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TG-DTA ANALYSIS OF SEWAGE SLUDGE, COAL, BIOMASS AND
THEIR MIXTURES
Authors: Agnieszka Kijo-Kleczkowska, Magdalena Szumera, Katarzyna Środa
("Rynek Energii" - 10/2015)
Keywords: sewage sludge, biomass, hard coal, co-combustion, TG-DTA of fuels
Abstract. The paper presents the thermal analyses TG-DTG-DTA of sewage sludge, coal and biomass and their
mixtures in different proportions, tested in air. Due to problems regarding the use and application of sewage
sludge, which are associated with increasingly restrictive legislation regarding the need for sewage sludge
storage in Poland beginning January 1, 2016, and because the waste has a gross heat of combustion greater than
6 MJ/kg d.m., its thermal disposal (combustion and co-combustion with other fuels: coal or biomass) is
extremely important. High combustion efficiencies can be achieved provided that the combustion process is
properly designed taking into account the properties of the fuel. Co-combustion of sewage sludge with coal leads
to an increase calorific of fuel and co-combustion of sewage sludge with biomass reduces CO2 emissions.
1. INTRODUCTION
Coal upgrading processes generate large quantities of fine and ultra-fine coal particles, usually
in the form of coal-water slurries. Thermal utilization of these wastes is a preferable option to
their neutralization [8]. Similarly is with utilization of sewage sludge. Combustion of sewage
sludge and their co-combustion with other fuels (coal, biomass) is the best option in their
utilization. High combustion efficiencies can be achieved provided that the combustion
process is properly designed to take into account the unique properties of the fuel. Because
biomass is considered as a carbon-neutral fuel, co-combustion of coal with sewage sludge and
biomass may cause lower the CO2 emission and decrease the cost of electricity.
Sewage sludge can be a full-valuable energy fuel. Earlier, sewage sludge must be subjected to
appropriate treatment: dewatering, compaction and drying. Tables 1 and 2 show the
characteristic of installations drying and combustion of sewage sludge in Poland [16].
Table 1. The characteristic features of the sewage sludge drying plant in Poland [16]
Characteristic feature Unit Value
The total number of installations - 19
The total efficiency Mg d.m./year about 80 000
The range water evaporation Mg H20/h 1 – 9.15
The range of the drying process % d.m. 18 - 95
The range of heat consumption rate kWhth/kg H20 0.75 – 1.3
The range of electrical energy ratio kWhel/kg H20 0.06 – 0.085
The range of obtained availability % 30 - 90
Energy product - natural gas, biogas, fuel oil
The range of the cost of energy product PLN/Mg d.m. 550 900
Table 2. The characteristic features of the sewage sludge combustion plant in Poland [16]
Characteristic feature Unit Value
The total number of installations - 11
The total efficiency Mg
d.m./year about 160 000
The range of capacity of sewage sludge combustion plant Mg
d.m./h 0.2 – 7.9
The range of the dry weight of sewage sludge subjected to the
furnace % d.m. 33 - 90
The range of obtained availability % 0.06 – 0.085
Development of ash / residue from flue gas cleaning - storage - solidifying
Type of furnace construction - grate (4) + fluid (7)
Type of exhaust gas cleaning system - dry with sodium
bicarbonate
In the paper [20] the authors highlighted, that the water content in the sewage sludge depend
on the dewatering process in the wastewater treatment plant. They stresses that the high water
content in the fuel is adverse for the combustion in a cement kiln, because:
temperature in the cement kiln may fall to temperature limit of cement clinker creation.
The result is a decline in the quality of the product;
large amount of evaporated water increases the flow of flue gases, which can overload the
installations for cleaning emitted gas, and ventilators.
Author of the paper [2] points out, that in 2011, participation of the co-combusted sewage
sludge, with respect to the alternative fuels, in the firm (cement group) “Cemex”, was only
1.5% (in total mass). Table 3 shows the amount of sewage sludge subjected to the co-
combustion process in the cement group “Cemex”, in the years 2009-2011.
