TECHNICAL FIELD

[0001] The present disclosure relates generally to pulp production and more particularly, to the production of pulp and other co-products from non-wood feedstock.

RELATED APPLICATIONS

[0002] This application claims priority from US application No. 63/350,276 filed June 8, 2022 entitled “DESILICATION AND MANUFACTURE OF LOW CARBON INTENSITY CHEMI-MECHANICAL NON-WOOD PULP AND CO-PRODUCTS”. For the purposes of the United States, this application claims the benefit under 35 USC §119 of US application No. 63/350,276 filed June 8, 2022 entitled “DESILICATION AND MANUFACTURE OF LOW CARBON INTENSITY CHEMI-MECHANICAL NON-WOOD PULP AND CO-PRODUCTS”, which is incorporated herein by reference in its entirety.

BACKGROUND

[0003] Pulp is an important material used in the industrial production of paper products. In traditional pulp production, wood is broken down by chemical or mechanical means to separate lignin from cellulose fibers. The cellulose fibers are then screened, cleaned, dried and collected as pulp in preparation for paper making. Traditional pulp production processes can have an adverse environmental impact by contributing to problems such as climate change and pollution. There is a need for sustainability in the paper industry by finding ways to reduce industrial pressure on forests, excessive water use, and fossil fuel consumption. For example, various non-wood feedstocks are being considered as candidate materials for pulp production. Unlike wood-based feedstock which is typically obtained through harmful environmental practices like deforestation, non-wood feedstock can be obtained from byproducts (e.g., wheat straw, a byproduct of harvesting wheat grains). [0004] However, while desirable from an environmental perspective, non-wood feedstock can be difficult to process compared to traditional wood-based feedstock for several reasons. First, non-wood feedstock tends to have less uniformity (e.g., straw feedstock can be overly long) and lower bulk density compared to wood-based feedstock, which can result in plugging of equipment and operational losses. Second, non-wood feedstock tends to have elevated fines content, which can negatively impact pulp properties and decrease mill productivity due to pulp drainage limitations. Third, non-wood feedstock tends to have elevated silica content, which can foul process equipment and impair their efficiency. The elevated silica content also impairs the lignin precipitation process required to improve the treatability of the effluent (i.e. , the elevated levels of silica present in the process liquor can co-precipitate in a colloidal form, thereby impairing filtration and making lignin removal difficult). In some cases, the lignin precipitation process is further impaired by the presence of hemicellulose in byproduct streams (e.g., black liquor), which can contribute to the formation of hydrogels. The above-noted issues generally result in an energy inefficient process with limited advantages over traditional wood-based pulp production.

[0005] In addition, pulp produced from non-wood feedstock using existing technologies tend to have slower drainage rates compared to traditional wood-based pulp. The slower drainage rates would require the non-wood pulp to be processed with larger equipment. Non-wood pulp also typically have lower tensile strength compared to wood-based pulp. High kappa chemi-mechanical non-wood pulps typically have higher lignin content compared to low kappa chemical non-wood pulps. The elevated lignin content and hornification (i.e., the inhibited reswelling of pulp fibres after drying) incurred during the flash drying of the non-wood pulps can reduce the strength properties of the pulp.

[0006] There is a need for methods and systems that address the aforementioned challenges associated with producing pulp from non-wood feedstock. There is a need for methods and systems that can process pulp filtrates efficiently from non-wood feedstock.

SUMMARY OF THE DISCLOSURE

[0007] In general, the present specification describes systems and processes for producing pulp and other bioproducts from non-wood feedstock. [0008] One aspect relates to a method for producing pulp from non-wood feedstock. The method comprises the steps of desilicating the non-wood feedstock to selectively remove silica therefrom, impregnating the desilicated non-wood feedstock to selectively separate lignin therefrom, mechanically refining the impregnated non-wood feedstock to obtain a pulp stream, performing oxygen-alkali treatment on the pulp stream to separate additional lignin from the pulp stream, and removing the separated lignin from the pulp stream. The desilication step is performed at a first temperature. The impregnation step is performed at a second temperature with an alkaline solution having low alkali charge. The oxygen-alkali treatment is performed at a third temperature.

