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Nitrous Oxide Emissions from Waste Incineration

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EU energy and environmental policy in waste management leads to increasing interest in developing methods for waste disposal with minimum emissions of greenhouse gases and minimum environmental impacts. From the point of view of nitrous oxide (N2O) emissions, waste incineration and waste co-combustion is very acceptable method of waste disposal. Two factors are important for attaining very low N2O emissions from waste incineration, particularly for waste with higher nitrogen content (e.g. sewage sludge, leather, etc.): temperature of incineration over 900°C and avoiding selective noncatalytic reduction (SNCR) de-NOx method based on urea. For reduction of N2O emissions retrofitting such plants to ammonia-based SNCR is recommendable. The modern selective catalytic reduction facilities for de-NOx at waste incineration plants are only negligible source of N2O.
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c
2006 Institute of Chemistry, Slovak Academy of Sciences
DOI: 10.2478/s11696-006-0016-x
REVIEW
Nitrous Oxide Emissions from Waste Incineration
a
K. SVOBODA,
b
D. BAXTER, and
c
J. MARTINEC
a
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic,
CZ-165 02 Prague, Czech Republic
e-mail: svoboda@icpf.cas.cz
b
European Commission, Joint Research Center, Institute for Energy, NL-1755 ZG Petten, The Netherlands
e-mail: david.baxter@jrc.nl
c
Department of Thermal and Nuclear Power Plants, Faculty of Mechanical Engineering,
Brno University of Technology, CZ-616 69 Brno, Czech Republic
Received 18 January 2005; Revised 8 June 2005; Accepted 15 June 2005
EU energy and environmental policy in waste management leads to increasing interest in de-
veloping methods for waste disposal with minimum emissions of greenhouse gases and minimum
environmental impacts.
From the point of view of nitrous oxide (N
2
O) emissions, waste incineration and waste co-
combustion is very acceptable method of waste disposal. Two factors are important for attaining
very low N
2
O emissions from waste incineration, particularly for waste with higher nitrogen content
(e.g. sewage sludge, leather, etc.): temperature of incineration over 900
C and avoiding selective
noncatalytic reduction (SNCR) de-NO
x
method based on urea. For reduction of N
2
O emissions
retrofitting such plants to ammonia-based SNCR is recommendable. The modern selective catalytic
reduction facilities for de-NO
x
at waste incineration plants are only negligible source of N
2
O.
Nitrous oxide is a greenhouse gas with a global
warming potential approximately 300 times higher
than CO
2
. Being chemically inert in the troposphere
with a long lifetime (110—150 years), N
2
O can reach
the stratosphere where it is destroyed by photolysis
forming NO, one of the species contributing to ozone
depletion. N
2
O contribution to global warming from
anthropogenic sources is estimated to be around 4—
6 % [1, 2]. N
2
O appears naturally in the atmosphere.
Oceans, tropic and temperate soils, are believed to
be the major natural sources of N
2
O. Nevertheless,
concentration of N
2
O has increased by around 13 %
since pre-industrial times [2]. The global (worldwide)
anthropogenic emissions [3] of N
2
O in 1990 (3.2 Tg ni-
trogen/year) account for 20 % of the estimated total
N
2
O emissions (14.9 Tg nitrogen/year, 1 Tg = 10
12
g).
The share of anthropogenic emissions by sector
varies according to world region and the specific eco-
nomic activities. Based on various studies [3—6] the
approximate contribution by sectors is as follows:
i) Agriculture and land use is the highest source of
anthropogenic N
2
O emissions (50—70 %);
ii) Industrial sources, mainly nitric acid and adipic
acid production accounts for 15—25 %;
iii) Stationary and mobile fossil fuel combustion
15—25 %.
Waste treatment and disposal is mentioned in the
previous inventories as a relatively less significant
anthropogenic source of N
2
O. Main activities with
potential N
2
O emissions are waste water treatment,
sewage sludge incineration, municipal solid waste in-
cineration, biomass combustion for energy production,
incineration of waste-based fuels with high content of
nitrogen, etc. Quantification of their contribution to
the anthropogenic N
2
O emissions is still difficult be-
cause they remain uncharacterized.
Agriculture and land use
N
2
O is formed in agricultural soils mainly by the
microbial process of nitrification and denitrification.
These processes are enhanced by an increase of avail-
able nitrogen in soils through N-fertilization, manure
application, atmospheric deposition, etc. Food needs
78
Chem.Pap.60(1)78—90 (2006)
c
2006 Institute of Chemistry, Slovak Academy of Sciences
NITROUS OXIDE EMISSIONS
in the coming years are expected to boost N-fertilizer
application and in turn to accelerate the N
2
O accu-
mulation in the atmosphere [7].
Biomass burning is another source of N
2
O, both
during the fire and by increasing soil N
2
O emissions
through stimulation of N-mineralization. Thus, the
conversion of tropical forest to agricultural land and
biomass burning in tropical areas have a great impact
on global N
2
O emissions [8]. Burning is also used to
rejuvenate old pasture. It is estimated that 40 % of all
savannahs are burned each year.
Fossil fuel combustion
Production of N
2
O from fossil fuel combustion is
associated with the oxidation of the fuel-nitrogen. It
is well known that N
2
Oisformedbytwodierent
pathways, i.e. through gas phase reaction of nitroge-
nous groups in the volatiles (HCN) and also through
the heterogeneously catalyzed oxidation of the char-
bound nitrogen species.
N
2
O emissions are highly temperature-sensitive, so
that N
2
O level in flue gas increases when temperature
decreases [9, 10]. In line with this, high-temperature
pulverized coal combustion technologies produce neg-
ligible N
2
O emissions (below 5 vol. ppm), while lower-
temperature fluidized bed combustion can produce
relatively high N
2
O emissions, even as high as 200
vol. ppm. In addition, it is known that NO
x
control
procedures can lead to elevation of N
2
O emissions as
a side effect.
Fossil fuel combustion in vehicles is another im-
portant source of N
2
O. It is generally accepted that
cars fitted with a “first generation” three-way cata-
lyst (TWC) for NO
x
abatement emit more N
2
Othan
old vehicles without catalyst. Measurements on vehi-
cles with fuel economies of 6—12 l/100 km [11] have
shown the N
2
O emission rate of 5—11 mg/km. It ap-
pears that N
2
O emissions are mostly formed during
the light-off phase, when the catalyst is warming up. If
the catalyst degrades (due to ageing), then the length
of the light-off phase could be extended and the pe-
riod over which N
2
O is emitted [12] is longer. Increase
of N
2
O emissions from deactivated catalysts is due to
a deterioration of the catalyst activity to decompose
N
2
O rather than an increase in catalyst activity in
converting NO to N
2
O [13].
