The effect of bio-drying pretreatment on heating values of municipal. solid waste. Huang Dandan 2,e, Liu Chang 2,f

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1 Advanced Materials Research Submitted: ISSN: , Vols , pp Accepted: doi: / Online: Trans Tech Publications, Switzerland The effect of bio-drying pretreatment on heating values of municipal solid waste Zhao Shuqing 1, 2,a, Huang Wenxiong 2,b, Yin Ran 2,c, Yuan Song 2,d, Huang Dandan 2,e, Liu Chang 2,f 1 Civil and Environmental Engineering School, Beijing University of Science and Technology, Beijing , China; 2 The Research Institute of Urban Construction, Beijing , China a zhaoshuqing@cucd.cn, b my_address@163.com, c yinran@cucd.cn, d yuansong@cucd.cn, e huangdandan@cucd.cn, f liuchang@cucd.cn Keywords: Bio-drying; water content; heating value; municipal solid waste (MSW) Abstract: Bio-drying process, degrading part of the easily biodegradable organic fraction contained in waste, produces heat that combined with an adequate airflow-rate, allows the fast evaporation of waste moisture content. Three ventilation methods were used to investigate the effect of bio-drying pretreatment on municipal solid waste (MSW) characteristics (including water content, mass, heating value and the gross energy capacity). The results show that the water content of MSW decreased from 61.5% to minimum 23.7% after 18 days bio-drying, whereas its lower heating value (LHV) increased an average of 168% from 5413 kj/kg. In the three trials, the highest LHV amounted to 15.8 MJ/kg, which achieved the LHV demand of solid recovery fuel (SRF). In bio-drying process, the average water removal rate was 80.6%. Smaller organic matter degradation led to a great decline of the water content; therefore, the decrease of gross energy capacity of MSW is not significant. Increasing attention is being focused on the potential hazards caused by waste landfill because of greenhouse gas emission and potential groundwater pollution. Hence, the European Union has developed Landfill Directive, which requires a reduction in the amount of biodegradable waste going to landfills, an increase in recycling, and improved energy recovery [1]. To achieve this goal, mechanical biological treatment (MBT) is receiving increased attention [2]. MBT-treated waste can be used to produce solid recovery fuels (SRF), which can not only replace traditional fossil fuels [2], but also reduce greenhouse gas emissions [3]. Conventional SRF production processes include the mechanical sorting of MSW, which separates high heating value components, such as plastic, using screening and winnowing. This method is widely applied in Europe, USA, and other countries [2]. However, MSW in many developing countries, e.g., China, is characterized by high water content (often more than 70%) and low heating value due to a relatively high proportion of food waste. In China, MSW contains high kitchen waste content, high water content, and low heating value [4]. As a result, if the MSW is treated directly by mechanical sorting, the high water content and the putrescible property greatly deteriorate the sorting efficiency of the waste or even invalidate the operation, thus mechanical sorting becomes inoperative. As an approach to MBT, bio-drying pretreatment used the thermal energy released from aerobic decomposition of readily degradable All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,02:17:46)

