Optimization of biomass pyrolysis reactor model and performance analysis of pyrolysis gas separation using Laval nozzle and absorption refrigeration cycle

With the growing demand for clean energy, biomass pyrolysis has attracted increasing attention for producing high-value carbon-neutral products, but pyrolysis gas treatment remains a key bottleneck. This study compares mixed-gas separation driven by pyrolysis gas pressure using a Laval nozzle and absorption refrigeration liquefaction. An accurate biomass pyrolysis reactor model was first developed to predict pyrolysis gas distribution. By incorporating Gibbs free energy minimization and the catalytic effect of biochar, the model reduced prediction deviation by 2.53% compared with the original thermodynamic model. Based on the separation model, Laval nozzle separation generated CO₂ microdroplets with an average mass fraction of 0.175 and reduced the CO₂ molar fraction in pyrolysis gas by 23.91%. The separated liquid phase reached a CO₂ molar fraction above 0.925, while requiring a small footprint and showing favorable economic feasibility, making it suitable for small-scale separation. In terms of exergy destruction, the Laval nozzle showed clear advantages: when only evaporator heat exchange was considered in the absorption refrigeration cycle, its exergy destruction was only 44.28% – 55.07% of that of absorption refrigeration liquefaction. Absorption refrigeration liquefaction required additional low-quality heat. For the optimal working-fluid pair and a pyrolysis gas flow rate of 1.0 kg h⁻¹, the maximum condensed liquid reached 0.5378 kg h⁻¹, 3.07 times that of Laval nozzle separation, with a COP of 0.6654 and low-quality heat consumption of 0.0778 kW. However, due to phase equilibrium limitations, the CO₂ purity was only 0.576 – 0.844, making it more suitable for large-scale separation.