J. Environ. Eng. Manage., 20(1), 9-17 (2010)
9
CO2 EMISSION FROM MUNICIPAL SOLID WASTE INCINERATOR: IPCC
FORMULA ESTIMATION AND FLUE GAS MEASUREMENT
Tsao-Chou Chen and Cheng-Fang Lin*
Graduate Institute of Environmental Engineering
National Taiwan University
Taipei 106, Taiwan
Key Words: Municipal solid waste, incineration, greenhouse gases, statistical analysis, carbon
dioxide
ABSTRACT
Although most of the air pollutants can be removed with air pollution control facilities, the
greenhouse gas (carbon dioxide) is unavoidably released into the atmosphere from the flue gas
during waste incineration. The amounts of CO2 emitted from municipal solid waste incinerators are
usually estimated following the guidelines set down by the Intergovernmental Panel on Climate
Change (IPCC). However, there is lack of studies investigating the difference between IPCC
estimation and actually measured values from the waste incinerators. In this study, the substantial
CO2 emissions from Taipei City's three incinerators were compared with the estimated figures with
IPCC Guidelines. In both cases, the seasonal emission data exhibit non-normal distribution.
Statistical analysis depicts a significant difference of 71% in the 95% confidence interval between
the emission estimated from the IPCC Guidelines and the flue gas measurements. The correlation
between the waste components and the emissions was evaluated as well. The results of the
correlation analysis for the components of waste material and CO2 emissions show that there is a
significant correlation between plastics and the IPCC estimated emission and there is a significant
correlation with garden trimmings for the measured CO2 emissions.
INTRODUCTION
Incineration is currently the preferred treatment
operation for municipal solid wastes (MSW). Theoretically and virtually, all the volatile carbon content
in MSW will be converted into CO2 emissions. During
incineration of MSW, most of the greenhouse gases
(GHGs) in the emissions consist of CO2 and N2O, of
which CO2 being higher and more important than N2O
[1-3]. CO2 emissions, in particular, have played the
most significant role on global warming [4]. In
densely populated urban areas, the scarcity of land and
a desire to promote waste disposal makes incineration,
the primary alternative of waste disposal. This makes
the CO2 emissions generated during the incineration
process and their effect on climate change an important issue for municipal authorities to tackle with
MSW management. By applying the life-cycle inventory method and incorporating a compensatory system
for waste, a unit function for emissions may be set up
to assess the amount of GHGs released by different
types of waste disposal schemes. Studies have shown
*Corresponding author
Email: [email protected]
that when the different types of waste disposal methods are ranked based on their contribution to GHG,
waste recycling will contribute to the smallest amount
of GHG emissions, while incineration is second lowest to waste recycling [1,5-7]. The CO2 released from
biomass materials (e.g. paper, food and wood waste)
during incineration is excluded from the GHG assessment [1-3] and this will help to lower GHG emissions. The recovery of residual metals from incineration and the conversion of thermal energy from the
burning process into electricity also offset GHG emissions. This may be due to displaced electric utility
generation and decreased energy consumption for the
production processes using recycled inputs [1]. The
Intergovernmental Panel on Climate Change (IPCC),
however, includes the fossil and biogenic CO2 emissions produced for waste incineration with energy recovery when assessing GHG emissions. Fossil CO2
emissions are included into the national emissions under the energy sector, while biogenic CO2 emissions
should be included in the energy sector and report as
an information item [3] for avoiding double counting
J. Environ. Eng. Manage., 20(1), 9-17 (2010)
10
activity data with energy sector. While there has a
separate set of guidelines for the CO2 emissions produced when waste disposal is used to generate energy,
there is still some difference in emission offset between the method adopted for calculating CO2 emissions and the method used by the U.S. Environmental
Protection Agency when looking at the conversion of
thermal energy from waste incineration into electricity
and the recovery of residual metal in the waste management sector. The default emission factors from the
IPCC formula was widely used by the most communities for assessing CO2 emissions during waste incineration. Hence, it is highly possible to perceive a significant discrepancy between the assessed emissions
and the actual emissions. The Kyoto Protocol requires
the signatory states to commit to a reduction in GHG
emissions and to set up a GHG emissions trading
scheme. This makes the question of a discrepancy between the CO2 emissions released by the MSW incinerator during the burning process and the IPCC assessed value an important one. Relevant data from cities with a comprehensive waste management system
are therefore needed in order to conduct an in-depth
study and analysis of IPCC estimates and actual CO2
emissions.
