US 20080072478 A1
A process for producing a fuel product from biomass that includes providing a biomass feedstock to a high-shear mixer, mixing the biomass feedstock in the mixer in the absence of oxygen and under conditions sufficient to undergo liquefaction, and re-circulating and blending the liquefied biomass with the biomass feedstock.
1. A process for producing a fuel product from biomass, comprising the following steps:
(a) providing a biomass feedstock to a high-shear mixer;
(b) mixing said biomass feedstock in said mixer in the absence of oxygen and under conditions sufficient for the biomass to undergo liquefaction, thereby forming liquefied biomass; and
(c) re-circulating and blending at least a portion of said liquefied biomass with said feedstock biomass.
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9. A process for producing a fuel product from organic-waste material, comprising the following steps:
(a) providing a organic-waste material feedstock to a high-shear screw extruder system;
(b) mixing said organic-waste material in the absence of oxygen in said extruder at a temperature of at least 650 degrees F. and at a pressure of at least 200 psi, thereby forming liquefied organic-waste material; and
(c) re-circulating and blending at least a portion of said liquefied organic-waste material with fresh feedstock organic-waste material.
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1. Field of the Invention
Enormous quantities of wood waste material are produced both by recycling and as byproducts of industrial and commercial activity. For example, it is estimated that about 5,000 lumber mills in the U.S. continuously generate sawdust and wasted wood at a rate of approximately ten percent of the processed lumber. Similarly, over 1,100 cotton gins in the U.S. produce gin waste in the form of cotton stalks, mostly lignocellulose, which have to be plowed into the ground in order to minimize insect damage. The lignocellulosic stalks of corn, wheat, other grains, hays, grasses, sugar cane bagasse, and soybeans are also produced in large quantities but, with the exception of sugar cane bagasse, they are largely left to waste because of the expense involved in collecting them.
Much potentially useful biomass waste is also available from dead wood in forests, which is typically destroyed by insects, microorganisms, or fires. Further, national forests have accumulated an excess of living biomass in the form of dense small trees, shrubs and pine needles that should be removed to save older, large trees from being destroyed in catastrophic wild forest fires. Moreover, solid waste from municipal sewage treatment plants consists of a sludge that contains organic material and toxic constituents that constitute a disposal problem. Similar wastes are produced by nearly 100,000 dairy operations in the U.S., which must continuously dispose of a mixture of bedding and manure, all organic material. Additional organic-waste material is produced in large quantities as waste from cattle, hog, chicken and turkey farms. Finally, it is estimated that approximately 280 million automotive tires are discarded annually in the U.S., ranging from 20 to 1,000 pounds in weight, which also represents a serious, continuing disposal problem.
Most of this waste material is currently being disposed of in landfills around the world. Approximately 300 million tons of solid waste is placed in about 3,500 landfills around the U.S. alone every year, about 70-80 percent of which is organic matter. Thus, it is clear that the magnitude of these organic wastes constitutes a serious environmental problem. As a result, increasingly stringent regulation of waste disposal practices are being imposed to satisfy environmental standards. Therefore, reutilization of these materials has become an important component of prudent industrial policy.
With the advent of energy sources being limited by OPEC in the 1970's, three programs were initiated to develop a source of domestic oil. They were 1) coal liquefaction, 2) oil shale mining, and 3) biomass liquefaction. Once the oil crisis calmed, these programs were scaled back. In the case of biomass liquefaction, two projects survived.
The first was run by the U.S. Department of Energy in Albany, Oreg. in the late 1970's. The liquefaction process consisted of mixing a carrier oil with low wood flour concentrations and introducing this slurry into a closed loop heated/pressurized system. The slurry was recycled at various pressures and temperatures for times from 12 hr. to several days. A key component was the use of a piston pump to pump and pressurize the slurry. While the oil made had O2 levels of 8 to 12% versus the 53% of the wood flour, two major problems were noted. The first was the inefficiency of the process. It took approximately three times as much energy as was gained. In addition, cycle times of days dictated that a very large plant must be utilized for the process to be economically viable. The second problem was the tendency of the piston pumps to plug on the wood flour. Thus, this liquefaction process was not reliable or economically sound.
The second project was funded by the U.S. Department of Energy with overview by Battelle Northwest. The main research was conducted by the Department of Chemical Engineering of the University of Arizona in Tucson. The pump problems experienced in the first project were solved by using a plastic extruder to pump and pressurize the slurry. Optimum slurry levels of 50/50 (wood mass/carrier oil mass) were found on single screw extruders and 65/35 on twin screw extruders (versus 12% wood mass/88% carrier oil mass used in the Albany project described above). Lower wood levels were easier to pump, while higher wood levels increased conversion quantities. A total of 57 runs explored process variables to determine their influence. Pressures up to 3000 psi and temperatures up to 400 degrees C. were evaluated. Superheated steam was injected into a tubular reactor just after the extruder. In addition, CO was injected to increase the hydrogenation of the created oil to make it more like petroleum. While these studies resulted in a better understood and efficient liquefaction process, the overall efficiency was still not economically viable enough for commercialization.
