Optimisation of efficiency and emissions in pellet burners

There is a trade-off between the emissions of nitrogen oxides (NOx) and of unburnt hydrocarbons and carbon monoxide (OGC and CO). Decreasing the excess air results in lower NOx emission but also increased emission of unburnt. The efficiency increases, as the excess air is decreased until the losses due to incomplete combustion become too high. The often-high NOx emission in today’s pellet burners can be significantly reduced using well-known techniques such as air staging. The development of different chemical sensors is very intensive and recently sensors for CO and OGC have been introduced on the market. These sensors may, together with a Lambda sensor, provide efficient control for optimal performance with respect to emissions and efficiency. In this paper, results from an experimental parameter study in a modified commercial burner, followed by Chemkin simulations with relevant input data and experiments in a laboratory reactor and in a prototype burner, are summarised. Critical parameters for minimisation of NOx emission from pellet burners are investigated in some detail. Also, results from tests of a new sensor for unburnt are reported. In conclusion, relatively simple design modifications can significantly decrease NOx emission from today’s pellet burners.