Polysaccharides Release in a Laboratory-Scale Batch Hydrothermal Pretreatment of Wheat Straw under Rigorous Isothermal Operation.

Felicia Rodríguez,Arturo Sánchez,Adrián Santiago-Toribio,Efrén Aguilar-Garnica

doi:10.3390/molecules27010026

Abstract

Hydrothermal pretreatment (HP) is an eco-friendly process for deconstructing lignocellulosic biomass (LCB) that plays a key role in ensuring the profitability of producing biofuels or bioproducts in a biorefinery. At the laboratory scale, HP is usually carried out under non-isothermal regimes with poor temperature control. In contrast, HP is usually carried out under isothermal conditions at the commercial scale. Consequently, significant discrepancies in the values of polysaccharide releases are found in the literature. Therefore, laboratory-scale HP data are not trustworthy if scale-up or retrofitting of HP at larger scales is required. This contribution presents the results of laboratory-scale batch HP for wheat straw in terms of xylan and glucan release that were obtained with rigorous temperature control under isothermal conditions during the reaction stage. The heating and cooling stages were carried out with fast rates (43 and −40 °C/min, respectively), minimizing non-isothermal reaction periods. Therefore, the polysaccharide release results can be associated exclusively with the isothermic reaction stage and can be considered as a reliable source of information for HP at commercial scales. The highest amount of xylan release was 4.8 g/L or 43% obtained at 180 °C and 20 min, while the glucan release exhibited a maximum of 1.2 g/L or 5.5%. at 160 °C/180 °C and 30 min.

Highlights

Produced at a rate of 980 million tons per year, wheat straw (WS) is currently one of the most abundant agricultural residues worldwide [1]
Experimental results expressed as polysaccharides release or, xylan and glucan releases were obtained from a laboratory-scale batch hydrothermal pretreatment (HP) of WS carried out under an overall isothermal operation
The main difference between the results described in this contribution and those already available is that they were obtained in rigorous isothermal conditions with a precise characterization of temperature trajectories in terms of heating and cooling rates and periods, as well as reaction times

Summary

Introduction

Produced at a rate of 980 million tons per year, wheat straw (WS) is currently one of the most abundant agricultural residues worldwide [1]. The heating time or heating rates is commonly considered as the key operating condition in thermochemical LCB pretreatments [25,26] In this contribution, experimental results expressed as polysaccharides release or, xylan and glucan releases were obtained from a laboratory-scale batch HP of WS carried out under an overall isothermal operation. The main difference between the results described in this contribution and those already available is that they were obtained in rigorous isothermal conditions with a precise characterization of temperature trajectories in terms of heating and cooling rates and periods, as well as reaction times. The %Gmr values do not show increases proportional to temperature as in other pretreatments, such as acid-catalyzed, that have been previously reported to favor glucan hydrolysis In this contribution, the resulting glucan monomers may be produced from superficial, fast-reactive glucans on WS. To have HP operating at rigorous isothermal conditions is fundamental to reach hydrogen and ethanol co-production via dark fermentation [32] and other eco-friendly fuel precursors [33]

Wheat Straw Characterization

Experimental HP Runs

Analysis on Xylan and Glucan Release

Findings

Conclusions