Article pubs.acs.org/EF
Burning Properties of Slurry Based on Coal and Oil Processing Waste Dmitrii O. Glushkov, Sergey Yu. Lyrshchikov, Sergei A. Shevyrev, and Pavel A. Strizhak* National Research Tomsk Polytechnic University, 30, Lenin Avenue, Tomsk, 634050, Russia ABSTRACT: The main physicochemical coefficients of the components and of the tested coal−water slurry containing petrochemicals (CWSP) have been collected. The viscosity, mixture stability, density, ash content, humidity, flash and ignition temperatures, and enthalpy of combustion of the tested CWSP are reported. The temperatures of combustion of fuel compositions with different concentrations of solid and liquid components have been measured, as well as the important parameters: the ignition time delays of droplets (radius 0.25−2.5 mm) and the times required to reach their complete burning. The study allows determination of the optimal fuel compositions yielding the shortest ignition time delays, the longest times to complete burning, the highest temperatures of combustion, and the maximum enthalpy of combustion. It is found that the viscosity of CWSP is dependent almost proportionally on the moisture content of components. It is shown that the calorific value of CWSP is dependent proportionally on the calorific value of components. It is revealed that the ignition delay times of CWSP are dependent significantly on the degree of coal metamorphism and the volatiles content, as well as on the heat of vaporization and the boiling point of added flammable liquids. Important experimental data are obtained, which allow prediction of the CWSP composition with the desired properties of components, for which the following characteristics are known: calorific value, moisture content, ash content, volatile content, heat of vaporization, boiling point, and other characteristics considered.
1. INTRODUCTION Over the past few years, there has been a sustained growth in production and consumption of solid fuel in the world. In particular, one can observe an increase in consumption of coal of varying degrees of metamorphism in such countries as Germany, Japan, China, the United States, India, and Russia.1−4 To date, much of produced coal is cleaned. For example, all produced coal is cleaned in Australia and the Republic of South Africa. On average, 70%−90% of production volumes are cleaned in the world. As a result, high-in-ash wastes are discarded annually. Their amount is estimated in the range of millions of tons.1−4 With the growth in coal production and coal cleaning, this huge amount will continue to grow. In the very near future, the issues of disposing such waste can become complex, especially in states with limited area. Over the past 20−25 years, the world’s scientific community has developed a group of waste-disposing technologies of coal processing. In particular, it is possible to highlight the preparation of coal−water slurry (CWS) and its burning in power plants,5−14 the processing of the resulting ash for use in the construction of buildings and roads,15,16 its application in chemical and petrochemical productions, and other uses. Surely, large amounts (millions of tons) of coal conversion waste emerging annually are difficult to recycle only for the construction, chemical, and petrochemical industries. The main direction of the processing of such substations is their utilization as the components of fuel suspensions and their burning in the combustion chambers of power plants. In addition to issues of utilization of high-in-ash waste of coal cleaning, modern coal−water fuels allow us to solve several economic and environmental problems, because of their improved environmental characteristics, compared to conventional solid coal and liquid fuels.5−7 Large-scale theoretical and experimental investigations have been conducted in different countries. They examined the © XXXX American Chemical Society
preparation and combustion of CWS compositions based on coal of varying degrees of metamorphism. The results of studies5−14 give the understanding of the basic laws of ignition and burning of CWS. References 5−14 describe the experimental techniques, physical and mathematical models, predictive mathematical models, methods and algorithms for numerical modeling, and the dependences of the main parameters of the process. Appropriate summary, conclusions, and recommendations were formulated. However, the modern ideas based on the studies presented in refs 5−14 are still limited, in terms of the influence of CWS components on ignition characteristics. Moreover, mixtures of CWS with various organic substances, e.g., scavenge oils, and petroleum production and processing waste, have not been fully investigated yet. Note that there is an increasing interest in combustion of CWS, biomass, and various waste materials in a fluidized bed.9,14,17−20 The main attention is traditionally paid to the completeness of fuel combustion, burning temperature stabilization, environmental indicators in flue gas analysis, and the issues of ash removal. A fairly large group of inert materials is indicated, which provide the specific conditions relevant to a fluidized bed. In the analysis of CWS, little attention is paid to ignition processes. Studies9,21−23 have shown that the ignition stage plays a crucial role for efficient burning of CWS. The experiments9,21 and simulations22,23 have shown that it is advisible to carry out research on the ignition of CWS on an example of single particles. Moreover, it was determined22,23 that ignition is possible at temperatures in a combustion chamber that are significantly lower than the traditionally Received: December 8, 2015 Revised: February 24, 2016
