Term Report
“Coal combustion, Pyrolysis and Gasification”
Submitted By:
Umar Pitafi
2K20-CHE-110
Awais Aslam
2K20-CHE-114
Jamshaid Sabir
2K20-CHE-117
Ahmad Fakhar
2K20-CHE-119
Submitted To:
Sir Subhan Azeem
Department of Chemical Engineering
TABLE OF CONTENTS
1
Coal combustion __________________________________________________________ 1
1.1
1.1.1
1.1.2
Introduction to Coal combustion ________________________________________________ 1
A trip down the memory lane _________________________________________________________ 2
Challenges Faced____________________________________________________________________ 2
1.2
Basic Process of Coal Combustion _______________________________________________ 2
1.3
Coal Combustion Technology __________________________________________________ 4
1.3.1 Coal Fixed-bed Combustion ___________________________________________________________ 4
1.3.1.1
Fixed Grate ____________________________________________________________________ 5
1.3.1.2
Moving Grate __________________________________________________________________ 6
1.3.2 Coal Suspending Combustion __________________________________________________________ 8
2
Pyrolysis _________________________________________________________________ 9
2.1
Introduction to Pyrolysis ______________________________________________________ 9
2.2
Biomass pyrolysis techniques __________________________________________________ 9
2.2.1
2.2.2
2.2.3
2.3
Slow pyrolysis _____________________________________________________________________ 10
Fast Pyrolysis ______________________________________________________________________ 10
Flash pyrolysis _____________________________________________________________________ 10
Types of biomass pyrolysis reactors ____________________________________________ 11
2.3.1 Small-scale pyrolysis devices for fundamental research ____________________________________ 11
2.3.1.1
Resistively heated micro-furnace or tube pyrolyzer___________________________________ 11
2.3.1.2
Resistively heated element pyrolyser ______________________________________________ 11
2.3.1.3
Curie-point filament pyrolyzer ___________________________________________________ 11
2.3.1.4
Laser pyrolyzer ________________________________________________________________ 12
2.3.1.5
Plasma pyrolysis reactor ________________________________________________________ 12
2.3.2 Pilot scale pyrolysis reactors _________________________________________________________ 13
2.3.2.1
Fixed Bed Reactor _____________________________________________________________ 13
2.3.2.2
Bubbling Fluidized Bed Reactor ___________________________________________________ 13
2.3.2.3
Circulating Fluidized Bed Reactor _________________________________________________ 13
2.3.2.4
Entrained flow reactor __________________________________________________________ 14
2.3.2.5
Vacuum Furnace Reactor________________________________________________________ 14
2.3.2.6
Ablative Reactor _______________________________________________________________ 14
2.3.2.7
Rotating Cone Reactor __________________________________________________________ 15
2.3.2.8
Auger Reactor ________________________________________________________________ 15
3
Gasification _____________________________________________________________ 16
3.1
Introduction to Gasification ___________________________________________________ 16
3.2
Fundamentals of Gasification: _________________________________________________ 17
3.3
Reactions and Transformation: ________________________________________________ 18
3.4
Detailed Gasification Chemistry: _______________________________________________ 19
3.5
Thermodynamics and Kinetics of gasification reactions: ____________________________ 21
3.6
Syngas Composition: ________________________________________________________ 22
3.7
Types of Commercial Gasifiers: ________________________________________________ 23
3.7.1
3.7.2
3.7.3
3.8
4
Fixed (Moving) bed Gasifier:__________________________________________________________ 23
Entertained Flow Gasifiers: __________________________________________________________ 25
Fluidized-Bed Gasifiers: _____________________________________________________________ 27
R&D for Gasifier Optimization/Plant Supporting System: ___________________________ 28
References ______________________________________________________________ 29
1 COAL COMBUSTION
1.1 INTRODUCTION TO COAL COMBUSTION
[1]
Coal is a fossil fuel, formed largely by the partial decomposition and ‘coalification’ of ancient
plants under high pressure of overburden at elevated temperature during the course of hundreds of
millions of years. Coal is inhomogeneous and mainly composed of combustible organic matter,
mineral matter, and moisture. Since the coal-forming time could be quite different for different
coals, a variety of coal types exist, corresponding to various stages of coalification. For
combustion, coal is classified as lignite, sub-bituminous, bituminous, and anthracite.
Lignite, the youngest coal, is brown to black in color, with a high volatile matter content and a
high moisture content. It also has a high ash-content and a low heating-value in comparison with
other types of coal. Sub-bituminous coal is black, similar to bituminous in color. It has a lower
moisture content than lignite but is still of relatively low heating value. Bituminous coal has a
volatile matter-content from high to medium and low moisture-content. It is easy to ignite and
burn-out, and its heating value is high. Anthracite, with the longest coalification age, is the oldest
of all coals. It is jet black in color, hard and brittle. Its moisture content is low and carbon content
is high. Anthracite has a high heating value but is difficult to ignite and burn out.[1]
Combustion:
Combustion is a rapid chemical reaction between fuel and oxygen. When combustible elements of
fuel combine with O2, heat energy comes out. During combustion combustible elements like
Carbon, Sulfur, Hydrogen etc. combine with oxygen and produce respective oxides. Combustion
may be defined as the rapid high temperature complicated chemical reaction of oxygen with
Carbon, Hydrogen and Sulphur of coal
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1.1.1 A trip down the memory lane
[1]
Coal is an important energy source for humankind. Coal combustion has been identified in some
of the earliest recorded history. According to Elliott and Yoke (1981), the Chinese used coal as
early as 1000 BC, while the Greeks and Romans made use of coal before 200 BC. By 1215 AD,
trade in coal had started in England. The pioneering uses of coal (e.g., coke, coal tars, gasification),
have advanced steadily since the late sixteenth century. Coal combustion technology has been
further developed since the late nineteenth century. The coal fixed-bed stoker system was invented
in 1822; the firing of pulverized coal occurred in the brick-kiln in 1831, and fluidized-beds were
invented in 1931 [1]
1.1.2 Challenges Faced
[3]
About 85% of the coal produced in the United States is used to produce about 55% of its
electricity. To continue using coal for electric power generation, we must make coal burn cleaner
and make coal-fired plants more efficient. The 1990 Amendments to the Clean Air Act require
drastic reductions in the levels of SOx, NOx, and particulate emissions. Another goal for the
industry is to significantly reduce CO2 emissions by increasing the thermal efficiency of the power
plants. For all these reasons, a lot of attention has recently focused on the development of efficient
and environmentally clean technologies for production of electricity. Various projects have already
demonstrated the commercial feasibility of low-emission boiler systems (LEBS), as well as highperformance power systems that will improve their thermal efficiency while reducing even further
the emissions of SOx, NOx, and particulates.[3]
1.2 BASIC PROCESS OF COAL COMBUSTION
[1]
Coal is an organic fuel. When heated, the organic matter of coal is pyrolyzed, and then evolves
as volatile. The remaining solid is a mixture of carbon and mineral matter, which is referred to as
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“char.” The combustion of coal is primarily the combustion of carbon as well as the volatile matter.
