Abstract:
The gasifier operates to mix a start up heat source with crude syngas combustion for driving gasification of waste. Combustion flue gas can be maintained above 650° C. until reaching a quench to prevent formation of dioxins. Excess heat is liberated through a heat recovery unit. The gasifier can operate in a batch mode to process small batches of waste efficiently for small installations, such as ships, apartment buildings, hospitals and residences.

Description:
TECHNICAL FIELD 
     This invention relates to waste treatment and to gasification of organic material. More specifically, it relates to an improved method and apparatus for the gasification of organic waste in a small-scale installation. 
     BACKGROUND 
     Municipal waste is most commonly sent to landfills or incinerated. For many municipalities, landfills are preferred over incineration due to lower infrastructure cost and reduced concerns about immediate air pollution resulting from incineration. The heat resulting from incineration can be used to generate electricity. 
     Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, are a group of polyhalogenated compounds which are significant because they act as environmental pollutants. Dioxins are produced in small concentrations when organic material is burned in the presence of chlorine, whether the chlorine is present as chloride ions or as organochlorine compounds. Dioxins occur as by-products in the municipal incineration of chlorine-containing substances, such as PVC (polyvinyl chloride). In incineration, dioxins can also form in the atmosphere above the stack as the exhaust gases cool through a temperature window of 650° C. to 250° C. Reducing the quantity of dioxins can be achieved through rapid quenching of the exhaust gases through this 400 degree window. Thus, the problem of dioxin formation is avoided in conventional gasification systems. 
     Municipal waste gasification plants are not in widespread use, as is the case for incinerators. Gasification plants are commonly used to convert coal into fuel gas. In gasification, the waste is heated in the presence of reduced oxygen or air supply so that a synthesis gas of hydrogen and carbon monoxide is produced. 
     The synthesis gas can contain tars and a secondary gasifier can be used to convert tars to synthesis gas. Before combusting the synthesis gas, it may be scrubbed to remove contaminants. The synthesis gas can then be used as a fuel supply for power generation or other energy needs. In PCT publication WO2008/044216 dated Apr. 17, 2008, there is described by the Applicant of the present application a waste disposal system that gasifies small batches of waste to produce a clean synthesis fuel gas. A Primary Gasifier chamber is heated by a combustion fuel source with a small amount of introduced air to produce a crude synthesis gas (containing tars) that is then passed to a secondary gasifier chamber that is heated by a combustion or electric fuel source to an optimum gasification temperature to produce synthesis gas essentially free of tars. The synthesis gas can be quenched and cleaned of other contaminants, such as acid gases, and then it can replace a conventional fuel source driving the gasification process as it becomes available. The synthesis gas is decontaminated and the combustion gases resulting from the synthesis gas are clean and can pass directly to the atmosphere as flue gas. When the batch process is completed, a small amount of carbonaceous residue, typically less than 10% of the original volume, and a small amount of various filtered contaminants remain. 
     SUMMARY 
     It has been discovered that the production of crude synthesis gas can be controlled so as to allow for efficient scrubbing of flue gases that contain the combustion products of the crude synthesis gas. A secondary gasification system can thus be avoided. In some embodiments, the crude synthesis gas is fed into a suitable high-temperature combustion zone that is equipped with a conventional heat source, typically a hydrocarbon fuel burner, where the synthesis gas is combusted and the hot combustion products are used to heat a gasification chamber. Scrubbing of the flue gas uses equipment that can process a predetermined flow rate of gas. Controlling the heating of the gasifier, as well as controlling the amount of process air allowed into the gasifier can ensure that the amount of crude synthesis gas produced will be within the capability of the scrubbing equipment. 
     It has also been discovered that combustion of the crude synthesis gas along with combustion of any additional fuel can be controlled to maintain the temperature of the flue gas throughout the heat exchange with the gasification chamber above 650° C. (preferably above 700° C.) until the flue gas is quenched and scrubbed. By controlling the flue gas from dropping below 650° C., the formation of dioxins is greatly reduced. This discovery is useful in the context of a small, batch mode gasifier as well as in the context of a larger, continuous-feed gasifier. 
