SOLID WASTE PROCESSING AND TREATMENT notes

Solid waste management
Solid waste processing and treatment
SHUBHAM SHARMA
Department of civil engineering
BGIET, SANGRUR

Processing Techniques and Equipment's
 Various processing techniques are available to improve the efficiency of solid waste
management systems. For example, to reduce storage requirements at medium- and high-rise
apartment buildings, both incineration and baling are used.
 In some cases, wastes are baled to reduce haul costs to the disposal site. At the disposal site,
solid wastes are compacted to use the available land effectively.
 Shredding is also used to improve the efficiency of disposal sites. The selection of processing
techniques for these purposes depends on the components of the overall waste management
system and, in most cases, is situation-specific.
 The goal is often to minimize the volume of waste sent to landfills, promote recycling, and
manage hazardous materials responsibly.

Purposes of Processing
 There are three main purposes of processing solid wastes:
 To improve the efficiency of solid waste management systems.
 To recover useful materials.
 To recover conversion products and energy.
 Recovery of Materials for Reuse:
o Components that are most amenable to recovery are those for which markets exist and
which are present in the wastes in sufficient quantity to justify their separation.
o Materials that have been recovered from solid wastes include paper, cardboard, plastic,
glass, ferrous metal, aluminium, and other residual nonferrous metal.

 Recovery of Conversion Products and Energy:
o Combustible organic materials can be converted to intermediate product and ultimately
to energy in a number of ways, including:
(1) incineration or direct combustion in power boilers to produce steam.
(2) pyrolysis produce a synthetic gas or liquid fuel, and
(3) biodegradation with and without sewage sludge to generate methane.
o What is important as a first step is to separate combustible organic materials from the
other solid waste components.
o Once they are separated, further processing is usually necessary before the materials can
be used for the production of power. Typically, they must be shredded and dried before
use.

Mechanical Volume Reduction
 Volume reduction is an important factor in the development and operation of most of solid waste
management systems. In most cities, vehicle equipped with compaction mechanisms are used for the
collection of solid wastes.
 To increase the useful life of landfills, wastes usually are compacted before being covered.
 Recently, high-pressure compaction systems have been developed to reduce landfill requirements.
 Low-Pressure Compaction: Typically, low-pressure compactors include those used at
apartments and commercial establishments used for waste paper and cardboard, and stationary
compactors used at transfer stations.
 High-Pressure Compaction: In most of these systems, specialized compaction equipment is
used to produce compressed solid wastes in blocks or bales of various sizes. When wastes are
compressed, their volume is reduced.
 Chemical Volume Reduction: Chemical volume reduction is a method, wherein volume
reduction occurs through chemical changes brought within the waste either through an addition of
chemicals or changes in temperature. Incineration is the most common method used to reduce the
volume of waste chemically, and is used both for volume reduction and power production. These
other chemical methods used to reduce volume of waste chemically include pyrolysis, hydrolysis.

Incineration of Municipal Wastes
 One of the most attractive features of the incineration process is that it can be used to reduce the
original volume of combustible solid wastes by 80 to 90 percent.
 In some of the newer incinerators designed to operate at temperatures high enough to produce a
molten material.
 Although the technology of incineration has advanced in the past two decades, air pollution
control remains a major problem in implementation.
 In addition to the use of large municipal incinerators, onsite incineration is also used at individual
residences, apartments, stores, industries, and hospitals.
 Many waste incinerators are also used to generate electricity as a useful by-product of the waste
incineration process.

Mechanical Size Reduction
 Size reduction is the term applied to the conversion of solid wastes as they are collected into smaller
pieces. The objective of size reduction is to obtain final product that is reasonably uniform and
considerably reduced in size in comparison to its original form.
 It is important to note that size reduction does not necessarily imply volume reduction. In some
situations, the total volume of the material after size reduction may be greater than that of the original
volume.
 In practice, the terms shredding, grinding, and milling are used interchangeably to describe mechanical
size-reduction operations. Wastes are shredded before they are baled. The disposal of shredded wastes
in landfills without the use of daily cover is another important application of size reduction.
 The types of equipment that have been used for reducing the size of and for homogenizing solid
wastes include small grinders, chippers, large grinders, jaw crushers, rasp mills, shredders, and hammer
mills.

