Ignition Characteristics of Coal–Water Slurry Containing

Article pubs.acs.org/EF

Ignition Characteristics of Coal−Water Slurry Containing Petrochemicals Based on Coal of Varying Degrees of Metamorphism Ksenia Yu. Vershinina, Geniy V. Kuznetsov, and Pavel A. Strizhak* National Research Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk, 634050, Russia ABSTRACT: This paper examines a coal−water slurry containing petrochemicals (CWSP) based on lignite, bituminous coal, anthracite, and typical wastecarbon residue obtained from the pyrolysis of tires. Waste motor oil is used as a liquid fuel component of the slurry. The study of CWSP ignition is performed via placing a fuel droplet on a low-inertia thermocouple junction (junction diameter is 0.1 mm; thermal inertia is less than 1 s) into the hot oxidant flow. The temperature and velocity of the oxidant flow vary in the range of 600−1200 K and 0.5−5 m/s, respectively. These ranges are typical for power plants in the large- and small-scale energy industry. The paper specifies the minimum oxidant temperatures which are sufficient for sustainable ignition, as well as the ignition delay times, the times for complete burnout of CWSP droplets (radii of 0.5−2 mm), and the maximum temperatures in the fuel combustion area. We discuss how the oxidant temperature and the component composition of CWSP influence the parameters listed above. Addition of a small amount of coal (with high content of volatiles) in a fuel composition can decrease the ignition temperature of CWSP based on bituminous coal and even anthracite by 50−150 K compared to that which is a traditional one for the energy sector. At the same time, the presence of coal of a high degree of metamorphism in CWSP leads to a substantial increase in the temperature in the fuel combustion zone, as well as in the duration of this process (as a result, more heat is released). The paper indicates the compositions which are characterized by minimum ignition lag, low temperature of ignition, and relatively high temperatures of combustion.

1. INTRODUCTION The technologies of preparation and combustion of coal−water slurry (CWS) in power plants began to develop intensively in the 70s of the 20th century in the United States, China, Japan, India, Germany, and some other countries. The CWS is a suspension prepared from water and pulverized coal or coal waste fuels (for example, filter cakes and coal sludge) with a small amount of chemical additives (plasticizers, stabilizers, and surfactants). The CWS has received a high interest due to two main reasons: (i) the need to recycle large amounts of coal sludge and waste from coal cleaning plants, and (ii) the need to replace gas, oil, and petroleum products by more accessible coal fuel. To date, the problem of coal waste recycling has not been fully solved.1−6 Moreover, the volumes of coal mining will be increased around the world (especially in China), according to the forecast for the next decade.1−6 Therefore, coal processing waste will be also annually increased. For this reason, the problem of its large-scale recycling is becoming more and more important across the world. One of the promising directions for the development of composite coal fuels is the preparation of a coal−water slurry containing petrochemicals (CWSP).7−11 Such a slurry has not only water and coal in its composition but also a flammable liquid of petroleum or natural origin (for example, oils, alcohols, resins, water−oil emulsions, fuel oil, oil sludge, etc.). Flammable liquids are added to the slurry composition to enhance the stability of fuel (segmental stability) and to change its energy characteristics. Note that, along with coal waste, flammable and toxic petroleum liquid waste is estimated in millions of tons per year,1−3 and it also requires recycling. In addition to the problem of recycling the waste of processing coal and oil, large-scale involvement of CWS and CWSP in the power industry will reduce a number of power © 2016 American Chemical Society

plants operating with coal. In turn, this will reduce the dangerous environmental impact.12−14 References 15 and 16 reported that combustion of CWS greatly reduces the emissions of NOx and SOx to the environment in comparison to conventional pulverized coal. There are prospects for reducing CO2 emissions by converting solid fuels into CWSP. As a result, this may partly solve the problem of greenhouse gases (global warming, global dimming, etc.). There are many studies (for example, refs 17−22) aiming to solve a rather complicated task, namely, to find the optimum liquid coal fuels with the required rheology, thermophysics, and thermokinetics. The complexity of this problem can be primarily explained by a variety of potential components, their thermal and mechanical properties, as well as a selection of the optimum ratio of components. In particular, coal of different grades (including low-grade lignite), coal sludge, and waste coal can be used as a coal combustible component for preparing CWSP. Waste water can be used as a noncombustible dispersion medium in addition to technological water cleaned from impurities.23,24 The list of combustible or flammable liquid waste (as the third component of CWSP) includes dozens of items (oil, solvents, cleaners, sludge, etc.). Note that many papers (for example, refs 25−29) report about the ignition and combustion of various CWS compositions. References 25, 28, and 29 examine the features of CWS combustion in a fluidized bed. This is a promising technology for combustion of biomass, waste, and liquid coal fuel. However, despite the relevance and expediency to use Received: April 27, 2016 Revised: July 25, 2016 Published: August 5, 2016 6808

DOI: 10.1021/acs.energyfuels.6b01016 Energy Fuels 2016, 30, 6808−6816

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Figure 1. Appearance of solid fuel particles from Russian Federation fields.

