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Thermal utilization (treatment) of plastic waste
Energy xxx (2015) 1e10
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Thermal utilization (treatment) of plastic waste Janusz Wojciech Bujak* skiego 6, 85-950 Bydgoszcz, Poland Polish Association of Sanitary Engineers, Division Bydgoszcz, Rumin
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2015 Received in revised form 26 June 2015 Accepted 27 June 2015 Available online xxx
This paper presents the results of a pilot study of a thermal utilization installation for incinerating plastic waste. The research was conducted on an industrial scale in a plant that manufactures plastic tape (used for warning, packing and masking purposes, among others). The system was considered in terms of three aspects: energy, environmental and economic. Due to the very high LCV (lower calorific value) of the waste, an innovative rotary combustion chamber (rotary kiln) was employed. The experimental installation was analysed in terms of the temperature distribution in the rotary kiln, secondary combustion (afterburner) chamber and heat recovery system. The thermal efficiency of the tested installation was determined. The emissions into the atmosphere were measured and compared with the applicable emission standards. Due to the nature of the waste, particular attention was paid to emission analysis of carbon oxide and volatile organic compounds. In terms of the economic aspect, fundamental economic indicators were found for the tested system to determine the profitability of its construction. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Plastic waste Rotary kiln Thermal treatment (incineration) of waste Air pollution Waste management
1. Introduction Waste management, i.e., the collection, transport, recovery and neutralization of waste, as well as the supervision of these activities and the locations where the waste is neutralized, are among the most important issues in terms of environmental protection. The importance of waste management continues to grow due to the progress of civilization and the increase in waste quantity [1]. Plastics are hailed as ‘‘one of the greatest innovations of the millennium’’ [2], and this reputation has certainly proven to be true. The facts that plastic does not undergo rotting or rusting processes, is lightweight, inexpensive and reusable give many perspectives for its future. In addition, using plastic materials contributes to saving energy and CO2 emissions. It is assumed that plastics replaced with other alternative materials require 22.4 million extra tons of crude oil when we take into consideration a life cycle perspective for using plastics. The total global production of plastics increased from approximately 1.3 MT (million tons) in 1950 to 245 MT in 2006. One of the most important elements of waste management is the thermal treatment process. This process involves the oxidation of waste through incineration, gasification or decomposition (such as pyrolysis). The main advantages of this type of process are as follows:
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- the possibility of converting waste into safe forms; - significant reduction in the weight and volume of waste; - the possibility of recovery of substantial amounts of heat. The incineration of plastic waste is highly beneficial because of its high LCV (lower calorific value). Table 1 [3] shows that the LCV of polyethylene is the same as the LCV of diesel fuel. In the case of waste with a high LCV, such as plastic waste, the process of thermal disposal involves gasification or pyrolysis. These processes [4] and their detailed reaction mechanisms and simulation models, including the elimination of redundant species and reactions, rate of products and reaction flux analysis and uncertainty analysis have been reported [5e17]. The common goal of the aforementioned waste incineration processes is to utilize the resulting energy in a way that is both simple and minimally burdensome for the environment. Heat recovery from such systems is achieved through special technological systems. Holmgren [18] described a heating system that uses waste heat supplied from various sources, using the city of Gothenburg € teborg) in Sweden as an example. He presented the results of (Go research on the use of waste heat from industrial plants and waste incinerators equipped with recovery steam boilers. He showed that heating networks can utilize waste heat that would otherwise have limited use. Mori, Kikegawa and Uchida [19] presented a waste heat recovery model for a waste incineration plant in Tokyo. Application of the model resulted in a reduction of carbon dioxide emissions by approximately 8%.
http://dx.doi.org/10.1016/j.energy.2015.06.106 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
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Table 1 LCV of plastics compared with conventional fuels. Item
LCV (MJ kg1)
Methane Gasoline Fuel oil Coal Polyethylene Mixed plastics Municipal solid waste
53 46 43 30 43 30e40 10
Additionally, Dean [20] described heat recovery from medical waste incineration systems at hospitals. He discussed the possibility of producing steam using waste heat from the incineration of medical waste. He showed that using a heat recovery system of that type significantly reduced the annual operating costs of the hospital boiler room. Bujak [21] presented experimental studies of the useful energy flux and thermal efficiency coefficient of an installation for utilizing thermal waste with heat recovery. The system consisted of a unit for preparing and loading waste, a combustion chamber, a thermoreactor chamber and a recovery steam boiler. The basic fuel included hospital waste, and natural gas was used as extra fuel. The average flux of waste incinerated was 100 kg/h. Heat recovery in the form of a usable energy flux included in the saturated steam was in the range of 600e800 kW (1000e1200 kg/h of saturated steam) at a load higher than 4 > 65%. The ratio of thermal efficiency oscillated between 56 and 62% depending on the load of the incinerator. The stream of useful energy and the energy efficiency ratio increased with increasing incineration load. This paper presents the results of a pilot study of a thermal utilization installation for incinerating plastic waste. The research was conducted on an industrial scale in a plant that manufactures plastic tape (used for warning, packing and masking purposes, among others). A rotating kiln of a special and untypical structure has been designed and constructed. Due to the very high capacity value of wastes (36 MJ/kg) and low content of non-flammable parts (minerals), the following length-to-diameter ratio was taken e 2:1. For the analysed case, the length of the rotating kiln was 4 m, and its diameter e 2 m. Typical rotating kilns have several-time greater lengths in relation to their diameter. For example, cement kilns have approx. 45 m in length and 3 m in diameter (15:1). A standard ratio for rotating kilns burning hazardous or medical wastes is no less than 4:1. The testing plant was tested for compliance with the requirements of the directive [22] on permissible emissions to the atmosphere, as well as for its thermal efficiency. Obtaining positive results and their use in practice may bring significant economic benefits. Implementation of such systems may be considerably more cost effective than typical solutions. A short rotating kiln means much lower cost of its construction and, in particular, lower cost in relation to the rooms where it is located. During the tests, also the maximum content of unoxidized organic compounds in slag and bottom ashes was controlled. It is of particular importance when a rotating kiln is short e a short pathway for burned components.
2. Management of plastic waste before and after system upgrade Fig. 1 shows a diagram of heat management in the plant before the system was upgraded. The plant used the heat for technological purposes, heating, ventilation and hot water.
Due to the nature of plastic tape production, heat is supplied in the form of thermal oil with parameters ranging from 230 C to 280 C and returns with temperatures from 210 C to 260 C. The primary and largest consumers of heat are the technological devices manufacturing the plastic tape. Table 2 shows the thermal power of the production equipment. Oil was heated in a local plant boiler fired with natural gas with a thermal power of Q ¼ 2000 kW. The plastic waste was exported to regional waste disposal facilities. Fig. 2 shows the thermal management structure after the system was upgraded. The upgrade included the design and construction of a special plastic waste incineration system based on a rotary kiln with co-current combustion. In typical incinerating installations that burn hazardous and medical waste, the aspect ratio between the length and the diameter of the rotary kiln is minimum 4:1. The experimental system under study uses a unique experimental facility construction of the combustion chamber (rotary kiln). Due to the very high LCV of the waste, the accepted aspect ratio was 2:1. (length of 4 m and diameter of 2 m). The heat recovery system was equipped with a special oil-tube boiler. Due to the proximity of the technological production lines and the incinerator, waste transport was reduced to the level of interdepartmental transfer. Simultaneously, the time between the creation of waste and its neutralization was reduced to a minimum. The system complies with the mandates imposed on the waste incineration process. The temperature in the secondary combustion (afterburner) chamber is continuously maintained above 1100 C. The required flue gas retention time in the secondary combustion (afterburner) chamber is 2 s. Due to the homogeneity of the postproduction waste, maintaining proper parameters for the combustion process is simplified. The maintenance of said parameters is also beneficial in terms of maintaining the effectiveness of the flue gas treatment system, thus complying with the standards concerning harmful emissions into the atmosphere. The advantage of the on-site facility location provides the possibility of a highly efficient use of the heat generated by the waste incineration process. Thermal oil heated in the oil-tube boiler is sent via pipeline to the processing and ventilation equipment or to heat exchangers. Thus, the local natural gas-fired boiler room can be eliminated. After the upgrade, that room serves only as a backup heat source. The value of this decreased (reduced) natural gas flux can be converted into a flux of pollutant and carbon dioxide emissions to the atmosphere. Thus, we obtain the effect of the socalled “avoided emission.” An additional advantage of the on-site location of the thermal treatment system of post-production waste is the elimination of export to external (regional) waste disposal facilities/contractors, which reduces the consumption of large quantities of diesel fuel or gas (petrol) for external transport. Therefore, the effect of “avoided emission” is not included in this article. 3. Description of technology and measurement system 3.1. Thermal recycling technology The investigated system of plastic waste incineration (Fig. 3) consists of the following elements: A. Loading system e plastic waste generated during production is delivered to the incinerator building. In the recycling plant waste is crushed/shredded and compressed. In such form, waste is then loaded in large containers with a capacity of 370 or 770 L. The container with its contents is automatically weighed and recorded by the computer system. It is then raised by the elevator above the loading chamber and completely emptied.
