Patent Publication Number: US-2004040677-A1

Title: Organic biomass paper pulping

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to methods of converting a variety of forms of biomass into pulp. More specifically, this invention relates to such methods which are highly energy efficient and economical, do not produce significant amounts of pollutants, and can be adapted to produce pulp suitable as paper pulp, bleachable and unbleached board pulp, and semi-chemical pulp.  
       [0002] The method of the invention provides a flow type process which uses a relatively low concentration of fiber to liquor following the reduction of agricultural waste biomass to short fiber bundles or suitable deciduous woods reduced to crushed flakes.  
       [0003] Variable pulp qualities related to yield, fiber structure, bleachability, etc., can be modified by changes in temperature, time and chemical concentration to meet the specific requirements for the various final pulp products.  
       BACKGROUND OF THE INVENTION  
       [0004] The failure of agricultural wastes to meet the process needs of modern pulping methods has inhibited their use in developed countries. Today, however, the growing needs for energy, reduction of pollution and production of edible material for both animals and humans demand that a new look be taken at the availability of biomass to meet those needs.  
       [0005] Conventional pulping processes have relied upon high temperatures (typically about 375° F.) and pressures (typically about 175 psi) as well as harsh chemicals which are used in high concentrations and must be recovered. These processes further require expensive digesters and boilers. Conventional pulp mills also must invest large amounts of money for both air and water pollution control.  
       [0006] As the need for fiber has grown, and the amount of timber available for cutting becomes limited, attempts have been made to use waste biomass agricultural residues (e.g., straws) for making pulp. Most of this material, however, consists of short fibers which are a mixture of hydrophilic and hydrophobic fibers which intermittently form dense knots. As such, it is not suitable for conventional pulping, which is designed to handle uniform densities of fibrous materials. To the extent that such material is used in conventional pulping methods, it has the same problems of expensive equipment and chemicals, toxicity and costly air and water pollution controls attendant conventional pulping using conventional fibrous material.  
       [0007] U.S. Pat. No. 4,652,341 discloses and claims an improved pulping process in comparison to these conventional methods outlined above. In this process, fibrous ligno-cellulosic material is contacted with a dilute aqueous solution of a nitrate ion source in the presence of an aluminum compound to nitrate the ligneous component of the material, then the resultant nitrated material is contacted with an alkaline extraction liquor to extract the nitrated ligneous component to provide softened, partially defibered cellulosic structures which subsequently are completely defibered. The aluminum compound in the nitrating step is provided in an amount sufficient to accelerate the nitration of the lignin, and the process can be carried out at atmospheric pressure and at temperatures below about 110° C.  
       [0008] Although the process of the &#39;341 patent has proved to be useful, further improvements are sought. Especially desirable are further improvements in chemical drainage, reduced chemical concentrations, and the minimization of pollution factors, as well as the development and introduction of a closed liquid system and simple pH control.  
       SUMMARY OF THE INVENTION  
       [0009] A method for pulping fibrous biomass comprising cellulose, hemicellulose and lignin components comprises  
       [0010] a) decorticating, shredding or crushing of the biomass;  
       [0011] b) concurrently with, or prior to, step a), contacting the biomass with an aqueous solution of a nitrate ion source at a concentration within the range of about 0.10% to about 3.0% in the presence of an aluminum compound to initiate nitration of the lignin component of the biomass;  
       [0012] c) soaking the partially nitrated biomass from step a) in an aqueous solution of a nitrate ion source at a concentration within the range of about 0.1%-3.0% in the presence of an aluminum compound, wherein the wt:wt ratio of said solution to said biomass is at least 15:1, essentially all of the biomass becomes impregnated with said solution and additional lignin component of the biomass is nitrated;  
       [0013] d) heating said solution of impregnated biomass to a temperature wherein nitration of said lignin component of said biomass is completed to a desired degree;  
       [0014] e) draining said nitrated biomass from said solution;  
       [0015] f) adding said nitrated biomass to an aqueous extraction solution of NH 4 OH, wherein the wt:wt ratio of aqueous solution:biomass is at least 15:1;  
       [0016] g) heating said solution of step f) to a temperature of about 100° C. to solubilize and extract the hemicellulose and nitrated ligneous component of said biomass and provide partially defibered cellulosic structures;  
       [0017] h) separating said partially defibered cellulosic structures from said extraction liquor; and  
       [0018] i) completely defibering said partially defibered cellulosic structures.  
