An Experimental Approached to Investigate Keys Operating Parameters for Thermal Destruction of Major Components of Simulated Infectious Waste

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1 An Experimental Approached to Investigate Keys Operating Parameters for Thermal Destruction of Major Components of Simulated Infectious Waste Paisan Letsalaluk 1, Woranuch Jangsawang 1 and Somrat Kerdsuwan,* 1 The Joint Graduate School of Energy and Environment, King Mongkut s University of Technology Thonburi, Bangkok, Thailand The Waste Incineration Research Center, Department of Mechanical Engineering, King Mongkut s Institute of Technology North Bangkok, Bangkok, Thailand Abstract: Infectious waste is discarded matter generated from medical activities and has a severe environmental impact if not properly disposed. Of all the available technologies for treating, incineration has been found to be the most effective choice for destroying the infectious and toxic components, volume, and weight reduction. It is also attractive aesthetically because the high temperature which exists within the incinerator not only destroys the pathogens in the waste but also destroys organic components of the waste that the community often finds objectionable when wastes are disposed by landfill. However, complete combustion requires sufficient air in the combustion chamber, sufficient temperatures in the combustion bed and combustion gas, sufficient time over which materials are exposed to a temperature profile and mixing that assures good contact of the waste/fuel with combustion air. Consequently, the success of waste incineration depends on keys operating parameters of incinerator. The objective of this study is to investigate experimentally keys operating parameters for thermal destruction of major components of infectious waste. Cotton 5.59%, plastic (PE) 11.3 %, rubber gloves.55 % and water 1.5% by weight, is burnt in a -kw controlled-air incinerator with a goal to study thermal destruction process. Preheated wall temperature and the weight of waste per batch which feed into PCC are the keys operating parameters to be investigated. The experiments were performed for the preheated PCC temperature varied from,,, C and initial temperature of secondary combustion chamber (SCC) started at C with the simulated infectious waste feed rate of 5, 7.5 and kg/batch, respectively. The variation of temperatures and the concentration of gaseous from the exit stack; carbon monoxide (CO) and oxygen (O ) were measured for analyzing. Results showed that higher preheated temperature directly effected on the burning period of a batch of waste but with the preheated temperature of C and the waste feed rate of 5 kg/batch gave a shorter burning period than other conditions. Evolution of SCC temperature followed to the evolution of PCC temperature because it used the devolatilized gaseous from PCC as fuel. The higher preheated temperature in PCC increased the devolatilized gaseous emission rate which was reacted with O in SCC. CO evolution from preheated temperature about C at the waste feed rate of 5 kg/batch is less than others conditions. The evolution of O is reversed to CO evolution; O concentration is reduced during CO concentration is increased. Thermogravimetry (TGA) tests have been conducted on the thermal decomposition of the major components of infectious waste in order to explain the results found from experiments. The results of tests showed that the decomposition of waste material significantly depended on the thermal destruction temperature. This study could indicate that keys operating parameters of an infectious waste incinerator is a preheating temperature of PCC and amount of waste feed per batch. Also, an optimization operating condition for this incinerator should be at about the preheated PCC of C and waste feed rate of 5 kg/batch. Results of study will be of benefit to further study of optimize operation and the development of controlled-air infectious waste incinerators. Keywords: Controlled-air Incinerator, Infectious Waste, Operating Parameters, Thermogravimetry. 