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Fig. 1. Waste management before plant upgrade.
Table 2 Summary of technological equipment. Machinery/device type
Thermal power [kW]
Temperature of thermal oil [0C]
Impregnator Coater (acrylic) Hot melt
1,000 400 600
260 220 140
When the loading chamber is full, horizontal hydraulic cylinder pushes the waste into the rotary kiln. The average frequency of loading is every 15 min, with the average weight of a load of 50 kg. B. Rotary kiln e built with a 2% incline sloping in the direction of the secondary combustion (afterburner) chamber where incineration of plastic waste takes place. The duration of the
incineration process depends on the LCV and the moisture content of the waste. Due to the very high LCV of the waste, a large diameter of the combustion chamber in relation to its length was used (diameter of 2 m and length of 4 m). As shown in Photo 1, the rotary kiln is short but has a large diameter. C. Secondary combustion (afterburner) chamber e where gases produced during the incineration process in the rotary kiln are afterburnt. This process is conducted at temperatures from 1100 C to 1200 C, and the minimum retention time of incinerated gases must not be less than 2 s. D. Recovery boiler e after leaving the reactor, flue gas at temperatures from 1100 to 1200 C heat the thermal oil to a temperature of approximately 280 C. With special pumps, thermal oil is sent to the processing equipment. The flue gases flowing through the heat recovery system are cooled to temperatures between 265 C and 280 C.
Fig. 2. Waste management after plant upgrade.
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Fig. 3. Schematic of the waste recycling system with heat recovery.
E. Flue gas cleaning system e multisectional bag filter to remove dust. In addition, this system is equipped with metering devices for dosing sorbent and urea.
3.2. Energy balance of the system The energy flows through the incinerator with a heat recovery boiler as previously described [21]. The final equation (1) is as follows:
E_ pw þ E_ awcch þ E_ ngcch þ E_ angcch þ E_ ngach þ E_ angach þ E_ agach X ¼ E_ ue þ E_ el (1)
where: E_ pw e chemical enthalpy flux of plastic waste (kW), E_ awcch e physical enthalpy flux of air used for waste incineration in the combustion chamber (kW), E_ ngcch e chemical enthalpy flux of natural gas supplied to the combustion chamber (kW), E_ angcch e physical enthalpy flux of air used for natural gas combustion in the combustion chamber (kW), E_ ngach e chemical enthalpy flux of natural gas supplied to the afterburner chamber (kW), E_ angach e physical enthalpy flux of air used for natural gas combustion in the afterburner chamber (kW), E_ agach e physical enthalpy flux of air used for afterburning gas combustion in the afterburner chamber (kW),
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Photo 1. Rotary kiln.
E_ ue e usable (effective) energy flux (kW), P ðE_ el Þ e total loss of energy fluxes (kW). The fluxes of energy supplied to the thermal plastic waste recycling system are shown on the left hand side of equation (1). As assumed in the design, the main flux of energy supplied to the system consists of plastic waste (E_ pw). The LCV of the supplied waste is so high that the incineration is stable. Therefore, under the circumstances, there is no need for additional fuel (natural gas). The burning process is autothermal. The values of the physical enthalpies of air fluxes supplied to the combustion chambers (E_ amw_ cch) and the secondary (aftercombustion) chamber (E ag-dch) change depending on the amount of air required for the correct incineration of waste, the aftercombustion of generated gases and the
ε¼
3.3. Energy efficiency coefficient The energy efficiency of a system for the thermal recycling of plastic waste can be determined using two methods: indirect or direct. The indirect method involves determining all components responsible for energy losses and is frequently performed via calculations (analytical method). The direct method involves measuring the energy flux supplied to the system and the heat flux transferred to the steam receivers (usable/effective heat). The energy losses constitute the difference between the energy flux supplied to the system and the effective heat produced by the system. This paper describes the energy efficiency coefficient by the direct method. The coefficient of energy efficiency is defined as follows:
E_ ue _ _ _ _ Epw þ Eamwcch þ Engcch þ Eangcch þ E_ ngdch þ E_ angdch þ E_ agdch
temperature of the environment from which the air is taken. It is assumed that the supply of additional fluxes of chemical enthalpy in the form of additional fuel (E_ ng-cch) and (E_ ng-dch) is necessary only in the system start-up period. The air enthalpy fluxes (E_ ang-cch, E_ angused for natural gas burning (supplied to the burners) depend on the temperature of the environment from which the air is taken and on the amount required for correct gas burning. The energy fluxes carried away from the waste recycling system are shown on the right hand side of equation (1). The use of a recovery boiler within the processing system allows the main energy flux carried away from the system to be the usable (effective) energy flux (E_ ue). Other fluxes of energy carried away from the system are the total energy losses S(E_ el), consisting of the losses of heat fluxes through the external walls of combustion chambers, after combustion chambers, concrete ducts and recovery boiler, as well as energy fluxes in the form of the physical and chemical enthalpy of ash.
(2)
3.4. Description of the measurement system The measurement system consists of three essential components (Fig. 4):
dch)
a) flux measurement of the following: ‒the chemical enthalpy of additional fuel fed into the rotary kiln and the afterburner chamber e using a GM flow meter manufactured by Vortex, with a measuring accuracy of 1.25%, ‒the mass of plastic waste, measured by electronic scales with 0.5% accuracy (SC1), b) temperature measurement using sensors with a measuring accuracy of 2.0% for the following: ‒ flue gas temperature at the beginning of (near the loading of waste) the rotary kiln (T1), ‒ flue gas temperature at the end of the rotary kiln (at the connection with the afterburner chamber) (T2),
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Fig. 4. Measurement system of the investigated installation.
c) ▪
▪
▪
‒ flue gas temperature at the end of the afterburner chamber (T3), ‒ flue gas temperature before the recovery boiler (T4), ‒ flue gas temperature after the recovery boiler (T5), ‒ inlet thermal oil temperature of the recovery boiler (T6), ‒ outlet thermal oil temperature of the recovery boiler (T7), continuous flue gas monitoring consisting of the following: measuring component: ‒ system of collection and transport of flue gas samples, ‒ system of dust measurement and the reference parameters (static pressure, temperature and flue gas velocity) necessary to perform calculations, ‒ measuring panel with analysers, treatment and computing component: ‒ measurement data concentrator processing data from the analysers and sensors while converting it from analogue to digital format, ‒ emission computer implementing the acquisition, archiving, verification and presentation of measurement data and the creation of graphs and generation of reports, auxiliary component: - panel with technical gases necessary for continuous system calibration.
The concentrations of pollutants emitted into the atmosphere were measured using the following methods: - FTe IR measuring method e based on the ability of polyatomic gas particles to absorb infrared radiation; the performed quantitative analysis involved the measurement of CO, SO2, NO, HCl, HF and H2O, - FID measurement method - based on flame-ionization detection; the analysis method was used to measure the concentration of total hydrocarbons, - measurement method based on a zirconia sensor, measuring the oxygen concentration in gases with a high content of combustible compounds and impurities resulting from high temperature (for example, in the flue gases); the zirconia probe consisted of a heated measuring cell, an electronic controller and a pneumatic unit supplying reference air.
The parameters measured by the system for the continuous monitoring of flue gas and their ranges and measurement errors are shown in Table 3.
