Batch-wise vs. continuous steam pretreatment of lignocellulosic materials

Pretreatment is one of the most essential steps for production of cellulosic ethanol to overcome recalcitrance of lignocellulosic biomass. The rigid structure must be modified to break down the lignin structure, remove the hemicelluloses and disrupt the crystalline structure of the cellulose to increase enzyme accessibility. An ideal method of pretreatment should result in high sugar yields, avoid formation of inhibitors and result in high digestibility of cellulose, and should, of course, be cost effective. Today, steam pretreatment (acid catalyzed) is one of the most common pretreatment methods both in lab-, demo- and commercial scale processes.

In lab-scale, batch-wise operation where the pretreatment usually takes place for a few minutes (not including filling and emptying time of the equipment) is most extensively used. However, in commercial scale a continuously operated steam pretreatment system will be the main option. Materials derived from batch or continuous pretreatment often have different performances in terms of digestibility and fermentability when treated at the same severity.

For scale-up purposes it is a vital importance to gather data that yield similar results in small-scale and commercial-scale operation. In this study, steam pretreatment of spruce and agricultural materials were performed batch-wise (10-litre scale) or continuously (20 kg/h). Similar parameters (severities) were tested on both equipment and the goodness of the pretreatment was assessed by enzymatic hydrolysis and potential fermentability of the resulting materials. The basis for comparison is provided by the yields from pretreatment and enzymatic hydrolysis, generation of inhibitory compounds and the potential for fermentation.