Thermal Processing of Coal in the Plasma Pulverized-Coal Burner

Introduction

Current trends of industry and economy development dictate their own rules the main of which is the efficient and economical use of fuel and energy resources including for the production of calorific and electric energy. It will enhance the economic growth of a separate enterprise and the industry in general.

The main enterprises producing electrical energy are thermal power stations which are running on solid fossil fuel – coal. These stations are reliable and easy to use. The increasing of the produced electricity final cost is influenced not only by the quality of coal but also by the quantity of boilers kindling. It is important because mazut is used as a start-up fuel. Mazut is also used to flame pulverized coal blend when the blend in the boiler is starting to pulsate for various reasons and the temperature begins to decline sharply so then it can go out.

Apart from mazut inflammation low temperatureand its high calorific value there is a number of disadvantages. It should be mentioned such disadvantages as the need for extra energy consumption to maintain mazut in the liquid state especially in winter; the negative impact of sulfur contained in mazut released in large quantities when it is burned on the main and auxiliary boiler equipments; increase of SOx emissions to the atmosphere. These disadvantages indicate the need for full or partial replacement of mazut to alternative sources of additional energy for highlighting the blend and kindling a boiler.

The Description of the Device and the Principle of its Operation

We propose to use a variant of the plasma-fuel system developed by us as one of the alternative technologies to replace mazut in large power boilers. Let’s consider the main advantages of such systems on the example of the plasma pulverized-coal burner shown in Figures 1 and 2 [1].

The plasma pulverized-coal burner (fig.1) is composed by two concentric and coaxially arranged tubes of different diameters, the inner tube 1 of the first stage and the outer pipe 2 of the second stage with the fixed electric arc plasmatron 4 in the butt end of the inner tube 1. The burner is provided with two separate adjustable air supply ducts: one – for supplying additional air into the inner tube 1 of the first stage, the second – for supplying additional air into the outer tube 2 of the second stage. Wherein the ducts for supplying air and fuel mixture and air are made in the form of tubes 6 and 7 with the tangential entry which arearranged perpendicularly to the concentric tubes: to the inner tube 1 and the outer tube 2. At the exit of the feeders 10 and 11 are installed dampers 18 and 19 for adjusting the air and fuel mixture consumption. The electric arc plasmatron 4 is installed along the horizontal axis of the inner tube 1. The power supply of the electric arc plasmatron 4 is provided from the source 23 on the power cables 24 and 25.

The power supply source 23 is a thyristor three-phase converter (rectifier) of AC to DC. Positive output of the source 23 via a cable 24 is connected with the plasmatron and the output with a minus sign – is connected with the plasmatron cable 25.

The plasma pulverized-coal burner is started as follows. The voltage is supplied from the power supply source 23 through the power cables 24 and 25 to the electric plasmatron 4. As a result in the plasmatron 4 the electric arc plasma is burnt up which unwinds and is blown into the inner tube 1 by air. The plasma torch 5 is formed at the output from the plasmatron 4. The pulverized coal from the feeder 10 is supplied to the first stage to the tube 1. The air and fuel mixture consumption supplied to the first stage is 20-40% of the total air and fuel mixture prepared in the burner. The pulverized coal is injected tangentially twisting spirally in the inner wall of the tube 1 and comes to the plasma torch 5 then is pulled to the outer tube 2. After that the pulverized coal in volume up to 80% from the total consumption through the burner is supplied by the air from the second feeder 11 also tangentially, and then is twisting spirally in the inner wall of the outer tube 2 of the second stage. Reacting with plasma prepared on the first stage air and fuel mixture, coal feed stream of the second stage partially is ignited and gasified. At the output from the pulverized-coal burner when mixing with the secondary air, all the coal feed stream is ignited to form the torch 22.

Power control of the burner is carried out by changing the air and fuel mixture consumption to the first and second stages and by changing the power of the plasmatron. The current control is carried out in the range from 100 to 300 A and the voltage from 200 to 300 V.

Figure 2 shows the operation of the plasma pulverized-coal burner in the mode of pulverized coal blend flame lighting. The prototypes for this burner and served are burners and the results of the investigations shown in the presented studies in detail [2, 3].

The Results of the Investigations and Their Discussion

At the initial stage of designing a complex of calculations is carried out through the programs “Terra” and “Plazmaugol” [4, 5] to determine the expected range of the initial input parameters (temperature, coal and the air consumption, power of the plasmatron and other thermodynamic parameters).