Waste treatment and disposal
Waste treatment and disposal activities are be-
lieved to be a noticeable source of N
2
O. However, very
little work on their characterization has been reported
so far. This makes difficult any assessment of the waste
management options in terms of N
2
O emissions.
Thermal treatment of solid waste materials achie-
ves a reduction of waste volume while producing heat
and/or power. Municipal solid waste (MSW) is one of
the main waste streams in terms of mass. N
2
Oemis-
sions from MSW incinerators vary according to the
combustion temperature, as for coal combustion [14].
However, due to the generally low nitrogen content
in the MSW, the emissions rarely exceed 20 vol. ppm
in any incineration technology (e.g. grate technology,
fluidized bed, rotary kilns). On the other hand, N
2
O
emissions can be enhanced when NO
x
abatement tech-
niques are used.
Another type of solid waste with increasing mass
production is sewage sludge. Thermal treatment (com-
bustion) of this waste is reported to lead to very high
N
2
O emissions, reaching 350 vol. ppm [15] at lower
temperatures of combustion. Thermal treatment of
other wastes with high nitrogen content, such as waste
leather, meat and bone meal (MBM), plastics, etc.
could lead to punctual high emissions of N
2
O. There-
fore an assessment of N
2
O emission from these pro-
cesses is necessary before disposal of such waste by in-
cineration is selected. Alternatively, abatement meth-
ods have to be identified and set in place.
N
2
O emission from waste biomass combustion
(agricultural residues, waste wood, demolition waste,
etc.) is also dependent on combustion temperature
and nitrogen content of the fuel. In principle, the emis-
sions are lower than in coal combustion due to lower
nitrogen content in waste biomass (0.12—0.60 mass %
(waf basis) compared to 1—2 mass % in coals). How-
ever, there are some exceptions of waste biomass, like
malt waste (4.99 mass % (waf)), coffee husks (1.6
mass % (waf)), with high nitrogen content.
As regards other waste management options, it can
be mentioned that composting of organic household
waste produces low N
2
O emissions. He et al. [16] mea-
sured N
2
O emissions from aerated composting and de-
tected a small peak of N
2
O(< 10 vol. ppm) during
the first day of treatment. Beck-Friis et al. [17] con-
cluded that more than 98 % of the nitrogen emissions
are NH
3
-related and less than 2 % are N
2
O-related.
Limited information has been found concerning N
2
O
emissions from waste landfills. Landfill leachate can
be [18] a source of N
2
O when it is recirculated to the
landfill. The same risk can arise when leachate is used
for the production of compost.
The purpose of this work is to collect com-
prehensive information concerning N
2
O emissions
from thermal treatment processes of various organic
waste materials: incineration and co-incineration of
waste, including municipal solid waste, sewage sludge,
nitrogen-containing waste, and waste biomass. The
knowledge gathered is useful to reduce uncertainties
inherent to the current N
2
O inventories. In addition,
it is essential to assess the potential need of abatement
measures to limit N
2
O emissions in future operation
of waste incineration plants.
N
2
O Emissions from MSW Incineration
Municipal solid waste (MSW) is relatively compli-
cated heterogeneous mixture of combustible and in-
Chem. Pap. 60 (1) 78—90 (2006) 79
K. SVOBODA, D. BAXTER, J. MARTINEC
Table 1. Comparison of Municipal Solid Waste Composition
from Thailand and UK [19]
Component Thailand UK
w(Paper)/% 13 31
w(Food waste)/% 39 25
w(Textiles, rubber, leather, and wood)/% 23 5
w(Plastics)/% 10 8
w(Metals)/% 8
w(Glass)/% 15 10
w(Other)/% 13
w(Ash raw)/% 13.4 22.3
w(Moisture raw)/% 58.4 32.4
HHV raw/(MJ kg
1
) 6.49 10.25
HHV dry/(MJ kg
1
) 15.59 15.17
w
i
(Ultimate analyses (dry basis))/%
Combustibles 67.8 67.00
Carbon 37.14 35.81
Hydrogen 5.41 4.82
Nitrogen 0.22 0.78
Sulfur 0.09 0.41
Chlorine 0.8 0.75
combustible (inorganic) materials. The organic frac-
tion of MSW consists of plastics, paper, textile, waste
biomass (wood, plants, etc.), and food residues. The
calorific value of the MSW is mostly dependent on two
factors: organic content and moisture. The usual lower
heat energy value is in the range 8—13 MJ/kg. The
composition of MSW, moisture content, and heat en-
ergy value is subjected to high seasonal and regional
variety [19, 20] and to long-term trends caused by var-
ious factors, e.g. better sorting of waste, introduction
of new packaging materials. In Asian countries, the
calorific value of MSW is generally lower due to the
high moisture content of the food waste fraction (see
Table 1). The moisture content coupled with calorific
value of the MSW affects significantly the combustion
process on the grate, maximum attainable local tem-
perature, and related emissions.
Nitrogen content of the municipal solid waste is
relatively low, the main sources being plastic materi-
als (polyamides, polyurethane, etc.), textile (acryloni-
trile, wool, etc.), and proteins from waste food. Typi-
cal content of nitrogen in MSW is 0.1—1 mass % (dry
basis). The usual content of nitrogen in MSW [19
21] from EU countries and USA is around 0.7 mass %,
dry basis. In Asian countries the content of nitrogen
is usually lower.
The prevailing technology for MSW incineration
in Europe and USA is grate furnace followed by
post-combustion chamber with secondary air injec-
tion. Other incineration technologies like rotary kiln
or fluidized bed-based technologies are still uncom-
mon and used rather in Japan for sorted MSW. Tech-
nologies based on alternative thermal treatments for
MSW (e.g. pyrolysis and gasification) are exception-
ally used in large scale [22]. The fuel nitrogen is under
such reducing conditions converted mostly to ammo-
nia, which can be relatively easily removed from the
resulting fuel gas.
Nitrous oxide emissions in municipal solid waste
incineration (MSWI) originate practically from fuel
nitrogen content in MSW. The mechanisms of NO
x
/
N
2
O formation and destruction in an MSW inciner-
ator are mainly related to the operating conditions,
in particular temperature, oxygen concentration, mix-
ing of gases and catalytic effects of walls, ash parti-
cles, etc. The optimum temperature range for mini-
mizing simultaneously CO, NO
x
,andN
2
O emissions
is estimated to be between 850
C and 950
C, depend-
ing on emission requirements. Although optimization
of emission is feasible, simultaneous minimization of
all regulated emissions (CO, NO
x
,N
2
O, SO
2
,HCl,
etc.) by applying only primary measures is in practice
hardly possible.
