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SIMPLOT AUSTRALIA PTY LTD
PROPOSED NATURAL GAS BOILERS
and COGENERATION PLANT
AIR QUALITY IMPACT STUDY
Environmental Dynamics Project ED5133 May 2011
SIMPLOT AUSTRALIA PTY LTD
PROPOSED NATURAL GAS BOILERS and COGENERATION PLANT
AIR QUALITY IMPACT STUDY
Contents Page
1.0 INTRODUCTION ..................................................................................................................... 1
2.0 SITE AND PROJECT DESCRIPTIONS ................................................................................ 2
3.0 AIR QUALITY STANDARDS ................................................................................................. 5 3.1 Nature of the Emissions .............................................................................................................. 5 3.2 In-Stack Concentration Standards ............................................................................................... 5 3.3 Workplace Air Quality Standards ............................................................................................... 6 3.4 Ambient Air Quality Standards ................................................................................................... 6
4.0 THE EMISSION DISPERSION MODEL .............................................................................. 8
5.0 MODEL INPUTS .................................................................................................................... 14 5.1 Source and Emission Characteristics......................................................................................... 14 5.2 Background Concentrations ...................................................................................................... 16 5.3 Summary of Model Inputs ......................................................................................................... 18
6.0 MODEL WIND PREDICTIONS ........................................................................................... 19 6.1 Surface Wind Climatology ........................................................................................................ 19 6.2 Wind Speed and Direction Frequency Distributions ................................................................. 20 6.3 Distribution of Stability Classes ................................................................................................ 22
7.0 EMISSION DISPERSION PREDICTIONS ......................................................................... 23 7.1 Present PM10 Emissions ............................................................................................................ 23 7.2 Dispersion Characteristics of the Proposed Cogeneration Stack ............................................... 24 7.3 Expected Future NOx Emissions ............................................................................................... 25
8.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................ 28
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 1
Environmental Dynamics May 2011
1.0 INTRODUCTION Simplot Australia Pty Ltd (“Simplot”) operates a potato processing plant at Ulverstone, Tasmania,
under a permit issued by the State Government’s Environment Protection Authority (EPA). Simplot
is planning to install a natural gas cogeneration plant on the west side of the plant. Simplot currently
operates three coal-fired boilers, and plans to convert two boilers to natural gas.
In March 2011, Simplot engaged Dr Steve Carter of Environmental Dynamics to assess the air quality
impact of the particulate emissions from the coal-fired boilers at present; the air quality impact of
emissions from the natural gas cogeneration plant and boilers; and to determine an appropriate height
for the cogeneration plant stack.
Dr Carter visited the site on 7 March 2011. An emission dispersion modelling strategy was agreed
with Dr Mike Power, EPA’s air quality modelling specialist, using the sophisticated wind and air
emission dispersion model, TAPM, with TAPM’s chemistry mode used to predict the fate of nitrogen
oxide emissions associated with natural gas combustion.
This report is structured as follows:
• Section 2 describes the plant and its neighbourhood, and the proposed project.
• Section 3 sets out the air quality standards.
• Section 4 describes the dispersion modelling methodology.
• Section 5 presents the estimated emission data.
• Sections 6 and 7 present the wind predictions, and the predicted air quality impact of the present and expected future emissions. and assesses compliance with the ambient air quality standards for the various contaminants.
• Section 8 sets out the study conclusions and recommendations.
Acknowledgements
Mike Power, EPA’s air modelling specialist, was kind enough to assist in developing the modelling
strategy, and to review the draft report. Many thanks also to David Owens and Garth Jackson of
Simplot Australia, Mark Gregory of Renewable Intelligence, Steven Hill and Leon Daych of SDA
Engineering, and Nick Holmes of RCR Energy.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 2
Environmental Dynamics May 2011
2.0 SITE AND PROJECT DESCRIPTIONS The Simplot potato processing plant in Ulverstone on the north-west coast of Tasmania, is located
about 3 km inland from the Bass Strait, at MGA94 coordinates of {5,442,800 m N; 429,500 m E}, or
a GDA latitude and longitude of {41° 09′47′′ S, 146° 09′ 35′′E}.
Figure 2.1 shows a plan aerial view of the Simplot site, which is just north of the Bass Highway, and
bounded to the west by the Leven River. Residential areas and the Ulverstone primary school are
located immediately north-east of the plant. Residences are also located immediately south-east of the
plant, and a sub-division is located south-west of the plant, on the south side of the Bass Highway.
Figure 2.1 Google Earth views of the Simplot potato processing plant.
Figure 2.2 shows low-level aerial views of the plant. The top photo looks looking north east towards
the Ulverstone CBD and Bass Strait. It shows the location of a) the three existing boiler stacks; b) the
proposed cogeneration plant; and c) several steam stacks serving the potato processing building. The
bottom photo looks south-west, showing the slightly elevated terrain on the south side of the Bass
Highway.
200 m N
Bass Hwy
Leven River
Residences
School
Residences
Simplot
Industrial
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 3
Environmental Dynamics May 2011
Figure 2.2 Aerial views looking north-east (top) and south-west (bottom) at the Simplot potato processing plant.
Proposed cogeneration plant
Simplot plans to install a 7.9 MW (electrical) gas turbine-generator and an associated 12 MW
(thermal) heat recovery steam generator. The facility will burn about 1,750 kg/h of natural gas at full
load, and will supply all the plant’s electrical power needs, and will produce enough steam to supply
about one-third of the plant’s process heat requirements.
N Bass Strait
Ulverstone
Proposed Cogen plant
location
Boiler stacks Order: 1, 2, 3 Stacks discharging
process steam only
Boiler stacks
Cogen plant
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 4
Environmental Dynamics May 2011
The gas turbine facility will be located in the north-west corner of the site, immediately west of potato
stores 5 and 6. This location is about as far as it is possible to be from the nearest residences on the
Simplot side of the Leven River, and the nearest residence on the far (i.e. west) shore of the river is
located over a kilometer away. The dispersion modelling results presented later in this report show
that is also a good location because the cogeneration plant stack is 100 m from the boiler stacks, and
its plume rarely reinforces the boiler stack plumes.
