Post on 23-Dec-2021
267
[Japanese Journal of Water Treatment Biology Vol.34 No.4,267-2771998]
Emission Characteristics of Greenhouse Gas N2O
from Sewage Sludge Incineration Process
MOTOYUKI MIZUOCHI1, KAZUAKI SATO2, YUHEI INAMORI
1 and MASATOSHI MATSUMURA3
1National Institute for Environmental Studies
/16-2, Onogawa, Tsukuba, Ibaraki,305-0053, Japan2Public Works Research Institute
/1, Asahi, Tsukuba, Ibaraki, 305-0006, Japan3Institute of Applied Biochemistry
, Tsukuba University
/1-1-1,Tennoudai, Tsukuba, Ibaraki,305-0006, Japan
Abstract
N2O is one of the important greenhouse gas, and it has been pointed out that its contribu-
tion to global warming will increase in the future. In the past, higher level concentration of
N2O emission was reported for some sewage sludge incinerators than at other fixed emission
sources including a solid waste incinerator. Therefore, we investigated the practical factors of
N2O emission from the sewage sludge incineration originated from the difference of incinerator
type and coagulant type used for dehydration. As a result, it was cleared that emitted amount
of N2O is strongly depended on the incineration temperature and the coagulant type used for
dehydration process. Amounts of N2O emitted from sewage sludge incineration in FY 1995 in
Japan was estimated at 4030t on a nitrogen basis and thought to more increase to the future.
Key words: Sewage sludge, Incineration, N2O, Coagulant, Sludge treatment
INTRODUCTION
Nitrous oxide(N2O)is, along with carbon
dioxide(CO2), a major greenhouse gas, and
because its radiative forcing per molecule is
greater than that of CO2, regardless of the fact that its present concentration is slight
compared with CO2, it has great latent
capacity to contribute to the greenhouse
effect. It is pointed out that the NZO share
of the greenhouse effect has increased,
particularly in recent years1). But much is still unknown concerning its sources and the
quantities emitted by each source, and there are many small scale sources2). The
Framework Convention on Climate Change
that took effect in March 1994 is premised
on the adoption of a basket approach that
sets reduction targets for all greenhouse
gasses using the global warming potential for emission control of greenhouse gasses. At the
Third Session of the Conference of the
Parties to the United Nations Framework
Convention on Climate Change (COP 3),
held in Kyoto, December 1997, targets were
set not only for CO2, but for CH4, N2O,
HFC, PFC, and SF63).
In 1995,1689 X 103t(dry weight)of
sewage sludge was produced in Japan 4),
and its production volume has been
268 Japanese J. Wat. Treat. Biol. Vol.34 No.4
increasing along with the spread of
wastewater systems 5). The percentage of this
sewage sludge incinerated stands at 63%,
indicating that the sewage sludge
incineration rate in Japan is far more than
the average of 11%in the nations of Europe
and 25%in the United States6). And of the
sewage sludge that is not incinerated,20
is converted to sludge cake and disposed of
at landfill sites. But considering the chronic
shortage of space for landfill in Japan, the
percentage of sewage sludge incinerated is forecasted to continue to rise.
On the other hand, cases of measurements
of N2O from sewage sludge incinerators in
higher concentrations than found at other
stationary emission sources such as solid
waste incinerators have been reported7,8
).Incinerators used to incinerate sewage
sludge are broadly classified as either
multiple-hearth incinerators or fluidized bed
incinerators. Fluidized bed incineration has
been rising in recent years. Sewage sludge
is incinerated as sludge cake, and the
coagulant used during dehydration is also
broadly classified as either polymer
coagulant, or as lime type coagulants
containing calcium hydroxide(Ca(OH)2)and
ferric chloride(FeCl3), and recently the use
of polymer coagulants has been increasing
because they are easier to maintain. In this
report, sludge dehydrated using polymer
coagulant will be called polymer cake and
sludge dehydrated with lime type coagulant
will be referred to as lime cake.