According to measurements in 10 incineration
plants in Japan [23], the concentration of N
2
O in flue
gas (without application of SNCR for de-NO
x
)isbe-
tween 0.5 and 8 vol. ppm. Variation of the emissions
depends mainly on combustion temperature and ni-
trogen content in the waste-fuel. In previous measure-
ments in various waste incinerators [24] the N
2
Ocon-
centrations in flue gas have been found to be lower
than 20 vol. ppm (frequently 4—10 vol. ppm). In Eu-
ropean countries [25] the nitrous oxide emissions mea-
sured in MSW incinerators were in the range of 1—12
mg/m
3
(i.e. 0.5—6 vol. ppm).
During combustion and incineration processes, a
significant correlation between N
2
O and CO emissions
has been observed, as well as a reverse trends in NO
x
and N
2
O emissions [24]. The N
2
O, NO, and CO emis-
sions are influenced by the presence of alkali, alkaline
earth metal compounds, and Fe
2
O
3
in the combustion
chamber [26]. Presence of such compounds generally
causes lower N
2
O emissions due to catalytic destruc-
tion of N
2
O [27].
Formation of N
2
O emissions in waste incineration
facilities is principally coupled with the methods ap-
plied for reduction of NO
x
emissions. Especially the
so-called selective noncatalytic reduction (SNCR) can
be relatively significant source of N
2
O emissions. The
conversion to N
2
O depends on the reagent for de-NO
x
reaction (i.e. urea or ammonia). In the case of urea,
the conversion of the reduced NO
x
to N
2
O can attain
about 30 %. Tsujimoto et al. [28] reported 20 % con-
version to N
2
O of the reduced NO
x
.Inthecaseof
ammonia, the maximum conversions of NO
x
to N
2
O
found are around 15 %.
The selectivity for N
2
O formation in selective cat-
alytic reduction (SCR) of NO
x
depends on compo-
sition and age of the catalyst, on operation temper-
ature, NO
2
/NO mole ratio, water vapour concentra-
tion, on deposition of ammonia salts on the catalyst,
etc. Generally, at lower operation temperatures and
80
Chem.Pap.60(1)78—90 (2006)
NITROUS OXIDE EMISSIONS
Table 2. Proximate and Elementary Analyses of the Wet, Semi-Dry, and Dry Sewage Sludge, Comparison with Coal [35, 15]
Analysis Wet sludge Semi-dried sludge Dry sludge Bituminous coal
Proximate
w(Water (raw))/% 76.0 68 13 6
w((Ash (wf))/% 51.8 31 45.0 35.4
w(Volatiles (waf))/% 92.4 90 92.7 43.8
w((NH
3
-N (waf))/% 3.2 0.9 1.19
LCV (raw)/(MJ kg
1
) 1.098 4.408 9.724 19.4
LCV (wf)/(MJ kg
1
) 4.567 13.775 20.984 32.112
w
i
(Ultimate (waf))/%
C 51.9 47.8 53 81.6
H 7.8 7.68 7.8 5.8
O 29.8 38.4 31.13 9
N 8.8 4.6 6.5 1.3
S 1.7 0.77 1.4 2.3
wf = water-free basis, waf = water- and ash-free basis.
active, fresh catalyst, the selectivity for N
2
Oforma-
tion is low (lower than 5 %). With significantly de-
activated de-NO
x
catalyst the conversions of NO
x
to
N
2
O in SCR can be higher, but maximally about 8 %.
Modern facilities for SCR are attaining conversions of
NO
x
to N
2
O on the level 1—2 % only, so their con-
tribution to N
2
O emissions from waste incineration is
only a few vol. ppm.
Biomass Combustion as Biofuel
A range of biomass types can be used as fuels,
such as wood, wood residues (bark, sawdust, odd-
sized pieces, demolition wood), agricultural residues
(straw from grains, husks from rice, coconuts, coffee,
stalks from maize or cotton, bagasse from sugar cane),
forestry and landscape conservation residues (thin-
nings and verge grass), and energy crops [29]. Typ-
ical nitrogen content in biomass between 0.12 mass %
and 0.60 mass % (waf basis) has been reported [29].
Some exceptions of biofuels with higher nitrogen con-
tent are malt waste (4.99 mass % (waf)), coffee husks
(1.6 mass % (waf)).
According to the literature [30] combustion of
wood or other biofuels scarcely produces N
2
Oemis-
sions as contrary to combustion of coals. This is pri-
marily due to the lower nitrogen content of biofuels
compared to coal (1—2 %). Low N
2
O emissions are
also due to the high content of volatile matter and wa-
ter vapour. The formation of HCN (the main N
2
Opre-
cursor) during the pyrolysis/devolatilization stages of
biomass combustion is less relevant than in the case of
coal [10]. The remaining bio-char has normally only a
small content of nitrogen, which could be transformed
into nitrous oxide. Moreover, the bio-char is very re-
active in reactions for reduction of N
2
O. As a result,
particularly in big-scale facilities at combustion tem-
peratures higher than 850
C, combustion of wood and
biomass produces only negligible N
2
O emissions (max-
imum 2—5 vol. ppm). On smaller combustion units
(e.g. stove combustion in houses) the N
2
OandCO
emissions can be relatively significant [31, 32].
Sewage Sludge (SS) Mono-Incineration
Sludge is formed during waste water treatment.
The quantity of sludge produced worldwide is rapidly
increasing. The sewage sludge production in Europe
is expected to reach over 8 million tons (dry basis) in
2005 [33]. Among the disposal techniques, agricultural
recycling is envisaged to be the most promising due
to the favourable characteristics of the sludge com-
pared to inorganic fertilizers. However, the presence
of impurities (heavy metals, persistent organic pollu-
tants, pathogens) is a difficulty that has to be further
properly solved. Incineration accounts for 19 % of the
sludge disposal in Germany [34]. Two main technolo-
gies are available for the sewage sludge, i.e. mono-
incineration (fluidized bed, multiple-hearth furnace)
and co-incineration (with coal, municipal solid waste
or in other processes). Sewage sludge is characterized
by high content of nitrogen (typically 5—8 mass %
(waf)). This might be expected to lead to high emis-
sions of NO
x
and N
2
O. Furthermore, substantial part
of the fuel-nitrogen is bound in the form of ammonia
compounds that may result in N
2
O emissions.
In general, three kinds of SS can be distinguished
according to the pretreatment steps: wet (mechani-
cally dewatered), dry sludge, and semi-dried sludge.
Table 2 shows the elementary analysis of the sludges
differing in water content (wet, dry, and semi-dry). As
can be seen, the wet sludge has higher N-content than
the dried sludge because part of the ammonia is re-
leased during the drying. The coal analysis data are
also included for comparison.
Fluidized Bed Combustion (FBC) of Pre-Dried
Sludge
The success of fluidized bed technology for sewage
Chem. Pap. 60 (1) 78—90 (2006) 81
K. SVOBODA, D. BAXTER, J. MARTINEC
Fig. 1. Comparison of N
2
O emissions from fluidized bed com-
bustion of dry sludge (
), semi-dried sludge ( ), and
wet sludge (
) [35, 15]. Dried sludge: freeboard temp.