 Jaw crusher  Hammer mills

Component Separation
 Component separation is a necessary operation in which the waste components are
identified and sorted either manually or mechanically to aid further processing:
 recovery of valuable materials for recycling;
 preparation of solid wastes by removing certain components prior to incineration, energy
recovery, composting and biogas production.
 The most effective way of separation is manual Hand Sorting:
A. At the source where solid waste are generated
B. At a transfer station
C. At a centralized processing station
D. At the disposal site.

 Air Separation
 This technique has been in use for a number of years in industrial operations for segregating
various components from dry mixture. Air separation is primarily used to separate lighter
materials (usually organic) from heavier (usually inorganic) ones. The lighter material may
include plastics, paper and paper products and other organic materials.
 Magnetic separation
 The most common method of recovering ferrous scrap from shredded solid wastes involves the
use of magnetic recovery systems. Ferrous materials are usually recovered either after shredding
or before air classification. When wastes are mass-fired in incinerators, the magnetic separator is
used to remove the ferrous material from the incinerator residue. Magnetic recovery systems
have also been used at landfill disposal sites.
 Screening
 Screening is the most common form of separating solid wastes, depending on their size by the
use of one or more screening surfaces. Screening has a number of applications in solid waste
resource and energy recovery systems. Screens can be used before or after shredding and after
air separation of wastes in various applications dealing with both light and heavy fraction
material.

 Suspended Type Permanent Magnetic Separator

Drying and Dewatering
 Drying and dewatering operations are used primarily prior incineration systems, with or
without energy recovery systems.
 These are also used for drying of sludges in wastewater treatment plants, prior to their
incineration or transport to land disposal.
 The purpose of drying and dewatering operation is to remove moisture from wastes and
thereby make it a better fuel.

Solid-waste treatment and disposal
 Once collected, municipal solid waste may be treated in order to reduce the total volume and weight
of material that requires final disposal. Treatment changes the form of the waste and makes it easier to
handle. It can also serve to recover certain materials, as well as heat energy, for recycling or reuse.

Biochemical Conversion of Waste
 Biochemical processes, like anaerobic digestion, can also produce clean energy in the form
of biogas which can be converted to power and heat using a gas engine.
 Anaerobic digestion is the natural biological process which stabilizes organic waste in the
absence of air and transforms it into biofertilizer and biogas.
 Anaerobic digestion is a reliable technology for the treatment of wet, organic waste.
 Organic waste from various sources is biochemically degraded in highly controlled, oxygen-
free conditions circumstances resulting in the production of biogas which can be used to
produce both electricity and heat.

Sanitary Land Filling or Controlled Tipping Method
 Land disposal is the most common management strategy for municipal solid waste. Refuse can be safely
deposited in a sanitary landfill, a disposal site that is carefully selected, designed, constructed, and
operated to protect the environment and public health.
 In this method of refuse disposal, refuse is carried and dumped into the low lying area (are marked as
the land fill site) under an engineered operation, designed and operated in an environmentally sound
manner, as not to cause any public nuisance or hazards to public health or safety.
 One of the most important factors relating to landfilling is that the buried waste never comes in contact
with surface water or groundwater. Engineering design requirements include a minimum distance
between the bottom of the landfill and the seasonally high groundwater table.
 Most new landfills are required to have an impermeable liner or barrier at the bottom, as well as a
system of groundwater-monitoring wells. Completed landfill sections must be capped with an
impermeable cover to keep precipitation or surface runoff away from the buried waste. Bottom and cap
liners may be made of flexible plastic membranes, layers of clay soil, or a combination of both.