This study was undertaken to survey the
amounts of N2O released from the
incineration of sewage sludge focused on
types of incinerators and dehydrating
coagulants used to enhance the dehydration
of the sewage sludge. Then based on the
results of the survey, the amounts of N2
O emitted from sewage sludge incinerators in
Japan were calculated and future release amounts were forecasted.
METHOD
The survey encompassed 14 incinerators at
ll treatment plants. Table l shows the
relationship between the types of incinerators
surveyed and the type of sludge cake
incinerated in them.
Exhaust gas was taken near the
incinerator outlets, and to study the effe cts
of exhaust gas treatment, it was also taken
from the outlets of the chimneys after it had
passed through exhaust gas treatment
systems. The collection of exhaust gas
samples, the measurement of the quantities
of exhaust gas, and the analysis of the
oxygen(O2), carbon monoxide(CO), CO2,
NOX, and HCN concentrations in the exhaust
gas were all performed in conformity with
JIS methods9). The N2O was analyzed by gas
chromatography e quipped with ECD. The
exhaust gas samples used for N2O analysis
were passed through magnesium perchlorate
(Mg2(ClO)4)and soda asbestos to remove
water(H2O)and sulphur dioxide(SO2)in
order to avoid the effects of the production
Table l The relationship between the types of incineators surveyed
and the type of sludge cake incinerated in them
Emission Characteristics of Greenhouse Gas N2O from Sewage Sludge Incineration Process 269
of N2O through the reaction of H2O, SO2,
and nitrogen monoxide(NO)in the exhaust
gas:areaction that is assumed to occur on
the interior surface of the sampling bag
(tedlar bag)while the gas sample is stored
in it10,11).
Fig.1 shows the sampling method. And to
avoid the effects of O2, CO2, and water that
are present in the gas and have a big effect
on the precision of N2O analysis, precut
mechanisms were installed on each gas
chromatography system and an adsorption
mechanism consisting of a pre-column filled
with magnesium perchlorate and soda
asbestos was installed in front of the injection
port. Table 2 shows the N2O measurement
conditions.
The volatile solids(VS), moisture content,
and nitrogen content of the sludge cake fed
into the surveyed incinerators were analyzed
according to the Wastewater Test Methods12).
RESULT AND DISCUSSION
State of Emissions from Sewage Sludge
Incinerators Table 3 shows the measurement
results. The N2O concentration shown in the
table is the concentration at smokestack
outlets. The N2O concentration ranged from
88to 430 ppmv in the case of polymer cake
incineration, and range from 31 to 99 ppmv
for lime cake. The N2O concentration in the
case of polymer cake incineration tended to
be higher fbr that incinerated in fluidized bed
incinerators than that was in multiple-hearth
incinerators.
The N2O conversion rate is the percentage
of nitrogen in the sludge cake converted to
N2O by the incineration, assuming that there
Fig.1 Sampling method for N2O analysis
Table 2 Analytical condition of N2O
270 Japanese J. Wat. Treat. Biol. Vol.34 No.4
Table 3 The measurement results of this investigation
*EGT:Exhaust Gas Treatment system
is no thermal formation of N2O from nitrogen
in the air during the incineration. In a
polymer cake incineration case, this rate
ranges from 5.9%to 18%and which is
higher than 1.3%to 3.5%obtained for lime
cake, as in the case of N2O concentration i n
the exhaust gas. And the rate obtained from
polymer cake incinerated in a fluidized bed
incinerator was also high, ranging from 6.1
to 18%.
Fig.2 shows the relationship between the
conversion rates of N2O and NOX. The NOX
conversion rate was found in the same way
as the N2O conversion rate. The NOX
concentration measured in exhaust gas
generated from fluidized bed incinerators
incinerating polymer cake was extremely low
at less than 5 ppmv, with the result that the
conversion rate to NOX ranged from 0.1
to 0.5%, far less than the conversion rate to
N2O. On the other hand, the NOx
concentrations at fluidized bed incinerators
incinerating lime cake were high at 100 and
200ppmv respectively and the conversion
rates were also high at 2.5%and 3%,
revealing that the conversion rate to N2O
traded off with the conversion rate to NOX.
The incineration temperature of sewage
sludge in ordinary fluidized bed incinerators
is approximately 800℃. It is known that
highly volatile nitrogenous compounds are
generated via HCN and NCO in combustion
of coal at near 800℃ of temperature, and
that HCN and NCO are precursors of N2
O of a homogeneous reaction in gaseous
phase13). It is also known that most of the
nitrogen constituent included in sewage
sludge becomes volatile under a temperature
near 800℃14), and it is assumed that a
reaction similar to that which occurs in the
coal combustion process occurs in the
sewage sludge combustion process. But it is
also claimed that in this reaction route, the
production of NO from NCO is dominant
because of the presence of oxides. It has
been reported that in the case of fluidized
combustion of coal, calcium oxide (CaO)
originally placed inside the furnace to restrict
the production of sulphur dioxide (SO2)has
an oxidizing action that limits the production
Emission Characteristics of Greenhouse Gas N2O from Sewage Sludge Incineration Process 271
Fig.2 Relationship between conversion rate of N2O and NOX. Symbols:○,fluidised
bed(polymer cake);●, fluidised bed(lime cake);□, multiple-hearth(polymer
cake);■, multiple-hearth(lime cake)
of N2O and inversely, encourages the
production of NO15). In the lime sludge case,
the average Ca(OH)2 content is about 30
% and combustion converts this to CaO that
acts as an oxide.
The efficiency of exhaust gas treatment is
the ratio of the quantity of N2O removed by
the exhaust gas treatment process to the
quantity of emitted N2O measured near the
incinerator outlet, assuming that the quantity
of N2O removed by the above process is the
difference between the quantity of emitted
N2O measured near the incinerator outlet
and the quantity of emitted N2O measured
after the exhaust gas treatment process.
The results on the N2O removal efficiency
were between 10%and 30%excluding a
negative result from treatment plant E.
Fig.3 shows the exhaust gas treatment
processes generally used in multiple-hearth
and fluidized bed incinerators. Because N2
O is relatively highly soluble in water, it is
assumed that the N20 is removed by
scrubbers that use a spray system to
perform gas-liquid contact in order to
eliminate dust or acidic gasses such as SO2
or HCI from exhaust gas in both fluidized
bed and multiple-hearth incinerators.
Because scrubbing wastewater from fluidized
bed incinerators at both treatment plants H
and J dissolved a large part of the N2O that
was removed by the exhaust gas treatment
processes, it was assumed that N2O removal
is primarily done by scrubbers. But because
scrubbing wastewater is normally returned
to a water treatment process that is operated
under aerobic conditions, the dissolved N2
O is assumed to be discharged again by
aeration without anaerobically biological
reduction to nitrogen gas.
Fig.4 shows the results of a study of the
effects of combustion temperature on the
N2O conversion rate. In an sewage sludge
incinerator, the interior furnace temperature
is monitored as the combustion temperature.
In the case of fluidized bed incinerator where
272 Japanese J. Wat. Treat. Biol. Vol.34 No.4
Fig.3 General exhaust gas treatment process for multiple hearth and fluidized bed incinerator
Fig.4 The effects of combustion temperature on the N2O conversion rate of investigated
fluidized bed incinerators. Symbols:○, polymer cake;●, lime cake
sludge is incinerated after being mixed with
sand shaped fluid medium, so the interior
furnace temperature is almost identical to the
combustion temperature. But in the case of
multiple-hearth incinerator where the sludge
cake fed into the incinerator is incinerated as
it moves to the bottom retaining its lumpy
form, the interior furnace temperature is
lower than that of the part where combustion
occurs. Therefore, Fig.4 presents only the
results for fluidized bed incinerators. The
measured temperature of combustion ranged
Emission Characteristics of Greenhouse Gas N2O from Sewage Sludge Incineration Process 273
from 770 to 860℃. And it was found that
in a polymer cake, the conversion rate
tended to decline as the combustion
temperature risen. It is reported that in the
case of fluidized bed coal combustion, N2
O formation is maximized near 730℃16), and the
same tendency was observed in the case of
fluidized combustion of sewage sludge.
Estimation of Annual Emissions of NaO by
Sewage Sludge lncineration A total of 241
sewage sludge incinerators were operated in
Japan in 1995, an o tese,142 were
fluidized bed incinerators, 64 were
multiple-hearth incinerators,12 were rotary
kilns, and 20 were step grate stoker
incinerators. Three were other kinds. Table 4
shows the results of finding the N2O
conversion rates in each type of incinerator
and of incinerated sludge cake type based on
the results listed in Table 3. The N20
conversion rates presented in Table 4 were
obtained by performing a simple averaging
of the results. The following formula was
applied to find the amounts of NZO emitted
from each treatment plant where sludge was
incinerated, and these were totalled to obtain
the estimation of annual amounts of N2
O emitted in Japan. Because no survey has
been performed for rotary kilns or for step
grate stoker incinerators, it was assumed
that N2O conversion rates in these
incinerators are identical to those for
fluidized bed incinerators and multiple-hearth
incinerators, respectively.
Emitted quantity=(total sludge cake
incinerated in a single year)X(1-sludge
cake water content)X(nitrogen content)X
(average N2O conversion rate)
The quantity of sludge cake incinerated
and its water content were based on
Sewerage Statistics17). The nitrogen content
was calculated based on a relational formula
for the VS and nitrogen content of sludge
cake found from results of a survey of sludge
cake at 59 wastewater treatment plants in
Japan18,19). The quantity of VS of each sludge
cake was also based on Sewerage Statistics.
Fig.5 shows the release amounts of N2O
from fluidized bed incinerators, multiple-hearth
incinerators, rotary kilns, and step grate
stocker incinerators organized by polymer
cake and lime cake. Fig.6 shows the
emission percentage in each incinerator type.
The results of this survey did not take into
account N20 removed by exhaust gas
treatment systems, particularly scrubbers.
However, because it is possible that the
N20 absorbed in a scrubbing water is
discharged again during the water treatment
process, the estimation of the annual release
amounts did not account for the reduction
performed by exhaust gas treatment
systems.
The total amounts of N2O emitted by
sewage sludge incineration in Japan during
1995is estimated at about 4,030 t on a
nitrogen base, and 77%of this was emitted
from fluidized bed incinerators. The
Table 4 N2O conversion rate used to estimate release amounts of N2O from
each type of incinerators
274 Japanese J. Wat. Treat. Biol. Vol.34 No.4
Fig.5 The release amounts of N2O from fluidized bed incinerators, multiple-hearth
incinerators, rotary kilns, and step grate stocker incinerators organized by polymer
cake and lime cake in Japan FY 1995. Symbols:★, polymer cake;★, lime cake
Fig.6 The emission percentage by incinerator type.
population served by wastewater treatment
systems in 1995 was 66.8 million people 4),
and 63%of generated sewage sludge was
incinerated. Therefore the quantity of N2O
that would be emitted annually per person,
if the sludge generated is incinerated, was
calculated as 96g assuming that the
quantity of sludge produced per member of
this population group is the same.
According to the“Japan's Second National
Communication under United Nations
Framework Convention on Climate
Change”20), which is a greenhouse gas
sources/sinks list presented to the
Intergovernmental Panel on Climate change
(IPCC)in 1997, the quantity of anthropoginic
N2O emitted in Japan in 1994 was estimated
as 70,000 ton a nitrogen base. Assuming that,
although the estimation year was 1995 for
sewage sludge incinerators and 1994 for
Emission Characteristics of Greenhouse Gas N2O from Sewage Sludge Incineration Process 275
Japan overall, there is no significant
difference between the two annual estimation
for N2O emission from incineration of sewage
sludge, the N2O emission from sewage sludge
incineration share was approximately 6% of
total annual N2O emission in Japan.
