Patent Application
20110197797
This invention relates to the regeneration and manufacture of activated carbon in a multiple hearth furnace system and utilization of ‘waste’ streams from said manufacture to the increase the ultimate efficiency thereof.
Activated carbon is a microcrystalline, nongraphitic form of carbon which has been processed to increase its porosity. Activated carbon is typically characterized by a large specific surface area, preferably by not necessarily from 400 to as high as 2500 m2/gram. This large surface area enables activated carbons to act as a very effective absorbent as a result of the high degree of surface reactivity. Favorable pore size makes this surface area accessible to gases and liquids. Generally, the larger the surface area of the activated carbon, the greater is its adsorption capacity. Activated carbons are used in processes to efficiently remove pollutants from liquid and gaseous streams.
Different kinds of raw materials have been made into activated carbons, including plant material, peat, lignite, soft and hard coals, tars and pitches, asphalt, petroleum residues and carbon black. Coal has been found to be a good raw material for the production of activated carbons.
The preparation of activated carbons generally involves two steps. During the first step, noncarbon elements are eliminated as volatile gases by pyrolytic decomposition of the starting material. Where the feed stock contains water, the first step results in the production of steam. Once ‘dry’, a portion of the carbon feed stock is removed through devolatilization. As much of the volatile portions of the feed stock as possible is removed with the goal of only fixed carbon (FC) remaining along with an unavoidable residue of ash. The ‘pores’ of the remaining carbon, i.e. the FC, have been exposed by the devolatilization of the feed stock.
The second step involves a gasification reaction occurring at high temperature. During this step, the diameter of the pores is enlarged, thus increasing the volume of the pores. Typical reactions taking place in the furnace include the following:
C+H2O→CO+H2
C+CO2→2CO
O2+H2→2H2O
O2+2CO→2CO2
CO+H2OCO2+H2
The H2O is introduced into the reaction in the form of steam, the C is primarily the FC resulting from the first step and the remaining reactants are free gaseous molecules.
Gasification converts the carbonized raw material into a form that contains the greatest possible number of randomly distributed pores of various shapes and sizes, and a final product with a high surface area.
Besides the activated carbon, outputs of the two steps described above include steam and volatile matter, both from the first step. It is known that steam may be brought from an area of a reaction where it is in excess to an area where it is required. U.S. Pat. No. 4,455,282 to Gerald Marquess and David J. Nell brought waste steam from a drying step into the oxidation step, where it was needed for the oxidation reactions.
Besides steam and volatile matter, ‘waste’ outputs from prior art activated carbon manufacturing include CO2, H2 and mixtures of organic vapors and solids of varying sizes. Exhausting these outputs to the atmosphere has become less and less feasible and/or desirable. Segregation of some fractions of such exhaust for recycling, e.g. water, and utilization of realizable chemical energy of other fractions is desirable for at least this reason.
A multiple hearth furnace is disclosed wherein a plurality of hearths are arranged in series. Some of the hearths form a drying section producing water vapor, some form a devolatilization section producing volatile gas and some define an activation section wherein chemical reactions take place that consume water vapor and CO2 and are, as a net, endothermic. Recycled gas from the drying section and devolatilization section pass through an outlet attached to an activation section inlet by a conduit external to the furnace, whereby the water vapor fraction is consumed in the chemical reactions of the activation section.
The furnace may also include a combustion chamber, in-line with the conduit, whereby the volatile gas fraction of the recycled gas is burned in the combustion chamber. A water conduit may be attached to the combustion chamber, whereby supplemental water vapor may be added to the combustion chamber and heated therein. A portion of the volatile gas fraction may be burned in the activation section.
The multiple hearth furnace may be provided with a recycling fan to optimize the flow of recycled gas through the conduit. Similarly, an exhaust fan may be connected to the drying section by an exhaust outlet, whereby water vapor and volatile gas not able to be recycled can be removed from the furnace. A cyclone, or other particulate capture devise may be used to capture and return fines to the activation zone.
