final report on biomass gasifier
TRANSCRIPT
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Chapter 1. Introduction to Gasification
1.1 What is Gasification
Gasification is a process that converts carbonaceous materials, such as coal,
petroleum, biofuel, or biomass, into carbon monoxide and hydrogen by reacting the
raw material at high temperatures with a controlled amount of oxygen and/or steam.
The resulting gas mixture is called synthesis gas or syngas and is itself a fuel.
Gasification is a method for extracting energy from many different types of organic
materials.
Gasification is a thermal conversion process in which solid or liquid fuel is
converted into a gaseous fuel. Contrary to combustion, gasification produces a gas
that is combustible.
Gasification can be considered as combustion with a shortage of oxygen. The
process is generally operated at the point where just enough oxygen is added to the
process that the heat generated equals the energy that is required to volatilize the
feedstock.
The advantage of gasification is that using the syngas is potentially more
efficient than direct combustion of the original fuel because it can be combusted athigher temperatures or even in fuel cells, so that the thermodynamic upper limit to the
efficiency defined by Carnot's rule is higher or not applicable. Syngas may be burned
directly in internal combustion engines, used to produce methanol and hydrogen, or
converted via the Fischer-Tropsch process into synthetic fuel. Gasification can also
begin with materials that are not otherwise useful fuels, such as biomass or organic
waste. In addition, the high-temperature combustion refines out corrosive ash
elements such as chloride and potassium, allowing clean gas production fromotherwise problematic fuels.
Gasification of fossil fuels is currently widely used on industrial scales to
generate electricity. However, almost any type of organic material can be used as the
raw material for gasification, such as wood, biomass, or even plastic waste.
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Gasification relies on chemical processes at elevated temperatures >700C,
which distinguishes it from biological processes such as anaerobic digestion that
produce biogas.
1.2
Why Gasification
One of the most compelling challenges of the 21st Century is finding a way to
meet national and global energy needs while minimizing the impact on the
environment. Gasification can help meet those challenges.
Gasification is a time-tested, reliable, and flexible technology that converts
carbonaceous materials, biomass, municipal waste, scrap tires and plastics into a clean
high energy gas.
The Power Hearth produces a clean, particulate-free gas that can be used to
fuel industrial boilers or to power internal combustion or turbine engines with
generators to produce megawatts of electricity.
Gasification does not involve combustion, (or burning), but instead is a
thermal chemical process that uses high temperature in a controlled environment, with
limited oxygen, to convert carbon-based materials directly into a high energy
producer gas. The gasification process breaks these materials down to the molecularlevel, so impurities can be easily and inexpensively removed.
The high-temperature combustion refines out corrosive ash elements
allowing clean gas production from otherwise problematic fuel sources.
Gasification can recover the energy locked in biomass and municipal solid
waste - converting those materials into valuable products and eliminating the need for
incineration or landfill.
Gasification has been reliably used on a commercial scale for more than 50
years in the refining, fertilizer, and chemical industries, and for more than 35 years in
the electric power industry.
Gasification produces electricity with significantly reduced environmental
impact compared to traditional technologies.
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Gasification plants bring good jobs to a community construction jobs
needed to build a plant, and well-paying permanent jobs needed to run the plant.
Compared to the old coal-burning plants, gasification can capture carbon
dioxide much more efficiently and at a lower cost. This capture technology is being
successfully used at gasification plants in the U.S. and worldwide.
Gasification is an investment in our energy future.
Gasification is not incineration. Incineration is the burning of fuels in an
oxygen-rich environment, where the waste material combusts and produces heat and
carbon dioxide, along with a variety of other pollutants. Gasification is the conversion
of feedstocks into their simplest molecules - carbon monoxide, hydrogen and methane
forming a syngas or producer gas that can be used for generating electricity or
producing thermal heat.
1.3 Gasification v/s Combustion
Gasification is not an incineration or combustion process. Rather, it is a
conversion process that produces more valuable and useful products from
carbonaceous material. Following table compares the general features of gasification
and combustion technologies. Both gasification and combustion processes convertcarbonaceous material to gases. Gasification processes operate in the absence of
oxygen or with a limited amount of oxygen, while combustion processes operate with
excess oxygen. The objectives of combustion are to thermally destruct the feed
material and to generate heat. In contrast, the objective of gasification is to convert the
feed material into more valuable, environmentally friendly intermediate products that
can be used for a variety of purposes including chemical, fuel, and energy production.
Elements generally found in a carbonaceous material such as C, H, N, O, S, and Cl
are converted to a syngas consisting of CO, H2, H2O, CO, N2, CH4, H2S, HCl,
COS, HCN, elemental carbon, and traces of heavier hydrocarbon gases. The products
of combustion processes are CO2, H2O, SO2, NO, NO2, and HCl.
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Table 1.3: Comparison between Gasification and Combustion
FEATURES GASIFICATION COMBUSTION
Purpose
Creation of valuable,usable products from waste orlower value material
Generation of heat ordestruction of waste
Process Type Thermal and chemical
conversion using no or limitedoxygen
Complete combustion usingexcess oxygen (air)
Raw Gas
Composition
(before gas
cleanup)
H2, CO, H2S, NH3, and
particulates
CO2, H2O, SO2, NOx, and
particulates
Gas Cleanup Syngas cleanup at
atmospheric to highpressures depending on thegasifier design
Treated syngas used forchemical, fuels, or powergeneration
Recovers sulphur species inthe fuel as sulphur orsulphuric acid
Clean syngas primarily
consists of H2and CO
Flue gas cleanup atatmospheric pressure
Treated flue gas isdischarged to atmosphere
Any sulphur in the fuel isconverted to SO2that mustbe removed using flue gascleanup systems, generatinga waste that must belandfilled.
Clean flue gas primarily
consists of CO2 and H2O
Ash/char or slag
handling
Low temperatureprocesses produce a charthat can be sold as fuel.
High temperatureprocesses produce slag, anon-leachable, non-hazardous materialsuitable for use asconstruction materials.
Fine particulates are
recycled to gasifier. Insome cases fineparticulates may beprocessed to recovervaluable metals.
Bottom ash and flyash arecollected, treated, anddisposed as hazardous wastein most cases.
Temperature 1400F 2700F 1500F 1800F
Pressure Atmospheric to high Atmospheric
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1.4 Advantages of Gasification over Combustion
1.4.1 Environmental
Gasification has inherent advantages over combustion for emissions control.
Emission control is simpler in gasification than in combustion because the
produced syngas in gasification is at higher temperature and pressure than the
exhaust gases produced in combustion. These higher temperatures and
pressures allow for easier removal of sulfur and nitrous oxides (SOX, and
NOX), and trace contaminants such as mercury, arsenic, selenium, cadmium,
etc. Gasification systems can achieve almost an order of magnitude lower
criteria emissions levels than typical current U.S. permit levels and +95%
mercury removal with minimal cost increase. In addition, gasification systems
require less water than other technologies.
1.4.2 Carbon Capture Utilization and Storage
Similar to the removal of other contaminants, gasification lends itself to
efficient carbon dioxide (CO2) removal because of the high temperature and
pressure of the produced syngas. Studies show that in CO2removal
applications, integrated gasification combined cycle (IGCC) plants are more
efficient than other commercial technologies. Captured CO2is prevented from
entering the atmosphere through either utilization or storage. The two most
common options are carbon dioxide enhanced oil recovery (CO2EOR),
and carbon sequestration. CO2EOR is a highly practical utilization strategy, in
which CO2is injected underground into mature oilfields to sweep residual oil,
where CO2is stored underground in the process. Carbon
sequestration involves injecting the CO2 into a deep geologic formation for
permanent storage.
1.4.3 Feedstock Flexibility
Several gasifier designs have been developed to accommodate various grades
of coal in addition to wastes and various types of biomass. Gasifier can also
handle pet coke and other refinery products. The potential for using more than
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one feedstock in a single facility reduces project risk and may extend the
project lifespan.
1.4.4 Product Flexibility
Gasification can be coupled with advanced turbine technology to produce
electricity in an IGCC plant. Syngas produced by gasification can also be
further processed into liquid fuels (diesel, gasoline, jet fuel, etc.), hydrogen
and synthetic natural gas, or a range of fertilizers or other high-
value chemicals including anhydrous ammonia, ammonium sulfate, sulfur,
phenol, naphtha and CO2as mentioned above, among many others. Also, slag
produced from coal ash can be used in the production of building materials
such as cement.
1.4.5 High Efficiency
IGCC power plants offer efficiencies similar to or better than other coal power
plants. Additionally, in a carbon dioxide capture and sequestration (CCS)
scenario, an IGCC power plant is much more efficient than a pulverized coal
combustion power plant. This is mainly due to the decreased energy required
to remove CO2 from the process streams in gasification as compared with a
pulverized coal combustion system
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1.5 Gasification Methods
Fig.1.5.1: Different gasification Methods
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Chapter 2. Literature Review
In today`s scenario of depleting conventional fossil fuels, biomass provides an
alternate source of energy. Gasification is a chemical process that converts
carbonaceous materials like biomass into useful convenient gaseous fuels or chemical
feedstock. The product gas of gasification has a calorific value unlike that of complete
combustion process. The present study is going to be focused on parametric analysis
and study of the mathematical model to predict the effect of usage of various types of
fuels in gasification process and also the usage of oxygen as a gasifying agent.
Prof. M. K. Chopraand Shrikant U. Chaudhariin their paper Performance of
Biomass Gasifier Using Woodhave studied the basics of gasification using wood
as feedstock, the sensitive analysis of produced Syngas and the study of the
composition. (International Journal of Advanced Engineering Research and
Studies)
B.V.Babu and Pratik N. Sethhave discussed about the effect of oxygen and steam
enrichment on biomass gasification. Equilibrium model for a downdraft gasifier is
solved using Engineering equation Solver. The effect on calorific value of the
producer gas is studied in detail. (Modeling & simulation of biomass gasifier: effect
of Oxygen enrichment and steam to air ratio, Chemical Engineering Department
Pillani)
Gasification by Dr. Samy Sadaka, et.al have discussed various gasification
process, gasification zones, types of gasification, gasification agents and gasification
applications are stated. The chemical equations governing gasification are used to
analyse the gasification model. Effect of bed temperature, bed pressure, bed height,particle size, moisture content in fuel has been stated in detail. Parametric analysis of
gasification parameter is done.
Guidelines for safe and eco-friendly Biomass Gasification by Intelligent
Energy Europefunded by European commissionhas identified HSE (health safety
and environment) issues regarding gasification. Risk assessment has been done for
safer gasification process and gasifier manufacturing and thus certain guidelines have
been laid down.
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Handbook for Biomass Downdraft Gasifier for Engine System by Solar
Research Institute U.Sis a practical guide to gasifier systems to design, test, operate
and manufacturing of small downdraft gasifiers( upto200kW capacity). Fuel testing,
gas analysis, other methodology current development and future research scopes are
stated in great detail.
Development of a Small Downdraft Biomass Gasifier for Developing Countries
a report by M. A. Chawdhurya and K. Mahkamovb designed developed and tested
a small downdraft biomass gasifier at the DUBLIN UNIVERSITY( UK). They
found out that for composition, moisture content and consumption of biomass
feedstock (3.1 kg/hr for wood chips, 2.9 kg/hr for pellets), temperature inside the
reaction zone (950-1150
o
C), primary air flow rate (0.0015 m3/s), exit temperature ofthe producer gas (180-220 oC) was measured. The main constituents of syngas
included nitrogen (50-56%), carbon monoxide (19-22%), hydrogen (12-19%), carbon
dioxide (10-12%) and a small amount of methane (1-2%). These results were used in
Engineering Equation Solver (EES) software to obtain the lower calorific value of
syngas (4424-5007 kJ/m3) and cold gas efficiency (62.5-69.4%) of the gasifier, which
were found close to the calculated values. Again the thermal efficiency was calculated
as 90.1-92.4%.
Sirigudi Rahul Rao,he has developed process model of gasifier in which air is used
as gasifying agent and bioreactor in Aspen Plus software. Using the developed model
studied the performance of the gasifier by manipulating the process variables and
characterizing the effect on gas quality and composition.
Anil K. Rajvanshi, the Director of Nimbkar Agricultural Research Institute,
Phaltan, Maharashtra, India, in his chapter on Biomass Gasification in the book
Alternative Energy in Agriculture, has deeply studied the effect and use of
gasification in agricultural sector and has discussed the opportunities and challenges
faced by our country in the successful commercialization of the same.
Ola Maurstad, Howard Herzog et. all of The Norwegian University of Science
and Technology (NTNU) and Massachusetts Institute of Technology (MIT), in
their paper titled Impact of coal Quality and Gasifier Technology on IGCC
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performancehave given a brief review on the significant importance of quality of
coal and technology behind construction of gasifier when syngas is used in an IGCC.
Suresh Babu of Gas Technology Institute of USA, in his paper Biomass
Gasification for Hydrogen Production-Process Description and Research Needs
has discussed the possible application of Syngas in the production of hydrogen and
the significance of the process along with the usage of hydrogen in industry.
The paper titled, Thermodynamic Analysis of a Coal-Based Combined Cycle
Power Plant authored by P. K. Nag and D. Raha of Mechanical Engineering
Department, IIT Kharagpur, have given a brief outlook into the thermodynamic
aspect of a power plant run on coal gasification cycle. The paper deals with the
comparison of gasification cycle with Brayton and Rankine cycle.
After detailed study of the above, it was found that there is a lack of use of biomass
gasifiers on a large scale in the country. Our country, whose villages house an
abundant supply of biomass, will surely benefit from an installation of a biomass
gasifier in the rural area which would use the village biomass as feedstock. The
biomass gasifier can be combined into an integrated gasification combined cycle
(IGCC) to produce electricity for benefiting villages with electricity shortage. The
syngas produced after proper refinement could be used to run IC engines for vehicles.
The raw syngas can replace conventional chulha and LPG as cooking gas. Thus for
successful use of syngas in above situation, it is necessary to study the composition of
the syngas as well as the impact of different input factors on the heating value of
syngas and its composition. Hence a parametric or sensitivity analysis is necessary
along with successful trial and production of syngas.
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Chapter 3. Types of gasifiers
A variety of biomass gasifier types have been developed. They can be grouped
into four major classifications: fixed-bed updraft, fixed-bed downdraft, bubbling
fluidized-bed and circulating fluidized bed. Differentiation is based on the means of
supporting the biomass in the reactor vessel, the direction of flow of both the biomass
and oxidant, and the way heat is supplied to the reactor. Table 3 lists the most
commonly used configurations. These types are reviewed separately below:
Table 3.1: Comparison of different Gasifier types
Gasifier Type Flow Direction Support Heat Source
Fuel Oxidant
Updraft FixedBed Down Up Grate Combustion ofChar
DowndraftFixed Bed
Down Down Grate Partialcombustion ofvolatiles
BubblingFluidized Bed
Up Up None Partialcombustion ofvolatiles andchar
CirculatingFluidized Bed
Up Up None Partialcombustion of
volatiles andchar
3.1 Updraft Fixed Bed Gasification:-
Also known as counterflow gasification, the updraft configuration is the oldest
and simplest form of gasifier; it is still used for coal gasification. Biomass is
introduced at the top of the reactor, and a grate at the bottom of the reactor supports
the reacting bed. Air or oxygen and/or steam are introduced below the grate and
diffuse up through the bed of biomass and char. Complete combustion of char takesplace at the bottom of the bed, liberating CO2 and H2O. These hot gases (~1000
oC)
pass through the bed above, where they are reduced to H2 and CO and cooled to
750oC. Continuing up the reactor, the reducing gases (H2 and CO) pyrolyse the
descending dry biomass and finally dry the incoming wet biomass, leaving the reactor
at a low temperature (~500oC). Examples are the PUROX and the Sofresid/Caliqua
technologies.
