• Energy Research

Alkaline pretreatment of lignocellulosic biomass has been intensively investigated but heavy water usage and environmental pollution from wastewater limits its industrial application. This study presents a pretreatment technique by in-situ injection of potassium hydroxide concentrations ranging from 0.8% to 10% (w/w) into the briquetting process of wheat straw and meadow grass. Results show that the biomethane yield and hydrolysis rate was improved significantly with a higher impact on wheat straw compared to meadow grass. The highest biomethane yield from wheat straw briquettes of 353mL.g-1 VS was obtained with 6.27% (w/w) potassium hydroxide injection, which was 14% higher than from untreated wheat straw. The hydrolysis rates of wheat straw and meadow grass increased from 4.27×10-2 to 5.32×10-2d-1 and 4.19×10-2 to 6.00×10-2d-1, respectively. The low water usage and no wastewater production make this a promising technology.

Anaerobic co-digestion of agricultural by-products or wastes with complementarity characteristics is commonly used to enhance methane yield. This study firstly explores the possibility of co-digesting grass and forb species (white clover, chicory and plantain) differing in nutrient composition in enhancing methane yield. This was examined with two inocula (a cattle manure-based inoculum and a grass-based inoculum) in a batch assay. Results showed that co-digesting grass and forbs synergistically enhanced methane yield potential on average by 31 L kg−1 volatile solids (+11%) and reduced lag phase time by 0.8 day in the grass-based inoculum, but not in the cattle manure-based inoculum. Mixtures containing plantain showed more consistent synergistic effect than chicory. Synergistic effects were attributed to more balanced nutrient composition (especially C/N ratio) in grass-forb mixtures. We demonstrate that anaerobic co-digestion of grass and forbs is feasible for enhancing methane yield, which promotes the utilization of multi-species grasslands for bioenergy production.

Abstract One of the main obstacles for a more widespread use of wheat straw for renewable energy production is the absence of a low-cost technology which promotes its biodegradation. The aim of this study was to determine whether methane yield from wheat straw could be improved by using alkali at low concentration and ambient temperature if the fiber fraction had previously undergone mechanical pretreatment. After combining mechanical and alkali pretreatment, a linear correlation between alkali concentration and hydrolysis of the fiber content, mainly hemicelluloses, was found. A positive effect by combining briquetting of wheat straw with sodium hydroxide addition was found with the hemicellulose content decreasing by up to 66%. In general, alkali improved the ultimate methane yield and the methane yield obtained after 30 days. Briquetting reduced the optimal alkali concentration needed to reach the maximal improvement in methane yield compared to merely shredding. The results proved that it is not necessary to work with the high alkali concentrations traditionally used to improve methane production if the straw is mechanically pretreated prior to alkali addition. Methane yield obtained after 30 days as well as net energy yield rose by around 40% when briquetting was combined with 2% sodium hydroxide. Therefore, a low concentration of alkali, especially sodium hydroxide, applied at ambient temperature can improve the energy and the economic balance of pretreatment.

Anaerobic digestion (AD) can be used as a stand-alone process or integrated as part of a larger biorefining process to produce biofuels, biochemicals and fertiliser, and has the potential to play a central role in the emerging circular bioeconomy (CBE). Agricultural residues, such as animal slurry, straw, and grass silage, represent an important resource and have a huge potential to boost biogas and methane yields. Under the CBE concept, there is a need to assess the long-term impact and investigate the potential accumulation of specific unwanted substances. Thus, a comprehensive literature review to summarise the benefits and environmental impacts of using agricultural residues for AD is needed. This review analyses the benefits and potential adverse effects related to developing biogas-centred CBE. The identified potential risks/challenges for developing biogas CBE include GHG emission, nutrient management, pollutants, etc. In general, the environmental risks are highly dependent on the input feedstocks and resulting digestate. Integrated treatment processes should be developed as these could both minimise risks and improve the economic perspective.

Abstract The present study investigated the feasibility of using pilot-scale anaerobic filter to treat by-product liquid (brown juice, BJ) from grass protein extraction in a green biorefinery, and produce biogas via anaerobic digestion without co-digestion substrate. Prior to feed into the un-heated anaerobic filter reactor, the BJ was warmed up to 55−60 °C in order to maintain the digestion temperature. The influence of retention time and feeding frequencies were tested in order to optimize the overall performance. The study demonstrated that anaerobic filter is feasible for converting BJ to biogas. The specific CH production reached 230 mL gCOD−1 d −1 while up to 80 % of influent COD was removed within a hydraulic retention time of 5.5 days. In addition, higher feeding frequencies enhanced the process stability as it narrowed the temperature variation and decreased the instant impact on buffer capacity.

In-situ hydrogen biomethanation is a promising technology to upgrade biogas. The efficiency of biomethanation relies on various parameters, e.g. gas supplement, temperature and hydrogenotrophic methanogens. Therefore, it is important to investigate the characteristics of in-situ hydrogen biomethanation under different conditions. In this study, two experiments (lasted for 91 days and 105 days) were carried out to investigate the impacts of feeding gas and operating conditions on performances of reactors and microorganisms. During the whole experiment, no obvious fluctuation of pH and limitation of gas-liquid mass transfer were found. Results showed that the hydrogenotrophic methanogenesis performed better at thermophilic condition, while the dominant archaea genera at mesophilic and thermophilic temperature was determined to be Methanobacterium and Methanothermobacter, respectively. The highest CH4 content (greater than 90%) was obtained when H2 and CO2 was feeding at ratio of 4:1 and Methanothermobacter was dominant. These findings can provide useful information for promoting hydrogen biomethanation.

Abstract In this work, ensiling of Festuca arundinacea (Tall Fescue, TF) prior to biogas production was studied and the effect of storage time, additives (pilot-scale trial) and mechanical pretreatment (full-scale trials) were investigated. In the pilot-scale test, TF was ensiled by adding two biological additives and stored for up to 9 months. In the full-scale test, two 7 ton TF piles (unchopped and chopped) were ensiled for 3 months. Both the ultimate CH4 potential and silage products were measured to evaluate the effect of ensiling. Results show that ensiling of TF resulted in well-preserved silages with low pH for most of the variants stored for up to nine months. The biological additives contained both hetero- and homo-fermentative lactic acid bacteria and enzymes showed a positive effect on both organic dry matter (ODM) preservation and biogas production. Full-scale ensiling presented silage quality with higher ODM losses if the grass is not cut prior to the ensiling process. In general, ensiling is an appropriate storage method for TF, allowing it to be used as a long-term feedstock for biogas plants.

Abstract The study investigated the effect of co-ensiling of cover crops (CC) and barley straw (BS) on biogas production. Blends containing chopped CC and BS (mix ratios of 1:0, 2:1, 3:1, 6:1; 10:1 and 19:1 (w:w)) were prepared and stored for 4 months prior to the biogas batch assay. Results show that CC is feasible for producing biogas with reasonable CH4 yield (330 mL CH4 gVS−1) and has good storability. Using of co-ensiling blends showed advantages over CC as it elevated the hydrolysis rate (k) from 0.024 to 0.061 d−1 and decreased the lag phase (from 5 to 0.8 days) during the thermophilic (51 °C) batch test. Synergistic effects were observed from both CH4 yield and hemi-cellulose removal. The finding provided an alternative strategy for integrated straw management and CC utilization for improved biogas production.