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Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces
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Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces S.V. Syrodoy, J.A. Kostoreva*, A.A. Kostoreva, L.I. Asadullina National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634050, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2019 Received in revised form 13 July 2019 Accepted 15 July 2019 Available online xxx
The results of experimental studies of the ignition processes of a large set of particles (from 100 to 500) of wood-coal composite fuels under conditions of high-temperature radiation-convective heating are presented. The ignition of wood-coal fuel mixture fuel particles was studied under the conditions of their free fall along the quill ceramic cylinder passage in a high-temperature oxidizing medium (air). Such a configuration provides the conditions that most closely match the heating conditions of actual furnaces of heat generating installations. The experiments have been carried out on the mixtures of particles of subbituminous coal (that is quite typical of coal-fired power industry) with the particles of woody biomass of three types: birch, pine and larch, which are widely spread in the territory of many states of the northern part of the Earth hemisphere. The recording of the heating and ignition processes of the fuel mixture particles was carried out with a high-speed Photron FASTCAM CA45 (500 fps) camera. The air temperature in the passage along which the particles moved varied in experiments from 873 K to 1273 K. The purpose of the experiments performed was the ignition delay time (tign). The main variable factors are the ambient temperature and the concentration of woody components. The consistent errors in determining the main characteristics of the process did not exceed 5%, the random ones did not exceed 13%. The size of coal particles in the experiments did not exceed 60 mm, and of the wood - 5 mm. It has been established that in such a fuel system one of the fuel components of the coal particle is “conducting” (or leading), and the second is “conducted” in the reaction of the thermochemical response of the fuel mixture with air oxygen. According to the results of experimental studies, the ignition delay times for the wood-coal particle mixtures have been stated. The increase in the concentration of smallsized coal particles in fuel mixtures from 0% to 100% leads to the decrease in the entire induction period (up to 20%) at the relatively cold temperatures. It was also found that with typical sizes of coal particles up to 60 mm, stable ignition of sufficiently large wood particles (up to 5 mm), present in the mixture, is provided. The conditions and characteristics of ignition of wood-coal mixtures hardly depend on the type of wood, which is the second component of the mixture. According to the results of the research conducted and following the requirements for optimizing the processes of fuel combustion, the use of mixed coal- and wood-based fuels in boilers of large and small power plants is justified. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Biomass Wood Experiment Ignition delay time
1. Introduction Over the past two decades, researchers in many countries have made great efforts to scientifically and technically substantiate the possibility to significantly increase the use of coal [1,2] (including low grade) [3,4] and biomass [5,6] to generate thermal and electric power. The main problems of using these two rather promising energy sources in high (steam boilers) and low (boilers) power engineering are: high concentrations of anthropogenic oxides and fly ash in coal combustion products [7,8] and relatively low heating capacity of typical
* Corresponding author. Lenin Avenue, 30, Housing 4, Audience 201, Tomsk, 634050, Russia. E-mail address: [email protected] (J.A. Kostoreva). https://doi.org/10.1016/j.joei.2019.07.007 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: S.V. Syrodoy et al., Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.07.007
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types of biomass compared to other energy sources [9,10]. In order to solve these problems, scientists in many countries are trying to justify not the use of homogeneous coal or biomass in power engineering (the most promising type of fuel is wood), but various composite fuels based on them [11,12]. For example, experimental [13,14] and theoretical [15] studies of the combustion of coal-water, organic-coal-coal [16] fuels, as well as mixtures of coal and wood particles [17,18] are intensively conducted. A rather large-scale positive effect from the burning of such composite fuels is the considerable reduction in anthropogenic emissions [17,18] (sulfur and nitrogen oxides, as well as fly ash), in conditions of moderate (in many practical cases) reduction of energy indicators of thermal power plants or small boiler plants [19,20]. From the technological point of view, the simplest are the technologies of burning crushed coal and dispersed wood in steam and hotwater furnaces [21,22]. But so far there have been few examples of power plants boilers operating on mixtures of coal and wood (no more than twenty [23]), although attempts to incinerate such mixtures have been made on hundreds of thermal power plants in many developed countries, for example [24e26]. The reason for this state of affairs is most likely the current-date lack of general theory (as well as the insufficient experimental data for practical use) of combustion and, in particular, of wood-coal mixtures ignition. It is important, however, that, the sawing and woodworking waste particles (their share reaches 15% in most cases) with typical sizes from 1 mm to 6 mm can be used as the second component of coalwood mixtures. Further grinding of such rather large particles (mainly sawdust chips) requires a large energy input. Therefore, one of the main problems arising from the analysis of the possibility of burning sawmill and woodworking waste in steam and water heaters in a coal mixture is to assess the conditions and the characteristics of the ignition processes of the wood biomass particles, which can be a hundred times larger in size than the coal dust formed after grinding in a kind of ball mill apparatus widely used in the power engineering. So, it is necessary to estimate the lag times of the ignition of large wood particles in relation to the coal ones. The aim of the work is to experimentally establish the basic laws of ignition of mixtures of coal and wood particles, including the dependences of the ignition delay time on the ambient temperature. In power systems it is possible to use the most different types of woody biomass, both softwood and hardwood. The most promising is the use of wood processing wastes, the price of which in many cases essentially comes down to loading and transportation costs. The burning of wood and, especially the hardwood grown specifically as an energy resource is also promising for a large group of Asian countries. It can be noted that the growth rate of the latter is several times higher than the similar characteristics of coniferous trees, which gives grounds