**4. MSW gasification technologies**

#### **4.1. Overview of existing gasification technologies**

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 before the treatment of residual waste. The main gasification syngas product contains carbon monoxide, hydrogen, and methane. Generally, the gas generated from gasification has a net calorific value (NCV) of 4–10 MJ/Nm3 . The calorific value of syngas from pyrolysis and gasification is lower than that of natural gas, which has a NCV of approximately 38 MJ/Nm3 [34]. As mentioned earlier, an important issue in using syngas in alternative thermal treatment facilities is a problem related to tar. The tar can cause blockages and other operational problems, and it is associated with plant failures and inefficiencies in many pilot and commercial-scale facilities. The application of the high-temperature secondary processing phase can be used to "crack" the tars and purify the syngas before applying the energy recovery systems. This process is referred to as "gas clean up" or "polishing," and can enable higher efficiency energy recovery than can be applied through other waste heat treatment processes.

Energos plant is using a mixture of post recycled MSW and industrial waste residue from material recovery facility as a feedstock. However, the amount of industrial waste is smaller compared with MSW. Before applying thermal treatment, using a low rpm and high torque shredder the feedstocks are shredded. After that ferrous metals are removed magnetically. Partial oxidation of the feedstock at sub-stoichiometric oxygen conditions (air to fuel ratio, k = 0.5–0.8), and combustion of the fixed carbon on the bed results in total organic carbon (TOC) of less than 3% from WTE ash in the first chamber of Energos process. In the adjoining chamber, the syngas generated during gasification are combusted completely, and the heat generated during combustion of the syngas is sent to the heat recovery system. During this process, temperatures climb up to 900 and 1000°C in the gasification chamber and oxidation chamber, respectively. All dioxins formed in this process are destroyed in combustion chamber and rapidly cooled in the heat recovery steam generator, which minimizes dioxin formation.

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 127

formation was also kept comparatively low in this process (around 25% of the EU limit).

A schematic diagram of the gasifier and the combustion chamber is shown in **Figure 6** [38].

Norwegian Environmental Agency and reported an 11% oxygen concentration.

After passing through the heat recovery steam generator, the flue gas enters into a dry flue gas cleaning system, which consists of a bag filter, an activated carbon injection, dry scrubbing with lime, and a filter dust silo. The lime absorbs the acidic compounds in the flue gas and the heavy metal molecules and activated carbon adsorb the dioxins. Emissions are continuously monitored. **Table 5** shows typical emission measurements at the Averoy Energos plant in Norway. These measurements were performed by TUV NORD Umweltschtz for the

NOx

**Figure 6.** Flowchart of a model Energos plant.

However, most commercial gasification facilities processing MSW-derived feedstock (SRF) utilize a secondary combustion chamber to burn the syngas and recover energy from a steam circuit, seeking to recover more energy. Other major products produced by gasification include solid residues of noncombustible materials (ash) that contain a small amount of carbon. Also, high-temperature plasma gasification technologies are being used at various stages of gasification process. Using this plasma technologies, tar-free clean syngas can be produced, as well as the ash can be fused into glassy or vitreous residue [35]. To recover high energy efficiency from hydrogen fuel cells attached with gasifiers and engines, different pathways are available. Waste to energy (WTE) processes are a combination of partial oxidation and volatilization of the contained organic compounds. The first gasification furnace is the combustion of the volatile gases and the steam generation of the second furnace. Japan is the world's largest producer of MSW gasification. However, the main technology used in Japan is the grate combustion of "as-received MSW," but there are more than 100 thermal treatment plants based on relatively novel processes such as direct smelting, the Ebara fluidization process, and melting process such as Thermoselect gasification [36, 37]. These processes produce glass fibers that are less hazardous than conventional WTE combustion processes and can be used advantageously in external landfills.

Transportation of as-collected MSW from one location to another is not permitted in Japan. Consequently, the grate combustion facilities are relatively small. In addition, the MSW is transported to a central WTE facility that serves as a SRF in local SRF facilities and in several communities. Additionally, all WTE facilities are used to vitrify their ash after combustion by means of electric furnaces, thermal plasma melting, or other means.

The following sections introduce several technologies available in worldwide.

#### **4.2. Energos grate combustion and gasification process**

The Energos grate combustion and gasification technology is currently operating one plant in Germany, six plants in Norway, and one in the UK. This technology was developed in Norway in the 1990s to provide an economical alternative to reducing greenhouse gas emissions such as those from gasoline. All operating plants handle MSW and commercial waste or industrial waste. The current operating plants range in capacity from 10,000 to 78,000 tons per year.

