Preliminary life cycle assessment of energy and carbon results of high ligno-cellulosic biomass pyrolysis coupled with anaerobic digestion

Lignin, the most abundant aromatic biopolymer on Earth, is extremely recalcitrant to degradation. Therefore, the presence of lignin in the biomass constrains and challenges the improvement of bioconversion techniques. This paper presents a preliminary life cycle assessment of an innovative combination of technologies to convert high ligno-cellulosic biomass into useful products, primary energy and fuels. The experimental process exploits residual biomasses with high ligno-cellulosic content for the production of biogas and consists on the combined use of thermo-chemical (pyrolysis) and biological (anaerobic digestion) pathways and would allow for a wider range of exploitable residual biomasses, higher yields, product diversification and other advantages for agro-energy industries and the environment. Residual biomass energy sources – such as manure and corn “stover” (cobs, leaves, etc.) or other by-products of farming or other activities – represent a low cost feedstock source that can be used for energy production. Moreover, the employment of residual biomass might be less controversial than those of harvested biomass, which raise environmental concerns or issues related to competition with food needs. These concerns and issues have resulted in growing interests in alternative, non-edible biomass resources. Corn stover, typically composed of 35−40% cellulose, 20−25% hemicellulose, and 15−20% lignin, is very abundant agricultural residue but its raw substrate is recalcitrant to bioconversion and this represents the major obstacle to its cost-effective exploitation. The experimental processing of corn stover through a pyrolytic pre-treatment provides a digestible feed for anaerobic bacteria producing biogas. The process provides also some valuable co-products, such as biochar and digestate which can be used as soil fertilizers. The experimentation has been conducted at a laboratory scale and is divided into three steps: 1) drying of corn stover; 2) pyrolysis of the corn stover, with the production of bio-oil, gas and biochar and 3) anaerobic digestion of bio-oil and gas, with the production of methane, CO2, digestate. The functional unit was defined as 10 t of dried corn stover, the system boundaries include the three above-mentioned steps. This first assessment has focused on the energy and carbon balance of the process. Calculations have been conducted with experimental data, when available, and with data from literature, always assuming a conservative approach, in particular regarding the amount of bio-oil obtainable from the pyrolysis of corn stover, pyrolysis energy consumption, heating requirements, methane yield. Results show a ratio between spent and produced energy always higher than 1, with a mean value of 2.6 ± 1.3. The pyrolytic pre-treatment allows for an 2.5 fold increase of the carbon available for bio-methanation. As for the C balance, apart from the avoided emissions due to fossil fuels replacement, a relevant contribution is given by biochar, which appears able to store C in soils and is widely studied for this property. Results have been compared to the conventional anaerobic digestion of corn stover.