It is known that the principal combustion process of coal involves four basic stages
i.
Formation of coal-oxygen complexes with evolution of heat.
ii.
Decomposition of these complexes with the generation of CO2 and H2O molecules
and formation of carboxyl (COOH), carbonyl (C=O) and phenolic -OH groups along
with more heat generation.
iii.
Decomposition of these groups to produce CO, CO2, H2, H2O and hydrocarbons such
as ethane, ethylene, propylene etc.
iv.
[2]
Decomposition of aliphatic structure with the formation of CO, CO2, and H2O.[1]
In low temperatures, the first step is developed faster than others. Oxygen molecules are diffused
through the pores into the internal surface and are attached to the coal surface by physical
adsorption. In this stage, the oxide layer formed due to the exposure of coal surface to the air,
prevents the diffusion of oxygen partially and oxidation rate is decreased with time. The reactions
between oxygen and coal are exothermic. The reaction rate increases as the temperature increases
and as a result coal reaches to ignition temperature at about 175°C with the firing of a flame. The
time required from the beginning of oxidation to reach the ignition temperature is called incubation
period.
Ignition point is the temperature at which the temperature of the combustible material should be
reached before it is combined with oxygen and combustion takes place.
For complete combustion to take place, sufficient time must be allowed before the temperature of
the gases is lowered below that point.
In complete combustion, the carbon combines with an equivalent amount of oxygen to form CO2.
Incomplete combustion occurs when coal does not unite according to the reaction stoichiometry.
In this type of combustion carbon monoxide, CO, may be formed which may be burned to carbon
dioxide by the reaction with more oxygen. The hydrogen and oxygen combine to produce H2O
vapor. Sulphur is converted to SO2, which in dissolution with water forms sulphuric acid. The
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amount of oxygen required for the combustion of a definite species or compound is fixed. Calorific
value of each combustible substance is unique for it. In practice it is impossible to obtain complete
combustion with the theoretical amount of air. In most of the cases, excess, amounting to double
or more than the theoretical supply of oxygen is required, depending upon the nature of the fuel to
be burned and the method of burning it. The reason for this is that it is impossible to bring each
molecule of oxygen in the air into intimate contact with the particles in the fuel that are to be
oxidized. It has been shown experimentally that coal usually requires 50 per cent more than the
theoretical net calculated amount of air, or about 18 kg per kg of fuel either under natural or forced
draft. If less than this amount of air is supplied, the carbon burns to carbon monoxide instead of
dioxide and its heat as complete combustion is not obtained.[2]
1.3 COAL COMBUSTION TECHNOLOGY
[1]
Coal combustion is extensively used for both industrial and domestic purposes. Boilers of power
plants, industrial boilers and heat kilns consume most of the world’s reserve of coal. The basic
coal combustion technology can be classified on the basis of the particle size of burning coal and
coal-feeding methods, which mainly include
i.
the coal fixed-bed combustion
ii.
coal suspending combustion
iii.
coal fluidized-bed combustion
1.3.1 Coal Fixed-bed Combustion
[1]
In early times, the coal fixed-bed combustion was the only known way of burning coal. The coal
bed is supported on a grate, which may be fixed or movable, and the air needed for combustion,
generally passes upward through the coal bed either by the chimney draught or by a fan. However,
as an exception, in some hand-fired domestic appliances the combustion air is drawn downward
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through the coal bed for eliminating smoke. In general, coal may be fed to the bed in the three
modes: overfeed, underfeed, and cross-feed.[1]
1.3.1.1 Fixed Grate
[1]
An overfeed fixed bed on a fixed grate is the simplest way of coal combustion. Fresh coal is
spread onto the surface of the burning coal bed manually or by a spreader. From the grate to the
bed surface the bed is divided into several zones based on combustion reactions that take place.
The combustion zones are shown in Figure 1. The fresh coal on the bed surface is heated rapidly
by the hot combustion gas and the radiation from the high-temperature flames and furnace walls.
It is advantageous to the ignition of the coal. The burning coal then descends in turn through the
reducing region and the oxidization region, becoming ash on the grate, and is finally removed.
Figure 1. Coal Fixed Bed Combustion
Process Description
The combustion air is generally supplied from the grate, flowing upwards through the fuel-bed.
The air is first heated by the coal ash, and then reacts with the high temperature coal char. The
combustion reaction produces CO2 and releases a large amount of heat, resulting in the rapid rise
in the bed temperature. The oxygen will be finally used up with the progressing of the oxidizing
reaction. The bed layer where the oxidizing reaction takes place is referred to as the oxidizing
layer, which is the highest temperature zone in the bed. If the thickness of the coal-bed is greater
than that of the oxidizing layer, a reducing layer will appear on top of the oxidizing layer, where
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CO2 can react with carbon at high temperature, producing CO. Therefore; different combustion
reactions may take place with different combustion products, depending on the bed thickness. For
this reason, two different combustion methods were designed accordingly, i.e., the shallow-bed
combustion and the thick-bed combustion.
In the shallow-bed combustion, the coal bed is about 100–150mm thick for bituminous, so there
is no occurrence of reducing reaction. All air needed in combustion is supplied from the bottom of
the bed. In the thick-bed method, the bed thickness is about 200–400 mm for bituminous, its
combustion air is provided separately. The primary air is provided from the bottom of the bed, and
the secondary air is provided over the bed to burn out the combustible gas produced by the bed.
The ratio of the primary air to the secondary air depends on the coal volatile content and the amount
of combustible gases. Coal can be fed not only onto the bed, but also under the bed.
