The prospects of burning coal and oil processing waste in slurry, gel, and solid state

Applied Thermal Engineering 156 (2019) 51–62

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Research Paper

The prospects of burning coal and oil processing waste in slurry, gel, and solid state

T

⁎

Ksenia Vershinina , Galina Nyashina, Vadim Dorokhov, Nikita Shlegel National Research Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk 634050, Russia

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

and oil wastes can effectively be • Coal burned together in slurry, gel and solid state.

low ignition delay times and • Fairly ignition temperatures are typical for solid fuel.

Minimum concentration of NO and • SO are typical for slurry. The relative efficiency is maximum for • waste-derived slurries. relative efficiency is minimum for • The waste-derived fuel in a solid state. x

x

A R T I C LE I N FO

A B S T R A C T

Keywords: Industrial waste Rapeseed oil Coal-water slurry Combustion Emissions Fuel effective utilization factor

This paper presents the experimental investigation results of ignition and combustion characteristics of fuels based on typical coal flotation waste which is formed and accumulated around the world in large volumes (about 200–300 million tons per year). We focus on the comparison of these characteristics for waste-derived fuels in different states: slurry, gel, and solid. We analyze the ignition delay times, minimum (threshold) ignition temperatures, combustion heat, concentrations of anthropogenic emissions, and the relative efficiency of fuels. We have determined that minimum ignition temperatures and shorter ignition delay times are achieved in the solid state (50–80% lower than in the slurry and gel state). The lowest concentrations of the most hazardous emissions are present in the liquid (slurry) state (NOx and SOx are 18–75% lower than from the combustion of coal in the solid state). The conditions were identified for the efficient use of coal and oil processing waste as part of composite fuels. The relative fuel effective utilization factor of waste-derived fuel mixtures in different states ranged from 0.11 to 45.5. It is maximal for fuels in the slurry state. The obtained data has been compared with the results of other studies.

1. Introduction The urgency of the problem of deep coal processing and more complete and efficient use of its products is caused by the escalation of global environmental [1], social and economic [2] problems due to the

increasing consumption of primary fossil energy resources (coal, oil, natural gas) [3–5]. In this respect, a significant negative contribution is made by using coal, the most dangerous power source in terms of environment (impressive volumes of cindery dumps and gaseous anthropogenic emissions). This is fully applicable to China, India, Russia,

Abbreviations: CWS, coal water slurry; CWSP, coal water slurry containing petrochemicals ⁎ Corresponding author. E-mail address: [email protected] (K. Vershinina). URL: http://hmtslab.tpu.ru (K. Vershinina). https://doi.org/10.1016/j.applthermaleng.2019.04.035 Received 24 December 2018; Received in revised form 22 March 2019; Accepted 8 April 2019 Available online 09 April 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature Drelative NOx Qas Std SOx Td Tg Tgmin

Ad ash level of dry sample, % Cdaf, Hdaf, Ndaf, Odaf fraction of carbon, hydrogen, nitrogen, oxygen in the sample converted to a dry ash-free state, % Ci cost of component, $/kg DiNOx relative parameters describing the heat of coal, CWS or CWSP combustion with regard to the cost of components and concentration of emissions NOx, MJ/$·ppm DiSOx relative parameters describing the heat of coal, CWS or CWSP combustion with regard to the cost of components and concentration of emissions SOx, MJ/$·ppm DiNOx&SOx relative parameters describing the heat of coal, CWS or CWSP combustion with regard to the cost of components

Vdaf τd

and concentration of emissions NOx and SOx, MJ/$·ppm relative complex parameter concentration of nitrogen oxides, ppm heat of combustion, MJ/kg fraction of sulfur in the sample converted to a dry state, % concentration of sulfur oxides, ppm temperature in the center of a slurry droplet (particle), °C temperature in the combustion chamber, °C minimum temperature in the combustion chamber sufficient for sustainable fuel ignition, °C yield of volatiles of filter cake converted to a dry ash-free state, s ignition delay time, s

Dmitrienko MA, et al. [24] show that a set of environmental, economic and energy performance parameters make CWSPs rather attractive, so their combustion can become more efficient than that of coal dust at real thermal power plants and boiler plants within several years. At the same time, the greater the installed power capacity of power units, the shorter the payback period of fuel preparation systems when using CWSP. Analyzing the results [12], we can conclude that oil sludge, oil refining waste, used industrial and domestic oils are the most attractive additives in terms of the volume available worldwide and the advantages obtained (ignition process acceleration, heat of combustion increase, etc.). In some experiments, these wastes were added to CWSPs to be burned as part of mixed fuel [25] or in the form of coal particles granulated with oil (using oil granulation to obtain low-ash coal-oil granulate) [26]. It is a relevant task to conduct a complex analysis of the combustion efficiency of waste-derived fuel mixtures (in the slurry, gel or solid state) based on coal processing waste and vegetable oils, used industrial oils or oil sludge. Coal-oil granulate production and combustion have been the focus of some studies including [27,28]. It is also necessary to note the experience of preparation and industrial combustion of waste-derived slurries described in the studies [20,29–31]. However, in these papers, coal of different types, fuel oil or specialized additives in the form of oils are used as solid or liquid combustible components. Within this research, we focus on producing coal-oil solid fuel (granulate) from coal flotation waste and vegetable and used industrial oils. All these components, unfortunately, are still hardly involved in the fuel and energy cycle. Using waste-derived fuel mixtures could solve several important problems: disposal of numerous industrial wastes and freeing areas occupied by them; expanding the scope of raw materials for energy generation and reducing mining rates (exhaustion of subsoil); recovery of waste and service water (it can be burned as part of fuel slurries); improving fire and explosion safety at power enterprises (ignition of slurries is impossible without specialized heating); lower anthropogenic emissions due to using water as part of slurries; longer service life due to lower temperatures of fuel combustion in combustion chambers. The purpose of this work is to experimentally determine the prospects of burning mixtures of coal processing and oil refining wastes in the slurry, gel, and solid states, taking into account the main energy, environmental, technical and economic parameters.