Table 3. The amount of sewage sludge subjected to the process of co-combustion in cement group Cemex,
in thousands. Mg, in 2009 [2]
2009 2010 2011 other
Cement plant „Chełm” 1719 5522 6251 13492
Cement plant „Rudniki” - 1047 1406 2453
The authors of the paper [21] highlighted that co-combustion of sewage sludge with other
fuels is the ideal solution, which uses the existing industrial facilities for disposal of the waste.
This solution brings a lot of benefits arising from the need lack of construction of costly
installation. The authors of this paper analyzed the addition of sewage sludge to coal, in an
amount of not more than 1%. This solution is the most promising way of using municipal
sewage sludge in the near future.
According to [15], the addition of sewage sludge to coal should be strictly controlled. In the
case of the co-combustion processes, an increase of participation of sewage sludge in the fuel
mixture, may cause increasing emissions of sulfur oxides and nitrogen oxides.
In the paper [14], authors analyzed the mixture of sewage sludge and sawdust, in a ratio of
80/20%. They observed, that the increase of moisture in the mixture reduces the content of
nitrogen, sulfur and chlorine in analyzed sample.
In the paper [22] authors suggested, that by mixing of dried sewage sludge with biomass can
be improved energy indicators of biomass as fuel. The study indicated the possibility of the
co-combustion a pine bark and sewage sludge, in the proportion of 20% to 40% (for biomass).
Sewage sludge is characterized by a high moisture and a high content of ash, low calorific
value and a significant content of harmful substances for environment (chlorine, sulfur,
metals) [5]. It follows that the sewage sludge combustion process is not indifferent to the
environment (emissions of CO, SO2, NOx, dust, PAHs, dioxins and furans and the existence
of heavy metals in ash). In the case of biomass, during combustion are emitted, mainly: CO,
HC, tar, PAHs, CxHy, coke particles, nitrogen oxides, heavy metals, dioxins and furans [18].
However, in the case of the coal combustion process additionally distinguished solid products
of combustion - slag and ash– Fig. 1 [12].
Fig.1. Emissions during fuels combustion [12]
Combustion of coal, biomass and sewage sludge, depend on the characteristics of the fuel, as
well as emission of different gases into the atmosphere. Note, however, that the proper
organization of conduct of fuels combustion and co-combustion ensures reducing of pollution
emission and allows you to optimize the process, and especially utilisation of waste like
sewage sludge.
In Poland, from 1 January 2016, a total ban on the storage of sewage sludge about a heat of
combustion above 6 MJ/kg d.m. will be implemented [17].
Choosing an appropriate method for sewage sludge disposal should depend on the properties
of sewage sludge. The physical, chemical, sanitary, and technological properties of sewage
sludge are important and change depending on the type and method of sewage treatment.
Because of its unusual properties, sewage sludge is different from coal and biomass. Sewage
sludge combustion is different because it includes incineration drying, devolatilization and the
combustion of volatiles and char, which contain an average of 50% mineral substances [10].
In contrast with the combustion of coal, the combustion of sewage sludge char is shorter
stage. This finding potentially results from the relatively slow thermal decomposition rate of
the waste and the small portion of coal in the sewage sludge char (approximately 10%).
Undoubtedly, this factor favours rapid char oxidation and sediments with high porosities [25].
The paper [9] presents combustion of sewage sludge, coal and biomass. It has been found,
that unlike coal and similarly to biomass, the course of combustion of sewage sludge is
determined not only by char combustion, but too by the step of devolatilization and
combustion of volatiles (Fig.2).
a)
b)
c)
Fig.2. Course of change of surface and centre temperature and mass of the briquette of: a) hard coal, b) sewage
sludge, c) energetic willow (I-heating and evaporation of moisture (1), II-devolatilization and volatile
combustion (2-4), III- char combustion (5), ash (6)) during combustion and visualization of process (research
stand: combustion chamber; t=8500C)
Increasing the amount of sewage sludge in Poland and the increasingly stringent legal
regulations in the field of environmental protection causes, that the combustion of sewage
sludge and their co-combustion with other fuels are very forward-looking and they should be
further studied.
