vgbe energy journal 7 (2022) - International Journal for Generation and Storage of Electricity and Heat

Emission footprint analysis of dispatchable gas-based power generation technologies
Emissions in
0ppmvd @ 15 % O 2
80
60
40
20
0
0 20 40 60 80 100
An efficiency characteristic is needed to convert
the full-load emission values to the selected
apples-to-apples metric, i.e., mg/
kWh el , over the entire load range. The underlying
efficiency characteristic used in the present
study was derived by combining available
information from gas turbine manufacturers
[17] and scientific literature [18], [19].
As a result, the NO X emission part-load characteristic
can be converted into the applesto-apples
metric. F i g u r e 7 shows the exemplary
results for a SC-GT with an emission
level of 15 ppmvd @ 15 vol.% O 2 and
electrical efficiency of 39 % at full load [20],
which equals 236 mg/kWh el . It should be
noted that F i g u r e 7 depicts the emission
behavior of the gas turbine over the entire
load range, including load points below the
minimum environmental load. At loads below
the MEL, emissions are no longer in
compliance with the legislation. Thus, operation
over extended periods is not permissible
at loads below the MEL.
While data on NO X emissions must be reported
to the EPA, reporting of other pollutantssuch
as CO or UHC emissions is not mandatory.
Since no other turbine data source
could be identified, a plausible part load CO
& UHC emission characteristic was derived
from the scientific literature [15], [21], [22].
The following trends are expected for CO:
––
Very low emissions at full load due to
complete combustion
––
Constant, low emissions at load reduction
until a particular load point (for CO typically
20-50 % of nominal load, for UHC
typically ~10-20 %-points lower as for CO)
Power in %
Fig. 6. Load-dependent GT NO X emissions in ppmvd.
Emission in mg kWh el
5000
4000
3000
2000
1000
regulated by
local
authority
Power in %
Fig. 7. Load-dependent GT NO X , CO, and UHC emissions in [mg/kWh el ].
NOx
CO el = 39 %
UHC
13. BlmSchV (NOx)
13. BlmSchV (CO)
regulated by
13. BlmSchV
0
0 25 50 75 100
––
Rapid and exponential increase towards
lower loads when diffusion-type pilot burners
are activated for flame stabilization
The expected results were used to benchmark
the derived CO part-load characteristics.
For UHC the same trend as for CO emissions
was assumed but shifted 10 %-points
towards the lower load in compliance with
the underlying scientific literature [15],
[21], [22]. The resulting trend curves parametrized
for CO and UHC emissions are displayed
in F i g u r e 7. The corresponding
full-load emission values are 19 mg/kWh el
for CO (2 ppmvd @ 15 vol.% O 2 ) and
33 mg/kWh el for UHC (TOC as C 3 H 8 ; 2 ppmvd
@ 15 vol.% O 2 ) based on an electrical
efficiency of 39 %. Additionally, the 13 th
BImSchV emission limits are displayed corrected
with the underlying part load efficiency
characteristic of a SC-GT.
For the present study, a somewhat optimistic
starting point (at 20 % relative load) for
the CO emission increase was chosen to represent
state-of-the-art gas turbines and comply
with the underlying scientific literature.
However, it should be mentioned that the
individual starting point varies manufacturer-
and engine-dependent.
For formaldehyde and PM emissions, loaddependent
part-load characteristics could
not be found in the publicly available literature.
Primarily, investigations on formaldehyde
emissions from gas turbine engines are
very scarce. This may be attributed to formaldehyde
emissions being typically very
low [21], although they account for the
highest share of hazardous air pollutants
(HAP) [23]. Formaldehyde forms as an early
intermittent species of methane oxidation
[24]. Thus, very low formaldehyde emissions
are expected under complete combustion
conditions, although a similar trend to
CO can be anticipated towards very low
loads [25]. However, no detailed information
on the formation process of formaldehyde
for reduced loads was found in the
available literature. As a result, for the present
study, formaldehyde emissions were
accounted for by a constant value over the
entire load range (3 mg/mN 3 [23], i.e.,
23 mg/kWh el for an electrical efficiency of
39 %). Since the available literature data is
not sufficient to model PM with satisfactory
accuracy, an averaged value over the entire
load range was assumed (1 mg/mN 3 [26],
i.e., 8 mg/kWh el for an electrical efficiency
of 39 %). This should be a conservative estimation
since PM emissions from gas turbines
are generally very low [27]. The corresponding
part load trend curves for both
PM and HCHO emissions are subsequently
derived by the application of the part-load
efficiency characteristic of a SC-GT.
3 Modeling approach for the
gas-based power plants
This section explains the underlying modeling
approach for aggregating individual
RICE or GTs into a power plant configuration,
the plant operation for a given load
profile, and the corresponding emission calculation.
For this purpose, an EXCEL-based
model framework was developed.
To highlight the different modes of operation
of gas-based power generation plants,
an exemplary “peaking” scenario and an exemplary
“baseload” scenario are examined
in detail. The two scenarios differ in power
plant configuration. The peaking scenario
comprises plant configurations that feature
multiple and rapid startups and shutdowns
as well as transient operations. Therefore,
one SC-GT and the corresponding number
of about 10 MW el RICE to reach the same
power output represent the plant configuration
of the peaking scenario. In contrast, the
baseload scenario features plant configurations
that focus on efficient power generation
close to full-load operation. Consequently,
a CC-GT and the corresponding
number of about 20 MW el RICE to reach the
same power output represent the plant configurations
of the baseload scenario. The
corresponding load profiles used for each
scenario are derived from publicly available
actual plant operation profiles of a CC-GT
power plant located in Germany 11 [37].
Since the real load profile of an aggregated
RICE power plant may exceed the transient
capabilities of conventional GTs, a load profile
was chosen that both technologies can
be operated with. In the case of high tran-
11
The load profiles used are originally from the
600 MW CC-GT power plant Lausward in Germany.
vgbe energy journal 7 · 2022 | 37