**3. Basics of gasification**

#### **3.1. Mechanism**

Combustion, gasification, and pyrolysis are thermal energy conversion processes available for the thermal treatment of solid wastes. **Figure 1** introduces all the potential pathways to convert MSW or biomass into different energy forms using thermal, mechanical, and biological processes. **Figure 2** shows the schematic diagram of syngas production and how to utilize the gas for various purposes such as power generation, creating chemicals by upgrading steps, and further biochemical processing before producing fuels or chemicals. As shown in these figures, different products are obtained from the application of these processes, and different energy and residual material recovery systems can be used in various types of technologies.

Gasification is a thermochemical conversion process of carbonaceous materials into gaseous product at high temperatures with the aid of gasification agent. The gasification agent (another gaseous compound) allows the feedstock to be quickly converted into gas by means of different heterogeneous reactions [6–9]. The gaseous product obtained during this process is called synthetic gas (syngas) or producer gas, and it mainly contains hydrogen, carbon monoxide, carbon dioxide, and methane. Also, a small amount of inert gases, hydrocarbons, tar, and gas pollutants can be found [10]. Based on the effect of gasification agent, gasification can be divided into two categories. If the gasification agent partially oxidizes the feed material it is called direct gasification. During direct gasification, to maintain the temperature of the process, oxidation reaction supplies the required energy. If the gasification process takes place without the aid of gasification agent it is called indirect gasification [7, 11]. Usually steam is used for indirect gasification as it is easily available. Moreover, it increases the hydrogen content in the producer gas [7].

As shown in **Figure 3**, two main gasification processes can be classified into direct and indirect gasification processes. Indirect gasification processes are conducted without air or oxygen injection. The heating value of the syngas is significantly affected by the presence of nitrogen. In the absence of nitrogen in indirect gasification process, the volumetric efficiency and higher heating value of producer gas both increases [12, 13]. Also, indirect gasification

**Figure 2.** Pathway of waste to energy (gases, fuels, chemicals) by gasification.

**Figure 1.** Pathways to convert MSW to different types of energy forms or chemicals through various conversion

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 119

processes.

**Figure 1.** Pathways to convert MSW to different types of energy forms or chemicals through various conversion processes.

**Figure 2.** Pathway of waste to energy (gases, fuels, chemicals) by gasification.

**3. Basics of gasification**

118 Gasification for Low-grade Feedstock

**Table 2.** Calorific values of various materials [4].

gen content in the producer gas [7].

Combustion, gasification, and pyrolysis are thermal energy conversion processes available for the thermal treatment of solid wastes. **Figure 1** introduces all the potential pathways to convert MSW or biomass into different energy forms using thermal, mechanical, and biological processes. **Figure 2** shows the schematic diagram of syngas production and how to utilize the gas for various purposes such as power generation, creating chemicals by upgrading steps, and further biochemical processing before producing fuels or chemicals. As shown in these figures, different products are obtained from the application of these processes, and different energy and residual material recovery systems can be used in various types of technologies. Gasification is a thermochemical conversion process of carbonaceous materials into gaseous product at high temperatures with the aid of gasification agent. The gasification agent (another gaseous compound) allows the feedstock to be quickly converted into gas by means of different heterogeneous reactions [6–9]. The gaseous product obtained during this process is called synthetic gas (syngas) or producer gas, and it mainly contains hydrogen, carbon monoxide, carbon dioxide, and methane. Also, a small amount of inert gases, hydrocarbons, tar, and gas pollutants can be found [10]. Based on the effect of gasification agent, gasification can be divided into two categories. If the gasification agent partially oxidizes the feed material it is called direct gasification. During direct gasification, to maintain the temperature of the process, oxidation reaction supplies the required energy. If the gasification process takes place without the aid of gasification agent it is called indirect gasification [7, 11]. Usually steam is used for indirect gasification as it is easily available. Moreover, it increases the hydro-

**Material Calorific value (BTU/lb) Ash content (wt.%) Moisture content (wt.%)**

Soft wood 6330 0.1 19 Fiberboard, 90% paper 7600 4.6 7.5 Damp wood 5690 1.2 27.5 Leather trimmings 7670 5.2 10.4 Cotton seed hulls 10,600 2.47 8.9 Sludge material (steel mill) 9150 24.5 1.9

