Thermochemical pretreatment of lignocellulose to enhance methane fermentation: I. Monosaccharide and furfurals hydrothermal decomposition and product formation rates

Abstract

Over a pH range 1–4 and temperatures from 170 to 230°C, the decomposition rates of xylose, gaiactose, mannose, glucose, 2‐furfural, and 5‐hydroxymethyl‐2‐furfural (5‐HMF) were pseudo first order. The effect of temperature and pH on the pseudo first‐order decomposition rate constants was modeled using the Arrhenius equation and acid‐base catalysis, respectively. Decomposition rates of the monosaccharides were minimum at a pH 2–2.5. Above pH 2.5, the monosaccharide decomposition was base catalyzed, with acid catalysis occurring at a pH of less than 2 for glucose. The furfurals were subject to acid catalysis at below ca. pH 3.5. The hydrothermal conversion of glucose to its decomposition products during thermochemical Pretreatment can be modeled as a combination of series and parallel reactions. The formation rates of identified soluble products from glucose decomposition, 5‐HMF and levulinic acid, were also functions of temperature and pH. The rate of 5‐HMF formation relative to glucose decomposition decreased as the pH increased from 2.0 to 4.0, with levulinic acid formation only detected when the pH was 2.5 or less. For glucose decomposition, humic solids accounted for ca. 20% of the decomposition products.