Energos plant is using a mixture of post recycled MSW and industrial waste residue from material recovery facility as a feedstock. However, the amount of industrial waste is smaller compared with MSW. Before applying thermal treatment, using a low rpm and high torque shredder the feedstocks are shredded. After that ferrous metals are removed magnetically. Partial oxidation of the feedstock at sub-stoichiometric oxygen conditions (air to fuel ratio, k = 0.5–0.8), and combustion of the fixed carbon on the bed results in total organic carbon (TOC) of less than 3% from WTE ash in the first chamber of Energos process. In the adjoining chamber, the syngas generated during gasification are combusted completely, and the heat generated during combustion of the syngas is sent to the heat recovery system. During this process, temperatures climb up to 900 and 1000°C in the gasification chamber and oxidation chamber, respectively. All dioxins formed in this process are destroyed in combustion chamber and rapidly cooled in the heat recovery steam generator, which minimizes dioxin formation. NOx formation was also kept comparatively low in this process (around 25% of the EU limit). A schematic diagram of the gasifier and the combustion chamber is shown in **Figure 6** [38].

After passing through the heat recovery steam generator, the flue gas enters into a dry flue gas cleaning system, which consists of a bag filter, an activated carbon injection, dry scrubbing with lime, and a filter dust silo. The lime absorbs the acidic compounds in the flue gas and the heavy metal molecules and activated carbon adsorb the dioxins. Emissions are continuously monitored. **Table 5** shows typical emission measurements at the Averoy Energos plant in Norway. These measurements were performed by TUV NORD Umweltschtz for the Norwegian Environmental Agency and reported an 11% oxygen concentration.

**Figure 6.** Flowchart of a model Energos plant.

before the treatment of residual waste. The main gasification syngas product contains carbon monoxide, hydrogen, and methane. Generally, the gas generated from gasification has a net

mentioned earlier, an important issue in using syngas in alternative thermal treatment facilities is a problem related to tar. The tar can cause blockages and other operational problems, and it is associated with plant failures and inefficiencies in many pilot and commercial-scale facilities. The application of the high-temperature secondary processing phase can be used to "crack" the tars and purify the syngas before applying the energy recovery systems. This process is referred to as "gas clean up" or "polishing," and can enable higher efficiency energy

However, most commercial gasification facilities processing MSW-derived feedstock (SRF) utilize a secondary combustion chamber to burn the syngas and recover energy from a steam circuit, seeking to recover more energy. Other major products produced by gasification include solid residues of noncombustible materials (ash) that contain a small amount of carbon. Also, high-temperature plasma gasification technologies are being used at various stages of gasification process. Using this plasma technologies, tar-free clean syngas can be produced, as well as the ash can be fused into glassy or vitreous residue [35]. To recover high energy efficiency from hydrogen fuel cells attached with gasifiers and engines, different pathways are available. Waste to energy (WTE) processes are a combination of partial oxidation and volatilization of the contained organic compounds. The first gasification furnace is the combustion of the volatile gases and the steam generation of the second furnace. Japan is the world's largest producer of MSW gasification. However, the main technology used in Japan is the grate combustion of "as-received MSW," but there are more than 100 thermal treatment plants based on relatively novel processes such as direct smelting, the Ebara fluidization process, and melting process such as Thermoselect gasification [36, 37]. These processes produce glass fibers that are less hazardous than conventional WTE combustion processes and can be

Transportation of as-collected MSW from one location to another is not permitted in Japan. Consequently, the grate combustion facilities are relatively small. In addition, the MSW is transported to a central WTE facility that serves as a SRF in local SRF facilities and in several communities. Additionally, all WTE facilities are used to vitrify their ash after combustion by

The Energos grate combustion and gasification technology is currently operating one plant in Germany, six plants in Norway, and one in the UK. This technology was developed in Norway in the 1990s to provide an economical alternative to reducing greenhouse gas emissions such as those from gasoline. All operating plants handle MSW and commercial waste or industrial waste. The current operating plants range in capacity from 10,000 to 78,000 tons per year.

means of electric furnaces, thermal plasma melting, or other means.

**4.2. Energos grate combustion and gasification process**

The following sections introduce several technologies available in worldwide.

cation is lower than that of natural gas, which has a NCV of approximately 38 MJ/Nm3

recovery than can be applied through other waste heat treatment processes.

. The calorific value of syngas from pyrolysis and gasifi-

[34]. As

calorific value (NCV) of 4–10 MJ/Nm3

126 Gasification for Low-grade Feedstock

used advantageously in external landfills.


**Table 5.** Emissions from Energos plant (at 11% oxygen) [38].

The reported availability of the Energos plants is approximately 90% (8000 hours per year, similar to a typical combustion WTE plant).

