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Comparison of fuel value and combustion characteristics of two different RDF samples
Waste Management xxx (2015) xxx–xxx
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Comparison of fuel value and combustion characteristics of two different RDF samples A. Sever Akdag˘ a,b, A. Atımtay a, F.D. Sanin a,⇑ a b
Department of Environmental Engineering, Middle East Technical University, 06800 Ankara, Turkey Department of Environmental Engineering, Hacettepe University, 06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 May 2015 Revised 27 July 2015 Accepted 25 August 2015 Available online xxxx Keywords: Combustion Proximate analysis RDF TGA Ultimate analysis
a b s t r a c t Generation of Municipal Solid Waste (MSW) tends to increase with the growing population and economic development of the society; therefore, establishing environmentally sustainable waste management strategies is crucial. In this sense, waste to energy strategies have come into prominence since they increase the resource efficiency and replace the fossil fuels with renewable energy sources by enabling material and energy recovery instead of landfill disposal of the wastes. Refuse Derived Fuel (RDF), which is an alternative fuel produced from energy-rich Municipal Solid Waste (MSW) materials diverted from landfills, is one of the waste to energy strategies gaining more and more attention. This study aims to investigate the thermal characteristics and co-combustion efficiency of two RDF samples in Turkey. Proximate, ultimate and thermogravimetric analyses (TGA) were conducted on these samples. Furthermore, elemental compositions of ash from RDF samples were determined by X-Ray Fluorescence (XRF) analysis. The RDF samples were combusted alone and co-combusted in mixtures with coal and petroleum coke in a lab scale reactor at certain percentages on energy basis (3%, 5%, 10%, 20% and 30%) where co-combustion processes and efficiencies were investigated. It was found that the calorific values of RDF samples on dry basis were close to that of coal and a little lower compared to petroleum coke used in this study. Furthermore, the analysis indicated that when RDF in the mixture was higher than 10%, the CO concentration in the flue gas increased and so the combustion efficiency decreased; furthermore, the combustion characteristics changed from char combustion to volatile combustion. However, RDF addition to the fuel mixtures decreased the SO2 emission and did not change the NOx profiles. Also, XRF analysis showed that the slagging and fouling potential of RDF combustion was a function of RDF portion in fuel blend. When the RDF was combusted alone, the slagging and fouling indices of its ash were found to be higher than the limit values producing slagging and fouling. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Municipal Solid Waste (MSW) is an unavoidable by-product of human activities. MSW generation increases with population growth and economic development as well as changes in lifestyles and in consumption patterns. Establishment of affordable, effective and precisely sustainable Municipal Solid Waste Management (MSWM) is crucial for sustainable development and promoting public health (Brunner and Rechberger, 2015; Cocarta et al., 2009; Santibañez-Aguilar et al., 2013). As a sustainable approach, ‘waste to energy’ strategies are gaining more attention. Production of Refuse Derived Fuel (RDF)/Solid Recovered Fuel (SRF) is one of the waste to-energy strategies that ⇑ Corresponding author. E-mail address: [email protected] (F.D. Sanin).
has been utilized in the past few decades to solve both waste and energy problems simultaneously (Rada and Andreottola, 2012). RDF is the segregated high calorific fraction of processed MSW (Gallardo et al., 2014; Nasrullah et al., 2015; Rada and Ragazzi, 2014). The use of RDF in the thermal processes has become popular and starts to receive wide attention in the world as production of RDF provides a dramatic decrease in space requirement and effectively utilizes the reusable energy of the solid waste (Gug et al., 2015; Patel et al., 2012; Zhou et al., 2013). Also, it is stated that its high energy content make RDF compatible with conventional fossil fuels (Garg et al., 2007). Proximate and ultimate analyses are conducted in many studies to assess thermal characteristics of RDFs (Ahn et al., 2013; Dunnu et al., 2010; Wagland et al., 2011). It is generally found that RDF samples have a low proportion of fixed carbon and a high amount of volatile matter when compared to other conventional fuels. In
http://dx.doi.org/10.1016/j.wasman.2015.08.037 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sever Akdag˘, A., et al. Comparison of fuel value and combustion characteristics of two different RDF samples. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.037
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A. Sever Akdag˘ et al. / Waste Management xxx (2015) xxx–xxx
addition, RDF samples contain higher moisture and have variable ash content (Ahn et al., 2013; Ferrer et al., 2005; Seo et al., 2010; Wagland et al., 2011; Wang et al., 2014). Additional observations on the combustion behavior of RDF are made by Thermogravimetric Analysis (TGA). It is shown by Seo et al. (2010) that when RDF amount in RDF-coal blends is higher, the rate of decomposition increases and also the residual char decreases because of the high volatile fraction in RDF. Investigating the bottom ash composition of RDF samples is also important in order to identify possible operational problems such as corrosion, slagging and fouling. The high presence of alkali metals in fuels is known to cause slagging, fouling and ash agglomeration; this is especially a concern with biomass originating fuels (Jenkins et al., 1998). As mentioned above, there are a number of studies about RDF in literature; however, none of these studies make an integrated approach in evaluating the thermal properties of RDF by combining all the related features. In addition, studies evaluating the ash composition by XRF and assessing this data for slagging and fouling potential of RDF are very limited. Therefore, the purpose of this work is to investigate the thermal characteristics and combustion efficiency of the RDF samples obtained from Material Recovery Facilities (MRFs) of two big cities in Turkey by a holistic approach. In this approach, the thermal characteristics and combustion patterns of RDF samples is intended to be evaluated by proximate and ultimate analysis, TGA, and lab-scale combustion reactor studies. Additionally, comparative analysis with conventional fuels is integrated into the study. The knowledge gap in literature on slagging and fouling potential of RDF is also expected to be filled by the results of this work.
prepared by mixing RDF with the conventional fuels at ratios of 3%, 5%, 10%, 20% and 30%. The samples (alone and blends) were prepared as pellets by a hand operated pellet press. The pellets had 15 mm diameter and about 2 mm height. Each sample (pellet) combusted was formulated to keep the calorific value constant as 4.184 kJ. Therefore, in this work, the RDF mixing ratio means the share of the energy input from the RDF in the total energy input from the fuel mixtures. RDF samples alone were also combusted without any coal or petroleum coke addition to observe the efficiency of RDF combustion alone. 2.3. Thermal characterization of RDF, coal and petroleum coke samples Proximate and ultimate analyses were made on the RDF, coal and petroleum coke samples. Then the calorific values (lower heat value) of all the fuels were determined. Furthermore, XRF analyses were made on the ashes of the RDF samples used. All experiments were done on triplicate samples. 2.3.1. Proximate analysis In proximate analysis, it is typically assumed that the fuel was composed of two types of carbon; volatile and fixed carbon. In this analysis, first, the moisture content of samples was determined according to ASTM method E790-08. Then, the volatile matter and ash content analyses were done in accordance with ASTM method E897-88 and ASTM method E830-87, respectively. Finally, the fixed carbon was calculated by subtracting the sum of percentages of volatile matter and ash content on dry basis from a total of hundred percent.