The aim of this paper is a comparative thermal analysis of various types of fuel-mixtures, with
respect to the basic fuels (biomass – energetic willow, sewage sludge and coal), in an air.
The thermal analysis of different fuels was analysed, among others, in papers
[4,7,11,13,23,24].
It has been found, among others, that biomass and sewage sludge demonstrate the specificity
of devolatilization. According to [23], thermal decomposition of wood already starts at the
temperature of approximately 2200C, while its particular components decompose in the
following temperatures:
hemicellulose: 220÷3200C,
cellulose: 320÷3700C,
lignin: 320÷5000C.
Qualitative difference between the thermal decomposition of hemicellulose (cellulose and
hemicellulose) and lignin was noticed - the first one undergoes "deeper" thermal
decomposition, producing more volatile products, while lignin gives more coke residues.
With regard to the TG/DTG measurements of sewage sludge, it is worth paying attention to
the paper [7]. Samples of three types of sewage sludge were heated in neutral atmosphere
with constant velocity 100C/min. The measurements made it possible to distinguish three
basic fuel degassing stages. It has been stated that initially the sludge loses its moisture, and
then there is a two-step degassing process of volatile parts, with various speed of their
separation. The first stage of heating, from ambient temperature to ca. 1100C is related to
separation of analytical moisture, included in the studied sewage sludge samples. In the range
of temperatures 130-2100C all the studied types of sludge demonstrated further sample mass
loss that can be interpreted as loss of water chemically related to fuel [1]. Some people
emphasise that drying fuels can be slightly delayed due to high heat of water vaporisation
[26]. Then, it is possible to observe sludge organic matter decomposition. In the scope of
temperatures 200-3300C the thermal decomposition of lignin and hemicellulose components
begins. In the scope of degassing temperatures 330-4000C cellulose decomposes [3]. In the
next stage the organic matter is decomposed, being the product of active sludge operation [4].
The most intensive degassing is observed in the temperature range of 210 to 4100C. In this
temperature range, cellulose undergoes decomposition, which originates from waste paper
and plant matter contained in sewage. According to [6] cellulose may constitute twenty to
thirty some per cent of sewage sludge dry mass. Therefore, it may be concluded that the main
component of the studied sludge are substances created in initial sludge. In this scope of
temperatures curves have a very similar course. The maximum peak finishes before the
sample reaches the temperature of 4000C. The next temperatures range (410-550
0C)
corresponds to volatile parts decomposition arising during active sludge operation on sewage
[3]. This stage indicates the greatest share in the sample of components arisen during active
sludge operation. It should also be emphasised that the sewage sludge contains clay minerals,
whose main component is kaolinite, whose decomposition to the so-called metakaolinite takes
place in the range of temperatures 450-7000C and involves dehydroxylation and water
separation [19]. The next stage of sludge decomposition takes place in the scope of
temperatures between 670 and 7500C, with maximum at 710
0C. This peak is certainly the
responsibility of calcium carbonate decomposition contained in the sewage sludge, added to
the sludge as part of hygienisation [1,3]. On this basis we can speak about the quantity of lime
added to sediment in different treatment plants. Samples of fuels subject to higher
temperatures demonstrated further mass loss. At this stage such significant peaks were not
observed, however, high processes temperature indicates that there is further decomposition
of mineral substances of the studied sewage sludge.
From the point of view of thermogravimetric studies, in the literature there are no research for
three-component mixtures and their comparison for basic fuels and two-component mixtures.
2. EXPERIMENTAL STUDIES
The basic fuels applied in research were: sewage sludge, hard coal and energetic willow.
Sewage sludge was created in wastewater treatment plant, which served large urban-industrial
agglomeration. Sewage sludge was made from a technological process in which after
compaction, activated sewage sludge with preliminary sediment goes to an anaerobic
fermentation chamber and is subjected to a stabilization process. Then, sewage sludge was
mechanically dehydrated and dried to form granules with a moisture content of less 10%.
All fuels were ground in a laboratory mill, and then they were sieved below 100 µm. Then,
mixtures given above were prepared according to the mass proportions.
