Environ. Sci. Technol. 2003, 37, 1025-1030
Optimizing Municipal Solid Waste Combustion through Organic and Elemental Carbon as Indicators STEFAN RUBLI,* HASAN BELEVI, AND PETER BACCINI Swiss Federal Institute for Environmental Science and Technology, 8600 Dubendorf, Switzerland
The temperature, redox conditions, and residence times of the solid waste on the grate and of the raw gas in the secondary combustion zone determine the mineralization processes of organics in municipal solid waste incinerators. An improved knowledge of the influence of these factors on the incineration process might help to optimize incinerators with regard to mineralization efficiency of organics. This paper presents a method for investigating the influence of process parameters on mineralization of organics to CO2 by using the elemental carbon (EC) and organic carbon (OC) concentrations in the solid residues as indicators. The results obtained by experiments in full-scale incinerators show that the EC concentration in these residues is a good indicator of oxygen supply, whereas the OC/EC ratio is a good indicator of temperature in and above the furnace bed. Very effective burnout of the bottom ash up to 0.95 g of TOC (EC + OC) per kilogram of dry matter (DM) and of the electrostatic precipitator (ESP) ash up to 0.24 g of TOC per kilogram of DM was achieved in a full-scale incinerator.
Introduction High mineralization of organic components of municipal solid waste is desired in municipal solid waste incinerators (MSWIs). The part of carbon transferred to the solid residues is determined as total carbon (TC). TC in solid residues includes carbonate carbon (CC) and total organic carbon (TOC). TOC is defined as the difference between TC and CC (1-3). Hence, TOC is the sum of organic carbon (OC) and elemental carbon (EC). The TOC can be characterized by different analytical methods (4, 5). For characterization, it is important to differentiate between EC and OC. EC is a product of incomplete combustion with a high molecular weight and a structure similar to graphite, whereas OC is composed of unburned organic matter as well as organic products formed during the combustion process (6, 7). As presented in a previous field study, the differentiation between OC and EC in sewage sludge incineration provides information on process conditions in the furnace (8). Observations suggested that EC is a good indicator of oxygen supply to the furnace bed and that the OC/EC ratio is an indicator of the temperature in the furnace bed. A pyrolysis-oxidation apparatus for quantitative determination of these two carbon species in incineration residues was also presented in this study (8). The temperature and redox conditions in and above the furnace bed, as well as the residence times of the solid waste * Corresponding author phone: +41-1-823-5138; fax: +41-1-8235226; e-mail:
[email protected]. 10.1021/es020579w CCC: $25.00 Published on Web 01/25/2003
2003 American Chemical Society
on the grate and of the raw gas in the secondary combustion zone, are the most important factors determining mineralization processes of organics in an incinerator. Improved knowledge of the influence of these factors on the incineration process might help to optimize incinerators with regard to the mineralization efficiency of organics. High mineralization is targeted in incinerators because organics can adversely influence the long-term behavior of incinerator residues in landfills (9, 10). This paper presents a method for investigating the influence of process parameters on the quality of incinerator bottom ash and electrostatic precipitator (ESP) dust using elemental and organic carbon concentrations in the solid residues as indicators.
