Patent Application
20120108860
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The present invention relates generally to the conversion of biomass to fuel range hydrocarbons.
Due to governmental legislation as mandated in the Renewable Fuels Standards (RFS), the use of renewable energy sources is becoming increasingly necessary to reduce emissions of carbon based fuels and provide alternatives to petroleum based energy and feedstock. One of the alternatives being explored is the use of biomass. Biomass is any carbon containing material derived from living or formerly living organisms, such as wood, wood waste, crops, crop waste, waste, and animal waste.
Pyrolysis is the chemical decomposition of organic materials by heating in the absence of oxygen or other reagents. Pyrolysis can be used to convert biomass (such as lignocellulosic biomass) into pyrolysis oil or so-called bio-oil. The bio-oils obtained by pyrolysis of biomass or waste have received attention recently as an alternative source of fuel.
Generally the pyrolysis of biomass produces four primary products, namely water, “bio-oil,” also known as “pyrolysis oil,” char, and various gases (H2, CO, CO2, CH4, and other light hydrocarbons) that do not condense, except under extreme conditions. For exemplary purposes, the pyrolysis decomposition products of wood from white spruce and poplar trees are shown in Table 1.
Fast pyrolysis is one method for the conversion of biomass to bio-oil. Fast pyrolysis is the rapid thermal decomposition of organic compounds in the absence of atmospheric or added oxygen to produce liquids, char, and gas. Fast pyrolysis affords operation at atmospheric pressure, moderate temperatures, and with low or no water usage. Pyrolysis oil yields typically range from 50-75% mass of input biomass and are heavily feedstock dependent.
The major advantage of these fuels is that these are CO2 neutral and contain a very low fraction of bonded sulfur and nitrogen. Thus, they contribute very little to the emission of greenhouse gases or other regulated air pollutants.
There has been a considerable effort in the past to develop pyrolysis processes for the conversion of biomass and waste to liquids for the express purpose of producing renewable liquid fuels suitable for use in boilers, gas turbines and diesel engines.
However, pyrolysis oil obtained from biomass fast pyrolysis process is chemical complex compounds comprising generally a mixture of water, light volatiles, and non-volatiles. They are in general of relatively low quality. As fuels they have a number of negative properties such as high acidity (lead to corrosion problem), substantial water content (usually in the range of 15% to 30%), variable viscosity, low heating values (about half that of the diesel fuel), low cetane number, etc. These negative properties are related to the oxygenated compounds contained in bio-oils that result in a 45% oxygen content. In general, the pyrolysis oil has total acidity number (TAN) value of approximately 100. The desired TAN value for transportation fuel is less than 10.
There has been a considerable effort in the past to address the high TAN problem by post treatment or upgrading the pyrolysis oils before they are used as a regular fuel. Most method essentially involves the removal of oxygen. Particular attention has been focused on hydrotreating using conventional petroleum catalysts, for example, cobalt-molybdenum or nickel-molybdenum on alumina to produce essentially oxygen-free naphthas. Since pyrolysis liquids typically contain between 30 to 50 wt % of oxygen, complete removal of oxygen requires a substantial consumption of hydrogen which represents a major and prohibitive cost.
Therefore, developing a new method or process for improving quality of pyrolysis oil would be a significant contribution to the art.
Generally speaking, this invention discloses a process for producing high quality pyrolysis oil from biomass by utilizing a torrefaction pretreatment step for biomass pyrolysis process wherein the pretreatment step improves the quality of the pyrolysis oil by reducing acidity.
The disclosed process comprises at least the following steps: a) a step of subjecting a biomass feedstock to a thermal treatment in a reactor A under a torrefaction reaction condition to produce an torrefied biomass feedstock; and b) a step of pyrolyzing the torrefied biomass feedstock in a reactor B under a pyrolysis reaction condition to form a pyrolysis oil product.