The composition of the fuel mixture was so chosen so as to investigate the impact of each
component on the combustion process. The following basic fuels and their mixtures were
used in research:
sewage sludge,
energetic willow,
hard coal,
energetic willow + sewage sludge (50%/50%) - effect of the addition of sewage sludge to
energetic willow on combustion process of fuel,
hard coal + sewage sludge (50%/50%) - effect of the addition of sewage sludge to hard
coal on combustion process of fuel,
hard coal + sewage sludge + energetic willow (50%/25%/25%) - effect of the addition of
mixture of sewage sludge and energetic willow to hard coal on combustion process of
fuel,
hard coal + sewage sludge + energetic willow (45%/10%/45%) - effect of the addition of
sewage sludge (10%) to mixture of hard coal and energetic willow on combustion process
of fuel,
hard coal + sewage sludge + energetic willow (37.5%/25%/37.5%) - effect of the addition
of sewage sludge (25%) to mixture of hard coal and energetic willow on combustion
process of fuel.
2.1. Characteristics of fuels used in researches
Table 4 presents the proximate and ultimate analyses of sewage sludge, hard coal and
biomass. It should be noted, that the content of volatile matter and carbon in fuels, and their
calorific value significantly affect on the combustion process.
Table 4. Proximate and ultimate analyses of basic fuels used in studies (d.m.)
Sewage sludge Energetic willow Hard coal
PROXIMATE ANALYSES
Moisture W % 4.94 7.42 10.08
Volatiles matter V % 51.44 69.65 28.91
Ash A % 36.44 2.23 11.07
LHV Q kJ/kg 12 574 16 828 23 488
ULTIMATE ANALYSES
Carbon C % 30.77 44.65 59.89
Hydrogen H % 3.92 6.12 3.62
Nitrogen N % 4.26 0.69 1.17
Oxygen O % 18.23 38.83 12.89
Total sulphur S % 1.44 0.06 1.71
2.2. Test stand and measurement methods
The research was conducted by means of STA 449 F3 Jupiter device by NETZSCH, being the
property of the Department Thermophysical Research Laboratory of the Faculty of Material
Science and Ceramics of the AGH University of Science and Technology, enabling
simultaneous thermal analysis with the use of two research techniques: Thermogravimetry
and Differential Thermal Analysis (TG-DTA), in relation to one sample (Fig. 3, Table 5).
Fig. 3. Test stand (STA 449 F3 Jupiter of NETZSCH)
Table 5. The measurement parameters
Type of the crucible / cover Al2O3 / no
Atmosphere // rate of gas flow Air // 40 ml/min.
Program of temperature 21-1400°C
The rate of heating 10°C/min.
Weight of sample before measurement: mg
Sewage sludge 50.0
photo: dr G. Grabowski
Biomass (energetic willow) 50.0
Hard coal 50.0
Energetic willow 50% + sewage sludge 50% 50.0
Hard coal 50% + sewage sludge 50% 50.3
Hard coal 50% + sewage sludge 25% + energetic willow 25% 50.1
Hard coal 45% + sewage sludge 10% + energetic willow 45% 50.0
Hard coal 37.5% + sewage sludge 25% +energetic willow 37.5% 50.2
2.3. Experimental studies results
Figures 4-11 illustrate the results of fuels thermal measurements TG/DTA performed in the
air atmosphere. In Tables 6-9 authors presented an analysis of process and the behaviour of
fuels at various stages of the process. Process start from 210C (0 s). Curves TG/DTG/DTA
have some peaks, the first - endothermic. The first peak indicates the fuel evaporation. After
the first peak it can be observed a sudden drop in fuel mass. The fuel is ignited by volatiles
matter. The final stage of the process is the char combustion. Combustion of volatiles and
char combustion are the exothermic reactions. The maximum rate of weight loss (DTG) is
connected with the peak of temperature (DTA), indicating the highest fuel reactivity. Fuels
with a high content of volatile exhibit high intensification of the devolatilization. For fuels
with a high carbon content, the dominant phase in the combustion process is a char
combustion. Sewage sludge and energetic willow have two peaks during devolatilization,
which demonstrates the specificity of process in case of these fuels.