A primary goal of this invention is the use of liquefied biomass to induce more efficient breakdown of biomass in the production of a combustible energy product. More particularly, the invention relates to a process for producing a fuel product from biomass that includes providing a biomass feedstock to a high-shear mixer, mixing the biomass material under conditions sufficient for the material to undergo liquefaction, and re-circulating and blending at least part of the liquefied biomass material with the feedstock.
In a preferred embodiment of the invention, the process of the invention takes advantage of the reactive nature of liquefied biomass material to induce the breakdown of a new feedstock of biomass by re-introducing the liquefied biomass back into the high-shear mixer.
According to these and other embodiments, the present invention involves the degradation of feedstock biomass, which preferably is organic-waste material, with a liquefied biomass produced by the direct liquefaction or fast pyrolysis of the feedstock biomass material. Such liquefied biomass is produced according to known liquefaction processes in the absence of oxygen at typical temperatures between about 230 and 370 degrees C. (about 450-700 degrees F.) and typical pressures between 200 and 3,000 psi. Alternatively, a liquid biomass product may also be produced by the process of fast pyrolysis, which is instead carried out at atmospheric pressure and at temperatures of 400-600 degrees C. (about 205-315 degrees F.) with a residence time of about two to five seconds, or at temperatures greater than 600 degrees C. with residence times of less than 0.5 seconds.
If desired, the liquid biomass so produced by either direct liquefaction or fast pyrolysis may be mixed with additives (such as the heavy ends of fast pyrolysis, petroleum asphalts, natural bitumens, oils from tar sands, oils from shales, heavy ends of coal liquefaction, petroleum pitch, and petroleum coke derived from petroleum delayed coking processes) in order to modify its characteristics to meet specific needs of particular applications, and the resulting mixture may further blended with one or more additional materials of choice.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.
This invention is based on the idea of utilizing liquid biomass produced by direct liquefaction or fast pyrolysis in a high-shear mixer to more efficiently produce combustible fuels. While other reactive liquids also can aid in liquefaction (such as unsaturated fatty acids/derivatives), utilizing liquid biomass to aid in the breakdown of solid biomass feedstock is more efficient.
As used in this disclosure, the term biomass refers in general to any organic-waste material that has been found to be suitable for conversion to liquid form by a process of liquefaction or fast pyrolysis. In particular, and without limitation, such biomass and organic-waste material are defined as organic material containing various proportions of cellulose, hemicellulose, and lignin; to manures; to protein-containing materials, such as soybeans and cottonseeds; and to starch-containing materials, such as grain flours. Hemicellulose is a term used generically for non-cellulosic polysaccharides present in wood. Finally, organic-waste material is intended to include rubber waste material (such as from tires), and bituminous wastes (such as from coal fines).
The term liquefaction, as used in this disclosure with reference to biomass, refers to direct-liquefaction and fast-pyrolysis processes by which a solid biomass is converted into liquid form. Such processes are well known in the art. For convenience, the liquid materials formed by liquefaction are referred to in the art and herein as “liquefied” materials, as distinguished from “liquified” materials formed by condensation from a vapor state. Direct-liquefaction processes provide high yields of liquid products from biomass by the application of sufficient pressure, typically in the range of 200 to 3,000 psi, in the absence of air, and at approximate temperatures in the 230-370 degree C. range. Fast pyrolysis processes, which also produce a liquid product from biomass, are instead carried out at atmospheric pressure and at temperatures of 400-600 degree C. with a residence time of about two to five seconds, or at temperatures greater than 600 degree C. with residence times of less than 0.5 seconds. It is noted that, in contrast, indirect-liquefaction processes first convert biomass to gases, which are then caused to react catalytically to produce liquids. The scope of this invention does not include liquids obtained by indirect liquefaction.
A key difference between the invention and the related art lies in the frictional heat generated in the high-shear mixer (e.g., in an extruder with the high shear screw). As used herein, “high shear” means a compression ratio that ranges from between about 1.25:1 to 4:1. In other words, the substance(s) being acted upon by, for example, an extruder screw, is compressed from between about 0.8 to about 0.25 its original volume. Thus, both heat and pressure in a highly localized area are present so that slow heat transfer from heating elements located outside the mixer is not a hindrance. The nature of a high shear screw can further be descriptively defined in two stages. The first allows movement of the free flowing biomass (e.g., wood chips) into a compressed area that pushes into stage two. The second stage is a high shear rotation of the extruder screw or similar device with a very small gap over a very small length of the screw. Depending on the design of the extruder or similar device, too much compression or too long a section of high sheer compression will cause the screw to lock up or stall the motor. Too large a gap or too slow a rotational speed will not result in liquefied biomass.