This is the underfeed mode. In this mode, burning coal moves in co-current flow with the
combustion air. The released volatile matter, moisture, and combustion air pass up through the bed
so that less smoke is emitted in part-load operations. The underfeed-stoker designed to burn
bituminous and anthracite for firing boilers and warm-air furnaces is automatic and often used for
residential purposes. In the stoker, coal is fed from a bin or hopper by a feed screw into the bottom
of a conical retort, through the inner and outer walls from which air from a motor-driven fan is
supplied for combustion. According to A. Ralph, the underfeed-stoker is not used for the firing of
huge boilers, because of the impossibility of building them large enough to burn coal at the
required rate. In the intermediate sizes, the stoker tends to lose its favorable position due to its
sensitivity to the caking and ash fusion characteristics of coal. Thus, the successful operation of
this type of stoker lies in the careful selection of the coal used on it and a conservative rating to
avoid a high rate of burning as well as an excellent mechanical design.[1]
1.3.1.2 Moving Grate
[1]
The chain-grate stoker is a typical automatic moving stoker. Its moving grate carries the coal
bed on it passing through the high temperature areas of furnaces. The coal overfeed mode combines
with the moving grate, forming a spreader stoker (Figure 2), in which coal is spread onto a moving
bed by a spreader. The moving grate moves from the rear wall to the front wall or vice versa
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depending on the type of the spreader. As in the case of fixed grate, most fresh coal falls onto the
burning coal bed, getting better ignition condition. Spreader stokers are of adaptability to a wide
range of types and sizes of coals, ability to respond quickly to the change of load, and relative
freedom from slag and deposit problems in the furnace or on the heating surface. The
disadvantages of this type of stoker are the tendency to excessive smoke emission at part loads,
high carryover of fly ash and cinder at high loads, which can be minimized by an over-jet, for
increasing the turbulence in the furnace to reduce smoke and by the use of dust collectors to reduce
the emission of fly ash from the stack.
Figure 2. Spreader Stoker
On a considerable number of these moving grates coal is fed not by spreaders, but from a hopper
under an adjustable guillotine-type gate. The gate controls the thickness of the coal bed. This is
referred to as the cross feed mode. In the moving grate, the coal bed carried by the grate moves
from the front wall to the rear wall in furnace, and the coal drying, devolatilization, volatile
combustion, char combustion and ash removing take place in the bed with the bed moving. The
combustion regions in the moving bed are shown in Fig. 3. The coal on the surface of the bed is
ignited by the radiation of the furnace, and the combustion is transmitted downwards. In general,
a front arch and a long rear arch are used to insure stable ignition and fine mixing of fuel and
oxygen. This type of equipment remains suitable for the plants which can be assured of a longtime
supply of a suitable coal, and which do not require rapid changes of load. The problems of smoke
at low loads and that of carry-over of fly ash are much less acute than they are with spreader
feeding of coal.[1]
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Figure 3. Guillotine Gate Stoker
1.3.2 Coal Suspending Combustion
[1]
Most modern coal-fired power stations burn pulverized coal, which is blown into the combustion
chamber of a power plant through a specially designed burner. The burner mixes air with the
powdered coal, which then burns in a flame in the body of the combustion chamber. This is
suspension combustion and in this type of plant there is no grate. Finely ground wood, rice husk,
bagasse, or sawdust can be burned in a similar way. Suspension firing requires a special furnace.
The size and moisture content of the biomass (wood) must also be carefully controlled. Moisture
content should be below 15% and the biomass particle size has to be less than 15mm. Suspension
firing results in boiler efficiency of up to 80% and allows a smaller sized furnace for a given heat
output. However, it also requires extensive biomass drying and processing facilities to ensure that
the fuel is of the right consistency. It also demands special furnace burners.
A small number of plants designed to burn biomass in this way have been built. The technology
is also of great interest as the basis for the co-firing of wood or other biomass with coal in
pulverized. Pulverized coal combustion taking place in a suspension phase was first used as a
means of firing cement kilns. In the 1920s, it began to be applied to power generation. From 1930
onwards, nearly all coal-fired power plants and large industrial boilers have been fired by
pulverized fuel rather than by stoker system because of the two principal advantages [1]
(1) The pulverized fuel combustion allows a wider range of coals than a stoker.
(2) In practice, the stoker is limited to a maximum output of about 30 MW
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2 PYROLYSIS
2.1 INTRODUCTION TO PYROLYSIS
Pyrolysis is the one of the most common methods in thermal conversion technology of biomass.
In pyrolysis, biomass is heated to moderate temperatures, 400-600oC, In the absence of
stoichiometric oxygen to produce oil that can be used as a feed stock in existing petroleum
refineries. This is a high throughput process that has a potential for requiring relatively low capital
investment. In gasification, biomass is heated to high temperatures, >700oC, to produce a synthesis
gas(H2 and CO), which can be converted in a catalytic step to liquid transportationfuels(mixed
alcohols, Fishcher-Tropsch fuels, methanol –to-gasoline etc.). This technology builds upon
decades of experience with gasification of coal. Both approaches have the potential advantages of
being relatively insensitive to feedstock type, both suffer from production of unwanted byproducts.
In pyrolysis, oxygen-containing compounds (aldehydes, ketones, phenolics and organic acids)
make the oil too unstable and acidic for introduction into existing pipelines, tankers and refineries.
2.2 BIOMASS PYROLYSIS TECHNIQUES
Depending on the operating conditions, the pyrolysis process can be divided into three sub classes.
Conventional slow pyrolysis, fast pyrolysis and flash pyrolysis. The range of important operating
parameters for pyrolysis processes is given in the following table. At present, the preferred
technology is fast or flash pyrolysis at high temperature with very short residence time.
Main operating parameters for pyrolysis process
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2.2.1 Slow pyrolysis
Slow pyrolysis is a conventional pyrolysis process whereby the heating rate is kept slow
(approximately 0.1-1 ˚C/s). This slow heating rate leads to higher char yield than the liquid and
gaseous products. Slow pyrolysis has been utilized for thousands of years primarily for the
production of charcoal. In slow wood pyrolysis, biomass is heated to ~500 ˚C. the vapour residence
time in the reactor, gas-phase products have ample opportunities to continue to react with other
products to form char.