Existing boilers
Simplot currently operates three coal-fired boilers, which together consume about 31,000 tonnes per
year of coal, that is hauled to Ulverstone from the Fingal Valley. Boilers 1 and 2 are identical, and
Boiler 3 is about half the size of the other two. The three boilers are just a few meters apart from each
other, and are served by stacks that are about 23 m high. Boiler stacks 1 and 2 are identical, and boiler
stack 3 is of a slightly different design. The stacks have a free air height of about 12 m above the
boiler house, which means the stack emissions clear the building wake effects fairly well.
Proposed boiler conversions to natural gas
It is proposed to convert boilers 1 and 3 to natural gas. Boiler 2 will continue to operate, using coal
fuel, until the cogeneration plant has been commissioned.
Plant operation
This air quality study assumes that the Simplot plant operates 24 hours a day throughout the year, with
both the natural gas fired boilers and the cogeneration facility operating at constant full-load.
In fact, the Simplot plant follows a production cycle of twelve days continuous production, followed
by a two day shutdown. In addition, there are two shutdowns of 2 and 3 weeks in the middle of the
year, and the end of the year respectively. The cogeneration facility is expected to operate near full-
load, with the load of the natural gas fired boilers adjusted to demand.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 5
Environmental Dynamics May 2011
3.0 AIR QUALITY STANDARDS
3.1 Nature of the Emissions The principal contaminant associated with coal-fired boiler emissions is particulate material. In-stack
concentration standards are set in terms of total particulates, while ambient air quality standards are
set in terms of PM10, which refers to particulates smaller than 10 μm in aerodynamic diameter. PM10
material behaves as a gas over the distances of interest to this air quality impact study.
The principal emission contaminants associated with the combustion of natural gas are nitrogen
oxides (NOx), mainly in the form of nitric oxide (NO) and nitrogen dioxide (NO2). The NO poses
only a low health risk, and does not have an ambient air quality standard. However, NO2 is an acidic
gas that is associated with respiratory health impacts, and it has strict ambient air quality standards.
Minor natural gas combustion contaminants include carbon monoxide (CO), sulphur oxides (SOx),
and very fine particulates. However, the mass emission rates of these minor contaminants are well
known to be far too low to require an air quality impact assessment, for all but a major facility such as
a power station.
Typical boiler emissions at stack discharge contain about 90% NO and 10% NO2, and this ratio is
assumed by this study. The NO is converted to NO2 as the emissions disperse away from the stack, an
oxidation process which requires ozone (O3), not oxygen, such that the conversion rate is limited by
the ability of the NO molecules to find and interact with background O3 present in the atmosphere.
The dispersion model used by this study was configured to explicitly take this process into account,
and its performance in this regard is known to be quite good, through ground-truthing work carried
out by Cement Australia at their Railton plant, in close consultation with EPA.
The combustion conditions of a gas turbine engine mean the cogeneration plant emissions are
expected to contain a much higher fraction of NO2. Indeed, in order to err on the side of caution, this
study assumes that the cogeneration plant’s NOx emissions are entirely in the form of NO2.
3.2 In-Stack Concentration Standards Schedule 1 of the Tasmanian Environment Protection Policy (Air Quality) 2004, commonly termed
the Air Quality EPP, specifies the following in-stack concentration limits.
a) 100 mg/Nm3 for total particulates and 500 mg/Nm3 for NOx, in coal-fired boiler emissions, with
concentrations adjusted to dry normal conditions (0 ºC, 101.325 kPa), and 7% oxygen (O2). The
NOx is measured as NO2.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 6
Environmental Dynamics May 2011
b) 350 mg/Nm3 for NOx in gas fired boiler emissions. The NOx is measured as NO2, with the
concentration adjusted to dry normal conditions (0 ºC, 101.325 kPa), and 7% oxygen (O2).
c) 90 mg/Nm3 for NOx in gas-fired turbine generator emissions, when the generator capacity is less
than 10 MW. The NOx is measured as NO2, with the concentration adjusted to dry normal
conditions (0 ºC, 101.325 kPa), and 15% oxygen (O2).
Ambient air quality vs in-stack standards
Prescribing an in-stack standard for a contaminant usually ensures the ambient air quality standards
will be met, but this is not guaranteed. An in-stack standard does not consider the cumulative impact
of multiple emission sources, nor does it consider stack height, building wake effects, or flow rate.
This latter is important, because the air quality impact of a contaminant depends on its mass emission
rate, not its in-stack concentration. A mass emission rate is calculated by multiplying the contaminant
concentration by the flow rate, under the same conditions (e.g. dry normal conditions).
3.3 Workplace Air Quality Standards Workplace air quality standards apply within the boundary of an industrial site. The Australian Safety
and Compensation Council prescribes workplace exposure standards for an 8 hour averaging period (a
working day), assuming exposure over a 40 hour working week. A short-term exposure limit for a 15
minute averaging period, may also be specified.
For non-toxic particulates, the exposure standard is 10 mg/m3. For NOx, the exposure standards are
5.6 mg/m3 (8 h) and 9.4 mg/m3 (15 min) for NO2; and 31 mg/m3 (8 hours) for NO.
3.4 Ambient Air Quality Standards The Air Quality EPP prescribes ambient air quality design criteria for various airborne contaminants.
For the purpose of this study, there is no difference between a design criterion and a standard. In
addition, the National Environment Protection (Ambient Air Quality) Measure (1994), termed the Air
Quality NEPM, prescribes ambient air quality standards for six “fingerprint” contaminants associated
with urban areas.
Table 3.1 sets out ambient air quality standards for the contaminants associated with the existing and
proposed emission sources at the Simplot plant. The Air Quality NEPM standards are more stringent
than the Air Quality EPP standards. They apply everywhere within urban areas with populations
greater than 25,000 people. Outside of such urban areas, in this case Ulverstone, the Air Quality
NEPM standards apply to any sensitive locations, such as a residence, and the Air EPP standards
apply to non-sensitive locations, such as the Leven River or bushland.