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The advantages of updraft gasification are:
Simple, low cost process
Able to handle biomass with a high moisture and high inorganic content (e.g.
municipal solid waste)
Proven technology
The primary disadvantage of updraft gasification is that Syngas contains 10-
20% tar by weight, requiring extensive syngas cleanup before engine, turbine or
synthesis applications.
Fig 3.1: Updraft Biomass Gasifier
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3.2 Downdraft Fixed Bed Gasification
Also known as cocurrent-flow gasification, the downdraft gasifier has the
same mechanical configuration as the updraft gasifier except that the oxidant and
product gases flow down the reactor, in the same direction as the biomass. A major
difference is that this process can combust up to 99.9% of the tars formed. Low
moisture biomass (
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Fig 3.2: Downdraft Biomass gasifier
3.3 Bubbling Fluidized Bed
Most biomass gasifiers under development employ one of two types of
fluidized bed configurations: bubbling fluidized bed and circulating fluidized bed. A
bubbling fluidized bed consists of fine, inert particles of sand or alumina, which have
been selected for size, density, and thermal characteristics. As gas (oxygen, air orsteam) is forced through the inert particles, a point is reached when the frictional force
between the particles and the gas counterbalances the weight of the solids. At this gas
velocity (minimum fluidization), bubbling and channeling of gas through the media
occurs, such that the particles remain in the reactor and appear to be in a boiling
state. The fluidized particles tend to break up the biomass fed to the bed and ensure
good heat transfer throughout the reactor.
The advantages of bubbling fluidized-bed gasification are:
Yields a uniform product gas
Exhibits a nearly uniform temperature distribution throughout the reactor
Able to accept a wide range of fuel particle sizes, including fines
Provides high rates of heat transfer between inert material, fuel and gas
High conversion possible with low tar and unconverted carbon
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The disadvantages of bubbling fluidized-bed gasification is that large bubble
size may result in gas bypass through the bed
Fig 3.3: Bubbling Fluidized Bed Gasifier
3.4 Circulating Fluidized Bed
Circulating fluidized bed gasifiers operate at gas velocities higher than the
minimum fluidization point, resulting in entrainment of the particles in the gas stream.
The entrained particles in the gas exit the top of the reactor, are separated in a cyclone
and returned to the reactor.
The advantages of circulating fluidized-bed gasification are:
Suitable for rapid reactions
High heat transport rates possible due to high heat capacity of bed material
High conversion rates possible with low tar and unconverted carbon
The disadvantages of circulating fluidized-bed gasification are:
Temperature gradients occur in direction of solid flow
Size of fuel particles determine minimum transport velocity; high velocities
may result in equipment erosion
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Heat exchange less efficient than bubbling fluidized-bed. Most of the gasifier
technologies described in this report employ a bubbling fluidized-bed or circulating
fluidized-bed system.
Fig 3.4: Circular Fluidized Bed gasifier
3.5 Updraft Gasification
A gasification reactor provides a method to provide gas-solid reactions in
which a gas stream passes through a bed of particles. If the particles remain fixed in
their positions, the equipment is called a fixed-bed reactor. In fact, the particles are
usually allowed to move without detaching from each other and therefore the process
is better classified as moving bed. The particles will not detach from each other if the
gasification agent velocity is less than the fluidization velocity.
Fixed bed gasification can be of updraft, downdraft or cross draft type. Since
there is an interaction of air or oxygen and biomass in the gasifier, they are classified
according to the way air or oxygen is introduced to the system. Here, only updraft
gasification is discussed because this is the basis of the design of the reactor in the
project. Figure 2.1 shows a schematic view of a possible gasifier configuration using
this technique. The particles of biomass, for instance wood chips are fed at the top of
the reactor and slowly move to the bottom where the residual ash is withdrawn. The
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combustion and gasification agents normally air is injected through the distributor at
the bottom.
In their downward movement, the biomass particles undergo the following
main processes: drying, devolatilization, gasification, and combustion. During the
conversion in a gasifier, there is no sharp delimitation between these regions. For
instance, a descending particle may be going through devolatilization in its outer
layers while inner layers are drying. A simplified sequence of events occurring in the
updraft gasifier is described as follows starting from the top of the fuel bed:
Fig 3.5: Updraft Biomass Fixed Bed Gasifier
3.5.1 Drying
During this event, the temperature of the wood chips is increased and
the moisture in the wood is evaporated by heat exchange between the wood
and the hot gas stream that is coming from the combustion zone.
3.5.2 Devolatization
The temperature of the dry wood chips is increased further and the
volatile products are released from the wood chips thereby leaving char. For
all biomass, volatiles represent a significant portion of the fuel and in
gasifiers; devolatilization provides part of the produced gases. The release of
volatiles is driven by increase of temperature. As the wood chips slowly
descend, the hot gases produced in the gasification and combustion zones
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exchange energy with the colder solid. Three main fractions are produced
during pyrolysis of biomass - light gases (H2, CO, CO2, H2O, and CH4), tar
(composed of relatively heavy organic and inorganic molecules that escape the
solid matrix as gases and liquid in the form of vapour) and char, the remaining
solid residue.
3.5.3 Gasification (Reduction)
After drying and devolatilization, the char enters the gasification zone
where carbon reacts with steam, carbon dioxide, and hydrogen. Endothermic
reactions in this section produce carbon monoxide and hydrogen. The slightly
exothermic reaction of hydrogen with carbon produces methane. The carbon
monoxide produced also reacts with water to produce hydrogen and carbon
dioxide in the water gas shift reaction. Differentiation between the gasification
zone and combustion zone is based on the presence or absence of oxygen. The
reactions that take place in this region of the gasifier can be represented
3.5.4 Combustion
The remaining char is burned, using oxygen from air in the feed gas
and leaving an ash residue. From the point of view of energy generation and
consumption, if taken as irreversible, the combination of exothermic reactions
involves an energy input of 394 MJ/kmol of carbon (calculated at 298 K) and
is mainly responsible for the energy requirements of the process. This energy
is used to promote and sustain the gasification and pyrolysis reactions, which
are mostly endothermic. In typical updraft gasifiers the following processes
take place at temperatures indicated in table:
Table 3.5.4.1: Temperatures range of various zones
Process Temperatures
1. Drying >423 K
2. Pyrolysis 423-973 K
3. Reduction 1073-1473 K
4. Combustion 973-1773K
The gas exiting from the top of the reactor consists of products of drying,
devolatilization and gasification processes. It contains a significant amount of tar and
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moisture and is at low temperatures between 473 K and 623 K because of the high
heat exchange between the solid and gas phases. Updraft gasifiers are useful for
producing gases to be burned at temperatures of above 473 K. At higher temperatures,
the tars do not condense and can easily be burnt in combustors (e.g. burners for
cooking). The high tar level makes them difficult to cleanfor other applications where
clean gas is required for example in internal combustion engines.
The major advantages of this type of gasifier are its simplicity in design, high
degree of controllability, high charcoal burn-out and internal heat exchange leading to
low gas exit temperatures and high gasification efficiencies because of the high heat
exchange. Also, because of the internal heat exchange the fuel is dried in the top of
the gasifier and therefore fuels with high moisture content (up to 50 % wb) can beused. Furthermore this type of gasifier can even process relatively small sized fuel
particles and accepts some size variation in the fuel feedstock.
Major drawbacks are the high amounts of tar and pyrolysis products, because
the pyrolysis gas does not pass through the combustion zone of the reactor. This is of
minor importance if the gas is used for direct heat applications, in which the tars are
simply burnt when above condensation temperature.
Table 3.5.4.2: Comparison between different types of gasifier
Sr.
No.
Gasifier
Type
Advantage Disadvantages
1. Updraft Small pressure drop Good thermal efficiency Little tendency towards
slag formation
Great sensitivity to tarand moisture andmoisture content of fuel
Relatively long timerequired for start up of ICengine
Poor reaction capabilitywith heavy gas load
2. Downdraft Flexible adaptation of gasproduction to load
Low sensitivity to charcoaldust and tar content of fuel
Design tends to be tall Not feasible for very small
particle size of fuel
3. Crossdraft Short design height Very fast response time to
load
Flexible gas production
Very high sensitivity toslag formation
High pressure drop
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3.6 Factors affecting Gasification
Studies have shown that there are several factors influencing the
gasification of wood. These include the following:
3.6.1
Energy content of Fuel
Fuel with high energy content provides easier combustion to sustain
the endothermic gasification reactions because they can burn at higher
temperatures. Beech wood chips have an energy content of approximately 20
MJ/kg. This is typical for most biomass sources and has been proved to be
easy to gasify.
