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Mechanical activation of micronized coal: Prospects for new combustion applications
Applied Thermal Engineering xxx (2014) 1e8
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Mechanical activation of micronized coal: Prospects for new combustion applications A.P. Burdukov a, b, V.I. Popov a, b, M.Yu. Chernetskiy b, *, A.A. Dekterev b, K. Hanjali c a, c a
Novosibirsk State University, Novosibirsk, Russia Kutateladze Institute of Thermophysics, SB RAS, Novosibirsk 630090, Russia c Delft University of Technology, Delft, The Netherlands b
h i g h l i g h t s Microgrinding coal in a special high-impact mill reduces the coal activation threshold. Mechanical activation makes coal easier to ignite and to burn in a bright compact flame like heavy oil. Activated coal can burn easier in self-igniting and self-sustained regime. Potential for replacing liquid fuel in a variety of industrial and power application.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2013 Accepted 31 December 2013 Available online xxx
The paper presents experimental studies of the effects of enhanced mechanical activation of microground coal on its inflammability and combustion efficiency. Experimental test of reactivity of pulverized coal milled in a new rotating impact disintegrator developed at the IT SB RAS (Novosibirsk, Russia) showed that the coal activation threshold was reduced by a factor of two or more, making it easier to ignite and burn more efficiently. The preliminary experiment in a pilot-scale 1 MW rig, followed by more detailed experimental and numerical investigations in a cylindrical swirl combustor of 5 MW thermal power, confirmed the earlier visual observations and measurements that the mechanically disintegrated micro-ground coal behaves much like heavy oil. This offers prospects for designing new coal-dust burner that will ensure ignition at much lower temperatures, more efficient and more stable combustion and lower NOx emission compared with coal of the same granulation (about 40 mm) obtained in conventional mills. The research offers new opportunities for replacing liquid fuels by coal in various combustion applications for heating and electricity production from coal as well as new prospects for efficient coal gasification. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Mechanical activation Micronized coal Pulverized coal combustion
1. Introduction Coal is expected to remain the mainstay of electricity generation and heating in many countries for the foreseeable future. However, its exploitation, processing and utilization will unavoidably need to adjust to meet the ever more stringent regulations on pollutant emission. The environmental constraints, together with the imminent increase in the fuel and energy prices on one hand and a decline of the quality of coal reserves on the other, call for search for new coal pre-processing and conversion methods and technologies which should ensure higher combustion efficiency, operational flexibility and easier control, all leading to diminishing adverse environmental impact. * Corresponding author. Tel.: þ7 383 335 66 84. E-mail address: [email protected] (M.Yu. Chernetskiy).
Among novel options in clean coal technologies, the use of microground (micronized) coal has been considered as one of the prospective routes to overcome various shortcomings in the conventional pulverized coal combustion that prevail worldwide in large coal-fired power plants and various industrial and heating installations. The past two decades have witnessed a number of projects aimed at examining various issues of combustion of micronized coal and its possible use for replacement of oil for ignition and combustion support of the common dust coal, or its use in the process of reburning. Briceland et al. [1] at the Institute of Gas Technology were among the first to indicate at potential advantages of combusting micro-ground coal in large installations. Freihaut et al. [2,3] investigated combustion properties of micronized coal and its use for intense combustion applications. Rosenberger et al. [4] discussed technical and environmental issues and implications of micronized coal to replace oil for the start-up and
1359-4311/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.12.081
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operation of boiler N 6 TPS Tilinois Power Havana (USA) using a special system of ultrafine grinding mill. More recent investigations focused on Micronized Coal Reburning (MCR) option as a means to reduce carbon in fly ash and NOx emission (e.g. Milliken Clean Coal Technology Demonstration Projects DOE/NETL-2000/1122, DOE/ NETL-2001/1148, EPA/625/R-96/001 [5e7]. Still more recently, Li et al. [8] reported on optimization of coal reburning in a 1 MW tangentially fired furnace. The benefits from coal micronization on combustion are generally well known and manifest among others in compact flames with faster ignition and more complete char burnout requiring less excess air. The origin of theses benefits is commonly attributed to the increase in the surface-to-volume ratio due to a decrease in the particle size, but also due to a more extended external surface of irregularly shaped particles and in some cases increased porosity, all depending on the type of mills used. Less is known about the change of the coal kinetic properties, especially the enhancement of coal reactivity due to mechanical activation, which greatly depends on the method of coal grinding. Welham and Chapman [9], Welhem et al. [10], investigated this issue, but focused on detecting and identifying the effects of long milling in a tumbling ball-mill. They found that a slow milling leads to an increase in coal reactivity and concluded that the effect cannot be attributed solely to the increase in the particle surface area. Similar studies were reported earlier by Khrenkova [11]. More recently, using a thermo-gravimetric analyzer, Yusupov et al. [12] reported a significant increase of reactivity of lignite and brown coal during milling in a centrifugal planetary mill. Burdukov et al. [13e16] pursued a design of a new mill referred here as disintegrator, featured by high mechanical impact but more adequate for industrial application than planetary mills. A thermo-gravimetric analysis of coal samples before and after milling confirmed that the impact coal crushing in the disintegrator enhances coal reactivity to the level comparable to that achieved in a laboratory-scale planetary mill considered by Yusupov et al. [12]. The enhancement of the reactivity of coal, desirably to the level of gas or fuel oil can improve the combustion efficiency and stability in various conventional installations as well as open prospects for innovative application at various scales. For example, the ignition of coal-fired boilers is often assisted with a gas or fuel-oil flame torch. The use of activated coal for ignition has already been proved possible when using the appropriate mechanical equipment. This trend is currently being developed at the IT SB RAS (Burdukov et al. [14]). This paper presents some results of investigation of the influence of the grinding method and type of mills on the enhancement of coal mechanical activation, and of the effects on the ignition and combustion of micronized coal. Presented are the results of experimental testing of combustion of three types of brown coal and two types of black coal, micro-grinding in two different mills.
The combustion of microground coal was tested in two pilot-scale rigs of thermal capacity of 1 MW and 5 MW. Next to confirming the successful autothermal (self-sustained) operation, the results provide useful guidelines for designing new concept of dust coal combustors. In addition, the experiments provide a database for testing of mathematical models for the computer simulation of the process (currently in progress), which could be used for design and optimization of new combustion and gasification devices using mechanically activated microground coal. 2. Coal preparation: the mills and mechanical activation In order to identify the particular effect of coal grinding methods on its kinetic characteristics we studied the changes in physical and chemical properties of several brown and black coals. mechanically dispersed in different mills of higher power density. Among a number of different types of mills with higher intensity of mechanical action, which can be found in the literature, the work here reported compares the results obtained with coals micronized in a vibratory centrifugal mill (VTSM-7), Fig. 1a, and in a disintegrator type developed at the ITP SBRAS (hereafter denoted as DIS), Fig. 1b, (Burdukov et al. [15]). Both mills produce microground coal of very similar granulations (for particle size distribution, see Sections 3 and 4 below), but because of different mechanical actions of the two mills, they have been shown by the thermogravimetric analysis to have different activation thresholds. It is noted that in both cases the initial (basic, parent) coal was that obtained from the conventional mills in the power plant Belowo. The intense mechanical impact in high-energy devices such as disintegrating mill shown in Fig. 1b causes not only a rapid decrease of the particle size (Table 2, Fig. 2) and increase in their surface but also enhances the reactivity of coal in the processes of subsequent thermooxidative destruction in the air under non-isothermal conditions. While the size of particles plays an important role, the method of grinding is crucial as coal dust of almost equal granulation may show no notable change in its reactivity as in the case of the vibrocentrifugal mill, Fig. 1a, which shows more or less the same reactivity as the initial coal ground in a conventional industrial mill. As noted in the Introduction, some other types of mills can also increase the coal reactivity, for example a planetary mill, which we considered earlier. However, while useful for laboratory exploration, the complex design and higher energy consumption makes this type of mill inconvenient for real-scale applications (Burdukov et al. [13]). The changes in physico-chemical characteristics of coal treated in the mills with high intensity of mechanical action were analyzed by testing the coal kinetic properties by thermogravimetric analysis of the coal sample in air flow (Burdukov et al. [15,16]). The observed effects of changing the activation energy for mechanical activation of coal can be summarised as follows. First, the influence of the type
Fig. 1. Views of the two types of coal mill. (a) e Vibrocentrifugal mill VCM), capacity up to 100 kg per hour; (b) e IT SB RAS disintegration mill (DIS), capacity up to 150 kg per hour.