       [0019] In a preferred embodiment, the heating of the acidic and alkali solutions of the biomass are carried out using microwave energy. The use of microwave energy provides quick, efficient, and even heating which can be closely controlled.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020] The method of the present invention presents an efficient and clean procedure for obtaining pulp from various forms of biomass. The method of this invention is an improvement of that used for making paper pulp described in U.S. Pat. No. 4,652,341. It now has been found that by suitable and precise preparation of the particular biomass of choice and specific selection of the choice of acid and alkali agents, their concentrations, reaction conditions and manner of heating, any of a wide variety of forms of biomass can be treated readily to provide pulp suitable for making a variety of products.  
       [0021] As used herein, the term “renewable, fibrous biomass,” “fibrous, ligno-cellulose biomass” or, simply, “biomass,” refers to herbaceous and woody plants and agricultural waste and residues. Agricultural waste includes agricultural residues, such as wheat, grass, oat, rice and barley straw, corn stover, sugar cane, bagasse and shrubs. Woody plants include relatively low or non-commercial type trees, such as poplar, alder, aspen and cottonwood) that need to be thinned from forests or plantations or farms specific to growing such types of deciduous wood.  
       [0022] A wide variety of forms of fibrous biomass can be used in the method of the present invention, including, without limitation, woody materials, such as aspen, bamboo and poplar, and grassy materials, such as straws, bagasse and kenaf. In addition, high yield shrub growth, such as coppice willow, and agricultural wastes, such as cotton stalks, sugar cane bagasse and corn stalks, can be used.  
       [0023] A critical first step in the method is correct fiber preparation. The biomass should be in a small-sized particulate form. It is important that the biomass not only be cut into small pieces that are relatively uniform, but that the pieces be decorticated and/or shredded or flaked into homogeneous longitudinal fiber structures, rather than simply chopped. The decortication and/or shredding or flaking is done to enhance penetration of the biomass particles by maximizing the amount of internal surface area that will be directly exposed to the nitrating agent, thus accelerating nitration of the lignin while maintaining the fibrous structure. Desirably, the biomass pieces are decorticated if they have central pith content covered by an outer structure or bark and then shredded or crushed or flaked. Hereinafter, the biomass will be referred to as “shredded;” it is to be understood that this term encompasses “crushed” and/or “flaked” as well.  
       [0024] One of skill in the art readily can develop a pre-pulping treatment protocol for any particular biomass feedstock to take into account any special characteristics of a particular feedstock. For example, wheat straw and wood flakes are both suitable sources for pulp, but they require different preparations. Wood flakes are hydrophilic and can be solvated and nitrated more easily than can wheat straw, which has both hydrophilic and hydrophobic regions; the latter takes much more time to solvate and nitrate. Wheat straw also contains random knotted fibers, known as nodes. In view of this combination of features, it is desirable to shred the straw and thus break up the nodes to reduce the time necessary to nitrate the lignin component of the straw.  
       [0025] Biomass such as corn stalks and sugar cane, both of which are useful as sources of pulp, are decorticated prior to cutting. With both of these feedstocks, it is the outer, fibrous structure that is retained and used in the pulping process. Kenaf also needs to be decorticated prior to pulping.  
       [0026] Bamboo is a woody structure and needs to be flaked for pulping. Cotton stalks are shrub material that desirably are cut and shredded. Determining the specific pretreatment steps necessary to decorticate, shred, and/or crush a specific type of biomass for use in the pulping process of this invention is well within the abilities of one of ordinary skill in the art.  
       [0027] There obviously can be significant physical differences among the various sources of possible starting materials. These can be minimized by the pretreatment steps selected. In addition to decortication and/or shredding, pretreatment also can include the addition of a low percent acid liquor to the biomass as it is being cut or shredded. The low acid liquor typically is added to the biomass in an amount that is approximately 20-30% by weight of the dry biomass being shredded, although this can vary (in some instances, upward of 100%) depending upon the specific biomass and type of shredder. This step is particularly useful when the feedstock is, for example, any of a variety of types of straw, as the acid liquor begins to penetrate the dense node structure, thus making the subsequent nitrating and alkalizing steps more efficient.  