Japan, Poland, the USA, and other countries, where coal processing efficiency cannot be considered high [6]: waste rock with an ash content of 60–70% and fine coal slime with 20–50% of ash are dumped in disposal areas. The amount of such slime annually amounts to at least 10–12% of the initial (mine) coal. Thus, irrecoverable losses of the extracted coal amount to considerable volumes [6,7]. For example, in Kemerovo region (Russia), an increase in the total mass of such waste annually exceeds 12–15 million tons. In addition to direct losses of extracted coal, stockpiling and storage of fine waste of solid fuel processing lead to serious environmental pollution. Most of the chemicals (flocculating agents, coagulants, etc.) used in the preparation process are adsorbed on fine coal particles and discharged with them into the environment, polluting nearby territories and water bodies [6-8]. There is also a significant problem with the use of lignite, whose reserves in Russia make up more than 30% of the world volumes [9]. Taking into account their high moisture content, oxidability, and low combustion heat, the use of lignite is actually limited. Long-distance transportation of lignite is economically unprofitable. Thus, increasing the value of final coal products by, e.g., involving unused coal slime at lignite mining sites is a significant scientific result [10,11]. Over the recent years, coal and oil processing wastes, as well as lignite, sludge, resins, coal tar residues, and other combustible components have been regarded as components of coal-water slurries containing petrochemicals (CWSP) to be involved in the energy cycle. The review article [12] highlighted the main problems (in the field of fuel slurry preparation, storage, transportation, and combustion) that restrain the development of fuel slurry technologies in the world. Potentially significant factors and processes that affect the ignition and combustion of waste-derived fuels have been identified: the ratio of components, coal dust particle size, oxidizer temperature, properties of components, method and duration of fuel preparation, etc. Not only the direct combustion of coal-water slurries (CWS) and CWSP, but also the production of syngas from these slurries is a relevant area for development [13]. Common disadvantages of CWS and CWSP combustion technologies can be neutralized using specialized additives, which sometimes allow to completely eliminate them and gain advantages. In particular, straw, sawdust, charcoal, vegetable oils, and waste are used to reduce the negative environmental indicators of burning CWS and CWSP and to intensify the ignition processes [14–17]. Used industrial and domestic oils, oil sludge, resins, and other combustible liquids are used to increase the combustion heat and temperature in the combustion chamber [18–20]. Metal powders, ceramics or sand are applied to stabilize the characteristics of slurry fuel combustion (speed of reactions, change of temperature in the combustion chamber, etc.) [21]. Some additives are recycled as CWSP components [22,23] without much improvement in their combustion performance. The role of additives is specific. They are applied to provide certain environmental, economic or energy performance characteristics.

2. Materials Coal flotation waste (filter cakes) and used industrial (turbine, transformer, automobile) oils are one of the largest waste categories in terms of accumulated volume [4,32]. Therefore, we opted for these wastes as fuel components. In this paper, the filter cake of nonbaking coal was chosen, as it has rather typical physical and chemical properties [33]. The water content and combustion heat of the filter cake of nonbaking coal in its initial (wet) state is about 43.5% and 16.42 MJ/ 52

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During a series of experiments, the temperature in the combustion chamber of the muffle furnace (Tg) was varied in the range of 600–1000 °C. Before each experiment, the measurement of the temperature sensor, built into the muffle furnace, was checked using a thermoelectric temperature transducer (type K, temperature range 0–1100 °C, systematic error ± 3 °C, response rate no more than 10 s). After the temperature stabilized, video recording started and a fuel droplet (particle) was moved into the furnace. The size (initial diameter) of droplets and fuel particles in the conducted research was about 2 mm. To determine this parameter, we saved a frame with a droplet (particle) before it entered the combustion chamber. Using a Tema Automotive software package, we measured the droplet (particle) size in four sections. The initial diameter was determined by the mean value [12,15,25]. The systematic error of size determination with the corresponding resolutions of video cameras did not exceed 4%. The results were compared if the deviation of the average radius did not exceed ± 0.15 mm. Since we placed the droplet (particle) on the thermocouple junction, we were able to measure the temperature inside the sample (Td) in its heating process. At a first approximation, the temperature at the “thermocouple junction – fuel” interface can be regarded as the temperature in the central part of the droplet (particle) [15,25]. The ignition delay and complete combustion times of the fuel droplet can be determined by the trend of the Td temperature change over time. The heterogeneous ignition of the coke residue, undoubtedly, begins from the particle surface. However, recording the droplet surface temperature for the investigated fuels of different compositions is difficult, because the droplet surface is transformed during the heating process, its position is not constant, and dispersions of solid particles are possible with some compositions. For this reason, using the temperature inside the droplet as a reference value is a fairly reasonable approach [25,33]. The ignition delay time (τd) and minimum ignition temperature (Tgmin) were chosen as parameters characterizing the ignition processes of the investigated fuels [25,33]. The ignition delay (τd) was the interval from the start of the droplet (particle) heating up until when the ignition criterion was satisfied, which required the following conditions to hold simultaneously: Td ≥ Tg and dTd/dτ ≥ 10 °C/s [25,33]. It is also possible to determine the ignition delay time using a video recording of the investigated process. The initiation of the combustion reaction is characterized by the appearance of a certain level of luminosity. Therefore, it was possible to determine the ignition criterion not only by the change in the reference value – the temperature in the droplet center – but also by the change in the luminosity of the fuel sample. Currently, this approach is widely used [33,37]. For example, in [37] the luminosity was used as a criterion for determining the moment of coal particle ignition. When using this approach, it is important to establish the boundaries defining the beginning and end of fuel burning. In this study, we relied on the experience of measuring the temperature inside the fuel droplet and the corresponding ignition criterion described above [25,33]. It suggests that the problem of determining the ignition delay time and combustion duration was solved using specialized software algorithms of Tema Automotive, which allows tracking the luminosity intensity of a fuel droplet (particle). For this, we used the Threshold software parameter, which allows to set the color gradient of the RGB color model in the observation area. According to this color

