Life Cycle Assessment of RDF Production from Aged MSW and its Utilization System

Transcription

1 Proceedings of the International Conference on Sustainable Solid Waste Management, 5-7 September 2007, Chennai, India. pp Life Cycle Assessment of RDF Production from Aged MSW and its Utilization System Dezhen Chen 1, Xulu Zhai 1 and Gongming Zhou 2 1 Thermal & Environmental Engineering Institute, Tongji University, Shanghai, China 2 National Engg. Research Center for Urban Pollution Control, Tongji University, Shanghai, China. chendezhen@mail.tongji.edu.cn,zhougm@mail.tongji.edu.cn ABSTRACT There are a large amount of aged municipal solid wastes (MSW) to be disposed in China and separating combustibles from those aged MSW to produce RDF is one of the effective ways of recovering energy. In this paper first the fresh and aged MSW have been characterized, and then different RDF utilization methods are proposed and compared economically; thirdly life cycle inventories of RDF production from aged MSW and the proposed utilization methods have been analyzed based on experimental data; and finally life cycle assessment (LCA) was performed to judge the environmental potential impacts of the whole process quantitatively. LCA results showed that potential impacts of RDF production and its two utilization systems are negative thus the two systems are environmentally beneficial. RDF incineration in the existing fresh MSW incinerator gave lower potential impacts than RDF gasification and generation in a new system, which means that the former process is super to the later both economically and environmentally. Also mass burn of combustibles in aged MSW was analyzed and this utilization was not recommended due to its high transportation consumption and higher emissions. Transportation consumes diesel whose LCA impacts are mainly human toxicity via soil (HTs); and RDF should be utilized in local area within 25km in diameter for gasification system or 29km in diameter for being auxiliary fuel in fresh MSW incinerators to ensure the whole process to be environment friendly. Keywords: Aged Municipal Solid Waste, RDF, LCA, Energy recovery 1.0 INTRODUCTION Municipal solid wastes (MSW) keep increasing in China in recent fifteen years, but they have not been treated and disposed properly and there exist a huge amount of aged MSW that wait for being further treated. Traditionally the disposal method is landfilling or dumping unsafely in some suburbs or rural areas, whose storage amounts to more than 6 billion tons and results in more than 200 cities surrounding with MSW, even now there are still a large amount of MSW disposed temporarily and unsafely; for example, in the year of 2005, the amount of MSW dumped unsafely in the whole country amounted to tons (China Construction Department, 2006). Of all the disposal methods, landfilling takes up about 80% of capacity and it will still be the main treatment method in China in the future. Many of the old landfill sites are not sanitary landfills due to economy problem, meanwhile new landfill sites are more and more difficult to find nowadays. To 406

2 Sustainable Solid Waste Management mine the aged MSW and rehabilitate the landfill to give space for fresh MSW is the easiest option for MSW disposal; but how to recycle the mined aged MSW needs exploration. On the other hand the high moisture content and non-enough lower heat value (LHV) of fresh MSW is a big problem for incineration treatment. If fresh MSW management can be coupled with disposal of aged MSW from landfill sites / dumpsites, then the LHV problem of fresh MSW can be solved. In this work production of refuse derived fuel (RDF) with combustibles from aged MSW was investigated; the potential utilization systems of RDF including using it to help fresh MSW incineration are evaluated with help of life cycle assessment. 2.0 FRESH AND AGED MSW CHARACTERISTICS AND THEIR LHV COMPOSITION The fresh MSW were sampled from an incinerator receiving hopper; and this sampling kept for half a year. Their moisture content, composition and LHV were analyzed and the results were showed in Figure1. The aged MSW were sampled from Laogang landfill and Sanlin dumping site. Their moisture contents, components and LHV were shown in Table 1 and Table 2. Moisture/Ratio 80.0% 75.0% 70.0% 65.0% 60.0% 55.0% 50.0% 45.0% 40.0% 35.0% 30.0% Moisture Ratio of LHV from plastics LHV Feb. Mar. Apr. May Jun. Figure 1 Overall Moisture Content, LHV and Ratio of LHV from Plastics in Fresh MSW Samples from Shanghai Changing with Month (Sampled in 2004) From Figure1 it can be seen that the moisture contents in fresh MSW samples were generally higher than 42%, and their LHV is lower than 6500kJ/kg in half a year. Furthermore LHV of fresh MSW was mainly contributed by plastics, which amounts to more than 50% (see Figure1); while plastics were believed to contribute to dioxins emission. On the other hand, investigation found that around 70% of plastics in fresh MSW is polyethylene and polypropylene and 40% of them can be easily separated for re-utilization (Fu DD, 2007). If plastics were separated, the LHV would be even lower. In practise auxiliary fuel such as oil is often needed for stable and complete combustion. The high soil contents of the aged MSW (Table 1) are the result of daily cover and decomposition of degradable components in fresh MSW. All the aged MSW samples gave off no smell, which is beneficial for component separation. Another important phenomena is that moisture content of those combustibles in aged MSW is much lower than those in fresh MSW; and if the combustibles listed in Table 1 undergoes any further treatment, their moisture contents decrease very fast. Therefore their LHV is higher and good quality of RDF can be produced with the combustibles from aged MSW LHV (kj/kg) 407

