Co-combustion of coal processing waste, oil refining waste and municipal solid waste: Mechanism, characteristics, emissions

Chemosphere 240 (2020) 124892

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Co-combustion of coal processing waste, oil refining waste and municipal solid waste: Mechanism, characteristics, emissions Dmitrii O. Glushkov*, Kristina K. Paushkina, Dmitrii P. Shabardin Heat and Mass Transfer Simulation Laboratory, National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634050, Russia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Co-combustion was investigated for filter cake with used oil and MSW.
Guaranteed ignition were established for 3 composite fuel groups at ambient temperatures 600e1,000  C.
Minimum and maximum ignition delay times for 2 mm droplets are 3 s and 25 s.
Сombustion temperatures 1,300  C for fuel with 10% of used oil promote the reduction of PCDD/Fs in flue gases.
Lower NOx and SOx emissions were established for fuel compositions based on filter cake and MSW.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 April 2019 Received in revised form 15 July 2019 Accepted 16 September 2019 Available online 17 September 2019

This experimental research studies co-combustion of wet coal processing waste (filter cakes) with typical municipal solid waste (wood, rubber, plastic, cardboard) and used turbine oil, as combustible components of composite liquid fuel. Ignition mechanisms and characteristics of single droplets of three fuel composition groups have been investigated in a motionless heated air with using a high-speed video recording system. Analyzing video frames, a physical model of the process under study was developed. The values of the guaranteed ignition delay times have been determined for three fuel groups with different compositions at the ambient temperature 600e1,000  C. The minimum values of ignition delay times are about 3 s, the maximum ones are about 25 s. In addition to the established difference in the ignition delay times, the various fuel compositions also differ in combustion temperatures. Maximum values reaching 1,300  C for compositions with 10% of used oil. It has also been determined that fuels with municipal solid waste are notable for lower nitrogen and sulfur oxide concentrations in flue gases as compared to filter cakes in initial state. Adding used oil to such fuel compositions increases the anthropogenic emissions but these worsening environmental characteristics do not exceed the regulated allowable limits of pollutants for solid fossil fuel combustion by thermal power plants. The obtained results are the backbone for the development of an environmentally friendly, cost- and energy-efficient co-combustion technology for municipal solid waste recovery by burning it as part of composite fuels, e.g., in boiler furnaces instead of coal. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Shane Snyder Keywords: Co-combustion Municipal solid waste Coal processing waste Oil refining waste Ignition delay time Combustion temperature

* Corresponding author. National Research Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk, 634050, Russia. E-mail address: [email protected] (D.O. Glushkov). URL: http://hmtslab.tpu.ru/ https://doi.org/10.1016/j.chemosphere.2019.124892 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

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1. Introduction At present, industrial and municipal waste pollution is one of the main environmental issues all over the world (Wang and Wang, 2013; De Souza et al., 2014; Funari et al., 2015; Dong et al., 2018). As reported by the Russian Federal Service for Supervision of Natural Resources, landfill sites in Russia alone store more than 94 billion tons of solid waste, occupying the area of over 150 thousand ha. Combustible wastes from petroleum and coal industries account for a considerable share (about 50%) among a wide variety of industrial wastes. Several billion tons of municipal solid waste (MSW) is produced annually throughout the world (World Bank, 2018). About 80% of the total quantity of MSW is combustible materials. In order to solve waste-related environmental problems in a comprehensive way, a promising strategy would be the combined recovery of industrial and municipal wastes by burning them as part of composite fuels at thermal power plants (TPPs). The cocombustion of typical industrial and municipal wastes instead of coal at TPPs will help resolve a number of issues. It will alleviate the environmental damage caused by unconditioned wastes and decrease the areas of landfill sites. It will also reduce the consumption of high-grade energy resources by TPPs and the cost of energy generation, as composite waste-based fuel costs dozens of times less than coal. Moreover, it will reduce the CO2, CO, NOx, and SOx emissions with flue gases into the atmosphere. Waste-based fuel compositions may differ from one another (Glushkov et al., 2016a, 2019; Nyashina et al., 2018a). A typical composition is a slurry based on moist coal processing waste with petroleumderived flammable liquids and fine combustible MSW. 1.1. Coal processing waste According to expert evaluations (BP, 2019; EIA, 2019), many countries with deposits of solid fossil fuels are facing the problem of environmental pollution with coal processing wastes. As a rule, coal is processed before long-distance transportation to reduce the environmental pollution with fine dust. The resulting amount of flammable moist wastes is at least 15% of the processed coal. Such wastes are called filter cakes (FC). FC is a mixture of fine coal (particle size about 100 mm) with water (mass fraction about 50%). Such combustible waste is a by-product of coal processing that can be used in thermal power engineering as fuel (Nyashina et al., 2017, 2018a). At coal washing plants, FC is usually stockpiled at open sites. Starting 1980, about 9.5 billion tons of FC has been accumulated globally without being recovered. The annual increment approximates 0.2 billion tons (Glushkov et al., 2019). Moisture evaporation leads to the pollution of large areas with coal dust and deterioration of the environmental situation in the nearby regions (Ramirez et al., 2017; Tang et al., 2018; Wang et al., 2018). Today, coal processing wastes are perceived as an energyproducing resource (Glushkov et al., 2016a; Wang et al., 2018; Kurgankina et al., 2019). However, due to their low heat of combustion (about 10 MJ/kg), the direct combustion of filter cakes is ineffective. One of the promising ways to involve coal processing wastes into the thermal power engineering is by using them as the main component in fuel slurries (Glushkov et al., 2016a, 2016c; Kurgankina et al., 2019) for heat and power industry facilities. 1.2. Waste flammable liquids The world accumulates about 45 million tons of used oils (Kline & Company, 2018) each year, of which only 15% is recovered (Khalladi et al., 2009). Most technologies of used oil recovery are based on the following treatment methods (Sakata et al., 1999; Coulon et al., 2005; Fuentes et al., 2007; Khalladi et al., 2009;