kg, respectively [25]. From the studies [24,25,34] we can conclude that vegetable and some used industrial oils are most attractive by environmental, economic, and energy performance parameters. In addition, the use of oils of different origin within one experiment allows analyzing the importance of this factor (in terms of changes in the scale of ignition and combustion parameters). Therefore, rapeseed and used turbine oil were selected. Tables 1 and 2 show the basic properties of the filter cake and oils [12,25,33]. We studied waste-derived fuel mixtures in three states (Fig. 1): (i) slurry; (ii) gel state; (iii) solid state. Fuels were prepared using a homogenizer in accordance with the methods from [25,33]. The components were mixed for 10 min to obtain a homogeneous mass, and the resulting composition was placed in a storage container. In this study, we added oil (petroleum and vegetable) to the filter cake so that the proportion of oil in the final mixture was either 10% or 20%. The share of filter cake in the initial slurry was 90% and 80%, respectively. Thus, the concentration of solid particles in the slurry was about 39% and 35%, respectively. Experiments with slurries (Fig. 1a) were carried out for 24 h after preparation. To obtain the second fuel type – gel fuel (Fig. 1b) – the slurry was stored in a container for three days (72 h). During this time, under the gravity force, solid particles of the filter cake began to precipitate. A moisture layer was formed on the fuel surface, and after that, it was removed with a syringe. The precipitate (dried slurry or gel) was the second type of investigated fuel (Fig. 1b). The third type of fuel is represented by solid particles (Fig. 1c). Special samples were made from the initial slurry in the form of granules with a size of about 2 mm (for studying ignition characteristics) and 20 mm (for studying emission characteristics). After that, the fuel was kept in the open air for 24 h at a temperature of 20 °C, pressure of 101.3 kPa and relative moisture content of 80%. The water content in the three types of fuels was 35–39% for slurry; 21–24% for gel fuel; and 10–12% for solid particles. It was determined by weighing the fuel (in the gel and solid states) and calculating the mass difference before and after storage and drying. The water content in the initial slurries was determined according to the water content of the filter cake as the main component of the fuels. 3. Experimental setup and methods Fig. 2 shows a scheme of the setup [15,34] used to study the ignition and combustion of waste-derived fuels. The main elements of the setup are a rotary muffle furnace, a coordinate mechanism for automatic movement of the fuel sample into the combustion chamber, a highspeed video camera (up to 105 fps), a gas analyzer, and a PC. Ignition and combustion characteristics of single slurry fuel droplets were determined according to the methods described in [15,25]. This approach was well tested in the study of new types of fuels to obtain basic experimental data before conducting full-scale tests (for example, in a pilot-scale boiler, where it is difficult to track combustion aspects due to failure to fully implement the video recording and thermocouple measurements) [15,25]. Due to its simplicity and reliability, this approach is often used to investigate slurry fuels [35] and solid fuels (for example, granulated wastewater sludge [36]). Auxiliary tools were used to generate fuel droplets/particles. Droplets of the wettest fuel (Fig. 1a) were generated by a dispenser. Solid particles (Fig. 1c) were initially formed in the shape of spherical particles using a miniature laboratory spatula. This tool was also used to form less viscous jelly fuel (Fig. 1b). The generated fuel droplet (particle) was placed on the junction of a fast-response thermocouple (type S, temperature range 0–1600 °C, systematic error ± 1 °C, response rate no more than 0.1 s, junction diameter 0.1 mm). The thermocouple was installed in the coordinate mechanism which was moved by a drive connected to the personal computer.

Table 1 Results obtained from proximate and ultimate analysis of filter cake of nonbaking coal in dry state [25]. Proximate analysis

53

Ultimate analysis (%)

Ad (%)

Vdaf (%)

Qas (MJ/kg)

Cdaf

Hdaf

Ndaf

Std

Odaf

21.2

16.09

26.92

90.13

4.255

2.31

0.441

2.77

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Table 2 Properties of oils. Sample

Density at 20 °C (kg/m3)

Moisture content (%)

Ash (%)

Flash point (°C)

Ignition temperature (°C)

Combustion heat (MJ/kg)

Used turbine oil [25] Rapeseed oil

868 911

– 0.28

0.03 0.03

175 242

193 –

44.99 39.52

Under identical initial conditions, 6 to 10 experiments were carried out within one measurement set. Then rough errors were eliminated and the experimental results were averaged. The results were taken into account if they differed from the mean values by no more than 2–3%. The remaining measurements were discarded in accordance with the processing algorithms of experimental data obtained in multifactor experiments. Fig. 1. Appearance of the investigated fuels: a – slurry in its original form; b – gel (after storage of original slurry); c –solid particles.

