Environmental aspects of converting municipal solid waste into energy as part of composite fuels

Accepted Manuscript Environmental aspects of converting municipal solid waste into energy as part of composite fuels

Dmitrii Glushkov, Kristina Paushkina, Dmitrii Shabardin, Pavel Strizhak PII:

S0959-6526(18)32468-5

DOI:

10.1016/j.jclepro.2018.08.126

Reference:

JCLP 13910

To appear in:

Journal of Cleaner Production

Received Date:

04 April 2018

Accepted Date:

13 August 2018

Please cite this article as: Dmitrii Glushkov, Kristina Paushkina, Dmitrii Shabardin, Pavel Strizhak, Environmental aspects of converting municipal solid waste into energy as part of composite fuels, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.08.126

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Environmental aspects of converting municipal solid waste into energy as part of composite fuels Dmitrii Glushkov*, Kristina Paushkina, Dmitrii Shabardin, Pavel Strizhak National Research Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk, 634050, Russia1 * Corresponding author. E-mail addresses: [email protected]. 1Heat

Mass Transfer Simulation Laboratory, URL: http://hmtslab.tpu.ru/.

Abstract The paper outlines the results of the experimental research into the ignition and combustion of composite liquid fuel with fine-grained particles of municipal solid wastes (MSW) added as a solid fuel component: wood, rubber, plastic, and cardboard. A relatively low concentration (about 10 wt.%) of these components in the fuel intensifies the ignition process, given that all other conditions are the same. The experimental research has been performed using two experimental setups. The first setup allows for researching the conditions of radiant heating of motionless fuel droplets (about 1 mm in size) in a muffle furnace when the temperature is in the range of 400–1000 °C. The second setup allows for implementing the conditions of convective heating of motionless fuel droplets (about 1 mm in size) in the flow of pre-heated air (at the rate of 3 m/s and the temperature in the range of 400–700 °C). The research findings include minimum temperatures required for the stable ignition of composite liquid fuel with added MSW. They also include the dependencies of ignition delay times on the temperature under different heating conditions. It has also been determined that fuels with MSW are notable for lower nitrogen oxide and sulfur oxide concentrations in gaseous combustion products as compared to fuels without municipal wastes. Maximum difference in the concentrations of NOx and SOx for such fuels reaches 70% and 45% (in absolute units, 125 ppm and 50 ppm, respectively). The results of analytical calculations of relative fuel performance coefficients provide good reasons for the prospective use of such compositions in thermal power engineering. These relative coefficients took values in the range of 1–3. The results obtained provide ground for developing a disposal technology for MSW that cannot be processed or recycled. The technology must be power-efficient as well as economically efficient and environmentally friendly. Keywords: municipal solid wastes, utilization, composite liquid fuel, ignition and combustion, power generation. 1. Introduction Tackling environmental pollution and municipal waste has been one of the main global problems lately. Solving this problem on the whole suggests dealing with groups of tasks. These include, in particular, a rational use of fossil fuels for electricity and heat production, as well as an environmentally friendly and energy efficient waste utilization. The amount of industrial and municipal solid waste 1

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produced annually exceeds the amount of that recovered. Therefore, research on this issue, focused on developing high-potential industrial technologies of converting industrial and municipal solid waste into energy as a part of composite fuels, is very important. Composite liquid fuels based on industrial and municipal wastes as one of alternative fuel types have numerous advantages compared to fossil fuels: they can be prepared from a wide range of combustible waste components with predictable environmental and energy performance indicators, and besides, they provide energy security and foreign exchange saving addressing environmental concerns, and socio-economic issues as well [1, 2]. 1.1. Classification of municipal solid wastes and their processing depth As of now, the problem of MSW processing and disposal is a globally pressing one [3–17]. The main components of MSW are: paper and cardboard (25–30% of total wastes volume); organic wastes (including food, 26–35%); metal and glass (5–12%); plastic (7–10%); wood, textiles and rubber (2–4% of each) [2–5]. Thus, the content of energy-yielding fractions (cardboard, paper, wood, textile, polymer wastes) is about 82% of the total volume of MSW. Annually, 1.3–1.6 billion tons of such wastes are generated globally [2–5]. About 75% of municipal wastes are not reclaimed or processed. Instead, they are stockpiled at landfill sites where they undergo slow thermal decomposition thus polluting the environment. According to experts, the impact of such wastes on the environment is comparable to that of coal-fired thermal power plants and automotive vehicles [6] for some indicators. In different countries, the degree of waste conversion is different [7]. From the European Union Waste Framework Directive, the most characteristic approaches could be highlighted to the disposal of solid wastes [8]: minimizing waste sources, reusing, recycling, composting, combustion and burial, burning as fuel to produce power. Modern combustion technologies allow for deriving up to 80% of energy contained in wastes [9]. Although combustion is aimed at reducing the volume of wastes, disposal through combustion has several typical drawbacks. The drawbacks are associated with the necessity of implementing complicated and costly smoke gas purification routines at waste combustion facilities. Those routines are aimed at reducing the concentration of harmful volatiles down to admissible concentration levels [10]. Despite the evident progress in solving the above problems, the transition from waste stockpiling and burial towards disposal and processing remains a pressing issue. In the USA, China, Russia, India and other countries, most MSW are stockpiled at dedicated landfill sites [10–14]. Authorized and unauthorized landfill sites occupy huge territories (tens of millions of hectares), and their area increases by 6–8% per annum. The environmental situation with landfill sites is further aggravated by the exclusion of huge territories from agricultural use and the pollution of soil, ground waters and atmosphere near populated areas [15]. At landfill sites, biochemical fermentation takes place. It generates biogas that comprises methane (30–50%) and carbon dioxide (50–70%). This leads to the contamination of local air, ground waters and soil. Apart from the negative local impact, the gases emitted during slow combustion 2

