategies for cultivating MA are considered and it is shown that their cultivation at biomethane plants, using CO₂ from biogas upgrading and the liquid fraction of digestate, is appropriate in photoautotrophic and, partially, mixotrophic modes. Based on a literature review, the optimal conditions for growing microalgae in a photoautotrophic mode are shown. A preliminary assessment of the mass balance of CO₂ and nutrients for growing microalgae on the digestate of biogas plants is performed. It is shown that to ensure the productivity of MA cultivation at the level of 5 g/(L·day), the liquid fraction of the digestate of a typical composition will be advisable to dilute with water in a ratio of 2-5% digestate and 95-98% water. This will ensure a sufficient amount of nutrients in the MA growth medium, as well as minimize the impact of turbidity and high ammonia concentrations what are present in the digestate.

The work provides an estimate of the required mass ratio of the MA and straw, at which the optimal C/N ratio for anaerobic fermentation is achieved at 19.8 and the dry matter content in the input mixture of substrates at 9.8%. When mixing MA with straw for further production of biogas from them, there is no need to concentrate microalgae.

Data on the cost of microalgae grown in open and closed systems are presented, as well as possible markets for MA and prices for MA in these markets. It is shown that closed types of photobioreactors should allow for a lower MA production cost and have lower energy requirements. A preliminary estimate of MA production cost in a flat-panel photobioreactor with a volume of 100 thousand L and a productivity of 5 g/(L·day) is presented. It is shown that with an estimated cost of MA cultivation of 0.26 $/kg and a price for biomethane from MA of 900 $/1000 m³, a positive economic effect can be expected with a profitability of 0.02 $/kg MA. However, for the cost-effective production of advanced biomethane from microalgae, the cost of their cultivation should be less than 0.2 euros/kg of dry matter of MA.

Considering the expected increase in the number of biomethane plants in Ukraine due to the opening of exports to EU countries, the technology of growing microalgae on the digestate of biogas plants has all the prerequisites for scaling up at the national level, which will allow increasing the potential for biomethane production and reducing greenhouse gas emissions.