Investigation of rice-straw-ash fouling/slagging and countermeasures using supplementary additives and co-firing with Si–Al-rich coal in a pilot-scale grate-fired combustor

Abstract

This study aimed to investigate the alkali-related problems arising from firing a Thai rice-straw/rice-husk mixture in a pilot-scale grate-fired combustor (150 kWth) and to evaluate different countermeasures in order to mitigate these problems, i.e., addition of kaolin, co-firing with anthracite, and the synergistic effect of combining these measures. Chemical fractionation analysis was used as a preliminary indication of the propensity for interaction between the ash-forming elements in order to predict the slagging and fouling tendencies of the fuels. The results agreed well with those of the combustion experiments and the morphological and microstructural analyses of the bottom-ash and deposit samples via SEM-EDX and elemental mapping. Firing pure biofuels was found not to provoke sintering/slagging but instead resulted in severe fouling. The high deposition flux and significant impediment to the heat transfer of the deposit probe were attributed to the accumulation of condensing gas KCl and the impact of Si-based particles on the probe, as well as the K2SO4 aerosols originating from gas-phase sulfation. All the countermeasures applied showed a promising ability to reduce the deposit formation and prevent the heat transfer from deteriorating, particularly with kaolin or anthracite, or both, at stoichiometric amounts. The success of these countermeasures can be credited to the incorporation of the K-containing species released from biomass into the kaolin and anthracite minerals (a byproduct from the crushing process), as well as to the formation of high-temperature-melting K–Al- silicates. Another topic of interest was the exploitation of anthracite for any option, which could completely extinguish Cl in the deposits and simultaneously generate K2SO4, which may help to reduce chlorine corrosion.