for reasonable conclusions [27,28] about the feasibility of growing, for example, lindens, eucalyptus, poplar and other types of energy wood on special plantations for subsequent burning in boiler furnaces for different purposes. In this regard, the actual task is to compare the ignition characteristics of mixtures of various wood species with coal and evaluate the ignition efficiency of such fuels based on hardwood wood. 2. Studied materials and experimental methods Experimental studies of the conditions and characteristics of subbituminous coal (rank of coal - D) fuel mixtures and three types of wood ignition processes (Table 1). Subbituminous coal is typical and is widely used in thermal power plants in many countries. As the second component of the mixtures, three types of wood growing in Western Siberia (birch, pine, larch) have been studied. These types of wood are widely spread in Europe, Asia and America [29] and are the typical representatives of the “energy biomass”. The main characteristics of the fuel components under the study are shown in Table 2. Technical characteristics of subbituminous coal are given in Table 3. The composition and characteristics of coal ash are given in Table 4. It is possible to determine the conditions and characteristics of the ignition of mixtures of coal and wood particles using several experimental methods differing in heating conditions, the number of particles and their position relative to the heating source. A portion of crushed materials can be located on the surface of a plate heated to high temperatures [30]. A thin layer of fuel particles is heated by a gas flow [31]. A metal particle heated to high temperatures falls on the surface of a layer of crushed fuel [32]. A portion of fuel is entered into the channel of a horizontally located cylinder heated to high temperatures [33]. In all these cases, the heating conditions are quite different from the conditions of the combustion chambers of steam and hot water boilers, in which particles of coal and wood soar in a stream of gases, which are a mixture of combustion products of the fuel and air heated to high temperatures. Experimental data on the ignition characteristics of particles of coal and wood that are hovering in the medium heated to high temperatures has not been published yet.
Table 1 Wood ecoal fuel compositions. Composition number
Coal
Wood
Rank
Percentage, %
Type
Percentage, %
N1
D
Birch
N2
D
N3
D
60 70 80 90 60 70 80 90 60 70 80 90
40 30 20 10 40 30 20 10 40 30 20 10
Larch
Pine
Please cite this article as: S.V. Syrodoy et al., Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.07.007
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Table 2 Component
Ср J/(kg $ K)
l W/(m $ K)
r kg/m3
Vdaf %
Cг %
Qт J/kg
Long flame coal (D) Birch Larch Pine
1150 1250 e 2300
0,116 0,15 0,13 0,14
1253 610 370 500
46 85 85 85
76 48,6 50,1 49,6
33,5$106 15$106 15,5$106 15,5$106
Where: Ср e specific heat capacity [J/(kg $ K)], l e heat conductivity [W/(m $ K)], r e density [kg/m3], Vdaf-volatile yield [%], Cг e carbon content [%], Qт e caloric content [J/kg].
Table 3 Technical characteristics of coal. As-fired fuel basis. Composition, % W rt
At
11,5
15,9
Srp
Sra
0,4
0,4
Cr
Hr
56,4
4,0
Nr
Or
1,9
9,9
Caloric content Q ri МДЖ/кг
Ash content Ad , %
Moisture content W, %
Volatile content V daf , %
21,90
18,0
4,5
40,5
Table 4 Composition and characteristics of subbituminous coal ash. Ash fusibility temperature, С
True liquid state of slag
Normal liquid slag removal onset
tA
tB
tC
t0 С
VA (t0 ,)Па∙с
t С
1160
1310
1440
1400
100
1600
20 Solid base deposits susceptibility no
Elemental composition per sulphate-free mass, % Al2O3 TiO2 Fe2O3 CaO SiO2
MgO
K2O
Na2O
Onset slagging temperature t
60,0
2,7
3,0
2,0
1000
20,6
0,9
6,8
4,0
VA ðtÞ Па с
A methodology for conducting experimental studies of the coal and wood particles ignition processes with reproduction of the conditions that most closely correspond to the conditions of steam and water-heating boilers have been developed. Experimental studies have been conducted on the stand, the scheme of which is shown in Fig. 1. The fuel particles were introduced into the channel of a hollow, vertically arranged ceramic cylinder, on the external surface of which an electric heater was attached. This configuration of the heating area has allowed to study the ignition processes of the aggregate (150e200 pcs.) of small-sized fuel particles under the conditions of their free fall in a high-temperature oxidizing environment and provided minimal errors (there are no fuel particle holders such as in Ref. [34]) simulating the real furnace conditions of the boiler unit. The particles of the charcoal mixture were sprayed using a pneumatic device into the channel of a ceramic cylinder heated to high temperatures (the maximum possible temperature of 1473K). The registration of the heat and mass transfer and fuel particles ignition processes was carried out with a high-speed Photron FASTCAM CA4 5 video camera (video speed up to 20,000 f/s). The temperature of the air inside the cylinder (Tg) in the experiments varied in a fairly wide range (from 870K to 1270K), corresponding to the ambient temperatures in the combustion chambers of the boiler units. The Tg measurements were performed with a thermoelectric converter (chromel-aluminum) with a junction diameter of 0.5 mm (with the inertia of no more than 3 s). The experiments were carried out only under the condition of achieving stationary Tg values in the entire channel along the height after complete heating of the air in the cylinder channel (Fig. 1). The time between the onset of heat exposure (the moment particles hit the combustion chamber) and the ignition (the appearance of a flame) was considered as the ignition delay (tign). The ignition delay time was determined after analyzing the videogram frames, as the difference between the moments of time when the mixture was introduced into the channel with air heated to high temperatures and the moment when the flame appeared near the surface of at least one particle. The consistent error in determining the main measured parameters (Тg, tign) was no more than 5%. The weight of the mixture was 1e2 g in a channel volume of 0.157 m3. This value corresponds to the real concentration of fuel particles achieved in wood-coal boiler units. The size of coal particles ranged from 40 to 60 mm. The characteristic dimensions of the second component of the mixture ranged from 3 to 5 mm. It should be noted that the most preferable (according to the estimates [35]) is the technology of burning wood-coal mixtures in the furnaces of steam or hot water boilers designed for burning coal. In this case, there is no need for a radical change in the systems of supplying fuel to the combustion chamber and of the fuel burning. However, if, during the preparation of coal, they are crushed in conventional fuel preparation systems to typical sizes of 40e60 mm (and even lower), then the wood can flow to the station in a different state (even if sawmill or woodworking waste is used). Therefore, one of the objectives of the experimental studies was to assess the conditions and characteristics of ignition of wood-coal mixtures with significantly different characteristic sizes of wood and coal particles (up to 100 times). It is obvious that chopping wood to micron-sized particles is most likely impractical. In this case, the energy costs of fuel preparation increases significantly. It is rational to burn wood biomass particles of 4e5 mm in size. In this case, their complete combustion in the furnaces of modern boilers is guaranteed and the costs of grinding are minimized. The error of thermocouples used to measure the temperature of the medium did not exceed 2K (according to the product certificate). A series of experiments (at least 15 experiments) have been conducted under identical conditions of fuel particles heating and feeding. Based on their results, random measurement errors of tign have been determined. The confidence interval for tign determining at a confidence level of 0.95 did not exceed 13%.