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at landfill sites are the main components of the greenhouse gas. The increasing concentration of this gas in the atmosphere leads to the intensification of global warming [16]. Conversion of waste to energy by its burning in fuel composition can be one of the effective solutions for problem of industrial and municipal waste utilization during transition period from dumping towards recycling. According to the global experience, waste burning [14, 17] will allow for disposing of already collected wastes that are not re-usable because of thermal decomposition, decay, etc. Thus, the development of techniques for recovering MSW in order to reduce the load of landfill sites and improving the environmental situation around such sites is a pressing issue. Normally, such problems are solved by simply burning a disperse mixture of source components thus generating heat and electricity [12]. However, due to the low calorific value of MSW [18] (as compared to traditional hydrocarbon fuels [19]) and high concentration of harmful gases (burning plastic and synthetic materials produces toxins and carcinogens [20]), complete replacement of hydrocarbon fuel with combustible wastes is not economically, environmentally or technically viable. 1.2. Combustion of municipal solid wastes In this study, we propose an alternative approach to solving the problem of waste utilization: the use of MSW as components of liquid composite fuels. Such fuels normally comprise three main components [21–24]: low-grade coal, water and used combustible liquid (transformer, turbine or engines oils, etc.). Composite fuel combustion processes are characterized by improved environmental and economical indicators, as compared to burning natural solid fuels. The reason behind this is high combustion efficiency of fuels comprising typical coal and oil processing wastes. This is because the reaction ability of the fuel is improved, thanks to two factors: (i) low-temperature activation of the fuel reactivity at the ignition stage; (ii) combustion intensification during the main reaction of oxidation when carbon from the fuel interacts with water vapor [21–24]. This composite fuel is characterized not only by relatively high combustion temperatures, comparable to those of solid fossil fuels, but also by low content of harmful substances in flue gases [25, 26], as compared to widely spread MSW combustion technologies. Based on the assessments in [27], the adding of 10–20% of typical MSW into composite liquid fuels will decrease the territories to be used as landfill sites by 20–30%. It will also provide for more economical use of nonrenewable hydrocarbon fuels burned to produce heat and electricity. The energy potential (more than 650·1018 J) of industrial and municipal wastes determines the prospects of their utilization by burning them as a composite fuel component. As of 2017, 9.5·109 tons of coal processing wastes, 50·109 tons of MSW, and 0.3·109 tons of used oils have been accumulated all over the world. Partial (50% of energy generation) replacement of coal in thermal power engineering by the equivalent (in terms of energy generation) amount of composite fuels will save about 1 billion tons/year of high-quality solid fossil fuels over 20 years (the regulated period of a safe operation of a boiler in thermal power engineering). During the same period, 24.24·109 tons of filter cakes, 5.76·109 3

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tons of MSW, and 0.36·109 tons of used oils will be utilized. Composite fuel burning in thermal power engineering will completely solve the problem of utilizing used oils accumulated by 2017, as well as that of coal processing waste produced annually. Additionally, it will reduce the amount of filter cakes accumulated by 2017 by 10%. The addition of MSW to composite fuels will ensure the disposal of up to 50% of the annual production. Therefore, the actual task is to study the regularities and characteristics of the physical and chemical processes that occur during the ignition and combustion of typical fuel compositions based on MSW, as well as the study of the environmental characteristics of flue gases and their comparison with the similar characteristics of fuel compositions without MSW additives. 1.3. Purpose and objectives of the research The purpose of this research is to experimentally study the processes of composite fuel droplet ignition and combustion with different additives selected among the most typical municipal solid wastes. Also, a complex analysis of the main environmental, energy and economic performance characteristics of burning such fuels will be performed; the characteristics will be compared to similar ones of natural solid fuels that are burned to produce about 40% of electricity and heat globally. The difference between the present study and other relevant ones is that we proposed recovering MSW not by directly burning it at dedicated waste-burning plants but by burning it as part of composite fuels at thermal power plants, generating power and minimizing hazardous emissions in the atmosphere. Developing the theoretical basis of such technology requires experimental and theoretical research. In this study, we have, for the first time, conducted an experimental research into patterns and characteristics of igniting fuel droplets of several compositions with typical MSW additives, as well as performed an analysis of environmental characteristics of gaseous combustion products of these fuels. 2. Experimental investigation 2.1. Materials preparation The research was performed on five fuel samples based on filter cakes of coking coal (type K) obtained from the Severnaya coal washing plant in Kemerovo region, Russian Federation. Such wastes are a byproduct of coal processing that can be used in thermal power engineering as fuel. When the produced coal is prepared for long hauls, it is washed with water to remove fine fractions. This prevents environmental contamination with coal dust from railroad trains with open carts; it also improves fire safety of the solid fuel when it is exposed to environmental effects. After the coal has been washed, the resulting liquid is left in tanks until fine particles (up to 80 µm in size) settle on the bottom. This residue is then let through press filters to remove excess liquid. The moist residue is the filter cake. The mass fraction of water is about 40%. At coal washing plants, filter cakes are stockpiled at open sites. This results in the contamination of large territories. 4