The Tugnuiskiy coal was used as the fuel on the calculations and field trials with the following parameters shown in Table 1

The calculation under the program “Terra” allows to define theoretically required amount of the air. Thus for 100 kg of the coal there should be 130.27 kg of the air (which corresponds to the concentration of the pulverized coal M=0.62). The calculation was made at the temperature range from 700 to 2500 ° C which revealed an effective temperature range which is 1100-1200°C. The further increase of the plasmatron injected power to raise the temperature is ineffective. Because with significant injected power of the plasmatron the temperature of the mixture is increasing slightly. In practice it is observed the outer surface of the coal particles melting which violates the fuel components output.

As a result of theoretical researches under the program “Plasmaugol” it has been determined the plasmatron effective power which is 70-90 kW depending on the fuel consumption. When the initiation of the mixture 30% forthe first stage and the activation with the help of the burning gas obtained for thefirst stage we can prepare the second part of the air and fuel mixture without a direct impact on the plasmatron. As a result of the calculations we obtained carbon oxide come on return from of the plasma pulverized-coal burner which is 612 kg/h, hydrogen – 11 kg/h, the degree of gasification is 65.5%.

Table 1: The coal characteristics from the Tugnuiskiy coal-mining field

The coal organic matter, %
Сc	67,3
Оc	8,9
Нc	3,3
Nc	0,7
Sc	0,4
The coal mineral matter,%
SiO2	11,33
TiO2	0,66
Al2O3	2,27
Fe2O3	2
CaO	1,53
MgO	0,68
K2O	0,48
Na2O	0,48
Wp	14
Aр	19,4
Vг	45
Qрн, kJ/kg	23030

 

As a result of conducted analyses it was implemented the ignition of the coal feed stream for the first and second stages, both together and separately only for the first stage. On return from of the plasma pulverized-coal burner there was emerged a steady-state flame. The tests were conducted outside the boiler unit. The plasmatron with – system automation: coal-, air-, electricity- and water supply system was installed on the burner.

Today our team are well experienced with the development, design and creating of plasma fuel systems for coal and pulverized-coal fired boilers of a different power. So in 2002 there were performed assembly operations and adjustment work at Ulan-Ude Central Heating and Power Plant-2 [6]. The researches on the “The test program of upgraded plasma burners with low emissions of NO_x was carried out on the boiler unit No. 2 of Ulan-Ude Central Heating and Power Plant-2. The boiler unit TPE-185 of E-160-1,4-250kt type with the steam-generating capacity of 160 tons of the steam per an hour and with six flat-plate burners.

The results of measurement tests showed up a direct dependence of the emissions content in the exhaust gases and the amount of the unburned carbon loss that on whether plasma systems are included or not, it can be seen on the diagram (Figure 3). Especially there is represented a dependence of the CO content. The amount decreasing is 33% at an average (Table 2) which means the fuel underburning decreasing.

Reduction of the NO_x emissions was 10.2% at an average. Such result is very significant if we take into consideration that only 25% of the pulverized coal flow through topping plasma burners (2 nozzles from 8 nozzles are operated). However the increase of the oxygen concentration in the exhaust gases increased up to 2.2% at an average thatoccurs probably due to the decrease of the fuel nitrogen formation.The exhaust gases temperature remained at the level of 128-131 °C. When plasma burners were started up the content of fuel remnants was decreasing in the ash from 14.2% to 3.4%, in the slag from 20.9% to 14.8%. After the plasmatrons were switched off in 15 minutes the content of fuel remnants in the ash was 4,8%, in the slag was 7,8%

Figure 1: The circuit schematic of the device and plasma pulverized-coal burner: Click here to View figure

 

1– the first stage; 2 – the second stage; 3 – the fixing arrangement of the plasmatron; 4 – the electric arc plasmatron; 5 – the plasma torch; 6 – the air supply duct of the first stage; 7 – the air and fuel mixture supply duct of the second stage; 8, 9 – the air and fuel mixture movement direction; 10, 11 – pulverized-coal bunkers; 12, 13 – air entrainment lines; 14, 15 – pulverized coal injection lines; 16, 17 – air shutters; 18, 19 – coal dumpers; 20 – air and fuel mixture movement on the first stage 21 – air and fuel mixture movement on the second stage; 22 – pulverized coal blend flame; 23 – power supply source; 24, 25 – power cables.

Figure 2: Plasma pulverized-coal burner in operation Click here to View figure

 

Figure 3: The dynamics of the nitrogen oxides and carbon monoxide concentrations changes in the exhaust gaseswhen tests of upgraded plasma burnerswere carried out   Click here to View figure

 

Table 2: Dynamics of changes in the concentrations of oxygen, nitrogen oxides, carbon monoxide in the exhaust gases and in the content of fuel remnants in the ash and the slag when testing plasma burners.