A
DOI: 10.1021/acs.energyfuels.5b02881 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels accepted at power plants (no less than 1000 K). Studies21 have demonstrated that it is possible to burn stably CWSP at temperatures substantially lower than 1000 K. The volume of low-grade oils coming out as processing products, and that could be used as flammable liquids (dielectric, turbine, motor, machine oils), is also measured in millions of tons per year.4 Many regions (countries) are interested in their utilization (Europe, Asia, Africa, South and North America). Studies of effective combustion technologies both for CWS and coal−water slurry containing petrochemicals (CWSP) are of current interest. In recent years, many experimental investigations have been conducted on the burning of CWSP droplets based on coal, waste oil, and oil− water emulsion. References 24−26 demonstrate that CWSP can be used as a prime fuel in many power plants. However, only high-grade coals of varying degrees of metamorphism were used as the basic components of CWSP in those experiments. It is advisible to use the waste from the processing and cleaning of these coals (e.g., filter cakes) when producing CWSP,27 since most of such coals are extensively exported by developed countries (for example, China, the United States, India, Russia). The volume of coals can grow by several factors by 2020.4 The improvement of the production of CWSP based on filter cakes, with the addition of waste oils of different origin, is a crucial point. It is important to determine how the different components of such fuel suspensions can affect the basic properties of the composite fuel (stability, viscosity, enthalpy of combustion, ignition inertia, combustion duration, and others). Until now, very few studies have been published on the ignition and combustion of CWSP of various compositions. Moreover, there are no data on the conditions of low-temperature ignition (below 800 K).28 It is thus very important to conduct experiments with different filter cakes and petrochemicalderived waste to feed the corresponding database. To expand the possible applications of the research results, we should enhance the fuel heat conditions from low temperature28 to more than 1200 K, appropriate to modern power plants, units, and installations. References 21 and 27 present the results of experimental studies of CWSP ignition under such conditions. Studies21,27 focus mainly on determining the minimum (limit) temperatures of stable ignition, ranges of change in ignition delay times, and the times of complete combustion of fuel droplets. However, the properties of CWSP based on various components have not been studied yet, as well as the influence of these properties on ignition characteristics. The objective of this study is to investigate experimentally the characteristics of CWSP obtained from filter cakes of bituminous coals and waste oils, as well as to determine the range of the main components’ concentration for energyefficient burning of such fuel compositions in power plants.
We used scavenged motor, transformer, and turbine oils, greasy and oily mixtures, and oil−water emulsion (from Gerasimovskiy deposit of the Tomsk region, Russia) as a liquid flammable component. Tables 2−5 (presented later in this paper) present the characteristics of studied filter cakes, coals, and flammable liquids. The plasticizer Neolas was added to all compositions in a relative mass concentration from 0.5% to 1% (Table 1 lists the characteristics of Neolas).
Table 1. Main Properties of the Plasticizer “Neolas” No.
parameter
value
1 2 3 4
appearance surfactant content pH of solution density @ 293 K
transparent liquid 25 wt % 6.5 954 kg/m3
We prepared CWSP with a homogenizer (Precision Mechanics, Model MPW-302). According to recommendations,24−27 the oil− water emulsion was prepared at the first stage. The components (water and oil or other flammable liquid was injected according to the required mass concentration) were added into a working glass of the homogenizer (capacity of 0.25 L) after previous weighting by an analytical balance (ViBRA, Model HT 84RCE). The duration of mixing the oil−water emulsion components was 4 min. We then injected the required filter cake mass in the glass of homogenizer containing the already-prepared suspension. The duration of this stage of slurry preparation was 10 min. These prepared fuel compositions were stored in 0.25 L airtight glass bottles. The measurements of the thermal and physical properties of combustion and ignition time delays of the prepared CWSP were conducted using the experimental setup described in Figure 1.