3 Life Cycle Assessment of RDF Production from Aged MSW and its Utilization System Table 1. Composition of Aged MSWs from Laogang Landfill and Sanlin Dumpsite (%Wt) Samples Plastics Rubber Wood &Bamboo Fabrics Paper Stone/glass/ tile Soil MSW from dumpsite MSW from landfill, MSW from landfill, MSW from landfill, Table 2. Moisture Contents in Different Components of Aged MSWs in Table 1 (%wt) Samples Plastics Rubber Wood &bamboo Fabrics Paper Overall moisture of combusti bles Overall LHV, kj/kg Rate of LHV from plastics MSW from dumpsite MSW from landfill, MSW from landfill, MSW from landfill, MINING OF AGED MSW AND RDF PRODUCTION Whether in dumping sites or in landfills, the aged MSW need mechanical excavation before retreatment and re-utilization; then the aged MSW undergo separation steps to separate combustibles from soil, stones, glass, bricks, etc. When using combustibles to produce RDF, they must be shredded to let blending combustibles and additives easier. Use of additives helps to control acidic gas emission, the procedure of making RDF from aged MSW can be shown in Figure 2: RDF briquetting Aged MSW Mining First separation Second separation Shredding Additive Soil & smallsized stone, etc Heavy and bigsized incombustibles Figure 2 Schematic Diagram of Making RDF from Aged MSW CaO 408

4 Sustainable Solid Waste Management No drying step is needed in Figure 2 due to the low moisture content of combustibles and also the fast evaporation speed. During the separation processes the moisture content can be decreased by more than 10% without heating. Take the 10 MSW from landfill in Table 1 for example, after two steps separation, the moisture content decreases to 15%; then CaO added by 8%wt of the total weight, LHV of the resulted RDF is 16950kJ/kg, which amounts to 3 times of that of fresh MSW in Shanghai, this RDF is the basis for later system comparison. The ultimate analysis of those combustibles in RDF is given in Table 3; it can be seen that contents of S and Cl are very high, therefore addition of CaO is very necessary when produce RDF. In Table 4 the material and energy consumptions of RDF production are given based on experimental data. Table 3. Ultilimate Analysis of Combustbiles in 10 Years Old MSW from Laogang Landfill (Dry Basis, Impurities Included) Component C % N % H % S % Cl % O % Plastic < Rubber < Wood & bamboo Clothes and fabric < Table 4. Material and Energy Consumption during Making RDF from Aged MSW Step Aged MSW mining Separation, shredding and blending Electricty: 0.75kwh/ton aged MSW RDF production Energy & material consumption Oil: 0.7kg/ton aged MSW Electricty: 0.7kwh/ton RDF CaO: 80kg/ton RDF The produced RDF has many potential applications. One important application is used as auxiliary fuel in fresh MSW incinerators. From Figure 1 it can be seen that LHV of fresh MSW is very low and it needs auxiliary fuel to maintain steady combustion. To replace presently-used auxiliary fuel (oil and coal) with RDF has many advantages; one of them is RDF can be applicable to both fluidized bed incinerators and grate-furnace incinerators. RDF can also be used as a clean energy after gasification. 4.0 COMPARISON OF RDF UTILIZATION SYSTEMS WITH LCA 4.1 The Systems for Comparison There are two competitive systems for RDF application: gasification and generation system (notated as G & G system) and using RDF as auxiliary fuel in an existing grate incinerator in Shanghai to replace fuel oil. For G & G system, the generation capacity is 1MW because this is a mature and safe capacity for biomass generation system; the generator could be inner combustion engine with capacity of 200KW; its efficiencies is around 26.5% (Yin XL, et al, 2000). From experiments the gasification efficiency is found to be 75.6%; therefore the comprehensive efficiency is 20.03%. If gas turbine is used, the efficiency could be higher, but the fuel gas scrubbing is very strict and expensive. For system using RDF as auxiliary fuel in fresh MSW incinerator, the capacity of incinerator is 350ton/day, the typical capacity in Shanghai; and gross generation efficiency is around 22% according to operation experiences. It is assumed that use of RDF won t change the generation efficiency. When the RDF mentioned above was gasified with air as gasification reagent, the fuel gas produced is 409

