Assignment
Topic;
Gasification
Subject;
Energy Engineering
Assigned by;
Sir Ayyaz Ahmad
Prepared by;
Junaid Anwar (2017-CH-716)
Talha Bhutta (2017-CH-715)
Semester;
6th
Date;
13-08-2020
MNS UET MULTAN
Introduction
Any technological process that can convert any carbon-based raw material such as
coal into fuel gas known as synthesis gas is called gasification.
Gasification process mostly occurs in a gasifier generally at a high temperature or
pressure vessel where oxygen or air or steam as gasifying agent are directly
contacted with the any carbon-based raw material causing a series of chemical
reactions to occur that convert the feed to syngas and ash or slag.
The syngas can be further converted or shifted by adding steam and reacting over
a catalyst in a water-gas-shift reactor to hydrogen and carbon dioxide (CO2).
Gasification is also a thermochemical process in which syn gas is produced by the
reactions between fuel and the gasification agent. The syngas is mainly composed
of CO, H2, N2, CO2, and some hydrocarbons (CH4, C2H4, C2H6, etc.). Very small
amounts of H2S, NH3, and tars may also be included.
Only a tiny amount of oxygen is required for Gasification which is combined with
steam and cooked under intense pressure. A gaseous mixture composed primarily
of carbon monoxide and hydrogen is produced by a series of reactions. This
syngas can be burned directly or used as a starting point to manufacture fertilizers,
pure hydrogen, methane or liquid transportation fuels.
Brief history
Scottish engineer William Murdoch produced syngas in sufficient quantity to light
his home in the late 1790s by using coal as a feedstock. The cities in Europe and
America began using syngas also known as "town gas" because at that time it was
used to light city streets and homes.
Gasification is making a comeback with a global climate crisis looming on the
horizon and power-hungry nations on the hunt for alternative energy sources.
World gasification capacity expected to grow by more than 70 percent by 2015.
Much of that growth will occur in Asia, driven by rapid development in China
and India. But the United States is embracing gasification, as well.
Processes of Gasification
Gasification is made up for five discrete thermal processes:
1) Drying
2) Pyrolysis
3) Combustion
4) Cracking
5) Reduction
Now we will discuss these in brief detail;
1) Drying
Drying is the removal of the moisture contents in the biomass before it enters
Pyrolysis. All the moisture needed to be removed from the fuel before any above
100°C processes happen.
All of the water in the biomass will get vaporized out.
Fuel with the high moisture content or poor handling of the moisture internally, is
one of the most common reasons for failure to produce clean gas.
2) Pyrolysis
Pyrolysis is the process to heat the raw biomass in the absence of the air to break
it down into charcoal and various tar gasses and liquids.
Once the temperature rises the above around the 240°C the biomass rapidly
decompose with heat.
The biomass breaks down into a combination of solids, liquids and gasses. The
solids that remain we commonly call charcoal. The gasses and liquids that are
released we collectively call tars.
3) Combustion
Combustion is the only net exothermic process in the gasification process.
All of the heat is recovered from the combustion process that drives drying,
pyrolysis, and reduction.
Combustion can be fueled by either the tar gasses or char from Pyrolysis.
Different reactor types use one or the other or both.
4) Cracking
Cracking is the process in which the break down of large complex molecules such
as tar into lighter gases take place by exposure to heat.
For the production of clean gas this process is crucial that is compatible with an
internal combustion engine because tar gases condense into sticky tar that can
cause rapidly fouling the valves of an engine.
To ensure the proper combustion cracking is also necessary because complete
combustion only occurs when combustible gases
thoroughly mix with oxygen. The high
temperatures produced decompose the large tar
molecules that pass through the combustion zone.
5) Reduction
The process of stripping the oxygen atoms from
combustion products of hydrocarbon molecules.
Reduction is the reverse process of combustion.
Reduction is the removal of oxygen from these
waste products at high temperature to produce
combustible gases.
Reduction in a gasifier is accomplished by passing carbon dioxide (CO 2) or water
vapor (H2O) across a bed of red hot charcoal (C). The carbon in the hot charcoal
is highly reactive with oxygen; it has such a high oxygen affinity that it strips the
oxygen off water vapor and carbon dioxide, and redistributes it to as many single
bond sites as possible.