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The research findings [22–27] allow for making a conclusion that filter cakes are a promising combustible component of composite liquid fuels: (i) no extra preparation of filter cakes (grinding, wetting, or drying) is required, a composite fuel is produced by means of mixing them with other solid or liquid components; (ii) a relatively uniform particle size distribution provides fuel compositions with high lamination resistance of solid and liquid components; (iii) a relatively high heating value (Table 1) can be further increased to that of thermal coals by adding 5–15% of a combustible liquid (used motor, turbine, transformer oils, etc.); (iv) a stable ignition at the ambient temperature exceeding that of coal dust ignition by 50–100 °C, as well as a subsequent stable combustion up to the complete burnout of the organic part; (v) they allow for a long-distance pipeline shipment (over hundreds and thousands of kilometers) and storage in tanks, which reduces fuel losses during transportation and environmental pollution with coal dust; (vi) they have higher fire safety properties than coal during fuel transportation and storage; (vii) a low content of NOx and SOx in flue gases versus the same characteristics of burning coal dust. That is why in this research filter cake (type K) was used as the main composite fuel component. The research has been performed on five different compositions: No. 1 – filter cake 100%; No. 2 – filter cake 90% + wood 10%; No. 3 – filter cake 90% + rubber 10%; No. 4 – filter cake 90% + plastic 10%; No. 5 – filter cake 90% + cardboard 10%. The mass fractions of the components are listed here. Their appearance is shown in Fig. 1.

filter cake

wood

rubber plastic Fig. 1. The appearance of fuel components.

cardboard

The size of the particles of fine-ground MSW is comparable to that of coal particles. The main properties of the fuel components are listed in Tables 1–3. The properties of the filter cake have been obtained for dry samples; to that end, they have been dried at 105 °C until the moisture has fully evaporated. Table 1. Properties of fuel components [18, 24, 28]. Proximate analysis. component

Wa (%)

Ad (%)

Vdaf (%)

Qas,V (× 106 J/kg)

filter cake

–

26.5

23.1

24.83

wood

20.0

2.0

83.1

16.45

rubber

2.0

1.8

67.4

33.50

plastic

2.0

0.2

99.5

22.00

cardboard

5.0

3.0

79.8

17.50

5

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Cdaf (%)

Hdaf (%)

Ndaf (%)

Sdaf (%)

Odaf (%)

filter cake

87.2

5.1

2.1

1.1

4.5

wood

50.3

6.0

0.2

0.1

43.4

rubber

97.9

1.2

0.3

0.6

–

plastic

66.7

7.9

–

–

25.4

cardboard

46.3

6.3

0.3

0.2

46.9

Table 3. Flash temperature and ignition temperature of fuel components. component

flash temperature, °C

ignition temperature, °C

filter cake

–

450

wood

230

340

rubber

–

350

plastic

306

415

cardboard

–

250

Fuel droplets were generated by Finnpipette Novus electronic dispenser with an ad-hoc tip (minimum and maximum dosage volumes are 1 µl and 10 µl, pitch variation is 0.1 µl). The size of droplets was about 1 mm. 2.2. Apparatus and experiment procedure The processes of the ignition and combustion of composite liquid fuel droplets have been researched using two experimental setups (Figs. 2 and 3). Setup No. 1 allows for implementing the conditions of radiant heating of motionless fuel droplets in a muffle furnace when the temperature is in the range of 400–1000 °C. Setup No. 2 allows for implementing convective heating conditions of similar fuel droplets in the flow of pre-heated air (at the rate of 3 m/s and the temperature in the range of 400–700 °C). The use of different experimental techniques allows for analyzing more thoroughly how different conditions of droplet interaction with the environment influence the characteristics of fuel ignition. When ignition conditions are close to threshold ones, the characteristics of the induction period will significantly depend on the patterns of phase transitions of different mechanisms of heat transfer to the liquid droplet [29].

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Fig. 2. Schematic of experimental setup No. 1.

Fig. 3. Schematic of experimental setup No. 2. The advantage of setup No. 1 lies in the wide range of possible temperature adjustments. The temperature can reach 1000 °C and more. Such temperatures are peculiar to solid fuel combustion processes taking place in boiler furnaces [30]. Under intensive heating of a fuel droplet, an increase in the oxidizer temperature over 1000 °C will have lesser effect on the ignition delay time; thus, 1000 °C has been taken as the limiting value of the temperature variation range. For setup No. 2, the upper limit of the oxidizer temperature variation range is 700 °C, which is due to the specifications of the heating equipment. Setup No. 2 allows for modeling the processes of fuel droplet interaction with pre-heated air 7

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flux in laboratory conditions; these conditions are the basis of industrial technologies of fuel combustion [31]. The main difference between the experimental setups (Figs. 2 and 3) lies in the use of different equipment for creating conditions necessary for igniting fuel droplets. In setup No. 1 (Fig. 2), Nabertherm R 50/250/13 rotary muffle furnace was used as such equipment. The inner diameter of the ceramic tube is 0.04 m and tube length is 0.45 m; the temperature variation range is 20–1200 °C; the temperature is adjusted by the signal of an integrated type S thermocouple. In setup No. 2 (Fig. 3) the flux of pre-heated air was generated by LEISTER LE 5000 HT heater (11 kW, maximum temperature at the output is 900 °C) and LEISTER ROBUST air fan (0.25 kW, air flow rate up to 1200 l/min, static pressure up to 8 kPa) in a glass cylinder (inner diameter 0.1 m, length 0.5 m). Air temperature at the heater output was adjusted by signals from the integrated type K thermocouple. Fuel droplets were fed into the heating zone using a positioning robotic arm (Figs. 2 and 3), and its velocity was not more than 0.5 m/s in order to prevent droplet deformation and sliding off the ceramic holder. In setup No. 1 (Fig. 2) the fuel droplet was fed into the ceramic tube along the tube symmetry axis through one of the side apertures. The ongoing processes were recorded using a high-speed camera aimed through the opposite aperture in the tube. In setup No. 2 (Fig. 3), the fuel droplet was introduced through an aperture in the wall of a glass cylinder across the air flux into the air flux until it reached the symmetry axis of the tube. The high-speed video camera is looking across the air flux. When experiments in the cross sections corresponding to the location of the fuel droplet were conducted (Fig. 2 and 3), the gaseous medium temperature Tg was monitored using the type K thermocouple (measured temperature range 0–1100 °C, accuracy ±0.004T for temperatures over 400 °C, time lag no more than 3 seconds). Also, before starting the experiments, the concentration of oxygen is monitored in the above cross-section (and it was about 20%). Testo 340 gas analyzer (accuracy ±0.1%, and measurement resolution 0.01%) was used for the measurements. When series of 5–7 experiments each were conducted at identical initial conditions, the following parameters were monitored: temperature (Tg) of the heated air, air velocity (Vg) at the convective heating, initial droplet radius (Rd). The processes taking place during fuel ignition and combustion were recorded using high-speed monochrome Phantom Miro M310 camera. Its main specifications are as follows: filming rate 3268 frames per second at maximum resolution 1280×800 pixels; pixel size 20 µm; minimal exposure 1 µs; 12 bit depth; 12 Gb memory. Video recordings of the experiments were analyzed using the Tema Automotive software. High-speed video recording system is used to conduct a detailed analysis of consistent patterns in the combustion process; it has also allowed for automatically calculating the ignition delay time (td). The values of td were determined from the evolution of the droplet luminance over time [32] by the Threshold algorithm implemented via a group of procedures of the Tema Automotive software [33–35].

