US20090158663A1

US20090158663A1 - Method of biomass gasification

Info

Publication number: US20090158663A1
Application number: US11/962,245
Authority: US
Inventors: Gregg Anthony Deluga; Vladimir Zamansky; Parag Prakash Kulkarni
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2007-12-21
Filing date: 2007-12-21
Publication date: 2009-06-25

Abstract

A method for the gasification of biomass, wherein the biomass feedstock is combined with a light hydrocarbon composition to form a slurry; followed by feeding the slurry to a gasifier to produce a fuel gas. In another embodiment, a method for the gasification of biomass is described. The method includes the steps of combining a biomass feedstock with water to form a slurry; feeding the slurry to a gasifier to produce a fuel gas; and injecting a light hydrocarbon into the gasifier, to generate gasification temperatures greater than about 900° C., by partial or complete combustion of the light hydrocarbon. In some other embodiments, the biomass gasifier product gas is coupled to a reformer, wherein a light hydrocarbon is injected to generate high temperatures.

Description

BACKGROUND

This invention generally relates to bio-fuels, and in particular to a method of carrying out biomass gasification.
Biomass is receiving increased attention as a renewable energy source, in this era of emphasis on energy security and increasing environmental concerns.
Multiple ways of utilization of biomass are being tried. Traditionally, the most common method of using the biomass has been to burn it, to provide heat energy. This heat energy is then used directly, or used to produce steam.
It has been demonstrated that gasification of biomass is a better option than burning it. Gasification produces a clean burning gas that can be easily utilized. The product gas can be used as a feedstock for production of value added fuels and chemicals. The burning of this gas in internal combustion engines or turbines offers higher efficiencies, as compared to steam devices.
Historically, gasification technology was used to convert coal into a combustible gas. This gas was in some places also distributed as coal gas or town gas. Biomass was traditionally used for direct firing, or was carbonized to convert it into charcoal. Charcoal was then further used as fuel directly, or was gasified to produce a combustible gas. Direct gasification of biomass has also been attempted with different gasifier configurations, in an attempt to produce a combustible gas from the biomass. Although gasification of biomass is a desired option, there are many difficulties encountered, due to the inherent nature of biomass. Moreover, there are operational difficulties associated with the process.
Biomass encompasses a wide spectrum of material. Within different types of biomass, on the elemental level, there is a wide variation in carbon, hydrogen, and oxygen content. Also, there are other components such as moisture and ash, and the content of these components varies widely in different feedstocks. These components do not add to the calorific value of the feedstock. Thus, high ash content, high moisture content, and high oxygen content, implicitly suggest feedstocks with lower calorific value as compared to coal—the most common solid fuel. There are many differences between coal and biomass as a feedstock for gasification. As an example, due to the higher moisture content often present in biomass, a considerable amount of energy is required for drying the biomass.
Biomass also has a larger amount of total volatile matter, as compared to coal. During gasification, the volatile matter is released at relatively lower temperatures. A large part of this volatile matter is responsible for the tar formation during gasification of biomass. Tar levels in the product gas also depend on the gasifier configuration. In an updraft fixed bed gasifier that operates between about 300° C. and about 1000° C., the product gas contains up to 35000 ppm tar. In a downdraft configuration, the tar level in product gas is comparatively low, but still contains about 500 and about 1000 ppm tar. Depending on the gasifier configuration, tars tend to deposit on the walls of gasification equipment, or get transported downstream with the product gas. The tar deposits create the need for frequent maintenance, and may also reduce the operating life of the gasification equipment. The carryover of tar can deposit on and block filters, pipes, valves and turbochargers, leading to a decrease in performance. In some instances, elaborate cleaning systems are required to address these problems.
Most of the biomass gasifiers commercially available today operate at or near atmospheric pressures. If these gasifiers are operated at elevated pressures, higher overall thermal efficiencies can be achieved. However pressurized biomass gasifiers have their own set of problems. One significant problem relates to the feeding of the biomass into the gasifier. The characteristics of biomass, such as low density, varying particle size, and varying moisture content, lead to problems like clumping and interlocking of particles of different sizes.
Handling and transportation of solid biomass feedstock may involve transportation through bins or bags. Frequently, mechanical transportation such as pneumatic transport, or mechanical transport such as belt conveyer, chain conveyer, screw conveyer, bucket elevator, or vibrating pans, are employed. Such transport systems are usually adequate under atmospheric pressure conditions. However if the gasification of biomass is carried out at an elevated pressure, it may be necessary to operate the biomass feeding system at a pressure higher than the gasifier itself. This may require use of systems such as lock hoppers or piston feeders. Many practical difficulties have been faced in maintaining such feeding systems at high pressures. Lock hopper systems do not have large turndown capability, and require the use of a considerable quantity of inert pressurizing gas for the lock hoppers or piston feeders.
Hydraulic conveying of solid particles has been successfully deployed for coal gasification at higher pressures. This typically involves forming a slurry of coal with water, and feeding the same to the gasifier. Slurry feeding has an advantage in that the solid suspension can be directly pumped into the gasifier. However, water slurries involve energy penalties of evaporating the water in the gasifier. A coal gasification unit is able to sustain such energy penalties due to the inherent, high energy content of coal. However, if a water slurry is used for feeding biomass feedstock to a pressurized biomass gasifier, it may not sustain the gasification, due to the lower energy content of biomass feedstock.
Thus, there is a need to develop new methods for efficiently gasifying biomass—especially at elevated pressure—while minimizing problems like tar formation.

BRIEF DESCRIPTION OF THE INVENTION

Briefly, some embodiments of the present invention provide for a method of biomass gasification wherein the biomass feedstock is combined with a light hydrocarbon composition to form a slurry, and the slurry is then fed to a gasifier to produce a fuel gas.
According to some embodiments of the present invention, a method for the gasification of biomass comprises the steps of combining a biomass feedstock with water to form a slurry, feeding the slurry to a gasifier to produce a fuel gas, and injecting a light hydrocarbon into the gasifier, to generate gasification temperatures greater than about 900° C., by partial or complete combustion of the light hydrocarbon.
According to some other embodiments of the present invention, a method for the gasification of biomass comprises the steps of gasifying a biomass feedstock in a gasifier to produce a product gas, feeding the product gas to a reformer section, injecting a light hydrocarbon into the reformer section, and generating temperatures of greater than about 1000° C., by partial or complete combustion of the light hydrocarbon, in the reformer section.

DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout the drawing, wherein:

FIG. 1 is a schematic of the process of biomass gasification according to some embodiments of the present invention.

DETAILED DESCRIPTION

As discussed in detail below, embodiments of the present invention describe a method for biomass gasification that allows for pressurized operation, with reduced tar formation.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.
Various steps in the process are represented in FIG. 1. The method of biomass gasification is generally shown as 10. Biomass feedstock 20 undergoes a feed preparation step 30, where it undergoes preprocessing. Step 30 may be an optional step in some embodiments. The pre-processed biomass feedstock 20 is then mixed with a light hydrocarbon composition 50, in slurry preparation step 40. Slurry preparation step 40 may be optional in some embodiments. The slurry is then directed to a site for gasification step 60. Gasification operation 60 broadly involves thermal processing of the feedstock.
Thermal processing involves processing of biomass feedstock by processes such as pyrolysis, partial oxidation, complete oxidation, or a combination of these processes. The term “Pyrolysis” refers to the heating of feedstock in the absence of any oxygen. “Partial oxidation” refers to the heating of feedstock in the presence of sub-stoichiometric oxygen. “Complete oxidation” refers to the heating of feedstock in the presence of stoichiometric or excess amounts of oxygen. Depending upon the configuration of the reactor in which the thermal processing is carried out, more than one of these reactions may be taking place in a single reactor. Hence, although the term gasification used herein refers predominantly to oxygen-starved reactions such as pyrolysis and partial oxidation, the conditions for complete oxidation may also be present in the gasification reactor. Gasification also involves reaction of the feedstock with steam.
In some embodiments, the light hydrocarbon 50 may be directly supplied to the gasification step 60. The product gas 65 from the gasification step 60 has a fuel value due to presence of carbon monoxide and hydrogen, which are products of partial combustion and pyrolysis. In some embodiments, the product gas 65 is taken through a reforming section 70, which is optional in some embodiments. At least a portion of light hydrocarbon 50 is directly introduced in the reforming section 70. The product gas out of reformer is referred to as fuel gas 75 hereinafter. The fuel gas 75 is further used in applications 80, which will be described below. The dotted lines connecting light hydrocarbon 50 to the blocks 40, 60 and 70 indicate that, light hydrocarbon 50 may be used in at least one of these sections, and the process described by the embodiments of the present invention employs one or more of these dotted line connections.
The term “biomass” is loosely used to cover many renewable energy sources. However, the term “biomass feedstock” 20, or “biomass” or “feedstock”, as used hereinafter, generally includes only the materials derived from plants. Thus, the term biomass feedstock 20 as used in this description and the appended claims includes materials such as wood and tree based materials, forest residues, agricultural residues and energy crops. The wood and tree materials and forest residues may include wood, woodchips, saw dust, bark or other such products from trees, straw, grass, and the like. Agricultural residue and energy crops may further include short rotation herbaceous species, husks such as rice husk, coffee husk, etc., maize, corn stover, oilseeds, residues of oilseed extraction, and the like. The oilseeds may be typical oil bearing seeds like soybean, camolina, canola, rapeseed, corn, cottonseed, sunflower, safflower, olive, peanut, and the like.
The feedstock 20 may also include material obtained from agro-processing industries such as the oil industry. Thus, the feedstock 20 can include a deoiled residue after extraction of oil from the oil seeds. These could be, for example, a deoiled soybean cake, deoiled cottonseed, deoiled peanut cake, and the like. The feedstock can include the gum separated from the vegetable oil preparation process—e.g. lecithin in the case of soybean. Gums are polysaccharides of natural origin, capable of causing a large viscosity increase in solution. Other types of gums, such as agar, dammar gum, Arabic gum, which may not be derived from oils, but directly from plants, may also be used as feedstock 20. The feedstock 20 may also include non-seed parts of trees that bear oil—coconut, palm, avocado, and the like. The feedstock 20 may include inedible varieties like linseed, castor, and the like. The biomass feedstock 20 may also include other tree-based products such as shells, e.g., coconut shell, almond shell, walnut shell, sunflower shell, and the like. Cellulosic fibers like coconut, jute, and the like, may also constitute all or part of feedstock 20. The biomass feedstock 20 may also include algae, microalgae, and the like. It could also include agro-products after preliminary processing. As an example, this might include feedstocks such as bagasse (obtained after juice removal from sugarcane), cotton gin trash, and the like. Thus, the biomass feedstock 20 as used herein includes materials that are formed as a result of photosynthesis. The feedstock 20 includes only plant biomass and does not generally include waste materials such as sewage or industrial waste.
Thus, although a closed set, biomass feedstock 20 involves a number of species. These species differ widely in various constituents, calorific value, physical characteristics and other features. As an illustration, the ash content in rice husk can be about 15-25%, but is only about 2% or less in wood. The moisture content of wood varies from about 10-60%, depending on the source, while the moisture content of groundnut shells is about 2-3%. The calorific value of these feedstocks also differs widely—e.g. the heating value of wood varies between about 10-20 MJ/kg, that of rice husk varies between about 13-14 MJ/kg, while that of bagasse is between about 8-10 MJ/kg.
In many instances, the presence of glycerides is another very significant characteristic of these plant-derived feedstocks 20. Glycerides are the esters of glycerol, the trihydric alcohol. Depending on the number of fatty acids glycerol is esterified with, the glycerides are classified as mono-, di- and tri-glycerides. Thus, triglycerides are compounds wherein glycerol is esterified with three fatty acids. The chemical formula for triglycerides is RCOO—CH₂CH(—OOCR′)CH₂—OOCR″, where R, R′, and R″ are long alkyl chains. The groups R, R′and R″ may all be different, two of them may be same, or all three may be same. As an illustration, soybean comprises glycerides of palmitic acid, stearic acid and arachidic acids, while palm contains glycerides of palmitic and oleic acid, and the like. In some embodiments, the feedstock 20 may include free fatty acids in addition to the glycerides. The feedstock 20 may contain sterols e.g. rapeseed and corn contain high amounts of campesterol and sitosterol.
In another embodiment, the biomass feedstock 20 includes plant-derived materials with a moisture content of less than about 20%. This includes multiple feedstocks, including but not limited to, rice husk, coffee husk, coconut shell, straw, and oil seed residues. The oil seed residues may in turn include various oil seeds, such as but not limited to, soybean, camolina, canola, rapeseed, corn, cottonseed, sunflower, safflower, olive, peanut, and the like.
In case of soybean, for example, the seeds are roasted to remove the hull. Oil is then extracted by pressing or other mechanical operations, followed by, or in parallel with, solvent extraction operations. The gum-like residue of the extraction, comprising lecithin and other hydrocarbons, can be employed as a feedstock for gasification. The process for extracting oil from other oil seeds is similar, and the residue so obtained will generally contain moisture at a level less than about 20%, by weight.
In another embodiment, the biomass feedstock 20 includes materials derived from plants, with an oxygen content of less than about 60 wt. %. Oxygen is a component that does not contribute to the calorific value of the feedstock. It has been observed that a higher percentage of oxygen—typically greater than about 10% can result in a higher percentage of tars. As per this embodiment, the biomass feedstock 20, preferably with an oxygen content of about 40 wt. % or less, is used for gasification. In a more preferred embodiment, the biomass feedstock 20 with an oxygen content of about 20 wt % or less is used. In yet another embodiment, a biomass feedstock 20, with an oxygen content of about 25% or less is used.
In one embodiment, the feedstock 20 includes at least about 30% plant biomass. In another embodiment, the feedstock includes up to about 50% plant biomass. In yet another embodiment, the plant biomass content could be between about 50-100%.
Other physical characteristics of feedstock 20 that affect thermal processing include shape and size. The shape and size varies, depending upon the source of feedstock 20. It is often desirable to have a sizing operation before any thermal processing of the feedstock 20.
The step 30 of feed preparation may involve more than one unit operation. The most common feed preparation operation is usually sizing. The sizing operation converts the biomass feedstock to a particle size range appropriate for thermal processing. The sizing operation may include cutting, grinding, attrition, shearing etc. In some instances the size reduction may be followed by the sorting of particles according to size ranges by use of operations such as sieving. The size of the particles used in thermal processing operations typically varies from about 50 mm (2 inches) to lower size levels, e.g., in the millimeter-to-micrometer range. The lower particle size results in better reaction rates in thermal processing operations. However, more energy is required for the size reduction itself. Thus, there is a balance involved in the particle size used for thermal processing, and the power required for size reduction. In the present invention, an average feedstock particle size of about 0.1 mm to about 20 mm is often preferred. In one embodiment, the particle size of feedstock 20 ranges from about 0.0015 mm to about 4 mm. In another embodiment, the particle size of feedstock 20 ranges from about 0.1 mm to about 2 mm. In another embodiment, the particle size ranges from about 1 mm to about 5 mm, and in some other embodiments, the particle size ranges from about 5 mm to 20 mm. In the case of feedstocks such as sawdust, the particles are of a lower size than the preferred size range. In such cases, the feedstock may be subjected to agglomeration, densification or briquetting, to meet the required size and density criteria, by increasing the average size of the feedstock particles.
Apart from the sizing operation, the feed preparation step 30 may involve other pre-processing steps, such as, but not limited to, moisture removal, volatile reduction, and carbonization. Drying or moisture removal can be a separate preprocessing step in locations where waste heat is available. The step is especially preferred in the case of high-moisture content feedstock, such as algae. In the case of other feedstocks with less than about 20% moisture, sufficient moisture removal can often occur in the pre-heating zone of the reactor in gasification step 60.
In some instances, the feed preparation step 30 may involve carbonization, wherein the feedstock is heated to a medium temperature of about 200° C. to about 400° C. This removes substantially all of the moisture and low volatile compounds from the feedstock 20. The volatiles removed from the biomass feedstock 20 may be condensed to a liquid—sometimes referred to as “pyrolysis oil”. This material has a good energy value that may be subsequently recovered. Usually, the volatile compounds in the biomass feedstock are responsible for the tar formation. Hence, the removal or reduction in quantity of the volatiles results in the desirable reduction of tar during gasification step 60.
As discussed earlier, there are difficulties involved in direct solid feeding to a pressurized gasifier. Hydraulic conveying and slurry feeding may be more readily carried out, but the slurrying of biomass feedstock with water involves high ‘energy penalties’ for evaporating the water used for slurrying. According to one embodiment of the current invention, as shown in FIG. 1, the biomass feedstock 20, after being treated/prepared in step 30, is then mixed with a light hydrocarbon 50, to form a slurry 55. The slurry 55 is then fed to the gasification step (unit) 60. The slurry feeding makes the feedstock “pumpable”, and reduces the problems discussed earlier. Slurrying with a light hydrocarbon, as provided by one embodiment of this invention, considerably improves the feeding process. The energy involved in the evaporation of light hydrocarbon 50 is much lower than energy required for the evaporation of water. The light hydrocarbon also undergoes partial or complete oxidation in the subsequent gasification step, to provide extra heat energy, resulting in higher temperatures in the gasification reactor.
The light hydrocarbon selected may be obtained from a petroleum refining operation or from any other source. In one embodiment, the light hydrocarbon 50 may be a petroleum fraction that distills up to about 160° C. Typically, this includes fractions commonly known as gasoline and “light ends”, which generally comprise C₁-C₇compounds, i.e., compounds involving 1 to 7 carbons in the hydrocarbon chain. In one embodiment, the liquid hydrocarbon includes lower boiling naphtha, and liquefied petroleum gas (LPG). In one embodiment, the light hydrocarbon 50 is a compound selected from alkanes or paraffins, from methane (C₁) to normal heptane (C₇). The hydrocarbon compound 50, may be either straight-chain or branched.
In another embodiment, the light hydrocarbon comprises saturated compounds, such as cycloalkanes. Non-limiting examples include cyclopentane, cyclohexane, or derivatives of these, e.g. methyl cyclopentane. In another embodiment, the light hydrocarbon 50 is a compound selected from alkenes or alkynes such as ethylene, propylene, butylene, and the like. In yet another embodiment, the light hydrocarbon 50 comprises aromatic compounds such as benzene, alkyl benzenes (e.g., toluene and xylene), naphthalene, and the like.
According to another embodiment of the current invention, the light hydrocarbon 50 may be a biologically-derived liquid fuel, such as but not limited to, pyrolysis oil and biodiesel. Generically, the light hydrocarbon 50 is obtained as a result of processing triglycerides.
In this embodiment, the slurry preparation step 40 provides a slurry 55 of biomass feedstock 20 with light hydrocarbon 50. Slurry 55 is a suspension of the feedstock particles in the liquid fuel. The slurry preparation step 40 usually involves intimate mixing of particles of the biomass feedstock 20 and the light hydrocarbon 50. The particles may be dispersed in the liquid by the action of an agitator or other mixing device. The slurry 55 is formed in a manner, which ensures that the feedstock particles are uniformly distributed in the suspension. Additives such as stabilizers, viscosifiers, suspension agents, or viscoelastic surfactants may be added to slurry 55 to maintain the suspension, and to improve its transport characteristics. In one embodiment, the slurry 55 has a solid content of about 5% to about 70%. In some embodiments, the solid content of the slurry is about 30% to about 90%. In some preferred embodiments, the solid content of the slurry is between about 40% and about 60%.
The gasifier in the gasification section 60 may be a fluidized bed gasifier, an entrained flow gasifier, or any other gasifier that can be configured to accept slurry feeds. Suitable gasifiers are known in the art. As an example, a description of some of them can be found in the “Kirk-Othmer Encyclopedia of Chemical Technology”—Fifth Edition, Volume 3, published in 2004, Biomass Energy, pages 693-699, which is incorporated herein by reference. For example, in an entrained flow gasifier, the gasifying agents such as oxygen, air, steam or combination of these fluidize the feedstock. The gas velocities are increased beyond minimum fluidization velocity, so that the materials are carried through the reactor. As described earlier, a pressurized biomass gasifier provides for better overall efficiencies. Accordingly, the gasifier is operated at high pressures, typically between about 10 bar and about 40 bar. In another embodiment, the gasifier is operated at about 30 bar.
During gasification, the light hydrocarbon 50 undergoes partial and complete combustion, due to the high temperature conditions in the gasifier. Thus, the regular in-flow of combustible light hydrocarbons with the slurry assists in sustaining the high temperatures in the gasifier.
The biomass component of slurry 55 is also partially oxidized in the gasifier, producing syngas, which comprises carbon monoxide (CO) and hydrogen (H₂). The syngas may also include carbon dioxide (CO₂) and other compounds formed by reactions taking place in the gasifier. The tar produced during gasification of the biomass feedstock 50 undergoes cracking, due to the high temperature prevalent in the gasifier. The cracking process converts tar into smaller hydrocarbons, which in turn are subjected to cracking or gasification, further adding to the syngas production. In the presence of light hydrocarbons, the gasification temperature is increased, and this increases the rate of cracking. Thus, the amount of tar coming out of the gasifier is reduced, and the product gas from the gasifier is much cleaner. The problems associated with the tar, such as those mentioned in the previous section, are thus alleviated.
In contrast with earlier described embodiments, another embodiment involves configuring preparation section 40 to combine the biomass feedstock 20 with water, to form a slurry 55. Various techniques or combinations of techniques can be used for combining the components, e.g., mixing, agitating, shaking, stirring and the like. The slurry 55 is then fed to the gasification section. Usually, the slurry may be relatively easily fed at a pressure higher than atmospheric pressure. The water-content of the slurry enhances gasification by providing enough water for the gasification reaction between feedstock 20 and water (i.e. steam). This embodiment is preferred when the water content in the feedstock is very low, e.g., below about 5% In another preferred embodiment, the use of a water slurry is preferred when the water content in the feedstock is below about 10%. In this embodiment, the light hydrocarbon 50 is injected into the gasifier directly, and assists in maintaining adequate gasifier temperatures, as explained above. Lower hydrocarbons such as methane (C₁) may also be used in this embodiment, in addition to other compounds as described in previous paragraphs.
In yet another embodiment, as shown in FIG. 1, by the dotted line to the gasification 60, the biomass feedstock 20 is sent to the feed preparation step 30, where it undergoes one or more processing steps as outlined above. After feed preparation, a slurry 55 is prepared in the slurry preparation section 40. The slurry preparation section 40 is configured to combine the biomass feedstock 20 with either water or light hydrocarbon 50, or with a combination of the light hydrocarbon and water, forming slurry 55. Additives like stabilizers may be added to stabilize the slurry 55. The slurry is then fed to the gasification section 60. Light hydrocarbon 50 is added to the gasification section 60. The presence of the light hydrocarbon leads to the generation of higher temperatures in the gasifier, and can therefore result in a reduction in the amount of tar carry over in the product gas 65, as discussed previously. Biomass-water slurries, which would otherwise make the biomass gasification unsustainable as described in an earlier section, can also be used in the scheme provided by this embodiment. The addition of light hydrocarbon 50 to the gasifier generates energy to evaporate the water in the slurry, thus making aqueous slurries of biomass a viable option. The feedstock slurries made partially or completely with light hydrocarbon 50 result in higher quantities of light hydrocarbons in the gasifier. This results in higher temperatures than those generated by the use of slurry with water alone, thus leading to further reduction of tar carryover. This embodiment is preferred for a biomass feedstock which has low or moderate moisture content, typically between about 5% to about 20%.
As described previously, relatively high gasifier temperatures are maintained in step 60, so that at least a substantial portion of the tar component is eliminated by cracking. In general, higher temperatures tend to increase the amount of tar removed. In some embodiments, it is preferred to operate the gasifiers at temperatures higher than about 1000° C. In some embodiments, the temperatures in the gasifier are maintained in the range from about 1000° C. to about 1400° C. In some preferred embodiments, the gasifier temperature is advantageously maintained between about 1300° C. and about 1400° C. Operation at even higher temperatures is also possible. For example, operation of the gasifier at temperatures of at least about 1700° C. results in tar levels of only 1 ppm in the product gas. However, operation at such high temperatures requires use of expensive refractory materials in the gasifier section, which may not be economically favorable. In one embodiment, it is found advantageous to crack at least about 90% of the tars generated in the gasifier.
Although this embodiment describes use of a slurry for a pressurized gasifier, it could also be employed in a gasifier operating at atmospheric pressures or any other pressures. In all of these variations, the presence of the hydrocarbon component in the slurry increases the gasifier temperature, and alleviates the tar problems as described above.
In yet another embodiment (as depicted in FIG. 1 by the stream of hydrocarbon 50 going to reformer 70), the gasification section 60 is operated at moderate temperatures, typically from about 700° C. to about 1000° C. Hence a conventional gasifier can be used in this embodiment. As described earlier, the conventional gasifier results in the generation of a tar laden product gas 65. This tar laden gas is then taken to a reformer section 70. In the reformer section 70, the light hydrocarbons are completely or partially oxidized to generate high temperatures—typically about 1000° C. or above. These higher temperatures result in the cracking of the tar components, thus reducing the tar problem, and resulting in a cleaner product gas 75.
In some embodiments, the use of reforming section 70 can be combined with the use of hydrocarbons, either for slurry preparation 40, or for the direct injection in gasification section 60, or both. Various advantages may arise from this embodiment. For example, there may be less of a need to use costly high temperature structural materials in the gasification section, due to lower operating temperatures prevalent therein.
In some embodiments, e.g., when the gasifiers are operating at atmospheric pressures, the reformer section 70 can be coupled with gasification section 60. In this manner, the tar problems associated with the conventional gasifiers may be reduced. Moreover, in the embodiments wherein conventional atmospheric pressure gasifiers are used, it may be possible to effectively operate a dry feeding system, i.e., without using a slurry. Hence, in such embodiments, the slurry preparation step 40 may be absent.
The following description relates to the configuration wherein fuel gas 75 emanates from reformer section 70. However, as described previously, reformer 70 may be an optional feature, and hence, the description below is equally applicable for product gas 65, emanating from the gasification section 60.
The fuel gas 75 emanating from the gasification section includes primarily CO and H₂, but may also include CO₂and water (H₂O), which are products of complete combustion. The fuel gas 75 may contain nitrogen (N₂), if air is used for gasification.
The fuel gas 75 may be used in multiple applications 80, such as electrical power generation, heat generation, and chemical production. The fuel gas 75 may be upgraded in energy content, by using a water gas shift reaction (WGS). WGS involves reacting the fuel gas 75 with steam in the presence of catalysts such as metal oxides (e.g., iron oxide and copper oxide). In a WGS process, the water molecule reacts with carbon monoxide to form hydrogen and carbon dioxide. Thus, the WGS technique removes carbon monoxide from the fuel gas, and adds hydrogen to it. Since the energy content of hydrogen is more than that of carbon monoxide, the WGS effectively increases the energy content of the fuel gas 75.
Often, the WGS reaction is followed by a CO₂removal stage, wherein the CO₂content of the fuel gas is removed by various techniques, such as but not limited to, membrane separation, pressure swing adsorption, temperature swing adsorption, chemical scrubbing, and the like. The resultant stream, which contains primarily hydrogen, can then be used for combustion in an internal combustion engine or a gas turbine, for generating mechanical or electrical power. The resultant stream may also be fed to fuel cells for the generation of power. The CO₂removal from fuel gas 75 is preferred for meeting carbon emission constraints. The resultant stream may also be used as a hydrogen source in chemical synthesis reactions. As non-limiting examples, the resultant stream may be used as a hydrogen source in the hydrogenation reaction of oils; in hydrotreating processes; for hydrodesulfurization; or for other reactions which consume hydrogen.
The fuel gas 75 may be directly used in applications like power generation, mechanical work, or chemical synthesis, without a WGS reaction. Typically the chemical synthesis reactions, such as the Fischer Tropsch synthesis reaction, are used to form synthetic hydrocarbons from synthesis gas. These reactions require the conditioning of fuel gas 75, so as to maintain a desired proportion of carbon monoxide and hydrogen. The appropriate ratio of these compounds can be achieved by selective removal of either of the compounds. For example, if the amount of carbon monoxide in fuel gas 75 is higher than the desired range, it may be selectively removed by membranes, or by preferential oxidation of CO. Alternatively, it may be subjected to the WGS reaction, which removes carbon monoxide from the fuel gas 75, and adds equivalent (molar) amounts of carbon dioxide and hydrogen to it. CO₂is subsequently removed by suitable techniques, leaving the fuel gas 75 with an altered proportion of carbon monoxide and hydrogen. Thus, by controlling the WGS reaction, the proportion of carbon monoxide and hydrogen can also be controlled.
When the amount of hydrogen in the fuel gas 75 is higher than the desired range, it may be removed by use of suitable separation techniques. It may also be removed by a reverse water gas shift reaction, wherein the carbon dioxide and hydrogen components are reacted in presence of suitable catalyst to form carbon monoxide and water. This may typically be practiced when there is a substantial amount of carbon dioxide in the fuel gas 75, in addition to high hydrogen. After the proper ratio of carbon-to-hydrogen has been achieved by these techniques, the fuel gas 75 may be used in the synthesis of value-added chemical materials. Non-limiting examples of such materials include methanol, synthetic gasoline, diesel, jet fuel and the like. In some embodiments, a part of these materials can be recycled as light hydrocarbons to form a slurry with the biomass feedstock.
In some embodiments, the fuel gas 75 may be used for heating applications. As an example, the fuel gas may be used to fire a heater to produce thermal energy. In another embodiment, the fuel gas may be used in a boiler to produce steam. The steam may be further used for heating purposes, process applications, or in a steam turbine, to produce power.
In some embodiments, the fuel gas 75 may be used in applications 80, at a desired location adjacent to the site of preparation. In other embodiments, the fuel gas can be transported to other sites (sometimes distant), for storage, further processing, or use in a selected application. Those skilled in the art are familiar with storage and transportation techniques for such materials.
Thus, the embodiments of present invention provide a method for the gasification of a biomass feedstock, with reduced tar formation and ease of operation at higher pressures. The method may also be used for biomass gasification at atmospheric pressures, to alleviate the tar problems. Certain embodiments of the present invention also provide methods to effectively prepare and use aqueous biomass slurries in gasification processes, minimizing or eliminating some of the drawbacks noted previously.
The use of a hydrocarbon to make a slurry, or injection into the gasifier as suggested by the embodiments of the present invention, may not be intuitive for several reasons. First, the cost involved can be very significant. Second, the hydrocarbons typically are not considered renewable resources. Hence a combination of a hydrocarbon with a renewable energy source such as biomass is typically not contemplated. However, as demonstrated by the current invention, the use of small quantities of hydrocarbon reduces the tar problem, which would otherwise be a major drawback in biomass gasification. Also as described previously, all hydrocarbons need not be based on petroleum feedstocks. As described above for some embodiments of the present invention, the light hydrocarbon may be derived from a biomass feedstock. It includes feedstocks like pyrolysis oil, or the fuels made from the synthesis gas produced in the gasification.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