3.6.2 Fuel Moisture content
Since moisture is in effect water, a non-burnable component in the
biomass, it is important that the water content be kept to a minimum. All water
in the feed stock must be vaporized in the drying phase before combustion
otherwise there will be difficulty in sustaining combustion because the heat
released will be used to evaporate moisture. Wood with low moisture content
can therefore be more readily gasified than that with high moisture. Wood
with high moisture content should be dried first before it can be used as fuel
for the gasifier. The beech wood chips used in the experiments have been
factory dried to a moisture content of 10% prior to packaging. This makes it
suitable as a fuel for the gasifier .Updraft gasifiers are also capable of
operating with fuels that have moisture contents of up to 50%.
3.6.3 Size Distribution of the Fuel
Fuel should be of a form that will not lead to bridging within the
reactor. Bridging occurs when unscreened fuels do not flow freely axially
downwards in the gasifier. Therefore particle size is an important parameter in
biomass gasification because it determines the bed porosity and thus the fluid-
dynamic characteristics of the bed. On the other hand, fine grained fuels lead
to substantial pressure drops in fixed bed reactors. The experimental
wood
chips are approximately 10 x 10 x 2 mm and regular in shape. This size is not
fine grained when compared to the micron scale and thus no substantial
pressure drops occur in the reactor.
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3.6.4 Temperature of the Reactor
There is a need to properly insulate the reactor so that heat losses are
reduced. If heat losses are higher than the heat requirement of the endothermic
reactions, the gasification reactions will not occur. The reactor in the
laboratory has been insulated with 50 mm of alkaline earth silicate to keep
heat losses minimal.
Chapter 4. Mathematical modelling and Thermodynamic
Equilibrium Model
In this project, the non-stoichiometric equilibrium method is used as the base
for the mathematical modelling of the gasifier. The method is particularly suitable for
fuels like coal, biomass, the exact chemical formula of which is not clearly known. In
non-stoichiometric modeling, no knowledge of a particular reaction mechanism is
required to solve the problem. In a reacting system, a stable equilibrium condition is
reached when the Gibbs free energy of the system is at the minimum. So, this method
is based on minimizing the total Gibbs free energy. The only input needed is the
elemental composition of the feed, which is known from its ultimate analysis.
General gasification reaction can be represented as:-
CHaObNc+ dH2O + e (O2+ 3.76 N2) a1H2+a2CO+a3CO2+a4CH4+ a5N2+ a6H2O
The combustion reactions:
1.C + O2 CO (-111 MJ/Kmol)*
2.CO + O2 CO (-283 MJ/Kmol)*
3.H2+ O2 H2O (-242 MJ/Kmol)*
Other important gasification reactions include:
4.C + H2O CO + H2 (+141 MJ/Kmol)* the Water-Gas Reaction
5.C + CO2 2CO (+172 MJ/Kmol)* the Boudouard Reaction
6.C + 2H2 CH4 (-75 MJ/Kmol)* the Methanation Reaction
Combustion reactions will result in completion under normal gasification
operating conditions. Under the condition of high carbon conversion, the three
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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 syngas
composition.
7.CO + H2O CO2+ H2 (-41 MJ/Kmol)* Water-Gas-Shift Reaction
8. CH4 + H2O CO2+ 3H2 (+206 MJ/Kmol)* Steam-Methane-Reforming
Reaction
(*Enthalpy of reaction. Under the sub-stoichiometric reducing conditions of
gasification, most of the fuels Sulphur is converted to hydrogen sulphide (H2S) and,
to a lesser degree, carbonyl sulphide (COS). Nitrogen in the feed is converted to
nitrogen (N2), with some ammonia (NH3) and a small amount of hydrogen cyanide
(HCN). Chlorine in the feed is primarily converted to hydrogen chloride (HCl). In
general, the quantities of sulphur, nitrogen, and chloride in the fuel are sufficiently
small that they have a negligible effect on the main syngas components of H 2and CO.
Relative to the thermodynamic understanding of the gasification process; its kinetic
behaviour 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 are known to have
catalytic activity on some of the gasification reactions. The kinetics of gasification is
as yet not as developed as is its thermodynamics. Homogeneous reactions occurring
in the gas phase can often be described by a simple equation, but heterogeneous
reactions are intrinsically more complicated.
Let A denote the air supply in kg dry air/kg dry fuel, F the amount of dry fuel
required to obtain one normal cubic meter of the gas (1 Nm3 of gas represents a
volume of 22.4 litre at NTP), and XCthe carbon content of the fuel (kg carbon/kg dry
fuel). Carbon is split between CO, CO2, and CH4. For 1 normal cubic meter of gas
produced, one can write the carbon molar balance between inflow and outflow
streams.
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4.1 Sample calculation of reaction balance from generalized equation
Assuming hydrogen balance;
Since during the reaction total moles of hydrogen will be same, the
summation of the number of moles of hydrogen on the right and left hand side
of the general equation will be same. Therefore moles of H2on left hand of the
equal to sign of the general equation will be the sum of hydrogen moles in
feedstock and those present in moisture. So the sum total is:
da 2/
Similarly the moles of H2 on right hand of the equal to sign of the
general equation will be the sum of hydrogen moles in constituents of product
gas i.e. in H2, CH4and H2O. So the sum total is:
641 *2 aaa
Equating both sides we get;
641 *22/ aaada
But while programming convenience we have considered a not as
the number of moles of atom of H2in feedstock but as the actual weight of H2
in feedstock (kg/kg of fuel). So to get moles of H2we will have to divide by
the molecular weight of each molecule. Molecular weight of hydrogen
molecule is two and that of water is eighteen. Thus the equation becomes
641 *2182
2/aaa
da
We now consider that we use 30 kg of feedstock to produce G Nm 3
of
syngas at NTP (1 bar pressure and 25
o
C temperature). As we know, 1Nm
3
of gas at NTP occupies 22.4 litres of volume the volume fraction for the each
constituent of product gas can be obtained by dividing it by 22.4. Finally the
above equation takes the form,
G
aaada
*4.22
*2
182
2/ 641
All the following reactions were obtained by elemental balancing by
following the above methodology.
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4.2 Elemental balancing
Carbon balance:
G
)VV(VX CHCOCOC
*4.2212
*3042
or
42**56 CHCOCOC VVVXG - - - (1)
Hydrogen balance:
Let S represent the total steam supplied as humidity associated with air
and added steam (kg steam/kg of dry fuel), and W represent the moisture
content of fuel (kg water/kg dry fuel).
We write the molar balance of H2as follows:
G
VVVWXS CHOHHH
*4.22
)*2(
1821830 422
or
422*2)**336()(**33.37 CHOHHH VVVXGWSG ...(2)
Oxygen balance:-
If Oa represents the mass fraction of oxygen in air and XO is the
oxygen content of the fuel (kg oxygen/kg dry fuel), hence the molar balance of
O2as follows:
G
VVVOaAWXS OHCOCOO
*4.22
)*5.0*5.0(
32
*
18321830 22
or,
OHCOCO
O
VVVOaAXGWSG
22*5.0*5.0
)*(**21)(**33.37
..(3)
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Nitrogen balance:
If XN is the nitrogen content of the fuel (kg nitrogen/kg dry fuel) and
Na is the mass fraction of nitrogen in air, the molar balance of N2gives:
G
VNaAX NN
*4.2228
*
2830 2
or
2)*(**24 NN VNaAXF (4)
The volume fractions of all constituents of the product gas add up to
1.0. We, therefore, also have:
122422
NOHCHHCOCO VVVVVV (5)
To estimate the values of the seven unknowns: VCO, VCO2, VH2, VH2O,
VN2, VCH4, and F total of seven equations are required. For this purpose,
besides the above five equations (Equation 1 to Equation 5), any two of the
equations can be assumed from the water-gas reaction, Boudouard reaction,
shift conversion and methanation to be in equilibrium. Working with Water-
gas reaction and Boudouard reaction was chosen. For the Boudouard reaction,
the equilibrium constant is:
2
2
COCOpb
PPK
where PCO is the partial pressure of CO, which is equal to volume
fraction of CO, (VCO* the pressure of the reactor, P)
22
*)(
*
)*( 22
CO
CO
CO
COpb
V
PV
PV
PVK (6)
Similarly, for the watergas reaction:
OHCOH
OH
COHpw
V
PVV
P
PPK
22
2
2 ***
(7)
Solving equations (1) to (7) equilibrium concentrations of gases are
found.