Please cite this article in press as: A.P. Burdukov, et al., Mechanical activation of micronized coal: Prospects for new combustion applications, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2013.12.081
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Table 2 Particles size classes in [%] of Kansko-Achinsk brown coal grade B-2 used in the experimental rig 1 MW, for the parent coal (basic sample, from Belowo power plant) and after microgrinding in the vibrocentrifugal mill (VCM) and in the disintegrator mill (DiS). Grinding method
Basic sample Vibrocentrifugal mill Disintegrator mill
Fig. 2. Sieve residue Res [%] (lines) of brown coal of B-2 grade from Kansk-Achinsk deposit. 1 e After disintegration mill (Dis); 2 e initial (basic) sample from a common power-plant mill. The size distribution and sieve residual after vibrocenrtifugal mill (VCM) is similar to 1.
of mills on the reactivity of coal, depending on the availability of the rate of oxidation of activated carbon after the mechanical treatment is the key factor. Second, the important prerequisite for utilizing the benefits of mechanical activation is to avoid the decay of reactivity enhancement due to reaction with atmospheric oxygen, which requires that the microgrinding should be performed in situ simultaneously with feeding coal into the burner and its combustion. These factors must be considered when designing devices for gasification and combustion of fine coal, or using activated micronized coal in torches for ignition of common pulverized coal in boilers, thus without using liquid fuel. 3. The combustion experiment in the 1 MW pilot-scale rig One of the main objectives of the experimental studies was the observation of ignition and combustion of coal of varying degrees of metamorphism, as well as achieving the autothermal operation in the pilot-scale rigs of capacity of 1 MW and 5 MW by burning coal prepared in the disintegrator mill type developed at IT SB RAS. In the preliminary experiment carried out in a smaller rig of 1 MW we considered two coals: brown coal of B-2 grade from Kansk-Achinsk deposit and black coal from Tihonsky deposit. Their chemical compositions are given in Table 1. An important prerequisite for a fair comparison of the reactivity of different coals tested is to eliminate possible effect of different particle sizes. Table 2 shows the measured classes of the average particle dimensions of brown coal grade B2 from Kansk-Achinsk deposit after grinding in the mills of both types. It is clear that the average size of particles is almost the same, about w40e42 mm. Thus, the reacting surface in both cases is practically the same and therefore the inflammation temperature threshold and combustion speed of both flames is expected to be identical if no change in coal reactivity occurs during milling. The sieve residuals distributions of the basic coal sample after the common power plant mill, and from the two micronizing mills, vibrocentrifugal and disintegrator mill for the same coal, are illustrated in Fig. 2. It is noted that for the coal microgrinded in the vibrocentrifugal and disintegrator mills, both the sieve residual (and also the size distribution, not
Size of particles, mm <2
2e20
20e50
50e100
>100
2.31 6.16
7.6 28.53 45.94
9.2 27.41 39.6
9.3 21.84 7.48
73.9 19.91 0.82
shown in Fig. 2) have very similar distributions as for the disintegrator [15]. Of course, the same or similar particle size distribution does not a guarantee the same surface-to-volume ratio as the milling in different mills may also change the effective surface due to possible deformation of particles shape and especially their porosity. In order to eliminate the particle surface-to-volume effect, the coal samples from the two mils have been subjected to a porosity analysis using the multipoint BET, BJH and deBohr’s methods [16]. These results showed that an increase in the surface-to-volume area due to high impact coal crushing in disintegrator was less than 15%, thus not significant. However, the thermogravimetric analysis showed that the activation energy of coal after disintegrator is almost twice lower than of coal microground in the virbocentrifugal mill, whish essentially shows no difference in reactivity from the parent coal sample [17]. Fig. 3 shows the activation energy in function of the degree of decomposition for the brown Kansk-Achinsk coal as obtained from the thermogravimetric analysis. The striking difference between the initial sample and coal from the disintegrator shows undoubtedly a significant reduction of the activation energy achieved in the disintegrator, especially at the first stage of decomposition, i.e. for the low values of the transformation rate.
Table 1 Chemical composition in [%] of coals used in the 1 MW rig experiment. Brown coal of B-2 grade from Kansk-Achinsk deposit; black coal from Tihonsky deposit. Coal
Wr
Ar
Vdaf
Cr
Hr
Sr
Nr
Qr [MJ/kg]
Brown (K-A B2) Black (Tihonsky)
39.0 17.8
7.3 18.3
48 30.2
37.6 47.3
2.6 3.05
0.4 0.25
0.4 1.3
13.03 16.9
Fig. 3. Activation energy of the basic sample (a) and of the mechanically-activated coal microground in disintegrator mill (b) for brown coal grade B2 from Kansk-Aschinsk deposit. (From thermogravimetric analysis).
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A view of the 1 MW combustion stand is shown in Fig. 4a and a schematic of the combustor (swirl burner, its extension e afterburner, and the furnace) with the microgrinder and coal feeding system in Fig. 4b. The experimental rig was constructed to embody the entire technological cycle of the combustion of dust-coal and next to the combustor and coal feeder shown in Fig. 4b, it includes also of a vapour generator for burner support, and a system of flue gas cleaning that utilizes an ash hydro-scrubber. The stand is equipped with photometric, multicomponent gas analyzing and platinum-rhodium thermocouples of type B of wire thickness of 0.3 mm with unshielded head, allowing one to perform real-time diagnostics and monitoring of the combustion process. The coalair mixture is ignited with a standard ignition-spark methane device or a plasmatron torch with a power up to 15 kW, later replaced by a simple propane-butane gas torch. For the activated coal of most types microground in the disintegrator the ignition devised was operated only during the first 1e2 min. Then the ignition device is switched off, and combustion continues in the self-sustained (autothermal) mode. The time within which particles get into the flame after leaving the mill is only about 1 s. This immediate use of activated coal prevents any possible reactivity decay that might occur due to atmospheric oxidation or possible relaxation of the activated molecular structures if coal was left for a longer time before being burnt. For non-activated coal from the vibrocentrifugal mill the ignition had to last longer (typically 5 min or more) to support combustion until the refractory walls inside the afterburner became sufficiently heated to ensure self-ignition. However, a notable difference between the combustion features of activated and non-activated coals is in the visual outlook of the flame and especially in the extent of their flame contours, which are clearly visible through the inspection window. While the flame of the activated coal is short and compact with a distinct front close to the afterburner exit, followed by a traceless bright zone, the flame of the non-activated coal is long with visible traces of burning particles and not very sharp flame front. Both contours are indicated in Fig. 4b. Fig. 5 shows the distribution of the temperatures along the centreline of the afterburner when using Kansk-Achinsk coal microground in the two mills, VCM and DIS. In both cases the temperature increases almost linearly, but there is a significant difference in the temperature values and their gradient along the streamwise direction. The activated DSM coal burns faster reaching about 1000 C at the location t8. A sharp flame-front contour appears already at the afterburner exit. In contrast, the nonactivated coal from the VCM burns slower reaching at location t8 only about 850 C, and continues to burn well into the furnace, as indicated by
Fig. 5. Temperature along the afterburner (Fig. 4b) measured by thermocouples t6, t7 and t8 for Kansk-Achinsk coal in combustion of non-activated coal from the vibrocentrifugal (VCM) mill (empty symbols) and of mechanically-activated microground coal from the disintegrating mill (DIS) (full symbols).
its contour in Fig. 4b. It is also noted that the ignition of the DIS coal lasted only about 1 min, whereas for ensuring a continuous combustion of the VCM coal the ignition torch was switched on for about 5 min. Fig. 6 shows the time evolution of the temperatures at locations t1, t6 and t8 for two activated coals milled in the disintegrator: brown coal grade B-2 from Kansk-Achinsk deposit, and black coal from Thonsky deposit. Both types of coal show similar performances: a stable ignition and autothermal combustion is maintained after turning off the ignition torch, as illustrated by the temperatures measured by thermocouples t1, t6 and t8. It is recalled that thermocouples 6 and 8 were located at the beginning and at the end of the afterburner and thermocouple t1 immediately after the ignition torch (2). As indicated by the peaks of the t1 thermocouple reading, a somewhat longer ignition period was required for black coal (about 3 min), presumably because of a larger content of ash and a smaller content of volatiles (see Table 1). It is noted in passing that a higher volatile content, as well as their rapid burnout at the same level of excess air, will reduce the concentration of NOx. In contrast, the full autothermal mode (without a support of the ignition torch) could not be achieved with any of the two coals tested if milled in the vibrocentrifugal mill.