2. Materials and methods 2.1. The material recovery facilities investigated and sampling The names of the cities that the RDF samples produced are not disclosed due to confidentiality reasons. So the two samples are denoted as RDF-A and RDF-B. In both facilities, MSW collected is processed in three lines. Fraction of waste that is paper, glass, plastics and metal are recovered, the biodegradable fraction (mostly food waste) is sent to composting and the remaining portion is processed as RDF. The MRFs schemes are given in Fig. 1. The RDFs produced in both plants are mainly made up of fractions of textile, paper and plastics. Major part of plastics is constituted by plastic bags, with the remaining fraction PET. The composition of RDF changes day to day but typical fractions are about 60% textile, 20% paper and 20% plastics. Since other organics including food waste is diverted to a composting facility in both plants, all remaining fraction exiting the RDF plant can be considered as combustible. The RDF products are recently being combusted in different cement factories. RDF samples, RDF-A and RDF-B, were taken on February, 2014 from the two material recovery facilities and were brought to the laboratory for analysis. The RDFs were mixed well prior to sampling in the facilities to obtain a representative sample. Upon arrival at the laboratory, the RDF samples were ground well by the use of a ball-mill to obtain homogenous samples and only the homogenized samples were used throughout the study. Care was given to collect well-mixed samples before any analysis to further ensure the homogeneity. The coal and petroleum coke samples used in the combustion studies were taken from a cement factory in Turkey. 2.2. Sample preparation for combustion In order to observe the effects of RDF addition to main fuels during combustion, RDF-coal and RDF-petroleum coke mixtures were
2.3.2. Ultimate analysis Carbon (C), oxygen (O), hydrogen (H), nitrogen (N) and sulfur (S) are the main chemical elements in a fuel. The chemical analysis is very important to calculate the material balance accurately. Thus, C, H, N, S and O content of the samples were determined by ultimate analysis conducted by Truspec Leco CHNS-932 analyzer. Samples were introduced to auto sampler of the analyzer and combusted at 950 °C, and then the content of C, N, S and H were measured simultaneously. Once the percentages of carbon, nitrogen, hydrogen, sulfur and ash were determined, the amount of oxygen was calculated by subtracting the total percentages of the mentioned elements from a hundred percent. All results were reported on dry basis. 2.3.3. Calorific values Calorific values of RDF samples were determined according to ASTM method E711-87 by using a Leco AC-500 model bomb calorimeter. The calorimeter was calibrated before each measurement by using benzoic acid as a standard. 2.3.4. X-Ray Fluorescence (XRF) analysis X-Ray Fluorescence (XRF) analysis was done by Spectro XLAB2000 PEDX-ray fluorescence spectrometer for the purpose of determining the inorganic content of ashes of RDF samples. The device was calibrated with the standard samples which are certified from U.S. Geological Survey (USGS). In the analysis, the loss of organics on ignition was achieved by fluxing the sample with sodium tetraborate and burning at 1100 °C; then the inorganic content of the sample remained was pelletized for further analysis. In this analysis Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn and Fe, which were the major inorganic elements in the samples, were determined. Then, the elements were converted to their element oxides to calculate the slagging/fouling indices of RDF samples.
Please cite this article in press as: Sever Akdag˘, A., et al. Comparison of fuel value and combustion characteristics of two different RDF samples. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.037
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Fig. 1. MRF schemes for RDF-A and RDF-B (left RDF-A, right RDF-B).
2.4. Thermogravimetric Analysis (TGA) of RDF samples Thermogravimetric analysis was carried out according to ISO 11358-1 method by using Perkin Elmer Pyris STA- 6000 model TGA instrument, which measures the sample weight as a function of time as temperature is increased according to a determined heating rate. The instrument also calculates and displays Differential Thermal Gravimetric Analysis (DTGA) signal (rate of weight loss) from weight loss vs. time signals. Using a constant heating rate of 10 °C/min in dry air atmosphere, the samples were heated from 25 °C to 950 °C. TGA and DTGA curves for RDF-A, RDF-B, coal and petroleum coke samples were obtained. 2.5. Lab-scale combustion experiments of RDF – coal and RDF petroleum coke mixtures A laboratory scale set up was established in order to test the cocombustion characteristics and efficiencies of RDF samples. In combustion experiments, pellets of RDF samples alone and in different mixtures with coal and petroleum coke were combusted in an electrically heated cylindrical quartz batch reactor with dimensions of 50 mm diameter and 1200 mm height. A schematic diagram of the experimental setup is given in Fig. 2. The reactor had a perforated plate made of quartz somewhere in the middle height of the reactor to hold the sample pellets. The pellets were combusted at the batch reactor where only one pellet was burned at a time at fixed temperature of 900 °C. The effect of RDF addition to the main fuels which were coal and petroleum coke was investigated by analyzing the flue gases during combustion experiments. At the beginning of each experiment, the cylindrical quartz reactor was heated first to 900 °C at which the combustion occurs. The two thermocouples, one of which located close to furnace wall and the other located inside the quartz reactor (combustion zone), allowed measuring the temperature of the furnace and the combustion zone of the reactor. Then, air was introduced from the bottom of the reactor when the temperature of the combustion zone reached 900 °C. The flow rate of air, which was 1000 cm3/min, was adjusted by a flow meter. Next, top seal of the reactor was opened in order to drop pelletized sample into the reactor and was closed rapidly. The pelletized sample instantly reached the combustion zone of the reactor. The combustion began and the gas analyzer started sampling the flue gas. 2.6. Flue gas analyzer The combustion gases, CO2, CO, NO, NO2 and SO2, were monitored by Madur Photon flue gas analyzer continuously. The
analyzer consisted of two units which were a conditioning unit and a gas analysis unit. In the conditioning unit called as PDG100, moisture in the flue gas was trapped and solid particles in the flue gas were filtered. Thus, the results of analysis were reported on dry basis. In the second unit, the conditioned flue gas was analyzed. The device measured the concentrations of O2 and CO2 gases with an Infrared (IR) cell as volume percentages; concentrations of other gases were measured electrochemically in ppm. 3. Results and discussions 3.1. Thermal characterization of RDF, coal and petroleum coke samples 3.1.1. Proximate/ultimate analysis of RDF, coal and petroleum coke samples and their calorific value determination The average results of proximate analysis and standard deviations obtained from the triplicate analyses are given in Table 1. In evaluating the results, it should be noted that there is difference in moisture and fixed carbon contents between RDF-A and RDF-B. This is because of the way the samples are processed in the facilities. The RDF-A sample was dried in its facility and then sent to us, whereas RDF-B plant had no drying unit, so sample was sent to us as it is and moisture content was determined by us as sampled. Besides, fixed carbon is the residue remaining after volatile matter disappears during the combustion process. RDF samples in this study were mainly composed of volatile matter (VM) which was 81.8% and 68.5% for RDF-A and RDF-B, respectively. As it is seen, the RDF-A has more volatile contents than RDF-B and thus its fixed carbon content is less. The reason of the high volatile matter content of RDF is that they contain many volatiles such as paper, plastic etc.; also, the reduction of the inert materials from MSW stream during the RDF production process increases the volatile matter content of RDFs. The volatile matter contents of RDF samples were higher than that of the coal and petroleum coke samples, and their fixed carbon contents were lower accordingly. Among all samples, the petroleum coke sample had the lowest volatile matter content and highest fixed carbon content. In addition, the ash content of RDF is important since it determines the calorific value and also the amount of residue that would be left behind upon combustion. In this respect, RDF-A and RDF-B were similar in ash content (between 12% and 15%) and ash contents of both were slightly lower than that of coal (17.2%). On the other hand, petroleum coke had very small ash content (0.7%) relative to the other three fuels. Carbon, hydrogen, and oxygen content of the fuels are very important since these elements constitute the main fuel fraction
Please cite this article in press as: Sever Akdag˘, A., et al. Comparison of fuel value and combustion characteristics of two different RDF samples. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.037
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Fig. 2. Lab scale combustion setup.
Table 1 Proximate analysis of RDF samples, coal and petroleum coke.
* **
Sample
Moisture (%)*
Volatile matter (%)
RDF-A RDF-B Coal Petroleum coke
1.6 ± 0.02 14.8 ± 0.09 4.3 ± 0.03 7.0 ± 0.12
81.8 ± 0.89 68.5 ± 0.52 29.3 ± 0.11 12.1 ± 0.17
**
Fixed carbon (%) 5.2 ± 0.52 16.6 ± 0.74 53.5 ± 0.12 87.4 ± 0.31
**
Ash (%)
**
12.9 ± 0.38 14.9 ± 0.64 17.2 ± 0.22 0.7 ± 0.01
% by weight, as collected. % by weight on dry basis.
of the waste. Nitrogen content is also required to be known because it determines the amount of NOx formation. Sulfur is another important element, amount of which should be known since the presence of sulfur causes the generation of acid gases SO2 and SO3 which contribute to air pollution and corrosion. In this context, ultimate analysis which gave the elemental composition of RDF, coal and petroleum coke samples were conducted and calorific values of the samples were determined all in triplicate samples and the results with the standard deviations are shown in Table 2. Ultimate analysis of the samples revealed that the carbon contents of RDF-B, coal and petroleum coke were close to each other. Carbon content of RDF-A was the lowest and that of the petroleum coke was the highest among the samples studied. The calorific values of RDF samples, which were 22,143.40 kJ/kg and 19,229.79 kJ/ kg on dry basis, for RDF-A and RDF-B, respectively, were close to each other and were a little lower than that of the coal used in this study. The petroleum coke had the highest calorific value among all the fuels due to its lower volatile matter content and higher carbon content. Also, it was seen that the volatile matter content decreases with an increase in the carbon content for all fuels. Sulfur had the lowest percentage among all the elements measured; in RDF-B the sulfur content was 0.45%, whereas sulfur was not detected in RDF-A. Nitrogen and hydrogen percentages in the samples were all lower than 10%. However, the content of hydrogen was higher in RDF samples than in coal and petroleum coke samples as seen in Table 2. The high hydrogen content of RDF samples may be linked to the high volatile matter content originating from the organic matter in the waste.