Cardboard, granulated 8592 12.3 6.4 Carbon residue 13,681 8.7 0.0 Wood waste, sawdust 7500 0.8 14 Nut shells 7980 1.75 11.85

Nitrile rubber 15,240 3.4

**3.1. Mechanism**

As shown in **Figure 3**, two main gasification processes can be classified into direct and indirect gasification processes. Indirect gasification processes are conducted without air or oxygen injection. The heating value of the syngas is significantly affected by the presence of nitrogen. In the absence of nitrogen in indirect gasification process, the volumetric efficiency and higher heating value of producer gas both increases [12, 13]. Also, indirect gasification

**Figure 3.** Direct and indirect gasification processes.

process decreases the cost of gas clean up and energy recovery by lowering the gas production rate. However, the process is quite complex and the investment cost is higher [7].

Pure oxygen gasification as direct gasification has same advantages over indirect gasification. However, the cost of producing pure oxygen is expected to account for more than 20% of the total cost of electricity production [14].

Generally, a gasification system is composed of three stages: (1) gasifier for useful producing syngas; (2) the syngas cleaning system for removal of pollutants and harmful compounds; (3) an energy recovery system such as a gas engine. Additionally, sub-systems are included to prevent environmental impacts such as air pollution, solid wastes, and wastewater.

of carbon and hydrogen, which are easily converted to combustible gases in volatiles, are included in the feedstock. The quantities, composition, and characteristics of chemical species released due to devolatilization are dependent on several factors such as original composition and structure of the waste, temperature, pressure, and heating rate imposed by particular reactor types. In devolatilization, various gas compositions are produced, and these gases are

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 121

• Many chemical reactions occur in a reducing environment that is in remarkably lower oxidation (25–50%) than stoichiometric oxidation. Following **Table 3**, in an auto-thermal gasification process, the partial oxidation of combustible gas, vapors, and char are controlled by the amount of air, oxygen, or oxygen-enriched air. Also, this heat is necessary for the thermal cracking of tar hydrocarbons and char gasification by steam, and carbon dioxide maintains the operation temperature of the gasifier. Following the enthalpy of reactions 1, 2, and 3 in **Table 3**, in auto-thermal gasification processes, about 28% of the carbon heating value is invested in CO production, and the remaining 72% of the carbon heating value is conserved in the gas. The heating value of gas is generally between 75 and 88% of the original fuel because it also contains some hydrogen. If this value were 50% or lower, gasification using coal, biomass, and waste would probably never have become such an interesting process [18]. On the other hand, in an allo-thermal gasification process, the heat is supplied by external sources that are using heated bed materials, burning chars or gases, and utilizing plasma touch. The specific

generated by the hydrogen and carbon in the waste [16, 17].

**Figure 4.** Main reactions and steps of gasification process.

#### **3.2. Chemistry**

#### *3.2.1. Process steps*

The gasification process of solid waste has endothermic and exothermic reactions, which are successive and repetitive [15, 16]. **Figure 4** describes the main reactants and steps of the gasification process.

• Heating and drying at about 160°C: In this stage, the moisture and steam from the feedstock are removed by the porous solid phase.

• Devolatilization (or pyrolysis or thermal decomposition) at about 700°C: This stage determines the thermal cracking reactions and conversion of heat and mass, including light permanent gases (such as H2 , CO, CO2 , CH4 , H2 O, and NH3 ), tar (condensable hydrocarbon vapors), and char (residue emitted after devolatilization). Vapors produced in this stage undergo thermal cracking to gas and char. In the case of MSW, as described in **Figure 4**, high contents

**Figure 4.** Main reactions and steps of gasification process.

process decreases the cost of gas clean up and energy recovery by lowering the gas produc-

Pure oxygen gasification as direct gasification has same advantages over indirect gasification. However, the cost of producing pure oxygen is expected to account for more than 20% of the

Generally, a gasification system is composed of three stages: (1) gasifier for useful producing syngas; (2) the syngas cleaning system for removal of pollutants and harmful compounds; (3) an energy recovery system such as a gas engine. Additionally, sub-systems are included to

The gasification process of solid waste has endothermic and exothermic reactions, which are successive and repetitive [15, 16]. **Figure 4** describes the main reactants and steps of the

