Patent Application
20080016769
The present invention is directed to the production of synthetic natural gas (predominantly methane) from carbonaceous materials, preferably biomass materials. Synthetic natural gas, also sometimes called “green gas” is a renewable gas from biomass with natural gas specifications. Therefore, it can be transported through the existing gas infrastructure, substituting for natural gas in all existing applications. Another advantage of green gas is that is carbon neutral. That is, using biomass as an energy supply will typically not result in a net CO2 emission since its source can be replanted and uses CO2 from the atmosphere during its growth period.
While this invention is applicable to a broad range of carbonaceous feedstocks including the traditional naturally occurring solid fossil fuels such as coal, peat, lignite, tar sands, and bitumen from oil shale, the preferred feedstocks for use in the present invention are biomass feedstocks Non-limiting examples of biomass feedstocks suitable for being converted in accordance with the present invention include trees such as red cedar, southern pine, hardwoods such as oak, cedar, maple and ash, as well as bagasse, rice hulls, rice straw, kennaf, old railroad ties, dried distiller grains, corn stalks and cobs and straw. Cellulosic materials are the more preferred biomass feedstocks, with wood and dried distillers grains being the most preferred. Biomass is typically comprised of three major components: cellulose, hemicellulose and lignin. Cellulose is a straight and relatively stiff molecule with a polymerization degree of approximately 10,000 glucose units (C6 sugar). Hemicellulose are polymers built of C5 and C6 sugars with a polymerization degree of about 200 glucose units. Both cellulose and hemicellulose can be vaporized with negligible char formation at temperatures above about 500° C. On the other hand, lignin is a three dimensional branched polymer composed of phenolic units. Due to the aromatic content of lignin, it degrades slowly on heating and contributes to a major fraction of undesirable char formation. In addition to the major cell wall composition of cellulose, hemicellulose and lignin, biomass often contains varying amounts of species called “extractives”. These extractives, which are soluble in polar or non-polar solvents, are comprised of terpenes, fatty acids, aromatic compounds and volatile oil.
In most instances the carbonaceous mateials used in the practice of the present invention will be found in a form in which the particles are too large for conducting through the tubes of the pyrolysis unit. Thus, it will usually be necessary to grind the carbonaceous material to an effective size. In this case, the carbonaceous material is ground, otherwise reduced in size, to a suitable size of about 1/32 inch to about 1 inch, preferably from about 1/16 inch to about ½ inch, and more preferably from about ⅛ inch to about ¼ inch. Grinding techniques are well know and varied, thus any suitable grinding technique and equipment can be used for the particular carbonaceous material being converted.
The type of pyrolysis preferred for use in the practice of the present invention is known as “fast pyrolysis” which is a thermal decomposition process that occurs at moderate temperatures with a high heat transfer rate to the carbonaceous particles and a short hot vapor residence time in the reaction zone. Several conventional reactor configurations have been used in the art, such as bubbling fluid beds, circulating and transported beds, vortex or cyclonic reactors, and ablative reactors. While all of these reactors have their advantages they are all faced with limitations, such as the tendency of fluid bed reactors to produce more gas and coke then the desired pyrolysis oil, the preferred pyrolysis product of the present invention. The pyrolysis reactor of the present invention contains a plurality of vertically oriented straight tubes within the enclosed reactor vessel which is heated by use of a suitable heating device, such as one or more burners.
The pyrolysis of biomass as practiced by the present invention produces a liquid product, pyrolysis oil or bio-oil that can be readily stored and transported. Pyrolysis oil is a renewable liquid fuel can be used for production of chemicals and liquid fuels, or as herein for the production of synthetic natural gas. As previously mentioned, synthetic natural gas is a very desirable product because it is derived from a renewable source and it can be used as a substitute for natural gas for all natural gas applications.