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Fig. 1. Experimental stand. 1 - high-speed video camera; 2 - particle sprayer; 3- high temperature oven; 4 - hollow ceramic cylinder; 5e electric heater; 6 - thermostat, 7 - a highspeed video camera mount, 8 - fuel particles.
It should be noted that in the experiments dry particles of all three types of biomass were used (they were dried before the experiments). In this connection, it can be noted that in real fuel supply systems of boiler plants, the temperatures is usually sufficient to remove moisture from the wood particle to a depth of 1e1.5 mm from its outer surface, even with high initial moisture content of the wood. Such heating depths, as shown in the results of studies [36], are sufficient for igniting a particle of several millimeters in size and for the subsequent steady burning.
3. Results and discussion Fig. 2 shows the footage of a typical videogram of the wood-coal mixture (based on coal and birch sawmill wastes) heating and ignition processes under conditions of high temperatures of the gaseous medium. It can be noted that in the initial period of time (the number of particles hitting the high-temperature chamber) the fuel mixture is a dispersed stream of relatively large wood particles (typical size up to 5 mm) and coal dust (with characteristic particle sizes of 40e60 mm). At the same time, the process of fuel ignition can be divided into two interrelated stages: at the first stage, the ignition of small-sized coal particles is initiated. The latter are a kind of “accelerator” of the thermochemical oxidation reaction of wood pyrolysis products. It can be noted that the period of time between the processes of ignition of
Fig. 2. Frames of a typical videogram of the charcoal mixture heating and ignition processes.
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coal and wood is insignificant (less than 0.001 s). Accordingly, we can conclude that the combustion of wood-coal mixtures can be carried out in a single combustion chamber using common burners with no need of grinding particles to micron sizes. Fig. 3 shows the dependences of the ignition delay times of the coal-wood mixtures under study on the ambient temperature. Such dependences are the main characteristics of the processes of ignition of solid [37], liquid [38], and various kinds of composite fuels [39]. The analysis of the tign (T) dependences shows that their form differs significantly from similar curves for solid [40] and liquid [41] fuels (the approximating experimental values correspond to linear dependencies rather than exponential ones). Such results may be due to the fact that when igniting a mixture of fine particles of coal and of almost 100 times larger in size wood particles, the latter start burning after the coal. However, the wood biomass particles significantly affect the conditions of heat exchange of coal particles with the ambient - they accumulate the energy, coming in the unit volume of the mixture. That is, in fact, for the ignition of a small-sized coal particle surrounded by significantly larger wood particles, more heat is needed compared to the ignition conditions of homogeneous coal dust of adequate size (curves 6 in Fig. 3). The results of the experiments performed are given below in a different format to better substantiate the answer to the question about the influence of woody biomass (wood species) on the conditions and characteristics of the charcoal mixture ignition. It is clearly seen in Fig. 4 that in a rather wide and practical range of the oxidizing environment temperature changes, the ignition delay times for charcoal mixtures do not exceed 0.65 s (Fig. 4). The tign values of subbituminous coal and wood (birch, pine, larch) mixtures differ by no more than 0.18 s (with minimum Tg ¼ 873K). With the increase of Tg (Fig. 4), the difference of tign decreases and with T ¼ 1273 K it does not exceed the confidence intervals for determining ignition delay times. It can be concluded, that the individual properties of wood do not have a significant effect on the conditions and characteristics of ignition of rather typical charcoal mixtures, as small particles of coal start burning first. These experimental data are most likely fundamental. Birch, pine and larch differ very significantly in their structure and properties [42] (Table 2). The experiments, however, show the steady ignition and burning of the charcoal mixture at high concentrations of each type of the biomass. This gives grounds to conclude that the influence of the type of wood on the characteristics and conditions of ignition of the composite coal and wood biomass based fuels is insignificant. In addition to the above, it should be noted that the three studied wood species used as the second component of the coal-wood mixtures significantly differ in the content of structurally bound moisture [16] (birch and pine differ the most). However, as experiments have shown, this moisture has practically no effect on the characteristics of the processes of ignition of the wood particles of these species and their subsequent burning. It can be concluded that water vapor released during the pyrolysis of birch, for example, does not almost inhibit the process of heating of the particle and the gas layer near its surface in which the combustion occurs (the interaction of the gaseous products of thermal decomposition of wood and the air heated to high temperatures). If we consider that birch is one of the most moisture-saturated wood species in its natural state, we can conclude that the conditions and characteristics of ignition of the vast majority of hardwood species under the heating conditions under consideration (typical for heat-and-power engineering of the countries) will correspond to those established by the results of the experiments. The analysis of the results of the studies performed also shows that the change in the concentration of one of the components of the mixture by 1.5e2 times leads to the change in the ignition delay times of no more than 0.3 s. In addition, the changes in tign in the organization of the combustion process do not have any significant effect on the stability of ignition and
Fig. 3. The dependence of the delay time of wood-coal mixtures ignition on the ambient temperature (a - birch/coal; b - pine/coal; c - larch/coal) at different coal concentrations: 1e0%, 2e60%, 3e70%, 4e80%, 5e90%, 6e100%.