In this case seven equations can be solved simultaneously. MATLAB
is used for solving these simultaneous equations.
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LCV of the syngas can be calculated in two ways depending upon the
usage of syngas.
The first method deals with the usage where methane is separated from
syngas as it may react to give methanol. So while calculating the LCV of the
syngas it is assumed that the tar is completely removed, the gas is scrubbed
and the only constituents in the gas that contribute to its heating value are
hydrogen and carbon monoxide.
LCVCO= (282.99 kJ/Nm3) "LCV of CO"
LCVH2= (241.83 kJ/Nm3) "LCV of H2"
LCVCH4= (802.34 kJ/Nm3); "LCV of CH4"
So the LCV of the gas is calculated by the formulae;
LCVsyngas = (LCVCO*VCO) + (LCVH2*VH2)
The other method does take into account the heating value of methane
which is appreciable. So the heating value of syngas is given by;
LCVsyngas = (LCVCO*VCO) + (LCVH2*VH2) + (LCVCH4*VCH4)
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Chapter 5. Analysis of Fuels
5.1 Proximate Analysis
The proximate analysis of coal was developed as a simple means of
determining the distribution of products obtained when the coal sample isheated under specified conditions. As defined by ASTM D 121, proximate
analysis separates the products into four groups: (1) moisture, (2) volatile
matter, consisting of gases and vapors driven off during pyrolysis, (3) fixed
carbon, the nonvolatile fraction of coal, and (4) ash, the inorganic residue
remaining after combustion. Proximate analysis is the most often used analysis
for characterizing coals in connection with their utilization. The actual method
of analysis is described below:
5.1.1 Moisture
Known weight of coal heated in silica crucible at 105-110 C for 1 hour.
%M = (Loss in wt./Original Wt.)*100
5.1.2 Volatile Matter
Dry coal is heated at 950 C for 7 minutes in furnace
%V = (Loss in wt./Original wt.)*100
5.1.3 Ash
Dry coal heated in platinum crucible at 400-700 C then ignite for
hour at 700 C, weigh the burnt material and repeat process until weight
of burnt material remains constant
5.1.4 Fixed Carbon
%FC = 100 (%M + %V + %Ash)
5.1.5 Goutels Formula
GCV = 4.187 * (82 * %FC + a * %V * %M) kJ/kg
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5.2 Ultimate Analysis
The ultimate analysis indicates the various elemental chemical
constituents such as Carbon, Hydrogen, Oxygen, Sulphur, etc. It is useful in
determining the quantity of air required for combustion and the volume and
composition of the combustion gases. This information is required for the
calculation of flame temperature and the flue duct design etc.
5.2.1 Carbon and Hydrogen
22 COOC
OHOH 222 22
Absorbers used are:
Anhydrous magnesium perchlorate or calcium chloride for H2O
Soda lime & potassium hydroxide for CO2
%H = (2/18) * (H2O wt./Coal wt.)
%C = (12/44) * (CO2wt./Coal wt.)
5.2.2 Nitrogen (Kjehldahls Method)
Coal + conc.H2SO4 with Sodium
acidicNHalkaliheatSONHHN 3424 )(62
%N = (vol. of acid consumed * normality of NaOH * 1.4)/wt. of coal
5.2.3 Sulphur
Burn known weight of coal completely
10 ml distilled water in bomb pot
Collect washing of bomb pot
Add BaCl2
44223222 2 BaSOBaClSOHOHSOOSOOS
BaSO4 is precipitate. Weigh it.
%S = (Wt. of BaSO4* 32 * 100)/(Wt. of coal * 233)
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5.2.4 GCV Equation
GCV = [(33.8 * C) + 144 * (H (O/8))] + (9.375 * S)
Fig 5.2.1: Copy of Report of Proximate Analysis of Coal
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Table 5.2: Ultimate Analysis of Coal
Fig 5.2.2: Analysis of Rice Husk
Fig 5.2.3: Analysis of Wood Pellets
Element (% by wt)
1. Carbon 60.121
2. Oxygen 10.020
3. Hydrogen 1.358
4. Nitrogen 0
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Chapter 6. Manufacturing of a Gasifier
6.1 Materials of Construction
Gasifiers are usually constructed from commercially available
materials such as steel pipe, sheet, and plate. Although the metal temperatures
encountered in well-designed air gasifiers do not usually exceed the softening
point of mild steel, certain stainless steels or inconel may give the extra
temperature resistance necessary for critical areas such as the grate, hearth, or
nozzles.
Some of the mild-steel components may suffer chemical corrosion in
certain parts. Corrosion is likely to occur in areas where water condenses or
collects since gasifier water often contains organic acids. In these instances,
the steel should be replaced by corrosion-resistant materials such as copper,
brass, epoxy lined steel, or stainless steel as required.
6.2 Methods of Construction
A gasifier is built much like a water heater, and the same methods of
construction are used. The workshop should be equipped with tools for
performing tasks such as shearing sheet metal, rolling cylinders and cones,
drilling, riveting, grinding, painting, sawing, tube cutting, and pipe threading.
An oxyacetylene torch is valuable for cutting and welding tasks, but an
arc welder is preferred for mild-steel welding.All seals must be made gas-tight; threaded and welded fittings are preferred at all points, and exhaust-
pipe-type gaskets can be used if necessary. High-temperature, anti-sieze pipe
dope should be used on all pipe joints. High-temperature applications will
require ceramic fiber or asbestos gaskets. Silicone sealant is appropriate attemperatures below 300C and rubber or Viton "O" rings and gaskets will
perform excellently at room temperature. The system should be leak-tested
before the initial startup, as well as after modifications.
6.3 Sizing and Laying out of Pipes
When designing a gasifier, it is important to keep the pressure drop in
the system as small as possible. Because there are unavoidable pressure drops
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associated with the gasifier, the cyclone separator, and the cleanup system, it
is very important to use adequately sized pipe. On the other hand, gas
velocities within the pipes should be adequate so that entrained solids will be
conveyed to their proper point of removal, rather than deposited inside the
pipe.
When laying out pipe connections for a gasifier system, it is important
to allow access to various parts that may require cleaning or adjustment.
6.4 Instruments and Control
The gasifiers of the past were crude, inconvenient devices. Today's
gasifiers are evolving toward safer, automated processes that make use of a
wide range of present-day instruments and controls6.5 Temperature
Thermocouples (such as chromel-alumel type K) should be used to
measure various gasifier temperatures, especially below the grate, as a check
for normal or abnormal operation. Temperatures at the grate should not exceed
800C; higher temperatures indicate abnormal function. Consequently, the
signal from the thermocouple can be used by a control system.
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6.6 Design of Gasifier
Desired output = 20-22 kW
As per the literature review,
1KW == 3 lb biomass
20KW == 30 kg coal
Let,
L = total height of gasifier
di= internal diameter;
pi= design pressure;
t = thickness of cylinder
Density of coal = 550 kg /cubic meter
Density = Mass/Volume
Now,
Volume = 2(/4)(di)2(L);
Assume,
Di= 200 mm
Substituting values we get,
L = 970 mm
Syt= 200 N/mm2 (Cast Iron, Design Data Book)
f.o.s = 1.5
ti= 10 mm
di= 200 mm
= 0.3
Substituting these values in the equations given below:
,FOS
Sytall , 1
)1(
)21(
2
iall
ialli
p
pdt
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barpi 3.12
Operating pressure
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DIMENSION: Height=450mm, Diameter =340mm.