3.1.2. Determination of ash composition of RDF samples by XRF analysis The elemental composition of ash is important for slagging and fouling concerns. There is a wide range of elements which contribute to slagging and fouling in combustion of refuse. XRF analysis was done to determine the inorganic elements, which formed the ash content of RDF samples. Using the result of this analysis, fouling and slagging indices of RDF samples were calculated. Results of XRF analysis are presented as element oxides on dry basis in Table 3. It can be seen in Table 3 that Si and Ca were the most abundant inorganic elements in ashes of RDF samples tested in this study. The Cl content in RDF ash samples was also high, which may originate from chlorine in PVC (polyvinyl chloride) plastics and waste paper in RDF content (Beckmann and Ncube, 2007; Dong et al., 2002; Tchobanoglous and Kreith, 2002). Moreover, Fe, K, Al and S are other inorganic elements in ashes found in high concentrations. Slagging and fouling indices were calculated for the RDF samples by using the formula given in Table 4. Particularly ‘‘Base to acid ratio” (B/A), and ‘‘sulfur ratio” (Rs) were used as slagging indices; ‘‘total alkalis” (TA), and ‘‘fouling index” (Fu) were used as fouling indices, as they were previously used in Teixeira et al. (2012). B/A ratio shows the presence of alkali and alkali-earth oxides in the sample relative to Al2O3, SiO2. TiO2 content have an important role in melt formation. Higher basic compounds, B, are assumed to decrease the melting temperature, while the acidic ones, A, increase the melting point. The sulfur ratio, Rs, is related to slagging potential of sample and it is calculated using the S content of the sample. The fouling index, Fu, which is based on
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Table 2 Ultimate analysis and calorific values of RDF samples, coal and petroleum coke.
* **
Sample
Carbon (%)*
Hydrogen (%)*
Nitrogen (%)*
Sulfur (%)*
Oxygen (%)*
Calorific Value (kJ/kg)**
RDF-A RDF-B Coal Petroleum coke
44.14 ± 1.09 56.47 ± 0.53 63.80 ± 0.15 68.12 ± 0.21
5.63 ± 0.37 8.96 ± 0.25 3.65 ± 0.02 3.63 ± 0.05
0.97 ± 0.31 1.50 ± 0.08 1.88 ± 0.04 1.94 ± 0.06
– 0.45 ± 0.02 0.55 ± 0.02 4.73 ± 0.09
36.26 ± 1.20 17.73 ± 0.17 12.94 ± 0.06 20.51 ± 0.11
22,143.40 ± 661.1 19,229.79 ± 151 25,641.23 ± 279.1 34,381.18 ± 299.2
% by weight on dry basis. Lower heating value on dry basis (kJ/kg).
Table 3 Inorganic element contents of RDF samples (% by wt.).
Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl K2O CaO TiO2 V2O5 Cr2O3 MnO Fe2O3
RDF-A
RDF-B
0.20 0.13 0.43 1.21 0.14 0.20 0.29 0.27 3.36 0.15 – – 0.01 0.08
0.06 0.12 0.42 1.54 0.11 0.24 0.32 0.44 2.72 0.13 – 0.01 0.01 0.23
Base-to-Acid ratio, gives more relevance to the alkaline elements which are the main agents of fouling. The results and limit values are given in Table 5. As seen from Table 5, the RDF ash samples have moderate to high slagging and fouling tendency when compared to limit values. Base to acid ratio (B/A) of both RDF ash samples were higher than ‘‘high slagging” class. Presence of alkaline elements in the RDF ash samples caused the high base to acid ratios, which were 2.3 and 1.8 for RDF-A ash and RDF-B ash, respectively. It should be noted that the values higher than 1 corresponds to high tendency of slagging. Fouling tendency of RDF ash samples was high when compared to limit value because of the high content of volatile inorganic components like Na2O, K2O as shown in ash composition table. Also, sulfur and phosphorus content of RDF samples are high, which makes nearly all of slagging and fouling indices higher than the limit values. It is worth to note that there are some studies in which slagging and fouling indices of coal is calculated (Teixeria et al., 2012; Park and Jang, 2011). As it can be seen from these studies, the slagging and fouling tendency of coal is lower when compared to RDF.
3.2. Co-combustion of RDF–coal and RDF–petroleum coke mixtures Fig. 3 is given as a sample graph showing the results of a batch combustion experiment run. In this figure, change in O2 and CO2
Table 4 Empirical relations for slagging and fouling tendency of ash composition (Park and Jang, 2011; Pronobis, 2005). Classification
Index
Formula
Slagging
Base-to-acid ratio (B/A)
Fe2 O3 þCaOþMgOþK2 OþNa2 O SiO2 þTiO2 þAl2 O3 B A xSðin dry fuelÞ
Sulfur ratio (Rs) Fouling
Total Alkalis (TA) Fouling ratio (Fu)
Na2 O þ K2 O B A xNa2 O þ K2 O
Fig. 3. Results of co-combustion of coal with 5% RDF-A.
concentrations in the flue gas with respect to time is shown on the left ordinate of the graphs. The flue gas analyzer measured the concentration of these two gases as volume percentages. CO, NO, NO2 and SO2 concentrations which were measured as ppm are shown on the right ordinate of the graph. Measurements of flue gas composition were performed with two seconds interval; therefore, the profiles of gases were plotted with respect to time. From the graph given in Fig. 3 and all other co-combustion experiments carried out in this study, it can be seen that a sharp decrease in oxygen concentration and increase in carbon monoxide and carbon dioxide concentration were observed in the flue gas as soon as the pellets were dropped into the reactor. This quick carbon monoxide formation is explained by high volatiles content in RDF samples. Because the volatile matter burns quickly as soon as the pellet is dropped to the reactor, this creates an unstable medium and results in high carbon monoxide concentration (Kobyashi et al., 2005). Another reason for high carbon monoxide concentration may be the temperature difference between the reactor and the fuel (pellet) at the beginning of the experiment. The pellet is at room temperature when it is dropped into the reactor. Incomplete combustion occurs until the pellet is heated up to the reactor temperature, which results in high carbon monoxide emission. Also, the fracture strength of the pellet has an indirect effect on CO emission. The more brittle pellets are fractured into small particles when they are dropped into the reactor and therefore they burn faster. This fast burning increases the emission of volatiles and thus causes an unstable medium (Kobyashi et al., 2005). On the average, oxygen level dropped to around 10% immediately and carbon dioxide rose to around 5–10%. Concentration of carbon monoxide depended on the RDF percentage in mixtures; it changed between 1000 ppm and 5000 ppm when RDF percentage was 3–5%; on the other hand, it increased up to 6000 ppm when RDF percentage was higher. The carbon monoxide concentration increased to even higher values and it could not be measured with the gas analyzer for the mixtures with 20–30% RDF content or when sole RDF was burned. From the graphs, it can also be seen that the carbon monoxide formation was lower in RDF-petroleum coke mixtures than the ones in RDF-coal mixtures.
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6 Table 5 Slagging and fouling indices calculated for RDF samples.
Slagging-fouling indices of RDF samples
Limit values (Park and Jang, 2011; Pronobis, 2005)
RDF-A
RDF-B
Low
Medium
High
Slagging index
B/A RS
2.3 0.5
1.8 0.3
<0.5 <0.6
0.5 < B/A < 1 0.6 < Rs < 2
>1 >2
Fouling index
TA Fu
0.5 1.1
0.5 0.9
<0.3 60.6
0.3 < TA < 0.4 –
>0.4 0.6 < Fu 6 40
Petroleum coke, with higher calorific value and carbon content, and lower volatile matter content than coal yielded better combustion characteristics in RDF mixtures. The efficiency of the combustion processes was calculated based on a carbon mass balance. The percentage of carbon that was captured as carbon monoxide and carbon dioxide was found by calculating the areas under the emission curves for all experiments. The carbon measured by the ultimate analysis was used for the calculation of the amount of total carbon in the feed. These calculations allowed us to check the degree of control that we had on the system as well as the accuracy of our analysis during the experiments. Calculations showed that carbon balance held within a maximum of ±20% deviation, where in majority of the combustions the deviation was much less than 10%. Then, the efficiencies of the combustion were determined by calculating the amount of carbon which transformed into carbon dioxide. The results of carbon combustion efficiency for RDF-A and RDF-B are shown in Figs. 4 and 5, respectively. Both Figs. 4 and 5 show that the combustion efficiency decreased with increasing RDF fraction. The replacement of coal and petroleum coke with RDF caused more carbon monoxide formation which was the indication of incomplete combustion. ANOVA tests were performed to interpret the significance of the results statistically. F-tests and multiple range tests were performed in the ANOVA test to compare the mean values of the combustion efficiencies of RDF samples for the 7 different levels (0%, 3%, 5%, 10%, 20%, 30%, and 100%) of RDF fractions. It was found from F-test that there was a statistically significant difference between the means of efficiency of RDF co-combustion with both petroleum coke and coal when different levels of RDF fractions were incorporated into the fuel at 95.0% confidence level. Multiple range test results indicated that there are no statistically significant differences between only coal or only petroleum coke combustion and co-combustion of coal or petroleum coke with the addition of 3%, 5% and 10% RDF. At these levels the primary fuel (coal or petroleum coke) seems to dominate the
Fig. 5. Combustion Efficiency of RDF-B with coal and petroleum coke.
