Article pubs.acs.org/EF
Ignition of the Coal−Water Slurry Containing Petrochemicals and Charcoal Geniy V. Kuznetsov, Sergey Yu. Lyrschikov, Sergey A. Shevyrev, and Pavel A. Strizhak* National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634050, Russia ABSTRACT: The lack of effective technologies to use coal processing waste and waste flammable liquids is a major research and development problem. To address this problem, we study the ignition characteristics and conditions for coal−water slurries (CWS) based on these wastes. The key challenge posed by the wastes under study is their poor reactivity (they require more power resources for sustainable ignition). Therefore, this paper suggests adding a highly reactive material (charcoal) in a low concentration to coal−water-slurry containing petrochemicals (CWSP) based on wastes. Individual CWSP droplets are studied in a model combustion chamber. The integral characteristics of the processes under study (ignition delay time and minimum oxidizer temperatures sufficient for ignition) are determined for a CWSP droplet of 0.5−2 mm in radius fixed at the junction of a low-inertia thermocouple using a high-speed (up to 105 fps) video camera and Tema Automotive software. We compare the ignition characteristics of CWSP compositions based on charcoal and brown coal as well as filter cakes (processing waste) of coking, low-caking, and non-baking coals in an oxidizer flow (temperature of 600−1000 K and velocity of 0.5−5 m/s). Charcoal is a promising additional component for the CWS and CWSP. In view of ecological, economic, and energy-related aspects, it is advisible to use charcoal as an additive to intensify the CWS and CWSP ignition. Even a small addition (within 10−15% in relative mass concentration) of this component may significantly reduce the limit (minimum) temperature of sustainable ignition and ignition delay time (inertia).
1. INTRODUCTION The use of the coal−water slurry (CWS) and coal−water-slurry containing petrochemicals (CWSP) in power engineering has sound environmental benefits compared to direct combustion of coal in a pulverized state.1−3 In particular, the use of these slurries may reduce harmful emissions of nitrogen oxides and sulfur into the atmosphere.4−6 Moreover, the preparation, storage, and transportation of such fuels provide environmental benefits as well as fire and explosion safety.7 The following raw materials can be used for the preparation of CWS and CWSP: (i) coal or coal processing waste, such as filter cakes with high moisture and ash content, (ii) fine fraction of coking and semi-coking, (iii) carbon residue of lowtemperature pyrolysis of tires, etc. The use of coal processing waste for the preparation of CWS and CWSP can greatly simplify the technological schemes of fuel preparation. Moreover, it may reduce production fines for environmental pollution by coal-cleaning waste.8−10 The substances listed above are of particular interest because their use may significantly reduce or even eliminate the primary cost for fuel preparation related to the grinding of coal to a specific particle size.1−3 Furthermore, fine coal has a relatively high ash content and, accordingly, low combustion heat. The authors of a previous study suggested the use of charcoal as a solid fuel component to enhance the combustion of the CWS and CWSP.11 Charcoal has a low ash content and relatively high combustion heat compared to various coals and their processing waste. The apparent advantage of using charcoal for heat production is the almost complete absence of sulfur compounds in the fuel structure, hence lower emissions of sulfur oxides in the environment. In other words, it may significantly improve the environmental situation in industrial regions. Furthermore, charcoal is a good adsorbent © 2016 American Chemical Society
and can help to minimize the emissions of various oxides during the combustion of CWS and CWSP. In particular, there is 30−40 times less sulfur in flue gases from the combustion of charcoal versus bituminous coal.11 The content of carbon dioxide during the combustion of charcoal is several times lower than that of bituminous coal (per ton of conventional fuel). Similar conclusions can be made for the fuel suspensions, such as the CWS and CWSP based on charcoal.11 The use of charcoal as a primary solid fuel component or as an additive contributes to the solution of the global problem, namely, the reduction of greenhouse gases.11 To date, very few experimental data have been published on the ignition and combustion of the fuel compositions containing charcoal particles.11 Therefore, it is of interest to investigate the integral ignition characteristics (first of all, ignition delay time and limit combustion temperature) for one and several CWSP droplets based on charcoal and various liquid combustible components. The purpose of this study is to determine experimentally the integral characteristics of ignition in the oxidant flow: ignition delay time and minimum temperature of sustainable combustion initiation. We will examine these characteristics through an example of single CWSP droplets with the addition of charcoal.