Experimental Section The experiments were accomplished in a full-scale facility for several purposes. The advantage of full-scale investigations is the direct connection of the results with realistic process conditions. Thus, no upscale is needed. Furthermore, the investigated incinerator offers a large database with a wide range of process parameters that are stored over a long period of time. This information is important in elucidating the correlation of the different process conditions and the organic indicators in the solid residues. Moreover, the investigated incinerator operates a furnace with three separately controlled grate sections and five zones of undergrate air injection. This system enables experiments with diverse operating parameters such as different residence times, temperatures, and oxygen distributions. The disadvantages of the experiments in full-scale incinerators are the varying solid waste properties and the difficult control of the process parameters during the experiments. To reduce these disadvantages to a minimum, the staff was instructed to mix the solid waste thoroughly and control the incineration process according to the suggested experimental process parameters. Incinerators. Samples of bottom ashes and electrostatic precipitator dust were taken in two different MSWIs. The experimental work was performed at a facility with a burning capacity of 10 t/h of unprocessed waste (incinerator 1). Incinerator 2 (8 t/h of unprocessed waste) was chosen to validate the results from incinerator 1. Incinerator 1 is equipped with three separately regulated water-cooled grates. This incinerator is coupled with sewage sludge combustion in a rotary kiln. The off-gas of sewage sludge incineration enters into the first zone of the sidewalls of the MSWI. Consequently, there are three kinds of air supply: undergrate air, air from the sidewalls, and secondary air (Figure 1). Undergrate air is supplied through five zones, and additional air from the sidewalls enters through the first four zones. The air entering from the first zone of the sidewalls (drying zone) consists of the raw gas from the coupled sewage sludge incinerator and is oxidized in the secondary zone of the MSWI. Undergrate airflow is coupled with the air supply from the sidewalls. Lower air supply from the sidewall, therefore, causes a higher undergrate air supply, even though steam production does not change much. Secondary air is injected at the transition to the after-burning chamber. Temperature is measured at different points above the furnace bed and in the after-burning chamber (Figure 1). The steam production of incinerator 1 is 31 t/h at a temperature of 395 °C; the steam pressure is 38 bar. Incinerator 2 is equipped with an air-cooled backward reciprocating grate with an inclination of 26°. The air supply is composed of undergrate and secondary air. The steam production of incinerator 2 is 31 t/h at 400 °C; the steam pressure is 40 bar. VOL. 37, NO. 5, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic diagram of the investigated furnace. Air is supplied through the grate (zones 1-5), the sidewalls (ASW zones 1-4), and the secondary zone. Temperature is measured above the grate (zones 1-5) and in the secondary combustion zone. Experiment 1. The aim of the first experiment was to show the correlation between the temperature in and above the furnace bed and the OC/EC ratio in the slag and ESP ash. For that reason, municipal solid waste was mixed with different percentages of organic scrap from car shredding plants (RESH). This residue has an average calorific value of 14-20 MJ/kg (11, 12), which is higher than that of municipal solid waste. Further characteristics are the smaller particle size compared to the municipal solid waste, the high dry matter content of 95%, and the high ignition temperature. The latter characteristic might inhibit ignition on the grate and might shift the flame position toward the burnout zone of the grate. An uncontrolled combustion process that results in an instability of the flame position might occur at higher RESH concentrations (>10%) and might influence the OC and EC concentrations in the bottom and ESP ashes. The experiment was divided into three time periods of 4 h each. The percentage of organic residues from car shredding plants was increased between each period from 2-3 wt % (first period) to 5-6 wt % (second period) to 10-11 wt % (third period). The crane operator prepared the different mixings by his own experience. Consequently, the different proportions result in a correspondingly large error ((20%) in concentrations. The incineration process during the experiment was controlled by steam production. It was suggested that steam production should range between 29 and 31 t/h. Experiment 2. The aim of this experiment was to elucidate the impact of different oxygen supply distributions across the grate on the organic indicators in the analyzed solid residues. Operating with five separately controlled air-supply zones, the position of the flame can be influenced by changing the air distribution across the grate. The experiment was 1026
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again divided into three time periods of 4 h each. Municipal solid waste without RESH served as fuel. The oxygen supply was controlled automatically by the steam production during the first period. This causes a high fluctuation of the undergrate air flow depending on the waste input rate, waste inhomogenity, and air supply from the sidewalls. In the second and third periods, undergrate air and air from the sidewalls were constantly injected in each zone. Low undergrate air and high sidewall air in the first two zones was injected during this period. Thus, the intention was to achieve a broader pyrolytic zone and a shortening of the oxidation zone on the grate. Consequently, the position of the fire should shift toward the end of the grate. In the third period, the adjusted air distribution should result in a short pyrolytic zone and a broader oxidative zone. A further dissimilarity between these two periods was the different distribution between the total undergrate air and the total air supply from the sidewalls. In the second period, the total undergrate air supply (15 000 N m3/h) was higher than the total air supply from the sidewalls (14 300 N m3/h). This was changed in the third period, where the undergrate air supply (13 800 N m3/ h) was lower than the air supply from the sidewalls (15 500 N m3/h). Secondary airflow was 5000 N m3/h during the second period and 6000 N m3/h during the third period. The higher secondary airflow in the third period was chosen to compensate for the lower undergrate air supply. The mean steam production should range between 24 and 26 t/h during the experiment. Sampling and Sampling Preparation. Sampling of ESP dust and bottom ash was divided into three periods of 4 h for each experiment. Samples were collected every hour to give four samples for each period. Bottom ash was collected
TABLE 1. Organic and Elemen tal Carbon Concentrations, OC/EC Ratios in the Bottom Ash of the Investigated MSWI, and Standard Deviations from the Average Values for the Periods of the First and Second Experiments first experiment OC (g/kg of DMa)
stdb (%)
EC (g/kg of DMa)
08:30-09:30 h 09:30-10:30 h 10:30-11:30 h 11:30-12:30 h
first period 0.051 ( 0.016 1.77 ( 0.27 0.036 ( 0.005 2.66 ( 0.23 0.084 ( 0.010 2.75 ( 0.31 0.067 ( 0.023 35 3.88 ( 0.07
14:00-15:00 h 15:00-16:00 h 16:00-17:00 h 17:00-18:00 h
second period 0.036 ( 0.003 0.67 ( 0.03 0.028 ( 0.006 1.29 ( 0.11 0.051 ( 0.003 2.51 ( 0.25 0.039 ( 0.002 25 1.06 ( 0.12
08:30-09:30 h 09:30-10:30 h 10:30-11:30 h 11:30-12:30 h
third period 0.182 ( 0.004 6.33 ( 0.06 0.082 ( 0.011 6.12 ( 0.13 0.075 ( 0.012 1.88 ( 0.16 0.020 ( 0.001 75 1.59 ( 0.02
a
DM ) dry matter.
b
second experiment stdb OC/EC stdb (%) ratio (%)
31 58 65
0.029 0.014 0.031 0.017 0.054 0.022 0.020 0.037 0.029 0.013 0.040 0.013
37
48
56
OC (g/kg of DMa)
stdb (%)
EC (g/kg of DMa)
06:00-07:00 h 07:00-08:00 h 08:00-09:00 h 09:00-10:00 h
first period 0.144 ( 0.039 1.36 ( 0.03 0.117 ( 0.003 1.52 ( 0.03 0.260 ( 0.014 5.56 ( 0.27 0.285 ( 0.004 41 2.61 ( 0.05
11:30-12:30 h 12:30-13:30 h 13:30-14:30 h 14:30-15:30 h
second period 0.139 ( 0.007 1.82 ( 0.15 0.132 ( 0.016 0.81 ( 0.02 0.118 ( 0.001 1.46 ( 0.03 0.115 ( 0.009 9.0 1.59 ( 0.17
15:30-16:30 h 16:30-17:30 h 17:30-18:30 h 18:30-19:30 h
third period 0.130 ( 0.013 3.16 ( 0.19 0.124 ( 0.020 1.86 ( 0.09 0.120 ( 0.004 2.87 ( 0.08 0.125 ( 0.004 3.0 1.73 ( 0.08
stdb OC/EC stdb (%) ratio (%)
70
0.107 0.077 0.047 0.109
35
31
0.076 0.163 0.081 0.072
44
30
0.041 0.067 0.042 0.072
29
std ) standard deviation of the values within the period.