Fig. 4. Curves TG-DTG-DTA for the sewage sludge combustion: a) with respect to the process temperature; b)
with respect to time of the process
Fig. 5. Curves TG-DTG-DTA for the energetic willow combustion: a) with respect to the process
temperature; b) with respect to time of the process
Fig. 6. Curves TG-DTG-DTA for the hard coal combustion: a) with respect to the process temperature; b) with
respect to time of the process
Fig. 7. Curves TG-DTG-DTA for the combustion of mixture: energetic willow 50% + sewage sludge 50%: a)
with respect to the process temperature; b) with respect to time of the process
Fig. 8. Curves TG-DTG-DTA for the combustion of mixture: hard coal 50% + sewage sludge 50%: a) with
respect to the process temperature; b) with respect to time of the process
Fig. 9. Curves TG-DTG-DTA for the combustion of mixture: hard coal 50% + sewage sludge 25% + energetic
willow 25%: a) with respect to the process temperature; b) with respect to time of the process
Fig. 10. Curves TG-DTG-DTA for the combustion of mixture: hard coal 45% + sewage sludge 10% + energetic
willow 45%: a) with respect to the process temperature; b) with respect to time of the process
Fig. 11. Curves TG-DTG-DTA for the combustion of mixture: hard coal 37.5% + sewage sludge 25% +
energetic willow 37.5%: a) with respect to the process temperature; b) with respect to time of the process
Table 6. Percentage change of fuels mass, analyzed at intervals of time at 20 min
Fuel Time of process, min
0-20 20-40 40-60 60-80 80-100 100-120
Sewage sludge Δm= 9.4% Δm= 29.7% Δm= 22.03% Δm= 2.12% - -
Energetic willow Δm= 7.02% Δm= 62.21% Δm= 26.5% - - -
Hard coal Δm= 2.56% Δm= 6.02% Δm= 37.28% Δm=
29.72%
Δm=
20.71% -
Energetic willow 50%
+ sewage sludge 50% Δm= 8.95% Δm= 45.95% Δm= 24.13% Δm= 0.01% - -
Hard coal 50%
+ sewage sludge 50% Δm= 4.97% Δm= 18.68% Δm= 29.06%
Δm=
25.06% Δm= 0.04% -
Hard coal 50% + sewage sludge 25%
+ energetic willow 25% Δm= 5.46% Δm= 25.56% Δm= 31.94%
Δm=
25.48% Δm= 0.01% -
Hard coal 45% + sewage sludge 10%
+ energetic willow 45% Δm= 6.2% Δm= 33.81% Δm= 33.73% Δm=21.15% - -
Hard coal 37.5% + sewage sludge
25% +energetic willow 37.5% Δm= 6.45% Δm= 35.15% Δm= 32.60%
Δm=
18.25% - -
Table 7. Percentage change of fuels mass, analyzed at intervals of temperature at 2000C
Fuel Temperature of process. oC
0-200 200-400 400-600 600-800 800-1000 1000-1200
Sewage sludge Δm= 8.21% Δm= 28.17% Δm= 24.34% Δm= 2.49% - -
Energetic willow Δm= 6.44% Δm= 59.29% Δm= 29.99% Δm= 0.01% - -
Hard coal Δm= 2.71% Δm= 3.66% Δm= 35.14% Δm=
31.05% Δm= 23.72% Δm= 0.01%
Energetic willow 50%
+ sewage sludge 50%
Δm=
8.06% Δm= 43.62% Δm= 27.07% Δm=0.29 % - -
Hard coal 50%
+ sewage sludge 50%
Δm=
4.46% Δm= 16.81% Δm= 27.92%
Δm=
27.23% Δm=1.39% -
Hard coal 50% + sewage sludge
25% + energetic willow 25%
Δm=
5.11% Δm= 23.32% Δm= 30.75% Δm= 27.9% Δm= 1.37% -
Hard coal 45% + sewage sludge
10% + energetic willow 45%
Δm=
5.89% Δm= 31.21% Δm= 33.05%
Δm=
24.73% Δm=0.01% -
Hard coal 37.5% + sewage sludge
25% +energetic willow 37.5%
Δm=
6.08% Δm= 32.80% Δm= 31.99%
Δm=
21.86% - -
Table 8. Parameters values of combustion process
Fuel
Mass rate of
combustion
[mg/s]
comb.