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Fig. 4. The dependencies of the delay time of wood-coal mixtures ignition on the ambient temperature (1 - pine, 2 - birch, 3 - larch) for different concentrations of components in the coal/wood system: a) 90/10; b) 80/20; c) 70/30; d) 60/40.
the subsequent combustion of the mixture. Therefore, we can conclude that the mixing of the two components can be carried out not at the stage of preparation of the fuel mixture, but directly in the combustion space. The local heterogeneity of the mixture will not play a big role in the initiation of the combustion. Even a small concentration of fine coal particles is sufficient, as experiments have shown, to start the combustion of wood particles, even hardwood, at relatively low temperatures (873 K). With increasing T, the influence of the concentrations of the components is minimized, because the tign value becomes very small (less than 0.2 s at T ¼ 1273 K). The analysis of the results of the experiments performed gives grounds for several main conclusions. 1. The comparison of typical ignition delay times for mixtures of particles of fairly common coal and typical wood species with similar ignition times for water-coal [36] and organo-coal [15] fuels shows significant advantages of wood-coal mixtures over liquid composite fuels in terms of the burning process arrangement. The ignition delay times for wood-coal fuels under identical heating conditions are ten or more times less than the tign values of composite liquid fuels. 2. When modeling the conditions and characteristics of ignition of wood-coal mixtures in the furnaces of steam and hot water boilers there is no need to use complex mathematical models of heat and mass transfer and chemical reaction processes similar to those describing the ignition of water-carbon drops [14,36] and organ-coal [15] fuels. 3. It also becomes impractical to analyze the conditions and characteristics of wood particles ignition systematically and theoretically using complex kinetic schemes and models of a structurally inhomogeneous medium, similar, for example, to [16]. 4. The concentration of relatively large particles of wood in the mixture has almost no effect on the values of tign (the smallest particles of coal always start to burn first). 5. An important outcome of the conducted experimental studies is the conclusion that during the burning of wood-coal mixtures in the furnaces of steam and hot water boilers there is no need to radically change the technology of fuel preparation, transportation and combustion. When the delay times are less than 0.6 s, the ignition of the fuel mixtures occurs stably and steadily in the studied range of changes in the main significant factors, which is quite typical for the power system. Of course, a good mixture of coal and wood components is important. However, this process can be well organized due to the proper placement of the injectors for joint or separate spraying of the components of the fuel mixture.
4. Conclusions Based on the results of experimental studies, we can conclude that the conditions and characteristics of ignition of the mixture of subbituminous coal small-sized particles (with the characteristic size of 40e60 mm) and fairly large wood particles (5 mm in size) are determined by the process of coal ignition that initiates the burning of wood in a very short period of time (less than 0.1 s). The analysis of the results obtained also allows to conclude, that the combustion of coal-wood mixtures in the furnaces of steam and hot water boilers is possible with typical sizes of wood particles up to 5 mm. This conclusion justifies the use of sawmill waste in the power system as a fuel component with characteristic dimensions up to 5 mm. Please cite this article as: S.V. Syrodoy et al., Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.07.007
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Qian, Theoretical and numerical study on ignition behaviour of coal dust layers on a hot surface with corrected kinetic parameters, J. Hazard Mater. 368 (2019) 156e162. [34] S.V. Syrodoi, G.V. Kuznetsov, A.V. Zakharevich, V.V. Salomatov, Influence of the temperature dependence of the thermophysical properties of coalewater fuel on the conditions and characteristics of ignition, Solid Fuel Chem. 51 (2017) 160e165. [35] D.O. Glushkov, G.V. Kuznetsov, P.A. Strizhak, Ignition of a composite propellant by a hot particle under conditions of a nonideal thermal contact, Russ. J. Phys. Chem. B 9 (2015) 631e636. [36] G.V. Kuznetsov, V.V. Salomatov, S.V. Syrodoy, Numerical simulation of ignition of particles of a coalewater fuel, Combust. Explos. Shock Waves 51 (2015) 409e415. [37] G.V. Kuznetsov, S.A. Yankovskii, Condition and characteristics in ignition of composite fuels based on coal with the addition of wood, Therm. Eng. 66 (2019) 133e137. [38] V.L. Strakhov, A.N. Garashchenko, G.V. 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Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces S.V. Syrodoy, J.A. Kostoreva*, A.A. Kostoreva, L.I. Asadullina National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634050, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2019 Received in revised form 13 July 2019 Accepted 15 July 2019 Available online xxx
The results of experimental studies of the ignition processes of a large set of particles (from 100 to 500) of wood-coal composite fuels under conditions of high-temperature radiation-convective heating are presented. The ignition of wood-coal fuel mixture fuel particles was studied under the conditions of their free fall along the quill ceramic cylinder passage in a high-temperature oxidizing medium (air). Such a configuration provides the conditions that most closely match the heating conditions of actual furnaces of heat generating installations. The experiments have been carried out on the mixtures of particles of subbituminous coal (that is quite typical of coal-fired power industry) with the particles of woody biomass of three types: birch, pine and larch, which are widely spread in the territory of many states of the northern part of the Earth hemisphere. The recording of the heating and ignition processes of the fuel mixture particles was carried out with a high-speed Photron FASTCAM CA45 (500 fps) camera. The air temperature in the passage along which the particles moved varied in experiments from 873 K to 1273 K. The purpose of the experiments performed was the ignition delay time (tign). The main variable factors are the ambient temperature and the concentration of woody components. The consistent errors in determining the main characteristics of the process did not