Table 1. Component Composition of Tested CWSP (Relative Mass Fractions)

power units because of its high humidity and ash content, as well as a large amount of volatiles in its structure.30,31 The latter is a main cause for high risk of fire and explosion of lignite during its transportation, loading, drying, and preparation.30,31 However, lignite can be effectively used for preparing CWS and CWSP to enhance the reactivity of the fuel composition, especially under low-temperature conditions (the oxidant temperature is less than 1000 K).32−34 Note that, to date, there is a lack of data on the combustion characteristics and conditions of CWS and CWSP consisting of coal of varying degrees of metamorphism (for example, refs 11 and 35). In particular, ref 35 put forward the idea that to prepare CWS from the mixture of low-reaction coal (anthracite) and highreaction lignite is reasonable. Reference 35 examines the rheological characteristics of a similar fuel composition. It shows35 that transportation of such a fuel composition is possible through pipelines. Moreover, the study35 specifies the ignition temperature, the duration of evaporation and burning of an individual CWS droplet. Reference 11 reports that an increasing concentration of enriched coal in CWSP based on waste coal leads to decreasing the time of combustion initiation by 10−15%. Thus, the creation of a fuel composition is of

CWSP in power plants, there is a rather limited number of the experimental and theoretical data indicating how different components influence the combustion characteristics of CWSP7−11 based on waste of processing coal and oil. One of the factors limiting the use of CWS and CWSP in an industrial scale is a lack of a common technology which will be applicable for the combustion of various fuel compositions in power boiler furnaces. Besides that, a significant disadvantage of CWS is high ignition delay associated with additional expenditure of heat and time for water evaporation (compared to the ignition of coal without liquids). Ignition delay also increases when using highly metamorphized coal with high useful-energy value, but low reactivity. The latter means that, for sustainable ignition, high temperatures are required. One of the ways to reduce the ignition delay of composite liquid fuel is to add such high-reaction substances as petroleum derivatives7−11 or high-reaction coal to its composition. Note that, despite the spread and large stocks of lignite (which is the most highly reactive type of coal) in many regions of the world, it is not in high demand in industry and power generation, in contrast to bituminous coal and anthracite. Usually, lignite is not a project (main) fuel in thermal power plants and other 6809

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equal to 2−3 h. However, ignition characteristics for CWSP droplets in laboratory conditions are quite close for these two methods of preparation. Therefore, for laboratory experiments, to apply mixing by using mixers (dispergators) as a least energy-consuming method of preparation is appropriate. The application of cavitators involves extra milling of coal in a water environment, destruction of molecules, and activation of coal particles. Coal decomposes into separate organic constituents with an active surface of particles and a large number of free organic radicals. A cavitator, as a rule, consists of a revolving drum, grinding balls, and rolls. Preliminarily, coal, oils, water, and grinding balls are weighed. Coal and balls are placed in the drum. Then, water and liquid combustible components are added in the drum. The drum is placed on rolls. Milling begins. The main difference of methods of CWSP preparation by using a cavitator and homogenizer is a physical impact of milling balls on coal particles or waste of its processing. Such impact leads to a change of rheological properties due to the additional decreasing size of coal particles. In the research, we applied a method of CWSP preparation with the use of a homogenizer. The fuel preparation procedure involves the following steps. Solid components are preliminarily processed in a rotary mill. Then, using special sieves, we select the particles with an average size of 80 μm. Carbon residue particles with an average size of 80 μm are obtained through low-temperature pyrolysis of tires. Milling dispersity is 80 μm according to the recommendations,7−11 which suggest that the particle size of a solid fuel component should be up to 100 μm. This size ensures longer keeping of structural homogeneity of CWS and CWSP. An analytical balance (measurement resolution is 10−5 g) enables us to weigh and to provide the required mass fractions of the used components (Table 1). A homogenizer (rotational speed range is 3000−25000 rev/min) is used to prepare the CWSP compositions. Two stages of mixing components take place. At the first stage, the preparation of an oil−water emulsion happens in a working glass of the homogenizer (capacity of 0.25 L) during 3−4 min. At the second stage, we add solid combustible components into the working glass of the homogenizer. The duration of mixing is 8−10 min. The prepared fuel is placed in a sealed container. An electronic dispenser (minimum and maximum dosage volumes are 1 and 10 μL, variation step is 0.1 μL) generates droplets. To control the initial mass (md) of droplets, each experiment includes their obligatory weighing. 2.3. Ignition of CWSP. Figure 2 shows a scheme of the experimental setup used to study the combustion initiation of individual CWSP droplets during convection heating. The setup is similar to that used in previous experiments.8−11 The main ignition characteristics of solid and slurry fuels are an ignition delay time, a minimum (limit) temperature of ignition, a minimum sufficient impulse of heat to ignite, dimensions of the ignition zone, a rate of growth of fuel surface temperature, a limit temperature gradient at the fuel surface, etc. In the research, we used the officially accepted ignition criteria8−11 based on temperature monitoring in a combustion area Td, calculating ignition delay times τd and threshold temperatures of oxidant Tgmin. The hot air flow is formed inside a hollow cylinder (feature 1 in Figure 2) made of heat-resistant quartz glass (inner diameter of 0.1 m, length of 1 m). The air fan (capacity is 0.25 kW, gas flow rate is not more than 1200 L/min, feature 2 in Figure 2) and heater (capacity is 11 kW, maximum temperature of gas is 1300 K, feature 3 in Figure 2) generate the hot flow. The temperature (Tg) and velocity (Vg) of the air flow vary between 0.5−5 m/s and 600−1200 K, respectively, by using a control unit (feature 4 in Figure 2). To measure a velocity, we use an anemometer (error ± 3%, measurement resolution is 0.1 m/s). To determine the volume concentration of oxygen in the oxidant flow, a gas analyzer (error ± 0.2%, measurement resolution is 0.01%) is