combustion yielding similar efficiencies. However, the test results showed that the significant decreases were observed as the RDF fraction increased from 10% to 20% in both coal and petroleum coke mixtures. Furthermore, the combustion efficiency also decreased when the RDF ratio in petroleum coke mixtures was increased from 20% to 30%. Also, the minimum efficiencies were observed when only RDF samples were combusted without any coal and petroleum coke addition. Furthermore, for the cocombustion experiments with coal, it was seen that there was no efficiency differences between the 20% and 30% mixing ratio. Also, the general combustion efficiencies of petroleum coke–RDF mixtures were higher than those of coal–RDF mixtures because of better fuel features of petroleum coke compared to coal. It is concluded that the combustion efficiencies decreased significantly if the RDF sample fraction in the mixtures were more than 10%. At this point it is worth to note that this percentage may change from one condition to the other since the combustion efficiency highly depends on the composition of RDF, which is variable even within a single facility, combustion conditions such as temperature and flowrate, reactor type and condition of samples (pellets or loose material). However, the general trends are expected to be independent of the working conditions. Therefore the results of this work should be looked upon by considering these facts. RDF addition to the fuel blends decreased the SO2 emission due to the lower sulfur content in the RDF samples than in the coal and petroleum coke samples, but it did not change NO emission appreciably. SO2 emission was higher in petroleum coke mixtures than coal mixtures since the sulfur content of petroleum coke was higher than coal. 3.3. Thermogravimetric Analysis (TGA) of RDF, coal and petroleum coke samples
Fig. 4. Combustion Efficiency of RDF-A with coal and petroleum coke.
TGA and DTG profiles evaluated in the combustion tests of RDFA and RDF-B and coal and petroleum coke are represented in Figs. 6 and 7, respectively.
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Fig. 6. TGA and DTG profiles of RDF-A and RDF-B (left RDF-A, right RDF-B).
Fig. 7. TGA and DTG profiles of coal and petroleum coke (left coal, right petroleum coke).
DTG curves of both RDF samples had shoulders/peaks which were caused by the big mass losses observed in three different regions. The first mass loss observed at the beginning the of the experiment, from ambient temperature to about 120 °C, is due to the loss of moisture and very light volatile matter content of the fuel. In this region there is a difference between RDF-A and RDF-B that matches nicely with the data in Table 1. The RDF-A has less moisture, which yields a smaller peak at around 100 °C compared to RDF-B. During the combustion process, the main decomposition of RDF samples took place in between temperatures of 200 °C and 600 °C. The rate of weight loss was maximum at around 320 °C for RDF-A and at around 330 °C for RDF-B in the dry air environment. Unlike the moisture loss region, more than one peak was observed in this region. In the RDF-B curves, three exothermic peaks were observed at 330 °C, 400 °C, and 510 °C; while, in RDF-A curves, there were two peaks at 320 °C and 500 °C. At these peak points the mass loss was significant. As mentioned in Piao et al. (2000), the presence of the shoulders shows that there are different volatile matter fractions in RDF samples; for example the existence of cellulosic materials and the plastics may produce different peaks during decomposition and combustion. When the decomposition temperatures and decomposition speeds of different types of volatile matters included in RDF samples are close to each other, less peaks are formed, which might be the case observed in RDF-A. For both RDF samples, a last peak occurred at about 650 °C. This last peak was due to the char combustion. Also the reactions between char and volatiles, which were coming from previous phases of the process, might be another cause of this last peak.
The combustion process in the experiments was complete at around 700 °C and the mass loss of RDF-A and RDF-B were found as 79.1%, 80.9%, respectively, which meant that about 20% of char remained at the end of the process. Fig. 7 presents the TG and DTG curves observed for coal and petroleum coke samples. Unlike the RDF combustion, the main decomposition of coal samples took place at higher temperatures (between 700 °C and 800 °C). The highest mass loss, which corresponded to the peak at 720°C, was mainly due to the volatile matter and char decomposition. At the end of the process the degraded mass was 53.3% of the original mass. The typical DTG curve characteristics for petroleum coke mentioned in Magdziarz and Werle (2014) were observed in the current study; there was only one peak between the temperatures of 500 °C and 700 °C due to the decomposition of all organic matter, loss of volatiles and char. The total weight loss in the combustion process was 35.8% and the maximum weight loss occurs at around 610 °C. The initial decomposition temperature of petroleum coke was the highest when compared to coal and RDF samples. Therefore, having the highest ignition and burnout temperatures, petroleum coke was the hardest fuel to ignite and burnout totally among the samples burnt in the current study. It was concluded from the thermogravimetric analysis that, the constituents of the fuel decomposes in solid-phase at the char combustion in coal and petroleum coke combustion; however, in RDF combustion, the decomposition occurs at early stages of the combustion in gas phase due to the low fixed-carbon and high volatile matter content of RDF. Also, it can be stated that the combustion mechanisms of volatile matters in RDF samples are complicated than that of char. Therefore, char combustion characteristics
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is not a criteria for RDF combustibility. However, volatile matter combustion is the most crucial parameter and should be investigated since the combustion duration, speed and mass loss of volatile matters affect the RDF combustibility. 4. Conclusion Results of this study have shown that calorific values of RDF samples on dry basis are close to that of coal and a little lower compared to petroleum coke used in this study. Although the calorific values of RDF samples are sufficient, the use of sole RDF samples in combustion processes is limited due to its combustion characteristics. While the RDF sample fraction in the fuel mixtures is increased (such more than 10%), the combustion characteristics of the fuel mixtures changes from char combustion to volatile combustion. In addition, CO emission increases and so the combustion efficiency decreases significantly when the RDF fraction in the mixture is higher than 10%. However, the co-combustion of RDF with lower ratios (such as 3%, 5%, and 10%) does not decrease the combustion efficiency significantly. Also, the minimum efficiency is observed at the combustion of pure RDF without any coal or petroleum coke fraction. RDF addition to the fuel blends decreases the SO2 emission due to the lower sulfur content in the RDF samples than in the coal and petroleum coke samples, but it does not change NO emission appreciably. Slagging and fouling intensity of RDF samples determined in this study indicates that the use of RDF samples in cocombustion processes needs to be kept in certain amounts. TGA results showed that the combustion mechanisms of volatile matters in RDF samples are complicated than that of char. The mass loss of RDF-A and RDF-B are found as 79.1%, 80.9%, respectively, which means about 20% of char remained at the end of the process. In coal and petroleum coke combustion, the constituents of the fuel decomposes in solid-phase at the char combustion; however, in RDF combustion, the decomposition occurs at early stages of the combustion in gas phase due to the low fixed-carbon and high volatile matter content of RDF. References Ahn, S.Y., Eom, S.Y., Rhie, Y.H., Sung, Y.M., Moon, C.E., Choi, G.M., Kim, D.J., 2013. Application of refuse fuels in a direct carbon fuel cell system. Energy 51, 447– 456. http://dx.doi.org/10.1016/j.energy.2012.12.025. ASTM Standard E711-88, 2004. Standard Test Method for Gross Calorific Value of Refuse-Derived Fuel by the Bomb Calorimeter. ASTM International, West Conshohocken, PA. ASTM Standard E790-08, 2004. Standard Test Method for Residual Moisture in a Refuse-Derived Fuel Analysis Sample. ASTM International, West Conshohocken, PA. ASTM Standard E830-87, 2004. Standard Test Method for Ash in the Analysis Sample of Refuse-Derived Fuel. ASTM International, West Conshohocken, PA. ASTM Standard E897-88, 2004. Standard Test Method for Volatile Matter in the Analysis Sample of Refuse-Derived Fuel. ASTM International, West Conshohocken, PA. Beckmann, M., Ncube, S., 2007. Characterisation of Refuse Derived Fuels (RDF) in reference to the Fuel Technical Properties. In: 26th Annual International Conference on Incineration and Thermal Treatment Technologies, IT3; Phoenix, AZ; United States. Brunner, P.H., Rechberger, H., 2015. Waste to energy – key element for sustainable waste management. Waste Manage. 37, 3–12.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Comparison of fuel value and combustion characteristics of two different RDF samples A. Sever Akdag˘ a,b, A. Atımtay a, F.D. Sanin a,⇑ a b
Department of Environmental Engineering, Middle East Technical University, 06800 Ankara, Turkey Department of Environmental Engineering, Hacettepe University, 06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 May 2015 Revised 27 July 2015 Accepted 25 August 2015 Available online xxxx Keywords: Combustion Proximate analysis RDF TGA Ultimate analysis