## **4.3. Ebara fluidized bed process**

The Ebara process (**Figure 7**) consists of partial combustion of shredded MSW in a fluidized bed reactor. The second furnace is where the gas produced in the fluidized bed reactor is combusted to generate temperatures up to 1350°C [36]. There is no oxygen enrichment. The largest application of the Ebara process is a three-line in Spain, with 900 tons per day.

at much higher heat efficiency than is possible in the conventional WTE plant using a steam

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 129

Recent research has shown there is a growing interest in plasma-assisted gasification of MSW. A plasma torch is a device that transforms electricity into heat by passing the current through a gas stream. Increased interest is focused on plasma-assisted gasification applied to the treatment of MSW. It might be a new way to increase WTE around the world. The Earth Engineering Center of Columbia University under the supervision of Professor Nickolas J. Themelis conducted a study of this technology. Plasma technology has long been used for the destruction of harmful materials such as asbestos, toxic wastes from hospitals, and surface coatings. Although plasma technology has been used for these purposes, its application in MSW has not yet been studied. Plasma-assisted gasification in the WTE process combines the partial oxidation of hydrocarbon in the MSW and the use of plasma. Using a relatively high voltage, high-current electricity is passed between two electrodes to create an electric arc. The inert gas is passed through the arc under pressure and is transferred to a closed container of waste, reaching a maximum temperature of 13,900°C in the arc heat. The temperature from the torch can reach 2760–4427°C. At this temperature, most types of waste are decomposed into gaseous elements, and complex molecules are separated into atoms. This arc decomposes

turbine.

**4.5. Plasma-assisted gasification WTE process**

**Figure 7.** Ebara fluid bed gasification process.

In the reactor, the ash overflow from the fluidized bed is separated using a vibrating screen whose screen size is 3–4 mm. Metal particles are unable to pass it, however, sand particles can. The bottom ash produced during this process cannot be used for pavement construction purpose; it must be melted with slag, which is the final solid product used in construction areas. The Spanish plant using the Ebara process produces approximately 560 kWh per SRF ton.

#### **4.4. Thermoselect gasification and melting process**

Many plants, ranging from grate combustion to the Japan steel[Fe] engineering (JFE) direct smelting process and the seven JFE Thermoselect plants with a total capacity of 2000 tons per day, are operated by the JFE steel company of Japan [37]. In order to enter the gas turbines or engines, which generate electricity, the syngas produced in Thermoselect furnaces requires purification. Compared to conventional grate combustion, the amount of processed gas per ton of MSW is low. However, cleaning the reducing gas is more complicated than cleaning combustion processed gas. The Thermoselect process also produces industrial oxygen used for partial oxidation and gasification of MSW using part of the generated electricity. There is the possibility that the syngas product can be burned in a gas turbine to generate electricity

**Figure 7.** Ebara fluid bed gasification process.

The reported availability of the Energos plants is approximately 90% (8000 hours per year,

TEQ) 0.1 0.001

**) Energos, Averoy**

The Ebara process (**Figure 7**) consists of partial combustion of shredded MSW in a fluidized bed reactor. The second furnace is where the gas produced in the fluidized bed reactor is combusted to generate temperatures up to 1350°C [36]. There is no oxygen enrichment. The

In the reactor, the ash overflow from the fluidized bed is separated using a vibrating screen whose screen size is 3–4 mm. Metal particles are unable to pass it, however, sand particles can. The bottom ash produced during this process cannot be used for pavement construction purpose; it must be melted with slag, which is the final solid product used in construction areas. The Spanish plant using the Ebara process produces approximately 560 kWh per SRF ton.

Many plants, ranging from grate combustion to the Japan steel[Fe] engineering (JFE) direct smelting process and the seven JFE Thermoselect plants with a total capacity of 2000 tons per day, are operated by the JFE steel company of Japan [37]. In order to enter the gas turbines or engines, which generate electricity, the syngas produced in Thermoselect furnaces requires purification. Compared to conventional grate combustion, the amount of processed gas per ton of MSW is low. However, cleaning the reducing gas is more complicated than cleaning combustion processed gas. The Thermoselect process also produces industrial oxygen used for partial oxidation and gasification of MSW using part of the generated electricity. There is the possibility that the syngas product can be burned in a gas turbine to generate electricity

largest application of the Ebara process is a three-line in Spain, with 900 tons per day.

similar to a typical combustion WTE plant).

**Table 5.** Emissions from Energos plant (at 11% oxygen) [38].

**Parameter EU limits (mg/Nm3**

Particulate matter 10 0.24 Hg 0.05 0.00327 Cd + Ti 0.05 0.00002 Metals 0.5 0.00256 CO 50 2 HF 1 0.02 HCl 10 3.6 TOC 10 0.2 NOx 200 42 NO3 10 0.3 SO2 50 19.8

**4.4. Thermoselect gasification and melting process**

**4.3. Ebara fluidized bed process**

Dioxins (ng/Nm3

128 Gasification for Low-grade Feedstock

at much higher heat efficiency than is possible in the conventional WTE plant using a steam turbine.

