## 3 Results  
### Operation with 100% LD-Slag  
The experiments with LD-slag worked well on a practical level. It proved easy to pneumatically transport the LD-slag to the material silo via an ordinary pressure discharge truck. Feeding fresh bed material and removing bottom ash via ash screw worked flawlessly. The pressure drop over the bed and presumably also the fluidization behaviour were similar to operation with sand. There were no tendencies to form agglomerations or other problems commonly occurring during operation of biomass boilers. However, the measured emission levels of CO especially (but also NO) were not satisfactory:  
* At startup with completely fresh LD-slag (day 2), the CO concentration was initially low. However, soon an increasing trend could be seen and after 7 h of operation the CO concentration was 100 ppm and still increasing. This is a considerably higher concentration than what would be expected for silica after a similar time of operation.
* In the next morning (day 3), the trend accelerated and the CO concentration soon exceeded 1200 ppm, which is outside the preferred measuring range of the instruments used. During operation with silica sand and wood fuel, such an increase in emissions would be expected to take place only after several days of operation without regeneration of bed material.  
It was deemed likely that the high emission levels were related to poor absorption of ash components. Typically, alkali metals and especially potassium (K) are considered the most critical ash components during combustion of biomass. Accumulation of alkali in fluidized bed boilers is well-known to create a number of difficulties, including bed sintering and deposits on super-heater tubes [21]. In smaller CFB boilers, saturation of ash components in the flue gas can also be associated with emissions. The mechanism is debatable, but one mechanism that has been suggested is that biomass char would burn in accordance with the shrinking sphere model, in which case an ash layer surrounding the char particle would make oxygen diffusion difficult [9]. It has also been reported in a study comprising experimental work and chemical modeling that alkali metals can act as a strong inhibitor of CO oxidation at atmospheric pressure and in the temperature range of 773-1373 K [22]. Due to the reactive nature of alkali metals, heterogeneous effects could also be a possibility. Addition of fresh LD-slag to the boiler resulted in a minor temporary reduction in CO emissions, which seemingly supports the hypothesis that LD-slag absorbs ash components that influence the combustion result, but only at a low level. This topic will be further elaborated below.  
Chalmers research boiler is a commercial production unit and sustained operation with CO emission levels above 1000 ppm were not acceptable to the plant operators (Akademiska Hus AB). Also, it seems reasonable to believe that the use of LD-slag as an alternative bed material in commercial boilers would not be favored if the emissions would increase in comparison to the use of silica sand. Therefore, it was decided to examine three potentially viable ways to achieve improved operational conditions with respect to emissions: (i) addition of elemental sulphur directly to the boiler; (ii) addition of aqueous ammonium sulphate to the cyclone outlet; or (iii) mixing LD-slag with silica sand.