**6. Environmental impacts**

#### **6.1. Air pollution**

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, tars, nitrogen and sulfur oxides, dioxins and furans, and particle materials. Various strategies can be adopted to control exhaust gas in the gasification process, and, as mentioned above, they are rigorously dependent on the adopted plant configurations, especially regarding the particular requirements of the specific energy conversion device. In any case, an obvious advantage in that air pollution control is possible not only at the reactor outlet but also at the exhaust gas outlet through a variety of approaches. Furthermore, the low levels of oxygen (ER ranges between 0.25 and 0.50) in the gasification process strongly inhibit the formation of dioxins and furans even though hydrogen chloride in the syngas must be managed if combustion for heat or power follows gasification. Recently collected emissions data indicate that gasification technology meets emission standards [52]. A synthesis of these data is shown in **Table 6**, together with the limits of the European Community and Japanese standards.

#### **6.2. Solid residue treatment**

It is important to report some considerations regarding the management of solid residues such as bottom ash and air pollution control (APC) residues to define the environmental performance of gasification-based WTE facilities. Depending on the type of waste and on the specific gasification technology, the type and composition of these residues differ greatly [22, 51, 53, 58]. **Table 7** reports some leaching tests carried out on the slags of two large-scale, high-temperature gasification units. All values are significantly lower than the emission standard, and the low impurity content of the slag and its good homogeneity make it possible to sell for a variety of uses such as aggregates in asphalt pavement mixtures. The metals recovered from the melting section can be also recovered during the chemical treatment of fly ash and then landfilled.

Therefore, it can be deduced that the amount of solid residues generated in the MSW gasifica-

Quality standard for soil (in agreement with Notification No. 46, Japanese Ministry of the Environment and the JIS-

Tests carried out in a Nippon Steel high-temperature shaft furnace with a capacity of 252 tons/day of MSW, bottom ash

Tests carried out in a JFE high-temperature shaft furnace plant having a capacity of 314 tons/day of RDF from MSW [32].

**Element (mg/L) Regulationa Measuredb Measuredc Korea standardd**

Gasification of Municipal Solid Waste http://dx.doi.org/10.5772/intechopen.73685 133

Cd < 0.01 < 0.001 < 0.001 < 0.03 Pb < 0.01 < 0.005 < 0.005 < 0.1 Cr6+ < 0.05 < 0.02 < 0.02 < 0.05 As < 0.01 < 0.001 < 0.005 < 0.05 T-Hg < 0.0005 < 0.0005 < 0.0005 < 0.001 Se < 0.01 < 0.001 < 0.002 — F < 0.8 — < 0.08 — B < 1.0 — < 0.01 —

In the gasification process, wastewater produced by the gas cooler and the wet scrubber containing many soluble and insoluble pollutants such as acetic acid, sulfur, phenol, and other organic compounds [10]. The insoluble matter in this wastewater is mainly composed of tar. The amount of wastewater generated by removing tar through the scrubber is about 0.5 kg/Nm3 of treated gas [60], and requires expensive treatment. There are also some minor problems such as high salt content and low pH associated with the wastewater generated in gasification process. However, these can be controlled easily by doing chemical precipitation and

"In the gasification plant Thermie Energy Farm, one of the three IGCC projects selected for funding by the European Union, the sequence of treatment for tar-rich wastewater is: (a) precipitation of sulfur by iron sulfate addition; (b) recovery of sulfur and dust by filtering; (c) disposal of filter cake; (d) stripping off gases and the major part of the hydrocarbons dissolved in the water; (e) partial evaporation of water and usage of condensate as scrubber make-up;

The recovered salts are treated through sanitary landfills because their potential for contamination is very low. The hydrocarbons and the recovered gas are decomposed and recovered as energy in the combustor [60, 62]. Recent trends due to difficulties in treatment and disposal are developing tar-free gasification technologies, but this is nonetheless possible only

and (f) discharge of evaporator blowdown to conventional bio-treatment" [60, 62].

tion process is reduced and the throughput at the landfill can be reduced.

**Table 7.** Results of some slag leaching tests in two high-temperature MSW gasifiers [30].

dRecycling standard of waste (No. 5 of enforcement regulations in waste management law in Korea).

from other MSW incinerators, and residues from recycling centers [59].

**6.3. Wastewater treatment**

Japanese Industrial Standard K0058).

a

b

c

neutralization [61].

for wastes with low contaminant content [10].


**Table 6.** Some certified emissions from waste gasification plants [30, 48, 52, 54–57].


a Quality standard for soil (in agreement with Notification No. 46, Japanese Ministry of the Environment and the JIS-Japanese Industrial Standard K0058).

b Tests carried out in a Nippon Steel high-temperature shaft furnace with a capacity of 252 tons/day of MSW, bottom ash from other MSW incinerators, and residues from recycling centers [59].

c Tests carried out in a JFE high-temperature shaft furnace plant having a capacity of 314 tons/day of RDF from MSW [32]. dRecycling standard of waste (No. 5 of enforcement regulations in waste management law in Korea).