2.2.2 Fast Pyrolysis
Fast pyrolysis uses much faster heating rates (about 10-200oC) and is considered as a better
process than slow pyrolysis for producing liquid or gases. In fast pyrolysis the liquid product yield
is higher since the fast heating rates allow the conversion of thermally unstable biomass
compounds to a liquid product before they form undesired coke. Typically, fast pyrolysis processes
produce 60-75 wt% of liquid bio oil,15-20%wt of solid char and 10-20 %wt of noncondensable
gases depending on the feed stock used. Fast pyrolysis occurs on the timescale of a few seconds
or less. Therefore, chemical reaction kinetics, heat and mass transfer processes and phase transition
phenomena play important role in product distributions. Among pilot scale reactors, fluidized bed
reactors are best suited for the process as they offer high heating rates, rapid devolatilization and
are easy to operate. Other reactors such as entrained flow reactors, circulating fluidized bed
reactors, rotating cone reactors etc. are also used for this purpose.
2.2.3 Flash pyrolysis
Flash pyrolysis is an improved version of fast pyrolysis, whereby the heating rates are very high,
>1000oC/s, with reaction times of few to several seconds. Present reactors for flash pyrolysis
include fluidized bed reactors, vacuum pyrolysis reactor, rotating cone reactor, entrained flow
reactor, ablative, vortex or blade, twin screw reactors. Entrained flow or fluidized bed reactors are
considered the best reactors for this purpose. Due to the rapid heating rates and short reaction
times, for better yields, this process requires smaller particle size compared to the other processes.
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2.3 TYPES OF BIOMASS PYROLYSIS REACTORS
2.3.1 Small-scale pyrolysis devices for fundamental research
2.3.1.1 Resistively heated micro-furnace or tube pyrolyzer
Micro furnaces provide a constantly heated isothermal pyrolysis zone into which samples are
introduced by a liquid syringe, solid plunger syringe, or in a little cup. Lack of control of the
temperature/time characteristics of the sample has made the continuous-mode pyrolyzer less
attractive for precise analytical work. However, the combination of a pulsed molecular beam
source with the continuous mode pyrolysis oven overcomes these control problems.
2.3.1.2 Resistively heated element pyrolyser
Filament pyrolyzers can acquire a controlled pyrolysis temperature extremely quickly. An initial
pulse of heating at high voltage produces a current through the metal filament causing it to heat
rapidly until the programmed pyrolysis temperature is achieved. The pyrolysis temperature is
maintained by reducing the voltage. The filament pyrolizer such as a Pt-coil pyrolizer appears to
be the mostly used among various commercial models of the pyrolysis reactors. Samples that are
soluble in a volatile solvent are pyrolysed using a ribbon probe. Those that are not heated using a
coil probe. While samples are added directly on to the ribbon probe, quartz tubes are used to hold
the samples before being inserted into the coil probe with regards to the later, the exact pyrolysis
reaction time is difficult to determine since the sample never come in to direct contact with the
filament.
2.3.1.3 Curie-point filament pyrolyzer
Curie point pyrolyzer utilizes ferro-magnetic metals to provide rapid and reproducible heating
conditions. The sample is positions on to the end of pyrolysis wire made from an appropriate ferromagnetic alloy. It is then inserted the pyrolyzer and rapidly heated using a high frequency induction
coil. The temperature ceases to rise when the curie-point of the metal has been reached. That is the
exact reproducible temperature at which ferro-magnetic material loses its magnetism. At this point
the temperature remains constant until the coil is switched off. In contrast to the micro furnace, the
rise time of curie-point pyrolyzer is much faster. However, the choice of different pyrolysis
temperatures is limited since they are determined by the curie point of available materials.
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2.3.1.4 Laser pyrolyzer
The Laser pyrolyser consists of a laser and the associated optical devices , the sample chamber and
cold trap. The laser is focused through microscope objective lense and the targeted area is
pyrolysed using either a continuous wave or a number of high energy pulses. The thermal
interaction between laser and material initiates a shock which in turn produce a range of pyrolysis
products. A variety of different laser can be used as a fragmentation source depending on the type
of material being pyrolysed. Lasers used for pyrolysis include Nd:YAG laser (1064nm), an rogan
ion laser(458-515 nm)and a ArF eximer laser(193nm) to name a few. The use of laser as a
fragmentation source has several advantages. Unlike the filament, curie-point and the furnace
pyrolyzer, laser pyrolysis requires the very little sample preparation or pretreatment since analysis
is performed directly on the solid polymer matter. The intense, short duration laser beam enables
rapid temperature rise times, followed by rapid cooling, reducing the potential for secondary
reactions between the pyrolysis products and therby simplifying interpretations of fragmentation
patterns. finally, the collimated nature of the laser beam enables focusing to achieve spatial
resolutions in targeting specific areas and layers of the sample.
2.3.1.5 Plasma pyrolysis reactor
Plasma pyrolysis reactor offers some unique advantages for biomass conversion, in comparison to
conventional pyrolysis at low temperatures and slow heating rates. The high energy density and
temperature associated with thermal plasmas and the corresponding fast reaction times provide a
potential solution for the problems that occur in conventional pyrolysis processes, such as low gas
productivity and the generation of heavy tarry compounds. Nevertheless, plasma pyrolysis of
biomass for energy and chemical production are seldom studied because of the high electrical
power consumption. In fact, the temperature initiated in thermal plasma (usually 2500- 9500 oC)
is much too high for biomass pyrolysis. Recently there was an study which resulted that this type
of plasma combines the high plasma reactivity and thermal efficiency with a medium temperature
(900-9500oC) and favours hydrocarbon cracking and thus increase the yield of syngas. This
method is therefore practical interest for the utilization of biomass material for the purpose of
syngas and char production.
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2.3.2 Pilot scale pyrolysis reactors
2.3.2.1 Fixed Bed Reactor
Fixed bed reactors were traditionally used for the production of charcoal. Poor and slow heat
transfer resulted in very low liquid charcoal yields. These gasifiers are divided into downdraft and
updraft fixed bed reactors. Their technology is simple, reliable and proven for fuels with a relative
uniform size. In a down draft fixed bed reactor, solid moves slowly down and a vertical shaft and
air introduced and reacts at a throat that supports the gasifying biomass. The solid and product gas
move downward in a co-current mode. A relatively clean gas is produced with low tar and usually
with high carbon conversion. In contrast, the updraft fixed bed reactor is characterized by solid
moving down a vertical shaft and contacting a counter-current mode. The product gas is very dirty
with high levels of tars although tar crackers have been developed to alleviate this problem.
