1. Introduction - Firenze University Press

(nitrogen) utilization. Five case studies have been proposed and analyzed. The first one is a
reference, lignite-fired oxy-fuel power unit. It is composed of boiler, flue gas recirculation
loop, steam cycle, air separation unit and CO2 purification and compression system. The
subsystems within the reference case have been only slightly integrated. It represents
therefore, the state of the art for oxy-fuel systems proposed in recently published
prefeasibility studies [3, 4]. Remaining cases are focused on ideas for efficiency
improvements like heat recovery from the flue gas to combusted oxygen, heat recovery from
the flue gas compression train to steam cycle, as well as, lignite drying by waste nitrogen
leaving the air separation unit. First two of these ideas have been already investigated in the
literature, while the use of waste nitrogen for coal drying seems to be innovative and
promising. The results dealing with heat recovery from the flue gas to combusted oxygen
have been presented in [2]. Authors concluded that it is better to use the waste heat to preheat
boiler feed water than oxidizer.
Considering the ASU-waste nitrogen as drying agent for lignite, it is important to mention,
that this gas is completely dry and have therefore higher potential for moisture absorption
than ambient air. Moreover, the risk of ignition and explosion in the dryer is minimized.
The last, fifth case summarize all structural improvements applied in previous cases.
2. Case studies
General assumptions for all analysed cases have been summarized in Table 1. As it has been
already mentioned case no. 1 is the reference.
Table 1. Analyzed case studies
Heat recovery from the
flue gas to combustion
oxygen
Heat recovery from the flue
gas compression train to
steam cycle
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Lignite drying by waste
nitrogen
Case 1 NO NO NO
Case 2 YES NO NO
Case 3 NO YES NO
Case 4 NO NO YES
Case 5 YES YES YES
2.1 Reference case (Case 1)
The flow sheet of the reference case has been presented in Fig. A1a and A1b in the appendix
A. The reference case structure composes of boiler, flue gas treatment and conditioning line,
CO2 compression system, ASU and supercritical steam cycle. The recirculation loop is of hot
and wet type which means that part of the flue gas which goes back to the furnace to control
the flame temperature has not been cooled down and the moisture has not been condensed. In
accordance to [2], oxy-fuel power units equipped in hot or cold recirculation loop do not
differ in energy efficiency.
Main parameters of the reference case plant have been presented in Table 2. Volumetric
content of oxygen in oxidizer entering the combustion chamber has been assumed to 23,5%
which enables for keeping similar adiabatic flame temperature and heat exchange rates in the
boiler furnace as for conventional air-combustion system [5].