**Table 7.** Results of some slag leaching tests in two high-temperature MSW gasifiers [30].

Therefore, it can be deduced that the amount of solid residues generated in the MSW gasification process is reduced and the throughput at the landfill can be reduced.

#### **6.3. Wastewater treatment**

tars, nitrogen and sulfur oxides, dioxins and furans, and particle materials. Various strategies can be adopted to control exhaust gas in the gasification process, and, as mentioned above, they are rigorously dependent on the adopted plant configurations, especially regarding the particular requirements of the specific energy conversion device. In any case, an obvious advantage in that air pollution control is possible not only at the reactor outlet but also at the exhaust gas outlet through a variety of approaches. Furthermore, the low levels of oxygen (ER ranges between 0.25 and 0.50) in the gasification process strongly inhibit the formation of dioxins and furans even though hydrogen chloride in the syngas must be managed if combustion for heat or power follows gasification. Recently collected emissions data indicate that gasification technology meets emission standards [52]. A synthesis of these data is shown in **Table 6**, together with the limits of the European Community and Japanese standards.

It is important to report some considerations regarding the management of solid residues such as bottom ash and air pollution control (APC) residues to define the environmental performance of gasification-based WTE facilities. Depending on the type of waste and on the specific gasification technology, the type and composition of these residues differ greatly [22, 51, 53, 58]. **Table 7** reports some leaching tests carried out on the slags of two large-scale, high-temperature gasification units. All values are significantly lower than the emission standard, and the low impurity content of the slag and its good homogeneity make it possible to sell for a variety of uses such as aggregates in asphalt pavement mixtures. The metals recovered from the melting section can be also recovered during the chemical treatment of fly ash and then landfilled.

> **Mitsui R21 Toyohashi, Japan**

400 tons/day

0.032 0.018 0.000051 0.0032 0.0008 0.006 0.1/0.1 —

**Energos Averoy, Norway**

400 tons/day **Plasco En. Ottawa, Canada**

100 tons/day **EC/ Japanese Standard**

**Korea Standard**

**6.2. Solid residue treatment**

132 Gasification for Low-grade Feedstock

**Nippon Steel Kazusa, Japan**

<sup>N</sup>(at 11% O2

200 tons/day **Thermoselect Nagasaki, Japan**

300 tons/day

)

**Ebara TwinRec Kawaguchi, Japan**

420 tons/day

**Table 6.** Some certified emissions from waste gasification plants [30, 48, 52, 54–57].

2.3 MWe 8 MWe 5.5 MWe 8.7 MWe 10.2 MWe —

Particulate 10.1 < 3.4 < 1 < 0.71 0.24 9.1 10/11 14.2 HCl < 8.9 8.3 < 2 39.9 3.61 2.2 10/90 16.7 NOx 22.3 — 29 59.1 42 107 200/229 106.8 SOx < 15.6 — < 2.9 18.5 19.8 19 50/161 85.5 Hg — — < 0.005 — 0.0026 0.0001 0.03/− 0.09

**Company, plant location**

Waste capacity

Power production

Dioxins/ furans, n-TEQ/m3 N

Emission, mg/m3

In the gasification process, wastewater produced by the gas cooler and the wet scrubber containing many soluble and insoluble pollutants such as acetic acid, sulfur, phenol, and other organic compounds [10]. The insoluble matter in this wastewater is mainly composed of tar. The amount of wastewater generated by removing tar through the scrubber is about 0.5 kg/Nm3 of treated gas [60], and requires expensive treatment. There are also some minor problems such as high salt content and low pH associated with the wastewater generated in gasification process. However, these can be controlled easily by doing chemical precipitation and neutralization [61].

"In the gasification plant Thermie Energy Farm, one of the three IGCC projects selected for funding by the European Union, the sequence of treatment for tar-rich wastewater is: (a) precipitation of sulfur by iron sulfate addition; (b) recovery of sulfur and dust by filtering; (c) disposal of filter cake; (d) stripping off gases and the major part of the hydrocarbons dissolved in the water; (e) partial evaporation of water and usage of condensate as scrubber make-up; and (f) discharge of evaporator blowdown to conventional bio-treatment" [60, 62].

The recovered salts are treated through sanitary landfills because their potential for contamination is very low. The hydrocarbons and the recovered gas are decomposed and recovered as energy in the combustor [60, 62]. Recent trends due to difficulties in treatment and disposal are developing tar-free gasification technologies, but this is nonetheless possible only for wastes with low contaminant content [10].
