Gasification is a process that converts organic or fossil based carbonaceous materials into carbon
monoxide, hydrogen and carbon
dioxide. This is achieved by reacting the
material at high temperatures (>700 °C), without combustion, with a
controlled amount of oxygen and/or steam. The resulting gas mixture is called syngas (from synthesis gas or synthetic gas) or producer gas and is itself a fuel. The power derived from gasification
and combustion of the resultant gas is considered to be a source of renewable
energy if the gasified compounds were
obtained from biomass.
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 at higher 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 gas engines,
used to produce methanol and hydrogen, or converted via the Fischer–Tropsch process
into synthetic fuel.
Gasification can also begin with material which would otherwise have been
disposed of such as biodegradable waste.
In addition, the high-temperature process refines out corrosive ash elements
such as chloride and potassium, allowing clean gas production from
otherwise problematic fuels. Gasification of fossil fuels is currently widely used on industrial scales to generate
electricity.
History
The process of producing energy
using the gasification method has been in use for more than 180 years. During
that time coal and peat were used to power these plants.
Initially developed to produce town gas for lighting & cooking in 1800s, this was replaced by
electricity and natural gas, it was
also used in blast furnaces but the
bigger role was played in the production of synthetic chemicals where it has been in use
since the 1920s.
During both world wars, especially the Second World
War, the need of gasification produced
fuel reemerged due to the shortage of petroleum. Wood gas generators,
called Gasogene or Gazogène, were used to power motor vehicles in Europe. By 1945 there were trucks, buses and agricultural machines
that were powered by gasification. It is estimated that there were close to
9,000,000 vehicles running on producer gas all over the world
Chemical reactions
In a gasifier, the carbonaceous
material undergoes several different processes:
The dehydration or drying process occurs at around 100°C. Typically the
resulting steam is mixed into the gas flow and may be involved with subsequent
chemical reactions, notably the water-gas reaction if the temperature is
sufficiently high enough (see step #5).The pyrolysis
(or devolatilization) process occurs at around 200-300°C. Volatiles are
released and char is produced, resulting in up to 70% weight loss for coal.
The process is dependent on the properties of the carbonaceous material and
determines the structure and composition of the char, which will then undergo
gasification reactions.
- The combustion process occurs as the volatile products and some of the char reacts with oxygen to primarily form carbon dioxide and small amounts of carbon monoxide, which provides heat for the subsequent gasification reactions. Letting C represent a carbon-containing organic compound, the basic reaction here is
- The gasification process occurs as the char reacts with carbon and steam to produce carbon monoxide and hydrogen, via the reaction
- In addition, the reversible gas phase water-gas shift reaction reaches equilibrium very fast at the temperatures in a gasifier. This balances the concentrations of carbon monoxide, steam, carbon dioxide and hydrogen.
In essence, a limited amount of
oxygen or air is introduced into the reactor to allow some of the organic
material to be "burned" to produce carbon dioxide and energy, which
drives a second reaction that converts further organic material to hydrogen and
additional carbon dioxide. Further reactions occur when the formed carbon
monoxide and residual water from the organic material react to
form methane and excess carbon dioxide (4 CO + 2 H2O --> CH4 + 3 CO2). This
third reaction occurs more abundantly in reactors that increase the residence
time of the reactive gases and organic
materials, as well as heat and pressure. Catalysts are used in more sophisticated reactors to improve reaction
rates, thus moving the system closer to the reaction equilibrium for a fixed
residence time.
Gasification
processes
Several types of gasifiers are
currently available for commercial use: counter-current fixed bed, co-current
fixed bed, fluidized bed,
entrained flow, plasma, and free radical.
Counter-current
fixed bed ("up draft") gasifier
A fixed bed of carbonaceous fuel
(e.g. coal or biomass) through which the "gasification agent" (steam,
oxygen and/or air) flows in counter-current configuration. The ash is either removed in the dry condition or as a slag. The slagging gasifiers have a lower ratio of steam to
carbon, achieving temperatures higher than the ash fusion
temperature. The nature of the gasifier means that the fuel must have high mechanical
strength and must ideally be non-caking so that it will form a permeable bed,
although recent developments have reduced these restrictions to some extent.