Almost every country has set environmental
regulations that limit the emission of pollutants in the
flue gas from incinerators when burning the waste.
Environmental authorities also set methods and procedures to measure concentrations of concerned pollutants. There is, however, little regulation of CO2
emissions that could provide actual amount of CO2
generated in a MSW incinerator. The objective of this
study is to investigate the difference between the
IPCC default values for calculating the CO2 emissions
in a MSW incinerator and the actual measured values
from the flue gas during actual operations.
MATERIALS AND METHODS
factors from 2006 IPCC Guidelines for National
Greenhouse Gas Inventories [3] were used in estimating the CO2 emissions over three successive years
(2005-2007) for the studied area and were compared
with the actual CO2 measurements in the flue gas released from Taipei City incinerators during regular
operation over the same period. Pre-processing, postprocessing and the environmental effects of GHGs
that might be indirectly emitted in the waste management system will not be considered. Electricity was
not treated as a substitute for emission volume in order to comply with the IPCC Guidelines method for
calculating CO2 emissions.
2. Estimation of IPCC CO2 Emissions from Flue Gas
The IPCC formula for calculating CO2 emissions
from waste incinerator uses the fossil carbon content
burnt in the waste as the input parameter. The product
of the oxidation factor and amount of fossil carbon
oxidized give the CO2 emission. The related data involve the waste components, total mass, dry matter
content, total carbon content, fossil carbon fraction
and oxidation factor. The collection, transportation
and incineration/disposal of waste in Taipei City are
all carried out by government authorities. The defined
categories of waste were household waste, garden
(yard) and park waste, and commercial/institutional
waste. This agrees well with the general definition for
MSW used in the IPCC Guidelines [3]. According to
CO2 emission estimation method suggested by the
IPCC Guidelines, this study used Tier 2a method [3]
to estimate the amount of CO2 emissions produced by
incineration after wastes were transported to the incinerator and were sampled to reveal the waste physical
composition. To facilitate the comparison of the CO2
emissions from each unit of waste incinerated, the estimation method was modified as follows:
CO2 Emissions = MSW
1. Data Acquisition
The Taipei City is the studied region in this research. The total city area is 271.8 km2 and the population is 2.6 million with an average population density of 9,700 people km-2. Three incinerators have
been built in Neihu, Muzha and Beitou since 1991.
The three incinerators have a capacity of handling
3,900 t d-1 of municipal refuse. All three city incinerators were equipped with air pollution control devices
(APCDs). Neihu and Beitou incinerators have semidry scrubbers, activated carbon and bag house, while
the Muzha incinerator has an electrostatic precipitator,
wet scrubber and catalytic oxidation unit. The main
function of the APCD is to reduce and remove particulates, acid gases [8,9], dioxins [10] and heavy
metals emitted into the atmosphere from the flue gas.
The MSW CO2 emissions formula and emission
∑ (WF
j
j
dm j CF j FCF j OF j )
44 3
10
12
(1)
where CO2 emissions in kg t-1 waste; MSW = mass of
municipal solid waste as wet weight incinerated, t
waste; WFj = fraction of waste type/material of component j in the MSW (as wet weight incinerated); dmj
= dry mass content in the component j of the MSW
incinerated, (fraction); CFj = fraction of carbon in the
dry matter (i.e., carbon content) of component j; FCFj
= fraction of fossil carbon in the total carbon of component j; OFj = oxidation factor, (default value for
MSW = 100%); 44/12 = conversion factor from C to
CO2; 103 = conversion factor from t to kg; j = component of the Taipei City’s MSW incinerated; and
∑ WF
j
j
= 1.
