ABSTRACT
The waste problem in Indonesia still causes many social and environmental problems, so the application of technology that is able to handle the high volume of waste is increasingly urgent. Thermal technology solutions in waste management that are available today still require feasibility studies to be applied in Indonesia. One thermal technology that has been widely applied in various countries is plasma gasification. Plasma gasification is an effective method for decomposing various organic and inorganic compounds into the basic elements of a compound. The plasma gasification process includes feed handling, plasma gasification, cooling, compression, and cleaning of synthetic gas. Plasma gasification can convert organic waste material into syanthetic gas that can be used to produce efficient energy and fulfill electricity needs.
Explore the revolutionary potential of plasma gasification
technology, a cutting-edge waste-to-energy solution that transforms solid waste
into renewable energy. This innovative process involves high-temperature plasma
breakdown, leading to syngas production and the creation of vitrified residue.
Not only does this approach address waste reduction and efficient waste
management, but it also contributes to environmental sustainability by
generating clean, renewable energy. Discover how plasma gasification plays a
crucial role in creating a sustainable future, reducing reliance on landfills,
and effectively treating various waste streams, including hazardous waste.
Solid waste, both biomass and plastic, in urban areas causes
social and environmental problems in almost all major cities in Indonesia. The
handling of urban waste in various landfills is not implemented consistently as
planned. Based on the website of the Directorate General of PPKL-Ministry of
Environment and Forestry, the amount of Indonesian waste in 2018 reached 66
million metric tonnes per year. The composition of Indonesia's waste is organic
waste (food waste, wood branches, leaves) of 57%, plastic waste of 16%, paper
waste of 10%, and others (metal, textiles, leather, rubber, glass) of 17%. The
average percentage of waste processed by composting for cities in Indonesia is
16.2%, or about 11 million metric tonnes per year. There is still 82% unmanaged.
Waste that is not managed properly will have a negative impact. It is important
to take seriously the decline in environmental quality due to waste.
There are several methods of waste management, such as open dumps, sanitary landfills, composting, and incineration. However, these types of waste management can lead to environmental problems, one of which is groundwater pollution. Environmental pollution can lead to an increased spread of disease, reduce environmental aesthetics, and have an impact on global warming.
Thermal technologies such as incineration, gasification, and pyrolysis used for waste treatment are the most effective because they can reduce the volume of waste, reduce the toxicity of waste, and produce valuable products that can be further processed. Currently, thermal and non-thermal plasma technologies are widely used in industrial applications, including waste treatment. Thermal plasma used in plasma gasification for processing is a safe and efficient option because it is done in an environmentally friendly way. Plasma gasification can treat a wide range of waste, including hazardous waste. The main advantage of plasma gasification is that it can convert waste into energy and help reduce dependence on fossil fuels. In addition, plasma gasification produces up to 100 times fewer dioxins than incineration.
RESEARCH METHODS
This research was conducted with literature reviews on the utilisation of solid waste and plamsa gasification technology used to convert solid waste into new renewable energy sources. Thermal waste-to-energy conversion technology has not been applied in Indonesia until now. However, there are currently more than 1200 waste-to-energy plants operating in more than 40 countries around the world (ISWA, 2016). These operating incineration technologies are equipped with adequate flue gas cleaning equipment and sophisticated combustion controls to meet the needs of stringent emission standards. In this journal, the author will try to explore appropriate technologies to overcome waste problems in Indonesia. One of the technologies that can be a solution to overcoming waste is the utilisation of plasma technology, or what is known as plasma gasification. This chapter will discuss solid waste conversion, plasma technology, and types of plasma.
A. Solid Waste Conversion
There are basically two alternative waste-to-energy processes, namely biological and thermal processes. The basic difference between the two is that the biological process produces biogases, which are then burned to produce power that will drive motors and generators, while the thermal process produces heat that can be used to generate steam and drive steam turbines and generators. Anaerobic digestion (biogas) or landfills are two methods for achieving the biological process. While the thermal process can be achieved in several ways, namely incineration, pyrolysis, and gasification,.
