Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED GASIFIER
The present invention relates to an apparatus and process for the gasification
of any
carbonaceous or other material of useable calorific value to produce a high
quality gas
preferably to fuel a reciprocating gas engine for the generation of
electricity
Sources of traditional fossil fuels and hydrocarbons have a finite life and
there is ever
growing pressure from environmental groups, as well as government authorities,
to clean up
the planet. There is also international pressure to suppress noxious emissions
that are causing
climate change. Waste to energy systems are known, but mainly rely upon
incineration with
high capital cost and production of large quantities of dirty ash, and are
increasingly becoming
unacceptable.
The present invention provides an efficient solution, at a relatively modest
capital cost.
The system provides significant improvements to well known, long established
technology, with
the advantage of allowing a modular and adaptable system to be custom built to
suit the
composition and quantity of the waste supply. The design has low maintenance
costs and
beats by a substantial margin all emission targets set by international and
domestic treaties
and agreements. A wide range of fuels can be processed including, but not
limited to, forestry
waste, Municipal waste after removal of metals, food waste including factory
processing waste,
sewage, animal waste and rubber tyres.
The process is one of continuous flow. The waste is dried and metals, if
present, are
extracted. Any plastics, glass etc can also be removed although this is not
essential. The waste
is then graded, with the oversize material being shredded and re-introduced.
The fuel thus
produced is then injected into an unique anaerobic gasifier and gasified at
about 800 C. The
gas is cooled and filtered to remove contaminants before being fed into gas
engines or gas
turbines for power generation. The solid residues from the gasifier together
with any oils and
tars are then introduced into a secondary gasifier to produce further gas and
heat for use in the
cycle. The minimal residues are converted to an inert vitrified slag for use
in the construction
industry. Hence the full process has no unusable residues.
The gasifier consists of a substantially horizontal, cylindrical reactor which
rotates
slowly within a refractory lined furnace vessel. The waste material is
indirectly heated in an
oxygen free atmosphere. The gas produced, after cooling and cleaning, can be
used to
generate "green" electricity via a gas engine or gas turbine. Thermal energy
produced also has
profitable uses.
The main feature of the design is the provision of an innovatory internal vane
arrangement which allows homogeneous distribution of the feed material over a
large area of
the retort. This exposes it quickly to the heat without the need for rapid
tumbling and agitation
that is used in competing processes. Furthermore "cold spots" are avoided,
thus increasing the
plant's ability to produce gas of a consistently good quality.
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As a result of the internal configuration, the design is also an improvement
on the
conventional rotating reactor design because it permits the use of a more
compact unit. A slight
increase in diameter allows the use of a shorter vessel. Thermal analysis
confirms that the
system provides the correct rate of heating needed to generate good quality
gas. The
refractory lining acts as a good heat sink and gives the required temperature
stability.
The design is robust and has the advantage that, whilst innovative, it draws
on proven
engineering principles and also avoids the problems associated with other
systems that
incorporate high speed moving parts.
By way of example only, a specific embodiment of the present invention will
now be
described with reference to the accompanying drawings in which:
Figure 1 is a schematic view of a gasification plant in accordance with the
present invention.
Figure 2 is a longitudinal section of the gasifier.
Figure 3 is a cross-section of the gasifier kiln.
Figure 4 is a diagrammatic view of the secondary gasifier showing the
principal flow patterns.
The dotted lines are not structural. They show flow boundaries.
Referring to figure 1, wet fuel is delivered via conveyer 13 to fuel hopper
14. From the
hopper the fuel is fed into the dryer 15 by a screw feeder. The fuel rolls
around the dryer and is
heated to evaporate off the moisture. The drying process also serves to
sterilize the fuel feed.
The dried fuel is then graded for size via a trommel 16 where the correctly
sized fuel passes
through and the oversized fuel goes onto the reject conveyer 22 where it is
delivered to a reject
skip 23 for further processing.
The correct sized dry fuel is transported via a conveyer 17, along with the
fuel that did
not require drying. Both fuels are stored in the dry fuel hopper 18.
The shredding of oversized dry fuel is carried out by selected equipment
suitable for the
material to be shredded (eg. tyres, dry industrial waste). On completion the
material is carried
by a separate conveyor and dropped onto conveyor 17. Oils and other liquid
fuels are stored in
tanks and pumped into the gasifier 19 or bio oil storage tank 28 to fuel a
secondary gasifier
depending on the fuel combinations being processed and the respective heat
balance.
The fuel is then fed via an elaborate purged feed system, to avoid the ingress
of air, into
the gasifier 19 where it is heated to separate the gas from the solid char.
The gas is cooled in
the gas quench unit 20 where it is also cleaned. The gas is then compressed
and stored in a
gas storage unit. Then it is used to fuel a gas engine to generate
electricity.