Musthafa, 2016): mechanical (filtration of solid particles and free water), thermophysical (evaporation and vacuum distillation), physical and chemical (coagulation and adsorption), and chemical. All the listed methods have serious limitations as to the performance of process plants (Kontorovich et al., 2014) and new energyintensive equipment is costly to develop and produce. Quite often, used oils are not disposed of but stored in tanks or drain reservoirs. This poses an environmental and fire hazard due to possible leaks of liquid wastes into soil and bodies of water (Anifowose and Odubela, 2018). However, waste flammable liquids can be recovered by burning them as part of composite fuels based on FC. Oils have high heat of combustion (about 40 MJ/kg). Adding 10e20% of this component to composite fuels increases their combustion temperature and heat of combustion (Glushkov and Strizhak, 2017; Nyashina et al., 2018a).

1.3. Municipal solid wastes Over 2 billion tons of MSW is produced annually throughout the world (World Bank, 2018). Combustible MSW (about 80% of the total quantity) on an industrial scale is mainly recovered by trash incineration plants (Moskvichev and Tugov, 2012; Jeswani et al., 2013; De Souza et al., 2014). They are relatively small enterprises with transient behavior of energy-generating equipment, thus the amount and quality of power produced from MSW combustion with heat of combustion 5e10 MJ/kg (Jimenez et al., 2011; Moskvichev and Tugov, 2012; Jeswani et al., 2013) do not allow them to be on a par with coal-fired TPPs and supply the electricity produced to the central power system (Pasek et al., 2013; Tugov, 2015). The known technologies of waste combustion can be divided into two types by their temperature regime (Makarichi et al., 2018; Istrate et al., 2019; Santos et al., 2019): low-temperature (600e900  C) and high-temperature (above 900  C). The lowtemperature combustion regime does not require costly technological equipment or additional high-quality fuels (such as natural gas) to maintain the operating temperature in the furnace. However, a large amount of toxic substances (PCDD/Fs, polyaromatic hydrocarbons) are emitted in this case (Chen et al., 2018). Expensive gas treatment systems should be used for the flue gases to comply with the regulations for harmful substance emissions into the atmosphere. This makes most of the trash incineration plants unprofitable and environmentally unfriendly (Singh and Basak, 2018). High-temperature MSW combustion technologies are considered to be environmentally safer, as the most hazardous substances (PCDD/Fs) fully decompose to elemental parts under relatively high process temperatures (Tuppurainen et al., 1998; Hatanaka et al., 2001; McKay, 2002; Tugov, 2015; Messerle et al., 2018). Therefore, there is no need for costly flue gas treatment systems (McKay, 2002; Hu et al., 2018; Mancini et al., 2019) in case of combustion temperature above 1,000  C, combustion residence time greater than 1 s, combustion chamber turbulence represented by a Reynolds number greater than 50,000. However, specialized thermal (Tugov, 2015) or electroplasma (Messerle et al., 2019) equipment is required to maintain a relatively high temperature in the furnace. The operation of such equipment is usually provided by highquality fuel or a larger amount of energy than is released from MSW combustion. Electroplasma technologies used for MSW treatment at a reactor temperature above 1,300  C have an obvious benefit in terms of environmental indicators. However, they have not been widely adopted in real practice and are used primarily for hazardous medical waste treatment (Block et al., 2015; Messerle et al., 2018).

