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A review of methods for the thermal utilization of sewage sludge: The Polish perspective
Renewable Energy 35 (2010) 1914–1919
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A review of methods for the thermal utilization of sewage sludge: The Polish perspective Sebastian Werle*, Ryszard K. Wilk Institute of Thermal Technology, Silesian University of Technology at Gliwice, 44-100 Gliwice, Konarskiego 22, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2009 Accepted 15 January 2010 Available online 11 February 2010
On the basis of demographic projections, it is estimated that the quantity of sewage sludge which will be produced in Poland between 2010 and 2018 will increase from 612.8 thousand tons (dry basis, d.b.) to 706.6 thousand tons (d.b.). Currently, the predominant method for the disposal of this sludge is its storage and agricultural application. However, the legislation taking effect in the next few years will effectively block these avenues of sewage-sludge disposal. Therefore, effective methods of thermal sewage-sludge utilization must be developed. Here we review the state of knowledge and technology in thermal methods for the utilization of municipal sewage sludge to obtain useful forms of energy such as pyrolysis, gasification, combustion, and co-combustion. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Sewage sludge Pyrolysis Gasification Combustion Co-combustion
1. Introduction Municipal sewage sludge is generated in wastewater treatment plants in the process of sewage-treatment [1,2]. The waste stream of sewage sludge is rapidly growing, generating wastes which require management in compliance with the law [3]. According to Ministry of the Environment of Poland, sewage sludge belongs to group No. 19, including the wastes from the treatment installation and the equipment for waste management, wastewater from wastewater treatment plants and from the treatment of drinking water and water for industrial purposes [4]. Poland is inhabited by 38.2 million people with an average population density of 122 persons per square kilometer, and has a territory of 322,577 km2, of which 311,904 km2 is occupied by land. In the year 2008, in 3090 (both municipal and industrial) Polish sewage-treatment plants more than 1100 thousand tons (d.b.) of municipal and industrial sewage sludge was produced. In 2008, wastewater treatment plants serviced only 63% of the population (87% in urban areas and 26% in rural areas, where about 39% of the population lives); by comparison, in the countries of Western Europe, more than 78% of the population is serviced by waste treatment plants. In Poland, only 456 cities and 559 rural communities possessed a modern wastewater treatment plant with enhanced nitrogen and phosphorus removal. In these plants, 918 hm3 of waste was treated, which accounts for 73% of the waste discharged through urban and rural
* Corresponding author. Tel.: þ48 32 237 29 83; fax: þ48 32 237 28 72. E-mail address: [email protected] (S. Werle). 0960-1481/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.01.019
sewerage systems. This ratio fluctuates around 70% in most countries. The average unit index of sewage sludge generated in Polish wastewater treatment plants amounts to 0.25 kg d.b./m3 of treated wastewater. Table 1 presents the quantities of municipal sewage sludge produced in Poland in the years 1999–2007 along with the anticipated amount in the future. According to the Polish Environmental Policy [5] and the objectives of the National Waste Management Plan 2010 (NWMP) [1] as well as the National Urban Wastewater Treatment Program (NUWTP) [2], the quantity of sewage treated in Poland is systematically increasing. A measurable effect of this is, first of all, the increasing proportion of the population being served by wastewater treatment, but also the growth of the amount of produced sewage sludge (and sewage). On the basis of demographic projections, it is estimated that the quantity of sludge which will be produced in Poland between the years 2010 and 2018 will increase from 612.8 thousand tons (d.b.) to 706.6 thousand tons (d.b.) [1]. Currently, the predominant method for the disposal of sewage sludge is its storage and agricultural application. Sewage sludge is deposited in landfill sites dedicated exclusively to this sludge, in lagoons or jointly with municipal waste. Fig. 1 presents the current structure of the sewage-sludge management system [1]. In terms of the commitments derived from the introduction of European Union (EU) Directives, this structure of sewage-sludge utilization in Poland is highly unfavorable. The main problems are the high percentage of stored sewage sludge and a lack of installations for its thermal utilization. Thermal processes can be used for the conversion of large quantities of sewage sludge (e.g., in large urban areas) into useful energy. Processes for thermal utilization of
S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919 Table 1 The amount of municipal sewage sludge produced in Poland in the period 1999– 2007 [1] (and the anticipated amount in the future). Year
Amount of sludge produced, thousand tons (d. b.)