The char is quenched in a water trough then fed by a screw conveyer to a dryer
29 then
stored in a hopper 30. Oils and tars that are carried over with the gas are
removed by an
extraction unit 27 and stored in storage vessel 28. From storage the oils are
used as a fuel
along with the char via burners 1 to fuel the secondary gasifier 2. The
combustion air used in
the secondary gasifier is taken from the dryer. This air is dried in 25 by the
forced draft fan 26
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then sent to the combusters1. Any slag that is produced in the secondary
gasifier pours out
from the secondary gasifier as a vitrified slag into a trough 32 where it is
removed into skips 33.
The hot gases from the secondary gasifier are treated to reduce NOx by NOx
suppression system 3. The temperature is then controlled at 4 before heating
the gasifier 5.
Having heated the gasifier the hot gases are guided via ducts 6 to a further
stage of
temperature control 7. From there they heat the dryer before being guided via
duct 9 to a filter
that collects any dust allowing the clean exhaust to pas to the chimney 12 via
an Induced
draft fan 11.
The preferred plant equipment is further described in the following sections.
The process takes any carbonaceous material, wood, plastic etc with a size
less than
16mm. Gas quality is initially determined by the overall temperature and
subsequently by the
gas temperature and gas residence time in the gasifier. By a process of rapid
heating in the
absence of air in the rotary kiln gasifier, the sorted waste is gasified to
produce significant
quantities of gas fuel. To maintain the safety and integrity of the gasifier a
pressure release
system is needed to enable the swift release of gas. As the gas will be at a
high temperature its
release into the atmosphere will cause it to self ignite. Thus this emergency
release will be
directed through a low-pressure flare immediately above the gasifier. A
nitrogen purge system
ensures safe operation during start up and shutdown.
Wet fuel is transported, by conveyor, to a storage hopper that has a
volumetric capacity
for 3 hours at maximum output. The fuel is fed by gravity into a screw feeder
then into the
dryer. This allows for control of the feed rate into the dryer. This unit will
improve the energy
efficiency of the overall process by using waste heat to drive off moisture.
In the dryer, the fuel will be dried to water content of less than 8%,
dependent on the
inlet water burden, though if this rises above 40% then the "dried" fuel
moisture content may
rise above 8% whilst the system stabilizes.
The dryer is a rotating kiln type dryer with internal fins to increase the
heating surface
area and to keep the fuel moving. Dryer temperature is controlled to maintain
a fuel
temperature of around 125'C to 140'C so as to minimize premature pyrolysis of
the waste. The
dryer is designed for fuel temperatures of 240'C. Temperature control is by
dilution air added to
the hot exhaust gases from the gasifier. These are thermally controlled to
supply the required
temperature to the dryer.
The dryer runs at a constant speed and the control variables are wet fuel feed
rate and
heating temperature. The fuel dwell time in the dryer is controlled by the
incline of the dryer and
is pre-set to 20 minutes. The dryer exhaust gases then pass through a ceramic
fabric filter for
removal of particulates. An Induced air fan draws the exhaust gases through
the dryer and
filter. The end of the dryer has a trommel that rejects the fuel of a size
greater than 5/8 of an
inch. The correctly sized fuel travels to the dry fuel hopper and the rejects
fall into a skip.
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The wet air from the dryer is drawn off. This air is then dried through
cooling to provide
dry air to the forced draft fan. The water collected is used as part of the
secondary gasifier de
NOx system
The dry fuel is stored in the dry fuel hopper above the gasifier. It has a 3-
hour supply of
fuel at maximum load. By gravity the fuel falls into one of 2 sets of
hydraulically actuated ram
loaders, where waste is charged incrementally to the gasifier. Each loader is
phased, one filling
whilst the other is feeding the gasifier. A purpose designed feeding mechanism
has been
provided which ensures a positive seal between the gasifier and atmosphere,
purge entrained
air from the fuel and positively feed the fuel to the gasifier.
Based on the principles previously outlined, the gasifier is a rotary kiln
consisting of a
rotating, slightly inclined metal shell or tube, which progressively
transports the fuel along its
length, and is contained within a refractory lined static metal shell. The
exhaust gas from the
secondary gasifier external to the kiln heats the tube.
A quench system cools the gas, and a gas clean up plant then ensures that the
gas is
suitably cleaned of contamination. An effluent plant neutralizes the effluent
streams from the
gas clean up system. The function of the gas clean up plant is to remove the
contaminants
from the gas stream. Cleaning is required to prevent contaminants from causing
a problem
within the downstream equipment such as rapid clogging of filters and
corrosion of gas engine
internals etc.