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According to the method [33–35], the thresholds were determined for gas-phase ignition of volatiles in the vicinity of the droplet and its heterogeneous ignition. The processes occurring during the induction period were recorded by a high-speed video camera. Software Tema Automotive with continuous tracking algorithm was used to determine the ignition moment [33–35]. A frame-by-frame analysis of a high-speed video record of the process under study allowed to define the ignition moment, and corresponding values of td. The ignition delay time is a period from a droplet injection into the heated air to the combustion initiation (in the gas phase or in the droplet surface). Events identification was implemented in Tema Automotive software by the image autotrigger. The autotrigger identifies the emergence of a flame in the gas region outside the droplet during the gas-phase ignition of volatiles. Inside the droplet, the autotrigger identifies the moment of the glow appearance on its surface. The tracking algorithm monitored the intensification values (from 0 to 255) of gray shades (from black to white) in the area of the video record. An intensity range of 220–255 corresponded to the sample combustion. The ignition moment was recorded when finding the value of this range. The systematic error when calculating td did not exceed 3%. Random errors for sets of 5–7 experiments under identical conditions did not exceed 10%. Besides the Tema Automotive software was used for monitoring the size of droplets (about 1 mm) via the initial frames of the video record (i.e. before the droplet was heated). Using Tema Automotive algorithms, four droplet sizes were automatically measured in four sections. The obtained values were averaged and droplet radius Rd was then calculated. Systematic error when determining Rd with the respective video recording resolution settings of the high-speed camera did not exceed 4%. Setup No. 1 (Fig. 2) is used to analyze the concentration of harmful gases in gaseous fuel combustion products. Testo 340 gas analyzer is mounted in place of the high-speed camera. The gas analyzer has four sensors: O2 (measuring range 0–25%, accuracy ±0.2%, resolution 0.01%), CO2 (measuring range 0–25%, accuracy ±0.2%, resolution 0.1%), CO (measuring range 0–10000 ppm, accuracy ±10%, resolution 1 ppm), SO2 (measuring range 0–5000 ppm, accuracy ±10%, resolution 1 ppm), NOx (measuring range 0–4000 ppm, accuracy ±5%, resolution 1 ppm). The method for measuring the content of anthropogenic gases produced by burning the fuel is similar to the one used in [25, 26]. A fuel sample weighing at least 1 g was placed inside the cylinder (dimensions d=10 mm, h=10 mm) made of stainless steel mesh. Such minimum admissible amount of fuel was chosen in accordance with the specifications of the gas analyzer used. To obtain reliable measurement results, minimum volumes of flue gases had to be supplied as a prerequisite. The cylinder with the fuel was positioned in the center of the ceramic tube of the muffle furnace using a robotic arm (Fig. 2). From the aperture in the opposite side, a gas analyzer probe was inserted into the ceramic tube to collect flue gases. To conduct experiments, both apertures in the ceramic tube (Fig. 2) were tightly sealed with thermal insulating material to prevent flue gases from escaping into the medium.

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At identical initial conditions, series of 4–6 experiments were conducted. The results of the experiments are presented as average experimental data values.

3. Results and discussion 3.1. Characteristics of ignition and combustion Figure 4 shows the frames of the video recordings that illustrate the ignition and combustion processes for the droplets of composite liquid fuels in the heated air flow. It was found that consistent patterns in the investigated processes for the four fuel compositions with the addition of 10% of MSW (No. 2–5) are similar to the patterns in the processes of ignition and combustion of the original filter cake (No. 1). The obtained result is explained by the determinant influence of this component on the consistent patterns in the physical and chemical processes taking place during fuel heating. It could be highlighted the following main stages of the interaction of composite liquid fuel droplet with the heated air flow: inert heating; moisture evaporation from the subsurface layer; thermal decomposition of flammable components (coal and MSW); mixing combustible gases with the oxidizer; gaseous mixture ignition and burnout; solid residue heating; heterogeneous ignition and combustion of the solid residue. The distinction of the investigated processes for compositions No. 1–5 (Fig. 4) lies in different duration of separate stages. We have sorted the components that were added to filter cakes in accordance with the ignition intensity of the resulting composition, in the ascending order: wood, rubber, plastic, cardboard. Composition No. 5 (filter cake 90% + cardboard 10%) shows minimum ignition delay times as compared to other compositions under identical heating conditions. Such a result was obtained because there is less moisture in the composite fuel with added MSW as compared to composition No. 1 (filter cake without combustible additives). In such conditions, less energy and time is spent on heating subsurface droplet layer, moisture evaporation and combustion initiation. Also, adding typical MSW leads to the emergence of a pronounced stage of gas-phase combustion in the droplet vicinity. This is caused by the majority of typical municipal wastes are peculiar for gas-phase combustion mode. When they are heated, thermal decomposition takes place and gaseous products react with the oxidizer. For the MSW investigated in this research, the share of non-combustible solid residue is much less than that for the filter cake (Table 1). Therefore, adding them into the fuel is the reason for gas-phase combustion in the droplet vicinity and reduced ash residue after the burnout of combustible components. Lesser ignition delay times for composition No. 5 based on filter cake and cardboard are explained through relatively low temperature values necessary for initiating cardboard combustion (Table 3), as compared to the similar characteristic of other components (in compositions No. 2–4).