2. FUEL COMPONENTS In this study, we used charcoal as a solid fuel component of CWSP (see Table 1). As liquid fuel components of CWSP, we used fuel oil, waste engine oil, and turbine oil (see Table 2). A wetting agent Received: September 5, 2016 Revised: November 7, 2016 Published: November 8, 2016 10886
DOI: 10.1021/acs.energyfuels.6b02245 Energy Fuels 2016, 30, 10886−10892
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Energy & Fuels Table 1. Results of Technical and Elemental Analyses of Carbon Components of CWSP technical analysis
elemental composition (%, daf)
coal
Ad (%)
Vdaf (%)
Qds,V (×106, J/kg)
C
H
(O + N + S)
charcoal from Chernogolovka, Russia brown coal from Krasnoyarsk region, Russia filter cake of gas coal from a washing plant, Kemerovo region, Russia
1.49 4.12 33.82
22.58 47.63 43.11
29.60 22.91 22.16
83.11 73.25 75.12
3.49 6.52 4.64
13.40 20.23 20.24
Table 2. Properties of Liquid Combustible Components of CWSP combustible liquid
density at 293 K (kg/m3)
moisture content (%)
ash content (%)
ignition temperature (K)
combustion heat (×106, J/kg)
waste motor oil fuel oil waste turbine oil
871 1000 868
0.28 6.12 Tg is achieved and the Td change rate is no less than 10 K/s. The systematic error in determining the time τd did not exceed 0.5 ms. Also, we determined the minimum temperature of sustainable ignition of CWSP (Tmin g ). The evaluation methods for systematic and random errors are similar to those described in studies.27,29 We have performed at least 6−10 experiments. This ensured the satisfactory repeatability of results under identical heating conditions. Because of a greater difference between the experimental data at lower oxidant temperatures, we had to conduct more experiments under these conditions.
5. RESULTS AND DISCUSSION The experiments have shown that the ignition stages of CWSP correspond well to modern ideas about the ignition of CWS, regardless of the component composition of CWSP.4−7,29−35 In particular, we observed the following processes during the experiments: (i) inert heating of the fuel droplet, (ii) evaporation of liquid components (oil and water) from the near-surface layer, (iii) thermal decomposition of the organic portion of coal in the near-surface layer of the particle, (iv) mixing of the products of thermal decomposition and evaporation with the oxidant, (v) combustion initiation of the emerging gas mixture, (vi) heating of the coke residue, (vii) heterogeneous ignition of the coke residue, and (viii) combustion of the coke residue. These stages are the same for various types of coal components and liquid combustible components. However, their duration is different. The maximum combustion temperature of the CWSP depends upon the liquid fuel component (see Figure 2) used in its composition. The videograms of the experiments indicate that the maximum temperature is reached during simultaneous gas-phase combustion of the mixture of evaporation products and volatiles as well as at the beginning of heterogeneous combustion of the coke residue. The highest peak temperature is typical of the CWSP compositions with turbine oil, whereas the lowest peak temperature is typical of the CWSP with fuel oil (see Figure 2). The peak combustion temperature is an
Figure 1. Scheme of the experimental setup:27 (1) hollow quartz cylinder, (2) blower, (3) air heater, (4) control unit, (5 and 6) thermocouples, (7) recorder, (8) minirobotic arm, (9) fuel droplet, (10) high-speed camera, (11) analytical balance, (12) computer, (13) duct, (14) exhaust ventilation, and (15) homogenizer.
4. IGNITION OF CWSP Figure 1 shows a scheme of the experimental setup developed for the study of the integral ignition characteristics of CWSP droplets.28 A blower (feature 2 in Figure 1) and a heater (feature 3 in Figure 1) generated an airflow with a temperature of Tg = 600−1000 K inside a hollow quartz cylinder (feature 1 in Figure 1). The values of Tg were monitored at three points along the symmetry axis of the cylinder 1 (with corresponding technological holes in its wall). We used the type10888
DOI: 10.1021/acs.energyfuels.6b02245 Energy Fuels 2016, 30, 10886−10892
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Energy & Fuels
Figure 2. Temperature trend (at Tg ≈ 780 K and Vg ≈ 3 m/s) at the droplet center (Rd ≈ 1 mm) of CWSP (30% coal, 70% water, and 10% combustible liquid) based on charcoal and different liquid combustible components.