4. Results and discussion 4.1. Ignition delay times and minimum ignition temperatures

model, the gradient value 255 corresponds to white; the gradient value 0 corresponds to black. It was assumed that the sample combustion corresponded to the 220–255 RGB range [33,36]. The ignition delay time τd was the interval from the moment the droplet (particle) was introduced into the combustion chamber and up until the Threshold parameter reached 220 in any point of the sample surface. Under identical initial conditions, 6 to 10 experiments were carried out within one measurement series. To determine the minimum ignition temperature, the temperature in the combustion chamber was varied with an increment of 5 °C. The measurements were repeated at least 6 times to record the ignition and combustion. Rough errors were eliminated by repeating the experiment. The concentrations of the main anthropogenic emissions of SOx and NOx were selected as parameters characterizing the environmental characteristics of fuel combustion [24,34]. For the emission measurement, we used a gas analyzer with the SOx and NOx measurement range of 0–2000 ppm and accuracy of 10 ppm [24,34]. Gaseous emission concentrations were recorded for 5 min. The flue gases formed during the fuel combustion passed through the gas sampling hose to the electrochemical sensors of the gas analyzer, where a quantitative and qualitative analysis of chemical compounds in a gaseous medium was performed. Continuous monitoring and recording of flue gas components were carried out using specialized EasyEmisson software [24,34]. The concentrations of sulfur and nitrogen oxides were not converted; for processing and analysis we used the values (ppm) directly measured by the gas analyzer in the process of burning fuels of different compositions.

Figs. 3 and 4 present the ignition characteristics of droplets and particles of waste-derived slurries, gel and solid fuels. Fig. 3a shows how the ignition response rate of droplets and solid particles changes with different combustion chamber temperature and fuel composition (the group with rapeseed oil is considered). Moisture content is, undoubtedly, the determining factor for the ignition of the investigated fuels. Three states (Fig. 1) of the same fuel (by type of components) have different water content. Therefore, wetter fuels – the initial slurry in the liquid state – are characterized by the highest ignition response rate. Lower moisture content in the fuel decreases the ignition delay times. Therefore, the lowest ignition response rate is typical of solid particles (Fig. 3a). This type of fuel ignites more easily than the initial slurry. The maximum differences in the ignition delay time of the slurry and solid fuel are on average 70–75%. These differences are most noticeable at low combustion chamber temperatures and decrease with a temperature growth of the external gas medium (Fig. 3a). The measured ignition delay times (Fig. 3a) are in good agreement with the experimental results from [36]. In this study, dry (water content is not more than 10%) sewage sludge was burned in the form of granules (diameter of 5–10 mm) at a temperature of 800–900 °C. The recorded ignition delay of volatiles varied in the range of 8–10 s on average. This range coincides very well with the time range of the heterogeneous ignition delay of all the fuels under study, with the exception of slurries (Fig. 3a). The ignition delay time of wet slurries does not fall into the selected mean range, since a rise in the moisture content increases the inert heating stage accordingly.

Fig. 2. Scheme of experimental setup. 54

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Fig. 3. Ignition delay times (a) and minimum ignition temperatures (b) of fuel droplets (particles): 1 – 90% filter cake, 10% rapeseed oil (slurry); 2 – 80% filter cake, 20% rapeseed oil (slurry); 3 – 90% filter cake, 10% rapeseed oil (gel state); 4 – 80% filter cake, 20% rapeseed oil (gel state); 5 – 90% filter cake, 10% rapeseed oil (solid state); 6 – 80% filter cake, 20% rapeseed oil (solid state).

Fig. 4. Ignition delay times (a) and minimum ignition temperatures (b) of fuel droplets (particles): 1 – 90% filter cake, 10% used turbine oil (slurry); 2 – 80% filter cake, 20% used turbine oil (slurry); 3 – 90% filter cake, 10% used turbine oil (gel state); 4 – 80% filter cake, 20% used turbine oil (gel state); 5 – 90% filter cake, 10% used turbine oil (solid state); 6 – 80% filter cake, 20% used turbine oil (solid state).

It has been established that an increase in the proportion of oil reduces the time of combustion initiation as well. Oil is a combustible component with a high calorific value, it easily evaporates and ignites (as compared to coal processing waste), which contributes to the accelerated ignition in the solid part of composite fuel. An increase in the oil share in the investigated fuels led to a proportional decrease in the water and solid component share. It is safe to say that at the stage of fuel ignition, the influence of oil vapor ignition and combustion on the subsequent ignition of the solid residue is predominant. Therefore, with an increase in the share of rapeseed oil, the ignition delay times decreased on average by 10–12% (for initial slurry) and by 10–55% (for gel slurry). It is interesting to note that an increase in the proportion of oil in the solid fuel did not lead to a significant decrease in the ignition response rate (Fig. 3a). The likely reason for this is that even a smaller amount of burning oil vapor and volatiles is enough to heat and ignite the dewatered coke residue, and an increase in the oil proportion does not significantly accelerate this process. The threshold (minimum required) temperatures to initiate the combustion of the fuel group under study (with rapeseed oil) ranged from 390 to 475 °C (Fig. 3b). A similar pattern was observed in the experiments: the ignition temperature decreased with decreasing fuel

moisture. This parameter was significantly different for the wettest (initial slurry) and driest fuels (solid particles). The maximum differences were 40–45 °C. The minimum ignition temperatures for solid particles with different rapeseed oil content (10% and 20%) were about 390 °C and 430 °C, respectively. These values are quite different from the ignition temperatures of granulated sewage sludge [36], as well as biomass and sewage-based pellets. According to the study [36], the ignition temperature of granules was 340–590 °C with the sample size variation from 5 mm to 10 mm and at the velocity of the heated stream of ≈2 m/s. Jiang L., et al. [38] used fuel compositions that included sewage sludge and plant components (Chinese fir, camphor, and rice straw). In order to evaluate the effect of biomass fraction (20–80%) on the ignition characteristics of fuels, a thermogravimetric analysis in an oxidizing atmosphere was carried out. The minimum ignition temperatures for these fuels were on average 220–290 °C [38]. An increase in the rapeseed oil share in the fuel reduced the ignition temperature by 40–45 °C for all the compositions under study (Fig. 3b). It is also important that close (with a difference of no more than 10 °C) ignition temperatures can be achieved by adding a larger proportion of oil to the wet slurry rather than by drying the fuel. For example, the 55

