Simulation and experiment of plasma ignition of low-grade coal

A plasma-coal burner is studied utilizing a model of plasma thermochemical preparation of coal for combustion, implemented in the PlasmaKinTherm program. For boiler start-up and coal combustion stabilization, plasma-coal burners do not require fuel oil or gas. The PlasmaKinTherm program combines thermodynamics and kinetics to describe the thermochemical preparation of fuel in the plasma-coal burner volume. The purpose of the simulation was to determine the conditions for plasma ignition of low-grade coal. A numerical study was carried out of the inﬂ uence of the plasmatron power on the ignition of the air mixture (coal + air). High-ash Ekibastuz coal was used in the calculations. The distributions of temperature and velocity of gas and coal particles and concentrations of products of plasma thermochemical preparation of coal for combustion along the length of the burner were calculated. As a result of the analysis of the processes of plasma ignition of coal, their main patterns were revealed, including the shift of the maximum temperatures and velocities of the products of thermochemical preparation of coal for combustion upstream (towards the plasma torch), as well as the fact that the maximum values of temperatures and velocities of the products do not depend on power plasmatron. At the plasmatron power determined by kinetic modeling, experiments were conducted to test and validate the ignition and combustion conditions for a highly reactive two-component fuel torch. The assumptions made during the development of the mathematical model were conﬁ rmed by comparing the calculations with experimental data.


Introduction
German thermal power plants are being restarted and new natural gas processing facilities are being built despite massive investments in green technologies and renewable energy.A growing number of investments are being made in coal power.The European Union's (EU's) transformation eff orts have reached an important milestone with this trend.Despite the best eff orts of the fashion environmental agenda, not everything is going as planned.Sustainable and green technologies have resulted in the expenditure of nearly 1 trillion euros.As a consequence, Berlin is switching openly to coal and gas now that its budget has hit rock bottom [1].
Only about a third of global electricity is generated by coal, which is the most carbon-intensive fossil fuel.While most countries are in the process of phasing out coal, it will remain important for metallurgy and power generation.There will be an increase in coalfi red power generation, as well as an increase in CO 2 emissions from power plants by over 2% per year.By 2030, coal power plants must be reduced by an annual average of 10% if net zero is to be achieved.According to the latest data, coal-fi red power plants continue to produce electricity at record levels for the second year in a row -around 10400 TWh.
According to the International Energy Agency, the amount of coal used to generate electricity in the Asia-Pacifi c region increased by about 3% in 2022 compared to 2021 due to extreme weather conditions and high natural gas prices.The number of coal power plants in China has continued to grow, and the nation's coal capacity in 2022 is expected to exceed its current level by 11GW.The energy sector is adopting this strategy because of energy security constraints, current economic benefi ts, and the tendency to combine modern energy sources with additional variable renewable energy sources.Due to extreme heat, India has increased coal-fi red power production.The coal-fi red power generation market is expected to grow by more than 8.5% by 2022.
During Russia's ongoing special military operation in Ukraine, the European Union and the United https://10.18321/cpc22(1)[27][28][29][30][31][32][33][34][35][36] Kingdom have again begun to use coal as an additional energy source to improve their energy security.The ongoing global energy crisis have forced the EU to take steps to improve energy security while facing a shrinking gas market and limited fuel supplies.In the UK and some EU countries, plans are being discussed regarding a return to full capacity of coal generation or delaying the date of the closure of coal-fi red thermal power plants.Coal-fi red power capacity is expected to grow by about 10 GW in the winters of 2022 and 2023 in Germany.It is estimated that EU and UK coal capacity will increase by 15% (19 GW) to 146 GW as a result of the approved plan [2].
Energy effi ciency improvements have always been driven by the cost-eff ectiveness of power generation and growing concerns about CO 2 emissions.Electricity is generated by directly converting the chemical energy of coal into thermal energy and then directly converting this into electrical energy.Thermal effi ciency has been improved by minimizing heat losses at low temperatures, reducing the irreversibility of the process of converting coal's chemical energy into thermal energy, and maximizing thermal energy production [3].
It is crucial for power generation to develop more effi cient, low-emission coal technologies in order to reduce carbon emissions.In spite of rapid advances in renewable energy and energy storage, fossil fuels will remain a signifi cant component of global energy consumption.A report by the International Energy Agency (IEA) estimates that coal will become the leading source of electricity production by 2040, surpassing all renewables except hydropower.World Energy Outlook, published by the International Energy Agency in late 2016, forecasts the construction of 730 gigawatts (GW) of coal-fi red power plants by 2040, mostly in developing nations.
Worldwide, coal-fi red power plants have an average effi ciency of 33 percent, according to the World Coal Association, which actively supports HELE (High-Effi ciency, Low-Emissions).For modern stations, this fi gure rises to 45%, while for «ready» stations, it is about 40%.By improving coal power plant effi ciency by just 1%, CO 2 emissions can be reduced by 2-3% [4].
Coal-fi red power plants continue to be a reliable and stable source of energy.The use of new innovative technologies at these stations is becoming increasingly important as a result of modern environmental problems.In addition to reducing harmful emissions, they also reduce the impact on the environment, which is hugely benefi cial for human health and the environment.These new technologies are environmentally and economically effi cient, which allows us to prevent environmental problems while also promoting the health and wellbeing of people and animals.In order to create an environmentally friendly and sustainable energy system, investing in this area and developing these technologies are essential.Developing plasma technologies in the energy sector can help overcome the energy crisis.In response to the increasing population growth and its standard of living, plasma technology has become increasingly popular and relevant for destroying solid and liquid wastes containing carbon, including toxic healthcare wastes [5][6][7][8].Thermal power plants can be repowered using plasma-assisted combustion to increase combustion effi ciency, decrease emissions, and, most importantly, limit unburned fuel [9].
The main goal of coal energy development is to develop new technologies to improve the effi ciency of solid fuel use, both environmentally and economically.It is one of the innovative technologies in this fi eld to utilize plasma ignition, stabilize combustion, and burn diff erent types of coal with plasma-coal burners (PCBs).Traditionally, fuel oil and gas have been used in fl aring and fi re-up boilers to stabilize the pulverized coal combustion.The new system eliminates the use of fuel oil and gas altogether.In plasma ignition and combustion stabilization (PICS), an air mixture containing coal dust and air is heated to the temperature necessary for coal volatiles to be released and coke residue to partially gasify.As a result, in the PCB coal is converted into a high reactive two-component fuel (HRTF) containing combustible gas and coke residue.With eff ective mixing with air, HRTF ignites and burns sustainably in boiler furnaces without further highly reactive fuel.Fig. 1 illustrates the PCB diagram.
PCB design involves using mathematical and physical models of thermochemical processes involving oxidizers and fuels, as well as heat and mass transfer.Using PICS technology in PCBs has been studied numerically and experimentally to improve environmental and energy effi ciency.A comparison is made between numerical simulations and experimental data using thermodynamic and kinetic models [10][11][12].