2 538 Environmental Protection and Resources Exploitation II organic matter to evaporate water in the waste, and the water content was removed by the forced ventilation [5]. Bio-drying process reduced the waste water content, and then increased the lower heating value of waste with no external energy consumption [6]. Furthermore, the method can easily stabilize the biodegradable organic fraction contained in waste [7]. Due to these advantages, bio-drying pretreatment process has become an important alternative to produce SRF using MSW [2]. Adani [8] carried out a laboratory study that showed that the degree of stabilization and the rate of bio-drying were inversely correlated: high bio-drying rates led to low stability of organic matters and high energy content of products. Rada [9] simulated the dynamic changing process of lower heating value during MSW bio-drying. Tambone et al. [7] reported the effects of bio-drying on municipal solid waste properties, including biological activity, net heating value, mass, etc. However, little research has been done in China on the waste characteristics and heating value variation of MSW with high water content during the bio-drying process. This study focused on MSW with high water content in China, and investigated the water content, lower heating value (LHV), and gross energy of waste under different ventilation conditions. This study provided an overview of parameters for optimizing the SRF production method, and the conditions under which the bio-drying method worked as pretreatment. Materials and Methods 1.1 Characteristics of the MSW feedstock The test was started in March, at an average room temperature of 20. All MSW used in this experiment was collected from garbage cans in a residential community in Chaoyang District, Beijing, China. After breaking the basic artificial package bags, large plastics were crushed to less than 300mm and mixed together. Based on the biodegradable characteristics, wastes were clssified into four different components: kitchen waste, plastic, paper, and other. Table 1 lists the characteristics of experimental waste. Tab. 1 Experimental waste components and water content Components Wet Weight Components (%) Water Content (%) Dry Weight Components (%) Volatile Solid Content (%) Kitchen Waste Plastic Paper Others Mixture Experimental Equipment Figure 1 is the schematic diagram of the experimental equipment used for this research. The reactor was a PVC cylinder with a height of 2.0m, an inner diameter of 0.8m. The outer layer of cylinder was covered by 30mm of polyurethane to insulate heat. A solid slab with a set inclination was placed at the bottom of the reactor, which was covered by gravels and connected to a PVC pipe with a diameter of 40mm to discharge leachate. The top of the solid slab was connected with a PVC ventilation pipe with a diameter of 40mm. The pipe was sealed at the end, and connected with a vortex fan and a gas rotameter at the inlet to control air volume ventilating. A disk with several holes (diameter: 5mm) was set at 200mm from the bottom. MSW was filled above the baffle, with a

3 Advanced Materials Research Vols filling height of 1.3m. To ensure uniformity of material properties in each reactor, the different categories of mixed waste in reserve was filled in batches, sequentially and equally. After filling waste in reactors, straw with a sufficient thickness were placed above the waste to absorb water vapor in ventilation and prevent the condensed water go back to waste pile. Fig. 1 Schematic diagram of the experimental equipment The top of the reactor was sealed tightly, and was equipped with two vent pipes connecting to a negative pressure ventilator to keep a slightly negative pressure at the outlet. To achieve the predetermined temperature in a short time (<1min), the air in the reactor was heated by resistance wires to the predetermined temperature and controlled by a voltage regulator. To monitor the temperature variation at the top, middle, and bottom of the pile, three temperature sensors were set at 1.0m, 0.7m, and 0.3m from baffles, respectively. 1.3 Experimental Setup and Operation Three parallel trials were conducted for this study. Trial IA was intermittently ventilated (10min/20min) at room temperature. Trial CA was intermittently ventilated (10min/20min) for the first three days, and then continuously ventilated after reaching 60 to rapidly reach the high temperature of piles. Trial HIA was intermittently ventilated (10min/20min) at 40 (by a heating device). Table 2 lists experimental parameters, including the material weight, ventilation conditions, ventilation temperature, and ventilation time, etc. 1.4 Test Methods The temperature of the waste pile was continuously monitored by a temperature sensor (Pt100) every 10min; data were recorded in a computer. Sampling every 3d while turning; the weight of each test sample was no less than 2kg. Sample water content (100±5 C, drying to a constant weight), pile weight (weighted by a platform scale), heating value (oxygen bomb combustion, PARR 1356, USA), and C/H/N (elemental analyzer CE-440) were measured. The water content, heating value, element, and other indicators were analyzed in a parallel test, which made the deviation of two results less than 10%. The measured value was the arithmetic average of two tests.

4 540 Environmental Protection and Resources Exploitation II Tab. 2 Aerobic condition of 3 trials Parameters Trial IA Trial CA Trial HIA Mass of Pile 198.4kg kg kg Ventilation Method 10min/20min Intermittent ventilation for 3d, continuous ventilation for 15d 10min/20min Temperature Room temperature Room temperature 40 Ventilation Rate 0.077m 3 /min.m m 3 /min.m m 3 /min.m 3 Time 18d 18d 18d Turning Frequency 3d 3d 3d Results 2.1 Temporal Evolution of Temperature Figure 2 shows the temporal evolution of temperature, continuously recorded by PLC at the center of the pile. The room temperature remained at around 20 C. The initial experimental temperature was close to the room temperature; it gradually rose to thermophilic phase (>60 C). Trial CA and trial HIA reached the thermophilic phase on the 2 nd day (considered to be fast); while Trial IA reached the target phase at the 3 rd day (considered to be slow). Fig. 2 Temporal evolution of temperature during bio-drying Figure 2 shows that three trials were staying at the thermophilic phase with the maximum temperature of 70 C for 18d, 12d, and 10d. This indicated that the biological reaction in waste piles was very active. Trial IA stayed relatively long in thermophilic phase and showed no significant decrease; its temperature stayed at around 60 C until the end of the test. The temperature of trial CA