Generally, pyrolysis requires that a feedstock have less than about 15% moisture content, but there is an optimization between moisture content and conversion process efficiency. The actual moisture content will vary somewhat depending on the commercial process equipment used. Since some of the biomass received for processing can have a moisture content from about 40 to 60% it will have to be dried before pyrolysis. Any conventional drying technique can be used as long as the moisture content is lowered to less than about 15% when mixed with the superheated steam. For example, passive drying during summer storage can reduce the moisture content to about 30% or less. Active silo drying can reduce the moisture content down to about 12%. Drying can be accomplished either by very simple means, such as near ambient, solar drying or by waste heat flows or by specifically designed dryers operated on location. Also, commercial dryers are available in many forms and most common are rotary kilns and shallow fluidized bed dryers.
This invention can be better understood with reference to the sole figure hereof. The carbonaceous feedstock is conducted via line 10 and superheated steam is conducted via line 12 to mixing zone Mix wherein the two are sufficiently mixed before being conducted via line 14 into pyrolysis process unit P. The superheated steam, which will be at a temperature from about 315° C. to about 700° C. acts as both a source of hydrogen as well as a transport medium. The amount of superheated steam to feedstock will be an effective amount. By effective amount we mean at least that amount needed to provide sufficient transport of the feedstock. That ratio of superheated steam to feedstock, on a volume to volume basis, will typically be from about 0.2 to 2.5, preferably from about 0.3 to 1.0. The temperature conditions for the pyrolysis reaction will be described later in detail. The steam is preferably introduced so that the feedstock is diluted to the point where it can easily be transported through the reactor tubes. Fluidization will typically result and can realize fluid pyrolysis by virtue of good contact among steam, feed polymers and heat decomposition products of carbonaceous material liberated in the gas phase.
The mixture of steam and feedstock, which will be at a temperature of above its dew point of greater than about 230° C., is fed to the pyrolysis reactor P via line 14 into a flow divider FD where it is distributed into the plurality of vertically oriented straight reactor tubes of effective internal diameter and length within a metal cylindrical vessel of suitable size. Flow divider FD can be any suitable design that will divide the feedstock substantially equally among the plurality of reactor tubes. The reactor tubes for the pyrolysis reactor are straight instead of coiled because the residence time needs to be very short in order to produce the maximum amount of oil without the production of an undesirable amount of gas. The temperature of the mixture entering the pyrolysis unit will be at least about 230° C. Typical internal diameters for the pyrolysis reactor tubes will be from about 2 to about 4 inches, preferably from about 2.5 to about 3.5 inches, and more preferably about 3 inches.
The feedstock passing though the pyrolysis reactor tubes is subjected to fast pyrolysis at temperatures from about 400° C. to about 650° C. and pressures from about 3 to 35 psig, preferably from about 5 psig to about 35 psig. The residence time of the feedstock in the pyrolysis reactor will be an effective residence time. By “effective residence time” we mean that amount of time that will result in the maximum yield of oil without excess gas make. Typically this effective amount of time for purposes of this invention will be from about 0.2 to about 7 seconds, preferably from about 0.3 seconds to about 5 seconds. The heating rate will be a relatively high heating rate of about 1,000° C. per second to about 10,000° C. per second. The high heating rate in the pyrolysis reactor of the present invention, at temperatures below about 650° C. and with rapid quenching, causes the liquid intermediate products of pyrolysis to condense before further reaction breaks down higher molecular weight species into gaseous products. The high reaction rates also minimize char formation, and under preferred conditions substantially no char is formed. At high maximum temperature, the major products is gas, thus the need for the present process to operate at low enough temperatures to maximize the production of pyrolysis oils.
Although the source of heat for the pyrolysis unit, as well as the reformer of the present invention, can be any suitable source, it is preferred that the source of heat be one or more burners B located at bottom of the pyrolysis and reforming process unit. Fuel for the burners B can be any suitable fuel. It is preferred that at least a portion of the fuel to the burners be obtained from the present process itself, such as the syngas produced in either the pyrolysis reactor or in the reformer. For example at least a portion of syngas stream 20 can be diverted via line 21 and used as a fuel to burners B. A portion of the syngas stream 20 can also be combined with syngas stream of line 30.
