## 3 Results
### Effect on CO Emissions during Reduced Air-to-Fuel Ratio  
Switching from sand as bed material to manganese ore should facilitate fuel conversion via the mechanisms described in Section 1.2 above, thus allowing for a reduced air-to-fuel ratio without increasing the emissions of unburnt such as CO. Summaries of the measured CO emissions during lambda-experiments can be found in Figures 5 and 6.  
Figure 4: Temperature profile over the boiler when operated on higher air-to-fuel-ratio (\(\approx\)1.17). Tbottom was in the range 855–875 \({}^{\circ}\)C and have been used as reference. For 50% Mn ore, data were for air-to-fuel ratio 1.13 since data for 1.17 are lacking.  
As was explained above, the results in Figures 5 and 6 are not totally intuitive. Two things are apparent:  
* By substituting sand with manganese ore, it was possible to achieve a substantial reduction in CO emissions compared to the sand reference. For low air to fuel ratios (>1.11), the measured effect on CO emissions varied from 70% reduction to 5% increase compared to a sand reference, depending on the degree of substitution with manganese ore. For higher air to fuel ratios the CO emissions were comparable or slightly higher. With 100% manganese ore as bed material, the emissions of CO and NO increased substantially, but in combination with secondary measure in form of sulphur feeding a reduction in CO-emissions of 60%-90% could be achieved for low air-to-fuel-ratios (>1.11).
* The positive effect is not a given. For experiments with partial substitution of sand, it can be seen that during the following days of operation (with 30% and 50% manganese ore) the CO emissions increased compared to the experiments with 10% manganese ore. For the experiments with 100% manganese ore, no positive effect is seen until active measures in form of sulphur feeding is taken. On the contrary, the CO emissions increases substantially compared to experiments with sand.  
Figure 5: Measured concentrations of CO (mg/nm\({}^{3}\), at 6% O\({}_{2}\)) at position kh2 in the convection path as a function of the air-to-fuel ratio for experiments with partial substitution of sand with manganese ore.  
Figure 6: Measured concentrations of CO (mg/nm\({}^{3}\), at 6% O\({}_{2}\)) at position kh2 in the convection path as a function of the air-to-fuel ratio for experiments with 100% substitution of sand with manganese ore.  
It is not straightforward to explain some of the phenomena from Figures 5 and 6. It is unexpected that CO emissions should decrease very considerably when adding 10% manganese ore, only for the effect to be smaller with 30%, negligible with 50% and greatly increase emissions with 100%.  
One explanation model is that the results are connected to poor removal of ash components. During normal operation about 30% of the silica sand used as bed material is replaced each day. In practice, this means that alkali metals and other ash components absorbed by the bed material is continuously removed. This regeneration procedure was not implemented during the experiments campaign and may have resulted in accumulation of ash components in the boiler, which could have had a negative impact on the final combustion stage which determines emissions. The motivation for implementing sulphur feeding the last two days of the campaign was to add an alternate mechanism for removal of potassium and check if this would have any effect, which it obviously had. This possible explanation model will be further discussed in Section 5 below.