Chen and Lin: CO2 Emission from MSW Incinerator
3. Sampling and Calculation of CO2 Emissions from
Flue Gas
Taiwan environmental regulations require MSW
incinerators to file reports as stationery sources on a
quarterly basis each year. The reports have to present
measurements for particulate, acid gases, heavy metals and dioxins [11]. As carbon dioxide emission is
not the required monitored parameter, CO2 emission
data were not directly obtainable. Therefore, CO2
emission was calculated by taking CO2 fractional
composition from quarterly flue gas measurements using the dry gas calibration values over 3 successive
years (2005-2007). The following formula was employed to calculate CO2 generation for a total of 36
samples.
CO2 (mg m-3) =
[CO 2 (%)]( 10 4 )[44 (g mol -1 )]
22.4 (L mol-1 )
(2)
CO2 (kg h-1) = [CO2 (mg m-3)][dry gas (Nm3 min-1)]
[60 (min h-1)][10-6 (kg mg-1)] (3)
The hourly CO2 emissions thus obtained is then divided by the hourly waste-combustion rate. These result in the measured CO2 emissions in the flue gas per
each metric ton of wet waste incinerated.
4. Data Processing and Analysis
Data collected for CO2 emissions of the three
Taipei City incinerators were on a quarterly basis
from 2005 through 2007 from the flue gas emissions.
The monitored values and estimation methods were
then used to generate 36 emission samples to facilitate
statistical analysis [12]. The emission unit used was
11
the CO2 emissions per ton of wet MSW (CO2 kg t-1
waste). Using the results of combustible fractions in
received waste [13-15], the combustible materials
burned at each incinerator and the actual flue gas
measurements from three incinerators, the quarterly
CO2 emissions data were sorted and statistically analyzed. All the statistical tests were carried out using
SPSS 11.0 (SPSS Inc., Chicago, USA).
RESULTS AND DISCUSSION
1. Analysis of Measured CO2 Emissions
The descriptive statistics of flue gas measurements and the fraction of combustible material in
waste over the past 3 years are shown in Table 1. Flue
gas measurements gave mean of 964 kg t-1 waste for
CO2 emissions from 36 samples with a standard deviation of 187 kg t-1, indicating significant variation
between the individual measurements. The amount of
CO2 emitted during incineration is affected by the differences in the burned waste components. Among the
waste categories, the greatest variations in individual
sample values was food waste with a standard deviation of 11.9 and the mean variance over 3 years was
23%. This shows that biogenic and fossil carbon fraction in the waste are the major uncertainty factors, due
to a variety of MSW [16]. In the IPCC Guidelines,
Taiwan is assigned to the South-Eastern Asia where
the default value for the food waste component is
43.5%. The actual food waste measurements from
Taipei City were therefore obviously lower than the
IPCC default at 47%. Comparative analysis shows
that the percentage of food waste in Taipei City MSW
is also lower than the IPCC's global default value of
39.5% (the lower and upper bounds of global food
Table 1. Statistics of the measured CO2 emission and Taipei City MSW components
95% Confidence Interval
for Mean
Lower
Upper
Std.
Deviation
Minimum
Maximum
1,027
187
635
1,630
38.0
3.0
44.4
5.3
9.5
3.4
22.6
0.0
56.7
12.0
4.8
23.4
19.7
1.4
0.5
3.2
19.4
17.4
0.4
8.6E-02
6.3
27.4
22.1
2.4
0.9
4.6
11.9
6.9
2.9
1.1
1.1
10.1
6.8
0.0
0.0
23.3
46.4
48.7
12.0
6.5
1.2
0.6
2.5
0.7
0.8
0.4
1.8
0.3
1.5
0.8
3.3
1.2
1.0
0.6
2.2
1.3
0.0
0.0
0.0
0.0
4.0
2.3
7.5
6.3
N
Mean
Measured CO2 emission
(kg t-1 waste)
36
964
901
Combustible fraction
Papers (wt%)
Textiles (wt%)
36
36
41.2
4.1
36
36
36
36
36
36
36
36
36
Garden trimmings (wt%)
Food waste (wt%)
Plastics (wt%)
Leather and rubber (wt%)
Others (wt%)
Incombustible fraction
Iron (wt%)
Other metal (wt%)
Glass (wt%)
Others (wt%)
J. Environ. Eng. Manage., 20(1), 9-17 (2010)
12
Expected normal
2
agrees with the results of the significance test and
probably does not follow normal distribution (Fig. 1a).