Fig. 4. A view of the 1 MW combustion rig (left) and a sketch of the main rig components. 1 e disintergrator microrginder, 2 e plasmatron (igniter), 3 e vortex burner, 4 e burner extension (afterburner), 5 e furnace, 6 e view window, 7 e flue gas exhaust, t6-t8 thermocouples (t6 at L ¼ 150 mm, t7 at L ¼ 510 mm; t8 at L ¼ 850 mm; t1 near the ignition torch). Visually observed flame contours in the furnace are denoted by dashed lines: grey thin line e non-activated coal; black thicker line e mechanically activated coal.
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Fig. 6. Time evolution of temperatures measured by thermocouples t1 (short dashed line), t6 (long dashed line), and t8 (full line) during ignition and the autothermal regime for activated coals. (a) e Kansk-Achinsk brown coal, B-2 grade; (b) e Tihonsky black coal.
4. The combustion experiment in a 5 MW semi-industrial rig A further exploration of the effects of coal activation on the combustion was subsequently carried out in a larger, semiindustrial-scale combustion rig of maximum power of 5 MW, Fig. 7. This rig was designed for the study the possibility of using mechanically-activated coal in industrial boilers and to serve as the first step in the investigation of different real-scale factors on the ignition and combustion stability. The combustor consists of two stages. The first stage is a cylindrical swirl burner of the interior diameter of 315 mm and a length of 1515 mm, with a tangential
feeding of microground coal premixed with the primary air through a vaneless spiral. The burner is followed by a coaxial cylindrical combustion chamber (furnace) of 1000 mm internal diameter and length of 2820 mm, equipped also with a vaneless spiral that provides a tangential entry of secondary air that reinforces the swirling motion, but could also be used for feeding additional coal of microor larger granulation. The burner spiral has an aperture for a gas torch (propane-butane from a bottle) used to ignite the coal-air mixture. In fact the burner is the main rig component that serves for the thermochemical preparation, ignition and self-sustained combustion of microground coal, which should then ensure the
Fig. 7. Experimental rig of 5 MW power. (a) e Schematic of the combustor; T1 to T8 denote thermocouples and G5-G7 the gas sampling apertures. (b) A view at the complete rig with an inset (top centre) showing the burner in autothermal operation as confirmed by a bright flame seen through view windows along the whole length.
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Table 3 The chemical composition of coals used in the 5 MW experiment, [%]. Wr
Ar
Vdaf
Cr
Hr
Sr
Nr
Qr [NJ/kg]
Long-flame Kuznetsk coal 20.0 12.43 42 53 3.8 0.27 1.7 20.5 Brown coal form 20.0 20.0 48.0 43.4 3.4 0.2 0.8 16.4 Anadyr deposit Black coal from 7.0 38.1 30.0 43.4 2.9 0.8 0.8 16.8 Ekibastuz deposit
continued combustion in the furnace without or with addition of non-activated dust coal that can be supplied to the furnace in realscale installations, as discussed briefly in Section 5. The latter option was not investigated in the present experiment, but nevertheless the secondary air was provided and its flow rate adjusted to ensure the complete burning of coal and its combustion products in the furnace. The combustion rig was equipped with an adjustable coal feed system with a disintegrator mill of a much larger capacity (1500 kg/h) than used for 1 MW rig, the primary and secondary air supplies, a dry-cyclone slag removal and a system for ash recovery. It is also equipped with a pressure controller in the combustion chamber. The combustion process was monitored by a set of Pt-Rho B-type thermocouples (0.3 mm wire thickness, placed in ceramic sheath with an open head) for measuring the temperature of the combusting products, and with a gas sampling probes connected to a multicomponent gas analyzer with real-time computer processing of the measured data. The experiments were conducted for three coals of different properties and degree of metamorphism, of which two are brown coals from different deposits (Kuznetsk and Anadyr) and the third is black coal (Ekibastuz deposit). The composition of the coals used is given in Table 3. Coal was microground and mechanically activated in the IT SB RAS disintegrator (DIS), Fig. 1b, ground immediately prior to combustion. The coal samples were subjected to spectral size analysis before and after grinding. The size distribution was determined from the residues on a set of sieves, and the spectral analysis was performed with an optical method using a software developed in the IT SB RAS. All three coals showed similar size spectra. As an illustration, the spectra of the size distributions of the Kuznetsk long-flame coal and Anadyr coal are shown in Fig. 8. All coals showed also a similar difference of the size distribution between the parent coal and the one ground in the disintegrator mill. In the present experiment the consumption of microground coal fed into the burner was 200 kg/h. The primary air flow rate
corresponded to the excess air ratio 0.3 at the temperature of 60 C. The coal-air mixture was ignited by a gas torch applied only during the initial period of typically 1e2 min after which the ignition torch was switched off and the operation assumed the so-called authothermal regime. The experiments were conducted typically for 10e15 min which was sufficient for detecting and confirming the autothermal regime. Of course, the experiment could last longer and at higher power, but such runs have not been pursued often due to high costs and needs for cooling of the rear end of the rig and of the exhaust flue gases before discharging to the ash removal system. The temperature was measured by eight thermocouples located as indicated in the rig schematic in Fig. 7a: thermocouple T1 was located in the first-stage spiral close to the ignition gas torch, T2, T3 and T4 were placed in the burner and T5, T6 and T7 in the furnace, all with the thermocouple heads protruding at about 80 and 100 mm (respectively in the burner and furnace) from the interior combustor wall. The last thermocouple, T8, was located at the burner centre at the same cross-section as T4, aimed at providing some information about the temperature in the core of the burner. Six apertures are provided for the flue gas sampling probe, Fig. 7a. We present now some results of the temperature measurements for various coals, which demonstrate the achievement of the autothermal regime. The focus is on the burner (the temperatures T1, T2, T3, T4), which provide sufficient information needed for diagnosing this regime. The results for a typical run for different coals are shown in Figs. 9e11, which illustrate the process evolution in time during an experimental campaign. First, one can see that during the ignition there is a sharp rise of temperature in the burner at all measured locations (T1-T4). However, the conditions for reaching the autothermal regimes and its features are different reflecting different coal compositions. For long-flame Kuznetsk coal (Fig. 9) the temperatures T2, T3 and T4 reach soon the stable and almost constant values of about 1600e1700 C in the established regime. Thermocouple T1 shows an initial increase to about 500 C, but after turning off the ignition gas torch (after 100 s) the temperature T1 dropped to about 100 C and remained almost unchanged for the rest of the operation. Thus, the experiments showed that the ignition of the mechanically activated long-flame Kuznetsk coal happens almost instantly after lighting with the gas torch, confirming that further ignition and combustion of coal were self-sustained by coal itself. This is denoted as the autothermal regime of combustion. The coal feeding was stopped after 500 s, when all thermocouples show a sharp fall off in the temperature. This coal seems to be very suited for autothermal combustion, presumably because of low ash and high
Fig. 8. Residue and size distribution of (a) e Kuznetsk long-flame coal and (b) Anadyr coal. 1 e after disintegrator mill, 2 e basic sample from Belowo power plant.
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Fig. 9. Time evolution of the temperatures measured by thermocouples T1 (short dashed line), T2 (long dashed line), T3 (full line), T4 (chain line) during ignition and autothermal regime for the activated long-flame Kuznetsk coal.
