Fuel Processing Technology 104 (2012) 80–89
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Characterisation of the properties of alternative fuels containing sewage sludge Małgorzata Wzorek ⁎ Department of Process Engineering, Opole University of Technology, ul. Mikołajczyka 5, 45–271 Opole, Poland
a r t i c l e
i n f o
Article history: Received 26 September 2011 Received in revised form 10 April 2012 Accepted 18 April 2012 Available online 10 June 2012 Keywords: Sewage sludge Fuel from waste Co-combustion process Cement rotary kiln
a b s t r a c t The paper presents the characteristics of the fuels which were prepared with the use of sewage sludge and other waste materials. Applicability of those fuels was analysed in the coal co-combustion processes, and in particular in the cement clinker manufacturing process. Three types of sludge-derived fuels were subjected to comparative analyses: the fuel which was prepared with the use of sewage sludge and coal slurry (PBS fuel), that involving sewage sludge and meat and bone meal (PBM fuel), and that in which sewage sludge was composed with sawdust (PBT fuel). Physical and chemical properties of those fuels were investigated, with special attention paid to their calorific values and physical properties. The results showed that the fuels manufactured with the use of waste materials offered the energy values which were satisfactory for the cement industry as specified for alternative fuels in that branch. The tests for physical properties revealed that such fuels may be subjected to mechanical handling operations in the transport and storage processes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The municipal sewage sludge makes the principal type of waste materials which are connected with sewage treatment. Its volume is evaluated to reach about 3% of the waste water which is subjected to treatment. Expansion of the sewerage systems and construction of new sewage treatment plants resulted in the rapid growth of the available volumes of the sludge, and consequently the problem of their utilisation was faced. That problem relates in particular to new EU member states, inclusive of Poland, where the sewerage systems have been expanded extensively. EU Directives [1,2] restricted considerably the use of the sewage sludge in agriculture and imposed a ban on their landfilling. Hence, it is necessary and urgent to seek solutions for safe neutralisation and disposal of those materials. Thermal methods for the sewage sludge utilisation attract more and more interest recently. The heat energy which is contained in the sludge may be recovered in the combustion processes, in incineration plants intended for the municipal sewage sludge only [3,4], or in cocombustion processes with other energy carriers within various industrial plants, e.g. in power plants, heat and power stations, municipal waste incineration plants, and in high-temperature processes which provide favourable conditions for thermal degradation of wastes [5–8]. The cement clinker burning process in particular is especially favourable for degradation of wastes. That advantageous situation
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results, among other things, from the high temperature level in a rotary kiln, where the feeds need to be heated up to about 1450 °C, where the heat exchange and mass transfer surface areas are high, and where there is no problem of incineration residue at all (the whole volume of ash makes a component of the clinker product). The cement industry makes use of various alternative fuels which are based on wastes, and these may substitute for as much as about 40% of conventional fuels. Additional sources of energy attract the attention of the cement industry since the clinker burning process is highly energy-consuming. The theoretical heat demand for clinker burning is about 1760 kJ/ kgclinker (dry production method) while the actual heat consumption ranges from 3000 to 4000 kJ/kgclinker [9,10]. Many years of experience of the cement producers in the use of alternative fuels make it possible to evaluate the applicability of wastes which are charged to the clinker burning process. In particular, the clinker quality is a matter of concern. Hence, physical–chemical properties were defined which should be offered by the alternative fuels in order to avoid any disturbances in the kiln operation and any negative effects on the clinker quality, and in order to control emissions of pollutants [11–13]. The wastes to be employed as fuels must meet a number of parameters which are required by the process itself. The alternative fuels should first of all show the acceptable level of the following performance properties: – performance as fuels, including their calorific values which make the decisive parameters for the amounts of the conventional fuel to be substituted,
M. Wzorek / Fuel Processing Technology 104 (2012) 80–89
– chemical composition, since the ash produced in the combustion process will then be absorbed by the clinker product, – physical properties, which define stability of such fuels in the transport, storage and kiln feeding operations. In order to satisfy the requirements for the wastes to be used as energy sources, both in the cement industry and in other industries, various types of wastes are more and more frequently combined together to yield blends which are adjusted, both from the viewpoint of their energy potentials and physical properties, to the intended utilisation processes. The wastes are subjected to consolidation, e.g. pelletizing, briquetting or granulation, and those processes produce fuels with a pre-defined form to make their transport and storage operations easier [14–17]. The processed wastes, in the form of granules or briquettes, can be stored safely with no risk of secondary environmental pollution. As they can be transported easily, the area of their potential outlets is expanded, too. The sewage sludge, and in particular the material after mechanical dehydration, does not carry much energy with it since its water content is 70–80% on average. Only after it is dried, or after it is utilised as a component in the fuel manufacturing processes, it can make an attractive source of energy. This paper provides a suggested route for the use of the sewage sludge in combination with other selected wastes, i.e. with coal slurry, with the meat and bone meal and with wastes from the wood industry. Those compositions could then be converted into granulated fuels with the performance properties as required by the cement industry. 1.1. Characteristics of fuel components 1.1.1. Sewage sludge The EU countries are estimated to produce about 8 million Mg (as dry matter) of municipal sludge per year. In Poland, the amount of stabilised sewage sludge reached 613,000 Mg (dry matter) in 2010, and that amount grows by about 30,000 Mg/year (dry matter) on average [18]. The physical–chemical properties of the municipal sewage sludge make that material hard to utilise from the technical point of view. That problem is essentially conditioned by high water contents in sewage sludge materials which exceed 99% for the green sludge and which can be reduced by mechanical dewatering to 80–65%. The sludge is greasy at such a high water content which makes the handling and transport operations hard to do. Moreover, the sludge contains biologically active substances; they are responsible for the specific stinking odour which is repulsive for the surroundings. The fraction of organic components makes the indication for the performance of sewage sludge as a fuel. Werther and Ogada [3] state that the primary sludge contains 3–5% of dry matter in which organic substances make 55–70%. After fermentation, the organic substances content is reduced to 40–55% (of dry matter) [7]. The scope of that decline affects the calorific value of the sludge. The literature reports [3,8,19] specify that the residue after methane fermentation offers the calorific value within 6.7–12.0 MJ/ kgdry matter, which is lower than for the primary sludge (13.30– 17.50 MJ/kgdry matter) and for the surplus activated sludge (15.00– 17.00 MJ/kgdry matter). The energy-related properties of sewage sludge samples are presented in Table 1. The municipal sewage sludge also carries the load of various organic micro-pollutants (like PAHs, PCBs, PCDDs and PCDFs) and pathogenic organisms. However, the most important and most numerous groups of micropollutants are formed by heavy metals. The waste water treatment process will transfer them to and accumulate in the sewage sludge material, and their contents in the sludge may vary within a wide range.
