Combustion improvements of upgraded biomass by washing and torrefaction

Different streams of low-grade biomass of different origins (road side grass, miscanthus, wheat straw and spruce bark) were upgraded to reasonable quality commodity fuels by significantly reducing the alkali and chlorine content, through washing with water in a first step and torrefaction in a second. The aim was to produce a cleaner fuel with a higher energy content for further densification. The final goal was to demonstrate the improvements attained by the upgrading process during combustion using the lab-scale combustion simulator facility of ECN part of TNO. Analyses of the original, washed and washed/torrefied samples showed that approximately 90% of Cl and up to 60-80% of K can be removed by this upgrading route; during the torrefaction step Cl and S are removed from the solids; during torrefaction K and Na are not significantly altered; the pre-wash step is crucial for the removal of K. Si and Ca remain largely in the solid fraction; post-wash seems to be a viable route to upgrade dry-type of biomasses; "straw" like material requires more time for washing than "grass" type. The combustion results showed that: NOx emissions are generally decreased after upgrading; High temperature chlorine corrosion can be effectively mitigated; Fine particulate matter (submicron/aerosols) formation is strongly reduced, effectively reducing the risk of alkali induced fouling. However, slagging tests revealed that pre-washing and torrefaction has minor impact on the slagging propensity of the fuel. Only, slightly reduced slagging is observable and therefore the use of mineral combustion additives, or smart blending with other fuels in order to further mitigate the slagging risks is strongly recommended.