1. INTRODUCTION Infectious waste is a discard waste originating from healthcare premises. It needs special treatment more than other wastes since they are contaminated by diseases. The infectious waste will cause much problem to human, animals and plants if it was discharged to the environment [1]. In the past, it was mixed with other wastes and discharged to the environment. The U.S. Environmental Protection Agency (EPA) recommends a number of treatment methods that are aimed at decontaminating hospital wastes to eliminate risk to health and the environment. Method for treated the infectious waste consist of steam sterilization, incineration, thermal inactivation, gas/vapor sterilization, chemical disinfecting and sterilization by radiation []. All of these methods can destroy pathogen in infectious waste but for incineration, it provides more advantage of greatly reduce volume and mass of the waste in the order respectively of 9 % and 7 % [3]. The incineration advantage is also to minimize transportation and disposal costs of the infectious waste. It is also attractive aesthetically because the high temperature which exists within the incinerator, not only destroys the pathogens in the waste but also destroys organic components of the waste that the community often finds objectionable when wastes are disposed by landfill. Corresponding author: Somrat_k@yahoo.com The goal of the combustion process is to complete combustion of the organic compound constituents in the waste. Gases emissions from combustion process are mainly composed of carbon dioxide, oxygen, nitrogen and water but also contain carbon monoxide, sulfur oxide, nitrogen oxides, hydrogen chlorine and small quantities of dioxins and furans. The production of these unwanted compounds is related to the combustion process and the fuel []. In order to protect the environment, more and more strict regulations are being implemented but the reduction of theses toxic emissions without a decrease in the thermal efficiency of the incinerators is currently the most important challenge. Complete combustion requires sufficient air in the combustion chamber, sufficient temperatures in the combustion bed and combustion gas, sufficient time over which materials are exposed to a temperature profile and mixing that assures good contact of the waste/fuel with combustion air [5]. So, the success of waste incineration depends on the optimum operating conditions of the operating parameters. The objective of study is to try to understand the combustion behavior of the incineration of the major components of infectious waste. The influence of preheated combustion chamber temperature and waste feed rate on the evolution of temperature and gaseous emission from the combustion of a controlled-air incinerator will be studied. The main objectives of this study are as followed: a) Study on combustion behavior in a controlled air 3

2 incinerator by study temperature variation in primary and secondary combustion chamber related to infectious waste feed rate. b) Study gaseous emission parameters and quantity, which emitted from the second combustion chamber. Gas Analyzer. EXPERIMENT PROTOCOL AND EQUIPMENT A -kw controlled-air incinerator, used in this work, is composed of two major parts (as shown is figure 1), primary and secondary combustion chambers. Combustion process in PCC will be sub-stoichiometric combustion and in SCC will be excess air combustion. An air blower was installed on top of SCC for supply air to PCC and SCC. A burner was installed at each combustion chamber for igniting the waste and maintaining the chamber temperature. Thermocouples type-k are used to measure temperature in the combustion chamber. They are installed near the exit of the combustion chamber to provide a representative temperature reading away from the flame zone, which can otherwise cause erratic temperature readings as well as damage to the thermocouple. The electrochemical gas analyzer is used to measure the concentration of combustion gases; Carbon monoxide (CO) and oxygen (O ), from the downstream of SCC. The combustion gaseous concentration and combustion chambers temperature were recorded by datalogger. Fig. 1 Experimental set-up. Major components of real infectious waste were used to simulate the infectious waste used for this study. It composes of cotton 57 %, syringe 11 %, rubber glove % and water % by weight. The experiment will not supply air to PCC in order to control the sub-stoichiometric condition but supply excess air about % to SCC. The preheating temperature of SCC is constant at C and preheating temperature of PCC will be vary from to C and with the waste feed rate at 5, 7.5 and kg per batch (See the experimental program in Table 1). Table 1 Experimental Program Parameter Unit Condition Condition Condition Condition Preheating PCC Temperature Degree C Feeding Rate kg/batch 5, 7.5, 5, 7.5, 5, 7.5, 5, 7.5, Preheating SCC Temperature Degree C Primary Air Supply % Secondary Air Supply % excess air 3. RESULTS AND DISCUSSIONS 3.1 The influence of Preheated Chamber Temperature on the