3. Plasma processing of hazardous waste

First in Ukraine, full-scale equipment for medical waste processing as well as another hazardous waste has been built by the E.O. Paton Electric Welding Institute of the National Academy of Sciences of Ukraine (NASU) and the Institute of Gas, NASU [3, 9]. Its fundamental advantage is using water steam-plasma as a gasification agent, which allows to obtain the gasification products of maximum calorific value. Mode of the equipment operation satisfies all the requirements of the Directive 2000/76/EC [15].

A new PT is employed for heating the material that is injected into the reaction arc chamber. Both average and local heat losses of PT elements are necessary to know when the device is operating under extreme conditions to select operating and cooling regimes. Operating characteristics of the PT plasma flow and parameters were determined from the heat conservation calculations while measuring voltage drop, gas flow rate and arc current strength in the circuit. The preference has been given to the PT with neutral, fixed average arc length and step-formed copper electrode [22]. This enabled to reduce arc shunting after anode step and ensured the stability of length of the arc in the wide diapason of gas flow rates and current variation. The employed plasma source also is different comparing to ordinary plasma torches with the conical expanded anode. The anode step also serves for reduction of the pressure drop in the discharge channel and to fix the arc in the stable position. The total PT length is 0.25 m, the insular part anode diameter is 0.03 m and the diameter of extended part of the anode is 0.04 m. The neutrode makes separate neutral section of the torch and is isolated from the anode. It is located between insulating rings made of thermal resistant glass textolite. Each ring is also used for tangential air supply and contains a pair of tangential-oriented blowholes (as GN, G1 and G3 in Figure 1) for the arc stabilization. The experimental equipment for producing arc plasma is comprised of rectifier for power supply, gas supply, water-cooling

The modified similarity theory has been applied for the analysis of operating and thermal characteristics and result generalization [22–24]. Voltage–current characteristic (VCC) of PT were generalized employing criterial equations and following expressions were established:

�0:<sup>55</sup> <sup>G</sup>

PG performance and thermal characteristics can be evaluated by its efficiency η indicating

d2 �0:<sup>14</sup>

ð Þ pd<sup>2</sup> 0

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ð Þ pd<sup>2</sup> <sup>0</sup> l d <sup>0</sup>

η ¼ GH UI ð Þ: (2)

St: (4)

: (1)

: (3)

2 Gd<sup>2</sup>

Generalization of the TC of PG is similar to generalization of the electric characteristic:

2 Gd<sup>2</sup>

1 � η <sup>η</sup> <sup>¼</sup> <sup>4</sup><sup>l</sup> d2

<sup>0</sup>:<sup>22</sup> <sup>G</sup>

Here U is arc voltage, I is arc current, G is total gas flow rate, d2 is anode diameter and p is

The research concludes that PT VCC depends on the following main factors: (i) radial and tangential injection of plasma-forming gas; (ii) gas flow rate of plasma-forming gas to produce the desired arc; (iii) arc chamber geometry and (iv) gas composition. The first factor was

d2 �0:<sup>12</sup>

systems and airing devices.

Ud<sup>2</sup>

what part of generated energy is transferred to gas:

1 � η

<sup>I</sup> <sup>¼</sup> <sup>1350</sup> <sup>I</sup>

<sup>η</sup> <sup>¼</sup> <sup>5</sup>:5∗10�<sup>3</sup> <sup>I</sup>

pressure. The value of η may be presented also as the Stanton number [23]:

The other type of experimental equipment for destruction of hazardous waste has been installed in Lithuanian Energy Institute (LEI) [11, 14]. It consists of a plasma jet reactor with DC arc plasma source capacity of up to 90 kW. The plasma process uses air, nitrogen, water vapor or their mixtures. The plasma-forming gas flow rate in the reactor reaches up to 2–7 g/s, the average exhaust mass temperature varies from 2800 to 3500 K. Experimental and numerical studies carried out upon the realization of the plasma decomposition process of organic and inorganic substances.

#### 3.1. Plasma sources

Arc plasma torch (PT) is a key element of the equipment. It was made according to the twoelectrode axial scheme with hollow copper electrodes. Compressed air and steam are used as the plasma-forming gases. PT ignition is carried out with air and then transition to steam occurs after the heating [9].

