Control system design of a biogas treatment and liquefaction plant

Abstract

The current energy scenario is transitioning towards renewable energy sources, oriented towards renewable electrical production to reduce global warming by cutting the use of fuels for electrical production. Reduction of fossil fuels is pushing the reconversion of existing heating and combustion engines to the battery electrical solution. Not all applications can be substituted by electrical driven solutions, like ship propulsion, heavy duty trucks or high temperature ovens. Production of a renewable and sustainable fuel is mandatory to implement a renewable energy solution for such applications. Renewable fuels can be obtained from biogas production (natural decomposition of organic matter) or synthetic methane / fuel from H2 obtained via a CO2 catalytic process. Both solutions require an upgrading process to separate CO2 from CH4. The CH4 has value as a fuel or energy vector and the CO2 has a value for industrial or alimentary purposes as well as H2 generation. New upgrading and polishing solutions have been developed in ADDsynergy, the commercial brand of CRYO INOX SL, part of the Pietro Fiorentini Group, an international company with a wide range of products and solutions for different applications including renewable energy. CRYO INCO is a company based on engineering and manufacturing solutions for Liquefaction, Distillation, Regasification and Fuelling stations from to the treated Natural Gas or Biogas and has the Patent Application for the Cryoupgrading (P6195EP00), the Cryopolish (P6207EP00), and the combined solution (P6206EP00). The first Bio-LNG or LBM & Food Grade LCO2 plant has been built in Spain in the Sologas waste management plant in As Somozas, Galicia. This plant is a pilot plant that integrates Cryoupgrading, Biomethane and CO2 liquefaction, Membrane Polishing and Cryopolish. A description of the plant is given, detailing each equipment and its functionalities. Specific process data is given via the PFD and H&M balance. The primary focus of this project is the development of the control system. Starting from the process description of each equipment, the plant is segmented into 8 different control systems: Biogas Pre-treatment, Cryoupgrading, Biomethane Cryopolish and Liquefaction, Mixed Refrigerant, LBM Storage Tank and Boil-off, LBM Transfer, LCO2 Storage and Transfer, and Fire & Gas and ESD. An overview of the control system explains the basis of its control as well as the definition of the most important tool: the control screen. The automatic control of the plant is designed via three key tools are used: Controllers, Automatic Sequences, and Cause & Effect Matrixes. Controllers are used to control specific variables like pressure or temperature. Automatic Sequences are used to start and stop a system or maintaining it operational. The Cause & Effect Matrix is a safety methodology to define the plant’s warning, alarms, and safety actions. For each system a description of its components is given with a control point of view, and the Controllers, Automatic Sequences and Cause & Effect Matrix are explained in detail. The implementation of the control system started during April 2024 and the Bio-LNG & LCO2 Liquefaction plant started operations in May 2024. The designed control system was implemented successfully. Cryoupgrading and Membrane Polishing have been tested and show positive results. Cryopolish has also shown positive results, but its control is highly complex, indicating that Membrane Polishing is a better solution. Three modifications have been proposed for the future: manual controller activation, semi-automatic sequence creation and a revamp of the Mixed Refrigerant System control.