Introduction to biomass combustion and pollutant reduction in wood stoves and boilers
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Introduction
to biomass combustion
and pollutant reduction
in wood stoves and boilers
Prof. Dr. Thomas Nussbaumer
Verenum Research, Zürich
Lucerne University of Applied Sciences and Arts, Horw
Swiss Federal Office of Energy, Delegate in IEA Task 32
Switzerland
IEA Webinar 06 May 2021
Verenum
Content
1. Intro: The role of bioenergy for
a) Energy Supply / Climate Change
b) Air Pollution
Verenum
World energy supply 1990 to 2017
Total in ktoe/a Share in %
Coal
Natural Gas
Nuclear
Hydro
Wind, Solar
olar
Hydro W ind&S Wind&Solar
Biofuels and waste
9.1%
Oil
https://www.iea.org/data-and-statistics 20.1.20
Facts or hypotheses
1. Solar energy has largest potential but causes need for energy storage
2. Wood from sustainable forestry is renewable and net CO2 free (and easily storable)
Verenum and can ideally complement solar energy as storable fuel
Target conflict
Global effect of smoke from biomass on mortality
Mio deaths per year
Verenum [OECD/IEA: Energy Poverty, IEA, Paris 2010]
Content
1. Intro
2. Pollutant formation with focus on Particulate
Matter (PM) and Organic Compounds (OC)
Verenum
Pollutants from biomass combustion
CO
+ hn SOA
VOC
EC + POA
PIA
PM10
hn Salts
SIA
NOx
CO2
Cfix
C
Ca, K, Cl, N
Ash
CaO, KCl, ..
Verenum
Products from Products from Incomplete Combustion Products from
Complete (PIC) Complete
Combustion Combustion
Solid Particulate Matter G a s p h a s e p o l l u t a n t s
Chimney
Droplets Gas
Biomass
Salts + Char
Soot COC VOC CO CO2 H2O NOX
> 10 µm < 100 nm
–H2 +O2 +O2
KCl, K2SO4 Ash PAH
oxides CaCO3 t< t> Gas phase
combustion
< 100 nm > 1–10 µm
in combustion chamber
T > 800° T < 800° T > 550°
Consecutive reactions
O2 = 0
by secondary air
+O2
Condensation CO
Secondary >700° Primary
Coagulation Tars Tars
Nucleation O2 = 0
CmHn +O2 +O2
CO, H2 T > 800° +CO2
Devolatilisation Pyrolysis Gasification
Solid fuel conversion
K+, Na+, Cl–, SO 42–, C
OH –, CO 32–, NO 3– CaCO3 (Char)
by primary air
u > 0.1 m/s T > 300° + O2
in fuel bed
Zn-, Mg-, Fe-, Al-oxides
Waste: Cu, Cr, Pb, (Cd) CxHyOz
Evaporation T > 100°
Evaporation Entrainment H2O
1 u > 0.1 m/s
2
T > 800° K, Na, Ca, S, Cl, N
Verenum
u Gas velocity, t Residence time, short/long COC: Condensable Organic Compounds
1 Solid-particle-path, 2 Solid-vapour-particle-path VOC: Volatile Organic Compounds T: [Evans and Milne, 1987], H 2: [Jess, 1996]
PM10 Atmosphere
PIA POA + EC/BC SOA
Solid Particulate Matter G a s p h a s e p o l l u t a n t s
Chimney
Droplets Gas
Biomass
Salts + Char
Soot COC VOC CO
100 000
[mg/Nm3]
(11% O 2)
10 000
CO
1 000
100
[Nussbaumer, T., Energy & Fuels,
10 17, No 6, 2003, 1510–1521, 17]
0 1 2 3 4 [–] 5 [Lauber, A.; Nussbaumer, T., 13th ETH-Con-
ference on Combustion Generated Nano-
Verenum λ
Excess Air Ratio l particles, June 22 – 24 2009, Zurich 2009]
Solid Particulate Matter G a s p h a s e p o l l u t a n t s
Droplets Gas
Biomass
Salts + Char
Soot COC VOC CO