interest when combining components ensuring not only a high calorific value (owing to high-calorie bituminous coal or anthracite) but also relatively high reactivity (by adding a combustible liquid and lignite). The objective of this study is to investigate experimentally the ignition characteristics of CWSP based on coals of varying degrees of metamorphism.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. Figure 1 shows the appearance of the solid components used for preparing fuel suspensions. Waste motor oil is one of the most typical petroleum wastes. It is, therefore, selected as a liquid fuel component of CWSP. Moreover, studies8−11 on the combustion of the CWSP containing different fuel oils illustrate the feasibility of using waste motor oil as an additive, because it intensifies the combustion of CWSP at lower oxidant temperatures. Table 1 specifies the composition and concentrations of tested fuel components. Tables 2−4 show the results of technical and elemental analysis of the components of tested slurry fuels.

Table 2. Results of Technical Analysis of Solid Fuel Components of CWSP component

Wa (%)

Ad (%)

Vdaf (%)

Qas,V (×106 J/kg)

lignite of grade B2 bituminous coal of grade D carbon residue from lowtemperature pyrolysis of tires anthracite

14.11 10.09

4.12 8.52

47.63 40.19

22.91 24.82

1.39

13.10

20.14

3.46

3.09

4.29

35.46

Table 3. Results of Elemental Analysis of Solid Fuel Components of CWSP component lignite of grade B2 bituminous coal of grade D carbon residue from low-temperature pyrolysis of tires anthracite

Cdaf (%)

Hdaf (%)

Ndaf (%)

Stdaf (%)

Odaf (%)

64.86 77.46

6.896 6.253

0.62 2.27

0.384 0.347

27.13 13.64

91.47

2.422

0.37

2.438

2.93

96.23

1.62

0.64

0.87

0.85

In the given paper, properties of CWSP components (elemental constituents, humidity, ash content, combustion heat, etc.) and prepared CWSP (density, humidity, ash content, viscosity, segmental stability, combustion heat, etc.) were determined by using methods described in ref 8. References to the relevant documents (methodologies, standards) are reported in the paper.8 2.2. Preparation of CWSP. Nowadays, the most widespread methods of CWSP preparation are a mixing by a homogenizer, a milling, and a mixing with water when cavitating. According with the first method, the duration of preparation of CWSP varies from 5−6 min to 10−15 min. The second method involves a mixing of components during several hours (duration can reach 6−8 h). Duration of preparation depends on required properties of CWSP (for instance, segmental stability or ignition characteristics). The analysis of refs 8−11 concludes that minimum ignition delays of CWSP droplets are achieved when operating a dispergator (mixer) and are equal to not less than 6−10 min. In contrast, for cavitators, this duration is

Table 4. Results of Characteristic Analysis of Liquid Fuel Component sample

density at 293 K (kg/m3)

moisture content (%)

ash content (%)

flash temperature (K)

ignition temperature (K)

combustion heat (×106 J/kg)

waste motor oil

871

0.28

0.78

405

491

44

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Figure 2. Schematic of the experimental setup.8−11 (Legend: 1, hollow quartz cylinder; 2, air fan; 3, heater; 4, remote control; 5, thermocouple; 6, low-inertia thermocouple; 7, temperature recorder; 8, minirobotic arm; 9, fuel droplet; 10, high-speed video camera; 11, computer; 12, crosscorrelation video camera; 13, dual solid-state laser; 14, laser generator; 15, synchronizer; 16, ductwork; 17, ventilation; 18, remote control of ventilation system; 19, analytical balance.