Fig. 11. Time evolution of the temperatures measured by thermocouples T1 (short dashed line), T2 (long dashed line), T3 (full line), T4 (chain line) during ignition and the autothermal regime for the activated black coal from Ekibastuz deposit.
carbon contents. Although not directly relevant for the present study that focuses on the burner, we note that the flame temperature at the exit of the burner entering the furnace is high enough to maintain combustion of gaseous components mixed with the secondary air in the furnace, as detected by the temperature measurements at the T5 to T7 locations (not shown here). Fig. 10 shows the time evolution of the temperature in the burner for combustion of the activated brown coal form Anadyr deposit. As seen in the figure, this coal required a considerably longer time to enter the autothermal regime. It was found experimentally that gas burner must be kept switched-on continuously for more than 400 s in order to establish the autothermal operation. Here the coal feeding was stopped after 15 min. The temperatures at all locations are considerably lower than in the previous case, reaching (and slightly exceeding) 1000 C only after about 800 s. Moreover, in this case the flame of the autothermal regime moves along the burner after switching off the gas torch, as illustrated by a sudden decrease in the temperature at point T2, shown by the dotted line in Fig. 10. This coal differs from the Kusnetsk coal (Fig. 9) only in relatively high ash content and somewhat (about 20%) lower calorific value Qr (see Table 3). Fig. 11 shows the time evolution of the temperature for yet another coal type, this time for activated black coal from Ekibastuz deposit. This type of coal is very difficult to ignite, presumably because of a very high ash contents and a low percentage of
volatiles. The figure shows that combustion of this type of coal can be self-sustained only by supporting it with a periodic switching on of the gas torch. But each following inclusion of a gas torch provides better coal ignition, most probably because of the warming of the refractory shielding in the burner. Unfortunately the coal feeding was stopped after about 24 min before full autothermal mode was established because of the rig overheating. However, the experiment indicated that in a longer campaign the autothermal mode could be achieved. Further experiments with Ekibastuz coal are needed to examine the prerequisites and possibly some special conditions and remedies to ensure reliable autothermal combustion of this and similar types of coal. 5. Prospect for new developments To close this discussion, we note that the above presented results can serve as the basis for designing new burners which will use mechanically-activated microground coals instead of gas or liquid-fuel for ignition and support of common or newly designed installations for pulverised coal combustion. Depending on the thermal power and type of coal, one can think of either a single or a double-stage combustor. An example of a possible design is a twostage burner that looks similar to the 5 MW rig described above. However, this time we would have a true two-stage combustor: the burner tube acts now as the first-stage (the “burner”) in which the autothermal regime is run using microground coal. The flame from the first-stage serves now for ignition and support of combustion of standard pulverized coal fed into the second stage (the “furnace”). We estimate that in total up to about 25% of total coal should be microground in a mill that ensures coal mechanical activation, and fed into the first stage (burner), whereas the remaining coal can be used in its initial standard granulation and fed into the secondstage (furnace). This concept is currently under investigation. 4. Conclusions
Fig. 10. Time evolution of the temperatures measured by thermocouples T1 (short dashed line), T2 (long dashed line), T3 (full line), T4 (chain line) during ignition and the autothermal regime for the activated brown coal form Anadyr deposit.
Experiments in two pilot-scale combustion rig of nominal thermal capacities of 1 and 5 MW (the latter being of a semiindustrial type) confirmed that several types of coals, when mechanically activated by microgrinding in a new disintegrator-type mill, can operate in self-igniting and self-sustained authothermal regime without the need for support by gas or liquid fuel. Ignition by a gas torch was applied only during a short initial period (1e2 min for Kuznetsk brown coal, 3 min for Tihonsky black coal and 5e7 min for Anadyr brown coal) to initiate combustion from a cold start.
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Visual observations of the flame showed that the flame of mechanically-activated coal is much shorter with a traceless afterburning zone compared with long flame with visible particle trajectories in burning of initial coal obtained from an industrial mill such as commonly used in power and industrial boilers. In comparison with the widely used pulverized coal, microgrinded coal ensures more efficient and stable combustion, lower NOx emission and lower deposition of ash and slag in the burner. The temperature and composition of gas mixture at the outlet of the burner (the first stage of the rig) showed to fulfil the necessary conditions for ignition of common (non-activated) coal that could be added to the afterburner combustion chamber, which in real full-scale installations can be the proper boiler furnace. The thermogravimetric analysis showed that the coal activation energy threshold can be reduced by a factor of two or more, depending on the type of coal, and in particular on the type of mill and the mode of grinding. In the cases considered, using the disintegrator with intense mechanical impact, the activation energy of Kansk-Achinsk and Kusnetsk long-flame coals was reduced on average by about 40e50% which showed still to be sufficient to ensure the autothermal regime. In a similar experiment at the same operating conditions but with coal microground in a vibrocentrifugal mill, although with the fractional size composition very close to that obtained in the disintegrator, the self-ignition and sustained combustion could not be achieved as manifested in some cases by the immediate temperature decay in the whole combustor after switching off the ignition gas torch. When the ignition was kept longer, in some cases the microground coal from the vibrocentrifugal mill was shown to run in the autothermal mode, but the flame was longer with visible particle trajectories, resembling to flames in common coal-dust burners. Thus, the type of the mills and effects, depending on the availability of the rate of oxidation of activated carbon after the mechanical treatment, is the key factor for enhancing the reactivity of coal. Related to the above, the second prerequisite for a successful autothermal operation is the concurrency of the microgrinding and combustion, which should prevent a decay in coal reactivity due to reaction with the atmospheric oxygen. These factors must be considered when designing devices for combustion and gasification of fine coal, including devices for ignition of coal in boilers without liquid fuel. The experiment confirmed the potential of mechanically activated microground coal to replace liquid fuels in various combustion applications for heating and electricity production.