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terms of its potential calorific value when burning in a boiler. The most attractive fuel by this criterion is, undoubtedly, the solid fuel with a high proportion of liquid combustible oil products (Figs. 5, 6). The combustion heat of the fuels under study (Figs. 5, 6) can be considered higher than the average combustion heat of typical waste and biomassbased fuels. For example, this parameter of dehydrated sewage sludge may vary in the range of 12–15 MJ/kg [36,38]; a wider range is characteristic of various plant agricultural wastes – on average, 7–18 MJ/kg [40,41]. By modifying the composition of the pellets [42] or, for example, co-combustion of different types of waste, it is possible to achieve sufficiently high values of combustion heat (for example, 21.7 MJ/kg for pellets from a mixture of biomass and dry sewage sludge [42]). This approach is also illustrated in this work (Figs. 5, 6). The following factors will reduce the calorific value of the wastederived fuel mixture: (i) an increase in the mass fraction of inert moisture; (ii) reducing the proportion of the liquid combustible component; (iii) use of vegetable oils (since petroleum liquids, especially oils, have a higher calorific value). It is estimated (Figs. 5, 6) that adding even 10% of rapeseed or used turbine oil to the wet coal processing waste (filter cake) increases the caloric content of 1 kg of the resulting fuel mixture by 2.31 MJ and 2.88 MJ, respectively. However, fuel combustion should be considered not only in terms of its energy performance. Technological, economic, and environmental aspects must be taken into account as well. Therefore, maximizing the calorific value and minimizing the ignition cost cannot be the only purpose of creating a fuel composition. It is important to consider that the combustion heat increase leads to higher heat density of heating surfaces, and potentially to an increase in the formation of harmful emissions. These aspects require investigation. It is also necessary to take into account that the technology of flame combustion is realizable only for slurries, and is impossible for granulated solid or jelly-like fuels that are most attractive by energy performance criteria. Experiments have shown that fuels based on wet coal flotation waste and oils have relatively high storage stability (they are not delaminated compared to typical CWS). A relatively small amount of moisture was separated in the fuel mixtures stored for three days in a sealed container (Table 3). There are two reasons for this. Firstly, coal flotation waste contains a small amount of reagents (coagulants, flocculants) introduced into coal during its washing. These reagents promote the binding of solid particles and prevent their precipitation. Secondly, the oil components also contribute to the binding, coating of solid particles, and creating a stable structure of the stored fuel. According to the observations (Table 3), both rapeseed and used turbine

initial slurry with 20% of rapeseed oil has the same ignition temperature as solid fuel containing 10% of oil (Fig. 3b). The parameters of the fuel ignition processes with the addition of used turbine oil are shown in Fig. 4. Similar patterns have been established for these fuels and for the compositions with rapeseed oil (Fig. 3). Higher combustion chamber temperature led to a decrease in the ignition response rate of all the compositions (Fig. 4a). Lower fuel moisture content (in the sequence “initial slurry – gel-like fuel – solid fuel”) produced the same result. However, the numerical values and scales of the ignition characteristics change were different (Figs. 3a, 4a). Compared to rapeseed oil, used turbine oil has lower flash and ignition temperatures and a higher calorific value (Table 2). Therefore, for the fuel with rapeseed oil, longer ignition delay times were recorded than for the fuel with the addition of turbine oil, provided that the same type of fuel samples were compared (for example, gel-like slurries). The differences in this comparison varied on average from 13% to 40%, depending on the fuel composition and heating temperature (Figs. 3a, 4a). Thus, the presence of components with lower boiling points, flash and ignition temperatures contributes to the implementation of lowtemperature gas-phase and heterogeneous combustion (as compared to conventional combustion processes of coal-water slurries). For comparison, Fig. 4a shows the curves of the ignition delay times of mechanically activated fuels based on filter cakes. These fuels (slurries) were prepared using the technology described in [39]. In the present study, there was no task to analyze the nature and aspects of physical and chemical activation processes for slurry fuels. However, it is necessary to give a brief description of such slurries. Activation of the original waste (filter cake) is carried out by demineralization of carboncontaining raw materials using the method of oil granulation and subsequent processing of raw materials by exposure to extreme influence (mechanical and ultrasonically-induced cavitation, short-pulse electro-hydraulic discharge) [39]. The final product is characterized by improved energy performance, as well as resistance to delamination during storage. It is advisable to compare the ignition characteristics for the investigated fuels and mechanically activated filter cakes. It is clear from Fig. 4a that the ignition delay times of the mechanically activated filter cake without additives and with the addition of turbine oil (10 wt %) are very low and varied in the experimental conditions in the range of 1.45–2.6 s (Fig. 4a). The ignition delay times closest to this range are typical of solid fuels containing used turbine oil. In comparison with the mechanically activated slurries, wet fuel slurries have rather high ignition delay times (at least 10 s, see Fig. 4a). Thus, wet mechanically activated slurries based on coal processing waste have great prospects in terms of minimizing the ignition costs. However, it is expensive to activate filter cakes. Therefore, an approach that does not imply deep processing of the feedstock with a simultaneous increase in the time spent on the fuel ignition is also reasonable. The diagram in Fig. 4b illustrates the minimum temperatures in the combustion chamber required to ignite fuel droplets and particles based on coal processing waste and used turbine oil. For the group of fuels under consideration, the ignition temperature varied in the range of 400–480 °C. The highest ignition temperature is characteristic of the slurry with a low concentration of waste oil product or without it at all. At high moisture content, the proportion of combustible mass in the fuel is lower. In such conditions, the concentration of volatile products of coal particles and oil vapors is not enough to realize the ignition conditions. Moreover, a large amount of heat is spent on endothermic evaporation. It is necessary to increase the temperature in the combustion chamber to meet the ignition conditions of all the fuel components. For similar reasons, the ignition temperature of dehydrated solid particles with a high proportion of turbine oil was the lowest (Fig. 4b). The highest combustion heat values of the investigated fuels are presented in Figs. 5 and 6. These values have been obtained additively, based on the mass fraction and combustion heat of each individual fuel component. It is possible to assess the prospects of a particular fuel in

