4. Plasma application in sewage sludge treatment

During sewage treatment, the main pollutants are separated as sewage sludge. Depending on the original pollution load of the water being treated, they may include the heavy metals in 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 accumulated on its territory [13].

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 thousand tons per year.

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 in Ref. [26]:

$$\mathbf{M(kg)} = \mathbf{0.35P(kWh)},\tag{8}$$

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 carboncontaining raw materials.

#### 4.1. Laboratory experiment

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 thermody-

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

• curved stream reactor for the treatment of gaseous, liquid and solid substances with small

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:

Nufd <sup>¼</sup> <sup>0</sup>:006Re<sup>0</sup>:<sup>86</sup>

Nufd <sup>¼</sup> <sup>0</sup>:0256Re<sup>0</sup>:<sup>8</sup>

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.

During sewage treatment, the main pollutants are separated as sewage sludge. Depending on the original pollution load of the water being treated, they may include the heavy metals in

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

4. Plasma application in sewage sludge treatment

fd : (5)

fd εl: (6)

<sup>ε</sup><sup>l</sup> <sup>¼</sup> <sup>1</sup>:48ð Þ <sup>x</sup>=<sup>d</sup> �0:<sup>15</sup>: (7)

particular, this applies to the high temperature thermal insulation of the reactor [9].

• steady ARC volume reactor, devoted for incineration of wide range of waste.

Three different plasma chemical reactors were designed in LEI:

• straight stream reactor for flue gas treatment;

solid dispersed particles and

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

The last-mentioned is under reconstruction.

namic calculations.

172 Gasification for Low-grade Feedstock

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


Table 3. Basic gasification products composition obtained from sewage sludge.

Quartz tube of inner diameter 3.2 cm and a length of 13 cm was used as a reactor model. It placed a portion of sewage sludge to be studied in the process of gasification. Aggregate data on the composition of treated dry products of gasification are presented in Table 3 [13].

With these data, an equation for the reaction involving carbon, hydrogen, oxygen and organic matter was determined:

$$2\,\mathrm{CH\_{2.483}O\_{0.530}} + 1.334\,H\_2O = 2.549H\_2 + 0.111CO + 0.876CO\_2 + 0.013CH\_4.\tag{9}$$

Gross equation of sewage sludge in this reaction correlates well with the results of independent chemical study in Ukraine for their composition.

Analyzing the results of this experiment, it should be noted its main disadvantage associated with the overall low efficiency of the gasification process, despite even a relatively high yield of hydrogen. Indeed, most of the carbon in process (9) is directed to the production of a ballast gas CO2, rather than a combustible CO. Thus, this experiment cannot be considered as too successful in terms of achieving the ultimate goal of the process – high energy efficiency.

The main reason for this result appears to be the low wall temperature of the reactor-quartz tube, which in these studies was 430–480�C. The two processes seem to contribute for the syngas production: the actual steam-plasma gasification of the raw material on the tube axis, where the temperature determined by the PT jet is quite high, and the so-called water gas shift reaction at the walls of the tube.

$$\rm{CO} + \rm{H}\_{2}\rm{O} \rightarrow \rm{H}\_{2} + \rm{CO}\_{2}.\tag{10}$$

4.2. Thermodynamic calculation of the gasification process using plasma technologies

At present, quite a lot of software tools have been developed and used for quantitative analysis of gasification processes. However, with all the advantages of numerical calculations, such publications leave "in shadow" basic physical and chemical regularities. Just the knowledge of their characteristics built a clear understanding of the analyzed process. In reality, the basis of the quantitative description of gasification lie very simple thermodynamic relations arising from the laws of Hess used in thermochemistry [28]. It should be borne in mind only the

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Following Refs. [29–31], the process of plasma-steam gasification can be represented by the

where QТ<sup>R</sup> = QR + ΔQ is the total thermal energy that is released as a result of the chemical reactions QR and due to some additional source of heat energy ΔQ (so far we do not necessarily associate it with the energy of the plasma jet QРL), so that the reaction mixture reaches the desired temperature ТР of the gasification products, w and m—the amount of water and oxygen, per 1 kmol of waste, n1, n2, n3, n4, n5 and n6 are the coefficients for the corresponding reaction products. Among the latter are gases, most often obtained in the composition of gasification products and soot. In this formula, the energy term in the form presented was introduced in our paper [29]. It allows to distinguish the role of an additional source of energy ΔQ in viewpoint of achieving the optimal, predictably perceived, temperature TP of the

The "ideal" process of plasma-steam gasification would correspond to the case when only H2 and CO would be present on the right side. Formally, it is possible to make many options of reaction (11) with various stoichiometric coefficients, including the relevant "ideal" process. However, in accordance with the second law of thermodynamics, nature chooses only such a path and the completion of the varying reactions, in which the principle of maximum entropy

Special software—"TERRA" thermodynamic calculations system is used for the conversion processes quantitative analysis with a glance of the accompanying reactions [32]. It also allows

Analysis of the process of plasma-steam gasification was made on a more optimal than (9)

to determine the necessary amount of energy expenditure for carrying out reactions.

dS ≥ dQ=T: (12)

CHxOy þ wH2O þ mO<sup>2</sup> ¼ n1H<sup>2</sup> þ n2CO þ n3CO<sup>2</sup> þ n4H2O þ n5CH<sup>4</sup> þ n6C þ QTR, (11)

4.2.1. Generalized reaction of gasification

gross equation in a sufficiently general form:

gasification process.

4.2.2. Plasma-steam gasification

is realized:

reaction:

features associated with the operation of the plasma source [29].

The optimal temperature for this reaction is just about 500�C [27]. This assumption is also supported by the very high content of CO2 in the reaction products in a small diameter quartz tube (Table 3), if compared with our experimental data in the full-scale reactor presented in Table 2.

Equally important and negative factor was also the low reaction rate of carbon in such a system, which exponentially depends on the temperature. As a result, a significant part of steam as gasifying agent passes a small reactor, not reacting with the raw material, which in general predetermines the low energy efficiency of the process.

