Life cycle inventory analysis on Bio-DME and/or Bio-MeOH products through BLUE tower process

Background, aims, and scope: In this study, we focused on the biomass di-methyl ether (Bio-DME) and the biomass methanol (Bio-MeOH) in BTL (Biomass to Liquid) fuels which might bring a solution on an energy storage and/or CO ₂ emissions abatement. For these fuels, our object is to estimate CO₂ emissions and energy intensities (the specific energy consumption in each sub-process) for the biomass liquefaction system, which is expanded to material's transportation, energy conversion and fuel transportation in detail. Materials and methods: In this paper, we estimated life cycle inventories (CO₂ emissions and energy intensities) on Bio-DME and Bio-MeOH using the bottom-up methodology. The system boundary consists of the pre-processing, the biomass liquefaction process and the fuel transportation. In order to evaluate the uncertainties in the pre-processing, the moisture content of biomass materials and the transportation distance to the plant were considered by the Monte Carlo simulation. Also, in the energy conversion process, our system is built up by gasification through the BLUE Tower (BT) process, with the liquefaction process. In addition, in the fuel transportation, we assumed that the liquefied fuel (Bio-DME/Bio-MeOH) was delivered to the end-users at a distance of 209 km round trip. Results: First, the assumptions for estimation of CO₂ emissions and energy intensities in the pre-processing were as follows: The biomass materials are the waste products from Japanese Cedar. Also, the uncertainties of moisture content and transportation distance are assumed to be 20 to 50 wt.% and 5 to 50 km, respectively. In the case that the reaction temperature and the pressure in the liquefaction plant were 210-290°C and 2.0-5.0 MPaA of DME/MeOH synthesis, the production rate of DME is 37,634 to 44,912 GJ/yr, and that of MeOH was 30,562 to 39,865 GJ/yr. These production rates are used as functional units of CO₂ emissions and energy intensities. Under these operational conditions, CO₂ emissions for the entire system are 42.3-64.6 g-CO₂/MJ-Fuel of Bio-DME, and 48.3-77.7 g-CO₂/MJ-Fuel of Bio-MeOH, respectively. Also, energy intensities are 1.22-1.65 MJ/MJ-Fuel of Bio-DME, and 1.42-2.03 MJ/MJ-Fuel of Bio-MeOH, respectively. Discussion: The effects on the operation conditions of temperature and pressure were as follows: In general, in the liquefaction synthesis, a good result (the liquefied fuel at a higher efficiency) is obtained as the pressure would be increased. Also, the temperature to encourage the reaction well is required to some extent. The specific CO₂ emissions would decrease as the temperature drops. Note that the minimum temperature of DME or MeOH synthesis is at least 210-220°C. Meanwhile, CO₂ emissions would decrease as the pressure increases. However, in the case of Bio-DME fuel, CO₂ emissions would be slightly worse. This implies that the production rate is not ideal, compared to the total energy consumption. In all conditions we analyzed, Bio-DME/MeOH production indicated that energy intensities were lower than 2.0 MJ/MJ-Fuel, and that there was a potential to mitigate CO₂ emissions. Conclusion: We focused on a small gasification-liquefaction plant from the perspective of biomass resources distribution. Although this circumstance might be unique to Japan, the plant scale for the biomass energy use would not be able to expand. Nowadays, the fuel of biomass to liquid (BTL) is exceedingly being counted on as a promising fuel, so as to reduce CO₂ emissions. However, when considering the collection area of biomass resources, the moisture content, the plant scale and the auxiliary power, it is implied that Bio-MeOH/DME does not always reduce CO₂ emissions on an LCA basis, compared to a conventional plant. Recommendations and perspectives: The LCA of the small scale biomass energy system we treated in our study is extremely significant to build a sustainable biomass energy system, since we presume that the utilization of biomass resources would compete with other strategies such as food production, etc. For future work, the feasibility studies on the wheel efficiency and/or the investigation of demand of DME/MeOH will be required from the perspectives of a diversity of energy sources, and/or alternative fuels use.