The emission data would therefore tend to follow a
lognormal distribution [3,18].
(a)
1
2. Analysis of Estimated CO2 Emissions
0
-1
-2
400
600
800
1000 1200 1400 1600 1800
Measured value (CO2 kg t-1 waste)
Expected normal
2
(b)
1
0
-1
-2
0
200
400
600
800
1000 1200 1400
Estimated value (CO2 kg t-1 waste)
Fig. 1. Q-plot of (a) measured and (b) estimated CO2
emissions in flue gas from incinerated MSW.
waste fractions were 34 and 45% respectively with a
confidence interval of 95%) [3].
The average amount of CO2 emissions (964 kg t1
), is less than 1,179 kg t-1 emitted in France [17].
Thus, it is reasonable to state that the default values
assigned by the IPCC on a regional basis for developed, developing and other countries around the world
are not justified.
To explore the distribution of measured CO2
emissions, the Shapiro-Wilk normal test was used, as
there were less than 50 samples available. The statistics value is found to be 0.932 with the significance
probability (p) of 0.044. The null hypothesis was rejected as the p value is less than 0.05. The skewness
was 1.086 (positively skewed) with the Kurtosis coefficient of 3.307 (leptokurtic), indicating that CO2
emissions follow a non-normal distribution. The
measured data from the 3-year period were sorted in
ascending order and tested using the normal probability plot. These results show that the cumulative probability of the observations diverged from the straight
line of cumulative probability distribution defined by
the normal distribution theory. These results show that
the distribution of measured CO2 emissions in flue gas
Using the IPCC Guidelines, the estimated CO2
emissions from 36 samples have a mean and standard
deviation of 567 and 187 kg t-1 waste, respectively.
The mean CO2 emission was lower than the measured
value of 397 CO2 kg t-1 waste, or 41% of the mean
measured emission. The individual variation between
the estimated values was also greater than the measured emissions. The statistics of the measured, estimated and calibrated flue gas CO2 emission is shown
in Table 2.
To investigate the distribution of the estimated
CO2 emissions based on MSW components, the same
hypothesis of normal distribution test was used here.
The Shapiro-Wilk normal test gave the statistic of
0.849. The null hypothesis is rejected as the significant probability p = 0.010 (< 0.05). The skewness thus
calculated was 2.071 (positively skewed) with a Kurtosis coefficient of 7.720 (leptokurtic), indicating that
the estimated CO2 emission from the IPCC Guidelines
does not follow a normal distribution. The estimated
emissions were also clustered more closely to the
mode and around the lower bound compared to the
measured emissions with some extreme data points
near the upper bound. The data have trouble breaking
the lower boundary of the emissions and tend to extend towards the high end as well. This kind of
skewed result is referred to in statistics as the "floor
effect". Here it is probably the result of the limits imposed by the waste categories' default emission factors.
When tested against the normal probability plot (Fig.
1b), the results show that most of the plots have a cumulative probability variance diverged from the normal distribution theory's cumulative probability distribution line. The divergence is also more obvious than
the distribution chart for the measured emissions (Fig.
1a), showing that the distribution of the CO2 emissions in the incinerators' flue gas estimated from the
components of the incinerated MSW agrees with the
results of the significance test that it does not follow
normal distribution model.
3. Analysis of CO2 Emissions from Incinerated
MSW
For the estimated and measured CO2 emissions
from the incinerators, first, the total amount of MSW
and the components of the waste incinerated quarterly
over 3 years were added up then multiplied against the
estimated and measured CO2 emissions for each ton of
MSW incinerated in each quarter and in each year.