Figure 3. Maximum temperatures of CWSP combustion (Tg ≈ 790 K) (a) and typical temperature change trend of CWSP droplet No. 5 (numbers of compositions correspond to Table 1) during interaction with hot air flow (b). approximation, Td can be regarded as the temperature at the center of the droplet. Using a high-speed camera (feature 10 in Figure 2) and Tema Automotive software, we determine the CWSP droplet size before interacting with the hot air flow.36−38 Measurement of six diameters in different sections takes place for each droplet. Then, the average radius Rd is determined. Systematic error of determination of Rd did not exceed 4% with respect to the corresponding resolution of the highspeed video recording. The initial size (radius) of droplets is ∼1 mm. The monochrome high-speed camera (sample rate is over 3000 frames per second at a resolution of 1280 × 800 pixels, feature 10 in Figure 2) records processes occurring during ignition of individual CWSP droplets. Processing videos of each experiment by algorithms of Tema Automotive software allows us to determine times of ignition delay (τd) and complete burnout (τc) for CWSP droplets. The mentioned algorithms detect the glow intensity of CWSP droplets (similar to the techniques described in previous experiments8−11). For this purpose, the software parameter Threshold was used. This parameter allows one to set up the RGB color model gradient in the observation area. According to this color model, the 255 gradient value corresponds to a white color, the 0 gradient value to black one. The

applied. The volume concentration of oxygen is 20.5% at the selected ranges of Tg and Vg. Importantly, such a concentration of oxygen is the most typical one for power plants. The cylinder (feature 1 in Figure 2) has three technological holes (diameter of 10 mm) along the axis of symmetry. The first and the third holes are used for installation of type-K thermocouples (temperature measurement range is 273−1373 K, systematic error ± 3 K, inertia is not more than 10 s, feature 5 in Figure 2). Inserting a CWSP droplet to the hot oxidant flow occurs through the central hole. The CWSP droplet is fixed on the low-inertia thermocouple junction (type R, temperature measurement range is 273−1873 K, systematic error ± 1 K, inertia is no more than 1 s, junction diameter is 0.1 mm, feature 6 in Figure 2) before interacting with the flow. The thermocouple (feature 6 in Figure 2) with a sample is placed to the minirobotic arm (8). The minirobotic arm moves the CWSP droplet in the oxidant flow and stops on the axis of the cylinder symmetry. The readings of thermocouples (features 5 and 6 in Figure 2) were transferred to the recorder (feature 7 in Figure 2), which provides monitoring the oxidant temperature Tg and the temperature at the interface of thermocouple junction/fuel composition (Td). In the first 6811

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Energy & Fuels RGB range of 220−255 conforms to combustion of the sample in the given research. Ignition delay time τd is considered as an interval from the beginning of interaction between the CWSP droplet and the hot flow of oxidant until the parameter Threshold reaches 220 at any point on the fuel sample surface. Parameter τc is a time from the beginning of interaction between the CWSP droplet and the hot flow of oxidant until its complete burnout (i.e., when Threshold becomes less than 220 over the entire surface of droplet). Additional experiments verified the procedure for determining τd and τc described above as in ref 9. We revealed the temperature change at the center of the CWSP droplet (Td) using a low-inertia thermocouple (feature 6 in Figure 2) and recorder (feature 7 in Figure 2). Times τd and τc were determined through Td. Ignition delay time τd is an interval from introducing the CWSP droplet to the oxidizer flow until the fulfillment of the ignition criterion,9 i.e., the simultaneous satisfaction of such conditions as Td > Tg and dTd/dτ > 10 K/s. The complete burnout of the sample (time τc) is characterized by the deviation of Td by not more than 0.05Tdmax relative to a steady value (at τ → ∞; Tdmax is a maximum droplet temperature during combustion). These experiments confirm the use of the parameter Threshold in Tema Automotive for detecting τd and τc characterizing the ignition of CWSP droplets of various compositions. To determine τd and τc, 6−10 experiments take place under identical heating conditions of droplets of the same CWSP. When deviating times of ignition delay or complete burnout of CWSP from the average value in a series more than 3%, data of such experiments were excluded from further analysis. As a rule, a number of such tests in a series of experiments equals to less than three. The minimum number of experimental values which are required for determining the average times τd is equal to six, and the maximum number is 10. Permissible variations of τd and τc did not exceed 3%. In further analysis, we considered τd and τc which are average values for the series of experiments. For determining maximum temperatures of combustion of CWSP droplets Tdmax, as a rule, 20−30 tests take place under identical heating conditions. Such a large number of tests is necessary for reliable determination of Tdmax taking into account the change of size (radius) of the droplets within 4%. We consider a permissible variation of Tdmax of not more than 10 K toward the average value in a series. When averaging Tdmax, results of 10 tests meeting the mentioned condition are used. The similar approach operates when determining Tgmin. In the further analysis, the average values of τd, τc, Tdmax, and Tgmin for the series of experiments were considered (Figures 3−5).