Forecasting Future Annual Emissions of NaO
by Sewage Sludge Incineration In the
December 1997, COP3 where agreement of
greenhouse gas reduction was established, the target period was set as 2008-2012, and
reduction rates set with 1990 as the
reference year.
In 1995,54 % of the population was
sewered in Japan4), and this percentage for
the last ten years has risen by between 1.5
and 2%per year. Assuming that it will
continue to rise about 2 % per year, the
percentage of sewered population will be 79
% by 2008. More than 60%of sewage sludge
produced is now incinerated, and considering
the pressing shortage of space for landfill
disposal, the percentage of sewage sludge
incinerated is sure to continue to rise.
About 10%of sewage sludge is now used
on green belt or agricultural land as compost
or dried sludge. Considering a such situation
about the sewage, it is calculated that 8,500
t of N2O on a nitrogen base will be emitted
as a product of sewage sludge incineration in
2008,assuming that the sewered population
grows at an annual rate of 2%as it does
now, that the percentage of sludge disposed
of on green belt and agricultural land
remains at its present 10%, and that the
remainder of the sludge produced is
incinerated. So in 2008, the year that the
greenhouse gas reduction target period
begins, sewage sludge incineration alone will
increase the total amount of anthropoginic
N20 emitted in Japan by about 6%over
levels in 1994.
CONCLUSIONS
The following results were obtained from
a survey of the state of N20 emissions from
14 sewage sludge incinerators at
11 treatment plants. A survey focused on
differences in the kinds of incinerators and
differences in the coagulants used at the
sewage sludge dehydration process.
(1)In the case of polymer cake, the
conversion rate to N2O ranged from 5.9
to 18%, and which was higher than the
1.3%to 3.5%for lime cake, and the same
conversion rate was particularly high
between 6.1%and 18%in cases when
polymer type cake was incinerated in
fluidized bed incinerators.
(2)The conversion rate was low for both
types of incinerator when lime cake was
incinerated. Therefore, it was concluded
that the calcium hydroxide (Ca(OH)2)
added to the sludge cake as a coagulant
during dehydration was transformed into
CaO by the combustion process and that
it acted as an oxidizer. In the fluidized bed
incinerator cases, in particular, this
resulted in the conversion to NZO being
replaced by conversion to NOX.
(3) In the case of fluidized bed incinerators,
the NZO conversion rate was found to be
in an inverse relationship with the
combustion temperature within a
combustion temperature range from 770 to
860℃.
(4)The total amounts of N2O emitted by
sewage sludge incineration in Japan in 1995
was estimated at 4,030 t on nitrogen base,
and 77%of this was discharged from
fluidized bed incinerators. The release
amounts per member of the population
served by the wastewater systems that
incinerated sludge was found to be 96g
per person per year.
(5)It was calculated that assuming that the
276 Japanese J. Wat. Treat. Biol. Vol.34 No.4
sewered population grows at an annual
rate of 2% as it does now, that the
percentage of sludge disposed of on green
belt and agricultural land remains at its
present 10%, and that the remainder of
the sludge produced is incinerated, in 2008,
which is the first year of the greenhouse
gas reduction target period established at
COP3,8,500 t of N2O on a nitrogen base
will be emitted from sludge incineration.
It is assumed that the percentage of
bsewage sludge incinerated in Japan will
continue to rise steadily. In dehydrator,
vacuum filter and filter press now using lime
type coagulants will be steadily replaced by
centrifugal separators using polymer type
coagulants at the same time as the fluidized
bed incinerator share of all incinerators also
climbs. Consequently, emissions of N2O by
wastewater treatment are forecast to climb
rapidly, requiring the introduction of N2O
emission control measures.
