Chemical looping combustion of biomass with negative co2 emissions

Resumen

Chemical Looping Combustion of biomass with negative CO2 emissions Abstract CO2 emissions to the atmosphere became an important environmental problem because of the effect on the global warming and consequently, the climate change. The climate change challenge demands a commitment of combined strategies between global institutions, governments and citizens. To reach the objectives set in the Paris Agreement (2015), greenhouse gas emissions need to be reduced. No technology is currently able to achieve the necessary reductions in greenhouse gas emissions on its own. Biomass represents an interesting alternative fuel for heat and power production as a carbon dioxide-neutral fuel. Moreover, if the CO2 generated during biomass combustion process is captured then negative-CO2 emissions would be reached and these are named bioenergy with Carbon Capture and Storage (BECCS) technologies. Chemical Looping Combustion (CLC) came up as one of the most promising CO2 capture technologies thanks to the low energy penalty of the CO2 separation and therefore, low-cost. CLC technologies are based in two interconnected fluidized bed reactors without gas mixing. The combustion takes place in the fuel reactor where the oxygen is supplied by a solid oxygen carrier, normally metal oxides. After being reduced, the oxygen carrier goes to the air reactor without gas mixing between reactors where it is oxidized again in air, and it is able for a new redox cycle starts. The main objective of this thesis is to evaluate the biomass combustion with CLC technologies (bioCLC). The research plan covers studies different at different scales, starting at lab scale, through a 1 kWth pilot plant, to the continuous operation in a 20 kWth plant to demonstrate the viability and optimizing the operation range for different low-cost oxygen carriers and different types of biomass residues. Iron based ores as well as manganese ores has been pointed as promising oxygen carriers because of their reactivity and low cost. Under In Situ Gasification Chemical Looping Combustion (iG-CLC) mode, the performance of the process was studied focusing on the effect of different important operating variables in a 1 kWth CLC unit. Values about 100% of carbon capture efficiency with the three types of biomass were obtained using ores as oxygen carrier and the total oxygen demand was reduced because of the different reactivity of each oxygen carrier and the operating conditions optimization. Also a synthetic iron-manganese mixed able to release molecular oxygen during the CLC process was used. The effect of the several operating variables was also studied finding a high dependency on the air excess and the temperature, both in the air reactor, on the CO2 capture efficiency and the total oxygen demand. About 100% of CO2 capture efficiency achieving values of total oxygen demand about 5%. The use of a mixed Cu-Mn oxide was also studied as a Chemical Looping with Oxygen Uncoupling (CLOU) oxygen carrier. High CO2 capture efficiencies were obtained and close to null total oxygen demand was achieved. The effect of the operating conditions on the performance of CLC was evaluated in a 20 kWth CLC unit using an iron ore as oxygen carrier. A Spanish biomass residue was tested achieving values close to 100% and total oxygen demands between 15-20% despite the low temperature used, supporting the consideration of the CLC process with biomass (bioCLC) as a promising bioenergy with Carbon Capture (BECCS) technology. One of the aspects to be considered in the combustion of biomass is the formation of NOx and the possible existence of tar in the gaseous product stream. This work compares the results obtained with two different chemical looping combustion modes, iG-CLC and CLOU. All these results show the feasibility of the integration of solid biofuels with CLC technologies achieving a negative emission technology able to produce an energy gain through a BECCS technology.