       [0028] Specifically, the shredded biomass can be sprayed with a solution of a low percent acid liquor nitrating agent so as to maintain the fibrous structure of the biomass while initiating nitrating action of the lignin. This pretreatment step desirably is carried out concurrently with the shredding of the biomass to facilitate the shredding as well as to initiate nitration of the lignin, but it also can be carried out before the shredding by soaking the biomass for an hour or more in the dilute acid solution. A preferred nitrating agent is HNO 3  within the range of about 0.10% to about 3.0%, preferably about 0.10% to about 1.0%, equivalent to the level used in the subsequent nitrating step described below. The acid concentration desirable for a particular material is dependent upon the absorption rate of the material and its lignin content. Barley grass straw, for example, rapidly absorbs water, whereas bamboo flakes, which are very dense, do not, and the lignin content of the two materials is not the same. One of skill in the art can determine a desirable acid concentration within the guidelines provided herein useful in the pretreatment of a specific fibrous biomass through routine experimentation. It is desirable to keep the acid concentration as low as possible; the fibrous structures can absorb up to six times their weight in moisture, and as the absorbed acid cannot easily be washed out, the more acid absorbed, the more alkali subsequently will be needed to neutralize it.  
       [0029] Desirably, the nitrating solution has a temperature within the range of about 70-80° C. whether it is sprayed onto the biomass or used to soak the biomass. It also is desirable that the nitrate ion source solution contains an aluminum-containing accelerator. A preferred accelerator is Al 2 (SO 4 ) 3 —18H 2 O, commonly known as alum. The accelerator serves to accelerate the action of the nitrating agent. Generally, the amount of alum required is at least about 0.06 parts, preferably 0.06 to 0.11 parts, by weight of each part of active HNO 3 .  
       [0030] This pretreatment step, not used in conventional pulping processes, helps maintain the fiber structure of the shredded biomass (the fiber bundles), which, in turn, helps maintain the efficiency of subsequent washings and drainage steps and helps reduce loss of fines, thereby increasing yields, as well as reducing ultimate chemical and water usage. In addition, pretreating with acid (spray or soak) prior to the shredding step makes the shredding process easier and more consistent, as the acid solution acts as a lubricant to the mechanical process.  
       [0031] Before the initial decorticating or shredding step, the biomass desirably is cut and shaken to eliminate dirt, metals and other debris that may be present.  
       [0032] The cut, decorticated and/or shredded, and acid-sprayed or soaked biomass then is introduced into a closed reaction vessel, preferably a hydrapulper, containing a solution of nitrating agent. The preferred nitrating agent is HNO 3 .  
       [0033] The nitrating agent in the hydrapulper generally is provided in solution at a concentration within the range of about 0.1% to about 3.0%, depending on the nature of the biomass feedstock and the required characteristics of the pulp. The lignin in straw of various types can be nitrated using a solution of about 0.10% nitrate ion source; the lignin in wood flakes typically is nitrated using about a 2% solution. These illustrations are intended as guidelines; the exact percentage used in a given process will be dependent upon the type and drainage characteristics of the fibrous biomass being treated, the temperature at which the nitration is carried out and any time limitations and the availability of mild agitation. The solution of nitrate ion source is provided in significant excess of the amount of biomass. Typically, the ratio of total nitrate ion solution to biomass is at least about 15:1, e.g., within the range of about 15:1 to about 20:1, and possibly as high as about 25:1. The ratio should be such that the resultant biomass solution can be agitated easily in the hydrapulper and can be quickly and precisely heated in the heating step described below.  
       [0034] The hydrapulper typically is a closed, vertical type hydrapulper which allows rapid drainage, ready access and instantaneous broad heating, such as from intermittent microwave energy, as discussed below. The hydrapulper can be insulated mild steel with a coating to prevent passage of microwave energy outside the container. Fumes that may be generated during the processing are recycled from the top of the hydrapulper to the bottom of the fluid/fiber mix.  