1999 2000 2002 2004 2005 2006 2007 2010 2018
354.0 359.8 435.0 476.0 486.1 501.3 533.4 612.8 706.6
sludge can be developed at existing installations (e.g., heating plants, power plants, or cement plants) or in newly built facilities. Thermal methods of sewage-sludge utilization should be preceded by dehydration and drying of sludge. In Poland, 98% of municipal sewage-treatment plants use a biological treatment, and, among them, 36% with enhanced biogenic removal (see Table 2). In the EU, 50% of sewage-treatment plants have anaerobic digestion, 18% incorporate aerobic digestion and 4% lime stabilization, whereas 24% of the plants undertake no sludge stabilization; the same tendencies are observed in Poland [6]. 2. Regulations and choices in sewage-sludge utilization methods As mentioned earlier, a significant reduction in sludge quantity is achieved through digestion. Additionally, there is a tendency in Europe to have the following hierarchy of sewage-sludge management priority: avoidance, minimization, recycling, and thermal methods with energy recovery and landfilling. There is a wide range of analyzed and proposed solutions for municipal sewage-sludge utilization. Nevertheless, there are serious legal constraints determining this choice. The first of these results from the application of the EU Directive 91/271/EEC (Urban Wastewater Treatment Directive) [7] and its amendment, Directive 98/15/EC. The most important wording of this directive is contained in Article 14, which says "sludge arising from wastewater treatment shall be re-used whenever appropriate". This sentence clearly formulates the aim of the directive in ordering the processing of sewage sludge. The implication of this document also expresses the necessity of building new
1915
Table 2 Wastewater treatment plants in Poland in 2008 as of December 31st. Municipal wastewatertreatment plants
Total
Mechanical Biological With increased biogenic removal (disposal)
Number 3090.0 60.0 9019.0 76.0 Capacity, dam3/24 h Population using waste 63.1 0.2 water treatment plants, % of total population
2233.0 1998.0 16.3
797.0 6945.0 46.6
sewerage systems and constructing and upgrading sewage-treatment plants, which would increase the mass flow of produced sewage sludge. The second act is the Sewage Sludge Directive 86/278/EEC [8], which introduces restrictions concerning the use of sewage sludge in agriculture. It mainly focuses on the concentrations of heavy metals in sewage sludge, and hence limits its agricultural and natural use. This directive was supplemented by entries appearing in the subsequent acts of the European Community, namely the so-called "Waste Directive" [9]. The Directive and the Regulation [10] firmly established rules for the problem of dealing with sewage sludge. A major consequence of the implementation of the next Directive – 99/31/EC on the landfilling of waste [11] were the Regulations of the Polish Ministry of Economy and Labor [12] and the Polish Ministry of Economy [13] which introduced a ban from the date of January 1, 2013, of the storage of sewage sludge with parameters as presented in Table 3. The most important parameter, taking into consideration the energy aspect, is parameter 3, which effectively eliminates the possibility of disposing of completely untreated sewage sludge in sites other than a hazardous-waste landfill. In view of the presented facts, there is a large and pressing need for the development of thermal methods for the disposal of sludge. According to a forecast suggested in National Waste Management Plan, the necessary amendments to the handling of the disposal of sewage sludge in Poland will be made as shown in Fig. 2 [1]. The present paper is written to review the state of the art in thermal technologies for sewage-sludge utilization taking into account the existing facilities and Polish exigencies. Thermal processes allow for a significant reduction in the weight and volume of transformed sewage sludge. The high heavymetal content in sewage sludge effectively reduces the possibility of its agricultural use. Additionally, the lack of land suitable for agricultural application in the vicinity of urban sewage-treatment
Fig. 1. Structure of the sewage-sludge disposal system in Poland, 2000–2007.
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S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919
Table 3 Polish criteria for the storage of sewage sludge in a non-hazardous waste landfill [13].
Table 4 Elementary dry composition of biologically stabilized sewage sludge 1, World (average); 2, Racibo´rz (POL); 3, Bia1ystok (POL); 4, Silesian Region (POL) 5, Turkey [17].
Parameter
Value limit
Component, wt %
Sludge 1
Sludge 2
Sludge 3
Sludge 4
Sludge 5
1
2
3
1 2 3
Overall organic carbon (OOC),% d.b. Loss at calcination (LOC),% d.b. Heating value, MJ/kg d.b.
5.0 8.0 Maximum 6.0
c h o n s cl w p
40.57 5.36 23.50 4.50 1.20 0.04 – 24.83
29.54 4.47 16.27 3.54 1.44 0.27 – 44.47
29.75 4.12 18.35 3.55 1.32 0.08 – 42.83
28.0–45.00 4.00–5.50 2.42–43.47 3.00–5.00 1.00–1.50 0.03–0.08 – 20.00–40.00
39.48 6.19 25.46 3.93 1.45 – – 23.49
Heating value, MJ/kg
15.10
11.13
7.56
7.00–15.00
12.00
plants also necessitates the development of thermal methods. The applicability of using thermal methods for sewage-sludge disposal is determined by its properties: e.g., the heat of combustion (calorific value) and composition (including moisture content). The heating value of raw sludge is about 17 MJ/kg; for activated sludge, about 15 MJ/kg, and for stabilized sludge (digested: anaerobic, aerobic, or lime stabilization) around 11 MJ/kg [2]. It is worth emphasizing here that any thermal method for the disposal of sewage sludge is usually preceded by a process of partial (to obtain 85% dry matter) or total (>85% dry matter) drying [14]. It is clear that sewage sludge should be concentrated (naturally on a drying bed or in lagoons, or mechanically using presses or centrifuges), stabilized (biologically or chemically), and dehydrated on filters [15]. The types of procedures performed on the sewage sludge depend firstly on the technological scheme of the treatment plant and secondly on the excepted mode of sewage-sludge utilization. These processes may be an integral part of the sewagetreatment plant or can be elements of the proposed installation for the thermal treatment of sewage sludge. 3. Characteristics of sewage sludge To determine the usefulness of sewage sludge for thermal transformation it is necessary to know its basic physical and chemical characteristics. The elementary composition of sewage sludge and the contents of trace elements and inorganic compounds depend on many factors, but it may play a central role for the country or region of the world. Table 4 shows an average sample of elementary stabilized dried sewage sludge [16]. Table 5 presents the trace-element contents of sewage sludge [16] and Table 6 the contents of basic inorganic compounds.
The National Urban Wastewater Treatment Program (NUWTP) is the largest program with regard to investment and the most costly from among all the tasks resulting from the implementation of the EU directives in the field of environmental protection [2]. In the period from now until 2015 it will require over 42 billion PLN (1 Euro ¼ 4.5 PLN) [18]. Implementation of the NUWTP will have significant effects on new investments in wastewater treatment. They will be modernized, extended, or built and the technology of sewage-sludge utilization will be taken in the consideration. Fig. 3 presents a map of Poland showing the currently treated and untreated wastewater. Here it is very important to note the implied need to invest in wastewater treatment plants, and thus for the management of the installation of sewage-sludge utilization methods. 4. Review of thermal methods of sewage-sludge utilization There are several thermal technologies available, both in the market and under development, for thermal processing of sewage sludge. These technologies can be classified in various ways. For the present discussion they have been grouped into three categories: mono-incineration, co-combustion and alternative processes (see Fig. 4). 4.1. Pyrolysis and gasification Generally it can be assumed that pyrolysis is a process of degradation (breakdown) of chemical molecules under the
Fig. 2. Forecast changes in the modes of dealing with sewage sludge in Poland [1].