This study applies the more stringent Air Quality NEPM standards everywhere.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 7
Environmental Dynamics May 2011
Contaminant ppm µg/m3 Period Reference Notes PM10 n/a
n/a
50
150
1 day
1 day
NEPM
EPP
5 days/year
100th percentile
NO2 0.12
0.16
0.03
246
328
61
1 hour
1 hour
1 year
NEPM
EPP
NEPM
1 day/year
99.9th percentile
–
Table 3.1 Ambient air quality NEPM standards and EPP design criteria. “Notes” gives the exceedences per year allowed by the NEPM, and the percentile to be compared to the EPP.
Compliance assessment
The ambient air quality standards refer to total concentrations, so the assessment must consider all
emission sources within the Simplot plant boundaries, in addition to the background concentrations of
the contaminants of interest.
It is standard practice to assess compliance with ambient air quality standards by using an emission
dispersion model to examine the fate of emissions over a year. This leads to 8,760 hourly ground
level concentration (GLC) predictions across a grid (and/or at specified receptor locations), which can
then be averaged as need be, for comparison to an air quality standard.
The Air Quality NEPM permits the one hour NO2 standard to be exceeded on one day a year, and a
common interpretation of this clause is to compare the second highest 1-hour GLC prediction to the
standard (i.e. the second highest of the 8,760 one hour GLC predictions made by a dispersion model
from a year of meteorology). For the Air Quality EPP, the 100th percentile is the 99.9th percentile is
the 9th highest GLC (1 h).
The Air Quality NEPM allows the PM10 (24 h) air quality standard to be exceeded on five days a year,
to allow for events such as bushfires. However, this study assesses compliance by examining the
maximum GLC (24 h) predictions.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 8
Environmental Dynamics May 2011
4.0 THE EMISSION DISPERSION MODEL Emission dispersion modelling procedures are well established, and this study’s methodology is quite
standard. Several computer models are in common use across Australia to predict emission dispersion,
and the CSIRO’s sophisticated model The Air Pollution Model (TAPM), which is used for this study,
is known to perform well for the Ulverstone region.
TAPM Version 4 was released in late 2008. The model has been verified for several Australian and
international datasets, and is described by papers available on CSIRO’s web site www.cmar.csiro.au,
including:
• Hurley, P., 2008. TAPM V4. Part 1: Technical Description, CSIRO Marine and Atmospheric Research Paper No. 25; and
• Hurley, P., Edwards, M., and Luhar, A., 2008. TAPM V4. Part 2: Summary of Some Verification Studies. CSIRO Marine and Atmospheric Research Paper No. 26.
Historically, the lack of site-specific meteorological data reduced the credibility of many dispersion
modelling exercises. Simple models, such as Ausplume, need to be provided with a meteorology file
derived from weather station data, and the nearest weather station that produces high quality hourly
meteorology is located at Devonport. This weather station is known to have a very similar surface
wind climate to Ulverstone but an Ausplume meteorology file prepared from weather station data does
not contain only observed wind data: it must also include hourly stability classes and mixing height
estimates that are inferred from upper air observations, such as cloud cover.
TAPM avoids this problem by using atmospheric physics to predict fully 3-D winds from synoptic
scale meteorological data gathered by the Bureau of Meteorology, from weather stations across
Australia. It is supported by data sets of land use, soil and vegetation, sea surface temperature, and
terrain elevation.
Previous studies have found that Ausplume and TAPM make similar predictions when applied to
industrial plants in the Ulverstone area. However, TAPM is the more sophisticated and credible
model, and sometimes predicts slightly higher ground level concentrations (GLCs). Ausplume, being
the simpler model, is sometimes believed to be more conservative model, but this is not always true.
Wind modelling
TAPM initially produces 3-D winds across a large outer grid which, for this study, was defined to
have 25 East x 25 North x 25 vertical grid points, centred on the Simplot plant. The horizontal grid
points were 30 km apart, and this 720 km x 720 km grid includes all of northern Tasmania. The
vertical grid points extend from a height of 10 m to 8 km, with the spacing increasing with height
since upper air winds are more uniform than near ground winds.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 9
Environmental Dynamics May 2011
TAPM then produces 3-D winds over four progressively smaller regions nested within the outer grid.
The inner grids also have 25 East x 25 North grid points, with separations of 10 km, 3 km, 1 km, and
300 m. The vertical grid is unchanged.
Figure 4.1 (top) shows part of the innermost wind prediction grid, with a grid spacing of 300 m, and a
horizontal extent of 7.2 km x 7.2 km, centred on the Simplot plant. Figure 4.1 (bottom) shows the
surface (10 m) winds over this grid predicted for 6 am on 2 January 2003. The year 2003 has been
identified by EPA as a year characterised by poor dispersion conditions for Tasmania, although a
recent project in the Ulverstone area found that TAPM makes similar predictions for other years.
Dispersion modelling strategy and prediction grids
TAPM models the dispersion of emissions from a point source, such as a stack, by releasing a steady
stream of discrete “PartPuffs” from the stack. Each PartPuff represents a small mass of contaminant,
and is carried away from the stack by the 3-D winds. Diffusive effects are modelled by allowing the
PartPuff to expand as a puff in the horizontal plane, and nudging it in the vertical direction after every
time step. This approach is an example of a “Lagrangean” method, meaning it is a “moving with the
flow” approach, rather like drifter buoys being used to gather information on ocean currents.
As the emissions disperse, TAPM switches from tracking PartPuffs to computing concentration
gradients and diffusion coefficients, and solves the advection-diffusion equation to predict the fate of
the emissions. This is an example of an “Eulerian” method, whereby emission dispersion predictions
are based on consideration of the entire grid, rather like a network of fixed current meters being used
to gather information on ocean currents, instead of the drifter buoys approach.
Contaminant concentration predictions are made across a pollution grid with grid spacings that can be
as small as 30 m. For this study, ground level concentration (GLC) predictions were made across a
2,100 m x 2,100 m grid with a 60 m grid spacing. At distances greater than several hundred meters
from the plant, TAPM’s finite-difference approach to computing concentration gradients depends on
the grid spacing, but the concentration prediction accuracy of the specified grid is sufficient to assess
air quality impacts at these “far field” distances.
