Gasification is a process that converts organic or fossil
fuel 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
Adler Diplomat 3 with gas generator (1941)
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 World War II, the
need of gasification produced fuel reemerged due to the shortage of
petroleum.[6] 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:
Pyrolysis of carbonaceous fuels
Gasification of char
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 {\rm C} + {\rm
O}_2 \rarr {\rm CO}_2
The gasification
process occurs as the char reacts with carbon and steam to produce carbon
monoxide and hydrogen, via the reaction {\rm C} + {\rm H}_2 {\rm O} \rarr {\rm
H}_2 + {\rm CO}
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. {\rm CO} + {\rm H}_2 {\rm O}
\lrarr {\rm CO}_2 + {\rm H}_2
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.[citation
needed] 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.[citation needed]
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
Pilot scale bubbling fluidized bed gasification setup to
gasify biomass or fossil fuels, University of Saskatchewan.
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.
Chemrec has developed a process for gasification of black
liquor.
Waste disposal
HTCW reactor, one of several proposed waste gasification
processes. According to the sales and sales management consultants KBI Group a
pilot plant in Arnstadt implementing this process has completed initial tests.
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.[18] This
facility came after widespread public opposition shelved plans for a similar
plant in Attleboro, Massachusetts. In addition, construction of a biomass
gasification plant was approved in DeKalb County, Georgia on June 14, 2011.
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. 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
Gasification plant Güssing, Austria (2006)
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. 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).
Examples of demonstration projects include:
Those of the
Renewable Energy Network Austria,[31] 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[32] and
4 MW of heat,[33] 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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