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1.4. Composite fuels

2. Experimental procedure

Based on the experimental and theoretical analysis (Glushkov et al., 2016b; Glushkov and Strizhak, 2017; Nyashina et al., 2018a, 2018b) of coal and oil processing waste, we drew a conclusion that using composite fuels in real practice is a high-potential area for thermal power engineering development. Depending on the component composition, such fuels have advantages over the common solid fossil fuels in terms of cost (Nyashina et al., 2017, 2018a) and harmful emissions. It has been established (Nyashina et al., 2017, 2018a, 2018b) that the concentrations of typical anthropogenic emissions from burning composite liquid fuels are lower than those from burning coal in its initial (dry) state: 40% lower for CO2, 20% lower for NOx, and 40% lower for SOx. Better environmental characteristics of flue gases from composite fuel combustion vs. those of coal combustion are conditioned by the following (Glushkov et al., 2018; Nyashina and Strizhak, 2018; Nyashina et al., 2018b). When coal fuel is burned, the NOx and SOx emissions are directly linked to the content of nitrogen and sulfur in the fuel. Adding any amount of water to the fuel slurry obviously reduces the total content of nitrogen and sulfur in the fuel composition, which contributes to lower concentration of the corresponding oxides in flue gases. Besides, the water in the fuel composition (30e50%) is an oxidizer in the composite fuel combustion, which intensifies coal burnout. Rapid water evaporation provides a finer distribution of the carbon basis due to microexplosions of slurry droplets. These effects lead to a larger surface of the fuel particle reaction with water vapor dissociation products and carbon oxide, which contributes to a decrease in NOx and SOx concentrations. During the thermal dissociation of water vapors at T > 1,000  C, free molecules of oxygen and hydrogen are released: 2H2O(g) % 2H2(g) þ O2(g) (Nernst, 1926; Jellinek and Kachi, 1984). The oxygen formed in the reaction intensifies combustion, whereas hydrogen and carbon monoxide participate in nitrogen and sulfur reduction reactions: 2NO þ 4H2 þ O2 / N2 þ 4H2O (reaction temperature over 200  C) (Efstathiou and Olympiou, 2017); 2NO þ 2CO / N2 þ 2CO2 (reaction is intense at over 1,000  C) (Daood et al., 2014); SO2 þ 3H2 / H2S þ 2H2O (reaction temperature over 600  C) (Feng et al., 2017); SO2 þ 3CO / 2CO2 þ COS (reaction temperature over 500  C) (Feng et al., 2017). Along with typical components (coal, water, oil), composite fuels may contain 10e20% of combustible MSW with a relatively high heat of combustion: paper 13e18 MJ/kg, wood 15e20 MJ/kg, plastic 20e40 MJ/kg, rubber 30e35 MJ/kg (Skodras et al., 2007; Chen, 2018; Glushkov et al., 2018, 2019). The combustion of such fuels should be experimentally studied at the fundamental level to explore the technical component of composite fuel combustion at coal-fired TPPs, substantiate the economic effect from saving highquality solid fossil fuels and evaluate the reduction of atmospheric pollution by flue gases. The main patterns of physical and chemical processes as well as their characteristics can be explored in laboratory conditions using single fuel droplets as an example. In actual practice, the characteristics of fuel combustion in a boiler furnace (minimum temperatures required for fuel ignition, combustion time and efficiency, as well as flame temperature) can be evaluated using the known characteristics of a single fuel droplet. In such conditions, one can determine the approximate values of the characteristics describing the popular flame combustion of fuel. The purpose of this research is to experimentally study the characteristics of single droplet ignition (ignition delay times) and combustion (temperature during combustion, environmental emissions) of a group of composite liquid fuel compositions which differ by the additional components of typical combustible MSW and their concentration.

2.1. Fuel preparation

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The research was performed on a group of composite fuel compositions based on filter cakes of coking coal from the Severnaya Coal Washing Plant that is situated in Kemerovo Region of the Russian Federation. To prepare composite liquid fuel compositions under study, we used a well-tested technique (Glushkov et al., 2018, 2019). Its scheme is presented in Fig. 1. Typical municipal solid waste (cardboard, wood, rubber, plastic) was ground separately with a cutting mill SW-2 (HT Machinery, JapanTaiwan). The samples obtained for different MSW were sieved through a sifter with a standard mesh size of 140 mm according to ISO 3310:2000. Thus, FC and MSW components with particle size under 140 mm were used to prepare three groups of fuel compositions. The first two groups of compositions were produced by mixing FC with MSW with different concentrations of the latter (10 and 20%). The third group of compositions consisted of FC, MSW, and used turbine oil. The last one increases the combustion temperature and thermal effect of process (Glushkov et al., 2016b; Glushkov and Strizhak, 2017; Nyashina et al., 2018b). The experiments were also carried out with basic compositions: FC without any additional components, and a mixture of FC with used turbine oil (without MSW). The fuel compositions are presented in Table 1. The fuel components were mixed in a vessel with a volume of 0.2 L using a DC-600RM mixer (HT Machinery, Japan-Taiwan) at 600 rpm for 15 min. The main characteristics of the composite fuel components are listed in Tables 2 and 3 (Glushkov et al., 2018, 2019). The FC characteristics were obtained for dry samples (before the analysis, they were dried at about 105  C until the moisture has fully evaporated). Higher heating values of fuel components (FC in dry state; MSW and used oil in initial wet state) were measured in a bomb calorimeter. The heat of combustion for fuel compositions was calculated analytically using the methods from (McAllister et al., 2011) and obtained values are presented in Table 1. Adding 10e20% of typical MSW to the fuel increases its heat of combustion by 5e40% (Table 1). This characteristic increases more significantly when used oil is added. The maximum difference in the heat of combustion is over 70% (see compositions No. 1 and No. 12 in Table 1). The theoretical estimate made illustrates the energy performance benefit of composite fuels with MSW and used oils (compositions No. 2 e No. 14) over composition No. 1 without these components. 2.2. Experimental technique The ignition and combustion of composite fuel single droplets have been researched using an experimental setup (Fig. 2). Single droplets were ignited in a motionless high-temperature air medium which was generated in the volume of a hollow ceramic tube (inner diameter 50 mm, length 500 mm) of a muffle furnace Loiplf50/500e1200 (Laboratory Equipment & Instruments ZAO, Russia). The range of temperature variation was 20e1,200  C, the values set were measured using an in-built type S thermocouple. The temperatures around the upper limit of the above mentioned range are characteristic of fuel combustion in boiler furnaces. In each set of 5e8 experiments conducted under identical initial conditions, the furnace was heated to a given temperature. After it stabilized, a fuel droplet with a diameter of about 2 mm was generated by an electronic dispenser (range 1e10 ml, pitch variation 0.1 ml) Finnpipette Novus (Thermo Scientific, USA), and deposited on a holder that was introduced into the furnace by a coordinate mechanism SPSh20-23017/2000Z (Mechatronic Product Factory ZAO, Russia) through one of the side apertures of the ceramic tube