5 Advanced Materials Research Vols drops significantly, as its ventilation time was longer. The temperature of trial HIA dropped rapidly after the 4 th turning, since the moisture was vaporized by high ventilation temperature quickly, and the resulting low water content limited organic matter degradation heat release; the exothermal heat from the organic matter degradation was less than that carried by ventilation, therefore, the temperature decreased. 2.2 Temporal Evolution of Water Content Due to the aerobic fermentation in bio-drying, the cell structure of waste biomass was destroyed and the field water-holding capacity decreased; as a result, a small amount of water may leak from the waste matrix. However, only a small amount of water was lost in this way; most of the water loss was contributed by convective evaporation [10]. In this study, no leachate was observed. Fig. 3 Temporal evolution of water content during bio-drying Figure 3 shows the temporal evolution of water content in waste during bio-drying. After the first 3d, the water content of waste only changed a little. At this period, the system was in the heating phase, and the microorganisms were in the lag phase. As such, the biological reaction was not active, the exothermal heat was lower, and most water was carried off by convective evaporation. Therefore, at the start of the test period, the temporal evolution of water content did not show a significant decrease. The water content in waste displayed a clear decline with the process of bio-drying. In particular, the water of trial CA and trial HIA was lost rapidly and fell to 23.72% and 24.50% at the end of the experiment, respectively, which was about 13% lower than that of Trial IA. This indicated that air ventilation and continuous ventilation under 40 can significantly increase the water removal rate and enhance the drying effect. Adani [8] stated that the water vapor removal rate was high, but the amount of biodegradation was low when the pile temperature was low during the aerobic biological treatment process. Compared with the other two trials, trial IA stayed in the thermophilic phase longer, hence, the water loss was slower. This was consistent with Adani's results [8]. 2.3 Temporal Evolution of Mass During the bio-drying process, MSW mass loss came from two sources. First, water was continuously vapored in ventilation; second, the easily biodegradable organic fraction contained in

6 542 Environmental Protection and Resources Exploitation II waste was degraded to CO 2 and water, as well as VOC, and was taken away by airflow. The exothermal heat from the organic matter degradation rose the temperature of the waste pile and thus facilitated the convective evaporation of water [5]. As such, the water removal rate was increased. Finally, a small amount of leachate may be discharged from the system; though in this study, no leachate was observed. Fig. 4 Mass variations of the MSW of trial IA during bio-drying Among the mass losses discussed above, the mass reduction caused by convective evaporation of water was the most significant, followed by the mass loss caused by organic matter degradation. In the three trials, after 18d bio-drying, the average water removal rate was 80.6%. Taking trial IA as an example, Figure 4 shows the mass losses of MSW of trial IA during bio-drying. Theoretically, plastic did not take part in the reaction and thus the mass of plastic did not obviously change. This finding was in agreement with the result shown in Figure 4. Water mass in waste decreased from kg to 34.38kg (i.e. about 72% of the total water was lost). In addition, the mass loss of kitchen waste (dry weight) represented a high proportion; after 18 days of bio-drying, about 48% of the dry weight was degraded. The degradation rate of the paper was small. 2.4 Temporal Evolution of Heating Value Heating value includes higher heating value (HHV) and lower heating value (LHV). The difference value between HHV and LHV is the latent heat in the water vapor condensation. In the process of bio-drying, the water content decreased continuously, while the weight of plastic changed slightly and resulted in a higher proportion of the waste. Due to the high caloric value of plastic, it contributed significantly to the HHV of the entire dry weight. Figure 5 is the temporal evolution of higher heating value of the non-plastic component during bio-drying. Figure 5 shows that the HHV of dry weight of non-plastic component increased at first, and then decreased. The initial heating value was about 15.9MJ/kg; it continuously increased to the peak value (21MJ/kg) and then decreased. One possible reason for this phenomenon was that the easily biodegradable organic fraction with short-chains was partially degraded at the beginning of bio-drying, thus the heating value of kitchen waste was increased. However, as the bio-drying progressed, a not easily biodegradable organic fraction with large molecules was also broken down, which leaded to the decrease of the heating value of dry weight. Among the three trials, trial IA took