An alternative embodiment of the multiple hearth furnace through which a feed stock containing water, ash, FC and volatile material passes, the furnace of the alternative embodiment has a similar arrangement of hearths. A devolatilization section outlet is attached to a conduit external to the furnace with a volatile gas valve between the devolatilization section outlet and the conduit and an activation section outlet is also attached to the conduit with an activation section valve between the activation section outlet and the conduit. The other end of the conduit is connected to a drying section inlet, whereby a controlled portion of the gas inside the furnace flows from the drying section, through the devolatilization section and into the activation section with a portion of the activation section gas and devolatilization section gas recycled to the drying section.
The alternative embodiment furnace may also have a combustion chamber, in-line with the conduit, between the valves and the drying section inlet, whereby a portion of the combustible [the gas contains volatiles and CO, H2, CH4 etc] gas fraction of the recycled gas is burned in the combustion chamber. A recycling fan in-line with the conduit may also be provided to optimize the flow of recycled gas through the conduit. Similarly, an exhaust fan may be connected to one or more sections by exhaust outlets, whereby gas not needed for recycling is removed from the furnace.
The alternative embodiment furnace may be provided with one or more monitors, e.g. temperature or humidity monitors, supplying data from which it can be determined whether the furnace is performing at an optimal level. The flow through the activation section valve and the devolatilization section valve may then be varied independently to alter the flow therethrough and the flow through the exhaust fan may also be varied such that the optimal level may be achieved.
Also disclosed is a method and apparatus for recycling and reusing the exhaust gas from an activated carbon producing furnace. The method includes utilizing a superheating chamber to heat the exhaust gas to a temperature at which condensable organics are transformed into non-condensable organics. Water vapor is removed from the exhaust gas, and the exhaust gas is compressed and mixed with a source of oxygen. The exhaust gas/oxygen source mixture is burned in a gas turbine for generating power.
The exhaust gas may be pre-heated prior to the step of superheating the exhaust gas and, since the superheated temperature is no longer needed, the pre-heating step may utilize extraneous heat contained in the superheated exhaust gas.
The step of removing water vapor from the gas can be achieved utilizing a condenser and/or a liquid gas separator.
In a preferred embodiment, the oxygen source is compressed atmospheric air.
The ultimate, though not required, goal is a mixture of non-condensable organics containing a high ratio of ethane and methane.
Referring to the drawings, there is shown a multiple hearth furnace 1 of generally cylindrical configuration constructed of a tubular outer steel shell 2, which is lined with heat resistant, insulating material 4. This furnace is provided with a plurality of burner nozzles 6, with one or more being provided on one or more of the hearths, as necessary, for initial start-up operation and for controlling the temperatures within the different regions of the furnace to carry out the particular processing desired. Any suitable type of fuel may be provided to the burners 6.
The feed stock is fed in through an input port 8 and is thereby placed on top hearth 10. The remainder of the moving parts of multiple hearth furnace 1 serve to transport the feed stock through the hearths, transforming it into activated carbon, which exits the system through outlet port 24. The multiple hearths shown in
From feed stock to activated carbon, as well as the intermediate and waste materials, the solids are moved through furnace 1 through a combination of gravity and pushing. The pushing is accomplished utilizing arms 32 mounted on a rotating central shaft 34. Each arm 32 contains a plurality of rabble teeth 36. During operation, the central shaft 34 rotates and the arms 32 move around the hearth. The rabble teeth 36 are angled with respect to the rabble arms 32 and positioned on the rabble arms 32 so as to result in a net advance of the solids in a radial direction. In
Thus, the feed stock to be processed enters the top of the furnace at an inlet 8 and passes downwardly through the furnace in a generally serpentine fashion alternately inwardly and outwardly across the hearths and is discharged at the bottom of the furnace, as indicated at 24.
Exhaust gases from the furnace are discharged from an outlet 44 at the top of the furnace 1. In the prior art, in order to support combustion, air was added at the bottom of the furnace. Additional air was added, as deemed necessary, in various other hearths throughout the furnace. An exhaust fan 46 could be fitted to encourage the upward flow through the furnace 1. The upward flow of hot gas can be some portion, or all, of the heat needed to dry the feed stock in drying section 26. The exhaust gases discharged through outlet 44 are, thus, removing the water vapor from the drying section 26.
