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Biomass fractionation

 

Extending the feedstock base of CIMV organosolv technology – towards a multi-feedstock BIOCORE biorefinery 

 

CIMV fractionation technology has previously been developed for cereal straw (barley and wheat) refining. One of the BIOCORE aims is to extend the range of biomass feedstocks that can be used. Therefore, the process conditions required to refine other feedstocks such as rice straw, SRC poplar and hardwood have been studied. To achieve this, various aspects of the CIMV process have been modified, including the mechanical preparation of the feedstocks and the actual organosolv process conditions.

 

Mechanical pre-processing

In organosolv technology, solvent diffusion is governed by particle size. Therefore, it is important to determine the appropriate mechanical pre-processing of different biomass feedstocks. Wheat straw is cut into 5-15 cm length wisps and then sieved before refining. To achieve this, an adjustable speed grinder without perforated grills is appropriate. This procedure is also applicable to rice straw and, more than likely, to all cereal straws. Regarding woody biomass ( hardwood or SRC poplar), it has been shown within the BIOCORE project that mechanical pre-processing should ideally procure a feedstock displaying an average particle size of 1 cm. Moreover, testing has revealed that the presence of bark in SRC poplar samples does not significantly compromise the efficiency of the refined organosolv process (at least with respect to cellulose and lignin extraction), indicating that debarking of SRC poplar is unnecessary.

 

Optimization of the organosolv refining

 

         The base case for CIMV organosolv technology is wheat straw refining. Reaction conditions in this case are described by a defined reaction time and temperature (approximately 100 °C) and a solvent system characterized by a fixed acetic acid:formic acid:water ratio. In BIOCORE, these conditions were successfully applied to rice straw refining and suitably adapted for refining of hardwood, including SRC poplar. Regarding hardwood, the major modifications to the organosolv process were increased reaction time and adjustments to the formic acid/acetic acid ratio, while the reaction temperature was unchanged. Young SRC poplar (2 year growth) could be refined using this protocol and the presence of bark had little or no influence on the quality of the cellulose stream. This finding is significant, because it provides energy savings linked to the fact that debarking is avoided. As expected, softwood was recalcitrant to CIMV technology, almost certainly due to the presence of resins, which impede solvent penetration. Nevertheless, the incorporation of small amounts of softwood (up to 10% w/w) into hardwoord feedstocks did not hinder organosolv refining. Thanks to a collective effort, BIOCORE researchers are now well on the way to finding a solution that will allow softwood to be used in the process. If successful, the CIMV technology will possess an almost unlimited capacity to use lignocellulosic biomass resources.

 

Upfront biomass refining technology

In period 3, a full mass balance analysis of the CIMV process performed by KULeuven and CIMV was completed. In particular, this revealed that the lignin recovery is maximal, and that the C5-rich hemicellulose fraction, which has been the focus of much attention in the project, contains over 60% dry weight sugars, of which approximately half are monomeric xylose. Moreover, regarding the C5-rich hemicellulose fraction, extensive analyses and round-robin testing provided considerable insight into the chemical composition of the fraction, revealing that some of the sugars are esterified, and in an accessory fashion, allowed BIOCORE partners to make significant advances in protocol harmonization.

Working on improving the benchmark organosolv technology, ECN developed a new, improved EtOH/water organosolv process route, which introduces an extra step before biomass pulping. Interestingly, this step removes proteins, waxes and other extractives from the feedstock and thus eliminates their influence on the fractionation process. As a result, the final cellulose pulp is characterized by higher purity and the lignins more closely resemble native lignin. A patent application has been filed for this new process route.

 

Investigating the valorization of process residues

All biorefinery systems are characterized by process residues, which can potentially constitute waste streams. To achieve optimal performance it is important to define the best valorization routes for process residues, in order to avoid the generation of waste that could induce negative cost effects, linked to its treatment and/or evacuation.

ECN’s analysis of the BIOCORE technology scheme has revealed two major types of residues. The first are “fines” from mechanical biomass pre-treatment, while the second are fermentation residues. Regarding fines, both wheat and rice straw fines were tested in different CHP systems in order to evaluate their potential for use as combustibles. Results show that wheat and rice straw fines are difficult fuels on a stand-alone basis, mainly due to a high level of alkalis. Combustion shows good burnout / efficiency of 99.5%. Recommendations are to: 1) co-fire with coal or with other biomass 2) add potassium binders, such as Al-Silicates. Low temperature gasification showed a lower burnout/ efficiency of 96%. Therefore, this technology could be used to produce gas for electricity production, perhaps using a MgO (dolomite) bed to prevent agglomeration.

 

Analysis and preliminary evaluation of 5 collected fly ashes of wheat, rice straw, straw mixtures and mixtures with coal by ECN indicate that fly ashes do not comply with EU criteria for use in concrete (EN-450-1). All evaluated fly ashes did comply with criteria for use as fertilizer in use in several EU member states. Finalization of the work (incl. technology selection) and an evaluation of the potential use of fermentation residues for anaerobic digestion and/or application in feed of fertilizer are in progress.