4. Test results 4.1. Test results for thermal waste recycling process Natural gas (additional fuel) was burned only during the startup the system, in order to reach a proper temperature in the rotating kiln and the afterburner chamber. This phase of the process system operation was not tested. Combustion of plastic waste without using natural gas was initiated after achieving the required temperatures. During the testing period (steady state) only the waste was incinerated. Fig. 5 shows the test results of the flue gas temperatures in a rotating combustion chamber. The flue gas temperature at the beginning of the chamber (T1) changed slightly over the study period. It fluctuated within the range of 922 C and 1011 C, with an average value of 985 C. The temperature at the end of the rotary kiln (T2) was slightly higher and fluctuated within the range of 993 C and 1048 C. The average value measured during the test
Table 3 Values measured by the system for continuous monitoring of flue gases and their ranges and measurement errors. Measured parameters
Measuring range
Measuring error (%)
Total dust, mg/m3n Total organic carbon, mg/m3n Chloride, mg/m3n Hydrogen fluoride, mg/m3n Sulphur dioxide, mg/m3n Carbon monoxide, mg/m3n Nitric oxide, mg/m3n Carbon dioxide, % Oxygen, % Humidity, % Flue gas flux, m3/h Flue gas temperature, C Static pressure, mbar
0e100 0e160 0e100 0e10 0e2000 0e700 0e2000 0e20 0e25 0e30 0e10,000 0e2000 0e1600
2.0 1.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 0.25
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T e m p e r a t u r e o f f l u e g a s e s [ 0C ]
1400
1300 1200 1100 1000 900 800 700
T1 - at the beginning of the rotary kiln
600
T2 - at the end of the rotary kiln T3 - after the discharge chamber
500
400
Period of time Fig. 5. Flue gas temperatures at the beginning and end of the rotary kiln and behind the discharge chamber.
was 1029 C. The temperature differences at the beginning and end of the rotary chamber were not significant. It is worth noting that very high temperatures occurred (T1) even in the front of the rotary kiln. The occurrence of these temperatures was due to the high LCV of plastic waste. The high temperature (T1) also demonstrates rapid combustion practically from the time of loading the waste into the rotary kiln. Nevertheless, the combustion process was quite stable due to the specially selected length and diameter ratios of the rotary combustion chamber. The stability of the combustion process is also proven by the highly uniform temperature (T1 to T2) over the entire length of the rotary kiln. The flue gas temperature behind the secondary combustion (afterburner) chamber (T3) fell within the range of 1030 C and 1384 C. However, the periods in which the temperature (T3) fell below 1100 C were very rare. During standard operation of this type of system, an auxiliary burner would turn on to maintain the minimum temperature above 1100 C. During the testing period, the auxiliary burner was turned off. Despite the lack of auxiliary
burner operation, the flue gas temperature in the discharge chamber was increasing. The reason was the air supply to the afterburner chamber, which initiated the combustion of yet unburnt gas in the rotating combustion chamber. Thus, a waste gasification process occurred in the rotary kiln. When analysing the temperature measurements in thermal waste processing, an autothermal combustion process (without additional fuel) was achieved in both chambers. Fig. 6 shows the actual concentrations of CO (carbon monoxide) and volatile organic compounds, expressed as TOC (total organic carbon), in the outgoing flue gas. Devices to sample the flue gas and to measure the mass flux of dust contained therein were located on the exhaust chimney. To conduct a detailed analysis, the above mentioned compounds were selected due to the problems in complying with emission standards during the burning of plastic waste. In the study period, the concentration of carbon monoxide in the flue gas ranged from 5 to 15 mg/m3n, with an average value of 11.2 mg/m3n. No significant peaks occurred, even during the
50
Emission
[ mg / m3 n ]
45
CO - carbon oxides
40
TOC - total organic carbon
35 30 25 20 15 10 5 0
Period of time Fig. 6. The concentration of total organic carbon and carbon monoxide in the flue gas at 11% oxygen, a temperature of 273.15 K and a pressure of 101.3 kPa (average one-minute concentration).
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Table 4 Existing standards and actual emissions to the atmosphere of the test technology at 11% oxygen, a temperature of 273.15 K and a pressure of 101.3 kPa (average daily emission limit). Type of compounds measured 3
Total dust, mg/m n Total organic carbon, mg/m3n Chloride, mg/m3n Hydrogen fluoride, mg/m3n Sulphur dioxide, mg/m3n Carbon monoxide, mg/m3n Nitric dioxide, mg/m3n
Emission standards
Actual emission
10.0 10.0 10.0 1.0 50.0 50.0 200.0
7.1 8.5 4.9 0.3 11.3 12.7 165.0
loading of waste. It should be noted that the loading of waste was performed using a lift. This loading method is called a periodic loading method. The average frequency of loading was every 15 min, the average weight of a load was 50 kg, and the average flux of waste incinerated was 200 kg/h. The concentration of TOC (total organic carbon) in the flue gas fluctuated between 5 and 10 mg/m3n, except for a few peaks with values reaching up to 50 mg/m3n. The peaks occurred during a few waste loading processes. Characteristically, they occurred during the same shift. This observation shows that the experience of workers operating such systems also affects the quality of combustion and emissions, especially when loading the waste. It should be emphasized that all the 30-min concentration averages were below the limit. The average concentration of TOC in the flue gases during the study period reached a value of 8.8 mg/m3n. Table 4 shows the existing standards and the actual emissions to the atmosphere resulting from plastic waste incineration over the study period. During the tests, also the maximum content of unoxidized organic compounds in slag and bottom ashes was controlled. The total content of organic coal did not exceed 3.0%, however it was close to the maximum value and amounted to 2.9%. The above results were obtained at the following rotary speed of the kiln e 2 rotations per hour. At greater speeds the slag was not totally burned e it contained more than 3% of coal. The average mass flux of slag produced during the test was 9.6 kg/h. It constituted 4.8% of the plastic waste charge. The graph in Fig. 7 shows that the flue gas temperatures when entering (T4) and leaving (T5) the thermal oil boiler are very different. The mean temperature difference measured in the study
period was 870.9 C. The exhaust gas temperature at the entrance to the thermal oil boiler during the study varied in the range of 907.7.0 to 1263.3 C, yielding an average value of 1156.1 C. The flue gas temperature at the outlet of the thermal oil unit fluctuated in the range of 265.2 C and 336.7 C, with an average value of 285.2 C. The average value measured during the test was 1029 C. As observed in Fig. 7, the temperature of the flue gas leaving the oil boiler gradually increased with time of operation. At the beginning of the test period, when the heat exchange surfaces were clean, the output temperature of the flue gas (T5) ranged from 265 C to 280 C. With the progressive pollution of the heating surfaces, the flue gas temperature increased. At the end of the test period, it reached a value in excess of 300 C. After exceeding the maximum value of T5max ¼ 350 C, the heat recovery boiler was cleaned. This situation occurred after the end of the study. The relatively high flue gas temperatures at the outlet of the thermal oil boiler resulted from the required parameters of the secondary heat carrier, namely heating oil. Due to the nature of plastic tape manufacturing, the required thermal oil temperatures at the outlet were dependent on technological needs. Fig. 8 shows the temperatures of the heating oil at the supply inlet and the return outlet from the processing systems, as measured during the study period. In analysing the operation of the oil boiler, we can see that the starting temperature of the oil (T7) varied in the range of 250.2 Ce290.4 C, with an average value of 272.4 C. The temperature of the heating oil returning from the processing equipment (T6) fluctuated in the range of 211.1 Ce268.4 C. The average value obtained in the test period was 249.5 C. The use of the heat recovery module resulted in the recovery of useful energy in the form of hot thermal oil (Table 5). The average flux of this energy reached E_ ue ¼ 1269.6 kW, and the thermal efficiency of the entire system (part of the combustion and heat recovery) was ƞ ¼ 65.2%. Using waste heat to satisfy heat needs reduces the consumption of fossil fuels burned. To illustrate directly, the use of the heat produced in the local plant boiler room was reduced by the amount of heat received from the recovery system. The value of the reduced heat flux can be converted to the classic fuel flux. The calculation accounts for the average flux of useful energy produced in the form of hot oil, E_ ue ¼ 1269.6 kW. The plastic tape plant that is the object
Fig. 7. The temperature of flue gases at the inlet and outlet of the thermal oil boiler.
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Fig. 8. The temperature of heating oil at the inlet and outlet of the thermal oil boiler.
Table 5 Thermal efficiency of the studied system.
Table 7 Input data e technical indicators.