We can identify the following reaction stages in the system of CWSP droplet/gas: (i) inert heating of the sample, (ii) evaporation of moisture from the near-surface layer (water and liquid nonflammable component), (iii) thermal decomposition of the organic part of coal in the near-surface layer of the droplet, (iv) mixing of combustible gas with an oxidizer, (v) ignition of gas−vapor mixture, (vi) heating up of carbon, and (vii) heterogeneous ignition of carbon and its subsequent combustion. Temperature trend lines in CWSP droplets (Figure 3b) correspond closely to modern concepts about processes of combustion of slurry fuels. These concepts exist owing to published experimental and theoretical investigations.39−42 In refs 39−42 as in the given research, temperature trend lines include parts illustrating stages of fuel combustion: inert heating, evaporation of water and liquid combustible component, yield of volatiles, gas-phase ignition of volatiles and vapors, heterogeneous ignition of coke-coal, etc. Moreover, in all studies, nonlinearity of temperature trend lines takes place. This behavior is due to simultaneous operation of phase transitions, chemical reactions, and heat and mass transfer. To compare quantitatively temperatures in droplets of fuel with those which are in refs 39−42 is difficult, because blend compositions for CWS and CWSP differ. In different regions of the world, properties of coals of one type significantly differ. Therefore, to compare ignition characteristics of CWSP based on coals of different types is important within one region. Such an approach will enable one to carry out an adequate analysis for finding blend compositions which are more effective toward the most important ignition characteristics. This task is solved within the given research. When the CWSP droplet burns, energy is intensively released. Therefore, the registered maximum temperatures of CWSP droplets exceed the oxidant temperature (see Figure 3). The experiments indicated that Tdmax can significantly differ depending on the composition of CWSP. The highest values of Tdmax (over 1020 K) are observed for CWSP compositions Nos. 1, 2, 4, and 5 (see Figure 3, Table 1). For CWSP compositions Nos. 3, 6, and 7, Tdmax is 950−980 K (see Table 1). The solid fuel component of composition No. 3 (see Table 1) is anthracite, which is characterized by the highest ignition temperatures among the coals of varying degrees of metamorphism. Recall that the oxidant temperature was 790 K in the experiments on determination of Tdmax. On the basis of the discussion above, we can assume that, when CWSP composition No. 3 (see Table 1) is heated, basically, vapor of waste motor oil is burned, whereas only a small fraction of anthracite starts to react. This leads to the low value of Tdmax. CWSP composition No. 6 (see Table 1) is also characterized by a relatively low Tdmax. The probable reason for this result is the relatively low combustion heat of the carbon residue obtained from pyrolysis of tires, which is a basis of CWSP composition No. 6 (see Table. 1). CWSP compositions in the experiments were prepared with almost equal mass concentrations of coal and water (see Table 1). Nevertheless, according to research,10 increasing water content in CWSP compositions will lead to growth of their ignition delay time. Burning time (i.e., interval from the moment of heterogeneous ignition of carbon to its complete burnout) will decrease when increasing the water amount, because the relative mass fraction of the combustible component becomes less due to the increased water content. Thus, the growth of water content in CWSP droplets leads to

3. RESULTS AND DISCUSSION Figure 3a illustrates the results of the experiments, where we determined the maximum combustion temperature (Tdmax) of CWSP droplets of various compositions. Figure 3b depicts a typical trend of temperature change of the CWSP droplet during heating, evaporation of moisture, and ignition of volatiles and carbon (the trend is similar to that presented in previous reports8−11). The oxidant temperature (Tg ≈ 790 K) in the experiments on determination of Tdmax was selected after a series of preliminary experiments. Most of the tested compositions were ignited at a temperature of about 800 K (except for the composition No. 3 based on anthracite (see Table 1)). On the other hand, at oxidant temperatures of over 800 K, the combustion of samples was completed within the time which is insignificantly different (only by a few times) from the response time of the thermocouple performing a function of the CWSP droplet holder. In this case, it was not possible to obtain a reliable trend of temperature change at the center of the droplet, as well as to detect Tdmax for the tested compositions. Recall that the trend illustrates the process from the moment of ignition of droplets until the end of its combustion. 6812

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lignite, which has the highest reaction activity among other tested coals (see Figure 1). When lignite is heated up, many volatiles are released. These substances mix with vapor of the combustible liquid (waste motor oil) and, thus, form a reactive combustible gas mixture which is capable to ignite at relatively low oxidant temperatures. Additional thermal energy is released during gas-phase combustion of the mixture. This energy heats the carbon residue of lignite and enhances its heterogeneous ignition. The solid combustible component of CWSP composition Nos. 5 and 7 (see Table 1) is coals of varying degrees of metamorphism: from lignite to anthracite. Since the limit ignition temperatures of these compositions are relatively low (about 720 K), one can assume that the lignite additive may enhance the ignition and subsequent combustion of bituminous coal and anthracite in the CWSP composition. Moreover, comparative analysis of Tgmin values for CWSP compositions Nos. 2 and 4 (Table 1) showed that the addition of lignite (with 5% mass concentration) to the CWSP based on coal reduces the minimum ignition temperature by almost 10% (see Figure 4). The highest Tgmin (over 1050 K) are typical for CWSP No. 3 (see Table 1). This can be explained by the high concentration of anthracite in this CWSP composition. Anthracite requires high oxidant temperatures for combustion initiation. After analyzing the maximum combustion temperatures (see Figure 3) and minimum ignition temperatures (see Figure 4) of different CWSP compositions, we can distinguish several fuel compositions, which are promising in terms of the efficiency of CWSP combustion in power plants. In particular, CWSP compositions Nos. 1, 2, 4, and 5 (see Table 1) are characterized by low limit ignition temperatures (not more than 810 K) and relatively high combustion temperatures (over 1020 K). We defined experimentally how times of ignition delay (see Figure 5a) and complete burnout (see Figure 5b) of CWSP compositions depend on an oxidant temperature. According to the experimental results on evaluation of Tgmin (see Figure 4), the values of τd and τc were determined in the oxidant temperature range of 600−1000 K. Figure 5 demonstrates that, when increasing the oxidant temperature, times of ignition delay and complete burnout of individual CWSP droplets exponentially decrease. This can be explained as follows. When the heat inflow to the surface of the CWSP droplet enhances, all the stages of its ignition and combustion are intensified: evaporation of moisture and flammable liquid, thermal decomposition of coal with yield of volatiles, formation of the combustible gas−vapor mixture with