Table 1: Thermal technology differences
|
Parameters |
Combustion |
Gasification |
Pyrolysis |
|
Temperature (oC) |
800 - 1450 |
500 - 1800 |
250 - 900 |
|
Atmosphere |
Water |
O2, H2O |
Inert, N2 |
|
Products |
>1 |
<1 |
0 |
|
- Gas Phase |
CO2, H2O, O2, N2 |
H2, CO, CO2, CH4, H2O |
CO, CO, H2O, N2, HC |
|
- Solid Phase |
Ash, slag |
Ash, slag |
Ash, coke |
|
- Liquid Phase |
- |
- |
Pyrolysis oil, water |
Plasma is the fourth form of the main substance that exists on this earth, in addition to solid, liquid, and gas. Plasma is an ionized gas produced from spontaneous electrical discharge. Plasma on a laboratory scale can be divided into two groups, namely high-temperature plasma (fusion plasma) and low-temperature plasma (gas discharge). The classification of various types of plasma can be seen in Table 2 below. High-temperature plasma means that all charges (electrons, ions, and neutral charges) are in a state of thermal equilibrium. Low-temperature plasma is further divided into thermal plasma (quasi-equilibrium plasma), which is in LTE (local thermal equilibrium) conditions, and non-thermal plasma or cold plasma (non-equilibrium plasma).
Table 2: Classification of plasma types
|
Plasma
Type |
Temperature |
Example |
|
High Temperature Plasma (Equilibrium
plasma) |
Te = Ti = Th; Tp = 106 - 108 K; ne ≥ 1020 m-3 |
Laser plasma fusion
|
|
Low
Temperature Plasma / thermal plasma (Quasi-equilibrium
plasma) |
Te ≈ Ti
≈ Th; Tp = 2 x 103 - 3 x 104 K;
ne ≥ 1020 m-3 |
Plasma arc; Atmospheric RF discharge |
|
Non-thermal
plasma (Non-equilibrium plasma) |
Te" Th; Tp ≈ 3
x 102 -
4.5 x 102 K;
ne ≈
1010 m-3 |
Corona discharge |
As noted earlier, plasma is often referred to as the fourth state of matter. Plasmas differ from other lower-energy states of matter, such as solids, liquids, and gases. The differences between the four states of matter can be seen in Table 3 below.
Table 3: Differences between Plasma and the other three types of matter
|
Material Type |
Solids |
Fluids |
Gas |
Plasma |
|
Example |
Es (H2O) |
Water
(H2O) |
Steam
(H2O) |
Ionised gas (H2→H+ + H+ +
2e )- |
|
Temperature |
< 0ºC |
0 < T
< 100ºC |
T > 100ºC |
Extremely
Hot
T > 100,000ºC |
|
Picture of Molecul es |
 |
 |
|
|
Plasma
is the material in nature that can support a successful fusion reaction. It is
a collection of high-density gas (107-1.032 m2) that is ionized and
exists within a sea of electrons. Plasmas are generated by providing enough
energy to release the electrons while maintaining a neutral total charge,
usually through the provision of thermal energy. The plasma temperature itself
is measured in units of electron volts (eV) and can reach 105 eV or the
equivalent of 1010 K. This temperature refers to electrons, while the
temperature of ions is usually much lower. Plasma, based on its temperature,
can be categorized into two types: hot plasma and cold plasma. Hot plasma is
used in fusion reactions, while cold plasma finds applications in gas
purification, surface treatment, waste management, and hydrocarbon
decomposition."
C. Thermal Plasma
It is a plasma that possesses high energy density, with a temperature similarity between heavy particles (atoms, molecules, ions) and electrons. Due to the significantly higher mobility, the energy imparted to the plasma is captured by the electrons, which are then transferred to the heavy particles through elastic collisions. Because of the high electron number density associated with operation at atmospheric pressure, the frequency of elastic collisions is very high, and thermal equilibrium is quickly reached.
Examples of thermal plasmas include plasmas generated from direct current (DC) or radio frequency (RF) currents. The advantages of thermal plasmas encompass high temperature, high intensity, non-ionizing radiation, and high energy density. The heat source can effectively address sharp surfaces and steep thermal gradients without being dependent on chemistry. While the upper limit of achievable temperature in fossil fuel combustion is around 2000°C, thermal plasma generated from electricity can reach temperatures of 20,000°C or more.
Thermal plasma reactors offer various advantages, including high quench rates that enable the production of specific gas and solid material compositions. They also require low gas flow rates (except for non-transfer plasma equipment) compared to fossil fuel combustion, thereby reducing the need for off-gas treatment.
Thermal plasmas
find applications in material processing due to their high energy density,
allowing them to heat, melt, and, in many cases, vaporize the materials being
treated. Additionally, thermal plasmas are used for chemical synthesis as they
serve as a source of reactive species at high temperatures. This is
particularly crucial in the preparation of pigments, high-purity synthetic
silica, high-purity ultra-fine ceramics, and inorganic powders.