5 Life Cycle Assessment of RDF Production from Aged MSW and its Utilization System shown in Table 5, it can be seen that H 2 is the main component, also concentration of methane and ethylene are very high in addition to CO. Table 5. Fuel Gas Composition when RDF Gasified (746, CaO Content 8%wt)* (%) H 2 O 2 N 2 CO 2 CO CH 4 C 2 H 4 C 2 H 6 C 3 H 6 NH 3 (ppm) H 2 S(ppm) *Gasification efficiency is 75.6%, LHV of the fuel gas 10.61MJ/m 3 When RDF is burnt in the laboratory tube furnace the emissions are shown in Table 6; at the same time the combustibles for making RDF is mass-burnt in the same furnace and the emissions are also gave in Table 6. The CO emissions are much higher than that from the practical incinerator in Table 6 is because the space of the testing furnace is not big enough for gas combustion. Data in Table 5 and Table 6 lay the basis for later comparison. Table 6. Pollutant Emissions during RDF Combustion in the Laboratory Tube Furnace and Compared with Emissions from Mass Burn and from Grate Furnace Incinerator Pollutants CO, NO X, HCl, SO 2, C x H y, RDF burn* 134/(2.3) 54/(1.0) 73.2/(1.664) 24.3/(0.97) 0.333/ (0.003) Mass burn* 360/(6.18) 56/(1.04) 345/(7.84) 81.1/(3.24) 1.2/(0.0108) 350ton/day grate furnace incinerator ** *α=1.5; Vy = 9.3Nm / kg ; in furnace of 850 ; **: O 2 =11%; Vy = 13.86Nm / kg MSW in average 4.2 Energy and Material Inventories of RDF Utilization Systems Table 7 and Table 8 list the energy and material (including pollutants) inventories of G&G generation system and incineration system respectively. For the G&G system, dry scrubbing system of fuel gas is adopted; after operation for a certain period the tar, fly ash on the surface of desulfurizer is washed with water and then the desulfurizer is regenerated with stream. The produced waste water is contaminated with tar and COD. The other pollutant emissions are referred to biomass gasification and generation experiences. Data in Table 8 are mainly based on the operation observatory of an incineration plant in Shanghai. Table 7. Input & Output Streams of RDF G&G System (Based on 1 Ton RDF) Input streams Unit Data Remark CaO Kg/t 5 Used for tar decomposition Water consumption Kg/t 1400 Used for cleaning the desulfurizer and equipment Steam of 0.6Mpa Kg/t 150 For regeneration of desulfurizer Electricity Kwh/t 94 Operation the whole system (Wu CZ,2006) Desulfurizer Kg/t 9.75 Taking into account of regeneration and aging Diesel Kg/t Consumed when starting the system 410