Description
Value
Technical analysis module
The average chemical enthalpy flux in waste, kW The average flux of useful energy e thermal oil, kW The energy efficiency of the system, %
1947.2 1269.6 65.2
Average hourly mass flux of incinerated plastic waste, kg/h Average system load, % Average hourly flux of generated heat, kW Average hourly flux of additional fuel for combustion chamber, m3n/h Average hourly flux of additional fuel for afterburner chamber, m3n/h Average LCV of plastic waste, MJ/kg Additional fuel type e natural gas
of this study had its own boiler fired with natural gas. The required production of thermal hot oil in the boiler room was correspondingly reduced. The subsequent reduction in the natural gas flux consumed was 149.4 m3n/h. (Table 6). On the other hand, incinerated plastic waste generated during production belongs to the group of contaminated or mixed plastics. Examples here may be various types of plastics and multi-material plastics contaminated with various kinds of adhesives. For this type of waste, energy inputs used to separate the pure polymer or monomer (chemical recycling) significantly exceed its calorific value. When burning this type of waste (energy recycling), we achieve the effect of avoided emissions in relation to chemical recycling also known as raw material recycling. Table 6 shows the reduction of greenhouse gas emissions as a result of energy recycling (incineration with heat recovery) in the examined system. In the analysed period, CO2 emissions were reduced by 298.7 kg/ h. Emissions of SO2, CO, NOx and dust were omitted. The effect of avoided emissions due to the elimination of transport to external facilities processing plastic waste (chemical recycling) was not included in Table 6.
200.0 100.0 1269.6 0.0 1.6 36.0 NG-50
specific solution, even if justified from a technical perspective, also yields economic benefits, considering macroeconomic, social and environmental conditions. Analysis of the economic viability of any venture should demonstrate whether the net financial result of implementing the investment will be sufficiently high and competitive when compared to other investment opportunities. Table 7 shows the basic input in the form of technical indicators. The capacity and performance of the plastic waste incineration, the thermal power recovery system and other technical parameters were determined based on the conducted research. Next, Table 8 provides basic input associated with economic aspects. In particular, these aspects define the capital and operating expenditures of the studied system. Table 9 shows the results of the investment profitability calculations. Economic analysis of this system showed that the SPB (simple payback period) of the capital expenditures incurred for its construction is 4.5 years. The NPV (net present value) of the project after taxes is greater than zero. In the case presented, after 15 years,
5. Economic aspects The primary objective of any business operation is its cost effectiveness. It is therefore necessary to determine whether a
Table 8 Input data e expenditures. Economic analysis module
Table 6 Reduction of greenhouse gas emissions. Description
Value
Flux of recovered energy (thermal oil), kW Reducing the volume flux of natural gas, m3n/h The reduction of CO2 emissions as a result of the recycling of energy, in kg/h
1269.6 149.4 298.7
CAPEX (capital expenditures), $ OPEX (operating expenditures), $ Price for recycling 1 kg of plastic waste, $ Price of 1 m3n of natural gas as additional fuel, $ Employment rate e number of people per 8 h Gross pay rate, $/person Repairs, $/year Other maintenance costs, $/year
3,000,000 651,000 0.2 0.6 2 3000 207,000 156,000
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Table 9 Calculations of profitability indicators. Profitability indicators Net present value (NPV), $ (for r ¼ 0, after 15 years) Simple payback (SPB), years Internal rate of return (IRR), %
2,647,196 4.5 16.1
its value is $ 2,647,196. The IRR (internal rate of return) of the project was 16.1%, which was higher than the expected 8.3%. Considering all of the above mentioned economic indicators, it is clear that the tested system is both profitable and attractive in terms of economic development. 6. Conclusions This paper presents the results of a pilot study of the installation of a thermal treatment (incineration) facility for plastic waste. The study was conducted in a plant that manufactures plastic tape (used for warning, packing and masking purposes, and others) The system was considered in terms of three aspects: energy, environmental and economic. In terms of energy, tests have shown the value of designing and building plastic waste incineration (thermal disposal) plants with heat recovery. This type of plant, in addition to performing its basic role (waste management), is also highly efficient in terms of heat production. The process of intense burning of wastes has already taken place at the beginning of the rotating kiln. The temperature of exhaust gases in this part of the kiln was very high, reaching in the testing period an average value of 985 C. Though the rotating kiln was short, it was possible to use its volume to ensure the maximum thermal performance. There was a stable and high temperature inside the cubic volume of the kiln. The use of a heat recovery module resulted in useful energy that was recovered in the form of thermal oil at a temperature of 280 C. The value of the energy flux averaged E_ ue ¼ 1269.6 kW, and the thermal efficiency of the entire system (incineration and heat recovery component) was h ¼ 65.2%. Despite high temperatures of exhaust gases at outlet of the recovery oil boiler resulting from the required parameters of heat oil, thermal efficiency of the whole system (burning and heat recovery part) was satisfactory and relatively high h ¼ 65.2%. Such good thermal efficiency was obtained owing to low losses of the heat flux through the external wall of the rotating kiln (short and wellinsulated) to the environment. At the same time, it was demonstrated that the assumed construction ratios of the rotating kiln guaranteed a proper and suitable level of thermal processing of wastes. At a sufficiently low rotary speed (2 rotations per hour), it was possible to obtain the total content of organic coal in slag and ashes below 3.0%. Analysis of the environmental aspect proved that the actual emissions to the atmosphere that resulted from the thermal treatment of plastic waste, in the experimental system, during the study period were lower than the current emission standards in force in the European Union. Due to the nature of the disposed waste, particular attention was paid to the emission analysis of carbon oxide and volatile organic compounds. In both cases, the average daily emission limit values fell below the limits. The same also applied to the average 30-min values. The use of the heat recovery system also resulted in the reduction of greenhouse gas emissions because the plant under study had its own boiler room fired with natural gas. The necessary hot thermal oil production by the plant boiler room was correspondingly reduced. The
consequent reduction of the natural gas flux consumed was 149.4 m3n/h. The reduction of CO2 emissions due to the use of energy recycling (incineration with heat recovery) was 298.7 kg/h. Emissions of SO2, CO, NOx and dust were omitted. In economic terms, economic fundamentals were set for the test system to determine the profitability of its construction. It has been demonstrated that the test system is both cost-effective and attractive from the economic perspective. The simple payback period (SPB) of capital expenditures for its implementation was 4.5 years. The NPV (net present value) of the project after taxes was greater than zero. Obtaining such a good economic result was possible due to, among others, low cost related to the construction of the room for the analysed technology. Owing to a small length of the rotating kiln, it was possible to reduce the room in size by 35%. The results prove that it is worth seeking optimum solutions, taking into account the specificity of burned wasted, which bring significant ecological, power and economic benefits.
References [1] Qu Y, Zhu O, Sarkis J, Geng Y, Zhong Y. A review of developing an e-wastes collection system in Dalian, China. J Clean Prod 2013;17:1175e82. [2] Panda AK, Singh RK, Mishra DK. Thermolysis of waste plastics to liquid fuel. A suitable method for plastic waste management and manufacture of value added productsda world prospective. Renew Sustain Energy Rev 2010;14: 233e48. [3] Scott G. Green polymers. Polym Degrad Stab 2000;68:1e7. [4] Miranda M, Cabrita I, Pinto F, Gulyurtlu I. Mixtures of rubber tyre and plastic wastes pyrolysis: a kinetic study. Energy 2013;58:270e82. [5] Leung DYC, Wang CL. Kinetic modeling of scrap tire pyrolysis. Energy Fuels 1999;13:421e7. [6] Chiarioni A, Reverberi AP, Fabiano B, Dovı VG. An improved model of an ASR pyrolysis reactor for energy recovery. Energy 2006;31:2460e8. [7] Mani M, Nagarajan G, Sampath S. Characterisation and effect of using waste plastic oil and diesel fuel blends in compression ignition engine. Energy 2011;36:212e9. [8] Li AM, Li XD, Li SQ, Ren Y, Shang N, Chi Y, et al. Experimental studies on municipal solid waste pyrolysis in a laboratory-scale rotary kiln. Energy 1999;24:209e18. [9] Wallman PH, Thorsness CB, Winter JD. Hydrogen production from wastes. Energy 1998;23:271e8. [10] Xianwen D, Xiuli Y, Chuangzhi W, Wennan Z, Ch Yong. Pyrolysis of waste tires in a circulating fluidized-bed reactor. Energy 2001;26:385e99. [11] Singhabhandhu A, Tezuka T. The waste-to-energy framework for integrated multi-waste utilization: waste cooking oil, waste lubricating oil, and waste plastics. Energy 2010;35:2544e51. [12] Mani M, Nagarajan G. Influence of injection timing on performance, emission and combustion characteristics of a DI diesel engine running on waste plastic oil. Energy 2009;34:1617e23. Tura cs T, Zse ly IG, Kramarics A nyi T. Kinetic analysis of mechanisms of [13] Kova complex pyrolytic reactions. J Anal Appl Pyrolysis 2007;79:252e8. [14] Conesa JA, Marcilla A, Font R, Caballero JA. Thermogravimetric studies on the thermal decomposition of polyethylene. J Anal Appl Pyrolysis 1996;36:1e15. [15] Lee JS, Kim SD. Gasification kinetics of waste tire-char with CO2 in a thermobalance reactor. Energy 1996;21:343e52. [16] Ranzi E, Dente M, Faravelli T, Bozzano G, Fabini S, Nava R, et al. Kinetic modeling of polyethylene and polypropylene thermal degradation. J Anal Appl Pyrolysis 1997;40e41:305e19. [17] Seungdo K, Kavitha D, Yu Tae U, Jung J, Song J, Lee S, et al. Using isothermal kinetic results to estimate kinetic triplet of pyrolysis reaction of polypropylene. J Anal Appl Pyrolysis 2008;81:100e5. [18] Holmgren K. Role of a district-heating network as a user of waste-heat supply €teborg. Appl Energy 2006;83:1351e67. from various sources e the case of Go [19] Mori Y, Kikegawa Y, Uchida H. A model for detailed evaluation of fossil-energy saving by utilizing unused but possible energy-sources on a city scale. Appl Energy 2007;84:921e35. [20] Dean K. Heat-recovery incinerator for a community hospital. ASHRAE J 1996;38:49e53. [21] Bujak J. Experimental study of the energy efficiency of an incinerator for medical waste. Appl Energy 2009;86:2386e93. [22] Directive 2010/75/EU of the European parliament and of the council of 24 November 2010 on industrial emissions (integrated pollution prevention and control).