exceed 5%, the random ones did not exceed 13%. The size of coal particles in the experiments did not exceed 60 mm, and of the wood - 5 mm. It has been established that in such a fuel system one of the fuel components of the coal particle is “conducting” (or leading), and the second is “conducted” in the reaction of the thermochemical response of the fuel mixture with air oxygen. According to the results of experimental studies, the ignition delay times for the wood-coal particle mixtures have been stated. The increase in the concentration of smallsized coal particles in fuel mixtures from 0% to 100% leads to the decrease in the entire induction period (up to 20%) at the relatively cold temperatures. It was also found that with typical sizes of coal particles up to 60 mm, stable ignition of sufficiently large wood particles (up to 5 mm), present in the mixture, is provided. The conditions and characteristics of ignition of wood-coal mixtures hardly depend on the type of wood, which is the second component of the mixture. According to the results of the research conducted and following the requirements for optimizing the processes of fuel combustion, the use of mixed coal- and wood-based fuels in boilers of large and small power plants is justified. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Biomass Wood Experiment Ignition delay time
1. Introduction Over the past two decades, researchers in many countries have made great efforts to scientifically and technically substantiate the possibility to significantly increase the use of coal [1,2] (including low grade) [3,4] and biomass [5,6] to generate thermal and electric power. The main problems of using these two rather promising energy sources in high (steam boilers) and low (boilers) power engineering are: high concentrations of anthropogenic oxides and fly ash in coal combustion products [7,8] and relatively low heating capacity of typical
* Corresponding author. Lenin Avenue, 30, Housing 4, Audience 201, Tomsk, 634050, Russia. E-mail address: [email protected] (J.A. Kostoreva). https://doi.org/10.1016/j.joei.2019.07.007 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: S.V. Syrodoy et al., Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.07.007
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types of biomass compared to other energy sources [9,10]. In order to solve these problems, scientists in many countries are trying to justify not the use of homogeneous coal or biomass in power engineering (the most promising type of fuel is wood), but various composite fuels based on them [11,12]. For example, experimental [13,14] and theoretical [15] studies of the combustion of coal-water, organic-coal-coal [16] fuels, as well as mixtures of coal and wood particles [17,18] are intensively conducted. A rather large-scale positive effect from the burning of such composite fuels is the considerable reduction in anthropogenic emissions [17,18] (sulfur and nitrogen oxides, as well as fly ash), in conditions of moderate (in many practical cases) reduction of energy indicators of thermal power plants or small boiler plants [19,20]. From the technological point of view, the simplest are the technologies of burning crushed coal and dispersed wood in steam and hotwater furnaces [21,22]. But so far there have been few examples of power plants boilers operating on mixtures of coal and wood (no more than twenty [23]), although attempts to incinerate such mixtures have been made on hundreds of thermal power plants in many developed countries, for example [24e26]. The reason for this state of affairs is most likely the current-date lack of general theory (as well as the insufficient experimental data for practical use) of combustion and, in particular, of wood-coal mixtures ignition. It is important, however, that, the sawing and woodworking waste particles (their share reaches 15% in most cases) with typical sizes from 1 mm to 6 mm can be used as the second component of coalwood mixtures. Further grinding of such rather large particles (mainly sawdust chips) requires a large energy input. Therefore, one of the main problems arising from the analysis of the possibility of burning sawmill and woodworking waste in steam and water heaters in a coal mixture is to assess the conditions and the characteristics of the ignition processes of the wood biomass particles, which can be a hundred times larger in size than the coal dust formed after grinding in a kind of ball mill apparatus widely used in the power engineering. So, it is necessary to estimate the lag times of the ignition of large wood particles in relation to the coal ones. The aim of the work is to experimentally establish the basic laws of ignition of mixtures of coal and wood particles, including the dependences of the ignition delay time on the ambient temperature. In power systems it is possible to use the most different types of woody biomass, both softwood and hardwood. The most promising is the use of wood processing wastes, the price of which in many cases essentially comes down to loading and transportation costs. The burning of wood and, especially the hardwood grown specifically as an energy resource is also promising for a large group of Asian countries. It can be noted that the growth rate of the latter is several times higher than the similar characteristics of coniferous trees, which gives grounds for reasonable conclusions [27,28] about the feasibility of growing, for example, lindens, eucalyptus, poplar and other types of energy wood on special plantations for subsequent burning in boiler furnaces for different purposes. In this regard, the actual task is to compare the ignition characteristics of mixtures of various wood species with coal and evaluate the ignition efficiency of such fuels based on hardwood wood. 2. Studied materials and experimental methods Experimental studies of the conditions and characteristics of subbituminous coal (rank of coal - D) fuel mixtures and three types of wood ignition processes (Table 1). Subbituminous coal is typical and is widely used in thermal power plants in many countries. As the second component of the mixtures, three types of wood growing in Western Siberia (birch, pine, larch) have been studied. These types of wood are widely spread in Europe, Asia and America [29] and are the typical representatives of the “energy biomass”. The main characteristics of the fuel components under the study are shown in Table 2. Technical characteristics of subbituminous coal are given in Table 3. The composition and characteristics of coal ash are given in Table 4. It is possible to determine the conditions and characteristics of the ignition of mixtures