Already in appearance of the gross equation, it follows that sewage sludge should have good energy characteristics, based on the ratio of the hydrogen and oxygen components in its composition [27]. In the further basic thermodynamic estimates, we selected a simple and convenient for estimation the gross sewage sludge equation in the form of CH2.5O0.5 for which an analysis of the processes of plasma-steam gasification is performed later.

#### 4.2. Thermodynamic calculation of the gasification process using plasma technologies

#### 4.2.1. Generalized reaction of gasification

Quartz tube of inner diameter 3.2 cm and a length of 13 cm was used as a reactor model. It placed a portion of sewage sludge to be studied in the process of gasification. Aggregate data on the composition of treated dry products of gasification are presented in Table 3 [13].

Components H2 CO CO2 CH4 %. vol. 71.8 0.1 24.7 0.4

With these data, an equation for the reaction involving carbon, hydrogen, oxygen and organic

Gross equation of sewage sludge in this reaction correlates well with the results of indepen-

Analyzing the results of this experiment, it should be noted its main disadvantage associated with the overall low efficiency of the gasification process, despite even a relatively high yield of hydrogen. Indeed, most of the carbon in process (9) is directed to the production of a ballast gas CO2, rather than a combustible CO. Thus, this experiment cannot be considered as too successful in terms of achieving the ultimate goal of the process – high energy efficiency.

The main reason for this result appears to be the low wall temperature of the reactor-quartz tube, which in these studies was 430–480�C. The two processes seem to contribute for the syngas production: the actual steam-plasma gasification of the raw material on the tube axis, where the temperature determined by the PT jet is quite high, and the so-called water gas shift

The optimal temperature for this reaction is just about 500�C [27]. This assumption is also supported by the very high content of CO2 in the reaction products in a small diameter quartz tube (Table 3), if compared with our experimental data in the full-scale reactor presented in

Equally important and negative factor was also the low reaction rate of carbon in such a system, which exponentially depends on the temperature. As a result, a significant part of steam as gasifying agent passes a small reactor, not reacting with the raw material, which in

Already in appearance of the gross equation, it follows that sewage sludge should have good energy characteristics, based on the ratio of the hydrogen and oxygen components in its composition [27]. In the further basic thermodynamic estimates, we selected a simple and convenient for estimation the gross sewage sludge equation in the form of CH2.5O0.5 for which

general predetermines the low energy efficiency of the process.

an analysis of the processes of plasma-steam gasification is performed later.

СО þ Н2О ! Н<sup>2</sup> þ СO2: (10)

СH<sup>2</sup>:<sup>483</sup>O<sup>0</sup>:<sup>530</sup> þ 1:334 H2O ¼ 2:549H<sup>2</sup> þ 0:111CO þ 0:876CO<sup>2</sup> þ 0:013CH4: (9)

matter was determined:

174 Gasification for Low-grade Feedstock

reaction at the walls of the tube.

Table 2.

dent chemical study in Ukraine for their composition.

Table 3. Basic gasification products composition obtained from sewage sludge.

At present, quite a lot of software tools have been developed and used for quantitative analysis of gasification processes. However, with all the advantages of numerical calculations, such publications leave "in shadow" basic physical and chemical regularities. Just the knowledge of their characteristics built a clear understanding of the analyzed process. In reality, the basis of the quantitative description of gasification lie very simple thermodynamic relations arising from the laws of Hess used in thermochemistry [28]. It should be borne in mind only the features associated with the operation of the plasma source [29].

Following Refs. [29–31], the process of plasma-steam gasification can be represented by the gross equation in a sufficiently general form:

$$\mathrm{CH\_xO\_y} + \mathrm{wH\_2O} + \mathrm{mO\_2} = \mathrm{n\_1H\_2} + \mathrm{n\_2CO} + \mathrm{n\_3CO\_2} + \mathrm{n\_4H\_2O} + \mathrm{n\_5CH\_4} + \mathrm{n\_6C} + \mathrm{Q\_{TR}} \tag{11}$$

where QТ<sup>R</sup> = QR + ΔQ is the total thermal energy that is released as a result of the chemical reactions QR and due to some additional source of heat energy ΔQ (so far we do not necessarily associate it with the energy of the plasma jet QРL), so that the reaction mixture reaches the desired temperature ТР of the gasification products, w and m—the amount of water and oxygen, per 1 kmol of waste, n1, n2, n3, n4, n5 and n6 are the coefficients for the corresponding reaction products. Among the latter are gases, most often obtained in the composition of gasification products and soot. In this formula, the energy term in the form presented was introduced in our paper [29]. It allows to distinguish the role of an additional source of energy ΔQ in viewpoint of achieving the optimal, predictably perceived, temperature TP of the gasification process.

The "ideal" process of plasma-steam gasification would correspond to the case when only H2 and CO would be present on the right side. Formally, it is possible to make many options of reaction (11) with various stoichiometric coefficients, including the relevant "ideal" process. However, in accordance with the second law of thermodynamics, nature chooses only such a path and the completion of the varying reactions, in which the principle of maximum entropy is realized:

$$\mathbf{dS} \succeq \mathbf{d} \mathbf{Q}/T. \tag{12}$$

Special software—"TERRA" thermodynamic calculations system is used for the conversion processes quantitative analysis with a glance of the accompanying reactions [32]. It also allows to determine the necessary amount of energy expenditure for carrying out reactions.

