from Sewage Sludge Incineration Process

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


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


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


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


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


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


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


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.

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