in the scoop of the crane for 1 h and was mixed with shovels by two persons for 15 min. Afterward, 10 kg was transferred to plastic bags. ESP ashes were also collected in periodic intervals of 10 min each from the chain conveyor to give hourly samples. All samples were dried at 105 °C for at least 12 h in the laboratory to determine the dry matter (DM) content. ESP dust was ground in hammer mills and sieved through a screen of 0.5 mm. The whole bottom ashes were ground in steel ball mills for 4 h and were also sieved through a 0.5-mm screen to separate bulky, nonferrous metals. Iron was separated by a permanent magnet. The weight of the metals was determined separately. No organic material was rejected. All samples were filled into plastic receptacles until analysis. In view of the heterogeneous bottom ash, the standard deviation of the OC and EC concentrations of the sampling was determined. The relative standard deviation (RSD) of the sampling for OC is 18% (0.133 ( 0.024 g/kg), and the RSD for EC is 11% (1.345 ( 0.150 g/kg) (13). The higher RSD for OC can be ascribed to the fact that the OC concentrations are 1-2 orders of magnitudes lower than the EC concentrations. The RSD of TOC in ESP ash sampling is 7% (14). Analytical Methods. Total carbon (TC) was determined by CNS analysis (CNS analyzer 1500, Carlo Erba Instruments, Italy). Carbonate carbon (CC) was analyzed by coulometric analysis (model 5011, Coulometrics Inc.). TOC was calculated by the difference between TC and CC. OC and EC were determined by a thermal method. The thermal differentiation of the organic species is based on the different oxidation temperatures and thermal stabilities of these species (15, 16). The apparatus consists of two furnaces in series that are connected by a quartz tube. The oxidation or pyrolysis takes place in the first furnace. The gaseous products are completely oxidized in the second furnace at 800 °C through the supply of additional oxygen. The oxidized gases are led to a nondispersive infrared detector. Details about the apparatus are described elsewhere (8, 17). The sample matrix is important for the oxidation temperatures of the organic species. Thus, OC (at 380 °C) and EC (at 580 °C) in bottom ashes were determined directly under oxidative conditions (13, 18). The quantitative analysis of OC and EC in ESP dust was obtained by pyrolyzing the samples under 800 °C for the determination of OC + CC, followed by the oxidation at 800 °C for the determination of EC. OC concentrations are calculated by the difference between OC + CC and CC, which was determined by coulometric analysis.
Results and Discussion OC/EC Ratio as a Temperature Indicator. Table 1 shows the OC and EC concentrations, as well as the OC/EC ratios, in the bottom ash during the three periods of the first and second experiments. The OC concentrations range between 0.020 and 0.180 g/kg of DM. The averages were 0.060 g/kg of DM in the first period, 0.039 g/kg of DM in the second period, and 0.090 g/kg of DM in the last period. The EC concentrations range between 0.67 and 6.33 g/kg of DM. The averages of the three time periods are 2.76, 1.38, and 3.98 g/kg of DM, respectively. Comparing these values with the limiting value for TOC concentrations of 50 g/kg of DM for direct disposal in landfills in Switzerland (19), the measured TOC concentrations are an order of magnitude lower. Thus, good mineralization of the organic carbon to CO2 was achieved during the experiment. The OC/EC ratio, which is postulated as an indicator of the temperature in the furnace bed, ranges between 0.013 and 0.054. Establishing a correlation between the average temperature of the second and third zone and the OC/EC ratios by comparing the data of the three time periods of this experiment is difficult because of the low OC/EC ratios in the bottom ashes. At these low OC levels, additional factors might have additional impacts on the ratios and might disguise the effect of temperature. An important factor could be the instability of the combustion process. An indicator of this instability is the increasing RSDs of the EC concentrations and the OC/EC ratios with increasing RESH content in the second and third periods (Table 1). In Figure 2, the temperature in the secondary combustion zone is compared with the OC and EC concentrations of the ESP dust. The columns are shifted by 15 min to the left because of the approximate time delay between temperature measurement and ESP collection. The lowest OC concentrations (no columns) are below the detection limit of 0.020 g/kg. The variation of the OC concentrations is more distinctive than that of the EC concentrations. The highest OC concentration was measured during the second period (0.56 g/kg). A reverse correlation between temperature and OC/EC ratio is observed in each period. Lower temperatures cause higher OC/EC ratios. To emphasize this correlation, gray circles indicate phases with lower temperatures. In these samples, the OC/EC ratios are higher than those in the remaining samples within the same sampling periods. The sampling intervals of 1 h are rather short to give very accurate VOL. 37, NO. 5, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Organic and elemental carbon concentrations in the ESP ashes compared to the temperatures in the secondary combustion zone during the first experiment.
