By Dr. B. Cooker, Chemical Processing Solutions, Consultant with Lee Enterprises Consulting
Cheap, plentiful lignocellulosic biomass (LCB), consisting of cellulose, hemicellulose and lignin, is deconstructed to release useful chemical intermediates in the bioeconomy. These include glucose from cellulose, glucose and other sugars from hemicellulose, and oxygenated benzene derivatives (OBTX) from lignin. Reviewed here are recent published developments of note in LCB deconstruction, emphasizing process simplicity, efficiency, and prospective economics.
Renewable plant material is derived from agricultural crops, forest products, the pulp/paper industry and from municipal solid waste (MSW). These large scale industries produce significant amounts of such raw materials for possible conversion to renewables. The plant materials are largely LCBs, containing cellulose (polyglucose), hemicellulose (heteropolymers of glucose and other sugars) and lignin (branched heteropolymer of propyl phenol and oxygenates). These components are intimately mixed in plants, down to cell scale (10 microns). Lignin is tough and resilient, giving structural rigidity. See refs. 1, 2. These same properties cause processing challenges in deconstructing the biomass to sugar and OBTX, efficiently, at high yield and low cost. The biomass-fed renewable chemicals industry historically focused on corn kernel to produce commercial ethanol. The kernels, mainly polyglucose starch, rapidly hydrolyze to glucose. LCB requires more intense deconstruction to obtain the intermediates.
Current LCB Deconstruction
Commercial Biofuel Plants Using Lignocellulosic Biomass
Ref. 3 is on commercial biofuel production, almost entirely to ethanol, in 19 plants, from diverse LCB sources. It shows use of two feed deconstruction/pretreatment technologies: 1. Thermochemical processing, followed by fermentation and/or chemical synthesis (5 plants) and 2. Enzymic hydrolysis (13 plants).
Other Reported Current LCB Deconstruction
Refs. 3 – 5 summarize the LCB pretreatment and deconstruction technologies which have been developed. Pretreatment accelerates saccharification, releasing polysaccharides and sugars. Refs. 3 – 5 indicate that heating under autogenous pressure at 80C to 200C, in aqueous pHs from acidic to alkaline, with enzymes simultaneously or sequentially at the lower temperature, currently dominates the practice on both pilot and commercial scales. Although such treatments promote conventional saccharification enzyme effectiveness, enzyme concentrations sufficient for high sugar yields are expensive. See ref. 5. Reported alternative and supplementary deconstruction techniques include: 1. Blocking enzyme-adsorbing sites on lignin with proteins, increasing enzyme availability to the polysaccharides. 2. Ions and chelating agents to dissolve biomass at or near ambient temperatures, minimizing thermally promoted byproducts and improving energy efficiency. 3. Powerful solvents, such as pyridine. See ref. 4 for the technological details.
The above suggests emphasizing non-chemical, physical deconstruction methods, avoiding deployment of chemical cofeeds. They add operating cost, can promote side reactions, and may need separation and recovery downstream. Possible physical deconstruction and pretreatment includes size reduction, intense shear, high pressure gradients and sonication. Once mechanical deconstruction is fully deployed, resort should be made, additionally, to elevated temperature and other means.
Proposed Hierarchy of LCB Deconstruction Means
The following hierarchy of LCB deconstruction means is proposed, from most favored to less favored: 1. Particle size reduction. 2. Shear. 3. Concentration. 4. Temperature. 5. Pressure. 6. Solvents. 7. Regenerable catalysts. 8. Reactants, reagents, consumed enzymes, pH modifiers and the like.
Recent LCB Deconstruction Developments of Note
Thermal, with Adjunct Mechanical Forces
A thermal LCB deconstruction method, with adjunct shear from pumped stream transfer, without chemical or other inputs was developed by Renmatix Inc. See refs. 6 – 11. The LCB included wood chips and corn stover. Their continuous Plantrose® process has two steps: 1. An aqueous slurry of ground LCB feeds a reactor at 200C, where the hemicellulose is solubilized, yielding the xylose and 2. The slurry feeds the supercritical water reactor, at approximately 375C, the cellulose depolymerizing to glucose. Ref. 12 reports the mean RT is only seconds for the desired conversion, because of the high thermal driving force. Renmatix states in ref. 12 that the residual solid lignin can be burned to generate process steam and power or chemically converted to phenolic intermediates.