Evolution of Temperature and Gaseous Emission a) Evolution of PCC Temperature 5 55 C C C Fig. Evolution of T 1 (5 kg feed rate). to maintain the desired SCC temperature with the minimum use of auxiliary fuel []. Results from figure show the evolution of the PCC temperature from a feed rate of 5 kg/batch. Temperature evolution with a preheated temperature of C occurs in shorter period, about 7.5 min, and the variation of temperature is about 5 C which is less than other preheating conditions. The shorter period of temperature evolution indicated the higher destruction rate of the infectious waste. The smaller variation preheating temperature of the PCC shall be an indicator of combustion stability in the chamber. The evolution of the PCC temperature from a feed rate of 7.5 and kg/batch is shown in figure 3 and. Higher preheating temperature produces shorter T 1 evolution than lower preheating temperature which also indicates the higher destruction rate of infectious waste. The evolution of temperature distribution in primary combustion chamber during the operation shall be the first important parameter to indicate the completeness of the combustion process. It is desirable to operate the PCC with temperature high enough to sustain combustion in the chamber and to generate sufficient volatile combustion gases and heat 37

3 C C C 1 5 C C C Deg. C Min Fig. 3 Evolution of T 1 (7.5 kg feed rate) C C C C 3 5 Fig. Evolution of T 1 ( kg feed rate). b) Evolution of SCC temperature The secondary combustion chamber temperature should high enough to destroy pathogens coming from PCC with flue gas. Also this temperature should high enough in order to sustain the complete combustion of off-gas. The results of experiment of the evolution of the SCC temperature (T ) from a feed rate of 5 kg/batch is shown in figure 5. Temperature evolution of the SCC from the preheating temperature of C occurs in shorter period, 7.5 min the same as the evolution of T 1. T evolutions are not slightly stable because of the variation rate of combustible gas (CO, primarily) from the PCC. However, the T evolution from preheated temperature of C is slightly stable due to slow rate of combustible gas from the PCC. Note that for the preheated PCC temperature about C has distinct results. This is because the combustion rate in the PCC at this temperature is very high. Results from figure and 7 show the evolution of the SCC temperature from a feed rate of 7.5 and kg/batch. We observe that at higher preheating temperature produces shorter the evolution of T than lower preheating temperature. 3 5 Fig. 5 Evolution of T (5 kg feed rate). 1 5 C C C 3 5 Fig. Evolution of T (7.5 kg feed rate) Fig. 7 Evolution of T ( kg feed rate). C C C C c) Evolution of Carbon monoxide Carbon monoxide is a product of incomplete combustion and can be used for combustion index to indicate the effectiveness of combustion process. Results from figure shows the evolution of carbon monoxide (CO) from a feed rate of 5 kg/batch. We can see that the evolution of CO depends on the preheating temperature. The higher the preheating temperature, the more evolution of CO. CO evolution curves from the preheating temperature of C and C are partly drop at the first part of the evolution because of the thermal decomposition process of the infectious waste (see section 3.). The evolution of CO from a feed rate of 7.5 and kg/batch is shown in figure 9 and. CO gas evolution also depends on the preheating temperature. The higher preheating temperature will result in higher CO evolution. Moreover, we can observe that the maximum peak 3

4 of CO for kg feed rate is about 3 times of the 5 kg feed rate. So, this feed rate is not suitable to burn this waste, nor the combustion air at this condition has to be regulated. 1 C C C % C C C C ppm 35 Fig. 11 Evolution of O (5 kg feed rate). Fig. Evolution of CO (5 kg feed rate). 3 c c c c % C C C ppm Fig. 9 Evolution of CO (7.5 kg feed rate). 3 3 C C C C Fig. Evolution of O (7.5 kg feed rate). C C C C ppm % Fig. Evolution of CO ( kg feed rate). d) Evolution of Excess Oxygen Excess Oxygen is the over supply oxygen coming out from combustion process and also could be used to observe the combustion phenomena. Results from figure 11 show the evolution of Oxygen (O ) with a feed rate of 5 kg/batch. We observe that the evolution of O for the preheating temperature of C reduces and increases faster than other preheating temperature. It may be because the waste is burnt under a suitable condition of the preheating temperature and waste feed rate (correspond to the small evolution time of T 1 and T which indicate rapid combustion process). Lower preheating temperature produce lower O evolution. We observe the same trend for the evolution of O from the waste feed rate of 7.5 and kg which is shown in figure and Fig. 13 Evolution of O ( kg feed rate). Evolution of excess O had a correlation with CO, T 1 and T. Figure 1 showed the correlation between the evolution of CO, O, T 1 and T for the initial preheating temperature of C with a waste feed rate of 5 kg/batch. The figure showed that T followed T 1 and the T 1 followed CO concentration but O concentration had a reverse effect. 39