The throughput for this type of gasifier is relatively low. Thermal efficiency
is high as the temperatures in the gas exit are relatively low. However, this
means that tar and methane production is significant at typical operation
temperatures, so product gas must be extensively cleaned before use. The tar
can be recycled to the reactor.
In the gasification of fine,
undensified biomass such as rice hulls, it is necessary to blow air into the reactor by means of a
fan. This creates very high gasification temperature, as high as 1000 C. Above
the gasification zone, a bed of fine and hot char is formed, and as the gas is
blow forced through this bed, most complex hydrocarbons are broken down into
simple components of hydrogen and carbon monoxide.
Co-current
fixed bed ("down draft") gasifier
Similar to the counter-current type,
but the gasification agent gas flows in co-current configuration with the fuel
(downwards, hence the name "down draft gasifier"). Heat needs to be
added to the upper part of the bed, either by combusting small amounts of the
fuel or from external heat sources. The produced gas leaves the gasifier at a
high temperature, and most of this heat is often transferred to the
gasification agent added in the top of the bed, resulting in an energy
efficiency on level with the counter-current type. Since all tars must pass
through a hot bed of char in this configuration, tar levels are much lower than
the counter-current type.
Fluidized
bed reactor
The fuel is fluidized in oxygen and steam or air. The ash is removed dry or as
heavy agglomerates that defluidize. The temperatures are relatively low in dry
ash gasifiers, so the fuel must be highly reactive; low-grade coals are
particularly suitable. The agglomerating gasifiers have slightly higher
temperatures, and are suitable for higher rank coals. Fuel throughput is higher
than for the fixed bed, but not as high as for the entrained flow gasifier. The
conversion efficiency can be rather low due to elutriation of carbonaceous material. Recycle or subsequent combustion
of solids can be used to increase conversion. Fluidized bed gasifiers are most
useful for fuels that form highly corrosive ash that would damage the walls of
slagging gasifiers. Biomass fuels generally contain high levels of corrosive
ash.
Entrained
flow gasifier
A dry pulverized solid, an atomized
liquid fuel or a fuel slurry is gasified with oxygen (much less frequent: air)
in co-current flow. The gasification reactions take place in a dense cloud of
very fine particles. Most coals are suitable for this type of gasifier because
of the high operating temperatures
and because the coal particles are well separated from one another.
The high temperatures and pressures also
mean that a higher throughput can be achieved, however thermal efficiency is
somewhat lower as the gas must be cooled before it can be cleaned with existing
technology. The high temperatures also mean that tar and methane are not
present in the product gas; however the oxygen requirement is higher than for
the other types of gasifiers. All entrained flow gasifiers remove the major
part of the ash as a slag as the operating temperature is well above the ash
fusion temperature.
A smaller fraction of the ash is
produced either as a very fine dry fly ash or as a black colored fly ash slurry. Some fuels, in
particular certain types of biomasses, can form slag that is corrosive for
ceramic inner walls that serve to protect the gasifier outer wall. However some
entrained flow type of gasifiers do not possess a ceramic inner wall but have
an inner water or steam cooled wall covered with partially solidified slag.
These types of gasifiers do not suffer from corrosive slags.
Some fuels have ashes with very high
ash fusion temperatures. In this case mostly limestone is mixed with the fuel
prior to gasification. Addition of a little limestone will usually suffice for
the lowering the fusion temperatures. The fuel particles must be much smaller
than for other types of gasifiers. This means the fuel must be pulverized,
which requires somewhat more energy than for the other types of gasifiers. By
far the most energy consumption related to entrained flow gasification is not
the milling of the fuel but the production of oxygen used for the gasification.
Plasma
gasifier
In a plasma gasifier a
high-voltage current is fed to a torch, creating a high-temperature arc. The
inorganic residue is retrieved as a glass-like substance.