6 Sustainable Solid Waste Management Input streams Unit Data Remark Output streams Remark Electricity %LHV Efficiency of gas engine multiplying gasification efficiency SO 2 Kg/t Concentration in flue gas: 26 mg/nm 3 NOx Kg/t Concentration in flue gas: 200mg/Nm 3 CO 2 Kg/t 465 Mineral carbon in RDF from plastics & rubber CO Kg/t 2.21 Concentration in flue gas: 1250mg/Nm 3 PM 10 Kg/t TSP emitted from engine: 6.7mg/Nm 3 Waste water Kg/t 1200 Produced by washing the fuel gas scrubber COD Kg/t Discharged together with waste water Tar Kg/t 0.45 Discharged together with waste water Table 8. Input & Output Inventory of using RDF in Incinerator as Auxiliary Fuel Input streams Unit Data Remark Ca(OH) 2 Kg/t 5.42 Used for removing acidic gases Water consumption Kg/t 450 Used for cleaning the equipment and cooling Activated carbon Kg/t 0.41 For absorbing the dioxins Electricity Kwh/t For operating the whole system Diesel Kg/t The saved oil consumption when RDF used Output streams Remark HCl Kg/t 0.5 Concentration in flue gas: 35.82mg/Nm 3 Electricity %LHV 22 Gross efficiency of generation SO 2 Kg/t 0.97 Concentration in flue gas: 69.43mg/Nm 3 NOx Kg/t 0.97 Concentration in flue gas: 70mg/Nm 3 CO 2 Kg/t 465 Mineral carbon in RDF from plastics & rubber Dioxins Kg/t 1.39E-10 Concentration in flue gas: 0.01ng/Nm 3 CO Kg/t Concentration in flue gas: 20mg/Nm 3 PM 10 Kg/t TSP emitted from bag filter: 2.853mg/Nm 3 HF Kg/t Concentration in flue gas decreased from 0.17 mg/nm 3 to 0.1mg/Nm 3 Waste water Kg/t 370 For washing and cooling Additionally, if RDF or combustibles should be transported with diesel truck, a truck with load of 12.5 tons is chosen, its diesel consumption is 38L/100km or L/ton/km. 4.3 Life Cycle Assessment of Different Utilization Systems and Discussion of the Results Life-cycle-assessment (LCA) is a holistic approach that covers all main activities related to the waste management system and translates the information into resource consumptions and potential 411

7 Life Cycle Assessment of RDF Production from Aged MSW and its Utilization System environmental impacts. Here LCA analysis is used to evaluate different RDF utilization system with help of the newly developed EASEWASTE (Environmental Assessment of Solid Waste Systems and Technologies) and suggestions were given to guide their choices. Framework and structure of EASEWASTE was described in detail by Kirkeby et al. (2006). Table 9 presents the important categories related to waste management technology and the normalization reference used to convert the individual potential impact categories into person equivalents (PE), which is an average value for the yearly contribution to that impact category by all the activities and consumptions of one person in Europe and can be replaced by the similar data in a country or a region considered. Figure 3 gives the calculated potential impacts of different categories connected with RDF production, RDF incineration and RDF G&G systems (RDF production step included in those two systems) in person equivalents (PE). Potential Impact Category Global Warming, 100 years Table 9. Potential Impact Categories Included in EASEWASTE Acronym Unit Physical basis Normalization reference EDIP97 GW100 kg CO 2 -eq. /person/yr Global 8700 Acidification AC kg SO 2 -eq. /person/yr Regional 74 Nutrient Enrichment NE kg NO - 3 -eq. /person/yr Regional 119 Human Toxicity, soil HTs m 3 soil /person/yr Regional 157 Human Toxicity, water HTw m 3 water /person/yr Regional Ecotoxicity, water chronic ETwc m 3 water /person/yr Regional Bulky waste BW kg/person/yr Regional 1350 Hazardous HW kg/person/yr Regional 20.7 Slag & Ash SA kg/person/yr Regional 350 Photochemical Ozone Formation, low and high NOx PhOI, PhOH kg C 2 H 4 -eq/person/yr Regional 25 The positive value of impacts in PE means the harmful impacts to the environment; while negative values mean saving the environmental impacts. It can be seen that RDF production process causes impacts to environment due to energy and material s consumption, especially for category of human toxicity via soil (HTs). But due to the power generated from the two RDF utilization systems, their potential impacts to the environment are generally negative, especially for incineration system, it is better than G&G system if transportation distance for two systems not considered. Table 10 gave the main potential impacts caused by transportation. The χ in the formula is the transportation distance in kilometer. It can be seen that similar to RDF production; human toxicity via soil (HTs) is the main impact to environment for transportation. When RDF G&G plant is 25.7km away or RDF incineration plant is 29.2km away, then the benefit of power generation is discounted by transportation for those two systems, so RDF should not be transported farther than those distances. When combustibles is mass burnt instead of RDF production, the transportation consumption for combustibles is 2.56 times of that of RDF, for the density of RDF is 2.56 times of that of combustibles loading on the truck even additives is not counted. So the transportation distance should be less than 11.4km for mass-burning 412