Flue gas, which will typically be comprised of CO2 and N2 is exhausted from the pyrolysis reactor via line 15 and the reaction products from the pyrolysis reactor are sent via line 16 to quench zone Q resulting in a mixture of liquid, gaseous and solid products. Most of the solids, which will typically be in the form of ash, will be collected from quench zone Q via line 17. The liquid product will be in the form of a pyrolysis oil and the gaseous product will be a syngas. The resulting liquid and gaseous products are conducted via line 18 to first separation zone Si wherein a syngas stream is separated from the pyrolysis oil and collected overhead via line 20 or a portion being diverted via line 21 to either or both of burners B. This syngas stream is comprised primarily of hydrogen, carbon dioxide, carbon monoxide, and methane. The pyrolysis oil stream, which may contain some remnants of char and ash formed during pyrolysis, is conducted via line 22 to reformer R along with an effective amount of superheated steam via line 23. It is preferred that reformer R contains a plurality of coiled reactor tubes within an enclosed reactor vessel heated by a suitable heating means, such as one or more burners.
At least a portion of the pyrolysis oil is converted to syngas in reformer R, which syngas is also composed primarily of hydrogen, carbon dioxide, carbon monoxide and methane. The inlet temperature of the feedstock and superheated steam entering reformer R will preferably be about 200° C. The exit temperature of the product syngas leaving reformer R via line 24 will typically be from about 850° C. and 1200° C., preferably between about 900° C. and about 1000° C. At a temperature of about 1100° C. and above and with a contact time of about 5 seconds, one obtains less than about 5 mole percent of methane and about 15 mole percent of CO2, which is an undesirable result. Pressure in the reformer is not critical, but it will typically be at about 3 to 500 psig. Also, it is preferred that the residence time in the reformer be from about 0.4 to about 1.5 seconds.
For any given feedstock, one can vary the proportions of hydrogen, carbon dioxide, carbon monoxide and methane that comprise the resulting syngas product stream as a function of the contact time of the pyrolysis oil feedstock in the reformer, the exit temperature, the amount of steam introduced, and to a lesser extent, pressure. Certain proportions of syngas components are better than others for producing synthetic natural gas, thus conditions should be such as to maximize the production of carbon monoxide and methane at the expense of hydrogen.
Returning now to the Figure hereof flue gas is exhausted from the reformer via line 23 and the product syngas stream from reformer R is conducted via line 24 to heat recovery zone HR1 where it is preferred that water be the heat exchange medium and that the water be passed as preheated steam to one or both of the pyrolysis reactor P or reformer R via lines 25 where it is further heated to produce at least a portion of the superheated steam used for both units. Heat Recovery zone HR1 can be any suitable heat exchange device, such as the shell-and-tube type wherein water is used to remove heat from product stream 24. From heat recovery zone HR1 the product syngas is passed via line 26 through second separation zone S2 which contains a gas filtering means and preferably a cyclone (not shown) and optionally a bag house (not shown) to remove at least a portion, preferably substantially all, of the remaining ash and other solid fines from the syngas. The filtered solids are collected via line 28 for disposal.
The filtered syngas stream is then passed via line 30 to water wash zone WW wherein it is conducted upward and countercurrent to down-flowing water via line 31. The water wash zone preferably comprises a column packed with conventional packing material, such as copper tubing, pall rings, metal mesh or other such materials. The syngas passes upward countercurrent to down-flowing water which serves to further cool the syngas stream to about ambient temperature, and to remove any remaining ash that may not have been removed in second separation zone S2. The water washed syngas stream is then passed via line 32 to oil wash zone OW where it is passed countercurrent to a down-flowing organic liquid stream to remove any organics present, such as benzene, toluene, xylene, or heavier hydrocarbon components via line 35 that may have been produced in the reformer. The down-flowing organic stream will be any organic stream in which the organic material being removed is substantially soluble. It is preferred that the down-flowing organic stream be a hydrocarbon stream, more preferably a petroleum fraction. The preferred petroleum fractions are those boiling in naphtha to distillate boiling range, more preferably a C16 to C20 hydrocarbon stream, most preferably a C18 hydrocarbon stream.