Feedstock
There are a large number of
different feedstock types for use in a gasifier, each with different
characteristics, including size, shape, bulk density, moisture content, energy
content, chemical composition, ash fusion characteristics, and homogeneity of
all these properties. Coal and petroleum coke are used as primary feedstocks
for many large gasification plants worldwide. Additionally, a variety of
biomass and waste-derived feedstocks can be gasified, with wood pellets and
chips, waste wood, plastics and aluminium, Municipal Solid Waste
(MSW), Refuse-derived fuel
(RDF), agricultural and industrial wastes, sewage sludge, switch grass,
discarded seed corn, corn stover and other crop residues all being used.
Waste
disposal
Waste gasification has several
advantages over incineration:
- The necessary extensive flue gas cleaning may be performed on the syngas instead of the much larger volume of flue gas after combustion.
- Electric power may be generated in engines and gas turbines, which are much cheaper and more efficient than the steam cycle used in incineration. Even fuel cells may potentially be used, but these have rather severe requirements regarding the purity of the gas.
- Chemical processing (Gas to liquids) of the syngas may produce other synthetic fuels instead of electricity.
- Some gasification processes treat ash containing heavy metals at very high temperatures so that it is released in a glassy and chemically stable form.
A major challenge for waste
gasification technologies is to reach an acceptable (positive) gross electric
efficiency. The high efficiency of converting syngas to electric power is
counteracted by significant power consumption in the waste preprocessing, the
consumption of large amounts of pure oxygen (which is often used as
gasification agent), and gas cleaning. Another challenge becoming apparent when
implementing the processes in real life is to obtain long service intervals in
the plants, so that it is not necessary to close down the plant every few
months for cleaning the reactor.
Environmental advocates have called
gasification "incineration in disguise" and argue that the technology
is still dangerous to air quality and public health. "Since 2003 numerous
proposals for waste treatment facilities hoping to use... gasification
technologies failed to receive final approval to operate when the claims of project
proponents did not withstand public and governmental scrutiny of key
claims," according to the Global Alliance for Incinerator Alternatives.One facility which operated from 2009-2011 in Ottawa had 29
"emissions incidents" and 13 "spills" over those three
years. It was also only able to operate roughly 25% of the time.
Several waste gasification processes
have been proposed, but few have yet been built and tested, and only a handful
have been implemented as plants processing real waste, and most of the time in
combination with fossil fuels.
One plant (in Chiba, Japan using the Thermoselect process) has been processing industrial waste since year 2000, but
has not yet documented positive net energy production from the process.
In the USA, gasification of waste is expanding across the country. Ze-gen is operating a waste gasification demonstration facility in
New Bedford, Massachusetts. The facility was designed to demonstrate gasification of
specific non-MSW waste streams using liquid metal gasification.This facility came after widespread public opposition
shelved plans for a similar plant in Attleboro, Massachusetts.
Also in the USA, plasma is being
used to gasify municipal solid waste, hazardous waste and biomedical waste at
the Hurlburt Field Florida Special Operations Command Air Force base.
PyroGenesis Canada Inc. is the technology provider.
Current
applications
Syngas can be used for heat
production and for generation of mechanical and electrical power. Like other
gaseous fuels, producer gas gives greater control over power levels when
compared to solid fuels, leading to more efficient and cleaner operation.
Syngas can also be used for further
processing to liquid fuels or chemicals.
Heat
Gasifiers offer a flexible option
for thermal applications, as they can be retrofitted into existing gas fueled
devices such as ovens, furnaces, boilers, etc., where syngas may replace fossil fuels. Heating
values of syngas are generally around 4-10
MJ/m3.
Electricity
Currently Industrial-scale
gasification is primarily used to produce electricity from fossil fuels such as
coal, where the syngas is burned in a gas turbine. Gasification is also used
industrially in the production of electricity, ammonia and liquid fuels (oil)
using Integrated Gasification Combined Cycles (IGCC), with the possibility of producing methane and hydrogen
for fuel cells. IGCC is also a more efficient method of CO2 capture
as compared to conventional technologies. IGCC demonstration plants have been
operating since the early 1970s and some of the plants constructed in the 1990s
are now entering commercial service.
