Energy from biomass - Lessons from European boilers

The steam temperature of biofuel boilers is limited by high temperature corrosion of superheater alloys in the ash deposit/flue gas environment. Advanced European biomass boilers combine design modifications, process changes and corrosion-resistant alloys to achieve substantially higher steam temperatures and efficiencies than U.S. biomass boilers. This review of design modifications and process changes is part of a US DOE-funded study of opportunities to increase heat recovery from renewable fuels. The role of advanced alloys in increasing superheater temperatures will be reviewed in a subsequent paper. Design modifications to reduce superheater corrosion include adding an "empty pass" between the furnace and the superheater, installing cool tubes to trap low melting temperature chlorine deposits ahead of the superheater, heating the final superheater in the recirculated fludizing medium of a CFB boiler, operating with a slagging superheater, designing superheaters for quick replacement, raising the superheater temperature above the dew point of the most corrosive deposits and installing an external superheater fired by a less-corrosive fuel. Process changes include diluting corrosive biomaterials with less-corrosive fuels, adding high sulfur fuels to convert alkali chlorides to lower melting temperature sulfates before they reach the superheater, washing chlorides out of agricultural residues and adding chemicals that convert alkali chlorides to aluminosilicates.