[0009] In some embodiments, the non-wood feedstock is desilicated with a chemical compound solution. The chemical compound solution may be sodium carbonate, sodium hydroxide or potassium hydroxide. In some embodiments, the non-wood feedstock is mechanically desilicated with a mechanical pulper. The enriched filtrate containing the separated silica may be removed from the stream of non-wood feedstock with a mechanical press.

[0010] In some embodiments, the first temperature is in the range of 50°C to 100°C. In some embodiments, the second temperature is in the range of 100°C to 130°C. In some embodiments, the third temperature is in the range of 95°C to 130°C. The second temperature may be higher than the first temperature. The third temperature may be lower than the second temperature. The third temperature may be higher than the first temperature.

[0011] In some embodiments, the impregnated non-wood feedstock is mechanically refined at medium to high consistency. For example, the impregnated non-wood feedstock may be mechanically refined at a consistency level in the range of 8% to 40%. In some embodiments, water is removed from the pulp stream after completion of the oxygen-alkali treatment. The water may be removed from the pulp stream by drying the pulp stream with superheated steam.

[0012] In some embodiments, the method comprises deconstructing the non-wood feedstock and cutting the deconstructed non-wood feedstock to a nominal length before desilicating the cut non-wood feedstock. The nominal length may be in the range of 40 mm to 100 mm. In some embodiments, the cut non-wood feedstock may be screened to remove fines before desilicating the screened non-wood feedstock and/or before impregnating the screened non-wood feedstock with an alkaline solution. The cut nonwood feedstock may be screened by a primary screen and a secondary screen. The primary screen and the secondary screen may be pyramid screens. The primary screen may have a first set of slots interspaced between a first set of rolls, with the width of each slot in the first set of slots defining a first internal roll opening. The secondary screen may have a second set of slots interspaced between a second set of rolls, with the width of each slot in the second set of slots defining a second internal roll opening. The first internal roll opening may be about 1 mm, and the second internal roll opening may be about 2 mm. In some embodiments, the screened non-wood feedstock is washed before it is desilicated

[0013] Another aspect relates to a method for producing pulp from non-wood feedstock comprising the steps of chemically treating a stream of the non-wood feedstock with a first alkaline solution, performing oxygen-alkali treatment on the stream of desilicated non-wood feedstock, and removing the separated silica and the separated lignin from the stream to obtain the pulp. The desilication is performed at a first temperature. The delignification is performed at a second temperature. The second temperature may be higher than the first temperature.

[0014] Another aspect relates to a system for producing pulp from non-wood feedstock. The system comprises an apparatus for desilicating the non-wood feedstock at a first temperature to selectively remove silica therefrom, and an impregnation tube for impregnating the non-wood feedstock with an alkaline solution at a second temperature to selectively separate silica therefrom. The alkaline solution has low alkali charge. The system comprises a mechanical refiner for refining the impregnated non-wood feedstock into a pulp stream, and a reaction tower for receiving the pulp stream. The reaction tower is configured to receive oxygen and steam to perform oxygen-alkali treatment on the pulp stream to separate lignin therefrom. The system may also comprise a post-processing apparatus connected to the reaction tower to remove the separated lignin from the pulp stream.

[0015] Additional aspects of the present invention will be apparent in view of the description which follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Features and advantages of the embodiments of the present invention will become apparent from the following detailed description, taken with reference to the appended drawings in which:

[0017] FIG. 1 is a flowchart of a process for producing pulp from non-wood feedstock according to an example embodiment.

[0018] FIG. 2 is a schematic diagram of an example embodiment of a system that may be used to implement aspects of the FIG. 1 process.

[0019] FIG. 3 depicts exemplary supplementary processes that can be incorporated or combined with the FIG. 1 process to produce other bioproducts from non-wood feedstock.

DETAILED DESCRIPTION

[0020] The description, which follows, and the embodiments described therein, are provided by way of illustration of examples of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not limitation, of those principles and of the invention.

[0021] Aspects of the present invention relate to systems and processes for producing pulp and, optionally, co-products (e.g., bioproducts) from non-wood feedstock. For the purposes of facilitating the description, the term “non-wood” is used herein to refer to products like straw and/or other agricultural residuals that remain after harvesting. The term “straw”, as used herein, includes but is not limited to: wheat straw, barley straw, oat straw, flax straw, rice straw, hemp, bamboo, miscanthus, sorghum, switchgrass, ryegrass, com stover, bagasse and banana tree.