2.3.2.2 Bubbling Fluidized Bed Reactor
Bubbling fluidized beds, biomass particles are introduced into a bed of hot sand fluidized by a
recirculated product gas. The high heat transfer rates from fluidized sand cause rapid heating of
biomass particles and some ablation by attrition with the sand particles occurs. The bubbling
fluidized bed gasifier is characterized good temperature control and high reaction rates. They have
greater tolerance to particle size range. They also have higher particulates and more moderate tar
levels in product gastar cracking catalysts can be added to the bed. The bubbling fluidized bed
pyrolyzer is characterized by simple construction and operation and is well understood technology.
They have good temperature control, limited turn-down capability and provide very efficient heat
transfer to biomass particles due to high solid density.
2.3.2.3 Circulating Fluidized Bed Reactor
For circulating fluidized bed reactors, biomass particles are introduced into a circulating fluidized
bed of hot sand. The recirculated product gas, sand, and biomass particles move together. The high
heat transfer rates from sand ensure rapid heating of biomass particles and ablation is more
prevalent than with regular fluidized beds. The circulating fluidized bed gasifier is characterized
by all features of the bubbling fluidized bed reactors in addition to higher cost at lower capacity.
They are in-bed catalytic processing and circulation of hot solids can cause erosion problems inside
the reactor vessel. The circulating fluidized bed pyrolizer is characterized by good temperature
control in the reactor. the residence time for the char is almost the same as for vapour and gas.the
Page | 13
char is attrited more due to higher gas velocities, which result in high char contents. The produced
char is typically separated by cyclone.
2.3.2.4 Entrained flow reactor
Entrained flow fast pyrolysis is in principle a simple technology. however, most of the
development of these reactors have not been as successful as had been hoped due to poor heat
transfer between the hot gas and soil particles. This reactor can result in very high heating rates of
the particles and the residence time can be varied from milli seconds to a few seconds. This gasifier
reactor is characterized by a simple design, but costly feed preparation is needed for woody
biomass. They require high gas flows in lower liquid yields.
2.3.2.5 Vacuum Furnace Reactor
For this reactor, biomass is thermally decomposed under reduced pressure. The vapours produced
are quickly removed from the vacuum and recovered as bio-oil as condensation. This pyrolysis
reactor is characterized by longer residence time of solid and short residence times. Other
important feature in this reactor includes ability to produce larger paricles than most fast pyrolysis
reactors and there is less char in the liquid product due to lower gas velocities. There is also no
requirement for carrier gas and the process is mechanically complicated. The typical liquid yields
for dry biomass feed obtained in this process are from 35 to 50%.
2.3.2.6 Ablative Reactor
The ablative reactor is characterized by high velocity impact of particle on a hot reactor wall,
achieved by centrifugal force or mechanically. High relative motion is achieved between particles
and the the reactor wall which is typically less than600 oC. the system is more intensive and the
process is mechanically driven so the reactor is so complex. The ablative pyrolysis reactors have
considerable advantages over conventional fluidized bed reactors,
1. No milling of the biomass is required, because the heat introduced as the particles are pyrolysed
as by the direct contact with the hot surface.
2. They have good heat transfer with high heating rates and relatively small contact surface
because compact design.
3. They have high energy and cost efficiency as no heating and cooling of fluidizing gases is
required.
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4. They allow installation of condensation units with a small volume, requiring less space at lower
costs.
2.3.2.7 Rotating Cone Reactor
The rotating cone reactor is a noval reactor type for flash pyrolysis of biomass with negligible char
formation, in which rapid heating and a short residence time of the solids can be realized. Biomass
materials like wood, rice husks or even olive stones can be pulverized and fed tot the rotating cone
reactor. Carrier gas requirements in the pyrolysis reactor are much less than for fluid bed and
transported bed systems. However, gas is needed for char burn off and for sand transport. Complex
integrated operations of three subsystems are required: Rotating cone pyrolyzer, riser for sand
recycling, and bubbling char combustor. Like other shallow transportedbed reactors relatively fine
particles are required to obtain a good liquid yield. The liquid yields of 60-70% on dry feed are
typically obtained. There is no large scale commercial implementation.
2.3.2.8 Auger Reactor
In an auger reactor, hot sand and biomass particles are fed at one end of a screw. The screw mixes
the sand and biomass and conveys them along, providing a good control of the biomass residence
time. This process does not dilute the pyrolysis products with a carrier or fluidizing gas. However,
sand must be reheated in a separate vessel and mechanical reliability is a concern. There is no large
scale commercial implementation. The advantage of this reactor include,
 Compact size
 No carrier gas required
 Lower processing temperatures (400oC)
The challenges in this are,:
1. Presence of moving parts in the hot zone,
2. Heat transfer at a large scale may be a problem
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3 GASIFICATION
3.1 INTRODUCTION TO GASIFICATION
Gasification is a technological process that can convert any carbonaceous (carbon-based) raw
material such as coal into fuel gas, also known as synthesis gas (syngas for short). Gasification
occurs in a gasifier, generally a high temperature/pressure vessel where oxygen (or air) and steam
are directly contacted with the coal or other feed material causing a series of chemical reactions to
occur that convert the feed to syngas and ash/slag (mineral residues).
Syngas is so called because of its history as an intermediate in the production of synthetic natural
gas. Composed primarily of the colorless, odorless, highly flammable gases carbon monoxide
(CO) and hydrogen (H2), syngas has a variety of uses. The syngas can be further converted
(or shifted) to nothing but hydrogen and carbon dioxide (CO2) by adding steam and reacting over
a catalyst in a water-gas-shift reactor. When hydrogen is burned, it creates nothing but heat and
water, resulting in the ability to create electricity with no carbon dioxide in the exhaust gases.
Furthermore, hydrogen made from coal or other solid fuels can be used to refine oil, or to make
products such as ammonia and fertilizer. More importantly, hydrogen enriched syngas can be used
to make gasoline and diesel fuel. Polygeneration plants that produce multiple products are
uniquely possible with gasification technologies. Carbon dioxide can be efficiently captured from
syngas, preventing its greenhouse gas emission to the atmosphere and enabling its utilization (such
as for Enhanced Oil Recovery) or safe storage.
