Assessing the impacts of short-term power generation

Kieran Laxen

Short-term operating plant:• Presentation topics

1. Assessment approach2. Model inputs and assumptions3. Consideration of impacts

• Applies to any limited operation plant• STORs• Backup generators• Emergency generators• CHP plant

• Discussing NOx emissions and NO2 impacts

STOR Short Term Operating Reserve

• Banks / array of generators • Used for peak demand or for periods of no wind / sola

energy production • Possibly 250 to 2500 hours per year (or up to 8760?)• Operate on Diesel or Gas (to be economic gas needs to operate for

longer hours)

• Currently no permit required – no control on operating hours

• Urban and rural locations (including within AQMAs)

STOR AssessmentsSome poor assessments being carried out• Crucial for a proper assessment to add generator impacts

to local baseline not just local background Background is averaged across a 1x1km grid square Baseline is the concentration at a receptor – for a receptor near a

road this will be higher than the background

• Annual mean impacts can be significant even with short-term operation

• Need to address short-term impacts using a probability approach

Problems faced - assessingThere is no official guidance in the UK in relation to development control on how to describe air quality impacts. Use IAQM/EPUK guidance.

Problems faced - Modelling• Constant operation or scheduled operation – typically

constant modelled. • Meteorological data – maximum of 5 years of modelled data• Constant emissions or variable emissions

• Be sceptical of any emissions data you’re given• Model at emission limits where nothing else is available.

• Chemistry?• Modelling road traffic?

Condition Specific Emissions Value Condition

NOx Emission rate (mg/Nm3) 903Standardised to 0degC, 101.325 kPa, measured oxygen (assumed to be 9% O2 content), wet (assumed to be 8%

moisture content).NOx Emission rate (mg/Nm3) 506 Normalised to 0 degC, 101.325 kPa, 15% O2, dry.NOx Emission rate (mg/Nm3) 1,793 Normalised to 0 degC, 101.325 kPa, 0% O2, dry.NOx Emission rate (g/kWhoutput) 3.7 -NOx Emission rate (g/s) 2.28 -

Example emissions• There are no requirements for stationary generators to meet EU Stage emission

limits (NRMM limits)! • Diesel generators / gas generators / dual fuel generators / abatement (SCR systems)• Emissions presented in the same units: dry flow, 0 degC, 101.325 kPa, 0% oxygen.• Based on typical generator combustion (~35-40% efficiency, 77% excess air, 9% O2,

8% moisture). Plant Emission Limit

(mg/Nm3)Comment

Diesel Generator (EU Stage 1) 4400 <9.2 g/kWhoutput

Diesel Generator (EU Stage 2) 2780 <6 g/kWhoutput

Diesel Generator (EU Stage 3) - For Plant <560 kWDiesel Generator (EU Stage 4) - For Plant <560 kW

Diesel Generator (Proposed EU Stage 5) 322 <0.67 g/kWhoutput

Gas Generator (500 mg/Nm3) 657 500 mg/Nm3 @ 5% O2 dryGas Generator (250 mg/Nm3) 329 250 mg/Nm3 @ 5% O2 dryGas Generator (25 mg/Nm3) 33 25 mg/Nm3 @ 5% O2 dry

MCPD Diesel 673 190 mg/Nm3 @ 15% O2 dryMCPD Gas (pre-31Dec17 operation) 673 190 mg/Nm3 @ 15% O2 dryMCPD Gas (post-31Dec17 operation) 337 95 mg/Nm3 @ 15% O2 dry

Warning – wrong emissionsEmission rates are provided at 5% O2, dry, 0 °C and 101.325 kPa.