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Table 1 Composition of biologically stabilized sewage sludge samples. Parameter
Unit
Proximate analysis HHV MJ/kg Water % a Ash % a Volatiles %
Sewage 1
Sewage 2
Sewage 3
Sewage 4
[20]
[21]
[22]
[23]
11.32–12.90 79.25–82.50 35.51–40.28 47.55–56.94
9.50 3.90a 53.80 42.80
9.09 76.20 36.62 50.10
13.34 8.50a 43.30 50.80
22.70 3.30 15.50 3.10 1.60 No data
21.81 3.66 18.07 3.94 1.10 0.057
30.10 4.12 17.84 3.79 0.85 No data
Ultimate analysis wt.% of dry matter C % 23.75–27.24 H % 3.37–3.72 O % 26.57–27.90 N % 3.98–4.37 S % 1.11–1.18 Cl % 0.85–0.17 HHV—high heat value. a In dry matter.
The heavy metal contents in various sewage sludge samples are presented in Table 2. The analytically established properties which are interesting for the energy industry demonstrate that the thermal utilisation (i.e. combustion) of the sewage sludge is possible only when its moisture content is low; the sludge may be used together with other fuels or it may be a component in a composition fuel product. 1.1.2. Meat and bone meal The volume of wastes of animal origin which are neutralised and disposed of annually in the EU is as high as 9 million t (data for the earlier fifteen countries), inclusive of about 2 million t/year in Germany and 2.4 million t/year in France. Poland processes about 685,000 t of animal wastes per year and that level is predicted to reach about 770,000 t over the next 5–7 years [26]. The animal by-products which are generated chiefly by the meat industry need special treatment because of the risk of biological pollution of the environment. They have to be utilised quickly due to the bacteriological hazard, repulsive stench and problems in storage. Since problems were faced recently in utilisation of animal meals (ban on feeding the meals to animals because of BSE), more attention was paid to the energy content of those materials. Energy-related properties of animal meals are presented in Table 3. Meals offer some properties which are favourable for their use in industrial combustion processes. These are: low water content, calorific value which is close to that of midrange hard coal, and sulphur content – below that of brown coal. The use of the meat-and-bone meals in the energy sector faces technical problems which result from the dust generated by those materials and from their fat content—they tend to stick all over the
Table 2 Contents of trace elements in sewage sludge samples. Metal
Sewage 1
Sewage 2
Sewage 3
Sewage 4
ppm
[20]
[21]
[24]
[25]
1236–57,994 28.0–53.56 1931–3503 36.34–64.89 2.88–9.30 1.96–33.39 104–193.5 2.77–6.72 0.91–2.57 0.15–0.64 1.03–3.09 0.06–1.13
23,586–26,000 106.0–380.0 2432–6100 20.0–49.5 10.9–40.0 16.0–50.0 80.0–800.0 6.2–15.3 1.99–2.50 No data No data 23.1–27.1
No data 66–2021 354–640 26–465 No data 37–179 80–2300 No data No data No data 2.3–10 No data
1000–154,000 10–990,000 101–49,000 13–223.0 11.3–2,490 23.2–36.5 204–1337 1.1–230 0.6–56 2.6–329 1–3.410 1.7–17.2
Fe Cr Zn Pb Co Ni Cu As Hg Tl Cd Sn
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M. Wzorek / Fuel Processing Technology 104 (2012) 80–89
Table 3 Properties of animal meals [27,28]. Parameter
Units
Table 5 Energy-related specification of Polish coal slurry [31].