S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919
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Table 5 Contents of trace elements in sewage sludge. Metal, ppm
Sludge 1
Sludge 2 (average results)
Arsenic Chromium Tin Zinc Cobalt Magnesium Copper Nickel Lead Mercury Selenium Iron
6.2–15.3 106.2–380.0 23.1–27.1 2,432.0–6,100.0 10.9–40.0 4,519.0–5,697.0 80.0–800.0 16.0–50.0 20.0–49.5 1.99–2.50 1.2–1.3 23,586.0–26,000.0
10.0 500.0 14.0 1,700.0 30.0 260.0 800.0 80.0 500.0 6.0 5.0 17,000.0
influence of a sufficiently high temperature (300–900 C [20]) in an anaerobic environment [21]. The main groups of products arising from the pyrolysis of sewage sludge are as follows [20,22]: A gas fraction, mainly consisting of hydrogen, methane, carbon dioxide, and carbon monoxide, along with a few minor gases; the calorific value of the pyrolytic gas is approximately 15 MJ/ m3 [23]. A solid fraction (pyrolytic coke); this also includes inert substances and dust with a significant content of heavy metals. A liquid fraction, consisting mainly of tars and oils, water, and organic compounds. The relative proportions among the various pyrolytic components depend mainly on the temperature and pressure of process and also on the turbulence in the reactor and many other factors. The oil–from-sludge technology is based on pyrolysis [18]. The essence of this process is to submit sewage sludge containing 95% dry matter to pyrolysis at 450 C for more than 30 min at atmospheric pressure. As a result, solid hydrocarbons and carbonization products (e.g., pyrolytic coke) are formed. Liquid hydrocarbons are also generated. The product can be used a raw material for use in many industries (e.g., petrochemical). A crucial process for the pyrolysis of sewage sludge is the Carver–Greenfield technology (C–R) leading to a refuse-derived fuel (RDF) [6]. This process provides for the possibility of simultaneously drying the sewage sludge before its combustion or gasification. The essence of the process is the mixing of raw sewage sludge with oil waste (e.g., used motor oil). The prepared mixture is passed through an evaporating system to remove all the water. After drying, the sludge is passed to a centrifuge to separate the liquid phase from the particles. The result is a solid waste and the liquid phase is returned to the installation for use as fuel. Gasification is the process of converting a solid fuel into a gas by treating the solid fuel in a generator with oxygen, air, and steam, or by other gasification methods [21]. As shown in Marrero et al. gasification of sewage sludge leads to a high-quality flammable gas that can be used for the generation of electricity or support such processes as the drying of sewage sludge [24]. An example of a gas composition resulting from the gasification of sewage sludge is presented in Table 7 [20,25]. This includes only the main combustible components; the others, depending on the gasification medium, are oxygen, carbon dioxide, or nitrogen.
Fig. 3. Wastewater requiring treatment in Poland [2].
The heating value of the gas after gasification varies around a value of 4 MJ/m3. The gas obtained can be used to generate electricity or to produce heat for the drying of sewage sludge [25]. An important aspect of the process of the gasification of sewage sludge is the production of hydrogen. Mathieu and Dubuisson presented a possibility for producing hydrogen by means of a high temperature gasification process [26]. There are many applications of the gasification process. Bien quotes a number of sewage-sludge gasification technologies [23]. The Krupp Uhde PreCon technology is one example. Here dry sewage sludge is gasified in a fluidised bed reactor. After the removal of the metallic and inorganic components, the crushed material is dried to a moisture content of 10% and put into a reactor at a temperature of 700–1000 C. In Poland, gasification of sewage sludge is still largely a prospective technology. It is associated with the extensive experience in biomass gasification; therefore, there it is natural to build on this experience. The installation by Zamer of an EKOD gasifier for the gasification and pyrolysis of sewage sludge is a good example. This fluidised bed reactor is able to utilize 1.7 t/h of sewage sludge [27]. The solid fraction from the gasification goes to pyrolysis and
Table 6 Contents of basic inorganic compounds in sewage sludge. Component
Na2O
MgO
Al2O3
CaO
TiO2
%a Component %a
0.42 Fe2O3 2.98
1.49 K2O 0.97
7.34 MnO 0.05
5.29 SiO2 18.9
0.36
a
As a percentage of all the mass of metal oxides in the dry weight.
Fig. 4. Main methods of thermal sewage-sludge disposal [19].