Figure 4. Ignition delay times of CWSP droplets (30% coal, 70% water, and 10% combustible liquid) depending upon the droplet size. The CWSP is based on charcoal and different liquid combustible components (at Tg ≈ 780 K and Vg ≈ 3 m/s).
average for waste engine oil. This effect is due to the properties of flammable liquids. It is almost directly proportional to the ignition temperature and the combustion heat of the liquid flammable component.
studies.27,29 This is due to the fact that the rates of the thermal decomposition of the organic part of coal, the evaporation of liquid combustible and non-combustible components, and the gas-phase and heterogeneous oxidation are nonlinearly (as a rule, exponentially) dependent upon the temperature. The wider the range of the oxidant temperature and the droplet size, the more pronounced are these effects. Note that the ignition delay times of the tested liquid combustible components are almost the same given the high oxidant temperatures and small droplet sizes. We determined the critical or limit temperature of the sustainable ignition of the CWSP based on charcoal, i.e., the minimum oxidant temperature to ensure the sustainable ignition of fuel. In particular, the critical (limit) temperatures are 728, 722, and 707 K for the tested compositions based on engine oil, turbine oil, and fuel oil, respectively. Table 5. Integral Ignition Characteristics for CWSP with Different Carbon Components carbon component of CWSP charcoal brown coal filter cake of coking coal filter cake of low-caking coal filter cake of non-baking coal
Figure 3. Ignition delay times of CWSP droplets (30% coal, 70% water, and 10% combustible liquid) depending upon the oxidant temperature. The CWSP is based on charcoal and different liquid combustible components (at Rd ≈ 1 mm and Vg ≈ 3 m/s).
Figures 3 and 4 illustrate the ignition delay times of the tested CWSP depending upon the oxidant temperature and the fuel droplet size, respectively. The ignition delay time of CWSP is slightly dependent upon the liquid fuel component. The difference between the ignition delay times is no more than 1.3−1.6 s, while the oxidant temperature changes in the range of 600−1000 K. The ignition delay time of the CWSP droplet decreases nonlinearly as the oxidant temperature Tg increases. The time τd grows exponentially as the droplet size increases. This behavior of the ignition delay times corresponds well to the modern concepts of the combustion theory and the results of
τd at 950 K
critical (minimum) temperature of sustainable ignition (Tmin g )
1.8−3.5 1.9−2.8 3.2−5.1
707−730 680−720 795−840
3.8−6.2
910−940
5.1−7.3
860−905
Table 5 shows the average values of ignition delay times and the critical ignition temperature for the CWSP based on charcoal and brown coal as well as filter cakes of coking, lowcaking, and non-banking coals. The data are based on the experimental results of the present studies and previous reports.27,29 The integral ignition characteristics of the CWSP based on charcoal are similar to those for the CWSP based on brown coal. However, the ignition characteristics of the CWSP based on charcoal are significantly different from those of the CWSP based on the filter cakes of bituminous coal with a high degree 10889
DOI: 10.1021/acs.energyfuels.6b02245 Energy Fuels 2016, 30, 10886−10892
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Energy & Fuels of metamorphism (see Table 4). The ignition delay times of the CWSP based on filter cakes are 2−4 times greater than those of the CWSP based on charcoal. The minimum temperature of sustainable ignition of the CWSP based on filter cakes is 100− 230 K higher than that of the CWSP based on charcoal.
Figure 6. Ignition delay times of the CWSP droplet depending upon (a) its size (at Tg ≈ 900 K and Vg ≈ 3 m/s) and (b) oxidant temperature (at Rd ≈ 1 mm).
coal starts to thermally decompose. This causes an increase in the ignition delay time of the CWSP based on the filter cakes of coal with a high degree of metamorphism. Figures 5 and 6 show that the difference between the integral ignition characteristics of fuels fades as the oxidant temperature increases and the CWSP droplet size decreases. Power boilers typically operate at the oxidant temperatures of no less than 1000 K. At the same time, the size of the droplets injected into a combustion chamber can be less than 1 mm. Therefore, the results illustrate the possibility of the real use of the CWSP prepared from coals and their processing waste with different qualities and degrees of metamorphism. Previous studies27,29 examined the effect of even a small addition of highly reactive brown coal to the CWSP based on bituminous coal or recycled coal. The addition of even 10% brown coal to the CWSP reduces the minimum temperature Tmin by 50−70 K, while the ignition delay times drop severalg fold. We can make the same conclusion for the CWSP having charcoal in its composition, because the ignition characteristics of the CWSP based on brown coal and charcoal are similar (see Table 5 and Figures 5 and 6).