Nearer the Simplot plant, the PartPuff tracking methodology means predictions are the same whatever
grid spacing is used, so the grid spacing simply needs to be small enough to adequately define the
pattern of predictions. Based on preliminary modelling, it was agreed with Dr Mike Power of EPA
that a 60 m grid spacing was sufficient to achieve this.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 10
Environmental Dynamics May 2011
Figure 4.1 The TAPM innermost wind grid, with 300m spacing. Top: A land/water view of the grid. Bottom: Surface winds at 6am on 2 January 2003. Only every other wind vector is shown. The Simplot plant is shown by the yellow square. The colours and contour values show the terrain elevations.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 11
Environmental Dynamics May 2011
Building wake effects
High above the ground, the wind is not affected by obstacles such as buildings. Closer to the ground,
a building influences wind patterns for a distance downwind that is roughly five times the lesser of the
building’s height and its width; and for a height up to about 2½ times the height of the building.
Figure 4.2 shows wind moving over a building, creating a “cavity zone” on the lee side of the
building, with turbulent effects extending for some distance downwind. If a stack’s height is less than
about 2½ times the height of the building, then its emissions can be entrained into the wind flow over
the building, and downwashed into the cavity zone, resulting in downwash fumigation.
Figure 4.2 Downwash fumigation.
TAPM uses building geometry data to calculate cavity and wake dimensions, and hence determine
overall wake meteorology and turbulence parameters. It uses the PRIME algorithm to model the fate
of emissions that are affected by these building wake effects. Figure 4.3 shows the building model
used for this study, based on the coordinates given in Table 4.1.
The Simplot plant covers an area of roughly 500m x 200m, but most of its buildings are not very high
and have fairly flat roofs. Potato storage buildings 5 & 6, adjacent to the proposed cogeneration plant,
are about 7.5 m high, so experience suggests that the cogeneration stack at least 11 m high is needed
in order for its emissions to avoid substantial downwash fumigation. The complex of buildings that
includes the boiler house is about 11.4 m high, and emissions from the 23 m high boiler stacks almost
entirely avoid downwash. Their plumes are visible, so this is easily confirmed by observation.
Cavity zone
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 12
Environmental Dynamics May 2011
Figure 4.3 Looking down (top) and north-east (bottom) at the “block model” that represents the plant buildings. See Table 4.1 for the building codes (A, B, etc). The heights of buildings with sloping roofs are set equal to the roof ridge line. The green dots in building C are the existing boiler stacks (Boilers 1 – 2 – 3 from west to east). The green dot west of building B is the stack of the proposed cogeneration plant.
B
C
D
A
G
E
F
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 13
Environmental Dynamics May 2011
A. 6.5 m high
Potato stores 1-4
m East
429,502
429,490
429,520
429,522
429,530
429,537
429,569
429,571
m North
5,442,873
5,442,809
5,442,805
5,442,820
5,442,820
5,442,852
5,442,846
5,442,861
E. 8.5 m high
Potato stores 17 & 18
m East
429,490
429,485
429,479
429,475
429,483
429,479
429,541
429,560
m North
5,442,803
5,442,785
5,442,785
5,442,770
5,442,768
5,442,751
5,442,738
5,442,792
B. 7.5 m high
Potato stores 5 & 6 429,266
429,262
429,352
429,358
5,442,912
5,442,873
5,442,858
5,442,895
F. 7.0 m high
Potato stores 9 & 10 429,593
429,582
429,644
429,653
5,442,858
5,442,800
5,442,790
5,442,848
C. 11.4 m high
Processing plant, boiler house, and cold store
429,253
429,247
429,290
429,285
429,320
429,318
429,412
429,414
429,432
429,442
5,442,863
5,442,818
5,442,811
5,442,788
5,442,781
5,442,768
5,442,751
5,442,762
5,442,757
5,442,833
G. 13.3 m high
Granule plant 429,384
429,382
429,371
429,369
429,380
429,378
429,393
429,399
5,442,908
5,442,889
5,442,891
5,442,878
5,442,876
5,442,863
5,442,861
5,442,906
D. 8.5 m high
Potato stores 7 & 8 429,502
429,490
429,502
429,571
429,578
5,442,910
5,442,878
5,442,873
5,442,861
5,442,895
Table 4.1 MGA 94 coordinates of the “block model” of the Simplot plant buildings.
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Environmental Dynamics May 2011
5.0 MODEL INPUTS
5.1 Source and Emission Characteristics Table 5.1 gives the stack and emission characteristics for the present three coal-fired boilers, based on
stack tests carried out in February 2011 by LEC Environmental Pty Ltd. The stack test reports are
available on request, but EPA already has them, and they are not appended to this report because the
focus of this study is on the air quality impact of the proposed situation, not the present situation.
There are no other significant sources of particulate emissions in the vicinity of the Simplot plant,
except for woodsmoke emissions from domestic woodheaters during the winter months.
Boiler 1 Boiler 2 Boiler 3 m East (MGA 94) 429,298 429,309 429,313
m North (MGA 94) 5,442,807 5,442,805 5,442,803
Height (m) 23.0 23.0 23.0
Area at test ports (m2)
Speed at test ports (m/s)
Flow rate (Am3/s)
Stack exit diameter (mm)
Exit speed (m/s)
1.350
7.00
9.45
1185
8.57
1.350
5.20
7.02
1185
6.37
0.950
4.70
4.47
1100
4.70
Temperature (ºC / K)
Moisture (g/Nm3)
Flow rate (Nm3/s) dry
TSP (mg/ Nm3) dry
NOx as NO2 (mg/ Nm3) dry
TSP MER (g/s)
NOx as NO2 MER (g/s)
NO / NOx (%) at discharge
172 / 445
41.4
5.51
132
155
0.727
0.855
0.90
160 / 433
53.6
4.16
96
152
0.397
0.632
0.90
145 / 418
51.8
2.77
20
179
0.055
0.499
0.90
7% oxygen
Oxygen (%)
PM10 (mg/ Nm3) dry
NOx as NO2 (mg/ Nm3) dry
10.9
195
214
8.2
107
166
11.3
32
254
Table 5.1 Stack characteristics at present.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 15
Environmental Dynamics May 2011
The flow rates and associated characteristics in Table 5.1 are average values. The total suspended
particulate (TSP) concentrations are the maximum values measured by two tests. The measurements
varied considerably between tests, perhaps due to boiler modulation, or in the case of boilers 1 and 2 it
may be due to the stack test ports being located in positions that do not comply with AS 4323.1.