[0022] The systems and processes may involve treating the non-wood feedstock with alkaline solutions and/or alkaline liquor in one or more steps. For the purposes of facilitating the description, the term “alkaline solution” is used herein to refer to a solution containing only inorganic dissolved solids of chemical compounds (e.g., Na ₂ CO3, NaOH, KOH, etc.). For the purposes of facilitating the description, the term “alkaline liquor” is used herein to refer to a solution containing both inorganic dissolved solids of chemical compounds (e.g., Na ₂ CO3, NaOH, KOH, etc.) and organic dissolved solids (e.g., lignin, cellulose, hemi-cellulose, etc.).

[0023] Illustratively, systems and processes described herein can overcome the challenges associated with the elevated silica levels of non-wood feedstocks to provide a low carbon intensity solution for the production of non-wood pulp. For example, systems and processes described herein can overcome problems associated with hornification during the pulp production process. By overcoming such problems, the non-wood pulp that is produced will have reduced fines contents and/or improved material properties. In addition, systems and processes described herein can provide improved effluent treatability, lignin precipitation, and filterability. This can help facilitate the valorization of straw fines and sulphur free lignin and silica as co-products like biofuel pellets, sodium silicate, etc. Systems and processes described herein can also increase drainage rates of non-wood pulp to allow for a reduction in the size of the equipment required to process the non-wood pulp efficiently.

[0024] FIG. 1 is a flowchart of a process 10 for producing pulp 4 from non-wood feedstock 2 (e.g., straw feedstock) according to one embodiment. Process 10 is a hybrid pulping process that utilizes a combination of chemical and mechanical means to produce pulp 4 from feedstock 2. Such hybrid processes take advantage of both the benefits of chemical pulping (e.g., selectivity and pulp strength) and the benefits of mechanical pulping (e.g., high yield) to provide a relatively low carbon intensity solution for the production of non-wood pulp. Process 10 may be utilized to produce pulp 4 from various different types of straw feedstock 2 and other types of non-wood feedstock. For the purposes of facilitating the description, process 10 is described herein in relation to straw feedstock 2.

[0025] Process 10 begins with optional straw preparation step 20. Straw preparation step 20 comprises separating fines 6 from straw feedstock 2 and removing the separated fines 6 as dry material. To remove fines 6 at step 20, straw feedstock 2 may be screened or otherwise passed through one or more layers of screens. The fines 6 removed from straw feedstock 2 can be optionally converted into useful co-products, such as a pellet fuel product, in supplementary processes (e.g., see FIG. 3).

[0026] In one embodiment, step 20 comprises pre-processing straw feedstock 2 before passing the pre-processed straw feed stock 2 through the screen(s). The pre-processing of straw feedstock 2 may include one or more of de-stacking, de-twining and/or deconstructing straw bales to provide loose straws. The loose straws may then be cut individually or in bulk to relatively uniform lengths. For example, the loose straws may be cut to a length in the range of about 40 mm to about 100 mm (e.g., 50 mm, in particular embodiments). Long straw strands may cause operational problems in non-wood pulping operations. Accordingly, it can be desirable to cut long straw strands to more nominal lengths to mitigate such problems. Cutting long straw strands to more nominal lengths can reduce the variability in the final pulp product, increase bulk density of feedstock 2, and/or improve the reliability of the overall pulping process 10.

[0027] To prepare the straw at step 20, the pre-processed feedstock 2 is transferred to a screening system. The pre-processed feedstock 2 may, for example, be transferred to the screening system through a conveyor belt system. The conveyor belt system may include a detection system that will reject the pre-processed straw feedstock 2 if residual twines are present. If the pre-processed straw feedstock 2 is rejected by the detection system, it may be returned for further pre-processing before it is transferred to the screening system.

[0028] The screening system may include one or more layers or stages of screens. Each layer of the screens may have openings that remove straw fines 6 from the feed stream of feedstock 2 (i.e. , straw fines 6 will be removed from feedstock 2 as they drop through the openings). Each layer of the screens may have different sized openings.

[0029] The screening system may optionally include a dust collection system for collecting dust in feedstock 2. The dust collection system may include dust hoods located at various transfer points for feedstock 2. The dust hoods may be connected to one or more draft fans or the like. The draft fan(s) may be operated to draw the dust into a baghouse, or the like, where the dust will be filtered out of the air. The dust collection system can help prevent the build-up of straw dust and fines 6 in the plant for housekeeping and fire prevention purposes.