= 850
C, FB temp. = 850
C. Wet sludge: freeboard
temp. = 850
C, FB temp. = 800
C. Semi-dried sludge:
freeboard temp. = 830
C, FB temp. = 870
C.
sludge combustion is related to advantages such as
the high turbulence and large surface area for heat
transfer, homogeneous combustion temperature, and
quick start-up and shut-down.
FBC of pre-dried sewage sludge [15] has exerted
high levels of N
2
O (250—350 mg m
3
) at oxygen
concentrations higher than 6 vol. % and combustion
temperatures about 850
C. NO
x
emissions measured
under these conditions were around 1000—1200 vol.
ppm [36]. The same authors [36] reported the effect
of temperature and air/fuel ratio on N
2
OandNO
x
emissions. Just like for coals, the increase of combus-
tion temperatures leads to higher NO
x
and lower N
2
O
emissions. On the other hand, NO
x
and N
2
O emissions
increase with an increase in excess air ratio.
FBC of Wet Sludge
Unlike pre-dried sludge, wet sludge appears not
to exhibit similar NO
x
characteristics as coals [36].
Firstly, very low NO
x
are measured during combus-
tion of wet sludge (less than 200 mg m
3
). Secondly,
wet sludge FBC exhibits an unusual NO
x
vs.O
2
trend;
NO
x
decreases as the oxygen concentration increases
and the effect of the combustion temperature is in-
significant. N
2
O emissions between 400 mg m
3
and
700 mg m
3
were measured during wet sludge FB
combustion [15]. These authors found that oxygen
concentration had little effect on N
2
O levels during
stationary fluidized bed combustion (see Fig. 1).
FBC of Semi-Dry Sludge
NO
x
emissions from semi-dried sludge are closer
to those of wet sludge although in some cases moder-
ately higher, reaching 400 mg m
3
. Like wet sludge,
NO
x
emissions exhibit a tendency to decrease with the
increase of O
2
concentration, whereas, there was no
strong dependence of N
2
O emissions on O
2
concentra-
Fig. 2. Vari ation of N
2
O emissions with freeboard tempera-
ture measured at large-scale sludge incineration plant
VERA in Germany [35].
tion as shown in Fig. 1. The combustion of semi-dried
sludge gave lower N
2
O emissions than wet sludge [35].
Thus, typical N
2
O concentrations of less than 250 mg
m
3
were measured. There was a good agreement be-
tween the results from the test rig and measurements
from large-scale combustors, with emissions ranging
between 30—300 mg m
3
.
N
2
O Reduction Techniques
As a result of the very high N
2
Oemissionlev-
els encountered during combustion of sewage sludge,
it might be necessary to apply N
2
O reduction tech-
niques. Studies in this direction reached the following
conclusions [15, 35, 37]:
Staged combustion of pre-dried sludge is effective
in lowering N
2
O emissions while such method is not ef-
fective for wet and semi-dried sludge. Varying the bed
temperature while keeping the freeboard temperature
constant has no influence on the N
2
Oemissions.
Raising the freeboard temperature proved to be
the most efficient way of reducing N
2
O emissions in
FBC of sewage sludge. According to measurements at
VERA incinerator (Fig. 2), N
2
O concentrations de-
creased from 160 mg m
3
to 20 mg m
3
as the free-
board temperature raised from 900 to 934
C.
Incineration of Waste with High Nitrogen
Content
Waste organic materials with high nitrogen content
could be susceptible to produce N
2
O emissions when
they are disposed by means of incineration. Examples
of such waste materials are leather waste, meat and
bone meal, some waste plastics, etc. They are briefly
treated here although very limited knowledge concern-
ing N
2
O emissions is currently at hand. The composi-
tion and heat energy value of some wastes with poten-
tial use for energy recovery by incineration are com-
pared and assessed in Table 3.
82
Chem.Pap.60(1)78—90 (2006)
NITROUS OXIDE EMISSIONS
Table 3. Typical Composition of Waste Organic Materials with High Nitrogen Content [38—41]
Polyamide-6 Polyuretane foam Urea-based glue MBM Leather waste
Proximate
w(Water (raw))/% 3.7 1.8 32.8 3.5 13.3
w(Ash (wf))/% 0.03 0.3 0.1 28.7 5.25
w(Volatiles (waf))/% 99.8 88.2 87.4 76.55
LHV (wf)/(MJ kg
1
) 28.70 27.00 13.40 17.00 18.30
w
i
(Ultimate (waf))/%
C 62.6 63.2 32.5 40.4 54.9
H 9.9 6.7 5.8 6.4 5.1
O 15.2 13.5 24.1 40.1* 19.2
N 12.1 6.6 37.4 7.8 14.4
S < .01 0.01 0.01 0.5 1.4
*By difference, LHV = lower heat energy value.
Footwear waste is an industrial waste with poten-
tial use for energy recovery. The heat energy value of
this waste is in the range 12.5—21 MJ/kg and the con-
tent of volatile matter is similar to common biomass
( 65 mass %). This waste leather is characterized by
the high nitrogen content (14 mass % (waf)) mainly
coming from the proteins (amino acids) of the ani-
mal skin. Bahillo et al. [39] explored the suitability
of fluidized bed combustion as a disposal option for
this kind of waste. They have found N
2
O emissions of
200—250 vol. ppm at FBC temperature about 850
C.
An important decrease of N
2
O emissions to values
about 100 vol. ppm was detected for combustion tem-
perature about 900
C.
Meat and bone meal (MBM) is being destroyed
in the EU due to the possibility of being responsi-
ble for the transmission of spongiform encephalopa-
thy (BSE). Thermal treatments, incineration or gasi-
fication, are considered to be appropriate for the de-
struction of BSE pathogens by allowing enough resi-
dence time and adequate oxygen supply. The MBM
is a fuel with high heat energy value (17 MJ/kg),
with nitrogen content around 7.8 mass % (waf). Ded-
icated incineration plants are being set up in Eng-
land, although the most common method seems to
be co-incineration, mainly in cement kilns [38]. Co-
combustion of Biomal (crushed animal rests) together
with wood chips, sorted municipal waste and peat has
also been tested in fluidized bed boilers [40]. It re-
sulted in relatively low NO
x
emissions and probably
low N
2
O emissions as well. To authors knowledge, no
specific studies concerning N
2
O emissions from MBM
combustion have been published. It could be presumed
that the high Ca-content present in this waste might
act as a catalyst in the reduction of N
2
OtoN
2
.
Nitrogen (up to 5 mass %) is present in the plastic
waste fractions of MSW [41]. Polymers with high ni-
trogen content are nylons/polyamides, polyurethane
foam, and urea-based glues widely used for wood.