     The gasifier operates to mix a start up heat source with crude syngas combustion for driving gasification of waste. Combustion flue gas can be maintained above 650° C. until reaching a quench to prevent formation of dioxins. Excess heat is liberated through a heat recovery unit. The gasifier can operate in a batch mode to process small batches of waste efficiently for small installations, such as ships, apartment buildings, hospitals and residences. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which: 
         FIG. 1  is a block diagram of the gasifier according a primary embodiment of the invention; 
         FIG. 2  is a sectional elevation view of a combustion chamber for producing combustion gases to be communicated with a heat exchanger of a gasification chamber according to a second embodiment; and 
         FIG. 3  is a more detailed diagram of the quench and scrubber system according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The process begins by loading waste, for example a standard garbage bag of waste, into the gasification chamber  12  (see  FIG. 1 ) though a loading hatch  14 . The vacuum pump  13  is turned on and the diesel burner  18  is lit. While in the present embodiment, diesel fuel is chosen, it will be appreciated that any heat source able to provide the required heat energy and temperature can be utilized. The oxygen content of the final exhaust stream is measured by an O 2  sensor  48 . The O 2  sensor  48  allows the process to control the oxygen in the combustion zone  21  to within regulated limits; usually 6%-12% in the flue gas. The gasification chamber  12 &#39;s pressure is kept at a fixed set point, slightly below atmospheric. The process air  26  is pulled into the chamber  12  by the negative relative pressure inside. The process air  26  is preheated by being pulled through a small heat exchanging channel in the combustion zone  21 . The flow rate is controlled by a proportional valve (part of source  22 ) and is monitored by a flow transmitter  24 . The flow rate is measured by the flow transmitter  24  and the valve is adjusted to decrease or increase the flow rate accordingly. In operation, the process air feed rate is controlled between 0 and 20 scfm and is used to either accelerate or decelerate the gasification process and thus control the amount of synthesis gas produced. Reducing synthesis gas production can be required when combustion of synthesis gas generates temperatures that reaches a set point in the flue gas. 
     Following start up, as the process heats up the gasification chamber  12 , the waste inside the chamber  12  begins breaking down into synthesis gas (syngas). The crude syngas  20  produced from the heated waste is fed into the combustion zone  21  through a port. The syngas begins to burn, resulting in an increased demand for O 2  in the combustion zone  21 ; this triggers a loop that opens the combustion air feed  28  to the combustion zone  21 . By measuring the O 2  content of the exhaust flow and the combustion zone temperature using sensors  34 , it is ensured that all syngas  20  is combusted fully. The exhaust stream O 2  content is kept at or above the set point established for complete combustion. A suitable set point can be between 6% and 12% O 2  content. The combustion air flow rate is monitored by a flow transmitter  30  and controlled by a proportioning valve in the source  28 . 
     The invention disclosed herein is related to other technologies aimedat modifying the chemical form of the waste and specifically those causingthe thermal oxidation of all organic waste. Pyrolysis, gasification andincineration (or combustion) are the main thermal processes used to modifythe chemical form of waste; they are well known and used widely around the world. There are numerous patents describing all manners of operatingthese processes (usually on a continuous basis rather than batch operation)to improve their efficiency, reduce their environmental impact, processdifferent types of waste, etc. 
     Pyrolysis operates in the complete absence of oxygen and thermallydecomposes the organic waste into a carbon based char and a mixture ofoils. It is not a process that is suitable for small-scale waste treatment. Neither the char nor the mixture of oils can be used by a small isolatedcommunity. As such, pyrolysis is a viable process only for relatively large-scale operations and for the treatment of specialty waste such as scrap tiresand plastics. 
     Gasification is a process in which a controlled amount of oxygen isused to convert the organic molecules in the waste into a synthesis gascontaining mostly carbon monoxide and hydrogen. The organic molecules within the waste, which are heated by both thehot internal reactor wall and the pre-heated process air, react with theoxygen in the process air to “gasify” into a mixture of carbon monoxide andhydrogen. Along with the products of gasification, nitrogen, which isintroduced into the gasifier as part of the process air, is found in thesynthesis gas. The final synthesis gas produced within the gasification zoneof the reactor is a volumetric mixture of 50% nitrogen, 25% hydrogenand 25% carbon monoxide. Under certain operating conditions, some of theorganic molecules decompose only partially to form smaller volatilehydrocarbons known as tars. 