PROCESS: Sheet metal is rolled and longitudinally welded to form a
cylinder.
6.7.7
ASH BED
MATERIAL: Cast Iron.
DIMENSION: Height=300mm, top length=600mm,
bottom length= 440mm.
PROCESS: Cast iron sheets are welded to form a shell
of the frustum.
Fig. 6.7.1: Updraft Gasifier
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Fig 6.7.2: Solid model for experimental setup (All dimensions are in mm)
6.8 Dimensions
6.8.1 BLOWER
MATERIAL: Cast Iron.
SPECIFICATION: 0.5hp, 12 cfm, 1500rpm.
6.8.2 STOPPER
MATERIAL: Cast Iron.
SPECIFICATION: 0.5 inch (3 no), 0.75 inch (2 no).
6.8.3 NOZZLE
MATERIAL: Brass.
SPECIFICATION: 0.5 inch long, 5 mm outlet diameter.
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6.8.4 GASKET
MATERIAL: RUBBER.
SPECIFICATION: 5mm thick, can sustain temperature up to 200oC
6.8.5 BURNER
MATERIAL: Stainless steel.
SPECIFICATION: 1mm outlet opening.
6.8.6 DISTRIBUTOR
MATERIAL: Cast iron.
SPECIFICATION: Inlet -1 inch, Outlet - 0.5 inch.
6.8.7 NUTS AND BOLTS
MATERIAL: Cast iron.
SPECIFICATION: M10 (12 no).
6.8.8 AIR TUBES
MATERIAL: Cast iron.
SPECIFICATION: Diameter 1 inch, 2 nos.
6.8.9 STEAM PIPES
MATERIAL: Cast iron.
SPECIFICATION: 35 psi, Diameter 0.5 inch, can sustain
temperature upto150
o
C.
6.8.10 PIPES
MATERIAL: Pipes.
SPECIFICATION: Diameter 0.5 inch.
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Photo 1: Actual Experimental Model
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6.9 Procedure to operate the gasifier:-
6.9.1 Clean the gasifier and place it at a clean place.
6.9.2 Fill the gasifier water jacket till the 3/4thmark of the glass tube.
6.9.3
Porous grainy ash must be filled in the gasifier. The ash must
rise to about 3-4in from the bottom and just below the steam
and air ports.
6.9.4 Now in a separate panel burn some quantity of coal. The coal
must be heated till it becomes red hot. Normally 2-3 kg of coal
must be used for this purpose.
6.9.5 Pour this coal into the gasifier keeping the air vents open. Start
the blower.6.9.6 Now fill the ash collector with water 2 inches below the steam
and air ports.
6.9.7 Connect the steam and gas temperature sensors to the
indicators.
6.9.8 After about 10 min pour about 5-7 kg of feedstock in the
gasifier. Close the air vents and also close the top portion of the
gasifier.
6.9.9 Now after 30 min pour another 5-7 kg of coal into the gasifier
into the gasifier through the hopper.
6.9.10 10 min later add another 10-12 kg of coal such that it fills the
gasifier completely below the gas outlet port. Coal at this stage
must be added in batches of 2-3 kg.
6.9.11Now wait for about 3-4 hrs for the gasifier to be stable and gas
to be produced. Hold a matchstick in front of the gas outlet
port. If it burns it signals that gas is produced.
6.9.12 In between pocking with an iron rod must be done through the
pocking hole provided. This avoids even distribution of coal
and avoids blockage to gas flow. Gas flow if blocked causes an
increase in back pressure and this may push out with great
force the feedstock out of the gasifier from the ash collector.
Such a case occurred during one of the trial. The pocking rod
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can also be used to check the level of red hot coal in gasifier.
This is done by inserting the rod in the gasifier through the
pocking hole and keeping it in the gasifier for about 5-10 min.
Remove the rod and hold a thin paper edge against it at the
bottom. The paper begins to burn, now move the paper slowly
in vertical direction. At some distance the paper will stop
burning, mark the position. The distance of the mark on the rod
from its bottom is the height of the red hot coal zone.
6.9.13 Experienced operators can reduce down the stabilisation time
drastically. Also they can detect the production of gas from the
smell of CO.
6.9.14 Inexperienced operator may need more time and more than a
couple of trials to get the desired output.
6.10 Precautions while operating the gasifier:
6.10.1 The gasifier must not be compactly packed with coal as it leads
to build up of back pressure.
6.10.2 In case of power failure or shut down of gasifier the air
distribution valve must be closed and not the blower directly. Ifthis care is not taken the syngas may flow back into the blower
and catch fire damaging the blower. Such case was found to
occur during one of the trial on the experimental setup.
6.10.3 Asbestos hand gloves must be worn while operating hopper
when the gasifier is working.
6.10.4Not much of the gas must be inhaled to detect syngas from the
odour of CO since higher concentration of CO is harmful and
causes nausea.
6.10.5 The water level in the water jacket must not be allowed to fall
below the 1/4 thmark and gasifier must not be operated without
water in the water jacket.
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Photo 2: Actual Syngas flame after production
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Chapter 7. Syngas Testing Methods and Applications
To have proper control on combustion process, an idea about complete or
incomplete combustion of fuel is made by the analysis of flue gas. Thus,
(i)
if the gases contain considerable amount of carbon monoxide, it
indicates that incomplete combustion is occurring (i.e. considerable
wastage of fuel is taking flue).
(ii)
if the flue gases contain a considerable amount of oxygen, it indicates
the oxygen supply is in excess, though the combustion may be
complete.
The analysis of Syngas is primarily done by two methods Gas
chromatography and Orsat Gas Analyzer
7.1 Gas Chromatography
Gas chromatography - specifically gas-liquid chromatography -
involves a sample being vapourised and injected onto the head of the
chromatographic column. The sample is transported through the column by
the flow of inert, gaseous mobile phase. The column itself contains a liquid
stationary phase which is adsorbed onto the surface of an inert solid.
Fig 7.1.1: Schematic diagram of a gas chromatograph
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Instrumental Components are describes as follows:
7.1.1 Carrier Gas
The carrier gas must be chemically inert. Commonly used gases
include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas
is often dependant upon the type of detector which is used. The carrier gas
system also contains a molecular sieve to remove water and other impurities.
7.1.2 Sample Injection Port
For optimum column efficiency, the sample should not be too large,
and should be introduced onto the column as a "plug" of vapour - slow
injection of large samples causes band broadening and loss of resolution. The
most common injection method is where a microsyringe is used to inject
sample through a rubber septum into a flash vapouriser port at the head of the
column. The temperature of the sample port is usually about 50C higher than
the boiling point of the least volatile component of the sample. For packed
columns, sample size ranges from tenths of a microliter up to 20 microliters.
Capillary columns, on the other hand, need much less sample, typically around
10-3L. For capillary GC, split/splitless injection is used.
Fig 7.1.2:Spit/Spitless Injection System
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The injector can be used in one of two modes; split or splitless. The
injector contains a heated chamber containing a glass liner into which the
sample is injected through the septum. The carrier gas enters the chamber and
can leave by three routes (when the injector is in split mode). The sample
vapourises to form a mixture of carrier gas, vapourised solvent and vapourised
solutes. A proportion of this mixture passes onto the column, but most exits
through the split outlet. The septum purge outlet prevents septum bleed
components from entering the column.
7.1.3 Columns
There are two general types of column packedand capillary(also
known as open tubular). Packed columns contain a finely divided, inert, solid
support material (commonly based on diatomaceous earth) coated with liquid
stationary phase. Most packed columns are 1.5 - 10m in length and have an
internal diameter of 2 - 4mm.
Capillary columns have an internal diameter of a few tenths of a
millimeter. They can be one of two types; wall-coated open tubular(WCOT)
orsupport-coated open tubular(SCOT). Wall-coated columns consist of a
capillary tube whose walls are coated with liquid stationary phase. In support-
coated columns, the inner wall of the capillary is lined with a thin layer of
support material such as diatomaceous earth, onto which the stationary phase
has been adsorbed. SCOT columns are generally less efficient than WCOT
columns. Both types of capillary column are more efficient than packed
columns.