The resulting syngas stream is conducted via line 34 to acid gas scrubbing zone AGS wherein acidic gases, preferably CO2 and H2S are removed. Any suitable acid gas treating technology can be used in the practice of the present invention. Also, any suitable scrubbing agent, preferably a basic solution can be used in the acid gas scrubbing zone AGS that will adsorb the desired level of acid gases from the vapor stream. It will be understood that it may be desirable to leave a certain amount of CO2 in the scrubbed stream depending on the intended use of resulting methane product stream from the methanation unit. For example, if the methane product stream is to be introduced into a natural gas pipeline, no more than about 4 vol. % of CO2 should be remain. If the methane product stream is to be used for the production of methanol, then at least that stoichiometric amount of CO2 needed to result in the production of methanol should remaing. One suitable acid gas scrubbing technology is the use of an amine scrubber. Non-limiting examples of such basic solutions are the amines, preferably diethanol amine, mono-ethanol amine, and the like. More preferred is diethanol amine. Another preferred acid gas scrubbing technology is the so-called “Rectisol Wash” which uses an organic solvent, typically methanol, at subzero temperatures. The scrubbed stream can also be passed through one or more guard beds (not shown) to remove catalyst poisoning impurities such as sulfur, halides etc. The treated stream is passed via line 36 from acid gas scrubbing zone AGS to methanation zone M. Methanation of syngas involves a reaction between carbon oxides, i.e. carbon monoxide and carbon dioxide, and hydrogen in the syngas to produce methane and water, as follows:
CO+3H2→CH4+H2O (1)
CO2+4H2→CH4+2H2O (2)
Methanation reactions (1) and (2) take place at temperatures of about 300° C. to about 900° C. in methanation zone M which is preferably comprised of two or more, more preferably three, reactors each containing a suitable methanation catalyst. The methanation reaction is strongly exothermic. Generally, the temperature increase in a typical methanator gas composition is about 74° C. for each 1% of carbon monoxide converted and 60° C. for each 1% carbon dioxide converted. Because of the exothermic nature of methanation reactions (1) and (2), the temperature in the methanation reactor during methanation of syngas has to be controlled to prevent overheating of the reactor catalyst. Also high temperatures are undesirable from an equilibrium standpoint and reduce the amount of conversion of syngas to methane since methane formation is favored at lower temperatures. Formation of soot on the catalyst is also a concern and may require the addition of water to the syngas feedstock.
A preferred way to control heat during the methanation reaction is use a plurality of reactors with heat removed between each reactor. Thus, methanation zone M preferably comprises a series of three adiabatic methanation reactors R1, R2 and R3. Each of these reactors is configured to react carbon oxide and hydrogen contained in the syngas in the presence of a suitable catalyst to produce methane and water, in accordance with the reactions (1) and (2) set forth hereinabove. Each of the methanation reactors includes a catalyst capable of promoting methanation reactions between carbon oxides and hydrogen in the syngas feedstock. Any conventional methanation catalyst is suitable for use in the practice of the present invention, although nickel catalysts are most commonly used and the more preferred for this invention. Such catalysts are, especially those containing greater than 50% nickel, are generally stable against thermal and chemical sintering during methanation of undiluted syngas streams. Alternatively, other stable catalysts that are active and selective towards methane may be used in the methanation reactors.
As previously mentioned because the methanation reaction is strongly exothermic, heat needs to be removed between reactors. Thus, heat recover zones HR2 and HR3 are used to remove heat from the stream as it passed from reactor R1 to reactor R2 and reactor R2 to reactor R3 respectively. Any suitable exchange device can be used, preferably a shell-and-tube type wherein water can be used to remove heat from the product stream. The water can then be recycled to one or both of 12 and 23 where it can be further heated to produce superheated steam. As can be appreciated from the above and as shown in the examples discussed below, the inlet and outlet temperatures of the streams entering and exiting methanation reactors R1-R3 can be controlled by varying the percentage of syngas being delivered to each of the reactors as well as how much heat is exchanged by heat exchangers HR2 and HR3. Typically, the inlet temperature of reactors R1 and R2 will be from about 400° F. to about 450° F. with an outlet temperature of about 500° F. to about 800° F. The third reactor, which will operate at a lower temperature than that of reactors R1 and R2 will have an inlet temperature of about 400° F. and an outlet temperature of about 500° F.