Composition and nitrogen content of the mentioned
materials is shown in Table 3. Investigation of com-
bustion of these plastic waste materials in fluidized
bed has confirmed NH
3
,HCN,andN
2
Oamongthe
decomposition products from the plastics.
Co-Combustion of Waste (MSW, Sewage
Sludge, and Biomass)
Substitution of a fraction of coal by another fuel
such as waste or biomass has an important effect on
operation conditions, e.g. temperature distribution,
ash properties (particle sticking and agglomeration
in FBC), and emissions (CO, NO
x
,N
2
O, SO
2
,HCl,
heavy metals, persistent organic pollutants, etc.) [31,
36, 40, 42—48]. Emissions during co-combustion of
coal with various waste and biomass in existing coal
power plants depend generally on waste/biomass com-
position (nitrogen, sulfur, heavy metals, and others)
and content of water. Table 4 compares typical com-
positions of bituminous and sub-bituminous (lignite)
coal with those of wood, MSW, and sewage sludge.
As can be seen, bituminous coal properties (volatiles,
nitrogen, ash contents, etc.) are substantially different
in comparison with waste or biomass-based fuels. Ni-
trogen content of mature, hard biomass (e.g. wood)
is typically very low. Growing biomass (e.g.leaves,
grass) and straw contains somewhat higher percent-
age (0.4—1 mass %) of nitrogen resembling more the
nitrogen content in coals. Nitrogen content of MSW is
similar or slightly lower than in coals. Very high val-
ues (4—10 mass %) of nitrogen are commonly found
in dry sewage sludge. Ash content of MSW and sewage
sludge is relatively high, comparable with lignite coals.
In the case of biomass, it is fairly low although it de-
pends on the type of biomass. Plant, grass and straw-
based biomass has generally higher ash content than
wood.
The effect of waste or biomass-based residues co-
combustion on N
2
O emissions is basically influenced
by i) waste—coal mass ratio, ii) waste composition, iii)
coal composition, particularly volatile and ash con-
tent, and iv) combustion technology (e.g.powdered
coal combustion, FBC, grate or stove combustion).
The combustion technology affects maximum temper-
Chem. Pap. 60 (1) 78—90 (2006) 83
K. SVOBODA, D. BAXTER, J. MARTINEC
Table 4. Typical Composition and Properties of Various Coal-Based and Waste-Based Fuels
Property of fuel (conditions) Hard, bituminous Brown, subbitum. Wood MSW Dried sewage
coal coal sludge
LHV (raw)/(MJ kg
1
) 27 15 12.4 12 10.6
w(Moisture (raw))/% 7.1 36 33 35 3
w(Volatiles (wf))/% 24.8 43 83.2 65 50.5
w(Ash (wf))/% 7.7 24 0.34 30 46.1
w(Fixed C (wf))/% 67.5 33 16.5 5 3.4
w
i
(Ultimate (wf))/%
C 72.5 53.7 48.7 33 25
H 5.6 4.4 5.7 4 4.9
N 1.3 0.8 0.13 0.6 5.2
S 1.0 0.63 0.04 0.2 0.7
Cl 0.1 0.016 < 0.03 1.0 0.1
O 11.1 16.6 45 31 17.7
Ash fusion temp./
C 1280 1200 1100 1150 1000
ature attained during the combustion process and res-
idence time of flue gas [30, 49—52].
As a thumb rule, co-combustion of mixtures of
biomass waste-based fuel and coal with energy input
of the biomass up to 10 % causes slight decrease of
N
2
O emission and only very mild or practically no
operational (mostly ash related) problems. The NO
x
and N
2
O emissions reduction is small in case of lig-
nite and subbituminous coals with high volatile con-
tent [53]. More significant effects are observed for N
2
O
emissions in co-combustion of biomass or waste with
bituminous coals [36, 51].
Principal combustion technologies that are used for
co-combustion of coal with waste, sewage sludge, and
biomass are: Pulverized coal combustion, cycloid com-
bustion, FBC (stationary, circulating, atmospheric,
and pressurized combustion), stove, grate, and rotary
kiln combustion (used mainly for special technologies,
e.g. production of cement). Below, there is a summary
of co-combustion studies of the various wastes and coal
by means of the mentioned technologies. Operation
scale is another important factor. Generally, CO and
N
2
O emissions measured in smaller stoves and smaller
FBC facilities are higher [31, 32, 35, 54] than in bigger
facilities with more uniform temperature distribution
in the furnace and longer residence time of flue gas in
the combustion/incineration chamber.
Co-Combustion of MSW with Coal
The co-combustion is technically difficult due to
the nature of MSW (bigger pieces, content of metals,
ceramics, glass, etc.). This is the reason why practi-
cally only co-combustion of coal with sorted MSW or
with refused derived fuels (RDF) containing mainly
plastics, paper, and textile has been studied [43, 48,
52, 55]. The RDF contains approximately 0.7 mass %
nitrogen and ash content is usually lower than 10
mass %. Due to special properties and composition of
MSW, the main technology applied for mixed MSW is
grate combustion with proper, sophisticated flue gas
cleaning [20]. Co-combustion of sorted MSW or RDF
in cement production rotary kilns is possible under
assumption that especially chlorine and heavy metals
contents are limited and guaranteed. Due to very high
temperature at least in the part of cement-producing
rotary kiln, the N
2
O emissions are practically negligi-
ble, but NO
x
emissions are very significant.
Co-Combustion of Biomass, RDF, and Sewage
Sludge in Pulverized Coal Combustion
Due to relatively high temperature in a pulverized
coal combustion furnace, the emissions of nitrous ox-
ide are very low. Spliethoff and Hein [44] found N
2
O
emissions below 10 vol. ppm during pulverized coal
co-combustion with biomass. The main parameters af-
fecting N
2
O emissions are nitrogen content in waste
biomass, construction of burners, and biomass feed-
ing system (separate or mixed with coal dust). Simi-
lar emission can be expected from RDF with nitrogen
content below 1 mass %. Slightly higher N
2
Oemis-
sions can be supposed in pulverized and cycloid co-
combustion of waste with high content of nitrogen
(e.g. dried sewage sludge, MBM, etc.).
Co-Combustion of Biomass and RDF
in Fluidized Bed (FB)
The generally observed trend in N
2
O emissions
from any fluidized bed-based co-combustion technol-
ogy of coal and mature biomass or RDF with low ni-
trogen content is reduction of N
2
O emissions [51, 43,
48]. The reduction of the nitrous oxide emissions is
more pronounced in the case of hard (bituminous coal)
co-combustion with biomass than for lignite coals with
higher content of volatiles. The effect of biomass FB
co-combustion on N
2
O emissions is weaker for reactive
lignite coals with high content of volatiles and with
coal ash containing catalytically active compounds
84
Chem.Pap.60(1)78—90 (2006)
NITROUS OXIDE EMISSIONS
Fig. 3. Dependence of N
2
O emissions in FB co-combustion of
wet sewage sludge (SS) with bituminous coal on frac-
tion of wet sewage sludge in the mixed fuel under dif-
ferent conditions for limestone desulfurization (effect of
Ca/S mole ratio) [57]. Ca/S = 0
; Ca/S = 3 .
