Biogas is nowadays getting more attention as a means for converting wastes and lignocelluloses to green fuels for cars and electricity production. The process of biogas production from N-methylmorpholine oxide (NMMO) pretreated forest residues used in a co-digestion process was economically evaluated. The co-digestion occurs together with the organic fraction of municipal solid waste (OFMSW). The process simulated the milling of the lignocelluloses, NMMO pretreatment unit, washing and filtration of the feedstock, followed by an anaerobic co-digestion, upgrading of the biogas and de-watering of the digestate. The process also took into consideration the utilization of 100,000 DW (dried weight) tons of forest residues and 200,000 DW tons of OFMSW per year. It resulted in an internal rate of return (IRR) of 24.14% prior to taxes, which might be attractive economically. The cost of the chemical NMMO treatment was regarded as the most challenging operating cost, followed by the evaporation of the washing water. Sensitivity analysis was performed on different plant size capacities, treating and digesting between 25,000 and 400,000 DW tons forest residues per year. It shows that the minimum plant capacity of 50,000 DW tons forest residues per year is financially viable. Moreover, different co-digestion scenarios were evaluated. The co-digestion of forest residues together with sewage sludge instead of OFMSW, and the digestion of forest residues only were shown to be non-feasible solutions with too low IRR. Furthermore, biogas production from forest residues was compared with the energy produced during combustion.

Biogas has been identified as one of the most cost - effective renewable fuels. In order to increase biogas production, yields from traditionally substrates either need to be improved or other alternative substrates must be made available for anaerobic digestion. Cellulose and lignocellulose rich wastes are available in large amounts and have great potential to be utilized for biogas production. This project focused on the opti mization of the pretreatment conditions when using the organic solvent N - methylmorpholine - N - oxide (NMMO) to enhance the methane yield from forest residues and straw. It also focused on a techno - economic evaluation of this pre - treatment technology. NMMO has previously been shown to be effective in dissolving cellulose and, as a consequence, in increasing the methane yield during the subsequent digestion. The goal of this project was to develop a technology that increases energy production from domestic subst rates in a cost - effective and environmentally friendly way. The treatment works well at lower temperatures (90°C), which means that water from the district heating system can advantageously be used in the treatment. The results showed that treatment with NMMO at 90°C doubles the methane yield from forest residues and increases the methane yield from straw by 50 %. For the techno - economic evaluation, the base case was assumed to be a facility with a capacity of 100 000 tones forest residues/year. After a w ashing and filtration step, the treated material will be utilized in a co - digestion process where 33% of the incoming material consists of forest residues and the rest is source - sorted household waste. The scale - up, process design, simulation and calculati ons were made using the software tool Intelligen SuperPro Design ® . The total investment costs were calculated to be about 145 million €, when forest residues or straw are to be used as raw material. Costs for operation (i.e. raw materials, energy, waste ma nagement, maintenance and personnel costs) were set against the incomes from the products (i.e. methane, carbon dioxide and the lignin - rich digested residue) to see if the process was profitable. The internal return rate (IRR), a parameter that indicates w hether a process is profitable or not, indicated that evaluated processes with capacities over 50 000 tons forest residues/year are profitable. However, co - digestion of forest residues with sewage sludge instead of household waste was not profitable. Both the laboratory results and the energy and economic calculations showed that the washing and filtration step is critical for the proposed process. The energy balance calculation resulted in an EROI value of 0.5, which means that the produced methane from fo rest residues counted up only the half of the energy needed for the treatment as well as NMMO separation and recycling. It is important to separate the NMMO well after the treatment, since remaining NMMO at concentrations higher than 0.002% were found to i nhibit the subsequent digestion step. Also it was showed out to be important that the washing step operates with small amounts of water to save energy within the NMMO recovery. A rotary vacuum filtration is therefore recommended for the washing and filtrat ion step, and a mechanical vapor design is recommended for the evaporation, saving up to 70 - 90% energy compared to a conventional design. Treatment of straw with recycled instead of fresh NMMO has also been tested and equal amounts of methane were obtain ed. After a well - functioning washing and filtration step, NMMO could not be detected in the digestate residue.