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S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919
Table 7 Typical contents of combustible components of a gas resulting from the gasification of sewage sludge. Component, wt. % vol Carbon monoxide Hydrogen Methane Ethane Acetylene
6.28–10.77 8.89–11.17 1.26–2.09 0.15–0.27 0.62–0.95
the gas from gasification can be burned in the power-stoker boilers popular in Poland or, after purification, used in combined heatingand-power (CHP) systems. 4.2. Combustion and co-combustion Sewage sludge can, because of its energy properties, be combusted or co-combusted. However, as a result of its relatively high contents of nitrogen and sulfur (see Table 4) when taking into consideration the possibility of combustion or co-firing of sewage sludge the impacts of NOx and SOx emissions should be carefully examined [28]. The risk of the emission of heavy metals, dioxins, furans, and ash should also be taken into consideration [29]. If the heating value of organic compounds in the sewage sludge is not sufficient to evaporate the water contained in it, the sewage sludge may be thermally processed only with additional fuel. Sewage sludge can be burned autothermally only in cases when it is dry, involving the necessity of prior drying [23]. Combustion and cocombustion of sewage sludge in a fluidised bed is a popular method for its thermal transformation. The papers [30–32] presented the results of the combustion of sewage sludge in a bubbling fluidized bed combustor. The emission of nitrogen oxides and ash comminution were analyzed with the aid of different and complementary experimental protocols. In bubbling fluidized bed combustors, granular sewage sludge is rapidly mixed due to the turbulence of the bed. As a result of the mechanical action of the grains, the ash agglomerates formed during combustion are fragmented. Rapid equalization of the temperature and the high heat-transfer coefficient result in an intense and even combustion process, ensuring low emissions of NOx. Shimizu and Toyono presented a study of a process for combustion and co-combustion of sewage sludge in a circulating fluidized bed [33]. In this type of bed, the gas from the reactor passes through the combustion chamber with a significantly higher velocity than in the bubbling fluidized bed; thus, a very high degree of mixing and uniform combustion are achieved. An important method for the utilization of sewage sludge is by cofiring in rotary kilns for the pyroprocessing of cement. Zabaniotou and Theofilou presented the results of studies on sewage-sludge combustion in this type of furnace [34]. Wet sludge was utilized, and the analysis was focused on the emission of heavy metals, especially mercury. The results showed that the use of sewage sludge did not result in exceeding emission standards. This was mainly due to the high temperature (up to 1800 C), providing for a total decomposition of organic matter [35]. Additionally, in this type of furnace, the gas-residence time is much longer than in the conventional equipment for the combustion of sewage sludge. Moreover, the combustion is affected in a highly alkaline environment so that the acidic components of the exhaust gases are chemically bound. Another technique for the management of sewage sludge is cocombustion in dust boilers. Stelmach and Wasilewski reported a Polish experimental co-firing of sewage sludge with coal in a type OP-230 dust boiler in a thermal-electric power station in Gdan´sk [36]. They concluded that co-combustion, with the addition of sludge at 1% of the total mass of fuel, does not require structural
changes to the boiler. Moreover, the process did not significantly affect the efficiency of the boiler. The authors drew the overall conclusion that the thermal utilization of sewage sludge is highly promising. Another Polish example of sewage-sludge co-combustion with coal was in a thermal-electric power station in Rzeszo´w. Tests with co-combustion of sewage sludge in a WR-25 stoker boiler were conducted [37]. Kotlicki presents properties of sludge originating from Polish sewage-treatment plants [38]. The purpose of an analysis is consideration of sewage sludge co-firing in power plant. The influence of sludge drying on its heat value is presented in simulation as well as possibilities to produce a renewable energy in power plant. There is concluded that sewage sludge may be in small quantities (a few percent) successfully burned in boilers without any negative consequences. In [39] environmental aspects of sludge from sewage-treatment plant treatment during co-combustion in energetic boilers are presented. Detailed study on thermal treatment of sludge during co-combustion in energetic boilers is given. The analysis of costs and benefits of application of those technologies are presented. On the basis of data obtained form heating plant Gliwice the possible quantity of sludge for utilization was calculated. The mass and energy flows during co-combustion were analyzed as well as economic aspects of the process. The results showed that the use of sewage sludge did not result in exceeding emission standards and operation costs of installation. Sewage sludge can also be co-combusted with lignite [40], wood [41], or municipal waste [22]. Municipal waste is an especially good material for co-combustion with sewage sludge. There exist a whole range of technologies for the co-combustion of sewage sludge with municipal wastes: wood boilers, fluidized bed boilers, furnaces for kilning bricks, cement furnaces or, as mentioned earlier, rotary furnaces [22]. One example is the Siemens Schwell–Brenna Technology, in which crushed wastes are mixed with sewage sludge [22]. In this method, a rotary kiln is used and the pyrolysis process is run at a temperature of 450 C. Post-process residues, consisting of about 30% of carbon, are supplied to the boiler, where they are burned together with the gas at a temperature of around 1300 C. The recovered heat is used, e.g., for the heating of the charge. 4.3. Combined processes Combined processes for the thermal utilization of sewage sludge are usually a combination of pyrolysis and gasification or combustion and gasification. The first example of these is the Thermoselect method, which is essentially an in-line process of pyrolysis and gasification of solid products [42]. As a result of the pyrolysis, pyrolytic coke is obtained, which is then gasified to obtain syngas. The Noelle Conversion is another example of a combination of gasification and pyrolysis [22]. In this technology, the batch is gasified under high pressure (>3.5 MPa) and high temperature (>2000 C). The SVZ technology [20] is an example of the combination of pyrolysis, gasification, and combustion. Dried sludge is crushed and ground, and then compressed and gasified at a temperature of 1300 C. Gases from the process, after cleaning to remove light oils and tars, are used as a raw material for the production of hydrocarbons, e.g., methanol. 5. Conclusions 1. According to the prognosis, the stream of produced watertreatment and sewage sludge in Poland will grow; this follows foremost from the lifestyle changes of our society, but is also due to the increased percentage of the population connected to the sewerage network.
S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919
2. The legislated limits will determine the choices for sewagesludge utilization; the disposal of sewage sludge in places other than hazardous-waste landfills, and even agricultural use will have to be replaced in the next few years by other methods. This is strong incentive to develop thermal methods of sewagesludge utilization. 3. The National Urban Wastewater Treatment Program is the largest with regard to investment and the most expensive from among all the tasks resulting from the implementation of the EU directives in the field of environmental protection. 4. Thermal methods are a promising alternative, which must (and will) prevail for several years. 5. Classic combustion of sewage sludge is well-known and controllable, but because of the emissions of nitrogen oxides, heavy metals, and other harmful compounds, it raises many questions and social objections and requires large investments for the purification of flue gases. 6. Co-combustion of sewage sludge with other natural resources (coal, lignite, or wood) or municipal waste has a good outlook and appears to be a satisfactory method for the management of sewage sludge. In adding only small quantities of sewage sludge in relation to the total mass of burned fuel, these methods do not require any additional investment. In Poland, circumstances would appear to indicate particular interest in the co-combustion of sewage sludge in dust and stoker boilers. To date, there are few Polish examples, but it is still a developing area of sewage-sludge utilization. 7. Alternative methods for the thermal utilization of sewage sludge (pyrolysis, gasification, or combined processes) are an important element in the wider problem of sludge disposal. There are many technologies that use gasification or pyrolysis (or a combination of these two). Their undoubted advantage, in addition to the disposal of sludge, is that it becomes possible to obtain a product that can be effectively used for the generation of energy. Polish conditions also appear to present a good opportunity to utilize this group of waste-disposal technologies.