#### 4.2.2. Plasma-steam gasification

Analysis of the process of plasma-steam gasification was made on a more optimal than (9) reaction:

$$\text{CH}\_{2.5}\text{O}\_{0.5} + 0.5\text{H}\_2\text{O} = \text{CO} + 1.75\text{H}\_2 \,\, + \text{Q}\_{\text{TR}}.\tag{13}$$

of reagents, in contrast to (13), contained also oxygen, and among the reaction products syngas

where K, L, M are coefficients that determine the content of components such as steam and oxygen, as well as the hydrogen one in the reaction products, respectively, under the stoichiometric reaction with respect to syngas production. Thus this reaction is stoichiometric as well as (13) for obtaining products of gasification as synthesis gas only. Nevertheless it has the most wide functional possibilities to achieve the best index of energy efficiency of the process as it allows varying the composition of the gasification agent. In determining the energy efficiency, naturally, the consumption of energy for oxygen production should also be taken into account. The range of possible specific energy consumption in the technological process of obtaining the oxygen itself is

one is realistic today. Quantitative index of energy efficiency of the conversion process is the ratio

PL þ РO<sup>2</sup>

of ~ 0.8 and for oxygen – РO2. WSG is the heat energy of syngas from 1 kg of the original raw mixture. In this form, it fully corresponds to the definition of energy saving (or energy efficiency) as energy costs (here, РPL<sup>J</sup> + РO2) per unit productivity (here the product is syngas of

The value of L = 0 in reaction (16) corresponds to the plasma-steam gasification (13), and the case ΔQ = 0 is usual steam-oxygen technology, although their opposition does not make sense. Indeed, from the point of view of the process chemistry, in both cases, oxygen atoms, characteristic of these technologies, and hydrogen atoms, originally included in the gasified sewage sludge, are present in the reaction. For the noted limit values of L, the coefficient K takes the values Kmax = 0.5 and Kmin = 0, respectively. However, generally speaking, the reactions (16) can also correspond to the intermediate values of the coefficients K and L. Simple functions are

For clarity, the function (18), which characterizes the oxygen content of L as a function of the amount of steam K introduced by the PT, is shown in Figure 3 as line 1. Line 4 represents the thermal power introduced into the reactor by a plasma jet at its nominal enthalpy of

PL = ΔQ/0.8 is the electricity consumption for the production of plasma jet by efficiency

<sup>η</sup> <sup>¼</sup> <sup>Р</sup><sup>С</sup>

determined on the basis of mass balances in reaction (16):

HPL = 3.6 kW�h/kg in accordance with equation

CH<sup>2</sup>:<sup>5</sup>O<sup>0</sup>:<sup>5</sup> þ KH2O þ LO<sup>2</sup> ! CO þ MH<sup>2</sup> þ QTR (16)

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. The first one corresponds to promising technologies, the second

1∙0:5 þ K þ 2L ¼ 1, or L ¼ 0:25 � 0:5K; (18)

1∙2:5 þ 2K ¼ 2M, or M ¼ 1:25 þ K: (19)

=WSG, (17)

components only were present:

chosen as PO2 = 0.35–1 kWh/m3

where Р<sup>С</sup>

energy WSG).

for oxygen

for hydrogen

The heat of combustion of sewage sludge QLSS required to determine the energy of the process is determined on the basis of Mendeleev's Eq. [27]:

$$Q\_{l\text{SS}} = -100 \cdot (81 \cdot \text{c}\_{\text{C}} + 246 \cdot \text{c}\_{H} - 26 \cdot (\text{c}\_{\text{O}} - \text{c}\_{\text{S}}) - 6 \cdot \text{c}\_{W}) \cdot 4.19 \text{ kJ/kg} \tag{14}$$

where сС, сH, cO, cS and cW are mass fractions of carbon, hydrogen, oxygen, sulfur and water. (Mendeleev's equation is an analog of the relations known in the Western scientific literature as Dulong or Milne equation.) As may be shown, this heat of combustion of sewage sludge is QLSS = �25.68 MJ/kg [33]. Following the law of Hess, its enthalpy of formation is ΔH<sup>0</sup> CH2.5O0.5 = �76.8 kJ/kg [33].

The thermodynamic analysis of the sewage sludge conversion process carried out in the TERRA software [32] allows to determine the composition of its gasification products as a function of temperature. As it turned out, both for the reaction (12) and for other considered reactions, it is characterized by the practical completion of the gasification processes at 1250 K. More strictly, the mass fraction of the traces of CO2, H2O and CH4 among the products of gasification at this temperature does not exceed 1–2%. As it turned out, the energy QРL(T), which must be additionally introduced with a plasma torch per 1 kg of reagent mass in (12), to reach this temperature, is 0.785 kWh/kg. This parameter allows to determine the productivity of the gasifier at a given power of the PT.

Knowing the calorific values for CO and H2, as well as the composition of the products obtained in the reaction (12), it is easy to determine the calorific value of the resulting syngas in this process WSG = 6.23 kWh/kg. It allows to define the energy output of the gasification plant and its energy efficiency on the basis of a comparison with the specific energy QР<sup>L</sup> introduced into the reactor.

The value WSG significantly exceeds the electricity consumption 0.785 kWh/kg by steam PT to produce 1 kg of syngas. Thus, even taking into account the relatively low efficiency of ηЕЕ ~ 0.3 of electricity generation, the energy consumption is much lower than the level of energy of syngas produced. Indeed, taking into account also the efficiency of the PT at ηPL = 0.8, this is enough to ensure the operation of the PT, since it exceeds the value of ΔQ = 0.785 kWh/kg:

$$W\_{\rm SG} \cdot \eta\_{\rm EE} \cdot \eta\_{\rm PL} = 6.23 \times 0.3 \times 0.8 = 1.5 \text{ kW} \text{h}/\text{kg} > 0.785 \text{ kWh/kg}. \tag{15}$$

It is good preconditions for the energy self-sufficiency of the sewage sludge processing and the production of additional energy to compensate the role of raw materials moisture and ash residue vitrification or for the production of electricity for external consumers.