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Fig. 1. Scheme of composite liquid fuel preparation.

Table 1 Composite liquid fuel compositions and their heat of combustion.

Table 3 Ultimate analysis of fuel components.

Group

No.

FCa

oil

wood

rubber

plastic

cardboard

Q (MJ/kg)

component

Cdaf (%)

Hdaf (%)

Ndaf (%)

Sdaf (%)

Odaf (%)

I

1 2 3 4 5 6 7 8 9 10 11 12 13 14

100% 90% 90% 90% 90% 80% 80% 80% 80% 90% 70% 70% 70% 70%

e e e e e e e e e 10% 10% 10% 10% 10%

e 10% e e e 20% e e e e 20% e e e

e e 10% e e e 20% e e e e 20% e e

e e e 10% e e e 20% e e e e 20% e

e e e e 10% e e e 20% e e e e 20%

10.78 11.29 13.05 11.88 11.43 11.80 15.31 12.98 12.07 14.11 15.12 18.64 16.30 15.40

FC wood rubber plastic cardboard oil

87.2 50.3 97.9 66.7 46.3 83.1

5.1 6.0 1.2 7.9 6.3 13.7

2.1 0.2 0.3 e 0.3 0.3

1.1 0.1 0.6 e 0.2 1.0

4.5 43.4 e 25.4 46.9 1.9

II

III

Q is heat of combustion. a e FC in initial wet state (moisture content 50%).

Table 2 Characteristics of fuel components. Proximate analysis. component a

FC wood rubber plastic cardboard oil

Wa (%)

Ad (%)

Vdaf (%)

Qas,V (MJ/kg)

e 20.0 2.0 2.0 5.0 0.3

26.5 2.0 1.8 0.2 3.0 0.8

23.1 e e e e 100.0

24.83 16.45 33.50 22.00 17.50 44.02

Ad is ash; Qas,V is higher heating value; Vdaf is volatile content; Wa is moisture content. a e in dry state.

Cdaf, Hdaf, Ndaf, Odaf, Sdaf are fractions of carbon, hydrogen, nitrogen, oxygen, and sulfur in the fuel component converted to a dry ash-free state.

along the tube symmetry axis (Fig. 2). The processes occurring during the induction period were recorded by a Phantom v411 high-speed camera (Vision Research, USA): 12 bit depth; 20 mm pixel size; 4,200 fps filming rate (at resolution 1280  800 pixels); 1 ms minimum exposure; 16 Gb memory; image-based auto-trigger. The group of parameters was controlled during the series of 5e8 experiments conducted under identical initial conditions: diameter (Dd) of the fuel droplet and temperature (Tg) in the muffle furnace. Software and hardware were used for high-speed video recording of investigated process. It has allowed for automatic calculating of the ignition delay time (td) and for detailed analysis of physical and chemical processes in the ignition and burnout. Tema Automotive software (Image Systems AB, Sweden) was used to calculate td values by the threshold algorithm (Glushkov et al., 2016b, 2018; Nyashina et al., 2018a, 2018b) that based on the evolution of the droplet luminance during induction period. The systematic error and standard deviation when calculating td did not exceed 3% and 10%, respectively. To estimate the scale of increase in the anthropogenic emission

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Fig. 2. Scheme of experimental setup.

concentration in case of co-combustion of fuel compositions with used oil and to analyze the influence of MSW in the composite fuel on the change of these characteristics, we performed an experimental research by a time-proven method (Glushkov et al., 2018; Nyashina and Strizhak, 2018; Nyashina et al., 2018b). The flue gases were analyzed using an experimental setup (Fig. 2). Testo 340 gas analyzer was mounted on the setup in place of the high-speed camera. The standard characteristics of four sensors are as follows: O2 (measuring range 0e25%, resolution 0.01%, accuracy ±0.2%), CO (measuring range 0e10,000 ppm, resolution 1 ppm, accuracy ±10%), NOx (measuring range 0e4,000 ppm, resolution 1 ppm, accuracy ±5%), SO2 (measuring range 0e5,000 ppm, resolution 1 ppm, accuracy ±10%). The CO2 concentrations (measuring range 0e25%, resolution 0.1%, accuracy ±0.2%) were calculated by the gas analyzer automatically by O2 concentrations:

CO2 ¼

CO2max ðO2base O2 Þ O2base

where CO2max is fuel-specific carbon dioxide value, %; O2base is oxygen reference value, %; O2 is measured oxygen content in flue gases, %. CO2max and O2base values were chosen according to recommendations in the instruction manual for Testo 340 gas analyzer. The following values were used: CO2max ¼ 18:4% and O2base ¼ 7%. The fuel sample was introduced into the pre-heated muffle furnace. Flue gases were collected by a gas analyzer probe inserted into the furnace. Both apertures of the ceramic tube were corked to prevent flue gases from escaping the furnace in the experiments. Several series of 3e5 experiments were conducted under identical initial conditions. The mass of fuel sample was no more than 1 g according to gas analyzer specifications about the minimum required quantity of fuel (volume of flue gases) necessary to obtain reliable measurement results. The obtained results are presented as average experimental data values.

3. Results and discussion 3.1. Characteristics of ignition and combustion The fuel compositions investigated in this study (see Table 1) was divided into 3 groups: I e FC 90% þ MSW 10%; II e FC 80% þ MSW 20%; III e FC 70% þ MSW 20% þ oil 10%. Fig. 3 presents typical video frames of ignition and combustion of a group of

composite liquid fuel droplets of different compositions. It has been established that the above mentioned compositions (with MSW and oil and without them) are characterized by the identical set of processes occurring during the induction period. Neither a variation of combustible MSW concentration in the range of 10e20%, nor the addition of a combustible liquid in the amount of up to 10% lead to a change in the composite liquid fuel ignition mechanism. The analysis of high-speed video recording made it possible to highlight the following main stages of interaction between a single droplet of composite liquid fuel with the motionless pre-heated air: inert heating; moisture evaporation from the nearsurface layer; thermal decomposition of solid combustible components (coal and municipal waste); mixing combustible gases with oxidizer vapors; gaseous mixture ignition and burnout; solid residue heating; heterogeneous ignition and combustion of the solid residue. The obtained result is explained by the decisive influence of a fine solid combustible component (FC) in the fuel mixture composition on the consistent patterns of physical and chemical processes taking place during fuel heating. Thus, Fig. 4 illustrates a physical model of ignition of composite fuel single droplet based on combustible industrial and municipal waste, universal for all the three groups of compositions. The video frames (Fig. 3) illustrate that at the ignition moment, the shape of the combustible gas mixture in the vicinity of the droplet is spherical. The size of combustible gas mixture zone in 2e3 times higher than the fuel droplet diameter. The lower the heated air temperature, the larger the size of the zone of combustible gases forming until the gas-phase ignition around the droplet. Another reason is the more mass fraction of components in the fuel composition with a higher content of volatiles. Under nearthreshold ignition conditions, the difference in the investigated processes is most distinct for the three groups of compositions with different component concentration and for compositions from one group with different MSW components. The difference is in the duration of process stages and their combination in general (Fig. 5). Fig. 5 presents three regions (highlighted in different colors) illustrating the variation ranges of the main characteristic e ignition delay time (td) at various air temperatures Tg ¼ 600e1,000  C, when all the composite liquid fuel compositions are reliably ignited. The curve demarcating the blue area above (I group of fuel compositions) corresponds to the ignition delay times of fuel composition No. 1. The curve demarcating the red area below (III group of fuel compositions) corresponds to the ignition delay times of fuel composition No. 14. The td values of other compositions

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Fig. 3. Video frames of ignition and combustion of 2 mm droplets of different fuel compositions at Tg ¼ 800  C: left droplet is fuel composition No. 1 from I group; middle droplet is fuel composition No. 9 from II group; right droplet is fuel composition No. 14 from III group.

Fig. 4. Scheme of physical model of industrial and municipal waste co-combustion: 1 e initial (saturated) fuel, 2 e heated motionless air, 3 e mixture of FC and MSW, 4 e diffusion zone of flammable gases and water vapors, 5 e evaporation front, 6 e gas phase ignition area, 7 e combustion front, 8 e solid residue.

Fig. 5. Regions (highlighted in color) of ignition delay times for three groups of composite liquid fuel compositions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

under study lie in between these two curves. It was established (Glushkov et al., 2018) that the duration of the induction period for fuel compositions with MSW is less than td of the FC without any additives. When wood is added into the FC, the ignition delay times are reduced by 5e7%; if rubber is added, they are reduced by 10e13%; if plastic, by 12e17%; and if cardboard, by 15e22%. Obtained results (Fig. 5) let us conclude that the air temperature Tg ¼ 600  C is the minimum necessary for the composite fuels ignition in conditions of co-combustion of coal processing waste, MSW, and combustible liquid. In the temperature range of 600e1,000  C the difference in the ignition delay time values does not exceed 25% for the three groups of fuel compositions. Less significant difference in the td values was established at Tg > 1,000  C. In such conditions the intensity of physical and chemical processes during the induction period is so high that the heat and mass transfer in the droplet and its vicinity does not influence significantly on the ignition characteristics. The ignition delay times for different fuel compositions are only marginally different (less than 5%) at the air temperature more than 1,000  C. We have sorted the fuel compositions under study in