Figure 5. Temperature trends at the center of the CWSP droplet with charcoal and brown coal as the (a) main component and (b) additional component (at Rd ≈ 1 mm, Tg ≈ 850 K, and Vg ≈ 3 m/s).
Figure 5 shows typical temperature trends of the CWSP droplet. The trends reflect different combustion characteristics of the compositions based on brown coal and charcoal. Figure 6 shows the ignition delay times of CWSP depending upon the oxidant temperature. The difference between the integral ignition characteristics of the tested carbon components (see Table 5 and Figures 5, 6) can be explained by the difference in the properties of various coals. The filter cakes of bituminous coal with a high degree of metamorphism have low reactivity and a high temperature of thermal decomposition with the yield of volatiles (in comparison to brown coal and charcoal). This increases the minimum temperature of sustainable ignition as well as the time of inert heating of the fuel droplet until the organic part of 10890
DOI: 10.1021/acs.energyfuels.6b02245 Energy Fuels 2016, 30, 10886−10892
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Energy & Fuels The experimental data indicate that charcoal can be used as an additive to CWS and CWSP. Charcoal may intensify fuel heating, gas-phase ignition of volatiles, and heterogeneous combustion of carbon. The ignition characteristics of CWS and CWSP based on charcoal and brown coal are rather similar (see Table 5). This expands the possibility of varying the highly reactive components of coal fuel suspensions. Furthermore, this opens up new possibilities for a modern energy feedstock.
6. CONCLUSION (1) The experimental studies have shown that charcoal can be an important component for CWSP. It has a low ash content and high combustion heat and, thus, intensifies the combustion of CWS fuels. Therefore, one can use the filter cakes of even the lowest grade coal with an addition of charcoal to intensify the combustion of the fuel composition. This expands the raw material base of possible CWSP components. (2) The CWSP based on charcoal has a rather low critical (minimum) temperature of sustainable ignition that is comparable to the CWSP based on brown coal. In the case of the CWSP based on low-grade carbon components (for example, filter cakes), the limit temperature can be considerably reduced by the addition of even a small amount (10−15%) of charcoal. (3) The ignition delay time of the CWSP based on charcoal is similar to that of charcoal. However, it greatly differs from the CWSP prepared from filter cakes of various coals. This allows one to create CWS with the desired combustion characteristics on the basis of various solid fuel components.
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Tmin = minimum air temperature required for ignition of g CWSP (K) Qas,V = enthalpy of combustion of the analytical sample of coal (J/kg) Vdaf = yield of volatiles of filter cake to a dry and ash-free state Vg = air velocity (m/s) Wa = humidity of the analytical sample of the filter cake in an air-dried state (%) τ = time (s) τd = ignition delay time (s)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Pavel A. Strizhak: 0000-0003-1707-5335 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Research into the characteristics of CWSP ignition processes was funded by the Russian Science Foundation (Project 15-1910003). 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 the National Research Tomsk Polytechnic University as one of the world-leading universities.
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NOMENCLATURE Ad = ash level of the dry sample (%) Cdaf = fraction of carbon in the sample converted to a dry and ash-free state (%) Hdaf = fraction of hydrogen in the sample converted to a dry and ash-free state (%) Ndaf = fraction of nitrogen in the sample converted to a dry and ash-free state (%) Odaf = fraction of oxygen in the sample converted to a dry and ash-free state (%) Rd = droplet radius (mm) Sdaf = fraction of sulfur in the sample converted to a dry and ash-free state (%) Td = temperature at the interface of the thermocouple junction/slurry (K) Tg = air temperature (K) 10891
DOI: 10.1021/acs.energyfuels.6b02245 Energy Fuels 2016, 30, 10886−10892
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DOI: 10.1021/acs.energyfuels.6b02245 Energy Fuels 2016, 30, 10886−10892