#### 4.2.3. Plasma-steam-oxygen gasification in stoichiometric mode

Significant increase of conversion efficiency can be achieved by the addition of oxygen into the process. At the first stage, an "ideal" conversion reaction was considered, in which the number of reagents, in contrast to (13), contained also oxygen, and among the reaction products syngas components only were present:

$$\text{CH}\_{2.5}\text{O}\_{0.5} + \text{KH}\_2\text{O} + \text{LO}\_2 \rightarrow \text{CO} + \text{MH}\_2 + \text{Q}\_{\text{TR}} \tag{16}$$

where K, L, M are coefficients that determine the content of components such as steam and oxygen, as well as the hydrogen one in the reaction products, respectively, under the stoichiometric reaction with respect to syngas production. Thus this reaction is stoichiometric as well as (13) for obtaining products of gasification as synthesis gas only. Nevertheless it has the most wide functional possibilities to achieve the best index of energy efficiency of the process as it allows varying the composition of the gasification agent. In determining the energy efficiency, naturally, the consumption of energy for oxygen production should also be taken into account. The range of possible specific energy consumption in the technological process of obtaining the oxygen itself is chosen as PO2 = 0.35–1 kWh/m3 . The first one corresponds to promising technologies, the second one is realistic today. Quantitative index of energy efficiency of the conversion process is the ratio

$$\eta = \left(\mathbf{P}\_{\rm PL}^{\rm C} + \mathbf{P} \mathbf{O}\_{2}\right) / \mathbf{W}\_{\rm SG} \tag{17}$$

where Р<sup>С</sup> PL = ΔQ/0.8 is the electricity consumption for the production of plasma jet by efficiency of ~ 0.8 and for oxygen – РO2. WSG is the heat energy of syngas from 1 kg of the original raw mixture. In this form, it fully corresponds to the definition of energy saving (or energy efficiency) as energy costs (here, РPL<sup>J</sup> + РO2) per unit productivity (here the product is syngas of energy WSG).

The value of L = 0 in reaction (16) corresponds to the plasma-steam gasification (13), and the case ΔQ = 0 is usual steam-oxygen technology, although their opposition does not make sense. Indeed, from the point of view of the process chemistry, in both cases, oxygen atoms, characteristic of these technologies, and hydrogen atoms, originally included in the gasified sewage sludge, are present in the reaction. For the noted limit values of L, the coefficient K takes the values Kmax = 0.5 and Kmin = 0, respectively. However, generally speaking, the reactions (16) can also correspond to the intermediate values of the coefficients K and L. Simple functions are determined on the basis of mass balances in reaction (16):

for oxygen

СH2:5O0:<sup>5</sup> þ 0:5H2O ¼ CO þ 1:75H2 þ QТ<sup>R</sup>: (13)

QlSS ¼ �100∙ð Þ 81∙с<sup>C</sup> þ 246∙с<sup>H</sup> – 26∙ð Þ cO � cS – 6∙cW ∙4:19 kJ=kg, (14)

The heat of combustion of sewage sludge QLSS required to determine the energy of the process

where сС, сH, cO, cS and cW are mass fractions of carbon, hydrogen, oxygen, sulfur and water. (Mendeleev's equation is an analog of the relations known in the Western scientific literature as Dulong or Milne equation.) As may be shown, this heat of combustion of sewage sludge is QLSS = �25.68 MJ/kg [33]. Following the law of Hess, its enthalpy of formation is

The thermodynamic analysis of the sewage sludge conversion process carried out in the TERRA software [32] allows to determine the composition of its gasification products as a function of temperature. As it turned out, both for the reaction (12) and for other considered reactions, it is characterized by the practical completion of the gasification processes at 1250 K. More strictly, the mass fraction of the traces of CO2, H2O and CH4 among the products of gasification at this temperature does not exceed 1–2%. As it turned out, the energy QРL(T), which must be additionally introduced with a plasma torch per 1 kg of reagent mass in (12), to reach this temperature, is 0.785 kWh/kg. This parameter allows to determine the productivity

Knowing the calorific values for CO and H2, as well as the composition of the products obtained in the reaction (12), it is easy to determine the calorific value of the resulting syngas in this process WSG = 6.23 kWh/kg. It allows to define the energy output of the gasification plant and its energy efficiency on the basis of a comparison with the specific energy QР<sup>L</sup>

The value WSG significantly exceeds the electricity consumption 0.785 kWh/kg by steam PT to produce 1 kg of syngas. Thus, even taking into account the relatively low efficiency of ηЕЕ ~ 0.3 of electricity generation, the energy consumption is much lower than the level of energy of syngas produced. Indeed, taking into account also the efficiency of the PT at ηPL = 0.8, this is enough to ensure the operation of the PT, since it exceeds the value of ΔQ = 0.785 kWh/kg:

It is good preconditions for the energy self-sufficiency of the sewage sludge processing and the production of additional energy to compensate the role of raw materials moisture and ash

Significant increase of conversion efficiency can be achieved by the addition of oxygen into the process. At the first stage, an "ideal" conversion reaction was considered, in which the number

residue vitrification or for the production of electricity for external consumers.

4.2.3. Plasma-steam-oxygen gasification in stoichiometric mode

WSG∙ηEE∙ηPL ¼ 6:23 � 0:3 � 0:8 ¼ 1:5 kWh=kg > 0:785 kWh=kg: (15)

is determined on the basis of Mendeleev's Eq. [27]:

CH2.5O0.5 = �76.8 kJ/kg [33].

176 Gasification for Low-grade Feedstock

of the gasifier at a given power of the PT.

introduced into the reactor.

ΔH<sup>0</sup>

$$1\text{-}0.5 + K + 2L = 1,\\
or \, L = 0.25 - 0.5K; \tag{18}$$

for hydrogen

$$1\text{-}2.5 + 2\text{K} = 2\text{M}, \text{or } \text{M} = 1.25 + \text{K}.\tag{19}$$

For clarity, the function (18), which characterizes the oxygen content of L as a function of the amount of steam K introduced by the PT, is shown in Figure 3 as line 1. Line 4 represents the thermal power introduced into the reactor by a plasma jet at its nominal enthalpy of HPL = 3.6 kW�h/kg in accordance with equation

