Browsing by Author "Laukkanen, Timo P., Dr., Aalto University, Department of Mechanical Engineering, Finland"

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Improving Municipal CHP Production Efficiency by Integrating Biomass Upgrading
(2016) Kohl, Thomas
School of Engineering | Doctoral dissertation (article-based)
Reducing CO2 emissions in order to abate climate change is one of the biggest challenges currently facing societies. The efficient and sustainable use of biomass conversion technologies for energy purposes is one important means of mitigating CO2 emissions while simultaneously reducing the need to import energy. However, compared to fossil fuels, biomass conversion per se suffers from lower energetic conversion efficiencies. New well-designed, deeply heat and material-integrated processes can help to overcome this obstacle. As an abundant but widely scattered resource, biomass should primarily be used locally or else upgraded before transport to central installations. Biomass-based, combined heat and power (CHP) production is an established and efficient technology that has proved it can compete with the conventional methods of energy generation under current tax regimes. However, increased use of biomass raises the price for limited biomass feedstock, and, in the future, it might hamper the affordability of municipal biomass-fired CHP plants. In order to react to the expected increase in competition for restricted biomass resources, communal CHP plants should be integrated with biomass upgrading processes that add valuable products to the portfolio. The objective of the thesis is to investigate the influence of integrating three different biomass upgrading processes, pelletising, torrefaction and fast pyrolysis, with a municipal CHP plant. In particular, their influence on important operational parameters, the energetic and environmental performance of the plant as well as the plant's finances, should be taken into account. With respect to the latter aspects of system's performance, a concise and significant assessment methodology that makes it easy to compare the processes yielding multiple products is developed and assessed in this study. The study shows that all three integration options are quite possible within the operational limits of the CHP plant. By utilising free boiler capacity during times of low district heating demand, high bio-product yields and greater district heating output can be realised, which in turn leads to improved primary energy (PE) efficiency and reduced CO2 emissions. Despite its lower energetic and exergetic efficiency, the integration of fast pyrolysis demonstrated the best economic performance. The thesis also concludes by arguing that the results obtained via thermal efficiency analysis do not add any new information to the results obtained via exergy and PE analyses. Exergy and PE analyses should be the preferred means for process assessment.