     To date, nearly all efforts to design a very small waste treatmentsystem have been based on incineration. In incineration, the organic wasteis mixed with excess air and a combustible fuel. Consequently, the waste iscompletely burned and all carbon is converted to carbon dioxide. 
     The diesel burner  18  is shut off when temperatures in the combustion region  21  crest a set point of roughly 1100 Celsius, and the combustion air feed  29  is past a certain feed rate, roughly 25 cubic feet per minute; which indicates that enough syngas is being generated to maintain combustion without the diesel. The temperature in the combustion zone  21  is kept constant by use of a two stage diesel burner  18  for heating and by controlling the conditions in the chamber  12 , including process air  22  feed rate, heating rate and mixing using mixer  16 , to manage the amount of syngas  20  produced. In the event of highly energetic syngas production, water can be sprayed using sprayer  32  onto chamber  12  and or into the combustion region  21  to control temperature. Water spray evaporates and absorbs heat from the gasses; it is re-condensed and collected in the scrubber  44 . 
     If the syngas quality falters and the temperature drops, then the diesel burner  18  can be relit to maintain sufficient temperatures for operation. To prevent formation of dioxins in the flue gas, the temperature of the flue gas is measured using sensor  38 , preferably at a point right before the quench  40 , and if the temperature drops below 650° C., then it is important to bring the temperature of the flue gas back up in order to avoid the potential formation of dioxins. For this purpose, the diesel burner  18  is restarted. Thus, temperature is measured in the combustion region  21  using sensor  34  and in the flue gas exhaust using sensor  38  prior to the quench  40 . 
     Dioxin levels were measured at 0.11 ng/m 3  when flue gas was in the temperature range of 560° C. and 610° C. In some municipalities the legal limit is as low as 0.1 ng/m 3  and this limit is expected to be respected by using temperatures above 610° C. 
     While in the embodiment described above, the flue gas temperature is monitored, it will be appreciated that this can be measured indirectly by measuring the chamber  12  temperature. 
     The water spray nozzle  32  serves to cool the chamber  12  during shut down and to ‘freeze’ the ash in the bottom to prevent excessive gasification of the carbon in the ash; also the nozzle  32  serves to inject water to lessen the quality of syngas exiting the primary if temperatures are found to be too high in the combustion chamber  21 . Temperatures in any chamber should not exceed 1250° C. The governing temperature within the gasification system is the temperature of the stainless steel at the bottom of the gasification chamber  12 . This temperature may not exceed the working temperature range of the material. This is nominally set at 775 degrees Celsius. 
     The rate of crude syngas  20  production is measured by three parameters. First, the amount of air  22 , 26  which is required to ensure syngas combustion (plus a fixed amount of excess air) is measured; by stoichiometric deduction the syngas production rate is estimated. The second parameter is the position of the vacuum bypass control valve  50 . Given that the combustion region  21  and the chamber  12  are kept at a constant pressure, the rate of crude syngas production can be measured by the position of the vacuum bypass control valve  50  (e.g. without relying on controlling process air source  22 ). Negative pressure (e.g. 90 kPa absolute) is maintained in the combustion chamber  12  by the liquid ring vacuum pump  42 . The degree of vacuum is controlled by the vacuum bypass valve  50 . It will be appreciated that other arrangements can be provided to control pressure. 
     As syngas  20  production begins, the pressure rises in the chamber  12  and the vacuum bypass control valve  50  closes to maintain enough suction to keep the pressure constant, the position of valve  50  is proportional to the syngas production rate. The third parameter is the temperature of chamber  12  (as measured by sensor  36 ); gasification can be estimated to begin at chamber temperatures above 350 degrees Celsius. 
