Large industrial boilers continue to grow in size. In the largest ones, the measurement systems no longer reach the inner parts of the combustion chambers. In addition, new measurement solutions are needed due to tightened regulations, new technology and use of renewable energy sources, such as biofuels. To measure in the inner parts of combustion chambers and to find new measurement methods, sensor ball propagating inside the combustion chamber was invented and patented. This research focused on the challenges in the implementation of such a sensor ball.

There are many technological issues to be solved before final use of the sensor ball. They concern communication possibilities inside the combustion chamber, the hovering of the sensor ball in different types of combustion, and the operating time of sensor balls in harsh environments such as combustion chambers and flames.

The preliminary research done and documented in this thesis concerns the following questions: What is the attenuation of radio and microwave signals inside the combustion chamber? What kind of electromagnetic noise is present in the chamber? What could be the operation time of a sensor in flames and what are the conditions for the hovering and propagation of the sensor ball inside the combustion chamber? Important issues such as receiving antennas, positioning of the sensor ball and feasible measurement solutions, which, can be interlinked with this kind of sensor, are mainly excluded from the study due to a lack of time and a need to limit the topic area.

The investigations done by calculations, modelling, simulations, and practical tests, yielded a number of results. First, the operation time of sensors balls in the boilers can be minutes. However, if the enclosure of the sensor ball is made from very good thermal insulation and the dissipation power of sensor electronics is not limited, the sensor can be destroyed more likely due to self-heating than an external high temperature. In addition, wires through the enclosure to sensor electronics can dramatically shorten the operation time of a thermally well-protected sensor. The hovering and free propagation of the sensor ball is possible in CFB boilers. If an operation time of about tens of seconds can be seen adequate, the sensor ball can also hover in many other types of boilers having adequate upward flows and suspension densities. Further, it was stated that communication is possible in the combustion environments, but demands special solutions. The main limitations to the communication are noise for wideband communication systems and attenuation for all wireless systems. It was theoretically observed that at least in the combustion area of a kraft recovery boiler there exists weak plasma due to strong ionization originating obviously from alkaline.

As a preliminary result, it was seen that due to the effects of flames and ionization on the radio and microwave signals, the positioning methods and techniques, which are based on flight times and signal strength indications, do not suit the mobile sensor operating in the combustion area. In the combustion area, the sensor must position itself. The selfpositioning solution is one of most important future research topics in parallel to e.g. the final communication solution and operating time extension.