D. Cold or Non-thermal Plasma
Cold or non-thermal plasmas have lower energy density, resulting in a significant temperature difference between electrons and heavier particles. Electrons, possessing sufficient energy, collide with the background gas, leading to dissociation, excitation, and low-level ionization without a considerable increase in gas enthalpy. Consequently, the electron temperature exceeds that of the heavy particles by several orders of magnitude, enabling the maintenance of discharge temperature at much lower levels, even at room temperature.
Cold or non-thermal plasmas produce active species that are more diverse and higher in energy than those commonly generated in chemical reactors. The presence of these active species allows processes to be carried out on the material's surface that cannot be achieved by other means or are impractical and uneconomical if conducted using alternative methods.
Application-wise, cold or non-thermal plasmas are used for local surface
modification, as ions, atoms, and molecules remain relatively cold and do not
cause thermal damage to the touched surface. This type of plasma is generated
in various types of incandescent discharge, low-pressure RF discharge, and
corona discharge, with energy densities ranging from 10-4 to tens of
watts per cm-3.
Currently,
cold or non-thermal plasma technology is widely employed by industries for
coating and etching processes, as well as for treating NOx and SOx exhaust
gases. Exhaust gases containing NOx and/or SOx are exposed to plasma, resulting
in the formation of radicals that initiate complex reactions, converting NOx
and/or SOx into specific products. This mechanism takes place in the NOx and/or
SOx removal plasma reactor. When the flue gas comes into contact with the
plasma, radicals are formed. Additive gases such as ammonia (NH3) or
hydrocarbons like methane (CH4) need to be introduced to generate
additional radicals, promoting particulate formation reactions. Furthermore,
the addition of these gases is tailored to the expected end product. Some
products resulting from flue gas treatment, such as ammonium nitrate (NH4)NO3,
can be utilized for fertilizers.
DISCUSSION
Plasma gasification is distinct from incineration as it does not involve the burning of waste. Instead, plasma gasification is the process of transforming carbon-based materials in an oxygen-deficient environment using an external high heat source (plasma). This process yields a fuel gas, commonly referred to as synthesis gas, which can be utilized for various applications. In this chapter, we will delve into the discussion of plasma gasification, including its methods and processes.
A. Plasma Gasification
It is an effective method for breaking down various organic and inorganic compounds into their basic elements, allowing for reuse and recycling. The pivotal component of a plasma gasification system is a plasma reactor, which can consist of one or more plasma torches. A plasma torch is formed by applying DC voltage to two electrodes and passing gas through them, resulting in a very high temperature ranging from 5,000 °C to 10,000 °C.
Currently, thermal plasma is the most
commonly used type of plasma for the plasma gasification process. The reactor
plasma operates under sub-stoichiometric conditions, without oxygen entering
the reactor plasma, preventing any combustion process. Consequently, the plasma
gasification system distinguishes itself from an incinerator or other
combustion furnace.
 |
| Figure 1. Simple Schematic of a Plasma
Gasification Reactor |
With temperatures reaching 10,000 °C, plasma
can decompose various toxic compounds within 1/1,000 seconds, eliminating the
formation of other compounds and the production of toxic gases typically
associated with incinerator combustion. These extreme temperatures are
achievable only with a plasma torch system, as high temperatures are required
to break down organic compound molecules into basic gas compounds such as
carbon monoxide and hydrogen. Similarly, inorganic compounds can be melted into
molten glass, which then crystallizes.
Plasma gasification units eliminate the need
for large landfills and address various environmental challenges by converting
municipal solid waste and hazardous waste into fuel. These units process
carbon-containing materials, including municipal solid waste and hazardous
bio-waste from hospitals, and yield two beneficial by-products: Synthesis Gas,
an energy-rich fuel used to generate 'green electricity' from a non-polluting
source, and a commercially useful inert solid substance commonly known as 'slag.'
This slag can be utilized as a road and building material. This method is
referred to as Plasma Gasification.
There are two methods used in plasma
gasification: 'plasma arc' and 'plasma torch.' 'Plasma arc' gasification units
operate on the same principle as arc-welding machines, where an electric arc is
formed between two electrodes. This high-energy arc produces a
high-temperature, highly ionized gas, enclosed in a chamber. Waste material is
introduced into the chamber, and the intense heat of the plasma causes the
breakdown of organic molecules (such as oils, solvents, and paints) into their
basic atoms. In a well-controlled process, these atoms form harmless gases,
such as CO2. Solids like glass and metals melt to create a frozen
lava-like material, in which toxic metals are encapsulated. With plasma arc
technology, there is no burning, incineration, or dust formation.