Acknowledgements This work was partly financed by the RF Government Grant No 11.G34.31.0046, Lead Scientist K. Hanjalic, NSU and SP-6672.2013.1. References [1] C.L. Briceland, M.J. Khinkis, R.T. Waibel, in: Proc. Int. Symp. On Combustion Diagnostics: From Fuel Bunker to Stack; Akron, Ohio, October 4e6, 1983, American Flame Ressearch Committee, Houston, TX, 1983. Paper no.10. [2] J.D. Freihaut, A. Vranos, W.M. Proscia, B. Knight, Investigation of the Combustion Properties of Micronized Coal: Technical Progress Report for the Period January 1, 1988eMarch 31, 1988, 1988. [3] J.D. Freihaut, W. Proscia, B. Knight, A. Vranos, H. Hollick, K. Wicks, Combustion Properties of Micronized Coal for High Intensity Combustion Applications, 1989. Technical Report, DOE/PC/80263eT24; UTRCe957253-F. [4] F. Rosenberger, Ch.J. Guilfoyle, W.O. Parker, Technical and economic feasibility for the application of micronized coal as replacement for no. 6 oil for start-up and low-load operation at Illinois Power Havana No. 6 Cycling Unit, in: Proc. American Power Conference vol. 53, 1991. Conference: 53, Chicago, USA, 29 Apre1 May 1991. [5] DOE/NETL, The Advanced Tangentially Fired Combustion Techniques for the Reduction of Nitrogen Oxides (NOx) Emissions from Coal-fired Boilers Demonstration Project, US Department of Energy and Nation Energy Technology Laboratory, 2000. DOE/NETL-2000/1122. [6] DOE/NETL, Micronized Coal Reburning Demonstration for NOx Control: A DOE Assessment, U.S. Department of Energy, National Energy Technology Laboratory, 2001. DOE/NETL-2001/1148. [7] EPA, Summary Report on Control of NOx Emissions by Reburning, US Environmental Protection Agency, 1996, p. 17. EPA/625/R-96/001. [8] S. Li, T. Xu, Q. Zhou, H. Tan, Sh Hui, H. Hu, Optimization of coal reburning in a 1 MW tangentially fired furnace, Fuel 86 (2007) 1169e1175. [9] N.J. Welham, P.G. Chapman, Mechanical activation of coal, Fuel Process. Technol. 68 (2000) 75e82. [10] N.J. Welham, V. Berbenni, P.G. Chapman, Increased chemisorption onto activated carbon after ball-milling, Carbon 40 (2002) 2307e2315. [11] T.M. Khrenkova, Mekhanokhimicheskaya Aktivatsiya Ugley, Nedra, Moscow, 1993 (in Russian). [12] T.S. Yusupov, I.G. Shumskaya, A.P. Burdukov, Effect of mechanical dispersion of lignite on its thermal decomposition, J. Min. Sci. 43 (5) (2007) 539e544. [13] A.P. Burdukov, V.V. Konovalov, T.S. Yusupov, Prospects for use of micronized coal in power industry, Therm. Sci. 6 (1) (2002) 29e42. [14] A.P. Burdukov, V.I. Popov, Investigation of pulverized coal combustion, in: Proceedings of Adv. Research Workshop NATO on Advanced Combustion and Aerothermal Technologies, Springer, 2007, pp. 131e140. [15] A.P. Burdukov, V.I. Popov, V.A. Faleev, Study of mechanically activated coal combustion, Therm. Sci. 13 (1) (2009a) 127e138. [16] A.P. Burdukov, V.I. Popov, T.S. Yusupov, M.Yu Chernetskiy, K. Hanjali c, Authothermal combustion of mechanically-activated micronized coal in a 5 MW pilot-scale combustor, Fuel (2014) (to appear). [17] A.P. Burdukov, V.I. Popov, I. Lomovsky, T.S. Yusupov, Effect of mechanochemical activation on coal combustion in torch, Chem. Sustainable Dev. 17 (2009b) 465e472.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Mechanical activation of micronized coal: Prospects for new combustion applications A.P. Burdukov a, b, V.I. Popov a, b, M.Yu. Chernetskiy b, *, A.A. Dekterev b, K. Hanjali c a, c a
Novosibirsk State University, Novosibirsk, Russia Kutateladze Institute of Thermophysics, SB RAS, Novosibirsk 630090, Russia c Delft University of Technology, Delft, The Netherlands b
h i g h l i g h t s Microgrinding coal in a special high-impact mill reduces the coal activation threshold. Mechanical activation makes coal easier to ignite and to burn in a bright compact flame like heavy oil. Activated coal can burn easier in self-igniting and self-sustained regime. Potential for replacing liquid fuel in a variety of industrial and power application.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2013 Accepted 31 December 2013 Available online xxx
The paper presents experimental studies of the effects of enhanced mechanical activation of microground coal on its inflammability and combustion efficiency. Experimental test of reactivity of pulverized coal milled in a new rotating impact disintegrator developed at the IT SB RAS (Novosibirsk, Russia) showed that the coal activation threshold was reduced by a factor of two or more, making it easier to ignite and burn more efficiently. The preliminary experiment in a pilot-scale 1 MW rig, followed by more detailed experimental and numerical investigations in a cylindrical swirl combustor of 5 MW thermal power, confirmed the earlier visual observations and measurements that the mechanically disintegrated micro-ground coal behaves much like heavy oil. This offers prospects for designing new coal-dust burner that will ensure ignition at much lower temperatures, more efficient and more stable combustion and lower NOx emission compared with coal of the same granulation (about 40 mm) obtained in conventional mills. The research offers new opportunities for replacing liquid fuels by coal in various combustion applications for heating and electricity production from coal as well as new prospects for efficient coal gasification. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Mechanical activation Micronized coal Pulverized coal combustion
1. Introduction Coal is expected to remain the mainstay of electricity generation and heating in many countries for the foreseeable future. However, its exploitation, processing and utilization will unavoidably need to adjust to meet the ever more stringent regulations on pollutant emission. The environmental constraints, together with the imminent increase in the fuel and energy prices on one hand and a decline of the quality of coal reserves on the other, call for search for new coal pre-processing and conversion methods and technologies which should ensure higher combustion efficiency, operational flexibility and easier control, all leading to diminishing adverse environmental impact. * Corresponding author. Tel.: þ7 383 335 66 84. E-mail address: [email protected] (M.Yu. Chernetskiy).
Among novel options in clean coal technologies, the use of microground (micronized) coal has been considered as one of the prospective routes to overcome various shortcomings in the conventional pulverized coal combustion that prevail worldwide in large coal-fired power plants and various industrial and heating installations. The past two decades have witnessed a number of projects aimed at examining various issues of combustion of micronized coal and its possible use for replacement of oil for ignition and combustion support of the common dust coal, or its use in the process of reburning. Briceland et al. [1] at the Institute of Gas Technology were among the first to indicate at potential advantages of combusting micro-ground coal in large installations. Freihaut et al. [2,3] investigated combustion properties of micronized coal and its use for intense combustion applications. Rosenberger et al. [4] discussed technical and environmental issues and implications of micronized coal to replace oil for the start-up and
1359-4311/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.12.081
Please cite this article in press as: A.P. Burdukov, et al., Mechanical activation of micronized coal: Prospects for new combustion applications, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2013.12.081
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operation of boiler N 6 TPS Tilinois Power Havana (USA) using a special system of ultrafine grinding mill. More recent investigations focused on Micronized Coal Reburning (MCR) option as a means to reduce carbon in fly ash and NOx emission (e.g. Milliken Clean Coal Technology Demonstration Projects DOE/NETL-2000/1122, DOE/ NETL-2001/1148, EPA/625/R-96/001 [5e7]. Still more recently, Li et al. [8] reported on optimization of coal reburning in a 1 MW tangentially fired furnace. The benefits from coal micronization on combustion are generally well known and manifest among others in compact flames with faster ignition and more complete char burnout requiring less excess air. The origin of theses benefits is commonly attributed to the increase in the surface-to-volume ratio due to a decrease in the particle size, but also due to a more extended external surface of irregularly shaped particles and in some cases increased porosity, all depending on the type of mills used. Less is known about the change of the coal kinetic properties, especially the enhancement of coal reactivity due to mechanical activation, which greatly depends on the method of coal grinding. Welham and Chapman [9], Welhem et al. [10], investigated this issue, but focused on detecting and identifying the effects of long milling in a tumbling ball-mill. They found that a slow milling leads to an increase in coal reactivity and concluded that the effect cannot be attributed solely to the increase in the particle surface area. Similar studies were reported earlier by Khrenkova [11]. More recently, using a thermo-gravimetric analyzer, Yusupov et al. [12] reported a significant increase of reactivity of lignite and brown coal during milling in a centrifugal planetary mill. Burdukov et al. [13e16] pursued a design of a new mill referred here as disintegrator, featured by high mechanical impact but more adequate for industrial application than planetary mills. A thermo-gravimetric analysis of coal samples before and after milling confirmed that the impact coal crushing in the disintegrator enhances coal reactivity to the level comparable to that achieved in a laboratory-scale planetary mill considered by Yusupov et al. [12]. The enhancement of the reactivity of coal, desirably to the level of gas or fuel oil can improve the combustion efficiency and stability in various conventional installations as well as open prospects for innovative application at various scales. For example, the ignition of coal-fired boilers is often assisted with a gas or fuel-oil flame torch. The use of activated coal for ignition has already been proved possible when using the appropriate mechanical equipment. This trend is currently being developed at the IT SB RAS (Burdukov et al. [14]). This paper presents some results of investigation of the influence of the grinding method and type of mills on the enhancement of coal mechanical activation, and of the effects on the ignition and combustion of micronized coal. Presented are the results of experimental testing of combustion of three types of brown coal and two types of black coal, micro-grinding in two different mills.