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Energy journal homepage: www.elsevier.com/locate/energy
Thermal utilization (treatment) of plastic waste Janusz Wojciech Bujak* skiego 6, 85-950 Bydgoszcz, Poland Polish Association of Sanitary Engineers, Division Bydgoszcz, Rumin
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2015 Received in revised form 26 June 2015 Accepted 27 June 2015 Available online xxx
This paper presents the results of a pilot study of a thermal utilization installation for incinerating plastic waste. The research was conducted on an industrial scale in a plant that manufactures plastic tape (used for warning, packing and masking purposes, among others). The system was considered in terms of three aspects: energy, environmental and economic. Due to the very high LCV (lower calorific value) of the waste, an innovative rotary combustion chamber (rotary kiln) was employed. The experimental installation was analysed in terms of the temperature distribution in the rotary kiln, secondary combustion (afterburner) chamber and heat recovery system. The thermal efficiency of the tested installation was determined. The emissions into the atmosphere were measured and compared with the applicable emission standards. Due to the nature of the waste, particular attention was paid to emission analysis of carbon oxide and volatile organic compounds. In terms of the economic aspect, fundamental economic indicators were found for the tested system to determine the profitability of its construction. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Plastic waste Rotary kiln Thermal treatment (incineration) of waste Air pollution Waste management
1. Introduction Waste management, i.e., the collection, transport, recovery and neutralization of waste, as well as the supervision of these activities and the locations where the waste is neutralized, are among the most important issues in terms of environmental protection. The importance of waste management continues to grow due to the progress of civilization and the increase in waste quantity [1]. Plastics are hailed as ‘‘one of the greatest innovations of the millennium’’ [2], and this reputation has certainly proven to be true. The facts that plastic does not undergo rotting or rusting processes, is lightweight, inexpensive and reusable give many perspectives for its future. In addition, using plastic materials contributes to saving energy and CO2 emissions. It is assumed that plastics replaced with other alternative materials require 22.4 million extra tons of crude oil when we take into consideration a life cycle perspective for using plastics. The total global production of plastics increased from approximately 1.3 MT (million tons) in 1950 to 245 MT in 2006. One of the most important elements of waste management is the thermal treatment process. This process involves the oxidation of waste through incineration, gasification or decomposition (such as pyrolysis). The main advantages of this type of process are as follows:
* Tel.: þ48 501541185; fax: þ48 523220853. E-mail address: [email protected].
- the possibility of converting waste into safe forms; - significant reduction in the weight and volume of waste; - the possibility of recovery of substantial amounts of heat. The incineration of plastic waste is highly beneficial because of its high LCV (lower calorific value). Table 1 [3] shows that the LCV of polyethylene is the same as the LCV of diesel fuel. In the case of waste with a high LCV, such as plastic waste, the process of thermal disposal involves gasification or pyrolysis. These processes [4] and their detailed reaction mechanisms and simulation models, including the elimination of redundant species and reactions, rate of products and reaction flux analysis and uncertainty analysis have been reported [5e17]. The common goal of the aforementioned waste incineration processes is to utilize the resulting energy in a way that is both simple and minimally burdensome for the environment. Heat recovery from such systems is achieved through special technological systems. Holmgren [18] described a heating system that uses waste heat supplied from various sources, using the city of Gothenburg € teborg) in Sweden as an example. He presented the results of (Go research on the use of waste heat from industrial plants and waste incinerators equipped with recovery steam boilers. He showed that heating networks can utilize waste heat that would otherwise have limited use. Mori, Kikegawa and Uchida [19] presented a waste heat recovery model for a waste incineration plant in Tokyo. Application of the model resulted in a reduction of carbon dioxide emissions by approximately 8%.
http://dx.doi.org/10.1016/j.energy.2015.06.106 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
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Table 1 LCV of plastics compared with conventional fuels. Item
LCV (MJ kg1)
Methane Gasoline Fuel oil Coal Polyethylene Mixed plastics Municipal solid waste
53 46 43 30 43 30e40 10
Additionally, Dean [20] described heat recovery from medical waste incineration systems at hospitals. He discussed the possibility of producing steam using waste heat from the incineration of medical waste. He showed that using a heat recovery system of that type significantly reduced the annual operating costs of the hospital boiler room. Bujak [21] presented experimental studies of the useful energy flux and thermal efficiency coefficient of an installation for utilizing thermal waste with heat recovery. The system consisted of a unit for preparing and loading waste, a combustion chamber, a thermoreactor chamber and a recovery steam boiler. The basic fuel included hospital waste, and natural gas was used as extra fuel. The average flux of waste incinerated was 100 kg/h. Heat recovery in the form of a usable energy flux included in the saturated steam was in the range of 600e800 kW (1000e1200 kg/h of saturated steam) at a load higher than 4 > 65%. The ratio of thermal efficiency oscillated between 56 and 62% depending on the load of the incinerator. The stream of useful energy and the energy efficiency ratio increased with increasing incineration load. This paper presents the results of a pilot study of a thermal utilization installation for incinerating plastic waste. The research was conducted on an industrial scale in a plant that manufactures plastic tape (used for warning, packing and masking purposes, among others). A rotating kiln of a special and untypical structure has been designed and constructed. Due to the very high capacity value of wastes (36 MJ/kg) and low content of non-flammable parts (minerals), the following length-to-diameter ratio was taken e 2:1. For the analysed case, the length of the rotating kiln was 4 m, and its diameter e 2 m. Typical rotating kilns have several-time greater lengths in relation to their diameter. For example, cement kilns have approx. 45 m in length and 3 m in diameter (15:1). A standard ratio for rotating kilns burning hazardous or medical wastes is no less than 4:1. The testing plant was tested for compliance with the requirements of the directive [22] on permissible emissions to the atmosphere, as well as for its thermal efficiency. Obtaining positive results and their use in practice may bring significant economic benefits. Implementation of such systems may be considerably more cost effective than typical solutions. A short rotating kiln means much lower cost of its construction and, in particular, lower cost in relation to the rooms where it is located. During the tests, also the maximum content of unoxidized organic compounds in slag and bottom ashes was controlled. It is of particular importance when a rotating kiln is short e a short pathway for burned components.
2. Management of plastic waste before and after system upgrade Fig. 1 shows a diagram of heat management in the plant before the system was upgraded. The plant used the heat for technological purposes, heating, ventilation and hot water.