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A review of methods for the thermal utilization of sewage sludge: The Polish perspective Sebastian Werle*, Ryszard K. Wilk Institute of Thermal Technology, Silesian University of Technology at Gliwice, 44-100 Gliwice, Konarskiego 22, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2009 Accepted 15 January 2010 Available online 11 February 2010
On the basis of demographic projections, it is estimated that the quantity of sewage sludge which will be produced in Poland between 2010 and 2018 will increase from 612.8 thousand tons (dry basis, d.b.) to 706.6 thousand tons (d.b.). Currently, the predominant method for the disposal of this sludge is its storage and agricultural application. However, the legislation taking effect in the next few years will effectively block these avenues of sewage-sludge disposal. Therefore, effective methods of thermal sewage-sludge utilization must be developed. Here we review the state of knowledge and technology in thermal methods for the utilization of municipal sewage sludge to obtain useful forms of energy such as pyrolysis, gasification, combustion, and co-combustion. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Sewage sludge Pyrolysis Gasification Combustion Co-combustion
1. Introduction Municipal sewage sludge is generated in wastewater treatment plants in the process of sewage-treatment [1,2]. The waste stream of sewage sludge is rapidly growing, generating wastes which require management in compliance with the law [3]. According to Ministry of the Environment of Poland, sewage sludge belongs to group No. 19, including the wastes from the treatment installation and the equipment for waste management, wastewater from wastewater treatment plants and from the treatment of drinking water and water for industrial purposes [4]. Poland is inhabited by 38.2 million people with an average population density of 122 persons per square kilometer, and has a territory of 322,577 km2, of which 311,904 km2 is occupied by land. In the year 2008, in 3090 (both municipal and industrial) Polish sewage-treatment plants more than 1100 thousand tons (d.b.) of municipal and industrial sewage sludge was produced. In 2008, wastewater treatment plants serviced only 63% of the population (87% in urban areas and 26% in rural areas, where about 39% of the population lives); by comparison, in the countries of Western Europe, more than 78% of the population is serviced by waste treatment plants. In Poland, only 456 cities and 559 rural communities possessed a modern wastewater treatment plant with enhanced nitrogen and phosphorus removal. In these plants, 918 hm3 of waste was treated, which accounts for 73% of the waste discharged through urban and rural
* Corresponding author. Tel.: þ48 32 237 29 83; fax: þ48 32 237 28 72. E-mail address: [email protected] (S. Werle). 0960-1481/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.01.019
sewerage systems. This ratio fluctuates around 70% in most countries. The average unit index of sewage sludge generated in Polish wastewater treatment plants amounts to 0.25 kg d.b./m3 of treated wastewater. Table 1 presents the quantities of municipal sewage sludge produced in Poland in the years 1999–2007 along with the anticipated amount in the future. According to the Polish Environmental Policy [5] and the objectives of the National Waste Management Plan 2010 (NWMP) [1] as well as the National Urban Wastewater Treatment Program (NUWTP) [2], the quantity of sewage treated in Poland is systematically increasing. A measurable effect of this is, first of all, the increasing proportion of the population being served by wastewater treatment, but also the growth of the amount of produced sewage sludge (and sewage). On the basis of demographic projections, it is estimated that the quantity of sludge which will be produced in Poland between the years 2010 and 2018 will increase from 612.8 thousand tons (d.b.) to 706.6 thousand tons (d.b.) [1]. Currently, the predominant method for the disposal of sewage sludge is its storage and agricultural application. Sewage sludge is deposited in landfill sites dedicated exclusively to this sludge, in lagoons or jointly with municipal waste. Fig. 1 presents the current structure of the sewage-sludge management system [1]. In terms of the commitments derived from the introduction of European Union (EU) Directives, this structure of sewage-sludge utilization in Poland is highly unfavorable. The main problems are the high percentage of stored sewage sludge and a lack of installations for its thermal utilization. Thermal processes can be used for the conversion of large quantities of sewage sludge (e.g., in large urban areas) into useful energy. Processes for thermal utilization of
S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919 Table 1 The amount of municipal sewage sludge produced in Poland in the period 1999– 2007 [1] (and the anticipated amount in the future). Year
Amount of sludge produced, thousand tons (d. b.)