Figure 1. Schematic of the experimental setup. (Legend: 1, hollow quartz cylinder; 2, air fan; 3, heater; 4, remote control; 5 and 6, thermocouples; 7, recorder; 8, minirobotic arm; 9, droplet; 10, highspeed video camera; 11, computer; 12, gas analyzer (O2); 13, ductwork; and 14, ventilation.)
2. EXPERIMENTAL PROCEDURE 2.1. Materials. The filter cakes obtained from the cleaning of the bituminous, low-caking, and nonbaking coal (coal cleaning plant of Kuznetsk Basin, Russian Federation) were used as the main components of CWSP. According to this method, at one of the stages of cleaning, the coal sludge suspension is thickened by surfaceactive substances, and a type of pulp is produced with a high concentration of solid particles suitable for filtration. The basic equipment for filtering is belt or frame filter presses, from which a large fraction of water is removed through a filter mesh. The wet waste formed in such a way is called a filter cake. The sizes of the coal particles in the CWSP component compositions are 80−100 μm. To provide these particle sizes, a set of special sieves is used.
2.2. Instruments and Procedure for Determination of Ignition Characteristics. The droplets of CWSP were produced by the dosing device Finnpipette Novus (minimum and maximum dosage volumes are 1 μL and 10 μL, pitch variation is 0.1 μL). We weighted the samples with an analytical balance ViBRA HT 84RCE (measurement resolution is 10−5 g) to monitor the initial mass in each experiment. The droplet diameter was determined using a high-speed camera (feature 10 in Figure 1) and Tema Automotive software. Six diameters and then the average value of radius Rd were measured and B
DOI: 10.1021/acs.energyfuels.5b02881 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Table 2. Results of Technical Sample (Analytical) Analysis of Filter Cakes and Bituminous Coal of Appropriate Grade sample of filter cake and grade of coal
Wa (%)
dry filter cake based on bituminous coal (the enrichment factory “Severnaya”) bituminous coal (the colliery “Berezovskaya”) dry filter cake based on low-caking coal (the enrichment factory “Chernigovskaya Koksovaya”) low-caking coal (the enrichment factory “Chernigovskaya Koksovaya”) dry filter cake based on nonbaking coal (the enrichment factory “Kaltanskaya Energeticheskaya”) nonbaking coal (the enrichment factory “Kaltanskaya Energeticheskaya”)
calculated for each image of the droplet. The systematic error in determining Rd by high-speed video recording did not exceed 4%. Initial sizes (radius) of droplets were from 0.25 mm to 2.5 mm before interacting with the hot air flow. The minirobotic arm (feature 8 in Figure 1) was used to inject the fuel mixture droplet hanging on a junction of thermocouple (feature 6) in the cylinder (feature 1). We used an S-type thermocouple with a temperature measurement range of 273−1873 K (the systematic error is ±10 K, the inertia is Tg and achieved a rate of change (Td) of no less than 10 K/s). The τb parameter is the time from the moment of igniting the coal coke until its complete burnout (characterized by the value of Td, using the stationary value corresponding to Tg). Taking into account the video recording rate of 103 frames per second, systematic errors in determining the values τd, τb, and τc do not exceed 0.5 ms. The techniques for evaluating the systematic and random errors in determining the basic parameters of ignition and combustion in the experiments are similar to those described in refs 29−32. 2.3. Instruments and Procedure for Determination of Fuel Properties. In accordance with the purpose of the study, we conducted the experiments to specify the main properties of different CWSP, as well as the properties of the components of these mixtures. The technical analysis of filter cakes and initial coals includes the
2.05 2.76 2.89
Ad (%)
Vdaf (%)
Qas,V (× 106 J/kg)
26.46 14.65 50.89 21.68 21.20 18.07
23.08 27.03 30.16 27.40 16.09 15.07
24.83 29.76 15.23 26.23 26.92 27.65
