2.1. Typical Industrial Layouts of Units for CLF Combustion
To date, a fairly large number of experimental setups and experimental-industrial units used for burning fuel slurries have been developed; for example, in China, there are more than 90 steam and power plants using CLF. Among the most common Russian plants burning slurry fuels are prismatic (straight-through) elongated furnace chambers of hot water and steam boilers without significant modifications, used for direct combustion of pulverized coal or fuel oil. As a rule, they are based on the principle of flame burning by fine-dispersed fuel spray through burners or nozzles [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10]. For example, the boiler (TPE-214) of Novosibirsk thermal power plant (Russia) was operated at burning of large volumes of CWS (over 350 × 10
3 m
3), implemented in conjunction with the technical project of the coal pipeline (262 km) “Belovo-Novosibirsk” (Russia, from 1989 to 1997) [
8]. This project was carried out on the basis of industrial research into the preparation and combustion of CWS at Belovskaya thermal power plant (Russia, from 1986 to 1987) on the basis of boilers PC-40-1 with steam capacity of 640 t/h, as well as TP-35 (
Figure 1) of Min-Kush thermal power plant based on Kavak brown coal (Kyrgyzstan) [
1,
8].
The team of the Institute of Thermophysics (Siberian Branch of the Russian Academy of Sciences) conducted thermal calculations and changed the design of the low power boiler (KE-10-14C) for CWS combustion. For this boiler, the vortex combustion mode was used (
Figure 2).
It is known that the combustion of fuel slurries based on solid (coal, coal processing waste) and liquid (water, waste oils, etc.) components is accompanied by an increase in the size of the ignition zone and a decrease in the temperature level due to the presence of liquid inert ballast. In this regard, there was a gradual transition to the vortex method of CLF burning (due to the angular circular swirl) as the most efficient combustion with the longest period of cyclicity of the soaring of fuel droplets in the reaction zone and high combustion efficiency.
For furnaces of direct-flow boiler (P-56GM) and drum boiler (BKZ-75-39FB), the tangential arrangement of burners and various forms of air blowing were applied, providing a vortex flow of combustion products and stable combustion of CWS [
3].
In turn, the gas and fuel oil boiler (DKVr 6.5/13GFO) was additionally equipped with muffle (cyclone) vortex furnace extensions. They were used for pre-ignition and flame combustion of fine-sprayed fuel slurry with its subsequent burnout in the main furnace of the boiler [
4]. For the reconstructed boiler DKVr-20-13 (at a transition from the layer burning of coal to the slurry one) the vortex combustion of CLF was numerically studied using the ANSYS Fluent 12 software [
5].
The study [
6] considered in detail the issues of modernization of steam and hot water boilers with installing cyclone furnace extensions operating on traditional fuels: coal, gas and fuel oil (
Figure 3).
The study [
7,
8] presents the results of designing an experimental setup (
Figure 4) for CLF burning. The stable CLF ignition for this furnace was carried out in the temperature range of 600–700 °C, and the temperature of the gases at the outlet of the cyclone furnace extension was 1090–1160 °C.
Studies [
9,
10] present the results of numerical calculations of coal-water fuel combustion in the adiabatic vortex combustion chamber obtained with ANSYS FLUENT software. These mathematical calculations to determine the optimal modes of combustion of CWS can improve the design accuracy of various boilers.
2.2. Typical Diagrams of Experimental Setups for Studying CLF Combustion
Efficient CLF combustion at thermal power plants or small boilers, as well as achieving the maximum efficiency of the power plant, is associated with the optimal organization of fuel ignition in the combustion chamber, stabilization of the flame combustion and achievement of the set temperature level. As a rule, it depends on the correctly chosen physical parameters (Tgmin is minimal ignition temperature, τd is the ignition delay time, τc is the time of complete combustion, Tdmax is the maximum temperature at the drop center during heating) for the relevant design calculations of furnace chambers. This requires a series of experimental studies on the combustion initiation of various CLF and the necessary conditions for their development.