     Once it is determined that syngas is produced, the combustion air  28 , 29  is injected immediately to ensure constant exhaust emissions at the exit  46 . Determination of the correct amount of air is calculated by the process controller  15  based on the parameters described above. The process controller  15  can comprise fixed circuitry, a microcontroller, FPGA, DSP, or a more complete computer programmed to perform the control loop function. Additionally, the controller  15  can also record measured values during operation and provide reports or logs of operation. The control loop for this step will now be described. 
     The transition from burning diesel fuel to burning syngas is done with the aid of a small pilot burner (part of burner  18 ) which burns diesel fuel. The small burner is located in the combustion region  21  at the entrance of the syngas  20  into the combustion zone  21 . The pilot flame is used to ensure that the syngas is combusted fully in the early stages of syngas production, when the quality of the syngas is less than optimal. This window of ‘poor’ syngas lasts approximately 10 minutes. 
     In one embodiment, the combustion region  21  has two burners, one main and one pilot burner. Alternatively, a two stage burner can be used. It is the main burner which is shut off when temperatures rise as mentioned above. The mixer  16  slowly turns the waste in the chamber  12  causing the gasification reaction to occur more rapidly and steadily by putting more waste in contact with the hot floor and process air  22 , 26  than would be the case if the waste were stagnant. Also process air channelling problems are avoided by keeping the waste in motion. A motor (not illustrated) is provided to turn the mixer  16 . The mixer motor is turned on by controller  15  when temperatures inside the chamber  12  reach 350° C. The mixer  16  should stay on at all times although it is possible to stop it as a way to reduce syngas production in the case that syngas production is too great. 
     An automated loop controls the resistance of the motor to jamming. If the motor and shaft force against an obstruction in the chamber  12 , the current to the motor will spike. This spike is monitored and will cause the motor to reverse directions. If the motor reverses direction three times in a minute, it shuts down and can be restarted manually. 
     The scrubber  44  is a wet scrubber. The water in the scrubber  44  becomes acidic due to the combustion exhaust passing through the scrubber. The acidity can be neutralized by the addition of liquid caustic soda into the recirculation line. A pH meter can be used to monitor the acidity in the scrubber and open a valve to the caustic tank which can be gravity fed into the recirculation line based on a set pH value, eg. 7. 
       FIG. 3  shows details of one embodiment of the quench  40 , pump  42  and scrubber  44  arrangement illustrating also a water circulation pump  43 , caustic reservoir  45 , and discharge filter  49 . The water used in the quench  40  and scrubber  44  is circulated using pump  43  through a heat exchanger  47 , namely a 50 kW heat recovery unit, i.e. corresponding to the water being cooled from 40° C. to 25° C. at a rate of 45 L/min. The cooled water is then used in the quench  40  and for the scrubber shower of the scrubbing tower  44 . While the 10% NaOH from reservoir  45  can be added at an amount of 0.5 litres per batch of waste, it will be appreciated that it can be added as a result of measuring pH, either manually or automatically under the control of the controller  15 . When the gasifier operates in a continuous mode with waste added as the processing is in progress, balancing pH of the recirculation loop is best done in a continuous way. Excess water in the recirculation loop is emptied through a drain valve and a filter  49 . The amount of water drained can be about 10 to 20 L per batch. The drain valve can be controlled automatically by controller  15  in response to water level sensors in the scrubber  44 . 
     In some embodiments, the level of water in the scrubber  44  should be kept constant. The liquid ring vacuum pump  42 , which controls the vacuum of the system, can rely on a constant flow from the recirculation line which is in turn fed by the scrubber water reservoir. The process generates water and some water is exhausted as vapor, the exact amounts may vary, hence the scrubber  44  is equipped with an automatic fill and drain which are controlled by level sensors. For example, two vibrating fork level sensors can be used in the scrubber. They send a signal when they are under water versus in air. The two forks are located 2 inches apart, one above the other. The desired water level is between the two sensors, thus the lower sensor is always under water and the upper sensor is always above water. If the lower sensor is exposed due to a decreasing water level, the fill valve will activate and fill the scrubber with fresh water until the lower sensor is submerged. If the upper sensor is submerged due to a rising water level, then the drain will open until the upper sensor is above water. 