Figure 3. The main regularities characterizing the stoichiometric mode of gasification of the sewage sludge in the function of the amount of water introduced into the reaction K with plasma-steam jet – molar and energy ratios (a) and energy consumption for oxygen production and energy efficiency indicators of process (b): 1—oxygen content L introduced into the reactor; 2—additional energy ΔQ, which should be introduced in volume to achieve the operating temperature; 3—the energy of the producing syngas WSG; 4, 4a—the energy introduced by the steam-plasma jet QPL with its enthalpy HPL = 3.6 and 0.72 kWh/kg, respectively; 5a and 5b—energy consumption for oxygen production at a specific consumption of energy 0.35 and 1 kWh/Nm3 , respectively; 6a and 6b—indicators of energy efficiency of the process at the corresponding specified energy consumption for the production of oxygen.

$$Q\_{\rm PL} = H\_{\rm PL} m H\_{\rm 2} O\_{\prime} \tag{20}$$

Area K < 0.17 corresponds the negative values ΔQ <0(Figure 3); this means that excess heat energy is released in the reaction zone, which can be used for the ash residue's vitrification. The level of energy consumption for this need is difficult to determine in general terms, but empirical ratio (13) is known for them. In this area, lines 6 characterizing the level of energy efficiency of the process η are indicated by a dashed line. This is emphasized by the fact that here the energy costs for maintaining the gasification process are negative. In other words,

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In the absence of a PT, the stoichiometric gasification regime according to the reaction (15) is realized for a single value K0 = 0.17, corresponding to the intersection of line 2, which characterizes the required energy level ΔQ with the coordinate axis. It, in turn, corresponds to the moisture content of sewage sludge of about 10%, if it is determined from the composition of the reagents on the left side of the reaction (15). This moisture value is characteristic just for the conditioning of sewage sludge, which are currently dried with the help of those or other

However, the range of values K = 0.17 and near it for practical operation of the gasifier should be excluded, because the software TERRA reveals a significant soot formation, which makes it unacceptable for gasification. Thus, solving also the problem of obtaining more high quality syngas, it is expedient to move along line 2 to its maximum value corresponding to the stoichiometric regime at K = 0.5 (Figure 3). The results obtained are presented in Table 4. As can be concluded, an increase in the amount of water introduced into the process L is twice, corresponds to a worsening of energy efficiency of the conversion process η by a factor three.

The introduction of a significant amount of energy with a plasma jet markedly worsens the indicator of the energy efficiency of the plant, as follows from Figure 3b and Table 4. Therefore, it is of interest to compare it with the non-stoichiometric regime, which can be easily

рO2 = 1 kWh/Nm3 0.09 0

рO2 = 1 kWh/Nm3 0.057 0.16

Table 4. Calculated parameters characterizing the stoichiometric process of conversion of sewage sludge at its humidity of 10% to synthesis gas using plasma technology, depending on the additional amount of water vapor introduced with

0.25 0.5

4.2.4. Non-stoichiometric mode of plasma-steam-oxygen gasification

Parameter K, arb.un.

L, arb.un. 0.125 0 ΔQ, kWh/kg 0.19 0.785 PPLC, кВт∙ч/кг 0.24 0.98 РО2, kWh/kg рO2 = 0.35 kWh/Nm<sup>3</sup> 0.03 0

WСГ, kWh/kg 5.79 6.23 η, arb.un. рO2 = 0.35 kWh/Nm<sup>3</sup> 0.046 0.15

there is an energy release.

drying technologies.

the plasma jet.

where mH2O corresponds to the mass of water in the jet injected per kg of reagents. This enthalpy value corresponds to the moderate operating mode of the PT used in Ref. [9]. In principle, the higher values of plasma enthalpy, corresponding to the forced operating regime of the steam PT, can be achieved.

It can be concluded that the introduction of oxygen in the stoichiometric mode of gasification with the use of plasma technologies corresponds to an increase in the energy efficiency of the process. As it follows from Figure 3b, the maximum value η in the process (which corresponds to the highest value of the additional energy ΔQ and, consequently, the worst energy efficiency) occurs exactly when the oxygen content is L = 0, for which K = 0.5 corresponds—that is, on the right side of each graph. On the contrary, the value η decreases with a gradual increase of the oxygen content L (i.e., moving to the left along the abscissa).

Area K < 0.17 corresponds the negative values ΔQ <0(Figure 3); this means that excess heat energy is released in the reaction zone, which can be used for the ash residue's vitrification. The level of energy consumption for this need is difficult to determine in general terms, but empirical ratio (13) is known for them. In this area, lines 6 characterizing the level of energy efficiency of the process η are indicated by a dashed line. This is emphasized by the fact that here the energy costs for maintaining the gasification process are negative. In other words, there is an energy release.

In the absence of a PT, the stoichiometric gasification regime according to the reaction (15) is realized for a single value K0 = 0.17, corresponding to the intersection of line 2, which characterizes the required energy level ΔQ with the coordinate axis. It, in turn, corresponds to the moisture content of sewage sludge of about 10%, if it is determined from the composition of the reagents on the left side of the reaction (15). This moisture value is characteristic just for the conditioning of sewage sludge, which are currently dried with the help of those or other drying technologies.

However, the range of values K = 0.17 and near it for practical operation of the gasifier should be excluded, because the software TERRA reveals a significant soot formation, which makes it unacceptable for gasification. Thus, solving also the problem of obtaining more high quality syngas, it is expedient to move along line 2 to its maximum value corresponding to the stoichiometric regime at K = 0.5 (Figure 3). The results obtained are presented in Table 4. As can be concluded, an increase in the amount of water introduced into the process L is twice, corresponds to a worsening of energy efficiency of the conversion process η by a factor three.