Combined
heat and power
In small business and building
applications, where the wood source is sustainable, 250-1000 kWe and new zero
carbon biomass gasification plants have been installed in Europe that produce
tar free syngas from wood and burn it in reciprocating engines connected to a
generator with heat recovery. This type of plant is often referred to as a wood
biomass CHP unit but is a plant with seven different processes: biomass
processing, fuel delivery, gasification, gas cleaning, waste disposal,
electricity generation and heat recovery.
Transport
fuel
Diesel engines can be
operated on dual fuel mode using producer gas. Diesel substitution of over 80%
at high loads and 70-80% under normal load variations can easily be achieved.[23] Spark ignition engines
and SOFC fuel cells can
operate on 100% gasification gas. Mechanical energy from the engines may be used for e.g.
driving water pumps for irrigation or for coupling with an alternator for
electrical power generation.
While small scale gasifiers have
existed for well over 100 years, there have been few sources to obtain a ready
to use machine. Small scale devices are typically DIY projects. However, currently in the United
States, several companies offer gasifiers
to operate small engines.
Renewable
energy and fuels
In principle, gasification can
proceed from just about any organic material, including biomass and plastic
waste. The resulting syngas can be combusted. Alternatively, if the syngas is
clean enough, it may be used for power production in gas engines, gas turbines
or even fuel cells, or converted efficiently to dimethyl
ether (DME) by methanol dehydration,
methane via the Sabatier reaction,
or diesel-like synthetic fuel via the Fischer–Tropsch process.
In many gasification processes most of the inorganic components of the input
material, such as metals and minerals, are retained in the ash. In some
gasification processes (slagging gasification) this ash has the form of a
glassy solid with low leaching
properties, but the net power production in slagging gasification is low
(sometimes negative) and costs are higher.
Regardless of the final fuel form,
gasification itself and subsequent processing neither directly emits nor traps greenhouse
gases such as carbon dioxide. Power
consumption in the gasification and syngas conversion processes may be
significant though, and may indirectly cause CO2 emissions; in
slagging and plasma gasification, the electricity consumption may even exceed
any power production from the syngas.
Combustion of syngas or derived
fuels emits exactly the same amount of carbon dioxide as would have been
emitted from direct combustion of the initial fuel.[dubious – discuss] Biomass
gasification and combustion could play a significant role in a renewable energy
economy, because biomass production removes the same amount of CO2
from the atmosphere as is emitted from gasification and combustion.[dubious – discuss] While
other biofuel technologies such as biogas and biodiesel are carbon
neutral, gasification in principle may run
on a wider variety of input materials and can be used to produce a wider
variety of output fuels.
There are at present a few
industrial scale biomass gasification plants. Since 2008 in Svenljunga, Sweden, a biomass gasification plant generates up to 14 MWth,
supplying industries and citizens of Svenljunga with process steam and district
heating, respectively. The gasifier uses biomass
fuels such as CCA
or creosote impregnated waste wood and other kinds of recycled wood to
produces syngas that is combusted on site.[27][28] In 2011 a similar gasifier, using the same kinds of fuels,
is being installed at Munkfors
Energy's CHP plant. The CHP plant will generate 2 MWe
(electricity) and 8 MWth (district
heating).[29][30]
Examples of demonstration projects
include:
- Those of the Renewable Energy Network Austria, including a plant using dual fluidized bed gasification that has supplied the town of Güssing with 2 MW of electricity, produced utilising GE Jenbacher reciprocating gas engines and 4 MW of heat, generated from wood chips, since 2003.
- Chemrec's pilot plant in Piteå that has produced 3 MW of clean syngas since 2006, generated from entrained flow gasification of black liquor.
- The US Air Force Transportable Plasma Waste to Energy System (TPWES) facility at Hurlburt Field, Florida.
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