Then, they were added up on a quarterly basis in order
Chen and Lin: CO2 Emission from MSW Incinerator
13
Table 2. Statistics of the measured, estimated and calibrated flue gas CO2 emission
95% Confidence
Interval for Mean
Lower
Upper
N
Mean
Std. Deviation
Minimum
Maximum
Measured CO2 emission
(kg t-1 waste)
36
964
901
1,027
187
635
1,629
Estimated CO2 emission
(kg t-1 waste)
36
567
503
630
187
238
1,343
Calibrated CO2 emission
(kg t-1 waste)
36
970
861
1,077
320
408
2,296
Table 3. Descriptive statistics for waste incinerated and CO2 emissions each quarter in Taipei City, 2005-2007
Waste incinerated (t)
Measured CO2 emission (t)
Estimated CO2 emission (t)
N
Total Amount
12
12
12
1,843,600
1,775,000
1,014,200
to calculate the estimated and measured CO2 emissions from the incineration of Taipei City MSW. The
formula is as follows:
∑ (IM
−3
pij W pij 10
)
(4)
ij
EEij =
∑ (IE
pij
W pij 10 −3
)
(5)
ij
where MEij = measured CO2 emission in flue gas after
MSW incineration (CO2 ton per quarter, i: year, from
2005-2007; j = quarter (1-4)); IMpij = measured CO2
emission in flue gas per ton of MSW from each incinerator each quarter in each year) (CO2 kg t-1 waste; p:
incinerator, Neihu, Muzha and Peitou); Wpij = amount
of MSW incinerated from each incinerator each quarter in each year (ton per quarter); 10-3 = conversion
factor from kg to t; EEij = estimated CO2 emission in
flue gas from incineration of MSW (CO2 ton per quarter); IEpij = estimated CO2 emission in flue gas per ton
of MSW from each incinerator each quarter in each
year (CO2 kg t-1 waste). The results from this study on
the amount of incinerated waste and their CO2 emission are summarized as descriptive statistics in Table 3.
The calculations and statistical analysis give 12
sample points for the amount of waste incinerated
each quarter as well as the measured and estimated
CO 2 emissions (Table 3). There was a total of
1,843,600 t of MSW incinerated in Taipei City. Using
the quarterly flue gas measurements and incineration
volume, the total CO2 emissions over the three years
was 1,775,000 t. The IPCC Guidelines uses a separate
method that calculates the total emissions based on the
components, default emission factors and incineration
volume. IPCC formula gave a total for CO2 emissions
of 1,014,200 t over the three-year period. A comparison of the two results shows that, while conditions
such as the MSW incineration volume and component
Std. Deviation
12,700
15,100
16,000
Measured emission
Estimated emission
180000
160000
Emission of CO2 in Ton
MEij =
Quarterly amount
Maximum
Mean
177,000
153,600
171,700
147,900
119,600
84,500
Minimum
140,200
124,400
62,000
140000
120000
100000
80000
60000
40000
20000
0
1st 2nd 3rd 4th 1st 2nd 3rd 4th 1st 2nd 3rd 4th
2005
2006
2007
Quarters
Fig. 2. Comparison of quarterly measured and estimated
CO2 emissions from incinerated MSW.
for each quarter over the 3 years are identical, both
CO2 emissions are different. Figure 2 shows quite clearly
that the measured CO2 emissions in flue gas were significantly higher than the values calculated using the
IPCC Guidelines. The difference in CO2 emissions over
the three years was as high as 760,800 t.
The mean measured CO2 emissions from the
three incinerators each quarter over three years compared to the mean CO2 emissions estimated using the
IPCC Guidelines was significantly lower by 397 kg t-1
of MSW incinerated. If the average amount of waste
incinerated in each quarter over the three years was
used as a reference, the emissions using the IPCC
Guidelines is, on average, 63,400 t less CO2 emissions
for each quarter, and 253,600 t less CO2 emissions
each year. According to the IPCC Guidelines, this difference in estimated CO2 emissions could, through the
emission exchange system, convert to 447,600 t of
MSW for incineration. This implies that there was a
14
J. Environ. Eng. Manage., 20(1), 9-17 (2010)
discrepancy in the amounts of MSW incineration/disposal. The major difference may be due to the
biogenic CO2 emissions in the incinerated waste not
being deducted from the measured CO2 emissions in
the flue gas. The IPCC, therefore, recommends the use
of field sites measurements for emissions in order to
reduce the uncertainty and discrepancy of CO2 emission [3,16].