degradation of time ignition characteristics. However, interest lies in that the threshold (limit) temperature of ignition decreased when increasing the water content. This occurs due to interaction of water vapors and carbon.26 Carbon of cokecoal interacts with water vapors with formation of low-reaction complexes intensifying ignition of coal residue.26 Moreover, the area of reacting agent increases,26 because during water evaporation, pressure arises which increases the intrapore space. In addition to the type and concentration of components, such parameters of oxidation as temperature and flow rate can influence ignition delay. In ref 34, when increasing the flow rate in the range of 0.5−5 m/s, times of ignition delay and complete burnout nonlinearly decrease within 40−60% due to the heat transfer enhancement in the system of oxidant/droplet surface. However, with increasing temperature of the gas environment (for instance, up to 900−1000 K), the influence of flow rate on ignition delay decreases. Deviations of τd become less than 20% when varying the flow rate in the range of 0.5−5 m/s. Thus, oxidant temperature plays a defining role for ignition and combustion. Figure 4 shows the minimum (limit) ignition temperatures (Tgmin) of the tested CWSP compositions. The experiments

Figure 4. Minimum temperatures of CWSP ignition (numbers of compositions correspond to Table 1).

revealed that relatively low oxidant temperatures (about 720 K) are sufficient for sustainable ignition of CWSP compositions Nos. 1, 5, and 7 (see Table 1). The combustible solid component of CWSP composition No. 1 (see Table 1) is a

Figure 5. Times of ignition delay (a) and complete burnout (b) of CWSP depending on the oxidizer temperature (numbers of compositions correspond to Table 1). 6813

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Energy & Fuels

combustion chamber. Using data about threshold (minimum) temperatures of oxidant Tgmin, an optimum mixture ratio and a type of components can be found. For the ecology, concentrations of SOx, NOx, and CO2 depend on temperature conditions in a combustion chamber. Monitoring temperatures of Td(τ) and Tdmax enables one to define compositions, for which, during combustion, Tdmax will be changed in a limited range. For economy, for instance, the ratio of energy output to a consumed fuel amount depends on the carbon content directly within a CWSP composition. In anthracite, the carbon content is maximum, and in lignite, this content is minimum. However, to ignite CWSP based on lignite, considerably less energy is required compared to CWSP based on anthracite. This fact is well observed when comparing Tgmin. The experimental data in Figures 3−5 indicate an objective information illustrating the reasonability of blending coals in CWSP to decrease expenditures for heating and ignition of fuel and to obtain a higher heat release during combustion. Since properties of coals in different regions of the world can considerably differ, then the mentioned approach is appropriate to apply for different countries in search of optimum conditions of CWSP combustion. In this case, the above-mentioned requirements for power engineering, ecology, and economy should be taken into account. Analysis of the results8,9,46 showed that conditions and characteristics of ignition of a CWSP droplet on a holder (for instance, on a thermocouple) and when soaring in a combustion chamber can differ. Primarily, specific features of ignition of soaring CWSP droplets can be noted during their collisions in a combustion chamber. These processes can lead to the additional energy release during ignition of droplets of fuel. Furthermore, to analyze differences of conditions and characteristics of ignition of soaring CWSP droplets based on various coals is appropriate in the future, taking into account the significant influence of dispersion and breakup of droplets. Coal concentration in a CWS can vary in the range of 40− 60%.17−26 In high concentrated CWS, the mass fraction of coal can reach 70−72%.27,47 The range of mass concentrations of liquid combustible components in a slurry fuel can vary from very low values (for instance, in ref 47, mass concentration of waste motor oil used as a stabilizing agent was 0.3%) to higher values. For coal−oil suspensions, the mass concentration of organic liquid (for instance, mazut, heavy crude oil, kerosene) varies in the range of 46−70%.8−15 Therefore, the results obtained in the given research (dependences for τd, τc, Tdmax, and Tgmin) are of particular interest for water−coal technologies with high content of solid fuel in suspensions. In addition to a group of external factors (for instance, oxidant temperature, pressure, rate of energy supply, oxygen content), grade and characteristics of coal (ash content, humidity, carbon content, etc.) strongly influence its ignition and combustion. During ignition, the content of volatiles in a coal is really important.48 Low-grade coals with high content of volatiles have an increased reactivity compared to coals of higher grade.49 In ref 50, the authors note the main rule: the lower degree of coalification the coal has, the lower temperature of ignition and the shorter burning time take place. Moreover, ref 48 includes the experimental dependence illustrating the nonlinear decrease of ignition temperature when increasing concentration of volatiles in the range of 0−60%. During combustion of coal coke, the degree of coalification is also important; however, in the given case, a greater influence has a structure of carbon residue. Coals with a low degree of