The combustion of microground coal was tested in two pilot-scale rigs of thermal capacity of 1 MW and 5 MW. Next to confirming the successful autothermal (self-sustained) operation, the results provide useful guidelines for designing new concept of dust coal combustors. In addition, the experiments provide a database for testing of mathematical models for the computer simulation of the process (currently in progress), which could be used for design and optimization of new combustion and gasification devices using mechanically activated microground coal. 2. Coal preparation: the mills and mechanical activation In order to identify the particular effect of coal grinding methods on its kinetic characteristics we studied the changes in physical and chemical properties of several brown and black coals. mechanically dispersed in different mills of higher power density. Among a number of different types of mills with higher intensity of mechanical action, which can be found in the literature, the work here reported compares the results obtained with coals micronized in a vibratory centrifugal mill (VTSM-7), Fig. 1a, and in a disintegrator type developed at the ITP SBRAS (hereafter denoted as DIS), Fig. 1b, (Burdukov et al. [15]). Both mills produce microground coal of very similar granulations (for particle size distribution, see Sections 3 and 4 below), but because of different mechanical actions of the two mills, they have been shown by the thermogravimetric analysis to have different activation thresholds. It is noted that in both cases the initial (basic, parent) coal was that obtained from the conventional mills in the power plant Belowo. The intense mechanical impact in high-energy devices such as disintegrating mill shown in Fig. 1b causes not only a rapid decrease of the particle size (Table 2, Fig. 2) and increase in their surface but also enhances the reactivity of coal in the processes of subsequent thermooxidative destruction in the air under non-isothermal conditions. While the size of particles plays an important role, the method of grinding is crucial as coal dust of almost equal granulation may show no notable change in its reactivity as in the case of the vibrocentrifugal mill, Fig. 1a, which shows more or less the same reactivity as the initial coal ground in a conventional industrial mill. As noted in the Introduction, some other types of mills can also increase the coal reactivity, for example a planetary mill, which we considered earlier. However, while useful for laboratory exploration, the complex design and higher energy consumption makes this type of mill inconvenient for real-scale applications (Burdukov et al. [13]). The changes in physico-chemical characteristics of coal treated in the mills with high intensity of mechanical action were analyzed by testing the coal kinetic properties by thermogravimetric analysis of the coal sample in air flow (Burdukov et al. [15,16]). The observed effects of changing the activation energy for mechanical activation of coal can be summarised as follows. First, the influence of the type
Fig. 1. Views of the two types of coal mill. (a) e Vibrocentrifugal mill VCM), capacity up to 100 kg per hour; (b) e IT SB RAS disintegration mill (DIS), capacity up to 150 kg per hour.
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Table 2 Particles size classes in [%] of Kansko-Achinsk brown coal grade B-2 used in the experimental rig 1 MW, for the parent coal (basic sample, from Belowo power plant) and after microgrinding in the vibrocentrifugal mill (VCM) and in the disintegrator mill (DiS). Grinding method
Basic sample Vibrocentrifugal mill Disintegrator mill
Fig. 2. Sieve residue Res [%] (lines) of brown coal of B-2 grade from Kansk-Achinsk deposit. 1 e After disintegration mill (Dis); 2 e initial (basic) sample from a common power-plant mill. The size distribution and sieve residual after vibrocenrtifugal mill (VCM) is similar to 1.
of mills on the reactivity of coal, depending on the availability of the rate of oxidation of activated carbon after the mechanical treatment is the key factor. Second, the important prerequisite for utilizing the benefits of mechanical activation is to avoid the decay of reactivity enhancement due to reaction with atmospheric oxygen, which requires that the microgrinding should be performed in situ simultaneously with feeding coal into the burner and its combustion. These factors must be considered when designing devices for gasification and combustion of fine coal, or using activated micronized coal in torches for ignition of common pulverized coal in boilers, thus without using liquid fuel. 3. The combustion experiment in the 1 MW pilot-scale rig One of the main objectives of the experimental studies was the observation of ignition and combustion of coal of varying degrees of metamorphism, as well as achieving the autothermal operation in the pilot-scale rigs of capacity of 1 MW and 5 MW by burning coal prepared in the disintegrator mill type developed at IT SB RAS. In the preliminary experiment carried out in a smaller rig of 1 MW we considered two coals: brown coal of B-2 grade from Kansk-Achinsk deposit and black coal from Tihonsky deposit. Their chemical compositions are given in Table 1. An important prerequisite for a fair comparison of the reactivity of different coals tested is to eliminate possible effect of different particle sizes. Table 2 shows the measured classes of the average particle dimensions of brown coal grade B2 from Kansk-Achinsk deposit after grinding in the mills of both types. It is clear that the average size of particles is almost the same, about w40e42 mm. Thus, the reacting surface in both cases is practically the same and therefore the inflammation temperature threshold and combustion speed of both flames is expected to be identical if no change in coal reactivity occurs during milling. The sieve residuals distributions of the basic coal sample after the common power plant mill, and from the two micronizing mills, vibrocentrifugal and disintegrator mill for the same coal, are illustrated in Fig. 2. It is noted that for the coal microgrinded in the vibrocentrifugal and disintegrator mills, both the sieve residual (and also the size distribution, not
Size of particles, mm <2
2e20
20e50
50e100
>100
2.31 6.16
7.6 28.53 45.94
9.2 27.41 39.6
9.3 21.84 7.48
73.9 19.91 0.82
shown in Fig. 2) have very similar distributions as for the disintegrator [15]. Of course, the same or similar particle size distribution does not a guarantee the same surface-to-volume ratio as the milling in different mills may also change the effective surface due to possible deformation of particles shape and especially their porosity. In order to eliminate the particle surface-to-volume effect, the coal samples from the two mils have been subjected to a porosity analysis using the multipoint BET, BJH and deBohr’s methods [16]. These results showed that an increase in the surface-to-volume area due to high impact coal crushing in disintegrator was less than 15%, thus not significant. However, the thermogravimetric analysis showed that the activation energy of coal after disintegrator is almost twice lower than of coal microground in the virbocentrifugal mill, whish essentially shows no difference in reactivity from the parent coal sample [17]. Fig. 3 shows the activation energy in function of the degree of decomposition for the brown Kansk-Achinsk coal as obtained from the thermogravimetric analysis. The striking difference between the initial sample and coal from the disintegrator shows undoubtedly a significant reduction of the activation energy achieved in the disintegrator, especially at the first stage of decomposition, i.e. for the low values of the transformation rate.
Table 1 Chemical composition in [%] of coals used in the 1 MW rig experiment. Brown coal of B-2 grade from Kansk-Achinsk deposit; black coal from Tihonsky deposit. Coal
Wr
Ar
Vdaf
Cr
Hr
Sr
Nr
Qr [MJ/kg]
Brown (K-A B2) Black (Tihonsky)
39.0 17.8
7.3 18.3
48 30.2
37.6 47.3
2.6 3.05
0.4 0.25
0.4 1.3
13.03 16.9
Fig. 3. Activation energy of the basic sample (a) and of the mechanically-activated coal microground in disintegrator mill (b) for brown coal grade B2 from Kansk-Aschinsk deposit. (From thermogravimetric analysis).
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A view of the 1 MW combustion stand is shown in Fig. 4a and a schematic of the combustor (swirl burner, its extension e afterburner, and the furnace) with the microgrinder and coal feeding system in Fig. 4b. The experimental rig was constructed to embody the entire technological cycle of the combustion of dust-coal and next to the combustor and coal feeder shown in Fig. 4b, it includes also of a vapour generator for burner support, and a system of flue gas cleaning that utilizes an ash hydro-scrubber. The stand is equipped with photometric, multicomponent gas analyzing and platinum-rhodium thermocouples of type B of wire thickness of 0.3 mm with unshielded head, allowing one to perform real-time diagnostics and monitoring of the combustion process. The coalair mixture is ignited with a standard ignition-spark methane device or a plasmatron torch with a power up to 15 kW, later replaced by a simple propane-butane gas torch. For the activated coal of most types microground in the disintegrator the ignition devised was operated only during the first 1e2 min. Then the ignition device is switched off, and combustion continues in the self-sustained (autothermal) mode. The time within which particles get into the flame after leaving the mill is only about 1 s. This immediate use of activated coal prevents any possible reactivity decay that might occur due to atmospheric oxidation or possible relaxation of the activated molecular structures if coal was left for a longer time before being burnt. For non-activated coal from the vibrocentrifugal mill the ignition had to last longer (typically 5 min or more) to support combustion until the refractory walls inside the afterburner became sufficiently heated to ensure self-ignition. However, a notable difference between the combustion features of activated and non-activated coals is in the visual outlook of the flame and especially in the extent of their flame contours, which are clearly visible through the inspection window. While the flame of the activated coal is short and compact with a distinct front close to the afterburner exit, followed by a traceless bright zone, the flame of the non-activated coal is long with visible traces of burning particles and not very sharp flame front. Both contours are indicated in Fig. 4b. Fig. 5 shows the distribution of the temperatures along the centreline of the afterburner when using Kansk-Achinsk coal microground in the two mills, VCM and DIS. In both cases the temperature increases almost linearly, but there is a significant difference in the temperature values and their gradient along the streamwise direction. The activated DSM coal burns faster reaching about 1000 C at the location t8. A sharp flame-front contour appears already at the afterburner exit. In contrast, the nonactivated coal from the VCM burns slower reaching at location t8 only about 850 C, and continues to burn well into the furnace, as indicated by
Fig. 5. Temperature along the afterburner (Fig. 4b) measured by thermocouples t6, t7 and t8 for Kansk-Achinsk coal in combustion of non-activated coal from the vibrocentrifugal (VCM) mill (empty symbols) and of mechanically-activated microground coal from the disintegrating mill (DIS) (full symbols).