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accordance with the td values in descending order (other conditions being equal): I group e FC 90% þ MSW 10%; II group e FC 80% þ MSW 20%; III group e FC 70% þ MSW 20% þ oil 10%. That means that the latter compositions feature the lowest ignition delay times. The highest td values are typical of FC in the initial wet state (composition No. 1 without any solid or liquid combustible components). Adding 10% of typical MSW to the composite fuel, increasing its concentration to 20%, and adding up to 10% of used turbine oil significantly decrease the duration of the fuel droplet heating until the gas-phase ignition moment (Fig. 5), other things being equal. The obtained result is explained by lower moisture content in fuel compositions with MSW, as compared to composition No. 1, and by the presence of a component (combustible liquid) that becomes chemically active during heating. In such conditions, less energy and time is spent on heating the nearsurface droplet layer, moisture evaporation (the endothermic effect is about 2 MJ/kg), and combustion initiation. Also, adding typical municipal waste to FC results in the emergence of a distinct phase of gas-phase combustion around the droplet due to a relatively high content of volatiles in MSW. Apart from the established difference in the ignition delay times, composite fuel compositions are also characterized by different combustion temperatures. This parameter is important for the analysis of both energy performance and environmental characteristics of the fuel combustion process. Fig. 6 presents the typical temperature variation curves in the combustion process of different composite liquid fuel compositions at Tg ¼ 800  C. The solid lines illustrate the temperature in the droplet center (T1), the dashed ones e around the droplet (T2), where the gas mixture burns out that forms in the evaporation of the liquid fuel component (if present) and thermal decomposition of solid components (FC and MSW). The distance between the T1 and T2 thermocouples corresponds to the initial fuel droplet diameter (2 mm). To visualize the explanation of differences in the position of solid and dashed curves in Fig. 6, we put a scheme in the bottom right-hand corner showing the location of thermocouples when the T1 and T2 temperatures are measured during the induction period. In accordance with the established pattern of physical and chemical processes (Figs. 3 and 4), the gas-phase ignition of evaporation products and thermal decomposition of fuel components first occur around the droplet. In Fig. 6, the gas-phase ignition moment (td) corresponds to the point of intersection of the corresponding dashed curves with the line Tg ¼ 800  C. That means that the heating of a relatively combustible gaseous mixture around the fuel droplet increases the intensity of the exothermic reaction. As a

Fig. 6. Typical temperature evolution of 2 mm droplets of different fuel compositions during induction period at Tg ¼ 800  C.

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result of thermochemical self-heating, the temperature of the gaseous mixture exceeds that of the hot air, which testifies to the gas-phase ignition. The maximum values of T2 reach 1,300  C for the third group of fuel compositions under study, containing used oil. According to (Tugov, 2015), when MSW is combusted at 1,300  C and above, all the hazardous gaseous substances decompose to elemental parts, which eliminates harmful emissions like PCDD/Fs in the composition of flue gases. The additional introduction of carbamide into the boiler furnace and a mixture of reagents (activated carbon and alkaline sorbent Ca(OH)2) into the wet/dry adsorber reduces the concentration of harmful substances in flue gases (Tugov, 2015). They are additionally cleaned in the filter to the values corresponding to the regulatory limits. An intense heat release during the gas mixture combustion heats up the relatively thin near-surface layer of the droplet, which leads to the heterogeneous ignition of the solid residue (see Fig. 4). The maximum rate of the temperature growth in the fuel droplet (solid lines in Fig. 6) corresponds to a time interval when the burnout rates of the gas mixture reach maximum values (extreme of dashed curves in Fig. 6). Under the conditions of the heterogeneous combustion of the solid residue, its layer-by-layer burnout occurs. The combustion front moves in the direction of the deep droplet layers (Fig. 4). The duration of all the processes from the moment of the gas-phase ignition to the burnout of the solid residue at Tg ¼ 600e1,000  C is 15e35 s. The T2 temperature decreases from the maximum value to the ambient temperature Tg less rapidly than the T1 does. This has the following reasons. After the gas mixture burns out, the solid residue combustion emits heat which is transferred from the heterogeneous exothermic decomposition zone into the environment. That is why the T2 temperature exceeds the Tg value over a long period of time. The solid curves in Fig. 6 show that the maximum temperatures (1,000e1,150  C) in the solid residue burnout of composite fuel compositions under study are close to the T2 values, which do not exceed 1,200  C. The results of the study (Fig. 6) suggest that compositions with a high concentration of MSW are recommended for practical application. Adding 10e20% of used oils to such compositions will have a positive effect on the reduction of the concentration of PCDD/Fs in flue gases (Tugov, 2015) due to an increase in the combustion temperature. Under such conditions, however, the concentration of the main anthropogenic emissions (carbon, nitrogen, and sulfur oxides) in flue gases is expected to rise, as compared to the same characteristics when composite fuels without used oils are burned (Nyashina et al., 2018b). It is generally agreed that combustion temperatures of 850  C and a gas residence time of 2 s or 1,000  C and a gas residence time of 1 s are necessary for total destruction of PCDD/Fs. Since decomposition increases exponentially with temperature and incineration temperature of 1,200  C, which is typical for chlorinated hydrocarbon wastes, requires a residence time measured in milliseconds for total destruction (McKay, 2002). The critical temperature zone for the maximum rate of PCDD/Fs from precursors such as chlorophenol is 350e400  C depending on the precursor concentration. Furthermore, the rate of PCDD/Fs formation from these precursors is 1 or 2 orders of magnitude greater than dioxin formation rates from conventional de novo synthesis. The maximum rate of dioxin formation from de novo synthesis is in excess of 300  C (Froese and Hutzinger, 1994; McKay, 2002; Mukherjee et al., 2016). Therefore, in order to minimize PCDD/Fs formation in this temperature zone it is necessary to cool the gases through the critical temperature zone as rapidly as possible (McKay, 2002). In general effective PCDD/Fs minimization can be achieved by applying the following (McKay, 2002): “In the first case, combustion temperature should be above 1,000  C, combustion residence