Parameter Provided Calculated (15% O2) Calculated (0% O2)NOx mg/Nm3 1782 661.2 2342.2O2 (%) 5 15 0

H2O vapour (%) 0 0 0Absolute Exhaust Pressure (Pa) 101325 101325 101325Temp (°C) 0.0 0.0 0.0

Parameter Actual Normalised (incorrectly)Temp (°C) 470.0 0.0O2 (%) 15.0% (assumed) 15.0% (assumed)H2O vapour (%) Unknown UnknownMass Flow Rate (kg/h) Unknown UnknownVolume (m3/min) 460.0 169.0Volume (m+/h) 27267.7 10141.0 (calculated)

Parameter Actual Normalised (correctly)Temp (°C) 470.0 0.0Proportion Air Above Stoichiometric (%) 95.0% 0.0%

O2 (%) 9.8% 0.0%H2O vapour (%) 7.3% 0.0%Mass Flow Rate (kg/h) 12971.2 6308.8Volume (m3/h) 27541.0 4620.3

Lambda settings above about 1.9 start to result in misfires. Lambda 1.9 is 90% excess air.Supplier assumes 15% O2 in actual flow because that is the reference condition used to compare plant. 15% O2 = >280% excess air. This would lead to misfires. So is not realistic.

Emission rate calculated by supplier: 1.86 g/s

Emission rate calculated correctly: 3.00 g/s

Assuming correct flow conditions:

Supplied Calculations:

NO2/NOx In-Stack Ratio (ISR) Database

Alpha databasehttps://www3.epa.gov/scram001/no2_isr_database.htm

<5% <10% <20% <30% <40% <50% <100%0

10

20

30

40

50

60

Diesel - Dataset size: 124Looking at the power of the generators, generators with >1000KW output are unlikely to be greater than 20% but there are some that are as high as 16%.

• If modelling chemistry…primary NO2?

Typical scenario

• Defra ‘mapped’ background NO2: 18-24 g/m3

• Significant local emission sources (e.g. road traffic)

Annual Mean – Impact descriptors Annual Mean Concentration At Receptor In Assessment Year

(g/m3)

Change in Concentration – NO2 (g/m3)

<0.2 0.2-0.6 0.6-2.2 2.2-4.0 >4.0

<30.2 Negligible Negligible Negligible Slight Moderate

30.2-37.8 Negligible Negligible Slight Moderate Moderate

37.8-41.0 Negligible Slight Moderate Moderate Substantial

41.0-43.8 Negligible Moderate Moderate Substantial Substantial

>43.8 Negligible Moderate Substantial Substantial Substantial

* Baseline will depend on the actual concentration and will need to be investigated

Process Contribution Band: 0.2 – 0.6 0.6 – 2.2 2.2 – 4.0 >4.0

Negligible <37.2 <28.0 - -

Slight adverse 37.2 - 40.4 28.0 - 35.6 <26.2  Moderate adverse >40.4 35.6 - 41.6 26.2 - 37.0 <33.8 *

Substantial adverse - >41.6 >37.0 >33.8 *

Baseline concentrations for defining impact descriptor:

Annual mean

0.6 to 2.2 g/m3 PC band – ‘moderate adverse’ if baseline is 35.6 to 41.6 g/m3

Typical emissions MCP emissions

Baseline:• Defra ‘mapped’ backgrounds (~24 g/m3) – all below 40 g/m3

• Monitoring? ~36 g/m3 – impact ‘moderate adverse’• Judgement? How many people are affected?

Approach for short-term impacts: ‘a lot of randomness’

• From the full 8760 value dataset, randomly select 250 values.

• Each time an hourly concentration is selected it is excluded from the dataset.

• Select the 19th highest value from the 250 independent values. This is a representative value for 19th highest concentration in the year.

Repeat this process 10,000 times or more!

Evaluate the 10,000 ‘years’ of calculated process contributions from randomly selected hours

Model a full year (accounting for worst-case modelling): output hour-by-hour data.

What to do with 10,000 short-term PCs!

Minimum (g/m3) 8Maximum (g/m3) 33095%ile (g/m3) 201

PC (g/m3) Count Percentage0-100 4288 42.9%

100-120 1294 12.9%120-160 2675 26.8%160-200 1211 12.1%

>200 532 5.3%

0 50 100 150 200 250 300 350 4000

100

200

300

400

500

600

700

800

900

Process contribution

Coun

t

Annual Mean Baseline (g/m3) ~36-37Short-term baseline (g/m3) ~72-74Headroom (g/m3) (200 – baseline) 128Total (g/m3) (95%ile to max) 326-458

Chance of PC >120 (g/m3) 44.2%

Consider the whole dataset of predicted short-term concentrations:• All below the headroom?