a b s t r a c t Generation of Municipal Solid Waste (MSW) tends to increase with the growing population and economic development of the society; therefore, establishing environmentally sustainable waste management strategies is crucial. In this sense, waste to energy strategies have come into prominence since they increase the resource efficiency and replace the fossil fuels with renewable energy sources by enabling material and energy recovery instead of landfill disposal of the wastes. Refuse Derived Fuel (RDF), which is an alternative fuel produced from energy-rich Municipal Solid Waste (MSW) materials diverted from landfills, is one of the waste to energy strategies gaining more and more attention. This study aims to investigate the thermal characteristics and co-combustion efficiency of two RDF samples in Turkey. Proximate, ultimate and thermogravimetric analyses (TGA) were conducted on these samples. Furthermore, elemental compositions of ash from RDF samples were determined by X-Ray Fluorescence (XRF) analysis. The RDF samples were combusted alone and co-combusted in mixtures with coal and petroleum coke in a lab scale reactor at certain percentages on energy basis (3%, 5%, 10%, 20% and 30%) where co-combustion processes and efficiencies were investigated. It was found that the calorific values of RDF samples on dry basis were close to that of coal and a little lower compared to petroleum coke used in this study. Furthermore, the analysis indicated that when RDF in the mixture was higher than 10%, the CO concentration in the flue gas increased and so the combustion efficiency decreased; furthermore, the combustion characteristics changed from char combustion to volatile combustion. However, RDF addition to the fuel mixtures decreased the SO2 emission and did not change the NOx profiles. Also, XRF analysis showed that the slagging and fouling potential of RDF combustion was a function of RDF portion in fuel blend. When the RDF was combusted alone, the slagging and fouling indices of its ash were found to be higher than the limit values producing slagging and fouling. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Municipal Solid Waste (MSW) is an unavoidable by-product of human activities. MSW generation increases with population growth and economic development as well as changes in lifestyles and in consumption patterns. Establishment of affordable, effective and precisely sustainable Municipal Solid Waste Management (MSWM) is crucial for sustainable development and promoting public health (Brunner and Rechberger, 2015; Cocarta et al., 2009; Santibañez-Aguilar et al., 2013). As a sustainable approach, ‘waste to energy’ strategies are gaining more attention. Production of Refuse Derived Fuel (RDF)/Solid Recovered Fuel (SRF) is one of the waste to-energy strategies that ⇑ Corresponding author. E-mail address: [email protected] (F.D. Sanin).
has been utilized in the past few decades to solve both waste and energy problems simultaneously (Rada and Andreottola, 2012). RDF is the segregated high calorific fraction of processed MSW (Gallardo et al., 2014; Nasrullah et al., 2015; Rada and Ragazzi, 2014). The use of RDF in the thermal processes has become popular and starts to receive wide attention in the world as production of RDF provides a dramatic decrease in space requirement and effectively utilizes the reusable energy of the solid waste (Gug et al., 2015; Patel et al., 2012; Zhou et al., 2013). Also, it is stated that its high energy content make RDF compatible with conventional fossil fuels (Garg et al., 2007). Proximate and ultimate analyses are conducted in many studies to assess thermal characteristics of RDFs (Ahn et al., 2013; Dunnu et al., 2010; Wagland et al., 2011). It is generally found that RDF samples have a low proportion of fixed carbon and a high amount of volatile matter when compared to other conventional fuels. In
http://dx.doi.org/10.1016/j.wasman.2015.08.037 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
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addition, RDF samples contain higher moisture and have variable ash content (Ahn et al., 2013; Ferrer et al., 2005; Seo et al., 2010; Wagland et al., 2011; Wang et al., 2014). Additional observations on the combustion behavior of RDF are made by Thermogravimetric Analysis (TGA). It is shown by Seo et al. (2010) that when RDF amount in RDF-coal blends is higher, the rate of decomposition increases and also the residual char decreases because of the high volatile fraction in RDF. Investigating the bottom ash composition of RDF samples is also important in order to identify possible operational problems such as corrosion, slagging and fouling. The high presence of alkali metals in fuels is known to cause slagging, fouling and ash agglomeration; this is especially a concern with biomass originating fuels (Jenkins et al., 1998). As mentioned above, there are a number of studies about RDF in literature; however, none of these studies make an integrated approach in evaluating the thermal properties of RDF by combining all the related features. In addition, studies evaluating the ash composition by XRF and assessing this data for slagging and fouling potential of RDF are very limited. Therefore, the purpose of this work is to investigate the thermal characteristics and combustion efficiency of the RDF samples obtained from Material Recovery Facilities (MRFs) of two big cities in Turkey by a holistic approach. In this approach, the thermal characteristics and combustion patterns of RDF samples is intended to be evaluated by proximate and ultimate analysis, TGA, and lab-scale combustion reactor studies. Additionally, comparative analysis with conventional fuels is integrated into the study. The knowledge gap in literature on slagging and fouling potential of RDF is also expected to be filled by the results of this work.
prepared by mixing RDF with the conventional fuels at ratios of 3%, 5%, 10%, 20% and 30%. The samples (alone and blends) were prepared as pellets by a hand operated pellet press. The pellets had 15 mm diameter and about 2 mm height. Each sample (pellet) combusted was formulated to keep the calorific value constant as 4.184 kJ. Therefore, in this work, the RDF mixing ratio means the share of the energy input from the RDF in the total energy input from the fuel mixtures. RDF samples alone were also combusted without any coal or petroleum coke addition to observe the efficiency of RDF combustion alone. 2.3. Thermal characterization of RDF, coal and petroleum coke samples Proximate and ultimate analyses were made on the RDF, coal and petroleum coke samples. Then the calorific values (lower heat value) of all the fuels were determined. Furthermore, XRF analyses were made on the ashes of the RDF samples used. All experiments were done on triplicate samples. 2.3.1. Proximate analysis In proximate analysis, it is typically assumed that the fuel was composed of two types of carbon; volatile and fixed carbon. In this analysis, first, the moisture content of samples was determined according to ASTM method E790-08. Then, the volatile matter and ash content analyses were done in accordance with ASTM method E897-88 and ASTM method E830-87, respectively. Finally, the fixed carbon was calculated by subtracting the sum of percentages of volatile matter and ash content on dry basis from a total of hundred percent.
2. Materials and methods 2.1. The material recovery facilities investigated and sampling The names of the cities that the RDF samples produced are not disclosed due to confidentiality reasons. So the two samples are denoted as RDF-A and RDF-B. In both facilities, MSW collected is processed in three lines. Fraction of waste that is paper, glass, plastics and metal are recovered, the biodegradable fraction (mostly food waste) is sent to composting and the remaining portion is processed as RDF. The MRFs schemes are given in Fig. 1. The RDFs produced in both plants are mainly made up of fractions of textile, paper and plastics. Major part of plastics is constituted by plastic bags, with the remaining fraction PET. The composition of RDF changes day to day but typical fractions are about 60% textile, 20% paper and 20% plastics. Since other organics including food waste is diverted to a composting facility in both plants, all remaining fraction exiting the RDF plant can be considered as combustible. The RDF products are recently being combusted in different cement factories. RDF samples, RDF-A and RDF-B, were taken on February, 2014 from the two material recovery facilities and were brought to the laboratory for analysis. The RDFs were mixed well prior to sampling in the facilities to obtain a representative sample. Upon arrival at the laboratory, the RDF samples were ground well by the use of a ball-mill to obtain homogenous samples and only the homogenized samples were used throughout the study. Care was given to collect well-mixed samples before any analysis to further ensure the homogeneity. The coal and petroleum coke samples used in the combustion studies were taken from a cement factory in Turkey. 2.2. Sample preparation for combustion In order to observe the effects of RDF addition to main fuels during combustion, RDF-coal and RDF-petroleum coke mixtures were
2.3.2. Ultimate analysis Carbon (C), oxygen (O), hydrogen (H), nitrogen (N) and sulfur (S) are the main chemical elements in a fuel. The chemical analysis is very important to calculate the material balance accurately. Thus, C, H, N, S and O content of the samples were determined by ultimate analysis conducted by Truspec Leco CHNS-932 analyzer. Samples were introduced to auto sampler of the analyzer and combusted at 950 °C, and then the content of C, N, S and H were measured simultaneously. Once the percentages of carbon, nitrogen, hydrogen, sulfur and ash were determined, the amount of oxygen was calculated by subtracting the total percentages of the mentioned elements from a hundred percent. All results were reported on dry basis. 2.3.3. Calorific values Calorific values of RDF samples were determined according to ASTM method E711-87 by using a Leco AC-500 model bomb calorimeter. The calorimeter was calibrated before each measurement by using benzoic acid as a standard. 2.3.4. X-Ray Fluorescence (XRF) analysis X-Ray Fluorescence (XRF) analysis was done by Spectro XLAB2000 PEDX-ray fluorescence spectrometer for the purpose of determining the inorganic content of ashes of RDF samples. The device was calibrated with the standard samples which are certified from U.S. Geological Survey (USGS). In the analysis, the loss of organics on ignition was achieved by fluxing the sample with sodium tetraborate and burning at 1100 °C; then the inorganic content of the sample remained was pelletized for further analysis. In this analysis Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn and Fe, which were the major inorganic elements in the samples, were determined. Then, the elements were converted to their element oxides to calculate the slagging/fouling indices of RDF samples.
