Biogas and Biomethane Production Under Continuous Operations

Global warming and climate change are imminent threats to the future of humankind. A shift from the current reliance on fossil fuels to renewable energy is key to mitigating the impacts of climate change. Biological raw materials and residues can play a key role in this transition through technologies such as anaerobic digestion. However, biological raw materials must also meet other existing food, feed and material needs. Green biorefinery is an innovative concept in which green biomass, such as grass, is processed to obtain a variety of protein products, value-added co-products and renewable energy, helping to meet many needs from a single source. In this study, an analysis has been conducted to understand the renewable energy potential of green biorefinery by-products and residues, including grass whey, de-FOS whey and press cake. Using anaerobic digestion, the biogas and biomethane potential of these samples have been analyzed. An analysis of the fertiliser potential of the resulting digestate by-products has also been undertaken. All the feedstocks tested were found to be suitable for biogas production with grass whey, the most suitable candidate with a biogas and biomethane production yield of 895.8 and 544.6 L/kg VS, respectively, followed by de-FOS whey and press cake (597.4/520.3 L/kg VS and 510.7/300.3 L/kg VS, respectively). The results show considerable potential for utilizing biorefinery by-products as a source for renewable energy production, even after several value-added products have been co-produced.

Anaerobic digestion (AD) is a bioprocess technology that integrates into circular economy systems, which produce renewable energy and biofertilizer whilst reducing greenhouse gas emissions. However, improvements in biogas production efficiency are needed in dealing with lignocellulosic biomass. The state-of-the-art of AD technology is discussed, with emphasis on feedstock digestibility and operational difficulty. Solutions to these challenges including for pre-treatment and bioaugmentation are reviewed. This article proposes an innovative integrated system combining alkali pre-treatment, temperature-phased AD and bioaugmentation techniques. The integrated system as modelled has a targeted potential to achieve a biodegradability index of 90% while increasing methane production by 47% compared to conventional AD. The methane productivity may also be improved by a target reduction in retention time from 30 to 20 days. This, if realized has the potential to lower energy production cost and the levelized cost of abatement to facilitate an increased resource of sustainable commercially viable biomethane.

This study considered the possibility of reducing the environmental footprint of paper and pulp industry by producing bioenergy from paper sludge by using process wastewater instead of fresh water, and reclaiming water trapped in paper sludge. Experimental studies are conducted with streams from three different pulp and paper mills (virgin pulp mill (VP), corrugated recycling mill (CR), tissue printed recycling mill (TPR)) for sequential bioethanol and biogas production with simultaneous reclamation of water from paper sludge (PS). Total energy yields of 9215, 6387, 5278 MJ/tonne dry PS for VP, CR and TPR, respectively, were obtained for ethanol-biogas production. Virgin pulp paper sludge gave the highest yield for ethanol and biogas in stand-alone processes (275.4 kg and 67.7 kg per ton dry PS respectively) and also highest energy conversion efficiency (55%) in sequential process compared with CR and TPR. Energy and environmental case study conducted on virgin pulp mill has proven the possibility of using paper sludge bioenergy to reduce energy demand by 10%, while reclaiming 82% of the water from the PS, reducing greenhouse gas emissions (GHG) by 3 times and producing solids suitable for land spreading.

The paper and pulp industry is one of the major industries that generate large amount of solid waste with high moisture content. Numerous opportunities exist for valorisation of waste paper sludge, although this review focuses on primary sludge with high cellulose content. The most mature options for paper sludge valorisation are fermentation, anaerobic digestion and pyrolysis. In this review, biochemical and thermal processes are considered individually and also as integrated biorefinery. The objective of integrated biorefinery is to reduce or avoid paper sludge disposal by landfilling, water reclamation and value addition. Assessment of selected processes for biorefinery varies from a detailed analysis of a single process to high level optimisation and integration of the processes, which allow the initial assessment and comparison of technologies. This data can be used to provide key stakeholders with a roadmap of technologies that can generate economic benefits, and reduce carbon wastage and pollution load.

Growth inhibition kinetics of a novel non-acetone forming butanol producer, Clostridium sporogenes BE01, was studied under varying concentrations of acetic and formic acids in rice straw hydrolysate medium. Both the organic acids were considered as inhibitors as they could inhibit the growth of the bacterium, and the inhibition constants were determined to be 1.6 and 0.76 g/L, respectively, for acetic acid and formic acid. Amberlite resins—XAD 4, XAD 7, XAD 16, and an anion exchange resin—Seralite 400 were tested for the efficient removal of these acidic inhibitors along with minimal adsorption of sugars and essential minerals present in the hydrolysate. Seralite 400 was an efficient adsorbent of acids, with minimal affinity towards minerals and sugars. Butanol production was evaluated to emphasize the effect of minerals loss and acids removal by the resins during detoxification.