determination of the fraction of moisture in the sample in an air-dry state (Wa), the ash level in a dry state (Ad), and the yield of volatiles from a dry ash-free state (Vdaf) by the international procedures.33−35 The viscosity of the prepared CWSP was determined by rotating a viscometer (measurement range is 10−1000 mPa s) at a temperature of 290 K, based on the results of three measurements, followed by calculation of the mean value (maximum measured error is ±15%). We determined the enthalpy of combustion (Qas,V) of an analytical sample of filter cake and petroleum products at constant volume by a calorimeter (Model IKA C 2000), according to the procedures described in the literature.36,37 To determine the enthalpy of combustion of the produced suspensions of CWSP, the sample was predried at a temperature of 378 K and its humidity level was defined. We then measured the enthalpy of combustion of the oven-dried sample of CWSP by the procedure discussed in the study.36 After that, the enthalpy of combustion was recalculated, based on the humidity of the produced fuel composition. The density was determined by a density meter (measurement range = 820−880 kg/m3, graduation scale mark = 1 kg/m3) for the liquid components of CWSP (waste oils, greasy mixture, and oil− water emulsion). The humidity and ash level were calculated using the procedures described in refs 38 and 39, and the flash point and the ignition temperature were determined using the procedures described in an earlier report.40 The flash point is the minimum temperature at which petroleum vapors form a mixture with air, which flashes when one holds a direct flame close to it. The ignition temperature is a minimum temperature, at which petroleum is ignited, when one holds a flame close to it, and it burns at least 5 s. Elemental constituents of the filter cake sample (according to the procedures described in refs 41 and 42) were determined using the Vario MICRO Cube device. We burned a sample of a known mass in an oxygen atmosphere at a temperature of 1373 K. The subsequent chromatographic separation of formed gases, as well as detection, was performed by a catarometer. Later, it was used to calculate the mass fraction of C, H, N, and S, using the instrument software (oxygen was determined by calculation) and recounted to a dry-ash-free state. We determined the phase stability of the studied CWSP by the procedure based on continuous monitoring of component separation in special glass transparent containers. We illuminated the containers six times a day (i.e., every 4 h) and determined the uniformity or separation of phases (birth and position of phase boundaries were monitored) according to a constancy of reflection and refraction coefficients of the light rays. The segmental stability of fuel suspensions was estimated by observing stratification. This method involves the measurement of the volume of the temporary water or oil−water ligament layer separated during storage and transportation. We observed the stratification of samples using measuring cylinders with a capacity of 50 and 1 mL of division cost. Fuel samples were placed into 50 mL measuring cylinders. After 1 h, the volume of the separated ligament (Vc) (more transparent after spotlight illumination) was determined visually. A sustainability index (Y1) was calculated using the formula Y1 =
Vc V0
with respect to the initial fuel volume V0. It was assumed that CWSP stratified, when Y1 reached the value of 0.05. Studies43−45 have described similar methods for determining the duration of preservation of structural stability of fuels. The main difference from the present C
DOI: 10.1021/acs.energyfuels.5b02881 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Table 3. Results of Elemental Sample (Analytical) Analysis of Filter Cakes and Bituminous Coal of Appropriate Grade sample of filter cake and grade of coal
Cdaf (%)
Hdaf (%)
Ndaf (%)
Sdaf (%)
Odaf (%)
bituminous coal (the colliery “Berezovskaya”) dry filter cake based on bituminous coal (the enrichment factory “Severnaya”) low-caking coal (the enrichment factory “Chernigovskaya Koksovaya”) dry filter cake based on low-caking coal (the enrichment factory “Chernigovskaya Koksovaya”) nonbaking coal (the enrichment factory “Kaltanskaya Energeticheskaya”) dry filter cake based on nonbaking coal (the enrichment factory “Kaltanskaya Energeticheskaya”)
87.20 79.79 87.47 77.30 90.13 87.97
5.090 4.486 5.039 4.783 4.255 4.104
2.05 1.84 2.15 1.93 2.31 2.23
1.022 0.868 0.444 0.326 0.441 0.526
4.46 12.70 4.77 15.32 2.77 5.03
study lies in the fact that the experiments43−45 defined Y1 not by the volume, but by the height of the separated ligament (the parameter Y1 was determined as a ratio of corresponding heights). It was defined that the stability, on average, lasted 7−9 days for the studied fuel compositions of CWSP, whereby the less stable composition was based on filter cake from nonbaking coal. Tables 2−5 present the defined characteristics of the studied filter cakes, coals, and flammable liquids.