[0030] In one embodiment, the screening system includes two levels of screens (i.e., a primary screen and a secondary screen). Each level of the screens may include slots interspaced between rolls. Each level of screens may be, for example, a pyramid roll screen. Each level of the screens may be configured to sort feedstock 2 based on length or other dimensions into an accepted stream (e.g., stream with straws having lengths greater than a threshold length) and a rejected stream (e.g., stream with straw fines). The screens may be configured, for example, by adjusting the size of the gaps between slots or spacing between the rolls. The size of the gaps between the slots or spacing between the rolls may be referred to herein as Internal Roll Opening (I O). The IRO may be adjusted before or during operation of the screening system. In particular embodiments, this feature makes it possible to optimize the screening process during operation. For example, the IRO may be varied to control the amount or size of fines 6 that are filtered out of feedstock 2.

[0031] In one embodiment, the fines 6 at the initial level of screening will fall between the inter-roll slots onto, for example, a fines conveyor belt to enter the rejected stream, while the “overs” (i.e., straws having lengths greater than the threshold length) will be discharged from the initial level to one or more subsequent levels of the screens. The one or more subsequent levels of screens may be configured to screen relatively larger fines 6 or fines 6 that would not have been screened out by the preceding level of screens. For example, the IRO of the first level of screens may be set at about ~1 mm and the IRO of the next or second level of screens may be set at about ~2 mm.

[0032] In one embodiment, the relatively larger fines 6 at the second level of screening will fall between the inter-roll slots onto, for example, the fines conveyor belt, while the overs will be discharged from the second level of screens.

[0033] The fines 6 that are removed from the feed by the second level screen may be collected along with fines 6 that are removed by the first level screen. The collected fines 6 may be stored or provided to a separate or supplementary systems for further processing. For example, the collected fines 6 may be transported to a straw pellet plant to produce biofuel pellets.

[0034] Since non-wood feedstock like straw feedstock 2 inherently have high concentrations of fines 6 that add little value to the pulp properties, it can be desirable to pre-process straw feedstock 2 in the manner described above during optional preparation step 20. By removing the fines 6 and/or straw dust from straw feedstock 2 as dry material, they can be more efficiently converted into value added co-products (e.g., biofuel pellets) that can be used, for example, to generate power.

[0035] The stream of straw feedstock 2 output by the screening system may be washed by a straw washing system before it is transported (e.g., by conveyor belt) to the next stage of processing. The washing system may be included as a part of the screening system or provided as a separate system. The washing system may wash the straw in hot water. Since agricultural feedstock typically has high concentrations of sand, grit and rocks due to the nature of the harvesting operations, it can be desirable to wash feedstock 2 to remove debris and contamination prior to the pulping operations. In addition, the washed straw can more uniformly absorb the alkaline cooking chemicals in the desilication and digesting process, as described elsewhere herein, resulting in more consistent pulp properties (i.e., lower kappa variability).

[0036] Optionally, the accepted stream of straw feedstock 2 may pass through a weight scale before it is desilicated in step 25. The weight scale may be configured to monitor the total mass flow rate of straw feedstock 2.

[0037] After completing straw preparation step 20, process 10 proceeds to step 25 where the accepted stream of feedstock 2 is treated with hot water or a chemical solution (e.g., an alkaline solution) to pre-filter or otherwise separate some of the silica 7 from the stream of feedstock 2. The chemical treatment may be conducted for a period of ten to thirty minutes or more. In some embodiments, the stream of feedstock 2 is heated and chemically desilicated with compounds like sodium carbonate, sodium hydroxide, potassium hydroxide, or the like. Feedstock 2 may also be treated with hot water without the addition of alkaline chemicals in step 25. In contrast to organic compounds like weak black liquor which cause lignin to co-precipitate with silica upon pH adjustment, the compounds used in desilication step 25 can preferentially separate silica 7 from the stream of feedstock 2.