Kinetic modeling
The purpose of the simulation is to determine the conditions for plasma ignition of low-grade coal.The criterion for stable ignition and combustion of the fuel mixture at the outlet of the PCB is a high content of combustible gases (CO and H 2 ), above 15%, and gas heating to a self-ignition temperature (>800 о С) [13].In PlasmaKinTherm, which was developed using TERRA and Plasma-Coal softwares, a thermodynamic approach is combined with the calculation of thermochemical transformations of primary coal degradation products in the gas phase to determine oxidation of coke residue carbon and volatile release kinetics.A plasma source is used to simulate the fl ow of coal and air particles in a twophase reaction channel.Particles and gas are evenly mixed as they enter the PCB (Fig. 1).Plasma fl ame heats the gas, which heats the particles.As the particles heat up, the kinetic mechanism begins to release volatile substances into the gas phase.Coal particles and ash (mineral mass) are assumed to be isothermal and inert components, respectively, in the model based on one-dimensionality and quasistationarity.The particles are also assumed not to interact with one another.When pulverized coal fuel interacts or mixes with plasma, it is heated by a fl ow of high-temperature gas.In the model, organic and mineral masses make up coal's composition.Coal's organic mass is made up of carbon and volatile substances (CH 4 , C 6 H 6 , CO, H 2 , H 2 O, CO 2 ).The kinetics of coal particle heating determines the process of volatile release.When volatile substances are released into the gas phase, a high-temperature gas fl ow reacts with them.An equilibrium model of local thermodynamics is implemented in TERRA for calculating such reactions in the gas phase.The thermodynamic method was used to calculate hightemperature thermochemical transformations in gas phase of primary coal degradation products based on the oxidation of the coke residue and the kinetics of volatile release.
In the numerical simulation of the PICS process, Ekibastuz coal was used in a straight-fl ow cylindrical PCB (Table 1).With its 40% ash content, 24% volatile yield, and 5.8% moisture content, Ekibastuz hard coal has a specifi c heat of combustion of 4000 kcal/kg.According to Table 1, coal has the following chemical composition.Based on the data in Table 2, a model composition for Ekibastuz coal was determined, wt.%: ash content -40, C -46.18, H 2 -2.63,H 2 O -1.84, CO -3.95, CO 2 -1.4,CH 4 -0.55,C 6 H 6 -3.45.
Coal dust consumption was 1000 kg per hour, while air consumption was 1429 kg/h, which implies a dust concentration of 0.7 kg/kg in the air mixture.The concentration of dust in the air mixture required to obtain a HRTF torch in work [13] was found to be suffi cient for Ekibastuz coal.An initial temperature of 27 о С is applied to the air mixture and the inner wall of the PCB, which is 3 m in length and 0.15 m in diameter.Calculations were performed for PCB plasmatron power of 40, 60, and 80 kW to determine PICS of coal.In Table 2, coal dust fractional composition is determined by sieve analysis.60 microns was the average diameter of coal particles.Figure 2-6 illustrate the results of the kinetic calculations.In Fig. 2, the gas phase compositions for 40, 60, and 80 kW plasmatron powers are compared along the length of the PCB.The process of ignition occurs for all plasmatron powers (Fig. 2(a), 2(b), and 2(c)).For all plasmatron powers, the curves of the products of plasma thermochemical preparation of coal (PTPC) for combustion are almost the same, but the contents are diff erent.A crucial indicator is the concentration of fl ammable components (CO+H 2 ).Small-scale extremes of the concentration curves of fl ammable gas (CO+H 2 ) were observed at the beginning of the channel (0.1-0.4 m): 1.4, 1.4, and 1.6%, respectively, for 40, 60, and 80 kW.At plasmatron powers of 40, 60, and 80 kW, the concentration of fl ammable components increases with increasing temperature, reaching 18.6, 29.4, and 41.7% at the exit from the PCB, respectively.These concentration values of fl ammable components signifi cantly exceed the level of their concentration required for stable ignition of the air mixture at the PCB outlet (>15%).The content of harmful gases is important from the viewpoint of the environment.Figure 2 shows a peak in CO 2 gas content at the half length of the PCB at 40, 60, and 80 kW power levels.Compared to traditional burners, these results are much more environmentally friendly.
