DISPOSAL OF BIOSOLIDS: A STUDY USING THERMOGRAVIMETRIC ANALYSIS Anthony R. Auxilio, Sunaina Dayal, Luguang Chen, Kawnish Kirtania and Sankar Bhattacharya* Department of Chemical Engineering, Monash University, Clayton, Victoria, 3800, Australia *Email: anthony.auxilio@monash.edu ABSTRACT This paper explores the potential use of municipal biosolids as a source of energetic fuel. The current major methods of biosolids processing and disposal and alternative thermal processing methods such as pyrolysis, gasification and combustion are reviewed. Thermogravimetric analyses on biosolids in our laboratory, at 5-20K/min heating rates and at temperatures up to 1100 o C were conducted. The sample was subjected to different atmospheric conditions (pyrolysis, combustion and gasification) and the thermogravimetric and differential thermogravimetric graphs were obtained and evaluated for reactivity and effects of O 2, CO 2 and steam. It was found that the reaction environment (gas composition) have profound effects on the combustion and gasification processes. As part of a larger project on beneficiation and use of sludge, this fundamental study sheds insights into biosolids pyrolysis, gasification and combustion characteristics, and has implications for design of reactors used for processing of such biosolids. Keywords: Biosolids, sludge, pyrolysis, gasification INTRODUCTION In the context of environmental engineering and waste management, sludge refers to the solids extracted in sewage treatment and consists of fecal matter, ground up food from garbage, silt, sand, bits of plants, algae, bacteria and chemical precipitates (Wang et al. 2008). Table 1 shows the sludge production and its utilization in most high-income non- OECD countries. It is interesting to note that Australia is one of those countries that have higher percentage for landfill disposal (76%) and no indication of any alternative processing for reduction of waste such as gasification. Figure 1 can be considered the basic sludge-processing scheme (Werther and Ogada 1999; Wang et al. 2008). The importance of water removal in the entire process cannot be underestimated because it serves two main purposes, namely, reduction of: (1) sludge volume and therefore reduction of transportation and handling costs (Qi et al. 2011), and (2) moisture content of sludge, which is a property that affects pyrolysis or gasification process (Fytili and Zabaniotou 2007). In 1991, the Water Environment Federation in the US defined sludge as the solids within the wastewater treatment plant, and out of it when this material is disposed of without any utilization, whereas, biosolids are the solids, in whatever form, that leave the treatment plant and are destined for some beneficial use. The term biosolids will be used in this paper.
Table 1 Biosolids production and utilization in high-income non-oecd countries (Wang et al. 2008). Country Dry metric Agricultural Landfill Incineration Ocean Other (t/yr 10 3 ) use (%) (%) (%) (%) (%) USA 5358 33.3 34 16.1 6.3 10.3 Germany 2700 27 54 14 5 Japan ~2000 25 75 UK 1107 42 8 14 30 13 France 852 60 20 20 Italy 816 33 55 4 8 Spain 350 50 35 5 10 Netherlands 323 26 50 3 2 19 Australia 300 9 76 2 13 Denmark 170 54 20 24 2 Belgium 200 29 55 15 1 Greece 48 10 90 Ireland 37 12 45 35 8 Portugal 25 11 29 60 Luxemburg 8 12 88 Figure 1 Schematic of basic sludge processing. New and tougher government legislation in and outside Australia over the last decade could make traditional disposals of biosolids less and less popular in the coming years. Thus, developing alternative and sustainable disposal methods becomes an indispensable action. Common methods of traditional and alternative disposal are briefly discussed below. Traditional disposal methods Agricultural use. Biosolids act as a good fertilizer depending upon their nitrogen and phosphorus content. Processes for treatment of wastewater consist of nitrification-