Renmatix announced collaboration with Total, BASF and UPM in 2015 and their facilities include the commercial scale. Their continuous thermal processing seems to yield a simple flow sheet, low reaction process mean residence times, high volume-specific rates and minimal chemical input.
Tetrahydrofuran (THF) Solvent, with Adjunct Mechanical Forces
University of CA, Riverside substituted solvent for enzymes in LCB deconstruction. See refs. 13 – 15, which report THF use as a water cosolvent, containing mineral acid, to deconstruct LCBs, including corn stover and wood. The PFD in ref. 14 shows THF-treated biomass solids in the water phase and lignin being 80+% in a separate THF liquid phase. The broken-down, solubilized lignin is isolated from the THF by solvent distillation and recycle. They obtained nearly pure solid lignin, potentially available for chemical conversion. The aqueous phase is enzyme-treated, completing the sugar release, with fermentation to ethanol. They claim the process removes about 90% of the lignin from the biomass and only 10% of the previous enzyme to sugar yield ratio load is needed, at 95% sugar yield. They state this process could be integrated with simultaneous saccharification fermentation (SSF), to produce ethanol, combining enzymic hydrolysis of biomass to sugars with sugar fermentation to ethanol, in one step. See the PFD in ref. 14. CogniTek, via its subsidiary MG Fuels, has purchased commercialization rights to the technology (ref. 15).
Mechanical Forces: Orifice Flow, Continuous Auger Processing
Ref. 16, filed by Edeniq Inc., discloses pumping ground LCB aqueous slurry through flow restrictions at high shear rates, with and without saccharification enzymes, inducing particle size reduction and faster, more complete sugar release than without high shear. The temperatures were less than 100C. Edeniq’s ref. 17 discloses use of continuous augers to saccharify LCBs, with enzymes present throughout, at <70C. The highest shear zones were between the screw flight tips and the barrel wall.
Lignin from LCB Deconstruction
OBTX and derivatives for the bioeconomy potentially originate in the aryl rings of lignin in LCB. Ref. 18 shows recent pulp and paper producer interest in using lignin from their black liquor process residue.
The bioeconomy chemical producers need efficient, scalable, low cost LCB deconstruction processes to yield renewable sugar and OBTX chemical intermediates. Deconstruction of plentiful, cheap LCBs from agriculture, forest products, pulp and paper and MSW receives much needed attention. Current methods are dominated by thermochemical conversion and enzymic saccharification. The hierarchy of LCB deconstruction promoters suggested here, from most to least favored, based on anticipated process simplicity and cost is: 1. Particle size reduction. 2. Shear. 3. Concentration. 4. Temperature. 5. Pressure. 6. Solvents. 7. Regenerable catalysts. 8. Reactants, reagents, enzymes, pH modifiers. Recent developments of note in relation to these processing factors have been reviewed.
About the Author: Dr. Bernard Cooker is a member of Lee Enterprises Consulting, the world’s premier bioeconomy consulting group, with more than 100 consultants and experts worldwide who collaborate on interdisciplinary projects, including those requiring the technologies discussed in this article. The opinions expressed herein are those of the author, and do not necessarily express the views of Lee Enterprises Consulting.
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2 “Producing Biofuels via the Sugar Platform”, C. E. Wyman, B. E. Dale, CEP, March, 2015, p. 45 – 51.
3 “Commercial-Scale Production of Lignocellulosic Biofuels”, T. Brown et al, CEP, March, 2015, p. 62
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6 “Renmatix Gets Total as New Investor”, C&EN, 9 March, 2015, p. 21.
7 “Biobased Assets Change Hands”, M. Bomgardner, C&EN, May 11, 2015 p. 4.
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9 “Green in 2015”, C&EN, August 24, 2015, p. 32 – 35.
10 C&EN, September 23, 2013, p. 19
11 “Bill Gates, Total Invest in Renmatix”, C&EN, September 19, 2016, p. 12
12 www.renmatix.com, Renmatix Inc. website
13 “Co-Solvent Pretreatment Reduces Costly Enzyme Requirements for High Sugar and Ethanol Yields from Lignocellulosic Biomass”, T. Y. Nguyen et al, ChemSusChem, doi:10.1002/cssc.201403045 (Feb 24, 2015).