Abstract
The new EU bioenergy sustainability criteria demonstrate that bioenergy sustainability can be a challenge (Schlegel and Kaphengst, 2007). In 1990s, the energy crops were the main source for biogas production in Europa; however, their competition with food production led these sources to be phased out as sustainable renewables. This makes alternative abundant bioenergy resources such as lignocellulosic materials increasingly interesting. Anaerobic digestion (AD) is a suitable waste management method in which renewable bioenergy can be produced from different feedstocks including lignocellulosic material. Even though the lignocellulose is a biomass with high energy content, it has rigid structure to be used in AD. To overcome this, a pre-treatment method is needed for the complete extraction of the energy in AD. Several pre-treatment methods have shown to be very effective independent of the type of lignocellulose in the biomass. Apart from assessing physical characteristics of lignocellulosic materials and their biogas production potential before and after pre-treatment, this review assesses the developed pre-treatment methods for lignocellulosic feedstocks for AD, and highlights the effectiveness, limitations and challenges of these techniques. This review discusses the possible strategies to implement a lignocellulosic-based biogas plant with optimised net cost and energy consumption through improving process design. Even though high energy yields from the harvested biomass is economically desirable, the solutions with the highest possible energy yield are not necessarily the ecologically best ones. Thermal pre-treatment appears to give the highest increases in methane yields, but the proper balance between high yields and the ecological fate of non-digested carbon containing materials (i.e. lignin in the AD digestate) needs to be further studied. Heat recovery and process integration will be needed to reduce inherent energy consumption in thermal pre-treatment.
Highlights
Biogas production through anaerobic digestion (AD) is a reliable source that replaced 36 Mtoe fossil fuel consumption in 2018 (IEA, 2020)
Lignocellulosic biomass can be seen as a suitable feedstock for bioenergy production as it is an available source and does not compete with food production; its rigid structure prevents the complete use of these energy sources (Paul and Dutta, 2018)
The purpose of this review can be split into two parts: to give an overview of the various pre-treatment methods available for lignocellulosic biomasses to investigate their potential in biogas production after a pre-treatment step; to assess how these methods can be integrated into existing and new AD plants in terms of economic cost, energy balance, digestability of the pretreated feedstock and storage
Summary
Biogas production through anaerobic digestion (AD) is a reliable source that replaced 36 Mtoe fossil fuel consumption in 2018 (IEA, 2020). It may be possible that lignocellulosic pre-treatment for AD is energetically unfavourable on an industrial scale in some cases This is due to the energy and pre-treatment processes required to maximise the hydrolysis potential of lignocellulosic-based feedstocks. This may suggest that the recalcitrant components (e.g., lignin) that are left over from bio-processes may be better used as soil fertilisers (Rahman et al, 2013). The purpose of this review can be split into two parts: to give an overview of the various pre-treatment methods available for lignocellulosic biomasses to investigate their potential in biogas production after a pre-treatment step; to assess how these methods can be integrated into existing and new AD plants in terms of economic cost, energy balance, digestability of the pretreated feedstock and storage. Unlike other recently published review papers that have been focused mainly on challenges and opportunities of pre-treatment techniques (Table 1), the current review paper in addition to dealing with the different pre-treatment methods, temped to comparing large number of experimental results in this field in order to give an overview of biogas production potential of different lignocellulosic materials before and after pre-treatment as well as selecting the most effective pre-treatment method for various feedstock categories
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