SCHMID energy solutions
Graph: [Schmid]
Verenum
Solid Particulate Matter G a s p h a s e p o l l u t a n t s
Droplets Gas
Biomass
Salts + Char
Soot COC VOC CO
Influence of combustion phase Influence of operation
Start-up (log wood boiler)
PM [mg/mn3] @13% O2
d )
t (ba
star
o d)
t (go
star
ry
iona
stat
CO [mg/m n3] @13% O 2
[Bäfver, L. et al, Biomass and [Good, J., Obermayr, D., Nussbaumer, T.,
Bioenergy 35 (2011) 3648–3655] 11. Holzenergie-Symposium, ETH Zürich 2010]
VerenumSolid Particulate Matter G a s p h a s e p o l l u t a n t s
Droplets Gas
Biomass
Salts + Char
Soot COC VOC CO
Toxicity low medium high
Influence of operation ... and technology
[P. Zotter et al., Environ. Sci. Technol.,
2019 Apr 2; 53(7): 3959-3968]
Verenum
[Klippel, N.; Nussbaumer, T., 15th Eur. Biomass Conf., Berlin 2007]Content
1. Intro
2. Pollutant formation
3. Measures for resid. wood combustion (RWC)
3.1 Primary measures
VerenumMeasure 1
e.g. in wood stoves
Diffusion
flame
1000°
O2 = 0
O2
[H.R. Christen, Allgemeine Chemie, 1971]
Verenum1-stage combustion
CO2, H2O, O2, N2
CO, CxHy
+ air at
l>1
O2 + N2
wood C H O
VerenumLimitations
Problem 6: Problem 4:
flame quenching gas tightness inside
Problem 3:
Problem 5: air tightness
heat release from
combustion (radiation)
Problem 2:
mixing of gas and air
Problem 1:
air distribution
VerenumLimitations
Problem 6:
flame quenching
Overfilling can cause quenching
VerenumAvoid quenching
3. hot combustion chamber
2. Mixing improvement
1. Quenching avoided
air
and (additionally and mandatory):
appropriate start-up ..
Verenum.. appropriate start-up: technology dependent,
e.g. for updraft as a rule: ignition from the top
[www.holzenergie.ch] [www.holzenergie.ch]
[T. Nussbaumer et al., EU Biomass Conference 2008] [H. Hartmann, 20th ETH-Conf. on Nanop., Zürich, 2016]
VerenumAppropriate fuel: technology dependent
Moisture content Log size
10,000 300
CO OGC Stove 1 Stove 2 Stove 3
mg/Nm³
(13 250
mg/Nm³
8,000
(13 % O2)
% O2)
211 Undiluted flue gas
Particle emission
Gaseous emission
200
6,000
150
123
4,000 105
85
100
65 74
2,000 34
50
22 21
0 0
0 5 10 15 20 25 30 35 %
40 45 n=3 n=3 n=3
Fuel moisture content 5x5 cm 7x7 cm 9x9 cm
Log size
Stove 1 Stove 2 Stove 3
[H. Hartmann, 20th ETH-Conf. on Comb. Gen. Nanoparticles, Zürich, 2016
and TFZ-Bericht 36, www.tfz.bayern.de] and additional info by
[R. Mack et al. Central European Biomass Conference, Graz 2020]
VerenumMeasure 2
e.g. in wood boiler
Diffusion Premixed
flame flame 1540°
1000° 1560°
O2 = 0 520°
O2 O2 > 0
300°
O2
[H.R. Christen, Allgemeine Chemie, 1971] [H.R. Christen, Allgemeine Chemie, 1971]
Verenum2-stage combustion: principle in downdraft boiler
wood CHoval
HO
Hoval
+ air with
l1 T > 800o
t > 0.3-0.5 s
CO2, H2O, N2
Verenum [Nussbaumer, Energy & Fuels, Vol. 17, No 6, 2003, 1510–1521, 17]Titel 2-stage combustion: examples in stoves
Bionic Fire Tiba (Sirius) Attika (Juna)
https://attika.ch/
Specht TwinFire Xeoos http://www.specht-ofen.de Tiba AG & Hochschule Luzern 2012
[Zotter et al. 2017, 21st ETH Conf.