Table 5.1 shows that the TSP concentrations in the boiler 1 and 2 stacks exceed the 100 mg/Nm3 limit
specified by the Air EPP (at the reference condition of 7% O2), but the TSP concentrations in the
boiler 3 stack are only about one-third the 100 mg/Nm3 limit.
This study conservatively assumes all the particulate material in the emissions to be PM10. This is
likely to be true in the case of boiler 3, which is served by a baghouse. However, boilers 1 and 2 are
served by less sophisticated air pollution control equipment (momentum separators) and their stack
emissions are likely to contain a significant amount of particulate material greater than PM10 size,
which helps explain the difference in measured in-stack TSP concentrations.
The NOx as NO2 concentrations are all well under the 500 mg/Nm3 limit specified by the Air EPP.
Transition period
There will be a transition period, during which the coal-fired boiler 2 will continue to operate while
the cogeneration plant is commissioned. The air quality impact of the emissions during this period
will be intermediate with respect to the present and final air quality impacts
Expected future emissions
Table 5.2 gives the stack and emission characteristics for the proposed situation, in which Simplot
operates two natural-gas fired boilers (1 and 3), and the cogeneration facility. Table 5.2 is based on
expected cogeneration facility emission data provided by SDA Engineering (contacts Leon Daysh and
Steven Hills), and expected boiler emission data provided by RCR Energy (contact Nick Holmes).
This information is subject to verification through post-commissioning stack tests, but worst-case
assumptions have been applied for the purpose of air quality impact assessment. The cogeneneration
plant and the two boilers are assumed to all run continuously throughout the year, at full load, but the
plant’s routine shut-downs mean there will be no operation on some 80-90 days of the year.
NOx emissions from the cogeneration plant are assumed to be entirely in the form of NO2, because
SDA Engineering reports that Siemens have measured very high (90% or higher) NO2 content in
emissions from cogeneration plants running at 75% load. However, it is expected that the Simplot
cogeneration plant will usually run near full (100%) load, for which NO2 is expected to be only about
28% of the NOx emissions.
NO2 is assumed to be 10% of the NOx emissions from the two natural gas boilers, which is a standard
assumption for ordinary natural gas combustion conditions.
Simplot Australia Pty Ltd: Proposed Natural Gas Boilers and Cogeneration Plant: Air Quality Impact Study Page 16
Environmental Dynamics May 2011
Boiler 1 Boiler 3 Cogen
m East (MGA 94) 429,298 429,313 429,245
m North (MGA 94) 5,442,807 5,442,803 5,442,880
Height (m) 23.0 23.0 13.0
Rating (MW)
Flow rate (Am3/s)
Temperature (ºC / K)
Moisture (% v/v)
Oxygen (% v/v)
Flow rate (Nm3/s)
Exit diameter (mm)
Exit speed (m/s)
16
9.50
176 / 450
??
2.5
5.77
1185
8.61
10
5.94
176 / 450
??
2.5
3.61
1100
6.25
7.6
37.39
159 / 432
7.45
13.97
21.87
1900
13.19
NOx as NO2 conc (mg/Nm3)
NOx as NO2 MER (g/s)
NO2 in NOx (%)
350
2.67
10
350
1.67
10
51
1.32
100
Table 5.2 Stack characteristics following switch to natural gas. The moisture content of the boiler emissions is not known, and dry flow is assumed.
Bypass stack
The cogeneration plant has a bypass stack, in addition to its normal stack. The bypass stack is mainly
used during plant start-up, ahead of engaging the heat recovery unit. It will be the same height and
diameter as the normal stack, and the two stacks are located just a few meters apart. The bypass stack
emissions will be much the same as those from the normal stack, but higher temperature, and thus
higher speed. The emissions will therefore have better initial plume rise and better dispersion than the
emissions from the normal stack, such that if the air quality impact of emissions from the normal stack
are acceptable, then emissions from the bypass stack will also be acceptable.
5.2 Background Concentrations Table 5.3 lists the background contaminant concentrations assumed by this study. In a semi-rural
setting, the background O3 concentration is the main parameter of importance to modelling the fate of
NOx emissions from the boiler stacks. The other parameters are only important in modelling NOx
dispersion in the presence of a photochemical smog. West Ulverstone has a fairly clean airshed, so
the chemical reactions essentially reduce to the NO + O3 → NO2 + O2 oxidation reaction.
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Airborne particles (APM) ≈ PM10 particulates 15 µg/m3
Nitrogen oxides (NOx as NO2) 2 ppb
Sulphur dioxide (SO2) 2 ppb
Smog reactivity (Rsmog) 0.3 ppb
Ozone (O3) 30 ppb
Airborne fine particles (AFM) ≈ PM2.5 particulates 5 µg/m3
Table 5.3 TAPM chemistry mode background concentration inputs.
Background NO2 concentration
The total NO2 concentration is the value that is compared to the ambient air quality standard, so the
background NO2 concentration must be added to the NO2 concentration at a given location that is due
to the stack emissions. CSIRO carried out NO2 measurements for 8 months in 2007/08, at the Ti-Tree
Bend air quality monitoring station in Launceston. The 70% percentile concentration of NO2 is often
set as the background concentration, and at Ti-Tree Bend was 10 μg/m3 (1 h).