• Heating and drying at about 160°C: In this stage, the moisture and steam from the feedstock

• Devolatilization (or pyrolysis or thermal decomposition) at about 700°C: This stage determines the thermal cracking reactions and conversion of heat and mass, including light perma-

O, and NH3

and char (residue emitted after devolatilization). Vapors produced in this stage undergo thermal cracking to gas and char. In the case of MSW, as described in **Figure 4**, high contents

), tar (condensable hydrocarbon vapors),

tion rate. However, the process is quite complex and the investment cost is higher [7].

prevent environmental impacts such as air pollution, solid wastes, and wastewater.

total cost of electricity production [14].

**Figure 3.** Direct and indirect gasification processes.

120 Gasification for Low-grade Feedstock

are removed by the porous solid phase.

, CO, CO2

, CH4 , H2

**3.2. Chemistry**

*3.2.1. Process steps*

gasification process.

nent gases (such as H2

of carbon and hydrogen, which are easily converted to combustible gases in volatiles, are included in the feedstock. The quantities, composition, and characteristics of chemical species released due to devolatilization are dependent on several factors such as original composition and structure of the waste, temperature, pressure, and heating rate imposed by particular reactor types. In devolatilization, various gas compositions are produced, and these gases are generated by the hydrogen and carbon in the waste [16, 17].

• Many chemical reactions occur in a reducing environment that is in remarkably lower oxidation (25–50%) than stoichiometric oxidation. Following **Table 3**, in an auto-thermal gasification process, the partial oxidation of combustible gas, vapors, and char are controlled by the amount of air, oxygen, or oxygen-enriched air. Also, this heat is necessary for the thermal cracking of tar hydrocarbons and char gasification by steam, and carbon dioxide maintains the operation temperature of the gasifier. Following the enthalpy of reactions 1, 2, and 3 in **Table 3**, in auto-thermal gasification processes, about 28% of the carbon heating value is invested in CO production, and the remaining 72% of the carbon heating value is conserved in the gas. The heating value of gas is generally between 75 and 88% of the original fuel because it also contains some hydrogen. If this value were 50% or lower, gasification using coal, biomass, and waste would probably never have become such an interesting process [18]. On the other hand, in an allo-thermal gasification process, the heat is supplied by external sources that are using heated bed materials, burning chars or gases, and utilizing plasma touch. The specific


dioxin strongly declines because of the oxidation reactions of the dioxin synthesis mechanism [19–21]. All gasification reactions except oxidation reactions create equilibrium. In fact, the final gas composition is determined by reaction rates and catalytic effects, rather than by the

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 123

Equivalent ratio (ER) is defined as the ratio of the actual amount of oxidant to stoichiometric oxidant for complete combustion. This parameter is the most important operating parameter in gasification process because it affects syngas composition, tar content, gas yield, and its chemical energy. The pyrolysis process is operated at close to ER zero, and the combustion process is operated at more than ER one for complete combustion. In **Figure 5**, the conversion of char in the gasification process at ER 0.25 to 0.35 appears to maximize even though these gasifiers and those that are used in large-scale commercial plants (following **Table 4**), namely, moving grate gasifiers [25] and fluidized bed gasifiers [26] operated with wet fuels, are operated at about ER 0.5. With a lower ER, the gas yield from char is reduced, and the tar in syngas increases while with a higher ER, the oxidation reactions in the gasification process improve

**Figure 5.** Syngas composition at chemical equilibrium as a function of ER for the gasification of wood at 1 atm [29].

chemical equilibrium after an infinite period of time [22–24].

**3.3. Operating and performance parameter**

*3.3.1. Equivalent ratio*

a Note that Cx Hy represents tars and, in general, the heavier fuel fragments produced by thermal cracking, and CnHm represents hydrocarbons with a smaller number of carbon atoms and/or a larger degree of unsaturation than CxHy.

**Table 3.** Main reactions in the heterogeneous and homogeneous phases during the solid waste gasification process.

gasification reactions are those taking place between the devolatilized solid waste (char) and gases excluding oxygen.