Bone meal
Meat meal
Blood meal
13.8–14.1 3.3–4.9 34.5–41.4
17.3–25.0 0.5–7.0 10.9–20.5
21.9–23.4 3.9–4.0 1.4–1.6
Ultimate analysis wt.% of dry matter C % 30.5–33.3 N % 3.5–51.8 H % 7.3–7.9 O % 0.1–0.3 S % 4.4–4.7
40.5–43.8 7.7–43.8 4.1–5.3 13.9–20.0 0.3–0.5
51.8–52.0 15.6 6.5 – 0.64
Proximate analysis LHV MJ/kg Water % Asha %
LHV—high heat value. a In dry matter.
transportation systems. Moreover, biological activity of meals will become evident under high humidity conditions: digestion processes will be initiated which will pose a health hazard to the operating personnel [28]. The energy value of the meat-and-bone meal suggests its utilisation in the combustion processes, and when used in combination with the municipal sludge, it may add to the energy advantages of the fuel produced for such a mix. 1.1.3. Wood industry wastes The woodworking companies are estimated to produce about 7.5 million m 3 of waste wood material per year, which makes 27% of the total timber harvested [29]. More than 63% of the wood waste come from sawmills. That group of wastes comprises first of all lump wastes in the form of various types of edgings, root swellings, and also sawdust and bark. Additional volumes of wastes come from the furniture industry (wastes of woodbase materials and of solid wood, wood dust, as well as sawdust and shavings). Properties of industrial wood wastes are presented in Table 4. Joint utilisation of the sewage sludge and wood biomass makes it possible to take advantage of the favourable legislation which promotes the growth of the renewable energy production. Hence, that outlet may help in eliminating the waste disposal problem. 1.1.4. Coal slurry In Poland, classless size grades of coal, inclusive of coal slurry, make a considerable group of selectively recovered wastes which are produced in the hard coal treatment and cleaning processes. Coal slurry is recovered from water-slurry cycles in coal mines, and Table 4 Properties of industrial wood wastes [29]. Parameter
Units
Proximate analysis LHVa, MJ/kg LHVb MJ/kg Water % Ashb %
Sawing wastes sawn Shavings from Dust from timber cutting sawn timber milling 6–10 18.5–20.0 45–60 0.5–2.0
Ultimate analysis wt. % of dry matter C % 43.5–50.7 N % 0.1–0.5 H % 6.2–6.4 S % b0.05 Bulk density kg/m3 LHV—low heat value. a Fresh material. b In dry matter.
250–350
Plywood wastes
13–16 19–19.2 5–15 0.4–0.5
15–17 19–19.2 5–15 0.4–0.8
15–17 19–19.2 5–15 0.4–0.8
43.5–50.0 0.1–0.5 6.2–6.4 b0.05
43.5–50.0 0.1–0.5 6.2–6.4 b 0.05
43.5–50.0 0.1–0.5 6–6.4 b0.05
80–120
100–150
200–300
Region
Lower-Silesian South–East East Central West South a
Water short-lived hygroscopic %
%
10.5–38 13–31 11.5–36 22–28 16–31 16–31
1.0–1.5 3.5–8.5 2.5–7.0 3.5–4.5 1.0–2.0 1.0–3.5
Asha %
Volatilesa %
LHV MJ/kg
27–34 23–24 12–30 19–31 17–29 16–26
21–27 37–38 36–39 35–37 27–41 32–37
19.80–24.60 16.95–20.90 18.30–25.40 19.20–23.40 21.10–28.90 22.80–27.60
In dry matter.
its volume reaches 6–16% of the total amount of coal-related wastes [30]. Coal slurry offers a relatively low calorific value, high ash and water contents, and a very low grain size (0 ÷ 1 mm). The properties of Polish coal slurry from different region of the country are presented in Table 5. Coal slurry makes a commercial product at present; it is offered as a separate size grade product which may be used in the professional power engineering, in fluidized-bed combustion processes. Coal slurry may be used in the production of coal concentrates, special fuels in the form of aqueous coal suspensions and/or briquettes, in soil consolidation, in reclamation of light soils, etc. [32]. Despite numerous possible outlets for coal slurry, most of its volume remains outside any commercial utilisation and it is transferred to settlers. 2. Experimental 2.1. Materials The sewage sludge for the study was obtained from the municipal mechanical-biological sewage-treatment plant with Population Equivalent of 225,000. In order to improve the properties of the sludge and to convert it into an alternative fuel which is applicable in the clinker process, the sludge was blended with other wastes. Because of the greasy consistence of the sludge, other components had to offer a low water content and a powdery structure. Moreover, their calorific values should be high enough for the final fuel to have the value of over 13 MJ/kg. The meat and bone meal was one of the components which were employed for the conversion of the sludge into fuel. Its grain size was ≥4 mm and its odour was strong. Broken up bone pieces and animal hair could be observed in the structure of the meal. Concentrated coal slurry was also used for blending. The size of its particles was b1 mm and its humidity was about 6%. The beech sawdust made the third component for fuel blending. Its grain size was b12 mm and it was received from the wood processing works. The energy properties and ash compounds of the fuel components are presented in Tables 6 and 7. Three types of alternative fuels were produced on the basis of the sewage sludge and other waste materials. In order to identify the produced fuels, individual symbols were attributed to them: − PBS—fuel which was based on the sewage sludge and coal slurry, − PBM—fuel obtained from the sewage sludge and meat and bone meal, − PBT—fuel which was produced with the use of sawdust. 2.2. Preparation of fuel samples and operating procedure The sludge and other waste materials were processed and converted into fuels in a demonstration stand as shown in Fig. 1, with
M. Wzorek / Fuel Processing Technology 104 (2012) 80–89
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Table 6 Energy properties of fuel components. Parameter
Units
Sewage sludge
Coal slurry
Meat and bone meal
Sawdust
Grain size/consistence
mm/–
Greasy
0–1
≥4
5–12
Proximate analysis LHV Moisture Volatilesa Asha
MJ/kg % % %
2.27/11.37a 80.10 49.57 38.70
23.53 6.46 45.19 24.33
17.45 5.27 68.82 23.75
17.50 7.31 64.60 6.16
59.57 4.30 1.29 8.68b 1.54 0.29
43.57 5.35 9.84 16.67b 0.47 0.35
50.94 5.61 0.00 37.28b 0.01 0.00
Ultimate analysis wt.% of C H N O S Cl a b
dry matter % 24.99 % 3.73 % 4.20 % 27.18b % 1.07 % 0.13
In dry matter. Calculated from the balance (difference).