[W. Wiest., 13. Holzenergie-Symposium, Zürich 2014] [T. Nussbaumer & P. Odermatt, TGA 5 2013, 54–58] on Comb. gen. Nanoparticles]
VerenumImprovements since 1975/80
Titel
Boilers Stoves
[BEST (formerly BLT), AUT] [HKI, GER]
Industrieverband Haus-, Heiz und Küchentechnik PM
CO PM
[L. Lasselsberger, BLT Wieselburg (A) 2016]] [V. Schmatloch, Kachelofen&Kamin, Januar 2018]
Approach: "Technical Guidelines for design of low emission
stoves, presentation 2 by Morten Warming (DTI, DK)
VerenumAnforderungen für tiefe Emissionen
„Gap“
between type-test and reality
chimney sweeper measurements
PM real-life
PM type-test
Conclusion: „no correlation“
[German Biomass Research Center –
Verenum Deutsches Biomasseforschungszentrum, DBFZ, Leipzig 2014]Anforderungen für tiefe Emissionen
„Gap“
between type-test and reality
Approach: "Real life" test methods:
presentation 3 by Gabriel Reichert (BEST, AUT)
Verenum [C. Schmidl et al., «Be Real» 14. Holzenergie-Symp., Zürich 2016]Content
1. Intro
2. Pollutant formation
3. Measures
3.1 Primary measures
3.2 Secondary measures
VerenumTitel Electrostatic
Catalysts Aerosols
Inserts from Biomass Precipitators (ESP)
Combustion
[Oekosolve
2019]
CO > 70% CO > 70%
Reduction
VOC > 40% VOC > 70%
PM 30% - 40% oxidizable PM PM 60% - 80%
[V. Schmatloch, EMPA [D. Jud, Oekosolve,
[G. Reichert et al., BEST, 13. [M. Aleysa, Fraunhofer IBP, CEBC Graz 2020]
Holzenergie-Symp., Zürich 2014] Dübendorf, 2004]
15. Holzenergie-Symp., 2018]
VerenumTitel Electrostatic
Catalysts Aerosols Inserts from Biomass
Precipitators (ESP)
Combustion
Pressure drop Temperature control needed
Aging Soot causes re-entrainment
Limited effect during cold start and/or limited availability
Periodic cleaning needed
a s ure s easures
nd ary me primary m bustion
Seco repla ce) lete com
t (but not o f in c omp
om plemen d in case
can c y are limite
ince the
s
VerenumContent
1. Intro
2. Pollutant formation
3. Measures
4. Consequences for low emissions
VerenumRequirements for low emissions
1. Appropriate ignition and start-up
2. Appropriate fuel (moisture, size, ash) and fuel amount
3. Technology with 2-stage combustion, secondary air and hot comb. chamb.:
Target for T T T: l = 1.5-2.5, T > 800o, t > 0.3-0.5 s
Options: - forced draft with ventilator
- inserts for mixing and/or catalytic effect
- combustion control
4. Optional secondary measure: ESP with monitoring
5. For boilers: heat storage tank
Remaining challenges: - cold start
- ambient conditions, mostly for natural draft
- operator influence
VerenumRequirements for low emissions
are valid for all combustion types
but:
- for automatic combustion safely applicable
(in small scale for wood pellets)
- for manual devices highly challenging
most important in real-life
VerenumPM emissions in small scale in real-life conditions
limit for boilers 20 mg/m3
with uncertainty 31 mg/m3:
pellet boilers: 64% meet emission limit
other boilers: 25% meet; 75% exceed
PM in mg/m3 @ 13 % O2
[A. Kather, N. Woltersdorf: Feinstaubemissionen aus biomasse-befeuerten
Verenum Kleinfeuerungsanlagen, IET, TUHH, Hamburg 2013]1. Intro
2. Pollutant formation
3. Measures
4. Consequences
5. Conclusions
Verenum1. RWC contributes to PM and OC hence their emissions need to be reduced
2. Primary measures are:
- combustion design
- ideal start-up and appropriate fuel
- ideal operation with appropriate air supply and high temperature
3. The technology has been improved by:
- thermal insulation, air tightness, secondary air and hot combustion ch.
- modern devices achieve low emissions on test-bench
4. Secondary measures can further reduce emissions,
but cannot replace primary measures
5. Main reason for high impact of RWC is the
«gap» between test-bench results and non-ideal operation in real life
6. Control systems and monitoring can assist to improve real-life operation
7. Information und inspections remain necessary
VerenumAcknowledgements
Swiss Federal Office of Energy
Swiss Federal Office for the Environment
Swiss National Research Foundation
Innosuisse and SCCER Biosweet
IEA Bioenergy Task 32
Info
www.verenum.ch
www.hslu.ch
www.holzenergie-symposium.ch
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