4. Analysis of CO2 Emissions and Component of
MSW
After statistically testing the distribution of
measured and estimated CO2 emissions, it is ascertained that the emissions were non-normal in their distribution and there are some obvious discrepancies in
practice. This study intended to determine if the estimated fossil carbon component in MSW significantly
affected the measured CO2 emissions in the flue gas
during incineration and if the amount of emissions exhibited any correlation with the components of MSW.
The non-parametric statistical test was used to test divergence and correlation. This avoids the prerequisite
assumption of normality and homogeneity of variance
used in parametric statistics. First, the variance between the measured and estimated CO2 emissions in
flue gas was tested. Wilcoxon signed-ranks test was
used in order to take the magnitude of the variance
and its positive and negative signs into account. Results of the test are shown in Table 4.
Table 4 shows that the estimated CO2 emissions
using the components of incinerated waste according
to IPCC Guidelines are lower than the actual measured CO2 emissions in flue gas in 34 samples (negative ranks row) and higher than the actual measured
CO2 emissions in 2 samples (positive ranks row). The
test converts the magnitude of the variance in estimated and measured emissions into absolute ranks.
Taking the 34 CO2 emissions with values lower than
the measured values' mean rank of 19.0 and the 2 CO2
emissions with values higher than measured values'
mean rank of 10.5, the variance between the means of
these two ranks were then tested if they were statistically significant. The test results are shown in Table 4.
The variance test of the two mean ranks gives a Z
value of -4.902, and double-sided testing gives a p
value of 0.000 (< 0.05), showing that the IPCC estimated emissions when compared to actual measured
emissions results in a visibly different and lower value
for CO2 emissions in flue gas. From the analysis, it is
justified to state that in practice MSW incineration
counted more CO2 emissions.
It is therefore necessary to understand how the
estimate produced using the IPCC Guidelines can be
corrected in order to match a better amount of CO2 released into the atmosphere from the flue gas after
MSW incineration. This will help preserve the fairness of the GHG emissions trading scheme. First, the
means for the measured CO2 emissions and the IPCC
Guidelines’ estimated emissions were established. The
difference between the upper and lower bounds for
both values (95% confidence interval) were divided
by the difference between the upper and lower bounds
for the IPCC estimates (95% confidence interval). The
resultant upper and lower bounds accounted for
around 63 and 79% of the original estimates, respectively. Using the difference between the two, one can
find the mean of 71% as a calibration factor. To determine if there was a difference between the calibrated CO2 emission and the measured CO2 emission
in flue gas after the correction factor is substituted into
the IPCC estimate, the 36 estimated CO2 emission
samples were each multiplied by a coefficient of 1.71
to correct the emission. The Wilcoxon signed-ranks
test was then used to analyze the variance between the
measured CO2 emission and calibrated CO2 emission.
Table 4. Wilcoxon signed-ranks test on CO2 emission in flue gas
N
Negative ranks
Positive ranks
Ties
Total
Negative ranks
Calibrated CO2 emission — Measured
Positive ranks
CO2 emission
Ties
Total
a
Estimated CO2 emission < Measured CO2 emission
b
Estimated CO2 emission > Measured CO2 emission
c
Estimated CO2 emission = Measured CO2 emission
d
Calibrated CO2 emission < Measured CO2 emission
e
Calibrated CO2 emission > Measured CO2 emission
f
Calibrated CO2 emission = Measured CO2 emission
g
Based on positive ranks.