Fig. 5. High calorific values of the investigated fuels: 1 – 90% filter cake, 10% rapeseed oil (slurry); 2 – 80% filter cake, 20% rapeseed oil (slurry); 3 – 90% filter cake, 10% rapeseed oil (gel state); 4 – 80% filter cake, 20% rapeseed oil (gel state); 5 – 90% filter cake, 10% rapeseed oil (solid state); 6 – 80% filter cake, 20% rapeseed oil (solid state); 7 – 100% filter cake. 56

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parameters (increases the inert period), it also leads to the effect of microexplosive fuel breakup, which will be discussed further. This effect is technologically important and consists in a spontaneous fragmentation of the initial fuel droplet (particle) during heating. After the dehydration of the fuel droplet (particle) top layer, the evaporation process of oil product and decomposition of the plant component and coal particles are accelerated. These processes proceed with energy consumption and transfer of part of the supplied energy into the droplet depth. As this stage proceeds, a combustible mixture is formed around the droplet. It consists of an oxidizing agent, decomposition products, and vapors of combustible liquid components. When the required concentration is reached, the mixture ignites in the gas phase. The heat release from the gas-phase flame (Figs. 7–12), intensifies the heating of unreacted components. With sufficient heating, heterogeneous ignition of the solid part of the fuel is realized. The described mechanism is common for all the investigated fuel compositions but differs by the duration of individual stages (Figs. 7–12). Fig. 8 shows that the process of fuel droplet ignition is accompanied by the formation of a finely dispersed cloud of burning particles and, presumably, burning volatile components. This effect was less pronounced for the slurry with a lower proportion of oil in the composition (Fig. 7) and for fuels in the gel and granular form (Figs. 10, 12). It can be concluded that the presence of oils in the composition of composite waste-derived fuels leads to the microexplosion effect. Another important condition of such effect is a sufficient share of water in the fuel since the boiling of both water and combustible liquids intensify it [43] during fuel heating. A higher temperature increases the effect of microexplosive fragmentation. The role of water (or rather, its rapid evaporation during heating) is very important for the microexplosive breakup of multicomponent fuel droplets. When the initial droplet (particle) breaks up, the specific surface reaction area increases significantly, facilitating the access of oxygen molecules to the combustible substance and intensifying the heating and burnout. It is for these reasons that the microexplosive fragmentation of fuel droplets is considered a phenomenon that can improve and facilitate the combustion process [44,45]. It should be noted that among all the fuel compositions examined, fragmentation was only observed in mixtures with a sufficiently high proportion of water (i.e. for slurries and gel fuels). Water, during its transition to the gaseous state, contributes to the buildup of pressure in the droplet (particle) and subsequent sharp release of flammable and water vapors with a spontaneous destruction of the droplet or particle. The recorded effect is in good agreement with the results of other studies. In particular, Burdukov AP, et al. [45] established that the effective mass combustion rate is higher for a coalwater slurry droplet (lignite, anthracite, and bituminous coal were used) than for a dehydrated particle. This effect is presumed [45] to be associated with the reaction of water vapors and carbon, as well as with the phenomenon of microexplosive breakup (partial or complete fragmentation) of a droplet at the stage of moisture evaporation and volatile release, due to which the oxidation surface area increases.

Fig. 6. High calorific values of the investigated fuels: 1 – 90% filter cake, 10% used turbine oil (slurry); 2 – 80% filter cake, 20% used turbine oil (slurry); 3 – 90% filter cake, 10% used turbine oil (gel state); 4 – 80% filter cake, 20% used turbine oil (gel state); 5 – 90% filter cake, 10% used turbine oil (solid state); 6 – 80% filter cake, 20% used turbine oil (solid state); 7 – 100% filter cake.

oils prevent the lamination of the slurry. Used turbine oil showed the best result. In such slurry, the smallest proportion of water was separated after three days of storage (Table 3).

4.2. Aspects of ignition process and combustion modes of waste-derived fuels Figs. 7–12 show typical video frames of the investigated combustion processes of waste-derived fuels in different states. Experiments with a slurry fuel droplet, gel fuel droplet, and a solid fuel particle were compared. At the first stage of the droplet (particle) heating, the evaporation rate of water and liquid combustible components increases. Already at this stage, it is possible to record the changes occurring in the considered system “fuel – heated air”. In particular, when moisture evaporates, the surface configuration of the fuel droplet changes, since the vapors, diffusing through the internal structure of the fuel, lead to the surface transformation, separation of small particles and changing the droplet surface appearance from glossy to matt. The higher the temperature in the combustion chamber, the faster these processes occur. The influence of water evaporation on the ignition and combustion processes of the fuel mixtures under study can be considered as one of the determining factors. The lower the water content in the fuel, the faster and less pronounced the dehydration stage, which is typical of gel slurry droplets and especially granulated fuel particles. The duration of inert heating of a slurry droplet is longer than that of dried fuels since water evaporation requires a sufficiently large energy supply compared to other endothermic heat effects in the “fuel – heated air” system. In particular, the evaporation heat of water is about 2 MJ/kg, that of petroleum oils is 160–350 kJ/kg, and the heat effect of the thermal decomposition of coal is 200–300 kJ/kg. Thus, the process of water evaporation is dominant due to its energy intensity, but with increasing temperature, the rate of water evaporation increases significantly and the contribution of this process to the total duration of the inert period (i.e. before the fuel ignition) becomes smaller. However, although the presence of a sufficient amount of water (approximately 30% and above) in the fuel mixture deteriorates the ignition