Due to the nature of plastic tape production, heat is supplied in the form of thermal oil with parameters ranging from 230 C to 280 C and returns with temperatures from 210 C to 260 C. The primary and largest consumers of heat are the technological devices manufacturing the plastic tape. Table 2 shows the thermal power of the production equipment. Oil was heated in a local plant boiler fired with natural gas with a thermal power of Q ¼ 2000 kW. The plastic waste was exported to regional waste disposal facilities. Fig. 2 shows the thermal management structure after the system was upgraded. The upgrade included the design and construction of a special plastic waste incineration system based on a rotary kiln with co-current combustion. In typical incinerating installations that burn hazardous and medical waste, the aspect ratio between the length and the diameter of the rotary kiln is minimum 4:1. The experimental system under study uses a unique experimental facility construction of the combustion chamber (rotary kiln). Due to the very high LCV of the waste, the accepted aspect ratio was 2:1. (length of 4 m and diameter of 2 m). The heat recovery system was equipped with a special oil-tube boiler. Due to the proximity of the technological production lines and the incinerator, waste transport was reduced to the level of interdepartmental transfer. Simultaneously, the time between the creation of waste and its neutralization was reduced to a minimum. The system complies with the mandates imposed on the waste incineration process. The temperature in the secondary combustion (afterburner) chamber is continuously maintained above 1100 C. The required flue gas retention time in the secondary combustion (afterburner) chamber is 2 s. Due to the homogeneity of the postproduction waste, maintaining proper parameters for the combustion process is simplified. The maintenance of said parameters is also beneficial in terms of maintaining the effectiveness of the flue gas treatment system, thus complying with the standards concerning harmful emissions into the atmosphere. The advantage of the on-site facility location provides the possibility of a highly efficient use of the heat generated by the waste incineration process. Thermal oil heated in the oil-tube boiler is sent via pipeline to the processing and ventilation equipment or to heat exchangers. Thus, the local natural gas-fired boiler room can be eliminated. After the upgrade, that room serves only as a backup heat source. The value of this decreased (reduced) natural gas flux can be converted into a flux of pollutant and carbon dioxide emissions to the atmosphere. Thus, we obtain the effect of the socalled “avoided emission.” An additional advantage of the on-site location of the thermal treatment system of post-production waste is the elimination of export to external (regional) waste disposal facilities/contractors, which reduces the consumption of large quantities of diesel fuel or gas (petrol) for external transport. Therefore, the effect of “avoided emission” is not included in this article. 3. Description of technology and measurement system 3.1. Thermal recycling technology The investigated system of plastic waste incineration (Fig. 3) consists of the following elements: A. Loading system e plastic waste generated during production is delivered to the incinerator building. In the recycling plant waste is crushed/shredded and compressed. In such form, waste is then loaded in large containers with a capacity of 370 or 770 L. The container with its contents is automatically weighed and recorded by the computer system. It is then raised by the elevator above the loading chamber and completely emptied.
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Fig. 1. Waste management before plant upgrade.
Table 2 Summary of technological equipment. Machinery/device type
Thermal power [kW]
Temperature of thermal oil [0C]
Impregnator Coater (acrylic) Hot melt
1,000 400 600
260 220 140
When the loading chamber is full, horizontal hydraulic cylinder pushes the waste into the rotary kiln. The average frequency of loading is every 15 min, with the average weight of a load of 50 kg. B. Rotary kiln e built with a 2% incline sloping in the direction of the secondary combustion (afterburner) chamber where incineration of plastic waste takes place. The duration of the
incineration process depends on the LCV and the moisture content of the waste. Due to the very high LCV of the waste, a large diameter of the combustion chamber in relation to its length was used (diameter of 2 m and length of 4 m). As shown in Photo 1, the rotary kiln is short but has a large diameter. C. Secondary combustion (afterburner) chamber e where gases produced during the incineration process in the rotary kiln are afterburnt. This process is conducted at temperatures from 1100 C to 1200 C, and the minimum retention time of incinerated gases must not be less than 2 s. D. Recovery boiler e after leaving the reactor, flue gas at temperatures from 1100 to 1200 C heat the thermal oil to a temperature of approximately 280 C. With special pumps, thermal oil is sent to the processing equipment. The flue gases flowing through the heat recovery system are cooled to temperatures between 265 C and 280 C.
Fig. 2. Waste management after plant upgrade.
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Fig. 3. Schematic of the waste recycling system with heat recovery.
E. Flue gas cleaning system e multisectional bag filter to remove dust. In addition, this system is equipped with metering devices for dosing sorbent and urea.
3.2. Energy balance of the system The energy flows through the incinerator with a heat recovery boiler as previously described [21]. The final equation (1) is as follows:
E_ pw þ E_ awcch þ E_ ngcch þ E_ angcch þ E_ ngach þ E_ angach þ E_ agach X ¼ E_ ue þ E_ el (1)
where: E_ pw e chemical enthalpy flux of plastic waste (kW), E_ awcch e physical enthalpy flux of air used for waste incineration in the combustion chamber (kW), E_ ngcch e chemical enthalpy flux of natural gas supplied to the combustion chamber (kW), E_ angcch e physical enthalpy flux of air used for natural gas combustion in the combustion chamber (kW), E_ ngach e chemical enthalpy flux of natural gas supplied to the afterburner chamber (kW), E_ angach e physical enthalpy flux of air used for natural gas combustion in the afterburner chamber (kW), E_ agach e physical enthalpy flux of air used for afterburning gas combustion in the afterburner chamber (kW),
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Photo 1. Rotary kiln.
E_ ue e usable (effective) energy flux (kW), P ðE_ el Þ e total loss of energy fluxes (kW). The fluxes of energy supplied to the thermal plastic waste recycling system are shown on the left hand side of equation (1). As assumed in the design, the main flux of energy supplied to the system consists of plastic waste (E_ pw). The LCV of the supplied waste is so high that the incineration is stable. Therefore, under the circumstances, there is no need for additional fuel (natural gas). The burning process is autothermal. The values of the physical enthalpies of air fluxes supplied to the combustion chambers (E_ amw_ cch) and the secondary (aftercombustion) chamber (E ag-dch) change depending on the amount of air required for the correct incineration of waste, the aftercombustion of generated gases and the
ε¼
3.3. Energy efficiency coefficient The energy efficiency of a system for the thermal recycling of plastic waste can be determined using two methods: indirect or direct. The indirect method involves determining all components responsible for energy losses and is frequently performed via calculations (analytical method). The direct method involves measuring the energy flux supplied to the system and the heat flux transferred to the steam receivers (usable/effective heat). The energy losses constitute the difference between the energy flux supplied to the system and the effective heat produced by the system. This paper describes the energy efficiency coefficient by the direct method. The coefficient of energy efficiency is defined as follows:
E_ ue _ _ _ _ Epw þ Eamwcch þ Engcch þ Eangcch þ E_ ngdch þ E_ angdch þ E_ agdch
temperature of the environment from which the air is taken. It is assumed that the supply of additional fluxes of chemical enthalpy in the form of additional fuel (E_ ng-cch) and (E_ ng-dch) is necessary only in the system start-up period. The air enthalpy fluxes (E_ ang-cch, E_ angused for natural gas burning (supplied to the burners) depend on the temperature of the environment from which the air is taken and on the amount required for correct gas burning. The energy fluxes carried away from the waste recycling system are shown on the right hand side of equation (1). The use of a recovery boiler within the processing system allows the main energy flux carried away from the system to be the usable (effective) energy flux (E_ ue). Other fluxes of energy carried away from the system are the total energy losses S(E_ el), consisting of the losses of heat fluxes through the external walls of combustion chambers, after combustion chambers, concrete ducts and recovery boiler, as well as energy fluxes in the form of the physical and chemical enthalpy of ash.
(2)
3.4. Description of the measurement system The measurement system consists of three essential components (Fig. 4):
dch)
a) flux measurement of the following: ‒the chemical enthalpy of additional fuel fed into the rotary kiln and the afterburner chamber e using a GM flow meter manufactured by Vortex, with a measuring accuracy of 1.25%, ‒the mass of plastic waste, measured by electronic scales with 0.5% accuracy (SC1), b) temperature measurement using sensors with a measuring accuracy of 2.0% for the following: ‒ flue gas temperature at the beginning of (near the loading of waste) the rotary kiln (T1), ‒ flue gas temperature at the end of the rotary kiln (at the connection with the afterburner chamber) (T2),
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Fig. 4. Measurement system of the investigated installation.
c) ▪
▪
▪
‒ flue gas temperature at the end of the afterburner chamber (T3), ‒ flue gas temperature before the recovery boiler (T4), ‒ flue gas temperature after the recovery boiler (T5), ‒ inlet thermal oil temperature of the recovery boiler (T6), ‒ outlet thermal oil temperature of the recovery boiler (T7), continuous flue gas monitoring consisting of the following: measuring component: ‒ system of collection and transport of flue gas samples, ‒ system of dust measurement and the reference parameters (static pressure, temperature and flue gas velocity) necessary to perform calculations, ‒ measuring panel with analysers, treatment and computing component: ‒ measurement data concentrator processing data from the analysers and sensors while converting it from analogue to digital format, ‒ emission computer implementing the acquisition, archiving, verification and presentation of measurement data and the creation of graphs and generation of reports, auxiliary component: - panel with technical gases necessary for continuous system calibration.