1999 2000 2002 2004 2005 2006 2007 2010 2018
354.0 359.8 435.0 476.0 486.1 501.3 533.4 612.8 706.6
sludge can be developed at existing installations (e.g., heating plants, power plants, or cement plants) or in newly built facilities. Thermal methods of sewage-sludge utilization should be preceded by dehydration and drying of sludge. In Poland, 98% of municipal sewage-treatment plants use a biological treatment, and, among them, 36% with enhanced biogenic removal (see Table 2). In the EU, 50% of sewage-treatment plants have anaerobic digestion, 18% incorporate aerobic digestion and 4% lime stabilization, whereas 24% of the plants undertake no sludge stabilization; the same tendencies are observed in Poland [6]. 2. Regulations and choices in sewage-sludge utilization methods As mentioned earlier, a significant reduction in sludge quantity is achieved through digestion. Additionally, there is a tendency in Europe to have the following hierarchy of sewage-sludge management priority: avoidance, minimization, recycling, and thermal methods with energy recovery and landfilling. There is a wide range of analyzed and proposed solutions for municipal sewage-sludge utilization. Nevertheless, there are serious legal constraints determining this choice. The first of these results from the application of the EU Directive 91/271/EEC (Urban Wastewater Treatment Directive) [7] and its amendment, Directive 98/15/EC. The most important wording of this directive is contained in Article 14, which says "sludge arising from wastewater treatment shall be re-used whenever appropriate". This sentence clearly formulates the aim of the directive in ordering the processing of sewage sludge. The implication of this document also expresses the necessity of building new
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Table 2 Wastewater treatment plants in Poland in 2008 as of December 31st. Municipal wastewatertreatment plants
Total
Mechanical Biological With increased biogenic removal (disposal)
Number 3090.0 60.0 9019.0 76.0 Capacity, dam3/24 h Population using waste 63.1 0.2 water treatment plants, % of total population
2233.0 1998.0 16.3
797.0 6945.0 46.6
sewerage systems and constructing and upgrading sewage-treatment plants, which would increase the mass flow of produced sewage sludge. The second act is the Sewage Sludge Directive 86/278/EEC [8], which introduces restrictions concerning the use of sewage sludge in agriculture. It mainly focuses on the concentrations of heavy metals in sewage sludge, and hence limits its agricultural and natural use. This directive was supplemented by entries appearing in the subsequent acts of the European Community, namely the so-called "Waste Directive" [9]. The Directive and the Regulation [10] firmly established rules for the problem of dealing with sewage sludge. A major consequence of the implementation of the next Directive – 99/31/EC on the landfilling of waste [11] were the Regulations of the Polish Ministry of Economy and Labor [12] and the Polish Ministry of Economy [13] which introduced a ban from the date of January 1, 2013, of the storage of sewage sludge with parameters as presented in Table 3. The most important parameter, taking into consideration the energy aspect, is parameter 3, which effectively eliminates the possibility of disposing of completely untreated sewage sludge in sites other than a hazardous-waste landfill. In view of the presented facts, there is a large and pressing need for the development of thermal methods for the disposal of sludge. According to a forecast suggested in National Waste Management Plan, the necessary amendments to the handling of the disposal of sewage sludge in Poland will be made as shown in Fig. 2 [1]. The present paper is written to review the state of the art in thermal technologies for sewage-sludge utilization taking into account the existing facilities and Polish exigencies. Thermal processes allow for a significant reduction in the weight and volume of transformed sewage sludge. The high heavymetal content in sewage sludge effectively reduces the possibility of its agricultural use. Additionally, the lack of land suitable for agricultural application in the vicinity of urban sewage-treatment
Fig. 1. Structure of the sewage-sludge disposal system in Poland, 2000–2007.
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S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919
Table 3 Polish criteria for the storage of sewage sludge in a non-hazardous waste landfill [13].
Table 4 Elementary dry composition of biologically stabilized sewage sludge 1, World (average); 2, Racibo´rz (POL); 3, Bia1ystok (POL); 4, Silesian Region (POL) 5, Turkey [17].
Parameter
Value limit
Component, wt %
Sludge 1
Sludge 2
Sludge 3
Sludge 4
Sludge 5
1
2
3
1 2 3
Overall organic carbon (OOC),% d.b. Loss at calcination (LOC),% d.b. Heating value, MJ/kg d.b.
5.0 8.0 Maximum 6.0
c h o n s cl w p
40.57 5.36 23.50 4.50 1.20 0.04 – 24.83
29.54 4.47 16.27 3.54 1.44 0.27 – 44.47
29.75 4.12 18.35 3.55 1.32 0.08 – 42.83
28.0–45.00 4.00–5.50 2.42–43.47 3.00–5.00 1.00–1.50 0.03–0.08 – 20.00–40.00
39.48 6.19 25.46 3.93 1.45 – – 23.49
Heating value, MJ/kg
15.10
11.13
7.56
7.00–15.00
12.00
plants also necessitates the development of thermal methods. The applicability of using thermal methods for sewage-sludge disposal is determined by its properties: e.g., the heat of combustion (calorific value) and composition (including moisture content). The heating value of raw sludge is about 17 MJ/kg; for activated sludge, about 15 MJ/kg, and for stabilized sludge (digested: anaerobic, aerobic, or lime stabilization) around 11 MJ/kg [2]. It is worth emphasizing here that any thermal method for the disposal of sewage sludge is usually preceded by a process of partial (to obtain 85% dry matter) or total (>85% dry matter) drying [14]. It is clear that sewage sludge should be concentrated (naturally on a drying bed or in lagoons, or mechanically using presses or centrifuges), stabilized (biologically or chemically), and dehydrated on filters [15]. The types of procedures performed on the sewage sludge depend firstly on the technological scheme of the treatment plant and secondly on the excepted mode of sewage-sludge utilization. These processes may be an integral part of the sewagetreatment plant or can be elements of the proposed installation for the thermal treatment of sewage sludge. 3. Characteristics of sewage sludge To determine the usefulness of sewage sludge for thermal transformation it is necessary to know its basic physical and chemical characteristics. The elementary composition of sewage sludge and the contents of trace elements and inorganic compounds depend on many factors, but it may play a central role for the country or region of the world. Table 4 shows an average sample of elementary stabilized dried sewage sludge [16]. Table 5 presents the trace-element contents of sewage sludge [16] and Table 6 the contents of basic inorganic compounds.
The National Urban Wastewater Treatment Program (NUWTP) is the largest program with regard to investment and the most costly from among all the tasks resulting from the implementation of the EU directives in the field of environmental protection [2]. In the period from now until 2015 it will require over 42 billion PLN (1 Euro ¼ 4.5 PLN) [18]. Implementation of the NUWTP will have significant effects on new investments in wastewater treatment. They will be modernized, extended, or built and the technology of sewage-sludge utilization will be taken in the consideration. Fig. 3 presents a map of Poland showing the currently treated and untreated wastewater. Here it is very important to note the implied need to invest in wastewater treatment plants, and thus for the management of the installation of sewage-sludge utilization methods. 4. Review of thermal methods of sewage-sludge utilization There are several thermal technologies available, both in the market and under development, for thermal processing of sewage sludge. These technologies can be classified in various ways. For the present discussion they have been grouped into three categories: mono-incineration, co-combustion and alternative processes (see Fig. 4). 4.1. Pyrolysis and gasification Generally it can be assumed that pyrolysis is a process of degradation (breakdown) of chemical molecules under the
Fig. 2. Forecast changes in the modes of dealing with sewage sludge in Poland [1].