The 'plasma arc' Plasma Gasification Unit
boasts a remarkably high crushing efficiency. This unit is robust, capable of
destroying any type of waste with or without pre-treatment, and produces a
stable waste form. The burned gas is cleaned by an off-gas system and oxidized
to CO2 and H2O in a ceramic oxidizer. There is minimal
potential for air pollution due to the use of electric heating in the absence
of free oxygen. The inorganic part of the waste is preserved in the form of
stable, yield-resistant slag.
In a 'plasma torch' Plasma Gasification Unit, an arc is formed between a copper electrode and molten slag or other electrodes with different polarities. Similar to the 'plasma arc' system, the plasma torch system exhibits a very high degree of destruction efficiency, is robust, and can treat any kind of waste or medium with or without pretreatment. The inorganic part of the waste is preserved in the form of stable, melt-resistant slag. The air pollution control system used is larger than the plasma arc system due to the need to stabilize the torch gas ('Plasma Gasification,' n.d.). Below is an illustration of how a plasma torch works.
 |
| Illustration of the plasma torch |
Working Plasma Gasification Process
There are several steps in the plasma gasification process, including feed handling, plasma gasification reaction, cooling unit, syngas compression and cleaning, and plasma gasification products. The first step in plasma gasification involves putting waste into a plasma converter or reactor. The resulting plasma, formed by ionized gases, is very hot, reaching up to 5000 °C. This plasma reactor operates without oxygen entering, preventing combustion. Therefore, plasma gasification does not burn waste like an incinerator but decomposes waste into its basic structure, resulting in synthetic gases and metal crusts that are generally harmless.
Generally, there are three reactions that occur in plasma gasification to produce synthesis gas (syngas) consisting of carbon monoxide and hydrogen gas. The first reaction is gasification or thermal cracking. In this process, large molecules are broken down into smaller and lighter gas molecules. This pyrolysis process produces hydrocarbon gas and hydrogen gas, typically forming radicals during various steps. The end result of this process is light hydrocarbons such as methane and hydrogen.
The second reaction in the syngas formation process is partial oxidation. Partial oxidation can produce carbon monoxide, and with a more complicated oxidation process, it can produce carbon dioxide and water. Carbon dioxide and water are the final products of an oxidation process.
The third reaction that occurs is the reforming
reaction, which is a combination of reactions during the gasification process.
For example, carbon can react with water to produce carbon monoxide and
hydrogen, or carbon can react with carbon dioxide to produce two carbon
monoxide molecules. This reforming reaction has the potential to form fuel gas.
The gasification process is controlled at a
plasma plume temperature of 4,000-5,000 °C (specifically for thermal plasma),
with a syngas temperature coming out of the reactor ranging from 1,250 to 1,450
°C. By maintaining these temperatures, it is possible to minimize the size of
the reactor and produce syngas as a fuel gas in large quantities. The reactions
formed can include CO, H2, CO2, CH4, H2O,
N2, O2, H2S, COS, HCN, NH3, S, SO2,
SO3, NO2, NO, Cl2, HCl, C2H2,
and solid carbon.
 |
| Plasma gasification based on Westinghause USA design |
Once the syngas exits the plasma gasifier, it requires cooling before undergoing cleaning to remove compounds such as hydrogen chloride (HCl), hydrogen cyanide (HCN), and ammonia (NH3). The cooling process occurs in two steps: the first at a high temperature of 900 °C, and the second at a low temperature of 450 °C using a convective heat exchanger. Both heat exchangers recover heat for the high-pressure steam used in the steam cycle.
Particulates are then removed through a wax
filter, and the syngas temperature is further reduced to 120 °C, where its heat
is recovered by the high-pressure steam from the steam cycle. At this point,
the syngas can be cleaned of HCl, HCN, and ammonia through venturi and scrubber
trays. To remove unwanted sulfur compounds, carbon sulfide (COS) is first
converted to hydrogen sulfide (H2S), which is then stripped from the
syngas through the ammonium process.
Waste, after undergoing a plasma gasification process, can be utilized as synthetic gas (syngas). Syngas can be used to generate electricity through a combination of power generation cycles. The electricity is derived from the syngas and steam formed during the conversion process. The clean, cold syngas is then piped into the power generation equipment in series with the steam turbine. The residual combustion from this power generation equipment is used to create steam in the residual heat control boiler. The cooled, environmentally friendly combustion residues in the boiler are then released into the air. The steam generated from both the gasification process and the residual heat control boiler is fed into the steam turbine to generate additional electricity.
Another product of the plasma gasification
process is non-organic waste, such as metals. The metals melt and collect at
the bottom of the combustion chamber into a crust, making them retrievable and
reusable for the metal industry or asphalt mixture.
CONCLUSIONS
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