Advantages
 This method is most simple and economical. No costly plant or equipment is required in
this method, as is required in other methods of incineration or pulverization.
 Separation of different kinds of refuse, as required in incineration method, is also not
required in this method.
 There are no residues or byproducts left out/evolved in this method, and hence no further
disposal is required; this being a complete method in itself.
 Low lying water-logged areas and odd quarry pits can be easily reclaimed and put to
better use. The mosquito-breeding places are also, thus, eliminated.

Disadvantages:
 Low lying depressions or dumping sites may not always be available ;or even if they are available
today, they may ultimately become scare or unavailable in future, since the production of solid waste
is a continuous process.
 There is a continuous evolution of foul gases near the fill site, especially during the times the refuse is
being dumped there. These gases may often be explosive in nature, and are produced by the
decomposing or evaporating organic matter. These gases, known as landfill gases, become a serious
environmental problem at sanitary landfill sites. These gases is need be estimated, properly disposed
off.
 Since the dumped garbage may contain harmful and sometimes carcinogenic non-bio-degradable
substances, such as plastics, unused medicines, paints, insecticides, sanitary napkins, etc., they may
start troubling on a later date, particularly during rainy season, when excess water seeping through
the area, may come out of the dump, as a coloured liquid, called leachate. This highly poisonous and
polluted leachate, containing organic compounds like chlorinated hydrocarbons, benzene, toluene,
xylene, etc.; is likely to seep to the underground water-table, to contaminate the ground water,
leading to diseases, like cholera, typhoid, polio, etc. In order to avoid such harmful effects, the
leachates may have to be scientifically assessed, collected, and disposal of.

Disposal of MSW (Refuse) by Composting
 Composting of refuse is a biological method of decomposing solid wastes. This
decomposition can be affected either under aerobic conditions, or under anaerobic
conditions, or both. The final end product, is a manure, called the compost or humus,
which is in great demand in European countries as fertilizer for farms.
 Basically, composting is considered to be an aerobic process, because it involves piling up
of refuse and its regular turning, either manually or by mechanical devices, so as to ensure
sufficient supply of air and oxygen during its decomposition by bacteria, fungi and other
microorganisms, like actinomycetes’.
 composting helps in Reducing the Waste Stream, Cuts Methane Emissions From
Landfills, Improves Soil Health and Lessens Erosion, Conserves Water, Reduces
Personal Food Waste

 Composting using vegetable waste

Process of composting
 Initially, the process starts with the mesophilic bacteria, which oxidise the organic matter (in the refuse) to
carbon dioxide and liberate heat. The temperature rises to about 45°C, and at this point, the thermophilic
bacteria take over and continue the decomposition. During this phase, the temperature further rises to
about 60°C, which has to be maintained for at least 3 days in order to destroy pathogenic bacteria.
 This temperature control is crucial, because optimal decomposition occurs between 55 and 60°C, but if the
temperature exceeds 60°C, decomposition slows down. In about 4 to 5 weeks, the easily biodegradable
fraction gets consumed and the temperature of the compost mass starts falling.
 Complete stabilisation occurs after the compost is allowed to cure for another 2 to 8 weeks. During the
active early decomposition phase, the thermophilic bacteria (mainly Bacillus, Clostridium and
Pseudomonas) act as the principal decomposers; while fungi (such as Mucor, Penicillium and
Aspergillus) are more active during the curing stage.
 The entire composting, thus, gets completed in about 3-4 months time. Volume reductions of the original
organic material of up to 50% are achieved under ideal conditions. The finally produced compost usually,
has Cathy smell and a dark brown colour.

 In India, the composting is practised in rural areas on the mixture of night soil
and refuse. Two methods, which are generally adopted here, are:
1. Indore process
2. Bangalore* process
Indore method of composting uses manual turning of piled up mass (refuse +
night soil), for its decomposition under aerobic conditions.
Bangalore method of composting, on the other hand, involves anaerobic
decomposition of wastes; and does not involve any turning or handling of the mass, and is,
hence, cleaner than the Indore method. This method is, therefore, widely adopted by
municipal authorities throughout India.