Fig. 1. Drum drier. 1—screw-conveyor feeder, 2—screw-conveyor motor, 3—control box, 4—drum motor, 5—discharge bin, 6—supply of drying air, 7—drum, 8—drying air outlet.
2.3. Methods
the use of a drum drier which had been designed especially for that purpose [33]. The fuel production method comprised two steps: pre-mixing the sludge with other waste materials at pre-defined ratios, and granulation and drying. The fuel granulate is formed in the feeding device (1). There is a screw conveyor there with the exchangeable die plate at its end. The cutting tool which is located downstream is driven by the screw conveyor shaft. The fuel material is forced through the die plate and cut off to form the extrudate. Depending on the size of the holes in the die plate, it is possible to control the diameter of fuel particles within 10–40 mm. The granulated fuel then enters the drum (7) for drying. There are vertical flight plates inside the drum which are arranged along 1/3 of the drum length (preliminary drying section). The fuel material becomes dry when it is contacted with the counter-current flow of the drying medium (hot air or hot combustion gas) at the temperature below 200 °C. After passing along the drum, the granulated fuel goes to the discharge bin (5). The produced fuels were left to stabilise over about 7 days, and then their physical-chemical properties were investigated. The grain size of the fuels was 35 mm and their compositions were as follows: PBS—60 wt.% of sewage sludge, 34 wt.% of coal slurry, and 6 wt.% of burnt lime; PBM—75 wt.% of sewage sludge, 24 wt.% of meat-and-bone meal, 1 wt.% of burnt lime; PBT—80 wt.% of sewage sludge, 19 wt.% of sawdust, 1 wt.% of burnt lime.
Table 7 Major ash compounds of fuel components. Parameter wt.%
Sewage sludge
Coal slurry
Meat and bone meal
Sawdust
SiO2 Al2O3 Fe2O3 CaO MgO P2O5 SO3 Mn3O4 TiO2 SrO Na2O K2O
24.26 6.18 12.88 31.62 1.33 13.00 5.49 0.29 0.54 0.15 0.41 1.16
47.36 28.83 9.57 4.23 2.26 0.78 3.55 0.10 1.21 0.09 1.02 2.89
13.61 1.55 0.72 42.79 3.09 33.79 0.64 0.02 0.06 0.04 1.92 0.86
16.34 3.16 1.43 38.89 7.44 2.06 2.62 2.02 0.20 0.08 0.60 11.06
The fuels obtained from the sewage sludge were subjected to numerous tests and investigations to find their properties which might affect the combustion process itself and the cement clinker process. Their physical properties were analysed as well. The energy properties, i.e. calorific value was conducted by using Oxygen Bomb Calorimeter KL-Mn, in accordance with PN-ISO 1928: 2002, the ash content—in accordance with PN-ISO-562:2002, and the volatile matter content—in accordance with PN-ISO-562:2000. The ultimate analyses of the samples were performed via Elementary Analyser Vario Macro EL. The flash point values for the fuels were found by the Marcusson method, up to the standard PN-82/C-04008, and characteristic temperatures for ash fusibility—according to PN-82/-04535:1982. The thermal and thermogravimetric analyses involved the differential thermal analyser NETZSCH, type STA 4009EP, within 0÷1400 °C and under the oxygen atmosphere. The chemical composition of ashes was analysed by using the ICP method, and the remaining component levels—using the PANalitical XRF method. The mass spectrometer ICP MS Perkin Elmer, ‘Elan’ 6100, was used to learn the heavy metal contents, while polychlorinated biphenyls (PCBs) were determined as the total of congeners: PCB 28, PCB 52, PCB 101, PCB 138 and PCB 180, with the use of the Agilent 6890N gas chromatograph which was equipped with the ECD detector. The total polycyclic aromatic hydrocarbons (PAHs) were analysed with the use of the liquid chromatography assembly: HPLC 1200 Series. The analysis yielded (PAHs) like: naphthalene, phenanthrene, anthracene, fluoranthene, benzo-(a)-anthracene, chrysene, benzo(a)-pyrene, benzo-(b)-fluoranthene, benzo-(k)-fluoranthene, benzo(g,h,i)-perylene. Within the physical properties of the fuels, the following parameters were studied: bulk density (according to PN-ISO 567), strength-bydropping, water resistance, water absorbability and frost resistance (these were tested according to the procedures as below). In the strength-by-dropping test, as per PN-G-04651, samples were dropped down twice from 1.5 m against a concrete surface. The strength-by-dropping factor was calculated from the formula: W ZR ¼
N1 ⋅100% N
where: N—number of granules which were subjected to testing [pcs] N1—number of granules which survived with no damage [pcs] The water absorbability test was based on finding the amount of water which had been absorbed by the fuel material under conditions
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M. Wzorek / Fuel Processing Technology 104 (2012) 80–89
as defined by the standard PN-G-04652. The fuel samples were left to stand in water over 24 h, and the difference between the sample weights after and before water absorption made the measure of this parameter. Another parameter which was studied was water-resistance, according to PN-G-04652. It is defined as the ratio (in percent) of the strength-by-dropping factor after absorption of water (the sample was immersed in water over 24 h) and the original value of that factor. In practice, that procedure was performed immediately after the water absorbability test. In the frost resistance test, according to the adapted standard PNEN-1367-1, the fuel samples were stored in a low temperature chamber over 50 cycles (i.e. 50 days). The number of cycles was selected to reflect the average period of negative temperature values in winter. One cycle involved freezing fuel samples down to − 18 ± 2 °C which was followed by thawing them up to 18 ± 2 °C. The samples were then subjected to the strength-by-dropping tests.
Table 8 Energy properties of sludge—derived fuels, hard coal and requirements of the cement industry. Parameter
Proximate analysis LHV MJ/kg HHV MJ/kg Moisture % a Volatiles % % Asha
3.1. Energy properties of sludge-derived fuels The low calorific value for any alternative fuel makes the principal parameter which is decisive for the share of the conventional fuel which may be substituted in the clinker manufacturing process. A good quality clinker can be produced only when adequate thermal conditions are maintained in individual sections of the kiln; and that is affected by the calorific value of the fuel employed and by the way of its combustion. The low calorific value of the alternative fuel should be close to that of the primary fuel (about 25 MJ/kg). Wastes are known, however, to offer lower heating values, which are considerably lower in many cases. The minimum low calorific value has been established therefore for alternative fuels (≥13 MJ/kg) which can assure both proper kiln operation and satisfactory quality of the clinker product. Fuels with such low calorific values may not be used in the clinker sintering zone (about 1450 °C) but only in the calcination zone where the temperatures of only about 900 °C need to be provided [34]. The heating potential of the sludge-derived fuels is shown in Table 8 and compared with the properties of hard coal and with the requirements as set by the cement industry for the alternative fuels. The fuels produced from the sewage sludge offer the heating value within 13–19 MJ/kg, hence they satisfy the minimum requirement of the cement industry. The heating values of the obtained fuels are comparable to that of RDF (Residue-Derived Fuel) which is produced on the basis of the flammable fraction of the municipal waste. Depending on its specific composition, RDF may offer variable calorific values which may fall within 15–20 MJ/kg, as per Genon et al. [36] but the values below 14 MJ/kg [37,38] are also possible. The fuels with such heating values may be used to substitute up to about 10% of the conventional fuel in the clinker burning process. Within the alternative fuels prepared in this study, the highest heating value was offered by PBS fuel, while the lowers value was found for the fuel which was based on the sludge and on sawdust fuel (PBT)—its calorific value was only one half of the value of hard coal (the calorific value of Polish hard coal equals to 20–29 MJ/kg). PBM and PBT fuels had high contents of volatiles (that figure is 2.4 ÷ 5.9% for coal) which suggested that their reactivity should be high: they should catch fire more easily and they should burn faster. When the figures in Table 8 are analysed and compared, it becomes apparent that hard coal and sludge-derived fuels have the same basic elemental composition. The differences, however, appear in the percentages for individual elements. The alternative fuels contain more oxygen and nitrogen, but their elemental carbon contents make only one half of that for hard coal (PBM and PBT fuels). In
PBS fuel
PBM fuel
PBT fuel
Hard coal [14]
Requirements of cement industry [35]
19.30 21.71 8.58 34.44 27.86
14.59 15.97 8.67 56.84 33.72
13.23 15.54 10.37 66.74 20.36
20.5–31.9 – 5.5–10.00 2.40–5.90 3.10–24.00
≥13 – b30 – b40
31.42 4.43 2.61 40.50b 0.65 0.03
64.1–72.5 4.00–5.60 2.60–12.80 No data 0.60–1.4 b 0.10
– – – – b2.50 b0.30
Ultimate analysis wt.% of dry matter C % 50.28 36.64 H % 3.91 4.12 N % 1.72 6.87 O % 15.01 17.95b S % 1.16 0.68 Cl % 0.06 0.02 a
3. Results and discussion
Units
b
In dry matter. Calculated from the balance (difference).
relation to hard coal, they contain less sulphur at the lower limit observed for coal (the sulphur content in coal is 0.6 ÷ 1.4%). Because of the strongly alkaline conditions in the rotary kiln, in the clinker production process, sulphur in the form of SO2 is chemically bounded to produce sulphates and it is discharged from the kiln together with the clinker product as well as with the cement kiln dust (CKD). The remainder is emitted in flue gases as SO2. The data presented by CEMBUREAU [39] show that the use of alternative fuels in cement kilns does not increase the total emissions of SO2 and other pollutants like: HCl, TOC, NOx. Emissions of SO2 from the clinker manufacturing process results predominantly from the sulphide contents in the natural raw materials contain volatile sulphur compounds (e.g. pyrite) which are used in the cement industry [40]. Hence, the sulphur content in alternative fuels is restricted because of the required clinker quality. Chlorine is another objectionable component of fuels. Its presence affects the emission of acid gases. Moreover, it is responsible for the so-called internal circulation between the sintering zone and the cyclone heater in the dry method for the clinker production. The solid material will consequently build up in cyclones, in pipelines and in the heater, which will upset the stable flow of gas and other material streams [41]. The chlorine content in a fuel may be responsible for the synthesis of dioxins and furans (PCDDs and PCDFs) under some specific combustion conditions. Hence, the combustion criteria have been established for the chlorinated organic compounds and polychlorinated biphenyls (PCBs). Said conditions have to be adhered to if those compounds are to be destroyed in the incineration process [42]. In the light of available scientific information and numerous measurements of PCDDs /PCDFs emissions, the modern European cement plants and those operated in other parts of the world, in which alternative fuels are used, release dioxins at the concentration below 0.1 ng TEQ/m 3 in their flue gases, i.e. below the permissible level [43]. Grochowalski [44] for example quoted the average concentration of dioxins in the flue gases from the clinker burning process for Polish cement plants below 0.02% ng TEQ/m 3. Previous studies [45,46] have established that partial substitution of conventional fuels with alternative fuels (including meat meal, used tires and waste materials with substantial chlorine contents) in the process of clinker burning does not lead to increase of level of emission of PCDDs/PCDFs. The chlorine content in all sludge-derived fuels is comparable and it makes about 15% of the level which is defined acceptable for the cement industry (b3%). For example, the chlorine content in the PASi fuel (fuel produced with the use of paper, sawdust and liquid wastes, i.e. solvents, paints and lacquers) amounts to 0.24% [35], in
M. Wzorek / Fuel Processing Technology 104 (2012) 80–89
the tire derived fuel (TDF) [47] that content is 0.4%, while in the RDF fuel—from 0.28 to 0.7% [36]. As mentioned earlier, ash from the fuel combustion process is absorbed by the clinker product. Hence, the amount of ash and its chemical composition are important from the viewpoint of the clinker quality. The chemical compositions: for the ash from the studied fuels and for the hard coal ash, are arranged for comparative presentation in Fig. 2. The ash materials from the sludge-containing fuels have different properties as compared to the hard coal ash. The former show much higher contents of CaO and lower contents of acidic components (SiO2 and Al2O3), which is specific for biomass-derived fuels [50], but they contain the same oxides as the raw materials which are used for the production of clinker. Special attention should be paid to the content of P2O5 in the ash from PBM, i.e. from the fuel prepared with the use of the sludge and meat and bone meal. The content of P2O5 reaches up to about 20%. A high content of P2O5 is undesirable since it may adversely impact the hydraulic activity of cement. According to Lea [51], the phosphorus content in clinker should be reduced to 0.2-0.3% only since higher phosphorus levels impair the clinker quality: they are catalytic to decomposition of alite—the principal phase component of clinker. Thus, the fuel which is based on sewage sludge and meat and bone meal (PBM fuel) may be used in the cement clinker process at the amount which is equivalent to about 5% of the total heat demand. In some European countries, however, where the meat and bone meal is used as a fuel, the P2O5 content in clinker may reach up to 0.5%. The alternative fuels may be substituents for 7% of conventional fuel then [34]. Besides the regular mineral components, the ash from PBS, PBM and PBT fuels contains also heavy metals. The heavy metal contents for sludge-based fuels are provided in Table 9. The results demonstrate that the levels of heavy metals contained in the studied fuels comply with the limits for hard coal. The highest concentrations were noted for manganese (PBM and PBT fuels), zinc (PBT fuel) and chromium (PBS fuel). The heavy metals which can be found in the fuels come predominantly from the sewage sludge, and their amounts in a fuel are contingent upon their contents in the sludge which was taken for the production. The low-volatility trace elements, i.e. Sr, Mn, Sn, Ba, Sn, As, Cr, Co, Cu, Zn, Sb, Mo, Ni and V, become completely embedded in the clinker structure. The American research revealed that those elements could be fixed in various clinker phases up to 99.9% [52]. The progress in the technology for co-combustion of alternative fuels, however, may be declared to be responsible for the increased levels of some heavy metals in clinker. That may be applicable for example to chromium,
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Table 9 Heavy metal contents in sludge-derived fuels and in hard coal. Heavy metal ppm
PBS fuel
PBM fuel
PBT fuel
Hard coal [14]
Requirements of cement industry [35]
Mn Cr Zn Pb Co Ni V Cu As Sn Tl Cd Hg
165 b 181 250 63.20 14.12 b25 1.14 58.88 1.53 0.478 0.048 1.25 0.987
350.9 22.08 446.1 19.43 3.89 4.92 2.53 43.84 0.997 0.030 0.051 0.434 0.421
319.3 93.68 881.2 24.93 6.27 15.92 3.10 71.77 2.254 0.028 0.089 1.690 0.859
4–1990 0–60 2–3560 2–370 0–140 0–130 2–100 0.5–50 0–170 0.02–1 – 0.1–3.0 1–10
Total content b2500
Cd + Tl + Hg b100
Hg b 10
cadmium orantimony [53]. The use of spent oils and solvents [35], and used tires [47], is considered to be the reason of increased zinc content in clinker. From the viewpoint of atmospheric pollution, not only sulphur and chlorine make the unwanted fuel components. The presence of volatile heavy metals (Hg, Cd and Tl) and polychlorinated biphenyls (PCBs) is also objectionable. Hence, limits have also been established for those components in alternative fuels. The content of volatile heavy metals (mercury, cadmium and thallium) is lowest in PBM fuel where it amounts to 0.9% of the acceptable limit. That level for PBS fuel is 2.28%, while for PBT fuel it is 2.69% of the allowable value. Table 10 shows the contents of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) in the produced alternative fuels. As regards the PCBs level, the limit value of 50 ppm was defined by Lafarge Cement. The PCBs contents were determined in the tested fuels and they were found to be very low, close to the determination limits in practice. The flash point and the ash fusibility temperature should be learned for the complete specification of a fuel. Those parameters are not critical if the fuel is intended for the production of cement clinker. In the power industry, however, they provide the information on the efficiency of the combustion process and on potential hazards in the operation of boilers. The flash point values, as measured by the Marcusson method, are presented in Fig. 3. The lowest flash point (~315 °C) is offered by PBT fuel which can be accounted for by the highest content of volatiles in that fuel. A low flash point value means quick vaporisation and carbonisation of a fuel, and violent combustion of fuel volatile components. The flash point for PBM fuel equals to ~345 °C, while it is ~415 °C for PBS. That difference results from the fact that the biomass components were used in the production of PBT and PBM fuels which offer lower flash point values than conventional fuels. The combustion process is affected adversely by softening and then melting of the slag and ash. The temperature level and range at which that phenomenon is faced is dependent on many factors. The chemical composition of ash, and the type of combustion atmosphere, make the most important factors. If those parameters are known, one may approximately assess the potential hazard which will result from covering the heat exchange surfaces in power boilers Table 10 PCBs and PAHs contents in sludge-derived fuels.