Once dried, the material is heated to about 1400° F. in the devolatilization section. The specific solids temperature required is a function of the feed material. All of the volatile material passes from the solids into the atmosphere inside the furnace 1. Only FC and ash remain. The FC moves into the activation section. In the activation section, the key chemical reactions are:
C+H2O→CO+H2(endothermic)
C+CO2→2CO(endothermic)
O2+H2→2H2O(exothermic)
O2+2CO→2CO2(exothermic)
CO+H2CO2+H2(reversible)
Note that there is no burning of the volatile material. This material is not present in the activation section 30 in the prior art, up flow design, it having flowed away from the activation section 30 and into the drying section 26 and out the exhaust outlet 44.
In addition, the vaporized volatile matter in this recycle stream, having flowed from the devolatilization section 28 into the drying section 26, is fuel. The vaporized volatile matter fraction of the recycle stream is injected into the gas space above the FC material in the activation section 30 and all or a portion is burned as fuel. Thus, the exhaust gas from drying section 26 and devolatilization section 28 are recycled and used as a source of free steam and fuel to add energy for the endothermic reactions of activation. A significant portion of the fuel used to create steam to be injected into the activation section 30 will be saved by use of the recycled steam. In addition, a significant portion of the fuel used to support the endothermic reactions in the activation section 30 will be replaced by the recycled fuel. Recycling fan 52, or other means, may be used to regulate flow of the recycle stream into the activation section 30.
Besides optimizing the steam concentration, combustion chamber 54 may be used to optimize burning of the fuel fraction of the recycled gas stream. The fuel fraction of the recycled gas stream contains the volatiles as well as H2 and CO from various chemical reactions within the furnace 1, particularly from the activation section 30.
Available down flow furnaces provide the heat required for the drying section 26 and devolatilizing section 28 either with fuel burners or by recycling hot gas from the hearths 18-20 of the activation section 30 into the drying 26 and devolatilization 28 sections. Furnace 1 of
As in
Actuatable valves 66 and the power to exhaust fan 46 are controlled such that the composition of the recycled gas passing through recycled gas fan 52 is controlled for multiple variables. That is, the fuel content (primarily derived from outlet 68 of devolatilization section 28) and the steam content (primarily derived from outlets 60, 62 of activation section 30) of the recycled gas are monitored and controlled by adjusting the flow through outlets 60, 62 and 68 by valves 66 and the exhaust flow through outlet 72 by the power supplied to exhaust gas fan 46.
Standard temperature, humidity, sampling and/or otherwise appropriate monitors may be located at any convenient location of any of the embodiments described herein. Data from these monitors may be used to optimize the drying, devolatilization and activation processes occurring in furnace 1. Such optimization may take the form of adjusting valves 66 to vary gas flows to/from various sections of the furnace as well as adjusting the power supplied to either of fans 46 or 52, particularly where the inlets to fans 46, 52 are not provided with a valve.
It is also possible to use raw materials such as old tires that are suitable for the manufacture of activated carbon, but which arrive void of water. In this embodiment, although the water can still be inserted into the process to generate the required steam, the energy to heat such water can still be derived from the process as explained above.
It is also noted that in an arrangement like that shown in
To one degree or another, the preceding methods and apparatus may have one or more outputs that are not useful (e.g. activated FC), immediately recycled (e.g. water vapor mixed with vaporized volatile matter) or harmless (e.g. pure water/water vapor). To an ever increasing degree depending upon the industrial application and geographic/jurisdictional location of the apparatus in question, simply exhausting anything but the purest and most benign byproduct of the present method and apparatus to the atmosphere is no longer an option. Even emissions of CO2, formerly considered almost as benign as pure H2O or air, is coming under increasing scrutiny.
Certainly, liquid H2O or air/H2O vapor mixed with CO2, H2 and mixtures of organic vapors and solids of varying sizes can be problematic to discard or recycle. The preceding embodiments sought to reuse and recycle these emissions to the greatest degree possible.
FGC&PG 80 is attached to the multiple hearth furnace 1 described above with only relevant outputs shown in
Gas exits dust collector 81 and is conveyed to heat exchanger 86 through conduit 84. The gas in conduit 84 contains relatively large fractions of water vapor, organics, carbon dioxide, hydrogen, carbon monoxide, tar and other particulates. The temperature of the gas in conduit 84 is of the order of about 950° F. The gas gains energy in heat exchanger 86, exiting with a temperature of about 1900° F.