The torrefaction reaction condition includes a temperature ranging from 180° C. to 350° C., a pressure ranging from atmospheric pressure to 500 psia, and a residence time ranging from 1 minute to 24 hours. The pyrolysis reaction condition includes a temperature ranging from 375° C. to 700° C., a pressure ranging from vacuum conditions (0.1 psia) to 1000 psia., and a residence time ranging from 0.1 to 200 seconds. The pyrolysis oil product according to the current invention has a TAN number between 80 and 200.
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Embodiments of the invention relate to a process to utilize a torrefaction pretreatment step for biomass pyrolysis process. This pretreatment improves the quality of the pyrolysis oil by reducing acidity. The inventive process shows that as a pretreatment to pyrolysis, resulting pyrolysis oil obtained from torrefied biomass has approximately 25% lower acetic acid than that from untorrefied biomass pyrolysis oil.
As used herein, the term “biomass” includes any renewable source (living or formerly living), but does not include oil, natural gas, and/or petroleum. Biomass thus includes but is not limited to wood, paper, crops, animal and plant fats, biological waste, algae, and the like.
According to one embodiment of the invention, there is disclosed a step of subjecting a biomass feedstock to a thermal treatment in a reactor under a torrefaction reaction condition to produce a torrefied biomass feedstock.
Torrefaction consists of a slow heating of biomass feedstock in an inert atmosphere to produce a solid with lower hemicellulose content, higher energy density, nearly moisture free (<3 wt %), and low resistance to fracture (brittle).
Any standard torrefaction reactor can be used to torrefy the biomass feedstock. Exemplary reactor configurations include without limitations augers reactor, ablative reactor, rotating cones reactor, fluidized-bed reactor, entrained-flow reactor, vacuum moving-bed reactor, transported-bed reactor, and fixed-bed reactor.
Any standard torrefaction reaction condition can be used to torrefy the biomass feedstock in the torrefaction reactor. A person skilled in the art can readily select a combination of temperature, pressure, and residence time that produces a torrefied product. In one embodiment, the torrefaction reaction condition includes a temperature ranging from 180° C. to 350° C., a pressure ranging from atmospheric pressure to 500 psia, and a residence time ranging from 1 minute to 24 hours. In another embodiment, the torrefaction reaction condition includes a temperature ranging from 220° C. to 280° C., a pressure ranging from 11 psia to 30 psia, and a residence time ranging from 5 to 20 minutes. In yet another alternative embodiment, the torrefaction reaction conditions include a temperature ranging from 180° C. to 350° C., a pressure ranging from 0.1 psia to 500 psia, and a residence time ranging from 1 minute to 24 hours.
A variety of catalysts may be used for torrefaction reaction. In some embodiments, torrefaction is carried out in the presence of a catalyst material selected from a group consisting solid acid catalyst such as ZSM5, solid base catalyst such as Hydrotalcite, silica catalyst such as Diatomite, silica-alumina catalyst such as Kaolin, Group B metal oxide catalyst such as Ammonium Molybdate, pyrolytic char and any combination thereof.
In some embodiments, the torrefaction reaction is carried out in the absence of diatomic oxygen in an inert gas atmosphere such as nitrogen, argon, steam, carbon oxides, etc. In some embodiments, the torrefaction reaction is carried out in a reducing gas atmosphere that comprises carbon monoxide. Also, torrefaction may be carried out with other reactants such as hydrogen, ammonia, etc.
The torrefied biomass according to various embodiments of the invention may be added to a pyrolysis reactor for further processing. In some embodiments, the torrefied biomass is pyrolyzed in a pyrolysis reactor under pyrolysis reaction conditions to form a pyrolysis oil product.
Pyrolysis, which is the thermal decomposition of a substance into its elemental components and/or smaller molecules, is used in various methods developed for producing hydrocarbons, including but not limited to hydrocarbon fuels, from biomass. Pyrolysis requires moderate temperatures, generally greater than about 325° C., such that the feed material is sufficiently decomposed to produce products which may be used as hydrocarbon building blocks.
Embodiments of the inventive process use any standard pyrolysis reactor providing sufficient heat to pyrolyze torrefied biomass feedstock, including without limitation, auger reactor, ablative reactor, a bubbling fluidized bed reactor, circulating fluidized beds/transport reactor, rotating cone pyrolyzer, vacuum pyrolyzer, and the like.