The linear DC arc heater was produced in LEI for heating air, nitrogen, steam or their mixtures up to 7000 K. It was connected to the reactor vessel. By achieving gas temperature over 4000 K, molecules of hazardous substances and waste decay to atoms, radicals, electrons and ions so that it appears ability to obtain simple combination of harmless chemicals. Several configurations of linear DC PT with hot cathode and step-formed anode were considered. As a sample, it could be mentioned PT 70 kW of power, with radial and tangential injection designed especially for the production of non-equilibrium plasma jet. Its analog was described elsewhere [22]. The novel PT (Figure 1) was manufactured and applied for the treatment of hazardous organic and inorganic compounds. It consists of a button type hafnium cathode, transitional copper anode for arc initiation 3, neutrode 5, insulation rings and step-formed copper anode 7. To increase the angular velocity of arc rotation, magnetic stabilization of flow was applied employing the coil 8 [22].

Figure 1. Schematic presentation of linear DC plasma torch. 1—Cathode junction with hafnium emitter; 2, 4, 6—Insulating rings with gas injection; 3—Intermediate anode; 5—Neutrode; 7, 9—Step-formed anode; 8—Magnetic coil.

A new PT is employed for heating the material that is injected into the reaction arc chamber. Both average and local heat losses of PT elements are necessary to know when the device is operating under extreme conditions to select operating and cooling regimes. Operating characteristics of the PT plasma flow and parameters were determined from the heat conservation calculations while measuring voltage drop, gas flow rate and arc current strength in the circuit. The preference has been given to the PT with neutral, fixed average arc length and step-formed copper electrode [22]. This enabled to reduce arc shunting after anode step and ensured the stability of length of the arc in the wide diapason of gas flow rates and current variation. The employed plasma source also is different comparing to ordinary plasma torches with the conical expanded anode. The anode step also serves for reduction of the pressure drop in the discharge channel and to fix the arc in the stable position. The total PT length is 0.25 m, the insular part anode diameter is 0.03 m and the diameter of extended part of the anode is 0.04 m. The neutrode makes separate neutral section of the torch and is isolated from the anode. It is located between insulating rings made of thermal resistant glass textolite. Each ring is also used for tangential air supply and contains a pair of tangential-oriented blowholes (as GN, G1 and G3 in Figure 1) for the arc stabilization. The experimental equipment for producing arc plasma is comprised of rectifier for power supply, gas supply, water-cooling systems and airing devices.

of Sciences of Ukraine (NASU) and the Institute of Gas, NASU [3, 9]. Its fundamental advantage is using water steam-plasma as a gasification agent, which allows to obtain the gasification products of maximum calorific value. Mode of the equipment operation satisfies all the

The other type of experimental equipment for destruction of hazardous waste has been installed in Lithuanian Energy Institute (LEI) [11, 14]. It consists of a plasma jet reactor with DC arc plasma source capacity of up to 90 kW. The plasma process uses air, nitrogen, water vapor or their mixtures. The plasma-forming gas flow rate in the reactor reaches up to 2–7 g/s, the average exhaust mass temperature varies from 2800 to 3500 K. Experimental and numerical studies carried out upon the realization of the plasma decomposition process of organic

Arc plasma torch (PT) is a key element of the equipment. It was made according to the twoelectrode axial scheme with hollow copper electrodes. Compressed air and steam are used as the plasma-forming gases. PT ignition is carried out with air and then transition to steam

The linear DC arc heater was produced in LEI for heating air, nitrogen, steam or their mixtures up to 7000 K. It was connected to the reactor vessel. By achieving gas temperature over 4000 K, molecules of hazardous substances and waste decay to atoms, radicals, electrons and ions so that it appears ability to obtain simple combination of harmless chemicals. Several configurations of linear DC PT with hot cathode and step-formed anode were considered. As a sample, it could be mentioned PT 70 kW of power, with radial and tangential injection designed especially for the production of non-equilibrium plasma jet. Its analog was described elsewhere [22]. The novel PT (Figure 1) was manufactured and applied for the treatment of hazardous organic and inorganic compounds. It consists of a button type hafnium cathode, transitional copper anode for arc initiation 3, neutrode 5, insulation rings and step-formed copper anode 7. To increase the angular velocity of arc rotation, magnetic stabilization of flow was applied

Figure 1. Schematic presentation of linear DC plasma torch. 1—Cathode junction with hafnium emitter; 2, 4, 6—Insulating

rings with gas injection; 3—Intermediate anode; 5—Neutrode; 7, 9—Step-formed anode; 8—Magnetic coil.

requirements of the Directive 2000/76/EC [15].

and inorganic substances.