4.3. Environmental parameters Figs. 13, 14 show the concentrations of sulfur and nitrogen oxides resulting from the combustion of the fuels under study in three states (slurry, gel and solid). Comparing the concentrations of sulfur and nitrogen oxides (Figs. 13, 14) in the process of burning slurries and dried fuels, it was established that as the fuel moisture content decreases, SOx and NOx

Table 3 Mass of moisture separated during storage of slurry relative to the initial fuel mass. 90% filter cake, 10% rapeseed oil

80% filter cake, 20% rapeseed oil

90% filter cake, 10% used turbine oil

80% filter cake, 20% used turbine oil

2.22%

1.68%

1.85%

0.73%

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Fig. 7. Video frames of ignition and combustion of the initial slurry droplet (90% filter cake, 10% rapeseed oil).

fuels with rapeseed oil (Figs. 13a, 14a). This is conditioned by the chemical composition of the combustible liquids. The sulfur content of turbine oils has a relatively small variation and most often is in the range of 0.5–1.1%; the sulfur content in rapeseed oil does not exceed 0.03% [48]. For the solid fuel, the difference between the compositions based on turbine and rapeseed oils does not exceed 15% on average; high temperatures in the case of burning waste-derived fuels in the solid state stimulated the oxidation of fuel sulfur. However, the difference in nitrogen oxide values was insignificant for all the three states under study when varying the type of oil (Figs. 13b, 14b). This is because the temperature mode has a greater impact on the emission of nitrogen oxides. The high calorific value of rapeseed and used turbine oils (Table 2) leads to a significant increase in the combustion zone temperature. An additional thermal effect of combustible liquids leads to a high-temperature mechanism of nitrogen oxidation in the combustion zone when high temperatures are reached (more than 1000 °C). Consequently, it can be concluded that when waste-derived slurry fuels in the granulated state are burned, the type of used combustible liquid and its mass concentration do not have a significant effect on the concentration of gaseous emissions in the combustion products, whereas the variation of these parameters for slurry fuels can change the values of SOx and NOx concentration in wide ranges from 5% to 50%. The comparison of the obtained results with the identical parameters for some other types of fuel indicates the acceptability and availability of the investigated fuel compositions. For example, the concentration of SOx and NOx in the combustion products of coal dust (at 1000 °C) was approximately 350 and 415 ppm, respectively [52]. These values exceed the established SOx emissions for all the fuels under study (Figs. 13a, 14a) and NOx for all the fuels, except the granulate (Figs. 13b, 14b). Ren X., et al. [53] determined the emission parameters of burning various biomass types (corn straw, sugar cane, rice husk, olive oil cake, etc.). The concentrations of harmful gas emissions were measured for each of the studied samples. The range of SO2 and NOx concentrations in the exhaust gases was 20–130 ppm and

emissions can increase to 65% and 75%, respectively. This result is explained by a significant influence of the water component concentration on the composition of the combustion products of wastederived slurry fuel. Water vapors reduce the temperature and rate of adiabatic combustion, which leads to a decrease in the formation of NOx. A low moisture level in the solid fuel leads to a regular temperature increase in the combustion zone, which intensifies the oxidation of nitrogen and sulfur, contained in the fuel and air, to the corresponding oxides. In addition, the decomposition of water caused by heating leads to the release of free oxygen and hydrogen molecules [34,46–48]. This transformation occurs by the equation 2H2O(g) ⇄ 2H2(g) + O2(g)) [49,50] at a temperature of about 1000 °C and above. Oxygen generated during the reaction intensifies the combustion process, and hydrogen acts as a reducing agent, helping to reduce the content of nitrogen and sulfur oxides in flue gases: SO2 + 3H2 → H2S + 2H2O (at temperatures above 600 °C) [48]; 2NO + 4H2 + O2 → N2 + 4H2O (at temperatures above 200 °C) [46]. For the three investigated aggregate conditions, an increase in the concentration of the liquid combustible component (from 10% to 20%), as exemplified by used turbine oil, can lead to an increase in the concentrations of sulfur and nitrogen oxides in the combustion products (Fig. 14). For example, at a temperature of 1000 °C, for the slurry, the concentration of SOx (NOx) increased by 25% (43%), for the gel-like fuel – by 15% (37.5%), for the solid fuel – by 5% (8%). It is obvious that as the water content of the fuel decreases, the influence of the combustible component content on the concentration of anthropogenic emissions decreases. Thus, varying the concentration of used turbine oil when burning waste-derived fuels in the solid state is possible in a wide range without significant harm to the environment. An increase in the mass concentration of rapeseed oil (Fig. 13), on the contrary, improves the environmental parameters of combustion, since the plant component of the fuel is non-toxic and biodegradable [51]. Replacing part of the fuel with a plant additive (rapeseed oil) decreases the total sulfur and nitrogen content of the mixture. The concentration of sulfur oxides in the combustion products of fuels in the slurry and gel states with turbine oil is higher than that of

Fig. 8. Video frames of ignition and combustion of the initial slurry droplet (80% filter cake, 20% rapeseed oil). 58

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Fig. 9. Video frames of ignition and combustion of gel slurry droplet (90% filter cake, 10% rapeseed oil).

