Ammonia measurement

challenges in SCR units

CEM-2014, 16.5.2014

Olli Antson, Kati Lehtoranta

VTT Technical Research Centre of Finland

Engines and Emissions team

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Ammonia measurement challenges in SCR units

Contents

Focus of the team´s research

Emission regulation in marine engines

SCR unit in emission reduction

Case examples on NH3 measurements

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Focus of the team´s research

High speed

enginesMedium speed

engines

Emission

measurement

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Emission Regulation in marine engines by IMO

It is well known that ship emissions contribute significantly to global emissions and to

local emissions at ports.

The regulation by International Maritime Organization (IMO) includes rules to decrease

emissions of sulphur compounds and nitrogen oxides. There is regulation also on

energy efficiency of ships which includes greenhouse gas emissions, and regulation on

VOC emissions.

These rules gradually set the maximum allowed sulphur percentage in fuel to 0.5%

globally by the year 2020, and to 0.1 % in specific emission control areas by the year

2015. Ships can also comply with these rules by installing an after-treatment system for

SOx removal.

Also NOx emissions are restricted gradually in marine diesel engines and the limits

referred as Tier I, II and III are given as specific emissions (g/kWh) based on the

maximum operating speed of the engine.

The current Tier II standard can be met by optimization of combustion process, but Tier

III standards which are planned to come into force in 2016 (postponed to 2021) are

expected to require an after-treatment unit for emission control.

The tight requirements set by IMO Tier III can be met by utilizing SCR technology

(Selective Catalytic Reduction).

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SCR – Selective Catalytic Reduction

Urea decomposition:

(NH2)2CO -->NH3 + HNCO

HNCO + H2O --> NH3 + CO2

Main NOx reduction reaction in SCR:

4NO + 4NH3 + O2 → 4N2 + 6H2O

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Case examples on NH3 measurements

Case 1: FTIR and impinger sampling (at VTT’s medium speed engine

laboratory)

Case 2: FTIR and in-situ laser method (at a customer´s medium speed

engine)

Case 3: FTIR and diode laser method (a gas engine equipped with a side-

flow SCR unit, at a customer´s SCR installation)

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Case 1: FTIR and impinger sampling at VTT’s medium

speed engine laboratory

Engine: Wärtsilä Vasa 4R32,1640 kW.

Fuel: HFO with 2.4% S

SCR: Extruded honeycomb with 35 cpsi, volume 40,5 dm3, V2O5 catalyst

Exhaust gas: 500 kg/h, temperature 260-340 °C

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Case 1: FTIR and impinger sampling at VTT’s medium

speed engine laboratory

Test SCR operation:

Measured NOx efficiency at exhaust T 260-340 °C

When temperature decreases

NOx efficiency decreases –

and ammonia slip formation

enhances (while keeping urea

dosing constant)

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FTIR sampling

FTIR

N2

Heated probe and sampling line,

Zero gas

Exhaust

gas

~ 3

00°C

Heated filter and pump

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Impinger sampling

Sample from hot exhaust gas

through a steel tube, sample flow

1,5-2 l/min, bubbled through 50 or

20 ml of water (in ice bath) with a

sampling time of 30-40 min.

Exhaust

gas

~ 3

00°C

Steel tube 200°C

P

0°CH2O

Exhaust

gas

~ 3

00°C

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FTIR and impinger sampling - Results

Results from 4 different cases,

FTIR as an average of the impinger

sampling time (error bars show the

measurement uncertainty)

→ Results at same level, or

FTIR gave lower values for these

extractive methods

Note. The impinger sampling in water

– all ammonia probably not measured

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Case 2: FTIR and in-situ laser method

Engine: Medium speed diesel engine

Fuel: HFO and LFO (Sulphur contents: HFO ~ 2.4%, LFO <0.05%)

SCR: V2O5 based catalyst

Ammonia slip measurement with FTIR and in-situ laser

FTIR 1

N2

Heated probe and sampling line,

Zero gas

Exhaust

gas

~ 3

00°C

Heated filter and pump

In-situ laser

FTIR 2

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FTIR and in-situ laser method – Results (LFO)

→ Similar ammonia

results with both

methods

(2 FTIR units were

utilized)

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FTIR and in-situ laser method – Results (HFO)

→ With FTIR clearly

lower ammonia values

are recorded

→ Sulphur related

reactions cause

interference to ammonia

measurement with FTIR

(extractive sampling)

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Case 3: FTIR vs. diode laser measurement

A gas engine equipped with a side-flow SCR unit.

The actual target of this field campaign was to compare different catalyst setups by

varying exhaust flow and urea feed, but it was also possible to compare the NH3 results

with different measurement methods.

This SCR unit contains several sampling points for extractive sampling.

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Case 3: FTIR vs. diode laser measurement

There is a systematic difference between the NH3 results. During a two week campaign

the laser device gave 15-30% larger values than FTIR instrument. A reason for these

differences could not be found.

The laser device was operated by using factory settings for NH3 analysis.

The calibration of FTIR instrument was carried out at laboratory before the measurement

period with reference gases (NH3 in N2). During the field measurements only zero gas

was tested daily.

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Ammonia measurement challenges in SCR units

Results

• Sampling line problems: gas phase reactions, sulphur related reactions

cause interference to ammonia measurement when sampling is

required (like for FTIR measurement)

→ Significant especially when using fuels with high sulphur

contents: measured NH3 slip levels are lower than actual levels

• Unresolved deviations between FTIR and laser instrument

Further developments

- Total sample collection and in-situ laser measurement

- Extractive sampling

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