Impact of Mineral Additives on Ash Fusion Temperatures and Deposit Tenacity during Entrained-Flow Combustion of Biomass

Abstract

Biomass combustion in boilers originally designed for coal presents substantial technical challenges, notably the lower ash fusion temperature (AFT) of agricultural crops, which causes increased ash deposition since biomass ash becomes sticky at lower temperatures than coal. This study investigated the type and optimal amount of additive required to increase the AFTs of switchgrass, miscanthus, and corn stover by blending mineral additives. FactSage predicted AFTs in both natural and blended fuels. Additives high in MgO and CaO, such as lime and calcite, were effective at 1–2 w% blend rates. Experiments with a 150 kW solid-fuel furnace measured ash deposition tenacity and its correlation to modeled AFTs. A temperature-controlled deposition probe, representing the heat transfer tubes in the convective pass, was installed to collect ash deposits. Lime, coal fly ash, and cement kiln dust additives at 1 w% and 2 w% were tested with miscanthus, switchgrass, and corn stover. Deposit tenacity was evaluated with an air jet mimicking a sootblower. Deposits from pure biomass fuels were 2–3 times stronger than coal. Adding additives raised AFT, resulting in deposits that were easier to remove, with up to a 73% reduction in deposit tenacity. Adding 2 w% coal fly ash or lime during miscanthus firing resulted in deposit tenacities similar to coal. All 2 w% additives yielded deposit tenacity comparable to coal when firing switchgrass. They minimally changed the AFT of corn stover but lowered deposit tenacity. FactSage predictions of AFT generally correlated inversely with measured deposition tenacity, despite models using a purely thermodynamic approach not representing complex deposition mechanisms.