In 1979, a new type of WCOT column was devised - theFused Silica
Open Tubular(FSOT) column. These have much thinner walls than the glass
capillary columns, and are given strength by the polyimide coating. These
columns are flexible and can be wound into coils. They have the advantages of
physical strength, flexibility and low reactivity.
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Fig 7.1.3: Cross section of Fused Silica Open Tube Column
7.1.4 Column Temperature
For precise work, column temperature must be controlled to within
tenths of a degree. The optimum column temperature is dependant upon the
boiling point of the sample. As a rule of thumb, a temperature slightly above
the average boiling point of the sample results in an elution time of 2 - 30
minutes. Minimal temperatures give good resolution, but increase elution
times. If a sample has a wide boiling range, then temperature programming
can be useful. The column temperature is increased (either continuously or in
steps) as separation proceeds.
7.1.5 Detectors
There are many detectors which can be used in gas chromatography.Different detectors will give different types of selectivity. A non-
selectivedetector responds to all compounds except the carrier gas, aselective
detectorresponds to a range of compounds with a common physical or
chemical property and aspecific detectorresponds to a single chemical
compound. Detectors can also be grouped into concentration dependant
detectorsand mass flow dependant detectors. The signal from a concentration
dependant detector is related to the concentration of solute in the detector, and
does not usually destroy the sample Dilution of with make-up gas will lower
the detectors response. Mass flow dependant detectors usually destroy the
sample, and the signal is related to the rate at which solute molecules enter the
detector. The response of a mass flow dependant detector is unaffected by
make-up gas.
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Table 7.1.5: Comparison between different types of Detector
Detector Type
Support
gases Selectivity Detectability
Dynamic
range
Flameionization
(FID)Mass flow
Hydrogenand air
Most organic cpds. 100 pg 107
Thermalconductivity
(TCD)Concentration Reference Universal 1 ng 107
Electron
capture(ECD)
Concentration Make-up
Halides, nitrates,nitriles, peroxides,
anhydrides,organometallics
50 fg 105
Nitrogen-phosphorus
Mass flowHydrogen
and airNitrogen,
phosphorus10 pg 106
Flamephotometric
(FPD)Mass flow
Hydrogenand air
possiblyoxygen
Sulphur,phosphorus, tin,boron, arsenic,
germanium,selenium,
chromium
100 pg 103
Photo-ionization
(PID)Concentration Make-up
Aliphatics,aromatics, ketones,esters, aldehydes,
amines,heterocyclics,
organosulphurs,some
organometallics
2 pg 107
Hallelectrolyticconductivity
Mass flowHydrogen,
oxygen
Halide, nitrogen,nitrosamine,
sulphur
The effluent from the column is mixed with hydrogen and air, and
ignited. Organic compounds burning in the flame produce ions and electrons
which can conduct electricity through the flame. A large electrical potential is
applied at the burner tip, and a collector electrode is located above the flame.
The current resulting from the pyrolysis of any organic compounds is
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measured. FIDs are mass sensitive rather than concentration sensitive; this
gives the advantage that changes in mobile phase flow rate do not affect the
detector's response. The FID is a useful general detector for the analysis of
organic compounds; it has high sensitivity, a large linear response range, and
low noise. It is also robust and easy to use, but unfortunately, it destroys the
sample.
Fig 7.1.5: Flame Ionisation Detector
7.2 Orsat Gas Analyzer
An Orsat gas analyser is a piece of laboratory equipment used to
analyse a gas sample (typically fossil fuel flue gas) for its oxygen, carbon
monoxide and carbon dioxide content. Although largely replaced by
instrumental techniques, the Orsat remains a reliable method of measurement
and is relatively simple to use. It was patented before 1873 by Mr. H Orsat.
7.2.1 Construction
Consists of a water-jacketed measuring burette, connected in series
to a set of three absorption bulbs, each through a stop-cock.
The other end is provided with a three-way stop-cock, the free end of
which is further connected to a U-tube packed with glass wool (for avoiding
the incoming of any smoke particles, etc.)
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The graduated burette is surrounded by a water-jacket to keep the
temperature of the gas constant during the experiment.
The lower end of the burette is connected to a water reservoir by
means of a long rubber tubing.
The absorption bulbs are usually filled with glass tubes, so that the
surface area of contact between the gas and the solution is increased.
The absorption bulbs have solutions for the absorption of CO2, O2
and CO respectively.
First bulb has potassium hydroxide solution (250g KOH in 500mL
of boiled distilled water), and it absorbs only CO2.
Second bulb has a solution of alkaline pyrogallic acid (25g
pyrogallic acid+200g KOH in 500 mL of distilled water) and it can absorb
CO2 and O2.
Third bulb contains ammonical cuprous chloride (100g cuprous
chloride + 125 mL liquor ammonia+375 mL of water) and it can absorb CO2,
O2 and CO.
Hence, it is necessary that the flue gas is passed first through
potassium hydroxide bulb, where CO2 is absorbed, then through alkaline
pyrogallic acid bulb, when only O2 will be absorbed ( because CO2 has
already been removed) and finally through ammonical cuprous chloride bulb,
where only CO will be absorbed
7.2.2 Method of Analysis
The whole apparatus is thoroughly cleaned, stoppers greased and
then tested for air-tightness.
The absorption bulbs are filled with their respective solutions to level
just below their rubber connections.
Their stop-cocks are then closed. The jacket and levelling reservoir
are filled with water.
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The three-way stop-cock is opened to the atmosphere and reservoir is
raised, till the burette is completely filled with water and air is excluded from
the burette.
The three-way stop-cock is now connected to the flue gas supply and
the reservoir is lowered to draw in the gas, to be analysed, in the burette.
The sample gas mixed with some air is present in the apparatus. So
the three-way stop-cock is opened to the atmosphere, and the gas expelled out
by raising the reservoir.
This process of sucking and exhausting of gas is repeated 3-4 times,
so as to expel the air from the capillary connecting tubes, etc.
Finally, gas is sucked in the burette and the volume of the flue gas is
adjusted to 100 mL at atmospheric pressure.
For adjusting final volume, the three-way stop-cock is opened to
atmosphere and the reservoir is carefully raised, till the level ofwater in it is
the same as in the burette, which stands at 100 mL mark.
The three-way stop-cock is then closed.
The stopper of the absorption bulb, containing caustic potash
solution, is opened and all the gas is forced into this bulb by raising the water
reservoir.
The gas is again sent to the burette.
This process is repeated several times to ensure complete absorption
of CO2 [by KOH solution].
The unabsorbed gas is finally taken back to the burette, till the level
of solution in the CO2 absorption bulb stands at the constant mark and then,
its stop-cock is closed.
The levels of water in the burette and reservoir are equalised and the
volume of residual gas is noted.
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The decrease in volume-gives the volume of CO2 in 100 mL of the
flue gas sample.
The volumes of O2 and CO are similarly determined by passing the
remaining gas through alkaline pyrogallic acid bulb and ammonical cuprous
chloride bulb respectively.
The gas remaining in burette after absorption of CO2, O2 and CO is
taken as nitrogen.
Fig 7.2.2: Orsat Gas Analyzer Apparatus
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Photo 3: Collection of Syngas in collection tube
Photo 4: Inside of Gasifier furnace (after 3 batches)
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7.3 Electrical Power (IGCC)
Electrical power generation by integrated gasification combined cycle
(IGCC) power plant using coal or refinery bottoms as a feedstock has proven
to be economical. In addition, IGCC for municipal waste, and biomassfeedstocks are realizing some commercial applications, and could potentially
develop a large foothold in the market if certain drivers develop as expected,
including energy price forecasts and more stringent greenhouse gas
requirements.