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Fig. 7. NOx and SOx concentrations in flue gases for three groups of composite liquid fuel compositions (for experimental samples of identical mass): a e I group (FC 90% þ MSW 10%); b e II group (FC 80% þ MSW 20%); c e III group (FC 70% þ MSW 20% þ oil 10%).

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time should be greater than 1 s, combustion chamber turbulence should be represented by a Reynolds number greater than 50,000. In the second category, very rapid gas cooling from 400 to 250  C should be achieved, semi-dry lime scrubbing and bag filtration coupled with activated carbon injection adsorption as end-of-pipe treatments can all play a role in prevention or minimization of dioxins in the final flue gas emission to the atmosphere”. The design of typical industrial coal-fired boilers (Basu, 1999) can provide all the necessary conditions to minimize PCDD/Fs concentrations in flue gases from the combustion of composite fuel. 3.2. Environmental emissions We have sorted MSW components that we added to the initial FC in accordance with the concentrations of NOx and SOx in combustion products, in descending order (Fig. 7): rubber, cardboard, wood, plastic. The corresponding results in normalized values (by heat of FC combustion) are presented in Fig. S1 (see supplementary material). The qualitatively normalized values are in good agreement with values obtained for experimental samples of identical mass (Fig. 7). This is because different compositions of composite fuels within one group have rather close values of the heat of combustion (Table 1). Among various fuel groups, the most significant difference in the heat of combustion is typical of III group (containing oil). According to Table 1, the values of Q differ by 5e25% within one fuel group and by 7e40% among all the compositions studied. FC has the lowest heat of combustion. Therefore, the more the heat of combustion of other fuel compositions differs from Q ¼ 10.78 MJ/kg (Table 1), the lower their normalized values (by heat of FC combustion) of NOx and SOx emission (Fig. S1) will be relative to the results shown in Fig. 7). Fuel compositions with plastic show minimum concentrations of nitrogen and sulfur oxides in flue gases as compared to other compositions under identical combustion conditions (Fig. 7 and Fig. S1). It was determined that for compositions No. 1 and No. 4 (or No. 8), the maximum difference in NOx concentrations is 50% (46% in normalized values), SOx e 40% (44% in normalized values), in absolute units e 150 ppm and 50 ppm, respectively. Such differences are quite significant because the maximum NOx and SOx concentrations in gaseous filter cake combustion products are about 340 ppm and 130 ppm (Fig. 7a), respectively. The content of nitrogen and sulfur in typical MSW is 2e10 times less than in the FC in initial state according to the element composition of the fuel components (Table 3). Thus, the mass fraction of FC is reduced at the preparation of I and II groups of fuel compositions due to addition of components (wood, rubber, plastic, cardboard) with less content of nitrogen and sulfur. Accordingly, as a result of burning such composite fuel under conditions identical to those when burning composition No. 1 (FC 100%), the main anthropogenic emissions (NOx and SOx) is lower (Fig. 7a and b). Compositions with adding 20% of MSW (II group of fuel compositions) are characterized by 5e10% (2e7% in normalized values) lower concentrations of NOx and SOx in flue gases than those with