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Fuel composition No. 1 (filter cake 100%)

t=0

1.5 s

3.0 s

4.5 s

5.5 s

6.0 s

Fuel composition No. 2 (filter cake 90% + wood 10%)

t=0

1.5 s

3.0 s

4.5 s

5.5 s

9.0 s

Fuel composition No. 3 (filter cake 90% + rubber 10%)

t=0

1.5 s

3.0 s

4.5 s

5.5 s

9.0 s

Fuel composition No. 4 (filter cake 90% + plastic 10%)

t=0

1.5 s

3.0 s

4.5 s

5.5 s

9.0 s

Fuel composition No. 5 (filter cake 90% + cardboard 10%)

t=0

1.5 s

3.0 s

4.5 s

5.5 s

9.0 s

Fig. 4. Ignition and combustion of composite liquid fuel under convective heating in the heated air flow at Tg=600 °C (experimental setup No. 2). Consistent patterns in the development of fuel droplet combustion under convective heating are characterized by the peculiarities of air flow around the droplet [36]. The processes of rapid droplet heating and moisture evaporation are taking place on the front side (i.e. the one facing the air flow). These processes occur in a less intense manner on the back side of the droplet in the region with lower air flow velocities. A local zone of droplet heterogeneous ignition is formed on the side facing the air flow (Fig. 4). After that, the process of thermal decomposition of fuel components intensifies. Gaseous products of 11

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thermal decomposition of the fuel mix with the air in the vicinity of the droplet. Frames showing the gasphase combustion stage (Fig. 4) illustrate that the incoming air flow separates from the droplet surface in the vicinity of droplet equator (i.e. in the direction of the air flow). Here vortexes are formed along the air flow direction. In this region, linear speed of the gas flow is significantly lower than that of the air stream flowing around the droplet, and the concentration of combustible gases is at maximum. Therefore, gasphase combustion of the gaseous mixture takes place in this region. Heterogeneous combustion is developing in parallel with gas-phase combustion (Fig. 4). The front of heterogeneous combustion is spreading from a local source over the droplet surface in the direction of the air flow movement. It was found that as a result of moisture evaporation from the subsurface layer of the droplet, the structure of its surface becomes rough (Fig. 4). In this case, a distinct protrusion facing the incoming air flow is a zone of local droplet ignition. The duration of heating an individual protrusion until the ignition of the carbonaceous residue is significantly smaller than the duration of the induction period when a smooth local ignition zone of the droplet is formed. The result obtained can be explained by the fact that droplets with smooth surface (unlike those with rough surface) are characterized by intense heat dissipation from the heating zone of the subsurface layer to the depth of the droplet. This leads to a prolonged inert heating stage. Under radiant heating, consistent patterns in the physical and chemical processes (Fig. 5) are similar to those determined for convective heating conditions (Fig. 4). However, rather a significant difference lies in the long duration of inert heating of the fuel droplet and in the formation of the ignition and combustion zone of thermal decomposition products in the oxidizer medium above the droplet. The result obtained is attributed to the absence of intensive air movement in the droplet vicinity. Because of the uniform droplet heating, the rate of thermal decomposition of combustible components is identical along the entire droplet surface. That is why gas-phase ignition of thermal decomposition products occurs in a uniform manner in the droplet vicinity. Then the combustion process takes place predominantly above the droplet, where, as a result of the thermogravitational convection, a stoichiometric ratio is established between the concentration of the combustible gaseous mixture components [37]. It can be noticed that the intensity of the flame in the frame with t=7.4 s (Fig. 5) is lower in the lower regions of the droplet and increases as we go vertically upwards from the droplet bottom. This indicates the presence of a rich premixed flame zone in this region, which occurs due to the relatively higher concentrations of the oxidizer, as compared to the fuel here [37]. The evaporation and the thermal decomposition intensity of the composite fuel components is relatively low. This causes the flame to be established nearer to the droplet surface. As compared to the conditions of convective heating, the conditions of radiant heat transfer are the reason for more uniform heating of the solid combustible residue. As a result, such a system is characterized by uniform heterogeneous ignition of the solid residue of thermal decomposition, without a pronounced zone of local ignition.

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Fuel composition No. 5 (filter cake 90% + cardboard 10%)

t=0

3.0 s

7.2 s

7.3 s

7.4 s

11.2 s

Fig. 5. Ignition and combustion of composite liquid fuel under radiant heating at Tg=600 °C (experimental setup No. 1). The conditions of radiant heating implemented using experimental setup No. 1 have allowed us to determine the influence of typical MSW (that are added into the filter cake) on the ignition delay time. Figure 6 shows the dependencies of ignition delay times for fuel compositions No. 1–5 in a wide range of air temperature Tg variations (450–1000 °C). Increasing the air temperature from 450 to 1000 °C leads to a decrease in ignition delay time of various fuel compositions from 14–20 to 1–3 s respectively (Fig. 6).

Fig. 6. The dependence of the fuel ignition delay times on the air temperature under radiant heating (experimental setup No. 1).