However, field measurements of NOx and NO2 by Cement Australia at Railton (the Environment
Division is familiar with these data) show that for non-urban areas in Tasmania the background
concentration is no more than about 4 μg/m3 (1 h), which is assumed by this study. Looking ahead,
the predicted total NO2 GLCs are well under the ambient air quality standard, so it is not important to
have a precise knowledge of the background NO2 concentration in west Ulverstone.
Background O3 concentration
Ozone concentrations reported in the West Australian 2007 State of the Environment Report, and on
the EPA Victoria’s website, suggest a 70th percentile hourly O3 concentration of 80 μg/m3 [37 ppb] is
appropriate for small cities such as Hobart and Launceston. A 2007 study for a power plant in the
Tamar Valley estimated a maximum hourly background O3 concentration of 90 μg/m3 [42 ppb] for
that area, which is rural in general but also home to several industries.
For the Simplot plant, data from the clean air monitoring station at Cape Grim in NW Tasmania
provide a good indication of appropriate O3 concentrations. Data for 2003 and 2004 provided by
CSIRO show monthly mean O3 concentrations ranging from 17 to 32 ppb [36 to 68 μg/m3] and peak
hourly O3 concentrations ranging from 29 to 66 ppb [62 to 141 μg/m3]. This study thus assumes a
background ozone concentration of 30 ppb [64 μg/m3] characterises the west Ulverstone area.
Background APM, FPM, SO2 and smog reactivity (Rsmog) concentrations
None of these background contaminant concentrations are important in this non-urban modelling
exercise, but need to be included in the model inputs. Typical values of APM (= PM10) and FPM
(=PM2.5) for rural Tasmania are used for the background concentrations of these contaminants.
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Rsmog is used as a surrogate for Volatile Organic Compounds (VOCs) in modelling the NO2 chemistry
associated with photochemical smog. Rsmog is a concentration that is defined as a reactivity coefficient
(~0.0067) multiplied by the VOC concentration. An Rsmog value of 0.3 ppb was used for the Railton
study, while Rsmog values of 0.5 ppb have been used for studies in Melbourne. Accordingly, an Rsmog
value of 0.3 ppb is used by this study. The model also needs to know the amount of Rsmog discharged
by the stacks themselves, and for this study the amount is set to zero.
5.3 Summary of Model Inputs Table 5.4 summarises the model inputs for the wind and dispersion predictions.
Model TAPM V4.0.4.
Default files Sim.def (winds) Simold.def (present emissions) SimNew.def (future emissions)
Meteorology 2003 with the 31 December 2002 used for model pre-prediction spin-up.
Terrain, land use. Geodata 9-sec DEM ~250 m resolution and soil type data Tas100mgrid.txt ~100 m resolution Vege.aus 3-min grid ~5 km resolution TasSVLU250m.txt ~250 m resolution Soil.aus 3-min grid ~ 5km resolution
Wind grid centres. 146° 09.5’ E, 41° 19.0’ S GDA 94 datum {429,392 m E, 5,442,400 m N} MGA 94 GDA 94 datum
Meteorology grids 25 x 25 horizontal grid points, all five grids 30 km, 10 km, 3 km, 1 km, 300 m resolution
25 vertical grid points. At {10, 25, 50, 100, 150, 200, 250, 300, 400, 500, 600, 750, 1000, 1250, 1500, 1750, 2000, 2500, 3000, 3500, 4000, 5000, 6000, 7000, 8000 m}. Levels 1750 m and above not included in output files.
Building profiles Building geometries given in Table 4.1.
Present sources Source and emission characteristics given in Table 5.1. Normal tracer mode used, with unit tracers (1 g/s) for all sources. Section 5.4 describes how predictions are obtained from these unit tracers.
Future sources Source and emission characteristics given in Table 5.2. NOx chemistry mode used. Background concentrations given in Table 5.3.
Pollution grid 36 E-W cols x 36 N-S rows. 60 m x 60 m spacing.
SW corner = {428,492 m E, 5,441,800 m N} MGA 94
Computation mode Eulerian + Lagrangean, for both present and future emissions.
Table 5.4 Summary of TAPM inputs.
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6.0 MODEL WIND PREDICTIONS
6.1 Surface Wind Climatology Figure 6.1 shows a plot of the terrain in the region of the Simplot plant, looking SW from Bass Strait,
and Figure 6.2 shows the 2003 surface (10 m) wind rose predicted by TAPM at the plant.
Figure 6.1 Regional terrain in the vicinity of the Simplot plant, looking SW from Bass Strait.
Modal wind and the sea breeze
The dominant signature in the wind rose shown in Figure 6.2 is westerly. This is because the modal
air flow across Tasmania is westerly, since weather systems move west to east across Tasmania.
Figure 6.1 shows this tendency towards westerly winds is enhanced by the broadly east-west trending
high ground that lies inland from Ulverstone.
The westerly modal wind signature is enhanced and modified by the sea breeze. The sea breeze
usually manifests itself in the late morning, and can persist through to the early evening. It generally
has a NW wind signature along the north coast of Tasmania, and certainly this is true at the Simplot
plant, given its coastal location, and the largely flat coastal terrain in the Ulverstone area.
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Figure 6.2 Surface 2003 wind rose predicted by TAPM at the Simplot plant. Calm conditions (< 0.5 m/s) occur less than 2% of the time.
Katabatic winds
The wind rose in Figure 6.2 also shows distinctive SW to SE wind signatures due to katabatic winds.
Katabatic winds are cold air drainage flows, produced by radiative cooling of high ground at night,
which in turn results in a layer of cold, dense air that flows down from the high ground towards the
coast. The katabatic wind can be fairly strong near the coast, and at Ulverstone the katabatic wind in
the early hours of the morning can be comparable in strength to the sea breeze (10 to 15 kts, or 5 to 8
m/s). The direction of katabatic winds is often aligned to the valleys that channel the air flow as it
moves towards the coast. This has not been examined closely in the case of the Simplot plant, but
Figure 6.1 shows two significant valley systems inland of Ulverstone.
6.2 Wind Speed and Direction Frequency Distributions Figure 6.3 shows the predicted frequency distributions of surface (10 m) wind speeds and directions
respectively. The NW coast of Tasmania is a fairly windy place: sea breezes and katabatic winds are
both stronger at the coast than inland, and the flat terrain in the Ulverstone area means the Simplot
plant is fully exposed to the modal westerly winds across Tasmania. The median (50th percentile)
predicted wind speed range is 2-3 m/s (~5 kts), and calm conditions occur less than 2% of the time.
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Figure 6.3 Predicted frequency distributions of wind speeds (top) and directions (bottom).
The performance of TAPM V4 in predicting winds has been assessed on a number of projects carried
out by the author (Dr Carter), by comparison to wind observations by local weather stations. The
performance is generally good, perhaps with a tendency to underpredict wind speeds. In particular,
TAPM wind predictions at Devonport airport have been compared to winds observed by the airport
weather station, and the predicted winds are a little lower than the observed winds, so the same is
likely true for TAPM wind predictions for the Ulverstone area. This is not considered to be a serious
modelling drawback.
Fr
eque
ncy
(%)
Wind direction (degrees from)
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6.3 Distribution of Stability Classes Figure 6.4 shows the frequency distribution of stability classes predicted by TAPM at the Simplot
plant. Stability class A refers to highly unstable conditions, associated with hot sunny days, with
substantial vertical mixing of the air due to vertical eddies. Stability classes B and C are moderately
unstable, and unstable, respectively. Stability class D refers to neutral conditions, under which a
parcel of air that is moved vertically by small perturbation, tends to stay at its new level. Stability
classes E and F refer to stable conditions, under which vertical mixing of the air is suppressed, for
example under conditions of a temperature inversion.
Figure 6.4 Predicted frequency distribution of stability classes.
A plume tends to cone under neutral conditions, and to fan out horizontally under stable conditions,
resulting in poor emission dispersion, particularly under stable conditions, because the emissions are
spread through a much smaller volume of air compared to unstable conditions. Figure 6.4 shows that
neutral and stable conditions occur between half and 2/3rds of the time at the Simplot plant, which is
expected since sea breezes and katabatic flows are fairly stable air flows.
Overall, the frequently neutral and stable atmospheric conditions in the Ulverstone area tend to result
in poor emission dispersion, but this is offset by the fact that low wind speeds are not frequent. In
other words, stronger winds mean the stack emissions are discharged into a greater volume of air per
second, which offsets the fact that the winds are often characterised by poor vertical mixing.
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7.0 EMISSION DISPERSION PREDICTIONS
7.1 Present PM10 Emissions Figure 7.1 shows the highest predicted PM10 GLCs (24 h) for the combined emissions from the three
existing boilers. As noted in Section 5, the predictions are based on the highest measurements of total
suspended particulate (TSP) material made in early 2011, assuming the TSP is entirely PM10. This is
probably accurate in the case of boiler 3, which is served by a baghouse, but boilers 1 and 2 are served
by less effective air pollution control equipment (momentum separators), and their emissions will
contain some larger material, and the ambient air quality standard only applies to PM10.
Figure 7.1 Maximum predicted GLCs (24 h) for PM10 emissions from the three present boiler stacks. The GLCs do not include a background PM10 concentration. Contours at 2, 5, 10, 15, 20, 30, 50 and 70 μg/m3.
Highest PM10 GLCs (24 h). 3 boilers at present. Max on grid ≈ 84 μg/m3. NEPM limit = 50 μg/m3.
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In Figure 7.1, the maximum predicted highest GLC is 84 μg/m3 (24 h), and occurs on-site, where
workplace exposure standards apply, and it is more than 100 times less than the non-toxic dust
exposure standard of 10 mg/m3 (8 h).
The maximum predicted off-site GLCs are 30 – 40 μg/m3 (24 h), which occur just beyond the site
boundaries. The concentration gradient is quite steep near the plant, and the GLCs rapidly fall off to
about 30 μg/m3 (24 h) at the nearest residences, immediately south of the plant. When the estimated
(winter) 15 μg/m3 (24 h) background PM10 concentration is added to these predictions, the resulting
overall GLCs are about 45 μg/m3 (24 h) at these residences, which meets the 50 μg/m3 (24 h) Air
Quality NEPM standard.
Moving away from the Simplot plant, the predicted PM10 GLCs continue to rapidly decrease, and the
maximum GLCs in the vicinity of the primary school are less than 10 μg/m3 (24 h), or about 25 μg/m3
(24 h) when the estimated background PM10 concentration is included.
Overall, it is concluded that the ambient air quality impact of worst-case PM10 emissions from the
combined three present coal-fired boilers is less than the Air Quality NEPM standard. The in-stack
TSP concentrations of boiler stacks 1 and 2 exceed the 100 mg/Nm3 limit specified by the Air Quality
EPP, but in-stack TSP concentrations are a poor guide to ambient air quality impact (see Section 3.2).
7.2 Dispersion Characteristics of the Proposed Cogeneration Stack A series of dispersion modelling exercises was carried out to assess the general emission dispersion
characteristics associated with the proposed cogeneration stack, with the following results.
1. The stack emissions have high vertical buoyancy and momentum fluxes, which results in a
good initial plume rise.
2. The cogeneration stack is located about 100 m NW of the boiler stacks, and the prevailing
winds are W to SW. When the wind is from the W to SW, the cogeneration stack plume does
not overlap the boiler stack plumes, except at some distance from the plant, after the plumes
have spread significantly horizontally. When the wind is from the NW or SE, the plumes
overlap more directly, but this does not happen often, and only NW winds are of concern
since a SE wind disperses the emissions over the Leven River.
3. The cogeneration stack discharges about 1.23 g/s of pure NO2, while the two gas boilers
together discharge about 0.23 g/s of NO2 and about 2.05 g/s of NO, which is rapidly converted
to NO2 to the extent allowed by the amount of O3 in the air. Overall, the cogeneration stack’s
impact on the air quality near the Simplot plant is comparable to the air quality impact of the
two boilers. In other words, neither emission source is dominant.