Fig. 2. Chemical analysis of ashes of sludge-derived fuels and hard coal [14,48,49].
Parameter ppm
PBS fuel
PBM fuel
PBT fuel
Requirements of cement industry [35]
PCBs PAHs
b0.05 9.1
b0.05 2.9
0.11 9.3
b50 –
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Fig. 3. Flash point values for selected fuels [14,48].
with slag. The sintering, softening and melting temperatures for sludge-derived fuels in the oxidising and semi-reducing atmospheres are presented in Table 11. In order to observe the changes which take place in fuels at high temperatures, the samples were subjected to differential thermal analysis (DTA) and thermogravimetric analysis (TG), and the findings are presented in Fig. 4. Thermograms of test fuels demonstrate similar behaviours for those fuels. A small loss in weight can be observed up to the temperature of 150 °C for every fuel, which is attributable to water evaporation. At about 300 °C, the highest exothermic peak was observed which was due to fuel ignition (PBM fuel—310 °C, PBT fuel—290 °C). Two peaks were observed, however, for PBS fuel: at 320 and at 360 °C, which were probably caused by fuel ignition and coal decomposition. The combustion process then takes place up to the temperature of 800 °C. Some small endotherm is apparent at about 840 °C which results from decomposition of calcium carbonate contained in the ash. The loss in weight for PBS fuel is no longer observed at 1200 °C and it amounts to 77%. The same for PBM fuel takes place at 920 °C (loss in weight of 55%), and for PBT fuel at 820 °C (loss in weight of 69.7%). 3.2. Physical properties of sludge-derived fuels The fuels produced from wastes, alike regular fuels, can be characterised by a number of quality parameters which are important from the viewpoint of applicability. Apart from power performance properties, special attention is also paid to the fuel stability, homogeneity of its composition, and its physical properties. The quality parameters represent the form of a fuel, like shape, particle size, bulk density and stability of the fuel. Strength-by-dropping, water resistance, frost resistance or water absorption of a fuel, these are the principal properties which define the possible performance of that fuel in transport and storage
Table 11 Fusibility of ash after combustion of sludge-derived fuels. Parameter Oxidising atmosphere
Semi-reducing atmosphere
Sintering point Softening point Melting point Pour point Melting point Softening point Melting point Pour point
Unit
PBS fuel
PBM fuel
PBT fuel
Hard coal [48]
°C °C °C °C °C °C °C °C
1110 1210 1260 1290 1010 1110 1200 1230
1110 1260 1400 1440 1080 1190 1340 1380
1260 1350 1530 1560 1110 1320 1500 1540
910–1180 1250–1360 1280–1500 1350–1500 950 1230 1400 1420
Fig. 4. TG and DTA profiles for sludge-derived fuels: a) PBS fuel, b) PBM fuel, c) PBT fuel.
operations. The measured values of the strength-by-dropping factor for the sludge-derived fuels are presented in Fig. 5. The fuel which was based on the sludge and meat and bone meal (PBM fuel) and that based on the sludge in combination with sawdust (PBT fuel) are characterised by similar and very high values of the strength-by-dropping factor: it is 97% for PBM fuel and it reaches as high as 100% for PBT fuel. Hence, those fuel samples did not suffer from any damage, cracking or disintegration in practice. The factor for the PBS fuel, however, was much lower and considerable destruction was noted for some part of samples in tests. The strength properties of the fuels were not impaired by a month-long storage, and they even improved slightly for PBM. Most wastes which are utilised as fuel components have capillary structures and they are hygroscopic—they absorb water from the environment. As declared by Wandrasz [14], the sorption potential of a material is dependent on numerous factors, inter alia on the specific properties which are defined by the molecular structure, chemical composition, etc. Sorption of water is also controlled by temperature
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Table 12 Bulk density for sludge-derived fuels.
Fig. 5. Strength-by-dropping factor for sludge-derived fuels.
and time. Through sorption of atmospheric humidity, the fuels may lose their mechanical strength. Moreover, the organic components of a fuel may undergo biological decomposition under storage conditions. Water resistance makes one of parameters which define the change in strength-by-dropping of a fuel due to water absorption. The water resistance findings for sludge-derived fuels are presented in Fig. 6. The water resistance tests showed that water was destructive for the strength-by-dropping that value decreased for all the tested fuels. PBM fuel was found to be least water resistant, and its strength-by-dropping was reduced dramatically after absorption of water. That parameter declined by 30% for PBM fuel and by 10% for PBS fuel. Frost resistance shows the effect of changing temperatures on fuel quality. With regard to frost resistance (Fig. 6), the impact of varying temperatures (and in particular of negative temperatures) was not so extensive on the structure of fuels as it was observed for water. After the frost resistance test, the strength-by-dropping of PBS fell down by 30%. The PBM and PBT fuels, on the other hand, were more resistant to that type of impacts as their strength was reduced on average by 10% only. The water absorbability is another parameter which is used to evaluate the fuel performance under storage conditions. That parameter defines the water absorption capacity under defined conditions. This parameter is especially interesting because of considerable initial water content in sludge-derived fuels. The highest water absorbability potential was noted for PBT fuel (120%). That can be accounted for by its content of sawdust which absorbs water quickly. The lowest imbibition level was measured for PBM fuel (70%) which again results from its composition, i.e. from the content of meat and bone meal and its properties. Bulk density is an essential parameter from the viewpoint of transport: that property tells us about the weight-to-volume ratio for a loose material (granular, finely broken up crystalline substance, etc.).