[min]
Tcomb.
[oC]
DTG max.evap.
[%/min]
TDTG max.evap.
[oC]
DTG
max.
[%/min]
TDTG max.
[oC]
DTA
max.
[μV/mg]
TDTA
max.
[oC]
Sewage sludge 0.45 75 774 0.7 112
2.08
1.43
0.26
282
472
705
1.14
1.42
351
523
Energetic
willow 0.82 59 613 1.03 98
8.82
8.16
281
317
1.31
1.27
388
580
Hard coal 0.49 100 1018 0.32 103 2.23 562 1.15
0.95
469
715
Energetic
willow 50% +
sewage sludge
50%
0.69 60 624 1.13 104 5.95 307 1.32
1.35
363
494
Hard coal 50%
+ sewage sludge
50%
0.49 82 844 0.48 100
1.16
1.49
1.66
311
472
562
1.35 499
Hard coal 50%
+ sewage sludge
25% + energetic
willow 25%
0.56 81 832 0.61 93 3.05
1.94
317
561
1.26
1.06
477
682
Hard coal 45%
+ sewage sludge
10% + energetic
willow 45%
0.61 79 810 0.78 96 4.23
1.88
318
563
1.27
1.12
363
708
Hard coal
37.5% + sewage
sludge 25%
+energetic
willow 37.5%
0.61 77 795 0.81 96 4.52
1.83
316
561
1.26
1.14
369
698
Mass rate of combustion= (m fuel – m ash) / comb.
where: m fuel – initial mass of fuel [mg], m ash – mass of ash [mg],
DTGmax.evap..- maximum evaporation rate of fuel, mg/min.,
Tmax.evap.- maximum temperature of fuel evaporation, ᵒC,
DTGmax. - maximum value of DTG, mg/min.,
TDTGmax.- temperature of maximum value of DTG, ᵒC,
DTAmax- maximum value of DTA, μV/mg,
TDTAmax - temperature of maximum value of DTA, ᵒC,
comb. – time of fuel combustion, s,
Tcomb. – temperature of fuel combustion, ᵒC.
Table 9. The change of rate of fuel mass loss to maximum value
Fuel Change of the fuel mass, reading from the curves TG/DTG Steps of combustion
Sewage sludge
t1 = 194oC
τ1 = 17.25min
DTG1=0.36%/min
t2 = 282.4oC
τ2 = 26.05min
DTG2=2.08%/min
∆t = 88.4oC
∆τ = 65.98min
∆m = 10.45%
Devolatilization,
volatiles combustion
Energetic willow
t1 = 193oC
τ1 = 17.16min
DTG1=0.02%/min
t2 = 281.46oC
τ2 = 25.95min
DTG2=8.82%/min
∆t = 88.46oC
∆τ = 8.79min
∆m = 13.44%
Devolatilization, volatiles
combustion
Hard coal
t1 = 264oC
τ1 = 24.21min
DTG1=0.13%/min
t2 = 562.01oC
τ2 = 54.02min
DTG2=2.23%/min
∆t = 298.01oC
∆τ = 29.81min
∆m = 31.45%
Devolatilization,
volatiles combustion,
char combustion
Energetic willow 50%
+ sewage sludge 50%
t1 = 190oC
τ1 = 16.82min
DTG1=0.20%/min
t2 = 307.02oC
τ2 = 28.51min
DTG2=5.95%/min
∆t = 117.02oC
∆τ = 11.69min
∆m = 21.86%
Devolatilization, volatiles
combustion
Hard coal Sobieski
50% + sewage sludge
50%
t1 = 208oC
τ1 = 18.57min
DTG1=0.20%/min
t2 = 561.57oC
τ2 = 53.98min
DTG2=1.66%/min
∆t = 353.57oC
∆τ = 35.41min
∆m = 38.36%
Devolatilization,
volatiles combustion,
char combustion
Hard coal Sobieski
50% + sewage sludge
25%
+ energetic willow
25%
t1 = 201oC
τ1 = 17.91min
DTG1=0.16%/min
t2 = 317.01oC
τ2 = 29.52min
DTG2=3.05%/min
∆t = 116.01oC
∆τ = 11.61min
∆m = 12.18%
Devolatilization, volatiles
combustion
Hard coal Sobieski
45% + sewage sludge
10%
+ energetic willow
45%
t1 = 200oC
τ1 = 17.82min
DTG1=0.09%/min
t2 = 318.05oC
τ2 = 29.61min
DTG2=4.23%/min
∆t = 118.05oC
∆τ = 11.79min
∆m = 16.88%
Devolatilization, volatiles
combustion
Hard coal Sobieski
37.5% + sewage
sludge 25% +energetic
willow 37.5%
t1 = 207oC
τ1 = 18.52min
DTG1=0.13%/min
t2 = 316.03oC
τ2 = 29.41min
DTG2=4.52%/min
∆t = 109.03oC
∆τ = 10.89min
∆m = 17.64%
Devolatilization, volatiles
combustion
t – temperature, 0C τ – time, min