The oxygen is more attracted to the bond site on the C than to itself, thus no free
oxygen can survive in its usual diatomic O2 form. All available oxygen will bond
to available C sites as individual O until all the oxygen is gone. When all the
available oxygen is redistributed as single atoms, reduction stops
CO2 is reduced by carbon to produce two CO molecules, and H 2O is reduced by
carbon to produce H2 and CO. Both H2 and CO are combustible fuel gases.
Major Reactions
Gasifier classification
Based on the gasification agents used, biomass gasification processes can
be divided into;
I. Air gasification (using air)
II. Oxygen gasification (using oxygen)
III. Steam gasification (using steam)
IV. Carbon dioxide gasification (using carbon dioxide)
V. Supercritical water gasification (using supercritical water)
Comparison of Gasification Agents
Oxygen gasification, steam gasification, carbon dioxide gasification, and
supercritical water gasification generally results in the higher HHVs of
syngas than those obtained by air gasification.
Air gasification is the most widely applied process because the gasification
agent is cheap, the reaction process is easy and the reactor structure is simple.
The gasification of biomass with the steam, carbon dioxide or supercritical
water the overall reaction is endothermic and external heating is required
during the whole gasification process.
The biomass gasification with air or oxygen, the overall gasification may be
endothermic or exothermic and these reactions can be controlled or changed
by varying the air or oxygen content.
Gasifier classification
Based on the gasifiers;
1) Fixed bed gasifiers (or moving bed gasifiers)
i. updraft
i. downdraft
ii. horizontal-draft
2) Fluidized bed gasifiers
i. bubbling fluidized bed
ii. circulated fluidized bed
iii. double circulated fluidized bed
3) Entrained flow gasifiers
Comparison of Gasifiers
1) Fixed bed gasifier
i. Small capacity (0.01–10 MW)
ii. Can handle large and coarse particles
iii. Low product gas temperature (450–650°C)
iv. High particulate content in gas product stream
v. High gasification agent consumption
vi. Ash is removed as slag or dry
vii. May result in high tar content (0.01–150 g/Nm3)
2) Fluidized bed gasifier
i. Medium capacity (1–100 MW)
ii. Uniform temperature distribution
iii. Better gas-solid contact
iv. High operating temperature (1000–1200°C)
v. Low particulate content in the gas stream
vi. Suitable for feedstocks with low ash fusion temperature
vii. Ash is removed as slag or dry
3) Entrained flow
i. Large capacity (60–1000 MW)
ii. Needs finely divided feed material
iii. Very high operating temperatures
iv. Not suitable for high-ash-content feedstocks
v. Very high oxygen demand
vi. Short residence time
vii. Ash is removed as slag
viii. May result in low tar content (negligible)
SYNGAS COMPOSITION
This can vary significantly depending on the feedstock and the gasification
process involved, however typically syngas is;
i. 30 to 60% carbon monoxide (CO)
ii. 25 to 30% hydrogen (H2)
iii. 0 to 5% methane (CH4)
iv. 5 to 15% carbon dioxide (CO2)
v. plus a lesser or greater amount of water vapor
vi. smaller amounts of the sulfur compounds hydrogen sulfide (H2S)
vii. carbonyl sulfide (COS)
viii. some ammonia and other trace contaminants
Gasification Applications
1. Microscale applications;
Generally, these gasifier applications gets classified for
lower power ranges between 1 – 7 kW. This is the range used by most of the farmers for
irrigation purpose in the developing countries.
2. Medium scale applications;
Generally, these gasifier applications gets classified for
power ranges between 30 -500 kW. These applications are widely used in the small to
medium agricultural industries and forestry industries. Generally, they are used in
sawmills, wood cutting industries, and in generating power. They can be used for
supplying power to the remote areas.
3. Large-scale applications;
Generally, these gasifier applications gets classified for
higher power ranges between 500 kW and above. Thus, they are costly and need the
utmost care while construction and delivery.
4. Other Applications;
As per the high demand of gasifiers and their applications, in
future, it can spread across various industries like
Pulp industries
Cement industries
Metallurgy
References
1. http://www.allpowerlabs.com/gasification-explained
2. https://www.netl.doe.gov/research/Coal/energy-
systems/gasification/gasifipedia/intro-to-gasification
3. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/gasification
4. https://ankurscientific.com/blog/2018/10/07/application-of-gasifie