of coal and wood particles using several experimental methods differing in heating conditions, the number of particles and their position relative to the heating source. A portion of crushed materials can be located on the surface of a plate heated to high temperatures [30]. A thin layer of fuel particles is heated by a gas flow [31]. A metal particle heated to high temperatures falls on the surface of a layer of crushed fuel [32]. A portion of fuel is entered into the channel of a horizontally located cylinder heated to high temperatures [33]. In all these cases, the heating conditions are quite different from the conditions of the combustion chambers of steam and hot water boilers, in which particles of coal and wood soar in a stream of gases, which are a mixture of combustion products of the fuel and air heated to high temperatures. Experimental data on the ignition characteristics of particles of coal and wood that are hovering in the medium heated to high temperatures has not been published yet.
Table 1 Wood ecoal fuel compositions. Composition number
Coal
Wood
Rank
Percentage, %
Type
Percentage, %
N1
D
Birch
N2
D
N3
D
60 70 80 90 60 70 80 90 60 70 80 90
40 30 20 10 40 30 20 10 40 30 20 10
Larch
Pine
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Table 2 Component
Ср J/(kg $ K)
l W/(m $ K)
r kg/m3
Vdaf %
Cг %
Qт J/kg
Long flame coal (D) Birch Larch Pine
1150 1250 e 2300
0,116 0,15 0,13 0,14
1253 610 370 500
46 85 85 85
76 48,6 50,1 49,6
33,5$106 15$106 15,5$106 15,5$106
Where: Ср e specific heat capacity [J/(kg $ K)], l e heat conductivity [W/(m $ K)], r e density [kg/m3], Vdaf-volatile yield [%], Cг e carbon content [%], Qт e caloric content [J/kg].
Table 3 Technical characteristics of coal. As-fired fuel basis. Composition, % W rt
At
11,5
15,9
Srp
Sra
0,4
0,4
Cr
Hr
56,4
4,0
Nr
Or
1,9
9,9
Caloric content Q ri МДЖ/кг
Ash content Ad , %
Moisture content W, %
Volatile content V daf , %
21,90
18,0
4,5
40,5
Table 4 Composition and characteristics of subbituminous coal ash. Ash fusibility temperature, С
True liquid state of slag
Normal liquid slag removal onset
tA
tB
tC
t0 С
VA (t0 ,)Па∙с
t С
1160
1310
1440
1400
100
1600
20 Solid base deposits susceptibility no
Elemental composition per sulphate-free mass, % Al2O3 TiO2 Fe2O3 CaO SiO2
MgO
K2O
Na2O
Onset slagging temperature t
60,0
2,7
3,0
2,0
1000
20,6
0,9
6,8
4,0
VA ðtÞ Па с
A methodology for conducting experimental studies of the coal and wood particles ignition processes with reproduction of the conditions that most closely correspond to the conditions of steam and water-heating boilers have been developed. Experimental studies have been conducted on the stand, the scheme of which is shown in Fig. 1. The fuel particles were introduced into the channel of a hollow, vertically arranged ceramic cylinder, on the external surface of which an electric heater was attached. This configuration of the heating area has allowed to study the ignition processes of the aggregate (150e200 pcs.) of small-sized fuel particles under the conditions of their free fall in a high-temperature oxidizing environment and provided minimal errors (there are no fuel particle holders such as in Ref. [34]) simulating the real furnace conditions of the boiler unit. The particles of the charcoal mixture were sprayed using a pneumatic device into the channel of a ceramic cylinder heated to high temperatures (the maximum possible temperature of 1473K). The registration of the heat and mass transfer and fuel particles ignition processes was carried out with a high-speed Photron FASTCAM CA4 5 video camera (video speed up to 20,000 f/s). The temperature of the air inside the cylinder (Tg) in the experiments varied in a fairly wide range (from 870K to 1270K), corresponding to the ambient temperatures in the combustion chambers of the boiler units. The Tg measurements were performed with a thermoelectric converter (chromel-aluminum) with a junction diameter of 0.5 mm (with the inertia of no more than 3 s). The experiments were carried out only under the condition of achieving stationary Tg values in the entire channel along the height after complete heating of the air in the cylinder channel (Fig. 1). The time between the onset of heat exposure (the moment particles hit the combustion chamber) and the ignition (the appearance of a flame) was considered as the ignition delay (tign). The ignition delay time was determined after analyzing the videogram frames, as the difference between the moments of time when the mixture was introduced into the channel with air heated to high temperatures and the moment when the flame appeared near the surface of at least one particle. The consistent error in determining the main measured parameters (Тg, tign) was no more than 5%. The weight of the mixture was 1e2 g in a channel volume of 0.157 m3. This value corresponds to the real concentration of fuel particles achieved in wood-coal boiler units. The size of coal particles ranged from 40 to 60 mm. The characteristic dimensions of the second component of the mixture ranged from 3 to 5 mm. It should be noted that the most preferable (according to the estimates [35]) is the technology of burning wood-coal mixtures in the furnaces of steam or hot water boilers designed for burning coal. In this case, there is no need for a radical change in the systems of supplying fuel to the combustion chamber and of the fuel burning. However, if, during the preparation of coal, they are crushed in conventional fuel preparation systems to typical sizes of 40e60 mm (and even lower), then the wood can flow to the station in a different state (even if sawmill or woodworking waste is used). Therefore, one of the objectives of the experimental studies was to assess the conditions and characteristics of ignition of wood-coal mixtures with significantly different characteristic sizes of wood and coal particles (up to 100 times). It is obvious that chopping wood to micron-sized particles is most likely impractical. In this case, the energy costs of fuel preparation increases significantly. It is rational to burn wood biomass particles of 4e5 mm in size. In this case, their complete combustion in the furnaces of modern boilers is guaranteed and the costs of grinding are minimized. The error of thermocouples used to measure the temperature of the medium did not exceed 2K (according to the product certificate). A series of experiments (at least 15 experiments) have been conducted under identical conditions of fuel particles heating and feeding. Based on their results, random measurement errors of tign have been determined. The confidence interval for tign determining at a confidence level of 0.95 did not exceed 13%.