Any standard pyrolysis reaction condition can be used to pyrolyze the torrefied biomass feedstock in a pyrolysis reactor. A person skilled in the art can readily select a combination of temperature, pressure, and residence time that produces a pyrolyzed product. In one embodiment, the pyrolysis reaction condition includes a temperature ranging from 375° C. to 700° C., a pressure ranging from vacuum condition to 1000 psig., and a residence time ranging from 0.1 to 200 seconds. In another embodiment, the pyrolysis reaction condition includes a temperature ranging from 425° C. to 525° C., a pressure ranging from atmospheric pressure to 300 psia., and a residence time ranging from 0.5 to 2 seconds.
A variety of catalysts may be used for the pyrolysis reaction. In one embodiment, the pyrolysis reaction is carried out in the presence of a catalyst material selected from a group consisting of solid acid catalyst such as ZSM5, solid base catalyst such as Hydrotalcite, silica catalyst such as Diatomite, silica-alumina catalyst such as Kaolin, Group B metal oxide catalyst such as Ammonium Molybdate, pyrolytic char and any combination thereof.
The pyrolysis oil product obtained according to some embodiments of the present invention has a TAN number between 80 and 200. The pyrolysis oil product obtained according to some other embodiments of the present invention has a TAN number between less than 20 and 50.
The following examples of certain embodiments of the invention are given. Each example is provided by way of explanation of the invention, one of many embodiments of the invention, and the following examples should not be read to limit, or define, the scope of the invention.
The comparison study of the process of torrefaction prior to pyrolysis has been performed in a micropyrolysis unit. The reactions were carried out at torrefaction temperatures ranging from 179° C. to 321° C. and pyrolysis temperatures ranging from 379° C. to 521° C. with no catalyst loading. In addition, a wide variety of biomass was tested including red oak, switchgrass, miscanthus, and corn stover pellets. Comparative pyrolysis tests were run without the torrefaction pretreatment at the same pyrolysis temperatures.
The experimental results indicating the reduction of acetic acid in the pyrolysis product due to torrefaction are shown as follows:
1Mass of acetic acid over mass of biomass
2Acetic acid peak area over total peak area by GC/MS
3Torrefaction-pyrolysis acetic acid level relative to pyrolysis acetic acid level
The result above shows that the acetic acid concentration in pyrolysis oil products was reduced by 18 to 39% with this pretreatment than that from un-torrefied biomass. The resulting pyrolysis oil would have a similar reduction in TAN (total acid number) value as ˜80% of the TAN is due to acetic acid in pyrolysis oils.
As discussed above, the pyrolysis oil obtained from biomass fast pyrolysis process is of relatively low quality. In general, pyrolysis oil has TAN value of approximately 100. The desired TAN value for transportation fuel is less than 10.
The results above shows that using torrefied biomass as a pretreated feed for pyrolysis helps reduce TAN (total acid number) of the pyrolysis oil product. The pretreatment by torrefaction according to the current invention helps to significantly reduce the TAN value of the pyrolysis oil product by 25%. This is mainly attributed to the release of acetic acid in the torrefaction step.
The step of pretreatment of torrefaction can be easily integrated with the pyrolysis step. The pretreatment step improves the quality of the feed quality of pyrolysis step and therefore results in higher quality of pyrolysis oil product including low TAN value. In addition, such integrated process reduces the operating cost and capital investment of post treatment process.
It should be noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the content clearly dictates otherwise. Thus, for example, reference to a composition containing “a compound” includes a mixture of two or more compounds. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. It should also be noted that, as used in this specification and the appended claims, the phrase “configured” describes a system, apparatus, or other structure that is constructed or configured to perform a particular task or adopt a particular configuration. The phrase “configured” can be used interchangeably with other similar phrases such as arranged and configured, constructed and arranged, constructed, manufactured and arranged, and the like.
Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims, while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents. In closing, it should be noted that each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiments of the present invention.
| Number | Date | Country | |
|---|---|---|---|
| 61407994 | Oct 2010 | US |
