Evaluating the risk of hydrate formation in CO2 pipelines under transient operation

In this work, the risk of hydrate formation is examined in buried pipelines transporting CO2 in liquid or dense-phase during normal conditions, system shut-in and startup, and rapid depressurization. When the pipeline operates under normal conditions, the flow model and hydrate phase boundary enable to determine the time and location where the CO2 stream enters the hydrate formation region. The fluid flow is described by a set of partial differential equations resulting from the conservation of mass, momentum, and energy. The hydrate formation conditions for pure CO2 in the presence of free water are fitted from experimental data. A case study is conducted for a 600 MW natural gas-fired power plant with CO2 recovery. The simulations are carried out for soil clay with a low (<4%), medium (4-20%), and high (>20%) moisture content. Results show that, when the CO2 is in a supercritical state (40 degrees C) at the pipe inlet and the soil has a low conductivity, the CO2 stream might operate outside the hydrate stability region, even in the presence of free water. In case of a system shut-in or startup, the risk of hydrates is low if the water content of the CO2 stream is below 250 ppm. In situations of rapid depressurization, even a low water content level might not be sufficient to avoid hydrates. (C) 2013 Elsevier Ltd. All rights reserved.