       [0035] The biomass is in the solution of the nitrate ion source for a time sufficient to allow essentially all of the biomass to be impregnated with, or absorb, the solution. The impregnation time varies depending upon the source of the biomass. Various types of straw can become impregnated relatively quickly; shredded straws can be impregnated within a few minutes to about an hour; crushed wood flakes typically take a longer period of time, e.g., from about 1 to about 3 hours, depending upon the density of the wood and the flake thickness and size. Full impregnation can add from about 5-7 times the dry weight of the biomass.  
       [0036] The absorption step generally is carried out at a temperature within the range of about 75° C. to about 100° C., preferably within the range of about 80° C. to about 85° C. Desirably, the solution is heated to and maintained at this temperature for the desired time using microwave energy.  
       [0037] As in the acid pretreatment step described above, this step is carried out in the presence of an aluminum-containing accelerator, such as alum, in the solution of the nitrate ion source. The accelerator serves to shorten the time needed to convert the lignin component of the biomass to an insoluble lignin nitrate and generally is present in about 0.06 parts to about 0.11 parts, by weight of each part of active HNO 3 .  
       [0038] Once the biomass has absorbed the aqueous solution, the biomass and remaining solution is heated with microwave energy to raise or maintain the temperature of the biomass to within the range of about 85° C. to about 100° C., preferably about 100° C., for a period of time sufficient to nitrate the lignin component of the biomass to a desired level. The desired completeness of the nitration of the lignin component of the biomass will depend upon the ultimate intended use of the pulp being produced. Paper making pulp does not require nitration of all the lignin present in the biomass. To attain and maintain required strength qualities it is necessary to vary the amount of lignin dissolved out of the pulp. Thus, the completeness of the nitration will vary, for example, between semichemical pulp and bleachable pulp. This is well understood by persons of ordinary skill in the art.  
       [0039] Generally, the desired heating can be accomplished within a period of about 1 to about 5 minutes if microwave heating is used. Although other types of heating can be used, such as steam injection, a steam bath, or electric heat, microwave heating is preferred. Microwave heating produces a very fast, uniform, even heating which, through intermittent application, allows a fairly flat temperature profile to be maintained during the heating period. Microwave heating also avoids the localized overheating that can occur with steam injection and accomplishes the desired nitration of the lignin more quickly than occurs with slow control heating by electricity. Microwave heating also can be turned on and off quickly and readily to easily maintain the temperature of the solution inside the hydrapulper at a desired level for a desired period of time. The microwave energy applied at this level of water solution does not affect the constituents of the cellulosic fiber.  
       [0040] Once the nitrating step has been completed, the nitrating agent is drained from the biomass and the biomass desirably is washed at least once with water to remove surface chemical. The biomass can be washed directly in the hydrapulper. The wash water preferably is hot, e.g., at a temperature of about 75°-100° C., and generally is used in an amount equivalent to that absorbed by the fiber. Desirably the wash water is brought to the desired temperature using microwave energy. Sufficient washing is indicated when the pH of the wash water is within the range of about 5-6, preferably about 6. Failure to achieve this pH level indicates poor pretreatment of the biomass or too much acid and will require undesirably high amounts of alkali in the subsequent step of the treatment process. The separated nitrating agent advantageously can be recycled by refreshing it through the addition of further nitrating agent, alum source and water as needed and then serving as nitrating agent for further biomass cooking. By recycling the liquor in this way, heat and chemicals are conserved. The acid wash water also desirably is recycled, such as for use in the pretreatment of a new batch of biomass or to adjust the pH value of the subsequent alkali stage black liquor.  
       [0041] The nitrated and washed biomass then is contacted with an alkaline extraction liquor. This step also conveniently is carried out in the hydrapulper in which lignin extraction occurred. The biomass is submerged in sufficient extraction liquor to form a slurry. The lignin nitrate readily is dissolved in basic aqueous media having an initial pH in the range of about 12.0 to about 13.0. The alkali concentration of the extraction solution must be sufficient for the solution to leach out, i.e., dissolve, most, if not all, of the nitrated ligneous material in the fibrous biomass plus neutralize any free acid in the fiber mix. The hemicellulose as well as silica and lipids present in the biomass also are extracted from the cellulose in this step. A highly preferred alkaline extraction liquor comprises NH 4 OH as the alkaline agent. The concentration of alkali generally is within the range of about 0.6% to about 4.0%.  
