Sulfite pretreatment for biorefining biomass
Inventors
Wang, Gaosheng • Pan, Xuejun • Zhu, Jun Yong • Gleisner, Roland L.
Assignees
US Department of Agriculture USDA • Wisconsin Alumni Research Foundation
Publication Number
US-9090915-B2
Publication Date
2015-07-28
Expiration Date
2029-04-17
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Abstract
The present invention relates to a method using sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL). More specifically, it relates to a sulfite-based chemical process for pretreating biomass in solutions to reduce access barriers of enzymes to the lignocellulose, resulting in efficient conversion through enzymatic saccharification.
Core Innovation
The present invention relates to a sulfite pretreatment method, referred to as Sulfite Pretreatment to Overcome Recalcitrance of Lignocellulose (SPORL), for treating biomass to overcome the physical and chemical barriers imposed by lignocellulose. The process uses sulfite or bisulfite chemicals in aqueous solutions at specified pH and temperature ranges to alter the chemical and physical structure of the biomass. This reduction in biomass recalcitrance facilitates enhanced access of enzymes to cellulose, leading to efficient conversion of cellulose into fermentable sugars through enzymatic saccharification.
The problem being addressed by the invention is the recalcitrance of lignocellulosic biomass, which is a major impediment to effective enzymatic cellulose bioconversion. Lignocellulose comprises cellulose, hemicellulose, and lignin forming robust and hydrolytically stable plant cell walls that resist degradation. Existing pretreatment methods typically require substantial size reduction of biomass feedstock to very small particle sizes prior to chemical pretreatment to achieve satisfactory cellulose conversion efficiencies but consume significant mechanical or thermal energy. Softwood biomass, which comprises a significant portion of renewable feedstock, is particularly challenging to pretreat effectively. Additionally, many existing pretreatment processes suffer from low cellulose digestibility, slow saccharification rates, and lack of mature commercial equipment, all of which limit commercialization.
The invention proposes a method that is effective in pretreating chip or chop size biomass directly without the need for extensive prior size reduction. SPORL pretreatment conditions, including sulfite or bisulfite dosage, pH adjustment, and temperature control, significantly remove hemicellulose, partially remove lignin without excessive delignification, and prevent lignin condensation by promoting lignin sulfonation. This process achieves over 90% cellulose conversion efficiency in a short enzymatic hydrolysis time with low enzyme loading. Moreover, the pretreatment alters biomass structure favorably to reduce energy consumption in subsequent mechanical size-reduction steps such as disk milling or hammer milling, thereby improving the overall efficiency and economics of biomass bioconversion.
Claims Coverage
The patent document includes one independent claim outlining the core inventive method and multiple dependent claims refining aspects of the process. Below are the main inventive features extracted from the independent claim and its dependents.
Sulfite or bisulfite pretreatment at controlled pH and temperature
A method of treating cellulose-containing woody biomass comprising hemicellulose and lignin by treating for 0.1 to 120 minutes with a sulfite or bisulfite base adjusted to an initial pH of about 1.5 to 5.0 at temperatures between about 150 to 250° C. The sulfite or bisulfite charge is greater than 0 and no more than 9% by weight based on oven-dried biomass. The pretreatment removes hemicellulose with only partial delignification.
Inclusion of mechanical size-reduction or steam explosion
The biomass can be mechanically size-reduced by disk milling or hammer milling or treated by steam explosion prior to or during the sulfite or bisulfite pretreatment step. Size reduction may occur before or after pretreatment, or pretreatment may be integrated with mechanical processing steps.
Use of sulfur dioxide with sulfite or bisulfite
The sulfite or bisulfite pretreatment base may comprise sulfur dioxide in addition to sulfite or bisulfite to enhance the pretreatment efficacy.
Specific mass ratio ranges for sulfite or bisulfite to biomass
The mass ratio between sulfite or bisulfite and biomass in the pretreatment is about 1:100 to 1:10 or more specifically about 1:40 to 1:15, controlling chemical dosage for optimal processing.
Subsequent enzymatic hydrolysis and fermentation or chemical conversion
Following pretreatment, enzymatic hydrolysis is conducted to convert cellulose to sugars. The sugars may then be fermented to ethanol or chemically converted to other biofuels or biochemicals.
The patent claims cover a biomass pretreatment process using sulfite or bisulfite at controlled pH and temperature conditions with limited chemical dosage to remove barriers to enzymatic cellulose conversion effectively. The claims include integration with mechanical size reduction or steam explosion and subsequent hydrolysis and conversion to biofuels or biochemicals, establishing a comprehensive method for biomass biorefining.
Stated Advantages
SPORL process achieves over 90% cellulose conversion efficiency for softwood biomass in short enzymatic hydrolysis times and with low enzyme loading.
The pretreatment can directly treat chip or chop-sized biomass without need for previous fine size reduction, greatly reducing mechanical or thermal energy consumption.
SPORL reduces the energy required in subsequent mechanical size-reduction steps by altering chemical and physical biomass structure.
The process produces lower levels of fermentation inhibitory compounds such as HMF and furfural compared to dilute acid pretreatment, improving fermentation suitability.
Use of sulfite or bisulfite promotes lignin sulfonation, increasing hydrophilicity of lignin and reducing enzyme inhibition by lignin.
The process uses conventional pulp and paper industry chemicals and equipment, potentially facilitating easier scale-up and commercialization than existing pretreatment methods.
Documented Applications
Conversion of lignocellulosic biomass including softwood, hardwood, agricultural residues, and waste paper into fermentable sugars via enzymatic saccharification.
Production of ethanol by fermenting sugars obtained from enzymatic hydrolysis of sulfite-pretreated biomass.
Chemical conversion of sugars obtained from the pretreatment into other biofuels, biochemicals, and bioproducts.
Integration with steam explosion and mechanical size-reduction processes such as disk milling or hammer milling to optimize biomass processing efficiency.
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