(e.g. CaO, Fe
2
O
3
, etc.) causing low emissions in FB
single combustion of the reactive coal [49, 53].
Very special kind of biomass waste is the newly
produced meat and bone meal (MBM) and other
animal waste such as “Biomal”(crushed, thermally
treated, pulpy animal waste). Co-combustion of both
kinds of animal waste differing substantially in wa-
ter content and calorific value is possible in fluidized
bed boilers. The relatively high content of nitrogen
in such waste (around 7 mass % in dry matter) can
lead to higher N
2
OandNO
x
emissions. But, as practi-
cal FB co-combustion experience with wood and sub-
bituminous coal has shown [40, 56], the NO
x
,CO,and
N
2
O emissions are below 100 mg (norm. m)
3
in FB
co-combustion with wood and lignite coal, probably
due to higher content of calcium and special thermal
behaviour of the animal waste nitrogen.
Co-Combustion of Sewage Sludge in
Atmospheric FB
Sewage sludge can be found in three different forms
concerning water content and pretreatment: wet, dry,
and semi-dry. N
2
O emissions from co-combustion of
wet and dry sewage sludge with coal and wood in FB
combustion are shown in Figs. 3 and 4.
Co-combustion of wet sludge with bituminous coal
in atmospheric circulating fluidized bed combustor
leads to an increase in N
2
O emissions [36, 57]. Main
parameters affecting N
2
O emission are: nitrogen con-
tent in the sludge, coal characteristics, coal ash com-
position, limestone addition for in situ desulfurization,
freeboard temperature distribution, and gas residence
time (gas velocity). Increasing bed and freeboard tem-
perature results in reduction of N
2
O emissions. Never-
theless, CO and N
2
O can be as significant as 100 vol.
ppm or even 200 vol. ppm.
Co-combustion of semi-dried and dried sewage
sludge in atmospheric fluidized bed is known to pro-
duce higher NO
x
emissions than in the case of FB
Fig. 4. Dependence of N
2
O emissions in FB co-combustion of
dry sewage sludge with bituminous coal and wood on
sludge supply expressed as fraction of total energy input
of the mixed fuel [46]. Coal-sewage sludge
; wood-
sewage sludge
.
co-combustion of wet sewage sludge [35, 36, 46]. The
N
2
O emissions in semi-dried sludge co-combustion are
generally lower than in wet sludge co-combustion, but
still on the levels 20—100 vol. ppm. FB co-combustion
of dried sewage sludge with bituminous coals leads to
slight elevation of N
2
O emissions on levels around 50
vol. ppm. Simultaneous wood and dried sewage sludge
FB co-combustion with bituminous coal can offer fur-
ther reduction of N
2
O emission on levels between 10
and 40 vol. ppm [46]. Similar results and effects can
be expected in FB co-combustion of dried sewage
sludge with lignite coals. Higher freeboard tempera-
ture (over 900—930
C) is proved to be a relatively
simple method for reduction of N
2
O emissions. N
2
O
concentrations below 20 vol. ppm in flue gas are at-
tainable at freeboard temperatures above 930
C. Un-
fortunately, NO
x
emissions are relatively high in FB
co-combustion with high dried sludge/coal mass ratio,
even under convenient air staging conditions.
Co-Incineration of Sewage Sludge in MSW
Incinerators
It is suitable and employed technology for thermal
destruction of sewage sludge especially in Germany,
where the gas cleaning in MSW incineration is suffi-
ciently dimensioned and flexible to manage higher con-
centrations of heavy metals and nitrogen oxides [36].
Due to high temperature in post combustion cham-
ber (typically over 900
C) the N
2
O emissions are only
slightly elevated and are generally low (below 20 vol.
ppm) in sewage sludge co-incineration with MSW.
SNCR as Source of N
2
O Emissions
Brief Description of SNCR and Factors Affect-
ing its Efficiency
Selective noncatalytic reduction (SNCR) is a wide-
spread secondary measure for NO
x
control. In this
Chem. Pap. 60 (1) 78—90 (2006) 85
K. SVOBODA, D. BAXTER, J. MARTINEC
process NO is reduced to N
2
, in the presence of O
2
,
by injection of a reducing agent, such as ammonia,
urea or cyanuric acid. The process is characterized by
a selectivity in the reaction pathways as shown by the
overall steps [58—60]
4NH
3
+4NO+O
2
4N
2
+6H
2
O(A)
4NH
3
+5O
2
4NO + 6H
2
O(B)
The selectivity towards N
2
or NO (reactions (A)or
(B)) mainly depends on temperature and gas compo-
sition. The optimum temperature for the reduction to
N
2
is between 900
C and 1000
C. Oxidation of NH
3
to NO becomes increasingly important when the max-
imum temperature is exceeded. Below the optimum
temperature window the selective reduction reactions
are too slow, resulting in undesired high ammonia slip.
Other factors affecting process efficiency are ammonia
stoichiometry to NO (i.e. NH
3
/NO mole ratio), con-
centrations of oxygen, CO and H
2
O in flue gas, and
residence time.
Sufficient residence time within the temperature
range is required for allowing chemical reactions and
for proper mixing between the reagent and flue gas.
Certainly, when reactions are fast, mixing may be
the rate-limiting step. The minimum residence time
is around 0.2— 0.5 s [58]. Residence times in excess of
1 s yield optimum NO
x
reductions.
Concerning the O
2
concentration, both very high
concentrations in flue gas (over 9 vol. %) and very low
ones (typically below 2 vol. %) were found to be an ob-
stacle for attaining high NO reduction efficiencies [59].
At increasing oxygen concentrations, the temperature
range for NO reduction is widened (mainly towards
lower temperatures), but the NO reduction potential
decreases [60].
Increasing CO concentration in flue gas shifts the
selectivity towards oxidation of NH
3
to NO, as well
as increases the kinetic rate of this reaction. At
higher CO concentrations, the optimum temperature
for SNCR is displaced towards lower temperatures
[61].
Conversion of NO
x
to N
2
O
Wojtowicz et al. [9] reported conversion ratios of
m(N
2
O)/m(NO
x
) between 5 % and 50 % for coal com-
bustion. However, the last figure seems high compared
with later studies. Thus, the maximum conversions of
NO
x
to nitrous oxide under well-controlled conditions
of SNCR with ammonia are below 5 % and in many
cases below 2 % [62].