       [0042] The amount of alkaline extraction liquor used should be sufficient to completely submerge or continuously contact the nitrated biomass so as to form a slurry and to dissolve out and effectively separate from the cellular fiber the nitrated lignin, hemicellulose and other, minor, components of the biomass. A typical wt:wt ratio of extraction liquors to nitrated biomass is at least about 15:1, can be as high as about 25:1, and preferably is within the range of about 15:1 to about 20:1. The amount of alkali added to reach a final pH of about 8 will vary based upon the percentage of acid used and the efficiency of the previous washing step.  
       [0043] The nitrated biomass in the alkaline extraction liquor is stirred to assure good contact between the biomass fibers and the alkaline liquor. The solution then is heated, again preferably by microwave energy, to an internal temperature in the range of about 90° C. to about 100° C., preferably about 100° C. This temperature is maintained for a period of time sufficient to ensure dissolution of the nitrated lignin and hemicellulose components. This can take from about 6 to about 11 minutes, up to about 30 minutes or longer, depending upon the nature of the biomass, the amount of nitrated lignin and the concentration and amount of the alkali solution. A pH of about 8.0 is a desired final pH.  
       [0044] Following treatment with the alkaline extraction liquor the fibrous cellulosic material typically is separated from all or most of the lignin-containing solution (hereinafter referred to as the “black liquor”) by drainage for wood chips or a high density press for straw and similar materials. Typically, as much of the black liquor is removed as possible, but for some semi-chemical pulps it is desirable to retain a portion of the black liquor with the pulp, as understood by persons of ordinary skill in the art.  
       [0045] The mild, short heating and non-pressure conditions of the current process are such that the individual fiber structure and small fiber chains can be maintained by controlled mechanical defiberization. For example, the softened structures can be subjected to the action of a double disc defibrator or a refining machine which will physically break apart the soft fiber bundles. The principal benefit of preserving fiber strength of either individual small fibers or chains of small fibers is that it enables one to manufacture higher strength paper out of the final pulp and improves drainage characteristics.  
       [0046] The mechanically defibered pulp then is conveyed to screens where any remaining outsize fiber bundles are screened out and discarded as rejects.  
       [0047] After the mechanical defibering, it is desirable to wash the fibers with water to remove surface alkali and black liquor unless it is desired to have a semi-chemical type pulp. The washing preferably is accomplished with a minimal amount of hot water, e.g., at a temperature of about 85° to about 100° C. The washing can be performed in any conventional manner, but preferably is carried out by use of a rotary washer. The separated and washed cellulosic pulp then can be used in any of the conventional paper mill operations for making fiber webs on machines to produce a wide variety of types of board or paper.  
       [0048] The black liquor separated from the cellulosic fibers contains dissolved ligneous components, hemicellulose components, lipids and, perhaps, silica. The specific content will vary depending upon the variety of biomass pulped, the percentage of acid chemicals used, the processing time and the presence of extraneous materials.  
       [0049] Extraneous materials, such as silica, if present in sufficient quantity (e.g., 10%) can be a significant detriment to further use of the black liquor. If desired, the lignin content can be separated by acidifying the black liquor and then precipitating the lignin.  
       [0050] The black liquor has a number of uses, apparent to persons of skill in the art. For semi-chemical and unbleached pulps, black liquor with pH adjustment can be added back to the pulp at the wet or size press stage to enhance color and sizing characteristics with additional mass, unless the liquor contains silica.  
       [0051] The production of bleachable pulps may make the black liquor contents sufficiently high in lignin and other solubles to warrant the concentration of the liquor or precipitation of the solubles for sale as a lignin source as sizing or an adhesive agent in the manufacture of particle board or for a variety of other end uses.  
       [0052] All black liquor, dilute or concentrated, can be used as a fertilizer or by urban waste disposal planners, since there are no polluting elements, either by introduction into the process or formed by chemical reaction. Fertilizer constituents such as potassium or phosphorus can be added as desired.  
       [0053] In summary, the black liquor obtained in the process of the present invention can be utilized with only minor modification in a variety of ways, as a binder, water repellant or size, extender for fortified rosin size, stiffening agent in paperboard, hygroscopic fertilizing additive, and retention agent in papermaking. Thus, essentially all of the initial starting biomass and process chemicals can be used in accordance with the present invention.  
       [0054] A further advantage of the present process is that it can be computer controlled. For any desired type of biomass, once initial standards for precise chemical concentrations, time and temperature, have been determined, the process can be controlled by computer, with the single exception of an initial test of drainage capability and residual acidity of the first stage nitrating action.  
       [0055] A further advantage of the present process is that the acid treatment, washing and alkali treatment can be carried out in the same reaction vessel. This simplifies the process and decreases costs, as less equipment is needed.  
       [0056] The invention is further illustrated by the following examples which are provided for illustrative purposes and is not intended to be limiting. 
     
    
    
     EXAMPLE 1  
     [0057] Pulping of Wheat Straw  
     [0058] Wheat straw is cut into pieces less than 2 inches long. Two hundred g. of the chopped wheat straw are weighed and placed in a microwavable pressure cooker.  
     [0059] Three liters of a 1% HNO 3  solution containing a 0.10% alum solution are prepared. The HNO 3  solution is heated to 80° C. using microwave energy.  
     [0060] Two hundred cc of the heated HNO 3  solution are sprayed onto the chopped straw. The sprayed sample then is passed through a refiner, such as a double disc refiner, with the aperture between the plates set so as to crush the nodes of the straw pieces.  
     [0061] The remaining nitric acid/alum solution (2.8 liters) is added to the pressure cooker and the pretreated, shredded straw is soaked for an hour. For larger, commercially sized lots of straw, a closed, tower type hydrapulper with a recycled gas off design would be used. The temperature of the straw—acid solution is brought to 100° C. and held at that temperature for 3 minutes using microwave energy.  
     [0062] The acid solution then is drained from the biomass. The drained nitrating solution can be recycled and additional acid and/or alum added as needed for use in the nitration of a subsequent batch of straw. The nitrated biomass is washed twice using 2 liters of boiling water each time. The used wash water also can be recycled and additional acid and/or alum added as needed for use in the nitration of a subsequent batch of straw.  
     [0063] Boiling water is added to the biomass, bringing the water/fiber ratio to about 15:1. The mixture is agitated, then the pH is adjusted to pH 12 with the addition of a dilute solution of NH 4 OH. The temperature of the resultant solution is adjusted to 100° C. with microwave energy and held at that temperature until the lignin nitrate component is dissolved. The final pH of the solution is at pH 8.  
     [0064] The foregoing step has dissolved the lignin nitrate component of the biomass. The cellulosic fibrous pulp remaining is removed from the cooking vessel. The black liquor, containing the dissolved lignin nitrate can be drained from the cellulosic pulp, or a portion of it can be retained with the pulp, depending upon the desired use of the pulp. The pulp then is screened and treated in accordance with conventional processes to form paper or board.  
     EXAMPLE 2  
     [0065] Pulping of Alder to Make Liner Board  
     [0066] Logs of alder are flaked to a maximum of 5 mm thickness. The flakes are crushed and screen sorted to a maximum size of 3 cm. The bark is removed by holding the flakes in a soaking tank of 1% nitric acid in a 10:1 ratio of acid solution to flakes at a temperature of 75-80° C. using microwave energy for a period of time, typically 15-30 minutes, sufficient to permit the differential in time of absorption of acid solution between bark and wood to show. This permits the bark to sink while the wood flakes float. A sample size of about 8 oz. (225 grams) of alder flakes is obtained for further processing.  
     [0067] A 0.20% nitric acid solution containing 10% of 0.02% concentration of alum is prepared.  
     [0068] The wood flakes are added to a pressure cooker and the acid solution is added in a 15:1 wt:wt ratio of solution:flakes. The vessel is closed and the contents heated to 100° C. using microwave energy. That temperature is maintained for two hours. The acid solution then is drained from the wood flakes. The flakes are washed two times with 1 liter of 100° C water.  
     [0069] The pH of the biomass is adjusted to pH 12 using a dilute aqueous solution of NH 4 OH. The wt:wt ratio of solution to biomass is 15:1. The solution is heated to a temperature of 100° C. and held at that temperature for 30 minutes to dissolve the nitrated lignin. The final pH of the solution is pH 8.  
     [0070] The black liquor is drained away. The partially defibered cellulose remaining is defibered and screened. The cellulose pulp is suitable for making linerboard.