Khí hóa chất thải rắn đô thị bằng Plasma nhiệt

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KHÍ HÓA CHẤT THẢI RẮN ĐÔ THỊ BẰNG PLASMA NHIỆT
GASIFICATION OF MUNICIPAL SOLID WASTE 
BY THERMAL PLASMA
Nguyễn Hữu Thành1
Tóm tắt:
Lượng chất thải rắn đang gia tăng do sự gia tăng dân số và các hoạt động phát triển kinh tế xã hội 
của con người. Xử lý chất thải rắn bằng plasma nhiệt đã trở thành công nghệ nổi bật hơn trong thập kỷ 
qua do các vấn đề về xử lý chất thải ngày càng gia tăng và vì nhận ra các cơ hội để tạo ra các sản phẩm 
phụ có giá trị. Plasma nhiệt khí hóa chất thải rắn với các giá trị tiêu cực thấp đã thu hút được sự quan 
tâm như là một nguồn cung cấp năng lượng và phát triển các quá trình công nghệ để xử lý chất thải rắn 
thậm chí là chất thải rắn đô thị (MSW). Tổng quan này trình bày một số nguyên tắc và đặc điểm vật lý, 
hóa học cơ bản của quá trình plasma nhiệt để khí hóa chất thải rắn đô thị.
Từ khóa: Plasma nhiệt khí hóa, xử lý chất thải rắn, chất thải rắn đô thị.
Abstract:
The quantum of solid waste is increasing due to increase in population and human socio-economic 
development activities. Thermal plasma solid waste treatment has over the past decade become a more 
prominent technology because of the increasing problems with waste disposal and because of the 
realization of opportunities to generate valuable co-products. Thermal plasma gasification of solid wastes 
with low negative values has attracted interest as a source of energy and spawned process developments 
for treatment of even municipal solid wastes (MSW). This review presents some of the basic physical 
and chemical principles and characteristics of thermal plasma process for the gasification of MSW
Keywords: Thermal plasma gasification, solid waste treatment, municipal solid wastes.
1. Introduction
Municipal solid has been defined differently by various countries. On a general perspective, MSW 
typically will consist of biodegradable waste, inert waste, electric and electronic, hazardous waste, 
toxic waste, medical waste and recyclable material. The composition of municipal solid waste varies 
depending on factors such as economic development, culture, climate and energy sources. As the global 
projection of MSW is expected to reached 2.2 billion tonnes per year in 2025, it will continue to be a 
major environmental issue facing countries worldwide especially in developing countries [1, 2]. The 
annual growth rate of global municipal solid waste is projected to be around 3.2–4.5% in developed 
nations and 2–3% in developing nations [3]. Sustainable and successful treatment of MSW should be 
safe, effective, and environmentally friendly. The primary components of the philosophy are (a) source 
reduction including reuse of products and on-site composting of yard trimmings, (b) recycling, including 
off-site (or community) composting, (c) combustion with energy recovery, and (d) disposal through 
landfill. Among them, landfill has been the practice most widely adopted. There are two main drawbacks 
of landfill. One is that surrounding areas of landfills are often heavily polluted since it is difficult to 
keep dangerous chemicals from leaching out into the surrounding land [4]. The other is that landfill can 
increase chances of global warming by releasing CH4, which is 20 times more dangerous as a greenhouse 
gas than CO2 [5]. Therefore, we must find a more environmentally friendly alternative to treat MSW. 
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A plasma is defined as a quasineutral gas of charged and neutral particles which exhibits collective 
behavior [6]. Plasma can be classified into non-thermal and thermal plasmas according to the degree of 
ionization and the difference of temperature between heavy particles and electrons [7, 8]. Thermal plasma 
can be characterized by approximate equality between heavy particle and electron temperatures and have 
numerous advantages including high temperature and high energy density [9]. Electrically generated 
thermal plasma can reach temperature of ~10,000 oC or more, whereas only an upper temperature limit 
of 2,000 oC can be achieved by burning fossil fuels [10]. Because of this reason, thermal plasma has been 
traditionally used in high temperature and large enthalpy processes [5].
Over the past decade, thermal plasma process has also been regarded as a viable alternative to 
treat highly toxic wastes, such as air pollutant control residues, radioactive, and medical wastes [5]. 
It has also been demonstrated that the thermal plasma process is environmentally friendly, producing 
only inert slag and minimal air pollutants that are well within regional regulations. Recently, a thermal 
plasma process for a gasification of MSW has been planned and constructed as a pilot program in 
commercial plants. The thermal plasma process employs extremely high temperatures in the absence or 
near-absence of O2 to treat MSW containing organics and other materials. The MSW is dissociated into 
its constituent chemical elements, transformed into other materials some of which are valuable products. 