The particulates are removed by physical separation, whilst the halides and
sulphurous
compounds are removed by chemical reaction. The plant also includes a
polishing filter to
remove trace compounds including dioxins, furans and heavy metals.
Gas and minimal liquid products exit the gasifier in the gaseous phase and
pass
through a quenching spray that reduces the gas temperature and saturates the
gas to its
adiabatic condition. This allows condensation of the oils and tars and
performs a degree of
solids removal.
Owing to the expected levels of condensable tars and oils it was decided that
a wet
quench would be more prudent. The condensates of the volatile hydrocarbons are
collected
and removed regularly to the bio oil storage tank. Operational experience will
determine the
actual frequency.
The design requires a use of chemicals based on the normal expected levels of
contaminants. This is very much dependent on the composition of the waste.
A gas compressor is situated before the carbon filters to ensure that the gas
is drawn
through the gas clean-up plant and is of sufficient pressure to pass through
the carbon filter
and then feed the gas engine and also to achieve the gas storage compressor's
inlet pressure.
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Liquors are collected from the plant and delivered into a liquor holding tank.
These will
include the blow down from the char quench system and spent materials from the
scrubbing
stage. The effluent will include chlorine, fluorine, sulphur and hydrogen
sulphide contaminants,
which are oxidised at stabilized pH levels by the addition of sodium hydroxide
and sodium
hypochiorite. The liquors are then injected into the secondary gasifier.
Chlorine compounds, a precursor to the formation of dioxins and furans, are
expected
to be present in the gas. However, their formation in the gasification process
will be minimal
owing to the absence of significant quantities of oxygen Nevertheless,
provision is made in the
gas clean up plant to remove chlorine compounds.
The transition time of the fuel from initial entry to ash removal is
determined by the
angle of inclination, the speed of rotation being pre-set. The angle of
inclination can be
adjusted manually. The gasifier is designed to heat up the fuel as quickly as
possible to the
pyrolysis temperatures in order to minimize carbon in the ash. The temperature
of the gas will
initially be determined by the temperature of, the fuel when the gas is given
off, and
subsequently by heat gained by the gas from the shell and from the ash being
tumbled by the
gasifier.
Solid ash or char residue from the gasifier is deposited into a water-cooled
receiver to
reduce the temperature. The char is then ground and transported via a screw
conveyor into the
char storage hopper. From here the char will be collected via bottom silo
empting rotary valves
through gravity and carried in the primary combustion air into a char
combustor.
The char, carbon and ash, from the gasifier is used as the primary fuel in the
secondary
gasifier, together with the tar and oil collected during the gas quench
process. Added to this will
be any concentrated effluent and the dust from the filters. The non-
combustibles are vitrified in
the secondary gasifier to produce a slag. This vitrified slag is used in the
construction industry.
A gas storage facility is also provided to smooth out variations in gas
quality caused by
changes in the waste stream. The storage tanks can also supply gas for short
periods when the
gasifier is not producing gas, for example during start-ups.
The gas is preferably fed into a gas engine, which drives an alternator to
generate
electrical power for export into the local grid network. The exhaust heat from
the gas engines is
added to the system to support the process.
The secondary gasifier is designed to form a molten slag from the ash products
encapturing pollutants and to produce a vitrified slag.
The refractory-lined vessel is fired by multiple fuel burners that can operate
on gas, fuel
oil, Including the bio oils and tars and the char from the gasifier.
As the fuel is injected into the secondary gasifier it is gasified and as the
gas burns
progressively as it travels down through the secondary gasifier it is
aerodynamically forced to
rotate at high speed.
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The secondary gasifier has a unique flow made up of two aerodynamic spirals,
one
inside the other. The outer spiral rotates downwards towards a vortex
collector cone, and as
the flames spin down the cone the rotation inverts into a vortex. This, causes
the remaining ash
to eject into the molten pool that collects and finally drains into a water
trough by gravity,
leaving cleaned gases to spiral through the ultra high temperature central
axis.
Centrifugal force retains the heavier un-gasified fuel with ash products and
other matter
around the periphery of the cylindrical chamber thus providing a longer
gasification/bum time
and thus reducing the emissions. This also cools the walls, which-run at less
than 900 C for
long refractory life. As the gases bum out and become rarefied at very high
temperature they
rotate towards the central outlet Circling the flames back on themselves and
blending layer
upon layer achieves regenerative combustion, which increases the temperature
on residence
period to several times that of conventional burners.
Immediately after the secondary gasifier, a water and urea injection system
reduces the
temperature and NOx levels. This system consists of a urea concentrate holding
tank and a
mixing tank where the water from the combustion air dryer is delivered.: The
mixing tank is
controlled to give the correct consistency of urea and water to minimize the
NOx levels.
