Analysis of Medical Waste Incinerator Performance Based on Fuel Consumption and Cycle Times ()
1. Introduction
This paper gives more insight to hospitals on the medical waste incineration process, and its major challenges, in particular fluctuations in fuel consumption and incineration cycle time. Medical wastes incinerated were segregated in two categories: sharps waste (containing needles, syringe and surgical blades). The other waste category included all waste such as pathological waste (human tissues, organs and body fluid), pharmaceutical waste (drugs, vaccines spoiled or expired), and chemical waste (detergents, dressing solutions), grouped together.
2. Literature Review
2.1. Incineration Background
This study involved assessment of the medical waste incineration which is a thermal waste treatment process that involves the combustion of organic substances contained in the waste materials at higher temperatures (850˚C - 1000˚C), a process which takes too long if the incinerator is not well designed and operated. The incineration process detoxifies medical waste by destroying most of the organic compounds contained in it and reduces the volume and weight of the waste leading to inert residual of solids, with an appreciable amount of fuel being consumed.
Incineration of medical waste converts the waste into essentially non-combustible solid residue or ash [1-4]. Other outputs include flue gas and heat. The ash is mostly formed by the inorganic constituents of the waste, and may take the form of solid lumps or particulates that can also be carried by the flue gas [1,2,4]. Thus, the flue gases must be cleaned of gaseous and particulate pollutants before they are dispersed into the atmosphere. In some cases, the heat generated by incineration can be recovered and used to generate hot water or electric power. The need to convert heat into power is critical in order to off-set fuel consumption costs. Waste incineration with energy recovery is one of several waste-to-energy (WtE) technologies such as gasification, pyrolysis and anaerobic digestion. For environmental protection purposes, incineration may also be implemented without energy recovery [5,6].
The main advantage of incineration over all other methods is the volume reduction, which is important in cities where space is scarce and landfill plots are not available. Incinerators reduce the solid mass of the original waste by 80% - 85% [7,8] and the volume (already compressed somewhat in garbage trucks) by 95% - 96%, depending on composition and degree of recovery of materials such as metals from the ash for recycling. This means that while incineration does not completely replace landfilling, it significantly reduces the necessary volume for disposal [9]. Incineration reduces the volume considerably but does not completely solve the problem because the ash that remains after the process must still be landfilled.
The main disadvantage of incineration is that emissions released into the environment are harmful [10]. Dioxins and furans, for example, are released through the incinerator stack and are carcinogenic. Trace metals are also released and these can cause respiratory problems [11]. These emissions, however, can be reduced to minimum through the use of baghouses and scrubbers when used as air pollution control devices [8,12,13]. The use of a scrubber can reduce the dioxin and furan emissions by a total of 86% [14].
A high level of technical competence is required in designing, operating and monitoring of any incineration facility in order to minimize fuel consumption and shorten the incineration cycle time [2]. This is because an incineration facility is an integrated activity involving a number of process operations (feed reception, control and preparation—actual combustion stage—treatment of combustion products, waste gases and residues). The options available within these process operations can be combined in various ways to meet the technical needs of a wide range of circumstances1. The major problem is, however, the changes in the incineration or combustion duration and lower temperature in the chambers, which causes continuous fuel flow into the burners. The heat loss due to excessive air supply lowers the temperature, also leading to excessive fuel consumption.
Exhaust gas temperatures from the incineration systems are typically as high as 1100˚C [10]. At these temperatures, most of the operating costs can be related to fuel costs. Waste heat recovery represents one method for off-setting the operating costs. A properly designed temperature control system is required so that the fuel supply is stopped once the pre-selected and set temperature is reached. The fuel is used up only when the temperature drops below this value. This provides a saving in fuel consumption in a long run, despite the high costs related to the installation of the control system. In any case, the total operating cost is high due to fuel [6], electricity, labour, maintenance and supervision. The major incinerator requirements are often referred to as the 3Ts: the Temperature must be high enough, there must be enough Turbulence in the combustion gas mixture (provided by use of electric blowers) and it must be held at these conditions for a long enough Time [15]. The time for incineration has a great impact on the incineration efficiency and fuel consumption, which forms a major part of the study presented in this paper. The time required to complete one combustion cycle is studied in details in this paper as cycle time. Although the cycle time can include waste loading and ash removal, these two operations has no significant effect on fuel and power consumption and hence are not strongly related to operating costs of the medical waste incinerator.
2.2. Fuel Consumption during Incineration
Another disadvantage of incinerators is excessive fuel consumption when there is no proper control of temperature and incineration cycle time. High fuel consumption occurs when the operator is trying to burn extremely moist waste, or when too much air is added to the system [6]. It should be noted that water must be evaporated from the wet waste before volatilization can occur. Since heat is not released from the waste until it starts to volatilize, the auxiliary burner must supply the extra energy needed, leading to high fuel consumption. To reduce fuel consumption, the high moisture waste must be limited in any particular load. If the combustion chambers have leaks, excess air will be introduced to the incinerator and increase fuel consumption. Air could enter the incinerator through doors that have become warped due to overheating, or through deformed seals or holes in the incinerator chamber, piping or connections due to corrosion. If excess air is introduced in the primary chamber, the volatile gases will be partially burned in the primary chamber and will not be available to heat the secondary chamber. If excess air enters the secondary chamber, the temperatures will drop and the burner will operate for longer periods even when the temperature control system is well functioning [16].