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4. Figure 7.2 shows predicted GLCs for a 1 g/s mass emission rate from the cogeneration plant
stack, for stack heights of 11 m, 13 m, 15 m, and 17 m. The predicted GLCs are normalised
with respect to the GLCs for a 13 m high stack. The figure shows that a 15 m high stack
would result in 10-15 % decrease in local air quality impact, while an 11 m high stack would
result in a 15% increase in local air quality impact.
Looking ahead, the predicted NO2 GLCs for a 13 m high cogeneration stack are only about
half the ambient air quality standard. Figure 7.thus 2 suggests that an 11 m high cogeneration
stack would be acceptable, with its emissions usually managing to avoid being entrained in
the building wake effects associated with air flow over the adjacent 7.5 m high potato store
buildings 5 & 6. However, the boiler house and processing plant buildings are about 11.4 m
high, and are close enough to the cogeneration plant that best practice and the need to be
conservative argues for a 13 m high cogeneration stack, and this is assumed in the predictions
presented in the next section.
Figure 7.2 Maximum GLCs vs cogeneration stack height for a unit (1 g/s) mass emission rate. The GLCs are normalised such that 100% corresponds to a 13 m high stack.
7.3 Expected Future NOx Emissions Figure 7.3 shows the 2nd highest predicted NO2 GLCs (1 h) due to combined NOx emissions from gas
fired boilers 1 and 3; and the proposed cogeneration plant. The predictions assume a cogeneration
plant stack height of 13 m, with pure NO2 emissions. The NO2 component of the boiler emissions is
assumed to be 10% of the total boiler NOx emissions.
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Figure 7.3 Second highest predicted GLCs (1 h) for NO2 emissions from the cogeneration plant stack and the two natural gas boiler stacks. The GLCs include a 4 μg/m3 background NO2 concentration. Contours at 40, 60, 70, 80, 90, 120 and 160 μg/m3.
The 2nd highest predicted off-site GLC (1 h) is about 120 μg/m3 (1 h), including the estimated 4 μg/m3
(1 h) background NO2 concentration. This is about half the Air Quality NEPM standard for NO2 of
246 μg/m3 (1 h), so the standard is easily met.
Figure 7.4 shows the 2nd highest predicted total NOx as NO2 GLCs (1 h), for the same emission
scenario, whereby all the NO is converted to NO2 and added to the NO2 GLCs predictions that are
shown in Figure 7.3.
2nd Highest NO2 GLCs (1 h). Future emissions. Max on grid ≈ 190 μg/m3. NEPM limit = 246 μg/m3.
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The 2nd highest predicted off-site GLC (1 h) occurs immediately north of the cogeneration plant , and
just exceeds the Air Quality NEPM standard for NO2 of 246 μg/m3 (1 h), as can be seen by the red
GLC 246 μg/m3 contour extending beyond the site boundary at that location. However, the standard
is for NO2, not total NOx as NO2, so Figure 7.4 simply shows that the Simplot plant’s future NOx
emissions have potential to cause a slight ambient air quality problem, but do not do so because the
conversion of NO to NO2 is limited by the amount of O3 in the atmosphere.
Figure 7.4 Second highest predicted GLCs (1 h) for total NOx as NO2 emissions from the cogeneration plant stack and the two natural gas boiler stacks. The GLCs include a 4 μg/m3 background NO2 concentration. Contours at 50, 75, 100, 150, 200, 246, and 300 μg/m3.
Compliance with on-site workplace exposure standards has also been examined, although the GLCs
are not shown. The maximum predicted highest NO2 GLC is 163 μg/m3 (8 h), about 34 times less
than the NO2 workplace exposure standard of 5.6 mg/m3 (8 h).
2nd Highest NOx as NO2 GLCs (1 h). Future emissions. Max on grid ≈ 491 μg/m3. NEPM NO2 limit = 246 μg/m3.
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8.0 CONCLUSIONS AND RECOMMENDATIONS This study has used the sophisticated emission dispersion model TAPM to examine the air quality
impact of emissions from the Simplot plant in west Ulverstone.
Present air quality
Stack tests carried out in early 2011 show that the in-stack TSP concentrations of boiler stacks 1 and 2
exceed the 100 mg/Nm3 limit specified by the Air Quality EPP. The exceedence can be by a factor of
about 2, with considerable variation in stack test results due to emission modulation that is likely due
to turbulent air flow at the stack test port locations, which do not comply with AS 4323.1. These two
boilers are served by momentum separators, which are no longer considered to be consistent with best
practice air pollution control equipment.
Boiler 3 is served by a baghouse, which is consistent with best practice expectations, and has stack
test ports that comply with AS 4323.1 Its measured in-stack TSP concentration is about one-third the
Air Quality EPP limit.
However, in-stack TSP concentrations are a poor guide to ambient air quality impact, and worst-case
modelling shows that the off-site ambient air quality impact of PM10 emissions from the combined
three present coal-fired boilers is less than the 50 μg/m3 (24 h) Air Quality NEPM standard, including
an assumed background PM10 concentration of about 15 μg/m3 (24 h), mainly associated with winter
domestic wood heater emissions.
Future air quality
TAPM’s chemistry mode has been used to predict the fate of worst-case NOx emissions from the
cogeneration plant and the two gas fired boilers, operating together. It is predicted that off-site NO2
GLCs will meet the ambient air quality standard of 246 μg/m3 (1 h) for NO2 specified by the Air
Quality NEPM, assuming background NO2 concentrations of 4 μg/m3 (1 h). These predictions assume
a cogeneration stack height of 13 m, which a series of modelling exercises found would best ensure
emissions largely avoid downwash by building wake effects, although an 11 m high stack would also
have been acceptable.
Recommendations
The cogeneration normal and bypass stacks should be at least 13 m high, and the normal stack should
have a sampling platform and test ports that comply with AS 4323.1. 75 mm ports with flanges are
suitable for NOx sampling. The project to convert two boilers to gas should also ensure that boiler
stack test ports comply with AS 4323.1, particularly regarding test port location in an area of straight
flow, with provision for safe access.
Post-commissioning stack tests should be carried out to confirm the worst-case emission parameters
assumed by this study (such test are usually required by EPA as a standard permit condition).