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It was found that for the stable ignition of fuels based on filter cakes with finely ground typical MSW, the air temperature has to be at least 450 °C. Although the temperatures required for igniting individual components of the mixture are significantly below 450 °C (Table 3), they are not determinant for the limiting temperature required for igniting the composite fuel. As noted above, the addition of different components influences the intensification of the ignition process. Figure 6 shows the differences between the ignition delay times. The duration of the induction period for fuel compositions with MSW is less than td of the filter cake without any additives. When wood is added into the filter cake, the ignition delay times are reduced by 7%; if rubber is added, they are reduced by 13%; if plastic, by 17%; and if cardboard, by 22%. This result indicates that the operation of heat generation equipment can be optimized by adding different solid combustibles to the fuel and varying their concentration. The differences in ignition delay times for different composite fuel compositions (Fig. 6) in a wide range of ambient air temperature can be explained by analyzing the MSW characteristics presented in Tables 1 and 3. The MSW under consideration is characterized of the gas-phase ignition mechanism. When a substance is heated up, its thermal decomposition occurs and flammable gases get heated, too. When the gas mixture reaches threshold concentrations and temperatures, ignition occurs. Thus, the higher the volatile content in MSW, the more rapidly the combustible gas mixture is formed, all the other conditions being equal. However, the intensity of thermal decomposition depends not only on the volatile content in the substance but also on its moisture content. The higher the moisture content, the less rapidly the heating of the fuel and its thermal decomposition occur. This is explained by the endothermic heat effect of water evaporation. When a wet combustible substance is heated, a great amount of energy supplied to the fuel is spent on the phase transition (2 MJ/kg). Further heating and thermal decomposition of the substance occur only after water evaporates. Therefore, ignition delay times of fuel composition No. 2 (filter cake 90% + wood 10%) are higher than those of fuel compositions No. 3–5 (see Wa in Table 1). The moisture content of rubber, plastic and cardboard is 2–5%, which is significantly lower than that of wood (20%). Therefore, this factor has no great effect on ignition delay times of fuel compositions No. 3–5. Apart from the above factors, the intensity of the gas-phase ignition of substances is characterized by flash and ignition temperatures (Table 3). The lower these temperature values, the lower the ambient temperatures required to heat up the fuel for the intensive thermal decomposition to start and the combustible gas mixture to form. Under identical conditions of heating fuel samples with different flash and ignition temperatures, substances with lower flash and ignition temperatures demonstrate more intensive thermal decomposition and, consequently, lower ignition delay times. Therefore, the comparison of characteristics of fuel compositions No. 3–5 in Table 3 shows that the ignition delay times of composite fuels with cardboard are lower than those of plastic and rubber, the former having the minimum ignition temperature. Also, in the course of the research, it was determined that at air temperatures above 1000 °C, the intensity of combustion initiation is high, and the heat and mass transfer have less impact on the ignition 14

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characteristics as compared to air temperatures Tg=450–1000 °C. In the case of radiant heating at Tg>1000 °C, the ignition delay times for the same fuel composition are only marginally different. For example, at Tg=1100 °C and Tg=1200 °C the difference td is less than 5%. For practical applications, the research findings of ignition of composite liquid fuels under convective heating (acquired from setup No. 2) bear the most interest. Figure 7 shows the dependencies of ignition delay times for fuel compositions No. 1–5 in a wide range of temperatures Tg (450–700 °C) of air flow. It has been determined that in the conditions of convective heating of fuel droplets, the temperature of the heated air flux at 450 °C is the minimum required for the stable ignition of the compositions of composite liquid fuels based on filter cakes with MSW. Increasing the temperature of air flow from 450 to 700 °C leads to a decrease in ignition delay time of various fuel compositions from 6.5– 7.5 to 2.5–3 s respectively (Fig. 7).

Fig. 7. The dependence of the fuel ignition delay time on the air temperature under convective heating (experimental setup No. 2). The research findings (Figs. 6 and 7) allow the conclusion that, irrespective of the droplet heating mechanism (convective or radiant), 450 °C is the threshold temperature required for initiating combustion. Also, the analysis of the results (Figs. 6 and 7) lets us for to deduce that under more intense heat exchange with the medium, at identical air temperatures, ignition delay times differ by 2–3 times. 15

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Apart from this fact and as compared to the results (Fig. 6), the maximum difference in td for compositions with different components is at least 15% (Fig. 7). The research findings allow for making an important practical conclusion that efficient utilization of typical MSW is possible at thermal power engineering facilities. Moreover, adding such components into the main fuel will contribute to the reduction of fuel consumption while excluding adverse effects on the power characteristics of the process (see combustion heat values in Table 1). 3.2. Environmental characteristics The concentration of harmful gases in composite fuel combustion products has been analyzed for two main components: NOx (Fig. 8) and SOx (Fig. 9). Increasing the temperature of ambient air from 700 to 1000 °C leads to a increase in NOx and SOx concentrations from 30–80 to 180–340 ppm (Fig. 8) and from 10–20 to 75–130 ppm (Fig. 9) respectively. When the fuel was burned with typical MSW used as fuel components, the concentration of carbon oxides in flue gases was not so different.

Fig. 8. The dependence of NOx concentration in flue gases of composite fuel on the air temperature at which the combustion process was initiated. In accordance with the element composition of the fuel components (Table 2), the content of nitrogen and sulfur in typical MSW is 2–10 times lower than that in the original filter cake. Thus, as a 16

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result of preparing the fuel composition, the mass fraction of the filter cake is reduced because a solid combustible component is added (wood, rubber, plastic, cardboard) that contains less nitrogen and sulfur. Accordingly, as a result of burning such fuel in conditions identical to those when burning composition No. 1 (filter cake 100%), the concentration of main anthropogenic emissions of NOx and SOx into the atmosphere is reduced. When adding MSW (for instance, plastic) into the fuel, the heat emitted during combustion is equivalent to that when burning filter cake without any additives (Table 1). In some cases, for example, when rubber is added, the heat effect of the fuel combustion process exceeds that of filter cake (Table 1). It can be concluded that when typical MSW is added into a composite fuel, it yields an equivalent amount of energy during combustion, while the concentration of main anthropogenic emissions in flue gases is lower.