168 Gasification for Low-grade Feedstock

occurs after the heating [9].

employing the coil 8 [22].

3.1. Plasma sources

The modified similarity theory has been applied for the analysis of operating and thermal characteristics and result generalization [22–24]. Voltage–current characteristic (VCC) of PT were generalized employing criterial equations and following expressions were established:

$$\frac{\text{L}I d\_2}{I} = 1350 \left( \frac{I^2}{\text{G} d\_2} \right)^{-0.55} \left( \frac{\text{G}}{d\_2} \right)^{-0.14} \left( p d\_2 \right)^0. \tag{1}$$

PG performance and thermal characteristics can be evaluated by its efficiency η indicating what part of generated energy is transferred to gas:

$$
\eta = \mathcal{G}H(\mathcal{U}\mathcal{U}).\tag{2}
$$

Generalization of the TC of PG is similar to generalization of the electric characteristic:

$$\frac{1-\eta}{\eta} = 5.5 \ast 10^{-3} \left(\frac{I^2}{Gd\_2}\right)^{0.22} \left(\frac{G}{d\_2}\right)^{-0.12} (pd\_2)^0 \left(\frac{l}{d}\right)^0. \tag{3}$$

Here U is arc voltage, I is arc current, G is total gas flow rate, d2 is anode diameter and p is pressure. The value of η may be presented also as the Stanton number [23]:

$$\frac{1-\eta}{\eta} = \frac{4l}{d\_2} \text{St.}\tag{4}$$

The research concludes that PT VCC depends on the following main factors: (i) radial and tangential injection of plasma-forming gas; (ii) gas flow rate of plasma-forming gas to produce the desired arc; (iii) arc chamber geometry and (iv) gas composition. The first factor was evaluated during the experimental investigation of gas flow rate at the constant and various values of PT. In the present and previous [22] studies when the radial injection is not applied, operating characteristics were observed as decreasing in the current range between 150 and 250 A. This follows as a result of dropping electric field intensity which linearly depends on the arc current. It was also established that voltage drop and electric field intensity linearly decrease with increasing of gas flow rate in the range of 7–<sup>10</sup> <sup>10</sup><sup>3</sup> and 5–<sup>8</sup> <sup>10</sup><sup>3</sup> kg s<sup>1</sup> . When the radial and tangential injection in different locations is used, the arc is strongly turbulized and a possibility to heat up much larger amount of gas in the PT of reduced dimensions is available. Consequently, the voltage drop in such PT increases up to 70% and the possibility for better control of plasma-forming process appears.

When tangential injection of plasma-forming gas is applied inside the PT anode, the character of operating characteristics is slightly dropping or remains as stabile. The impact of gas flow rate, anode diameter and arc current on plasma generated electric characteristics and thermal efficiency for similar PT are described in Refs. [22, 23, 25]. It is important to notice that static PT characteristics may be also slightly rising with increase of arc current strength.

The present measurements over 120 experiments were carried out varying with the help of resistors arc current strength and injected air flow rate G1 and G3. Some geometrical PG characteristics and ranges of experiments carried out are summarized in Table 1.

#### 3.2. Plasma chemical reactors

Technologically, the conversion process is carried out in a flow reactor. It has a metal case and is lined with the layer of fireproof and heat-insulating materials on the inside (Figure 2). PT

> electrical power reaches up to 160 kW with efficiency coefficient = 0.7–0.8. The equipment also includes lock-chamber for the periodic load of the packed medical waste, steam generator, power supply of up to 500 volts and a current up to 350 A, as well as the system for the gas quenching and cleaning. General view of equipment as well as PT is shown in paper [9].