5 T1 T p CO - - Temp T1 and T (C) CO (ppm) % O O ( % ) Mass (%), %min Time (x.5 min) - 3 Fig. 1 Correlation between T 1, T, O and CO at preheating PCC temperature of C, 5 kg/batch. Fig. 1 Cotton decomposition. 3. Thermal Decomposition Characteristics of Major Components of Infectious Waste Thermogravimetric Analysis (TGA) was conducted to examine the thermal decomposition of major components of infectious waste i.e., cotton, rubber glove, PE syringe (Polyethylene Plastic, PE) and mixed of these components. The temperature in TGA can be varied from ambient temperature up to O C. In this study, the samples were subjected to temperature ramps in an inert gas (nitrogen) in the temperature range from O C to O C. Nitrogen at a flow rate of 5 ml/min. was used as the inert gas at atmosphere and the heating rate was kept at O C/min. The mass evolution of these samples was determined as a function of temperature. TGA has been extensively used to determine the devolatilization of materials [7,]. The results obtained from the four samples using Thermogravimetric Analysis (TGA) are shown in Figure 1 to 19. They show the mass evolution of the sample as a function of the temperature. TGA curves for cotton and rubber glove (see Figure 1 to 17) show that the thermal decomposition process occurs in one stage and their pyrolysis have some residue left. The thermal destruction of PE syringe (Figure 1) occurs in one stage and its pyrolysis does not result in any residue. The thermal destruction of mixed sample between cotton, rubber glove and PE syringe occurs in two stage and there are some residue left (Figure 19). TGA was made in order to obtain information on the overall kinetics of decomposition for solid waste materials. The temperature for pyrolysis depends on type of waste and heat of pyrolysis depends on the chemical structure of the material. The weight loss due to the volatile released during pyrolysis exhibits itself as an endothermic reaction. The temperature for the beginning of pyrolysis and maximum decomposition for all waste types are showed in Table Mass(%) - 3 Fig. 17 Rubber glove decomposition Fig. 1 PE syringe decomposition Mass(%) Mass (%) - 3, %min - -, %min - - -, %min - Fig. 19 Mixed waste decomposition. 3

6 Table 1 Pyrolysis temperature and maximum temperature of decomposition rate Pyrolysis Temperature ( C) Temperature at Maximum Decomposition Rate ( C/min) Cotton 35 Rubber glove PE Syringe Mixed Waste, , 59.9 From the results of pyrolysis information, it is shown that cotton was devolatized at the lowest temperature ( C) followed by rubber glove (3 C) and PE syringe (3 C). When these wastes were mixed, the pyrolysis temperature occurred in two steps at C and 37 C. Due to pyrolysis temperatures of cotton and rubber glove were closely, the first shoot of mass loss from mixed wastes was probably due to thermal destruction of cotton and rubber glove. When pyrolysis temperature achieved about 37 C which nearly devolatilized temperature of PE syringe, the second shoot began. For the combustion of infectious waste in a controlled-air incinerator, the mixed waste was charged into PCC. The preheating temperature of PCC is acted as a heat source for pyrolysis process. The waste receives heat and begins to release devolatilzed gaseous and flow into SCC. The amount of gaseous and the released rate depend on pyrolysis temperature which in this circumstance, the preheating temperature of PCC. Referred to Figure ( 5 kg waste feed/batch) which showed the emission of CO from SCC during experiments and one can observed that there were two peak of CO. The first peak of CO should corresponded to the devolatilized process of cotton and rubber glove and the second peak, the PE syringe. At and C, the concentration of CO was lower which indicated that the release rate of devolatilized gas from PCC was also low, however, the time for thermal destruction of waste was longer. The preheating temperature about C seemed good enough for operating of this incinerator because CO concentration was quite low and it took shorter time to burn. In contrast, at C of preheating temperature, the concentration of CO increased. This might be due to too high pyrolysis temperature and the combustion (with low amount of oxygen due to sub-stoichimetric condition) began, consequently, this operating condition should be avoided. 