Transport of Liquid CO2 Derived from Steelworks Off-Gases

Abstract

Steelworks are a hard-to-abate CO2 producing industry but large reductions are necessary to achieve goals set by the European Union. A promising option to mitigate CO2 emissions is to separate the CO2 from off-gas streams and subsequently move it to permanent storage. This thesis investigates the energy requirements and costs of liquefying and transporting CO2-rich streams to permanent storage sites.The study focuses on ship transport of liquid CO2 derived from HIsarna off-gas. The phase behavior of various gas streams with increasing purities of 95.5, 97.5 and 99.5 vol% CO2 is analyzed with NIST REFPROP in the pressure and temperature range of ship transport to assess the effects of impurity content. These synthetic CO2-rich mixtures are composed of typical impurities from the HIsarna steelmaking process with or without subsequent heat recovery from combusting CO and H2. The impurity composition for both cases consists of N2, CO and H2 or N2 and O2 respectively. Aspen HYSYS was utilized to configure models of open and closed liquefaction systems which were used to simulate the various synthetic mixtures to find the effects of the input streams on the energy requirements. The costs of this liquefaction equipment are analyzed by sizing it with the Aspen Process Economic Analyzer and subsequently using various cost correlations. The transportation costs were calculated for the scales of 100, 200 and 400 kt of liquefied product per year with various correlations from literature based on the selected shipping conditions.Results show that increasing the CO2 volume fraction will decrease energy requirements and operational costs for both open and closed liquefaction systems. The closed liquefaction system showed lower energy requirements and operational costs for all investigated mixtures compared to the open liquefaction system. Furthermore it was found that the effects of the different impurity cases, N2, CO and H2 or N2 and O2, are relatively small. The capital costs of liquefaction equipment are found to be higher for the closed liquefaction system and slightly decrease with CO2 purity. An increase of CO2 purity also creates a reduction in operational and capital costs for transport infrastructure. Total operational costs are found to scale linearly with yearly capacity while capital costs increase strongly sublinearly.The findings provide insight into the effects of CO2 purity, impurity components, liquefaction systems and process scale on the operational and capital costs of liquefaction and transport. These results can be combined with research on CO2 separation costs to find a preferred CO2 storage route and support CO2 abatement in the steel industry.