From heat exchanger 86, the gas is conveyed to superheat chamber 90. Air is added to the gas to promote combustion and the temperature of the mixture is raised to approximately 2100° F. by combustion in the superheater. The purpose of the superheat chamber is to raise the temperature of the gas high enough to cause larger particles and large molecular weight organic molecules to decompose to smaller organics, with ethane (C2H6) and methane (CH4) being the ultimate, though not required, goal.
The high temperature of the gas exiting the super-heater 90 is not necessary for later steps and may, therefore, be used for the source of heat for heat exchanger 86. The superheated gas enters the top of heat exchanger 86 and is used to raise the temperature, as mentioned previously, of the gas from conduit 84. Thus, the now extraneously hot gas from superheater 90 is used to preheat the gas going into the superheater 90 to minimize the amount of energy that needs to be expended in superheater 90 to raise the gas to the decomposition temperature.
The gas exiting heat exchanger 86 will be a slightly hotter than the gas entering the heat exchanger 86 from conduit 84, e.g. approximately 1200° F. This gas still contains large fractions of water vapor, carbon monoxide and hydrogen, but as much of the organics/tar as possible will have been decomposed into non-condensable, smaller molecule organics. This gas is passed through a condenser 96 in which it is cooled, e.g. by air or water, down to approximately 110° F. Water exits the condenser 96 through conduit 104, after cleaning, this water can be used in other apparatus needing water, e.g. furnace 1. The remaining gas is conveyed to liquid gas separator 102 to separate as much of the remaining condensed water vapor as possible; from whence it is added to conduit 104. The gas may now, most accurately for this FGC&PG 80 apparatus, be considered fuel gas. After a final pass through particulate control filter 108, from which dust particles are removed through conduit 110, the gas enters compressor 112. Compressed fuel gas exits compressor 112 at a pressure appropriate for a gas turbine 116, e.g. 500 psi. Atmospheric air may be utilized as an oxygen source for the gas turbine 116. Air is supplied to compressor 106 from the atmosphere through line 100. The pressurized air and fuel gas are combined and ignited in the combustor 114 element of gas turbine 116. The result of this combustion process is a great increase in the volume and temperature of the fuel gas/air combination. This combustion product is the input to the blades of gas turbine 116, causing the blades to be displaced and exert torque on shaft 118. Shaft 118 may be attached to AC generator 118, resulting in the generation of AC power.
Thus, FGC&PG 80 reduces a potentially problematic gaseous mixture of air, H2O, CO2, H2, CO, organics and solid particulates of varying sizes to pure water, collected dust, electrical energy and emissions from gas turbine 116. The water can be utilized in any number of ways, including in multiple hearth furnace 1 for production of activated carbon. Although CO2 is, ultimately, exhausted from FGC&PG 80 as exhaust from gas turbine 116, the overall energy system emits a lower ratio CO2 to KWh than burning coal directly to energy. The improvement being a fraction of the carbon present in the original coal is removed from the process as activated carbon. In addition, the relative purity of the compressed fuel gas entering combustor 114, i.e. in contrast to burning pure coal, results in minimal less desirable emissions from turbine 116.
The temperatures provided in the description of FGC&PG 80 are exemplary. The actual temperatures will be partially dependent upon the temperature of the gas in conduit 84, which will be fixed by the temperatures in various zones of furnace 1 and the ratio of exhaust from each zone; and partially dependent upon the temperature needed in superheat chamber to efficiently decompose molecules and particles.
Although certain particular embodiments of the invention are herein disclosed for purposes of explanation, various modifications thereof, after study of this specification, will be apparent to those skilled in the art to which the invention pertains.
It is noted that, while the invention has been described with reference to various embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Further, although the invention has been described herein with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed herein; rather, the invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.
Those skilled in the art, having the benefit of the teachings of this specification, may achieve numerous modifications thereto and changes may be made without departing from the scope and spirit of the invention in its aspects.