Based on Fig. 4-11 and Tables 6-9 it can be concluded that:
The intense change of the mass of sewage sludge in the initial stages of the process (0-40 s,
0-4000C) based on the fuel evaporation and devolatilization.
The rapid mass loss of energetic willow in the next stage of the process (20-40 s, 200-
4000C) is due to the high content of volatile compounds in the fuel.
The intense change in the mass of coal in the range (40-60 s, 400-6000C) confirms that the
combustion of the fuel determined by char combustion.
The addition of sewage sludge to energetic willow accelerates the first stage of the process
(0-20 s, 0-2000C) in relation to the pure energetic willow, but slows down the next step
(20-40 s, 200-4000C) and leads to a slight prolongation the total time of the process.
The addition of sewage sludge to hard coal intensifies the initial stages of the process (0-40
s, 0-4000C), in relation to the pure coal and causes shortening the process time.
The addition of mixture (sewage sludge 25% + energetic willow 25%) to hard coal
intensifies the process in the initial stages (0-40 s, 0-4000C) and leads to a reduction in
combustion time, compared to coal.
The increase in the share of sewage sludge in a mixture of coal and willow, from 10% to
25%, leads to the intensification of the initial stages (0-40 s, 0-4000C) and slightly
reduction of combustion time.
In the case of sewage sludge and energetic willow can be seen more than one peak in the
DTG curve, demonstrating the specificity of devolatilization of this type of fuel.
The maximum rate of mass loss of sewage sludge, energetic willow and hard coal takes
place, respectively, at temperatures: 2820C (devolatilization and volatiles combustion);
2810C (devolatilization and volatiles combustion) and 562
0C (char combustion).
The addition of sewage sludge to energetic willow leads to an increase of the temperature,
at which occurs the maximum speed of the process, in reference to energetic willow.
The addition of sewage sludge to coal decreases the temperature, at which occurs the
maximum speed of the process, in reference to hard coal.
The addition of mixture (sewage sludge 25% + energetic willow 25%) to hard coal
decreases the temperature, at which occurs the maximum speed of the process, in reference
to hard coal.
The increase of sewage sludge in a mixture of hard coal and energetic willow: (hard coal
45% + sewage sludge 10% + energetic willow 45%) to (hard coal 37.5% + sewage sludge
25% +energetic willow 37.5%) decreases the temperature, at which occurs the maximum
speed of the process.
The increase of carbon in the basic fuel causes an increase of temperature, at which the
DTA reaches a maximum value.
The addition of sewage sludge to a mixture of energetic willow and hard coal (hard coal
50% + sewage sludge 25% + energetic willow 25%) decreases the temperature, at which
the DTA reaches a maximum value, in reference to hard coal.
The increase of sewage sludge in a mixture of hard coal and energetic willow: (hard coal
45% + sewage sludge 10% + energetic willow 45%) to (hard coal 37,5% + sewage sludge
25% +energetic willow 37,5%) decreases the temperature, at which the DTA reaches a
maximum value.