#### *3.2.2. Gasification reactions*

The gasification reactions have various reactions, but **Table 3** shows just three independent gasification reactions: the water-gas reaction, the Boudard reaction, and hydrogasification. In the gasifier, where there is no more carbon in the feedstock, only two reactions are produced: the water-gas shift reaction, which is the combination of the water-gas and Boudard reactions, and methanation, which is the combination of the water-gas and hydrogasification reactions. These reactions are a simple framework related to reactants and products of H, N, O, S, etc. in the feedstock [16]. Also, CO is produced instead of CO2 , H2 instead of H2 O, and for other elements, H2 S instead of SO2 , and NH3 or HCN instead of NO. Moreover, the formation of dioxin strongly declines because of the oxidation reactions of the dioxin synthesis mechanism [19–21]. All gasification reactions except oxidation reactions create equilibrium. In fact, the final gas composition is determined by reaction rates and catalytic effects, rather than by the chemical equilibrium after an infinite period of time [22–24].

#### **3.3. Operating and performance parameter**

#### *3.3.1. Equivalent ratio*

gasification reactions are those taking place between the devolatilized solid waste (char) and

represents hydrocarbons with a smaller number of carbon atoms and/or a larger degree of unsaturation than CxHy.

**Table 3.** Main reactions in the heterogeneous and homogeneous phases during the solid waste gasification process.

1 C + ½O2 → CO −111 MJ/kmol Carbon partial oxidation 2 CO + ½O2 → CO2 −283 MJ/kmol Carbon monoxide

O −242 MJ/kmol Hydrogen oxidation

O → CO + H2 +131 MJ/kmol Waster-gas reaction

O → CO2 + H2 −41 MJ/kmol Water-gas shift reaction

O → CO + 3H2 +206 MJ/kmol Steam methane reforming

O −227 MJ/kmol Methanation

Hy → qCnOm + rH2 Endothermic Dehydrogenation

represents tars and, in general, the heavier fuel fragments produced by thermal cracking, and CnHm

H2 Endothermic Carbonization

H2 Endothermic Dry reforming

H2 Exothermic CnHm partial oxidation

)H2 Endothermic Steam reforming

3 C + O2 → CO2 −394 MJ/kmol Carbon oxidation

2

10 C + 2H2 → CH4 −75 MJ/kmol Hydrogasification

12 C + CO2 → 2CO +172 MJ/kmol Boudard reaction

2

2

The gasification reactions have various reactions, but **Table 3** shows just three independent gasification reactions: the water-gas reaction, the Boudard reaction, and hydrogasification. In the gasifier, where there is no more carbon in the feedstock, only two reactions are produced: the water-gas shift reaction, which is the combination of the water-gas and Boudard reactions, and methanation, which is the combination of the water-gas and hydrogasification reactions. These reactions are a simple framework related to reactants and products of H, N, O, S, etc.

, H2

instead of H2

oxidation

or HCN instead of NO. Moreover, the formation of

O, and for other

in the feedstock [16]. Also, CO is produced instead of CO2

, and NH3

S instead of SO2

gases excluding oxygen.

14 pCx

Hy

a

Note that Cx

15 CnHm → nC + m/

Oxidation reactions

122 Gasification for Low-grade Feedstock

4 H2 + ½O2 → H2

Gasification reactions involving steam

2

O2 → nCO + m/

O → nCO + (n + m/

2

5 CnHm + n/

6 C + H2

7 CO + H2

8 CH4 + H2

9 CnHm + nH2

Gasification reactions involving hydrogen

11 CO + 3H2 → CH4 + H2

Gasification reactions involving carbon dioxide

13 CnHm + nCO2 → 2nCO + m/

Decomposition reactions of tars and hydrocarbons<sup>a</sup>

*3.2.2. Gasification reactions*

elements, H2

Equivalent ratio (ER) is defined as the ratio of the actual amount of oxidant to stoichiometric oxidant for complete combustion. This parameter is the most important operating parameter in gasification process because it affects syngas composition, tar content, gas yield, and its chemical energy. The pyrolysis process is operated at close to ER zero, and the combustion process is operated at more than ER one for complete combustion. In **Figure 5**, the conversion of char in the gasification process at ER 0.25 to 0.35 appears to maximize even though these gasifiers and those that are used in large-scale commercial plants (following **Table 4**), namely, moving grate gasifiers [25] and fluidized bed gasifiers [26] operated with wet fuels, are operated at about ER 0.5. With a lower ER, the gas yield from char is reduced, and the tar in syngas increases while with a higher ER, the oxidation reactions in the gasification process improve

**Figure 5.** Syngas composition at chemical equilibrium as a function of ER for the gasification of wood at 1 atm [29].


*3.3.5. Carbon conversion efficiency*

from it (secondary measures) [24, 27].