[0038] To preferentially separate silica 7 from feedstock 2, desilication step 25 may be conducted under atmospheric pressure at temperatures in the range between 50°C to 100°C. If a strong alkali (e.g., NaOH) is used in step 25, then a relatively low reaction temperature (e.g., ~60°C) may be sufficient to prioritize silica removal over delignification reactions. If a weak alkali (e.g., Na ₂ CO3) or hot water is used in step 25, then a relatively high reaction temperature (e.g., ~90°C) may be required to prioritize silica removal over delignification reactions. In some embodiments, desilication step 25 is conducted at relatively low consistency levels or consistencies in the range of 1% to 5%.

[0039] In some embodiments, the chemical desilication of feedstock 2 is supplemented with mechanical desilication to provide a chemi-mechanical process in step 25. In such embodiments, desilication step 25 may comprise mechanically pulping the stream of feedstock 2 after it has been heated and chemically reacted (e.g., in alkaline solution) to mechanically separate more silica 7 from feedstock 2.

[0040] Upon completion of desilication step 25, a relatively large amount of silica 7 will be dissolved in the alkaline desilication liquor and selectively removed from the stream of feedstock 2 as shown in FIG. 1. The silica enriched filtrate (i.e. , the liquor containing the dissolved silica) may be removed from the stream of feedstock through a mechanical press. In addition, trace metals (e.g., copper, iron, nickel, manganese, other transition metals, and the like) may also be separated from the stream of feedstock 2 and removed with the silica enriched filtrate upon completion of step 25.

[0041] Since it is generally difficult to precipitate or separate lignin from feedstock 2 in the presence of silica (i.e., silica can act as a protective barrier for the lignin to hamper the delignification process), it can be desirable to remove silica 7 from the stream of feedstock 2 (e.g., as sodium silicate liquor) before delignification. In addition, it is generally desirable to avoid mixing lignin with silica in black liquor (i.e., the by-product obtained from the process of digesting feedstock into pulp by removing lignin and other extractives from the feedstock to free the cellulose fibers), to the extent possible, since mixtures of lignin and silica can be difficult to manage in supplementary processes. Conducting desilication step 25 in advance of impregnation step 30 can also help improve the drainage properties of the final pulp product.

[0042] After completing desilication step 25, process 10 proceeds to impregnation step 30 where feedstock 2 is heated and impregnated with compounds such as sodium carbonate, sodium hydroxide, potassium hydroxide, or the like. In some embodiments, impregnation step 30 is optional.

[0043] In contrast to conventional processes operated at high temperature and with aggressive alkali to remove lignin directly from the feedstock, impregnation step 30 is typically performed at a relatively low temperature (e.g., below 130° C) and with alkaline impregnation solution that has a relatively low alkali charge (e.g., low alkali soda). This prioritizes, at step 30, the selective removal of lignin 8A from feedstock 2 and the preparation of fibers for further processing downstream. [0044] Compared to desilication step 25, impregnation step 30 is typically performed under relatively higher pressures and/or at relatively higher temperatures. In some embodiments, impregnation step 30 is performed under pressures in the range of 1 bar to 11 bar. In some embodiments, impregnation step 30 is performed at temperatures between 100°C to 130°C. In some embodiments, impregnation step 30 is performed at consistencies in the range of 8% to 30%. The alkali compounds used in step 30 can swell the fibers of feedstock 2 to make them more accessible to further refining and delignification downstream.

[0045] In addition to selective lignin removal and fiber preparation, impregnation step 30 may also result in the further separation and removal of trace metals (e.g., including but not limited to the transition metals of copper, iron, nickel, manganese, or the like) from feedstock 2. This can be advantageous since high levels of transition metals in pulp can negatively impact the performance and selectivity of oxygen-alkali based delignification step 50.

[0046] In some embodiments, impregnation step 30 is performed in an impregnation tube. The impregnation tube may be connected to a digester blow tank. In some embodiments, impregnation step 30 is performed for approximately fifteen (15) to twenty- five (25) minutes. In some embodiments, impregnation step 30 comprises mechanically pressing feedstock 2 after it has been heated and impregnated in alkaline solution to squeeze or mechanically remove lignin 8A from feedstock 2. Upon completion of impregnation step 30, a relatively large amount of lignin 8A will be dissolved in the alkaline impregnation liquor and removed from the stream of feedstock 2 as black liquor 8 (as shown in FIG. 1). For example, between 60% to 70% of the lignin originally found in feedstock 2 may be removed therefrom upon completion of step 30.