>> no risk• A small number greater than the headroom?

>> some risk• Lots greater than the headroom?

>> high risk

Environment Agency Modelling

https://consult.defra.gov.uk/airquality/medium-combustion-plant-and-controls-on-generators/https://en.wikipedia.org/wiki/Hypergeometric_distribution

Probability and Risk Environment Agency has used hypergeometric distribution Probability of 200 µg/m3 being exceeded more than 18 times a year,

depending on the number of operational hours Acceptable risk:

the EA is saying that a 5% chance is acceptable AQC screens on a 1% chance

Model the percentile that would lead to chance

Mutually exclusive selections

• Dataset size – 24 values. 7 Ys and 13 Ns.

• You randomly select four of the values, and after each value is selected that value is not available to select again.

• What is the probability that two of the selected four are Y?

• Hypergeometric distribution!

Dataset - 1

Dataset - 2

Dataset - 3

Dataset - 4

Dataset - 5

Dataset -Selected

Y Y << randomly selected 1st N N N N NY Y Y Y YN N N N NN N N N NN N N N N << randomly selected 4th N N N N NY Y Y Y YN N N N NY Y Y Y YY Y Y Y YN N N N NN N N N NN N N << randomly selected 2nd Y Y Y Y YN N N N NY Y Y Y YN N N N NN N N N << randomly selected 3rd N N N N NY Y Y Y YN N N N NY Y Y Y Y

Hypergeometric distribution

drawn not drawn totalExceedenc

e k = 19 K − k = 353 K = 372

Not exceedenc

en − k = 231 N + k − n − K = 8157 N − K = 8388 (m)

Total n = 250 N − n = 8510 N = 8760

 Contingency table, 250 hours of operation:

How to calculate. Excel? ’R’ commands:

372 exceendence hours in full dataset before the is a 1% chance of 19 being selected

The 95.75 %ile

No chemistry PC – 35% conversion Chemistry PC – 16% primary NO2

For 250 hours operation the 1% chance of an exceedence is associated with 372 hours in a full year above 200 µg/m3 (a 95.75th percentile = 372 / 8760)

In other words, we model for a full year and if the 95.75th percentile (including baseline) is less than 200 µg/m3 then there is less than a 1% chance of the objective being exceeded and the impact is ‘not significant’.

Assessed percentile

There are different percentiles for different operational hours and for difference associated probabilities

Hours of operation Number of hourly exceedences in full dataset for <1% probability of exceedence Percentile

250 372 95.75%500 186 97.88%1000 94 98.93%1500 64 99.27%2000 49 99.44%

Hours of operation Number of hourly exceedences in full dataset for <5% probability of exceedence Percentile

250 443 94.94%500 222 97.47%

1000 112 98.72%1500 75 99.14%2000 57 99.35%

Probability distribution vs calculated random selection

Minimum (g/m3) 7.5Maximum (g/m3) 329.595%ile (g/m3) 20198%ile (g/m3) 222

94.94%ile 95.75%ileChemistry - 16% primary NO2 186 g/m3 221 g/m3

Hypergeometric distribution is clearly more optimistic than the random selection approach

PC (g/m3) Count Percentage0-100 4288 42.9%

100-120 1294 12.9%120-180 3476 34.8%180-200 410 4.1%

>200 532 5.3%

<5% probability which the EA use is too optimistic The 94.94th %ile is not in the top 5% of concentrations

AQC use <1% probability criterion which is in the top 5%

vs

Final noteMedium Combustion Plan Directive

1 to 50 MWth plant

Transposed into UK law by December 2017 Emission limits from Dec 2018 for new plant and 2025 or 2030 for

existing plant Unclear who will Permit the plant – EA or LA

Siting Near or in AQMAs In suburban / residential areas Low level: 3-7 m release – near receptors Hospitals Horizontal exhaust

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