Estimated CO2 emission — Measured
CO2 emission
34a
2b
0c
36
21d
15e
0f
36
Mean
rank
19.0
10.5
17.3
20.1
Sum of
ranks
645
21
Z test
Asymp.
Sign (2-tailed)
-4.902g
0.000
-0.487g
0.626
364
302
Chen and Lin: CO2 Emission from MSW Incinerator
The result showed that the two mean ranks returned a
variance test result Z of -0.487. In double-sided test,
the p value was 0.626 (> 0.05). This did not reach the
0.05 required for significance. The null hypothesis
that the estimated and the calibrated emission had
equivalent mean ranks can therefore be accepted. This
test indicated that the measured CO2 emission and the
IPCC Guidelines' estimated emission multiplied by
the calibration factor of 1.71 had no significant variance as shown in Table 4.
When comparing the mean measured CO2 emission of 964 kg t-1 waste and the mean calibrated CO2
emission of 970 kg t-1 waste, the variance between the
two is 6 CO2 kg t-1 waste, or 0.6% of the mean measured CO2 emission. This shows that there is no statistical difference between the calibrated CO2 emission
data and the measured CO2 emission in flue gas. To
facilitate the comparison of the descriptive statistics
for calibrated emission with the measured emissions,
the data were listed in the calibrated CO2 emissions
shown in Table 2.
The IPCC CO2 emission was estimated based on
the components of the MSW. There are uncertainty
factors involved when collecting and analyzing samples of waste components [16]. For better understanding of the causes of the variance, the degree of association between CO2 emission and waste component
was analyzed. Using the Spearman rank order correlation coefficient, the correlation between the two was
calculated as shown in Table 5.
The correlation coefficients in Table 5 form a
correlation matrix. The correlation coefficient rs was
calculated by taking the pair-wise difference in two
variables for each sample value to analyze their correlation. The result is between -1 and 1, and the closer
the value is to ±1, the higher the level of correlation.
From Table 5, it may be seen that the significant correlation exists between the measured CO2 emission in
the incinerated MSW and its components in the following categories: garden trimmings and food waste.
The two correlation coefficients are 0.401 positive
correlation and -0.493 negative correlation, with the
15
variance of 16 and 24%, respectively, suggesting a
medium level of correlation. With CO2 emission calculated using the IPCC Guidelines, significant correlation existed for food waste, others and plastics. The
correlation coefficients were -0.524, -0.540 and 0.985,
with variances of 27, 29 and 97% respectively. Food
waste and others both have a medium degree of negative correlation, while plastics have high degree of
positive correlation. According to the IPCC Guidelines, plastics default dry matter content, total carbon
content and fossil carbon fraction values are 100, 75
and 100%, respectively. The estimated CO2 emission
above and the correlation test with MSW content indicate that the magnitude of estimated CO2 emissions
using the IPCC Guidelines is affected by the default
values for dry matter content and carbon content. This
is the reason why plastics with a default dry matter
content and carbon fossil fraction of 100%, have a
high degree of correlation with the emissions. The explained variation between the two is 97% as well, so
the higher plastic content in the waste components
translates to more statistical significance when calculating the estimated CO2 emissions. For the measured
CO2 emissions in flue gas, however, if biogenic CO2
was not excluded then the plastics correlation coefficient is just 0.137, its explained variation just 1.9%
and is not statistically significant. At the same time, a
significant positive correlation existed between the
measured CO2 emission and the MSW component for
garden trimmings. This, however, was assigned a fossil carbon fraction of 0% in the IPCC Guidelines. And
while the explained variation between two are just
16%, it still exhibits a medium degree of significant
correlation statistically. If the fraction of garden trimmings in MSW components increases then their statistical significance on the measured CO2 emission in
flue gas would increase as well. The only category to
show statistical significance for both measured and estimated CO2 emissions in the correlation with MSW
components is the food waste. The rest of the components have a negative correlation. The variation was
24 and 27% respectively. According to IPCC Guide-
Table 5. Correlation coefficient between MSW component and CO2 emissions
Measured CO2
emission (rs)
Sig. (2-tailed)
R2
Papers
Textiles
Garden
trimmings
Food waste
Plastics
Leather
and rubber
Others
0.244
0.152
0.059
0.178
0.300
0.032
0.401*
0.015
0.160
-0.493**
0.002
0.243
0.137
0.427
0.019
-0.161
0.347
0.026
-0.250
0.141
0.063
-0.524**
0.001
0.274
0.985**
0.000
0.970
-0.124
0.471
0.015
-0.540**
0.001
0.292
Estimated CO2
0.043
emission (rs)
0.084
0.805
0.626
Sig. (2-tailed)
0.002
0.007
R2
*Correlation is significant at the 0.05 level (2-tailed).