its subsequent ignition, heating up of coke residue, as well as ignition and combustion of carbon. Figure 5 illustrates that ignition delay times for CWSP composition No. 2 grow significantly nonlinearly when reducing the oxidant temperature Tg. The similar conclusion can be made about complete burnout time, because τc includes value τd. As a consequence, values τc and τd are higher than the same parameters in Figure 5 for other CWSP compositions. In particular, this is noticeable at an oxidant temperature of 800− 900 K. CWSP composition No. 2 contains the milled bituminous coal of grade D. According to Figure 4, this component leads to the growth of the threshold (limit) temperature of ignition. At an oxidant temperature of about 800 K, the composition No. 2 stably ignited. At temperatures of 800−850 K, the duration of such stages as the inert fuel heating and the yield of volatiles were maximum compared to all the compositions containing low-reaction lignite. Differences of durations of stages of the inert heating for compositions with lignite and bituminous coal reached several seconds. Therefore, time τd in Figure 5 for composition No. 2 was noticeably higher. The similar conclusion can be made for composition No. 3 with anthracite. However, for composition No. 3, a minimum temperature of ignition was higher than 1000 K. Therefore, Figure 5 omits curve No. 3. The behavior of dependences τc(Tg) and τd(Tg) denotes the correlation between stages of metamorphism (degrees of coalification) and characteristics of heating and ignition. The more volatiles are contained in coal, the more quickly it heated and ignited. Increasing the degree of coalification (anthracite has the highest degree) leads to the growth of limit temperature of ignition and the combustion heat. However, in this case, the combustion rate decreases. Importantly, water and liquid combustible component in the CWSP composition only enhance this effect through endothermic phase transitions. Experiments demonstrated that, when increasing Tg due to the heat transfer enhancement in the system of gas environment/fuel, the influence of the CWSP composition on its ignition characteristics significantly decreases (see Figure 5). Despite this, the maximum deviation of the curves for times of ignition delay and complete burnout (see Figure 5) is observed for composition No. 4 (Table 1). This is the most noticeable in the oxidizer temperature range of 730−850 K. Most likely, decreasing ignition delay of CWSP composition No. 4 (see Table 1) in this temperature range is explained by the presence of the highly reactive additive (lignite with a mass fraction of 5%). The decreased times of complete burnout for this composition are due to the dispersion of lignite, as well as the possible incomplete burnout of the organic part of bituminous coal in the oxidant temperature range of 730−850 K. Decreasing the temperature in a combustion chamber slows down formation of toxic oxides (especially, NOx).43−45 This is due to decreasing rates of characteristic chemical reactions. When decreasing the oxidant temperature, water evaporation rates nonlinearly decrease, and water vapors minimize emissions of NOx and SOx. Therefore, the temperature condition is critically important to minimize emissions of pollutants. The experimental results on the influence of various coals on the main ignition characteristics (τd, τc, Tdmax, and Tgmin) enable one to conclude that CWSP with various compositions of coals can be promising for power engineering, ecology, and economy. For power engineering, first of all, heat release depends on the combustion heat of fuel and temperature in a 6814