50–750 ppm, respectively [53]. When burning pellets made of coal and sewage sludge, the following emission ranges were recorded in the combustion conditions in a test boiler at 800 °C and with varying sample sizes: 110–175 ppm for SO2 and 370–540 for NOx [26]. The differences in the emission characteristics of the fuels studied in this work and the results obtained by other researchers are explained by different properties of the fuels and differences in the experimental approaches. However, in general, it can be said that the measured ranges (Figs. 13, 14) of the SOx and NOx concentrations are in good agreement with the measurements of other studies [26,52,53].

Fig. 11. Video frames of ignition and combustion of solid particle (90% filter cake, 10% rapeseed oil).

4.4. Relative fuel effective utilization factors It is possible to estimate the benefits of using the fuels under study vs. conventional boiler fuel (coal) by calculating the relative factors [52,54]. They take into account the combustion heat (Figs. 5, 6), fuel costs, and environmental characteristics of combustion (Figs. 13, 14). The paper [54] provides a description and justification of several methods for calculating relative indicators for new types of fuels. In the present paper, one of the considered approaches has been chosen. The following expressions were used [52,54]: DcwspNOx = Qas, V cwsp/(Ccwsp · NOx_cwsp); DcwspSOx = Qas, SOx_cwsp); DcwspNOx&SOx = DcwspNOx · DcwspSOx; Drelative = DcwspNOx&SOx/DcoalNOx&SOx,

V

Fig. 12. Video frames of ignition and combustion of solid particle (80% filter cake, 20% rapeseed oil).

components assuming zero cost of water, since process and sewage water can be used to prepare composite slurry fuels. The cost of coal flotation waste (filter cakes) is assumed to be zero, only transportation costs are taken into account (0.0058 $/kg). The estimates (Fig. 15) indicate that waste-derived fuels in the slurry state containing used turbine oil are the most effective. As moisture decreases, the calculated parameter decreases due to improved environmental combustion characteristics. However, even in the solid state, the particles of waste-derived fuels based on coal processing and oil refining waste are 4.5–11.3 times as effective as coal dust (for conventional coal dust, Drelative = 1). The fuels with the addition of rapeseed oil, despite the acceptable combustion heat and relatively low SOx and NOx emissions, are characterized by low efficiency indicators (Fig. 15), which is associated with a high cost of rapeseed oil as compared to coal and used turbine oil. The

cwsp/(Ccwsp ·

where Qas, V – combustion heat, MJ/kg; C – cost, $/kg; NOx, SOx – concentrations of anthropogenic emissions, ppm. The concentrations of anthropogenic emissions for subsequent comparative calculations were selected at Tg ≈ 1000 °C (Figs. 13, 14). For coal dust under similar combustion conditions, we obtained the following values: SOx ≈ 340 ppm, NOx ≈ 500 ppm. The cost of pulverized nonbaking coal is assumed to be 0.04 $/kg. The cost of rapeseed and used turbine oils was 0.8 and 0.09 $/kg, respectively. The cost of fuel mixture was determined in proportion to the concentration of the

Fig. 10. Video frames of ignition and combustion of gel slurry droplet (80% filter cake, 20% rapeseed oil). 59

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Fig. 13. Concentration of sulfur (a) and nitrogen oxides (b) emissions of waste-derived fuels: 1 – 90% filter cake, 10% rapeseed oil (slurry); 2 – 80% filter cake, 20% rapeseed oil (slurry); 3 – 90% filter cake, 10% rapeseed oil (gel state); 4 – 80% filter cake, 20% rapeseed oil (gel state); 5 – 90% filter cake, 10% rapeseed oil (solid state); 6 – 80% filter cake, 20% rapeseed oil (solid state).

Fig. 14. Concentration of sulfur (a) and nitrogen oxides (b) emissions of waste-derived fuels: 1 – 90% filter cake, 10% turbine oil (slurry); 2 – 80% filter cake, 20% turbine oil (slurry); 3 – 90% filter cake, 10% used turbine oil (gel state); 4 – 80% filter cake, 20% used turbine oil (gel state); 5 – 90% filter cake, 10% used turbine oil (solid state); 6 – 80% filter cake, 20% used turbine oil (solid state).

Fig. 15. Relative effective utilization factors of fuels based on filter cake mixed with oils vs. coal dust.

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set of the characteristics under study does not allow this fuel to significantly exceed the indicators of coal dust due to a negative economic factor. However, the slurry fuel with turbine oil (Fig. 15, column 3) is characterized by a very high efficiency indicator, even though its combustion heat is 1.43 times as low as that of coal. The positive effect, in this case, is achieved through relatively low emissions and the use of only low-cost waste. The performed calculations demonstrate a great potential of the fuels under study, but do not take into account many aspects of technological innovation costs when switching coal-fired plants to new waste-derived fuels. Coal still remains an affordable alternative to natural gas and is likely to remain one of the main fuels for energy companies around the world. Using rapeseed oil instead of used industrial oils in the composition of waste-derived fuels makes it possible to improve energy performance indicators (ignition temperature, combustion rate, and ignition delay) without increasing the environmental load only in the case of slurries. Gel and solid fuels with the addition of rapeseed oil are inferior to coal more than 9 times (Drelative ranges from 0.11 to 0.74). In addition, the use of such components makes energy production more expensive in comparison with the use of oil refining waste, which costs 6–10 times as low.

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