In a preferred embodiment of the present invention, the step of recovering at least a part of generated heat and/or at least a part of waste heat in the regeneration zone and effectively utilizing the recovered heat is further provided. The recovered heat can be effectively utilized, for example, for drying and heating of the biomass feedstock and the generation of steam as the gasifying agent.
The product stream from the methanation unit will be comprised predominantly of methane. That is, it will contain at least about 75 vol. %, preferably at least about 85 vol. %, and more preferably at least about 95 vol. % methane. If the methane product stream is to be introduced into a natural gas pipeline, then it must meet the specification requirements for the pipeline. Such a specification for most pipelines, with respect to CO2 content will be less than about 4 volume percent. If the methane product stream is to be used for the production of methanol, then higher amounts of CO2 will be required. The product methane stream is preferably introduced into a natural gas pipeline and utilized at any downstream facility. One such facility if preferably a plant that converts the methane to syngas then to other products, such as alcohols, transportation fuels, or lubricant base stocks. If it is desired to produce syngas from the methane produced in the methanation unit M, then any suitable process can be used that convert methane or natural gas to syngas. Preferred methods include steam reforming and partial oxidation. More preferred is steam reforming. Steam reforming of methane is a highly endothermic process and involves following reactions:
Main reaction
CH4+H2O→CO+3H2 −54.2 Kcal per mole of CH4 at about 800° C. to about 900° C.
Side reaction
CO+H2O→CO2+H2 +8.0 kcal per mole of CO at about 800° C. to about 900° C.
CO2 reforming of methane: It is also a highly endothermic process and involves the following reactions:
Main reaction
CH4+CO→2CO+2H2 −62.2 kcal per mole of CH4 at about 800° C. to about 900° C.
Side reaction: Reverse water gas shift reaction
CO2+H2→CO+H2O −8.0 kcal per mole of CO2 at about 800° C. to about 900° C.
The steam reformer will preferably be one similar to reformer R hereof, which is a coiled tubular reactor. Preferred steam reforming catalysts are nickel containing catalysts, particularly nickel (with or without other elements) supported on alumina or other refractory materials, in the above catalytic processes for conversion of methane (or natural gas) to syngas is also well known in the prior art. Kirk and Othmer, Encyclopedia of Chemical Technology, 3rd Ed., 1990, vol. 12, p. 951; Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, pp. 186 and 202; U.S. Pat. No. 2,942,958 (1960); U.S. Pat. No. 4,877,550 (1989); U.S. Pat. No. 4,888,131 (1989); EP 0 084 273 A2 (1983); EP 0 303 438 A2 (1989); and Dissanayske et al., Journal of Catalysis, vol. 132, p. 117 (1991).
The catalytic steam reforming of methane, or natural gas, to syngas is a well established technology practiced for commercial production of hydrogen, carbon monoxide and syngas (i.e., a mixture of hydrogen and carbon monoxide). In this process, hydrocarbon feed is converted to a mixture of H2, CO and CO2 by reacting hydrocarbons with steam over a supported nickel catalyst such as NiO supported on alumina at elevated temperature (850° C. to 1000° C.) and pressure (10-40 atm) and at steam to carbon mole ratio of 2-5 and gas hourly space velocity of about 5000-8000 per hour.
This process is highly endothermic and hence it is carried out in a number of parallel tubes packed with a catalyst and externally heated by flue gas to a temperature of 980° C. to about 1040° C. (Kirk and Othmer, Encyclopedia of chemical Technology, 3rd, Ed., 1990, vol. 12, p. 951, Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, p. 186).
This is based on Provisional Application 60/832803 filed Jul. 24, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60832803 | Jul 2006 | US |