Utilization of urea instead of ammonia as reducing
agent leads to a higher portion of NO converted into
N
2
O (typically > 10 %). In terms of the formation
mechanism, studies suggested that HNCO (one of the
products of the initial decomposition of urea) reacts
according to the following sequence [14]
HNCO + OH NCO+H
2
O(C)
NCO+NO N
2
O+CO (D)
HNCO + H NH
2
+CO (E)
Comparison of urea and cyanuric acid as reducing
agents shows that urea generates less N
2
O under
equivalent conditions. This is consistent with the pre-
vious reaction mechanism, as all cyanuric acid is con-
verted to HNCO after injection.
Under poorly controlled SNCR conditions, such as
too high temperature with oxidation of surplus ammo-
nia and with quick quenching of gases (low residence
times for thermal destruction of N
2
O) the conversions
to N
2
O may attain 20—25 %. The use of promot-
ers (CO, C
2
H
6
) to shift the optimum temperature to
lower one appears to increase the fraction of NO con-
verted into N
2
O [14].
Selective Catalytic Reduction (SCR) as
Source of N
2
O Emissions
Brief Description of SCR and Factors Affecting
its Efficiency
Depending on nitrogen content in waste or biomass
and on combustion conditions, particularly tempera-
ture and the NO
x
concentrations in flue gas can vary
in a broad range. For combustion of waste/biomass
with high content of nitrogen (above 1—2 mass %)
and for combustion processes with temperatures above
1000
CtheNO
x
concentrations in flue gas can be rel-
atively high, over 300 mg (norm. m)
3
[36]. This is
especially true for FB combustion or co-combustion of
dry sewage sludge or some waste with high N-content
and low moisture content (< 10 %). For reduction
of NO
x
(NO+NO
2
) emissions either selective non-
catalytic reduction (SNCR) or selective catalytic re-
duction (SCR) by ammonia can be used. The SCR
achieves a more efficient reduction of NO
x
,withcom-
mon conversions of NO
x
between 80 % and 90 %.
The usual position of SCR in the flue gas cleaning
system of an MSW incinerator is behind wet gas clean-
ing system (scrubbers + demister). Only in excep-
tional cases it is located behind electrostatic precipita-
tor (ESP). The position of SCR behind wet cleaning of
flue gas has advantage of lower concentration of dust,
heavy metals, SO
2
, and HCl. It means, under such
conditions, the catalyst in SCR technology will be less
deactivated (poisoned) by deposition of fine dust, de-
position of volatile compounds of heavy metals (esp.
arsenic compounds), and by deposition of ammonium
salts, like ammonium sulfate and bisulfate.
A disadvantage of the position of SCR behind
wet gas cleaning lies in requirements on reheating
86
Chem.Pap.60(1) 78—90 (2006)
NITROUS OXIDE EMISSIONS
of gases. The older (former) generation of de-NO
x
catalysts (mostly based on TiO
2
—V
2
O
5
—WO
3
)had
usual range of operating temperature between 250
C
and 350
C. The newer generation of SCR catalysts is
characterized by lower operating temperature about
200
C (with less energy necessary for reheating of
gases). Such low-temperature SCR is usually coupled
with lower emissions of N
2
O, but on the other hand,
relatively low temperatures can bring some difficulties
with formation and deposition of ammonium salts on
the surface of the catalyst.
Overall Reactions for NO
x
Reduction and Side
Reactions Leading to N
2
O Formation
Selective catalytic reduction of NO
x
is mainly
based on reactions (F )and(G) where, particularly
at temperatures between 200
C and 250
C, the reac-
tion (G) is faster than reaction (F ) [63, 64].
4NH
3
+4NO+O
2
4N
2
+6H
2
O(F)
4NH
3
+2NO+2NO
2
4N
2
+6H
2
O(G)
The NO
2
/NO mole ratio increases with decreasing
temperature, but for the sufficiently high NO
2
/NO
ratio catalytic oxidation is needed upstream the SCR
catalyst [64]. In laboratory studies a number of side
reactions in SCR leading to N
2
O formation have been
identified. Temperature is one of the most important
parameters influencing the potential mechanisms.
At high temperatures (> 350
C), where the frac-
tion of NO
2
is low and at low space velocities of gas,
N
2
O can be formed mainly according to reaction (H)
[65]. For SCR catalyst based on V
2
O
5
—TiO
2
the se-
lectivity of N
2
O formation from NO
x
is increasing un-
der temperatures over 350
C.
4NH
3
+4NO+3O
2
4N
2
O+6H
2
O(H )
In addition, ammonia can be partially oxidized to N
2
O
at temperatures over approximately 350
C according
to the following reaction [65]
4NH
3
+4O
2
2N
2
O+6H
2
O(I )
At intermediate temperatures (250—350
C) the fol-
lowing reactions in the presence of NO
2
can be sup-
posed to be potentially significant source of N
2
O
6NH
3
+8NO
2
7N
2
O+9H
2
O(J)
4NH
3
+4NO
2
+O
2
4N
2
O+6H
2
O(K )
At low temperatures (< 250
C) and higher NO
2
/NO
ratio, reaction mechanism involving transient forma-
tion of NH
4
NO
3
and its decomposition to N
2
and NO
is supposed [65]
2NH
3
+2NO
2
NH
4
NO
3
+N
2
+H
2
O(L)
NH
4
NO
3
+NO
2
3NO+2H
2
O(M )
At temperatures below 200
C the formation and de-
position of solid ammonium nitrate on SCR catalyst
according to reaction (L) was experimentally demon-
strated [63]. Presence of water vapour and slow heat-
ing rates causes preferable formation of HNO
3
and
NH
3
in decomposition of NH
4
NO
3
(i.e.lowerN
2
Ofor-
mation selectivity)
NH
4
NO
3
HNO
3
+NH
3
(N )
Conversion of NO
x
to N
2
O in Big-Scale SCR
Units
The catalytic activity of catalysts based on TiO
2
V
2
O
5
—WO
3
depends mainly on content, interactions,
surface, and catalytic properties of vanadium pen-
toxide (V
2
O
5
). Higher concentrations of V
2
O
5
in the
catalyst usually cause higher catalytic activity in de-
NO
x
, but simultaneously higher sensitivity to deac-
tivation at expositions to higher temperatures (espe-
cially above 400
C). At higher concentrations of V
2
O
5
in the catalyst, it seems that the conversion of NO
x
to N
2
O in SCR is higher [66].
The conversion of nitrogen from NO
x
to N
2
Oin
SCR is dependent mainly on temperature [65, 67], cat-
alyst ageing (deactivation), and water vapour concen-
tration in the gas [68]. At temperatures 250—300
C
the conversions of nitrogen from NO
x
to N
2
Oarebe-
low 5 %, at temperatures below 250
C(formodern
new catalysts) the conversion to N
2
Oislowerthan
3%.InmodernSCRunitswithfreshcatalystand
water vapour concentration above 4 vol. % the con-
version of NO
x
to N
2
O is commonly below 1 % [24,
68]. The conversions of NO
x
to N
2
O in SCR units are
summarized in Table 5.
