Integrating fast pyrolysis reactor with combined heat and power plant improves environmental and energy efficiency in bio-oil production

Abstract

Abstract Biomass as a renewable energy source is an alternative to fossil fuels. Due to its lower energy density compared to fossil fuels, different ways of converting biomass into an energy-dense fuel are sought after. Fast pyrolysis of biomass into pyrolysis oil represents such method, allowing wider substitution of fossil fuels. Pyrolysis is an energy-intensive process but its efficiency can be improved by integration of pyrolysis reactor into a combined heat and power plant. In this study environmental analyses are conducted on the production of 50 000 tons of pyrolysis oil from wood to substitute for heavy fuel oil. The pyrolysis oil is produced either in a stand-alone reactor or as an integrated part of a combined heat and power plant. The combined heat and power plant is wood and peat fueled, producing of 820 GWh of energy. The cradle-to-grave emissions of CO2-eq, NOx, SO2, and PM are calculated by using two methods: 1) life cycle assessment and 2) the Tool for Sustainability Impact Assessment. Reductions in environmental loads of over 75% in (fossil) CO2-eq and over 90% in NOx, SO2, and PM can be achieved when heavy fuel oil is replaced by pyrolysis oil. The pyrolysis is 20% more efficient if integrated with CHP production. The potential reductions vary several percent between the applied methods because of different system boundaries. Nevertheless, integrating the pyrolysis reactor into a combined heat and power plant appears as the best option irrespective of the method used. Extending the system boundaries with the life cycle inventories results in significant increases of emissions in many processes; however, combustion in plants has a dominant role in overall emissions but is not subject to the boundary extensions, and thus, the totals of emissions between the methods are nearly the same.