2.3. Incineration Cycle Time
The capacity of an incineration process should be calculated based on overall cycle time for the process to complete all the necessary operations. The cycle time cannot include delays introduced by the operator. Only legitimate process steps constrained by the equipment and techniques employed to operate the process are valid in an assessment of cycle time. The total mass charged during the period is divided by the total cycle time, and the resultant number gives the operating rate or incinerator capacity in kg/h. It is this number which should then be compared with any capacity [17]. In this study, however, the capacity was established based on combustion times only. Thus, the incinerator capacity defined in this study can also be regarded as the combustion rate for the medical waste.
When the incinerator is charged with the appropriate mix and quantity of waste, the operator should close the door, ensure all interlocks are engaged, and start the burn cycle. The burn cycle should not be interrupted by opening the charging door until after the burn is complete and the unit has cooled down. No additional waste should be added to the primary chamber unless the incinerator is equipped with an appropriate ram feed device.
In this study, the incineration cycle time is defined as the time taken from the start to the end of combustion (when the chamber has been cooled to about 250˚C - 300˚C). The rate of combustion can be slowed by reducing the quantity of under-fired air. Moreover, the rate of combustion depends on the type of waste charged and composition. In this study, sharps waste and other medical waste were categorized to study the effect of waste composition on the cycle times and diesel oil consumption.
2.4. Loading Rate for Incinerators
The waste can be loaded into the incinerator as batches or in a continuous manner. For facilities incinerating more than 26 tons of waste per year, dual chamber controlled air incinerators are the recommended configuration. To establish the quantity of waste incinerated (defined as incinerator capacity), the basic measurement for every incinerator site must be the quantity of waste charged to the incinerator during the year. Because the incinerator is limited to a fixed quantity of waste on every charge, each load should be recorded separately, and the quantities totaled for the year, and preferably weekly and monthly. Such data will also assist the owner in determining waste generation rates in the specific health facility. Incineration plants can be in operation 24 hours a day which allows for increased net waste disposal per day. In hospitals, however, most of the incinerators operate for few hours per day, due low waste generation. However, fuel consumption during hours of operation must be examined. Meanwhile, the duration of combustion could also be minimized to reduce fuel consumption by proper control of the 3Ts [10].
2.5. Medical Waste Incineration Process Studied
For the medical waste incinerator studied, the waste is fed in batches into the incinerator primary chamber, where flames around 950˚C burn the waste in multiple stages [15], as summarized in Table 1. As the waste is burned, ash is produced which is collected for later disposal in a landfill. These systems are capable of incinerating a wide range of wastes and, when properly maintained and op-
Table 1. Design features of the batch medical waste incinerator studied.
erated, will achieve emissions of PCDD/F and mercury below the level of most national and international standards. This system is equipped with an after burner connected to the chimney and sized to provide a residence time of at least one second at a temperature higher than 900˚C, to ensure complete combustion and minimize PCDD/F emissions [16]. The primary chamber, however, should be operated in the temperature range specified by the manufacturer (typically 500˚C to 800˚C).
3. Materials and Methods
3.1. Design Features of the Assessed Incinerator
The old Temeke hospital incinerator was a simple facility comprising of a cylindrical combustion chamber with an opening for waste feeding and a chimney for smoke outlet, as shown in Figure 1. The secondary burner (or afterburner) is connected to the chimney, which acts as a secondary chamber. The waste is loaded on the grate and ashes are collected below the grate. Such units are not suitable in terms of combustion efficiency and environmental acceptance. The first chamber performs pyrolytic destruction of the waste and final combustion of gases takes place in the secondary chamber. This incinerator was designed mainly for destroying placentas from labor wards, but due to scarcity of incineration facilities, it was used for destroying all medical waste generated in a district hospital, which implies that the incinerator was being overloaded.
The fuel consumption was alarmingly high, about 20 - 40 L/h, which necessitated investigation. The maximum temperature was about 700˚C only in the secondary chamber, but the primary chamber temperature was only about 400˚C - 500˚C. The incinerator had the capacity to burn only about 10 placentas per day, but was usually loaded from 55 to 214 kg of mixed medical waste due to lack of incineration facilities. Lack of air pollution control device made the whole equipment less useful. The
Figure 1. Design features of the assessed incinerator.
incinerator was located in densely populated area, which made the smoke problems to be a continuous nuisance to the nearby community. Other incinerator problem in relation to its design was fluid leakage (blood) from the drying placentas in the primary chamber onto the floor which caused aesthetic view and odorous environment.
3.2. Experimental Model Formulation
Figure 1 shows the process flow sheet for the double chamber incinerator used in this study. In this model, the input parameters studied include: Sharps waste loaded in primary chamber, Sw(t); Other waste loaded in primary chamber, Ow(t); Total diesel oil consumed per cycle by the two burners, Do(t), and the incineration cycle time, Tc(t), where t denotes time in days. The quantity of air through the burners (stoichiometric air) and blower (excess air) were not determined. The multi-variable time series recorded for N = 654 days, can be expressed as per Equation (1), in which, each parameter was recorded separately: