Effect of kaolin addition on ash characteristics of palm empty fruit bunch (EFB) upon combustion

Palm empty fruit bunch (EFB), a by-product of the palm oil industry, is being recognized as one of the most potential kinds of biomass for energy production in Thailand. However, it has been reported that, in combusting EFB in boilers, some compounds evolving from abundant alkali metals in EFB into gas-phase condense and deposit on low-temperature surfaces of heat exchange equipment, causing fouling and corrosion problems. To come up with a solution to impede the deposition, kaolin, which is abundant in kaolinite (Al2Si2O5(OH)4), is employed to capture the alkali metal vapours eluding from the combustion region. The experiments were designed to simulate the combustion situations that may take place when kaolin is utilized in two different approaches: premixing of kaolin with EFB prior to combustion and gas-phase reaction of volatiles from EFB with kaolin. The amounts of kaolin used were 8% and 16% by weight based on dry weight of EFB, which were equivalent to one and two times of the theoretical kaolin requirement to capture all potassium originally present in the EFB. The furnace temperatures used for EFB combustion were 700-900 °C and ashes were analyzed by XRF and XRD. The results revealed that, under the kaolin premixing condition, 8% kaolin addition was sufficient to capture the potassium compounds at low temperature, i.e. 700 and 800 °C. However, when the temperature was increased to 900 °C, 16% kaolin addition was needed to completely capture the potassium compounds. The results from gas-phase experiments showed that kaolin can capture volatile potassium at maximum 25% at 900 °C. The XRD results showed, for both experimental cases, the evidence of formation of the high melting temperature potassium-alumino-silicates, which confirmed the reaction of potassium compounds with kaolin. The study also suggests that the premixing method is better than the other because of its higher overall capture efficiency.