> Figure 2. Schematic presentation of plasma jet reactor for treatment hazardous waste. (a) Stream reactor with: 1—Plasma torch; 2—Plasma torch and feeder connecting section; 3—Window for observation and measurement; 4—Layer of Zr2O3; 5—Cooling section (five units). (b) Construction of thermocouple's junction: 1—Thermocouple; 2—Frame; 3—Layer of

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The lock-chamber for medical wasteload is located in the upper part of the reactor. Unit management does not involve the full loading of the total reactor space with wastes. This is important for gasification products, if they move through a thick layer of raw materials, not to

Table 2 presents the composition of the basic gasification products obtained from the medical waste in the equipment for plasma-steam gasification [9]. In these experiments, organic wastes of such average composition have been studied: 60% of cellulose C6H10O5 + 30% of plastics

Components H2 CH4 CO CO2 C2H4 C2H2 C3H6 H2S H2O Other %. vol. 49.89 1.99 35.25 2.52 3.37 3.92 0.45 0.13 1.92 0.63

be cooled below 1100C [15].

insulating cover; 4—layer of ceramic cover.

based on polyethylene (–CH2–CH2–)n + 10% water.

Table 2. Basic gasification products composition obtained from medical waste.


Table 1. Plasma source technical parameters.

Efficiency of Plasma Gasification Technologies for Hazardous Waste Treatment http://dx.doi.org/10.5772/intechopen.74485 171

evaluated during the experimental investigation of gas flow rate at the constant and various values of PT. In the present and previous [22] studies when the radial injection is not applied, operating characteristics were observed as decreasing in the current range between 150 and 250 A. This follows as a result of dropping electric field intensity which linearly depends on the arc current. It was also established that voltage drop and electric field intensity linearly decrease with increasing of gas flow rate in the range of 7–<sup>10</sup> <sup>10</sup><sup>3</sup> and 5–<sup>8</sup> <sup>10</sup><sup>3</sup> kg s<sup>1</sup>

When the radial and tangential injection in different locations is used, the arc is strongly turbulized and a possibility to heat up much larger amount of gas in the PT of reduced dimensions is available. Consequently, the voltage drop in such PT increases up to 70% and

When tangential injection of plasma-forming gas is applied inside the PT anode, the character of operating characteristics is slightly dropping or remains as stabile. The impact of gas flow rate, anode diameter and arc current on plasma generated electric characteristics and thermal efficiency for similar PT are described in Refs. [22, 23, 25]. It is important to notice that static PT

The present measurements over 120 experiments were carried out varying with the help of resistors arc current strength and injected air flow rate G1 and G3. Some geometrical PG

Technologically, the conversion process is carried out in a flow reactor. It has a metal case and is lined with the layer of fireproof and heat-insulating materials on the inside (Figure 2). PT

Power, P (kW) 33–78 Arc current, U (A) 175–245 Arc voltage, I 160–335

plasma torch 15–23 cathode 1.1–1.53 ignition section 1.08–2.16 neutrode – anode 13.0–19.3

cathode, GN 0.54–1.0 neutrode, G1 – anode, G2 1.85–7.6 Plasma jet average mass temperature (K) 3460–5200

) 0.16–0.18

the possibility for better control of plasma-forming process appears.

3.2. Plasma chemical reactors

170 Gasification for Low-grade Feedstock

Cooling water flow rate, Gv (kg s<sup>1</sup>

Water temperature increment (deg):

Source gas flow rate (kg s<sup>1</sup>

):

Table 1. Plasma source technical parameters.

characteristics may be also slightly rising with increase of arc current strength.

characteristics and ranges of experiments carried out are summarized in Table 1.

.

Figure 2. Schematic presentation of plasma jet reactor for treatment hazardous waste. (a) Stream reactor with: 1—Plasma torch; 2—Plasma torch and feeder connecting section; 3—Window for observation and measurement; 4—Layer of Zr2O3; 5—Cooling section (five units). (b) Construction of thermocouple's junction: 1—Thermocouple; 2—Frame; 3—Layer of insulating cover; 4—layer of ceramic cover.

electrical power reaches up to 160 kW with efficiency coefficient = 0.7–0.8. The equipment also includes lock-chamber for the periodic load of the packed medical waste, steam generator, power supply of up to 500 volts and a current up to 350 A, as well as the system for the gas quenching and cleaning. General view of equipment as well as PT is shown in paper [9].

The lock-chamber for medical wasteload is located in the upper part of the reactor. Unit management does not involve the full loading of the total reactor space with wastes. This is important for gasification products, if they move through a thick layer of raw materials, not to be cooled below 1100C [15].

Table 2 presents the composition of the basic gasification products obtained from the medical waste in the equipment for plasma-steam gasification [9]. In these experiments, organic wastes of such average composition have been studied: 60% of cellulose C6H10O5 + 30% of plastics based on polyethylene (–CH2–CH2–)n + 10% water.