     The scrubber water temperature is an important parameter that affects the suction capabilities of the liquid ring vacuum pump  42 . The scrubber water should not exceed 50 degrees Celsius. The scrubber water is re-circulated to the quench  40 . The quench mixed temperature should not exceed 60 degrees Celsius. The ‘mixed temperature’ is the temperature of the flow of exhaust gas and quench water between the quench  40  and the ring pump  42 . All the water in the system is closed loop and a heat exchanger  47  is used to remove any excess heat being built up in the recirculating water. 
     The heat recovered from unit  47  can be used for building air heating purposes (either direct hot water radiators or in a forced air furnace), for heat storage, for pre-heating hot water, as a heat source for heat pumps, or other industrial uses. 
     It will be appreciated that the heat exchanger  47  removes most of the heat generated by the whole gasification system, as the resulting exhaust gas is cooled to about 40° C. This low temperature exhaust can facilitate installation of the gasification system, since conventional heating or kitchen ventilation ducts can be used to vent exhaust gas to the outside of the building instead of using high temperature chimney ducts. Also, the single point for heat recovery is simpler than recovering heat in a gasification system that uses a secondary gasifier and cleans the synthesis gas, since heat then can be recovered both in the syngas scrubber and then separately in the flue gas. 
     It will be appreciated that many variations from the above-described embodiments are possible. For example, the burner can be substituted by a variety of heat sources, and in some cases propane or natural gas will be preferred as a combustible fuel over diesel. 
     The combustion zone  21  can be substituted for a combustion chamber  21 ′ as shown in  FIG. 2 . This can be done to increase the capacity of the system with regards to the burning of syngas and consequently the waste processing capacity of the system. The combustion chamber  21 ′ receives the syngas  20  from the gasification chamber  12 . The syngas  20  is drawn into the chamber  21 ′ by virtue of it being at a lower pressure than the gasification chamber  12 . The chamber  21 ′ has a cylindrical mixing region  21   a  that receives the flow of syngas  20  tangentially to spiral and flow into a burning region  21   b  where combustion air from port  29  is injected in an ignition region. The chamber  21 ′ is provided with a refractory lining  21   c , and suitable insulation  21   d . The mixing region  21   a  is where the syngas enters the chamber and is put into contact with the pilot burner, namely part of the diesel burner  18 . The syngas  20  is heated and mixed in the mixing region. The syngas is then forced into contact with the combustion air  29  where it ignites and burns. The exhaust  21   e  from the combustion chamber  21 ′ is directed into a heat exchanging region surrounding chamber  12 , where the heat from the exhaust gas is transferred back to the process. 
     Additional details of a gasifier system is found in co-pending U.S. patent application Ser. No. 12/445,455 filed Apr. 13, 2009, the content of which is incorporated herein by reference. 
     The gasification system shown in the above embodiments is a small scale device that operates on batches of waste, for example, a single, large, domestic garbage bag of waste. With a system of this scale, the amount of syngas produced is sufficient to drive the gasification process for a large part of a batch cycle, however, for most domestic waste, there is not significant excess of synthesis gas to be used for other purposes. The present invention can be applied to larger installations or to continuous gasification systems. In these cases, there can be significant excess syngas produced. It will be appreciated that crude synthesis gas to be used for purposes other than driving the gasification process can be processed by a secondary gasifier and scrubbed so as to produce a clean syngas supply, in addition to using the crude syngas as a fuel separate from the combustion providing heat to the gasification chamber. 
     When waste is continuously fed into chamber  12 , as can be done using a chute or feed screw mechanism, it is possible to control the amount of waste placed in the chamber  12  to keep the temperature of the chamber above, for example, 650 C, so that flue gas remains sufficiently high. Alternatively, a solid fuel source, such as a suitable coal could be initially added to the chamber  12  so that the start-up phase can be below 600° C. without production of dioxins. Once temperatures in the chamber  12  reach about 650° C., the addition of waste can proceed without producing dioxins in the flue gas. 
     The present invention has been described with regard to preferred embodiments. The description as much as the drawings were intended to help the understanding of the invention, rather than to limit its scope. It will be apparent to one skilled in the art that various modifications may be made to the invention without departing from the scope of the invention as described herein, and such modifications are intended to be covered by the present description.