denitrification phases that lead to phosphorus and nitrogen presence in biosolids (Werther and Ogada 1999). Other materials of agricultural value could also be present. Advantages of this method of disposal include the recycling of organic materials and replacement of artificial fertilizers (Lundin et al. 2004) and the relative cost efficacy compared with other methods (Coker and Carlton-Smith 1986). However, controlled application is necessary to prevent a steep increase in the concentration of heavy metals and the introduction of synthetic organic chemicals in agricultural land (Tebbutt 1998). In addition, apart from storage problems, biosolids are continuously produced throughout the year but only required for agricultural applications a few times during the year (Fytili and Zabaniotou 2007). Land-filling. Biosolids are discarded to landfills by either co-disposal with other municipal waste or mono-disposal (Werther and Ogada 1999). Land-filling is used for land reclamation (Tebbutt 1998). However, disposal through this method poses risks of contamination to ground water. In addition, existing landfill sites are nearing full capacity with diminishing potential of construction of new sites (Caputo and Pelagagge 2001). Other issues include handling and stability problems owing to the physical properties of biosolids (Werther and Ogada 1999). Sea dumping. Controlled disposal in deep water sites has been practiced for several years apparently without any considerable adverse environmental effects (Tebbutt 1998). However, water pollution regulations in many countries have changed over the years to prohibit this practice, resulting in the search for other disposal options (Werther and Ogada 1999). Combustion (incineration). This process may be termed disposal if the process is used in an essentially ash-free liquids or solids. The advantages of this process are: volume reduction, detoxification, environmental impact mitigation, regulatory compliance, energy recovery, stabilization in landfills and sanitation. The disadvantages are: cost, operating problems, staffing problems, secondary environmental impacts (e.g. air emissions, waterborne emissions and residue impacts), public reaction and technical risk (Niessen 2002). Alternative disposal methods Pyrolysis. Conversion of biosolids, which may have a large amount of heavy metals or toxic chemicals, to oil is technically feasible, but the capital and running costs are high. The world s first full-scale oil from biosolids demonstration facility was operated at the Subiaco Wastewater Treatment Plant in Perth, Australia (Vasileski 2007). The chemical reactions taking place in pyrolysis are thermal cracking and condensation reactions. This process is endothermic and the final products occur in three phases. The gaseous fraction is the non-condensable gas (methane, CO 2 and small amounts of other gases). The liquid fraction consists of char, oil, water and substances like acetic acid, acetone and methanol. The solid fraction is primarily char (pure carbon) and inert materials in small concentrations (Fytili and Zabaniotou 2007). These products can be used as fuel or disposed of. The oil can also be used as raw material for chemical industries (Werther and Ogada 1999). Since heavy metals are concentrated in a solid carbonaceous residue, pyrolysis tends to produce less pollutants compared with combustion (Fytili and Zabaniotou 2007). The relative proportions of the three fractions of pyrolysis products vary and depend upon temperature, pressure, reactor turbulence, reactor residence time and effluent characteristics (Werle and Wilk 2010).