its contour in Fig. 4b. It is also noted that the ignition of the DIS coal lasted only about 1 min, whereas for ensuring a continuous combustion of the VCM coal the ignition torch was switched on for about 5 min. Fig. 6 shows the time evolution of the temperatures at locations t1, t6 and t8 for two activated coals milled in the disintegrator: brown coal grade B-2 from Kansk-Achinsk deposit, and black coal from Thonsky deposit. Both types of coal show similar performances: a stable ignition and autothermal combustion is maintained after turning off the ignition torch, as illustrated by the temperatures measured by thermocouples t1, t6 and t8. It is recalled that thermocouples 6 and 8 were located at the beginning and at the end of the afterburner and thermocouple t1 immediately after the ignition torch (2). As indicated by the peaks of the t1 thermocouple reading, a somewhat longer ignition period was required for black coal (about 3 min), presumably because of a larger content of ash and a smaller content of volatiles (see Table 1). It is noted in passing that a higher volatile content, as well as their rapid burnout at the same level of excess air, will reduce the concentration of NOx. In contrast, the full autothermal mode (without a support of the ignition torch) could not be achieved with any of the two coals tested if milled in the vibrocentrifugal mill.
Fig. 4. A view of the 1 MW combustion rig (left) and a sketch of the main rig components. 1 e disintergrator microrginder, 2 e plasmatron (igniter), 3 e vortex burner, 4 e burner extension (afterburner), 5 e furnace, 6 e view window, 7 e flue gas exhaust, t6-t8 thermocouples (t6 at L ¼ 150 mm, t7 at L ¼ 510 mm; t8 at L ¼ 850 mm; t1 near the ignition torch). Visually observed flame contours in the furnace are denoted by dashed lines: grey thin line e non-activated coal; black thicker line e mechanically activated coal.
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Fig. 6. Time evolution of temperatures measured by thermocouples t1 (short dashed line), t6 (long dashed line), and t8 (full line) during ignition and the autothermal regime for activated coals. (a) e Kansk-Achinsk brown coal, B-2 grade; (b) e Tihonsky black coal.
4. The combustion experiment in a 5 MW semi-industrial rig A further exploration of the effects of coal activation on the combustion was subsequently carried out in a larger, semiindustrial-scale combustion rig of maximum power of 5 MW, Fig. 7. This rig was designed for the study the possibility of using mechanically-activated coal in industrial boilers and to serve as the first step in the investigation of different real-scale factors on the ignition and combustion stability. The combustor consists of two stages. The first stage is a cylindrical swirl burner of the interior diameter of 315 mm and a length of 1515 mm, with a tangential
feeding of microground coal premixed with the primary air through a vaneless spiral. The burner is followed by a coaxial cylindrical combustion chamber (furnace) of 1000 mm internal diameter and length of 2820 mm, equipped also with a vaneless spiral that provides a tangential entry of secondary air that reinforces the swirling motion, but could also be used for feeding additional coal of microor larger granulation. The burner spiral has an aperture for a gas torch (propane-butane from a bottle) used to ignite the coal-air mixture. In fact the burner is the main rig component that serves for the thermochemical preparation, ignition and self-sustained combustion of microground coal, which should then ensure the
Fig. 7. Experimental rig of 5 MW power. (a) e Schematic of the combustor; T1 to T8 denote thermocouples and G5-G7 the gas sampling apertures. (b) A view at the complete rig with an inset (top centre) showing the burner in autothermal operation as confirmed by a bright flame seen through view windows along the whole length.
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Table 3 The chemical composition of coals used in the 5 MW experiment, [%]. Wr
Ar
Vdaf
Cr
Hr
Sr
Nr
Qr [NJ/kg]
Long-flame Kuznetsk coal 20.0 12.43 42 53 3.8 0.27 1.7 20.5 Brown coal form 20.0 20.0 48.0 43.4 3.4 0.2 0.8 16.4 Anadyr deposit Black coal from 7.0 38.1 30.0 43.4 2.9 0.8 0.8 16.8 Ekibastuz deposit
continued combustion in the furnace without or with addition of non-activated dust coal that can be supplied to the furnace in realscale installations, as discussed briefly in Section 5. The latter option was not investigated in the present experiment, but nevertheless the secondary air was provided and its flow rate adjusted to ensure the complete burning of coal and its combustion products in the furnace. The combustion rig was equipped with an adjustable coal feed system with a disintegrator mill of a much larger capacity (1500 kg/h) than used for 1 MW rig, the primary and secondary air supplies, a dry-cyclone slag removal and a system for ash recovery. It is also equipped with a pressure controller in the combustion chamber. The combustion process was monitored by a set of Pt-Rho B-type thermocouples (0.3 mm wire thickness, placed in ceramic sheath with an open head) for measuring the temperature of the combusting products, and with a gas sampling probes connected to a multicomponent gas analyzer with real-time computer processing of the measured data. The experiments were conducted for three coals of different properties and degree of metamorphism, of which two are brown coals from different deposits (Kuznetsk and Anadyr) and the third is black coal (Ekibastuz deposit). The composition of the coals used is given in Table 3. Coal was microground and mechanically activated in the IT SB RAS disintegrator (DIS), Fig. 1b, ground immediately prior to combustion. The coal samples were subjected to spectral size analysis before and after grinding. The size distribution was determined from the residues on a set of sieves, and the spectral analysis was performed with an optical method using a software developed in the IT SB RAS. All three coals showed similar size spectra. As an illustration, the spectra of the size distributions of the Kuznetsk long-flame coal and Anadyr coal are shown in Fig. 8. All coals showed also a similar difference of the size distribution between the parent coal and the one ground in the disintegrator mill. In the present experiment the consumption of microground coal fed into the burner was 200 kg/h. The primary air flow rate
corresponded to the excess air ratio 0.3 at the temperature of 60 C. The coal-air mixture was ignited by a gas torch applied only during the initial period of typically 1e2 min after which the ignition torch was switched off and the operation assumed the so-called authothermal regime. The experiments were conducted typically for 10e15 min which was sufficient for detecting and confirming the autothermal regime. Of course, the experiment could last longer and at higher power, but such runs have not been pursued often due to high costs and needs for cooling of the rear end of the rig and of the exhaust flue gases before discharging to the ash removal system. The temperature was measured by eight thermocouples located as indicated in the rig schematic in Fig. 7a: thermocouple T1 was located in the first-stage spiral close to the ignition gas torch, T2, T3 and T4 were placed in the burner and T5, T6 and T7 in the furnace, all with the thermocouple heads protruding at about 80 and 100 mm (respectively in the burner and furnace) from the interior combustor wall. The last thermocouple, T8, was located at the burner centre at the same cross-section as T4, aimed at providing some information about the temperature in the core of the burner. Six apertures are provided for the flue gas sampling probe, Fig. 7a. We present now some results of the temperature measurements for various coals, which demonstrate the achievement of the autothermal regime. The focus is on the burner (the temperatures T1, T2, T3, T4), which provide sufficient information needed for diagnosing this regime. The results for a typical run for different coals are shown in Figs. 9e11, which illustrate the process evolution in time during an experimental campaign. First, one can see that during the ignition there is a sharp rise of temperature in the burner at all measured locations (T1-T4). However, the conditions for reaching the autothermal regimes and its features are different reflecting different coal compositions. For long-flame Kuznetsk coal (Fig. 9) the temperatures T2, T3 and T4 reach soon the stable and almost constant values of about 1600e1700 C in the established regime. Thermocouple T1 shows an initial increase to about 500 C, but after turning off the ignition gas torch (after 100 s) the temperature T1 dropped to about 100 C and remained almost unchanged for the rest of the operation. Thus, the experiments showed that the ignition of the mechanically activated long-flame Kuznetsk coal happens almost instantly after lighting with the gas torch, confirming that further ignition and combustion of coal were self-sustained by coal itself. This is denoted as the autothermal regime of combustion. The coal feeding was stopped after 500 s, when all thermocouples show a sharp fall off in the temperature. This coal seems to be very suited for autothermal combustion, presumably because of low ash and high
Fig. 8. Residue and size distribution of (a) e Kuznetsk long-flame coal and (b) Anadyr coal. 1 e after disintegrator mill, 2 e basic sample from Belowo power plant.
