Concept for a net-zero energy self-supply biorefinery: wheat straw study case

Abstract

Developing sustainable biorefineries is an urgent matter to switch to a sustainable society. The main goal of biorefineries is the complete valorization of biomass towards material applications and using the remaining non-valorized fractions to supply the process with energy. Parts of the pulp fraction after pretreatment, the hemicellulose sugar fraction, and a part of the raw material can be used to generate the required energy, either via coupling with biogas production and/or via thermochemical use of the biomass. We want to examine the concept of the "energy self-sufficient biorefinery" using wheat straw as a study case for the intermediate refining into hemicellulosic sugars, lignin and a cellulose-enriched pulp. The scope of this concept stays within the primary biorefining with the balance limit of the production of intermediates. Different scenarios were proposed for the valorization of the products and further usage of non-valorized streams for biogas or direct energy production. This biorefinery concept was simulated in Aspen Plus V10.0, Aspen Energy Analyzer was used for the calculation of the energy requirements. Then, a fraction of the feedstock was used for direct energy production through combustion. The valorization of the cellulose-enriched pulp either through biogas or thermochemical valorization proved not to be enough to cover the energy demand from such a biorefinery concept. However, the biogas stage increases the available product-palette from the biorefinery. Using the hemicellulosic hydrolysate increased the biogas potential; yet, a combined heat-power system using the biogas was not enough to cover the energy demand. The inclusion of an additional step for thermochemical production of energy directly from the feedstock shown to cover the energy demand and this way the total amount of actual biomass required for an energy-self-supply biorefinery was calculated. The additional biomass for energy production ranged between 30-80% additional biomass, depending on the chosen-product configuration. This is an initial step into the conceptualization of net zero biorefineries, and based on the obtained results, further optimization can be performed to couple the techno-economic and environmental performance of each configuration to find optimal configurations.