Indore method
 This method of composting uses manual turning of piled up mass (refuse + night soil), for its
decomposition under aerobic conditions. In this method, layers of vegetable wastes and night soil
are alternatively piled in depths of about 7.5 to 10 cm each, to a total depth of about 1.5 m in a
trench; or above the ground to form a mound called a windrow.
 A windrow is a long mound or stack of the organic MSW (mixed with cattle dung and human excreta
if needing disposal) dumped on land in a height of about 2.5m to 3m wide at the base. Most
windrows are conical in cross section and about 50m in length. The composting waste is aerated by
periodically turning the waste mix in the windrow, or in the trench, as the case may be. The manual
turning with a pitchfork can be adopted at smaller installations ; while at large plant, mechanical
devices like self-propelled over cab loaders, rotary ploughs, etc may be used to turn the refuse
once or twice per week, which serve to introduce oxygen and to control the temperature. This
process of turning is continued for about 4 to 5 weeks, during which time, the readily biodegradable
organics are consumed. The waste compost mass is finally allowed to cure for another 2 to 8 weeks
without any turning. The entire composting process, thus takes about 3-4 months time to complete,
after which the compost becomes ready for being taken out for use or for sale.

Bangalore method
 It involves anaerobic decomposition of wastes and does not involve any turning or handling
of the mass, and is, hence, cleaner than the Indore method. This method is, therefore, widely
adopted by municipal authorities throughout India.
 The refuse and night so soil, in this method, are, therefore, piled up in layers in an underground
earthen trench (about 10 m x 1.5 m x 1.5 m). This mass is covered at its top by layer of earth
of about 15 cm depth, and is finally left over for decomposition. Within 2 to 3 days of burial,
intensive biological action starts taking place, and organic matter begins to be destroyed.
 Considerable heat gets evolved in the process, which raises the temperature of the
decomposing mass to about 75°C. This heat prevents the breading of flies by destroying the
larvae. After about 4 to 5 months (depending upon the season), the refuse gets fully stabilised
and changes into a brown coloured odourless innocuous powdery mass, called humus.
This humus is removed from the trenches, sieved on 12.5 mm sieves to remove stones, broken
glass brickbats, etc., and then sold out in the market as manure. The empty trenches can again
be used for receiving further batches of refuse.

Carbon-nitrogen ratio
 C/N of the input material in the compost heap is an important factor for the bacterial
activity to continue, since the bacteria use nitrogen for building there cell structures(as
proteins) and carbon for food (as) energy):the anaerobic bacteria develop in this digestion,
use up carbon about 40-50 times faster than they use up nitrogen.
 Hence for the proper development of the anaerobic digestion, C/N ratio of the digestive
material should be between 30 to 50 to optimum digestion .

Vermi-composting
 Vermi-composting uses the natural composting process of decomposition of biodegradable
organic matter by the soil bacteria- as in ordinary composting technique described earlier,
but takes the assistance of cultured earth worms, that are now produced commercially.
 These earth worms do help in quicker decomposition of the organic matter.
 The method helps in adopting the composting technique in individual bungalows and
institution, in waste, to dispose of domestic waste and more particularly for dispositions of
the yard and garden wastes, particularly the leaves and grass clippings, which cannot be
thrown away with MSW in countries like USA

The various steps involved in applying the Vermi composting technique at
individual domestic level are summarised below:
1. Dig a small pit-about 0.5 m square and 1m deep.
2. Line the pit with straw or dried leaves and grass.
3. Organize the disposal of organic domestic waste (such as vegetable wastes) into the pit as
and when generated.
4. Introduce a culture of worms that is now produced commercially.
5. Cover the pit contents daily, by sprinkling of dried leaves and soil every day.
6. Water the pit once or twice a week to keep it moist.
7. Turn over the contents of the pit every 15 days.
8. In about 45 days, the waste will be decomposed by the action of the microorganisms.
9. The produced humus (soil) in the pit is fertile and rich in soil nutrients. It can, hence, be used
in the garden.