Temperature curves in Fig. 3(a)-3(c) are qualitatively similar.The temperature of the gas and coal particles increases along the length of the PCB.For example, for power 60 kW the gas temperature reaches a maximum (Tg=1849 о С) at a length of 1.38 m, but coal particle temperature does not reach its maximum along the entire length of the fl ow part (X=3 m), but the fi ne fraction of 10 μm in size reaches a maximum temperature (1415 о С) at a length of 2.06 m.Except for fractions 1 and 2, the temperature of the gas in the PCB is higher than the temperature of all coal fractions.Temperature diff erences between gas and particle temperatures at exit from the PCB increase with increasing size of coal fractions (Table 2).Carbon oxidation reaction of large coal particles results in reduced heat release due to the slower heating of large particles compared to small ones.In the exiting PCB, the temperature of particle fractions 1 and 2 exceeded the gas temperature by 11 and 13 degrees, respectively, due to the carbon oxidation reaction on the coal surface.Converging trends in gas and particle temperatures indicate that gas and particles tend to thermal equilibrium at the PCB outlet (3 m).In general, from Fig. 3  a plasmatrоn power of 40, 60 or 80 kW is suffi cient to heat and ignite particles, since at the output of the PCB the gas temperature is higher than the ignition temperature (800 о С).
According to Fig. 4, the gas temperature is suffi cient to ignite the coal particles for all calculated variants.As the plasma torch power increases, the gas temperature shifts toward the plasma source at the beginning of the PCB channel (X=0 m).The range of maximum gas temperatures is quite narrow and lies between 1409 and 1849 о С.
Velocity curves in Figures 5(a)-5(c) are qualitatively similar.Over the length of the PCB, coal particles and gas and move at an increasing speed.For example, at plasmatron power of 60 kW, gas and coal particles are accelerated along its length of the PCB from 13 m/s to their maximum velocity (Fig. 5(b)).The maximum gas velocity is 84.3 m/s over a channel length of 1.44 m, decreasing to 62.1 m/s at the exit of the PCB.The velocity curves of coal particles accelerated by gas do not have an extremum at the length of the PCB considered.In fraction 1, coal particles reach 75 m/s at a 1.7 m PCB length.At the PCB output, coal particles of other fractions have maximum velocities of 72.7, 72.7, 64.6, and 58.9 m/s (curves 2-5).Gas and coal particle velocities at the exit of the PCB are signifi cantly higher than air mixture velocities at the exit of conventional coal burners (usually 20-30 m/s).Converging trends in gas and particle velocities indicate that gas and particles tend to dynamic equilibrium at the PCB outlet (3 m).
There is an extremum in the gas velocity curves at plasmatron powers of 40-80 kW (curves 1-3) (Fig. 5(a)-5(c)).In response to an increase in plasmatron power, the gas velocity shifts towards the plasma source as its maximum velocity increases (Fig. 6).Note that the maximum gas velocity varies between 64.9 and 84.28 m/s.Plasmatron powers of 40, 60, and 80 kW resulted in maximum gas velocity at lengths of 1.74, 1.64, and 1.44 m.Gas velocity followed the same sequence as temperatures at the burner exit.
At 40 and 60 kW, coal fraction velocities at the PCB exit exceed 50 m/s, which is signifi cantly faster than air mixture speeds during traditional solid fuel combustion.Plasmatron power fractions exceed 15 m/s throughout the PCB, eliminating coal particle separation during horizontal dust pipeline transportation.During traditional solid fuel  2. combustion, the coal fraction exits the PCB at a speed greater than 50 m/s for all plasmatron powers.In the horizontal PCB, particle velocities are higher than 15 m/s throughout, preventing particle separation.A signifi cant increase in the speed of the HRTF at the output of the PCB, compared to the speed of the air mixture at the exit of the burner during traditional combustion of solid fuel, increases the range of the HRTF torch.
Using kinetic modeling results, experiments were conducted on the PICS process in PCB.