FIGURE 3. OC/EC ratios in the bottom ashes versus the average temperature of the second and third zones above the grate from the first and second experiments. The data points are average hourly values. An approximate time delay of 90 min between sampling of the bottom ash and measurement of the temperature was taken into consideration. Three data points are not considered: the first and second samples of the third period of the first experiment because of the high EC concentrations caused by the instability of the flame position (high RESH content) and the third sample of the second experiment because of the high EC concentration caused by a very fast reduction of primary air supply. correspondence, but a phenomenological background is observed. This correlation is also observed in Figure 3, where the OC/EC ratios in the bottom ashes of the first and second experiments are compared to the mean temperature of the second and third zones of the furnace (burning zone). The correlation coefficient is R2 ) 0.75. If one considers only the first experiment, the mentioned instability of the flame position and the high heterogeneity of the grate would result in a lower correlation coefficient. However, the high temperature difference between the experiments shows the correlation between the temperature and the OC/EC ratios. The correlation supports the hypothesis that the OC/EC ratio in the bottom ash is an indicator of the temperature in furnace bed. Furthermore, the figure also illustrates that the OC/EC ratio increases by an order of magnitude (from 0.01 to 0.10) for a temperature reduction of 100 °C. EC Concentration as an Indicator of Oxygen Supply. Air distribution was changed in the second experiment to elucidate the correlation between the oxygen supply and the EC concentrations in the bottom ash and ESP dust. In the first period between 6:00 and 10:00 h (Figure 4), the combustion process was controlled by the steam production. Hence, the fluctuation of the undergrate air supply was very 1028
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strong during this period. Nevertheless, the average EC concentration in the bottom ash was low at 2.76 g/kg of DM, but the hourly average concentrations of EC varied between 1.36 and 5.56 g/kg of DM. A sharp decline in the steam production took place at about 7.50 h. This was achieved by reducing the total primary air supply (undergrate air plus air supply from the side walls). The waste flow reduction was much slower than the air supply reduction. This caused a low oxygen supply to the furnace bed because of the high amount of waste on the grate. Consequently, a higher EC concentration of 5.56 g/kg resulted in the third sample (8:30-9:30 h), despite the relatively high ratio of undergrate air to steam production. In the second period (11:30-15:30 h), the average OC and EC concentrations were very low, with values of 0.13 and 1.42 g/kg of DM, respectively, although the average undergrate air supply was lower than in the first period. In the third period, the average EC concentration was 2.41 g/kg of DM (OC ) 0.13 g/kg of DM), which is higher than the average concentration in the second period, where the undergrate air supply was higher (14 800 N m3/h). Steam production fluctuated heavily during the third period because of the varying mass throughput of the furnace. A smaller part of the fluctuation might also have been caused by a changing energy content of the waste. The gray circles in
FIGURE 4. Organic and elemental carbon concentrations in the bottom ashes compared to the steam production and undergrate air supply during the second experiment. Black columns: EC concentrations. Gray columns: OC concentrations.
FIGURE 5. EC concentrations in the bottom ashes versus the average ratios of undergrate air supply to steam production from the second and third experiments. The data points are average hourly values. Figure 4 indicate phases with higher steam production, which means higher charging of the furnace. Because of the constant undergrate air injection and the higher charging, the oxygen supply of the waste bed was lower during the sampling periods of 15:30-16:30 and 17:30-18:30 h compared to the other phases. Consequently, the EC concentrations in the bottom ashes are higher during phases with lower oxygen supplies. Figure 5 shows the correlation between the EC concentration and the ratio of undergrate air to steam production of the second experiment. The third sample of the first period is not considered because of the special conditions on the waste bed, which are related to the previously mentioned reduction of the steam production at 7:50 h (Figure 4). The correlation coefficient of R2 ) 0.67 shows the relationship of the EC concentration to the ratio of undergrate air to steam production, which is a measure of oxygen supply of the waste bed. Consequently, the EC concentration as an indicator reacts to short-term changes in the combustion process of about 1 h but also to changes of 4 h and longer. These longer periods are especially important because they represent the process conditions that are of interest for process optimization in actual facilities. The EC concentration in the ESP ashes exhibited behavior similar to that of the EC concentration in the bottom ash
