Off-Design of a Small-Scale Liquefaction Plant Operating With Biomethane

Liquefied natural gas is recently playing an important role in the heavy-duty vehicle and marine fuel market due to the low ambient impact, easy onboard storage, and low commercial prices. Biomethane from biogas upgrading can integrate the LNG market by providing the added value of an almost zero carbon footprint fuel. The production rate of biomethane is distributed in various anaerobic digestion plants, and daily amounts are limited to a few tons per day, with a substantial variability depending on digester feed. For this reason, small-scale liquefaction systems are requested to convert biomethane into bio-LNG. The Joule-Brayton reverse cycle is a promising solution for small-scale plants due to its simplicity and ease of regulation. The control strategy of this plant is important since small-scale installations are characterized by relatively high specific consumption that might increase when the system is operated in part-load or off-design conditions. For this reason, the comparison between two control strategies is proposed in this study: variable rotating speed control strategy and inventory control are compared and assessed. A steady-state off-design model of the plant was implemented in Aspen Hysys by considering the behavior of all the main system components, including compressors, turbine, intercoolers and aftercoolers, turbine, and cold-box. Typical analytical relations for heat exchanger off-design were considered as well as typical characteristic maps for the fluidmachinery. The two control systems were introduced in the model by implementing the control equations at the steady-state. Results showed that inventory control allows the system to achieve better performance and results to be more flexible. Variable rotating speed control strategy led to surge issues at low small biomethane production and low ambient temperature. By considering a plausible biomethane production profile, inventory control allows the specific consumption to be reduced by 4.3 %, and liquefied biomethane production increases by about 131 t with respect to variable speed control.