Experiment
PICS experiments were conducted to test and confi rm the conditions for stable ignition and combustion of the HRTF torch at PCB output, determined by kinetic modeling, and to compare calculations and experimental results.Experiments were performed on a PCB (Fig. 7).The plasmatron 3 was placed at the beginning of the PCB (the cylinder-lined section 2).Coal dust from hopper 4 is mixed with air from centrifugal fan 6 to create an air mixture.The fan provides an air fl ow of 1430 kg/h and, accordingly, an initial speed of the air mixture in the PCB of 15 m/s.In the PCB, the air mixture interacts with the fl ame from the plasmatron 3. Plasma fl ames  are created by blowing air between electrodes and heating it to the plasma state.Plasmatrons produce fl ames with temperatures exceeding 5000 о С [13].PCB with a diameter of 0.15 m and a length of 3 m was used for the experiments.The electrical power of the plasmatron was 70 kW.The effi ciency of the plasmatron was 85%.Table 1 shows the coal used in the experiments (Ekibastuz coal).By sieve analysis, coal dust fractional composition is shown in Table 2.An air mixture temperature of 27 о С resulted in coal and air consumption of 1000 and 1430 kg per hour, respectively.After commissioning, two experiments on PICS were carried out.It was the primary objective of the experiments to ignite the air mixture with plasma and to obtain a stable HRTF torch at the PCB outlet, as well as to determine the composition, the temperature, and the degree of carbon gasifi cation of the HRTF torch.An air mixture with 0.7 kg/kg dust concentration was established and maintained in the experiments.As a result of these conditions, an HRTF torch was obtained, which self-ignited and burned steadily at the PCB outlet (Fig. 8).As a general rule, it is more diffi cult to ignite and burn a HRTF torch in the open air than in the furnace volume of the boiler with an organized secondary air supply.HRTF torch combustion in the open air is more challenging.4-5 m was the length of the HRTF torch.Digital pyrometer Ircon Ultrimax Plus UX10P was used to measure the torch's temperature, which ranged from 600 to 3000 о C. Taking into account the temperature range in which the measurement is made is important when determining the measurement error.As a result, measuring temperatures below 1500 о С may result in an error of up to 0.5% of the measured value, higher temperatures may result in an error of up to 1%, and temperatures over 2000 о С may result in an error of as much as 2% of the measured value.One degree is the minimum temperature resolution.Even in the case of ignition of high-ash Ekibastuz coal, the HRTF torch's temperature varied between 1150 and 1180 о С, ensuring stable ignition and combustion when mixed with air oxygen.With the help of a specialized heat-resistant probe, gas samples were collected for analysis.Figure 9 illustrates the sampling scheme.Gas samples were taken using the following steps.When the centrifugal fan 2 is turned on, coal dust is supplied from the hopper 3 and an air mixture is formed, ignited by the plasma fl ame.A stainless steel probe with a diameter of 5mm was attached to a section of the PCB in the volume of the HRTF torch.By using a gas extraction syringe 6, a high-temperature fi lter 4 and three-way valve 5 were used to collect the required amount of gas.The fi lter is made of replaceable superfi ne basalt fi ber.The Chromatek-Gazochrom-2000 gas chromatograph was connected to the syringe containing the sample in order to determine its composition.
There is no slag on PCB inner surfaces, which is the main advantage of this technology.The coal dust concentration in the air mixture is kept at 0.7 kg/kg, preventing solid fuel combustion in PCB and limiting the PICS process temperature below the coal ash softening temperature (here, Ekibastuz coal).Ash from Ekibastuz coal softens at t 3 =1250 о С.As a result of this process, the solid residue was passed through a high-temperature fi lter with a replaceable fi ller 4 to determine the residual carbon content (Fig. 9).An absorption gravimetric method was used to determine the carbon content of condensed PICS products.