The concentrations of pollutants emitted into the atmosphere were measured using the following methods: - FTe IR measuring method e based on the ability of polyatomic gas particles to absorb infrared radiation; the performed quantitative analysis involved the measurement of CO, SO2, NO, HCl, HF and H2O, - FID measurement method - based on flame-ionization detection; the analysis method was used to measure the concentration of total hydrocarbons, - measurement method based on a zirconia sensor, measuring the oxygen concentration in gases with a high content of combustible compounds and impurities resulting from high temperature (for example, in the flue gases); the zirconia probe consisted of a heated measuring cell, an electronic controller and a pneumatic unit supplying reference air.
The parameters measured by the system for the continuous monitoring of flue gas and their ranges and measurement errors are shown in Table 3.
4. Test results 4.1. Test results for thermal waste recycling process Natural gas (additional fuel) was burned only during the startup the system, in order to reach a proper temperature in the rotating kiln and the afterburner chamber. This phase of the process system operation was not tested. Combustion of plastic waste without using natural gas was initiated after achieving the required temperatures. During the testing period (steady state) only the waste was incinerated. Fig. 5 shows the test results of the flue gas temperatures in a rotating combustion chamber. The flue gas temperature at the beginning of the chamber (T1) changed slightly over the study period. It fluctuated within the range of 922 C and 1011 C, with an average value of 985 C. The temperature at the end of the rotary kiln (T2) was slightly higher and fluctuated within the range of 993 C and 1048 C. The average value measured during the test
Table 3 Values measured by the system for continuous monitoring of flue gases and their ranges and measurement errors. Measured parameters
Measuring range
Measuring error (%)
Total dust, mg/m3n Total organic carbon, mg/m3n Chloride, mg/m3n Hydrogen fluoride, mg/m3n Sulphur dioxide, mg/m3n Carbon monoxide, mg/m3n Nitric oxide, mg/m3n Carbon dioxide, % Oxygen, % Humidity, % Flue gas flux, m3/h Flue gas temperature, C Static pressure, mbar
0e100 0e160 0e100 0e10 0e2000 0e700 0e2000 0e20 0e25 0e30 0e10,000 0e2000 0e1600
2.0 1.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 0.25
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T e m p e r a t u r e o f f l u e g a s e s [ 0C ]
1400
1300 1200 1100 1000 900 800 700
T1 - at the beginning of the rotary kiln
600
T2 - at the end of the rotary kiln T3 - after the discharge chamber
500
400
Period of time Fig. 5. Flue gas temperatures at the beginning and end of the rotary kiln and behind the discharge chamber.
was 1029 C. The temperature differences at the beginning and end of the rotary chamber were not significant. It is worth noting that very high temperatures occurred (T1) even in the front of the rotary kiln. The occurrence of these temperatures was due to the high LCV of plastic waste. The high temperature (T1) also demonstrates rapid combustion practically from the time of loading the waste into the rotary kiln. Nevertheless, the combustion process was quite stable due to the specially selected length and diameter ratios of the rotary combustion chamber. The stability of the combustion process is also proven by the highly uniform temperature (T1 to T2) over the entire length of the rotary kiln. The flue gas temperature behind the secondary combustion (afterburner) chamber (T3) fell within the range of 1030 C and 1384 C. However, the periods in which the temperature (T3) fell below 1100 C were very rare. During standard operation of this type of system, an auxiliary burner would turn on to maintain the minimum temperature above 1100 C. During the testing period, the auxiliary burner was turned off. Despite the lack of auxiliary
burner operation, the flue gas temperature in the discharge chamber was increasing. The reason was the air supply to the afterburner chamber, which initiated the combustion of yet unburnt gas in the rotating combustion chamber. Thus, a waste gasification process occurred in the rotary kiln. When analysing the temperature measurements in thermal waste processing, an autothermal combustion process (without additional fuel) was achieved in both chambers. Fig. 6 shows the actual concentrations of CO (carbon monoxide) and volatile organic compounds, expressed as TOC (total organic carbon), in the outgoing flue gas. Devices to sample the flue gas and to measure the mass flux of dust contained therein were located on the exhaust chimney. To conduct a detailed analysis, the above mentioned compounds were selected due to the problems in complying with emission standards during the burning of plastic waste. In the study period, the concentration of carbon monoxide in the flue gas ranged from 5 to 15 mg/m3n, with an average value of 11.2 mg/m3n. No significant peaks occurred, even during the
50
Emission
[ mg / m3 n ]
45
CO - carbon oxides
40
TOC - total organic carbon
35 30 25 20 15 10 5 0
Period of time Fig. 6. The concentration of total organic carbon and carbon monoxide in the flue gas at 11% oxygen, a temperature of 273.15 K and a pressure of 101.3 kPa (average one-minute concentration).
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Table 4 Existing standards and actual emissions to the atmosphere of the test technology at 11% oxygen, a temperature of 273.15 K and a pressure of 101.3 kPa (average daily emission limit). Type of compounds measured 3
Total dust, mg/m n Total organic carbon, mg/m3n Chloride, mg/m3n Hydrogen fluoride, mg/m3n Sulphur dioxide, mg/m3n Carbon monoxide, mg/m3n Nitric dioxide, mg/m3n
Emission standards
Actual emission
10.0 10.0 10.0 1.0 50.0 50.0 200.0
7.1 8.5 4.9 0.3 11.3 12.7 165.0
loading of waste. It should be noted that the loading of waste was performed using a lift. This loading method is called a periodic loading method. The average frequency of loading was every 15 min, the average weight of a load was 50 kg, and the average flux of waste incinerated was 200 kg/h. The concentration of TOC (total organic carbon) in the flue gas fluctuated between 5 and 10 mg/m3n, except for a few peaks with values reaching up to 50 mg/m3n. The peaks occurred during a few waste loading processes. Characteristically, they occurred during the same shift. This observation shows that the experience of workers operating such systems also affects the quality of combustion and emissions, especially when loading the waste. It should be emphasized that all the 30-min concentration averages were below the limit. The average concentration of TOC in the flue gases during the study period reached a value of 8.8 mg/m3n. Table 4 shows the existing standards and the actual emissions to the atmosphere resulting from plastic waste incineration over the study period. During the tests, also the maximum content of unoxidized organic compounds in slag and bottom ashes was controlled. The total content of organic coal did not exceed 3.0%, however it was close to the maximum value and amounted to 2.9%. The above results were obtained at the following rotary speed of the kiln e 2 rotations per hour. At greater speeds the slag was not totally burned e it contained more than 3% of coal. The average mass flux of slag produced during the test was 9.6 kg/h. It constituted 4.8% of the plastic waste charge. The graph in Fig. 7 shows that the flue gas temperatures when entering (T4) and leaving (T5) the thermal oil boiler are very different. The mean temperature difference measured in the study
period was 870.9 C. The exhaust gas temperature at the entrance to the thermal oil boiler during the study varied in the range of 907.7.0 to 1263.3 C, yielding an average value of 1156.1 C. The flue gas temperature at the outlet of the thermal oil unit fluctuated in the range of 265.2 C and 336.7 C, with an average value of 285.2 C. The average value measured during the test was 1029 C. As observed in Fig. 7, the temperature of the flue gas leaving the oil boiler gradually increased with time of operation. At the beginning of the test period, when the heat exchange surfaces were clean, the output temperature of the flue gas (T5) ranged from 265 C to 280 C. With the progressive pollution of the heating surfaces, the flue gas temperature increased. At the end of the test period, it reached a value in excess of 300 C. After exceeding the maximum value of T5max ¼ 350 C, the heat recovery boiler was cleaned. This situation occurred after the end of the study. The relatively high flue gas temperatures at the outlet of the thermal oil boiler resulted from the required parameters of the secondary heat carrier, namely heating oil. Due to the nature of plastic tape manufacturing, the required thermal oil temperatures at the outlet were dependent on technological needs. Fig. 8 shows the temperatures of the heating oil at the supply inlet and the return outlet from the processing systems, as measured during the study period. In analysing the operation of the oil boiler, we can see that the starting temperature of the oil (T7) varied in the range of 250.2 Ce290.4 C, with an average value of 272.4 C. The temperature of the heating oil returning from the processing equipment (T6) fluctuated in the range of 211.1 Ce268.4 C. The average value obtained in the test period was 249.5 C. The use of the heat recovery module resulted in the recovery of useful energy in the form of hot thermal oil (Table 5). The average flux of this energy reached E_ ue ¼ 1269.6 kW, and the thermal efficiency of the entire system (part of the combustion and heat recovery) was ƞ ¼ 65.2%. Using waste heat to satisfy heat needs reduces the consumption of fossil fuels burned. To illustrate directly, the use of the heat produced in the local plant boiler room was reduced by the amount of heat received from the recovery system. The value of the reduced heat flux can be converted to the classic fuel flux. The calculation accounts for the average flux of useful energy produced in the form of hot oil, E_ ue ¼ 1269.6 kW. The plastic tape plant that is the object
Fig. 7. The temperature of flue gases at the inlet and outlet of the thermal oil boiler.