3.3 The influence of Waste Feed Rate on the Gaseous Emission In this section, we will determine the influence of waste feed rate on the gaseous emission. The results of experiment show in figure. We observe that for the lower feed rate (5 kg/batch) the combustion process produces small amount of CO for all preheated temperature. This should be because at this small amount of waste, the PCC can sustain the combustion reaction causing in the chamber. In contrast, for higher waste feed rate (7.5 and kg/batch), the lower preheated temperature (, and C) produce less concentration of CO than for higher preheated temperature. The reason is that, at 7.5 and kg of waste per batch, the combustion reaction with high preheated temperature occurs very rapid and the SCC is too small to change the off-gas to be the completed combustion product. This operating condition should be avoid. CO (ppm) kg/batch 7.5kg/batch kg/batch Fig. Average evolution of CO with the PCC temperature.. CONCLUSIONS The keys operating conditions for the operation of the controlled-air incinerator compose of the waste feed rate and the preheating temperature of the primary combustion chamber. They are significant to the evolution rate of CO from the PCC. The waste feed rate of 5 kg/batch and the preheating temperature C are a good condition for operates this -kw incinerator. This experiment can be summarized that the 7.5 and kg/batch waste feed rate give more CO evolution than the 5 kg/batch waste feed rate. The preheating temperature of PCC is a factor of the evolution of CO. Higher preheating temperature accelerate pyrolysis reaction of the simulated infectious in the PCC. Consequently, the evolution of CO shall be controlled by controlling the preheating temperature. The preheating temperature between C and C could be good condition to operate this incinerator. Even the waste feed rates and the preheating temperature is major influence factor for operate this incinerator, however, others operating factor such as air supply rate and preheating temperature of the secondary combustion chamber and consistency of the waste are also influence to the evolution of CO and O from the simulated infectious waste. From experiments could indicate that an optimum operation condition of this incinerator is at primary combustion chamber temperature of C with the waste feed rate of 5 kg/batch. This operating conditions give higher destruction rate of the waste, and lower CO concentration. ACKNOWLEDGEMENTS The authors would like to express their grateful to the Joint Graduate School of Energy and Environment for funding support and the Waste Incineration Research Center, 311

7 Department of Mechanical Engineering, Faculty of Engineering, King Mongkut s Institute of Technology North Bangkok for facilities support to success the study. REFERENCES [1] Borowsky, A.R. and Fleischaue, P.D. (1993), Medical Waste Disposal - What's new?. Paper No. 93-TP-.3, th Annual Meeting and Exhibition, Air & Waste Management Assoc., Denver, CO, June [] US EPA. (199), Medical Waste Management in the United States : First Interim Report to Congress. Office of Solid Wastes, EPA/53-SW-91a, Washington, D.C. [3] Niessen,W.R. (1995), Combustion and Incineration Process :Application in Environmental Engineering, Marcel Dekker Inc., New York, USA. [] Hasselris, F. (199), Effect of Waste Composition and Charging Cycle on Combustion Efficiency of Medical Waste and Other Solid Waste Combustors, ASME National Waste Processing Conference, Detroit, USA. [5] T.J. Chang, 1991, Implementation of Regional Biomedical Waste Incineration Facilities, Paper No.91-3., th Annual Meeting and Exhibition, Air & Waste Management. Association, Vancouver, B.C., June 1-1. [] US EPA. (199), Handbook: Operation and Maintenance of Hospital Medical Waste Incinerators EPA/5/-9/, Office of Air Quality Planning and Standards Research Triangle Park, NC. [7] Koufopanos C.A. et.al. (199), Kinetic Modelling of The Pyrolysis of Biomass and Biomass Components, The Cananian Journal of Chemical Engineering, Vol.7, pp 75-. [] Jinno, D., Gupta, A.K and Yoshikawa K. (), Thermal Decomposition Characteristics of Several Key Components in Solid Wastes, Proceeding of Incineration and Thermal Treatment, New Orleans. 3