S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919
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Table 5 Contents of trace elements in sewage sludge. Metal, ppm
Sludge 1
Sludge 2 (average results)
Arsenic Chromium Tin Zinc Cobalt Magnesium Copper Nickel Lead Mercury Selenium Iron
6.2–15.3 106.2–380.0 23.1–27.1 2,432.0–6,100.0 10.9–40.0 4,519.0–5,697.0 80.0–800.0 16.0–50.0 20.0–49.5 1.99–2.50 1.2–1.3 23,586.0–26,000.0
10.0 500.0 14.0 1,700.0 30.0 260.0 800.0 80.0 500.0 6.0 5.0 17,000.0
influence of a sufficiently high temperature (300–900 C [20]) in an anaerobic environment [21]. The main groups of products arising from the pyrolysis of sewage sludge are as follows [20,22]: A gas fraction, mainly consisting of hydrogen, methane, carbon dioxide, and carbon monoxide, along with a few minor gases; the calorific value of the pyrolytic gas is approximately 15 MJ/ m3 [23]. A solid fraction (pyrolytic coke); this also includes inert substances and dust with a significant content of heavy metals. A liquid fraction, consisting mainly of tars and oils, water, and organic compounds. The relative proportions among the various pyrolytic components depend mainly on the temperature and pressure of process and also on the turbulence in the reactor and many other factors. The oil–from-sludge technology is based on pyrolysis [18]. The essence of this process is to submit sewage sludge containing 95% dry matter to pyrolysis at 450 C for more than 30 min at atmospheric pressure. As a result, solid hydrocarbons and carbonization products (e.g., pyrolytic coke) are formed. Liquid hydrocarbons are also generated. The product can be used a raw material for use in many industries (e.g., petrochemical). A crucial process for the pyrolysis of sewage sludge is the Carver–Greenfield technology (C–R) leading to a refuse-derived fuel (RDF) [6]. This process provides for the possibility of simultaneously drying the sewage sludge before its combustion or gasification. The essence of the process is the mixing of raw sewage sludge with oil waste (e.g., used motor oil). The prepared mixture is passed through an evaporating system to remove all the water. After drying, the sludge is passed to a centrifuge to separate the liquid phase from the particles. The result is a solid waste and the liquid phase is returned to the installation for use as fuel. Gasification is the process of converting a solid fuel into a gas by treating the solid fuel in a generator with oxygen, air, and steam, or by other gasification methods [21]. As shown in Marrero et al. gasification of sewage sludge leads to a high-quality flammable gas that can be used for the generation of electricity or support such processes as the drying of sewage sludge [24]. An example of a gas composition resulting from the gasification of sewage sludge is presented in Table 7 [20,25]. This includes only the main combustible components; the others, depending on the gasification medium, are oxygen, carbon dioxide, or nitrogen.
Fig. 3. Wastewater requiring treatment in Poland [2].
The heating value of the gas after gasification varies around a value of 4 MJ/m3. The gas obtained can be used to generate electricity or to produce heat for the drying of sewage sludge [25]. An important aspect of the process of the gasification of sewage sludge is the production of hydrogen. Mathieu and Dubuisson presented a possibility for producing hydrogen by means of a high temperature gasification process [26]. There are many applications of the gasification process. Bien quotes a number of sewage-sludge gasification technologies [23]. The Krupp Uhde PreCon technology is one example. Here dry sewage sludge is gasified in a fluidised bed reactor. After the removal of the metallic and inorganic components, the crushed material is dried to a moisture content of 10% and put into a reactor at a temperature of 700–1000 C. In Poland, gasification of sewage sludge is still largely a prospective technology. It is associated with the extensive experience in biomass gasification; therefore, there it is natural to build on this experience. The installation by Zamer of an EKOD gasifier for the gasification and pyrolysis of sewage sludge is a good example. This fluidised bed reactor is able to utilize 1.7 t/h of sewage sludge [27]. The solid fraction from the gasification goes to pyrolysis and
Table 6 Contents of basic inorganic compounds in sewage sludge. Component
Na2O
MgO
Al2O3
CaO
TiO2
%a Component %a
0.42 Fe2O3 2.98
1.49 K2O 0.97
7.34 MnO 0.05
5.29 SiO2 18.9
0.36
a
As a percentage of all the mass of metal oxides in the dry weight.
Fig. 4. Main methods of thermal sewage-sludge disposal [19].
