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Anaerobic digestion of municipal waste is a promising technology for renewable energy production, notably for methane (CH4) production. Existing reactor designs have limitations that prevent efficient conversion and high throughput, especially of substrates high in suspended solids. Here, we introduce a novel compartmentalized reactor design encompassing controlled feed flow over fixed microbial biofilms for high rate CH4 production from enzymatically pre-hydrolyzed municipal solid waste. In a 240 liter working volume reactor, the biofilm generation was completed in less than 60 days and CH4 production was minimum ˜700 L day-1 in the ensuing months with a Hydraulic Retention Time (HRT) of 10 days or lower. When the organic load was gradually increased, a reactor productivity of 6 LCH4 Lreactor-1 day-1 was achieved. At this point, the reactor processed 20.8 gCOD Lreactor-1 day-1 at a HRT below 5 days, maintaining above 70% of the maximal Chemical Oxygen Demand (COD) conversion rate. Such fast conversion rates and high yields are far beyond what has been reported for other reactor designs, and are a crucial prerequisite for industrial realization of renewable biogas production from municipal solid waste and other organic waste streams.

Efficient methods for lignin characterisation are increasingly important as the field of lignin valorisation is growing with the increasing use of lignocellulosic feedstocks, such as wheat straw and corn stover, in biorefineries. In this study, we characterised a set of authentic lignin biorefinery samples in situ with no prior purification and minimal sample preparation. Lignin chemical formulas and lignin Fourier transform infrared (FTIR) spectra were extracted from mixed spectra by filtering out signals from residual carbohydrates and minerals. From estimations of C, H and O and adjustment for cellulose and hemicelluloses contents, the average chemical formula of lignin was found to be C9H10.2O3.4 with slight variations depending on the biomass feedstock and processing conditions (between C9H9.5O2.8 and C9H11.1O3.6). Extracted FTIR lignin spectra showed many of the same characteristic peaks as organosolv and kraft lignin used as benchmark samples. Some variations in the lignin spectra of biorefinery lignin residue samples were found depending on biomass feedstock (wheat straw, corn stover or poplar) and on pretreatment severity, especially in the absorbance of bands at 1267 and 1032 cm−1 relative to the strong band at ~1120 cm−1. The suggested method of FTIR spectral analysis with adjustment for cellulose and hemicellulose is proposed to provide a fast and efficient way of analysing lignin in genuine lignin samples resulting from biorefineries.

Effects of treatment time, citrate concentration, temperature, and pH on Si extraction from wheat straw prior to hydrothermal pretreatment were investigated for maximising Si removal and biomass recovery before biomass refining. With citrate, an almost linear negative correlation between Si content in the residual biomass and treatment temperature was observed up to 170 °C, yielding a Si removal of up to 97.7%. This high Si removal came at the expense of a low mass yield (down to 45%) in the insoluble lignocellulosic fraction. Optimum process conditions for high Si removal and high total mass yield were: 100 mM sodium citrate, 130 °C, 60 min, 2% w/v solids, and pH of ∼6.5 during extraction. Using the proposed process conditions, silica removal of up to 77% was achieved with a mass yield of 72.8%. This Si removal from the insoluble lignocellulosic fraction did not affect the enzymatic cellulose hydrolysis, neither negatively nor positively.

Elemental analysis by X-ray fluorescence spectrometry (XRF) of solid samples from a biorefinery process was performed to study the behaviour of mineral elements in a process involving hydrothermal pretreatment of biomass (wheat straw, corn stover, sugarcane bagasse, palm oil empty fruit bunches, poplar) followed by enzymatic hydrolysis and fermentation. For all the different biomasses, the biorefinery process concentrated silicon, aluminium, and calcium in the solid fraction, while potassium and magnesium were solubilised in the process and removed from the solid fraction. Sodium concentrations were in general low and they only increased in case of addition during the process. No general tendencies were observed for phosphorus, sulphur, and iron concentrations. A prerequisite for XRF elemental analysis was defining an average chemical formula for the organic matrix of process biomass samples. Based on ultimate elemental analysis of all biomasses, the formula for biomass was C6H8.4O3.5, which was used for all types of samples (raw biomass, pretreated biomass, and lignin residue) and can be used in future XRF analysis of samples of similar process and biomass feedstock as those used in this study.

In this study 26 new oligophenol cellulase inhibitors were discovered from wheat straw pretreatment liquors. By consideration of the reaction mechanisms for their formation it is proposed that these oligophenols are formed during hydrothermal biomass pretreatment by pentose self-condensation reactions involving aldol condensations, 1,4 additions to α,β unsaturated carbonyl compounds, 3-keto acid decarboxylations and oxidations. Furthermore, pentose reactions with phenolic lignin components are suggested. The identification of the central role of xylose in the reaction routes for oligophenolic inhibitor formation led to the solution to protect the reactive anomeric center in xylose. It is shown that protection of the anomeric center in in situ generated xylose with ethylene glycol monobutyl ether, during pretreatment of wheat straw, reduces the level of oligophenols by 73%. The results pave the way for implementation of new types of reactions that hinder inhibitor formation in lignocellulosic biomass processing.

Gasification of spent lignin pellets was used to obtain a gas suitable for energy production. Spent lignin was obtained from second-generation cellulosic ethanol demo plant using wheat straw as feedstock. Gasification of lignin did not give rise to any feeding problems, thus no significant changes were needed in the existing gasification installation. The rise of temperature and steam flow rate favoured the formation of H 2 , while hydrocarbons (C n H m ) and tar contents decreased. The increase of equivalent ratio (ER) also decreased hydrocarbons and tar contents, but syngas higher heating value (HHV) was reduced. The use of natural minerals improved lignin gasification. The presence of dolomite led to the highest H 2 and to the lowest C n H m and tar contents. Results obtained at bench-scale were confirmed at pilot-scale, as similar trends were obtained. However, as the residence time in pilot gasifier was higher, greater gas yields with higher H 2 and CH 4 concentrations were obtained, while tar contents decreased. After syngas hot cleaning and upgrading, the final syngas composition showed to be suitable for a wide range of applications (e.g. energy production and synthesis of chemicals), since it was substantially enriched in hydrogen, whereas tar and heavier gaseous hydrocarbons were completely destroyed.