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adding 10% of MSW (I group of fuel compositions). It should be mentioned, when adding MSW (wood, plastic, cardboard) into the fuel composition, the heat released during cocombustion is comparable to that when burning FC in initial state (Table 1). In some cases (for example, when rubber is added), the heat effect of the composite fuel combustion significantly exceeds that of the FC combustion without any additives (Table 1). It can be concluded that when typical MSW are added into a composite fuel, on the one hand it yields an equivalent amount of energy during the combustion of a samples with the identical mass, on the other hand the concentration of the main anthropogenic emissions is lower (Fig. S1). When used oil was added to the fuel (III group of the fuel compositions), NOx and SOx concentrations were 18e22% and 10e12% higher, respectively (Fig. 7c) than the same characteristics of the fuel compositions (Fig. 7b) without a combustible liquid (II group of the fuel compositions) for experimental samples of identical mass. In normalized values (by heat of FC combustion) the opposite result was obtained: NOx and SOx concentrations lower in 0.5e1.5% and 2e8% for III group of fuel compositions in comparison (Fig. S1c) with II group of the fuel compositions (Fig. S1b). In this case the scale of environmental emissions improvement is lower in comparison with the concentration (10%) of oil added into fuel composition. The negative environmental effect of adding used oil (for experimental samples of identical mass) is conditioned by its chemical composition, namely, sulfur and nitrogen compounds, as well as by an increase in the combustion temperature resulting in the intensification of oxidation reactions of sulfur and nitrogen. However, a 10% concentration of turbine oil, e.g., in the fuel composition shortens the coke residue ignition delay times, because the gas-phase combustion heats both coal decomposition products and combustible liquid vapors. Moreover, it also reduces the ignition temperature by 20e50  C. Consequently, a decrease in the temperature may compensate for a possible growth in the concentrations of NOx and SOx emissions at the initial stage of combustion. Besides, adding a liquid combustible component leads to a higher temperature in the combustion zone, which promotes the reduction of PCDD/Fs content in the gaseous combustion products and improves the ash residue characteristics. Generalizing the results of this research and known experimental studies (Nyashina et al., 2017; Nyashina and Strizhak, 2018), we can draw a conclusion that adding a liquid combustible component to the composite fuel can contribute to a growth of anthropogenic emission concentrations e sulfur and nitrogen oxides. However, in comparison with dry coals or FC in initial wet state, the deterioration of environmental performance due to adding used oils is not significant (Table 4). That is why composite liquid fuels based on combustible industrial and municipal waste are promising energy resources for thermal power engineering. Using such fuels in real practice, on the one hand, reduces the consumption of high-quality solid fuel, which can otherwise be used in the chemical industry for the synthesis of new materials, and on the other hand, provides a solution to the environmental

Table 4 Allowable emissions of pollutants by power plants (Tugov, 2015) and research findings. harmful substance

power plants for MSW combustion (EU 94/67/EEC), daily average values

power plants for solid fuel combustion (RF state standard GOST 50831-95), research values at a ¼ 1.4 findingsa

solid particles carbon monoxide, CO nitrogen oxides, NOx sulfur oxides, SOx

10 mg/m3 50 mg/m3 (43 ppm)

150e250 mg/m3 300e400 mg/m3 (258e344 ppm)

e 180e370 ppm

200 mg/m3 (160 ppm)

300e640 mg/m3 (340e513 ppm)

190e400 ppm

50 mg/m3 (19 ppm)

1200e1400 mg/m3 (450e526 ppm)

75e135 ppm

a

e the ranges presented were obtained by generalizing the results (Fig. 7) for three groups of composite liquid fuel compositions.

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problems of industrial waste and MSW recycling and reduction of anthropogenic emissions into the atmosphere by coal-fired thermal power plants. Therefore, the obtained results serve as a foundation for the development of an environmentally friendly as well as cost- and energy-efficient co-combustion technology for industrial waste and MSW recycling by burning it as part of composite fuels in boiler furnaces instead of coal. 4. Conclusions 1. We have validated the sustainable ignition and combustion of composite fuel droplets based on combustible industrial (filter cake, turbine oil) and municipal (wood, rubber, plastic, cardboard) waste up to their complete burnout in the conditions typical of boiler furnaces. Ignition delay times for 14 fuel compositions (divided into 3 groups: I e FC 90% þ MSW 10%; II e FC 80% þ MSW 20%; III e FC 70% þ MSW 20% þ oil 10%) have been established. The minimum values are about 3 s and the maximum values are about 25 s at the ambient temperatures ranging from 600 to 1,000  C. 2. The differences in the ignition delay times (up to 20%) for various compositions of composite fuel are more distinct under the near-threshold ignition conditions. We have also determined the differences in the temperatures during combustion, with their maximum values reaching 1,300  C for compositions with 10% of used turbine oil. 3. The main anthropogenic emissions in gaseous combustion products have been analyzed. The concentrations of carbon oxides from the combustion of fuels with various MSW components differ less significantly, than those of nitrogen and sulfur oxides: CO2 e 16e18%, CO e no more than 370 ppm. When various MSW is added to the composite fuel, the maximum difference in NOx concentrations is 50% (46% in normalized values), SOx e 40% (44% in normalized values) or 150 ppm and 50 ppm, respectively, in absolute units. Such differences are rather significant. The maximum concentrations of NOx and SOx in flue gases of filter are 340 ppm and 130 ppm, respectively. 4. The more MSW there is in the composite fuel, the lower the concentrations of NOx and SOx in gaseous combustion products, as compared to the initial FC. Adding used oil to such fuel compositions results in higher concentrations of anthropogenic emissions. For compositions with 20% of MSW, the concentrations of NOx and SOx are 5e10% (2e7% in normalized values) lower than for the corresponding compositions with 10% of MSW. In the case of adding used oil, the concentrations of NOx and SOx are 18e22% and 10e12% lower than for the corresponding compositions without a combustible liquid for samples of the identical mass. In normalized values (by heat of FC combustion) NOx and SOx concentrations lower in 0.5e1.5% and 2e8% for the fuel compositions with a combustible liquid in comparison with the corresponding compositions without oil. Acknowledgments The reported research was funded by Russian Foundation for Basic Research and the Government of the Tomsk region of the Russian Federation [grant number 18-43-700001]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.124892.

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