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Fig. 1. MRF schemes for RDF-A and RDF-B (left RDF-A, right RDF-B).
2.4. Thermogravimetric Analysis (TGA) of RDF samples Thermogravimetric analysis was carried out according to ISO 11358-1 method by using Perkin Elmer Pyris STA- 6000 model TGA instrument, which measures the sample weight as a function of time as temperature is increased according to a determined heating rate. The instrument also calculates and displays Differential Thermal Gravimetric Analysis (DTGA) signal (rate of weight loss) from weight loss vs. time signals. Using a constant heating rate of 10 °C/min in dry air atmosphere, the samples were heated from 25 °C to 950 °C. TGA and DTGA curves for RDF-A, RDF-B, coal and petroleum coke samples were obtained. 2.5. Lab-scale combustion experiments of RDF – coal and RDF petroleum coke mixtures A laboratory scale set up was established in order to test the cocombustion characteristics and efficiencies of RDF samples. In combustion experiments, pellets of RDF samples alone and in different mixtures with coal and petroleum coke were combusted in an electrically heated cylindrical quartz batch reactor with dimensions of 50 mm diameter and 1200 mm height. A schematic diagram of the experimental setup is given in Fig. 2. The reactor had a perforated plate made of quartz somewhere in the middle height of the reactor to hold the sample pellets. The pellets were combusted at the batch reactor where only one pellet was burned at a time at fixed temperature of 900 °C. The effect of RDF addition to the main fuels which were coal and petroleum coke was investigated by analyzing the flue gases during combustion experiments. At the beginning of each experiment, the cylindrical quartz reactor was heated first to 900 °C at which the combustion occurs. The two thermocouples, one of which located close to furnace wall and the other located inside the quartz reactor (combustion zone), allowed measuring the temperature of the furnace and the combustion zone of the reactor. Then, air was introduced from the bottom of the reactor when the temperature of the combustion zone reached 900 °C. The flow rate of air, which was 1000 cm3/min, was adjusted by a flow meter. Next, top seal of the reactor was opened in order to drop pelletized sample into the reactor and was closed rapidly. The pelletized sample instantly reached the combustion zone of the reactor. The combustion began and the gas analyzer started sampling the flue gas. 2.6. Flue gas analyzer The combustion gases, CO2, CO, NO, NO2 and SO2, were monitored by Madur Photon flue gas analyzer continuously. The
analyzer consisted of two units which were a conditioning unit and a gas analysis unit. In the conditioning unit called as PDG100, moisture in the flue gas was trapped and solid particles in the flue gas were filtered. Thus, the results of analysis were reported on dry basis. In the second unit, the conditioned flue gas was analyzed. The device measured the concentrations of O2 and CO2 gases with an Infrared (IR) cell as volume percentages; concentrations of other gases were measured electrochemically in ppm. 3. Results and discussions 3.1. Thermal characterization of RDF, coal and petroleum coke samples 3.1.1. Proximate/ultimate analysis of RDF, coal and petroleum coke samples and their calorific value determination The average results of proximate analysis and standard deviations obtained from the triplicate analyses are given in Table 1. In evaluating the results, it should be noted that there is difference in moisture and fixed carbon contents between RDF-A and RDF-B. This is because of the way the samples are processed in the facilities. The RDF-A sample was dried in its facility and then sent to us, whereas RDF-B plant had no drying unit, so sample was sent to us as it is and moisture content was determined by us as sampled. Besides, fixed carbon is the residue remaining after volatile matter disappears during the combustion process. RDF samples in this study were mainly composed of volatile matter (VM) which was 81.8% and 68.5% for RDF-A and RDF-B, respectively. As it is seen, the RDF-A has more volatile contents than RDF-B and thus its fixed carbon content is less. The reason of the high volatile matter content of RDF is that they contain many volatiles such as paper, plastic etc.; also, the reduction of the inert materials from MSW stream during the RDF production process increases the volatile matter content of RDFs. The volatile matter contents of RDF samples were higher than that of the coal and petroleum coke samples, and their fixed carbon contents were lower accordingly. Among all samples, the petroleum coke sample had the lowest volatile matter content and highest fixed carbon content. In addition, the ash content of RDF is important since it determines the calorific value and also the amount of residue that would be left behind upon combustion. In this respect, RDF-A and RDF-B were similar in ash content (between 12% and 15%) and ash contents of both were slightly lower than that of coal (17.2%). On the other hand, petroleum coke had very small ash content (0.7%) relative to the other three fuels. Carbon, hydrogen, and oxygen content of the fuels are very important since these elements constitute the main fuel fraction
Please cite this article in press as: Sever Akdag˘, A., et al. Comparison of fuel value and combustion characteristics of two different RDF samples. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.037
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Fig. 2. Lab scale combustion setup.
Table 1 Proximate analysis of RDF samples, coal and petroleum coke.
* **
Sample
Moisture (%)*
Volatile matter (%)
RDF-A RDF-B Coal Petroleum coke
1.6 ± 0.02 14.8 ± 0.09 4.3 ± 0.03 7.0 ± 0.12
81.8 ± 0.89 68.5 ± 0.52 29.3 ± 0.11 12.1 ± 0.17
**
Fixed carbon (%) 5.2 ± 0.52 16.6 ± 0.74 53.5 ± 0.12 87.4 ± 0.31
**
Ash (%)
**
12.9 ± 0.38 14.9 ± 0.64 17.2 ± 0.22 0.7 ± 0.01
% by weight, as collected. % by weight on dry basis.
of the waste. Nitrogen content is also required to be known because it determines the amount of NOx formation. Sulfur is another important element, amount of which should be known since the presence of sulfur causes the generation of acid gases SO2 and SO3 which contribute to air pollution and corrosion. In this context, ultimate analysis which gave the elemental composition of RDF, coal and petroleum coke samples were conducted and calorific values of the samples were determined all in triplicate samples and the results with the standard deviations are shown in Table 2. Ultimate analysis of the samples revealed that the carbon contents of RDF-B, coal and petroleum coke were close to each other. Carbon content of RDF-A was the lowest and that of the petroleum coke was the highest among the samples studied. The calorific values of RDF samples, which were 22,143.40 kJ/kg and 19,229.79 kJ/ kg on dry basis, for RDF-A and RDF-B, respectively, were close to each other and were a little lower than that of the coal used in this study. The petroleum coke had the highest calorific value among all the fuels due to its lower volatile matter content and higher carbon content. Also, it was seen that the volatile matter content decreases with an increase in the carbon content for all fuels. Sulfur had the lowest percentage among all the elements measured; in RDF-B the sulfur content was 0.45%, whereas sulfur was not detected in RDF-A. Nitrogen and hydrogen percentages in the samples were all lower than 10%. However, the content of hydrogen was higher in RDF samples than in coal and petroleum coke samples as seen in Table 2. The high hydrogen content of RDF samples may be linked to the high volatile matter content originating from the organic matter in the waste.