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Table 7 Typical contents of combustible components of a gas resulting from the gasification of sewage sludge. Component, wt. % vol Carbon monoxide Hydrogen Methane Ethane Acetylene
6.28–10.77 8.89–11.17 1.26–2.09 0.15–0.27 0.62–0.95
the gas from gasification can be burned in the power-stoker boilers popular in Poland or, after purification, used in combined heatingand-power (CHP) systems. 4.2. Combustion and co-combustion Sewage sludge can, because of its energy properties, be combusted or co-combusted. However, as a result of its relatively high contents of nitrogen and sulfur (see Table 4) when taking into consideration the possibility of combustion or co-firing of sewage sludge the impacts of NOx and SOx emissions should be carefully examined [28]. The risk of the emission of heavy metals, dioxins, furans, and ash should also be taken into consideration [29]. If the heating value of organic compounds in the sewage sludge is not sufficient to evaporate the water contained in it, the sewage sludge may be thermally processed only with additional fuel. Sewage sludge can be burned autothermally only in cases when it is dry, involving the necessity of prior drying [23]. Combustion and cocombustion of sewage sludge in a fluidised bed is a popular method for its thermal transformation. The papers [30–32] presented the results of the combustion of sewage sludge in a bubbling fluidized bed combustor. The emission of nitrogen oxides and ash comminution were analyzed with the aid of different and complementary experimental protocols. In bubbling fluidized bed combustors, granular sewage sludge is rapidly mixed due to the turbulence of the bed. As a result of the mechanical action of the grains, the ash agglomerates formed during combustion are fragmented. Rapid equalization of the temperature and the high heat-transfer coefficient result in an intense and even combustion process, ensuring low emissions of NOx. Shimizu and Toyono presented a study of a process for combustion and co-combustion of sewage sludge in a circulating fluidized bed [33]. In this type of bed, the gas from the reactor passes through the combustion chamber with a significantly higher velocity than in the bubbling fluidized bed; thus, a very high degree of mixing and uniform combustion are achieved. An important method for the utilization of sewage sludge is by cofiring in rotary kilns for the pyroprocessing of cement. Zabaniotou and Theofilou presented the results of studies on sewage-sludge combustion in this type of furnace [34]. Wet sludge was utilized, and the analysis was focused on the emission of heavy metals, especially mercury. The results showed that the use of sewage sludge did not result in exceeding emission standards. This was mainly due to the high temperature (up to 1800 C), providing for a total decomposition of organic matter [35]. Additionally, in this type of furnace, the gas-residence time is much longer than in the conventional equipment for the combustion of sewage sludge. Moreover, the combustion is affected in a highly alkaline environment so that the acidic components of the exhaust gases are chemically bound. Another technique for the management of sewage sludge is cocombustion in dust boilers. Stelmach and Wasilewski reported a Polish experimental co-firing of sewage sludge with coal in a type OP-230 dust boiler in a thermal-electric power station in Gdan´sk [36]. They concluded that co-combustion, with the addition of sludge at 1% of the total mass of fuel, does not require structural
changes to the boiler. Moreover, the process did not significantly affect the efficiency of the boiler. The authors drew the overall conclusion that the thermal utilization of sewage sludge is highly promising. Another Polish example of sewage-sludge co-combustion with coal was in a thermal-electric power station in Rzeszo´w. Tests with co-combustion of sewage sludge in a WR-25 stoker boiler were conducted [37]. Kotlicki presents properties of sludge originating from Polish sewage-treatment plants [38]. The purpose of an analysis is consideration of sewage sludge co-firing in power plant. The influence of sludge drying on its heat value is presented in simulation as well as possibilities to produce a renewable energy in power plant. There is concluded that sewage sludge may be in small quantities (a few percent) successfully burned in boilers without any negative consequences. In [39] environmental aspects of sludge from sewage-treatment plant treatment during co-combustion in energetic boilers are presented. Detailed study on thermal treatment of sludge during co-combustion in energetic boilers is given. The analysis of costs and benefits of application of those technologies are presented. On the basis of data obtained form heating plant Gliwice the possible quantity of sludge for utilization was calculated. The mass and energy flows during co-combustion were analyzed as well as economic aspects of the process. The results showed that the use of sewage sludge did not result in exceeding emission standards and operation costs of installation. Sewage sludge can also be co-combusted with lignite [40], wood [41], or municipal waste [22]. Municipal waste is an especially good material for co-combustion with sewage sludge. There exist a whole range of technologies for the co-combustion of sewage sludge with municipal wastes: wood boilers, fluidized bed boilers, furnaces for kilning bricks, cement furnaces or, as mentioned earlier, rotary furnaces [22]. One example is the Siemens Schwell–Brenna Technology, in which crushed wastes are mixed with sewage sludge [22]. In this method, a rotary kiln is used and the pyrolysis process is run at a temperature of 450 C. Post-process residues, consisting of about 30% of carbon, are supplied to the boiler, where they are burned together with the gas at a temperature of around 1300 C. The recovered heat is used, e.g., for the heating of the charge. 4.3. Combined processes Combined processes for the thermal utilization of sewage sludge are usually a combination of pyrolysis and gasification or combustion and gasification. The first example of these is the Thermoselect method, which is essentially an in-line process of pyrolysis and gasification of solid products [42]. As a result of the pyrolysis, pyrolytic coke is obtained, which is then gasified to obtain syngas. The Noelle Conversion is another example of a combination of gasification and pyrolysis [22]. In this technology, the batch is gasified under high pressure (>3.5 MPa) and high temperature (>2000 C). The SVZ technology [20] is an example of the combination of pyrolysis, gasification, and combustion. Dried sludge is crushed and ground, and then compressed and gasified at a temperature of 1300 C. Gases from the process, after cleaning to remove light oils and tars, are used as a raw material for the production of hydrocarbons, e.g., methanol. 5. Conclusions 1. According to the prognosis, the stream of produced watertreatment and sewage sludge in Poland will grow; this follows foremost from the lifestyle changes of our society, but is also due to the increased percentage of the population connected to the sewerage network.
S. Werle, R.K. Wilk / Renewable Energy 35 (2010) 1914–1919
2. The legislated limits will determine the choices for sewagesludge utilization; the disposal of sewage sludge in places other than hazardous-waste landfills, and even agricultural use will have to be replaced in the next few years by other methods. This is strong incentive to develop thermal methods of sewagesludge utilization. 3. The National Urban Wastewater Treatment Program is the largest with regard to investment and the most expensive from among all the tasks resulting from the implementation of the EU directives in the field of environmental protection. 4. Thermal methods are a promising alternative, which must (and will) prevail for several years. 5. Classic combustion of sewage sludge is well-known and controllable, but because of the emissions of nitrogen oxides, heavy metals, and other harmful compounds, it raises many questions and social objections and requires large investments for the purification of flue gases. 6. Co-combustion of sewage sludge with other natural resources (coal, lignite, or wood) or municipal waste has a good outlook and appears to be a satisfactory method for the management of sewage sludge. In adding only small quantities of sewage sludge in relation to the total mass of burned fuel, these methods do not require any additional investment. In Poland, circumstances would appear to indicate particular interest in the co-combustion of sewage sludge in dust and stoker boilers. To date, there are few Polish examples, but it is still a developing area of sewage-sludge utilization. 7. Alternative methods for the thermal utilization of sewage sludge (pyrolysis, gasification, or combined processes) are an important element in the wider problem of sludge disposal. There are many technologies that use gasification or pyrolysis (or a combination of these two). Their undoubted advantage, in addition to the disposal of sludge, is that it becomes possible to obtain a product that can be effectively used for the generation of energy. Polish conditions also appear to present a good opportunity to utilize this group of waste-disposal technologies.
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