#### 4.2.4. Non-stoichiometric mode of plasma-steam-oxygen gasification

QPL ¼ HPLmH2O, (20)

, respectively; 6a and 6b—indicators of energy efficiency of the process at the corresponding

where mH2O corresponds to the mass of water in the jet injected per kg of reagents. This enthalpy value corresponds to the moderate operating mode of the PT used in Ref. [9]. In principle, the higher values of plasma enthalpy, corresponding to the forced operating regime

Figure 3. The main regularities characterizing the stoichiometric mode of gasification of the sewage sludge in the function of the amount of water introduced into the reaction K with plasma-steam jet – molar and energy ratios (a) and energy consumption for oxygen production and energy efficiency indicators of process (b): 1—oxygen content L introduced into the reactor; 2—additional energy ΔQ, which should be introduced in volume to achieve the operating temperature; 3—the energy of the producing syngas WSG; 4, 4a—the energy introduced by the steam-plasma jet QPL with its enthalpy HPL = 3.6 and 0.72 kWh/kg, respectively; 5a and 5b—energy consumption for oxygen production at a specific consumption of

It can be concluded that the introduction of oxygen in the stoichiometric mode of gasification with the use of plasma technologies corresponds to an increase in the energy efficiency of the process. As it follows from Figure 3b, the maximum value η in the process (which corresponds to the highest value of the additional energy ΔQ and, consequently, the worst energy efficiency) occurs exactly when the oxygen content is L = 0, for which K = 0.5 corresponds—that is, on the right side of each graph. On the contrary, the value η decreases with a gradual increase

of the oxygen content L (i.e., moving to the left along the abscissa).

of the steam PT, can be achieved.

specified energy consumption for the production of oxygen.

energy 0.35 and 1 kWh/Nm3

178 Gasification for Low-grade Feedstock

The introduction of a significant amount of energy with a plasma jet markedly worsens the indicator of the energy efficiency of the plant, as follows from Figure 3b and Table 4. Therefore, it is of interest to compare it with the non-stoichiometric regime, which can be easily


Table 4. Calculated parameters characterizing the stoichiometric process of conversion of sewage sludge at its humidity of 10% to synthesis gas using plasma technology, depending on the additional amount of water vapor introduced with the plasma jet.

realized for the same value of L0 = 0.165, as in the stoichiometric regime at the point K0 = 0.17, but at K > 0.17. Therefore, it is advisable to introduce excess oxygen into the reactor.

In this case, in order to optimize the plasma-steam gasification process of sewage sludge, the next reaction was analyzed:

$$\rm CH\_{2.5}O\_{0.5} + \rm HH\_2O + LO\_2 \rightarrow CO + MH\_2 + ECO\_2 + DH\_2O + Q\_{\rm TR} \tag{21}$$

energy consumption for vitrification, would lead to overheating of the internal volume of the

The consumption of electrical energy for the production of a plasma jet with a much lower enthalpy—0.72 kWh/kg is also shown in Figure 3a. Without even carrying out detailed calculations, it can be concluded that the use of a less powerful PT would lead to an improvement of the energy efficiency of the process, since it is the level of energy expenditure for the operation

The calculated data of Table 6 can be useful for assessing the efficiency of the sewage sludge gasification installation, depending on the presence of the mineral mass, which requires vitrification, in its composition. For this, it should be taken into account that at K = 0.5, the next

(when recalculating to electrical energy to power PT). To determine the permissible content of the mineral part in the initial sewage sludge, it is necessary to use the relation (7). If there is a mineral mass in the composition of sewage sludge at a rate of M per 1 kg, the amount of excess energy produced is converted into electric energy, which will be P(1 - M), and it, in turn, can be

where the difference in parentheses characterizes the amount of syngas obtained from 1 kg of the mixture. It follows that M = 0.17 kg. Thus, the data of the last column of Table 6 for the

НР<sup>L</sup> = 0.72 kWh/kg 0.03 0.12

рO2 = 1 kWh/Nm<sup>3</sup> 0.114 0.1

рO2 = 1 kWh/Nm<sup>3</sup> 0.028 0.033

Table 6. Calculated parameters characterizing the non-stoichiometric process of sewage sludge conversion with its 10% humidity in syngas with oxygen content L0 = 0.165 using plasma technology depending on the amount of water steam

consumed for vitrification according to (7). Hence we can define M:

Parameter K, arb. un.

L0, arb. un. 0.165 0.165 ΔQ, kWh/kg 0.04 0.09 QPL, kWh/kg НР<sup>L</sup> = 3.6 kWh/kg 0.16 0.58

РО2, kWh рO2 = 0.35 kWh/Nm<sup>3</sup> 0.04 0.035

kNS, arb. un. 0.934 0.807 WSG\*, kWh/kg 5.44 5.11 η, arb. un. рO2 = 0.35 kWh/Nm<sup>3</sup> 0.015 0.023

P ¼ ð Þ QPL � ΔQ =0:8 ¼ ð Þ 0:58 � 0:09 =0:8 ¼ 0:6 kWh=kg (23)

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181

M kg ð Þ¼ 0:35Pð Þ 1 � M , (24)

0.25 0.5

gasifier.

of the PT that determines its effectiveness.

excess energy P is introduced into reactor:

K>K0 introduced with plasma jet.

This gasification mode is called "non-stoichiometric", as there are the products of partial combustion of sewage sludge—CO2 and H2O—among the products of gasification. In determining the parameters of the process in the non-stoichiometric mode of gasification, it should be taken into account that, in addition to the syngas, the ballast components are formed from the unit mass of the initial reagents. In other words, the correction factor should be taken:

$$\mathbf{W\_{SG}^\*} = [(\mathbf{mCO} + \mathbf{mH\_2})/(\mathbf{mCO} + \mathbf{mH\_2} + \mathbf{mH\_2O})] \\ \mathbf{W\_{SG}} = \mathbf{k\_{NS}W\_{SG}} \tag{22}$$

where kNS is the non-stoichiometric coefficient.

Recall that, in principle, it is possible to compose many variants of the reaction (21) with the different stoichiometric coefficients. However, in fact, only those are actually realized where maximum entropy principle is satisfied (see Eq. (12)). Examples of the resulting compositions of gasification products are shown in Table 5.

Using these data, the parameters of non-stoichiometric gasification regimes for K = 0.25 and 0.5 were calculated (Table 6).