Gasification offers an alternative to more established ways of converting feedstock’s
like coal, biomass, and some waste streams into electricity and other useful products. The
advantages of gasification in specific applications and conditions, particularly in clean generation
of electricity from coal, may make it an increasingly important part of the world's energy and
industrial markets. The stable price and abundant supply of coal throughout the world makes it the
main feedstock option for gasification technologies going forward. The technology's placement
markets with respect to many techno-economic and political factors, including costs, reliability,
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availability and maintainability (RAM), environmental considerations, efficiency, feedstock and
product flexibility, national energy security, public and government perception and policy, and
infrastructure will determine whether or not gasification realizes its full market potential.
The graphic below is a representation of a gasification process for coal, depicting both the
feedstock flexibility inherent in gasification, as well as the wide range of products and usefulness
of gasification technology.
3.2 FUNDAMENTALS OF GASIFICATION:
Gasification is a partial oxidation process. The term partial oxidation is a relative term which
simply means that less oxygen is used in gasification than would be required for combustion (i.e.,
burning or complete oxidation) of the same amount of fuel. Gasification typically uses only 25 to
40 percent of the theoretical oxidant (either pure oxygen or air) to generate enough heat to gasify
the remaining unoxidized fuel, producing syngas. The major combustible products of gasification
are carbon monoxide (CO) and hydrogen (H2), with only a minor amount of the carbon completely
oxidized to carbon dioxide (CO2) and water. The heat released by partial oxidation provides most
of the energy needed to break up the chemical bonds in the feedstock, to drive the other
endothermic gasification reactions, and to increase the temperature of the final gasification
products.
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3.3 REACTIONS AND TRANSFORMATION:
The chemistry of gasification is quite complex and is accomplished through a series of physical
transformations and chemical reactions within the gasifier. Some of the major chemical reactions
are shown in the diagram below. In a gasifier, the carbonaceous feedstock undergoes several
different processes and/or reactions:
 Dehydration – Any free water content of the feedstock evaporates, leaving dry material and
evolving water vapor which may enter into later chemical reactions.
 Pyrolysis – This occurs as the feedstock is exposed to rising temperature in the gasifier.
Devolatization and breaking of the weaker chemical bonds occurs, releasing volatile gases such as
tar vapors, methane, and hydrogen, along with producing a high molecular weight char which will
undergo gasification reactions.
 Combustion – The volatile products and some of the char react with limited oxygen to form carbon
dioxide (CO2), carbon monoxide (CO), and in doing so, provide the heat needed for subsequent
gasification reactions.
 Gasification – The remaining char reacts with CO2 and steam to produce CO and hydrogen (H2).
 Water-gas-shift and methanation – These are separate reversible gas phase reactions taking place
simultaneously based on gasifier conditions. These are minor reactions which play a small role
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within in the gasifier. Depending on the desired product, the syngas may undergo further watergas shift and methanation processing downstream from the gasifiers.
3.4 DETAILED GASIFICATION CHEMISTRY:
The chemical reactions of gasification can progress to different extents depending on the
gasification conditions (like temperature and pressure) and the feedstock used. Combustion
reactions take place in a gasification process, but, in comparison with conventional combustion
which uses a stoichiometric excess of oxidant, gasification typically uses one-fifth to one-third of
the theoretical oxidant. This only partially oxidizes the carbon feedstock. As a "partial oxidation"
process, the major combustible products of gasification are carbon monoxide (CO) and hydrogen,
with only a minor portion of the carbon completely oxidized to carbon dioxide (CO2). The heat
produced by the partial oxidation provides most of the energy required to drive the endothermic
gasification reactions.
Within a gasification process, the major chemical reactions are those involving carbon, CO, CO 2,
hydrogen (H2), water (steam) and methane (CH4), as follows:
The combustion reactions:
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1.
C + ½ O2 → CO
2.
CO + ½ O2 → CO2
3.
H2 + ½ O2 → H2O
(-111 MJ/kmol)
(-283 MJ/kmol)
(-242 MJ/kmol)
Other important gasification reactions include:
4. C + H2O ↔ CO + H
"the
Water-Gas
Reaction"
Boudouard
Reaction"
Methanation
Reaction"
(+131 MJ/kmol)
5. C + CO2 ↔ 2CO
"the
(+172 MJ/kmol)
6. C + 2H2 ↔ CH4
"the
(-75 MJ/kmol)
With the above, the combustion reactions are essentially carried out to completion under normal
gasification operating conditions. And, under the condition of high carbon conversion, the three
heterogeneous reactions (reactions 4 to 6) can be reduced to two homogeneous gas phase reactions
of water-gas-shift and steam methane-reforming (reactions 7 and 8 below), which collectively play
a key role in determining the final equilibrium synthesis gas (syngas) composition.
7. CO + H2O ↔ CO2 + H2
"Water-Gas-Shift
Reaction"
(-41 MJ/kmol)
8. CH4 + H2O ↔ CO2 + 3 H2
"Steam-Methane-Reforming
Reaction"
(+206 MJ/kmol)
In the low-oxygen, reducing environment of the gasifier, most of the feedstock’s sulfur coverts to
hydrogen sulfide (H2S), with a small amount forming carbonyl sulfide (COS). Nitrogen chemically
bound in the feed generally converts to gaseous nitrogen (N2), with some ammonia (NH3), and a
small amount forming hydrogen cyanide (HCN). Chlorine is primary converted to hydrogen
chloride (HCl). In general, the quantities of sulfur, nitrogen, and chloride in the fuel are sufficiently
small that they have a negligible effect on the main syngas components of H2 and CO. Trace
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elements associated with both organic and inorganic components in the feed, such as mercury,
arsenic and other heavy metals, appear in the various ash and slag fractions, as well as in gaseous
emissions, and need to be removed from the syngas prior to further use.
The table summarizes the main transformations of solid fuel constituents to gaseous species in both gasification and
combustion. This shows clearly the marked differences between gasification (resulting in syngas) and combustion
(resulting in exhaust gas).
