Maryam M. Kabir

Techno-economical study of biogas production improved by steam explosion pretreatment

Economic feasibility of steam explosion pretreatment for improvement of biogas production from wheat straw and paper tube residuals was investigated. The process was simulated by Aspen plus ®, and the economical feasibility of five different plant capacities was studied by Aspen Process Economic Analyzer. Total project investment of a plant using paper tube residuals or wheat straw was 63.9 or 61.8 million Euros, respectively. The manufacturing cost of raw biogas for these two feedstocks was calculated to 0.36 or 0.48 €/m(3) of methane, respectively. Applying steam explosion pretreatment resulted in 13% higher total capital investment while significantly improved the economy of the biogas plant and decreased the manufacturing cost of methane by 36%. The sensitivity analysis showed that 5% improvement in the methane yield and 20% decrease in the raw material price resulted in 5.5% and 8% decrease in the manufacturing cost of methane, respectively.

Biogas production from lignocelluloses by N-methylmorpholine-N-oxide (NMMO) pretreatment: Effects of recovery and reuse of NMMO

The effects of N-methylmorpholine-N-oxide (NMMO) pretreatment on barley straw and forest residues were investigated for biogas production. The pretreatments were performed at 90°C with 85% NMMO for 3-30h. The best pretreatment conditions resulted in 100% improvement in methane yield during the subsequent digestion compared to that of the untreated lignocelluloses. Methane yields of 0.23 and 0.15Nm(3) CH4/kg VS were obtained from barley straw and forest residues, respectively, corresponding to 88% and 83% of the theoretical yields. In addition, the effects of the pretreatment with recovered and reused NMMO was also studied over the course of five cycles. Pretreatment with recycled NMMO showed the same performance as the fresh NMMO on barley straw. However, pretreatment of forest residues with recycled NMMO resulted in 55% reduction in methane yield.

Experimental and economical evaluation of bioconversion of forest residues to biogas using organosolv pretreatment.

The methane potential of forest residues was compared after applying organic solvent, i.e., acetic acid, ethanol, and methanol pretreatments using batch anaerobic digestion (AD). The pretreatments were performed at 190 °C with 50% (V/V) organic solvent for 60 min. The accumulated methane yields after 40 days of AD from pretreated forest residues were between 0.23 and 0.34 m(3) CH4/kg VS, which shows a significant improvement compared to 0.05 m(3) CH4/kg VS, from untreated forest residues. These improvements count up to 50% increase in the methane yields from the pretreated substrates based on expected theoretical yield from carbohydrates. Among the organic solvents, pretreatments with acetic acid and ethanol led to highest methane yields, i.e., over 0.30 m(3) CH4/kg VS. However, techno-economical evaluation showed, pretreatment with methanol was more viable financially. The capital investments of the plant operating 20,000 tons of forest residues varied between 56 and 60 million USD, which could be recovered in less than 8 years of operation.

Biogas from Lignocellulosic Materials

Methane production via anaerobic digestion is a steadily growing industry in Europe and all over the world. Biomethane reduces the demand for fossil fuels, since it can be used for the production of power and heat or converted to vehicle fuel. Anaerobic digestion is a renewable energy technology; however, it can also be considered as a low-cost environmental-friendly waste management process, since it reduces the emission of greenhouse gases (GHGs), while it stabilizes the wastes. Currently, mainly the organic fraction of household waste, food waste, sewage sludge, manure, and energy crops is used for biogas production; nevertheless, there are a wide range of other organic substrates which can be utilized for biogas production. Among the organic matters, lignocellulosic materials have a great potential. Great abundance worldwide and carbohydrate-rich contents make them an attractive feedstock for biofuel production. Currently, anaerobic digestion of energy crops is widespread; however, biogas production from lignocellulosic residuals and wastes is still under investigation. This chapter focuses on anaerobic digestion of lignocellulosic materials. It explains the anaerobic digestion process and the current technologies used for crops digestion. It also summarizes the biogas potential of different lignocellulosic materials and the latest research on pretreatments to improve the methane yield. Finally, this chapter compares anaerobic digestion of lignocellulosic materials with energy production from these kinds of materials through thermochemical processes.

Dry anaerobic digestion of lignocellulosic and protein residues

Utilisation of wheat straw and wool textile waste in dry anaerobic digestion (AD) process was investigated. Dry-AD of the individual substrates as well as co-digestion of those were evaluated using different total solid (TS) contents ranging between 6 to 30%. Additionally, the effects of the addition of nutrients and cellulose- or protein-degrading enzymes on the performance of the AD process were also investigated. Dry-AD of the wheat straw resulted in methane yields of 0.081 – 0.200 Nm3CH4/kgVS with the lowest and highest values obtained at 30 and 21% TS, respectively. The addition of the cellulolytic enzymes could significantly increase the yield in the reactor containing 13% TS (0.231 Nm3CH4/kg VS). Likewise, degradation of wool textile waste was enhanced significantly at TS of 13% with the addition of the protein-degrading enzyme (0.131 Nm3CH4/kg VS). Furthermore, the co-digestion of these two substrates showed higher methane yields compared with the methane potentials calculated for the individual fractions at all the investigated TS contents due to synergetic effects and better nutritional balance.