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Fig. 1. Experimental stand. 1 - high-speed video camera; 2 - particle sprayer; 3- high temperature oven; 4 - hollow ceramic cylinder; 5e electric heater; 6 - thermostat, 7 - a highspeed video camera mount, 8 - fuel particles.
It should be noted that in the experiments dry particles of all three types of biomass were used (they were dried before the experiments). In this connection, it can be noted that in real fuel supply systems of boiler plants, the temperatures is usually sufficient to remove moisture from the wood particle to a depth of 1e1.5 mm from its outer surface, even with high initial moisture content of the wood. Such heating depths, as shown in the results of studies [36], are sufficient for igniting a particle of several millimeters in size and for the subsequent steady burning.
3. Results and discussion Fig. 2 shows the footage of a typical videogram of the wood-coal mixture (based on coal and birch sawmill wastes) heating and ignition processes under conditions of high temperatures of the gaseous medium. It can be noted that in the initial period of time (the number of particles hitting the high-temperature chamber) the fuel mixture is a dispersed stream of relatively large wood particles (typical size up to 5 mm) and coal dust (with characteristic particle sizes of 40e60 mm). At the same time, the process of fuel ignition can be divided into two interrelated stages: at the first stage, the ignition of small-sized coal particles is initiated. The latter are a kind of “accelerator” of the thermochemical oxidation reaction of wood pyrolysis products. It can be noted that the period of time between the processes of ignition of
Fig. 2. Frames of a typical videogram of the charcoal mixture heating and ignition processes.
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coal and wood is insignificant (less than 0.001 s). Accordingly, we can conclude that the combustion of wood-coal mixtures can be carried out in a single combustion chamber using common burners with no need of grinding particles to micron sizes. Fig. 3 shows the dependences of the ignition delay times of the coal-wood mixtures under study on the ambient temperature. Such dependences are the main characteristics of the processes of ignition of solid [37], liquid [38], and various kinds of composite fuels [39]. The analysis of the tign (T) dependences shows that their form differs significantly from similar curves for solid [40] and liquid [41] fuels (the approximating experimental values correspond to linear dependencies rather than exponential ones). Such results may be due to the fact that when igniting a mixture of fine particles of coal and of almost 100 times larger in size wood particles, the latter start burning after the coal. However, the wood biomass particles significantly affect the conditions of heat exchange of coal particles with the ambient - they accumulate the energy, coming in the unit volume of the mixture. That is, in fact, for the ignition of a small-sized coal particle surrounded by significantly larger wood particles, more heat is needed compared to the ignition conditions of homogeneous coal dust of adequate size (curves 6 in Fig. 3). The results of the experiments performed are given below in a different format to better substantiate the answer to the question about the influence of woody biomass (wood species) on the conditions and characteristics of the charcoal mixture ignition. It is clearly seen in Fig. 4 that in a rather wide and practical range of the oxidizing environment temperature changes, the ignition delay times for charcoal mixtures do not exceed 0.65 s (Fig. 4). The tign values of subbituminous coal and wood (birch, pine, larch) mixtures differ by no more than 0.18 s (with minimum Tg ¼ 873K). With the increase of Tg (Fig. 4), the difference of tign decreases and with T ¼ 1273 K it does not exceed the confidence intervals for determining ignition delay times. It can be concluded, that the individual properties of wood do not have a significant effect on the conditions and characteristics of ignition of rather typical charcoal mixtures, as small particles of coal start burning first. These experimental data are most likely fundamental. Birch, pine and larch differ very significantly in their structure and properties [42] (Table 2). The experiments, however, show the steady ignition and burning of the charcoal mixture at high concentrations of each type of the biomass. This gives grounds to conclude that the influence of the type of wood on the characteristics and conditions of ignition of the composite coal and wood biomass based fuels is insignificant. In addition to the above, it should be noted that the three studied wood species used as the second component of the coal-wood mixtures significantly differ in the content of structurally bound moisture [16] (birch and pine differ the most). However, as experiments have shown, this moisture has practically no effect on the characteristics of the processes of ignition of the wood particles of these species and their subsequent burning. It can be concluded that water vapor released during the pyrolysis of birch, for example, does not almost inhibit the process of heating of the particle and the gas layer near its surface in which the combustion occurs (the interaction of the gaseous products of thermal decomposition of wood and the air heated to high temperatures). If we consider that birch is one of the most moisture-saturated wood species in its natural state, we can conclude that the conditions and characteristics of ignition of the vast majority of hardwood species under the heating conditions under consideration (typical for heat-and-power engineering of the countries) will correspond to those established by the results of the experiments. The analysis of the results of the studies performed also shows that the change in the concentration of one of the components of the mixture by 1.5e2 times leads to the change in the ignition delay times of no more than 0.3 s. In addition, the changes in tign in the organization of the combustion process do not have any significant effect on the stability of ignition and
Fig. 3. The dependence of the delay time of wood-coal mixtures ignition on the ambient temperature (a - birch/coal; b - pine/coal; c - larch/coal) at different coal concentrations: 1e0%, 2e60%, 3e70%, 4e80%, 5e90%, 6e100%.