A second key difference is the returning of liquefied biomass to the mixer to react in the high-shear zone immediately with the solid biomass feedstock (e.g., wood) to form more liquefied biomass.
In a preferred embodiment, the high-hear mixer includes a screw extruder system. As well understood in the art of extrusion, very viscous liquid materials can barely be poured from a container by gravity. Hence, when such materials are placed under high shear rates (wherein one layer is moved rapidly away from its adjacent layer), internal friction is generated within the viscous liquid, which in turn generates heat. Thus, the energy of the electric motor turning the extruder screw under high torque is converted into frictional heat, which causes the viscous liquid to rise in temperature with a resulting lower viscosity.
A pilot plant with two different biomass processing designs has been constructed to overcome several of the deficiencies of the earlier liquefaction processes. The first was a loop reactor which utilized previously created liquefied biomass to preheat the incoming biomass feedstock. The second was a loop reactor as above with external heating elements that provided a preheating step. The combination of the preheating and recirculation of the liquefied biomass created a “digestion effect” missing in earlier designs/processes. In other words, the incoming feedstock biomass is degraded more readily because the liquefied biomass (and especially the pre-heated liquefied biomass) is very reactive. This allowed much greater output rates and lower energy costs per barrel, since the mixer has to utilize less external heat to attain the threshold temperature at which liquefaction takes place. An additional benefit is the flashing of any remaining water in the biomass.
In contrast, previous liquefaction processes needed extra heat (such as from superheated steam) to achieve the same results, albeit at a much slower rate. Since commercial plants will use chopped biomass (e.g., wood chips) rather than “flour,” the cost associated with grinding is reduced or eliminated. The pre-digested chip is easily smeared/sheared in the mixer. In addition, since the new treated/exposed surface area of is virtually the same as flour, the chip diameter can be increased to match the feed section of the size of the mixer used. High shear screws, such as those used in screw-type extruders, can accelerate the break down of the chips to liquefied biomass. The optimum efficiency (energy out/energy in) for the process of the invention has been found to be 4/1 vs. 1/1 and 0.3/1 for previous processes.
The aggressiveness of reactivity of the liquefied biomass grows at higher temperatures, with ester, alcohol, acid, and anhydride functional groups all thought to play a role. The use of high shear extruder screws increases the rate of production of bio oil by dramatically increasing the wood surface area which helps more quickly expose the wood to the ambient heat. In addition, the work shear produced by the screw creates extra heat above that supplied by the external heater source alone.
A preferred continuous process would recycle bio oil (liquefied biomass) at >600 degrees F and infuse it with a steady stream of newly metered biomass (e.g., wood) at 50-350 psi. This defines a continuous digestion step. The excess oil would be collected as final product prior to the slurry being introduced into the extruder. The wood/oil slurry preferably is heated to a temperature over 250 degrees. The extruder would heat the mix to >600 deg. F. and pressurize to over 1000 psi. The mix would then be depressurized to allow the steam and CO2 to separate from the bio oil and be used to dry the incoming wood. Alternatively, the steam could be used to perform mechanical work, such as will a steam engine. A less efficient process (semi-continuous batch) would still recycle the liquefied biomass but would use alternating vessels to mix the hot oil and dry wood. Thus, while the slurry from one is fed to the extruder, the other(s) would be filled with hot bio oil and wood. When the first vessel is empty, the activity would reverse.
The rate of conversion is a function of rate of digestion, temperature, pressure, shear level, and residence time (length and linear rate) in the reactor. The equilibrium of conversion is dependent on these and supplemental hydrogenation. For example, introducing CO to the mix to react with the water in the wood to produce hydrogen via the water shift reaction.
Moreover, the chemical reduction of the liquefied biomass (i.e., hydrogenation) in a secondary step preferably is performed to get maximum heat content and distillation potential of the produced fuel.
As seen in
As well understood in the art of extruders, heating takes place in the feed and compression zones of the extruder by surface friction upon the wall of the extruder barrel 34. Liquefaction of the biomass feedstock 36 is attained by providing a suitable air environment (pressure devoid of oxygen) and temperature by heat transfer through the barrel 34 using sources of heat generation, such as electrical heater 44. Other, indirect sources of heating may be used, such as induction or radiant heat, and the heat provided by the heated liquefied biomass 40 also is utilized.