Analyzing the results presented in Table 6, it should be borne in mind that they are not energetically self-consistent. Indeed, with an oxygen content L0 = 0.165, a relatively small additional thermal energy ΔQ = 0.04–0.09 kWh/kg is required. Table 6 also shows the energy introduced with a jet of a PT operating in our ordinary energetic mode with enthalpy НР<sup>L</sup> = 3.6 kWh/kg [9] and – for comparisons – in a much less intense mode HPL = 0.72 kWh/kg. One can conclude by comparing the values of ΔQ and НР<sup>L</sup> between each other, that in this regime one can confine ourselves to a low-power PT. Otherwise, the excess energy of the plasma torch can be used to vitrify the ash residue. Thus, the final analysis causes a significant decrease in the value η compared with the data in Table 4. Here it should be taken into account that when working with moist sewage sludge, the energy introduced by the PT is proportional to ΔK=K-K0. The introduced thermal energy levels at K = 0.5 exceed the noted values ΔQ and, in the absence of


Table 5. Composition of gasification products in non-stoichiometric mode with oxygen content L0 = 0.165 according to the calculated data in the TERRA software.

energy consumption for vitrification, would lead to overheating of the internal volume of the gasifier.

realized for the same value of L0 = 0.165, as in the stoichiometric regime at the point K0 = 0.17,

In this case, in order to optimize the plasma-steam gasification process of sewage sludge, the

This gasification mode is called "non-stoichiometric", as there are the products of partial combustion of sewage sludge—CO2 and H2O—among the products of gasification. In determining the parameters of the process in the non-stoichiometric mode of gasification, it should be taken into account that, in addition to the syngas, the ballast components are formed from the unit mass of the initial reagents. In other words, the correction factor should be taken:

Recall that, in principle, it is possible to compose many variants of the reaction (21) with the different stoichiometric coefficients. However, in fact, only those are actually realized where maximum entropy principle is satisfied (see Eq. (12)). Examples of the resulting compositions

Using these data, the parameters of non-stoichiometric gasification regimes for K = 0.25 and 0.5

Analyzing the results presented in Table 6, it should be borne in mind that they are not energetically self-consistent. Indeed, with an oxygen content L0 = 0.165, a relatively small additional thermal energy ΔQ = 0.04–0.09 kWh/kg is required. Table 6 also shows the energy introduced with a jet of a PT operating in our ordinary energetic mode with enthalpy НР<sup>L</sup> = 3.6 kWh/kg [9] and – for comparisons – in a much less intense mode HPL = 0.72 kWh/kg. One can conclude by comparing the values of ΔQ and НР<sup>L</sup> between each other, that in this regime one can confine ourselves to a low-power PT. Otherwise, the excess energy of the plasma torch can be used to vitrify the ash residue. Thus, the final analysis causes a significant decrease in the value η compared with the data in Table 4. Here it should be taken into account that when working with moist sewage sludge, the energy introduced by the PT is proportional to ΔK=K-K0. The introduced thermal energy levels at K = 0.5 exceed the noted values ΔQ and, in the absence of

The water content in reagents, the mole fraction of K Composition of gasification products, wt.

0.25 0.84 0.089 0.034 0.032 0.5 0,.69 0.083 0.11 0.117

Table 5. Composition of gasification products in non-stoichiometric mode with oxygen content L0 = 0.165 according to

СО Н<sup>2</sup> СО<sup>2</sup> Н2О

CH2:<sup>5</sup>О<sup>0</sup>:<sup>5</sup> þ KН2O þ LО<sup>2</sup> ! CO þ MH2 þ EСО<sup>2</sup> þ DН2О þ QTR: (21)

SG ¼ ½ � ð Þ mCO þ mH2 =ð Þ mCO þ mH2 þ mH2O WSG ¼ kNSWSG, (22)

but at K > 0.17. Therefore, it is advisable to introduce excess oxygen into the reactor.

next reaction was analyzed:

180 Gasification for Low-grade Feedstock

W<sup>∗</sup>

were calculated (Table 6).

the calculated data in the TERRA software.

where kNS is the non-stoichiometric coefficient.

of gasification products are shown in Table 5.

The consumption of electrical energy for the production of a plasma jet with a much lower enthalpy—0.72 kWh/kg is also shown in Figure 3a. Without even carrying out detailed calculations, it can be concluded that the use of a less powerful PT would lead to an improvement of the energy efficiency of the process, since it is the level of energy expenditure for the operation of the PT that determines its effectiveness.

The calculated data of Table 6 can be useful for assessing the efficiency of the sewage sludge gasification installation, depending on the presence of the mineral mass, which requires vitrification, in its composition. For this, it should be taken into account that at K = 0.5, the next excess energy P is introduced into reactor:

$$P = (Q\_{\rm PL} - \Delta Q)/0.8 = (0.58 - 0.09)/0.8 = 0.6 \text{ kWh/kg} \tag{23}$$

(when recalculating to electrical energy to power PT). To determine the permissible content of the mineral part in the initial sewage sludge, it is necessary to use the relation (7). If there is a mineral mass in the composition of sewage sludge at a rate of M per 1 kg, the amount of excess energy produced is converted into electric energy, which will be P(1 - M), and it, in turn, can be consumed for vitrification according to (7). Hence we can define M:

$$M(\text{kg}) = 0.35P(1 - M),\tag{24}$$

where the difference in parentheses characterizes the amount of syngas obtained from 1 kg of the mixture. It follows that M = 0.17 kg. Thus, the data of the last column of Table 6 for the


Table 6. Calculated parameters characterizing the non-stoichiometric process of sewage sludge conversion with its 10% humidity in syngas with oxygen content L0 = 0.165 using plasma technology depending on the amount of water steam K>K0 introduced with plasma jet.

index of the energy efficiency of the gasification equipment are valid up to 17% of the mineral content in sewage sludge to be vitrified.