 Vermi- composting using earthworms

Thermochemical Conversion of Waste
 The three principal methods of thermochemical conversion of waste are combustion in
excess air, gasification in reduced air, and pyrolysis in the absence of air.
 The most common technique for producing both heat and electrical energy from
household wastes is direct combustion.
 Combined heat and power (CHP) or cogeneration systems, ranging from small-scale
technology to large grid-connected facilities, provide significantly higher efficiencies than
systems that only generate electricity.
 Combustion technology is the controlled combustion of waste with the recovery of heat
to produce steam which in turn produces power through steam turbines.

Incineration
 The process of burning wastes in large furnace in the presence of air is known as incineration. Burning is a
very effective method of reducing the volume and weight of solid waste, though it is a source of
greenhouse gas emissions. In modern incinerators the waste is burned inside a properly designed furnace
under very carefully controlled conditions.
 Incineration can reduce the volume of uncompacted waste by more than 90 percent, leaving an inert
residue of ash, glass, metal, and other solid materials called bottom ash.
 The combustible portion of the waste combines with oxygen, releasing mostly carbon dioxide, water
vapour, and heat.
 The gaseous by-products of incomplete combustion, along with finely divided particulate material called
fly ash, are carried along in the incinerator airstream. Fly ash includes cinders, dust, and soot.
 In order to remove fly ash and gaseous by-products before they are exhausted into the atmosphere,
modern incinerators must be equipped with extensive emission control devices. Such devices include
fabric baghouse filters, acid gas scrubbers, and electrostatic precipitators. Bottom ash and fly ash are
usually combined and disposed of in a landfill. If the ash is found to contain toxic metals, it must be
managed as a hazardous waste.

 Municipal solid-waste incinerators are designed to receive and burn a continuous supply of
refuse. A deep refuse storage pit, or tipping area, provides enough space for about one day of
waste storage.
 The refuse is lifted from the pit by a crane equipped with a bucket or grapple device. It is then
deposited into a hopper and chute above the furnace and released onto a charging grate or
stoker.
 The grate shakes and moves waste through the furnace, allowing air to circulate around the
burning material. Modern incinerators are usually built with a rectangular furnace, although
rotary kiln furnaces and vertical circular furnaces are available. Furnaces are constructed of
refractory bricks that can withstand the high combustion temperatures.
 Combustion in a furnace occurs in two stages: primary and secondary. In primary combustion,
moisture is driven off, and the waste is ignited and volatilized. In secondary combustion, the
remaining unburned gases and particulates are oxidized, eliminating odours and reducing the
amount of fly ash in the exhaust. When the refuse is very moist, auxiliary gas or fuel oil is
sometimes burned to start the primary combustion.

 In order to provide enough oxygen for both primary and secondary combustion, air must be thoroughly
mixed with the burning refuse. Air is supplied from openings beneath the grates or is admitted to the area
above. The relative amounts of this under fire air and overfire air must be determined by the plant
operator to achieve good combustion efficiency. A continuous flow of air can be maintained by a natural
draft in a tall chimney or by mechanical forced-draft fans.

While operating the incinerators, the following points should be
carefully observed:
 The charging should be thorough and rapid.
 Each batch of refuse entering the furnace should be well mixed, and the proportion of fuel in
the charge be adjusted to provide complete combustion, and proper temperature.
 Refuse containing 80% garbage and 20% rubbish, will normally burn without any auxiliary
fuel, if air supplied for combustion is pre-heated to about 150°C. Whereas, the refuse
containing 50 to 60% garbage and 40 to 50% rubbish will burn satisfactorily, without any
pre-heated air. If the percentage of garbage is less than 50%, flashy fire may result.
 When moisture contents of the refuse are high, such as during monsoons, auxiliary fuels, like
wood, coal, or oil will be required.
 The minimum temperature in the combustion chamber should be sufficient (normally larger
than 670°C), so as to incinerate all organic matter and oxidise foul smelling gases. If steam is
to be generated, a temperature of about 1000°C is required to be produced in the
combustion chamber.