The change of rate of fuel mass loss to the maximum value, in case of hard coal, takes
place during the char combustion.
The addition of sewage sludge and energetic willow to hard coal causes that, the change of
rate of fuel mass loss to maximum value takes place during devolatilization and volatiles
combustion.
3. CONCLUSIONS
The Polish energy sector relies on hard coal. However, other biomass or waste fuel solutions
are needed, including sewage sludge.
The experimental studies performed for this study resulted in the following conclusions.
1. A high volatile content in the fuel intensifies combustion process.
2. The addition of the sewage sludge to energetic willow accelerates the first stage of the
process in relation to the energetic willow, but slows down the next step and leads to a slight
prolongation the total time of the process.
3. The addition of the sewage sludge to hard coal intensifies the initial stages of the process,
in relation to the coal, and causes shortening the process time.
4. The maximum rate of mass loss of sewage sludge and energetic willow takes place during
devolatilization and volatiles combustion and the maximum rate of weight loss of hard coal
takes place during char combustion.
5. The increase of sewage sludge in a mixture of hard coal and energetic willow, from 10% to
20%, decreases the temperature, at which occurs the maximum speed of the process, and at
which the DTA reaches a maximum value.
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ANALIZA TG-DTA OSADÓW ŚCIEKOWYCH, WĘGLA, BIOMASY ORAZ ICH
MIESZANEK
Słowa kluczowe: osady ściekowe, biomasa, węgiel kamienny, współspalanie, analiza TG-DTA paliw
Streszczenie. W pracy zaprezentowano porównawczą analizę termiczną TG-DTG-DTA osadów ściekowych
oraz węgla i biomasy, a także ich mieszanin w różnych proporcjach, przeprowadzoną w atmosferze powietrza.
Ze względu na problem wykorzystania i zastosowania osadów ściekowych, związany chociażby z
zaostrzającymi się przepisami prawnymi dotyczącymi składowania tych odpadów o cieple spalania powyżej 6
MJ/kg s.m., od 1 stycznia 2016r., niezwykle istotna staje się ich termiczna utylizacja (spalanie i współspalanie z
innymi paliwami: węglem czy biomasą). Odpowiednio zaplanowany przebieg spalania paliwa, uwzględniający
jego właściwości, gwarantuje uzyskanie wysokiej sprawności procesu. Współspalanie osadów ściekowych z
węglem prowadzi do wzrostu kaloryczności paliwa (w odniesieniu do samych odpadów), a ich współspalanie z
biomasą powoduje redukcję emisji CO2.
Kijo-Kleczkowska Agnieszka, dr hab. inż., prof. nadzw., Politechnika Częstochowska,
Wydział Inżynierii Mechanicznej i Informatyki, Instytut Maszyn Cieplnych, e-mail:
[email protected]; członek Sekcji Spalania Komitetu Termodynamiki i Spalania PAN;
działalność naukowa: badania mechanizmu i kinetyki procesu spalania oraz współspalania:
paliw węglowych, biomasy, mułów węglowych oraz osadów ściekowych, w strumieniu
powietrza oraz w warstwie fluidalnej; termodynamika; wymiana ciepła; ochrona środowiska.
Szumera Magdalena, dr inż., adiunkt, Akademia Górniczo-Hutnicza im. St. Staszica,
Wydział Inżynierii Materiałowej i Ceramiki, Katedra Ceramiki i Materiałów Ogniotrwałych,
e-mail: [email protected], działalność naukowa: chemia ciała stałego, badania nad
materiałami dla ochrony i kształtowania środowiska w aspekcie otrzymywania, poznania ich
struktury oraz właściwości.
Środa Katarzyna, mgr. inż., doktorant, Politechnika Częstochowska, Wydział Inżynierii
Mechanicznej i Informatyki, Instytut Maszyn Cieplnych, e-mail:
[email protected], działalność naukowa: badanie właściwości i spalania
osadów ściekowych, współspalanie osadów ściekowych z paliwami węglowymi i biomasą,
ochrona środowiska, energetyka.