Table 2. Basic gasification products composition obtained from medical waste.

The main physical result of this experimental exploration was a possibility of self-power supply by syngas with gas-diesel engine system taking into account even low efficiency of electricity production ~30%. This fact was verified in Section 4.2 on the ground of thermodynamic calculations.

In general, the previous experience of using this equipment has confirmed the correctness of the basic technical solutions laid down therein. However, it also revealed some shortcomings of individual design solutions. They demand the revision process of further development. In particular, this applies to the high temperature thermal insulation of the reactor [9].

Three different plasma chemical reactors were designed in LEI:


The last-mentioned is under reconstruction.

We have assumed the plasma flow has been characterized as optically thin. The transport coefficients and thermodynamic properties depend only on the temperature and pressure. The plasma flow in the reactor is also characterized with extremely high temperature gradients and recirculating turbulent flow with wall confinement. The flow inside the chamber was separated. Heat transfer characteristics in the entrance region of the reactor in this case of sudden expansion for the region of x/d < 0.4 could be described by the following equations:

$$\mathrm{Nu}\_{\acute{f}d} = 0.006 \mathrm{Re}\_{\acute{f}d}^{0.86}. \tag{5}$$

their composition. The Kyiv wastewater treatment plant (known as Bortnychi station of aeration) processes municipal and industrial sewage and run-off rain water. It accepts 9000 m<sup>3</sup> wastewater per day on an average. At present, 9 million tone of sewage sludge are accumu-

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Centralized wastewater treatment plants in Lithuania produce relatively small amounts of sewage sludge. The annual amount of dry sewage sludge produced in Lithuania is up to 50

The special problem of this waste is heavy metals in its compound [16, 17]. The presence of these pollutants prevents the burial of sewage sludge and substantially limits its use in agriculture and forestry. A similar situation occurs when certain wastes (e.g., industrial, medical, military and sewage sludge) are destroyed in special devices known as incinerators, which leads to the formation of relatively high toxic waste in ash. Toxic residues (ash, slag, sediment of filters and sedimentation tanks) can be easily placed on landfills in case they were first immobilized and converted to non-leachable products. If these residues are heated to a very high temperature, then their main components, including minerals and toxic heavy metals, melt and take on a glassy appearance. This requires temperatures above 1700 K, which are not available in the most incinerators, but are easily achieved in plasma reactors [21]. The system of plasma vitrification of ash produces a chemically stable and mechanically strong substrate. After vitrification, this mineral product looks like a vitreous, similar in structure to basalt lava (even superior to basalt by mechanical strength); its main components are oxides of silicon, aluminum and calcium in the form of chemically inactive compounds that are resistant to washing. The effectiveness of this technology is convincingly confirmed by the data on the

example of vitrification of the ash residue in a medical incinerator, given in Ref. [21].

A simple empirical estimate of the energy inputs required for the vitrification process is given

where M is the mass of the vitrified product and P is the electrical energy consumed in the process. It is quite simple and allows you to calculate the energy required for the gasifier, regardless of the thermodynamic calculations associated with the conversion of carbon-

The equipment for hazardous waste processing created at the Institute of Gas, NASU was presented shortly above. Its fundamental advantage is using of water steam-plasma PT up to 160 kW of capacity. Nevertheless, such powerful and complex equipment cannot be used for laboratory studies to optimize the gasification processes of different types of carboncontaining raw materials. That is why relatively low-power industrial steam PT "Multiplaz

M kg ð Þ¼ 0:35P kWh ð Þ, (8)

lated on its territory [13].

thousand tons per year.

in Ref. [26]:

containing raw materials.

4.1. Laboratory experiment

3500" up to 3.5 kW has been used in this research.

For the region of x/d > 0.4 described by the equation for entrance region of the pipe:

$$\mathbf{Nu\_{fd}} = 0.0256 \mathbf{Re\_{fd}^{0.8} \varepsilon\_{\mathrm{l}}}.\tag{6}$$

Here ε<sup>l</sup> is the entrance factor, equal:

$$
\varepsilon\_l = 1.48(\text{x}/d)^{-0.15}.\tag{7}
$$

Nu and Re are Nusselt and Reynolds criterions, respectively. Index fd means that Nu and Re are calculated according to the flow conditions in the entrance and reactor channel diameter.