3.1.2. Determination of ash composition of RDF samples by XRF analysis The elemental composition of ash is important for slagging and fouling concerns. There is a wide range of elements which contribute to slagging and fouling in combustion of refuse. XRF analysis was done to determine the inorganic elements, which formed the ash content of RDF samples. Using the result of this analysis, fouling and slagging indices of RDF samples were calculated. Results of XRF analysis are presented as element oxides on dry basis in Table 3. It can be seen in Table 3 that Si and Ca were the most abundant inorganic elements in ashes of RDF samples tested in this study. The Cl content in RDF ash samples was also high, which may originate from chlorine in PVC (polyvinyl chloride) plastics and waste paper in RDF content (Beckmann and Ncube, 2007; Dong et al., 2002; Tchobanoglous and Kreith, 2002). Moreover, Fe, K, Al and S are other inorganic elements in ashes found in high concentrations. Slagging and fouling indices were calculated for the RDF samples by using the formula given in Table 4. Particularly ‘‘Base to acid ratio” (B/A), and ‘‘sulfur ratio” (Rs) were used as slagging indices; ‘‘total alkalis” (TA), and ‘‘fouling index” (Fu) were used as fouling indices, as they were previously used in Teixeira et al. (2012). B/A ratio shows the presence of alkali and alkali-earth oxides in the sample relative to Al2O3, SiO2. TiO2 content have an important role in melt formation. Higher basic compounds, B, are assumed to decrease the melting temperature, while the acidic ones, A, increase the melting point. The sulfur ratio, Rs, is related to slagging potential of sample and it is calculated using the S content of the sample. The fouling index, Fu, which is based on
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Table 2 Ultimate analysis and calorific values of RDF samples, coal and petroleum coke.
* **
Sample
Carbon (%)*
Hydrogen (%)*
Nitrogen (%)*
Sulfur (%)*
Oxygen (%)*
Calorific Value (kJ/kg)**
RDF-A RDF-B Coal Petroleum coke
44.14 ± 1.09 56.47 ± 0.53 63.80 ± 0.15 68.12 ± 0.21
5.63 ± 0.37 8.96 ± 0.25 3.65 ± 0.02 3.63 ± 0.05
0.97 ± 0.31 1.50 ± 0.08 1.88 ± 0.04 1.94 ± 0.06
– 0.45 ± 0.02 0.55 ± 0.02 4.73 ± 0.09
36.26 ± 1.20 17.73 ± 0.17 12.94 ± 0.06 20.51 ± 0.11
22,143.40 ± 661.1 19,229.79 ± 151 25,641.23 ± 279.1 34,381.18 ± 299.2
% by weight on dry basis. Lower heating value on dry basis (kJ/kg).
Table 3 Inorganic element contents of RDF samples (% by wt.).
Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl K2O CaO TiO2 V2O5 Cr2O3 MnO Fe2O3
RDF-A
RDF-B
0.20 0.13 0.43 1.21 0.14 0.20 0.29 0.27 3.36 0.15 – – 0.01 0.08
0.06 0.12 0.42 1.54 0.11 0.24 0.32 0.44 2.72 0.13 – 0.01 0.01 0.23
Base-to-Acid ratio, gives more relevance to the alkaline elements which are the main agents of fouling. The results and limit values are given in Table 5. As seen from Table 5, the RDF ash samples have moderate to high slagging and fouling tendency when compared to limit values. Base to acid ratio (B/A) of both RDF ash samples were higher than ‘‘high slagging” class. Presence of alkaline elements in the RDF ash samples caused the high base to acid ratios, which were 2.3 and 1.8 for RDF-A ash and RDF-B ash, respectively. It should be noted that the values higher than 1 corresponds to high tendency of slagging. Fouling tendency of RDF ash samples was high when compared to limit value because of the high content of volatile inorganic components like Na2O, K2O as shown in ash composition table. Also, sulfur and phosphorus content of RDF samples are high, which makes nearly all of slagging and fouling indices higher than the limit values. It is worth to note that there are some studies in which slagging and fouling indices of coal is calculated (Teixeria et al., 2012; Park and Jang, 2011). As it can be seen from these studies, the slagging and fouling tendency of coal is lower when compared to RDF.
3.2. Co-combustion of RDF–coal and RDF–petroleum coke mixtures Fig. 3 is given as a sample graph showing the results of a batch combustion experiment run. In this figure, change in O2 and CO2
Table 4 Empirical relations for slagging and fouling tendency of ash composition (Park and Jang, 2011; Pronobis, 2005). Classification
Index
Formula
Slagging
Base-to-acid ratio (B/A)
Fe2 O3 þCaOþMgOþK2 OþNa2 O SiO2 þTiO2 þAl2 O3 B A xSðin dry fuelÞ
Sulfur ratio (Rs) Fouling
Total Alkalis (TA) Fouling ratio (Fu)
Na2 O þ K2 O B A xNa2 O þ K2 O
Fig. 3. Results of co-combustion of coal with 5% RDF-A.
concentrations in the flue gas with respect to time is shown on the left ordinate of the graphs. The flue gas analyzer measured the concentration of these two gases as volume percentages. CO, NO, NO2 and SO2 concentrations which were measured as ppm are shown on the right ordinate of the graph. Measurements of flue gas composition were performed with two seconds interval; therefore, the profiles of gases were plotted with respect to time. From the graph given in Fig. 3 and all other co-combustion experiments carried out in this study, it can be seen that a sharp decrease in oxygen concentration and increase in carbon monoxide and carbon dioxide concentration were observed in the flue gas as soon as the pellets were dropped into the reactor. This quick carbon monoxide formation is explained by high volatiles content in RDF samples. Because the volatile matter burns quickly as soon as the pellet is dropped to the reactor, this creates an unstable medium and results in high carbon monoxide concentration (Kobyashi et al., 2005). Another reason for high carbon monoxide concentration may be the temperature difference between the reactor and the fuel (pellet) at the beginning of the experiment. The pellet is at room temperature when it is dropped into the reactor. Incomplete combustion occurs until the pellet is heated up to the reactor temperature, which results in high carbon monoxide emission. Also, the fracture strength of the pellet has an indirect effect on CO emission. The more brittle pellets are fractured into small particles when they are dropped into the reactor and therefore they burn faster. This fast burning increases the emission of volatiles and thus causes an unstable medium (Kobyashi et al., 2005). On the average, oxygen level dropped to around 10% immediately and carbon dioxide rose to around 5–10%. Concentration of carbon monoxide depended on the RDF percentage in mixtures; it changed between 1000 ppm and 5000 ppm when RDF percentage was 3–5%; on the other hand, it increased up to 6000 ppm when RDF percentage was higher. The carbon monoxide concentration increased to even higher values and it could not be measured with the gas analyzer for the mixtures with 20–30% RDF content or when sole RDF was burned. From the graphs, it can also be seen that the carbon monoxide formation was lower in RDF-petroleum coke mixtures than the ones in RDF-coal mixtures.
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6 Table 5 Slagging and fouling indices calculated for RDF samples.
Slagging-fouling indices of RDF samples
Limit values (Park and Jang, 2011; Pronobis, 2005)
RDF-A
RDF-B
Low
Medium
High
Slagging index
B/A RS
2.3 0.5
1.8 0.3
<0.5 <0.6
0.5 < B/A < 1 0.6 < Rs < 2
>1 >2
Fouling index
TA Fu
0.5 1.1
0.5 0.9
<0.3 60.6
0.3 < TA < 0.4 –
>0.4 0.6 < Fu 6 40
Petroleum coke, with higher calorific value and carbon content, and lower volatile matter content than coal yielded better combustion characteristics in RDF mixtures. The efficiency of the combustion processes was calculated based on a carbon mass balance. The percentage of carbon that was captured as carbon monoxide and carbon dioxide was found by calculating the areas under the emission curves for all experiments. The carbon measured by the ultimate analysis was used for the calculation of the amount of total carbon in the feed. These calculations allowed us to check the degree of control that we had on the system as well as the accuracy of our analysis during the experiments. Calculations showed that carbon balance held within a maximum of ±20% deviation, where in majority of the combustions the deviation was much less than 10%. Then, the efficiencies of the combustion were determined by calculating the amount of carbon which transformed into carbon dioxide. The results of carbon combustion efficiency for RDF-A and RDF-B are shown in Figs. 4 and 5, respectively. Both Figs. 4 and 5 show that the combustion efficiency decreased with increasing RDF fraction. The replacement of coal and petroleum coke with RDF caused more carbon monoxide formation which was the indication of incomplete combustion. ANOVA tests were performed to interpret the significance of the results statistically. F-tests and multiple range tests were performed in the ANOVA test to compare the mean values of the combustion efficiencies of RDF samples for the 7 different levels (0%, 3%, 5%, 10%, 20%, 30%, and 100%) of RDF fractions. It was found from F-test that there was a statistically significant difference between the means of efficiency of RDF co-combustion with both petroleum coke and coal when different levels of RDF fractions were incorporated into the fuel at 95.0% confidence level. Multiple range test results indicated that there are no statistically significant differences between only coal or only petroleum coke combustion and co-combustion of coal or petroleum coke with the addition of 3%, 5% and 10% RDF. At these levels the primary fuel (coal or petroleum coke) seems to dominate the
Fig. 5. Combustion Efficiency of RDF-B with coal and petroleum coke.
