Abstract
Nitrogen is by-product from cryogenic air separation processes used for oxygen production for metallurgy and oxygen-enriched combustion purposes. If the gases are delivered from air separation unit (ASU) in liquid phase, liquid nitrogen (LN2) can be used as energy accumulator for stabilization of electrical grid system with large share of renewable energy sources. When the energy demand is high and not enough electricity is generated in power plants, energy accumulated inLN2may be recovered in a cryogenic power cycle. In this research complete exergy analysis of liquid nitrogen direct expansion cycle and combined direct expansion/Brayton cycle was performed.
Highlights
Oxygen is used on large scale in metallurgy since the beginning of 20th century
It is possible to use nitrogen as energy accumulator, if air ingredients are collected from the air separation unit (ASU) in liquid form
Oxygen can be used for industrial and power generation purposes, while liquid nitrogen can be stored in cryogenic vessel
Summary
Oxygen is used on large scale in metallurgy since the beginning of 20th century. In recent years the use of oxygen in power generation is investigated. The most mature technology of oxygen production is cryogenic air separation. It is possible to use nitrogen as energy accumulator, if air ingredients are collected from the air separation unit (ASU) in liquid form. When the demand for electricity is low, the energy can be used for air separation and. Oxygen can be used for industrial and power generation purposes, while liquid nitrogen can be stored in cryogenic vessel. When the demand for electrical energy is high, stored liquid nitrogen can be expanded in LN2 exergy recovery system to produce electricity. Two configurations of such systems were analyzed in this paper. As a cryogenic fluid with temperature much lower than ambient, LN2 has large exergy. Specific exergy of liquid nitrogen at 1 bar is equal to 769kJ/kg and exergy density (exergy per unit volume) - 620kJ/m3
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