**Correlation is significant at the 0.01 level (2-tailed)
-0.078
0.652
0.006
16
J. Environ. Eng. Manage., 20(1), 9-17 (2010)
lines, food waste was assigned a default value of 38%
for total carbon content in percent of dry weight. This
is the lowest default value out of all of the MSW
components, showing that food waste has a lower total
carbon content compared to other MSW components.
Therefore, food waste, compared to other waste materials of equivalent mass, produces significantly less
CO2 emissions after incineration. Hence, this phenomenon is presented in the correlation test for both
measured and estimated CO2 emissions.
When estimating CO2 emissions, the IPCC
Guidelines eliminates the CO2 emissions from food
waste, garden trimmings and paper as they are biogenic in origin. While for measured CO2 emissions in
flue gas, incinerating food waste contributes significantly less CO2 emissions when compared to other
waste categories. Garden trimmings provide significantly higher contribution of CO2 emissions while paper exhibits less significance difference. These might
be the main causes for the difference between the
IPCC estimation and the actual measured values in
Taipei City.
trimmings and plastics therefore merits further study
because they achieved statistically significant correlation with the measured and estimated CO2 emissions.
This would reduce the greater tendency for this type
of waste to release CO2 emissions directly into the atmosphere during incineration.
To avoid excessive divergence between the estimated emissions and the actual measured CO2 emissions in flue gas and help maintain the fairness of future international emission trading schemes, and to
better manage and understand the amount of carbon
dioxide emitted into the environment, the 95% confidence interval values from the descriptive statistics of
waste components' combustible fraction (Table 1) can
be used for comparison. If the component fraction
used for the estimation falls within the mean confidence interval, then multiply the total CO2 emissions
estimated using the IPCC Guidelines by the calibration factor of 1.71. This will provide a better estimate
of the actual total CO2 emissions released into the atmosphere from MSW incineration.
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CONCLUSIONS AND RECOMMENDATIONS
When no flue gas emission measurements are
available, IPCC Guidelines offers default emission
factors to generate a quick and convenient way of estimating the CO2 emissions from waste incineration.
The CO2 estimate produced using the IPCC method,
however, is prone to differences between the actual
emissions. In this study, the measured CO2 emissions
in flue gas from MSW incinerators in Taipei City
were compared with the estimated emissions using
IPCC formula. The results show that both data follow
non-normal distribution and the emissions are significantly different in terms of their statistical and practical significance. For a confidence interval of 95%, the
IPCC estimates are significantly different from the
measured emissions in flue gas by 71%. As for waste
categories that affect CO2 emissions of actual incinerating operations, garden trimmings show a positive
correlation while the food waste shows a negative correlation. The variations for the two are 16 and 24% respectively. As for the waste categories that exhibit a
significant discrepancy based on the IPCC Guidelines’
default emission factor for incinerated waste and their
estimated emissions, food waste and the other components show a negative correlation with a variation of
27 and 29%, respectively. Plastics exhibit a high degree of positive correlation with an explained variation of 97%. The fraction of food waste, garden trimmings and plastics in MSW components therefore are
key factors affecting the CO2 emissions in the flue gas.
The CO2 emissions contributed by these types of
waste in the flue gas are probably the main cause of
the difference between measured and estimated emissions. The choice of disposal methods for garden
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Manuscript Received: May 11, 2009
Revision Received: August 23, 2009
and Accepted: September 9, 2009