CO2　pipeline　leaks　associated　with　intense　throttling　effects　generate　flows　of　dry　ice　particles　that　tend　to　coalesce　and　deposit　on　pipe　walls,　leading　to　blockages.　Adopting　molecular　dynamics　simulations,　the　collisions　of　nanoscale　dry　ice　particles　were　studied　to　reveal　the　coalescence　mechanism　at　the　molecular　level.　Through　systematic　analysis　of　morphological　evolution　during　dry　ice　particle　collisions　at　varying　velocities,　two　coalescence　mechanisms　were　identified:　intermolecular　force-induced　adhesion　under　low-velocity　condition　(vp=100　m/s)　and　interfacial　melting-recrystallization-driven　coalescence　at　mid-velocities　(200　m/s　≤　vp≤600　m/s).　High-velocity　collisions　(vp≥700　m/s)　were　observed　to　trigger　complete　particle　melting,　accompanied　by　significant　molecular　dissipation,　which　suppressed　recrystallization　and　led　to　the　formation　of　unstable　disordered　aggregates.　Quantitative　investigation　reveals　that　under　low-to-mid　velocity　conditions,　the　coalescence　degree　dominated　by　phase　transitions　strengthens　with　increasing　velocity.　At　lower　velocities,　larger　particles　exhibit　weaker　coalescence　due　to　stronger　interfacial　molecular　confinement　by　potential　fields,　while　at　higher　velocities,　increased　particle　size　amplifies　momentum　transfer,　thereby　enhancing　coalescence.　This　study　elucidated　the　collision-coalescence　mechanisms　of　dry　ice　particles　at　the　molecular　level,　providing　a　theoretical　foundation　for　enhancing　the　safety　of　CO2　pipeline　engineering.　©　2025　The　Institution　of　Chemical　Engineers