3.5 THERMODYNAMICS AND KINETICS OF GASIFICATION REACTIONS:
Gasification reactions are reversible. The direction of the reaction and its conversion are subjected
to the constraints of thermodynamic equilibrium and reaction kinetics. The combustion reactions
C + ½ O2 → CO
(-111 MJ/kmol)
CO + ½ O2 → CO2
(-283 MJ/kmol)
H2 + ½ O2 → H2O
(-242 MJ/kmol)
essentially go to completion (to the right). The thermodynamic equilibrium of the gasification
reactions
CO + H2O ↔ CO2 + H2
"Water-Gas-Shift Reaction" (-41 MJ/kmol)
CH4 + H2O ↔ CO2 + 3 H2
"Steam-Methane-Reforming Reaction" (+206 MJ/kmol)
are relatively well defined and collectively impose a strong influence on the thermal efficiency
and the produced syngas composition of a gasification process. Thermodynamic modeling has
been a useful tool for estimating key design parameters for a gasification process, for example:
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 Calculating of the relative amounts of oxygen and/or steam required per unit of coal feed.
 Estimating the composition of the produced syngas.
 Optimizing the process efficiency at various operating conditions.
Other deductions concerning gasification process design and operations can also be derived from
the thermodynamic understanding of its reactions. Examples include:
 To produce a syngas with a low methane content, a high temperature and substantial amount of
steam in excess of the stoichiometric requirement are required.
 Gasification at very high temperature, on the other hand, will increase oxygen consumption and
decrease the overall process efficiency.
 To produce a syngas with a high methane content (see discussion of synthetic natural gas
production), gasification needs to be operated at low temperature (~700°C), but the methanation
reaction kinetics will be poor without the presence of a catalyst (see discussion of catalytic
gasification).
 There is considerable advantage to carry out gasification under pressure. At a typical entrained
flow gasifier operation temperature of ~2,700°F (1,500°C), the syngas composition shows very
little change as a function of operating pressure (Higman, 2008), but significant savings in
compression energy and cost reduction from using smaller equipment can be realized.
Relative to the thermodynamic understanding of the gasification process, its kinetic behavior is
more complex. Very little reliable kinetic information on coal gasification reactions exists, partly
because it is highly depended on the process conditions and the nature of the coal feed, which can
vary significantly with respect to composition, mineral impurities, and reactivity. Certain
impurities, in fact, are known to have catalytic activity on some of the gasification reactions.
3.6 SYNGAS COMPOSITION:
Depending on the feedstock and the gasification process involved; however typically syngas is

30 to 60% carbon monoxide (CO),

25 to 30% hydrogen (H2),

0 to 5% methane (CH4),

5 to 15% carbon dioxide (CO2),
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plus a lesser or greater amount of water vapor, smaller amounts of the sulfur compounds hydrogen
sulfide (H2S), carbonyl sulfide (COS), and finally some ammonia and other trace contaminants.
3.7 TYPES OF COMMERCIAL GASIFIERS:
Although there are various types of gasifiers (gasification reactors), different in design and
operational characteristics, there are three main gasifier classifications into which most of the
commercially available gasifiers fall. These categories are as follows:
 Fixed-bed gasifiers (also referred as moving-bed gasifiers)
 Entrained-flow gasifiers
 Fluidized-bed gasifiers
Commercial gasifiers of GE Energy, CB&I E-Gas™ and Shell SCGP are examples of entrainedflow types. Fixed-or moving-bed gasifiers include that of Lurgi and British Gas Lurgi (BGL).
Examples of fluidized-bed gasifiers include the catalytic gasifier technology being commercialized
by Great Point Energy, the Winkler gasifier, and the KBR transport gasifiers.
3.7.1 Fixed (Moving) bed Gasifier:
Fixed- or moving-bed gasifiers commonly operate at moderate pressures (25-30 atmospheres).
Feedstocks in the form of large coal particles1 and fluxes are loaded into the top of the refractorylined gasifier vessel and move slowly downward through the bed, while reacting with high oxygen
content gas introduced at the bottom of the gasifier that is flowing countercurrently upward in the
gasifier. The basic configuration is the same as seen in the common blast furnace. Reactions within
the gasifier occur in different "zones". In the "drying zone" at the top of the gasifier, the entering
coal is heated and dried, while cooling the product gas before it leaves the reactor. The coal is
further heated and devolatized by the higher temperature gas as it descends through the
"carbonization zone". In the next zone, the "gasification zone", the devolatized coal is gasified by
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reaction with steam and carbon dioxide. Near the bottom of the gasifier, in the "combustion zone",
which operates at the highest temperature, oxygen reacts with the remaining char.
Moving-bed gasifiers operate in two different modes. In the dry-ash mode of operation (e.g., Lurgi
dry ash gasifier), the temperature is moderated to below the ash-slagging temperature by reaction
of the char with excess steam. The ash below the combustion zone is cooled by the entering steam
and oxidant (oxygen or air) and produced as a solid ash. In the slagging mode of operation
(e.g., British Gas/Lurgi or BGL gasifier), much less steam is used, and as the result, a much higher
temperature is achieved in the combustion zone, melting the ash and producing slag.
Moisture content of the fuel is the main factor which determines the discharge gas temperature.
Lignite, which has very high moisture content, produces raw gas at a temperature of around 600°F.
Lower moisture bituminous coal produces gas temperatures of over 1000°F. Typically, the product
gas leaving the gasifier is quenched by direct contact with recycle water to condense and remove
tars and oils. After quench, heat can be recovered from the gas by generation of low pressure
steam.
Characteristics
Moving-bed gasifiers share the following characteristics:
 Simplicity of gasifier configuration and operation
 High equipment efficiency
 Relatively low oxidant (oxygen or air) requirement
 Less complex feedstock preparation with the use of coarse coal particles
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 Product gas at relatively low temperatures, thus no need for expensive high-temperature heat
recovery equipment
 Feedstock flexibility: suitable to handle coals with high reactivity and moisture
 High "cold-gas" thermal efficiency, when the heating value of the produced hydrocarbon liquids
is accounted for
 High methane content in product gas
 Limited ability to handle coal fines
 Caking coals require design modifications to the gasifier
 Long feedstock residence time in gasifier and slag flow characteristics require carefully controlled
feed size distribution for proper operation
 Hydrocarbon liquids such as tars and oils are produced; increased effort to clean produced gas if
it is used for applications other than direct heating
 Explosion hazard without careful process monitoring
3.7.2 Entertained Flow Gasifiers:
In entrained-flow gasifiers, fine coal feed and the oxidant (air or oxygen) and/or steam are fed cocurrently to the gasifier. This results in the oxidant and steam surrounding or entraining the coal
particles as they flow through the gasifier in a dense cloud. Entrained-flow gasifiers operate at
high temperature and pressure—and extremely turbulent flow—which causes rapid feed
conversion and allows high throughput. The gasification reactions occur at a very high rate (typical
residence time is on the order of few seconds), with high carbon conversion efficiencies (9899.5%). The tar, oil, phenols, and other liquids produced from devolatization of coal inside the
gasifier are decomposed into hydrogen (H2), carbon monoxide (CO) and small amounts of light
hydrocarbon gases. Entrained-flow gasifiers have the ability to handle practically any coal
feedstock and produce a clean, tar-free syngas. Given the high operating temperatures, gasifiers of
this type melt the coal ash into vitreous inert slag.