**4. MSW gasification technologies**

**4.1. Overview of existing gasification technologies**

*3.3.7. Other parameters*

syngas (Nm3

waste).

*3.3.6. Tar content*

verted to carbon in the syngas such as CO, CO2

in a landfill) as well as the chemical efficiency of the process.

Other parameters are the heating value of the syngas (kJ/Nm3

Carbon conversion efficiency (CCE) is defined as how many carbons in the waste gets con-

Ccarbon\_syngas)/(Qwaste Ccarbon\_waste); Ccarbon\_waste is the carbon fraction in the waste and Ccarbon\_syngas is the carbon fraction in the syngas with no dust or tar. This parameter shows the amount of the unconverted portion, which has to be treated by other processes or sent for disposal (such as

In the case of tar, if possible, the content and composition of the tar is analyzed. These tars, which are condensable heavy hydrocarbon materials, including oxygen-containing hydrocarbons and polyaromatic hydrocarbons, are an important parameter because they cause problems in all gasification processes, including the end of process [33]. The occurrence of excessive slag in boilers can cause blockages, corrosion, and also reduces the overall efficiency of the process. The amount of other metals and refractory surfaces increase and can also causes of ruin reforming catalysts, sulfur removal system, ceramic filters, etc. Also, if these tars are removed by a wet system using water, the tar is just moved from the gas to the water, and this water changes to wastewater with a loss of chemical energy of the gas and the generation of hazardous wastewater. Therefore, the content and composition of tar in syngas is an important factor in determining the energy conversion device that can be utilized, taking into consideration the cleaning system, and the technical and economic performance. These cleaning systems can be applied inside the reactor (primary measures) and/or downstream

/kg waste), and the specific energy production, that is, the chemical energy

of the syngas produced by the mass unit of waste fed to the gasification process (kJ/kg

Gasification can be considered as a process between pyrolysis and combustion in that it involves the partial oxidation of the material. This means that oxygen is injected but not enough to cause complete combustion. The temperatures are typically above 650–800°C. Although this process is mostly exothermic, it may be required to initialize and maintain the gasification process.

Raw MSW is not appropriate for the gasification process, so generally a separation is needed, including mechanical homogenization and the separation of glass, metals, and inert materials

, CH4 , C2 H6 , C3 H8

, etc., that is, CCE = (Qsyngas

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 125

), the flow rate of the specific

b This value can increase to about 10 MJ/m3 <sup>N</sup> in oxygen gasification processes.

**Table 4.** Typical ranges of operating and process performance parameters in air/ oxygen-enriched gasification of MSW [30].

and the heating value of syngas is reduced; this could cause incomplete combustion in a flare or syngas combustor, which is generally downstream from the gasifier [24, 27, 28].

#### *3.3.2. Reactor temperature*

Temperature profile along the reactor is another important characteristic of both allo-thermal (indirectly heated) gasifier and auto-thermal (directly heated) gasifier. The reactor temperature profile is considered as a state variable of the process, and it is affected by different parameters, such as ER, residence time, waste chemical energy, composition, inlet temperature of the gasifying medium, quality of the reactor insulation, etc. Moreover, the state of the bottom ash and the content of tar in the syngas can also be determined by the temperature profile of the reactor [24, 27].

#### *3.3.3. Residence time*

Generally, the residence time of gases and waste in the reactor is determined by the reactor type and design. Also, a fixed reactor type and design have limitations in terms of varied residence times: for example, the superficial gas velocity is varied in a fluidized bed and the velocity of grate elements is varied in the moving grate reactor [25, 31, 32].

#### *3.3.4. Cold gas efficiency*

Cold gas efficiency (CGE) is defined as the ratio between the heating value of the syngas produced and the heating value of the feedstock fed into the gasification process, that is, CGE = (Qsyngas LHVsyngas)/(Qwaste LHVwaste). This is called "cold gas efficiency" since it does not take into account the gas sensible heat but only its potential chemical energy, that is, those related to the combustion heats of obtained syngas and fed waste.

