Biorefineries convert biomass or biological materials into fuels, energy, chemicals and/or bioproducts (King, 20120). Many biorefining technologies exist or are under development. Most prevalent at present are biorefineries for the production of fuels, such as biodiesel from vegetable oils, and alcohol from grain, sugar cane or from lignocellulosic sources. Chemicals generated by biorefineries may provide the building blocks for the chemical industry, referred to as green platform chemicals, which replace fossil fuel generated platform chemicals (Cherubini and Strømman, 2011). Biorefineries generate these green platform chemicals purposely or as a by-product stream and in either case, these may be valuable products.
Natural biopolymers and renewable sources of fuels and chemicals are increasingly important environmentally and economically (King, 2010). Renewable resources are a means of reducing our dependence on conventional fossil fuels for fuel and chemicals. Renewable resources can provide for basic chemical constituents that are needed for many industries, such as monomers for plastics. Biomass can provide fuel and chemicals along with many specialized products such as cellulose for foods and paper, which cannot be obtained in any other manner.
Biomass typically refers to and any material of biological origin, living or dead, and includes, but is not limited to, plant parts, fruits, vegetables, plant processing waste, chaff, grain, meals, straw, hemp, grasses, oat hulls, rice hulls, corn, corn husks, cotton gin waste, weeds, aquatic plants, hay, forestry products, wood chips, wood waste, wood pulp, pulping byproducts, paper, paper products, paper waste, or peat. Hydrocarbons such as coal, lignite coal, oil, heavy oil or tar may also be considered biomass materials in some instances.
The primary constituents of lignocellulosic biomass are lignin, hemicellulose and cellulose, of which lignin comprises about 6% to 40% by weight. Lignin carries proportionately more of the recoverable energy in biomass. However, recalcitrance of lignin is a serious impediment in the biorefining of lignocellulosic biomass. Lignin is heterogeneous and lacks a primary defining structure, comprising chains of aromatic and oxygenate constituents forming larger molecules that are not easily treated by most currently available processes.
Lignocellulosic biomass is typically comprised of about 38% to 70% of cellulose by weight depending on source, with hardwoods and hemp straw containing higher levels of cellulose. Hemicellulose content in biomass is variable ranging from about 10% to 30%, with higher amounts found in agriculture-sourced biomass such as wheat straw and oat hulls.
It is well-known in the art (Sixta, 2006) to process biomass such as wood and other lignocellulosic material to obtain cellulose through well known processes such as Kraft pulping and bleaching processes such as elemental chlorine free (ECF) and total chlorine free (TCF) bleaching. Wood chips are digested in a Kraft digester to produce brown pulp which has a kappa number (K) of about 25, which is an indication of the residual lignin content or bleachability of the pulp. The brown pulp is screened and then passed through an oxygen delignification process, followed by usually several steps of hydrogen peroxide bleaching at alkaline pH and filtering and drying, to reduce the K to less than about 5, and produce Kraft bleached pulp.
It is well known to those in the art (Sixta et al., 2006) that it is critically important to avoid presence of transition metals during the hydrogen peroxide bleaching process. Reactive oxygen species, particularly hydroxyl radicals, generated through the Fenton reaction cause oxidative damage of the cellulose affecting pulp quality. For this reason, the pulping industry commonly employs chelating agents in the bleaching process to capture transition metals and prevent or minimize the Fenton reaction with hydrogen peroxide.
Microcrystalline cellulose (MCC) is a valuable biopolymer used in the food and pharmaceutical sectors and in industrial applications such as in oil, gas and mining. The predominant industrial process for generating MCC is well established (U.S. Pat. Nos. 2,078,446; 2,978,446 and 3,146,168). The process exposes highly pure cellulose such as dissolving grade alpha cellulose or Kraft pulp to a strong mineral acid digest, followed by a physical size reduction. Digestion with hydrochloric or sulfuric acid removes amorphous domains within cellulose fibrils, leaving fragments of cellulose fibrils with high crystallinity. However, the yield of industrial production is low (as low as 30%). Size range of MCC is variable and can be from 30 to about 100 microns and higher. The MCC then is processed and sorted to achieve specific ranges in size and form depending on the desired application. MCC can be further processed such as through blending with attriting aids (U.S. Pat. No. 6,037,380), grinding, homogenization, microfluidization or treatment with ultrasound to achieve smaller sizes, including less than about 1 micron, to generate solutions with colloidal properties. The predominant production process for MCC using acid hydrolysis is expensive due to high capital and operating costs, and the use of corrosive mineral acids is problematic with respect to safety and environment.
Microfibrillated cellulose (MFC), also known as cellulose nanofibrils and microfibrils, is a cellulose pulp where extensive defibrillation of the cellulose fibrils has occurred by mechanical delamination. The diameter of the fibrils is from about 5 to 60 nm, and the length can be several microns long. No acid digestion takes place and these fibrils do not have increased crystallinity compared to the parent material and are not considered to be crystalline cellulose. Mechanical delamination of the fibrillar structure in MFC production can be enhanced by increasing the friction of the fibrils through oxidation of cellulose fibers using a transition metal salt and hydrogen peroxide (U.S. Patent 2006/0289132 A1), persulfate salts (U.S. Pat. No. 5,580,974) or TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) (US Patent 2010/0233481; Saito et al., 2007). The major drawback of the MFC production process is its very high energy requirement to achieve the required physical delamination (700-1400 MJ kg−1 vs. <7 MJ kg−1 for TEMPO and NCC; Isogai et al., 2011).
The most common process for generating nanocrystalline cellulose (NCC) is similar to that of MCC production, consisting of digestion with a strong mineral acid (such as 64% sulfuric acid), followed by mechanical size reduction (Klemm et al., 2011). Diverse parent materials can be used but wood pulp is predominant. Nanocrystalline cellulose fragments (also known as whiskers, nanowhiskers or nanocrystals) are generated with variable sizes reported in the literature (widths from 5 to 70 nm and lengths from 100 to several thousand nm). Physical properties of NCC are strongly influenced by source of parent material, the type of acid used in digest (hydrochloric or sulfuric), charge and dimensions. Several mechanical size reduction processes can be used following the acid digest such as ultrasonic treatment (Filson and Dawson-Andoh, 2009; Klemm et al., 2011), cryogenic crushing and grinding, and homogenization such as fluidization, which also increase yield. NCC may also be generated from MCC using strong mineral acid hydrolysis followed by separation by differential centrifugation, which results in a narrow size distribution of the NCC (Bai et al., 2009). The use of strong mineral acid hydrolysis for the production of NCC either from biomass sources or from MCC encounters the same economic, environmental and safety limitations as for the production of MCC.
TEMPO oxidation may be used to produce NCC with high carboxylate content and high dispersion in water (Isogai et al., 2011). Hirota et al. (2010) demonstrated high yield of NCC from mercerized wood cellulose oxidized using TEMPO at pH 4.8 for 1-5 days followed with ultrasound treatment. The TEMPO oxidation of MCC generated by acid hydrolysis from wood cellulose or mercerized cellulose resulted in lower yield of NCC with lower carboxylate content and comparatively lower dispersion in water.
Oxidation of biomass from renewable sources in a one-step procedure with ammonium persulfate has been reported to generate NCC with a high degree of carboxylation (WO 2011/072365 A1; Leung et al., 2011). The yields of NCC from hemp, flax, wood and MCC were 36%, 28%, 36% and 84%, respectively.
The value of refining lignocellulosic biomass into primary constituents and platform chemicals may be significantly enhanced with new, preferably environmentally friendly, processes that may increase yield, generate novel or improved end products, and/or are low cost, safe and non-polluting.