Merits of using incineration
 This is the most sanitary method of refuse disposal, and ensures complete destruction of
pathogenic bacteria and insects.
 There is no odour trouble or dust nuisance.
 Some cost can be recovered by selling the steam power and clinkers.
 The disposal site can be conveniently located within the city near the outskirts, and
transportation problems sorted out easily.
 It requires very less space for refuse disposal.

Demerits and Limitations.
 It is a very costly method, and requires a lot of technical knowledge.
 Solid wastes to be burnt should have a high calorific value.
 Smoke, odour, and ash nuisance may result due to the improper and incompetent operation
of the plant, particularly if substances like plastics, giving high calorific value to the wastes,
are present in the wastes.
 Transport vehicles are required in slightly large numbers, as there may occur delays in their
emptying near the incinerators.

Gasification
 Gasification processes involve the reaction of carbonaceous feedstock with an oxygen-
containing reagent, usually oxygen, air, steam or carbon dioxide, generally at temperatures
in excess of 800°C.
 It involves the partial oxidation of a substance which implies that oxygen is added but the
amounts are not sufficient to allow the fuel to be completely oxidised and full combustion to
occur. The process is largely exothermic but some heat may be required to initialise and
sustain the gasification process.
 The main product is a syngas, which contains carbon monoxide, hydrogen and methane.
 Syngas can be used in a number of different ways, for example:
1. Syngas can be burned in a boiler to generate steam which may be used for power
generation or industrial heating.
2. Syngas can be used as a fuel in a dedicated gas engine.
3. Syngas, after reforming, may be suitable for use in a gas turbine
4. Syngas can also be used as a chemical feedstock.

 Schematic of MSW Gasification and power generation plant

Pyrolysis
 Upon heating in closed containers in oxygen free atmosphere, most of the organic substances of solid
waste can be split through a combination of thermal cracking and condensation reactions into gaseous,
liquid and solid fractions. This process is known as pyrolysis or thermal pyrolysis.
 In contrast to the combustion process which is highly exothermic (releasing heat on burning in the
presence of oxygen), the pyrolysis is highly endothermic (consuming heat). That is why, this process is
also known as destructive distillation.
 When the organic solid waste is pyrolyzed, we obtain the following three types of products at different
stages or temperatures:
• a gas stream, which primarily contains hydrogen, methane, CO, CO, and other gases, depending upon
the organic character of the solid waste being pyrolyzed.
• a liquid fraction, consisting of a tar and/or an oil stream, which is a liquid at room temperature and is
found to contain chemicals such as acetic acid, acetone, and methanol.

• a solid fraction, consisting of charcoal like product of almost pure carbon plus any inert
material that may have entered the process.
• The respective quantum's of three end products (i.e. gas, oil and charcoal) is found to
depend upon the temperature of pyrolysis. Under conditions be maximum gasification, the
energy content of the resulting gas is found to be about 26000 kJ/m3 and that of resulting
oil to be about 23000 kJ/kg.
• Pyrolysis may be used for reducing the quantities of sludge produced in a water or
wastewater treatment plan, before their ultimate disposal by methods like landfill and land
application.

Energy recovery
 There is a wide range of Waste to energy technologies, biochemical and thermochemical, for the
conversion of solid waste into energy (steam or electricity). Fuels such hydrogen, natural gas, synthetic
diesel and ethanol can be utilized.
 The biochemical route, in the case of MSW, refers to anaerobic digestion, which consists of controlled
decomposition by microbes to reduce the organic material. Biochemical processes are used in the
treatment of waste with high percentages of biodegradable organic matter and high moisture content.
Methane, fuel for electricity generation, steam and heat can be produced.
 One of the disadvantages of the biological treatment is the preprocessing required to separate MSW.
Biochemical conversion of waste can be grouped into four categories: anaerobic digestion/fermentation,
aerobic digestion, composting, and landfill gas power (LFG). These technologies are the most
economic and environmentally safe means of obtaining energy from MSW.