Gasification. In this process, biosolids are converted into primarily gas by treatment in a generator with O 2, air and steam. The product obtained is high-quality combustible gas (CO, H 2, CH 4, C 2 H 6 and C 2 H 2 ), which has numerous uses such as generation of electricity and production of heat for drying of biosolids. The heating value of this gas is estimated to be around 4 MJ/m 3 (Werle and Wilk 2010). Prior to gasification, the biosolids are dried to reduce the moisture, thereafter; the dried biosolids are thermally decomposed. Since gasification is a net reductive process, issues such as SO X and NO X emissions, heavy metals and fly ash are eliminated (Fytili and Zabaniotou 2007). Objectives of the study Alternative disposal methods involving pyrolysis, gasification and combustion presents a potentially more sustainable path for processing of biosolids with energy recovery. This study, as part of a larger project on beneficiation of biosolids, assesses the pyrolysis, gasification and combustion characteristics of typical biosolids generated in Victoria using thermogravimetric analysis. Results from this study will then be used for bench-scale experiments where the gaseous and any liquid products will be thoroughly analysed using chromatography to assess the extent of recovery. MATERIALS AND METHODS Biosolids The biosolids used in this work was anaerobic digested sludge from a municipal wastewater treatment plant in Victoria. The as-received biosolids was filter pressed and stored at 4 o C to minimize microbial activity. Sample of this filter-pressed biosolids were placed overnight in an oven at 105 o C, then the dried samples were ground into powder using mortar and pestle and stored in a dessicator. Samples for any subsequent analysis were taken from these ground and dried samples in which the ultimate and metals analysis are shown in Table 2 and Table 3 respectively. Table 2 Ultimate analyses of biosolids (%DB, weight) as measured by CHNS analyser. C H N S O Ash Percent 33.79 4.80 4.89 0.57 15.95 40.57 Table 3 Metal analyses of biosolids as measured by Inductively Coupled Plasma Atomic Emission Spectroscopy. Tl As Cd Cr Cu Pb Ni Se Zn Hg Concentration (ppm) < 10 3.0 1.6 44 640 35 23 < 2 1000 0.6 Experiments and Sample Characterization A NETZSCH STA 449 F3 Jupiter simultaneous analyser was used to carry out thermogravimetric (TG) and differential thermo-gravimetric analyses (DTG). These tests were conducted in conditions of combustion, gasification and pyrolysis, which were
created by using different atmospheres. Sample size of less than 106 µm and weight of approximately 50 mg was used in each run. Heating rates were varied between 5, 10 and 20 K/ min at temperatures to 1100 o C. The following gas compositions (vol. %) were used: 100 % pure N 2, 5 % O 2 95 % N 2, 10 % O 2 90 % N 2, 20 % O 2 80 % N 2, 20 % CO 2 80 % N 2, 20 % steam 80 % N 2, 20% steam 20% CO 2 60% N 2. Respectively, these are labelled as: 100N 2 (pyrolysis condition); 5O 2 95N 2, 10O 2 90N 2, 20O 2, (combustion conditions); 20CO 2, 20ST, 20ST20CO 2 60N 2 (gasification conditions). C, H, N and S contents in biosolids were determined by Perkin Elmer PE 2400 Element Analyzer. Metal analyses were carried out using Inductively Coupled Plasma - Atomic Emission Spectroscopy (IPC-AES). RESULTS AND DISCUSSION Pyrolysis. We did not observe significant difference in the weight loss among the three heating rates used 5 K/min, 10 K/min and 20 K/min. Therefore, all subsequent analyses and plots made here are based on 10 K/min. Figure 2 shows the TG and DTG profiles of biosolids under pyrolysis condition (100 % pure N 2 ) at a heating rate of 10K/min. The results show that dehydration (5 % mass loss) took place below 200 o C but the major weight loss occurred between 200-600 o C (55 % mass loss) due to de-volatilization or thermal degradation of organic matter. It is clear in the DTG curve that at least five decompositions took place but the major ones appeared in the region between 250-450 o C where depolymerization is likely to have taken place, with a slight shoulder peak around 384 o C. This demonstrates the heterogeneity of the material and that complex and simultaneous processes must have taken place. When the temperature reaches about 900 o C, complete decomposition started, which is attributable to the decomposition of minerals present in biosolids. Overall, several decompositions, including depolymerisation and secondary reactions occurred throughout the overall pyrolysis process (Yu et al. 2002; (Scott et al. 2006; (Yanfen and Xiaoqian 2010). Effect of O 2 concentration. Figures 2a and 2b also show the TG and DTG profiles of biosolids for different amount of ambient oxygen (5O 2 95N 2, 10O 2 90N 2 and 20O 2 ). The presence of oxygen is expected to accelerate the overall rate of reaction. In fact, at the same heating rates, it can be observed from the TG curves that as the amount of oxygen increases, the mass loss profile shifts to a lower temperature starting at a temperature of 465 o C. At different heating rates but similar oxygen concentration (Figure 3), the lower the heating rate, the steeper was the TG curve; at the same time, the DTG peaks shifted towards a lower temperature. It must be noted that the optimum oxygen amount where the maximum TG loss of ~ 67 % was observed is at 10O 2 90N 2 (see Fig. 2a). The DTG (rate change of mass, %/min) curves, figure 2b, present a significant effect of oxygen level. There are multiple and sharper peaks representing multiple decompositions with different levels of oxygen in the reactant gas mix. A first sharp peak appears around 325 C, another one appears between 465-500 C, and a much smaller one at 825 C. This means several reaction zones to consider while calculating the kinetic parameters, which in turn have ramifications for reactor design. During these preliminary experiments, we have not sought to identify the compounds decomposing at the different temperatures. This research will be carried out later.