From the analysis of world practice on the study of combustion initiation of slurry fuel droplets, it should be noted that the most well-known methods are [
11,
12]. It is believed that the most widespread is the experimental approach, implemented by suspending a single fuel droplet on the thermocouple junction, its further placing in a heated medium (heated air, combustion products, their mixture) and recording the temperature change of the droplet (
Figure 5a,b). In turn, holders made of other materials (thin metal wire, quartz thread, ceramic rod) are also often used to register fuel ignition characteristics’ parameters [
13,
14]. There are methods for studying ignition and combustion of CLF on a hot surface (conductive heating) [
15] or in a muffle furnace (radiant heating [
16]) (
Figure 5c,d). In rare cases, local energy sources heated to a certain temperature (metal disks, etc.), laser pulse (for gasification), as well as spark discharge energy are used.
All the known aforementioned experimental approaches are far from the real combustion processes of thermal power plants because all of them use contact with the heated surface. Often holders lead to changes in the heat transfer conditions in the suspended fuel droplet [
13,
14]. One may observe processes of the heat sink from the droplet to the holder and an additional energy flow through the holder to it. These heat transfer processes are different from real conditions. As for experiments with fuel droplets freely falling through a cylindrical channel of heated air, there are restrictions associated with a small residence time of the fuel droplet in the combustion chamber (a cylindrical channel of limited length is used, see
Figure 6a [
17,
18]). As noted above, the CLF burnout requires a long residence time of fuel droplets in the furnace space, i.e., directly in the active combustion zone.
The study of CLF ignition and combustion in these thermal power plants (large and small boilers) is difficult and limited. This is due to rather high rates of physical and chemical processes, occurring in the boiler furnace, and the impossibility of visual recording of combustion and ignition of fuel droplets in the entire volume of the core zone (
Figure 6b). Individual viewing windows allow monitoring of CLF combustion processes in a narrow region (
Figure 6c).
In this regard, it is advisable to develop a model combustion chamber, which would allow, on the one hand, bringing the conditions of fuel ignition and combustion to the furnaces of real boilers, and, on the other hand, visualizing these processes for soaring CLF droplets in a swirling flow of heated air with their direct video recording in real time. It is expedient to develop and manufacture a model combustion chamber from optically transparent quartz glass based on the results of calculations of geometric dimensions.
2.3. Designing the Model Combustion Chamber
In the study of the characteristics of ignition and combustion of soaring CLF droplets, in contrast to the stationary suspended droplets [
11,
12,
13,
14], the necessary parameters for the soaring of a droplet of the fuel composition were estimated. The aim of the evaluation was to determine the geometric dimensions of the expanding part of the quartz tube (with conical inlet and outlet channels), where the droplet would be ignited by the heated air flow. A model cone-shaped combustion chamber has been developed as a promising design (in terms of manufacturing complexity, placement in the laboratory and compliance with the conditions of fuel combustion at TPP). This design allows keeping the drop in the given range of heights by changing air flow pressure (due to the pressure difference over the chamber height) in the vertical direction, and, thus, changing the residence time of the fuel droplet in the combustion core. The initial data for the calculation are presented in
Table 1.
The geometric dimensions of the chamber are calculated by the method of determining the soaring of a single droplet, which assumes the equality of forces of aerodynamic drag of the droplets and the gravitational forces in the ascending air flow.
When calculating the conditions of CLF droplet soaring, the following were assumed:
- 1
The coefficient of droplet sphericity (spherical shape factor) φ = 0.73 [
19].
- 2
Droplet motion in the vertical direction in the range h = 0–120 mm (height of the calculated cone-shaped chamber).
- 3
Properties of the component composition of CLF (density, ash, etc.) are subject to the additivity rule, and they can be determined using the relevant properties of the components.