The organic components are transformed into syngas, which is mainly composed of H2 and CO and 
inorganic components are vitrified into inert glass-like slag [5].
2. Characteristics of thermal plasma process for the treatment of MSW
Thermal plasma for wastes treatment has received great attention recently to meet the contemporary 
needs to solve problems with increasing environmental pollutions. Compared with commonly used 
combustion methods for waste treatment, thermal plasma provides the following advantages [5]: (1) 
high energy density and temperatures, and the correspondingly fast reaction times, offer the potential 
for a large throughput with a small furnace. (2) High heat flux densities at the furnace boundaries lead 
to fast attainment of steady state conditions. This allows rapid start-up and shutdown times compared 
with other thermal treatments such as incineration. (3) Only a small amount of oxidant is necessary to 
generate syngas, therefore, the gas volume produced is much smaller than with conventional combustion 
processes and so is easier and less expensive to manage. These characteristics make thermal plasma 
process an ideal alternative to conventional methods of solid waste treatment.
There are three kinds of processes inside the thermal plasma furnace for solid waste treatment. First 
is pyrolysis (without O2) of gaseous, liquid, and solid waste in a thermal plasma furnace with plasma 
torches. Second is gasification (O2-starved) of solid waste containing organic compounds to produce 
syngas (H2 + CO). Last is vitrification of solid wastes by transferred, non-transferred, or hybrid arc 
plasma torch according to electric conductivity of substrate. Processes being considered importantly 
for the treatment of solid wastes are gasification and vitrification; this is due to the energy recovery and 
volume reduction. The gasification process is an old industrial process that uses heat in an O2 - starved 
environment to break down carbon based materials into fuel gases. It is closely related to combustion 
and pyrolysis, but there are important distinctions between them. Gasification is similar to starved-air 
burning because O2 is strictly controlled and limited so that the feedstock is not allowed to be completely 
burned as heat is applied. Instead of combusting, the raw materials go through the progress of pyrolysis, 
producing char and tar. The char and tar are broken down into syngas, mainly composed of H2 and 
CO, as the gasification process continues. The global gasification reaction is written as follows; waste 
material is described by its ultimate analysis (CH
x
O
y
) [11]:
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CH
x
O
y 
+ wH2O + mO2 + 3.76mN2 → aH2 + bCO + cCO2 + dH2O + eCH4 + fN + gC (1) 
Where w is the amount of water per mole of waste material, m is the amount of O2 per mole of waste, 
a, b, c, d, e, f and g are the coefficients of the gaseous products and soot (all stoichiometric coefficients 
in moles). This overall equation has also been used for the calculation of chemical equilibrium occurring 
in the thermal plasma gasification with input electrical energy [11]. The concentrations of each gas have 
been decided depending on the amount of injected O2, H2O, and input thermal plasma enthalpy. The 
detailed main reactions are as follows [5, 11, 12]:
 CH4 + H2O → CO + 3H2 (CH4 decomposition-endothermic) (2)
 CO + H2O → CO2 + H2 (Water gas shift reaction-exthermic) (3) 
 C + H2O → CO + H2 (Heterogeneous water gas shift reaction-endothermic) (4) 
 C+CO2 → 2CO (Boudouard equilibrium-endothermic) (5)
 2C + O2 → CO (6)
The H2 and CO generated during the gasification process can be a fuel source. Therefore, plasma 
gasification process has been combined with many other technologies to recover energy from the 
syngas [5].
3. Characteristics of thermal plasma process for the gasification of MSW
Combustion can play a number of important roles in an integrated MSW management system as 
follows [5]: it can (1) reduce the volume of waste, therefore preserving landfill space, (2) allow for 
the recovery of energy from the MSW, (3) permit the recovery of minerals from the solid waste which 
can then be reused or recycled, (4) destroy a number of contaminants that may be present in the waste 
stream, and (5) reduce the need for the “long-hauling” of waste.
The recovery of energy from MSW combustion typically involves the conversion of solid waste to 
energy resulting in the generation of electricity from the recovered heat, and/or the generation of hot 
water or steam to use for community-based industrial, commercial or residential heating applications. 
