Patent Application
20140283439
The present invention relates to a process for optimizing a biomass feedstock for gasification for the production of syngas. The biomass feed, which is preferably a lignocellulosic material, is subjected to controlled torrefaction so that the biomass only undergoes a weight loss of about 10% to 15% on a dry ash free basis. This increases the energy density and friability of the so treated biomass and results in higher efficiency on subsequent gasification.
A substantial amount of research and development is being done to reduce our dependency on petroleum-based energy and to move us toward more sustainable energy sources, such as wind energy, solar energy, and biomass. Of these three sustainable energy sources, biomass has received significant attention. One reason for this is because biomass is widely available in many forms. Also, the economy of agricultural regions that are growing crops can greatly benefit from biomass to transportation fuel plants. Thus, governmental bodies in agricultural regions are very supportive of proposed biomass plants. In order to convert biomass to high value products, such as transportation fuels, one promising process is to first convert the biomass to syngas by gasification. Gasification is a well-known process for producing synthesis gas (syngas), which is a gas mixture containing varying amounts of carbon monoxide and hydrogen as the major components.
Various types of gasifier designs are known. The most common type of gasifier used in biomass gasification is believed to be an up-draft fixed-bed design (counter-current) design, in which air, oxygen and/or steam flows upward through a permeable bed of biomass and counter-currently to the flow of ash and other byproducts of the reaction. Typical up-draft gasifiers have significant technical shortcomings. First, the introduction of air into the hot gasification chamber partly combusts the biomass, resulting in a lower overall heating value compared to gasifiers that employ indirect heating. Second, if air is used as the gasification agent, nitrogen in the air which is a diluent, reduces the energy content per unit volume of output gas, making the output gas less useful for such things as gas turbines, for storage, and for subsequent chemical processing. Third, tars and phenolic hydrocarbons produced in an up-draft gasifier require removal to reduce emissions, avoid fouling of a gas turbine, and avoid catalyst poisoning when used to create liquid fuels. The removal equipment adds to system complexity and size, with the result that for economic reasons the gasifier is usually limited only to large installations. Because biomass is a low-energy content fuel and is dispersed geographically, a large-scale gasifier requires transport and storage of the biomass, which negatively affects the economic payback for the system.
A more advantageous type of gasifier for biomass is a fluidized bed gasifier whereby a volume of gas, such as steam and air and/or oxygen is passed through a bed of biomass with sufficient velocity to create a fluidized-bed of biomass particles. This mode of operation is advantageous to up-draft fixed bed gasifiers in that the fluidized bed allows for a more uniform temperature distribution within the gasifier and thus can result in a higher syngas yield and a reduction in unwanted by-products, such as tar and soot. This type of gasifier suffers from a requirement that the particles of biomass must be reduced in size by several orders of magnitude that can consume a significant amount of energy—in some cases up to 30% of the energy contained in the biomass itself.
In view of the above, there is a need for biomass gasification processes and equipment that are economically practical for use at medium- to small-scale installations, including direct sources of biomass such as agricultural operations (for example, farms), factories in which biomass materials are starting materials and/or byproducts (for example, paper mills, ethanol plants, etc.), bioplants, and small towns or villages. There is also a need for biomass pretreatment processes that result in a more efficient and economical feed for gasification. One such pretreatment is torrefaction. Torrefaction is a relatively mild pretreatment of biomass at a temperature from about 200° C. to about 350° C. The properties of the biomass are changed to increase its heating value, reduce its tendency to degrade during storage, and make it more friable and hence easier to mill. Conventional torrefaction processes are used to produce a densified product that can be used in place of or in conjunction with coal.
While both torrefaction and gasification of biomass are well known, there is still a need in the art for improved processes combining these two technologies that can lead to a more efficient and economical process for converting biomass to transportation fuels.
In accordance with the present invention there is provided a process for preparing a biomass feedstream for gasification, which process comprises:
Any biomass feedstock can be used in the practice of the present invention. Preferred are plant biomass feedstocks, typically referred to as lignocellulosic feedstocks, which are generally comprised of cellulose, hemicellulose, and lignin. Non-limiting examples of plant, or lignocellulosic, feedstocks include non-woody plant biomass, cultivated crops, such as, but not limited to, grasses, for example, but not limited to, C4 grasses, such as switchgrass, cord grass, rye grass, miscanthus, reed canary grass, or a combination thereof, or sugar processing residues such as bagasse, or beet pulp, agricultural residues, for example, soybean stover, corn stover, rice straw, rice hulls, barley straw, corn cobs, wheat straw, canola straw, rice straw, oat straw, oat hulls, corn fiber, recycled wood pulp fiber, sawdust, hardwood, for example aspen wood and sawdust, softwood, or a combination thereof. Further, the lignocellulosic feedstock may include cellulosic waste material such as, but not limited to, newsprint, cardboard, sawdust, and the like. For urban areas, the best potential plant biomass feedstock includes yard waste (e.g., grass clippings, leaves, tree clippings, and brush) and vegetable processing waste.
Lignocellulosic feedstock can include one species of fiber or alternatively, lignocellulosic feedstock can include a mixture of fibers that originate from different lignocellulosic feedstocks. Furthermore, the lignocellulosic feedstock can comprise fresh lignocellulosic feedstock, partially dried lignocellulosic feedstock, fully dried lignocellulosic feedstock or a combination thereof. In general, the term “biomass” as used herein includes all of the terms, plant biomass, liqnocellulosic, cellulosic, and hemicellulosic. It is preferred that the biomass used in the practice of the present invention comprised at least about 30 wt. % cellulose/hemicelluloses, based on the total weight of the biomass.
The biomass is preferably dried, then reduced to an effective size before the torrefaction pretreatment step. Technologies for drying biomass are well known in the art and include both passive, as well as active drying. Passive drying is preferred for cost considerations, but depending on the nature of the biomass, passive drying may not be able to reduce the moisture level to a level acceptable for being fed to a gasifier. Therefore, some form of active drying, such as the use of an external heat source and blowers may be needed.
It is preferred that the biomass, after drying, contain no more than about 2.0 wt. %, preferably no more that about 1.5 wt. %, and more preferably no more than about 1.0 wt. % water, based on the total weight of the biomass after drying. The biomass is subjected to a size reduction step to reduce it to a size suitable for drying and torrefaction. It is preferred that the size reduction step result in a biomass feed having an average particle size of about 0.1 inch to about 3 inches, preferably from about 0.25 inches to 1.5 inches. If any of the biomass is to be sent directly to a gasifier than it is preferred that the average particle size of that portion should be from about 1 to 500 microns, preferably from about 150 microns to 300 microns. The fibrous structure of the biomass makes it very difficult and costly to reduce its particle size. Non-limiting examples of mechanical size reduction equipment include rotary breakers, roll crushers, jet mills, cryogenic mills, hammer mills, impactors, tumbling mills, roller mills, shear grinders, and knife mills. Hammer mills are preferred for the practice of the present invention.
The biomass is subjected to torrefaction after size reduction which results in an increase in its carbon content, a reduction in oxygen content, and the substantial elimination of storage hygroscopicity and degradation. Torrefaction is a process of exposing biomass to heat in the absence of air or oxygen between about 200 to about 350° C., preferably from about 225° C. to about 320° C., more preferably about 260° C. to about 300° C. Torrefaction results in the removal of the majority of volatile species as well as a fraction of the oxygen in the biomass and results in a more friable material with increased energy density that tends to be hydrophobic and less prone to rotting. The products of torrefaction are a solid char material, water vapor, and a number of organic species including carbon dioxide, carbon monoxide, acetic acid, formaldehyde, acetaldehyde, as well as a number of other compounds in lower concentration. Torrefaction has been proposed as a means to make biomass more closely resemble coal and as a means to make the transport and storage of biomass more viable due to the increased density and greater storage stability. There has been some work that has examined if torrefaction should be used as a preprocessing step for gasification. It is conventional wisdom that the loss in overall energy in the biomass from torrefaction nullifies any potential benefit that torrefaction would have on the gasification process (typically the weight loss from torrefaction outweighs the energy density gain).
This prior art ignores several critical points. First, gasification experiments were typically done at extremely high temperature (1200 to 1300° C.) which is outside the range of temperature where biomass is typically gasified. At such high temperature conditions, even very stable tars or methane will react and thermodynamic equilibrium will favor the production of CO over CO2. In addition, the water gas shift reaction is fairly facile at these conditions so any CO2 produced directly via any direct decomposition route would rapidly equilibrate to the levels predicted by equilibrium. Biomass gasification is typically practiced at much lower temperatures (650 to 950° C.) where tar and methane are much more stable and CO2 is more thermodynamically favorable than at high temperature. In addition, while it is true that torrefaction does result in less mass fed to the gasifier and hence potentially lower yields to carbon monoxide, this is only true if the additional mass would have been converted to product and not byproducts. Similarly, while it is true that the weight loss from torrefaction is typically greater than the energy density gain from torrefaction, the real metric to examine is the yield to carbon monoxide vs. weight loss from torrefaction as the yield to product is not constant across all degrees of torrefaction.
The inventors hereof have found that torrefaction can have a significant benefit on gasification by recognizing three important factors that have been overlooked in prior work coupling the benefits of torrefaction with gasification:
This third point is most important given that the majority of the literature treats torrefaction as a rate process whereby shorter residence times can be traded off for higher temperatures. This is true only to a degree in that the maximum temperature must be kept below the decomposition temperature of cellulose in order to maximize yield to product. As such, the temperature must be maintained below about 320° C. to ensure that the cellulose does not begin to decompose and the residence time must be selected such that the majority of volatile species are removed from the biomass.
A group of 160 different biomass samples was examined including grasses, various wood species, and various agricultural residues (e.g. corn stover) and found that the optimum weight loss from torrefaction to eliminate volatile species and maximize product yield varies between 11 and 14 wt. % on a dry, ash free basis.
Further, torrefaction is capable of producing a relatively uniform pretreated biomass from a wide variety of biomass materials. If desired, the severity of the torrefaction process can be altered to produce a torrefied product having substantially the same energy content as that produced from a completely different biomass feedstock. This has advantages in the design of a gasifier feed system and greatly simplifies the gasifier operation with respect to controlling the H2:CO ratio in the resulting syngas. In addition, by selectively removing the carboxylates in the torrefaction unit, it is believed that less methane will be produced and fewer tars will be formed during gasification by reactions between aldehydes and carboxylic acids formed by the decomposition of hemicellulose and acids and phenols derived from lignin.
Torrefied biomass retains a high percentage of the energy content of the initial biomass feedstock (ca. ˜90%). Gaseous products produced during torrefaction are typically comprised of condensable and non-condensable gases. The condensable gases are primarily water, acetic acid, and oxygenates such as furfural, formic acid, methanol, and lactic acid. As previously mentioned, it is preferred that the biomass feedstock be dried prior to torrefaction to facilitate use of the condensable oxygenates as a heating fuel (typically having a heating content greater than 65 BTU/SCF). The non-condensable gases are comprised primarily of carbon dioxide and carbon monoxide, but may also contain small amounts of hydrogen and methane.
The present invention will be better understood with reference to the
The resulting dried torrefied biomass solids from 120 are conducted from dryer 120 to torrefaction reactor 130, via line 38. The vapor phase from the drying reactor is exhausted through line 16, which is split into stream 17 which is preferably vented, and 18 which is conducted to combustion zone 160. The portion of stream 16 that is vented is managed to help control the temperature of combustion zone 160.
A vapor phase product (tor-gas) and torrefied solids are produced in the torrefaction reactor 130. The torrefaction reactor 130 is operated in a non-oxidizing atmosphere and at a temperature from about 200° C. to about 350° C., preferably from about 225° to about 320° C., and more preferably at a temperature of about 250° to about 300° C. The resulting tor-gas will typically be comprised of primarily water vapor with additional amounts of methane, methanol, acetol, CO, CO2, furfural, and low carbon organic acids such as formic acid, acetic acid, and lactic acid. The torrefaction reactor requires a sweep gas to ensure that reaction products are removed from the reactor and is preferably flowed in counter-current mode. The tor-gas exits the torrefaction reactor 130 via line 24 and is split into two portions; line 26 which is recycled to torrefaction reactor 130 to act as the sweep gas and via line 25 which to the combustor 160. The portion of stream 24 that is sent to the combustor is controlled to maintain an effective flow of sweep gas to the torrefaction reactor. By effective amount we mean that amount needed that would be effective for sweeping the torrefaction zones of tor-gas product. Any excess vapor phase fraction from stream 24 is sent to the combustor 160.
Any type of torrefaction reactor can be used in the practice of the present invention. Non-limiting example of such types of torrefaction reactors include continuous reactors, non-limiting example that include horizontal moving bed reactors, fluid bed reactors, and jet mill reactors. It is preferred that the reactor be a moving bed reactor. If a fluid bed reactor is used it is preferred that a cyclone be used to separate fines from the resulting product vapor phase that is passed via line 24. The separated fines can be returned to the fluid bed.
Excess tor-gas stream 25, and a residual flue gas stream 18 are combined with blown air via line 20 that is produced by passing air at low pressure via 19 through a blower 180. The blown air, excess tor-gas, and residual flue gas are combusted in the combustor 160 at an effective temperature to destroy at least 99% of the volatile organic components of the tor-gas and produce a heated flue gas 32.
A portion of the heated flue gas 34 is returned to the dryer 120 to provide at least a portion of the heat needed to maintain the desired dryer temperature. The remaining portion of the flue gas 33 is directed heat exchanger 170 whereby a heat transfer fluid 35 is heated, resulting in a cooled flue gas stream 37 and heated heat transfer fluid stream 36.
The final torrefied biomass solids exit the torrefaction reactor 130 via line 46 and are passed to torrefied biomass bin 200 where they are metered via second metering valve 41 to storage or directly to a gasification unit (not shown) for the production of syngas. An optional milling step 210 can be used in the event the particle size of the torrefied biomass exiting second torrefaction reactor 130 is too large for the intended gasifier. Any suitable milling apparatus can be used to reduce the size to a fluid gasification size.
It will be understood that a jet mill torrefaction reactor can be used as the torrefaction reactor. If a jet mill torrefaction reactor is used then there will be no need for a milling step prior to gasification since the particle size of the biomass exiting a jet mill reactor will be well within the acceptable particle size for fluid bed gasification.
This application is a Continuation-in-Part of Non-Provisional application U.S. Ser. No. 13/491,496 filed Jun. 7, 2012 which is a Continuation-in-Part of Non-Provisional application U.S. Ser. No. 12/825,887 filed Jun. 29, 2010 now U.S. Pat. No. 8,282,694 issued Oct. 9, 2012 which was based on Provisional Application U.S. Ser. No.61/295,358 filed Jan. 15, 2010.