Fig. 9. The dependence of SOx concentration in flue gases of composite fuel on the air temperature at which the combustion process was initiated. The results shown in Figs. 8 and 9 are in good agreement with ultimate analysis data (Table 2) of composite fuel components. We have sorted the components that were added to filter cakes in accordance with the concentration of NOx and SOx, in descending order: rubber, cardboard, wood, plastic. Composition No. 4 (filter cake 90% + cardboard 10%) shows minimum ignition delay times as compared to other compositions under identical combustion conditions. It was determined that for compositions No. 17

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1 and 4, maximum difference in NOx concentrations is 70%, and in SOx concentrations, it was 45%. In absolute units, these differences are 125 ppm and 50 ppm, respectively. Such differences are rather significant because maximum NOx and SOx concentrations in gaseous filter cake combustion products are about 300 ppm and 130 ppm (Figs. 8 and 9), respectively. When fuel ignition and combustion occur at high temperatures, the concentration of anthropogenic components in flue gases increases. When the temperature of the oxidizer medium is in the range of 700– 1000 °C, the concentration of NOx and SOx increases by 4–6 times for all the compositions of composite liquid fuels under study. These results are due to the increased intensity and depth of interaction of the initial fuel components with the oxidizer. In the range of relatively high temperatures, the rate of nitrogen and sulfur oxidation reaction is also high. This is the reason behind rather significant difference between the results obtained at Tg=700–1000 °C (Figs. 8 and 9). 3.3. Relative performance indicators of composite fuels with added wastes To evaluate the viability of the use of typical MSW as components of composite liquid fuels, we have performed a comprehensive analysis that takes into account economic, environmental and energy output aspects. Dmitrienko et al. [38] have presented two approaches to the evaluation of the main advantages of the use of composite liquid fuels instead of coals. These evaluations relied on relative environmental, engineering, economic and energy output indicators of using such fuels as compared to coal. The first approach is based on the simultaneous consideration of the determined weight coefficients (environment, economy and energy). The ratios of anthropogenic emissions for coals and high-potential composite liquid fuels were determined [38]. After that, the resulting ratios were determined for all harmful emissions. In addition, the dependences of these ratios on the combustion temperature were obtained, and the ratios of heat of combustion were calculated for different fuel samples. The economic indicators were calculated taking into account the average market value of the components in the fuels under study. The second approach lies in calculating a complex coefficient describing the amount of energy (MJ) produced by combustion in relation to the cost (USD) of fuel slurry and concentration of the main harmful emissions (ppm). That was the approach used in this paper for assessing the prospects of the practical application of composite fuels based on filter cakes with added MSW. The calculations have been performed for five fuel compositions using the following expressions [38]: DCF+MSW NOx = Qas, V CF+MSW / (CCF+MSW ∙ NOx_CF+MSW); DCF+MSW SOx = Qas, V CF+MSW / (CCF+MSW ∙ SOx_CF+MSW); DCF+MSW NOx&SOx = DCF+MSW NOx ∙ DCF+MSW SOx, where D is the performance indicator; Qas,V is thermal effect of the fuel combustion process (Table 1), MJ/kg; 18

C is cost, USD/kg;

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NOx is nitrogen oxide concentration (Fig. 8), ppm; SOx is sulfur oxide concentration (Fig. 9), ppm; CF+MSW indexes are composite fuel with added MSW, NOx – nitrogen oxide, SOx – sulfur oxide. Maximum concentrations of anthropogenic emissions (Figs. 8 and 9) at Tg=1000 °C were used in the calculations. The costs of different fuel compositions were calculated in proportion to the components concentration. The main contribution into the cost of coal processing wastes (filter cakes) and MSW (wood, rubber, plastic, cardboard) was assumed to be the expenses incurred during transportation from the storage site to the fuel preparation site – 0.0058 USD/kg. A relative performance indicator was introduced for the visual representation of the advantages of using fuels with added MSW. It is equal to the ratio of the performance of composite fuel with added MSW to the performance of composite fuel without additives: Drelative = DCF+MSW NOx&SOx / DCF NOx&SOx. The results of the calculations are shown in Fig. 10. For composition No. 1 (filter cake 100%), Drelative = 1. For compositions No. 2–5, Drelative varies from 1.5 to 2.5. The performance indicators of fuel compositions No. 2–4 with those of pure filter cakes were compared to analyze the effectiveness of MSW adding to composite liquid fuels. The values of Drelative > 1 for fuel compositions No. 2–4 illustrate the potential of using the technology of MSW disposal by burning them as an additive to liquid composite fuels.

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Fig. 10. The relative indicators of real-life performance of composite fuels with different compositions. The research findings (Fig. 10) allow the conclusion that from the viewpoint of the complex performance indicator (taking into account the economic, environmental and energy aspects), compositions No. 3 (filter cake 90% + rubber 10%) and No. 5 (filter cake 90% + cardboard 10%) have the highest priority. Nevertheless, the performance of all the compositions of composite liquid fuels with added MSW is higher than that of filter cakes by 1.5–3 times (Fig. 10). 4. Prospects of waste disposal by burning them as additives to composite fuels The development and introduction of technologies of burning composite liquid fuels with typical MSW will allow for the industrial disposal of such wastes, as their annual global production is 1.3–1.6 billion tons [3–5], and 70–80% of that amount is not re-claimed. New approach provides for several positive effects: (i) Reducing the area of landfill sites, thanks to the scheduled disposal of unreclaimed MSW that cannot be used as secondary raw materials due to thermal decomposition, decay, etc. (ii) Reduced environmental pollution, thanks to the MSW disposal as part of an environmentally friendly thermal power engineering technology. (iii) Saving on high-quality solid fossil fuels by reducing their consumption by thermal power engineering through the replacement by an equivalent amount of MSW. 20