In the first of these cases, the energy efficiency index in the entire range of moisture content K in the reacting mixture does not exceed 0.1 (Figure 4a). Even better is the efficiency index of the steam-gasification, that is, in the second case, when its value does not exceed 0.05. However, one should realize that in the reactor space the vitrification and gasification zones are not so separated in space that some of the energy of the plasma jet is not consumed by the gasification processes. Therefore, we believe that, in general, the proposed technology can ensure the energy efficiency of the gasification process for sewage sludge with an index not

Thus, practically all cases presented in Figure 4, the consumption of syngas for the electricity generation by means of a gas-diesel power station is only a fraction of the total volume of its

In the variant represented by the last equation, this part is only 30% of the energy for the synthesis gas obtained (in deriving these relations, Eq. (17) was used). Accordingly, the remaining part of it can be spent, for example, for the production of electrical energy to external consumers, which will facilitate the commercialization of this development. Thus, in the variant proposed, the processing technology corresponds well to the general idea of

It should be emphasized that the sensitivity of the estimates has been obtained from the selected composition of carbon-containing gasified raw materials. Therefore, further development of these studies requires variation of this composition, as well as more strictly quantitative fraction of the mineral component of the sewage sludge. The same applies to other types of hazardous waste. This part of the publication is the methodological basis for such an analysis. In accordance with this, the role of the plasma part of the technology can also increase or, conversely, decrease. Nevertheless, especially for multi-purpose installations, its role from the

numerous publications in the world literature, known as the Waste-to-Energy.

point of view of the environmental safety of the process remains unchanged.

treatment plant based on plasma-steam-oxygen technology

5. The state of design and construction of the shaft reactor for waste

In 2017, the Institute of Gas of the National Academy of Sciences of Ukraine completes the execution of the state order for development of steam-plasma technology for the processing of sewage sludge with the support of the Ministry of Education and Science of Ukraine. The result will be a reactor module for waste treatment based on plasma-steam-oxygen technology, which can become the core of plants for the recycling of hazardous waste: bottom sediments of aeration stations of urban water purification systems, unsorted solid household wastes (they are dangerous because of the risk of entering into their composition of chlorinated compounds), medical waste, overdue pesticides and chemical treatments for plants, etc. The module is designed in such a way as to ensure its payback through the production of electrical

SG=ηEE ≈ 0:1ηW<sup>∗</sup>

SG=0:3 ≈ 0:33W<sup>∗</sup>

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183

SG: (25)

PO<sup>2</sup> <sup>þ</sup> <sup>Δ</sup>QPL=0:8Þ=ηEE <sup>¼</sup> <sup>η</sup>W<sup>∗</sup>

worse than 0.1.

production

5.1. Features of the project

A more rigorous problem of the non-stoichiometric gasification regime, self-consistent with respect to energy consumption, is also considered. It was solved on the basis of varying the values of L in the reaction (21) for a given value of K. The value of L was determined at which the compensation of the emerging thermal energy deficit ΔQ is attained due to the energy of the plasma jet introduced with the indicated quantity K of water steam at a certain enthalpy. In other words, it was determined at which values of L the condition ΔQ(L) – QPL = 0 is reached.

The main regularities, which ultimately represent the efficiency of the non-stoichiometric gasification process with a small enthalpy of HPL = 0.72 kWh/kg of the plasma jet and in its absence, that is, for wet bottom sludge are shown in Figure 4.

Figure 4. The main regularities characterizing the energy efficiency of non-stoichiometric modes of sewage sludge gasification as a function of the amount of water vapor K introduced into the reaction with the enthalpy of the plasma jet HPL = 0.72 kWh/kg (a), and also, in its absence, for wet sewage sludge (b): 1—the oxygen content L introduced into the reactor; 2—additional energy ΔQ, which should be introduced into the volume to reach the operating temperature, equal to the energy introduced by the steam-plasma jet QPL (the latter—with the exception of wet sewage sludge); 3—the energy of the syngas WSG\*; 4—coefficient of nonstoichiometrykNS; 5a and 5b—energy consumption for the production of oxygen at a specific consumption of 0.35 and 1 kWh/Nm3 , respectively; 6a and 6b—energy efficiency index of the process at the indicated energy inputs for the production of oxygen, respectively.

In the first of these cases, the energy efficiency index in the entire range of moisture content K in the reacting mixture does not exceed 0.1 (Figure 4a). Even better is the efficiency index of the steam-gasification, that is, in the second case, when its value does not exceed 0.05. However, one should realize that in the reactor space the vitrification and gasification zones are not so separated in space that some of the energy of the plasma jet is not consumed by the gasification processes. Therefore, we believe that, in general, the proposed technology can ensure the energy efficiency of the gasification process for sewage sludge with an index not worse than 0.1.

Thus, practically all cases presented in Figure 4, the consumption of syngas for the electricity generation by means of a gas-diesel power station is only a fraction of the total volume of its production

$$\left| \mathrm{PO}\_{2} + \Delta Q\_{\mathrm{PL}} / 0.8 \right\rangle / \eta\_{\mathrm{EE}} = \eta \mathrm{W}\_{\mathrm{SG}}^{\*} / \eta\_{\mathrm{EE}} \simeq 0.1 \eta \mathrm{W}\_{\mathrm{SG}}^{\*} / 0.3 \simeq 0.3 \mathcal{S} \mathrm{W}\_{\mathrm{SG}}^{\*}.\tag{25}$$

In the variant represented by the last equation, this part is only 30% of the energy for the synthesis gas obtained (in deriving these relations, Eq. (17) was used). Accordingly, the remaining part of it can be spent, for example, for the production of electrical energy to external consumers, which will facilitate the commercialization of this development. Thus, in the variant proposed, the processing technology corresponds well to the general idea of numerous publications in the world literature, known as the Waste-to-Energy.

It should be emphasized that the sensitivity of the estimates has been obtained from the selected composition of carbon-containing gasified raw materials. Therefore, further development of these studies requires variation of this composition, as well as more strictly quantitative fraction of the mineral component of the sewage sludge. The same applies to other types of hazardous waste. This part of the publication is the methodological basis for such an analysis. In accordance with this, the role of the plasma part of the technology can also increase or, conversely, decrease. Nevertheless, especially for multi-purpose installations, its role from the point of view of the environmental safety of the process remains unchanged.
