Improvements on the liquefaction of a pipeline CO₂ stream for ship transport

Abstract

Ship transport and pipeline transport of CO₂ are considered to be viable options for large scale CCS. While pipeline transport is usually recommended for shorter distances, ship transport might also be considered for these distances in the early stage of CCS due to its high flexibility and low capital expenses. Ship transport is usually carried out in liquefied state at a temperature around −50 °C and low pressures. Thus, an efficient liquefaction process is required. Unlike other works on CO₂ liquefaction, it is assumed that the CO₂ has been transported by pipeline before it is transported by ship. Consequently, the CO₂ is expanded instead of being compressed and purification is not necessary as it has already been carried out before pipeline transport. Basic 1-stage, 2-stage and 3-stage closed cycle liquefaction processes are modelled, showing that a significant reduction of the minimum specific energy demand can be obtained by employing a multi-stage cascade design. The energy demand for the 2-stage base process is approximately 39% lower than for the 1-stage process. The energy demand can be further reduced by approximately 13% with a 3-stage process. Four measures of optimisation have been analysed: For the CO₂ stream, energy recovery with a liquid expander and a two phase expander are investigated. For the refrigeration cycle, the use of an aftercooler and the replacement of the cascade heat exchanger by a phase separator is studied. It can be seen that the energy demand can be reduced by approximately 30%–40% with respect to the base process if those four measures are implemented. The impact of the optimisations is influenced by the impurity concentrations of the CO₂ stream. Besides liquefaction, the specific electrical and thermal energy demands for the injection of CO₂ have been calculated. The electrical energy demand is proportional to the wellhead pressure and was found to be between 1.9 kWh/t CO₂ and 7.8 kWh/t CO₂. The thermal energy demand is between 21.0 kWh/t CO₂ and 30.5 kWh/t CO₂ , increasing for higher wellhead pressures and impurity concentrations.