Biomethane grid injection or biomethane liquefaction: A technical-economic analysis

The gradual reduction of subsidies for electricity production from biogas and the raising interest of bio-methane as an integration to natural gas market force the biogas plant owners to choose alternative solutions for biogas exploitation. In this study, two solutions for biomethane distribution have been compared: biomethane liquefaction and grid injection. The analysis was carried out as a function of gas connection cost, electric tariff, selling price and type of expander adopted in the liquefaction cycle (radial turbines or screw expanders). A nitrogen Joule-Brayton reverse cycle was considered for liquefaction. A detailed model of the cycle was developed in Aspen Hysys and optimized to minimize the energy specific consumption. Results show that expander efficiency has a key role in the liquefaction scenario. Screw expanders lead to a specific consumption 1.45 times higher than radial turbines but reduce capital costs by a factor of 1.39. Biomethane grid injection is the preferable solution in terms of investment risk if the connection cost is below 500 k$, independently on the electricity price. As far as it concerns the profit, liquefaction with radial turbines is preferable up to electricity price of 0.23 $/kWh. A sensitivity analysis on product selling prices shows that biomethane grid injection is always the most profitable solution when connection costs are low. For higher connections cost, liquefaction with radial turbines is the best solution for minimizing the investment risk and maximizing the profit for most of the combination of selling prices.