Please cite this article in press as: A.P. Burdukov, et al., Mechanical activation of micronized coal: Prospects for new combustion applications, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2013.12.081
A.P. Burdukov et al. / Applied Thermal Engineering xxx (2014) 1e8
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Fig. 9. Time evolution of the temperatures measured by thermocouples T1 (short dashed line), T2 (long dashed line), T3 (full line), T4 (chain line) during ignition and autothermal regime for the activated long-flame Kuznetsk coal.
Fig. 11. Time evolution of the temperatures measured by thermocouples T1 (short dashed line), T2 (long dashed line), T3 (full line), T4 (chain line) during ignition and the autothermal regime for the activated black coal from Ekibastuz deposit.
carbon contents. Although not directly relevant for the present study that focuses on the burner, we note that the flame temperature at the exit of the burner entering the furnace is high enough to maintain combustion of gaseous components mixed with the secondary air in the furnace, as detected by the temperature measurements at the T5 to T7 locations (not shown here). Fig. 10 shows the time evolution of the temperature in the burner for combustion of the activated brown coal form Anadyr deposit. As seen in the figure, this coal required a considerably longer time to enter the autothermal regime. It was found experimentally that gas burner must be kept switched-on continuously for more than 400 s in order to establish the autothermal operation. Here the coal feeding was stopped after 15 min. The temperatures at all locations are considerably lower than in the previous case, reaching (and slightly exceeding) 1000 C only after about 800 s. Moreover, in this case the flame of the autothermal regime moves along the burner after switching off the gas torch, as illustrated by a sudden decrease in the temperature at point T2, shown by the dotted line in Fig. 10. This coal differs from the Kusnetsk coal (Fig. 9) only in relatively high ash content and somewhat (about 20%) lower calorific value Qr (see Table 3). Fig. 11 shows the time evolution of the temperature for yet another coal type, this time for activated black coal from Ekibastuz deposit. This type of coal is very difficult to ignite, presumably because of a very high ash contents and a low percentage of
volatiles. The figure shows that combustion of this type of coal can be self-sustained only by supporting it with a periodic switching on of the gas torch. But each following inclusion of a gas torch provides better coal ignition, most probably because of the warming of the refractory shielding in the burner. Unfortunately the coal feeding was stopped after about 24 min before full autothermal mode was established because of the rig overheating. However, the experiment indicated that in a longer campaign the autothermal mode could be achieved. Further experiments with Ekibastuz coal are needed to examine the prerequisites and possibly some special conditions and remedies to ensure reliable autothermal combustion of this and similar types of coal. 5. Prospect for new developments To close this discussion, we note that the above presented results can serve as the basis for designing new burners which will use mechanically-activated microground coals instead of gas or liquid-fuel for ignition and support of common or newly designed installations for pulverised coal combustion. Depending on the thermal power and type of coal, one can think of either a single or a double-stage combustor. An example of a possible design is a twostage burner that looks similar to the 5 MW rig described above. However, this time we would have a true two-stage combustor: the burner tube acts now as the first-stage (the “burner”) in which the autothermal regime is run using microground coal. The flame from the first-stage serves now for ignition and support of combustion of standard pulverized coal fed into the second stage (the “furnace”). We estimate that in total up to about 25% of total coal should be microground in a mill that ensures coal mechanical activation, and fed into the first stage (burner), whereas the remaining coal can be used in its initial standard granulation and fed into the secondstage (furnace). This concept is currently under investigation. 4. Conclusions
Fig. 10. Time evolution of the temperatures measured by thermocouples T1 (short dashed line), T2 (long dashed line), T3 (full line), T4 (chain line) during ignition and the autothermal regime for the activated brown coal form Anadyr deposit.
Experiments in two pilot-scale combustion rig of nominal thermal capacities of 1 and 5 MW (the latter being of a semiindustrial type) confirmed that several types of coals, when mechanically activated by microgrinding in a new disintegrator-type mill, can operate in self-igniting and self-sustained authothermal regime without the need for support by gas or liquid fuel. Ignition by a gas torch was applied only during a short initial period (1e2 min for Kuznetsk brown coal, 3 min for Tihonsky black coal and 5e7 min for Anadyr brown coal) to initiate combustion from a cold start.
Please cite this article in press as: A.P. Burdukov, et al., Mechanical activation of micronized coal: Prospects for new combustion applications, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2013.12.081
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Visual observations of the flame showed that the flame of mechanically-activated coal is much shorter with a traceless afterburning zone compared with long flame with visible particle trajectories in burning of initial coal obtained from an industrial mill such as commonly used in power and industrial boilers. In comparison with the widely used pulverized coal, microgrinded coal ensures more efficient and stable combustion, lower NOx emission and lower deposition of ash and slag in the burner. The temperature and composition of gas mixture at the outlet of the burner (the first stage of the rig) showed to fulfil the necessary conditions for ignition of common (non-activated) coal that could be added to the afterburner combustion chamber, which in real full-scale installations can be the proper boiler furnace. The thermogravimetric analysis showed that the coal activation energy threshold can be reduced by a factor of two or more, depending on the type of coal, and in particular on the type of mill and the mode of grinding. In the cases considered, using the disintegrator with intense mechanical impact, the activation energy of Kansk-Achinsk and Kusnetsk long-flame coals was reduced on average by about 40e50% which showed still to be sufficient to ensure the autothermal regime. In a similar experiment at the same operating conditions but with coal microground in a vibrocentrifugal mill, although with the fractional size composition very close to that obtained in the disintegrator, the self-ignition and sustained combustion could not be achieved as manifested in some cases by the immediate temperature decay in the whole combustor after switching off the ignition gas torch. When the ignition was kept longer, in some cases the microground coal from the vibrocentrifugal mill was shown to run in the autothermal mode, but the flame was longer with visible particle trajectories, resembling to flames in common coal-dust burners. Thus, the type of the mills and effects, depending on the availability of the rate of oxidation of activated carbon after the mechanical treatment, is the key factor for enhancing the reactivity of coal. Related to the above, the second prerequisite for a successful autothermal operation is the concurrency of the microgrinding and combustion, which should prevent a decay in coal reactivity due to reaction with the atmospheric oxygen. These factors must be considered when designing devices for combustion and gasification of fine coal, including devices for ignition of coal in boilers without liquid fuel. The experiment confirmed the potential of mechanically activated microground coal to replace liquid fuels in various combustion applications for heating and electricity production.
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Please cite this article in press as: A.P. Burdukov, et al., Mechanical activation of micronized coal: Prospects for new combustion applications, Applied Thermal Engineering (2014), http://dx.doi.org/10.1016/j.applthermaleng.2013.12.081