As the biomass feedstock 36 and liquefied biomass 40 is heated and mixed in the extruder 30, internal friction, called viscous dissipation, further heats the mixture. The additional heat energy is created by the electrical power of the motor driving the screw 32 and constitutes energy converted into frictional heat. The helical screw 32 of the extruder 30 also generates pressure by exerting a drag flow upon the mixture. The screw 32 furthermore moves the biomass material down the barrel 34 and to a supply line 46 and/or storage container. As well understood in the art, viscous dissipation is the process by which heat is generated in a highly viscous fluid by the dissipative action of shearing forces acting on the fluid, such as can be produced in extruders. See R. B. Bird, W. E. Stewart and E. N. Lightfoot, “Transport Phenomena,” J. Wiley & Sons (1960), pp. 276-279; and S. Middleman, “Fundamentals of Polymer Processing,” McGraw-Hill Book Co. (1977), pp. 131-137 and 371.
Another embodiment of an extruder for use in a process according to the invention is shown in
The present invention is based in part on the fact that all organic-waste materials also contain reactive chemical groups. Lignocellulosic material, the major component of trees, shrubs, stalks, grasses, and growing vegetation in general, contains cellulose and hemicellulose molecules with two reactive hydroxy groups. These groups react readily with other organic groups, especially aldhehydes. Therefore, such feedstock material is suitable raw material for combination with liquefied biomass, which leads to a more efficient liquefaction process.
The liquefied biomass produced by direct liquefaction can have different chemical compositions and properties, depending on the liquefaction conditions. For example, different tar-like products were obtained by the direct liquefaction of Douglas Fir wood operating at about 3,000 psi and temperatures in the 324-350 degree C. range (about 615-660 degree F.) in the presence of a synthesis gas (67% carbon monoxide and 33% hydrogen). The resulting products varied from 3.2 to 18.1 wt percent in oxygen content and from 13,300 to 16,530 Btu/lb in heating value. Obviously, different raw materials would also yield different liquefied biomass, which may vary in consistency from tar-like products to light oils. As one skilled in the art would readily appreciate, similar differences exist in the liquefied biomass obtained by fast pyrolysis.
Thus, it is well known that any biomass, especially lignocellulosic material, can be converted into a heavy tar or oil by applying heat and pressure in the process, while retaining most of the heating value of the biomass feedstock in a more concentrated form. Water and carbon dioxide are driven off the biomass to make it more like a petroleum crude oil. For the purposes of this invention, the temperature and pressure can be adjusted to give a very viscous liquid product, which can be pumped at 150 degrees C. (about 302 degrees F.) but is a brittle solid at ambient temperatures. Test data show that the high molecular weights of the cellulosic and hemi-cellulosic portions of the biomass are degraded to lower molecular weight aromatic and aliphatic ethers, alcohols, hydrocarbons and a variety of other chemicals.
The following examples illustrate the invention with regard to organic-waste material.
A mixture of 50% liquefaction biomass oil and 50% white pine screened to feed a 1.25 inch Davis Standard extruder was heated to 300 F for 2 hours. The mixture was then run through the extruder with a 4:1 compression ratio and converted to any oil with a heat of combustion of 14,000 BTU's. With 2 parts recycled liquefied biomass (the produced oil) and two parts wood by weight, approximately one part of new oil was produced (i.e., oil in addition to that re-circulated back to mix with new feedstock).
A run similar to Example 1 was performed, except with dried grass as the feedstock. The ratio of 2/2 (oil/feedstock) gave 3 parts oil, with 2 parts oil returned to mix with the feedstock biomass and one part stored for combustion as a fuel. The oil had a heat of combustion of 12,700 BTU's.
A mixture of 2 parts wood (dry pine) and 1 part re-circulated bio oil gave 1 part of new bio oil using a 28 mm Werner & Pfleiderer twin screw extruder under the conditions used in Example 1.
Note that in all cases, the oxygen level goes from approximately 53% net weight in the solid biomass feedstock to between 15 and 28% in the liquefied biomass (oil).
A repetition of the runs described in Examples 1-3 with a low-shear screw and low compression ratio failed to make any liquefied biomass, even at a barrel temperature of 650 F.
A repetition of the runs described in Examples 1-3 with unsaturated fatty acids instead of re-circulating bio oil also produced new oil, but not as efficiently.
A mixture of 49% liquefaction biomass oil, 1% tertiary butyl peroxy benzoate (TBPB), and 50% white pine screened to feed a 1.25 inch Davis Standard extruder was heated to 170 F for 2 hours. The mixture was then run through the extruder with a 4:1 compression ratio and converted to an oil with a heat of combustion of 14,000 BTU's. With 2 parts recycled liquefied biomass (the produced oil) and two parts wood by weight, approximately one part of new oil was produced (i.e., oil in addition to that re-circulated back to mix with new feedstock). Alternatively, the TBPB can introduced into the high-shear area of the mixer as a coating prior to, or concurrent with, the addition of the oil and wood.
Various changes in the details and components that have been described may be made by those skilled in the art within the principles and scope of the invention herein described in the specification and defined in the appended claims. Therefore, while the present invention has been shown and described herein in what is believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent processes and products.