Biogas from lignocellulosic biomass: A techno-economic study of pretreatment with NMMO
Borås and its municipally owned energy company Borås Energy and Environment Ltd (BEMAB), who has played an active role in this project, plans to increase its production of biogas for renewable vehicle fuel from today’s 2 million Nm3 to 4.7 million Nm3 until 2016.
The organic materials that are currently available for biogas production are limited and new materials are needed to achieve an increased production of biogas.
Waste fractions with a high content of lignocellulosic biomass have a great potential to be utilized for biogas production, due to their globally high accessible amount. These materials, such as wood, straw, forest residues to name a few examples, however, are persistent to microbiological degradation in the biogas reactor. To solve this problem a suitable pretreatment step is required where the material is broken up, and the compact structure breaks down.
One of the biggest challenges when trying to utilize lignocellulose-rich fractions in an anaerobic digestion process is to find a cost effective way to make this type of substrate available for biodegradation. To achieve this goal it is necessary to coordinate efforts across multiple disciplines and fields.
This project focused on techno-economic evaluation of a chemical pretreatment technology using an organic solvent, NMMO (N-methylmorpholine-N-oxide). NMMO has been previously shown to be effective in dissolving cellulose, reducing crystallinity and thereby increasing the methane yield during the subsequent digestion. Two lignocellulose-rich fractions with different composition and character were examined. The results showed that treatment with NMMO doubles the methane yield from forest residues and increases the methane yield from straw by 50 percent. The effects of the pre-treatment on the structure and composition of these materials leading to increased biogas production were also investigated. Finally, an economic evaluation was performed, where a biogas production including NMMO-pretreatment of forest residues has been designed and scaled up to industrial scale and the financial potential of this process was determined.
The study was limited to evaluating treatment effects in laboratory scale by measuring biomethane potential during the subsequent batch digestion assays. Process design and economic evaluation were then performed for a co-digestion process in which NMMO-pretreated forest residues were utilized as one of the substrates. The evaluation was performed using an advanced process simulator program, Intelligen SuperPro Designer.
Both the experimental results and the energy and economic calculations showed that the critical step in the process is the washing and filtration step after the treatment. It is therefore important to separate the NMMO carefully after the treatment, since as small amounts of NMMO residues as concentrations greater than 0.002 percent were found to inhibit the subsequent digestion step.
On the other hand, it is also important to operate the washing step with as small amounts of water as possible to reduce the energy demand.
In the future, it would be interesting to experimentally investigate how the co-digestion process utilizing lignocellulose-rich fractions works. How does the treated material affect other substrates? What is the optimal mixture of substrates in order to achieve a stable and robust digestion process when cellulose and lignocelluloses-rich waste fractions are used in a co-digestion process?