Biogas production from organic materials can be used as a renewable vehicle fuel, provide heat and generate electricity and can thereby reduce the greenhouse gas emissions. This thesis focuses on the biogas production based on lignocelluloses. There is an abundant availability of lignocelluloses, constituting 50% of the total biomass worldwide. However, the biomass recalcitrance limits the microbial degradation as well as the biogas production from these types of materials. In the present work different pretreatment methods have been performed in order to decrease the biomass recalcitrance and improve the biogas production. Steam explosion pretreatment, together with the addition of sodium hydroxide and hydrogen peroxide, has been performed on lignocellulosic-rich paper tube residuals. The pretreatment has resulted in methane yields of up to 493 NmL/gVS, which is an increase by 107% compared with untreated material. Furthermore, the use of an organic solvent, N-methylmorpholine-N-oxide (NMMO), was evaluated as a pretreatment method for spruce (both chips and milled), rice straw, and triticale straw. The NMMO pretreatment resulted in 202, 395, 328, and 362 NmL CH4/g carbohydrates produced of these substrates, respectively, corresponding to an increase of between 400-1,200% compared with the untreated version of the same material. Moreover, the paper tube residuals have been co-digested with an unstable nitrogen-rich substrate mixture, mainly based on municipal solid waste. The addition of the lignocellulosic-rich paper tubes in a co-digestion process showed stabilizing effects and prevented the accumulation of volatile fatty acids with a subsequent reactor failure. Additionally, synergistic effects have been found leading to between 15-33% higher methane yields when paper tubes were added to the co-digestion process compared with the yields calculated from the methane potentials of the two substrates. Substrate characterization analysis can be used to study the changes on the lignocellulosic components after the pretreatment, relating the changes to the performance in the anaerobic digestion. Increased accessible surface area, measured by the Simons’ stain and the enzymatic adsorption methods, as well as decreased crystallinity, determined by using the Fourier Transform Infrared Spectroscopy, can all be linked to improved biogas production after pretreatment. Finally, the NMMO pretreatment on forest residues has been financially evaluated for an industrial scale process design. The base case that was evaluated simulated a case where pretreated forest residues were co-digested with the organic fraction of municipal solid waste to obtain optimal nutritional balance for the anaerobic digestion. This process has been found to be economically feasible with an internal rate of return of 20.7%.

This study deals with the addition of paper tube residuals to a nitrogen rich mixture of organic waste obtained from industrial and municipal activities. This nitrogen rich mixture, called buffer tank substrate (BTS) in the following text, is utilized in a large scale biogas plant. The effects were investigated in semi-continuous co-digestion processes and variations in operational conditions were studied. The addition of paper tubes had stabilizing effects and prevented the failure of the process and made it possible to decrease the hydraulic retention time from 25 to 20 days. Furthermore, synergetic effects were found, with 15-34% higher methane yields, when paper tubes were co-digested with BTS. Moreover, steam explosion pretreatment of the paper tube waste with the addition of 0-2% NaOH were evaluated by batch digestion experiments. Increasing the NaOH concentrations used in the pretreatment resulted in increasing methane yields, with the highest of 403 Nml/gVS methane production corresponding to an increase by 50% compared to that when untreated paper was digested (268 Nml/gVS). The long-term effects of this best pretreatment were further investigated by continuous co-digestion experiments leading to higher methane yield when pretreated paper tubes were utilized in the co-digestion process compared to untreated.

Softwoodspruce (chips and milled), ricestraw and triticale (a hybrid of rye and wheat) straw, were pretreated with N-methylmorpholine-N-oxide (NMMO or NMO) prior to anaerobic digestion to produce biogas. The pretreatments were performed at 130 °C for 1–15 h, and the digestions continued for six weeks. The digestions of untreated chips (10 mm) and milled (<1 mm) spruce, ricestraw and triticalestraw resulted in 11, 66, 22 and 30 Nml CH4/g raw material. However, the pretreatments have improved these methane yields by 400–1200%. The best digestion results of the pretreated chips and milled spruce, ricestraw and triticalestraw were 125, 245, 157 and 203 Nml CH4/g raw material (or 202, 395, 328 and 362 Nml CH4/g carbohydrates) respectively, which correspond to 49, 95, 79 and 87% of the theoretical yield of 415 Nml CH4/g carbohydrates. Although the experiments were carried out for six weeks, one and a half weeks was enough to digest the materials.

Different substrate characteristic analyses have been studied on rice and triticale straw pretreated with NMMO (N-methylmorpholine-N-oxide) prior to biogas production. Simons’ stain, water retention value (WRV), and enzymatic adsorption were used to measure the change in the accessible surface area of the lignocellulosic substrates. FTIR was used to measure the change in cellulosic crystallinity and Time-of-Flight-Secondary-Ion-Spectroscopy (ToF-SIMS) to measure the ratio of cellulose to lignin on the sample surface. All methods showed increased accessible surface area and a decrease in crystallinity after the pretreatments. These qualities were linked to improved biogas production. In the future, the tested methods could replace the time-consuming methane potential analysis to predict the methane production of lignocellulosic materials. Simons’ stain, enzymatic adsorption, and crystallinity measurement by FTIR can be regarded as the recommended methods for the prediction of the improved biogas production as a result of the pretreatment.

Paper tube residuals, which are lignocellulosic wastes, have been studied as substrate for biogas (methane) production. Steam explosion and nonexplosive hydrothermal pretreatment, in combination with sodium hydroxide and/or hydrogen peroxide, have been used to improve the biogas production. The treatment conditions of temperature, time and addition of NaOH and H2O2 were statistically evaluated for methane production. Explosive pretreatment was more successful than the nonexplosive method, and gave the best results at 220 °C, 10 min, with addition of both 2% NaOH and 2% H2O2. Digestion of the pretreated materials at these conditions yielded 493 N ml/g VS methane which was 107% more than the untreated materials. In addition, the initial digestion rate was improved by 132% compared to the untreated samples. The addition of NaOH was, besides the explosion effect, the most important factor to improve the biogas production.