100 5O 2 95N 2 10O 2 90N 2 20O 2 80 100N 2 Mass % 60 40 (a) Mass rate change (%/min) 3.0 2.5 2.0 1.5 1.0 0.5 (b) 5O 2 95N 2 10O 2 90N 2 20O 2 100N 2 0.0 Figure 2 TG (a) and DTG (b) curves for biosolids at heating rate of 10 K/min under pyrolysis and different oxygen concentrations (5O 2 95N 2, 10O 2 90N 2, 20O 2 ).
100 5O 2 95N 2 10O 2 90N 2 80 20O 2 Heating rate = 5 K/min Mass % 60 40 (a) 100 5O 2 95O 2 10O 2 90N 2 80 20O 2 Heating rate = 10 K/min Mass % 60 40 (b) 100 5O 2 95N 2 10O 2 90N 2 80 20O 2 Heating rate = 20 K/min Mass % 60 40 (c) Figure 3 TG curves for biosolids at heating rates of (a) 5 K/min, (b) 10 K/min and (c) 20 K/min for different oxygen concentrations (5O 2 95N 2, 10O 2 90N 2, 20O 2 ). Effect of CO 2. Figure 4 shows the TG and DTG profiles of biosolids when 20 % of carbon dioxide, 20% steam and when both were introduced into the atmosphere. The general profile of the TG curves is similar for all three cases. The effect of the kinetics of the decomposition, which resulted in a delayed decomposition, could also be observed in both TG and DTG curves when the experiment was conducted at different heating rates (not shown). As shown in Fig. 4, two main decomposition reactions occurred at 285 o C and
900 o C. The decomposition at 900 o C was not observed under a partial oxygen atmosphere (see Fig. 2) but was observed in the pyrolysis condition (see Fig. 2) and is less defined. The fact that the mass change at 900 o C is greater under CO 2 than in pure pyrolysis condition means that a Boudouard reaction (C + CO 2 = 2CO) (Tomita and Ohtsuka 2004) may likely have taken place. 100 20CO 2 20ST 90 20ST20CO 2 60N 2 Heating rate = 10 K/min 80 Mass % 70 60 50 40 (a) 30 2.5 (b) 20CO 2 20ST Mass rate change (%/min) 2.0 1.5 1.0 0.5 20ST20CO 2 60N 2 Heating rate = 10 K/min 0.0 Figure 4 TG (a) and DTG (b) curves for biosolids at heating rate of 10 K/min under gasification (20CO 2, 20ST and 20ST20CO 2 60N 2 ) atmospheres. Effect of steam. Figure 4 also shows the TG and DTG profiles of biosolids when 20 % of steam was introduced into the atmosphere. Compared with 20CO 2, the second main decomposition occurred at a lower temperature of 835 o C and the TG mass loss (62 %) is marginally lower. This indicates that the addition of steam enhances the gasification process by increasing its reactivity thus requiring lower temperature for the reaction. Combined effect of CO 2 and steam. Figure 4 also shows the TG and DTG profiles of biosolids when both steam and carbon dioxide were introduced into the atmosphere at the
same amount (20ST20CO 2 60N 2 ). The second main decomposition still occurred at about 835 o C, similar to the 20ST, but the TG mass loss is actually the average of 20ST and 20CO 2, i.e., 64 %. Overall, similar to our experience of pyrolysis and gasification of lower-rank coals, gasification of these biosolids appear to be more affected by steam than CO 2. More experiments at different levels of steam and CO 2 are planned to identify their effects in detail. Nevertheless these information are important for reactor design for thermally treating biosolids. Reactivity calculations. From the previous calculations, it is clear that experimental decomposition is not correlated to a single reaction; reactivity calculations need to be made in different reactions zones for these biosolids and considering multiple reactions. Nevertheless, simple reactivity calculations can shed important insights before detailed investigation can be undertaken. The reactivity (R) can be calculated