UEF // University of Eastern Finland

Liekkipäivät, Espoo, 23.10.2018

Heikki Lamberg1, Olli Sippula1, Jorma Joutsensaari2, Mika Ihalainen1, Jarkko Tissari1, Anna Lähde1, Jorma Jokiniemi1

1Fine Particle and Aerosol Technology Laboratory, Department of Environmental and Biological Sciences, Kuopio, University of Eastern Finland

2Aerosol Physics, Department of Applied Physics, Kuopio, University of Eastern Finland

Study of high-temperature oxidation of wood combustion

particles using tandem differential mobility analysis

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Motivation1. Small-scale wood combustion is a major source of particulate emissions causing health and

climate effects –particularly logwood combustion important source

2. Improvements in combustion technology of logwood stoves, such as air-staging, havedecreased emissions of CO and organic compouds (both particulate and gaseous), but is ineffective for soot (Nuutinen et al. 2014)*

3. Also modern automatic wood-fired boilers emit soot when operated at partial load(Lamberg et al., 2011)

4. What are the requirements for gas-phase oxidation of soot in residential wood combustionappliances?

*Nuutinen et al. 2014 Biomass & Bioenergy, 67, 167-178.

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Primary combustionchamber

Post-combustionchamber

Motivation– What temperature vs. residence time is

needed for wood combustion soot oxidation?

• Does soot structure affect the oxidation ?

• Does inorganic coumpounds have an effect ?

• Can efficient soot oxidation be achieved in a post-combustion chamber?

Post-combustionchamber

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Experiment setup

•High-temperature tandem differentialmobility analyzer (DMA) system*

– Combustion appliance

• Pellet boiler (nominal and low load)

• Wood stove

– Dilution

– Stabilization

• Reduces fluctuation in emissions

– Particle classification in DMA1

• 40 nm, 100 nm, 200 nm with pellet; 90 nmwith wood stove

– High-temperature reactor (up to 950 °C)

• Ø 28 mm silicon carbide tube, 50 cm heated

– Particle size measurement using the SMPS (scanning mobility particle sizer)

4*similar HT-TDMA system has been used, for example, inHiggins et al. 2002. J. Chem. Phys. A, 106, 96-103.

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Reactor

Operation of the HT-TDMA system

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Combustion emissions

•Low load

– 7 kW (25 kW boiler)

– CO 178 mg/MJ

– PM1 16.3 mg/MJ

•Nominal load

– CO 63 mg/MJ

– 12.2 mg/MJ

– Typical modern pellet boiler

•Wood stove

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NominalLow

output StoveSoot 2 50 60OC 4 13 30K 25 16Cl 17 6

SO4 25 1.8NO3 <dl 1.8

Fe 4 0.8Na <dl 0.7

Zn 1.3 0.7

Chemical composition (%)

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Pellet combustion, classified 40 nm

Low load Nominal load

25 °C460 °C

• Size decreases already at 460 °C

• Size decreases already at 460 °C

• Even at 810 °C, particle size hasdecreased only by 40%

Mainly ash?

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Pellet combustion (nominal load) classified 40 nm particles

40 nm particles aremainly ash in both cases

Nominal load

Thermodynamic equilibrium composition of ash compounds in PM (FactSage 6.2)

• Eg. Shrinking of particles from 40 nm to 30 nm at 660 °C correspond to 58% decrease in particle volume

• This can be explained by the volatilization of alkali chlorides and decomposition of carbonates

• ZnO is thermally stable

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Pellet combustion, classified 100 and 200 nm

Low load, 100 nmNominal load, 100 nm

• Both inorganics and sootin 100 nm particles

• Not much organics

• More soot than in nominal load

• Both inorganics and soot

• Higher content of inorganics

1 µm

Low load, 200 nm

• More soot compared to 100 nm particles

• Some inorganics attached to soot agglomerates

• Organic matter not well visiblewith TEM

500 nm

Inreasing soot content

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860 °C710 °C

460 °C25 °C

Pellet combustion, classified 100 nm and 200 nm

Low load 100 nmNominal load, 100 nm

710 °C

25 °C

460 °C

Low load 200 nm

860 °C

710 °C

460 °C

25 °C

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Pellet combustion, classified 100 nm and 200 nm

Low output, 100 nmNormal combustion, 100 nm

• Size decrese below 460 °C

– KCl or organic matter

– Not clear in TEM

• Double mode reflects to particles with different chemicalcontent or structure

– 510 – 570 °C

• Biggest change in 460 – 710 °C

• Some soot left in 710 °C, but notat 860 °C

• Soot particles oxidizedalready at 460 °C

• Two modes between 660 –860 °C

• Only spherical particles at 860 °C

– ZnO

Low output, 200 nm

• No size decrese below 460 °C

• Double mode reflects to particleswith different chemical contentsor structures

– 570 – 610 °C

• Higher relative size change than40 nm and 100 nm

– more soot in 200 nm

– but required higher temperature

• different soot structure or

• lower/different inorganiccontent

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Relative size changes of the tested particles

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• The biggest change at 550-610 °C (pellet) and 460-810 °C (stove)

• Inorganics evaporate in hightemperature

– But their main effect is the catalyticeffect on soot oxidation

• More inorganics, nominal load

– Smaller relative size change

– Size in 710 °C about 43 – 48 nm (100 nm and 200 nm)

• When more soot, low load

– Certain temperature required

– Size in 710 °C about 28 nm (100 nmand 200 nm)

• Zn was found to be the last elementpresent

– TEM-EDS at 860 °C

LO, 40 nmLO, 100 nmLO, 200 nmNO, 40 nmNO, 100 nmNO, 200 nmStove, 90 nm

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Size changes of combustion particles with similar residence times

(1.3-1.4 seconds at peak temperature)

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• The measured size decreases were fitto a model based on modifiedArrhenius expression, according to Higgins et al. 2002*

– Decrease in particle diameterdescribed as a function of reactortemperature

– Only for temperature range at whichparticle size clearly and monotonically decreased

• 360-660 °C low output

• 260-710 °C normal combustion

• 460-860 °C wood stove

Oxidation kinetics

14*Higgins et al. 2002. J. Chem. Phys. A, 106, 96-103.

dDp/dt= particle diameter decrease rate

T= temp

R = universal gas constant

A = pre-exponential factor

Ea= activation energy

Temperature profiles and residencetimes were taken into account.

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Oxidation kinetics – Approximate time required for 90%

reduction of soot mass (assuming particle-size independent mass conversions)

15

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Conclusions and practical implications of the results• Pellet boiler:

– Not a major source of emissions

– 1 sec at 700-750 °C result in about 90% soot oxidation

– Inorganic species catalyze soot oxidation

-> Low load operation soot emissions couldbe efficiently removed via post-combustionzone design

• Wood stove:

– Important souces of atmosphericpollutants

– 1 sec at 800 °C result in about 70% soot oxidation

– Easier to oxidize than fossil fuelsoot (?)

– Soot emissions could be decreasede.g. via using insulating materialsin post-combustion zone.

• Measurements could be improvedby:

– Measuring particle effectivedensities to get more reliablemass

– High temperature samplingof fresh soot directly fromcombustion chamber wouldbe more realistic

Thank you!

uef.fi

H. Lamberg, O. Sippula, J.Joutsensaari, M. Ihalainen, J.Tissari, A. Lähde, J. Jokiniemi(2018).Analysis of high-temperatureoxidation of wood combustionparticles using tandem-DMAtechnique. Combustion and Flame191, 76–85.

This work was supportedby Maj and Tor NesslingFoundation (201500069 & 201600029)&Academy of Finland: NABCEA-project (296645)