Acknowledgments
The authors would like to express their
deep gratitude to all those wastewater
treatment plant personnel who assisted them
by obtaining exhaust gas samples from
sewage sludge incinerators and permitting
the authors to view daily reports on sludge
treatment that were indispensable for the
analyses performed as part of this research
study.
REFERENCES
1) Ramanathan, V., Cicerone, R.J., Singh, H.
B.,and Kiehl, J.T.: Trace gas trends and
their potential role in climate change,
Journal of Geophysica Research, 90,
5547-5566 (1985)
2) IPCC: Radiative forcing of climate
change (1994)
3) Kyoto Protocol to the United Nations
Framework Convention on Climate
Change (1977.12)
4) Japan Sewage Works Association (1997)
Sewerage Statistics in Japan, FY 1995
5) Sewerage and Sewage Purification
Department, Ministry of Construction
(1977) Sewerage in Japan, 19976) Matthews, P.,(ed.): A global atlas of
wastewater sludge and biosolids use and
disposal, IAWQ scientific and technical
report No.4 (1997)7) Iwasaki, Y., Tatsuichi, S. and Ueno, H.
Measurement of N2O emission from
stationary sources and studies on the
formation, Journal of Japan Society of Air
Pollution,27, 85-91 (1992)8) Yasuda, K., Takahashi, M., Yajima, I. and
Kaneko, M.: The emission behavior of
nitrous oxide caused by sewage sludge
incineration, 5, 142-150 (1994)9) Japanese Standards Association: JIS
Handbook for Environmental
Determination 1993 (1993)10) Muzio, L.J., Teague, M.E., Kramlich, J.C.,
Cole, J.A., McCarthy, J.M. and Lyon, R.K
Errors in grab sample measurements of
N2O from combustion sources, JAPCA,
39, 287-293 (1989)11) Lyon, R.K., Kramlich, J.C. and Cole, J.A.
: Nitrous oxide: Sources, sampling and
science policy, Environmental Science
and Technology, 23, 392-393 (1989)12) Japan Sewage Works Association
Wastewater Test Method (1984)13) Moritomi, H., Suzuki, Y., Kido, N., Ikeda,
M., Suzuki, K. and Torikai, K.: N2O formation mechanisms on coal
combustion, Kagaku Kogaku Ronbunsyu,
20, 849-856 (1994)14) Suzuki, Y. and Kido, N.: Circulating
fluidized bed combustion of sewage
sludge, Proceedings of Annual Meeting
of Japan Society of Energy FY 1996
(1996)15) Hupa, M. and Kilpinen, P.: Gas phase
Emission Characteristics of Greenhouse Gas N2O from Sewage Sludge Incineration Process 277
formation and destruction of N2O,5th
International Workshop on Nitrous Oxide
Emissions, Tsukuba, Japan(1992)
16) Suzuki, Y., Moritomi, H., Kido, N., Ikeda,
M.,Suzuki, K. and Torikai, K. N2
O: formation from char and heterogeneous
reactions with char and CaO, 5th
International Workshop on Nitrous Oxide
Emissions, Tsukuba, Japan(1992)
17) Soete, G.G.: Nitrous oxide from
combustion and industry:Emission and
control, International Workshop
“Methane and nitrous oxide”,
Amersfoort, Netherlands(1993)
18) Murakami, T., Kubo, J. and Suzuki, K.:
Study on thermal characteristic of
sewage dewatered sludge(I), Journal of
Japan Sewage Works Association,23
(265) 38-47 (1986)
19) Murakami, T., Kubo, J. and Suzuki, K.:
Study on thermal characteristic of
sewage dewatered sludge(II), Journal of
Japan Sewage Works Association,23
(266) 27-36 (1986)
20) Japan's Second National Communication
under United Nations Framework
Convention on Climate Change(1997)
(Submitted 1998.6.3)
(Accepted 1998.7.13)