DOI: 10.1021/acs.energyfuels.6b01016 Energy Fuels 2016, 30, 6808−6816

Article

Energy & Fuels metamorphism have a more complex structure compared to coals of higher degree of coalification. Therefore, the coal coke has a better oxidability.51 The main horizons which open when developing technologies of CWSP fuel burning involve solution of the three tasks: minimizing waste of oil and coal processing owing to their use as components of fuels; increasing fire and explosion safety of coal boiler stations by omitting stages of drying the milled coal and handling the pulverized coal, and decreasing emissions of SOx, NOx, CO2, etc. The third task is the most important for the world community, because emissions when burning fuels around the world lead to negative large-scale consequences: greenhouse effect, resettlement of peoples, health problems of the population, and the extinction of fauna and flora. The solution of such the tremendous task as decreasing emissions of pollutants when burning CWSP is the most important vector of development of composite fuels. Optimizing CWSP ignition is a one of the first steps in the given direction. In particular, variation of solid (coals, products, and waste of coal processing) and liquid (waste oils, oily sludge, oil refinery waste) combustible components and their concentrations enables one to change conditions and characteristics of CWSP combustion in wide ranges. As a consequence, concentrations of emissions of SOx, NOx, CO2, and others will be changed significantly. According to the research results, optimum CWSP compositions can be found when varying coals. Even adding a small amount of coals of different degrees of coalification can change τd, τc, Tdmax, and Tgmin significantly. On the basis of refs 8−11 and 34, optimum compositions and concentrations of CWSP components can be obtained by varying the combustible liquids (waste oils, water−oil emulsions, mazut, coal tar, etc.). References 8−11 and 34 show that ignition delay time and maximum temperature of CWSP combustion can be changed more than 30% even when adding 7−10% of liquid combustible component. The optimality of fuel compositions can be estimated according to various criteria: maximum duration of keeping the segmental stability (no stratification), necessary viscosity for transporting CWSP through pipelines, minimum ignition delay time, minimum threshold temperature of ignition, maximum heat release, minimum concentrations of emissions of pollutants, etc. Certainly, to create a fuel composition, which is the best according to all the possible criteria of optimality, is almost impossible. Moreover, such fuel will be optimum only for one region. For other regions of the world, due to different properties of components, fuel compositions can differ considerably. The main result of the research is that, using the known coal grades which characterize different degrees of coalification, characteristics of CWSP ignition can be changed in wide ranges.

(2)

(3)

(4)

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ignition temperature can significantly reduce emissions.43−45 The experimental results showed that the stable ignition of most of the tested CWSP compositions can be implemented at an oxidant temperature of less than 800 K. We may thus conclude that it is reasonable to vary the oxidant temperature in the range of 600−1000 K, when burning the CWSP in an industrial scale. This approach allows one not only to optimize the costs in industrial power facilities and to extend the lifetime of power equipment but also to reduce the environmental load by improving the environmental performance of power plants operation. In this study, a change in fuel composition did not have a substantial influence on ignition delay and combustion duration of individual CWSP droplets. The maximum difference between the values of τd did not exceed 6 s for different compositions in the entire temperature range considered. A similar conclusion can be made for time τc. The results of the study illustrate the prospects of using coal of varying degrees of metamorphism for preparing CWSP compositions. This approach allows one to reach an acceptable ratio between the expenditures for combustion initiation and combustion heat of CWSP under conditions of low-temperature ignition (Tg is less than 1000 K). To reduce the ignition delay of CWSP at Tg < 1000 K is possible not only by the addition of coals with a low degree of metamorphism but also through the variation of fuel oil concentration in the CWSP composition.11 Moreover, by increasing the concentration of water, reduction of the minimum oxidant temperature sufficient for stable ignition of CWSP can be achieved.8,10 Thus, the possibility exists of changing the characteristics and conditions of stable ignition of CWSP by varying the type and concentration of all fuel components.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research into the characteristics of CWSP ignition processes was funded by Russian Science Foundation (project 15−19− 00003). The optimization of the combustion of fuels based on coal and oil processing waste was performed within the framework of the strategic plan for the development of National Research Tomsk Polytechnic University as one of the world-leading universities.

4. CONCLUSIONS

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(1) Significant reduction of minimum temperatures required for stable ignition of composite liquid fuel is possible. This can be achieved by varying the concentration and type of CWSP components. Use of low-grade coal with a high volatile content as a component intensifying the ignition of CWSP at lower gas temperatures is reasonable. According to the experimental data, the addition of lignite (with mass concentration of 5%) in CWSP based on bituminous coal reduces the minimum ignition temperature by nearly 7−10%. Decreasing the 6815

NOMENCLATURE A = ash level of dry sample, % Cdaf = fraction of carbon in the sample converted to a dry ash-free state, % Hdaf = fraction of hydrogen in the sample converted to a dry ash-free state, % Ndaf = fraction of nitrogen in the sample converted to a dry ash-free state, % Odaf = fraction of oxygen in the sample converted to a dry ash-free state, % d

DOI: 10.1021/acs.energyfuels.6b01016 Energy Fuels 2016, 30, 6808−6816

Article

Energy & Fuels Stdaf = fraction of sulfur in the sample converted to a dry ashfree state, % md = initial mass of CWSP droplet, g Rd = radius of droplet, mm Td = temperature at the interface of thermocouple junction/ slurry, K T d max = maximum temperature at the interface of thermocouple junction/slurry, K Tg = air temperature, K Tgmin = minimum air temperature required for ignition of CWSP, K Qas,V = enthalpy of combustion of analytical sample of coal, (×106 J/kg) Vdaf = yield of volatiles of coal to a dry ash-free state Vg = air velocity, m/s Wa = humidity of analytical sample of filter cake in an air-dry state, % τ = time, s τc = time of complete burnout, s τd = ignition delay time, s

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DOI: 10.1021/acs.energyfuels.6b01016 Energy Fuels 2016, 30, 6808−6816