CWSP compositions have similar changes in the laws of viscosity. This can be explained by the fact that, by increasing the temperature of the suspension, its viscosity decreases. On the other hand, by increasing the storage time of CWSP, its viscosity increases. For example, let us consider the following composition: 89.5% of filter cake based on low-caking coal, 10% of oil−water emulsion, 0.5% of plasticizer. Its viscosity changes with the growth of the temperature: 449 mPa s at 290 K, 437 mPa s at 310 K, and 390 mPa s at 330 K. For the compositions with filter cake based on bituminous coal and nonbaking coal (Table 6), the viscosity varies slightly less (within 10%) as the temperature increases from 290 K to 330 K than in the case of low-caking coal. If we increase the storage time (without stirring), the viscosity of the same composition (89.5% of filter cake based on low-caking coal, 10% of oil− water emulsion, 0.5% of plasticizer) will be 449 mPa s after 1 h, 456 mPa s after 24 h, 468 mPa s after 48 h, and 493 mPa s after 72 h. For other CWSP compositions (Table 6), the relative viscosity varies similarly. Based on these results, we may conclude that the viscosity of CWSP can be kept at the required level, even after relatively long-term storage without periodic stirring. This can be achieved by changing the temperature of the CWSP. If it is not possible, it is advisible to stir CWSP (to stabilize viscosity) before putting the composition into the combustion chamber. Enthalpy of combustion of a fuel as-received is one of the main characteristics of the produced CWSP. We measured the enthalpy of combustion for different fuel compositions in a calorimetric bomb. We found out that the enthalpy of combustion can be calculated using the concentration of the fuel components and its particular enthalpy of combustion (Q = ∑qiwi). In this case, the deviation is smaller than 2% (Table 6). The elemental composition of CWSP can be computed in a similar way. The obtained results illustrate that the components directly influence the highlighted properties of fuel compositions. We specified the properties of our CWSP produced from various filter cakes and liquid combustible components. We established the influence of these properties on ignition delay time and time of complete burnout. Figure 3 compares the results for the different CWSPs. It is shown that the ignition delay time is the most impacted by the utilization of filter cake. The periods (τd) can differ by more than 50% at comparable temperatures of ignition.
Table 4. Investigation Results of Composition and Heat of Combustion of Filter Cakes in an Initial State (after Obtainment at the Enrichment Factory) sample of filter cake filter cake based on bituminous coal (the enrichment factory “Severnaya”) filter cake based on low-caking coal (the enrichment factory “Chernigovskaya Koksovaya”) filter cake based on nonbaking coal (the enrichment factory “Kaltanskaya Energeticheskaya”)
proportion of dry substance (%)
Qrs,V (× 106 J/kg)
56.5
14.03
62.1
9.46
60.9
16.4
3. RESULTS AND DISCUSSION Tables 6−8 present the results of technical and elemental analysis of studied fuel compositions (to compare with similar characteristics of separate components of fuels). We conducted experimental investigations to determine the properties of produced CWSP (Table 6). The viscosity of prepared mixtures was determined to be proportional to the water content (see Figure 2). Note that there is a significant increase in the viscosity of mixtures based on motor oil (synthetical). This effect is more obvious for CWSP from filter cake based on bituminous coal, for which we observed the agglomeration of filter cake particles with the formation of bigger fuel droplets. Most probably, this is due to particular properties of the filter cake (correlation of organic and mineral mass) and motor oil, which is the presence of various welding rods behaving as flocculants. This is similar to the process used at oil granulating during coal cleaning.46 Also note that CWSP based on motor oil had the highest stability. We performed the experiments on determining the viscosity of CWSP at different storage times and temperatures. All Table 5. Properties of Combustible Liquids sample of combustible liquid
density at 293 K (kg/m3)
humidity (%)
ash level (%)
flash temperature (K)
ignition temperature (K)
Qas,V (× 106 J/kg)
waste motor oil waste turbine oil waste transformer oil greasy and oily mixture oil−water emulsion
871 868 877 861 869
0.28