using the equation: where m 0 is the initial ash-free mass of biosolids (Borrego and Alvarez 2007). Figure 5a shows the reactivity of biosolids under pyrolysis condition, varying oxygen concentrations (5O 2 95N 2, 10O 2 90N 2 and 20O 2 ) and under different gasification conditions (20CO 2, 20ST and 20ST20CO 2 60N 2 ) at a heating rate of 10K/min. In the initial stage of de-volatilization (~ 250 o C), pyrolysis has shown higher reactivity compared with partial oxygen atmosphere regardless of the amount of oxygen used. When effects of different levels of oxygen are compared, 20O 2 showed the highest reactivity at ~ 250 o C. However, at around 450 o C when volatiles decomposition is taking place, the reactivity of 20O 2 is the greatest followed by the cases of 5O 2 95N 2 and 10O 2 90N 2, while that of pyrolysis is the lowest. This anomalous behaviour with O 2 is being further investigated. In regards to the gasification conditions (Fig. 5b), at lower temperature (~ 250 o C), the addition of steam enhances the reactivity during volatile evolution while the addition of carbon dioxide impedes it. In fact, when both steam and CO 2 were added and at the same volumetric concentration (20%), the resulting reactivity is the average of steam and CO 2 added separately. Moreover, at the second main decomposition reaction between 825 and 900 o C, the effect of steam enhancing the reactivity is very pronounced in that the temperature at peak reactivity decreases from 900 o C to around 835 o C. This particular result is significant because it indicates that gasification of biosolids can be done at a lower temperature by adding steam instead of carbon dioxide. Nevertheless, the use of CO 2, particularly if it is recycled from an integrated system, may have a beneficial effect on the gas composition emitted from such a process. Subsequently, we also attempted isoconversional kinetic analysis using Flynn-Wall- Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods to obtain the values of the activation energy (E α ). The calculated activation energies were similar, and ranged between 300 and 10 kj/mol. Clearly, multiple and independent reactions and zones need to be considered in obtaining accurate values of activation energy.
8 (a) 5O 2 95N 2 10O 2 90N 2 Reactivity (s -1 ) 6 4 20O 2 100N 2 Heating rate = 10 K/min 2 0 8 (b) 20ST20CO 2 60N 2 20CO 2 6 20ST Heating rate = 10 K/min Reactivity (s -1 ) 4 2 0 Figure 5 Reactivity of biosolids at (a) pyrolysis and varying oxygen concentration and (b) at different gasification conditions (20CO 2, 20ST and 20ST20CO 2 60N 2 ). CONCLUSIONS AND ON-GOING WORK This initial investigation reinforces the fact that pyrolysis and gasification using steam, CO 2 and oxygen is an alternative treatment method for biosolids. The other major conclusions are; (1) Major mass loss occurs between 200-600 o C, due to drying and devolatilisation. However, under gasification conditions another significant mass loss in the temperature range of 600-900 o C occur, which is attributed to the Boudouard and water-gas reactions. (2) The presence of steam and oxygen does augment the processing reaction while the presence of CO 2 has an impeding effect under the same conditions. However, steam has clearly a more dominant effect on reactivity of these biosolids. The ongoing work includes modelling of the reactivity data considering multiple independent reactions and distributed activation energy, and measurements on biosolids
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