78 / JOURNAL OF ENVIRONMENTAL ENGINEERING / JANUARY 2001 CHARACTERISTICS OF ENERGY FLOW IN MUNICIPAL SOLID WASTE INCINERATOR By Moo Been Chang 1 and Chien Kun Huang 2 ABSTRACT: The energy flow of an existing municipal solid waste incinerator (MWI) was analyzed for better understanding of the energy balance and efficiency in a typical MWI located in Taiwan. The MWI investigated is of mass-burn type and is equipped with electrostatic precipitators followed by wet scrubbers for removing air contaminants from gas streams. The results of energy balance analysis indicate that the rate of energy input is 73,000 MJ/h based on the lower heating value of 7,600 kJ/kg of municipal solid wastes for single incineration unit. Furthermore, energy distribution analysis demonstrates that the heat loss of incinerator, heat absorbed by boiler, heat loss of air pollution control devices, and heat discharge from stacks are 3.8%, 68.8%, 11.1%, and 15.1% of total energy input, respectively. The sum of heat absorbed by boiler and the heat discharged with the flue gas via stack accounts for more than 80% of the total energy input. INTRODUCTION Space for sanitary landfills is extremely expensive and dif- ficult to find in Taiwan, therefore, the government plans to build more than 30 large-scale incinerators to treat more than 90% of the municipal solid waste (MSW) generated on the island by the year 2002. The advantages of incineration in- clude the effective reduction of the mass and volume of MSW, destruction of organic pollutants and recovery of the energy contained in MSW. During the combustion process, the tem- perature of the incineration chamber is typically maintained at 900–1,000°C for complete combustion of MSW and to avoid generation of thermal NO x . Most of the energy recovered from the waste gas heats the boiler water to generate high-pressure steam. The proximate analysis of incoming MSW indicates that it contains 48.6% moisture, 11.5% ash, and 39.9% combustible materials. Furthermore, results of elemental analysis of the combustible materials indicate that it contains 21.3% carbon, 3.2% hydrogen, 14.1% oxygen, 1.1% nitrogen, 0.1% sulfur, and 0.3% chlorine and the actual lower heating value (LHV) is approximately 7,600 kJ/kg-waste [Taipei Environmental Protection Bureau (EPB) 1998]. According to the Taiwan En- vironmental Protection Agency (EPA) report on waste heat re- covery of large-scale municipal solid waste incinerators (MWIs) (Table 1), the electricity generated by existing (MWIs) located in Taiwan varies from 140 kWh to 450 kWh for in- cinerating one metric ton MSW (Taiwan EPA 1998). Morris (1996) indicated that typical MWIs captured only about 15% of materials intrinsic heat value of MSW to generate steam and spin turbines due to the limitation of energy efficiency. Haith (1998) indicated that the electrical power generated by burning MSW was 561 kWh/t MSW combusted. Charles and Kiser (1995) also reported that the electricity efficiency for existing MWIs in the United States was 550–660 kWh/t MSW. Based on the above information, the electricity effi- ciency of MWIs in Taiwan is significantly lower than that in the United States. The lower energy efficiency observed in Taiwan’s MWIs could be partly attributed to the significantly higher moisture content in MSW (48.6% in this case) in ad- 1 Grad. Inst. of Envir. Engrg., National Central Univ., Chungli, Taiwan 320, Republic of China. 2 Grad. Inst. of Envir. Engrg., National Central Univ., Chungli, Taiwan 320, Republic of China. Note. Associate Editor: Susan Powers. Discussion open until June 1, 2001. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this tech- nical note was submitted for review and possible publication on August 13, 1999. This technical note is part of the Journal of Environmental Engineering, Vol. 127, No. 1, January, 2001. ASCE, ISSN 0733-9372/ 01/0001-0078–0081/$8.00 + $.50 per page. Technical Note No. 21704. dition to the difference in heating values of MSWs in two countries. Taiwan is a wet country with an average rainfall of 2.6 m/yr. Besides, the percentage of population served with sanitary sewers in Taiwan is still low (less than 10%) and most kitchen wastes are not processed with home grinders for trans- porting through the sewer system. As a result, the MSW in Taiwan contains more garbage with high moisture content compared with other developed countries. In this technical note, analysis of energy flow based on de- sign/field data in a large-scale MWI located in northern Taiwan has been carried out to gain better insights of the energy dis- tribution within an incinerating unit in a typical MWI. The approach adopted and information collected in the present study can provide environmental engineers with a good ref- erence for conducting relevant analysis. METHOD Assumptions The incinerator investigated is of mass-burn type and is equipped with electrostatic precipitators (ESPs) followed by wet scrubbers (WSs) for removing particulate and gaseous contaminants from flue gas. This incinerator is of step-recip- rocating grate type and has been operating since the early 1990s. Three incineration units, three boilers, one steam tur- bine, and three sets of APCDs are included in the plant. The volume of single combustion chamber is approximately 190 m 3 . The loading of combustion chamber is designed as 400 MJ/m 3 /h. Approximately 80% of all air required for combus- tion is supplied to the grate as primary air and the other 20% or so is supplied to the lower part of the secondary combustion chamber based on the design criteria (Mechanical 1990). Part of the energy recovered from MSW incineration is used to reheat the flue gas stream at the stack inlet to prevent the white smoke. The design capacity of single incineration unit is 300 t MSW per day based on the designed LHV of 5,600 kJ/kg. However, the actual LHV of MSW in Taiwan has increased to approximately 7,600 kJ/kg in recent years, so the maximum incineration capacity is kept at 240 t MSW per day to prevent the incinerator from damage. The boiler was designed to op- erate at low pressure and temperature (2,450 kPa and 280°C). The average residues percentage of MSW incinerated is ap- proximately 18.5% by mass based on the operating data (Tai- wan EPA 1998). The mass ratio of bottom ash to fly ash is 6:1 (Modification 1998; Retrofitting 1998). In the present study, it is assumed that the basic combustion reactions in- clude: (1) C + O 2 → CO 2 ; (2) H 2 + 1/2 O 2 → H 2 O; (3) Cl 2 + H 2 O → 2 HCl + 1/2 O 2 ; (4) N 2 is the only speciation of nitrogen; and (5) S + O 2 → SO 2 . The theoretical air require- ment is equal to 2.26 Nm 3 /kg-waste (2.91 kg-air/kg-waste) J. Environ. Eng., 2001, 127(1): 78-81 Downloaded from ascelibrary.org by VIT University on 09/06/17. Copyright ASCE. For personal use only; all rights reserved.