(Table 2). The average EC concentration of the first period was 0.814 ( 0.109 g/kg of DM. The corresponding air supply, consisting of sidewall and secondary air, was 22 030 ( 1160 N m3/h, but the fluctuation was very intense. The average EC concentration of the second period was 0.530 ( 0.096 g/kg of DM, although the mean air supply was lower at 20 060 ( 590 N m3/h. The lower concentration might be a consequence of the constantly injected air. This is supported by the very low average concentration of 0.265 ( 0.029 g/kg of DM during the last period, where the constant air supply was 21 000 ( 80 N m3/h. Accordingly, the EC concentration in the ESP dust is also a good indicator of the oxygen conditions above the waste bed and in the secondary combustion zone. Validation. To validate the results, incinerator 2 was chosen. It has a completely different furnace design with an air-cooled backward reciprocating grate that has an inclination of 26°. The OC and EC concentrations and ratio, as well as parameters such as the oxygen content in the raw gas, steam production, and furnace temperature above the waste bed, are listed in Table 3. The OC concentrations and the OC/EC ratios (except for sample 4) are 1-2 orders of magnitude higher than the values in incinerator 1, where the experiments were carried out. The EC concentrations are of the same order of magnitude (except for VOL. 37, NO. 5, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. EC Concentrations in ESP Ash of the Second Experiment and the Corresponding Secondary Air Supply and Air Supply from the Sidewalls EC (g/kg)
secondary air (N m3)
air from sidewalls (N m3)
07:15-08:15 h 08:15-09:15 h 09:15-10:15 h 10:15-11:15 h average
first period 0.947 ( 0.044 6590 ( 757 0.735 ( 0.004 5980 ( 215 0.715 ( 0.029 6350 ( 290 0.857 ( 0.025 5790 ( 950 0.814 ( 0.109 6180 ( 510
16 750 ( 2190 14 530 ( 590 15 890 ( 520 16 220 ( 2400 15 850 ( 1420
12:45-13:45 h 13:45-14:45 h 14:45-15:45 h 15:45-16:45 h average
second period 0.541 ( 0.024 5230 ( 160 0.489 ( 0.006 5300 ( 10 0.432 ( 0.003 5630 ( 330 0.657 ( 0.003 6300 ( 140 0.530 ( 0.096 5630 ( 210
14 220 ( 250 14 500 ( 20 14 450 ( 490 14 580 ( 450 14 440 ( 240
16:45-17:45 h 17:45-18:45 h 18:45-19:45 h 19:45-20:45 h average
third period 0.270 ( 0.001 6300 ( 10 0.294 ( 0.013 6300 ( 15 0.225 ( 0.006 6330 ( 50 0.269 ( 0.003 6400 ( 15 0.265 ( 0.029 6330 ( 25
14 750 ( 60 14 730 ( 50 14 800 ( 10 14 800 ( 15 14 770 ( 35
TABLE 3. Validation: Organic and Elemental Carbon Concentrations, OC/EC Ratios, and Process Parameters of the Facility Equipped with a Backward Reciprocating Grate bottom O2 content steam furnace ash OC EC OC/EC in the raw gas production temp sample (g/kg) (g/kg) ratio (vol %) (t/h) (°C) 1 2 3 4
5.62 2.10 4.34 1.36 2.33 1.79 1.81 30.0
2.68 3.19 1.30 0.06
8.5 8.7 9.1 4.0
34.5 33.0 32.5 37.0
910 880 900 960
sample 4). The results show the impact of the different grate system and can be explained by the following characteristics. The furnace bed is higher on this grate type compared to the grates with lower inclinations. As a consequence, the heating of the furnace bed by thermal radiation is reduced. In combination with the higher undergrate air supply, the average waste bed temperature is expected to be lower on this grate type. For that reason, the OC concentrations and OC/EC ratios are higher in this facility. The advantage of this grate type is the thorough mixing of the waste, which allows for a very effective supply of oxygen to the waste bed. Thus, the EC concentrations are as low as in the other facility. The EC concentration in sample 4 is very high because of an overcharging of the grate by waste (115% of allowed steam production). The undergrate air supply was too low, which resulted in reductive conditions in and
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above the waste bed and in a high EC content. At the same time, the temperature increased in the waste bed because of the high waste input. Therefore, the OC/EC ratio in the last sample was very low. The results from this incinerator support the conclusions drawn from the experiments in incinerator 1. In summary, the use of organic indicators permits improved elucidation of the process conditions in the furnace. The experiments showed that optimization of the incineration process regarding mineralization of organics could be achieved. A very effective burnout of the bottom ash up to 0.95 g of TOC (EC + OC) per kilogram of dry matter (DM) and of the ESP ash up to 0.24 g of TOC per kilogram of DM was achieved in a full-scale incinerator.
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Received for review February 5, 2002. Revised manuscript received December 20, 2002. Accepted December 23, 2002. ES020579W
