8 Figures, 6 Tables.-- © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Chemical looping combustion (CLC) processes combined with CO2 sequestration and sustainable management of biomass represent a promising BioEnergy with Carbon Capture and Storage (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. The advantage of the CLC technology compared to other CO2 capture technologies is that only fuel-N contributes to nitrogen oxides formation. Moreover, scarce information is available about tar formation in CLC. Thus, this work focuses on these two aspects and compares the results obtained when two different chemical looping combustion modes are used, namely In Situ Gasification Chemical Looping Combustion (iG-CLC) and Chemical Looping with Oxygen Uncoupling (CLOU). Important differences were observed depending on the combustion mode. In both cases most of the fuel-N appeared as N2 in the fuel reactor. However, in iG-CLC more than 94% of the nitrogen measured in the fuel reactor was N2 independently of the biomass used. These percentages under the CLOU mode were lower. In this case, low amounts of N2O were also detected, although it decreased to almost zero at 850 °C. In the air reactor, NO was found and its concentration remained below the legal limit for NOx emissions in power installations with all the types of biomass tested and operating modes. Tar species and concentrations found at the fuel reactor outlet stream were different under the two combustion modes. About 2.5–3.7 g/Nm3 total tar could be found at 980 °C burning under iG-CLC mode, mostly naphthalene. On the contrary, insignificant tar amounts were found in CLOU. The authors thank the Spanish Ministry of Economy and Competitiveness (MINECO) for the funding received from the projects ENE2014-56857-R and ENE2016-77982-R as well as FEDER for the financial support. A. Pérez-Astray thanks MINECO for the BES-2015-074651 pre-doctoral fellowship co-financed by the European Social Fund. I. Adánez-Rubio acknowledges MINECO and Universidad de Zaragoza (UNIZAR) for the post-doctoral grant awarded (FJCI-2015-23862). T. Mendiara thanks for the ‘‘Ramón y Cajal’’ post-doctoral contract awarded by MINECO. Peer reviewed