7 Advanced Materials Research Vols the shortest time to reach the peak value, which may be related to the higher degradation rate of organic matter. Fig. 5 Temporal evolution of higher calorific value of non-plastic component during bio-drying Fig. 6 Temporal evolution of higher heating value of MSW during bio-drying Figure 6 shows the temporal evolution of MSW HHV during bio-drying, which was similar to the trend shown in Figure 5, but the overall heating value was improved 3~4 MJ / kg. Looking across the three trials, trial IA took the shortest time to reach the peak value. This was because the organic matter degradation rate was high, and the plastic occupied a relatively high proportion in unit weight. LHV is the most effective indicator for evaluating the heating value during waste incineration. Figure 7 is the temporal evolutions of the LHV of wet weight during bio-drying. The figure shows that the LHVs of wet weight of all reactors rose with bio-drying, which reversed the temporal evolution of water content in Figure 3. This implied a negative correlation between LHV and water content. The LHV of wet weight at feeding was 5413 kj/kg and it exceeded or approached 7000 kj/kg at the 6 th day, which met the heating value requirement of economic incineration [11]. At the 18th day, the LHVs of three trials were 12.0 MJ/kg, 14.8 MJ/kg,15.8 MJ/kg, an improvement of

8 544 Environmental Protection and Resources Exploitation II 140%, 191%, and 173%, respectively. All values reached the heating value required to produce solid recovered fuel (SRF) [12]. Fig. 7 Temporal evolution of material lower heating value during bio-drying 2.5 Temporal Evolution of Energy Capacity of MSW During bio-drying, the water content and MSW mass decreased, but the LHV increased; correspondingly, the gross energy capacity (the product of LHV and MSW mass) also varied. Figure 8 compares MSW gross energy capacities of three trials during bio-drying. Due to the degradation of partial organic matter in bio-drying, the gross energy capacity should be gradually decreased. In Figure 8, after 18 days of bio-drying, the gross energy capacity of trial CA slightly increased. This may be because of the bad uniformity of MSW and sampling deviation. The gross energy capacities of both trial IA and trial HIA showed a decreasing trend in the range of 8%~10%. Although organic matter was degraded in the process, a rapid drop of water content in MSW was achieved with the price of smaller amount of organic matter degradation during bio-drying. In the process, the energy capacities of plastic and paper remained constant or less changed, which were much larger than the energy loss resulted from the organic matter degradation. Therefore, the reduction of gross energy capacity was insignificant compared with total energy capacity. In fact, the theoretical heat loss can be calculated by measuring the LHV of kitchen waste and the degradation weight. The calculated result was close to the measured value. For a certain scale of waste incineration plant, bio-drying pretreatment can achieve a great reduction of the combustion capacity and investment of incineration plant, but the total energy production decreased insignificantly, if bio-drying products were used for incineration. Conclusions (1) In the process of high water content MSW bio-drying, the water content and mass of MSW decrease constantly, thus waste reduction was achieved. After 18 days of bio-drying, the water content of MSW decreased from 61.5% to 23.7 % (minimum). Average 80.6% water removal was achieved during bio-drying. (2) After 18 days of bio-drying, the LHVs of three trials increased by an average of 168%. The LHV of product, generated by continued ventilation and continuous ventilation under 40, reaches up to kj/kg and kj/kg, respectively. The product can be used as SRF.

Advanced Materials Research Vols. 1010-1012 545 (3) Bio-drying obtained a great reduction of water content of MSW by degrading fewer amount of organic matter.

