Abstract:
Gas Turbines (GTs) are used extensively in pipelines to compress gas at
suitable points. The primary objective of this study is to look at CO2 return
pipelines and the close coupling of the compression system with advanced
prime mover cycles.
Adopting a techno-economic and environmental risk analysis (TERA) frame
work, this study conducts the modelling and evaluation of CO2 compression
power requirements for gas turbine driven equipment (pump and compressor).
The author developed and validated subroutines to implement variable stators
in an in-house GT simulation code known as Variflow in order to enhance the
off-design performance simulation of the code. This modification was achieved
by altering the existing compressor maps and main program algorithm of the
code. Economic model based on the net present value (NPV) method, CO2
compressibility factor model based on the Peng-Robinson equation of state and
pipeline hydraulic analysis model based on fundamental gas flow equation were
also developed to facilitate the TERA of selected GT mechanical drives in two
case scenarios. These case scenarios were specifically built around
Turbomatch simulated GT design and off-design performance which ensure that
the CO2 is introduced into the pipeline at the supercritical pressure as well as
sustain the CO2 pressure above a minimum designated pressure during
transmission along an adapted real life pipeline profile.
The required compression duty for the maximum and minimum CO2 throughput
as well as the operation site ambient condition, guided the selection of two GTs
of 33.9 MW and 9.4 MW capacities. At the site ambient condition, the off design
simulations of these GTs give an output of 25.9 MW and 7.6 MW respectively.
Given the assumed economic parameters over a plant life of 25 years, the NPV
for deploying the 33.9 MW GT is about £13.9M while that of the 9.4 MW GT is
about £1.2M. The corresponding payback periods (PBPs) were 3 and 7 years
respectively. Thus, a good return on investment is achieved within reasonable
risk. The sensitivity analysis results show a NPV of about £19.1M - £24.3M and
about £3.1M - £4.9M for the 33.9 MW and 9.4 MW GTs respectively over a 25 -
50% fuel cost reduction. Their PBPs were 3 - 2 years and 5 - 4 years
respectively. In addition, as the CO2 throughput drops, the risk becomes higher
with less return on investment. In fact, when the CO2 throughput drops to a
certain level, the investment becomes highly unattractive and unable to payback
itself within the assumed 25 years plant life. The hydraulic analysis results for
three different pipe sizes of 24, 14 and 12¾ inch diameters show an increase in
pressure drop with increase in CO2 throughput and a decrease in pressure drop
with increase in pipe size for a given throughput. Owing to the effect of elevation
difference, the 511 km long pipeline profile gives rise to an equivalent length of
511.52 km. Similarly, given the pipeline inlet pressure of 15 MPa and other
assumed pipeline data, the 3.70 MTPY (0.27 mmscfd) maximum average CO2
throughput considered in the 12¾ inch diameter pipeline results in a delivery
pressure of about 15.06 MPa. Under this condition, points of pressure spikes
above the pipeline maximum operating allowable pressure (15.3 MPa) were
obtained along the profile. Lowering the pipeline operating pressure to 10.5
MPa gives a delivery pressure of about 10.45 MPa within safe pressure limits.
At this 10.5 MPa, over a flat pipeline profile of same length, the delivery
pressure is about 10.4 MPa. Thus, given the operating conditions for the dense
phase CO2 pipeline transmission and the limit of this study, it is very unlikely
that a booster station will be required. So also, compressing the CO2 to 15 MPa
may no longer be necessary; which eliminates the need of combining a
compressor and pump for the initial pressure boost in order to save power. This
is because, irrespective of the saving in energy, the increase in capital cost
associated with obtaining a pump and suitable driver far outweighs the extra
expense incurred in acquiring a rated GT mechanical drive to meet the
compression duty.