[0047] After impregnation step 30, process 10 proceeds to refining step 40. In refining step 40, the desilicated and impregnated feedstock 2 is subject to medium to high consistency refining, where consistency is defined as the weight percentage of solids in the pulp slurry mixture (or in other words, the proportion of the solids in the pulp slurry mixture, as measured by weight). The level of refining of the desilicated and impregnated feedstock 2 may be medium consistency refining (e.g., 8% to 12% consistency), medium-high consistency refining (e.g., 13% to 19% consistency), or high consistency refining (e.g., 20% to 40% consistency) in particular embodiments. The medium to high consistency refining may be implemented by a mechanical refiner. The mechanical refiner may be, for example, a blowline refiner. The mechanical refiner may include a stator and a rotor that are operable to drive one or more pairs of plates to move in a synchronized fashion. The plates may be made of metal or other suitable materials. In some embodiments, refining step 40 is performed for approximately one to three minutes. The refining action can help de- fibri Hate the chemically treated feedstock 2 before additional lignin 8B is removed from the refined pulp stream in delignification step 50.

[0048] In contrast to conventional mechanical pulping techniques that are performed at low consistency (e.g., 3% to 4% consistency), medium to high consistency refining step 40 can encourage more fiber to fiber contact and less fiber to metal contact. This allows the desilicated and impregnated feedstock 2 to be refined into a pulp stream in a gentler fashion.

[0049] After refining step 40, process 10 proceeds to a further delignification step 50. Delignification step 50 is typically performed at temperatures in the range of 95°C to 130°C although other temperatures are also possible. Delignification step 50 comprises performing oxygen-alkali treatment on the pulp stream formed from mechanically refining feedstock 2 in step 40 to separate more lignin 8B therefrom. The oxygen-alkali treatment is performed by adding oxygen, alkali, and steam into the refined pulp stream. In one embodiment, the amount of added oxygen is about 1% to 3% by weight and the amount of added alkali is about 2% to 10% by weight. Examples of suitable alkali compounds include but are not limited to: sodium hydroxide, potassium hydroxide, and other strong alkali compounds. In one embodiment, a supplemental compound such as hydrogen peroxide is added to the oxygen and alkali stream during step 50. Additional compounds like magnesium sulphate may also be utilized to enhance the selectivity of the oxidation reaction.

[0050] Compared to impregnation step 30, delignification step 50 is typically conducted at lower temperatures, lower pressure levels (e.g., 2 bar to 7 bar), and lower consistency (e.g., 8% to 12%). In general, delignification step 50 is more selective in removing lignin over the other carbohydrates (e.g., cellulose and hemicellulose) that may remain in the nonwood fibers contained in feedstock 2. Conducting delignification step 50 may, in some cases, help produce a final pulp product 4 that is brighter and/or has enhanced L*a*b* (i.e. , lightness, red/green value, blue/yellow value) color attributes. [0051] The oxygen-alkali treatment performed in step 50 may also shift the silica remaining in the pulp (i.e. , residual silica that was not removed in step 25) onto the pulp fibres and out of the alkaline process liquors. The oxygen-alkali treatment may be performed in a reaction tower, or the like. In particular embodiments, the oxygen-alkali treatment is performed for a duration of approximately thirty (30) to sixty (60) minutes.

[0052] In one embodiment, an inline booster pump, or the like, is configured to maintain stage pressure and to transport the refined pulp to a downstream blow tank, or the like, where a combination chemical/steam mixer facilitates the addition of oxygen and steam to the refined pulp stream before it enters the reaction tower. The pressure may be maintained at, for example, about 4 bar to 6 bar.

[0053] After delignification step 50, process 10 proceeds to a post-processing step 60. In post-processing step 60, the lignin 8B separated from the pulp stream during step 50 is removed from the pulp stream. In one embodiment, step 60 comprises washing the pulp stream to remove the black liquor 8 containing lignin 8B from the pulp. In one embodiment, the ratio between the amount of lignin removed in step 30 and the amount of lignin removed in step 60 is about 2:1. For example, between 20% to 30% of the lignin originally found in feedstock 2 may be removed therefrom upon completion of step 50.

[0054] Lignin 8A, 8B removed from the pulp stream may be collected for use in one or more supplementary processes (e.g., supplementary process 80 in FIG. 3). Post-processing step 60 may also include one or more stages of pulp screening. Pulp screening may include the removal of fines from the pulp produced in step 50. The removal of fines, both primary and secondary, after the oxygen-alkali treatment improves the drainage rate of the pulp and reduces the surface area of the dewatering equipment required to process the pulp. This can lead to capital cost savings and improved pulp properties.

[0055] After completing step 60, a high kappa pulp stream is typically obtained (e.g., a pulp stream having a kappa number that is greater than 30). In the pulp and paper industry, the kappa number is a dimensionless indicator of the bleachability of pulp. The kappa number is approximately proportional to the residual lignin content of the pulp. The kappa number may be defined as the product of a constant and percentage lignin content (e.g., Kappa Number ~ 6.578*L, where L is percentage lignin content). In some embodiments, the pulp stream has a kappa number that is greater than 30. For example, the pulp stream may have a kappa number that is in the range of 40 to 50 for some applications (e.g., unbleached paper towels). A high kappa number with a low standard deviation can be a good indicator of high uniformity in the pulp stream.

[0056] Due to higher pulp kappa numbers of the pulp stream and the independent removal of silica 7 and lignin 8A, 8B (i.e. , removal of silica 7 in step 25, and removal of lignin 8A, 8B in steps 30, 50), the effluent properties of black liquor 8 can be improved with reduced levels of biochemical oxygen demand (BOD), chemical oxygen demand (COD), and toxicity.

[0057] The above-described steps of process 10 can, alone or in combination, help enhance the selective removal of lignin 8A, 8B from the straw while reducing the amount of silica 7 in the impregnation and oxygen-alkali cooking liquor. For example, the targeted delignification in step 50 combined with fines removal in the filtrate in step 60 provides the pulp stream with increased freeness characteristics and improved pulp properties. In addition, reduced processing temperatures requirements (e.g., in both impregnation step 30 and delignification step 50) reduces the carbon intensity required to produce a non-wood fibre with properties that make the fibre acceptable for use as pulp.

[0058] After post-processing step 60, the pulp stream is dried in step 70 to form the final pulp product 4. In some embodiments, the pulp stream is moved to a high density storage before it is transported to a pulp drying and finishing area. The pulp drying and finishing area may include one or more of the following: one or more drying sections (e.g., complete with a fan and cyclone separator), a pulp cooling stage, a discharge air scrubber, a gas burner, a reboiler, and a superheater. In such embodiments, the pulp stream may be fed from the high density storage to presses (e.g., twin roll presses (TRP)) that operate in parallel to dewater the pulp stream). The dewatered pulp (e.g., pulp with greater than 48% consistency) from each TRP may be metered into a pressure-specifying sensory device (PSSD) by a rotary valve.

[0059] In some embodiments, drying of the pulp at step 70 is accomplished with the use of superheated steam. The water that is evaporated from the pulp stream through the superheated steam treatment may be removed from the dryer and used for process heating. For example, the water may be used to heat the stream of feedstock 2 in impregnation step 30 and/or delignification step 50. Under steady state operating conditions, the superheated steam dryer used in step 70 can create about 2.8 Bar of steam. This steam can displace boiler steam, low grade steam, and high grade hot water (e.g., heated to 130°C). The integration of these energy streams into the pulping process can help lower the carbon footprint relative to other thermal drying processes. In addition, the use of superheated steam can provide improvements in pulp properties of high kappa pulps. For example, superheated steam dried high kappa pulps are typically bulkier, stronger, tougher and stiffer than conventional flash dried pulps. Superheated steam dried high kappa pulps can also have a more desirable color (e.g., a darker or richer color) compared to conventional flash dried pulps.

[0060] Process 10 may be implemented by various apparatuses which collectively form a chemi-mechanical system for producing pulp 4 from non-wood feedstock 2. The system may be located or otherwise provided within a pulp production plant 200 (e.g., see FIG. 3).

[0061] FIG. 2 is a schematic diagram of an example embodiment of a system 100 that may be used to implement processes of the type shown in FIG. 1 . System 100 comprises an impregnation tube 102 located upstream of a blow tank 104. Impregnation tube 102 may be designed or otherwise adapted to perform step 30 in process 10. Impregnation tube 102 has a chamber for impregnating washed straw, or other feedstock 2 prepared in step 20 and desilicated in step 25, with steam and one or more alkali compounds. Blow tank 104 may be operated to transfer the desilicated and impregnated feedstock 2 from the chamber of impregnation tube 102 to refiner 106 at desirable pressures and/or consistency levels.