9 Advanced Materials Research Vols (3) Bio-drying obtained a great reduction of water content of MSW by degrading fewer amount of organic matter. Because of lower degradation of components with higher LHV such as plastic and paper, the gross energy capacity reduces insignificantly. Fig. 8 Comparisons of MSW gross energy capacities during bio-drying Acknowledgements This work was financially supported by National Key Technology R&D Program of China (No. 2012BAC25B03). References [1] EC. Council Directive, 1999/317 EC of 26 April 1999 on the Landfill of Waste. Official Journal of the European Communities. [2] Velis C. A., Longhurst P J, Drew G H, et al. Production and Quality Assurance of Solid Recovered Fuels Using Mechanical Biological Treatment (MBT) of Waste: A Comprehensive Assessment[J]. Critical Reviews in Environmental Science and Technology, 2010,40(12): [3] Morris J. Bury or burn North America MSW? LCAs provide answers for climate impacts & carbon neutral power potential [J]. Environmental science & technology, 2010,44(20): [4] Chen Z. L., Li X. B., Zhou L., et al.study on composting of MSW pre-treatment [J].Environmental Sanitation Engineering (in Chinese), 2004, 12(1): [5] Velis C. A., Longhurst P. J., Drew G. H., et al. Biodrying for mechanical biological treatment of wastes: A review of process science and engineering [J]. Bioresource Technology, 2009,100(11): [6] Sugni M., Calcaterra E., Adani F. Biostabilization biodrying of municipal solid waste by inverting air-flow[j]. Bioresource Technology, 2005,96(12): [7] Tambone F., Scaglia B., Scotti S., et al. Effects of biodrying process on municipal solid waste properties[j]. Bioresource technology, 2011,102(16):

10 546 Environmental Protection and Resources Exploitation II [8] Adani F., Baido D., Calcaterra E., et al. The influence of biomass temperature on biostabilization biodrying of municipal solid waste[j]. Bioresource Technology, 2002,83(3): [9] Rada E. C., Franzinelli A., Taiss M., et al. Lower heating value dynamics during municipal solid waste bio-drying[j]. Environmental technology, 2007,28(4): [10] Shao L. M., Jin T. F., He P. J., et al..influence of the addition of recycled products on aerobic biological treatment for municipal solid waste [J]. Chinese Journal of Environmental Engineering (in Chinese) 2008, 2(9): [11] Li A. M., Li D. F., Xu X. X.,A discussion on pretreatment to improve MSW incineration [J].Chinese Journal of Environmental Engineering (in Chinese), 2008, 2(6): [12]Committee E. Solid recovered fuels Specifications and classes (DD CEN/TS 15359:2006) [R]. London: 2006.

11 Environmental Protection and Resources Exploitation II / The Effect of Bio-Drying Pretreatment on Heating Values of Municipal Solid Waste / DOI References [2] Velis C. A., Longhurst P J, Drew G H, et al. Production and Quality Assurance of Solid Recovered Fuels Using Mechanical-Biological Treatment (MBT) of Waste: A Comprehensive Assessment[J]. Critical Reviews in Environmental Science and Technology, 2010, 40(12): / [5] Velis C. A., Longhurst P. J., Drew G. H., et al. Biodrying for mechanical-biological treatment of wastes: A review of process science and engineering [J]. Bioresource Technology, 2009, 100(11): /j.biortech [6] Sugni M., Calcaterra E., Adani F. Biostabilization-biodrying of municipal solid waste by inverting airflow[j]. Bioresource Technology, 2005, 96(12): /j.biortech [7] Tambone F., Scaglia B., Scotti S., et al. Effects of biodrying process on municipal solid waste properties[j]. Bioresource technology, 2011, 102(16): /j.biortech [8] Adani F., Baido D., Calcaterra E., et al. The influence of biomass temperature on biostabilizationbiodrying of municipal solid waste[j]. Bioresource Technology, 2002, 83(3): /S (01) [9] Rada E. C., Franzinelli A., Taiss M., et al. Lower heating value dynamics during municipal solid waste bio-drying[j]. Environmental technology, 2007, 28(4): /

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