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Fig. 8. The temperature of heating oil at the inlet and outlet of the thermal oil boiler.
Table 5 Thermal efficiency of the studied system.
Table 7 Input data e technical indicators.
Description
Value
Technical analysis module
The average chemical enthalpy flux in waste, kW The average flux of useful energy e thermal oil, kW The energy efficiency of the system, %
1947.2 1269.6 65.2
Average hourly mass flux of incinerated plastic waste, kg/h Average system load, % Average hourly flux of generated heat, kW Average hourly flux of additional fuel for combustion chamber, m3n/h Average hourly flux of additional fuel for afterburner chamber, m3n/h Average LCV of plastic waste, MJ/kg Additional fuel type e natural gas
of this study had its own boiler fired with natural gas. The required production of thermal hot oil in the boiler room was correspondingly reduced. The subsequent reduction in the natural gas flux consumed was 149.4 m3n/h. (Table 6). On the other hand, incinerated plastic waste generated during production belongs to the group of contaminated or mixed plastics. Examples here may be various types of plastics and multi-material plastics contaminated with various kinds of adhesives. For this type of waste, energy inputs used to separate the pure polymer or monomer (chemical recycling) significantly exceed its calorific value. When burning this type of waste (energy recycling), we achieve the effect of avoided emissions in relation to chemical recycling also known as raw material recycling. Table 6 shows the reduction of greenhouse gas emissions as a result of energy recycling (incineration with heat recovery) in the examined system. In the analysed period, CO2 emissions were reduced by 298.7 kg/ h. Emissions of SO2, CO, NOx and dust were omitted. The effect of avoided emissions due to the elimination of transport to external facilities processing plastic waste (chemical recycling) was not included in Table 6.
200.0 100.0 1269.6 0.0 1.6 36.0 NG-50
specific solution, even if justified from a technical perspective, also yields economic benefits, considering macroeconomic, social and environmental conditions. Analysis of the economic viability of any venture should demonstrate whether the net financial result of implementing the investment will be sufficiently high and competitive when compared to other investment opportunities. Table 7 shows the basic input in the form of technical indicators. The capacity and performance of the plastic waste incineration, the thermal power recovery system and other technical parameters were determined based on the conducted research. Next, Table 8 provides basic input associated with economic aspects. In particular, these aspects define the capital and operating expenditures of the studied system. Table 9 shows the results of the investment profitability calculations. Economic analysis of this system showed that the SPB (simple payback period) of the capital expenditures incurred for its construction is 4.5 years. The NPV (net present value) of the project after taxes is greater than zero. In the case presented, after 15 years,
5. Economic aspects The primary objective of any business operation is its cost effectiveness. It is therefore necessary to determine whether a
Table 8 Input data e expenditures. Economic analysis module
Table 6 Reduction of greenhouse gas emissions. Description
Value
Flux of recovered energy (thermal oil), kW Reducing the volume flux of natural gas, m3n/h The reduction of CO2 emissions as a result of the recycling of energy, in kg/h
1269.6 149.4 298.7
CAPEX (capital expenditures), $ OPEX (operating expenditures), $ Price for recycling 1 kg of plastic waste, $ Price of 1 m3n of natural gas as additional fuel, $ Employment rate e number of people per 8 h Gross pay rate, $/person Repairs, $/year Other maintenance costs, $/year
3,000,000 651,000 0.2 0.6 2 3000 207,000 156,000
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Table 9 Calculations of profitability indicators. Profitability indicators Net present value (NPV), $ (for r ¼ 0, after 15 years) Simple payback (SPB), years Internal rate of return (IRR), %
2,647,196 4.5 16.1
its value is $ 2,647,196. The IRR (internal rate of return) of the project was 16.1%, which was higher than the expected 8.3%. Considering all of the above mentioned economic indicators, it is clear that the tested system is both profitable and attractive in terms of economic development. 6. Conclusions This paper presents the results of a pilot study of the installation of a thermal treatment (incineration) facility for plastic waste. The study was conducted in a plant that manufactures plastic tape (used for warning, packing and masking purposes, and others) The system was considered in terms of three aspects: energy, environmental and economic. In terms of energy, tests have shown the value of designing and building plastic waste incineration (thermal disposal) plants with heat recovery. This type of plant, in addition to performing its basic role (waste management), is also highly efficient in terms of heat production. The process of intense burning of wastes has already taken place at the beginning of the rotating kiln. The temperature of exhaust gases in this part of the kiln was very high, reaching in the testing period an average value of 985 C. Though the rotating kiln was short, it was possible to use its volume to ensure the maximum thermal performance. There was a stable and high temperature inside the cubic volume of the kiln. The use of a heat recovery module resulted in useful energy that was recovered in the form of thermal oil at a temperature of 280 C. The value of the energy flux averaged E_ ue ¼ 1269.6 kW, and the thermal efficiency of the entire system (incineration and heat recovery component) was h ¼ 65.2%. Despite high temperatures of exhaust gases at outlet of the recovery oil boiler resulting from the required parameters of heat oil, thermal efficiency of the whole system (burning and heat recovery part) was satisfactory and relatively high h ¼ 65.2%. Such good thermal efficiency was obtained owing to low losses of the heat flux through the external wall of the rotating kiln (short and wellinsulated) to the environment. At the same time, it was demonstrated that the assumed construction ratios of the rotating kiln guaranteed a proper and suitable level of thermal processing of wastes. At a sufficiently low rotary speed (2 rotations per hour), it was possible to obtain the total content of organic coal in slag and ashes below 3.0%. Analysis of the environmental aspect proved that the actual emissions to the atmosphere that resulted from the thermal treatment of plastic waste, in the experimental system, during the study period were lower than the current emission standards in force in the European Union. Due to the nature of the disposed waste, particular attention was paid to the emission analysis of carbon oxide and volatile organic compounds. In both cases, the average daily emission limit values fell below the limits. The same also applied to the average 30-min values. The use of the heat recovery system also resulted in the reduction of greenhouse gas emissions because the plant under study had its own boiler room fired with natural gas. The necessary hot thermal oil production by the plant boiler room was correspondingly reduced. The
consequent reduction of the natural gas flux consumed was 149.4 m3n/h. The reduction of CO2 emissions due to the use of energy recycling (incineration with heat recovery) was 298.7 kg/h. Emissions of SO2, CO, NOx and dust were omitted. In economic terms, economic fundamentals were set for the test system to determine the profitability of its construction. It has been demonstrated that the test system is both cost-effective and attractive from the economic perspective. The simple payback period (SPB) of capital expenditures for its implementation was 4.5 years. The NPV (net present value) of the project after taxes was greater than zero. Obtaining such a good economic result was possible due to, among others, low cost related to the construction of the room for the analysed technology. Owing to a small length of the rotating kiln, it was possible to reduce the room in size by 35%. The results prove that it is worth seeking optimum solutions, taking into account the specificity of burned wasted, which bring significant ecological, power and economic benefits.
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Please cite this article in press as: Bujak JW, Thermal utilization (treatment) of plastic waste, Energy (2015), http://dx.doi.org/10.1016/ j.energy.2015.06.106