#### *3.3.5. Carbon conversion efficiency*

Carbon conversion efficiency (CCE) is defined as how many carbons in the waste gets converted to carbon in the syngas such as CO, CO2 , CH4 , C2 H6 , C3 H8 , etc., that is, CCE = (Qsyngas Ccarbon\_syngas)/(Qwaste Ccarbon\_waste); Ccarbon\_waste is the carbon fraction in the waste and Ccarbon\_syngas is the carbon fraction in the syngas with no dust or tar. This parameter shows the amount of the unconverted portion, which has to be treated by other processes or sent for disposal (such as in a landfill) as well as the chemical efficiency of the process.

#### *3.3.6. Tar content*

and the heating value of syngas is reduced; this could cause incomplete combustion in a flare

<sup>N</sup> in oxygen gasification processes.

**Table 4.** Typical ranges of operating and process performance parameters in air/ oxygen-enriched gasification of MSW

<sup>N</sup> 4–7b

Temperature profile along the reactor is another important characteristic of both allo-thermal (indirectly heated) gasifier and auto-thermal (directly heated) gasifier. The reactor temperature profile is considered as a state variable of the process, and it is affected by different parameters, such as ER, residence time, waste chemical energy, composition, inlet temperature of the gasifying medium, quality of the reactor insulation, etc. Moreover, the state of the bottom ash and the content of tar in the syngas can also be determined by the temperature

Generally, the residence time of gases and waste in the reactor is determined by the reactor type and design. Also, a fixed reactor type and design have limitations in terms of varied residence times: for example, the superficial gas velocity is varied in a fluidized bed and the

Cold gas efficiency (CGE) is defined as the ratio between the heating value of the syngas produced and the heating value of the feedstock fed into the gasification process, that is, CGE = (Qsyngas LHVsyngas)/(Qwaste LHVwaste). This is called "cold gas efficiency" since it does not take into account the gas sensible heat but only its potential chemical energy, that is, those

velocity of grate elements is varied in the moving grate reactor [25, 31, 32].

related to the combustion heats of obtained syngas and fed waste.

or syngas combustor, which is generally downstream from the gasifier [24, 27, 28].

 Equivalence ratio, − 0.25–0.35<sup>a</sup> Waste low heating value, MJ/kgwaste 7–18

 Carbon conversion efficiency, % 90–99 Cold gas efficiency, % 50–80

 Net electrical efficiency, % 15–24 Specific net energy, kWh/twaste 400–700

*3.3.2. Reactor temperature*

Operating parameters

a

b

[30].

Process performance parameters

124 Gasification for Low-grade Feedstock

Syngas low heating value, MJ/m3

This value can increase to about 10 MJ/m3

This value is typically equal to 0.50 in moving grate gasifiers.

profile of the reactor [24, 27].

*3.3.3. Residence time*

*3.3.4. Cold gas efficiency*

In the case of tar, if possible, the content and composition of the tar is analyzed. These tars, which are condensable heavy hydrocarbon materials, including oxygen-containing hydrocarbons and polyaromatic hydrocarbons, are an important parameter because they cause problems in all gasification processes, including the end of process [33]. The occurrence of excessive slag in boilers can cause blockages, corrosion, and also reduces the overall efficiency of the process. The amount of other metals and refractory surfaces increase and can also causes of ruin reforming catalysts, sulfur removal system, ceramic filters, etc. Also, if these tars are removed by a wet system using water, the tar is just moved from the gas to the water, and this water changes to wastewater with a loss of chemical energy of the gas and the generation of hazardous wastewater. Therefore, the content and composition of tar in syngas is an important factor in determining the energy conversion device that can be utilized, taking into consideration the cleaning system, and the technical and economic performance. These cleaning systems can be applied inside the reactor (primary measures) and/or downstream from it (secondary measures) [24, 27].

#### *3.3.7. Other parameters*

Other parameters are the heating value of the syngas (kJ/Nm3 ), the flow rate of the specific syngas (Nm3 /kg waste), and the specific energy production, that is, the chemical energy of the syngas produced by the mass unit of waste fed to the gasification process (kJ/kg waste).