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(iv) Efficient use of investments to develop cutting-edge industrial thermal power technologies and upgrading thermal power plants. The message in the last statement implies that the disposal of MSW by burning and producing heat and electricity is an intermediate stage between the incineration of wastes (without energy production) and re-using wastes as raw materials. That is why the development and introduction of radically new technologies and the construction of new industrial facilities that will be used for a relatively short time period (10–15 years) is not economically feasible. In the recent years, much attention is paid to the gasification of different substances and materials (low-quality fossil fuels, biomass, MSW, etc.) and the development of respective industrial technologies for producing syngas [1, 2, 39–44]. The technology for producing heat and electricity from burning syngas is characterized by minimal content of harmful substances (CO, CO2, NOx, SOx) in flue gases; their concentration does not exceed the one observed when burning natural gas. Therefore, in terms of environmental indicators, the performance of burning syngas obtained by the gasification of MSW is higher than that when burning typical composite liquid fuels with such wastes added as fine-dispersed solid components. However, currently, the practical use of syngas production technologies is limited. “The major barrier that has prevented the widespread uptake of advanced gasification technologies for treating MSW has been the higher ash content in the feed making the gasification operation difficult. In addition, high amounts of tar and char contaminants in the produced gas make it unsuitable for power production using energy efficient gas engines or turbines” [40]. Currently, the most successful approach that has been practically implemented is biomass gasification [41–44]. However, from the perspective of negative environmental impact, MSW are the most dangerous type as their industrial disposal technologies are underdeveloped. The use of syngas in thermal power engineering instead of widely spread solid fossil fuels or liquid composite fuels will entail the implementation of costly supplementary procedures to add an industrial waste gasification unit into the process or install piping and tanks for transporting and storing syngas produced at the facilities. Some measures are also required to comply with explosion and fire safety standards. Besides, the gasification of MSW is rather a power-intensive technology as is. Normally, rather high temperature (900 °C) in an inert gas medium is maintained inside the reactor [39, 40] for rapid thermal decomposition of solids. Such conditions are power-intensive. That is why the disposal of MSW by combustion as composite liquid fuel additives is a practicable and promising approach.

5. Conclusions (i) Large volumes of unreclaimed MSW are determinant for the prospect of their combustion as additives to composite liquid fuels. The development of efficient technologies for burning such fuels is characterized by positive environmental and economic effects. That is why it has been experimentally discovered consistent patterns and necessary conditions for the ignition of composite liquid fuel droplets 21

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under convective and radiant heating. The fuels were based on filter cakes with 10% of typical MSW – wood, rubber, plastic, cardboard. (ii) The main stages have been highlighted for mutually dependent physical and chemical processes: inert heating of a droplet; moisture evaporation from the subsurface layer; thermal decomposition of flammable components (coal and MSW); combustible gases mixing with the oxidizer; gaseous mixture ignition and burnout; heating of the solid residue; the heterogeneous ignition and combustion of the solid residue. (iii) It has been found that 450 °C is the minimum air temperature required for the stable ignition of composite liquid fuel droplet. Wood, rubber, plastic, and cardboard (in the ascending order) intensify the ignition of the compositions under study when added to the cake. The 90% filter cake + 10% cardboard composition shows the shortest ignition delay times of all the compositions tested under identical heating conditions. The maximum difference of 15% is observed between ignition delay times for compositions with different components under convective droplet heating (air temperature range 450–700 °C) and 22%, under radiant heating (air temperature range 450–1000 °C). In the case of radiant heating at Tg>1000 °C, the ignition delay times for the same fuel composition are only marginally different (less than 5% at td≈2.5 s). (iv) When typical MSW are added into a composite fuel, it yields an equivalent amount of energy during combustion, while the concentration of main anthropogenic emissions in flue gases is lower. Maximum difference in the concentrations of NOx and SOx for such fuels reaches 70% and 45%, respectively. In absolute units, these differences are 125 ppm and 50 ppm. This result characterizes rather a significant impact of additives made of MSW on the reduction of the concentration of nitrogen and sulfur oxides in flue gases. This is because maximum NOx and SOx concentrations in the gaseous products of filter cake combustion are about 300 ppm and 130 ppm, respectively. (v) The obtained results are the basis for developing upgrade routines for solid and liquid fuel combustion technologies currently used in thermal power engineering for the co-disposal of MSW. In particular, it is possible to optimize the modes of operation of heat-generating equipment by adding different solid combustible components into the fuel and adjusting their concentration. From the viewpoint of the complex performance indicator (taking into account the economic, environmental and energy aspects), compositions No. 3 (filter cake 90% + rubber 10%) and No. 5 (filter cake 90% + cardboard 10%) have the highest priority. Nevertheless, the performance of all the compositions of composite liquid fuels with added MSW is 1.5–3 times higher than that of filter cakes. Acknowledgments The reported research was funded by Russian Foundation for Basic Research and the government of the Tomsk region of the Russian Federation, grant No. 18-43-700001.

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ACCEPTED MANUSCRIPT Highlights - At least 1.3 billion tons of municipal solid wastes are produced annually, 80% of which is not processed - The disposal of typical wastes by adding them to composite liquid fuels is a promising approach - Burning such fuels yields an equivalent amount of energy with NOx and SOx concentrations reduced by 70% and 45% - 450 °C is the minimum temperature required for the ignition of a typical fuel composition - A 10% mass fraction of MSW in fuel composition improves ignition by at least 15%