Conventional combustion technologies include mass burn incineration. On the basis of chemical analysis, 
the average composition of combustible materials in MSW can be expressed by the formula C6H10O4 
[13]. When this hypothetical compound is combusted with air, the reaction is [13]:
C6H10O4 + 6.5O2 + (24.5N2) → 6CO2 + 5H2O + (24.5N2) ∆H = -6.5 MWh /ton (7)
Although, incineration technology has been widely utilized to reduce the total volume of waste 
and recover the energy from MSW, the emissions of pollutants such as NO
x
, SO
x
, HCl, harmful organic 
compounds, and heavy metals are high. Another problem is the serious corrosion of the incineration 
system by alkali metals contained in solid residues and fly ash [14]. Thermal plasma technology has been 
applied for the treatment of MSW as an alternative to solve these problems [5].
Thermal plasma technology can make extremely high temperatures in the absence of or near-absence 
of O2, with MSW containing organics and other materials. Organics are converted into syngas and other 
materials dissociated into constituent chemical elements that are then collected and vitrified to produce 
an inert glass-like slag; most of the heavy and alkali metals (with the exception of mercury, zinc and 
lead, which can vaporize at high temperatures and be retained in fly ash and syngas) are retained in the 
vitrified slag. The vitrified slag obtained after cooling can be used as construction materials. The simple 
gasification reaction of MSW using thermal plasma can be expressed as follows [13]:
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C6H10O4 + 3O2 → 3CO + 3CO2 + 4H2 + H2O ∆H = -1.3 MWh/ton (8)
The principal product of plasma gasification of MSW is a low to medium calorific value syngas 
composed of CO and H2 as shown in equation (8). This gas can be burned to produce heat and steam, 
or chemically scrubbed and filtered to remove impurities before conversion to various liquid fuels or 
industrial chemicals. Syngas combusts according to the following equations [13]:
3CO + 4H2 + 3.5O2 → 3CO2 + 4H2O ∆H = -1.5 MWh/ton (9)
Table 1 shows the important differences mentioned above between incineration and thermal plasma 
gasification [5]. Main differential factors between them are amount of added O2 and temperature inside a 
furnace. Incinerators have designed to maximize CO2 and H2O, indicating complete combustion, however 
thermal plasma treatment system is designed to maximize CO and H2, indicating incomplete combustion. 
These complete and incomplete combustions have been controlled using added O2 amounts. Incinerators 
add a large quantity of excess air, but thermal plasma treatment systems add a limited quantity of O2. 
Therefore, inside of incineration furnace is an oxidizing environment, causing the generation of NOx 
and SO
x
, but inside of thermal plasma process is a reducing environment, prohibiting the generation 
of NO
x
 and SO
x
. Temperature of incineration furnaces is around 800 oC which is below an ash melting 
point. This makes inorganic materials contained in MSW to convert to bottom and fly ash. However, 
temperature of thermal plasma processes is around 1,400 oC, which is above an ash melting point. This 
makes inorganic materials contained in MSW to convert to vitrified slag which can be utilized as a 
source of construction materials.
Table 1: Comparison between the incineration and thermal plasma gasification processes 
for MSW treatment [5]
Differential factors Incineration process Thermal plasma process
Definition - Mass burning process - Gasification process
Amount of O2 - Designed to maximize CO2 and H2O 
- Added large quantity of excess air 
- Oxidizing environment 
- Generating NO
x
 and SO
x
- Designed to maximize CO and H2 
- Added limited quantity of O2 
- Reducing environment 
- Prohibiting the generation of NO
x
 and 
SO
x
Temperature - Operating at temperature below ash 
melting point 
- Inorganic materials are converted to 
bottom ash and fly ash 
- Bottom ash and fly ash are collected, 
treated, and disposed as hazardous 
wastes.
- Operating at temperature above ash 
melting point 
- Inorganic materials are converted to 
glassy slag and fine particulate matter 
- Slag is non
- Leachable, nonhazardous and suitable 
for use in construction materials
4. Conclusions
Thermal plasma technology is proven method for generating high temperatures at atmospheric 
pressure, which is not achievable by burning fuels. Thermal plasma gasification processes convert 
organics contained in MSW into syngas, and dissociate other materials into constituent chemical 
elements that are then collected and vitrified to produce an inert glass-like slag retaining most of the 
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heavy and alkali metals from the waste. The vitrified slag can be used as construction materials. In 
addition, NO
x
 and SO
x
 are not emitted due to O2-starved conditions inside the thermal plasma furnace. 
Therefore, thermal plasma processes are an environmentally friendly alternative for the gasification 
of MSW.
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Date of receipt: 20/02/2020
Review date: 04/3/2020
Date acceptedfor posting: 31/3/2020 1 
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