[0062] A press 105 is located between impregnation tube 102 and refiner 106. Press 105 may be located upstream or downstream of blow tank 104. In some cases, it may be advantageous to position press 105 upstream of blow tank 104 to reduce the energy consumption required to treat black liquor 8 in subsequent processes. In the embodiment illustrated in FIG. 2, press 105 is provided between blow tank 104 and refiner 106 to squeeze or mechanically remove pressate carrying the separated lignin 8A from the rest of the stream carrying the desilicated and impregnated feedstock 2. The pressate may be stored in a tank 107 before it is transferred to supplementary systems for further processing (e.g., see process 80 in FIG. 3).

[0063] Refiner 106 receives the desilicated and impregnated feedstock 2 from blow tank 104. Refiner 106 may be designed to perform step 40 in process 10. In the example embodiment illustrated in FIG. 2, refiner 106 also receives one or more alkali compounds to support the mechanical refining. The alkali compounds provided to refiner 106 may be the same compounds as those provided to impregnation tube 102, or they may be different compounds in other embodiments. Refiner 106 may be suitably configured to refine the impregnated feedstock 2 at medium to high consistency.

[0064] Dilution conveyor 108 is located downstream of refiner 106. Dilution conveyor 108 may be operated to transfer the refined pulp stream to oxygen alkali reactor 110 where step 50 is performed. In one embodiment, system 100 comprises a mixer 109 located between dilution conveyor 108 and reactor 110. Mixer 109 may be operated to mechanically mix the pulp stream with steam and/or oxygen, and deliver the mixture to reactor 110.

[0065] Downflow tower 112 is located downstream of oxygen alkali reactor 110. Downflow tower 112 is designed to act as a storage buffer between reactor 110 and additional screening and pulp washing apparatuses located further downstream (not shown). Downflow tower 112 includes a port for receiving a dilution filtrate and a chamber for mixing the dilution filtrate with the pulp. Since pulp can be difficult to pump at higher consistencies, downflow tower 112 can provide the additional reaction time needed to dilute the pulp to relatively lower consistencies before pumping the diluted pulp to the screens and pulp washing apparatuses.

[0066] A wide range of variations and additions are possible within the scope of the present invention. These include supplementary processes that may be performed in conjunction with the main process 10 of producing pulp 4 described herein. FIG. 3 depicts some exemplary supplementary processes 80, 82, 84 that can be incorporated or combined with the process 10 to produce useful co-products from non-wood feedstock 2.

[0067] In one exemplary supplementary process, a lignin precipitation process 80 is incorporated to convert black liquor 8 produced in process 10 to useful materials for electronics applications (e.g., supercapacitors) and/or chemical applications (e.g., chemical emulsions, chemical adhesives, etc.). In another exemplary supplementary process, a thermal process 82 (e.g., a hydrothermal liquefaction process) is incorporated to produce low carbon transportation fuels from fines 6 and black liquor 8. In another exemplary supplementary process, a pelletization process 84 is incorporated to produce biofuel pellets from fines 6. In another exemplary supplementary process, a silica precipitation process 90 is incorporated to convert silica compounds (e.g., sodium silicate) produced in desilication step 25 to materials like silicic acid for use in additional high value products.

[0068] The examples and corresponding diagrams used herein are for illustrative purposes only. Different configurations and terminology can be used without departing from the principles expressed herein.

[0069] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the scope of the invention. The scope of the claims should not be limited by the illustrative embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole. For example, various features are described herein as being present in “some embodiments” or in “one embodiment”. Such features are not mandatory and may not be present in all embodiments. Embodiments of the invention may include zero, any one or any combination of two or more of such features. This is limited only to the extent that certain ones of such features are incompatible with other ones of such features in the sense that it would be impossible for a person of ordinary skill in the art to construct a practical embodiment that combines such incompatible features. Consequently, the description that “some embodiments” possess feature A and “some embodiments” possess feature B should be interpreted as an express indication that the inventors also contemplate embodiments which combine features A and B (unless the description states otherwise or features A and B are fundamentally incompatible).