Challenges in Kinetic Parameter Determination for Wheat Straw Pyrolysis

Abstract

Wheat straw is a renewable agricultural by-product that is currently underutilized in the production of bioenergy and bioproducts due to its high ash content, as well as high transport costs due to its low volumetric energy density. The thermogravimetric analysis of this material produces derivative curves with a single broad peak, making it difficult to identify the three conventional pseudo-components (cellulose, hemicellulose, and lignin), which is resolved using the second derivative to determine inflection points. Model-fitting methods and isoconversional methods were applied to determine the degradation kinetics of wheat straw at two different particle sizes, as well as that of a reference feedstock (beech wood), and the obtained values were used to divide the degradation curves to be compared to the experimental data. Seven different pyrolysis reaction networks from the literature were given a similar treatment to determine which provides the best estimation of the actual pyrolysis process for the case of the feedstocks under study. The impact of the potassium content in the feedstock was considered by comparing the original pathway with a modification dependent on the experimental potassium content and an estimated optimum value.

Modeling Fast Pyrolysis of Waste Biomass: Improving Predictive Capability

“…The mass balance closure was also reported, due to the stoichiometric unbalancing present in the reaction networks in the literature: not only the deviation in the product distribution (vs experimental balance results) but also that in the fraction of condensable bio-oil associated compounds. Finally, the results reported by Fonseca et al for the deviation of the derivative fit of each model are also presented here. All of these values are reported in Table .…”

“…On a second series of tests, variants of the best performant cases of Table , Ranzi 08 + Trendewicz – Exp and RAC + Trendewicz – Opt were contrasted. The variants consider the use of the kinetic parameters determined by Fonseca et al and alternative hemicellulose degradation pathways (production of xylosan vs low molecular weight compounds), contrasting hardwoods and grasses by Debiagi et al The results are presented in Table . Here, a similar analysis as the one made for Table can be easily made.…”

“…■ RESULTS AND DISCUSSION Primary Pyrolysis. Simulations were performed using Aspen Plus, using the best results reported by Fonseca et al, 36 contrasting CSTR and PFR models. The mass balance closure was also reported, due to the stoichiometric unbalancing present in the reaction networks in the literature: not only the deviation in the product distribution (vs experimental balance results) but also that in the fraction of condensable bio-oil associated compounds.…”

“…Fonseca et al 36 examined the ability of various reaction networks to predict the thermal degradation behavior of two different biomasses, beech wood, and wheat straw while incorporating the effects of potassium content using the Trendewicz 35 modification. All networks performed well when modeling the degradation of beech wood, but not so well when modeling that of wheat straw, mostly due to the high ash content of the latter material.…”

“…As can be seen, a very common solution is Gibbs minimization, , which features an obvious downgrade of being unfeasible to predict the composition of the condensates . Another alternative is the implementation of reaction networks, such as the ones evaluated by Fonseca et al, also successfully implemented by different authors. ,, Black box systems and stoichiometric reactors feature the obvious upside of being easy to set up based on experimental data but do not allow for the prediction of different system behaviors under different operating conditions.…”