combustion yielding similar efficiencies. However, the test results showed that the significant decreases were observed as the RDF fraction increased from 10% to 20% in both coal and petroleum coke mixtures. Furthermore, the combustion efficiency also decreased when the RDF ratio in petroleum coke mixtures was increased from 20% to 30%. Also, the minimum efficiencies were observed when only RDF samples were combusted without any coal and petroleum coke addition. Furthermore, for the cocombustion experiments with coal, it was seen that there was no efficiency differences between the 20% and 30% mixing ratio. Also, the general combustion efficiencies of petroleum coke–RDF mixtures were higher than those of coal–RDF mixtures because of better fuel features of petroleum coke compared to coal. It is concluded that the combustion efficiencies decreased significantly if the RDF sample fraction in the mixtures were more than 10%. At this point it is worth to note that this percentage may change from one condition to the other since the combustion efficiency highly depends on the composition of RDF, which is variable even within a single facility, combustion conditions such as temperature and flowrate, reactor type and condition of samples (pellets or loose material). However, the general trends are expected to be independent of the working conditions. Therefore the results of this work should be looked upon by considering these facts. RDF addition to the fuel blends decreased the SO2 emission due to the lower sulfur content in the RDF samples than in the coal and petroleum coke samples, but it did not change NO emission appreciably. SO2 emission was higher in petroleum coke mixtures than coal mixtures since the sulfur content of petroleum coke was higher than coal. 3.3. Thermogravimetric Analysis (TGA) of RDF, coal and petroleum coke samples
Fig. 4. Combustion Efficiency of RDF-A with coal and petroleum coke.
TGA and DTG profiles evaluated in the combustion tests of RDFA and RDF-B and coal and petroleum coke are represented in Figs. 6 and 7, respectively.
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Fig. 6. TGA and DTG profiles of RDF-A and RDF-B (left RDF-A, right RDF-B).
Fig. 7. TGA and DTG profiles of coal and petroleum coke (left coal, right petroleum coke).
DTG curves of both RDF samples had shoulders/peaks which were caused by the big mass losses observed in three different regions. The first mass loss observed at the beginning the of the experiment, from ambient temperature to about 120 °C, is due to the loss of moisture and very light volatile matter content of the fuel. In this region there is a difference between RDF-A and RDF-B that matches nicely with the data in Table 1. The RDF-A has less moisture, which yields a smaller peak at around 100 °C compared to RDF-B. During the combustion process, the main decomposition of RDF samples took place in between temperatures of 200 °C and 600 °C. The rate of weight loss was maximum at around 320 °C for RDF-A and at around 330 °C for RDF-B in the dry air environment. Unlike the moisture loss region, more than one peak was observed in this region. In the RDF-B curves, three exothermic peaks were observed at 330 °C, 400 °C, and 510 °C; while, in RDF-A curves, there were two peaks at 320 °C and 500 °C. At these peak points the mass loss was significant. As mentioned in Piao et al. (2000), the presence of the shoulders shows that there are different volatile matter fractions in RDF samples; for example the existence of cellulosic materials and the plastics may produce different peaks during decomposition and combustion. When the decomposition temperatures and decomposition speeds of different types of volatile matters included in RDF samples are close to each other, less peaks are formed, which might be the case observed in RDF-A. For both RDF samples, a last peak occurred at about 650 °C. This last peak was due to the char combustion. Also the reactions between char and volatiles, which were coming from previous phases of the process, might be another cause of this last peak.
The combustion process in the experiments was complete at around 700 °C and the mass loss of RDF-A and RDF-B were found as 79.1%, 80.9%, respectively, which meant that about 20% of char remained at the end of the process. Fig. 7 presents the TG and DTG curves observed for coal and petroleum coke samples. Unlike the RDF combustion, the main decomposition of coal samples took place at higher temperatures (between 700 °C and 800 °C). The highest mass loss, which corresponded to the peak at 720°C, was mainly due to the volatile matter and char decomposition. At the end of the process the degraded mass was 53.3% of the original mass. The typical DTG curve characteristics for petroleum coke mentioned in Magdziarz and Werle (2014) were observed in the current study; there was only one peak between the temperatures of 500 °C and 700 °C due to the decomposition of all organic matter, loss of volatiles and char. The total weight loss in the combustion process was 35.8% and the maximum weight loss occurs at around 610 °C. The initial decomposition temperature of petroleum coke was the highest when compared to coal and RDF samples. Therefore, having the highest ignition and burnout temperatures, petroleum coke was the hardest fuel to ignite and burnout totally among the samples burnt in the current study. It was concluded from the thermogravimetric analysis that, the constituents of the fuel decomposes in solid-phase at the char combustion in coal and petroleum coke combustion; however, in RDF combustion, the decomposition occurs at early stages of the combustion in gas phase due to the low fixed-carbon and high volatile matter content of RDF. Also, it can be stated that the combustion mechanisms of volatile matters in RDF samples are complicated than that of char. Therefore, char combustion characteristics
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is not a criteria for RDF combustibility. However, volatile matter combustion is the most crucial parameter and should be investigated since the combustion duration, speed and mass loss of volatile matters affect the RDF combustibility. 4. Conclusion Results of this study have shown that calorific values of RDF samples on dry basis are close to that of coal and a little lower compared to petroleum coke used in this study. Although the calorific values of RDF samples are sufficient, the use of sole RDF samples in combustion processes is limited due to its combustion characteristics. While the RDF sample fraction in the fuel mixtures is increased (such more than 10%), the combustion characteristics of the fuel mixtures changes from char combustion to volatile combustion. In addition, CO emission increases and so the combustion efficiency decreases significantly when the RDF fraction in the mixture is higher than 10%. However, the co-combustion of RDF with lower ratios (such as 3%, 5%, and 10%) does not decrease the combustion efficiency significantly. Also, the minimum efficiency is observed at the combustion of pure RDF without any coal or petroleum coke fraction. RDF addition to the fuel blends decreases the SO2 emission due to the lower sulfur content in the RDF samples than in the coal and petroleum coke samples, but it does not change NO emission appreciably. Slagging and fouling intensity of RDF samples determined in this study indicates that the use of RDF samples in cocombustion processes needs to be kept in certain amounts. TGA results showed that the combustion mechanisms of volatile matters in RDF samples are complicated than that of char. The mass loss of RDF-A and RDF-B are found as 79.1%, 80.9%, respectively, which means about 20% of char remained at the end of the process. In coal and petroleum coke combustion, the constituents of the fuel decomposes in solid-phase at the char combustion; however, in RDF combustion, the decomposition occurs at early stages of the combustion in gas phase due to the low fixed-carbon and high volatile matter content of RDF. References Ahn, S.Y., Eom, S.Y., Rhie, Y.H., Sung, Y.M., Moon, C.E., Choi, G.M., Kim, D.J., 2013. Application of refuse fuels in a direct carbon fuel cell system. Energy 51, 447– 456. http://dx.doi.org/10.1016/j.energy.2012.12.025. ASTM Standard E711-88, 2004. Standard Test Method for Gross Calorific Value of Refuse-Derived Fuel by the Bomb Calorimeter. ASTM International, West Conshohocken, PA. ASTM Standard E790-08, 2004. Standard Test Method for Residual Moisture in a Refuse-Derived Fuel Analysis Sample. ASTM International, West Conshohocken, PA. ASTM Standard E830-87, 2004. Standard Test Method for Ash in the Analysis Sample of Refuse-Derived Fuel. ASTM International, West Conshohocken, PA. ASTM Standard E897-88, 2004. Standard Test Method for Volatile Matter in the Analysis Sample of Refuse-Derived Fuel. ASTM International, West Conshohocken, PA. Beckmann, M., Ncube, S., 2007. Characterisation of Refuse Derived Fuels (RDF) in reference to the Fuel Technical Properties. In: 26th Annual International Conference on Incineration and Thermal Treatment Technologies, IT3; Phoenix, AZ; United States. Brunner, P.H., Rechberger, H., 2015. Waste to energy – key element for sustainable waste management. Waste Manage. 37, 3–12.
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