 In thermochemical conversion, both biodegradable and nonbiodegradable matters contribute to the
energy output. Incineration, gasification and pyrolysis are types of thermochemical conversion
processes, which are fundamental and necessary components of a comprehensive and integral urban
solid waste management system.
 The main advantages of thermochemical processes include lower masses and volumes of waste,
decrease in the space occupied by landfills, destruction of organic pollutants such as halogenated
hydrocarbons, and decrease in the emission of GHGs due to anaerobic decomposition. When
considering the life cycle, the use of waste as a source of energy generates less environmental impacts
than other conventional energy sources.
 With incineration, the energy value of waste can be recovered; however, pyrolysis and gasification can
be utilized to recover the chemical value of waste. The derived chemical products, in some cases, can
be utilized as inputs in other processes or as secondary fuels.
 With the conversion of MSW into fuels, higher calorific values are obtained along with more
homogeneous physical and chemical compositions, lower levels of pollutants and ashes, less excess air
required for combustion, and better conditions for storage, handling, and transportation. Therefore, it
is recommended to establish a balance between increasing production costs and the potential
reduction of costs associated with designing and operating the system.

Energy recovery through thermochemical process
 Incineration processes can also offer the possibility of recovering the energy,
mineral or chemical content of waste. Due to the heterogeneous nature of waste, some
differences with respect to conventional fossil fuel power plants have to be considered in the
energy conversion process. The efficiency of a coal burning cycle is generally around 40%,
while the efficiency of a garbage incineration cycle varies between 20 and 25%, if
operating in a cogeneration mode, and up to 25–35% in the case of power production only .
In general, fuel quality (i.e., waste) and other technical conditions (e.g., plant size, low
temperature sources, etc.) limit the electrical efficiency of incinerators. This means that more
than 70–80% of the heat generated by waste combustion is rejected to the environment.
 The conversion efficiency of steam energy into electricity increases with higher steam
temperatures and pressures. However, when increasing steam temperature, the heat transfer
surfaces are submitted to severe high-temperature corrosion, caused by metal chlorides in
the ash particles deposited on the gas tubes and by high concentrations of chlorine and
sulphur in MSW. Most chlorines are present in plastics (e.g., PVC), while fluorine's are
present in polytetrafluoroethylene (PTEF), along with other inorganic compounds.

Gasification is the thermal conversion of carbon-based material into a mixture of
combustible gases, called syngas. Gasification is used to convert solid materials such as
coal, coke, biomass and solid waste into a gas, with average composition 15–30% CO, 12–
40% H, and 4.5–9% CH4. The lower heating value (LHV) of syngas is between 4 and 13
MJ/Nm3, depending on the oxidizing agent used in gasification, operating conditions,
among other factors . From the syngas gas produced, different chemical intermediate
products can be obtained, with different industrial uses. Energy can also be obtained, in the
form of power, heat or biofuel. Gasification temperature is one of the most important
operation parameters that affects the performance of the process, due to the balance
between endothermic and exothermic reactions involved.
 compared different thermochemical conversion processes, and verified that gasification
technology is the best choice considering energy and environmental perspectives.
Gasification has attracted attention and gained importance in recent years, presenting
higher energy efficiency and being friendlier to the environment.

Pyrolysis is the thermal degradation of organic material in an oxygen-deficient
atmosphere at approximately 400–900°C, producing gas, liquid and solid products. The
yield and composition of the products are influenced by a range of pyrolysis process
parameters, including the type of waste, reactor system, gas residence time, contact time,
heating rate, temperature, pressure ranges, and presence of catalysts.
 Due to the different operation conditions, pyrolysis can be classified into three main
categories: slow, fast and flash pyrolysis.
 Pyrolysis is a promising technology and is currently utilized in many regions of the world
for MSW disposal and energy generation. The objective of MSW pyrolysis is to treat waste,
reduce its volume and associated hazards, destroying potentially harmful substances.
Pyrolysis can also involve energy recovery from waste, in the form of heat, steam,
electricity, or fuel (e.g., oil, char, and gas).

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