Patent Application
20070294937
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.
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:
Referring to
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 then sent to the combusters 1. 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 10 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 ⅝ of an inch. The correctly sized fuel travels to the dry fuel hopper and the rejects fall into a skip.
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.
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 hypochlorite. 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.
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/burn 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 burn 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.
Whilst the invention has been described in detail in terms of a specific embodiment thereof, it will be apparent that various changes and modifications can be made by one skilled in the art without departing from the spirit and scope thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0411643.0 | May 2004 | GB | national |
| 0500620.0 | Jan 2005 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB05/01768 | 5/10/2005 | WO | 11/22/2006 |