The content О2, %
	The plasmatron is switched off	The plasmatron is started up	The change, %
1 sample	8	8,1
	7,8	8,2
	8	8,1
	7,93	8,13	-2,52
2 sample	8	7,8
	7,8	8,3
	8	8,2
	7,93	8,1	-2,1
3 sample	8,7	8,2
	8,5	8,4
	8,2	8,3
	8,47	8,3	1,97
The average value 2,31%                                                                         (value 1,97% excluded)
The content СО, мg/м3
	The plasmatron is switched off	The plasmatron is started up	The change, %
1 sample	18	8
	17	10
	19	10
	18	9,33	48,15
2 sample	14	11
	15	11
	17	11
	15,33	11	28,26
3 sample	15	13
	17	13
	18	12
	16,67	12,67	24
The average value 33%
The content NOx, мg/м3
	The plasmatron is switched off	The plasmatron is started up	The change, %
1 sample	656	551
	648	548
	684	571
	662,67	556,67	16
2 sample	606	558
	618	562
	594	572
	606	564	6,93
3 sample	630	562
	618	583
	595	561
	614,33	568,67	7,43
The average value 10,2%
The fuel remnants in the ash and the slag
		In the ash, %	In the slag, %
The plasmatron is switched off	1420	6,5	20,9
The plasmatron is started up	1440	3,4	14,8
The plasmatron is switched off	1505	4,8	7,8
The plasmatron is started up	1530	2,9	17,5
The plasmatron is started up	1620	2,9	17,1

 

Conclusion

During the tests the plasma burner effectiveness is proved for kindling a boiler and inflammation pulverized coal blend flame. Kindling and inflammation were carried out in the standard operating mode but mazut is not supplied to the burner.

As a result of theoretical and experimental investigations of the plasma pulverized-coal burner comparing it with known plasma systems it stands to mention the following advantages of the burner:

Simplified construction (there isn’t a scroll and the flow swirl is achieved by using petals installed inside the tube at the dust flow entrance);

Inside the burner coal plasma-thermal pretreatment is implemented with an adjustment coefficient of the air excess in supplied air and fuel mixture stream which improve the process of the electric chemical fuel preparation and to install this burner to any fuel systems of pulverized-coal fired boilers operated in power plants.

So the process of the electric chemical fuel preparation is divided into two processes: in the inner tube it occurs directly this process itself and in the intertubular space at the beginning it occurs thermal fuel preparation and only then the it occurs mixing of the two flows with followed electric chemical preparation of the main dust flow.

The inner tube production from an ordinary steel makes cheaper a production and the cost of such burner.

The plasma pulverized-coal burner construction allows processes of air supply control, pulverized coal control, temperature control and control of the length of the pulverized coal blend flame in the boiler furnace operate completely automatically.

All of the above-mentioned make it possible to statethat the developed plasma pulverized-coal burner is an effective tool to ignite the pulverized coal blend flame not using mazut and can be used in pulverized-coal fired boilers of a medium and high power.

References

1. Buyantuev S. L.; Shishulkin S.U.; Zonkhoev G.B.;Starinsky I.V.; Khmelev A.B. The plasma pulverized-coal burner. The patent of the RF No. 2543648 as of January 10, 2014 published on March 10, 2015, report .No. 7
2. Buyantuev, S.L.; Zonkhoev, G.B.; Shishulkin, S.Yu.;Starinsky, I.V.The plasma thermal processing and preparation of coals for combustion in the energy and the housing and public utilities. Ulan-Ude: publishing house ESSTU, 2014, . 164
3. Buyantuev, S.L.; Shishulkin, S.Yu. The plasma thermal processing and preparation of coals for combustion. Ulan-Ude: publishing house ESSTU, 2003, 176.
4. Zonkhoev, G.B. Improvement of the fuel use effectiveness and environmental specifications of power plants pulverized-coal fired boilers using modified plasma burner, PhD thesis, ESSTU, Ulan-Ude, 2011.
5. Shishulkin, S.Yu. The plasma thermal pretreatment of solid fuels for burning based on the modular two-stage unit, PhD thesis, ESSTU, Ulan-Ude, 2009.
6. Buyantuev, S.L.; Elisafenko, A.V.; Legostaev, S.M.; others. The using of plasma pulverized-coal burner for pollutant emissions reductions and technical and economic index increasing of a bolier unit such as TPE-185 of the Ulan-Ude Central Heating and Power Plant-2. The energy industry; 2009, 3, 13-15.

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