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Fig. 4. The dependencies of the delay time of wood-coal mixtures ignition on the ambient temperature (1 - pine, 2 - birch, 3 - larch) for different concentrations of components in the coal/wood system: a) 90/10; b) 80/20; c) 70/30; d) 60/40.
the subsequent combustion of the mixture. Therefore, we can conclude that the mixing of the two components can be carried out not at the stage of preparation of the fuel mixture, but directly in the combustion space. The local heterogeneity of the mixture will not play a big role in the initiation of the combustion. Even a small concentration of fine coal particles is sufficient, as experiments have shown, to start the combustion of wood particles, even hardwood, at relatively low temperatures (873 K). With increasing T, the influence of the concentrations of the components is minimized, because the tign value becomes very small (less than 0.2 s at T ¼ 1273 K). The analysis of the results of the experiments performed gives grounds for several main conclusions. 1. The comparison of typical ignition delay times for mixtures of particles of fairly common coal and typical wood species with similar ignition times for water-coal [36] and organo-coal [15] fuels shows significant advantages of wood-coal mixtures over liquid composite fuels in terms of the burning process arrangement. The ignition delay times for wood-coal fuels under identical heating conditions are ten or more times less than the tign values of composite liquid fuels. 2. When modeling the conditions and characteristics of ignition of wood-coal mixtures in the furnaces of steam and hot water boilers there is no need to use complex mathematical models of heat and mass transfer and chemical reaction processes similar to those describing the ignition of water-carbon drops [14,36] and organ-coal [15] fuels. 3. It also becomes impractical to analyze the conditions and characteristics of wood particles ignition systematically and theoretically using complex kinetic schemes and models of a structurally inhomogeneous medium, similar, for example, to [16]. 4. The concentration of relatively large particles of wood in the mixture has almost no effect on the values of tign (the smallest particles of coal always start to burn first). 5. An important outcome of the conducted experimental studies is the conclusion that during the burning of wood-coal mixtures in the furnaces of steam and hot water boilers there is no need to radically change the technology of fuel preparation, transportation and combustion. When the delay times are less than 0.6 s, the ignition of the fuel mixtures occurs stably and steadily in the studied range of changes in the main significant factors, which is quite typical for the power system. Of course, a good mixture of coal and wood components is important. However, this process can be well organized due to the proper placement of the injectors for joint or separate spraying of the components of the fuel mixture.
4. Conclusions Based on the results of experimental studies, we can conclude that the conditions and characteristics of ignition of the mixture of subbituminous coal small-sized particles (with the characteristic size of 40e60 mm) and fairly large wood particles (5 mm in size) are determined by the process of coal ignition that initiates the burning of wood in a very short period of time (less than 0.1 s). The analysis of the results obtained also allows to conclude, that the combustion of coal-wood mixtures in the furnaces of steam and hot water boilers is possible with typical sizes of wood particles up to 5 mm. This conclusion justifies the use of sawmill waste in the power system as a fuel component with characteristic dimensions up to 5 mm. Please cite this article as: S.V. Syrodoy et al., Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.07.007
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Funding The study has been carried out by the grant from the Russian Science Foundation (project No. 18-79-10015). References [1] Jun Deng, Shuai-Jing Ren, Yang Xiao, Qing-Wei Li, Chi-Min Shu, Thermal properties of coals with different metamorphic levels in air atmosphere, Appl. Therm. Eng. 143 (2018) 542e549. [2] Ashley Arigoni, Alexandra Newman, Cameron Turner, Casey Kaptur, Optimizing global thermal coal shipments, Omega 72 (2017) 118e127. [3] Pratima Meshram, B.K. Purohit, M.K. Sinha, S.K. Sahu, B.D. Pandey, Demineralization of low-grade coal e a review, Renew. Sustain. Energy Rev. 41 (2015) 745e761. [4] Michalis Agraniotis, Christian Bergins, Malgorzata Stein-Cichoszewska, Emmanouil Kakaras, High-efficiency pulverized coal power generation using low-rank coals, in: Low-Rank Coals for Power Generation, Fuel and Chemical Production, 2017, pp. 95e124. 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Please cite this article as: S.V. Syrodoy et al., Ignition of wood and coal particle mixtures in conditions of steam and water boiler furnaces, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.07.007