7.4 Coal-to-Liquids
The synthesis gas (syngas) created by gasificationonce impurities
such as sulfur and mercury are removedcan be turned into liquid fuels and
chemicals via the Fisher-Tropsch process or other processes. Since impurities
are removed earlier in the process, these ultra-clean liquid fuels burn with
much fewer emissions than conventional diesel fuel. Environmental
considerations, national energy concerns, and global oil markets could play a
role in the development of these applications.
7.5 Coal-to-SNG (Synthetic Natural Gas) and Hydrogen
Syngas produced by gasification can also be used for the production of
synthetic natural gas (SNG) by a process called methanation. SNG is identical
to natural gas and is capable of the same applications. The future of the natural
gas market will play a large role in driving this application of gasification.
Syngas refinement using a water-gas shift process can be used to produce
hydrogen. This may become a significant gasification technology application
if hydrogen infrastructures and markets become established.
7.6 Coal-to-Chemicals
Gasification offers a means of converting coal to a variety of useful
products including fertilizers, ammonia, and the manufacture of plastics.
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7.7 Industrial Applications
The chemical and physical conversion characteristics of gasification
also allow for more specialized applications in a wide range of industries,
particularly in the production of electricity, chemical byproducts, and
hydrogen.
7.8 Distributed Generation/Biomass
Gasification of biomass holds potential for distributed power
generation and small-scale syngas production. Some groups are looking at
gasification of biomass for power, transportation fuels, and even cooking fuel
in remote locations.
7.9 Co-Generation
Gasification of multiple products by one plant has the potential to
change the way we view energy production. The ability to produce multiple
products allows plant management to optimize profits based on market
conditions and can improve plant efficiency, economics, and decrease overall
environmental impact versus multiple plants to each produce one product.
7.10
Integrated Gasification Fuel Cell (IGFC)
The Fuel Cells technology area, part of DOE's Advanced Energy
Systems R&D Program is working to develop and demonstrate high
efficiency, fuel flexible solid oxide fuel cells (SOFCs) and coal-based SOFC
power generation systems for large (greater than 100 MW) integrated
gasification fuel cell (IGFC) power plants. Fully integrated IGFC power plants
have the potential to achieve greater than 60 percent net efficiency, near-zero
air emissions (CO2capture greater than 99 precent) and minimal water
consumption.
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Chapter 8. Parametric Analysis and Results
8.1 Program Codes
Fig 8.1: Programming window for 1Nm3Synags
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Fig 8.2: Programming window for 30 kg Feedstock
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8.2 Effect of pressure on syngas composition
To observe the effect of pressure on product gas composition by
MATLAB model, pressure is varied up to 100 bar. There is increase in
methane and CO2 content in the synthesis gas with increasing pressure. The
yield of synthesis gas drops with pressure, whereas the heat content yields
increases (reflecting the higher methane content). Figure 15.1 (c) shows the
trend of LCV and operating pressure. The LCV of syngas increases with
increase in pressure.
Fig 8.2.1: Effect of pressure on syngas composition.
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Fig 8.2.2: Effect of pressure on syngas composition.
Fig 8.2.3: Effect of pressure on LCV of syngas.
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8.3 Effect of steam to fuel ratio on syngas composition
The hydrogen in syngas increases with an increase in steam to fuel
ratio but the carbon monoxide level drops with a rise in the steam to fuel ratio.
An increase in steam to fuel ratio in gasifier enhances the shift reaction in
which carbon monoxide converts into carbon dioxide with the presence of
steam. Therefore a rise in both hydrogen and carbon dioxide contents with the
expense of carbon monoxide.
Fig 8.3 Effect of Steam to fuel ratio on syngas composition.
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8.4 Effect of air to fuel ratio on syngas composition
The content of hydrogen, carbon monoxide, and methane in syngas
impacts on heating value of the syngas. At lower air to fuel ratio and up to
0.25, the syngas consists of high methane traces. The carbon monoxide
content in syngas is maximum at 0.20.25 air to fuel ratio.
Fig 8.4.1: Effect of air to fuel ratio on syngas composition
Fig 8.4.2: Effect of air to fuel ratio on syngas composition
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8.5 Effect of Oxygen Enrichment
Fig.15.4 (a) shows how the composition of gas changes with oxygen
fraction in the air for an oxygen factor of 0.3 to 0.8. Mostly all variations of
the molar fractions versus oxygen fractions are more or less linear. The mole
fraction of N decreases with increasing oxygen fraction as expected. The
composition of methane produced is very low. The percentage of hydrogen in
the fuel gas increases continuously with oxygen fraction for an increase of
oxygen fraction. It is interesting to know that carbon dioxide and water vapour
percentages are also increasing as nitrogen percentage is decreasing. In
producer gas, nitrogen, which is an inert, reduces and other component
fractions would increase as is evident from figure. Fig. 15.4 (b) shows a
significant increase in the calorific values of fuel gas by increasing the oxygen
fraction. Calorific value increases nonlinearly from 425 kJ/Nm3to 975 kJ/Nm3 for an
increment of oxygen fraction 0.25 to 0.7.This increment is due to increase in the
amount of CO and of H2.
Fig 8.5.1: Effect ofoxygen enrichment on Syngas composition.
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Fig 8.5.2: Effect ofoxygen enrichment on LCV.
8.6 Effect of air to fuel ratio on LCV of syngas
The content of hydrogen, carbon monoxide, and methane in syngas
impacts on heating value of the syngas. The LCV of syngas is high at
relatively low air to fuel ratios. The hydrogen content in biomass decreases
sharply with an increase in air to fuel ratio reducing the LCV. LCV of syngas
increases with increase in pressure and decrease in steam to fuel ratio. The use
of air with enriched oxygen also increases the heating value of the gas.
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Fig8.6 Effect of air to fuel ratio on LCV of syngas
8.7 Effect of steam to fuel ratio on LCV of syngas
Figure shows trend of LCV and steam to fuel ratio. With increase in
steam to fuel ratio the LCV of syngas decrease considerably from 4750kJ/Nm3
to 2125 kJ/Nm3 between steam to fuel ratio 0.2 to 1.8 due to increase in
hydrogen percentage. So, to get high heating value low value of steam to fuel
ratios are recommended.
Fig 8.7 Effect of steam to fuel ratio on LCV of syngas
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8.8 Result for Coal
Fig 8.8.1: Result for 1Nm3Syngas
Fig 8.8.2: Result for 30 kg coal
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8.9 Result for Rice Husk
Fig 8.9.1: Result for 1Nm3Syngas
Fig 8.9.2: Result for 30 kg rice husk
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8.10 Result for Wood Pellets
Fig 8.10.1: Result for 1Nm3Syngas
Fig 8.10.2: Result for 30 kg wood pellets
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Chapter 9. Future Prospective
9.1 Integrated Gasification Combined Cycle
An Integrated Gasification Combined Cycle (IGCC) technology allows coal
to be used to generate power as cleanly as natural gas.
IGCC technology has three basic components. In the gasification phase (1),
heat, pressure, pure oxygen and water are used to break coal down into its
component parts and convert it into a clean synthetic gas (syngas). The syngas is
cleaned before it can be converted into substitute natural gas (SNG) which
eventually fuels the power turbines. Remaining particulates are removed from the
syngas in the particulate scrubber (2). Carbon monoxide is converted to carbon
dioxide (CO2) by adding steam in shift vessel (3). The gasification process makes it
possible to capture most of the mercury (silver), sulfur (yellow)and carbon dioxide
(CO2) in the syngas (4). The captured CO2will be transported via pipeline for use
in enhanced oil recovery or storage in a saline geologic reservoir (5).
The IGCC plant then converts the syngas into substitute natural gas (SNG or
methane), through a process called methanation (6). The SNG, which is relatively
high in energy content, powers two gas turbines. Excess heat contained in the
exhaust from those turbines then heats water to power a steam turbine (7). Thishigh-efficiency approach is known as combined-cycle. The higher energy content
of the SNG (as compared with syngas) improves the efficiency of the power
production.
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Fig 9.1.1: Integrated Gasification Combined Cycle
Because the SNG is a clean fuel, nitrogen oxide (NOx) also can be
reduced considerably during and after combustion. The results are
substantially lower emissions compared to