8 Sustainable Solid Waste Management the combustibles in incinerators. Considering the emissions increase when combustibles are mass burnt (suggested by data in Table 6), the transportation distance should be even shorter for the utilization to be beneficial. But it is very rare in big cities that the distances between incineration plants and landfills or dumpsites is less than 11.4km. So just from point of transportation it is enough to conclude that mass burn of combustibles is not as applicable as RDF does. Person equivalents (PE) GW100 ETwc HTs PhOI BW HW Ets HTs AC Hta NE PhOH SA RDF production RDF incineration RDF G&G s ys tem Potential impact categories Figure 3 Tonnage Environmental Impacts for the Two RDF Utilization Systems Table 10. Impacts of Transportation to the Environment Category GW100 ETwc HTs HW & HTa PE χ χ χ χ CONCLUSION Fresh and aged municipal solid wastes (MSW) were analyzed and characterized for their composition and lower heat values (LHV). It was found that fresh MSW are characterized with high moisture content and low LHV, while combustibles in aged MSW are mainly plastics and of lower moisture content therefore can be made into refused derived fuel (RDF). The two systems for RDF utilization, namely RDF gasification and generation system and RDF incineration as auxiliary fuel in fresh MSW incinerators were compared economically and environmentally. The environmental comparison was finished with help of life cycle assessment based on life cycle inventory analysis of the two systems. The LCA results showed that using RDF as auxiliary fuel in incinerators is more beneficial than RDF G&G system both from economic and environmental aspects. Transportation consumes diesel and causes bad impacts to environment so the transportation distances should be controlled. When massburn of combustibles in aged MSW is adopted, the transportation distances should be less than 11.4 km and it is not as flexible as RDF utilization systems. ACKNOWLEDGEMENTS This project is supported by Asian regional research program on environmental technology sustainable solid waste landfill management in Asia funded from Sweden International Development Agency (Sida). 413

9 Life Cycle Assessment of RDF Production from Aged MSW and its Utilization System REFERENCES China Construction Department.City & Town Environment and Sanitation Plan for the Eleventh Five Year. Document No.[2006]243, valid from 2006, 8 th, Oct, Beijing. Fu, D., Study on recycling plastics from MSW for Anti-slip board and pipe products. Master thesis, Tongji University, Shanghai (Mar. 2007). Kirkeby J. T., Bhander G. S., Birgisdottir H., Hansen T. L, Hauschild M. and Christensen T. H. Environmental assessment of solid waste systems and technologies: EASEWASTE. Waste Management & Research, 24: pp.3-15(2006). Wu CZ, Ma LL, Chen Y. State of - art of biomass gasification & power generation. China Energy Science & Technology, pp.76-79(2006). Yin XL,Wu CZ,Zheng XP,et al.design and operation analysis on middle-sized biomass gasification and power generation system. Acta Energiae Solaris Sinica,21(3), pp (2000) 414