The calculation method for droplet soaring is identical [
20]. The air flow rate along the channel section, at which a single drop passes into a soaring state, is the rate of soaring [
21]. It corresponds to the beginning of the destruction of the monodisperse soaring layer. At that,
where ε is the porosity (relative fraction of volume not filled with solid phase).
is the bulk density, and
is the body density (drop).
The soaring rate may be determined by Equations [
20]:
where Re
vit is the Reynolds criterion, ϖ
vit is the rate of soaring, m/s;
dd is the drop diameter, m;
,
are the density (kg/m
3) and dynamic viscosity coefficient (Pa·s) of air; and
Ar is the Archimedes criterion.
Density and dynamic coefficient of air viscosity (
Table 2) are taken at a temperature of 823 K [
22].
Archimedes’ criterion is calculated by the expression [
20]:
where
is the kinematic coefficient of the medium viscosity, m
2/s; and
is the dynamic coefficient of the medium viscosity, Pa·s.
Droplet size (equivalent diameter):
Considering the deviation from the spherical shape the drop size will be [
19]:
To calculate the Archimedes’ criterion, it is necessary to determine the density of the CLF droplet. According to the reference data [
23,
24], the density of coal dust in the composition of CLF is 1700 kg/m
3 (water content of the initial drop is ≈43.5%).
The density of CLF droplet in the initial state:
In the future, with the known elemental composition of the used solid fuel, the droplet density is specified according to [
25]:
where
is the density of the organic mass of the fuel;
Cp,
is the percentage of carbon and hydrogen in the fuel; and
is the ash content per dry mass of fuel.
The criterion of Archimedes for air temperature of 823 K:
High pressure vortex fan Leister Robust provides the maximum air flow rate Vv = 1200 L/min at 293 K, which corresponds to the air velocity of 4–5 m/s in the channel with a diameter of 80 mm.
Air velocity in the channel:
When air temperature in the channel is 823 K, considering its density of 0.43 g/L, the mass air flow rate:
The air velocity in the channel with a diameter of 80 mm is:
Thus, the required slurry velocity is provided, and the CLF drop can move vertically in the chamber along the expanding part of the cone (
Figure 7). In the calculation it was assumed that the smaller diameter of the cone corresponds to the diameter of the quartz tube. The maximum height of the cone will take 120 mm (due to the limitations of the size of the experimental stand). It was believed that the droplet soaring rate of 6.98 m/s corresponds to the region with a smaller cone diameter (80 mm).
Let us consider the final state of the droplet (to determine the angle of the cone opening)—complete burnout of organic mass with forming the ash envelope. For this intermediate state of the droplet, its diameter (
dd) corresponds to 1.095 mm (model of the retained ash envelope [
19]). Ash content (A
d) of the initial drop of CLF is 25%.
The ash envelope density in a droplet with a diameter of 1.095 mm will be:
where
is the true ash density (in the range of 2100–2400 for Kuznetsk coals) [
26].
The Archimedes’ criterion:
At this rate and air flow rate of 0.0588 m
3/s the diameter of the cone is:
Consequently, the larger diameter of the cone is 137 mm. The height of the cone was taken earlier as 120 mm. Thus, the opening angle of the cone is about 24 degrees (
Figure 7).
Due to technological limitations for casting the cone-shaped chamber from transparent optical quartz glass, the dimensions of the real combustion chamber have been changed (
Figure 7b). As a result, the large diameter of the cone has almost doubled (258 mm). The dimensions of the input and output channels and the cone opening angle remain unchanged. To control the temperature in the combustion chamber by chromel-aluminum thermocouple, as well as the input and discharge of CLF drops, there are two technological holes with a diameter of 11 mm in its side part. The chamber is made with a total height of 325 mm. The volume of the combustion chamber is 6 L. This allowed expanding the limits of permissible rates of soaring of CLF droplets (for a combination of a single, small group and a flow of droplets).