Fig. 6. Water resistance, frost resistance and water absorbability.
Type of fuel
Bulk density kg/m3
PBS fuel PBM fuel PBT fuel Wooden pallets [14] Brown coal briquettes [14]
433 325 520 528 700–725
In order to be able to efficiently transport, store and use (i.e. burn) fuels, the compaction processes are employed which are especially applicable to biomass and agricultural wastes [54–56]. They are intended to increase the bulk density of solid fuels up to at least 250 kg/m 3 [57]. The bulk density values for granular or briquetted wastes are much higher. The bulk density values for sludge-derived fuels against those of other fuels as per [14] are provided in Table 12. The sewage sludge fuels are presented in Fig. 7. To recapitulate, the fuels produced with the use of the sewage sludge have the strength properties which make them stable in the transport, storage and handling operations. However, it is necessary to protect the fuels against rainfall and other negative atmospheric impacts under temporary storage conditions immediately before their use, i.e. before charging them to a furnace. Such fuels can be stored just in the open (the use of some roofing is recommended then) or in a closed storage accommodation.
Fig. 7. Sludge-derived fuels: a) PBS fuel, b) PBM fuel, c) PBT fuel.
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4. Conclusions In the paper it is suggested to utilise the sewage sludge by mixing it together with other properly selected wastes, i.e. with under-grade sized coal, with wastes from animal waste utilisation plants and with wood wastes. Those components can be converted into granulated fuels which meet the requirements of the cement industry. That route will make it possible to utilise the sludge as a source of energy, in an environmentally friendly way. The useful “value” of the sewage sludge can be improved by the admixture(s) of other waste(s) so that the sludge becomes applicable in the production of energy, if the energy properties of the sludge grade(s) are not unacceptably low. The fuels can be obtained in this way which offers the pre-assumed parameters. The parameters can be tailored to make the fuels fit for the co-combustion process with coal. Moreover, when processed together with other wastes, the sludge is converted into a granulated fuel which eliminates its disadvantageous consistence. The produced fuel can be transported and stored easily which expands its potential applicability. The produced fuels offer the physical-chemical properties which meet the requirements of the cement industry. The calorific values of PBM and PBT fuels fall within 13–15 MJ/kg, which classifies those materials as low-grade alternative fuels. The corresponding value for PBS is higher, above 19 MJ/kg (medium-grade fuel). When the content of harmful components is considered which adversely affect the clinker burning process, i.e. chlorine, sulphur and volatile heavy metals, the sludge-derived fuels satisfy the requirements of the cement industry for alternative fuels. The tests for physical properties demonstrated that the fuels may be subjected to mechanical handling operations which are connected with their transport, loading and unloading, etc. Definitely better strength parameters are offered by PBM and PBT, i.e. by the fuels prepared from the sludge and meat and bone meal, and from the sludge and sawdust. Destructive effects of water on strength-by-dropping were observed for all test fuels. The fuels are characterised by high water absorbability and they should be protected against atmospheric precipitation for short-time intermediate storage. The fuels which were produced with the use of the sludge may be employed in the clinker burning process instead of hard coal, and they may be charged to rotary kilns by means of the alternative fuel transport and feeding systems which exist in cement plants. Applicability of the sludge-derived fuels, however, may be much more extensive since their physical-chemical parameters make it possible to use such fuels in other co‐combustion processes with coal. References [1] Commission of European Communities. Council Directive 99/31/EC of 26 April 1999 on the Landfill of Waste, 1999. [2] Commission of European Communities. Council Directive 86/278/EEC of 4 July 1986 on the Protection of Environment and in Particular of the Soil when Sewage Sludge is used in Agriculture, 1986. [3] J. Werther, T. Ogada, Sewage sludge combustion, Progress in Energy and Combustion Science 25 (1995) 55–119. [4] T. Murakami, Y. Suzuki, H. Nagasawa, T. Yamamoto, T. Koseki, Combustion characteristic of sewage sludge in an incineration plant for energy recovery, Fuel Processing Technology 90 (2009) 778–783. [5] S. Stelmach, R. Wasilewski, Co-combustion of dried sewage sludge and coal in pulverized coal boiler, Journal of Material Cycles and Waste Management 10 (2008) 110–115. [6] M. Sanger, J. Werther, T. Ogada, NOx and N2O emission characteristics from fluidized bed combustion of semi-dried municipal sewage sludge, Fuel 80 (2001) 167–177. [7] P. Stasta, J. Boran, L. Bebar, P. Stehlik, J. Oral, Thermal processing of sewage sludge, Applied Thermal Engineering 26 (2006) 1420–1426. [8] S. Wetle, R. Wilk, A review of methods for the thermal utilization of sewage sludge. The Polish perspective, Renewable Energy 35 (2010) 1914–1919. [9] P. Alsop, The cement plant operations handbook, International Cement Review, 2001. [10] M. Carvalho, D. Madicate, Theoretical energy requirement for burning clinker, Cement and Concrete Research 29 (1999) 695–698.
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