Experimental and canculation results comparison
A comparison of the experimental results and calculations of PICS is presented in Table 3. Experimental and numerical studies are compared based on identical initial data (plasmatron power, coal and air consumption, PCB geometric parameters and dust concentration in the air mixture).According to the Table 3, the HRTF temperature varies within a narrow range of 1170-1180 о С and diff ers by no more than 1% from the calculated temperature.There was little diff erence between the calculated value (41.7%) and the actual value (32.1% to 37.1%) of combustible components (CO + H 2 ).There was a variation between 53.8 and 60.5% in the degree of carbon gasifi cation, which diff ered only by 1% from the calculated value.The discrepancy between experimental and numerical values of key PICS parameters is not greater than 24%.In multicomponent heterogeneous plasma systems, such a discrepancy between experimental and calculated data is to be expected given the complexity of plasma two-phase PICS processes.In addition, PlasmaKinTherm is suitable for engineering calculations of PCB, as it adequately refl ects the basic laws of PICS.

Conclusion
In order to calculate plasma-coal burners and describe the patterns of plasma fl ame interaction with the air mixture under various initial conditions of the process, the PlasmaKinTherm program and the plasma thermochemical preparation model for combustion of pulverized coal fuel are developed.This model describes the PICS process along a PCB by tracking temperature distributions, velocity distributions, and concentrations of PICS products based on thermodynamic and kinetic approaches.
In this work, the operating parameters of the PCB were numerically studied, the distribution of temperature and velocity of coal particles and gas, and the concentrations of PCB products along its length.When using a plasmatron power in the range of 40-80 kW, stable ignition of high-ash coal is ensured at an air mixture fl ow rate of 2430 kg/h.At the output of the PCB, high temperatures (>1050 о С), velocities (>50 m/s) and concentrations of fl ammable components (>18.6%) are observed, suffi cient for stable ignition of the HRTF.
When plasmatron power is increased, the temperatures and velocities maxima of PTPC products shift upstream (towards the plasma source).

Fig. 2 .
Fig. 2. Change in the composition of the gas phase at a plasmatron power of 40 kW (a), 60 kW (b) and 80 kW (c) along the length of the PCB.

Fig. 3 .
Fig. 3. Change in the temperature of coal fractions at a plasmatron power of 40 (a), 60 (b) and 80 kW (c) along the length of the PCB: 1-5 -size fractions of coal particles according to Table2.

Fig. 6 .
Fig. 6.Change in gas phase velocity at a plasmatron power of 40 kW (a), 60 kW (b) and 80 kW (c) along the length of the PCB.

Table 1 .
Composition and technical characteristics of Ekibastuz coal, in mass percentages.

Table 3 .
PCB parameter comparison between experimental and numerical valuesAt this a narrow range of maximum temperatures and velocities was observed, and the plasmatron's power had little eff ect on them.PICS experiments were conducted to test and confi rm the conditions for stable ignition and combustion of the HRTF torch at PCB output, determined by kinetic modeling, and to compare calculations and experimental results.Analysis of comparison of calculation results with experimental data confi rms that the assumptions made during the development of the model refl ect reality, and the PlasmaKinTherm program can be used for the calculation and design of plasma-coal burners.