In my opinion, the BRISK2 project is an excellent opportunity for researchers in the field of thermochemical conversion of biomass to conduct experimental measurements in state-of-the-art facilities. I am an Associate Professor in the Universidad Carlos III de Madrid (Spain), department of Thermal Engineering and Fluid Mechanics. My research is focussed on the application of the fluidized bed technology to build reactors capable of holding thermochemical conversion reactions of biomass. After checking the facilities offered by the partners of the BRISK2 project, I decided to contact VTT, where a fluidized bed reactor was available to conduct measurements of biomass combustion, monitoring the pollutant emissions associated to the process. 

When I arrived at VTT in Jyväskylä (Finland), everything was ready to start with the experimental measurements from the very first day. I have always been working together with my hosts Juho Kauppinen, Teemu Soini and Toni Pikkarainen during my 7 days research stay at VTT. Their enthusiastic experimental work and their complete availability for discussion of the results made easy the usually arduous work in the laboratory.  As a consequence of the hard work of all the team at VTT, we could run a total of 23 different combustion tests in only 7 working days. We could even increase the number of tests programmed before the stay thank to some new ideas that emerged while discussing the results obtained during the first few days. 

The combustion tests at VTT were conducted using two feedstocks: Cynara cardunculus L. (a lignocellulosic energy crop) and previously dried sewage sludge. For each feedstock, three different particles where used to conform the bed, namely sepiolite, silica sand, and braunite. These particles differ in density, obtaining a lighter bed for the sepiolite particles and a heavier bed for the braunite. Therefore, the biomass particles suffer descending buoyancy forces in the sepiolite bed, whereas in the braunite bed, they are subjected to ascending buoyancy forces. Moreover, the braunite particles are oxygen carriers, which can also affect the biomass combustion process. For each feedstock and bed material, two different combustion temperatures and gas velocities were tested. 

During the tests, some agglomerates were formed when burning Cynara cardunculus L. at the higher temperature, which may be attributed to the alkali content of cardoon, which contributes to reduce the melting temperature of the ash generated after combustion. In contrast, no agglomeration problems were detected while burning sewage sludge, however, mass accumulation in the bed was perceived by an increase of the bed pressure drop, due to the high ash content of sewage sludge. The comparison of the results obtained for different bed materials showed an improvement of the combustion, i.e., a reduction of the CO emission, in the sepiolite bed, where the combustion of biomass occurs inside the dense bed due to buoyancy forces, increasing the residence time of the exhaust gas in the reactor. An effect of the oxygen carrier capability of braunite was also noticed in the tests conducted for high air velocities.