#### **4.5. Plasma-assisted gasification WTE process**

Recent research has shown there is a growing interest in plasma-assisted gasification of MSW. A plasma torch is a device that transforms electricity into heat by passing the current through a gas stream. Increased interest is focused on plasma-assisted gasification applied to the treatment of MSW. It might be a new way to increase WTE around the world. The Earth Engineering Center of Columbia University under the supervision of Professor Nickolas J. Themelis conducted a study of this technology. Plasma technology has long been used for the destruction of harmful materials such as asbestos, toxic wastes from hospitals, and surface coatings. Although plasma technology has been used for these purposes, its application in MSW has not yet been studied. Plasma-assisted gasification in the WTE process combines the partial oxidation of hydrocarbon in the MSW and the use of plasma. Using a relatively high voltage, high-current electricity is passed between two electrodes to create an electric arc. The inert gas is passed through the arc under pressure and is transferred to a closed container of waste, reaching a maximum temperature of 13,900°C in the arc heat. The temperature from the torch can reach 2760–4427°C. At this temperature, most types of waste are decomposed into gaseous elements, and complex molecules are separated into atoms. This arc decomposes the waste with a device known as a plasma converter to a molecular gas and solid waste (slag). This process is for net generators of electricity depending on the composition of the input waste, and the amount of waste sent to landfills is reduced.

lower the compression ratio of diesel engine [40, 44]. Due to the lower heating value (LHV), untransformed engines show superior performance than engines converted to gas. Nevertheless, a correctly modified modern engine can allow more than 25% of the net power output [44]. The engine has the advantage of being stronger than gas turbines, and it is more resistant to pollutants [10]. Nevertheless, when the gas is compressed into a turbocharger, the same condition as in the gas turbine will occur [10, 44]. The main disadvantage of the gas engine is that the efficiency achieved using the combined cycle mode is low, and the economies of scale are poor [10].

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 131

The power plants that build on advanced combined cycle gas turbines could enable an efficiency rate of approximately 60% [45]. Due to the consumption for gas pre-treatment, the effective net electrical output is below 40% [46, 47]. In fact, gas turbines allow extremely low levels of pollutants, mainly tar, alkali metals, sulfur, and chlorine compounds [10]. The chemical recovery cycle is an exciting and novel option. In this case, pre-treatment of the gas, which usually uses the tar or the catalytic cracking process of the steam reforming process, needs the energy content in the turbine exhaust gas [14, 48]. Typical gas turbines should be suitable for low LHV, for easy start-up, the burner must allow dual fuel operation, and a longer combus-

Environmental performance in a MSW thermal treatment technology is important for the feasibility of the whole process. Recent research [51, 52] has shown that the operation of thermochemical and biochemical solid waste conversion processes poses little risk to human health or the environment compared to other commercial processes. Biochemical processes and those of anaerobic digestion have gained a wider acceptance in recent years [53]. The strong opposition to gasification processes from environmental organizations is the result of misunderstanding that these processes are only minor variations of incineration. As mentioned above, an important difference is that gasification is an intermediate process for producing fuel gas that can be used for various purposes. The most common process these days is the use of syngas for the production of on-site electricity and/or thermal energy, but there is a potential for chemical and fuel production due to the gasification of MSW, and this is possibly a true goal in the near future. The type of indirect combustion process discussed above is already emphasized in several important aspects that make it different from conventional incineration. Moreover, it makes air pollution control easier and cheaper compared with the conventional combustion processes. Although exhaust gas cleanup of thermochemical conversion processes is easier compared with incineration process, still a proper process and emission control system design is required to satisfy the safety and health requirements. The producer gas obtained from gasification process includes various air pollutants that must be controlled before being discharged to outside. These include hydrocarbons, carbon monoxide,

tion chamber is needed to improve CO emissions control [49, 50].

**5.3. Gas turbine**

**6. Environmental impacts**

**6.1. Air pollution**

For MSW processing, a plasma torch can be used to gasify the solids, dissolve volatile gases, and electrify ash into slag and metal globules. A syngas product can be used to produce synthetic fuels or electricity in a gas engine or turbine generator. As mentioned in the previously discussed earth engineering center (EEC) study, the technology is a Westinghouse plasma owned by Alter NRG, Plasco Energy Group, Europlasma, and the In EnTec Process [39]. A major benefit to grate combustion is a dramatic reduction in process gas flow (up to 75%). In addition, the reducing atmosphere in the gasification process should reduce NOx emissions more than in the grate combustion process. However, this study showed that the cost of capital per ton of capacity is the same as that of grate combustion. Since electricity is used for high-temperature gas, energy production per ton of raw material is not expected to be higher than that of combustion. In a system such as the Alter NRG gasification process, it is expected to generate approximately 0.6 MWh/ton of MSW. Finally, the availability of these plants is different from the combustion process WTE plants (8000 hours annually).