The fine coal feed can be fed to the gasifier in either a dry or slurry form. The former uses a lock
hopper system, while the latter relies on the use of high-pressure slurry pumps. The slurry feed is
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a simpler operation, but it introduces water into the reactor which needs to be evaporated. The
result of this additional water is a product syngas with higher H2 to CO ratio, but with a lower
gasifier thermal efficiency. The feed preparation system needs to be evaluated along with other
process design alternatives for a particular application.
The high temperatures involved in this type of gasification tend to shorten the life of system
components, including gasifier vessel refractory. Also, it may be necessary to add fluxes or blend
feedstock parameters to achieve good slagging characteristics.
Characteristics
Entrained-flow gasifiers typically exhibit the following characteristics:
 Fuel flexibility; can accept a variety of solid feedstocks
 Large oxidant requirements
 Can either be oxygen or air blown, but most commercial plants are oxygen blown
 Uniform temperature within the reactor
 Slagging operation
 Short reactor residence time
 High carbon conversion, but low cold gas efficiency
 High level of sensible heat in product gas, heat recovery is required to improve efficiency
 Environmentally most benign; produced syngas consists of mainly H2, CO and carbon dioxide
(CO2) with trace amount of other contaminants which can be removed downstream of the reactor;
glassy slag is inert and easily disposed
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3.7.3 Fluidized-Bed Gasifiers:
Fluidized-bed gasifiers suspend feedstock particles in an oxygen-rich gas so the resulting bed
within the gasifier acts as a fluid. These gasifiers employ back-mixing, and efficiently mix feed
coal particles with coal particles already undergoing gasification. To sustain fluidization, or
suspension of coal particles within the gasifier, coal of small particles sizes (<6 mm) is normally
used. Coal enters at the side of the reactor, while steam and oxidant enter near the bottom with
enough velocity to fully suspend or fluidize the reactor bed. Due to the thorough mixing within
the gasifier, a constant temperature is sustained in the reactor bed. The gasifiers normally operate
at moderately high temperature to achieve an acceptable carbon conversion rate (e.g., 90-95%) and
to decompose most of the tar, oils, phenols, and other liquid byproducts. However, the operating
temperatures are usually less than the ash fusion temperature so as to avoid clinker formation and
the possibility of de-fluidization of the bed. This, in turn means that fluidized-bed gasifiers are best
suited to relatively reactive coals, low rank coals, and other fuels such as biomass.
Some char particles are entrained in the raw syngas as its leaves the top of the gasifier, but are
recovered and recycled back to the reactor via a cyclone. Ash particles, removed below the bed,
give up heat to the incoming steam and recycle gas. At startup, the bed is heated externally before
the feedstock is introduced.
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Characteristics
Fluidized-bed gasifiers may differ in ash conditions (dry or agglomerated/slagging) and in design
configurations for improving char use. Also, depending on the degree of fluidization and bed
height, these types of reactors sometimes are also named as circulating fluidized bed reactors,
and/or transport reactors.
Fluidized-bed gasifiers display these characteristics:
 Load flexibility and high heat transfer rates
 Fuel flexibility, can gasify a wide range of feedstocks
 Moderate oxidant and steam requirements
 Uniform, moderately high temperature throughout the gasifier
 Higher cold gas efficiency than entrained-bed gasifiers, but lower carbon conversion
 Extensive char recycling is required
3.8 R&D FOR GASIFIER OPTIMIZATION/PLANT SUPPORTING SYSTEM:
Gasifiers operate under demanding conditions, presenting several challenges especially in regard
to materials, where not only is the gasifier itself—more specifically the refractories—under severe
physical and chemical stress, but so are any devices inserted to monitor and control the gasification
process.
Availability is very important to the economics of a gasification plant. A shut-down gasifier halts
synthesis gas (syngas) production and, therefore, final product output (electricity, liquid fuels,
etc.). With current, state-of-the-art technology, many integrated gasification combined cycle
(IGCC) designs incorporate a spare gasifier in order to achieve acceptable overall plant
availability, even though this entails a higher capital cost. With continued operating experience
and research, it is believed that an online availability of 85–95 percent in utility applications—and
95 percent for chemical production and other applications, can be achieved by a gasification plant,
but currently, most plants cannot achieve that without redundancy or fuel backup:
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 Tampa Electric Integrated Gasification Combined-Cycle Project – operating commercially since
1996, 82% gasifier availability and 74% plant availability.
 Wabash River Coal Gasification Repowering Project – operating commercially since 1995,
reaching 79.1% plant availability, and 76% gasifier availability.
 Great Plains Synfuels Plant – operating since 1984, 98.7% days with production (i.e., not-zero
production), although this is not traditionally how availability is measured.
 Elcogas SA IGCC, Puertollano, Spain – operating since 1998, provided 78.6% gasifier availability,
and 48% overall plant availability as of 2007.
Research and development is being conducted to increase the availability of the gasifier and
decrease the cost of operation and maintenance, thereby substantially optimizing gasifier
operation. Examples include advanced materials development for refractory and the development
of a reliable, practical and cost-effective means of monitoring real-time temperature in the gasifier
through advances in sensors and instrumentation.
In addition to development of technologies such as advanced refractories and sensors, current
research efforts also include development of gasifiers for low-rank coal, creating models to better
understand the kinetics and particulate behavior of fuel inside a gasifier, and developing practical
solutions to mitigate the plugging and fouling of syngas coolers.
4 REFERENCES
1.
2.
3.
4.
Coal, Oil, Shale, Natural Bitumen, Heavy oil and Peat – vol I – Coal Combustion Xinnglin Shen
Mechanism and optimization of coal Combustion
Fuel and energy lecture slide
Research paper on Gasification by US Department of energy NETL
(https://netl.doe.gov/research/Coal/energy-systems/gasification/gasifipedia/intro-togasification)
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