From various catalyst and carrier compositions
the best performance and lowest N
2
O concentrations
have been attained by V
2
O
5
—WO
3
—TiO
2
-based cat-
alysts [67]. For low-temperature SCR (< 200
C) var-
ious new catalysts on the basis of Fe—Mn oxides
on TiO
2
and Mn—Ce—O
x
oxides have been tested
[69, 70]. The new catalysts proved very low N
2
O
formation selectivities. At low temperatures (150—
200
C) carbon impregnated with Cu or Cu—Fe ex-
hibits very high catalytic activity in SCR reduction
of NO
x
, higher than conventional catalysts [71], but
the big-scale applications and selectivities for N
2
Ofor-
mation are not described. Zeolitic catalysts [72—74]
for destroying of N
2
O at higher temperatures are in-
tended mainly for industrial processes (e.g. produc-
tion of nitric acid). Their working temperature is
normally above 300
C or even above 350
C. From
the point of view of application in modern waste
Chem. Pap. 60 (1) 78—90 (2006) 87
K. SVOBODA, D. BAXTER, J. MARTINEC
Table 5. Approximate Conversions of NO
x
to N
2
O [67, 68]
Temperature Conversion of NO
x
Notice, catalyst, conditions, etc.
range/
CtoN
2
OinSCR
x(NO
x
)/mole %
< 180 > 1 Inconvenient temperature range, formation and deposition of
ammonium salts.
180—220 0.7—2 Low-temperature SCR, higher NO
2
/NO ratio needed, N
2
O
emission increases with NO
2
/NO ratio over 0.6.
220—300 1—3 Classical SCR in MSWI high energy needed for gas reheat-
ing, older generation of SCR catalysts.
300—400 < 5 Inconvenient SCR for MSW and waste incineration (higher
costs for gas reheating, higher N
2
O emissions).
incineration units the needed temperature seems to
be too high. Another disadvantage of such catalysts
is sensitivity to some acid gases (e.g.SO
2
,HCl,
etc.).
Side Effects of SCR of NO
x
The SCR can simultaneously oxidize reactive hy-
drocarbons and partly oxidize even benzene, toluene,
and PAH, where the conversions depend on catalyst
composition, operating temperature, etc. [75—77]. At
lower temperatures (< 180
C) depending on NO
2
and
NH
3
concentrations in flue gas [64] ammonium nitrate
can be formed and deposited within catalyst. Simul-
taneously, at presence of SO
2
in the gas at concentra-
tions above about 10 mg/m
3
the probability of forma-
tion and deposition of ammonium bisulfate and sulfate
increases. In the case of SO
3
(H
2
SO
4
) presence in flue
gas, even very small concentrations (1—3 vol. ppm)
can contribute to formation of the ammonium salts
and their deposition. The deposited ammonium salts
are decomposed by reaction with NO under absence
of NH
3
in gas
2NO+
1
/
2
O
2
+2NH
4
HSO
4
2N
2
+3H
2
O+2H
2
SO
4
(O)
Such decomposition, as it is obvious, leads tem-
porarily to higher sulfuric acid concentration in flue
gas and higher corrosion in the units (e.g. heat ex-
changers) behind the SCR.
Multifunctional Catalytic Filters (MCF’s)
The MCF’s are designed for simultaneous reduc-
tion of dust emissions together with catalytic destruc-
tion of NO
x
, volatile organic compounds (VOC) and
polychlorinated organic compounds (PCDD/F) emis-
sions [78—80]. The catalyst dispersed into the filter
structure, i.e. fibers, grains or felt, has to be multi-
functional, often modified in comparison with com-
mon SCR catalyst. Catalytic oxidation of VOC’s can
be elevated by inclusion of Ni, Cu, Mn or other cat-
alytically active metals. There are two basic kinds of
MCF’s used in MSW and other waste incineration fa-
cilities:
a) Teflon (PTFE) based felt filters with membrane
[78, 81]. Such filters have operating temperature typ-
ically between 180
C and 230
C.
b) Ceramics fiber based MCF’s [80, 82] with possi-
ble operating temperature between 220
C and 350
C.
Advantage of such ceramic MCF’s is possibility of re-
moval of deposits of volatile inorganic/organic com-
pounds from the filters by short-time heating to tem-
peratures over 500
C.
The MCF’s have usually somewhat different op-
timum temperature range for SCR of NO
x
and de-
struction of PCDD/F and/or VOC’s. Therefore, their
operating temperature is mostly a matter of compro-
mise and acceptable conversions of all catalytic de-
structions. The selectivities for N
2
O formation in such
MCF’s are not stated in open literature, but probably,
the conversions to N
2
O are relatively low.
In reality, in big, modern plants for MSWI the SCR
of NO
x
contributes typically only by a few vol. ppm
to elevation of N
2
O concentrations in flue gas [24].
CONCLUSION
1. MSWI and biomass incineration in big-scale fa-
cilities with proper controlling of the combustion pro-
cess are negligible sources of N
2
O emissions.
2. Combustion of biomass and waste in small (esp.
stove, grate, and FB) facilities without secondary com-
bustion chamber and at lower freeboard temperature
leads usually to higher CO and N
2
O emissions [31,
32].
3. Mono-combustion and co-combustion of sewage
sludge and waste with high N-content (> 3mass%)
under temperatures lower than approx. 900
C(typi-
cally FB combustion and co-combustion) can be rel-
atively significant source of N
2
O emissions. Temper-
atures over 900
C cause substantial decrease of N
2
O
emissions and they are widely practically employed for
reduction of N
2
O emissions.
4. Co-combustion of common biomass and RDF in
pulverized and FB coal combustion facilities leads to
88
Chem.Pap.60(1) 78—90 (2006)
NITROUS OXIDE EMISSIONS
reduction of N
2
O emissions, particularly for bitumi-
nous coals with lower content of volatiles.
5. SNCR and SCR of NO
x
by ammonia under care-
fully controlled conditions is elevating only slightly the
N
2
O emissions in MSW, waste and biomass combus-
tion units. The conversions of NO
x
to N
2
O are usually
lower than 5 % (or the N
2
O concentration contribu-
tion is in units of vol. ppm).
6.SNCRofNO
x
by urea is practically the only sig-
nificant source of “de novo” created N
2
O emissions in
MSW and waste/biomass incineration facilities. Nev-
ertheless, the conversions of NO
x
to N
2
O are usually
lower than 20 % (usual contribution in content in flue
gas being in tens of vol. ppm).
Acknowledgements. The authors appreciate nancial sup-
ports of the European Commission and the Gr ant Agency of
AS CR, Project No. A 4072201.
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