1. A method for the gasification of biomass, comprising the following steps:

combining a biomass feedstock with a light hydrocarbon composition to form a slurry; and

feeding the slurry to a gasifier to produce a fuel gas.

2. The method of claim 1, wherein the oxygen content in the biomass feedstock is less than about 60%.

3. The method of claim 1, wherein the water content in the biomass feedstock is less than about 20%.

4. The method of claim 1, wherein the biomass feedstock comprises at least about 30% of plant biomass.

5. The method of claim 1, wherein the biomass feedstock comprises a pre-processed feedstock.

6. The method of claim 1, wherein the biomass feedstock comprises a residue of oilseed extraction.

7. The method of claim 1, wherein the biomass feedstock comprises a residue of soybean oilseed extraction.

8. The method of claim 1, wherein the light hydrocarbon has a boiling point of up to about 160° C.

9. The method of claim 1, wherein the light hydrocarbon comprises C₁to C₇hydrocarbons.

10. The method of claim 1, wherein the light hydrocarbon is obtained from the processing of triglycerides.

11. The method of claim 1, wherein the light hydrocarbon is obtained as a byproduct of oil processing.

12. The method of claim 1, wherein the gasifier comprises an entrained flow gasifier.

13. The method of claim 1, wherein the fuel gas is further subjected to a water-gas shift reaction, so as to increase the hydrogen content of the fuel gas.

14. The method of claim 1, wherein the fuel gas is used in a power generation process

15. The method of claim 1, wherein the fuel gas is used in chemical synthesis.

16. A method for the gasification of deoiled soybean cake, comprising:

combining the deoiled soybean cake with at least a light hydrocarbon to form a slurry;

wherein the light hydrocarbon has a boiling point up to about 160° C.; and

feeding the slurry in a pressurized gasifier to produce a fuel gas.

17. A method for the gasification of biomass, comprising:

combining a biomass feedstock with water to form a slurry;

feeding the slurry to a gasifier to produce a fuel gas; and

injecting a light hydrocarbon into the gasifier, to generate gasification temperatures greater than about 900° C., by partial or complete combustion of the light hydrocarbon.

18. The method of claim 17, wherein the oxygen content in the biomass feedstock is less than about 60%.

19. The method of claim 17, wherein the water content in the biomass feedstock is less than about 20%.

20. The method of claim 17, wherein the biomass feedstock comprises plant biomass.

21. The method of claim 17, wherein the biomass feedstock comprises deoiled soybean residue.

22. The method of claim 17, wherein the light hydrocarbon is derived from triglycerides.

23. A method for the gasification of biomass, comprising:

gasifying a biomass feedstock in a gasifier to produce a product gas;

feeding the product gas to a reformer section;

injecting a light hydrocarbon into the reformer section; and

generating temperatures of greater than about 1000° C., by partial or complete combustion of the light hydrocarbon, in the reformer section.

24. The method of claim 23, further comprising a slurry preparation section, wherein said biomass feedstock is slurried with at least one of water or said light hydrocarbon.